Why Not Glycine Electrochemical Biosensors?

Abstract

Glycine monitoring is gaining importance as a biomarker in clinical analysis due to its involvement in multiple physiological functions, which results in glycine being one of the most analyzed biomolecules for diagnostics. This growing demand requires faster and more reliable, while affordable, analytical methods that can replace the current gold standard for glycine detection, which is based on sample extraction with subsequent use of liquid chromatography or fluorometric kits for its quantification in centralized laboratories. This work discusses electrochemical sensors and biosensors as an alternative option, focusing on their potential application for glycine determination in blood, urine, and cerebrospinal fluid, the three most widely used matrices for glycine analysis with clinical meaning. For electrochemical sensors, voltammetry/amperometry is the preferred readout (10 of the 13 papers collected in this review) and metal-based redox mediator modification is the predominant approach for electrode fabrication (11 of the 13 papers). However, none of the reported electrochemical sensors fulfill the requirements for direct analysis of biological fluids, most of them lacking appropriate selectivity, linear range of response, and/or capability of measuring at physiological conditions. Enhanced selectivity has been recently reported using biosensors (with an enzyme element in the electrode design), although this is still a very incipient approach. Currently, despite the benefits of electrochemistry, only optical biosensors have been successfully reported for glycine detection and, from all the inspected works, it is clear that bioengineering efforts will play a key role in the embellishment of selectivity and storage stability of the sensing element in the sensor.

Electrodetection of small molecules by conformation-mediated signal enhancement

“…To demonstrate the excellent and tuneable selectivity of our biosensor platform, we have used amino acids as target analytes as they present a high level of similarity and are known to be very difficult to discriminate between by established approaches. 11 Thus, the selectivity of our glycine and leucine sensing platforms was assessed against a panel of other common amino acids, including polar, non-polar, small, and bulky amino acids. An initial validation using optical (FRET) sensing showed that Cys-LeuFS was selective for leucine, with some weaker affinity for isoleucine and valine, while Cys-GlyFS was selective for glycine, with some weak affinity for leucine and valine (Fig.…”

“…[1][2][3][4][5][6][7][8] Amongst established biomarker sensing techniques, which include fluorescence, enzyme-linked immunosorbent assays (ELISAs), and surface plasmon resonance, electrochemical approaches offer several desirable advantages such as miniaturization, low cost, and fast response times. [9][10][11] Electrochemical sensors are also capable of achieving very high sensitivity, with attomolar limits of detection having been attained by nanostructuring electrode surfaces using metal/metal oxide nanoparticles, carbon nanotubes, graphene, or conducting polymers. [12][13][14] Together, these characteristics have led to the widespread adoption of electrochemical sensing platforms in point-of-care (POC) and selfmonitoring applications such as blood glucose monitoring for diabetes.…”

“…10 Despite their potential, the performance of electrochemical sensing platforms for sensing small molecule biomarkers (< 1 kDa) is comparatively worse than for macromolecular biomarkers. 11,15,16 In direct electrochemical sensing platforms, where the analyte is directly oxidized or reduced at the electrode surface, the high redox stability of small molecules requires large overpotentials to activate redox reactions involved in sensing.. Small molecules are also less structurally complex than macromolecules. Together, these factors reduce sensor selectivity.…”

“…10,11 Electrochemical biosensors, which use a biological recognition element coupled to an electrochemical transduction principle, solve this issue of selectivity by relying on the ligand specificity of biomolecules. 10,13,17 Catalytic biosensors make use of an enzyme or other redox mediators such as metal complexes 11 to perform a secondary electron transfer reaction with the analyte. However, these biosensors are difficult to generalize (with some exceptions) 18 and can produce undesirable side-products that can interfere with sensing.…”

“…Given the high specificity of SBPs and their ease of modification toward a target small molecule, our biosensor platform overcomes the issue of poor specificity and the need for complex electrode modifications inherent to previously introduced electrochemical small analyte detection platforms. 11 In doing so, we have developed the first family of electrochemical sensors that harness a protein conformational change to sensitively detect a small molecule analyte.…”