Performance Enhancement of a Microfabricated Resonator Using

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Performance enhancement of a microfabricated resonator using electrospun nanoporous polymer wire Seokyung Hwang, Wuseok Kim, Heewon Yoon, and Sangmin Jeon ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00461 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Performance enhancement of a microfabricated resonator using electrospun nanoporous polymer wire Seokyung Hwang, Wuseok Kim, Heewon Yoon, and Sangmin Jeon * Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk, Republic of Korea

Abstract A nanoporous polymethylmethacrylate (PMMA) wire was prepared by electrospinning under high humidity and attached between two prongs of a microfabricated quartz tuning fork (QTF). Exposure of the QTF to ethanol vapor caused a frequency shift due to a decrease in the modulus of the PMMA wire, and the frequency change increased as the concentration of ethanol vapor increased. The nanoporous wire-coated QTF exhibited higher sensitivity and faster response time than a plain wire-coated QTF, which was attributed to the high surface area and pore networks facilitating the transport of ethanol molecules inside the PMMA wire. Keywords: Microresonators, Quartz tuning fork (QTF); Electrospinning; Porous wire; Gas sensor

* Corresponding author. E-mail: [email protected], Phone number: +82.54.279.2392.

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Gas sensors such as alcohol sensors for preventing drunk driving and environmental sensors for monitoring hazardous compounds in air and water are frequently encountered in our daily lives. Gas sensors based on semiconducting metal oxides measure the electrical changes caused by the adsorption of gas molecules on the sensor surface.1-3 Although the metal oxide gas sensors have many advantages that are sensitive, compact and robust, they require high temperature and power for operation. Recently, conducting polymer-based gas sensors have attracted much attention as a promising alternative to the metal oxide gas sensors because they can be operated at room temperature.4-6 However, their wide applications are limited because most polymers are not conductive. To take advantage of polymer-based gas sensors that can use numerous types of polymers as sensing materials, it is essential to measure changes in the nonelectrical properties of polymers, such as changes in mass and mechanical strength caused by gas adsorption. Microfabricated

resonating

sensors

including

microcantilevers,

quartz

crystal

microresonators (QCM), and quartz tuning forks (QTF) are sensitive to adsorption-induced changes in mass and surface stress, which can be calculated by measuring the change in the resonance frequency of the microresonator during gas adsorption.7-10 Among these microresonators, QTF possesses unique properties that originate from its geometry comprising two vibrating prongs with a specific gap. QCMs and microcantilevers can measure the changes in mass and stress occurred only on the sensor surface, whereas QTFs can measure changes in the mass and stress of a free-standing sample attached between the two prongs. The free-standing sample, for instance a polymer wire or a polymer membrane, is stretched and compressed by the two vibrating prongs and the change in the mechanical properties of the sample can be measured from the change in the resonance frequency.11-15 Tao et al. prepared a polymer wire using a tip-drawing method and attached the wire on the two prongs of a QTF. The sensitivity of the polymer wire-coated QTF was significantly


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improved compared to the plain QTF but the response time of the sensor was low owing to the slow diffusion of gas molecules into the polymer. They addressed this problem by reducing the diameter of a small portion of the wire.13 However, this approach using a focused ion beam under vacuum is expensive and the tip-drawing method is not suitable to produce polymer wires with even thickness. In this paper, we solved these problems by attaching a nanoporous polymer microwire onto the prongs of a QTF. Nanoporous polymethylmethacrylate (PMMA) wires were prepared by an electrospinning method under high humidity. Evaporation of the volatile solvent during electrospinning cools the polymer wire, leading to the condensation of moisture into water droplets. The presence of a non-solvent (water) induces the phase separation of the polymer solution and produces nanoporous PMMA wires.16-17 Upon exposure of the porous wire-coated QTF (pw-QTF) to ethanol vapor, the resonance frequency of the QTF decreased due to the ethanol absorption-induced decrease in the modulus of the PMMA wire, which was proportional to the concentration of ethanol vapor. The pw-QTF exhibited higher sensitivity and shorter response time than the plain PMMA wire-coated QTF (w-QTF), which was attributed to the high surface area and pore networks facilitating the transport of ethanol molecules inside the PMMA wire.

Experimental section Materials QTFs with a resonance frequency of 32.76 kHz and a spring constant of 13 kN m-1 were obtained from ECS Inc (Olathe, Kansas). They are 200 µm wide, 600 µm thick, and 3400 mm long. PMMA (Mw = 350,000 g mol-1) and dichloromethane (DCM) were purchased from Aldrich (Saint Louis, MO). N,N-dimethylformamide (DMF) was obtained from Samchun (Seoul, Korea) and used without further purification. Deionized water (18.3 MΩ-cm) was


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purified using a reverse osmosis water system (Human Corporation, Korea). Ethanol was purchased from Merck (Darmstadt, Germany).

Electrospinning of plain and nanoporous PMMA microwires PMMA was dissolved in DCM/DMF (4:1, v/v) to make a 13 wt% solution, which was loaded into a 10 mL plastic syringe. The distance between the nozzle and the collector was maintained at 15 cm. Relative humidity (RH) was controlled at 30% for the plain wire and 60% for the nanoporous wire.16 The applied voltage was controlled 5–6 kV and the flow rate was fixed at 20 μL min-1. The electrospun PMMA wires were collected over a gap formed between two separated aluminum plates to obtain freestanding wires.18 The suspended wire was then transferred onto the prongs of a QTF using an optical microscope and a threedimensional translational stage. (Figure S1)

QTF Measurements A multifunctional data acquisition board (PCI-6251, National Instruments, Austin, TX) and a homebuilt LabVIEW program were used to measure the resonance frequencies and Q factors of the QTFs. Dry nitrogen was passed through a gas bubbler containing ethanol or water to produce solvent vapor stream. The solvent vapor stream was mixed with dry nitrogen and passed through the flow cell. Mass flow controllers (Brooks Instrument, MA) were used to control the flow rate and the total flow rate was maintained at 100 mL min-1. All experiments were conducted at room temperature.

Results and Discussion Figure 1a and b show scanning electron microscopy (SEM) images of the plain PMMA wires at different scales. The plain wires were obtained by electrospinning the PMMA


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solution at low humidity (RH = 30 %). The average diameter of the wires was measured to be 2.37 µm ± 0.20 µm. The cross-sectional SEM image of the wire in the inset of Figure 1b shows that the plain wire has a smooth surface and dense core. Figure 1c and d show SEM images of the nanoporous PMMA wires that were produced by electrospinning the PMMA solution at high humidity (RH = 60%). The average diameter of the nanoporous PMMA wires was 2.25 µm ± 0.42 µm and the cross-sectional SEM image of the wire (inset of Figure 1d) shows surface pores and inner cavities. Nanoporous structures are formed due to phase separation of the polymer solution during electrospinning. DCM begins to evaporate before DMF because of its high volatility. The evaporation of volatile solvent during electrospinning cools the polymer wire, leading to the condensation of moisture into water droplet. Note that water mixes with DMF but is a poor solvent for PMMA. The presence of non-solvent (water) in the PMMA wire induces phase separation and produces nanoporous structures.16-17

Figure 1. SEM images of (a) the plain PMMA wire and (b) its magnified image. Inset shows the cross-sectional SEM image of the plain PMMA wire. SEM images of (c) the nanoporous PMMA wire and (d) its magnified image. Inset shows the cross-sectional SEM image of the nanoporous PMMA wire. Figure 2a and b show optical microscopy images of the QTFs coated with plain and nanoporous wire, respectively. The diameter of the plain and nanoporous wires were measured to be 2.08 µm and 2.26 µm, respectively. Note that the distance between the two


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prongs was 200 µm. The resonance frequency change ( ∆f ) in a QTF is affected by the effective stiffness ( ∆k ) and the mass loading ( ∆m ),19 f0  f   ∆k ∆m  ∆f =  0   − ∆k ≈ m0  2 k 0  2   k0

(1)

where k0, f0, and m0 are the spring constant (13 kN m-1), resonance frequency (32.76 kHz), and mass (~2.7 mg) of the bare QTF, respectively. According to Eq. 1, owing to the addition of a 2 μm-thick PMMA wire (~2 ng), the frequency should decrease by 0.01 Hz if the spring constant remains unchanged. However, the attachment of the PMMA wire between the two prongs of the QTF induced an increase in the frequency. Figure 2c shows that the resonance frequency of the bare QTF (32.76 kHz) increased to 32.88 kHz upon plain-wire coating and to 32.82 kHz upon porous-wire coating, suggesting that the stiffness effect dominated the mass loading effect. The changes in the spring constant can be converted to the apparent modulus of the PMMA wire as follows,13

E=

2Lk0 L ∆f ≈ ∆k Af0 A

(2)

where A and L are the cross-sectional area and length of the wire, respectively. The apparent modulus was calculated to be 5.5 GPa for the plain wire and 2.5 GPa for the nanoporous wire. The nanoporous wires have a lower modulus than the plain wires due to the presence of surface pores and inner cavities, which makes the wires mechanically weak.


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Figure 2. Optical microscopy images of QTFs with (a) a plain wire and (b) a nanoporous wire. (c) Changes in the resonance frequency peaks after attaching the wire. (black: bare QTF, red: w-QTF, blue: pw-QTF).


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Figure 3a shows time-dependent changes in the resonance frequencies of the bare QTF, w-QTF, and pw-QTF upon exposing them to a series of ethanol vapor concentrations: 1% 3% 5% 10% 15% 20% 25%. After exposing the QTFs to ethanol for 40 min, dry nitrogen was injected for 1 h to remove the ethanol vapor. Removal of ethanol vapor causes frequency recovery, which confirms that ethanol adsorption is reversible. The bare QTF shows negligible change to all ethanol concentrations owing to its low sensitivity to mass (~50 ng Hz-1).20 In contrast, the decrease in the resonance frequencies of w-QTF and pw-QTF were greater than that of the bare QTF and increased with the concentration of ethanol vapor. This frequency change is attributed to a decrease in the modulus of the PMMA wires induced by the ethanol absorption.13 No further change in the frequency was observed for pw-QTF above the ethanol vapor concentration of 20%, indicating that the PMMA wire became saturated with ethanol. Note that the frequency change of pw-QTF is much faster than that of w-QTF. The response time (i.e., the time required for the frequency to fall to 63.2%) for pw-QTF upon exposure to a 20% ethanol atmosphere is measured to be 5 min, which is more than 6 times faster than that of w-QTF (34 min). This fast response time indicates that the diffusion of gas molecules is much faster for the nanoporous wire than for the plain wire due to its high surface area.14 Figure 3b compares the frequency shifts of the three QTFs with ethanol vapor concentration. The pw-QTF shows a frequency change even in a 1% ethanol atmosphere, while the response of w-QTF in the same conditions is negligible. The frequency change of pw-QTF is 60 times larger than that of w-QTF and 1170 times larger than that of the bare QTF when exposed to a 15% ethanol atmosphere. The sensitivity difference decreases as the ethanol concentration increases because the nanoporous wire becomes saturated above a 20% ethanol atmosphere.


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Figure 3. (a) Time-dependent changes in the resonance frequencies of the bare QTF (black square), w-QTF (red circle), and pw-QTF (blue triangle) upon exposure to ethanol vapor. The concentrations of ethanol vapor in a series of measurements were varied sequentially: 1%, 3%, 5%, 10%, 15%, 20%, and 25%. The inset shows a magnification of the red dotted circle area. (b) Changes in the resonance frequencies of the bare QTF (black square), w-QTF (red circle), and pw-QTF (blue triangle) as a function of ethanol vapor concentration. Dashed curves were made to guide the eye.

A control experiment was conducted to examine the influence of moisture on the measurement. Figure 4 shows that the frequency changes of a bare QTF, w-QTF, and pwQTF were measured with time after exposure to different concentrations of water vapor. As with ethanol, negligible changes were observed for the bare QTF regardless of the relative humidity. In contrast, the frequency changes of w-QTF and pw-QTF were not as pronounced as those for ethanol. Further, the response time of both wire-coated QTFs is 3 min, which is faster than that with ethanol, and the frequency change is fully saturated at each concentration. These differences were related to the low affinity of PMMA with water. PMMA is only slightly swelled by water (4.5%) while alcohols can swell PMMA approximately eight times more than water (38%).21 These results confirm that the sensitivity and selectivity of a polymer wire-coated QTF gas sensor can be tuned by changing the polymer properties.11


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0 ∆ f (Hz)

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-1

0

200

400

Time (min)

Figure 4. Time-dependent changes in the resonance frequencies of the bare QTF (black square), w-QTF (red circle), and pw-QTF (blue triangle) upon exposure to water vapor. The concentrations of water vapor in a series of measurements were varied sequentially: 7%, 12%, 17%, and 22%.

Conclusions In summary, we improved the performance of a QTF sensor by attaching a nanoporous polymer wire between two prongs of the QTF. The resonance frequency of the pw-QTF was measured as a function of the ethanol and water contents of its environment and compared with that of w-QTF. Compared to w-QTF, pw-QTF showed higher sensitivity and faster response time to ethanol, which was attributed to the high surface area and pore networks facilitating the transport of ethanol molecules inside the PMMA wire. Although the performance of the developed sensor can be further improved by replacing polymers or employing a sensor array, the strategy presented here offers a method to enhance the sensitivity and response time of polymer wire-coated QTF sensors for specific target solvents. In addition to sensor applications, the polymer-coated QTF can be used to study the adsorption of gas molecules into various polymer wires.


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Supporting Information Instrumental setup for porous wire-mounted QTF

Acknowledgement This work was supported by the Industrial Strategic Technology Development Program (10070241, Fabrication of High Bulk Porous Composite Sorbent with Pulp Support for Removal of Oil and Heavy Metals by Wet-laid Process) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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(3), 233-236. 11. Sampson, S. A.; Date, K. S.; Panchal, S. V.; Ambrale, A.; Datar, S. S., Investigation of QTF based gas sensors. Sensors and Actuators B: Chemical 2015, 216, 586-594. 12. Kim, W.; Yun, M.; Lee, S.; Jeon, S., Enhanced sensitivity of quartz tuning fork sensors using electrospun polymer wires. RSC Advances 2016, 6 (37), 31131-31134. 13. Boussaad, S.; Tao, N. J., Polymer Wire Chemical Sensor Using a Microfabricated Tuning Fork. Nano Letters 2003, 3 (8), 1173-1176. 14. Yun, M.; Lee, S.; Yim, C.; Jung, N.; Thundat, T.; Jeon, S., Suspended polymer nanobridge on a quartz resonator. Applied Physics Letters 2013, 103 (5), 053109. 15. Lee, S.; Yun, M.; Jeon, S., A tunable microresonator sensor based on a photocrosslinking polymer microwire. Applied Physics Letters 2014, 104 (5), 053506. 16. Bae, H.-S.; Haider, A.; Selim, K. M. K.; Kang, D.-Y.; Kim, E.-J.; Kang, I.-K., Fabrication of highly porous PMMA electrospun fibers and their application in the removal of phenol and iodine. Journal of Polymer Research 2013, 20 (7), 158. 17. Dayal, P.; Liu, J.; Kumar, S.; Kyu, T., Experimental and Theoretical Investigations of Porous Structure Formation in Electrospun Fibers. Macromolecules 2007, 40 (21), 7689-7694. 18. Li, D.; Wang, Y.; Xia, Y., Electrospinning of Polymeric and Ceramic Nanofibers as Uniaxially Aligned Arrays. Nano Letters 2003, 3 (8), 1167-1171. 19. Rai, A.; Tsow, F.; Nassirpour, S.; Bankers, J.; Spinatsch, M.; Pete He, M.; Forzani, E. S.; Tao, N. J., Selective detection of sulfur derivatives using microfabricated tuning forkbased sensors. Sensors and Actuators B: Chemical 2009, 140 (2), 490-499. 20. Waszczuk, K.; Gula, G.; Swiatkowski, M.; Olszewski, J.; Herwich, W.; Drulis-Kawa, Z.; Gutowicz, J.; Gotszalk, T., Evaluation of Pseudomonas aeruginosa biofilm formation using piezoelectric tuning fork mass sensors. Sensors and Actuators B: Chemical 2012, 170, 7-12. 21. Yu, Y.; Kieviet, B. D.; Kutnyanszky, E.; Vancso, G. J.; de Beer, S., CosolvencyInduced Switching of the Adhesion between Poly(methyl methacrylate) Brushes. ACS Macro Letters 2015, 4 (1), 75-79.


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Figure 1. SEM images of (a) the plain PMMA wire and (b) its magnified image. Inset shows the crosssectional SEM image of the plain PMMA wire. SEM images of (c) the nanoporous PMMA wire and (d) its magnified image. Inset shows the cross-sectional SEM image of the nanoporous PMMA wire. 356x267mm (150 x 150 DPI)


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Figure 2. Optical microscopy images of QTFs with (a) a plain wire and (b) a nanoporous wire. (c) Changes in the resonance frequency peaks after attaching the wire. (black: bare QTF, red: w-QTF, blue: pw-QTF). 109x227mm (300 x 300 DPI)


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Figure 3. (a) Time-dependent changes in the resonance frequencies of the bare QTF (black square), w-QTF (red circle), and pw-QTF (blue triangle) upon exposure to ethanol vapor. The concentrations of ethanol vapor in a series of measurements were varied sequentially: 1%, 3%, 5%, 10%, 15%, 20%, and 25%. The inset shows a magnification of the red dotted circle area. (b) Changes in the resonance frequencies of the bare QTF (black square), w-QTF (red circle), and pw-QTF (blue triangle) as a function of ethanol vapor concentration. Dashed curves were made to guide the eye. 34x14mm (300 x 300 DPI)


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Figure 4. Time-dependent changes in the resonance frequencies of the bare QTF (black square), w-QTF (red circle), and pw-QTF (blue triangle) upon exposure to water vapor. The concentrations of water vapor in a series of measurements were varied sequentially: 7%, 12%, 17%, and 22%. 42x35mm (300 x 300 DPI)


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Table of content (TOC) 54x30mm (300 x 300 DPI)


Performance Enhancement of a Microfabricated Resonator Using

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