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Highly sensitive hydrazine chemical sensor based on ZnO nanorods field-effect transistor† Rafiq Ahmad,a Nirmalya Tripathy,a Da-Un-Jin Jungb and Yoon-Bong Hahn*ab
Received 26th October 2013, Accepted 22nd November 2013 DOI: 10.1039/c3cc48197b www.rsc.org/chemcomm
A highly sensitive hydrazine chemical sensor has been fabricated based on a field-effect transistor (FET) by growing vertically-aligned ZnO nanorods directly on silver electrodes. The FET sensor showed a high sensitivity and a low limit of detection (LOD) of 59.175 lA cm
2
lM
1
and B3.86 nM, respectively. This demonstrates a cost effective and low power consuming FET strategy for the detection of hydrazine.
Hydrazine (N2H4), a well-established and proven advantageous chemical species is used in multidisciplinary applications as an emulsifier, corrosion inhibitor, antioxidant, photographic developer, pesticide and insecticide, plant growth regulator, etc.1 However, repeated usage of hydrazine in high doses results in short- and long-term exposure related potential hazards to all living races and the environment such as lethal damage to the central nervous system, kidney and liver.2 Moreover, the Environmental Protection Agency (EPA) has classified it as a human carcinogen. To address this problem, various current detection methods have been reported including chromatography, electrochemical, chemiluminescence and various types of spectroscopy, but for the electrochemical method, there is still a challenge to enhance the electron transfer rate over the surface of the working electrode for the sensors.3 Therefore, directly grown nanostructures on the desired electrodes could be promising for the reliable and effective identification of toxic chemical species by electrochemical and current–voltage (I–V) characteristics. ZnO nanostructure-based sensors have recently aroused much interest owing to the multifunctionality of ZnO such as its wide band gap (B3.37 eV), high exciton binding energy (60 meV), biocompatibility, high electron feature and good electrochemical properties, simple and cost-effective synthesis, optical transparency, and so forth.4
Furthermore, various ZnO nanostructures have been used for the fabrication of electrochemical hydrazine sensors.5 Mostly, these conventional ZnO based electrodes were fabricated by post-pasting of separately synthesized ZnO nanostructures using Nafion/binder, which did not enable robust mechanical adhesion and electrical contact between the nanostructured ZnO and electrode.6 According to Fang et al., Nafion/binder forms a ‘partially blocked’ electrode array system that is likely to coat at least part of the ZnO nanostructured surface.7 This ultimately results in passivation of the modified electrode and a decreased electro-analytical performance of the electrode. In this work, for the first time, we fabricated a FET based hydrazine sensor by directly growing vertically aligned ZnO NRs on a Si/SiO2 substrate between the source–drain using an aqueous method. The key advantage of our work is that the directly grown ZnO NRs, which not only establish tight contacts with the electrode surface, also ensure an extremely large surface area and fast electron transport (Fig. S1, ESI†). Fig. 1 shows the schematic of the fabrication process of the ZnO NRs FET. First, a cleaned Si(100)/SiO2 (200 nm) wafer was taken as the device substrate. A thin layer of PMMA was then spin-coated on Si/SiO2 and baked. The pattern of the source–drain electrode on a mask was transferred through electron beam lithography and development. In the next step,
a
Dept. of BIN Fusion Technology, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 561-756, Republic of Korea. E-mail: [email protected] b School of Semiconductor and Chemical Engineering, and Semiconductor Physics Research Center, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 561-756, Republic of Korea † Electronic supplementary information (ESI) available: Adhesion test, transfer characteristic and sensing performance comparison. See DOI: 10.1039/ c3cc48197b
1890 | Chem. Commun., 2014, 50, 1890--1893
Fig. 1
Schematic illustration of device fabrication process.
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Fig. 3 (a) Schematic illustration of overall experimental setup. (b) I–V characterization of the FET device in the absence (black-dot) and presence of 5 mM hydrazine (red-dots) in 0.01 M PBS (pH = 7.0).
Fig. 2 (a) Top-view FESEM image; (b) XRD pattern; (c) TEM image of a single ZnO NR; (d) HRTEM image. Inset in figure a and c show a crosssectional FESEM image and a SAED pattern of the ZnO NRs, respectively.
Ag (B100 nm) was deposited onto the exposed area and formed the electrode pads after a lift-off process in acetone. Thereafter, a patterned region was formed by lithography between the source– drain. The seed layer of ZnO (B60 nm) was then sputtered onto the patterned region and was followed by lift off in acetone. Then, the vertically aligned ZnO NRs were grown on the patterned region in solution using an equal molar mixture of Zn(NO3)26H2O (0.05 M) and HMTA (0.05 M).4j The general morphology of the as-synthesized ZnO NRs was examined by field emission scanning electron microscopy (FESEM), which confirmed that the synthesized NRs are grown uniformly in a high density (Fig. 2a). The cross-sectional view (inset in Fig. 2a) shows that the densely grown ZnO NRs are vertically aligned to the substrate. The typical length and average diameters of the NRs are B1.2 mm and B40–80 nm, respectively. This clearly confirms that the uniform vertical alignment of the ZnO NRs is due to the pre-sputtered thin film of the ZnO seed layer. The crystallinity of the ZnO NRs was investigated by XRD with Cu-Ka radiation (l = 1.54178 Å) in the range of 30–601 with an 81 min 1 scanning speed (Fig. 2b). All the observed peaks in the pattern are wellmatched with the reported values of wurtzite hexagonal ZnO ( JCPDS Card No. 75-1526). The strongest peak at 34.21 is attributed as ZnO (0002) in the pattern confirming that the NRs grow along the [0001] direction in preference. Further, the detailed morphological and structural characterizations of the ZnO NRs were characterized by TEM equipped with HRTEM (Fig. 2c). The TEM observations are fully consistent with the observed FESEM images in terms of the morphology and dimensionality of the as-synthesized ZnO NRs. The FFT pattern in Fig. 2d shows the single crystallinity of the NRs grown along the [0001] direction. Also, the corresponding SAED pattern of the as-grown NRs (inset in Fig. 2c) is in accordance with the HRTEM. Prior to device characterization, the source–drain electrodes of the fabricated FET device were covered by a PDMS layer to reduce the leak current and eliminate the effect of the metal-nanorods
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contact region. The electrical characterizations of the as-fabricated FET hydrazine sensor were performed by a data acquisition system for real-time data measurement (HP 4155A semiconductor parameter analyser) using a typical I–V technique. To detect hydrazine in a phosphate buffered solution (PBS, pH = 7.0), 100 mL of various concentrations of hydrazine solution was deposited on a pre-defined area by PDMS to make sure there was the same coverage for different solutions during each measurement. Then, the drain current (ID) dependence on drain–source voltage (VDS) was measured in the absence/presence of the hydrazine solution at a fixed gate-source voltage (VGS = 0.1 V) provided through a Ag/AgCl reference electrode. The overall experimental setup is schematically shown in Fig. 3a. The current responses of the FET sensor with/without hydrazine are shown in Fig. 3b along with the real device image in the inset. It is clear that the current value substantially increased with hydrazine compared to without it, owing to the electro-oxidation of hydrazine. Moreover, the on/off ratio of the sensor was calculated as B104 in the presence of 5 mM hydrazine at VGS = 0.1 V (Fig. S2, ESI†). This suggests that the FET sensor exhibits excellent sensing performance towards hydrazine due to the good electrocatalytic and fast electron transfer properties of the ZnO NRs. The principle of charge transfer between a nanostructure and its surface-adsorbed species has been utilized to enable the hydrazine chemical sensing. At first, upon treatment with PBS, the dissolved-oxygen gets adsorbed onto the ZnO NR surfaces and the surface reactions are promoted in the liquid state to generate ionic species (O2 , OH , etc.), which absorb electrons from the conduction band and desorb to the ZnO NR surfaces.8 Further, the reaction mechanism between hydrazine and the active OH species may take place according to the following equation:4a N2H4 + 5/2OH - 1/2N3 + 1/2NH3 + 5/2H2O + 2e
(1)
Furthermore, these chemisorbed ionic species deplete the surface electron states which leads to a decrease in conductance of the sensor or an increase in resistance due to the formation of an electron depletion layer at the ZnO NR surfaces.9 This increase in current is ascribed to the discharge of trapped electrons into the conduction band during the catalytic reaction of hydrazine.10 To understand the detailed sensing performance of the fabricated FET hydrazine sensor, different concentrations of hydrazine were prepared in 0.01 M PBS (pH = 7.0) solutions and
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Fig. 4 (a) I–V responses with increasing hydrazine concentration (1 nM– 70 mM) and (b) their corresponding calibration curve, i.e. response current vs. hydrazine concentration (mM) in log scale. It shows linearity with a high regression coefficient (R2) value of 0.9342. Error bars show a standard deviation of 3.38% for three different electrodes.
the device transfer characteristics were recorded at these concentrations of hydrazine. As shown in Fig. 4a, the current gradually increases with an increase in hydrazine concentration (1 nM to 70 mM), which implies a decline in resistance of the device as the resistance is inversely proportional to the current. This behaviour is probably attributed to the increase in ionic strength of the solution with the addition of different concentrations of hydrazine that might produce more electrons. Noticeable current changes at a higher potential range (i.e., 0.2–1.0 V) based on hydrazine concentration are obtained. For the calculation of sensitivity and linear range, first, a calibration curve was plotted by taking an average of the currents over a range of potentials spanning from 0.2–1.0 V. Then, the sensitivity of fabricated FET sensor was calculated by taking the slope of current-concentration profile. From Fig. 4b, a linear behaviour in the range of 1 nM–60 mM with a high R2 (0.9342) was obtained, indicating a high degree of accuracy throughout the assay range. The FET sensor showed a high sensitivity of 59.175 mA cm 2 mM 1, which is higher in the context of a wide linear range than the various amperometric based hydrazine sensors (Table S1, ESI†).4a,5b–d,7,11 The LOD was calculated based on the standard deviation (SD) of response and the slope of the calibration curve (S), i.e. LOD = 3.3(SD/S) = B3.86 nM. Fig. 5a shows the I–t characteristics of the sensor at VDS = 0.1 V and VGS = 0.1 V. It can be clearly seen that the sensor response (ID) gradually increases with an increase in hydrazine concentration and finally reaches a saturation level, ascribed to the saturation of the electrocatalytic surface of the ZnO NRs. Next, we calculated the sensor performance from the calibration curve (Fig. 5a, inset) showing very similar sensing performances
Fig. 5 (a) I–t characteristics and (b) interference test of the fabricated ZnO NRs based FET. The inset in (a) shows the calibration curve.
1892 | Chem. Commun., 2014, 50, 1890--1893
Communication
to that of the I–V based, with a response time of B4 s. Additionally, an experiment was performed in the presence of interfering species to demonstrate the FET sensor selectivity (Fig. 5b). The hydrazine presence shows an immediate sensor response, however the addition of interfering species comparatively shows negligible effects. Further, the sensor showed a significant response with subsequent hydrazine additions. Thus, a consensus has been reached that our proposed FET sensor is highly selective for hydrazine. To evaluate the reproducibility, three devices were fabricated and their sensing experiments were performed. The sensor showed response with a relative standard deviation of 3.38%. The long-term storage stability of the sensor was determined with respect to the storage time. After each experiment, the sensor was washed with the buffer solution. The long-term storage stability of the sensor was tested for 8 weeks. The sensor retained 96.4% of its initial sensitivity for up to 6 weeks. After 6 weeks, the response gradually decreased, which might be due to a loss of catalytic activity. The above results clearly suggest that the sensor could be used for more than 1 month without any significant loss in sensitivity. In summary, we have fabricated a FET hydrazine sensor based on solution grown vertically-aligned ZnO NRs. The FET sensor exhibited a good sensitivity (59.175 mA cm 2 mM 1), a low LOD (B3.86 nM), and a linear dynamic range (1 nM–60 mM). These encouraging results, with the low-cost and ease of fabrication, gives an opportunity to FET based sensors to present themselves as a promising method for hydrazine detection. This work was supported in part by the Pioneer Research program (2012-0001039) and the Industrial Technology Research Infrastructure Program (N0000004) through the National Research Foundation funded by the Ministry of Science, ICT & Future Planning and by the Ministry of Trade, Industry and Energy (Korea), respectively.
Notes and references 1 (a) S. Garrod, M. E. Bollard, A. W. Nicholls, S. C. Connor, J. Connelly, J. K. Nicholson and E. Holmes, Chem. Res. Toxicol., 2005, 18, 115–122; (b) E. H. Vernot, J. D. MacEwen, R. H. Bruner, C. C. Haus and E. R. Kinkead, Fundam. Appl. Toxicol., 1985, 5, 1050–1064; (c) V. R. Choudhary, C. Samanta and P. Jana, Chem. Commun., 2005, 5399–5401. 2 Y. Y. Liu, I. Schmeltz and D. Hoffman, Anal. Chem., 1974, 46, 885–889. 3 (a) G. E. Collins, Sens. Actuators, B, 1996, 35, 202–206; (b) S. M. Golabi and H. R. Zare, Electroanalysis, 1999, 11, 1293–1300; (c) A. Savafi and M. A. Karimi, Talanta, 2002, 58, 785–792; (d) C. Bourdillon, M. Delamar, C. Demaille, R. Hitmi, H. Moiroux and J. Pinson, J. Electroanal. Chem., 1992, 336, 113–123; (e) N. Maleki, A. Safavi, E. Farjami and F. Tajabadi, Anal. Chim. Acta, 2008, 611, 151–155; ( f ) Z. Zhao, G. Zhang, Y. Gao, X. Yang and Y. Li, Chem. Commun., 2011, 47, 12816–12818. 4 (a) A. Umar, M. M. Rahman, S. H. Kim and Y.-B. Hahn, Chem. Commun., 2008, 166–168; (b) R. Ahmad, N. Tripathy and Y.-B. Hahn, Biosens. Bioelectron., 2013, 45, 281–286; (c) Y.-B. Hahn, R. Ahmad and N. Tripathy, Chem. Commun., 2012, 48, 10369–10385; (d) R. Ahmad, N. Tripathy, J.-H. Kim and Y.-B. Hahn, Sens. Actuators, B, 2012, 174, 195–201; (e) Y.-B. Hahn, Korean J. Chem. Eng., 2011, 28, 1797–1813; ( f ) A. Rafiq, N. Tripathy and Y.-B. Hahn, Sens. Actuators, B, 2012, 169, 382–386; (g) K. N. Han, C. A. Li, M.-P. N. Bui, X.-H. Phama and G. H. Seong, Chem. Commun., 2011, 47, 938–940; (h) N. Tripathy, A. Rafiq, H.-S. Jeong and Y.-B. Hahn, Inorg. Chem., 2012, 51, 1104–1110; (i) R. Ahmad, N. Tripathy and Y.-B. Hahn, J. Nanosci. Lett., 2014, 4, 18–23; ( j) A. Qurashi, J. H. Kim and Y.-B. Hahn, Electrochem. Commun., 2012, 18, 88–91; (k) T.-K. Hong, N. Tripathy, H.-J. Son, K.-T. Ha, H.-S. Jeong and Y.-B. Hahn, J. Mater. Chem. B, 2013, 1, 2985–2992.
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Communication 5 (a) C. Zhang, G. Wang, Y. Ji, M. Liu, Y. Feng, Z. Zhang and B. Fang, Sens. Actuators, B, 2010, 150, 247–253; (b) Y. Ni, J. Zhu, L. Zhang and J. Hong, CrystEngComm, 2010, 12, 2213–2218; (c) S. K. Mehta, K. Singh, A. Umar, G. R. Chaudhary and S. Singh, Electrochim. Acta, 2012, 69, 128–133; (d) J. Liu, Y. Li, J. Jiang and X. Huang, Dalton Trans., 2010, 39, 8693–8697. 6 (a) X. C. Xiang, Y. Chi, G. B. Xiang and F. S. Jiang, Chin. Sci. Bull., 2013, 58, 2563–2566; (b) H. Acar, R. Garifullin, L. E. Aygun, A. K. Okyay and M. O. Guler, J. Mater. Chem. A, 2013, 1, 10979–10984; (c) J. Wang, Chem. Rev., 2008, 108, 814–825.
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ChemComm 7 B. Fang, C. Zhang, W. Zhang and G. Wang, Electrochim. Acta, 2009, 55, 178–182. 8 M. M. Rahman, S. B. Khan, A. Jamal, M. Faisal and A. M. Asiri, Chem. Eng. J., 2012, 192, 122–128. 9 Z. Fan, D. Wang, P.-C. Chang, W. Y. Tseng and J. G. Lu, Appl. Phys. Lett., 2004, 85, 5923–5925. 10 S. Majumdar, Mater. Sci.-Pol., 2009, 27, 123–129. 11 (a) A. Umar, M. M. Rahman and Y. B. Hahn, J. Nanosci. Nanotechnol., 2009, 9, 4686–4691; (b) S. Ameen, M. S. Akhtar and H. S. Shin, Talanta, 2012, 100, 377–383.
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Highly sensitive hydrazine chemical sensor based on ZnO nanorods field-effect transistor† Rafiq Ahmad,a Nirmalya Tripathy,a Da-Un-Jin Jungb and Yoon-Bong Hahn*ab
Received 26th October 2013, Accepted 22nd November 2013 DOI: 10.1039/c3cc48197b www.rsc.org/chemcomm
A highly sensitive hydrazine chemical sensor has been fabricated based on a field-effect transistor (FET) by growing vertically-aligned ZnO nanorods directly on silver electrodes. The FET sensor showed a high sensitivity and a low limit of detection (LOD) of 59.175 lA cm
2
lM
1
and B3.86 nM, respectively. This demonstrates a cost effective and low power consuming FET strategy for the detection of hydrazine.
Hydrazine (N2H4), a well-established and proven advantageous chemical species is used in multidisciplinary applications as an emulsifier, corrosion inhibitor, antioxidant, photographic developer, pesticide and insecticide, plant growth regulator, etc.1 However, repeated usage of hydrazine in high doses results in short- and long-term exposure related potential hazards to all living races and the environment such as lethal damage to the central nervous system, kidney and liver.2 Moreover, the Environmental Protection Agency (EPA) has classified it as a human carcinogen. To address this problem, various current detection methods have been reported including chromatography, electrochemical, chemiluminescence and various types of spectroscopy, but for the electrochemical method, there is still a challenge to enhance the electron transfer rate over the surface of the working electrode for the sensors.3 Therefore, directly grown nanostructures on the desired electrodes could be promising for the reliable and effective identification of toxic chemical species by electrochemical and current–voltage (I–V) characteristics. ZnO nanostructure-based sensors have recently aroused much interest owing to the multifunctionality of ZnO such as its wide band gap (B3.37 eV), high exciton binding energy (60 meV), biocompatibility, high electron feature and good electrochemical properties, simple and cost-effective synthesis, optical transparency, and so forth.4
Furthermore, various ZnO nanostructures have been used for the fabrication of electrochemical hydrazine sensors.5 Mostly, these conventional ZnO based electrodes were fabricated by post-pasting of separately synthesized ZnO nanostructures using Nafion/binder, which did not enable robust mechanical adhesion and electrical contact between the nanostructured ZnO and electrode.6 According to Fang et al., Nafion/binder forms a ‘partially blocked’ electrode array system that is likely to coat at least part of the ZnO nanostructured surface.7 This ultimately results in passivation of the modified electrode and a decreased electro-analytical performance of the electrode. In this work, for the first time, we fabricated a FET based hydrazine sensor by directly growing vertically aligned ZnO NRs on a Si/SiO2 substrate between the source–drain using an aqueous method. The key advantage of our work is that the directly grown ZnO NRs, which not only establish tight contacts with the electrode surface, also ensure an extremely large surface area and fast electron transport (Fig. S1, ESI†). Fig. 1 shows the schematic of the fabrication process of the ZnO NRs FET. First, a cleaned Si(100)/SiO2 (200 nm) wafer was taken as the device substrate. A thin layer of PMMA was then spin-coated on Si/SiO2 and baked. The pattern of the source–drain electrode on a mask was transferred through electron beam lithography and development. In the next step,
a
Dept. of BIN Fusion Technology, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 561-756, Republic of Korea. E-mail: [email protected] b School of Semiconductor and Chemical Engineering, and Semiconductor Physics Research Center, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 561-756, Republic of Korea † Electronic supplementary information (ESI) available: Adhesion test, transfer characteristic and sensing performance comparison. See DOI: 10.1039/ c3cc48197b
1890 | Chem. Commun., 2014, 50, 1890--1893
Fig. 1
Schematic illustration of device fabrication process.
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Fig. 3 (a) Schematic illustration of overall experimental setup. (b) I–V characterization of the FET device in the absence (black-dot) and presence of 5 mM hydrazine (red-dots) in 0.01 M PBS (pH = 7.0).
Fig. 2 (a) Top-view FESEM image; (b) XRD pattern; (c) TEM image of a single ZnO NR; (d) HRTEM image. Inset in figure a and c show a crosssectional FESEM image and a SAED pattern of the ZnO NRs, respectively.
Ag (B100 nm) was deposited onto the exposed area and formed the electrode pads after a lift-off process in acetone. Thereafter, a patterned region was formed by lithography between the source– drain. The seed layer of ZnO (B60 nm) was then sputtered onto the patterned region and was followed by lift off in acetone. Then, the vertically aligned ZnO NRs were grown on the patterned region in solution using an equal molar mixture of Zn(NO3)26H2O (0.05 M) and HMTA (0.05 M).4j The general morphology of the as-synthesized ZnO NRs was examined by field emission scanning electron microscopy (FESEM), which confirmed that the synthesized NRs are grown uniformly in a high density (Fig. 2a). The cross-sectional view (inset in Fig. 2a) shows that the densely grown ZnO NRs are vertically aligned to the substrate. The typical length and average diameters of the NRs are B1.2 mm and B40–80 nm, respectively. This clearly confirms that the uniform vertical alignment of the ZnO NRs is due to the pre-sputtered thin film of the ZnO seed layer. The crystallinity of the ZnO NRs was investigated by XRD with Cu-Ka radiation (l = 1.54178 Å) in the range of 30–601 with an 81 min 1 scanning speed (Fig. 2b). All the observed peaks in the pattern are wellmatched with the reported values of wurtzite hexagonal ZnO ( JCPDS Card No. 75-1526). The strongest peak at 34.21 is attributed as ZnO (0002) in the pattern confirming that the NRs grow along the [0001] direction in preference. Further, the detailed morphological and structural characterizations of the ZnO NRs were characterized by TEM equipped with HRTEM (Fig. 2c). The TEM observations are fully consistent with the observed FESEM images in terms of the morphology and dimensionality of the as-synthesized ZnO NRs. The FFT pattern in Fig. 2d shows the single crystallinity of the NRs grown along the [0001] direction. Also, the corresponding SAED pattern of the as-grown NRs (inset in Fig. 2c) is in accordance with the HRTEM. Prior to device characterization, the source–drain electrodes of the fabricated FET device were covered by a PDMS layer to reduce the leak current and eliminate the effect of the metal-nanorods
This journal is © The Royal Society of Chemistry 2014
contact region. The electrical characterizations of the as-fabricated FET hydrazine sensor were performed by a data acquisition system for real-time data measurement (HP 4155A semiconductor parameter analyser) using a typical I–V technique. To detect hydrazine in a phosphate buffered solution (PBS, pH = 7.0), 100 mL of various concentrations of hydrazine solution was deposited on a pre-defined area by PDMS to make sure there was the same coverage for different solutions during each measurement. Then, the drain current (ID) dependence on drain–source voltage (VDS) was measured in the absence/presence of the hydrazine solution at a fixed gate-source voltage (VGS = 0.1 V) provided through a Ag/AgCl reference electrode. The overall experimental setup is schematically shown in Fig. 3a. The current responses of the FET sensor with/without hydrazine are shown in Fig. 3b along with the real device image in the inset. It is clear that the current value substantially increased with hydrazine compared to without it, owing to the electro-oxidation of hydrazine. Moreover, the on/off ratio of the sensor was calculated as B104 in the presence of 5 mM hydrazine at VGS = 0.1 V (Fig. S2, ESI†). This suggests that the FET sensor exhibits excellent sensing performance towards hydrazine due to the good electrocatalytic and fast electron transfer properties of the ZnO NRs. The principle of charge transfer between a nanostructure and its surface-adsorbed species has been utilized to enable the hydrazine chemical sensing. At first, upon treatment with PBS, the dissolved-oxygen gets adsorbed onto the ZnO NR surfaces and the surface reactions are promoted in the liquid state to generate ionic species (O2 , OH , etc.), which absorb electrons from the conduction band and desorb to the ZnO NR surfaces.8 Further, the reaction mechanism between hydrazine and the active OH species may take place according to the following equation:4a N2H4 + 5/2OH - 1/2N3 + 1/2NH3 + 5/2H2O + 2e
(1)
Furthermore, these chemisorbed ionic species deplete the surface electron states which leads to a decrease in conductance of the sensor or an increase in resistance due to the formation of an electron depletion layer at the ZnO NR surfaces.9 This increase in current is ascribed to the discharge of trapped electrons into the conduction band during the catalytic reaction of hydrazine.10 To understand the detailed sensing performance of the fabricated FET hydrazine sensor, different concentrations of hydrazine were prepared in 0.01 M PBS (pH = 7.0) solutions and
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Fig. 4 (a) I–V responses with increasing hydrazine concentration (1 nM– 70 mM) and (b) their corresponding calibration curve, i.e. response current vs. hydrazine concentration (mM) in log scale. It shows linearity with a high regression coefficient (R2) value of 0.9342. Error bars show a standard deviation of 3.38% for three different electrodes.
the device transfer characteristics were recorded at these concentrations of hydrazine. As shown in Fig. 4a, the current gradually increases with an increase in hydrazine concentration (1 nM to 70 mM), which implies a decline in resistance of the device as the resistance is inversely proportional to the current. This behaviour is probably attributed to the increase in ionic strength of the solution with the addition of different concentrations of hydrazine that might produce more electrons. Noticeable current changes at a higher potential range (i.e., 0.2–1.0 V) based on hydrazine concentration are obtained. For the calculation of sensitivity and linear range, first, a calibration curve was plotted by taking an average of the currents over a range of potentials spanning from 0.2–1.0 V. Then, the sensitivity of fabricated FET sensor was calculated by taking the slope of current-concentration profile. From Fig. 4b, a linear behaviour in the range of 1 nM–60 mM with a high R2 (0.9342) was obtained, indicating a high degree of accuracy throughout the assay range. The FET sensor showed a high sensitivity of 59.175 mA cm 2 mM 1, which is higher in the context of a wide linear range than the various amperometric based hydrazine sensors (Table S1, ESI†).4a,5b–d,7,11 The LOD was calculated based on the standard deviation (SD) of response and the slope of the calibration curve (S), i.e. LOD = 3.3(SD/S) = B3.86 nM. Fig. 5a shows the I–t characteristics of the sensor at VDS = 0.1 V and VGS = 0.1 V. It can be clearly seen that the sensor response (ID) gradually increases with an increase in hydrazine concentration and finally reaches a saturation level, ascribed to the saturation of the electrocatalytic surface of the ZnO NRs. Next, we calculated the sensor performance from the calibration curve (Fig. 5a, inset) showing very similar sensing performances
Fig. 5 (a) I–t characteristics and (b) interference test of the fabricated ZnO NRs based FET. The inset in (a) shows the calibration curve.
1892 | Chem. Commun., 2014, 50, 1890--1893
Communication
to that of the I–V based, with a response time of B4 s. Additionally, an experiment was performed in the presence of interfering species to demonstrate the FET sensor selectivity (Fig. 5b). The hydrazine presence shows an immediate sensor response, however the addition of interfering species comparatively shows negligible effects. Further, the sensor showed a significant response with subsequent hydrazine additions. Thus, a consensus has been reached that our proposed FET sensor is highly selective for hydrazine. To evaluate the reproducibility, three devices were fabricated and their sensing experiments were performed. The sensor showed response with a relative standard deviation of 3.38%. The long-term storage stability of the sensor was determined with respect to the storage time. After each experiment, the sensor was washed with the buffer solution. The long-term storage stability of the sensor was tested for 8 weeks. The sensor retained 96.4% of its initial sensitivity for up to 6 weeks. After 6 weeks, the response gradually decreased, which might be due to a loss of catalytic activity. The above results clearly suggest that the sensor could be used for more than 1 month without any significant loss in sensitivity. In summary, we have fabricated a FET hydrazine sensor based on solution grown vertically-aligned ZnO NRs. The FET sensor exhibited a good sensitivity (59.175 mA cm 2 mM 1), a low LOD (B3.86 nM), and a linear dynamic range (1 nM–60 mM). These encouraging results, with the low-cost and ease of fabrication, gives an opportunity to FET based sensors to present themselves as a promising method for hydrazine detection. This work was supported in part by the Pioneer Research program (2012-0001039) and the Industrial Technology Research Infrastructure Program (N0000004) through the National Research Foundation funded by the Ministry of Science, ICT & Future Planning and by the Ministry of Trade, Industry and Energy (Korea), respectively.
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Chem. Commun., 2014, 50, 1890--1893 | 1893
