Nanoscale Electrocatlyst for Chemicalnd Biolgical Sensor

Abstract

This dissertation describes scientific research that covers preparation and use of two classes of designed inorganic hybrid nanostructures (nanoparticles and graphene) as electrocatalysts in interfacial molecular and biomolecular electron transfer and electrocatalysis. Emphasis is on both fundamental properties of the nanostructures and on their use in chemical and biological sensing devices at the single-molecular scale. The project is focused on eletroactive (or redox) Prussian blue nanoparticles (PBNPs) as well as on graphene and quantum dots (QDs). The results have offered significant interests in understanding of fundamental electronic properties of hybrid nanomaterials and their potential applications in next generation ultra-sensitive chemical sensors and biosensors. As the first man-made coordination compound, Prussian Blue (PB) has a long history dating back over 300 years ago (the first synthesis in 1704). This interesting material has recently been used broadly as an electron transfer (ET) catalyst for new chemical and biological sensors. We have initiated efforts in synthesis, functional characterization and applications of PB in novel nanostructured forms focused on controlled-size PB nanoparticles (PBNPs). Fast and reversible interfacial PBNP ET on an AU (111) electrode surface modified with functional alkanethiols was detected. In terms of ET characteristics, assembled on a solid surface in a two-dimensional array these nanoparticles behave like a large redox molecule such as a metalloenzyme in the eletrocatalytic reduction of H2O2 and glucose oxidation, suggesting a tunneling mechanism. Furthermore, the interfacial electrochemical ET and electrocatalysis of PBNPs immobilized on Au(111)-electrode surfaces modified by variable-length differently functionalized thiol-based self-assmbled molecular monolayers (SAMs) have been explored. The SAMs are terminated by positively (-NH3+) and negtively charged groups (-COO-) as well as by neutral hydrophobic groups (-CH3). In addition, three-dimensional networks of cross-linked [Poly(ethyleneimine) (PEI)] onto which PBNPs are attached by electrostatic forces have been fabricated and assembled on single-crystal Au(111) electrode surfaces to enhance eletrocatalysis resulting from the 3D nanostructure skeleton. In situ AFM of the different assembled layers in the electrolyte solution shows molecular scale structures on the surface, in keeping with electrochemical behavior. To increase the conductivity of PBNPs as an active electrode, reduced graphene oxide (RGO) and PBNPs were exploited as building ingredients to prepare with increased electrical conductivity and functional variability nanohybrid electrocatalysts, which are further transformed into free-standing graphene papers. PBNPs doped graphene paper shows highly efficient electrocatalysis towards reduction of hydrogen peroxide and can be used as flexible chemical sensors for potential applications in detection of hydrogen peroxide or/and other organic peroxides. The as-prepared PBNPs-RGO paper is further capable of biocompatible accommodation of enzymes for development of freestanding enzyme based biosensors and as a potentially platform for electrocatalytic energy conversion. Graphene/PBNPs paper obtained using Au filter substrate shows high electrical conductivity, outstanding mechanical strength, excellent thermal stability, and structural uniformity. This type of paper has good electrochemical performance. Strong electrocatalysis property has, furthermore, been achieved using these mixed materials. This variety of flexible active electrodes can be freely used for electrocatalytic monitoring H2O2, glucose or other analytes. A new method based on one-step oil/water (O/W) two phase induced interfacial self-assembly of RGO nanosheets into graphene nanofilms has been refined. The method shows high efficiency for incorporation of CdSe quantum dots (QDs) into nanofilms to prepare QDs doped graphene nanostructures. Confined QDs interact closely with RGO to promote and to significantly enhance photoinduced electron or energy transfer between RGO and CdSe QDs, which could hold promise for their applications as photovoltaic materials in solar cells.