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Smartphone-Enabled Aerosol Particle Analysis Device

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The health effects of suspended particulate matter (PM) in the air are well documented; however, there is a lack of convenient tools to recognize and quantify PM onsite. Here, we design and fabricate a portable PM analysis system to realize onsite aerosol particle analysis. The system contains a micromachined virtual impactor (VI), a thermophoretic deposition chip and a smartphone-based portable imaging device. Silicon dioxide (SiO2) particles and polystyrene sphere (PLS) particles are used to verify the micro-sized VI with an accurate cut-off diameter of 2 μm (PM2). After separation, the fine particles are thermally deposited on a replaceable film. Then, a smartphone connected with a hand-held optical microscope is applied to directly image and analyze the deposited particles with the assistance of a self-developed Android application; thus, the size and distribution of the particles can be acquired immediately. The PM analysis device is successfully applied to analyze the particle content from a smoking cigarette as a real complex sample demonstration. Considering the compact integration of the particle separation, collection, detection and analysis components, the reported PM analysis device is promising for point-of-need outdoor and indoor air quality monitoring.
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Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000.
Digital Object Identifier 10.1109/ACCESS.2019.Doi Number
Smartphone-Enabled Aerosol Particle Analysis
Device
Yanna Li,1 Wei Pang,*1 Chen Sun,2 Qiang Zhou,2 Zuzeng Lin,1Ye Chang,2 Quanning Li,2
Menglun Zhang,*1, and Xuexin Duan*2,3
1State Key Laboratory of Precision Measuring Technology & Instruments,
2 College of Precision Instrument and Opto-Electronics Engineering, Tianjin University,
3Nanchang Institute for Microtechnology of Tianjin University, Tianjin 300072, China
Corresponding author: Xuexin Duan (e-mail: xduan@tju.edu.cn).
The authors gratefully acknowledge financial support from the Natural Science Foundation of China (NSFC Nos. 91743110, 61674114, 21861132001), the
National Key R&D Program of China (2017YFF0204600), Tianjin Applied Basic Research and Advanced Technology (17JCJQJC43600), the Foundation
for Talent Scientists of Nanchang Institute for Microtechnology of Tianjin University, and the 111 Project (B07014).
ABSTRACT The health effects of suspended particulate matter (PM) in the air are well documented;
however, there is a lack of convenient tools to recognize and quantify PM onsite. Here, we design and
fabricate a portable PM analysis system to realize onsite aerosol particle analysis. The system contains a
micromachined virtual impactor (VI), a thermophoretic deposition chip and a smartphone-based portable
imaging device. Silicon dioxide (SiO2) particles and polystyrene sphere (PLS) particles are used to verify
the micro-sized VI with an accurate cut-off diameter of 2 μm (PM2). After separation, the fine particles are
thermally deposited on a replaceable film. Then, a smartphone connected with a hand-held optical
microscope is applied to directly image and analyze the deposited particles with the assistance of a self-
developed Android application; thus, the size and distribution of the particles can be acquired immediately.
The PM analysis device is successfully applied to analyze the particle content from a smoking cigarette as a
real complex sample demonstration. Considering the compact integration of the particle separation,
collection, detection and analysis components, the reported PM analysis device is promising for point-of-
need outdoor and indoor air quality monitoring.
INDEX TERMS virtual impactor, particulate matter, smartphone, Android application
I. INTRODUCTION
Particulate matter (PM) suspended in the atmosphere is
harmful to human health. In particular, particles with
diameters less than 2.5 µm (PM2.5) are capable of
penetrating deeply into respiratory systems and causing
serious health problems, such as cardiorespiratory mortality
[1] and cardiovascular disease [2]. It has been reported that
a 4.34 μg/m3 increase in PM2.5 may lead to a 138%
increase in the risk of Alzheimer’s disease [3]. In addition,
tobacco particles are the most frequently inhaled PM in our
daily life [4]. Developing tools that can provide rapid PM
analysis is generally required. In recent years, increasing
attention has been paid to the development of portable
devices or systems to provide onsite PM analysis including
the particle size distribution [5]-[6], mass concentration [7]-
[10], surface structure [11]-[12] and composition [13]-[15].
The current methods for PM analysis can be generally
classified as methods based on gravimetric analysis and
light scattering [16]. Gravimetric analysis uses a mass
balance [17]-[18] to weigh the PM after offline collection
and separation by filter membranes of different sizes. A
similar approach based on a quartz crystal microbalance
(QCM) [19] can acquire the weight of PM in real time.
However, this approach has a high cost and cannot achieve
portable detection. Light scattering is the other most used
PM detection method [20]-[21]. When laser light is incident
on a particle, the particle surface will scatter the light in a
particular direction. By collecting the scattered light, the
size and number of the particles can be read out directly.
Some portable equipment based on light scattering for
PM2.5 or PM10 detection has been commercially available
in the market. However, the scattered light is largely
affected by the particle concentration, and the superposition
of signals from particle mixtures will strongly disturb the
real signals. [22] This error will hinder an accurate
quantification of real samples. In addition, the optical
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI
10.1109/ACCESS.2019.2930776, IEEE Access
VOLUME XX, 2017 9
lenses of the equipment are easily contaminated by the
particles, which often requires a calibration and ablution
after a period of use. Thus, there is a large demand for an
easy-to-operate, portable and reliable method for point-of-
need microparticle quantifications. To achieve an accurate
quantification of particle mixtures, particles have to be
separated on demand. The inertial impactor is one of the
most popular methods to separate PM in air [23], and the
virtual impactor (VI), which is a subclass of the inertial
impactor, has been reported recently [24]-[26]. Due to its
higher separation efficiency, lower particles loss, and
relatively small size [27], the VI possesses enormous
potential for portable PM detection.
With the rapid development of microprocessors and
microcameras, a current trend in developing portable
analytical devices is the use of a smartphone as a portable
imaging device. Cellphone-integrated colorimetric chemical
detection [28] portable microscopy [29], point-of-care
biomedical systems [30]-[31], and immunosensors have
been reported [32]. The use of smartphones as analytical
devices [33] will enable users to have access to portable
and cost-effective sensing at any time and any place.
The motivation of this work is to develop a low-cost,
portable and onsite device, which could count the number
of the particles and provide their size distributions. In this
work, a simple portable camera (attachment of the
smartphone) is used to image the particles. As such portable
camera cannot provide enough resolution images to
distinguish different sized particles, a VI is required to
separate the particles by their size first before image
analysis to provide accurate counting.
II. THEORETICAL FOUNDATIONS AND SIMULATION
A. System Design
Fig. 1A shows the whole separation chip, which consists of
three bonded layers. In addition, Fig. 1B displays the
overall working flow of the aerosol particle detection. First,
different sized particles were initially separated in the VI
with a cut-off diameter of 2 µm (separation). After
separation, the separated fine particles were deposited on
the membrane on the bottom wafer by thermophoresis
(deposition). The temperature gradient induced by the two
layers introduced a force to push the fine particles deposited
on the cold substrate. Next, a smartphone-based hand-held
microscope was applied to directly image the collected
samples (imaging). Finally, a self-developed application
(APP) was employed to acquire the microparticle
information, such as the number and size distributions. Fig.
1C shows the real sensor system, the machined VI, the PM
sensor, and the imaging system.
FIGURE 1. (A) Schematic of the PM separation chip. (B) The working flow of the PM analytical system. (C) The VI for particle separation with microfluidic
channels. (D) The PM sensor contains a thermal electrode with the resistance of 28 Ω. (E) The imaging system, including a hand-held microscope and a
smartphone.
The particle sensor chip consists of three bonded layers.
The main layer is a VI for mixed particle separation. The
VI microfluidic channel is fabricated by micromachined
technology with a size of 70 mm × 28 mm × 3.5 mm (Fig.
1C), and the main channel width (D1) and the height (H) are
etched to a size of 1 mm × 1 mm. The distance (S) of the
major flow channel and the width (D2) of the minor channel
are 1.4 mm and 1.5 mm, respectively (all the parameters are
shown in Fig. 2.). The VI chip consists of an air inlet (3.5
mm in diameter), two air outlets (3.5 mm in diameter), and
one rectangular opening (the length and width are 4.5 mm ×
8 mm.). For simplicity, we denote the inlet channel flow as
A, the minor channel flow as B, and the major channel flow
as C. The top layer is a fused quartz wafer that serves as a
transparent cap for the air-microfluidic channels, with a
metal electrode as a thermophoretic heater (Fig. 1D). A
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VOLUME XX, 2017 9
400-nm-thick gold (Au) layer together with 20 nm of Cr
were evaporated on the top of the quartz layer to achieve a
resistance of 28 Ω, and the other two resistance electrodes
in the rectangular dotted box are spare. The bottom layer is
a glass slide with a small piece of filter membrane on the
slide. The imaging system consists of a compact 100X
portable microscope magnifier (CONN ISSUER; OMA-
100+AX1), a smartphone (which can be any commercial
smartphone with a 1200 Megapixel camera), and a special
holding device to connect the mobile phone (Fig. 1E, the
whole magnifier setup is shown in supporting information
(SI) Fig. S1.). A developed smartphone APP is used to
perform the image analysis and particle quantifications.
(The source code of the APP can be downloaded at the
GitHub page https://github.com/TJUbiomems/Microsphere,
and for detailed information of the workflow of the
Microsphere APP, refer to SI Fig. S2.).
The silicon dioxide (SiO2) particles and polystyrene sphere
(PLS) particles with diameters of 2 µm and 5 µm were
purchased from Aladdin (99.9% purity for all particles).
B. Theoretical Calculation and Simulation of the VI.
SiO
2
particles with diameters of 2 µm and 5 µm are
used in the theoretical computation. For simplicity, we
denote the SiO
2
particles with diameters of 2 µm and 5
µm as PS
2
and PS
5
, respectively. Fig. 2 shows a
schematic diagram of the VI. The inlet air flow carries
the coarse and fine particles, which are accelerated
through an injection nozzle. The coarse particles move
along the minor flow channel (the straight channel).
The fine particles with less mass will move to major
flow channels (the two side channels). The aerosol
particles are divided into different flow channels based
on their movement inertia in the VI. In addition, the
inertias of the particles are determined by their mass,
which can be further characterized by their
aerodynamic diameter [34].
FIGURE 2. The schematic diagram of the VI.
When the particle collection efficiency of a certain diameter
in the major channel reaches 50%, the diameter is defined as
the cut-off diameter of the VI, which is denoted as d50 [35].
d50 can be approximated as
Cp
QC
stkTD
d
)(9 50
2
1
50 =
(1)
where η is the dynamic viscosity of air, D1 and T are the
width and depth of the VI inlet channel, ρp is the particle
density, Q is the volumetric flow rate through the inlet, and
Stk50 is the Stokes number, which is in the range of 0.479-
0.59 for a rectangular inlet [36]. The Cunningham
correction factor Cc, for particles with diameters larger than
1 μm, is defined as
d
Cc
52.21 +=
(2)
where d is the particle diameter and λ is the length of the
mean free path of air. The 50% cut-off diameter (d50) is
normally employed as the key operating indicator in a size-
selective device [37]-[38]. In this work, the VI is designed to
separate PM2; thus, the 50% cut-off diameter is 2 µm.
The Reynolds number (Re) is another important
parameter for the VI. The flow is laminar for Re <
2000 and turbulent for Re > 4000 [39]. Re is
dimensionless in any consistent system of units and is
calculated by
UW
Ra
e
=
(3)
where ρ is the density of air, U is the average flow velocity
through the acceleration nozzle, and μ is the viscosity of the
air.
According to equations (1-2), Cc is calculated to be
1.066528, and Q is calculated to be 660 ml/min.
We use finite element analysis (FEA) to simulate the
aerosol particle flow state in the VI and select the flow
particle tracking module, and we use a laminar flow module
to build the simulation physical field. The initial width and
height of the inlet are set to be 1 mm × 1 mm, the ratio of
the distance (S) to the major channel width (D1) is set to 1.4,
and the ratio of the minor channel width (D2) to the major
channel width (D1) is at 1.5. The air flow is treated as a
three-dimensional (3D), steady, incompressible, and
laminar flow. The boundary conditions are no-slip
conditions for all of the walls (more simulation parameter
information is given in SI Table S1.).
According to equations (1-2), Cc is calculated to be
1.066528, and Q is calculated to be 660 ml/min. Fig. 3
shows the FEA simulation results. From the particle
trajectories in the VI (Fig. 3a, where Q is 660 ml/min),
approximately 50% of PS2 pass through the minor flow
channel, the other 50% pass through the major flow channel;
meanwhile, nearly all PS5 pass through the minor flow
channel, and the particle streamlines are clearly exhibited in
the inset of Fig. 3a. In Fig. 3b, the PS5 collected in the
minor flow channel reach a plateau with a sharp increase at
the beginning. This means that all large particles have
enough inertial force to overcome the Stokes drag force [39]
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI
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VOLUME XX, 2017 9
of the air flow. In contrast, the amount of PS2 increases
gradually. Additionally, Fig. 3b shows that the 50% cut-off
diameter of 2 µm is reached when Q is set at 660 ml/min,
which is in good agreement with the above theoretical
calculation. In Fig. 3c, the collection efficiency (N1/N)
increases almost linearly with the minor flow ratio (Q1)
with Q = 660 ml/min, and the 50% collection efficiency of
PS2 can be obtained when r = 0.1. As shown in Fig. 3d, the
simulation data lie on the fitted curve, and its S-shape
verifies the reasonable design of the VI. The 50% cut-off
diameter of the VI is 2 µm.
FIGURE 3. FEA simulation results of the SiO2 particles. (A) Particle
trajectories generated with an inlet flow of Q = 660 ml/min and r = 0.1 (B)
Measured particle collection efficiency in the major flow channel
corresponding to the Q. (C) The collected efficiency in the major flow
channel corresponding to r when Q = 660 ml/min. (D) The S-shape fitted
curve shows the cut-off diameter is 2 µm.
III. EXPERIMENTAL SECTION
A. THE VI SEPARATION RESULTS WITH SiO2
PARTICLES
SiO2 particles were used to verify the VI separation
performance. (The overall aerosol particle generation setup
is shown in SI Fig. S3, the air sampler is reusable since a
filter is mounted in front of the VI to prevent the clogging
in the air-microfluidic channel.). By adjusting the inlet Q
and the suction of the vacuum pump, the optimal flow
parameters and separation efficiency were acquired. During
the operation, the total aerosol Q was set to 600 ml/min-800
ml/min, and the Reynolds number ranged from 700 to 934.
The minor-to-total flow ratio was fixed at r = 0.1. Fig. 4 (a-j)
shows the particle collection efficiency of the VI. As Q
increased from 600 ml/min to 800 ml/min, both PS2 (Fig. 4
(a-e)) and PS5 (Fig. 4 (f-j)) gradually moved from the major
flow channel C towards to the minor flow channel B;
however, PS5 changed more rapidly than PS2. After the
particles flowed in from the inlet channel, the special
structure of the VI forced the particles to divide and flow
into two channels, and particles with different diameters
flowed into different channels based on their inertial force
and the Stokes drag force. As Q increased, the inertial force
of larger particles increased more than the drag force, with
the result that PS5 had enough inertial force to overcome the
Stokes drag force and maintain the original path flowing
into the minor flow channel. Meanwhile, the inertial force
of PS2 increased slowly due to their small diameters. The
Stokes drag force of air was dominant when the inlet flow
velocity increased for fine particles; therefore, their
trajectories changed slowly.
FIGURE 4. The experimental data with SiO2 particles: (a-j) the particle collection efficiency with Q ranging from 650 ml/min to 750 ml/min. (k-l) The
normalized collection efficiencies of PS2 and PS5, respectively. (m) The particle loss of the VI corresponding to Q.
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VOLUME XX, 2017 9
Due to the instability of the aerosol particle generator, the
concentration of the particles fluctuates within a certain
range, but the overall trend of the collection efficiency is
obvious. Fig. 4 (k-m) illustrates this result. From the bar
chart, the VI achieves a 50% cut-off diameter of 2 µm at Q
= 700 ml/min. This experimental result of the Q is
consistent with our calculation result, with a different of
only approximately 5.7%. We calculated the loss efficiency
of the VI, and it was found that the particle loss will be
minimized as Q is closer to the cutting point flow rate (QC).
As shown in Fig. 4 m, the particle loss of PS5 was
minimized at Q = QC = 700 ml/min, and the particle loss of
PS2 was minimized at Q = 750 ml/min, very close to QC.
B. THE VI SEPARATION RESULTS WITH PLS
PARTICLES
To further verify the compatibility of the VI, PLS particles
were used to verify the separation ability. Here, for simplicity,
PLS particles with diameters of 2 µm and 5 µm are denoted
as PP2 and PP5, respectively. Because the density of the PLS
particle is 1050 kg/m3, by equation (1), we can calculate that
Q is approximately 1134 ml/min (Stk50 is in the range of
0.479-0.59). The experimental results are shown in Fig. 5,
and a cut-off diameter of 2 µm is achieved for PLS particles
when Q = 1230 ml/min and Re = 1324. In addition, from Fig.
5m, we can obtain the same conclusion that as Q is closer to
QC, the particle loss is minimized.
Figure 5. The experimental data with PLS particles: (a-j) the particle collection efficiency corresponding to Q ranging from 1030 ml/min to 1430 ml/min. (k-l)
The normalized collection efficiencies of PP2 and PP5, respectively. (m) The PLS particle loss of the VI corresponding to Q.
C. THERMOPHORETIC DEPOSITION AND
QUANTIFICATION OF THE PARTICLES.
A FEA simulation was adopted to investigate the particle
thermophoretic deposition in a real operating atmosphere. A
plate-to-plate thermal precipitation simulation model was
designed based on the real sensor parameters and actual air
flow conditions. (The simulation details and explanations are
shown in SI Fig. S4 Fig. S6.). In the end, a 3 V applied
voltage was adopted in the experiment. (The analysis process
is shown in SI Fig. S7 Fig. S8). Under the action of the
thermophoretic force, the particles were deposited on the
bottom layer. While, the deposition time is well controlled as
well to collect enough particles for the analysis. After the
sampling, we recorded a picture of the particle deposition
area on the bottom layer and directly analyzed the image
using the developed cellphone APP. Fig. 6A shows a typical
result of the SiO2 particles (the sampling time is 20 s, and
aerosol particles spray out from the generator at a velocity of
6 mm/h). The result clearly reveals that there are 452
particles smaller than 2.50 µm, 280 particles between 2.50
µm and 5.0 µm, and 1 large particle with a diameter between
5.50 µm and 5.57 µm (from the left picture of Fig. 6A). In
addition, the particle size distribution can be easily acquired
by the APP analysis (from the right picture of Fig. 6A.). If
we adjust the particle diameter range by sliding the button
below the diameter range configuration box, the different
diameters of the particle analysis can be displayed (SI Fig.
S9).
To verify the accuracy of our PM measurement, we
compared the measurement system with a commercial
particle measurement instrument (OPC, LIGHTHOUSE,
SOLAIR 1110). We denote the VI with the photographic
analysis as PA and the optical counter method as OPC. In
addition, SiO2 particles with diameters of 2 µm and 5 µm
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VOLUME XX, 2017 9
were used in the comparison experiment (inlet rate of Q =
700 ml/min and a sampling time of 20 s). In Fig. 6B, the
injection velocity of the sample for the x-axis is the spray out
rate of the aerosolized particles in the powder cylinder of the
powder generator (PALAS, RGB 1000). With a faster
injection speed, a higher powder concentration will be
reached. As the concentration of the sample particles changes,
the trend of the variation in the PA measurement results is
consistent with that of the OPC measurement data. We also
verified the measurement repeatability of the PA method (see
Fig. 6C, injection velocity of 6 mm/h). The measurement
shows that the data of PS2 fluctuate, and the difference in PS2
between the maximum (group 1) and minimum (group 3) is
36.2%, while this difference is 37.5% in the OPC method
(see the PS2 data of the major channel flow C in Fig. 4c,
orange color). At the same time, the variation in the PS5
results in the five groups of the PA method has no large
difference, which is rather comparable to the results of the
OPC method (see the PS5 data of the major channel flow C in
Fig. 4c, orange color.). The comparison experiments
illustrate that the fluctuation of the PA calculation results is
the same as that with the OPC method. All the data have
proved that the PA method has high accuracy in detecting the
aerosolized particles.
FIGURE 6. (A) The processed results of the PS2 and PS5 photographs by a self-developed APP. (B) The comparison results between the PA method
and OPC method. (C). The repeatability test of the PA method.
D. INTEGRATION AND APPLICATION
Furthermore, in order to manufacture a real portable particle
analysis device for personal use, we developed a prototype
system by integrating the PM separation chip, a minipump,
needle valves, batteries and gas pipes in a resin box (10 cm
×7 cm×11 cm) (Fig. 7A). The PM sensing device was then
applied for real sample detection. Here, a smoking cigarette
(Zhong Nanhai, China) was used as the sampling resource.
Fig. 7B shows the recorded particle distributions of the
smoking cigarette. (The details of the operation process are
shown in SI Video1 Video 4.). After the cigarette burned
for 2 mins, 79.16% of the particles were distributed between
2 μm - 3 μm, and the particles larger than 5 μm only
accounted for 0.37%. This suggests that most of the smoke
particles are below PM2.5 and proves that the developed PM
sensing device and the system can be applied for real
particles analysis.
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10.1109/ACCESS.2019.2930776, IEEE Access
VOLUME XX, 2017 9
Figure 7.
(A) The assembled device for aerosolized particle detection. (B) The particle number concentration distribution of a smoking
cigarette
.
IV. CONCLUSION
A compact and portable PM sensing device integrated with a
particle separation chip and smartphone-based imaging
device is reported in this work. The experimental results
confirm the good separation performance of the PM sensor
with two types of particles, and the results are consistent with
our theoretical calculations. A smartphone attached to a
simple hand-held microscope and a cell phone APP are
developed to image and analyze the separated particles
directly. The particle number and size distributions of real
particle samples can be quantified precisely without extra
calibrations. The PM sensing device (70 mm × 28 mm × 3.5
mm) is compact, portable, accurate, and economical. On the
other hand, due to the pixel resolution limitation of the
smartphone and the 100X magnification ability of the
portable microscope, the device cannot distinguish particle
sizes smaller than 1 µm. However, we also believe that the
system is preferable for personal environmental monitoring.
In addition, the system holds great promise for various point-
of-need applications involving particle quantifications,
especially for heavily polluted environments.
ACKNOWLEDGMENT
The authors gratefully acknowledge financial support from
the Natural Science Foundation of China (NSFC Nos.
91743110, 61674114, 21861132001), the National Key R&D
Program of China (2017YFF0204600), Tianjin Applied
Basic Research and Advanced Technology
(17JCJQJC43600), the Foundation for Talent Scientists of
Nanchang Institute for Microtechnology of Tianjin
University, and the 111 Project (B07014).
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