The main aim of this thesis is to develop novel highly sensitive, membrane-free, inexpensive one-way oxygen sensors that work in neutral solutions at or near 0 V vs. Ag/AgCl to avoid interference from oxidizing or reducing agents in water or blood samples. The theoretical background is to use chemically-modified electrodes with metalloporphyrins as catalysts for dioxygen reduction at or near 0 V vs. Ag/AgCl . Three approaches are employed to prepare metalloporphyrin-derived electrodes....[ Read more ]

The main aim of this thesis is to develop novel highly sensitive, membrane-free, inexpensive one-way oxygen sensors that work in neutral solutions at or near 0 V vs. Ag/AgCl to avoid interference from oxidizing or reducing agents in water or blood samples. The theoretical background is to use chemically-modified electrodes with metalloporphyrins as catalysts for dioxygen reduction at or near 0 V vs. Ag/AgCl . Three approaches are employed to prepare metalloporphyrin-derived electrodes.

1. The first approach is based on a traditional chemical modification technique and commercial voltammetric electrodes. Iron(III)-tetra(3-methoxy-4-hydroxy-phenyl) porphyrin chloride (FeTMHPP) is dip-coated, dropwise-coated, and electrochemically-polymerized on four different bare electrodes: glassy carbon (GCE), graphite (GE), gold (AuE), and platinum electrodes (PtE), respectively. Their electrocatalytic properties for dioxygen reduction are characterized and compared. These chemically-modified electrodes demonstrate different electrocatalytic behavior for dioxygen reduction in PBS pH7.0. It is demonstrated for the first time that only electrochemically polymerized poly-FeTMHPP film on bare PtE has a highIy electrocatalytic property for dioxygen reduction in PBS pH7.0 with a cathode peak potential, E_pc, ranging from 0 mV to +I50 mV vs. Ag/AgCl which is controlled by the polymeric film thickness. The kinetic results show that this system has a four electron transfer mechanism for dioxygen reduction. The results are transferred to a thick film chip oxygen sensor. The poly-FeTMHPP/Pt chip oxygen sensor shows clearly that it has a potential to be a practical oxygen sensor. The high sensitivity, the fast response (5 s) and the good operational data are the most interesting features. The dynamic range of the poly-FeTMHPPPt oxygen sensor between 50 and 350 μM covers the normal dissolved oxygen range of waste water and blood samples. The detection limit of 337 pM for dissolved oxygen in PBS pH7.0 allows a highly sensitive measurement. If the oxygen sensor is designed to be a disposable chip for a 1-shot-measurement, the present stability is already sufficient. In this case automated manufacturing will guarantee the reproducibility. The stability of poly-FeTMHPP/Pt oxygen sensor must be improved, however, if the sensor is intended to be used repeatedly.

2. In order to improve the stability of the sensor, we also demonstrate a new approach to chemical copolymerization of FeTMHPP with tetraethyl orthosilicate (TEOS) for preparing a chemically-modified electrode on bare PtE by the sol-gel method. The advantage of this approach is that a heat treatment step at high temperature is not necessary for forming a stable structure. This electrode shows an extremely high stability after more than 1,000 CV potential scans in PBS pH7.0. After iron in FeTMHPP is replaced by cobalt, the CoTMHPP-doped sol-gel/Pt electrode shows not only an extremely high stability but also a very high oxygen reduction peak potential in cyclic potential scans. After 1000 cyclic potential scans, its dioxygen reduction peak current has a deviation of smaller than 5%. However, after the combination of CoTMHPP with TEOS, the sensitivity is reduced to half compared with poly-FeTMHPPPt electrode, but it is ca. 50% higher than that for FeTMHPP-doped sol-geVPt electrode. TEM test results show that CoTMHPP in these modifications has a multi-crystalline structure.

3. The last approach is to design a ternary chemically-modified electrode, a CoTMHPP-doped sol-gel/Pt/carbon electrode system. After a thin Pt film is sputtered or electrochemically deposited on GCE, GE and CFE, chemical copolymerization of CoTMHPP with TEOS by the sol-gel method is applied to this composite electrode. The merit of this approach is that the less expensive carbon electrodes (GCE, GE, and CFE) can be modified to have a positive E_pc, for dioxygen reduction in PBS pH7.0. In comparison to the electrochemically-deposited Pt particle (Pt-e) modified carbon electrodes, physically-coated Pt film (Pt-s) modified carbon electrodes are also used as base electrodes for CoTMHPP-doped sol-gel film oxygen sensors. The experimental results demonstrate that Pt-s/GCE and Pt-s/GE composite electrodes can not only replace Pt electrode as base electrodes to reduce its price but also improve the sensitivity of the resulting electrodes. This result will be deciding for developing a mass-producible screen-printed interference-free oxygen sensor.

For these approaches it is clearly shown that the interference of both organic and biomaterial substances, i.e., ascorbic acid and acetaminophen, are greatly reduced for the measurement of dioxygen at near 0 V vs. Ag/AgCl in PBS pH7.0.

Summarizing, the feasibility of the concept of constructing a novel highly sensitive, extremely stable, inexpensive, and interference-free oxygen sensor working at or near 0 V vs. Ag/AgCl in a neutral solution is clearly demonstrated.