Chemically modified field effect transistors: The effect of ion-pair association on the membrane potentials

Antifouling improvement in Pb2+ ion selective electrodes by using an environmentally friendly capsaicin derivative

Biofouling is a critical issue for ion selective electrodes (ISE) in complex aqueous systems, seriously compromising the analytical performance of the electrodes (i.e., stability, sensitivity, and lifetime). Herein, an antifouling solid lead ion selective electrode (GC/PANI-PFOA/Pb²⁺-PISM) was successfully prepared by adding propyl 2-(acrylamidomethyl)-3,4,5-trihydroxy benzoate (PAMTB), an environmentally friendly capsaicin derivative, into the ion-selective membrane (ISM). The presence of PAMTB caused no loss in the detection performance of GC/PANI-PFOA/Pb²⁺-PISM (e.g., detection limit (1.9 × 10^-7 M), response slope (28.5 ± 0.8 mV/decade), the response time (20 s), stability (8.6 ± 2.9 μV/s), selectivity and no water layer), whilst imparting an excellent antifouling effect with an antibacterial rate of 98.1% when the content of PAMTB in the ISM was 2.5 wt%. Further, GC/PANI-PFOA/Pb²⁺-PISM maintained stable antifouling properties, excellent potential response, and stability even after soaking in a high-concentration bacterial suspension for 7 days.

A feasible, fast and reliable method for estimating ion-site association constants in plasticized PVC ion-selective electrode membranes

A feasible, fast and reliable method for estimating ion association constants in PVC plasticized membranes of ion-selective electrodes from potentiometric data has been theoretically and experimentally substantiated. The method is based on the established fact of complete dissociation of salts of quaternary ammonium cations R₄N ⁺ An^‒ (except for those containing methyl substituents at the nitrogen atom) in a membrane plasticized with o-nitrophenyl octyl ether (o-NPOE). Therefore, the boundary potential at the interface of the membrane with an aqueous solution of R₄N⁺ depends only upon the concentrations of the corresponding solution and the ion exchanger in the membrane and is independent of the presence of a lipophilic ionic additive (LIA), which makes it possible to use such ions as reference ones in the internal filling solution. If the ions studied (i⁺) are capable of forming ion associates with the ion exchanger, then the introduction of LIA into the membrane will lead to a decrease in the concentration of free i⁺ ions and to a corresponding increase in the boundary potential, from which the ion association constant can be directly calculated. The results obtained agree with the known literature data and the results of quantum chemical calculations. The prospective of applying the proposed method to the study of other membrane compositions is discussed.

General description of the simultaneous response of potentiometric ionophore-based sensors to ions of different charge

The response of ion-selective membrane electrodes is usually still described with the semiempirical Nicolskii-Eisenman equation although it cannot correctly reproduce experimental data if ions of different charges are involved. The recently published self-consistent model that was derived for two ions of any charges is now extended to any number of ions of any charge. One single explicit equation is here given for the first time for any number of monovalent, divalent, and trivalent ions. Deviations relative to the Nicolskii-Eisenman equation are shown to be especially high at low interferences and bias calibrations if done in a mixed solution of a target sample as well as multivariate calibrations with sensor arrays.

Poly(vinyl) chloride membrane copper-selective electrode based on 1-phenyl-2-(2-hydroxyphenylhydrazo)butane-1,3-dione

1-Phenyl-2-(2-hydroxyphenylhydrazo)butane-1,3-dione (H(2)L) was used as an effective ionophore for copper-selective poly(vinyl) chloride (PVC) membrane electrodes. Optimization of the composition of the membrane and of the conditions of the analysis was performed, and under the optimized conditions the electrode has a detection limit of 6.30×10(-7) M Cu(II) at pH 4.0 with response time 10s and displays a linear EMF versus log[Cu(2+)] response over the concentration range 2.0×10(-6) to 5.0×10(-3) M Cu(II) with a Nernstian slope of 28.80±0.11 mV/decade over the pH range of 3.0-8.0. The sensor is stable for 9 weeks and exhibits good selectivity with respect to alkali, alkali earth and transition metal ions (e.g. Na(+), K(+), Ba(2+), Ca(2+), Zn(2+), Cd(2+), Co(2+), Mn(2+), Ni(2+), Fe(2+), Al(3+)) in the 3.0-8.0 pH range. It was successfully applied for the direct determination of copper(II) in zinc, aluminum and nickel based alloys, in soils polluted by oil, and as an indicator electrode for potentiometric titration of copper ions with EDTA.

Potassium selective chemically modified field effect transistors based on AlGaN/GaN two-dimensional electron gas heterostructures

We investigate the use of the AlGaN/GaN high electron mobility transistor (HEMT) as a novel transducer for the development of ion-selective chemically modified HEMT sensors (ChemHEMTs). For this, polyvinyl chloride (PVC) membrane doped with ion-selective ionophores is deposited onto the area of the gate for the chemical recognition step, while the AlGaN/GaN HEMT is used as the transducer. In particular, the use of a valinocycin doped membrane with thickness of 50 microm generates a sensor with excellent analytical characteristics for the monitoring of K(+). The K(+)-ChemHEMT has sensitivity of 52.4 mV/pK(+)in the linear range of 10(-5) to 10(-2)M, while the detection limit is in the order of 3.1 x 10(-6)M. Also, the sensor shows selectivity similar to valinomycin-based ISEs, while the signal stability over time and the measurement to measurement reproducibility are very good.

A Novel Membrane Sensor for Histamine H1-Receptor Antagonist "Fexofenadine"

The construction and general performance of thirteen new polymeric membrane sensors for the determination of fexofenadine hydrochloride based on its ion exchange with reineckate, tetraphenylborate and tetraiodomercurate have been studied. The effects of membrane composition, type of plasticizer, pH value of sample solution and concentration of the analyte in the sensor internal solution have been thoroughly investigated. The novel sensor based on reineckate exchanger shows a stable, potentiometric response for fexofenadine in the concentration range of 1 x 10(-2) - 2.5 x 10(-6) M at 25 degrees C that is independent of pH in the range of 2.0 - 4.5. The sensor possesses a Nernstian cationic slope of 62.3+/-0.7 mV/concentration decade and a lower detection limit of 1.3 x 10(-6) M with a fast response time of 20 - 40 s. Selectivity coefficients for a number of interfering ions and excipients relative to fexofenadine were investigated. There is negligible interference from almost all studied cations, anions, and pharmaceutical excipients, however, citrizine that has a structure homologous to that of fexofenadine was found to interfere. The determination of fexofenadine in aqueous solution shows an average recovery of 99.83% with a mean relative standard deviation (RSD) of 0.5%. Direct potentiometric determination of fexofenadine in tablets gave results that compare favorably with those obtained by standard spectrophotometric methods. Potentiometric titration of fexofenadine with phosphomolybdic acid as a titrant has been monitored with the proposed sensor as an end point indicator electrode.

Evaluation of the separate equilibrium processes that dictate the upper detection limit of neutral ionophore-based potentiometric sensors

The upper detection limit of polar ionophore-based ion-selective electrode membranes is predicted by utilizing the coextraction constant of dissociated electrolyte, the stability constant of the ionophore, and the membrane composition. The coextraction constant of dissociated electrolytes into the polar poly(vinyl chloride) membrane plasticized with o-nitrophenyl octyl ether (PVC-NPOE) is here measured by a novel approach. The sandwich membrane technique is utilized, with one membrane segment containing a lipophilic cation exchanger and the other containing an anion exchanger. This yields information about the coextraction constant and the free ion concentrations of the electrolyte in the two segments. Predictions correlate quantitatively with the upper detection limit observed for ion-selective electrodes based on the ionophores valinomycin, tert-butylcalix[4]arene tetraethyl ester, and calcimycin. The difficulties of the prediction of the upper detection limit for nonpolar poly(vinyl chloride) membranes plasticized with bis(2-ethylhexyl sebacate) (PVC-DOS) due to ion association are discussed in detail. A thermodynamic cycle experiment with a series of sandwich membranes shows that the principal processes governing the upper detection limit of PVC-DOS membranes are identical to those for the PVC-NPOE membranes. However, the stability of the ion pairs between the ionophore-metal ion complexes and the extracted anion are different from that of ion pairs formed between the same anion and the lipophilic anion exchanger. This makes it difficult to quantitatively predict the upper detection limit on the basis of simple apparent coextraction and complexation data alone. The approach reported herein is useful not only for mechanistic purposes but also to shed light onto the many cases where coextraction effects need to be understood but are not directly experimentally accessible.