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Highly Responsive PEG/Gold Nanoparticle Thin Film Humidity Sensor via Inkjet Printing Technology Chun-Hao Su, Hsien-Lung Chiu, Yen-Chi Chen, Mazlum Yesilmen, Florian Schulz, Bendix Ketelsen, Tobias Vossmeyer, and Ying-Chih Liao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03433 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019
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Highly Responsive PEG/Gold Nanoparticle Thin Film Humidity Sensor via Inkjet Printing Technology Chun-Hao Su,1 Hsien-Lung Chiu,1 Yen-Chi Chen,1Mazlum Yesilmen,2 Florian Schulz,2 Bendix Ketelsen,2 Tobias Vossmeyer,2 and Ying-Chih Liao1* 1 Department
2
of Chemical Engineering, National Taiwan University, Taipei, Taiwan
Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg,
Germany ABSTRACT In this study, a highly responsive humidity sensor is developed by printing gold nanoparticles (GNP) grafted with a hygroscopic polymer. These GNPs are inkjet printed to form a uniform thin film over an interdigitated electrode with controllable thickness by adjusting the printing parameters. The resistance of the printed GNP thin film decreases significantly upon exposure to water vapor and exhibits a semi-log relationship with relative humidity (RH). The sensor can detect RH variations from 1.8% to 95% with large resistance changes up to 4 order of magnitude with no hysteresis and small temperature dependence. In addition, with a thin thickness, the sensor can reach absorption equilibrium quickly with response and recovery times of ≤ 1.2 and ≤ 3 seconds, respectively. The fast response to humidity changes also allows the GNP thin film sensor to distinguish signals from intermittent humidification/dehumidification cycles with a frequency up to 2.5 Hz. The printed * Author to whom the correspondence should be addressed. Telephone: 886-2-3366-9688, e-mail:
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sensors on flexible substrates show little sensitivity to bending deformation and can be embedded in a mask for human respiratory detection. In summary, this study demonstrates the feasibility of applying printing technology for the fabrication of thin film humidity sensors, and the developed methodology can be further applied to fabricate many other types of nanoparticle based sensor devices.
Keywords Humidity sensor, gold nanoparticle, inkjet printing, poly(ethylene glycol), respiratory monitoring
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Introduction Humidity measurement is important to various applications such as goods storage, cellular biology, environmental monitoring, agriculture, and biomedical analysis1. To detect humidity changes and to convert the humidity measurements into electric or optical signals, a variety of humidity sensing technologies have been developed2, such as resistive and capacitive sensors3, gravimetric sensors4, optics5, and surface acoustic wave technology6. In addition, many sensing materials have also been discovered or synthesized for humidity detection, such as carbon7-10, metal oxides11-17, 2D materials 18-20, and polymers21-26. Among these methods and materials, polymer based resistive humidity sensors
possess the advantages of simple structure, easy preparation, low cost, high sensitivity, rapid response, and compatibility with modern integrated circuit technology, and therefore have been used widely. Despite the aforementioned advantages of resistive humidity sensors, several challenges still remain regarding polymeric humidity sensors. First, detection at low relative humidity is still a problem for polymer based resistive humidity sensors. Due to the extremely low electrical conductivity of polymers in dry atmosphere, it is technically difficult to detect the resistance change at very low humidity, and therefore the range of detection is limited. Next, in order to cope with the low conductivity of the humidity sensing materials, thick polymer films are needed for resistance measurements. The large thickness leads to long water vapor absorption and desorption times when the humidity changes, and thus these humistors usually have long response and recovery times. To
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increase the conductivity of these sensing materials and to reduce the thickness of resistive humidity sensors, conductive materials have been added to the sensing films. Li et al. added carbon black to poly(4-vinylpyridine) to reduce the resistance at low humidity27. However, although the sensing range is extended (0 to 97 RH%), the sensitivity for humidity also decreases after the addition of carbon black. Khan et al. prepared metal nanoparticle loaded poly(ethylene glycol) (PEG) thin films for humidity sensing over a wide range from 0 to 100 RH%28. Their results show that the inclusion of gold or silver nanoparticles into a hygroscopic polymer matrix can greatly improve the conductivity of nanometer thin films and provide good sensitivity for humidity measurements. On the other hand, Zhang et al. used a layer-by-layer self assembly method to prepare polyaniline (PANI)/graphene oxide (GO) films for humidity sensing, and the results show high sensitivity, fast response/recovery characteristics, and good repeatability 29. More recently, Kano et al. utilized inorganic colloidal silicon nanocrystals as a humidity-sensitive thin film 30. The nanocrystal thin film with nanometer thickness shows great sensitivity and a dynamic response time down to 40 ms. These frontier studies show that the reduced thickness of humidity sensors can significantly reduce the response times. Thus, sensitive thin films with controllable conductivity and thickness are necessary for resistive humidity sensors with improved response characteristics. Despite the advances in sensing material synthesis, challenges still remain in the manufacturing process for humidity sensing thin films. In general, the sensing layers of polymeric resistive humidity
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sensors are usually fabricated by spin coating to produce thin films with nanometer thickness23, 30. The polymer film cannot be deposited directly onto the desired area and leads to vast material waste during the manufacturing process. To resolve these issues, inkjet printing technology, a direct writing method, provides accurate liquid deposition, and has been widely used to fabricate thin film patterns with negligible loss of materials31-32. With the advantages of pattern formation and flexible processability, inkjet printing technology have been widely used to print thin film sensors on miniaturized circuits3337.
However, inkjet printing with nanoparticles requires stable nanoparticle inks with long term
stability. Thus, polymer grafting methods38-39 are widely adopted to avoid nanoparticle aggregation or enhance nanoparticle ink stability. Recently, Schulz et al.39 utilized a two-layer strategy to greatly stabilize GNPs in aqueous solutions. The inner layer composed of mercaptoundecanoic acid (MUA) can create a high-density protection layer on GNPs, while the outer PEG layer provides sufficient steric stabilization for GNPs in aqueous solutions. Because the great ink stability and the hygroscopic nature of exposed PEG layers, this two-layer approach will be adopted here to prepare GNP inks for printed humidity sensing thin films. In this study, GNPs grafted with hygroscopic polymers are used as sensing material for resistive humidity sensor. The GNPs, which were synthesized using established protocols,
39-40
offer
advantages of stable dispersity, a well-defined and tunable surface chemistry, and very high chemical stability. By utilizing inkjet printing technology, uniform thin films with controllable thickness down
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to the sub-micrometer scale will be deposited in a confined area on flexible substrates. The contained gold nanoparticles facilitate electron transfer and thus result in a measurable resistance for these printed films with submicron thickness. The conductivity of the hygroscopic polymers will increase when exposed to water vapor, and thus one can detect humidity change by measuring the electrical resistance of printed thin films. Moreover, because of the reduced film thickness, fast water absorption/desorption is achieved and is observed as fast dynamic electric signal responses. The sensitivity, repeatability, response and recovery time with potential applications of the printed sensors will be carefully examined to demonstrate the feasibility of printed sensors for quick and accurate humidity detection.
Experiments Materials
Tetrachloroauric(III) acid (≥99.9% trace metal basis), trisodium citrate dihydrate (≥99.0%), and 11mercaptoundecanoic acid (95%) (MUA) were ordered from Sigma-Aldrich; ethanol absolute (≥99.9%) was obtained from VWR Chemicals. Ethylenediaminetetraacetic acid tetrasodium salt hydrate (EDTA) and citric acid monohydrate (≥99.5%) were obtained from Merck. α-Methoxypoly(ethylene glycol)ω-(11-mercaptoundecanoate) (PEGMUA, 2 kDa) was synthesized as described previously.41
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Deionized (DI) water with a resistivity of 18.2 MΩ cm was used. Regenerated cellulose (RC) filters (pore size: 0.2 µm) were obtained from Carl Roth GmbH.
Synthesis of functionalized PEGMUA/MUA-GNP ink
An aqueous citrate-stabilized GNP suspension was prepared following our previously published procedure40, scaled up to a total volume of 2 L. The GNPs were then functionalized with PEGMUA/MUA via ligand displacement39. The PEGMUA/MUA mixture used for the displacement reaction was prepared by mixing 6 mL of an aqueous solution containing 161 mg (80.5 µmol) PEGMUA with 3 mL of an ethanolic solution containing 17.4 mg (79,8 µmol) MUA. The resulting mixture corresponded to a 1:1 molar ratio of PEGMUA/MUA. The total volume of 9 mL ligand mixture was added rapidly to the stirred solution of citrate stabilized GNPs (volume: ~2 L). After stirring overnight at room temperature the solution was filled into centrifugation flasks and centrifuged at 10 °C and 17700 g for 2 h. The supernatant containing excess ligands and reaction byproducts was discarded and the obtained pellets of PEGMUA/MUA functionalized GNPs were dispersed in purified water and pooled. The centrifugation procedure was repeated at least two times. Finally, the GNP dispersion was passed through a syringe filter (0.2 µm, RC) and a total volume of 5 mL purified GNP dispersion was obtained. The GNP particle concentration was 0.53 µM, as determined via the method of Haiss et al. 42, corresponding to a gold mass concentration of 5.8 mg/mL.
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The inkjet GNP ink was prepared by adding DI water into the GNP solution and the final GNP concentration was 1.93 mg/mL.
Fabrication of printed thin film sensors
Interdigitated carbon electrodes were printed using a robotic dispensing system (DT-200F; Dispenser Tech Co., Ltd.). A stainless steel nozzle with an inner diameter of 0.06 mm (34G) was used to print the conductive carbon paste (Nissin EM Co. Ltd., Japan) over a polyethylene terephthalate (PET) thin film (Universal Films, Japan). The PEGMUA/MUA-GNP ink was deposited over the interdigitated electrodes on the PET thin film, which was placed on a heating stage at an operating temperature of 45 °C, by using an inkjet printing system (JetLab 4, MicroFab Technologies Inc.). Droplets of 55 μm diameter were ejected from a 50 μm piezoelectric nozzle at a frequency of 800 Hz at a speed of 6 m/s. A dot spacing of 35 μm was used for all the printed GNP thin film patterns.
Instrumentation
Transmission electron microscopy (TEM) measurements were performed using a JEOL JEM-1011 instrument operated at 100kV. The morphologies of PEGMUA/MUA-AuNPs thin films were observed with a scanning electron microscope (SEM, Nova NanoSEM 230). The UV-vis absorbance measurements were carried out with a spectrophotometer (Jasco V-670). GNP sizes in aqueous suspension were measured by dynamic light scattering (DLS, Nano ZS, Malvern Instruments, UK).
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Measurements
The printed sensors were placed in an environmental chamber (Terchy Tech. Ltd., HRM-80FA, Nantou, Taiwan) for humidity detection tests. The operating ranges of the chamber were between 1045°C and 35- 95 RH%. The resistance of the printed sensors was measured by electric meter (DM2630, HOLA). The repeatability test was performed by using an electrochemical workstation (CHI 6271E, CHI, USA). Due to slow humidity elevation rate and control difficulties at low humidity (
