Rongning Liang

Potentiometric Sensing of Neutral Species Based on a Uniform-Sized Molecularly Imprinted Polymer as a Receptor

(Figure Presented) Sensing changes: A uniform-sized molecularly imprinted polymer (MIP), employed as a receptor, can be incorporated into a polymeric membrane used as an ion-selective electrode for the Potentiometrie sensing of neutral species (see scheme). Changes in the binding sites of the MIP in the membrane phase induced by recognition of the analyte are measured by using an indicator ion that has a molecular structure similar to that of the analyte.

Trace-Level Potentiometric Detection in the Presence of a High Electrolyte Background

Polymeric membrane ion-selective electrodes (ISEs) have become attractive tools for trace-level environmental and biological measurements. However, applications of such ISEs are often limited to measurements with low levels of electrolyte background. This paper describes an asymmetric membrane rotating ISE configuration for trace-level potentiometric detection with a high-interfering background. The membrane electrode is conditioned in a solution of interfering ions (e.g., Na(+)) so that no primary ions exist in the ISE membrane, thus avoiding the ion-exchange effect induced by high levels of interfering ones in the sample. When the electrode is in contact with the primary ions, the interfering ions in the membrane surface can be partially displaced by the primary ions due to the favorable ion-ligand interaction with the ionophore in the membrane, thus causing a steady-state potential response. By using the asymmetric membrane with an ion exchanger loaded on the membrane surface, the diffusion of the primary ions from the organic boundary layer into the bulk of the membrane can be effectively blocked; on the other hand, rotation of the membrane electrode dramatically reduces the diffusion layer thickness of the aqueous phase and significantly promotes the mass transfer of the primary ions to the sample-membrane interface. The induced accumulation of the primary ions in the membrane boundary layer largely enhances the nonequilibrium potential response. By using copper as a model, the new concept offers a subnanomolar detection limit for potentiometric measurements of heavy metals with a high electrolyte background of 0.5 M NaCl.

A simple approach for fabricating solid-contact ion-selective electrodes using nanomaterials as transducers

A simple and robust approach for the development of solid-state ion-selective electrodes (ISEs) using nanomaterials as solid contacts is described. The electrodes are fabricated by using the mixture of an ionic liquid (IL) and a nanomaterial as intermediate layer, formed by melting the IL. Tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) is chosen as an model of IL to provide strong adhesion between the inner glassy carbon electrode and the intermediate layer. Nanomaterials including single-walled carbon nanotubes (SWCNTs) and graphene were used as active ion-to-electron transducers between the glassy carbon electrode and the ionophore-doped ISE membrane. By using the proposed approach, the solid-contact Cu(2+)- and Pb(2+)-selective electrodes based on ETH 500/SWCNTs and ETH 500/graphene as transducers, respectively, have been fabricated. The proposed electrodes show detection limits in the nanomolar range and exhibit a good response time and excellent stability.

Potentiometric detection of chemical vapors using molecularly imprinted polymers as receptors

Ion-selective electrode (ISE) based potentiometric gas sensors have shown to be promising analytical tools for detection of chemical vapors. However, such sensors are only capable of detecting those vapors which can be converted into ionic species in solution. This paper describes for the first time a polymer membrane ISE based potentiometric sensing system for sensitive and selective determination of neutral vapors in the gas phase. A molecularly imprinted polymer (MIP) is incorporated into the ISE membrane and used as the receptor for selective adsorption of the analyte vapor from the gas phase into the sensing membrane phase. An indicator ion with a structure similar to that of the vapor molecule is employed to indicate the change in the MIP binding sites in the membrane induced by the molecular recognition of the vapor. The toluene vapor is used as a model and benzoic acid is chosen as its indicator. Coupled to an apparatus manifold for preparation of vapor samples, the proposed ISE can be utilized to determine volatile toluene in the gas phase and allows potentiometric detection down to parts per million levels. This work demonstrates the possibility of developing a general sensing principle for detection of neutral vapors using ISEs.

Mussel-Inspired Surface-Imprinted Sensors for Potentiometric Label-Free Detection of Biological Species

Using sensors to quantify clinically relevant biological species has emerged as a fascinating research field due to their potential to revolutionize clinical diagnosis and therapeutic monitoring. Taking advantage of the wide utility in clinical analysis and low cost of potentiometric ion sensors, we demonstrate a method to use such ion sensors to quantify bioanalytes without chemical labels. This is achieved by combination of chronopotentiometry with a mussel-inspired surface imprinting technique. The biomimetic sensing method is based on a blocking mechanism by which the recognition reaction between the surface imprinted polymer and a bioanalyte can block the current-induced ion transfer of an indicator ion, thus causing a potential change. The present method offers high sensitivity and excellent selectivity for detection of biological analytes. As models, trypsin and yeast cells can be measured at levels down to 0.03 U mL-1 and 50 CFU mL-1 , respectively.

Hydrophilic molecularly imprinted polymers for selective recognition of polycyclic aromatic hydrocarbons in aqueous media

A hydrophilic molecularly imprinted polymer (H-MIP) for selective recognition of polycyclic aromatic hydrocarbons in an aqueous medium has been developed in which a hydrophilic functional co-monomer is used to improve the surface hydrophilicity. By using phenanthrene as a model, the proposed H-MIP exhibits remarkably improved selectivity in aqueous solution.

Soluble Molecularly Imprinted Polymer-Based Potentiometric Sensor for Determination of Bisphenol AF

Molecularly imprinted polymer (MIP)-based polymeric membrane potentiometric sensors have been successfully developed for determination of organic compounds in their ionic and neutral forms. However, most of the MIP receptors in potentiometric sensors developed so far are insoluble and cannot be well dissolved in the polymeric membranes. The heterogeneous molecular recognitions between the analytes and MIPs in the membranes are inefficient due to the less available binding sites of the MIPs. Herein we describe a novel polymeric membrane potentiometric sensor using a soluble MIP (s-MIP) as a receptor. The s-MIP is synthesized by the swelling of the traditional MIP at a high temperature. The obtained MIP can be dissolved in the plasticized polymeric membrane for homogeneous binding of the imprinted polymer to the target molecules. By using neutral bisphenol AF as a model, the proposed method exhibits an improved sensitivity compared to the conventional MIP-based sensor with a lower detection limit of 60 nM. Moreover, the present sensor exhibits an excellent selectivity over other phenols. We believe that s-MIPs can provide an appealing substitute for the traditional insoluble MIP receptors in the development of polymeric membrane-based electrochemical and optical sensors.

Potentiometric flow injection system for determination of reductants using a polymeric membrane permanganate ion-selective electrode based on current-controlled reagent delivery

A polymeric membrane permanganate-selective electrode has been developed as a current-controlled reagent release system for potentiometric detection of reductants in flow injection analysis. By applying an external current, diffusion of permanganate ions across the polymeric membrane can be controlled precisely. The permanganate ions released at the sample-membrane interface from the inner filling solution of the electrode are consumed by reaction with a reductant in the sample solution thus changing the measured membrane potential, by which the reductant can be sensed potentiometrically. Ascorbate, dopamine and norepinephrine have been employed as the model reductants. Under the optimized conditions, the potential peak heights are proportional to the reductant concentrations in the ranges of 1.0×10(-5) to 2.5×10(-7)M for ascorbate, of 1.0×10(-5) to 5.0×10(-7)M for dopamine, and of 1.0×10(-5) to 5.0×10(-7)M for norepinephrine, respectively with the corresponding detection limits of 7.8×10(-8), 1.0×10(-7) and 1.0×10(-7)M. The proposed system has been successfully applied to the determination of reductants in pharmaceutical preparations and vegetables, and the results agree well with those of iodimetric analysis.

Modeling the response of a control-released ion-selective electrode and employing it for the study of permanganate oxidation kinetics

Although polymeric membrane ion-selective electrodes (ISEs) based on outward ion fluxes have been found analytically useful, there is still a lack of a theoretical framework for this detection system. In this study, we attempted to model the response of this kind of permanganate ISE and employed this ISE to analyze the rapid MnO4−/H2O2 reaction. This response is attributed to H2O2 oxidation with MnO4− that is released from the inner solution to the membrane surface layer. The results show that the experimental data can be fitted well to the proposed model that is elucidated mathematically from the viewpoint of chemical kinetics. The second-order rate constant is determined at a near neutral pH and is in agreement with the acid dissociation law to provide the specific value of 370 M−1 s−1. The kinetic mechanism was then investigated by performing DFT calculations. Via analysis of the Mn–O bond length and the HOMO orbital, it has been found that the studied redox system functions similarly as the so-called hydrogen abstraction mechanism with an energy barrier of 24.5 kcal mol−1. This study is considered to be the first report on the simulation of MnO4− attack at the O–H bond. On the basis of the transition state theory and previous studies on MnO4− attack at the CC and C–H bonds, the relationship between the experimental rate constant and computational energy barrier is finally constructed. The result indicates the validity of our proposed method and makes the control-released ISE a very promising platform to study the kinetics.

An all-solid-state imprinted polymer-based potentiometric sensor for determination of bisphenol S

An all-solid-state polymeric membrane potentiometric sensor for determination of bisphenol S has been developed by using the imprinted polymer as the receptor and a nanoporous gold film as the solid contact. The sensor has a linear concentration range of 0.1 to 2 μM with a detection limit of 0.04 μM.