A novel microgravimetric immunosensor has been developed usingTiO2 nanoparticle-modified immunoassay and silver enhancement reaction. An antibody-conjugatedTiO2 nanoparticle is bound to the AFP antigen immobilized on a quartz resonator. When thenanoparticles are exposed to UV light in a silver nitrate solution, the photocatalyticreduction of silver ions results in the formation of metallic silver onto the nanoparticles andinduces a decrease in the resonance frequency. The frequency change by this photocatalyticreduction reaction is three orders of magnitude larger than the change by antigen bindingalone. The efficiency of the photocatalytic reaction has been found to increase with thefraction of anatase crystallites in the nanoparticles and the concentration of theAgNO3 solution. The results highlight the potential of the photocatalytic nanoparticles for thedetection of low concentrations of target molecules using gravimetric sensors.