The inhibition of photocatalytic activity of rutile nanoparticles via surface encrustation with La(OH)₃ quantum dots

Abstract

Strategic materials for UV filters focus on creating substances that effectively block UV radiation while ensuring safety and stability. This study demonstrates the use of La(OH)₃ quantum dots (QDs) to modify the surface of rutile TiO₂ nanoparticles, aiming to reduce their photocatalytic activity under UV exposure, thus improving sunscreen safety. In-depth material characterisation techniques, including X-ray diffraction and transmission electron microscopy were carried out and UV filtering properties were confirmed through UV-visible spectroscopy and evaluation of photocatalytic activity (PCA). It was demonstrated that La(OH)₃ QDs modulated the surface properties of TiO₂, leading to lower PCA compared to uncoated TiO₂. The most effective composite contained TiO₂ modified with 5 wt% La(OH)₃ paving the way for its potential use in sunscreen applications.

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Introduction

Titanium dioxide, TiO₂, is successfully employed in sunscreen formulations due to both an inherent ability to scatter and reflect incident UV at particle sizes over 200 nanometres while being able to efficiently absorb incident UV due its semiconductor bandgap at nanoparticulate size‡.¹ However, untreated this absorption is the cause of the photocatalytic activity (PCA) of nano-TiO₂.² This property is fundamental to many applications but intuitively detrimental to the application to sunscreen because generation of reactive oxygen species (ROS) can damage skin cells, accelerate ageing^3–6 and damage DNA.^7,8

In Europe, the SCCS have developed guidelines for the safe inclusion of titanium dioxide nanoparticles in sunscreens.⁹ The guidelines are based upon two basic tenets: exclusion of anatase and elimination of photocatalytic activity. A limited range of commercial rutile nano-TiO₂ products are available that satisfy these SCCS requirements, and these are produced by either doping the rutile crystal structure (replacing titanium atoms with one of several compatible cations) or by surface treatment with an insulator (silica being the most common). Currently, continuously coated commercial nanoparticles contain 10–20 wt% oxides or hydroxides of materials such as Al, Si or Zr.^10,11

The quantum dots encrustation process is less complicated and cost-effective than the afore-mentioned methods. Furthermore, using quantum dots rather than continuous coating of the TiO₂ nanoparticle is advantageous in that it has little effect on absorption efficiency and choice of a suitable material should impart no colour. Lanthanum compounds are white, and therefore potentially well suited to this application^12,13 unlike ceria and bismuth, which lead to coloured products.^14,15 Therefore, in this work, surface modification of commercial rutile nanoparticles with lanthanum hydroxide, La(OH)₃, quantum dots is reported for the first time and has been shown to effectively mediate PCA with potential practical importance as a UV filter in sunscreens.

The results demonstrate that the La(OH)₃ QD-encrusted rutile TiO₂ nanoparticles exhibit significantly reduced photocatalytic activity compared to bare rutile TiO₂ while maintaining excellent UV absorption properties. This approach offers a promising strategy for developing safer and more effective UV filters in a cost-effective and industry-scalable methodology.

Materials and methods

Regents and materials

All chemicals were analytical grade from commercial sources. TiO₂ (rutile nanoparticles<100 nm), La₂O₃ (≥99.9%), 30% NH₄OH, 4-{bis[4-(dimethylamino)phenyl]methylidene}-N,N-dimethylcyclohexa-2,5-dien-1-iminium chloride (crystal violet), and ethanol were purchased from Sigma Aldrich. 69% HNO₃ was obtained from Merck.

Experimental methods

La(OH)₃ QD@TiO₂ nanocomposites were synthesised via a conventional precipitation technique to yield samples with three different concentrations of La/Ti: 5% w/w, 10% w/w and 20% w/w. Firstly, 200 mg of TiO₂ were suspended in 100 mL deionised (DI) water and dispersed by magnetic stirring and ultrasonication. The amount of La₂O₃ required to give the desired La/Ti ratio was dissolved in 20% HNO₃ with magnetic stirring at 60 ± 1 °C. The lanthanum solution was carefully added to the TiO₂ solution with stirring and then 1 M NH₄OH was added dropwise to this mixture until the pH was within the range of 8–9. The resulting precipitate was isolated by centrifugation, decanting and discarding the supernatant. In similar fashion, the product was washed three times with deionised water and twice with ethanol, each time by resuspension and centrifugation at 10 000 rpm for 5 min. The product was then dried at 85 °C overnight^13,16 and ground to a fine white powder using a mortar and pestle.

The product was characterised and tested using an array of typical analytical techniques as described in ESI.†

Results and discussion

The X-ray powder diffraction (XRD) patterns for each sample are collected and indexed using ICDD references for La(OH)₃ (ICDD reference 04-005-8587) and TiO₂ (ICDD reference 996-900-9084), as presented in Fig. 1(a). La(OH)₃ peaks are not significantly observed, as peaks may overlap with TiO₂, and the amount of La(OH)₃ present in the sample compared to TiO₂ is small and presents lower crystallinity than TiO₂ (see Fig. S1, ESI†).

Fig. 1 (a) XRD patterns, (b) UV visible spectra, (c) relationship between the relative concentration of CV and UV irradiation time of commercial TiO₂, P25 and 5, 10 and 20 wt% ^QDLa(OH)₃@TiO₂ nanocomposites, (d) average HaCaT cell viability following 24 h treatment with TiO₂ (rutile) and 10 wt% ^QDLa(OH)₃@TiO₂ at 25, 50 and 100 mg L⁻¹ and (e) upon 15 min simulated solar irradiation, expressed as percentage of viable cells with respect to IC₀ control (no UV).

Fig. 1(b) shows that commercial TiO₂ has a peak absorption at 335 nm, confirming it to be an effective UVB absorber. The composites exhibit lower UV absorbance and improved transparency in the visible region, possibly due to the reduced TiO₂ surface area exposed to the incoming radiation. A slight decrease in absorption with increasing La(OH)₃ can be explained by the results of the Brunauer Emmett–Teller (BET) surface area analysis (Table 1). However, as the concentration of La increases, the surface area decreases until 10 wt% La(OH)₃ and then increases again at 20 wt% La(OH)₃. This could be due to La(OH)₃ thoroughly coating the TiO₂ nanoparticle at higher concentrations and forming clusters on the surface. On the other hand, the selective absorbance in the UVB range remains consistent across the samples. The fluorescence spectra in Fig. S2 (ESI†) shows that the emission peak at 681 nm, is highest for pure TiO₂ with the intensity decreasing as La loading increases. This indicates that the surface encrustation with La(OH)₃ induces fluorescence quenching effects, which deactivates the excited state of TiO₂, inhibiting photocatalysis.¹⁷ The cytotoxicity of the commercial rutile nanoparticles and the surface-treated nanocomposite materials was determined using the MTT assay in the absence of simulated sunlight, as presented in Fig. 1(d). For assessment of phototoxicity of the materials, cell viability assays with HaCaT have been performed in the presence of simulated sunlight as presented in Fig. 1(e). A reduction in cell viability was observed with TiO₂ (rutile) nanoparticles or 10 wt% nanocomposite in the absence of radiation throughout the tested concentrations, suggesting cytotoxicity from possible oxidative stress-induced cell damage. However, no significant change in the cytotoxicity is observed with increasing concentration. Following 15 min exposure of HaCaT cells to simulated solar radiation, a concentration-dependent reduction in cell viability was observed. At low concentrations, the TiO₂ (rutile) nanoparticles tested here provided protection against UV radiation, which suggests that the UV absorption efficiency prevails over free radical formation. However, at higher concentrations, they induced cell death most likely due to ROS generation as a direct consequence of photocatalysis. On the other hand, with 10 wt% nanocomposites, the higher the concentration, the higher the cell viability suggesting that the treatment with La(OH)₃ supported the suppression of photocatalysis of TiO₂.

Table 1 Mean crystallite sizes, zeta potential, BET surface area, calculated bandgap energies, and photocatalytic kinetics for commercial TiO₂ and prepared composites

Sample	Crystallite size (nm)	Zeta potential (mV)	BET surface area (m^2 g^−1)	Bandgap (eV)	CV degradation (%)	Rate constant k (min^−1 × 10^−3)
P25	27.0 ± 3.0	NA	47.8 ± 0.5	2.68 ± 0.08	93.6 ± 0.5	65.4 ± 0.2
Commercial rutile	26.9 ± 0.8	1.1 ± 0.1	22.8 ± 0.8	2.52 ± 0.04	35.3 ± 1.1	6.7 ± 0.3
5 wt% ^QDLa(OH)3@TiO2	25.9 ± 2.2	5.6 ± 1.3	20.5 ± 0.9	2.42 ± 0.03	25.2 ± 2.4	4.8 ± 0.4
10 wt% ^QDLa(OH)3@TiO2	20.7 ± 2.6	3.6 ± 1.0	19.5 ± 0.7	2.51 ± 0.05	24.0 ± 0.9	4.6 ± 0.4
20 wt% ^QDLa(OH)3@TiO2	24.0 ± 0.5	5.6 ± 0.6	21.3 ± 0.6	2.56 ± 0.10	11.3 ± 1.6	1.8 ± 0.3

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The degradation of dyes has been previously ascribed to a pseudo-first-order reaction using the Langmuir–Hinshelwood model.¹⁵Fig. 1(c) shows that P25 exhibits the highest photocatalytic activity, followed by commercial rutile nanoparticles. A suppression of photocatalytic activity is observed for La(OH)₃ encrusted samples. P25, a common anatase-rutile composite UV filter has degraded the CV dye completely within the 60 minutes period. Comparatively, the commercial rutile has a 35% degradation rate. 5%, 10% and 20 wt% La(OH)₃ encrusted TiO₂ nanocomposites reduced photocatalysis by 10%, 11% and 24%, respectively. This reduction in photocatalysis may be attributed to the change in surface properties of TiO₂. As seen from Fig. 1(b), the reduced UV absorption can decrease the photoexcitation of electrons. Furthermore, the increased positive surface charge, indicated by increasing zeta potentials in Table 1, could improve the stability of the composite nanoparticles, influencing the interactions with other materials such that it hinders the adsorption of the organic molecules and reduces the efficiency of the degradation process. On the other hand, La(OH)₃ encrustation can lead to improved charge separation. This specific system could act as a recombination centre, inhibiting photogenerated electrons and holes and reducing overall photocatalysis. Furthermore, increased zeta potential reduces the agglomeration of nanoparticles, making stable homogeneous colloid systems possible. This is a desired quality in sunscreen formulations, as agglomeration can lead to a whiter appearance in the sunscreen when applied on the skin.

Electron microscopy was employed to visualise La(OH)₃ attachment to the TiO₂ surface (see Fig. S3, ESI†).

The SEM image in Fig. 2 shows evidence of the formation of QDs on the TiO₂ nanoparticle surface at 5 wt% of La(OH)₃.

Fig. 2 Scanning electron microscopy (SEM) images of (a) and (b) pure TiO₂ (c) and (d) 5 wt% ^QDLa(OH)₃@TiO₂.

The high-angle annular dark field (HAADF) image in Fig. 3(a) shows the crystalline TiO₂ structure and the amorphous nature of La(OH)₃ at the surface edge, further confirmed with the FFT inset. Fig. 3(b) shows the inverse FFT and profile of the selected area in (a) used to calculate the lattice spacing of the TiO₂ structure, which was found to be 0.32 nm and was well-aligned with corresponding d spacing for (110) plane of rutile. On the other hand, further verification was needed to identify the amorphous La(OH)₃. As seen from the HAADF image in Fig. 3(c), the overlapping region was used for EELS analysis. The EELS colour map with TiO₂ region in red and La(OH)₃ region in blue is presented in Fig. 3(c) inset. Accordingly, detailed EELS analysis in Fig. 3(d) shows identified Lanthanum N and M edges along with Ti L and O K edges for the selected region in Fig. 3(c), confirming the La(OH)₃ formation on specific areas on the TiO₂ surface.

Fig. 3 Microscopic analysis of the sample. (a) High magnification scanning transmission electron microscope (STEM) image with inset of fast Fourier transform (FFT) pattern, (b) inverse FFT and profile in the inset of the selected area in (a). (c) STEM image of overlapping regions with inset of electron energy loss colour map and (d) electron energy loss spectra (EELS) identifying lanthanum N and M edges.

Conclusion

In summary, this study demonstrated surface encrustation of commercial rutile nanoparticles with La(OH)₃ QDs via a cost-effective, industry-scalable method. The samples with a 5% and 10% (w/w) La/Ti ratio resulted in successful suppression of photocatalysis while also displaying good retention of UV absorbance. Hence, prepared nanocomposites show potential for application as UV filters in sunscreens.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge using facilities within the Australian National Fabrication Facility Node and Dr Lachlan Smillie's assistance at the UOW electron microscopy centre.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tb00114e

‡ At nanoparticulate size, TiO₂ is estimated to absorb >95% of incident UV, while scattering/reflecting <5%

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