Label-free sensors

In contrast to SPFS, SPR, and SPDS are tools that can study biomolecular interactions without external labels. They share the same category of label-free biosensors with the reflectometry interference spectroscopy (RIfS) [46], waveguide spectroscopy [47], quartz crystal microbalance (QCM) [48], micro-cantilever sensors [49], etc. Although the label-free sensors cannot compete with SPFS in terms of sensitivity [11], they are however advantageous in avoiding any additional cost/time in labeling the molecules. In particular, the label-free detection concept eliminates undue detrimental effects originating from the labels that may interfere with the fundamental interaction. In this sense, it is worthwhile to develop and improve such sensors as instruments complementary to those ultra-sensitive sensors that require labels. 

High-refractive-index materials enable the investigation of bioreactions at the surface within the evanescent field. Various sensor systems were developed with the advantage of high sensitivity and measurement in the presence of the sample without any rinsing. The sensor principles can be divided into label-free sensor systems where the determination of the effective refractive index is the pivotal parameter and systems based on surface-confined fluorescence excitation where the marker molecule is solely detected. 

A potential concern with label-free sensors such as those we have been describing is that (paradoxically) the detection is an inferred process, that is, if one observes a shift in a spectrum, does that really correspond to the capture of a biomolecule, and if so, does the amount of the shift correspond with what one expects based on theory DeLouise and Miller set out to test that question in the context of mesopor-ous silicon sensors in a series of papers focused on correlating the optical response to loading of the enzyme GST, a parameter that could be measured independently... 

The optical ring resonator, as a class of evanescent label-free sensors, has been extensively explored in recent years [5] and has shown advantages over other evanescent label-free sensors. The resonant nature of light in a ring resonator... 

A large number of biosensors are based on modified versions of field-effect transistors (FETs), and in combination with silicon nanowires (SiNWs) they form versatile, label-free sensors that have been used for the detection of proteins, single viruses, and DNA [6-11]. 

The results obtained with PE multilayers as well as DNA on top of the capacitive EIS sensor could verify their feasibility as transducer for a label-free detection of adsorption, binding, and interactions of charged macromolecules. Nevertheless, our experiments do not enable us to clearly distinguish between the contributions in the signal generation from each of the mechanisms discussed in sections 7.3 and 7.4. Probably, both basic mechanisms, namely, the intrinsic charge of molecules and the ion-concentration redistribution in the intermolecular spaces or in the multilayer, affect the sensor signal by superposition. 

MJ. Schoning, M.H. Abouzar, S. Ingebrandt, J. Platen, A. Offenhauser, and A. Poghossian, Towards label-free detection of charged macromolecules using field-effect-based structures scaling down from capacitive EIS sensor over ISFET to nano-scale devices, in Mater. Res. Soc. Symp. Proc. 915, 0915-R05-04 (2006). 

Label-Free Biochemical Sensors Based on Optical Microresonators... 

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