Abstract We present here the behavior of the resonance frequency of porous anodic alumina cantilever arrays during water vapor adsorption and emphasize their possible use in the micromechanical sensing of humidity levels at least in the range of 10-22%. The sensitivity of porous anodic aluminium oxide cantilevers (Δf/Δm) and the humidity sensitivity equal about 56 Hz/pg and about 100 Hz/%, respectively. The approach presented here for the design of anodic alumina cantilever arrays by the combination of anodic oxidation and photolithography enables easy control over porosity, surface area, geometric and mechanical characteristics of the cantilever arrays for micromechanical sensing. 133

Alexey Klimenko

Alexey Lukashin

Andrey Eliseev

Olga Boytsova

Vasiliy Lebedev

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1332
Nanomechanical humidity detection
through porous alumina cantilevers
Olga Boytsova*1,2, Alexey Klimenko1, Vasiliy Lebedev1, Alexey Lukashin1
and Andrey Eliseev1
Full Research Paper Open Access
Address:
1Lomonosov Moscow State University, 119992, Moscow, Russia and
2Kurnakov Institute of General and Inorganic Chemistry of The
Russian Academy of Science, 119991, Moscow, Russia
Email:
Olga Boytsova* - boytsova@gmail.com
* Corresponding author
Keywords:
anodic aluminium oxide; atomic force microscopy (AFM); cantilever
arrays; humidity; mechanical sensor; porous alumina
Beilstein J. Nanotechnol. 2015, 6, 1332–1337.
doi:10.3762/bjnano.6.137
Received: 18 September 2014
Accepted: 20 May 2015
Published: 16 June 2015
This article is part of the Thematic Series "Self-assembly of
nanostructures and nanomaterials".
Guest Editor: I. Berbezier
© 2015 Boytsova et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
We present here the behavior of the resonance frequency of porous anodic alumina cantilever arrays during water vapor adsorption
and emphasize their possible use in the micromechanical sensing of humidity levels at least in the range of 10–22%. The sensitivity
of porous anodic aluminium oxide cantilevers (Δf/Δm) and the humidity sensitivity equal about 56 Hz/pg and about 100 Hz/%, res-
pectively. The approach presented here for the design of anodic alumina cantilever arrays by the combination of anodic oxidation
and photolithography enables easy control over porosity, surface area, geometric and mechanical characteristics of the cantilever
arrays for micromechanical sensing.
Introduction
The last two decades have seen a surge in resonant micro- and
nanomechanical engineering raised by the creation of high-
performance and cost-effective detection systems. Highly
elastic silicon and, in contrast, soft polymer cantilevers were
implemented to chemical sensors with resonance shift assay
[1-3]. To date, the determination of trace amounts of adsorbed
biological, chemical and other multiplex substances through
micromechanical sensing still holds a tremendous potential
[4-8]. To improve the shifts of the resonant frequency one needs
to enlarge the active surface area of the sensor while preserving
its mechanical stiffness. This necessitates the use of the porous
systems with “open through porosity”. One promising porous
system exhibiting all the necessary characteristics is anodic
aluminium oxide (AAO). Its huge surface area offers an enor-
mous number of binding sites.
The adsorption of chemicals on the surface of the cantilever
manifests itself by the shift of the resonance frequency in
Beilstein J. Nanotechnol. 2015, 6, 1332–1337.
1333
Figure 1: SEM images of a six-cantilevers array (2 μm thickness) obtained through chemical photolithography: (a) general view, (b) porous structure
of AAO cantilever with 30 nm average pore diameter and 105 nm average interpore distances and (c) pore size distribution.
dynamic vibration mode. The loading mass can be calculated
from the change of resonance frequencies after (ƒ1) and before
(ƒ0) adsorption as follows [9,10]:
(1)
Here, the spring constant k is given by k = (E·w·t3)/(4·l3), where
w, t and l are the width, thickness and length of the cantilever
and E is the Young’s modulus of the cantilever [11]. According
to Equation 1, the more molecules are adsorbed on the surface
of a cantilever, the larger is the shift of the resonance frequency.
Therefore AAO cantilevers hold a great promise for the devel-
opment of micromechanical sensor arrays.
In the present work we concentrated on the elaboration of
experimental methods for the fabrication of anodic alumina
cantilever arrays and their testing for adsorption of water
vapors.
Results and Discussion
The proposed procedure for the fabrication of cantilever arrays
combines anodic oxidation of aluminium foil with a conven-
tional photolithography process (Figure 1).
Using the lithography approach for AAO pattern formation
mostly becomes possible due to selective and anisotropic
etching of anodic alumina films on aluminium in basic solu-
tions. On the other hand anodic oxidation itself enables easy
control over channel diameter and interpore distance of the
porous film over a wide range (pore diameter from 15 to
200 nm; interpore distance from 50 to 500 nm, film thicknesses
up to 200 micrometers) [3]. Moreover the thickness of AAO
films can be regulated precisely by using conventional electro-
chemistry methods by controlling the total electric quantity and
assuming a current efficiency of 90% [12]. Thus, the proposed
approach enables easy control over porosity, surface area, and
geometric characteristics of cantilever arrays which provide
wide opportunities for the design of micromechanical sensors
with specific mechanical response.
To predict the behavior of the cantilever in the gas/liquid phase
we investigated alterations of the amplitude–frequency charac-
teristics when changing pressure and humidity. The frequency
response of elastic beam is strongly dependent on the fluid it is
vibrating in [2]. In vacuum, when the vapors are absent, the
measurement of the resonance frequency is routinely made
compared to measurements in viscous media. At first we
emphasized the damping effect on cantilever vibration for
porous AAO and standard Si cantilevers explored in the real
system (air) and in model (vacuum). The amplitude–frequency
profiles of rectangular cantilevers made of Si and porous anodic
aluminium oxide are provided in Figure 2. Their characteristics
are summarized in Table 1. The Q factor for both types of
beams significantly increases after change air media to vacuum.
Natural shifts to high-frequency region are observed for the
porous cantilever as well for the Si cantilever.
For comparison we also studied the effect of the water content
on the vibration of the cantilever from the AAO membrane
obtained at 40 V in 0.3 M oxalic acid. Figure 3 shows the
experimental results using a cantilever with a length of 800 μm
and 10% porosity.
The resonant frequency decreases from 117.7 kHz at 10% to
116.5 kHz at 21% humidity. Changing the humidity from 21 to
10% increases the resonant frequency by 1120 Hz and quality
factor of the AAO cantilever increases from 61 to 82, respect-
ively (Figure 3).
Using Equation 1 and the Young’s modulus of AAO of
340 GPa one can evaluate the quantity of water adsorbed onto
the anodic alumina surface. The calculation gives a result of
Δm = 20 pg at a sensitivity Δf/Δm of 56 Hz/pg. On the other
hand, the amount of absorbed water can be estimated from the
Langmuir monolayer absorption isotherm. Taking into account
the total area of the cantilever of 1.74·10−6 m2 (1.58·10−6 m2
Beilstein J. Nanotechnol. 2015, 6, 1332–1337.
1334
Table 1: Shifts of resonance frequency and quality factors Q for Si and AAO cantilevers under different pressure (vacuum and air, humidity 22%).
cantilever material Si AAO
condition air vacuum (10−5 mbar) air vacuum (10−5 mbar)
Q factor 350 3230 39 52
resonant frequency f, kHz 322.01 322.79 177.12 181.02
Figure 2: a) Frequency responses of Si rectangular cantilever (2 μm
thick, 100 μm long and 30 μm wide) in the air (black line) and in
vacuum (red solid line). The frequency shift is 780 Hz. b) Frequency
responses of AAO cantilever (2 μm thick, 800 μm long and 100 μm
wide) in the air (black line) and in vacuum (red solid line). The shift
here is 3900 Hz.
interior and 1.63·10−7 m2 exterior area) and the density of the
active sites of 4.8 nm−2 the maximal mass of a water mono-
layer adsorbed onto cantilever surface is 250 pg. This results in
an increase of the experimental water surface coverage of about
8% within the experimental humidity levels.
From the Langmuir sorption isotherm a humidity change from
10 to 22% should result in growth of the surface coverage by
Figure 3: Frequency responses of AAO rectangular cantilever (2 μm
thick, 800 μm long and 100 μm wide) at relative humidities of 10 and
21% (1120 Hz shift).
8.9%, which fits well to the obtained value. However, despite
the good agreement between the estimations we should stress
that they can be far from being realistic due to a strong chem-
ical interaction of water molecules with AAO surface and pore
curvature.
Figure 4 illustrates the resonant frequency dependence on air
humidity. The experimental points were obtained by measuring
the resonant frequency with decreasing humidity levels from 22
to 10% (at 298 K).
After each humidity decrement the system was left to reach
equilibrium for 10–30 min. Equilibration criterion was resonant
frequency creep below 1 Hz/min. The curve indicates a linear
behavior approximated by f = 118.99·(1 + 0.00198H)−1/2 (red
line), which correlates well with Equation 1. The humidity
sensitivity of the AAO cantilever in the linear range estimated
by the slope of the experimental curve equals about 100 Hz/%.
Hysteresis was not observed for the considered range close to
equilibrium. The resonance frequency for the same value of
humidity measured after a cycle of increasing and decreasing
humidity is well reproduced within the error of 30 Hz. We
believe this error corresponds to an inaccuracy of setting up the
desired humidity value and a slight deviation from equilibrium
during measurements. The plateau at low humidity levels
Beilstein J. Nanotechnol. 2015, 6, 1332–1337.
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Figure 5: Photomask (1); cantilever arrays as formed on Al-foil (2); free-standing anodic alumina cantilevers (3).
Figure 4: Resonance frequency of the AAO cantilevers (2 μm thick,
800 μm long and 100 μm wide) as function of relative humidity
from 10 to 22%. Red line: approximation of the linear part as
f = 118.9972·(1 + 0.00198H)−1/2. Inaccuracy of frequency detection is
below the size of the symbols.
demonstrates a moisture detection limit of 10%. This likely
occurs due to residual native water absorbed on the AAO
surface, which cannot be removed without heating. Linear
behavior of environmental humidity on the resonance frequency
of the cantilever oscillation allows us to use AAO cantilevers as
humidity sensors at least in the humidity range of 10–22%.
Conclusion
The proposed combination of anodic oxidation and photolithog-
raphy processes enables the successful formation of porous
alumina cantilevers with desired geometric characteristics.
Because of the high surface area of the pores the cantilevers ex-
hibit an exceptional sensitivity to absorbed mass. The reso-
nance frequency shift in water vapor absorption experiments
over a humidity range of 10–22% fits well to the monolayer
adsorption isotherm. A strong response to environmental
change enables the use of AAO cantilever arrays in microsen-
sors with ultra-low detection limits. The study has to be consid-
ered as an initial experiment to focus on the profound under-
standing of micromechanical behavior of anisotropic porous
materials having a high specific surface area, such as anodic
alumina films.
Experimental
Preparation of the cantilever array
AAO layer formation was carried out in 0.3 M H2C2O4 (98%,
Aldrich) at a constant voltage of 40 V. The electrolyte was
pumped through the two-electrode cell by a peristaltic pump,
and its temperature was kept constant (2 °C) during anodization.
The films with a thickness of 2 μm were obtained by control-
ling the total electric charge. To define the shapes of the
cantilever array the pattern of the photolithographic mask was
directly transferred onto the AAO surface (spin-coat with S1818
microposit, 6 s at 500 rpm and than 20 s at 5000 rpm, 5 min of
drying at 110 °C, 315 mJ of UV radiation [13]). Then unmasked
AAO was etched away with 0.5% KOH for 10 min. The mask
was removed after drying the samples at 160 °C for 10 min with
acetone. Finally the remaining aluminium was dissolved in
0.5 M CuCl2 acidic solution (5 vol % HCl). The size of the
obtained microcantilevers equals 800 × 100 × 2 µm3 (Figure 5).
One should note that these dimensional characteristics can be
simply tuned by choosing lithographical mask with the desired
cantilever shapes and varying of anodization charge. Films were
characterized by optical (Nikon Eclipse 600pol) and scanning
electron (LEO Supra 50VP) microscopes. The density of anodic
alumina was determined by measuring sample hydrostatic
weight.
Resonance frequency measurement
Mechanical measurements with air, vacuum and wet media
were carried out by AFM (NTEGRA Aura) in the range of
Beilstein J. Nanotechnol. 2015, 6, 1332–1337.
1336
0–600 kHz. The experimental setup used in this study is shown
on Figure 6. The AFM is equipped with vacuum chamber
Ar-flow controllers as well as humidity and temperature
sensors.
Figure 6: Scheme of measurement cell in atomic force microscope
with an integrated optical read-out.
The desired moisture level was obtained by controlling the
humidity at 25 °C with saturated salt-water and CaCl2 dryer.
Saturated salt-water was used to reach high relative humidity in
the microscope chamber. After stabilization of humidity equi-
librium in the system we start measurements of the resonance
frequency over a CaCl2 dryer down to the lowest possible
humidity level. The microscope determined the upper limit of
humidity level. Typically around 10 min have been needed to
stabilize the cantilever to a fixed baseline. Humidity levels were
varied in the range of 10–22%. An example of the resonance
spectrum of porous alumina cantilevers detected with the AFM
is shown in Figure 7.
Figure 7: Spectrum of lateral (in-plane) modes corresponding to an
alumina cantilever 2 μm thick, 800 μm long and 100 μm wide.
The arrays were supported by Si tips before measurements. The
reflectivity of the surface of anodic aluminium oxide is suffi-
cient for detecting vibrations without additional metal reflec-
tive coating. According to the exhibited behavior, adequate
level of sample reproducibility has been demonstrated by the
fabrication procedure described. The frequency does not shift
for different points along the one cantilever. In contrast, inten-
sity and a uniform vibration were indicated along the whole
cantilever independently from where the AFM beam was
detected. The magnitude of the maximal resonance frequency
(about 545 kHz) predictably decreases when the optical beam of
AFM moves to base of array. Experimentally we established the
excitation of mechanical vibrations perpendicular to the
cantilever surface as torsion. The thickness of cantilever arrays
influences on stiffness and may also cause an increasing in reso-
nance frequency: from 500 kHz for 2 μm thick cantilevers to
670 kHz for 30 μm thick cantilevers.
Acknowledgements
The work was partially supported with the grant of RFBR
13-08-01382 and by M. V. Lomonosov Moscow State Univer-
sity Program of Development. The authors would like to thank
Dr. K. S. Napolskii for help with discussion and insightful
comments.
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Nanomechanical humidity detection through porous alumina cantilevers

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Nanomechanical humidity detection through porous alumina cantilevers

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