The objective of this thesis is to develop high performance electrochemical sensors with high sensitivity and selectivity for the determination of biologically important redox-active species. Various electrocatalysts of nanometer size were immobilized onto electrode surfaces by new techniques such as layer-by-layer self-assembly. The properties of the modified electrodes were characterized by a line of physical techniques. The applications of our modified electrodes in biosensors were systematically explored in both chemical and biological media. _{2 2}...[ Read more ]

The objective of this thesis is to develop high performance electrochemical sensors with high sensitivity and selectivity for the determination of biologically important redox-active species. Various electrocatalysts of nanometer size were immobilized onto electrode surfaces by new techniques such as layer-by-layer self-assembly. The properties of the modified electrodes were characterized by a line of physical techniques. The applications of our modified electrodes in biosensors were systematically explored in both chemical and biological media.

Chapter 2 describes preparation, characterization and application of the electrodes modified with nano-scale mixed-valence transition metal oxide films, including hydrous ruthenium oxide (RuO₂⋅2H₂O) and molybdenum oxide films. The films were characterized by voltammetry, in-situ spectroelectrochemistry, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The GCE-(RuO₂⋅2H₂O)_n film exhibits an excellent electrocatalytic property in the oxidation of ascorbic acid and oxidation/reduction of hydrogen peroxide. Analytical applications of the GCE-(RuO₂⋅2H₂O)_n as sensors for ascorbic acid and glucose are demonstrated.

Chapters 3 and 4 present the syntheses, characterization, and applications of the inorganic-inorganic multilayer films self-assembled alternatively from polyoxometalates (POM) and transition metal ammine complexes (MN). Negatively charged POM and positively charged MN were layer-by-layer self-assembled onto glassy carbon and ITO-coated electrode substrates (S) by electrostatic effect and hydrogen bond. The S-(POM/MN)_n, films were characterized by absorption spectrophotometry such as ultraviolet (UV-vis) and infrared (IR), cyclic voltammetry, XPS, SEM and transmission electron microscopy (TEM). FT-IR spectra further demonstrated the layer-by-layer self-assembly of alternating layers and the hydrogen bond interaction between molecules.

Films assembled from lacunary Dawson-type POM and inorganic MN show partiicularly interesting properties. Two novel non-metal oxide-substituted POMs (P₂W₁₇SO₆₁^4- and P₂W₁₇PO₆₁^5-) were electrochemically synthesized for the first time. Various transition metal ion and lanthanide (or actinide) ions can be incorporated into lacunary POMs by electrochemical method, which is a simple but powerful method to synthesize new polyoxometalates.

In chapter 5, the very stable multilayer films have been developed from POMs and organic macrocycles such as MPP, MPc, and ME. In such films, P₂W₁₈ species displays different electrochemical and electrocatalytic properties depending on the nature of the assembled cations, PtEN and RhEN, a typical synergetic effect in film caused by the cations.

Chapter 6 focuses mainly on the development of biosensors for L-ascorbate and nitric oxide based on the films described in chapter 5. The films prepared from Chromium(III)-tera(4-N,N,N-trimethylanilinium-porphyrin (Cr(III)TTMeAP) and POM displayed a strong catalytic effect in the oxidation of L-ascorbic acid. The amperometric determination of L-ascorbic acid can be carried out at +0.02 V (near 0 V) versus SCE, resulting in significant elimination of many interferences from real samples. A good linear response in the concentration range of 1.0 x 1.0^-7 to 1.0 x 10^-3 M was achieved. The detection limit reached 5.0 x l0^-8 M. Such electrode were successfully employed in the analysis of L-ascorbic acid in mouse hippocampus, cerebellum and blood samples and the results were in good agreement with those obtained by other reference method.

In addition, the films fabricated from MPP (or MPc) and POM can be employed to construct electrochemical sensors for several electroactive species at physiological pH. Particularly, GCE-(P₂W₁₈/CuPc)_n multilayer film exhibits an enhanced electrocatalytic activity towards the NO oxidation, as well as the O₂ and H₂O₂ reduction. In this system, we combined together the electrocatalytic reduction properties of P₂W₁₈ to the O₂ and H₂O₂ and the electrocatalytic oxidation properties of CuPc to NO. The sensor was successfully utilized to monitor NO release from rate brain slice when a stimulator for NOS was injected.

Chapters 7 to 11 describes the preparation of electrodes modified with noble metal nanoparticles (Pd, Pt, Rh, Au). The electrochemical behavior of NO and N₂O at these CMEs in aqueous media were systematically studied.

A new NO sensor with high sensitivity, excellent selectivity and good repetitivity was prepared by (a) the electrochemical deposition of noble metal nanoparticles (Pt, Pd, Au or Rh) onto bare electrodes or Nafion-coated electrodes, (b) the overoxidation of polypyrrole film electropolymerized onto electrode surfaces. These CMEs exhibit excellent catalytic oxidation or/and reduction of NO at nanaomolar concentration.

A N₂O sensors at physiological pH based on the enhanced electrocatalytic reduction of N₂O at the noble metal nanoparticles deposited at electrodes. These CMEs provide a class of sensitive probes for the electrochemical detection of N₂O with a low detection limit and a wide linear response range from 24.3 μM to 1.94 mM. A mechanism is proposed to account for the electrocatalytic reduction of N₂O on the CMEs, which adequately explains our experimental observations.

A novel and significantly-improved sensitive and selective glucose biosensor has been fabricated based on immobilizing glucose oxidase onto a thiocyanide (SCN^-) monolayer adsorbed noble metal nanoparticle films. Due to the strong adsorption of SCN^- anion, background current at the SCN^-/M/GCE (M = Rh, Pt or Pd) is remarkably reduced, leading to a substantial improvement of sensitivity in the response to hydrogen peroxide, compared with that at a M/GCE. The biosensor possesses a rapid linear response to glucose concentration from 5.0 x 10^-6 to 1.2 x 10^-3 M, and a low detection limit of 1.0 x 10^-6 M (at the signal-to-noise ratio = 3).

Finally, our sensors were applied to study the molecular basis of clinical effect and treatment mechanism of acupuncture and drugs in vascular dementia (VaD), by measuring the levels of NO, L-AA, glucose, hydrogen peroxide and protein levels in normal, diseased VaD and treated rat models. The availability of these data allows us to gain a better understanding of VaD and its treatment by acupuncture and drugs at molecular level.