Modern Directions for Potentiometric Sensors

Silk Fibroin-Regulated Nanochannels for Flexible Hydrovoltaic Ion Sensing

The evaporation-induced hydrovoltaic effect based on ion-selective nanochannels can theoretically be employed for high-performance ion sensing; yet, the indeterminate ion-sensing properties and the acquisition of high sensing performance are rarely explored. Herein, a controllable nanochannel regulation strategy for flexible hydrovoltaic devices with highly sensitive ion-sensing abilities is presented across a wide concentration range. By multiple dip-coating of silk fibroin (SF) on an electrospinning nylon-66 nanofiber (NNF) film, the surface polarity enhancement, the fibers size regulation with a precision of ≈25 nm, and the nanostructure firm binding are achieved simultaneously. The resultant flexible freestanding hydrovoltaic device exhibits an open circuit voltage up to 4.82 V in deionized water, a wide ion sensing range of 10-7 to 100 m, and ultrahigh sensitivity as high as 1.37 V dec-1, which is significantly higher than the sensitivity of the traditional solid-contact ion-selective electrodes (SC-ISEs). The fabricated flexible ion-sensitive hydrovoltaic device is successfully applied for wearable human sweat electrolyte sensing and for environmental trace-ion monitoring, thereby confirming the potential application of the hydrovoltaic effect for ion sensing.

Design and application of potentiometric sensors for the determination of mefenamic and phenylanthranilic acids

Characteristics, performance and applications of potentiometric membrane sensors are described for the determination of mefenamic and phenylanthranilic ions. Ion associates of mefenamic, ClO₄^-, and phenylanthranilic ions with crystal violet (counter-cation) as ion exchange sites have been used as ionophores in the plasticized one- and two-layer membrane ion-selective electrodes. The LOD is reported to be 8.4 × 10^-5 M for mefenamic acid, and 5.1 × 10^-5 M for phenylanthranilic acid. The cations of basic dyes (crystal violet) are characterized by significant delocalization of the positive charge and polarizability. This may explain the better selectivity of the developed sensors. These sensors were used for the direct assay of mefenamic and N-phenylanthranilic acids in model solutions and applications studied in commercial pharmaceutical preparations.

An electrochemical biosensor for the detection of aflatoxin B1 based on the specific aptamer and HCR biological magnification

Aflatoxin B1 (AFB1) is a highly toxic mycotoxin, which causes severe acute or cumulative poisoning. Therefore, it is important to develop sensitive and selective detection methods for AFB1 for the safety of food and medicinal herbs. Herein, we have developed a "signal-on" electrochemical aptasensor based on the high specificity of the aptamer and hybridization chain reaction (HCR) biological amplification for AFB1 detection. In this work, thiol-modified complementary DNA (cDNA) immobilized on the surface of a gold electrode (GE) served as an initiator DNA. When AFB1 was present, it competed with the cDNA for binding to the aptamers, which resulted in the detaching of aptamers from the cDNA-aptamer duplexes. Then, the single-stranded cDNA acted as an initiator to trigger the HCR signal amplification. Therefore, long double-stranded DNA (dsDNA) products were produced, which could load large amounts of methylene blue (MB) molecules to generate a distinct electrochemical signal. Under the optimized conditions, the proposed electrochemical aptasensor achieved the ultrasensitive detection of AFB1 with a linear detection range of 0.01-100 pg mL^-1, and a limit of detection (LOD) down to 2.84 fg mL^-1. Furthermore, the electrochemical aptasensor was successfully applied for detecting AFB1 in corn and two kinds of traditional Chinese medicine samples, indicating the potential value for AFB1 detection in practical samples.

History of Cobaltabis(dicarbollide) in Potentiometry, No Need for Ionophores to Get an Excellent Selectivity

This work is a mini-review highlighting the relevance of the θ metallabis(dicarbollide) [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- with its peculiar and differentiating characteristics, among them the capacity to generate hydrogen and dihydrogen bonds, to generate micelles and vesicles, to be able to be dissolved in water or benzene, to have a wide range of redox reversible couples and many more, and to use these properties, in this case, for producing potentiometric membrane sensors to monitor amine-containing drugs or other nitrogen-containing molecules. Sensors have been produced with this monoanionic cluster [3,3'-Co(1,2-C₂B₉H₁₁)₂]^-. Other monoanionic boron clusters are also discussed, but they are much fewer. It is noteworthy that most of the electrochemical sensor species incorporate an ammonium cation and that this cation is the species to be detected. Alternatively, the detection of the borate anion itself has also been studied, but with significantly fewer examples. The functions of the borate anion in the membrane are different, even as a doping agent for polypyrrole which was the conductive ground on which the PVC membrane was deposited. Apart from these cases related to closo borates, the bulk of the work has been devoted to sensors in which the θ metallabis (dicarbollide) [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- is the key element. The metallabis (dicarbollide) anion, [3,3'-Co(1,2-C₂B₉H₁₁)₂]^-, has many applications; one of these is as new material used to prepare an ion-pair complex with bioactive protonable nitrogen containing compounds, [YH]_x[3,3'-Co(1,2-C₂B₉H₁₁)₂]_y as an active part of PVC membrane potentiometric sensors. The developed electrodes have Nernstian responses for target analytes, i.e., antibiotics, amino acids, neurotransmitters, analgesics, for some decades of concentrations, with a short response time, around 5 s, a good stability of membrane over 45 days, and an optimal selectivity, even for optical isomers, to be used also for real sample analysis and environmental, clinical, pharmaceutical and food analysis.

Evaluation of mesoporous borosilicate glass-ceramic composites as frits in reference electrodes

The development of new mesoporous frits for reference electrodes to overcome the limitations of cross-contamination and screening effect is essential for many electrochemical measurements. Available frit-based reference electrodes (e.g., mesoporous, microporous) still suffer from cross-contamination and/or errors in electrochemical measurements. In this work, a mesoporous glass-ceramic composite is prepared to mitigate such limitations. Mesoporous glass-ceramic frits were prepared from low-cost materials (i.e., borosilicate and kaolin) at relatively low temperatures (750-850 °C). The prepared glass-ceramic frits were characterized using scanning electron microscopy (SEM), impedance measurements, and nitrogen sorption isotherms. The developed mesoporous glass-ceramic composites are characterized by a high chemical resistance against corrosive materials and a low thermal expansion. Reference electrodes constructed with the developed mesoporous glass-ceramic frits exhibited a low flow rate of 0.002 ± 0.001 to 0.41 ± 0.06 μL h^-1 and high potential stability as well as very small potential drift of -2.4 ± 0.2 to -4.9 ± 0.2 μV h^-1. Mesoporous glass-ceramic based reference electrodes exhibited average potential variations of 13 ± 3 mV over the concentration range of 1 mM to 0.1 M KCl. This indicates that mesoporous glass-ceramic frit-based reference electrodes exhibited a much lower flow rate compared to available microporous frit-based reference electrodes. Moreover, the developed mesoporous ceramic-based reference electrodes exhibited a 4-15-fold improvement in potential variations and a large improvement in potential stability in comparison with the reported mesoporous-frit-based reference electrodes.

A ratiometric electrochemical strategy based on Fe (III) and Pt (IV) for immobilization-free detection of Escherichia coli

A new ratiometric electrochemical strategy for immobilization-free detection of Escherichia coli (E. coli) was constructed by using a capture DNA-polyaniline/copper ferrite nanoparticles/graphene oxide (cDNA-PANI/CuFe₂O₄/GO) composite as capture probes, which has a high specific surface area and good magnetic properties. Then trigger DNA/Au nanoparticles (tDNA/Au NPs) were used as signal amplification labels, and Pt (IV) and Fe (III) were chosen as the signal probes. In the presence of targets, the sandwich format among cDNA-PANI/CuFe₂O₄/GO, E. coli and auxiliary DNA (aDNA) was realized by using the aptamer recognition system. Then, the tDNA/Au binding could be anchored on the sandwich format due to the principle of base complementation between unpaired aDNA and tDNA. And the unbounded tDNA of tDNA/Au NPs could bind an amount of Pt (IV). After separation using a magnet, a handful of unbound Pt (IV) which remained in the supernatant reacted with a large number of Fe (III) ions, leading to a markedly increased I_Fe(III)/I_Pt(IV) value. Oppositely, the sandwich format could not appear in the absence of targets, and even the tDNA/Au could not be immobilized on it. So, the redox reaction between a large amount of Pt (IV) residue in the supernatant and Fe (III) was significantly successful, causing a low I_Fe(III)/I_Pt(IV) value. Under optimal conditions, we found that I_Fe(III)/I_Pt(IV) was linearly related to the logarithmic E. coli concentration with a low limit of detection (1.862 × 10³ cfu mL^-1). This devised ratiometric electrochemical method may develop into a powerful and effective means for the detection of E. coli in real samples, which may also be developed as a universal tool for another microorganism.

Dibenzo-18-Crown-6-based Carbon Paste Sensors for the Nanomolar Potentiometric Determination of Daclatasvir Dihydrochloride: An Anti-HCV Drug and a Potential Candidate for Treatment of SARS-CoV-2

Daclatasvir dihydrochloride (DAC) is an anti-hepatitis C virus (HCV) drug that has recently proven to be a promising candidate for the treatment of SARS-CoV-2. Still, there is a lack of sensitive potentiometric methods for its determination. In this work, carbon paste sensors based on dibenzo-18-crown-6 (DB18C6) were fabricated and optimized for the sensitive and selective potentiometric determination of DAC in Daclavirocyrl® tablets, serum, and urine samples. The best performance was obtained by two sensors referred to as sensor I and sensor II. Both sensors exhibited a wide linear response range of 5×10⁻⁹ − 1×10⁻³ mol/L, and Nernstian slopes of 29.8 ± 1.18 and 29.5 ± 1.00 mV/decade, with limits of detection, 4.8×10⁻⁹ and 3.2×10⁻⁹ mol/L, for the sensors I and II, respectively. Sensors I and II displayed fast response times of 5–8 and 5–6 s, respectively, with great reversibility and no memory effect. Moreover, the sensors exhibited a lifetime of 16 days. For the study of sensors morphology and elucidation of the interaction mechanism, the scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (¹H NMR) techniques were performed. A selectivity study was performed, and the proposed sensors exhibited good discrimination between DAC and potentially coexisting interferents with sensor II displaying better selectivity. Finally, sensor II was successfully applied for the determination of DAC in the above-mentioned samples, with recovery values ranging from 99.25 to 101.42%, and relative standard deviation (RSD) values ranging from 0.79 to 1.53% which reflected the high accuracy and precision.

Evaluation of PAMAM Dendrimers (G3, G4, and G5) in the Construction of a SPR-based Immunosensor for Cardiac Troponin T

An immunosensor was developed using a SAM of an alkanethiol associated with PAMAM(G4) dendrimers based on surface plasmon resonance (SPR) to enhance the sensitivity for troponin T detection in blood samples. The feasibility of using three-dimensional platforms based on dendrimers for the development of immunosensors was demonstrated by evaluating three different generations of these dendrimers (G3, G4, and G5) to detect troponin T. The results showed the efficiency of these 3D platforms in anchoring biomolecules, amplifying the detection of troponin T. The sandwich assay showed good performance for troponin T detection, using secondary monoclonal antibodies, in the concentration range of 5 - 300 ng mL^-1 (0.14 - 8.67 nmol L^-1), R² = 0.991, with the LOD of 3.6 ng mL^-1. The sandwich assay's applicability was demonstrated by evaluating a secondary polyclonal antibody's performance in the concentration range of 3 - 30 ng mL^-1, R² = 0.998, with the LOD of 0.98 ng mL^-1. The immunosensor was applied to determine troponin T in blood plasma samples from healthy patients, with an average recovery of 88 to 104%. The performance of the SPR-based immunosensor indicates reliable results and is expected to contribute to the rapid diagnosis of heart attack, with reduced costs.

Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends

This article presents a comprehensive overview of recent progress in the design and applications of solid-contact ion-selective electrodes (SC-ISEs).

Why ammonium detection is particularly challenging but insightful with ionophore-based potentiometric sensors – an overview of the progress in the last 20 years

An overview of ionophore-based electrodes for ammonium sensing critically analyzing contributions in the last 20 years and with focus in analytical applications.

In-field determination of soil ion content using a handheld device and screen-printed solid-state ion-selective electrodes

Small-holding farmers in the developing world suffer from sub-optimal crop yields because they lack a soil diagnostic system that is affordable, usable, and actionable. This paper details the fabrication and characterization of an integrated point-of-use soil-testing system, comprised of disposable ion-selective electrode strips and a handheld electrochemical reader. Together, the strips and reader transduce soil ion concentrations into to an alphanumeric output that can be communicated via text message to a central service provider offering immediate, customized fertilizer advisory. The solid-state ion-selective electrode (SS-ISE) strips employ a two-electrode design with screen-printable carbon nanotube ink serving as the electrical contacts for the working and reference electrodes. The working electrode comprises a plasticizer-free butyl acrylate ion-selective membrane (ISM), doped with an ion-selective ionophore and lipophilic salt. Meanwhile, the reference electrode includes a screen-printed silver-silver chloride ink and a polyvinyl-butyral membrane, which is doped with sodium chloride for stable reference potentials. As a proof of concept, potassium-selective electrodes are studied, given potassium’s essential role in plant growth and reproduction. The ISE-based system is reproducibly manufactured to yield a Nernstian response with a sub-micromolar detection limit (pK⁺ of 5.18 ± 0.08) and near-Nernstian sensitivity (61 mV/decade) in the presence of a 0.02 M strontium chloride extraction solution. Analysis of soil samples using the printed electrodes and reader yielded a correlation coefficient of 𝑅² = 0.89 with respect to values measured via inductively coupled plasma atomic emission spectroscopy (ICP-AES). The reliable performance of this system is encouraging toward its deployment for soil nutrient management in resource-limited environments.

Functionalization of micro- and nano-apertures with chromate-selective solvent polymeric membrane

A new miniaturization approach to create micro- and nanoscale ion selective electrodes (ISEs) was demonstrated and the concept tested with an environmentally relevant chromate-selective membrane consisting of 7.7:62.2:31.1 wt % Aliquat336:2-NPOE:PVC. Apertures of 100 nM and 30 microM dimensions were made using MEMS fabrication techniques and functionalized through a macroscale application of solvent polymeric membrane. Performance studies for the microscale ISE showed a response slope of -58.6+/-5.6 mV decade(-1) and limit of detection (LOD) of 2.1 x 10(-5)+/-1.1 x 10(-5) M, versus -65.2+/-4.2 mV decade(-1) and 1.8 x 10(-5)+/-6 x 10(-6) M for the nanoscale ISE. This was consistent with control studies with carefully conditioned coated wire electrodes, which demonstrated a response slope of -61.7+/-2.4 mV decade(-1) and a LOD of 3.0 x 10(-6)+/-1 x 10(-6) M. Response times for the best micro- and nanoscale ISEs were in the 10-20 s timeframe. Electrical resistance measurements were in the GOmega range for the microscale ISEs and nanoscale ISEs. Appropriate ISE geometry was confirmed through AFM measurements and calculations based on electrical properties for micro- and nanoscale apertures. These micro- and nanoscale ISEs are expected to have significant impact in the field of microscale analytical processes.