Constructions And Applications Of Quantum Dots Based Fluorescent Probes And Sensors

Quantum dots?QDs?are a class of biological photonics nanomaterials with unique optical properties such as high fluorescence quantum yield,resistance to photo-bleaching,broad-band absorption,narrow-band and tunable emission,as well as electronic properties of proning to charge/energy transfer,and chemical property of easy modification.Therefore,theQDs-based fluorescent sensing systems,with the advantages of design flexibility,high sensitivity and specificity,and cost-effectiveness,have been widely used for the development of biological analysis and disease diagnosis methods.The first-generation "always-on" and the second generation "off-on" fluorescent sensing systems based onQDs are single-emission intensity-based systems,which are vunarable of the interference from environmental factors and susceptible to false positive results.TheQDs-based ratiometric fluorescent sensing systems,allowing the measurement of changes in the ratio of the fluorescence intensities at two or more well-resolved wavelengths,have offered the new-generation sensing systems,pocessing the advantages of improved accuracy and reliablity as well as good visualization capability.As such,theQDs-based ratiometric fluorescent sensing systems are appealing for analytical application in complex biological environments.In this thesis,we developed dual-colorQDs-based ratiometic fluorescent probes/sensors with diverse nanostructures,and sysmatically studied their signal transduction mechanisms.The developed probes/sensors were qualified to be used for highly sensitive,visual detection of nuleic acids/metal ions.Additionally,one of the ratiometic fluorescent probes/sensors was developed for logic operations as a keypad lock.The main contents are as follows:1.A quantum dot-ligand?QD-L?ratiometric fluorescent probe is developed for visual,selective detection of double-stranded nucleic acid?ds DNA?.This probe comprises of a dual-emissionQDs nanohybrid with a "core-satelite" structure formed by electrostatically assembling rQD@SiO₂ and gQD,as the signal displaying unit,and a ligand?L?,dipyrido[3,2-a:2',3'-c]phenazine?dppz?,attached onto the gQD surface via phase transfer,as fluorescence quencher of gQD.The specific dsDNA/dppz binding can break up theQDL system,displacing the ligand from the gQD surface and restoring the gQD emission.Since the red emission of rQD@SiO₂ stays constant,the probe is successfully used for quantification of ds DNA with the ratiometric fluorescence.Furthermore,with the complementary DNA?c DNA?as a template,this probe also is capable of detecting single nucleotide polymorphism?SNP?of rs1801133 gene fragment.Importantly,depending on the amount of ds DNA/SNP,the probe displays continuous color change from orange to green,realizing high-contrast visual detection.2.A ratiometric fluorescent nanosensor for visual detection of Pb²⁺ and pH is constructed,using graphene oxide?GO?as substrate,silica-encapsulated green-emissionQD?gQD@SiO₂?and aptamer-modified red-emissionQD?apt-rQD?as dual fluorophores.The sensor featuring a "sesame biscuit"-like structure is controllably assembled via adsorbing gQD@SiO₂ onto the edge of GO by ionic interaction,followed by absorbing apt-rQD onto the GO surface via the ?-stacking interaction.The nanosensor shows the characteristic of the nonquenched green fluorescence due to the silica encapsulation and quenched red fluorescence owing to nanomaterials surface energy transfer?NSET?.The nanosensor responds to Pb²⁺/pH in ratiometric fluorescence: the red fluorescence varied upon analytedriven "G4—single strand" conformational changes of the aptamer,whereas the green one remained constant.Under optimized conditions,the nanosensor is demonstrated to be capable of quantifying Pb²⁺ with a detection limit of 11.7 p M,as well as pH with a sensing resolution of 0.1 pH unit.More importantly,the ratiometric fluorescent sensor facilitates visualization of analytes in a distinct "traffic light" manner,which is exemplified by semiquantification of exogenous Pb²⁺ in living cells.To further demonstrate practicality,fluorescent test strips are fabricated by immobilizing the nanosensor on paper.The test strips display "traffic light"-type fluorescence-color changes,with the capacity for on site,nakedeye detection of Pb²⁺ in real samples,as well as point-of-care pH testing in routine urinalysis.3.A dual-color quantum dots?QDs?-based binary fluorescence resonance energy transfer?Bi FRET?nanosensor is controllably assembled,with positively charged dual-colorQDs as dualdonors,negatively charged poly?d A?or ds DNA as the spacer,and meso-tetra?4-sulfonatophenyl?porphine dihydrochloride?TSPP?as the acceptor.Due to the spectral overlaps between emissions of dual-colorQDs and absoption bands of TSPP,Bi FRET?FRET-1 and FRET-2?occurs.The biopolymer coating minimizes the interfacial thickness to be ?2 nm,which results in improved efficiencies of FRET-1 and FRET-2.On the basis of Zn???chelation-induced spectral modulation,efficiencies of FRET-1 and FRET-2 are oppositely tuned,realizing the ratiometric detection of Zn???with a limit of detection of 1 nM.The nanosensor either in solution or on a paper substrate displays continuous color changes from yellow to bright green toward Zn???,exhibiting excellent visualization capability.Furthermore,the nanosensor is successfully used to semi-quantitatively visualize exogenous Zn???in living cells.4.By conjugating TSPP to "core-satellite" dual-color silica-encpsulatedQDs nanohybrid,a superstructured nanodevice with the "gQD@SiO₂/bQD@SiO₂-TSPP" configuration is constructed and developed for the kepad lock logic operation with ratiometric fluorescence as the output.The operation mechanism of the nanodevice relies on the modulations of bQD?donor?-TSPP?acceptor?FRET: the inputs in serial can cause continuous structural changes of the acceptorfrom Zn???TSPP to TSPP,and then to Fe???TSPP or vice versa,inducing the wavelength shift of Soret band absorption of Zn???TSPP,and consequently the changes in its overlap with the donor emission.For the unlock operation,N,N,N',N'-tetrakis?2-pyridylmethyl?ethylenediamine?TPEN,Zn???chelator?,Fe???/Fe???,and UV light are s inputed in serial,and the nanodevice outputs blue emission.For the lock operation,the sequential inputs are ascorbic acid?Fe???chelator?,Zn???and UV light,and green emission is the output.By means of centrifugation,the nanodevice-based keypad lock operations reveal high resettability.Furthermore,paper keypad lock and cell keypad lock are developed by immobilizing the logic device on paper substrate or incubating with living cells,respectively.