Advances in the definition of a drop-based functionalization protocol for CMOS-compatible MEMS biosensors

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 (ARXPS). 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 . Then, a sample containing several MEMS resonators underwent a similar procedure. The amino coated resonators were then exposed to a drop of solution containing an oligonucleotide specifically designed to link to a portion of human MGMT (methylguanine-DNA methyltransferase) mRNA, and subsequently to its FITC fluorescent labeled complementary target (again in drop form). 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. Both the XPS and fluorescence data suggest that the drop-based protocol can be successfully used for the bioactivation of MEMS-based biosensors.