To demonstrate the feasibility of a fast, lowenergy breath diagnosis method, hydrogen sensors utilizing Joule self-heating were developed. The sensors consisted of suspended graphene films functionalized with Pd nanoparticles as sensing layers and utilized self-heating to achieve fast responses and humidity robustness with low energy consumption. Thanks to nanoscale point contacts between the graphene and Au electrodes, heat dissipation to the electrodes was greatly suppressed. The application of an appropriate voltage bias increased the graphene temperature to 180 °C with a low power consumption per unit graphene width of 0.93 mW/μm. Because of the temperature increase caused by the Joule self-heating of the graphene, the sensors responded to parts per million level H 2 , and a response time of 15 s was achieved at a H 2 concentration of 100 ppm. At temperatures over 100 °C, the sensor response realized by self-heating was lower than that by heating using an external heater. The response reduction was due to suppressed charged-carrier scatterings with H-induced potentials under high electric fields in the self-heated graphene. Finally, we demonstrated voltage-controlled multimolecular detection by self-heating. At a two-terminal voltage difference of 0.1 V, Pd-functionalized graphene responded not to H 2 but to humidity. Meanwhile, at 0.9 V, the sensor responded to 10 ppm of H 2 despite great humidity variations in the background. Temperature change by self-heating is much faster and requires less energy than that by heating using an external heater. Fast, low-energy multimolecule detection realized by self-heating paves the way for mobile, low-power, real-time molecular sensors for use in health diagnosis applications.

Recently low-power, small gas sensors have been strongly demanded to realize "super smart society." In particular, ammonia: NH 3 sensors are expected to be key devices for breath diagnosis. However, it is difficult for NH 3 sensors to obtain high selectivity against hydrogen: H 2 , since conventional metal-oxide gas sensors respond to any reducing gases. In this study, Co-porphyrin functionalized graphene sensors were fabricated, and selective NH 3 sensing was realized by the selective metal-ligand bond of Co with NH 3 . The sensor successfully detected sub-ppm NH 3 , while it showed no response to high concentration H 2 . Furthermore, we investigated the effect of humidity on the Co-porphyrin functionalized graphene NH 3 sensor. We demonstrated the detection of low concentration NH 3 even under rapid humidity changes. We considered that the key sensing mechanism of our sensor is charge transfer to graphene by the electronic structure change of Co-porphyrin-graphene complexes due to NH 3 adsorption on Co-porphyrin.

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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.