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Fabrication of Ag-Cu2O/reduced graphene oxide nanocomposites as SERS substrates for in situ monitoring of peroxidase-like catalytic reaction and biosensing Yue Guo, Hai Wang, Xiaowei Ma, Jing Jin, Wei Ji, Xu Wang, Wei Song, Bing Zhao, and Chengyan He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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ACS Applied Materials & Interfaces

Fabrication of Ag-Cu2O/reduced graphene oxide nanocomposites as SERS substrates for in situ monitoring of peroxidase-like catalytic reaction and biosensing Yue Guo,† Hai Wang,‡ Xiaowei Ma,† Jing Jin,† Wei Ji,§ Xu Wang,† Wei Song,*† Bing Zhao,† and Chengyan He‡ †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University,

Changchun130012, P.R. China.*E-mail: [email protected] ‡

China Japan Union Hospital, Jilin University, 126 XianTai Street, Changchun 130033, P. R.

China. §

School of Chemistry, Dalian University of Technology, Dalian 116023, P. R. China

Keywords: surface-enhanced Raman scattering, peroxidase-like catalysis, fingerprints, glucose, biosensing, detection ABSTRACT: Highly sensitive biosensors are essential in medical diagnostics, especially for monitoring the state of an individual’s disease. An ideal way to achieve this objective is to analyze human sweat secretions by non-invasive monitoring. Due to low concentrations of target analytes in human secretions, fabrication of ultra-sensitive detection devices is a great challenge. In this work, Ag-Cu2O/reduced graphene oxide (rGO) nanocomposites are prepared by a facile

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two-step in situ reduction procedure at room temperature. Ag-Cu2O/rGO nanocomposites possess intrinsic peroxidase-like activity and rapidly catalyze oxidation of the peroxidase substrate 3, 3’, 5, 5’-tetramethylbenzidine (TMB) in the presence of H2O2. On the basis of the excellent SERS properties and high peroxidase-like activity of the Ag-Cu2O/rGO nanocomposites, the catalytic oxidation of TMB can be monitored by SERS. This approach can detect H2O2 and glucose with high sensitivity and distinguish between diabetic and normal individuals using glucose levels in fingerprints. Our work provides direction for designing other SERS substrates with high catalytic activity and the potential for application in biosensing, forensic investigation, and medical diagnostics. 1. INTRODUCTION During the past few decades, artificial enzyme mimics have attracted much attention due to the application in biotechnology, food processing, chemical industry, and environmental science.1-5 Compared to natural enzyme mimics, artificial ones exhibit advantages of tunable activity, high stability, low cost, and ease of preparation and purification. Among various possibilities, peroxidase mimics have created widespread interest due to their potential application in the detection of H2O2 and glucose, when used as biosensors in biological systems.6,7 Since the first report on the peroxidase-like activity of Fe3O4 nanoparticles,8 many nanomaterial-based mimics have been developed using carbon, noble metal, metal oxide, and chalcogenide nanoparticles.9-16 To further improve the catalytic activity of these materials, composites with synergistic peroxidase-like properties have been studied.17-21 For example, Lu and co-workers prepared conductive polyaniline (PANI)/Cu9S5 composite nanowires, which exhibited enhanced catalytic activity over Cu9S5 nanoparticles and PANI alone as peroxidase mimics.17 Improved peroxidase-like catalysis may arise from a synergistic effect between the


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Cu9S5 and PANI components. Similar results have been observed for systems based on noble metal/graphene, noble metal/oxide, CuS/graphene, and CNT/ZnFe2O4 nanocomposites, etc.18-21 It is crucial to monitor peroxidase-like catalytic processes to understand the kinetics and mechanism of their reaction on catalyst surfaces. Analytical techniques that have been applied for this task include electrochemical methods, colorimetric analysis, optical sensing, and surfaceenhanced Raman scattering (SERS).22-25 SERS offers the advantages of high sensitivity and selectivity that facilitate in situ monitoring and identification of analytes such as H2O2. For example, Jung et al. fabricated a superhydrophobic Au-modified Cu/Ag sheet as a SERS substrate and peroxidase mimic for biosensing H2O2 with a detection limit of 10-6 M.26 Recently, we demonstrated a facile direct reduction method for preparing core-shell Ag@CDs nanoparticles with excellent peroxidase-like activity that enables in situ monitoring of TMB in the presence of H2O2.27 This approach allows for the detection of H2O2 in amounts as low as 1.6×10-8 M using SERS. Although several SERS substrates with peroxidase-like catalytic activity have been developed for H2O2 detection, fabricating new nanomaterials with SERS capability and peroxidase-like activity for detecting other analytes remains a challenge. Identification of latent fingerprints is important in forensic investigations. However, it is difficult to visualize incomplete or intentionally defaced fingerprints. However, much useful information can be obtained by analyzing the constituents and specific contaminants within these forensic traces. For example, residual drugs may be retained in the fingerprints of drug addicts, and glucose may be detected in the fingerprints of individuals with severe diabetes. Therefore, a crucial objective is the detection and determination of specific contaminants and metabolites within the fingerprints of individuals.


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In this report, we describe a two-step in situ reduction procedure for preparing ternary AgCu2O/rGO nanocomposites. These materials can serve as SERS substrates and nanocatalysts, thus providing the opportunity to monitor peroxidase-like catalytic processes. A simple approach for the sensitive detection of H2O2 and glucose using SERS is demonstrated based on the activity of the catalysts. The most important contribution of this work is that the methodology can distinguish between diabetic and normal individuals using glucose levels detected within a fingerprint, which can be used in wearable devices for in-situ detection of the glucose level of diabetics.28, 29 We anticipate that Ag-Cu2O/rGO nanocomposites also will have applications in biosensing, forensic investigations, and medical diagnostics. 2. EXPERIMENTAL SECTION 2.1. Chemicals Copper nitrate trihydrate (Cu(NO3)2•3H2O) is purchased from Xilong Chemical Co.Ltd. pAminothiophenol (PATP) and glucose oxidase (GOx) were obtained from Sigma-Aldrich. Silver nitrate (AgNO3) and dimethyl sulfoxide (DMSO) are purchased from Sinopham Chemical Reagent Co.Ltd. Hydrazinium hydrate (N2H4•H2O, 70%-80%), 3,3,5,5-tetramethylbenzidine (TMB) is obtained from Aladdin (Shanghai, China). Hydrogen peroxide (H2O2) and glucose are from Beijing Chemical Works. 2.2. Characterization Transmission microscope (TEM, JEM-2100F) is used to characterize the morphologies of the as prepared samples at 200 kV. X-ray diffraction (XRD) patterns are obtained with a PANalytical B.V. (Empyrean) diffractometer using Cu-Kα radiation. X-Ray photoelectron spectroscopy (XPS) data are taken from a Thermo ESCALAB 250 photoelectron spectrometer with Al as the excitation source. UV-vis spectra are obtained from Shimadzu UV-3600 UV-Via-


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NIR spectrophotometer. Infrared spectroscopy (FT-IR) data are measured on a Bruker-Vertex 80v spectrometer (detector: DTGS; resolution: 4 cm-1; number of scans: 32; sample preparation strategy: KBr tablet). PATP of SERS detection is measured by Renishaw-1000 spectrometer with a He/Ne laser as excitation line of 532 nm, the laser power is typically 10.7 mW. Catalytic reaction of Raman spectra is measured with by a LabRAM ARAMIS Smart Raman Spectrometer with a HeNe laser as the excitation line of 633 nm. Data acquisition time is 30 s accumulation for one measurement. The Raman band of a Si wafer at 520.7 cm-1 is used to calibrate the spectrometer. The weight percentage of Ag and Cu2O in the nanocomposites is determined by inductively coupled plasma (ICP) atomic emission spectrometric analysis (Agilent 725). 2.3. Fabrication of ternary Ag-Cu2O/rGO nanocomposites Firstly, we have synthesized GO from natural graphite powder via a modified Hummer's and Offeman's method.30,31 Then Ag-Cu2O/rGO nanocomposites are prepared through a two-step in situ reduction approach. In a typical procedure, 50 mg of GO is added into 100 mL of distilled water under sonication for about 1 h to obtain a uniform dispersion of GO. After that, 0.05 g of Cu(NO3)2•3H2O is dissolved in 50 mL of water and mixed with 3 mL of GO dispersion (0.5 mg•mL-1). The mixture is stirred for another 2 h. Then 1.0 mL of N2H4•H2O (0.5 mM) is dropwise added into above-mentioned dispersion under continuous stirring for 1.5 h. Finally, Cu2O/rGO nanocomposites are separated from the reaction system by centrifugation at 5000 rpm for 10 min. The collected solid product is washed for three times with water and ethanol, respectively. Then, the obtained powder is dried in a vacuum oven at 60 oC for 12 h. In order to synthesize Ag-Cu2O/rGO nanocomposites, 15 mL of AgNO3 (5 mM) is dropwise added into Cu2O/rGO dispersion (0.2 mg·mL-1, 40 mL) under continuous stirring for 1 h. The synthesized Ag-Cu2O/rGO nanocomposites are separated from the aqueous solution by


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centrifugation. The collected solid product is washed for three times with water and ethanol, respectively. Then, the obtained powder is dried in a vacuum oven at 60 oC for 12 h. 2.4. SERS properties of PATP on the surface of Ag-Cu2O/rGO nanocomposites For detection of SERS activity, PATP molecules are used as Raman probe. 1mg of AgCu2O/rGO nanocomposites is dissolved in 1 mL of water. Then 40 µL of Ag-Cu2O/rGO suspension is mixed in 40 µL of PATP solution with different concentration from 10-5 - 10-9 M for 40 min to study the detection sensitivity. SERS spectra are collected directly from the mixing solution containing Ag-Cu2O/rGO suspension and PATP. Before SERS detection, the mixed solution is under ultrasonic condition for about 1 min. The SERS measurements are performed by Renishaw-1000 spectrometer with He/Ne laser as excitation line of 532 nm and the laser power at samples position is typically 10.7 mW. 2.5. Catalytic activity measurement by UV-vis spectroscopy 100 µL of TMB (3 mM) and 100 µL of H2O2 (1 M) are mixed in a reaction volume of 2.7 mL of acetate buffer (pH 4.0). Then, 100 µL of Ag-Cu2O/rGO nanocomposites dispersion (1 mg•mL1

) is added. The mixed solution is incubated at room temperature for 30 min. The resulting

reaction solution is subjected to UV-vis measurements. 2.6. SERS monitoring of peroxidase-like catalytic oxidation of TMB in the presence of H2O2 by Ag-Cu2O/rGO nanocomposites and the detection of H2O2 and glucose 3.6 mg of TMB is dissolved in 1 mL of dimethyl sulfoxide (DMSO) solvent and diluted to 3 mM using acetate buffer solution (pH = 4.0). 10 µL of 3 mM TMB and 10 µL of 1 mg•mL-1 AgCu2O/rGO nanocomposites dispersion are mixed together. Then 10 µL of 3×10-2 M H2O2 is added into the above mixture to monitor the peroxidase-like catalytic oxidation of TMB. For the detection of H2O2, 10 µL of H2O2 with different concentrations (final concentration is 10-3 - 10-8


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M) are added into the above mixture of TMB and Ag-Cu2O/rGO nanocomposites dispersion. For the detection of glucose, 0.5 mg of Ag-Cu2O/rGO NPs is added into 500 µL of 1.2 mg•mL-1 GOx aqueous solution under ultrasonic irradiation and then stirred for about 6 h. After that, the solution is centrifuged and washed with the water twice, and then redispersed into 500 µL of water. Then 40 µL of the above solution, 40 µL of 3 mM TMB and 40 µL of glucose with different concentrations are incubated at 37 oC for 25 min to measure the concentrations of glucose. 2.7. SERS detection for the distinguishing of diabetic patients The clean glass slide is ultrasonically cleaned by sequentially immersing the substrate in ethanol and deionized water. After being dried under flowing N2, diabetic patients or normal people wash their hands with water and then wait for 30 min, the fingerprints can be pressed onto the clean glass slide, and then place in the 3 ~ 5 oC refrigerator. After that, 100 µL of 3 mM TMB and 100 µL of the above mentioned mixture of Ag-Cu2O/rGO NPs and GOx are incubated at 37 oC for 25 min to measure the fingerprints. 3. RESULTS AND DISCUSSION 3.1. Preparation and characterization of Ag-Cu2O/rGO nanocomposites The overall scheme for fabricating the Ag-Cu2O/rGO nanocomposites is presented in Figure 1. In the first step of the synthesis, Cu2O particles are prepared on the surface of rGO by a reduction reaction. During this process, the generation of Cu2O is accompanied with conversion of GO to rGO with the assistance of hydrazine. Ag nanoparticles are then deposited on the Cu2O by a sacrificial template route.32 Figures 2a and b contain TEM images of the Cu2O/rGO and AgCu2O/rGO nanocomposites, respectively. The rGO sheets exhibit a typical rippled and crumpled structure. The Cu2O/rGO nanocomposites contain cubical around 400 nm Cu2O particles


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partially enwrapped by rGO sheets. After addition of AgNO3, Ag nanoparticles with a size of tens of nanometers are deposited directly on the surface of the Cu2O/rGO nanocomposites.

Figure 1. Schematic illustration of synthetic procedure of Ag-Cu2O/rGO nanocomposites. Figure S1 confirms the observation that the Ag-Cu2O nanocomposites are attached to directly to the rGO. An enlarged TEM image shows that the Ag nanoparticles are selectively deposited on the Cu2O surfaces, which indicates their formation by the sacrificial templating of Cu2O (Figure 2c). The Ag nanoparticles remain attached to the Cu2O/rGO surface throughout prolonged sonication during the preparation of the TEM specimen, which indicates their intimate interfacial contact between the Ag nanoparticles and Cu2O. Figure 2d shows a distinct set of visible lattice fringes in the HRTEM image of the Ag-Cu2O/rGO nanocomposites. The 0.233 nm fringe corresponds to the (111) plane of cubic Ag,33 whereas the 0.245 nm fringe is consistent with the (111) plane of cubic Cu2O.34 Figure S2 shows elemental Cu, Ag, O, and C mappings of the Ag-Cu2O/rGO composites. The Ag nanoparticles are dispersed primarily on the surface of the Cu2O particles. Figure S3 shows the EDX spectrum of the Ag-Cu2O/rGO nanocomposites, which demonstrates that these materials are composed of Cu, Ag, C, and O and have been synthesized successfully. The weight percentages of Ag and Cu2O in the Ag-Cu2O/rGO nanocomposites are also determined by ICP measurement, which gives values of 11% and 74%, respectively.


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Figure 2. TEM images of (a) Cu2O/rGO and (b, c) Ag-Cu2O/rGO nanocomposites, (d) HRTEM image of the prepared Ag-Cu2O/rGO nanocomposites. Figure 3a shows the XRD patterns of GO, Cu2O/rGO, and Ag-Cu2O/rGO. The sharp peak at 12.2° in the GO pattern corresponds to the (001) plane of GO. Prominent diffraction peaks at 29.5, 36.4, 42.2, 52.4, and 62.3° in the Cu2O/rGO pattern are readily indexed to the (110), (111), (200), (220), and (311) lattice planes, respectively, of cubic Cu2O. In addition to the diffraction peaks of Cu2O in the Ag-Cu2O/rGO sample, signals at 38.1, 44.2, 66.4, and 77.4° can be indexed to the (111), (200), (220), and (311) reflections, respectively, of the fcc structure of Ag. This establishes the formation of Ag nanoparticles on the surface of the Cu2O/rGO nanocomposites. In the above examples, no diffraction peaks of graphene are observed due to the low crystallinity of rGO compared with that of Cu2O and Ag. Raman spectroscopy is used to further characterize the presence of rGO in the Ag-Cu2O/rGO nanocomposites. Figure 3b shows the Raman spectra of GO, Cu2O/rGO, and Ag-Cu2O/rGO. A G band at 1604 cm-1 and a D band at 1349 cm-1 are observed for the GO sample. After reduction by hydrazine, the G band of rGO shifts from 1604 to 1601 cm-1 due to conversion of the isolated double bonds in GO to conjugated double bonds in rGO. The ID/IG intensity ratios of Cu2O/rGO


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(0.92) and Ag-Cu2O/rGO (0.94) are greater than that of GO (0.70), which indicates that the population of oxygen-containing functional groups has been reduced by hydrazine reduction and that the conjugated graphene network has been re-established.35 In the Cu2O/rGO nanocomposites, four new peaks associated with Cu2O36 appear at 124, 147, 218, and 628 cm-1. The intensity of these peaks is decreased in the Ag-Cu2O/rGO nanocomposites owing to the partial conversion to Ag nanoparticles via the sacrificial template approach.

Figure 3. (a) XRD pattern of GO, Cu2O/rGO and Ag-Cu2O/rGO nanocomposites ; (b) Raman spectra of GO, Cu2O/rGO and Ag-Cu2O/rGO nanocomposites. FT-IR spectra of the GO, Cu2O/rGO, and Ag-Cu2O/rGO nanocomposites are shown in Figure S4. The stretching vibration of the GO hydroxyl groups appears at 3415 cm-1. Peaks at 1726 and 1622 cm-1 are assigned to C=O carbonyl stretching and to skeletal vibrations of graphitic domains, respectively. The above peaks are absent from the Cu2O/rGO nanocomposites. The appearance of a peak at 632 cm-1 is assigned to the Cu–O stretching vibration in the Cu2O phase. The intensity of the 632 cm-1 peak diminishes in the Ag-Cu2O/rGO nanocomposites, because of replacement of Cu2O by Ag nanoparticles.


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Figure 4. XPS spectra of the prepared Ag-Cu2O/rGO nanocomposites: (a) C1s XPS spectrum, (b) O1s XPS spectrum, (c) Cu2p XPS spectrum and (d) Ag3d XPS spectrum. X-ray photoelectron spectroscopy (XPS) is used to characterize the composition and chemical state of the Ag-Cu2O/rGO nanocomposites. Figure 4 shows the C, O, Cu, and Ag signals of the product. The C1s spectrum of rGO can be deconvoluted into peaks centered at 284.6, 286.1, and 287.8 eV, which are assigned to C−C, C−O, and C=O groups, respectively.37 The O 1s profile can be divided into three peaks at 530.7, 531.7, and 532.8 eV. The first two originate from the lattice oxygen of Cu2O and adsorbed oxygen, respectively. The last peak is assigned to C−O groups and the lattice oxygen of CuO.38,39 The CuO is due to the oxidation of Cu2O on its surface.40 The XPS spectrum also contains Cu 2p3/2 and Cu 2p1/2 peaks at 952.6 and 932.37 eV, which indicate the formation of Cu2O particles. Weaker signals at 933.5 and 953.7 eV are the result of Cu2O oxidation.40 No CuO phase is detected in the XRD pattern, which suggests that


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only trace amounts of CuO are present on the Ag-Cu2O/rGO surface. Peaks centered at 368.5 and 374.5 eV are assigned to Ag 3d5/2 and Ag 3d3/2, respectively, and demonstrate the formation of pure metallic Ag.41 Surface-enhanced Raman scattering of Ag-Cu2O/rGO nanocomposites is investigated with PATP as the reporter probe. The SERS spectra in Figure 5 shows well-defined Raman bands characteristic of PATP molecules adsorbed at different concentrations on Ag-Cu2O/rGO nanocomposites. Peaks at 1078 (C−S stretching), 1144 (C–H bending) and 1390 , 1436 cm-1 (C−H bending and C−C stretching) are observed in the SERS spectra.42 It is obvious that PATP molecules on the surface of Ag-Cu2O/rGO nanocomposites as SERS substrates display an intensive enhancement. The inset of Figure 5 shows the variation of SERS intensity as a function of PATP concentration. The signals at 1078 and 1144 cm-1 are not affected by background, and their intensities decrease with decreasing concentration of the probe molecule. PATP bands are observed at concentrations as low as 10-9 M, which demonstrates the excellent SERS behavior of the Ag-Cu2O/rGO nanocomposites.

Figure5. SERS spectra of PATP molecules with different concentrations adsorbed on AgCu2O/rGO nanocomposites. The inset shows the linear variation between SERS intensity of bands at 1078 and 1144 cm-1and –lg[PATP].


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3.2. Peroxidase-like catalytic oxidation of TMB in presence of H2O2 by Ag-Cu2O/rGO NPs with SERS monitoring Ag-Cu2O/rGO nanocomposites are not only good SERS substrates, but also efficient peroxidase-mimic nanocatalysts. The peroxidase-like activity of Ag-Cu2O/rGO NPs is demonstrated by the model oxidation of TMB in the presence of H2O2. As shown in the inset in Figure 6, a blue color is generated by mixing TMB, H2O2, and Ag-Cu2O/rGO nanocomposites in pH 4.0 HOAc/NaOAc buffer. No absorption peak is observed for systems containing only TMB, TMB+H2O2, or TMB+Ag-Cu2O/rGO. As shown in the inset in Figure S5, the UV-vis spectrum of the TMB/H2O2/Ag-Cu2O/rGO system displays higher peroxidase-like catalytic activity than TMB/H2O2/Cu2O/rGO, TMB/H2O2/Cu2O and TMB/H2O2/rGO. This result indicates that AgCu2O/rGO nanocomposites hugely accelerate the reaction of TMB with H2O2 and are good peroxidase-like nanocatalysts for the oxidation of TMB in the presence H2O2.

Figure 6. UV-Vis spectra of (a) TMB, (b) TMB+ H2O2, (c) TMB+Ag-Cu2O/rGO nanocomposites, and (d) TMB+ H2O2/Ag-Cu2O/rGO solution after mixing for 30 min. Inset: visual color change of the corresponding solution.


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Because Ag-Cu2O/rGO nanocomposites function as both SERS substrates and peroxidase-like nanocatalysts, they provide an opportunity for SERS monitoring of TMB oxidation in the presence of H2O2. Figure S6 shows the changes in SERS intensity with time of TMB molecules on the surface of Ag-Cu2O/rGO nanocomposites after addition of H2O2. Bands at 1189, 1335, and 1605 cm-1 appear in the SERS spectra after introduction of H2O2 that are assigned to CH3 bending, inter-ring C−C stretching, ring stretching and C−H bending modes.43 These bands correspond to formation of the charge transfer complex (CTC) and radical cation of TMB (TMB+), which is consistent with the UV-vis absorption results.44 The SERS intensities of these bands increase with time and change little after 13 min, which indicates that TMB is oxidized gradually by Ag-Cu2O/rGO nanocomposites in the presence of H2O2. By comparison, the Raman spectrum of native TMB molecules has also been showed in Figure S7a, the typical bands at 1418 cm-1 have been observed, which can be assigned to unoxidized diamine.44 As the TMB solution with a high concentration can also be partially oxidized by the high concentration of H2O2. We have also studied the changes of the SERS spectra of TMB molecules with high concentration of H2O2 in the absence of Ag-Cu2O/rGO nanocomposites. As shown in Figure S7b, the typical bands of 1191, 1337, and 1610 cm-1 that are assigned to CH3 bending, inter-ring C−C stretching, ring stretching and C−H bending modes of CTC and TMB+ also appear with increasing time. However, the intensities of these bands are much weaker than those with SERS substrates, and the typical band at 1418 cm-1 of TMB molecules is still observed, indicating that only a few part of TMB molecules are oxidized in the absence of Ag-Cu2O/rGO nanocomposites. Figure S8 also shows the changes of Raman spectra of Ag-Cu2O/rGO nanocomposites in the presence of H2O2. It is found that the typical bands of rGO are found at around 1337 and 1603 cm-1. And the intensity ratio of these two bands almost do not change with increasing time. The


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XPS spectrum also demonstrates the stability of Ag-Cu2O/rGO nanocomposites in the presence of H2O2 (Figure S9). We also find that Ag-Cu2O/rGO nanocomposites show good catalytic stability and recyclability, because they can be separated from the system after reaction. Recyclability is studied in the presence of 400 µL of 3×10-3 M H2O2. After reaction, Ag-Cu2O/rGO nanocomposites are washed three times sequentially with water and ethanol and used again in the catalytic reaction. Figure S10 shows the SERS spectra of TMB in the presence of Ag-Cu2O/rGO nanocomposites and H2O2 after 13 min after three repetitions. The SERS intensities do not change much indicating that the Ag-Cu2O/rGO nanocomposites are good reusable catalysts and SERS substrates. It is easy to determine H2O2 by this reaction, because the SERS intensity depends on the H2O2 concentration. Figure S11 shows SERS spectra of oxidized TMB catalyzed by Ag-Cu2O/rGO nanocomposites in the presence of different concentrations of H2O2. The SERS intensities at 1189, 1335, and 1605 cm-1 corresponding to CH3 bending, inter-ring C−C stretching, ring stretching and C−H bending modes of CTC and TMB+ increase with the concentration of H2O2. A linear correlation is observed between intensities of SERS peak and the logarithm of the H2O2 concentration. The inset in Figure S11 shows the good linear relationship (r = 0.9743) of the SERS intensities at 1189 and 1605 cm-1 versus the logarithm of the H2O2 concentration. Concentrations as low as 10-8 M H2O2 can be detected by this method. Accurate determination of glucose is crucial in the field of diabetes diagnosis.45 Because glucose oxidase (GOx) catalyzes the oxidation of glucose to gluconolactone and H2O2, we design a SERS method to detect glucose by catalytic oxidation of TMB in the presence of GOx and glucose. The intensities of the CH3 bending mode, inter-ring C–C stretching mode, and ring


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stretching and C–H bending modes at 1189, 1335, and 1605 cm-1 increase with time for 26 min, but no further after 28 min. This indicates that almost all TMB has been oxidized at this time (Figure 7, Figure S12).

Figure 7. SERS spectra of TMB molecules on the surface of Ag-Cu2O/rGO nanocomposites in the presence of GOx and glucose with increasing time. Because the signal intensity depends on the concentration of glucose, we have conducted a determination of glucose using SERS. Figure 8 shows SERS spectra of TMB molecules on the Ag-Cu2O/rGO nanocomposites surface in the presence of different concentrations of glucose. The SERS intensities at 1189, 1335, and 1605 cm-1 increase with increasing glucose concentration. As depicted in Figure S13, a linear correlation between intensity and the logarithm of glucose concentration is obtained from 10-2 to 10-8 M. Considering the influence of interfering substances, it is estimated that this approach can detect glucose at concentrations as low as 10-8 M.


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Figure 8. SERS spectra of oxidized TMB molecules on the surface of Ag-Cu2O/rGO substrates in the presence of glucose from 10-2 to 10-8 M ) .The right is enlarged spectra in the 1180 – 1200 cm-1 regions)

Figure 9. SERS intensities at 1189 and 1605 cm-1 at a concentration of 10 mM glucose and 100 mM galactose, mannose, fructose, and sucrose). The error bars represent the standard deviation of three measurements. Selectivity is one of the most important biosensor parameters. The SERS response based on the peroxidase-like catalytic reaction in this study is highly specific for glucose. As shown in Figure 9, the intensities of the bands at 1189 and 1605 cm-1 in the presence of 10 mM glucose are


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much greater than those of sucrose at concentrations as great as 100 mM. This result indicates that glucose can be detected in the presence of galactose, mannose, and fructose with high selectivity. We have also studied the SERS response for the detection of glucose in diluted serum. As shown in Figure S14, serum has little effect on the determination of glucose and the signals of the oxidized TMB are consitent with the SERS response in the absence of serum. We have the possibility of distinguishing diabetic from normal individuals by detecting glucose levels in their fingerprints based on SERS monitoring using the peroxidase-like catalytic reaction. Figure 10 shows SERS spectra of TMB molecules on the surface of fingerprints of diabetic and normal persons in the presence of the Ag-Cu2O/rGO nanocomposites and GOx. By monitoring the peak at 1189 cm-1, we find obvious differences between the response of diabetic and normal individuals, which illustrates that a simple approach is available for diagnosis of this disease state. As shown in Figure 10, obvious band at 1189 cm-1 is observed for a diabetes even with a blood sugar level as low as 10.50 mM, while this peak is not found for a normal person with a blood sugar level of 5.56 mM. Therefore, we have developed a simple method of diabetes detection based on peroxidase-like catalysis and SERS.


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Figure 10. SERS spectra of oxidized TMB molecules on the surface of fingerprint from diabetes patients (Blood sugar level is 19.69, 14.84 and 10.50 mM, respectively) and normal person (Blood sugar level is 5.56 mM) in the presence of Ag-Cu2O/rGO substrates and GOx. The bottom line is blank test without fingerprint. On the right is schematic representation of SERS detection of fingerprint by using Ag-Cu2O/rGO nanocomposites as SERS substrate. 4. CONCLUSION Ternary Ag-Cu2O/rGO nanocomposites are fabricated by a facile two-step in situ reduction procedure at room temperature. Straightforward reduction of AgNO3 by a sacrificial Cu2O template resulted in the selectively deposition of Ag nanoparticles on the Cu2O surface. The AgCu2O/rGO nanocomposites are efficient SERS substrates and excellent peroxidase-like nanocatalysts. We have used the nanocomposites as SERS substrates to monitor peroxidase-like


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catalysis for the detection of H2O2 and glucose with high sensitivity. The strategy developed in this work shows promise in the near future for disease diagnosis and forensic investigation. ASSOCIATED CONTENT Supporting Information Characterization of Ag-Cu2O/rGO NPs and the SERS properties for the monitoring the peroxidase-like catalytic reaction. Summary of SEM images, EDX mapping, EDX spectrum, FTIR spectra, XPS spectra, Raman spectra and SERS properties. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]. Tel/Fax: (+86)-0431-85168473

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources National Natural Science Foundation of China (no. 21473068, 21327803, 21611130173, 21603021, 81572082). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work were supported by the National Natural Science Foundation of China (nos. 21473068, 21327803, 21611130173, 21603021, 81572082). ABBREVIATIONS rGO, reduced graphene oxide; NPs, nanoparticles; SERS, surface-enhanced Raman scattering; TMB, 3,3’,5,5’-tetramethylbenzidine; GOx, glucose oxidase. REFERENCES (1)

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