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The absorption spectrum of the amyloid beta 1-40 peptide (Abeta(1-40)) conjugated to gold colloidal suspension of 15, 20, 30, and 40 nm size were examined under temperature ranging from 5 to 50 degrees C. As the pH was externally altered repetitively between pH 4 and 10, Abeta(1-40)-coated 20 nm gold colloid nanoparticles exhibited a reversible color change at the entire temperature range tested in this study except for 5 +/- 0.2 degrees C. This reversible change may be due to the fact that hydrophilic Abeta(1-40) evolves between a three-dimensional network containing mainly beta-sheet and alpha-helices, and an intermediate of this process implies a reversible step reported as initiation of the fibrillogenesis in Alzheimer's disease. When other nanosize particles were investigated, Abeta(1-40)-coated 30 and 40 nm colloids exhibited the reversible color change when temperature was lowered to 18 +/- 0.2 and 6 +/- 0.2 degrees C, respectively. This specific and unique size and temperature dependence in reversible color change strongly suggests that the noncovalent intrinsic intermolecular potential formed between the nanocolloidal surface and each Abeta(1-40) monomer conjugated at the surface drives the process.