Citrate-capped gold nanoparticles were prepared by chemical reduction of HAuCl4 using citrate as the reducing agent and stabilizer. The synthesized nanoparticles showed an intense localized surface plasmon resonance absorption band at 520nm. Uniformly distributed spherical gold nanoparticles with average particle diameter size of 12.2±1.65nm were verified through Transmission Electron Microscopy (TEM) analysis. The stability of the gold nanoparticles was affected by pH and Cu2+. Aggregates are formed at pH < 3 due to surface charge neutralization of the citrate-capped gold nanoparticles. With Cu2+, aggregation is being initiated at a certain concentration. Visual observations, UV-Vis spectroscopy and TEM analysis were used to characterize the aggregated system. At optimum parameters (i.e., wavelength, reaction time, and pH), a calibration curve for Cu2+ determination was constructed. The calibration curve has good linearity (r2=0.9822) and sensitivity of 0.0036 A.U./mM for the linear range from 2.5mM to 100mM. Reproducible absorbance readings (r.s.d.= 1.56-6.15%, n=6) for the linear concentration range of 2.5mM to 100mM were obtained. The detection limit for Cu2+ determination method is 5.0 mM.