Polymer nanoparticles integrated with ESIPT modules for sensing cysteine based on modulation of their tautomeric emission

Abstract

Excited-state intramolecular proton transfer (ESIPT)-exhibiting polymer nanoparticles (e-PNPs) were prepared using 2,6-diformyl-4-methylphenol and guanidine hydrochloride. The e-PNPs bear abundant aromatic o-OH, CN and NH₂ groups which are responsible for ESIPT modules. By incorporating ESIPT units into nanostructures, the ESIPT moieties can achieve an appropriate hydrophobic environment for the ESIPT reaction, enabling e-PNPs to show strong ESIPT-fluorescence. Upon the addition of Cu(II), the tautomeric fluorescence (t-FL) of e-PNPs is significantly quenched because of the coordination of Cu(II) with e-PNPs. The presence of cysteine (Cys) can elicit the t-FL recovery of e-PNPs–Cu(II) complexes due to the chelation interaction between Cu²⁺ and the sulfhydryl and amino groups of Cys. Based on this signalling mechanism, the e-PNPs–Cu(II) sensing system was used to detect Cys, and the linear range and limit of detection of the nanosensing system were 0–6.3 μM and 1.8 nM, respectively. As expected, this method was successfully applied to the determination of Cys in rabbit serum and plasma samples with satisfactory recovery ranging from 97.4 to 105.6%. Moreover, e-PNPs–Cu(II) based nanoprobes were employed for imaging Cys in live cells, which demonstrated their potential values for bioanalytical applications.