Electrochemical Microsensor Designing And Application

Microelectrode is any electrode whose characteristic dimension is comparable to or smaller than the diffusion layer thickness,δ, under the given experimental conditions,when a steady state or a pseudo steady state (cylindrical electrodes) is attained. It is now conventionally assumed that a microelectrode has dimensions of tens of micrometers or less, down to submicrometer range.If the geometric dimensions of a electrode become progressively smaller, then the behavior of the electrode begins to depart, quantitatively and qualitatively,from that of a large electrode which can be approximated by an electrode of infinite dimension. These differences are caused by changing conditions of the mass transport from the bulk of solution toward the electrode and have several important practical implications, such as a decreased ohmic drop of potential, IR, fast establishment of a steady-state signal, increased current density due to enhanced mass transport at the electrode boundary, increased signal-to-noise ratio and neglectable perturbation to sample due to small dimension of the microelectrodes. These effects make sufficiently small electrodes advantageous in many areas of electroanalytical chemistry(e.g., measure in high resistance system or high speed and transient reaction) and biochemistry (in situ lectroanalytical measurements on living organisms).The development in microelectronics, micromechanics technology and material science made their intense study and application further enhanced. Microelectrodes of various geometries have been prepared mechanically or lithographically. In addition to single electrodes, microelectrode arrays have been prepared and used.Microelectrode is modified through the course, surface of which is optimize in the molecule level in order to acquire desired function. Modified microelectrode,which combined advantage of microelectrode with one of modified electrode, becames a activitied study domain in electrochemical chemistry, electroanalytical chemistry and other discipline currently. Microstructure of recognize function were taken into attention by many scholars due to their good prospect of application in combinatorial chemistryand high throughput screening.And microelectrodes were used to fabricate functionated microstructure or microdevice on several kind of matrix surface through local modification of function substance. Current signal was enhanced and consequently analytical sensitivity was improved if microelectrodes assembled properly.It's based on the followed principle: when cathode and anode are placed very closely, especially 0.1～10μm, redox cycle can been established because substances reduced on cathode can diffuse to anode and be oxidized to initial state,go round and round,as the result that Faraday current is magnified.So it is possible to detect reversible trace substances under the conditions a great deal of irreversible substances coexist.Due to the reason mentioned above,some innovative and initial work on the fabrication of microelectrodes, electrochemical monitoring and detection,and microelectrode electrochemistry have been done in my master program.The main work are summarized as follows:1.A solid-stated Cu/CuSe microelectrode was prepared by cathodic deposition of selenium and subsequent formation of a thin layer of CuSe on a copper wire 15μm in diameter. The electrochemical behavior of the electrode was studied in order to define its selective response to cupric ions. The dimensions and response time (< 2s) allowed use of this electrode in the measuring influxes as well as effluxes of copper (II) ions in Gingko roots. Copper (II) fluxes showed marked spatial and temporal features and exhibited an oscillatory pattern in time.2.Fabrication and characterization of a novel microelectrode based on iridium oxide film is presented. In this method, a uniform iridium oxide film is coated on the surface of an iridium metal wire through electrochemical deposition by cyclic voltammetry. The proposed method allows excellent control of the deposited amount and the rate of the iridium oxide film growth. The microelectrode exhibits very promising pH sensing performance, with a super Nernstian response in the tested pH range of 0 to 14. The sensitivity of the proposed device is 71.93 mV/pH and matches well with the estimated value within a 0.93 mV/pH. The potential response is fast, with a 95% response time obtained in less than 0.2 s for all pH changes. The open-circuit potential of the microelectrode is almost drift-free, with an average variation over time in calibration slopes as small as 0.025mV/day. Therefore, the present microelectrode is very suitable for continuous pH measurement without the need of frequent calibration. In this paper, the mechanism of iridium oxide film response to pH is discussed. The microelectrode have also exhibited an excellent analytical performance for oxidation of hydrogen peroxide. These studies indicated that, the iridium oxide modified microelectrode could be prepared for use as pH and H₂O₂ sensor.3.Scanning Electrochemical Microscopy (SECM) was used to deposit and visually characterize localized precipitate microarrays of copper hexacyanoferrate and iron hexacyanoferrate on glassy carbon electrode (GCE), respectively. Dissolution of a sacrificial Cu microelectrode and stripping of Fe pre-deposited on a platinum microelectrode generated relevant metal cations in the gap between the microelectrode and the GCE, which precipitated some [Fe(CN)₆]^4- anions that were generated simultaneously by reduction of [Fe(CN)₆]^3- at the GCE. By moving the microelectrode in the"skip-dip deposition"mode, precipitate-dot microarrays of copper hexacyanoferrate (CuHCF) and iron hexacyanoferrate(FeHCF), typically of ²5-μm diameter for each dot, was fabricated, respectively. The diameter and thickness of disk-shaped precipitate dot could be modified by changing the concentration of K₃[Fe(CN)₆] or the deposition time. The deposited metal hexacyanoferrate microstructures show catalytic activity for the oxidation of dopamine and the reduction of hydrogen peroxide, respectively, which were characterized visually by SECM.4. A new model of ferricyanide-mediated electron transfer was presented. Based on proposed principle, novel disposal uric acid and glucose strip were fabricated with screen-printing graphite two-electrode system and potassium ferricyanide as electron mediator. Both strips exbibit excellent linear responses to detecting substance in a wide measurement potential,-0.6⁺0.6V, which be adjusted at will according to concentration of detecting substance and level of interference. Statistical analysis shows the result of detection by Strip and conventional method have preferable relativity.