Drop-coating silanization of silicon substrates as a step towards the fabrication of CMOS-based MEMS biosensors

MEMS-based biosensors are a promising new platform for the delivery of diagnostic services close to the point of care, where issues like reliability, ease of use, and low cost are of primary importance. The intrinsically parallel nature of MEMS fabrication allows for very low unitary cost of elementary MEMS components. As an added benefit, the reuse or modification of standard Complementary MOS (CMOS) technologies allows the coexistence on the same silicon chip of MEMS components and the driving and conditioning circuitry. In the case of biosensors, specific issues related to the bio-activation of the sensor surface and its compatibility with on-chip MEMS and electronics have to be taken into account. While standard biofunctionalization procedures normally involve immersion of the sample in the required solutions, this approach may not be feasible for silicon chips containing mechanically sensitive MEMS components and electronic circuits. Moreover, the bio-coating technique must be compatible with sensor package and wiring. In this work, the use of drop-coating as a substitute to immersion for the creation of bioactive surfaces on MEMS sensors is investigated. The target sensor platform is a CMOS-based resonant sensor based on the microbalance principle. Preliminarily, a test to verify the effectiveness of the functionalization protocol was performed: test silicon dioxide surfaces were cleaned in an ammonia-based hydroxylation solution, and silanized through drop-coating with an aqueous-based APTES (amino-propyl-triethoxysilane) solution as the preliminary step towards the deposition of a bioactive layer. The surfaces were studied by means of conventional and angle resolved x-ray photoelectron spectroscopy. The spectroscopic characterization confirmed that the resulting surface chemical composition was not significantly different upon the two alternative processing approaches: both the atomic percentages values and the outermost layer in-depth distribution of the functionalities are comparable for the two approaches. Subsequently, a sample containing several MEMS resonators [4] underwent a similar procedure. The amino coated resonators were then exposed to a solution containing an oligonucleotide specifically designed to link to a portion of human MGMT (methylguanine-DNA methyltransferase) mRNA [6], and subsequently to its FITC fluorescent labeled complementary target. A comparison between this sample and a reference sample, not exposed to the target, shows a clear fluorescence signal and can interpreted as the occurrence of a specific binding between probe and target.