Scientists at University of Illinois at Chicago (UIC) are taking the unique approach of using the natural stimuli responses in bacterial spores to create high-accuracy humidity sensors. Spores have a hygroscopic membrane that allows water to flow in and out of the cell as the environmental humidity changes, making them magnitudes more responsive than polymers commonly used in humidity sensors.
By integrating graphene quantum dots (GQDs) on the surface of the spores, scientists can measure the changes in electron tunneling between each dot, or the bio-device’s conductivity, as the transmembrane hydraulic pressure causes the spore to shrink or expand. The spores could eventually be used in the human body to monitor organ function, cancer status, and hydration.
Osmosis, or the transport of water along a concentration gradient, occurs in living cells with a water-permeable membrane. Water osmoses from high to low water concentration, until equilibrium is achieved between bodies. So, the spore intakes water when the environmental water concentration becomes higher than that of the spore, and loses water when its water content is higher than that of the environment. All the while, hydraulic pressure relative to the spore’s interior and its environment increases as the spore expands, and decreases as the spore shrinks. The scientists were able to relate these changes in relative pressure to changes in conductivity.
The scientists placed a graphene-covered spore on a silica-on-silicone chip, spanning two gold/chromium electrodes, separated by 5 microns. They ran a bias voltage (35 meV) across the spore to spur electron transport between the graphene dots. The scientists varied humidity by passing N2 gas around the device. Meanwhile, they measured the change in conductivity due to the changes in electron tunneling distance between GQDs, as the spore shrank and expanded. As the spore shrinks, spacing between the GQDs also shrinks, facilitating the transport of electrons between dots.
In conclusion, a 300 Torr (about a thousandth of atmospheric pressure) change in relative pressure due to altering humidity causes a 1.63-nm change in electron tunneling between graphene dots. This yields an impressive five-fold change in conductivity. Because spores are so responsive, they hold lots of potential as bio-electromechanical humidity sensors.