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 × 2 μm³ were measured to be ∼7.2 μm/pH units and 0.1 μm/mM of glucose concentration. First order swelling pressures of 2550 and 1580 Pa were also calculated from the deflection measurements. Extreme extrapolated sensitivities of 1 nm/1.4 × 10^-4 pH unit and 1 nm/0.01 mM of glucose concentration can be achieved using an optical lever detection method similar to the ones employed in the AFMs (resolution of 1 nm). The measured response times for these sensors were several seconds (typically < 10 s).