Ion Sensor Research Articles

Diacylated 1,10‐diaza‐18‐crown‐6 as an Alternative Template for Fluorescent Sensors of Calcium Ions

ABSTRACTThe performance of many known fluorescent sensors for Ca2+ is pH‐dependent. We set out to address this by preparing a novel calcium sensor based on diacylated 1,10‐diaza‐18‐crown‐6 and containing no basic centres. The sensor is sensitive to sub‐millimolar levels of Ca2+ and partially tolerant of an aqueous environment. However, it suffers from a lack of selectivity over some other divalent cations, and retained pH sensitivity in the acidic range. The relations between these properties and the sensor's structure are discussed.

Templates-Built Structural Designs for Piezoelectrochemical Pressure Sensors.

Self-powered sensors, capable of detecting static and dynamic pressure without an external power source, are pivotal for advancements in human-computer interaction, health monitoring, and artificial intelligence. Current sensing technologies, however, often fall short of meeting the growing needs for precise and timely pressure monitoring. This article introduces a novel self-powered pressure sensor utilizing electrochemical reactions. The sensor's ion conduction path and internal resistance adjust in response to external stress across a broad range. Its three-dimensional structure, crafted by using a simple template on the electrolyte, enables the efficient and cost-effective detection of various mechanical stimuli. This device not only achieves an optimized power density of approximately 2.34 mW cm-2─surpassing most existing technologies─but also features excellent flexibility, quick response, and recovery times (0.15 and 0.19 s respectively); high durability (2000 cycles); and a broad sensing range (0.23-20 kPa). Moreover, it serves as an ionic touchpad, enhancing data collection and recognition, and integrates seamlessly with a mouthpiece for accurate, real-time monitoring of respiratory activities. This innovative sensor offers minimal cost and simple process requirements while providing multifunctional capabilities for energy harvesting and pressure sensing, marking a significant step forward in the design of next-generation sensors.

Development of a potentiometric sensor for mercury (II) ion using cerium (IV) tinmolybdophosphate

ObjectivesThe primary objective of this report is to develop a heperopolyacid salt, Cerium (IV) tinmolybdophosphate (CeSnMoP), with distinctive attributes that significantly enhance its ion exchange capacity. Through deliberate adjustments in temperature, pH, and volume ratios, we have carefully prepared a range of CeSnMoP samples. One sample exhibiting an ion exchange capacity (IEC) of 5.06 ± 0.2 meq gm-1 has been identified for further extensive analysis. The second objective was to develop the potentiometric sensor by using the synthesised sample therefore it was transformed into an electrode incorporating PVC as binder material and validated as a potentiometric sensor for Mercury ions which can work as variable media.Data descriptionInstrumental analyses, such as XRD, IR, TGA, SEM and EDS, were used to elucidate the compound’s structural aspects. Distribution coefficient (Kd) studies highlighted the compound's pronounced selectivity towards Hg2+ ions. This catalyst was further utilized as an electro-active substance for detecting Hg2+ ions in an external solution. Epoxy resin played the role of a binder in the electrode formulations. The electrode, comprising a membrane with 50% exchanger material, demonstrated superior performance. This selected membrane exhibited a wide operational concentration range of 1 × 10–6 M – 1 × 10–1 M of Hg2+ ions for quantitative analysis of unknown samples of mercury ions. The lower detection limit for the calibration curve was recorded up to 2 × 10–8 M from 1 to 10–1 M. The electrode effectively sensed this metal ion within the pH range of 3.74–7.51 and exhibited a lifespan exceeding 8 months.

Surface Blocking-Based Potentiometric Biosensor for Detection of E. coli ATCC 15597 Using Phage MS2 as a Receptor.

Nowadays, using a potentiometric ion sensor to achieve detection of biological analytes is still a big challenge, since these analytes usually do not yield a measurable ion signal. To address this challenge, a simple and robust potentiometric sensing protocol based on a delayed Nernstian response is proposed for the label-free detection of biological analytes. The proposed sensor platform is composed of two layers: the surface recognition layer and the indicator-ion-selective electrode (ISE) membrane layer. It is based on a surface blocking mechanism in which the surface recognition interactions between the surface recognition element and the target can impede the diffusion of the indicator ion from the aqueous phase to the sensing membrane phase to reach the final Nernstian-response equilibrium, thus resulting in a delayed potential change. Such a potential change could be used to measure the concentration of a biological target in the sample. Thus, a sensing system was designed by using phage MS2, its host bacterium Escherichia coli ATCC 15597(abbreviated as E. coli H), and a solid-contact butyrylcholine ISE as a surface recognition element, a target, and an indicator electrode, respectively. This new concept offers a simple, sensitive, and extremely selective potentiometric method for detection of E. coli H with a detection limit of 100 CFU mL-1. It can be expected that the present approach may pave the way to using ISEs to detect various important nonionic biological targets in clinical and environmental applications.

Naked eye detection of anions: Dihydrazone receptor-based visual sensing

In this study, we designed and synthesized a novel colorimetric sensor for fluoride ions by integrating a thienodicarbohydrazide framework with salicylaldehyde, resulting in a unique bis-hydrazone-phenol receptor. This sensor exhibited exceptional selectivity for fluoride and acetate ions in nonaqueous environments, with detection visible to the naked eye. Upon interaction with fluoride ions, the receptor underwent a significant red shift in its UV–Visible absorption spectrum, moving from an absorption maximum at 332 nm–380 nm, signalling the recognition event. Acetate ions induced a similar shift. The binding interactions were thoroughly investigated, with a Jobs plot revealing a 1:1 receptor-anion complex formation with fluoride ions and a 1:2 complex with acetate ions. Further validation came from 1H NMR titration, which confirmed the presence of hydrogen bonding interactions between the receptor and the ions. The receptor demonstrated remarkable binding constants of 5.2 × 10⁴ M−2 for acetate and 1.6 × 10⁴ M−1 for fluoride ions, highlighting its strong and selective affinity. Fluorescence titrations further confirmed the receptor's potential as a chemo sensor, showing distinct emission changes in the presence of fluoride and acetate ions. This study not only underscores the receptor's capability for visual detection of fluoride ions in non-aqueous systems but also paves the way for its application in diverse analytical and environmental settings.

An Optical Au3+ Sensor Based on layer-by-layer PEI/PAA-Rho thin Films on ITO.

This study presents the development of a sensitive and selective gold ion (Au3+) sensor utilizing layer-by-layer (LbL) assembled thin films composed of polyethylenimine (PEI) and poly (acrylic acid) (PAA) conjugated with rhodamine (Rho). The first study revealed that the polymeric sensors (PAA-Rho) demonstrated significant selectivity and sensitivity in their colorimetric and fluorescence responses to Au3+ compared to other metal ions. In their spirolactam form, the polymeric sensors were non-fluorescent but could selectively transform into the fluorescent ring-opened amide form upon interaction with Au3+ ions, resulting in fluorescence enhancement and observable color changes. Common co-existing metal ions showed negligible interference in the detection of Au3+. The LbL sensor exhibited a linear increase in absorbance with the addition of bilayers, confirming successful film deposition. Surface morphology analysis using SEM, along with structural confirmation via ATR-FTIR and XRD, further validated the sensor's capability to detect cation. Results demonstrated that the LbL sensor exhibited selectivity for Au3+ ions within the range 1 × 10-6 to 1 × 10-3 M. This approach offers an easily understandable and intrinsically sensitive means for detecting Au3+ ions in both environmental and biological applications.

Two new Cd(II)-based coordination polymers derived from flexible linker: Highly sensitive and selective “turn off-on” fluorescent sensors for ferric and dichromate ions

Coordination polymers (CPs) are peculiar multidimensional materials that find utility as luminescent sensors for ions and molecules existing in wastewater discharge. Herein, two new Cd(II)-based CPs with formula [Cd(L)(phen)]·0.5H2O (1) and [Cd(L)(2,2'-bipy)] (2) (H2L = 3,3'-((1,2-phenylenebis(methylene))bis(oxy))dibenzoic acid, phen = 1,10-phenanthroline and 2,2'-bipy = 2,2'-bipyridine) have been synthesized and characterized. The single crystal X-ray analysis for 1 reveals that, Cd(II)-based centre adopts an octahedral geometry and is coordinated to four carboxyl O atoms from three L2- ligands and two N atoms from phen molecules. While, in CP 2, the Cd(II) centre is seven coordinated, which is completed by five carboxylate O atoms from two distinct L2− ligands and two N atoms from the 2,2-bipy ligand. Both 1 and 2 have been used as turn-off fluorescence sensor for selective and sensitive detection of Fe3+ and Cr2O72-. The CP 1 exhibited superior selectivity and sensitivity than 2 with limit of detection (LOD) 1.732×10-5 M and 5.996×10-6 M against Fe3+ and Cr2O72- respectively. Interestingly, the emissive behaviour of 1 get restored on adding 2-hydroxybenzoic acid (2-hyd acid) (turn on) due to the competitive formation of 2-hyd acid @Fe3+ or 2-hyd acid@Cr2O72- and concomitant release of 1. The fluorescence recovery selectivity of 2-hyd acid over other antioxidants has been attributed to the synergistic effect of high EHOMO value and a small space steric hindrance. The sensing properties of the MOF have been addressed with the help of Hirshfeld surface and density functional theory analyses.

Spectroscopic and Computational Study of Vanillin-Dipyrrin Ligand that Capable of Colorimetric Sensor for Zinc and Copper Ions

Phenolic-dipyrrin is a promising colorimetric chemosensor due to its tautomeric formation (phenolic-hemiquinone forms). In methanol solution, the vanillin-dipyrrin compound exists as a hemiquinone species, whereas the complexes exist in a phenolic form. This molecule showed a red-magenta color in methanol, in contrast to yellowish allyl-protected-dipyrrin. In the presence of zinc or copper ions, the vanillin-dipyrrin solution turned into a pale-yellow complex, similar to that of protected-dipyrrin. A computational study of the ligands and complexes showed a close relevance, where the energy transition between the highest molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the ligand in the hemiquinone form had a lower energy transition, and the two complexes showed slightly different transition energies. Apart from naked-eye color changes, vanillin-dipyrrin showed limits of detection of 9.2 × 10–7 M and 4.8×10−7 M for zinc and copper ions, respectively.

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.

A fabrication strategy for millimeter-scale, self-sensing soft-rigid hybrid robots.

Soft robots typically involve manual assembly of core hardware components like actuators, sensors, and controllers. This increases fabrication time and reduces consistency, especially in small-scale soft robots. We present a scalable monolithic fabrication method for millimeter-scale soft-rigid hybrid robots, simplifying the integration of core hardware components. Actuation is provided by soft-foldable polytetrafluoroethylene film-based actuators powered by ionic fluid injection. The desired motion is encoded by integrating a mechanical controller, comprised of rigid-flexible materials. The robot's motion can be self-sensed using an ionic resistive sensor by detecting electrical resistance changes across its body. Our approach is demonstrated by fabricating three distinct soft-rigid hybrid robotic modules, each with unique degrees of freedom: translational, bending, and roto-translational motions. These modules connect to form a soft-rigid hybrid continuum robot with real-time shape-sensing capabilities. We showcase the robot's capabilities by performing object pick-and-place, needle steering and tissue puncturing, and optical fiber steering tasks.

Air-Writing Recognition Enabled by a Flexible Dual-Network Hydrogel-Based Sensor and Machine Learning.

Accurate air-writing recognition is pivotal for advancing state-of-the-art text recognizers, encryption tools, and biometric technologies. However, most existing air-writing recognition systems rely on image-based sensors to track hand and finger motion trajectories. Additionally, users' writing is often guided by delimiters and imaginary axes which restrict natural writing movements. Consequently, recognition accuracy falls short of optimal levels, hindering performance and usability for practical applications. Herein, we have developed an approach utilizing a one-dimensional convolutional neural network (1D-CNN) algorithm coupled with an ionic conductive flexible strain sensor based on a sodium chloride/sodium alginate/polyacrylamide (NaCl/SA/PAM) dual-network hydrogel for intelligent and accurate air-writing recognition. Taking advantage of the excellent characteristics of the hydrogel sensor, such as high stretchability, good tensile strength, high conductivity, strong adhesion, and high strain sensitivity, alongside the enhanced analytical ability of the 1D-CNN machine learning (ML) algorithm, we achieved a recognition accuracy of ∼96.3% for in-air handwritten characters of the English alphabets. Furthermore, comparative analysis against state-of-the-art methods, such as the widely used residual neural network (ResNet) algorithm, demonstrates the competitive performance of our integrated air-writing recognition system. The developed air-writing recognition system shows significant potential in advancing innovative systems for air-writing recognition and paving the way for exciting developments in human-machine interface (HMI) applications.

A Highly Sensitive, Low Creep Hydrogel Sensor for Plant Growth Monitoring

Ion−conducting hydrogels show significant potential in plant growth monitoring. Nevertheless, traditional ionic hydrogel sensors experience substantial internal creep and inadequate sensitivity, hindering precise plant growth monitoring. In this study, we developed a flexible hydrogel sensor composed of polyvinyl alcohol and acrylamide. The hydrogel sensor exhibits low creep and high sensitivity. Polyvinyl alcohol, acrylamide, and glycerol are crosslinked to create a robust interpenetrating double network structure. The strong interactions, such as van der Waals forces, between the networks minimize hydrogel creep under external stress, reducing the drift ratio by 50% and the drift rate by more than 60%. Additionally, sodium chloride and AgNWs enrich the hydrogel with conductive ions and pathways, enhancing the sensor’s conductivity and demonstrating excellent response time (0.4 s) and recovery time (0.3 s). When used as a sensor for plant growth monitoring, the sensor exhibits sensitivity to small strains and stability for long−term monitoring. This sensor establishes a foundation for developing plant health monitoring systems utilizing renewable biomass materials.

Ion sensors based on organic semiconductors acting as quasi-reference electrodes

Thin-film devices that transduce the chemical activity of ions into electronic signals are essential components in various applications, including healthcare diagnostics and environmental monitoring. Combinations of organic semiconductors (OSCs) and ion-selective materials have been explored for developing solution-processable ion sensors. However, the necessity of reference electrodes (REs) and operational stability in ion-permeable OSCs have posed questions regarding whether reliable measurements with thin-film components are attainable with OSCs. Herein, we report electric double-layer transistors (EDLTs) with OSCs in single-crystal forms for ion sensing. Our EDLTs demonstrated high operational stability, with a one-to-one relationship between the source electrode potential and device resistance, and served as quasi-REs (qRE). When our EDLT is served as qRE, its drift was as small as 0.5 mV/h and comparable to that of commonly employed REs. In our system, the semiconductor-electrolyte interface is self-passivated by the alkyl chains of OSCs in single-crystal structures, with the two-dimensional transport layer appearing unaltered upon gating. EDLT arrays with ion-selective and nonselective liquid junctions enable ion concentration sensing without a conventional RE. These findings provide opportunities to develop thin-film devices based on OSCs for easy integration and reliable measurements.

3D-Printed Transducers for Solid Contact Potentiometric Ion Sensors: Improving Reproducibility by Fabrication Automation.

3D printing technology has become attractive in the development of electrochemical sensors as it offers automation in fabrication, customization on-demand, and reproducibility, among other features. Nonetheless, to date, solid contact potentiometric ion sensors have remained overlooked using this technology. Thus, the novelty of this work relies on demonstrating for the first time the usefulness of the multimaterial 3D printing approach to manufacture potentiometric ion-selective electrodes. The significance is indeed twofold. First, we discovered that by using the polyethylene terephthalate glycol (PETg) and polylactic acid-carbon black (PLA-CB) filaments together with a rational electrode design containing a well to accommodate the ion-selective membrane, a tight seal among all of the sensing materials is obtained. Importantly, this has mainly impacted the electrode-to-electrode reproducibility (ERSD0 ± 3 mV). Second, 75 ready-to-use electrodes can be printed in less than 3.5 h in a completely automated manner at a cost of ∼0.32 €/sensor. This feature may positively impact the suitability of further scaled-up production as well as the possibility of application in low-resource contexts. Overall, the presented outcomes are expected to encourage certain research directions to adopt using multimaterial 3D-printing approaches for producing highly reproducible solid contact potentiometric ion-selective electrodes, but are not restricted to them.

Synthesis of fluorometric carbon nano dots(CNDs) for selective sensing of biologically important Fe3+ and Cu2+ metal ions and evaluating their antioxidant capacity.

In this research, CNDs were prepared by a green and cost effective method using Cinnamomum Tamala (bay leaf) as carbon sources. TEM, UV, FTIR, ZETA Potential, PL and Fluorescence methods were used to characterize the produced CNDs and the average particle size is 3.42nm. This research was conducted on the development of fluorescent sensors for various metal ions, including Fe3+, Cu2+, Zn2+, Ni2+, Pb2+, Cr3+, Mg2+, Na+ 1 and Cd2+. The CNDs demonstrated selective sensing of biologically important Fe+ 3 and Cu+ 2 metal ions. The CNDs antioxidant assay was tasked with DPPH• radical scavenging properties. CNDs made from Cinnamomum Tamala had the highest DPPH free radical scavenging activity at 100mg/L (42.06%) with the IC50 of 130.68mg/L. The outcome implies that Indian spices are among the best materials for optical metal ion detection and sensing, and they also have therapeutic benefits.

Preparation of glutathione-Cu/Ag bimetallic nanoparticles for temperature and metal ion Co2+ sensing

In this work, copper silver bimetallic nanoparticles (Cu/Ag BNPs) with strong fluorescence were prepared using glutathione (GSH) as a reductant and protectant. The range of fluorescence signal of GSH-Cu/Ag BNPs to ambient temperature is 30∼90 °C has a sensitive response and can be used as a temperature sensor. The high fluorescence intensity of GSH-Cu/Ag BNPs at around 30 °C indicates that the detection of Co2+ does not require harsh temperature conditions, which could be very convenient for future practical detection work. In addition, glutathione in Cu/Ag BNPs and the synergistic effect of Cu and Ag nanoparticles in Cu/Ag BNPs can be used as a metal ion sensor for Co2+ fluorescence detection.

A comprehensive investigation of cholesteric liquid crystal interpenetrating polymer networks for metal ion sensor technology

This study outlines the fabrication process of a photonic polymer cholesteric liquid crystal (PCLC) engineered for dynamic response to external stimuli. The methodological framework includes the precise templating of PCLC onto an interpenetrating polymer network (IPN) following selective UV crosslinking and the careful removal of nonreactive mesogens. The sensor component of the PCLC film is composed of poly(acrylic acid) hydrogel. A key aspect of the process involves the formation of micro-holes through acetone treatment to enhance sensitivity. Additionally, functionalization with potassium hydroxide (KOH) improves the detection of calcium ions (Ca2+) by disrupting hydrogen bonds and facilitating the formation of potassium carboxylate salt. Combining the inherent properties of PCLC with the hydrogel’s responsiveness to metal ions results in a sophisticated sensor designed to detect Ca2+ ions through the modulation of PCLC pitch, creating distinct transmission bands. The manuscript provides a detailed quantitative analysis of stimulus-induced changes in PCLC wavelength, using measured spectra to refine and optimize process conditions. The resulting PCLC sensor demonstrates exceptional sensitivity (9.30 nm/mM) within the 1–15 mM Ca2+ ion concentration range, along with excellent specificity. This cost-effective, user-friendly, and colorimetric sensing modality holds significant promise for various applications in analytical chemistry.

Heterocyclic Hemipiperazines: Multistimuli-Responsive Switches and Sensors for Zinc or Cadmium Ions.

Advance in the design of molecular photoswitches - adapters that convert light into changes at molecular level - opens up exciting possibilities in preparing smart polymers, drugs photoactivated inside humans, or light-fueled nanomachines that might in the future operate in our bloodstream. Hemipiperazines are recently reported biocompatible molecular photoswitches based on cyclic dipeptides. Here we report a multistimuli-responsive hemipiperazine-based switch that reacts on light, solvents, acidity, or metal ions. Its photoequilibration is controlled by the intramolecular hydrogen bonding pattern. The compound can be used as a mid-nanomolar photoswitchable fluorescent sensor for zinc and cadmium ions, applicable to monitor environmental pollution in real time.

Effect of oxygen content in CuO x nanofilms on chloride ion detection for ion sensor applications

Sensors for detecting chloride ions have been required for routine monitoring of industry and human health. This study proposes a concept of an ion sensor based on CuO x nanofilms with different oxygen contents. The CuO x -based sensors exhibited an increase in DC current for those with low oxygen content and a decrease for those with high oxygen content following exposure to a chloride ion solution. AC impedance analysis suggested differential reactions of chloride ions in the bulk and surface regions of CuO x , dependent on the oxygen content. For the CuO x -based sensors with a ratio of 0.78 oxygen to copper atoms at chloride ion concentrations of 10−1000 ppm, the sensitivity in the bulk region calculated from AC impedance was 61−2926, which was higher than the sensitivity of 1.3−2.6 calculated from DC impedance. Finally, CuO x -based sensors demonstrated identifiability for chloride ions compared to sodium and calcium ions.

A novel organic chromo-fluorogenic optical sensor for detecting chromium ions

Sensing trivalent chromium ion (Cr(III)) is widely applied in different areas, such as clinical analysis, marine, environmental monitoring, or even chemical industry applications. Cr(III) has a significant role in the physiological process of human life. It is classified as an essential micronutrient for living organisms. Herein, we developed and designed a novel optical Cr(III) ions sensor film. The investigated sensor has a relatively small dynamic range of 1.24 × 10−3 to 0.5 μM. We report a highly sensitive optical sensor film for Cr(III) ions based on diethyl 3,4-diaminothieno[2,3-b]thiophene-2,5-dicarboxylate (3D) probe. The optical characteristics of the chemical probe exhibit substantial emission at 460 nm under 354 nm excitation. Besides, the interaction of the Cr(III) ions with 3D involves a complex formation with a 2:1 (metal: ligand) ratio, which is convoyed by the main peak enhancement that centered at 460 nm of 3D, and the main peak is red-shifted to 480 nm. The easily discernible fluorescence enhancement effect is a defining characteristic of the complexation reaction between the 3D probe and Cr(III). On the basis of the substantial fluorescence mechanism caused by the formation of a (Cr(III)-3D complex, which inhibits the photo-induced electron transfer (PET) process, the devised optical sensor was proposed. This film exhibits exceptional sensitivity and selectivity due to its notable fluorescence properties, stock shift of less than 106 nm, and detection capabilities at a significantly low detection limit of 0.37 × 10−3 μM. The detection procedure is executed by utilizing a physiological pH medium (pH = 7.4) with a relative standard deviation RSDr (1 %, n = 3). In addition, the 3D sensor demonstrates a high degree of affinity for Cr(III), as determined by the calculation of its binding constant to be 1.40 × 106. We present an impressive optical sensor that is constructed upon a three-dimensional molecule.