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Published in Micro & Nano Letters
Received on 30th July 2007
Revised on 29th November 2007
doi: 10.1049/mnl:20070054
ISSN 1750-0443
Formation of ultrathin hydrogel films on
microcantilever devices using electrophoretic
deposition
H. Du S. Kondu H.-F. Ji
Department of Chemistry, Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana71272, USA
E-mail: hji@chem.latech.edu
Abstract: Uniform hydrogel films from micro/nano-hydrogel particles were prepared on the silicon wafers and
microcantilevers. These films were assembled on the substrates by using electrophoretic deposition (EPD)
method. The microcantilevers coated by such films responded to the environmental pH changes, indicating
that the EPD could be used as a new method for modifying microcantilevers.
1
Introduction
Microcantilevers are simple micromachined devices with
typical dimensions of 0.2 – 1 mm thickness, 20 – 100 mm
width and 100 – 500 mm length. They are commonly
fabricated from silicon or silicon nitride using wellestablished batch processes that involve photolithographic
patterning and a combination of surface and bulk
micromachining. Microcantilevers used as probes in
atomic force microscopy (AFM) to translate small forces
into topographic images were initially converted into a
platform for a new class of sensors in 1994. Although
being among the simplest of structures, they have proved
to be a cost-effective and ultrasensitive sensing device for
chemicals and biological species in air and solutions [1].
Microcantilevers undergo bending as a result of
molecular adsorption or absorption by confining these
processes to one side of the cantilever. One focus of
microcantilever sensing is to develop a novel surface
modification approach to increase the microcantilever
bending amplitudes and thus further improve
sensitivities. Self assembled monolayer [2], self assembled
multilayer [3], surface conjugation chemistry [4], spin
polymer coatings [5] and so on have been widely used for
microcantilever surface modifications. Recently, those
modified by stimuli-responsive hydrogels have shown
significant bending amplitudes compared to other
technologies [6, 7].
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Hydrogels are polymeric materials consisting of a mixture
of monomers, cross-linking agents and initiators. After
polymerisation, hydrogels obtain a flexible threedimensional network through the cross-linking of the
monomers with the aid of cross-linking agents. Hydrogels
have received a great deal of attention because of their wide
range of chemical and biological applications, attributed to
their good biocompatibility and volume change property in
response to physical or chemical stimuli in the
environment, such as temperature [8], pH, ionic strength
[9] and other chemical and biological species [10].
Hydrogels thus have found wide applications in cell
cultures, cell immobilisation, tissue engineering [11],
biological and chemical sensing [12], drug delivery [13],
intelligent coating [14] and so on. For microcantilever
sensing applications, the challenge is to develop adequate
surface coating techniques to achieve gel immobilisation on
the sensor platform with high stability and reproducibility.
Furthermore, an ultrathin film is required for fast responses
since the gel-swelling time is proportional to the gel
thickness [15]. Bashir et al. [7] reported a pH sensor with
ultrahigh sensitivity based on a microcantilever structure
with a lithographically-defined cross-linked copolymeric
hydrogel. Silicon-on-insulator wafers were used to fabricate
cantilevers on which a polymer consisting of
poly(methacrylic acid) with poly(ethylene glycol)
dimethacrylate was patterned using free-radical ultraviolet
(UV) polymerisation. The whole process is complicated and
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a cleanroom as well as expensive equipment is needed. On the
other hand, the thin layer of the mixture of the monomers
spin-coated on the microcantilever surface is very easily
evaporated before polymerisation is initiated under
irradiation of a UV lamp, even within several minutes.
Thus, although hydrogel modified microcantilevers have
shown much enhanced microcantilever bending amplitudes
and sensitivity, the direct surface modification process, that
is, polymerisation of the hydrogel on the microcantilever
surfaces, is tedious and problematic [6, 7].
One indirect approach would be to develop hydrogel
microspheres or nanoparticles, and assemble these particles
on the microcantilever surface in an organised manner by
electrostatic
attraction
and
other
intermolecular
interactions, which is commonly called layer-by-layer (LbL)
self-assembly technique [16, 17]. However, our initial
investigation showed that the LbL technique did not
produce a continuous, reproducible ultrathin film on a
microdevice, such as a microcantilever.
In this work, we report the development of a uniform,
compact, ultrathin hydrogel film on a microcantilever
sensor device, and possibly many other microdevices, using
the electrophoretic deposition (EPD) technique.
EPD has been extensively used for fabricating thin films
from suspensions of nano- or micro-sized particles. The
EPD technique offers precise control of film thickness,
uniformity and deposition rate. However, EPD had only
been used on hard particles, including ceramic particles
[18], colloidal gold particles [19], polystyrene particles [20]
and so on. In this report, we expand the application of
EPD to soft particles, such as hydrogel microspheres and
nanoparticles.
2
Experimental
In our experiments, we used commercially available silicon
MCLs (Veeco Instruments, Santa Barbara, CA). The
dimensions of the V-shaped silicon MCLs were 180 mm in
length and 2 mm in thickness. One side of these cantilevers
was covered with a thin film of chromium (3 nm) and
followed by a 20 nm layer of gold, both deposited by
e-beam evaporation. Since gold film does not stick well to
SiO2 , the thin chromium layer was used to improve adhesion.
The hydrogel micro-particles employed in this paper,
polyacrylamide, were synthesised by precipitation
polymerisation from acrylamide (AAm), methacrylic acid
(MAc) and methylene bisacrylamide (MBAAm) in ethanol
according to Nakazawa et al. [21]. The as-prepared microparticles were treated ultrasonically over 30 min and then
left over night. The supernatant liquid consisted of nanoparticles with an average diameter of 100 nm (Fig. 1).
These as-prepared micro-/nano-particles were negatively
charged.
Micro & Nano Letters, 2008, Vol. 3, No. 1, pp. 12 – 17
doi: 10.1049/mnl:20070054
Figure 1 SEM images of a Si plate surface coated by
hydrogel nanoparticles
The plate was firstly coated with a three bilayer of (PEI/PSS) plus
one PEI layer at the outmost surface and then exposed to a gel
nanoparticle solution in water for 1 h Average size of the
hydrogel particles was about 100 nm
A pair of parallel plate electrodes (50 nm thick Au on 5 nm
Cr on Si (100)) with lateral dimensions of about 15 mm 5 mm served as electrodes in the EPD. As for the anode,
in order to increase the adherence between the hydrogel
particles and the surface of the anode, the gold surface of
the anodic electrodes was further treated with
5.0 1025 M 11-mercapto-1-undecanoic acid in alcohol
overnight and then coated with three alternative layers of
poly(ethylenimine) (PEI)/poly(styrenesulfonate) (PSS)
through the LbL technique [16]. The time interval for
multilayer formation was 20 min. The concentrations of the
PEI and PSS were 1.5 and 3.0 mg/mL, respectively and
the electrodes were placed vertically with a distance of
5 mm between them. For microcantilever deposition, a thin
platinum plate served as the cathode (2 mm 2 mm)
and the tip of the microcantilever was the anode. The gold
surface of the microcantilever was treated with 5.0 1025
M 11-mercapto-1-undecanoic acid and three PEI/PSS
bi-layers as mentioned above. The platinum plate and
microcantilever tip were carefully assembled against a teflon
Figure 2 Schematic illustration of the electrode assembly
for microcantilever EPD
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spacer and fixed using a ‘o’ ring. The gap between the two
electrodes was about 2 mm. Fig. 2 schematically illustrates
this assembly.
3
Results and discussion
In the EPD, the electrodes were immersed into 20 ml of a
diluted suspension of micro-/nano-particles of the
hydrogel. The solvent of the suspension is 95% ethanol and
the concentration is 0.5 wt% for nano-particles and
2 wt% for micro-spheres. The deposition process was
performed using a constant voltage mode at room
temperature. The applied voltage varied from 0.5 to 10 V
using a power source. No gas evolution was observed at
either electrode during the deposition process. After EPD,
the samples were removed and gently rinsed with distilled
water three times and then dried under nitrogen gas for
measurements. Fig. 3 shows the hydrogel thin film
obtained through EPD of hydrogel nanoparticles at a 2 V
applied potential for various times.
The scattered gel nanoparticles on a surface can not be seen
under an optical microscope 5 min after EPD. However, the
particles started merging with each other and a twodimensional (2D) network appeared on the substrate after
15 min and the blank area in the 2D network was gradually
reduced. A uniform and continuous thin hydrogel film was
complete in 35 min. This observation was consistent with
Böhmer’s results on the EPD of micro-sized polystyrene
(PS) latex particles [22, 23] and Pt nano-particles [24],
where ‘cluster– cluster aggregation’ of the PS and Pt
nanoparticles was observed.
The films prepared under different potentials suggested
that it takes a longer time to achieve a continuous film
under lower potentials. When the applied voltage was
increased to 5 V, no obvious ‘cluster– cluster aggregation’
was observed, the EPD process was faster and the
formation of a uniform film was nearly complete in 10 min.
These films prepared from the gel nanoparticles were
uniform and continuous. No defects or pinholes were
observed. The thickness of these films was between 70 and
100 nm as measured by AFM (Fig. 4), suggesting that the
film was made of a monolayer of hydrogel particles.
It is noteworthy that the prepared continuous hydrogel
films were very stable in water. The film kept intact when
the potential was cut out and did not fall apart even when
the applied field was reversed. This phenomenon
contradicts Böhmer’s observation on the EPD behaviour of
PS latex particles [21]. During the PS deposition, Böhmer
observed that the PS latex particle cluster became a less
ordered structure and the film eventually broke up when
the electric field was withdrawn. It was also observed that
the process was accelerated when the voltage was reversed.
The EPD process of 3 mm gel microspheres is shown in
Fig. 5. The gel film prepared on the microcantilevers under
the same conditions showed the same results as those on
the relatively larger Si plates.
Figure 3 Optical microscope images of the EPD deposited films from 100 nm gel nanoparticles at 2V (Magnifications are
750)
a
b
c
d
e
For 2 min
For 8 min
For 15 min
For 25 min
For 35 min
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showed reproducible results. The expected observation
showed that the hydrogel film produced using EPD
was an easy and reliable approach for modification of
microcantilever sensors.
For the 200 nm deflection, the surface stress change is
calculated to be 2.87 N/m on the cantilever according to
Stoney’s equation
Dz ¼
Figure 4 AFM image of a film of 100 nm gel particles
deposited on a Si plate through EPD at 2 V for 35 min
The average thickness of the gel film was approximately 100 nm
A microcantilever coated with a layer of 3 mm-thick
hydrogel film (Fig. 6a) through EPD process was used for
sensing validation. The bending responses of the hydrogel
coated cantilever with a change of environmental pH were
detected. The experimental setup and the principle of the
cantilever deflection measurement were reported previously
[25]. Fig. 6b shows the cantilever response to varied pH in
a series of 0.001 M phosphate buffer solutions. The
cantilever bent by 200 nm when the pH changed from 7.0
to 4.0. Five cantilevers prepared under similar conditions
3(1 n)L2
ds
Et 2
(1)
where Dz is the observed deflection at the end of the
cantilever, n and E the Poisson’s ratio (0.2152) and the
Young’s modulus (155.8 GPa) for the silicon substrate,
respectively, t and L the thickness (1 mm) and length
(180 mm) of the cantilever, respectively, and ds the
differential stress on the cantilever. This surface stress is a
significant number.
Many of the reported microcantilever sensors have
relatively small bending amplitudes (,20 nm). For silicon
microcantilever sensors, typically, it requires hours to reach
a good baseline (,1 nm noise level) before each test. This
long waiting time is the main obstacle for commercialising
microcantilever technology. One solution is to increase
surface stress, that is, the bending amplitude, upon
analyte – protein interaction. Since a 10 nm noise-level
baseline can be readily achieved in seconds, the deflection
signal s is required to be .30 nm for user-friendly devices.
Figure 5 Microscopic imagery of the gel films
a-e Optical microscope images of the EPD deposited film from 3 mm gel microspheres at 5 V (Magnifications are 750)
a For 2 min
b For 10 min
c For 15 min
d For 25 min
e For 35 min
f SEM image of a EPD deposited film from 3 mm gel particles at 10 V for 35 min
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doi: 10.1049/mnl:20070054
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[4] GUPTA A., AKIN D., BASHIR R.: ‘Single virus particle mass
detection using microresonators with nanoscale
thickness’, Appl. Phys. Lett., 2004, 84, pp. 1976– 1978
[5] LANG H.P., BALLER M.K., BERGER R., ET AL .: ‘An artificial nose
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Figure 6 Optical image and response of hydrogel-coated
cantilever
a Optical image of a hydrogel film coated microcantilever
Hydrogel film was produced by EPD of 3 mm gel particles under
10 V for 30 min
b Response of hydrogel coated cantilever to the different pH
phosphate buffer solutions
Hydrogel films provide a coating material that dramatically
increases the bending amplitude of cantilevers. This work
demonstrated that hydrogel film can be readily formed on
microcantilevers by using an EPD approach.
4
Conclusion
In summary, we demonstrated a convenient and reliable
approach based on the EPD process to deposit a uniform
and continuous hydrogel thin film on microcantilever
devices. The bending responses of hydrogel coated
microcantilever with a change in environmental pH were
observed, demonstrating the feasibility of this hydrogel film
for microsensor development. This hydrogel film formation
approach may be used on many other microdevices for
various applications.
5
Acknowledgment
This work was supported by NSF Sensor and Sensor
Network ECS-0428263 and NSF MRI grant 0618291.
6
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