Shape-Controlled Synthesis of Gold Nanoprism and Nanoperiwinkles

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J. Phys. Chem. C 2007, 111, 15146-15153

Shape-Controlled Synthesis of Gold Nanoprism and Nanoperiwinkles with Pronounced Electrocatalytic Activity Bikash Kumar Jena and C. Retna Raj* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India ReceiVed: March 26, 2007; In Final Form: August 6, 2007

We report the one-pot shape-controlled aqueous synthesis of triangular gold nanoprisms (GNPMs) and nanoperiwinkles (GNPWs) and their electrocatalytic activity toward reduction of oxygen and oxidation of methanol. The GNPMs and GNPWs were synthesized using 5-hydroxytryptamine (HT) as a reducing/stabilizing agent at room temperature. The UV-visible absorption spectrum of the nanostructured colloidal particles exhibits transverse and longitudinal surface plasmon bands at 535 and 900 nm, respectively. The transmission electron microscopy measurement shows that the nanostructured particles have prism and periwinkle-like morphology. The concentration of HT controls the shape and morphology of the nanostructured particles. The GNPMs have the size of 70-110 nm whereas GNPWs have the size of 150-230 nm. The X-ray diffraction profiles of GNPMs and GNPWs reveal that the nanoparticles are composed of mainly a Au(111) lattice plane. The GNPMs and GNPWs were self-assembled on a three-dimensional silicate network derived from (3-mercaptopropyl)trimethoxysilane, and their spectral and electrochemical properties have been investigated. The UV-visible diffuse reflectance spectral measurement shows that both GNPM and GNPW retain their morphology on the silicate network. The nanoparticles on the silicate network show excellent electrocatalytic activity toward oxidation of methanol and reduction of oxygen. The electrocatalytic activity of GNPMs and GNPWs is higher than the spherical gold nanoparticles.

Introduction Nanosized metal and semiconductor particles play an important role in many different areas due to their unique properties. The electronic, optical, optoelectronic, magnetic, and catalytic properties of these nanostructured particles can be conveniently tuned by controlling the size and shape.1 Because the catalytic properties depend on size, shape, and morphology, various approaches have been employed to obtain metallic nanostructures.2 Synthesis of anisotropic nanoparticle-like triangular gold nanoprism (GNPM) is of great interest because of its unique properties. The field enhancement effect near the triangle tips of metal nanoprism makes it useful in atomic force microscopy (AFM) and scanning tunneling microscopy (STM);3 they are also very promising for medicinal applications.4 The methodologies available for the synthesis of these nanostructured particles are difficult and time-consuming, and they often require structure-regulating reagents or techniques.5,6 The control of crystal morphology by proteins has been observed in a biological system.7 Recently, proteins, biological extracts, and small biomolecules have been used for the synthesis of metal nanoparticles.5e,8 Dong and co-workers synthesized the hexagonal and truncated triangular shaped single-crystalline Au nanoparticles by using L-amino acids without any templates.9 Sastry and co-workers recently reported the biological synthesis of triangular nanoprism using the lemongrass extract.5e However, in these approaches, the desired nanostructure was obtained after a very long time (6-12 h).5e,9 Although the optical properties of these anisotropic nanoparticles are well studied, the electrochemical and electrocatalytic properties have not been investigated. * Corresponding author. E-mail: [email protected].

The deliberate tailoring of electrochemical interfaces with nanostructured metal and semiconductor particles have gained enormous interest in the development of electrochemical nanoscale devices.10,11 Particularly the gold (Au) nanoparticle has attracted much attention in many different areas such as catalysis, biosensing, etc. Although bulk Au is a poor catalyst, recently it has been demonstrated that nanosized Au particles have excellent catalytic activity.12 The high catalytic activity of these nanosized particles are attributed to the large surfaceto-volume ratios and the existence of special binding sites on the surface of the particles. The electrocatalytic property of Au nanoparticles has been investigated by different groups.13-19 Zhong et al. explored the possible utilization of monolayerprotected nanosized Au particles for the oxidation of methanol in alkaline solution.15 Ohsaka and co-workers have extensively studied the electrocatalytic behavior of electrochemically deposited Au nanoparticles on different conducting support toward reduction of oxygen.16 It is generally observed that the catalytic property of the nanostructured metal particles largely depends on the size and shape.20 Li and Shi have demonstrated that the electrocatalytic behavior of electrochemically grown Au particles depend on their shape. The electrochemically deposited flowerlike Au nano structure showed higher electrocatalytic activity than that of the pine cone and sheet structures.21 Very recently, Ogumi and co-workers have shown the electrocatalytic application of Au nanoparticles in the development of direct methanol fuel cell.22 Significant enhancement in the electrocatalytic activity of Pt-Ru/C was noticed in the presence of nanosized Au particles. The promotional effect of Au nanoparticles in the electrooxidation of methanol has also been observed recently.23 Our group is interested in exploring the electrocatalytic and electroanalytical application of nanostructured metal particles.

10.1021/jp072363s CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

Shape-Controlled Synthesis of GNPMs and GNPWs Recently we have shown the potential application of spherical nanoparticles in the electrochemical sensing of different biomolecules, glucose and NADH, and the development of biosensors.14 Because the catalytic properties greatly depend on the size, shape, and morphology, it would be promising to explore the electrocatalytic properties of anisotropic nanostructures. The studies on the electrocatalytic oxidation of methanol and reduction of oxygen have received considerable interest mainly because of the energy-related applications such as fuel cell technology.24 Development of an efficient catalyst for direct methanol fuel cell (DMFC) is one of the key steps in achieving high efficiency. Precious metal platinum (Pt) has been extensively used as a catalyst in DMFC.24,25 However, the major problem associated with the Pt-based catalyst is the poisoning by adsorbed CO generated during the oxidation of methanol.24,26 To overcome the problem of poisoning, various multicomponent catalysts like Pt-Ru have been widely used.27 The development of a highly efficient non Pt catalyst that can overcome the problem of poisoning is a challenging task. It has been shown recently that Au electrode does not undergo poisoning during methanol oxidation by the electrogenerated CO.28,29 Considering the importance of these electrocatalytic reactions and the catalytic properties of nanostructured Au particles, in the present investigation we demonstrate a rapid eco-friendly one-pot approach for the shape-controlled synthesis of GNPMs and Au nanoflowers with periwinkle-like morphology at room temperature using the neurotransmitter 5-hydroxytryptamine (HT) and their electrocatalytic activity toward reduction of oxygen and oxidation of methanol. The nanoparticles were self-assembled on a sol-gel-derived silicate network, and their electrocatalytic activity has been investigated. Experimental Materials. HT, (3-mercaptopropyl)trimethoxysilane (MPTS), and HAuCl4 were obtained from Sigma-Aldrich and used as received. All other chemicals, unless mentioned otherwise, used in this investigation were of analytical grade. All the solutions were prepared with Millipore water. Carbon-coated copper grids were obtained from Pelco International for transmission electron microscopy (TEM) measurement. Instrumentation. The UV-visible spectra of the colloidal nanoparticles were recorded on a Shimadzu UV-1601 spectrophotometer. The UV-visible diffuse reflectance spectra (DRS) of GNPMs and gold nanoperiwinkles (GNPWs) on the silicate network were measured with a Shimadzu UV-2401 PC spectrophotometer. TEM images were obtained from a transmission electron microscope (JEOL JEM 2010 electron microscope) operating at 200 kV. The samples were obtained by dropping 2 µL of colloidal solution on the carbon-coated copper grids. The X-ray diffraction profile was obtained with the Phillips X’pert PRO X-ray diffraction unit using Ni-filtered Cu KR (λ ) 1.54 Å) radiation. Electrochemical measurements were performed using a two-compartment three-electrode cell with a polycrystalline Au working electrode (0.031 cm2), a Pt wire auxiliary electrode, and Ag/AgCl (3 M NaCl) reference electrode. Cyclic voltammograms were recorded using a computer-controlled CHI643B electrochemical analyzer. All the electrochemical experiments were performed in argon atmosphere. Synthesis of Nanostructured Au Particles. All glassware used in the synthesis of colloidal nanoparticles was cleaned with freshly prepared Aqua Regia and rinsed thoroughly with water. The colloidal nanoparticles were synthesized by mixing HAuCl4 and HT at room temperature. In a typical synthesis, 10 mL of aqueous solution of HAuCl4 (0.3 mM) was stirred for 2 min,

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15147 SCHEME 1: Schematic Illustration of the GNPM Electrode

and then 150 µL of HT (20 mM) was added to the solution. The stirring was continued for 30 min, and the resulting nanocolloid was stored at 4 °C. The spherical gold nanoparticles (SGNs) of 5-6 nm diameters were synthesized according to our earlier procedure.14 Self-Assembling of Nanoparticles on the Silicate Network. The self-assembling of nanostructured particles on the silicate network for electrochemical experiments was done according to our previous report.14 Briefly, the MPTS sol was prepared by mixing MPTS, methanol, and water (as 0.1 M HCl) in the molar ratio of 1:3:3, respectively, and stirred vigorously for 30 min. The well-polished and electrochemically cleaned polycrystalline Au electrode (0.031 cm2) was soaked in MPTS sol for 10 min. MPTS sol self-assembles on the polycrystalline Au electrode and exists as a sol-gel 3-D silicate network.14,30 This silicate network modified electrode was then soaked in the colloidal GNPMs or GNPWs for 18 h. Hereafter the MPTS network modified electrode and GNPMs and GNPWs selfassembled electrodes will be referred as MPTS, GNPM, and GNPW electrodes, respectively. A schematic illustration of the GNPM electrode is shown in Scheme 1. These electrodes were rinsed repeatedly with water and kept in phosphate buffer solution (PBS) before being subjected to electrochemical experiments. For UV-visible DRS measurements, the goldcoated coverslip was first modified with the silicate network, and then GNPMs and GNPWs were self-assembled on the network as described earlier. For X-ray diffraction (XRD) measurements, the microscopic glass slide or coverslip was cleaned well with acetone and water and soaked in a methanolic solution of MPTS monomer for about 18 h. The MPTS monomer modified glass slide or coverslip was washed with a copious amount of water and then soaked in colloidal nanoparticles for 6 h. Results and Discussion The progress of the formation of Au nanoparticles was followed by UV-visible spectroscopy. A total of 3 mL of HAuCl4 (0.3 mM) and 45 µL of HT (20 mM) were mixed together and introduced into the quartz cell. The absorbance spectra were recorded at the time interval of 1 min. Figure 1A shows the optical absorption spectra obtained for HAuCl4, HT, and the colloidal nanoparticles. The absorption band at 298 nm for HAuCl4 completely disappears upon the addition of HT and the color of HAuCl4 changes to red, indicating the reduction of Au3+ and the formation of Au nanoparticles. The colloidal nanoparticle shows two distinct plasmon absorption bands centered at 535 and 900 nm (Figure 1A, spectrum c). The

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Figure 2. Representative HRTEM images of Au nanoparticles obtained by the reduction of aqueous HAuCl4 (0.3 mM) with HT (0.3 mM). Reaction time: (a) 5, (b) 10, and (c) 30 min. Insets shows the single triangular GNPM and selected area electron diffraction pattern.

Figure 1. (A) Optical absorption spectra obtained for (a) HT (0.3 mM), (b) HAuCl4 (0.3 mM), and (c) colloidal nanoparticles. (B) Optical absorption spectra illustrating the progress of the formation of GNPMs. [HAuCl4] ) [HT] ) 0.3 mM. The spectra were recorded at the time interval of 1 min.

progress of the growth of nanoparticles was monitored in real time by UV-visible spectroscopy (Figure 1B). A monotonic increase in the absorbance at 535 nm and the appearance of a new shoulder-like band at the higher wavelength side (∼850 nm) has been observed during the course of the reaction. The position of the surface plasmon band at 535 nm does not change with time, although a gradual increase in the absorbance was observed. However, with time, the position of the new band shifts to the higher wavelength side, with an observable increase in the absorbance, and then it stabilizes at 900 nm after 30 min. The kinetics of the formation of nanoparticles was monitored by measuring the absorbance at 535 and 900 nm with time (Supporting Information). The absorbance at 900 nm increases with time at the early stage of the reaction, and it slowly decreases after 15 min; the absorbance at this wavelength stabilizes within 30 min. On the other hand, absorbance of the band at 535 nm reached the saturation level in 30 min. These time-dependent spectral features indicate the formation of either spherical nanoparticles that aggregate with time, anisotropic nanoparticles, or the combination of both.5,31 Figure 2 displays the representative HRTEM images obtained at three different stages of the reaction. At the early stage of the reaction (5-10 min), formation of near-spherical nanoparticles were observed (Figure 2a,b). These nanoparticles have the size distribution between 25 and 35 nm. After 10 min of reaction, these spherical nanoparticles develop into nanoprisms with triangular or hexagonal shapes (Figure 2c). The number

of triangular nanoparticles formed is higher than the other nanostructured particles. Many of the nanoprisms have regular edges with an angle of 60° for triangles and 120° for hexagons between the adjacent sides. The size of nanotriangles range from 70 to 110 nm. The selected area electron diffraction pattern was obtained by aligning the electron beam perpendicular to the planar surface of the GNPM. The hexagonal nature of these pattern spots clearly shows the single-crystalline nature of GNPM (Figure 2 inset). The energy dispersive spectrum (EDS) confirms that the GNPMs consist only of Au (Supporting Information). The anisotropic Au nanoparticles are known to exhibit two characteristic bands corresponding to the transverse and longitudinal plasmon absorption.1a The longitudinal plasmon absorption band is tunable with aspect ratio of the nanoparticles from the visible to the near-infrared region.1a,2c,32 The triangular gold nanoparticles are known to show two distinct bands corresponding to the transverse and longitudinal absoprtion.5e The longitudinal band is a strong function of edge length.32 In the present case, the bands observed at 535 and 900 nm correspond to the transverse and longitudinal plasmon absorption, respectively. The growth of nanostructured particles is highly sensitive to the concentration of HT. By slightly changing the concentration of HT, we were able to produce nanostructured particles with different morphology. We have examined the influence of the concentration of HT at a fixed concentration of the precursor (0.3 mM). Figure 3 presents the optical spectra of Au nanoparticles obtained at different concentrations of HT. At lower concentration of HT (0.075 mM), a significant decrease in the absorbance at 298 nm due to HAuCl4 and the appearance of a small hump at ∼535 nm was observed. However, when the concentration of HT was increased in the range from 0.15 to 0.3 mM, two distinct bands corresponding to the transverse and longitudinal absorption, due to the growth of GNPMs, have been observed (Figure 3c,d). When the concentration of HT exceeds 0.3 mM, the longitudinal band at 900 nm completely disappears whereas the transverse band at 535 nm shifts to the longer wavelength side and its absorbance substantially decreases (Figure 3e,f). The shift in the band position at 535 nm and

Shape-Controlled Synthesis of GNPMs and GNPWs

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15149 SCHEME 2: Mechanism for HT-Induced Formation of Au Nanoparticlea

a ‘a’ and ‘b’ represent the redox reaction corresponding to the voltammetric peaks shown in Figure 7.

Figure 3. Optical absorption spectra obtained for the Au nanoparticle at different concentrations of HT. [HT]: (a) 0, (b) 0.075, (c) 0.15, (d) 0.3, (e) 0.45, and (f) 0.6 mM. [HAuCl4]: 0.3 mM. Reaction time: 30 min. Inset shows the photographs of the corresponding colloidal nanoparticles.

Figure 4. TEM images obtained at different concentrations of HT. (a) Periwinkle nanoparticles [HT]: 0.45 mM. (b) Aggregated nanoparticles [HT]: 0.6 mM. Selected GNPWs are shown at the bottom. Other experimental conditions are the same as Figure 3.

decrease in the absorbance at higher concentrations of HT can be ascribed to the aggregation or growth of nanoparticles of different morphology. Figure 4 shows the TEM images obtained for the nanoparticles synthesized at two different concentrations of HT. When the concentration of HT is >0.3 mM, we observed two distinct features: nanocrystals with periwinkle-like morphology and aggregated nanoparticles. Interestingly, the growth of nanoperiwinkles of different shapes has been observed when the concentration of HT was 0.45 mM (Figure 4a). The GNPWs have the size distribution between 150 and 230 nm. Further increase in the concentration of HT results in the formation of aggregated nanoparticles (Figure 4b). In agreement with the optical spectra (Figure 3f), the TEM images clearly show the aggregation of nanoparticles at higher concentrations of HT (g0.6 mM). These results indicate that it is possible to tune the morphology and shape of the nanostructured particles by controlling the concentration of HT. The influence of the concentration of AuCl4- on the formation of nanostructured particles has also been investigated at a fixed concentration of HT (0.3 mM) by spectral measurement

(Supporting Information). The optical spectrum shows two bands (∼535 and 900 nm) at the equimolar concentration of AuCl4and HT (0.3 mM each). The band at 900 nm shifted to ∼787 nm when the concentration of AuCl4- was increased to 0.45 mM. At a higher concentration of the precursor (0.6 mM), we observed a broad band at ∼560 nm; the band corresponding to AuCl4- at ∼300 nm does not disappear completely, suggesting that all the AuCl4- has not been reduced. On the other hand, at a lower concentration of AuCl4- we observed an ill-defined broad band at ∼560 nm. All these results further indicate the facts that the growth of GNPM is sensitive to the concentration of precursor and HT and that equimolar concentration is required for the growth of GNPM. Willner and co-workers have reported the catacholamine neurotransmitters mediated growth of Au nanoparticles and their quantification by optical method.8b The catacholamine neurotransmitters and their metabolites induce the growth of spherical shape nanoparticles, and they do not induce the formation of anisotropic nanoparticles.8b In the present investigation, we observed the growth of GNPMs and GNPWs by the indoleamine neurotransmitter. The formation of nanostructured particles is attributed to the oxidation of HT by HAuCl4. As shown in Scheme 2, reduction of HAuCl4 occurs due to the transfer of electrons from HT to the metal ion, resulting in the formation of Au0. The metallic Au then undergoes nucleation and growth with time to form GNPMs or GNPWs. It is welldocumented in the literature that the oxidation of 5-hydroxyindoles generates a free radical.36,37 The radical generated by the oxidation of HT can undergo a chemical reaction in aqueous solution to yield different products. The reaction of radical strongly depends on the experimental condition. Hydroxylated dimers and tryptamine-4,5-dione (TAD) (Scheme 2) are the major products in neutral and acidic pH.37 These oxidation products are expected to adsorb on the surface of the nanoparticles (vide supra). Characterization of Self-Assembled GNPMs and GNPWs on the Silicate Network. To examine the electrocatalytic properties of the nanostructured particles, we have selfassembled GNPMs and GNPWs on a thiol-functionalized silicate network (Scheme 1). Figure 5 displays the UV-visible diffuse reflectance spectra obtained for GNPMs and GNPWs selfassembled on the silicate network modified gold-coated coverslip. As in the case of colloidal solution, the GNPMs on the silicate network show two bands at 535 and 900 nm corresponding to the transverse and longitudinal surface plasmon bands, respectively. On the other hand, the GNPWs on the

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Figure 5. UV-visible diffuse reflectance spectrum of (a) silicate network, (b) GNPWs, and (c) GNPMs on the silicate network modified goldcoated cover slip. Inset shows the UV-visible spectrum obtained for colloidal (a) GNPMs and (b) GNPWs.

Figure 6. XRD pattern obtained for (a) GNPMs and (b) GNPWs.

silicate network exhibit a band at ∼540 nm, which is very similar to the spectra obtained for the colloidal GNPWs. These spectral features confirm that the GNPMs and GNPWs retain their morphology on the silicate network. The XRD pattern obtained for the self-assembled GNPMs and GNPWs shows four peaks corresponding to the (111), (200), (220), (311) planes of a face centered cubic lattice of Au (Figure 6). In both the cases, the peak corresponding to the (111) plane is more intense than the other planes. The close examination of the XRD profile shows that the ratio between the intensities of the (200) and the (111) diffraction peaks for GNPMs and GNPWs are 0.33 and 0.25, respectively. These values are much lower than the conventional value (0.52),33 demonstrating that the (111) plane is the predominant orientation. The intensity of the planes of GNPW is relatively higher than GNPM. The sizes of the GNPMs and GNPWs were also calculated from X-ray line broadening analysis by applying the Scherrer formula.34 The sizes obtained for GNPMs and GNPWs are ∼89 and ∼161, respectively, and are in close agreement with TEM data. The GNPM and GNPW electrodes show a broad oxidation wave in the potential range from +0.32 to 0.4 V and a reduction peak at ∼0.17 V in alkaline pH (0.1 M KOH) (Supporting Information), corresponding to the formation of surface oxides

Figure 7. Voltammetric response obtained for the GNPM electrode in 0.1 M PBS of pH 7.2. Scan rate: 100 mV/s.

and their reduction.15c Such voltammetric response was not observed for the MPTS electrode, revealing that the voltametric features of GNPM and GNPW electrodes are due to the presence of nanostructured Au particles on the silicate network. The area of the electrochemically accessible GNPM and GNPW has been measured by chronoamperometry and cyclic voltammetry using Fe(CN)63-/4- redox couple. The area has been calculated using Cottrel and Randles-Sevcik equations.35 The surface area of GNPMs and GNPWs on the silicate network was calculated to be 0.0285 and 0.0263 cm2, respectively. The GNPM and GNPW electrodes show two reversible voltammetric peaks (‘a’ and ‘b’) characteristic of a surface-confined redox species at ∼ -0.04 and -0.2 V in neutral PBS (Figure 7). These reversible voltammetric responses can be ascribed to the redox reaction of the oxidation products of HT (Scheme 2) that adsorbed on the nanoparticle surface. The voltammetric peak ‘a’ at -0.2 V is very small and rather broad whereas the peak ‘b’ at -0.04 V is well defined. Note that the magnitude of peak current at -0.04 V is 0.3 V) potential,29,38 which indicates the high electrocatalytic activity of the nanosized particles. In the absence of methanol, both GNPM and GNPW electrodes show a broad oxidation wave at ∼0.4 V and a sharp reduction wave at ∼0.17 V (Supporting Information), and these voltammetric peaks are ascribed to the formation of Au oxide and its reduction (vide supra).15a,c The reduction wave observed at ∼0.17 V disappears in the presence of methanol, supporting the involvement of surface oxides in the electrocatalysis; the surface oxides function as an efficient electron-transfer mediator in the oxidation process.15a,c The oxygen-containing species, surface oxide, generated by the process of premonolayer oxidation, is responsible for the observed catalytic effect. The involvement of surface oxides in the electrocatalytic reaction has already been demonstrated for the polycrystalline Au electrode.15,39 The electrocatalytic activity of GNPW toward reduction of oxygen and oxidation of methanol is significantly higher than GNPM. The current density for the reduction of oxygen and oxidation of methanol on the GNPW electrode is relatively higher than that on the GNPM. The high catalytic activity can be explained by considering the surface morphology of the nanostructured particles. Close examination of the XRD profile obtained for GNPM and GNPW reveals that the intensity of

15152 J. Phys. Chem. C, Vol. 111, No. 42, 2007 220 and 311 planes are significantly high in the case of GNPW. The intensity of these two planes is very weak in the case of GNPM. The higher Miller index crystals have surface irregularities,40 and the Au atom in (220) and (331) planes have higher unsaturation than the (111) or (200) planes. The high Miller index surfaces have been composed of low-index terraces linked by kinked steps of monatomic height. The particle having high Miller index planes consisting of steps and kinks are known to significantly influence the reactivity of the catalyst surface.41-43 In the present investigation, we suggest that the high catalytic activity of GNPW can be due to the flower-like morphology. It is interesting to note that the electrocatalytic activity of GNPMs and GNPWs toward oxygen and methanol is higher than the SGNs. In the case of oxygen reduction, the SGN electrode shows only one peak (Figure 8c) at (∼ -0.21 V), which is 45 mV more negative than that on GNPM and GNPW electrodes. On the GNPM and GNPW electrode, the reduction starts at much less negative potential than that on the SGN electrode. The oxidation of methanol on the SGN electrode occurs at 0.32 V (Figure 9c), which is 60-90 mV more positive than that at the GNPM and GNPW electrodes. Moreover, the peak current for the reduction of oxygen and oxidation of methanol at the GNPM and GNPW electrodes is significantly higher than that at SGN electrode. One can argue that the higher current obtained could be due to the high surface area44 of GNPMs and GNPWs. To compare the electrocatalytic activity of GNPM and GNPW with SGN, we have normalized the catalytic current with the surface area. Interestingly, the current density calculated for the reduction of oxygen and oxidation of methanol at GNPM and GNPW electrode is 1.5-1.7 times higher than that at SGN electrode, indicating that GNPMs and GNPWs have higher electrocatalytic activity than SGN. We suggest that the shape and morphology of the anisotropic nanostructured particles could be reason for the high catalytic activity. Conclusion In summary, we have shown a simple approach for the rapid room temperature synthesis of GNPMs and GNPWs in aqueous solution using indoleamine neurotransmitter without any structureregulating reagents/technique. The growth of nanostructured particles depends on the concentration of HT. Concentration of HT is the key factor in controlling the shape and morphology of the nanocrystals. This work can be significant in further understanding the biomolecule-directed synthesis of nanostructured particles and the natural process of biomineralization. Because GNPMs show a longitudinal band at the longer wavelength side, it may also find application in infraredabsorbing optical coating. GNPMs and GNPWs immobilized on a sol-gel-derived silicate network shows excellent electrocatalytic activity toward reduction of oxygen and oxidation of methanol. The electrocatalytic activity of GNPMs and GNPWs is higher than that of SGN. Acknowledgment. This work was supported by grants from CSIR, DST, and NST, New Delhi. Authors thank Professor S. K. Ray, Department of Physics and Meteorology, IIT Kharagpur, for XRD measurements. The authors are grateful to Dr. Asim Bhowmik, Indian Association for the Cultivation of Sciences, Kolkata, for DRS measurements. Supporting Information Available: Additional information available as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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