An electrochemical biosensor for the determination of the acephate pesticide was developed based on algal modified silica coated ZnO quantum dots (QDs). The surface active alkaline phosphatase enzyme on the Chlorella sp. algal cell wall dephosphorylates substrate p-nitrophenyl phosphate (pNPP) and release p-nitrophenol (pNP), which being electroactive gets oxidized on the modified working electrode to generate current equivalent to the amount of pNP released from the substrate. The presence of acephate would inhibit the formation of pNP and hence decrease in current, which indirectly corresponds to the concentration of the pesticide present. The biosensor responds linearly in the concentration range of 10⁻¹¹ M to 10⁻³ M for acephate and showed that QDs strongly influenced the biosensor performance as they increased sensitivity and improved the detection limit.