Polymeric membrane ion-selective electrode based on potential-modulated ion transfer: ultrasensitive measurement of oceanic pH

The application of a potentiometric pH electrode in ocean acidification observation is still a challenge due to its poor sensitivity to small pH changes. Herein, a simple approach to remarkably improve the detection precision of a polymeric membrane ion-selective electrode is proposed based on the potential-modulated ion transfer mechanism. The present sensing strategy displays highly sensitive responses to small pH changes for seawater analysis with a precision of 5 μpH, which is 2 orders of magnitude lower than that of the conventional pH electrode.

Photoelectrochemical conversion for ion-selective electrodes based on CdS semiconductor film

BACKGROUND

This study presents a novel photoelectrochemical (PEC) conversion method for ion-selective electrodes (ISEs) based on CdS semiconductor film. The motivation stems from the need to enhance the sensitivity and precision of ISEs for various analytical applications.

RESULTS

We synthesized CdS film on FTO conductive glass via a hydrothermal method and utilized this electrode as the working electrode. Under visible light irradiation, CdS generated photocurrent that is proportional to its applied voltage within a large potential window of ∼0.80 V. Ascorbic acid (AA) effectively inhibited electron-hole complexation, enhancing photocurrent stability. Potential modulation from ISEs acting as the reference electrode further regulated photocurrent generation, demonstrating excellent sensitivity and linearity for a wide range of ion concentrations. The method was validated by detecting serum calcium levels, showing agreement with traditional ISEs potentiometry and ICP-OES methods.

SIGNIFICANCE

This photoelectrochemical conversion strategy offers a promising approach for sensitive and accurate ion detection, with potential applications in clinical diagnostics and environmental monitoring.

A photoelectrochemical biosensor based on self-calibration platform of carbon-rich plasmonic probe with near-infrared driving signal amplification

Exploring the photochemical (PEC) method induced by low-energy light source makes great significance to achieve high stability and accurate analysis. A sensing platform driven by near-infrared (NIR) light was designed by making the biochemically encoded carbon rich plasmonic hybrid (CPH) probe, the peptide@C-Mo2C. The inherent plasmonic effect of C-Mo2C CPH can directly absorb NIR light, thus starting effective electronic-hole pairs separation. Moreover, the photothermal effect of C-Mo2C CPH also promoted the reaction yield of photothermal catalyst reaction on sensing interface to assist the PEC signal amplification. In the presence of target trypsin, it cleaves the peptides, resulting in the release of peptide@C-Mo2C probe from interface, which leads to a relative decrease in PEC signal. More importantly, a self-calibration system consisting of two independent PEC test channels attempted to eliminate the influence of background signal and baseline drift. The test channel was used to specify the recognition target, while the blank channel was used as a reference. Therefore, the signal difference between two channels was recorded, so as to obtain results with less error and higher stability. In this NIR driven PEC sensor, the carbon rich probe with direct and efficient NIR light conversion promoted the sensitivity and a self-calibration system guaranteed the stability which provided innovative thoughts for developing ingenious PEC sensor.

Carbon fiber-based multichannel solid-contact potentiometric ion sensors for real-time sweat electrolyte monitoring

Solid-contact ion-selective electrodes (SC-ISEs) feature miniaturization and integration that have gained extensive attention in non-invasive wearable sweat electrolyte sensors. The state-of-the-art wearable SC-ISEs mainly use polyethylene terephthalate, gold and carbon nanotube fibers as flexible substrates but suffer from uncomfortableness, high cost and biotoxicity. Herein, we report carbon fiber-based SC-ISEs to construct a four-channel wearable potentiometric sensor for sweat electrolytes monitoring (Na+/K+/pH/Cl-). The carbon fibers were extracted from commercial cloth, of which the starting point is addressing the cost and reproducibility issues for flexible SC-ISEs. The bare carbon fiber electrodes exhibited reversible voltammetric and stable impedance performances. Further fabricated SC-ISEs based on corresponding ion-selective membranes disclosed Nernstian sensitivity and anti-interface ability toward both ions and organic species in sweat. Significantly, these carbon fiber-based SC-ISEs revealed high reproducibility of standard potentials between normal and bending states. Finally, a textile-based sensor was integrated with a solid-contact reference electrode, which realized on-body sweat electrolytes analysis. The results displayed high accuracy compared with ex-situ tests by ion chromatography. This work highlights carbon fiber-based multichannel wearable potentiometric ion sensors with low cost, biocompatibility and reproducibility.

Fast and sensitive coulometric signal transduction for ion-selective electrodes by utilizing a two-compartment cell

Here, we propose a fast and sensitive coulometric signal transduction method for ion-selective electrodes (ISEs) by utilizing a two-compartment cell. A potassium ion-selective electrode (K⁺-ISE) was connected as reference electrode (RE) and placed in the sample compartment. A glassy carbon (GC) electrode coated with poly(3,4-ethylenedioxythiophene) (GC/PEDOT), or reduced graphene oxide (GC/RGO), was connected as working electrode (WE) and placed in the detection compartment together with a counter electrode (CE). The two compartments were connected with an Ag/AgCl wire. The measured cumulated charge was amplified by increasing the capacitance of the WE. The observed slope of the cumulated charge with respect to the change of the logarithm of the K⁺ ion activity was linearly proportional to the capacitance of the GC/PEDOT and GC/RGO, estimated from impedance spectra. Furthermore, the sensitivity of the coulometric signal transduction using a commercial K⁺-ISE with internal filling solution as RE and GC/RGO as WE allowed to decrease the response time while still being able to detect a 0.2% change in K⁺ concentration. The coulometric method utilizing a two-compartment cell was found to be feasible for the determination of K⁺ concentrations in serum. The advantage of this two-compartment approach, compared to the coulometric transduction described earlier, was that no current passed through the K⁺-ISE that was connected as RE. Therefore, current-induced polarization of the K⁺-ISE was avoided. Furthermore, since the GCE/PEDOT and GCE/RGO (used as WE) had a low impedance, the response time of the coulometric response decreased from minutes to seconds.

Redox probe-based amperometric sensing for solid-contact ion-selective electrodes

The transformation from the traditional potentiometric response of an ion-selective electrode (ISE) to other signal readout is promising to realize the potential signal amplification. In this work, the redox probes, including ferrocyanide/ferricyanide (Fe(CN)₆^3-/4-), hexaammineruthenium (Ru(NH₃)₆³⁺) and ferrocene derivatives, were introduced to read out the potentiometric response for the solid-contact Ca²⁺-ISE. The mechanism is that the oxidation current of the redox probe on a glassy carbon electrode is modulated by the potential of the ISE through changing the concentrations/activities of Ca²⁺ under the control of the constant applied potential. Results show that the linear range and the slope sensitivity for detection Ca²⁺ by using the amperometric signal based on Fe(CN)₆^4-/3- redox probe are adjustable through changing the applied potentials. Moreover, the redox probe-based amperometric signal for the solid-contact Ca²⁺-ISE is found to be related to both of the types of the redox probes and the electrode areas. This work provides a convenient and general method for translating the potential response at mV grade to the amperometric signal at the μA level, and is promising for detection of ions with high sensitivity by using the ISEs.

Translating potentiometric detection into non-enzymatic amperometric measurement of H2O2

The developments of alternative signal readout strategies for the ion-selective electrodes (ISEs) are necessary in order to break through the limitation of the Nernst equation. In this work, a simple, convenient and easily operated strategy based on the non-enzymatic amperometric measurement of H₂O₂ is proposed to read out the potentiometric responses for the ISEs. The proposed amperometric signal readout based on H₂O₂ is carried out in a two compartment electrochemical cell configuration containing a detection cell and a sample cell, physically connected by a salt bridge. A glassy carbon (GC) electrode is placed in the detection cell to monitor the oxidation current of H₂O₂, and an ISE is placed in the sample cell to act as both the reference electrode and the potentiometric sensor for obtaining the ion activities. The oxidation of H₂O₂ is induced by a constant potential applied between the GC electrode and the ISE, and subsequently modulated by the potential change of the ISE in the presence of the primary ion. The proposed amperometric signal readout based on H₂O₂ shows the satisfied slope sensitivity and detection limit, which are better than or compared to those for the potentiometric responses for the ISEs. This work provides a general strategy for transforming the potential response of the ISEs into the amperometric readout, and is promising for detection of cations (eg., Ca²⁺) and anions (eg., NO₃^-) with high sensitivity and excellent selectivity.

A critical review on the use of potentiometric based biosensors for biomarkers detection

Potentiometric-based biosensors have the potential to advance the detection of several biological compounds and help in early diagnosis of various diseases. They belong to the portable analytical class of biosensors for monitoring biomarkers in the human body. They contain ion-sensitive membranes sensors can be used to determine potassium, sodium, and chloride ions activity while being used as a biomarker to gauge human health. The potentiometric based ion-sensitive membrane systems can be coupled with various techniques to create a sensitive tool for the fast and early detection of cancer biomarkers and other critical biological compounds. This paper discusses the application of potentiometric-based biosensors and classifies them into four major categories: photoelectrochemical potentiometric biomarkers, potentiometric biosensors amplified with molecular imprinted polymer systems, wearable potentiometric biomarkers and light-addressable potentiometric biosensors. This review demonstrated the development of several innovative biosensor-based techniques that could potentially provide reliable tools to test biomarkers. Some challenges however remain, but these can be removed by coupling techniques to maximize the testing sensitivity.

An enzyme linked aptamer photoelectrochemical biosensor for Tau-381 protein using AuNPs/MoSe2 as sensing material

Alzheimer's disease is a worldwide health problem and it has attracted extensive attention. Tau protein is an important biomarker in the pathogenesis of Alzheimer's disease. Herein, we devise a in situ enzyme catalysis generating electron donor photoelectrochemical (PEC) biosensor for Tau-381 protein. Tau-381 protein aptamer is immobilized onto the surface of AuNPs/MoSe₂ nanosheets modified electrode. In the presence of Tau-381 protein, an aptamer-protein duplex is formed. Meanwhile, the Tau-381 antibody and the protein G/AP (protein G labeled with alkaline phosphatase) are captured with the affinity interaction between Tau-381 protein and Tau-381 antibody, Tau-381 antibody and protein G/AP. The electron donor, ascorbic acid, is in situ produced by the catalyzing of ascorbic acid 2-phosphate in the PEC detection solution. As a result, low blank noise and strong photocurrent response are engendered. The photocurrent response is related to the concentration of Tau-381 protein. The detection range of Tau-381 protein is from 0.5 fM to 1.0 nM with detection limit of 0.3 fM. This in situ generating electron donor PEC biosensor can detect various targets by simply alternating antibody, antigen, or aptamer commercially. Thus, this work represents a simple and general sensing protocol.