Quantum Dot @Polypetide Nanogel Ratiometric Fluorescent Sensor For Cadmium Ion Detection

Quantum dots(QDs)are a class of fluorophores with unique optical properties including high brightness,high optical stability,wide-band absorption and narrow-band emission.Aptamer,DNAzyme and aptazyme belong to functional nucleic acids.Fluorescent nanobiosensors are readily constructed by modifying quantum dots with functional nucleic acids,and widely used for sensitive detection of various targets.Polypeptides are compounds formed by dehydration condensation of 10～100 amino acid molecules.Polypeptides can be self-assembled to nanogel that features three-dimensional network,adjustable porous structure and target-enrichment effect,providing a biocompatible carrier for nanosensors.Photonic crystal structure and plasma structure provide local electromagnetic enhancement substrates for nanosensors.In this thesis,we have developed a quantum dot-spherical nucleic acid nanosensor,further combining with NG nanocarrier and plasma-photonic crystal hybrid-structured substrate,for highly sensitive detection of cadmium ion(Cd2+).The main research contents are as follows:1.Controlled preparation of quantum dot-spherical nucleic acid(QD-SNA).Red-emission water-soluble quantum dot was first prepared.The QD-SNA was then prepared by coupling the QD with aptazyme(triple-helix hairpin)and its substrate(BHQ-modified hairpin DNA)via EDC/NHS reaction.The as-prepared QD-SNA showed a spherical structure,and the QD fluorescence was in a quenching state.2.Aptazyme(bipedal walker)-mediated signal amplification of QD-SNA.The aptazyme was specifically designed to have a "Zn2+DNAzyme-Cd2+aptamerZn2+DNAzyme" structure.In the absence of target Cd2+,the aptazyme was hybridized with a DNA sequence to form a triple-helix hairpin where the dual Zn2+DNAzyme’s catalytic activities were deactivated.In the presence of target ion,the aptamer favorably binds with Cd2+,driving the aptazyme separated from the QD-SNA and simultaneously the dual Zn2+DNAzyme’s catalytic activities were activated.The dual Zn2+DNAzyme can shear the neighboring substrates and self-drive the aptazyme walk in a bipedal manner,leading to the separation of BHQ from the QD-SNA and the recovery of QD fluorescence.The kinetics of bipedal walker on QD-SNA was monitored by total internal reflection inverted fluorescence microscopy(TIRF),and the walking time was determined to be 30 min.3.Controllable construction of gQD/rQD-SNA@PNG ratiometric fluorescent nanosensor and its use for sensitive Cd2+detection.With silica nanosphere as the core,the preparation of gQD/rQD-SNA@PNG nanosensor was accomplished with three steps:electrostatic adsorption of rQD-SNA on the surface of silica core,electrostatic adsorption of PC10A(an engineered polypetide)and self-assembly into a PNG shell around the silica core,and loading oil-soluble green-emission quantum dots(gQDs)into the hydrophobic zone of PNG shell by ultrasonic method.The as-prepared gQD/rQD-SNA@PNG nanosenor had a core-shell structure with a particle size of～158 nm and a surface charge of-29.4 mV,and showed dual emission.Based on the signal amplification of QD-SNA and target enrichment of PNG,the nanosensor can be used to sensitively detect Cd2+,with a detection limit(3σ)of～1 pM.Based on the dual QD emission,the nanosensor produced ratiometric fluorescence as a function of target concentration,featuring high-fidelity and visual detection.4.Construction of paper analysis device for on-site and smart phone detection of Cd2+.Using silica photonic crystal and gold nanoparticles-anchored silica photonic crystal as template,respectively,photonic crystal-structured and phonic crystal-plasma hybrid-structured substrates were prepared by nitrocellulose(NC)filling and hydrofluoric acid etching.Two kinds of PADs were,respectively,prepared by adsorbing gQD/rQD-SNA@PNG nanosensors on the above substrates.The PAD with the photonic crystal structured substrate achieved the detection limit of～0.1 pM,attributed to slow light effect of photonic crystal.The PAD with the phonic crystal-plasma hybrid-structured substrate realized the ultra-sensitive detection of Cd2+,and the detection limit was as low as～10 fM.This is attributed to the cooperation of slow light effect of photonic crystal and local surface plasmon resonance of AuNPs.Owing to the ultrahigh sensitivity,the PADs were demonstrated to be capable of on-site and smart phone detection of Cd2+in real samples(lake water,drinking water,serum).