Direct application strategy to immobilise a thioctic acid self-assembled monolayer on a gold electrode

Immobilisation of a self-assembled monolayer (SAM) onto an electrode surface is often achieved by immersing it in a solution for over 24 h. A biological or biologically derived recognition component can then be linked to the SAM in fabricating a biosensor. This time consuming immobilisation step can be a drawback in biosensor development, especially when repeated preparations of the biosensor are required. In this work, an alternative immobilisation strategy involving the direct application of a known quantity of the ethanolic solution of the alkanethiol, thioctic acid, on a gold electrode surface was studied. The solution was left to dry at room temperature for approximately 20 min. Comparable results including the relative percentage decrease in double layer capacitance, the surface coverage and the percentage of binding to the bacterial protein, Protein A, were obtained relative to those obtained with SAM formed by the immersion method. Shewhart's statistical analysis technique was applied to examine the stability in terms of the relative percentage decrease in double layer capacitance. In these tests, within 99.7% confidence control limits, only a 1% deterioration was observed over a 3-month period. Therefore, all these results have demonstrated that the direct application method yields a stable thioctic acid SAM on a gold electrode surface with characteristics similar to those obtained with an immersion method. However, formation of a SAM using direct application can be achieved within a significantly shorter period of time compared to immersion method. (C) 2003 Elsevier B.V. All rights reserved.