In vitro synthesis of CdTe quantum dots on graphene and their biosensor applications

Abstract

Recently, the public health concerns are one of the great social challenges of the twenty-first century. Modern society is demanding that all of environmental and bodily functions are monitored by technological communication devices such as electronic and photonic sensors, actuators, and data transfer instruments that can record signal and information in order to protect human life. In this response to the needs of highly demanding sensitivity, accuracy, and reliability requirements, biosensors are key elements for the advancement of science and the improvement of human health. Biosensor devices are allowing scientists and medical doctors to detect biological chemicals such as enzymes, antibodies and disease-representing biomarkers, and thus providing early diagnosis and improving medical treatments. However, the lack of stability, fouling, low efficiency, and non-specific reaction are problems that are now being faced to solve for achieving high performanced devices. Recently, much recent attention has been drawn toward the design of advanced biosensor based on nanomaterials in order to overcome the above mentioned hot issues, but the erstwhile achievements are inferior to meet up modern demands. Development of nanohybrid materials as nanoscale building blocks is essential for the constructions of highly advanced technology devices such as field effect transistors, sensors, electronics, and energy conversion and storage devices. In this context, semiconductor nanoparticles and carbon nanostructures are a particularly promising candidate to serve as ideal building blocks for the design of optical and electrochemical devices. The outstanding features of quantum dots (QDs), along with quantum yields, tunable photoluminescence, and optoelectronics, have lead to the intensive studies and the realization of numerous applications. Recent scientic and technological breakthroughs of QDs are significantly depending on how to couple semiconductors with carbon nanostructures due to the enhancement of photoconductivity and synergistic effects of nanohybrids. Recently, graphene has been emerged as 2D functional supporting materials for various nanoparticles (NPs), owing to its high electronic and thermal conductivities, remarkable mechanical properties, and large surface area. Tremendous efforts in synthesis approaches of nanohybrids have been mostly placed on two strategies

direct growth of NPs on the surface graphene and employ of dual functional organic linkers between graphene and NPs. However, there are limitations inherent in these methods: tedious experimental process, chemical and thermal treatment requirements, and pre-synthesize of QDs for linker-based methods

tribulations of the size, array density, and position control of NPs for direct growth methods. In this regard, there is a need for the development of an environmentally friendly and straightforward synthetic method for the design and fabrication of QDs/carbon nanostructure nanohybrids. In this thesis, we demonstrated that biological strategy for synthesis of graphene/CdTe nanohybrids by using metal binding proteins (MT-PCS). MT-PCS proteins herein are attractive biotemplates and structure-guiding components for synthesis and assembly of NPs by integrating them into graphene materials, due to their sequence programmability, selective molecular recognition ability, and multifunctionality. As geometrical 2D support for QDs, the water-soluble GOs were obtained by strong oxidation from commercial graphite and then assembled with MT-PCS through hydrophobic and hydrophilic interaction under controlled pH. The crystalline CdTe QDs were in-situ synthesized onto the obtained MT-PCS/GO nanohybrids. The specific peptide sequences of MT-PCS selectively trapped $Cd^{2+}$ and $Te^{2-}$ ions and then directly nucleated and grew the CdTe QDs on the surface of MT-PCS/GO nanohybrids without any reducing chemicals. A uniform size and spontaneous 2D organization of QDs were accomplished by the α-helical structures of MT-PCS adsorbed on 2D GOs. The chemical characterization of QD/MT-PCS/GO nanohybrids was comprehensively analyzed by XPS and FT-IR spectra, respectively. The fluorescence quenching features of CdTe QDs on MT-PCS/GO enabled them to apply optical biosensors for detecting glucose molecules. The QD/MT-PCS/GO nanohybrids showed the better reliability compared to the pristine CdTe QDs. In addition, the QD/RGO nanohyrids through post-chemical treatment of the QD/MT-PCS/GO served as advanced electrode materials for fabrication of electrochemical biosensors. In particular, the direct electron transfer between electrode and redox center in enzymes through traversal of electron vacancies within QDs provide opportunities to construct a “mediatorless” electrochemical biosensor. The QD/RGO hybrids were employed to fabricate electrochemical glucose biosensor with a flow injection system. The GOx/QD/RGO/GCE shows rather fast electron transfer, and high sensitivity (1.395 mA mM??1), and rapid response (5 s) with detection limit of 0.239 mM than the GOx/QD/GC electrode. From these results, this in vitro biosynthesis strategy coupled with 2D supports allowed for facile, eco-friendly, and controllable synthesis of QDs, which could become a general, versatile method to synthesis and assembly of nanomaterials.