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+.

Response Patterns of Chromoionophore-Based Bulk Optodes Containing Lipophilic Electrolytes: Toward Background-Independent pH-Sensing

Despite a number of advantages of ion-selective optical sensors (optodes), their practical application is limited by their response mechanism, which leads to the dependence of the signal on the activity of two ions (analyte ion and the so-called reference ion) in the solution at the same time. Here, we show that the introduction of a lipophilic electrolyte into the polymeric optode membrane allows assessing the ionic activity of H+ cations regardless of the concentration of the background electrolyte containing a hydrophilic cation, with NaCl as an example of such an electrolyte. For the first time, the applicability of this approach is proven theoretically utilizing the numerical simulation of optode response. A correlation between the interfacial potential stability and the single-ion optical response is established. The predicted optical response is independent of background cation concentration to a significant extent. Theoretical conclusions are supported by experimental data obtained with chromoionophore-based optodes doped with various lipophilic electrolytes, including ionic liquids, by thin-film spectrophotometry and macrophotography coupled with digital color analysis. Most of the experimental sensor characteristics, such as the response range and its median, as well as its independence from the background electrolyte concentration are in quantitative agreement with the proposed theoretical description.

Perspective on fluorescence cell imaging with ionophore-based ion-selective nano-optodes

Inorganic ions are ubiquitous in all kinds of cells with highly dynamic spatial and temporal distribution. Taking advantage of different types of fluorescent probes, fluorescence microscopic imaging and quantitative analysis of ion concentrations in cells have rapidly advanced. A family of fluorescent nanoprobes based on ionophores has emerged in recent years with the potential to establish a unique platform for the analysis of common biological ions including Na⁺, K⁺, Ca²⁺, Cl^-, and so on. This article aims at providing a retrospect and outlook of ionophore-based ion-selective nanoprobes and the applications in cell imaging.

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.

Ionophore-Based Potassium Selective Fluorescent Organosilica Nano-Optodes Containing Covalently Attached Solvatochromic Dyes

Fluorescent nanoprobes containing ionophores and solvatochromic dyes (SDs) were previously reported as an alternative to chromoionophore-based nano-optodes. However, the small-molecular SDs are prone to leakage and sequestration in complex samples. Here, we chemically attached the SDs to the surface of organosilica nanospheres through copper-catalyzed Click chemistry to prevent dye leakage. The nano-optodes remained well responsive to K+ even after exposure to a large amount of cation-exchange resin, which acted as a sink of the SDs. The potassium nanoprobes exhibited a dynamic range between 1 μM to 10 mM and a good selectivity thanks to valinomycin. Preliminary sensing device based on a nylon filter paper and agarose hydrogel was demonstrated. The results indicate that the covalent anchoring of SDs on nanospheres is promising for developing ionophore-based nanoprobes.

Ion-Induced Phase Transfer of Cationic Dyes for Fluorescence-Based Electrolyte Sensing in Droplet Microfluidics

Fluorescence-based sensing in droplet microfluidics requires small sample volumes, allows for high-throughput assays, and does not suffer from photobleaching as each flowing sensor is only scanned one time. In this paper, we report a selective and sensitive fluorescence-based ion-sensing methodology in droplet microfluidics using a T-junction PDMS chip. The oil stream is doped with sensor ingredients including an ionophore, a cation exchanger, and a permanently cationic fluorophore as the optical reporter. Electrolyte cations from the aqueous sample are extracted into oil segments and displace the cationic dyes into aqueous droplets. Laser-induced fluorescence of the two immiscible phases is collected alternately, which is in clear contrast to most other ion-selective optode configurations such as nanoparticle suspensions that rely on mixed optical signals of two phases. The cation exchanger, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, is found to dramatically enhance the dye emission in the nonpolar sensing oil by preventing ion-pairing interactions and aggregations of the dye molecules, providing new insights into the mechanism of cationic dye-based ion sensors. The high dye brightness allows us to use low concentrations of sensing chemicals (e.g., 10 μM) in the oil and attain high sensitivity for detection of ions in an equal volume of sample. Using valinomycin as the ionophore and methylene blue as the dye, K⁺ is detected with a response time of ∼11 s, a logarithmic linear range of 10^-5 to 10^-2 M, a 20-fold total fluorescence response, >1000-fold selectivity against other electrolyte cations, and negligible cross-sensitivity toward the sample pH. The K⁺ concentration in untreated and undiluted whole blood and sweat samples is successfully determined by this microfluidic sensing method without optical interference from the droplet sample to the sensing oil. Detection of other ionic analytes can be achieved using the corresponding ionophores.

Plasticizer-Free Thin-Film Sodium-Selective Optodes Inkjet-Printed on Transparent Plastic for Sweat Analysis

A novel strategy to functionalize transparent flexible plastic films with an optical ion-sensing layer using an inkjet-printing technology is described. The hydrophobic sensing chemicals that include a sodium ionophore, a lipophilic proton chromoionophore, and a lipophilic ion-exchanger are co-deposited onto substrates such as transparent polyester film sheets in the absence of any plasticizer and/or hydrophobic polymer matrix. The inkjet-printing process enables the formation of optode films with nanoscale thickness/roughness that readily facilitate interfacing with aqueous samples. Using a smartphone detector, the colorimetric response of the optodes is shown to reach 95% of equilibrium values within 100 s in response to different concentrations of sodium ions, which is more rapid than traditional ion-selective optodes based on plasticized PVC films as the sensing layer. The new optodes also exhibit high selectivity to Na⁺ over interfering ions including K⁺, Ca²⁺, and Mg²⁺. Chemical leaching experiments show that the highly hydrophobic optode components remain in place on the plastic substrate surface. Hence, excellent sensor stability and fully reversible optical responses are obtained, which is essential for potential continuous monitoring applications. Further testing of the sensors with undiluted human sweat samples is shown to yield accurate values for sodium concentrations. Therefore, the use of plasticizer-free ion-selective optode nanolayers that enable highly selective ion sensing on a clear plastic support is likely to expand the range of available chemical sensors suited for preparing wearable real-time sweat analysis devices.

Plasticizer-free and pH-independent ion-selective optode films based on a solvatochromic dye

A layer of a solvatochromic dye, an ionophore, and an ion-exchanger deposited on a Nylon membrane enables highly selective colorimetric and fluorometric ion sensing. This new platform does not suffer from interference from the sample pH and does not require a plasticizer to dissolve the sensing chemicals.

Si-corrole-based fluoride fluorometric turn-on sensor

We present here a new type of fluoride ion optode, constituted by a highly lipophilic PVDF porous membrane modified with a liquid receptor layer containing the emission-active Si corrole F[Formula: see text] selective ionophore. For the optimized composition of the receptor layer, in acidic solutions an increase of Si-corrole emission was observed by increasing fluoride ion concentration, a behavior different from most porphyrinoid-based optical sensors. An observed linear dependence of the Si corrole emission intensity (read at 635 nm) was within the range 10[Formula: see text] to 10[Formula: see text] M of fluoride ions.

A potentiometric sensor based on modified electrospun PVDF nanofibers – towards 2D ion-selective membranes

Core–shell modified nanofiber mats were used as ion-selective membranes for the first time.

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.

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.

A tunable detection range of ion-selective nano-optodes by controlling solvatochromic dye transducer lipophilicity

A range of ionic solvatochromic dye (SD) transducers for use in ion-selective emulsified optical sensors are introduced and characterized.

Electrospun nanofiber supported optodes: scaling down the receptor layer thickness to nanometers – towards 2D optodes

A novel type of optode sensor is proposed using electrospun nanofibers as the supporting inert material.