Ion-selective Optodes Research Articles

Facile Preparation Method of a Disposable Capillary Biosensor Using an Ion-selective Optode Membrane and a Dissolvable Enzyme Membrane and Its Application to Urea Sensing

Abstract A facile preparation method of a disposable capillary biosensor immobilizing a plasticized poly(vinyl chloride) (PVC) membrane ion-selective optode containing a lipophilic pH indicator dye and an ionophore, and a dissolvable poly(ethylene glycol) (PEG) membrane containing an enzyme is developed. The method involves the sequential introduction of membrane cocktail solution followed by air introduction. Repeating this process allowed facile preparation of a capillary biosensor comprised of two layer structure, which can be used for single-step biosensing just by sipping the sample with capillary action. Here a urea biosensor was prepared as a proof-of-concept.

Fluorescent Nano-Optodes for Glucose Detection

We have designed fluorescent nanosensors based on ion-selective optodes capable of detecting small molecules. By localizing the sensor components in a hydrophobic core, these nanosensors are able to monitor dynamic changes in concentration of the model analyte, glucose. The nanosensors demonstrated this response in vitro and also when injected subcutaneously into mice. The response of the nanosensors tracked changes in blood glucose levels in vivo that were comparable to measurements taken using a glucometer. The development of these nanosensors offers an alternative, minimally invasive tool for monitoring glucose levels in such fields as diabetes research. Furthermore, the extension of the ion-selective optode sensor platform to small molecule detection will allow for enhanced monitoring of physiological processes.

Surface Area Effects on the Response Mechanism of Ion Optodes: A Preliminary Study

The relationship between the surface-to-volume ratio and the response mechanism of polymeric ion probes (ion optodes) is not well understood. In this work, the surface-to-volume ratio of ion optodes was systematically increased in an attempt to characterize this relationship. Several different batches of ion-selective optodes were fabricated via the solvent displacement method using sodium ionophore X, BME-44, and ETH 1001 for sodium-, potassium-, and calcium-selective optodes, respectively. Dilution of the membrane cocktail with varying amounts of an organic solvent provided a convenient tool to control the resulting particle size distribution. Specifically, ion optodes of five different size distributions were fabricated. An apparent shift of the response function on the pH scale was observed for optodes with identical composition that differed in terms of size. There was a strong correlation between the calculated specific surface area and the apparent ion-exchange constant for all three types of ion optodes. However, there was an indication that selectivity does not substantially correlate with the optode size. We hypothesize that the observed effect is caused by surface phenomena which contribute to the overall optode response. The results reported here may raise a word of caution about the application of established response models, which were developed for macroscopic ion optodes, toward probes at micrometer and submicrometer scales.

Fabrication of Micrometer and Submicrometer-Sized Ion-Selective Optodes via a Solvent Displacement Process

Microsphere-based ion optodes represent a promising and versatile tool to measure ionic activities in confined samples. The reported methods of micro- and nanosphere optode fabrication, however, suffer from various degrees of complexity. We propose a large-scale fabrication of polymeric ion-selective optodes using a solvent displacement method. Plasticized poly(vinyl chloride) along with the optode components was dissolved in a solvent miscible with water. Injection of a polymer solution into a stirred aqueous phase containing a surfactant causes spontaneous emulsification. This technique does not require additional preparation steps and allows one to control the composition of the sensor matrix precisely. Several factors affecting the particle size distribution are examined such as composition of continuous and disperse phases. The concentration of the polymer in the organic solvent and the choice of the solvent nature allowed us to control the particle size distribution within 200 nm-30 microm. The concentration and the nature of the surfactant had a little influence on the particle size distribution. We fabricated three different batches of ion-selective optodes using chromoionophore I, lipophilic ion-exchanger and sodium ionophore X, BME-44, and ETH 5234 for sodium, potassium, and calcium optodes, respectively. The sensors were fully functional with excellent selectivity to interfering ions.

The Composition Dependence Selectivity Changes by Plasticizer at the Cation Sensors Based on Tetracycline Antibiotics

The performances of tetracycline based cation selective polymeric membrane electrodes of many sets with different plasticizers were investigated as the selectivity of ion-selective electrodes and optodes are greatly influenced by membrane solvent and also controlled by plasticizers. The membrane 1 with Bis(2-ethylhexyl) sebacate (DOS) and additive shows good potentiometric performance toward Ca2+ (slope: 27.8 mV per decade; DL: -4.52) including selectivity. Contrastingly, membrane 4 with Dibutyl phthalate (DBP) shows near-Nernstian response, it has also shown the best measuring range and detection limit for Ca2+ (29.5 mV and -5.10) and Mg2+ (24.4 mV and -5.04) and the least selectivity has been also observed between Ca2+ and Mg2+. When the membrane 1 and 4 were used together to flow system, we could determine the concentration of Ca2+ and Mg2+, simultaneously.

Polymerized Nile Blue derivatives for plasticizer-free fluorescent ion optode microsphere sensors

Lipophilic H +-selective fluorophores such as Nile Blue derivatives are widely used in ISE-based pH sensors and bulk optodes, and are commonly dissolved in a plasticized matrix such as PVC. Unfortunately, leaching of the active sensing ingredients and plasticizer from the matrix dictates the lifetime of the sensors and hampers their applications in vivo, especially with miniaturized particle based sensors. We find that classical copolymerization of Nile Blue derivatives containing an acrylic side group gives rise to multiple reaction products with different spectral and H +-binding properties, making this approach unsuitable for the development of reliable sensor materials. This limitation was overcome by grafting Nile Blue to a self-plasticized poly( n-butyl acrylate) matrix via an urea or amide linkage between the Nile Blue base structure and the polymer. Optode leaching experiments into methanol confirmed the successful covalent attachment of the two chromoionophores to the polymer matrix. Both polymerized Nile Blue derivatives have satisfactory pH response and appropriate optical properties that are suitable for use in ion-selective electrodes and optodes. Plasticizer-free Na +-selective microsphere sensors using the polymerized chromoionophores were fabricated under mild conditions with an in-house sonic microparticle generator for the measurement of sodium activities at physiological pH. The measuring range for sodium was found as 10 −1–10 −4 M and 1–10 −3 M, for Nile Blue derivatives linked via urea and amide functionalities, respectively, at physiological pH. The observed ion-exchange constants of the plasticizer-free microsphere were log K exch = −5.6 and log K exch = −6.5 for the same two systems, respectively. Compared with earlier Na +-selective bulk optodes, the fabricated optical sensing microbeads reported here have agreeable selectivity patterns, reasonably fast response times, and more appropriate measuring ranges for determination of Na + activity at physiological pH in undiluted blood samples.

Absorbance characterization of microsphere-based ion-selective optodes

Ionophore-based microsphere sensors are characterized here in transmission mode. These sensors contain a lipophilic ionophore for the analyte cation, a chromoionophore for recognizing H +, and a lipophilic cation-exchanger. They function on the basis of an ion-exchange equilibration step where an increased concentration of analyte ion leads to increased level of extraction into the bulk of the microsphere, expelling protons in return and deprotonating the chromoionophore. Since the path length is variable across the microsphere, such bead-based sensors are normally characterized in fluorescence mode. In this paper, the response of the sensing microspheres is calculated from the ratio of transmitted light intensities at the absorbance peak maxima of the protonated and unprotonated forms of the chromoionophore. At a fixed position of the particle, the resulting responses are found to be independent of light scattering, incident light intensity and the shape or size of the microsphere. The responses of potassium-selective microspheres obtained by this method agree quantitatively with corresponding fluorescence-based data.

Development of a disposable mercury ion-selective optode based on trityl-picolinamide as ionophore

A disposable ion-selective optode for mercury based on trityl-picolinamide (T-Pico) as neutral ionophore was developed. The sensing layer consist of plasticised PVC incorporating T-Pico as a selective ionophore for Hg 2+, ETH 5418 as a chromoionophore, and potassium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate as lipophilic salt. The measurement principle is based on an ion-exchange mechanism. When the optode membrane is introduced into a water sample for 5 min, there is a colour change from red to blue, depending on the mercury concentration (pH 4.7), making it possible to use the absorbance at 665 nm as the analytical signal. The optode membrane response to Hg 2+ was not fully reversible; however, it reveals a very good selectivity to many cations including alkali, alkaline earth, transition, and heavy metal ions. The detection limit for Hg 2+ is 5.0 × 10 −7 M at pH 4.7. The response characteristics of the sensor including dynamic range, reproducibility, response time, and lifetime are discussed in detail. This sensor was used for the determination of mercury in environmental water samples with satisfactory recovery.

New hexamethine–hemicyanine dyes for the development of integrated optochemical sensors

New far-visible absorbing anilino-cyanine dyes have been synthesised for future application as chromoionophores in integrated waveguide absorbance optodes based on bulk optodes. The effect of the heterocycle, of the substitution of the heterocyclic nitrogen and of the type of heptamethine central ring on the pKa values (4.3-8.2 in ethanol-water solutions and 9.5-11.0 in plasticised PVC membranes), on the spectroscopic characteristics of the dye and on photostability is discussed. pH-selective bulk optodes have been formulated as a first approach to develop ion-selective optodes, and sensitivity, repeatability, lifetime and response time have been determined. The dyes show good analytical behaviour for use as chromoionophores for the development of ion-selective optodes. Reversible (80-87%), fast (tr90%=0.94-2.28 min) and pH-sensitive membranes (slopes of 0.09-0.23 DeltaAbs.pHdec-1, absorbance range 0.19-0.53) have been obtained. Moreover, they exhibit good spectroscopic features for employment with integrated optochemical sensors: absorption maxima of the acidic species in plasticised PVC membranes matched those of 650-670-nm LEDs, high molar absorption coefficients (epsilonacidic=3.5x10(4)-9.3x10(4) L mol-1 cm-1 and epsilonbasic=1.9x10(4)-6.7x10(4) L mol-1 cm-1) and fluorescence.

테트라싸이크린 항생제를 담체로 이용한 막전극의 전위차 특성

본 연구에서는 테트라싸이크린 항생제를 양이온선택성 전극의 담체로 사용한 막전극을 제작하고 알칼리 및 알칼리 토금속에 적용하여 전위차 감응도를 조사하였다. 제작된 전극은 지용성 첨가제의 유무와 관계없이 <TEX>$Ca^{2+}$</TEX> 이온에 대해서 거의 이상적인 전위차 감응도를 나타내었다. 또한 <TEX>$Mg^{2+}$</TEX>를 제외한 다른 양이온에도 이상적인 네른스트값에 80% 이상의 감응도와 <TEX>$10^{-5}$</TEX> 이하의 검출한계를 나타내었다. 수명, 감응시간, 재현성 등도 이온선택성 전극에 응용하기에 문제가 없었다. The main component governing selectivity in ion-selective electrodes and optodes is the ionophore. For this reason, a member of natural products that possess selective ion-binding properties have long been sought after. By applying this principle, the performance of tetracycline used as neutral carriers for cation selective polymeric membrane electrode was investigated. The cation ion-selective electrode based on tetracycline gave a good Nernstian response of 26.6 mV per decade for calcium ion in the activity range <TEX>$1x10^{-6}M$</TEX> to <TEX>$1x10^{-2}M$</TEX> with and without lipophilic additives. The optimized cation ion-selective membrane electrodes displayed very comparable potentiometric responses to various mono and di-valent cations of alkali and alkaline earth metal ions except <TEX>$Mg^{2+}$</TEX>.

Photoresponsive Ion-Selective Optical Sensor

A novel concept of photoresponsive ion-selective optical sensor is described. Photochemical reactions can be utilized to generate and control ion fluxes in an ion-selective optode in the same manner as nonequilibrium electrochemical methods have been used in ion-selective electrodes. In contrast to their equilibrium counterparts, the photoresponsive pH-selective ion optodes are sensitive to both the buffer capacity of the sample and activity of hydrogen ions. Active optical probes are especially attractive for intracellular applications because they can be fabricated as submicron-sized beads. Common optical techniques, such as fluorescence microscopy and flow cytometry, can be combined with active ion probes with only minor modification of the existing experimental setup.

Logit linearization of analytical response curves in optical disposable sensors based on coextraction for monovalent anions

The application of a decimal logistic transformation to the sigmoidal calibration curve of ion-selective bulk optodes for the determination of anions based on hydrophobic membranes containing neutral ionophore and chromoionophore is formally established and, consequently, a wide linear calibration function is obtained. The problems derived from the use of a sigmoidal curve in the calibration are therefore solved and the linear dynamic range is increased. The general equation resulting from the logistic transformation is discussed considering the stoichiometric factors for monovalent anions, and the linearization of the theoretical fit to experimental data was checked for two real cases. The strategy was applied to the determination of chloride and nitrate using disposable sensors for different types of waters (tap, well, stream, rain, snow and sea), validating the results against a reference procedure. This new linear calibration proposed for anion determination increases the linear dynamic range up to six orders of magnitude. The main advantage is that it is possible to directly quantify samples with very different analyte contents in a fast and simple way. The methods are easy to use and eliminate the need for prior treatment of the sample.

Establishment of wide-range linear response curves in bulk optode sensors for cations based on ion exchange

A linear curve is obtained when a decimal logistic transformation is applied to the sigmoidal calibration curve of ion-selective bulk optodes for the determination of cations based on ionophore–chromoionophore chemistry. In this way, problems derived from the use of a sigmoidal shape calibration functions are solved and the linear dynamic range is increased. The method is easier to use and eliminates the need for prior treatment of the sample. The influence of different parameters on the linearization of the theoretical model was studied: charge of the analyte, ionophore:chromoionophore ratio and complex stoichiometry, and the goodness of the linearization of the theoretical fit to experimental data for two experimental systems was checked. The procedure was applied to the determination of potassium and hardness using disposable sensors in different types of waters (tap, well, stream, rain, snow and sea), validating the results against a reference procedure. This new linear calibration proposed for cation determination increases the linear dynamic range up to 7–8 orders of magnitude. The main advantage is that it is possible to directly quantify samples with very different analyte contents in a fast and simple way.

Quantitative determination of heparin levels in serum with microtiter plate-format optode

A new assay method has been developed for the quantitative determination of heparin in serum using a microtiter plate-format optode (MPO). Heparin and proton in physiological sample are favorably co-extracted into the solvent polymeric optode membrane containing both cationic lipophilic additive, tridodecylmethyl ammonium chloride (TDMAC), and proton-selective ionophore, 3-hydroxy-4-(4-nitrophenylazo)-phenyloctadecanoate (ETH 2412), resulting in the absorbance change of the membrane to varying heparin levels. The optimized MPO composition contains low polymer-to-plasticizer ratio compared to those of conventional ion-selective optodes or electrodes, i.e., poly(vinyl chloride) (20.0)/dioctylsebacate (76.3)/ETH 2412 (1.7)/TDMAC (1.0) (wt.%): it resulted in a quantitative response to heparin from 0 to 15 unit/mL in serum with high sensitivity. The heparin–protamine titration on the MPO could provide rapid and precise determination of heparin. It was shown that the heparin levels in serum sample could be determined from the rate of absorbance change over time (Δ A/Δ t); this method was more effective than the direct absorbance measurement in minimizing the interferences from color and turbidity of serum samples. MPO has been developed as a high throughput and convenient disposable sensing device, and may find a wide application in the determination of polyions and charged macromolecules.

Integration of multiple-ion-sensing on a capillary-assembled microchip

Multiple-ion-sensing functions are integrated on a capillary-assembled microchip (CAs-CHIP). Since the CAs-CHIPs are fabricated by embedding various chemically functionalized square capillaries onto a lattice PDMS channel plate having same channel dimensions as outer dimensions of square capillaries, integration of parallel multiple-ion-sensing is easily realized. Here, three ion-sensing capillaries are prepared and used for integrating these functions onto a single microchip. Ion-sensing square capillaries (sodium, potassium, calcium) are prepared by attaching ion-selective optode membranes to inner wall of capillaries, and are characterized in terms of response time, response range, and ion selectivity. Finally, fully characterized ion-sensing capillaries are embedded into PDMS channel plate in parallel to fabricate a multiple-ion-sensing chip. The CAs-CHIP-based strategy is promising for integrating multiple chemical sensing functions onto a single microchip.

Development of a PVC-membrane ion-selective bulk optode, for UO22+ ion, based on tri-n-octylphosphine oxide and dibenzoylmethane

A novel uranyl ion-selective bulk optode membrane, incorporating tri-n-octylphosphine oxide for cation recognition and a lipophilic chromoionophore dibenzoylmethane, has been prepared. The PVC membrane composition was optimized to result in the widest working concentration range. The response range of the proposed optode is 4.1 x 10(-6) to 2.0 x 10(-4) mol L(-1) UO2(2+). The probe works at pH 4.0. Ion interference is low and selectivity, reproducibility, and stability are good.

A copolymerized dodecacarborane anion as covalently attached cation exchanger in ion-selective sensors.

The traditional cation exchangers used in ion-selective electrodes and optodes are tetraphenylborate derivatives, which are generally adequate for most analytical applications but may in some cases suffer from decomposition by acid hydrolysis, oxidants, and light. Recently, halogenated dodecacarboranes were found to be improved cation exchangers in terms of lipophilicity and chemical stability. This forms the basis for the convenient covalent attachment of the cation exchanger to the polymeric backbone of the sensing material. This is a challenge that has not satisfactorily been solved and which is especially important in view of developing ultraminiaturized sensing arrays. Here, a C-derivative of the closo-dodecacarborane anion (CB(11)H(12)(-)) with a polymerizable group was synthesized as a chemically stable cation exchanger. This new derivative was copolymerized with methyl methacrylate and decyl methacrylate (MMA-DMA) to fabricate a plasticizer-free polymer with cation-exchange properties. This polymer could be conveniently blended with traditional plasticized poly(vinyl chloride) or with noncrosslinked methacrylic polymers to give solvent cast films that appear to be clear and homogeneous and that could be doped with ionophores. Optode leaching experiments supported the covalent grafting of the carborane anions. Ion-selective membranes and optode thin films were evaluated in terms of response function, response time, and selectivity. In all cases, the new material exhibited behavior similar to free tetraphenylborate derivative-based membranes. As a result of these studies, an all-polymeric plasticizer-free calcium-selective membrane was fabricated on the basis of the covalently attached carborane, a recently introduced grafted calcium ionophore, and an MMA-DMA polymer matrix. The resulting ion-selective electrodes showed Nernstian response slopes and rapid response times, demonstrating that covalent grafting of all sensing components is a feasible approach to the development of ion sensors.

Ionophore-based ion-selective potentiometric and optical sensors.

This paper describes ion-selective electrodes (ISEs) and optodes with a focus on the fundamental mechanisms, response characteristics, and recognition elements (ionophores) described to date for these sensors. The topics covered review the most frequently encountered ideas about ionophore-based sensors, including the theoretical and experimental parameters that influence response, typical ionophore structures, useful membrane compositions, and applications. Since these sensors are applicable in a number of fields, such as clinical, environmental, and process monitoring, as well as more novel approaches such as microfluidic-based systems and micro/nanoprobes, this article provides descriptions that are oriented toward a more general audience. Furthermore, a large portion of this review describes the ionophore component itself in hopes of inspiring ideas in readers of how novel molecular architectures with new ionic targets or improved selectivity can be developed.

Optical biosensor for urea with improved response time

An optical biosensor for urea measurements was developed. The operation of the sensor is based on the well-known urease enzyme-catalyzed hydrolysis of urea. The ammonium ions liberated in the reaction are detected with an ion selective optode membrane containing nonactin as ion selective ionophore and ETH 5294 chromoionophore in a thin (1 μm) plasticized poly(vinylchloride) film. The basic sensing element was home made of a microscope glass slide, a HeNe laser light source, photodiode light detector and light in coupling, de-coupling elements. The transducer membrane and the enzyme containing reaction layer were sandwich-cast with spin coating onto the surface of the sensing slide. The attenuation of the laser light propagating inside the glass wave-guide was used as signal for urea measurements. With this arrangement membranes provided good sensitivity (0.05 absorption unit when going from 0.1 to 1 mM urea) and short (16–20 s) response time. Taking advantage on the improved response time, flow injection urea measurements were made in the 0.01–2 mM concentration range. Thirty sample/hour analysis-rate, good peak-to-peak reproducibility (RSD=0.02) and recovery (95–104%) was achieved with buffer diluted urea solutions. Applications for the analysis of real samples are planned to do in the future.

Fluorescent Ion-Selective Optode Membranes Incorporated onto a Centrifugal Microfluidics Platform

The development of an integrated analysis system for small ions based on ion-selective optodes and centrifugal microfluidics is reported. The performance of this system was evaluated through five-point calibration plots for two types of optode membranes, one being cation-selective and the other anion-selective, which were incorporated into a microfluidics platform on which fluid motion is induced via angular rotation. Additionally, the application of the microfluidic platform to ion analysis is studied via a two-point calibration protocol used to quantify an unknown sample. Calibrant solutions are delivered from reservoirs fabricated onto the platform to a measuring area that contains the optode membrane, with a change in membrane fluorescence being monitored. This work demonstrates the first instance of a microfluidic-based analysis system with detection based on ion-selective optode membranes monitored with fluorescence transduction. Furthermore, in addition to employing a standard excitation source where a fiber-optic probe is coupled to a tungsten-halogen lamp, laser diodes such as those employed in portable CD/DVD players were studied as excitation sources to enhance the observed fluorescence signals.