Light-weight molecular sensors have attracted growing interests in recent years, since they have a potential to be used for easy, fast health condition check. Some molecules in breath are consider as markers of some diseases. For example, acetone is an indicator of diabetes mellitus. Nonanal is consider to be one of the biomarkers of lung cancer. If we can realize small-size and low-power sensors that detect a specific molecule selectively, the sensors will be integrated with mobile terminals such as smart phone and the concentration of several molecules in breath can be checked regularly for a long time of period. The long-time monitoring of changes in biomarker concentrations could enable extremely early-stage diagnosis of diseases. Therefore, the development of sensors to detect molecules in expired air is strongly demanded. In this work, we will introduce two type of nanoscale hydrogen sensors. One is nanoscale Pt film sensors that accurately detect the concentration of hydrogen even at high relative humidity. The other is suspended graphene sensor functionalized with Pd nanoparticles that can detect both hydrogen and water selectively depending on the bias voltage applied to the sensor.

Hydrogen is an important biomarker for the human digestive system and its concentration ranges from sub-ppm to several-hundred ppm. However, accurate detection of ppm-level hydrogen in breath is difficult due to competing detection of water which is included in breath with high concentration. Here, we fabricated Pt thin films that respond to hydrogen in air at concentrations as low as 500 ppb [1]. In both dry and humid air, these films have almost identical response to hydrogen, i.e., their resistance decreases linearly with increasing hydrogen concentration regardless of relative humidity. Even at high relative humidity, these Pt thin films can detect ppm-level hydrogen. Based on the chemical kinetics, namely the adsorption and desorption of hydrogen and oxygen, the sensor response is quantitatively described by relating the hydrogen surface coverage to the magnitude of electron scattering at the Pt surface. The proposed model successfully reproduces the effects of hydrogen concentration and time on the sensitivity, particularly at hydrogen concentrations below 20 ppm.

Another hydrogen sensor consisting of suspended graphene functionalized Pd nanoparticles will be also demonstrated. Various sensors using Pd as a sensing layer or as a catalyst of carbon materials have been proposed because Pd has selectivity to H₂. However, most of them are affected by relative humidity variations. Therefore, high temperature operations for humidity robustness have been studied utilizing not only external heaters but also Joule heating (self-heating). However, most studies on self-heating focused on sensor response time and recovery time. No attention has been paid to self-heating-induced changes in the physical properties and device parameters except for temperature. Here, the effects of self-heating on Pd-functionalized suspended graphene sensors were investigated. Self-heating was realized by nanoscale point contacts between the graphene and Au electrodes, which suppressed heat dissipation to the electrodes. By applying an appropriate voltage bias, the graphene temperature was increased up to 180 °C. At temperatures over 100 °C, the sensor response realized by self-heating was lower than that by external heater heating. The response reduction was due to suppressed charged-carrier scatterings with H-induced potentials under high electric fields in the self-heated graphene. In addition, it should be noted that this sensor can detect hydrogen at high bias condition thanks to self-heating-induced high temperature and that it can detect water at low bias conditions because of easy adsorption of water on sensing materials. This multi-functionality of the sensors will be also promising for mobile applications where space availability is extremely limited.

Reference

[1] T. Tanaka et al., Sensors and Actuators B, 258, p913, 2018.