Impact of Surface Roughness on the Self-Assembling of Molecular Films onto Gold Electrodes for Label-Free Biosensing Applications

The properties of molecular films assembled over gold electrode surfaces are prone to reflect fabrication characteristics. Here we show the importance of controlling electrode surface characteristics of a gold interface destined to label-free electrochemical biosensing applications and how to decrease the effect of the roughness on the properties of the interface by using redox composite materials within 5 to 10 nm in thickness. This controlling of surface characteristics allows us to fabricate reproducible, disposable printed circuit board microelectrodes for label-free electrochemical capacitive assays, which are specifically aimed at applications in point-of-care molecular diagnostic devices. We demonstrate that both the electrochemical roughness factor and the mechanical roughness root mean square of the surface are useful parameters for the quality control of molecularly assembled films and the chemical modification of the surface. For instance, the measured ionic capacitance of gold interfaces is linear as a function of an electrochemical roughness factor lower than ca. 2. On the other hand, roughness factor values higher than ca. 2 lead to a random ionic capacitance outcome. A similar trend of presenting reproducible or irreproducible electrochemical behaviour, wherein a roughness threshold exists, is perceived for charge transfer resistance whether the interfacial redox activity is blocked with a dielectric molecular layer. It was also demonstrated that redox reversibility of gold interfaces is sensitive to the roughness root mean square. Finally, we also confirmed that if the roughness is controlled below a well-defined threshold or if a film thickness (within 5 to 10 nm) is assembled over the surface, the sensing properties are reproducible in micro-fabricated printed circuit board electrodes used in assaying C-reactive protein in human serum. This is an important upshot that permit the microfabrication of cost-effective microelectrodes for label-free electrochemical capacitive biosensors, within the capability to be further integrated into Lab-on-PCB microsystems.