A colorimetric biosensor for convenient quantification of ethanol and methanol is described. The biosensor utilizes a 'one-pot' nanocomposite consisting of Fe3O4 magnetic nanoparticles (MNPs) and alcohol oxidase (Al Ox) simultaneously entrapped in large pore sized mesocellular silica. Al Ox immobilized in the silica generates H2O2 in the presence of alcohol in a sample, which subsequently activates MNPs in the mesopores of the silica to convert a colorimetric substrate into a colored product. Using this strategy, a target alcohol was specifically detected through a very convenient colorimetric signal resulting from the combined reactions. This strategy enabled successful sensing of ethanol and methanol in a linear concentration range from 100 to 500 mu M with a detection limit as low as 25 mu M by employing 2,2'-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) diammonium salt (ABTS) as a peroxidase substrate. Along with excellent reusability via simple magnetic capturing, enhanced operational stability was achieved by the nanocomposite system. The present nanocomposite would serve as a novel platform for rapid and convenient analysis of alcohol.