An ultrasensitive chemical microsensor based on self-aligned dry-patterned environmentally sensitive hydrogels

In this paper, we report on the fabrication and testing of an ultrasensitive chemical microsensor based on an environmentally sensitive hydrogel actuated cantilever beam. A single-mask dry-etching technique was used to pattern two kinds of hydrogels (pH and glucose sensitive) underneath the beam without requiring any alignment (self-aligned) and photoinitiators. The hydrogels swell in response to an increase in the pH or glucose concentration, thereby deflecting the microcantilevers by pushing it up. Typical sensitivities for a sensor having dimensions of 500×20×2μm³ were measured to be ∼ 7.2μm/pH units and 0.1μm/mM of glucose concentration. First order swelling pressures of 2550 and 1580Pa were also calculated from the deflection measurements. Extreme extrapolated sensitivities of 1nm/1.4×10^-4 pH unit and 1nm/0.01mM of glucose concentration can be achieved using an optical lever detection method similar to the ones employed in the AFMs (resolution of 1nm). The measured response times for these sensors were several seconds (typically < 10s).