Fluorescence resonance energy transfer fluorescent polymer dots without conventional chromophores: Synthesis, emission mechanism and applications as Cu2+ probe and fluorescent ink

Recently, the non-aromatic fluorescent materials without benzene rings in their structures have aroused widespread interest. In this work, a citric acid functionalized polyethyleneimine oligomer, CA-PEI, was synthesized through acyl chloride method. In CA-PEI, the polyethyleneimine was bonded to citric acid molecule with a molar ratio of 1:1 through an amide bond, which was confirmed by NMR, FTIR, GPC and XPS. The results indicated that CA-PEI had dual excitation and dual emission properties, and the two emission centers were derived from n-sigma* and n-pi* transition, respectively. CA-PEI was used to synthesize the light-emitting polymer dots N-PDs through a hydrothermal reaction. Interestingly, compared with CA-PEI, N-PDs had dual excitation and single emission properties, and its Stoke's shift was as high as 193 nm. This can be attributed to the compact structure of N-PDs, which promoted the fluorescence resonance energy transfer (FRET) process. Importantly, N-PDs can serve as excellent probes in response to Cu2+, with a detection limit as low as 2.5 nmol L-1. The response mechanism involves the formation of a copper amine complex between Cu2+ and the PEI part of N-PDs, which then quenches the fluorescence emission of N-PDs through the fluorescence inner-filter effect. N-PDs as well were applied as an encryptable fluorescent ink.