Capacitance-Based Biosensor for the Measurement of Total Loss of L-Amino Acids in Human Serum during Hemodialysis

In this paper, we present a biosensor based on a gold nanoparticle (AuNP)-modified Pt electrode with an adjusted membrane containing cross-linked L-amino acid oxidase for the detection and quantification of total L-amino acids. The designed biosensor was tested and characterized using the capacitance-based principle, capacitance measurements after electrode polarization, disconnection from the circuit, and addition of the respective amount of the analyte. The method was implemented using the capacitive and catalytic properties of the Pt/AuNP electrode; nanostructures were able to store electric charge while at the same time catalyzing the oxidation of the redox reaction intermediate H₂O₂. In this way, the Pt/AuNP layer was charged after the addition of analytes, allowing for much more accurate measurements for samples with low amino acid concentrations. The combined biosensor electrode with the capacitance-based measurement method resulted in high sensitivity and a low limit of detection (LOD) for hydrogen peroxide (4.15 μC/μM and 0.86 μM, respectively) and high sensitivity, a low LOD, and a wide linear range for L-amino acids (0.73 μC/μM, 5.5 μM and 25-1500 μM, respectively). The designed biosensor was applied to measure the relative loss of amino acids in patients undergoing renal replacement therapy by analyzing amino acid levels in diluted serum samples before and after entering/leaving the hemodialysis apparatus. In general, the designed biosensor in conjunction with the proposed capacitance-based method was clinically tested and could also be applied for the detection of other analytes using analyte-specific oxidases.