二步電沉積法制備Au/氧化石墨烯復合薄膜作為SERS基底

徐玲　姚愛華＊,1,　胥巖　王德平1,

(1同濟大學先進土木工程材料教育部重點實驗室，上海200092)

(2同濟大學材料科學與工程學院，上海200092)

二步電沉積法制備Au/氧化石墨烯復合薄膜作為SERS基底

徐玲2姚愛華＊,1,2胥巖2王德平1,2

(1同濟大學先進土木工程材料教育部重點實驗室，上海200092)

(2同濟大學材料科學與工程學院，上海200092)

采用二步電沉積方法在Ti片表面制備了Au-氧化石墨烯(Au-GO)復合薄膜，通過XRD、SEM、XPS等對薄膜的組成、結構和形貌進行了表征，并以羅丹明6G(R6G)為探針分子，對Au-GO/Ti基底的SERS活性進行了表征。結果顯示，Au納米顆粒尺寸約為60 nm，均勻、致密分布于GO表面，該基底顯示出較高的SERS活性，對R6G分子的檢測極限可達～10-10mol·L-1，增強因子高達約106，且基底顯示出良好的穩(wěn)定性，在冰箱中存放90 d后，SERS活性僅降低30%左右。

金；氧化石墨烯；表面增強拉曼散射；復合薄膜

Surface-enhanced Raman scattering(SERS)is a non-destructive,ultrasensitive and powerful analytical technique,which has been widely used in the identification and detection of chemical and biological molecules,gas sensing,and food and environmental monitoring[1].To date,various SERS-active substrates with high Raman enhancementfactors(EFs)have been successfully developed.In particular,noble metal nanoparticles(Ag and Au)of various sizes and shapes have shown the most promise as SERS substrates dueto their considerable electromagnetic contribution to SERS enhancement[2].Graphene and its derivatives (such as graphene oxide,GO,and reduced graphene oxide,rGO)demonstrate several advantages as SERS-active substrates for ultrasensitive Raman detection, such as additional chemical enhancement,quenching molecule fluorescence,and a high adsorption capacity toward target molecules[3-4].Compared to graphene,GO can be produced at higher yields,and is therefore more suitable for practical applications.GO possesses abundant oxygen-containing functional groups that serve as active sites for metal nanoparticles immobilization.Additionally,it exhibits high affinity toward various compounds(especially aromatic species) throughπ-πstacking and electrostatic interaction, thus bringing analytes in close proximity to the substrate,and facilitating electromagnetic and chemical enhancement[4].A recent study has shown that GO allows for a higher degree of chemical enhancement than pristine graphene because its oxygen-containing groups(carboxyl,epoxy,carbonyl,and hydroxyl)have a larger polarizability and stronger local dipole moments[5].

Recently,hybrid films consisting of graphene layers and metallic nanostructures have been developed as SERS substrates to achieve high sensitivity,good reproducibility and long-term stability.Two approaches were usually adopted for fabrication of the hybrid films including transferring graphene to a prepared metallic nanostructure and fabricating metallic nanoparticles on grapheme-coated substrates[6].For example, Zhao et al.[7]deposited a 4-nm-thick Au film on graphene grown on Cu foils by plasma sputtering,achieving an EF of～10-6for R6G and a low detection limit of 0.1 nmol·L-1for Sudan III.Zhu et al.[8]transferred graphene onto wet-chemical synthetic Au nanovoid arrays for detection of R6G with EFs of～104.In these cases,complex experimental conditions and multiple synthesis steps were usually required.By contrast, electrodeposition is a simple,scalable,and environmentally friendly process.Moreover,this approach eliminates chemical contamination during substrate fabrication and allows for control of the size and coverage of the metal NPs deposited conducting surfaces.

Herein,a highly sensitive SERS-active Au/GO hybrid film was fabricated using a simple,scalable and environmentally friendly two-step electrodeposition process.In this technique,GO layers were first deposited onto a Tisheetby electrophoretic deposition from an aqueous GO suspension.Subsequently,Au NPs were simultaneously electrochemically reduced and deposited on the rGO layers.The sensitivity, reproducibility,and stability of the resultant Au NP-rGO hybrid film were characterized employing R6G as a probe molecule.

1　Experimental

1.1Chemicals

HAuCl4·4H2O,KH2PO4,ethanoland acetone were purchased from Shanghai Aladdin Chemical Reagent Company.Graphene oxide(GO)was purchased from Nanjing XFNANO Materials Tech.Co.Ltd.Titanium sheets(99.5%purity)were purchased from Beijing Chemical Works.All reagents were analytical grade and used without further purification.All aqueous solutions were prepared with double-distilled water (18.2 MΩ·cm).

1.2Two-step electrodeposition of GO and Au nanoparticles on Tisheet

Prior to deposition,Ti sheets of 17×17×0.5 mm3were cleaned sequentially with ethanol,acetone and deionized water for 10 min in an ultrasonic bath.The sheets were then chemically polished by immersion in a mixture of HF and HNO3acids(VHF:VHNO3=1:1.2)for 30 s and rinsed in deionized water.Electrophoretic deposition of GO on the Ti sheet was performed in a two-electrode cell with a Ti sheet as the anode and a Pt sheet as the cathode.The distance between the two electrodes was 1 cm.The cleaned Ti sheets were immersed in a 10 mL suspension of GO(0.5 mg·mL-1), and a DC voltage of 3 V was applied for 400 s to depositGO onto the surface ofthe Tisheet.

Electrodeposition of Au nanoparticles on the GO-deposited Ti sheets was performed on a CHI660D electrochemistry station(Shanghai CH Instruments,China)using a three-electrode system.The GO-deposited Ti sheet was used as working electrode,Pt wire as counter electrode and Ag/AgCl(3 mol·L-1KCl)as reference electrode.Electrodeposition process was performed by applying a potentialof-5 V for 400 s in an aqueous solution containing 75 mmol·L-1KH2PO4and 6 mmol·L-1HAuCl4.For comparison,Au NPs were directly deposited on bare Ti sheet under the same conditions.

1.3Characterization

The morphologies of GO,Au,and Au-GO hybrid films were observed using a field emission scanning electron microscope(FESEM,Hitachi S-2360).The crystal phase was characterized using X-ray powder diffraction(XRD,Rigaku Dmax2550)with Cu Kα radiation(λ=0.154 18 nm)operated at40 kV and 40 mA.The elemental composition of the films was analyzed by X-ray photoelectron spectroscopy(XPS, Escalab250Xi,ThermoScientfic,USA)using a 500 μm-diameter beam ofmonochromatic Al Kαradiation. SERS measurements were conducted using a Renishaw inVia micro-Raman spectrometer with He-Ne laser excitation at 532 nm.Prior to measurement,the asprepared SERS substrates were immersed for 12 h in freshly prepared aqueous solutions of R6G at various concentrations,then thoroughly rinsed with deionized water and dried under nitrogen flow.The SERS spectra were collected from at least 5 random locations with an accumulation time of 10 s,and then averaged.

Fig.1(a)XRD pattern of Au-GO/Ti substrate;(b)UV-Vis absorption spectra of GO suspension, GO/Ti and Au-GO/Ti substrates

2　Results and discussion

2.1Preparation and characterization of Au-GO hybrid films

The crystal structure and phase composition of the Au-GO/Ti were analyzed by XRD.As shown in Fig.1(a),in addition to the diffraction peaks of the Ti metal phase(JCPDS No.44-1294),the film exhibited four prominent peaks located at 38.3°,44.5°,64.7° and 77.7°,which can be assigned to(111),(200), (220),and(311)crystalline planes ofthe face-centered cubic(fcc)of Au(JCPDS No.04-0784),indicating the successfuldeposition ofmetallic Au NPs.No diffraction peaks corresponding to GO could be clearly identified. To valid the result,different samples fabricated from different deposition times of GO were measured independently,and the results were the same for all samples.This suggests that the absence of layerstacking regularity of GO or relatively low GO content in the hybrid film[9].Fig.1(b)displays the UV-Vis absorption properties of GO,GO-deposited Ti sheets (GO/Ti)and Au-GO/Ti.GO exhibited a maximum absorption peak at 228 nm and a shoulder peak at 304 nm,corresponding to theπ-π*transition of aromatic C-C bands and the n-π*transition of C=O bands,respectively[10].In addition to the characteristic absorption peaks of GO,the GO/Ti substrate exhibited a new absorption peak at 340 nm,likely originating from Ti oxides,as electrodeposition process is knownto promote the formation of oxide layers on the surface of Ti sheets[11].After electrodeposition of Au NPs,a strong absorption peak at～500 nm was observed, corresponding to the surface plasmon absorption of Au NPs.

SEM images ofthe Tisheetand the GO-deposited Ti sheet confirmed the formation of GO layers on the Ti sheet(Fig.2(a),(b)).It can be clearly seen in Fig.2 (b)that some wrinkles or folded GO stacks were deposited on the smooth Ti surface.After the deposition of Au NPs,the GO-deposited Ti sheet exhibited a homogenous distribution of spherical Au NPs with average diameters of～60 nm on its surface(Fig.2(c)). Noteworthily,the present two-step electrodeposition method achieved uniform coverage of Au NPs over a large substrate area,as seen in a low magnification SEM image of the Au-GO hybrid film(Fig.2(e)).For comparison,Au nanoparticles were also deposited on a bare Ti sheet using the same deposition parameters. As shown in Fig.2(d),the Au NPs had an average size of～50 nm,which is slightly smallerthan those deposited on the GO-deposited Ti sheet.By comparing Fig.2(c) with(d),it can be concluded that the uniformity and coverage of the Au NPs were significantly improved by the presenceofGO layers.Thisismainly attributed to the functional groups and surface charges of GO, which provide active nucleation sites for Au3+ions to form Au NPs.The good uniformity and high coverage of Au NPs on the GO-deposited Ti sheet provide a structuralbasis for highly sensitive Raman detection.

XPS analysis was employed to further confirm the elemental compositions and corresponding chemical states of the prepared Au-GO hybrid films. The Au-GO/Ti substrate consisted of C,O,Au,Ti elements,as shown in Fig.3(a).Fig.3(b)shows the high-resolution spectra of Au4f from Au-GO/Ti and Au/Tisubstrates.The double peaks at84.0 eV(Au4f7/2) and 87.7 eV(Au4f5/2)with a value difference(3.7 eV) indicate the presence of metallic Au.Clearly,the Au-GO/Ti substrate exhibited a slight shift in Au4f peaks to a higher binding energy(0.2 eV)compared to Au/ Ti substrate.This may be attributed to electron transfer from GO to Au NPs due to the higher work function of Au(5.1 eV[12])compared to GO(＞4.5 eV, depending on the amountofoxygen functionalities[13])[14].

Fig.2 SEM images of the surfaces of the(a)Ti sheet,(b)GO/Ti substrate,(c)Au-GO/Ti substrate(high magnification), (d)Au/Ti substrate,and(e)Au-GO/Ti substrate(low magnification)

The Raman spectra of Au-GO/Ti substrate was compared to that of the GO/Ti substrate.As shown in Fig.4,the Raman spectrum of GO/Ti presented characteristic D and G bands at1 345 and 1 589 cm-1, which are attributed to the breathing mode ofκ-point and the stretching mode of E2gphonon of sp2carbon atom,respectively[15].Upon electrodeposition of Au nanoparticles,the Raman signal of GO was significantly amplified due to the strong electromagneticfield enhancement induced by the localized surface plasmon resonance(LSPR)effectof Au nanoparticles.

Fig.3(a)Full XPS spectrum of Au-GO/Ti substrate;(b)high-resolution Au4f spectra from Au-GO/Ti and Au/Ti substrates

Fig.4 SERS spectra of Au-GO/Ti and GO/Ti substrates

Fig.5 SERS spectra of 2.5×10-5mol·L-1R6G adsorbed on different substrates

2.2SERS performance of the Au-GO/Ti substrates

R6G was employed as a probe molecule to evaluate and compare the SERS activity of asprepared GO/Ti,Au/Ti and Au-GO/Ti substrates.Fig. 5 shows the SERS spectra of 2.5×10-5mol·L-1R6G adsorbed on different substrates.While no Raman signals from R6G were recognized on a bare Ti sheet, substantial Raman enhancement was observed on Au/ Ti,GO/Tiand Au-GO/Tisubstrates.The Raman peaks exhibited by these substrates corresponded well with previous reports[6-7].The Au-GO/Ti substrate exhibited the highest SERS sensitivity,with approximately 3-and 6-fold enhancement of the Raman peak intensity of R6G,compared to the Au/Ti and GO/Ti substrates, respectively.The high SERS sensitivity of the Au-GO/ Ti substrate suggested a synergistic effect between the Au nanoparticles and GO rather than simple sum of the effects ofthe individualcomponents.

Au-GO hybrid films were further evaluated to determine the concentration dependency of its SERS performance.The substrates were immersed in a series of aqueous solutions with R6G concentrations ranging from 2.5×10-5to 2.5×10-11mol·L-1and the SERS spectra were recorded at an excitation wavelength of 532 nm.The Raman signal intensity of R6G gradually decreased with decreasing R6G concentrations(Fig.6(a)).However,all characteristic peaks exhibited high signal quality and could be clearly identified even at a low concentration of 2.5× 10-10mol·L-1,implying that the Au-GO/Ti substrates possess high sensitivity for the detection of target analytes.The average enhancement factor(EF)for R6G was calculated according to the followingequation[16]:

where ISERSand IRSrepresent peak intensities of the SERS spectra obtained from 2.5×10-5mol·L-1R6G on the Au-GO/Ti substrate and 2.5×10-2mol·L-1R6G on a quartz substrate,respectively.NSERSand NRSare the numbers of R6G molecules excited by the laser beam on the Au-GO/Ti and quartz substrates,respectively. Here two Raman peaks at 612 and 1 362 cm-1were selected for calculating the EFs.The average EF value ofthe Au-GO/Tisubstrate was found to be 2×106,which was approximately two orders ofmagnitude larger than that of the Au/Ti substrate.The EF and detection limit of the Au-GO hybrid film were comparable to or even higher than previous reported values for Au NPsgraphene hybrid films[7-8,16].

Fig.6 SERS spectra of Au-GO/Ti substrates loaded with different R6G with concentrations

Three main factors may explain the high SERS activity of the Au-GO hybrid films:the enrichment effect of the SERS substrate toward the probe molecules,electromagentic contribution and chemical contribution.In the presentstudy,GO contributed to a strong SERS effect by several mechanisms.First,the GO layers provided a perfect two-dimensional flat surface,with large surface area and negative surface charges.Thus GO was an ideal support for the uniform deposition of Au nanoparticles with relatively high coverage,effectively increasing the amountof hot spots on the substrate,and resulting in strong electromagnetic coupling between neighbouring Au nanoparticles and increased electromagnetic enhancement.Additionally,graphene and its derivatives are known to have a chemical enhancement effect on SERS signal due to theπ-π stacking and charge transfer between graphene and the probe molecules[3-4].Because of the short-range effect of chemical enhancement,a small substratemolecule distance is necessary for a considerable SERS enhancement.Good affinity between the GO and R6G allows for charge transfer between them,and thereby facilitates the chemical enhancement effect of GO.Furthermore,the strong interaction between the GO layers and Au nanoparticles is crucial for their synergistic effect.XPS analysis(Fig.3)showed that upon deposition of Au nanoparticles on the GO surface,electrons were transferred from the GO to the Au nanoparticles driven by their different work functions,and the transferred electrons may redistribute and gather on the upper surface of the Au NPs[17]. This process not only contributes to the surface plasmon excitation of Au nanoparticles,but also enhances the charge transfer from the electron-rich Au nanoparticles to R6G,thus magnifying the overall Raman signal.

To evaluate the reproducibility ofthe as-prepared Au-GO/Ti substrate,the spot-to-spot and substrate-tosubstrate variation were quantified by determining the relative standard deviation(RSD)of the Raman peak intensity of adsorbed R6G(2.5×10-7mol·L-1).Fig.7(a) shows the intensity of the Raman peak at 1 362 cm-1collected from ten randomly selected positions on the substrate.The RSD was 16.4%,indicating that the Au-GO/Ti substrate was homogeneous and generated reproducible Raman signals due to the uniformly distributed hot spots.To test the substrate-to-substrate reproducibility,five Au-GO/Tisubstrateswere randomly selected and loaded with 2.5×10-7mol·L-1R6G,and a series of SERS spectra were measured at five different positions on each substrate to obtain the average intensity of the Raman peak at 1 362 cm-1.The RSD was 18.5%,thus confirming that there was low variability between the different prepared Au-GO/Ti substrates.

Fig.7 Evaluation of the reproducibility of Au-GO/Ti substrates:(a)Spot-to-spot variation of the Raman peak intensity at 1 362 cm-1collected from ten randomly selected positions on a substrate;(b)Substrate-to-substrate variation of average Raman peak intensity from five randomly selected substrates

The stability of the Au-GO/Ti substrates was assessed by determining the Raman peak intensity of adsorbed R6G(2.5×10-7mol·L-1)after storage in the refrigerator for 0,30,or 90 d.The spectra in Fig.8 showed that the Raman signal intensity of R6G decreased to approximately 8%and 30%of its initial value after 30 and 90 days of storage,respectively. These results indicate that the Au-GO/Ti substrate is highly stable and produces reliable SERS measurements.Based on the above results,the Au-GO/Ti is a promising substrate for highly sensitive molecule detection.

Fig.8 SERS spectra of R6G absorbed on Au-GO/Ti substrate after storage in the refrigerator for 0, 30 and 90 d,respectively

3　Conclusions

A simple and environmentally friendly two-step electrodeposition process was developed to fabricate a large-area SERS-active Au-GO hybrid film.In this process,GO layers were first electrophoretically deposited onto a Ti sheet.Au NPs with an average size of～60 nm were then uniformly deposited on the surface of GO.This hybrid film exhibited higher SERS activity toward R6G probe molecules compared to the Au nanoparticles or GO alone,indicating the synergistic effect of GO and Au nanoparticles. Furthermore,the Au-GO/Ti substrate demonstrated high SERS sensitivity,with an EF of～10-6and a detection limit of～10-10mol·L-1for R6G,and good stability.Therefore,this study presents an alternative route for the fabrication of high-performance SERS-active substrates.

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Two-Step Electrodeposition Process for Fabrication of Au-Graphene Oxide Hybrid Films as SERS Substrates

XU Ling2YAO Ai-Hua＊,1,2XU Yan1WANG De-Ping1,2
(1Key Laboratory of Advanced Civil Engineering Materials,Ministry of Education,Tongji University,Shanghai 200092,China)
(2School of Materials Science and Engineering,Tongji University,Shanghai 200092,China)

Two-step electrodeposition process was developed to prepare Au nanoparticle-graphene oxide(Au-GO) hybrid film as large-area surface-enhanced Raman scattering(SERS)substrates.The composition,microstructure and morphology of the resultant hybrid film were characterized by XRD,SEM and XPS.Meanwhile,SERS activity ofthe Au-GO/Tisubstrates was also evaluated using rhodamine 6G(R6G)as probe molecules.The results showed that Au nanoparticles with an average size of～60 nm were uniformly deposited on the surface of GO.The substrates exhibited strong and uniform SERS response toward R6G with a detection limit of～10-10mol·L-1and an enhancement factor of～106.Furthermore,the Raman signal intensity of R6G only decreased to approximately 30%of its initial value after 90 days of storage,indicating the Au-GO/Ti substrate is highly stable and produces reliable SERS measurements

Au;graphene oxide;surface-enhanced Raman scattering;hybrid film

O614.123；TB333

A

1001-4861(2016)12-2183-08

10.11862/CJIC.2016.280

2016-06-19。收修改稿日期：2016-09-30。

上海市自然科學基金(No.13ZR1444200)及中央高校基本科研業(yè)務費專項基金資助項目。

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