Potentiometric pH Response Research Articles

Directly Using Ti3C2Tx MXene for a Solid-Contact Potentiometric pH Sensor toward Wearable Sweat pH Monitoring.

The level of hydrogen ions in sweat is one of the most important physiological indexes for the health state of the human body. As a type of two-dimensional (2D) material, MXene has the advantages of superior electrical conductivity, a large surface area, and rich functional groups on the surface. Herein, we report a type of Ti3C2Tx-based potentiometric pH sensor for wearable sweat pH analysis. The Ti3C2Tx was prepared by two etching methods, including a mild LiF/HCl mixture and HF solution, which was directly used as the pH-sensitive materials. Both etched Ti3C2Tx showed a typical lamellar structure and exhibited enhanced potentiometric pH responses compared with a pristine precursor of Ti3AlC2. The HF-Ti3C2Tx disclosed the sensitivities of -43.51 ± 0.53 mV pH-1 (pH 1-11) and -42.73 ± 0.61 mV pH-1 (pH 11-1). A series of electrochemical tests demonstrated that HF-Ti3C2Tx exhibited better analytical performances, including sensitivity, selectivity, and reversibility, owing to deep etching. The HF-Ti3C2Tx was thus further fabricated as a flexible potentiometric pH sensor by virtue of its 2D characteristic. Upon integrating with a solid-contact Ag/AgCl reference electrode, the flexible sensor realized real-time monitoring of pH level in human sweat. The result disclosed a relatively stable pH value of ~6.5 after perspiration, which was consistent with the ex situ sweat pH test. This work offers a type of MXene-based potentiometric pH sensor for wearable sweat pH monitoring.

Anodic-cathodic formation of pH-sensitive TiO2-MoOx films on titanium

Abstract A new type of solid-state metal-oxide pH sensor based on TiO2-MoOx films on titanium has been fabricated by plasma electrolytic oxidation in the anodic-cathodic mode. The structural features of the films have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). According to XPS, the surface layers of the resulting films contain up to 5 at.% Mo in the form of oxides (MoO3, Mo2O5 or Mo4O11) and the near-surface layers additionally contain MoO2. The potentiometric pH response of the TiO2-MoOx electrode reveals near-Nernstian behavior, i.e. a linear working range from pH 2 to 12 with a slope of about 64 ± 2 mV/pH. The mixed films are characterized by very fast response time in alkali and acids solutions shorter than 5 s. The response is not affected by the direction of the pH change and the presence of K+ and Na+ ions. The resulting electrodes have been tested as pH sensors in acid-base titration. The relative error in the determination of NaOH using Ti/TiO2-MoOx electrode does not exceed 2.5%. The possibility of using electrodes based on TiO2-MoOx films to accurately determine the alkalinity of industrial waters is shown. In addition, this sensor offers some analytical characteristics, such as high sensitivity, good reproducibility and simple preparation procedure. Compared to a glass electrode, the Ti/TiO2-MoOx electrode has several advantages, including low cost and high stability.

Fabrication and Use of Dual‐function Iridium Oxide Coated Gold SECM Tips. An Application to pH Monitoring above a Copper Electrode Surface during Nitrate Reduction.

AbstractThe fabrication of a gold microelectrode modified with iridium oxide film (IrOx) and its use as tip with a dual function in SECM experiments is reported. The defective structure of the coating onto the microelectrode surface was used as strategy to combine the advantages of both amperometric (for current‐distance determination) and potentiometric (for pH sensing) SECM operation modes. Approach curves, using oxygen and hexaammineruthenium(III) as redox mediators, were obtained without significant loss of the performance and reproducibility of the potentiometric pH response. This allowed the precise positioning of the proposed tip above a substrate in SECM experiments and, subsequently, to monitor pH at the substrate surface. The IrOx modified microelectrode was applied successfully in SECM experiments involving the local proton consumption during the nitrate reduction at a copper cathode surface.

Characterization and application of a new pH sensor based on magnetron sputtered porous WO3 thin films deposited at oblique angles

In this communication we report about an outstanding solid-state pH sensor based on amorphous nanocolumnar porous thin film electrodes. Transparent WO3 thin films were deposited by reactive magnetron sputtering in an oblique angle configuration to enhance their porosity onto indium tin oxide (ITO) and screen printed electrodes (SPE). The potentiometric pH response of the nanoporous WO3-modified ITO electrode revealed a quasi-Nernstian behaviour, i.e. a linear working range from pH 1 to 12 with a slope of about ⿿57.7mV/pH. pH detection with this electrode was quite reproducible, displayed excellent anti-interference properties and a high stable response that remained unaltered over at least 3 months. Finally, a pH sensor was developed using nanoporous WO3-modified screen printed electrode (SPE) using a polypyrrole-modified Ag/AgCl electrode as internal reference electrode. This full solid state pH sensor presented a Nernstian behaviour with a slope of about ⿿59mV/pH and offered important analytical and operation advantages for decentralized pH measurements in different applications.

Electrochemical Characterisation of a Real-Time pH Sensor

Real-time monitoring of pH levels is important for many reasons. Abnormal tissue pH is an indicator of altered cellular metabolism in illnesses such as stroke and cancer. Tumour pH, for example, is more acidic than that of normal tissue in both animals and humans due to elevated levels of anaerobic and aerobic glycolysis in tumours1. In the brain, the release of neurotransmitters result in a series of metabolic processes causing an influx of various ions that result in pH changes in the surrounding matrix2. Transient local pH changes in the brain are important markers of neuronal activity that can be used to follow metabolic processes which underlie the biological basis of behaviour, learning and memory3. In this study a carbon paste electrode (CPE) was modified with a quinone containing diazonium salt by electrochemical deposition. Various deposition conditions were employed to give the optimum and most efficient method. Shown in Figure 1 is a SEM micrograph of a modified CPE alongside the mechanism for the electrodeposition of the diazonium salt onto the electrode. Figure 2 shows a bare CPE cycled in a PBS solution compared to a modified CPE cycled in the same solution. This indicates that the diazonium salt was successfully deposited onto the electrode surface. Potentiometric pH responses of the modified electrodes showed a response slope of −60.36 ± 0.89 mV/pH, n = 23, as shown in Figure 3. The electrode stability was tested for a period of up to 28 days. Modified electrodes were also subjected to treatment with physiologically relevant proteins and lipids to investigate any implications for real-time monitoring of brain pH.

Potentiometric Response Characteristics of Oxide-Coated Gallium Electrodes in Aqueous Solutions

This paper reports on the potentiometric response characteristics of gallium electrodes coated with native or anodic oxide layers in aqueous solutions. Solid metallic gallium electrodes were prepared by cooling liquid gallium monoliths supported either on planar silicon substrates or incorporated within glass capillaries. These metallic gallium electrodes showed potentiometric pH responses with a slope of ca. -38mV/pH in the pH range between 3 and 9. The potentials measured could originate from redox processes involving surface oxide that was formed as a result of reactions between the metallic gallium and water (and/or OH−), as observed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Similar potentiometric measurements were examined for planar and capillary-incorporated gallium electrodes that were anodized in ice-cooled aqueous H2SO4 at 10V. Although the resulting nanoporous anodic oxide-coated electrodes did not initially show potentiometric pH responses, they exhibited pH responses with a smaller response slope (ca. -30mV/pH; pH 3∼9) after potentiometric measurements in a solution of pH ≤ 3. This observation could be explained by the acid-induced dissolution of an oxide-based barrier layer that passivated the underlying gallium electrode. This explanation was compatible with the observation of an additional gallium oxide layer over the nanoporous anodic oxide after the potentiometric pH measurements. Interestingly, in contrast to the metallic gallium electrodes, the anodic oxide-coated electrodes exhibited potentiometric Nernstian responses to SO42− at ≥ 10−4M, reflecting the presence of sulfate doped into the oxide during the anodization. These results provide fundamental knowledge required to design miniaturized electroanalytical probes based on gallium electrodes.

Preparation of Self-Organized Nanoporous Anodic Gallium Oxide and Its Potentiometric Behavior

In this presentation, we will discuss the anodic formation of self-organized nanoporous gallium oxide from solid gallium metal and its potentiometric behavior. Metallic gallium has been utilized to fabricate µm-scale electrodes within polydimethylsiloxane (PDMS)-based microchannels [1,2] and glass capillaries [3] by taking advantage of its low melting point (ca. 30 °C). These electrodes can be prepared by freezing liquid gallium introduced into the space of interest at low temperature. The resulting gallium electrodes have been applied for conductivity [2] and amperometric [3] detection. Recently, we have found the formation of self-organized nanoporous structures by anodizing solid gallium metal [4], as with aluminum and other valve metals (e.g., Ti) [5]. The nanoporous structures could be obtained by anodizing solid gallium metal in ice-cooled 4 and 6 M aqueous H2SO4 at 10 V and 15 V. The diameters of the resulting nanopores have a narrow diameter distribution (relative standard deviation: ca. 10-20%), and could be controlled in the range of 18-40 nm by varying the H2SO4 concentration and anodization voltage. The EDX data of the materials indicate that the nanoporous materials were based on gallium oxide (Ga2O3) with sulfate as an impurity introduced during the anodization. Such nanoporous structures could be formed on the surface of solid gallium metal introduced at the end of a glass capillary, indicating the capability to integrate nanoporous monoliths with microfluidic channels.More recently, we have investigated the potentiometric pH responses of the nanoporous anodic oxide-coated gallium electrodes [6]. This project was initiated under the hypothesis that, due to the presence of the impurity sulfate, the physicochemical properties of the anodic gallium oxide is different from that of common gallium oxide that are known as a wide bandgap semiconductor (~ 4.8 eV). The oxide-coated gallium electrodes exhibited potentiometric pH responses (i.e., a potential increase at lower pH) only after potentiometric measurements in a solution of pH ≤ 3. This observation suggests that the anodic oxide layer is not conductive enough for reliable potentiometric measurements, and thus needs to be dissolved in an acidic solution to measure a potential defined by redox reactions involving the underlying gallium electrode. The response slope was smaller than that measured at gallium electrodes coated with much thinner native oxide layers, reflecting the sluggish redox reaction at the anodic-oxide coated electrodes. The corrosion of the gallium oxide and gallium metal limits their applicability for analytical devices. These results provide fundamental knowledge required to develop miniaturized electrodes based on nanoporous anodic oxide-coated gallium. This work was primarily funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DE-FG02-12ER16095). The authors also acknowledge Terry C. Johnson Center for Basic Cancer Research and Targeted Excellence Funds of Kansas State University for partial financial support of this work.

A single-walled carbon nanotube thin film-based pH-sensing microfluidic chip

A novel microfluidic pH-sensing chip was developed based on pH-sensitive single-walled carbon nanotubes (SWCNTs). In this study, the SWCNT thin film acted both as an electrode and a pH-sensitive membrane. The potentiometric pH response was observed by electronic structure changes in the semiconducting SWCNTs in response to the pH level. In a microfluidic chip consisting of a SWCNT pH-sensing working electrode and an Ag/AgCl reference electrode, the calibration plot exhibited promising pH-sensing performance with an ideal Nernstian response of 59.71 mV pH(-1) between pH 3 and 11 (standard deviation of the sensitivity is 1.5 mV pH(-1), R(2) = 0.985). Moreover, the SWCNT electrode in the microfluidic device showed no significant variation at any pH value in the range of the flow rate between 0.1 and 15 μl min(-1). The selectivity coefficients of the SWCNT electrode revealed good selectivity against common interfering ions.

Effect of Nanoporous Structure on Enhanced Electrochemical Reaction

Geometric factors affecting the enhanced electrocatalysis on nanoporous Pt (L2-ePt) were examined by electrochemical methods and computer simulations. The experimental results revealed that the electrochemical enhancement of O2 and H2O2 does not come only from expansion of the active surface area (so-called roughness factor, fR) of L2-ePt. The presence of extra contribution was verified by the fact that significant enhancement in electrocatalytic reactions remained even after the effect of the fR was eliminated from the electrochemical redox behavior of O2 and H2O2 on L2-ePt electrodes. Not only the voltammetric observation but also potentiometric pH responses of L2-ePt suggested the presence of unique nanoporous effects other than the surface enlargement in regard to heterogeneous electrochemical reactions. L2-ePt showed near Nernstian behavior, faster response time, and less hysteresis even if the real surface area was smaller than that of flat Pt. Increased residence time near the electrode surface due to extremely confined space of nanoporous structure was proposed as possible origins and examined by the Monte Carlo simulations of simple model electrodes. The theoretical approaches indicated that long residence time of reactant at electrode surface by confinement effect of the nanoporous environment well accounted for the experimental results.

Effects of polypyrrole modified electrode functionalization on potentiometric pH responses

Two functionalized polypyrrole films, poly(11-N-pyrrolylundecanoic) acid and poly(N-undecylpyrrole), were used as sensitive layers for pH sensors. The functionalized pyrrole monomers were first synthesized before carrying out their electropolymerization onto non-oxidizable electrodes. The deposits were characterized by SEM imaging, XPS and PM-IRRAS to study surface structure and composition. Poly(11-N-pyrrolylundecanoic) acid coating yielded very promising results as sensitive layer in pH sensors. Indeed, the potentiometric responses from pH 4.0 to 9.0 appeared linear, reversible, stable in time over a period of 1 month and highly selective for H + ions. This behavior was attributed to carboxylic acid groups which are sensitive to proton concentration changes. This was confirmed by the fact that poly(N-undecylpyrrole) film led to potentiometric responses being pH dependent only from pH 6.0 to 9.0 with low sensitivity. Poly(11-N-pyrrolylundecanoic) acid is thus promoted to be used as sensitive layer in pH sensors.

A solid-state pH sensor based on WO 3-modified vertically aligned multiwalled carbon nanotubes

In this communication, a novel solid-state pH sensor based on WO 3/MWNTs nanocomposite electrode will be reported. WO 3 nanoparticles were homogeneously coated on vertically aligned MWNTs by magnetron sputtering. Potentiometric pH response of the WO 3/MWNTs electrode in Britton–Robinson buffers revealed a linear working range from pH 2 to12 with a slope of about −41 mV pH −1 and a response time less than 90 s. The stability of the electrode remained over a month. Moreover, the WO 3/MWNTs electrode displayed excellent anti-interference property. Compared to conventional pH sensors, the pH sensor based on WO 3/MWNTs nanocomposite electrode also showed excellent reproducibility, high stability and superb selectivity.

A novel pH sensor based on hydroquinone monosulfonate-doped conducting polypyrrole

Abstract Functionalized polypyrrole (PPy) with hydroquinone monosulfonate (HQS) incorporated as the dopant has been prepared by a simple one-step electropolymerization of pyrrole at a stainless steel electrode from aqueous solution. Potentiometric pH responses of the HQS-doped PPy electrodes showed a response slope of −50.54 ± 1.67 mV/pH (28 °C), a linear working range of pH 2–12 and a response time less than 100 s. The electrode stability was maintained over the period of a month. Compared with other PPy-based pH electrodes reported previously, the HQS-doped PPy electrode shows significantly improved potentiometric response slope, response time, reproducibility and stability. Interference studies with several ions showed minimal effects on the potentiometric response of the modified electrode. A combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectrometry (ToF-SIMS) was performed to investigate the surface composition and characteristics of the electrodes, including the chemical basis of the electrode performance. Cyclic voltammetry revealed the expected electrochemical response of the HQS, which was electroactive in response to pH changes. HQS-doped PPy shows excellent potential as a novel pH sensor, incorporating both the electroactive species and conducting support in an integrated form, for a variety of applications requiring pH monitoring.

Conditioning treatment to improve the potentiometric pH response of polydiphenylamine modified electrodes

The potentiometric pH response of gold electrodes, modified with polydiphenylamine (PDPA) films was studied. These were obtained by electropolymerization of saturated diphenylamine in 3.7 M HClO 4 solutions by cycling the working electrode potential between −0.10 and 0.65 V versus saturated calomel electrode (SCE). The PDPA modified electrodes were subjected to a conditioning treatment consisting of a sequence of electrochemical procedures performed in 1 M H 2SO 4 solutions. This treatment improves the linearity and sensitivity of the pH response and enlarges to 0 ≤ pH ≤ 11 the measuring range. The potential versus pH slope is 0.041 V/pH unit; this value is due to the fact that the conditioning treatment produces an effective p K distribution that is much wider that in the as made PDPA films.

Potentiometric pH responses of fibrillar polypyrrole modified electrodes

The potentiometric pH responses of novel fibrillar polypyrrole modified electrodes were studied. The effects of the film thickness and the scan rate were examined. The results show that the film thickness has little effect on the potentiometric response for the one-dimensional growth pattern of polypyrrole. This makes it easy to obtain electrodes with the same potentiometric pH response slope. The scan rate influences the potentiometric pH response. The highest response slope is 53.0mV/pH, which was measured on the electrode prepared at a scan rate of 25mV/s.

PH sensitivity of polyaniline and its substituted derivatives

The pH sensitivity of electrically conducting polyaniline (PANI) and its derivatives poly( o-methylaniline) (P oMeANI), poly( o-ethylaniline) (P oEtANI), poly( o-propylaniline) (P oPrANI) and poly( N-methylaniline) (PNMeANI) have been studied with potentiometry and UV–vis spectroscopy. The electropolymerisation of PANI was carried out either in hydrochloric acid (HCl), methanesulphonic acid (MSA), benzenesulphonic acid (BSA) or dodecylbenzenesulphonic acid (DBSA). It is shown that the pH sensitivity of PANI and its derivatives depend on the substituent and the size of the acid anion that is used in the electropolymerisation. PANI membranes prepared in HCl show a selective and slightly super-Nernstian potentiometric pH response (62.4±0.9 mV per pH; pH 2–9), while PNMeANI, P oPrANI and PANI prepared with DBSA have a strongly suppressed pH response. The hysteresis effect observed in the UV–vis measurements for all membrane types studied restrict the use of PANI-based optical pH sensors to the pH range of only 5–8.

Potentiometric pH response of polymer membranes incorporated with ion-exchangers.

Potentiometric pH response of polymer membranes incorporated with ion-exchangers.

Effects of polymer thickness on the potentiometric pH responses of polypyrrole modified glassy carbon electrodes

The potentiometric pH response of electropolymerized polypyrrole modified glassy carbon electrodes as a function of polymer film thickness was examined. Electrodes coated with thicker polypyrrole films showed mixed potentiometric responses toward both cationic and anionic species, while thinner polypyrrole films (<2 μm) showed good linear response to pH changes for the pH range between 2 and 12. The possible mechanism for such behavior was discussed.

The improved potentiometric pH response of electrodes modified with processible polyaniline. Application to glucose biosensor

Processible polyaniline (PCPAn) modified electrodes are characterized by an advanced potentiometric pH response in comparison to those based on regular polyaniline. Glassy carbon electrodes modified with PCPAn by dip-coating exhibited a fully reversible potentiometric response of approximately 90 mV pH–1 over the range pH 3–9. Such significantly higher potentiometric responses of PCPAn modified electrodes compared to existing devices is explained on the basis of the thermodynamics of polyaniline redox reactions. The potentiometric biosensor for glucose based on processible polyaniline has been developed using a non-aqueous enzymology approach for enzyme immobilization. In the model solution, which mimics blood serum, the biosensor was useful for glucose detection over the concentration range 0.1–30 mM and the maximum response value reached was ≈80 mV. The advanced potentiometric response of PCPAn modified electrodes provides their application for sensor and biosensor development.

Potentiometric pCO2 sensor using polyaniline-coated pH-sensitive electrodes

A very simple method of constructing a differential pCO2 sensor device is described, using polyaniline-based pH electrodes in place of the pH-ISFET-based system employed previously. The polyaniline film-coated Pt electrode was shown to exhibit not only an enhanced potentiometric pH response but also a greatly reduced oxygen sensitivity compared with the uncoated Pt electrode. The proposed differential system employs two identical polyaniline electrodes to make a pCO2 probe and a reference electrode. Both electrodes are made by coating the polyaniline surface with a gas-permeable silicone rubber membrane doped with valinomycin. The pCO2 electrode is covered with a hydrogel-based recipient layer inside the gas-permeable membrane whereas the reference electrode does not have the hydrogel layer. In this sensor configuration, the emf differences between the pCO2 and the reference electrodes serve as analytical signals and hence the ion responses caused by the two parts of the outer gas-permeable membranes cancel out. The polyaniline-based gas sensor system was shown to provide better emf stability than the uncoated Pt electrode-based counterpart.

The potentiometric pH response of chromoionophores used in optical sensors

Recently, we reported the advantageous properties of some H+-selective electrodes based on a novel class in ionophores [1] which were mainly used as chromoionophores in optical sensors. The performances of solvent polymeric membranes containing three similar phenoxazine derivatives (ETH 2439, ETH 5294 and ETH 5350, commercial products from Fluka AG, Buchs, Switzerland) were studied and compared with H+-selective glass electrodes and membranes with tridodecylamine (TDDA) as the ionophore. These electrodes showed the expected properties (linear range, selectivity and response time) except for the one with the membrane containing ETH 5350. A considerable loss of this ionophore from the membrane phase cast with polar (2-nitrophenyl-octylether, o-NPOE) or nonpolar (bis(1-butylpentyl)adipate, BBPA) plasticizer into the sample solution was observed. Consequently, we reported that electrodes constructed with the ETH 5350 carrier were not satisfactory for pH measurement [1]. It can be estimated from the constitution of the three phenoxazine derivatives that their partition coefficients between the organic membrane and water are rather similar. Judging from a theoretical estimation according to the Hansch parameters [2], ETH 5350 tends to be even more lipophilic than the other two phenoxazines shown in Fig. 1. Therefore the observed differences in the dissolu-