Silicon nitride surfaces as active substrate for electrical DNA biosensors

Miniaturization and integration are fundamental steps to fabricate smart and easy-to-use DNA biosensors to be massively exploited. Among the several materials, silicon nitride, extensively used as passivation layer in the microelectronics industry, is a good candidate for the development of biosensors employing electrical transduction. We report an exhaustive study on chemical grafting of DNA oligonucleotides on Si3N4 surfaces and electrical testing for DNA detection. In particular, we developed an optimised grafting protocol on Si3N4 surfaces characterised by contact angle, ellipsometry, Atomic Force Microscopy, and Transmission Electron Microscopy. A comparison with thermally grown SiO2 surfaces chemically treated with the same immobilization protocol was also carried out. DNA size electrical detection was carried out by immobilizing oligonucleotide probes on MIS (Metal Insulator Semiconductor)-like capacitors and measuring the characteristic capacitance voltage (C-V) curves upon hybridization with both oligo-perfect match (DNA PM) and PCR product targets. The correlation between DNA size and C-V shift has been evaluated. The analytical performances at various PM target concentrations showed a sensitivity value of 0.048 V/nM with a LoD (Limit of Detection) of about bout 2.0 nM and LoQ (Limit of Quantification) of 6.5 nM. For PCR product target, the measured sensitivity was 0.043 V/ng ml−1 with a LoD of about 2.7 ng/μl and LoQ of 8.8 ng/μl.