Influence of the Molecular Orientation and Ionization of Self-Assembled Monolayers in Biosensors: Application to Genosensors of Prostate Cancer Antigen 3

Abstract

Electrochemical sensing depends on the immobilization of biorecognition elements and charge transfer to the electrode, which can be tuned and sometimes controlled using self-assembled monolayers (SAMs) as matrices. In this study, we show that an efficient immobilization of an ssDNA probe to detect the prostate cancer antigen 3 biomarker is achieved even for less organized SAMs provided that the terminal groups were ionized. These conclusions were reached by comparing the biosensors made with 11-mercaptoundecanoic acid (11-MUA) prepared in ethanol and thioglycolic acid (mercaptoacetic acid: MAA) prepared in ethanolic and aqueous solvents. The highest sensitivity with electrochemical impedance spectroscopy was observed for sensors assembled over MAA monolayers on gold prepared in ethanol and water (at pH 12.5), with limits of detection of 1.2 × 10–9 and 1.7 × 10–9 mol L–1, respectively. The biosensors made on MAA films prepared in water at pH 7.0 and 2.5 were not efficient because they are mostly terminated by protonated carboxylic acid groups, according to sum-frequency generation vibrational and polarization modulation-infrared reflection-absorption (PM-IRRAS) spectroscopies, which prevent an adequate attachment of the biomarker. Well-organized SAMs of 11-MUA also showed good detection performance, but the limit of detection was not as low, 2.1 × 10–9 mol L–1, probably due to its long chains that decrease the charge transfer. Therefore, the matrix fabrication method can control the molecular assembly of biosensors and hence their performance.