Multiple electrochemical detection technique

Electrochemical detectors were reported used by 21% of the respondents to the detector survey (47). Electron transfer processes offer highly sensitive and selective methods for detection of solutes. Various techniques have been devised for this measurement process, with the most popular being based on the application of a fixed potential to a solid electrode. Potential pulse techniques, scanning techniques, and multiple electrode techniques have all been employed and can offer certain advantages. Two excellent reviews of electrochemical detection in flowing streams have appeared (59,60), as well as a comprehensive chapter in a series on liquid chromatography (61). 

Different miniaturized flow injection analyzers have been constructed using spectrophotometry as a leading detection technique. Besides, electrochemical techniques such as amperometry and potentiometry with chemically modified solidstate electrodes and tubular membrane-based ISEs, respectively, have been proved to be well adapted to multiparametric measurements of inorganic species present in wastewater, using multiple sensor arrays. 

The most commonly-used detectors are those based on spectrophotometry in the region 184-400nm, visible ultraviolet spectroscopy in the region 185-900nm, post-column derivativisation with fluorescence detection (see below), conductivity and those based on the relatively new technique of multiple wavelength ultraviolet detectors using a diode array system detector (described below). Other types of detectors available are those based on electrochemical principles, refractive index, differential viscosity and mass detection. 

Microdialysate samples have been analyzed using a variety of nonseparation-based analytical techniques including immunoassay, biosensors, and MS [1-4]. The main limitation to the use of these methods is that they are typically restricted to the measurement of a single analyte. For more complex samples, the detection of multiple substances is usually necessary. In this case, the dialysate sample is normally analyzed by conventional chromatographic or electrophoretic separation methods employing optical, electrochemical, or mass spectrometric modes of detection [5]. 

The SNIFTIRS approach is clearly related in some ways to the EMIRS technique, in that it does involve potential modulation, (but not lock-in detection), albeit at a much lower frequency ca. 0.01-to 0.02 Hz [83, 85,137], than the ca. 10 Hz typically employed in EMIRS experiments. Consequently, SNIFTIRS is also restricted to electrochemical systems that are essentially reversible over the timescale of the poterttial modulation, but has proved extremely sensitive, and is generally reported as being surface specific, only detecting potential-induced changes in adsorbed species [119, 138]. One specific exception to this generalization is where 1 and 2 are chosen such that the species of interest are fiiUy adsorbed at one potential, and fiiUy desorbed at the other potential [16, 85]. This led Weaver and Corrigan [139] to coin the general acronym PD IRS, for those approaches that involve multiple reference spectra as well as multiple sample spectra. Thus, Fig. 11(d) is an example of the PDIRS approach, as is Fig. 11(e), in which the sample potential is sequentially decreased... 

In prospective study, more innovative devices need to be developed for multiplex analysis. Furthermore, even though lateral flow in 2-D paper sheet has been mostly elucidated based on fiber structure and surface energy, there are still limited study on vertical flow in a 3-D paper device. It was reported that a 3-D paper device coupled with electrochemical electrodes fabricated on two filter paper sheets for detection of multiple cancer markers based on chemiluminescence, which prevents cross-talk between adjacent detection zones. Therefore, 3-D paper sensors with versatile structure need to be further explored to realize multiple anal5d e detection using multiple techniques. 

Scanning electrochemical microscopy (SECM, see Chapter 12) is another microelectrode technique that has been used at the cellular surface. Briefly, with SECM a microelectrode (UME, see Chapter 6) functions as a scanning probe that detects local electrochanical activity. When the UME is rastered over a sample, electrochemical data is recorded at multiple positions and an image is constructed based on the local electrochemical properties of the area of interest SECM has been thoroughly reviewed (19,20,52, Chapter 12 of this... 

Voltammetry is the most versatile technique in electrochemical analysis (Protti, 2001). In voltammetric technique, both the current and the potential are measured and recorded. The position of peak current is related to the specific chemical, and the peak current density is proportional to the concentration of the corresponding species. A remarkable advantage of voltammetry is the low noise, which can endow the biosensor with higher sensitivity (Bard and Faulkner, 2001). In addition, voltammetry is able to detect multiple compounds, which have different peak potentials, in a single electrochemical experiment, thus offering the simultaneous detection of multiple analytes. The graph of current... 

Remarkable developments on microfluidic devices have been made over the last decade and a promising future for this technology is expected. Eurthermore, new electrochemical techniques for microfluidic applications have been presented over the last few years. The detection of single or multiple analytes on the same device making use of different electrochemical techniques (amperometric, potentiometric, conductimetric, or electrochemiluminiscence) microfluidic transport based on... 

---