Label-Free Biosensing of DNA in Microfluidics Using Amorphous Silicon Capacitive Micro-Cantilevers

Biosensing uses biorecognition reactions to detect molecules of interest such as biomarkers of disease, food contaminants or environmental agents. Label-free detection techniques, due their simplicity, are promising for point-of-care applications. In this paper, label-free biosensing of DNA is demonstrated with amorphous silicon micromechanical cantilevers operating in the stress-mode. The sensors are fabricated on glass substrates in a bottom electrode configuration, for capacitive readout, and are integrated with microfluidic channels for reproducible fluid handling. The amplitude noise in the capacitance measurement is ~20 aF in air and ~731 aF in water, corresponding to minimum detectable tip deflections of 42.8 nm in air and 17.2 nm in water. The sensor functionalization is performed using gold-thiol chemistry under optimized conditions, resulting in a DNA probe density of 8.50 × 10
¹²
molecules cm
^-2
, corresponding to a coverage of 34 % of the gold surface and an average spacing between molecules of ~3.4 nm. With this system, target DNA with the sequence of microRNA-145, a cancer-associated biomarker, is detected with a limit of detection of 63 nM (corresponding to a mass of ~613 fg). This type of sensors can be multiplexed to allow the detection of multiple targets and integrated with portable electronics for point-of-use applications.