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Monolayer-Capped Cubic Platinum Nanoparticles for Sensing Nonpolar Analytes in Highly Humid Atmospheres Ekaterina Dovgolevsky,§,† Gady Konvalina,§,†,‡ Ulrike Tisch,† and Hossam Haick*,†,‡ Department of Chemical Engineering, TechnionsIsrael Institute of Technology, Haifa 32000, Israel and Russell Berrie Nanotechnology Institute, TechnionsIsrael Institute of Technology, Haifa 32000, Israel ReceiVed: June 23, 2010; ReVised Manuscript ReceiVed: July 13, 2010
We report on the feasibility of cubic Pt nanoparticles (NPs) capped with four representative organic ligands, viz. oleylamine (ODA), 11-mercaptoundecanol, 11-mercaptoundecanoic acid, and benzylmercaptan, for sensing gaseous nonpolar analytes in humid atmospheres. Chemiresistors based on cubic Pt NPs with nonpolar ligands show a very large increase in resistance upon exposure to nonpolar analyte vapors, combined with a low sensitivity to polar analyte vapors, especially to water. The sensing mechanism can be understood in terms of analyte-induced changes in the NP-NP core distance and changes in the permittivity of the medium between the NPs. The sensing capabilities of the Pt NP chemiresistors for nonpolar molecules in highly humid atmospheres are demonstrated by dosing an ODA-capped cubic Pt NP sensor with air mixtures containing low octane concentrations and high humidity levels that are typical for many applications. The simple construction, low cost, stability, fast response, and high sensitivity to nonpolar molecules, together with the low sensitivity to water vapor, are promising features for sensing applications in real confounding atmospheres. 1. Introduction The use of monolayer-capped metal nanoparticles (MCNPs) in chemiresistors facilitates the implementation of reliable gas sensors that have fast response time, low output impedence and that are small in size and weight.1-4 MCNP-based chemiresistors can be operated at room temperature or slightly above, thereby enabling easy device integration and low power operation.5 Furthermore, this type of chemiresistors would enable on-chip integration of sensors array and mass production of portable microanalysis systems with integrated read-out electronic at low costs.4,6 Recent progress in synthesis techniques has allowed the preparation of MCNPs of nearly any chemical composition, capped with a wide variety of molecular ligands.1,2,7-10 Moreover, it is possible to vary rather freely the particles’ sizes and shapes, and, therefore, the structural properties of the nanopores in MCNP films.11-17 For applications in chemiresistors, this grants control over the interparticle distance, as well as over the dielectric environment of the metal cores in MCNP films.1,9,18-21 For sufficiently small currents, the Ohmic resistance, R, of a MCNP film, can be expressed using an activated electron tunneling model:20,22
E e2 and Ea ) RT 4πεrε0r
( )
R ∝ exp(βδ) · exp
(1)
where β is the tunnelling decay constant, δ is the edge-to-edge separation of the metal cores, R is the gas constant and Ea is the activation energy for electron transfer, which is essentially the Coulomb energy associated with the charging two initially * To whom correspondence should be addressed. § These authors contributed equally to this work. † Department of Chemical Engineering. ‡ Russell Berrie Nanotechnology Institute.
neutral cores, e is the electronic charge, εr is the dielectric constant of the surrounding medium, ε0 is the permittivity of free space, and r is the particle radius. During exposure to analyte vapor, the filling of empty pores between the MCNPs with analyte molecules can: (i) induce swelling of the MCNP films and (ii) change the permittivity of the medium between the NPs’ metal cores. Equation 1 shows that the resistance increases exponentially as the core-core separation (δ) increases, whereas increasing the permittivity εr results in a reduction of the energy barrier required for charge carrier formation and, therefore, a decrease in resistance. Analyte-induced swelling and changes in the dielectric environment of the NPs, which may have opposite effects on the charge transport, seem to be the major contributors to the sensing response of the MCNP chemiresistors.23-28 Hence, by tailoring the chemical and physical properties of MCNPs one can obtain the desired sensitivity and selectivity for a particular sensing application.1,29-34 Until recently, MCNP-based chemiresistors were fabricated exclusively from spherical MCNPs. However, for some applications, the use of 3D assemblies of spherical MCNPs in a chemiresistive platform can be a disadvantage because, for example, of their limited swellability. The voids between adjacent spherical MCNPs, which make up 30-60% of the total volume of a 3D spherical NPs film,18,22 can host analyte molecules during the exposure process, but do not (or hardly) contribute to the swelling-induced sensing signal. Moreover, the interface contacts between adjacent NPs, at which analyte molecules adsorb and induce changes in the resistivity, are relatively small compared with the total surface area of an individual monolayer-capped spherical NP. The relatively small interface contacts could also suppress efficient electron transfer and, therefore, increase the signal-to-noise ratio (cf., Scheme S1 in the Supporting Information (SI)). Recently, we have shown that 3D films made of cubic Pt NPs capped with organic monolayers provide significantly higher sensitivity toward various analytes in the gas phase than
10.1021/jp105810w 2010 American Chemical Society Published on Web 07/30/2010
Cubic Pt NPs for Sensing Nonpolar Analytes the equivalent films made of spherical NPs.18 This was explained by the higher swellability of the cubic NP films, which was confirmed through thickness monitoring during exposure to selected analytes, using spectroscopic ellipsometery. In this work, we explore the use of chemiresistive films of cubic Pt NPs stabilized with four different organic ligands for sensing nonpolar analytes in the gas phase and in highly humid atmospheressmainly towards practical applications for medical diagnostics (e.g., lung, breast and liver cancer, chronic renal failure, etc.) via breath samples (see refs 35-42). Cubic Pt NPs stabilized with nonpolar ligands show a large positive response to representative nonpolar analytes, combined with low sensitivity to polar analytes and water. The sensing mechanism is explained in terms of analyte-induced swelling of the NP film and/or changes in the permittivity of the medium between the NPs. The sensing capabilities for nonpolar analytes in highly humid atmospheres are tested directly by dosing the sensors with air mixtures containing low concentrations of nonpolar analyte and high humidity levels, which are typical for many applications. 2. Experimental Section 2.1. Materials. Platinum acetylacetonate (97% purity), oleylamine (ODA), benzylmercaptan (BZM), 11-mercaptoundecanol (MUOH), and 11-mercaptoundecanoic acid (MUA) were purchased from Sigma-Aldrich. Analyte vapors were generated from decane, octane, hexane, ethanol, and ethyl benzene, also purchased from Sigma-Aldrich. Water vapor was obtained from 18 MΩ cm resistivity deionized (DI) water, purified with a Millipore Nanopure water system. 2.2. Synthesis and Structural Characterization of the Monolayer-Capped Cubic Pt NPs. In a typical synthesis, 0.028 g (0.071 mmol, 3.6 mM) of precursor (platinum acetylacetonate) was dissolved in 20 mL of toluene. To this solution, 13 equiv. of ODA was added as the surfactant.43 The platinum precursor was then decomposed under a hydrogen pressure of 3 bar at 55 °C in a pressure reaction vessel (Fischer-Porter bottle) for 20 h. The final Pt NPs precipitated in the reaction bottle were collected by centrifuging and dissolved in dichloromethane solvent. Cubic Pt NPs stabilized by BZM, MUOH, and MUA were synthesized, using the ligand exchange method, from preprepared ODA-capped cubic Pt NPs with corresponding capping ligands in tetrahydrofuran (THF) or dichloromethane solvents. In particular, an excess of incoming thiol, MUOH or MUA was added to the solution of ODA-capped Pt NPs in THF (3 mL). The solutions were constantly stirred for a few days to allow for a more complete ligand conversion. During the ligand exchange process, the final Pt NPs precipitated from THF solution. The final precipitate was washed through a repeated ultrasonic redispersion-centrifugation process to remove unbound ligands. At last, the precipitate was dissolved in ethanol (EtOH). Note, that the precipitation from THF solution and solubility in EtOH indicate that the major part of the ODA ligands were replaced with MUOH, or, alternatively, MUA. In the case of BZM-capped cubic Pt NPs, an excess of incoming BZM was added to the solution of ODA-capped cubic Pt NPs in dichloromethane (3 mL). After a few days of exchange reaction, the final Pt NPs were washed through repeated ultrasonic redispersion-centrifugation and finally dissolved in dichloromethane. Shape, size, and monodispersity of the resulting Pt NPs were monitored using TEM. Samples for TEM were prepared by drop casting 5 µL of diluted NPs solution onto 200-mesh carboncoated copper grids under ambient conditions. TEM images of
J. Phys. Chem. C, Vol. 114, No. 33, 2010 14043 the Pt NPs were obtained with a transmission electron microscope (Model CM120, Philips, The Netherlands) operated at 120 kV. 2.3. Chemiresistor Fabrication and Response Measurements. Chemiresistors based on thin films of monolayer-capped cubic Pt NPs (thickness ≈ 150 nm) were prepared by drop casting the toluene solution containing the NPs on interdigitated microsensor electrodes (IME, ABTECH). The IME patterns were combined from 25 pairs of Au electrodes of 20 µm width and 20 µm spacing on a glass substrate. The baseline resistance of the monolayer-capped cubic Pt NP films in the absence of analytes was between 5.6 and 180.1 kΩ. The sensors were mounted into a custom PTFE circuit board with 20 separated sensor sites. Afterward, the board was mounted inside a stainless steel test chamber with a volume of less than 100 cm3. An Agilent Multifunction switch 34980 controlled by USB was used to choose the active sensor at a given time. A Stanford Research System SR830 DSP lock-in amplifier controlled by an IEEE 488 system was used to supply the AC voltage signal and measure the corresponding current or resistance of the NP-coated IMEs. The sensing responses presented in this study were obtained at a fixed AC voltage (200 mV, 500 Hz). The entire system was controlled by a custom Labview program. Oil-free, purified air, obtained from a compressed air source, which had a baseline RH in the range 8.1-8.9% and an organic contamination of 5 months period, with (4% arbitrary fluctuations around the baseline resistance of the freshly prepared samples.
mixtures as a function of the mixture’s RH. These results show clearly that octane can be detected at low concentrations in an atmosphere of high humidity. The response to mixtures containing octane at Pa/Po ) 0.029 is consistently higher than the one containing octane at Pa/Po ) 0.010, which, in turn, is higher than the response to pure water vapor for all levels of RH.
We attempted to quantify the net contribution of the low amounts of octane to the sensing response in the following way. We subtracted the response to water alone from the response to the mixture, at three levels of RH. The water response at the measured RH levels of each mixture (Table 2) was computed using a spline fit. Figure 4c shows the mean values of (∆R/ Rb)Mixture - (∆R/Rb)Water versus the octane concentration of the different mixtures. The error bars represent the standard deviation. Every mean value and corresponding standard deviation was calculated from three mixtures with the same octane content but different RH. The net response of the ODA-capped cubic Pt NP sensor to octane increased in good approximation linearly with the octane concentration. Note, however, that this experimental data did not enable us to determine whether the combined response was simply the sum of the responses to the individual vapors, or whether the presence of one analyte blocked or enhanced the detection of the other. Here we used only one of the ODA-capped cubic Pt NP sensor that were exposed to individual analytes in section 3.1. Furthermore, the experiment described here was carried out 5 months after the one described in section 3.1, and the sensor was stored in ambient environment. The baseline and responses of sensors based on the same monolayer-capped cubic Pt NPs are qualitatively similar, but the responses’ magnitude may vary slightly from sensor to sensor. This is most probably due to experimental fluctuations in the morphology of the drop-casted films and/or different shelf lives of the used sensors.5 The sensitivity of the ODA-capped cubic Pt NP sensor to high concentrations of water vapor depends much stronger on the experimental setup than the sensitivity to octane. The response to the highly concentrated water vapor was not negligible throughout the whole RH range of the experiment (Figure 4b). The sensitivity to water, in terms of the slope of (∆R/Rb)Water versus RH, slightly decreased for intermediate RH levels around 50% (Figure 4b). Interestingly, the sensitivity to octane in the presence of water could be greatly improved by using an alternative setup, which evacuated the test chamber during analyte exposures. We found that the response differed both in magnitude and time evolution (see Figure S3, SI). A further advantage of using the vacuum setup is that the response showed less experimental scatter under repeated exposure than in the flow chamber, especially in the case of mixtures containing high RH. The slope of (∆R/Rb)Mixture - (∆R/Rb)Water versus Pa/Po was smaller for the vacuum chamber than the flow chamber, which would indicate a lower sensitivity to octane (Figure S3c, SI). However, since the responses’ standard deviations for the vacuum setup were much smaller, the overall sensitivity to octane was higher when using the vacuum setup. On the basis of the slope and average standard deviation, the estimated detection limits of octane in the presence of water vapor at concentration levels between 8% and 80% RH were 160 ppm and 40 ppm for the flow and vacuum setups, respectively. Finally, the response to water vapor in the vacuum setup was generally lower in magnitude than in the flow setup and the sensitivity was almost negligible. After a shallow incline
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at low RH levels, (∆R/Rb)Water reached a plateau and stayed almost constant from ∼20% RH to at least 80% RH (Figure S3b, SI). Hence, the ODA-capped cubic Pt NP sensor was almost insensitive to water vapor over the whole range of possible RH levels in realistic ambient atmospheres, if the exposure chamber was evacuated to intermediate vacuum levels around 1 Torr between exposures to analyte mixtures in ambient (moist) air. The difference in the performance of monolayercapped cubic Pt NP sensors when operated in a flow or vacuum setup is not yet well understood, but might be of practical importance for future applications. More extensive studies to clarify this issue are currently underway and will be published elsewhere. 4. Conclusions We explored the use of chemiresistive thin films of 17 ( 0.5 nm cubic Pt NPs stabilized with four different organic ligands in sensors for nonpolar analytes. Viability matching between specific capping ligands, viz. ODA, BZM, MUOH, and MUA, and the targeted analytes was taken into account to control the sensor-analyte interactions. The sensors showed an unprecedented large positive response to representative nonpolar analyte vapors, viz. hexane, octane and decane, and nonassociating (polar) ethyl benzene. Moreover, all tested monolayercapped cubic Pt NPs showed low sensitivity to polar analyte (ethanol) and water. The sensing mechanism was explained in terms of two competing processes: analyte-induced changes in the NP separation and changes in the permittivity of the medium between the NPs. Furthermore, we confirmed the sensing capabilities of such chemiresistors for nonpolar analytes in high humidity environments by dosing an ODA-capped cubic Pt NP sensor with binary mixtures of octane at low concentration and realistic amounts of water vapor. We found that operating the sensor in a vacuum setup, which allows the evacuation of the exposure chamber, at intermediate vacuum levels around 1 Torr before and after analyte exposure greatly improves its performance. The simple construction, low cost, stability, fast response, and high sensitivity of the monolayer-capped cubic Pt NP chemiresistors to nonpolar molecules, combined with their low sensitivity to water vapor, are promising features for sensing applications in real confounding atmospheres with high and/or variable levels of humidity. These include applications in novel medical diagnostic approaches such as breath testing,35,36 as well as applications for homeland security,56 such as the detection of explosives in airports or public spaces. Acknowledgment. The research was funded by the Marie Curie Excellence Grant of the European Commission’s FP6 program (Grant No. 1006752), and the Technion’s Russell Berrie Nanotechnology Institute (Grant No. 76517704/14). H.H. is a Knight of the Order of Academic Palms and holds the Horev Chair for Leaders in Science and Technology. Supporting Information Available: Schematic illustration of swelling-induced mechanism in NPs film, TEM image of cubic Pt NPs, and responses of ODA-capped cubic Pt NPs under various humidity and analytes conditions. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (2) Haick, H. J. Phys. D 2007, 40, 7173–7186. (3) Roeck, F.; Barsan, N.; Weimar, U. Chem. ReV. 2008, 108, 705– 725.
Dovgolevsky et al. (4) Aswal, D. K.; Gupta, S. K. Science and Technology of Chemiresistor Gas Sensors; Nova Science Publishers, Inc.: Hauppauge, NY, 2007. (5) Joseph, Y.; Guse, B.; Vossmeyer, T.; Yasudaa, A. J. Phys. Chem. C 2008, 112, 12507–12514. (6) Scho¨ning, M. J. Sensors 2005, 5, 126–138. (7) Fendler, J. H. Chem. Mater. 1996, 8, 1616–1624. (8) Brust, M.; Kiely, C. J. Colloid. Surf. A 2002, 202, 175–186. (9) Caruso, F.; Sukhorukov, G. Multilayer Thin Films 2003, 331–362. (10) Lim, S. K.; Ban, K. S.; Kim, Y.-H.; Kim, C. K.; Yoon, C. S.; Jin, S. Appl. Phys. Lett. 2006, 88, 163102/1–163102/3. (11) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782– 6786. (12) Lim, B.; Jiang, M.; Tao, J.; Camargo, P. H. C.; Zhu, Y.; Xia, Y. AdV. Funct. Mater. 2008, 18, 1–12. (13) Chen, S.; Ingrma, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098–2101. (14) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648– 8649. (15) Lisiecki, I. J. Phys. Chem. B 2005, 109, 12231–12244. (16) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. J. Phys. Chem. B 2005, 109, 13857– 13870. (17) Dovgolevsky, E.; Haick, H. Small 2008, 4, 2059–2066. (18) Dovgolevsky, E.; Tisch, U.; Haick, H. Small 2009, 5, 1158–1161. (19) Lim, S. I.; Zhong, C.-J. Acc. Chem. Res. 2008, 42, 798–808. (20) Wang, L.; Luo, J.; Schadt, M. J.; Zhong, C.-J. Langmuir 2010, 26, 618–632. (21) Lee, K.; Duchamp, M.; Kulik, G.; Magrez, A.; Seo, J. W.; Jeney, S.; Kulik, A. J.; Forro´, L.; Sundaram, R. S.; Brugger, J. J. Appl. Phys. 2007, 91, 173112. (22) Zabet-Khosousi, A.; Dhirani, A. Chem. ReV. 2008, 108, 4072–4124. (23) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H.G.; Wessels, J. M.; Wild, U.; Knop-Gericke, A.; Su, D.; Schogl, R.; Yasuda, A.; Vossmeyer, T. J. Phys. Chem. B 2003, 2003, 7406–7413. (24) Ibanez, F. J.; Zamborini, F. P. J. Am. Chem. Soc. 2008, 130, 622– 633. (25) Andres, R. P.; Bielefeld, J. D.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; M, W. J.; Osifchin, R. G. Science 1996, 273, 1–7. (26) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 4514–4515. (27) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465–11472. (28) Toda, M.; Joseph, Y.; Berger, R. J. Phys. Chem. 2010, 114, 2012– 2017. (29) Gomez-Romero, P. AdV. Mater. 2001, 13, 163–174. (30) Penn, S. G.; Hey, L.; Natan, M. J. Curr. Opin. Chem. Biol. 2003, 7, 609–615. (31) Roy, D.; Fendler, J. AdV. Mater. 2004, 16, 479–508. (32) Han, L.; Shi, X.; Wu, W.; Kirk, F. L.; Luo, J.; Wang, L.; Mott, D.; Cousineau, L.; Lim, S. I. I.; Lu, S.; Zhong, C. J. Sens. Actuators B 2005, 106, 431–441. (33) Wang, L.; Shi, X.; Kariuki, N. N.; Schadt, M.; Wang, G. R.; Rendeng, Q.; Choi, J.; Luo, J.; Lu, S.; Zhong, C.-J. J. Am. Chem. Soc. 2007, 129, 2161–2170. (34) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578–1586. (35) Peng, G.; Tisch, U.; Adams, O.; Hakim, M.; N., S.; Billan, S.; Abdah-Bortnyak, R.; Kuten, A.; Haick, H. Nat. Nanotechnol. 2009, 4, 669– 673. (36) Barash, O.; Peled, N.; Hirsch, F. R.; Haick, H. Small 2009, 5, 2618– 2624. (37) Peng, G.; Tisch, U.; Haick, H. Nano Lett. 2009, 9, 1362–1368. (38) Haick, H.; Hakim, M.; Patrascua, M.; Levenberg, C.; Shehada, N.; Nakhoul, F.; Abassi, Z. ACS Nano 2009, 3, 1258–1266. (39) Peng, G.; Trock, E.; Haick, H. Nano Lett. 2008, 8 (11), 3631– 3635. (40) Xue, R.; Dong, L.; Zhang, S.; Deng, C.; Liu, T.; Wang, J.; Shen, X. Rapid Commun. Mass Spectrom. 2008, 22, 1181–1186. (41) Amann, A.; Spanel, P.; Smith, D. Mini-ReV. Med. Chem. 2007, 7, 115–129. (42) Breath Analysis for Clincal Diagnosis and Therapeutic Monitoring; Amann, A.; Smith, D., Eds.; World Scientific Publishing Co. Pte. Ltd.: Singapore, 2005. (43) Ren, J.; Tilley, R. F. J. Am. Chem. Soc. 2007, 129, 3287–3291. (44) Different devices with the same coating showed similar results, within (5% deviations, mostly much higher than the device-to-device variations. Therefore, the observed discrimination effects can be attributed to the influence of the organic coating. (45) Different devices with the same coating showed similar results, within (5% deviations, mostly much higher than the device-to-device variations. Therefore, the observed discrimination effects can be attributed to the influence of the organic coating.
Cubic Pt NPs for Sensing Nonpolar Analytes (46) Bennett, M. E.; Alexander, W. A.; Lu, J. W.; Troya, D.; Morris, J. R. J. Phys. Chem. C 2008, 112, 17272–17280. (47) PCA finds projection weights for sensor response data that maximize total response variance in principal components, where the dimension capturing most sensor variance is given by PC1, and the dimension capturing the second most variance (subject to being orthogonal to PC1) is given by PC2, etc. (48) Ohshimal, H.; Kamaya, H.; Yoshida, T. U., I. Colloid Polym. Sci. 1986, 264, 817–822. (49) Kuzuya, T.; Tai, Y.; Yamamuro, S.; K., S. Sci. Technol. AdV. Mater. 2005, 6, 84–90. (50) Wuelfing, W. P.; Murray, R. W. J. Phys. Chem. B 2002, 106, 3139– 3145. (51) Mathias, S.; de Carvalho Filho, E.; Cecchini, R. G. J. Phys. Chem. 1961, 65, 425–427.
J. Phys. Chem. C, Vol. 114, No. 33, 2010 14049 (52) Wybourne, M. N.; Clarke, L.; Berven, C. A.; Hutchison, J. E.; Brown, L. O.; Mooster, J. L. Mater. Res. Soc. Symp. Proc. 2001, 582, H13.4/ 1-H13.4/6. (53) Brzozowska, M.; Oberts, B. P.; Blanchard, G. J.; Majewski, J.; Krysinski, P. Langmuir 2009, 25, 9337–9345. (54) Steinecker, W. H.; Rowe, M. P.; Zellers, E. T. Anal. Chem. 2007, 79, 4977–4986. (55) Phillips, M.; Gleeson, K.; Hughes, J. M. B.; Greenberg, J.; Cataneo, R. N.; Baker, L.; McVay, W. P. Lancet 1999, 353, 1930–1933. (56) Lareau, R. In Electronic Noses & Sensors for the Detection of ExplosiVes; Gardner, J. W., Yinon, J., Eds.; Springer: Netherlands, 2004. (57) Severin, E. J.; Lewis, N. S. Anal. Chem. 2000, 72, 2008–2015. (58) Sotzing, G. A.; Briglin, S. M.; Grubbs, R. H.; Lewis, N. S. Anal. Chem. 2000, 72, 3181–3190.
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