Two-Dimensional Nanomaterials For Biosensing And Photosensing

Two-dimensional nanomaterials with atomic thickness are emerging recently as their large surface-to-mass ratio and their unique physical properties,where the graphene and molybdenum disulfide(MoS2),as the representatives,have been intensively explored for the electronics,photoelectronics and biosensing.Because of high surface-to-volume ratio and high carrier mobility of graphene,the graphene field-effect transistor(GFET)functionalized with the aptamer,a single-strand DNA with high specificity to the target molecule,is well suited for label-free and sensitive detection of biological objects.However,existing GFET nanosensors are mostly limited to the well-conditioned buffer or pretreated or diluted physiologic fluids to avoid the nonspecific adsorption of molecule onto graphene surface,which could cause major difficulties in quantitative measurement of target molecules.Meanwhile,although GFET hold great potential of the GFET nanosnesors for binding kinetics,the effect of environment conditions such as salt-ionic strength and temperature have not investigated yet.In addition,direct detection of low-molecular-weight and low-charge molecules has remained a challenge for GFET nanosensors.This paper aims to address the above issues by including the following contents:(1)Biomarker detection in human serum.Graphene surface is sequentially functionalized with Polyethylene glycol(PEG)and aptamer,as a results,the aptamer specifically recognizes target molecules while the PEG effectively reject the nonspecific adsorption of background molecules in serum onto the graphene surface.Besides,PEG polymer increases the effective Debye length in vicinity of graphene surface,facilitating the biomarker detection.These characteristics yielded from the surface functionalization scheme enable graphene nanosneosrs for biomarker detection in physiological fluids.(2)Binding kinetics characterization for aptamer-biomarker interactions.The binding kinetics of aptmaer and target molecules as well as its dependence on temperature and the salt ions of Na+and Mg2+are investigated with GFET nanosensors.Kinetic and thermodynamic parameters at different conditons are quantified,where the underlying mechanisms are studied.(3)Detection of low-molecular-weight and low-charge molecules enabled by a long-chain DNA aptamer.When binding to target molecules,the long-chain aptamer experiences a large change in the conformational structure.As the DNA strand is heavily negative-charged,such a conformational structure change causes appreciable variation in conductance of graphene.As such,the direct dection of target molecules can be achived with the proposed long-chain aptamer.In contrast to the zero-band graphene,transition metal dichalcogenides(TMDs)molybdenum disulfide(MoS2)has a favorable bandgap that allows for strong light-matter interactions.Particularly,the direct bandgap of monolayer MoS2 exhibit strong photoluminescence as a result of emission of photogenerated electron-hole pairs.The high adsorption coefficient also features high photocurrent efficiency.These characteristics can be explored for both biosensing and photosensing purposes including the following contents:(1)Graphene/MoS2 heterostructure for multimodal biosensing.The graphene/MoS]heterostructure integrates different sensing modalities,which for the first time is exploered for the photoelectrochemical multimodal sensing of biomolecules in a single device.The multimodal sensing scheme offers low detection limit while significantly extends the linear sensing dynamic range.(2)Enhanced photosensing of MoS2 photodetector via defect healing enabled by mild oxygen plasma.As MoS2 yielded from either chemical vapor deposition(CVD)or mechanical exfoliation trypically exists sulfur vacancies.We treat MoS2 surface with mild oxygen plasma,and the adsorption of irradiated oxygen ions to vacancy sites improves the carrier mobility.Hence,the photocurrent efficiency of MoS2 photodetector is enhanced.