Amperometric immunosensors based on rigid conducting immunocomposites.

Novel polishable immunosensors based on rigid biocomposite materials have been constructed. These biocomposites contain graphite powder, rabbit IgG, and methacrylate or epoxy resins. This material acts as a reservoir for the biological molecules and as a transducer at the same time. In order to study the potential analytical properties of this new type of material, a competitive binding assay was developed to determine the RIgG present in a sample with the aid of goat anti-rabbit IgG labeled with alkaline phosphatase. Using phenyl phosphate as a substrate, the phenol produced by the enzymatic reaction was amperometrically detected at 800 mV (vs Ag/AgC1). The surface of the immunosensor can be regenerated by simply polishing, obtaining fresh immunocomposite ready to be used in a new competitive assay.

Validation of an automatic urea analyser used in the continuous monitoring of hemodialysis parameters.

The validation of an automatic urea analyser used in the monitoring of hemodialysis processes is reported. The analyser can indirectly determine dialysis parameters as dialysis delivery (KT/V) and protein catabolism (PCRn). These parameters are useful for the prescription and optimization of hemodialysis. The analyser, based on a previously-reported flow-injection analytical biosystem, was connected on-line to the effluent of a dialysis machine during several hemodialysis sessions. The urea concentration data were continuously processed and dialysis parameters were obtained in quasi real time by means of the integration of an adjusted time-dependent exponential function. These values were compared with those obtained by applying the methods traditionally employed in hospital laboratories. The evaluation comprised 24 data sets from several patients of different gender and age. No significant differences were found between the KT/V and PCRn results obtained with the usual method and those results produced by the analyser proposed here.

On-line monitoring of urea in effluent liquid during haemodialysis.

An analytical system specially built for on-line urea monitoring is reported. Measurements are carried out in the effluent of a haemodialysis machine. The measuring system employs the dialyser inflow stream as a carrier solution channel in a continuous fashion. The analyser periodically samples the outflow stream of the dialyser by means of an automatic injection valve. The analyser features a bioreactor consisting of immobilized urease and a gas-diffusion module. It is through this module that the urea is converted to ammonia gas which is transferred to another carrier channel, this transports the ammonium ion to a tubular, all-solid-state, ion-sensitive electrode. A timer controls the transport, injection, the measuring and the recording subsystems. The analyser has been used during actual haemodialysis sessions. Urea clearances were also measured in batch, using conventional spectrophotometric clinical equipment. The correlation between both methodologies was sufficient to confirm the usefulness of the developed on-line analyser to monitor the optimal length of haemodialysis sessions.

Covalent binding of urease on ammonium-selective potentiometric membranes.

As part of the development of disposable urea bioselective probes, the covalent binding of urease on ammonium-selective potentiometric membranes has been assessed. Nonactin/bis(1-butylpentyl)adipate/poly(vinylchloride) (PVC) membranes, directly applied to an internal solid contact (conductive epoxy-graphite composite), has been used as a support for covalent immobilization of urease. Two types of all-solid-state construction process have been assayed: thin layers of cellulose acetate (CA) were coated on the PVC ammonium-selective membranes (type 1) and blends of PVC and CA at various ratios were used as ammonium-selective membrane matrices (type 2). Urease was covalently attached to CA via aldehyde groups. These groups were created on the polysaccharide with sodium periodate to which the enzyme was immobilized through a spacer (hexamethylenediamine). The viability of both types of probe for the determination of ammonium ions was assessed after each step of the activation process. Results indicated that type 2 potentiometric probes are altered after the treatment with sodium periodate. Good results were obtained with type 1 probes. Their dynamic concentration range of response to urea was from 2 x 10(-5) to 0.01 M with a sensibility of 50 mV/decade.

Chloride determination in serum by a flow-injection analysis precipitation pseudo-titration technique using a flow-through all-solid-state silver electrode.

A simply constructed, tubular, all-solid-state, flow-through silver electrode for flow-injection analysis (F.I.A.) is described. Use of a single line manifold accommodating the silver electrode, with a low level of silver ion (5 x 10(-4) M) in the carrier stream, is a useful method to determine chloride in serum, by means of a precipitation pseudo-titration F.I.A. technique. The sampling frequency is about 60/h.