Ionophore-based ion-selective optical nanosensors operating in exhaustive sensing mode

The impact of zwitterionic surfactants on optode-based nanosensors via different fabrication approaches and sensing mechanisms

In this work, we explored the impact of zwitterionic surfactants, sulfobetaine 16 (SB-16) and a PEG-phospholipid conjugate (DSPE-PEG), on nanosensor performance. We fabricated four sensors (for Na+, K+, Al3+, and O2) and examined how these surfactants influenced various aspects, including fabrication methods, sensing mechanisms, and the incorporation of nanomaterials. Our results highlighted SB-16's role in enhancing selectivity in ion-exchange sensors (Na+ and K+) while maintaining sensitivity akin to its PEG counterpart. The liquid-liquid extraction based sensors (Al3+) were unaffected by surfactant choice, while the O2 sensors that operate via collisional quenching exhibited reduced sensitivity with SB-16 when compared to its PEG-based counterpart. Additionally, the SB-16 sensors proved adaptable to different fabrication approaches (SESE - single emulsion solvent evaporation and FNP - flash nanoprecipitation), showcased good cell viability and maintained a functional lifetime of at least five days. Furthermore, via the use of quantum dots, we showed that it is possible to incorporate other nanomaterials into the sensing phase of SB-16 sensors. Future investigations could target enhancing the pH stability of zwitterionic surfactants to further advance their applicability in sensor technologies.

Reversible Hydrophobic Deep Eutectic Solvent-Based Uranyl-Sensing Optode Film in Aqueous Streams: Color Transformation and Reusability

A hydrophobic deep eutectic solvent (HDES)-based optode was designed for the preconcentration and determination of the UO22+ ion in aqueous media using spectroscopic techniques [energy-dispersive X-ray fluorescence (EDXRF) and solid-state absorption]. The optode was developed by incorporation of HDES (tri-n-octyl phosphine oxide and decanoic acid in an equimolar ratio), tri-(2-ethylhexyl) phosphate, and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol into a cellulose triacetate matrix. Characterization studies were carried out using different techniques to understand the roles of HDES as a plasticizer, UO22+ extractant, and Br-PADAP immobilizer. Uptake studies revealed that the optimal pH was 3 and sorption followed the type II adsorption isotherm. Uranium in the U-sorbed optode can be directly analyzed over a large concentration range of 0.021 × 10-3-2.1 × 10-3 Mol L-1 using EDXRF. The optode film exhibited a linear dynamic range of 0.84 × 10-6-84 × 10-6 Mol L-1 for uranium, with a lowest limit of detection of 0.084 × 10-6 Mol L-1 by colorimetric analysis. This optode-based method was employed for seawater analysis for its UO22+ concentration without any matrix separation, and the concentration was found to be 1.30 ± 0.06 × 10-8 Mol L-1. The optode exhibited better selectivity for UO22+ in the presence of various cations including Sr2+ and Cs+ in an aqueous medium. Compared to other prevailing optical sensors, this optode performed better in terms of key factors like pH, equilibration time, reusability, and detection limit.

Light-induced Delivery of Charged Species using Ion-selective Core-Shell Nanoparticles

Controlled release systems have gained considerable attention owing to their potential to deliver molecules, including ions and drugs, in a customized manner. We present a light-induced ion-transfer platform consisting of a dispersion of nanoparticles (NPs, ~300 nm) with the conductive polymer poly(3-octylthiophene-2,5-diyl) (POT) in the core and a potassium (K+)-selective membrane in the shell. Owing to the photoactive nature of POT, POT NPs can be used for a dual purpose: as a host for positively charged species and as an actuator to trigger the subsequent release. POT0 and doped POT+ coexist in the core, allowing K+ encapsulation in the shell. As POT0 is photo-oxidized to POT+, K+ is released to the (aqueous) dispersion phase to preserve the neutrality of the NPs. This process is reversible and can be simultaneously assessed using the native fluorescence of POT0 and via potentiometric measurements. The NP structure and its mechanism of action were thoroughly studied with a series of control experiments and complementary techniques. Understanding the NP and its surrounding interactions will pave the way for other nanostructured systems, facilitating sophisticated applications. The delivery of ionic drugs and interference/pollutant catching for advanced sensing/restoration will be considered in future research.

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+.

Nanoemulsion-based silver ion-selective optode based on colorimetrically silver ion-responsive ionic liquid-based dye

In this study, we describe the fast-responsive nanoemulsion (NE)-based silver ion (Ag⁺)-selective optode based on colorimetrically silver ion-responsive ionic liquid-based dye (ILD). The ILD comprises purely functional sensing molecules, a protonated cationic merocyanine dye (KD-M13-H⁺) and an anionic Ag⁺ ionophore (BDM-SO₃^-), and thus, it can be used for highly sensitive silver ion (Ag⁺) sensing due to the extremely high content of dye in the organic phase (ionic-liquid phase). However, during the Ag⁺ sensing, the cationic merocyanine dye is converted into electrically neutral form by deprotonation of the dye, which leads to the conversion of liquified dye into solid form in the organic phase, which makes the response time slower when ILD is used for poly(vinyl chloride) (PVC) membrane-based ion-selective optode, especially for sensing of high Ag⁺ concentration. To solve this problem, we focused on the use of the nano-emulsification technique. The response time of the ILD-based nanoemulsion (NE) was considerably shorter (1 s) compared to that of the ILD-based PVC membrane (a few minutes) owing to the large surface area and excellent diffusivity of the emulsion. The ILD-based NE contained a very high dye concentration (833 mmol kg^-1) and exhibited approximately 12 times higher sensitivity than that of the plasticizer-based conventional NE. In the cation measurements, the ILD-based NE responded to Ag⁺ via a cation-exchange mechanism and demonstrated a highly selective response to Ag⁺ (log [Formula: see text] = - 3.0). ILD-NE was successfully applied to the detection of spiked Ag⁺ in a tap water sample with recoveries of 98 - 103% with a relative standard deviation (RSD) of less than 5%. In comparison with NE based on non-ionic ionophores without charge, NE based on BDM-SO₃^- responded to lower Ag⁺ concentrations owing to the effect of negative charge on the binding property. The novel ILD-based NE was capable of highly sensitive, rapid, and selective Ag⁺ sensing, providing potential for analytical devices applicable to high-performance on-site analysis.

Response Mechanism of Hyperpolarization-Based Polyion Nanosensors

The last decade has witnessed a rapid development of nano- and microparticle-based optical ion sensors, including ion-selective optodes (ISOs). While the application of nano-ISOs has shown promising performance for sensing inorganic ions, polyion sensing using nanoscale ISOs has encountered significant interference in complex samples such as blood plasma. Recently, we have reported on a new polyion sensing principle that operates through a novel mechanism to overcome this challenge. The new sensing mechanism showed improved characteristics not observed with conventional ion-exchange type sensors, but the precise mechanism of operation remained thus far unclear. This paper aims to clarify how protamine, the arginine-rich target polycation, behaves during optical signal transduction to give dramatically improved selectivity. Based on thermodynamic data, sensor performance and ζ-potential analysis, two discrete phases of protamine extraction are identified. Initially, protamine extracts into the bulk nanosensor phase, a process that is concurrent with the optical signal change. This is then followed by protamine accumulation onto the nanosensor surface, which starts only upon saturation of the optical signal change. The data indicate that the improved selectivity is due to the inability of small ions to form a sufficiently strong interaction with an active sensing ingredient, DNNS^-. Any exchange of one inorganic cation for another therefore remains optically silent, suppressing matrix effects. Moreover, the recognition of protamine is shown to be an exhaustive extraction process, making the response independent of the nature and concentration of the initial small cation in the nanosensor phase.

Sensors in a Flash! Oxygen Nanosensors for Microbial Metabolic Monitoring Synthesized by Flash Nanoprecipitation

Flash nanoprecipitation (FNP) is an efficient and scalable nanoparticle synthesis method that has not previously been applied to nanosensor fabrication. Current nanosensor fabrication methods have traditionally exhibited poor replicability and consistency resulting in high batch-to-batch variability, highlighting the need for a more tunable and efficient method such as FNP. We used FNP to fabricate nanosensors to sense oxygen based on an oxygen-sensitive dye and a reference dye, as a tool for measuring microbial metabolism. We used fluorescence spectroscopy to optimize nanosensor formulations, calibrate the nanosensors for oxygen concentration determination, and measure oxygen concentrations through oxygen-sensitive dye luminescence. FNP provides an effective platform for making sensors capable of responding to oxygen concentration in gas-bubbled solutions as well as in microbial environments. The environments we tested the sensors in arePseudomonas aeruginosa biofilms andSaccharomyces cerevisiae liquid cultures─both settings where oxygen concentration is highly dependent on microbial activity. With FNP now applied to nanosensor fabrication, future nanosensor applications can take advantage of improved product quality through better replicability and consistency while maintaining the original function of the nanosensor.

Recent improvements to the selectivity of extraction-based optical ion sensors

Optical sensors continue to demonstrate tremendous potential across a wide range of applications due to their high versatility and low cost. This feature article will focus on a number of recent advances made in improving the performance of extraction-based optical ion sensors within our group. This includes the progress of anchored solvatochromic transduction to provide pH and sample volume independent optical responses in nanoemulsion-based sensors. A recent breakthough is in polyion sensing in biological fluids that uses a novel indirect transduction mechanism that significantly improves the selectivity of dinonylnaphthalenesulfonate-based protamine sensors and its potential applications beyond polyion sensing. The role of particle stabilizers in relation to the response of emulsified sensors is shown to be important. Current challenges in the field and possible opportunities are also discussed.

Selectivity remains a constant challenge in the development of optical extraction-based sensors. Fortunately, there are several mechanistic and compositional changes with the potential to improve selectivity without developing new ionophores.

Chloride ion-selective dye liquid nanoemulsion: improved sensor performance due to intermolecular interactions between dye and ionophore

Ionophore-based dye liquid nanoemulsion sensors exhibiting rapid response, high selectivity, and high sensitivity to chloride were developed. Intermolecular interactions within emulsion contributed to the background suppression.

Nanoparticle-Based Liquid-Liquid Extraction for the Determination of Metal Ions

Traditional liquid phase extraction techniques that use optically responsive ligands provide benefits that enable cost-efficient and rapid measurements. However, these approaches have limitations in their excessive use of organic solvents and multistep procedures. Here, we developed a simple, nanoscale extraction approach by replacing the macroscopic organic phase with hydrophobic polymeric nanoparticles that are dispersed in an aqueous feed. The concentration of analytes in polymeric nanoparticle suspensions is governed by similar partition principles to liquid-liquid phase extraction techniques. By encasing optically responsive metal ligands inside polymeric nanoparticles, we introduce a one-step metal quantification assay based on traditional two-phase extraction methodologies. As an initial proof of concept, we encapsulated bathophenanthroline (BP) inside the particles to extract then quantify Fe2+ with colorimetry in a dissolved supplement tablet and creek water. These Fe2+ nanosensors are sensitive and selective and report out with fluorescence by adding a fluorophore (DiO) into the particle core. To show that this new rapid extraction assay is not exclusive to measuring Fe2+, we replaced BP with either 8-hydroxyquinoline or bathocuproine to measure Al3+ or Cu+, respectively, in water samples. Utilizing this nanoscale extraction approach will allow users to rapidly quantify metals of interest without the drawbacks of larger-scale phase extraction approaches while also allowing for the expansion of phase extraction methodologies into areas of biological research.

Chemosensory Optode Array Based on Pluronic-Stabilized Microspheres for Differential Sensing

Differential sensing techniques are becoming nowadays an attractive alternative to classical selective recognition methods due to the “fingerprinting” possibility allowing identifying various analytes without the need to fabricate highly selective binding recognition sites. This work shows for the first time that surfactant-based ion-sensitive microspheres as optodes in the microscale can be designed as cross-sensitive materials; thus, they are perfect candidates as sensing elements for differential sensing. Four types of the newly developed chemosensory microspheres—anion- and cation-selective, sensitive toward amine- and hydroxyl moiety—exhibited a wide range of linear response (two to five orders of magnitude) in absorbance and/or fluorescence mode, great time stability (at least 2 months), as well as good fabrication repeatability. The array of four types of chemosensitive microspheres was capable of perfect pattern-based identification of eight neurotransmitters: dopamine, epinephrine, norepinephrine, γ-aminobutyric acid (GABA), acetylcholine, histamine, taurine, and phenylethylamine. Moreover, it allowed the quantification of neurotransmitters, also in mixtures. Its selectivity toward neurotransmitters was studied using α- and β-amino acids (Ala, Asp, Pro, Tyr, taurine) in simulated blood plasma solution. It was revealed that the chemosensory optode set could recognize subtle differences in the chemical structure based on the differential interaction of microspheres with various moieties present in the molecule. The presented method is simple, versatile, and convenient, and it could be adopted to various quantitative and qualitative analytical tasks due to the simple adjusting of microspheres components and measurement conditions.

Protamine/heparin optical nanosensors based on solvatochromism

Optical nanosensors for the detection of polyions, including protamine and heparin, have to date relied upon ion-exchange reactions involving an analyte and an optical transducer. Unfortunately, due to the limited selectivity of the available ionophores for polyions, this mechanism has suffered from severe interference in complex sample matrices. To date no optical polyion nanosensors have demonstrated acceptable performance in serum, plasma or blood. Herein we describe a new type of nanosensor based on our discovery of a “hyper-polarizing lipophilic phase” in which dinonylnaphthalenesulfonate (DNNS⁻) polarizes a solvatochromic dye much more than even an aqueous environment. We have found that the apparent polarity of the organic phase is only modulated when DNNS⁻ binds to large polyions such as protamine, unlike singly charged ions that lack the cooperative binding required to cause a significant shift in the distribution of the polarizing DNNS⁻ ions. Our new sensing mechanism allows solvatochromic signal transduction without the transducer undergoing ion exchange. The result is significantly improved sensitivity and selectivity, enabling for the first time the quantification of protamine and heparin in human plasma using optical nanosensors that correlates with the current gold standard analysis method, the anti-Xa factor assay.

Novel optical nanosensors for the selective detection of the polycationic protamine based on solvatochromic signal change allow one to detect heparin in plasma.

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.

Paper-Based Optode Devices (PODs) for Selective Quantification of Potassium in Biological Fluids

This paper describes the design, fabrication, and feasibility of paper-based optode devices (PODs) for sensing potassium selectively in biological fluids. PODs operate in exhaustive mode and integrate with a handheld, smartphone-connected optical reader. This integrated measuring system provides significant advantages over traditional optode membranes and other paper-based designs, by obtaining a linear optical response to potassium concentration via a simple, stackable design and by harnessing a smartphone to provide an easy-to-use interface, thus enabling remote monitoring of diseases.

Enzyme-responsive fluorescent nanoemulsion based on lipophilic dye liquid

An enzyme-responsive fluorescent nanoemulsion (NE) based on lipophilic dye liquid (LDL) was developed for alkaline phosphatase (ALP). The response mechanism of the NE involved enzymatic reactions and simultaneous extraction of anions. The LDL-based NE exhibited 3.8 times higher sensitivity than plasticizer-based conventional NE. Detection limit and response range were 2.7 (U L-1) and 5-50 (U L-1), respectively. The response time was reduced to less than half that of the LDL-based membrane.

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.

Poly(acrylic acid) Hydrogel Microspheres for a Metal-Ion Sensor

Monodispersed cross-linked poly(acrylic acid) (PAA) droplets (PAA X-droplets), prepared using the microfluidic method with in situ ultraviolet curing, were used as small spherical sensors to simultaneously detect both Ca²⁺ and Mg²⁺ in human saliva and serum. The PAA X-droplet treated with KOH (PAA_KOH X-droplet) was used as a reference droplet because of its highly swollen state. The PAA_KOH X-droplets shrunk in response to the presence of divalent metal ions (Ms) by forming a bridged structure of COO-M-OOC. The sizes of the PAA_KOH X-droplets were precisely and dynamically monitored in the poly(dimethylsiloxane) (PDMS) channel with passing time when the aqueous metal-ion solutions were flowing at a controlled flow rate. The sizes of the PAA_KOH X-droplets continuously decreased to the saturated constant size. The saturated size of the PAA_KOH X-droplet did not change; however, the speed of size reduction increased with an increase in the concentration of the divalent metal ion. The saturated size was studied using the saturated diameter ratio (R_sat-dia) with respect to the initial diameter of the PAA_KOH X-droplet before the metal-ion treatment, and the speed of the size reduction was investigated using the inverse time to reach half the saturated diameter reduction (T_1/2^-1). Ca²⁺ and Mg²⁺ exhibited R_sat-dia values of 75.9 and 83.6%, respectively, when the flow rate was 5 μL min^-1, regardless of the metal concentration. The T_1/2^-1s for the Ca²⁺ and Mg²⁺ linearly increased with an increase in their concentrations. The R_sat-dia of the aqueous Ca²⁺/Mg²⁺ mixture solution had a linear relationship with φ [= C_Ca/(C_Ca + C_Mg), where C_Ca and C_Mg are the molar concentrations of Ca²⁺ and Mg²⁺, respectively]. The T_1/2^-1 of the aqueous Ca²⁺, Mg²⁺ mixture solution was calculated by adding the individual T_1/2^-1s of pure aqueous Ca²⁺ and Mg²⁺ solutions. Using the R_sat-dia and T_1/2^-1 of the Ca²⁺/Mg²⁺ mixture aqueous solution, the individual C_Ca and C_Mg in the mixture solution were successfully calculated. This method was applied to the human saliva and serum in which the major metal ions are Ca²⁺ and Mg²⁺, and other metal ions existed in undetectable amounts by the PAA_KOH X-droplets. This method is simple, cost-effective, and highly accurate and solves the hurdles of separating the interference effect of a Mg²⁺ ion when a Ca²⁺ ion is measured in biofluids.

In Situ Measuring Partition Coefficient at Intact Nanoemulsions: A New Application of Single-Entity Electrochemistry

We report a new application of the single-entity electrochemistry (SEE) to in situ measure a partition coefficient at intact nanoemulsions (NEs). The partition coefficient at intact NEs is the most crucial physicochemical property to determine the uptake of delivery molecules inside NEs. It, however, has not been unequivocally elucidated by currently existing techniques based on ex situ measurements. Herein, we apply the single-entity electrochemistry (SEE) to directly and quantitatively measure the partition coefficient at NEs in situ. In this work, we use NEs featured with amphiphilic triblock copolymer (Pluronic F-127) as a model system to extract/preconcentrate 2-aminobiphenyl (2-ABP) dissolved in the water and demonstrate a new application of SEE to in situ quantitatively estimate the amounts of 2-ABP distributed into each intact NE. Our SEE measurements reveal that the partitioning is governed by extraction of 2-ABP inside NEs rather than its adsorption on the NE surface, and this extraction is remarkably efficient with up to ∼8 orders of magnitude of the preconcentration factor, thus leading to the unprecedentedly large partition coefficient of 1.9 (±1.4) × 10¹⁰. This result implies that not only the thermodynamic distribution but also the intermolecular interaction of extracted compounds inside NEs could play a significant role in the apparent partition coefficient (P = 1.9 (±1.4) × 10¹⁰). The experimentally determined partition coefficient was validated by molecular dynamics (MD) simulations with showing a stabilizing role of intermolecular interaction in the partitioned system. We further verified our methodology with other compounds exhibiting aromatic properties, e.g., ferrocenemethanol. Significantly, our new approach can be readily applicable to investigate practical NEs commercially marketed for drug, food, and cosmetics.

A lipophilic ionic liquid-based dye for anion optodes: importance of dye lipophilicity and application to heparin measurement

Herein, a fully lipophilic ionic liquid (IL) comprising a lipophilic fluorescein anion and a trihexyltetradecylphosphonium cation was synthesized and used as the plasticizer for a plasticized poly(vinyl chloride) (PVC) membrane optode. Systematic investigation of the alkyl chain length of the fluorescein anion proved the significance of lipophilicity for obtaining the reversible absorbance measurements. A PVC membrane fabricated with the synthesized lipophilic IL was observed to comprise an unusually high dye concentration (915 mmol kg^-1) and exhibited good sensitivity as well as response time in its sensor performance. The sensitivity of the presented PVC membrane was 26-fold higher than that of a conventional optode membrane with the same membrane thickness and the same lipophilic dye of typical dye content (1 wt%). The response time was observed to be >120-fold faster by using a significantly thinner PVC membrane (approx. 140 nm). Heparin is known to be a polyanionic anticoagulant, and the presented PVC membrane exhibited an extremely fast response (20-150 seconds) to the heparin in diluted serum within the required concentration region. Thus, the lipophilic IL-based dye could significantly improve the sensor performance in conventional optodes, especially for an analyte showing slow diffusion, such as macromolecular heparin.

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.

From Molecular and Emulsified Ion Sensors to Membrane Electrodes: Molecular and Mechanistic Sensor Design

Selective molecular ion probes are often insoluble in water and require a hydrophobic solvent environment for strong and selective binding, which runs counter to the desire of utilizing them in a homogeneous solution. This Account aims to guide the reader on how such molecules, often coined ionophores, can be harnessed to design exceptionally useful optical and electrochemical sensors. We start here with some historical context on the design of such ionophores and continue with the explanation of the response mechanism of optical and potentiometric sensors and the role of combined components to build a robust ion sensor. This Account is addressed to nonspecialist readers and for this reason avoids extensive use of equations or theoretical considerations. The interested reader should turn to the original literature for further reading. Emulsified optical sensors are introduced as an initial example. Here, multiple reagents are confined in an attoliter sensing nanodroplet of the organic phase, immiscible with the aqueous sample phase. In this case, the ionophore molecules may retain their high affinity and selectivity to the target ion and the aqueous sample phase does not have to be modified. Emulsified optical sensors allow one to achieve the selective chemical sensing of ions, even with optically silent ionophores. Such ionophore-based nanodroplets are also discussed as a useful novel class of complexometric titration reagents and optical end point indicators with unique selectivities. We then turn our attention to potentiometric sensing probes and briefly discuss the unique opportunity of a direct characterization of ion-ionophore complexation properties offered by membrane electrodes. A carbonate-selective membrane electrode containing a highly selective tweezer-type ionophore with trifluoroacetophenone functional groups is then used as an example for the construction of a robust all-solid-state sensor. This potentiometric probe, in combination with a pH electrode, can directly measure PCO₂ in freshwater lakes, demonstrating a dramatically improved response time relative to traditional sensors equipped with a gas-permeable membrane. In recent years, new sensing modes and electrode designs have been introduced to expand the application scope of ionophore-based potentiometric sensors. Membrane electrodes containing ionophores are placed under dynamic electrochemistry control to give important progress in the field. We specifically highlight our recent works by membranes that are controlled by chronopotentiometry (controlled current) for speciation analysis, by ion transfer voltammetry on thin sensing films for multianalyte detection, by exhaustive coulometry for potentially calibration-free sensors and with coulometric membrane pumps for the selective delivery of reagents.

Equipment-Free Detection of K+ on Microfluidic Paper-Based Analytical Devices Based on Exhaustive Replacement with Ionic Dye in Ion-selective Capillary Sensors

A distance-based analysis of potassium ion (K⁺) is introduced that is performed on a microfluidic paper-based analytical device (μPAD) coupled to an ion-selective capillary sensor. The concept is based on two sequential steps, the selective replacement of analyte ion with an ionic dye, and the detection of this dye in a distance-based readout on paper. To achieve the first step, the capillary sensor holds a poly(vinyl chloride) (PVC) membrane film layer plasticized by dioctyl sebacate (DOS) that contains the potassium ionophore valinomycin, a lipophilic cation-exchanger and the ionic indicator Thioflavin T (ThT) on its inner wall. Upon introduction of the sample, K⁺ in the aqueous sample solution is quantitatively extracted into the film membrane and replaced with ThT. To convert the ion exchange signal into a distance-based analysis, this solution was dropped onto the inlet area of a μPAD to flow the ThT along a channel defined by wax printing, resulting in the electrostatic binding of ThT to the cellulose carboxylic groups. The initial amount of K⁺ determines the amount of ThT in the aqueous solution after ion-exchange, and consequently the distance of ThT-colored area reflects the sample K⁺ concentration. The ion exchange reaction was operated in a so-called "exhaustive sensing mode" and gave a distinct response in a narrow range of K⁺ concentration (1-6 mM) that cannot be achieved by the classical optode sensing mode. The absence of hydrogen ions from the equilibrium competition of the capillary sensor contributed to a complete pH-independence, unlike conventional optodes that contain a pH sensitive indicator. A very high selectivity for K⁺ over Na⁺ and Ca²⁺ has been confirmed in separate solutions and mixed solutions tests. K⁺ measurements in pooled serum samples at concentrations between 2 and 6 mM are successfully demonstrated on a temperature controlled support.

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.

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.

A method for estimating intracellular ion concentration using optical nanosensors and ratiometric imaging

Optical nanoparticle (NP)-based sensors have been widely implemented as tools for detection of targeted ions and biomolecules. The NP sensing platform offer a modular design that can incorporate different sensing components for greater target specificity and the ability to tune the dynamic range, as well as encapsulation of multiple dyes to generate a ratiometric signal with varying spectra. Despite these advantages, demonstrating quantitative ion imaging for intracellular measurement still possess a major challenge. Here, we describe fundamentals that enable intracellular validation of this approach using ion-selective nanosensors for investigating calcium (Ca²⁺) as a model ion. While conventional indicators can improve individual aspects of indicator performance such as Kd, wavelength, and ratiometric measurements, the use of NP sensors can achieve combined benefits of addressing these issues simultaneously. The nanosensor incorporates highly calcium-selective ionophores and two fluorescence indicators that act as signal transducers to facilitate quantitative ratiometric imaging. For intracellular Ca²⁺ application, the sensors are fine-tuned to physiological sensing range, and live-cell imaging and quantification are demonstrated in HeLa cells loaded with nanosensors and their responsiveness to carbachol-evoked store release (~400 nM). The current nanosensor design thus provides a promising sensing platform for real-time detection and optical determination of intracellular ions.

Optimizing calcium selective fluorimetric nanospheres

Recently it was shown that optical nanosensors based on alternating polymers e.g. poly(maleic anhydride-alt-1-octadecene) were characterized by a linear dependence of emission intensity on logarithm of concentration over a few of orders of magnitude range. In this work we focus on the material used to prepare calcium selective nanosensors. It is shown that alternating polymer nanosensors offer competitive performance in the absence of calcium ionophore, due to interaction of the nanospheres building blocks with analyte ions. The emission increase corresponds to increase of calcium ions contents in the sample within the range from 10(-4) to 10(-1) M. Further improvement in sensitivity (from 10(-6) to 10(-1) M) and selectivity can be achieved by incorporating calcium ionophore in the nanospheres. The optimal results were obtained for core-shell nanospheres, where the core was prepared from poly(styrene-co-maleic anhydride) and the outer layer from poly(maleic anhydride-alt-1-octadecene). Thus obtained chemosensors were showing linear dependence of emission on logarithm of calcium ions concentration within the range from 10(-7) to 10(-1) M.