Paper-based plasticizer-free sodium ion-selective sensor with camera phone as a detector

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.

Microfluidic paper-based analytical device with a preconcentration system for the measurement of anionic surfactants using an optode detector

The paper discusses the development of a low-cost, simple, and sensitive on-site measurement system for anionic surfactants (AS), specifically sodium alkylbenzene sulfonate (LAS), using a microfluidic paper-based analytical device (μPAD) with an optode as a detector. The need arises due to the discharge of AS-containing wastewater into natural environments, posing risks to aquatic organisms. Traditional methods for AS measurement have drawbacks like the use of harmful solvents, time-consuming procedures, and the need for expensive equipment, prompting the exploration of alternative approaches. The μPAD incorporates a sample solution preconcentration system using filter paper modified with chitosan. The optode, a chemical sensor detecting analytes optically, is employed for AS detection. When a large volume of AS is added to a positively charged modified filter paper with chitosan, the AS is adsorbed and concentrated on the filter paper. The concentrated AS is eluted with a small volume of alkaline solution, and the eluted AS is detected by the optode in the μPAD. The μPAD with preconcentration provides improved sensitivity and a broader range of detection compared to the method without preconcentration. In the present μPAD method, linear alkylbenzenesulfonate (LAS) in the concentration range of 0.1 to 10 μmol dm-3 was measured. The developed μPAD offers advantages such as portability, cost-effectiveness, and negligible interference from coexisting substances in environmental water samples. The μPAD method was applied for the determination of LAS in tap water and river water.

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 (5 min vs. 15 and 50 min). 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^-5 M 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.

Digital printing of selective and reversible ion optodes on fabrics: toward smart clothes for epidermal chemical sensing

While wearable chemical sensors often rely on electrochemical techniques, optical chemical sensors coupled with a smartphone or a miniaturized camera represent an attractive approach to the monitoring of sweat composition. In this paper, we modify real sports fabrics such as polyester-spandex fabrics with rational combinations of sensing chemicals including a pH indicator, an ion exchanger, and an ionophore via one-step inkjet printing. Highly selective and fully reversible pH optodes as well as Na⁺- and K⁺-selective optodes are obtained only when the most hydrophobic sensing chemicals are used (e.g., sodium ionophore VIII vs. sodium ionophore VI). These sensors exhibit large color-based responses that can be readily identified by naked eye or analyzed via an iPhone app. Their dynamic ranges well cover the physiological sweat concentrations of the analytes. Compared to most other sensors created on garments, our fabric-based optodes are cost-effective, mass-reproducible by the digital printing technology currently used in the textile industry, and do not significantly compromise the essential properties of fabrics such as flexibility, stretchability, wickability, and breathability.

A colorimetric paper-based optode sensor for highly sensitive and selective determination of thiocyanate in urine sample using cobalt porphyrin derivative

In this work, a highly sensitive colorimetric paper-based optode for the determination of thiocyanate in urine samples was developed for the first time. The cocktail solution of the optode was composed of 5,10,15,20-tetrakis(4-octyloxyphenyl)porphyrin cobalt(II) complex (L), tridodecylmethylammonium chloride (TDMACl), 2-nitrophenyl octyl ether, and polyvinyl chloride as an ionophore, an ion exchanger, a plasticizer, and a polymer, respectively. The paper-based optode responded to thiocyanate by increasing the blue component in the RGB index and a visible change, with the naked-eye, of the optode color from pink to green was observed. From the central composite design, the optimized conditions that yielded the highest sensitivity were 4.70 mmol/kg TDMACl and 13.75 mmol/kg L. The developed optode sensor was highly selective and responded to thiocyanate over other anions, with a working range of 0.001-5 mM and with a coefficient of determination (R²) of 0.9915. The limits of detection using naked-eye and camera were determined to be 50.0 μM and 1.26 μM, respectively. In addition, the LOD and LOQ estimated from the standard deviation of the blank were 0.65 and 1.87 μM, respectively. Furthermore, this sensor was successfully applied to the detection of thiocyanate in urine samples from non-smokers and smokers. The results were in good agreement with the standard ion chromatography (IC) technique. This developed paper-based optode sensor was simple, low-cost, portable, and easy to use as a sensing device without any complicated instrument.

Colorimetric and fluorescent turn-on detection of chloride ions with ionophore and BODIPY: Evaluation with nanospheres and cellulose paper

Here, we present the optical detection of chloride ions with co-extraction based anion-selective optodes containing a BODIPY derivative (BDP-OH) and the ionophore In(OEP)Cl. Spectroscopic studies suggested that BDP-OH and In(OEP)Cl formed an adduct BDP-O-In(OEP), which was converted to BDP-OH and In(OEP)Cl upon increasing sample Cl^- concentration, and induced signal changes in both fluorescence and absorbance modes. The method was evaluated in polystyrene-graft-poly(ethylene oxide) (PS-PEO) nanospheres (ca. 40 nm in diameter) and on cellulose paper. In contrast to Cl^- probes based on fluorescence quenching, the nanospheres exhibited turn-on fluorescence and ratiometric absorbance responses to a tunable Cl^- concentration range (10 μM-1 M). Through fluorescence microscopy, the nanospheres were able to respond to Cl^- concentration changes in HeLa cells. Cellulose paper-based Cl^- optodes with colorimetric response were successfully used to measure Cl^- in artificial sweat, providing a potential analytical tool for clinical diagnosis of cystic fibrosis.

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.

Paper-based cation-selective optode sensor containing benzothiazole calix[4]arene for dual colorimetric Ag+ and Hg2+ detection

A new paper-based analytical device based on bulk ion-selective optodes (ISOs) for dual Ag⁺ and Hg²⁺ detection has been developed. A plasticized PVC hydrophobic phase composed of 25,27-di(benzothiazolyl)-26,28-hydroxycalix[4]arene (CU1) as an ion-selective ionophore, potassium tetrakis(4-chlorophenyl)borate as an ion-exchanger and chromoionophore XIV as a lipophilic pH indicator was entrapped in the pores of cellulose paper. This paper strip showed higher selectivity for Ag⁺ and Hg²⁺ over common alkali, alkaline earth and some transition metal ions with a color change from blue to yellow. With the proposed sensor, Ag⁺ and Hg²⁺ can be measured with the range of 1.92 × 10^-6 to 5.00 × 10^-3 M for Ag⁺ and 5.74 × 10^-7 to 5.00 × 10^-5 M for Hg²⁺ with a limit of detection of 1.92 × 10^-6 M for Ag⁺ and 5.74 × 10^-7 M for Hg²⁺. The proposed sensor was successfully applied to determine the amount of mercury in various water sources and the amount of silver in cleaning product samples containing silver nanoparticles (AgNPs). The results were in good agreement with inductively couple plasma-optical emission spectrometric measurements (ICP-OES).

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.

Hue- and Chromaticity-Based Exploration of Surface Complexation-Induced Tunable Emission from Non-Luminescent Quantum Dots

Herein we report the use of a hue parameter of HSV (Hue, Saturation and Value) color space-in combination with chromaticity color coordinates-for exploring the complexation-induced luminescence color changes, ranging from blue to green to yellow to white, from a non-luminescent Fe-doped ZnS quantum dot (QD). Importantly, the surface complexation reaction helped a presynthesized non-luminescent Fe-doped ZnS QD to glow with different luminescence colors (such as blue, cyan, green, greenish-yellow, yellow) by virtue of the formation of various luminescent inorganic complexes (using different external organic ligands), while the simultaneous blue- and yellow-emitting complex formation on the surface of non-luminescent Fe-doped ZnS QD led to the generation of white light emission, with a hue mean value of 85 and a chromaticity of (0.28,0.33). Furthermore, the surface complexation-assisted incorporation of luminescence properties to a non-luminescent QD not only overcomes their restricted luminescence-based applications such as light-emitting, biological and sensing applications but also bring newer avenues towards unravelling the surface chemistry between QDs and inorganic complexes and the advantage of having an inorganic complex with QD for their aforementioned useful applications.

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.

Reference Electrodes with Ionic Liquid Salt Bridge: When Will These Innovative Novel Reference Electrodes Gain Broad Acceptance?

In this paper, we raise questions that researchers have to ask if they intend to replace a conventional reference electrode with an ionic liquid-based reference electrode and try to answer these questions based on our experiences and literature data. Among these questions, the most important is which ionic liquid should be used. However, beyond the chemical composition of the ionic liquid, to realize all the potential benefits of ionic-liquid based reference electrodes, there are additional, equally important considerations. Through examples we will show the importance of the (i) purity of the ionic liquid and the consequences of deviations from its stoichiometric salt composition, (ii) form of implementation of the ionic liquid-based reference electrode membrane (free-flowing salt bridge, or ionic liquid embedded in a membrane), (iii) membrane/gelling agent material and its composition, and (iv) experimental conditions (steady state or flowing conditions) under which it will be used. Finally, we recommend methods to test the performance criteria of the ionic liquid-based reference electrodes.

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.

Fully inkjet-printed distance-based paper microfluidic devices for colorimetric calcium determination using ion-selective optodes

Although the determination of calcium ions (Ca²⁺) is of high importance to monitor water hardness, currently available devices for on-site analysis suffer from a lack of user-friendliness and sensitivity. This work demonstrates fully inkjet-printed and low-cost microfluidic paper-based analytical devices (μPADs) for the simple naked-eye colorimetric determination of calcium ions (Ca²⁺) in drinking and tap water samples. The quantification of Ca²⁺ relies on visual readout of the length of a colour-changed detection channel modified with ionophore-doped ion-selective optode nanospheres (nano-optodes), eliminating the requirement of a scanner or a camera. All fabrication steps for deposition of assay reagents have been performed by means of a simple desktop thermal inkjet printer, which is expected to contribute to highly batch-to-batch reproducible device preparation. The detectable Ca²⁺ concentrations between 0.05 mmol L^-1 and 5 mmol L^-1 cover the range recommended by the International Organization for Standardization (0.05-2.5 mmol L^-1) and the World Health Organization (WHO) guideline for Ca²⁺ quantification in drinking water (less than 5 mmol L^-1). The lowest concentration of Ca²⁺ detectable by the naked eye was found to be 0.05 mmol L^-1, which is below the value achieved with previously reported paper-based devices. μPAD quantified Ca²⁺ concentrations in tap or drinking waters were within 15% error of the results obtained with a classical complexometric titration. Hence, distance-based μPADs relying on nano-optodes are sensitive and reproducible tools for equipment-free on-site assaying of Ca²⁺ in real samples.

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.

Implementation of a plasticized PVC-based cation-selective optode system into a paper-based analytical device for colorimetric sodium detection

On the example of a colorimetric sodium assay, this work demonstrates the implementation of a classical cation-exchange optode relying on an ionophore-doped plasticized PVC membrane into a paper-based analytical device (PAD). An ion-selective optode (ISO) system has been arranged into a vertically-assembled PAD (vPAD) integrating a pH-buffering function. Capillary force-driven sample liquid transportation through the paper matrix enabled pH-adjustment prior to the optical detection of the analyte cation. Functionalized paper layers with inkjet-deposited ISO membranes were combined with whole device lamination to attain a stable ion-exchange equilibrium required for the theoretical behavior of ISOs. Whole device lamination limited rapid evaporation of sample liquid on vPADs to avoid an increase of target concentration. Sigmoidal response curves between 10^-5 and 1 M of Na⁺ at pH 5.0-7.0 have been confirmed on vPADs, following the theory defined by the cation-exchange equilibrium reaction. Finally, the influence of the cellulosic paper substrate matrix acting as a cation-exchanger on the optode response behavior has been evaluated and compared with conventional plastic film optodes.

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

Blood electrolyte measurements play important roles in clinical diagnostics. Optical ion sensors as simple and elegant as a mercury thermometer are in high demand. We present here an analytical method to quantify potassium ions in undiluted human blood and plasma by measuring the distance or the rate of the color propagation. The sensor was composed of K⁺-selective nanospheres embedded in an agarose hydrogel where mass transport was diffusion controlled. The sensor's color-changing rate and the distance of color propagation depended linearly on the logarithm of K⁺ activity. A theoretical model was established and fully supported the experimental findings. This work lays the foundation of a new family of optical ion sensors for direct determination of common blood electrolytes.

Integrated Distance-Based Origami Paper Analytical Device for One-Step Visualized Analysis

An integrated distance-based origami paper analytical device (ID-oPAD) is developed for simple, user friendly and visual detection of targets of interest. The platform enables complete integration of target recognition, signal amplification, and visual signal output based on aptamer/invertase-functionalized sepharose beads, cascaded enzymatic reactions, and a 3D microfluidic paper-based analytical device with distance-based readout, respectively. The invertase-DNA conjugate is released upon target addition, after which it permeates through the cellulose and flows down into the bottom detection zone, whereas sepharose beads with larger size are excluded and stay in the upper zone. Finally, the released conjugate initiates cascaded enzymatic reactions and translates the target signal into a brown bar chart reading. By simply closing the device, the ID-oPAD enables a sample-in-answer-out assay within 30 min with visual and quantitative readout. Importantly, bound/free probe separation is achieved by taking advantage of the size difference between sepharose beads and cellulose pores, and the downstream enzymatic amplification is realized based on the compatibility of multiple enzymes with corresponding substrates. Overall, with the advantages of low-cost, disposability, simple operation, and visual quantitative readout, the ID-oPAD offers an ideal platform for point-of-care testing, especially in resource-limited areas.

An Ionophore-Based Anion-Selective Optode Printed on Cellulose Paper

A general anion-sensing platform is reported based on a portable and cost-effective ion-selective optode and a smartphone detector equipped with a color analysis app. In contrast to traditional anion-selective optodes using a hydrophobic polymer and/or plasticizer to dissolve hydrophobic sensing elements, the new optode relies on hydrophilic cellulose paper. The anion ionophore and a lipophilic pH indicator are inkjet-printed and adsorbed on paper and form a "dry" hydrophobic sensing layer. Porous cellulose sheets also allow the sensing site to be modified with dried buffer that prevents any sample pH dependence of the observed color change. A highly selective fluoride optode using an Al^III -porphyrin ionophore is examined as an initial example of this new anion sensing platform for measurements of fluoride levels in drinking water samples. Apart from Lewis acid-base recognition, hydrogen bonding recognition is also compatible with this sensing platform.

Thermochromic Ion-Exchange Micelles Containing H+ Chromoionophores

Thermochromic composites constitute a classical subfamily of stimuli responsive materials. We report here the thermochromic effect in Pluronic F-127 (F127) micelles containing hydrophobic ion-exchanger and H⁺ chromoionophores. The highly versatile and reversible thermochromism is attributed to the temperature-induced hydration-dehydration of the peripheral layer of the micelles, which in turn controls the ion-exchange process between the core and the periphery of the micelles. The color typically changes abruptly within 3-5 °C, and the color transition temperature can be tuned within 5-25 °C upon varying the F127 concentrations. This work lays the foundation of a new variety of thermochromic materials involving ion-exchange.

Ionophore-Based Titrimetric Detection of Alkali Metal Ions in Serum

While the titrimetric assay is one of the most precise analytical techniques available, only a limited list of complexometric chelators is available, as many otherwise promising reagents are not water-soluble. Recent work demonstrated successful titrimetry with ion-exchanging polymeric nanospheres containing hydrophobic complexing agents, so-called ionophores, opening an exciting avenue in this field. However, this method was limited to ionophores of very high affinity to the analyte and exhibited a relatively limited titration capacity. To overcome these two limitations, we report here on solvent based titration reagents. This heterogeneous titration principle is based on the dissolution of all hydrophobic recognition components in a solvent such as dichloromethane (CH₂Cl₂) where the ionophores are shown to maintain a high affinity to the target ions. HSV (hue, saturation, value) analysis of the images captured with a digital camera provides a convenient and inexpensive way to determine the end point. This approach is combined with an automated titration setup. The titrations of the alkali metals K⁺, Na⁺, and Li⁺ in aqueous solution are successfully demonstrated. The potassium concentration in human serum without pretreatment was precisely and accurately determined as 4.38 mM ± 0.10 mM (automated titration), which compares favorably with atomic emission spectroscopy (4.47 mM ± 0.20 mM).

New bis(azobenzocrown)s with dodecylmethylmalonyl linkers as ionophores for sodium selective potentiometric sensors

Novel biscrowns 1 and 2 were synthesized from 13-membered azobenzocrown ethers containing bromoalkylenoxy chains in para position relative to the azo group. The synthesized diester molecules are dodecylmethylmalonic acid derivatives differing by the linker length. The synthesized compounds have the potential of being used as sodium ionophores in ion-selective electrodes. They were characterized and used as ionophores in classic and miniature, solid contact (screen-printed and glassy carbon) membrane ion-selective electrodes. Compound 3, a similar monoester derivative of 13-membered azobenzocrown, was synthesized and used in membrane electrodes for comparison. Lipophilicity of new ionophores was determined by TLC. Lipophilicity of bis(azobenzocrown)s was found to be within the range of logPTLC = 12-13. It was observed that the particularly important selectivity coefficients logKNa,K determined for new electrodes, being logKNa,K = -2.5 and -2.6 (SSM, 0.1 M), are better than those of the electrodes featuring seven out of the nine commercially available sodium ionophores. It was concluded that the ionophore 1 creates, in acetone, with sodium iodide, complex of 1:1 stoichiometry (sandwich complex) with stability constant (logK) ca. 3.0.

Coupling dye-integrated polymeric membranes with smartphone detection to classify bacteria

We report the use of a colorimetric plastic-based device to discriminate four pathogenic bacteria: Klebsiella pneumoniae, Proteus vulgaris, Proteus mirabilis and Escherichia coli.