In Situ Synthesis of Silver Nanospheres, Nanocubes, and Nanowires

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In situ synthesis of silver nanosphere, nanocube and nanowire over boron doped graphene sheets for SERS application and enzyme-free detection of hydrogen peroxide Anju K Nair, Kala Moolepparambil Sukumaran, Sabu Thomas, Dr. Didier Rouxel, Subbiah Alwarappan, and Nandakumar Kalarikkal Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02005 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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In situ synthesis of silver nanosphere, nanocube and nanowire over boron doped graphene sheets for SERS application and enzyme-free detection of hydrogen peroxide Anju K Nair1,2, Kala Moolepparambil Sukumaran2, Sabu Thomas1,3, Didier Rouxel4, Subbiah Alwarappan5,*, Nandakumar Kalarikkal1,6* 1

International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam-686 560, Kerala, India 2 Department of Physics, St Teresas’s College Ernakulam-682011, Kerala, India 3 School of Chemical Sciences, Mahatma Gandhi University, Kottayam-686 560, Kerala, India 4 Institut Jean Lamour-UMR CNRS 7198, Facult´e des Sciences et Techniques, Campus Victor Grignard-BP 70239, 54506, Vandoeuvre-les-Nancy Cedex, France 5 CSIR- Central Electrochemical Research Institute (CSIR-CECRI) Karaikudi – 630 003, India (e.mail: [email protected]) 6 School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam-686 560, Kerala, India Email: [email protected]

Abstract An effective in-situ synthesis strategy for the preparation of silver nanostructures (nanosphere, nanocube and nanowire) on the surface of boron doped graphene (BG) is demonstrated. Further, these functional nanomaterials are employed for the surface-enhanced Raman scattering (SERS) and non-enzymatic electrochemical detection of H2O2 performed. Results confirmed the superior performance of BG-Ag nanostructures as better SERS platform. Of various geometries of silver nanoparticle employed in this work, we found that the Ag nanocube over BG (BG-AgNC) presents outstanding SERS performances for detecting 4-mercaptobenzoic acid (4-MBA) with a limit of detection of 1.0 × 10-13 M. Furthermore, the BG-AgNC exhibits excellent capability to detect melamine as low as 1.0×10-9 M. Electrochemical results confirmed BG-AgNW based platform exhibits a superior biosensing performance towards H2O2 detection. The enhanced performance is due to the presence of graphene that improved the conductivity and provided more active sites. The synthesis of doped graphene with metallic nanoparticles described in this work is expected to be a key strategy for the development of an efficient SERS and electrochemical sensor due to its simplicity, cost-effectiveness, long-term stability and better reproducibility. Key words: In-situ synthesis, silver nanostructures, boron doped graphene, SERS,

H O sensing

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1. Introduction Plasmonic nanostructures have received significant scientific and industrial interests for diverse applications due to their unique optical and chemical properties, such as energy conversion, SERS, catalysis, biological and electrochemical sensing.1,2–4 All these above mentioned properties and applications strongly rely on the surface morphology, size, edge composition, and crystal structure of the metallic nanoparticles. Amongst various metallic nanoparticles, AgNPs with well-defined shapes has practical applications in various fields due to their high plasmonic efficiency, biocompatibility, excellent conductivity and electromagnetic field enhancement in the entire visible region.5 Unfortunately, bare Ag nanoparticles suffer from drawbacks such as low stability and reproducibility due to colloidal agglomeration and thereby limiting its application in electrochemical and SERS sensing. Therefore, effective measures are needed to minimize the agglomeration and to maintain the high activity of metal nanoparticles. As a result, effective organization of noble metal NPs onto efficient well-organized assemblies is vital to understand their extraordinary application possibilities.

Anchoring the metal nanoparticles on to a suitable support is an efficient strategy to avoid the coalescence and agglomeration of the NPs. Support matrices with very large surface area can improve the adsorption ability and provide more active sites. Of various available support materials, graphene which is considered as a sp2 hybridized carbon network, is an ideal candidate as support matrix for metal nanostructures with improved charge transfer, superior mechanical strength, exceptionally high surface area and extraordinary thermal stability.6’7 Metal nanoparticles-graphene assemblies generally exhibit innovative and fascinating optical and electrical properties due to the association and interaction between graphene and metal nanoparticles which is not observed in bare nanoparticles.8 In the meantime, graphene could be utilized to vary the plasmonic properties of metal nanostructures and thereby it is a useful material for diverse plasmonic applications.9 The supporting nanomaterials with huge surface area offer plentiful active spots for assembling metal nanocrystals, which nucleate the growth of nanostructures.10 Moreover, graphene/metal NPs hybrids exhibit potential SERS capabilities due to the electromagnetic field enhancement from the metal nanostructures and the chemical enhancement arising the graphene as a result of the strong π-π interactions in graphene.11 For example, SiO2@Ag@GO nanocomposites11, AgNCs@GO assemblies12, graphene/silver nanohybrid13, reduced graphene oxide supported gold nanostars14, silver ACS Paragon Plus Environment

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dendrites- reduced graphene oxide15, Ag/graphene/Au nanohybrids16 etc. have been demonstrated as excellent SERS substrates for the trace level detection of toxic organic dyes and chemicals thereby demonstrating the usefulness of the graphene/metal nanoparticle hybrids. Besides, the improved SERS action, metal NPs/graphene nanostructures also possess the ability to enhance the electrocatalytic reduction of hydrogen peroxide.17–20 There are numerous reports on decorating metal nanostructures on graphene surfaces. However, there are no reports till date on the effect of hetero atoms (boron, nitrogen, sulfur etc) doped graphene-Ag nanostructures for SERS and electrochemical sensing. Graphene doped with heteroatoms is a standout amongst other available strategies to modify its electronic and electrochemical properties.21,22 Recently, boron-doped graphene (BG) is very well researched due to its exciting properties in the areas of fuel cells, super capacitors and batteries.23–25 Boron doping in carbon matrix is reported to create several lattice-defects on the adjacent sites and perturbs the charge distribution, which could initiate the charge exchange between neighbouring carbon atoms and improve the electrocatalytic activity.22 For the synthesis of BG, a variety of methods have been reported including chemical vapour deposition (CVD)26, arc discharge27, thermal treatment23, and solvothermal treatment28 of graphite oxide in the presence of borides. Recently, B doping is achieved by thermally treating the mixture of boron oxide (such as B2O3 and H3BO3) or boron halides with graphite at a high temperature. Thermal treatment is also a very common and a simple strategy to realize doped graphene. The other advantage of this method is that the extent of doping can be easily controlled. It is well known that BG can be synthesized by thermally annealing GO in the presence of B2O3, where B atoms that stems from B2O3 vapour can replace carbon atoms in the graphene thereby resulting in the doped graphene lattice. Similarly, H3BO3 is first converted into B2O3 followed by diffusing B2O3 vapour into the graphene nanosheets. Moreover, due to the high electrical conductivity, better biocompatibility, very large specific surface area, exceptional thermal stability, and superior catalytic ability, BG sheet is preferred as an electrode material for the electrochemical and SERS detection of biomolecules.29-27 However, the usefulness of AgNPs incorporated BG for the specific electrochemical detection of toxic molecules and biological targets is not known till date.

Till date, there are numerous methods available for the integration of nanoparticles over graphene/doped graphene sheets.30 The preparation methods can be mainly categorized as exsitu synthesis and in-situ growth process.31’32 The ex-situ synthesis generally follows the premodifications of nanoparticles or graphene, followed by physical mixing.33,32 This approach ACS Paragon Plus Environment

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results in the non-uniform distribution of the nanoparticles. Moreover, it creates low coverage density as well as stacking of graphene sheets. However, the in-situ protocol usually tunes the size and morphology of the nanoparticles due to the very high interaction amongst graphene and newly created metal atom nuclei.17 This strategy also involves the direct nucleation and development of NPs on graphene. Moreover, metal NPs–graphene exhibits huge surface area and powerful van der Waals interaction involving the graphene lattice and metal NPs, which considerably decrease NPs aggregation. Further, the strong interfacial interactions improve the stability of metal NPs.34 In our previous work, a simple procedure enabling selective growth of silver nanowires over BG sheets, resulting in the formation of BG-AgNW continuous network structure, for enhanced ORR performance in fuel cells has been reported.35

To the best of our knowledge, there are no reports available till date on the synthesis of BGAg nanostructure hybrids for the electrochemical detection of H2O2 and SERS sensing. Herein, an in-situ preparation strategy has been adopted to prepare Ag nanostructures with different morphologies (sphere, cube and wire) on the surfaces of BG sheets. Moreover, the prepared BG-Ag nanostructures exhibited enhanced SERS properties and non-enzymatic electrochemical detection of hydrogen peroxide. The typical probe molecule 4mercaptobenzoic acid (4-MBA) that has been investigated with surface enhanced Raman scattering was chosen as the model to confirm the impact of the substrate which suggests that BG with Ag nanocube substrate is ultrasensitive and exhibits high SERS enhancement factor (EF). Moreover, this SERS substrate was utilized for the trace detection melamine (1.0 ×10-9 M). The resulting BG-AgNW hybrid shows remarkable electrochemical sensing towards the detection H2O2. Therefore, the as-prepared hybrid nanostructure is a suitable platform for SERS and electrocatalysis. 2. Materials and Methods 2.1 Materials Silver nitrate (AgNO3), boron oxide (B2O3), poly vinyl pyrrolidone (PVP, Mw ~55,000), Graphite powder, silver trifluoro acetate (CF3COOAg), sodium borohydride (NaBH4), 4-mercaptobenzoic acid (HSC6H4CO2H), ethylene glycol (EG), and melamine (C3H6N6) were obtained from Sigma- Aldrich. Polyethylene glycol (PEG), hydrogen peroxide (H2O2) and sodium chloride (NaCl) were obtained from Alfa Aeser and the synthesis is carried out using Millipore water.


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2.2 Preparation of Boron-doped graphene (BG) Graphene oxide (GO) is prepared by the modified Hummer’s method.36 For the preparation of BG, GO and boron oxide (B2O3) were grinded in the 1:5 ratio and the whole sample is placed in a tubular furnace and heated to 900⁰C gradually with an uninterrupted argon gas flow for 3 hrs.23 In order to remove unreacted B2O3, the obtained sample BG is then treated with 3.0 M NaOH for 2 hrs. After the washing steps, BG is dried in vacuum at 60oC. 2.3 Preparation of Ag nanosphere over boron doped graphene (BG-AgNS) Briefly, 10.0 mg of BG is taken in 20.0 mL of PEG and sonicated for 1 hr. To this mixture, PVP (222 mg) is added and heated at 80⁰C. To this, 0.5mL of 0.5 M AgNO3 is added and transferred to a 100 mL Teflon-lined autoclave and heated at 160 °C for 24 hrs. After centrifugation, the product obtained is BG-AgNS. 2.4 Preparation of Ag nanocube over boron doped graphene (BG-AgNC) 10.0 mg of BG is added to 6.0 mL of EG in a 100 mL round-bottomed flask and heated under magnetic stirring in an oil bath preset to 150⁰C for 40 min, followed by the introduction of N2 for 15 min. Then sodium sulphide (Na2S) (80 µL, 3 mM in EG) is added to the solution followed by the addition of poly vinyl pyrrolidone (PVP-55, MW ≈ 55 000, 1.5 mL, 20 mg/mL in EG). After 10 min, silver nitrate (AgNO3) (0.5 mL, 48mg/mL in EG) is added to the mixture within a period of 20 s, and heated for an additional 15 min. Upon quenching the reaction with ice bath Ag-nanocubes are formed. This step is then followed by centrifugation and washing and the nanocubes are re-dispersed and stored in water.37 2.5 Preparation of Ag nanowire over boron doped graphene (BG-AgNW) synthesis of BG-AgNW is already reported by us.35 In brief, 10.0 mg of BG is added in 20.0 mL of EG and sonicated for 1 hr. To this, 0.668 g of PVP is added. This sample is then heated to 170o C and then 0.01 g of KBr, 0.02 g of NaCl and finally 0.2793 g of AgNO3 was added. The final solution is kept at 170⁰ C for 6 hrs. The product is then centrifuged and washed repeatedly to remove any unreacted materials. 2.6 Material characterization XRD is perfomed using Bruker X-ray powder diffractometer.

Transmission electron

microscopy (TEM) is performed using a JEOL JEM 2100 transmission electron microscope with an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FESEM) of the samples is performed by a field emission SEM system (FEI Quanta 400


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ESEM FEG). X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe ULVAC) and UV-Vis absorption analysis (Agilent CARY) are performed for further characterizations of the samples. The SERS Raman spectra are recorded with Confocal Raman, WITec equipped with a 100x objective (NA=0.5) with an excitation wavelength of 532 nm. The laser power at the samples is typically in the range of 10–20 mW. The data acquisition time was 10s.

2.7 SERS measurements Confocal Raman (WITec instrunment) is employed for obtaining SERS spectra at an excitation wavelength at 532 nm for both 4- mercaptobenzoic acid (4-MBA) and melamine (MA) detection (acquisition time is 10s). For SERS, 10 µL of the as-prepared BG-Ag samples are subjected to physisorption by drop cast method on the silicon wafer (~1 cm2). 4MBA (1.0 ×10-5 M, 10 µL) and MA (1.0 ×10-4M, 10 µL) in ethanol are spread over it and dried at room temperature. These stock solutions are then diluted to prepare different concentrations of analytes. In order to confirm the reproducibility of each sample, measurements are taken at different positions. Herein, eight random positions on BG-AgNC are chosen to ensure the reproducibility and stability of the SERS-based substrates. 3. Results and discussion The growth of Ag nanostructures over BG sheets is confirmed by absorption spectroscopy. Graphene oxide (GO) exhibits two absorption peaks one at 230 nm and the other at 304 nm which corresponds to the π- π* bonding of C-C interaction and n- π* interaction of C=O bonds respectively (Figure S1).3 The absorption spectra of BG, BG-AgNS, BG-AgNC and BG-AgNW samples are all shown in Figure 1a. After the reduction, the peak gradually changed from 230 nm of GO to 248 nm of BG. This indicates that the electronic configuration of BG sheets is reinstated.38 The BG-AgNS sample shows a surface plasmon resonance (SPR) peak at 428 nm which suggests that the AgNSs with an average diameter of ~30-40 nm is present on the BG surface. The BG-AgNC also displays a SPR band at 418nm alongside the absorption band of BG (281nm). The BG-AgNW exhibits three absorption peaks at 271, 350 and 386 nm corresponding to the absorption peaks of BG and SPR peaks of AgNWs respectively.39


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Figure 1. (a) UV-Vis absorption spectra of BG, BG-AgNS, BG-AgNC and BG-AgNW (b) XRD patterns of GO, BG and BG-Ag nanostructures (c) Raman spectra of BG and BG-Ag nanostructures Figure 1b represents the typical XRD patterns of GO, BG, BG-AgNS, BG-AgNC and BGAgNW. The XRD spectrum of GO shows a peak at 2θ = 10.2⁰, corresponding to the C(001) lattice with an interplanar distance of 0.85 nm.40 After the BG formation, the peak at 10.2⁰ disappeared and a new broad peak at 25.47⁰ is appeared due to the (002) diffraction plane of BG.35 This evidenced the formation of BG after the annealing process41. Besides the C(002) peak, the hybrids BG-AgNS, BG-AgNC and BG-AgNW exhibit peaks at 2θ = 38.1°, 44.2°, 64.5°, and 77.5° corresponding to the (111), (200), (220), and (311) fcc crystalline planes of Ag (ICDD No: 04-0783) confirming the formation of pure crystalline Ag nanostructures on the boron doped graphene surface.37 Further, the carbon diffraction planes of BG-AgNS, BGAgNC and BG-AgNW shifted towards 23.9⁰, 24.1⁰ and 24.8⁰ respectively, confirming the successful inclusion of Ag nanostructures. Herein, the Ag nanostructures can act as spacers and separate the graphene layers. This increases the interlayer spacing of BG sheets and as a result peak broadening is noticed.42’43 The above results confirm the successful growth of Ag nanostructures over the BG surface. Raman spectroscopy is further employed to examine the structural characteristics of the BGAg nanostructure hybrids. The Raman spectrum of GO exhibits three Raman peaks centred at 1358, 1590 and 2683 cm−1, corresponding to D, G and 2D bands respectively(Figure S2).44


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The Raman spectra of BG, BG-AgNS, BG-AgNC and BG-AgNW are shown in Figure 1c. The main distinction amongst BG and graphene oxide is the extent of disorder. The amount of disorder can be clearly identified from the D bands and G bands intensity ratio (ID/IG). The calculated (ID/IG) ratio of GO and BG are 0.98 and 1.002, respectively implies the creation of structural defects in the BG lattice.45,46 The calculated ID/IG ratio of BG-AgNS (1.02), BGAgNC (1.04) and BG-AgNW (1.01) is found to increase with the insertion of AgNPs of different morphology. Incorporating AgNPs onto the graphene matrix creates several defects on the graphene lattice and thereby the ID/IG ratio increases which again confirms the distribution of Ag nanostructures over BG surface.

The wide scan X-ray photoelectron spectroscopy (XPS) spectra of BG-AgNS, BG-AgNC and BG-AgNW shown in Figure S3 confirms the presence of elements C, B and Ag (their elemental percentage are given in Table S1). The atomic percentage of B in BG-AgNS, BGAgNC and BG-AgNW are estimated to be 1.1, 1.02 and 1.5% respectively. The deconvoluted C1s spectrum of BG-AgNS shows five different peaks due to C-B (284.0 eV), C=C (284.6 eV), C-C (285.2 eV), C-O (286.3 eV), O-C=O and C-O-B (288.5 eV) respectively. These values confirm boron doping onto the graphene matrix resulting in BG lattice47 (Figure 2a). A Similar deconvoluted C1s spectrum is noticed for BG-AgNC and BGAgNW (Figure 2d & g). The hybrids BG-AgNS, BG-AgNC and BG-AgNW contain boron atoms that predominantly exist as BC3 with neighbouring C atoms and are revealed in the deconvoluted B1s spectrum (Figure 2 b, e &h). Moreover, boron-oxy-carbides is also present in all the BG-Ag nanostructures (peak centred at ~191 eV confirm this).47’48 Figure 2c, f & i reveal the resultant Ag 3d spectrum of BG-AgNS, BG-AgNC and BG-AgNW respectively, and the peaks displayed in ~368.5 eV (Ag 3d5/2) and 374.5 eV (Ag 3d3/2) confirms the formation of Ag nanostructures on BG surface without any impurities.42


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Figure 2. XPS deconvoluted (a) C1s (b) B1s and (c) Ag3d of BG-AgNS (d) C1s (e) B1s and (f) Ag3d of BG-AgNC and (g) C1s (h) B1s and (i) Ag3d BG-AgNW The structure and morphology of the hybrid samples are examined by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Figure 3 is the FESEM images of BG and BG-Ag nanostructure hybrids. The FESEM image of BG sheets shows a rippled surface with wrinkles (Figure 3a). From the FESEM images of boron doped graphene-Ag nanostructures, it is obvious that Ag nanoparticles are uniformly distributed in the BG matrix. The oxygen-containing functional groups of BG favour the growth and anchoring of nanomorphotypes over the graphene sheets. Further, the oxygencontaining functional groups are responsible for the covalent bonding that assists to shape the homogeneous growth of Ag nanostructures over BG surface. Figure 3b is the typical FESEM image of the BG-AgNS (AgNSs are uniformly dispersed on the BG surface). The magnified FESEM image (inset of Figure 3b) reveals that there are dense AgNS having diameter ~30-40 nm on the surface of the BG-AgNS structure. Figure 3c is the FESEM image of BG-AgNC. From the FESEM image of BG-AgNC, it is evident that the Ag nanocubes of edge length


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~40-50 nm are homogeneously dispersed all over the BG matrix. Figure 3d and the inset shows the FESEM image and HR FESEM of BG-AgNW. From the image, it is evident that the long AgNWs are uniformly inserted in the graphene surface. The nanowires having an average length up to 10 ± 3 µm and diameter of 60 ± 5 nm are completely wrapped within BG sheets. Moreover, the growth of Ag nanostructures on BG sheets is very high thereby suggesting that the boron doping efficiently increases the interface amongst BG and Ag nanostructures.

Figure 3. FESEM images of (a) BG (b) BG-AgNS (c) BG-AgNC and (d) BG-AgNW; inset of (b), (c) and (d) represents the high magnification images of BG-AgNS, BG-AgNC and BG-AgNW respectively

The TEM image of the BG-AgNS reveals that the BG surface is completely covered with silver nanospheres (Figure 4a). The diameter of AgNS is found to be ~30-40 nm and is homogeneously distributed over BG. Figure 4a (inset) is the SAED pattern of BG-AgNS which confirms the high crystallinity of BG-AgNS. Figure 4d shows the high resolution TEM image of the edge between AgNS and BG sheets. A d-spacing of 0.24 nm corresponds to the (111) plane of AgNS is evident in the HRTEM analysis. The TEM images of BG-AgNC confirm that the Ag nanocubes with an average edge length of ~ 40-50 nm are


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homogeneously dispersed in graphene matrix (Figure 4b). The HRTEM image of BG-AgNC (Figure 4e) shows that AgNC has an inter planar spacing of 0.203 nm, corresponding to the (200) planes of FCC Ag nanocube.49 It is evident from figure 4c that the diameter of AgNWs is ~50-60 nm and the AgNWs are efficiently dispersed on boron-doped graphene sheets. From the HRTEM image of BG-AgNW, well-resolved lattice fringes with a spacing of 0.24 nm are clearly noticed, analogous to the (111) lattice of AgNWs.

Figure 4. TEM images of (a) BG-AgNS (b) BG-AgNC (c) BG-AgNW; inset of (a), (b) and (c) represents the SAED patterns of BG-AgNS, BG-AgNC and Bg-AgNW respectively and (d), (e) and (f) are HRTEM images of BG-AgNS, BG-AgNC and BG-AgNW respectively

3.1 Applications of BG-Ag nanostructures in SERS Two probe molecules 4-mercaptobenzoic acid (4-MBA) and melamine (MA) are selected to calcuate the SERS activity and sensitivity of BG-Ag nanostructures. Figure 5a displays the comparative SERS spectra of 1.0 ×10-5 M 4-MBA with different Ag nanostructures encapsulated by boron doped graphene (BG-AgNC, BG-AgNS and BG-AgNW) and bare Ag nanostructures. Herein, Raman spectra of bulk 4-MBA is presented as the reference. The main peaks at 1080 and 1586 cm-1 corresponds to ν (CC) ring breathing and stretching modes of 4-MBA.50,50


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The SERS intensity of 4-MBA peaks of as prepared BG-AgNP hybrids is drastically increased upon comparison with bare Ag nanostructures. Upon comparison with other BGAg nanostructures (BG-AgNS and BG-AgNW), the BG-AgNC exhibited the highest Raman signal due to the high density of silver nanocubes assembled on the BG surface. Of all, BGAgNC exhibited the highest SERS signal which is ~ 3 fold higher than AgNC, ~4 fold higher than AgNW and ~11 times than AgNS at 1080 cm-1. The highest Raman signal noticed with BG-AgNC is ascribed to the larger number of edge facets of the cubic nanostructures. Ag nanocubes exhibits sharp corners which significantly enhances the Raman scattering cross sections of analytes.51 The enhanced SERS effects of BG-AgNP hybrids is due to the combined activity of BG and silver nanostructures. The BG serves as a 2D matrix for the adsorption of silver nanostructures and the embedded Ag nanostructures is the major factor for the SERS improvement of 4-MBA52. Subsequently, the D and G band intensity originating from BG in BG-Ag nanostructure hybrids are masked and the distinct Raman peaks of 4-MBA are noticed. This is in agreement with the literature reports.53,54 The SERS signals are extremely high even for the low MBA concentration. The above facts confirm that these substrates are very efficient and sensitive for SERS detection. Moreover, the SERS intensity of BG-AgNC is stronger than BG-AgNS and BG-AgNW. SERS enhancement ability of a substrate is calculated based on the following equation52.

EF= (ISERS/ IBULK) × (NBULK/NSERS) where ISERS and IBULK are the Raman intensities of the peak of 4-MBA at 1080 and 1586 cm-1. NBULK and NSERS correspond to the number of molecule of the bulk 4-MBA and surface-adsorbed molecules exposed to the laser illumination, respectively. While ISERS and IBULK correspond to the SERS and normal Raman intensity of the bands of 4-MBA at 1080 and

1586 cm-1. For the EF calculation, we have assumed that the 4-MBA molecules are

uniformly distributed on the SERS active substrate and NSERS can be estimated from the average surface density of 4-MBA and the area of the laser spot. In our experiment, 10 µL of 1×10-5 M 4-MBA solution was added on the SERS substrate, after drying, a circular spot with the diameter of 2.70 mm was formed. The average surface density of 4-MBA is estimated to be 8.73 × 10-22 mol/µm2. The laser spot area is estimated from the diameter of the laser beam (~ 1 µm). The calculated NSERS value is 4.75 × 105 mol. Considering the density of bulk 4MBA (1.57 g/cm3) and the penetration depth of the laser beam as ~ 2 µm, NBULK was calculated as 8.29 × 109 mol. ACS Paragon Plus Environment

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The calculated SERS EF of all the samples is given in Table 1. A significant SERS improvement is observed in the BG-silver nanohybrids than the bare samples. For our BGAgNC SERS substrate, the EF for 4-MBA is estimated to be approximately 5.11 × 106 at 1080 cm-1 and 5.20 × 106 at 1586 cm-1, which is much better than the value of AgNC alone (1.75×106 at 1080 cm-1 and 1.91× 106 cm-1). The BG-AgNC exhibits the highest SERS enhancement. Ag nanocubes generate more intense local electromagnetic fields around the sharp edges of the nanocubes. Moreover, the very high specific surface area of BG provides additional active sites to adsorb more number of analyte molecules and thus generates better charge transfer to activate chemical enhancement. Therefore, the coupling of localized surface plasmon resonance between gapped metal nanoparticles creates more electromagnetic hotspots while chemical enhancement from BG generates huge SERS enhancement. In order to calculate the sensitivity of BG-AgNC towards 4-MBA detection, the Raman spectra is obtained by varying the 4-MBA concentration from 1.0 ×10-6 M to 1.0 ×10-13 M. The BG-AgNC exhibited a gradual decrease in the Raman signal intensity with decrease in the concentration. When the 4-MBA concentration is 1×10−13 M, the signature Raman peaks corresponding to 4-MBA at 1080 and 1582 cm−1 is still seen which confirms its excellent sensitivity (Figure 5b). Therfore, the as-developed BG-AgNC SERS substrate shows excellent sensitivity suitable for the detection of the 4-MBA at a concentration as low as 1×10−13 M better than other reported substrates.55’56’57’58’60 Figure 5c shows the calibration plots for the SERS intensities of the distinctive bands of 4MBA at various concentrations (1.0 ×10-6 M to 1.0 ×10-10 M) adsorbed on BG-AgNC. The calibration plots are linear with regression (R2) value of 0.996 and 0.997 at 1080 and 1586 cm-1 respectively. From the results, it is evident that the prepared BG-AgNC hybrid is appropriate for application as an extremely useful SERS-active substrate.


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Figure 5. (a) SERS spectra of 4-MBA (1.0 ×10-5 M) on BG-silver nanostructures and bare nanostructures (b) SERS spectra of 4-MBA at different concentrations ranging from 1.0 ×10-6 M to 1.0 ×10-13 M on the fabricated BG-AgNC (c) Calibration curves corresponding to average SERS signal intensities of the 4-MBA peaks at 1080 and 1586 cm-1 vs 4-MBA concentrations (d) SERS spectra of 4-MBA (1×10-6 M) collected at eight random pointsof the BG-AgNC sample Table 1. The calculated SERS enhancement factor (EF) of all the samples for 4-MBA Samples

EF at 1080 cm-1

EF at 1582 cm-1

BG-AgNC

5.11 × 106

5.20 × 106

BG-AgNW

4.22 ×106

4.45 × 106

BG-AgNS

2.99 ×106

3.44 × 106

AgNC

1.75×106

1.91×106

AgNW

1.33×106

1.46×106

AgNS

3.6×105

4.1×105


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In order to ensure the reproducibility of BG-AgNC substrate, SERS spectra of 4-MBA (1.0 × 10-6 M) on the substrate are collected from eight random points under identical experimental conditions (Figure 5d). The BG-AgNC exhibits a better reproducibile Raman spectra at these random points. Further, the calculated relative standard deviation (RSD) value is 2.0% for the peak heights at 1080 cm-1, which is better than the literature reports for reduced graphene oxide-Au hybrid and RGO/AgNP SERS substrates.52’53 The average RSD value lower than 12% reveals the good reproducibility of the BG-AgNC.59 The increase in the intensity of all characteristic peaks of 4-MBA is due to the synergistic effect between silver nanostructures and BG. The BG sheets with the very high surface area can act as an excellent adsorbent for 4-MBA and the graphene structure doped with hetero atoms can diminish the surface energy of the hybrid60, which significantly enhances Raman scattering signals. In addition, the Ag nanostructures provides charge transfer between the 4MBA, AgNPs and BG in the 3D framework. The results are consistent with a report in which Kong et.al studied the effect of B substituted graphene in SERS detection of pyridine using DFT calculation. They reported that B-doped graphene positively affects SERS which enhances the electronic interaction between the SERS substrate and the analytes thereby resulting in a chemisorption at the interface.61 Moreover, the presence of B atom promotes charge transfer process by introducing new excited states to the system.61 Subsequently, benefits from the BG hotspots and the assembly of AgNCs, the BG-AgNC hybrid exhibits a better SERS activity than the BG, BG-AgNS and BG-AgNW towards 4-MBA detection. The nanocubes decorated in BG matrix exhibit high SERS activity due to the improved electromagnetic (EM) fields that occur at the sharp edges and corners.52 Moreover, graphene possess enormous surface area thereby adsorbing number 4-MBA available for the detection thereby bestow more intense Raman peaks.

The BG-silver nanostructures are further used for the SERS detection of melamine (MA). Melamine (2, 4, 6-triamino-1, 3, 5-triazine) is a nitrogen-rich organic material and is widely utilized to make resin. In recent times, because of its minimal cost and high nitrogen content, MA has been illegally added to milk products to increase up the total protein contents. However, huge-dose intake can affect the kidney functioning and results in the mortality of infants. Therefore, sensitive and accurate sensing of trace amounts of melamine is very important for the food safety applications62. The SERS spectra of MA (1.0 × 10-4 M) on BG, BG-AgNS, BG-AgNC and BG-AgNW hybrid substrates are shown in Figure 6a. The MA shows an intense peak at 683 cm-1 corresponding to the ‘ring breathing II’ mode in MA.63,64 It ACS Paragon Plus Environment

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should be noted that there is not any considerable SERS enhancement seen at the bare BG substrate. However, BG-AgNS, BG-AgNC and BG-AgNW exhibits noticeable SERS enhancement towards MA detection. The BG-AgNC displays the maximum Raman intensity. The observed increase in the SERS activity is due to the synergistic effect involving BG and Ag nanocubes. The enhancement factor (EF) of BG-AgNC at 683 cm-1 is evaluated to be 1.16 × 105 which is significant than BG-AgNW (8.88 × 104) and BG-AgNS (6.34 × 104). The as-prepared BG-AgNC displays higher SERS enhancement capability than BG-AgNS and BG-AgNW based on the signal at 683 cm-1. Figure 6b depicts the SERS spectra of MA at various concentrations (1.0 × 10-5 to 1.0 × 10-9 M) on the BG-AgNC hybrid. The most intense peak at 683 cm-1 is seen in all the samples. These results confirm that BG-AgNC SERS substrate can detect MA up to a concentration of 1.0 ×10-9 M.

Therefore, the developed BG-AgNC substrate finds potential application in

food screening.64 For long term applications, reproducibility of the SERS substrates should be confirmed. In order to ensure this the Raman spectra of MA on the BG-AgNC substrate were collected from eight random points (Figure 6c) under identical experimental conditions. The calculated RSD value for the main SERS peak intensities of BG-AgNC is found to be below 4 % and thereby it confirms that the BG-AgNC nanostructures have excellent reproducibility in SERS experiments.65 Further the usefulness of SERS in real milk samples is demonstrated. As shown in Figure 6d, the characteristic peak of MA (1.0 × 10-8 M) is noticed in the milk sample. The detection limit and enhancement factor obtained using BG-AgNC in this work for MA is better than the literature reports.66’67’68


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Figure 6. (a) SERS spectra of BG-Ag nanostructures towards MA detection (1.0 ×10-4 M) (b) SERS spectra of BG-AgNC towards MA detection at various concentrations (1.0 ×10-5 M to 1.0 ×10-9 M) (c) SERS spectra of BG-AgNC sample collected at ten different positions for the detection of MA (1.0 × 10-5 M) (d) SERS spectra obtained from milk sample containing the BG-AgNC hybrid (a) before and (b) after the addition of 1.0 ×10-8 M melamine

The SERS results are in excellent agreement with our BG-Ag nanostructures that exhibits outstanding SERS activity The improved SERS activity is due to the following reasons (i) boron doping reinforced an excellent plasmonic coupling of Ag nanostructures and graphene. thereby resulting in a significant electrical field improvement and results in enhanced sensitivity; (ii) in situ growth of Ag nanostructures (nanosphere, nanocube and nanowire) over BG synergizes the large surface area of graphene, electrocatalytic activity of boron doped graphene and Ag nanostructures (iii) uniform distribution of boron atoms on the graphene lattice interacts with carbon atoms and thereby decrease the surface energy of the SERS substrate as well as facilitating the charge transfer between graphene and AgNPs.


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SERS activity depends mainly on the charge transfer mechanism induced by the energy gap between the substrate Fermi level and the LUMO of the analyte.

3.2 Non-enzymatic detection of H2O2 The non-enzymatic detection of H2O2 is performed at BG, BG-AgNS, BG-AgNC and BGAgNW modified glassy carbon electrodes. The cyclic voltammograms (CVs) of BG, BGAgNS, BG-AgNC and BG-AgNW electrodes in N2 saturated 1.0 mM H2O2 / 0.1 M PBS at 50 mV/s are depicted in Figure 7a. From the results it is evident that the BG-AgNW electrode exhibits a high peak current value of −75 µA at −0.25 V which is superior than BG-AgNS (−56 µA, −0.32 V) and BG-AgNC (−63 µA, −0.30 V) electrode. Upon comparison, it is evident that the BG electrode (−28 µA, −0.35 V) exhibited the smaller current towards H2O2 detection. Results indicated that the presence of B-G improved the electrocatalytic activity of the bare AgNS, AgNC and AgNW towards H2O2 detection. Figure 7b is the CV of the BG-AgNW electrode in N2 saturated 1.0 mM H2O2 /0.1 M PBS at various scan rates. The plot of peak current vs scan rate is found to be linear (scan rates from 10 to 100 mV/ s) thereby confirming that the H2O2 reduction is a surface-controlled process.44


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Figure 7. (a) Cyclic Voltammograms of bare GC, RG, BG, BG-AgNS, BG-AgNC and BGAgNW modified GCEs in 0.1 M PBS (pH = 7.2) containing 1.0 mM H2O2 (b) Cyclic Voltammograms of BG-AgNW modified GCEs in 0.1 M PBS containing 1.0 mM H2O2 at different scan rates from 10 mV/s to 100 mV/s and the inset shows their corresponding linear fit (c) Amperometric response of BG modified GCE towards H2O2 in 0.1 M PBS at +0.4 V, the amplified response is presented in lower inset (d) plot of current vs. [H2O2] corresponding to BG (e) Amperometric response of BG-AgNW modified GCE towards H2O2 in 0.1 M PBS at +0.4 V, the amplified response is presented in lower inset (f) current vs. [H2O2] corresponding to BG-AgNW


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The chronoamperometric response of the BG and BG-AgNW electrodes in N2 saturated 0.1 M PBS solution corresponding to the additions of H2O2 (−0.40V vs. Ag/AgCl) is hsown in Figure 7c & 7e respectively. The current response of the BG-AgNW hybrid is very rapid and stable (attaining the stable current within 5s) upon the addition of H2O2. Figure 7d and 7f depicts the calibration plots for BG and BG-AgNW respectively. The BGAgNW electrode exhibited a linear response for the addition of H2O2 (0.01 mM to 25 mM). The limit of detection (LOD) is found to be 0.31 µM (S/N ratio is 3) for BG-NWs. Table 2 lists the performance of BG-AgNW and BG modified GCEs towards H2O2 detection. The rapid response of BG-AgNW is due to the synergistic effect as a consequence of enhanced surface area of the BG and the involvement of the uniformly distributed Ag nanowires. The BG-AgNW exhibits the best sensing performance with an excellent limit of detection and significantly better sensitivity than the bare BG electrode. 69 Table 2. Comparison of the performance of s BG and BG-AgNW electrodes towards the H2O2 detection Sample

Sensitivity (mM− 1 cm−2)

Detection range

Limit of Detection

(mM)

(µM)

BG

174.8

0.01-11

1.50

BG-AgNW

338.1

0.01-25

0.31

Figure 8. (a) Nyquist Plot corresponding to BG, BG-AgNS, BG-AgNC and BG-AgNW in 0.1 M KCl including 5.0 mM [Fe (CN) 6]3-/4- (b) Amperometric response at the BG-AgNW electrode following the addition of 0.1 mM H2O2, 0.2 mM UA, 0.2 mM AA, 0.2 mM glucose and 0.2 mM DA in 0.1 M PBS (pH 7.2) at -0.40 V under N2 atmosphere.


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The electrochemical impedance measurements has been carried out to investigate the charge transfer property of the electrodes designed in this work (Figure 8a). The BG-AgNW exhibits a smaller semicircle (diameter) suggesting the low charge-transfer resistance (Rct) of BGAgNW hybrid. The calculated Rct value for BG, BG-AgNS, BG-AgNC and BG-AgNW are 148 Ω, 110 Ω, 90 Ω and 72 Ω respectively. From the results, it is evident that the presence of BG improved the electron donating ability of the AgNWs through an excellent connection between AgNW-graphene and therby synergistically facilitating the electrocatalytic performance.70,70. Moreover, the defect sites created due to boron doping faciliates the rapid electron transfer through the graphitic network thereby confirming the usefulness of BGAgNW as an excellent electrocatalyst for H2O2 reduction66. The selectivity of our BG-AgNW electrode toward H2O2 detection is analysed in the presence of ascorbic acid (AA, 0.2mM), uric acid (UA, 0.2mM), dopamine (DA, 0.2mM) and glucose (0.2mM) (Figure 8b). These results demonstrated that BG-AgNW exhibit a superior sensing performance for H2O2 sensing owing to its excellent electrocatalytic performance even in the presence of other interfering analytes. All the above superior performances signify that the BG-AgNW electrode displays an outstanding electrocatalytic deduction of hydrogen peroxide and the BG-AgNW electrode’s sensing action is comparable to the sensing activity of other electrode materials, are given in Table 3. The reproducibility of the BG-AgNW electrode for H2O2 sensing is determined from the RSD values shows less than 4% for five estimations on a similar electrode. It can be concluded that the BG sheets decorated with AgNWs have excellent applications on H2O2 sensing and qualifies as a high-quality electrochemical biosensor.

Table 3. Comparison of our present work for H2O2 sensing with previous literatures Electrode

LOD (µM)

Sensitivity

Linear range

(µAmM−1)

(mM)

References

Pt-N-graphene/ITO

0.34

61.23

0.001–1

71

Au/CNT/Polyaniline/GCE

0.4

152.29

0.22–8.82

72

Ag-CNT

1.6

-

0.05 - 0.5

73

Ag dentrites

0.5

7.39 µA/mM

0.005–12

74

Ag/DNA NPs

9.0

38.3

0.05–1.2

75

BG-AgNW

0.31

338.1

0.01-25

This work


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The enhanced electrochemical reduction of H2O2 at BG-AgNW hybrid is due to the synergistic effect that stems from the exceptional electrocatalytic activity of silver nanowires and the very high surface area of BG associated with open porous structure.76 Moreover, the AgNWs acted as a nanospacer between BG sheets which further prevents the restacking of BG sheets. As a result, more active spots are created on the surface BG-AgNWs that preferably adsorbs more molecules and are available at the electrode surface for the electrochemical reduction thereby providing a greater magnitude of peak currents. Further, the AgNW acted as a nanoconnecter responsible for excellent conduction between the BG and the electrode surface thereby improving the performance of the BG-AgNW electrode.77

4. Conclusions In summary, this work demonstrated a cost effective and an efficient strategy to synthesize Ag nanostructures with different morphology (sphere, cube and wire) over boron doped graphene sheets. The SERS activity of the BG-Ag nanostructures was demonstrated using important analytes such as 4-MBA and melamine. From the results, it is evident that the BGAgNC hybrid exhibits the highest SERS activity and sensitivity towards the detection of 4MBA and MA respectively. Moreover, the as-developed BG-AgNW platform displayed excellent electrocatalytic activity towards H2O2 detection and is found to be linear in the range of 0.01 mM to 25 mM (R2 = 0.999). We believe that this method is very effective for the synthesis of Ag nanostructures with different morphology over BG sheets for applications in SERS and electrocatalysis.

Acknowledgements The authors acknowledge AKN DBT MSUB (BT/PR-4800//INF/22/152/2012 dated 22/03/2012) for financial support. The authors acknowledge SAIF MGU for Raman characterizations and Mr. Anu A.S., IIUCNN, MGU for HRTEM characterization. The authors NK and ST acknowledge funding from DST-Government of India through the Nano mission, PURSE and FIST Programs, and UGC – Govt. of India for the SAP program. The author NK also acknowledges financial support from KSCSTE-Govt. of Kerala through a project. SA acknowledges CSIR-CECRI for the grant OLP 0088. Reference (1)

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35x15mm (300 x 300 DPI)


In Situ Synthesis of Silver Nanospheres, Nanocubes, and Nanowires

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