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Ecotoxicology and Human Environmental Health

Aging influences on the biokinetics of functional TiO2 nanoparticles with different surface chemistries in Daphnia magna Wenhong Fan, Huiting Lu, and Wen-Xiong Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04392 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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Environmental Science & Technology

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Aging influences on the biokinetics of functional TiO2 nanoparticles with different surface chemistries in Daphnia magna Wenhong Fana, Huiting Lua,b, Wen-Xiong Wangb,c* a

School of Space and Environment, Beihang University, Beijing 100191, P. R. China

b

Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

c

Marine Environmental Laboratory, HKUST Shenzhen Research Institute, Shenzhen 518057, PR China

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*Corresponding author, [email protected]

1

ACS Paragon Plus Environment


22 23

ABSTRACT Nanoparticles functionalized with various surface capping moieties are now widely used

24

in different fields, thus there is a major need to understand the behavior and fate of these

25

nanoparticles in the environments. The present study investigated the biokinetics of fresh

26

titanium dioxide nanoparticles (TiO2 NPs) or TiO2 NPs aged under an artificial sunlight (16 h

27

light: 8 h dark) for 1, 3, and 5 days, respectively. Two commercial functionalized TiO2 NPs

28

(with SiO2 coating or SiO2 and polydimethylsiloxane coating) were employed in this study.

29

Dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FT-IR) and contact

30

angle (CA) measurements demonstrated that the surface properties had changed due to the

31

degradation during aging. The biokinetic parameters including dissolved uptake and

32

depuration rate constant as well as bioconcentration factors were calculated by a biokinetic

33

model. All the biokinetic parameters were significantly dependent on the aging process.

34

Further data analysis showed that the CA of the TiO2 NPs affected the uptake rate constant

35

and the fast compartmental efflux, and both CA and hydrodynamic diameter affected the fast

36

compartmental efflux. These results were due to the changes of corresponding indexes during

37

the aging process. Our work highlighted the necessity of monitoring the physicochemical

38

indexes of functionalized NPs during aging in evaluation of their environmental risks.

39 40

Key word: TiO2 nanoparticles; aged; surface functional; biokinetics

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43 44 45

TOC

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46 47

INTRODUCTION Titanium dioxide nanoparticles (TiO2 NPs) are widely used due to their unique

48

physicochemical properties.1 The estimated production of TiO2 NPs is now 7,800-38,000

49

tons/year in the USA,2 and 550-5,500 tons/year elsewhere.3 The numerous uses of TiO2 NPs

50

will eventually lead to their entry into aquatic environment, either directly or indirectly.4, 5 In

51

fact, TiO2 NPs were the first NPs detected in natural water,6 and there has been considerable

52

attention on the possible risks associated with their environmental exposure.7

53

Numerous studies have evaluated the toxicities of TiO2 NPs in various aquatic organisms

54

including algae, invertebrates, and fish.8, 9 In the majority of these earlier studies, TiO2 NPs

55

were freshly suspended in waters by various methods. There has been very little attention on

56

the aging process of NPs in ecotoxicological studies. Nevertheless, in real scenarios,

57

nanoparticles may not be in contact with aquatic organisms immediately after their releases

58

into aquatic environment.10 Multiple transformations of NPs during the release process may

59

lead to unpredictable outcomes different from their fresh counterparts. Indeed, a few limited

60

ecotoxicological studies suggested that the aged NPs showed different toxicities compared to

61

the “fresh ones”.11, 12 Furthermore, TiO2 NPs are usually produced with capping moieties

62

such as alumina and silane coupling agents in various functional application,13 and such

63

surface functionalization of TiO2 NPs introduces an additional factor in regulating their

64

toxicity to aquatic organisms. Therefore, it is vital to evaluate the potential risks of the

65

functional TiO2 NPs (TiO2 FNPs). A few studies have investigated the physicochemical

66

properties of TiO2 FNPs along with aging and showed that the by-products after aging

67

process were different from their fresh counterparts.14 Moreover, the by-products of TiO2

68

FNPs could influence the toxicity to organisms including Daphnia magna, Lumbricus

69

terrestris, and Danio rerio.15-17 However, these studies were generally limited to hydrophobic

70

commercial TiO2 FNPs namely T-Lite SF (BASF), while little attention has been given to

71

other TiO2 FNPs. In our recent work, we demonstrated that the byproducts of three

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commercial TiO2 nanoparticles with different capping moieties underwent a capping

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moieties-dependent transformation during the aging process.18 However, to our knowledge,

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the impacts of changing surface chemistries of other TiO2 FNPs (hydrophilic TiO2 FNPs or

75

other capping moieties TiO2 FNPs) on the ecotoxicity are still unknown.

76

Bioaccumulation of nanoparticles has received considerable attention due to its importance

77

in determining the toxicity of the potential hazard materials.19 The biodynamic model,

78

originally used to study trace metal accumulation in aquatic organisms, has been utilized to 4


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study NPs (AgNPs, CuO NPs, ZnO NPs).20, 21 The unidirectional uptake and efflux rates from

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water absorption or dietary ingestion can be measured by using the biokinetic model.

81

Although biokinetic parameters of other NPs have been reported, measurements of these

82

parameters for TiO2 NPs are rare. Bourgeault et al. detected TiO2 NPs bioaccumulation in

83

zebra mussels by using 47Ti labeling.22 Recently, Fan et al. reported that six commercially

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available TiO2 NPs with different sizes and surface properties showed significantly different

85

bioconcentration.23 However, more information about the accumulation and depuration

86

behaviors of TiO2 NPs under various conditions is needed.

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Thus, the goal of this study was to evaluate the biokinetics of TiO2 FNPs during aging

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processes from waterborne exposure. Two commercial TiO2 FNPs with different surface

89

chemistries (hydrophilic or hydrophobic) were chosen. Daphnia magna were used as model

90

organism. The biokinetics parameters including uptake rate constant (ku), efflux rate constant

91

(ke) and bioconcentration factor (BCF) were calculated. This study contributes to a better

92

understanding of the influence of aging on biokinetics of TiO2 FNPs and can facilitate to

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predict the risks of TiO2 FNPs.

94 95 96

MATERIALS AND METHODS Test organisms and water. Daphnia magna used in this study had been maintained in our

97

laboratory for over 15 years. The daphnids were raised in GF/C (Whatman) filtered creek

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water (N 22°20′11.3″, E 114°15′59.4″). The water was refreshed every two days. Green

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algae Chlamydomonas reinhardtii were provided as food daily at a density of 105 cells mL-1.

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All D. magna used in this study were 14 days old. In all experiments, the simplified Elendt

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M7 medium (SM7) were used. The test media contained different concentrations of CaCl2,

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MgSO4, K2HPO4, KH2PO4, NaNO3, NaHCO3, Na2SiO3, H3BO3, and KCl.24 Animals were

103

not fed during the whole experiment.

104 105

TiO2 NPs and characterization. Two types of commercial TiO2 NPs with different

106

capping moieties (named H and S, respectively) used in this study were purchased from Xuan

107

Cheng Jing Rui New Material Company Limited (China). The properties of the commercial

108

TiO2 samples are shown in Table 1. According to the manufacturer, H (with SiO2 coating)

109

and S (with SiO2 and polydimethylsiloxane coating) were hydrophilic and hydrophobic,

110

respectively. The image of the TiO2 NPs in SM7 was obtained by transmission electron

111

microscopy (TEM, JEOL 2010F, accelerating voltage 200 kV). The elemental contents were 5



112

measured by energy dispersive spectroscopy (EDS). For TEM sample preparation, TiO2 NPs

113

were dispersed in SM7 medium followed by sonication for 20 min (50 W/liter at 40 kHz).

114

X-ray diffraction (XRD) was used for the crystal phase identification, and was obtained at

115

room temperature using RigaKu D/max-3C.

116 117

Preparation of aged TiO2 NPs. The TiO2 NPs samples were aged by exposing a 500 mg/L

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solution of the NPs powder (in ultrapure water) to an artificial sunlight (16 h light: 8 h dark)

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for 1, 3, and 5 days, respectively. A sodium discharged lamp (OSRAM, 400 W) was used for

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the irradiation. Suitable content of the water was added to the reactor due to the loss of

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evaporation under irradiation. The suspension after aged procedure was directly used as stock

122

solution. The size distribution and zeta potential of TiO2 NPs (1 mg/L) in the suspension were

123

measured by dynamic light scattering (DLS, Brookhaven Instruments Corporation, USA). All

124

the TiO2 NPs suspensions were subjected to sonication for 20 min (50 W/liter at 40 kHz)

125

immediately prior to the DLS measurement and the following biokinetics experiments.

126

After aging, suitable content of the suspensions was transferred to centrifuge tube. Then

127

the suspensions were centrifuged three times at 10000 rpm for 1 h. The sediments were freeze

128

dried, and the dry particles were examined by Fourier transform infrared spectroscopy

129

(FT-IR). The wettability of the TiO2 NPs was evaluated by contact angle (CA) measurements

130

(Data Physics, Germany). All measurements were done in triplicate. The contact angles were

131

measured immediately after contact of the water drops with the NPs.

132

For convenience, samples available from H and aged for 1, 3, and 5 days were named H1,

133

H3, and H5, respectively. Similarly, samples available from S and aged for 1, 3, and 5 day

134

were named S1, S3, and S5, respectively. Accordingly, fresh H and S were named H0 and S0,

135

respectively.

136 137

Uptake and efflux of fresh and aged TiO2 NPs in D. magna. In the uptake experiment, D.

138

magna were cultured in clean SM7 for 3 h before the uptake experiment to remove the food

139

retained in their gut lines. Then D. magna were transferred into different nominal

140

concentrations (1 mg/L or 10 mg/L) of 0, 1, 3, or 5 d aged TiO2 NPs with a density of 10

141

mL/individual. All treatments were conducted in triplicates. A control test without TiO2 NPs

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was also conducted under the same conditions. At the time of 20, 40, 60, 120 min after

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exposure, ten D. magna were transferred to clean SM7 and rinsed for three times. Then they

144

were collected on polycarbonate membranes (25 mm, Millipore, USA) and subjected to Ti 6


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concentration measurements.

146

In the efflux experiment, the 0, 1, 3, or 5 d aged TiO2 NPs were added to SM7 to achieve

147

nominal concentrations of 1 mg/L or 10 mg/L. D. magna were exposed to different TiO2 NPs

148

solution for 2 h at a density of 10 mL/individual, with three replicates for each treatment. A

149

control test without TiO2 NPs was also conducted under the same conditions. After that, the

150

organisms were collected and rinsed in clean SM7 solution. Then the D. magna were

151

transferred into new exposure chambers containing clean SM7 solution. The depuration

152

lasted for 48 h. At 3, 6, 12, 24 and 48 h, ten D. magna were collected and rinsed. D. magna

153

were not fed during the whole uptake and efflux experiments.

154

After exposure, all D. magna were dried at 80 °C to a constant weight, and then transferred

155

into a digestion tube and digested in 68% HNO3 and concentrated sulfuric acid–ammonium

156

sulfate.25 The Ti concentration was determined by ICP-OES (Optima 5300DV, USA). To

157

evaluate the accuracy of the digestion experiment, the blank and a spiked sample containing

158

Ti at 10 mg L–1 were analyzed every 15 samples. The Ti recoveries of the spiked sample were

159

approximately 90 to 112%. Meanwhile, a standard addition test was carried out. A certain

160

amount of TiO2 standard solution (PerkinElmer, USA) were added to digestion tube to obtain

161

a solution of 10 mg/L. These samples were digested as described above. The TiO2 recovery in

162

these samples ranged from 93% to 107%.

163 164

Calculation of biokinetics parameters. The biokinetic parameters were calculated by

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biodynamic model according to previous studies.26 If the nanoparticles are available only

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from waterborne phase, the accumulation of NPs in the organisms (C, mg/g) with time can be

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expressed by the following equation: dC/dt = ku×Cw - ke×C

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(1)

169

Where, ku (L/g/h) is the uptake rate constant, Cw (mg/L) is the TiO2 NPs concentration in

170

water, ke (/h) is the efflux rate constant. When the test time is short, it can be considered that

171

the value of ku×Cw is far greater than the value of ke×C, and then the component ke×C of

172

equation (1) can be ignored. Therefore, the accumulation of NPs in organisms with time can

173

be calculated as:

174

dC/dt = I = ku×Cw

(2)

175

Where, I (µg/g/h) is the influx rate. If the organisms are not exposed to NPs, the component

176

ku×Cw of equation (1) can be ignored. Equation (1) can be modified to:

177

dC/dt = - ke×C

(3) 7



178 179

Under steady-state condition (dC/dt = 0), the accumulation of NPs in the organisms (Css) is calculated as:

Css = ku×Cw / ke

180 181 182

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(4)

The bioconcentration factor (BCF) is defined as the quotient of TiO2 NPs concentration in organisms to that in water:19 BCF = Css / Cw

183

(5)

184

Therefore, BCF can be calculated from equation (4) and (5):

185

BCF = ku / ke

(6)

186 187

Data analysis. Statistical analyses of data were performed using SPSS software (IBM®

188

SPSS® Statistics 19.0). One-way analysis of variance (ANOVA), simple linear regression

189

and stepwise multiple linear regression analysis were used when necessary.

190 191 192

RESULTS Characterization of fresh and aged TiO2 NPs. As shown in Figure S1, all the observed

193

peaks were attributed to rutile, thus the two types of TiO2 NPs were indeed pure rutile as

194

described by the manufacturers. Consistent with the information provided by manufacturer,

195

sample H and S were hydrophilic and hydrophobic, respectively (Figure S2). From the TEM

196

observation (Figure 1A, B), the sizes of these NPs were less than 50 nm, and both tended to

197

aggregate in water. The hydrodynamic diameter and zeta potential of fresh H and S were 291

198

± 58.4 and -25.0 ± 1.72 mV, 409 ± 60.0 nm and -18.5 ± 1.51 mV, respectively (Table 2). The

199

hydrodynamic diameters of both TiO2 NPs increased with aging. After the NPs were aged for

200

5 days, the sizes of H and S increased to 689 ± 53.1 and 748 ± 66.6 nm, respectively.

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Meanwhile, the zeta potentials of the two TiO2 NPs decreased after aging, and were -16.8 ±

202

0.64 mV and -13.1 ± 1.47 for H5 and S5, respectively. Figure 1 C - F and Table 2 also

203

demonstrate that surface properties of the samples were in agreement with the information

204

provided by the manufacturer. The fresh H was a hydrophilic material with a water CA of

205

10.9 ± 0.4°, and the fresh S was a hydrophobic material with a water CA of 142.1 ± 0.9°.

206

After aging for 5 d, H was still hydrophilic, and its CA increased to 35.5 ± 2.2°, while the

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hydrophobic S was transformed to hydrophilic with a water CA of 52.7 ± 2.3°.

208

As shown in Figure S3 A, the observed broad peak at 966 cm-1 in the spectrum of H0 was

209

ascribed to stretching vibrations of Si-OH or Si-O groups superimposed to Si-O-Ti stretching.

210

The peak at 807 cm-1 and 1208 were due to Si–O–Si stretching vibration. However, these 8


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peaks were weak and even disappeared after aging. Meanwhile, new absorption peaks at

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1070, 1170, 1431 and 1624 cm-1 were observed. The first three peaks corresponded to

213

Si-O-Si anti-symmetric stretching, Si–O–Ti stretching vibrations, Ti–OH deformation

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vibration, respectively. The peak observed at ∼1645 cm−1 was attributable to the bending

215

mode of adsorbed water. 27-29 For sample S, although the peak at 792, 1083 and 2948 cm-1

216

were observed in all samples, the peak at 1268 cm-1 in the spectrum of S0 gradually

217

disappeared with increasing aging (Figure S3 B). The peak around 2948 cm-1 corresponded

218

to CHx vibration, and the peak at 1268 cm-1 was due to the stretching mode of the

219

SiCH2R-residuals.14 At the same time, new peaks at 1157 and 1218 cm-1 corresponding to

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Si-O-Ti and Si-O-Si stretching vibrations become increasingly clear with aging. Similar to

221

sample H, peaks at 1425 and 1624 cm-1 were observed in the spectrum of S1, S3 and S5.

222 223

Uptake experiments. The uptake of TiO2 NPs in D. magna was evaluated by exposure to

224

fresh or aged TiO2 NPs solutions for 2 h. The Ti content in D. magna in the control group

225

were close to zero (data not shown). However, in the TiO2 exposure treatments, the

226

accumulated Ti in D. magna increased linearly along with the exposure time at two exposure

227

concentrations (Figure 2). The influx rate and ku exposed at 1 mg/L and 10 mg/L were

228

subsequently calculated (Table 3 and Table S1). Obviously, the exposure concentration

229

affected the ku values of all TiO2 NPs samples. Other things being equal, the ku value obtained

230

at a TiO2 concentration of 1 mg/L was generally higher than that obtained at a TiO2

231

concentration of 10 mg/L. Take H3 for instance, its ku value was 6.64 ± 0.34 L/g/h at TiO2

232

concentration of 1 mg/L, higher than that at 10 mg/L exposure (3.85 ± 0.23 L/g/h).

233

Meanwhile, an aging-dependent impact on the ku value was also observed to both TiO2

234

samples. The ku value of H gradually increased with aging. In comparison, ku for S was

235

gradually reduced with aging. Therefore, H0 and S0 exposure generally resulted in the lowest

236

and highest ku, while H5 and S5 exposure resulted in comparable ku value. For example, the

237

ku values of fresh H and S were 4.38 ± 0.39 L/g/h and 9.75 ± 0.26 L/g/h at TiO2 concentration

238

of 1 mg/L, respectively. After 5 days of aging, the ku values of H and S were 6.81 ± 0.59

239

L/g/h and 6.84 ± 0.49 L/g/h, respectively.

240 241

Efflux experiments. D. magna were exposed to fresh or aged TiO2 NPs for 2 h, and then

242

depurated for 2 days. The Ti content in D. magna in the control group was close to zero

243

during efflux experiment (data not shown). The TiO2 NPs retained in daphnids from all TiO2 9



244

exposure treatments declined rapidly within the first 6 h, and then declined gradually during

245

the rest of depuration (Figure 3). These results suggested that the efflux of TiO2 NPs from

246

daphnids in both treatments was fitted by a two-compartmental elimination model.30 The ke1

247

and ke2 are defined as the fast and slow efflux rate constants, respectively. Then ke1 was

248

calculated according to data within the first 6 h, and ke2 was calculated according to data from

249

the rest of depuration time.

250

As shown in Table 3, Fresh H and S usually showed the highest and lowest ke1 values at

251

two exposed concentrations, respectively. Compared to fresh H, ke1 values from H after aging

252

decreased. On the contrary to H, the ke1 values from S increased with aging. Notable that the

253

ke1 values of fresh H were significantly different from that of fresh S at two exposed

254

concentrations (p < 0.05). However, the ke1 values of H5 and S5 at the same exposed

255

concentration were comparable (p > 0.05).

256

The efflux rate constants ke2 from all TiO2 samples are shown in Table 3. At 1 mg/L, the

257

ke2 values of H decreased with increasing aging days (from 0.016 ± 0.000 /h to 0.010 ±

258

0.001/h), while there was almost no obvious difference between ke2 values of S (p > 0.05). At

259

10 mg/L, conversely, there was almost no obvious difference between ke2 values of H (p >

260

0.05), while the ke2 values of S decreased with increasing aging days (from 0.008 ± 0.001 /h

261

to 0.015 ± 0.001/h).

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Accumulation of Ti in D. magna at the end of efflux experiments are displayed in Table S2.

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The accumulated TiO2 NPs in daphnids from all treatments were more than 25% after 48 h of

264

depuration. In some treatments (e.g., S0), nearly 50% of Ti still remained in the daphnids.

265

These results suggested that the elimination of TiO2 NPs was difficult for D. magna.

266

Moreover, fresh H and S usually showed the lowest and highest accumulation in all TiO2 NPs

267

treatments, and there were statistically significant differences between them (p < 0.05). After

268

5 days of aging, the aged TiO2 NPs treatments showed significant differences from their

269

corresponding fresh treatments (p < 0.05), and showed no difference between each other (p >

270

0.05).

271 272

BCF values of TiO2 NPs. The bioconcentration factors of the TiO2 NPs (BCF = ku / ke2)

273

with different aging days are shown in Table 3. The BCF values of H increased with aging

274

days at 1 mg/L (from 2.77 ± 0.39 × 105 L/kg to 6.67 ± 0.96 × 105 L/kg). However, there were

275

no differences between BCF values from H with different aging days (p > 0.05). Meanwhile,

276

the BCF values of S decreased with aging days at the two exposure concentrations. Besides, 10


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it was noteworthy that although the BCF value of fresh S was 3 times greater than that of the

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corresponding fresh S at both exposure concentrations, the BCF values of S after 5 days

279

aging were close to that of H5.

280 281

DISCUSSION

282

Characterization of TiO2 NPs along with aging. The two TiO2 NPs used in this study

283

displayed different surface properties and were generally used in hydrophilic or oleophilic

284

commercial cosmetics. After 5 days of aging, the surface wettability of H remained

285

hydrophilic along with aging, while the values of CA increased (Table 2). Meanwhile, the

286

surface wettability of fresh S turned from hydrophobic to hydrophilic (CA: 142.1 ± 0.9° to

287

52.7 ± 2.3°, Table 2). Haapanen et al. reported that Ti/Si ratio had a significant effect on the

288

water CA.31 Wang et al. also reported that the methoxy groups of the silane could condense

289

with surface hydroxyl groups from TiO2 and thus affect the CA of the TiO2 NPs.32 Thus, we

290

speculated that H also suffered from degradation to a certain extent along aging. Data from

291

FT-IR provided further evidence. The weaken/disappeared peaks and the new peak observed

292

in the spectrum of H or S suggested that the structures of aged TiO2 samples were different

293

from the fresh counterparts. Furthermore, data from this study showed that the hydrodynamic

294

diameters of H and S increased with aging, and their absolute values of zeta potentials

295

decreased (Table 2). Previous study found that SiO2-coated TiO2 NPs or silane-functional

296

TiO2 NPs usually generated lower values of point of zero charge (PZC) than the pure TiO2

297

NPs, and the ratio of Si to Ti determined the value of PAC.33 The coated NPs dispersed

298

readily when compared with the non-coated NPs.34 Thus, we can conclude that the surface

299

properties had changed due to the degradation during aging.

300 301

Uptake of TiO2 NPs in D. magna. All TiO2 NPs treatments displayed higher ku values at 1

302

mg/L than that at 10 mg/L (Table 3), which suggested that concentration could affect the

303

uptake of TiO2 NPs. For filter-feeding animals such as D. magna, direct ingestion was the

304

dominant route for waterborne exposure. Daphnids could easily ingested smaller particles of

305

0.4 – 40 µ m in diameter.35 TiO2 NPs tend to aggregate in solution, and the hydrodynamic

306

diameter of TiO2 NPs would increase with time. Moreover, previous studies showed that

307

either at the beginning of experiment or after some period of experiment, the hydrodynamic

308

diameter of TiO2 NPs at higher concentration was generally higher than that at lower

309

concentration.36, 37 Therefore, it could be speculated that the probability for TiO2 NPs to form 11



310

large agglomerates may be higher at higher concentration. The aggregated size may have

311

exceeded the filter meshes of D. magna, and explained the lower ku values at 10 mg/L.

312

The ku values of TiO2 NPs with different aging days varied even at the same exposure

313

concentration, suggesting that the uptake of TiO2 NPs was related to aging. We have

314

demonstrated that the two TiO2 FNPs used in present study suffered degradation after aging,

315

and their physicochemical properties displayed significant changes after aging. To further

316

investigate the relevant potential factors, the relationship between the ku values and each

317

physicochemical index was analyzed by stepwise multiple linear regression analysis. Results

318

suggested that only CA influenced the ku values (r2=0.861, p=0.001 at 1 mg/L and r2=0.962,

319

p=0.000 at 10 mg/L). As illustrated in Figure 4A, the ku value increased proportionally with

320

increasing CA at both exposure concentrations (p < 0.001), which indicated that CA was

321

closely related to the uptake of TiO2 NPs. Surface hydrophobicity of NPs has been shown to

322

affect the uptake rate of NPs in organisms in many previous studies.38, 39 When NPs come to

323

contact with the animals in aquactic environments, interactions occur at the membrane

324

interface consisting of a phospholipid bilayer.40 The hydrophilic NPs were apt to absorb at the

325

membrane surface, and hydrophobic NPs reside into the bilayer.41, 42 As a result, increasing

326

hydrophobicity of NPs always generated higher uptake rate, and vice versa.43, 44 In this

327

experiment, fresh H and S were hydrophilic and hydrophobic, respectively. After aging 5

328

days, the hydrophilicity of H weaken as compared to fresh H, and the surface porpery of S5

329

even turned to hydrophilic (Figure 1 and Table 2). Such changes of CA during the aging

330

period might induce differences in surfaces porperies among TiO2 NPs samples, and

331

eventually caused the differences in ku values.

332 333

Efflux of TiO2 NPs in D. magna. The efflux pattern of all treatments accorded with a

334

typical two-compartmental model,30 suggesting that aging did not appear to influence the

335

efflux pattern. Meanwhile, the efflux rate was different for TiO2 NPs samples with different

336

aging days. The relationships between physicochemical index related to aging and efflux rate

337

were similarly investigated using stepwise multiple linear regression analysis. Again, the

338

results suggested that only CA was related to ke1 values (r2=0.905, p=0.000 at 1 mg/L and

339

r2=0.685, p=0.011 at 10 mg/L). As illustrated in Figure 4B, negative correlation was

340

observed between CA and ke1 value (p < 0.05). Meanwhile, data analysis also suggested that

341

both CA and hydrodynamic diameter were related to the ke2 values (r2=0.947, p=0.001 at 1

342

mg/L and r2=0.939, p=0.001 at 10 mg/L). 12


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Previous toxicity studies have investigated the factors influencing the depuration of NPs

344

from organisms. Surface wettability of nanoparticles is regarded as one of these factors.45 The

345

hydrophilic NPs were slowly accumulated in the organisms and readily eliminated from

346

organisms, while the hydrophobic NPs tend to penetrate into the circulatory system of

347

organisms and hence distributed to various organs.46 Moreover, large particles could form

348

deposits on the gut surface and hard to be cleared by the daphnids.47, 48 However, how these

349

factors affect the depuration of NPs from filter-feeding animals is speculative.

350

Two-compartmental model is the typical efflux model of filter animals. The fast exchanging

351

pattern is generally related to the rapid depuration of non-assimilated NPs, while the slow

352

exchanging pattern corresponded to NPs retention in peritrophic membrane, debris or

353

microvilli.30, 49, 50 The significant influence of CA and hydrodynamic diameter of TiO2 NPs

354

may be due to the different interactions between NPs and internal organs of organisms.

355 356

Influence of aging on the BCF values of TiO2 NPs. Bioaccumulative substances are

357

generally considered those with BCF over 5000.51 According to Table 3, all the TiO2 NPs are

358

bioaccumulative substances. Meanwhile, the significant differences between the BCF values

359

of 5 days aged TiO2 NPs and its fresh counterparts (p < 0.05) indicated that aging was related

360

to the BCF values. Similar stepwise multiple linear regression analysis was conducted for the

361

possible correlation between physicochemical index during aging and BCF. Among these

362

factors, the BCF was significantly correlated with the CA (r2=0.750, p=0.005 at 1 mg/L and

363

r2=0.859, p=0.001 at 10 mg/L). As illustrated in Figure 5, the BCF values increased with the

364

increase of CA at two exposure concentrations. BCF is the quotient of ku and ke. Therefore,

365

based on discussion of ku and ke values, it could be concluded that the degradation during

366

aging led the change of BCF values.

367 368

Environmental implication. FNPs usually produce excellent integration and an improved

369

interface compared to pure NPs due to the change of surface chemistry.13 However, the

370

coating of FNPs may undergo degradation with time, and correspondingly physicochemical

371

properties of NPs could also change.10, 18 Therefore, it is difficult to predict their fate and

372

toxicity of aged FNPs simply based on the fresh FNPs without a detailed characterization of

373

the surface properties during aging.11, 52 Data analysis from this study showed that the indexes

374

such as CA changed during aging and may eventually led to various biokinetics between TiO2

375

NPs samples. In previous study, other surface properties relevant indexes such as surface 13



376

affinity and log Kow were also used to investigate the toxicity caused by change or surface

377

properties.53, 54 Therefore, more attention should be given to monitor the physicochemical

378

indexes of FNPs which may change along with time in order to fully evaluate their

379

environmental risks.

380

The biokinetics of two commercial TiO2 FNPs with different surface chemistries were

381

investigated in the present study. Data from DLS and CA demonstrated that the surface

382

structures of the two TiO2 NPs changed with aging. The accumulated Ti in D. magna in all

383

treatments also increased with time under two exposure concentrations. Efflux experiments

384

showed that the efflux pattern of all treatments followed a two-compartmental depuration.

385

Biokinetics parameters including ku, ke1, ke2, and BCF were influenced by aging and

386

concentration. Further data analysis showed that CA could affect the ku and ke1 values, and

387

both CA and hydrodynamic diameter could affect the ke2 values. These results were due to the

388

changes of corresponding indexes during aging. This study highlighted the importance of

389

monitoring the physicochemical indexes of FNPs along with aging.

390 391

ASSOCIATED CONTENT

392

Supporting Information

393

Influx rate of the TiO2 samples (Table S1). The X-ray diffraction spectrums of the TiO2 NPs

394

samples (Figure S1). Photos of the two TiO2 NPs in ultrapure water (A, C) and SM7 (B, D).

395

(A), (B): H; (C), (D): S (Figure S2). FT-IR spectra of two TiO2 NPs along with aging days:

396

(A) H; (B) S (Figure S3). This material is available free of charge via the Internet at

397

http://pubs.acs.org.

398 399

AUTHOR INFORMATION

400

Corresponding Authors

401

*E-mail: [email protected].

402 403

ACKNOWLEDGMENTS

404

This work was supported by the National Natural Science Foundation of China (21577116 to

405

W.-X. Wang, and 51290283 and 51378041 to W. Fan), and by the Basic Research Program of

406

Shenzhen Science, Technology and Innovation Commission (JCYJ20170413173434280 to

407

W.-X. Wang). 14


Page 14 of 26

Page 15 of 26


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18


Page 18 of 26

Page 19 of 26


547 548

Table 1. Properties of the commercial TiO2 samples nomenclature

H

S

crystalline phase

rutile

rutile

coating

SiO2

SiO2, polydimethylsiloxane

primary crystallite size (nm)

30

30

surface property

hydrophilic

hydrophobic

50±30

50±30

Ti: 73.15

Ti: 74.85

O: 20.52

O: 19.62

Si: 6.34

Si: 5.52

2

−1

BET surface area (m g ) a

Elemental contents (%)

549

a

elemental contents were measured by energy dispersive spectroscopy (EDS).

550 551

19



Page 20 of 26

552

Table 2. Hydrodynamic diameter, zeta potential and contact angle of the commercial TiO2

553

samples during aging procedure. Sample

Hydrodynamic diameter (nm)

Zeta potential (mV)

Contact angle (°)

H0

291.30 ± 58.35

-25.03 ± 1.72

10.9 ± 0.4

H1

361.03 ± 32.32

-20.47 ± 1.93

25.5 ± 3.1

H3

486.93 ± 50.00

-18.03 ± 0.23

33.1 ± 4.5

H5

689.47 ± 53.11

-16.83 ± 0.64

35.5 ± 2.2

S0

409.33 ± 59.94

-18.50 ± 1.51

142.1 ± 0.9

S1

651.00 ± 78.57

-15.57 ± 0.78

109.7 ± 2.2

S3

662.40 ± 150.85

-14.33 ± 1.31

59.1 ± 4.9

S5

748.33 ± 66.55

-13.10 ± 1.47

52.7 ± 2.3

554

20


Page 21 of 26


555

Table 3. Toxicokenetic parameters of the TiO2 NPs samples. Different letters indicate

556

statistically significant differences (p < 0.05, one-way ANOVA) among the TiO2 treatments at

557

the same exposed concentration. TiO2

1 mg/L ku (L/g/h)

ke1(/h)

ke2(/h)

BCF(105 L/kg)

H0

4.38 ± 0.39 a

0.086 ± 0.009 a

0.016 ± 0.000 a

2.77 ± 0.39 a

H1

6.44 ± 0.19 b

0.078 ± 0.009 ab

0.015 ± 0.001 a

4.21 ± 0.24 ab

H3

6.64 ± 0.34 b

0.074 ± 0.007 b

0.013 ± 0.002 b

5.15 ± 0.93 ab

H5

6.81 ± 0.59 b

0.070 ± 0.007 bc

0.010 ± 0.001 c

6.67 ± 0.96 bc

S0

9.75 ± 0.26 c

0.040 ± 0.002 d

0.008 ± 0.002 cd

10.71 ± 1.97 d

S1

8.50 ± 0.19 d

0.059 ± 0.005 e

0.006 ± 0.002 e

14.50 ± 3.89 e

S3

7.84 ± 0.13 d

0.063 ± 0.001 ce

0.008 ± 0.001 de

9.46 ± 1.11 cd

S5

6.84 ± 0.49 b

0.064 ± 0.001 ce

0.010 ± 0.001 cd

6.91 ± 1.18 c

558 TiO2

10 mg/L ku(L/g/h)

ke1(/h)

ke2(/h)

BCF(105 L/kg)

H0

3.01 ± 0.28 a

0.075 ± 0.010 a

0.014 ± 0.001 ab

2.15 ± 0.70 a

H1

3.20 ± 0.25 a

0.068 ± 0.009 ab

0.013 ± 0.001 b

2.39 ± 0.13 ab

H3

3.85 ± 0.23 b

0.058 ± 0.003 bc

0.014 ± 0.001 ab

2.72 ± 0.20 abc

H5

4.09 ± 0.25 bc

0.052 ± 0.004 c

0.016 ± 0.002 a

2.63 ± 0.49 abc

S0

6.19 ± 0.09 d

0.042 ± 0.004 d

0.008 ± 0.001 c

7.59 ± 0.41 d

S1

5.81 ± 0.10 d

0.051 ± 0.006 cd

0.013 ± 0.001 b

4.40 ± 0.38 e

S3

4.39 ± 0.13 ce

0.052 ± 0.001 c

0.014 ± 0.002 ab

3.20 ± 0.51 c

S5

4.21 ± 0.09 be

0.060 ± 0.002 bc

0.015 ± 0.001 ab

2.81 ± 0.18 bc

559 560

21



561

562 563

Figure 1. TEM images of the two types of TiO2 nanoparticles: (A): H; (B): S; (C)-(J): contact

564

angles of water droplets on the two TiO2 nanoparticles during aging procedure. H: (C) - (F);

565

S: (G) – (J). Fresh nanoparticles: (C), (G); photoperiod 1 days: (D), (H); photoperiod 3 days:

566

(E), (I); photoperiod 5 days: (F), (J).

567

22


Page 22 of 26

Page 23 of 26


24 H0 S0 H1 S1 H3 S3 H5 S5

18

12

6

(A)

0 0

30

60

90

120

150

-1 Accumulated Ti in the D.magna (mg g dry wt)

-1 Accumulated Ti in the D.magna (mg g dry wt)

568

150 H0 S0 H1 S1 H3 S3 H5 S5

120 90 60

(B)

30 0 0

30

Time (min)

60

90

120

150

Time (min)

569 570

Figure 2. Accumulated Ti in Daphnia magna exposed to low (A) and high (B) TiO2

571

nanoparticles concentrations (1 and 10 mg/L, respectively) over 4 hours. Values are mean ±

572

SD (n=3).

573

23



Page 24 of 26

574 e5

e5

H0 H1 H3 H5

TiO2-retained in daphnids (%)

TiO2-retained in daphnids (%)

(A)

e4

S0 S1 S3 S5

e4

H0 H1 H3 H5

S0 S1 S3 S5

e3

(B)

e3 0

12

24

36

48

60

0

12

Time (min)

24

36

48

60

Time (min)

575 576

Figure 3. Retention of TiO2 nanoparticles in D. magna during 2 days of depuration in SM7

577

after being exposed to different TiO2 nanoparticles concentrations (1 and 10 mg/L,

578

respectively). Values are mean ± SD (n=3).

579 580

24


Page 25 of 26


581

12 p=0.001, r2=0.861 ku(L/g/h)

9

6 p=0.000, r2=0.962 3 1 mg/L 10 mg/L

0 0

40

80 120 Contact angle (° )

160

0.10

0.08 ke1(/h)

p=0.000,r2=0.904 0.06

p=0.011,r2=0.683

0.04

1 mg/L 10 mg/L

0.02 0 582

40

80 120 Contact angle (° )

160

583

Figure 4. Relationships between the contact angle and the ku (A) or ke1 (B) values. Mean ±

584

standard deviation (n = 3).

585

25



Page 26 of 26

586

20 1 mg/L 10 mg/L

5

BCF(10 L/kg)

15

p=0.005, r2=0.750 10

5

p=0.001, r2=0.859 0 0

40

80

120

160

Contact angle (° )

587 588

Figure 5. Relationship between the contact angle and the BCF at 1 mg/L (A) or 10 mg/L

589

exposure concentration. Mean ± standard deviation (n = 3).

26


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