Contruction Of Several Upconversion Fluorescence Nanosensors And Their Applications

In recent years, upconversion nanoparticles(UCNPs) doped with certain rare earth ions have attracted considerable attention compared with the traditional organic fluorophores. UCNPs as fluorescent biolabels, possess several unique qualities, such as attractive optical and chemical features, low cytotoxicity, large stokes shifts, greater tissue penetration, high resistance to photobleaching, blinking and photochemical degradation. These excellent fluorescence properties make the UCNPs widely employed in biolabeling and biosensing. More importantly, compared with organic fluorophores and QDs, the excitation light of UCNPs(NIR light) can not be absorbed by biological samples, which could eliminate the autofluorescence by biological samples, thus providing an enlarged signal-to-background ratio and improved sensitivitie. In this work, upconversion fluorescent materials have been prepared by hydrothermal synthesis process based on the previous researches. Some sensors have been built using these materials for ultrasensitive detection with inexpensive, label-free, simplicity, good selectivity, high stability and capability of anti-interference. The main contents are summarized as follows:1. A novel label-free fluorescence nanosensor was developed for ultrasensitive detection of protamine and heparin based on fluorescence resonance energy transfer(FRET) between NaYF4:Yb,Er upconversion nanoparticles(UCNPs) and gold nanoparticles(AuNPs). The FRET system was formed by the electrostatic adsorption of AuNPs on the surface of UCNPs and the fluorescence of UCNPs was significantly quenched. When protamine was added to the mixture of UCNPs-AuNPs, the AuNPs interacted with protamine and then desorbed from the surface of UCNPs and aggregated, which result in the recovery of the fluorescence of UCNPs. Upon addition of both protamine and heparin, the FRET system formed owing to the stronger interaction between heparin and protamine than that with AuNPs, which leads to a marked fluorescence quenching of UCNPs. The concentration of protamine and heparin were proportional to the changes of the fluorescence of UCNPs. The low detection limit of 6.7 ng/mL and 0.7 ng/mL for protamine and heparin, respectively. Simultaneously, the measurement of protamine and heparin in serum can be achieved, suggesting the nanosensor can be used in a complex biological sample matrix.2. A novel label-free upconversion fluorescence resonance energy transfer(UC-FRET) biosensor for ultrasensitive detection of organophosphorus pesticides based on the electrostatic interaction between NaYF4:Yb,Er upconversion nanoparticles(UCNPs) donor and gold nanoparticles(AuNPs) acceptor was developed. The fluorescence of UCNPs can be significantly quenched by the AuNPs attached to the surface of UCNPs through electrostatic interaction. The detection mechanism is based on the facts that AuNPs quench the fluorescence of UCNPs and organophosphorus pesticides(OPs) inhibit the activity of acetylcholinesterase(AChE) which catalyzes the hydrolysis of acetylthiocholine(ATC) into thiocholine. Under the optimized conditions, the logarithm of the pesticides concentration was proportional to the inhibition efficiency. The detection limits of parathion-methyl, monocrotophos and dimethoate reached 0.67, 23, and 67 ng/L, respectively. Meanwhile, the biosensor shows good sensitivity, stability, and could be successfully applied to detection of OPs in real food samples, suggesting the biosensor has potentially extensive application clinic diagnoses assays.3. A simple fluorescence method for detection of uric acid(UA) based on NaYF4:Yb3+, Tm3+ upconversion nanoparticles(UCNPs) was developed. The proposed method is based on the fact that uricase could oxidize uric acid to allaintoin and hydrogen peroxide which could oxidize O-phenylenediamine(OPD) to the oxidized OPD(oxOPD). The fluorescence of UCNPs can be significantly quenched by oxOPD through inner filter effects(IFE). Under the optimized conditions, the UA concentration was proportional to the changes of the fluorescence intensity of UCNPs. The low detection limit of 6.7 μΜ for uric acid. More importantly, this method has potential in practical application to detect uric acid in human serum, suggesting the nanosensor can be used in a complex biological sample matrix.