A tunable microresonator sensor based on a photocrosslinking polymer microwireSeongjae Lee, Minhyuk Yun, and Sangmin Jeon

Citation: Applied Physics Letters 104, 053506 (2014); doi: 10.1063/1.4864272 View online: http://dx.doi.org/10.1063/1.4864272 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An energy-efficient readout circuit for resonant sensors based on ring-down measurement Rev. Sci. Instrum. 84, 025005 (2013); 10.1063/1.4792396 Stress-based resonant volatile gas microsensor operated near the critically buckled state J. Appl. Phys. 111, 104517 (2012); 10.1063/1.4720473 High resolution microresonator-based digital temperature sensor Appl. Phys. Lett. 91, 074101 (2007); 10.1063/1.2768629 Nonconductive polymer microresonators actuated by the Kelvin polarization force Appl. Phys. Lett. 89, 163506 (2006); 10.1063/1.2362590 Ultimate limits to inertial mass sensing based upon nanoelectromechanical systems J. Appl. Phys. 95, 2682 (2004); 10.1063/1.1642738

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A tunable microresonator sensor based on a photocrosslinking polymermicrowire

Seongjae Lee, Minhyuk Yun, and Sangmin Jeona)

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang,South Korea

(Received 12 December 2013; accepted 22 January 2014; published online 6 February 2014)

A polyvinylcinnamate (PVCN) microwire was attached between the two tines of a quartz tuning

fork (QTF) to form a polymer bridge. Exposure of a PVCN wire-connected QTF to ethanol vapor

decreased the modulus of the wire, resulting in a decrease in the resonance frequency. The

resonance frequency and Q factor of the resonator were measured as a function of the ethanol

vapor concentration. The photocrosslinking of the PVCN wire enhanced the sensitivity of the

QTF sensor and offered a facile route to developing a sensor with a tunable resonance frequency.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4864272]

The absorption of gas molecules into a polymer induces

substantial changes in the physical properties of the polymer,

including the mechanical strength and electrical resistance.1–6

Because the changes may be easily measured at room tempera-

ture, polymer-based gas sensors are considered to be a promis-

ing alternative to commercially available inorganic gas sensors

that require high temperatures for operation.7,8 Most polymer-

based gas sensors utilize conducting polymers and measure

changes in the electrical resistance upon gas absorption;3–6

however, the applicability of conducting polymer-based gas

sensors is quite limited because most polymers are not conduc-

tive. By contrast, changes in the mechanical properties of a

polymer, such as the modulus, depend strongly on the affinity

between a polymer and the gas molecules. The gas-sensitive

modulus of nearly all polymers can be used for sensing

applications.

Changes in the mechanical properties of polymers can be

measured using microresonators such as quartz crystal micro-

balances (QCMs), microcantilevers, or quartz tuning forks

(QTFs).9–14 Microresonators vibrate at resonance frequencies

that are sensitive to changes in mass or stress due to the

absorption of gas molecules. By measuring changes in the res-

onance frequency, the mass of absorbed gas molecules or the

change in the stress of the resonator can be obtained. The

mass sensitivities of the various microresonators follow the

order: microcantilevers > QCMs>QTFs.15–17 The mass sen-

sitivity of a typical QTF with a resonance frequency of 32

kHz is just�50 ng/Hz and is unsuitable for use in gas sensors.

The QTF has a unique geometry consisting of two

vibrating tines with a specific gap that permits the formation

of a free-standing polymer membrane or wire bridge

between the tines.13,14 The absorption of gas molecules into

the polymer bridge induces a change in the modulus of the

polymer bridge and affects the resonance frequency. The

suspended structure facilitates the diffusion of gas molecules

into the polymer and improves the sensitivity of the QTF.

Boussaad and Tao further improved the sensitivity of a poly-

mer microwire-connected QTF by reducing the diameter of

the polymer wire using a focused ion beam,13 but it required

an expensive vacuum process.

In this study, we used a photocrosslinkable polymer to

form a microwire on a QTF and improved the sensitivity of

the resulting sensor by photocrosslinking the polymer wire.

An increase in the polymer wire’s modulus due to photocros-

slinking increased the tension between the tines of the QTF

and amplified the frequency change measured during gas

absorption. This approach also offers a facile route to de-

velop a microresonator with a tunable resonance frequency.

Polyvinylcinnamate (PVCN) was obtained from Aldrich

(Saint Louis, MO) as a photocrosslinkable polymer. The

weight-average molecular weight (Mw) of PVCN was stated

by the manufacturer to be 200 000 g/mol. Tetrahydrofuran

(THF) was purchased from Aldrich and was used to prepare

a 10 wt. % PVCN solution. QTFs with 250 lm in width,

600 lm in thickness, and 3400 lm in length were purchased

from ECS, Inc., International (Kwangmyung, Korea). The

resonance frequency and spring constant of an uncoated

QTF were 32.758 kHz and 13 kN/m, respectively. A

home-built system consisting of a digitizer (NI PXI-5114), a

multiplexer (NI PXI-2593), and a function generator (NI

PXI-5406) was used to measure the resonance frequencies of

the QTFs. The phase and admittance of the QTF were

obtained using Labview software. The resonance frequency

and Q factor of the QTF were calculated from Lorentzian

curve fits to the conductance spectra, which were measured

in situ during the absorption of ethanol vapor by the PVCN

wire at room temperature. The concentration of ethanol

vapor was controlled by adjusting the flow rates of the dry

and wet nitrogen gas. The total flow rate of the gas stream

was fixed at 100 ml/min and wet nitrogen was obtained by

passing dry nitrogen through a gas bubbler containing pure

ethanol.

Figure 1(a) shows a schematic illustration of the proce-

dure used to prepare a polymer wire on a QTF. A sharp glass

tip was used to pull a viscous PVCN solution to form a wire

between silicon wafers. The wire was transferred onto a QTF

using a 3-D translational stage with monitoring using an op-

tical microscope. A tiny amount of THF was dropped onto

the tines of the QTF to partially dissolve and glue the wire

a)Author to whom correspondence should be addressed. Electronic mail:

jeons@postech.ac.kr

0003-6951/2014/104(5)/053506/4/$30.00 VC 2014 AIP Publishing LLC104, 053506-1

APPLIED PHYSICS LETTERS 104, 053506 (2014)

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onto the QTF. The diameter of the PVCN wire in Figure 1(b)

was measured to be �3 lm. The polymer wire-connected

QTF was placed inside a homebuilt flow cell (2.2 ml) for sub-

sequent measurements. Figure 1(c) shows the conductance

spectra before and after attaching the PVCN wire to the QTF

under a nitrogen atmosphere. A resonance peak appeared at

32.758 kHz before attachment and this peak shifted to

32.840 kHz after attachment. The change in the resonance fre-

quency (Df) of the QTF can be affected by the change in the

effective stiffness (Dk) and the change in the mass (DM)18

Df ðk;MÞ ¼ f02

� �Dk

k0

� DM

M0

� �; (1)

where f0, M0, and k0 represent the resonance frequency,

mass, and spring constant of the bare QTF, respectively. The

increase in the resonance frequency after attachment indi-

cated that the mass effects were negligible compared to the

stiffness effects. The mass of the attached PVCN wire was

calculated from the dimensions of the optical microscopy

image and was found to be �1 ng, whereas the mass of the

bare QTF was 2.7 mg. Assuming that the spring constant did

not change after attachment of the wire, the frequency change

due to the mass loading was calculated from Eq. (1) to be

0.01 Hz. A control experiment was conducted in which the

attached wire was cut using a razor blade. The resonance fre-

quency of the QTF with the PVCN wire residue was found to

be almost identical to that of the bare QTF, indicating that the

change in resonance frequency after attachment was induced

solely by the change in the spring constant of the QTF.

Figure 2(a) shows the changes in the resonance fre-

quency of the PVCN wire-coated QTF upon exposure to a se-

ries of concentrations of ethanol vapor: 1%! 3%! 5%!7% ! 10% ! 15% ! 20%. After 10 min of exposure to

each vapor, the ethanol vapor was removed by flowing dry

nitrogen into the flow cell. A change in the resonance fre-

quency was measured only for the cases in which the concen-

tration of ethanol vapor exceeded 10%. The dashed line

shows the frequency change in the QTF measured after cut-

ting the polymer wire using a razor blade and exposing the

system to the same series of ethanol vapor concentrations.

Negligible changes in the frequency were observed for the

QTF with a broken wire during the absorption of ethanol

vapor, indicating that the frequency change was induced not

by the change in mass but by the change in the wire modulus.

The attachment of a polymer bridge was found to

enhance the sensitivity of the QTF to gas absorption substan-

tially; however, the sensitivity requires further improvement.

To this end, the PVCN wire-attached QTF was exposed to UV

light. Figures 2(b)–2(d) show the changes in the resonance

frequency of the PVCN wire-coated QTF as a function of etha-

nol vapor concentrations after UV irradiation for 60 s, 120 s, or

300 s, respectively. As the UV irradiation time increased,

changes in the resonance frequency of the QTF in the presence

of gas were found to shift toward lower concentrations of etha-

nol vapor because the PVCN wire modulus increased due to

photocrosslinking, which increased the tension between the

tines of the QTF and amplified the modulus change in the wire

during the absorption of ethanol vapor. Further photocrosslink-

ing of the PVCN wire was found to degrade the sensitivity

because the PVCN wire became too stiff.19

Note that the dynamic ranges of the resonator were quite

narrow, regardless of the degree of photocrosslinking. The

resonance frequency of the uncrosslinked resonator was

observed to change at ethanol vapor concentrations between

10% and 20%, and the resonator photocrosslinked for 300 s

was observed to change between 1% and 10%. Beyond this

range, the frequency change became saturated, and no fur-

ther changes were observed because the PVCN wire modulus

was too small to undergo a frequency change. The resonance

frequency of the QTF after saturation was nearly identical to

that measured from the bare QTF, indicating that the PVCN

wire did not affect the vibrational frequency of the QTF once

saturated by ethanol vapor. By contrast, the frequency

change within the dynamic range was very large compared

to the noise level (0.01 Hz). This property is useful for qual-

ity control applications, such as imposing tight controls over

the alcohol content in food or drink.

FIG. 1. (a) Schematic diagram illustrating the preparation of the PVCN

wire-coated QTFs. (b) Optical microscopy image of a PVCN wire-attached

QTF. The diameter of the wire was measured to be �3 lm. (c) The reso-

nance frequency peaks before (black) and after (red) attaching the PVCN

wire to the QTF.

053506-2 Lee, Yun, and Jeon Appl. Phys. Lett. 104, 053506 (2014)

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The changes in the Q factors of the resonators were

measured simultaneously with the changes in the resonance

frequency. The Q factor of a microresonator is defined as the

ratio of the energy stored to the energy dissipated per cycle

and can be calculated from

Q factor¼ resonance frequency

FWHM; (2)

where FWHM is the full width at half maximum. A higher Qfactor implies a lower rate of energy dissipation relative to

the oscillation frequency. Figure 3(a) shows that the Q factor

of the uncrosslinked PVCN wire-connected QTF decreased

as the ethanol vapor concentration increased. The decrease

in the Q factor was attributed to an increase in the dissipation

of the PVCN wire upon the absorption of ethanol vapor.

Figures 3(b)–3(d) show the Q factor of the PVCN wire-

connected QTF after UV irradiation for 60 s, 120 s, or 300 s,

respectively. As with the resonance frequency, larger changes

in the Q factor were observed at lower concentrations of etha-

nol vapor as the UV irradiation time was increased; however,

FIG. 2. Variations in the resonance fre-

quency of a PVCN wire-coated QTF as

a function of the ethanol vapor concen-

tration. The concentration of ethanol

vapor was increased stepwise over a

series of measurements: 1% ! 3% !5% ! 7% ! 10% ! 15% ! 20%.

The PVCN wire was cured under UV

exposure for (a) 0 s, (b) 60 s, (c) 120 s,

or (d) 300 s.

FIG. 3. Variations in the Q factor of a

PVCN wire-coated QTF as a function

of the ethanol vapor concentration.

The ethanol vapor concentration was

increased stepwise over a series of

measurements: 1% ! 3% ! 5% !7% ! 10% ! 15% ! 20%. The

PVCN wire was cured by UV exposure

for (a) 0 s, (b) 60 s, (c) 120 s, or (d)

300 s.

053506-3 Lee, Yun, and Jeon Appl. Phys. Lett. 104, 053506 (2014)

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the Q factor decreased upon exposure to lower concentrations

of ethanol vapor and increased upon exposure to higher con-

centrations. The ethanol vapor concentration at which the Qfactor increased roughly coincided with the ethanol vapor

concentration at which the wire modulus reached a minimum.

This result indicated that an increase in the Q factor at higher

concentrations of ethanol vapor resulted from the small mod-

ulus of the PVCN wire. Once the wire had been saturated

with ethanol vapor, the PVCN wire connected to the QTF did

not affect the vibrational frequency of the QTF (i.e., damping

decreased), and the Q factor approached the original Q value

of the bare QTF (12 000).

Figures 4(a) and 4(b) show the variations in the reso-

nance frequency and Q factor, respectively, of the PVCN

wire-connected QTF under dry nitrogen as a function of the

UV irradiation time. The modulus of the PVCN wire

(EPVCN) could be obtained from13,14

EPVCN ¼2Lk0

Af0

Df � L

ADk; (3)

where L and A are the length and cross-sectional area of the

wire, respectively. Note that Dk and Df represent the changes

in the spring constant and resonance frequency due to the

attachment of the PVCN wire to the QTF, respectively. The

modulus of the uncrosslinked PVCN wire under a nitrogen

atmosphere was calculated from Eq. (3) to be 2.20 GPa,

whereas the modulus of the PVCN wire after photocrosslinking

for 300 s was calculated to be 2.35 GPa. The corresponding Qfactor of the PVCN wire-connected QTF increased from 4300

to 4700 after photocrosslinking. Although the relative changes

in the modulus and Q factor were less than 10% of the original

values measured in this experiment, the sensitivity of the QTF

improved significantly.

In summary, we developed a tunable resonator sensor

by attaching a photocrosslinking polymer wire to a microfab-

ricated QTF. The resonance frequency and Q factor of the

QTF were measured simultaneously as functions of the etha-

nol vapor concentration and the degree of polymer photo-

crosslinking. The sensitivity and Q factor of the resonator

improved upon photocrosslinking, which increased the ten-

sion between the tines of the QTF and amplified the modulus

change of the wire during the absorption of ethanol vapor.

This method offers a facile route not only to enhancing the

sensitivity of the sensor but also to developing a sensor with

a tunable resonance frequency.

This research was supported by Basic Science Research

Program through the National Research Foundation of Korea

(NRF) funded by the Ministry of Education (NRF-2011-

0011246)

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variations in the resonance frequency of a PVCN wire-coated QTF as

functions of the ethanol vapor concentration and photocrosslinking time.

FIG. 4. Variations in (a) the resonance frequency (modulus) and (b) the Q fac-

tor of the PVCN wire-coated QTF as a function of the UV irradiation time.

053506-4 Lee, Yun, and Jeon Appl. Phys. Lett. 104, 053506 (2014)

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