Introduction
Over the last few decades, nanotechnology, which covers a compilation of
technologies and methods for manipulating material on the nanoscale (i.e.,
nanomaterial or nanoparticle (NP)), has been attracting immense attention in
society and has been hailed by some scientists as the next industrial
revolution. The possible interests in nanotechnology originate mainly from
the novel properties and characteristics of the nanomaterials, which are not
the same as bulk materials and may be unpredictable and unimagined as scale
effects (Maynard, 2007; Hullman, 2007). Thus, this technology has rapidly
been developed and used across a variety of industries (e.g., electronics,
medicine, cosmetics, pharmaceuticals, food packaging, household appliances,
and national defense), leading to increased economic growth and new job
vacancies (Bekker et al., 2013).
Although nanotechnology provides society with enormous feasibilities,
questions have also been raised about uncertainties concerning the risks to
and potential health effects of released NPs on the environment. The
formation and release of the NPs into indoor environments and workplaces can
occur through both incidental (i.e., unintentional NPs) and planned
manufacturing processes (i.e., engineered NPs (ENPs)) (Balbus et al., 2007).
Moreover, much higher awareness should be given to the workers, who
manufacture and handle NPs directly in large quantities during the line
productions (Brouwer, 2010). Thus, a direct-reading airborne NP mass concentration detector
is very useful for the assessment of personal-
and location-dependent monitoring in workplaces and indoor environments.
For individual NP mass monitoring, the complete system relies on mini
portable devices that can be held and carried easily by the workers.
Currently, the already developed NP mass sensors based on
microelectromechanical systems (MEMS) need to be operated at high flow
velocity (50– 150 m s-1) using an external pump (Schmid et al.,
2013) or partial vacuum requiring bulky, heavy, and expensive vacuum tools
(Hajjam et al., 2011). In contrast, our first generation of the
cantilever-based airborne NP detector (CANTOR-1) evaluated
in this work has a small size, a low weight, an appropriate flow rate, an
atmospheric pressure working condition, and low-cost components. This device
had been successfully tested to prove the NP mass sensing principle with
detected ENPs down to ∼ 20 nm in diameter (Wasisto et al., 2013a).
However, these measurements were done offline, where the ENP sampling and the
sensor characterization were performed separately and controlled
manually. Thus, some supporting
components were needed to be added to the device. Recently, we reported on a
frequency tracking system based on a phase-locked loop (PLL) circuit and a
high voltage (HV) module based on a DC amplifier for nanoparticle mass
concentration monitoring in real time (Wasisto et al., 2014). This new device
(CANTOR-1) was successfully tested in normal indoor ambience, showing the
ability to detect, e.g., cigarette smoke. However, the quantitative detection
of NPs was impeded by coarse particulate matter, which was also deposited on
the cantilever. To ensure that CANTOR-1 monitors only NPs, coarse particles
have to be removed from the air passing through the NP sampler. Furthermore,
to give a quantitative NP concentration read-out, CANTOR-1 had to be
calibrated under defined ambient conditions.
Therefore, in this paper, we address a first quantitative test and evaluation
of CANTOR-1 in real-time ENP measurements. For this purpose, measurements
were performed under a defined ENP exposition in a sealed chamber and with
standard stationary aerosol monitoring equipment; i.e., a fast mobility
particle sizer (FMPS) was used to provide reference data for the
time-dependent ENP concentration. Membrane filters and impactors were
described to prevent interfering coarse particulate matter from entering the
sampler. A recycling process of ultrasonic wet cleaning is also performed to
regenerate the sensor after being fully loaded by ENPs during long-term
exposure.
Direct-reading cantilever-based airborne nanoparticle detector
Main components
Figure 1a shows the principle of the direct-reading CANTOR-1. Particle-laden
air is flown through a filter, which removes coarse particulate matter,
towards a cantilever biased to a negative potential (VEP) by
which the positively charged fraction of the airborne nanoparticles is
attracted. The mass added to the cantilever by the attached nanoparticles
lowers the resonant frequency of the cantilever, which is driven by a
piezostack (Vdr) and read-out (ΔV) using an integrated
piezoresistive strain gauge supplied by V0.
In principle, the CANTOR-1 system comprises two main components (i.e., a
miniaturized electrostatic aerosol sampler and a piezoresistive resonant
silicon cantilever mass sensor). The first main module is a cylindrical
miniaturized electrostatic NP sampler made of aluminum material and designed
as a tube with a diameter of 20 mm and a length of 45 mm (Fig. 1b). The
tube consists of three single aluminum parts joined by threads denoted as
inlet part, middle part, and outlet part, respectively. Aiming to direct the
aerosol flow against the cantilever positioned on the tube axis, the inlet
part was conically shaped with an inclination of 15∘ to the tube axis
(Fig. 1c). In the middle part of the tube, the second module of a silicon
cantilever resonator was mounted by double-sided tape on a small ceramic
printed circuit board (PCB) to integrate it with the piezoelectric stack
actuator (Fig. 1d). Gold bonding microwires of 30 µm were used for
getting electrical contact between cantilever and board. To generate stable
aerosol flow of 0.68 L min-1, a small fan (MF10A03A, SEPA Europe GmbH)
was mounted at the outlet part of the tube. To operate the NP sampler, a
voltage of 3.5 V from either a battery pack or a power supply (HP E3631A)
was used. This voltage, however, needed to be amplified first up to 0.5 kV
using a DC voltage amplifier (i.e., a factor of ∼ 140 higher).
(a) 2-D and (b) 3-D schematics, and
(c) photographs of the handheld CANTOR-1 showing (d) the
main components of its middle part. (e) SEM image of the silicon
piezoresistive cantilever resonator.
The employed second module is a silicon cantilever (Vcant=2750×100×50 µm3, mcant=32.04 µg, fcant=9.4 kHz), which has a full square
Wheatstone bridge on its clamped end as a piezoresistive element and works
based on the strain-to-resistivity change to read the sensor signal output
(Fig. 1e) (Wasisto et al., 2013b). The cantilevers are fabricated by
utilizing silicon bulk micromachining processes (i.e., photolithography,
thermal oxidation, dopant diffusion/implantation, and inductively coupled
plasma (ICP) cryogenic deep reactive ion etching (cryo-DRIE)). Figure 2
depicts the fabrication process steps of the self-sensing piezoresistive
silicon cantilever resonators, which can be described in detail as follows.
The fabrication was started with a preparation of (100)-oriented n-type
bulk silicon wafers (3–5 Ωcm) with a thickness of 300 µm.
In contrast to piezoelectric AlN/Si cantilever resonators (Sökmen et al.,
2010), which use SOI for the device fabrication, bulk silicon wafers are more
preferable for the current sensor fabrication considering their lower price,
high stability, high mechanical Q factor, and high degree of freedom for
the geometrical resonant cantilever design. At the beginning, a wafer was
placed in a furnace to grow a thermal silicon dioxide (SiO2) thin layer
on the silicon surface. A 400 nm SiO2 layer was obtained within 1 h at
a temperature of 1100 ∘C.
Subsequently, the oxidized wafer coated with a photoresist mask was
patterned utilizing UV lithography. Buffered hydrofluoric acid solution was
then used to open the areas for the definition of p-type piezoresistors.
Afterwards, to create a full piezoresistive Wheatstone bridge on the Si
wafer, boron implantation or diffusion was performed.
Furthermore, to form p+ feed lines to the Wheatstone bridge and
improve contact formation, additional boron diffusion or implantation was
used. For the implanted and diffused wafers, the standard deviations of the
measured resistivity from the desired value were ∼ 0.6 and
∼ 4.1 % in proportion to their lateral doping distribution,
respectively. Typical values of the junction depth and the surface doping
concentration amounted to ∼ 4.5 µm and
1.5–3.0 × 1018 cm-3, respectively, representing a
tradeoff between a large piezoresistive coefficient π44≈1 GPa-1 and a low temperature coefficient around -3×10-3 ∘C-1 (Cho et al., 2008; Peiner et al., 2008).
Since an ohmic contact was needed for electrostatic NP collection, a
phosphorus diffusion/implantation was then carried out and placed in an
n+-type well close to one of the p+-type piezoresistor contacts.
Next, to create the backside membranes on the bottom side of the wafer,
the selected bottom oxide layers were patterned by UV lithography and
buffered Hydrofluoric acid (HF) etching.
Through the oxide openings, a potassium hydroxide solution (KOH) at elevated
temperatures or cryo-DRIE was introduced to etch the silicon down to a
residual thickness of 25–50 µm. This process is very critical
because it determines the cantilever thickness and thus its operating
resonant frequency. From the experiments, a tolerance of
±0.5 µm (i.e., 1 to 2 %) was normally obtained using KOH
etching.
Subsequent to the opening of the SiO2 contact holes, a top-side
metallization was then deposited by 300 nm aluminum electron-beam
evaporation. Furthermore, to provide large-enough landing areas for
non-permanent electrical connections using spring-loaded contact pins, large
contact pads were provided (i.e., designed as a 0.75×1 mm2
large area).
To finish the fabrication process, the cantilevers were
lithographically patterned and released by cryo-DRIE from the front side of
the samples. A dry etcher (SI 500 C, SENTECH Instruments GmbH, Berlin,
Germany) was used along with a photoresist serving as the etching mask.
The fabrication process flow of the self-sensing piezoresistive
silicon cantilever resonators showing (a) thermal oxidation,
(b) p-type piezoresistor creation, (c) p+-type
contact formation, (d) phosphorus diffusion, (e) backside
membrane wet etching, (f) metallization, and (g) cantilever
free release dry etching.
Microparticle filtration components
In addition, to filter out the undesired microparticles, which could possibly
approach the cantilever, two filtration stages were also developed and
integrated on the NP sampler head of CANTOR-1. From the market, commercially
available grids for transmission electron microscopy (TEM) might be a
solution for cavity diameters down to 0.6 µm. However, those grids
were very fragile, leading to difficult mounting and use in our NP sampling
system. Therefore, the first stage of microparticle filtration was to use a
home-built silicon microfilter with a grid diameter of 2.5 µm. Its
manufacturing processes are illustrated in Fig. 3, which can be described as
follows.
Silicon dioxide (SiO2) was thermally grown on a silicon wafer to be
employed as a grid mask by subsequent lithography and HF dips. A second
lithography is done to prepare the rim on the front side of the filter.
By using ICP cryo-DRIE, the holes with a diameter of 2.5 µm
were transferred into the silicon wafer.
Before structuring the back side of the wafer, its front side had to be
firstly protected by a resist layer to prevent mechanical deformation because
of harsh contact with the aluminum plate carrier. Moreover, this process flow
had been selected to prevent grid artifacts (i.e., a mask undercut that was
expected due to the impaired release of the etching-induced heat through a
membrane).
The backside etching was performed to reduce the membrane thickness down
to ∼ 4 µm. This step is important to determine the ability of
the aerosol to pass through the filter channels, because creating too thick a
membrane will provide a lower aerosol flow rate, which is a disadvantageous
effect for the NP sampling system.
After stripping off the photoresist and SiO2 layers, the silicon
microfilter had been finally realized.
Fabrication process for the Si microfilter, including
(a) SiO2 masking, (b) ICP-RIE, (c) protective
coating, (d) back side etching, and (e) cleaning using an
HF dip.
To check on the utility of the fabricated silicon microfilter shown in
Fig. 4a, a preliminary simple test was carried out in the ENP exposure
assessment prior to the real-time monitoring of carbon aerosol ENPs
(Sigma-Aldrich Chemie GmbH). From scanning electron microscope (SEM)
investigation, it is clearly shown that, by placing the microfilter at the
opening inlet of the CANTOR-1 sampler head, the microparticles and their
agglomerates (dP > 2.5 µm) had been rejected. Thus,
only the ENPs (dP≤2.5 µm) could pass through the
filter holes and subsequently reach the cantilever surfaces (Fig. 4b).
SEM images of the (a) Si microfilter used in CANTOR-1 and
(b) the collected carbon ENPs on the cantilever that could pass
through the filter holes.
To further reject the smaller microparticles
(dP > 1 µm), an impactor made of aluminum was also
made and placed in between the microfilter and the cantilever sensor, acting
as the second large particle filtration stage in the system (Fig. 5a). In
other studies, this impaction technology had also been used for various
devices and described in detail (Mehdizadeh et al., 2013; Schmid et al.,
2013). For our Al impactor, its location is adjustable, which influences the
velocity of the aerosol flow. By integrating this impactor, microparticles of
diameters in the range of 1 µm < dP≤2.5 µm are expected to be impacted, and no longer in the aerosol
flow stream. Figure 5b shows as proof microparticles captured on the impactor
during a typical particle exposure experiment. Nevertheless, the maximum
aerosol velocity in the CANTOR-1 system was set to 8–20 m s-1 to
avoid any disturbances to the operating resonators during the aerosol NP
detection. The airflow through the sampler modeled using COMSOL Multiphysics 4.3b is depicted in Fig. 5c. The used
simulation module is particle tracing.
(a) Homebuilt aluminum impactor and (b) impacted
microparticles on its surface. (c) 3-D COMSOL simulation result
showing the aerosol flow velocity in a color gradient representation ranging from 0 (blue) to 20 m s-1
(red) inside the sampler.
System integration
Besides the main components (i.e., an NP sampler with its mounted cantilever
resonator) and the microparticle filtration stages (i.e., a silicon
microfilter and aluminum impactor), there are three other supporting modules
(i.e., electronic circuit, power supply, and data acquisition control
system), which need to be integrated to build a complete CANTOR-1 system
(Fig. 6). In the electronic circuit module, the frequency tracking system
based on PLL and HV modules based on a DC amplifier are realized and involved
to track the resonant frequency of the silicon cantilever in real time and
amplify the input voltage from 3.5 V to 0.5 kV in the NP sampler,
respectively (Wasisto et al., 2014). During particle sampling, a negative
high voltage of -0.5 kV is set to one of the cantilever electrodes, which
is connected with the bulk contact of the silicon cantilever to generate an
electric field over the sensor. Hence, the positively charged and uncharged
NPs will be attracted and polarized to the cantilever surfaces by
electrophoresis and dielectrophoresis mechanisms, respectively. The detailed
mechanism of the airborne NP deposition in an electrostatic sampler had been
described by Krinke et al. (2002), who had modeled the NP trajectory as a
force balance on a single NP approaching the substrate.
Schematic of the partially integrated CANTOR-1 showing all its
components (i.e., a silicon cantilever resonator mounted in an NP sampler,
power supply, electronic circuits, and data acquisition control system).
Additionally, a combination of two small electronic components (i.e., an
Arduino UNO microcontroller and a Seeedstudio relay shield) was also used to
automatically control the periodic switching between NP sampling and resonant
frequency measurement phases. The yielded response time of this relay shield
was in milliseconds. This fast switching control had overcome the previously
raised issue in the software development (i.e., a time delay of about 6–7 s
due to the communication overhead between measuring instruments and the
notebook computer) (Wasisto et al., 2014). The power sources used in CANTOR-1
originate from the two power supplies (HP 3631A) to supply the different
power requirements of the NP sampler (i.e., 3.5 V), the cantilever resonator
Wheatstone bridge (i.e., 1 V), and the PLL circuit (i.e., ±18 V).
In general, the operating process of CANTOR-1 can be divided into two phases,
which are run alternately. First, the resonant frequency of CANTOR-1 will be
tracked with a time resolution of 1 s under the condition when the PLL
integrated with the cantilever sensor is switched on and the NP sampler is
switched off by relay. Next, after the PLL integrated with the cantilever
sensor has been turned off and the sampler head is subsequently turned on, NP
sampling begins to be performed. It should be noted that, during the entire
switching, a digital multimeter (HP 34401A) is always kept in reading status
to continuously observe the resonant frequency change, which will be further
converted into detected NP mass concentration in a LabVIEW-installed PC.
In its present configuration, CANTOR-1 consists of all components for
real-time ENP monitoring except those for data acquisition control, logging,
and display, for which a laptop PC is used. Thus, CANTOR-1 is more than a lab
demonstrator, but can also be used as a portable indoor ENP monitor
comparable in weight (less than 1 kg, including the PC) with
state-of-the-art instrumentation (e.g., an Aerasense nanotracer of 750 g,
Buonanno et al., 2014, or a Testo miniDisc of 670 g, Fierz et al., 2011). A
fully integrated system, which will include all necessary components in one
base unit of a total weight of < 400 g, will be developed as the second
version (CANTOR-2).
Detector performance
Test aerosol nanoparticle generation
After integrating all components into a complete system, CANTOR-1 was then
assessed in carbon ENP exposures (Sigma-Aldrich Chemie GmbH), which were
performed in a test chamber under typical workplace conditions (i.e., V=1 m3, T=23 ∘C, RH = 30 %, and p=1 atm), as depicted in Fig. 7a. The generation of stable test aerosols was
started by nebulizing a suspension of ENPs in a solution of water/ethanol or
water/isobutanol using a constant output atomizer (TSI 3076, TSI Inc.).
Inside the atomizer, the incoming compressed air swelled through an orifice
forming a high-velocity jet. The liquid solution was then pulled into the
atomizing part through a vertical channel and subsequently atomized by the
jet. Large droplets were removed by impaction on the barrier opposite to the
jet. Meanwhile, the excess liquid was drained at the bottom part of the
atomizer to the closed reservoir. As a result, fine spray could leave the
atomizer through a channel at the top. By using this technique, the average
particle size of the produced aerosol could be varied from 20 to 300 nm.
(a) Schematic of the aerosol ENP generation setup in a test
chamber, involving an atomizer, a diffusion dryer, a fan, and an FMPS.
(b) Typical generated particle number and mass concentrations for
carbon airborne ENPs measured by FMPS as a function of time.
However, it should be noted that the produced aerosols from the atomizer were
still wet. Thus, they had to be dried in a diffusion dryer. In this device, a
water trap was incorporated into its inlet to collect coarse water droplets.
Moreover, this device consists of two concentric cylinders as the main parts
(i.e., inner and outer parts) formed by wire screen and acrylic cylinders,
respectively. The space between those two cylinders was filled with a volume
of round-shaped silica gel, which acted as a desiccant for having a strong
affinity with water molecules and maintaining a dry atmosphere at the tube.
During the flowing of the wet aerosol through the inner cylinder, water vapor
would diffuse into the silica gel through the wire screen. At the outlet of
the dryer, the dried aerosol ENPs can be obtained, which were then flown into
a sealed glass chamber (cf. Fig. 7a). To circulate the aerosol inside the
chamber (i.e., dynamic condition), a fan was used under a clean air supply of
12 L min-1 and put close to one of the inner chamber walls.
Furthermore, in situ monitoring of ENP number concentration and size
distribution was realized by a fast mobility particle sizer (FMPS, TSI 3091,
TSI Inc.) with a time resolution of 1 s (Fig. 7b). The FMPS worked
principally based on the electrical aerosol spectrometer (EAS) (Tammet et
al., 2002). The incoming ENPs, which were firstly charged in a unipolar
corona charger, were classified and measured with an array of 22
electrometers, where each channel corresponds to a defined electrical
mobility bandwidth. Thus, by knowing the ENP charge levels, the measured
currents of the electrometers can be extracted into ENP number size
distributions (6–523 nm). Summing all measured ENP numbers will yield the
total concentration of the flowing aerosols inside the chamber. Moreover, by
assuming the ENPs are in a spherical shape, their mass concentration can be
calculated. The typical stable generated carbon aerosols comprise two size
modes of NP diameters of ∼ 20 and ∼ 120 nm, respectively, at
total number concentrations on the order of
104 pt cm-3.
Real-time engineered nanoparticle exposure assessments
The performance of the complete system of CANTOR-1 was evaluated in two
real-time ENP exposure tests with different purposes (i.e., a minimum NP
sampling investigation and a device continuation, respectively). In the first
test, the setup of ENP detection was performed according to the specified
time (i.e., three cycles comprising 5 min measuring states and varied
sampling states (10, 8, 6, 4, 2, 1, and 0.5 min, respectively) for each
cycle). CANTOR-1 was automatically switched to the measuring status to track
its resonant frequency, whenever the time of the ENP sampling had expired
(Fig. 8a).
After experiencing three cycles of repeated exposure tests of carbon aerosol
ENPs with a stable concentration of 4.9 ± 1.2×104 pt cm-3 in a sequence of periodic states (i.e., consecutive sampling and
measuring), a real-time behavior of CANTOR-1 can then be observed. The
increasing mass loading of the deposited ENPs had caused a linear drop in
cantilever resonant frequency during a total sampling time of 94.5 min
(Fig. 8b). From the analyzed results, the collected ENP mass amounted to
ΔmENP=2×mcant×ΔfENP/fcant=26.88 ng, which corresponded to a
frequency shift of ΔfENP=-3.95 Hz. Thus, a mass
sensitivity of CANTOR-1 of SENP=ΔfENP/ΔmENP=-0.15 Hz ng-1 could be calculated under this
condition. Furthermore, from this test, it was shown that there was a
necessary waiting time (i.e., ∼ 1–2 min) of the resonant frequency
measurement before CANTOR-1 could operate under a stable condition, which was
related to the PLL capturing time. Moreover, the ability of CANTOR-1 to
detect the airborne ENP within only 1 min sampling had been confirmed,
although some unstable data points were also present, which could be
attributed to an unwanted lift-off of already deposited ENP fractions.
Next, differentiating from the first online test where the measurement was
done in only a day, the second exposure assessment of CANTOR-1 was performed
on two subsequent days to investigate the continuation of device use.
Moreover, even though, in the first test of online measurement, CANTOR-1
could already detect carbon ENPs with only 1 min sampling time resolution,
in the second test, CANTOR-1 was set at 5 min sampling time and 3 min
measuring time to better interpret the data (i.e., a sufficient frequency
shift). Thus, in total, CANTOR-1 will need 8 min to obtain a single data
point of the ENP monitoring. This value is well below the 10–15 min period
recommended for short-term aerosol exposure measurement (Duarte et al.,
2014).
(a) The tracked resonant frequency and (b) its
corresponding collected ENP mass monitored with CANTOR-1 during the first
test of carbon ENP sampling.
For the first day of the second online test, CANTOR-1 was operated only to
measure the collected ENP mass in a stable carbon aerosol concentration of
2.1 ± 0.3×104 pt cm-3 (i.e.,
33 ± 7 µg m-3). Therefore, the concentration inside the
chamber after the ENP exposure termination (i.e., aerosol evacuating process)
was not recorded. As expected, the similar effect to that obtained from the
first online test was also clearly seen in this experiment (Fig. 9a). The
linear decrease in the CANTOR-1 resonant frequency (i.e.,
-0.07 ± 0.02 Hz min-1) was proportional to the increase in the
collected ENP mass (i.e., 0.46 ± 0.11 ng min-1), with an ENP
sampling efficiency of 2.1 ± 0.5 %. In the last period of the
measurement, the resonant frequency of CANTOR-1 before being switched off was
9422.03 ± 0.04 Hz, which was also identical for the initial value on
the next measurement day.
On the second day of the second online test, CANTOR-1 was kept operational to
measure the ENP mass concentration from the early beginning of the exposure
up to the end of the chamber evacuation of ENPs. However, at a point where
the proof of stable ENP mass concentration has been sufficient, the flowing
of the dispersed aerosols to the chamber was stopped. This action was used to
validate the CANTOR-1 performance in terms of its sensitivity to the
transitions of the different ENP concentrations from low to high exposure
levels and in the opposite way. It can be obviously seen from Fig. 9b that
the CANTOR-1 resonant frequency gradually decreased as the aerosol ENPs
started to be injected into the chamber. This frequency reduction became
linear (i.e., -0.05 ± 0.03 Hz min-1, corresponding to a
collected mass of 0.34 ± 0.09 ng min-1 with an ENP sampling
efficiency of 1.1 ± 0.6 %) after the aerosols reached their stable
condition of 2.7 ± 0.2×104 pt cm-3 (i.e.,
46 ± 1 µg m-3). However, after exposure termination,
the CANTOR-1 resonant frequency did not degrade into lower values anymore.
Instead, it stayed at almost constant values of 9418.93 ± 0.13 Hz,
meaning that there were no ENPs being sampled on the cantilever surfaces.
There was even a slight decrease in total sampled ENP mass. It could be a
condition where the already deposited ENPs or their agglomerates were being
lifted off again as the cantilever sensor of CANTOR-1 kept vibrating.
Detector calibration
Although CANTOR-1 had already exhibited good performance in the two online
ENP mass monitoring tests as a microbalance, it still became a necessity to
calibrate this home-built device with a standard NP monitoring instrument in
well-defined aerosols to recalculate the measured data in the standard unit
for aerosol mass concentration (i.e., µg m-3). Thus, during
measurement on the second day of the second online test, the results obtained
by CANTOR-1 are also calibrated with FMPS (TSI 3091) (cf. Figs. 7b and 9b).
The calibrated ENP mass concentration (Cm_CANTOR-1) in
µg m-3 can be calculated using
The tracked resonant frequency of CANTOR-1 during the second test of
carbon ENP sampling and its corresponding collected ENP mass for
(a) day 1 and (b) day 2. On day 2, CANTOR-1 was kept
operational after ENP exposure termination.
Cm_CANTOR-1µgm-3=CFT,RH,p×ΔfΔtHzmin-1,
where CF is the calibration factor in terms of the interferences
from temperature, relative humidity, and pressure. Meanwhile, Δf/Δt is the resonant frequency shift per time unit of CANTOR-1 (i.e.,
Hz min-1). However, prior to the determination of CF, the ENP number
concentration measured by FMPS (i.e., Cn_FMPS) must first be
converted into ENP mass concentration (i.e., Cm_FMPS) by
assuming that the size-distributed particles are in a spherical shape. The CF
value (i.e., µg min (m3 Hz)-1), in this case, is given by
CF=Cm_FMPS_avgXΔf/Δt_avgµgmin(m3Hz)-1,
where Cm_FMPS_avg and XΔf/Δt_avg are the averaged value of Cm_FMPS under
stable conditions and its corresponding mean value of the CANTOR resonant
frequency shift per time unit, respectively.
The tracked resonant frequency and its corresponding ENP mass
concentration calibrated towards FMPS (TSI 3091) monitored with CANTOR-1
during the second day test of real-time carbon ENP sampling.
Figure 10 shows the calibrated ENP mass concentrations measured by CANTOR-1
in comparison with those measured by FMPS. In this case, the CF used for
CANTOR-1 was 815 µg min (m3 Hz)-1. During 1 h assessment of a
stable carbon ENP mass concentration of 46 ± 1 µg m-3
measured by FMPS, the precision of the calibrated ENP concentrations measured
by CANTOR-1 was found to be < 55 %, which was taken as the deviation of each measured data point.
Moreover, by multiplying the minimum frequency shifts that can be resolved
with CF, a limit of detection (LOD) of < 25 µg m-3 was
obtained for this detector. Regardless of some drawbacks of CANTOR-1 (i.e.,
time-consuming sensor preparation and a low ENP sampling efficiency of
1.07 %), the overall experimental results have verified the good
sensitivity and working status of CANTOR-1, because in the real workplace,
this detector aims to be used as a first alert for informing the workers in
regards to the danger caused by the unexpected excessive concentration of the
ENPs in air that can occur suddenly during their working shifts.
(a) Typical unstable resonant frequency signal and
(b) surface condition of a fully ENP-loaded CANTOR-1.
Detector robustness
To test the robustness of CANTOR-1 in terms of its operating lifetime,
long-term use during a carbon ENP exposition of ∼ 1–7×104 pt cm-3 was performed with a recycled silicon piezoresistive
cantilever integrated into CANTOR-1 within 7 workdays. It was found that the
signal of CANTOR-1 started to get unstable only after 43.5 h of ENP sampling
as compared to the normal device performance (Fig. 11a). Thus, ∼ 40 h
of continuous operations can be considered the lifetime of CANTOR-1 under
typical mass concentrations in the ambience at workplaces of
20–120 µg m-3. For the corresponding frequency shift and
collected ENPs, we found -79.15 Hz and 0.54 µg, respectively.
Using the cantilever mass of mcant≈32 µg, we
conclude that the operating life of CANTOR-1 ends when the mass of deposited
ENPs reaches a value of ∼ 2 % of the cantilever weight. This fact
was supported by SEM, showing the surface of the cantilever heavily loaded by
carbon ENPs (Fig. 11b). Almost all of the deposited ENPs exhibit an
agglomerate shape. Thus, the unstable signal of CANTOR-1 might be explained
by a loss of mass by detaching of large ENP agglomerates of high-enough
inertia.
To regenerate the fully ENP-loaded cantilever, a relatively simple but
efficient wet cleaning method was demonstrated using an ultrasonic cleaner
with acetone solution or deionized (DI) water inside. This ultrasonic cleaning technique had been implemented
in several heavily used cantilever sensors with varied geometries (i.e., down
to nanoscaled devices) and exposure levels, which had always obtained high
cleaning efficiencies up to ∼ 99 % (Wasisto et al., 2013c, d). During
the high pressure stage of the cleaning process using a Bandelin Sonorex
Ultrasonic bath TK 52 (f=35 kHz), the formed bubbles imploded, releasing
enormous amounts of energy attacking every surface; hence, after 0.5–2 min,
the deposited ENPs were almost completely detached from the cantilever
surface. Figure 12a and b depict the surface conditions of the heavily
ENP-loaded cantilever of CANTOR-1 before and after the ultrasonic wet
cleaning process, respectively. Nevertheless, considering their potentially
low fabrication cost (i.e., fabricated using bulk silicon instead of SOI
wafers), the cantilevers may also be considered a disposable component. Since
spring-loaded contact pins are used, wire-bonding is not required for
electrical contact to the cantilever die. Thus, it can be replaced easily
with either a new or a regenerated one after some period of the ENP sampling.
The surface conditions of fully ENP-loaded CANTOR-1
(a) before and (b) after ultrasonic cleaning.
Comparison between the developed partially integrated CANTOR-1 and
the other currently researched micro/nanomechanical particle detectors.
Developer, reference
Measurement principle
LOD andparticle sampling time
Notes
DU, USA (Hajjam et al., 2011)
Direct deposition in a partial vacuum and thermally actuated silicon resonator
3 µg m-3, 10 s
Partial vacuum, microscope and large pump needed, microparticle detection, no sensor recycling process
BSAC, LBNL, and EPA, USA (Paprotny et al., 2013)
Thermophorectic precipitator and film bulk acoustic resonator (FBAR)
2 µg m-3, 10 min 10 µg m-3, 4 min
High particle loss, fan stack needed, non-integrated electronic circuitries, no sensor recycling process
DTU, Denmark (Schmid et al., 2013)
Inertial impactor and nanomechanical resonant filter fiber
< 1 µg m-3, 1 s
High particle loss, external pump needed, bulky measurement tools required (i.e., lock-in amplifier and laser Doppler vibrometer), no sensor recycling process
McGill University, Canada (Morris et al., 2014)
Impactor and quartz crystal microbalance (QCM)
5.2 µg m-3, 30 min
Bulky measurement tools and vacuum pump needed, microparticle detection, no sensor recycling process
IHT TU Braunschweig and Fraunhofer WKI, Germany (this work)
Electrophoretic precipitator and self-sensing silicon cantilever resonator actuated with a piezoelectric actuator (CANTOR-1)
25 µg m-3, 1–5 min
Partially integrated, miniaturized sampler, two-stage microparticle filtrations, sensor recycling process demonstrated, sensor calibrated, digital multimeter and power supply required
Detector comparison
To compare the developed CANTOR-1 with the other up-to-date MEMS-/NEMS-based
particle detectors worldwide, Table 1 has summarized their important data
(i.e., measurement principle, LOD, particle sampling time, and notes). From
this comparison, it is clearly shown that only CANTOR-1 uses the
electrophoretic sampling method. The other prototypes from the University of
Denver (DU, USA; Hajjam et al., 2011), the Berkeley Sensor and Actuator
Center (BSAC, USA; Paprotny et al., 2013), the Technical University of
Denmark (DTU, Denmark; Schmid et al., 2013), and McGill University (Canada;
Morris et al., 2014), employ impaction and thermophoretic methods. The
slowest response is exhibited by quartz crystal microbalance (QCM) from
McGill University, with a 30 min sampling time. Moreover, the detected
targets of the devices from McGill University, DU, and BSAC were large
particles (i.e., microparticles). They do not have any filtering systems for
smaller particles (i.e., NPs). Some of them even had to be operated with a
large vacuum pump and the optical method to yield the aerosol flows and read
out the signal, which are not applicable for development of low-cost wearable
sensor systems. Although most of them provide slightly better LODs than
CANTOR-1, they have not demonstrated a method to remove the deposited
particles and recycle their sensors. Furthermore, for the nanowire device
from DTU, it is expected that the sensor will be fully loaded by NPs within
only a few minutes. As an improvement to CANTOR-1, its next generation (i.e.,
CANTOR-2) will be developed as a fully integrated system, where all sensing
components and their supporting electronics will be miniaturized and packed
into handy-format housing. Hence, it can be worn easily by workers in
nanotechnology industries during their working shifts. According to the
paradigm shift reported for air pollution monitoring towards lower-cost,
easy-to-use, portable, and direct-reading sensors, there is further
commercial potential for ENP monitoring to be expected in the near future
(Kumar et al., 2015; Snyder et al., 2013).
Biographies
Hutomo Suryo Wasisto received the Bachelor of
Engineering degree in electrical engineering from Gadjah Mada University,
Indonesia, the Master of Engineering degree in semiconductor engineering from
Asia University, Taiwan, and the Doktor-Ingenieur (Dr.-Ing.) degree in
electrical engineering with the Summa Cum Laude honor from the Technische Universität Braunschweig, Germany, in 2008,
2010, and 2014, respectively. Currently, he is working as a senior research
scientist at the Institute of Semiconductor Technology (IHT), TU Braunschweig
and the Laboratory for Emerging Nanometrology (LENA), Germany, where his main
interests are in the fields of micro-/nano-electromechanical systems
(MEMS/NEMS)-based silicon cantilever technology for personal aerosol
nanoparticle sensing devices and high-speed measurements of surfaces of
microstructures with high aspect ratios. He has published more than 40 papers
in international scientific journals and conference proceedings. He has also
been the recipient of the best paper award and the second best young
scientist poster award at the 8th IEEE International Conference on Nano/Micro
Engineered and Molecular Systems (IEEE NEMS 2013) in Suzhou, China, and the
26th European Conference on Solid-State Transducers (Eurosensors 2012) in
Krakow, Poland, respectively. In 2014, he received the
Walter-Kertz-Studienpreis (Walter Kertz Study Award) for his excellent
doctoral dissertation and achievements of scientific studies at the interface
between physics, electrical engineering, and information technology from the
TU Braunschweig, Germany.
Stephan Merzsch received the Diplomingenieur
(Dipl.-Ing.) degree in electrical engineering from the Braunschweig
University of Technology, Braunschweig, Germany, in 2008. Since 2008, he has
been a PhD student at the Institute of Semiconductor Technology (IHT), TU
Braunschweig, Germany. His main interests are semiconductor sensors,
microsystems technology, and nanoimprint techniques for airborne nanoparticle
sensors. Currently, he is working for Infineon Technologies AG, Munich,
Germany.
Erik Uhde received his diploma in chemistry (1994)
and later his PhD (physical chemistry, 1998) from the Braunschweig University
of Technology, Germany. In 1995, he started as a project manager at
Fraunhofer WKI. Since 2001, he has been the deputy head of the Department of
Material Analysis and Indoor Chemistry, which specializes in the
characterization of gaseous and solid air pollutants indoors. He is a member
of national and international standardization groups in the field of indoor
air quality.
Andreas Waag received his diploma as well as PhD
degree in physics from the University of Würzburg, Germany, in 1985 and
1990. In 1996, he got the Gaede Award of the German Vacuum Society for the
development of novel II–VI materials for blue–green laser diodes. Since
2003, he has been a full professor at TU Braunschweig, University of
Technology, and head of the Institute of Semiconductor Technology (IHT), with
activities in the field of oxides and nitrides for optoelectronics, as well
as sensor and joining technology.
Erwin Peiner received the Diplom-Physiker and PhD
degrees from the University of Bonn, Germany, in 1985 and 1988, respectively.
In 2000 he received the venia legendi for semiconductor technology from the
Faculty of Mechanical and Electrical Engineering of the TU Braunschweig.
Currently, he is the leader of the semiconductor sensors and metrology group
at the Institute of Semiconductor Technology (IHT) of the TU Braunschweig. He
has published more than 250 papers in international journals and conference
proceedings. He has been the project coordinator of the collaborative NanoExpo
project funded by the German Federal Ministry of Education and Research
(BMBF).