Label-free approaches

In the first approach, the antibody Abj captures the target microbe, which is interrogated using the antibody labeled with enzyme. The substrate becomes converted to an electro-chemically active product measured at the electrode. In the second approach, the captured microbe impedes the redox probe to access the electrode surface. Impedance (or capacitance, or decrease in the electron transfer) can be directly measured. Label-free approaches are simpler, involve a lower number of steps, and decrease commonly both, analysis time and cost. However, sensitivity usually becomes enhanced when a label (especially when amplification occurs) is involved. In next subsections, both approaches are commented in more detail. 

Faradaic and non-Faradaic impedance spectroscopies are effective methods to probe the interfacial electron-transfer resistance at a functionalized electrode. In Faradaic impedance sensors, the interaction between a biological receptor and a target species 

The components of the circuit are as follows R, (Warburg impedance resulting from the diffusion of ions from the bulk electrolyte to the electrode), (double-layer capacitance, mainly dependent on the electrode area), and Since R and represent bulk properties of the electrolyte solution and diffusion features of the redox probe in solution, they are not affected by physicochemical transformation occurring at the surface of electrode. Only Qi and R, which can be estimated, depend on the surface transformations. 

Similar to EIS, SWV (square-wave voltammetry) is another frequency-dependent electrochemical technique that could also be used in label-free Faradaic immunosensing [167]. In this case, a train of potential pulses is superimposed on a staircase potential signal with the latter centered between a cathodic pulse and an anodic pulse of the same amplitude. During each cathodic pulse, the analyte diffuses to the electrode surface and it is immediately reduced. During the anodic pulse, analyte that was just reduced is reoxidized. The current is sampled just before and at the end of each pulse and the current difference between these two points is then plotted against the staircase potential in a SW voltammogram. A linear potential scan in SWV is faster than EIS record and a familiar peak-shaped signal is more easily interpreted. 

Apart from the possibilities of label-free determination, impedance and capacitance measurements are usually employed for characterization purposes of surfaces later applied in labeled amperometric approaches (see comments inTable 9.1). 

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F. F., Direct detection of tuberculosis infection in blood serum using three optical label free approaches, Sens. Actuators B Chem. 2008, 129, 934 940... 

Electrochemical transduction of the hybridisation event can be classified into two categories label-based and label-free approaches. The label-based approach can be further subdivided into intercalator/groove binder, non-intercalating marker, and NP. The label-free approach is based on the intrinsic electroactivity of the DNA purine bases or the change in interfacial properties (e.g., capacitance and electron transfer resistance) upon hybridisation [49],... 

LABELING AND LABEL-FREE APPROACHES FOR PROTEIN QUANTIFICATION... 

Figure 12.25 Label-free approach to DNA detection using CE-CL analysis (A) Detection of ssDNAs that can fold into the G-quartet structure (B) Detection of ssDNAs that can form the G-quartets with the templet ssDNA. [From T. Li et al. (2007b), with permission.]...

Li, T., Li, B. L., Dong, S. J. (2007a). Adaptive recognition of small molecules by nucleic acid aptamers through a label-free approach. Chem Euro J 13, 6718-6723. 

Electrochemical AC impedance measurements provided another label-free approach to DNA hybridization detection on a DNA probe-doped polypyrrole film on MWCNT-modified electrodes... 

The two foregoing modification approaches produce devices in response to impedance in two different ways, capacitive and resistive, as described above. Both are label-free approaches with the potential directly to detect antigens and haptens in sandwich or competitive immunoassay formats. However, they are both somewhat limiting in their overall sensitivity. Non-label-free impedimetric approaches that offer enhancements in sensitivity are the use of electroactive polymer layers and the use of redox-active and/or nanoparticle labels. 

Tab e 9.2 Examples of electrochemical immunosensors related to food analysis with label-free approaches... 

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