The purpose of this paper is to develop a sensitive and low-cost colorimetric method for detection of melamine using simple circuitry. The stable p-nitroaniline-modified silver nanoparticles (Ag NPs) were synthesized by a zero-length covalent coupling chemistry, which showed sensitivity and selectivity towards melamine. The melamine induces the aggregation of nanoparticles due to electron donor-acceptor interaction between melamine and p-nitroaniline at the Ag NP interface, resulting in a shift in the surface plasmon band and a consequent color change of the Ag NPs from yellow to blue. The color change was monitored using UV-vis spectrophotometry. More importantly, this optical method was successfully utilized to a practical infant formula sample. As low as 0.1 ppm melamine in infant formula could be distinguished upon a color change of solution in two minutes.