|Case Study: Gold Electrode Preparation and Characterization|
|A company developing a biosensor based on self-assembled monolayers (SAMs) on a gold electrode
approached BEST-Bio to help solve a problem they were having with irreproducibility in their manufacturing process. Some of their sensors were
failing in performance tests, and the failure appeared to be correlated to specific batches of electrodes. BEST-Bio was tasked to develop an assay
to screen batches of electrodes in terms of performance criteria. |
|BEST-Bio analyzed the process flow and realized that the only quality control test for the biosensor was the
functional test after full assembly. It is well known in the literature that the surface properties of the gold affect the final properties of the SAM and
therefore the ultimate performance of the sensor. Therefore, BEST-Bio proposed that the company begin testing the electrodes at earlier stages of the manufacturing
process, i.e., before and after the SAM is formed on the gold electrode. |
|Electrode Testing Before SAM Formation|
|One of the primary requirements for the formation of a high quality, densely-packed SAM on gold is a smooth surface.
To assess the smoothness of the gold electrodes, BEST-Bio proposed the use of cyclic voltammetry to measure the effective electrochemical surface area. Briefly,
the electrode was scanned to +1.6 V vs. SCE and then back to -0.1 V vs. SCE. In the anodic sweep, oxygen chemisorbs onto the gold, forming a 1:1 oxide layer.
In the subsequent cathodic sweep, this oxide layer is reduced, resulting in a large cathodic peak. Integration of this peak allows calculation of the effective
electrochemical surface area of gold. Comparison of the effective electrochemical surface area to the theoretical value gives an estimate for the surface roughness.|
|In addition to testing the gold electrode properties before the formation of the SAM, BEST-Bio adapted a literature
protocol for achieving a reproducible surface at that stage of the manufacturing process. The protocol involved applying sequential pulses of anodic and cathodic
potential to the electrode for a prescribed amount of time. After an optimized number of pulses, the surface roughness was observed to achieve a minimum, offering
the ideal surface for SAM formation.|
|Electrode Testing After SAM Formation|
|BEST-Bio also developed a quantitative assay for the quality of the SAM employing a standard electrochemical technique.
Briefly, a linear potential sweep was performed in the cathodic direction, and the desorption of the SAM was observed at roughly -1.0 V vs. SCE. The area under the
desorption peak directly correlated with the quality of the SAM, and therefore offered a QC tool that directly related to the final performance of the sensor.|