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. 2025 Dec 31;11(1):1114–1124. doi: 10.1021/acsomega.5c08376

PEG Surface Modification of Microcrystalline Cellulose@Nanotitanium Dioxide Core–Shell Composite to Improve Sunscreen Performance

Huizhong Yu †, Junxian Xie †,‡, Lan Yao ‡,*, Haitao Yang †,‡,*
PMCID: PMC12809339  PMID: 41552543

Abstract

Polyethylene glycol (PEG) is commonly used to prevent particle aggregation of nanotitanium dioxide (nano-TiO2) through steric hindrance. The particle aggregation of microcrystalline cellulose@nano-TiO2 core–shell sunscreen composite (MCC@nano-TiO2) makes the powder difficult to grind and also boosts the probability of nano-TiO2 peeling off during grinding. It is essential to inhibit agglomeration by surface treatment to avoid adverse effects in cosmetic applications. First, the structural and UV protection capability of the composite was investigated before and after the complete coverage of MCC by nano-TiO2. Then, different amounts of PEG were added in the later stages of composite preparation to fix PEG onto the particle surfaces. Systematic examinations were then conducted to investigate the effects of PEG modification on structure and properties. Results reveal that at an MCC:nano-TiO2 mass ratio of 60:40, a 6% PEG modification significantly enhances the dispersibility and UV protection by altering the nano-TiO2 shell structure. Notably, PEG is exposed on the particle surface, which markedly increases UV reflection and scattering, thereby extending the UVA protection range. The critical absorption wavelength (λc) was 376 nm. PEG modification also minimizes agglomeration and potential release of nano-TiO2 particles. This study lays a foundation for the industrialization of MCC@nano-TiO2 and provides new ideas for the surface modification of core–shell nano-TiO2 materials.

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1. Introduction

Nanotitanium dioxide (nano-TiO2) exhibits excellent UV absorption capability and is widely used in sunscreen cosmetics. The physicochemical properties of nano-TiO2 depend on the material’s crystal form, size, morphology, and microstructure. Effectively and accurately controlling the crystal form and morphology of nano-TiO2 has always been a key focus in nano-TiO2 material research. , Rutile nano-TiO2 is stable at high temperatures, not easily transformed or decomposed, and has a high refractive index, strong UV reflection and scattering capabilities, and a broad protection range. Its photoactivity is weaker than that of anatase under aerobic conditions, and it exhibits better photochemical stability. Therefore, rutile nano-TiO2 is mainly used in the field of cosmetics. , Additionally, when the crystal form of nano-TiO2 is a mixed phase with rutile and anatase, a relative content of rutile (WR) greater than 70% demonstrates better photochemical stability. −

Agglomeration can affect the morphology and structure of nano-TiO2, which in turn influences its performance and stability of nano-TiO2 in sunscreens. Therefore, nano-TiO2 usually needs to be surface-treated or combined with dispersants to improve its dispersibility and mitigate the adverse effects of agglomeration in cosmetic applications. Common surface treatment methods can be divided into inorganic and organic coatings. Inorganic coatings mainly involve depositing a layer of oxides, such as silica, alumina, and zirconia, on the nano-TiO2 surface to enhance photostability, reduce photocatalytic activity, and prevent agglomeration. Organic coatings primarily involve the physical adsorption and chemical bonding of organic molecules, such as silane coupling agents, titanate coupling agents, and surfactants, on the surface to improve compatibility with the dispersion medium and prevent agglomeration. The surfactants commonly include anionic, nonionic, and cationic types. Anionic and cationic surfactants mainly prevent particle agglomeration by providing surface charges, such as cetyltrimethylammonium bromide (CTAB). Nonionic surfactants mainly prevent particle aggregation through steric hindrance, such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG).

PEG is a macromolecular polymer that changes its physical form from a viscous colorless liquid to a waxy white solid as its molecular weight increases. The PEG modification of inorganic nanoparticles mainly takes advantage of its hydrophilicity, steric hindrance effect, biocompatibility, and functionalization capability. Introducing a PEG layer onto the surface of nanoparticles can enhance their dispersion and stability, and also provide interfaces for further functionalization. − PEG contains both hydrophilic and hydrophobic groups. The terminal hydroxyl groups (−OH) of PEGs can be selectively oxidized, thereby enabling PEG to link with a variety of functional end groups. This enhances the adaptability of nanoparticles in various applications.

PEG is commonly used for the surface functionalization of nano-TiO2. PEG molecular chains extended in a serpentine manner in aqueous solutions, readily forming hydrogen bonds with the surface −OH groups of hydroxide colloids. PEG first underwent physical adsorption on the particle surface by polyvalent supramolecular interactions. This physical adsorption can facilitate the subsequent chemical adsorption. The adsorption of PEG on the particle surface formed a hydrophilic layer, which played a role in steric hindrance. Zhang et al. found that PEG-coated nano-TiO2 had good dispersibility in water. Moreover, as the molecular weight of PEG increased, the dispersibility and stability were enhanced, and the particle size decreased. The steric hindrance increased with the elongation of PEG molecular chains, and the number of PEG molecules coating the TiO2 surface was reduced. Furthermore, the PEG molecular chains on the TiO2 surface can passivate its surface activity, hindering the transfer of photogenerated electrons and reactive oxygen species, thereby reducing its photovoltaic and photocatalytic properties. ,

Moreover, steric hindrance affected TiO2 phase transformation. , Zhou et al. prepared nano-TiO2 with PEG-1000 and PEG-6000. PEG-1000 produced pure rutile, while PEG-6000 yielded anatase–rutile mixed crystals. Higher TiCl4 concentration increased rutile proportion but kept crystallite size smaller than the PEG-1000 group. PEG-1000 formed a “serpentine” monolayer, while PEG-6000 formed a thicker coating, inhibiting phase transformation and preventing crystallite aggregation at high temperatures. Furthermore, the PEG addition sequence impacted particle size, morphology, and crystallization. Adding PEG before synthesis yielded uniform particle size.

The evidence presented above indicates that the surface modification of nano-TiO2 by PEG is beneficial for its application in sunscreens. It not only improves the dispersion and biocompatibility of nano-TiO2 but also reduces its photocatalytic activity. Moreover, compared with single hydrophobic treatments (such as silane and fatty acid modifications), PEG modification offers the advantage of additional functionalization options (e.g., hydrophilic or lipophilic modifications) to better adapt to different formulation systems and application scenarios.

The micron-sized microcrystalline cellulose@nano-TiO2 core–shell sunscreen composite (MCC@nano-TiO2) combines broad-spectrum sun-protection performance with the potential for large-scale production. MCC@nano-TiO2 is prepared using TiCl4 as the Ti4+ source through a low-temperature hydrolysis method in a strongly acidic environment. This method can produce well-crystallized rutile nano-TiO2 without high-temperature calcination. As a result, it not only conserves energy but also effectively reduces carbon emissions. ,, However, the dehydration condensation of −OH groups on the particle surfaces leads to hydrogen bond formation during the drying process, which causes MCC@nano-TiO2 to be prone to particle aggregation. This not only makes the powder difficult to grind but also boosts the probability of nano-TiO2 peeling off while grinding. This study aims to introduce PEG onto the core–shell MCC@nano-TiO2 particle surfaces to inhibit particle aggregation. First, with the nano-TiO2 content slightly increased, the structural and UV protection properties of the composite before and after the complete coverage of MCC by nano-TiO2 were investigated. The MCC:nano-TiO2 mass ratios were 66:34, 64:36, 62:38, 60:40, 58:42, and 56:44. Then, different amounts of PEG (relative to the nano-TiO2 mass) were added in the later stages of composite preparation to fix PEG onto the particle surfaces at MCC:nano-TiO2 ratios of 60:40 and 56:44. Systematic examinations were conducted to examine the impact of PEG modification on the structure and properties. This study optimizes the preparation process of the sunscreen material through PEG surface treatment, laying a foundation for its industrialization. Meanwhile, it provides new ideas for the surface modification of core–shell nano-TiO2 materials.

2. Experimental Methods

2.1. Materials

Titanium tetrachloride (TiCl4), microcrystalline cellulose (MCC), hydrochloric acid (HCl) and ammonium hydroxide (NH3·H2O) were supplied by Sinopharm. Hand cream (Yuanyina, 30g/tube) was purchased from Taobao. The above chemicals were employed with no additional purification.

2.2. Synthesis of MCC@nano-TiO2

The substrate was an MCC suspension (4–20 μm). The MCC@nano-TiO2 samples were synthesized using low-temperature hydrolysis under strongly acidic conditions. The details of the MCC suspension preparation and the fabrication process of the composite were described in a previously reported protocol. The structure of the nano-TiO2 shell exerts a decisive influence on the MCC@nano-TiO2 performance. A systematic comparison was first conducted to elucidate the effects of minor increases in the nano-TiO2 content on the structure and performance of the composite before and after the MCC surface was covered by nano-TiO2. This group of samples was synthesized at pH = 0.4 and 35 °C with MCC:nano-TiO2 ratios of 66:34, 64:36, 62:38, 60:40, 58:42, and 56:44. After a 3-day sealed reaction, the MCC@nano-TiO2 powder was obtained by centrifugation, washing, drying, and grinding.

PEG for surface treatment was added during the later stage of MCC@nano-TiO2 synthesis. The untreated sample (without PEG) were prepared as a control under the following conditions: pH = 0.4, 35 °C, and MCC:nano-TiO2 = 60:40. PEG modification was carried out under the same conditions, the pH was adjusted to 2.0 after a 2-day reaction, and then PEG solutions containing 2, 4, and 6% PEG (relative to the mass of nano-TiO2) were added. Continue the reaction for another day, the MCC@nano-TiO2 powder was obtained by centrifugation, washing, drying, and grinding. Moreover, another set of samples was synthesized following the same protocol at an MCC:nano-TiO2 ratio of 56:44.

2.3. Characterization

The morphology and structure of MCC were thoroughly examined, the detailed information can be found in the previous report. Characterization of MCC@nano-TiO2 encompassed assessments of morphology and elemental distribution, particle-size distribution, chemical structure, thermal stability, and UV protection capability. Detailed procedures are provided in the Characterization section of the Supporting Information. Special note: Due to the insufficient remaining quantity of Yuanyina hand cream after the UV performance tests on the first set of samples, an additional tube from the same brand was used for the evaluations of the PEG-treated samples.

3. Results and Discussion

3.1. Morphology and Elemental Distribution

As displayed in Figure a–c, nano-TiO2 particles were observed in the SEM images of the 64:36, 60:40, and 56:44 samples. Several particles were observed in the image of the 64:36 sample. The particle in the lower-right corner was coated with flake-like nano-TiO2 crystals, and the larger particle in the center retained exposed cellulose microfibrils despite partial nano-TiO2 attachment. In the SEM image of the 60:40 sample, particles retained the short rod-like morphology and had formed a structure in which a nano-TiO2 layer with extremely fine grains completely encapsulated the MCC. The 56:44 sample also developed a complete core–shell structure, but its surfaces became rougher. Clear C, O, and Ti signals were detected on the cross-sectional of MCC@nano-TiO2 particles (Figure d,e). However, on the surface of the 64:36 particle, the C signal was markedly stronger than the Ti and O signals. Combined with the SEM results, it indicated that the nano-TiO2 layer in the 64:36 sample did not fully cover the MCC surface. In contrast, on the 60:40 particle, the Ti signal was much stronger than the C and O signals. Within the central regions of these MCC@nano-TiO2 particles, there was no significant difference in Ti intensity, which further demonstrated that a complete core–shell structure was achieved only when the mass of nano-TiO2 reached 40%.

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Morphology of samples at MCC:nano-TiO2 ratios of 64:36, 60:40, and 56:44. (a–c) SEM images; (d, e) EDS line scans.

As shown in Figure , the surface morphology of the PEG-treated sample was altered. The surface of the PEG-free (0% PEG) sample at MCC:nano-TiO2 = 60:40 was densely coated with fine nano-TiO2 grains, completely encapsulating the MCC. In contrast, the nano-TiO2 shells of PEG-treated samples exhibited different morphologies and diverse roughness levels. Notably, the 6% PEG sample showed a clear platy cluster stacking, whereas the surfaces of the 2 and 4% PEG samples were coated with dense nano-TiO2 grains, yielding a relatively compact structure. At an MCC:nano-TiO2 ratio of 56:44, the PEG-free sample displayed successive layers of nano-TiO2 grains and a few nano-TiO2 agglomerates loosely attached to the surface. While the 2% PEG sample presented a smoother surface with a more conformably adhered nano-TiO2 layer. This was likely because, during the reaction, PEG chains adopted a serpentine conformation in water and interact with the Ti­(OH)4 via hydrogen bonding, forming a structure that surface nano-TiO2 was tightly linked by PEG in the end. Conversely, the 4 and 6% PEG samples exhibited a more pronounced granular texture, with the MCC@nano-TiO2 particles being decorated by fine nano-TiO2 agglomerates. For the 6% PEG sample, the surface holes formed by nano-TiO2 cluster stacking were smaller.

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(a–d) SEM images of MCC@nano-TiO2 samples treated with 0, 2, 4, and 6% PEG at a MCC:nano-TiO2 ratio of 60:40; (e–h) at a ratio of 56:44.

3.2. Particle Size Distribution

The particle size increased significantly after nano-TiO2 loading (Figures and S1a,b). SPAN value (SPAN = (d90 – d10)/d50) (d10, d50, and d90: the particle sizes corresponding to the cumulative percentage distribution of 10, 50, and 90%) is an indicator of the uniformity of particle size distribution. The lower the SPAN value, the narrower the distribution. SPAN values of samples were presented in Supporting Information Table S1. The results showed that increasing nano-TiO2 content worsened the particle size uniformity, with the 60:40 sample exhibiting the poorest uniformity. Additionally, PEG-treated MCC@nano-TiO2 exhibited reduced particle size uniformity.

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Particle size distribution of MCC@nano-TiO2 samples treated with 0, 2, 4, and 6% PEG at MCC:nano-TiO2 ratios of 60:40 and 56:44. (a, c) particle size distribution; (b, d) the area-weighted mean particle size (D [3, 2]), the volume-weighted mean particle size (D [4, 3]), the particle sizes corresponding to the cumulative percentage distribution of 10, 50, and 90% (d10, d50, and d90).

The distribution peak around 100 μm (Figures a,c and S1a) was attributed to particle agglomeration within MCC@nano-TiO2, primarily resulting from hydrogen-bond formation through the dehydration of surface −OH groups. The peak observed at 1 μm corresponded to free nano-TiO2 agglomerates within MCC@nano-TiO2, and a lower peak intensity indicated a reduced content of nano-TiO2 agglomerates. The addition of PEG exerted varying effects on the peak intensity. As depicted in Figure b,d, PEG modification significantly increased mean particle size (area-weighted: D [3, 2], volume-weighted: D [4, 3]) of MCC@nano-TiO2, along with marked increases in d10, d50, and d90. It was likely because PEG molecular chains adopted a serpentine conformation in aqueous media during the reaction, forming hydrogen bonds of varying strengths with surface Ti­(OH)4.

3.3. Chemical Structure

In the XRD patterns (Figures a,b and S1c), the diffraction peaks at 2θ = 27.34, 36.42 and 54.22° correspond to the (110), (101) and (211) planes of rutile TiO2, respectively, matching PDF card 01-089-0552. The peaks at 2θ = 25.44, 37.88 and 48.12° correspond successively to the (101), (004) and (200) planes of anatase TiO2, in agreement with PDF card 01-073-1764. With increasing nano-TiO2 content, the crystalline form of nano-TiO2 shell evolved from a mixed phase to pure rutile phase and then reverted to a mixed phase, and the 64:36 sample corresponded to the pure rutile phase (Supporting Information Figure S1c). For the mixed phase sample, the grain size of anatase (101) crystal planes (DA101) value first increased and then decreased, the grain size of rutile (110) crystal planes (DR110) and WR values first decreased and then increased (Supporting Information Figure S1d). This reaffirmed that a sufficient quantity of Ti4+ must participate in the reaction to yield MCC@nano-TiO2 in the pure rutile form. The XRD patterns in Figure a,b exhibited distinct characteristic peaks of rutile TiO2. In addition, the XRD patterns of the 60:40 2% PEG, 60:40 6% PEG, and 56:44 4% PEG samples also showed weak characteristic peaks corresponding to anatase TiO2. At an MCC:nano-TiO2 ratio of 60:40, for the PEG-treated samples, the DR110 value fluctuated slightly compared with the PEG-free sample (Figure c). The DA101 value of 2% PEG sample showed no significant difference from the 60:40 sample in the first set of samples. In contrast, the 6% PEG sample exhibited a marked reduction to only 2.5 nm, which was attributed to the steric hindrance effect caused by extended PEG molecules in water. At MCC:nano-TiO2 = 56:44, for the PEG-treated samples, the DR110 value of the mixed phase 4% PEG sample was significantly larger than the others. The results demonstrated that PEG modification altered the crystal structure of nano-TiO2 shell. Due to the preparation procedure, PEG primarily acted on the surface of nano-TiO2 shell. PEG modification had a smaller effect on the nano-TiO2 shell crystal structure at MCC:nano-TiO2 = 56:44 than at 60:40.

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MCC@nano-TiO2 samples treated with 0, 2, 4, and 6% PEG at MCC:nano-TiO2 ratio of 60:40 and 56:44: (a, b) XRD patterns; (c, d) relative content of rutile (WR), and grain sizes of rutile (110) and anatase (101) crystal planes (DR110 and DA101); (e, f) Raman spectra.

In the FT-IR spectra (Supporting Information Figures S1e and S2), the peaks at 1454 and 1061 cm–1 corresponded to Ti–O–C bonds, whereas the peak at 1381 cm–1 was associated with Ti–O–Ti bonds. The stretching and bending vibration bands observed in the 695–470 cm–1 region were characteristic absorption peaks of TiO2, attributable to Ti–O bonds. It indicated that the nano-TiO2 in the MCC@nano-TiO2 was firmly bonded to the MCC surface via Ti–O–C linkages. , However, due to the extremely low PEG loading, no characteristic absorption peaks associated with PEG were observed in FT-IR spectra.

Additionally, Raman spectra clearly revealed the surface crystal phase of the nano-TiO2 shell (Figures e,f and S1f). The B1g (117.9 cm–1), Eg(1) (240.7 cm–1), Eg(2) (444.3 cm–1) and A1g (611.2 cm–1) represented the typical vibrational modes of rutile Ti–O band. , The Eg(1) (160.4 cm–1) represented the typical vibrational modes of anatase Ti–O band. , It showed that when MCC was completely encapsulated by nano-TiO2 (MCC:nano-TiO2 = 60:40), both PEG-free and PEG-treated samples were covered by an anatase nano-TiO2 layer. However, the anatase proportion on the nano-TiO2 shell surfaces of the PEG-treated samples was significantly higher than that of the PEG-free sample. After MCC was encapsulated by nano-TiO2 (MCC:nano-TiO2 = 56:44), a small amount of anatase existed on the surfaces of the 2% PEG and 4% PEG samples, while the nano-TiO2 shell of the 6% PEG sample was retained rutile phase. The presence of surface anatase nano-TiO2 in the PEG-free sample was attributed to a small amount of uncrystallized Ti­(OH)4 remaining at the end of the reaction, which transformed into anatase due to the absence of an acidic environment in the subsequent process. Moreover, PEG modification inhibited rutile formation through steric hindrance imposed by macromolecules, consequently decreasing its relative content. Results indicated that PEG modification at MCC@nano-TiO2 = 56:44 exerted a smaller influence on the surface crystal structure of nano-TiO2 shell than that at 60:40.

The TEM images of the MCC@nano-TiO2 sample treated with 6% PEG at an MCC:nano-TiO2 ratio of 60:40 (Figure a,b) revealed well-dispersed particles. PEG modification was conducive to the dispersion of composite particles. The EDS element distribution revealed that Ti elements were uniformly distributed but concentrated in the central area (Figure d–g). Combined with the HAADF-STEM image (Figure c), it is confirmed that MCC@nano-TiO2 with a uniformly nano-TiO2 shell has been successfully synthesized. The HRTEM image revealed lattice fringes corresponding to the rutile (110) plane with an interplanar spacing of 0.3271 nm (matching PDF card: 01-089-0552) and the anatase (101) plane with 0.3502 nm (PDF card: 01-073-1764) (Figure h). Moreover, the diffraction spots in the SAED pattern also confirm that MCC@nano-TiO2 is polycrystalline, with its nano-TiO2 shell comprising a mixed phase (Figure i).

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TEM results of MCC@nano-TiO2 treated with 6% PEG at MCC:nano-TiO2 ratio of 60:40. (a, b) TEM images of a group of dispersed MCC@nano-TiO2 particles; (c) HAADF-STEM image; (d–g) EDS element distribution; (h) HRTEM analysis; (i) SAED pattern.

The XPS survey of MCC@nano-TiO2 samples revealed C, O, and Ti signals (Figures a and S3a). The surface Ti atomic percentages (Ti/%) of the 64:36, 60:40, 56:44 samples were 5.50, 5.85, and 6.56%, respectively. The corresponding Ti/C atomic ratios were 0.14, 0.13, and 0.18. The Ti/C atomic ratio of the 56:44 sample was higher than that of the 60:40 sample. For the PEG-treated sample group, the surface Ti atomic percentages (Ti/%) of the 60:40 0% PEG, 60:40 6% PEG, 56:44 0% PEG, and 56:44 2% PEG samples were 5.00, 2.43, 6.03, and 5.27%, respectively. The corresponding Ti/C atomic ratios were 0.12, 0.06, 0.14, and 0.12. PEG modification led to a significant reduction in Ti% and Ti/C atomic ratio. Especially, at a 60:40 MCC:nano-TiO2 ratio, the Ti atoms in MCC@nano-TiO2 surface were largely shielded by the PEG macromolecules, leaving only a small fraction exposed after treatment with 6% PEG. However, at a 60:40 MCC:nano-TiO2 ratio, only a small fraction of the Ti atoms in the composite surface were masked by PEG macromolecules after treatment with 2% PEG.

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XPS spectra of the 60:40 0% PEG, 60:40 6% PEG, 56:44 0% PEG, 56:44 2% PEG samples. (a) XPS survey spectra; High resolution XPS spectra of (b) C 1s, (c) O 1s, (d) Ti 2p.

The binding energies of the C–OH and O–C–O bonds on the MCC surface have a strong correlation with hydrogen bonding. It decreased slightly after nano-TiO2 coating, and the total amount of these two bonds was markedly reduced. It aligned with the previous conclusion that loading nano-TiO2 onto the MCC surface lowered the surface chemical bond energies (Figures b and S3b). ,, The types of oxygen bonds in MCC@nano-TiO2 are consistent (Figures c and S3c). Within this composite, oxygen atoms coordinated to Ti exist in two bonding configurations: Ti–O–Ti and Ti–O–C. The nano-TiO2 is firmly anchored to MCC through Ti–O–C bonds. ,, As the nano-TiO2 content increased, the fitted peak-area ratio (A%) of the Ti–O–Ti bond rose, whereas that of the Ti–O–C bond declined. The A% ratios of Ti–O–Ti to Ti–O–C (Ti–O–Ti/Ti–O–C)­for the 64:36, 60:40 and 56:44 samples were 0.90, 1.17 and 3.27, respectively (Supporting Information, Table S2). It revealed that the Ti atoms on the composite surface shifted from predominantly participating in Ti–O–C linkages toward greater involvement in Ti–O–Ti connections with increasing nano-TiO2 content. For the PEG-treated samples, a new peak appears at 532.7 eV, corresponding to the C–O bond of PEG. The Ti–O–Ti/Ti–O–C ratio exhibited distinct trends after PEG modification, which was attributed to the degree of exposure of PEG on the MCC@nano-TiO2 surface (Table ). The Ti 2p peak was split into two components, corresponding to the Ti 2p3/2 and Ti 2p1/2 peaks of Ti4+ ions. The Ti atoms in MCC@nano-TiO2 were in the +4 oxidation state.

1. Ti/C, Data of O 1s XPS Spectra.

    O 1s
samples Ti/C Ti–O–Ti Ti–O–C –OH C–O C–OH
60:40 0% PEG 0.12 529.96 531.71 532.49   533.03
19.3 15.4 24.8 40.4
60:40 6% PEG 0.06 530.02 530.88 531.89 532.72 533.24
20.1 20.6 35.0 17.1 13.5
56:44 0% PEG 0.14 529.93 530.08 532.34   532.94
3.7 17.2 24.6 54.5
56:44 2% PEG 0.12 529.76 530.73 532.08 532.76 533.17
8.3 2.7 15.3 14.35 11.3
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a

Binding energy/eV.

b

A/%.

3.4. UV Protection Capability

The UV protection capability of the MCC@nano-TiO2 was displayed in Figures and S4. When the nano-TiO2 content ranged from 34 to 44%, all powders showed similar absorption curves and higher UV absorption than P25 across the entire spectrum, except for the 60:40 sample with its relatively low rutile content (Supporting Information Figure S4a). The UV absorbing capacity of the cream increased progressively with higher nano-TiO2 loading. Although the 64:36 sample contained pure rutile nano-TiO2, incomplete surface coating of the particles may partially explain its reduced absorption relative to other samples. Moreover, the UV protection capability of nano-TiO2 hinged on its hiding power; the reduced content therefore undermined performance by hiding power. , Both the PEG-free and PEG-treated powders exhibited significantly higher absorption intensities in the UVB and UVA ranges compared to P25 powder, with their UVA absorption capacity markedly surpassing that of P25 (Figure a,b). Compared with the untreated sample, the PEG-treated samples showed a reduced absorption in the UVB range, whereas its absorption in the UVA range remained essentially unchanged. Notably, the 60:40 6% PEG sample exhibited relatively high absorption approximately 400 nm. According to the XPS results, the nano-TiO2 surface in this sample exhibited more extensive coverage by PEG macromolecule, with this coating likely serving as the primary reason for the enhanced absorption. For the creams, PEG modification resulted in weaker absorption across the entire UV spectrum. These creams exhibited weaker absorption than the P25 cream at wavelengths below approximately 350 nm, but stronger absorption above 350 nm (Figure c,d).

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UV protection of MCC@nano-TiO2 samples treated with 0, 2, 4, and 6% PEG at MCC:nano-TiO2 ratio of 60:40 and 56:44. (a, b) UV absorption spectra of powders; (c, d) UV absorption spectra of creams; (e, f) the critical absorption wavelength (λc) of the powder, the in vitro sun protection factor (SPFin vitro) and the initial UVA protection factor (UVAPF0) of the cream.

The critical absorption wavelength (λc) values of the MCC@nano-TiO2 powder were all greater than 370 nm, demonstrating excellent broad-spectrum UV protection. As showed in Supporting InformationFigure S4c, both the in vitro sun protection factor (SPFin vitro) and the initial UVA protection factor (UVAPF0) rose with increasing nano-TiO2 content. However, the ratio of UVAPF0 to SPFin vitro (UVAPF0/SPFin vitro) showed a decreasing trend. For the 56:44 sample, the SPFin vitro was slightly below P25, and the UVAPF0/SPFin vitro ratio was 0.625, which was higher than that of P25. It indicated that the MCC@nano-TiO2 composite exhibited the best broad-spectrum UV protection performance at a 56:44 MCC:nano-TiO2 ratio.

PEG-treated samples still show broad-spectrum UV protection. Notably, the 60:40 6% PEG sample exhibited the broadest-spectrum UV protection performance, its λc value reached 376 nm. Although the MCC@nano-TiO2 creams exhibited higher UVA absorption intensities compared with the P25 cream, the SPFin vitro values were predominantly governed by UVB absorption. Consequently, P25, which showed stronger UVB absorption, achieved a higher SPFin vitro value than MCC@nano-TiO2. However, the SPFin vitro values of the PEG-treated samples decreased, the extent of reduction varying with the amount of PEG added.

3.5. The Effect of PEG Modification on MCC@nano-TiO2

The refractive index differences at the internal and external interfaces of MCC@nano-TiO2, together with the UV reflection of MCC, facilitated the UV light localization within the nano-TiO2 shell, thereby enhancing UV protection capability. The UV protection of MCC@nano-TiO2 was related to the structure of the nano-TiO2 shell. In pure rutile and mixed phase MCC@nano-TiO2, the porous structure formed by the stacking of irregular platy rutile crystallites significantly increases the effective scattering volume. This enhancement led to improved UV reflection and scattering, thereby providing better protection against UVA. Moreover, the greater refractive index difference between the inner and outer surfaces of the rutile phase composite resulted in stronger UV localization within the nano-TiO2 shell, thereby achieving more efficient UV absorption. As a result, the composite could significantly enhance its ability to reflect, scatter, and absorb UV light, thus providing excellent broad-spectrum UV protection. ,

Results revealed that PEG modification significantly influenced the structure of the nano-TiO2 shell, thereby affecting the performance of MCC@nano-TiO2. The entire MCC surface was exactly covered by a nano-TiO2 layer at a 60:40 MCC:nano-TiO2 ratio. At this ratio, the nano-TiO2 shell of the PEG-free sample consisted of a structure in which a very small amount of anatase covered the rutile phase. After 6% PEG modification, the proportion of anatase in the outermost layer of the nano-TiO2 shell increased and its crystallite size decreased markedly. PEG was added in the later stages of composite preparation to attach it to the particle surfaces. The PEG chains unfurled in a snake-like fashion within the aqueous medium. Upon the addition of PEG to the reaction system, these extended chains underwent physical adsorption on the particle surface through polyvalent supramolecular interactions. They subsequently formed hydrogen bonds with the −OH groups on the particle surface. One terminus of the PEG molecule became anchored to the particle surface, whereas the opposite end either projected outward into the bulk solution or folded back to lie in intimate contact with the same surface, giving rise to a uniformly hydrophilic film. In the subsequent reaction, Ti4+ ions continuously hydrolyzed to form Ti­(OH)4, which associated with PEG and ultimately resulted in PEG chains entwining and encapsulating the nano-TiO2, as shown in Figure . XPS results revealed that after treatment with 6% PEG, both the Ti% and the Ti/C ratio on the sample surface were markedly reduced, providing strong support for the conclusion. Moreover, the results of SEM and TEM morphology analysis as well as Ti element distribution indicated that the surface modification carried out by adding PEG in the later stages of composite formation did not affect the overall uniformity of the nano-TiO2 shell.

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Effect of PEG modification on the nano-TiO2 shell structure.

After the PEG macromolecules had bound to the surface of MCC@nano-TiO2, a pronounced steric-hindrance effect arose. During the ensuing hydrolysis, the resulting Ti­(OH)4 was isolated by the PEG chains and thus prevented from assembling into bulk rutile phase. Consequently, an extremely fine-grained, yet densely packed, anatase structure was ultimately formed. Notably, the λc increases significantly. This was because PEG macromolecules (refractive index: 1.47) were exposed on the particle surface, forming a more disordered medium with nano-TiO2 (rutile and anatase phases) that had complex refractive index differences at the internal and external interfaces. This enhanced UV reflection and scattering, thereby broadening UVA protection. However, PEG molecules had partially coated the surface, resulting in a reduction in exposed nano-TiO2, which thereby weakened the UV absorption performance of MCC@nano-TiO2. ,,

Nano-TiO2 totally coated the MCC surface at a 56:44 MCC:nano-TiO2 ratio, forming a pure rutile shell. Compared with the 60:40 group, the PEG amount (relative to nano-TiO2 mass) was higher here. Upon binding to the particle surface, the PEG macromolecules exerted a stronger steric hindrance effect. XPS results indicated that treatment with 2% PEG slightly reduced the Ti% and Ti/C ratio on the sample surface. This suggested that the newly formed Ti­(OH)4 subsequently reacted with PEG during the process. PEG was tightly encapsulated by the nano-TiO2 tightly, leaving only a small fraction exposed. While the λc remains unchanged, the compact hybrid layer of PEG and nano-TiO2 weakened the UV absorption capability. This indicated that the coverage of PEG by nano-TiO2 was detrimental to the UV performance of MCC@nano-TiO2.

TEM images revealed that PEG modification, at a 60:40 MCC:nano-TiO2 ratio and under 6% PEG, imparted excellent dispersibility to MCC@nano-TiO2. This result was attributed to the markedly increased surface hydrophilicity conferred by exposed PEG chains, which effectively inhibited particle agglomeration. The PEG layer that wrapped and entangled the nano-TiO2 on particle surfaces acted as a protective shield, greatly reducing nano-TiO2 detachment during grinding, limiting the formation of agglomerates within the sample, and therefore diminishing the chance of nano-TiO2 release. In addition, it also enhanced the biocompatibility of MCC@nano-TiO2 and enabled its further functionalization. − Moreover, it has proved that MCC@nano-TiO2 exhibited better photostability than pure nano-TiO2 (Degussa P25) in the previous research. , The coating of nano-TiO2 with a PEG layer reduced its surface activity, inhibited the transfer of photogenerated electrons and reactive oxygen species, thereby decreasing its photocatalytic performance. , This meant that in MCC@nano-TiO2, the exposed PEG on the surface was conducive to further reducing its photocatalytic activity, thus enhancing its photostability. In conclusion, PEG modification not only enhanced the stability and uniformity of MCC@nano-TiO2 in sunscreen applications, thereby ensuring the continuous effectiveness of its sun protection, but also effectively reduced the potential risks that nano-TiO2 may pose to human health and the environment, thereby enhancing the safety of the product.

4. Conclusions

PEG modification significantly affects the dispersibility, structure, and performance of MCC@nano-TiO2. Notably, the exposure of PEG on the particle surface markedly enhances the UV protection effect. Nano-TiO2 layer exactly covered the entire MCC surface at a 60:40 MCC:nano-TiO2 ratio. After treatment with 6% PEG (relative to nano-TiO2 mass), the steric hindrance of the macromolecules led to an increase in the anatase content in the outer layer of the nano-TiO2 shell and a marked reduction in crystallite size. PEG is exposed on the particle surface, creating an interface with complex refractive index contrast against both rutile and anatase nano-TiO2. This markedly increases UV reflection and scattering, thereby extending the UVA protection range (λc = 376 nm) while simultaneously delivering excellent dispersibility. Moreover, PEG encapsulation and entanglement around nano-TiO2 on the particle surface significantly reduced nano-TiO2 agglomerates in MCC@nano-TiO2, thereby diminishing the chance of nano-TiO2 release and mitigating the potential health and environmental risks associated with nano-TiO2. Furthermore, exploring the potential applications of PEG-modified materials in various sunscreen formulations is expected to drive the development of sunscreen products that are both safer and more effective.

Supplementary Material

ao5c08376_si_001.pdf (735.7KB, pdf)

Acknowledgments

The authors are grateful for the support of the National Nature Science Foundation of China (NSFC, No. 21978074) and Foundation of Hubei Provincial Key Laboratory of Green Materials for Light Industry (No. 201907B01, 201806A03).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c08376.

  • Characterization section; the particle size distribution and crystal structure of MCC@nano-TiO2 at different MCC:nano-TiO2 ratios (Figure S1); FT-IR spectra of MCC@nano-TiO2 samples with PEG modification (Figure S2); XPS spectra of MCC@nano-TiO2 prepared at different mass ratios at 35 °C at pH = 0.4 (Figure S3); UV protection of MCC@nano-TiO2 prepared at different mass ratios (Figure S4); SPAN values of samples (Table S1); Ti/C, data of O 1s XPS spectra (Table S2) (PDF)

H. Yu: Writing–original draft, Data curation, Conceptualization, Methodology. J.X.: Writing–review and editing. L.Y.: Writing–review and editing, project administration. H. Yang: Funding acquisition, Conceptualization.

The authors declare no competing financial interest.

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Supplementary Materials

ao5c08376_si_001.pdf (735.7KB, pdf)

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