BACKGROUD: This study describes the construction of an electrochemical formaldehyde biosensor based on poly(glycidyl methacrylate-co-3-methylthienyl methacrylate)/formaldehyde dehydrogenase/polypyrrole [poly(GMA-co-MTM)/FDH/PPy] composite film electrode. Formaldehyde dehydrogenase (FDH) was chemically immobilized via the epoxy groups of the glycidyl methacrylate (GMA) side chain of the polymer. Formaldehyde measurements were conducted in 0.1 mol L1, pH 8 phosphate buffer solution (PBS) including 0.1 mol L1 KCl, 0.5 mmol L1 of NAD+ (cofactor of the enzyme) and 1 mmol L1 of 1,2-napthoquinone-4-sulfonic acid sodium salt (NQS) as mediator with an applied potential of 0.23 V (vs. Ag/AgCl, 3 mol L1 NaCl). Analytical parameters of the biosensor were calculated and discussed. The biosensor was tested in rain water samples. RESULTS: Sensitivity was found to be 15 000 per mmol L1 (500 nA ppm1) in a linear range between 0.1 ppm and 3 ppm (3.3100 mu mol L1). A minimum detectable concentration of 4.5 ppb (0.15 mu mol L1) (S/N = 3) with a relative standard deviation (RSD) of 0.73% (n = 5) was obtained from the biosensor. Response time of the biosensor was very short, reaching 99% of its maximum response in about 4 s. The biosensor was also tested for formaldehyde measurements in rain water samples. Formaldehyde concentrations in samples were calculated using the proposed biosensor with recovery values ranged between 92.2 and 97.7% in comparison with the colorimetric Nash method. CONCLUSION: The poly(GMA-co-MTM)/FDH/PPy) electrode showed excellent measurement sensitivity in comparison with other formaldehyde biosensor studies. Strong chemical bonding between the enzyme and the copolymer was created via the epoxy groups of the composite film. The proposed biosensor could be used successfully in rain waters without a pretreatment step. (c) 2012 Society of Chemical Industry