Electrochemical factors affecting sensitivity

A label-free electrochemical DNA biosensor based on 4,4 -diaminoazobenzene (4,4 -DAAB) and multiwalled carbon nanotube (MWNT)-modified GCE for short DNA sequences related to HBV hybridization detection was presented by Li et al. [92]. DPV was used to investigate hybridization event. The decrease in the peak ciurent of 4,4 -DAAB was observed on hybridization of probe with the target. This electrochemical approach was sequence specific as indicated by the control experiments, in which no peak eurrent change was observed when a non complementary DNA sequence was used. Numerous factors affecting the target hybridization were optimized to maximize the sensitivity. Under optimal conditions, this sensor showed a good calibration range between 7.94 xlO M and 1.58 xlO M, with HBV DNA sequence deteetion limit of 1.1 xlO M. 

Fu, L.J., Zhang, H.R, Wu, Y.R, Wu, H.Q., Holze, R. 2005. Surface active sites An important factor affecting the sensitivity of carbon negative electrode material towards humidity. Electrochem. Solid-State Lett. 8 A456-A458. 

The fundamental reason for the uneven distribution of reactions is that the rate of electrochemical reactions on a semiconductor is sensitive to the radius of curvature of the surface. This sensitivity can either be associated with the thickness of the space charge layer or the resistance of the substrate. Thus, when the rate of the dissolution reactions depends on the thickness of the space charge layer, formation of pores can in principle occur on a semiconductor electrode. The specific porous structures are determined by the spatial and temporal distributions of reactions and their rates which are affected by the geometric elements in the system. Because of the intricate relations among the kinetic factors and geometric elements, the detail features of PS morphology and the mechanisms for their formation are complex and greatly vary with experimental conditions. 

An overview concerning the enzyme-based electrochemical biosensors published during the last 5 years for heavy-metal determinations is reported. Their sensitivity and selectivity toward inhibitors and the factors (pH, buffer, enzyme, and inhibitor concentrations) affecting the analytical characteristics of these biosensors are also discussed. 

The surface of the silicon crystal, no matter how it is finished, will have a certain number of lattice defects, which tend to dissolve preferentially resulting in formation of etch pits and other features. Terraces and steps of various sizes are inevitable consequences of anisotropic dissolution of the surfaces misoriented from the (111) surface. Also, a silicon surface, whether initially smooth or not, in HF solutions, has an intrinsic tendency to roughen and form micropores governed by sensitivity of the electrochemical reactions on a semiconductor electrode to surface curvature. Furthermore, the two groups of factors shown in Fig. 7.57 may affect each other. For example, the initial lattice inhomogeneities may provide the sites for deposition whereas localized deposition may enhance the development of etch features such as pits or hillocks. 

Laval etal. (1984) bound LDH covalently to electrochemically pretreated carbon. The enzyme was fixed by carbodiimide coupling simultaneously with anodic oxidation of the electrode surface. The total amount of immobilized LDH was determined fluorimetrically after removal from the electrode and hydrolysis. The authors found that at a maximal enzyme loading of 13 pmol/cm2 six enzyme layers are formed. The immobilization yield was about 15%. The kinetic constants, pmax and. Km, were not affected by the immobilization. The obtained enzyme loading factor of 10-3 indicates that diffusion in the enzyme layer was of minor influence on the response of the sensor. The layer behaved like a kinetically controlled enzyme membrane, i.e., the NADH oxidation current was proportional to the substrate concentration only far below Km- With increasing enzyme loading the sensitivity for NADH decreased due to masking of the electrode surface. 

Fig. 1 shows typical cyclic voltammogr ams with and without large excess of CD. The shape of the voltammogr am is sensitive to the heterogeneous electron transfer rate[8] between the substrate and electrode, and also to the dissociation and formation rates. If the electron transfer is reversible, i.e., the concentrations of the electroactive species at the electrode surface are determined by the Nernst equation, and if the relative contribution of the latter factor is small, the cyclic voltammogram shows a typical shape[8] with reversible electron transfer and no chemical reaction. In this case, the electrochemical response is purely controlled by the diffusion process of the substrate, CD and the complex. The peak separation in this case is 57 mV at 25°C, which is not affected by addition of CD to the electrolyte solution. This situation is usually attained by the use of slow scan rates[9] in CV. Since complexation reaction can be assumed to remain at equilibrium everywhere in the diffusion layer in the present circumstance, the apparent diffusion coefficient, D from the voltammogram in the presence of CD is written as... 

Although UV-Vis detector is the most widespread detection system, fluorescence or electrochemical detectors have also proven adequate for flavonoid analysis. However, the lack of active fluorescent or electrochemical groups in many flavonoids may affect the accuracy of these detection methods. MS is a powerful, highly sensitivity technique. Its use is spreading among food scientists because of its effectiveness in the identification of flavonoids and their applicability for quantitative analysis. Common sources of error in flavonoid analysis may derive from nonlinearity of the detector, which should be checked since many detectors are linear over only one or two decades. In addition, the different response factors of different flavonoids should be taken into account since detectors usually are not equally sensitive to aU components of a mixture. 

Many of the factors discussed previously for the SIR test are equally applicable to the corrosion test. The spacing between the anode and cathode will affect the corrosion rate because it affects the electric field driving the electrochemical migration. Temperature and humidity both provide accelerated conditions as described previously. Contamination will be more strongly coupled to electrochemical migration, which results in the increase in corrosion factor over time. This test is so sensitive that corrosion can be measured electrically before corrosive residues are visible microscopically. This results be-... 

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