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Highlighting work on a 2D surface enhanced Raman
scattering (SERS) array presented by Prof. Dongpeng Yan, Prof. Min Wei and co-workers at the State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China.
Surface enhanced Raman scattering based on Au nanoparticles/layered double hydroxide ultrathin ? lms A two-dimensional SERS ? lm based on Au nanoparticles and layered double hydroxide nanosheets has been fabricated, which shows an enhanced electromagnetic ? eld and can be widely used in the detection of various analytes.
See Dongpeng Yan, Min Wei et al .,
Mater . Chem . C , 2015, 3, 5167.
This journal is ?The Royal Society of Chemistry 2015J.Mater.Chem.C,2015,3,5167--5174|5167
Cite this:J.Mater.Chem.C,2015,3,5167
Surface enhanced Raman scattering based on Au nanoparticles/layered double hydroxide ultrathin films ?
Rui Tian,a Mingwan Li,a Haiyan Teng,a Heng Luo,a Dongpeng Yan*ab and Min Wei*a
Surface-enhanced Raman scattering (SERS)is an e?ective characterization method which has shown broad applications in the field of detection and identification.In this work,we describe the facile fabrication of SERS ultrathin films (UTFs)based on a two-dimentional architecture via layer-by-layer assembly of hygroscopic sodium polyacrylate (PAAS)modified Au nanoparticles (NPs)and MgAl layered double hydroxide (LDH)nanosheets.The resulting Au@PAAS/LDH UTFs possess a periodic long-range ordered layered structure and dense immobilization of Au NPs within the LDH interlayer microenvironment.The distance of Au NPs in the x–y plane reaches as low as B 2.16nm,and thus the coupling of Au–Au NPs enabled an enhanced electro-magnetic field and improved SERS effect.For example,a detection limit as low as 10à9M could be obtained
for Rhodamine 6G.Moreover,the obtained UTFs could be adapted to various solvents and dye molecules,confirming the versatility and the sensitivity of the 2D UTFs.In addition,the UTF exhibits satisfactory reusability and high stability at high temperature and various pH values.Therefore,this work highlights a high-efficiency SERS film material with facile preparation,which guarantees their further application in environmental detection and analysis.
1Introduction
Surface-enhanced Raman scattering (SERS)is an ultrasensitive spectroscopic characterization technique,which has been widely used in label-free detection and identification of pollutants,biomolecules,and chemical residues.1–5The properties of SERS-active materials play a key role in determining the SERS sensitivity and effectiveness,which have been regarded as the research focus in this field.6,7Conventional SERS-active materials can be classi-fied into two categories:metallic nanoparticles (NPs)in a colloidal solution (such as Au,Ag and Cu)8,9and roughened metallic surfaces (e.g.,films fabricated through electron beam lithography,chemical etching,mechanical deformation and electroplating).10,11However,the former often suffers from the formation of large conglomerations of NPs,while the latter has been limited by the complicated fabrication process and unstable nature of
the nanoparticles.12Therefore,the development of suitable species and/or new approaches toward the facile fabrication of Raman-active materials with the advantages of distinct signals,good reproducibility and high stability in the surrounding medium remains a challenging goal.1,13–15
Au NP is a well-known SERS-active medium,which has promising applications in the field of detection and analysis.16,17To achieve high SERS signal and sensitivity,control over ordered degree and uniform particle distribution of Au NPs is of crucial importance,1,18–20as a result of the NP–NP couplings.21,22To meet this requirement,one effective way is to assemble Au NPs with a suitable solid building block to obtain an ordered structure and morphology,and an idealized building block can also be regarded as an effective nano-spacer for efficient NP–NP couplings in both horizontal and vertical directions.23,24Layered double hydroxides (LDHs),with the general formula of [M II 1àx M III x (OH)2]z +A n àz /n áy H 2O (M II and M III are di-and trivalent metal ions respectively,and A n àis a guest anion),are one family of inorganic layered matrix with tunable versatility in chemical composition and high stability.25–31Recently,the exfoliated LDH nanosheet has been applied as a building block for the fabrication of various multifunctional thin films with advanced performance.32,33In our previous work,we reported several LDH nanosheet-based materials for fluorescence imaging,34intelligent display 35and optical detection applications.36The confined environment imposed by the 2D LDH nanosheets usually affords the as-prepared
films
a
State Key Laboratory of Chemical Resource Engineering,
Beijing University of Chemical Technology,Beijing 100029,P.R.China.E-mail:yandongpeng001@https://www.wendangku.net/doc/3b10292552.html,,weimin@https://www.wendangku.net/doc/3b10292552.html,;Fax:+86-10-64425385;Tel:+86-10-64412131b
Key Laboratory of Theoretical and Computational Photochemistry,Ministry of
Education,College of Chemistry,Beijing Normal University,Beijing 100875,China ?Electronic supplementary information (ESI)available:Morphology of Au NPs;assembly process and morphological characterization of (Au@PAAS/LDHs)n (n =0–10)UTFs;Raman tests of LDH nanosheets and drop-casted samples;loading and unloading properties of the UTFs.See DOI:10.1039/c4tc02714k
Received 27th November 2014,Accepted 1st April 2015DOI:10.1039/c4tc02714k
https://www.wendangku.net/doc/3b10292552.html,/MaterialsC
Journal of
Materials Chemistry C
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with superior performance compared with the pristine solution and solid forms.This thus inspires us to fabricate a new type of 2D-organized SERS-active material via the assembly of Au NPs with LDH nanosheets,which may take advantages of both components:(i)vertically and horizontally dense immobilization of Au NPs can be achieved via the layer-by-layer assembly approach,in which the short distance between Au NPs would promote the formation of ‘‘hotspots’’;37,38(ii)the incorporation of Au into the rigid LDH matrix would provide a high stability and reusability so as to perform highly-efficient SERS detection.In this work,we demonstrate the fabrication of (Au@PAAS/LDH)n ultrathin films via a two-step procedure (Scheme 1):(1)modification of Au NPs (particle size:14nm)with hygroscopic sodium polyacrylate (PAAS)to obtain Au@PAAS;(2)assembly of Au@PAAS with positively-charged LDH nanosheets via a layer-by-layer method to obtain (Au@PAAS/LDH)n UTFs.The as-prepared UTFs display a periodic long-range ordered structure and dense immobilization of Au NPs within the LDH interlayer microenviron-ment.The distance between adjacent Au NPs can reach as short as 2.16nm in the horizontal direction (Scheme 1B),which results in a largely enhanced electromagnetic field.The vibration of molecules in this electromagnetic field can be further amplified,which provided a detection limit of Rhodamine 6G (R6G)as low as 10à9M.Moreover,these UTFs can be adapted in the detection and identification for various solvents and analytes.In addition,largely-improved reusability and remarkable stability can also be achieved for these sophisticated UTFs.Therefore,a facilely fabricated SERS material with finely tuned uniformity and high coupling between particles can be obtained,which realizes the optimized electromagnetic field enhancement,and guarantees their further application in the fields of real-time and on-site detection.
2Experimental section
Materials
Hydrogen tetrachloroaurate hydrate (HAuCl 4á3H 2O,499.9%)was purchased from Aldrich Co.Ltd.Analytical-grade chemicals
including formamide,Mg(NO 3)2á6H 2O,Al(NO 3)3á9H 2O,urea,sodium citrate,sodium polyacrylate (PAAS),NaNO 3and HNO 3were purchased from Aladdin Chemical.Co.Ltd.Deionized and de-CO 2water was used in all the experimental processes.
Preparation of Au NPs
The aqueous synthesis of sodium citrated reduced Au NPs was performed according to a reported method.39,40In a typical synth-esis,100ml of HAuCl 4á3H 2O (1.0mmol)was heated to boiling under stirring,followed by addition of 10ml of sodium citrate (38.8mmol).The solution was stirred under boiling for 10min,and at room temperature for another 10min.The obtained Au NP colloidal suspension was cooled to room temperature and stored at 41C.
Fabrication of (Au@PAAS/LDH)n UTFs
The process of synthesis and exfoliation of MgAl-LDH was similar to the procedure described in our previous work.34,350.1g of MgAl-LDH was shaken in formamide (100ml)for 24h to produce a colloidal suspension of exfoliated MgAl-LDH nanosheets.41The quartz glass substrate was cleaned by immersion in a concentrated H 2SO 4–30%H 2O 2(7:3,v/v)solution for 30min.The Au@PAAS solution was obtained by adding PAAS (0.5g L à1,2ml)solution into an Au NP colloid solution (8ml).In order to fabricate the (Au@PAAS/LDH)n films,the substrate was dipped in a colloidal suspension (1g L à1)of LDH nanosheets for 10min followed by washing thoroughly,and then immersed into the Au@PAAS solution for another 10min.Multilayer films of (Au@PAAS/LDH)n were fabricated by alternate deposition of LDH nanosheets and Au@PAAS aqueous solution for n cycles.Finally,the UTFs were rinsed and washed thoroughly with deionized water and dried in a flow of nitrogen gas.Loading and unloading of analytes
Taking Rhodamine 6G (R6G)as the example for Raman reporter,the UTFs were dipped into an R6G solution (pH =10)for 2h to load dyes onto UTFs for Raman test,and the unloading process was carried out by dipping the UTFs into water (pH =10)for another 2h.
Sample characterization
Transmission electron microscopy (TEM)images were recorded on a JEOL JEM-2100TEM with an accelerating voltage of 200kV.The UV-vis absorption spectra were collected on a Shimadzu U-3000spectrophotometer,with a slit width of 1.0nm.X-ray di?raction patterns (XRD)of the UTFs were recorded using a Rigaku 2500VB2+PC di?ractometer under the conditions:40kV,50mA,Cu K a radiation (l =0.154056nm)with step-scanned in steps of 0.041(2y )in the range from 0.51to 21using a count time of 10s per step.The morphology of thin films was investigated using a scanning electron microscope (SEM Hitachi S-3500),and the accelerating voltage applied was 20kV.The surface roughness data were obtained using a NanoScope IIIa atomic force microscope (AFM)from Veeco Instruments.The SERS measurements were performed at room temperature on a LabRAM ARAMIS Raman system using a 633nm helium–neon laser line as the excitation source.The laser spot size was B 1m
m
Scheme 1(A)Preparation of a Au@PAAS precursor;(B)schematic representa-tion of the assembly process;(C)and (D)are the finite-di?erence time-domain (FDTD)simulation in x–y and x–z planes.
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with an incident power of 0.325mW and an acquisition time of 1s.The light impinging on the multilayer structures is propagating along z and polarized in the x direction.The accumulation time and laser power were the same for all Raman spectra.The fluorescence spectra were performed on a RF-5301PC fluorospectrophotometer with an excitation wavelength of 500nm.The fluorescence emission spectra range from 520–650nm,and both the excitation and emission slits are set to be 10nm.
3Results and discussion
Assembly of the (Au@PAAS/LDH)n UTFs
The well-dispersed Au NP colloidal suspension with a particle size of 14nm can be synthesized by the reduction of a HAuCl 4solution (Fig.S1in ESI ?),and a typical absorption band at 520nm is observed as a result of plasmon resonance (Fig.S2,ESI ?).39An attempt for the direct assembly of Au NPs with LDH nanosheets was carried out firstly,but a serious aggregation and non-uniform distribution of Au NPs are observed (Fig.S3,ESI ?).Alternatively,Au NPs were firstly modified with PAAS to obtain Au@PAAS with a better dispersion,which was further used to assemble with LDH nanosheets via a layer-by-layer method.The multilayer assembly process of the (Au@PAAS/LDH)n UTFs was monitored by UV-vis absorption spectroscopy.As PAAS shows no absorption (Fig.S3,ESI ?),the absorption band at B 520nm is attributed to plasmon resonance absorption of Au NPs in the UTFs (Fig.1A).The band intensity increases linearly with the increase of bilayer number n (Fig.1C),indicating a stepwise and regular film growth procedure,which can be further confirmed by the visible color evolution (Fig.1B).Moreover,the initial molar ratio of Au NPs to PAAS was carefully studied,and the typical UV-vis spectra for (Au@PAAS/LDH)10UTFs with variable Au/PAAS ratios are displayed in Fig.S4(ESI ?).It shows that the absorbance increases linearly with the increasing ratio of Au/PAAS,suggesting that the quantity of assembled Au NPs could be finely tuned within the UTF,which may further tailor the related electromagnetic field around Au NPs.The structural and morphological studies of the (Au@PAAS/LDHs)n UTFs
The structural and morphological properties of (Au@PAAS/LDHs)n UTFs were further studied.The small angle XRD pattern (Fig.2A)
displays a basal reflection at 2y =0.5251,indicating a periodic long-range ordered superlattice structure with a repeating thickness of 16.81nm.The obtained spacing distance is close to the monolayer arrangement of Au NPs (14nm)in the interlayer region of LDH nanosheets (0.48nm).In this way,a thin and rigid LDH nanosheet is employed as a confined nanospacer between Au NPs in the adjacent layers and the distance between Au NPs in the vertical direction can be estimated to be B 2.8nm (Scheme 1B).The typical top-view SEM images of these UTFs (Fig.2B and Fig.S5,ESI ?)reveal a homogeneous and uniform surface morphology.The NPs exhibit dense immobilization and uniform arrangement in the x–y plane,and the average distances are about 84.5,53.1,35.9,21.2,9.5and 2.16nm between adjacent Au NPs as the Au :PAAS ratio ranges from 0.25to 4as shown in TEM images (Fig.S6,ESI ?).The obtained structure may facilitate the formation of ‘‘hotspots’’which is beneficial to their coupling for the enhancement of the near-field intensity.The AFM topographical images (2m m ?2m m)of the (Au@PAAS/LDHs)n UTFs are illustrated in Fig.2C and Fig.S7(ESI ?),in which the well-dispersed Au NPs are observed with a high density.The root-mean square (RMS)roughness ranges from 3.60–10.26nm (Table S1,ESI ?),indicating the continuous and smooth surface of these UTFs.The UTF thickness can be estimated based on their side-view SEM images (Fig.2D and Fig.S8,ESI ?),from which an approximately linear increase of film thickness as a function of n was observed.An average thickness increment of 16.40nm per bilayer cycle was obtained (Table S1,ESI ?),which is consistent with the XRD observations (16.81nm).The results further confirm the uniform and periodic layered structure of these UTFs throughout the fabrication process.SERS performances
The field enhancement of (Au@PAAS/LDH)n UTFs was further studied by detecting their Raman intensity.In this work,R6G,which possesses well-established vibrational features (Table S2,ESI ?),was employed as the target analyte.The laser line of 633nm which does not disturb the fluorescence of R6G
was
Fig.1(A)UV-vis absorption spectra,(B)photographs,and (C)linear correlation between absorbance at 520nm and number of layers for the (Au@PAAS/LDHs)n (n =0–10)UTFs (the molar ratio of Au :PAAS is 4:
1).Fig.2The structural and morphological characterization of (Au@PAAS/LDH)10UTF:(A)XRD pattern,(B)top-view SEM image,(C)tapping-mode AFM topographical image,and (D)side-view SEM image.
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chosen for all the SERS measurements.The Raman spectrum of (Au@PAAS/LDH)10UTF was recorded (Fig.S9,ESI ?),which shows relatively weak vibrational peaks compared with that of R6G.Fig.S10(ESI ?)displays the Raman signals of polar light irradiated from di?erent polarization directions.It is found that the Raman signals from various directions show equivalent intensity at 609,1310,1360and 1649cm à1,respectively,which indicates that the LDH nanosheets have no influence on SERS performance of Au NPs.As a result,LDH nanosheets serve as a favorable matrix for the good dispersion and regular stacking of Au NPs for the investigation of the near-field enhancement e?ect.Moreover,(Au@PAAS/LDH)n UTFs with various Au/PAAS molar ratios were further studied (Fig.3).Compared with the bare Au NPs on substrates which exhibit low SERS e?ect and inhomogeneity (Fig.S11,ESI ?),the increase of the Au/PAAS molar ratio has e?ectively promoted the Raman intensity.Raman mapping in Fig.3indicated that good uniformity can be obtained for a ratio of 2to 4.Thus,UTF with Au/PAAS ratio of 4:1possesses better near-field enhancement e?ect and SERS signal,and the good uniformity enabled its practical use for subsequent tests.
Raman signals of R6G (10à3M)over the (Au@PAAS/LDH)n UTFs were studied in comparison with that of the bare glass substrate.Notable enhancement of the Raman signals of R6G is observed on the UTFs,and the signal intensities with R6G loaded on (Au@PAAS/LDH)10UTF were B 200times stronger than those obtained on glass (Fig.S12,ESI ?).In the study of SERS performance for the (Au@PAAS/LDH)n UTFs with a single layer (n =1)and multilayer (n 41),the multilayer (n =10)UTF exhibited much more enhanced intensity (Fig.S13,ESI ?)due to the accumulated signal.In addition,to quantitatively measure the enhancement capability of the (Au@PAAS/LDH)10UTF,SERS spectra for the UTF treated with various R6G solutions (concentration:from 10à3M to 10à9M)were recorded (Fig.4).The detection limit in the (Au@PAAS/LDH)n UTF system can reach as low as 10à9M,which shows a significant promotion
for detection compared with the pure glass substrate (10à3M).Such a low detection limit can be related to the enhanced electromagnetic field induced by the plasmonic couplings of densely arranged Au NPs in the vertical direction.And this detection limit is slightly higher or comparable with those of the previous reported systems for the detection of R6G by Au NPs (10à8M to 10à9M).42–46
A 3D finite-di?erence time-domain (FDTD)simulation was adopted by a commercial FDTD package (EMpro 2011.07)to study the spatial distribution of the electric fields,and a laser wavelength of 633nm propagates along the z direction and polarized in the x direction.With Au–Au distances about 2.16nm in the x–y plane and 2.8nm for the x–z cross section,regular ‘hot spots’between the neighboring Au NPs in the x–y plane and a certain degree of enhancement in the x–z cross section can be observed (Scheme 1C and D and Fig.S14,ESI ?).47,48The abundant ‘hot spots’in the x–y plane may be responsible for the sensitive SERS detection of analytes.More-over,several factors would influence the distribution of induced dipoles and electromagnetic field,such as the direc-tion of polarization light,total size of the nano array,distance and coupling effect between adjacent nanoparticles.49–51From this viewpoint,we investigated the effect of coupling from adjacent nanoparticles,considering both the incident light and the intense field induced by the surrounding nanoparticles (Fig.S15and S16,ESI ?).It is found that the asymmetric arrangement of nanoparticles in the x or y direction induces an inhomogeneous field distribution;while a more uniform distribution is obtained by adding nanoparticles in the bound-ary or extending the size of the nano array.
The local EM enhancement factors is estimated to be B 9.38?104,roughly as the fourth power of the local field:G loc =[E loc (l )/E 0(l )]4.52–54In addition,the experimental enhancement factor has also been calculated based on the reported expression:G =(I SERS /I Raman )?(N Raman /N SERS ),where I SERS and I Raman are the Raman signal intensity in the SERS spectra and blank Raman spectra;N Raman and N SERS are the number of molecules in the range of the laser spot area.
55
Fig.3Raman mapping of UTFs with various Au/PAAS molar ratios:(A)Au :PAAS =0.25:1,(B)Au :PAAS =0.5:1,(C)Au :PAAS =1:1,(D)Au :PAAS =2:1,(E)Au :PAAS =3:1,(F)Au :PAAS =4:
1.
Fig.4SERS spectra of R6G with various concentrations over (Au@PAAS/LDH)10UTF:(a)10à3M,(b)10à4M,(c)10à5M,(d)10à6M,(e)10à7M,(f)10à8M and (g)10à9M.
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Therefore,the value of I SERS /I Raman is about 200at 1508cm à1as displayed in Fig.S12(ESI ?),and N Raman and N SERS are supposed to be 0.945?108and 0.802?106in the volume of laser spot.56,57Accordingly,the EF of (Au@PAAS/LDH)10UTF is B 2.36?104,which is in accordance with the calculated EF (B 9.38?104).Therefore,the (Au@PAAS/LDH)10UTF possesses strong electric fields at the nanogaps between Au NPs,and can be used in SERS applications.
Application in the detection of various analytes
In actual cases,various analytes are often dispersed in di?erent media (such as organic and aqueous solutions).This situation requires a versatile sensor toward various analytes in the aqueous or organic phase.In this regard,the application scope of (Au@PAAS/LDH)10UTF was further studied in several typical solvents (acetone,formamide,1,2-dichloroethane (DCE),ethanol and water)and toward di?erent analytes (methylene blue,congo red,crystal violet,acid red and nile blue).Fig.5A shows the SERS performance of R6G in di?erent solvents,whose vibrational peaks (609,770,1310,1360,1508and 1649cm à1)are clearly detected for all these samples.Moreover,to identify the versatility and the sensitivity of the UTFs,several analytes were also detected.Taking methylene blue as an example,Raman peaks at 448,1188,1397and 1627cm à1are observed as typical vibration for the drop-casted sample on glass (Fig.S17A,ESI ?).A concentration down to 10à9M can be measured by the (Au@PAAS/LDH)10UTF with obvious Raman vibration peaks from methylene blue (Fig.5B).Furthermore,congo red,crystal violet,
acid red and nile blue are detected as well (the reference sample (ca.10à2M)fabricated by a drop-casting method are shown in Fig.S17B àE,ESI ?).Their detection limit values are 10à9M for congo red,10à10M for crystal violet,10à4M for acid red and 10à10M for nile blue,respectively (Fig.5C–F).Therefore,the application range of (Au@PAAS/LDH)n UTFs can be readily extended to various analytes regardless of molecular binding;particularly,the conjugated molecules containing C Q C bonds,aromatic rings and/or heterocyclic rings may provide a better signal and lower detection limit due to their strong Raman vibration.
To further extend the Au@PAAS/LDH towards SERS application in real analyte systems,the SERS performance of (Au@PAAS/LDH)10has also been analysed for the typical chemical residues in food.Fishery antiseptics (malachite green,a forbidden antiseptic in the seafood industry)and ectoparasiticide (thiram,a commonly ectopar-asiticide in seed and crop)have been taken as examples.Fig.S18A (ESI ?)shows the Raman characteristic vibration of thiram powder,and vibration bands at 395,560,928and 1380cm à1can be assigned to the characteristic stretching of S–S,S Q S,C Q S and C–N,respec-tively.SERS measurement of different concentrations of thiram in a mixture of water and formamide by (Au@PAAS/LDH)10UTF has been shown in Fig.6A,in which the 10à7M can be detected for this stable UTF.This detection limit is almost the same as the previous reported values.58–61Moreover,SERS measurements for malachite green were also performed and a concentration down to 10à12M can be detected (Fig.6B),in which the characteristic vibration bands are located in the same positions as the powdered form (Fig.S18B,ESI ?).This result is much lower than the general detection limit in previous literature.62–65Therefore,these (Au@PAAS/LDH)n UTFs can be readily extended to other systems for the detection of low-concentration chemical residues,which may pave an effective way for real SERS application.
Moreover,the dye molecules (such as R6G)are also one of the most common chemical residues in real systems,such as in paints.Herein,the trace detection of the R6G dye used in paints has been further studied.Fig.7shows the detailed procedure and the Raman spectrum detected in this real application.After dissolving the R6G in paints in water and loading it using (Au@PAAS/LDH)10UTF,the characteristic Raman shifts assigned to R6G at 609,770,1310,1360,1508and 1640cm à1can be clearly observed.Therefore,this Au@PAAS/LDH system can be potentially used for the trace detection of residues in the printing and dyeing
fields.
Fig.5(A)SERS spectra for the (Au@PAAS/LDH)10UTF treated with R6G (10à3M)in di?erent solvents ((a)ethanol,(b)water,(c)dichloroethane,(d)formamide,(e)acetone);SERS detection toward various analytes with di?erent concentrations:(B)methylene blue,(C)congo red,(D)crystal violet,(E)acid red,(F)nile
blue.
Fig.6SERS detection toward chemical residue analytes with di?erent concentrations:(A)thiram ((a)10à3M,(b)10à4M,(c)10à5M,(d)10à6M and (e)10à7M),and (B)malachite green ((a)10à6M,(b)10à7M,(c)10à8M,(d)10à9M,(e)10à10M,(f)10à11M and (g)10à12M).
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Reusability and stability tests
If the Au@PAAS/LDH UTF has the ability to load/unload analytes in a controlled fashion,the serious problem for traditional solid-state SERS materials—irreversible binding of the substrate with the analyte—can be avoided.And the loaded analyte can be concentrated in the interlayer of LDH due to the confinement e?ect.66,67After the (Au@PAAS/LDHs)10UTF was immersed in an R6G solution (pH =10)to load the dye for Raman test,pure water was used to rinse the UTF to remove the excess R6G from the surface.The aim of this operation is to ensure the loaded R6G is incorporated into the UTF at the ‘‘hotspot’’formed by adjacent Au NPs.The fluorescence emission spectra of the dye-loaded UTF reveal a strong emission at B 550nm (Fig.8A)originating from the luminescence of https://www.wendangku.net/doc/3b10292552.html,parison on the fluorescence spectra of (Au@PAAS/LDH)10,(Au/LDH)10and (PAAS/LDH)10UTF shows that the employment of PAAS enables the quantitative loading of R6G into the UTF,while UTF without PAAS cannot capture the R6G molecules in the aqueous solution e?ectively (Fig.S19,ESI ?).Additionally,release of the incorporated R6G was measured by the release kinetics of R6G in water at pH =10(Fig.S20and S21,ESI ?),indicating a complete unloading process.The complete loading and unloading process can also be verified by SERS measurements before and after loading of R6G (Fig.8B)with the obviously observed vibrations at
characteristic bands.The reversible tests were also carried out in di?erent solvents (acetone,formamide,DCE and ethanol)containing R6G.The results reveal that these organic solvents are suitable for the loading and partial unloading of R6G;while water (pH =10)can achieve an almost complete release of R6G within the UTF (Fig.8C).Reusability,which is a key factor in practical applications,36was also demonstrated by immersing the UTF in an R6G solution and water (pH =10)alternatively (Fig.8D).And the complete loading and unloading process for six cycles can be observed,which is in accordance with reversible adsorption behavior in polyelectrolyte LbL films.68–71Therefore,addition of PAAS into Au NPs can effectively capture the low-concentration analytes into the interlayer environment of LDHs in a reversible manner.
The morphology change of the (Au@PAAS/LDHs)10UTF was monitored by AFM images during a complete loading–unloading process.The original UTF surface (Fig.S22A,ESI ?)is smooth with a RMS roughness of 9.7nm,while a relative rough surface is observed after loading of R6G (Fig.S22B,ESI ?),accompanied with an increase in RMS roughness (12.6nm).After unloading R6G using water,a relatively smooth surface is recovered with a RMS roughness of 10.1nm (Fig.S22C,ESI ?).The reversible changes in morphology indicate that the embedding and removal of R6G impose no obvious influence on the structure of (Au@PAAS/LDHs)n UTF.72
The stability of the obtained (Au@PAAS/LDH)10UTF was further studied by monitoring its UV-vis spectrum at elevated temperature and various pH values.Upon heating the (Au@PAAS/LDH)10UTF at 601C from 0to 30min,a slight decrease in absorption intensity at B 520nm is observed without red-shift or broadening (Fig.9A).This phenomenon demonstrated that high temperature did not cause the oxidation or aggregation of Au NPs,which may be attributed to the protection of LDH nanosheets.33,34Similar results are also obtained when the UTFs are treated with di?erent pH values (Fig.9B):no significant change in absorbance at B 520nm is observed,indicating that the (Au@PAAS/LDH)10UTF remains stable in the pH range 5–14.
4Conclusions
In summary,ordered (Au@PAAS/LDH)n UTFs were fabricated by a facile assembly of inorganic LDH nanosheets with sodium polyacrylate (PAAS)modified Au NPs on quartz substrates.The UTFs exhibit a periodic long-range heterogeneous structure
and
Fig.7The detailed procedure of R6G detection in paints,and the obtained Raman spectrum by SERS of (Au@PAAS/LDH)10
UTF.
Fig.8Reversibility test of (Au@PAAS/LDH)10UTF:(A)relative fluores-cence emission spectra,(B)SERS measurements ((a)loading,(b)unloading,(c)reloading),(C)relative fluorescence intensity of R6G in di?erent solvents ((a)acetone,(b)formamide,(c)DCE,(d)water,(e)ethanol;black:loading in the solvent,red:unloading in the solvent,blue-unloading in water at pH =10),and (D)fluorescence intensity of the Au@PAAS/LDH UTF treated by an R6G solution and H 2O
alternatively.
Fig.9Stability test of the (Au@PAAS/LDH)10UTF recorded by UV-vis absorption spectra:(A)heat treatment at 601C for various times,and (B)absorbance at 520nm at di?erent pH values.
P u b l i s h e d o n 03 A p r i l 2015. D o w n l o a d e d b y B e i j i n g U n i v e r s i t y o f C h e m i c a l T e c h n o l o g y o n 22/05/2015 09:04:24.
This journal is ?The Royal Society of Chemistry 2015J.Mater.Chem.C,2015,3,5167--5174|5173
uniform morphology.Tight arrangement of Au NPs with about 2.16nm and 2.8nm in x–y and x–z planes can be constructed using LDH nanosheets,giving rise to an enlarged electromagnetic field for SERS detection.The concentrated dye molecules in the (Au@PAAS/LDH)n UTFs can be influenced by the generated electro-magnetic field,and a concentration down to 10à9M for R6G could be successfully detected due to the intensified vibration of mole-cules.This system was also studied for di?erent solvents (acetone,formamide,1,2-dichloroethane (DCE),ethanol and water)and various dye molecules (methylene blue,congo red,crystal violet,acid red,nile blue,thiram and malachite green),further confirming the versatility of the UTFs.Moreover,the as-prepared UTFs exhibit decent reusability and stability at high temperature and various pH values,which guarantees their practical recycling applications.Therefore,it is anticipated that this facile and cost-e?ective strategy for fabrication of near-field enhanced materials would facilitate potential applications in the detection and identification of the commonly used analyte molecules.
Acknowledgements
This work was supported by the 973Program (Grant No.2014CB932104),the National Natural Science Foundation of China (NSFC),Beijing Municipal Natural Science Foundation (Grant No.2152016),and the Fundamental Research Funds for the Central Universities (YS1406).M.Wei appreciates the financial aid from the China National Funds for Distinguished Young Scientists of the NSFC.
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