Photochromic performance of hydrogel based on deep eutectic solvent induced water soluble Cu-doped WO₃ hybrids with antibacterial property

Highlights

• Deep eutectic solvents (DESs) as a co-solvent was employed to prepare Cu-doped WO₃ photochromic materials.

• The fabricated P(AM-HEA)-Cu-WO₃-X photochromic hydrogels exhibit rapid photoresponse and enhanced self-fading rate.

• The deep-blue colored P(AM-HEA)-Cu-WO₃-25 % hydrogel could come back to its colorless state within 40 min.

• The mechanism of photochromic and self-fading behaviors for P(AM-HEA)-Cu-WO₃-X hydrogels was detailed analyzed.

• The remarkable antibacterial performance and cell viability possessed in hydrogels also confirmed.

Abstract

A novel photochromic and self-bleaching hydrogel was prepared based on the Cu-doped tungsten oxide (WO₃) hybrid encapsulated in the co-polymer matrix polymerized using acrylamide (AM) and hydroxyethyl acrylate (HEA) as monomers. The Cu-WO₃-25 % hybrid was generated by introducing Cu species into the lattice of WO₃ with hexagonal-like prismatic structure and acted as light absorber. Given that WO₃ and Cu-doped WO₃ hybrids possess superior water solubility by using water and deep eutectic solvents (DESs, ChCl:EG) containing sufficient hydrogen bond as co-solvent, the high transparency of 90 % of the hydrogels was achieved. As compared with P(AM-HEA)-WO₃-based hydrogel, the P(AM-HEA)-Cu-WO₃-X hydrogel exhibited the enhanced coloration degree under UV irradiation. The P(AM-HEA)-Cu-WO₃-25 % photochromic hydrogel changed from colorless to blue immediately once illuminated by UV light, and turns into dark-blue with the irradiation time going on. The self-bleaching process indicates it only takes 40 min for the P(AM-HEA)-Cu-WO₃-25 % hydrogel to return to the original colorless transparent state. Furthermore, the hydrogel systems exhibit efficient antibacterial activities against bacteria of both E. coli and S. aureus, accompanying with outstanding biocompatibility. As such, the fabricated hydrogel demonstrates rapid photoresponse, self-bleaching process, superior antibacterial performance, and cell viability, showing a great potential application as stimuli-responsive materials.

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 (H_xW^VI_1-xW^V_xO₃). 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.