Contruction Of NaYF₄ Upconversion Fluorescence Nanosensor And Their Applications

Owing to the unique photo-physical properties, rare-earth ions doped upconversion nanoparticles（UCNPs） have attracted extensive attention in the recent few years. Compared with conventional fluorescent labels, the unique luminescence mechanism of UCNPs possess several advantages such as improved detection sensitivity owing to no autofluorescence, deeper NIR light penetration into biological causing minimum photodamage to living organisms, good chemical and physical stability, and low toxicitywhich endow their potential applications in bio-medical field, biological luminescent labels and drug delivery carriers. In this work, we prepared upconversion fluorescent materials and built three biosensors for ultrasensitive detection of biomolecule. The main work is as follows: 1. A nanosensorbased on fluorescence resonance energy transfer（FRET） between upconversion nanoparticles（UCNPs） and gold nanoparticles（AuNPs） was developedfor melamine detection. The positively charged UCNPs as donor and the negatively charged AuNPs as acceptor bound together via electrostatic interaction, which caused the fluorescence quenching of UCNPs. Upon addition of melamine, Au NPs were released from the surface of UCNPs and aggregation due to the N-Au interaction between melamine and AuNPs, which results in the fluorescence of UCNPs gradually recovered. Under the optimalconditions including media pH（7.0）, the concentration of AuNPs（1.23 nM） and incubation time（12 min）, the fluorescence enhanced efficiency shows a linear response to the melamine concentration ranging from 32 to 500 nM with a detection limit of 18 nM. Compared with other fluorescence methods, the fluorimetric nanosensor shows high sensitivity of 0.9677, ease of operation and can be used for the determination of melamine in raw milk samples. 2. A novel nanosensor for tyrosine based on photoinduced electron-transfer（PET） between NaYF4:Yb, Tm upconversion nanoparticles（UCNPs） and melanin-like polymers. Melanin-like films were obtained from catalytic oxidation of tyrosine by tyrosinase, and deposited on the surface of UCNPs, and then quenched the fluorescence of UCNPs. Under the optimized conditions, the fluorescence quenching of UCNPs showed a good linear response to tyrosine concentration in the range of 0.8- 100 mM with a detection limit of 1.1 mM. Meanwhile, it shows good sensitivity, stability and is successfully applied to the detection of tyrosine in real serum. 3. A nanosensorbased on fluorescence resonance energy transfer（FRET） between upconversion nanoparticles（UCNPs） and gold nanoparticles（AuNPs） was developedfor H₂O₂ and L-lactate detection. Poly（vinylpyrrolidone）（PVP） acted as a mild reductant for the formation of Au seeds from HAuCl4, and it served also as acapping agent for bothAu seeds. When lactalase/ L-lactate or H₂O₂ was added, Au seeds were enlarged into AuNPs. Thenthe fluorescence of UCNPs was quenched by AuNPs through FRET. The biosenosr shows a good linear response toL-lactate and H₂O₂ concentration with a detection limit of 0.65 mM and 2.67 mM, respectively. It is successfully applied to the detection of L-lactate in real serum.