Ion-selective Electrode Research Articles

Development of a hydroponic device using potassium Ion-Selective electrode and neural network technology

The rapid development of modern agricultural biotechnology and information technology has become a key driver of modern agricultural growth. The demand for ion detection is prevalent in all aspects of agriculture. This study details the research on an ion sensor agricultural system using potassium (K) ion-selective electrodes (ISEs). The main emphasis is on the application of artificial neural networks (ANNs) for driving ISE development of accuracy. ISEs based on solid contact with metal oxides, specifically manganese dioxide, are studied. Moreover, the design details of an ion signal acquisition device using the NI-9205 acquisition card and LabVIEW are presented. Finally, based on experimental results of nutrient solution samples, ANNs show better performance than the multiple linear regression method. The mean relative error of K-ISE detection was maintained within 4%. This study thoroughly discusses ISE technology to promote its adoption in agriculture.

Evaluation of the Level of Electrolytes in Children with Steroid Sensitive or Steroid Resistant Nephrotic Syndrome

Background: Childhood idiopathic nephrotic syndrome is one of the most common conditions pediatric nephrologists encounter globally. Nephrotic syndrome is characterized by proteinuria, hyperlipidemia, and edema. The degree and duration of proteinuria have an impact on serum electrolyte levels. However, local data is limited. Objectives: To assess and compare the degree of electrolyte imbalance and its relationship to kidney function indicators during relapse and remission in children with idiopathic nephrotic syndrome. Methods: In this case-control study, blood samples were collected from 80 Iraqi children with an age range of (2-14) years. They were divided into three groups: Group I (20 individuals with steroid-sensitive nephrotic syndrome (SSNS)), Group II (20 individuals with steroid-resistant nephrotic syndrome (SRSN)), and Group III (40 healthy individuals as the control group). Serum electrolyte levels (Na, Ca, Cl, and K) were measured by an ion-selective electrode (9180 electrolyte analyzer). Blood creatinine and urea were measured by a Cobas c311 autoanalyzer during the relapse and remission phase. The patients were clients of the pediatric nephrology consultation center at the Children's Teaching Hospital / Baghdad Medical City and Al-Batoul Teaching Hospital for Women and Children from 15 February to 20 August 2022. The controls were healthy children whose medical history was reviewed to eliminate a history of kidney disease and underwent a comprehensive physical examination. Controls were recruited from a network of family, friends, relatives, and the National Autism Center/ Child Protection Teaching Hospital affiliated with Medical City/Baghdad. Results: Serum Calcium levels showed a clear decrease in all SSNS and SRSN patients compared to the control group. The levels of Sodium and Chloride were significantly lower than the control group during the relapse phase. The results of the relapse phase of SRNS patients indicated higher serum potassium concentration compared with the control group and the SSNS patient group, with a statistically significant difference). Conclusion: All children with idiopathic nephrotic syndrome had hypocalcemia in the relapse and remission phase. SRNS cases had hyperkalemia, Sodium, and chloride fluctuated between low levels during the relapse phase and normal levels during the remission phase.

Towards mass-production of ion-selective electrodes by spotting: Optimization of membrane composition and real-time tracking of membrane drying

Solid-contact ion-selective electrodes present tools for rapid assessment of charged species within a vast number of samples. Commercially available point-of-care electrochemical sensing systems are most often produced by screen-printing. However, this method is not applicable to polymeric membrane deposition, due to the mismatch of the membrane physical characteristics to those intended for screen printing. Herein, an industrial automated dispensing machine was used for fast and controlled deposition of small volumes of ion-selective membrane. Compared to the previously published literature, the dry solute membrane content was not altered. Rather, we optimized the solvents for the ISM preparation, thus significantly lowering the required overall number of membrane deposition steps. It was shown that dry membrane uniformity strongly depends on the solvent carrier system, with the best uniformity for the membrane prepared in a solvent mixture consisting of equal volumes of THF and cyclohexanone. The volume of a single membrane deposit (spot) was estimated based on colorimetric absorbance measurements to be 0.20 μL. Already with a single spot of the ion-selective membrane, an adequate potentiometric response to potassium ions was obtained, supporting the efficiency of the production technique. Evaporative membrane drying was investigated for the first time using time-resolved impedance spectroscopy, giving useful information on the influence of the solvent composition to the characteristics of the evaporative time profiles. With this approach, the overall production and drying of the described devices takes less than 30 min.

Self-powered microfluidic-based sensor for noninvasive sweat analysis

This study introduces a novel self-powered, microfluidic sweat sensing system. The device features a flexible thermoelectric system that enables efficient on-demand sweat extraction and data transmission. Both the thermoelectric generator (TEG) and the microfluidic system are thoroughly evaluated through numerical simulations and experimental testing. The device integrates electrochemical sensors for glucose, pH, and ion detection, along with solid-liquid triboelectric nanosensors to measure sweat rate. It demonstrates high selectivity for target analytes. Sweat, extracted via iontophoresis with carbagel, interacts with electrodes coated with ion-selective membranes (ISM) and glucose oxidase, producing an open-circuit potential. This selective detection of analytes results in distinct output differences. Additionally, the device delivers stable results within 20 minutes, reducing analysis time while maintaining a strong correlation with blood biomarker concentrations. This partially flexible, noninvasive system has potential applications in monitoring electrolyte loss and glucose imbalances through sweat.

Supramolecular-Coordinated Nanofiltration Membranes with Quaternary-Ammonium Cyclen for Efficient Lithium Extraction from High Magnesium/Lithium Ratio Brine

Ion-selective membranes (ISM) with sub-nanosized pore channels hold significant potential for applications in saline wastewater treatment and resource recovery. Herein, novel synergistic ion channels featuring bi-periodic structures were constructed through the coordination of functional Cyclen (quaternary_1,4,7,10-tetraazacyclododecane, Q_Cyclen) and Cu2+-m-Phenylenediamine (Cu2+-MPD) to develop supramolecular membranes for lithium extraction. The exterior quaternary ammonium-rich sites exhibit a significant Donnan exclusion effect, resulting in tremendous mono/divalent (Li+/Mg2+) ion selectivity; while the interior regular-confined channels of Cyclen yield a fast vehicular pathway, facilitating water molecules and Li+ ion-selective transport. The optimized membrane exhibited an increased water permeance of 19.2 L·m-2·h-1·bar-1 and simultaneously promoted Li+/Mg2+ selectivity (achieving a selectivity of 18.5 under a Mg2+/Li+ mass ratio of 30), surpassing the trade-off limit of conventional nanofiltration membranes. Due to the acquired excellent Li+/Mg2+ selectivity, lithium extraction from simulated salt-lake brines was successfully achieved through a two-stage nanofiltration process, reducing the Mg2+/Li+ mass ratio from 40 to 1.1. This work validates the applicability of macrocyclic with intrinsic sub-nanosized channels and desired multifunctionality for developing high-performance ISM for efficient lithium separation and beyond.

Reverse iontophoresis sensing electrode for joint detection of pH, NH4+, and lactic acid

With the rapid development of integrated electronic circuit technology, biological fluid-electricity integrated detection systems have gradually become a research hotspot. Researchers have integrated biological fluid electrical detection methods into circuits and developed small, low-power, portable detection devices equipped with sensing electrodes, which can achieve continuous monitoring of human health parameters. In recent years, the integration of reverse iontophoresis technology that can be used to extract body fluids with electrochemical sensors has opened up the possibility of flexible, portable biochemical sensing with excellent detection sensitivity. Herein, we present a method for extracting biofluids based on reverse iontophoresis technology, coupled with electrochemical sensing techniques for the simultaneous detection of various biomarkers in body fluids. The multi-channel sensing electrode was modified layer by layer using nitrogen-doped graphene (N-Gr), ion-selective membrane, or lactate oxidase for rapid and sensitive detection of pH (3–8), ammonium ion (NH4+) (0.1 mM–50 mM), and lactic acid (1 mM–50 mM). Subsequently, an integrated system for electrical stimulation extraction and sensing analysis based on reverse iontophoresis was established and experimentally tested. Experimental results demonstrated that the multi-channel joint detection sensing electrode has excellent sensing linearity, specificity, repeatability, and long-term stability. Finally, a smartphone-based WeChat applet was developed, which can realize parameter setting, function selection, and result display of sensor detection. In this study, reverse iontophoresis technology was used to collect body fluids, and the important biomarkers pH, NH4+, and lactic acid in the body fluids were detected. Overall, this research presents an integrated detection system and a multi-channel sensing scheme for the detection of important biochemical markers in bodily fluids, thereby providing potential value for health monitoring applications.

Efficient Li+/Mg2+ separation by two-dimensional montmorillonite membrane with stable channel structure through ion exchange and metal–organic coordination strategy

Two-dimensional (2D) nanochannel membranes stacked by nanosheets are promising membrane separation materials. However, intrinsic swelling properties of 2D membrane have emerged as a significant challenge to establish stable nanochannels and achieve high separation performance. Inspired by the natural ion exchange property of montmorillonite (MMT), we constructed a two-dimensional MMT membrane with robust nanochannels by exchanging Fe3+ into interlayer of the membrane, and further achieved efficient separation of Li+/Mg2+ through coordination manipulation of tannic acid (TA) and Fe3+. Swelling was dramatically prevented by strong interlayer interaction between exchanged Fe3+ and nanosheets resulting in a 32-fold increase in anti-swelling properties. Furthermore, experiments and simulations revealed that TA could rapidly chelate with Fe3+ to form a dense hydrophilic functional layer on the membrane interface, which endowed MMT membranes with enhanced mechanical strength, cation transport and recognition, as well as anti-fouling properties. Consequently, the resulting MMT membranes showed the Li+/Mg2+ selectivity as high as 9.98 with a fast Li+ permeation rate (0.108 mol m-2h−1), which exceeded most of the previously reported membranes. This study proposed a novel strategy to diminish the trade-off effect among stability, ion flux, and selectivity of membranes, and inspired the development of next-generation ion selective separation membranes.

Solid Contact Microfabricated Electrode for Point-of-Care Electrochemical Determination of a Parasitic Infection Managing Medication Based on a Transducer Layer Prussian Blue Analogue Decorated with Multi-Walled Carbon Nanotubes

Point-of-care diagnostics (POC), often known as on-site testing has evolved as a quick and accurate methodology for analyzing and therapeutic monitoring of drugs. The aim of this study is to provide cost-effective, simple, portable, reliable, and ecofriendly methodology for determination of Tinidazole (TD); potentiometric sensors are found to be an ideal fit for achieving these goals. An advanced microfabricated ion selective electrode is provided employing two-steps procedure. The first is selecting the most selective ionophore for TD determination while the second is to improve the stability and detection limit upon doping the nanoparticles as ion to electron transducer layer between the microfabricated copper electrode and the ion selective media as it can decrease potential drift from 8.0 to ∼1.0 mV h−1. Nernstian potentiometric results were obtained for TD in range of concentration 1.0 × 10−3 to 1.0 × 10−8 M, its slope was −57.43 mV/decade with lower detection limits 2.55 × 10−9 M. Essential values of the adopted sensor are fast and stable response time (9 ± 2 s). The provided potentiometric approach succeeds to asses TD in its pure and pharmaceutical forms, furthermore in different biological fluids with satisfactory results.

Boosting lithium/magnesium separation performance of selective electrodialysis membranes regulated by enamine reaction

Monovalent cation exchange membranes (MCEMs) have progressively played an important role in the field of ion separation. However, according to transition state theory (TST), synchronously tuning the enthalpy barrier (△H) and entropy barrier (△S) for cation transport to improve ion separation performance is challenging. Here, the enamine reaction between the -NH- and -CHO groups is applied to regulate the subsequent Schiff-base reaction between the -CHO and -NH2 groups, which reduces the positive charges of the selective layer but increases the steric hindrance. The increased -T△S (△S term) for cation transport plays an important role in improving Li+/Mg2+ separation performance. The optimal positively-charged glutaraldehyde@piperazine/polyethyleneimine assembled membrane (M-Glu@PIP/PEI) has a perm-selectivity (Li+/Mg2+) of 31.83 with a Li+ flux of 1.87 mol·m-2·h-1, surpassing the Li+/Mg2+ separation performance of state-of-the-art monovalent ion selective membranes (MISMs). Most importantly, the selective electrodialysis (S-ED) process with M-Glu@PIP/PEI can be directly applied to treat simulated salt-lake brines (SLBs), and its superior Li+/Mg2+ separation performance and operational stability enables 74.44% of the lithium resources with a Li+ purity of 34.02% to be recovered. This study presents new insights into the design of high-performance MCEMs for energy-efficient resource recovery.

Highly stable and antifouling solid-contact ion-selective electrode for K+ detection in complex system based on multifunctional peptide and conductive MOF

Highly stable and antifouling solid-contact ion-selective electrode for K+ detection in complex system based on multifunctional peptide and conductive MOF

Surface functionalized porous MXene (Ti3C2Tx)/Polyethyleneimine membrane for efficient osmotic energy conversion

The development of ion-selective membranes is crucial for achieving efficient osmotic power conversion in reverse electrodialysis system. However, balancing ion selectivity and ion permeability in membranes remains a challenge, limiting improvements in energy conversion efficiency for practical applications. In this study, we develop efficient cation exchange membranes by integrating versatile aromatic hydroxyl groups with MXene nanosheets and polyethyleneimine (PEI) into their lamellar structure. This approach holds promise for achieving non-swelling, high selectivity, and enhanced power density for osmotic energy conversion. The abundance positive surface charge within PEI molecules, combined with the 2D sub-nanochannels of MXene nanosheets, facilitates the biomimetic replication of channel size, chemical groups, and adjustable charge density in the resulting membranes. The fabrication of these membranes is easily achieved through simple hydrothermal, functionalization, and surface-charged techniques, suggesting promising scalability. These membranes exhibit remarkable stability against swelling in aqueous solutions for up to 25 days and demonstrate a high selectivity of 0.95 and a superior output power density of 18.3 W/m2 in artificial river water and seawater.

Ion-selective electrode based on polyurethane-immobilized di-(2-ethyl hexyl) phosphoric acid for low-concentration aqueous Pb2+ detection and quantification

This study used a polyurethane (PU)-based ion selective electrode (ISE) immobilized with di-(2-ethyl hexyl) phosphoric acid (D2EHPA) to detect and quantify Pb2+ ions at low concentrations (10−10 to 10−1 M) in aqueous solution. PU, synthesized from castor oil (Ricinus communis L.), was utilized as the ISE membrane matrix. The ISE demonstrated a sensitivity of 26.24 ± 0.12 mV/decade, a detection limit of 1.44 × 10−7 M, and a response time of 10 s. We maintained the measurement stability for up to six days of storage.The results of Pb2+ quantification produced by ISE were compared with the results using atomic absorption spectroscopy (AAS), indicating similar Pb2+ concentrations in both artificial and real wastewater samples (calculated t-value < theoretical t-value). Therefore, this ISE is suitable for detecting trace levels of Pb2+ and quantifying them with high accuracy.

Tap water filtration and purification usage and their impact on the concentrations of fluoride and other minerals – A community-based study

ObjectiveThis study investigated the prevalence of water filtration and purification system (WFPS) use among residents of central Indiana (USA) and determined the effects of WFPS on the concentrations of fluoride, calcium, magnesium, potassium, and sodium in tap water. MethodsA census-based questionnaire collected data on demographics, water use, and water sources. Participants were also asked to provide water samples from their tap water or the WFPS they used. Water samples were analyzed using ion-specific electrodes (fluoride) and atomic absorption spectrometry (metals). Mineral concentration comparisons between water sources used nonparametric tests; questionnaire associations were testing using correlations, chi-square tests, and nonparametric tests. ResultsOne hundred and one participants completed the study, of which 71 % used some type of WFPS. Blacks were less likely to use WFPS than Asian or White participants (p = 0.045). Those with bachelor's degrees or higher were more likely to use WFPS (p = 0.003). The most used WFPS were pitcher filters (31 %), water softeners (21 %), reverse osmosis systems (11 %), faucet-mounted filters (4 %), and whole-house carbon filters (1 %). Reverse osmosis systems resulted in the lowest mineral concentrations (median, ppm; F-0.08, Ca-2.30, Mg-0.46, Na-4.60, P-0.35). Pitcher filters were largely comparable to unfiltered tap water. Water softeners resulted in the highest sodium concentrations (78.40 ppm). ConclusionA large proportion of study participants use WFPS, with pitcher filters being the most common. Reverse osmosis systems had the most significant impact on reducing mineral levels in tap water, while pitcher filters do not adversely affect mineral concentrations. Clinical SignificanceUnderstanding how different WFPS affect the various minerals in tap water is essential for helping consumers in choosing the right system and for oral care providers to guide patients on water consumption and the need for fluoride supplementation, especially for those at high risk of dental caries.

All-solid-state K+ sensing array based on Au@polystyrene nanocomposites.

All-solid-state ion selective electrodes (ASS-ISEs) areeasy to miniaturize and array, meeting the needs of home sensing devices. However, ASS-ISEs still faces challenges in accuracy and stability due to basic potential changes caused by non-specific adsorption of charged background compositions and the complex electrode preparation steps. To this end, our group successfully subtracted the background signal by integrating a self-calibrating channel in the sensing array and simplified the electrode preparation steps by preparing multi-functional PS-Au nanocomposites. However, the uniformity and gold content of PS-Au nanocomposites are difficult to control, so Au@PS nanocomposites are prepared as sensor materials in this paper to further reduce the differences between batches of electrodes. K+ Au@PS sensing array can be obtained by directly dropping Au@PS nanocomposites on the screen-printed carbon electrodes (SPCEs), which shows a near Nernstian behavior in the range 1.0 × 10-3M to 0.3M and good reproducibility in real sample testing. The detection results by K+ Au@PS sensing array for K+ in human morning urine agreed well with that tested by ICP-AES, which make the K+-ASS-ISE suitable for home health monitoring.

Real-time and multiplexed detection of sodium and potassium ions using PEDOT:PSS OECT microarrays integrated with ion-selective membranes

In recent years, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT: PSS)-based organic electrochemical transistors (OECTs) have been intensively studied, and various applications have been explored, such as neural interfaces, cell electrophysiological recording and wearable electronics. Furthermore, PEDOT: PSS OECTs have gained attention for monitoring ion concentrations, such as sodium (Na+) and potassium (K+), across various fields, including environmental monitoring, clinical diagnostics, food quality control and even sweat monitoring. However, multiplexed sensing of such ions with high sensitivity and selectivity is still challenging. Therefore, this work presents a multiplexed sensing of ions with high sensitivity and selectivity utilizing PEDOT: PSS OECTs integrated with ion-selective membranes (ISMs). First, a microscale fabrication method for manufacturing OECT arrays based on PEDOT: PSS is described, and then the integration of Na+ and K+ISMs on OECT microarrays using the spin coating technique is introduced. A microfluidic cell was integrated with the OECTs for real-time measurement of Na+ and K+ ion concentrations in a controlled flow-through manner. Following this, the influence of the membrane composition and thickness on the sensing performance of the OECTs was statistically investigated. The OECTs displayed a consistent linear response towards the detected ion type in a concentration range from 1 to 100 mM, while exhibiting notable selectivity against the undesired ions. Our results indicate that a reduction in membrane composition and thickness leads to an increase in sensitivity, with the drawback of a decrease in selectivity for ion concentration detection. We optimized the ISMs for both ion types and utilized them on one and the same array for multiplexed monitoring of Na+ and K+ ion concentrations.

Phosphate Ion-Selective Electrode Based on Electrochemically Modified Iron.

The significance of the detection of phosphate ions is immense in the realms of chemistry, biology, medicine, environment, and industry. The detection of phosphate ions is currently mainly reliant on blue molybdenum colorimetry, which is accurate but requires sample pretreatment, intricate operation, and a high price tag. Consequently, it is essential to create a sensor with superior efficiency, precision, straightforward functioning, and instantaneous online detection. This study has designed and created an electrochemical modification based on an iron metal electrode for this purpose. Cyclic voltammetry was used to initially ascertain the potential (-0.57 V) necessary for constant potential electrolysis. Employing a constant potential electrolysis method, the iron oxide and its phosphate were modified onto the surface of the iron electrode to enable reaction with phosphate ions. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to characterize and analyze the morphology and elemental composition of Fe-PME, elucidating how it responds to phosphate ionation. The two-electrode system was then utilized for the evaluation of the phosphate ion response of Fe-PME at pH 4. Fe-PME's response to phosphate ions is demonstrated by the results, ranging from 10-5 to 0.1 M and with a slope of -52.8 mV dec-1. Fe-PME exhibited satisfactory results when compared to the conventional blue colorimetry of molybdenum.

Measuring insect osmoregulation in vitro: A reference guide

Osmoregulation is influenced by a wide variety of biotic and abiotic variables, and maintenance of systemic osmoregulatory homeostasis is critical to insect fitness. Because insects are so small, accurately quantifying renal organ function is technically challenging, and often requires specialized equipment. On top of this, nearly a century of toiling in the laboratory has led to a wide and still growing variety of methods that can be difficult for novice researchers to disentangle. Here, we provide a reference guide for the most used in vitro approaches in the study of insect osmoregulation, including the Ramsay assay, Ussing chamber, epithelial potential measurement, scanning ion-selective electrode technique, and hindgut assays. Along the way, we highlight the history of each methodological innovation.

A Novel Sulfonated Polyimide Composite Membrane Containing a Sulfonated Porous Material for All-Vanadium Redox Flow Batteries.

To improve the battery efficiency and cycling stability of sulfonated polyimide (SPI), a polyphosphazene with built-in -SO3H moieties (PP-SO3H), which is a porous covalent organic framework (COF) material, is facilely synthesized by the polymeric combination of hexachlorocyclotriphosphazene (HCCP) and p-diaminobenzenesulfonic acid. Due to its tunable pore size and flexible molecular design, the COF material can address the trade-off between the conductivity and the ion permeability of ion exchange membranes well, thereby improving the ion selectivity of membranes. The experimental results show that the SPI/PP-SO3H composite membrane has an excellent conductivity (up to 114.8 mS cm-1); the ion selectivity of the SPI/2% PP-SO3H membrane is 11.69 × 104 S min cm-3, which is 2.18 times higher than that of the SPI base membrane. PP-SO3H also improves the SPI membrane's mechanical strength, and the effect of PP-SO3H on SPI intermolecular interactions is analyzed by surface electrostatic potential (ESP) theoretical calculations. The Coulombic efficiency (CE) of the SPI/2% PP-SO3H membrane is 98.92%, the energy efficiency (EE) is 84.1% at a current density of 100 mA cm-2, and the self-discharge time of the SPI/2% PP-SO3H membrane is 3.5 times compared with the SPI base membrane. To measure the cycling stability of the composite membrane, the SPI/2% PP-SO3H membrane is cycled in the VRFB for more than 400 cycles, which is more stable than that of the SPI base membrane. These results show that SPI/2% PP-SO3H composite membranes are viable for VRFB applications.

Influences of marine biofouling on the potential responses of polymeric membrane ion-selective electrodes

Using polymeric membrane ion-selective electrodes (ISEs) for reliable long-term marine environmental observations is still a challenge due to the formation of biofilms on electrode surfaces. Unfortunately, the contributions of the specific biofilm components to sensor failure have not been studied. Herein, the influences of the specific biofilm components on the performance of the neutral carrier-based polymeric membrane ISEs are systematically investigated in terms of sensitivity, selectivity, and stability. More importantly, the spatial distributions of these foulants in the fouled sensing membrane phase are studied by confocal laser scanning microscopy for the first time to give insight into the fouling mechanisms. Three typical biochemicals (i.e., proteins, polysaccharides, microbial lipids) in biofilms are selected as the representative foulants. The poly (vinyl chloride)-based CO32--ISEs and K+-ISEs/Ca2+-ISEs have been selected as the model sensors for detection of anions and cations, respectively. Experiments show that depending on the type of the biofoulants, the adsorption of the foulants on the polymeric membrane surface and/or their extraction into the organic membrane phase could occur and induce a change in the potential responses. This study provides a deep insight into the influences of the specific biofilm components on the polymeric membrane ISEs, which is of importance for development of the polymeric membrane marine sensors with high antifouling capabilities for long-term marine monitoring.

High-Performance Electrochemical Creatinine Sensors Based on β-Lead Dioxide/Single-Walled Carbon Nanotube Electrodes.

Creatinine is an important biomarker of kidney function and muscular metabolism. In this paper, we developed the β-lead dioxide/single-walled carbon nanotube (β-PbO2/CNT) and the β-PbO2/CNT ion-selective electrode (β-PbO2/CNT/ISE), which were used as highly sensitive potentiometric sensors for creatinine detection. The fabricated electrodes exhibited highly pH-sensitive characteristics due to the synergistic effect of the electrochemical properties of CNT and β-PbO2. Moreover, an ammonium-ion-selective membrane coating allowed the β-PbO2/CNT electrode to be NH4+-selective for direct detection of the ammonium ion. By exploiting the electrochemical characteristics of these electrodes, the creatinine assay was established through the one-step selective conversion of creatinine by creatinine deiminase, in which the OH- and NH4+ generated by the enzymatic reaction were detected using β-PbO2/CNT and β-PbO2/CNT/ISE electrodes as pH- and NH4+-responsive sensors, respectively. The total creatinine assay can be completed within ∼5 min. The assay results from β-PbO2/CNT and β-PbO2/CNT/ISE showed excellent sensitivity values of -75.56 and 64.62 mV in the detection range of 10-400 μM with a fast response (20 s), and the limits of detection were calculated to be 0.06 and 0.13 μM, respectively. Moreover, the developed creatinine sensor showed high selectivity against 11 interfering bio/chemical species with negligible interferences (selectivity coefficient <10-4) and excellent repeatability (>97% within 25 cycles) and long-term stability for 4 weeks of storage. In addition, the feasibility and practicality of the device were successfully demonstrated in human serum tests, with recoveries of 95-104% for PbO2/CNT and 92-110% for PbO2/CNT/ISE.