Fluorescent Sensors Based On Metal Nanoclusters And Carbon Dots: Preparation, Combination And Sensing Application

The present work is concerned with fluorescent metal nanoclusters and carbon dots. We concentrate on the design and synthesis of a series of fluorescent materials including metal nanoclusters and carbon dots via the synthetic stratagies of biomineralization and pyrolysis reaction. The structures, spectral characteristics, chemical stability and analytical performances of these fluorescent materials were systematically investigated. Furthermore, combining the surface functionalization and design principle of optical sensors with the properties of fluorescent nanomaterials, we aimed to fabricate sensitive and selective sensing platforms for the detection of several small organic molecules and heavy metal ions in food and environment, facilitating their applications in biological/chemical and environmental systems. The main works are summarized as follows.（1） Sensing of kojic acid based on fluorescent copper nanoclusters（CuNC）: We utilized bovine serum albumin-capped copper nanoclusters to develop a new fluorometric method for the identification and determination of kojic acid（KA） with good sensitivity and selectivity. The fluorescence of CuNC was quenched effectively by KA, but not by the possible coexisting substances. Based on multiple spectroscopic studies, we propose that selective binding of KA to copper ions of CuNC promotes the formation of copper kojate on the surface of CuNCs, which statically quenches CuNC fluorescence.（2） Sensing of phytic acid based on lysozyme stabilized gold nanoclusters（Lys-AuNC） with fluorescence and resonance light scattering（RLS） detection modes: Lys-AuNC were employed as a dual-mode sensor for the detection of phytic acid. We focused on the changes in fluorescence and RLS spectra of Lys-AuNC, and further investigated the effects of phytic acid on the changes in size distribution and morphology of Lys-AuNC. Various measurements revealed that the intruduction of phytic acid could induce the formation of large aggregates containing numbers of dispersed Lys-AuNC, and thus lead to the fluorescence quenching and significant enhancement of RLS intensity.（3） ―Off-on‖ fluorescent sensing of phytic acid based on carbon dots（CDs）: Fluorescent carbon dots（CDs） were synthesized via a one-step pyrolysis method using citric acid as the carbon source and lysine as the surface passivation reagent. The fluorescence of the CDs was found to be effectively quenched by ferric（Fe（III）） ions with high selectivity via a photo-induced electron transfer（PET） process, arising from the complexation of Fe（III） ions with carboxyl and aromatic hydroxyl groups on the surface of the CDs. Upon addition of phytic acid to the CDs/Fe（III） complex dispersion, the fluorescence of the CDs was significantly recovered depending on the concentration of phytic acid. According to the fluorescence lifetimes, zeta potentials and resonance light scattering spectra, we expect that the introduction of phytic acid results in the release of Fe（III） ions from the CDs/Fe（III） complex because PA has a higher affinity for Fe（III） ions compared to CDs. Furthermore, we developed an ―off-on‖ fluorescence assay method for the detection of phytic acid using CDs/Fe（III） as a fluorescent probe.（4） Dual emissisive ratiometric fluorescent sensing platforms: A dual emissive CDs@SiO2/AuNCs hybrid sphere was synthesized by covalently linking the separately synthesized highly fluorescent bovine serum albumin stabilized gold nanoclusters onto the surface of the amino functionalized CDs@SiO2 spheres by using the EDC chemistry. The synthesized dual emissive hybrid spheres were further characterized and developed as ratiometric fluorescence probe for the determination of Hg²⁺ with high selectivity. Furthermore, combining the merits of dual emissive fluorescence analysis and molecular imprinting technique, molecular imprinted layer-coated dual emissive nanohybrid spheres were fabricated and used for nitrophenol sensing. The hybrid structure possessed the selectivity of molecular imprinting technique and the sensitivity of fluorescence analysis as well as well-defined morphology.