Nanoparticles that are responsive to multiple stimuli allow more precise control over the aggregation process and thus the structures and properties of the resulting aggregates. Au nanoparticles functionalized by poly-L-histidine (PLH), a simple polypeptide with a pK(a) value (approximately 6.2) around the physiological pH, are sensitive to pH and temperature simultaneously. The dual stimuli, pH and temperature, must act in a cooperative way to switch the hydrophobic beta-sheet structure of PLH and thus trigger the assembly/disassembly of the Au nanoparticles. The aggregation process and thus the conformation change of PLH can be well recognized by the color change of the Au nanoparticles by naked eyes.