Signal enhancement methods

A useful approach to improve the sensitivity of genosensors involving the use of Au-NPs consists of performing silver or even gold precipitation onto immobilized Au-NP-DNA conjugates. Nevertheless, the achieved improvements seem to be a compromise 

Analyte Modified Electrode Substrate Detection Technique Detection Limit Reference 

Synthetic DNA strands Quartz crystal Conductance 5.0 fmol 74 

CaMV35S 35S promoter from cauliflower mosaic virus CPE carbon paste electrode EIS electrochemical impedance spectroscopy GCE as carbon electrode PAT phosphinothricin acetyltransferase SNFETs field-effect transistors based on sin e-walled carbon nanotube networks. 

Silver enhancement is based on the reduction of silver ions from one solution (usually the enhancer) by another (the initiator) in the presence of Au-NPs [77]. The reduction reaction causes silver to build up preferentially on the surface of the Au-NPs, giving rise to a core-shell structure. An illustrative example is the work from Bonanni etal. [78]. They used streptavidin-coated Au-NPs and silver enhancement kits to amplify the impedimetric signal generated in a biosensor detecting the DNA hybridization event. The scheme displaying the sensor preparation procedure is shown in Fig. 4.13. A good reproducibility was achieved (RSD lower than 8.5%), the detection limit being 11.8 pmol. 

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Significant progress in signal enhancement methods for the central transition has been achieved by the implementation of double frequency sweeps (DFS) [62]. The basic idea of DFS, applicable for both static and MAS experiments, is to invert simultaneously the STs so that the populations of the outer spin levels are transferred to the CT energy levels before they are selectively excited (Fig. 4). 

RAPT signal-enhancement method (top) and a simple two-pulse Hahn-echo experiment (bottom). Both spectra were acquired at 11.75T, with the same experimental conditions. Reproduced from Cahill et al., ° Copyright 2009, with permission from Wiley. 

Similarly to previous detection approaches, recent works on the development of DNA electrochemical biosensors based on the use of Au-NPs and signal enhancement methods to improve the sensitivity, are summarized in Table 4.6. 

Table 4.6. Gold nanoparticle-based electrochemical biosensors using signal enhancement methods... 

The problems involved in quantitative analysis using NMR spectroscopy, have been discussed by several authors and it is evident that it still causes a lot of problems as especially pointed out by Hays55 in his excellent review on the subject. Thus in liquid state NMR spectroscopy the quantitative estimation of atoms and groups involves the use of normal analytical method. In the case of solid state NMR spectroscopy, however, the application of the cross-polarization technique results in signal enhancements and allows repetition rates faster than those allowed by the carbon C-13 Tl. Therefore, the distortion of relative spectral intensities must always be considered a possibility, and hence quantitative spectra will not always be obtained. 

It is also important to note that matrix-related effects, either signal enhancement or more commonly signal suppression, can have a pronounced effect on quantitative measurements. Based on these observations, the use of isotope-labeled standards is helpful to achieve accurate analytical measurement data on the diastereoisomers. Several methods found in the open literature include use of both 13C-labeled and d18-labeled surrogates as recovery and/or instrument standards [118],... 

Zeolite X and A with Gd(III) complexed by 8-hydroxyquinoline (8-HQ) was prepared either by the introduction of Gd(III)(8-HQ)2Cl complex into the zeolite synthesis gel or by stirring the Gd(III)-exchanged zeolite X in an ethanolic solution of 8-HQ (85). In vitro MRI studies showed signal enhancement (Ti shortening). However, the data cannot be compared with the previous results obtained with Gd(III)-loaded, but uncomplexed zeolites (because of partial complexation of Gd(III), different methods for determining the relaxivity enhancement, etc.)... 

It is also possible to screen catalysts according to their selectivities (regio- and stereoselectivity), their stabilities, and rates, since kinetic investigations can be carried out as well [23]. In addition, the application of especially designed NMR pulse sequences further extends the possibilities of the PHIP method [19]. For all of these applications, the main advantage of PHIP is the remarkable signal enhancement which is combined with a spectroscopic in situ method of observation. 

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