Oxidative and Toxicological Evolution of Engineered Nanoparticles
with Atmospherically Relevant Coatings
Qifan Liu,† John Liggio,*,† Dalibor Breznan,‡ Errol M. Thomson,‡ Premkumari Kumarathasan,*,§

Renaud Vincent,‡ Kun Li,† and Shao-Meng Li†

†Atmospheric Science and Technology Directorate, Science and Technology Branch, Environment Canada, 4905 Dufferin Street,
Toronto, Ontario M3H 5T4, Canada
‡Inhalation Toxicology Laboratory, Healthy Environments and Consumer Safety Branch, Health Canada, 0803C Tunney’s Pasture,
Ottawa, Ontario K1A 0K9, Canada
§Analytical Biochemistry and Proteomics Laboratory, Healthy Environments and Consumer Safety Branch, Health Canada, 0803C
Tunney’s Pasture, Ottawa, Ontario K1A 0K9, Canada

*S Supporting Information

ABSTRACT: The health impacts associated with engineered
nanoparticles (ENPs) released into the atmosphere have not
been adequately assessed. Such impacts could potentially arise
from the toxicity associated with condensable atmospheric
secondary organic material (SOM), or changes in the SOM
composition induced by ENPs. Here, these possibilities are
evaluated by investigating the oxidative and toxicological
evolution of TiO2 and SiO2 nanoparticles which have been
coated with SOM from the O3 or OH initiated oxidation of α-
pinene. It was found that pristine SiO2 particles were
significantly more cytotoxic compared to pristine TiO2
particles. TiO2 in the dark or under UV irradiation
catalytically reacted with the SOM, increasing its O/C by
up to 55% over photochemically inert SiO2 while having negligible effects on the overall cytotoxicity. Conversely, the
cytotoxicity associated with SiO2 coated with SOM was markedly suppressed (by a factor of 9, at the highest exposure dose)
with both increased SOM coating thickness and increased photochemical aging. These suppressing effects (organic coating and
photo-oxidation of organics) were attributed to a physical hindrance of SiO2-cell interactions by the SOM and enhanced SOM
viscosity and hydrophilicity with continued photo-oxidation, respectively. These findings highlight the importance of
atmospheric processes in altering the cytotoxicity of ENPs.

■ INTRODUCTION

Engineered nanoparticles (ENPs) are used extensively in a vast
range of consumer goods and industrial processes due to their
novel physical, chemical, and biological properties.1 In
particular, nanosized titanium dioxide and nanosized silicon
dioxide (denoted TiO2 and SiO2, respectively) are the two
most commonly manufactured and frequently used ENPs
globally.2 TiO2 is well-known as a semiconductor material used
in products such as solar cells,3 coatings,4 sunscreens,5 and
water purification agents.6 SiO2 is widely used in chemical-
mechanical polishing7 and as an additive to cosmetics,1

pharmaceuticals,8 and toothpaste.9 The estimated global
annual production of nano-TiO2 and SiO2 is 83 500−88 000
and 82 500−95 000 tons in 2010, respectively,10 and is
expected to increase into the foreseeable future.11

ENPs are likely to be released into the environment during
their manufacture, packaging, from accidental spills, and from
the degradation of products in which they are contained.2

Given the global widespread use of TiO2 and SiO2, their

release into the environment could pose a significant risk to
humans, particularly since a number of studies have
demonstrated their toxicity.12−15 For example, subacute
exposure of mice to 2−5 nm TiO2 nanoparticles caused a
significant lung inflammatory response within the first 2 weeks
of exposure.13 In addition, a significant decrease in cell viability
was observed for A549 human lung cancer cells exposed to 46
nm SiO2 nanoparticles.14 Instillation of amorphous SiO2

nanoparticles caused significant inflammatory changes in rat
lungs.15 Together, these results suggest that TiO2 and SiO2 are
capable of generating adverse health outcomes on their own,
due to their pro-oxidative and pro-inflammatory properties.
However, ENPs including TiO2 and SiO2 are not likely to

remain physically or chemically unchanged upon entering

Received: December 6, 2018
Revised: February 18, 2019
Accepted: February 22, 2019
Published: February 22, 2019

Article

pubs.acs.org/estCite This: Environ. Sci. Technol. 2019, 53, 3058−3066

© 2019 American Chemical Society 3058 DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

D
ow

nl
oa

de
d 

vi
a 

H
E

A
L

T
H

 C
A

N
A

D
A

 o
n 

M
ar

ch
 2

2,
 2

02
4 

at
 1

7:
19

:4
9 

(U
T

C
).

Se
e 

ht
tp

s:
//p

ub
s.

ac
s.

or
g/

sh
ar

in
gg

ui
de

lin
es

 f
or

 o
pt

io
ns

 o
n 

ho
w

 to
 le

gi
tim

at
el

y 
sh

ar
e 

pu
bl

is
he

d 
ar

tic
le

s.

pubs.acs.org/est
http://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.est.8b06879
http://dx.doi.org/10.1021/acs.est.8b06879


various environmental media. Indeed, changes in the surface
characteristics of ENP have been observed in soil and aquatic
environments.16,17 In the atmosphere, the role of ENPs may be
2-fold; they (TiO2) can potentially serve as a photoinitiator
and/or reactive site for free-radical mediated reactions of
gaseous species,18 converting gases to particulate products, or
they (TiO2 and SiO2) may act as seeds for the condensation of
low volatility products originating from the gaseous oxidation
of various volatile organic compounds (VOCs) present in the
atmosphere.19 Ultimately, both processes are likely to result in
ENPs that are coated and/or internally mixed with secondary
organic material (SOM). With continued atmospheric aging,
the interaction between the underlying nanoparticle and the
condensed SOM may profoundly alter the physical and
chemical properties of both the nanoparticle core and/or the
overlying SOM. The association between nanoparticle
physiochemical properties and various biological end points20

as well as the observation that secondary organic aerosols
(SOA) alone can induce a cellular response21,22 suggests that
the interaction between nanoparticle and SOM coatings has
the potential to alter the toxicological effects associated with
particles containing both.
While limited evidence of this effect exists within soil/

aquatic ecosystems,23,24 no information exists on the impact of
atmospheric aging and transformations of TiO2 and SiO2,
despite inhalation from ambient air being recognized as an
important future ENP exposure pathway.25 In a previous study,
the adsorption of organic gases onto single wall carbon
nanotubes (SWCNTs) during ambient air exposures was
suggested as the cause of an observed reduction in the overall
SWCNTs cytotoxicity.26 However, the exact chemical
composition, precursor source, atmospheric age, and the
amount of adsorbed materials onto those SWCNTs was not
determined. Such findings highlight a need to systematically
study the relationship between the amount and nature of
atmospherically relevant organic coatings on more common
ENPs and their potential for corresponding air quality and
health impacts.
In the current study, TiO2 and SiO2 nanoparticles were

systematically coated by SOM from the reactions of α-pinene
with O3 or OH radicals, simulating an exposure equivalent to
photochemical aging times of 0.8−8.0 days in the atmosphere.
Monitoring of the SOM oxidative evolution in these
experiments was combined with simultaneous in vitro
cytotoxicity analysis, across a range of SOM coating thickness
and photochemical age. The results here provide a first insight
into the impact of TiO2 and SiO2 exposed to the atmosphere,
on both the oxidative evolution of the overlying SOM, and the
corresponding effect on cellular toxicity end points. The
present findings can advance the understanding on atmospheri-
cally transformed nanoparticles of all types, in support of
associated risk assessment efforts.

■ EXPERIMENTAL SECTION
O3 Oxidation Experiments. The O3 oxidation of α-pinene

was performed in a dark flow tube reactor in the presence of
pre-existing TiO2 or SiO2 using an experimental approach
described previously26 and in the Supporting Information (SI,
Figure S1A). TiO2 (Sigma-Aldrich, #677469) and SiO2 (US
Research Nanomaterials Inc., #3438) nanoparticles were
suspended in ultrapure water at a concentration of 1.5 g L−1,
followed by 20−40 min of sonication. The particles used in the
experiments were generated by atomization of the above

solution (TSI, model 3076), dried through a diffusion dryer
(TSI, model 3062), and exposed to α-pinene and O3 (Ozone
Solutions, model TG-10). The relatively monodisperse TiO2
and SiO2 seed particles were generated with similar size
distributions (Figure S2), having mode mobility diameters of
approximately 120 and 150 nm, respectively. Gaseous α-pinene
(70 ppb) was produced by passing a gas flow of 20 mL min−1

over a permeation tube in a temperature-controlled oven. The
O3 exposure was converted to an equivalent atmospheric
exposure representing 0.5−8.0 days by assuming an average
tropospheric O3 concentration of 30 ppb.27 Gas-phase α-
pinene and O3 concentrations were monitored using a high-
resolution time-of-flight proton transfer reaction mass
spectrometer (HR-ToF-PTR-MS, Ionicon Analytik) and an
O3 analyzer (2B Technologies, model 202), respectively.
Particle size distributions were measured using a scanning
mobility particle sizer (SMPS). An Aerodyne HR-ToF-AMS28

was used to determine the bulk chemical composition of the
organic fraction of the coated ENPs, including the elemental
ratios of H/C and O/C as described previously.29 A shift in the
unimodal particle size distribution upon the addition of SOM
demonstrated that the nanoparticles were approximately evenly
coated with negligible new particle formation (Figure S3).
Assuming the nanoparticles were evenly coated resulted in a
calculated coating thickness ranging from 4 to 20 nm (achieved
by varying the α-pinene concentration). SOM coated ENPs
from each experiment were collected on 47 mm Teflon filters
at the exit of the reactor for subsequent cytotoxicity analysis
(Table S2). The specific types of ENP used in this work are
provided in Table S1.

OH Radical Oxidation Experiments. The TiO2 and SiO2
particles generated above were also exposed to SOM produced
from the photo-oxidation of α-pinene by OH radicals in a
manner described previously26,30 and in Figure S1B. OH
radicals were generated by the UV photolysis of O3 at 254 nm
in the presence of water vapor. Upon achieving a steady-state
concentration of O3 and ENPs, gaseous α-pinene (18 ppb) was
introduced into the photochemical reactor. In offline
calibrations, the OH radical concentration was systematically
varied by changing the voltages applied to the UV lamp
(Jelight) between 0 and 120 V, and was determined using CO
as a tracer compound (see SI). The OH exposures, quantified
by measuring the loss of CO via its reaction with OH (1.54 ×
10−13 cm3 molecules−1 s−1 at 298 K),31 were in the range of 1.0
× 1011−1.0 × 1012 molecules cm−3 s. Assuming an average
atmospheric OH concentration of 1.5 × 106 molecules cm−3,32

this experimental exposure is equivalent to 0.8−8.0 days of
atmospheric photochemical oxidation.33 The resultant ENPs
coated with SOM (1.5−7.5 nm coating thickness) were also
analyzed for their cytotoxicity (Table S2).

Cell Culture and Cytotoxicity Assays. J774A.1 murine
macrophage cells (ATCC) were maintained as previously
described.34 For experiments, J774A.1 cells were seeded in 96-
well plates (∼0.32 cm2 surface area) at 20 000 cells well−1, in
100 μL DMEM (phenol red-free (Hyclone)), with 10% heat-
inactivated FBS (Hyclone), 50 μg mL−1 gentamicin (Sigma-
Aldrich) and incubated for 24 h prior to particle exposure.
Stock suspensions of ENPs were prepared at 1 mg mL−1 in
serum-free DMEM (containing 50 μg mL−1 gentamicin),
briefly vortexed, and sonicated for 5 min using a Branson 2510
water bath sonicator with ice present. The particle stocks were
prepared newly for each experimental repeat. Particle standards
EHC6802 (Ottawa urban dust), Si12 (Sigma-Aldrich, 16 ± 3.1

Environmental Science & Technology Article

DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

3059

http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://dx.doi.org/10.1021/acs.est.8b06879


nm SiO2), and Aeroxide P25 (Evonik Industries, 21 ± 5 nm
TiO2) were included in the assays.
For cell dosing, working ENP suspensions were diluted in

DMEM as required, sonicated for an additional 2 min, briefly
vortexed, and dosed to cell monolayers at 10, 30, and 100 μg
cm−2 of well surface area (96-well plate) in 100 μL of DMEM
+ 10% FBS. The equivalent exposure concentration was 16, 50,
and 160 μg mL−1 of DMEM + 5% FBS (final conc.). The cells
were incubated (37 °C, 5% CO2, 95% RH) for 24 h prior to
the integrated cytotoxicity assay. “No-cells” wells containing
particle suspensions only were included in the experiments to
test for potential nanoparticle interference with the assays.
Furthermore, supernatants and cell lysates utilized for the
cytotoxicity analysis were clarified by centrifugation to remove
any traces of ENPs.
The integrated cytotoxicity assay was conducted to obtain

the cytotoxicity profile of the J774A.1 cells exposed to the
ENPs. The assay tested for cell membrane integrity (LDH
release, CytoTox 96, Promega), energy metabolism (cellular
ATP, ViaLight Plus, Lonza), and redox state (resazurin
reduction, Alamar Blue, Invitrogen), from the same exposure
experiment. The integrated cytotoxicity assay was performed as
previously described.35 All experiments were conducted three
times (n = 3) with duplicate samples per experiment. The
cytotoxicity data were normalized within an experiment for all
doses (including zero dose control), to the grand mean value
of all zero dose controls, to obtain fold effect (FE) for each
particle dose. These data were also analyzed using an
independent sample t-test and a two-way ANOVA (SPSS
Statistics) with dose and coating thickness or dose and
photochemical age as factors, as given in Tables S2 and S3.

■ RESULTS AND DISCUSSION
Impact of ENP Seed on SOM: O3 Oxidation of α-

Pinene. While the HR-ToF-AMS is not able to detect or
quantify the underlying ENP core, its utility is in the ability to
characterize the oxidation state of the SOM in real-time as a
measure of the oxidative evolution. This is typically achieved
through the analysis of various fragment ratios and the direct
determination of the O/C ratios.36,37 For example, the
measured fraction of the total AMS organic signal contributed
by C2H3O

+ ( f C2H3O
+) and CO2

+ ( f CO2
+) ion is often used as a

measure of the oxidation state of SOA.37 These metrics are
similarly used here to provide insight into the effect of the
underlying ENP seed on the oxidation state of α-pinene SOM
condensed onto the surface of TiO2 (denoted SOMTi) and

SiO2 (denoted SOMSi). It is important to evaluate the ability of
an ENP core to alter the oxidation state of SOM during
atmospheric aging processes, as increased organic oxygenation
has been linked to changes in the organic aerosol toxicity,38

and has a significant impact on climate and air quality by
strongly affecting the phase state, optical properties, hygro-
scopic growth, and cloud condensation nucleus (CCN) activity
of aerosol particles.39−41

The evolution of f C2H3O
+ and f CO2

+ for SOMTi and SOMSi as
a function of O3 exposure is shown in Figure 1A. For both
types of ENP, increased O3 exposure, equivalent to increased
aging time in the atmosphere, results in a decrease in f C2H3O

+

and increase in f CO2
+ respectively, reaching a constant fraction

at an O3 exposure of 2.8 × 1017 molecules cm−3 s
(approximately equivalent to 4 days of ambient exposure).
The C2H3O

+ ion is linked to non-acid containing oxygenates
(alcohols, aldehydes, ketones), while CO2

+ ion is predom-
inantly due to the presence of acidic groups.36 The decrease in
f C2H3O

+ and corresponding increase in f CO2
+ at low O3

exposures (0.5−2.0 days) is consistent with a conversion of
oxidation products containing carbonyl functional groups
(higher f C2H3O

+) to products containing carboxylic acid groups
(higher f CO2

+ and lower f C2H3O
+) and thus a higher overall

oxygen content of the SOM. For example, campholenic
aldehyde, a known α-pinene ozonolysis product which
contains a double bond, can undergo further reaction with
O3 to form terpenylic aldehyde, which can be further oxidized
in the particle phase to form terpenylic acid.42 This is also
reflected in the O/C evolution for these experiments shown in
Figure 1B which varies from (0.39 ± 0.01) to (0.42 ± 0.01)
depending upon the exposure time and seed type and is
consistent with literature values (0.33−0.52) of SOA generated
from the dark ozonolysis of α-pinene at an O3 exposure of 1.4
days.43 Given the absence of NOx in these experiments which
serves to propagate radical production and enhance secondary
chemistry, once all α-pinene and oxidation products containing
double bonds are consumed, it is expected that the O/C
should remain relatively constant at high O3 exposures (4−8
days; Figure 1B).
With the exception of low photochemical age experiments

(0.8 days OH exposure), the type of seed particle has a
negligible effect on overall SOM formation yields from O3
or OH oxidation (see Figure S4, SI). However, there are
observable differences in the degree of SOM oxidation
between experiments of different particle core types. Figure
1B indicates that the presence of TiO2 (in O3 experiments)

Figure 1. Oxidative evolution of SOM formed in the dark ozonolysis of α-pinene condensed onto TiO2 and SiO2 nanoparticles. Evolution of
f C2H3O

+ and f CO2
+ (A), and O/C ratios (B) as a function of equivalent aging time (3.5 × 1016−5.6 × 1017 molecules cm−3 s O3 exposure). The

equivalent ambient exposures are calculated assuming a global mean concentration of 30 ppb O3.
27 The dashed lines are guides for the eye only.

Environmental Science & Technology Article

DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

3060

http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://dx.doi.org/10.1021/acs.est.8b06879


slightly increases the oxygen content of the SOM compared to
the SOM formed in the presence of photochemically inactive
SiO2. This is attributed to dark reactions which occur on the
surface of TiO2. Although the majority of previous studies have
focused on the photoactivity of TiO2,

3−6,18,44 limited data
suggest that organic oxidation induced by TiO2 in the dark is
also possible, particularly for carbonyl containing species.45−47

For example, exposure of gaseous methacrolein (MAC) to
TiO2 under dark conditions resulted in the oxidation of MAC
to carboxylate on the TiO2 surface,47 while MAC was only
physically adsorbed onto SiO2.

48 It was hypothesized that the
reactivity of TiO2 toward MAC was caused by oxygen
vacancies on the TiO2 surface, which were often involved in
the catalytic reactivity of TiO2.

44 Similar dark reactions were
also observed during the heterogeneous interaction between
gaseous aldehydes (HCHO and CH3CHO) and TiO2, where
the oxidative adsorption converted aldehyde to carboxy-
late.45,46 The propensity of carbonyl oxidation to occur on
the surface of TiO2 described above is consistent with the
slight increase in O/C for the SOM studied here. This is
particularly relevant for α-pinene SOM formed through O3
oxidation, as it is known to contain significant carbonyl
functionality.49

Impact of ENP Seed on SOM: OH Oxidation of α-
Pinene. The oxidative evolution of TiO2 and SiO2 seeded
SOM formed from the OH initiated photo-oxidation of α-
pinene is shown in Figure 2A,B. Similar to the results for SOM
formed from O3 oxidation (Figure 1A), the f CO2

+ generally
increases and f C2H3O

+ decreases as a result of increased
photochemical aging (see Figure 2A). The relative change of
f CO2

+ in OH experiments (101−276%) is significantly higher
compared to the O3 experiments (10−12%), since OH can
easily induce further oxidation by converting first-generation α-
pinene product to later-generation product; while O3 has a
much lower reactivity, it can only react with limited products
which contain a double bond (e.g., campholenic aldehyde42).
During the initial OH exposure, the measured photochemical
age profile for f C2H3O

+ of SOMSi and SOMTi follow different
trends, with the f C2H3O

+ of SOMSi initially increasing whereas
the f C2H3O

+ of SOMTi begins at a higher relative fraction and
continually decreases with photo-oxidation. This suggests a
progression from less oxidized species in SOMSi with a higher
contribution to f C2H3O

+, to more oxidized products with a
lower contribution to f C2H3O

+ as has been noted during the
formation of α-pinene derived SOA.33 A similar overall
oxidative evolution to that of O3 experiments (i.e., O/C,
Figure 1B) is shown in Figure 2B for OH initiated SOM
formation. In this case, the O/C of the SOM are in the range
of 0.43−1.20 at OH exposures of 1.0 × 1011−1.0 × 1012

molecules cm−3 s (0.8−8.0 days), with an O/C at 0.8 days
of OH exposure similar to that of SOA from α-pinene photo-
oxidation in smog chambers.50 The higher range of O/C
for OH initiated experiments (for both particle seed types)
compared to O3 experiments is consistent with the formation
of later-generation SOM products which can be easily achieved
in the presence of OH but not O3, as mentioned above.
The difference in oxygen content between SOMTi and

SOMSi from OH initiated oxidation is significant. Under
otherwise identical conditions, the O/C for SOMTi at a
given OH exposure is consistently higher (11−55%) than that
of SOMSi (Figure 2C), which is also reflected in the f CO2

+

curvature of Figure 2A. A higher oxygenation level for SOMTi
can be attributed to the unique photochemical surface

reactivity of TiO2, which is known to participate in interfacial
photocatalytic reactions,44 while a SiO2 surface is chemically
inert. More specifically, TiO2 irradiated by UV light (λ < 390
nm) results in the generation of electron−hole pairs (e−/h+) in
the conduction and valence bands, respectively.18 The

Figure 2. Oxidative evolution of SOM formed in the OH oxidation of
α-pinene condensed onto TiO2 and SiO2 nanoparticles. Evolution of
f C2H3O

+ and f CO2
+ (A), O/C ratios (B), and the relative percentage

difference in O/C (SOMTi vs SOMSi) (C), as a function of equivalent
aging time (1.0 × 1011−1.0 × 1012 molecules cm−3 s OH exposure). A
and C: experiments were conducted under the 254 nm UV light
irradiation. The equivalent ambient exposures are calculated assuming
a global mean concentration of 1.5 × 106 molecules cm−3 OH.32 The
dashed lines are guides for the eye only.

Environmental Science & Technology Article

DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

3061

http://dx.doi.org/10.1021/acs.est.8b06879


photogenerated h+ may participate in the direct oxidation of
organics (details are provided in the SI).51 Additionally, e−/h+

can further react with water and oxygen to produce reactive
oxygen species (ROS) such as OH radicals, superoxide radical
anions, and hydrogen peroxide, all of which can indirectly
initiate photocatalytic organic reactions.18 In the case of TiO2
seeded experiments here, α-pinene may be oxidized by three
different oxidizing agents, i.e., OH from the gas-phase
photolysis of O3, photogenerated h+, and ROS formed on
the TiO2 surface. Conversely, for the SiO2 seeded experiments;
oxidation from OH radicals generated in the gas-phase is the
only possible reaction pathway. As a consequence, the
oxidation level of the organics coated on TiO2 remains higher
than that on SiO2 throughout the entire OH exposure range.
It should also be noted that the oxygen content difference

between SOMTi and SOMSi is not uniform across all
photochemical ages, as depicted in the relative increase in
the measured O/C for SOMTi over SOMSi (Figure 2C). This is
in contrast to Figure 1B (O3 experiments), where the oxygen
content difference is relatively stable (∼2%) over the entire O3
exposure range (0.5−8.0 days). At 0.8 days OH exposure,
SOMTi has an O/C which is 55% higher than that of SOMSi.
This relative enhancement quickly becomes stable and less
prominent (∼11%) with increasing OH exposures (4.4−7.8
days) which is likely due to the changing contribution of TiO2
photoreactivity. The results here suggest that at low OH
exposures (0.8−2.5 days), the TiO2 surface photochemistry
plays a larger role in the oxidation of SOM, while at high OH
exposures (4.4−7.8 days), OH produced from the gaseous
photolysis of O3 is the dominant photochemical pathway.
Additional insight into the chemical evolution differences
between these two ENP seeded SOM experiments is obtained
through the analysis of organic fragment groups, which is
shown in Figure S5 and discussed in the SI.
In the ambient atmosphere, a portion of the UV light in the

spectral region of 290−400 nm can reach the Earth’s surface,
while UV light with wavelengths below 290 nm (used here to
generate OH from O3) is completely absorbed by the O3
layer.52 Hence, to more realistically simulate the atmos-
phere, OH initiated photo-oxidation experiments were also
performed using 302 nm UV lamp irradiation which is both
present in the atmosphere and able to activate TiO2.

18 As
expected,53 due to the lower photochemical activity of TiO2 at
302 nm UV light irradiation (vs 254 nm), the measured O/C
of SOMTi (302 nm) across all photochemical ages is lower
than that of SOMTi (254 nm) but higher than that of SOMSi
(254 nm), as shown in Figure 2B. While lower than the O/C at
254 nm, the results of Figure 2B at 302 nm demonstrate that a
catalytic oxidation of SOM due to the TiO2 surface remains
likely under realistic atmospheric irradiation wavelengths.
Impacts on Cytotoxicity. Particle samples of pristine

(uncoated) and atmospherically transformed ENPs (from
various O3 and OH initiated oxidation experiments) were
subjected to in vitro cytotoxicity assays (see Experimental
Section) to assess the impact of atmospheric organic coatings
on the overall particle toxicity (ENP + coating). No
interference of ENPs with the assays employed was detected
from the “no cell” well assessments. The cytotoxicity results for
pristine and organic coated TiO2 and SiO2 particles are shown
graphically in Figures S6−S8. Figure S6 clearly indicates that
pristine SiO2 particles are significantly more cytotoxic
compared to pristine TiO2 particles (see sample S0 vs T0 in
Figure S6), as demonstrated by the elevated release of LDH (a

measure of cell membrane permeability), decreased ATP
content (a measure of cellular energy state), and decreased rate
of resazurin reduction (a measure of cellular redox status) at
100 μg cm−2 dosage. This is consistent with a recent study of
SiO2.

34 Conversely, with respect to the assays used here,
exposure to uncoated TiO2 particles (T0, T1, and P25) did not
produce overt cytotoxicity across the dose range assessed. This
is also consistent with a previous toxicity study on TiO2.

54

Furthermore, organic coatings of SOM from α-pinene
oxidation (via O3 or OH) onto TiO2 also results in a
negligible change in cytotoxicity (T0−T3 and Ta−Td),
regardless of reaction conditions (see Figure S7 and Table
S2). This suggests that α-pinene SOM on its own, regardless of
its formation pathway (OH vs O3) or its photochemical age, is
relatively non-cytotoxic when assessed with the currently used
assays. In contrast, SOM coated SiO2 exhibits clear organic
coating and aging effects on cytotoxicity, as will be discussed
below.
The results of the cytotoxic assays for SiO2 (at 100 μg cm−2

dosage) are shown in Figure 3 and 4 for experiments which

vary either the estimated organic coating thickness only (same
photochemical age; Sa and Sc), or the estimated photochemical
age only (same estimated coating thickness; Sa and Sb). A clear
difference between the cytotoxicity of uncoated and organically
coated SiO2 formed via O3 oxidation is observed (Figure 3). In
this case an estimated coating thickness of 7.5 nm results in a
12% decrease in the LDH leakage and an 80% and 214%
increase in the mean cellular ATP content and resazurin
reduction rate, respectively (see S1 vs S2 in Figure 3). A similar
relationship is observed for samples coated with SOM from
the OH oxidation of α-pinene (see S1 vs Sa or S1 vs Sc in Figure
3). Such results indicate that the cytotoxic potency of uncoated
SiO2 is strongly suppressed by the SOM coating, regardless of
its formation pathway (OH or O3). Further, the cytotoxicity

Figure 3. Cytotoxicity results (LDH, ATP, and resazurin reduction
assays; at 100 μg cm−2 dosage) from the exposure of J774A.1 murine
macrophages to pristine and SOM coated SiO2 formed from O3
and OH oxidation of α-pinene (estimated coating thickness 1.5−7.5
nm). All values are presented as average fold effect over control ±
standard error (n = 3 experiments). S1: Pristine SiO2; S2: SOM coated
SiO2 formed from O3 oxidation (estimated coating thickness 7.5 nm;
equivalent aging time 14 days); Sa: SOM coated SiO2 formed from
OH oxidation (estimated coating thickness 1.5 nm; equivalent aging
time 0.7 days); and Sc: SOM coated SiO2 formed from OH oxidation
(estimated coating thickness 7.5 nm; equivalent aging time 0.7 days).

Environmental Science & Technology Article

DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

3062

http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://dx.doi.org/10.1021/acs.est.8b06879


suppression is proportional to the estimated coating thickness
as indicated by the higher cytotoxicity of Sa (SOM thickness
1.5 nm) compared to that of Sc (SOM thickness 7.5 nm) in
Figure 3. These observations have been further verified via a
two-way ANOVA analysis and an independent sample t-test
(Table S2 and S3), which indicates that all the results
presented above are statistically significant (p-value <0.05).
Given that the SOM itself was not overtly cytotoxic (see
above), a reduction of the SiO2 toxicity (by a factor of 9, at the
highest exposure dose) is interpreted as a physical inability of
the cells to interact directly with the SiO2 core which is
consistent with the results of a previous study on the
atmospheric exposure of SWCNTs.26 It should be noted that
the cytotoxicity suppression observed here was associated with
a 24 h cell exposure. The effect of cell exposures longer than 24
h on cytoxicity was not assessed.
Photochemical aging is another factor that may affect the

cytotoxicity of ENP during their residence time in the
atmosphere. In the atmosphere and in these experiments,
continued photochemical aging results in a progressively more
oxidized SOM (see above). The impact of oxidative aging on
the overall cytotoxicity is also observable in Figure 4. We note
that O3 experiments result in minimally aged SOM (Figure
1B), and thus aging effects on cytotoxicity are restricted to
oxidation experiments. At an estimated coating thickness of 1.5
nm, the cytotoxicity of the coated SiO2 decreases with
increased photochemical aging time, as reflected in the higher
cytotoxicity of Sa (0.7 days OH exposure) relative to that of Sb
(8.0 days OH exposure) for all assays in Figure 4. This may be
explained by an increased SOM viscosity with continued
photochemical aging. Increases in the viscosity of SOM as a
result of photochemical aging is known to occur.55,56 In the
current OH aging experiments, the O/C ratio increases with
increasing OH exposures (Figure 2B), likely also increasing the
SOM viscosity. In the context of these experiments, a higher
viscosity is likely to provide a further physical barrier between

the SiO2 core and the cells, resulting in a lower cytotoxicity,
which is consistent with sample Sb having a weaker SiO2-cell
interaction (i.e., lower cytotoxicity) compared to Sa (Figure 4).
However, this photochemical aging effect (i.e., viscosity effect)
becomes insignificant for SiO2 with a larger coating thickness,
as reflected in the similar cytotoxicity of Sc (7.5 nm coating
thickness, 0.7 days OH exposure) relative to that of Sd (7.5 nm
coating thickness, 8.0 days OH exposure) for all assays in
Figure S8. The difference in the impact of photochemical aging
on coated SiO2 cytotoxicity in these two scenarios (1.5 and 7.5
nm coating thickness) suggests that with a small coating
thickness (1.5 nm, Sa and Sb), the SiO2 core may partially
interact with cells during exposure. As a result, the SOM
coating of Sb (1.5 nm, higher viscosity) may act as a stronger
barrier compared to that of Sa (1.5 nm, lower viscosity) during
the SiO2-cell exposure. However, at large coating thickness
(7.5 nm, Sc and Sd), the SOM coating acts as a sufficient SiO2-
cell barrier regardless of its photochemical age (i.e., viscosity).
In addition to changes in SOM viscosity, cytotoxicity
suppression associated with increases in hydrophilicity
(brought upon by photochemical aging) may also be possible.
Such an effect would be consistent with previous studies, which
have similarly shown that hydrophobic coatings on Si (created
by thermal carbonization of the Si surface) are more strongly
cytotoxic than hydrophilic coatings in in vitro exposures.57 ,58

The current results are also consistent with previous reports of
the toxicity of ambient particles. For example, it was found that
the leaching of water-soluble components from ambient
particles enhanced their cytotoxicity in A549 human lung
epithelial cells in vitro,59 consistent with the proposed notion
here that the photo-oxidation of SOM coatings (i.e., increased
viscosity and hydrophilicity) reduced the ENP cytotoxicity. In
addition, animal inhalation studies have also revealed a
contrast in the potency of ambient particles and their water-
extracted counterpart, in that the extracted and total particles
induced an acute and a blunted lung inflammatory response,
respectively.60 Further study is warranted to understand the
initiating molecular events leading to these two different
outcome paths (see SI for additional details).
While an alteration in the nature of interactions between

cells and SiO2 nanoparticles is likely caused by the
modification of surface properties (via SOM coatings in the
current study), the exact mechanisms for reduced cytotoxicity
remain to be clarified. The impact of SOM coatings on the
cytotoxicity suppression here may be even larger since target
cellular doses in the assays are sensitive to particle dispersion in
the cell culture medium, and Dynamic Light Scattering (DLS)
measurements suggest that SOM coatings slightly improved
dispersion of particles in the cell culture medium (see SI).
In the current study, the RH in the flowtube reactor was

kept at a constant value of 35%. Further study is warranted to
assess the impact of RH (water) since it is known that water
has complex impacts (promoting vs inhibiting) on TiO2
surface photochemistry.61,62 In addition, we note that the
ENP physicochemical properties may change upon particle
collection on the filter and resuspension in a cell culture
medium (i.e., the experimental procedure used here), thus
affecting cytotoxicity results. While a recent study has clearly
suggested that filter resuspension exposure is an appropriate
approach for the cytotoxicity analysis of isoprene-derived SOA
and would yield results similar to direct deposition exposure
(i.e., air−liquid interface (ALI) exposure),22 the potential for
sampling artifacts (e.g., particle agglomeration) cannot be

Figure 4. Cytotoxicity results (LDH, ATP, and resazurin reduction
assays; at 100 μg cm−2 dosage) from the exposure of J774A.1 murine
macrophages to pristine and SOM coated SiO2 formed from OH
oxidation of α-pinene (same estimated coating thickness, equivalent
aging time 0.7−8.0 days). All values are presented as average fold
effect over control ± standard error (n = 3 experiments). S1: Pristine
SiO2; Sa: SOM coated SiO2 (equivalent aging time 0.7 days, estimated
coating thickness 1.5 nm); and Sb: SOM coated SiO2 (equivalent
aging time 8.0 days, estimated coating thickness 1.5 nm).

Environmental Science & Technology Article

DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

3063

http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
http://dx.doi.org/10.1021/acs.est.8b06879


completely ruled out in the current experiments. Considering
this, future ALI exposure experiments38,63,64 which can avoid
such sampling artifacts are necessary to give additional insight
into the cytotoxicity of coated ENPs.
The present results indicate that the release of TiO2

nanoparticles into the atmosphere can potentially have impacts
on the chemical nature and evolution of SOM formed in the
atmosphere via increases in the SOM oxygen content, caused
by dark and photo-induced catalytic reactions of TiO2. Such
effects may have the potential to alter atmospheric organic
aerosol properties with implicit impacts on various air quality
processes. However, there is also clear evidence that neither
the pristine TiO2 particles nor the SOM resulted in overt
cytotoxicity as determined from three different assays in this
model system. At the same time, our results indicate that
atmospherically relevant organic coatings associated with SiO2
and their photochemical aging (at a small coating thickness)
may reduce the overall in vitro cytotoxicity of SiO2 in J774
mouse macrophage cells. Future in vivo animal exposure
experiments are necessary to understand the long-term fate of
these transformed particles. Considering that no other toxicity
data are presently available with respect to atmospherically
relevant coatings and aging of ENPs, the present work supports
the ongoing risk assessment of ENPs. Given the physical
nature of the toxicity suppression observed here (i.e., coating
thickness dependence), it is possible that the cytotoxicity of
other ENPs will be similarly reduced upon atmospheric
exposure, reducing the overall risks associated with accidental
or future ENPs releases. Further study is warranted,
particularly with respect to the impact of other SOM
precursors such as those from industrial, tobacco, and
consumer products on the current health end points and
those representative of various other biological processes. The
experimental approach and the observations outlined here offer
a novel way to understand the complexity of physicochemical
properties of ambient particles associated with diverse adverse
health impacts, and to perhaps bridge the vast database on
adverse health impacts of ambient pollutants with the less
understood emerging risk from released engineered nanoma-
terials into the environment.

■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.est.8b06879.

Details associated with the dark and photochemical flow
tube reactors, other experimental details, SOM for-
mation yields, TiO2 surface photochemistry, organic
fragment groups analysis of α-pinene photo-oxidation
SOM, and in vitro assay results (PDF)

■ AUTHOR INFORMATION
Corresponding Authors
*Phone: 1-416-739-4840; e-mail: John.Liggio@canada.ca.
*Phone: 1-613-218-4530; e-mail: premkumari.kumarathasan@
canada.ca.
ORCID
Qifan Liu: 0000-0003-0033-5313
John Liggio: 0000-0003-3683-4595
Dalibor Breznan: 0000-0001-9457-9866
Kun Li: 0000-0003-2970-037X

Shao-Meng Li: 0000-0002-7628-6581
Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors would like to thank Patrick Lee and Gang Lu for
their technical support with the AMS. This research was
financially supported by the Chemicals Management Plan
(CMP) of Canada.

■ REFERENCES
(1) Mitrano, D. M.; Motellier, S.; Clavaguera, S.; Nowack, B. Review
of nanomaterial aging and transformations through the life cycle of
nano-enhanced products. Environ. Int. 2015, 77, 132−147.
(2) Keller, A. A.; McFerran, S.; Lazareva, A.; Suh, S. Global life cycle
releases of engineered nanomaterials. J. Nanopart. Res. 2013, 15, 1692.
(3) O’regan, B.; Graẗzel, M. A low-cost, high-efficiency solar cell
based on dye-sensitized colloidal TiO2 films. Nature 1991, 353
(6346), 737−740.
(4) Gaźquez, M. J.; Bolívar, J. P.; Garcia-Tenorio, R.; Vaca, F. A
review of the production cycle of titanium dioxide pigment.Mater. Sci.
Appl. 2014, 5 (7), 441.
(5) Smijs, T. G.; Pavel, S. Titanium dioxide and zinc oxide
nanoparticles in sunscreens: focus on their safety and effectiveness.
Nanotechnol., Sci. Appl. 2011, 4, 95−112.
(6) Blanco-Galvez, J.; Fernańdez-Ibañ́ez, P.; Malato-Rodríguez, S.
Solar photocatalytic detoxification and disinfection of water: recent
overview. J. Sol. Energy Eng. 2007, 129 (1), 4−15.
(7) Kim, N. H.; Ko, P. J.; Choi, G. W.; Seo, Y. J.; Lee, W. S.
Chemical mechanical polishing (CMP) mechanisms of thermal SiO2
film after high-temperature pad conditioning. Thin Solid Films 2006,
504 (1−2), 166−169.
(8) Qian, K. K.; Bogner, R. H. Application of mesoporous silicon
dioxide and silicate in oral amorphous drug delivery systems. J. Pharm.
Sci. 2012, 101 (2), 444−463.
(9) Allen, D.; Battista, G.; Petrone, D.; Petrone, M.; Chaknis, P.;
DeVizio, W.; Volpe, A. The clinical efficacy of Colgate Total Plus
Whitening Toothpaste containing a special grade of silica and Colgate
Total Fresh Stripe Toothpaste in the control of plaque and gingivitis:
a six-month clinical study. J. Clin. Pediatr. Dent. 2002, 13 (2), 59−64.
(10) Keller, A. A.; Lazareva, A. Predicted releases of engineered
nanomaterials: from global to regional to local. Environ. Sci. Technol.
Lett. 2014, 1 (1), 65−70.
(11) Robichaud, C. O.; Uyar, A. E.; Darby, M. R.; Zucker, L. G.;
Wiesner, M. R. Estimates of upper bounds and trends in nano-TiO2
production as a basis for exposure assessment. Environ. Sci. Technol.
2009, 43 (12), 4227−4233.
(12) Srivastava, V.; Gusain, D.; Sharma, Y. C. Critical review on the
toxicity of some widely used engineered nanoparticles. Ind. Eng. Chem.
Res. 2015, 54 (24), 6209−6233.
(13) Grassian, V. H.; O’Shaughnessy, P. T.; Adamcakova-Dodd, A.;
Pettibone, J. M.; Thorne, P. S. Inhalation exposure study of titanium
dioxide nanoparticles with a primary particle size of 2 to 5 nm.
Environ. Health Perspect. 2007, 115 (3), 397−402.
(14) Lin, W.; Huang, Y. W.; Zhou, X. D.; Ma, Y. In vitro toxicity of
silica nanoparticles in human lung cancer cells. Toxicol. Appl.
Pharmacol. 2006, 217 (3), 252−259.
(15) Evans, S. A.; Al-Mosawi, A.; Adams, R. A.; BeŕuBe,́ K. A.
Inflammation, edema, and peripheral blood changes in lung-
compromised rats after instillation with combustion-derived and
manufactured nanoparticles. Exp. Lung Res. 2006, 32 (8), 363−378.
(16) Boxall, A. B.; Tiede, K.; Chaudhry, Q. Engineered nanoma-
terials in soils and water: how do they behave and could they pose a
risk to human health? Nanomedicine 2007, 2 (6), 919−927.
(17) Navarro, E.; Baun, A.; Behra, R.; Hartmann, N. B.; Filser, J.;
Miao, A. J.; Quigg, A.; Santschi, P. H.; Sigg, L. Environmental

Environmental Science & Technology Article

DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

3064

http://pubs.acs.org
http://pubs.acs.org/doi/abs/10.1021/acs.est.8b06879
http://pubs.acs.org/doi/suppl/10.1021/acs.est.8b06879/suppl_file/es8b06879_si_001.pdf
mailto:John.Liggio@canada.ca
mailto:premkumari.kumarathasan@canada.ca
mailto:premkumari.kumarathasan@canada.ca
http://orcid.org/0000-0003-0033-5313
http://orcid.org/0000-0003-3683-4595
http://orcid.org/0000-0001-9457-9866
http://orcid.org/0000-0003-2970-037X
http://orcid.org/0000-0002-7628-6581
http://dx.doi.org/10.1021/acs.est.8b06879


behavior and ecotoxicity of engineered nanoparticles to algae, plants,
and fungi. Ecotoxicology 2008, 17 (5), 372−386.
(18) Chen, H.; Nanayakkara, C. E.; Grassian, V. H. Titanium
dioxide photocatalysis in atmospheric chemistry. Chem. Rev. 2012,
112 (11), 5919−5948.
(19) Lee, J.; Donahue, N. M. Secondary organic aerosol coating of
synthetic metal−oxide nanoparticles. Environ. Sci. Technol. 2011, 45
(11), 4689−4695.
(20) Nel, A.; Xia, T.; Mad̈ler, L.; Li, N. Toxic potential of materials
at the nanolevel. Science 2006, 311 (5761), 622−627.
(21) Tuet, W. Y.; Chen, Y.; Fok, S.; Champion, J. A.; Ng, N. L.
Inflammatory responses to secondary organic aerosols (SOA)
generated from biogenic and anthropogenic precursors. Atmos.
Chem. Phys. 2017, 17 (18), 11423−11440.
(22) Arashiro, M.; Lin, Y. H.; Sexton, K. G.; Zhang, Z.; Jaspers, I.;
Fry, R. C.; Vizuete, W. G.; Gold, A.; Surratt, J. D. In vitro exposure to
isoprene-derived secondary organic aerosol by direct deposition and
its effects on COX-2 and IL-8 gene expression. Atmos. Chem. Phys.
2016, 16 (22), 14079−14090.
(23) Simonin, M.; Guyonnet, J. P.; Martins, J. M.; Ginot, M.;
Richaume, A. Influence of soil properties on the toxicity of TiO2

nanoparticles on carbon mineralization and bacterial abundance. J.
Hazard. Mater. 2015, 283, 529−535.
(24) Yang, S. P.; Bar-Ilan, O.; Peterson, R. E.; Heideman, W.;
Hamers, R. J.; Pedersen, J. A. Influence of humic acid on titanium
dioxide nanoparticle toxicity to developing zebrafish. Environ. Sci.
Technol. 2013, 47 (9), 4718−4725.
(25) Li, N.; Georas, S.; Alexis, N.; Fritz, P.; Xia, T.; Williams, M.;
Horner, E.; Nel, A. Why ambient ultrafine and engineered
nanoparticles should receive special attention for possible adverse
health outcomes in humans. J. Allergy Clin. Immunol. 2016, 138 (2),
386−396.
(26) Liu, Y.; Liggio, J.; Li, S. M.; Breznan, D.; Vincent, R.; Thomson,
E. M.; Kumarathasan, P.; Das, D.; Abbatt, J.; Antiñolo, M.; Russell, L.
Chemical and toxicological evolution of carbon nanotubes during
atmospherically relevant aging processes. Environ. Sci. Technol. 2015,
49 (5), 2806−2814.
(27) Hinz, F. Ground-Level Ozone in the 21st Century: Future Trends,
Impact and Policy Implications; Royal Society Science Policy Report:
London, 2008; 59/148.
(28) DeCarlo, P. F.; Kimmel, J. R.; Trimborn, A.; Northway, M. J.;
Jayne, J. T.; Aiken, A. C.; Gonin, M.; Fuhrer, K.; Horvath, T.;
Docherty, K. S.; Worsnop, D. R.; Jimenez, J. L. Field-deployable, high-
resolution, time-of-flight aerosol mass spectrometer. Anal. Chem.
2006, 78 (24), 8281−8289.
(29) Canagaratna, M. R.; Jimenez, J. L.; Kroll, J. H.; Chen, Q.;
Kessler, S. H.; Massoli, P.; Hildebrandt Ruiz, L.; Fortner, E.; Williams,
L. R.; Wilson, K. R.; Surratt, J. D.; Donahue, N. M.; Jayne, J. T.;
Worsnop, D. R. Elemental ratio measurements of organic compounds
using aerosol mass spectrometry: characterization, improved calibra-
tion, and implications. Atmos. Chem. Phys. 2015, 15 (1), 253−272.
(30) Liu, Y.; Liggio, J.; Harner, T.; Jantunen, L.; Shoeib, M.; Li, S.
M. Heterogeneous OH initiated oxidation: a possible explanation for
the persistence of organophosphate flame retardants in air. Environ.
Sci. Technol. 2014, 48 (2), 1041−1048.
(31) Liu, Y.; Sander, S. P. Rate constant for the OH + CO reaction
at low temperatures. J. Phys. Chem. A 2015, 119 (39), 10060−10066.
(32) Mao, J.; Ren, X.; Brune, W. H.; Olson, J. R.; Crawford, J. H.;
Fried, A.; Huey, L. G.; Cohen, R. C.; Heikes, B.; Singh, H. B.; Blake,
D. R.; Sachse, G. W.; Diskin, G. S.; Hall, S. R.; Shetter, R. E. Airborne
measurement of OH reactivity during INTEX-B. Atmos. Chem. Phys.
2009, 9 (1), 163−173.
(33) Lambe, A.; Onasch, T.; Massoli, P.; Croasdale, D.; Wright, J.;
Ahern, A.; Williams, L.; Worsnop, D.; Brune, W.; Davidovits, P.
Laboratory studies of the chemical composition and cloud
condensation nuclei (CCN) activity of secondary organic aerosol
(SOA) and oxidized primary organic aerosol (OPOA). Atmos. Chem.
Phys. 2011, 11 (17), 8913−8928.

(34) Breznan, D.; Das, D. D.; O’Brien, J. S.; MacKinnon-Roy, C.;
Nimesh, S.; Vuong, N. Q.; Bernatchez, S.; DeSilva, N.; Hill, M.;
Kumarathasan, P.; et al. Differential cytotoxic and inflammatory
potency of amorphous silicon dioxide nanoparticles of similar size in
multiple cell lines. Nanotoxicology 2017, 11 (2), 223−235.
(35) Kumarathasan, P.; Breznan, D.; Das, D.; Salam, M. A.; Siddiqui,
Y.; MacKinnon-Roy, C.; Guan, J.; de Silva, N.; Simard, B.; Vincent, R.
Cytotoxicity of carbon nanotube variants: a comparative in vitro
exposure study with A549 epithelial and J774 macrophage cells.
Nanotoxicology 2015, 9 (2), 148−161.
(36) Ng, N.; Canagaratna, M.; Jimenez, J.; Chhabra, P.; Seinfeld, J.;
Worsnop, D. R. Changes in organic aerosol composition with aging
inferred from aerosol mass spectra. Atmos. Chem. Phys. 2011, 11 (13),
6465−6474.
(37) Ng, N. L.; Canagaratna, M. R.; Zhang, Q.; Jimenez, J. L.; Tian,
J.; Ulbrich, I. M.; Kroll, J. H.; Docherty, K. S.; Chhabra, P. S.;
Bahreini, R.; Murphy, S. M.; Seinfeld, J. H.; Hildebrandt, L.;
Donahue, N. M.; DeCarlo, P. F.; Lanz, V. A.; Prev́ôt, A. S. H.;
Dinar, E.; Rudich, Y.; Worsnop, D. R. Organic aerosol components
observed in Northern Hemispheric datasets from Aerosol Mass
Spectrometry. Atmos. Chem. Phys. 2010, 10 (10), 4625−4641.
(38) Chowdhury, P. H.; He, Q.; Lasitza Male, T.; Brune, W. H.;
Rudich, Y.; Pardo, M. Exposure of lung epithelial cells to
photochemically-aged secondary organic aerosol shows increased
toxic effects. Environ. Sci. Technol. Lett. 2018, 5 (7), 424−430.
(39) Pajunoja, A.; Hu, W.; Leong, Y. J.; Taylor, N. F.; Miettinen, P.;
Palm, B. B.; Mikkonen, S.; Collins, D. R.; Jimenez, J. L.; Virtanen, A.
Phase state of ambient aerosol linked with water uptake and chemical
aging in the southeastern US. Atmos. Chem. Phys. 2016, 16 (17),
11163−11176.
(40) Cappa, C. D.; Che, D. L.; Kessler, S. H.; Kroll, J. H.; Wilson, K.
R. Variations in organic aerosol optical and hygroscopic properties
upon heterogeneous OH oxidation. J. Geophys. Res. 2011, 116 (D15),
204.
(41) Massoli, P.; Lambe, A. T.; Ahern, A. T.; Williams, L. R.; Ehn,
M.; Mikkila, J.; Canagaratna, M. R.; Brune, W. H.; Onasch, T. B.;
Jayne, J. T.; Petaja, T.; Kulmala, M.; Laaksonen, A.; Kolb, C. E.;
Davidovits, P.; Worsnop, D. R. Relationship between aerosol
oxidation level and hygroscopic properties of laboratory generated
secondary organic aerosol (SOA) particles. Geophys. Res. Lett. 2010,
37 (24), L03805.
(42) Kahnt, A.; Iinuma, Y.; Mutzel, A.; Böge, O.; Claeys, M.;
Herrmann, H. Campholenic aldehyde ozonolysis: a mechanism
leading to specific biogenic secondary organic aerosol constituents.
Atmos. Chem. Phys. 2014, 14 (2), 719−736.
(43) Chen, Q.; Liu, Y.; Donahue, N. M.; Shilling, J. E.; Martin, S. T.
Particle-phase chemistry of secondary organic material: modeled
compared to measured O:C and H:C elemental ratios provide
constraints. Environ. Sci. Technol. 2011, 45 (11), 4763−4770.
(44) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 photocatalysis and
related surface phenomena. Surf. Sci. Rep. 2008, 63 (12), 515−582.
(45) Xu, B.; Zhu, T.; Tang, X.; Shang, J. Heterogeneous reaction of
formaldehyde on the surface of TiO2 particles. Sci. China: Chem.
2010, 53 (12), 2644−2651.
(46) Finkelstein-Shapiro, D.; Buchbinder, A. M.; Vijayan, B.;
Bhattacharyya, K.; Weitz, E.; Geiger, F. M.; Gray, K. A. Identification
of binding sites for acetaldehyde adsorption on titania nanorod
surfaces using CIMS. Langmuir 2011, 27 (24), 14842−14848.
(47) Zhao, Y.; Huang, D.; Huang, L.; Chen, Z. Hydrogen peroxide
enhances the oxidation of oxygenated volatile organic compounds on
mineral dust particles: a case study of methacrolein. Environ. Sci.
Technol. 2014, 48 (18), 10614−10623.
(48) Chen, Z.; Jie, C.; Li, S.; Wang, H.; Wang, C.; Xu, J.; Hua, W.
Heterogeneous reactions of methacrolein and methyl vinyl ketone:
Kinetics and mechanisms of uptake and ozonolysis on silicon dioxide.
J. Geophys. Res. 2008, 113, D22303.
(49) Yu, J.; Cocker, D. R.; Griffin, R. J.; Flagan, R. C.; Seinfeld, J. H.
Gas-phase ozone oxidation of monoterpenes: gaseous and particulate
products. J. Atmos. Chem. 1999, 34 (2), 207−258.

Environmental Science & Technology Article

DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

3065

http://dx.doi.org/10.1021/acs.est.8b06879


(50) Zhao, D. F.; Kaminski, M.; Schlag, P.; Fuchs, H.; Acir, I. H.;
Bohn, B.; Has̈eler, R.; Kiendler-Scharr, A.; Rohrer, F.; Tillmann, R.;
Wang, M. J.; Wegener, R.; Wildt, J.; Wahner, A.; Mentel, Th. F.
Secondary organic aerosol formation from hydroxyl radical oxidation
and ozonolysis of monoterpenes. Atmos. Chem. Phys. 2015, 15 (2),
991−1012.
(51) Richard, C. Regioselectivity of oxidation by positive holes (h+)
in photocatalytic aqueous transformations. J. Photochem. Photobiol., A
1993, 72 (2), 179−182.
(52) Chadysǐene, R.; Girgzďiene, R.; Girgzďys, A. Ultraviolet
radiation and ground-level ozone variation in Lithuania. J. Environ.
Eng. Landsc. 2005, 13 (1), 31−36.
(53) Ardizzone, S.; Bianchi, C.; Cappelletti, G.; Naldoni, A.; Pirola,
C. Photocatalytic degradation of toluene in the gas phase: relationship
between surface species and catalyst features. Environ. Sci. Technol.
2008, 42 (17), 6671−6676.
(54) Vuong, N. Q.; Goegan, P.; Mohottalage, S.; Breznan, D.;
Ariganello, M.; Williams, A.; Elisma, F.; Karthikeyan, S.; Vincent, R.;
Kumarathasan, P. Proteomic changes in human lung epithelial cells
(A549) in response to carbon black and titanium dioxide exposures. J.
Proteomics 2016, 149, 53−63.
(55) Saukko, E.; Lambe, A.; Massoli, P.; Koop, T.; Wright, J.;
Croasdale, D.; Pedernera, D.; Onasch, T.; Laaksonen, A.; Davidovits,
P.; Worsnop, D.; Virtanen, A. Humidity-dependent phase state of
SOA particles from biogenic and anthropogenic precursors. Atmos.
Chem. Phys. 2012, 12 (16), 7517−7529.
(56) Shiraiwa, M.; Li, Y.; Tsimpidi, A. P.; Karydis, V. A.; Berkemeier,
T.; Pandis, S. N.; Lelieveld, J.; Koop, T.; Pöschl, U. Global
distribution of particle phase state in atmospheric secondary organic
aerosols. Nat. Commun. 2017, 8, 15002.
(57) Santos, H. A.; Riikonen, J.; Salonen, J.; Mak̈ila,̈ E.; Heikkila,̈ T.;
Laaksonen, T.; Peltonen, L.; Lehto, V. P.; Hirvonen, J. In vitro
cytotoxicity of porous silicon microparticles: effect of the particle
concentration, surface chemistry and size. Acta Biomater. 2010, 6 (7),
2721−2731.
(58) Shahbazi, M. A.; Hamidi, M.; Mak̈ila,̈ E. M.; Zhang, H.;
Almeida, P. V.; Kaasalainen, M.; Salonen, J. J.; Hirvonen, J. T.; Santos,
H. A. The mechanisms of surface chemistry effects of mesoporous
silicon nanoparticles on immunotoxicity and biocompatibility.
Biomaterials 2013, 34 (31), 7776−7789.
(59) Vuong, N. Q.; Breznan, D.; Goegan, P.; O’Brien, J. S.; Williams,
A.; Karthikeyan, S.; Kumarathasan, P.; Vincent, R. In vitro
toxicoproteomic analysis of A549 human lung epithelial cells exposed
to urban air particulate matter and its water-soluble and insoluble
fractions. Part. Fibre Toxicol. 2017, 14 (1), 39.
(60) Vincent, R.; Kumarathasan, P.; Goegan, P.; Bjarnason, S. G.;
Gueńette, J.; Beŕube,́ D.; Adamson, I. Y.; Desjardins, S.; Burnett, R.
T.; Miller, F. J.; Battistini, B. Inhalation toxicology of urban ambient
particulate matter: acute cardiovascular effects in rats. Res. Rep. Health
Eff. Inst. 2001, 104, 5−54.
(61) Ostaszewski, C. J.; Stuart, N. M.; Lesko, D. M.; Kim, D.;
Lueckheide, M. J.; Navea, J. G. Effects of coadsorbed water on the
heterogeneous photochemistry of nitrates adsorbed on TiO2. J. Phys.
Chem. A 2018, 122 (31), 6360−6371.
(62) Henderson, M. A. A surface science perspective on TiO2
photocatalysis. Surf. Sci. Rep. 2011, 66 (6−7), 185−297.
(63) Gaschen, A.; Lang, D.; Kalberer, M.; Savi, M.; Geiser, T.;
Gazdhar, A.; Lehr, C.-M.; Bur, M.; Dommen, J.; Baltensperger, U.;
Geiser, M. Cellular responses after exposure of lung cell cultures to
secondary organic aerosol particles. Environ. Sci. Technol. 2010, 44
(4), 1424−1430.
(64) Savi, M.; Kalberer, M.; Lang, D.; Ryser, M.; Fierz, M.; Gaschen,
A.; Ricka, J.; Geiser, M. A novel exposure system for the efficient and
controlled deposition of aerosol particles onto cell cultures. Environ.
Sci. Technol. 2008, 42 (15), 5667−5674.

Environmental Science & Technology Article

DOI: 10.1021/acs.est.8b06879
Environ. Sci. Technol. 2019, 53, 3058−3066

3066

http://dx.doi.org/10.1021/acs.est.8b06879