Poly(luminol) based sensor array for determination of dissolved chlorine in water

An optical sensor for determination of free chlorine content of drinking water was prepared and tested. The function of the sensor is based on detecting chemiluminescence signal provided by thin immobilized poly(luminol) reagent layer. The poly(luminol) reagent film was prepared by electropolymerization of luminol onto planar indium-tin-oxide (ITO) electrode. Different methods, like electrode potential cyclization (cyclic voltammetry, CV), pulsed potential electrolysis (pulsed amperometry, PA) and potentiostatic electrolysis (constant potential electrolysis, PSE) were employed for preparation of the poly(luminol) layer. The chemoluminescence (chemiluminescence) of the differently prepared films was investigated in the poly(luminol) - hypochlorite - hydrogen peroxide reaction. Highest luminescence signal was obtained by the films prepared with CV. Poly(luminol) layers deposited with pulsed potential showed 80% less luminescence while almost no signal was obtained in case of films made with constant potential technique. The effects of the buffer composition and pH on the analytical properties of the electro polymerized sensing layer were investigated. The lower concentration limit of free chlorine detection was 5 × 10−7 M in phosphate buffer at pH = 8.0. It was found that the chemiluminescence signal decreased significantly when hypochlorite concentrations over 1 mM were applied. An array of 24 micro wells was fabricated on ITO glass slab of about microscope slide size. The individual micro wells had identical volume and the poly(luminol) layer immobilized on their bottom had identical activity. The wells could be used for “single shot” determination of free chlorine content of drinking water. The long storage stability, the simple measurement procedure and low feasible concentration range makes the array an attractive analytical tool. Its applicability was proved measuring dissolved chlorine concentration of tap water samples.