Capturing of Nano-TiO2 from Complex Mixtures by Bisphosphonate

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Capturing of Nano-TiO2 from Complex Mixtures by Bisphosphonate Functionalized Fe3O4 Nanoparticles Ling-Feng Shen, Yi-Zhou Zhu, Peng-Fei Zhang, and He-Fang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02446 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 14, 2017

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ACS Sustainable Chemistry & Engineering

Capturing of Nano-TiO2 from Complex Mixtures by Bisphosphonate Functionalized Fe3O4 Nanoparticles Ling-Feng Shen, † Yi-Zhou Zhu, ‡ Peng-Fei Zhang,‡ and He-Fang Wang †*

†

Research Center for Analytical Sciences, College of Chemistry, Nankai University. Key

Laboratory of Biosensing and Molecular Recognition, State Key Laboratory of Medicinal Chemical Biology and ‡ State Key Laboratory of Elemento-Organic Chemistry Institute, 94 Weijin Road, Tianjin 300071, China

*E-mail: [email protected]

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ACS Paragon Plus Environment


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Abstract Selectively capturing of nanosized titanium dioxide (nano-TiO2) from complex mixtures is a very significant but very formidable task both for enriching the analytical methodology and removing/recycling of nano-TiO2. We presented a novel sustainable strategy for such task by using the bisphosphonate functionalized Fe3O4 (BP-Fe3O4) nanoparticles as the high selective absorbents toward nano-TiO2. The BP-Fe3O4 exhibited high equilibrium adsorption capacity (Qe) toward nano-TiO2 (2.45 g g-1 for 60 nm anatase TiO2), but negligible Qe toward nano-SiO2 (0.01 g g-1 for 15 nm SiO2). The BP-Fe3O4 also had good adsorption capability toward nano-TiO2 in complex liquid media and in the presence of coexisted 10-500 times of nano-SiO2. This capturing strategy enables the green pretreatment for quantitative analysis of nano-TiO2 in complex real samples with the spike recoveries of 69.9-99.0% and low detection limit of 0.02 ppm, and offers great convenience for identification of the crystal forms and even morphology of nano-TiO2 existed in complex real samples. Besides, this capturing strategy holds great potentials for separating or removing nano-TiO2 from environment and the sustainable recycling of nano-TiO2. Key words: magnetical separation, titanium dioxide, bisphosphonate, ferriferrous oxide, selective capturing

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Introduction Nanosized titanium dioxide (nano-TiO2) has been widely used as additives in pigments, cosmetics, toothpastes, pharmaceutics, foods, and implanted biomaterials.1 Nano-TiO2 is also the superstar in photocatalysis and solar cells.1 The global production of TiO2 was about 7.5 million tons in 2014, and only in pigments, the annual consumption was approximately 4.6 million tons.2 Such big production and consumption has raised serious safety concerns, since many researches have demonstrated that nano-TiO2 has adverse effects on living cells and condition-dependent toxicity to different organism groups.3-12 The International Agency for Research on Cancer has classified nano-TiO2 as a Group 2B carcinogen.10 As the extensive usage of nano-TiO2 is inevitable, the novel strategies for effective capturing of nano-TiO2 from complex mixtures are of great significance to analyze the nano-TiO2 in complex samples and to minimize their negative effect to ecosystem and human health. To date, the analysis (including the content determination and the size characterization) of nano-TiO2 in cosmetic and food products has aroused wide attentions.13-21 Among those literatures, the combination of solvent extraction and centrifugation or the harsh mixed-acid digestion was usually used as the sample pretreatment. The strategy aiming at the green pretreatment and the massively capturing of nano-TiO2 from complex matrix, to the best of our knowledge, has not been reported yet. Herein, we proposed a novel strategy for high selectively and quantitatively capturing nano-TiO2 from complex sample matrix including solid mixtures by using the bisphosphonate functionalized Fe3O4 (BP-Fe3O4) nanoparticles as adsorbents (Scheme 1). In this design, the bisphosphonate groups were responsible for the selective capturing 3



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nano-TiO2 owing to the strong binding affinity of phosphonate to TiO2,22 while the superparamagnetic Fe3O4 nanoparticles23 guaranteed the convenient and high efficient separation or removal of nano-TiO2 from complex mixtures (Scheme 1a). Owing to the reverse interaction of bisphosphonate and TiO2, the nano-TiO2 captured by BP-Fe3O4 could be easily released upon desorption by 1% aqueous ammonia (Scheme 1b). Note that in this work, the magnet was set aside (perpendicular to the gravity direction) to the containers of nanoparticle mixtures including BP-Fe3O4 (Scheme 1b) for excluding the gravity settling of nanoparticles. To the best of our knowledge, this is the first exploration for highly selective and quantitative capturing nano-TiO2 from complex mixtures based on the simple but strong chemical interactions (the high binding affinity between bisphosphonate groups and nano-TiO2).

a

Dopamine, N2

Fe3O4

O

50 oC stirring

O O

NH2 CS , Et N 2 3 25 oC ultrasonic

O

H N

H N

S BP-Fe3O4

H N

O

S

O

O OH P OH alendronic acid C P OH 25 oC HO O ultrasonic

S-+Et3NH

I2, Et3N Ice bath ultrasonic

HO

b

O

N C S

O

BP-Fe3O4 Nano-TiO2 Other Nanoparticles Vessel Magnet

Scheme 1. Schematic diagrams for (a) synthesis of BP-Fe3O4 nanoparticles and (b) capturing of nano-TiO2 from complex mixtures by BP-Fe3O4.

Experimental Section

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Reagents. All the analytical grade reagents were used directly. FeCl3⋅6H2O, CH3COONa, polyethylene glycol (MW 2,000), NaOH and HCl were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). Dopamine hydrochloride, nano-TiO2 (99%, 25, 60 and 100 nm, anatase; 60 nm, rutile), SiO2 (15 nm), humic acid (HA) and ethylene glycol were bought from Aladdin (Shanghai, China). Ethanol was bought from Kangkede Fine Chemical Research Institute (Tianjin, China). CS2, ethylacetate and triethylamine were from Benchmark Chemical Reagent Limited Company (Tianjin, China). Iodine was purchased from Pharmaceutical Company (Tianjin, China). Alendronic Acid was from Tokyo Chemical Industry (Shanghai, China). The cylindrical NdFeB magnet (diameter 3 cm, thickness 2 cm) was used throughout all the experiments. The real samples including fruit jelly, flour, and sunscreen were bought from the local supermarket. Apparatus. The morphology of Fe3O4 and BP-Fe3O4 nanoparticles were observed on a JEOL 100 CXII transmission electron microscope (TEM) working at 200 kV accelerating voltage. The magnetic properties were measured using a LDJ 9600-1 vibrating sample magnetometer (LDJ Electronics Inc., Troy, MI, USA) at 298 K by cycling the field from −70 to 70 kOe. The X-ray diffraction (XRD) patterns were recorded with a Rigaku D/max-2500 X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation (λ=1.5418 Å). The elemental percentage composition of BP-Fe3O4 and the contents of nanoparticles including

nano-TiO2 and

nano-SiO2

were

determined

by

inductively

coupled

plasma-atomic emission spectroscopy (ICP-AES) (IRIS Advantage, Thermo, USA). The wavelength of nano-TiO2 and nano-SiO2 was 323.452 and 251.612 nm respectively, and 5



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that for P and Fe was set at 214.914 and 259.940 nm. The comparative analysis of the real samples via the mixed-acid digestion/ICP-AES method was done by No. 46 Research Institute of China Electronics Technology Group Corporation using microwave (MARS 240/50, CEM, USA) for digestion and ICP-AES (725-ES, Varian, USA) for measurement of Ti. This digestion/ICP-AES method was validated using GBW 01655 stainless steel labelled with Ti 0.0093%, and the measured data were (0.0093 ± 0.0002)% with three replicates of 0.0092%, 0.0091% and 0.0095%. Synthesis of Fe3O4 and BP-Fe3O4 Nanoparticles. Fe3O4 nanoparticles were prepared by a slightly-modified procedure.24 For synthesizing BP-Fe3O4, four steps were involved. Firstly, in a three-neck flask, 10 mL aqueous dispersion of Fe3O4 nanoparticles (200 mg) was mixed with 40 mL aqueous solution of dopamine hydrochloride (500 mg) by magnetic stirring. The flask was heated in 50 oC water bath under stirring with protection of N2 for 3 hours, then the magnetic-harvested products were washed with water for 3 times and adjusted pH to 7.0 with NaOH solution for the last washing. Secondly, CS2 (200 µL) was added into the mixture of aqueous dispersion of the above dopamine-Fe3O4 solution (16 mL) and triethylamine (40 µL), and the resultant mixture was sonicated for 15 minutes at 25 oC, then the magnetic-harvested products were washed with ethanol for 3 times. Thirdly, to the dispersion of all the above solid products in ethylacetate (10 mL), triethylamine (200 µL) was added and the mixtures were sonicated at 4 oC ice-bath. Iodine (204 mg) was slowly added into the reactants within 15 minutes and the reactions were kept for 1 hour under ultrasonic in ice-bath. Then the products were magnetic-harvested and washed by ethylacetate and ethanol for 3 times respectively, and the solid was kept in 6


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ethanol (2 mL) for use. Fourthly, the aqueous solution (30 mL) of alendronic acid (280 mg) were sonicated for 15 minutes, then the above 2 mL ethanol solution of solid products was added and followed by adjusting pH to 10 with NaOH solution. The reactants were sonicated for 3 hours, then the solid were magnetic-harvested and washed with water for 3 times (adjusted pH to 3 with HCl solution for the last water washing) and ethanol for 3 times respectively. The resultant final products were dried in vacuum overnight. Static Adsorption Curves of Nano-TiO2 and Nano-SiO2. All the stock dispersions of the nanoparticles at 1.0 g L-1 were freshly made and ultrasonicated for 5 minutes before use. For the adsorption curves, 0.16 mL of BP-Fe3O4 or Fe3O4 (1.0 g L-1) was added into 3.0 mL aqueous dispersions of pure nanoparticle (with various concentrations C0), and the resultant mixtures were ultrasonicated for 6 minutes. Then an external cylindrical NdFeB magnet was placed aside at 2 cm of distance to the above mixtures (perpendicular to the gravity direction) for 10 minutes, and the content of the nanoparticles in decantate (Cd) after magnetic separation was determined by ICP-AES. The equilibrium adsorption capacity (Qe in g g-1) was calculated as (C0-Cd)×3.16/0.16. The static adsorption curves were made by plotting the Qe (calculated from series C0) against C0, and the maximum Qe was the value in flat part of the static adsorption curves. Desorption of Nano-TiO2 from BP-Fe3O4. 1% aqueous ammonia (3 mL) was added into the magnetic-harvested mixtures of nano-TiO2 and BP-Fe3O4 and ultrasonicated for 6 min, and the content of nano-TiO2 in the desorption solution after magnetic separation was determined by ICP-AES. The desorption rate (in percentage) was calculated as the amount ratio of nano-TiO2 in desorption solution and in the adsorbed part by BP-Fe3O4. The 7



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desorption solution after magnetic removal of BP-Fe3O4 was also centrifuged and the harvested dried white solid was used for XRD measurements. Reusage of BP-Fe3O4 for Adsorption of Nano-TiO2。0.16 mL of BP-Fe3O4 (1.0 g L-1) was added into 3.0 mL aqueous dispersion of anatase nano-TiO2 (60 nm, 0.07 g L-1), and the resultant mixtures were ultrasonicated for 6 minutes. The adsorption and desorption steps were the same as the above-mentioned. The magnetic-harvested BP-Fe3O4 was directly used for next adsorption of nano-TiO2. The same procedure was repeated for two times. Adsorption of Nano-TiO2 in Complex Liquid Media. The anatase TiO2 of 60 nm was used as the example. The procedure was the same as the static adsorption curves, and the difference was the complex liquid media instead of ultrapure water were used to make dispersions of nano-TiO2. The liquid media included the aqueous solutions of electrolytes (NaCl, MgCl2 and CaCl2) at the concentrations of 50, 100, 150 and 200 mM, the aqueous solutions of NaCl at 100 mM with pH 4-12 (adjusted with NaOH and HCl solution), and the aqueous solution of HA at 0.14, 0.35, 0.7, 3.5 and 7 g L-1. The Qe in the presence of HA was calculated from the desorbed amount of nano-TiO2 from BP-Fe 3O4 as massive water-insoluble HA in the decantate would block the channel of ICP-AES instrument. Competitive Adsorption of Nano-TiO2 and Massive Nano-SiO2. 0.16 mL of BP-Fe3O4 (1.0 g L-1) was added into 3.0 mL aqueous dispersions containing 0.07 g L-1 of nano-TiO2 and 0, 0.7, 3.5, 7, 35 g L-1 of nano-SiO2 respectively, and the resultant mixtures were ultrasonicated for 6 minutes. Then an external cylindrical NdFeB magnet was placed aside at 2 cm of distance to the above mixtures (perpendicular to the gravity direction) for 8


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10 minutes. Decant the dispersions and wash the magnetic-harvested solid with water for three times. Then 3 mL of 1% aqueous ammonia was added into the washed solid and the resultant mixtures were ultrasonicated for 6 min, followed by the magnetic separation of BP-Fe3O4, and the content of the nanoparticles in the desorbed dispersions was determined by ICP-AES. Co-adsorption of Anatase and RutileTiO2 by BP-Fe3O4. 0.16 mL of BP-Fe3O4 (1.0 g L-1) was added into 3.0 mL aqueous dispersions (0.115 g L-1) containing anatase and rutile TiO2 at different mass percentages (0, 25, 50, 75 and 100). The resultant mixtures were ultrasonicated for 6 minutes, then an external cylindrical NdFeB magnet was placed aside at 2 cm of distance to the above mixtures (perpendicular to the gravity direction) for 10 minutes. The magnetic-harvested solid was washed with water for three times. The above procedures were repeated many times, and all the washed solids were collected and dried in vacuum, and finally were directly used for XRD measurements. The XRD of the standard anatase/rutile mixtures and the magnetic-harvested dried solids was qualitatively and quantitatively compared. Quantitative Capturing of Nano-TiO2 from Real Samples. Three kinds of real samples were involved, namely, fruit jellies, flours and sunscreens. First, the content of nano-TiO2 in the real samples was determined by ICP-AES via two different sample pretreatments, i. e., the standard nitric acid-hydrogen peroxide-hydrofluoric acid digestion method16 and desorption of nano-TiO2 captured by BP-Fe3O4. For the digestion method, the samples of fruit jellies, flours and sunscreens (0.5 g) were digested respectively with a mixture of nitric acid (2.5 mL), hydrogen peroxide (2.5 mL) and hydrofluoric acid (0.5 9



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mL) in muffle furnace at 200 oC for 50 min. After cooled to room temperature, the digests were made up to 50 mL with ultrapure water. Blank digest was prepared in the same way. For the desorption of nano-TiO2 captured by BP-Fe3O4 method, fruit jelly, flour and sunscreen (0.25 g) were dispersed in 50 mL of ultrapure water respectively under ultrasonication for 30 min. Into the 3.0 mL of sample dispersions, 0.16 mL of BP-Fe3O4 (1.0 g L-1) was added and the resultant mixture was ultrasonicated for 6 min, then an external cylindrical NdFeB magnet was placed aside at 2 cm of distance to the above mixtures (perpendicular to the gravity direction) for 10 minutes. Decant the dispersions and wash the magnetic-harvested solid with ultrapure water for three times. Then 3 mL of 1% aqueous ammonia was added into the washed solid and the resultant mixtures were ultrasonicated for 6 min, followed by the magnetic separation of BP-Fe3O4, and the content of nano-TiO2 in the desorbed dispersions was determined by ICP-AES with calibration of the standard curves acquired in 1% aqueous ammonia. For the spike recoveries, the standard nano-TiO2 (60 nm, anatase) was added into the above-mentioned real sample dispersions and ultrasonicated for 30 min (to get the spiked samples), then the total nano-TiO2 (Ct, including the spiked amount (Cs) and the original content (Co) of nano-TiO2) was determined using the above-mentioned desorption of nano-TiO2 captured by BP-Fe3O4 method. The spike recoveries were calculated as (Ct-Co)/Cs in percentage.

Results and Dicussion Synthesis and Characterization of BP-Fe3O4. The synthesis and purify procedure was very simple and straightforward (Scheme 1a). First, the dopamine was anchored onto 10


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Fe3O4 nanoparticles by the strong binding of dihydroxyl to Fe,25 and then the amino group of dopamine was transferred into isothiocyanate group via the reaction with carbon disulfide, triethylamine, and iodine,26 and finally the alendronic acid was added to form BP-Fe3O4 through the addition reaction of the amino group of alendronic acid and the isothiocyanate group. The prepared BP-Fe3O4 nanoparticles were characterized by XRD, magnetic hysteresis curve, TEM, FTIR and ICP-AES. The BP-Fe3O4 nanoparticles displayed the typical face center cubic phase of Fe3O4 (JCPDC No. 19-0629, Figure S1a in Supporting Information). Besides, the BP-Fe3O4 nanoparticles were superparamagnetism (Figure S1b) with high saturation magnetization of 71.0 emu g-1(at 298 K) and negligible coercivity and remanence. Such high saturation magnetization and superparamagnetism enabled convenient and high efficient harvesting and re-dispersion of BP-Fe3O4 during the adsorption and desorption of nano-TiO2. The TEM image (Figure S1c-d) showed the uniform spherical BP-Fe3O4 with the average diameter of 299 ± 26.3 nm. In the FTIR spectrum of BP-Fe3O4 (Figure S1e), the 3410 and 3253 cm-1(N-H vibration), 1542 cm-1 (coupled vibration of N-H and C-N), 1264 cm-1 (stretching vibration of C-N), 1047 cm-1 (vibration of PO32-) and 1090 cm-1 (C=S band) suggested the successful functionalization as hinted in Scheme 1a. The ICP-AES analysis revealed that BP-Fe3O4 contained 1.81% of P and 66.67% of Fe.

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a

b

BP-Fe3O4

1.6 1.2

-1

Qe (g g )

-1

Qe (g g )

Fe3O4

0.8

2.5

BP-Fe3O4

2.0

Fe3O4

1.5 1.0 0.5

0.4

0.0 0.0 0.00

0.02

0.04

0.06

0.08

0.00

0.03

-1

0.06

0.12

6.0 BP-Fe3O4 4.8

Fe3O4 -1

Qe (g g )

8 -1

d

BP-Fe3O4

10

0.09

2

2

c

-1

CTiO (g L )

CTiO (g L )

Qe (g g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6 4 2

Fe3O4

3.6 2.4 1.2

0 0.00

0.18

0.36

0.54

0.72

0.0 0.00

-1

0.06

0.12

0.18

0.24

-1

CTiO (g L )

CTiO (g L )

2

2

Figure 1. The static adsorption curves of nano-TiO2 with different sizes and crystal forms by BP-Fe3O4 and Fe3O4: (a-c) 25, 60, and 100 nm, anatase; (d) 60 nm, rutile. All the adsorptions were in the ultrapure water. Static Adsorption of Nano-TiO2 by BP-Fe3O4 and Fe3O4. To verify the effect of bisphosphate on BP-Fe3O4 nanoparticles on the capturing of nano-TiO2, we compared the static adsorption curves toward nano-TiO2 by BP-Fe3O4 and Fe3O4. As shown in Figure 1, BP-Fe3O4 nanoparticles displayed large equilibrium adsorption capacity (Qe in g g-1) toward the nano-TiO2 with different sizes and crystal forms. For the same crystal form, the Qe was gradually increased with the increase of the sizes of nano-TiO2 (Figure 1a-c). For the different crystal forms of nano-TiO2 with the same size, the Qe was also different (Figure 1b vs 1d). The largest Qe was 10.42 g g-1 for the anatase TiO2 of 100 nm. In contrast, the Fe3O4 without bisphosphate groups had very low Qe for all kinds of nano-TiO2 tested. The reason for the different Qe toward different nano-TiO2 was not well-understood; however, the much larger Qe of BP-Fe3O4 against Fe3O4 toward nano-TiO2 proved the bisphosphate was the holding hand of nano-TiO2.

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Desorption of Nano-TiO2 from BP-Fe3O4 and Reusage of BP-Fe3O4. Desorption of the adsorbed nano-TiO2 from BP-Fe3O4 was firstly investigated, as it is important for the re-usage of BP-Fe3O4 and harvesting of pure nano-TiO2 from mixtures. The desorption rate was larger than 95% when 1% aqueous ammonia was used as the desorption reagent. Resultantly, the Qe of nano-TiO2 remained invariable for 3 times of reusages (Figure S2). The nano-TiO2 harvested after desorption from BP-Fe3O4 had the same XRD pattern as the fresh nano-TiO2 (Figure S3), demonstrating the possibility to get the clean nano-TiO2 via BP-Fe3O4 capturing. Adsorption of Nano-TiO2 by BP-Fe3O4 in Complex Liquid Media. To evaluate the robust of BP-Fe3O4 for adsorption of nano-TiO2, we tested the adsorption capability in different liquid media (Figure 2). Take the anatase TiO2 of 60 nm as the example. First, the aqueous solution of different electrolytes at different concentrations was concerned (Figure 2a). The Qe of nano-TiO2 at 0.115 g L-1 almost remained invariable in the presence of the most common electrolytes such as NaCl, MgCl2 and CaCl2 in the concentration range of 50−200 mM. Second, the aqueous media at different pHs was evaluated (Figure 2b). All the Qe of nano-TiO2 at 0.115 g L-1 in the pH range of 4−12 remained at the levels of higher than 2.0 g g-1. The Qe was slightly increased with the increase of pH in the range of 4−9, and the highest Qe was observed at pH 9 (2.75 g g-1). At pH higher than 9, the Qe was gradually decreased. The pH effect on Qe was most possibly ascribed to the surface states of BP-Fe3O4 and nano-TiO2. Third, the effect of humic acid on the Qe was also considered (Figure 2c) as humic acid was always coexisted in the real samples. The Qe for nano-TiO2 at 0.07 g L-1 was gradually decreased with the increase amount of coexisted humic acid, 13



however, the Qe in the presence of 0.14 g L-1 of humic acid was 0.79 g g-1, and Qe was still higher than 0.20 g g-1 even in the presence of 7 g L-1 of humic acid. As the content of humic acid in natural water (10 mg L-1) and soil (less than 1%) was relatively low, the BP-Fe3O4 was applicable for the adsorption or removal of nano-TiO2 from the natural water or soil samples.

b 3.5

2.5

3.0

-1

Q e (g g )

a 3.0

-1

Qe (g g )

2.0 1.5

2.0

1.0 NaCl MgCl2

0.5 0.0

c

1.5

2.5

1.5

CaCl2

0

50

100

150

-1

1.0

200

4

6

Celectrolyte (mmol L )

8

10

12

pH

1.2 -1

Qe ( g g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.9 0.6 0.3 0.0 0

0.14

0.35

0.7

3.5

7

-1

CHA (g L )

Figure 2. Adsorption of nano-TiO2 (60 nm, anatase) by BP-Fe3O4 in complex liquid media: Qe of nano-TiO2 at 0.115 g L-1 in the presence of (a) different electrolytes (NaCl, MgCl2 and CaCl2) at 50, 100, 150 and 200 mM respectively; (b) NaCl at 100 mM with pH 4-12 (adjusted with NaOH and HCl solution); and (c) Qe of nano-TiO2 at 0.07 g L-1 in the presence of HA at 0.14, 0.35, 0.7, 3.5 and 7 g L-1 respectively. The Qe in the ultrapure water was also included for comparison. Capturing of Nano-TiO2 from Solid Mixtures. To further explore the capability of BP-Fe3O4 for capturing of nano-TiO2 from the solid mixtures, we compared the static adsorption curves of nano-TiO2 and nano-SiO2 (Figure 3a) and examined the competitive adsorption of nano-TiO2 in the presence of massive nano-SiO2 (Figure 3c-d). The much larger Qe of nano-TiO2 (2.45 g g-1) against nano-SiO2 (0.01 g g-1) in Figure 3a revealed that BP-Fe3O4 owned the great adsorption selectivity of nano-TiO2 over nano-SiO2, being 14


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ascribed to the high affinity between the TiO2 and the bisphosphonate group of BP-Fe3O4. The fine selectivity over nano-SiO2 could be more intuitively seen from the photographs in Figure 3b. At the very beginning of adsorption (0 min), the dispersion of the mixtures of nano-TiO2 (or nano-SiO2) and BP-Fe3O4 were both brown (the mixed color of white nano-TiO2 or SiO2 and black BP-Fe3O4). After 10 min capturing and magnetic separation, however, the vessel of nano-TiO2 became clear and transparent, and the vessel side close to the magnet had the brown solid layer. In contrast, the vessel of nano-SiO2 was still filled with white dispersion at 10 min, and the vessel side close to the magnet showed the black solid. This pictures comparison clearly proved BP-Fe3O4 could capture most of the nano-TiO2, but could not capture nano-SiO2. Note that in the left vessels of each photo only containing the pure nano-TiO2 or nano-SiO2, the white dispersions were all observed during the 10 min for both nano-TiO2 and nano-SiO2, revealing the gravity settling of either nano-TiO2 or nano-SiO2 was negligible.

Figure 3. The selectivity for BP-Fe3O4 to capture nano-TiO2 (60 nm, anatase) over nano-SiO2 (15 nm) (a) The static adsorption curves of nano-TiO2 and nano-SiO2 by BP-Fe3O4; (b) photographs 15



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for the adsorption of nano-TiO2 and nano-SiO2 by BP-Fe3O4 in the presence of cylindrical NdFeB magnet at 0 and 10 min (the left white vessels in each photo only containing pure nano-TiO2 or nano-SiO2, were used for excluding the gravity settling of nanoparticles); (c) Capturing of nano-TiO2 (0.07 g L-1) over the coexisted nano-SiO2 (0, 0.7, 3.5, 7 and 35 g L-1 for entries 1-5, respectively); (d) The mass percentages of nano-SiO2 and nano-TiO2 before and after being captured by BP-Fe3O4 corresponding to entries 2-5 in Figure 3c. For the competitive adsorption, 0.07 g L-1 of nano-TiO2 coexisted with the massive nano-SiO2 (0.70−35.0 g L-1) was adsorbed by BP-Fe3O4, and the concentrations of the nanoparticles in desorption dispersions were examined. Figure 3c plotted the concentrations of nano-TiO2 and nano-SiO2 before and after the adsorption, while Figure 3d displayed the accordingly corresponding mass percentages of nano-TiO2 and nano-SiO2. In the presence of nano-SiO2, the amount of nano-TiO2 captured by BP-Fe3O4 (reflected by the desorption amount) was gradually decreased with the increasing of coexisted nano-SiO2, however, 28.14% of nano-TiO2 could still be captured even in the presence of 500 times of nano-SiO2 (Figure 3c). Furthermore, after the capturing by BP-Fe3O4, the mass percentages of nano-TiO2 in the mixtures were greatly increased (Figure 3d), e. g. from 9.09% to 76.27% in the case of 10 times of the coexisted SiO2, and from 0.20% to 28.94% even in the case of 500 times of the coexisted nano-SiO2. This data further demonstrated the superior capturing capability of nano-TiO2, which would find promise application in removal of nano-TiO2 from environment or recycling of nano-TiO2. Note that in our experiment, the magnet was set aside (perpendicular to the gravity direction) to the containers of nanoparticle mixtures including BP-Fe3O4, thus the higher capturing amount of 60 nm TiO2 over 15 nm SiO2 exactly proved the fantastic capturing capability of nano-TiO2, as 60 nm TiO2 sank faster than 15 nm SiO2 did. The coatings on the surface of nano-TiO2 would affect the capturing efficacy, but the BP-Fe3O4 nanoparticles could still capture the nano-TiO2 as long as there was any 16


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site of titatium on TiO2 (data not shown). Quantitatively Capturing of Nano-TiO2. To explore whether BP-Fe3O4 could capture nano-TiO2 quantitatively, we examined the co-adsorption of anatase and rutile nano-TiO2 (Figure S4). XRD was used for quantitating the adsorbed nano-TiO2 as different crystal forms had different XRD patterns. As shown in Figure S4a and S4b, the peaks of BP-Fe3O4 did not interfere with the characteristic peaks of anatase at 25 degree and rutile at 28 degree, thus the magnetic harvested mixtures of BP-Fe3O4 and nano-TiO2 were used directly for XRD measurements (Figure S4d). The relative ratios of the peak-intensities at 25 and 28 degree in Figure S4d were almost in according to the standard mixtures of anatase and rutile TiO2 in Figure S4c, elucidating that BP-Fe3O4 could quantitatively capture nano-TiO2. Consequently, the BP-Fe3O4 could not only selectively capture the nano-TiO2 from complex mixtures, but also offered the great convenience for quantifying the different crystal forms of nano-TiO2 and for observing the morphology (e. g., shape, size or even lattice line) of the captured nano-TiO2 by TEM (Figure S5 and S6). To further examine the quantitative capturing of nano-TiO2, we tested the capturing results and spike recoveries in real samples including fruit jellies, flours, and sunscreens (Table 1). First, the contents of nano-TiO2 in those samples were determined by ICP-AES via two different sample pretreatments, namely, the standard nitric acid-hydrogen peroxide-hydrofluoric acid digestion16 method and desorption of nano-TiO2 captured by BP-Fe3O4. Data in Table 1 showed that the contents in all real samples agreed well with the two pretreatments, demonstrating that BP-Fe3O4 was reliable for quantitative capturing of nano-TiO2 from real samples. Second, the capturing recoveries of the nano-TiO2 spiked 17



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real samples were in the range of 69.9−99.0%, further proving that the BP-Fe3O4 could quantitatively capture the nano-TiO2 from the complex solid matrixes. It was worth noting that the proposed desorption of nano-TiO2 captured by BP-Fe3O4 method was free-of-digestion in hazardous acids and thus highly convenient, just using aqueous dispersions of nano-TiO2 as standard samples for the calibration curves (the nano-TiO2 of different sizes had the similar sensitivity, Figure S7). Besides, BP-Fe3O4 capturing method could offer the great convenience for directly TEM observing the morphology of nano-TiO2 existed in real samples (e.g., Figure S6, the anatase nano-TiO2 captured from fruit jelly). Table 1. Analytical Results and Spike Recovery (Mean ± SD, n=3) for Determination of nano-TiO2 in Fruit Jelly, Flour, and Sunscreen Samples Samples

TiO2 content (ppm) This method a The standard method b

fruit jelly-1

376 ± 1

367 ± 33

fruit jelly-2

103 ± 4

105 ± 8

flour-3

11 ± 1

10 ± 3

flour-4

11 ± 7

10 ± 3

sunscreen-5

2 ± 0.15

2 ± 0.2

sunscreen-6

333 ± 5

350 ± 17

Spiked （ppm） 2 5 10 0.5 2.5 5 0.5 0.75 1 0.5 0.75 1 0.5 0.75 1 2 5 10

a

Recovery (%) 69.9 ± 5.6 78.4 ± 3.9 76.3 ± 5.2 75.6 ± 3.6 78.2 ± 5.6 81.4 ± 4.3 80.5 ± 1.0 76.1 ± 5.6 82.0± 2.0 85.8 ± 1.8 90.6 ± 1.0 88.4 ± 2.0 89.2 ± 1.3 97.8 ± 2.2 99.0 ± 2.0 79.2 ± 4.8 85.5 ± 0.1 86.9 ± 2.0

the detection limit was 0.02 ppm (calculated as the equivalent content in the measured sample solutions according to 3σ/k, where σ was the standard deviation of the blank (n=5), k was the slope of the standard calibration). b Real samples were pretreated using the standard nitric acid-hydrogen peroxide-hydrofluoric 18


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digestion method.

In summary, we presented the novel strategy of the selectively capturing nano-TiO2 from complex matrixes by using BP-Fe3O4 as the absorbents. The BP-Fe3O4 exhibited much higher equilibrium adsorption capacity toward nano-TiO2 over nano-SiO2, and BP-Fe3O4 also had good adsorption capability toward nano-TiO2 in complex liquid media and in the presence of coexisted 10-500 times of nano-SiO2. This capturing strategy enabled the green pretreatment for quantitative analyse of nano-TiO2 in complex real samples with spike recoveries of 69.9-99.0% and low detection limit of 0.02 ppm, and offered great convenience for identification of the crystal forms and even morphology of nano-TiO2 existed in complex real samples. Besides, this capturing strategy held great potentials for separating or removing nano-TiO2 from environment and the sustainable recycling of nano-TiO2. Acknowledgements

The National Natural Science Foundation of China (No. 21575070, 21435001, 21175073) and the Tianjin Natural Science Foundation (No.13JCYBJC17000) were gratefully acknowledged. Supporting Information Available Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. Reference 1.

Dulger, M.; Sakallioglu, T.; Temizel, I.; Demirel, B.; Copty, N. K.; Onay, T. T.; Uyguner-Demirel, C. S.;

Karanfil, T., Leaching potential of nano-scale titanium dioxide in fresh municipal solid waste. Chemosphere 2016, 144, 1567-1572. 2.

https://en.wikipedia.org/wiki/Titanium_dioxide. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.

Tomankova, K.; Horakova, J.; Harvanova, M.; Malina, L.; Soukupova, J.; Hradilova, S.; Kejlova, K.;

Malohlava, J.; Licman, L.; Dvorakova, M.; Jirova, D.; Kolarova, H., Reprint of: Cytotoxicity, cell uptake and microscopic analysis of titanium dioxide and silver nanoparticles invitro. Food Chem. Toxicol. 2015, 85, 20-30. 4.

Wilson, C. L.; Natarajan, V.; Hayward, S. L.; Khalimonchuk, O.; Kidambi, S., Mitochondrial

dysfunction and loss of glutamate uptake in primary astrocytes exposed to titanium dioxide nanoparticles. Nanoscale 2015, 7 (44), 18477-18488. 5.

Song, Z.-M.; Chen, N.; Liu, J.-H.; Tang, H.; Deng, X.; Xi, W.-S.; Han, K.; Cao, A.; Liu, Y.; Wang, H.,

Biological effect of food additive titanium dioxide nanoparticles on intestine: an in vitro study. J. Appl. Toxicol. 2015, 35 (10), 1169-1178. 6.

Srikanth, K.; Pereira, E.; Duarte, A. C.; Ahmad, I.; Rao, J. V., Assessment of cytotoxicity and

oxidative stress induced by titanium oxide nanoparticles on Chinook salmon cells. Environ. Sci. Pollut. R 2015, 22 (20), 15579-15586. 7.

Warheit, D. B.; Donner, E. M., Risk assessment strategies for nanoscale and fine-sized titanium

dioxide particles: Recognizing hazard and exposure issues. Food Chem. Toxicol. 2015, 85, 138-47. 8.

Warheit, D. B.; Brown, S. C.; Donner, E. M., Acute and subchronic oral toxicity studies in rats with

nanoscale and pigment grade titanium dioxide particles. Food Chem. Toxicol. 2015, 84, 208-224. 9.

Giovanni, M.; Tay, C. Y.; Setyawati, M. I.; Xie, J.; Ong, C. N.; Fan, R.; Yue, J.; Zhang, L.; Leong, D. T.,

Toxicity Profiling of Water Contextual Zinc Oxide, Silver, and Titanium Dioxide Nanoparticles in Human Oral and Gastrointestinal Cell Systems. Environ. Toxicol. 2015, 30 (12), 1459-1469. 10. Zhang, X.; Li, W.; Yang, Z., Toxicology of nanosized titanium dioxide: an update. Arch. Toxicol. 2015, 89 (12), 2207-17. 11. Fang, T.; Yu, L. P.; Zhang, W. C.; Bao, S. P., Effects of humic acid and ionic strength on TiO2 nanoparticles sublethal toxicity to zebrafish. Ecotoxicology 2015, 24 (10), 2054-66. 12. Chen, Z.; Wang, Y.; Zhuo, L.; Chen, S.; Zhao, L.; Luan, X.; Wang, H.; Jia, G., Effect of titanium dioxide nanoparticles on the cardiovascular system after oral administration. Toxicol. Lett. 2015, 239 (2), 123-130. 13. Krystek, P.; Tentschert, J.; Nia, Y.; Trouiller, B.; Noel, L.; Goetz, M. E.; Papin, A.; Luch, A.; Guerin, T.; de Jong, W. H., Method development and inter-laboratory comparison about the determination of titanium from titanium dioxide nanoparticles in tissues by inductively coupled plasma mass spectrometry. Anal. Bioanal. Chem. 2014, 406 (16), 3853-3861. 14. Shaw, B. J.; Ramsden, C. S.; Turner, A.; Handy, R. D., A simplified method for determining titanium from TiO2 nanoparticles in fish tissue with a concomitant multi-element analysis. Chemosphere 2013, 92 (9), 1136-1144. 15. Lim, J. H.; Sisco, P.; Mudalige, T. K.; Sanchez-Pomales, G.; Howard, P. C.; Linder, S. W., Detection and Characterization of SiO2 and TiO2 Nanostructures in Dietary Supplements. J. Agric. Food Chem. 2015, 63 (12), 3144-3152. 16. Nischwitz, V.; Goenaga-Infante, H., Improved sample preparation and quality control for the characterisation of titanium dioxide nanoparticles in sunscreens using flow field flow fractionation on-line with inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 2012, 27 (7), 1084-1092. 17. Soto-Alvaredo, J.; Dutschke, F.; Bettmer, J.; Montes-Bayon, M.; Profrock, D.; Prange, A., Initial results on the coupling of sedimentation field-flow fractionation (SdFFF) to inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) for the detection and characterization of TiO2 20


Page 20 of 22

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanoparticles. J. Anal. At. Spectrom. 2016, 31 (7), 1549-1555. 18. Dan, Y. B.; Shi, H. L.; Stephan, C.; Liang, X. H., Rapid analysis of titanium dioxide nanoparticles in sunscreens using single particle inductively coupled plasma-mass spectrometry. Microchem. J. 2015, 122, 119-126. 19. Benitez-Martinez, S.; Lopez-Lorente, A. I.; Valcarcel, M., Determination of TiO2 nanoparticles in sunscreen using N-doped graphene quantum dots as a fluorescent probe. Microchimica Acta 2016, 183 (2), 781-789. 20. Menneveux, J.; Wang, F.; Lu, S.; Bai, X. S.; Motto-Ros, V.; Gilon, N.; Chen, Y. P.; Yu, J., Direct determination of Ti content in sunscreens with laser-induced breakdown spectroscopy: Line selection method for high TiO2 nanoparticle concentration. Spectrochim. Acta B 2015, 109, 9-15. 21. Lopez-Heras, I.; Madrid, Y.; Camara, C., Prospects and difficulties in TiO2 nanoparticles analysis in cosmetic and food products using asymmetrical flow field-flow fractionation hyphenated to inductively coupled plasma mass spectrometry. Talanta 2014, 124, 71-78. 22. Thingholm, T. E.; Larsen, M. R., The Use of Titanium Dioxide for Selective Enrichment of Phosphorylated Peptides. Methods Mol. Biol. 2016, 1355, 135-46. 23. Essinger-Hileman, E. R.; Popczun, E. J.; Schaak, R. E., Magnetic separation of colloidal nanoparticle mixtures using a material specific peptide. Chem. Commun. 2013, 49 (48), 5471-5473. 24. Zhou, J.; Meng, L.; Feng, X.; Zhang, X.; Lu, Q., One-Pot Synthesis of Highly Magnetically Sensitive Nanochains Coated with a Highly Cross-Linked and Biocompatible Polymer. Angew. Chem. Int. Ed. 2010, 49 (45), 8476-8479. 25. Liu, Y.; Chen, T.; Wu, C.; Qiu, L.; Hu, R.; Li, J.; Cansiz, S.; Zhang, L.; Cui, C.; Zhu, G.; You, M.; Zhang, T.; Tan, W., Facile Surface Functionalization of Hydrophobic Magnetic Nanoparticles. J. Am. Chem. Soc. 2014, 136 (36), 12552-12555. 26. Nath, J.; Patel, B. K.; Jamir, L.; Sinha, U. B.; Satyanarayana, K. V. V. V., A one-pot preparation of cyanamide from dithiocarbamate using molecular iodine. Green Chem. 2009, 11 (10), 1503-1506.

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For Table of Contents Use Only

BP-Fe3O4 Nano-TiO2 Other Nanoparticles Vessel Magnet

Capturing of Nano-TiO2 from Complex Mixtures by Bisphosphonate Functionalized Fe3O4 Nanoparticles Ling-Feng Shen, † Yi-Zhou Zhu, ‡ Peng-Fei Zhang,‡ and He-Fang Wang †* Synopsis: Selectively capturing nano-TiO2 by bisphosphonate-functionalized-Fe3O4 nanoparticles was promising for green sample pretreatment, removing nano-TiO2 from environment and recycling of nano-TiO2.

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Capturing of Nano-TiO2 from Complex Mixtures by Bisphosphonate

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