Abstract
In this present study, the investigation about pH sensorial properties of WO3, via sol-gel, was evaluated by Voltammetry and Extended Gate Field Effect Transistor techniques. The X-ray diffractogram indicates the presence of a lamellar structure, d = 0.69 nm, resulting in WO3.2H2O. From Scanning Electron Microscopy of WO3.2H2O was observed a process corresponding to the delamination which consists of irregular stacking with rounded platelets. The WO3.2H2O was investigated as a pH sensor in the pH range 2–12, by the EGFET and Voltametry techniques presenting a sensitivity of 52 mV/pH and 60 mV/pH, respectively. These results can indicate that both Voltammetry and EGFET techniques present values close to the theoretical limit (59.2 mV/pH) as well as the material is a promising candidate for applications as a pH sensor and as disposable biosensor in the future.
tungsten oxide; pH sensor; voltammetry; EGFET
1 Introduction
The search for simple, speedy and inexpensive analytical tests utilizing chemical compounds at
very low concentrations has caused a growing need for the development of electrochemical
sensors11. Song KS, Nakamura Y, Sasaki Y, Degawa M, Yang JH and Kawarada H. pH-sensitive
diamond field-effect transistors (FETs) with directly aminated channel surface. Analytica
Chimica Acta. 2006; 3-8:573-574.,22. Silva GM, Lemos SG, Pocrifka LA, Marreto PD, Rosario AV and Pereira EC.
Development of low-cost metal oxide pH electrodes based on the polymeric precursor method.
Analytica Chimica Acta. 2008; 616(1):36-41. http://dx.doi.org/10.1016/j.aca.2008.03.019.
PMid:18471481
http://dx.doi.org/10.1016/j.aca.2008.03....
. For example, the biosensors on basis of the pH sensor are
widely utilized in the medical instruments. In order to obtain an improvement in the sensitivity
Bergveld proposed the Ion-Sensitive Field-Effect Transistor (ISFET)33. Bergveld P. Development of an ion-sensitive solid-state device for
neurophysiological measurements. IEEE Transactions on Biomedical Engineering. 1970; 17:70-71.
http://dx.doi.org/10.1109/TBME.1970.4502688.
http://dx.doi.org/10.1109/TBME.1970.4502...
. The development of the ISFET mimics a commercial Metal Oxide
Semiconductor Field Transistor (MOSFET) in which the metal gate electrode is removed, in order
to expose the underlying insulator layer to the solution. Based on ISFET, Van Der Spiegel et
al.44. Van Der Spiegel J, Lauks I, Chan P and Babic D. The extended gate chemically
sensitive field effect transistor as multi-species microprobe. Sensors and Actuators. 1983;
4:291-298. http://dx.doi.org/10.1016/0250-6874(83)85035-5.
http://dx.doi.org/10.1016/0250-6874(83)8...
, introduced another structure named
Extended Gate Field Effect Transistor (EGFET) which has a more flexible shape compared to ISFET,
and also presents better long-term stability, since the ions from the chemical environment are
excluded from any region close to the FET gate insulator55. Chou JC, Chiang JL and Wu CL. pH and procaine sensing characteristics of
extended-gate field-effect transistor based on indium tin oxide glass. Japanese Journal of
Applied Physics. 2005; 44(7A):4838-4842.
http://dx.doi.org/10.1143/JJAP.44.4838.
http://dx.doi.org/10.1143/JJAP.44.4838...
. Recently, several thin films have been widely used as the sensing
material of the EGFET pH sensors, such as carbon nanotubes66. Silva GR, Matsubara EY, Corio P, Roselen JM and Mulato M. Carbon felt/carbon
nanotubes/pani as ph sensor. Materials Research Society Proceedings. 2007; 1018:EE1410-1.
http://dx.doi.org/10.1557/PROC-1018-EE14-10.
http://dx.doi.org/10.1557/PROC-1018-EE14...
, SnO2[7]7. Batista PD, Mulato M, Graeff CFO, Fernandez FJR and Marques FD. SnO extended
gate field-effect transistor as pH sensor. 2Brazilian Journal of Physics. 2006;
36(2a):478-481. http://dx.doi.org/10.1590/S0103-97332006000300066.
http://dx.doi.org/10.1590/S0103-97332006...
,
ZnO[8]8. Batista PD and Mulato M. ZnO extended-gate field-effect transistors as H
sensors. pApplied Physics Letters. 2005; 87(14):143508-143510.
http://dx.doi.org/10.1063/1.2084319.
http://dx.doi.org/10.1063/1.2084319...
, V2O5
xerogel99. Guerra EM and Mulato M. Synthesis and characterization of vanadium
oxide/hexadecylamine membrane and its application as pH-EGFET sensor. Journal of Sol-Gel
Science and Technology. 2009; 52(3):315-320.
http://dx.doi.org/10.1007/s10971-009-2062-7.
http://dx.doi.org/10.1007/s10971-009-206...
,
V2O5/HDA[10]10. Guerra EM, Silva GR and Mulato M. Extended gate field effect transistor
using V2O5 xerogel sensing membrane by sol-gel method. Solid State
Sciences. 2009; 11(2):456-460.
http://dx.doi.org/10.1016/j.solidstatesciences.2008.07.014.
http://dx.doi.org/10.1016/j.solidstatesc...
,
V2O5/WO3[11]11. Guidelli EJ, Guerra EM and Mulato M. Ion sensing properties of
vanadium/tungsten mixed oxides. Materials Chemistry and Physics. 2011; 125(3):833-837.
http://dx.doi.org/10.1016/j.matchemphys.2010.09.040.
http://dx.doi.org/10.1016/j.matchemphys....
, V2O5/TiO2[1212. Guidelli EJ, Guerra EM and Mulato M. Vanadium and titanium mixed oxide
films: synthesis, characterization and application as ion sensor. Journal of the
Electrochemical Society. 2012; 159(6):J217-J222.
http://dx.doi.org/10.1149/2.053206jes.
http://dx.doi.org/10.1149/2.053206jes...
,1313. Guidelli EJ, Guerra EM and Mulato M. E V2O5/WO mixed
oxide films as pH-EGFET sensor: sequential re-usage and fabrication volume analysis.
3ECS Journal Solid State Science and Technology. 2012; 1(39):N39-N44.
http://dx.doi.org/10.1149/2.007203jss.
https://doi.org/10.1149/2.007203jss...
].
However, the search for improved materials as an alternative for ion-sensing membranes still
remains in this field of study. An alternative to ion-sensing membranes used in pH sensors is
the tungsten oxide (WO3) thin film obtained by the sol-gel route. WO3 thin
film presents the hopping conduction between W6+ and W5+ or W5+
and W4+ that may result in not only high conductivity but also high carrier
mobility1414. Li X, Zhang Q, Miao W, Huang L, Zhang Z and Hua Z. Development of novel
tungsten-doped high mobility transparent conductive In2O thin films.
3Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films. 2006;
24(5):1866-1869. http://dx.doi.org/10.1116/1.2333572.
http://dx.doi.org/10.1116/1.2333572...
. The conduction of WO3
thin films is very anisotropic, and across the W–O–W layers it is proposed to be due to, mainly,
protons which move through hydrogen bonds1111. Guidelli EJ, Guerra EM and Mulato M. Ion sensing properties of
vanadium/tungsten mixed oxides. Materials Chemistry and Physics. 2011; 125(3):833-837.
http://dx.doi.org/10.1016/j.matchemphys.2010.09.040.
http://dx.doi.org/10.1016/j.matchemphys....
.
Research on WO3 thin films has generated significant interest because of their use in
Semiconducting Metal Oxide (SMO)-based sensors for the detection of gaseous adsorbates1515. Lee D, Nam K and Lee D. Effect of substrate on NO2-sensing
properties of WO3 thin film gas sensors. Thin Solid Films. 2000; 375(1-2):142-146.
http://dx.doi.org/10.1016/S0040-6090(00)01261-X.
http://dx.doi.org/10.1016/S0040-6090(00)...
and because they are amenable to
microfabrication techniques1616. Wang G, Ji Y, Huang X, Yang X, Gouma PI and Dudley M. Fabrication and
characterization of polycrystalline WO3 nanofibers and their application for ammonia sensing.
The Journal of Physical Chemistry B. 2006; 110(47):23777-23782.
http://dx.doi.org/10.1021/jp0635819. PMid:17125339
http://dx.doi.org/10.1021/jp0635819...
.
The aims of this work are to explore WO3 thin films as ion-sensitive membranes with charges in pH solutions, to find out the sensor sensitivity, and to verify its suitability as a pH sensor. Finally, the used pH-sensor sensitivity should be verified and the results obtained for the EGFET and Voltammetry techniques should be compared.
2 Experimental
The tungsten oxide gel, WO3.nH2O, was prepared from sodium tungstate
(NaWO3, VETEC), by the ion exchange method (ion-exchange resin Dowex-50X8) as
described in the literature1111. Guidelli EJ, Guerra EM and Mulato M. Ion sensing properties of
vanadium/tungsten mixed oxides. Materials Chemistry and Physics. 2011; 125(3):833-837.
http://dx.doi.org/10.1016/j.matchemphys.2010.09.040.
http://dx.doi.org/10.1016/j.matchemphys....
. Acid solutions
were obtained by percolating 0.1M of NaWO3 aqueous solutions through a cationic
ion-exchange resin. Upon standing at room temperature (24 °C) for 2 weeks, the solutions were
polymerized, leading to a viscous yellow gel of WO3. The WO3 gel was
deposited on an indium tin oxide/polyethylene terephthalate (ITO/PET) substrate and dried at
room temperature, leading to the formation of a thin film.
The X-Ray Diffraction (XRD) data were recorded on a SIEMENS D5005 diffractometer using a graphite monochromator and CuKα emission line (1.541 Å, 40 kV, 40 mA). After that, samples in the film form deposited onto a glass plate were employed, and the data were collected at room temperature over the range 2° ≤ 2θ ≤ 50°, with a step of 0.020°.
Scanning Electron Microscopy (SEM) was carried out on a ZEISS microscope EVO 50 model operating at 20 kV. A thin gold coating (≈ 20Å) was applied to the sample using a Sputter Coater – Balzers SCD 050. The length of platelets particles was found using the software ImageTools®.
The electrical response of the sensor was measured using varying pH solutions, and the curves were obtained by an Agilent 34970A parameter analyzer. The electrode containing the film was dipped into the buffer solution, at room temperature, for 5 minutes, prior to the electrical measurement.
Voltammograms were measured using an AUTOLAB (Metrohm) model PGSTAT 302N (Nova 1.10) potentiostat/galvanostat interfaced with a computer. The conventional electrode arrangement was used, which consisted of PET/ITO as the working electrode, a platinum wire auxiliary electrode, and a saturated calomel electrode (SCE) as the reference. The WO3 thin film was deposited on the electrode surface by evaporating approximately 5 ml of the suspension at room temperature (24 °C). The electrode containing the film was dipped into the buffer solution at room temperature, for 5 minutes, prior to the electrical measurement. The buffer solutions used as supporting electrolyte had pH = 2, 4, 6, 7, 8, 10 and 12 each one.
3 Results and Discussion
The X-ray diffraction pattern of a tungsten oxide is shown in Figure l. It exhibits series of
harmonic peaks, that can be indexed as 0k0, according to the
WO3.nH2O[17]17. Chemseddine A, Babonneau F and Livage J. Anisotropic WO. 3·nH2O layers
deposited from gels. Journal of Non-Crystalline Solids. 1987; 91(2):271-278.
http://dx.doi.org/10.1016/S0022-3093(87)80311-3.
http://dx.doi.org/10.1016/S0022-3093(87)...
. Besides
that, the 0k0 index corresponds to the stacking of platelike particles along a
direction perpendicular to the substrate. Additionally, the peaks also indicate that the
lamellar structure of the WO3 is maintained after the drying process as observed in
Figure 2. Based on Bragg’s law, the basal spacing,
d, between the platelets (d = 0.69 nm) suggests that water
molecules (n = 2) are intercalated between these platelets resulting in
WO3.2H2O, as also observed in the literature1717. Chemseddine A, Babonneau F and Livage J. Anisotropic WO. 3·nH2O layers
deposited from gels. Journal of Non-Crystalline Solids. 1987; 91(2):271-278.
http://dx.doi.org/10.1016/S0022-3093(87)80311-3.
http://dx.doi.org/10.1016/S0022-3093(87)...
. Despite of cristallinity, the diffraction peaks of the
WO3.2H2O present high intensity and narrow peaks suggesting that this
material has a highly crystalline structure.
Figure 2 shows the scanning electron micrographs of the WO3. In this image was observed a process corresponding to the delamination of the stacked inorganic sheets. The crystallites of the material consist of irregular stacking with rounded platelets with a length of around 10–20 nm and thickness less than 1.0 nm, consistent to what is showed in Figure 1.
Figure 3 shows the drain current (IDS) as a
function of the voltage between transistor gate and source (VGS) for the
WO3, in contact with a buffer solution. It is observed that the threshold voltage
shift depends upon the pH value, i.e., the threshold voltage shifts from the left to the right
with increasing pH values. This dislocation towards higher voltages as a function of pH is due
to the sensing properties that are associated to the potential-determining ions in the buffer
solution (i.e., H+ and OH− ions). As the pH values of the buffer solution
are increasing, there is a threshold voltage shift positively because of the decreasing surface
potential. Based on this fact, the sensitivity of pH sensors can be extracted from the threshold
voltage shift from Equation 1 and 2, i.e.1818. Chien Y-S, Tsai W-L, Lee I-C, Chou J-C and Cheng H-C. A novel ph sensor of
extended-gate field-effect transistors with laser-irradiated carbon-nanotube network. IEEE
Electron Device Letters. 2012; 33(11):1622-1624.
http://dx.doi.org/10.1109/LED.2012.2213794.
http://dx.doi.org/10.1109/LED.2012.22137...
,
EGFET response in linear region (drain–source current as a function of gate–source voltage) using WO3.
From Figure 3, an IDS value of 350 μA was
adopted for determination of the sensitivity and the corresponding VGS values were
plotted as a function of the pH for all the samples (Figure
4). Based on the experimental results shown in Figure
3, the sensitivity of the EGFETs using ID = 350 mA and VDS of 0.3
V was 52 mV/pH. It is possible to note that the sensitivity is very close to the theoretical
limit1919. Temple-Boyer P, Launay J, Humenyuk I, Conto T, Martinez A, Beriet C, et al.
Study of front-side connected chemical field effect transistor for water analysis.
Microelectronics and Reliability. 2004; 44(3):443-447.
http://dx.doi.org/10.1016/j.microrel.2003.10.001.
http://dx.doi.org/10.1016/j.microrel.200...
. Besides that, Figure 4 shows that the EGFET exhibits good linearity.
Sensitivity of the WO3 EGFET pH sensor for buffer solutions with pH varying from 2 up to 10, for the linear region.
The behavior of the voltammogram and its correlation with the pH response were also
accompanied and it is showed in Figures 5 and 6. Each exchange of buffer solution to a higher pH can result
in altering the concentration of one of the species involved in the reaction, thus resulting in
a shift in the redox potential. Figures 5 and 6 show the voltammograms as a function of pH, in buffered
electrolytes. It was used a sweep rate slow (20 mV/s) in order to ensure that the main features
of the voltammogram were reversible. The anodic peak region was selected, in order to ensure
that the main features were preserved and to facilitate voltammogram analyses. The equation for
the electrode potential of this half-reaction can be rearranged. Figure 6 shows that the potential shifts to more negative values as the pH increases as
predicted by the Nernst equation in Equation
32020. Walczak MM, Dryer DA, Jacobson DD, Foss MG and Flynn NT. pH-dependent redox
couple: illustrating the nernst equation using cyclic voltammetry. Journal of Chemical
Education. 1997; 74:1195-1197. http://dx.doi.org/10.1021/ed074p1195.
http://dx.doi.org/10.1021/ed074p1195...
.
Voltammograms of WO3 in buffer solution with pH values ranging from 2 to 12 with magnification to guide the eyes.
The pH dependence of potential (E) can also be seen in Figure 7, where E is plotted against the pH. The slope of the line is 60 mV/pH. From the Nernst equation, the slope of the plot of E vs. pH should be 59.2 mV/pH, which is very close to the values obtained from the EGFET study. Therefore, the EGFET-pH sensor based on tungsten oxide is a promising method and it might be suitable to be used as a device in disposable sensors.
The sensorial capability occurs at the oxide/solution in both Voltammetry and EGFET analyses.
When a potential is applied, charge may arise by the adsorption/desorption of H+ ions
in the interface modified in both sign and intensity, depending on the pH of the solution in
contact with the sample. Thus, this charge assumes that the Nernst equation relates the total
double layer potential drop to the activity in solution of H+ (or OH-)
i.e. the potential-determining (p.d.) ions2121. Yates DE, Levine S and Healey TW. Site-binding model of the electrical
double layer at the oxide/water interface. Journal of the Chemical Society, Faraday Transition
1: Physical Chemistry in Condensed Phases. 1974; 70:1807-1818.
http://dx.doi.org/10.1039/F19747001807.
http://dx.doi.org/10.1039/F19747001807...
.
From this point of view, both the EGFET and voltammetric sensors exhibit the same
electrochemical mechanism involving charged interfaces and binding sites of H+ and
OH-. These sites can be protonated or deprotonated, leading to a surface charge
which is dependent on the pH of the electrolyte, and controls the surface potential99. Guerra EM and Mulato M. Synthesis and characterization of vanadium
oxide/hexadecylamine membrane and its application as pH-EGFET sensor. Journal of Sol-Gel
Science and Technology. 2009; 52(3):315-320.
http://dx.doi.org/10.1007/s10971-009-2062-7.
http://dx.doi.org/10.1007/s10971-009-206...
,2222. Huang BR and Lin TC. Leaf-like carbon nanotube/nickel composite membrane
extended-gate field-effect transistors as pH sensor. Applied Physics Letters. 2011; 99:023108.
http://dx.doi.org/10.1063/1.3610554.
http://dx.doi.org/10.1063/1.3610554...
. In the voltammetry is possible to sweep a pontential to the regions
which the electroactive species exist as oxidized (Ox) as well as, followed to reduced species
(Red) and when the ratio [Ox]/[Red] approaches to zero it is called isoelectric point or zero
(IEP).
During the variation of pH values it was observed a occurrence of a protonation/deprotonation
mechanism in the IEP that was previously described in the literature1313. Guidelli EJ, Guerra EM and Mulato M. E V2O5/WO mixed
oxide films as pH-EGFET sensor: sequential re-usage and fabrication volume analysis.
3ECS Journal Solid State Science and Technology. 2012; 1(39):N39-N44.
http://dx.doi.org/10.1149/2.007203jss.
https://doi.org/10.1149/2.007203jss...
,2323. MacDonald DE, Rapuano BE and Schniepp HC. Surface oxide net charge of a
titanium alloy: comparison between effects of treatment with heat or radiofrequency plasma glow
discharge. Colloids and Surfaces B: Biointerfaces. 2011; 82(1):173-181.
http://dx.doi.org/10.1016/j.colsurfb.2010.08.031. PMid:20880672
http://dx.doi.org/10.1016/j.colsurfb.201...
. Based on the protonation/deprotonation mechanism, it is possible to
consider that Reaction (1) in pH < IEP, pH = IEP and pH > IEP, respectively:
Based on the reaction 1, the voltammetric and EGFET studies, Figures 4 and 6, the threshold voltage shift depends on the pH value and could be explained by the history of the discharge (deprotonation) process of the film. In summary, it was possible to demonstrate that the effect of the solution pH variation would alter the concentration of one of the species involved in the reaction and result in a shift in the redox potential in the voltammetric and in the EGFET studies according to the Nernstian behavior.
4 Conclusion
The synthetic route to prepare WO3, via sol-gel, was successful. From the diffractogram was possible to identify the formation of platelike particles, as well as, lamellar structure. The SEM confirmed the X-Ray driffraction data, indicating irregular stacking with rounded platelets with a length of around 10–20 nm and thickness less than 1.0 nm. Both the EGFET and voltammetric sensors exhibited the same electrochemical mechanism. Therefore, as a sensing film on a pH-EGFET configuration, as well as, voltammetry, the WO3 material demonstrated a linear behavior and a high sensitivity (52 mV/pH and 60 mV/pH) for the pH range 2-12, close to the theoretical limit (59.2 mV/pH). Both techniques gave very close results. Therefore, these results suggest that the material is a good candidate as a pH sensor and may be even further employed as a biosensor for urea and glucose detection.
Acknowledgements
This work was supported by FAPESP, CNPq, INEO and FAPEMIG Brazilian agencies.
References
-
1Song KS, Nakamura Y, Sasaki Y, Degawa M, Yang JH and Kawarada H. pH-sensitive diamond field-effect transistors (FETs) with directly aminated channel surface. Analytica Chimica Acta. 2006; 3-8:573-574.
-
2Silva GM, Lemos SG, Pocrifka LA, Marreto PD, Rosario AV and Pereira EC. Development of low-cost metal oxide pH electrodes based on the polymeric precursor method. Analytica Chimica Acta. 2008; 616(1):36-41. http://dx.doi.org/10.1016/j.aca.2008.03.019. PMid:18471481
» http://dx.doi.org/10.1016/j.aca.2008.03.019 -
3Bergveld P. Development of an ion-sensitive solid-state device for neurophysiological measurements. IEEE Transactions on Biomedical Engineering. 1970; 17:70-71. http://dx.doi.org/10.1109/TBME.1970.4502688.
» http://dx.doi.org/10.1109/TBME.1970.4502688 -
4Van Der Spiegel J, Lauks I, Chan P and Babic D. The extended gate chemically sensitive field effect transistor as multi-species microprobe. Sensors and Actuators. 1983; 4:291-298. http://dx.doi.org/10.1016/0250-6874(83)85035-5.
» http://dx.doi.org/10.1016/0250-6874(83)85035-5 -
5Chou JC, Chiang JL and Wu CL. pH and procaine sensing characteristics of extended-gate field-effect transistor based on indium tin oxide glass. Japanese Journal of Applied Physics. 2005; 44(7A):4838-4842. http://dx.doi.org/10.1143/JJAP.44.4838.
» http://dx.doi.org/10.1143/JJAP.44.4838 -
6Silva GR, Matsubara EY, Corio P, Roselen JM and Mulato M. Carbon felt/carbon nanotubes/pani as ph sensor. Materials Research Society Proceedings. 2007; 1018:EE1410-1. http://dx.doi.org/10.1557/PROC-1018-EE14-10.
» http://dx.doi.org/10.1557/PROC-1018-EE14-10 -
7Batista PD, Mulato M, Graeff CFO, Fernandez FJR and Marques FD. SnO extended gate field-effect transistor as pH sensor. 2Brazilian Journal of Physics. 2006; 36(2a):478-481. http://dx.doi.org/10.1590/S0103-97332006000300066.
» http://dx.doi.org/10.1590/S0103-97332006000300066 -
8Batista PD and Mulato M. ZnO extended-gate field-effect transistors as H sensors. pApplied Physics Letters. 2005; 87(14):143508-143510. http://dx.doi.org/10.1063/1.2084319.
» http://dx.doi.org/10.1063/1.2084319 -
9Guerra EM and Mulato M. Synthesis and characterization of vanadium oxide/hexadecylamine membrane and its application as pH-EGFET sensor. Journal of Sol-Gel Science and Technology. 2009; 52(3):315-320. http://dx.doi.org/10.1007/s10971-009-2062-7.
» http://dx.doi.org/10.1007/s10971-009-2062-7 -
10Guerra EM, Silva GR and Mulato M. Extended gate field effect transistor using V2O5 xerogel sensing membrane by sol-gel method. Solid State Sciences. 2009; 11(2):456-460. http://dx.doi.org/10.1016/j.solidstatesciences.2008.07.014.
» http://dx.doi.org/10.1016/j.solidstatesciences.2008.07.014 -
11Guidelli EJ, Guerra EM and Mulato M. Ion sensing properties of vanadium/tungsten mixed oxides. Materials Chemistry and Physics. 2011; 125(3):833-837. http://dx.doi.org/10.1016/j.matchemphys.2010.09.040.
» http://dx.doi.org/10.1016/j.matchemphys.2010.09.040 -
12Guidelli EJ, Guerra EM and Mulato M. Vanadium and titanium mixed oxide films: synthesis, characterization and application as ion sensor. Journal of the Electrochemical Society. 2012; 159(6):J217-J222. http://dx.doi.org/10.1149/2.053206jes.
» http://dx.doi.org/10.1149/2.053206jes -
13Guidelli EJ, Guerra EM and Mulato M. E V2O5/WO mixed oxide films as pH-EGFET sensor: sequential re-usage and fabrication volume analysis. 3ECS Journal Solid State Science and Technology. 2012; 1(39):N39-N44. http://dx.doi.org/10.1149/2.007203jss.
» https://doi.org/10.1149/2.007203jss -
14Li X, Zhang Q, Miao W, Huang L, Zhang Z and Hua Z. Development of novel tungsten-doped high mobility transparent conductive In2O thin films. 3Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films. 2006; 24(5):1866-1869. http://dx.doi.org/10.1116/1.2333572.
» http://dx.doi.org/10.1116/1.2333572 -
15Lee D, Nam K and Lee D. Effect of substrate on NO2-sensing properties of WO3 thin film gas sensors. Thin Solid Films. 2000; 375(1-2):142-146. http://dx.doi.org/10.1016/S0040-6090(00)01261-X.
» http://dx.doi.org/10.1016/S0040-6090(00)01261-X -
16Wang G, Ji Y, Huang X, Yang X, Gouma PI and Dudley M. Fabrication and characterization of polycrystalline WO3 nanofibers and their application for ammonia sensing. The Journal of Physical Chemistry B. 2006; 110(47):23777-23782. http://dx.doi.org/10.1021/jp0635819. PMid:17125339
» http://dx.doi.org/10.1021/jp0635819 -
17Chemseddine A, Babonneau F and Livage J. Anisotropic WO. 3·nH2O layers deposited from gels. Journal of Non-Crystalline Solids. 1987; 91(2):271-278. http://dx.doi.org/10.1016/S0022-3093(87)80311-3.
» http://dx.doi.org/10.1016/S0022-3093(87)80311-3 -
18Chien Y-S, Tsai W-L, Lee I-C, Chou J-C and Cheng H-C. A novel ph sensor of extended-gate field-effect transistors with laser-irradiated carbon-nanotube network. IEEE Electron Device Letters. 2012; 33(11):1622-1624. http://dx.doi.org/10.1109/LED.2012.2213794.
» http://dx.doi.org/10.1109/LED.2012.2213794 -
19Temple-Boyer P, Launay J, Humenyuk I, Conto T, Martinez A, Beriet C, et al. Study of front-side connected chemical field effect transistor for water analysis. Microelectronics and Reliability. 2004; 44(3):443-447. http://dx.doi.org/10.1016/j.microrel.2003.10.001.
» http://dx.doi.org/10.1016/j.microrel.2003.10.001 -
20Walczak MM, Dryer DA, Jacobson DD, Foss MG and Flynn NT. pH-dependent redox couple: illustrating the nernst equation using cyclic voltammetry. Journal of Chemical Education. 1997; 74:1195-1197. http://dx.doi.org/10.1021/ed074p1195.
» http://dx.doi.org/10.1021/ed074p1195 -
21Yates DE, Levine S and Healey TW. Site-binding model of the electrical double layer at the oxide/water interface. Journal of the Chemical Society, Faraday Transition 1: Physical Chemistry in Condensed Phases. 1974; 70:1807-1818. http://dx.doi.org/10.1039/F19747001807.
» http://dx.doi.org/10.1039/F19747001807 -
22Huang BR and Lin TC. Leaf-like carbon nanotube/nickel composite membrane extended-gate field-effect transistors as pH sensor. Applied Physics Letters. 2011; 99:023108. http://dx.doi.org/10.1063/1.3610554.
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Publication Dates
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Publication in this collection
Jan-Feb 2015
History
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Received
17 Nov 2013 -
Reviewed
09 Sept 2014