Renewable urea sensor based on a self-assembled polyelectrolyte layer

Abstract

A renewable urea sensor based on a carboxylic poly(vinyl chloride) (PVC–COOH) matrix pH-sensitive membrane has been proposed, in which a positively charged polyelectrolyte layer is first constructed by using a self-assembly technique on the surface of a PVC–COOH membrane, and urease, with negative charges, is then immobilized through electrostatic adsorption onto the PVC–COOH membrane, by controlling the pH of the urease solution below its isoelectric point. The response characteristics of the PVC–COOH pH-sensitive membrane and the effects of experimental conditions have been investigated in detail. Compared with conventional covalent immobilization, the urea sensor made with this self-assembly immobilization shows significant advantage in terms of sensitivity and ease of regeneration. The potential responses of the urea sensor with self-assembly immobilization increase with the urea concentration over the concentration range 10⁻⁵–10⁻¹ mol l⁻¹, and the detection limit is 0.028 mmol l⁻¹. Moreover, this type of urea sensor can be repeatedly regenerated by using a simple washing treatment with 0.01 mol l⁻¹ NaOH (containing 0.5 mol l⁻¹ NaCl) and 0.01 mol l⁻¹ HCl. The urease layers and the polyelectrolyte layers on the PVC–COOH membrane are removed, the potential response of the sensor to urea solutions of different concentrations returns nearly to zero, and another assembly cycle of urease and polyelectrolyte can then be carried out.