Label-free measurement

Surface plasmon resonance (SPR) technique had become popular in interaction studies between biological molecules (1). It is an optical biosensor, and the interactions can be detected by SPR angle shift or reflection light intensity. In typical SPR measurement, one of pair interacting biomolecules was immobilized on a gold chip, and another was flowed over the chip as its solution. There are two major advantages in SPR assay (a) real time evaluations on kinetics studies and (b) label-free measurements. 

The surface plasmon resonance (SPR) is the analytical technique widely applied in the field of characterizing adsorption to surfaces. The big advantage of this method is the high sensitivity in detecting substances adsorbed on a surface (Ipg/pm of protein molecules) and the possibility of label-free measurements also in the in situ mode. The resulting graph is presented in the arbitrary units and called sensogram. 

Labeled immunosensors are derived from the immunoassay technology. This type of sensor, like the fluorescence, is expensive, and makes real-time measurements impossible. Immunosensors for direct, label-free measurements of various markers are attractive, as they also provide real-time monitoring. Optical immunosensors have been the most studied, but electrochemical immunosensors might offer at least the same detection range and provide a less-complicated instrumentation. 

Maehashi et al. (2007) used pyrene adsorption to make carbon nanotubes labeled with DNA aptamers and incorporated them into a field effect transistor constructed to produce a label-free biosensor. The biosensor could measure the concentration of IgE in samples down to 250 pM, as the antibody molecules bound to the aptamers on the nanotubes. Felekis and Tagmatarchis (2005) used a positively charged pyrene compound to prepare water-soluble SWNTs and then electrostatically adsorb porphyrin rings to study electron transfer interactions. Pyrene derivatives also have been used successfully to add a chromophore to carbon nanotubes using covalent coupling to an oxidized SWNT (Alvaro et al., 2004). In this case, the pyrene ring structure was not used to adsorb directly to the nanotube surface, but a side-chain functional group was used to link it covalently to modified SWNTs. 

The LPL catalytic assay measures the hydrolysis of a [14C[- or [3H]-triolein emulsion producing the 14C- or 3H -labeled free oleic acid [6]. The 14C- or 3H-labeled oleic acid is isolated by a selective extraction procedure and its radioactivity is determined by liquid scintillation counting [40]. Lipase activity is calculated as nanomoles of oleic acid released per minute per milliliter of postheparin plasma [41]. 

Li et al. [61] reported a novel method using a sequence-specific label-free DNA sensors based on silicon nanowires (Si-NWs) by measuring the change in the conductance. Kelley s group [62] developed a gold nanowire array (Au-NW) in 15-20 nm diameter range and this array was used for electrochemical DNA detection with the help of the elect-rocatalytic reporter systems, Ru(NH3)6+ and Fe(CN). ... 

Label-free detection of ligand-aptamer interaction was also demonstrated by means of impedance spectroscopy technique [52,53]. Simultaneously, Radi et al. [52] and Rodriguez et al. [53] reported application of Faradaic impedance spectroscopy (FIS) in detection of interaction of proteins with DNA aptamers. The detection method is based on the measurement of resistance in presence of redox mediator Fe(CN)6-In absence of target protein, the negatively charged aptamer repulse the redox mediator molecules from the sensor surface. In a paper by... 

Another label-free optical detection method—FTIR-ATR—has been applied for detection of thrombin by means of DNA aptamers [73], The antithrombin DNA aptamer previously developed by Tasset et al. [17] was immobilized covalently onto Si surface using UV irradiation method. As a quantitative measure, the area of N-H and CH2 bands was used. This method allowed to detect thrombin with a sensitivity around 10 nmol/L. The specificity of binding of protein to aptamer was also investigated using DNA with no binding site for thrombin. It has been noted that for effective binding study by FTIR-ATR method, the concentration of protein should be kept lower than 100 nmol/L. 

The biosensors can be divided into nonlabelled or label-free types, which are based on the direct measurement of a phenomenon occurring during the biochemical reactions on a transducer surface and labelled, which relies on the detection of a specific label. Research into label-free biosensors continues to grow [13] however labelled ones are more common and are extremely successful in a multitude of platforms. 

In this section an overview of the numerous methods and principles for the discrimination of enantiomers is given. First, the interaction principles of the polymer-based methods adapted from chromatographic procedures are illustrated. The discrimination of enantiomers was achieved some decades ago by using different types of stationary materials, like cyclodextrins or polymer-bonded amide selectors. These stationary-phase materials have successfully been appointed for label-free optical sensing methods like surface plasmon resonance (SPR) or reflectometric interference spectroscopy (RIfS). Furthermore, various successful applications to optical spectroscopy of the well-established method of fluorescence measurements for the discrimination of enantiomers are described. 

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