1
403
Figure 2. Vapor response isotherms of QCM sensors coated
with 10 ¯g of (a) 1- and (b) 2-capped Au nanoparticles to toluene
Figure 1. Frequency changes of QCM sensors coated with
0 ¯g of 1-capped Au nanoparticles exposed to 2000-ppm VOC
vapors measured at 20 °C. The inset is a schematic illustration of
-capped Au nanoparticle. The size of the Au nanoparticle and
the thickness of calixarene layer were estimated from a TEM
image and a computer-generated molecular model, respectively.
(
), n-octane ( ), acetone ( ), and ethanol ( ).
1
of the sensor responses of the QCMs coated with 1-capped Au
nanoparticles responding to exposure to 2000-ppm VOC vapors.
The QCMs responded with a decrease in the resonant frequency
within 30 s of switching the carrier gas flow from pure nitrogen
to VOC vapor. The decrease in resonant frequency indicates an
increase in the film’s mass through the adsorption of VOCs
within the nanoparticle films. When the carrier gas was changed
to pure nitrogen, the frequency returned to the original value
within 20 s, which indicates fast adsorption and desorption
processes of the VOCs in the nanoparticle films. The reversi-
bility and repeatability of the sensor responses were confirmed
by applying the VOC vapors for three consecutive exposures in
each experiment.
The response isotherms for the QCMs coated with 1- and 2-
capped Au nanoparticles for four VOCs are shown in Figure 2.
Both nanoparticle films display linear dependencies of the
adsorption amount for the four VOCs on the VOC’s vapor
pressures P/P0. The slope differences of the P/P0 dependence
are due to the differences in chemical affinity between the
nanoparticle film and VOCs as well as the molecular properties
of the VOCs such as their molecular volume. The slope for the
toluene sensing in the QCM coated with 1-capped nanoparticles
is 2.2 times that compared to that with 2-capped nanoparticles,
indicating a higher sensitivity of 1 for aromatic toluene. The
differences in molecular interaction of the VOCs at the surface
of nanoparticles affected the sensor sensitivity.
The QCM responses can be increased by increasing the
amount of sensing films. However, the lower resonance
propagation of sensing films leads to unstable oscillation due
to the contribution of the viscoelastic effect of coated materials.
The QCMs with different film thicknesses of 1-capped nano-
particles and 5,11,17,23-tetra(tert-butyl)-25,26,27,28-tetrakis(2-
ethylhexyloxyl)calix[4]arene (3) were prepared by changing the
rotation speed of the spin-coating processes and the concen-
tration of the solutions. While the sensor response of 3 linearly
increased less than 15 ¯g upon the exposure to 2000-ppm of
toluene vapor, the QCMs coated with 3 above 15 ¯g showed an
unstable oscillation under the same voltage. In contrast, the
QCMs with 1-capped nanoparticles exhibited a linear increase
when increasing the amount of sensing films until 40 ¯g. In a
QCM resonator, the acoustic wave propagates through the bulk
of the quartz crystal in a direction perpendicular to the surface.
In the case of nanoparticle films, the generated wave from the
surface of QCM can propagate within the sensing film and the
1
Surface-capped Au nanoparticles were prepared by reduc-
17
tion of HAuCl with NaBH in the presence of 1 or 2. 1- and 2-
4
4
capped Au nanoparticles can dissolve in many organic solvents
such as tetrahydrofuran (THF), CH2Cl2, toluene, and DMF.
The average diameter and standard deviation of 1-capped Au
nanoparticles are 2.1 and 0.5 nm as determined from the
transmission electron microscopic images (Figure S1 in Sup-
22
porting Information; SI ). Absorption spectrum of 1-capped Au
nanoparticle in THF displayed a surface plasmon band at around
5
20 nm, also suggesting the formation of small Au nanoparticles
2
2 18
(Figure S2, SI ). Thermogravimetric analyses (TGA) of
1
3
- and 2-capped gold nanoparticles showed weight losses of
7.3 and 38.2% at 550 °C, indicating that the density of cone-
shaped 1 on the surface of Au nanoparticles is slightly lower
than that of monomeric 2. The numbers of ligand on Au
nanoparticles can be estimated from the average diameter of Au
nanoparticles determined from TEM images and the weight
losses from TGA. The numbers of ligand are estimated to be 29
1
9
and 150 for 1- and 2-capped Au nanoparticles, respectively.
The thiol terminates in 1 were bonded to the Au nanoparticles
and formed a dense packing of calixarenes onto the surface of
Au nanoparticles (Figure 1).
Au nanoparticle films were deposited onto the surface of
QCMs by spin-coating 1 wt % THF solutions of 1- or 2-capped
Au nanoparticles. Packing Au nanoparticles can form a porous
structure with a large active surface area. When a 10 mg of
2
nanoparticle film is deposited on the surface of QCMs (0.19 cm
gold electrode area), the nanoparticle film has roughly 50 times
the surface area of comparable nonporous films. QCMs coated
with 1- and 2-capped Au nanoparticles were set into a
temperature-controlled measurement chamber and the sensing
properties of these modified QCMs (AT-cut quartz crystal,
9
MHz operation frequency) were investigated by monitoring the
frequency changes when the films were exposed to toluene,
n-octane, acetone, and ethanol vapors.20 These solvents have
comparable vapor pressures and represent analytes ranging from
nonpolar to polar VOCs. All the measurements were conducted
at a sensor temperature of 20 °C to avoid any temperature-
dependent frequency changes. Figure 1 shows the time courses
Chem. Lett. 2011, 40, 14021404
© 2011 The Chemical Society of Japan