Article pubs.acs.org/est
Cite This: Environ. Sci. Technol. 2018, 52, 2863−2869
A Triple Functional Approach To Simultaneously Determine the Type, Concentration, and Size of Titanium Dioxide Particles Bin Zhao, Tianxi Yang, Zhiyun Zhang, Michael E. Hickey, and Lili He* Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: The large-scale manufacturing and use of titanium dioxide (TiO2) particles in food and consumer products significantly increase the likelihood of human exposure and release into the environment. We present a simple and innovative approach to rapidly identify the type (anatase or rutile), as well as to estimate, the size and concentration of TiO2 particles using Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS). The identification and discrimination of rutile and anatase were based on their intrinsic Raman signatures. The concentration of the TiO2 particles was determined based on Raman peak intensity. Particle sizes were estimated based on the ratio between the Raman intensity of TiO2 and the SERS intensity of myricetin bound to the nanoparticles (NPs), which was proven to be independent of TiO2 nanoparticle concentrations. The ratio that was calculated from the 100 nm particles was used as a cutoff value when estimating the presence of nanosized particles within a mixture. We also demonstrated the practical use of this approach when determining the type, concentration, and size of E171: a mixture that contains TiO2 particles of various sizes which are commonly used in many food products as food additives. The presence of TiO2 anatase NPs in E171 was confirmed using the developed approach and was validated by transmission electron micrographs. TiO2 presence in pond water was also demonstrated to be an analytical capability of this method. Our approach shows great promise for the rapid screening of nanosized rutile and anatase TiO2 particles in complex matrixes. This approach will strongly improve the measurement of TiO2 quality during production, as well as the survey capacity and risk assessment of TiO2 NPs in food, consumer goods, and environmental samples.
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INTRODUCTION Titanium dioxide (TiO2) is widely used as a white pigment in paints, coatings, papers, plastics, cosmetics, personal-care substances, and food products.1 TiO2 is often used as a sunscreen in cosmetic and personal-care products, constituting as much as 10% of the product mass.2 Food-grade TiO2 (E171) has been implemented into a number of food products as a whitening or anticaking agent.3 The U.S. FDA restricts the application of E171 in foods, capping the concentration at 1% of the food product’s overall weight. Although TiO2 nanoparticles (NPs) of this size are not added deliberately, it is reported that as much as 36% of the TiO2 particles found in E171 are equal to, or smaller than, 100 nm.2 The widespread application of TiO2 could unavoidably lead to human exposure and release to the environment. Modern studies continue to highlight the potential risks of TiO2 NPs to human health and the environment,4−7 serving as a major basis for public health concerns. Several techniques such as X-ray diffraction (XRD), electron microscopy (EM), and electrospray-differential mobility analyses (ES-DMA) have been used to characterize TiO2 particles (crystal type and size distribution).2,8,9 However, these techniques cannot be used routinely for the rapid © 2018 American Chemical Society
screening of large sample quantities. Single-particle inductively coupled plasma mass spectrometry (ICP-MS) can achieve sensitive size determination of TiO2 particles10 but the technique is limited by the need for complicated procedures and high-level expertise by the user. ICP-MS is also unable to differentiate between anatase and rutile TiO2 particles. Until now, no single technique has been able to rapidly and simultaneously determine the type (anatase or rutle), concentration, and size (nanosized or microsized) of TiO2 particles. Estimating the presence of NPs in a complex mixture system remains to be a great challenge. In a prior study, we developed a novel method to extract and detect TiO2 NPs from milk11 based on surface-enhanced Raman spectroscopy (SERS).12−16 Myricetin (MYC)a natural flavonoidwas used to modify the surface of TiO2 and assist in their extraction. The SERS signals of MYC were used to establish a standard curve for the concentration quantification of TiO2 NPs. In this study, we further developed Received: Revised: Accepted: Published: 2863
October 23, 2017 January 6, 2018 January 31, 2018 January 31, 2018 DOI: 10.1021/acs.est.7b05403 Environ. Sci. Technol. 2018, 52, 2863−2869
Article
Environmental Science & Technology
repeated three independent times to ensure reproducibility in data collection. The identification and discrimination of rutile and anatase were based on their intrinsic Raman signatures. The TiO2 particle concentrations were determined based on the standard curve which was constructed based on the highest intrinsic Raman peak and known concentrations. Particle sizes were estimated based on the ratio (R) between the Raman intensity of TiO2 and the SERS intensity of MYC molecules which were bound to the particles. The R calculated from the 100 nm particles was used as a cutoff value when estimating the presence of nanosized particles within a mixture. The developed approach was used to determine the type, concentration, and size of E171, which contains different sizes of TiO2 particles. DLS, SEM, and Statistical Analysis. All anatase and rutile TiO2 particles were characterized based on dynamic light scattering (DLS) and scanning electron microscopy (SEM). DLS analyses were achieved after ultrasonic treatment for 15 min when 800 μL of each TiO2 dilution (0.02 g/L) was measured with a Nano-ZS Zetasizer (Malvern Instruments). Ultrapure water served as the DLS matrix. Each SEM sample was prepared by dropping 5 μL of TiO2 suspension (0.2 g/L) onto a silicon base. Each sample was dried at room temperature prior to SEM imaging (JEOL JSM-6320F). A transmission electron microscopy (JEOL JEM-2000FX) was used to characterize E171. The TEM sample was prepared by dropping 10 μL of E171 suspension (0.2 g/L) onto copper grids coated by an unbroken carbon film and dried at room temperature, followed by TEM measurement. Statistical analyses for the size distribution of E171 was carried out using the ImageJ software. Ten TEM images were rendered for analyses wherein more than 100 particles were counted in each image.
the method to simultaneously identify and discriminate the type, concentration, and size of TiO2 particles using Raman spectroscopy and SERS. To the best of our knowledge, this is the first reported use of SERS for the rapid determination of the type, concentration, and size of TiO2 particles. The developed approach would be applied to monitor the quality of TiO2 during production, as well as to facilitate the survey and risk assessment of TiO2 NPs in foods, consumable substances, and environmental samples.
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MATERIALS AND METHODS Chemicals and Materials. All TiO2 particles, 30, 100, and 800 nm (nominal size provided by manufacturer) rutile and anatase TiO2 were purchased from US Research Nanomaterials Inc. Food-grade TiO2 (E171, purity 99%) was purchased from Precheza a.s. (Czech Republic). MYC was purchased from Quality Phytochemicals LLC. All other chemicals were purchased from Sigma-Aldrich (St. Louis, USA) unless stated otherwise. All aqueous solutions were prepared with ultrapure water (18.2 MΩ·cm) from the Thermo Scientific Barnstead Smart2Pure Water Purification System. Binding of MYC with TiO2 Particles. All TiO2 particle aqueous stock suspensions (1g/L), rutile TiO2, anatase TiO2, and E171, were prepared by dispersing TiO2 particles in ultrapure water (pH 5.4) by sonication (Branson 2800) for 15 min followed by dilution to a desired concentration with ultrapure water prior to use. One millimolar MYC solution was made with ethanol. For the binding of MYC on TiO2 particles, 100 μL of each TiO2 suspension (0.4 g/L) was mixed with an equal volume of MYC (1 mM), respectively. The final concentration of TiO2 was 0.2 g/L. After incubation for 1 h at room temperature with gentle shaking, the mixture of TiO2 and MYC was centrifuged at 17 000g for 5 min and the supernatant was discarded. The washing step was repeated three times to completely wash away MYC molecules that were not bound to TiO2 particles. The sediment was redispersed with 10 μL of ultrapure water. All above experimental steps are schematically illustrated in Figure S1 of the Supporting Information. To establish standard curves, a series of TiO2 nanoparticles (30 nm) at different final concentrations (rutile, 0.005−0.2 g/L; or anatase, 0.002−0.2 g/L) were tested as described above. For the study of particle concentration effects, 0.02 g/L TiO2 suspension was also tested in addition to the 0.2 g/L TiO2 suspension. Environmental analyses were performed using pond water that was collected from the UMass Campus Pond which was directly mixed with three types of TiO2 particles, including 0.2 g/L E171 and 0.02 g/L anatase and rutile TiO2 NPs (30 nm), without any further treatment, respectively. MYC was added to these spiked samples following the procedures described previously. The particles were then recovered from the redispersion. Raman Measurements. Two microliters of redispersed TiO2 bound with MYC was dropped onto a gold slide and airdried. The prepared samples were immediately measured using a DXR Raman microscope (Thermo Scientific, Madison, USA) equipped with a 780 nm laser and a 20× microscope objective. All Raman and SERS spectra were obtained with a 5.0 mW laser power and a 50 μm slit aperture for 2 s acquisition time. OMINC 9.0 software (Thermo Scientific) was used for Raman data acquisition and analyses. Five spots were randomly selected for each sample under the microscope and scanned with the range of 100−1900 cm−1. Each experiment was
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RESULTS AND DISCUSSION Size Identification of Rutile and Anatase TiO 2 Particles. Scanning electron microscopy (SEM) and dynamic light scattering (DLS) were used to confirm the particle sizes which were reported by the product manufacturers. As shown in Figure S2, the measured sizes of 30 and 100 nm anatase and rutile TiO2 were roughly consistent with claimed sizes by manufacturer. However, the anatase and rutile TiO2 with nominal size of 800 nm showed an actual size of 200 and 300 nm, respectively. DLS analyses indicated that the agglomerates of TiO2 particles were present in aqueous suspensions (Table S1). Characterization of Rutile and Anatase TiO2 NPs Using Raman and SERS. We first investigated Raman spectra of TiO2 NPs and SERS spectra of MYC adsorbed on 30 nm TiO2 particles to discriminate each type. As shown in Figure 1A, anatase TiO2 NPs exhibited intrinsic characteristic Raman peaks at 144, 396, 514, and 636 cm−1 (curve a). In contrast, rutile TiO2 NPs showed intrinsic Raman peaks at 607 and 450 cm−1 (curve c). Anatase and rutile TiO2 NPs could therefore be distinguished based on their intrinsic Raman signatures. We could also differentiate anatase and rutile TiO2 NPs based on the SERS spectra of flavonoid-adsorbed NPs. Flavonoids have a strong binding affinity to TiO2 NPs through coordination interactions among catechol groups with Ti atoms.17 MYC was selected as the binding ligand molecule based on our previous study which demonstrated its strongest binding affinity with anatase TiO2, when compared to other flavonoids like luteolin and quercetin.11 The binding affinity of 2864
DOI: 10.1021/acs.est.7b05403 Environ. Sci. Technol. 2018, 52, 2863−2869
Article
Environmental Science & Technology
adsorbed to anatase TiO2 NPs exhibited a series of SERS peaks which are characteristic of MYC at 1389, 1480, and 1615 cm−1 (curve b). The SERS peaks of MYC on rutile TiO2 NPs were observed at similar locations with a slight shift (curve d). The major SERS peaks of MYC at 1615, 1480 cm−1 (1482 cm−1 for rutile) and 1389 cm−1 (1365 cm−1 for rutile) were assigned to CO stretching motion in combination with C2C3 stretches, B-Ring CH in-plane bending, and A-Ring breath. The SERS effect from TiO2 NPs was attributed to the TiO2-tomolecule charge-transfer mechanism.18,19 These MYC peaks do not have any overlap with the intrinsic Raman peaks of TiO2 NPs. Compared with their own Raman spectra, both anatase and rutile TiO2 NPs bound with MYC display decreased signal intensity (14.3% for anatase based on its peak at 144 cm−1; 14.3% for rutile based on its peak at 450 cm−1), probably due to the washing step leading to the loss of particles. Nevertheless, we can still distinguish anatase and rutile NPs clearly with SERS spectra of MYC-adsorbed NPs. Establishing Standard Curves for Rutile and Anatase TiO2 Quantification. We tested a series of rutile and anatase TiO2 NPs (30 nm) concentrations, using MYC as a binding ligand, and obtained their respective SERS spectra. The Raman peak intensity of MYC-adsorbed rutile and anatase NPs was dependently related to the concentration of NPs (Figure S3). As low as 0.005 g/L rutile and 0.002 g/L anatase NPs could be detected. On the basis of the highest intrinsic Raman peak of the NPs, 450 cm−1 for rutile and 144 cm−1 for anatase, standard curves were then established for rutile and antase TiO2 quantification using a log 10−log 10 model (Figure 1B,C). Rutile TiO2 showed a strong linear response from 0.005 to 0.2 g/L and the limit of detection (LOD) was 0.005 g/L. LOD in this study was defined as the lowest detectable concentration of TiO2, whose Raman intensity was higher than the threshold that was equal to the blank sample (without TiO2) signal plus three standard deviations. The stand curve for rutile TiO2 quantification is log Y = 0.49 log X + 2.92, where X is the nominal concentration of rutile TiO2 NPs and Y is Raman intensity at 450 cm−1. The coefficient of determination is 0.987. Anatase TiO2 likewise exhibited good linear relationship from 0.002 to 0.2 g/L and the LOD was 0.002 g/L. The stand curve for anatase TiO2 quantification is log Y= 0.66 log X + 4.28, where X is the nominal concentration of anatase TiO2 NPs and Y is Raman intensity at 144 cm−1. The coefficient of determination is 0.968. Characterization of TiO2 Particles of Varied Sizes Using SERS. Three different sizes of rutile (30, 100, and 300 nm) and anatase (30, 100, and 200 nm) TiO2 particles were studied. The SERS spectra of rutile TiO2 particles with different sizes are shown in Figure 2A which were normalized according to the intensity of the TiO2 peak at 450 cm−1. All of the SERS spectra show both Raman peaks of rutile TiO2 at 607 and 450 cm−1 and SERS peaks of MYC at 1615, 1482, and 1365 cm−1. It was determined that the normalized intensity of SERS peaks of MYC at 1615, 1482, and 1365 cm−1 increased gradually with decreasing particle size. We further calculated the ratio between the Raman intensity of rutile TiO2 at 450 cm−1 and the SERS intensity of MYC at 1615 cm−1 and defined it as factor R. As shown in Figure 2B, R increased as particle size increased. For 30, 100, and 300 nm rutile TiO2 particles, R is 2.166 ± 0.144, 6.580 ± 0.461, and 11.019 ± 1.738, respectively. A strong correlation was found between factor R and the size of rutile TiO2 particles (Figure 2C). The regression equation is y = 3.7 ln(x) − 10.4 and its coefficient of determination is 0.999.
Figure 1. (A) Raman spectra of anatase (a) and rutile (c) TiO2 NPs. SERS spectra of MYC-adsorbed anatase (b) and rutile (d) TiO2 NPs. “M” next to the peaks indicates SERS peaks of MYC bound on rutile or anatase TiO2. “R” and “A” indicate intrinsic Raman peaks of rutile and anatase TiO2, respectively. Both anatase and rutile TiO2 NPs are 30 nm. (B) Calibration curve for the quantification of rutile TiO2 using a log 10−log 10 model based on the Raman peak of rutile TiO2 at 450 cm−1. (C) Calibration curve for the quantification of anatase TiO2 using a log 10−log 10 model based on the Raman peak of anatase TiO2 at 144 cm−1.
MYC was also challenged with multiple interferences to prove its ability to bind on the TiO2 in a complex matrix.11 MYC 2865
DOI: 10.1021/acs.est.7b05403 Environ. Sci. Technol. 2018, 52, 2863−2869
Article
Environmental Science & Technology
Figure 2. (A) Normalized SERS spectra of rutile TiO2 particles of different sizes, 300 (blue curve, R300), 100 (green curve, R100), and 30 nm (red curve, R30). The concentration of all three rutile particles was 0.2 g/L. (B) Histogram of factor R corresponding to different sizes of rutile TiO2 particles. CV: Cutoff value. (C) Plot of factor R versus size of rutile TiO2 particles.
Figure 3. (A) Normalized SERS spectra of anatase TiO2 particles of different sizes, 200 (blue curve, A200), 100 (green curve, R100), and 30 nm (red curve, R30). The concentration of all three anatase particles was 0.2 g/L. (B) Histogram of factor R corresponding to different sizes of anatase TiO2 particles. CV: Cutoff value. (C) Plot of factor R versus size of anatase TiO2 particles.
Figure 4. (A) Histogram of factor R for 30, 100, and 300 nm rutile TiO2 particles of high (0.2 g/L) and low (0.02 g/L) concentration. (B) Histogram of factor R for 30, 100, and 200 nm anatase TiO2 particles of high (0.2 g/L) and low (0.02 g/L) concentration.
the peak intensity of anatase TiO2 at 144 cm−1. By comparison, SERS peaks of MYC at 1615 and 1389 cm−1 on 30 nm TiO2 NPs were clearly observed, while it was found to emit a very weak peak for 200 nm microsized particles. We then calculated the factor R, the ratio of TiO2 peak intensity at 144 cm−1 to MYC peak intensity at 1615 cm−1, for 30, 100, and 200 nm anatase TiO2 particles (Figure 3B). The R for 30, 100, and 200 nm anatase TiO2 particles is 10.916 ± 0.427, 36.910 ± 1.213, and 56.849 ± 5.082, respectively. A strong correlation was
Because of its size dependence, R can potentially be used for the size discrimination and determination. Furthermore, the cutoff value (CV) of rutile TiO2 particles was determined to be 6.58 using R value of 100 nm rutile TiO2 NPs which may be used as a meaningful screening benchmark for the presence of rutile TiO2 NPs in a sample. We also demonstrated the capacity of SERS for the differentiation of nanosized and microsized anatase TiO2 NPs (Figure 3A). The Raman intensity was normalized according to 2866
DOI: 10.1021/acs.est.7b05403 Environ. Sci. Technol. 2018, 52, 2863−2869
Article
Environmental Science & Technology obtained between factor R and the size of anatase TiO2 particles (Figure 3C); as demonstrated by the regression equation, y = 21.9 ln(x) − 63.7, and the coefficient of determination, 0.997. The CV of anatase TiO2 particles was determined to be 36.91 using the R value of 100 nm anatase TiO2 NPs which can be used for screening the presence of anatase TiO2 NPs in a sample. The particles agglomeration status did not affect the correlation between the R value and the size of individual particles for either rutile or anatase TiO2. This was mainly due to the fact that our measurement was based on the surface modification and that centrifugation was used to concentrate the particles at the end. Therefore, the initial agglomeration did not impact the final results significantly. Evaluation of the Concentration Effect on R. The effects of particle concentration on R were further investigated for both rutile and anatase TiO2 particles (Figure S4). We selected 0.2 and 0.02 g/L as the models of high-concentration and low-concentration TiO2. Each SERS spectra was first normalized according to the intensity of the rutile TiO2 peak at 450 cm−1. Both high and low particle concentrations of rutile TiO2 expressed about the same SERS peak intensity for particle-adsorbed MYC regardless of particle size. R values were calculated based on the ratio between Raman intensity of rutile TiO2 at 450 cm−1 and SERS peak intensity of MYC at 1615 cm−1 for comparison (Figure 4A). It was clear that the R value for each particle size was almost the same, indicating the independence of particle size with respect to particle concentration. It was also demonstrated that R is independent of anatase particle concentration (Figure 4B). The strong correlation between the R-value and particle size, as well as the independence of R with respect to particle concentration, demonstrates the potential of this strategy when estimating the size of TiO2 particles. More sizes of particles are needed to be tested to establish more statistically convincing standard curves for size prediction. The accuracy of the prediction also depends on the size distribution: the more uniform the size distribution, the more accurate the prediction. In a mixture that contains particles of various sizes, the cutoff R valuebased on the 100 nm particlescan be used as a benchmark when determining the presence of the nanosized particles within the mixture. Application of the Approach for E171 Analysis. E171a mixture with different sizes of anatase TiO 2 particleswas selected as a model due to its wide use in many food products which could result in human exposure and entrance of this material into the environment. As shown in Figure 5, the SERS spectrum of MYC-adsorbed E171 particles clearly shows intrinsic Raman peaks of anatase TiO2 at 144, 396, 514, and 636 cm−1, indicating that E171 is composed of anatase TiO2 particles. On the basis of the intensity of the peak at 144 cm−1 (mean = 6390), the concentration was determined to be 0.191 g/L according to the standard curve that was established for anatase TiO2. This was consistent with the spiked value (0.2 g/L). The recovery is around 95.5%. SERS peaks of MYC which were enhanced by TiO2 particles were also observed at 1615, 1480, and 1389 cm−1. The factor R ratio of TiO2 peak intensity at 144 cm−1 to MYC peak intensity at 1615 cm−1was calculated and determined to be 31.949 ± 1.797. The R value of E171 was less than 36.91 (the CV value for anatase TiO2 particles), suggesting that E171 contains anatase TiO2 NPs. TEM confirmed the existence of 32%
Figure 5. SERS spectrum of MYC-adsorbed E171 particles. The spiked concentration of E171 was 0.2 g/L. R value for E171 was 31.949 ± 1.797. Inset: Representative TEM image of E171.
nanosized TiO2 particles in E171 (Figure 5, inset) in good agreement with a previous study.2 Application of the Approach for Analysis of TiO2 Spiked Pond Water. We further challenged the developed method by analyzing pond water that was spiked with three types of TiO2 particles, including E171, 30 nm anatase, and 30 nm rutile. As shown in Figure 6, characteristic Raman peaks of
Figure 6. TiO2 detection in pond water. (a) Raman spectra of pond water; (b−d) SERS spectra of MYC-modified E171, anatase, and rutile TiO2. The spiked concentration of E171 was 0.2 g/L. Thirty nanometer anatase and rutile TiO2 was spiked into pond water to prepare 0.02 g/L samples, respectively.
TiO2 were clearly observed for MYC-modified E171 (144 cm−1), anatase (144 cm−1), and rutile (450 and 607 cm−1) TiO2. On the basis of these Raman signatures of TiO2, the type of TiO2 in the sample could clearly be determined. In contrast, pond water itself exhibited negligible background signals. The mean concentration of E171 and anatase TiO2 was calculated to be 0.182 and 0.021 g/L, respectively, based on the intensity of Raman peak at 144 cm−1 according to the standard curve established for anatase TiO2 above. The percentage recovery value (% RV) of E171 and anatase TiO2 was (91.3 ± 7.9)% and (105.1 ± 9.1)%. The mean concentration of rutile TiO2 was determined to be 0.019 g/L based on the intensity of Raman peak at 450 cm−1 according to the standard curve established for rutile TiO2 above. The % RV of rutile TiO2 was (95.7 ± 6.5)%. We calculated R values based on the ratio between the 2867
DOI: 10.1021/acs.est.7b05403 Environ. Sci. Technol. 2018, 52, 2863−2869
Article
Environmental Science & Technology Raman intensity of TiO2 (anatase at 144 cm−1, rutile at 450 cm−1) and SERS peak intensity of MYC at 1615 cm−1. The mean R value of E171, anatase, and rutile TiO2 was 30.25, 11.12, and 2.02, respectively. These results are aligned with those of the standard samples which were tested in pure water (31.949 ± 1.797, 10.916 ± 0.427, and 2.166 ± 0.144), suggesting that the sizes of TiO2 particles within real samples were predicted successfully. The robust accuracy of this method, when screening real samples, was likely attributed to the strong and specific binding of MYC with TiO2 particles which were not interfered by complex components within the sample (e.g., sugar, protein, oil, and salt).11 Taken together, we have demonstrated the capability of the developed method for TiO2 analyses in real environmental samples. It was reported that the concentration of TiO2 nanoparticles was 13.6 ppm in the municipal sewage.20 For the food products, the concentrations of TiO2 were identified to be ranging from 20 to 9000 ppm.21 We also demonstrated the TiO2 in donuts could be easily detected using Raman technique.22 For animal model study, the anatase TiO2 nanoparticles were found in the caudal lung lobe of rats at a concentration of 1200 ppm.23 Although the sensitivity of the developed method is not as great as inductively coupled plasma-mass spectrometry (ICP-MS, 600 ng of Ti),23 5 mg/L (5 ppm) for rutile, and 2 mg/L (2 ppm) for anatase TiO2 are competent for the detection of TiO2 in the environment, food, and biological samples. In summary, we developed a novel approach for simple, fast (
