New Biosensing Research Based On Graphene-type Nanomaterials For The Detection Of PH And Biological Thiols

With the rapid development of modern science and technology,biosensor has become a new hot area of research in the 21 st century.Biosensor is a multidisciplinary crossing analysis device with the advantages of high selectivity,high sensitivity,fast response,low cost and strong anti-interference ability.At present,biosensors have widely been applied to medical diagnosis,food engineering,microbiological detection and drug screening.In addition,with the development of nanotechnology,nanomaterials have aroused great interest due to their unique optical,electrical,and mechanical properties,which have brought new opportunities for the development of biosensors.Compared with the traditional biosensor,the novel biosensor based on nanomaterials has higher sensitivity,selectivity,and response rate.In recent years,carbon nanotubes,graphene,gold nanoparticles,quantum dots,DNA nanostructures,and nanocomposites have had a significant impact on the design and improvement of biosensors,which have showed great potential in the field of biosensing.Based on graphene-type nanomaterials,gold nanoparticles and functional nucleic acid molecular probes,we developed novel optical biosensors for the detection of pH and biological thiols that realized the analysis of actual samples and living cell imaging.The research contents are summarized as follows:In biological systems,pH value plays an essential role in protein folding,ion transport,enzyme catalysis,proliferation and apoptosis.In Chapter 2,we developed a simple programmable pH-responsive DNA sensor,by combining several triplex DNA nanoswitches with different pH dependencies,based on grahene oxide(GO)-mediated adsorption of double and single stranded fluorescence DNA probes and selective quenching of organic fluorophore.In this sensing system,we designed two fluorophore-labled DNA probes,each containing a certain content of TAT(the T-A-T base pairing in the parallel triplets)so as to undergo a conformation change under different pH values.Under acidic conditions the DNA probes were able to fold into triplex DNA structures,which would not be absorbed on GO nanosheets.Thus,the probe retained a strong fluorescence signal.By contrast,under alkaline conditions the probes have a single stranded tail,which allows them to be absorbed on the GO with substantially quenched fluorescence.By mixing these two probes in 1:1 molar ratio,we obtained a wider response ranges in the pH range of 5.0 to 8.0,which covers the pH ranges in varying biological systems with high signal-to-background ratio and fast response.Moreover,the unique system could be further extended to combining more DNA probes for monitoring pH values in even wider ranges according to demands of specific applications.Biological thiols including cysteine(Cys),homocysteine(Hcy),and glutathione(GSH)play crucial roles in various physiological processes in living systems,such as posttranslational modifications,biocatalysis,metal binding and xenobiotic detoxification.In Chapter 3,as graphene quantum dots(GQDs)serve as both reducing agent and stabilizer,we synthesize the nanohybrids of gold nanoparticle and GQDs(termed as AuNPs@GQDs)by in situ growing AuNPs on the surface of GQDs without adding any reducing agent and surfactant for sensing biological thiols(biothiols)in living cells.The fluorescence of GQDs in this nanohybrids can be efficiently quenched due to surface energy transfer.When biothiols is added in the system,the quenched fluorescence of GQDs in this nanohybrids can be significantly restored due to the stronger affinity of biothiols to AuNPs than GQDs,and thus the weaker affinity of GQDs to AuNPs will result in the dissociation of GQDs from surface of AuNPs.In the meantime,the colour of solution gradually changes from purplish red to colourless.This strategy realized ultrasensitive detection of bithiols with colorimetric and fluorometric readout.Furthermore,the nanosensor was used to monitor the level of biothiols in living cells.The nanohybrids have shown the great potential application for monitoring the change of biothiols in living cells.