Ion-selective Optodes Research Articles

Exploiting the pH-Cross Sensitivity of Ion-Selective Optodes to Broaden Their Response Range.

While the pH cross-sensitivity of chromoionophore-based ion-selective optodes (ISOs) has often been regarded as a significant limitation, this paper demonstrates how this apparent drawback can be transformed into a beneficial feature. The response range of chromoionophore-based ISOs shifts proportionally with changes in the sample pH. Thus, integrating them with a stable pH gradient across the optode surface, such as those provided by immobilized pH gradient (IPG) gels, allows for significant enhancement of the effective measuring range of chromoionophore-based ISOs while preserving their maximum sensitivity. We show that the measuring range of sodium-selective chromoionophore-based optodes can be increased up to 2.5 log units when used with commercially available IPG gels. This improvement in measuring range is directly correlated with the pH difference in the pH gradient across the optode, suggesting that even greater enhancements are possible with more substantial pH gradients. Furthermore, this approach is not confined to sodium-selective optodes but can be readily adapted to other ion-selective chromoionophore-based optodes, broadening their potential applications and impact in the field of chemical sensing. This work paves the way for the development of more versatile and highly sensitive optodes across a broad range of analytes, leveraging the pH cross-sensitivity as a tool for enhanced performance.

Innovations in ion-selective optodes: a comprehensive exploration of modern designs and nanomaterial integration.

In recent years, ion-selective optodes (ISOs) have remarkably progressed, driven by innovative modern designs and nanomaterial integration. This review explored the development of modern ISO by describing state-of-the-art strategies to improve their sensitivity, selectivity, and real-time monitoring capacity. The review reported the traditional membrane based-optodes, and investigated the latest research, current design principles, and the use of essential components, such as ionophores, indicator dyes, polymer membranes, and nanomaterials, in ISO fabrication. Special attention was given to nanomaterials (e.g., quantum dots, polymer dots, nanospheres, nanorods and nanocapsules) and particularly on how rare earth elements can further enhance their potential. It also described innovative ISO designs, including wearable optodes, smartphone-based optodes, and disposable paper-based optodes. As the pursuit of highly sensitive, selective, and adaptable ion sensing devices continues, this summary of the current knowledge sets the stage for upcoming innovations and applications in different domains (pharmaceutical formulations, medical diagnosis, environmental monitoring, and industrial applications).

Conducting polymer functionalization in search of advanced materials in ionometry: ion-selective electrodes and optodes.

Functionalized conducting polymers (FCPs) have recently garnered attention as ion-selective sensor materials, surpassing their intrinsic counterparts due to synergistic effects that lead to enhanced electrochemical and analytical parameters. Following a brief introduction of the fundamental concepts, this article provides a comprehensive review of the recent developments in the application of FCPs in ion-selective electrodes (ISEs) and ion-selective optodes (ISOs), particularly as ion-to-electron transducers, optical transducers, and ion-selective membranes. Utilizing FCPs in these devices offers a promising avenue for detecting and measuring ions in various applications, regardless of the sample nature and composition. Research has focused on functionalizing different conducting polymers, such as polyaniline and polypyrrole, through strategies such as doping and derivatization to alter their hydrophobicity, conductance, redox capacitance, surface area, pH sensitivity, gas and light sensitivity, etc. These modifications aim to enhance performance outcomes, including potential stability/emission signal stability, reproducibility and low detection limits. The advancements have led to the transition of ISEs from conventional zero-current potentiometric ion sensing to innovative current-triggered sensing approaches, enabling calibration-free applications and emerging concepts such as opto-electro dual sensing systems. The intrinsic pH cross-response and instability of the optical signal of ISOs have been overcome through the novel optical signal transduction mechanisms facilitated by FCPs. In this review, the characteristics of materials, functionalization approaches, particular implementation strategies, specific performance outcomes and challenges faced are discussed. Consolidating dispersed information in the field, the in-depth analysis presented here is poised to drive further innovations by broadening the scope of ion-selective sensors in real-world scenarios.

Plasticizer-based and polymer-free ion-selective optodes on cellulose paper

Paper-based ion-selective optodes (ISOs) allow for low-cost ion measurements using widely accessible optical detectors such as smartphones, representing an attractive analytical platform for medical testing and environmental monitoring. Previously reported paper-based ISOs rely on plasticized PVC membranes or PVC/plasticizer-free adsorption layers containing hydrophobic sensing chemicals. In this paper, we studied a new format of paper-based ISOs, in which a plasticizer phase containing sensing chemicals is deposited onto cellulose paper without using a hydrophobic polymer like PVC. Compared to ISOs without plasticizers and polymers, the plasticizer-based ISOs show much higher pH sensitivity when using chromoionophore I and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as the sensing chemicals. When the sensing layer further contains a solid ionophore with high molecular rigidity, the plasticizer-based ISOs for cations also exhibit enhanced sensitivity than the plasticizer-free ISOs. However, when the ionophore is a liquid at room temperature or has a long alkyl chain, the plasticizer-based and plasticizer-free ISOs have comparable responses because the ionophore behaves as a pseudo-plasticizer. Compared to ISOs with both plasticizers and polymers, the elimination of hydrophobic polymers like PVC makes inkjet printing more efficient and preserves the capillary action of the porous paper. By using the newly launched Samba cartridge with smaller nozzles, the plasticizer-based and polymer-free optodes for sodium, potassium, and calcium ions are prepared by inkjet printing for the first time. In an example application, the inkjet-printed plasticizer-based and polymer-free ISOs are used for colorimetric measurements of urine calcium.

Advancing 2D imaging of ammonium for biological application: Planar and nanoparticle-based ion-selective optodes

This study investigates the development and optimization of ammonium-selective chromoionophore-based optodes for two-dimensional (2D) imaging with conventional cameras. Ammonium, vital in various fields like agriculture, ecology, and health, requires accurate measurement. Traditional methods have limitations, leading to the exploration of optical sensors, or optodes, for direct imaging of ammonium gradients. The research covers planar and nanoparticle designs, exploring absorbance and fluorescence readouts with commercial cameras. The study extends to repeatability, response time, and selectivity of the optodes. Planar optodes, utilizing chromoionophores under different background colors, demonstrate improved characteristics with the addition of TiO2. The impact of plasticizer choice on response characteristics, including hysteresis, is explored. Nanoparticle designs using PVC and F127 are compared, with autoclaving identified as the superior sterilization method. The work effectively demonstrates 2D real-time NH4+ visualization using optode nanoparticles in agar gels. Finally, a proof-of-concept was performed to showcase a biological application for identifying urease-producing pathogens (Bacillus subtilis) without relying on pH changes. Overall, this study provides a comprehensive exploration of ammonium-selective optodes with promising applications in environmental monitoring and biological studies.

Optical fiber chemical sensing of anionic surfactants using a microfluidic polymer chip with embedded ion-selective fluorescence optode.

This study introduces a novel microfluidic polymer chip system that employs an embedded anionic surfactant (AS) ion-selective fluorescence optode (AS fluorescence optode) as a detector for measuring AS. The AS fluorescent optode comprises a lactone form of rhodamine B (L-RB) embedded in 2-nitrophenyl octyl ether plasticized poly (vinyl chloride) membrane. The AS fluorescence optode demonstrated a linear correlation between fluorescence intensity peak heights and AS concentrations within the range of less than 20µM under optimal flow conditions. The limit of detection for AS was approximately 0.06µM. The microfluidic system was utilized to measure AS levels in environmental samples, such as river water and tap water.

Effect of spontaneous degradation of ion-selective optodes receptor materials on sensors performance. Where is the catch?

Interest in application of biodegradable polymers as matrix for ion-selective sensors, including optodes is likely to increase. However, finding an alternative for plasticized poly(vinyl chloride) or polyacrylates, requires among others consideration if spontaneous changes in these materials affect the analytical performance of sensors. In this work we study ion-selective optodes prepared using biodegradable matrix: p-polycaprolactone and acetyl tributyl citrate using classical constituents: optically silent ionophore and chromoionophore as optical transducer. The effect of spontaneous changes of sensor constituents on its performance was studied. It was confirmed that hydrolysis of receptor is occurring leading to change of the matrix composition. Surprisingly it was found that the key factor affecting performance of sensor is spontaneous transformation of the dye embedded within it, typically considered as stable within the sensor matrix. N-octadecanoyl-Nile blue, i.e. chromoionophore I, undergoes acidic hydrolysis leading to formation of Nile blue and Nile red, and ultimately resulting in decreased sensor performance. The hydrolysis of chromoionophore in plasticized polycaprolactone matrix is significantly affected by incorporation of carbon dioxide to the solution containing nanoprobes. As model sensors hydrogen or potassium ions selective optodes were tested.

(Invited) Calibration-Free Ion-Selective Electrodes and Optodes for Decentralized Electrolyte Monitoring

Ionophore-based ion-selective sensors have been used in hospital analyzers for decades to measure electrolytes. Recently, there has been growing interest in affordable and accessible electrolyte measurements via at-home sensors, wearable sensors, transdermal sensors, and implantable sensors. However, the commercial success of ion-selective sensors in these new sensing modalities is hindered by the need for technically demanding on-site calibration. In fact, all ionophore-based sensors in clinical analyzers need to be calibrated with standard solutions at the point of use. We are developing calibration-free ionophore-based sensors, including ion-selective electrodes and optodes, to enable decentralized electrolyte measurements for the management of chronic heart, renal, and parathyroid diseases.In the first approach, we have designed self-calibrating solid-contact ion-selective electrodes. Due to microheterogeneities and variabilities in multiple phases and interfaces of the indicator and reference electrodes, the absolute electromotive force (EMF) varies among identically fabricated and stored sensors. We create an EMF baseline that corrects for the EMF variabilities by connecting two electrodes using an appropriately formulated ionic conductor. Upon the introduction of a sample, such as a drop of blood, the EMF change relative to this baseline allows for ion measurements with high sensor-to-sensor consistency in the absence of any calibration protocols. Surprisingly, when the cross-sectional area, viscosity, and formulation of the ionic conductor are precisely controlled and the ionic conductor is insulated from the sample, its presence does not compromise the Nernstian response slope of the primary ion in the sample. In other words, this built-in ionic bridge serves for one-point calibration before the sample addition but does not need to be removed for the sample testing, representing a novel self-calibration strategy for electrochemical sensors.In the second approach, we have for the first time functionalized the oil stream of droplet microfluidics with sensing chemicals to convert oil segments to sensors for adjacent aqueous droplets. Ionophores, ion exchangers, and dyes are dissolved in the oil to form highly selective fluorescent ion sensors. Analyte ions from the aqueous droplets are extracted into the oil segments to alter their fluorescence and absorbance. The composition of all oil segments is highly consistent, as they are from the same homogenous solution, avoiding the need for any on-site calibration. As the optical property of the oil segment is measured in a fashion that is spatially and temporally separated from the aqueous sample, this biphasic sensing method works for complicated samples such as undiluted whole blood.In the third approach, we have designed a home-use sensor based on a calibration-free oil sensor contained in a capillary tube. Only one segment of sensing oil is used for analysis of one segment of sample of a few microliters. By continuously alternating the relative position of the oil and the sample controlled by a stepper motor, efficient mass transfer between two phases is obtained. The color of the oil segment is acquired and analyzed by a smartphone to indicate the electrolyte concentration in an aqueous sample. The accuracy and precision of an at-home calcium monitor in this format have been validated in > 50 human blood samples.

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 (1s) 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 (833mmolkg-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.

Ionophore based optical sensors using hydrophilic polymer matrix – Ratiometric, pH independent ion-selective optodes

Classical approach for cation selective optode sensing requires high lipophilicity of receptor matrix and optical transducer dye, its application is restricted to relatively narrow pH range. We propose an alternative approach to ion-selective optodes, benefiting from rather hydrophilic polymer - plasticized polycaprolacton. This results in presence of lipophlic (plasticizer) and hydrophilic (sample/polycaprolactone) domains within the nanoprobes proposed. In consequence, this approach allows benefiting from alternative optical transducer: Nile blue dye and its unique properties and in consequence to benefit from new optical transduction mechanism allowing ratiometric sensing with no restriction for the sample pH. The response mechanism involves ionophore and ion pair composed of cation-exchanger and Nile blue cation dissolved in the plasticizer phase. Analyte incorporation to the phase occurs on expense of dye cation release to the hydrophilic phase. Ultimately a red staining of plasticizer phase is observed due to incorporation of neutral dye molecules formed in course of spontaneous hydrolysis occurring in water phase. Thus herein proposed optodes offer observable emission change at two wavelengths (increase of intensity emission as well as decrease) allowing ratiometric sensing in a broad concentration range. Moreover, herein proposed approach is applicable both in acidic and alkaline media, also in the absence of buffer, thus it mitigates significant constrain of classical optodes. As model system potassium or calcium selective nanospheres were prepared.

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.

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+ (10-6M to 0.1M) and Na+ (10-4M to 0.2M) 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.

Fully inkjet-printed paper-based Pb2+ optodes for water analysis without interference from the chloramine disinfectant.

We developed a paper-based colorimetric sensor for facile and cost-effective detection of Pb2+ in drinking and environmental water samples. The Pb2+ ion-selective optodes are fabricated by inkjet printing of ionophore, chromoionophore, and ion exchanger on cellulose paper. Pb2+ in water samples induces deprotonation of the pH chromoionophore and changes the optode color, which is acquired and analyzed by a smartphone. The paper-based optode without any plasticizer or polymer has a dynamic range and selectivity comparable to those of traditional optodes using PVC polymer and/or plasticizer. Furthermore, the response time of the plasticizer/polymer-free paper-based optode is much shorter than those of plasticized PVC-based optodes on paper and glass (5min vs. 15 and 50min). Moreover, the plasticizer/polymer-free optode preserves the water-wicking capability of porous cellulose paper, allowing for the design of pump-free microfluidic devices. Chloramine, a widely used disinfectant in drinking water, was found to be a strong and generic interference species for heavy metal ion detection via ion-selective optodes. A fully inkjet-printed lateral-flow paper-based device consisting of a sodium thiosulfate-based chloramine elimination zone and a plasticizer/polymer-free sensing zone was designed for Pb2+ detection in tap water disinfected by chloramine. The dynamic range of the Pb2+ sensor may be shifted from the current 10-6 to 10-5M to lower concentrations by using stronger ionophores, but this work lays a foundation for the design of paper-based heavy metal ion sensors without detrimental interference from disinfectants.

Fluorescence Anisotropy as a Self-Referencing Readout for Ion-Selective Sensing and Imaging Using Homo-FRET between Chromoionophores.

Fluorescence anisotropy has been widely used in developing biosensors and immunoassays, by virtue of the self-reference and environment-sensitive properties. However, fluorescence anisotropic chemical sensors on inorganic ions are limited by the total anisotropy change. To this end, we demonstrate here fluorescence anisotropic ion-selective optodes based on the homo-FRET (Förster resonance energy transfer) of the crowded chromoionophores. The conventional fluorescence on-off mode is transformed into the anisotropic mode. Variation of the target ion concentration changes the inter-chromoionophore distance in the organic sensing phase, leading to different extents of homo-FRET and steady-state anisotropy. A theoretical model is developed by coupling homo-FRET and anisotropy. Anisotropic detections of pH, K+, and Na+ are demonstrated as examples based on the different ionophores for H+, K+, and Na+, respectively. Further, fluorescence imaging of the nano-optodes, plasticized poly(vinyl chloride) sensing films, and live cells are demonstrated using a homemade fluorescence anisotropic imaging platform. The results form the basis of an ion-selective analytical method operating in the fluorescence anisotropic mode, which could potentially be applied to other fluorescence on-off probes based on homo-FRET.

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.

Ion-Selective Optodes Integrated Paper-Based Device for Simultaneous Colorimetric Quantification of Salivary Na<sup>+</sup>, K<sup>+</sup> and Ca<sup>2+</sup>

The quantification of electrolytes in biological fluids plays an important role for evaluating the human health condition. This work proposes a disposable and simple microfluidic paper-based analytical device (μPAD) for the quantitative colorimetric detection of salivary Na+, K+ and Ca2+, relying on ion-selective optodes (ISOs). Nanoscale ISOs composed of pluronic F-127 surfactant-based micelles were used as a colorimetric indicator system, reducing the assay time compared to the previously reported bulk membrane approach. Devices consist of three functionalized paper layers assembled by lamination, with the optical signal read out from the bottom layer. Saliva was chosen as the biological specimen because it is painlessly collected and easy-to-handle, which makes it suitable for POC assays. Simultaneous Na+, K+ and Ca2+ detection by a single μPAD was successfully confirmed by using aqueous metal ion samples with various concentrations of Na+, K+ and Ca2+ within the biologically relevant range. The simultaneous detection of the three cationic species enabled the correction of the three ion quantities by eliminating the interference from other ions using the calculated selectivity coefficients. For proof-of-concept, Na+ quantification in a human saliva sample was conducted by the developed μPADs. The calibration curve for this assay was prepared by applying artificial saliva to the μPADs. The concentrations of Na+ in spiked human saliva were also measured by the μPADs. The obtained recovery rates were 78-132 %, which indicates the quantification of the added Na+ in the real sample matrix. Finally, this μPAD was successfully integrated into a smartphone-based assay, which enables salivary electrolyte quantification in resource poor settings. The developed μPADs satisfy the criteria of low-cost (< $0.3/device), rapidity (< 5 min) and simple analytical procedure compared to the conventional ion-selective electrode (ISE) method.

Ion-Selective optodes: Alternative approaches for simplified fabrication and signaling

This review highlights recent advances in ion-selective optodes (ISOs), an ionophore-based optical chemical sensor. Progress in the development of new optode components, fabrication on various novel substrate materials, simplified signaling methods are summarized. Paper-based ISOs, hydrogels and other integrated sensing devices have enabled cost-effective sensing by reading from colorimetric response and distance change. Signal transduction based on photoactivation, upconverting, chemiluminescence and delayed luminescence have offered solutions to potential background optical interference. The intrinsic pH cross-response from chromoionophore-based ISOs have been overcome with the exhaustive sensing mode, solvatochromic dyes and other charged molecules. Various applications of ISOs including cellular and in vivo monitoring with nano-optodes are also discussed.

Unintended Changes of Ion-Selective Membranes Composition-Origin and Effect on Analytical Performance.

Ion-selective membranes, as used in potentiometric sensors, are mixtures of a few important constituents in a carefully balanced proportion. The changes of composition of the ion-selective membrane, both qualitative and quantitative, affect the analytical performance of sensors. Different constructions and materials applied to improve sensors result in specific conditions of membrane formation, in consequence, potentially can result in uncontrolled modification of the membrane composition. Clearly, these effects need to be considered, especially if preparation of miniaturized, potentially disposable internal-solution free sensors is considered. Furthermore, membrane composition changes can occur during the normal operation of sensors—accumulation of species as well as release need to be taken into account, regardless of the construction of sensors used. Issues related to spontaneous changes of membrane composition that can occur during sensor construction, pre-treatment and their operation, seem to be underestimated in the subject literature. The aim of this work is to summarize available data related to potentiometric sensors and highlight the effects that can potentially be important also for other sensors using ion-selective membranes, e.g., optodes or voltammetric sensors.

Ion-selective reversing aggregation-caused quenching - Maximizing optodes signal stability

An alternative optical signal transduction mechanism for ion-selective optodes is proposed. The nanostructural sensors benefit from ion-selective reversing aggregation caused quenching yielding turn-on, bright and highly stable optical signals. Selective incorporation of analyte results in transformation of the polymer dye from aggregate to a micelle structure, affecting spatial arrangement of chromophore groups in the nanostructure. Formation of micelles, induced by ion-selective interactions, is coupled with pronounced increase of emission due to decrease of aggregation caused quenching, characteristic for dispersed phase formation. The formed micelles are highly stable in solution, offering constant in time (days scale) emission signal. The important difference from other known systems is that the analyte binding induced change does not affect the chromophore group, but occurs in distant, terminal position of the side chain of the polymer.As a model system calcium selective optodes have been prepared. Thus obtained probes were characterized with broad analyte concentration range (from 10−7 to 10−3 M) emission signal increase. The turn-on response was observed within broad range of pH (6.3–8.9), with no sign of optical signal deterioration during 5 days contact with the analyte or more than two weeks storage.