BPEI-Based N-Doped Carbon Dots with Sensitive and Selective Cu²⁺ Ion-Sensing Ability

In this research, we examined the potential sensor characteristics of branched polyethyleneimine (BPEI)-derived carbon dots (CDs) synthesized using BPEI as a nitrogen source and citric acid (CA) as a carbon source, specifically for the recognition of various metal ions. Among the BPEI CDs produced with different amounts of BPEI to CA BPEI:CA ratios of 0.5:1, 1:1, and 2:1 w/w, named as BPEI^0.5 CD, BPEI¹ CD, and BPEI² CD, respectively. The BPEI^0.5 CD, which contains the least BPEI, exhibited the highest fluorescence intensity: 50,300 a.u. in a 0.6 mg/mL solution were recorded as λ_em: 420 nm at λ_ex: 360 nm and 600 V PMT voltage with 5 nm of slit width for both excitation and emission. We investigated the fluorescence variations in BPEI CD-based CDs in 2 mL solutions containing Cd²⁺, Co²⁺, Cu²⁺, Ni²⁺, and Pb²⁺ metal ions at various concentrations. Amongst these metal ions, the most pronounced sensitivity was noted for Cu²⁺ ions with a limit of detection (LOD) value of 0.39 ppm. For BPEI CDs created with BPEI:CA ratios of 0.5:1, 1:1, and 2:1 w/w, the sensitivity to Cu²⁺ ions increased with a higher BPEI ratio, with a LOD value of 0.30 ppm recorded for BPEI² CDs. Moreover, Cu²⁺ ion solutions were prepared from various salts, including chloride, acetate, nitrate, and sulfate; aside from some fluorescence variation observed for BPEI^0.5 CDs, no significant difference in BPEI CD fluorescence change was observed with the use of the various salt solutions of Cu²⁺ ion. In quenching experiments conducted on mixtures of Cd²⁺, Co²⁺, Cu²⁺, Ni²⁺, and Pb²⁺ metal ions with Cu²⁺, it was noted that BPEI CDs displayed selectivity for Cu²⁺ ions. Furthermore, the structures of BPEI CDs have been effectively utilized in real water samples, such as tap water and seawater, demonstrating a quenching capability of over 65% in the presence of 50 ppm Cu²⁺ ions.