This study presents a novel fluorescent and colorimetric dual-response sensor based on nitrogen and sulfur co-doped carbon dots (N/SCDs) for the detection of Cu²⁺ ions. The N/SCDs were synthesized using p-phenylenediamine and 2-mercaptobenzothiazole, which exhibited bright green fluorescence and good water solubility. In an aqueous system, the N/SCDs showed significant fluorescence quenching at 512 nm upon the presence of Cu²⁺, transforming from colorless to yellow. The sensor demonstrated a linear range of 5 to 400 μM with detection limits of 0.215 μM in fluorescence and 0.225 μM in colorimetric detection. The N/SCDs exhibited high selectivity towards Cu²⁺, with no significant interference from other metal ions. The sensor was also effective in monitoring Cu²⁺ in tap and lake water, with recovery rates ranging from 89.6% to 113.1%. This dual-modal sensor offers a promising method for sensitive and selective Cu²⁺ detection in real samples.This study presents a novel fluorescent and colorimetric dual-response sensor based on nitrogen and sulfur co-doped carbon dots (N/SCDs) for the detection of Cu²⁺ ions. The N/SCDs were synthesized using p-phenylenediamine and 2-mercaptobenzothiazole, which exhibited bright green fluorescence and good water solubility. In an aqueous system, the N/SCDs showed significant fluorescence quenching at 512 nm upon the presence of Cu²⁺, transforming from colorless to yellow. The sensor demonstrated a linear range of 5 to 400 μM with detection limits of 0.215 μM in fluorescence and 0.225 μM in colorimetric detection. The N/SCDs exhibited high selectivity towards Cu²⁺, with no significant interference from other metal ions. The sensor was also effective in monitoring Cu²⁺ in tap and lake water, with recovery rates ranging from 89.6% to 113.1%. This dual-modal sensor offers a promising method for sensitive and selective Cu²⁺ detection in real samples.