Omnidirectional brightness and UV-shielding bifunctional pigments via CeO₂ photonic crystal microbeads

Abstract

Photonic crystal microbeads with symmetrical structures exhibit non-iridescent colors, making them valuable as stable and vibrant colorants. However, their brightness is limited by small refractive index (RI) contrast. Enhancing this contrast is key to achieving more vivid structural colors. Herein, CeO₂@SiO₂ photonic crystal microbeads (10–30 µm) with a high RI were successfully fabricated by the microfluidic method. By tuning the diameter of CeO₂@SiO₂ colloidal particles from 160 nm to 265 nm, blue, green, and orange-red microbead pigments were successfully fabricated, exhibiting vivid structural colors across a wide viewing angle range of 0° to 75°. Interestingly, films composed of these microbead pigments and polymers showed bright, angle-independent colors and strong UV absorption. These CeO₂@SiO₂ microbeads can not only be used as pigments for bright, omnidirectional structural colors but also have potential applications in various fields that require UV-shielding, such as color lenses, decorative coatings, and cosmetics.

---

Introduction

Compared to traditional dyes and pigments, photonic crystal (PC) structural colors offer superior light fastness and greater environmental friendliness.^1–4 However, most conventional PCs are fabricated as films and exhibit angle-dependent structural colors.^5–9 Therefore, for applications such as pigments, developing PCs with wide viewing angles and other additional functions remains essential.^10–12 Nowadays, several methods have been employed to produce angle-independent PCs. For instance, amorphous arrays^13–16 and symmetrical structures^17–20 are designed to minimize spectral shifts caused by changes in viewing angles. However, the amorphous colloidal crystals are commonly dull in color because of light scattering.²¹ So, PC microbeads have attracted significant attention^22,23 due to their brilliant and angle-independent structural colors that originate from their long-range ordered structures and symmetrical arrangements. Meanwhile, their colors can be easily tuned across the visible spectrum by adjusting the colloidal particles' size.^24,25 Most PC microbeads are made from SiO₂—a low-refractive-index material with limited color brightness. Fortunately, the RI contrast can be enhanced by using high-refractive-index colloidal particles, thereby producing brighter structural colors.^26,27

As high-refractive-index colloidal particles, TiO₂, Cu₂O and ZnO are widely used in commercial products and scientific research.^28–31 However, their strong photocatalytic activity and toxicity might cause skin cell damage, limiting their applications.³² In contrast, CeO₂—a metal oxide^33,34 with a high RI (∼2.3)—offers a promising alternative due to its much weaker photocatalytic activity.³⁵ Additionally, it exhibits broader spectral-range UV-shielding properties compared to commercial UV-absorbing materials.³⁶ Therefore, using CeO₂ as the raw material for nanoparticle microbeads could integrate bright omnidirectional structural coloration with effective UV protection, making it a promising candidate for colorful UV-shielding pigments.

Herein, we report a CeO₂@SiO₂ microbead pigment with bright angle-independent structural color and UV-shielding properties. CeO₂ colloidal particles with size ranging from 120 nm to 190 nm were coated by a 10 nm to 70 nm thickness of SiO₂ by the Stöber method. Then, the PC microbead structure was built by self-assembly of CeO₂@SiO₂ colloidal particles within water droplets in an O/W emulsion via the microfluidic method. The ordered structure and high RI contrast rendered them bright colors, and colorful PC microbeads were realized by changing the particle size of CeO₂ and the thickness of the SiO₂ shell. Their reflection peaks remain stable within an observation angle range of 0–75° in air. Simultaneously, films consisting of these PC microbeads and polymers exhibit excellent UV-shielding properties. In addition to serving as vibrant colorants, these CeO₂-based PC microbead pigments also function as effective UV-shielding materials, demonstrating great potential for use in UV-protective pigments and cosmetic applications.³⁷

Materials and methods

Materials

Ce(NO₃)₃·6H₂O (99.5%), polyvinyl pyrrolidone (PVP, M_W = 29 000), ethylene glycol (EG, 99%), hexane, poly (ethylene glycol) diacrylate (PEGDA), tetraethyl orthosilicate (TEOS, 98%), arginine, Span80, NH₃·H₂O (28%) and kerosene were purchased form Aladdin Co. Ltd.

Synthesis of CeO₂@SiO₂ colloidal particles

CeO₂ particles were synthesized by high-temperature polyol reaction²⁶ with SiO₂ as seeds. SiO₂ seeds were fabricated by hydrolysis of TEOS (5.5 mL) using arginine (0.087 g) as the surfactant in water (87 mL). Take 169 nm CeO₂ for example, PVP (12.5 g), EG (100 mL), seed (1250 µL), and Ce(NO₃)₃·6H₂O (40 g) were placed in a 250 mL three-necked flask, and then the reactants were dissolved under mechanical stirring at 65 °C. After that, the transparent solution was heated to 155 °C under a N₂ atmosphere, and then the solution turned dark brown and finally became a pale yellow suspension during the 1.5 h reaction. After that, the CeO₂ particles were obtained by centrifugation and then washed 3 times with ethyl alcohol. The size of the CeO₂ particles was adjusted to 155 nm by increasing the seed dosage to 2000 µL, and 183 nm CeO₂ particles were obtained by decreasing the seed dosage to 300 µL. CeO₂@SiO₂ colloidal particles were obtained by the Stöber method with CeO₂ particles serving as seeds and tetraethyl orthosilicate (TEOS) as the silica source (Fig. 1a). A typical synthesis process of CeO₂@SiO₂ particles is as follow: 0.5 g CeO₂ particles, 180 mL ethyl alcohol, 20 mL H₂O, and 7 mL NH₃·H₂O were placed in a 250 mL three-necked flask and sonicated for 2 h for homo-dispersion; then, 1 mL TEOS was injected into the reactor under magnetic stirring at 25 °C. After a 4 h reaction, the CeO₂@SiO₂ colloidal particles were collected by centrifugation and washed 3 times with ethyl alcohol.

Fig. 1 (a) Synthesis procedure for CeO₂@SiO₂ colloidal particles. (b) TEM images of CeO₂ and CeO₂@SiO₂ colloidal particles. (c) Schematic flow diagram of the fabrication steps for the CeO₂@SiO₂ microbeads via a microfluid device.

Fabrication of the PC microbeads

The PC microbeads with uniform sizes were fabricated using a cross-shape microfluidic device (Fig. 1c). A 10 wt% aqueous solution of CeO₂@SiO₂ colloidal particles was used as the dispersed phase, and kerosene containing 2 wt% Span 80 was used as the continuous phase. Two injection pumps independently regulated the flow rates of the continuous and dispersed phases through syringes. At the cross of the microfluidic chip, the continuous phase sheared the dispersed phase, generating uniform microdroplets. Afterward, the emulsion was poured into a 10 cm diameter Teflon evaporating dish, and the water within the droplets was slowly evaporated by heating the dish to 40 °C for 48 h. Finally, the PC microbeads were obtained after being washed with hexane three times and dried.

Fabrication of a colored film consisting of PC microbeads and polymer

To prepare a blue film containing 5 wt% microbeads, 0.5 g of PC microbeads was added to 9.5 g of PEGDA with 1 wt% 2-hydroxy-2-methylpropiophenone as the photoinitiator. The mixture was gently shaken and stirred to ensure uniform dispersion. It was then placed in a vacuum chamber to fully infiltrate the PEGDA into the interspaces of the PC microbeads. Finally, the mixture was sandwiched between two glass slides separated by 0.5 mm tape and cured under UV light for 1 minute.

Characterization

The diameter of the colloidal particles and PC microbeads was characterized with SEM (Axia ChemiSEM) and TEM (Tecnai G220 S-Twin). About 100 particles were randomly measured from the images, and the polydispersity index (PDI) was calculated as the standard deviation divided by the average particle size. The zeta potentials of the nanoparticles were measured in water using a Malvern Nano-ZS90. PC microbeads were observed using an optical microscope (Axio Scope 5) in reflection mode and the reflection spectrum was collected by the fiber-coupled spectrometer. The UV-Vis absorption curves were measured by a UV-vis spectrophotometer (Cary 5000). A photo of the membrane and microbeads was taken by a camera (MI 11).

Results and discussion

Synthesis and characterization of CeO₂@SiO₂ colloidal particles

The synthesis process for CeO₂@SiO₂ colloidal particles is shown in Fig. 1a. Firstly, CeO₂ colloidal particles were synthesized via a high-temperature polyol reaction, and the particle size was controlled by adjusting the amount of silica seeds. The SEM images and polydispersity index (PDI) of the CeO₂ colloidal particles showed that the monodispersed CeO₂ colloidal particles with diameters of 122 nm (PDI = 0.062), 155 nm (PDI = 0.057), 177 nm (PDI = 0.067), and 183 nm (PDI = 0.052) were fabricated successfully (Fig. S1).

Next, a series of CeO₂@SiO₂ with different CeO₂ core sizes and SiO₂ shell thicknesses were fabricated by the Stöber method, as shown in Fig. S2 and S3. After silica coating, the morphological integrity of CeO₂@SiO₂ is significantly improved and their PDI is significantly lower than that of uncoated CeO₂, indicating enhanced monodispersity. The TEM images of the CeO₂@SiO₂ colloidal particles before and after coating (Fig. 1b) also revealed that the surface of CeO₂ was rough and there were obvious grain structures, whereas the surface of CeO₂@SiO₂ possesses a uniform SiO₂ shell; thus, the degree of sphericity was improved. Simultaneously, the zeta potential analysis showed that the calcined CeO₂ particles had a nearly neutral surface potential (0 mV), weakening electrostatic repulsion and hindering ordered self-assembly. After SiO₂ coating, the zeta potential increased to −41.6 mV, aligning with SiO₂ colloidal particles (∼−40 mV), thus enhancing electrostatic stabilization and guaranteeing the self-assembly.

Fabrication and characterization of tricolor microbeads via microfluidics

PC microbeads were synthesized in a W/O emulsion system with uniform dispersed-phase droplets using a microfluidic chip under microscopic supervision, using kerosene (2 wt% Span 80) as the continuous phase and an aqueous dispersion of CeO₂@SiO₂ (10 wt%) as the dispersed phase. The prepared microbeads exhibited a uniform particle size, and after water evaporation, they shrank and showed vibrant structural colors (Fig. 2a). The diameter of the microbeads was controlled by tuning the flow rate ratio of the continuous phase and dispersed phase within a microfluidic chip with fixed size. By adjusting the ratio of the continuous phase and dispersed phase (Fig. S4) to 5 µL min⁻¹: 150 µL min⁻¹, 7.5 µL min⁻¹: 150 µL min⁻¹, 15 µL min⁻¹: 150 µL min⁻¹, microbeads with particle sizes of approximately 10 µm, 20 µm, and 30 µm were successfully produced, respectively.

Fig. 2 Optical photographs of evaporative assembly and characterization of tricolor CeO₂@SiO₂ microbeads. (a) Droplet generation via a microfluidic process and photos of microbeads before and after water evaporation. (b) SEM images for the cross section of CeO₂@SiO₂ microbeads with different diameters (I) 11.37 µm and (II) 21.02 µm. (c) Photographs and SEM images of the tricolor microbeads in ethanol. (d) Reflectance spectra and optical microscope photographs (scale bars: 10 µm) of the tricolor microbeads.

Typically, structural colors originate from the self-assembly of the colloidal particles. Therefore, the effects of evaporation temperature on microbead assembly were investigated. As seen in the SEM image (Fig. S5), the ordered structure of the microbeads' surface was smashed when the evaporation temperature rose from 20 °C to 60 °C. The 2D-FTT analysis also showed that the ordered structure on the surface disappeared when the temperature was increased to 60 °C. Therefore, 40 °C was chosen as the evaporation temperature to balance the evaporation rate and the stability of the microbeads.

Definitely, the color of the microbeads depends solely on the diameter of the CeO₂@SiO₂ colloidal particle, regardless of the microbead size. Meanwhile, the center of the microbeads showed a brighter and whiter color under a microscope when the size of the microbeads is greater than 20 µm, especially in a higher RI polymer environment. This may be attributed to insufficient ordering during the assembly of larger microbeads, resulting in increased light scattering. The cross-sectional SEM (Fig. 2b) reveals that the microbeads' internal assembly consists of a more disordered core region and a well-ordered outer shell. Regardless of the microbead size, the thickness of the ordered shell remains approximately 2 µm. Thus, as the size of the microbeads increases, the disordered core region expands, leading to stronger light scattering and a more obvious white center. Therefore, the size of the microbeads should be below 20 µm to achieve a brighter color.

The relationship between the structure color of the microbeads and the size of the CeO₂@SiO₂ colloidal particles was further investigated. According to Bragg's formula;

where λ_max is the reflection peak wavelength, θ is the angle of incidence light, D is the center distance between colloidal particles, and n_eff is the overall effective RI, which is calculated by the following equation:
where f_p is the volume fraction of colloidal particles, n_p is the RI of colloidal particles, and n_env is the RI of the surrounding environment.

CeO₂@SiO₂ colloidal particles with similar sizes can be obtained from different diameter CeO₂ cores by coating SiO₂ of different thicknesses. For example, 10 µm microbeads were fabricated using CeO₂@SiO₂ particles with core sizes of 155 nm and 183 nm, which reached 295 nm and 281 nm after coating with SiO₂, as shown in the microscope photos (Fig. S6). It was observed that microbeads composed of 155 nm CeO₂ core appeared whiter than microbeads composed of 183 nm core, with less distinct structural colors, although the final CeO₂@SiO₂ particle sizes were similar. This may be due to the n_p of the CeO₂@SiO₂ particles with 183 nm CeO₂ cores being approximately 1.59, while that with 155 nm cores was about 1.57. In other words, a thicker SiO₂ shell could lower the overall RI of the CeO₂@SiO₂ particles. The lower the RI contrast between the colloidal particles and the surroundings, the weaker the reflection light, resulting in a whiter appearance. Accordingly, to maintain a favorable surface potential for self-assembly, while preserving the high n_p needed for vibrant structural colors, CeO₂@SiO₂ colloidal particles with a thinner SiO₂ coating were optimal.

Based on the experimental results mentioned above, photonic crystal microbeads with blue, green, and red colors were prepared using 155 nm CeO₂ coated up to 160 nm, and 183 nm CeO₂ coated up to 210 nm and 265 nm, as shown in Fig. 2c. When dispersed in ethanol, the microbeads exhibited distinct colors; meanwhile, blue microbeads red-shift to green, while the green microbeads appear yellow. This is because the RI of the environment becomes larger and n_eff becomes larger, so the λ_max of the microbeads is red-shifted. As shown in the SEM images, the surface of the microbeads is arranged orderly with few defects. Thus, the microbeads demonstrate bright structural colors under a microscope (Fig. 2d), and the reflection peaks of tricolor are 450 nm, 520 nm, and 635 nm. Meanwhile, the CeO₂@SiO₂ microbeads exhibit brighter colors compared to SiO₂ microbeads (Fig. S7). This is attributed to the greater RI contrast between CeO₂@SiO₂ (1.60–1.91) and air, compared to that of SiO₂ (1.46) and air. As a result, CeO₂@SiO₂ microbeads reflect more light and display more vivid colors than their SiO₂ counterparts.

Characterization of angle-independent structure colors

Due to the spherical shape and internally isotropic particle arrangement, the microbeads exhibit optical isotropy. As shown in Fig. 3a, ambient light irradiates into the photonic crystal microbeads from all directions. Then, the light was reflected because of the photonic band gap. The eye perceives the same composition of reflected light when observing photonic crystal beads from different angles; thus, the microbeads exhibit omnidirectional structural colors under ambient light.

Fig. 3 Scheme and measurement of angle dependency. (a) Schematic illustration of different viewing angles on PC microbeads. (b) Photographs and optical microscope photographs (scale bars: 20 µm) of tricolor microbeads in air. (c) A series of photographs (bars: 1 cm) and reflection spectra of tricolor films with the observation angle from 0° to 75°.

As shown in Fig. 3b, the dried microbeads exhibited apparent structure colors and the color remained unchanged when the microbeads were observed at different viewing angles. The orange-red, blue and green photonic crystal microbeads exhibited bright colors with the viewing angle changing from 0° to 75°. The orange-red structural color was diminished by the reduced RI contrast. Specifically, the required core particle size of 265 nm and a minimum SiO₂ shell thickness of 45 nm resulted in a high-refractive-index core volume fraction of only ∼25%, lowering the theoretical RI to approximately 1.60. Additionally, the blue light generated by incoherent and multiple scattering from individual particles interfered with the photonic bandgap color, further weakening the observed hue. Compared to the orange-red microbeads, the blue and green microbeads exhibited more vivid colors. This is because they are composed of colloidal particles approximately 160 nm and 210 nm in size, respectively, and required only a 20 nm SiO₂ coating to meet the spectral requirements. As a result, their theoretical refractive indices were approximately 1.91 and 1.66, which are higher than that of orange-red microbeads, contributing to the enhanced color brightness.

Just like many SiO₂ microbeads, the bonding between the CeO₂@SiO₂ colloidal particles that make up the microbead structure is still very weak, so the microbeads could break up easily; for example, ultrasound could gradually distribute the microbeads. Therefore, fixation and encapsulation of the microbeads are very important for their application. PEGDA—known for its good flowability and easy UV-induced polymerization—was used to fix the microbeads. Tricolor films were prepared by mixing the CeO₂@SiO₂ microbeads with PEGDA and treating them in a vacuum for 2 h; meanwhile, PEGDA was sufficiently infiltrated into the interior of the microbeads by capillary force. After that, the mixture was filled into two pieces of glass separated by tape and cured under UV light for one minute. As shown in Fig. 3c, photos of tricolor films at different angles demonstrate that their color remains unchanged with changing observation angles, exhibiting no iridescence. By varying the detection angle from 0° to 75°, the reflection peaks of the tricolor films remained unchanged.

UV-shielding properties of the PC microbead film

CeO₂ was capable of absorbing UV light at wavelengths ranging from 220 nm to 400 nm,³⁴ and the absorption range was larger than general UV absorbers. As shown in Fig. 4a, the absorption spectra of the PEGDA film with varying mass fractions of CeO₂@SiO₂ microbeads were analyzed and compared to the blank PEGDA film. Below 290 nm, the addition of CeO₂@SiO₂ microbeads had little impact on UV absorption, as the blank PEGDA film effectively blocked all UV light in this range.

Fig. 4 UV-absorption characteristics of CeO₂@SiO₂ microbeads. (a) UV absorption spectra of CeO₂@SiO₂ microbead (5–15 wt%) polymer films. (b) UV absorption spectra of CeO₂@SiO₂ microbead films with different colors. (c) Visualization of the UV-shielding properties of the PC microbead film.

When the wavelength exceeds 290 nm, the film containing CeO₂@SiO₂ microbeads exhibits significantly higher UV absorption ability than the blank film. This effect is particularly pronounced in the 310–340 nm range, where the UV-blocking capability is obvious. As the mass fraction of CeO₂@SiO₂ microbeads increased from 5% to 15%, the absorption ability was enhanced with a higher microbead content for wavelengths above 340 nm, and for UV in the 290–340 nm range it was completely absorbed. The absorption peak³⁴ at around 330 nm is due to the transfer of electrons within CeO₂ from O 2p2⁻ to Ce 4f4⁺, which strongly absorbs energy in this frequency interval. For wavelengths above 340 nm, the increased absorption intensity may result from the higher microbead mass fraction, which enhances light scattering due to the RI contrast. Thus, the UV absorption property was enhanced with increasing mass fractions of microbeads.

To investigate the UV-shielding properties of different colored films, the UV absorption spectra of PEGDA films containing approximately 5 wt% of tricolor microbeads of 10 µm are shown in Fig. 4b. The green and blue films exhibit similar UV absorption spectra in the 290–340 nm range, effectively blocking all UV light. However, for wavelengths above 340 nm, a little difference in absorption intensity is observed. This may be caused by the different CeO₂ core sizes in the CeO₂@SiO₂ colloidal particles that form the microbeads. Thus, it can be concluded that the color difference of the photonic crystal microbeads has very little influence on the absorption in the UV region, and all of them have excellent UV absorption properties.

To further visualize the UV-shielding performance of the CeO₂@SiO₂ microbead film, a UV activated ink—designed to change color from white to purple under UV light—was patterned onto black paper, as shown in Fig. 4c. Upon direct UV exposure, the color of the pattern changed within 10 seconds (Movie S1). However, when the CeO₂@SiO₂ microbead film was placed over the pattern to block UV light, the color remained unchanged even after one minute of exposure (Movie S2). Furthermore, when only half of the pattern was covered by the film, only the uncovered portion exhibited the color change.

Conclusions

In this paper, CeO₂@SiO₂ PC microbead pigments were fabricated successfully via a microfluidic method and then immobilized in polymer films, which showed impressive omnidirectional brightness and UV-shielding bifunctional properties. Firstly, CeO₂ colloidal particles of three different diameters were synthesized successfully and coated by SiO₂ in different thicknesses for better assembly. Then, CeO₂@SiO₂ PC microbeads with diameters ranging from 10–30 µm were fabricated via a microfluidic device by tuning the flow ratio of the continuous phase and dispersed phase. Blue, green, and orange-red PC microbeads were obtained from CeO₂@SiO₂ colloidal particles with diameters of 160 nm, 210 nm, and 265 nm, which exhibited bright, angle-independent structural colors due to their ordered photonic structures and geometric symmetry. Finally, films composed of these microbeads and polymers were fabricated, and exhibited vivid and omnidirectional colors. Since CeO₂ effectively absorbs UV light, the prepared films exhibited excellent UV-absorption properties. Thus, these CeO₂@SiO₂ PC microbead pigments are promising for future applications in the field of UV-shielding coatings or cosmetics.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. Supplementary information: optical microscope images and scanning electron microscope image of particles and beads. See DOI: https://doi.org/10.1039/d5tc03512k.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22238002), the Research and Innovation Team Project of Dalian University of Technology (DUT2022TB10) and the Fundamental Research Funds for the Central Universities (DUT22LAB610).

Notes and references

1. J. H. Kim, J. B. Kim, Y. H. Choi, S. Park and S.-H. Kim, Small, 2022, 18, 2105225.
2. L. R. P. Areias and J. P. S. Farinha, Dyes Pigment, 2022, 200, 110153.
3. S. H. Han, Y. H. Choi and S.-H. Kim, Small, 2022, 18, 2106048.
4. P. Yazhgur, N. Muller and F. Scheffold, ACS Photonics, 2022, 9, 2809–2816.
5. M. Xiao, Z. Hu, Z. Wang, Y. Li, A. D. Tormo, N. Le Thomas, B. Wang, N. C. Gianneschi, M. D. Shawkey and A. Dhinojwala, Sci. Adv., 2017, 3, e1701151.
6. J. B. Kim, J.-W. Kim, M. Kim and S.-H. Kim, Small, 2022, 18, 2201437.
7. Y. Chen, Z. Chen and J. Wei, ACS Appl. Nano Mater., 2023, 6, 14702–14709.
8. Y. Yang, Y. Chen, Z. Hou, F. Li, M. Xu, Y. Liu, D. Tian, L. Zhang, J. Xu and J. Zhu, ACS Nano, 2020, 14, 16057–16064.
9. D. Yang, W. Luo, Y. Huang and S. Huang, ACS Omega, 2019, 4, 528–534.
10. T. H. Zhao, G. Jacucci, X. Chen, D.-P. Song, S. Vignolini and R. M. Parker, Adv. Mater., 2020, 32, 2002681.
11. B. Li, C. Ouyang, D. Yang, Y. Ye, D. Ma, L. Luo and S. Huang, J. Colloid Interface Sci., 2021, 604, 178–187.
12. W. Niu, X. Wang, Y. Zheng, S. Wu, M. Hua, Y. Wang, X. Zhang, A. I. Y. Tok, X. He and S. Zhang, Small, 2020, 16, 2003638.
13. J. Fan, X. Cai, H. Chen, L. Wu, X. Dong, W. Zhang, Y. Qiao, Z. Meng and L. Qiu, Chem. Eng. J., 2022, 446, 136450.
14. I. Lee, D. Kim, J. Kal, H. Baek, D. Kwak, D. Go, E. Kim, C. Kang, J. Chung, Y. Jang, S. Ji, J. Joo and Y. Kang, Adv. Mater., 2010, 22, 4973–4977.
15. X. Wen, X. Lu, C. Wei, J. Li, Y. Li and S. Yang, ACS Appl. Nano Mater., 2021, 4, 9855–9865.
16. K. Ohno and Y. Mizuta, ACS Appl. Polym. Mater., 2020, 2, 368–375.
17. D. Kou, R. Lin, C. Li, S. Zhang and W. Ma, Chem. Eng. J., 2022, 430, 132805.
18. Z. Hu, N. P. Bradshaw, B. Vanthournout, C. Forman, K. Gnanasekaran, M. P. Thompson, P. Smeets, A. Dhinojwala, M. D. Shawkey, M. C. Hersam and N. C. Gianneschi, Chem. Mater., 2021, 33, 6433–6442.
19. R. Shanker, S. Sardar, S. Chen, S. Gamage, S. Rossi and M. P. Jonsson, Nano Lett., 2020, 20, 7243–7250.
20. S.-I. Lim, E. Jang, D. Yu, J. Koo, D.-G. Kang, K. M. Lee, N. P. P. Godman, M. E. E. McConney, D.-Y. Kim and K.-U. Jeong, Adv. Mater., 2023, 35, 2206764.
21. J. Zhang, J. Zhang, Y. Ou, Y. Qin, H. Wen, W. Dong, R. Wang, S. Chen and Z. Yu, Small, 2021, 17, 2007426.
22. J. Zhai, N.-X. Zhang, F. Li, C. Liu, G.-X. Li, X.-Q. Yu, Q. Li and S. Chen, J. Mater. Chem. C, 2025, 13, 3475–3481.
23. R. Ohnuki, S. Isoda, M. Sakai, Y. Takeoka and S. Yoshioka, Adv. Opt. Mater., 2019, 7, 1900227.
24. G. H. Lee, S. H. Han, J. B. Kim, D. J. Kim, S. Lee, W. M. Hamonangan, J. M. Lee and S.-H. Kim, ACS Appl. Polym. Mater., 2020, 2, 706–714.
25. S. K. Nam, E. Amstad and S.-H. Kim, ACS Appl. Mater. Interfaces, 2023, 15, 58761–58769.
26. Q. Fu, H. Zhu and J. Ge, Adv. Funct. Mater., 2018, 28, 1804628.
27. Y. Liu, C. Shao, Y. Wang, L. Sun and Y. Zhao, Matter, 2019, 1, 1581–1591.
28. T. Yu, J. Bi, W. Wei and X. Su, J. Mater. Chem. C, 2022, 10, 17451–17471.
29. Y. Fang, L. Chen, Y. Zhang, Y. Chen and X. Liu, Opt. Mater., 2023, 138, 113724.
30. C.-H. Liu, C.-H. Hsu, W.-T. Hsu, W.-C. Li, C. Chang and H.-P. Lin, Surf. Interfaces, 2024, 48, 104230.
31. X. W. Su, L. Q. Zhu, W. P. Li, H. C. Liu and H. Ye, RSC Adv., 2020, 10, 4554–4560.
32. T. O. Kozlova, D. N. Vasilyeva, D. A. Kozlov, I. V. Kolesnik, M. A. Teplonogova, I. V. Tronev, E. D. Sheichenko, M. R. Protsenko, D. D. Kolmanovich, O. S. Ivanova, A. E. Baranchikov and V. K. Ivanov, Molecules, 2024, 29, 2157.
33. T. Montini, M. Melchionna, M. Monai and P. Fornasiero, Chem. Rev., 2016, 116, 5987–6041.
34. N. Ditlopo, N. Sintwa, S. Khamlich, E. Manikandan, K. Gnanasekaran, M. Henini, A. Gibaud, A. Krief and M. Maaza, Sci. Rep., 2022, 12, 3468.
35. A. Miri, S. A. Birjandi and M. Sarani, J. Biochem. Mol. Toxicol., 2020, 34, e22475.
36. N. Ahmad, S. Rasheed, M. I. Nabeel, W. Ahmad, A. Mohyuddin, S. G. Musharraf, M. Najam-ul-Haq, Z. K. Ghouri and D. Hussain, Langmuir, 2023, 39, 11571–11581.
37. I. S. Raja, N. Duraipandi, M. S. Kiran and N. N. Fathima, RSC Adv., 2016, 6, 100916–100924.

---