Ultrasensitive Nanostructured Platform for the Electrochemical Sensing of Hydrazine Bikash Kumar Jena and C. Retna Raj* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India ReceiVed: January 5, 2007; In Final Form: February 22, 2007 Ultrasensitive electrochemical detection of hydrazine using nanosized Au particles self-assembled on a sol- gel-derived 3D silicate network is described. The citrate-stabilized gold nanoseeds (GNSs) were self-assembled on the thiol groups of the silicate network, which was preassembled on a polycrystalline Au electrode. The size of the GNSs on the network was enlarged by a seed-mediated growth approach, and the GNSs were characterized by UV-visible spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and electrochemical measurements. The enlarged nanoparticles (GNEs) on the silicate network have a size distribution between 70 and 100 nm and behave as a nanoelectrode ensemble. This nanostructured platform is highly sensitive toward the electrochemical oxidation of hydrazine. A very large decrease in the overpotential (∼800 mV) and significant enhancement in the peak currents with respect to the bulk Au electrode were observed without using any redox mediator. The nanostructured platform shows excellent sensitivity with an experimental detection limit (S/N ) 11) of 200 pM. The electrocatalytic properties of the nanostructured platform are strongly dependent on the particle coverage on the silicate network. This sensing platform is very stable and can be used for the continuous monitoring of hydrazine. The ultrasensitive nature of the sensor is ascribed to the existence of nanoelectrode ensembles. Introduction The deliberate tailoring of electrochemical interfaces with nanostructured metal and semiconductor particles has gained enormous interest with respect to the development of electro- chemical nanoscale devices. 1,2 The nanostructured metal par- ticles play a key role in catalytic and electrocatalytic reactions. 2-4 The catalytic efficiency of nanosized metal particles mainly depends on (i) the surface-to-volume ratio of the metal particle and (ii) the electronic interactions between the particles and the reactant molecules. The nanoparticles are very different from their bulk counterparts, and it has been shown that their catalytic activity originates from their quantum-scale dimensions. 5 Nano- sized gold (Au) particles are of increasing importance in different fields such as catalysis, sensors, microelectronics, and biological recognition. 2,6-8 Although bulk Au is considered to be a poor catalyst, recent works have revealed that nanosized Au particles have excellent catalytic activities. 9 For instance, nanosized Au particles supported on an oxide surface show very high catalytic activity toward carbon monoxide (CO) oxidation in the gas phase; bulk Au is largely inert to this reaction. 3,10 The electrocatalytic activities of nanosized Au particles toward CO, methanol, oxygen, NADH, glucose, and catechol have been demonstrated. 4,10-14 Hydrazine and its methyl derivatives are carcinogenic and hepatotoxic and have low threshold limit values of 10 ppb. 15 They are widely used as (i) high-energy propellants in rockets and spacecraft by the military and aerospace industries and (ii) fuel for zero-emission fuel cells. 16 It has been reported that hydrazine has been implicated in terrorist incident. 17 The development of sensitive methods for the detection of hydrazine is essential because of its importance in industry and its toxicity. Numerous methods such as spectrophotometry, fluorimetry, potentiometry, chromatography, voltammetry, amperometry, etc., have been reported for the quantification of trace amount of hydrazines. 18-26 Voltammetric methods are based on the oxidation of hydrazine 23-25 and have received considerable attention as high sensitivity can be achieved easily. However, the high overpotential required for oxidation is a major concern with electrochemical methods. Mediator-modified electrodes have been widely used to decrease the overpotential for the oxidation of hydrazine. 23-25 The major problem with mediator- modified electrodes is their lack of long-term stability because of the leaching of mediator from the electrode surface. Recently, Pt, Pd, and Cu-Pd nanoparticles have been utilized for the electrocatalytic oxidation of hydrazine. 27-30 Despite the fact that these nanosized metal particles have high catalytic activities, oxidation occurs at more a positive potential, and the detection limit is well above the threshold level of hydrazine. To the best of our knowledge, nanostructured Au particles have not been used for the electrocatalytic detection of hydrazine. Micro-/nanoelectrode ensembles have been widely used for the development of electrochemical sensors. They show many advantages over conventional macroelectrodes such as increased mass transport, decreased influence of solution resistance, low detection limits, and better signal-to-noise ratios. 31 In principle, the electroanalytical detection limit at an micro-/nanoelectrode ensemble can be much lower than that at an analogous macrosized electrode because the ratio between the faradaic and capacitive currents is higher. 31,32 Our group is interested in the development of electrochemical sensors based on nanostructured materials. 13 In an effort to develop a sensitive platform for the voltammetric detection of hydrazine, we have utilized nanosized Au particles in a sol-gel network derived from 3-(mercapto- propyl)trimethoxysilane (MPTS). Because the nanoparticle- modified electrodes behave like ensembles of nanoelectrodes, ultrasensitive detection of hydrazine at low potentials can be achieved. * Corresponding author. E-mail: crraj@chem.iitkgp.ernet.in. Fax: 91- 3222-282252. 6228 J. Phys. Chem. C 2007, 111, 6228-6232 10.1021/jp0700837 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007