Core-Shell NaHoF4@TiO2 NPs: A Labeling Method to Trace Engineered Nanomaterials of Ubiquitous Elements in the Environment.

Understanding the fate and behavior of nanoparticles (NPs) in the natural environment is important to assess their potential risk. Single particle inductively coupled plasma mass spectrometry (spICP-MS) allows for the detection of NPs at extremely low concentrations, but the high natural background of the constituents of many of the most widely utilized nanoscale materials makes accurate quantification of engineered particles challenging. Chemical doping, with a less naturally abundant element, is one approach to address this; however, certain materials with high natural abundance, such as TiO2 NPs, are notoriously difficult to label and differentiate from natural NPs. Using the low abundance rare earth element Ho as a marker, Ho-bearing core -TiO2 shell (NaHoF4@TiO2) NPs were designed to enable the quantification of engineered TiO2 NPs in real environmental samples. The NaHoF4@TiO2 NPs were synthesized on a large scale (gram), at relatively low temperatures, using a sacrificial Al(OH)3 template that confines the hydrolysis of TiF4 within the space surrounding the NaHoF4 NPs. The resulting NPs consist of a 60 nm NaHoF4 core and a 5 nm anatase TiO2 shell, as determined by TEM, STEM-EDX mapping, and spICP-MS. The NPs exhibit excellent detectability by spICP-MS at extremely low concentrations (down to 1 × 10-3 ng/L) even in complex natural environments with high Ti background.

Introduction

The past few decades have witnessed significant advances in nanotechnology, from the controlled synthesis of nanomaterials to their applications in nanomedicine,[1,2] energy harvesting and storage,[3,4] and soil and water remediation.[5,6] Nanosafety and nanotoxicology have emerged as new research topics in response to increasing concerns regarding the potential adverse effects on humans and the environment exposed to nanomaterials intentionally, or inadvertently.[7,8] As one of the few nanoparticles (NPs) that have already been widely used in industry for decades, TiO2 NPs have been heavily produced for a wide range of applications, such as pigments, sunscreens, cosmetics, medical implants, self-cleaning surfaces, photovoltaics, photocatalysts, antifogging surfaces, and wastewater treatment.[9,10] Because of this prevalence, it is crucial to understand the fate of engineered TiO2 NPs in the environment to assess their risk and control pollution. Indeed, TiO2 NPs have been predicted to have the highest environmental occurrence of all engineered NPs, and have been found in treated wastewater, sewage sludge, surface waters, sludge-treated soils, and sediments.[11]

Environmental concentrations of Ti are strongly influenced by geogenic sources. In rivers with high concentrations of suspended matter (6.0–140.6 mg/L), the fraction of suspended Ti reached 62.3–88.6% (1.0–7.5 mg/g in terms of dry mass) with a strong correlation between the mass of suspended matter and the concentration of suspended Ti.[12] X-ray fluorescence spectrometry determined the Ti content of soil samples from Ti mining sites to range from 0.47 to 2.80%, but the Ti was found to be of geogenic origin with no anthropogenic input.[5] Though some efforts have been devoted very recently to discrimination of engineered TiO2 from natural Ti-bearing NPs by a multielement detection approach,[13] this high natural background makes quantification of released and bioaccumulated concentrations of engineered TiO2 NPs extremely challenging in the absence of some functionalization of the NPs to facilitate their discrimination. Labeling approaches proposed to date, for a range of NP compositions, have included radiolabeling,[14] stable isotope enrichment,[15] chemical doping with a low-abundance element,[16] or barcoding with DNA fragments.[17,18] Each of these potential approaches has advantages and challenges, with cost and the scale at which the NPs can be produced being the major drawback of all. For TiO2 NPs, stable isotope labeling with 47Ti has been successfully applied for detection of the bioaccumulation of NPs in zebra mussels (Dreissena polymorpha) exposed for 1 h at environmental concentrations via water (7–120 μg/L of 47TiO2 NPs) and via their food (4–830 μg/L of 47TiO2 NPs mixed with 1 × 106 cells/mL of cyanobacteria).[15] Chemical doping is a promising approach to achieve a large amount (grams compared to milligrams for radiolabeling) of labeled NPs at an affordable cost. However, introducing new cations into the lattice of host materials may alter their physical and chemical properties, even if the concentration of the dopant is low enough that the crystal structure remains unchanged.[19] It was reported that strong structural inhomogeneity, and even a phase transition, can be induced when there is a large difference in size between the substituted cations and the host cations.[20,21] A mixed-phase material, rather than a homogeneous solid solution, could be obtained because of unsuccessful doping. An alternative approach is to make a core of the tracer element surrounded by a shell of the material of interest. This core–shell approach is preferred for toxicological and environmental fate studies, because the material that comes into contact with the environment or living organisms will be the surface material and should be a close analogue of the undoped material, assuming that factors such as NP density are not significantly altered and appropriate crystal phase/morphology can be obtained.

ICP-MS was recently adopted as a means to detect NPs at ultra low concentration,[22] thanks to the capacity for element-specific analysis and the low detection limits (down to ng/L). However, ICP-MS fails to differentiate between engineered NPs composed of high abundance elements (e.g., Ti) and their natural counterparts.[23,24] To ensure that only engineered TiO2 NPs are identified in complex media, Ho core–TiO2 shell (NaHoF4@TiO2) NPs were synthesized with the low-abundance element Ho used as a chemical marker. The core–shell design was proposed to achieve a high dopant concentration for better detection while retaining the structural integrity of the NPs being investigated via the shell. NaYF4 has been intensively investigated as a host for up-converting fluorescent materials with tunable particle sizes being demonstrated through the use of small NPs acting as nucleating seeds.[25] As an analogue of NaYF4, it was expected that the size of NaHoF4 NPs could be similarly controlled, to achieve particles above the size limit for spICP-MS detection (i.e., >20 nm).[24] For this reason, NaHoF4 was selected as the marker core, although it is dissimilar from the TiO2 shell both in structure and in composition.

TiO2-coated NPs can be synthesized via a hydrothermal process,[26−29] or a sol–gel reaction on the NP surface,[30] and Caruso et al. even proposed a layer-by-layer method for coating TiO2 onto polymer NPs.[31] Unfortunately, the hydrothermal conditions, or ultralow concentrations, make these approaches unfavorable for large-scale synthesis, a prerequisite for NPs for environmental studies. A sol–gel approach derived from the Stöber method has also been reported recently for TiO2 coating;[32] however, this method was less effective for the coating of TiO2 onto dissimilar nanostructures[33] and a post-thermal treatment was needed to achieve a crystalline TiO2 layer.

In this work, we developed a templating method for the large-scale synthesis of NaHoF4@TiO2 core–shell NPs, in which a sacrificial Al(OH)3 layer was deposited onto the NaHoF4 NP surface and then etched by HF or other fluorinated species via the hydrolysis of TiF4.

Result and Discussion

The synthesis of NaHoF4@TiO2 NPs involved four steps (see the Experimental Section for further details). A typical procedure can be described briefly as follows. First, NaHoF4 core NPs were obtained by thermolysis of Ho(CF3COO)3 and NaCF3COO in a mixed solvent consisting of a high boiling point solvent, 1-octadecene, and a coordinative solvent, oleylamine (Figure A), adapted from the approach published for NaYF4 NPs and its analogues.[25] Al(OH)3 was then deposited onto the NaHoF4 NP surfaces to improve colloidal stability in polar solvents such as ethanol or water.[34] An aqueous solution of TiF4 was introduced to the NaHoF4@Al(OH)3 NPs dispersion along with polyvinylpyrrolidone (PVP, Mw = 360 000) before the ethanol/water solvent system was heated to 60 °C with stirring and maintained at this temperature for 24 h. A subsquent addition of ammonia–water was followed by reflux at 100 °C for 2 h. Finally, the white product was isolated by centrifugation, washed with ethanol and water, and kept in ultrapure water.

Figure 1

(A) A schematic illustration of the synthetic route to produce NaHoF4@TiO2NPs; and (B) X-ray powder diffraction pattern of NaHoF4@TiO2 NPs produced using an Al(OH)3 template. NaHoF4, JCPDS no. 00–049–1896; TiO2, JCPDS no. 01–075–2545.

The X-ray powder diffraction (XRD) pattern shown in Figure B confirmed the presence of hexagonal phase NaHoF4 and tetragonal phase (anatase) TiO2. Compared to the NaHoF4 core NPs, however, an obvious broadening effect was observed for the TiO2 phase, indicating a very small crystal size. Transmission electron microscopy (TEM) images in Figure revealed a rough NP surface after coating with TiO2, as well as an increase in the mean particle size from 61.4 to 68.6 nm. It was also noted that the size distribution of the NPs broadened, reflected by the fact that the standard deviation increased to 16.8 nm from 6.4 nm. Energy-dispersive X-ray (EDX) spectroscopy was utilized to further confirm the coexsitence of Ti and Ho (Figure S1, Supporting Information), as well as the core–shell structure of the NPs. Scanning transmission electron microscopy EDX (STEM-EDX) mapping allowed for the elemental distribution of the NPs to be determined. The elements from the NaHoF4 core particle (F, Ho and Na) were observed to be encompassed by the Ti and O from the external TiO2 shell (Figure A–F). In addition, more Ti was detected at the edges of the NP than in the core. Elemental line profiling was also done using STEM-EDX across a single core–shell NP to map its cross-sectional distibution of elements (Na, Ho, F, Ti and O). As observed in the elemental mapping, higher counts for Ti and O were detected at the periphery of the NPs (approximately 5 nm in thickness), while stronger signals from F and Ho from the NaHoF4 core were evident in the middle of the NP, clearly demonstrating that the NaHoF4 NPs were coated with a layer of TiO2 (Figure G–L). A thickness of ca. 5 nm for the TiO2 shell layer was consistent with the 9 nm increase in average particle size observed by TEM (Figure ). Note that despite the use of an Al(OH)3 template in this work, no Al was detected for the product of NaHoF4@TiO2 by EDX, as is evident from Figure .

Figure 2

(A) TEM image and (B) size distribution of NaHoF4 core NPs, (C) TEM image and (D) size distribution of NaHoF4@TiO2NPs. 331 NaHoF4 NPs were counted for size analysis, yielding a median size and average size of 61.4 nm. 214 NaHoF4@TiO2 NPs were counted for size analysis, yielding a median size of 69.7 nm and an average size of 68.6 nm.

Figure 3

(A–F) STEM-EDX spectroscopy maps of an ∼69 nm NaHoF4@TiO2 core–shell NP, confirming the coexistence of the Ho core and the TiO2 shell, and (G–L) STEM-EDXline profiles of a NaHoF4@TiO2 NP highlighting the elemental distribution of the Ho and F core with the TiO2 shell.

NaHoF4 NPs obtained in organic solvents could not be used directly for TiO2 coating via a hydrolytic approach in ethanol, because they were inevitably covered by oleyamine and were thus dispersible only in nonpolar solvents; therefore, surface modification was required to make them dispersible in polar solvents. Additionally, there is a lack of interaction between the hydrophobic organic layer of the NaHoF4NPs and the TiO2 crystallite, which is unfavorable for the heterogeneous nucleation of TiO2 on the NaHoF4 surface.[35,36] The deposition of an Al(OH)3 layer not only removes the surface bound oleylamine, but also imposes a highly positive surface charge onto the NaHoF4 NPs,[34] providing a stable colloid in ethanol with a concentration up to 2 mg/mL. Once the TiF4 solution was added into the NPs suspension, an external Al(OH)3 layer moves the equilibrium of hydrolysis toward the formation of Ti(OH)4 by reacting with the consequential HF or through anion exchange with TiF62–, TiF5– or other fluorinated species.[37] As a result, the Al(OH)3 layer was etched and a Ti gel formed around the NaHoF4NPs. Ammonia–water was subsequently introduced to catalyze the condensation of the Ti gel to form a TiO2 layer on the NP surface. PVP was then used to protect the newly formed NaHoF4@TiO2 core–shell NPs from potential aggregation. A decrease in the hydrodynamic size was observed by dynamic light scattering (DLS) after the condensation triggered by addition of ammonia–water (Figure S2), thus confirming the loss of the Al(OH)3 layer.

Some of the NaHoF4@TiO2 NPs showed a significantly different morphology and smaller size in comparison to the NaHoF4 core NPs before coating (Figure S3), and particle size analysis by TEM also showed a broader size distribution after coating with TiO2 (Figure ). These results led to a hypothesis that NaHoF4 NPs were not stable in the presence of H+ or Al3+ since these ions could break Ho–F bonds, resulting in the formation of H–F or Al–F bonds.[37] This is supported by their bond dissociation energies (Al–F 675 kJ/mol, H–F 569 kJ/mol and Ho–F 540 kJ/mol). Increasing the temperature or polarity of the solvent would encourage the dissolution of NaHoF4. Indeed, NaHoF4 NPs appeared to be less stable in dimethyl sulfoxide (DMSO) than in ethanol. Only TiO2 NPs were observed by TEM and a weak NaHoF4 signal was detected by XRD for the product when using DMSO instead of ethanol as the solvent during the shell formation stage and on increasing the synthesis temperature to 160 °C (Figures S4 and S5).

Because of the intrinsic mismatch of the NaHoF4 and TiO2 lattices, TiO2 tends to grow on the NaHoF4 NP surface via a granule mode to form a rough layer consisting of small particles, minizing the free energy of system. As shown in the TEM images (Figures C and 4), the NaHoF4@TiO2 core–shell NPs exhibited a rough surface after coating with TiO2. High-resolution TEM images revealed that the outer TiO2 layer is formed from small TiO2 NPs with a size less than 5 nm. This is also reflected by the broadening of the diffraction peaks in the XRD pattern (Figure B). The (011) facets of TiO2, with a d-spacing of 3.52 Å, were observed in HRTEM, and its corresponding diffraction peak at 25.3° appeared as the strongest peak in the XRD pattern, confirming the presence of anatase TiO2. The bandgap of NaHoF4@TiO2 was determined as 3.7 eV (Figure S6), slightly larger than the typical value of 3.20 eV for anatase TiO2, although this likely reflects the influence from the ultrasmall particle size of TiO2.

Figure 4

HRTEM images of NaHoF4@TiO2 core–shell NPs, showing the anatase phase of TiO2 at the surface of the core–shell NPs.

The formation of a core–shell configuration is not only thermodynamically dependent on the interfacial energy between the core and shell materials but also sensitive to kinetic factors including the reaction rate, temperature and the amount of NPs serving as crystal seeds. Because of a lower critical free energy, heterogeneous nucleation requires a lower chemical potential than homogeneous nucleation.[35,38] In other words, a higher concentration (supersaturation) of the soluble crystallite is needed for homogeneous nucleation. Therefore, there is a concentration window to form the hybrid material, above the critical level for hetergeneous but below the level for homogeneous nucleation. As one specfic example of a hybrid material, NaHoF4@TiO2 NPs are more likely to form if the concentration of TiO2 crystallite falls within this concentration window during the condensation process. Excess ammonia–water would lead to a fast condensation process, and a high concentration of TiO2 crystallite if there are not enough NaHoF4 NP seeds to consume them from the solution phase. Pure TiO2 NPs, instead of core–shell structures, would form as a result of an homogeneous nucleation. However, an insufficient amount of ammonia could not trigger the condensation or ensure a reasonable time scale for the reaction. Our results indicated that NaHoF4@TiO2 core–shell NPs can be obtained when the relative amounts of ammonia, ethanol and water are 2 mL:740 mL:60 mL, respectively. A mixture of NaHoF4 NPs and ultrasmall TiO2 particles was achieved if more than 2.5 mL of 35% ammonia–water was used (Figure S7). A colorless gel covered product was obtained if no or less than 1 mL of ammonia–water was added.

The challenges for synthesizing core–shell NPs on the scale required for field environmental fate experiments were thus to avoid the heterogeneous nucleation at a high concentration and the fact that a hybrid structure is thermodynamically less favorable than the formation of two separate homogeneous NPs. Unlike reactive Ti precursors, such as alkoxides, an elevated temperature or a high pH value is required to speed up the hydrolysis of TiF4[39] or to facilitate the crystallization of TiO2.. Currently, TiO2-coated materials with improved crystallinity are typically synthesized by a hydrothermal approach with an extremely low concentration to avoid the formation of unwanted pure TiO2 NPs at elevated temperatures[28,40] (Table ). In this study, the Al(OH)3 layer plays an important role when synthesizing NaHoF4@TiO2 NPs on a large scale. In addition to providing the NPs with excellent colloidal stability, it also serves as a sacrificial template to confine the hydrolysis and condensation process of TiF4 to within the space surrounding the NaHoF4NPs (Figure A), thereby helping to preclude the formation of pure TiO2 NPs. The rate of TiF4 hydrolysis was accelerated at a relatively low temperature (60 °C), without altering the pH value, because of the presence of the Al(OH)3 layer preventing the condensation process. No product was isolated after the reaction was held at reflux for 24 h, in the absence of ammonia–water, and a gel-like product was recovered by centrifugation at 6000g for 20 min after stirring at 60 °C for 24 h following adjustment to a pH of 4 (Figure S8), confirming the mechanism and the useful window for optimal core–shell NP synthesis.

Table 1

Summary of Synthetic Conditions and Scales for the Different Approaches Investigated for TiO2 Coating of the NaHoF4 Core NPs

entry	amount of core NPs	amount of ti precursor	conditions	ref
1	8.9 mg of Au	90 mL of 2.7 mmol/L TiF4 aqueous solution	hydrothermal at 180 °C for 48 h	(26)
2	30 mg of α-Fe2O3	30 mL of 5.4 mmol/L TiF4 aqueous solution	hydrothermal at 180 °C for 3 h	(27)
3	100 mg of NaYF4(Yb, Tm)	28 mL of 5.7 mmol/L TiF4 aqueous solution	hydrothermal at 180 °C for 3 h	(28)
4	10 mg of Cu2O	25 mL of 0.3 mmol/L TiF4 aqueous solution	hydrothermal at 180 °C for 0.5 h	(29,41)
5	2.7 mg of Ag	46 mL of 0.5 mmol/L Titanium tetraisopropoxide (TTIP) solution in ethanol	hydrolysis of TTIP in a mixture of H2O and ethanol at room temperature for few minutes	(29,30)
6	1.9 mg of polystyrene	2 mL of 0.125 wt %Titanium bis (ammonium lactato) di- hydroxide solution	multisteps involved for coating TiO2 on PS NPs, followed by calcining at 900 °C on N2 for 4 h then on O2 for 8 h	(30,31)
7	α- Fe2O3	100 mL of 40 mmol/L titanium butoxide solution in ethanol	stirring in ethanol for 18–24 h, followed by drying at 100 °C overnight and calcining at 500 °C for 2 h	(32)
8	SiO2
9	graphene oxide
10	Fe3O4
11	NaYF4(Yb, Tm)/ Fe3O4	20 mL of 7.7 mmol/L titanium diisopropoxide bis(acetylacetonate) solution in ethanol	Stirring at 25 °C for 24 h, followed by drying at 60 °C and calcining at 500 °C for 3 h	(33)
12	1600 mg of NaHoF4	800 mL of 31.3 mmol/L TiF4 aqueous solution	in ethanol/water (740:60) at 60 °C for 2 h, then at 100 °C for 18 h	this work

Large-scale synthesis of TiO2-coated NPs could be potentially achieved via a sol–gel approach,[32] despite the fact that a calcination process up to 500 °C is required to obtain a crystallized TiO2 phase, with the consequent risk that larger particle aggregates may form because of the sintering effect. This approach requires preliminary NP seeds to be colloidally stable in a basic environment, which makes it inapplicable for positively charged or polymer-coated NPs. Positively charged NPs would lose their stability with addition of the ammonia–water catalyst, whereas NPs stabilized by functional polymer could remain colloidally stable in a basic environment, but the polymer layer being tightly bonded to the surface may hinder the coating with TiO2 (Figure S9). PVP was used in this work, and it was expected to interact with NPs via weak van der Waals forces. Our results suggested that this losely bound polymer layer did not affect the TiO2 coating. Because of the weak interaction between PVP polymer and the NPs, PVP can be readily removed by washing with water or replaced by other coatings (especially under highly alkaline conditions), allowing the surface to be made more representative of the TiO2 particles used in food, commestics and other applications.[42,43]

As expected, the NaHoF4@TiO2 core–shell NPs indeed showed superior detectability on spICP-MS even in the presence of a high background of Ti up to the μg/L regime (Figure ). Our results indicated that the ionic Ti levels in river (canal) water is up to 300 ppb, 100 times higher than the amount present in ultrapure water. In addition, Ti-containing particles were also observed in blank river water using spICP-MS. However, no Ho-containing particles, and very limited ionic Ho (<0.5 ppb), were detected either in river water or ultrapure water (Figure S10). To simulate the measurement conditions in an environmental study (e.g, quantification of release, accumulation, environmental transformations, or presence in effluent (or sludge) following treatment in a wastewater treatment plant, for example), we diluted the NaHoF4@TiO2 NP suspension (stock concentration, ca. 1.5 mg/mL) 100 million times with ultrapure water or river (canal) water, yielding a particle concentration of ca. 20 471 NPs/mL or a mass concentration of Ho of 4.5 ng/L as measured by spICP-MS. With the low background of Ti and Ho in ultrapure water, spICP-MS exhibited an excellent capacity to detect both the Ti and Ho components of the NaHoF4@TiO2 NPs, and mathematically provided an equivalent mean size (from the equivalent spherical volumes) of 68.0 and 54.8 nm for the NaHoF4 core and TiO2 shell, respectively (Figure E, F) using the density of anatase TiO2 bulk material (3.9 g/cm3) and a calculated density for NaHoF4 (3.99 g/cm3, see calculation in the Supporting Information). As the number of Ho- and Ti-containing particles detected were comparable, we can assume that the Ho and Ti components detected by spICP-MS come from the same core–shell NPs. This leads to an overall NP size of 78.2 nm for the NaHoF4@TiO2 NPs (Figure S11, see calculation in the Supporting Information), and subsequently a thickness of 5.1 nm for TiO2 shell, which is very close to the value given by the size analysis of TEM images (4.2 nm) and by the element mapping by EDX (5 nm). Despite the slightly larger diameter achieved by spICP-MS than by TEM both for NaHoF4 (68.0 nm vs 61.4 nm) and for NaHoF4@TiO2 (78.2 nm vs 69.7 nm), the results obtained by these two methods are convergent, if taking into account the fact that an underestimated value could be given by size analysis on TEM due to the low contrast (electron density) of TiO2 and the nonspherical shape of particles, while an overestimated size could be yielded by spICP-MS if the actual density of the NaHoF4 core is higher than the calculated value.

Figure 5

spICP-MS results of NaHoF4@TiO2 NP dispersions in ultrapure (UP) water and river water. (A) Real-time Ho signal from NaHoF4@TiO2 suspension in river water; (B) real-time Ti signal from NaHoF4@TiO2 suspension in river water; (C) real-time Ho signal from NaHoF4@TiO2 suspension in ultrapure water; (D) real-time Ti signal from NaHoF4@TiO2 suspension in ultrapure water; (E) size distribution of NaHoF4 component detected by spICP-MS; and (F) size distribution of TiO2 component detected by spICP-MS. Stock suspensions of NPs were diluted 100 million times with ultrapure water and river water, respectively, from ca. 1.5 mg/mL to ca. 15 ng/L for spICP-MS measurements. River water was collected from the Worcester and Birmingham Canal, near the University of Birmingham, and was used without filtration.

Not unexpectedly, spICP-MS was no longer able to detect the Ti component of the NaHoF4@TiO2 NPs in river water, because of the much higher abundance of background Ti than in ultrapure water (Figure A and B); however, the Ho component of NaHoF4@TiO2 NPs was easily detectable in the river water. Real signal intensity did not show much difference in river water or in ultrapure water, in terms of the frequency (particle number) and the intensity of the Ho peak (particle size) (Figure C, D). More importantly, the size of the Ho component (in the form of the NaHoF4 core NP) detected under the different conditions (ultrapure water and river water) are the same, 68.0 nm (Figure E). A similar result was obtained across a wide NaHoF4@TiO2 NP concentration range both in ultrapure water and in river water (data now shown). In addition to the low abundance of the marker element (Ho), the long-term physical and chemical stability of the marker NP (NaHoF4) is also crucial, because the leaching of Ho would result in an underestimated value for TiO2 in the environmental samples. Only negligible ionic Ho (0.015 mg/mL) was detected in the suspension of NaHoF4@TiO2 NPs (ca 60–100 mg/mL), even after storage for over 14 months, which could be partially attributed to the core–shell structure, wherein the TiO2 shell provides a barrier to the release of Ho.

These results demonstrated that spICP-MS is a sensitive and reliable technique to monitor Ti-containing NPs in complex environmental samples using Ho as a marker. This strategy could be extended to spICP-MS detection of NPs containing other nanomaterials containing elements of high natural abundance such as iron or zinc.

Conclusion

A novel approach to gram-scale synthesis of NaHoF4@TiO2 core–shell NPs was achieved, as a new strategy to detect NPs containing elements of high natural abundance such as Ti in complex environmental samples by spICP-MS. The deposition of an Al(OH)3 layer around the Ho core was crucial for the synthesis of NaHoF4@TiO2 NPs, not only because of the excellent colloidal stability it provided in ethanol or water, but also because of the hydrophilic surface necessary for the effective TiO2 deposition and coating. More importantly, the Al(OH)3 layer acted as a sacrificial template which facilitated the separation of the hydrolysis and condensation of TiF4 and confined these processes to the immediate vicinity of the NaHoF4 NP surface, allowing for the deposition of the TiO2 shell onto the NaHoF4 NP surface. Even when using these approaches, the TiO2 grew in a particular mode to form a noncontinuous phase on the NaHoF4 NPs, which minimized the surface energy at the interface because of their mis-matching lattice energies, resulting in NaHoF4@TiO2 NPs with rough surfaces. Although they were dissimilar in structure, the affinity of Ti to F is very high such that strong chemical interaction between TiO2 and NaHoF4 was expected and observed.

Due to the Al(OH)3 layer, this approach allowed for the large scale synthesis of NaHoF4@TiO2 NPs, enabling their application in environmental studies of TiO2 NP fate and behavior. The core–shell structure was confirmed by high-resolution TEM and STEM-EDX mapping, as well as by spICP-MS. We demonstrated that these core–shell NPs remain detectable by spICP-MS in the presence of a high background of Ti despite the NPs being present at an extremely low concentration. The introduction of a low abundance element (Ho) as a tracer, without altering the structure of the particles, provided an effective solution for the detection of engineered TiO2 NPs in the environment. This methodology will benefit research in nanotoxicology and ecotoxicology, and could also be a potential solution to the challenges of detecting other engineered NPs of high abundance elements such as Zn and Fe in the environment.

Experimental Section

Materials

All chemicals and solvents were purchased from Sigma Aldrich and used without further purification. Ultrapure water (18.2 MΩ cm at 25 °C) was obtained from a MiliQ purification system. River water was collected from the Worcester and Birmingham Canal near the University of Birmingham (UoB), and used immediately after collection without filtration.

Characterization and Synthesis of the NPs

Unless stated otherwise, all characterization was performed at UoB. X-ray powder diffraction data was collected on a Bruker D8 advance diffractometer with a copper target (λ = 1.5406 Å, 40 kV, 30 mA). All samples were prepared by drying 0.5 mL of aqueous solution onto an Si zero background holder in air. The parameters for a typical experiment are as follows: starting angle (2θ), 20°; stop angle, 80; step size, 0.02026°; time/step, 0.8 s; no. of scans, 3030; time of scanning, 42 min and 25 s. Hydrodynamic size and zeta potential were measured on a Zetasizer Nano ZS ZEN 3600 from Malvern. Single-particle ICP-MS (spICP-MS) data were obtained on a PerkinElmer NexION 350X. Transmission electron micropscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy and elemental mapping were carried out at the Nanoscale and Microscale Research Centre, University of Nottingham on a JEOL2100F transmission electron microscope operating at 200 kV (field emission electron gun source, information limit 0.19 nm). EDX mapping was perfomed using an Oxford instruments XMax 80 T silicon drift detector with INCA Energy 250 Microanalysis system in conjunction with the JEOL digital STEM system.

Step 1: Synthesis of NaHoF4 NPs

Ho(CF3COO)3 was obtained by dissolving Ho2O3 in trifluoroacetic acid (ca. 30% w/w) at 90 °C followed by removal of solvent on a rotary evaporator to obtain a pink powder. Ho(CF3COO)3 (8 mmol, 4.0 g) and NaCF3COO (11.8 mmol, 1.6 g) were dissolved in a 250 mL round-bottom flask containing oleayamine (40 mL) and 1-octadecene (40 mL) before being heated to 120 °C for 30 min in vacuo. After flushing with N2 three times, 70 mL of the solution was removed by syringe and the rest of the solution in the flask was put on a preheated metal bath with stirring at 310 °C under an atmosphere of N2. The 70 mL aliquot was slowly injected back into the system over a 30 min period with continuous stirring under N2. Once all of the solution had been transferred, the temperature was lowered to 300 °C and the reaction system held at this temperature for 1 h before being cooled to room temperature. NaHoF4 NPs precipitated from the solution by the addition of ethanol (200 mL), and were isolated by centrifugation prior to their redispersion in hexane (300 mL).

Step 2: Synthesis of NaHoF4@Al(OH)3 NPs

To the NaHoF4 NP dispersion in hexane (300 mL), was added oleylamine (2 mL) with stirring at room temperature. The dispersion remained clear afterthe addition of a diethyl ether solution containing AlCl3(5 mL, 1 g/mL). After stirring for 10 min, water (5 mL) was added dropwise, and the clear dispersion became more and more opaque to form a white cloudy but stable colloid. NPs were precipitated out by addition of acetone (300 mL), and collected by centrifugation.

Step 3: Synthesis of NaHoF4@TiO2 NPs

The NaHoF4@Al(OH)3 NPs were dispersed in ethanol (740 mL) in the presence of PVP (4 g, Mw = 360 000). An aqueous solution of TiF4 (60 mL, 25 mmol) was quickly added into the ethanol dispersion of NaHoF4@Al(OH)3 NPs under stirring at 60 °C, resulting in a gradual color change from pink to a yellow-green within 5 min. After stirring at 60 °C overnight, the dispersion became slighly milky, indicating the formation of the Ti gel. The system was brought to reflux by heating to 100 °C. After the quick addition of ammonia–water (2 mL, 35% w/w), the solution became cloudy. A white product was achieved by centrifugation at 6000g for 30 min, which was subsequently washed with ethanol and water, and finally stored in water. The yield of NaHoF4@TiO2 NPs was calculated to be approximately 70% in terms of Ti.

spICP-MS Analysis of NaHoF4@TiO2 NPs

The NPs were diluted 100 million-fold using Milli-Q or canal water to obtain the final concentration for analysis. The dilution was chosen from a preliminary dilution test with dilution ranging from 10 000 to 10 000 000 000, from which it was found that 100 million-fold dilution brought the NP concentration to within 5000–200 000 particles mL–1 (this being the desired range for spICP-MS analysis). The elements within the NPs were analyzed sequentially, and the river (canal) water samples were run last in order to avoid any carryover effects that may occur due to the high background of Ti in the river water. The instrument was calibrated using PerkinElmer Setup Solution. Ti and Ho were calibrated using ionic solutions obtained from Aristar and PerkinElmer, prepared as a dilution series to form a calibration curve. Finally, the transport effiency was calculated using 20 and 40 nm gold NPs obtained from Nanocomposix and gold ionic solution from Aristar.