In Vivo Measurement of Calcium Ion with Solid-State Ion-Selective Electrode by Using Shelled Hollow Carbon Nanospheres as a Transducing Layer

Attomolar-sensitive milk fever sensor using 3D-printed multiplex sensing structures

The diagnosis of milk fever or hypocalcemia in lactating cows has a significant economic impact on the dairy industry. It is challenging to identify asymptomatic subclinical hypocalcemia (SCH) in transition dairy cows. Monitoring subclinical hypocalcemia in milk samples can expedite treatment and improve the health, productivity, and welfare of dairy cows. In this study, an attomolar-sensitive sensor is developed using extrusion-based 3D-printed sensing structures to detect the ratio of ionized calcium to phosphate levels in milk samples. The unique geometries of the lateral structure of 3D-printed sensors, along with the wrinkled surfaces, provide a limit of detection down to the attomole (138 aM) concentration of the target analyte. The calcium-to-phosphate ratio in milk samples not only provides early disease indications but also enables on-site testing. This highly selective test is validated using real milk and blood samples, and the results are compared with those of commercial meters. This fast response (~10 s) low-cost sensor opens a promising tool for the farm-side diagnostic of dairy cows that can promote best practice management of dairy cows.

In Vivo Electrochemical Biosensing Technologies for Neurochemicals: Recent Advances in Electrochemical Sensors and Devices

In vivo electrochemical sensing of neurotransmitters, neuromodulators, and metabolites plays a critical role in real-time monitoring of various physiological or psychological processes in the central nervous system. Currently, advanced electrochemical biosensors and technologies have been emerging as prominent ways to meet the surging requirements of in vivo monitoring of neurotransmitters and neuromodulators ranging from single cells to brain slices, even the entire brain. This review introduces the fundamental working principles and summarizes the achievements of in vivo electrochemical biosensing technologies including voltammetry, amperometry, potentiometry, field-effect transistor (FET), and organic electrochemical transistor (OECT). According to the elaborate feature of sensing technology, versatile strategies have been devoted to solve critical issues associated with the sensing of neurochemicals under an intricate physiological environment. Voltammetry is a universal technique to investigate electrochemical processes in complex matrices which could realize the miniaturization of electrodes, while amperometry serves as a well-suited approach offering high temporal resolution which is favorable for the fast oxidation-reduction kinetics of neurochemicals. Potentiometry realizes quantitative analysis by recording the potential difference with reduced invasiveness and high compatibility. FET and OECT serve as amplification strategies with higher sensitivity than traditional technologies. Furthermore, we point out the current shortcomings and address the challenges and perspectives of in vivo electrochemical biosensing technologies.

Complexation Behavior and Clinical Assessment of Isomeric Calcium Ionophores of ETH 1001 in Polymeric Ion-Selective Membranes

Calcium ions are crucial in numerous physiological processes, and their precise measurement is important for many clinical diagnostics and therapeutic interventions. Traditional detection methods, such as atomic absorption spectroscopy, cannot meet clinical requirements, as they measure total calcium instead of the clinically relevant ionized form (Ca2+). Ion-selective electrodes (ISEs) provide a convenient, accurate, and specific method for Ca2+ determination. Here, two isomeric calcium ionophores (R,R)-calcium ionophore I, also known as ETH 1001, and (R,S)-calcium ionophore I are synthesized and characterized for the analysis of whole blood samples with the Eaglenos blood gas analyzer (model: EG-i30) equipped with test cartridges containing screen-printed electrodes. (R,R)-Calcium ionophore I demonstrated excellent precision in whole blood samples, achieving an average bias of -2.2% compared with the available gold standard. On the other hand, the (R,S) isomer was not satisfactory, exhibiting a bias of up to -20%. Ion transfer voltammetry at thin membrane films gave information about the complex stoichiometry, complex formation constants, and ion selectivity for the two isomeric ionophores. A stoichiometry of the Ca2+-ionophore complex was confirmed to be 1:2 for both ionophores, while the (R,R) isomer gave 3.4 orders of magnitude larger complex formation constants and a modestly higher selectivity. While these data are valuable, the poor performance of membranes containing the (R,S) isomer is not directly apparent from the fundamental binding characteristics. It may be caused by interference from lipophilic blood sample components and/or surface adsorption processes, suggesting that routine selectivity characterizations of membranes containing selective ionophores are insufficient to assess their usefulness in clinical applications.

Advancing Multi-Ion Sensing with Poly-Octylthiophene: 3D-Printed Milker-Implantable Microfluidic Device

On-site rapid multi-ion sensing accelerates early identification of environmental pollution, water quality, and disease biomarkers in both livestock and humans. This study introduces a pocket-sized 3D-printed sensor, manufactured using additive manufacturing, specifically designed for detecting iron (Fe2+), nitrate (NO3 -), calcium (Ca2+), and phosphate (HPO4 2-). A unique feature of this device is its utilization of a universal ion-to-electron transducing layer made from highly redox-active poly-octylthiophene (POT), enabling an all-solid-state electrode tailored to each ion of interest. Manufactured with an extrusion-based 3D printer, the device features a periodic pattern of lateral layers (width = 80 µm), including surface wrinkles. The superhydrophobic nature of the POT prevents the accumulation of nonspecific ions at the interface between the gold and POT layers, ensuring exceptional sensor selectivity. Lithography-free, 3D-printed sensors achieve sensitivity down to 1 ppm of target ions in under a minute due to their 3D-wrinkled surface geometry. Integrated seamlessly with a microfluidic system for sample temperature stabilization, the printed sensor resides within a robust, pocket-sized 3D-printed device. This innovation integrates with milking parlors for real-time calcium detection, addressing diagnostic challenges in on-site livestock health monitoring, and has the capability to monitor water quality, soil nutrients, and human diseases.

General In Situ Engineering of Carbon-Based Materials on Carbon Fiber for In Vivo Neurochemical Sensing

Developing real-time, dynamic, and in situ analytical methods with high spatial and temporal resolutions is crucial for exploring biochemical processes in the brain. Although in vivo electrochemical methods based on carbon fiber (CF) microelectrodes are effective in monitoring neurochemical dynamics during physiological and pathological processes, complex post modification hinders large-scale productions and widespread neuroscience applications. Herein, we develop a general strategy for the in situ engineering of carbon-based materials to mass-produce functional CFs by introducing polydopamine to anchor zeolitic imidazolate frameworks as precursors, followed by one-step pyrolysis. This strategy demonstrates exceptional universality and design flexibility, overcoming complex post-modification procedures and avoiding the delamination of the modification layer. This simplifies the fabrication and integration of functional CF-based microelectrodes. Moreover, we design highly stable and selective H+, O2, and ascorbate microsensors and monitor the influence of CO2 exposure on the O2 content of the cerebral tissue during physiological and ischemia-reperfusion pathological processes.

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.

Recent advances in nanomaterial-based solid-contact ion-selective electrodes

Solid-contact ion-selective electrodes (SC-ISEs) are advanced potentiometric sensors with great capability to detect a wide range of ions for the monitoring of industrial processes and environmental pollutants, as well as the determination of electrolytes for clinical analysis. Over the past decades, the innovative design of ion-selective electrodes (ISEs), specifically SC-ISEs, to improve potential stability and miniaturization for in situ/real-time analysis, has attracted considerable interest. Recently, the utilisation of nanomaterials was particularly prominent in SC-ISEs due to their excellent physical and chemical properties. In this article, we review the recent applications of various types of nanostructured materials that are composed of carbon, metals and polymers for the development of SC-ISEs. The challenges and opportunities in this field, along with the prospects for future applications of nanomaterials in SC-ISEs are also discussed.

Simple and sensitive electrochemical sensing of amethopterin by using carbon nanobowl/cyclodextrin electrode

Resulted from the severe side effects, the development of inexpensive, simple and sensitive method for amethopterin (ATP, an antineoplastic drug) is very important but it still remains a challenge. In this work, low cost nanohybrid composed of carbon nanobowl (CNB) and β-cyclodextrins (β-CD) (CNB-CD) was prepared with a simple autopolymerization way and applied as electrode material to develop a novel electrochemical sensor of ATP. Scanning-/transmission-electron microscopy, Fourier transform infrared spectrum, photographic image and electrochemical technologies were utilized to characterize morphologies and structure of the as-prepared CNB and CNB-CD materials. On the basic of the coordination advantages from CNB (prominent electrical property and surface area) and β-CD (superior molecule-recognition and solubility capabilities), the CNB-CD nanohybrid modified electrode exhibits superior sensing performances toward ATP, and a low detection limit of 0.002 μM coupled with larger linearity of 0.005-12.0 μM are obtained. In addition, the as-prepared sensor offers desirable repeatability, stability, selectivity and practical application property, confirming that this proposal may have important applications in the determination of ATP.

Highly Stable Solid Contact Calcium Ion-Selective Electrodes: Rapid Ion-Electron Transduction Triggered by Lipophilic Anions Participating in Redox Reactions of CunS Nanoflowers

Solid contact (SC) calcium ion-selective electrodes (Ca2+-ISEs) have been widely applied in the analysis of water quality and body fluids by virtue of the unique advantages of easy operation and rapid response. However, the potential drift during the long-term stability test hinders their further practical applications. Designing novel redox SC layers with large capacitance and high hydrophobicity is a promising approach to stabilize the potential stability, meanwhile, exploring the transduction mechanism is also of great guiding significance for the precise design of SC layer materials. Herein, flower-like copper sulfide (CunS-50) composed of nanosheets is meticulously designed as the redox SC layer by modification with the surfactant (CTAB). The CunS-50-based Ca2+-ISE (CunS-50/Ca2+-ISE) demonstrates a near-Nernstian slope of 28.23 mV/dec for Ca2+ in a wide activity linear range of 10-7 to 10-1 M, with a low detection limit of 3.16 × 10-8 M. CunS-50/Ca2+-ISE possesses an extremely low potential drift of only 1.23 ± 0.13 μV/h in the long-term potential stability test. Notably, X-ray absorption fine-structure (XAFS) spectra and electrochemical experiments are adopted to elucidate the transduction mechanism that the lipophilic anion (TFPB-) participates in the redox reaction of CunS-50 at the solid-solid interface of ion-selective membrane (ISM) and redox inorganic SC layer (CunS-50), thereby promoting the generation of free electrons to accelerate ion-electron transduction. This work provides an in-depth comprehension of the transduction mechanism of the potentiometric response and an effective strategy for designing redox materials of ion-electron transduction triggered by lipophilic anions.

Single-cell calcium monitoring of Caco-2 cell co-cultured with intestinal microbiome through carbon fiber based potentiometric microelectrode

The Caco-2 cells were used as intestinal epithelial cell model to illustrate the hyperuricemia (HUA) mechanism under the co-culture of the imbalanced intestinal microbiome in this work. The uric acid (UA) concentration in the HUA process was monitored, and could be up to 425 μmol/L at 8 h co-cultured with the imbalanced intestinal microbiome. Single-cell potentiometry based on ion-selective microelectrode was used to study extracellular calcium change, which is hypothesized to play an important role in the UA excretion. The potential signal of the calcium in the extremely limited microenvironment around single Caco-2 cell was recorded through the single-cell analysis platform. The potential signal of sharp decrease and slow increase followed within a few seconds indicates the sudden uptake and gradually excretion process of calcium through the cell membrane. Moreover, the value of the potential decrease increases with the increase of the time co-cultured with the imbalanced intestinal microbiome ranging from 0 to 8 h. The Ca2+ concentration around the cell membrane could decrease from 1.3 mM to 0.4 mM according to the potential decrease of 27.0 mV at the co-culture time of 8 h. The apoptosis ratio of the Caco-2 cells also exhibits time dependent with the co-culture of the imbalanced intestinal microbiome, and was 39.1 ± 3.6 % at the co-culture time of 8 h, which is much higher than the Caco-2 cells without any treatment (3.9 ± 2.9 %). These results firstly provide the links between the UA excretion with the apoptosis of the intestinal epithelial cell under the interaction of the imbalanced intestinal microbiome. Moreover, the apoptosis could be triggered by the calcium signaling.

Pinecone derived hierarchical carbon nanostructure as a transducer in a solid-state ion-selective electrode for in vivo analysis of calcium ion

Monitoring extracellular calcium ion (Ca2+) chemical signals in neurons is crucial for tracking physiological and pathological changes associated with brain diseases in live animals. Potentiometry based solid-state ion-selective electrodes (ISEs) with the assist of functional carbon nanomaterials as ideal solid-contact layer could realize the potential response for in vitro and in vivo analysis. Herein, we employ a kind of biomass derived porous carbon as a transducing layer to prompt efficient ion to electron transduction while stabilizes the potential drift. The eco-friendly porous carbon after activation (APB) displays a high specific area with inherit macropores, micropores, and large specific capacitance. When employed as transducer in ISEs, a stable potential response, minimized potential drift can be obtained. Benefiting from these excellent properties, a solid-state Ca2+ selective carbon fiber electrodes (CFEs) with a sandwich structure is constructed and employed for real time sensing of Ca2+ under electrical stimulation. This study presents a new approach to develop sustainable and versatile transducers in solid-state ISEs, a crucial way for in vivo sensing.

A Potentiometric Dual-Channel Microsensor Reveals that Fluctuation of H2 S is Less pH-Dependent During Spreading Depolarization in the Rat Brain

Spreading depolarization (SD) is one of the most common neuropathologic phenomena in the nervous system, relating to numerous diseases. However, real-time monitoring the rapid chemical changes during SD to probe the molecular mechanism remains a great challenge. We develop a potentiometric dual-channel microsensor for simultaneous monitoring of H2 S and pH featuring excellent selectivity and spatiotemporal resolution. Using this microsensor we first observe real time changes of H2 S and pH in the rat brain induced by SD. This changes of H2 S are completely suppressed when the rat pre-treats with aminooxyacetic acid (AOAA), a blocker to inhibit the H2 S-producing enzyme, indicating H2 S fluctuation might be related to enzyme-dependent pathway during SD and less pH-dependent. This study provides a new perspective for studying the function of H2 S and the molecular basis of SD-associated diseases.

Ion-Selective Micropipette Sensor for In Vivo Monitoring of Sodium Ion with Crown Ether-Encapsulated Metal-Organic Framework Subnanopores

In vivo sensing of the dynamics of ions with high selectivity is essential for gaining molecular insights into numerous physiological and pathological processes. In this work, we report an ion-selective micropipette sensor (ISMS) through the integration of functional crown ether-encapsulated metal-organic frameworks (MOFs) synthesized in situ within the micropipette tip. The ISMS features distinctive sodium ion (Na+) conduction and high selectivity toward Na+ sensing. The selectivity is attributed to the synergistic effects of subnanoconfined space and the specific coordination of 18-crown-6 toward potassium ions (K+), which largely increase the steric hindrance and transport resistance for K+ to pass through the ISMS. Furthermore, the ISMS exhibits high stability and sensitivity, facilitating real-time monitoring of Na+ dynamics in the living rat brain during spreading of the depression events process. In light of the diversity of crown ethers and MOFs, we believe this study paves the way for a nanofluidic platform for in vivo sensing and neuromorphic electrochemical sensing.

Ion-Electron Transduction Layer of the SnS2-MoS2 Heterojunction to Elevate Superior Interface Stability for All-Solid Sodium-Ion Selective Electrode

The high selectivity and fast ion response of all-solid sodium ion selective electrodes were widely applied in human sweat analysis. However, the potential drift due to insufficient interfacial capacitance leads to the deterioration of its stability and ultimately affects the potential accuracy of ion analysis. Designing a novel ion-electron transduction layer between the electrode and the ion selective membrane is an effective method to stabilize the interfacial potential. Herein, the SnS2-MoS2 heterojunction material was constructed by doping Sn in MoS2 nanosheets and used as the ion electron transduction layers of an all-solid sodium ion selective electrode for the first time, achieving the stable and efficient detection of Na+ ions. The proposed electrode exhibited a Nernst slope of 57.86 mV/dec for the detection of Na+ ions with a detection limit of 10-5.7 M in the activity range of 10-6-10-1 M. Via the electronic interaction at the heterojunction interfaces between SnS2 and MoS2 materials, the micro-nanostructure of the SnS2-MoS2 heterojunction was changed and SnS2-MoS2 as the ion-electron transduction layer acquired excellent capacitance (699 μF) and hydrophobicity (132°), resulting in a long-term potential stability of 1.37 μV/h. It was further proved that the large capacitance and high hydrophobicity of the ion-electron transduction layer are primary reasons for the excellent stability of the all-solid sodium ion selective electrode toward Na+ ions.

Effect of Ion Identity on Capacitance and Ion-to-Electron Transduction in Ion-Selective Electrodes with Nanographite and Carbon Nanotube Solid Contacts

The use of large surface area carbon materials as transducers in solid-contact ion-selective electrodes (ISEs) has become widespread. Desirable qualities of ISEs, such as a small long-term drift, have been associated with a high capacitance that arises from the formation of an electrical double layer at the interface of the large surface area carbon material and the ion-selective membrane. The capacitive properties of these ISEs have been observed using a variety of techniques, but the effects of the ions present in the ion-selective membrane on the measured value of the capacitance have not been studied in detail. Here, it is shown that changes in the size and concentration of the ions in the ion-selective membrane as well as the polarity of the polymeric matrix result in capacitances that can vary by up to several hundred percent. These data illustrate that the interpretation of comparatively small differences in capacitance for different types of solid contacts is not meaningful unless the composition of the ion-selective membrane is taken into account.

A Silicon-Based Field-Effect Biosensor for Drug-Induced Cardiac Extracellular Calcium Ion Change Detection

Calcium ions participate in the regulation of almost all biological functions of the body, especially in cardiac excitation-contraction coupling, acting as vital signaling through ion channels. Various cardiovascular drugs exert their effects via affecting the ion channels on the cell membrane. The current strategies for calcium ion monitoring are mainly based on fluorescent probes, which are commonly used for intracellular calcium ion detection (calcium imaging) and cannot achieve long-term monitoring. In this work, an all-solid-state silicone-rubber ion-sensitive membrane was fabricated on light-addressable potentiometric sensors to establish a program-controlled field-effect-based ion-sensitive light-addressable potentiometric sensor (LAPS) platform for extracellular calcium ion detection. L-type calcium channels blocker verapamil and calcium channel agonist BayK8644 were chosen to explore the effect of ion channel drugs on extracellular calcium ion concentration in HL-1 cell lines. Simultaneously, microelectrode array (MEA) chips were employed to probe the HL-1 extracellular field potential (EFP) signals. The Ca2+ concentration and EFP parameters were studied to comprehensively evaluate the efficacy of cardiovascular drugs. This platform provides more dimensional information on cardiovascular drug efficacy that can be utilized for accurate drug screening.

Robust Potentiometric Microelectrodes for In Situ Sensing of Ion Fluxes with High Sensitivity

Simple, reproducible, and reliable preparation of robust potentiometric microelectrodes is both challenging and of great importance for noninvasive real-time ion sensing. Herein, we report a simple strategy for the large-scale synthesis of nickel cobalt sulfide (NiCo2S4) nanowire arrays grown on carbon fibers for potentiometric microelectrodes. The highly uniform NiCo2S4 nanowire array serving as a transduction layer can provide a high capillary pressure and viscous resistance for loading the ion sensing membrane and exhibit a large redox capacitance for improving the stability. An all-solid-state lead-selective microelectrode, which presents a detection limit of 2.5 × 10-8 M in the simulated soil solution, was designed as a model for noninvasive, in situ, and real-time detection of ion fluxes near the rice root surface. Importantly, the microsensor enables sensitive detection of trace-level ion-fluxes. This work provides a simple yet general strategy for designing potentiometric microelectrodes.

A Fiber Sensor for Long-Term Monitoring of Extracellular Potassium Ion Fluctuations in Chronic Neuropsychiatric Diseases

The extracellular potassium ion concentration in the brain exerts a significant influence on cellular excitability and intercellular communication. Perturbations in the extracellular potassium ion level are closely correlated with various chronic neuropsychiatric disorders including depression. However, a critical gap persists in performing real-time and long-term monitoring of extracellular potassium ions, which is necessary for comprehensive profiling of chronic neuropsychiatric diseases. Here, a fiber potassium ion sensor (FKS) that consists of a soft conductive fiber with a rough surface and a hydrophobic-treated transduction layer interfaced with a potassium ion-selective membrane is found to solve this problem. The FKS demonstrates stable interfaces between its distinct functional layers in an aqueous environment, conferring an exceptional stability of 6 months in vivo, in stark contrast to previous reports with working durations from hours to days. Upon implantation into the mouse brain, the FKS enables effective monitoring of extracellular potassium ion dynamics under diverse physiological states including anesthesia, forced swimming, and tail suspension. Using this FKS, tracking of extracellular potassium ion fluctuations that align with behaviors associated with the progression of depression over months is achieved, demonstrating its usability in studying chronic neuropsychiatric disorders from a new biochemical perspective.

Stability Enhancement of Galvanic Redox Potentiometry by Optimizing the Redox Couple in Counterpart Poles

Potentiometry based on the galvanic cell mechanism, i.e., galvanic redox potentiometry (GRP), has recently emerged as a new tool for in vivo neurochemical sensing with high neuronal compatibility and good sensing property. However, the stability of open circuit voltage (E_OC) outputting remains to be further improved for in vivo sensing application. In this study, we find that the E_OC stability could be enhanced by adjusting the sort and the concentration ratio of the redox couple in the counterpart pole (i.e., indicating electrode) of GRP. With dopamine (DA) as the sensing target, we construct a spontaneously powered single-electrode-based GRP sensor (GRP_2.0) and investigate the correlation between the stability and the redox couple used in the counterpart pole. Theoretical consideration suggests that the E_OC drift is minimum when the concentration ratio of the oxidized form (O₁) to the reduced form (R₁) of the redox species in the backfilled solution is 1:1. The experimental results demonstrate that, compared with other redox species (i.e., dissolved O₂ at 3 M KCl, potassium ferricyanide (K₃Fe(CN)₆), and hexaammineruthenium(III) chloride (Ru(NH₃)₆Cl₃)) used as the counterpart pole, potassium hexachloroiridate(IV) (K₂IrCl₆) exhibits better chemical stability and outputs more stable E_OC. As a result, when IrCl₆^2-/3- with the concentration ratio of 1:1 is used as the counterpart, GRP_2.0 displays not only an excellent E_OC stability (i.e., 3.8 mV drifting during 2200 s for in vivo recording) but also small electrode-to-electrode variation (i.e., the maximum E_OC variation between four electrodes is 2.7 mV). Upon integration with the electrophysiology, GRP_2.0 records a robust DA release, accompanied by a burst of neural firing, during the optical stimulation. This study paves a new avenue to stable neurochemical sensing in vivo.

Real-time calcium uptake monitoring of a single renal cancer cell based on an all-solid-state potentiometric microsensor

Introduction: In addition to many cellular processes, Ca2+ is also involved in tumor initiation, progression, angiogenesis, and metastasis. The development of new tools for single-cell Ca2+ measurement could open a new avenue for cancer therapy.

Methods: The all-solid-state calcium ion-selective microelectrode (Ca2+-ISμE) based on carbon fiber modified with PEDOT (PSS) as solid-contact was developed in this work, and the characteristics of the Ca2+-ISμE have also been investigated.

Results: The Ca2+-ISμE exhibits a stable Nernstian response in CaCl2 solutions in the active range of 1.0 × 10−8 - 3.1 × 10−3 M with a low detection limit of 8.9 × 10−9 M. The Ca2+-ISμE can be connected to a patch clamp to fabricate a single-cell analysis platform for in vivo calcium monitoring of a single renal carcinoma cell. The calcium signal decreased significantly (8.6 ± 3.2 mV, n = 3) with severe fluctuations of 5.9 ± 1.8 mV when the concentration of K+ in the tumor microenvironment is up to 20 mM.

Discussion: The results indicate a severe cell response of a single renal carcinoma cell under high K+ stimuli. The detection system could also be used for single-cell analysis of other ions by changing different ion-selective membranes with high temporal resolution.

Micro- and nano-devices for electrochemical sensing

Electrode miniaturization has profoundly revolutionized the field of electrochemical sensing, opening up unprecedented opportunities for probing biological events with a high spatial and temporal resolution, integrating electrochemical systems with microfluidics, and designing arrays for multiplexed sensing. Several technological issues posed by the desire for downsizing have been addressed so far, leading to micrometric and nanometric sensing systems with different degrees of maturity. However, there is still an endless margin for researchers to improve current strategies and cope with demanding sensing fields, such as lab-on-a-chip devices and multi-array sensors, brain chemistry, and cell monitoring. In this review, we present current trends in the design of micro-/nano-electrochemical sensors and cutting-edge applications reported in the last 10 years. Micro- and nanosensors are divided into four categories depending on the transduction mechanism, e.g., amperometric, impedimetric, potentiometric, and transistor-based, to best guide the reader through the different detection strategies and highlight major advancements as well as still unaddressed demands in electrochemical sensing.

Graphical Abstract

A Portable Smartphone-Based System for the Detection of Blood Calcium Using Ratiometric Fluorescent Probes

Hypocalcemia is a disease that adversely affects the production and reproduction of dairy cows. A portable device for rapid bovine blood calcium sensing has been growing in demand. Herein, we report a smartphone-based ratiometric fluorescence probe (SRFP) platform as a new way to detect and quantify calcium ions (Ca²⁺) in blood serum. Specifically, we employed a cost-effective and portable smartphone-based platform coupled with customized software that evaluates the response of Ca²⁺ ions to ratiometric fluorescence probe in bovine serum. The platform consists of a three-dimensional (3D) printed housing and low-cost optical components that excite fluorescent probe and selectively transmit fluorescence emissions to smartphones. The customized software is equipped with a calibration model to quantify the acquired fluorescence images and quantify the concentration of Ca²⁺ ions. The ratio of the green channel to the red channel bears a highly reproducible relationship with Ca²⁺ ions concentration from 10 μM to 40 μM in bovine serum. Our detection system has a limit of detection (LOD) of 1.8 μM in bovine serum samples and the recoveries of real samples ranged from 92.8% to 110.1%, with relative standard deviation (RSD) ranging from 1.72% to 4.89%. The low-cost SRFP platform has the potential to enable campesino to rapidly detect Ca²⁺ ions content in bovine serum on-demand in any environmental setting.

Carbon composite thermoplastic electrodes integrated with mini-printed circuit board for wireless detection of calcium ions

Here, a smartphone-based portable sensing system is developed for real-time detection of Ca²⁺ ions in a variety of biofluids. A solid-contact calcium-selective electrode (Ca²⁺-ISE) consisting of an ion-selective membrane (ISM), carbon black nanomaterial and polystyrene-graphite nanoplatelets as a solid contact was fabricated. The polyvinylchloride (PVC)-based ISM was optimized using different plasticizers and ion-exchangers. Under optimized conditions, the solid contacts were electrochemically characterized by electrochemical impedance spectroscopy (EIS), chronopotentiometric and potentiometric measurements. The Ca²⁺-ISE showed a Nernst response with a slope of 31.2 ± 0.6 mV/decade in the concentration range from 0.1 M to 10^-4 M Ca²⁺ with a limit of detection (LOD) of 1.0 × 10^-5 M. In addition, the ISEs exhibited good selectivity to Ca²⁺ ions over various interfering electrolytes and metabolites. The Ca²⁺-ISEs were applied in human urine and, artificial serum and cerebrospinal fluid samples. The ISEs showed good recoveries between 90 and 105%, indicating potential applicability of these electrodes in biological fluids. The portable lab-made potentiometer provides wireless data signaling and transmission to a smartphone and final Ca²⁺ concentration display due to its customized software. Therefore, the developed smartphone-based sensing platform offers low cost (< $25), rapid, user-friendly detection of Ca²⁺ especially in resource-limited areas.

Automated regional citrate anticoagulation based on online monitoring of ionized calcium concentration: Proof of concept

BACKGROUND

Regional citrate anticoagulation (RCA), a complex and effective technique, is recommended as the anticoagulation of choice for continuous renal replacement therapy. One of its key objectives is to keep the ionized calcium in the targeted range. In this study, we aimed to develop an automated RCA based on online monitoring of the ionized calcium concentration and closed-loop feedback.

METHODS

We constructed calcium-selective electrodes with liquid inner contact, which measured a potentiometric signal as the output. We tested the responses, stability, and selectivity of the electrodes in flowing fluid containing calcium chloride. We compared the measurement accuracy between the electrodes and an i-STAT system in vivo. Moreover, we established closed-loop feedback using a proportional-integral-derivative controller model. We performed simulated automated RCA both in vivo and in vitro.

RESULTS

The electrode gave a Nernstian response to the variation of ionized calcium concentration. It showed high stability and a relatively short response time. Changes in the fluid flow rate, solution PH, and addition of metal ions including Mg²⁺ and K⁺ did not interfere with the measurements of ionized calcium. These measurements in whole blood by the electrode were very close to those assessed by the i-STAT system. The feedback control system responded quickly to an abnormal ionized calcium concentration and regulated the infusion rates of calcium or citrate to maintain the concentration of ionized calcium within the targeted range.

CONCLUSIONS

We successfully trialed automated RCA, which may help simplify the complexities of RCA in the future.

In vivo monitoring of calcium ions in rat cerebrospinal fluid using an all-solid-state acupuncture needle based potentiometric microelectrode

Acupuncture needles are regarded as ideal materiel for the development of microelectrodes for in vivo sensing. In this work, an all-solid-state ion-selective microelectrode (ISμE) has been developed by coating a calcium ion-selective membrane on an acupuncture needle tip with a diameter of less than 80 μm, which is modified with poly(3,4-ethylenedioxythiophene)-poly(sodium 4-styrenesulfonate) as solid contact. The proposed Ca²⁺-ISμE shows a Nernstian response toward Ca²⁺ in the range from 1.0 × 10^-6 to 3.1 × 10^-3 M with a slope of 30.8 ± 0.9 mV/decade (R² = 0.999), and the detection limit is 1.2 × 10^-7 M. The Ca²⁺-ISμE has been used for in vivo monitoring of the calcium changes in rat cerebrospinal fluid (CSF) under the injury of spinal cord transection. The results demonstrate that the calcium concentration in CSF increases sharply from the normal level of 20.6 ± 1.72 μM (n = 3) to 133.2 ± 7.63 μM (n = 3) with a severe fluctuation after spinal cord damage. Thus, the proposed Ca²⁺-ISμE is available for in vivo monitoring of calcium ions with high temporal resolution and flexibility. The detection system can be extended to measure other ions in CSF by changing different ion-selective membranes.

Development of an Electrochemical Dual H2S/Ca2+ Microsensor and Its In Vivo Application to a Rat Seizure Model

A dual electrochemical microsensor was fabricated for concurrent monitoring of hydrogen sulfide (H₂S) and calcium ions (Ca²⁺), which are closely linked important signaling species involved in various physiological processes. The dual sensor was prepared using a dual recessed electrode consisting of two platinum (Pt) microdisks (50 μm in diameter). Each electrode was individually optimized for the best sensing ability toward a target analyte. One electrode (WE1, amperometric H₂S sensor) was modified with electrodeposition of Au and electropolymerized polyaniline coating. The other electrode (WE2, all-solid-state Ca²⁺-selective electrode) was composed of Ag/AgCl onto the recessed Pt disk formed via electrodeposition/chloridation, followed by silanization and Ca²⁺-selective membrane loading. The current of WE1 and the potential of WE2 in a dual sensor responded linearly to H₂S concentration and logarithm of Ca²⁺ concentration, respectively, without a crosstalk between the sensing signals. Both WE1 and WE2 presented excellent sensitivity, selectivity (log⁡KH2S,iAmp≤-3.5, i = CO, NO, O₂, NO₂^-, AP, AA, DA, and GABA; and log⁡KCa2+,jPot≤-3.2, j = Na⁺, K⁺, and Mg²⁺), and fast response time with reasonable stability (during ca. 6 h in vivo experiment). Particularly, WE2 prepared using a mixture of two ionophores (ETH1001 and ETH129) and two plasticizers (2-nitrophenyl octyl ether and bis(2-ethylhexyl) sebacate) showed a very shortened response time (t_R to attain the ΔE/Δt slope of 0.6 mV/min = 3.0 ± 0.2 s, n ≥ 10), a critically required factor for real-time analysis. The developed sensor was utilized for simultaneous real-time monitoring of H₂S and Ca²⁺ changes at the brain cortex surface of a living rat during spontaneous epileptic seizures induced by a cortical 4-aminopyridine injection. The dynamic changes of H₂S and Ca²⁺ were clearly observed in an intimate correlation with the electrophysiological recording of seizures, demonstrating the sensor feasibility of in vivo and real-time simultaneous measurements of H₂S and Ca²⁺.

Materials Chemistry of Neural Interface Technologies and Recent Advances in Three-Dimensional Systems

Advances in materials chemistry and engineering serve as the basis for multifunctional neural interfaces that span length scales from individual neurons to neural networks, neural tissues, and complete neural systems. Such technologies exploit electrical, electrochemical, optical, and/or pharmacological modalities in sensing and neuromodulation for fundamental studies in neuroscience research, with additional potential to serve as routes for monitoring and treating neurodegenerative diseases and for rehabilitating patients. This review summarizes the essential role of chemistry in this field of research, with an emphasis on recently published results and developing trends. The focus is on enabling materials in diverse device constructs, including their latest utilization in 3D bioelectronic frameworks formed by 3D printing, self-folding, and mechanically guided assembly. A concluding section highlights key challenges and future directions.

Manipulating Particle Chemistry for Hollow Carbon-based Nanospheres: Synthesis Strategies, Mechanistic Insights, and Electrochemical Applications

Hollow carbon-based nanospheres (HCNs) have been demonstrated to show promising potential in a large variety of research fields, particularly electrochemical devices for energy conversion/storage. The current synthetic protocols for HCNs largely rely on template-based routes (TBRs), which are conceptually straightforward in creating hollow structures but challenged by the time-consuming operations with a low yield in product as well as serious environmental concerns caused by hazardous etching agents. Meanwhile, they showed inadequate ability to build complex carbon-related architectures. Innovative strategies for HCNs free from extra templates thus are highly desirable and are expected to not only ensure precise control of the key structural parameters of hollow architectures with designated functionalities, but also be environmentally benign and scalable approaches suited for their practical applications.In this Account, we outline our recent research progress on the development of template-free protocols for the creation of HCNs with a focus on the acquired mechanical insight into the hollowing mechanism when no extra templates were involved. We demonstrated that carbon-based particles themselves could act as versatile platforms to create hollow architectures through an effective modulation of their inner chemistry. By means of reaction control, the precursor particles were synthesized into solid ones with a well-designed inhomogeneity inside in the form of different chemical parameters such as molecular weight, crystallization degree, and chemical reactivity, by which we not only can create hollow structures inside particles but also have the ability to tune the key features including compositions, porosity, and dimensional architectures. Accordingly, the functionalities of the prepared HCNs could be systematically altered or optimized for their applications. Importantly, the discussed synthesis approaches are facile and environmentally benign processes with potential for scale-up production.The nanoengineering of HNCs is found to be of special importance for their application in a large variety of electrochemical energy storage and conversion systems where the charge transfer and structural stability become a serious concern. Particular attention in this Account is therefore directed to the potential of HCNs in battery systems such as sodium ion batteries (NIBs) and potassium ion batteries (KIBs), whose electrochemical performances are plagued by the destructive volumetric deformation and sluggish charge diffusion during the intercalation/deintercalation of large-size Na⁺ or K⁺. We demonstrated that precise control of the multidimensional factors of the HCNs is critical to offer an optimized design of sufficient reactive sites, excellent charge and mass transport kinetics, and resilient electrode structure and also provide a model system suitable for the study of complicated metal-ion storage mechanisms, such as Na⁺ storage in a hard carbon anode. We expect that this Account will spark new endeavors in the development of HCNs for various applications including energy conversion and storage, catalysis, biomedicine, and adsorption.

Natural Leukocyte Membrane-Masked Microelectrodes with an Enhanced Antifouling Ability and Biocompatibility for In Vivo Electrochemical Sensing

Probing chemical information in the central nervous system is essential for understanding the molecular mechanism of brain function. Electrochemistry with tissue-implantable carbon fiber electrodes (CFEs) provides a powerful tool for monitoring the dynamics of neurochemicals in a subsecond time scale; however, the implantation of CFEs into brain tissue immediately causes the nonspecific adsorption of proteins on electrode surfaces. This process can dramatically impact the performance of the electrochemical method in terms of reduced sensitivity and accuracy. Herein, we report a strategy to minimize the electrode biofouling by masking CFEs with leukocyte membranes (LMs). We find that the LM masking endows CFEs with a highly hydrophilic surface that gains a high resistance to nonspecific protein adsorption. The electrode reactivity to target molecules decreases by a small degree due to the membrane coating, but the sensitivity loss of the LM-masked CFEs is greatly lessened even after in vivo implantation for 8 h. This study offers a new method of microelectrode modification by natural cell membranes for sustained sensing performance during long-term in vivo analysis.

Solid-Contact Ion-Selective Electrodes: Response Mechanisms, Transducer Materials and Wearable Sensors

Wearable sensors based on solid-contact ion-selective electrodes (SC-ISEs) are currently attracting intensive attention in monitoring human health conditions through real-time and non-invasive analysis of ions in biological fluids. SC-ISEs have gone through a revolution with improvements in potential stability and reproducibility. The introduction of new transducing materials, the understanding of theoretical potentiometric responses, and wearable applications greatly facilitate SC-ISEs. We review recent advances in SC-ISEs including the response mechanism (redox capacitance and electric-double-layer capacitance mechanisms) and crucial solid transducer materials (conducting polymers, carbon and other nanomaterials) and applications in wearable sensors. At the end of the review we illustrate the existing challenges and prospects for future SC-ISEs. We expect this review to provide readers with a general picture of SC-ISEs and appeal to further establishing protocols for evaluating SC-ISEs and accelerating commercial wearable sensors for clinical diagnosis and family practice.

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).

In Vivo Electrochemical Sensors for Neurochemicals: Recent Update

In vivo electrochemical sensing based on implantable microelectrodes is a strong driving force of analytical neurochemistry in brain. The complex and dynamic neurochemical network sets stringent standards of in vivo electrochemical sensors including high spatiotemporal resolution, selectivity, sensitivity, and minimized disturbance on brain function. Although advanced materials and novel technologies have promoted the development of in vivo electrochemical sensors drastically, gaps with the goals still exist. This Review mainly focuses on recent attempts on the key issues of in vivo electrochemical sensors including selectivity, tissue response and sensing reliability, and compatibility with electrophysiological techniques. In vivo electrochemical methods with bare carbon fiber electrodes, of which the selectivity is achieved either with electrochemical techniques such as fast-scan cyclic voltammetry and differential pulse voltammetry or based on the physiological nature will not be reviewed. Following the elaboration of each issue involved in in vivo electrochemical sensors, possible solutions supported by the latest methodological progress will be discussed, aiming to provide inspiring and practical instructions for future research.