Density functional theory (DFT) calculations are employed to examine the adsorption behaviors of cysteine on the gold surface using Au6 and Au8 species as model reactants. Computed results show that cysteine molecules prefer to bind with gold clusters via the S-atom of the thiol group in vacuum and thiolate group in water. The gas-phase adsorption energies are around 20.2 kcal/mol for Au6 and 24.4 kcal/mol for Au8. In water environment, such values are slightly reduced for Au6 (19.6 kcal/mol), but increased a little more for Au8 (25.6 kcal/mol). As a result, if a visible light with a frequency of ν ≈ 6 × 10^14 Hz (500 nm) is applied, the time for the recovery of Au6 and Au8 from the most stable complexes will be about 0.38 and 9.3 × 10^3 s, respectively, at 298 K in water. The Au6 is in addition found to benefit from a larger change of energy gap that could be converted to an electrical signal for detection of cysteine.