Fluorescence sensing systems for gold and silver species

Cite this:DOI:10.1039/c4cs00328d

Fluorescence sensing systems for gold and silver species

Subhankar Singha,?Dokyoung Kim,?Hyewon Seo,Seo Won Cho and Kyo Han Ahn*

Besides the noble physical appearance of gold and silver,their novel chemical properties attracted the modern technology for various industrial,chemical and biological uses including medical applications.The widespread use of gold and silver,however,can cause potential hazards to our environment.Therefore,suitable detection methods are a prerequisite for the evaluation of their harmful e?ects as well as for studying their beneficial biological properties.Due to the several advantages over the

conventional analytical methods,the fluorescence detection of gold and silver has become an active research area in recent years.In this review,we provide an overview of the reported fluorescent detection systems for gold and silver species,and discuss their sensing properties with promising features.The future scope of developments in this field of research is also mentioned.

1.Introduction

Noble metals that are resistant to corrosion and oxidation in moist air include gold,silver,ruthenium,rhodium,osmium,iridium,platinum and palladium.Among those,gold and silver

have fascinated mankind since the earliest days of technological innovation,due to their lustrous appearance,malleability and noble characters,being used in currency coins,jewelry and ornaments,high-value tableware and utensils.Besides the physical appearance,gold and silver possess several unique chemical properties,which attracted the recent attention of their use for various purposes including chemical and bio-logical applications.The widespread use of gold and silver,however,can cause adverse e?ects to the environment as well as in biological systems.Accordingly,suitable detection methods are required for evaluation of the adverse e?ects as well as

for

Department of Chemistry and Center for Electro-Photo Behaviours in Advanced Molecular Systems,Pohang University of Science and Technology (POSTECH),77Cheongam-Ro,Nam-Gu,Pohang,790-784,Republic of Korea.E-mail:ahn@postech.ac.kr;Fax:+

82-54-279-5877Subhankar Singha

Dr Subhankar Singha received his BS in 2005from Midnapore College,and MS in 2007from Indian Institute of Technology (IIT),Kharagpur,India.He obtained his PhD in Organic Chemistry from POSTECH,Republic of Korea,in 2013under the supervision of Professor Kyo Han Ahn on the topic of ‘‘Studies on donor–accep-tor type fluorophores and two-photon probes for bioimaging application’’.He is now working

in the same group as a post-doctoral researcher.His research interest is focused on the development of two-photon probes for small molecules associated with signal

transductions.

Dokyoung Kim

Dr Dokyoung Kim received his BS from the Department of Chemistry at Soongsil University in 2006and his PhD in Organic Chemistry from POSTECH in 2014under the supervision of Professor Kyo Han Ahn on the topic of ‘‘Development of two-photon absorbing materials and fluorescent probes for bio-imaging’’.He is now working in the University of California,San Diego under the supervision of Professor Michael J.Sailor

(2015-present),as a post-doctoral researcher.His research interest is focused on the development of two-photon absorbing materials and organic–inorganic hybrid materials for investigating diseased-associated biological processes.

?Contributed equally to this work.

Received 30th September 2014DOI:10.1039/c4cs00328d

https://www.wendangku.net/doc/9599574.html,/csr

Chem Soc Rev

P u b l i s h e d o n 27 J a n u a r y 2015. D o w n l o a d e d b y N a n k a i U n i v e r s i t y o n 30/01/2015 10:40:47.

investigating their beneficial biological e?ects.The conventional analytical methods for gold and silver species are mostly useful for ex vivo analysis but pose certain limitations for in vivo analysis.For the latter purpose,small-molecular fluorescence sensing systems o?er tools of choice.

In this review,we briefly introduced chemical properties and biological utilities of gold and silver species separately, followed by a short discussion on the conventional detection systems and the advantages of the fluorescence detection methods.The main part of this review is focused on the reported fluorescence detection systems for gold and silver species,which are discussed with their sensing properties. Some unique approaches of colorimetric detection systems are also included.

2.Gold

2.1.Gold species

2.1.1.Chemical properties.Gold(Au)is a group11transi-tion metal in the periodic table,with the atomic number79.Chemically,gold is one of the least reactive chemical elements and can possess several oxidation states:commonly0,+1,and +3and rarelyà1,+2,+4,and+5states.1

Au(I)complexes have the electron configuration of[Xe]4f145d10 and usually form linear compounds with sp hybridization at gold. For example,AuCl,a commercially available source of Au(I),exists as a polymeric chain with m-Cl ligands.AuCl is stable to air and moisture,but can undergo reduction to Au(0)slowly along with formation of Au III2Cl6.Au(I)shows soft metal ion character,which prefers the soft donor atom such as sulfur and phosphine over hard atoms such as nitrogen and oxygen.2,3Au(III)complexes have electron configuration of[Xe]4f14d8and usually show square planar structures with four ligands.Au(III)is a hard metal ion and thus favors a hard donor atom such as nitrogen and oxygen, in contrast to Au(I).AuCl3,a commercially available source of Au(III),exists as a dimer(Au2Cl6)with m-Cl ligands.The cheapest commercial source of Au(III)is MAuCl4á2H2O(M=Na or K),which can be prepared from chlorauric acid(HAuCl4),which,in turn,is prepared from metallic gold by oxidizing with chlorine or with aqua regia.4

Additionally,gold has a nuclear spin of3/2,but because of a very low sensitivity and a quadrupole moment,only a few79Au spectra in a highly symmetric environment have been reported. However,the diamagnetic character of both Au(I)and Au(III) allows the monitoring of catalysis reactions by NMR.5 Gold and its complexes are fascinating metal species widely used in catalysis,surface chemistry,materials and theoretical investigations.Au shows the strongest relativistic e?ect among related transition metal elements.6,7Thus,Au has6s orbitals contracted whereas5d orbitals expanded.Both Au(III)and cationic Au(I)species hence show superior Lewis acidity,in particular toward alkynes,activating them toward nucleophilic addition. The strong alkynophilicity of gold complexes has been explored in various chemical transformations.Since the seminal report by Teles et al.in1998that cationic Au(I)–phosphine

complexes Seo Won Cho

Seo Won Cho received her BS in

2013from the Department of

Chemistry at Chonnam National

University.She is currently

pursuing her PhD study at the

Department of Chemistry,

POSTECH,under the supervision

of Professor Kyo Han Ahn.She is

working on the development of

two-photon absorbing materials

and their applications in bio-

imaging.

Kyo Han Ahn

Professor Kyo Han Ahn received

his BS from Seoul National

University in1980and his PhD

in Organic Chemistry from KAIST

in1985.After working for the

Yuhan pharmaceutical company

shortly,he moved to the

Department of Chemistry at

POSTECH in1986.He worked

with Professor Kyriacos C.

Nicolaou at the University of

Pennsylvania(1988),Professor

Elias J.Corey at Harvard

University(1995),and Professor

Michael J.Sailor at the University of California at San Diego

(2002),as visiting scholar during his sabbatical leaves.His

current research interests include luminescent materials,

molecular probes and imaging agents,and

bioconjugation.

Hyewon Seo

Dr Hyewon Seo received her BS

from the Department of

Chemistry at Kyung Hee

University in2009and her PhD

in Organic Chemistry from

POSTECH in2014under the

supervision of Professor Kyo Han

Ahn on the topic of‘‘Development

of reaction-based fluorescent

probes for gold ions’’.She is

now working in Samsung SDI as

a researcher.

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catalyzed hydration of alkynes,8a‘‘gold-rush’’has begun in homogenous catalysis,leading to discovery of many types of new chemical transformations.9–11

Along with the gold-rush in searching for chemical conver-sions catalyzed or promoted by gold species,gold nanoparticles below5nm size or so have been exploited for the chemical reactions on the surface.12Furthermore,gold nanoparticles (AuNPs)of r100nm size have received tremendous attention in various fields including chemistry,biology,and clinical chemistry,for their advantageous chemical and photophysical properties given by the large surface-to-volume ratio,high stabi-lity in the light or biological media,and tunable photophysical properties that can be readily tuned by surface modifications.13

2.1.2.Biological utilities.Au(I)complexes have long been used to treat rheumatoid arthritis.14They are known to inhibit

protein tyrosine phosphatases(PTPs),presumably through reversible binding to the active site cysteine residue.15Soft Au(I)complexes readily coordinate to soft sulfur and phosphorous donor atoms.Gold–thiol complexes such as solganol,auranofin and sanocrysin are representative gold-based drugs for the treat-ment of several diseases including asthma,malaria,HIV,and brain lesions(Fig.1).16–18

Au(III)is isoelectronic with platinum(II)and forms square planar complexes in general.After the discovery of cisplatin,platinum anticancer complexes have been extensively investigated,leading to several anticancer drugs.19,20Research activity on other organo-metallic therapeutic agents has led to investigation of gold com-plexes as anticancer agents.Despite the interesting medicinal properties of gold ions,their soluble salts such as gold chloride are known to cause damages to the liver,kidneys,and the peripheral nervous systems.Some Au(III)complexes are known to tightly bind certain enzymes,DNA or other biomolecules,distur-bing a variety of cellular processes.21–24

Recently,Au(I)species coupled with organic fluorophores also have been utilized for bioimaging purpose to find out potential drug candidates.In general,Au(I)complexes have broad physiologically therapeutic value,25whereas Au(III)com-plexes show toxic e?ects to the biosystems.26In the reducing cellular environment,Au(III)complexes are expected to produce Au(I)and metallic gold.

Due to the widespread application of gold species,it is highly desirable to develop suitable detection methods espe-cially those that are applicable in biological systems to monitor the gold mediated physiological processes.

2.1.

3.Optical properties.Gold-coordination complexes show biological activities as well as unique optical properties. The nature of the coordinating ligands dictates the lumines-cence properties of d10Au(I)complexes.27–29Au(I)species can form monometallic and dimetallic complexes(Fig.2)with phosphine,indole-phosphine,alkynyl,pyridine,N-heterocyclic carbenes(NHCs),pyrazole,etc.30–32Some gold complexes show luminescence in the solid state as well as in aqueous solution: for example,Au(I)–triphenylphosphine tris(sulfonate)showed emissions at494nm and515nm in the solid state and in aqueous solution,respectively.33

The application of luminescence properties of Au(I)com-plexes for bioimaging purpose is limited,because these com-plexes do not possess the requisite solution state luminescence that allows bioimaging by confocal fluorescence microscopy (CFM).34To overcome this drawback,at present,the develop-ment of Au(I)complexes for bioimaging purpose is pursued by functionalization of Au(I)with fluorescent ligands,typically known organic fluorophores.35,36Ott and Pope investigated a series of linear Au(I)complexes incorporated with an ancillary phosphine,alkynyl,thiolated fluorophore,or anthraquinone (Fig.3).37–39These Au(I)species coupled with known organic fluorophores have been investigated for bioimaging applications as well as for potential abilities as drug candidates.40

2.1.4.Conventional detection methods.The conventional analytical methods for the detection of gold species mostly rely on the various instrumental techniques such as flame atomic absorption spectrometry(FAAS),graphite furnace atomic absorption spectrometry(GFAAS),41–43inductively coupled plasma atomic emission spectrometry(ICP-AES),44inductively coupled plasma mass spectrometry(ICP-MS),45,46and electro-chemical assay.47However,these traditional methods

require Fig.1Au(I)–thiolate drugs used for rheumatoid

arthritis.

Fig.2Some luminescent monometallic and homometallic Au(I)

complexes.

Fig.3Au(I)–fluorophore complexes for bioimaging application.

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complicated sample preparation and separation procedures prior to analysis,in addition to use of expensive instrumentation with well-trained operators.48Some of the methods need chromato-graphic instrumentation,which has poor quantitative reprodu-cibility especially for the determination of trace amounts of gold ions.

In this point of view,the fluorescence spectroscopy with an appropriate sensing molecule,a fluorescence probe or a chemo-sensor,would provide a highly desirable detection method for gold species.The fluorescence method is widely used for sensing metal species because of its high sensitivity and facile operation.49,50Importantly,fluorescence sensing systems that allow in vivo and in vitro cellular imaging provide reliable tools for studying biological processes involving the target analytes as well as for clinical diagnosis and monitoring of diseases.

Here,we have overviewed recent studies aimed at the fluores-cence sensing of gold ions or gold nanoparticles in some cases,by small molecule probes or chemosensors.We have avoided the use of ‘‘sensor’’to indicate the fluorescence sensing systems,as the terminology is used by analytical chemists only for those sensing systems that are capable of continuous monitoring of analytes.2.2

Fluorescence sensing systems for gold species

Known fluorescent probes for gold ions can be classified into two categories according to the sensing mechanism:(1)the reaction-based and (2)coordination-based probes.The reaction based probes can be further divided into two,depending on whether the sensing reaction involves activation of the alkyne bond or not.According to the classifications,we overviewed the known fluor-escent probes by grouping them into three categories.

2.2.1.Fluorescence sensing of gold species through activation of the alkyne bond.Catalytically active gold complexes have common oxidation states of +1and +

3.Typical sources of gold complexes such as AuCl and AuCl 3have strong alkynophilicity,activating the alkyne bond toward oxygen,nitrogen,and even carbon-based nucleophiles.This reaction characteristic inspired several groups to develop the first reaction-based fluorescence sensing systems for gold species.In the late 2009and early 2010,Yoon and Kim,Ahn,and Tae groups concurrently disclosed rhodamine-based alkyne systems as the reaction-based fluores-cence sensing systems for gold species,respectively (Fig.4).Previously,a related ring-opening followed by cyclization reaction was used for the development of fluorogenic and chromogenic sensing systems for Hg(II ),Pb(II )and Ag(I )ions by Tae,Yoon,and Ahn groups,respectively.51–54Coordination of AuCl 3or AuCl to the alkyne bond triggered a series of bond reorganizations,involving the rhodamine-spirolactam ring-opening and intramolecular cyclization reactions to yield the heterocyclic ring (Fig.4).The ring-opening reactions accompanied by turn-on fluorescence and colorless-to-pink color changes,enabling the first fluorescence detection of the gold species with no appreciable interference from potentially competing metal species including Mg(II ),Ba(II ),Al(III ),Cr(II ),Mn(II ),Fe(II ),Fe(III ),Co(II ),Ni(II ),Pd(II ),Pt(II ),Cu(II ),Ag(I ),Zn(II ),Cd(II ),Hg(II ),and Pb(II ).

The excellent selectivity of the probes indicated that the alkyne activation by gold species is a highly promising sensing

strategy.Indeed,this sensing strategy was adopted by others in a number of papers followed.

The rhodamine B derivative 1(Fig.4)was independently evaluated in EtOH/HEPES bu?er (1:1,v/v,pH =7.4)55or in CH 3CN/PBS bu?er (1:1,v/v,pH =7.2),56showing similar sensing properties except the di?erent sensitivity in the case of AuCl.This latter discrepancy seemed to be caused by di?erent experimental conditions where AuCl might dispropor-tionate into Au(III )and Au(0)species [3Au(I )(aq)-Au(III )(aq)+2Au(0)].57AuCl 3is hygroscopic and hence should be handled in a glove box for the quantitative sensing purpose.

Similar sensing properties were obtained by the rhodamine 6G-based probe 2(Fig.4)in PBS bu?er (pH =7.4,containing 1%MeOH).58

According to the sensing mechanism of probe 1(Fig.4),the vinyl gold intermediate 1a underwent an unexpected transfor-mation into the formyloxazole P1as the major product,rather than producing the methyloxazole P2that was expected to be formed through the usual proto-deauration followed by double bond migration processes.59A subsequent mechanistic study was carried out by Ahn and co-workers using N -(propargyl)-benzamides as model compounds,which led to characterization of the corresponding vinyl gold intermediates and their reactivity depending on media.It was found that the vinyl gold inter-mediate generated from N -(propargyl)benzamide underwent the proto-deauration process in organic media to produce the corresponding methyloxazole as the major product,whereas,in aqueous media,it took the unusual reaction route to produce the corresponding formyloxazole as the major product.56

Kim and co-workers subsequently reported another type of alkyne activation approach to sense gold species,60in which

a

Fig.4Fluorescence sensing systems of gold species,probes 1–3,which are based on the alkyne activation,and sensing mechanisms for probes 1and 3,respectively.

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phenyl ring participated in a gold ion-mediated cyclization.The aryl alkynoate 3(Fig.4)thus underwent gold ion-promoted hydroarylation to produce a fluorescent coumarin,enabling turn-on sensing of Au(III )ions.A small response from Ag(I )was observed among various typical metal ions screened.The poor sensitivity of the probe toward AuCl reflects its lower Lewis acidity compared with AuCl 3toward the carbon–carbon triple bond.A drawback of the sensing scheme is the slow reaction rate,requiring more than one day for signal saturation with 10equivalents of AuCl 3in ethanol.

The pioneering studies concurrently reported by the several research groups ignited subsequent research e?orts toward the development of fluorescence sensing systems for gold species based on chemical transformations.

In late 2010,Peng and co-workers reported N -propargyl-naphthalimide 4(Fig.5),which selectively sensed Hg(II )or Au(III )ions depending on the reaction media and pH.61In HEPES bu?er (pH =7.4,containing 0.05%DMSO),the propargyl group in the probe underwent a regioselective oxymercuration reaction promoted by Hg(II )ions and produced the corre-sponding keto compound,which caused the change in the maximum fluorescence emission band of the probe from 543nm to 486nm.Under the same conditions,various other metal ions caused slight changes in the emission spectra.In contrast,the probe showed similar ratiometric response only toward Au(III )ions when the reaction medium was changed to MeOH/H 2O (95:5,v/v)and the pH to 9.0.Under the test conditions,it required about 5h for Hg(II )and 12h for Au(III )ions to yield the product,respectively.

Later Yoon and co-workers reported N -propargyl-1,8-naphthalimide 5(Fig.5),which also responded to Au(III )ions with ratiometric fluorescence change.62A Au(III )–acyl adduct was proposed as the sensing product,based on 1H NMR analysis,the conversion of which induced fluorescence emission shift from 527nm to 471nm.Addition of a surfactant,cetyltrimethyl ammonium chloride (CTAC,50m M),to a solution of probe (20m M)in PBS buffer (pH 7.4,containing 4%EtOH)accelerated the sensing reaction from ca.160min (buffer alone)to ca.40min (in the presence of CTAC)for completion.Under the optimized sensing conditions,the probe showed ratiometric response only toward Au(III )ions and no response toward other metal species.The probe was used for fluorescent imaging of gold ions in HeLa cells.

Lin and co-workers reported a FRET (fluorescence resonance energy transfer)sensing system based on N -(3-phenylpropargyl)-rhodamine thiolactam 6containing a BODIPY dye at the N -propargyl site,which sensed Au(III )ions through the alkyne activation strategy.The resulting FRET probe 7(Fig.5)selectively sensed Au(III )among various metal species with a ratiometric fluorescence change.63When probe 7was titrated with AuCl 3in EtOH/phosphate bu?er (7:3,v/v,pH =7.0),the BODIPY emission band at 514nm decreased while the rhodamine emission band at 594nm increased upon excitation at 470nm.Also,the FRET probe was used for determination of an unspecified amount of gold nanoparticles (AuNPs)in a commercial sample.For quanti-fication,AuNPs were pretreated with aqua regia to produce gold ions,and then the resulting solution was extensively diluted before sensing with the probe.

Song and co-workers reported a new type of alkyne activa-tion approach to sense gold ions,which involved an intra-molecular hydroamination reaction.A BODIPY dye containing a 20-ethynylbiphenyl-2-amine analogue,probe 8(Fig.5),thus underwent Au(III )-mediated cyclization in EtOH/PBS bu?er (1:1,v/v,pH =7.4)to signal turn-on fluorescence change.64The conversion rate was a little faster compared to that observed by the propargylamine-derived rhodamine,but requiring still more than an hour for signal saturation.Both Hg(II )and Pd(II )ions interfered the sensing reaction,which caused a

substantial

Fig.5Fluorescent probes (4–14)for gold ions,based on alkyne bond activation and their sensing mechanisms.

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reduction in the fluorescence intensity in competition experi-ments.The probe was used for fluorescent imaging of gold ion HeLa cells.

Emrullahog

?lu and co-workers reported a rhodamine-derived sensing system for gold ions,probe 9(Fig.5),which underwent the spirolactam ring-opening triggered by a gold ion-promoted alkyne activation to give turn-on fluorescence response.65The conversion was e?ected only by Au(III )ions among various other metal species examined in CH 3CN/HEPES bu?er (1:1,v/v,pH =7.0).The probe was used for fluorescent imaging of Au(III )ions in HCT-116cells.

Chen and co-workers reported N -propargyl-coumarin amide 10that selectively sensed Au(III )ions with a ratiometric fluores-cence change (Fig.5).66The formation of the 5-formyloxazole ring from the N -propargyl amide in the presence of Au(III )ions,an established cyclization reaction in aqueous media,caused an emission shift from 474nm to 512nm owing to a change in the intramolecular charge transfer (ICT).67Again,this alkyne activation approach also showed little response from other metal species examined.

Although the aforementioned alkyne activation approaches have been demonstrated to be powerful strategies to develop fluorescent probes for gold ions,in particular,with very high selectivity,potential drawbacks such as the high percentage of an organic co-solvent used,the slow reaction rate,or inter-ference from side reactions remained to be addressed.For example,the gold sensing process with probe 1could also produce two types of nonfluorescent compounds in approxi-mately 30–40%yields of the total mass.Simple hydration of the acetylenic moiety promoted by Lewis acidic gold species could compete with the spirolactam ring-opening process.Further-more,an unusual ring-closing process was found to compete with the sensing process,producing a tricyclic vinylgold(III )species that was nonfluorescent.68The formation of such nonfluorescent side products would be dependent on the sensing conditions such as the reaction medium,69tempera-ture,concentration,etc.,which would undermine the reliability of the quantification data.Also,the formation of nonfluores-cent side products would lead to lowering of the detection limit.Ahn and co-workers disclosed a new sensing scheme for gold species,in which the reaction site was separated from the signaling unit to alleviate the side reactions observed in the rhodamine-based probe 1.A fluorescein bis-(2-ethynyl)-benzoate,probe 11(R =2-ethynylbenzoyl)thus prepared (Fig.5),underwent Au(III )-promoted ester hydrolysis,which accompanied the turn-on fluorescence change in HEPES bu?er (pH =7.4,containing 0.25%DMSO).68Other possible side reactions including the simple hydration of the ethynyl moiety were found to be negligible.The conversion was quite fast and completed within 1h at ambient temperature.The sensing scheme was also highly selective to Au(III )among various metal ions examined,except Hg(II )that showed a minor interference.A similar sensing scheme was concurrently disclosed by Patil and co-workers.A fluorescein mono-(2-phenylethynyl)-benzoate,probe 12(R =2-(phenylethynyl)benzoyl),thus sensed Au(I )ions in CH 3CN/PBS bu?er (1:1,v/v,pH =7.4)with a turn-on

fluorescence change (Fig.5).70Other metal species showed negligible interference.Fluorescence images of A549cells pre-incubated with AuCl were obtained using the probe,but with limited resolution because of the poor emission behaviour of the hydrolyzed product,fluorescein mono-methyl ether,as noted by Ahn and co-workers.68

The aforementioned ester-type probes such as 11and 12pose limitations in bioimaging owing to the ubiquitous esterase activity in living systems.71Recently,Ahn and co-workers disclosed a novel approach to suppress the side reactions in gold-sensing systems based on alkyne activation as well as to avoid the esterase activity observed in the ester-type probes.To enhance reactivity of the original probe 1and,hence,to suppress the side reactions observed,they raised steric strain around the rhodamine lactam moiety by substituting the N -propargyl group in the original probe 1with N -(2-ethynylphenyl).As-prepared probe 13(Fig.5)thus underwent the Au(III )-mediated ring-opening reaction faster than the case of probe 1,with about 4.5times larger pseudo-first-order rate constant [k obs =0.13min à1for 13,k obs =0.029min à1for 1;measured in CH 3CN/PBS bu?er (1:1,v/v,pH =7.0)at 251C],which was ascribed to the ground-state elevation.72The rate acceleration in turn suppressed the competing alkyne hydration that was a non-signaling process,in addition to the hydroarylation side reaction observed in probe 1.As a result,the probe showed very high sensitivity,enabling detection of Au(III )ions down to the 0.5ppb level.Furthermore,a FRET sensing system (14a and 14b )was constructed by introdu-cing a naphthalimide dye to the N -phenyl ring of probe 13(Fig.5).Interestingly,the FRET system showed even faster response to gold ions than its parent compound 13;the fluorescence change was almost complete within 20min when the probe at 10m M was treated with an equivalent amount of AuCl 3in CH 3CN/PBS bu?er (1:9,v/v,pH =7.4)at 251C.It should be noted that most of the known probes based on alkyne activa-tion show a rather slow response toward gold ions.Under the titration conditions,an emission band of the naphthalimide at 520nm diminished while that of the rhodamine at 587nm increased,showing ratiometric response.

The probe also sensed (CH 3CN)Au[P(t -Bu)2(2-biphenyl)]SbF 6,a stabilized gold(I )species,with a linear ratiometric change depending on the concentration of gold ions.One of the FRET probes,probe 14b ,was used to obtain fluorescence images of the probe itself (green emission)and Au(III )ions (red emission)in N2A cells that were pre-incubated with AuCl 3.

In this section,we overviewed fluorescent probes for gold ions based on the alkyne activation approach.Other sensing characteristics are summarized in Table 1.

2.2.2.Other types of reaction-based sensing systems for gold species.In 2011,Lin and co-workers disclosed that a condensation product between rhodamine 6G hydrazide and phenyl isocyanate,the compound 15(Fig.6),underwent fast hydrolysis to the parent rhodamine promoted by Au(III )ions,which resulted in a turn-on fluorescence change.73This con-version is interesting considering that a related condensation product with phenyl isothiocyanate forms a heterocyclic ring promoted by Hg(II )ions,making it a turn-on sensing system.74

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The chemical conversion in PBS bu?er (pH 7.4,containing 0.3%DMF)proceeded fast only in the presence of Au(III )among various other metal species examined.The probe was used for fluorescent imaging of HeLa cells pre-incubated with AuCl 3.The corresponding analogue derived from rhodamine B hydra-zide showed slower response.

Later,fluorescent sensing systems based on another type of Au(III )-promoted hydrolysis reaction were also reported.

Table 1

Sensing characteristics of probes 1–14

Compd Selectivity Sensitivity,LOD;a response time Bioimaging data Others

Ref.1(Yoon)

Au(III )

63ppb in EtOH/H 2O (1:1,v/v)Imaging of HaCaT cells [Au(III )]

DMSO or EtOH/HEPES bu?er (1:1,v/v,pH =7.4)55

Saturation:30min (10eq.)

Chemodosimeter (LOD:100ppb in EtOH–HEPES buffer system)

NMR and mass analysis for the product 1(Ahn)Au(I ),Au(III )

0.4ppm

Not reported CH 3CN/PBS bu?er (1:1,v/v,pH =7.2)56

Saturation:20min (3eq.)NMR and mass analysis for product Reaction mechanism studied

2Au(III )

50nM

Imaging of HeLa cells [Au(III )]

Rhodamine 6G fluorophore

58

Saturation:80min (2eq.)PBS bu?er (containing 1%MeOH),pH =7.4NMR analysis for the product

3Au(III )64ppb

Imaging of HaCaT cells [Au(III )]In EtOH

60

Saturation:416h (10eq.)Au(III )-catalyzed hydroarylation reaction Slow reaction

4Au(III )Not reported

Not reported

MeOH/H 2O (95:5,v/v,pH =9.0)61

Saturation:20min (36eq.)Ratiometric

NMR and mass analysis for the product 5Au(III )

8.44m M

Imaging of HeLa cells [Au(III )]PBS bu?er (containing 4%EtOH),pH =7.462

Saturation:60min (5eq.)Surfactant (CTAC)for the enhanced rate Ratiometric

6Au(III )Not reported

Not reported

EtOH/PBS bu?er (7:3,v/v,pH =7.0)63

Saturation:not reported Turn-on type

No Hg(II )-mediated ring-opening 7Au(III )LOD:37m M

Not reported EtOH/PBS bu?er (7:3,v/v,pH =7.0)63

Saturation:15min (3eq.)FRET system

Quantitative detection of AuNPs

8Au(I ),Au(III )

63ppb

Imaging of HeLa cells [Au(III )]

EtOH/PBS bu?er (1:1,v/v,pH =7.4)64

Saturation:60min (5eq.)Turn-on type (PET process)

Au(III )-catalyzed intramolecular hydroamination 9Au(III )0.6ppm

Imaging of HCT-116cells [Au(III )]CH 3CN/HEPES bu?er (1:1,v/v,pH =7.0)65

Saturation:60min (5eq.)Monitoring gold ions in synthetic samples NMR and mass analysis for the product 10Au(III )44m M

Not reported

DMF/HEPES bu?er (6:4,v/v,pH =7.4)66

Saturation:20min (5eq.)Ratiometric

Mass analysis of the product

11Au(III )

0.4m M

Imaging of HeLa cells (fail to get imaging)HEPES bu?er (containing 0.25%DMSO),pH 7.468

Saturation:60min (2eq.)Sensing properties:R 1=H,R 2=2-ethynylbenzoyl Limits for bioimaging due to esterase hydrolysis 12Au(I )Not reported

Imaging of A549cells [Au(III )]Sensing properties:R 1=Ph,R 2=Me derivatives 70

Saturation:30min (10eq.)CH 3CN/PBS bu?er (1:1,v/v,pH =7.4)Esterase resistance:porcine liver esterase 13Au(I ),Au(III )0.5ppb

Not reported

CH 3CN/HEPES bu?er (3:7,v/v,pH =6.0)72

Saturation:60min (1eq.)Oxazine product formation (no side product)DFT calculation

14Au(I ),Au(III )

Not reported

Imaging of N2A cells (14a :low permeability)(14b :good permeability)

CH 3CN/PBS bu?er (1:9,v/v,pH =7.4)72

Saturation:20min (1eq.)

FRET system Low cytotoxicity

a

LOD:limit of detection.

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Emrullahog

?lu and co-workers reported a dual sensing system for Au(III )and Hg(II )ions,probe 16(Fig.6),which is composed of rhodamine B and BODIPY dyes linked together by an hydrazide imine bond.75The imine bond in the probe was hydrolyzed by Au(III )ions in CH 3CN/HEPES bu?er (1:1,v/v,pH =7.0)to give two separate dyes,the rhodamine thiohydrazide and formyl-BODIPY,which can be monitored in the pink and green emission channels,respectively.In the case of Hg(II )ions,the imine hydrolysis did not occur,but the rhodamine thiolactam ring was opened and thus could be monitored in the red channel.The hydrolysis proceeded fast only in the presence of Au(III )ions among various other metal species examined.The probe was used for fluorescent imaging of A549cells pre-incubated with AuCl 3.

The same group applied the imine hydrolysis approach to develop a BODIPY-based fluorescence probe for Au(III )ions.The imine bond in probe 17thus underwent hydrolysis promoted by Au(III )ions in EtOH/phosphate bu?er (1:1,v/v,pH =7.0)to give fluorescence turn-on response (Fig.6).76The conversion proceeded fast only in the presence of Au(III )and Au(I )ions,and other metal species including Hg(II )did not cause any appreci-able change.The probe was used for fluorescent imaging of A549cells pre-incubated with AuCl 3.

Other types of chemical conversions were utilized for sen-sing Au(III )ions.Kim and co-workers disclosed an interesting approach to sense Au(III )ions,which was based on two conver-sions:(1)generation of AuNPs from AuCl 3in HEPES bu?er 77–79

and (2)carbon–iodide (C–I)bond cleavage on the surface of AuNPs.80,81

They found that AuCl 3readily underwent reduction to give AuNPs in HEPES bu?er (pH =7.0)at room temperature,but not in other bu?er solutions such as Tris,PBS,and deionized water.They selected an iodo-substituted BODIPY dye,probe 18,to demonstrate the sensing scheme (Fig.6).82Upon treatment with AuCl 3,a EtOH/HEPES bu?er (95:5,v/v,pH =7.0)solution of probe 18,which is nonfluorescent,emitted green fluores-cence (l em =510nm)with linearly increasing intensity depending on the concentration of the gold species.The conversion occurred only by Au(III )and not by other metal species including Au(I ).A bromo analogue of the probe showed a much slower conversion by AuCl 3under otherwise the same conditions,but caused a significant change at higher temperature (651C).

Chang and co-workers disclosed a new sensing scheme for Au(III )species,which was based on a desulfurization reaction.83A thiocoumarin probe 19thus underwent Au(III )-promoted desulfurization 84to give the corresponding keto analogue,the conversion of which gave turn-on fluorescence change in addition to colour change from pink to yellowish green (Fig.6).Since the conversion was invented to sense Hg(II )ions originally by Czarnik and co-workers,85N ,N ,N 0,N 0-tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN),a masking agent,was required to suppress interference from other metal species,in particular,Hg(II ).As a result,probe 19in the presence of TPEN underwent the chemical conversion in CH 3CN–acetate buffer (pH =7.4)only by Au(III ),and other metal species gave no appreciable change.2.2.3.Coordination-based sensing strategy.Along with the reaction-based approach,the coordination-based sensing approach has also been used for the development of fluores-cent probes for metal cations.A few reports on the fluorescent probes for Au(III )species are known at the moment (Fig.7).In 2012,Lin and co-workers reported a pseudo-coordination-based sensing system for gold ions,probe 20(Fig.7).The probe sensed Au(III )through a metal-promoted rhodamine ring-opening process,which could be reversed by addition of an excess

amount

Fig.6Other types of reaction-based fluorescent probes (15–19)for gold ions and their sensing

mechanisms.

Fig.7Coordination-based fluorescent probes (20–22)for gold ions.

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of cyanide;hence,the probe is not a really ‘‘reversible’’sensing system.86The probe selectively sensed Au(III )among other metal species including Hg(II ).The result is interesting because a simple rhodamine hydrazide was invented to sense Hg(II )originally by Czarnik and co-workers,as noted by the authors.The Job plot indicated that the probe formed a 1:1complex with the Au(III )ion.The selectivity data were examined in EtOH/H 2O (3:7,v/v),not in a bu?er solution,using 20equivalents of metal species.The probe was used for fluorescent imaging of HeLa cells pre-incubated with AuCl https://www.wendangku.net/doc/9599574.html,ter,Algi and co-workers reported that 2,5-dithienylpyrrole selectively responded to Au(III )ions in the fluorescence quenching mode,probe 21(Fig.7).87Among various metal species evaluated,only Au(III )caused gradual quenching of the fluorescence at 440nm depending on its amount.The alkynophilicity of gold ion is used in the development of a coordination-based fluorescent probe.In 2014,Rashatasakhon and co-workers disclosed that a 1,8-naphth-alimide dye connected with ferrocene through an acetylene bridge,probe 22(Fig.7),selectively responded to Au(III )among various other metal species in the fluorescence turn-on mode.88The sensing characteristics of other types of reaction-based and coordination-based sensing systems are summarized in Table 2.2.3.

Summary and perspectives of gold detection

Since the reaction-based approach to fluorescently detect gold species was proven to be e?ective by several groups concurrently,various types of reaction-based fluorescence probes have been developed as overviewed above.As a result,now we can readily

quantify gold species in homogenous samples below the ppb level,simply by fluorimetry,with no appreciable interference from other metal species.Application of such fluorescence probes to bioimaging of gold species in living systems is of importance to investigate their biological e?ects.Although a number of probes were applied to fluorescence imaging of gold species in cells,use of them to tackle real biological issues is far from reality at the moment.In the case of in vivo assay,the typical gold species such as AuCl 3will react with biothiols in cells to form the corresponding Au(I )species that would have much lower Lewis acidity and thus may not activate the existing probes.A highly sensitive probe can even sense a stable gold species such as (CH 3CN)Au[P(t -Bu)2(2-biphenyl)]SbF 6,72but it is not known whether other types of gold species conceivable in living systems could activate the probe or not.Therefore,assess-ment of fluorescent probes toward other types of gold species,rather than AuCl or AuCl 3,is necessary,including studies on the fate of gold species in living systems.Also,a probing system for gold nanoparticles in living systems is of great interest,con-sidering that nowadays gold nanoparticles are widely used for bioimaging,photothermal therapy,drug delivery,and so on.

3.Silver

3.1.

Silver species

3.1.1.Chemical properties.Silver (Ag)is a group 11transi-tion metal in the element periodic table,with the atomic

Table 2

Sensing characteristic information of probe 15–22

Compd Selectivity LOD a and response time

Bioimaging data Other conditions

Ref.15

Au(I ),Au(III )

74nM

Imaging of HeLa cells incubated with Au(III )PBS bu?er (containing 0.3%DMF),pH =7.4

73

Saturation:1min (10eq.)Hydrolysis of acylsemicarbazide to carboxylic acid Monitoring gold ions in synthetic samples 16Au(III )44nM

Imaging of A549cells incubated with Au(III )EtOH/PBS bu?er (1:1,v/v,pH =7.0)75

Saturation:30min (20eq.)C Q N bond hydrolysis reaction

Au-ion binding ligand moiety study

17Au(III )65nM

Imaging of A549cells incubated with Au(III )CH 3CN/HEPES bu?er (1:1,v/v,pH =7.0)76

Saturation:o 1min (2eq.)Di?erential detection of Hg(II )and Au 3+

Au(III )-catalyzed hydrolysis of the C Q N moiety

18

AuNPs [from Au(III )]0.19nM

Not reported

EtOH/HEPES bu?er (5:95,v/v,pH =7.0)82Saturation:60min (4500eq.)In situ AuNPs generation and C–I bond cleavage on the surface

19Au(III )(TPEN)

11m M

Not reported

CH 3CN/acetate bu?er (1:1,v/v,pH =4.7)83

Saturation:1min (20eq.)

TPEN:Hg(II )masking agent

Au(III )-catalyzed desulfurization of thiocarbonyl 20Au(III )

48nM Imaging of HeLa cells incubated with Au(III )EtOH/H 2O (3:7,v/v)

86

Saturation:not reported (20eq.)1:1complex (reversed with excess cyanide)NMR and mass analysis of the product

21Au(III )

Not reported Not reported Phosphate bu?er (containing 0.15%CH 3CN,pH =7.2)87Saturation:not reported (8eq.)Coordination-based

Fluorescence turn-o?mode

22

Au(I ),Au(III )

95ppb

Not reported

CH 3CN/PBS bu?er (1:1,v/v,pH =8.0)

88

Saturation:10min (100eq.)

Coordination to carbon–carbon triple bond

a

LOD:limit of detection.

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number 47.Being a transition metal,silver shows multiple oxidation states (+1,+2and +3)in addition to the elemental state,but their chemistry is almost exclusively that of the ‘+1’state with the electron configuration of [Kr]4d 10.Due to the completely filled 4d subshell,Ag(I )forms diamagnetic com-plexes with a variety of coordination geometries including linear,two-coordinate complexes,planar three-coordinate com-plexes and tetrahedral four-coordinate complexes.Although the soft metal ion character of Ag(I )favored its coordination with the soft donor atom such as sulfur,the coordination complexes of Ag(I )with hard atoms such as nitrogen and oxygen are also observed with heterocyclic ligands (pyridine,bipyridine,phenanthroline)and macrocyclic ligands (crowns,cryptands,porphyrins).89,90

Silver has been also used in the organometallic 91and organocatalysis chemistry.92–94Although compared to the other transition metal elements,silver containing compounds are rare in organometallic chemistry.The p -bonded Ag(I )–olefin metal complex study by Winstein and Lucas in 193895is one of the earliest landmarks in organosilver chemistry.96Similarly,Ag(I )also has high tendency to form Ag(I )–arene complexes.Although the Ag(I )–carbene complexes were difficult to isolate due to the low stability,97still silver complexes are used extensively as catalysts for the reactions in which carbenes are believed to be the reaction intermediates.98Synthesis of a bis(carbene)silver(I )organometallic polymer was also possible using 1,2,4-triazole-3-,5-diylidene as a building block (Fig.8a).99

Due to the highest electrical and thermal conductivity among the metals,silver has widespread applications and the broad prospects in the electronic industry,and photographic and imaging industry also.

3.1.2.Biological utilities.Silver,along with most of its complexes,is toxic to bacteria,algae,and fungi while it is the least toxic for humans among the elements with such anti-bacterial e?ects,called the oligodynamic e?ect.100–102Silver complexes and silver nanomaterials irreversibly bind to the key enzyme systems in the cell membranes of pathogens:they used as antiseptics during the common medical processes to avoid external infections.103,104Silver sulfadiazine (silvadene),a silver containing antibacterial drug (Fig.8b),is well known as a topical cream on burns.

Besides the various applications of silver,much attention has been paid to the negative impact of Ag(I )on the environ-ment,especially on organisms.It is believed that Ag(I )can bind to the amine,imidazole,carboxyl groups of various metabolites such as high molecular weight proteins and metallothionein in tissues of cytosol fractions and enzymes such as sulphydryl enzymes to inactivate.105–108Ag(I )can interact with and displace

essential metal ions like Ca 2+and Zn 2+in hydroxyapatite in bones.Repeated silver exposure to human body may cause blood silver (argyria)and urine silver excretion,cardiac enlargement,growth retardation and degenerative changes in the liver.109Excessive Ag(I )intake may damage skin and eyes through long-term accumulation of insoluble precipitates.110

Although several possible roles of Ag(I )in biological systems have been proposed,such as interaction and inactivation of vital enzymes,binding to DNA,interaction with the cell membrane,and interference with electron transport,still the mechanism of the antimicrobial activity of Ag(I )has not been clarified because of a lack of a suitable detection and imaging system.

3.1.3.Conventional detection methods.Similar to the conventional gold detection methods,the detection of silver species also includes various instrumental techniques such as flame atomic absorption spectrometry (FAAS),graphite furnace atomic absorption spectrometry (GFAAS),inductively coupled plasma atomic emission spectrometry (ICP-AES),inductively coupled plasma mass spectrometry (ICP-MS),and electro-chemical assay.111–115Other than those instrumental techniques,extraction methods using molecular receptors or chelating ligands are also employed for Ag(I )detection.116,117

Compared with the conventional analytical methods,the fluorescence detection methods are highly advantageous as mentioned earlier.49,50Also,there is a great demand for imaging the distribution of silver ion in cellular processes with deeper penetration and higher 3D spatial selectivity.In this review,we overviewed the fluorescent and colorimetric silver detection systems with respect to their promising features and sensing schemes.3.2.

Fluorescent and colorimetric detection of silver ions

Based upon the sensing mechanism and platform,the fluores-cent and colorimetric detection systems for silver ions are mainly classified into three types:(1)coordination based systems,(2)reaction based systems and (3)others (quantum dots,nanoparticles,polymers and oligonucleotides based).3.2.1.Coordination-based systems.Due to soft Lewis acid character of Ag(I ),it has a high tendency to coordinate with soft Lewis base such as sulfur atoms.118,119Hence,silver chemo-sensors have been developed mainly based on the metal coordination to sulfur containing ligands.Other heteroatoms such as nitrogen and oxygen containing ligands are also used to modulate the binding and signaling in silver ion detection.The coordination based systems are described here under three subgroups,namely:(i)coordination to sulfur containing ligands,(ii)coordination to non-sulfur containing ligands,and (iii)excimer based chemosensors.

3.2.1.1.Coordination to sulfur donor-containing ligands.The sulfur containing ligands used for the development of Ag(I )fluorescence probes are either cyclic or acyclic ligands,which are discussed separately.

3.2.1.1.1.Cyclic ligands.In 1985,Oue and co-workers first reported that benzothiacrown ether 23(Fig.9a)had

high

Fig.8(a)Synthesis of a bis(carbene)silver(I )organometallic polymer.(b)Structure of an antibacterial drug,silvadene.

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selectivities for silver ion over other heavy metal ions in cation extraction experiments.120They also carried out silver ion spectrofluorimetry by using ion-pair extraction system with thiacrown ether 23as the cation receptor and eosin Y as the fluorescent anion.121Later,introduction of a nitrogen atom in the receptor and direct attachment with a fluorophore led to a fluorescence chemosensor (Fig.9b).In the absence of metal ions,this chemosensor emitted little due to the photo-induced electron transfer (PET)122from the amine lone pair to the excited state of the fluorophore.Upon binding of metal ions,the PET process was blocked and fluorescence was turned-on.Based on this sensing strategy,several fluorescence probes for Ag(I )were developed which are described here with their special features.

In 2000,Rurack et al.first introduced a thiaazacrown receptor into boron dipyrromethene dye to develop a turn-on type fluorescence Ag(I )probe 24(Fig.10).123The probe itself emitted weak fluorescence in CH 3CN/H 2O (1:3,v/v).Upon binding of metal ions with the receptor,probe 24emitted strong fluorescence towards Ag(I )as well as Hg(II )with 1:1binding stoichiometries for both https://www.wendangku.net/doc/9599574.html,ter,Lin et al.reported compound 25(Fig.10)in which the phenyl ring in probe 24was changed to ortho -methyl substituted one.124The detection limit obtained with probe 25was one order of magnitude lower than the case of 24.With the addition of 1.1ppm of Ag(I )(8.42m M)into the CH 3CN/H 2O (4:1,v/v)solution,drastic enhancement on the fluorescence intensity (4300-fold)of 25was observed at

509nm.Other metal ions exerted no influence on the fluores-cence intensity of 25except Hg(II ),which showed only less than 2-fold increased intensity at 1.1ppm (4.5m M)concentration.But when the ion concentration increased to 4.0ppm,the Hg(II )ion (16.47m M)also showed a significant fluorescent enhance-ment by more than 600-fold on the fluorescence intensity of 25.Although probes 24and 25showed high sensitivity towards Ag(I ),the interference from the thiophilic metal ion (Hg(II ))remained to be overcome.In the meantime,Schmittel and coworkers also developed iridium(III )and ruthenium(II )complexes 26and 27(Fig.10)with the aza-dithia-dioxa crown-ether receptor for Ag(I ).125Both compounds 26and 27exhibited selective luminescence enhancement toward Ag(I )in CH 3CN/H 2O (1:1,v/v).The emission enhancement factors observed with probes 26and 27were 3.4and 0.3respectively,indicative of better performance of the former over the latter.All other metal ions showed negligible fluorescence enhancement.However,the addition of Hg(II )quenched the emission of both the probes.Hence,the development of a selective fluorescence Ag(I )probe without interference from soft metal ions such as Hg(II )remained as a challenging issue.

For the selective detection of Ag(I )over other soft transition metal ions such as Hg(II ),Cd(II ),Cu(II )and Tl(I ),Shamsipur and co-workers developed a fluorimetric optode membrane based on dansyl dye 28(Fig.10).126The sensing device detected Ag(I )over a wide concentration range (5.0?10à7to 1.7?10à2M)with fast response (o 40s)in the fluorescence quenching mode.

The fluorescence detection based on such fluorescence quenching rather than enhancement is disadvantageous for a high signal output as well as for bioimaging applications.A naphthalimide dye attached with the tetrathia-aza-crown ether,compound 29(Fig.10),showed a similar quenching behaviour toward Ag(I )(F =0.04),owing to an intramolecular d–p interaction between the fluorophore and Ag(I )bound.Weakly fluorescent compound 29(F =0.06)in EtOH/buffer (1:4,v/v)showed about 5-fold fluorescence enhancement (at 532nm,F =0.30)upon treatment with Hg(II ).When this Hg(II )-bound complex of 29was treated with Ag(I ),its fluores-cence was quenched.127

The issue of fluorescence quenching due to Ag(I )complexa-tion was later solved by Yoon and Spring group.128Usually,chemosensors based on aza-thia-crown ether receptors show high binding selectivity for Ag(I )but signal selectivity for Hg(II ),because Ag(I )can quench or silence the fluorescence through enhanced spin-orbital coupling,energy or electron transfer processes.They introduced a carbonyl group between the 1,8-naphthalimide dye and the receptor moiety,leading to a naphthalimide-based probe 30(Fig.11).This minor change resulted in fluorescence enhancement of probe 30in the presence of Ag(I ).The conversion of amine to amide functionality resulted in the elevation of the oxidation potential of the fluorophore,and also in sterically blocking of the interaction between the bound Ag(I )and the naphthalimide fluorophore.Significantly,the addi-tion of Ag(I )to the probe induced fluorescence increase by around 14-fold whereas the addition of Hg(II )caused fluorescence increase by 6-fold when measured in CH 3CN/HEPES

bu?er

Fig.9(a)Structure of a thiacrown ether 23,and (b)its sensing mecha-nism for Ag(I

).

Fig.10Structures of fluorescent chemosensors 24–29.

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(1:1,v/v).Another approach towards Ag(I )selective turn-on type probe 31was reported by Lee and co-workers.They introduced a di?erent spacer in between the receptor and fluorophore (Fig.11).129The maximum chelation-enhanced fluorescence e?ect as high as 150-fold (F =0.26)was observed in the presence of Ag(I )with the probe in EtOH/HEPES bu?er (1:1,v/v,pH =7.2).

Although probes 30and 31sensed Ag(I )with high sensitivity and a quite good selectivity in the fluorescence turn-on mode,the high content of organic solvent (50%)would limit their use for bioimaging purposes.

Hu et al.reported a biocompatible and e?cient probe 32(Fig.11),which showed enhanced fluorescence towards Ag(I )in HEPES bu?er (pH =7.4).130The probe 32selectively exhibited large fluorescence enhancement towards Ag(I )over Hg(II )or other metal ions with a fast response (o 2min).The selectivity towards Ag(I )over Hg(II )was explained by a di?erence in the frontier molecular orbital energies calculated.The fluorescence increase at 578nm showed a linear response to 0.5–5m M Ag(I )with a detection limit of 1.0?10à7M,which reached the standards of US EPA and World Health Organization (WHO)for drinking water (5.0?10à7M).The probe was also used for imaging of Ag(I )in living cells by fluorescence confocal microscopy:the MCF-7cells incubated with the probe only exhibited almost no fluorescence.By contrast,the cells stained with both the probe and Ag(I )showed an obvious fluorescence in the cytoplasm and nucleolus (organelle inside the nucleus).Lodeiro and co-workers reported that structurally related,aza-trithia-cycles attached to the anthracene fluorophore through a methylene linker resulted in fluorescence quenching upon binding to an Ag(I )ion,the result of which inform us that the type of linker as well as the aza-thia-cyclic ligand can govern the sensing property of structurally related probes.131

The response behaviours of the turn-on type probes are highly dependent on experimental conditions such as the probe concentration,environmental media,excitation power,etc.Moreover,without response calibration curves under similar conditions,quantification of the analyte is di?cult.For the quantification purpose,the ratiometric sensing is highly desirable for the easy quantification of Ag(I )by avoiding those experimental e?ects.132,133Accordingly,in 2010,Jiang and co-workers reported the furoquinoline derived Ag(I )probe 33(Fig.12),which showed ratiometric behaviour based on the intramolecular charge transfer (ICT)mechanism.134The compound,which showed a large Stokes

shift (about 173nm),responded to Ag(I )with a 50nm red-shift in the emission band and with high a?nity (log K =7.21?0.07in ethanol).The quantum yield of the probe 33–Ag(I )complex in ethanol was calculated to be 0.18,which is slightly higher than that of the probe itself (F =0.14).The nitrogen atom in the furoquinoline moiety was involved in the coordination with silver ions,inducing ICT.The interaction between the silver ion and the nitrogen atom in the furoquinoline moiety enhances the electron-withdrawing ability of furoquinoline that leads to enhanced ICT and thus the red shift was observed.

Recently,another approach towards the ratiometric sensing of Ag(I )has been disclosed by Berdnikova and co-workers.The bis(styryl)pyridinium derivatives 34–36(Fig.12)containing two identical nonconjugated chromoionophores with azadithia-crown ether residues thus gave ratiometric optical response toward Ag(I )and Hg(II )over other metal ions examined.135Compounds 34–36showed the emission maxima around the 630nm region in acetonitrile and fluorescence quantum yields in the order of https://www.wendangku.net/doc/9599574.html,plex formation with a cation led to hypsochromic shifts of both absorption and emission spectra owing to interaction of the cation with the lone pair electron of the crown ether nitrogen atom in the dye molecule.Nevertheless,the Ag(I )complexes of 34–36emitted much weaker fluorescence than Hg(II )complexes.Moreover,the ratio-metric probes 33–36were examined only in non-biocompatible organic media,such as EtOH or CH 3CN.

A ratiometric Ag(I )probe 37(Fig.12)containing the quino-lone moiety similar to that of probe 33was reported by Jiang and co-workers.136The probe displayed a weak fluorescence band centered at 565nm (F =0.05)in MES bu?er solution (pH =6.0).Upon addition of Ag(I ),the emission band from the probe gradually decreased with simultaneous rising of a new emission band at 481nm (F =0.28).A plot of the fluorescence intensity ratio at the two wavelengths (I 481/I 565)showed a significant increase from 0.19to 4.99(up to 26-fold).The resultant complex Ag(I )–37displayed a ratiometric and highly selective response to iodide over other anions due to the favorable precipitation of AgI.

3.2.1.1.2.Acyclic ligands.Development of fluorescence Ag(I )probes based on acyclic receptors containing sulfur donor atoms was first reported by Ishikawa et al.137They studied a series of acyclic podands,open-chain crown

compounds,

Fig.11Structures of chemosensors 30–32that responded to Ag(I )in the fluorescence enhancement

mode.

Fig.12Structures of the ratiometric fluorescent probes 33–37.

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based on sulfur donors that are linked to a fluorophore through nitrogen.It was concluded that the stability of the Ag(I )complexes increased with the number of the sulfur donors in the receptor part of the probe.Among the several acyclic ligands containing compounds,both the compounds 38and 39(Fig.13a)possessed superior binding ability with Ag(I ).Accordingly,several fluorescence Ag(I )probes were developed with the general structure feature shown in Fig.13b.

In 2005,Akkaya and coworkers reported a bis(BODIPY)compound containing the N -phenyl-9-aza-3,6,12,15-tetrathiahepta-decane receptor,138probe 40(Fig.14),which showed a ratiometric fluorescence change toward Ag(I )with a large pseudo-Stokes’shift.139When excited at 480nm,the probe dissolved in THF showed very weak residual emission near 550nm but strong emission centered at 671nm,as a result of the excitation energy transfer from the left BODIPY dye to the right,p -extended styryl BODIPY dye that emitted in the red region (Fig.14).The binding of Ag(I )with the podand moiety resulted in a blue shift of the red emission band (41nm)with an emission intensity ratio (I 630/I 671)change from 0.25to 1.42.Remarkably,the other metal ions including Hg(II )caused only negligible changes in the emission spectrum.The binding constant between the probe and Ag(I )was determined to be as high as 1.7?105M à1.These sensing properties were reported only in THF,a non-biocompatible medium.

Fluorescence detection of Ag(I )under physiological condi-tions (50mM HEPES bu?er,pH =7.2,and 0.1M KNO 3)was later reported by Iyoshi et al.with a rosamine-based probe 41(Fig.14).140The probe had a negligible fluorescence quantum yield (F o 0.005)in the absence of Ag(I )due to the PET quenching process from the aniline nitrogen to that of the

excited xanthene dye.Upon the addition of Ag(I ),however,the fluorescence intensity of 41increased around 35-fold (F =0.13)at 574nm.A smaller enhancement in the fluorescence intensity was observed in the presence of Cu(I )and Cu(II ).Other transi-tion metals including Hg(II )had no e?ect on the fluorescence change.The binding mode of Ag(I )with the chelator moiety of 41was identified by a crystal structure of the silver complex,which showed a trigonal-planar coordination geometry in which the three sulfur atoms occupied the metal center.The 1

H NMR experiments suggested that the aniline nitrogen was associated with the Ag(I )center in the solution,which might inhibit the PET process.

Indolic sulfur–oxygen donor half-crown ethers were also reported to selectively respond to the Ag(I )ion but in the fluorescence quenching mode.141

Two-photon (TP)probes for Ag(I )were developed based on sulfur containing metal chelates.Two-photon probes can provide higher spatial resolution in bioimaging compared with one-photon probes.Furthermore,the low-energy near-infrared excitation light enables deeper tissue penetration and also alleviates the photobleaching and autofluorescence issues.142In 2008,Huang et al.reported the first two-photon probe (Fig.14)for Ag(I ),compound 42.143Soon after,they also reported another two-photon probe 43(Fig.14)with improved sensing properties.144The strong two-photon fluorescence of both the compounds 42(d =1120GM,TP excitation wavelength =810nm)and 43(d =950GM,TP excitation wavelength =790nm)was suppressed upon complexation with Ag(I )in CH 3CN.The suppression in the absorption and emission intensity upon addi-tion of Ag(I )indicated that the large p -electron density of the conjugated systems decreased owing to the binding-induced charge transfer from the aniline nitrogen to Ag(I ).Both the compounds showed a notable selectivity to Ag(I )over other potentially competing metal ions.Moreover,probe 43was successfully used for bioimaging of Ag(I )in live cells.The epithelial cells incubated with the probe in mixed HEPES bu?ered saline/EtOH/DMSO/CrEL (polyoxyethylene castor oil)(20:35:30:15,v/v/v/v)solution showed uniform and bright orange red fluorescence when fluorescently imaged by two-photon microscopy (TPM).In contrast,the probe treated epithelial cells,after incubation with the HEPES bu?er (pH =7.0)solution containing Ag(I ),showed quenched fluorescence.A two-photon probe that shows turn-on or even better ratiometric response to Ag(I )has yet to be discovered,which would provide a powerful tool for imaging of the metal ions in living tissue.

Other than the above mentioned sulfur-containing podands (Fig.13),a few ligands that contain one sulfur donor atom are known such as thiourea 44,thiofuran 45,and thiosemi-carbazide 46,which also showed strong binding a?nities with Ag(I )when assisted with non-sulfur hetero atom donors (N or O atoms)at appropriate positions.

Fu and co-workers reported the thiourea 44that sensed Ag(I )in the fluorescence enhancement mode (Fig.15).145The probe displayed 14-fold enhanced fluorescence (l em =385nm)upon binding with Ag(I )in MeOH/HEPES bu?er (3:1,v/v).Job’s plot indicated the formation of a 1:2complex between the

probe

Fig.13(a)The structures of the chemosensors 38and 39.(b)The general structure of fluorescent Ag(I )probes containing acyclic S-donor

ligands.

Fig.14Structures of fluorescent chemosensors 40–43.

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and Ag(I )with an association constant of 1.8?108M à2.Among other metal ions examined,interference from Hg(II )(caused 6-fold enhancement)was observed.

Pitchumani and co-workers reported the thiophene 45that also sensed Ag(I )with fluorescence enhancement (Fig.15).The probe selectively responded to Ag(I )among various metal ions screened in MeOH/bu?er (1:1,v/v,pH =7.4).146The fluores-cence enhancement was attributed to an increase in intra-molecular charge transfer (ICT)upon binding with Ag(I ),supported by a red shift (44nm)of the probe’s emission band that appeared at 359nm.

The binding of Ag(I )with the thiosemicarbazide moiety was also used to develop the naphthalimide based probe 46(Fig.15).147The probe selectively responded to Ag(I )with 60-fold enhanced fluorescence (l em =540nm)among various metal ions including Hg(II )in EtOH/HEPES (4:1,v/v,pH =6.5).Two emission peaks at 428nm and 540nm were observed,the former of which was ascribed due to the restriction of C Q N bond reorganization upon metal chelation.The increase in the fluorescence intensity was attributed to the inhibition of the PET process from the thiosemicarbazide group upon metal binding.Along with the sulfur-containing functional groups mentioned above,combination of sulfur donor(s)with or without assistance of other heteroatom donor(s)was found to be e?ective for the selective sensing of Ag(I ).Yoon and co-workers reported the thiomorpholine-containing fluorescein 47that sensed Ag(I )in the fluorescence turn-o?mode (Fig.16).148The fluorescence of the probe was quenched upon binding with Ag(I )along with a color change from a light yellow to pink in DMSO/HEPES bu?er (5:95,v/v,pH =7.4).It was proposed that Ag(I )chelates with the sulfur donor and the hydroxyl group of fluorescein.

Anand et al.reported a readily accessible disulfide 48(Fig.16)containing an anthracene fluorophore,which sensed

Ag(I )in the fluorescence turn-on mode with high selectivity over other metal cations in EtOH/HEPES bu?er (1:9,v/v,pH =7.4).149Upon addition of Ag(I ),the absorption band of the probe at 389nm was red-shifted by 26nm,resulting in a color change from yellow to colorless.Moreover,the probe emitted weakly (F =0.012)due to the PET blocking and the C Q N bond isomerization,but strongly emitted at 440nm (F =0.24)upon addition of Ag(I ).The probe was found to bind with Ag(I )in a 1:1stoichiometry,with the association constant and the detection limit of 6.407?102M à1and 2.797?10à7M,respectively.The reversibility of the interaction between the probe and Ag(I )was also confirmed by the addition of Na 2S into the solution containing the probe and Ag(I ),which resulted in weak fluorescence from the dissociated probe.

A supramolecular Ag(I )sensing system was reported by Iki and co-workers.150A bimetallic complex between Tb(III )ions and a photon-absorbing thiacalix[4]arene formed a supramolecular coordinate complex with Ag(I ),complex 49(Fig.16),enabling fluorescence detection of Ag(I )at nanomolar concentrations (3.2?10à9M;0.35ppb).The sensing properties of this system were found to be originated from the supramolecular complex formation,and not from the thiacalixarene or Tb(III )individu-ally,demonstrating the first ‘‘supramolecular approach’’to sense Ag(I ).

The aforementioned chemosensors indicate that both the cyclic as well as acyclic S-donor containing ligands are e?ective for the fluorescence detection of Ag(I ).A direct comparison revealed that cyclic ligands generally bind Ag(I )more strongly than the acyclic analogues,plausibly due to the additional stabilization by the macrocyclic e?ect.129But,in general,the chemosensors based on acyclic S-donor ligands show a higher degree of selectivity towards Ag(I ),plausibly by reduced binding a?nity to the competing metal ions.

3.2.1.2.Coordination to non-sulfur donor containing ligands.Several fluorescence probes for Ag(I )have been developed based on N and O-donor containing ligands.Additional stabilization by Ag(I )–p coordination was also observed for the Ag(I )-complexes in a few cases.

In 2002,Yoon and coworkers reported two pyrazole containing anthracene,50and 51,that sensed Ag(I )in the fluorescence turn-o?mode (Fig.17).151The probe 50responded to both Ag(I )and Cu(II )ions with fluorescence quenching,whereas the probe 51displayed selective fluorescent quenching only with Ag(I ).The 1,8-isomer 50showed about 100-fold stronger binding a?nity toward Ag(I )than the 9,10-isomer 51,but the former showed only a minor fluorescence change (0.3-fold)whereas the latter showed a larger change (20-fold),upon binding with Ag(I ).The larger change observed with 51was explained by the p –cation interaction involved only in this case.

Several other Ag(I )selective probes that also contain N-donors only (52and 53)152,153or contain both N-and O-donors (54,55,and 56)154,155(Fig.17)are known;however,all of which showed fluorescence quenching upon binding with Ag(I ),either due to the heavy metal e?ect (for 52and 54)or the reduction of ICT in the fluorophores attached (for 53,55,and 56

).

Fig.15Structures of the fluorescent chemosensors 44–46

.

Fig.16Structures of the fluorescent probes 47,48and a supramolecular complex 49.

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Kim and Yoon groups reported calixarene derivatives 57and 58bearing an azacrown ether as the Ag(I )binding site and pyrene as the signaling part (Fig.18).156The probes contain an addi-tional crown ether,which can bind to other metal ions such as K(I )(57)or Cs(I )(58).In ethanol,upon the addition of Ag(I ),the chelation enhanced fluorescence (CHEF e?ect)was observed for both 57and 58owing to the binding of Ag(I )at the azacrown site with a strong binding constant (K a =6.0?103M à1).But,after the addition of K(I )to the solution containing 57and Ag(I )(10eq.),chelation enhanced fluorescence quenching (CHEQ)was observed due to the complexation of K(I )into the crown

ether site which induced the decomplexation of Ag(I )from the azacrown site due to metal–metal ion repulsion.In the case of 58that contained the crown-6moiety for binding with Cs(I ),the fluorescence from 58–Ag(I )complex gradually decreased upon addition of the cesium ion (0–10eq.).The results established that the source of the binding selectivity towards Ag(I )comes from the calixarene and azacrown moieties.

Several other fluorescence probes (59–62)for Ag(I )have been developed based on the PET mechanism of fluorescence signaling (Fig.18).Among those,the naphthalimide-based probe 59(Fig.18)developed by Qian and co-workers is capable of bioimaging.157

The probe selectively responded to Ag(I )with turn-on fluorescence change at 533nm among various other metal ions in EtOH/Tris bu?er (4:6,v/v,pH =7.5).The Job plot indicated the formation of a 1:1complex between the probe and Ag(I ),and the association between them was determined to be 9.0(?0.5)?105M à1.

It was proposed that the N-and O-atoms of 8-alkoxyquinoline as well as the N-atoms of pyridine and piperazine moiety are responsible for metal binding.The probe was used to obtain bright green fluorescence images of HeLa cells that were pre-incubated with Ag(I ).

Use of high selenophilicity of Ag(I )was explored by Huang et al.to develop the fluorescence probe 60(Fig.18).158The probe sensed Ag(I )with enhanced fluorescence (4-fold),plausibly through inhibition of PET quenching.The probe showed excellent selectivity toward Ag(I )over other competing metal ions,especially Cu(II )and Hg(II )that are common inter-fering cations in many cases.

Amine associated PET quenching of lanthanide lumines-cence was also used by Dang et al.,to develop a fluorescence probe for Ag(I ).159Thus,the Tb(III )complex of the ligand 61(Fig.18)in methanol exhibited partially quenched fluorescence due to the PET process from the ‘‘free’’nitrogen lone pairs.However,upon addition of Ag(I )the terbium complex showed a noticeable fluorescence due to the binding of the nitrogen atoms with Ag(I ).Interestingly,other metal ions such as Na(I ),K(I ),Ca(II ),Ba(II ),and Zn(II ),etc.resulted in no increase in the emission intensity or a faint fluorescence,despite their binding interactions with the nitrogen atoms as suggested by the corres-ponding absorption spectral changes.

Hundal and co-workers reported the benzene-based tripodal imine 62(Fig.18)that sensed Ag(I )with fluorescence enhance-ment.160The probe emitted weakly at 413nm,plausibly due to PET quenching.Upon addition of Ag(I ),however,the fluores-cence was restored by 4-fold in CH 3CN/HEPES bu?er (8:2,v/v).Other than the PET mechanism,the charge transfer (CT)mechanism was also used to develop fluorescent chemosensors for Ag(I ),as exemplified by 63–67(Fig.19).

These molecules have two aromatic moieties,which can form CT complexes by intramolecular stacking.Introduction of Ag(I ),which can interact with the aromatic rings through cation–p interactions,would disturb the CT complexes and thus induce a fluorescence change.

The azine compounds 63and 64(Fig.19)reported by Bharadwaj and co-workers,exhibited weak emission with

an

Fig.17Structures of chemosensors 50–56that show Ag(I )-induced fluorescence

quenching.

Fig.18Structures of fluorescent Ag(I )chemosensors 57–62that are based on the PET process.

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absorption band due to intra-ligand charge transfer (ILCT)in the absence of a metal ion in THF.161Upon addition of Ag(I )ions,both the azine molecules emitted strongly (400-fold)at 487nm (for 63)and 560nm (for 64)with the binding constant (log b )of 10.32and 10.54,respectively.Only Cu(I )showed a small enhancement in the emission among other metal ions examined.The emission enhancement is due to the enhanced intramolecular charge transfer (ICT)in the presence of the metal ion that withdraws electron density from the donor amine moiety,along with the breaking of the CT complex by metal coordination.X-crystal structures of the Ag(I )-complexes reveal that,in each case,one Ag(I )is bonded to one N-atom of the azine moiety on each side.Each metal ion also showed weak bonding interactions with two C-atoms of the benzene ring.

Later,Pandey and co-workers reported the binuclear zinc(II )complex 65(Fig.19)that showed selective ‘‘on–off–on’’fluores-cence switching behaviour in the presence of Cu(II )and Ag(I ),respectively.162The complex emitted strong fluorescence in CH 3CN/Tris buffer (3:7,v/v,pH =7.0),which was quenched upon addition of Cu(II )but enhanced upon addition of Ag(I ).The fluorescence quenching in the presence of Cu(II )was attributed to the metal-to-ligand charge transfer (MLCT),whereas the fluorescence enhancement in the presence of Ag(I )was attribu-ted to the ligand-to-metal charge transfer (LMCT).The weekly emitting Cu(II )complex fluoresced strongly in the presence of Ag(I ),which suggested that Ag(I )could interact with the ligand more strongly than Cu(II );this was further supported by the higher association constant (log K a =8.05)observed for Ag(I )than that of Cu(II )(log K a =3.29).The chemosensor 65was also demonstrated to act as a molecular keypad lock system.

Another example of CT complexes was reported by Shi et al.,compounds 66and 67,which consisted of two triazolyl coumarin dyes installed onto the glucose (Fig.19).These compounds exhib-ited enhanced fluorescence in the presence of Ag(I )in an aqueous solution.163It was suggested that the two triazolyl coumarins coordinate with one silver ion through both the carbonyl groups of coumarin and one of the triazole nitrogens,causing CHEF.

The quantum yields of 66in water increased from 0.04to 0.10upon binding to Ag(I ).The compound 66showed high selectivity towards Ag(I )over other metal cations and was used for the fluorescence imaging of Ag(I )ions in Hep-G2cells.Due to the Lewis acid character of Ag(I ),it has a tendency to form Ag(I )–p type interaction with electron rich systems.Hence,Ag(I )selective fluorescence probes have been also developed based on the Ag(I )–p interaction,which could not only enhance the binding affinity but also modulate the signaling process.The tetra-dansylated diphenyl glycoluril 68(Fig.20)selec-tively sensed Ag(I )with the fluorescence enhancement.164An NMR study suggested the close proximity of Ag(I )to the dansyl aromatic moiety,plausibly through a cation p -type interaction.As a result,Ag(I )provided certain rigidity to the dansyl moieties,which would cause decrease in the non-radiative deactivation rate of the probe and thus increase in the fluorescence inten-sity.Fluorescence response of the probe in the presence of other metal ions was almost negligible compared to that of with Ag(I ).In 2013,Chao and co-workers reported the gold(I )acetylide complex 69(Fig.20)that also selectively sensed Ag(I )through the Ag(I )–p interaction.165Upon addition of Ag(I )into a solution of the probe in DMSO,around 18-fold enhancement in the emission intensity at 472nm was observed under excitation at 311nm,together with luminescence color change from blue-violet to blue-green.

The s -bonded Au–acetylide group usually emit from the 3

(pp *)excited state,which may be perturbed upon binding of Ag(I )at the acetylide group through Ag(I )–p interaction:it was suggested that the flexible nature of the probe would allow for conformational changes upon Ag(I )binding,which would bring the three arms close to each other and thus increase the possibility of forming intramolecular interactions among three Au atoms.The binding stoichiometry and affinity of the probe (log K )with Ag(I )were determined to be 1:1and 4.56?0.21in DMSO,respectively.The detection limit for Ag(I )was estimated to be 1.0?10à6mol dm à3.The control experiments were also carried to indicate that the acetylide groups and tripodal structures were responsible for the binding of Ag(I

).

Fig.19Structures of the charge transfer based chemosensors 63–67

.

Fig.20Structures of fluorescent Ag(I )chemosensors 68–71that are based on the Ag(I )–p interaction.

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The Ag(I )–p interaction could also give additional stability during the complexation,as exemplified by the fluorescence probes 70and 71that were reported by Ojida and co-workers (Fig.20)166Both the probes showed high selectivity to Ag(I ),which was ascribed due to the additional stability by the Ag(I )–arene interaction that was supported by X-ray crystallo-graphy and NMR analysis for the Ag(I )complex.A notable feature is that both the probes show ratiometric fluorescence changes toward Ag(I )in MeOH/HEPES buffer (1:1,v/v,pH =7.4).Upon addition of Ag(I ),the emission band of 70at 524nm shifted to 554nm (D l em =30nm)with the intensity ratio (I 554/I 524)change from 0.32to 11.0.The binding stoichiometry was determined to be 1:1by the Job plot,the binding constant was K a =4.53?105M à1,and the detection limit for Ag(I )was 65.6nM.The probe 71that has a diaza-18-crown ether also behaved similarly,but with inferior sensing properties (D l em =10nm:from 526nm to 536nm;I 536/I 526=0.77:from 0.74to 1.51;K a =2.89?105M à1).Some dyes show enhanced emission upon aggregation,which is called the aggregation-induced emission enhancement (AIE or AIEE).Thus,AIE fluorophores usually exhibit weak fluorescence in solution,but they become strongly emissive in the aggregate state due to the restriction of the rotational freedom.The AIE phenomenon has been explored in the development of fluores-cence probes in recent years.Among the AIE fluorophores,tetraphenylethylene (TPE)has received much attention due to synthetic feasibility and good photophysical properties.

Liu et al.reported the adenine-tagged TPE 72as a Ag(I )-selective AIE probe (Fig.21).167The emission band of the chemosensor at 470nm increased gradually with the increasing amount of Ag(I )in THF/H 2O (1:5,v/v).The fluorescence enhancement was attributed to the selective coordination of the adenine moieties with Ag(I )ions,leading to the formation of coordination complexes that formed aggregates due to the low https://www.wendangku.net/doc/9599574.html,ter,Ye et al.reported another AIE-based chemo-sensor 73(Fig.21)that sensed Ag(I )selectively with a ratiometric fluorescence change.168The chemosensor emitted rather weakly (at 435nm);however,after addition of Ag(I ),a new emission band at 485nm appeared with increasing intensity up to the point where the ratio of [Ag(I )]/[73]is below or equal to 2:1.The binding stoichiometry between 73and Ag(I )was 1:2.

As mentioned above,several strategies have been employed to develop Ag(I )selective fluorescence probes with improved sensing properties.All of the aforementioned probes exhibited absorption and emission bands in the UV-vis region (200–600nm)where some biomolecules also absorb light.As a result,autofluorescence

from the biomolecules could hamper the signaling process when those probes are used for bioimaging.Moreover,the use of strong energy light (from UV-vis region)could damage the cells and tissues.Also due to lower penetration ability,tissue imaging is difficult to achieve using the ‘‘UV-vis dyes’’.To avoid these problems,it is highly desirable to use a fluorophore which can be excited in the near-infrared (NIR)region of 650–950nm.

Wong and co-workers reported the expanded porphyrin[26]-hexaphyrin 74(Fig.22)that responded to Ag(I )with absorption and emission changes in the NIR wavelength region.169In the presence of Ag(I ),a light reddish color of the expanded porphyrin in MeOH turned to purple and then to blue;correspondingly,the absorption maximum at 543nm gradually decreased and a new band at 568nm appeared.In the emission spectra,a sharp decrease in the emission at 1050nm was observed upon the addition of Ag(I ),which was supposed to bind with the pyrrolic nitrogens.Among the other metal ions examined,Cu(II )and Hg(II )also caused a decrease in the NIR emission band.

Zheng et al.reported a heptamethine dye containing an adenine group,compound 75,which selectively sensed Ag(I )with a ratiometric fluorescence change (Fig.22).170The com-pound displayed two emission peaks,at 546nm and 731nm,in MeOH/H 2O (1/4,v/v,pH =5.4succinic acid–NaOH buffer).When Ag(I )was added to the solution,the emission band at 731nm decreased while that at 546nm increased,along with the solution color change from blue to pink.A 2:1complex between 75and Ag(I )was observed,which suggested adenine–Ag(I )–adenine binding interaction.The chemosensor exhibited excellent sensitivity towards Ag(I )with a detection limit of 34nM (ca.4ppb).The ratiometric response for Ag(I )was only observed in the acidic pH region (o 6.0).

In 2013,Li et al.reported another type of heptamethine-based chemosensor,compound 76(Fig.22),which sensed Ag(I )at neutral pH.171Upon addition of Ag(I )to the chemosensor in EtOH/HEPES bu?er (1:40,v/v,pH =7.0),the absorption band at 764nm decreased and a new band at 530nm increased,showing a colour change from blue to light red.

Accordingly,the emission band at 758nm decreased and that at 565nm increased,with the emission intensity ratio (I 565/I 758)changing from 5.95to 191in response to variation

of

Fig.21Structures of the AIE-based fluorescent chemosensors 72and 73for Ag(I

).

Fig.22Structures of the NIR fluorescent chemosensors 74–76.

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Ag(I )concentration from 0to 20m M.Among other metal ions examined,only Ag(I )caused the largest emission change from pH 6.5to 7.5with a response time of less than 3min.The detection limit was determined to be 2.0?10à7M.It was proposed that binding of Ag(I )with the piperazine moiety would shorten the pull–push p -conjugation system of the cyanine moiety,causing a large hypsochromic shift in the absorption and emission maxima as observed.

3.2.1.3.Excimer-based chemosensors.A host molecule or fluorescent chemosensor could be structurally fine-tuned to yield a self-assembled sandwich-like dimeric structure in the ground-state,called an excimer,through coordination events by an analyte.In the opposite way,a fluorescent chemosensor that is initially in the excimer state may be converted to a monomer upon binding with an analyte.Such excimer-based chemosensors may o?er ratiometric changes between the emission intensities from the monomer and the excimer.172The formation of excimer is highly dependent on the relative proximity between the fluorophores that have flat aromatic moieties.For example,pyrene,a flat and hydrophobic fluorophore,is highly suscep-tible to form the excimer.As a result,most of the excimer based chemosensors are mainly based on the pyrene fluorophore,except for a few examples of naphthalene.The excimer-based Ag(I )chemosensors thus can be mainly classified according to two sensing modes:(i)the monomer-to-excimer conversion,and (ii)the excimer-to-monomer conversion.

3.2.1.3.1.Monomer-to-excimer conversion.The excimer-based chemosensors of this type usually exhibit initially a strong monomeric emission from the dye,which upon binding with Ag(I )gradually decreases with concomitant appearance of an excimer emission at the longer wavelength.Accordingly,the ratio of excimer emission intensity to that of monomer emission intensity was used for the ratiometric sensing of Ag(I ).This type of probes thus consists of a fluorophore,mostly pyrene or naphthalene as the signaling moiety and a di?erent Ag(I )-specific receptor moiety (Fig.23).

The first example belongs to this category is the heterocyclic compound 77(Fig.23)reported by Yang et al.173The N,O-atoms of the heterocycle of 77were supposed to bind to Ag(I ),forming a 2:1complex between 77and Ag(I )in EtOH/H 2O (1:1,v/v,pH =7);the binding event induced the excimer band from the pyrene moieties.A structurally related one,the compound 78(Fig.23),was reported by Zhang et al.,which also showed the pyrene excimer emission upon binding Ag(I )in methanol.174Lee and co-workers also reported a structurally related but water-soluble chemosensor 79(Fig.23),175which was used to sense Ag(I )as well as silver nanoparticles.Due to the presence of the peptide moiety,the chemosensor was able to perform in an aqueous solution containing 1%of DMF or in an aqueous solution at pH 7.4.Upon addition of Ag(I ),the monomer emission bands of 79(at 378and 395nm)decreased while the excimer band at 480nm increased,with the intensity ratio (I 480/I 380)change from 0.0012to 0.4358(ca.363-fold).Among the other metal ions examined,Hg(II )induced a significant decrease in the monomer emission bands while a small increase

in the excimer band.Interestingly,it also showed a ratiometric response to AgNPs,with a ratiometric change in the emission intensities at 500and 378nm in aqueous solution.Ratiometric bioimaging of intracellular Ag(I )in HeLa cells was carried out with the probe:blue fluorescence from the probe was changed to green fluorescence in the presence of Ag(I ).Before treatment with Ag(I ),HeLa cells were washed with a 20mM HEPES bu?er solution (pH =7.4)containing NaNO 3,instead of NaCl,to avoid possible precipitation of Ag(I )ions as AgCl due to the high concentration of cellular chloride ions.A similar approach was reported by Liu et al.to sense Ag(I ),176where the compound 80(Fig.23)formed an excimer through the coordination between Ag(I )and two adenine (A)moieties in di?erent molecules.

Other than the pyrene fluorophore,naphthalene was also used as the signaling moiety in a few reports.In 2001,Glass

and

Fig.23Structures of fluorescent Ag(I )chemosensors 77–84based on the monomer-to-excimer formation.

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co-workers demonstrated a cooperative binding-induced excimer formation using two pinwheel systems.177

The systems contained four ethylenediamine moieties,which cooperatively interacted with Ag(I )and brought the attached fluorophores in close proximity to induce the excimer emission.Among several fluorophores tested only two fluoro-phores could induce the excimer band,namely naphthyl-sulfonanilide 81and pyrenyl acetanilide 82(Fig.23).

Later,Zhang et al.reported chemodosimeter 83(Fig.23)that contained two naphthyl-imidazolium moieties linked to a benzene ring.178The high coordinating ability of Ag(I )by the N-heterocyclic carbene led to selective sensing of Ag(I )over other heavy transition metal ions examined in CH 3CN/CH 2Cl 2(1:1,v/v).Coordination of Ag(I )to the benzoimidazolium moieties was suggested to prevent the PET quenching pathway from the excited naphthalene ring to the benzoimidazolium moiety.Formation of an excimer between the pyridyl moieties was also observed with compound 84(Fig.23)upon binding with Ag(I ),as demonstrated by Rao and co-workers.179A tetrahedral complex where Ag(I )bound with the four pyridine nitrogens was estab-lished by DFT computations and X-ray crystallographic data.3.2.1.3.2.Excimer-to-monomer conversion.An opposite mode to the above section is that a chemosensor initially remains in an excimer form due to the strong p –p interaction between the two arene fluorophores,but undergoes conforma-tional change upon binding Ag(I )to form the monomer form.Accordingly,the ratio of monomer emission intensity to that of excimer emission intensity enables the ratiometric sensing.Several chemosensors of this type containing pyrene fluoro-phore,85–91,have been developed through fine tuning of the receptor moieties (Fig.24).

Liu et al.developed a BINOL-pyrene derivative 85(Fig.24)by incorporating two pyrene rings through the triazole link that acted as the metal binding site.180The chemosensor showed the excimer-to-monomer ratiometric emission change upon binding with Ag(I )in MeOH/H 2O (200:1,v/v).No appreciable spectral change was observed upon addition of other metal ions except Hg(II ),which caused fluorescence quenching both for the monomer and excimer emissions.Wang et al.reported the bis(pyridylamine)derivative 86(Fig.24)that sensed Ag(I )with little inference from Hg(II )as well as other metal ions in DMSO/HEPES bu?er (1:1,v/v,pH =7.4).181

Excimer-based chemosensors have been also developed based on the calix[4]arene sca?old,as demonstrated by the chemosensors 87–91(Fig.24).The thiacalix[4]arene derivative 87(Fig.24)reported by Kumar and co-workers,displayed ratio-metric response with significant monomer emission enhance-ment at 377nm and excimer quenching at 470nm upon addition of Ag(I ).182Although the chemosensor possessed high sensitivity towards Ag(I ),it showed strong quenching behaviour towards Fe(III ).Concurrently,Yamato and co-workers also reported two Ag(I )-selective fluorescent chemosensors,88and 89(Fig.24),thiacalix[4]arene derivatives containing two pyrene rings through a triazole link.183Both of them,however,suffered from quenching interference from heavy metal ions such as

Cu(II )and Hg(II ).Later,Chung and co-workers reported another type of calixarene-based chemosensor 90(Fig.24),which showed higher metal ion sensitivity compared with the related compound 91(Fig.24).184Upon the addition of 10equivalents of Ag(I )in MeOH/CHCl 3(98:2,v/v),the excimer emission of probe 90(l max =476nm)decreased while the monomer emission (at 379and 398nm)increased.This ratiometric response towards Ag(I )was also observed even in 10%aqueous methanol solution.The efficient sensing properties of probe 90were ascribed due to the specific orientation of the lower-rim triazolylpyrenes,which played an important role in binding with Ag(I ).The chemosensor 90was not only more sensitive among the calix[4]arene-derived excimer-based chemosensors,but also it displayed reduced inter-ferences from the heavy metals (Hg(II ),Cu(II ))compared to others.In general,the excimer based chemosensors which showed monomer-to-excimer formation upon binding of Ag(I )seem to be more e?cient with respect to the selectivity over the

competing

Fig.24Structures of fluorescent Ag(I )chemosensors 85–91based on the monomer-to-excimer formation.

Chem Soc Rev Review Article

P u b l i s h e d o n 27 J a n u a r y 2015. D o w n l o a d e d b y N a n k a i U n i v e r s i t y o n 30/01/2015 10:40:47.

heavy metal ions compared to those chemosensors based on the excimer-to-monomer formation.

3.2.2.Reaction-based sensing systems.The chemosensors described above sense Ag(I)mainly through a reversible coordi-nation with the recognition/receptor part,of which association and dissociation with Ag(I)transduce a fluorescent signal from the fluorophore part.

In general,such receptor-based chemosensors are desirable for time-dependent monitoring of analytes in biological systems. However,it should be noted that the kinetically inert metal–ligand complexation requires an external,strong ligand to reverse the binding process,which would limit use of such metal-coordination based probes for time-dependent monitor-ing.Another potential limitation of the receptor-based chemo-sensors is the interference from other metal ions such as Hg(II) that compete with Ag(I)in binding to the receptor moiety.Also, the concentration range governed by the binding equilibrium may limit their practical use.One way to realize very high selectivity and high sensitivity of chemosensors for metal ions is the chemical reaction-based approach,so called the reaction-based or reactive chemosensors,or chemodosimeters.By exploring an analyte-specific chemical conversion to induce the fluorescence change,an irreversible but highly selective and sensitive chemosensor can be realized.Accordingly,many fluorescent chemosensors for metal cations have also been developed;however,it is only in recent years that the reaction-based approach for the fluorescent chemosensors for Ag(I)has received much attention from chemists.49,185 The colorimetric,not fluorimetric,detection of Ag(I)in aqueous solution was reported by Sun and co-workers,which was based upon the Ag(I)mediated oxidation of3,30,5,50-tetramethylbenzidine(TMB)92(Fig.25).186When a solution of92was treated with Ag(I)at room temperature for30min,a colour change from colourless to blue colour was observed with the appearance of three strong absorption peaks centered at 371,457,and656nm,respectively.The colour change was attributed to the oxidation of TMB by Ag(I)to form92a.No such colour change was observed in the cases of other metal ions except Fe(III);the interference was solved by using a chelating agent for Fe(III).The detection limit for Ag(I)was estimated to be as low https://www.wendangku.net/doc/9599574.html,ter,Gonza′lez-Fuenzalida et al.applied92for the in situ quantification of Ag(I)in the presence of AgNPs.187 This simple detection method was used to detect the Ag(I)ions adsorbed on the AgNP surface during nanoparticle formation. AgNPs were prepared by three di?erent strategies,one photo-chemical and two thermal.In all the cases,the yellow solution of AgNPs was turned to blue upon addition of92.This colour change indicated the presence of adsorbed Ag(I)ions on the AgNP surface,which was quantified at a confidence level of95%.

Czarnik and co-workers reported that the anthracene-thioamide93sensed Hg(II)as well as Ag(I)with fluorescence enhancement(Fig.26).85This is the first example where Ag(I)is fluorescently sensed through a chemical reaction,here desul-furization.The thioamide group acted as a fluorescence quencher;hence,its conversion to the amide group promoted by Ag(I)ions in water resulted in55-fold enhancement of the emission band center at413.5nm.

Later,the desulfurization strategy was adopted by Tsuka-moto and co-workers,to develop a coumarin-based system94 (Fig.26)that also detected Hg(II)and Ag(I)in the fluorescence turn-on mode.188In a phosphate bu?er(pH7.0)containing 0.1%DMSO,the N-acetylthioureido compound that was non-fluorescent underwent desulfurization upon treatment with Hg(II)or Ag(I),the conversion of which resulted in strong fluorescence at480nm(400-fold enhancement)within2min. The detection limit for Ag(I)was found to be1ppb.

The reaction-based approach to selectively sense Ag(I)was first disclosed by Ahn and co-workers in2009.54Thus,the N-iodoethyl-rhodamine lactam95(Fig.27)underwent the spiro-lactam ring-opening that was promoted by the coordination of Ag(I)to the iodide in EtOH/H2O(20:80,v/v),producing the ring-opened product95a.This chemical conversion was specific to Ag(I)among many metal ions examined,demon-strating the powerfulness of the reaction-based approach to realize complete analyte selectivity.The conversion accom-panied with a colour change from colourless to pink as well as a turn-on type fluorescence change with an orange emission band at558nm.The probe gave a good linear response toward Ag(I)in the concentration range of0.1–5.0m M,with the detec-tion limit of14ppb.They further demonstrated that such a reaction-based probe can be used to detect AgNPs quantita-tively for the first time,by combining the redox conversion of AgNPs to Ag(I)in the presence of H2O2and phosphoric acid. The probe was applied for the simple quantification of AgNPs in consumer products such as a hand sanitizer gel and a fabric softener.

On the basis of the strong a?nity of Ag(I)and Hg(II)ions toward selenium,Ma and co-workers developed the rhodamine B seleno-lactone96(Fig.28)as a fluorescence probe for imaging Ag(I)and Hg(II)in live cells.189The strong a?nity of selenium towards silver and mercury accelerated the rhodamine ring-opening followed

by Fig.25Ag(I)-mediated oxidation of3,30,5,50-tetramethylbenzidine(TMB)92

.

Fig.26Fluorescence sensing of Ag(I)/Hg(II)based on desulfurization of

thioamide93and thiourea94.

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