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Nanoscale, Low-energy Molecular Sensors for Health Care and Environmental Monitoring
Ken Uchida, Takahisa Tanaka
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DOI: 10.22661/AAPPSBL.2019.29.3.16

Nanoscale, Low-energy Molecular Sensors for Health Care and Environmental Monitoring

KEN UCHIDA1,2 AND TAKAHISA TANAKA2
1 DEPARTMENT OF ELECTRONICS AND ELECTRICAL ENGINEERING, KEIO UNIVERSITY
2 DEPARTMENT OF MATERIALS ENGINEERING, THE UNIVERSITY OF TOKYO

ABSTRACT

In the era of the internet of things (IoT), low-energy, small-size sensors are in strong demand. Low-energy physical sensors such as gyro, sound, and optical sensors have been integrated in mobile electrical terminals such as smart phones. However, chemical sensors such as gas sensors have not been implemented in small-size electronic systems, due to issues regarding size and energy consumption. In this feature article, two types of recently developed molecular sensors are introduced; a voltage-controlled multimolecular sensor consisting of Pd-functionalized, suspended graphene, and a Pt nanosheet, which can detect ppm-level hydrogen in expired air, are presented.

INTRODUCTION

Currently, numerous electrical products are connected to the internet. Some may download the newest software from servers and others may provide global positioning system (GPS) information to improve services. We have observed great progress in artificial intelligence (AI) technologies, by which numerous kinds of valuable information for individuals and for society at large are inferred from various kinds of data. In order to enhance the accuracy of the inference, the quantity as well as the variety of the data are critical. Therefore, various kinds of sensors are expected to be implemented in every electrical product that has internet access capabilities. We anticipate that their data will be utilized actively in the "big data" societies of the future. In fact, low-power physical sensors such as temperature, gyro, and optical sensors have been already integrated into mobile terminals such as smart phones. However, chemical sensors for mobile terminals are still under development and have not been integrated in mobile devices or devices driven by energy harvesters, because of their relatively large size and energy consumption.

In this feature article, we will introduce two types of sensors recently developed by our group; a Pd-functionalized graphene sensor activated by Joule heating [1], and a Pt nanosheet sensor [2] that can detect hydrogen in breath, are presented.

PD-FUNCTIONALIZED GRAPHENE SENSORS

The analysis of the molecules in human breath is a promising diagnostic technique because a number of compounds in human breath are considered to be related to various kinds of diseases [3-5]. The H2 concentration of breath is a good indicator of disorders in small intestine, including bacterial overgrowth, colonic fermentation, abnormal fermentation, and carbohydrate intolerance. [3,6-8] The typical H2 concentration ranges from a few ppm to several hundred ppm. However, breath contains many disturbing substances, such as a high concentration of water. [6,9] Thus, to develop an easy, ubiquitous, H2-based breath diagnosis method, H2 sensors should be able to detect low and wide ranges of H2 concentrations, should be small, and should show humidity robustness and low power consumption.

Recently, we fabricated Pd-functionalized suspended graphene sensors, where heat transfer from the self-heated graphene to the substrate was successfully avoided. The graphene was suspended on the electrodes using a polydimethylsiloxane (PDMS) stamp method. [10,11] By utilizing Joule heating within the suspended graphene, we explored the possibility of selective sensing of hydrogen/humidity by applying an appropriate bias voltage in realistic gas environments, in which small changes in the H2 concentration and large variations in the relative humidity (RH) were induced.

 

Fig. 1: Schematic of fabricated Pd-functionalized graphene sensor. The graphene was suspended over two electrodes; one is a source that is grounded and the other is a drain that is biased with drain voltage (VD).

Figure 1 shows the fabricated Pd-functionalized graphene sensor. The 300-nm-thick SiO2 thermally grown on the p-type Si substrate was used as the substrate. After the formation of Ti/Pt/Au electrodes on SiO2, HOPG was exfoliated using Nitto tape (SPV224-R) and placed on commercially available PDMS (Gel-Film짰 WF-20횞4 6mil). The PDMS was used to transfer the graphene on the electrodes. The graphene was transferred by a PDMS stamping method. After cleaning, 0.3-nm-thick Pd was deposited and agglomerated by annealing at 400 째C for 30 min. The suspension was confirmed by scanning electron microscopy (SEM) and the multi-layer structure and high quality of suspended graphene was checked by Raman spectroscopy. In addition, using transmission electron microscopy (TEM), we confirmed that Pd was placed on graphene as nanoparticles (NPs) and that there were eight graphene layers. One might consider eight-layer graphene to be too thick to work with as a transducer. However, it is reported that the electrical conductivity of eight-layer graphene can be well modulated by external electric fields. [12] Therefore, eight-layer graphene can work as a transducer of hydrogen-induced changes.

 

Fig. 2: (a) Temperature dependence of the sensor response. The device was heated by an external heater. (b) Sensor response as a function of time at various drain voltages.

 

Fig. 3: Temperatures as a function of self-heating power. Experimental temperatures were extracted by comparing VD-dependent and temperature-dependent sensor responses. Simulation data were obtained using multi-physics finite element method simulator with graphene-gold thermal contact resistance of 2500 關m2 쨌 K/mW.

Figure 2a shows the sensor response, which is defined by the resistance change (ΔR) relative to the original resistance (R0), to 100-ppm H2 as a function of time at various operating temperatures from room temperature (RT) to 180 째C. The sensor response increases as the operating temperature increases to 135 째C, due to the promoted dissociative adsorption of H2. However, the sensor response decreases at 180 째C because of the desorption of hydrogen from Pd. Figure 2b shows the time dependence of the sensor response to 100-ppm H2, where the device was operated at various VD from 0.01 V to 1.1 V at RT. The sensor response increases as VD increases to 0.8 V, and then gradually decreases as VD continues to increase from 0.9 V to 1.1 V. This tendency of the VD-dependent sensor response is almost the same as that of the temperature-dependent sensor response, which clearly suggests that VD-induced self-heating was successfully achieved. To calibrate the graphene channel temperature when the self-heating technique was utilized, the sensor responses to 100 ppm of H2 using the self-heating technique and hot chuck were compared. We also verified the relationship between the input power and temperature in the graphene sensor using numerical simulations. The simulation data agree well with the experimental data as shown in Figure 3. Figure 3 demonstrates that a temperature of 100 째C was achieved by small power consumption of 1 mW by Joule heating.

 

Fig. 4: (a) Room-temperature time dependence of the sensor response to a relative humidity (RH) change from 50% to 80% at VD of 0.01 V. (b) Room-temperature time dependence of the sensor response to 10 ppm of H2 with a RH increase from 50% to 80% at VD of 0.1 V (upper) and 0.9 V (lower).

Finally, voltage-controlled multi-molecule detection by self-heating was demonstrated. Figure 4a shows the time dependence of the sensor response to humidity at a VD of 0.01 V. The reference and test gases were humid airs with relative humidity (RH) of 50% and 80%, respectively. The sensor resistance decreased under the higher humidity of 80%, because water molecules were adsorbed on the oxidized Pd and acted as acceptors (hole donors) for the graphene operating in the hole regime. As a result, the graphene resistance was decreased. Figure 4b shows the sensor response to 10 ppm H2 balanced with RH-80% humid air. Humid air with a RH of 50% was used as the reference gas. As demonstrated in Figure 4b, the sensor function can be switched by changing VD. The upper figure shows that at a VD of 0.1 V, at which the sensor operates at RT, the sensor responded to humidity. On the other hand, the lower figure shows that at a VD of 0.9 V, at which sensor operates at approximately 135 째C, the sensor responded to 10 ppm H2 even when a large RH variation from 50% to 80% occurred simultaneously. Therefore, self-heating successfully prevented the effect of humidity and resulted in the detection of a low concentration of H2. These results led to the conclusion that the sensor function can be changed using applied voltages, thanks to the self-heating effects as shown in Figure 5. This multi-functionality realized with self-heating is extremely useful as it reduces the space requirements for sensors in small electrical terminals.

 

Fig. 5: Schematics illustrating the concept of a voltage-controlled multifunctional molecular sensor utilizing Joule heating of nanomaterials.

PT-NANOSHEET HYDROGEN SENSORS

We have developed another hydrogen sensor that utilizes a Pt nanosheet. The experimental results indicate that the Pt nanosheet sensors can detect ppm-level hydrogen in expired air. The hydrogen and oxygen adsorption/desorption kinetics on the Pt surface were utilized to quantitatively model the sensor response of the Pt nanosheets, based on the surface hydrogen coverage ratio.

 



Fig. 6: (a) Schematics of Pt nanosheet sensor. (b) Cross-sectional transmission electron microscopy (TEM) image of a Pt nanosheet.

Pt nanosheets were deposited using the electron beam deposition method on Si substrates covered with a 300-nm-thick SiO2 layer on the top. Aluminum electrode formation followed. Figure 6a shows the schematic of the sensor structure. Figure 6b shows the sectional TEM image of the Pt nanosheets. Polycrystalline Pt films with cracks were observed.

 

Fig. 7: (a) Sensor response of the Pt nanosheet. (b) Hydrogen concentration dependence of the sensor response. (c) Time dependence of the sensor response to expired air.

The sensor response, which is defined by the electrical current change (ΔI) relative to the original electrical current (I0), of a Pt nanosheet was found to be robust against humidity, as shown in Figure 7a. The substrate was heated at 150 째C. Compared to these Pt nanosheets, Pd nanosheets are much less robust as hydrogen sensors. For hydrogen concentrations from 500 ppb to 200 ppm, the response of the Pt nanosheet sensor was measured at RH = 0%, 50% and 90%. The same linear sensor response as a function of hydrogen concentration was obtained as shown in Figure 7b. Since room air typically contains 550 ppb hydrogen [13], expired air contains more hydrogen than typically in the atmosphere. Thus, the response of the Pt nanosheets tested here indicates sensor response sufficient for hydrogen detection in expired air. Furthermore, sensor response under expired air is shown in Figure 7c. Apparent increases of the sensor response just after the lunch as well as seven hours after the lunch were observed. These increases of sensor response are correlated with increases of hydrogen after ingesting foods; it is known that hydrogen concentrations inc.rease approximately six hours later when food is taken by examinees [14,15]. Therefore, this experiment clearly demonstrates that the present Pt nanosheet sensor responded to low-level hydrogen in air expired by a healthy human.

 



Fig. 8: Comparison of sensor response as a function of time between experimental data (symbols) and simulated data (lines).

We simulated the dependence of the sensor response on hydrogen concentration and time, by taking into account the time-dependent molecular coverage change [2]. The time-dependent sensor response at a hydrogen concentration of lower than 20 ppm was successfully reproduced by our model, as shown in Figure 8. However, at higher hydrogen concentrations, the calculated time dependence slightly deviates from the experimental data. We consider that the deviation was caused by catalytic water formation, which is not taken into account in our present model, on the Pt surface. At a hydrogen concentration of less than 20 ppm, the surface coverage change is dominated by oxygen desorption. Therefore, the sensor response is precisely predicted by adsorption and desorption of hydrogen and oxygen, which are fully considered in the model. The robustness against humidity in hydrogen sensing is attributed to the small contribution of water to the hydrogen surface coverage.

CONCLUSION

In this article, two types of molecular sensors that we recently developed were introduced; a voltage-controlled multimolecular sensor consisting of Pd-functionalized, suspended graphene, and a Pt nanosheet that can detect ppm-level hydrogen in expired air were presented. In the Pd-functionalized, suspended graphene sensor, Joule heating is successfully utilized as a low-energy activator of chemical reactions. Although the Pt nanosheet sensor introduced in this article was heated up by an external heater, the same Joule heating method should be applicable to lower the energy consumption of sensors. We believe that these small-size, low-energy sensors will become extremely useful in the "big data" societies of the future.

References

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Ken Uchida is a professor at the School of Engineering, The University of Tokyo. He received his B.S., M.S. and PhD degrees from The University of Tokyo in 1993, 1995, and 2002, respectively. He has studied carrier and thermal transports in nanoscaled materials and has developed advanced transistors and molecular sensors.

Takahisa Tanaka is a research associate at the School of Engineering, The University of Tokyo. He received his bachelor's, master's and doctorate degrees in engineering from Keio University in 2010, 2012 and 2015, respectively. His current area of research interest is the characterization of nanostructured materials applied to LSI and sensors.

 
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