Research On Novel Chem-/Bio-sensors Based On Quantum Dots And Graphenes And Their Applications In Environment

It is of great significance to develop new techniques and methods in realizing real-time, rapid, highly sensitive and selective analysis of pollutants in environment, which will play an important role in monitoring the implementation of water pollution control measures and further guaranteeing the drinking water security. Generally, the traditional methods, such as instrumental analysis, are expensive, time-consuming, complicated and necessary for technical skills, which limit their further applications. The development of nanomaterials and nanotechnology with their unique advantages and performance opens new opportunities for the detection of pollutants in water. Among these, quantum dots and graphene-based materials have been widely used to develop the functional nano-devices due to their unique optical, electrical and structural properties. However, these sensing methods still suffer from low sensitivity and poor selectivity originated from the defects in the sensing mechanism. In order to solve the above issues, we have explored the novel methods and principles in nanosensors, and designed several new chem-/bio-sensors sensors based on quantum dots and graphenes for rapid, real-time, highly sensitive and selective detection of environmental heavy metal ions, toxic compounds and biologically important molecules. Several works are as follows:(1) A simple photoillumination procedure was developed to produce thiol-capped CdTe quantum dots with high quantum yield (60%). Based on it, a novel layer-by-layer self-assembly technology was developed to immobilize the quantum dots on ITO substrate. It was found that the solid-phase of quantum dot allowed sensitive and selective determination of Cu(Ⅱ) ions. Furthermore, we realized the "on-off-on" detection of Cu(Ⅱ) ions at low concentration levels (0.02-1.0μM). It was observed that the quenched fluorescence could recover to more than95%after four cycles, which overcame the shortcomings of the non-renewable of colloidal quantum dots in the ion detection, thus possessing important application values compared with that in traditional solution assays; A novel ratiometric fluorescent sensor was developed based on inorganic luminescent CdTe nanorods and organic calcein blue through the cation-exchange reaction, which realized the ultrasensitive detection of Cu(Ⅱ) ions with a detection limit of0.13nM. The sensitivity of the ratiometric sensor was enormously enhanced60-fold in comparison with the traditional quantum dot-based sensor. Furthermore, this method can be applied to design other fluorescent sensors for a wide range of metal ions. (2) A novel graphene/CdTe quantum dot-based fluoroimmunoassay was developed by taking advantage of the unique two-dimensional structure and the excellent fluorescence quenching efficiency of grapheme, which realized the highly sensitive detection of the biomarker-a fetoprotein. The detection limit was determined to be0.15ng/mL, which was much lower than that of previously reported quantum dot-based fluoroimmunoassay; A novel graphene-based homogenous fluorescence-based immunoassay was developed for rapid and sensitive detection of MC-LR in water samples. The whole assay time was less than35min, and the detection limit was calculated to be0.14μg/L; A novel fluorescent aptasensor was developed for sensitive and sensitive detection of OTC in environmental samples based on RCA technique and graphene-based molecular beacon. The detection limit was calculated to be0.87nM for OTC assay. Moreover, the donor-acceptor distance was estimated to be223A, which significantly broke the distance limit (100A) in traditional fluoroimmunoassay based on energy transfer mechanism. Therefore, graphene can be used as novel acceptor in the design of fluorescent biosensors with high performance.(3) A graphene/T4DNA ligase-based fluorescent sensor was designed by taking advantage of the different adsorption affinity of graphene for single-stranded DNA and double-stranded DNA, which realized the rapid and highly sensitive detection of the single nucleotide polymorphism (SNP) compared with traditional methods. The results indicated that it was possible to accurately determine SNP with frequency as low as2.6%within40min. Furthermore, the presented method opened new opportunities for the development of other biosensing platforms for DNA methylation, DNA damage and multiple nucleases activity assay.(4) A promising and controllable internal method was developed to regulate the interaction between graphene sheets and DNA based on oxidative DNA damage, which also provided novel basis for the design of other extended graphene/DNA-based sensing platforms. Furthermore, we designed a novel graphene/DNAzyme-based catalytic beacon for Cu(Ⅱ), which allowed the highly sensitive and selective detection of Cu(Ⅱ) in aqueous solution. The detection limit was determined to be0.365nM, which was much lower than that of previously reported DNAzyme-based fluorescent sensors. Base on it, a promising graphene/DNAzyme-based label-free fluorescent sensor for Cu(Ⅱ) detection was developed by means of the unique luminescent property of extrinsic fluorophore GelRed, which avoided the complicated operation and weak performance of DNAzyme in traditional DNAzyme-based sensors.(5) Fine control over the surface charge density of colloidal graphene is possible through the control of salt concentration (NaCl or MgCl2) in the solution, thus resulting in the spontaneously assembled stacked-graphene, the average size distribution of which could be increased with the increase of salt concentration. Furthermore, this type of graphene-derived material possessed great capabilities in the capturing of various DNAs independent of their structure compared with the traditional colloidal graphene. Based on it, we designed a novel graphene/DNA-based fluorescent sensor for rapid and real-time screening genotoxic chemicals, such as Cu(II) ions and organic flavonoids, not only avoiding the complex operation and off-line detection in traditional electrochemistry-based sensors, but also improving the signal transduction, thus broadening the graphene-based applications in environemtal monitoring.(6) A highly active graphene/Au-NPs-based peroxidase mimetic was constructed in combination with the unique electronic property of graphene and the intrinsic catalytic activity of Au-NPs. It was observed that the calculated values of reaction rate for our catalyst with TMB as substrate was about1.3times higher than those for commercial HRP. Based on it, a novel peroxidase mimetic/aptamer-based colorimetric sensor was developed for rapid and highly sensitive detection of nucleic acids, hepatitis C virus, insulin and enzyme activity. Furthermore, this convenient approach can be applicable to the other extended label-free aptamer-based sensors for colorimetric detection of a broad range of analytes.In combination with the needs and development of water quality monitoring, the unique properties of quantum dots and graphenes, and various technologies in molecular biology, biochemistry and immunology, we have designed several novel nanosensors for typical pollutions in water environment, realized the regulation of the sensing performance, revealed the detection mechanism, and illustrated the intrinsic relationship between the interfacial reaction and sensing mechanism in functional nanomaterials, which greatly promoted the application of functional nanomaterials in environmental monitoring.