Potential Sensing Platform of Silver Nanoparticles Embedded in

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Potential Sensing Platform of Silver Nanoparticles Embedded in Functionalized Silicate Shell for Nitroaromatic Compounds Govindhan Maduraiveeran and Ramasamy Ramaraj* Centre for Photoelectrochemistry, School of Chemistry, Madurai Kamaraj University, Madurai-625 021, India A simple and new method to grow a pentagonally twinned structure of silver-silicate core-shell nanoparticles in aqueous environment at room temperature and its application in nitrobenzene (NB) sensing is described here. Silver-silicate core-shell nanoparticles were obtained by one-step synthesis using N-[3-(trimethoxysilyl)propyl]ethylene diamine (EDAS) as a reducing/stabilizing agent and cetyltrimethylammonium bromide (CTAB) as the growing agent for the growth of silver nanoparticles (Agnps). The silver-silicate core-shell nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), scanning electron microscope (SEM), UV-visible absorption, emission, excitation, and electrochemical measurements. The electrochemical studies of silversilicate core-shell nanoparticles modified electrode showed the silver nanoparticle’s oxidation potential and their corresponding reduction potential at 0.24 and -0.16 V, respectively. The optical and electrochemical applications silicate-shell stabilized silver nanoparticles were established toward nitrobenzene. The optical sensing of nitrobenzene by silver-silicate core-shell nanoparticles studied using absorption and emission spectral methods showed experimentally determined lowest detection limits (LOD) of 1 and 10 µM, respectively. Silver-silicate core-shell nanoparticles showed excellent electrocatalytic activity toward the reduction of nitrobenzene. The electrochemical sensor showed the lowest detection limit (LOD) of 2.5 nM toward nitrobenzene sensing. The development of newer methods for the controlled synthesis of metal nanoparticles with different shape and size has attracted major attention during the past decade.1 The size and shape of metal nanoparticles are extremely important parameters because they substantially affect the physical and chemical * Corresponding author. Phone: +91-452-2459084. E-mail: ramarajr@ yahoo.com. (1) (a) Branham, M. R.; Douglas, A. D.; Mills, A. J.; Tracy, J. B.; White, P. S.; Murray, R. W. Langmuir 2006, 22, 11376–11383. (b) Huang, C.-C.; Yang, Z.; Lee, K.-H.; Chang, H.-T. Angew. Chem., Int. Ed. 2007, 46, 6824–6828. (c) Muto, H.; Yamada, K.; Miyajima, K.; Mafune, F. J. Phys. Chem. C 2007, 111, 17221–17226. (d) Chen, Z.; Zu, Y. Langmuir 2007, 23, 11387–11390.

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as optics,12 catalysis,13 and biological diagnostics.14 Particularly, the fluorescent type nanoparticles have been used as indicators in biological applications such as imaging.15 Silver nanoparticles exhibited antimicrobial properties, and these are used in biosensors and electrooptical applications.16 Because of these properties, the incorporation of silver nanoparticles into various matrixes has intensively been investigated in order to extend their utility in materials, biomedical, and sensor applications.17 The applications of silver nanoparticles are connected with the stability of their dispersions allowing to prevent the aggregation process.18 The aqueous dispersions of silver nanoparticles with enhanced stability can be obtained by two types of protection mechanism ((i) based on steric repulsion and (ii) based on electrostatic repulsion).19 The steric repulsion displays the stabilizing effect with the help of immediate adsorbed polymer or ionic surfactant molecules at the phase-interphase and the balance between the attractive and repulsive forces is dependent on the adsorbed layer thickness.20 The electrostatic repulsion displays the surface charge of the dispersed phase and this can be enhanced by the ionic surfactant to provide the electrostatic protection for stabilizing the dispersed metal nanoparticles.21 The encapsulation of metal nanoparticles is achieved by a variety of substrates such as metal oxides, organically modified silicates, polymers shaped as thin films, spheres, fibers, dendrimers, and so forth.22,23 Among these substrates, the silicate materials have attracted much attention (12) (a) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824– 830. (b) Hakamada, M.; Mabuchi, M. Nano Lett. 2006, 6, 882–885. (c) Patra, A.; Hebalkar, N.; Sreedhar, B.; Radhakrishnan, T. P. J. Phys. Chem. C 2007, 111, 16184–16191. (13) (a) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663– 12676. (b) Manas, M.-M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638–643. (c) Selvaraju, T.; Ramaraj, R. Electrochim. Acta 2007, 52, 2998–3005. (d) Maduraiveeran, G.; Ramaraj, R. Electrochem. Commun. 2007, 9, 2051–2056. (e) Ramaraj, R. J. Chem. Sci. 2006, 118, 593–600. (14) (a) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547–562. (b) Berry, V.; Gole, A.; Kundu, S.; Murphy, C. J.; Saraf, R. F. J. Am. Chem. Soc. 2005, 127, 17600–17601. (15) (a) Aslan, K.; Wu, M.; Lakowicz, R. J.; Geddes, D. J. Am. Chem. Soc. 2007, 129, 1524–1525. (b) Ow, H.; Larson, D. R.; Srivastava, M.; Baird, B. A.; Webb, W. W.; Wiesner, U. Nano Lett. 2005, 5, 113–117. (c) Wang, L.; Yang, C.; Tan, W. Nano Lett. 2005, 5, 37–43. (16) (a) Wang, A.-Q.; Chang, C.-M.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 18860–18867. (b) Frederix, F.; Friedt, J.-M.; Choi, K.-H.; Laureyn, W.; Campitelli, A.; Mondelaers, D.; Maes, G.; Borghs, G. Anal. Chem. 2003, 75, 6894–6900. (c) Zhang, J.; Li, X.; Sun, X.; Li, Y. J. Phys. Chem. B 2005, 109, 12544–12548. (17) (a) Ahmed, I.; Ready, D.; Wilson, M.; Knowles, J. C. J. Biomed. Mater. Res. A 2006, 79, 618–626. (b) Jiang, S. Q.; Newton, E.; Yuen, C. W.; Kan, C. W. Text. Res. J. 2006, 76, 57–65. (c) Tarimala, S.; Kothari, N.; Abidi, N.; Hequet, E.; Fralick, J.; Dai, L. L. J. Appl. Polym. Sci. 2006, 101, 2938–2943. (18) (a) Shrivastava, S.; Bera, T.; Roy, A.; Singh, G.; Ramachandrarao, P.; Dash, D. Nanotechnology 2007, 18, 225103–225112. (b) Teeguarden, J. G.; Hinderliter, P. M.; Orr, G.; Thrall, B. D.; Pounds, J. G. Toxicol. Sci. 2007, 95, 300–312. (19) Kvitek, L.; Panacek, A.; Soukupova, J.; Kolar, M.; Vecerova, R.; Prucek, R.; Holecova, M.; Zboril, R. J. Phys. Chem. C 2008, 112, 5825–5834. (20) (a) Hunter, R. J. Double Layer Interaction and Particle Coagulation. In Foundations of Colloid Science, 2nd ed.; Oxford University Press: New York, 2001; p 635. (b) Chou, K. S.; Lai, Y. S. Mater. Chem. Phys. 2004, 83, 82– 88. (c) Luo, C.; Zhang, Y.; Zeng, X.; Zeng, Y.; Wang, Y. J. Colloid Interface Sci. 2005, 288, 444–448. (21) (a) Yu, D. B.; Yam, V. W. W. J. Phys. Chem. B 2005, 109, 5497–5503. (b) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B 2000, 104, 8333–8337. (22) (a) Khushalani, D.; Hasenzahl, S.; Mann, S. J. Nanosci. Nanotechnol. 2001, 1, 129–132. (b) Chakrabarti, K.; Whang, C. M. Mater. Sci. Eng., B 2002, 88, 26–34. (c) Bharathi, S.; Nogami, M.; Ikeda, S. Langmuir 2001, 17, 7468–7471.

because of their stability, reusability, safer operations, easy scale up, and good catalytic properties.24 Studies on the nitroaromatics show their detrimental effects on human health and on environment as toxic and mutagenic substances.25 Most of the nitroaromatics have originated from the production of explosives,26 although other sources of contamination include industry,27 coal fly ash,28 cigarette smoke,29 etc. The sensing of nitrobenzene has generated significant interest in the creation of secure environment and in other applications.29 To meet the requirements of rapid warning and field deployment, more-compact and low-cost instruments coupled with smaller sensing probes are highly desirable for facilitating the task of onsite monitoring of nitrobenzene. A number of different methods based on different principles have been developed in the past for the detection of compounds. Among these methods, optical and electrochemical-based sensors were generally employed due to their sensitivity, selectivity, simplicity, and low-cost instrumentation.30 Herein, we report the use of an environment benign solvent throughout the preparation of silver-silicate core-shell nanoparticles by using N-[3-(trimethoxysilyl)propyl]-ethylene diamine (EDAS) as the reducing/stabilizing agent. It is important to develop a simple and time-saving procedure for the synthesis of silver-silicate core-shell nanoparticles in aqueous medium without using any hazardous reducing agent. The sensor applications of silver-silicate core-shell nanoparticles were demonstrated by using optical (absorbance and emission) and electrochemical methods. To the best of our knowledge, this is the first time a report of the synthesis of silver-silicate core-shell nanoparticles in aqueous solution without using external reducing agent and its applications in nitrobenzene sensing based on three different analytical methods is presented. At the modified electrode, silversilicate core-shell nanoparticles were in electrical contact with each other and this composite structure was used as an electrochemical sensor for nitrobenzene. The performance of optical and electrochemical sensors based on silver nanoparticles with respect (23) (a) Jiang, T.; Chen, W.; Zhao, F.; Liu, Y.; Wang, R.; Du, H.; Zhang, T. J. Appl. Polym. Sci. 2005, 98, 1296–1299. (b) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223–2253. (c) Li, L.; Cao, X.; Yu, F.; Yao, Z.; Xie, Y. J. Colloid Interface Sci. 2003, 261, 366– 371. (24) (a) James, A. S.; Stoermer, R. L.; Sha, M.-Y.; Keating, C. D. Langmuir 2007, 63, 1984–1988. (b) Kumar, A.; Pushparaj, V. L.; Murugesan, S.; Viswanathan, G.; Nalamasu, R.; Linhardt, R. J.; Nalamasu, O.; Ajayan, P. M. Langmuir 2006, 22, 8631–8634. (c) Schottner, G. Chem. Mater. 2001, 13, 3422– 3435. (25) (a) Hrapovic, S.; Majid, E.; Liu, Y.; Male, K.; Luong, J. H. T. Anal. Chem. 2006, 78, 5504–5512. (b) Hilmi, A.; Luong, J. H. T. Environ. Sci. Technol. 2000, 34, 3046–3050. (26) Rodgers, J. D.; Bunce, N. J. Water Res. 2001, 35, 2101–2111. (27) Hartter, D. R. Toxicity of Nitro Compounds; Hemisphere Publishing Group: Washington, DC, 1985. (28) Rosenkranz, H. S.; Mermelstein, R. Mutat. Res. 1983, 114, 217–267. (29) (a) Rosenkranz, H. S.; McCoy, E. C.; Sanders, D. R.; Butler, M.; Kiriazide, D. K.; Mermelstein, R. Science 1980, 209, 1039–1043. (b) Wolfbeis, O. S. J. Mater. Chem. 2005, 15, 2657–2669. (c) Singh, S. J. Hazard. Mater. 2007, 144, 15–28. (30) (a) Kortum, G. Treatise on Electrochemistry; Elsevier Pub. Co.: Amsterdam, The Netherlands, 1965; p 497. (b) Bharathi, S.; Fishelson, N.; Lev, O. Langmuir 1999, 15, 1929–1937. (31) (a) Frattini, A.; Pellegri, N.; Nicastro, D.; Sanctis, O. Mater. Chem. Phy. 2005, 94, 148–152. (b) Rocha, T. C. R.; Winnischofer, H.; Westphal, E.; Zanchet, D. J. Phys. Chem. C. 2007, 111, 2885–2891. (c) Chang, R. Physical Chemistry for the Chemical and Biological Sciences; University Science Books: Sausalito, CA, 2000; Chapter 12, pp 445-447.

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to sensitivity, linear range, and stability toward nitrobenzene sensing was evaluated and discussed. EXPERIMENTAL SECTION Materials. Nitrobenzene and N-[3-(trimethoxysilyl)propyl]ethylene diamine were from Merck. Cetyltrimethylammonium bromide (CTAB) was from Aldrich. Silver nitrate was from the SRL. All other chemicals, unless mentioned otherwise, used in the present work were of analytical grade. Doubly distilled water was used for all experiments. Glass slides (76 mm × 26 mm × 1 mm) were used to record absorption and emission spectra for silver-silicate core-shell nanoparticles in the film state. All glasswares were thoroughly cleaned with aqua regia (3:1 HNO3/ HCl) and rinsed extensively with double distilled water before use. All the experimental solutions were prepared using 0.1 M phosphate buffer solution (PBS) at pH 7.2. Instrumentation. UV-visible absorption spectra were recorded for silver-silicate core-shell nanoparticles in colloidal and film states using an Agilent 8453 diode array spectrophotometer. The silver-silicate core-shell film was prepared by casting a known volume of solution on a glass plate. The silver-silicate core-shell nanoparticles were characterized using a transmission electron microscope (TEM) (JEOL-JEM 2000Fx-II) operating at an accelerating voltage of 200 kV high-resolution EM (HREM) using a charge-coupled device (CCD) camera attached to the TEM. The sample for high-resolution transmission electron microscopy (HRTEM) measurement was prepared by placing a drop of silver-silicate core-shell solution on a carbon-coated copper grid and drying at 25 °C. The X-ray diffraction (XRD) analysis was carried out by PANalytical (X’per PRO) with Ni filtered monochromatic Cu KR (1.5406 Å, 2.2 KW Max.) X-ray. The measurement sample for XRD was prepared by casting the silver-silicate core-shell nanoparticles on a glass plate and allowing them to dry at room temperature. The diffracted X-ray intensities were recorded in the 2θ range from 31° to 65°. Scanning electron microscopic (SEM) measurement was performed with a Hitachi (model S-3400N). Emission spectra were recorded on a Jasco FP-6300 spectroflourimeter. Electrochemical measurements were performed using a three-electrode cell with a glassy carbon (GC) working electrode (electrode area, 0.07 cm2), a Pt wire auxiliary electrode, and a Ag/AgCl reference electrode. The cyclic voltammograms were recorded using EG&G PAR 283A potentiostat/galvanostat controlled by M270 software. Before each experiment, the working electrode (GC) was twish polished with alumina slurry (0.5 µm) followed by sonication for 3 min in double distilled water. All the electrochemical experiments were recorded in deaerated 0.1 M phosphate buffer solution (PBS) at pH 7.2. Safety Considerations. Nitrobenzene and other nitroaromatic compounds are highly toxic and dangerous and should be handled with care. Skin and eye contact, accidental inhalation, and ingestion should be avoided. Aqua regia is a powerful oxidizing agent, and it should be handled with care. Synthesis of Silver Nanoparticles. In a typical experiment, a 5 mL mixture of silver nitrate (1 mM), N-[3-(trimethoxysilyl)propyl]-ethylene diamine (0.1 M) and cetyltrimethylammonium bromide (1 mM) was taken in aqueous solution and the mixture was stirred vigorously for 30 min. The vigorous stirring led to the formation of a light yellow color solution. The final color of 7554

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Figure 1. Absorption spectra of silver nitrate upon the addition of EDAS (a) and after stirring the reaction mixture for 30 min (b) and silver nitrate alone (c). Inset: Photographs taken before (A) and after (B) stirring the reaction mixture (Ag+ ) 1 mM, EDAS ) 0.1 M, and CTAB ) 1 mM).

the mixture changed to dark yellow-bright brownish at longer time. To monitor the growth process of silver nanoparticles, absorption spectra were recorded in real time by an absorption spectrophotometer. Upon the completion of the formation silver nanoparticles, the solution was kept at room temperature for a day. The resulting silver nanoparticles solution was stored at room temperature. RESULTS AND DISCUSSION The silver nanoparticles were prepared at room temperature (25 °C) by using a mixture of silver nitrate, N-[3-(trimethoxysilyl)propyl]-ethylene diamine, and cetyltrimethylammonium bromide in aqueous solution. This solution mixture was vigorously stirred for 30 min and a light yellow color developed during stirring. Figure 1 shows the absorption spectra obtained during the mixing of EDAS and CTAB with silver nitrate. The formation of silver nanoparticles was visually observed during the reduction of Ag+ ions from the change in color from colorless (before reduction) (curve a in Figure 1) to yellow (after reduction) (curve b in Figure 1). The inset in Figure 1 displays the photographs of the solution mixture before (A) and after (B) stirring. The intensity of color gradually increased at longer time. The slow growth of silver nanoparticles was monitored by an absorption spectrophotometer. The absorption intensity observed at 298 nm due to silver nitrate (curve c in Figure 1) decreased with a slight increase in the absorption intensity around the ∼400-500 nm region (curve a in Figure 1) upon the addition of EDAS and CTAB due to the interaction of Ag+ with aminosilane31a present in the reaction mixture. Absorption spectra were recorded with a time interval of 5 min and the observed spectra are shown in Figure 2. After vigorous stirring of the mixture, the solution’s color changed to light-yellow indicating the formation of silver nanoparticles. The so formed silver nanoparticles show the surface plasmon resonance (SPR) band at 413 nm (curve b in Figure 1 and Figure 2). A monotonic increase in the absorption band at 413 nm was observed during the course of the reaction, and the corresponding photographs taken before and after the growth of silver nanoparticles in solution are shown in Figure 2 (inset). The position of the SPR band observed at 413 nm did not change with time, although a gradual increase in the absorbance was observed. The

Figure 3. HRTEM image of silver-silicate core-shell nanostructures.

Figure 2. Absorption spectra showing the formation of silver nanoparticles in 375 min (in a solution mixture of 1 mM AgNO3, 0.1 M EDAS, and 1 mM CTAB) recorded at a time interval of 5 min. Inset: Photographs of the solutions taken before (a) and after (b) growth of silver nanoparticles.

kinetics of chemical reaction31b,31c were monitored in the present study by following the increase in absorbance of silver nanoparticles with time at 413 nm, and the rate constant was calculated as 0.5 µM s-1. A relationship exists between the nanoparticle size and SPR band position and band broadening with an increase in the particle size.32 We have not observed any significant change in the SPR band during the growth of the silver nanoparticles (Figure 2), which shows the formation and stabilization of silver nanoparticles. These observations reveal the dual role played by the aminosilane as a reducing agent and as a stabilizer for silver nanoparticles during the formation of silver-silicate core-shell nanoparticles in aqueous medium. The -NH2 groups of EDAS based silicate sol-gel can stabilize metal nanoparticles, while the presence of only one of these functional groups is not sufficient to accomplish this task. The aminosilane complexes with Ag+ ion to form the respective amine-silver complex depending on the amine concentration. The silver-amine complex is not labile, and the Ag+ ion exhibits much lower affinity to alkyl amines.30b Upon the reduction of Ag+ ion, the amine groups immediately cap the surface of the Ag moieties. Although at pH ∼