Photochromic performance of hydrogel based on deep eutectic solvent induced water soluble Cu-doped WO3 hybrids with antibacterial property
Graphical abstract
A novel fast photoresponse and self-fading photochromic hydrogel system was fabricated based on Cu-doped WO3, which prepared by employing DESs as co-solvent. The photochromic hydrogel displayed remarkable antibacterial ability and non-cytotoxicity, great potential in the applications of stimuli-responsive materials such as smart windows, repeatable erasing paper and photochromic sensors.
Introduction
As one kind of smart materials, photochromic hydrogels that are experienced coloration process under specific light irradiation and self-fading upon light removed, have gained enormous attentions due to the advantages of stimuli response, easy operation and functionality, thus showing widespread applications in sensors, information storage, and rewritable paper [1], [2], [3]. Accordingly, the presence of an oxidant (e.g., O2) significantly results in the fading behavior after coloration [4]. Hydrogels are three-dimensional (3D) cross-linked polymer networks which contain a group of polymeric materials and the hydrophilic structure can offer the possibility of holding large volume of liquid in swollen state [5], [6]. The 3D networks structure of hydrogel can provide channels for water and oxygen to pass through, which are necessary for the coloration and bleaching enhancement of the photochromic materials [7], [8].
In addition to the native properties of 3D network, high water content and flexibility, high antibacterial activity and good biocompatibility are also critical for photochromic hydrogels in practical applications of smart windows, sensors, rewritable paper and information storage devices [9], [10]. Therefore, the hydrogels fabricated by employing monomer with inherent antibacterial property is a feasible strategy to develop antibacterial hydrogel [11]. Further, it is well known that the positively-charged amino groups bearing on polymers can be electrostatically adsorbed with negatively-charged teichoic acid in bacterial cell membrane, further damaging the integrity of cell wall and leading to the death of bacteria [12], [13]. Among many polymers, acrylamide (AM) is widely utilized to prepare the hydrogels owing to its good biocompatibility and excellent permeability to both hydrophobic and water-soluble solutes, as well as the existed amino groups offer antimicrobial properties [14]. Moreover, AM is a water-soluble monomer with a large number of amide groups in its structure, which is easy to form hydrogen bonds and to be chemically modified, leading to high structure stability [15], [16]. Therefore, researchers have prepared pAM hydrogels with various characteristics by introducing functional groups and nanomaterials. In this regard, pAM hydrogels have been widely used in water treatment, biomedicine, waste treatment and other fields [17].
Photochromic materials have been widely investigated as they are quite attractive in various fields. They can be divided into organic and inorganic photochromic materials according to the differences in the structure and coloration mechanism [18]. The former mainly contains spiropyran, azobenzene and diarylethene [19], while the latter includes transition metal oxides (TMOs) and metal halides as well as rare earth complexes [20]. The inorganic photochromic TMOs, including tungsten oxide (WO3), molybdenum oxide (MoO3), titanium dioxide (TiO2), vanadium pentoxide (V2O5), and niobium pentoxide (Nb2O5) with good stability and cost efficiency compared to photochromic materials based on organic units have been widely studied [21], [22]. Among them, WO3 has drawn more researchers’ attention owing to the adjustable wide energy band gap (2.4–3.6 eV) and regarded as one of the most important candidates since discovered by Deb et al [23]. A coloration change from colorless to deep blue or sometimes brown is induced when exposed WO3 to UV irradiation or sun light, and this coloration change is as a result of the valence state reduction from W6+ to W5+ and/or W4+, which is then combined with H+ and formed tungsten bronzes (HxWVI1-xWVxO3). On the reverse, the W5+ is oxidized to W6+ with the assistance of oxygen, resulting in the bleaching process [24]. Slow photoresponse speed to the light stimulation, poor reversibility and limited applicable wavelength range are main challenges faced by most of WO3-based photochromic materials due to its native property of low charge carries separation efficiency, which definitely hinder their developments [24]. To improve the photochromic activities of WO3, a number of strategies have been developed, including structural tunable, heteroatom doping and the combination of WO3 with other material (such as noble and/or rare earth metals, or TMOs) [25], [26]. It is found that structure with large specific surface area-to-volume ratios (such as nanosheets, nanodots, flakes or hierarchical) may provide special electron transfer channels and shorten the ionic pass through length for species anchored on surface, resulting in the rapid decomposition of surface water molecular under UV stimulation, accelerating proton intercalation, and leading to a quick response in the coloration process [27]. Moreover, coloration behaviors under visible light can be apparently observed when WO3 particles possess quantum size diameter since the existence of the quantum size effect and the absorption peaks shift to the visible light ranges [25]. Recently, the metal-doping, such as Al, Cu, Mo, and Ti, can act as a simple yet efficient method to improve the photochromism effect of WO3 [28], [29]. The doped-metals may lead to a partially atomic substitution or diffused into the WO3 lattice, which not only can reduce the band gap of WO3, but also can change the crystal structure and the phase composition [30]. In some cases, elemental doping may result in a defect energy level, further affecting the charge transportation and enhancing the photochromic behavior [31]. Thereby, the fabrication of WO3-based heterojunctions with other semiconductors can efficiently enhance the photochromic activities of WO3. The semiconductor-coupling plays a role in the separation of electron–hole pairs generated under photo-stimulation due to different positions of valence band and conduction band, as well as reducing the reconnection of electrons and holes, improving the photochromic efficiency [20]. Herein, Cu is selected as a companion for WO3 not only due to its low cost compared with noble metals (Au, Ag, or Pt), but also because of its unique abilities to the enhancement of visible light absorption and the photocatalytic activity under UV light irradiation, as well as the antibacterial activity. Moreover, the presence of proton donors (such as H2O and methanol) in the surrounding environment would also affect the photochromic behaviors [32]. Accordingly, the above mentioned results can broaden researchers’ horizon and offer more strategies to improve the photochromic properties of WO3 based materials.
The preparation techniques for WO3 mainly include sol–gel strategy, hydrothermal, and chemical vapor deposition (CVD) methods [16]. And the solution-based synthesis procedures have been employed more widely owing to the advantages of mild conditions, easy operation, low cost, and adjustable morphologies [17], [18]. Deep eutectic solvents (DESs) as a novel class of green and sustainable solvents have been rapidly developed since first reported by Abbot in 2003 [33]. An increasing number of new DESs have been prepared by many researchers owing to easy preparation, inexpensive, and biodegradable. DESs are a mixture of one provide hydrogen-bond acceptor (HBA) and another provide hydrogen-bond donor (HBD) based on the strong hydrogen-bond interaction, in which the formed eutectic mixtures with a fusion point lower than that of either sole component (generally lower than 100 ℃) [34]. HBA is often a quaternary ammonium salts, while HBD comprises amines, carboxylic acids, urea, glycerol or carbohydrates [35]. It is worth noting that as the optimal substitute for ionic liquids (ILs), DESs possess some advantages over the traditional organic solvents, such as wide liquid range, excellent thermal stability, high conductivity, small vapor pressure, and non-or less toxicity [36], [37]. Moreover, -abundant –OH groups in DESs endow the as-prepared materials with excellent water dispersity. They have been widely applied in a variety of applications such as electrochemistry, catalysis, gas absorption, and high purity extraction and separation [38], [39]. In addition, DESs, as a functional medium, can be employed as precursors for the fabrication of different materials with unique properties. For example, Sun and co-workers designed a hybrid PtCu nanocluster on multi-walled carbon nanotubes (MWCNTs) through a one-step hydrothermal reduction method in DESs medium without adding any surface controlling agent. PtCu nanoparticles were uniformly distributed on MWCNTs and the cluster nanostructure was formed due to the agglomeration. Thanks to the improvement of charge transportation between the electrocatalytic moisture and the MWCNT matrix, the as-prepared PtCu ANCs/MWCNTs exhibited superior MOR catalytic property and long-term stability in acid medium [40]. Wu’s group used a ternary DESs composed of urea, melamine and metal chlorides as precursors for the preparation of a series of g-C3N4/metal oxide via a simple one step thermolysis method, in which the metal oxide was uniformly supported on g-C3N4 nanosheet. The effective interfacial interconnection between 2D g-C3N4 nanosheet and α-Fe2O3 nanoparticles, resulting in the highly photocatalytic activity for g-C3N4/40 wt% Fe2O3, and giving the maximum sufficiency for ammonia generation reached 4380 µmol L-1h−1 [41]. Thereby, the exploration of DESs not only can reduce the risk and cost of experiments, but also can provide more possibilities to prepare materials with unique performance.
Herein, inspired by the advantages of metal doping and the unique structure possessed in hydrogel, we developed a photochromic hydrogel system based on the Cu-doped WO3 hybrids. The preparation of the photochromic hydrogel was divided into two parts (scheme 1). First of all, Cu2+ was picked as light absorber for hybrids construction via a facile hydrothermal method by employing DESs as co-solvent, and Cu2+ also played a critical role in promoting the department of the electron-hole generated under photostimulation by interfacial charge transfer between the continuous energy levels of solids and synergistic effects existed in different metal species. The DESs solvent provided sufficient − OH group for the prepared materials, endowing them with excellent water solubility and further leading to the fabrication of hydrogels with high transparency. Secondly, hydrogel was utilized as the matrix, which on the basis of poly(hydroxyethyl acrylate) (pHEA) and pAM. The microstructure and composition of the as-prepared Cu-doped WO3 hybrids were studied through instrumental analysis, such as Fourier transform infrared (FT-IR), X-ray diffraction (XRD), Field emission scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The photochromic property and self-bleaching behavior of hydrogels were investigated by ultraviolet–visible (UV–vis) spectra and the mechanism was also analyzed. The results indicated a very fast photoresponsive rate of the Cu-doped WO3-based hydrogel, and the Cu doping not only efficiently deepened the coloration degree of P(AM-HEA)-Cu-WO3-25 % hydrogel, but also greatly promoted the self-bleaching process owing to a new charge transfer channel was formed through the W-O-Cu bond, endowing a fading time of 40 min. Moreover, the fabricated hydrogels also possessed remarkable antibacterial performance and cell viability. Therefore, the hydrogel system constructed in this work has potential applications in the fields of repeatable self-erasing paper and solar energy devices.
Section snippets
Chemicals and reagents
Choline chloride (ChCl, 98.0 %), ethylene glycol (EG, 99.0 %), acetic acid (AA, 99.5 %), sodium tungstate dihydrate (Na2WO4·2H2O, AR ≥ 99.5 %), and cupper chloride (CuCl2, 99.0 %), were all purchased from Sinopharm Chemical Reagent Co. ltd. HEA and AM were obtained from Shanghai Macklin Biochemicals Co. ltd. Moreover, N,N'-methylene-bis(acrylamide) (MBAA) and potassium persulphate (KPS) were purchased from Aladdin Industrial Co. ltd. (Shanghai, China), respectively. Escherichia coli (E. coli)
Basic characterization of WO3 and the series Cu-doped WO3 hybrids
The macrostructures and morphologies of the as-prepared WO3 and Cu-doped WO3 hybrids were investigated by FE-SEM technique. As displayed in Fig. S1a, WO3 is composed of hexagonal-like prismatic structure with uneven size. The morphology of Cu-WO3-5 % hybrids with 5 % Cu dosage is similar with WO3, and the observed cuboid-like structure anchored on the surface of hexagonal-like WO3 might attributed to the formation of Cu or CuxO (Fig. S1b). The FE-SEM images of Cu-WO3-15 % and Cu-WO3-25 %
Conclusions
In summary, we reported the construction of a new photochromic hydrogel based on Cu-WO3-25 % hybrids with fast coloration and self-bleaching rates. The co-solvent of DES and water favored the solubility of the final product, resulting in a colorless hydrogel with high transparency. The reversible photochromic hydrogel changed from colorless to deep-blue under UV irradiation for 3 min and returned to its original colorless state after 40 min in room temperature and air environment. Notably, the
CRediT authorship contribution statement
Bingbing Cui: Writing – original draft, Formal analysis. Chuanpan Guo: Formal analysis. Guodong Fu: Writing – review & editing, Supervision, Resources. Zhihong Zhang: Writing – review & editing, Supervision.
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.
Acknowledgements
This work was supported by the National Natural Science Foundation of China under Grant 52073059.
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