Full Length ArticleEngineering highly transparent UV-shielding films with disassembled polydopamine oligomers as light adsorber
Graphical abstract
Introduction
Due to the inherent high-energy characteristic, ultraviolet (UV) radiation has been widely applied in medical diagnosis and therapy [1], disinfection [2], food, and sewage treatments [3], [4], polymer processing, etc. However, UV radiation shows some obviously negative effects on organisms and polymer materials. Excessive exposure to UV radiation can bring severe damages on human skin cells and tissues as well as lead to the breakage of covalent bonds and the destruction of structures of polymer materials [5]. In this context, it is essential to provide sufficient protection for organisms and polymeric materials from UV overexposure. Therefore, it is significant to develop highly efficient UV-shielding films to decrease the UV light transmittance for many applications, such as vehicle windows, food packaging, contact lenses, and biomedical product protection.
The UV-shielding films are usually made from the incorporation of UV light absorbers and polymers, in which UV light absorbers and polymers are used to shield the UV light and provide the matrix of the films, respectively. The UV light absorbers can be divided into two categories: inorganic oxides and organic filters. Inorganic oxides, such as nano-TiO2, SiO2, ZnO, and Al2O3, etc., can effectively dissipate the UV light through absorbing, scattering, and reflecting [6], [7], [8], [9]. However, owing to their inherent wide band gaps, their UV absorption ability is insufficient. Meanwhile, conventional inorganic oxides possess photocatalytic activity and the generated free-radicals can induce undesirable photo-degradation of polymeric materials [10], [11]. Compared to inorganic oxides, organic absorbers (e.g., benzophenone, benzotriazole, salicylate, lignin, etc.) have more efficient UV-shielding performances as well as higher miscibility with polymer matrix [12], [13]. For example, Mehta et. al reported a gelatin film doped with ionic liquids and the typically prepared film was able to absorb nearly 100% UV light within the wavelength of 200–350 nm [14]. Zhang et al. developed a type of PVA/lignin nano-micelle composite films with excellent vapor barrier and UV-shielding properties [15]. However, the transparency of this film is not high, which impedes the high-efficiency application of the UV-shielding films. Hence, developing highly efficient UV-shielding films while maintaining high transparency remains a major challenge, although organic nanomaterials have great potential as light absorbers in the fabrication of safe, low-cost, biodegradable, high-performance UV-shielding materials.
Polydopamine (PDA), a typical synthetic melanin, can be easily obtained through spontaneous self-polymerization of dopamine under weak alkaline conditions [16], [17], [18], [19], [20], [21]. Owing to its unique chemical structures and the inherent active groups (e.g., NH2 and OH), PDA possesses lots of functions similar to natural melanin including photoprotection [22], [23], anti-oxidation [24], thermoregulation, biocompatibility [25], [26], and biodegradation [27]. It was confirmed that PDA is capable of dissipating UV radiation through non-radiative decay [28], [29], just as natural melanin in the body protects the skin tissues from UV light. This fantastic property confers upon PDA with the potential as a UV absorber [30], [31]. Actually, Dong et al. reported a sepia eumelanin (SE)-doped UV-shielding composite film that can block most UV light below 300 nm with a 0.5 wt% of SE [32]. They also used PDA nanoparticles (NPs) and hollow PDA as light absorbers to improve the UV shielding performance of films [33]. Furthermore, it was reported that PDA is prone to disassembly and can disintegrate into oligomers under alkaline conditions due to its supramolecular character. This is favorable for addressing the trade-off between UV-shielding ability and transparency in the UV-shielding films as dPDA exhibited distinct UV absorption based on their attenuated inter-and intra-molecular conjunction effects after disassembly [34], [35], [36]. In addition, the characteristics of low molecular weight and small size of dPDA are advantageous to their homogeneous distribution in the polymer matrix. Thus, it is highly possible to prepare a high-performance UV-shielding film with high transparency using dPDA oligomers as UV light absorber, while has yet to be performed up to now.
This work aims to prepare highly transparent UV-shielding films developing disassembled PDA oligomers as UV light absorber. Herein, the dPDA oligomers were prepared by disassembling the PDA NPs in strong alkaline solution, and used for preparing the highly transparent UV-shielding film by blending with polyvinyl alcohol (PVA). The UV–vis absorbance characteristics and the micro-structures of dPDA oligomers were analyzed. The UV-shielding properties and the visible light transmittance of the prepared dPDA/PVA nanocomposite films were investigated in detail. The relationship between preparation, structure and photophysical properties of the prepared UV-shielding films was studied in detail to elucidate their absorption mechanism. To the best of our knowledge, this is the first work to achieve the preparation of highly transparent UV-shielding films using dPDA.
Section snippets
Materials and reagents
Dopamine hydrochloride (DA-HCl), polyvinyl alcohol-1788 (PVA), titanium dioxide (TiO2), and methylene blue (MB) were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH), hydrochloric acid (HCl,~36%), glycerol and ethyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were analytical grade and used without further treatments.
Preparation of dPDA oligomers
In a typical synthesis, 180 mg of DA-HCl were fully dissolved in 90 mL of deionized (DI) water at 50 ℃, followed by addition of NaOH solution
Results and discussion
Polydopamine (PDA) is a well-known melanin-like nanomaterial which has been applied in a variety of fields, such as biomimetic adhesives, biomaterials, drug delivery, photoelectric device, Li-ion batteries, catalysts, etc. [20]. Despite the wide application of PDA, its molecular structure and formation mechanism seem difficult to be elucidated unambiguously and thus this problem keeps controversial so far. But it has been evidently proved that PDA is a supramolecular aggregates with dopamine
Conclusions
In summary, we successfully prepared dPDA oligomers by alkaline-induced disassembly of PDA NPs and explored the feasibility of dPDA as a promising UV absorber used in highly efficient UV-shielding film. The disassembly process of PDA in the strongly alkaline solution not only leads to the destruction of conjugated effects and intermolecular hydrogen bonds, but also triggers the complicated oxidation reactions. The dPDA oligomers prepared from PDA under alkaline conditions can be better
CRediT authorship contribution statement
Jingyu Lu: Conceptualization, Methodology, Resources, Data curation, Formal analysis, Investigation, Methodology, Writing - original draft. Jinchao Fang: Methodology, Software, Writing - original draft. Jiaqi Li: Conceptualization, Resources, Writing - review & editing. Liping Zhu: Funding acquisition, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The National Natural Science Foundation of China (Grant No. 51828301, 51773175 and 51973185) and the Fundamental Research Funds for the Central Universities.
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