Magnetic-Field-Driven Extraction of Bioreceptors into Polymeric Membranes for Label-Free Potentiometric Biosensing

A novel biosensing strategy on the dynamic and on-site detection of Vibrio coralliilyticus eDNA for coral health warnings

Heat stress and coral diseases are the predominant factors causing the degradation of coral reef ecosystems. Over recent years, Vibrio coralliilyticus was identified as a temperature-dependent pathogen causing tissue lysis in Pocillopora damicornis and one of the primary pathogens causing bleaching and mortality in other corals. Yet current detection techniques for V. coralliilyticus rely primarily on qPCR and ddPCR, which cannot meet the requirements for non-invasive and real-time detection. Herein, we developed an effective electrochemical biosensor modified by an Au-MoS2/rGO (AMG) nanocomposites and a specific capture probe to dynamically detect V. coralliilyticus environment DNA (eDNA) in aquarium experiments, with a lower limit of detection (0.28 fM) for synthetic DNA and a lower limit of quantification (9.8 fg/µL, ∼0.86 copies/µL) for genomic DNA. Its reliability and accuracy were verified by comparison with the ddPCR method (P > 0.05). Notably, coral tissue started to lyse at only 29 °C when the concentration of V. coralliilyticus increased abruptly to 880 copies/µL, indicating the biosensor could reflect the types of pathogen and health risks of corals under heat stress. Overall, the novel and reliable electrochemical biosensing technology provides an efficient strategy for the on-site monitoring and early warning of coral health in the context of global warming.

Chronopotentiometric sensors for antimicrobial peptide-based biosensing of Staphylococcus aureus

Charged antimicrobial peptides can be used for direct potentiometric biosensing, but have never been explored. We report here a galvanostatically-controlled potentiometric sensor for antimicrobial peptide-based biosensing. Solid-state pulsed galvanostatic sensors that showed excellent stability under continuous galvanostatic polarization were prepared by utilizing reduced graphene oxide/poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (rGO/PEDOT: PSS) as a solid contact. More importantly, the chronopotentiometric sensor can be made sensitive to antimicrobial peptides with intrinsic charge on demand via a current pulse. In this study, a positively charged antimicrobial peptide that can bind to Staphylococcus aureus with high affinity and good selectivity was designed as a model. Two arginine residues with positive charges were linked to the C-terminal of the peptide sequence to increase its potentiometric responses on the electrode. The bacteria binding-induced charge or charge density change of the antimicrobial peptide enables the direct chronopotentiometric detection of the target. Under the optimized conditions, the concentration of Staphylococcus aureus can be determined in the linear range 10-1.0 × 105 CFU mL-1 with a detection limit of 10 CFU mL-1. It is anticipated that such a chronopotentiometric sensing platform is readily adaptable to detect other bacteria by choosing the peptides.

Rapid antibiotic screening based on E. coli apoptosis using a potentiometric sensor array

Phenotypic antimicrobial susceptibility testing enables reliable antibiotic screening but requires multiple strategies to identify each phenotypic change induced by different bactericidal mechanisms. Bacteria apoptosis with typical phenotypic features has never been explored for antibiotic screening. Herein, we developed an antibiotic screening method based on the measurement of antibiotic-induced phosphatidylserine (PS) exposure of apoptotic bacteria. Phosphatidylserine externalization of E. coli that can be widely used as an apoptosis marker for antibiotics with different antibacterial mechanisms was explored. A positively charged PS-binding peptide was immobilized on magnetic beads (MBs) to recognize and capture apoptotic E. coli with PS externalization. Apoptotic E. coli binding led to the charge or charge density change of MBs-peptide, resulting in a potential change on a magneto-controlled polymeric membrane potentiometric sensor. Based on the detection of apoptotic E. coli killed by antibiotics, antibiotic screening for different classes of antibiotics and silver nanoparticles was achieved within 1.5 h using a potentiometric sensor array. This approach enables sensitive, general, and time-saving antibiotic screening, and may open up a new path for antibiotic susceptibility testing.

Polymeric membrane potentiometric antibiotic sensors using computer-aided screening of supramolecular macrocyclic carriers

Carrier-based polymeric membrane potentiometric sensors are an ideal tool for detecting ionic species. However, in the fabrication of these sensors, the screening of carriers still relies on empirical trial- and error-based optimization, which requires tedious and time-consuming experimental verification. In this work, computer-aided screening of carriers is applied in the preparation of polymeric membrane potentiometric sensors. Molecular docking is used to study the host-guest interactions between receptors and targets. Binding energies are employed as the standard to screen the appropriate carrier. As a proof-of-concept experiment, the antibiotic ciprofloxacin is selected as the target model. A series of supramolecular macrocyclic receptors including cyclodextrins, cucurbiturils and calixarenes are chosen as potential receptors. The proposed sensor based on the receptor calix[4]arene screened by molecular docking shows a lower detection limit of 0.5 μmol L-1 for ciprofloxacin. It can be expected that the proposed computer-aided screening technique of carriers can provide a simple but highly efficient method for the fabrication of carrier-based electrochemical and optical sensors.

Modern Potentiometric Biosensing Based on Non-Equilibrium Measurement Techniques

Modern potentiometric sensors based on polymeric membrane ion-selective electrodes (ISEs) have achieved new breakthroughs in sensitivity, selectivity, and stability and have extended applications in environmental surveillance, medical diagnostics, and industrial analysis. Moreover, nonclassical potentiometry shows promise for many applications and opens up new opportunities for potentiometric biosensing. Here, we aim to provide a concept to summarize advances over the past decade in the development of potentiometric biosensors with polymeric membrane ISEs. This Concept article articulates sensing mechanisms based on non-equilibrium measurement techniques. In particular, we emphasize new trends in potentiometric biosensing based on attractive dynamic approaches. Representative examples are selected to illustrate key applications under zero-current conditions and stimulus-controlled modes. More importantly, fruitful information obtained from non-equilibrium measurements with dynamic responses can be useful for artificial intelligence (AI). The combination of ISEs with advanced AI techniques for effective data processing is also discussed. We hope that this Concept will illustrate the great possibilities offered by non-equilibrium measurement techniques and AI in potentiometric biosensing and encourage further innovations in this exciting field.

Deep Learning-Enhanced Potentiometric Aptasensing with Magneto-Controlled Sensors

Bioelectronic sensors that report charge changes of a biomolecule upon target binding enable direct and sensitive analyte detection but remain a major challenge for potentiometric measurement, mainly due to Debye Length limitations and the need for molecular-level platforms. Here, we report on a magneto-controlled potentiometric method to directly and sensitively measure the target-binding induced charge change of DNA aptamers assembled on magnetic beads using a polymeric membrane potentiometric ion sensor. The potentiometric responses of the negatively charged aptamer, serving as a receptor and reporter, were dynamically controlled and modulated by applying a magnetic field. Based on a potentiometric array, this non-equilibrium measurement technique combined with deep learning algorithms allows for rapidly and reliably classifying and quantifying diverse small molecules using antibiotics as models. This potentiometric strategy opens new modalities for sensing applications.

Carbon-Based Transducers for Solid-Contact Calcium Ion-Selective Electrodes: Mesopore and Nitrogen-Doping Effects

Solid-contact ion-selective electrodes (SC-ISEs) exhibit great potential in the detection of routine and portable ions which rely on solid-contact (SC) materials for the transduction of ions to electron signals. Carbon-based materials are state-of-the-art SC transducers due to their high electrical double-layer (EDL) capacitance and hydrophobicity. However, researchers have long searched for ways to enhance the interfacial capacitance in order to improve the potential stability. Herein, three representative carbon-based SC materials including nitrogen-doped mesoporous carbon (NMC), reduced graphene oxide (RGO), and carbon nanotubes (CNT) were compared. The results disclose that the NMC has the highest EDL capacitance owing to its mesopore structure and N-doping while maintaining high hydrophobicity so that no obvious water-layer effect was observed. The Ca²⁺-SC-ISEs based on the SC of NMC exhibited high potential stability compared with RGO and CNT. This work offers a guideline for the development of carbon-material-based SC-ISEs through mesoporous and N-doping engineering to improve the interfacial capacitance. The developed NMC-based solid-contact Ca²⁺-SC-ISE exhibited a Nernstian slope of 26.3 ± 3.1 mV dec^-1 ranging from 10 μM to 0.1 M with a detection limit of 3.2 μM. Finally, a practical application using NMC-based SC-ISEs was demonstrated through Ca²⁺ ion analysis in mineral water and soil leaching solutions.

Beyond Nonactin: Potentiometric Ammonium Ion Sensing Based on Ion-selective Membrane-free Prussian Blue Analogue Transducers

The determination of ammonium ions (NH₄⁺) is of significance to environmental, agriculture, and human health. Potentiometric NH₄⁺ sensors based on solid-contact ion selective electrodes (SC-ISEs) feature point-of-care testing and miniaturization. However, the state-of-the-art SC-ISEs of NH₄⁺ during the past 20 years strongly rely on the organic ammonium ionophore-based ion selective membrane (ISM), typically by nonactin for the NH₄⁺ recognition. Herein, we report a Prussian blue analogue of copper(II)-hexacyanoferrate (CuHCF) for an ISM-free potentiometric NH₄⁺ sensor without using the ionophores. CuHCF works as a bifunctional transducer that could realize the ion-to-electron transduction and NH₄⁺ recognition. CuHCF exhibits competitive analytical performances regarding traditional nonactin-based SC-ISEs of NH₄⁺, particularly for the selectivity toward K⁺. The cost and preparation process have been remarkably reduced. The theoretical calculation combined with electrochemical tests further demonstrate that relatively easier intercalation of NH₄⁺ into the lattices of CuHCF determines its selectivity. This work provides a concept of the ISM-free potentiometric NH₄⁺ sensor beyond the nonactin ionophore through a CuHCF bifunctional transducer.

Towards potentiometric detection in nonaqueous media: Evaluation of the impacts of organic solvents on polymeric membrane ion-selective electrodes

Polymeric membrane ion-selective electrodes (ISEs) have been widely used in various fields including clinical diagnosis, environmental monitoring and industrial analysis. Although most samples of analytical interest measured by the ISEs are aqueous solutions, the applications of these electrodes in nonaqueous media are often inevitable. Unfortunately, so far, little has been known about the extent to which the properties of the ISEs could be affected by the organic solvents. Herein, the feasibility for the applications of the polymeric membrane ISEs in nonaqueous media has been investigated. A polymeric membrane Ca²⁺-ISE is chosen as a model of potentiometric sensors. Four typical water miscible organic solvents (three protic solvents: ethanol, acetic acid, and methanol, and one aprotic dipolar solvent: acetonitrile) are used as the representative examples. Experiments show that the aprotic solvent acetonitrile has the strongest destructive ability towards the sensing performance of the ISE in terms of Nernstian slope and selectivity coefficient. Moreover, the effect on the sensing performance depends on the kind of the protic solvent, the immersion time and the polarity of the membrane plasticizer. We believe that the obtained results could promote further applications of the polymeric membrane ISEs in the organic solvent-containing samples, which could significantly extend the application scope of the ISEs.

Solid-Contact Potentiometric Anion Sensing Based on Classic Silver/Silver Insoluble Salts Electrodes without Ion-Selective Membrane

Current solid potentiometric ion sensors mostly rely on polymeric-membrane-based, solid-contact, ion-selective electrodes (SC-ISEs). However, anion sensing has been a challenge with respect to cations due to the rareness of anion ionophores. Classic metal/metal insoluble salt electrodes (such as Ag/AgCl) without an ion-selective membrane (ISM) offer an alternative. In this work, we first compared the two types of SC-ISEs of Cl- with/without the ISM. It is found that the ISM-free Ag/AgCl electrode discloses a comparable selectivity regarding organic chloride ionophores. Additionally, the electrode exhibits better comprehensive performances (stability, reproducibility, and anti-interference ability) than the ISM-based SC-ISE. In addition to Cl-, other Ag/AgX electrodes also work toward single and multi-valent anions sensing. Finally, a flexible Cl- sensor was fabricated for on-body monitoring the concentration of sweat Cl- to illustrate a proof-of-concept application in wearable anion sensors. This work re-emphasizes the ISM-free SC-ISEs for solid anion sensing.

Solid-Contact Ion Sensing Without Using an Ion-Selective Membrane through Classic Li-Ion Battery Materials

The solid-contact ion-selective electrodes (SC-ISEs) are a type of potentiometric analytical device with features of rapid response, online analysis, and miniaturization. The state-of-the-art SC-ISEs are composed of a solid-contact (SC) layer and an ion-selective membrane (ISM) layer with respective functions of ion-to-electron transduction and ion recognition. Two challenges for the SC-ISEs are the water-layer formation at the SC/ISM phase boundary and the leaking of ISM components, which are both originated from the ISM. Herein, we report a type of SC-ISE based on classic Li-ion battery materials as the SC layer without using the ISM for potentiometric lithium-ion sensing. Both LiFePO₄- and LiMn₂O₄-based SC-ISEs display good Li⁺ sensing properties (sensitivity, selectivity, and stability). The proposed LiFePO₄ electrode exhibits comparable sensitivity and a linear range to conventional SC-ISEs with ISM. Owing to the nonexistence of ISM, the LiFePO₄ electrode displays high potential stability. Besides, the LiMn₂O₄ electrode shows a Nernstian response toward Li⁺ sensing in a human blood serum solution. This work emphasizes the concept of non-ISM-based SC-ISEs for potentiometric ion sensing.