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High-performance humidity sensors utilizing dopamine biomolecule-coated gold nanoparticles
Sensors and Actuators B 191 (2014) 204–210
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
High-performance humidity sensors utilizing dopamine biomolecule-coated gold nanoparticles夽 Ho-Cheng Lee, Chun-Yi Wang, Che-Hsin Lin ∗ Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University, Kaohsiung 804, Taiwan, ROC
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Article history: Received 11 April 2013 Received in revised form 27 September 2013 Accepted 29 September 2013 Available online 8 October 2013 Keywords: Gold nanoparticles Dopamine Resistance-based Humidity sensor Hydrophilic biomolecule
a b s t r a c t This study presents a simple process for producing resistance-based humidity sensors utilizing dopamine (DA)-coated gold nanoparticles (AuNPs) as the sensing material. Highly hydrophilic dopamine biomolecules are physically bonded to 4–6 nm AuNPs to enhance humidity sensing performance. Results show that the DA-coated AuNPs have good humidity sensing performance in the range of 20–90% R.H. The measured resistance response is more than 1900 times greater than sensors using the same AuNPs without a DA coating. The developed humidity sensor shows rapid response time for water adsorption (5 s) and desorption (10 s). Moreover, a 3-day long-term measurement at low medium and high humidity ranges also showed that the developed sensor has good stability. The method developed in this study provides a simple and inexpensive method to produce high-performance humidity sensors by using DA-coated AuNPs. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Moisture distributed in nature is normally measured as a physical quantity called humidity, which is usually divided into absolute and relative humidity. The ability to accurately measure changes in relative humidity is necessary in industrial and daily life applications [1]. Furthermore, being able to precisely control humidity by using high-performance humidity sensors is important for many applications in such agriculture, industry, healthcare, and the storage of many items [2,3]. Although many commercial humidity sensors have been developed, they suffer from a number of drawbacks including slow response rate, low sensitivity, complex fabrication processes and a low degree of stability. Since the first miniaturized humidity sensor was fabricated in 1994 [4], many miniaturized humidity sensors have been reported via a variety of working mechanisms and different sensing materials. Humidity sensors with higher sensitivity and faster time response are the competing factors for developing these sensors. In general, sensing materials with large specific surface areas were usually used to enhance the sensing performance of the humidity sensors. In general, typical sensing materials for commercial available humidity sensors can be divided into metal oxides [5,6], ceramics
夽 The preliminary results of this paper were reported at the conference of IEEE Sensors, October. 29–31, 2012, Taipei, Taiwan. ∗ Corresponding author. Tel.: +886 7 5252000 4240; fax: +886 946 526044. E-mail address: [email protected] (C.-H. Lin). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.09.104
[7,8], and polymers [9,10]. However, most miniaturized commercial humidity sensors may suffer from the disadvantages of lower sensing performance or slower response time. Since using these humidity sensors for the industrial applications, it is not durable for operating with a high temperature environment or achieving the lower power consumption. In addition, the conventional solid state humidity sensors is usually require a waiting period of more than 30 s to allow the sensor to reach a stable water adsorption state [11]. Alternatively, polymer-based humidity sensing materials, such as polyethyleneimine (PEI), usually require a longer response time of 4–6 min [12]. The humidity sensor with a nano-porous film composed of polycarbonate and cellulose also required a long period of time (4–8 min) for water adsorption [13]. Since the polymer-based sensing materials exhibit a long response time, they may not meet the requirement for immediate and rapid humidity measurements. Therefore, it is important to develop humidity sensors with rapid sensing response. Moreover, to enhance the ability of analyzing the measured response by humidity variation is also one of the important factors for developing high performance humidity sensors. Nohria et al. [14] presented a resistive-type humidity sensor, which was measured by the poly-aniline film that has been self-assembled on an interdigital electrode. The resistance responses only showed the resistance change of 700 in the humidity range of 50–90% R.H. Yao et al. [15] also reported a novel material for humidity sensing, which used the core-shell structure of polyvinyl alcohol (PVA) coated gold nanoparticles as the sensing material. However, the sensing performance for this new material only showed the capacitance change of 0.66 nF in the humidity range of 11.3–93%
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R.H. Therefore, it is difficult to achieve the accurate measurement for the small humidity change with these humidity sensors. A number of recent reports indicated that the nano-materials have shown great potential for developing high performance humidity sensors due to the large specific surface area for water adsorption. Kuang et al. [16] developed a humidity sensor made by a SnO2 nanowire as the humidity sensing material. The water molecules substituted the pre-adsorbed of oxygen molecules on the SnO2 surface and changed the electrical property of the sensing film. Thus, the increased conductivity was easily used as the indicator to measure the humidity variation in the range of 30–85% R.H. Alternatively, polyimide doped with the deliquesce salt of magnesium chloride was reported for developing high performance humidity sensors. Great capacitance response was obtained in the humidity range of 15–95% R.H. [16–19]. Therefore, sensing materials based on nanostructures can significantly improve the conductivity of the typical humidity sensors. Moreover, gold nanoparticles (AuNPs) have been well known for having large surface-to-volume ratio and widely used for chemical and biochemical sensor applications. Many studies have also reported that the interdigital electrodes coating with the nano-particles can provide a large electric conductivity change for vapor sensing [20–22]. Besides, AuNPs have been used as the sensing materials for humidity sensor, such the ligand capped of AuNPs was also reported for high performance measurement of the humidity variation [23,24]. Since the surface property of the AuNP is considered to be hydrophobic, the surface requires a modification toward the hydrophilic property for using a humidity sensing material. Dopamine (DA) is known as a neuron transmission molecule that transfers transmitting neuron signals in the synapse. This biomolecule is considered to be highly hydrophilic due to the exposed of amino and hydroxyl groups. Xi et al. [25] developed DA for surface modification of hydrophobic co-polymer membranes to significantly improve surface hydrophilicity. Furthermore, DAAuNPs have even been reported to have an ability for surface modification of the PDMS micro-channel, which solved the difficulties in controlling electro-osmotic flow and residue [26]. Therefore, the DA-AuNPs can absorb water molecules on the big surface area and shows the potential for high performance humidity measurement. This study provides a novel highly hydrophilic molecule of dopamine (DA) for surface modification of AuNPs to be used for a humidity sensing material. The proposed sensor exhibits a highsensitive and high-performance of humidity measurements, due to the large specific surface area of the AuNPs can provided the better humidity sensing performance. In addition, the hydrophilic nature of the modified DA molecules on the AuNP surface provides better moisture adsorption. The mechanism for enhancing the sensing performance of developed sensor is clearly discussed and the sensing performance is also systematically investigated. The surface morphology, effects of temperature, the influence of the number of DA-AuNP coating layers, time response, and long term stability of the proposed humidity sensor are presented in this study. Hence, the proposed humidity sensor presents a material for sensitive humidity variation measurements with rapid response times.
2. Working principle of the humidity sensor This work presents a novel humidity sensing material for developing a resistance-based humidity sensor. The measurement of humidity variation in this work was performed using 25 pairs of planar interdigital electrodes with a total surface area of 8 mm × 8 mm. The proposed humidity sensor exhibits a fast response time, low noise to signal ratio and repeatability. Since
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the measured change in resistance is significantly correlated to the humidity variation, this work provides a novel material to enhance the moisture adsorption ability in resistive-type humidity sensors. The working principle of the proposed humidity sensor is based on the hydrophilic amine groups of the exposed DA molecules on the AuNP surface, which have the affinity to adsorb water molecules as shown in Fig. 1(A). Because the water molecules dissociate into H+ and OH− at room temperature, Eq. (1) provides evidence of the link between H+ and R-NH2 − . When the chip is placed in a high humidity environment, the increase of existing OH− leads to increased conductivity in the chip. This dynamic behavior of localized charges on DA-modified AuNP has been reported [27]. Hence, the more water molecules that have been adsorbed on the DA-AuNP, the lower resistance that can be measured from the proposed sensor chip. R-NH2 − +H2 O → R-NH3 + OH−
(1)
Furthermore, surface modification of DA molecules on the AuNP surface can greatly enhance sensor sensitivity due to the higher concentration of the exposed DA on the AuNP surface, increasing water adsorption. Before humidity measurements using the proposed humidity sensor are made, it is essential to know the water adsorption ability of the sensing material. Fig. 1(B) presents the contact angle of the sensor surfaces of AuNPs with and without the DA coating. The obvious variation between each surface treatment shows that the contact angle without coated DA was 45◦ , while the coated surface achieved the low contact angle of 15◦ . Results indicated the surface energy significantly increased by coating with DA. Thus, the hydrophilic property of the sensor surface was greatly enhanced, and the sensitivity for the humidity sensing was improved. Therefore, the exposed DA molecules on the AuNP surface play an important role in water adsorption ability. 3. Materials and methods 3.1. Reagent Hydrogen tetrachloroaurate (HAuCl4 ·3H2 O) (99.99%), tetra-noctylammonium bromide (98%), and 4-(dimethylamino)pyridine (DMAP) were purchased from Alfa Aesar® and sodium borohydride (NaBH4 ) were supplied by Acros® . Dopamine hydrochloride (DA) was purchased from Sigma–Aldrich® . In this study, deionized and distilled water was used throughout for the preparation of DA-coated AuNP colloids. 3.2. Preparation of the gold nanoparticles for sensing materials First, a volume of 20.0 L aqueous gold-salt solution (1 M) was added to 30.0 mL toluene dissolved with 15 mM tetraoctylammonium bromide (TOAB) and stirred for 3 min [28]. Then, 10 mL NaBH4 reduction agent (0.4 M) was added into the toluene bath of gold-salt solution and stirred for 10 min. At this time, AuNPs with diameters around 13 nm immediately occurred and a ruby red color was present in top layer of toluene. Second, 1.0 mL of the toluenebased synthesized AuNPs was extracted and added into a 1.0 mL 4-(dimethyl-amino)pyridine (DMAP) solution (0.2 M) for the AuNP phase transfer, which causes the DMAP molecule to be adsorbed onto the nanoparticle surface for crossing over the water/toluene phase boundary [29]. Finally, 0.5 mL DA solution was added into the DMAP-AuNP colloids and stirred for 10 min. The DA molecules replaced the DMAP molecules on the AuNP surface and produced the developed humidity sensing material of DA-AuNPs. Fig. 2(A) presents the TEM image of the prepared AuNPs in 1 M DA solution, where the diameter of the particles was 4–6 nm. Fig. 2(B) presents the FESEM (JEOL, JSM-6700F) image showing the DA-AuNPs on the sensor substrate. Note that the measured size for the DA-AuNPs
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Fig. 1. (A) Schematic illustration of proposed DA-AuNPs humidity sensor working principle. (B) A photo of the contact angle of the sensor surface coated AuNPs with and without DA surface modification.
was around 10 nm after Pt coating. The coating of DA molecules on AuNPs surface can be verified using the electrochemical approach which has been reported in our recent report [30]. 3.3. Fabrication of the humidity sensor chip Fig. 3 illustrates the simplified fabrication process for the proposed humidity sensor chip. A low-cost glass substrate was used for fabricating the sensing electrodes by the photolithography etching process. Initially, Al/Cr conductive metal layers with thicknesses of 2000 A˚ and 500 A˚ were deposited on the glass substrate by a pulse-DC sputtering system (Fig. 3A). The interdigital electrode structures of 50 m in width and spacing were then patterned on the Al/Cr layer by the standard photolithography process (Fig. 3B). The chip was placed in aluminum and chromium etching solution to complete the pattern transfer (Fig. 3C). Finally, the aluminum interdigital electrodes were then completed by removing the excess photoresist (Fig. 3D). Since the sensing electrodes had been patterned, a commercial spray gun was used to fabricate the proposed humidity sensing film by uniformly spraying 100 L of the prepared DA-AuNP solution on the chip at a fixed angle (Fig. 3E). The DA-AuNPs were then adsorbed on the chip surface after 30 min of solvent evaporation at room temperature (Fig. 3F), which completed the developed humidity-sensing chip. They were then preserved in dry cabinets.
placed in the commercial environmental test chamber (THS-100, KSON Instrument, Taiwan) at 25 ◦ C and pressure of 1 atm. The climate chamber was capable of generating a stable humidity and temperature levels within the range of 15–90% R.H. and 10–90 ◦ C. The variation of the humidity measurement was set at 10% R.H. and measured for 30 minutes. The resistance change at different humidities was measured from the impedance analyzer (Instek LCR-821, Good-Will, Taiwan). The measured voltage was set at 1 V and the frequency at 10 kHz. The measured humidity data from the proposed sensor chip were then collected with a sampling rate of 1 Hz via a standard RS232 interface. 4. Results and discussion In this study, the novel material DA-AuNP was used for a resistive-type humidity sensor. The measured resistance response (Rresistance ) is defined as the resistance variation for the humidity change, as shown in Eq. (2), where R0 is the recorded resistance of initial humidity measurement, and R is the resistance change during the measurement. The sensitivity (S) for the developed humidity sensor is defined as the amount of resistance response change per unit of the humidity variation, shown in Eq. (3). Resistance response (Rresistance ) = R %R.H.
R R0
(2)
3.4. Experimental setup
Sensitivity (S) =
The developed DA-AuNP humidity sensor was characterized using the experimental setup shown in Fig. 4. The sensor was
Fig. 5 presents the measured resistance response for humidity variation in the range of 30–90% R.H. at 25 ◦ C, using the sensor
(3)
Fig. 2. (A) TEM image of the synthesis AuNPs added with 1 M DA solution. The diameter of AuNPs was 4–6 nm. (B) FE-SEM image of DA-AuNPs coating on the sensor surface.
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Fig. 3. A simplified fabrication process for the developed humidity sensor chip.
spread of the AuNPs coated with different DA concentrations. The mole fraction of DA and AuNPs was calculated for 1.7 × 104 . Note the optimal DA concentration for the proposed humidity sensor is shown in the results. With the significant change shown in measured resistance, it is clear that the sensor coated with AuNPs coated with more than 1 mM DA shows good sensitivity for humidity variation. Note that the decrease in the resistance change can be measured from the increase in humidity variation. The results illustrate that the AuNPs with a coating of 1 M of DA exhibit the optimum sensitivity for the proposed humidity sensor. Compared to typical resistive-type humidity sensors, the proposed DA-AuNP humidity sensor exhibits a sensitive and clear humidity variation response due to the DA molecules having the capability of water adsorption. Fig. 6 presents the measured change in resistance of sensors prepared with different surface coatings, those coated with AuNPs only, DA-AuNPs, and the non-coated electrode. Results show a significant difference in the cycling ascent
and descent resistance responses between each different coating of humidity sensors. Note that the non-coated electrode sensor shows only little resistance variation in measured humidity from 20 to 90% R.H. However, the obvious resistance changes shown in Fig. 6 indicate that coated DA-AuNPs have a larger sensitivity for measuring humidity variation. Fig. 6 also shows that the resistance response using the sensor coated with DA-AuNPs was 13,904%/%R.H., while in contrast those coated only with the AuNPs was 7.1%/%R.H, a difference of 1958 times greater than with the AuNP coating only. These results clearly indicate that the proposed DA-AuNP-based humidity sensor provides more sensitive performance for humidity measurement. The excellent sensitivity toward humidity variation of the novel DA-AuNPs is confirmed by measurements of resistance variation in Fig. 7, which shows the effect of numerous DA-AuNP coating layers. However, the number of DA-AuNP layers exhibits no effect on the measured resistance variation, with more than one DA-AuNP layer
Fig. 4. Experiment setup for characterizing the proposed humidity sensor. Note that the sampling frequency was set 1 Hz and the frequency for impedance measurement was 10 kHz.
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Fig. 5. Measured resistance of the humidity sensor coated with AuNPs under different DA concentration surface modification at 25 ◦ C. The mole fraction of DA and AuNPs was calculated for 1.7 × 104 .
Fig. 6. Measured resistance changed of the humidity sensor coated with AuNPs, DA-Au-NPs and the blank electrode, respectively (@ 25 ◦ C).
showing the same slope for resistance response as the sensor coated with only one layer of DA-AuNP. Moreover, because it is essential to know the effect of environmental temperature on the humidity sensors, Fig. 8 presents the resistance response at temperatures of
Fig. 7. Measured resistance changed for the humidity range of 30–90% R.H. by different DA-AuNPs coating layers (@ 25 ◦ C).
Fig. 8. Measured resistance changed for the humidity range of 30–90% R.H. by different working temperatures.
25 ◦ C, 30 ◦ C, 35 ◦ C, 40 ◦ C and 45 ◦ C. Results show that under the low humidity environment of 20% R.H., the resistance exhibits no difference at different temperatures. In a high humidity environment, only a small variation in the resistance response is evident in Fig. 8, indicating that temperature affects the measurement sensitivity only marginally. Hence, the proposed humidity sensor is capable of working under the temperature range of 25–45 ◦ C. These results indicate that the proposed DA-AuNP sensors do indeed provide an inexpensive and sensitive sensing material for humidity sensor applications. Fig. 9 illustrates the measured response time for water adsorption and desorption using the developed humidity sensor. The measured relative humidity was set in the range of 30–80% R.H. at 25 ◦ C and calculated the time constant for the resistance response. It is clear that the developed humidity sensor has rapid response times for both water adsorption (5 s) and desorption (10 s). This is due to the large specific surface area of DA-AuNPs. Compared to typical humidity sensors based on polymer sensing films for water absorption, the proposed humidity sensor has a faster response time for measuring humidity variation. Moreover, because longterm stability is also an important concern for developing such sensors, in this study the DA-AuNP humidity sensor was tested for long-term stability while set with 30% R.H., 60% R.H. and 90% R.H. at
Fig. 9. Measured time response and recovery time curves of the DA-AuNPs humidity sensor. The sensor presented a fast time response of water adsorption (5 s) and desorption (10 s) to achieve the time constant of resistance response. Note that the condition for this measurement was with the humidity range of 30–90% R.H. at 25 ◦ C.
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Fig. 10. Long-term stability test (24 h) by the developed DA-AuNPs sensor for humidity of 30% R.H., 60% R.H., 90% R.H. The resistance variation of each step is 1.05%, 3.24% and 2.51% (temperature: 25 ◦ C).
25 ◦ C for 24 h of each R.H. level. After this 72-h long-term stability test, the result shown in Fig. 10 indicates that the developed sensor exhibits good stability under each different humidity condition. The standard deviation of the measured resistances for each different humidity conditions is lower than 3.24%. It is also noted that the stability of the climate chamber used for this test is 2% in the specification. The measured variation might be caused by the system variation from the climate chamber, indicating that the proposed DA-AuNP humidity sensor has good stability for long-term humidity measurement. These results show that the developed humidity sensor exhibits the potential for the reliability necessary in highperformance humidity sensors. 5. Conclusions This study successfully developed a remarkable humidity sensor for the measuring range of 20–90% R.H. at 25 ◦ C. The highly hydrophilic dopamine biomolecule is physically bonded onto 4–6 nm AuNPs to enhance the humidity sensing performance. The measured resistance response is up to 1958 times greater than a sensor using the same AuNPs without DA coating. Moreover, the developed humidity sensor shows rapid response times for water adsorption (5 s) and desorption (10 s). The temperature coefficient was tested as well. A 3-day long-term measurement at 30, 60 and 90% R.H. environments demonstrates the good stability of the developed sensor. Acknowledgement The authors would like to thank the financial support provided by the National Science Council of Taiwan. References [1] E. Traversa, Ceramic sensors for humidity detection—the state-of-the-art and future-developments, Sens. Actuators B 23 (1995) 135–156. [2] N. Yamazoe, Y. Shimizu, Humidity sensors—principles and applications, Sens. Actuators B 10 (1986) 379–398. [3] L.T. Chen, C.Y. Lee, W.H. Cheng, MEMS-based humidity sensor with integrated temperature compensation mechanism, Sens. Actuators A 147 (2008) 522–528. [4] G. Gerlachb, K. Sagera, A piezoresistive humidity sensor, Sens. Actuators A 43 (1994) 181–184. [5] M. Anbia, S.E.M. Fard, Humidity sensing properties of Ce-doped nanoporous ZnO thin film prepared by sol–gel method, J. Rare Earths 30 (2012) 38–42. [6] M. Parthibavarman, V. Hariharan, C. Sekar, High-sensitivity humidity sensor based on SnO2 nanoparticles synthesized by microwave irradiation method, Mater. Sci. Eng. C: Mater. 31 (2011) 840–844.
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Biographies Ho-Cheng Lee received the B.S. and M.S. degree in Mechanical Engineering from Chung Hua University, Taiwan, in 2006 and 2008, respectively. His master study is focused on the micro-structure fabrication by the hot embossing process on glass substrate. He is currently a Ph.D. student in the Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University. His current research interests are in nano-technologies, microfluidic systems, bio-sensors, MEMS fabrication. Chun-Yi Wang received the B.S. and M.S. degree in Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University, Taiwan, in 2009 and 2012, respectively. His master study is focused on developing the high performance of gas sensors and humidity sensors, which has been published in IEEE MEMS and Sensors Conference.
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Che-Hsin Lin received the B.S. degree in chemical engineering from National Taiwan University, Taiwan, in 1994 and the M.S. and Ph.D. degrees in biomedical engineering from National Cheng Kung University in 1996 and 2002, respectively. His master study focused on bio-ceramics and bio-mechanics. He then worked on MEMS for
bio-analytical applications in his Ph.D. He is currently an associate professor in the Department of Mechanical and Electro-Mechanical Engineering, National Sun YatSen University. His research interests are in MEMS fabrication technologies, bioMEMS, microfluidic systems and applications of atmospheric plasma.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
High-performance humidity sensors utilizing dopamine biomolecule-coated gold nanoparticles夽 Ho-Cheng Lee, Chun-Yi Wang, Che-Hsin Lin ∗ Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University, Kaohsiung 804, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 11 April 2013 Received in revised form 27 September 2013 Accepted 29 September 2013 Available online 8 October 2013 Keywords: Gold nanoparticles Dopamine Resistance-based Humidity sensor Hydrophilic biomolecule
a b s t r a c t This study presents a simple process for producing resistance-based humidity sensors utilizing dopamine (DA)-coated gold nanoparticles (AuNPs) as the sensing material. Highly hydrophilic dopamine biomolecules are physically bonded to 4–6 nm AuNPs to enhance humidity sensing performance. Results show that the DA-coated AuNPs have good humidity sensing performance in the range of 20–90% R.H. The measured resistance response is more than 1900 times greater than sensors using the same AuNPs without a DA coating. The developed humidity sensor shows rapid response time for water adsorption (5 s) and desorption (10 s). Moreover, a 3-day long-term measurement at low medium and high humidity ranges also showed that the developed sensor has good stability. The method developed in this study provides a simple and inexpensive method to produce high-performance humidity sensors by using DA-coated AuNPs. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Moisture distributed in nature is normally measured as a physical quantity called humidity, which is usually divided into absolute and relative humidity. The ability to accurately measure changes in relative humidity is necessary in industrial and daily life applications [1]. Furthermore, being able to precisely control humidity by using high-performance humidity sensors is important for many applications in such agriculture, industry, healthcare, and the storage of many items [2,3]. Although many commercial humidity sensors have been developed, they suffer from a number of drawbacks including slow response rate, low sensitivity, complex fabrication processes and a low degree of stability. Since the first miniaturized humidity sensor was fabricated in 1994 [4], many miniaturized humidity sensors have been reported via a variety of working mechanisms and different sensing materials. Humidity sensors with higher sensitivity and faster time response are the competing factors for developing these sensors. In general, sensing materials with large specific surface areas were usually used to enhance the sensing performance of the humidity sensors. In general, typical sensing materials for commercial available humidity sensors can be divided into metal oxides [5,6], ceramics
夽 The preliminary results of this paper were reported at the conference of IEEE Sensors, October. 29–31, 2012, Taipei, Taiwan. ∗ Corresponding author. Tel.: +886 7 5252000 4240; fax: +886 946 526044. E-mail address: [email protected] (C.-H. Lin). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.09.104
[7,8], and polymers [9,10]. However, most miniaturized commercial humidity sensors may suffer from the disadvantages of lower sensing performance or slower response time. Since using these humidity sensors for the industrial applications, it is not durable for operating with a high temperature environment or achieving the lower power consumption. In addition, the conventional solid state humidity sensors is usually require a waiting period of more than 30 s to allow the sensor to reach a stable water adsorption state [11]. Alternatively, polymer-based humidity sensing materials, such as polyethyleneimine (PEI), usually require a longer response time of 4–6 min [12]. The humidity sensor with a nano-porous film composed of polycarbonate and cellulose also required a long period of time (4–8 min) for water adsorption [13]. Since the polymer-based sensing materials exhibit a long response time, they may not meet the requirement for immediate and rapid humidity measurements. Therefore, it is important to develop humidity sensors with rapid sensing response. Moreover, to enhance the ability of analyzing the measured response by humidity variation is also one of the important factors for developing high performance humidity sensors. Nohria et al. [14] presented a resistive-type humidity sensor, which was measured by the poly-aniline film that has been self-assembled on an interdigital electrode. The resistance responses only showed the resistance change of 700 in the humidity range of 50–90% R.H. Yao et al. [15] also reported a novel material for humidity sensing, which used the core-shell structure of polyvinyl alcohol (PVA) coated gold nanoparticles as the sensing material. However, the sensing performance for this new material only showed the capacitance change of 0.66 nF in the humidity range of 11.3–93%
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R.H. Therefore, it is difficult to achieve the accurate measurement for the small humidity change with these humidity sensors. A number of recent reports indicated that the nano-materials have shown great potential for developing high performance humidity sensors due to the large specific surface area for water adsorption. Kuang et al. [16] developed a humidity sensor made by a SnO2 nanowire as the humidity sensing material. The water molecules substituted the pre-adsorbed of oxygen molecules on the SnO2 surface and changed the electrical property of the sensing film. Thus, the increased conductivity was easily used as the indicator to measure the humidity variation in the range of 30–85% R.H. Alternatively, polyimide doped with the deliquesce salt of magnesium chloride was reported for developing high performance humidity sensors. Great capacitance response was obtained in the humidity range of 15–95% R.H. [16–19]. Therefore, sensing materials based on nanostructures can significantly improve the conductivity of the typical humidity sensors. Moreover, gold nanoparticles (AuNPs) have been well known for having large surface-to-volume ratio and widely used for chemical and biochemical sensor applications. Many studies have also reported that the interdigital electrodes coating with the nano-particles can provide a large electric conductivity change for vapor sensing [20–22]. Besides, AuNPs have been used as the sensing materials for humidity sensor, such the ligand capped of AuNPs was also reported for high performance measurement of the humidity variation [23,24]. Since the surface property of the AuNP is considered to be hydrophobic, the surface requires a modification toward the hydrophilic property for using a humidity sensing material. Dopamine (DA) is known as a neuron transmission molecule that transfers transmitting neuron signals in the synapse. This biomolecule is considered to be highly hydrophilic due to the exposed of amino and hydroxyl groups. Xi et al. [25] developed DA for surface modification of hydrophobic co-polymer membranes to significantly improve surface hydrophilicity. Furthermore, DAAuNPs have even been reported to have an ability for surface modification of the PDMS micro-channel, which solved the difficulties in controlling electro-osmotic flow and residue [26]. Therefore, the DA-AuNPs can absorb water molecules on the big surface area and shows the potential for high performance humidity measurement. This study provides a novel highly hydrophilic molecule of dopamine (DA) for surface modification of AuNPs to be used for a humidity sensing material. The proposed sensor exhibits a highsensitive and high-performance of humidity measurements, due to the large specific surface area of the AuNPs can provided the better humidity sensing performance. In addition, the hydrophilic nature of the modified DA molecules on the AuNP surface provides better moisture adsorption. The mechanism for enhancing the sensing performance of developed sensor is clearly discussed and the sensing performance is also systematically investigated. The surface morphology, effects of temperature, the influence of the number of DA-AuNP coating layers, time response, and long term stability of the proposed humidity sensor are presented in this study. Hence, the proposed humidity sensor presents a material for sensitive humidity variation measurements with rapid response times.
2. Working principle of the humidity sensor This work presents a novel humidity sensing material for developing a resistance-based humidity sensor. The measurement of humidity variation in this work was performed using 25 pairs of planar interdigital electrodes with a total surface area of 8 mm × 8 mm. The proposed humidity sensor exhibits a fast response time, low noise to signal ratio and repeatability. Since
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the measured change in resistance is significantly correlated to the humidity variation, this work provides a novel material to enhance the moisture adsorption ability in resistive-type humidity sensors. The working principle of the proposed humidity sensor is based on the hydrophilic amine groups of the exposed DA molecules on the AuNP surface, which have the affinity to adsorb water molecules as shown in Fig. 1(A). Because the water molecules dissociate into H+ and OH− at room temperature, Eq. (1) provides evidence of the link between H+ and R-NH2 − . When the chip is placed in a high humidity environment, the increase of existing OH− leads to increased conductivity in the chip. This dynamic behavior of localized charges on DA-modified AuNP has been reported [27]. Hence, the more water molecules that have been adsorbed on the DA-AuNP, the lower resistance that can be measured from the proposed sensor chip. R-NH2 − +H2 O → R-NH3 + OH−
(1)
Furthermore, surface modification of DA molecules on the AuNP surface can greatly enhance sensor sensitivity due to the higher concentration of the exposed DA on the AuNP surface, increasing water adsorption. Before humidity measurements using the proposed humidity sensor are made, it is essential to know the water adsorption ability of the sensing material. Fig. 1(B) presents the contact angle of the sensor surfaces of AuNPs with and without the DA coating. The obvious variation between each surface treatment shows that the contact angle without coated DA was 45◦ , while the coated surface achieved the low contact angle of 15◦ . Results indicated the surface energy significantly increased by coating with DA. Thus, the hydrophilic property of the sensor surface was greatly enhanced, and the sensitivity for the humidity sensing was improved. Therefore, the exposed DA molecules on the AuNP surface play an important role in water adsorption ability. 3. Materials and methods 3.1. Reagent Hydrogen tetrachloroaurate (HAuCl4 ·3H2 O) (99.99%), tetra-noctylammonium bromide (98%), and 4-(dimethylamino)pyridine (DMAP) were purchased from Alfa Aesar® and sodium borohydride (NaBH4 ) were supplied by Acros® . Dopamine hydrochloride (DA) was purchased from Sigma–Aldrich® . In this study, deionized and distilled water was used throughout for the preparation of DA-coated AuNP colloids. 3.2. Preparation of the gold nanoparticles for sensing materials First, a volume of 20.0 L aqueous gold-salt solution (1 M) was added to 30.0 mL toluene dissolved with 15 mM tetraoctylammonium bromide (TOAB) and stirred for 3 min [28]. Then, 10 mL NaBH4 reduction agent (0.4 M) was added into the toluene bath of gold-salt solution and stirred for 10 min. At this time, AuNPs with diameters around 13 nm immediately occurred and a ruby red color was present in top layer of toluene. Second, 1.0 mL of the toluenebased synthesized AuNPs was extracted and added into a 1.0 mL 4-(dimethyl-amino)pyridine (DMAP) solution (0.2 M) for the AuNP phase transfer, which causes the DMAP molecule to be adsorbed onto the nanoparticle surface for crossing over the water/toluene phase boundary [29]. Finally, 0.5 mL DA solution was added into the DMAP-AuNP colloids and stirred for 10 min. The DA molecules replaced the DMAP molecules on the AuNP surface and produced the developed humidity sensing material of DA-AuNPs. Fig. 2(A) presents the TEM image of the prepared AuNPs in 1 M DA solution, where the diameter of the particles was 4–6 nm. Fig. 2(B) presents the FESEM (JEOL, JSM-6700F) image showing the DA-AuNPs on the sensor substrate. Note that the measured size for the DA-AuNPs
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Fig. 1. (A) Schematic illustration of proposed DA-AuNPs humidity sensor working principle. (B) A photo of the contact angle of the sensor surface coated AuNPs with and without DA surface modification.
was around 10 nm after Pt coating. The coating of DA molecules on AuNPs surface can be verified using the electrochemical approach which has been reported in our recent report [30]. 3.3. Fabrication of the humidity sensor chip Fig. 3 illustrates the simplified fabrication process for the proposed humidity sensor chip. A low-cost glass substrate was used for fabricating the sensing electrodes by the photolithography etching process. Initially, Al/Cr conductive metal layers with thicknesses of 2000 A˚ and 500 A˚ were deposited on the glass substrate by a pulse-DC sputtering system (Fig. 3A). The interdigital electrode structures of 50 m in width and spacing were then patterned on the Al/Cr layer by the standard photolithography process (Fig. 3B). The chip was placed in aluminum and chromium etching solution to complete the pattern transfer (Fig. 3C). Finally, the aluminum interdigital electrodes were then completed by removing the excess photoresist (Fig. 3D). Since the sensing electrodes had been patterned, a commercial spray gun was used to fabricate the proposed humidity sensing film by uniformly spraying 100 L of the prepared DA-AuNP solution on the chip at a fixed angle (Fig. 3E). The DA-AuNPs were then adsorbed on the chip surface after 30 min of solvent evaporation at room temperature (Fig. 3F), which completed the developed humidity-sensing chip. They were then preserved in dry cabinets.
placed in the commercial environmental test chamber (THS-100, KSON Instrument, Taiwan) at 25 ◦ C and pressure of 1 atm. The climate chamber was capable of generating a stable humidity and temperature levels within the range of 15–90% R.H. and 10–90 ◦ C. The variation of the humidity measurement was set at 10% R.H. and measured for 30 minutes. The resistance change at different humidities was measured from the impedance analyzer (Instek LCR-821, Good-Will, Taiwan). The measured voltage was set at 1 V and the frequency at 10 kHz. The measured humidity data from the proposed sensor chip were then collected with a sampling rate of 1 Hz via a standard RS232 interface. 4. Results and discussion In this study, the novel material DA-AuNP was used for a resistive-type humidity sensor. The measured resistance response (Rresistance ) is defined as the resistance variation for the humidity change, as shown in Eq. (2), where R0 is the recorded resistance of initial humidity measurement, and R is the resistance change during the measurement. The sensitivity (S) for the developed humidity sensor is defined as the amount of resistance response change per unit of the humidity variation, shown in Eq. (3). Resistance response (Rresistance ) = R %R.H.
R R0
(2)
3.4. Experimental setup
Sensitivity (S) =
The developed DA-AuNP humidity sensor was characterized using the experimental setup shown in Fig. 4. The sensor was
Fig. 5 presents the measured resistance response for humidity variation in the range of 30–90% R.H. at 25 ◦ C, using the sensor
(3)
Fig. 2. (A) TEM image of the synthesis AuNPs added with 1 M DA solution. The diameter of AuNPs was 4–6 nm. (B) FE-SEM image of DA-AuNPs coating on the sensor surface.
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Fig. 3. A simplified fabrication process for the developed humidity sensor chip.
spread of the AuNPs coated with different DA concentrations. The mole fraction of DA and AuNPs was calculated for 1.7 × 104 . Note the optimal DA concentration for the proposed humidity sensor is shown in the results. With the significant change shown in measured resistance, it is clear that the sensor coated with AuNPs coated with more than 1 mM DA shows good sensitivity for humidity variation. Note that the decrease in the resistance change can be measured from the increase in humidity variation. The results illustrate that the AuNPs with a coating of 1 M of DA exhibit the optimum sensitivity for the proposed humidity sensor. Compared to typical resistive-type humidity sensors, the proposed DA-AuNP humidity sensor exhibits a sensitive and clear humidity variation response due to the DA molecules having the capability of water adsorption. Fig. 6 presents the measured change in resistance of sensors prepared with different surface coatings, those coated with AuNPs only, DA-AuNPs, and the non-coated electrode. Results show a significant difference in the cycling ascent
and descent resistance responses between each different coating of humidity sensors. Note that the non-coated electrode sensor shows only little resistance variation in measured humidity from 20 to 90% R.H. However, the obvious resistance changes shown in Fig. 6 indicate that coated DA-AuNPs have a larger sensitivity for measuring humidity variation. Fig. 6 also shows that the resistance response using the sensor coated with DA-AuNPs was 13,904%/%R.H., while in contrast those coated only with the AuNPs was 7.1%/%R.H, a difference of 1958 times greater than with the AuNP coating only. These results clearly indicate that the proposed DA-AuNP-based humidity sensor provides more sensitive performance for humidity measurement. The excellent sensitivity toward humidity variation of the novel DA-AuNPs is confirmed by measurements of resistance variation in Fig. 7, which shows the effect of numerous DA-AuNP coating layers. However, the number of DA-AuNP layers exhibits no effect on the measured resistance variation, with more than one DA-AuNP layer
Fig. 4. Experiment setup for characterizing the proposed humidity sensor. Note that the sampling frequency was set 1 Hz and the frequency for impedance measurement was 10 kHz.
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Fig. 5. Measured resistance of the humidity sensor coated with AuNPs under different DA concentration surface modification at 25 ◦ C. The mole fraction of DA and AuNPs was calculated for 1.7 × 104 .
Fig. 6. Measured resistance changed of the humidity sensor coated with AuNPs, DA-Au-NPs and the blank electrode, respectively (@ 25 ◦ C).
showing the same slope for resistance response as the sensor coated with only one layer of DA-AuNP. Moreover, because it is essential to know the effect of environmental temperature on the humidity sensors, Fig. 8 presents the resistance response at temperatures of
Fig. 7. Measured resistance changed for the humidity range of 30–90% R.H. by different DA-AuNPs coating layers (@ 25 ◦ C).
Fig. 8. Measured resistance changed for the humidity range of 30–90% R.H. by different working temperatures.
25 ◦ C, 30 ◦ C, 35 ◦ C, 40 ◦ C and 45 ◦ C. Results show that under the low humidity environment of 20% R.H., the resistance exhibits no difference at different temperatures. In a high humidity environment, only a small variation in the resistance response is evident in Fig. 8, indicating that temperature affects the measurement sensitivity only marginally. Hence, the proposed humidity sensor is capable of working under the temperature range of 25–45 ◦ C. These results indicate that the proposed DA-AuNP sensors do indeed provide an inexpensive and sensitive sensing material for humidity sensor applications. Fig. 9 illustrates the measured response time for water adsorption and desorption using the developed humidity sensor. The measured relative humidity was set in the range of 30–80% R.H. at 25 ◦ C and calculated the time constant for the resistance response. It is clear that the developed humidity sensor has rapid response times for both water adsorption (5 s) and desorption (10 s). This is due to the large specific surface area of DA-AuNPs. Compared to typical humidity sensors based on polymer sensing films for water absorption, the proposed humidity sensor has a faster response time for measuring humidity variation. Moreover, because longterm stability is also an important concern for developing such sensors, in this study the DA-AuNP humidity sensor was tested for long-term stability while set with 30% R.H., 60% R.H. and 90% R.H. at
Fig. 9. Measured time response and recovery time curves of the DA-AuNPs humidity sensor. The sensor presented a fast time response of water adsorption (5 s) and desorption (10 s) to achieve the time constant of resistance response. Note that the condition for this measurement was with the humidity range of 30–90% R.H. at 25 ◦ C.
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Fig. 10. Long-term stability test (24 h) by the developed DA-AuNPs sensor for humidity of 30% R.H., 60% R.H., 90% R.H. The resistance variation of each step is 1.05%, 3.24% and 2.51% (temperature: 25 ◦ C).
25 ◦ C for 24 h of each R.H. level. After this 72-h long-term stability test, the result shown in Fig. 10 indicates that the developed sensor exhibits good stability under each different humidity condition. The standard deviation of the measured resistances for each different humidity conditions is lower than 3.24%. It is also noted that the stability of the climate chamber used for this test is 2% in the specification. The measured variation might be caused by the system variation from the climate chamber, indicating that the proposed DA-AuNP humidity sensor has good stability for long-term humidity measurement. These results show that the developed humidity sensor exhibits the potential for the reliability necessary in highperformance humidity sensors. 5. Conclusions This study successfully developed a remarkable humidity sensor for the measuring range of 20–90% R.H. at 25 ◦ C. The highly hydrophilic dopamine biomolecule is physically bonded onto 4–6 nm AuNPs to enhance the humidity sensing performance. The measured resistance response is up to 1958 times greater than a sensor using the same AuNPs without DA coating. Moreover, the developed humidity sensor shows rapid response times for water adsorption (5 s) and desorption (10 s). The temperature coefficient was tested as well. A 3-day long-term measurement at 30, 60 and 90% R.H. environments demonstrates the good stability of the developed sensor. Acknowledgement The authors would like to thank the financial support provided by the National Science Council of Taiwan. References [1] E. Traversa, Ceramic sensors for humidity detection—the state-of-the-art and future-developments, Sens. Actuators B 23 (1995) 135–156. [2] N. Yamazoe, Y. Shimizu, Humidity sensors—principles and applications, Sens. Actuators B 10 (1986) 379–398. [3] L.T. Chen, C.Y. Lee, W.H. Cheng, MEMS-based humidity sensor with integrated temperature compensation mechanism, Sens. Actuators A 147 (2008) 522–528. [4] G. Gerlachb, K. Sagera, A piezoresistive humidity sensor, Sens. Actuators A 43 (1994) 181–184. [5] M. Anbia, S.E.M. Fard, Humidity sensing properties of Ce-doped nanoporous ZnO thin film prepared by sol–gel method, J. Rare Earths 30 (2012) 38–42. [6] M. Parthibavarman, V. Hariharan, C. Sekar, High-sensitivity humidity sensor based on SnO2 nanoparticles synthesized by microwave irradiation method, Mater. Sci. Eng. C: Mater. 31 (2011) 840–844.
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Biographies Ho-Cheng Lee received the B.S. and M.S. degree in Mechanical Engineering from Chung Hua University, Taiwan, in 2006 and 2008, respectively. His master study is focused on the micro-structure fabrication by the hot embossing process on glass substrate. He is currently a Ph.D. student in the Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University. His current research interests are in nano-technologies, microfluidic systems, bio-sensors, MEMS fabrication. Chun-Yi Wang received the B.S. and M.S. degree in Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University, Taiwan, in 2009 and 2012, respectively. His master study is focused on developing the high performance of gas sensors and humidity sensors, which has been published in IEEE MEMS and Sensors Conference.
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Che-Hsin Lin received the B.S. degree in chemical engineering from National Taiwan University, Taiwan, in 1994 and the M.S. and Ph.D. degrees in biomedical engineering from National Cheng Kung University in 1996 and 2002, respectively. His master study focused on bio-ceramics and bio-mechanics. He then worked on MEMS for
bio-analytical applications in his Ph.D. He is currently an associate professor in the Department of Mechanical and Electro-Mechanical Engineering, National Sun YatSen University. His research interests are in MEMS fabrication technologies, bioMEMS, microfluidic systems and applications of atmospheric plasma.