Morphology-transforming AuNPs-based fluorescent probe for ultra-low sensitive detection of levofloxacin in urine samples at pH 7.0 through excimer formation

Fluorescent metal nanoparticles (MNPs) have garnered considerable interest in the realm of healthcare applications. Within this domain, we present a novel fluorescent sensor: Pamoic acid (PA) functionalized excitation-dependent gold nanoparticles (PA@AuNPs), designed to ascertain levofloxacin (LF) concentrations in human urine samples under pH 7.0 conditions. The synthesis of PA@AuNPs was achieved via a chemical reduction method employing PA as both a reducing and stabilizing agent. Validation of the successful synthesis of PA@AuNPs was conducted utilizing a suite of analytical techniques including X-ray diffraction (XRD), ultraviolet–visible absorption spectroscopy (UV–Vis), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Microscopic examination via field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) elucidated the spherical morphology of PA@AuNPs, with an average diameter ranging from 10 to 15 nm. Furthermore, atomic force microscopy (AFM) revealed an average surface roughness of 20.72 nm for PA@AuNPs. Thermal analysis results divulge that PA@AuNPs are highly stable up to 370 °C. The PA@AuNPs have shown more than 10-fold higher selectivity for LF in comparison with other commercially available antibiotics. The fluorescence emission intensity of PA@AuNPs (quantum yield (ΦF) 3.36 %) has shown enhanced redshift (436 → 497 nm; ∼61 nm) (λex 353 nm) upon the sequential addition of LF (0 → 100 µM) and achieved a lower detection limit (LoD = 36 nM; LoQ = 121 nM; Ka = 1.0457 × 104 M−1) with increased ΦF 18.96 %. This radiative process and fluorescence enhancement were confirmed by time-correlated single photon counting (TCSPC) measurement (4.89 → 5.36 ns). Interestingly, upon sensing LF, PA@AuNPs changed their morphology from spherical to octahedron and displayed an excimer formation through intermolecular π-π stacking. Furthermore, the stability of the complex, PA@AuNPs•LF which formed through coordination, by restoring the fluorescence intensity of PA@AuNPs•LF (turn on) in the presence of ethylenediamine tetraacetic acid (EDTA) (turn off) is demonstrated. The formation of the PA@AuNPs•LF complex was further confirmed by XRD analysis and FTIR spectroscopy. The HRTEM and FESEM validate that PA@AuNPs•LF have octahedral morphology with an average size of 20–30 nm, and the obtained moiré fringes in TEM images are due to the overlap of LF on the surface of PA@AuNPs. The AFM analysis divulges that PA@AuNPs•LF had a larger surface roughness (Sa = 26.91 nm). The experimentally attained zeta potential of PA@AuNPs (–12.6 mV) was further decreased (–17.6 mV) in the presence of LF, which confirms the interaction between PA@AuNPs and LF. The cell viability (82 %) in L929 Fibroblast cells augmented the biological applications of PA@AuNPs. The practicability of the fluorescent sensor probe was demonstrated in human urine samples with recovery ranges from 97.56 to 104.38 %.