Abstract: A rotating disc electrode (4 mm diameter) based on Kel-F tubing filled with a tetrathiafulvalene-p-tetracyanoquinodimethane (TTF-TCNQ)/Si oil paste (5:8) mixed with xanthine oxidase (15:1), was combined with a Ag/AgCl reference electrode and a Pt wire counter electrode to form an amperometric biosensor. The response to hypoxanthine in pyrophosphate buffer was described by the enzyme-kinetic Michaelis-Menton formalism with good correlation from -100 to +300 mV at pH 6.1-8.8. Xanthine oxidase was oxidized by at least-two mediator species by a homogenous mechanism. The sensor could be operated in air-saturated solution, and operation in flow analysis systems is recommended. The calibration graphs were linear for up to 3 µM-hypoxanthine and the detection limit was ~10 nM. The response time was ~10 s. Ascorbate did not interfere for measurements made at 9 Hz, -75 mV and pH 6.7. The sensor paste was stable for 3 months at -30°C.

Abstract: A PTFE disc with a central 1 mm hole was press-fitted into a length of Kel-F tubing (4 mm i.d.), leaving a cavity 3 mm deep. This cavity and the central hole were filled with a paste of tetrathiafulvalene-p-tetracyanoquinodimethane (organic conductor)/high temperature silicone oil (5:8) and the surface of the paste was smoothed flat. Electrical contact with the paste was with a Pt wire and the electrode was attached to a stirrer. The enzyme peroxidase (5 mg) was made lipophilic by reaction with caprylic aldehyde (0.1 ml) and the mixture was dried in a vacuum desiccator. The product was dispersed in 0.1 mL of 0.1 M sodium pyrophosphate buffe of pH 6 and a 10 µL portion was applied to the paste electrode and allowed to dry in air. The electrode was mounted in a 100 mL cell with a Ag/AgCl reference electrode and a Pt counter electrode. All solution were made in the same buffer, kept at 25°C and stirred with the working electrode at 25 rpm. Current from the reduction of H2O2 was measured at -0.5 V. The working range was 0.01-4 µM-H2O2 but the response decreased over time due to intermittent exposure to open-circuit conditions; it was more stable in a flow injection system. Response time was ~10 s.

Abstract: Four different nitrite-selective optode membranes were studied. Each membrane consisted of a nitrite-selective ligand and four different H+-selective chromoionophores dissolved in a plasticized poly(vinyl chloride) bulk medium. The membranes' analytical parameters differ with respect to optical spectra, dynamic range and detection limit. The unique features of each membrane are discussed and the versatility of the optical sensors based on the membranes is evaluated. The response functions of the optical sensors, derived from the changes in the absorbance and/or fluorescence of the membranes with varying nitrite concentration, are described by a theoretical model based on coextraction equilibria. The dynamic range of the sensors extended from 0.5 to 5000 mg L-1 nitrite with a detection limit of 0.24 mg L-1. The response time, 95, of the membranes placed in a continuous flow measuring cell was 1.2 min when changing from 0.5 mg L-1 nitrite to 0.05 mg L-1, and 3.6 min in the opposite direction. The selectivity coefficients determined by the separate solution method were: 10^-3.3 for nitrate, 10^-3.7 for chloride, 10^-3.0 for bromide, 10^-3.8 for sulphate, and 10^-5.0 for phosphate. Various applications of these optode membranes are discussed.