Ion Sensor Research Articles

A Review on Soft Ionic Touch Point Sensors

A touch sensor is an essential component in meeting the growing demand for human‐machine interfaces. These sensors have been developed in wearable, attachable, and even implantable forms to acquire a wide range of information from humans. To be applied to the human body, sensors are required to be biocompatible and not restrict the natural movement of the body. Ionic materials are a promising candidate for soft touch sensors due to their outstanding properties, which include high stretchability, transparency, ionic conductivity, and biocompatibility. Here, this review discusses the unique features of soft ionic touch point sensors, focusing on the ionic material and its key role in the sensor. The touch sensing mechanisms include piezocapacitive, piezoresistive, surface capacitive, piezoelectric, and triboelectric and triboresistive sensing. This review analyzes the implementation hurdles and future research directions of the soft ionic touch sensors for their transformative potential.

Development of an optode-type chloride ion sensor utilizing optical fiber with silver film

This study delves into the development and assessment of a new chloride ion sensor employing optical fiber technology. It investigates the response characteristics of Ag-coated optode sensors, with a specific focus on film thickness and concentration dependency. Experimental results indicate that while film thickness offers sensitivity advantages, they are susceptible to fluctuations and detachment, thereby impacting reliability. To address this, the study suggests methods for correcting intensity fluctuations and improving sensor performance through calibration curves and spectrum measurements. Moreover, experiments demonstrate potential for enhancing sensitivity and resolution at low concentrations. These results highlight the superiority of the optode sensor method over existing techniques, particularly in sensitivity and adaptability for chloride ion monitoring. The study significantly contributes to sensor technology advancement, offering improved precision and reliability for environmental monitoring and industrial process control.

NeMeHg, genetically encoded indicator for mercury ions based on mNeonGreen green fluorescent protein and merP protein from Shigella flexneri.

The detection of mercury ions is an important task in both environmental monitoring and cell biology research. However, existing genetically encoded sensors for mercury ions have certain limitations, such as negative fluorescence response, narrow dynamic range, or the need for cofactor supplementation. To address these limitations, we have developed novel sensors by fusing a circularly permutated version of the mNeonGreen green fluorescent protein with the merP mercury-binding protein from Gram-negative bacteria Shigella flexneri. The developed NeMeHg and iNeMeHg sensors responded to mercury ions with positive and negative fluorescence changes, respectively. We characterized their properties in vitro. Using the developed biosensors, we were able to successfully visualize changes in mercury ion concentration in mammalian cultured cells.

Compositional Metrics of Fast and Slow Alfvénic Solar Wind Emerging from Coronal Holes and Their Boundaries

We seek to understand the composition and variability of fast solar wind (FSW) and slow Alfvénic solar wind emerging from coronal holes (CHs). We leverage an opportune conjunction between Solar Orbiter and Parker Solar Probe (PSP) during PSP Encounter 11 to include compositional diagnostics from the Solar Orbiter Heavy Ion Sensor as these variations provide crucial insights into the origin and nature of the solar wind. We use potential field source surface and magnetohydrodynamic models to connect the observed plasma at PSP and Solar Orbiter to its origin footpoint in the photosphere and compare these results with the in situ measurements. A very clear signature of a heliospheric current sheet crossing as evidenced by enhancements in low first ionization potential (FIP) elements, ion charge state ratios, proton density, low Alfvénicity, and polarity estimates validates the combination of modeling, data, and mapping. We identify two FSW streams emerging from small equatorial CHs with low ion charge state ratios, low FIP bias, high Alfvénicity, and low footpoint brightness, yet anomalously low alpha particle abundance for both streams. We identify high-Alfvénicity slow solar wind emerging from the overexpanded boundary of a CH having intermediate alpha abundance, high Alfvénicity, and dips in ion charge state ratios corresponding to CH boundaries. Through this comprehensive analysis, we highlight the power of multi-instrument conjunction studies in assessing the sources of the solar wind.

Carbon quantum dots with ultra-high quantum yield for versatile turn-on sensor of gluten and cyanide Ions

In this work, we reported the synthesis of carbon quantum dots (CQDs) with high fluorescent quantum yield (90.7 %). These nanoparticles were applied in a turn-off/turn-on sensor able to detect cyanide ions. In this sensing strategy, Fe3+ ions were added to the aqueous suspension of CQDs to quench the fluorescence of the system, which was recovered from the presence of cyanide ions. Furthermore, a sensor array was developed using the LDA chemometric tool, to classify different species present in cookie formulations, including gluten. The turn-off/turn-on sensor proved to be sensitive and effective for detecting cyanide ions, with a detection limit of 3.77 ppm. This high sensibility and selectivity allowed quantifying the analyte in a real sample of tap water.

Uncovering the sensing mechanism of a zinc ion sensor: Fluorescence enhancement induced by the elimination of the TICT state

Precise detection of zinc ion is of fundamental importance in the fields of environment protection and food safety. A comprehensive understanding of the sensing mechanism will help to the design of such sensors. The detailed photophysical process of a zinc ion sensor as well as the sensing mechanism are uncovered with the aid of density functional theory (DFT) and time-dependent density functional theory (TDDFT). Both the ground state and first excited state potential energy surfaces (PES) of the sensor are carefully explored to reveal the photo-physical process of the sensor. Excited state intramolecular proton transfer (ESIPT) is observed on the S1 state PES. Then, the twist motion of C=N double bond is triggered after the ESIPT process, which leads to a twisted intramolecular charge transfer (TICT) state. This TICT state is found to make the sensor non-emissive. With the addition of Zn2+, the TICT state is eliminated which greatly enhances the fluorescence of the sensor and achieves zinc ion detection. The interaction of the sensor with Cd2+ and Hg2+ are also explored, which well explains the good selectivity of the sensor.

Self-powered smart pressure sensors by stimuli-responsive ion transport within layered hydrogels

Self-powered ionic sensors represent an emerging class of device capable of detecting and converting external stimuli, such as pressure and temperature, into electrical outputs without the need for an external power source. However, they often suffer from relatively low sensitivity, and their detection capability is typically limited to the magnitude of external stimulus. In this study, we present a smart pressure sensor consisting of nanowire electrodes and three-component ionic hydrogels with distinct stiffnesses and ion selectivities. Our hydrogel device effectively converts external pressure into a convective flow of cations, which are efficiently captured by the nanowire electrodes, yielding remarkable electrical outputs. In addition, upon local compression, the hydrogel device induces a convective flow that guides cations to the nearest electrode with minimal frictional force. This creates local inhomogeneous ionic distributions around the electrodes, enabling the detection of the local position and directional movements of the applied pressure without the need for additional electrical circuits. By enabling the simultaneous detection of pressure magnitude, local position, and movement, our device is capable of encoding intricate movements into distinct electrical outputs, thereby presenting a novel avenue for the development of advanced self-powered sensors with enhanced capabilities.

A solid-state sensor based on nanosilica/methylcellulose composite and chromoionophore 8-carboxamidoquinoline derivative as an optode pellet for Zinc(II) ion

Zinc is known to play a central role in the immune system that creates a resistance barrier against viral infection. A naked-eye zinc(II) (Zn2+) ion sensor of 2-oxo-2-(quinolin-8-ylamino)acetic acid (OQAA) as a recognition probe was designed and synthesized. By inserting functional groups of amide and carboxylic acid into a conjugated molecule of 8-aminoquinoline, Zn2+ ion (borderline acid) coordinated favorably with aromatic nitrogen atoms (borderline bases) and N-amide. Characterization of OQAA was done by FTIR, 1H NMR, 13C NMR, UV–vis, and ESI-MS. The Zn2+ ion chemosensor was fabricated by immobilizing OQAA chromogenic ionophore onto a nanosilica/methylcellulose (Si/MC) composite pellet to form a solid-state pellet sensor. A distinguishable color change from pale light-yellow to yellowish-brown of the optode pellet was observed in the presence of Zn2+ ions. Optimization of the pellet-based Zn2+ ion sensor was carried out with a fiber optic reflectance spectrophotometer at 0.25 M OQAA in tris buffer (pH 7.4). The optical fiber sensor demonstrated a linear reflectance response between 0.8 M and 2.4 M Zn2+ ion (R²=0.9741) at λmax=635 nm with a limit of detection (LOD) of 3.6 × 10−2 M and response time of 5 min. The reflectance sensor exhibited good selectivity towards Zn2+ ion over other competitive heavy metals e.g., iron(II) (Fe2+), nickel(II) (Ni2+), cobalt(II) (Co2+), copper(II)(Cu2+) and cadmium(Cd2+) ions. Additionally, the proposed Zn2+ ion sensor showed high shelf-life stability of 75 days operational duration with 80.0 % initial optical response remaining. The quick, easy, and uncomplicated preparation of the OQAA-modified Si/MC (OQAA-Si/MC) composite pellet novel material gave a reproducible reflectance signal towards the determination of Zn2+ ion concentration with a low relative standard deviation (RSD) obtained at ∼1.0 % (n=12), and was reusable three times for assay of Zn2+ ion (RSD=1.3 %). The optical chemosensor imparts remarkable color change towards the analysis of Zn2+ ions, and could offer direct and fast in-situ visual inspection of Zn2+ ion levels in complex samples without an instrument.

Biogenic fabrication of a gold nanoparticle sensor for detection of Fe3+ ions using a smartphone and machine learning.

In recent years, smartphones have been integrated into rapid colorimetric sensors for heavy metal ions, but challenges persist in accuracy and efficiency. Our study introduces a novel approach to utilize biogenic gold nanoparticle (AuNP) sensors in conjunction with designing a lightbox with a color reference and machine learning for detection of Fe3+ ions in water. AuNPs were synthesized using the aqueous extract of Eleutherine bulbosa leaf as reductants and stabilizing agents. Physicochemical analyses revealed diverse AuNP shapes and sizes with an average size of 19.8 nm, with a crystalline structure confirmed via SAED and XRD techniques. AuNPs exhibited high sensitivity and selectivity in detection of Fe3+ ions through UV-vis spectroscopy and smartphones, relying on nanoparticle aggregation. To enhance image quality, we developed a lightbox and implemented a reference color value for standardization, significantly improving performance of machine learning algorithms. Our method achieved approximately 6.7% higher evaluation metrics (R 2 = 0.8780) compared to non-normalized approaches (R 2 = 0.8207). This work presented a promising tool for quantitative Fe3+ ion analysis in water.

High-Strength and High-Conductivity Core-Sheath Hydrogel Long Fibers for Stretchable Ionic Strain Sensors

High-Strength and High-Conductivity Core-Sheath Hydrogel Long Fibers for Stretchable Ionic Strain Sensors

A novel multifunction of wearable ionic conductive hydrogel sensor for promoting infected wound healing

Conductive hydrogels have attracted widespread attention in applications of artificial biological tissues and ionic skin. Herein, a highly ionically conductive and biocompatibility hydrogels that integrate sensing, low swelling, and antimicrobial are prepared by the composition of chitosan (CS) chains, tannic acid (TA), polyacrylic acid (PAA), and metal cations. The hydrogels with high conductivity had tensile strain repeatable adhesion, wide strain detection, biocompatibility, and motion sensing capabilities. Particularly, in the aspect of hydrogel strain sensing based on ion conduction, it has high nonliearity (GF=2.05) with a large strain (20 %-1400 %), and excellent cycle stability. In addition, owing to the low-swelling effect and dipole-dipole interaction between the existence of free catechol groups of the zwitterionic group and TA in the zwitterionic sulfobetaine methacrylate (SBMA), the hydrogels display repeatable and controllable adhesiveness. Meanwhile, the covalent cross-linking between AA and SBMA synergistically enhances the mechanical properties of the hydrogels (169 kPa, 1474 %). Furthermore, as demonstrated in the L929 mouse wound infection model, the results indicated that the prepared hydrogel group had antimicrobial activity and reduced inflammation, thereby accelerating wound healing. The biocompatible and mutli-functional conductive hydrogels provide a new method for the design of novel type ionic skin device.

One-pot fabrication of aluminium ion anchoring boric acid-carbon dots hybrid system and its highly selective fluorescence sensing for fluorine ion in water environment

Fluorine is one of the essential trace elements for human body, which is essential for calcification process in living organisms. In the paper, a “off–on” fluorescence sensor for fluorine ion based on aluminium ion and boric acid codoped carbon dots was synthesized and characterized. According to fluorescence analysis, the water-soluble sensor exhibited highly selective and sensitive for fluorine ion. In sensing process, F- could bind to Al3+ after the addition of fluorine ion, which resulted in the decomposition of Al-BA-CDs system, then intense yellow fluorescence signal appeared. Moreover, the fluorescence sensor exhibited low detecting limit (5.0 nanomolar level) by fluorescence titration spectrum. In addition, according to evaluate the fluorescence sensing properties of F- in HepG2 cell and zebra fish, this job greatly extended the use of F- sensor from aluminium ion and boric acid codoped carbon dots in organism. The work also provided an interesting idea for fabricating water-soluble fluorescence sensor for F-.

Resource utilization of sewage sludge as a nitrogen self-doped carbon quantum dot for specific detection of Fe3+ and Hg2+

With the widespread application of sewage biological treatment technology, a large amount of sewage sludge is generated during the sewage treatment process, which not only poses a threat to the environment but also results in wastage of C and N resources. In this study, the nitrogen self-doped fluorescent carbon dots (N-CDs) with an average particle size of 3.43 nm were synthesized green through a two-stage hydrothermal method. It was found that N-CDs can detect Fe3+ through internal filtration effect (IFE), with a linear detection range of 0–250 μM and a detection limit of 2.207 μM. Furthermore, N-CDs can also serve as a “turn off–on” probe for monitoring Hg2+, with a linear detection range of 2–10 μM and a detection limit of 0.989 μM. Interestingly, oxygen-containing groups were shown to be sensitive to Hg2+, which has been demonstrated for the first time. Moreover, the practical application of N-CDs as a double ion sensor in the detection of Fe3+ and Hg2+ has achieved good results for real power plant wastewater.

A Biodegradable Fiber Calcium Ion Sensor by Covalently Bonding Ionophores on Bioinert Nanoparticles.

Implantable sensors, especially ion sensors, facilitate the progress of scientific research and personalized healthcare. However, the permanent retention of implants induces health risks after sensors fulfill their mission of chronic sensing. Biodegradation is highly anticipated; while; biodegradable chemical sensors are rare due to concerns about the leakage of harmful active molecules after degradation, such as ionophores. Here, a novel biodegradable fiber calcium ion sensor is introduced, wherein ionophores are covalently bonded with bioinert nanoparticles to replace the classical ion-selective membrane. The fiber sensor demonstrates comparable sensing performance to classical ion sensors and good flexibility. It can monitor the fluctuations of Ca2+ in a 4-day lifespan in vivo and biodegrade in 4 weeks. Benefiting from the stable bonding between ionophores and nanoparticles, the biodegradable sensor exhibits a good biocompatibility after degradation. Moreover, this approach of bonding active molecules on bioinert nanoparticles can serve as an effective methodology for minimizing health concerns about biodegradable chemical sensors.

Synthesis of Phenol-Hydrazide-Appended Tetraphenylethenes as Novel On-Off-On Cascade Sensors of Copper and Glutathione.

This study reports the synthesis of novel fluorescent probes, phenol-hydrazide-appended tetraphenylethenes (TPEs I and II), and explores their photochemical properties. The probes exhibit aggregation-induced emission (AIE) in increasing water content, as observed using fluorescence spectroscopy. Further investigation with UV-vis and fluorescence techniques revealed their potential as ion sensors. Both TPE I and TPE II act as "turn-off" sensors for Cu2+ ions, showing decreased fluorescence intensity in their presence. Their limit of detection (LOD) and association constant (K a) for Cu2+ were found to be comparable at 747 nM/597 nM, and 2.46 × 105 M-1/2/1.78 × 105 M-1/2, respectively. Moreover, the quantum yields of TPE I and TPE II were also calculated and found to be 0.651 and 0.325, respectively. Interestingly, these probes also function as "turn-on" sensors for glutathione (GSH) in the presence of copper. This means their fluorescence can be reversibly switched off and on by alternating CuCl2 and GSH additions. Moreover, the LOD values for GSH with TPE II-Cu2+ were calculated to be 544 nM. In addition, the investigation also employed visual analysis to assess the color alterations of TPEs on filter paper and in real water samples. Overall, this research introduces promising new probes with potential applications in copper ion detection and biomolecule glutathione sensing in real water samples.

Unveiling the fast adsorption and desorption of heavy metals on/off nanoplastics by real-time in-situ potentiometric sensing

Nanoplastics (<1 μm) can serve as a transport vector of environmental pollutants (e.g., heavy metals) and change their toxicities and bioavailabilities. Up to date the behaviors of adsorption and desorption heavy metals on/off nanoplastics are largely unknown. Herein, polymeric membrane potentiometric ion sensors are proposed for in-situ assessment of the real-time kinetics of heavy metal adsorption and desorption on/off nanoplastics. Results show that nanoplastics can adsorb and release heavy metals in a fast manner, indicating their superior ability in transferring heavy metals. The adsorption behaviors are closely related to the characteristics of nanoplastics and background electrolytes. Particle aggregation and increases in salinity and acidity suppress the adsorption of heavy metals on nanoplastics. The desorption efficiencies of different heavy metals are Pb2+ (31 %) < Cu2+ (40 %) < Cd2+ (97 %). Our proposed method is applicable for the detection of the plastic pollutants with size <100 nm and of the samples with high salinities (e.g., seawater). This work would provide new insights into the assessment of environmental risks posed by nanoplastics and heavy metals.

Luminescent Silicon Nanocrystals as Metal Ion Sensors.

At relatively low concentrations in aqueous solution, Fe3+, Fe2+, Cu2+, and Ni2+ quench the photoluminescence (PL) of the undecenoic acid-capped silicon (Si) nanocrystals. The PL could be restored by adding a chelating agent, such as ethylenediaminetetraacetic acid (EDTA), to remove the ions. Fe3+ and Cu2+ also significantly increase the PL lifetime. Other metal ions, including Cd2+, Mn2+, Pb2+, Zn2+, In3+, K+, and Ca2+, had no effect on the Si nanocrystal PL. The limits of detection (LODs) for Fe3+ and Cu2+ of 370 and 150 nM, respectively, are low enough for metal ion sensing applications.

Explanation based on thermodynamic parameters regarding effect of sensing film thickness and amount of graphene oxide on sensor performance in aniline, N-phenylglycine and graphene oxide based electrochemical heavy metal ion sensor

Background: To construct a heavy metal ion sensor, selectivity and sensitivity are the key important parameters to be taken care of. In our earlier work, film thickness and amount of graphene oxide (GO) content in a novel composite ANGO, synthesized from aniline, N-phenylglycine and GO was varied and sensing parameters including sensitivity, limit of detection (LOD), thermodynamic parameter which includes -∆Gad and charge transport parameter including barrier width (BW), d, of charge transfer based on Simmon’s model were evaluated and compared and an LOD of 800 ppt for Cd2+ was achieved using square wave voltammetry (SWV) withstanding interference from several ions. Methods: In this work, thermodynamic factors such as -∆Gad, ∆H, reorganization energy, partition coefficient and solvated ionic radius were used to explain the sensor performance with respect to film thickness and amount of GO. All the parameters were analyzed for different film thicknesses and amount of GO and a correlation was achieved. Finally, effect of electrochemical surface area of different polyaniline-based material on thermodynamic properties of detection process of Cd2+ was studied. Results: The variation of the thermodynamic properties for Cd2+ sensing with respect to film thickness and amount of GO were examined. Similarly, variation of thermodynamic properties for polyaniline based different sensing materials were examined. Correlation coefficients were developed from the thermodynamic parameters and the d values to explain the underlying mechanism behind improved sensor performance. Conclusions: This study can provide information on the thermodynamic properties which can be predicted from BW technique. The correlation coefficients would help in designing polyaniline based novel sensing film material with the need of lesser number of experiments.

Rapidly prepared and screened supramolecular fluorescent sensors for the detection of metal ions

Fluorescent sensors capable of identification and quantitation of metal ions are of great importance for biological and environmental sciences. Supramolecular fluorescent sensors are designed and prepared based on the complexation of host-dye and ligand–metal ion. Two hosts, one commercially-available dye and eight ligands are used to form 16 potential three-component supramolecular sensors, which are screened to discover workable sensors. Among the workable sensors, one fluorescent sensor is capable of the identification of Cu2+ from nine other metal ions. Other suitable fluorescent sensors may be used for the quantitative analysis of metal ions in buffered solution, and metal sensing in live cells. This new analytical method provides an approach to rapidly generate sensors for different metal ions.

A structure-switching electrochemical aptamer sensor for mercury ions based on an ordered assembled gold nanorods-modified electrode

In this paper, an electrochemical aptamer sensor with high sensitivity and selectivity was proposed for the detection of mercury ion (Hg2+). The ordered assembled gold nanorods-modified screen-printed electrode (AuNRs/SPE) was used to improve the sensor with higher sensitivity and data reproducibility. The sensor was designed based on the specific recognition of Hg2+ with aptamer to form T-Hg2+-T binding structure. Two aptamer chains were applied in the designed sensor, aptamer S1 with terminal thiolate group was self-assembled on the AuNRs/SPE via Au–S bond, and complementary S2 labeled with ferrocene (Fc) would generate a strong redox electrical signal. The addition of Hg2+ would mediate the folding of the T-base of S1 to form a hairpin structure, resulting in the dissociation of the Fc-labeled S2 from the sensing system and a significant reduction in the redox current. The electrochemical properties of the sensor were further investigated using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The effects of aptamer reaction time, concentration, and target incubation time on the sensor response were also investigated. The results showed that the sensor had a sensitive response to Hg2+ at concentrations ranging from 1 pM to 1 μM, with a detection limit of 0.3 pM. Furthermore, the proposed sensor was applied in the determination of Hg2+ in real samples, demonstrating its potential practical value.