Non-Equilibrium Diffusion Controlled Ion-Selective Optical Sensor for Blood Potassium Determination

Ultrasensitive optical detection of strontium ions by specific nanosensor with ultrahigh binding affinity

The release and escape of radioactive materials has posed tremendous threats to the global environment. Among various radioactive elements, 90Sr has attracted growing attention due to its long half-life and its tendency to accumulate in bone tissue. Nonetheless, the concentration of 90Sr in radioactive waste is exceedingly low, far below the detection limits of currently available strontium-targeting chemical sensors. Herein, we propose an optical nanosensor (Sr2+-nanosensor) that exhibits an ultra-low detection limit of 0.5 nM, surpassing the 90Sr in the treated radioactive water from the Fukushima. The sensor offers wide sensing range of eight orders of magnitude, rapid response of less than 10 s, and high selectivity against 31 common ions. These excellent performances are attributed to a specific ligand (Sr2+-ligand) for Sr2+ recognition. The Sr2+ is found to be bound by six oxygen atoms from the Sr2+-ligand with a stability constant at least two orders higher than that of other traditional ligands. This study offers invaluable insights for the design of Sr2+-sensing methodologies as well as a technique for detecting trace amounts of environmental radioactive pollution.

Comparative analysis of a bulk optode based on a valinomycin ionophore and a nano-optode in micelles with pluronic F-127 for the quantification of potassium in aqueous solutions

In this work, two types of optical sensors were prepared for the quantification of potassium: the bulk optode (BO) and nano-optode (NO). The BO was prepared using three main components: the ionophore valinomycin, the ion exchanger tetrakis(4-chlorophenyl) potassium borate (K-TCPB), and the chromoionophore ETH 5294 (CHI). The optimal composition was found to be in a ratio of [1 : 1 : 1]. The NO was prepared by miniaturizing the BO through sonication in surfactant Pluronic F-127. The working range for the linear calibration model of BO was from 10-6 to 1.0 M K+ with a LODBO = 0.31 μM, meanwhile for NO was from 10-4 to 1.0 M K+ with a LODNO = 30.3 μM. Both optodes were tested for selectivity towards K+ in the presence of alkaline and alkaline earth ions, with a selectivity coefficient > 1.0. Furthermore, precision and stability studies of BO and NO were performed for three levels of K+ concentrations, 10-6, 10-3, 1.0 M for BO and 10-4, 10-2, 1.0 M for NO, showing a good homogeneity of the NO in the whole concentration range. However, an excessive variability was obtained for BO at 1.0 M K+. Therefore, the NO represents a potential tool for quantification of K+.

Near-Infrared Fluoride Sensing Nano-Optodes and Distance-Based Hydrogels Containing Aluminum-Phthalocyanine

Fluoride ions are highly relevant in environmental and biological sciences, and there is a very limited number of established fluoride chemical sensors. Previous fluoride-selective optodes were demonstrated with metal-porphyrin as the ionophore and required a chromoionophore for optical signal transduction. We demonstrate here novel optical fluoride sensing with nano-optodes containing an aluminum-phthalocyanine complex (AlClPc) as the single active sensing component, simplifying the conventional ion-selective optodes approach. The fluoride nano-optodes were interrogated in the absorbance and fluorescence modes in the near-infrared region, with absorption around 725 nm and emission peaks at 720 and 800 nm, respectively. The nano-optodes exhibited a lower detection limit around 0.1 μM and good selectivity over a range of common anions including ClO4-, Cl-, Br-, I-, SO42-, NO3-, and AcO-. Furthermore, the nano-optodes were physically entrapped in agarose hydrogels to allow distance-based point-of-care testing (POCT) applications. The 3D networks of the agarose hydrogel were able to filter off large particulates in the samples without stopping fluoride ions to reach the nano-optodes. The fluoride concentrations in real samples including river water, mineral water, and groundwater were successfully determined with the distance-based sensing hydrogel, and the results agreed well with those from commercial fluoride electrodes. Therefore, the results in this work lay the groundwork for the optical detection of fluoride in environmental samples without very sophisticated sample manipulation.

Ultrasensitive Ionophore-Based Liquid Sensors for Colorimetric Ion Measurements in Blood

The self-monitoring of electrolytes using a small volume of capillary blood is needed for the management of many chronic diseases. Herein, we report an ionophore-based colorimetric sensor for electrolyte measurements in a few microliters of blood. The sensor is a pipet microtip preloaded with a segment of oil (plasticizer) containing a pH-sensitive chromoionophore, a cation exchanger, and an ionophore. The analyte is extracted from the sample into the oil via a mixing protocol controlled by a stepper motor. The oil with an optimized ratio of sensing chemicals shows an unprecedentedly large color response for electrolytes in a very narrow concentration range that is clinically relevant. This ultrahigh sensitivity is based on an exhaustive response mode with a novel mechanism for defining the lower and higher limits of detection. Compared to previous optodes and molecular probes for ions, the proposed platform is especially suitable for at-home blood electrolyte measurements because (1) the oil sensor is interrogated independent of the sample and therefore works for whole blood without requiring plasma separation; (2) the sensor does not need individual calibration as the consistency between liquid sensors is high compared to solid sensors, such as ion-selective electrodes and optodes; and (3) the sensing system consisting of a disposable oil sensor, a programmed stepper motor, and a smartphone is portable, cost-effective, and user-friendly. The accuracy and precision of Ca2+ sensors are validated in 51 blood samples with varying concentrations of total plasma Ca2+. Oil sensors with an ultrasensitive response can also be obtained for other ions, such as K+.

A K+-sensitive photonic crystal hydrogel sensor for efficient visual monitoring of hyperkalemia/hypokalemia

Potassium ions (K⁺) play crucial roles in many biological processes. Abnormal K⁺ levels in the body are usually associated with physiological disorders or diseases, and thus, developing K⁺-sensitive sensors/devices is of great importance for disease diagnosis and health monitoring. Herein, we report a K⁺-sensitive photonic crystal hydrogel (PCH) sensor with bright structural colors for efficient monitoring of serum potassium. This PCH sensor consists of a poly(acrylamide-co-N-isopropylacrylamide-co-benzo-15-crown-5-acrylamide) (PANBC) smart hydrogel with embedded Fe₃O₄ colloidal photonic crystals (CPCs), which could strongly diffract visible light and endow the hydrogel with brilliant structural colors. The rich 15-crown-5 (15C5) units appended on the polymer backbone could selectively bind K⁺ ions to form stable 2 : 1 [15C5]₂/K⁺ supramolecular complexes. These bis-bidentate complexes served as physical crosslinkers to crosslink the hydrogel and contracted its volume, and thus reduced the lattice spacing of Fe₃O₄ CPCs and blue-shifted the light diffraction, and finally reported on the K⁺ concentrations by a color change of the PCH. Our fabricated PCH sensor possessed high K⁺ selectivity and pH- and thermo-sensitive response performances to K⁺. Most interestingly, the K⁺-responding PANBC PCH sensor could be conveniently regenerated via simple alternate flushing with hot/cold water due to the excellent thermosensitivity of the introduced PNIPAM moieties into the hydrogel. Such a PCH sensor provides a simple, low-cost and efficient strategy for visualized monitoring of hyperkalemia/hypokalemia, which will significantly promote the development of biosensors.

A cyanine dye probe for K+ detection based on DNA construction of G-quadruplex

Potassium ion (K⁺) plays an important role in the maintenance of cellular biological process for human health. Thus, the detection of K⁺ is very important. Here, based on the interaction between thiamonomethinecyanine dye and G-quadruplex formation sequence (PW17), K⁺ detection spectrum was characterized by UV-Vis spectrometry. The single-stranded sequence of PW17 can fold into G-quadruplex in the presence of K⁺. PW17 can induce a dimer-to-monomer transition of the absorption spectrum of cyanine dyes. This method shows high specificity against some other alkali cations, even at high concentrations of Na⁺. Further, this detection strategy can realize the detection of K⁺ in tap water.

Optical Detection of Heparin in Whole Blood Samples Using Nanosensors Embedded in an Agarose Hydrogel

Point-of-care quantification of the anticoagulant heparin still remains a significant clinical challenge as the reference method (colorimetric anti-factor Xa assay) cannot be performed in whole blood. Our group recently put forth the novel optical nanosensing principle using an ionic solvatochromic dye as a signal transducer. These nanosensors demonstrated significantly improved selectivity and sensitivity compared to ion-exchange-type polyion nanosensors and enabled protamine/heparin quantification in blood plasma samples. However, because the readout is absorbance-based, they are still not suitable for whole blood measurements. To overcome the background absorbance of blood, the nanosensors were here embedded in an agarose hydrogel capable of filtering out red blood cells while allowing plasma components to diffuse into the gel. Calibration curves for both protamine and heparin were successfully obtained in buffer, undiluted plasma, and undiluted whole blood using different colorimetric image analysis methods and a simple experimental setup.

Surface PEGylation of ionophore-based microspheres enables determination of serum sodium and potassium ion concentration under flow cytometry

We present here an ionophore-based ion-selective optode (ISO) platform to detect potassium and sodium concentrations in serum through flow cytometry. The ion-selective microsensors were based on polyethylene glycol (PEG)-modified polystyrene (PS) microspheres (PEG-PS). Ratiometric response curves were observed using peak channel fluorescence intensities for K⁺ (10^-6 M to 0.1 M) and Na⁺ (10^-4 M to 0.2 M) with sufficient selectivity for clinical diagnosis. Due to the matrix effect, proteins such as albumin and immunoglobulin caused an obvious increase in response for serum sample determination. To solve this problem, 4-arm PEG chains were covalently attached onto the surface of PS microspheres through a two-step reaction, which improved the stability and combated pollution of microspheres. As a preliminary application, potassium and sodium concentrations in human serums were successfully determined by the PEG-PS microsensors through flow cytometry.

Recent advances in polymeric nanostructured ion selective membranes for biomedical applications

Nano structured ion-selective membranes (ISMs) are very attractive materials for a wide range of sensing and ion separation applications. The present review focuses on the design principles of various ISMs; nanostructured and ionophore/ion acceptor doped ISMs, and their use in biomedical engineering. Applications of ISMs in the biomedical field have been well-known for more than half a century in potentiometric analysis of biological fluids and pharmaceutical products. However, the emergence of nanotechnology and sophisticated sensing methods assisted in miniaturising ion-selective electrodes to needle-like sensors that can be designed in the form of implantable or wearable devices (smartwatch, tattoo, sweatband, fabric patch) for health monitoring. This article provides a critical review of recent advances in miniaturization, sensing and construction of new devices over last decade (2011-2021). The designing of tunable ISM with biomimetic artificial ion channels offered intensive opportunities and innovative clinical analysis applications, including precise biosensing, controlled drug delivery and early disease diagnosis. This paper will also address the future perspective on potential applications and challenges in the widespread use of ISM for clinical use. Finally, this review details some recommendations and future directions to improve the accuracy and robustness of ISMs for biomedical applications.

Ruthenium bipyridine complexes as electrochemiluminescent transducers for ionophore-based ion-selective detection

We report here a method to determine target ion concentrations (with Na⁺ as a model) based on ionophores and electrochemiluminescence (ECL). Ruthenium bipyridine complexes were released from thin polymeric films (plasticized poly(vinyl chloride) also containing a sodium ionophore) into the sample solution following an explicit ion-exchange process (between Na⁺ and the ruthenium complex). Two signal transducers, tris(2,2'-(pCF₃)bipyridine)ruthenium(II) (Ru(p-CF₃-bpy)₃²⁺) and tris(2,2'-bipyridyl)dichlororuthenium(II) (Ru(bpy)₃²⁺), were examined using the sensing film, with the latter providing a more sensitive detection range (ca. 1 to 100 μM) than that of the more hydrophobic one (0.01 to 1 mM). While the ionophore (Na⁺ ionophore X) offered excellent selectivity to the method, the ruthenium complexes made the measurements independent of the sample pH. Furthermore for complex biological samples such as blood serum, an indirect approach of measuring the ECL of the remaining ruthenium complexes helps avoid background matrix interference to the ECL production at the working electrode, making the ECL method more attractive for real complex samples.

Hydrogel-Based Optical Ion Sensors: Principles and Challenges for Point-of-Care Testing and Environmental Monitoring

Hydrogel is a unique family of biocompatible materials with growing applications in chemical and biological sensors. During the past few decades, various hydrogel-based optical ion sensors have been developed aiming at point-of-care testing and environmental monitoring. In this Perspective, we provide an overview of the research field including topics such as photonic crystals, DNAzyme cross-linked hydrogels, ionophore-based ion sensing hydrogels, and fluoroionophore-based optodes. As the different sensing principles are summarized, each strategy offers its advantages and limitations. In a nutshell, developing optical ion sensing hydrogels is still in the early stage with many opportunities lying ahead, especially with challenges in selectivity, assay time, detection limit, and usability.

Colorimetric ratiometry with ion optodes for spatially resolved concentration analysis

The deprotonation degree of the lipophilic pH indicator dye (chromoionophore) in ionophore-based ion optodes (so-called bulk optodes) has traditionally been measured spectrophotometrically. This makes it difficult to obtain spatially resolved concentration information, for example in the study of heterogenous systems. This article reports on a new colorimetric method that relies on a ratiometric image analysis. The acquision of image data allows one to map the deprotonation degree in two dimensions, which in turn is used to obtain the spatially-resolved ion concentration of the image. Using the detection of potassium as an example, the deprotonation degree data calculated on the basis of image analysis correlate quantitatively with those from spectrophotometry. They showed no dependence on the type of camera used in spite of their different gamma correction values and spectral sensitivities, as expected from theory. As an example, the method is successfully applied to the pixel level analysis of an ensemble of pictures acquired at different times to spatially and temporally observe potassium ion diffusion into an agarose gel containing a potassium-selective optical sensor microemulsion.

Upconverting ion-selective nanoparticles for the imaging of intracellular calcium ions

Upconverting ion-selective nanoparticles that emit light at the near-infrared region are prepared here. The transport of calcium ions induces the deprotonation of the incorporated chromoionophore (P6) through ion exchange resulting in an increase in the emission of UCNPs for the detection of intracellular calcium ions.

Ionophore-based pH independent detection of ions utilizing aggregation-induced effects

Ionophores have been integrated into various electrochemical and optical sensing platforms for the selective detection of ions. Previous ionophore-based optical sensors rely on a H⁺ chromoionophore as the signal transducer and consequently, suffered from a pH cross-response. pH independent methods were proposed very recently by utilizing the solvatochromic dyes or the exhaustive mode. Here, we report a pH independent sensing principle based on nanospheres containing ionophores. As the ion-exchange occurs, the signal transducer undergoes aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ), leading to a dramatic change in fluorescence intensity. The principle was evaluated on different ionophores including those selective for K⁺, Na⁺, Ca²⁺, and Pb²⁺. The nanospheres were also introduced into microfluidic chips and successfully applied for the determination of sodium and potassium ion concentrations in diluted blood serum and urine samples.

Quick stabilization of capillary for rapid determination of potassium ions in the blood of epilepsy patients by capillary electrophoresis without sample pretreatment

Mutations in the potassium channel genes may be linked to the development of epilepsy and affect the blood potassium levels. Therefore, accurate determination of potassium in the blood will be critical to diagnose the cause of epilepsy. CE is a competent technique for the fast detection of multiple ions, but complicated matrices of a blood sample may cause significant variation of migration times and the peak shape. In this work, a procedure for rapid stabilization of the capillary inner surface through preflushing of a blood sample was employed. The process takes only 40 min for a capillary and then it can be used for more than 2 weeks. No pretreatment of the blood sample or other surface modification of the capillary is needed for the analysis. The RSDs of the migration time and peak area were reduced to 1.5 and 5.1% from 12.6 and 14.5%, respectively. The proposed method has been successfully applied to the determination of the potassium contents in the blood sample of patients with epilepsy at different stages. The recoveries of potassium ions in these blood samples are in a range from 86.5 to 104.5%.

Nanodiagnostics to monitor biofilm oxygen metabolism for antibiotic susceptibility testing

A method utilizing oxygen-sensitive nanosensor technology to monitor the oxygen consumption dynamics of living biofilms as they are exposed to antibiotics. This method provides information on the MBIC as well as kinetic response.

Inkjet-printed pH-independent paper-based calcium sensor with fluorescence signal readout relying on a solvatochromic dye

A challenge for paper-based cation sensors relying on classical carrier-based ion-selective optodes (ISOs) is their pH-cross response caused by the use of H⁺-sensitive chromoionophores as optical signal transducers. This work demonstrates fully pH-independent fluorescence-based calcium detection with a paper-based plasticizer-free ISO. To achieve a pH-independent assay, a solvatochromic dye (SD) instead of a traditional H⁺-sensitive chromoionophore has been applied to the paper-based ISO by means of inkjet printing technology. The detection principle depends on an ionophore-driven phase-transfer ion-exchange reaction between target cations and the positively charged SD, which no longer involves H⁺ in the optical signal transduction process. The developed paper-based ISOs with the SD resulted in Ca²⁺ concentration-dependent response curves not affected by the sample pH (pH 6.0, 7.0, and 8.0). The dynamic range obtained for Ca²⁺ detection was from 10^-5 to 1 mol L^-1 with a detection limit of 19.3 μmol L^-1. Additionally, excellent selectivity derived from the used ionophore has been confirmed. As a simple practical application, the determination of Ca²⁺ in mineral water has been achieved without the pH-buffering process required for conventional cation-exchange ISOs.

Red-Emitting Fluorescence Sensors for Metal Cations: The Role of Counteranions and Sensing of SCN- in Biological Materials

The spatiotemporal sensing of specific cationic and anionic species is crucial for understanding the processes occurring in living systems. Herein, we developed new fluorescence sensors derived from tetrapyrazinoporphyrazines (TPyzPzs) with a recognition moiety that consists of an aza-crown and supporting substituents. Their sensitivity and selectivity were compared by fluorescence titration experiments with the properties of known TPyzPzs (with either one aza-crown moiety or two of these moieties in a tweezer arrangement). Method of standard addition was employed for analyte quantification in saliva. For K⁺ recognition, the new derivatives had comparable or larger association constants with larger fluorescence enhancement factors compared to that with one aza-crown. Their fluorescence quantum yields in the ON state were 18× higher than that of TPyzPzs with a tweezer arrangement. Importantly, the sensitivity toward cations was strongly dependent on counteranions and increased as follows: NO₃^- < Br^- < CF₃SO₃^- < ClO₄^- ≪ SCN^-. This trend resembles the chaotropic ability expressed by the Hofmeister series. The high selectivity toward KSCN was explained by synergic association of both K⁺ and SCN^- with TPyzPz sensors. The sensing of SCN^- was further exploited in a proof of concept study to quantify SCN^- levels in the saliva of a smoker and to demonstrate the sensing ability of TPyzPzs under in vitro conditions.

Recent Developments in Nanosensors for Imaging Applications in Biological Systems

Sensors are key tools for monitoring the dynamic changes of biomolecules and biofunctions that encode valuable information that helps us understand underlying biological processes of fundamental importance. Because of their distinctive size-dependent physicochemical properties, materials with nanometer scales have recently emerged as promising candidates for biological sensing applications by offering unique insights into real-time changes of key physiological parameters. This review focuses on recent advances in imaging-based nanosensor developments and applications categorized by their signal transduction mechanisms, namely, fluorescence, plasmonics, MRI, and photoacoustics. We further discuss the synergy created by multimodal nanosensors in which sensor components work based on two or more signal transduction mechanisms.

Distance and Color Change Based Hydrogel Sensor for Visual Quantitative Determination of Buffer Concentrations

We present here an innovative platform for the determination of pH buffer capacity based on FITC-dextran loaded hydrogels. Optical signals from the pH-sensitive hydrogels were analyzed by simple parameters including distance and color change. The methodology was validated on five different buffer systems and exhibited wide linearity (0.1 to 100 mM), good batch-to-batch reproducibility, high versatility, and resistance to background ionic strength changes. Experimental results also fit well with a theoretical model based on numerical simulation. Preliminary application in carbonate alkalinity determination of seawater proved very successful. This hydrogel buffer concentration sensor is fundamentally different from conventional acid-base titrations, brings minimum perturbation to samples, and shows great potential in real applications.

Novel color standards for digital color analysis of optochemical sensor arrays

The indicator-based polymeric color standards for color referencing in digital color analysis (DCA) of optical chemical sensors (optodes) are proposed. In the novel standards, the colors referring to the actual absorption bands of the protonated and deprotonated forms of the indicator are mixed in constant proportions. The standards are based on the lipophilic pH-indicators: ETH5350 and ETH2439, commonly used in optodes, and the lipophilic electrolyte TBATBB. The dependence of the standard color on the TBATBB concentration in the optode is established and found to be linear. The standard colors remain unchanged upon varying the solution pH and the nature and the concentration of the electrolyte. Calibration curves of the indicator pH-optodes obtained in horse serum and referenced to the developed standards demonstrate lower error to span ratio, broader span and higher sensitivity as compared to the same data processed with the conventional gray standard. The colorimetric signal of the pH-optodes array measured in serum sample and referenced to the developed standards allowed accurate determination of the sample pH thus demonstrating practical prospects of the proposed color standards.

A rapid point-of-care optical ion sensing platform based on target-induced dye release from smart hydrogels

We report here a rapid and versatile metal ion analytical platform based on the dye release from hydrogels entrapping ion-selective microdroplets.

Rhodamine dye transfer from hydrogel to nanospheres for the chemical detection of potassium ions

Smart hydrogels incorporating various functional nanomaterials are becoming popular tools for chemical sensing.

Electrochemical aptasensor based on a potassium ion-triggered DNA conformation transition and self-assembly on an electrode

A sensitive and selective electrochemical aptasensor was developed for the detection of potassium ions based on a simple sensing principle and straightforward operation.

Imaging Sodium Flux during Action Potentials in Neurons with Fluorescent Nanosensors and Transparent Microelectrodes

Sodium flux plays a pivotal role in neurobiological processes including initiation of action potentials and regulation of neuronal cell excitability. However, unlike the wide range of fluorescent calcium indicators used extensively for cellular studies, the choice of sodium probes remains limited. We have previously demonstrated optode-based nanosensors (OBNs) for detecting sodium ions with advantageous modular properties such as tunable physiological sensing range, full reversibility, and superb selectivity against key physiological interfering ion potassium. (1) Motivated by bridging the gap between the great interest in sodium imaging of neuronal cell activity as an alternative to patch clamp and limited choices of optical sodium indicators, in this Letter we report the application of nanosensors capable of detecting intracellular sodium flux in isolated rat dorsal root ganglion neurons during electrical stimulation using transparent microelectrodes. Taking advantage of the ratiometric detection scheme offered by this fluorescent modular sensing platform, we performed dual color imaging of the sensor to monitor the intracellular sodium currents underlying trains of action potentials in real time. The combination of nanosensors and microelectrodes for monitoring neuronal sodium dynamics is a novel tool for investigating the regulatory role of sodium ions involved during neural activities.

Graphene Quantum Dots Integrated in Ionophore-Based Fluorescent Nanosensors for Na+ and K

To enrich the recipes of ion-selective nanosensors, graphene quantum dots (GQDs) were integrated into ionophore-based fluorescent nanosensors with exquisite selectivity and high sensitivity for Na⁺ and K⁺. The unique property of GQDs gave the nanosensors ultrasmall size (ca. 10 nm), high brightness, good biocompatibility, and potential pH sensing possibility. At pH 7.4, the sensors exhibited a detection range from 0.1 mM to 1 M for Na⁺ and from 3 μM to 1 mM for K⁺. The nanosensors were successfully applied to blood serum and urine samples. Chemically induced intracellular sodium concentration change in HeLa cells was also qualitatively monitored.

Colorimetric Calcium Probe with Comparison to an Ion-Selective Optode

Design strategies for small molecular probes lay the foundation of numerous synthetic chemosensors. A water-soluble colorimetric calcium molecular probe inspired by the ionophore-based ion-selective optode is presented here with a tunable detection range (around micromolar at pH 7). The binding of Ca²⁺ resulted in the deprotonation of the probe and thus a significant spectral change, mimicking the ion-exchange process in ion-selective optodes. The 1:1 exchange between Ca²⁺ and H⁺ was confirmed with Job's plot. Computational studies revealed possible monomer and dimer forms of the probe-Ca²⁺ complexes.