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J. Phys. Chem. C 2007, 111, 6228-6232
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 solgel-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 electrochemical nanoscale devices.1,2 The nanostructured metal particles 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 Nanosized 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, * Corresponding author. E-mail:
[email protected]. Fax: 913222-282252.
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 hydrazine23-25 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 mediatormodified 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-(mercaptopropyl)trimethoxysilane (MPTS). Because the nanoparticlemodified electrodes behave like ensembles of nanoelectrodes, ultrasensitive detection of hydrazine at low potentials can be achieved.
10.1021/jp0700837 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007
Electrochemical Sensing of Hydrazine
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Experimental Section Electrochemical measurements were performed using a twocompartment three-electrode cell with a nanoparticle-modified polycrystalline Au working electrode, a Pt wire auxiliary electrode, and a Ag/AgCl (3 M NaCl) reference electrode. Cyclic voltammograms were recorded using a computer-controlled CHI643B electrochemical analyzer. Scanning electron microscopic measurements were performed with a JEOL JEM 6700F field-emission scanning electron microscope. X-ray diffraction analysis of the nanoparticles was carried out with a Phillips X’pert PRO X-ray diffraction unit using Ni-filtered Cu KR (λ ) 1.54 Å) radiation. UV-visible diffuse reflectance spectra were recorded with a Shimadzu UV-2401 PC spectrophotometer. Citrate-stabilized nanosized gold seeds (GNSs) were prepared according to our previous report.13a The colloidal GNSs show an absorption band at ∼518 nm corresponding to the surface plasmon resonance. The nanostructured platform was developed according to our previous report.13c A polycrystalline Au electrode with a geometrical surface area of 0.031 cm2 was used as the substrate for the fabrication of the nanostructured platform. The MPTS sol was prepared by dissolving MPTS, methanol, and water (as 0.1 M HCl) in a molar ratio of 1:3:3 and stirring the mixture vigorously for 30 min. The electrochemically cleaned polycrystalline Au electrode was soaked in 0.5 mL MPTS sol for 10 min for the formation of a 3D sol-gel network on the electrode surface.13c The existence of -SH groups and a 3D silicate network was confirmed by FTIR measurements. The characteristic band for the -SH group and the asymmetric vibrational stretching for -Si-O-Si- were observed at 2560 and 10001200 cm-1, respectively.13c As the -SH groups are distributed throughout the network, the nanoparticles can be conveniently chemisorbed inside and on the surface of the network. The thusprepared MPTS-network-modified electrode was soaked in colloidal GNSs for 18 h for the chemisorption of Au nanoseeds onto the thiol tail groups of the 3D sol-gel network. The characteristic band at 2560 cm-1 for the -SH group disappeared upon the chemisorption of GNSs onto the thiol groups.13c The GNSs on the sol-gel network were enlarged by a seed-mediated growth approach.33 Typically, the GNS self-assembled electrode was soaked in a solution containing 0.3 mM NH2OH and 0.3 mM AuCl4-, and the solution was shaken constantly for 20 min. For field-emission scanning electron microscopy (FESEM) and diffuse reflectance spectroscopy (DRS) measurements, the goldcoated cover slip was first modified with the MPTS sol to obtain a sol-gel network. For XRD measurements, the MPTSmonomer-modified glass slide or coverslip was used. The selfassembly of GNSs and the seed-mediated growth were carried out as described earlier. The electrodes modified with MPTS network, GNSs, and enlarged nanoparticles (GNEs) are referred to hereafter as MPTS, GNS, and GNE electrodes, respectively. Results and Discussion Characterization of the Nanostructured Platform. The size and surface morphology of the Au nanoparticles were examined by FESEM. Figure 1 presents an FESEM image obtained for the nanostructured platform. The nanoparticles have a size distribution between 70 and 100 nm and have a nearly spherical shape. The nanoparticles are randomly distributed throughout the network and can be considered as ensembles of nanoelectrode. The distance between the particles is not uniform, and the existence of close-packed nanoelectrodes is clearly seen in the FESEM image. The nanoparticles on the surface of the network do not undergo aggregation during hydroxylamine
Figure 1. FESEM image of the nanostructured platform.
Figure 2. Cyclic voltammograms for Fe(CN)6 4-/3- redox couple in 0.1 M KCl on (a) MPTS, (b) GNS, and (c) GNE electrodes. Scan rate ) 100 mV/s.
seeding; this was further confirmed by recording the diffuse reflectance spectra. The diffuse reflectance spectrum of GNEs on the silicate network shows only one band at ∼540 nm, which is red-shifted by ∼20 nm with respect to the signal for colloidal GNSs, because of the enlargement in size (Supporting Information). The XRD measurements show four peaks corresponding to the (111), (200), (220), and (311) planes for GNEs on the sol-gel network (Supporting Information). The peak corresponding to the Au(111) plane is more intense than the others, indicating that the (111) plane is the predominant orientation.13b The areas of the electrochemically accessible GNSs and GNEs on the silicate network were calculated by cyclic voltammetry. The cyclic voltammograms of GNS and GNE electrodes in 0.1 M KOH were recorded in the potential window of -0.2 and +0.5 V (Supporting Information). The amounts of charge consumed during the reduction of electrochemically formed gold oxide were obtained by integrating the areas under the reduction peaks. The surface areas of the GNS and GNE electrodes were calculated to be 0.03 and 0.076 cm2, respectively; the reported value of 400 µC/cm2 was used in the calculation.34 The cyclic voltammetric responses of the MPTS, GNS and GNE electrodes were examined by taking the Fe(CN)64-/3redox couple in neutral solution (Figure 2). The MPTS-modified electrode showed a sluggish voltammetric response, with a ∆Ep value of ∼200 mV, indicating that the MPTS network on the electrode surface hinders the electron-transfer kinetics. On the other hand, a significant increase in the peak current associated with a decrease in the ∆Ep value was noticed at GNS electrode. The peak current and ∆Ep value observed at the GNS electrode
6230 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Jena and Raj
Figure 3. Voltammetric response of the (a) MPTS sol-gel network modified, (b) GNS, and (c) GNE electrodes toward hydrazine (0.2 mM). Scan rate ) 10 mV/s. The inset shows the voltammogram obtained for the oxidation of hydrazine on a polycrystalline Au electrode. Supporting electrolyte: 0.1 M phosphate buffer solution (PBS) of pH 9.2.
are very close to those of the unmodified polycrystalline Au electrode, indicating that the nanoparticles on the silicate network favor the electron-transfer reaction over the redox reaction. In the case of the GNE electrode, a significant increase in the peak current was observed (Figure 2c) because of the increase in the surface area. The shapes of the voltammograms obtained at the GNS and GNE electrodes were not sigmoidal as would be expected from either an individual nanodisk or a widely spaced nanodisk array at the sweep rate employed. Peakshaped voltammograms were observed for both electrodes, indicating the overlap of diffusion layers at the individual elements of the electrode to produce a diffusion layer that is linear to the geometrical surface area of the electrode (vide infra). Electrocatalytic Oxidation of Hydrazine. Figure 3 presents the voltammograms obtained for the oxidation of hydrazine at MPTS, GNS, and GNE electrodes. The MPTS electrode does not show any characteristic response for the oxidation of hydrazine in the potential window used. However, as can be readily seen from Figure 3, well-defined voltammetric responses were obtained for the oxidation of hydrazine on the GNS and GNE electrodes at less positive potentials. Interestingly, hydrazine oxidation on the GNE electrode occurred at -0.15 V, which is 800 mV less positive than that on a bulk Au electrode. It is quite surprising to observe such a very large decrease in the overpotential in the absence of any redox mediator to shuttle the electrons. To the best of our knowledge, such a large decrease in the overpotential has not been observed at any electrodes. The onset potential for the oxidation of hydrazine on the GNE electrode is much more negative (approximately -0.3 V) than the actual oxidation potential, suggesting that the nanoparticles efficiently catalyze the oxidation process. The bulk Au electrode shows a broad oxidation peak (Figure 3 inset) at more positive potential (>0.65 V), presumably because of sluggish electron-transfer kinetics. The large decrease in the overpotential associated with a substantial increase in the peak height reflects a faster electron-transfer reaction on the GNE electrode owing to the high catalytic effect of the nanostructured Au particles. The faster electron transfer leads to a sharper and more well-defined peak. Polycrystalline and single-crystalline Au electrodes have been used for the oxidation of hydrazine.35 The oxidation of hydrazine is a pH-dependent process that involves four electrons and five H+ ions at pH < pKa of hydrazine. A Tafel slope of 83-104 mV dec-1 has been reported for the oxidation of hydrazine on unmodified Au electrodes.35 In the present case, the Tafel slope was obtained at the foot of
Figure 4. Cyclic voltammograms obtained for the oxidation of hydrazine (0.2 mM) at different θp values. Other experimental conditions are the same as in Figure 3. θp ) (a) 0.93, (b) 1.56, (c) 2, (d) 2.33, and (e) 2.45.
the oxidation wave by sweeping the potential at a low scan rate and was 84 mV dec-1. This value is in close agreement with the value already reported in the literature.35 Particle-Coverage-Dependent Electrocatalytic Response. It is very interesting to note that the oxidation of hydrazine at the nanoparticle-modified electrodes greatly depends on the nanoparticle coverage (θp) on the silicate network. To examine the influence of θp on the electrocatalytic oxidation of hydrazine, electrodes were prepared at different hydroxylamine seedings. The θp value on the silicate network was changed by controlling the seeding time. θp was obtained by taking the ratio of the surface area of nanoparticles on the network to the geometrical surface area of the underlying electrode surface.11 Figure 4 illustrates the effect of GNE coverage on the silicate network in the electrocatalytic oxidation of hydrazine. Two interesting features were observed from the voltammograms: (i) a marked negative shift in the peak potential and (ii) a significant decrease in the current density with increasing θp on the silicate network. For instance, a negative shift in the oxidation potential of ∼150 mV was noticed as θp was increased from 0.93 to 2.45. The current density at low θp was 1.2 times larger than that at higher coverage. The negative shift in the peak potential implies that the GNE particles on the silicate network facilitate the electrontransfer kinetics for the oxidation process. These results can be explained by using relevant theories of microelectrode ensembles or partially blocked electrodes.36 The voltammetric response at an array or ensemble of micro-/nanoelectrodes depends on the distance between the electrode elements and the time scale (scan rate).32a,37 In the case of the total overlap regime, the diffusion layers at the individual elements of nanoelectrode ensemble overlap to produce a diffusion layer that is linear with respect to the entire geometrical area of the nanoelectrode ensemble. In such a case, peak-shaped voltammograms are obtained. As shown in Figure 3, peak-shaped voltammograms were obtained at both the GNS and GNE electrodes, indicating the overlap of the diffusion layers of individual elements on the electrode surface. It has been demonstrated theoretically and experimentally that the increase in the θp value on the electrode surface results in a negative shift in the peak potential and a change in the shape of the voltammogram from sigmoidal/pseudo-sigmoidal to peak-shaped.11,36b The negative shift in the peak potential and the change in shape of the voltammogram are due
Electrochemical Sensing of Hydrazine
Figure 5. Amperometric traces obtained on the GNE electrode for the sensing of hydrazine upon successive injections of (A) 2 nM and (B) 200 pM hydrazine into the stirred PBS at regular intervals. The electrode was polarized at -0.1 V. The inset shows the corresponding calibration plot.
to the change in the diffusion pattern. Kumar and Zou recently observed such voltammetric features for the oxidation of CO at a Au-nanoparticle-modified ITO electrode and explained the result by considering the electrode (ensemble electrode) as a partially blocked electrode.11 In the present investigation, the negative shift of 150 mV in the hydrazine oxidation potential as θp was increased from 0.93 to 2.45 is ascribed to the change in the diffusion pattern. When θp on the electrode surface increases, the diffusion pattern is expected to change from mixed nonlinear and linear diffusion to mostly linear diffusion. The high current density at lower coverage can be ascribed to the increased mass transport to the electrode surface. Amperometric Sensing of Hydrazine. One of the objectives of the present investigation was to utilize the nanoparticle-based platform for the electrochemical sensing of ultratrace levels of hydrazine under optimized conditions. The peak current for the oxidation of hydrazine increases with increasing solution pH, and the maximum current was obtained at pH 9.2 (Supporting Information). The voltammetric peak is rather broad at pH > 9.2. Because the peak current is high in alkaline solutions of pH 9.2, sensing of hydrazine was performed at this pH. Figure 5 displays the amperometric response of the GNE electrode toward hydrazine in the nano- and picomolar ranges. The GNE electrode was polarized at -0.1 V, and aliquots of hydrazine were injected into a stirred supporting electrolyte solution. A rapid increase in the current was noticed after each addition of hydrazine into the supporting electrolyte solution, and a steady-
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6231 state response was attained within 3 s. The GNE electrode is highly sensitive, and the amperometric response is very stable and offers a linear dependence over a wide range of hydrazine concentrations (200 pM-5 mM). The sensitivity and limit of detection (S/N ) 11) were found to be 0.02 ( 0.0015 µA/nM and 200 pM, respectively. To the best of our knowledge, this is the first time such a very low detection limit has been achieved by the electrochemical method. It is worth comparing the analytical performance of the GNE electrode with the those of available microdisk array or nanoparticle-modified electrodes.29,30 Compton and co-workers reported a detection limit of 1.8 µM at a potential of ∼0.1 V (SCE) on a Pd-plated boron-doped diamond microdisk array electrode.29 Yang et al. observed a detection limit of 270 nM with a copper-palladium alloy nanoparticle-plated electrode at an applied potential of 0.2 V (Ag/AgCl) under hydrodynamic conditions.30 However, our GNE electrode could successfully detect hydrazine in the picomolar range at -0.1 V, indicating that the analytical performance of the GNE electrode is superior to that of existing electrodes. The high sensitivity can be explained by considering the nanostructured platform as an ensemble of nanoelectrodes. As discussed earlier, the analytical detection limit at such an ensemble electrode will be much higher than that at the analogous macrosized electrode. The enhancement in the detection limit arises from an improved ratio between the faradaic and capacitive currents (S/N ratio).31b,32b It is well-documented in the literature that the capacitive current at a micro-/nanoelectrode ensemble will be much lower than that at a macrosized electrode of similar area. In order to confirm this idea, the capacitances of unmodified Au and nanoparticlemodified electrodes were estimated from the cyclic voltammograms obtained at different scan rates in 0.1 M PBS. The capacitances of the unmodified Au, GNS, and GNE electrodes were calculated to be 94, 23, and 14.5 µF/cm2, respectively. The capacitance of the GNE electrode is significantly lower than those of the other electrodes, which strongly supports the above assessment of the ensemble behavior of the GNE electrode. The long-term storage and operating stability of a sensor are essential for the continuous monitoring of hydrazine. The stability of the present sensor was evaluated by using the same sensor for 20 repetitive measurements in a supporting electrolyte solution containing 5 µM hydrazine. This electrode was again used for another 20 repeated measurements after 24 h. Interestingly, the peak potential and peak current for the oxidation of hydrazine remained almost the same in both sets of experiments. The coefficient of variation in the peak current for the two sets of experiment was calculated to be 0.18%, confirming that the electrode is very stable and can be used for repeated measurements. To further ascertain the operating stability of the present sensor, the amperometric response of the sensor in a supporting electrolyte solution containing 30 nM hydrazine was recorded, and we found that the sensor showed a stable response for a long time (Supporting Information). The long-term storage stability of the sensor was evaluated by measuring the electrode response for a period of 15 days. The electrode was kept in PBS between measurements on every day over the period of 15 days. No observable change in the peak current or peak potential was noticed for 9 days (Supporting Information); an 11% decrease in the peak current was noticed after 9 days, demonstrating that the sensor is highly stable and can be successfully used for a long time.
6232 J. Phys. Chem. C, Vol. 111, No. 17, 2007 Conclusions In conclusion, we have demonstrated the electrocatalytic activity of nanostructured Au particles toward the oxidation of hydrazine. The electrocatalytic oxidation of hydrazine occurs at -0.15 V, which is ∼800 mV less positive than the value for conventional bulk Au electrodes. The nanostructured platform was successfully used for the electrocatalytic sensing of hydrazine at less positive potentials for the first time without any redox mediator. This sensor is highly sensitive and stable for 15 days. Compared to the existing hydrazine sensors, this nanoparticle-based sensor is highly sensitive and stable and provides a very low detection limit and a wide linear response. The high sensitivity can be explained in terms of the relevant theories of micro-/nanoelectrode ensembles. This study demonstrates the electroanalytical application of nanosized Au particles and the development of a highly sensitive nanostructured platform. This platform can be successfully used for the fabrication of sensing devices. Acknowledgment. This work was supported by grants from the Council of Scientific and Industrial Research [No. 01 (1895)/ 03/EMR-II] and the Department of Science and Technology (No. SR/S5/NM-80/2006), New Delhi, India. We thank Dr. Asim Bhowmik, Indian Association for the Cultivation of Sciences, Kolkata, India, for DRS measurements. Supporting Information Available: UV-visible DRS spectra, XRD profile, voltammetric response of nanostructured platform in 0.1 M KOH, voltammetric response of hydrazine at different pHs, amperometric trace demonstrating operatng stability, plot showing the long-term storage stability. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem. 2000, 1, 18-52. (2) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042-6108. (3) Haruta, M. Catal. Today 1997, 36, 153-166. (4) Zhong, C.-J.; Maye, M. M. AdV. Mater. 2001, 13, 1507-1511 and references cited therein. (5) Alivisatos, A. P. Science 1996, 271, 933-937. (6) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (7) Storhoff, J. J.; Mirkin, C. A. Chem. ReV. 1999, 99, 1849-1862. (8) Daniel, M. -C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (9) Biswas, P. C.; Nodasaka, Y.; Enyo, M.; Haruta, M. J. Electroanal. Chem. 1995, 381, 167-177. (10) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 16471650.
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