Nanofiber Ion-Selective Membrane-Coated Carbon Paper All-Solid-State Sensors: One Stone, Two Birds.

Potentiometric sensors with nanostructural ion-selective membranes were prepared and tested. Electrospun nanofiber mats were applied in novel all-solid-state sensors, using carbon paper as an electronically conducting support. For the sake of simplicity, application of a solid contact layer was avoided, and redox-active impurities naturally present in the carbon paper have proven to be effective as ion-to-electron transducers. Application of a nanostructural ion-selective membrane requires an innovative approach to combine the receptor layer with the support. The nanofiber mat portion was fused with carbon paper in a hot-melt process. Applying temperature close to 120 °C for a short time (3 s) allowed binding the nanostructural ion-selective membrane with carbon paper, without significant changes in the nanofiber structure. This process was conveniently performed together with the lamination of the carbon paper support. The thus obtained, potentially disposable sensors were characterized as exhibiting highly reproducible potential readings in time as well as between sensors belonging to the same batch. The benefits of the application of nanostructural ion-selective membranes include shorter equilibration time, lower detection limit, and significantly lower material consumption. However, the nanostructural membrane is characterized by a higher electrical resistance, which is attributed to higher porosity.

Nanofiber-Supported Palladium Nanocubes-Toward Highly Active and Reusable Catalyst.

Electrospun nanofibers were used to support palladium nanocubes, resulting in a highly active, stable, and reusable catalyst. The system proposed herein offers significant advantages compared to catalysts in the form of nanoparticles suspension. The porous, solvent permeable structure of the nanofiber mat ensures uniform and stable time distribution of palladium nanoparticles; preventing coalescence and allowing multiple use of the catalyst. The proposed cross-linked poly(vinyl alcohol) nanofiber mat loaded with Pd nanocubes during the nanofiber preparation step is a macroscopic structure of intrinsically nanostructural character of the catalyst that can be easily transferred between different solutions without compromising its effectiveness in consecutive cycles. Thus, obtained system was characterized with high catalytic activity as tested on a model example of 4-nitrophenol (4-NP) reduction by NaBH4 to 4-aminophenol (4-AP). It is shown that loading nanofibers with Pd nanocubes during electrospinning resulted in a significantly more stable system compared to surface modification of obtained nanofibers with nanocube suspension.

Finding a perfect match of ion-exchanger and plasticizer for ion-selective sensors.

Application of neutral ionophore based ion-selective sensors requires presence of ion-exchanger in the receptor phase, silently assuming that it is not only soluble but also dissociates to ions in the applied plasticizer. Although for typically applied ion-selective membrane constituents (plasticizers - ion-exchanger pairs) dissociation of ion-exchangers to ions is proven by theoretical (or close to) performance of resulting sensors, search for alternative plasticizers or ion-exchangers requires a method allowing estimation of the match of properties of involved compounds. In this work we propose a simple optical approach allowing estimation of ion-exchanger interactions with plasticizer. The results were confirmed by conductivity studies of model plasticizers solutions. The estimated dissociation constants of model ion-exchangers in plasticizers used are in excellent agreement with the results of optical studies. It was shown that solubility coupled with poor dissociation to ions of ion-exchanger affects performance of the resulting ion-selective membrane. Rational choice of properties of ion-exchanger and plasticizer allows finding a perfect match of the two, that results in improvements in performance of sensors (e.g. detection limits). As model sensors potassium and sodium ion-selective electrodes with poly(vinyl chloride) (PVC) based membranes, plasticized with classical plasticizer bis(2-ethylhexyl sebacate) or biodegradable alternative acetyl tributyl citrate, were prepared and studied using selected ion-exchangers.

Ion-selective membrane plasticizer leakage in all-solid-state electrodes - an unobvious way to improve potential reading stability in time.

The effect of leakage of the plasticizer from the ion-selective membrane into the ion-to-electron transducer of all-solid-state potentiometric sensors is considered for the first time. The plasticizer can be transferred to the transducer phase, either during ion-selective membrane application or later; in both cases, its presence can affect the performance of the sensors. Clearly, this effect is most pronounced if the transducer is dispersible in the plasticizer. Towards this end, it is shown that application as the transducer of plasticizer dispersable poly(3-hexylthiophene) compared to typically used (non-dispersible) poly(3-octylthiophene) results in sensors offering higher reproducibility of recorded potentials equal to ±1.4 mV and ±2.5 mV, respectively (within-day test, n = 6). Although poly(3-hexylthiophene) was also found in the membrane in the solvent dispersed, neutral emission active form, the analytical parameters of poly(3-hexylthiophene) based sensors including selectivity were improved or comparable to those of classical poly(3-octylthiophene) transducer sensors.

Bypassed ion-selective electrodes - self-powered polarization for tailoring of sensor performance.

Potentiometric ion-selective sensors are attractive analytical tools as they have simple apparatus and facile use; however their analytical parameters cannot be easily tuned. To tailor the performance of these sensors, application of instrumental control - electrochemical trigger - is usually required. The proposed approach offers a self-powered instrument-free alternative. It benefits from a spontaneous redox process for the ion-selective electrode bypassed by a zinc wire and a resistor connected in series. Spontaneous oxidation of zinc induces charge flow and the accompanying reduction of the solid contact material of the sensor, magnitude of the current and finally the potential of the electrode can be controlled by adjusting the bypass resistance. The ultimate result of the proposed approach is qualitatively equivalent to recording sensor response under polarized electrode potentiometry conditions, however, it does not require application of a galvanostat. The change in the magnitude of the resistance connected can be used to tailor analytical parameters such as detection limit, linear response range, and selectivity of the sensor. As a model example, potassium-selective all-solid-state sensors with a polypyrrole solid contact were used.

3D-Drawn Supports for Ion-Selective Electrodes.

A new concept of easy to make, potentially disposable potentiometric sensors is presented. A thermoprocessable carbon black-loaded, electronically conducting, polylactide polymer composite was used to prepare substrate electrodes of user's defined shape/arrangement applying a 3D pen in a hot melt process. Covering of the carbon black-loaded polylactide 3D-drawn substrate electrode with a PVC-based ion-selective membrane cocktail results in spontaneous formation of a zip-lock structure with a large contact area. Thus, obtained ion-selective electrodes offer sensors of excellent performance, including potential stability expressed by SD of the mean value of potential recorded equal to ±1.0 mV (n = 6) within one day and ±1.5 mV (n = 6) between five days. The approach offers also high device-to-device potential reproducibility: SD of mean value of E0 equal to ±1.5 mV (n = 5).

Dual Sensitivity─Potentiometric and Fluorimetric─Ion-Selective Membranes.

Classical application of ion-selective membranes is limited to either electrochemical or optical experiments. Herein, the proposed ion-selective membrane system can be used in both modes; each of them offering competitive analytical parameters: high selectivity and linear dependence of the signal on logarithm of analyte concentration, high potential stability in potentiometric mode, or applicability for alkaline solutions in optical mode. Incorporation of analyte ions into the membrane results in potentiometric signals, as in a classical system. However, due to the presence of lipophilic positively charged ions, polymer backbones, full saturation of the membrane is prevented even for long contact time with solution. The presence of both positively charged and neutral forms of conducting polymers in the membrane results in high stability of potential readings in time. Optical signal generation is based on polythiophene particulates dispersed within the ion-selective membrane as the optical transducer. An increase of emission is observed with an increase of analyte contents in the sample.

Cubosome Based Ion-Selective Optodes-Toward Tunable Biocompatible Sensors.

We report here on a new generation of optical ion-selective sensors benefiting from cubosomes or hexosomes-nanostructural lipid liquid phase. Cubosome as well as hexosome optodes offer biocompatibility, self-assembly preparation, high stability in solution, and unique, tunable analytical performance. The temperature trigger reversibly changes the lipid nanoparticle internal structure-changing analyte access to the bulk of the probe and ultimately affecting the response pattern. Thus, cubosome or hexosome optodes are highly promising alternatives to conventional polymeric based optical nanoprobes.

Emission Intensity Readout of Ion-Selective Electrodes Operating under an Electrochemical Trigger.

We report for the first time on in situ transduction of electrochemical responses of ion-selective electrodes, operating under non-zero-current conditions, to emission change signals. The proposed novel-type PVC-based membrane comprises a dispersed redox and emission active ion-to-electron transducer. The electrochemical trigger applied induces a redox process of the transducer, inducing ion exchange between the membrane and the solution, resulting also in change of its emission spectrum. It is shown that electrochemical signals recorded for ion-selective electrodes operating under voltammetric/coulometric conditions correlate with emission intensity changes recorded in the same experiments. Moreover, the proposed optical readout offers extended linear response range compared to electrical signals recorded in voltammetric or coulometric mode.

Induced ion-pair formation/ de-aggregation of rhodamine B octadecyl ester for anion optical sensing: Towards ibuprofen selective optical sensors.

A novel approach is explored to result in anion selective nanostructural optodes, that do not require the presence of selective ionophore. The sensing principle proposed is based on interactions of polarity sensitive dye with anions, leading to change of the chromophore group environment, resulting in increase of emission for increase of analyte concentration. To induce interactions of the analyte with the dye precise matching of properties of analyte and receptor is required. It is shown that the careful balancing of composition of nanostructural probes allows fine tuning of linear response range to cover lower concentration range. The model analyte studied was ibuprofen, due to its clinical and environmental relevance, lack of ionophore. As model probes rhodamine B octadecyl ester based nanostructures were prepared and applied. For optimized system turn-on responses were obtained for ibuprofen concentration change from 10-4.3 M to 10-2 M, with no effect of other anti-inflammatory drugs such as naproxen or salicylate.

Insights into Primary Ion Exchange between Ion-Selective Membranes and Solution. From Altering Natural Isotope Ratios to Isotope Dilution Inductively Coupled Plasma Mass Spectrometry Studies.

Although ion-selective electrodes have been routinely used for decades now, there are still gaps in experimental evidence regarding how these sensors operate. This especially applies to the exchange of primary ions occurring for systems already containing analyte ions from the pretreatment step. Herein, for the first time, we present an insight into this process looking at the effect of altered ratios of naturally occurring analyte isotopes and achieving isotopic equilibrium. Benefiting from the same chemical properties of all isotopes of analyte ions and spatial resolution offered by laser ablation and inductively coupled plasma mass spectrometry, obtaining insights into primary ion diffusion in the preconditioned membrane is possible. For systems that have reached isotopic equilibrium in the membrane through ion exchange and between the membrane phase and the sample, quantification of primary ions in the membrane is possible using an isotope dilution approach for a heterogeneous system (membrane-liquid sample). Experimental results obtained for silver-selective membrane show that the primary ion diffusion coefficient in the preconditioned membrane is close to (6 ± 1) × 10-9 cm2/s, being somewhat lower compared to the previously reported values for other cations. Diffusion of ions in the membrane is the rate limiting step in achieving isotopic exchange equilibrium between the ion-selective membrane phase and sample solution. On the contrary to previous reports, quantification of silver present in the membrane clearly shows that contact of the membrane with silver nitrate solution of concentration 10-3 M leads to pronounced accumulation of silver ions in the membrane, reaching almost 150% of ion exchanger amount. The magnitude of this effect increases for higher concentration of the electrolyte in the solution.

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.

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. Keeping the overall macroscopic size of the sensing element the same as for classical plasticized poly(vinyl chloride) membranes, herein the proposed nanofiber based systems resulted in ultrathin (<10 nm) recognition layers with the total area nearly 3 orders of magnitude larger and the surface to volume ratio close to 7.5 × 107. Thus, for the first time close to 2D potentiometric receptors were obtained. Formation of thin and continuous liquid recognition layers on nanofibers was confirmed by XPS studies. The nanofiber based ion-selective mats used in the classical internal-solution arrangement were characterized with analytical parameters - the slope and detection limit well comparable to those for classical plasticized poly(vinyl chloride) based membranes. Despite the novel arrangement of the ion-selective layer and its nanometric thickness, the reproducibility of the recorded potentials, studied for more than 30 days, was high. Using confocal microscopy it was shown that electrolyte transport through porous nanofibers' mat phase is the rate limiting step in conditioning of the receptor layer. The estimated electrolyte diffusion coefficients for the nanofiber phase are close to 10-10 cm2 s-1, and thus are orders of magnitude lower compared to values characterizing ion transport through classical poly(vinyl chloride) based membranes. The truly nanostructural character of nanofiber ion-selective mats is visible in chronoamperometric experiments. It was shown that a core-shell nanofiber mat behaves as an array of nanoelectrodes - individual nanofibers. Thus, the novel nanofiber based architecture of ion-selective mats brings also a new quality to the current based electrochemistry of ion-selective sensors.

Quantifying plasticizer leakage from ion-selective membranes - a nanosponge approach.

The spontaneous process of release of plasticizers from membranes typically used in ion-selective sensors is an effect which limits the lifetime of sensors and comes with a risk of safety hazards. We use a nanosponge approach to look at the magnitude of this problem, quantifying the resulting contents of the plasticizer in solution. This novel method takes advantage of the spontaneous partition of the plasticizer (released and present in solution) into nanoparticles loaded with a solvatochromic dye. As a result, nanoparticles are transformed into capsules. This process is coupled with the turn-on fluorescence intensity change of the dye embedded in nanostructures, proportional to analyte concentration in the ppm range, providing insight into plasticizer contents in the solution. It was found that the spontaneous release of the plasticizer is dependent on its nature as well as the presence of an ionophore and ion-exchanger. For a typical ion-selective membrane composition the leakage effect results in up to 20 ppm of 2-nitrophenyl octyl ether found in solution after 12 h contact. On the other hand, for a less polar plasticizer - bis(2-ethylhexyl) sebacate, although the presence of an ionophore and ion-exchanger also increases the amount of the compound released from the membrane, its concentration in solution does not exceed 2 ppm after 12 h. The conclusions presented herein can be important not only for designing robust sensors but also for end-user safety. The results obtained for ion-selective membranes were equal within the range of experimental errors with those obtained using a liquid chromatography coupled with mass spectrometry (LC MS) approach, confirming the high analytical potential of the nanosponge approach.

Tailoring polythiophene cation-selective optodes for wide pH range sensing.

A challenge in turn-on mode operating optodes application is elimination of pH sensitivity. The polyoctylthiophene based optodes response mechanism is not involving hydrogen ions exchange, thus paves the way for optodes applicable for sensing in wide pH range. We report on nanoptodes that can be used both in alkaline and in acidic pH range inaccessible for classical systems using pH sensitive dyes as transducers. The proposed sensors offer 6 orders of magnitude broad, linear dependence of emission intensity on logarithm of analyte (K+ or Ca2+) concentration, in turn on mode. However, the slopes of calibration plots are to some extent dependent on solution pH and oxygen presence/absence. It is shown that this effect is resulting from changes of solution redox potential maintained mainly by dissolved oxygen. The solution redox potential affects the oxidation state of the polymer and thus amount of the neutral, fluorescent form of polyoctylthiophene, ultimately affecting performance of the sensor especially in acidic pH. Tailoring composition of polyoctylthiophene optodes, including hydrogen binding compound in the polymer phase, effectively diminishes the effect of pH change on sensitivity of proposed optodes, as shown on model examples of potassium and calcium sensors. Thus polyoctylthiophene based nanosensors show equally high sensitivity for analyte cations concentration change both in acidic (pH = 4) and alkaline (pH = 9.2) media.

Self-Powered Cascade Bipolar Electrodes with Fluorimetric Readout.

Bipolar electrodes working in a self-powered mode are a basis for the development of easy to use electrochemical-optical sensors, these systems are very promising due to their simplicity and no need of external polarization. However, the self-powered mode can be used only in cases when the redox potential difference of reactions occurring at opposite poles of the electrode is sufficiently high. To overcome this limitation, we propose the development of a system working spontaneously, but involving two bipolar electrodes, forming a cascade system. One of electrodes ("driving" electrode) works in self-powered mode and triggers charge transfer processes in the second ("sensing") bipolar electrode. For the sensing electrode, an electrochemical process of an analyte occurs at one pole, accompanied by a complementary process at the second pole, inducing an optical (fluorimetric) analytical signal. This concept was successfully tested on a model system of a sensing bipolar electrode with a platinum electrode participating in oxidation of an analyte, l-ascorbic acid, connected with electrode coated by poly(3-octylthiophene), where reduction of the polymer results in formation of fluorimetrically active neutral form. As the driving system, bipolar electrodes with zinc wire as one pole, characterized by a low redox potential, were used.

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. The proposed arrangement offers the possibility of a significant extension of the surface area of the probe while also minimizing the thickness of the receptor layer. This novel, close to 2D, optode configuration results in a sensor free from limitations related to analyte transport in the receptor phase. Unlike other formats, low analyte ion concentrations (10-8-10-5 M) were recorded, which are typically inaccessible for other formats of optodes, with a linear dependence of the emission signal on the logarithm of the analyte concentration. This effect results from a significant exhaustion of the analyte in the sample close to the interface with the sensor. On the other hand, as the ionophore surface concentration in the receptor was close to saturation, for a high concentration of the analyte (>10-5 M) in solution, the optode responses were observed with a sigmoidal dependence of the emission intensity on the logarithm of analyte concentration, independent of the applied ionophore contents in the 2D receptor phase. It was also shown that the response of the nanofiber supported liquid optode layer is reversible for the sigmoidal response range.

Multiwalled Carbon Nanotubes-Poly(3-octylthiophene-2,5-diyl) Nanocomposite Transducer for Ion-Selective Electrodes: Raman Spectroscopy Insight into the Transducer/Membrane Interface.

An approach to overcome drawbacks of well-established transducer materials for all-solid-state ion-selective electrodes is proposed; it is based on the formulation of the nanocomposite of multiwalled carbon nanotubes (MWCNTs) and poly(3-octylthiophene-2,5-diyl) (POT), in which the polymer is used as a dispersing agent for carbon nanotubes. Thus, the obtained material is characterized with unique properties that are important for its application as solid contact in ion-selective electrodes, including high: electronic conductivity, capacitance, and lipophilicity. Performance of the obtained all-solid-state electrodes was studied using a standard approach as well as Raman spectroscopy to allow insight into distribution of the transducer material within the sensor phases: the membrane and the transducer. Application of the composite prevents unwanted partition of POT to the membrane phase, thus eliminating the risk of alteration of the sensor performance due to uncontrolled change in the membrane composition.

Ultrasmall self-assembly poly(N-isopropylacrylamide-butyl acrylate) (polyNIPAM-BA) thermoresponsive nanoparticles.

Reported poly(N-isopropylacrylamide) (poly(NIPAM)) thermoresponsive systems do not preserve their structure and shape regardless applied temperature. Poly(NIPAM) modified with lipophilic units of butylacrylate (BA) is expected to form spontaneously nanospheres stable at a broad range of temperature. Moreover, it should be possible to introduce solvatochromic dyes to the spheres for optical evaluation of the system and designing thermometers at the nanoscale. In this study, poly(NIPAM-BA) polymer used in nanoprecipitation process formed stable nanospheres, as shown by scanning transmission electron microscopy (STEM), dynamic light scattering (DLS), and zeta potential analysis. As a model compound, Nile Red was introduced to the structures allowing fluorometric investigation and confocal imaging. The nanoparticles were stable in solution both below and above polymer transition temperature. However, as expected for thermoresponsive polymer, the diameter of nanospheres changed from about 30 nm at 10 °C to about 150 nm at 20 °C. For dye loaded spheres this process was coupled with pronounced change in emission. For low temperatures nanostructures existed as ultra-small highly lipophilic particulates, whereas at higher temperatures their diameter and hydrophilicity increased. In consequence the dye was extruded from spheres at low temperatures as a shell layer, this process was fully reversible within the temperatures range from 5 to 30 °C. Freezing of the nanospheres resulted in irreversible change in morphology allowing monitoring of transient sample freezing. Forming poly(NIPAM-BA) spheres loaded with solvatochromic dyes were found as a facile technique for designing optical nanothermometer.