Unconventional fluorescence behaviour of ε-polylysine: Selective and sensitive detection of Cu2+ in real water samples, test paper strips, living cells, and anticounterfeiting applications

Herein, we report the clustering-triggered emission (CTE) behaviour of ε-PL and its various applications. The CTE properties of ε-PL in solution were confirmed from the SEM analysis, DLS data, and the observation of the Tyndall effect. We explored these unique and unconventional photophysical characteristics of ε-PL for the selective detection of Cu2+ with a LOD of as low as 0.45 ppb (7.11nM), which is much less than the safe limit set by the WHO for Cu2+ in drinking water (2 ppm). The quantum yield of ε-PL reduced from 0.24 to 0.008 upon Cu2+ addition, and the quenching was found to be dynamic in nature. Further, ε-PL was able to detect Cu2+ in real water samples (tap and lake water) as well as in living cells (MCF-7). For real-time applications, ε-PL-coated paper strips and silica gel were fabricated for rapid and convenient detection of Cu2+. The anti-counterfeiting application of ε-PL has also been demonstrated. These findings highlight an efficient method for the selective detection of Cu2+ by ε-PL, an unconventional fluorescent polymer without any chemical modifications, and its real-time applications in Cu2+ sensing and as an anti-counterfeit material. This potentiates its use in agriculture, waste management, medicine, and related fields.