Electrochemical Study Of CdS Nanoparticles And Superoxide Dismutase Based On Self Assembled Monolayers

Self assembled monolayers (SAMs) have been considered as an efficient method to fabricate various interfaces with a plenty of research and application purposes because it can easily provide a terminus tunable and structural concise interfaces, which are very favorable for further modification .Biosensors can be fabricated through fixing the biologically active species (such as enzyme, antibody, cell etc.) to the surface of electrode by surface self-assembly monolayers (SAMs) technology. Their potential applications in the fields of chemistry, biology, clinic diagnestics as well as environmental monitoring have been proved. Superoxide dismutases(SODs) have been known to the ubiquitously distributed in aerobic organisms and to play an important role in cell protection mechanisms against oxidative damage from reactive oxygen species. Some natural obtained SODs are used in the determination of O₂^- where O₂^- can be sensed with the SOD-based biosensors. The copper complex moiety in SOD has been well documented as the active site for the catalytic dismutation of O₂^- . Histidine derivatives modified electrodes coordinate with copper ion to form Model compound of Superoxide dismutase, MSOD in short, providing a good method to investigate the catalytic dismutation of O₂^- by the SOD.In our work, two aspects concerning self assembled monolayers have been conducted in this thesis: 1) immobilization of CdS nanoparticles on the Au electrodes modified by SAM and their photoelectrochemical property measurement;and 2) in situ constructing MSOD on the gold electrode for the catalytic dismutation of O₂^- The main contents are listed as the following:1. Chemically modified CdS nanopartilces(Q-CdS) were prepared in AOT/heptane/water inverse micelles. DCC was used as the coupling reagent to form amide linkage between the terminal carboxylic group of the self-assembled monolayer of 3-MPA on gold and the amino group of the chemically modified CdS nanoparticles, forming a monolayer of CdS nanoparticles. Then glutaraldehyde used as cross-linkers to form self-assembled multilayers of CdS nanoparticles. The properties of the assembled CdS particles were determined by ultraviolet visible and fluorescence spectroscopy, as well as SEM. The results show that the diameter of the CdS nanoparticles is about 3-5nm.Electrochemical cyclic-voltammetry (CV) and electrochemical impendence spectroscopy (EIS) are employed to characterize the structures and properties of the self-assembled CdS nanoparticles and their photoelectrochemical properties have also been discussed. The results show the Q-CdS modified with 2-mercaptoethanesulfonate has greater photocurrent than that without it.2. A histidine-rich tripeptide (Cys-His-His, briefly CHH) and its mixture with other dilute molecules are constructed onto gold electrodes to form SAMs, and these modified electrodes are coordinated with copper ion. The surface coverage of copper ion for pure CHH SAM is 3.91xlO"nmol/cm2, and the surface coverage of tripeptide CHH is 4.67xlO"11mol/cm2. Based on the concentration of the CHH and Cu2+ on the surface of the electrode, we can calculate out the molar ratio of two components of histidine and Cu2+ is 2.4 I 1.For mixed SAMs of the tripeptide (CHH), dilute media molecules such as cysteamine, MPA, and 2-Mercaptoethanol, respectively, are used. All the SAMs are subjected to coordinate with Cu2+ to form the metal complexes interfacially. With the decrease of the amount of CHH tripeptide, the immobilized amount of copper decreases accordingly. For the different terminal groups of the dilute media molecules, the coordinated amounts of copper ion are different. The amount of copper decreases with the increase of the molar fraction of the medium molecular with -COOH or -OH as their terminal group. The situation is different for the medium molecular with -NH2 as its terminal group, in a certain molar fraction, a higher amount of coordinated copper can be obtained. This may be due to the different surface charge and coordinative ability of different terminal groups.3. By use of the stepwise construction method, the tripeptide Gly-Gly-His with the N-terminal Gly attached to the carboxyl group of the mixed thioctic acid/hexanethiol SAM on gold electrode surface. Where the carboxylic acid group of thioctic acid is used to anchor the in situ formed tripeptide, and hexanethiol, in the mixed self assembled monolayer, is used as the media molucules to decrease the steric crowding located on the reaction sites of the carboxylic acid group. Electrochemical cyclic voltammetry and impedance are employed to investigate the formation of mixed self assembled monolayers and the stepwise construction of the tripeptide as well as the interfacial coordination of copper ion to the tethered peptide. Results show when the ratio ofthioctic acid/hexanethiol is 1:1, the tripeptide in the self assembled monolayer has amaximum coverage as demonstrated by the coordinated sum of copper ion at theinterface. At the end, the superoxide dimutase-like activity of this kind of immobilizedcopper complex was presented, meaning the possibility for fabrication of immobilizedmetalloenzyme mimics by this kind of stepwise construction of various metal complexes.4. Xanthine and xanthine oxidase system is used to generate O2^-, and the cyclicvoltammetry is employed to investigate the SOD-like activity of MSOD immobilized onthe Au electrode. When the dismutation occurres, both anodic and cathodic peakcurrentsof the MSODs confined on the electrode increase due to the dismutation of superoxideoccuring at the interface of the constructed peptide-metal complexes. These modifiedelectrodes have the SOD-like activity towards O2 -according to the results of theexperiments.