Novel Biosensor Method Based On MnO₂-Nanosheet-Modified Upconversion Nanparticles

Upconversion nanparticles ?UCNPs? emerged as novel biolabelling probes. Compared with conventional fluorescent labels, such as organic dyes or quantum dots, UCNPs can convert near infrared light into visible light through a multi-photon mechanism, with low energy light, low background, high and stable luminous intensity, emission wavelength can be adjusted by controlling the composition and so on, this advantages make it has been widely used in biology detection. Now, UCNPs is used for biological markers and biological detection mainly through constructing fluorescence resonance energy transfer ?FRET? system. At this FRET system, UCNPs is energy donor and energy receptor mainly includes organic dyes, carbon nanoparticles,gold nanoparticles,grapheme,poly amine polymer.But the synthesis of this FRET system is complex and some of energy receptors are toxicity, which limits the biological applications of UCNPs. To settle this problem, we choose MnO₂ nanosheets as the energy receptor to formation of MnO₂nanosheets modified upconversion nanoparticle assemblies, which dominated by electrostatic interactions. The synthesis of this FRET system is simple and nontoxic. Based on this, the three parts of paper about MnO₂ nanosheets modified upconversion nanoparticle assemblies are as follows:1. MnO₂ nanosheets modified upconversion nanoparticle for sensitive turn-on fluorescence detection of H2O2 and glucose in blood. The principle was as follows: Firstly, we get hydrophobic NaYF4:Yb,Tm/NaYF4 UCNPs by solvothermal method. Secondly, we prepare hydrophilic nanoparticle by ligand oxidation. Then we prepare MnO₂ nanosheets modified upconversion nanoparticle assemblies dominated by electrostatic interactions. Because MnO₂ nanosheets have a broad absorption band from 250 to 500 nm, which leads to significant spectral overlap between the blue emission of UCNPs and the absorption of MnO₂, the upconverted luminescence of UCNPs can be effectively quenched by MnO₂ nanosheets coated on the surface of UCNPs. This MnO₂-induced quenching effect can be reversed by adding a small amount of H2O2. It would be the result of the H2O2-mediated reduction of MnO₂ to Mn2+, leading to the decomposition of the MnO₂ nanosheets accompanied by the upconverted luminescence recovery. The fluorescence intensity at 450 nm was linearly correlated with the H2O2 concentration in the range of 5-150?M and 180-350?M, respectively, and the detection limit was 0.9 ?M. Moreover, because glucose can be oxidized by oxygen ?O2? to produce H2O2 in the presence of glucose oxidase ?GOx?, a sensitive and cost-effective upconversion fluorescent assay can be achieved for the monitoring of blood glucose through detecting the enzymatically generated H2O2. The fluorescence intensity at 450 nm was linearly related to the glucose concentration in the range of 5-230?M and 250-400?M, respectively, and the detection limit was 3.7 ?M. The applicability of this MnO₂-modified UCNPs nanosystem for blood glucose monitoring in real samples with high selectivity.2. MnO₂ nanosheets modified upconversion nanoparticle for sensitive fluorescence detection of organophosphate. The principle was as follows:the quenched fluorescence of MnO₂ nanosheets modified upconversion nanoparticle assemblies can be restored after treated with thiocholine?TCh?, the catalytic hydrolysis products of acetylthiocholine?ATCh? by AChE. It would be the result of the decomposition of the MnO₂ reduced to Mn2+ by TCh. In the presence of OPs, the activity of AChE is inhibited by OPs, which preventing the generation of TCh and the FRET system is still existed, thus the fluorescence of UCNPs is still quenched. The effective paraoxon and malathion assay could be achieved by measuring the fluorescence intensity of UCNPs at 450 nm. The detection limit were 0.0199 ng ml/1 and 0.072 ?g ml/1 respectively.3. MnO₂ nanosheets modified upconversion nanoparticle for rapid turn-on fluorescence detection of ascorbic acid. The principle was as follows:the quenched fluorescence of MnO₂ nanosheets modified upconversion nanoparticle assemblies can be restored after treated with ascorbic acid. It would be the result of the ascorbic acid reduction of MnO₂ to Mn2+, leading to the decomposition of the MnO₂ nanosheets accompanied by the upconverted luminescence recovery. The fluorescence intensity at 450 nm was linearly related to the ascorbic acid concentration in the range of 100-300 ?M, and the detection limit was 95.69 ?M.