Structural Degradation at the Surface of a TiO2-Based Nanomaterial

Mar 11, 2010 - Environmental Science: Nano 2018 5 (1), 191-202 ... Journal of Nanoparticle Research 2018 20 (1), ..... Journal of Polymer Research 201...
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Environ. Sci. Technol. 2010, 44, 2689–2694

Structural Degradation at the Surface of a TiO2-Based Nanomaterial Used in Cosmetics ´ L A N I E A U F F A N , * ,†,‡ ME ´ RO MAXIME PEDEUTOUR,† JE ˆ M E R O S E , †,‡ A R M A N D M A S I O N , †,‡ F A B I O Z I A R E L L I , § D A N I E L B O R S C H N E C K , †,‡ ´ L I N E B O T T A , †,‡ CORINNE CHANEAC,| CE P E R R I N E C H A U R A N D , †,‡ ´ RO JE ˆ M E L A B I L L E , †,‡ A N D J E A N - Y V E S B O T T E R O †,‡ CEREGE UMR 6635 CNRS/Aix-Marseille Universite´, Europoˆle de l’Arbois, 13545 Aix-en-Provence, France, International Consortium for the Environmental Implications of Nanotechnology iCEINT, Europoˆle de l’Arbois, 13545 Aix-en-Provence, France, Fe´de´ration de Recherche des Sciences Chimiques de Marseille, ave Escadrille Normandie Niemen 13397 Marseille, France, and LCMC de Paris, UMR 7574 CNRS/UPMC, Jussieu, 75252 Paris, France

Received December 11, 2009. Revised manuscript received February 19, 2010. Accepted February 24, 2010.

A number of commercialized nanomaterials incorporate TiO2 nanoparticles. Studying their structural stability in media mimicking the environment or the conditions of use is crucial in understanding their potential eco-toxicological effects. We focused here on a hydrophobic TiO2 nanoparticle-based formulation used in cosmetics: T-Lite SF. It is composed of a TiO2 core, coated with two successive protective layers of Al(OH)3, and polydimethylsiloxane. Soon after contact with water (pH ) 5, low ionic strength), the T-Lite SF becomes hydrophilic and form aggregates. During this aging, 90%wt of the total Si of the organic layer is desorbed, and the PDMS remaining at the surface is oxidized. The Al(OH)3 layer is also affected but remains sorbed at the surface. This remaining Al-based layer still protects from the production of superoxide ions from thephotoactive/phototoxicTiO2 coreinourexperimentalconditions.

Introduction Nanotechnology-based materials and products have moved well beyond the laboratory. More than 800 of these products are on the market (www.nanotechproject.org) of which 50% are incorporated in cosmetics, clothing, and personal care products. Nanomaterials are undoubtedly present in our everyday life, but the societal acceptance of nanotechnologies is subject to studies ensuring the absence of detrimental impacts (1, 2). Among all the nanomaterials (i.e., materials nanostructured in the bulk, materials with nanostructure(s) at the * Corresponding author phone: +33 442 971 543; fax: +33 442 971 559; e-mail: [email protected]. Corresponding author address: CEREGE UMR 6635 CNRS/Aix-Marseille Universite´, Europoˆle de l’Arbois, 13545 Aix-en-Provence, France. † CEREGE UMR 6635 CNRS/Aix-Marseille Universite´. ‡ International Consortium for the Environmental Implications of Nanotechnology iCEINT. § Fe´de´ration de Recherche des Sciences Chimiques de Marseille. | LCMC de Paris, UMR 7574 CNRS/UPMC. 10.1021/es903757q

 2010 American Chemical Society

Published on Web 03/11/2010

surface, and materials containing nanoparticles bound to a surface or suspended in liquids and solids as well as airborne nanoparticles) (3) only two categories received particular attention in eco-toxicity studies (4): the pure nanoparticles e.g. fullerenes, nano-oxides, metallic nanoparticles suspended in liquids (5, 6), and the airborne nanoparticles (7, 8). However, in most of the everyday life applications, nanoparticles are surface modified, embedded in the final product, and do not come into direct contact with the consumer or the environment. Moreover, surface modification, adsorption, and other transformation processes may alter nanoscale structures with unknown consequences for living organisms. Consequently, studying the environmental behavior (e.g., structural alteration, dispersion in the aquatic environment) of commercialized nanomaterials and their (eco)toxicological impacts is one of the challenges in nanotoxicology. Many commercialized nanomaterials incorporate TiO2 nanoparticles (9). At the nanoscale, TiO2 is transparent to visible light, highly UV absorbent, has an iridescent quality, and is a photocatalyst. These properties lead to its widespread use in paints, cosmetics, self-cleaning materials, or wastewater treatments (10). In most of these applications, nanoTiO2 are surface-modified. For example, in sunscreen lotions, they are coated with inorganic and organic layers to facilitate their dispersion in the lotion and suppress unwanted toxic effects related of the TiO2 core (e.g., photo-oxidation reaction or generation of reactive oxygen species (ROS)). Exposure to the nanomaterials in these sunscreens can occur i) directly via skin contact and ii) indirectly after washing off the nanocompounds from the skin into to the aquatic environment followed by aging and trophic transfer. Indeed the specific properties can be modified because of immersion in water or abrasion with beach sand (11). During an indirect exposure scenario, the surface-modified nano-TiO2 present in sun protection creams will end up in water and natural systems. In this context, a main issue is the ability of the “protective” coating to withstand UV illumination and aggressive molecules. If the surface layer is removed, the photoactive nano-TiO2 will be in direct contact with living organisms including the skin (12) and will be able to generate genotoxicity and/or phototoxicity (13). Consequently, coating nano-TiO2 does not guarantee its innocuity after long-term aging, and formulations appearing safe initially may become hazardous during or after its intended use. The present paper is focused on the alteration of a regular TiO2-based formulation used in cosmetics (TLite SF) for which potential harmful effects have already been studied (12). Two specific points are addressed here: (i) what is the stability of the protective coating layers (Al- and Sibased) under relevant environmental conditions and (ii) does the alteration of the protective layer lead to the generation of toxic chemical species such as the ROS?

Experimental Section Commercialized TiO2-Coated Nanoparticles. The T-Lite SF nanoparticles manufactured by BASF are the Ti-based commercial product used in the present study. They are used in cosmetic applications for their transparency, compatibility, and dispersibility (www.cosmetics-europe.basf.de). It consists of TiO2 core (14-16 nm) coated with an Al(OH)3 layer and an outer layer of polydimethylsiloxane (PDMS (C2H6OSi)n). The Al(OH)3 layer protects against the photocatalytic effect of TiO2. The PDMS is used to disperse the T-Lite SF in the lotion and is considered to be inert and nontoxic. According to the manufacturer, the overall length of the T-Lite SF reaches 50 nm with a width of 10 nm (Figure 1). Analysis by XRD VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. (Left) transmission electron microscopy of T-Lite SF at pH 5. (Right) hydrodynamic diameters of the T-Lite SF suspended in water under stirring and daylight for 3 h. pH 5; [NaCl] ) 0.01 mol/L; 25 °C; [TiO2]initial ) 1 g/L. indicates the presence of one major TiO2 crystalline phase. The XRD patterns are in agreement with the crystallographic d-spacing of rutile with a slight contribution of anatase (data not shown). The chemical composition of the initial T-Lite SF powder was measured after alkaline dissolution. 250 mg of the T-Lite SF powder was mixed with 500 mg of LiBO2 in a graphite crucible and melted at 1100 °C during 25 min. The liquid mixture was then cooled down at ambient temperature to get an amorphous glass, which was then dissolved in 100 mL of HCl (2.4 N). The recovered solution was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using a Jobin Ultima-C spectrometer. The total Al and Si-content of the initial T-Lite SF powder is measured about 4 ( 1 %wt and 2 ( 0.5 %wt, respectively. Experimental Design. All the experiments were performed in polyethylene containers to avoid any release of Si from glass beakers. For the alteration, 1 g/L of the T-Lite SF powder was suspended in 250 mL of a 0.01 M NaCl solution. The suspensions were magnetically stirred during the indicated period of time (6 h, 24 h, or 48 h) in the dark or under artificial daylight using the HQI-BT lamp OSRAM (E40, 400W). HQIBT lamps present a uniform spectral intensity between 425 and 650 nm. More information about the light quality is available in ref 14. Until the end of the alteration, the pH was controlled and adjusted to pH ) 5 or 7 or 9 using HCl (0.1 M) and/or NaOH (0.1 M) solutions. For the samples exposed to daylight, the volume was kept constant to avoid any change in the liquid/volume ratio, and the temperature was maintained at 24 ( 4 °C. After alteration, the hydrodynamic diameter of the suspended T-Lite SF was measured using dynamic light scattering with a Nanotrac 250 (Microtrac, North Largo, FL) equipped with an optic fiber probe assembly. This technique based on the backscattered intensity allows the measurement of the hydrodynamic diameter in highly concentrated suspensions (20%). The suspensions were then ultracentrifuged (50000 rpm, 1 h). The chemical composition (dissolved Si and Al species) of the supernatant (liquid phase of the centrifugation) was analyzed by ICP-AES. The pellet (solid phase of the ultracentrifugation) was resuspended for 1 h in 250 mL of deionized water at the same pH and ionic strength than the alteration process and ultracentrifuged (50000 rpm, 1 h). This rinsing process was repeated twice. After the last ultracen2690

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trifugation, the pellet containing the altered T-Lite SF was freeze-dried for further characterization. Characterization of the Crystalline Structure of the Altered T-Lite SF. Attenuated total reflection-fourier transform infra-red (ATR-FTIR) spectroscopy was used to characterize the PDMS layer after alteration. Spectra were recorded on a Nexus (Thermo electron) spectrometer with an ATR diamond with a round probing area of 4 mm. 128 scans from 400 to 4000 cm-1 were taken at 4 cm-1 resolution. All data were corrected for the H2O vapor spectra and for the depth of penetration of the IR incident beam using the OMNIC software. Nuclear magnetic resonance (NMR) was used to characterize the PDMS (13C and 29Si NMR) and Al(OH)3 layers (27Al NMR). All solid-state NMR spectra were obtained on a Bruker Avance WB-400 spectrometer. 27Al MAS spectra were recorded at 104.3 MHz using single pulse at 90°, a spin rate of 12 kHz, and 512 scans. 13C CPMAS spectra were obtained at 100.7 MHz, 10 kHz spin rate, and 8k scans. 29Si CPMAS spectra were obtained at 79.5 MHz, 7 kHz spin rate, and 7k scans. ROS Measurement (Superoxide). The reduction of 2,3bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) allows for specific targeting and measurement of superoxide anions (15, 16) when it is combined with superoxide dismutase (SOD), a quencher of superoxide which cells utilize for protection (17). XTT reduction by O2•- results in the formation of XTT-formazan producing an absorption peak at 470 nm that can be used to quantify the relative amount of superoxide present. Experiments were performed on two individual suspensions: (i) TiO2 nanoparticles (18) and (ii) altered T-Lite SF resuspended in water (conditions of alteration: pH 5, daylight, 48 h) at a concentration of 25 mg/L and 130 mg/L, respectively. 10 mL of each suspension was magnetically stirred and exposed to daylight for up to 200 min in the presence of XTT and SOD at a concentration of 100 µM and 25 U/mL, respectively. Measurements were performed on a Cintra 10 spectrometer.

Results Suspension of T-Lite SF. Due to the PDMS coating, the T-Lite SF are initially hydrophobic. However, in less than 3 h under mild stirring, they are suspended in the aqueous solution and form aggregates with a bimodal size distribution centered on ∼400 nm and ∼1140 nm (Figure 1). This suspension was observed in the dark and under artificial daylight. The

FIGURE 2. Release of Si from the PDMS layer and Al from the Al(OH)3 layer during the T-Lite SF alteration: (A) as a function of pH after 48 h and (B) as a function of time at pH 5. [NaCl] ) 0.01 mol/L; 25 °C; [TiO2]initial ) 1 g/L.

FIGURE 3. 29Si and 13C NMR of the T-Lite SF after 48 h of alteration in water under daylight and in the dark. pH 5; [NaCl] ) 0.01 mol/ L; 25 °C; [TiO2]initial ) 1 g/L. For easiest comparison, the 29Si spectra of the T-Lite SF is superimposed to the spectra of pure PDMS. The intensity is normalized. chemical stability of the PDMS and Al(OH)3 protective layers was assessed during the T-Lite SF alteration with time (6 h, 24 h, and 48 h) and at several pH values (5, 7, and 9). The release of dissolved Si and Al species was used as a chemical indicator for the desorption/dissolution of the PDMS and Al(OH)3 coatings. For both Si and Al, the amounts in solution under the experimental conditions are slightly different and pH sensitive (Figure 2A). The release of Si coming from the desorption of the PDMS is time-dependent and sensitive to the illumination regime, whereas the dissolution of Al from the Al(OH)3 layer remains low even after 48 h (Figure 2B). The highest release of both Si and Al is observed in the dark at pH 5 with a maximum of 18 ( 2 mg/L of Si and 2 ( 0.2 mg/L of Al (Figure 2A). This corresponds to 90 ( 10 %wt of the initial Si content and 5 ( 0.5 %wt of the initial Al content of the T-Lite SF. Special attention was given to the T-Lite SF systems altered at pH 5 for which the dissolution of the Al-layer is the most important.

Characterization of the Protective PDMS Layer after Alteration of the T-Lite SF. We investigate changes in the structure of the polymeric layer remaining at the surface of the T-Lite SF by looking at the surface functional groups using ATR-FTIR and 29Si and 13C CP-MAS NMR. The 29Si NMR data of the nonaltered product reveal interesting features (Figure 3). The 29Si NMR resonance of the main chainSiO(CH3)2-repeat unit of the isolated polymer at -22 ppm (19) is detected on the signal of the T-Lite SF along with a broad peak centered at -55 ppm which is attributed to SiO-Al linkages with the underlying Al(OH)3 coating. Bandwidths and shoulders on the resonance indicate a nonunique binding environment. From peak area measurements, the proportion of Si linked to Al is estimated at 40%. Indeed, the presence of methyl groups throughout the PMDS structure allows a semiquantitative interpretation of the CP-MAS data. Upon alteration, the strong decrease in the 29Si NMR signal (see signal/noise ratio) is related to the release of Si from VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. ATR-FTIR spectra of the T-Lite SF after 6 h, 24 h, and 48 h of alteration in water under daylight (top) and in the dark (bottom). pH 5; [NaCl] ) 0.01 mol/L; 25 °C; [TiO2]initial ) 1 g/L. T-Lite SF. The PDMS remaining at the particle surface is significantly altered as evidenced by the relative peak intensities and areas showing a diminished contribution of the non-Al linked Si atoms. On the ATR-FTIR spectra (Figure 4), the characteristic band at 1258 cm-1 attributed to CH3 bending modes (20, 21) is clearly visible as a sharp peak on the T-Lite SF spectrum. However, the usually strong IR absorptions around 2906 and 2963 cm-1 (symmetric and asymmetric stretching of CH3) observed in pure PDMS are detected in the forms of weak peaks on the spectrum of the T-Lite SF. This surprising feature has been reported previously for PDMS adsorbed/deposited onto various surfaces (22–24) and is attributed to polymer orientation on the surface and layer thickness. The large absorption band from ca. 950-1100 cm-1 corresponds to Si-O-Si vibrations modes as described previously (20, 21). Upon alteration, the IR band at 1154 cm-1 corresponding to asymmetric stretching vibrations of oxygen atoms in Si-O bonds increases (25). The position of this new band, which further broadens the wide absorption region of the Si-O vibration, suggests an evolution toward more polymerized Si species (26). The same trend is observed with and without artificial daylight with different rates: after 24 h under illumination the 1154 cm-1 band is clearly visible, whereas it is only a shoulder in the dark. Upon alteration, the decrease of the δCH3 signal at 1258 cm-1 associated with the Si-CH3 units goes along with the increase of the CH3 and CH2 stretching bands between 2800 and 3000 cm-1 and their associated bending modes at 1458 and 1377 cm-1. This is attributed to the appearance of a newly formed organic phase, weakly linked to the surface since it is removed by additional rinsing (see vanishing CH3 and CH2 IR signals, Supporting Information). These findings are confirmed by the loss of the -0.9 ppm resonance of the 13C NMR spectra (Figure 3) attributed to the methyl group of the PDMS (27). The fate of the PDMS polymer on the particle is illumination-dependent. Alteration in the dark leads to a progressive disappearance of the IR vibrations associated with organics and a strong loss of the 13C NMR signal. In addition, small peaks at 21.2, 25.8, and 43.7 ppm 2692

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are detected and correspond to the aliphatic degradation products of the PDMS. The same peaks of degradation residue are detected with the particles aged under artificial daylight, but their intensity is over an order of magnitude higher representing around 100% of the signal. Characterization of the Protective Al(OH)3 Layer after Alteration of the T-Lite SF. 27Al NMR was used to investigate changes in the structure of Al(OH)3 layer. Two features are characteristic of the T-Lite SF before aqueous aging (Figure 5): a strong signal at σ ) 6 ppm located in the six coordinate chemical shift region of Al (Aloctahedral) and a peak at σ ) 63 ppm corresponding to Altetrahedral (28). After alteration in the dark or under daylight, a significant decrease of the signal at δ ) 63 ppm is observed. Before alteration ∼9% of the entire signal is attributed to Altetrahedral vs 2-3% after alteration. This highlights that 73 ( 6% of the Altetrahedral disappear during alteration. Characterization of the Nano-TiO2 Core and the Superoxide Generation Assessment. X-ray diffraction and XANES (X-ray absorption near edge structure) were used to study the structural behavior of the nano-TiO2 core after alteration of the T-Lite SF. Upon alteration, no significant differences are observed on the long- and short range order of the nano-TiO2 core (data not shown). We compared the superoxide production by altered T-Lite SF with a suspension of bare nano-TiO2. The physicochemical characterization of these pure nano-TiO2 particles is available in the Supporting Information. The absorbance data given in Figure 6 were converted into XTT-formazan production rates. The photoactivity of bare nano-TiO2 is obvious (Figure 6), and SOD completely suppressed the XTT-formazan production validating the generation of O2-•. However no significant amount of O2-• is generated in presence of altered T-Lite SF in our experimental conditions.

Discussion Nano-TiO2 is known for its efficient UV absorption and its ability to generate ROS such as O2-•, HO•, and H2O2 (29). To decrease the potential harmful effects due to their photoactivity, nano-TiO2 is coated. Reference 13 shows that many coated-TiO2 nanoparticles used in cosmetics formulations produce substantial lipoperoxidation. The authors highlighted a large variability in the efficiency of the coatings/ protective layers during a direct exposure but did not investigate the indirect exposure route. Nevertheless, an indirect exposure after aging of the material in aqueous media may be a significant contamination source. Soon after contact with water (pH 5, low ionic strength), the T-Lite SF becomes hydrophilic albeit initially hydrophobic due to the PDMS coating (Figure 1). While insoluble in water, the PDMS is rapidly degraded and/or desorbed during the T-Lite SF alteration as shown by the loss of its IR and NMR signatures. Up to 90 ( 10 %wt of the initial Si content is released into solution. The photochemical degradation of PDMS in the presence of oxygen has been largely discussed in the literature (30, 31). The surface silanol groups Si-OH are formed by abstraction of hydrogen followed by the reaction of the methylene radical Si-CH2 with oxygen and further rearrangement leading to [Si-O(OH)] structural units (20). Another possibility of the generation of [Si-O(OH)] is a bond break in the main chain of the PDMS (20). It is likely that similar PDMS degradation occurs at the surface of the T-Lite SF particles. The structure of the residual organic fraction on the surface of the particles appears identical irrespective of the illumination regime, and the relative diminution of the 29Si NMR signal of the -SiO(CH3)2- units suggests a shortened Si-O-Si backbone. However, the amounts of Si measured in solution (Figure 2) are higher in the dark than under artificial daylight. This may be a consequence of

FIGURE 5. 27Al NMR of the T-Lite SF after 48 h of alteration in water under daylight and in the dark. pH 5; [NaCl] ) 0.01 mol/L; 25 °C; [TiO2]initial ) 1 g/L.

FIGURE 6. Assessment of ROS generation by the altered T-Lite SF during 48 h under daylight at pH 5. XTT (2,3-bis(2-methoxy-4-nitro5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) combined with SOD (superoxide dismutase) were used to target and measure the O2-• anions. Measurements were performed at 470 nm. Bars indicate the standard deviation from the mean (n ) 3). accelerated degradation kinetics under artificial daylight leading to an enhanced precipitation of polymerized [Si-O(OH)] units (32) in solution or at the surface of the altered-T-Lite SF (solubility of silica is about 0.010-0.012%) (33). While the PDMS layer is strongly desorbed during aging, the Al(OH)3 layer is more stable. Only 5 ( 0.5 %wt of the initial Al content is dissolved leading to the release of 2 ( 0.2 mg/L of Al in solution. The theoretical solubility of Al(hydr)oxide is known to be low at pH 5 (for instance, the saturation index of gibbsite at 25 °C and 0.01 M NaCl is estimated about 0.26 µM at pH ) 5, 0.015 µM at pH ) 7, and 0.84 µM at pH ) 9), with slow dissolution kinetics ranging from 10-12 to 10-11 mol · m-2 · s-1 (pH ) 4.5) (34). All conclude that the release of 2 ( 0.2 mg/L of Al in our experiment is not only controlled by the solubility of the Al-based layer (35). However, the Al appears closely linked to the Si atoms at the surface of the particles via Si-O-Al bonds. Upon alteration 75% of the Altetrahedral signal disappeared (Figure 5), and significant amounts of Si are released from the particles (Figures 2 and 4). This suggests that the Al linked to the PDMS adopts a tetrahedral coordination. We hypothesize that the Altetrahedral is drawn into solution during the desorption of the PDMS. Nevertheless, the altered T-Lite SF do not generate superoxide in our experimental conditions, whereas bare nano-TiO2 do even at lowest concentration (Figure 6). The remaining Al-based layer at the surface after alteration prevents the chemical interactions between the Ti atoms of the surface of the nano-TiO2 core and the O2 and/or H2O

molecules from the solution. This inhibits the promotion of electron-/hole+ of the nano-TiO2 core and the ROS generation. This present study deals with short-term alteration experiments under mild conditions. Future work will include more aggressive alteration conditions (e.g., long-term alteration, stronger UV exposure, presence of organic mater, extended range of high ionic strengths, and pH values). This study needs to be considered as a first experimental approach examining the modifications of the physicochemical properties of nanomaterials in different exposure scenarios. This is a prerequisite for meaningful eco-toxicological investigations since changes in the surface properties of these nanomaterials will modify their dispersion and fate in aqueous environments (36).

Acknowledgments This program has been funded by the French national programs NANOALTER (INSU/EC2CO/CYTRIX) and AGING NANO & TROPH (ANR-08-CESA-001). The French Atomic Energy Commission (CEA) and National Center for Scientific Research (CNRS) are acknowledged for their support and funding of the international Consortium for the Environmental Implications of Nanotechnology. We thank Michel Guiliano and Jean-Paul Ambrosi for their respective help during the ATR-FTIR and ICP-AES measurements.

Supporting Information Available Additional ATR-FTIR spectra and physicochemical characterization of the bare TiO2 nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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