Evolution of large stokes shift and non-radiative energy transfer phenomenon in sustainable blue-fluorescent CQDs upon subnanomolar detection of Acebrophylline

Abstract

Carbon quantum dots (CQDs) have emerged as powerful fluorescent sensors for identifying harmful compounds in environmental and biological samples, due to their robust and adjustable emission characteristics. In this study, we explore CQDs (size ∼ 3 nm), as a probe, derived from Walnut Shell (WS) biomass waste for detecting Acebrophylline (AB), a respiratory disease medicine. From the selectivity studies, the calculated energy transfer between the CQDs (10 mM; donor) and AB (10 mM; acceptor) was found to be 64 %, attributed to the formation of a ground state complex, CQDs + AB. The CQDs demonstrated high selectivity and sensitivity to AB in concentrations between 1–100 μM with a detection limit of 0.142 nM (R² = 0.991, K_a = 1.39194 × 10⁻³ M⁻¹). Time-correlated single-photon counting (TCSPC) experiments validated the static quenching of CQDs (3.46 → 3.71 ns) when exposed to AB. The proposed detection method was successfully applied for detecting AB in human urine samples with a good recovery percentage (81 to 123 %; RSD ca. 1 %). After AB sensing, changes in the CQDs’ crystalline nature, elemental composition, and chemical state were examined using XRD, XPS, and FTIR spectroscopy. Microscopy imaging techniques (FESEM, HRTEM, and AFM) confirmed morphological changes of CQDs from spherical to agglomerated with an average diameter of approximately 14 nm. Additionally, the impact of time, pH, and interferons on AB sensing was investigated. In vitro anti-inflammatory activity and in vivo bioimaging studies on zebrafish were also performed. This study highlights several advantages, including a cost-effective and eco-friendly approach for healthcare applications.