Channel electrodes amperometric detection

C. -Ch.J. Lai, Ch.-h. Chen and F.-H. Ko, In-channel dual-electrode amperometric detection in electrophoretic chips with a palladium film decoupler, J. Chromatogr. A, 1023 (2004) 143-150. 

To avoid deterioration of the Au/Cr electrode, C-electrodes have been used. Dual C-fiber electrodes were constructed on PDMS. The separation channel (25 pm wide and 50 pm deep) was fabricated on the top plate. Two C fibers (33 pm dia.) were inserted into the PDMS channels (35 pm wide and 35 pm deep) fabricated on the bottom plate. Consecutive injections (up to 41) could be performed before the electrode was cleaned with a bipolar square wave voltage. The LOD of catechol was found to be 500 nM. With the use of the C electrode, peptides (e.g., Des-Tyr-Leu-enkephalin), which formed stable Cu(II) and Cu(III) complexes, could be detected [762]. Dual-electrode amperometric detection also allowed the positive detection of two peptides [763]. 

The graphical representation of this protocol is shown schematically in Fig. 10.15. Signals from two amperometric electrodes, representing channel 1 (blue) and channel 2 (red) detect to electroactive species, which is delivered to them with frequency modulation of, for example, 1 Hz. The experiment is performed in the benchtop fluid setup shown in Fig. 10.16. The first interesting observation is the presence of higher harmonics in the coherence spectrum. They arise as the effect of nonsinusoidal modulation. A pure sine wave would transform to the frequency domain as a single line. Any other waveform of the same frequency will contain higher harmonics in the spectrum. 

Actually, there are reports on transforming the negative effect of separation electric field into new detection approaches. A potentiostat-less detection scheme for amperometric detection in CE based on the use of microband array electrodes situated in the CE electric field has been proposed [53] as well as the use of an indirect amperometric detection with a carbon fibre in-channel configuration [54]. In this case, the potential difference induced by the CE separation electric field produces a change in the reduction potential of oxygen, which can be used to determine nonelectroactive analytes. 

Carbon fiber electrodes ( 100 nm dia.) were also used to characterize the diffusion between adjacent stream zones at the interface between a micro fluidic system (16 channels 50 im wide, 57 im deep, separated by a 22- im wall) and a large volume using 10 mM ferrocyanide and amperometric detection [756]. It was reported that when the carbon fiber electrode was used in a PDMS chip, the in-channel format gave better peak symmetry than the end-channel format [757]. 

In the placement of the working electrode in amperometric detection, the electrode cannot be too close or too far from the channel end. Why ... 

Channel electrode methodology has been used to investigate calcite dissolution in aqueous solution at around pH 3 where the process is thought to be limited by the rate of transport of H+ to the crystal surface [197-199, 209, 220,223, 225,226, 230, 254-257]. Amperometric detection was used in the form of a platinum electrode held at a potential corresponding to the transport-limited reduction of H+ [10 3 M], The detector electrode was positioned downstream of a dissolving calcite crystal and, with the notation of the previous section, the cell geometry was defined by the parameters xu = 0.469 cm, xA = 0.512 cm. Figure 65 shows a plot of [H+1°°)... 

Several research groups implemented carbohydrate analysis on-chip with direct detection of underivatized sugar molecules. Electrochemical detection is the most attractive approach, as it offers reasonable sensitivity and selectivity, and it is ideally suited for microchip format. Schwarz et al. [203] developed amperometric detection of sugars using microfabricated copper electrode. They separated fructose, sucrose, and galactose in 70 s on a glass chip with 50-p,m wide and 20-p,m deep microchannel and double tee injection geometry. The detection was based on Teflon-coated platinum wire plated with copper and inserted in the end of the separation channel etched in a conical shape. The detection limit down to 1 JtM was achieved. Hebert and coworkers [204] reported an... 

FIGURE 41.4 Examples of electrophoresis microchip fabricated in PT. (A) Microdevice with cupper tape electrodes (a) positioned externally over the channel for C D measurements. Point b is a representation for the solution reservoirs. (Reprinted from do Lago, C.L., Silva, H.D.T., Neves, C.A., Brito-Neto, J.G.A. and Fracassi da Silva, J.A., Anal. Chem., 75, 3853, 2003. With permission.) (B) Microdevice layout for end-channel amperometric detection. S, SW, and b represent, sample, sample waste, and buffer reservoirs, respectively. (Reprinted from do Lago, C.L., et al.. Anal. Chem., 75, 3853, 2003. and Coltro, W.K.T., et al.. Electrophoresis, 25, 3832, 2004. With permission.)... 

Amperometric Detection, Fig, 2 Five independent channels - all three electrodes are positioned on-channel. Electrode and channel widths are 50 and 100 mm, respectively, and the chaimel height is 50 mm. WE, RE, and CE stand for working, reference, and auxiliary electrodes, respectively reproduce from [3]... 

Amperometric Detection, Fig. 3 (a) Representative current response for the oxidation of 1.0 mmol/Ll hydro-quinone (HQ) in 0.1 mol L NaNOa using a microchip with design 1. To obtain this current profile, channels were first filled with 0.1 mol L 1 NaNOs electrolyte. The supporting electrolyte was then sequentially replaced by solutiems containing HQ. (b) Current differences between injections (quadruplicate), measured at the diffusion condition, present a linear relationship in the range of 1.0-4.0 mmol/L f HQ. Other conditions, three Au electrode cell potential, t-800 mV against Au pseudo-reference reproduce from [3]... 

In amperometric detection, the placement of the working electrode in CE/EC detection systems can be divided into three different categories, off-channel (or column), end-channel (or column), and in-channel detection. The purpose of this distinction is to indicate the location of the EC electrodes with respect to the CE electrodes and field (Fig. 4). This is an important consideration because of the possible interactions that might occur, as mentioned above, between the EC and much larger CE electric fields. 

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