Self-calibration of a polyaniline nanowire-based chemiresistive pH sensor

doi:10.1016/j.mee.2013.10.014

Microelectronic Engineering

Volume 116, 25 March 2014, Pages 26-32

https://doi.org/10.1016/j.mee.2013.10.014 Get rights and content

Highlights

•
Polyaniline nanowire network-based chemiresistive pH sensor was fabricated.
•
A three step mechanism for self-calibrating pH measurement is proposed.
•
The proposed method was able to measure the pH in the range 1–6.
•
Repeatable pH measurements were achieved even under the influence of degradation.

Abstract

This paper reports on a new method to measure the pH of the solution with repeatability and low hysteresis using a polyaniline nanowire-based chemiresistive sensor. Polyaniline’s natural tendency to irreversibly lose its conductivity, especially when frequently exposed in solutions with pH greater than 4, has prevented its advancement as a reusable pH sensor. In the presented method, the polyaniline is electrochemically reduced prior to each measurement in order to reset the doping level of the polymer. Then, the potential of the polyaniline is linearly scanned, from a fully reducing (V_R) to a partially oxidizing (V₀) potential, and the resulting current waveforms are used to determine the pH. Here, we demonstrate that by comparing the current values at V₀, the solution pH in the range 1–3 can be determined, and by comparing the peak values of the current responses from the two consecutive voltage sweep cycles, the pH of the solutions in the range 4–6 can be distinguished. Experimental results show that the proposed technique gives repeatable pH measurements even as the polyaniline nanowires undergo conductivity degradation.

Graphical abstract

Introduction

Conducting polymer nanowires (CPNWs) have emerged as a viable alternative to metal or oxide-based nanowires due to many advantages that CPNWs offer especially in sensor applications. These advantages include mechanical flexibility, low cost, ease of synthesis, relatively benign synthesis environment, and site-specific growth capability [1], [2]. Among the reported conducting polymer nanowires, a polyaniline nanowire is an excellent candidate to be used as a nanowire-based sensor because of its unique chemical properties including reversible redox chemistry, simple doping mechanism, and large conductivity change upon protonation [3], [4], [5], [6], [7]. In particular, its ability to change its conductivity by a few orders of magnitude upon pH change can be exploited for developing a conductometric or chemiresistive type sensors. Chemiresistive sensors have the benefit of being simple in structure and easy to be miniaturized to give high density and high throughput sensor arrays. Polyaniline nanowires are well-suited for chemiresistive sensors because of their unique one dimensional nanostructure leading to large conductance change upon pH change and large surface area-to-volume ratio resulting in high sensitivity [8], [9]. Hence, much work has been done for the implementation of polyaniline nanowire-based sensors demonstrating the feasibility as chemiresistive sensors for the detection of pH [10], [11], [12], [13], gas [14], [15], and biomolecules [16], [17] with good sensitivity and fast response.

The two main factors that determine the conductivity of polyaniline are its redox state and the degree of protonation also known as the doping level [18]. The redox state of polyaniline can vary from the fully reduced leucoemeraldine to the fully oxidized pernigraniline with partially oxidized emeraldine state in between. Both leucoemeraldine and pernigraniline form of polyaniline are electrically insulating, and only the partially oxidized emeraldine form exhibits conductivity when protonated due to the formation of delocalized radical cations which give polyaniline its conductive nature [33], [34]. As the doping level increases, more radical cations are formed on the polymer backbone structure to increase its conductivity. In other words, lowering the pH of the environment increases the conductivity of polyaniline. However, polyaniline is known to be completely insulating in neutral pH or in base solution.

Although polyaniline has demonstrated the potential to be used as an effective chemiresistive sensing material, polyaniline has three inherent problems: loss of conductivity in neutral or base environment [10], [11], conductivity degradation over time, and hysteresis in conductivity by the previous redox state. Much effort has been put into developing the modified polyaniline material that extends its electroactivity to the neutral pH region with success [19], [20], [21], [22], [23], however the degradation and hysteresis issues have been overlooked.

The conductivity of polyaniline irreversibly decreases over time when it is exposed to high pH solutions (e.g. pH 4 or higher) due to inevitable degradation of the polymer [10], [24], [25]. This degradation of conductivity cannot be restored even if the polymer is re-exposed to a strong acid solution. Possible causes for this irreversible conductivity degradation are structural damage due to mechanical stretching or twisting of the polymer chain caused by electrostatic charge of the dopant, and structural damage caused by the generation of quinone–hydroquinone couples [24], [25]. Therefore, if the sensor is to be used repeatedly, the conductivity of polyaniline must be calibrated frequently to compensate for the conductivity degradation.

The conductivity of polyaniline is known to exhibit a hysteresis effect, which is evident from the current vs. potential sweep characteristics [16], [26], [27], [28] where, for a fixed pH, the current response to the potential sweep in the positive direction is different from that to the reverse sweep of the potential. This indicates that polyaniline has a ‘memory effect’ where its conductivity is influenced by the previous redox state. The existence of hysteresis is more closely related to having a different level of doping at a given electrochemical potential of the polymer [29] and has been attributed to structural changes caused by Coulombic repulsions [28]. Therefore in order to obtain a hysteresis-free reading of the conduction current of polyaniline, one must initialize or reset the doping level of the polyaniline prior to each measurement such that the device would yield a reliable current reading. In this paper, we present a polyaniline-based chemiresistive pH sensor with a self-calibrating mechanism that gives repeatable measurements by minimizing the degradation and hysteresis effect.

Section snippets

Working principle

A polyaniline nanowire-based chemiresistive sensor consists of a reference electrode, a counter electrode, and two working electrodes connected through a polyaniline nanowire network as depicted in Fig. 1(a). A silver/silver chloride (Ag/AgCl) electrode is used as the reference electrode and all voltages are given with respect to this reference unless otherwise noted. A potentiostat is used to maintain the potential of the polyaniline at V_B. A differential voltage of V_D between the two working

Device fabrication and experimental methods

A microscale polyaniline nanowire-based chemiresistive sensor was designed and fabricated as shown in Fig. 4. The gold electrodes were patterned with conventional photolithography on glass slides. The device contains four separate electrodes, an on-chip miniaturized silver/silver chloride reference electrode, a counter electrode, and two working electrodes separated by a 5 μm gap. This gap was eventually bridged by the growth of a polyaniline nanowire network on the working electrodes and the

Results and discussion

A scanning electron microscope (SEM) image of the synthesized polyaniline nanowires is shown in Fig. 5. The elongated one dimensional morphology of polyaniline shows the high surface area of the nanowire network, which is suitable for sensor applications. High porosity of polyaniline network facilitates the diffusion of protons and anions into the network for faster response of the sensor compared to a conventional two dimensional film layer [14], [35].

The fabricated sensor was allowed to be

Conclusions

In conclusion, a chemiresistive pH sensor based on polyaniline nanowires has been developed and a new approach to obtaining a repeatable measurement covering a pH range from 1 to 6 has been demonstrated. The presented self-calibrating mechanism avoids the need to reset the device via a rinsing step. Two parameters have been suggested for use to measure pH: P₁, the ratio of the peak current values of the two consecutive potential sweep cycles, and P₂, the ratio of the current at V_B = V₀ with

References (35)

M.A. Bangar et al.
Thin Solid Films
(2010)
D. Zhang et al.
Mater. Sci. Eng., B
(2006)
A.A. Syed et al.
Talanta
(1991)
F. Patolsky et al.
Mater. Today
(2005)
A. Mulchandani et al.
Curr. Opin. Biotechnol.
(2011)
A.G. MacDiarmid et al.
Synth. Met.
(1995)
L.V. Lukachova et al.
J. Electroanal. Chem.
(2003)
T. Kobayashi et al.
J. Electroanal. Chem. Interfacial Electrochem.
(1984)
M. Nechtschein et al.
Synth. Met.
(1989)
V. Gupta et al.
Electrochem. Commun.
(2005)

M.A. Bangar et al.

Anal. Chem.

(2009)

J. Wang et al.

Adv. Polym. Technol.

(2013)

A.G. MacDiarmid et al.

Faraday Discuss.

(1989)

W.-S. Huang et al.

Faraday Trans.

(1986)

E.W. Paul et al.

J. Phys. Chem.

(1985)

W.W. Focke et al.

J. Phys. Chem.

(1987)

J. Wang et al.

Nano Lett.

(2004)

Cited by (28)

Fabrication of LaF<inf>3</inf> passivated porous silicon pH sensor by deep eutectic solvent based novel chemical route
2024, Sensing and Bio-Sensing Research
Lanthanum fluoride (LaF₃) and other rare-earth halides are commonly used in sensor and optical equipment. A deep eutectic solvent (DES) based LaF₃ passivated porous silicon (PS) sensor for sensing the pH of buffer solution has been fabricated in this research. An electrochemically etched PS structure has been prepared and passivated with novel DES-based LaF₃ to fabricate pH sensor. For the deposition of LaF₃ thin film to create heterostructure devices, a variety of complex and costly processes have been employed. In a quest to find a facile technique to fabricate a pH sensor, this research presents the justification of sensing characteristics of DES-based spin-deposited LaF₃/PS structure. Energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analyses both supported the almost stoichiometric and highly crystalline nature of LaF₃. Scanning electron microscopy (SEM) was used to confirm the uniform morphological structure. The sensing characteristics of fabricated pH sensors have been confirmed by studying the capacitance-voltage (CV) characteristics of prepared structure in a different buffer solution with different pH values. From the experimental results, it was observed that DES-based LaF₃ passivated PS structure shows voltage shift at the inflexion point of CV characteristics for different pH solutions. In light of this, it can be said that a pH sensor for sensing the pH levels of any buffer solution may be fabricated by depositing novel DES-based LaF₃ both inside the pores of PS and on top of it using the spin coating approach.
Au-decorated electrochemically synthesised polyaniline-based sensory platform for amperometric detection of aqueous ammonia in biological fluids
2022, Electrochimica Acta
Citation Excerpt :
Artificial saliva was used to simulate a biological fluid and a source of different ions (K+, Cl−, Na+, Ca2+, PO43+), which could affect the PANI. As PANI is a pH-sensitive material [57] two artificial salivas with different pHs were tested. The recovery rates for both solutions were comparable.
Ammonia (NH₃) present in biological fluids is biomarker for several disease states. In this work the well-known interaction between polyaniline (PANI) and NH₃ was used for the fabrication of an amperometric sensory platform to detect aqueous NH₃ at neutral i.e. biological pH. Understanding PANI's electrochemical synthesis and redox behaviour in acidic and neutral media was used to determine the NH₃ detection mechanism. The latter is based on the PANI deprotonation reaction, NH₄⁺ oxidation and follow-up PANI reduction and oxidation. Chronoamperometry was applied as an NH₃-detection method in a 50 µL background electrolyte in which a 1 µL sample was injected, triggering instantaneous changes to the current. The sensory platform's detection limit, based on pure electrochemically synthesised PANI (PANI_el) (24.64 µM), was reduced 17 times (1.44 µM) with the addition of 20 nm Au nanoparticles. This Au-decorated PANI_el sensory platform showed excellent reversibility, reusability and the possibility for continuously cycled NH₃ measurements, with a recovery rate of 90–99.5% for artificial saliva samples of different pHs. This demonstrates its suitability for more complex samples. The developed sensory platform presents potential from the industry point of view, as a base for an NH₃ batch injection analysis and for preventive home medical self-care.
Well-established materials in microelectronic devices systems for differential-mode extended-gate field effect transistor chemical sensors
2016, Microelectronic Engineering
Citation Excerpt :
The use of the described materials in an electronic circuit designed to form an EGFET sensor in the differential mode of operation shows their various possibilities for use in microelectronics systems. Those materials are advantageous for use due to their processing and fabrications techniques in miniaturized systems, as described for PANI in [30]. The use of an interesting sensor with well-established materials is proposed in a differential mode pH instrumental amplifier extended gate field effect transistor (D-IA-EGFET) system based on microelectronic devices.
Well-established materials commonly used in microfabrication industry were studied as chemical sensing elements of extended gate field effect transistor (EGFET) chemical sensors. Microelectronic components were used in the differential mode of operation. Three differential pairs of films made of polyaniline (protonated and non-protonated), fluorine doped tin oxide and titanium dioxide were used: PANI-EB × PANI-ES, PANI-EB × FTO and FTO × TiO₂. Good linearity was observed for all cases in the pH range 3 to 8. The differential measurements were also performed as a function of: i) temperature in the range 30 °C to 50 °C, ii) buffer concentration from 1 mM to 400 mM, and iii) time, for three hours. Hysteresis measurements for the 5-2-5-8-5 pH loop cycle were also conducted. The pair composed of polyaniline in the protonated and non-protonated form presented the best results with a sensitivity of 37 ± 4 mV/pH, good linearity, and stable response against variation of temperature and buffer concentration. A similarity in ion-sensing mechanisms and electrical properties of the single films is necessary for the fabrication of good and stable differential sensors as will be discussed. These materials can be satisfactorily used in miniaturized electronic sensor systems, thus promising great advances in the field.
Inkjet printing of conductive polymer nanowire network on flexible substrates and its application in chemical sensing
2015, Microelectronic Engineering
Citation Excerpt :
Polyaniline exhibits maximum conductivity under a strongly acidic environment (low pH) and loses its conductivity at neutral or basic environment (pH 7 or higher). Therefore chemiresistive polyaniline-based pH sensors have been extensively studied and documented [8–10]. Inkjet printing of polyaniline nanoparticles and nanograins have been previously reported and the development of an ammonia sensor was demonstrated [4,11].
This work reports an inkjet printing technique for patterning a conducting polymer nanowire network on a flexible film for applications in chemical sensing. The novelty of this work is in the patterning capability of polymer nanowires to form a conducting path. Polyaniline nanowires were chemically synthesized in an aqueous solution and a surfactant was added to lower the surface tension which enabled the printing of the nanowires using a commercially available inkjet printer. The nanowire network-based patterns were printed on a flexible transparency film, and its morphology characterization, patterning ability as well as the electrical properties were investigated. Finally, as a proof-of-concept, a fully-printed chemical sensors were developed by using the proposed printing technique on flexible films. Two types of sensors were fabricated: a pH sensor and a hydrogen peroxide sensor. The results demonstrate that the developed sensors can be utilized as a low cost, disposable, and easily printable chemical sensors. The proposed technology may find applications in the development of a simple print-and-use biochemical sensing kit for potential use in point-of-care diagnostics.
Multi-analyte detection of chemical species using a conducting polymer nanowire-based sensor array platform
2015, Sensors and Actuators, B: Chemical
Citation Excerpt :
More importantly, the conductivity of polyaniline is affected by the proton concentration, i.e., the pH, whereby the polymer exhibits maximum conductivity when exposed to the strong acidic environment (low pH), however loses its conductivity entirely at neutral or basic environment (high pH). In other words, polyaniline can be utilized as a chemiresistive sensor whose conductance measurement can be used as a pH indicator [13–16]. However, polyaniline-based chemiresistive sensing of other chemical species has been a challenge since its conductivity is minimally influenced by most chemical species.
This paper proposes a novel sensing platform for a conducting polymer nanowire-based chemiresistive sensor array for multi-analyte detection capability. In order to create a cross-reactive sensor array with each individual sensor having partial selectivity, distinct catalytic nanoparticles are incorporated into each sensing element resulting in a unique response from each sensor when exposed to a specific analyte. Based on the chemiresistive signal responses from the sensor array, a pattern recognition algorithm is applied to simultaneously differentiate one chemical species from another within a sample containing a mixture of analyte species. As a demonstration of the concept, the proposed sensor array was tested for the simultaneous classification and quantification of three chemical species: ascorbic acid, dopamine, and hydrogen peroxide. A highly conductive polyaniline nanowire network was used as a generic chemiresistive sensing platform for the device. Each sensor in the array responded distinctly with partial selectivity to each analyte, and hence provided a unique “fingerprint” response for the given composition of the sample solution. The results suggest that this technique can be applied either as an electronic nose or as an electronic tongue for the simultaneous classification and quantification of individual species within a mixture of a large number of chemical analyte.
High performance supercapacitor based on multilayer of polyaniline and graphene oxide
2015, Synthetic Metals
Citation Excerpt :
Conducting polymers have received great research interest in the fields of nanoscience and nanotechnology due to their high conductivity and redox behavior [1,2]. They are widely used for variety of applications such as light emitting display devices, energy conversion, supercapacitors, and sensors [3–8]. Among various conducting polymers, polyaniline (PANI) is quite attractive because of its different electronic and electrochemical properties in the reduced and oxidized states.
Multilayer films of polyaniline (PANI) and graphene oxide (GO) were deposited on indium tin oxide (ITO) electrode for supercapacitor application. The graphene oxide was synthesized using chemical method. The X-ray diffraction study confirms the formation of graphene oxide. The supercapacitive behavior of the PANI and PANI–GO electrodes were studied using cyclic voltammetry (CV) and galvanostatic charge–discharge techniques in 1 M H₂SO₄ solution. The electrochemical analysis confirms that the multilayer film has improved specific capacitance compared to pure polyaniline film. The specific capacitance of 201 F/g and 429 F/g was achieved for PANI and PANI–GO multilayer electrode. High electrochemical performance of the PANI–GO electrode could be due to increasing active sites for the deposition of polyaniline provided by large surface areas of graphene oxide sheets and the synergistic effect between polyaniline and graphene oxide. These results demonstrated the importance and great potential of graphene oxide in the development of high-performance energy-storage system based on polyaniline.

View all citing articles on Scopus

View full text

Self-calibration of a polyaniline nanowire-based chemiresistive pH sensor

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Working principle

Device fabrication and experimental methods

Results and discussion

Conclusions

Thin Solid Films

Mater. Sci. Eng., B

Talanta

Mater. Today

Curr. Opin. Biotechnol.

Synth. Met.

J. Electroanal. Chem.

J. Electroanal. Chem. Interfacial Electrochem.

Synth. Met.

Electrochem. Commun.

Anal. Chem.

Adv. Polym. Technol.

Faraday Discuss.

Faraday Trans.

J. Phys. Chem.

J. Phys. Chem.

Nano Lett.