super-Nernstian Response Research Articles

Amorphous ZnSn x O y Fabricated at Room-Temperature for Flexible pH-EGFET Sensor

In this article, the zinc-tin oxide (ZnSn x O y ) based sensing membrane fabricated on a flexible polyethylene terephthalate substrate at room temperature as an extended-gate field-effect transistor (EGFET) is investigated for pH-sensing applications. The ZnSn x O y -based EGFET sensor showed a super-Nernstian pH response (70.79 mV/pH) with very good linearity (0.997), a low hysteresis voltage (6 mV), and a low drift rate (0.25 mV/h). The high pH sensing behavior is attributed to the redox reaction of amphoteric zinc oxide (ZnO) in different pH solutions. Furthermore, good flexibility and bendability were shown by the ZnSn x O y -based EGFET sensor after 400 bending cycles.

Microstructural properties and sensitive performance of TbTaxOy sensing membranes for electrolyte-insulator-semiconductor pH sensors

The sensitivity and stability of sensing film material are mainly factors to impede the practical applications of ion-selective field effect transistor (ISFET) or electrolyte-insulator-semiconductor (EIS) pH sensors. Metal oxide films have demonstrated great advances on achieving high-performance pH sensors. However, some metal oxide films with low sensitivity and poor reliability still hinder the biochemical, environmental and medical applications. Therefore, choosing a proper sensing film material plays a very important role for the fabrication of highly sensitive and reliable sensors. In this article, the novel TbTaxOy sensing membrane demonstrating a super-Nernstian pH response and excellent stabililty was developed for a solid-state pH sensor. We investigated the relationship between sensing performance and microstructural property of the TbTaxOy sensing membranes deposited onto Si substrates through a reactive co-sputtering system and subsequently post-deposition annealing (PDA) at four temperatures (from 600°C to 900°C). To check the influence of PDA treatment on the microstructural properties of the TbTaxOy films, atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were employed to manifest the morphological, microstructural, and chemical features, respectively, of these films. Among these PDA temperatures, the TbTaxOy EIS device after annealing at 900 °C exhibited the best sensing performances including, the super-Nernstian sensitivity (67.04 mV/pH), the lowest drift rate (0.12 mV/h), and the smallest hysteresis voltage (1 mV). Probably, this PDA temperature can enhance the surface roughness, form the stoichiometry of TbTaO4 film, and reduce the crystal defects. The super-Nernstian response may be contributed to a variation in the oxidation state of Tb ion changing from Tb3+ to Tb2+, thereby generating two H+ ions and one electron transferred in the chemical reaction. Moreover, this TbTaxOy sensing film demonstrates better sensing performance compared to previously fabricated sensing materials available in the literature.

Iridium oxide nanoparticles modified carbon fiber microelectrodes for pH detection

<p indent=0mm>pH detection plays an important role in agriculture, food processing, environment protection and disease diagnosis. Electrochemical methods are widely used in pH sensing owing to their fast response, high sensitivity and simple to operate. But commercial pH sensors can only work in relatively stable in vitro environment rather than in vivo environment because of their large size and fragile proton sensitive membrane. In this work, we modify iridium oxide nanoparticles on the carbon fiber microelectrode, and fabricate a miniature, implantable solid pH sensor. The pH sensor has super-Nernstian pH response (its sensitivity can reach 65.5 mV/pH), wide linear range (pH≈2–12) and high stability. Finally, we implant the pH sensor into the fruit tissue and achieve the detection of pH in the fruit.

Super-Nernstian pH Sensor Based on Anomalous Charge Transfer Doping of Defect-Engineered Graphene.

The conventional pH sensor based on the graphene ion-sensitive field-effect transistor (Gr-ISFET), which operates with an electrostatic gating at the solution-graphene interface, cannot have a pH sensitivity above the Nernst limit (∼59 mV/pH). However, for accurate detection of the pH levels of an aqueous solution, an ultrasensitive pH sensor that can exceed the theoretical limit is required. In this study, a novel Gr-ISFET-based pH sensor is fabricated using proton-permeable defect-engineered graphene. The nanocrystalline graphene (nc-Gr) with numerous grain boundaries allows protons to penetrate the graphene layer and interact with the underlying pH-dependent charge-transfer dopant layer. We analyze the pH sensitivity of nc-Gr ISFETs by adjusting the grain boundary density of graphene and the functional group (OH-, NH2-, CH3-) on the SiO2 surface, confirming an unusual negative shift of the charge-neutral point (CNP) as the pH of the solution increases and a super-Nernstian pH response (approximately -140 mV/pH) under optimized conditions.

A Potentiometric Sensor Based on a Copper(II) Ion Composite for Vitamin C Determination in Urine

We developed an all-solid-state sensor for monoascorbate ion based on a composite containing copper(II) ions (5 wt %), graphite and epoxy resin. The sensor presented super-Nernstian response at room temperature under continuous bubbling of oxygen gas in sample. The detection limit of the sensor was 0.02 mM and its linear range was between 0.05 and 1.5 mM with upper limit of quantification equal to 3 mM. The leaching out effect of the active component from composite was estimated less than 11 µg of copper(II) sulfate per day under continuous diffusion process in deionized water. Photomicrographs of the electrode surface did not show the presence of salt particles on the surface indicating that copper(II) ions could be intercalated in graphene layers. Attenuated total reflectance infrared spectrum demonstrated this possibility. The sensor was successfully applied to the in vitro determination of L-ascorbic acid in urine.

Kinetic Description of the Membrane-Solution Interface for Ion-Selective Electrodes.

The theoretical models for ISEs almost exclusively assume thermodynamic equilibrium at the membrane/solution-phase boundary. In this report, we present a new, congruent model which combines first-order reaction kinetics of ion-exchange at the phase boundary and diffusional mass transport in the adjoining phases in the continuity equation. The influence of the rate constant in the new kinetic model has significant impact on the predicted transients corresponding to instantaneous change in the sample solution composition. The simulated transients generated with the new model coincide with the transients recorded in common potentiometric experiments, e.g., with transients recorded upon step change in the primary or interfering ion concentrations. The simulated transients also align well with previously published transients representing special cases of potentiometry (e.g., super-Nernstian response, non-Nernstian responses in the presence of highly interfering ions). The implementation of the kinetic model for simulating the transients in the water layer test also resulted in a better agreement with the experiments compared to the previous models.

Super-Nernstian Tafel slopes: An origin in coupled homogeneous kinetics

Simulations are reported relating to the predicted Tafel slopes of voltammograms recorded for a heterogeneous electrochemically reversible electron transfer process coupled to non-linear homogeneous chemical kinetics. Tafel plots are shown to be markedly non-linear with potential, indicating the essential need to be analysed via simulation. In the case of the EC2 reaction, for a simple one-electron reversible electron transfer, the following second order kinetics lead to Tafel slopes with apparent transfer coefficients notably above unity, and so gain an apparent super-Nernstian response. Simulations for a ‘titration’ reaction mechanism, A + e− ⇌ B, B + C → Y are also performed and the effects of electrode radius and analyte concentration on the Tafel slope are investigated and discussed.

Sponge and Paper Substrates Coupled with Ion Sensors Utilized in Microfluidic Sampling of High Solid-to-Liquid Ratio Samples

The on-site analysis of any ion concentrations in environmental samples faces many challenges, such as need of sample manipulations and use of a non-portable analytical instrumentation that in turn results in higher uncertainties of the determination and the same time the delay between sampling and the actual measurement. On the other hand, wearable sensors gain more and more attention for instant monitoring of clinically relevant analytes. The portable and relatively low-cost ion-selective electrodes (ISEs) have been extensively studied in context of on-site analysis of ions. However, their potentiometric response majorly depends on the physico-chemical state of an ion-selective membrane (ISM) and the protocol of the analysis. In the situation of nature samples containing high solid-to-liquid ratio, paper substrates and textile were investigated before to integrated with the ISEs to protect the ISM from mechanical damage and the same time deliver micro volumes of liquid to the electrode [1, 2]. Nevertheless, measurements of pH and heavy metals were found to be affected by side reactions and nonspecific adsorption of metal ions to the sampling substrates.In this study, method of pre-modification of paper substrates was investigated to eliminate the detrimental super-Nernstian response otherwise observed [3]. Potentiometric determination of heavy metals in environmental samples (Cd2+ and Pb2+) utilizing paper based microfluidic sampling was studied with a commercial crystalline membrane solid-state Cd2+-and a solid-contact Pb2+-ion selective electrodes. Pre-treatment of paper substrates in the solutions containing the salt with cation the same as the ISEs primary cation, successfully eliminated super Nernstian response. Moreover, the concentration dependent sorption-desorption of metal ion to/from paper substrates was found as possible interferences of using microfluidic paper-based sampling in heavy metal analysis. Then, the amount of heavy metal ions binding onto different substrates was detected.Owning to the lowest heavy metal sorption capacity of PU sponge, microfluidic sponge-based sampling was developed and studied as novel sampling and sample handling substrate to serve as alternative for solution sampling for ion analysis. Possibility of direct measurements of ions in such high solid-to-liquid ratio samples as mud, sludge, wet soil were explored [4].

On the super-Nernstian potentiometric response of InN/InGaN quantum dots

An explanation is given of the super-Nernstian response of InN/InGaN quantum dots (QDs) in potentiometric ion- and biosensing, beating the fundamental thermodynamic kT/e limit. It follows from the unique surface- and quantum properties of the QDs. The resulting quantum mechanical partition of electrons inside and outside of the QDs implies fractional electron transfer in an artificial chemical redox half-reaction of the QDs upon ion attachment or electron transfer to the QDs.

Ultrasensitive dopamine detection of indium-zinc oxide on PET flexible based extended-gate field-effect transistor

With the aim of a simple, label-free, and highly specific dopamine detection, we proposed an indium-zinc oxide (InZnxOy) sensing film on flexible PET using a simple sol-gel method for an extended gate field-effect transistor (EGFET) sensor. In this study, X-ray diffraction, atomic force spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the structural, morphological, and chemical features, respectively. The InZnxOy based EGFET sensor showed a super Nernstian pH response of 61.53 mV/pH with an excellent linearity of 0.995 (in the range of pH 2–12), a low hysteresis voltage of ∼ 3 mV (in the pH loop of 7→4→7→10→7) and a small drift rate of 3.52 mV/h (in the pH 7). Correspondingly, to investigate the impact of mechanical bending on the pH performances of InZnxOy based flexible EGFET sensor, a couple of bending tests, which include different bending radius and bending cycles, were performed. The device showed better mechanical durability up to 1500 bending cycles for pH sensitivity. In order to achieve the label-free detection of dopamine, the InZnxOy on PET-based EGFET was functionalized using a synthetic receptor named 4-carboxylphenylboronic acid, for the covalent binding of dopamine on the sensing surface. The sensitivity of dopamine is 10.69 mV/log(dopamine) with linearity 0.998 within the dopamine concentration range of 1 fM–1 nM and the limit of detection value is 0.523 fM. In addition, this InZnxOy based EGFET biosensor showed excellent selectivity to dopamine over other interfering chemicals. Our proposed InZnxOy EGFET biosensor was successfully applied to detect dopamine level in the real samples like rat blood serum and rat brain.

A Minimally Invasive Microsensor Specially Designed for Simultaneous Dissolved Oxygen and pH Biofilm Profiling.

A novel sensing device for simultaneous dissolved oxygen (DO) and pH monitoring specially designed for biofilm profiling is presented in this work. This device enabled the recording of instantaneous DO and pH dynamic profiles within biofilms, improving the tools available for the study and the characterization of biological systems. The microsensor consisted of two parallel arrays of microelectrodes. Microelectrodes used for DO sensing were bare gold electrodes, while microelectrodes used for pH sensing were platinum-based electrodes modified using electrodeposited iridium oxide. The device was fabricated with a polyimide (Kapton®) film of 127 µm as a substrate for minimizing the damage caused on the biofilm structure during its insertion. The electrodes were covered with a Nafion® layer to increase sensor stability and repeatability and to avoid electrode surface fouling. DO microelectrodes showed a linear response in the range 0–8 mg L−1, a detection limit of 0.05 mg L−1, and a sensitivity of 2.06 nA L mg−1. pH electrodes showed a linear super-Nernstian response (74.2 ± 0.7 mV/pH unit) in a wide pH range (pH 4−9). The multi-analyte sensor array was validated in a flat plate bioreactor where simultaneous and instantaneous pH and DO profiles within a sulfide oxidizing biofilm were recorded. The electrodes spatial resolution, the monitoring sensitivity, and the minimally invasive features exhibited by the proposed microsensor improved biofilm monitoring performance, enabling the quantification of mass transfer resistances and the assessment of biological activity.

Super Nernstian pH response and enzyme-free detection of glucose using sol-gel derived RuOx on PET flexible-based extended-gate field-effect transistor

In this paper, we fabricated a sol-gel derived RuOx, with low-temperature thermal oxidation, on polyethylene terephthalate (PET) flexible based extended-gate field-effect transistor (EGFET) as a pH sensor for the assessment of glucose for the first time. Structural identification, surface morphology, and film composition of the sensing membrane were investigated by X-ray diffraction, atomic force microscopy, and X-ray photoelectron microscopy, respectively. The RuOx on PET based EGFET sensor exhibited a pH sensitivity of 65.11 mV/pH in the wide range of pH 2–12 with an excellent linearity of 0.999. This sensor showed good reversibility in terms of a lower hysteresis voltage of ∼1 mV and a better stability for 12 h named as drift rate ∼2.08 mV/h. The RuOx-based EGFET sensor also demonstrated a high selective response towards H+. In addition, the pH performace of this RuOx EGFET flexible sensor kept unchanged after 500 repeated bending cycles. Subsequently, the RuOx on PET based EGFET was functionalized by 4-carboxyphenyl boronic acid (4-CPBA), which is a synthetic receptor for covalent attachment of glucose on the sensing surface. The sensitivity of glucose was 6.89 mV/mM with the linearity of 0.993 within the glucose concentration from 1 to 8 mM. The concentration of serum glucose measured by our 4-CPBA functionalized RuOx EGFET biosensor is comparable to that determined by commercial blood glucose monitoring system. In addition, this RuOx EGFET biosensor exhibited an elevated reference voltage shift response to glucose compared with other saccharides (e.g. fructose, sucrose and mannose).

Iridium and Ruthenium oxide miniature pH sensors: Long-term performance

The characteristics of electrodeposited Iridium oxide films (EIROF), and Ruthenium oxide films (RUOF) under normal and annealed conditions have been investigated. RUOFs were sputter deposited onto a substrate whilst EIROF sensors were made by electrodeposition. The adhesion of the RUOF sensors was investigated on Platinum and Titanium surfaces. The electrical impedance of the two metal oxides was measured in PBS demonstrating that both films have up to a 100 fold higher electrical double layer capacitance than bright Platinum electrodes. The EIROF films show super-Nernstian response, but the formal potential shows a continuous drift with time of approximately 2 mV day−1, measured over 40 days. Both RUOF sensors show approximately Nernstian behaviour. Short term pH precision of the annealed RUOF falls within a confidence interval of ± 0.1 pH, and stability is maintained within a confidence of ± 0.24 pH for up to 147 days. The un-annealed RUOF maintains the same precision but only for 20 days. Compared to EIROF sensors, no drift correction is required for the annealed RUOF making this metal oxide suitable for long term measurements where re-calibration is not possible.

Microscopic model for pH sensing mechanism in zinc-based nanowalls

Zinc based nanostructures are very promising material for pH sensing since they allow the realization of low-cost, sustainable, and high sensitivity electrodes. The pH sensitivity reported in literature for different ZnO nanostructures spreads from sub- to super-Nernstian, with the microscopic mechanism behind the H+ detection often unrevealed. In this work we synthesize by hydrothermal process and thermal annealing two zinc based nanowalls (NWLs) consisting of layered hydroxide zinc nitrate Zn5(OH)8(NO3)2·2H2O (as grown) and zinc oxide ZnO (annealed) phases, respectively. Scanning electron microscopy, micro Raman spectroscopy, X-ray photoemission spectroscopy were used to characterize the morphology and structure of NWLs. Electrochemical chronopotentiometric analysis in standard buffer solutions (pH 4 to 9) was used to study the response towards pH. Despite the two zinc based NWLs have the same morphology (interconnected sheets 10-20 nm thin, 1.4–1.7 μm wide), truly different behavior as pH sensitive electrodes are evidenced. As grown NWLs show a super-Nernstian response (+83.7 mV/decade), whereas annealed NWLs show a sub-Nernstian response (+27.1 mV/decade). The data are satisfactorily modeled by considering the crystallographic structures and assuming that layered hydroxide zinc nitrate NWLs is sensitive to only H+ (with two simultaneous and independent mechanisms), while zinc oxide NWLs is simultaneously and independently sensitive to both H+ and OH−. These data and the proposed modeling are useful to further develop the pH sensitivity of electrodes based on ZnO nanostructures.

Structural Properties and Sensing Characteristics of YbTaO4 Sensing Membranes

In this paper, we developed YbTaO4 sensing membranes for use in a solid-state electrolyte–insulator–semiconductor (EIS) pH sensor. Impact of rapid thermal annealing (RTA) treatment on the structural and sensing properties of the YbTaO4 sensing membranes deposited through reactive co-sputtering onto Si substrates was investigated. X-ray diffraction, atomic force microscopy, and X-ray photoelectron spectroscopy were employed to examine the structural, morphological, and chemical features, respectively, of the YbTaO4 sensing films annealed at three RTA temperatures (700, 800 and 900 °C). The YbTaO4 EIS sensor annealed at the 800 °C exhibited a super-Nernstian response of 71.17 mV/pH within the pH range of 2–12. It also showed the lowest hysteresis voltage (<1 mV) and the lowest drift rate (0.22 mV/h) among the these RTA temperatures. Presumably, the optimal annealing temperature improved the stoichiometry of YbTaO4 film and increased its (-131)-oriented nanograin size. Figure 1

PH dependent voltammetry at partially oxidized palladium and its potential for voltammetry-based pH sensing

We studied the effects of pH on voltammetry at partially oxidized palladium. Below pH7.2, a cathodic process is seen in cyclic voltammetry whose peak potential as well as the potential of the inflection point preceding the peak are pH dependent with super-Nernstian pH response in both phosphate buffer (−90mV/pH) and phosphate-buffered saline (−160mV/pH). Above pH7.2, a pH-independent secondary electrochemical process, hysteresis and/or a drop in sensitivity were observed. The mechanism of pH-dependent voltammetry is linked to the reduction of Pd(II) oxide and is modulated by adsorbed and partially protonated phosphate anions. The potential for pH sensing via voltammetry was evaluated in dynamic measurements, and even without a reference electrode when the microelectrode surface was partially covered with silver whose sharp anodic peak is not pH-dependent and therefore, can be used as internal reference. Practical pH measurement with voltammetry requires further research and optimization of material and stabilization protocols in the future.

Inorganic salt modified paper substrates utilized in paper based microfluidic sampling for potentiometric determination of heavy metals

Inorganic salt modified paper substrates were developed and utilized in microfluidic paper based sampling coupled with potentiometric ion sensors for the determination of heavy metals, such as Cd2+ and Pb2+. The application of paper based sampling without any paper substrate pre-treatment were characterized with super Nernstian response, while the paper substrates with inorganic salt pre-treatment were characterized with standard Nernstian response. The application of inorganic salt modified paper substrates was found advantageous when the cation of the salt in the conditioning solution was the same as the primary cations of the ion-selective electrodes (ISEs). A relatively high concentration of inorganic salts during pre-treatment of the paper substrates, namely Cd(NO3)2 or Pb(NO3)2 with subsequent measurement of cadmium(II) and lead(II), by Cd2+- and Pb2+-ISEs, respectively was needed to sustain Nernstian response of sensors. It was also found that paper substrates facilitate sorption of metal ions onto the paper substrates, with stronger binding strength given to Pb2+ over Cd2+. Moreover, the metal-paper interactions suggest concentration dependent sorption-desorption of metal ion, which can have direct limitations of the use of paper based analytics for the determination of low analyte concentration of heavy metals.

Enhanced Performance Stability of Iridium Oxide-Based pH Sensors Fabricated on Rough Inkjet-Printed Platinum.

Today, electrochemical sensors are used for a broad range of applications. A fundamental challenge is still the achievement of long-term sensor stability by ensuring good adhesion between the deposited sensing layer and the substrate material, e.g., a metal electrode. Until now, the most applied strategy to overcome this problem is to increase the surface roughness of the metal layer by mechanical etching or by electroplating of additional material layers, which both imply an increase in manufacturing steps and thus the final cost of the overall device. Alternatively, to overcome these adhesion problems, we propose the direct printing of a novel platinum nanoparticle ink, which is compatible with low-cost additive digital inkjet and with flexible low-cost substrates. This water-based platinum ink has two unique features: it leads to highly rough surfaces, which promotes the adhesion of deposited sensing material, and it is a highly low-temperature curing ink, compatible with polymeric substrates that cannot withstand high temperatures. Based on this concept, we report about a long-term stable and highly sensitive solid-state pH sensor functionalized by anodic electrodeposited iridium oxide on a rough nanostructured platinum printed layer. The sensors showed an excellent reproducibility with a linear super-Nernstian response (71.3 ± 0.3 mV/pH unit) in a wide pH range (pH 2-11). Long-term stability tests for over 1 year of application demonstrate an excellent mechanical sensor layer stability, which is correlated to the distinct roughness of the printed platinum layer. This novel approach is useful to simplify the fabrication process and with that the sensor costs.

Naphthocage: A Flexible yet Extremely Strong Binder for Singly Charged Organic Cations

We report a quite flexible naphthol-based cage (so-called "naphthocage") which adopts a self-inclusion conformation in its free state and is able to bind singly charged organic cations extremely strongly ( Ka > 107 M-1). Ion-selective electrodes prepared with this naphthocage show a super-Nernstian response to acetylcholine. In addition, the highly stable complex (1010 M-1) between ferrocenium and the naphthocage can be switched electrochemically, which lays a basis for its application in stimuli-responsive materials.

Solid-contact potentiometric sensors for reliable automatic quantification of 2,4-dichlorophenol (2,4-DCP) as a food taint

Sensitive, selective, and simple solid-contact potentiometric sensors were fabricated and characterized for 2,4-dichlorophenol (2,4-DCP) quantification. Membrane sensors based on the use of tridodecylmethylammonium chloride (TDMAC) and Aliquat 336 as anion exchangers were distributed in plasticized polyvinyl chloride (PVC) matrix. They revealed super-Nernstian responses with slopes of −70.3 and −78.2 over linear ranges 5.8 × 10−7–1.0 × 10−3 M and 3.3 × 10−6–1.0 × 10−3 M and with detection limits of 0.07 and 0.11 µg ml−1, for TDMAC and Aliquat 336, respectively. The sensors were compared with conventional sensors. The effect of plasticizer on the potentiometric response and diverse foreign common ions effect were examined. Sensors of both types showed a long life span, good selectivity for 2,4-DCP over a wide assortment of other common ions, long-term stability, high reproducibility, and rapid response. Validation tests illustrated good performance characteristics of the recommended sensors by evaluating the lower detection limit, range, accuracy, precision, repeatability, and between-day-variability. The sensors were effectively applied for the persistent monitoring of 2,4-DCP using a flow injection system. The sensors were applied to the 2,4-DCP potentiometric determination in spiked urine and fish samples. Recoveries ranged from 90% to 110%.