Direct transduction methods

Another strategy to differentiate the signal of the single strand from that of the double strand is based on the use of a protein that binds specifically to the ssDNA, preventing the oxidation of the guanine in single strands of DNA [16]. 

The great advantage of this type of detection is to avoid the use of marks or indicators of hybridization, simplifying the experimental procedure. However, the detection based on the electroactivity of bases gives rise to a lack of sensitivity. Various proposals based on the use of oxidation products of adenine as catalysts of NADH oxidation [17], or those based in the use of mediators for the oxidation of bases, with ruthenium complex [18, 19] or osmium complex [20] have been proposed in order to get an amplification of the signal and thereby improve the sensitivity. 

However, these methods induce an irreversible process preventing multiuse and are limited by the adenine and guanine content. 

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The properties of high specificity and a wide applicability with many analytes have led to the widespread use of immunoanalytical techniques. The benefits of electrochemical sensors include technical simplicity, speed, and convenience via direct transduction to electronic equipment. Combining these two systems offers the possibility of a convenient assay technique with high selectivity. Because of the complexity of immunoassay methods, such devices have not yet found widespread use. Nevertheless, electrochemical immuno-sensors offer the potential for fast, simple, cost-effective analysis of many... 

Individual nanopores or nanopore arrays can be developed into a sensor to detect a species of interest. The transduction methods described earlier (e.g., I-V response or current-time (I-t) response) can be applied to sense/detect an analyte. For instance, changes in the observed rectification response of a nanoporous system can be used to indicate and identify the presence of an analyte. - Often, this change in ICR ratio can be attributed to a disruption in the surface charge of the nanopore as the analyte of interest binds to the pore. Direct sensing of electroactive molecules can be accomplished electrochemically with nanoporous electrodes chronoamperometry and voltammetry measurements are most often applied with these systems for detection. 3 2.i83 However, the most typical measurement utilized in the application of nanopores as sensors is that of resistive-pulse sensing in which current blockades occur as a molecule passes through a pore and alters the conductivity. " For a detailed list of analytes that have been detected via different nanoporous platforms and measurement techniques, please refer to Table 11.3. 

The fluorescence properties of this and related indicators change markedly when Ca2+ is bound (Figure 15.19). These compounds can be introduced into cells to allow measurement of the available Ca2+ concentration in real time through the use of fluorescence microscopy. Such methods allow the direct detection of calcium fluxes and diffusion within living cells in response to the activation of specific signal-transduction pathways (Figure 15.20). 

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