Anomalous ingestion of copper has significant adverse effects and shows acute toxicity in living organisms. Recently, photoelectrochemical (PEC) method has attracted much attention as a platform for a Cu2+ ion sensor because of its high sensitivity, selectivity, low-cost, and accurate selection compared to other conventional methods. In this work, stepwise hydrothermal and in situ chemical approaches for synthesizing cadmium sulfide nanoparticles (CdS NPs) for decorating gold quantum dots (Au QDs) are presented, along with notable PEC performance. The amount of Au QDs loaded on the CdS NPs had a significant influence on the PEC performance. CdS NPs-Au QDs-2 with 1.0 mmol % Au QDs demonstrated an exceptional photocurrent density of 350.6 μA cm–2, which was 3.7-, 2.2-, and 2.0-fold higher than those of CdS NPs, CdS NPs-Au QDs-1 (0.75 mmol %), and CdS NPs-Au QDs-3 (1.25 mmol %), respectively. Femtosecond transient absorption dynamics of the ground state recovery showed a buildup time of 243 fs for Au and 268 fs for CdS, which were assigned to cooling of the photoexcited electrons. For CdS NPs-Au QDs, the transient spectrum was dominated by a signal from CdS with no contribution from Au. The fast buildup dynamic was absent in CdS-Au, indicating a rapid transfer of the photoexcited electrons from CdS to Au before cooling down. Unquestionably, the CdS NPs-Au QDs-2 photoelectrode response upon Cu2+ detection showed the lowest limit of detection of 6.73 nM in a linear range of 0.5–120 nM. The selectivity of CdS NPs-Au QDs-2 toward Cu2+ ions in lake and tap water was also studied, which suggested that CdS NPs-Au QDs-2 is promising as a photoactive material for PEC-based environmental monitoring and analysis.