A low cost instrumentation for the characterization of the electrical capacitance of an interface towards the characterization of self-assembled monolayers

The implementation of a large number of diagnostic screening tests requires a paradigm change from the classical hospital-centered analysis lab, towards a decentralized point-of-care testing that is expected to foster the birth of personalized medicine, in the near future. Low-cost, automatic fool-proof testing instrumentation and parallelized analyses will represent key technologies for the implementation of such epochal change. Most parallelized surface-bound assays are nowadays performed with strategies that derive directly from ‘classical’ techniques, involving complex multi-step procedures that rely heavily on biochemical reactions optimized for the labelling of analytes that bind to surfaces. In a more modern (and still largely experimental) approach, most of the assay specificity relies on the specific binding of an analyte to its probing molecule that has been covalently immobilized on a designed interface. The detection of such binding is then revealed with label-free techniques that are sensitive to a change of a physical property related to the binding of the analyte material, such as a change in dielectric constant, mass, ion conductivity. In order for such strategy to be successful, it is at least required that i) the probe molecules are highly specific, exposed efficiently to the surface and bound stably, ii) the solid-liquid interface is stable over time in the conditions of the assay, iii) the measuring technique is sensitive enough. As an additional set of desired features, the entire process should be amenable of parallelization, be easy-to-use and to make, be inexpensive. Furthermore, the possibility to provide real-time data is a plus. In this communication, we report the design, implementation and characterization of an innovative instrumentation for the automatic measurement of the electrical capacitance of an electrode-solution interface. Such instrumentation, employing a principle similar to chronoamperometry, can work in parallel on a large number of measuring micro-electrodes embedded in a microfluidic system, and can provide real-time data. In our characterization, the instrumentation can be made to apply a very low measuring voltage (10 to 20 mV steps) that does not disrupt the fragile monolayers of biological probe molecules. It sports a low measuring noise (1 % over a 10 nF capacitance). Such instrumentation was employed to study the assembly and substitution of self-assembled monolayers (SAM) on a clean gold surface, and it proved sensitive enough to follow such processes in real-time. Furthermore, we have optimized the preparation of mixed functional SAM that can be used to immobilize and expose biological probe molecules (oligonucleotides or proteins) to a solution. The electrical capacitance measurements on such interface is remarkably stable (less than 1 pF/min). Such low noise and high stability represent promising features towards the use of the implemented instrumentation and SAM for point-of-care diagnostic assays.