These complications, among many others, have seen a trend towards non-enzymatic-based electrochemical protein sensors. electrochemical protein sensors. Several electrochemical detection approaches have been exploited. Basically, these have fallen into two categories: labeled and label-free detection systems. The former rely on a redox-active signal from a reporter molecule or a label, which changes upon the interaction of the target protein. In this review, we discuss the label-free electrochemical detection of proteins, paying particular emphasis to Boc-D-FMK those that exploit intrinsic redox-active amino acids. and a physicochemical detector component (detection of Tyr phosphorylation can be performed in a rapid and cost-effective format . Using this principle, we detected the inhibition of Tyr phosphorylation using a small molecule. Using DPV in conjunction with multi-walled carbon nanotube-modified SPCEs, we determined the activity of c-Src non-receptor protein tyrosine kinase, p60c-Src, in combination with its highly specific substrate peptide, Raytide. Tyr kinase reactions were also performed in the presence of an inhibitor, 4-amino-5-(4-chlorophenyl)-7- ( em tert /em -butyl)pyrazolo[3,4- em d /em ]pyrimidine (PP2) (Figure 2) . Open in a separate window Figure 2. Schematic illustration for the label-free detection of tyrosine-kinase catalysed peptide phosphorylation. The peptides that are conjugated with a magnetic bead (MB) contain a single phosphorylation site such as tyrosine (Tyr). Since Tyr has intrinsic electro-activity, the current response from its voltammetric oxidation is monitored. Under optimized conditions, Tyr residue is phosphorylated in the presence of a tyrosine kinase and ATP. During phosphorylation, the phosphate group at the -position of ATP is transferred to the hydroxyl group of Tyr. The intrinsic electro-activity of Tyr is lost upon phosphorylation and the current response decays with the increasing concentration of the tyrosine kinase. Aggregation Boc-D-FMK of -synuclein has been detected based on the redox-active Tyr and Cys residues. The authors used constant current chronopotentiometric stripping analysis (CPSA) to measure hydrogen evolution (peak H) catalyzed by -synuclein at hanging mercury drop electrodes (HMDE) and square-wave stripping voltammetry (SWSV) to measure Tyr oxidation at carbon paste electrodes (CPE). Aggregation-induced changes in peak H at HMDE were relatively large in strongly aggregated samples, suggesting that this electrochemical signal may find use in the analysis of early stages of -synuclein aggregation. Native -synuclein could be detected down to subnanomolar concentrations by CPSA . The same group successfully detected a metallothionein from rabbit liver by CPSA in conjunction with HMDE , and using a phytochelatin-modified electrode, they were successful in detecting cadmium and zinc ions . This highlights the versatility of proteins as recognition elements, serving not only for other macromolecules but also for small molecules such as heavy metals. Directly capturing the possible configuration of biomolecules, and/or their involved interactions with other molecules, without a molecular recognition element is truly a remarkable progress. Although they enable quick and simple initial investigation into whether direct label-free detection is possible or not, they have a profound limitation. They cannot be used, successfully in complex sample matrices, where various protein molecules are present. Label-free protein detection is, therefore, commonly achieved by employing biomolecules with high affinity for the target protein. This ensures much improved specificity, especially when dealing with a more complex sample matrix such as urine, cerebral spinal fluid (CSF), and serum, which contains high levels of serum albumin and immunoglobulins. In this review, we will discuss antibody-based and aptamer-based electrochemical protein Boc-D-FMK sensors that utilise label-free strategies. 3.?Antibody-based protein detection Immunosensors exploit the interaction between an antibody (Ab), Ets2 synthesised in response to the target molecule, an antigen (Ag). Antibodies can be formed, when they are attached to an immunogen carrier such as serum albumin. There are two types of Abs: polyclonal and monoclonal. Polyclonal antibodies (pAb) have an affinity for the target antigen, and are directed to different binding sites, with different binding affinities. Monoclonal antibodies (mAb), on the other hand, are identical, because they are produced from one type of immune cell. They have higher sensitivity and selectivity than pAb, and are, therefore, preferred. Antibody binding sites are located at the ends of two arms (Fab units) of the Y-shaped protein. The tail end of the Y (aka Fc unit) contains species-specific structure, commonly used as an antigen for production of Boc-D-FMK species-specific Abs. The antibody is used as the recognition layer in biosensor development. There exists a handful of general immunosensor formats Boc-D-FMK (Figure 3) . Open in a separate window Figure 3. Schematic illustration for the general immunosensor formats. (A).