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A versatile fabrication of upconversion nanophosphors with functional-surface

A versatile fabrication of upconversion nanophosphors with functional-surface tunable ligands ?

Jing Zhou,Liming Yao,Chunyan Li and Fuyou Li *

Received 14th April 2010,Accepted 27th June 2010DOI:10.1039/c0jm01041c

Rare earth upconversion nanophosphors (UCNPs)have been recognized as a promising new class of biological luminescent labels for ?uorescence imaging.However,little work has been reported on the successful application of UCNPs in ?uorescence imaging so far.The major problem with UCNPs arises from the dif?culties in obtaining water-soluble UCNPs bearing appropriate surface functional groups.To solve this problem,herein we report a general procedure for the one-pot preparation of surface-functionalized UCNPs by a modi?ed polyol route assisted by difunctional ligands containing a carboxyl group,such as 11-aminoundecanoic acid (ADA),poly(ethylene

glycol)bis(carboxymethyl)ether (PEGBA)and folic acid (FA).The success of this one-pot method was con?rmed by FT-IR and 1H NMR spectroscopy.Due to the amino and carboxyl groups pointing outward,both amino-functionlized UCNPs (UCNPs-ADA)and carboxyl-functionlized UCNPs (UCNPs-PEGBA)are particularly suitable for coupling reactions with biological molecules and are potential candidates for bioimaging labels.By means of laser scanning upconversion luminescence microscopy,FA-functionlized UCNPs (UCNPs-FA)have been demonstrated to be effective in targeting folate-receptors overexpressing cancer cell lines.It is possible to design other surface-functionalized UCNPs by changing other bifunctional ligands with a carboxyl group.This strategy is general and facile for frabricating surface-functionalized UCNPs to potentially be used as bioimaging agents.

Introduction

Upconversion luminescence (UCL)involves conversion of two or more low energy photons,usually near infrared (NIR)(typi-cally 980nm),to higher energy visible emission through multiple photon absorption or energy transfer.1UCL is a rather unusual process and can only occur in materials in which multi-phonon relaxation processes are not predominant.In rare-earth compounds,the 4f or 5f electrons are ef?ciently shielded and are thus not strongly involved in the metal-to-ligand bonding.As a consequence,lanthanide compounds become important materials with UCL properties.1–11In comparison to conven-tional luminescent labels of several hundred nanometre (organic dyes,12metal complexes 13and quantum dots 14),nanosize upconversion nanophosphors (UCNPs)doped with Er 3+,Tm 3+,and/or Ho 3+1–11as ?uorescent labels exhibit deep penetration of NIR radiation,5absence of auto?uorescence of biological tissues,4–6minimal photodamage to biological tissues and large anti-Stokes shift.4As a result,UCNPs have recently been applied in bioimaging.3–6

For UCNPs to be successfully used as luminescent labels,they must exhibit good aqueous solubility and suitable surface func-tionality (i.e.,coating surface groups with carboxyl groups (–COOH),amino groups (–NH 2),or mercapto groups (–SH))to

allow further conjugation with biologically active molecules.Typically,two-step controllable synthetic strategies are used to convert the hydrophobic UCNPs into hydrophilic ones.These strategies include surface silanization,3b ,7polymer and organic coating,8a surface ligand oxidation process,3a ,9and ligand exchage.10Such two-step synthetic strategies are usually associ-ated with some intrinsic limitations,such as relatively compli-cated synthesis processes and post-treatment procedures.Recently,the groups of Zhang and Liu have reported a one-step approach for preparing water-soluble and compatible nano-crystals using polyethylenimine (PEI)as a surfactant.11Our group developed a modi?ed hydrothermal microemulsion route to prepare water-soluble and amine-functionlized UCNPs.5b These one-step methods are limited to synthesizing only amine-functionalized UCNPs.

We are interested in developing a versatile surface-modi?ca-tion method for the preparation of functional surface-tunable UCNPs.In the present study,we reported a versatile and general one-pot synthesis approach for functional-surface tunable UCNPs via a modi?ed polyol method assisted with functional-ized ligands containing a carboxylic acid,such as 11-amino-undecanoic acid (ADA),poly(ethylene glycol)bis (carboxymethyl)ether (PEGBA)and folic acid (FA),as shown in Scheme 1.This novel strategy is based on surface ligand exchange of diethylene glycol (DEG)coated UCNPs with carboxylic ligands because of the stronger coordination ability of the –COOH with lanthanide ions compared to the hydroxyl group (OH).The method conventionally provides functional surface-tunable UCNPs containing functional groups of –NH 2

Department of Chemistry &Advanced Materials Laboratory,Fudan University,220Handan Road,Shanghai,200433,P.R.China.E-mail:fyli@https://www.wendangku.net/doc/4b13154889.html,;Fax:+862155664621;Tel:+862155664185

?Electronic supplementary information (ESI)available:Supplementary Figs S1–11.See DOI:10.1039/c0jm01041c PAPER https://www.wendangku.net/doc/4b13154889.html,/materials |Journal of Materials Chemistry

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or –COOH.Moreover,the UCNPs containing the target mole-cule folic acid (UCNPs-FA)were easily synthesized and were further tested for application in targeted bioimaging.

Experimental section

Materials

All of the chemicals used were of analytical grade and were used without further puri?cation.Deionized water was used throughout.NaF,oleic acid (OA),11-aminoundecanoic acid (ADA),diethylene glycol (DEG),ethanol,ninhydrin,sodium acetate,acetate,ethylene glycol,n -propylalcohol and ascorbic acid were purchased from Sinopharm Chemical Reagent Co.China.Folic acid (FA)and poly(ethylene glycol)bis(carboxy-methyl)ether (PEGBA,MW 600)were obtained from Aldrich.Rare earth oxides Y 2O 3(99.999%),Yb 2O 3(99.999%),and Er 2O 3(99.999%)were purchased from Beijing Lansu Co.China.Rare earth chlorides (LnCl 3,Ln:Y,Yb,Er)were prepared by dissolving the corresponding metal oxide in 10%hydrochloric solution at elevated temperature and then evaporating the water completely.Ninhydrin (0.6g)was dissolved in mixed solvents of n -propy-lalcohol (30mL),ethylene glycol (60mL)and sodium acetate buffer (10mL,pH ?5.4)in order to obtain ninhydrin hydrate.Synthesis of diethylene glycol-coated UCNPs (UCNPs-DEG)All of the doping ratios of Ln 3+in our experiments were molar.Typically,in a polyol-mediated procedure for the preparation of NaYF 4:Yb,Er nanophosphors,15NaF was used to provide sodium ions and ?uoride ions simultaneously.20mL of DEG was added into a 100mL round-bottomed ?ask with a re?ux condenser,0.15g NaF was dispersed in DEG under vigorous stirring to form a clear solution.The mixture was then heated to 200 C under stirring under a ?ow of Ar.Then,the DEG solution (25mL)of 0.675mmol rare earth chlorides (LnCl 3,Ln:78mol%

Y,20mol%Yb,2mol%Er)was injected into the mixture con-taining NaF.The reaction mixture was stirred for 3h at 200 C.Synthesis of surface-functionalized UCNPs

After 3h,the above reaction mixture was cooled to 100 C.DEG solution (10mL)containing 3.375mmol oleic acid (OA)or 11-aminoundecanoic acid (ADA)or poly(ethylene glyco-l)bis(carboxymethyl)ether (PEGBA)or folic acid (FA)and 5.0mmol of NaOH was then injected into the mixture.The reaction mixture was stirred vigorously for 1h at 200 C under a ?ow of Ar.The system was allowed to cool to room temper-ature naturally,whereupon the pure powder was separated via centrifugation at a speed of 12000rpm and washed with water and then with anhydrous ethanol several times to remove the chloride ions and DEG,and it was then dried in a vacuum.Finally,DEG/OA,DEG/ADA,DEG/PEGBA and DEG/FA co-coupled UCNPs (de?ned as UCNPs-OA or UCNPs-ADA or UCNPs-PEGBA or UCNPs-FA,respectively)were obtained.Characterization

The size and morphologies of UCNPs were determined at 200kV using a JEOL JEM-2010F low to high resolution transmission electron microscope (HR-TEM).Samples of the UCNPs were prepared by placing a drop of dilute dispersion in water and chloroform respectively on the surface of a copper grid.Powder X-ray diffraction (XRD)measurements were taken with a Bruker D4X-ray diffractometer (Cu K a radiation,l ?0.15406nm).Fourier transform infrared (FT-IR)spectra were obtained from samples in KBr pellets using an IRPRESTIGE-21spec-trometer (Shimadzu).1H NMR spectra were recorded on a Var-ian Mercury 400spectrometer.Proton chemical shifts were reported in ppm down?eld from tetramethylsilane (TMS).Upconversion luminescence spectra were measured with an Edinburgh LFS-920?uorescence spectrometer using an external 0–1W adjustable continuous wave laser (980nm,Connet Fiber Optics,China)as the excitation source,instead of the Xenon source in the spectrophotometer.UV-visible absorption spectra were measured using a Shimadzu UV-2550ultraviolet-visible-near infrared spectrophotometer.Dynamic light scattering (DLS)and zeta potential experiments were carried out on an ALV-5000spectrometer goniometer equipped with an ALV/LSE-5004light scattering electronic and multiple tau digital correlator and a JDS Uniphase He–Ne laser (632.8nm)with an output power of 22mW.The size distribution was measured at 25 C with a detection angle of 90 .Quantitative analysis of amine (–NH 2)moieties of amino-functionlized UCNPs (UCNPs-ADA)

The ninhydrin analysis method 16was used to quantitatively analyze amine moieties on UCNPs-ADA.Dilutions of standard alanine (3.0?10à4M,0.05,0.10,0.15,0.25,0.40,0.50,0.60,0.70mL)in sodium acetate buffer (pH ?5.4)were placed in a series of 10mL volumetric ?asks.Then,0.95,0.90,0.85,0.75,0.60,0.50,0.40,0.30ml of sodium acetate buffer (pH ?5.4)were added to the volumetric ?asks,respectively.1.5mL ninhydrin hydrate and 0.05mL ascorbic acid (0.3%)were put into each ?ask,and were also added to 1mL sodium acetate

buffer

Scheme 1A schematic illustration of the one-pot synthesis of functional surface-tunable upconversion nanophosphors,(A)as-prepared UCNPs-DEG;(B1)amphiphilic OA-functionalized UCNPs (UCNPs-OA);(B2)NH 2-functionalized UCNPs (UCNPs-ADA);(B3)COOH-functional-ized UCNPs (UCNPs-PEGBA);(B4)FA-functionalized UCNPs (UCNPs-FA).

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(pH ?5.4)containing the dried UCNPs-ADA (10mg).The test ?asks were heated in boiling water in a beaker for 15min,stirring gently.When the yellow ninhydrin turned purple (known as Ruhemann’s purple),the ?asks were cooled to room temperature and then the solvents in the ?asks were diluted to 10mL.The amine groups were quanti?ed according to the Ruhemann’s purple quanti?cation protocol based on the measurement of the absorption of the purple complex solution at l ?570nm.For greater accuracy,the sample was tested three times.Cell culture

A human nasopharyngeal epidermal carcinoma cell line (K

B cell)

and a human breast adenocarcinoma cell line (MCF-7cell)were provided by Shanghai Institutes for Biological Sciences (SIBS),Chinese Academy of Sciences (CAS,China).The KB Cells were grown in RPMI 1640(Roswell Park Memorial Institute’s medium)supplemented with 10%FBS (fetal bovine serum)at 37 C and 5%CO 2.The MCF-7cells were grown in MEM supplemented with 10%FBS and 1%insulin (10mL:400U)at 37 C and 5%CO 2.Cytotoxicity of UCNPs-FA

In vitro cytotoxicity was measured by performing methyl thia-zolyl tetrazolium (MTT)assays on the KB cells.Cells were seeded into a 96-well cell culture plate at 5?104/well,under 100%humidity,and were cultured at 37 C and 5%CO 2for 24h;different concentrations of UCNPs-FA (0,62.5,125,250and 500m g mL à1,diluted in RPMI 1640)were then added to the wells.The cells were subsequently incubated for 4h or 24h at 37 C under 5%CO 2.Thereafter,MTT (10m L;5mg mL à1)was added to each well and the plate was incubated for an additional 4h at 37 C under 5%CO 2.After the addition of 10%sodium dodecyl sulfate (SDS,100m L/well),the assay plate was allowed to stand at room temperature for 12h.The optical density OD570value (Abs.)of each well,with background subtraction at 690nm,was measured by means of a Tecan In?nite M200monochromator-based multi-function microplate reader.The following formula was used to calculate the inhibition of cell growth:

Cell viability (%)?(mean of Abs.value of treatment group/mean Abs.value of control)?100%.

Laser scanning upconversion luminescence imaging

KB cells and MCF-7cells (5?108/L)were plated on 14mm glass coverslips and allowed to adhere for 24h.Subsequently,cells were washed with phosphate buffer solution (PBS).To study the targeted imaging of the folic acid-conjugated UCNPs (UCNPs-FA),the KB cells (FR-positive tumor cells)were incubated in a serum-free medium containing 150m g mL à1UCNPs-FA for 1h at 37 C under 5%CO 2,and then washed with suf?cient PBS to remove excess nanoparticles.To con?rm the receptor-mediated uptake,a probe control (UCNPs-DEG)and a cell control (FR-negative MCF-7tumor cells)were also tested under the same https://www.wendangku.net/doc/4b13154889.html,petition experiments were conducted where the KB cell culture was treated for 1h at 37 C with 50m L of 1mM folic acid solution and then with 150m g mL à1UCNPs-FA.

Laser scanning upconversion luminescence imaging was per-formed with our previously reported laser scanning luminescence microscope with an Olympus FV1000scanning unit.4Cells were excited by a CW infrared laser operating at 980nm (Connet Fiber Optics,China).A 40?oil-immersion objective lens was used and the upconversion luminescence signals were detected in the red channel (640–680nm).

Results and discussion

Synthesis and characterization of UCNPs-DEG

The UCNPs-DEG in the absence of the carboxyl group (–COOH)could be easily dispersed in water and ethanol to form a transparent colloidal solution,indicating the presence of a hydroxyl group (–OH)of DEG and adsorbed H 2O on the surface of the UCNPs.From the low magni?cation transmission electron microscope (TEM)images of UCNPs-DEG (Fig.1A),it can be seen that the as-prepared UCNPs-DEG samples consist of nanosize particles with short pillar-like morphology,that are somewhat agglomerated and probably stick to one another.The composition and crystallinity of the as-prepared DEG-capped UCNPs (UCNPs-DEG)were investigated by X-ray diffraction (XRD)(Fig.2A).The results of XRD indicate that these samples crystallized well,and all of the diffraction peaks’positions and intensities for NaYF 4are in good agreement with the data for the reference cubic phase (JCPDS card No.77-2042).The broad-ening of the diffraction peaks ascribed to a -NaYF 4distinctly indicates the nanocrystalline nature.Therefore,XRD patterns revealed that the a -NaYF 4nanocrystals formed were of high purity.In a further investigation,high-resolution transmission electron microscope (HR-TEM)analysis (Fig.1C)clearly reveals

the lattice fringes with a constant spacing of 3.2 A

ascribed to

the Fig.1TEM images of (A)UCNPs-DEG and (B)UCNPs-OA samples and (C)HR-TEM images of UCNPs-DEG

samples.

Fig.2XRD patterns of (A)UCNPs-DEG,(B)UCNPs-OA samples,and (C)the standard patterns of pure cubic NaYF 4(JCPDS card No.77-2042).

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(111)lattice planes of cubic NaYF4,indicative of the high crys-tallinity of these particles.Such well-crystallized surface-func-tionalized nanoparticles were obtained due to the high temperature during the synthesis process.

Synthesis and characterization of UCNPs-OA

It is well known that the–COOH group has a stronger coordi-nation ability with rare earth ions.Generally,the surface group of the UCNPs obtained through the polyol method is the–OH group.Therefore,it is reasonable to expect that the surface–OH ligand of the UCNPs is exchanged with the carboxylic compounds.Herein,we chose oleic acid(OA),a well-known surface ligand,as an example of the carboxylic compounds to exchange the surface ligand of UCNPs-DEG.

The successful process of the above ligand-exchange reaction

was con?rmed using Fourier transform infrared(FT-IR)spec-troscopy.As can be seen in Fig.3A,for the as-prepared UCNPs-DEG,the broad bands are consistent with previously reported data for DEG-coated nanocrystals.17The peaks at3444and 1634cmà1are due to the stretching vibration of the–OH and adsorbed H2O,and the bands at1463and746cmà1are assigned to the scissoring and rocking vibrations of the CH2groups, respectively.The peak at1063cmà1can be attributed to the stretching vibration of the C–O bond in https://www.wendangku.net/doc/4b13154889.html,pared with UCNPs-DEG,the FT-IR spectrum(Fig.3B)of UCNPs-OA displayed some new peaks at3007,2926,2855and1558cmà1.A characteristic band at3007cmà1is attributed to the?C–H stretching vibration.2926and2855cmà1correspond to asym-metric(n as)and symmetric(n s)stretching vibrations of methylene (CH2)in the long alkyl chain of OA;1558cmà1is associated with the asymmetric(n as)stretching vibrations of the carboxylic group of the bound oleic acid.These new peaks are similar to those of our previously reported OA-coated UCNPs,9indicating the presence of OA on the surface of the UCNPs.The peak at 3430cmà1,found in the spectrum of the UCNPs-OA sample,is associated with the O–H stretching vibration of DEG and adsorbed H2O.These results demonstrate that DEG and OA had co-coated on the surface of the UCNPs.

The presence of the OA coating on the surface of the UCNPs was further veri?ed by1H NMR measurements(Fig.4).To avoid interference from the dopant ions(Yb3+and Er3+)with the1H NMR signal,herein,pure NaYF4nanoparticles were selected for the1H NMR investigation.The1H NMR spectrum(Fig.4A)of UCNPs-DEG dispersed in D2O displayed peaks at d?3.67and 3.60ppm,attributed to the–CH2–bonds of DEG.d?1.87ppm was assigned to the–OH of DEG,indicating the presence of the surface ligands of DEG.The1H NMR spectrum of the UCNPs-OA dispersed in CDCl3was also recorded(Fig.4B).New peaks at d?0.83ppm,assigned to–CH3,and d?1.25and1.56ppm corresponded to the–CH2–bonds of the long alkyl chain and peaks centered at 2.01and 5.34ppm attributed to the –CH2COOH and–CH2]CH2–moieties suggest that OA was apparently coating the surface of the UCNPs.Moreover,the resonance peaks of DEG in the1H NMR spectrum of UCNPs-OA were shifted to low-?eld in comparison to those in the spectrum of UCNPs-DEG.On the basis of the above-described FT-IR and1H NMR results,it can be deduced that DEG and OA are co-coating the surface of UCNPs-OA.

The surface modi?cation had no obvious effect on the phases of the UCNPs,so the obtained UCNPs-OA were still cubic phase (JCPDS card No.77-2042)(Fig.2B).In the presence of OA,the surface properties of the obtained UCNPs were

changed

Fig.41H NMR spectra of(A)UCNPs-DEG dissolved in D2O and(B)

UCNPs-OA dissolved in CDCl3

Fig.5Room temperature UCL spectra of UCNPs-DEG(black line,1

mg mLà1)dissolved in water and UCNPs-OA(red line,1mg mLà1)

dissolved in chloroform.The UCNP samples were excited with a CW980

nm laser(power z800mW).Inset:the corresponding photographs show

UCNPs-OA dissolved in chloroform in daylight(left)and excited with

a CW980nm laser(right).

signi?cantly during our modi?ed polyol process.The high degree of dispersibility in Fig.1B may indicate that the surface of the nuclei was covered by oleic acid (OA)right after formation,which would limit the growth of particles and stabilize them against agglomeration.The average length and width of the particles in water are 30and 15nm,respectively.Owing to the presence of long chains of OA and hydrophilic groups of DEG on their surface (Scheme 1B),the resultant UCNPs-OA were amphiphilic.Moreover,as shown in Fig.5inset,the UCNPs-OA can disperse in chloroform in contrast to the hydrophilic UCNPs-DEG,proving that the UCNPs-OA are amphiphilic and are coated by OA and DEG.

Room temperature upconversion ?uorescence spectra of the UCNPs-DEG and UCNPs-OA under CW excitation at 980nm are shown in Fig.5.The prominent emission bands in the visible region are readily assigned to Er 3+transitions from 4F 9/2(between 635and 685nm),4S 3/2(between 534and 658nm),2H 11/2(between 515and 534nm)to the 4I 15/2ground state,respectively.The peak around 669nm (red emission)is much higher than peaks in the green emission range,which explains the nano-particles appearing red to the eye when pumped by the 980nm laser (Fig.5inset).

Synthesis and characterization of amine-functionalized and carboxylic acid-functionalized UCNPs

To establish the generality of the described synthetic method,we have extended our effort to the synthesis of UCNPs with DEG and other bifunctional ligands with a carboxyl group,such as 11-aminoundecanoic acid (ADA),poly(ethylene-glycol)bis(carboxymethyl)ether (PEGBA)and folic acid (FA).ADA contains not only a carboxyl group,but also an amino functional group,which can be further conjugated with biomolecules for bioimaging.ADA is likely to coat the surface of UCNPs owing to the strong coordination ability of the –COOH group with lanthanide ions.The success of the one-pot synthesis of the UCNPs-ADA was con?rmed by FT-IR and 1H NMR spectra (Figs S1and S2?).In the FT-IR spectra (Fig.S1),UCNPs-ADA exhibit a broad band at about 3431cm à1,corre-sponding to the O–H and/or N–H stretching vibration.The 2928and 2855cm à1transmission bands are respectively assigned to the asymmetric (n as )and symmetric (n s )stretching vibrations of methylene (CH 2)units in the long alkyl chain.The broad band at 1567cm à1is assigned to the primary amine group.Moreover,the peaks at 1459and 1411cm à1indicate carbonyl and C–C stretching vibrations,respectively.Moreover,in the 1H NMR spectrum of UCNPs-ADA samples (Fig.S2?),the resonance peaks of both DEG and ADA were observed,revealing that the ADA had been successfully conjugated onto the UCNPs.XRD patterns demonstrated that the phases of UCNPs-ADA was cubic NaYF 4(reference cubic phase JCPDS card No.77-2042)(Fig.S3?).The obtained UCNPs-ADA can emit red light under 980nm excitation (Fig.S4?).Because free amine moieties were found on the surface of UCNPs-ADA,their content was further determined by the ninhydrin method.16The amine content of the UCNPs-ADA was determined to be 9.62?10à6mol g à1(Fig.S5?).Owing to the amine on the surface of UCNPs-ADA,these functional nanoparticles are suitable for the conjugation of biomolecules and as potential labels for bioimaging.

PEGBA was chosen as a dicarboxylic ligand because the layer of PEG chains can dramatically reduce the uptake of nano-particles into the reticuloendothelial system (RES),which rapidly recognizes and ingests most injected nanoparticles as foreign substances.18PEGBA modi?cation on the UCNPs’surface was also investigated by FT-IR and 1H NMR measurements.Herein,as shown in Fig.3A,UCNPs-DEG did not exhibit absorption bands at 2928and 2858cm à1normally assigned to the asym-metric (n as )and symmetric (n s )stretching vibrations of methylene (CH 2)units in the long alkyl chain of PEGBA.After modi?ca-tion of the UCNPs with PEGBA,absorption peaks derived from PEGBA were clearly observed despite repeated ultracentrifuga-tion treatments (Fig.S6?).Additionally,the 1627cm à1trans-mission band is attributed to the asymmetric (n as )stretching vibrations of the carboxylic group of the bound PEGBA (Fig.S6?).Moreover,a new peak (d ?3.87ppm)in the 1H NMR spectrum (Fig.S7?)was assigned to the –CH 2–bonds beside the carboxylic groups of PEGBA.Both FT-IR and 1H NMR spectra indicated that the PEGBA ligands had bound to the surface of the UCNPs.

To further con?rm the conjugation of PEGBA onto the UCNPs’surface,the charge on the UCNPs-PEGBA’s surface was estimated.Fig.S8shows the zeta potential of the UCNPs-PEGBA.?After the PEGylation of UCNPs,the free polymer in the solution was separated by repeated ultracentrifugation and washed with water and anhydrous ethanol several times,and the UCNPs were then dissolved in water.The zeta potential of the UCNPs-DEG was about 20.1mV,the charge of UCNPs-PEGBA had decreased to about 5.41mV after modi?cation with PEGBA.The results of zeta potential analysis indicated that the charge on the UCNPs’surface can be attributed to the carboxyl group of bound PEGBA.

The dispersion stability of UCNPs-PEGBA was then investi-gated by dynamic light scattering (DLS)measurement.As shown in Fig.S9A and S9C,?the size distribution of the UCNPs-DEG is about 139.7nm,because they are somewhat agglomerated and probably stick to one another,as can be seen from the TEM measurements (Fig.1A).The hydrodynamic diameter of UCNPs-PEGBA (53.3nm)is expected to be smaller than the diameter of UCNPs-DEG,as the length of the ligands on the UCNPs’surface is also taken into account.These results of zeta potential and DLS analysis indicated that PEGBA effectively modi?ed the UCNPs elecrostatically to improve their dispersion stability.

Targeted bioimaging application of folic acid (FA)-functionalized UCNPs (UCNPs-FA)

The above mentioned data con?rmed that this modi?ed polyol strategy assisted by a carboxylic ligand is a versatile synthetic method for synthesizing functional surface-tunable UCNPs.Herein,we further developed this strategy for targeted bio-imaging.In light of the fact that folic acid (FA)contains two carboxyl groups per molecule and can easily bond to lanthanide ions,herein,FA was chosen as the target molecule.Moreover,FA is a high-af?nity ligand of the folate receptor (FR)and has attracted wide attention as a targeting agent,owing to its high stability,compatibility with both organic and aqueous solvents,nonimmunogenic character,low cost,and ability to conjugate

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with a wide variety of molecules.It can target FR overexpressing cancer cells lines(such as KB cells)because of its ability to react with this high af?nity,membrane-anchored protein.19

The success of the one-pot synthesis of UCNPs-FA was con?rmed by FT-IR spectrum.In Fig.S10,?the peaks at 3444and3290cmà1are due to the O–H and/or N–H stretching vibrations,and the2927and2923cmà1transmission bands are respectively assigned to the asymmetric(n as)and symmetric(n s) stretching vibrations of methylene(CH2)units in FA.The bands at1592,1554,1346and1178cmà1are assigned to the appearance of a benzene ring.The peak at744cmà1indicates the existence of secondary N–H.The1H NMR data also demonstrate that the FA was successfully conjugated onto the UCNPs(Fig.S11?). The resulting UCNPs-FA can be well-dispersed in water.The stability of the UCNPs-FA was further investigated under different NaCl concentrations(Fig.6).The UCNPs-FA were dispersed in varying salt solutions of NaCl,in the range 0.1–1.2mol Là1.Only a slight change in the upconversion luminescence emission of the UCNPs-FA was observed.Taken together,these results demonstrate that the FA-coated nano-particles are robust under varying salt solution concentrations and that surface modi?cation has no effect on the optical properties.

With regard to their potential application as bioprobes in bioimaging,it is necessary to evaluate the cytotoxicity of these nanoparticles.UCNPs-FA were chosen to be tested for their cytotoxicity on the basis of the reduction activity of MTT.The particles were delivered over a range of dosages(0–500m g mLà1 in our experiment)to a folate receptor(FR)positive KB cells using an in vitro toxicology assay kit(TOX-1,Sigma),as shown in Fig.7.It was demonstrated that after24h of incubation,more than85%of the cells survived,even at a high dose concentration of500m g mLà1.On the basis of the above-described MTT results,it can be deduced that the target group functionalized UCNPs-FA is biocompatible and nontoxic to living cells.Their low cytotoxicity implies that UCNPs-FA can potentially serve as ?uorescent probes for?uorescence imaging.

Further practical application of UCNPs-FA in luminescence-targeted imaging was investigated by utilizing our laser scanning upconversion luminescence microscope(LSUCLM)equipped with CW excitation at980nm.4To evaluate the target recogni-tion capability of UCNPs-FA and limit the non-receptor medi-ated endocytosis of these nanoparticles,KB(FR-positive)and MCF-7(FR-negative)cells were incubated in a serum-free medium containing UCNPs-FA(150m g mLà1)at37 C for1

Fig.7In vitro cell viability of KB cells incubated with different

concentrations(0,62.5,125,250and500m g mLà1)of UCNPs-FA for4or

24h at37 C.Error bars denote the standard

deviation.

Fig.8Upconversion luminescence and bright-?eld images of KB cells

measured on our modi?ed laser scanning upconversion luminescence

microscope equipped with a CW980nm laser:(A)KB cells(FR-positive)

incubated with150m g mLà1UCNPs-FA for1h at37 C;(B)cell control:

MCF-7cells(FR-negative)incubated with UCNPs-FA;(C)material

control:KB cells incubated with UCNPs-DEG under the same condi-

tions;(D)competition experiment:KB cells incubated with free folic acid

(50m L,1mM)for1h at37 C and then incubated with150m g mLà1

UCNPs-FA for1h at37 C.UCL signals were collected at640–680nm.

The merges of UCL images and bright?eld images are also shown.

(Fig.8A and B).For comparison,KB cells were also incubated with non-FA conjugated UCNPs-DEG (150m g mL à1)(Fig.8C)under otherwise identical conditions.KB cells were incubated with excess free FA (50m L;1mM)for 1h at 37 C and then incubated with UCNPs-FA (150m g mL à1)for 1h at 37 C (Fig.8D).

As shown in Fig.8A2,a strong UCL signal was observed in the KB cells under CW excitation at 980nm,whereas the MCF-7cells showed weak luminescence (Fig.8B2)due to the KB cells expressing a higher amount of folate receptor than the MCF-7cells.In addition,bright-?eld images of the cells were also taken (Fig.8A1and 8B1).The merge of UCL and bright?eld images (Fig.8A3and 8B3)revealed that upconversion luminescence originated from the surface of the KB cells,which should be attributed to the highly speci?c interaction between FA on the UCNPs-FA and FR on the KB cells.In contrast,both KB cells incubated with UCNPs-DEG (Fig.8C2)and cells in the competition experiment (Fig.8D2)showed a weak signal in the UCL images,suggesting non-speci?c binding of these nano-particles to the cells.These results establish that UCNPs-FA selectively accumulated on the surface of the KB cells and could be used as targeting agents for targeted imaging of KB cells with overexpressed FR.

Moreover,it should be noted that no auto?uorescence signal could be found in the UCL image of KB cells labeled with UCNPs-FA.Further quanti?cation analysis of the UCL signal across the line reveals a perfect signal-to-noise ratio with extremely high UCL intensity surpassing the predetermined detection threshold (counts >4095,region-1and region-3)and no background ?uorescence (counts $0,region-2),as demon-strated in Fig.9.Such an extremely high signal-to-noise ratio of UCNPs could hardly be obtained with conventional ?uorescent probes,which lack this unique upconversion luminescence mechanism.These results reveal that UCNPs could be used as novel biological luminescence labels with no background ?uorescence.

Conclusions

In conclusion,we have demonstrated a new and ef?cient proce-dure for the one-pot preparation of surface-functionalized upconversion nanophosphors by a polyol strategy assisted with

difunctional ligands containing a carboxylic group.The successful coating with difunctional ligands containing a carboxylic group was con?rmed by the FT-IR and 1H NMR spectra.The free surface amine content of the ADA-modi?ed UCNPs allows for further conjugation with biomolecules for bioimaging.PEGBA-modi?ed UCNPs have carboxyl groups protruding from the surface,which limits the size of the nano-particles.The resulting FA-conjugated nanoparticles are able to target FR-overexpressing cancer cells with no background ?uo-rescence and are promising candidates for targeted probes in bioimaging.Moreover,the generality of our one-pot method can be extended to the synthesis of UCNPs with DEG and other functional ligands with a carboxyl group,which opens up new perspectives for the synthesis of surface-functionalized ligands for biolabeling and targeted imaging.

The authors thank the ?nancial support from National Natural Science Funds for Distinguish Young Scholar (20825101),National Science Foundation of China (20775017),NCET-06-0353,and Shanghai Leading Academic Discipline Project (B108).

Notes and references

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