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. 2026 Oct 5. Online ahead of print. doi: 10.1039/d6ra08092h

Preparation and anti-ultraviolet properties of vermiculite-supported zinc–titanium layered double hydroxide composites

Yiwen Shen a, Feng Ren a, Xinyu Yang a,✉, Yongjun Feng b,✉, Weiliang Tian a,b,✉
PMCID: PMC13635899  PMID: 42835449

Abstract

Excessive ultraviolet (UV) radiation exposure carries severe hazards to human health, creating an urgent demand for high-performance UV absorbers. Herein, vermiculite (VMT)-supported zinc–titanium layered double hydroxides (ZnTi-LDHs) were prepared. Five types of VMT-based organic–inorganic UV-absorbing composites were further constructed via intercalation assembly by introducing organic UV absorbers during the in situ growth of ZnTi-LDHs on VMT. The chemical structure, micromorphology, and UV-shielding capacity of the as-prepared materials were systematically characterized. The results reveal that bare ZnTi-LDHs display microspherical morphology with diameters of 1–3 µm, and feature porous frameworks with heterogeneous pore sizes, mainly distributed at 1.07 µm. Furthermore, ZnTi-LDHs are positively charged with a potential of +33.2 mV. For the five VMT-based organic–inorganic composites, ZnTi-LDH nanosheets grow perpendicularly onto VMT lamellae, yielding a compact, homogeneous three-dimensional network. When used as functional additives in sunscreen formulations, these composites exhibit outstanding UV absorption capability. UV-vis spectroscopic measurements record a maximum absorbance of 2.74. In ex vivo UV exposure assays using porcine skin, the sunscreen prolonged the onset of obvious skin discoloration to 380 min. Hydrophobicity tests also verify that all five sunscreen formulations possess hydrophobic characteristics. Collectively, these results demonstrate that the as-synthesized composites are highly promising candidates for cosmetic applications.

By incorporating organic UV absorbers during the in situ growth of ZnTi-LDHs on vermiculite (VMT), five VMT-based organic–inorganic UV-absorbing composites were constructed, ZnTi-BHA-LDHs/VMT exhibits the optimal UV absorption performance.graphic file with name d6ra08092h-ga.webp

1. Introduction

With the rapid advancement of industrialization and urbanization, the continuous depletion of the atmospheric ozone layer has led to a significant increase in surface ultraviolet (UV) radiation.1 This not only accelerates material aging and ecological imbalance but also markedly raises the risk of human diseases such as skin cancer and photoaging.2–4 In light of the limitations of traditional organic and inorganic anti-UV materials in terms of safety, stability, and user experience, developing novel, efficient, stable, and biocompatible UV protection systems has become a hotspot in scientific research.5–7

Layered double hydroxides (LDHs) have garnered widespread interest among researchers due to their excellent UV blocking properties.8–11 However, current studies on the UV absorption applications of LDHs predominantly focus on MgAl-LDHs or ZnAl-LDHs types, while the ZnTi-LDHs exhibit superior UV absorption performance have not attracted sufficient attention from the research community.12–15 Moreover, incorporating VMT into LDHs can further enhance the UV absorption capability of the material. VMT is a typical 2 : 1 phyllosilicate with permanent negative layer charges, enabling efficient cation exchange and adsorption of cationic species. In contrast, LDHs represent a class of clays with positively charged host layers, which favor the intercalation and adsorption of anionic compounds.16,17 Based on the principle of positive and negative charge attraction, VMT and LDHs possess the conditions for combination.18 By growing LDHs on VMT sheets via hydrothermal crystallization, secondary functional materials can be constructed, thereby expanding the application field of VMT as a novel material. Tian et al.19 employed an in situ epitaxial growth technique to prepare MgAl-layered double hydroxides (MgAl-LDHs) on both the outer and interlayer surfaces of natural mineral VMT. The synthesis of MgAl-LDHs/VMT was achieved through lattice matching between VMT and MgAl-LDHs. Compared with conventional MgAl-LDHs, the hierarchical-structured MgAl-LDHs/VMT exhibits superior adsorption and regeneration performance for Cr(vi). This enhancement is attributed to its porous three-dimensional framework, which significantly increases the specific surface area and provides efficient transport channels.

Combining organic and inorganic ultraviolet absorbers to prepare organic–inorganic hybrid UV-absorbing materials can not only prevent direct contact between organic absorbers and human skin but also effectively mitigate the strong photosensitivity associated with inorganic absorbers, thereby delivering superior sun protection performance. Li et al.20 employed an anion exchange method to intercalate and assemble the organic ultraviolet absorber para-aminobenzoic acid (PABA) into the interlayers of ZnTi-LDHs, successfully constructing an organic–inorganic nanocomposite material, ZnTi-PABA-LDHs. The study indicates that while maintaining high crystallinity, the interlayer confinement effect of this composite significantly enhances the thermal stability of PABA and effectively suppresses the catalytic activity of ZnTi-LDHs. These characteristics suggest that ZnTi-PABA-LDHs holds promising application potential in the field of safe sunscreen materials. Chai et al.21 employed an anion-exchange method by reacting a 2-naphthylamine-1,5-disulfonic acid solution with a ZnAl-LDHs nitrate precursor, achieving the ordered intercalation of an anionic ultraviolet absorber into the interlayers of LDHs. XRD and TG characterizations revealed that the intercalated product exhibits significantly enhanced thermal stability, confirming the reinforcing effect of the LDHs interlayer confinement structure on the thermal stability of the ultraviolet absorber.

This study successfully prepared ZnTi-LDHs/VMT by introducing titanium element into the layered structure of LDHs, and systematically analyzed its unique UV absorption performance. Furthermore, an intercalation assembly method was employed to incorporate various common organic UV absorbers, such as salicylic acid (BHA), benzoic acid (BZA), ethylhexyl methoxycinnamate (EHM), 2-hydroxy-4-methoxybenzophenone (HMB), and phenolphthalein (PPN), into the interlayers of the LDHs, thereby constructing a series of VMT-based organic–inorganic composite UV-absorbing materials. By comparing their UV absorbance, the superior UV absorption performance of this class of materials was revealed. And we have also explored the potential application of these VMT-based organic–inorganic UV-blocking materials in the field of sunscreen cosmetics.

2. Experiment

2.1. Materials

The raw VMT was obtained from Xinjiang Yuli Xinlong Vermiculite Co., Ltd. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), titanium tetrachloride (TiCl4), hydrochloric acid (HCl), ammonia solution (NH3·H2O), and urea (NH2CONH2) were supplied by Beijing Chemical Co., Ltd. Glyceryl monostearate (C21H42O4), 2-hydroxybenzoic acid (BHA), benzoic acid (BZA), and phenolphthalein (PPN) were obtained from Shanghai Shanpu Chemical Co., Ltd. Ethylhexyl methoxycinnamate (EHM) and 2-hydroxy-4-methoxybenzophenone (HMB) were obtained from Shanghai McLean Biochemical Technology Co., Ltd. Glycerol (C3H8O3), stearic acid (C18H36O2), and ethyl alcohol (C2H6O) were purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. Disodium ethylenediaminetetraacetate (EDTA-2Na) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. Camel oil was supplied by Alar Silk Road Camel Team Agricultural Science and Technology Development Co., Ltd.

2.2. Synthesis of materials

2.2.1. Preparation of the VMT suspension

20 g of VMT was subjected to high-temperature expansion at 900 °C for 5 min. The obtained expanded VMT was washed five times with distilled water to remove impurities such as mica and sand. The cleaned expanded VMT was dried in an oven at 80 °C for 12 h. Then, 5 g of the dried expanded VMT was added to a beaker containing 1000 mL of distilled water and sheared using a high-speed shear homogenizer at 8000 rpm. The shear treatment was performed for 10 min, followed by ultrasonication for 10 min. The shear and ultrasonication steps were alternately repeated for a total duration of 4 h. The final VMT suspension was reserved for further use.

2.2.2. Preparation of ZnTi-LDHs/VMT

The TiCl4 solution was prepared firstly, 110 µL of 1 mol per L HCl solution was transferred into a 5 mL centrifuge tube, followed by 110 µL of TiCl4 solution. The mixture was shaken until the foam disappeared, resulting in a pale-yellow liquid. The same procedure was repeated five times to obtain approximately 1.1 mL of TiCl4 solution for later use. This step was performed to suppress the hydrolysis of TiCl4.

The specific preparation method of ZnTi-LDHs/VMT is as follows: 0.22 mL of the prepared TiCl4 solution, 2.38 g of Zn(NO3)2·6H2O, and 3.0 g of urea were dissolved in 50 mL of distilled water and stirred for 10 min. Then, 30 mL of the VMT suspension was added, and stirring was continued for another 10 min. The mixture was then transferred to Teflon-lined stainless-steel autoclave and heated at 120 °C for 24 h to achieve hydrothermal crystallization. After the reaction, the autoclave was allowed to cool naturally to room temperature. The precipitate was collected by centrifugation and washed six times alternately with water and ethanol. Finally, the solid product was dried overnight at 60 °C to obtain ZnTi-LDHs/VMT samples. The preparation flow chart is shown in Fig. 1.

Fig. 1. Schematic diagram of the ZnTi-LDHs/VMT preparation.

Fig. 1

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2.2.3. Preparation of VMT-based organic–inorganic UV-blocking materials

Solution A was prepared as follows: by dissolving 0.22 mL TiCl4, 2.38 g Zn(NO3)2·6H2O, and 3.0 g urea in 50 mL distilled water, then, 30 mL of the VMT suspension was added, followed by stirring for 10 min. Solution B was prepared by dispersing 0.32 g of PPN in 10 mL of absolute ethanol under ultrasonication. After mixing Solution A and Solution B, stirring was continued for another 10 min. The obtained mixture was then transferred into Teflon-lined stainless-steel hydrothermal autoclave and maintained at 120 °C for 24 h. After cooling naturally to room temperature, the precipitate was collected by centrifugation and washed six times alternately with deionized water and ethanol. Finally, the product was dried overnight at 60 °C to obtain the ZnTi-PPN-LDHs/VMT. Following the same procedure, ZnTi-BHA-LDHs/VMT, ZnTi-BZA-LDHs/VMT, ZnTi-EHM-LDHs/VMT, and ZnTi-HMB-LDHs/VMT samples were prepared by adding 0.14 g of BHA, 0.12 g of BZA, 0.29 g of EHM, and 0.23 g of HMB, respectively, during the preparation of Solution B.

2.2.4. Preparation of sunscreen samples

The formulation of the sunscreen is presented in Table S1, and the steps for sunscreen preparation are as follows: 7 g of camel oil, 2 g of glyceryl monostearate, and 3 g of stearic acid were placed in a beaker and heated in an 80 °C water bath until melted, the mixture was stirred rapidly for 5 min. Then, 1 mL of emulsifier and 1 mL of glycerol were added to the molten mixture, followed by continued stirring for another 5 min. Subsequently, 1 g of EDTA-2Na was dissolved in 10 mL of 80 °C distilled water and slowly dripped into the obtained mixture. While stirring, approximately 50 mL of distilled water was gradually added, and the mixture was cooled to room temperature to obtain the base cream. During the base cream preparation process, 5 wt% of VMT nanosheets, ZnTi-LDHs, or the UV-blocking materials synthesized in this study was added. The mixture was stirred for 15 min, after which heating was discontinued. When the temperature dropped to 30 °C, 0.1 mL of 10% methylparaben solution was added, and stirring until the mixture reached room temperature to obtain the final product.

2.3. Characterization method

The ultraviolet absorption curves of the sample powder and dispersion solution were measured using the UV-8000 s spectrophotometer (Shanghai Metash), where the scanning range was set to 200–800 nm with a scanning slit width of 0.1 nm. XRD analysis was conducted using a Bruker D8 diffractometer (Germany), with a scanning rate of 5° min−1. Morphology was characterized by SEM (Zeiss Sigma 300, Germany). The functional groups of the prepared samples were analyzed using a mid-infrared spectrometer (PerkinElmer Frontier, USA). The particle size distribution of the prepared samples was determined using a laser particle size analyzer (Liaoning Dandong Bettersize Instruments Ltd, China) and deionized water was used as the dispersion medium. The zeta potential was measured using a Malvern Zetasizer (ZEN3700), the pH of the tested solutions was adjusted to 7, and the concentration was 0.1 g L−1 in deionized water.

2.4. Ex vivo porcine skin UV test

The UV protection performance of the samples was evaluated using an ex vivo porcine skin model under a 40 W UV lamp (ZF-20D). The lamp provided UV irradiation in the wavelength range of 320–400 nm. The distance between the UV lamp and the surface of the porcine skin was maintained at 15 cm. The samples were uniformly applied to the exposed porcine skin at a dosage of 2 mg cm−2, with an effective exposed area of 1 cm2. Untreated porcine skin was used as the control. The skin specimens were continuously irradiated, and changes in skin color were recorded at predetermined time intervals. The UV protection time was defined as the irradiation time required for the first visually detectable discoloration of the porcine skin. Each sample was tested independently in triplicate, and the average value was reported.

3. Results and discussion

3.1. Characterization

Fig. 2a clearly displays the layered structure of expanded VMT. The uniform interlayer expansion resulting from heating at 900 °C provides an optimal foundation for the subsequent exfoliation process. The inset optical photograph shows that the expanded VMT resembles leeches in macroscopic morphology, with lengths approximately 1 cm.22Fig. 2b illustrates the exfoliated VMT, which exhibits a nanosheet-like structure with all dimensions below 1 µm. The corresponding inset photograph further demonstrates that these VMT nanosheets are densely and uniformly dispersed in solution.23 As shown in Fig. 2c, the synthesized ZnTi-LDHs possess a microspherical morphology, with diameters ranging from approximately 1 to 3 µm.24 The corresponding inset photograph reveals that these microspheres are composed of interconnected nanosheets, more importantly, the ZnTi-LDHs nanosheets form pore structures with varying sizes, which can serve as pathways for mass transport. After compositing ZnTi-LDHs with VMT nanosheets. As shown in Fig. 2d, the ZnTi-LDHs and VMT can be clearly distinguished by their different surface morphologies. The ZnTi-LDHs exhibit a nanosheet-like morphology and are distributed in close association with the VMT substrate, while exposed VMT regions are also observed. Based on the elemental mapping images in Fig. S1, the distribution of C, Mg, and Si derived from the VMT component is uniform. Similarly, Ti and Zn from the ZnTi-LDHs phase are also homogeneously dispersed. These results indicate that the ZnTi-LDHs/VMT functional material can be successfully fabricated via the in situ growth method.25 Moreover, as presented in Table S2, all characteristic elements corresponding to VMT and LDHs are detected. The Zn2+/Ti4+ atomic ratio is approximately 4 : 1, which aligns well with the stoichiometric ratio employed for the synthesis of ZnTi-LDHs.

Fig. 2. (a) SEM image of expanded VMT (the inset shows an optical image of the expanded VMT); (b) SEM image of exfoliated VMT nanosheets (the inset shows an optical image of the VMT nanosheets suspension); (c) SEM image of ZnTi-LDHs (the inset shows a higher-magnification SEM image of ZnTi-LDHs); (d) SEM image of ZnTi-LDHs/VMT.

Fig. 2

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Fig. 3 shows the SEM images of ZnTi-LDHs/VMT intercalated with different organic compounds, including ZnTi-BHA-LDHs/VMT, ZnTi-BZA-LDHs/VMT, ZnTi-EHM-LDHs/VMT, ZnTi-HMB-LDHs/VMT, and ZnTi-PPN-LDHs/VMT. It can be observed that all five organic–inorganic UV-blocking materials exhibit a vertically oriented sheet-like structure.26 These ZnTi-LDHs nanosheets grow vertically on the VMT flakes, forming a flower-like architecture. From the high-magnification SEM images, it is evident that between the vertically aligned layers of ZnTi-LDHs, some granular substances are present, this might be the result of morphological changes caused by organic intercalation.27

Fig. 3. SEM images of (a) ZnTi-BHA-LDHs/VMT, (b) ZnTi-BZA-LDHs/VMT, (c) ZnTi-EHM-LDHs/VMT, (d) ZnTi-HMB-LDHs/VMT and (e) ZnTi-PPN-LDHs/VMT; (f–j) are the corresponding high-magnification images.

Fig. 3

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Fig. 4a presents the XRD patterns of VMT, ZnTi-LDHs, and ZnTi-LDHs/VMT. The diffraction peaks at 8.86°, 17.76°, and 26.74° are assigned to the (003), (006), and (009) crystal planes of VMT, respectively.28 The peaks at 35.91°, 45.33°, and 59.56° correspond to the (0012), (0015), and (218) crystal planes of the inorganic matrix. For bare ZnTi-LDHs, the intense, sharp (003) characteristic reflection with symmetric full width at half maximum emerges at 13.2°. The (006) and (009) reflections are located at 23.92° and 34.81°, respectively, demonstrating the well-crystallized nature of the LDH structure. Apart from the major diffraction peaks of ZnTi-LDHs, weak reflections near 22° and 31° can be ascribed to trace Zn(OH)2 impurity in the sample. The XRD pattern of ZnTi-LDHs/VMT shows a smooth, stable baseline and retains the main characteristic peaks originating from both ZnTi-LDHs and VMT, verifying that ZnTi-LDHs were successfully loaded onto the VMT substrate. Fig. 4b shows the FTIR spectra of VMT, ZnTi-LDHs and ZnTi-LDHs/VMT. In the spectrum of VMT, the absorption bands at 3427 cm−1 and 1645 cm−1 correspond to the O–H stretching vibration and H–O–H bending vibration, respectively. Bands observed at 1000 cm−1, 680 cm−1, and 450 cm−1 are attributed to the stretching vibrations of Si–O–Si, Si–O–Al, and Si–O–Mg bonds. In the ZnTi-LDHs spectrum, peaks at 1502 and 1383 cm−1 correspond to the vibrations of interlayer carbonate anions in the LDH structure. These results further confirm the successful loading of ZnTi-LDHs onto VMT.29Fig. 4c shows the XRD patterns of the five organic–inorganic UV-blocking materials, the (003), (009), and (0012) crystal plane peaks of VMT and the (003), (006), and (009) crystal plane peaks of ZnTi-LDHs can be found in the XRD patterns of each material. In addition, the corresponding d(003) interplanar spacing of the LDH was calculated using Bragg's law (nλ = 2d sin θ), where n is the diffraction order (n = 1), λ denotes the wavelength of Cu Kα radiation (0.15406 nm), d is the interplanar spacing, and θ is the Bragg diffraction angle. The (003) reflection of pristine ZnTi-LDHs/VMT appears at 2θ of 13.2°, giving a d(003) spacing of 0.670 nm. Bycontrast all five organic–inorganic composites display their (003) reflection at 2θ of 12.54°, corresponding to a calculated d(003) value of 0.705 nm, an increase in the layer spacing indicates the insertion of organic molecules. Fig. 4d depicts the FTIR spectra of the UV-blocking materials. The absorption peaks at 3427 cm−1 correspond to the vibrations of structural hydroxyl groups. Characteristic vibration peaks of the Si–O–Si, Si–O–Al, Si–O–Mg appear at 1000 cm−1, 680 cm−1, and 450 cm−1, respectively. Additionally, the peaks at 1502 cm−1 and 1383 cm−1 are from the vibrations of interlayer carbonate anions in the LDHs structure and the intercalated organic molecules. Collectively, these results confirm the successful intercalation of the organic UV absorber into the interlayer space of the LDH.

Fig. 4. (a) XRD patterns of VMT, ZnTi-LDHs, and ZnTi-LDHs/VMT; (b) FTIR spectra of VMT, ZnTi-LDHs, and ZnTi-LDHs/VMT; (c) XRD patterns and (d) FTIR spectra of samples.

Fig. 4

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As shown in Fig. 5a, the particle size distribution of the VMT dispersion ranges from 60 nm to 340 nm, with a dominant peak around 204 nm. The particle size of ZnTi-LDHs is mainly distributed at 1.07 µm. The particle size of ZnTi-LDHs/VMT falls within 230 nm to 800 nm. The small particle sizes of all three materials indicate that the particle size distribution of the ZnTi-LDHs/VMT composite lies between that of VMT and ZnTi-LDHs, indicating the successful formation of a structurally integrated new system through their combinations. As shown in Fig. 5b, VMT exhibits a negative charge with a potential of −38.2 mV, while ZnTi-LDHs are positively charged with a potential of +33.2 mV. ZnTi-LDHs/VMT also shows a negative charge, with a zeta potential of −21 mV. The relatively high absolute zeta potential values indicate good colloidal stability of their respective dispersions.

Fig. 5. (a) Particle size diagram of VMT, ZnTi-LDHs and ZnTi-LDHs/VMT; (b) zeta potential diagram.

Fig. 5

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3.2. Comparison of the ultraviolet absorption properties of ZnTi-LDHs/VMT

Fig. 6a shows the solutions and corresponding solid powders of VMT, ZnTi-LDHs, and ZnTi-LDHs/VMT, respectively. The solutions of VMT, ZnTi-LDHs, and ZnTi-LDHs/VMT are homogeneous. When irradiated with a laser pointer, a bright light path appears in each solution, demonstrating the Tyndall effect, which confirms the uniform and stable nature of the three solutions. It can also be observed that the VMT powder exhibits a pale-yellow color, the ZnTi-LDHs powder appears pure white, and the ZnTi-LDHs/VMT powder displays a light gray hue, indicating that the color is changed after the growth of ZnTi-LDHs on VMT. All three powders are fine in texture, which is beneficial for their subsequent application in sunscreen and cosmetic formulations. Based on the UV absorption spectra shown in Fig. 6b, it can be observed that VMT exhibits light absorption across the wavelength range of 200–700 nm, with relatively strong absorption in the UV region (200–400 nm) and weaker absorption in the visible region (400–760 nm). The UV absorption spectrum of ZnTi-LDHs indicates that its absorption is mainly concentrated in the UV region, with higher absorbance values than VMT in this range. The spectrum of ZnTi-LDHs/VMT shows enhanced UV absorption within the 200–400 nm range, demonstrating a greater UV blocking effect compared to pure ZnTi-LDHs. Moreover, ZnTi-LDHs/VMT still retains noticeable absorption in the visible region, which can be attributed to the presence of VMT, as VMT itself possesses absorption ability in the visible range. Additionally, the pale-yellow color of VMT contributes to its partial shielding effect against visible light.

Fig. 6. (a) Optical photographs of VMT, ZnTi-LDHs, ZnTi-LDHs/VMT solutions and their corresponding solid powders; (b) UV absorption spectra of the VMT, ZnTi-LDHs, and ZnTi-LDHs/VMT.

Fig. 6

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Fig. S2 presents the UV absorption spectra of ZnO, TiO2, and ZnTi-LDHs/VMT, and their corresponding band gap plots.30 The absorbance of ZnTi-LDHs/VMT in the UV region is consistently higher than that of ZnO and TiO2. This enhancement is attributed to the strong UV shielding effect of the layered double hydroxide structure, which significantly increases absorption intensity below 400 nm. Additionally, it can be observed that ZnTi-LDHs/VMT has a lower band gap value of 2.88 eV, which is lower than that of ZnO (3.26 eV) and TiO2 (3.15 eV). It should be noted that band-gap magnitude alone cannot determine photon-absorption efficiency, because the real light-absorbing performance is jointly governed by multiple factors including micro-morphology, specific surface area, crystal defects and light-scattering effects. In our system, the enhanced UV-shielding capability arises from the combined contributions of band-structure modulation induced by Ti4+ doping and the hierarchical three-dimensional architecture of the VMT-supported composite.31

Fig. S3 shows the UV absorption spectra of MgAl-LDHs, ZnAl-LDHs, and ZnTi-LDHs/VMT and their corresponding band gap plots. ZnTi-LDHs/VMT exhibits strong UV absorption not only in the visible light region but also significantly higher UV absorbance in the UV region compared to MgAl-LDHs and ZnAl-LDHs. This enhancement is attributed to the introduction of titanium, which promotes photon absorption and facilitates electron transitions within the material. Furthermore, the band gap of ZnTi-LDHs/VMT is also lower than that of MgAl-LDHs (5.13 eV) and ZnAl-LDHs (4.87 eV).

3.3. Analysis of ultraviolet absorption properties of organic–inorganic UV-blocking materials

Fig. 7 shows the particle size distribution of the prepared organic–inorganic UV-blocking materials. Approximately 90% of the particle sizes of ZnTi-BZA-LDHs/VMT, ZnTi-EHM-LDHs/VMT, and ZnTi-HMB-LDHs/VMT are below 1 µm, while about 90% of the particle sizes of ZnTi-BHA-LDHs/VMT and ZnTi-PPN-LDHs/VMT are below 2 µm. All five materials exhibit small particle sizes. Such characteristics provide a skin-friendly, non-greasy feel when applied in sunscreen cosmetics, making them desirable for practical use.

Fig. 7. Particle size diagrams of samples.

Fig. 7

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Fig. 8 shows the UV absorption spectra of ZnTi-LDHs/VMT intercalated with different molar amounts of organic compounds. As seen in Fig. 8a, ZnTi-HMB-LDHs/VMT exhibits strong UV absorption in the range of 200–400 nm and high transparency in the visible region, effectively blocking UV radiation while allowing transmission of visible light beneficial to human health, thus meeting physiological requirements.32 Similar trends are observed in Fig. 8(b–e), indicating that these materials possess excellent UV-blocking performance and are suitable for cosmetic applications. From Fig. 8b, it can be noted that when the amount of BZA increases from 0.001 mol to 0.0015 mol, the UV-blocking performance improves significantly, while further increase in BZA content leads to a gradual enhancement in UV absorption. Fig. 8c demonstrates that as the amount of PPN increases, the UV-blocking performance gradually rises overall, and the absorption range of ZnTi-PPN-LDHs/VMT is broadened. Fig. 8d shows that increasing the amount of EHM from 0.001 mol to 0.002 mol results in a gradual improvement in UV absorption. However, when the amount is increased from 0.002 mol to 0.0025 mol, the enhancement is marginal, whereas a further increase from 0.0025 mol to 0.003 mol leads to a more pronounced improvement, indicating a dose-dependent effect. EHM is widely used in sunscreens due to its non-toxicity, colorlessness, odorlessness, and good skin compatibility.33,34 According to Fig. 8e, the UV-blocking performance of ZnTi-BHA-LDHs/VMT increases in a gradient manner with the addition of BHA, showing a relatively wide variation range, suggesting favorable potential for practical application in sunscreen formulations.

Fig. 8. The UV absorbance spectra of ZnTi-LDHs/VMT intercalated with different molar amounts of organic compounds (a) HMB; (b) BZA; (c) PPN; (d) EHM; (e) BHA.

Fig. 8

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3.4. Performance evaluation of organic–inorganic UV-blocking materials

The synthesized UV-blocking materials were incorporated into a base sunscreen cream at a concentration of 5 wt% for UV-Vis spectrophotometric testing. Fig. S4 presents the sunscreen samples prepared with different types of UV-absorbing materials. Samples A and B correspond to sunscreens containing 5 wt% ZnTi-BZA-LDHs/VMT and 5 wt% ZnTi-PPN-LDHs/VMT, respectively. Visually, both samples appear white and uniform in color. Samples C, D, and E are sunscreens containing 5 wt% ZnTi-HMB-LDHs/VMT, 5 wt% ZnTi-EHM-LDHs/VMT, and 5 wt% ZnTi-BHA-LDHs/VMT, respectively; these three samples are not pure white but exhibit light gray tones that gradually deepen. The corresponding UV absorption spectra of the five sunscreen samples reveal that all samples exhibit good light transmittance in the visible region while also showing high absorbance values in the UV region.

As listed in Table 1, the maximum UV absorbance values of the five sunscreens are 1.39, 1.94, 2.21, 2.51, and 2.74, respectively. The sunscreens were centrifuged at 2000 rpm for 10 min to evaluate their stability. No phase separation was observed in any of the five formulations, indicating good centrifugal stability. Subsequently, thermal stability was tested by placing the samples in an oven at 40 °C for 24 h. After cooling to room temperature, all sunscreens showed no significant changes and no oil–water separation, demonstrating good heat resistance.35,36 Finally, cold stability was assessed by storing the samples at −10 °C for 24 h and allowing them to return to room temperature. No noticeable changes or water exudation were observed, confirming good low-temperature stability.37 In summary, all five sunscreen formulations exhibited excellent performance in stability tests, combined with high UV absorbance, suggesting that they would provide effective sun protection in practical applications.

Table 1. Properties for the sunscreens containing 5 wt% of different UV-blocking materials.

Sample A λ max Centrifugal stability Thermal stability Cold stability
A 1.39 No phase separation No oil–water separation No water exudation
B 1.94 No phase separation No oil–water separation No water exudation
C 2.21 No phase separation No oil–water separation No water exudation
D 2.51 No phase separation No oil–water separation No water exudation
E 2.74 No phase separation No oil–water separation No water exudation
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Contact angle measurements were performed on the prepared base sunscreen cream and the five sunscreen formulations to evaluate their hydrophilic/hydrophobic properties. Fig. S5a shows the contact angle of the base sunscreen cream, which is 0°, indicating that the base cream is hydrophilic and therefore suitable for individuals with hydrophilic skin. Fig. S5(b–f) present the contact angles of sunscreens containing 5 wt% of ZnTi-BZA-LDHs/VMT, ZnTi-PPN-LDHs/VMT, ZnTi-HMB-LDHs/VMT, ZnTi-EHM-LDHs/VMT, and ZnTi-BHA-LDHs/VMT, respectively. All five sunscreen formulations exhibited substantially enhanced hydrophobicity, with water contact angles of 129°, 131°, 134°, 135°, and 136°, respectively.

The prepared sunscreens were applied in a practical test to evaluate their ultraviolet blocking performance. Two pieces of porcine skin with similar thickness and color were selected for each group. One piece was coated with the sunscreen formulated in this study, while both were exposed under a 40 W UV lamp. The color change of the skin was monitored, and the time until a noticeable color change occurred was recorded. Each group was tested three times for error analysis. As shown in Fig. 9a, the distinct color change on sunscreen-coated porcine skin under UV irradiation is appeared. Fig. 9b shows the color change occurred in the pig skin at approximately 20 min in the blank control group without any sunscreen application. Group A to C correspond to skins coated with sunscreens containing 5 wt% of VMT, ZnTi-LDHs, and ZnTi-LDHs/VMT, respectively, the recorded times for noticeable color change from white to light yellow were 170 min, 210 min, and 270 min, respectively.

Fig. 9. (a) The vitro evaluation of the VMT, ZnTi-LDHs and ZnTi-LDHs/VMT sunscreen tested on porcine skin; (b) color change time of pig skin. A: sunscreen with 5wt% VMT, B: sunscreen with 5wt% ZnTi-LDHs, C: sunscreen with 5wt% ZnTi-LDHs/VMT.

Fig. 9

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Fig. 10 illustrates the time required for noticeable color changes to occur on porcine skin coated with different sunscreens under UV lamp exposure. Group A serves as the blank control group, Groups B to F correspond to skins coated with sunscreens containing 5 wt% of ZnTi-BZA-LDHs/VMT, ZnTi-PPN-LDHs/VMT, ZnTi-HMB-LDHs/VMT, ZnTi-EHM-LDHs/VMT, and ZnTi-BHA-LDHs/VMT, respectively, the recorded times for noticeable color change were 230 min, 270 min, 330 min, 360 min, and 380 min, respectively. The relatively superior performance of ZnTi-BHA-LDHs/VMT may be related to the molecular structure of BHA. BHA contains an aromatic ring with phenolic hydroxyl and methoxy substituents, which can contribute to UV absorption through electronic transitions and provide antioxidant activity. When combined with the UV-screening effect of ZnTi-LDHs and the layered VMT framework, these characteristics may contribute to enhanced UV attenuation and consequently delay the UV-induced color alteration of porcine skin. The higher the UV absorbance, the longer the skin UV protection time. Enhanced UV absorbance of the composites generally translates to prolonged skin UV protection, as a greater proportion of incident UV radiation is attenuated before reaching the skin surface. These results demonstrate that the application of sunscreen provides significant protection to porcine skin by effectively absorbing UV radiation, thereby achieving excellent UV-blocking performance. However, there are several limitations in the present work. Only the porcine skin model was adopted for the UV protection test, long-term skin irritation and in vivo safety evaluations have not been performed. Moreover, the test environment can not fully mimic real outdoor conditions.

Fig. 10. Color change time of pig skin (A: blank, B to F correspond to sunscreens containing 5 wt% of ZnTi-BZA-LDHs/VMT, ZnTi-PPN-LDHs/VMT, ZnTi-HMB-LDHs/VMT, ZnTi-EHM-LDHs/VMT, and ZnTi-BHA-LDHs/VMT).

Fig. 10

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4. Conclusions

In this study, a series of sunscreen formulations based on VMT, ZnTi-LDHs, and ZnTi-LDHs/VMT composites were systematically developed and comprehensively characterized. The sunscreens based on ZnTi-LDHs/VMT composites exhibited significantly enhanced performance. Taking advantage of the interlayer anion tunability of LDHs, five organic–inorganic UV-blocking materials were further prepared via intercalation assembly, namely ZnTi-BZA-LDHs/VMT, ZnTi-PPN-LDHs/VMT, ZnTi-HMB-LDHs/VMT, ZnTi-EHM-LDHs/VMT, and ZnTi-BHA-LDHs/VMT. These organic–inorganic UV-blocking materials were incorporated into a base sunscreen at a concentration of 5 wt% to obtain novel sunscreen formulations. The five sunscreen formulations exhibited maximum UV absorbance values of 1.39, 1.94, 2.21, 2.51, and 2.74, respectively. In ex vivo UV-protection tests using porcine skin, the time required for a noticeable color change reached 230, 270, 330, 360, and 380 min, respectively. Moreover, the as-prepared composites endow sunscreen coatings with high water contact angle, showing hydrophobic surface characteristics that are favourable for mitigating water-induced wash-off. Collectively, these results indicate that the ZnTi-LDHs/VMT composites successfully integrate the structural stability of VMT with the excellent UV-shielding capability of ZnTi-LDHs. Although, the prepared composite shows great potential as a UV-shielding candidate material for sunscreen-related research. Further biosafety assessments are required before practical cosmetic application. For future research, further work will focus on optimizing the loading amount of organic UV absorbers, performing systematic photostability and biocompatibility tests, carrying out real-skin assessments, and exploring the incorporation of these ternary composites into commercial sunscreen matrices toward practical applications.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-OLF-D6RA08092H-s001

Acknowledgments

This work was supported by the Nanjiang Technology Project (2023AB028); President's Fund Major Project Cultivation Special (TDZKZD202502); Science and Technology Program (No. 2024DA012); Talent Development Plan for Top Scientific and Technological Leading Talents Project (XJRC-2025-BTKJ-PY-KJLJ-007), and the presidential research fund of Tarim University (TDZKSS202127).

Data availability

All data generated or analyzed during this study are included in this published article.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra08092h.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA08092H-s001
RA-OLF-D6RA08092H-s001.pdf (1.5MB, pdf)

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra08092h.

Articles from RSC Advances are provided here courtesy of Royal Society of Chemistry

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