Mesoporous Solid and Yolk–Shell Titania Microspheres as Touchless Colorimetric Sensors with High Responsivity and Ultrashort Response Times

Abstract

Touchless user interfaces offer an attractive pathway toward hygienic, remote, and interactive control over devices. Exploiting the humidity generated from fingers or human speech is a viable avenue for realizing such technology. Herein, titania microspheres including solid and yolk−shell structures with varying microstructural characteristics were demonstrated as high-performance, ultrafast, and stable optical humidity sensors aimed for touchless control. When water molecules enter the microporous network of the microspheres, the effective refractive index of the microsphere increases, causing a detectable change in the light scattering behavior. The microstructural properties of the microspheres, namely, the pore characteristics, crystallinity, and particle size, were examined in relation to the humidity-sensing performance, establishing optimum structural conditions for realizing humidity-responsive wavelength shifts above 100 nm, near full-scale relative humidity (RH) responsivity, ultrashort response times below 30 ms, and prolonged lifetimes. These optimized microspheres were used to demonstrate a colorimetric touchless sensor that responds to humidity from a finger and a microcontroller-based detector that translates the moisture pattern from human speech to electrical signals in real time. These results provide practical strategies for enabling humidity-based touchless user interfaces.

Respiratory Monitoring by Ultrafast Humidity Sensors with Nanomaterials: A Review

“…Limitations in the responsive RH scale up to 53% and lifetime up to several RH cycles were later dramatically improved up to ~85% and 90 cycles, respectively. This was achieved by increasing the mesopore sizes from <2 to ~7 nm and transitioning the amorphous titania particles into a polycrystalline phase that stabilizes the surface chemistry [ 111 ]. The enlarged pores provide two benefits: one is an increased ability to uptake water that increases the refractive index contrast with the surrounding and hence produces a larger change in scattering for improved optical responsivity, and the other is an enhanced diffusivity that improves the diffusion kinetics for faster response [ 112 ].…”

“…The dotted line represents the condition of equal response ( τ res ) and recovery time ( τ rec ). The reported values are dispersed from 100 to 0.01 s. Ultrafast response and recovery times shorter than 0.1 s are achieved by resistive-type [ 54 , 64 , 67 ], capacitive-type [ 129 , 130 ], and optical-type humidity sensors [ 82 , 110 , 111 ]. Frequency- and voltage-type sensors have relatively longer response and recovery times.…”

“…Water molecules interact strongly with a hydrophilic surface. As this can increase the output sensitivity to humidity change, most of the reported nanomaterials in fast-response humidity sensors are hydrophilic such as oxide nanomaterials (graphene oxide [ 54 , 102 , 103 , 105 , 143 , 144 , 145 ], silicon dioxide [ 16 , 39 , 47 , 52 ], titanium oxide [ 48 , 51 , 53 , 110 , 111 , 120 , 129 ]) and polymers [ 41 , 42 , 43 , 59 , 64 , 67 , 146 ]. Decreasing the adsorption energy of water molecules on the surface can be important for desorbing the molecules spontaneously when the environmental humidity decreases.…”

“…Duan et al summarized recent advancements in humidity sensors for human-body-related observations [ 17 ]. The recognition of verbal words has been reported from breath air [ 54 , 104 , 111 , 135 , 165 ]. Studies on phonation and voice patterns have been carried out by using strain sensors and pressure sensors on the neck of the subject [ 28 ].…”

“…Therefore, it is possible to recognize spoken words through moisture patterns from the breath. Jarulertwathana et al reported a comparison of moisture patterns from a fast-response humidity sensor and corresponding sound waveforms [ 111 ]. Large amplitudes of the sound waveforms correspond to the intensity of moisture patterns, as shown in Figure 18 a.…”