Electrochemical, detection response

The electrochemical detection of pH can be carried out by voltammetry (amper-ometry) or potentiometry. Voltammetry is the measurement of the current potential relationship in an electrochemical cell. In voltammetry, the potential is applied to the electrochemical cell to force electrochemical reactions at the electrode-electrolyte interface. In potentiometry, the potential is measured between a pH electrode and a reference electrode of an electrochemical cell in response to the activity of an electrolyte in a solution under the condition of zero current. Since no current passes through the cell while the potential is measured, potentiometry is an equilibrium method. 

Electrochemical detection is sensitive and selective, and it gives useful information about polyphenolic compounds in addition to spectra obtained by photodiode array detectors. Differences in electrochemically active substituents on analogous structures can lead to characteristic differences in their voltammetric behavior. Because the response profile across several cell potentials is representative of the voltammetric properties of a compound, useful qualitative information can be obtained using electrochemical detection (Aaby and others 2004). 

A significant peak must be obtained at range settings of 500 nA to make oxidative electrochemical detection feasible (try both mobile phases and different electrode materials if no response is obtained). 

Aromatic and sulfur-containing amino acids were separated by HPLC, and subjected to post-column UV irradiation before electrochemical detection with GCE vs AgCl/Ag electrodes. The analytes showed different behavior during lamp off and on periods. Thus, for example, tyrosine (46) and tryptophan (47) showed inherent electrochemical response at +0.80 V, but none at +0.60 V however, on turning on the UV lamp they showed sensitive response at both potentials126. 

One of these is electrochemical detection, which can be used with traditional CE as well as with the microchip design. Electrochemical detection generally provides good sensitivity and bulk property response (conductivity, potentiometry), and can be selectively tuned to a certain class of compounds (amperometry). ... 

An electrochemical sensor using an array microelectrode was tested for the detection of allergens such as mite and cedar pollen (Okochi et ah, 1999). Blood was used in the assay and the release of serotonin, a chemical mediator of allergic response, which is electrochemically oxidized at the potential around 300 mV, was monitored for electrochemical detection by cyclic voltammetry. 

An interesting feature of the electrochemical detection is its relatively small variation in sensitivity for various substances for which it responds. This relatively constant molar response is due to the small number of electrons, usually two or three, involved in electrochemical reactions. This feature is very convenient in trace analysis, because the analyst can predict the sample size, dilutions, and other manipulations drat must be used to produce the desired analytical sensitivity. 

Figure 27.18 Common configuration for postcolumn reactors with electrochemical analysis. (A) LC-chemical reaction-EC. Postcolumn addition of a chemical reagent (for example, Cu2+ or an enzyme). (B) LC-enzyme-LC. Electrochemical detection following postcolumn reaction with an immobilized enzyme or other catalyst (for example, dehydrogenase or choline esterase). (C) LC-EC-EC. Electrochemical generation of a derivatizing reagent. The response at the second electrode is proportional to analyte concentration (for example, production of Br2 for detection of thioethers). (D) LC-EC-EC. Electrochemical derivatization of an analyte. In this case a compound of a more favorable redox potential is produced and detected at the second electrode (for example, detection of reduced disulfides by the catalytic oxidation of Hg). (E) LC-hv-EC. Photochemical reaction of an analyte to produce a species that is electrochemically active (for example, detection of nitro compounds and phenylalanine). Various combinations of these five arrangements have also been used. [Reprinted with permission from Bioanalytical Systems, Inc.]...

In recent years, the unicellular nature of planktonic algae has been exploited for the construction of whole-cell based biosensors capable of real-time response on critical change of the aquatic ecosystems caused by pollutant emissions. Most of the proposed devices are based on the electrochemical detection of the inhibiting effect on the photosynthetic activity of algae and cyanobacteria exerted by some toxicants. 

Immobilization of bioactive material on/in the electrode allows combining bio-reaction selectivity with sensitivity of electrochemical detection. Irrespective of reaction in the biosensor, the electrochemical response is measured, in particular, as current at the given potential (amperometric sensor) or electrode potential (potentiometric sensor). 

The group of Ruhr pursued an approach where enzymes were only immobilized on specific areas of the electrode. The electrochemical detection was performed on the unmodified regions of the same electrode leading to faster response times of the sensor [58]. The authors used SECM to show the different kinetics at the modified and unmodified regions of the sensor surface [57]. SECM was (among other techniques [58,70]) used for microderivatization of the surface [63]. 

Glassy carbon working electrodes were used in electrochemical detection of pentazocine while no response was measured in one study at an operating potential of 0.8 V (94). Table 12 summarizes the HPLC column and mobile phase conditions utilized in reported investigations. 

In another report published near the same time, this group demonstrated the electrochemical detection of electroactive enantiomers, d- and L-3,4-dihydroxypheny-lalanine (DOPA) and (R)- and (.S )-A, A /-dimcthylfcrroccnylcthylamine (FcN) in imprinted sol-gel-derived thin films.68 To improve response times, the imprinted films were made to be very thin, — 70 nm, which is about 3 to 10 times thinner than typical sol-gel-derived films. The functional monomer used to imprint DOPA was PTMOS (Fig. 20.2) because it was expected to exhibit tt-tt and hydrophobicity interactions. For the ferrocene derivative, both PTMOS (Fig. 20.2) and carboxyethylsila-netriol sodium salt (CTES) were used for hydrophobicity and tt-tt interactions and electrostatic interactions between the amine and carboxy groups, respectively.68... 

Acetylcholineesterase and choline oxidase Enzymes were co-immobilized on chemically preactivated immun-dyne polyamide membrane (thickness 120 pm, size cut-off 3 pm). Applying 20 pL of 1% ChO solution in 0.1 M-phosphate buffer (pH 6). Assay was based on electrochemical detection of the generated h2o2. The response time was 2min. Detection limit was 50 nM and the response was rectilinear up to 20 pM. [71]... 

Another sensor based on a fiber-optic-based spectroelectrochemical probe uses a DNA/ethidium bromide system to take advantage of the biological recognition processes [92]. The concept of immobilizing electrochemical reagents on the end of an optical fibre is a useful addition to the field of bioanalytical sensors. Before this development, optical and electrochemical detection of DNA were performed separately. Optical and electrochemical detection of DNA are suitable for a DNA detection system [93, 94] and these techniques will enable a production of a cheap DNA biosensor with a rapid and quantitative response. 

Electrochemical devices have proven very useful for sequence-specific biosensing of DNA. Electrochemical detection of DNA hybridization usually involves monitoring a current response under controlled potential conditions. The hybridization event is commonly detected via the increased current signal of a redox indicator (that recognizes the DNA duplex) or from other hybridization-induced changes in electrochemical parameters (e.g., conductivity or capacitance). Modern electrical DNA hybridization biosensors and bioassays offer remarkable sensitivity, compatibility with modern microfabrication technologies, inherent miniaturization, low cost (disposability), minimal power requirements, and independence of sample turbidity or optical pathway. Such devices are thus extremely attractive for obtaining the sequence-specific information in a simpler, faster, and cheaper manner, compared to traditional hybridization assays. 

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