Medical Engineering & Physics 28 (2006) 978–981 Optical biodetection of cadmium and lead ions in water Saharudin Haron a , Asim K. Ray b,∗ a Department of Chemical Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia b Nanotechnology Laboratories, The Department of Materials, Queen Mary College, University of London, Mile End Road, London E1 4NS, UK Received 28 April 2006; accepted 28 April 2006 Abstract A three layer waveguiding silicon dioxide (SiO 2 )/silicon nitride (Si 3 N 4 )/SiO 2 structure on silicon substrate was proposed as an optically efficient biosensor for calibration of heavy metal ions in drinking water. The catalytic activities of urease and acetylcholine esterase (AchE) were inhibited by the presence of cadmium (Cd 2+ ) and lead (Pb 2+ ) ions. The detection limit as low as 1 ppb was achieved by employing the technique of total reflection at the interface between the Si 3 N 4 core and composite polyelectrolyte self-assembled (PESA) membranes containing cyclotetrachromotropylene (CTCT) as an indicator. © 2006 IPEM. Published by Elsevier Ltd. All rights reserved. Keywords: Optical biosensors; Enzyme reactions; Heavy metal ions 1. Introduction Lead in drinking water is one of the sources of lead poi- soning [1]. On average, about 10–20% of a child’s population in USA suffer from lead contamination of drinking water but this figure rises to as high as 60% for those who drink baby formulas and juices, which are mixed with water. Excess lead in the human body can cause serious damage to the brain, kidneys, nervous system and red blood cells [2]. The consumption of 250 ppm lead for 3–6 weeks through drink- ing water was found to significantly decrease the number of spontaneously active dopamine neurons [3]. Similarly, peo- ple can also be exposed to cadmium pollution by drinking contaminated water. Cadmium occurs naturally at low lev- els in foods. Stomach irritation is commonly observed after ingestion of food or water contaminated with high levels of cadmium. The safety margin for the normal intake of cadmium is about 1–3 g/day. Nausea, vomiting, diarrhoea, muscle cramps, salivation, sensory disturbances, liver injury, convulsions, shock and renal failure may be symptoms of short-term exposure of cadmium. Kidney stones can form as ∗ Corresponding author. E-mail address: a.k.ray@qmul.ac.uk (A.K. Ray). a result of consuming high levels of cadmium in drinking water. The damage to bone and blood can be caused by a lifetime exposure of cadmium [4]. Lead and cadmium were successfully separated from the organic compounds using a differential pulse polarography with a CaCl 2 electrolyte varying the concentration from 0.1 to 5 mol l -1 [5]. An accurate real-time measurement and quan- tification of heavy metal ions was performed using a high- resolution differential surface plasmon resonance technique. The difference in the resonance angles from the reference and sensing areas of the sensing membrane was found to be specific to the binding of the metal ions onto the sensing area coated with properly selected peptides [6]. It has been shown recently that attenuated total reflection (ATR) in a sil- icon nitride (Si 3 N 4 ) planar waveguide can be employed to monitor enzyme activity. A 190 nm thick Si 3 N 4 core layer is sandwiched between two 1.5 m thick SiO 2 cladding lay- ers (see Fig. 1). The refractive indices are typically n 1 ≈ 1.46 and n 2 ≈ 2 for SiO 2 and Si 3 N 4 layers, respectively and the evanescent field volume attained is, therefore, reasonably high. The sensitivity of the technique was found to be higher than that obtained from the UV–vis absorption spectroscopy by two orders of magnitude [7]. The sensing window on the top of Si 3 N 4 layer was coated with a polyelectrolytically 1350-4533/$ – see front matter © 2006 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.medengphy.2006.04.004