Two-dimensional MXene nanosheets (types Ti3C2Tx and Ti2CTx) as new ion-to-electron transducers in solid-contact calcium ion-selective electrodes

Optimized detection of calcium ion in serum using constant potential coulometry with metastable liquid-liquid contact doping enhanced PEDOT: PSS ink

Highly stable calcium ion selective electrodes (Ca2+-ISEs) were developed by drop-casting a layer of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) as an ion-to-electron transfer layer onto Au electrode. The conductive PEDOT: PSS ink was prepared using a metastable liquid-liquid contact (MLLC) doping method, which induced phase separation, removed excess PSS, and significantly enhanced charge transfer kinetics and conductivity. The resulting Ca2+-ISEs exhibited excellent electrochemical performance. Potentiometric studies revealed a significant sensitivity of 33.1 ± 0.98 mV/decade (N = 3) with a high potential stability of 3.16 ± 2.53 μV/h. Importantly, Ca2+-ISEs combined with the constant potential coulometry method, the lower detection limit was optimized to 8.527 × 10-8 M (LOD = 3σ/s, N = 3). The performance of the Ca2+-ISE system was evaluated in inactivated fetal bovine serum using constant potential coulometry, demonstrating the highest measurement accuracy compared to potentiometric and chronoamperometric. The enhanced PEDOT: PSS-MLLC based Ca2+-ISEs combined with the constant potential coulometry method developed in this research demonstrate considerable potential for clinical applications in blood ion analysis.

Next-Generation Potentiometric Sensors: A Review of Flexible and Wearable Technologies

In recent years, the field of wearable sensors has undergone significant evolution, emerging as a pivotal topic of research due to the capacity of such sensors to gather physiological data during various human activities. Transitioning from basic fitness trackers, these sensors are continuously being improved, with the ultimate objective to make compact, sophisticated, highly integrated, and adaptable multi-functional devices that seamlessly connect to clothing or the body, and continuously monitor bodily signals without impeding the wearer's comfort or well-being. Potentiometric sensors, leveraging a range of different solid contact materials, have emerged as a preferred choice for wearable chemical or biological sensors. Nanomaterials play a pivotal role, offering unique properties, such as high conductivity and surface-to-volume ratios. This article provides a review of recent advancements in wearable potentiometric sensors utilizing various solid contacts, with a particular emphasis on nanomaterials. These sensors are employed for precise ion concentration determinations, notably sodium, potassium, calcium, magnesium, ammonium, and chloride, in human biological fluids. This review highlights two primary applications, that is, (1) the enhancement of athletic performance by continuous monitoring of ion levels in sweat to gauge the athlete's health status, and (2) the facilitation of clinical diagnosis and preventive healthcare by monitoring the health status of patients, in particular to detect early signs of dehydration, fatigue, and muscle spasms.

Ultrasensitive electrochemical microRNA-21 detection based on MXene and ATRP photocatalytic strategy

A Ti3C2TxMXene-based biosensor has been developed and the photocatalytic atom transfer radical polymerization (photo ATRP) amplification strategy applied to detect target miRNA-21 (tRNA). Initially, Ti3C2TxMXene nanosheets were synthesized from the Ti3AlC2 MAX precursor via selective aluminum etching. Then, functionalization of Ti3C2TxMXene nanosheets with 3-aminopropyl triethoxysilane (APTES) via silylation reactions to facilitate covalent bonding with hairpin DNA biomolecules specifically designed for tRNA detection. Upon binding with the tRNA, the hairpin DNA liberated the azide (N₃) group, initiating a click reaction to affix to the photo ATRP initiator. Through the ATRP photoreaction, facilitated by an organic photoredox catalyst and light, a significant amount of ferrocenyl methyl methacrylate (FMMA) monomer was immobilized on the electrode. Therefore, the electrochemical signal is amplified. The electrochemical efficacy of the biosensor was assessed using square wave voltammetry (SWV). Under optimized conditions, the biosensor demonstrated remarkable sensitivity in detecting tRNA, with a linear detection range from 0.01 fM to 10 pM and a detection limit of 2.81 aM. The findings elucidate that the developed biosensor, in conjunction with the photo ATRP strategy, offers reproducibility, stability, and increased sensitivity, underscoring its potential applications within the experimental medical sector of the biomolecular industry.

Self-assembly of a AuNPs/Ti3C2 MXene hydrogel for cascade amplification of microRNA-122 biosensing

A three-dimensional (3D) self-assembled AuNPs/Ti3C2 MXene hydrogel (AuNPs/Ti3C2 MXH) nanocomposite was prepared for the fabrication of a novel microRNA-122 electrochemical biosensor. The 3D hydrogel structure was gelated from two-dimensional MXene nanosheets with the assistance of graphite oxide and ethylenediamine. MXene hydrogels supported the in situ formation of Au nanoparticles (AuNPs) that predominantly exploring the (111) facet, and these AuNPs are utilized as carriers for hairpin DNA (hpDNA) probes, facilitating DNA hybridization. MXene acted as both a reductant and stabilizer, significantly improving the electrochemical signal. In addition, the conjugation of PAMAM dendrimer-encapsulated AuNPs and H-DNA worked as an ideal bridge to connect targets and efficient electrochemical tags, providing a high amplification efficiency for the sensing of microRNA-122. A linear relationship between the peak currents and the logarithm of the concentrations of microRNA-122 from 1.0 × 10-2 to 1.0 × 102 fM (I = 1.642 + 0.312 lgc, R2 = 0.9891), is obtained. The detection limit is  0.8 × 10-2 fM (S/N = 3). The average recovery for human serum detection ranged from 97.32 to 101.4% (RSD < 5%).

A novel and ultrasensitive electrochemical immunosensor based on nanocellulose-Ti3C2Tx@ZrO2 nano framework for the detection of ovalbumin

A versatile and highly sensitive sensing platform based on nanocellulose/MXene/ZrO2 nano framework has been developed at the surface of a glassy carbon electrode (GCE) for detecting ovalbumin (Ova). To create this innovative nano framework, dialdehyde groups were introduced onto the surface of cellulose nanofibers (CNFs), which were then decorated with MXene nanosheets and nanostructured zirconia. Nanocellulose/MXene/ZrO2 nano framework was used as electrochemical mediator and immobilization environment that provided the large surface area and 197 % increment in the electrochemical signal which allowed the Ova detection in the femtomolar range. Ovalbumin antibody was immobilized on the surface of dialdehyde cellulose nanofiber through covalent bonding between amino groups of Ova and dialdehyde groups of CNFs. The assembly process of nano framework, anti-Ova, and Ova antigen were characterized using electrochemical approaches (CV and DPV). The fabricated immunosensor is further applied to DPV detection of Ova and it demonstrated a linear response to Ova antigen in the linear range of 0.01-1000 pg/mL. With optimal experimental conditions, the detection limit, quantification limit and sensitivity of Ova were found to be 1.1 fg/mL, 0.01 pg/mL and 0.1497 μA pg/mL cm-2, respectively. The fabricated immunosensor exhibited high selectivity, reproducibility, and interference resistance and achieved excellent recoveries in real food samples spiked with Ova, indicating its potential applicability in food safety monitoring.

MXene-Coated Ion-Selective Electrode Sensors for Highly Stable and Selective Lithium Dynamics Monitoring

Lithium holds immense significance in propelling sustainable energy and environmental systems forward. However, existing sensors used for lithium monitoring encounter issues concerning their selectivity and long-term durability. Addressing these challenges is crucial to ensure accurate and reliable lithium measurements during the lithium recovery processes. In response to these concerns, this study proposes a novel approach involving the use of an MXene composite membrane with incorporated poly(sodium 4-styrenesulfonate) (PSS) as an antibiofouling layer on the Li+ ion selective electrode (ISE) sensors. The resulting MXene-PSS Li+ ISE sensor demonstrates exceptional electrochemical performance, showcasing a superior slope (59.42 mV/dec), lower detection limit (10-7.2 M), quicker response time (∼10 s), higher selectivity to Na+ (-2.37) and K+ (-2.54), and reduced impedance (106.9 kΩ) when compared to conventional Li+ ISE sensors. These improvements are attributed to the unique electronic conductivity and layered structure of the MXene-PSS nanosheet coating layer. In addition, the study exhibits the long-term accuracy and durability of the MXene-PSS Li+ ISE sensor by subjecting it to real wastewater testing for 14 days, resulting in sensor reading errors of less than 10% when compared to laboratory validation results. This research highlights the great potential of MXene nanosheet coatings in advancing sensor technology, particularly in challenging applications, such as detecting emerging contaminants and developing implantable biosensors. The findings offer promising prospects for future advancements in sensor technology, particularly in the context of sustainable energy and environmental monitoring.

A Floating Capsule Electrochemical System for In Situ and Multichannel Ion-Selective Sensing

Free-floating electrochemical sensors are promising for in situ bioprocess monitoring with the advantages of movability, a lowered risk of contamination, and a simplified structure of the bioreactor. Although floating sensors were developed for the measurement of physical and chemical indicators such as temperature, velocity of flow, pH, and dissolved oxygen, it is the lack of available electrochemical sensors for the determination of the inorganic ions in bioreactors that has a significant influence on cell culture. In this study, a capsule-shaped electrochemical system (iCapsuleEC) is developed to monitor ions including K+, NH4+, Na+, Ca2+, and Mg2+ based on solid-contact ion-selective electrodes (SC-ISEs). It consists of a disposable electrochemical sensor and signal-processing device with features including multichannel measurement, self-calibration, and wireless data transmission. The capacities of the iCapsuleEC were demonstrated not only for in situ measurement of ion concentrations but also for the optimization of the sensing electrodes. We also explored the possibility of the system for use in detection in simulated cell culture media.

Bipotential-resolved electrochemiluminescence biosensor based on Bi2S3@Au nanoflowers for simultaneous detection of Cd(II) and ampicillin in aquatic products

In this paper, an electrochemiluminescence (ECL) biosensor was constructed using Bi₂S₃@Au nanoflowers as the based nanomaterial and Au@luminol and CdS QDs as independent ECL emission signal respectively. As the substrate of the working electrode, Bi₂S₃@Au nanoflowers improved the effective area of electrode and accelerated electron transfer rate between gold nanoparticles and aptamer, provided a good interface environment for the loading of luminescent materials. Then, the Au@luminol functionalized DNA2 probe was used as an independent ECL signal source under positive potential and recognized Cd(II), while the CdS QDs functionalized DNA3 probe was used as an independent ECL signal source under negative potential and recognized ampicillin. The simultaneous detection of Cd(II) and ampicillin in different concentrations are realized. This sensor not only has good selectivity and high sensitivity in real sample detection, but also open up a novel way to construct multi-target ECL biosensor for simultaneous detection.

Monitoring of trace aquatic sulfonamides through hollow zinc-nitrogen-carbon electrocatalysts anchored on MXene architectures

Herein, we designed and fabricated hollow N-doped carbon polyhedrons with atomically dispersed Zn species (Zn@HNCPs) through a topo-conversion strategy by utilising metal-organic frameworks as precursors. Zn@HNCPs achieved efficient electrocatalytic oxidation of sulfaguanidine (SG) and phthalyl sulfacetamide (PSA) sulfonamides through the high intrinsic catalytic activity of the Zn-N₄ sites and excellent diffusion from the hollow porous nanostructures. The combination of the novel Zn@HNCPs with two-dimensional Ti₃C₂T_x MXene nanosheets resulted in improved synergistic electrocatalytic performance for the simultaneous monitoring of SG and PSA. Therefore, the detection limit of SG for this technique is much lower than those of other reported techniques; to the best of our knowledge, this is the first detection approach for PSA. Moreover, these electrocatalysts show promise for the quantification of SG and PSA in aquatic products. Our insights and findings can serve as guidelines for the development of highly active electrocatalysts for application in next-generation food analysis sensors.

A dual-modal electrochemical aptasensor based on intelligent DNA Walker with cascade signal amplification powered by Nb.BbvCI for Pb2

As a unique nanomachine, DNA Walker can move continuously along a specific orbit to amplify signal. Therefore, based on DNA Walker and endonuclease assisted signal amplification strategy, a novel dual-mode visual electrochemical aptasensor was constructed for the detection of Pb²⁺. Ceric dioxide@mesoporous carbon (CeO₂/CS)@AuNPs not only could improve the conductivity of sensing interface but also could fix the aptamer. DNA Walker moved on the surface of the electrode to realize the pairing with the Ag-γFe₂O₃/cDNA probe, forming a special base sequence that could be spliced by the Nb.BbvCI. Under the action of endonuclease Nb.BbvCI, the Ag-γFe₂O₃/cDNA probe was continuously sheared and the amount on the electrode was decreased to amplify the signal. Besides, the nanoenzyme of Ag-γFe₂O₃ could catalyze 3'3'5'5'-tetramethylbenzidine (TMB) to blue color realizing the visual detection of Pb²⁺. The sensor has been successfully applied to the visual and accurate rapid detection of Pb²⁺ in aquatic products. The fabricated method of the sensor open up a new way for visual and accurate the detection of environmental pollutants.

Ion-selective electrodes based on laser-induced graphene as an alternative method for nitrite monitoring

Nitrite is an important food additive for cured meats; however, high nitrite levels pose adverse health effects to humans. Hence, monitoring nitrite concentration is critical to comply with limits imposed by regulatory agencies. Laser-induced graphene (LIG) has proven to be a scalable manufacturing alternative to produce high-performance electrochemical transducers for sensors. Herein, we expand upon initial LIG studies by fabricating hydrophilic and hydrophobic LIG that are subsequently converted into ion-selective sensors to monitor nitrite in food samples with comparable performance to the standard photometric method (Griess method). The hydrophobic LIG resulted in an ion-selective electrode with improved potential stability due partly to a decrease in the water layer between the electrode and the nitrite poly(vinyl) chloride-based ion-selective membrane. These resultant nitrite ion-selective sensors displayed Nernstian response behavior with a sensitivity of 59.5 mV dec-1, a detection limit of 0.3 ± 0.1 mg L-1 (mean ± standard deviation), and a broad linear sensing range from 10-5 to 10-1 M, which was significantly larger than currently published nitrite methods. Nitrite levels were determined directly in food extract samples of sausage, ham, and bacon for 5 min. These sensor metrics are significant as regulatory agencies limit nitrite levels up to 200 mg L-1 in finished products to reduce the potential formation of nitrosamine (carcinogenic compound). These results demonstrate the versatility of LIG as a platform for ion-selective-LIG sensors and simple, efficient, and scalable electrochemical sensing in general while demonstrating a promising alternative to monitor nitrite levels in food products ensuring regulatory compliance.

2D MXene-Based Biosensing: A Review

MXene emerged as decent 2D material and has been exploited for numerous applications in the last decade. The remunerations of the ideal metallic conductivity, optical absorbance, mechanical stability, higher heterogeneous electron transfer rate, and good redox capability have made MXene a potential candidate for biosensing applications. The hydrophilic nature, biocompatibility, antifouling, and anti-toxicity properties have opened avenues for MXene to perform in vitro and in vivo analysis. In this review, the concept, operating principle, detailed mechanism, and characteristic properties are comprehensively assessed and compiled along with breakthroughs in MXene fabrication and conjugation strategies for the development of unique electrochemical and optical biosensors. Further, the current challenges are summarized and suggested future aspects. This review article is believed to shed some light on the development of MXene for biosensing and will open new opportunities for the future advanced translational application of MXene bioassays.

Detection of Heavy Metal Ions by Ratiometric Photoelectric Sensor

In recent years, heavy metal pollution has become increasingly serious. Heavy metals exist in an environment mainly in the form of ions (heavy metal ions, HMs). They can contaminate food, water, soil, and the atmosphere, leading to serious harm to plants and animals. With high bioavailability and nonbiodegradability, HMs can accumulate through biomagnification. Consequently, heavy metal pollution has become the cause of many fatal diseases threatening human health and ecological environment. Therefore, the accurate detection of HMs is vital and necessary. In this paper, the harm and limit standards of heavy metals were systematically summarized and the common analysis methods were overviewed and compared. Specifically, the latest research progress of ratiometric photoelectric sensor, including optical and electrical sensor, were mainly described. The research status and advantages and disadvantages of a photoelectric sensor were summarized. Furthermore, the future directions were proposed, which provided the reference for the further research and application of the ratiometric photoelectric sensor.

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.

Electrochemiluminescence biosensor for determination of lead(II) ions using signal amplification by Au@SiO2 and tripropylamine-endonuclease assisted cycling process

MXene@Au as the base and Au@SiO₂ as signal amplification factor were used for constructing an ultrasensitive "on-off" electrochemiluminescence (ECL) biosensor for the detection of Pb²⁺ in water. The use of MXene@Au composite provided a good interface environment for the loading of tris(2,2-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) on the electrode. Based on resonance energy transfer, the Au (core) SiO₂ (shell) (Au@SiO₂) nanoparticles stimulate electron transport and promote tripropylamine (TPrA) oxidation. The luminescence effect of Au@SiO₂ was five times that of AuNPs and SiO₂ nanomaterials alone, and the ECL intensity was greatly improved. In addition, Pb²⁺ activated the aptamer to exert its endonuclease activity, which realized the signal cycle amplification in the process of Pb²⁺ detection. When Pb²⁺ was added, the ECL signal weakened, and the Pb²⁺ concentration was detected according to the decreased ECL intensity. Under optimized experimental conditions, this aptamer sensor for Pb²⁺ has a wide detection range (0.1 to 1 × 10⁶ ng L^-1) and a low detection limit (0.059 ng L^-1). The relative standard deviation (RSD) of the sensor is 0.39-0.99%, and the recovery of spiked standard is between 90.00 and 125.70%. The sensor shows good selectivity and high sensitivity in actual water sample analysis. This signal amplification strategy possibly provides a new method for the detection of other heavy metal ions and small molecules.

Room-temperature high-precision printing of flexible wireless electronics based on MXene inks

Wireless technologies-supported printed flexible electronics are crucial for the Internet of Things (IoTs), human-machine interaction, wearable and biomedical applications. However, the challenges to existing printing approaches remain, such as low printing precision, difficulty in conformal printing, complex ink formulations and processes. Here we present a room-temperature direct printing strategy for flexible wireless electronics, where distinct high-performance functional modules (e.g., antennas, micro-supercapacitors, and sensors) can be fabricated with high resolution and further integrated on various flat/curved substrates. The additive-free titanium carbide (Ti₃C₂T_x) MXene aqueous inks are regulated with large single-layer ratio (>90%) and narrow flake size distribution, offering metallic conductivity (~6, 900 S cm⁻¹) in the ultrafine-printed tracks (3 μm line gap and 0.43% spatial uniformity) without annealing. In particular, we build an all-MXene-printed integrated system capable of wireless communication, energy harvesting, and smart sensing. This work opens a door for high-precision additive manufacturing of printed wireless electronics at room temperature.

High-precision printing of flexible wireless electronics has not been achieved before. Here, the authors leverage a room-temperature direct printing strategy to realize an all-MXene-printed integrated system capable of wireless communication, energy harvesting, and smart sensing.

Superhydrophobic Functionalized Ti3C2Tx MXene-Based Skin-Attachable and Wearable Electrochemical pH Sensor for Real-Time Sweat Detection

Sweat pH is a critical indicator for evaluating human health. With the extensive attention on the wearable and flexible biosensing devices, the technology for the monitoring of human sweat can be realized. In this study, a sensitive, miniaturized, and flexible electrochemical sweat pH sensor was developed for the continuous and real-time monitoring of the hydrogen-ion concentration in human sweat. A flexible electrode was fabricated on the poly(ethylene terephthalate) (PET) substrate by a simple and low-cost screen-printing technology, which was based on the integration of fluoroalkyl silane-functionalized Ti₃C₂T_x (F-Ti₃C₂T_x) and the polyaniline (PANI) membrane technology instead of the traditional ion-sensitive membrane. The surface functionalization strategy for Ti₃C₂T_x with perfluorodecyltrichlorosilane can provide environmental stability. Functionalized Ti₃C₂T_x (F-Ti₃C₂T_x) was doped with PANI to obtain improved responsiveness, sensitivity, and reversibility. The constructed microsize, portable, and wearable F-Ti₃C₂T_x/PANI pH sensor aimed to real-time monitor the pH value of human sweat during exercise. On-body sweat pH monitoring for females and males, respectively, exhibited high accuracy and continuous stability compared with ex situ analyses. This study thus offers a facile and practical solution for developing a highly reliable MXene-based mini-type pH sensor to realize the online monitoring of human sweat pH.

Design of an amperometric glucose oxidase biosensor with added protective and adhesion layers

Enzymes have demonstrated great potential in the development of advanced electroanalysis devices due to their unique recognition and catalytic properties. However, unsatisfactory stability and limited electron communication of traditional enzyme sensors seriously hinder their large-scale application. In this work, a simple and effective method is proposed to improve the stability of enzyme sensors by using sodium hyaluronate (SH) as a protective film, MXene-Ti₃C₂/Glucose oxidase (GOD) as the reaction layer, and chitosan (CS) /reduced graphene oxide (rGO) as the adhesion layer. Results demonstrate that the repeatability of the designed sensor increased by 73.3% after improving the adhesion between the reaction layer and the current collector and that its response ability was greatly enhanced. Moreover, the long-term stability of the electrode surface with SH protective film proved to be superior than that without protective film, which suggests that this design can effectively improve the overall performance of the enzyme biosensor. This work proposed a multi-tier synergistic approach for improving the reliability of enzyme sensors. Graphical abstract Our proposed protective and adhesion layer can greatly improve the stability of enzyme sensor and realize the rapid detection of glucose in serum sample.

Highly reproducible solid contact ion selective electrodes: Emerging opportunities for potentiometry - A review

The solid contact ion-selective electrodes (SC-ISEs) have been extensively studied in the field of ion sensing as they offer the possibility of miniaturization, are relatively inexpensive in comparison to other analytical techniques and allow straightforward and routine analyses of ions in a number of clinical, environmental and industrial process samples. In recent years, significant interest has grown in the development of SC-ISEs with well-defined interfacialpotentials at the membrane, solid contact, and substrate electrode interfaces. This has resulted in interesting SC-ISEs exhibiting high electrode-to-electrode potential reproducibility, for those made in a single batch of electrodes, some approaching or exceeding those observed in liquid-contact ISEs. The advancement in the potential reproducibility of SC-ISEs has been partially achieved by scrutinizing insufficiently reproducible fabrication methods of SC-ISEs, or by introducing novel control measures or modifiers to components of the ISEs. This paper provides an overview of the methods as well as the challenges in establishing and maintaining reproducible potentials during the fabrication and use of novel SC-ISEs. The rules outlined in the works reviewed may form the basis of further development of cost-effective, user-friendly, limited calibration or calibration-free potentiometric SC-ISEs to achieve reliable ion analyses here and now.

Structure, synthesis, and sensing applications of single-walled carbon nanohorns

Single-walled carbon nanohorns (SWCNHs), a type of tapered carbon nanomaterials, are generally prepared by laser ablation method, arc method, and Joule heating method without the addition of metal catalysts, which makes them pure and environmentally friendly. The obtained aggregates of SWCNHs mainly have three different types of structure, dahlia-like, bud-like, and seed-like. Over the past few decades, they have been widely used in the fields of energy, medicine, chemistry, and sensing. The SWCNHs-based sensors have shown high sensitivity, rapid response, and excellent stability, which are mainly attributed to the excellent electrical conductivity, large electrochemical window, large specific surface area, and mechanical strength of SWCNHs. In this review, we systematically summarizes the structures, synthesis methods, and sensing applications of SWCNHs, including electrochemical sensors, photoelectrochemical sensors, electrochemiluminescence sensors, fluorescent sensors, and resistive sensors. Moreover, the development prospects of SWCNHs in this field are also discussed.

Flexible potentiometric pH sensors for wearable systems

There is a growing demand for developing wearable sensors that can non-invasively detect the signs of chronic diseases early on to possibly enable self-health management. Among these the flexible and stretchable electrochemical pH sensors are particularly important as the pH levels influence most chemical and biological reactions in materials, life and environmental sciences. In this review, we discuss the most recent developments in wearable electrochemical potentiometric pH sensors, covering the key topics such as (i) suitability of potentiometric pH sensors in wearable systems; (ii) designs of flexible potentiometric pH sensors, which may vary with target applications; (iii) materials for various components of the sensor such as substrates, reference and sensitive electrode; (iv) applications of flexible potentiometric pH sensors, and (v) the challenges relating to flexible potentiometric pH sensors.

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).