Article pubs.acs.org/Langmuir
Loss of Siloxane Monolayers from GaN Surfaces in Water Christina Arisio,† Catherine A. Cassou,‡ and Marya Lieberman*,† †
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States Department of Chemistry, University of California, Berkeley, California 94720, United States
‡
S Supporting Information *
ABSTRACT: Gallium nitride, with a thin passivating layer of Ga2O3, has been functionalized with octadecyltrichlorosilane (OTS) and aminopropyltriethoxysilane (APTES) self-assembled monolayers (SAMs). Water contact angles, atomic force microscopy, and X-ray photoelectron spectroscopy were used for characterization of the bare and functionalized surfaces. The SAMs are stable in acetonitrile, but both the hydrophobic OTS SAM and the hydrophilic APTES SAM completely desorb after 1−24 h of immersion in water and common buffers. The concentration of gallium in solution after a clean GaN chip is immersed in water is consistent with dissolution of roughly one monolayer of interfacial gallium oxide. Dissolution of this oxide layer could account for the loss of SAMs from GaN surfaces.
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INTRODUCTION Gallium nitride has shown much promise as a material for biosensor applications. It is used in high electron mobility transistors (HEMTs), in which the conductive channel is a twodimensional electron gas (2DEG) that lies between the interface of two semiconductors of differing band gaps.1,2 The conductivity of the 2DEG is strongly affected by nearby electrical charges. Therefore, ions can act as a gate for the conductive channel of the HEMT.3 The native oxide that forms on the surface of GaN in ambient conditions allows for direct surface functionalization. A variety of self-assembled monolayers (SAMs) based on siloxane precursors reacting with this surface oxide have been used to create sensing devices by providing a chemical link between the semiconductor and specific analytes.4−9 Many of these HEMT devices are intended to measure target molecules in an aqueous environment. Therefore, the stability of functionalized GaN surfaces in water is critical for the lifetime of the sensor.
OTS molecules evenly distributed across the Ga2O3 surface at early time points. The islands continue to grow, forming complete monolayer coverage within 3.5 days. Water contact angles confirm the growth of an OTS monolayer as the surface hydrophobicity increases from ∼10° for the bare clean surface to a maximum angle of 117° obtained for a full SAM. Aminopropyltriethoxysilane (APTES) SAMs (see Figure S4 of the Supporting Information) were grown by soaking a clean GaN chip in a 1−2% (v/v) solution of APTES in 18 MΩ H2O for 30 min. The growth mechanism involves solution phase hydrolysis and partial aggregation as well as surface binding, leading to a much faster but rougher growth process. Monolayer Stability Tests. Fully functionalized GaN samples were soaked in various solvents, water, and common buffers. Contact angle measurements (Figure 1) show that OTS SAMs were stable in acetonitrile for longer than 3 days, but they were unstable in all aqueous solutions examined. The SAMs in 18 MΩ H2O and pH 9 buffer began to desorb after 24 h, and the samples showed a steady decline in contact angle over the following 2 days. At pH 7, substantial monolayer loss occurred within a few hours of soaking in buffer. AFM imaging (Figure 2) provides visual confirmation of the loss of the monolayer. Figure 2a shows an OTS monolayer that was soaked in pH 7 buffer for approximately 30 min. This image shows the break-up of the monolayer from the smooth compact surface seen in Figure S2d of the Supporting Information to a rough surface with small OTS island patches. After 2.25 h, the sample resembles bare, unfunctionalized GaN, although the contact angle indicates that some OTS probably remains on its surface. Even after long soaking times, small
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RESULTS Growth of Siloxane SAMs on GaN. GaN samples underwent a rigorous cleaning procedure (see the Materials and Methods) to remove surface contamination. The water contact angle for the cleaned GaN surface was very hydrophilic, measuring less than 10°. Atomic force microscopy (AFM) imaging (see Figure S1 of the Supporting Information) of the clean bare GaN surface shows classic GaN dislocations, which appear as black pits or holes in the surface, as well as step features, which appear as diagonal lines across the image. Octadecyltrichlorosilane (OTS) SAMs were grown from 5 mM OTS in dry CHCl3/Isopar G. AFM imaging of the surface over 5 days (see Figure S2 of the Supporting Information) shows that growth occurs by island formation, with small patches of © 2013 American Chemical Society
Received: March 6, 2013 Published: March 27, 2013 5145
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Figure 1. Contact angle monitoring of OTS monolayer stability over time in various solvents shows loss of hydrophobic behavior.
X-ray Photoelectron Spectroscopy (XPS). XPS monitoring of the GaN surface was performed to provide confirmation of SAM growth and subsequent removal. Overlays of Ga 2p region-specific scans of the clean, unfunctionalized GaN surface seen in Figure 3a and the surface fully functionalized with siloxane SAMs show an attenuation of signal from Ga atoms (Figure 3b). The signal strength from Ga increases after buffer removal of the monolayers (Figure 3c). The percent concentration ratio of carbon 1s to gallium 2p (C/Ga) is 2:1 for Piranha-etched GaN (indicating adventitious carbon contamination). This ratio increases to 2.5:1 for a full APTES SAM and 6.5:1 for a full OTS SAM. The carbon/gallium ratio decreases significantly for the surface after buffer removal of OTS, at 0.34:1.
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DISCUSSION
Previously, our group has detailed the slow growth of ultrasmooth OTS monolayers on Si from dry organic solvents.22 In this study, AFM and water contact angles showed a similar slow growth process for OTS on GaN. However, the siloxane monolayer is unstable in water. Data obtained from XPS corroborate with AFM imaging and contact angle measurements for the growth and loss of the OTS and APTES SAMs. Given that the covalent bond of the siloxane to the surface is very strong,10,11 we hypothesized that dissolution of the interfacial gallium oxide layer was the cause of monolayer loss in aqueous environments. If the oxide beneath the SAM was dissolving, the monolayer would not only be lost but trace amounts of gallium should appear in the solution. Inductively coupled plasma−mass spectrometry (ICP−MS) was used to look for this gallium. The detection limit of ICP−MS was found to be approximately 4 pg/g (0.004 ppb), which is wellable to detect the concentration of gallium expected from dissolution of a single layer of gallium oxide on a 1 × 1 cm chip into 5 mL of water (Scheme 1). A Piranha-etched 10 × 10 mm GaN chip was soaked in 5 mL of ultra-pure water or pH 7 buffer for 24 h, after which the solution was analyzed for Ga by ICP−MS, using standard
Figure 2. AFM images of the loss of an OTS monolayer after soaking in pH 7 buffer for various lengths of time (t): (a) t, 30 min; contact angle, 70°; root mean square (RMS), 0.285 nm; and coverage, 42.1 ± 1.9% and (b) t, 2.25 h; contact angle, 68°; RMS, 0.175 nm; and coverage, 11.3 ± 1.7%.
amounts of the GaN surface remain covered by small islands, giving a bulk contact angle of 40−50°. Cleaning with Piranha acid recovers a smooth bare surface with a hydrophilic contact angle of less than 10°. The instability of hydrophilic (contact angle ∼ 65°) APTES monolayers on GaN (see Figure S5 of the Supporting Information) was similar to that of hydrophobic OTS monolayers. Substantial damage to the organic film was produced by a few hours of immersion in pure water or common buffers, and after 2 days of immersion, a contact angle of 20° was recovered, indicating complete loss of the monolayer. Similar instability was observed for stearic acid monolayers; see Supporting Information. 5146
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Figure 3. X-ray photoelectron spectra of (a) comprehensive survey scan of Piranha-cleaned GaN, (b) Ga 2p region-specific scan of GaN after Piranha etching and full SAM growth with OTS and APTES, and (c) Ga 2p region-specific scan of GaN with a full OTS SAM and the same sample after buffer removal of the SAM.
Scheme 1. Calculation of the Ga Concentration in Solution for the Loss of One Atomic Layer of Oxide from the Surface of a 10 × 10 mm Solid GaN Semiconductor Chip in 1 g of Watera
a
The calculation assumes a β monoclinic structure for Ga2O3.13
additions to calibrate the gallium concentrations. The Ga concentration for the solid samples was found to be 5.2 ± 1 ppb (0.264 ± 0.05 ppb/mm2 of chip area) for 18 MΩ H2O and 2 ± 0.3 ppb (0.100 ± 0.015 ppb/mm2 of chip area) for pH 7
buffer. The estimated concentration for the loss of one atomic layer of gallium from Ga2O3 (assumes β-monoclinic structure) from a 100 mm2 chip in 5 mL of water is 5 ppb or 0.25 ppb/ mm2. The data suggest that an equivalent of one layer of oxide 5147
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ethanol, then acetone, and followed by a final rinse with ultra-pure H2O. The chips were then etched for 20 min with a Piranha acid solution of 1:3 30% H2O2 (Fisher)/H2SO4 (concentrated, Fisher), rinsed with ultra-pure water, and dried with flowing nitrogen gas. The root-mean-square (RMS) roughness for a 1 μm2 image of Piranhaetched GaN is approximately 0.160−0.210 nm. Variations in this value predominantly depend upon the number of lattice dislocations captured in an image. Samples were used for SAM growth within 1−2 h of the cleaning treatment to reduce recontamination of the surface. Monolayer Deposition. To avoid excess surface contamination, samples were immediately used after cleaning. All solutions were made in 20 mL glass scintillation vials (Kimble), which were cleaned by the procedure mentioned above and dried in an oven. Each chip was placed in its own vial to avoid sample contamination. Carboxylic acid monolayers were grown by soaking a chip in 5−10 mL of 1 mM stearic acid (J. T. Baker) in absolute ethanol for a maximum time of 2 h. The functionalized samples were rinsed with ethanol and dried with nitrogen. APTES (Gelest, Inc.) monolayers were grown by soaking a chip in 5−10 mL of 2% APTES in ultra-pure water for a maximum time of 1 h. Because APTES tends to hydrolyze over time as a result of moisture from lab ambient, a fresh supply of APTES (less than 6 months old) was used for making solutions in water. After growth, the samples were rinsed with ultra-pure water and dried with nitrogen. OTS (90%, Aldrich) monolayers were grown from dry solution conditions in a drybox (nitrogen environment). Special care was taken to dry all solvents used because it was previously determined that dry conditions promote the formation of an ultra-smooth monolayer.22 Aluminum oxide (AAO, 50−200 μm, activated, Acros Organics) was dried overnight in an oven to remove any absorbed water and then used in a column to dry the OTS solvents. The AAO was first “wet” with CHCl3 (99.8%, Aldrich). A 1:4 mixture of CHCl3/Isopar G (Exxon Mobil) was then dried by passing through the column and collected into a flask with molecular sieves (8−12 mesh beads, Aldrich). The solvents were placed under vacuum and transferred along with sample vials and GaN chips into a drybox. GaN samples were soaked in 5−10 mL of a 5 mM OTS solution, which was made by adding OTS to the 1:4 solvent mixture. OTS monolayers were allowed to grow over a period of 5 days. The functionalized GaN chips were rinsed with CHCl3, then with ultra-pure water, and dried with nitrogen. A timed study was performed on monolayer formation by removing a GaN sample from the OTS solution at various time points. Once removed, the sample was rinsed with CHCl3 and dried with N2 gas. Water contact angles were measured on the sample, and atomic force microscope imaging was performed. The sample was then placed back into the OTS solution to continue SAM growth. Piranha etching after stability testing allowed GaN samples to be used for regrowth of another siloxane monolayer without any complications. However, multiple growth, removal, and acid cleanings of a single GaN sample do eventually damage the GaN surface. Stability and Solubility. Monolayer stability tests were carried out by soaking a functionalized GaN sample in 5 mL of solvent (aqueous and non-aqueous) for 4 days. Stability was tested in ultrapure water, pH 7 buffer (H3PO4/NaOH, Fisher), pH 9 buffer (B(OH)3/KCl/NaOH, Fisher), and acetonitrile (dry, Fisher). Powdered gallium(III) oxide stability tests were performed by placing 100 g of Ga2O3 (powder, Aldrich) in 50 mL of ultra-pure water or pH 7 buffer. The mixtures were stirred on a stir plate for 2 days and allowed to settle by gravity. The saturated solutions were decanted and filtered by passing through a syringe filter (0.45 μm, polypropylene, VWR). The solution was then centrifuged at 10 000 rpm for 30 min, decanted, and given a final syringe filtration. The solution was analyzed using inductively coupled plasma−optical emission spectroscopy (ICP−OES). The powder was found to be soluble in both water and buffer at around 1−10 ppm. The broad range in solubility could be due to undissolved oxide crystals that are small enough to pass through the 0.45 μm polypropylene syringe filter. Solid gallium oxide stability tests were carried out by placing a clean GaN chip in 5 mL of
has been lost. GaN may be stable to bulk etching in aqueous solutions,12 but its surface oxide is not. Repeat solubility tests have shown that the time necessary for complete loss of an OTS SAM in pH 7 buffer can vary from 1 to 24 h. The inconsistent length of time for the loss of an OTS monolayer is, we believe, due to the protection afforded by the hydrophobic chains of OTS. A well-packed OTS monolayer is more water-resistant than one that has a lot of imperfections in the packing of the OTS molecules. The more holes there are in the monolayer, the faster water can reach the underlying oxide and dissolve it. n-Type GaN that is derivatized with long-chain siloxanes has been shown to undergo photocatalytic cleavage of the SAM molecules under ultraviolet (UV) illumination.14 Under 254 nm of light, the contact angle of a dry SAM/GaN sample decreased from 110° to under 25° in about 0.5 h, and this time was decreased to a mere 1 min when the sample was illuminated under water. Damage to the SAM required bandgap illumination and was believed to involve C−C bond cleavage reactions. Ambient laboratory light only caused a slight decrease in the contact angle value, which saturated at 90°. It is possible that C−C bond cleavage events could create disorder or pinholes in the SAM, which would enable water to reach the oxide layer more easily, but cleavage of C−C bonds in the SAM would not account for the presence of the equivalent of a monolayer of dissolved gallium after the loss of the SAM. Our results show that the gallium oxide on which these siloxane SAMs are anchored is innately unstable in water. The use of III−V semiconductors that contain surface Al and, thus, a native aluminum oxide layer can provide a solution to the monolayer instability problem, because Al2O3 is very robust in aqueous environments. Al-terminated nitrides that can be used in HEMT structures include AlN or AlGaN, and some examples of HEMT structures with these materials exposed to the solvent interface are in use.15 Additionally, very thin GaN caps that are grown on top of Al−nitride structures may have reinforced gallium oxide stability if Al intercalates into the surface oxide. Preliminary tests of APTES on AlGaN/GaN HEMT structures with a 2 nm GaN cap show promising stability in aqueous solutions.
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CONCLUSION
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MATERIALS AND METHODS
The surface of gallium nitride can be functionalized with siloxane SAMs, but the SAMs are not stable in aqueous solvents. After soaking in aqueous solutions for periods of hours to days, there is a loss of water contact angle hydrophobicity, a loss of monolayer islands by AFM, and an increase in the XPS Ga signal. GaN biosensing applications predominantly involve biological mediums that are aqueous environments, and the instability of the gallium oxide surface in such environments will limit sensor lifetime and usage. GaN does have other technological applications,16−18 in which nonaqueous SAM functionalization may be used.19−21
Sample Preparation. All glassware and tweezers were rinsed with acetone (Fisher), then boiled in ethanol (absolute, Aaper) for 30 min, and given a final rinse with ultra-pure (18 MΩ, milli-Q) H2O. Gallium nitride samples were provided by Huili Grace Xing, Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN. The samples were approximately 10 × 10 mm chips, consisting of 2 μm thick metal organic chemical vapor deposition (MOCVD) grown n-type GaN on a sapphire substrate. The chips were rinsed with 5148
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ultra-pure water or pH 7 buffer for 2 days. The chip was removed, and the solution was analyzed using ICP−MS. Instrumentation. Contact angle measurements were taken using a Krüss G10 goniometer. All measurements were taken using ultra-pure water and an advancing drop. Reported values are an average of four locations on a single chip. AFM was performed using a Nanoscope IIIa Multimode Instrument (Digital Instruments) in tapping mode. Silicon cantilevers were used (spring constant, 40 N/m; resonant frequency, 300 kHz; Vista Probes). Image analysis was performed using the Nanoscope III software. XPS was performed using a Kratos XSAM800 instrument with an Al Kα source and a 0° take-off angle. Samples were mounted via carbon tape to the metal sample slugs of the instrument. Data acquisition was carried out at ultra high (×10−8 Torr) vacuum. Data analysis was performed using Kratos Vision II software. All peaks were calibrated against the C 1s (284 eV) peak. ICP−OES was performed with a Perkin-Elmer Optima 3300 XL instrument. Gallium emission wavelengths were scanned at 417 and 294 nm. A National Institute of Standards and Technology (NIST)-traceable Ga parent standard in 4% HNO3 was used to make all calibration standards. Standards used were between 0.1 and 4.0 ppm. All samples, including blanks, were acidifed to contain 4% HNO3, and the samples were calibrated by three-point standard addition after blank subtraction. Water samples were tested with a blank of 4% HNO3 in ultra-pure water, and buffer samples were tested with a blank of 4% HNO3 in pH 7 buffer. ICP−MS was performed with Thermo Finnigan Element 2 high-resolution ICP−MS. As with ICP−OES experiments, all samples were acidified to contain 4% HNO3. Calibration was performed using standard additions (spiked at levels of 0, 2.44, and 6.98 ppb) of a NIST-traceable gallium solution. The detection limit was calculated using 3 times the standard deviation for the average Ga concentration of a 4% HNO3-acidified blank. Machine drift was monitored using 75As as an internal standard (∼30 ppb). Potential interferences were monitored by tracking the measured 69Ga/71Ga ratio (1.507 in nature) and using medium-resolution mass discrimination.
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(4) Baur, B.; Steinhoff, G.; Hernando, J.; Purrucker, O.; Tanaka, M.; Nickel, B.; Stutzmann, M.; Eickhoff, M. Chemical functionalization of GaN and AlN surfaces. Appl. Phys. Lett. 2005, 87, 263901. (5) Baur, B.; Howgate, J.; von Ribbeck, H.-G.; Gawlina, Y.; Bandalo, V.; Steinhoff, G.; Stutzmann, M.; Eickhoff, M. Catalytic activity of enzymes immobilized on AlGaN/GaN solution gate field-effect transistors. Appl. Phys. Lett. 2006, 89, 183901. (6) Baur, B.; Steinhoff, G.; Hernando, J.; Purrucker, O.; Tanaka, M.; Nickel, B.; Stutzmann, M.; Eickhoff, M. Chemical functionalization of GaN and AlN surfaces. Appl. Phys. Lett. 2005, 87, 263901. (7) Petoral, R. M.; Yazdi, G. R.; Spetz, A. L.; Yakimova, R.; Uvdal, K. Organosilane-functionalized wide band gap semiconductor surfaces. Appl. Phys. Lett. 2007, 90, 223904. (8) Cimalla, I.; Will, F.; Tonisch, K.; Niebelschutz, M.; Cimalla, V.; Lebedev, V.; Kittler, G.; Himmerlich, M.; Krischok, S.; Schaefer, J.; Gebinoga, M.; Schober, A.; Friedrich, T.; Ambacher, O. AlGaN/GaN biosensorEffect of device processing steps on the surface properties and biocompatibility. Sens. Actuators, B 2007, 123, 740−78. (9) Kang, B. S.; Wang, H. T.; Ren, F.; Pearton, S. J. Electrical detection of biomaterials using AlGaN/GaN high electron mobility transistors. J. Appl. Phys. 2008, 104, 031101. (10) Wasserman, S.; Tao, Y.; Whitesides, G. Structure and reactivity of alkylsiloxane monolayers formed by reaction of alkyltrichlorosilanes on silicon substrates. Langmuir 1989, 5, 1074−1087. (11) Arranz, A.; Palacio, C.; Garcia-Fresnadillo, D.; Orellana, G.; Navarro, A.; Munoz, E. Influence of surface hydroxylation on 3aminopropyltriethoxysilane growth mode during chemical functionalization of GaN surfaces: An angle-resolved X-ray photoelectron spectroscopy study. Langmuir 2008, 24, 8667−8671. (12) Zhuang, D.; Edgar, J. H. Wet etching of GaN, AlN, and SiC: A review. Mater. Sci. Eng., R 2005, 48, 1−46. (13) Geller, S. J. Crystal structure of β-Ga2O3. Chem. Phys. 1960, 33, 676−684. (14) Howgate, J.; Schoell, S. J.; Hoeb, M.; Steins, W.; Baur, B.; Hertrich, S.; Nickel, B.; Sharp, I. D.; Stutzmann, M.; Eickhoff, M. Photocatalytic cleavage of self-assembled organic monolayers by UVinduced charge transfer from GaN substrates. Adv. Mater. 2010, 22, 2632. (15) Wen, X. J.; Schuette, M. L.; Gupta, S. K.; Nicholson, T. R.; Lee, S. C.; Lu, W. Improved sensitivity of AlGaN/GaN field effect transistor biosensors by optimized surface functionalization. IEEE Sens. J. 2011, 8, 1726−1735. (16) Lopez-Gejo, J.; Navarro-Tobar, A.; Arranz, A.; Palacio, C.; Munoz, E.; Orellana, G. Direct grafting of long-lived luminescent indicator dyes to GaN light-emitting diodes for chemical microsensor development. ACS App. Mater. Interfaces 2011, 3, 3846−3854. (17) Simpkins, B S; McCoy, K M; Whitman, L J; Pehrsson, P E Fabrication and characterization of DNA-functionalized GaN nanowires. Nanotechnology 2007, 18, 355301. (18) Ismail, M. N.; Goodrich, T. L.; Ji, Z. X.; Ziemer, K. S.; Warzywoda, J.; Sacco, A. Assembly of titanosilicate ETS-10 crystals on organosilane-functionalized gallium nitride surfaces. Microporous Mesoporous Mater. 2009, 118, 245−250. (19) Kim, H.; Colavita, P. E.; Metz, K M.; Nichols, B. M.; Sun, B.; Uhlrich, J.; Wang, X.; Kuech, T. F.; Hamers, R. J. Photochemical functionalization of gallium nitride thin films with molecular and biomolecular layers. Langmuir 2006, 22, 8121−8126. (20) Lian, C.; Xing, H.; Wang, C. S.; McCarthy, L.; Brown, D. DC characteristics of AlGaAs/GaAs/GaN HBTs formed by direct wafer fusion. IEEE Electron Device Lett. 2007, 28, 8−10. (21) Stine, R.; Simpkins, B. S.; Mulvaney, S. P.; Whitman, L. J.; Tamanaha, C. R. Formation of amine groups on the surface of GaN: A method for direct biofunctionalization. App. Surf. Sci. 2010, 256, 4171−4175. (22) Wang, Y.; Lieberman, M. Growth of ultra-smooth octadecytrichlorosilane self-assembled monolayers on SiO2. Langmuir 2003, 19, 1159−1167.
ASSOCIATED CONTENT
S Supporting Information *
AFM topographic image of clean GaN, characterization of the growth of OTS on GaN, characterization of growth and loss of APTES on GaN, and rapid loss of stearic acid SAM on GaN in water. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS GaN samples were provided by Huili Xing (University of Notre Dame). We thank the Center for Environmental Science and Technology and Dr. Antonio Simonetti for assistance with the ICP-OES and ICP-MS analyses.
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REFERENCES
(1) Pearton, S. J.; Kang, B. S.; Kim, S. K.; Ren, F.; Gila, B. P.; Abernathy, C. R.; Lin, J. S.; Chu, S. N. G. GaN-based diodes and transistors for chemical, gas, biological and pressure sensing. J. Phys.: Condens. Matter 2004, 16, R961−R994. (2) Stutzmann, M.; Garrido, J. A.; Eickoff, M.; Brandt, M. Direct biofunctionalization of semiconductors: A survey. Phys. Status Solidi 2006, 203, 3424−3437. (3) Steinhoff, G.; Hermann, M.; Schaff, W. J.; Eastman, L. F.; Stutzmann, M.; Eickhoff, M. pH response of GaN surfaces and its application for pH-sensitive field-effect transistors. Appl. Phys. Lett. 2003, 83, 177. 5149
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