Surface porosity-boosted fluorescence performance of N,N′-Bis(salicylidene)-1,3-propanediamine nanoparticles for thiabendazole detection: Sensing mechanism by DFT

Cluster-assembled materials based on small organic molecules (SOMs) have attracted much attention as appropriate platforms for different functional explorations due to their large surface-to-volume ratio. However, the self-association propensity of SOMs induces increased surface energy, surface passivation, and altered surface tension, limiting their large-scale applications. Suitable structural modification would be attempting this issue to achieve the highest level of surface stability. To this end, we fabricate SOMs-based fluorescent organic nanoparticles (ONPs) using N,N′-Bis(salicylidene)-1,3-propanediamine (BSPD); its surface stability and porosity were modified by introducing Na⁺ ions in BSPD-ONPs. Results from FTIR, UV–vis, Raman, XRD, and SEM show that there is enhanced crystalline behavior observed with the presence of Na; the micrographs obtained from atomic force microscopy (AFM) before and after introducing Na showed an improved surface morphology of BSPD-ONPs with highly ordered granulated particles with a size of 25 nm. Which is observed as, with the involvement of Na⁺ ions, the roughness of the particles reduced from 258 nm to 33 nm, and the average density of 0.560/μm increased to 1.136/μm², indicating that the increased compactness of the particles and the improvement of the photophysical character. These surface-modified ONPs acted as a high-performance turn-on fluorescence sensor towards thiabendazole (TBZ) detection (detection limit of 6.4 × 10⁻⁴ μM) in aqueous solution. The sensing mechanism of TBZ by BSPD-ONPs/Na was proposed by analyzing molecular energies involved in host-guest interaction through DFT, showing that the TBZ-recognition system works by a metal ion-induced hyperconjugation effect between the localization of σ electrons of the imine group in BSPD and sp² nitrogen at the imidazole and thiazole rings of TBZ, which resulted in an elevated fluorescence emission.