The Hercules beetle, native to the Central and South American rainforests, changes its shell colors when the level of the relative humidity in its environment changes. The beetle's shell reflects an olive-green or yellow color at low humidity and a nearly-black color at higher humidity—a result of changes in light refraction. 

Taking inspiration from the Hercules beetle's shell, a team of researchers from the Pohang University of Science and Technology (POSTECH) has fabricated a real-time "ultrafast" humidity-sensing optical sensor.

How a typical optical humidity sensor operates. Image used courtesy of MDPI

A Key Instrument in Optics: The Fabry-Pérot Interferometer 

To develop this sensor, the researchers looked to a common instrument in optics, the Fabry-Pérot interferometer (or "etalon"). An etalon features two partially-silvered surfaces in parallel. In the cavity between these two surfaces, optical waves can reflect back and forth—only when they are in resonance with it. With multiple beams interfering in the cavity, the interferometer can offer extremely high resolution.

Diagram of a Fabry-Perot interferometer. Image from Stigmatella aurantiaca [CC BY-SA 3.0]

In contrast, the sensor the POSTECH researchers used included gaps between metal nanoparticles, creating a number of membrane-like pores. These pores afforded the prototype a speed 10,000 times faster than conventional Fabry-Pérot interferometer-based optical sensors.

This sensor operates like many other colorimetric indicators for relative humidity measurement: as the relative humidity level changes, the colorimetric indicator exhibits different colors—red, green, blue—that are observable to the naked eye.

Researchers Use a Highly Water-sensitive Etalon

The researcher's novel colorimetric sensor was based on a tunable Fabry-Pérot-like resonator. This resonator included a top layer of disordered metal nanoparticles and a bottom layer of metallic mirrors. In between these two layers was a gas-responsive medium.

When the Fabry-Pérot-like resonator came in contact with external humidity, it created a resonant frequency from an internal hydrogel film. The resonant frequency depended on the thickness and the refractive index of the hydrogel film.

In response to changing humidity, the chitosan hydrogel swells and contracts.Image used courtesy of Science Advances

The POSTECH researchers claim that this chitosan (a biodegradable polymer) hydrogel film can quickly swell and contract like a super sponge: it can absorb water around it at nearly 1,000 weight percentage of its dried weight. This highly-absorbant quality comes from the hydrogen bonding between water molecules and the chitosan polymer. 

Close-up view of a metallic nanoparticle hydrogel-mirror etalon under a humid and dry state surrounding. Image used courtesy of POSTECH

Humidity-sensitive Optical Sensors for Security

With this humidity-sensitive optical sensor, the researchers envision a number of applications in security—from 3D QR codes to anti-counterfeit use cases. Lead researcher Professor Rho specified that security codes using such water-responsive color pixels can be integrated into security tags for electronic devices, ID cards, passports, and banknotes sensitive to humidity. 

The optical sensor could even be employed in gas and water vapor detection, the team posits; with the prototype's fast colorimetric sensing capabilities, it could be used to sense monoatomic gases such as hydrogen and oxygen, volatile organic compounds, and toxic gases such as hydrogen sulfide and nitrogen oxide.

This research was bankrolled by Samsung Research Funding and its Incubation Center for Future Technology.