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Sunlight-driven photochromic hydrogel based on silver bromide with antibacterial property and non-cytotoxicity
Chemical Engineering Journal 375 (2019) 121994
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Sunlight-driven photochromic hydrogel based on silver bromide with antibacterial property and non-cytotoxicity
T
Mengmeng Kanga, Yaoyao Denga, Olayinka Oderindea, Fangfang Sub, Wenjing Maa, Fang Yaoa, ⁎ ⁎ Guodong Fua, , Zhihong Zhangb, a
School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing, Jiangsu Province 211189, China Henan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou University of Light Industry, No. 166, Science Avenue, Zhengzhou, Henan Province 450002, China
b
HIGHLIGHTS
silver bromide-based photochromic • Ahydrogel was designed and fabricated. and PVP were used to main• Glycerol tain photochromism of the hydrogel. photochromic process of the fab• The ricated hydrogel was studied and
GRAPHIC ABSTRACT
Mimicking sunlight-driven glass, a novel photochromic hydrogel system was fabricated based on silver bromide. PVP and glycerol were employed to achieve repeatability and fast photochromism. The photochromic hydrogel displayed remarkable antibacterial ability and non-cytotoxicity, which can be as photochromic contact lenses, ornaments and artificial intelligent devices. Hence, photochromic hydrogel is a new direction in the field of silver bromine-based materials.
concluded.
hydrogel shows excellent anti• The bacterial property and non-cytotoxicity.
ARTICLE INFO
ABSTRACT
Keywords: Photochromic hydrogel Antibacterial Non-cytotoxic Silver bromide Glycerol
A novel photochromic hydrogel system was designed and fabricated based on silver bromide and cuprous bromide in poly(hydroxyethyl acrylate-co-polyacrylamide) hydrogel matrix. The first time we overcome the atoms side reaction and migration of halides in opening hydrogel system to realize photochromism. Here, polyvinyl pyrrolidone (PVP) and glycerol were employed as a surfactant and sustainable co-solvent, respectively, whose synergistic effect make AgBr endows the hydrogel with photochromic stability and repeatability. The photochromic hydrogel can be changed reversibly between light-blue and brown color for least 15 times in air under a simulated solar light irradiation. Both UV–vis absorption (approximately 408 nm) and the valence states of the metal ions were studied and verified the conversion between Ag+ and Ag0. The process of color changing was studied and the fabricated hydrogel with a half-life fading period (t1/2) was 6.78 min under a simulated solar light irradiation. Reasonably, the as-fabricated photochromic hydrogel displayed remarkable antibacterial ability to both Gram-positive- and Gram-negative bacterium with the Ag ions in the system. Interestingly, due to the little dosage of AgBr and CuBr in the hydrogel, it also possessed excellent non-cytotoxicity property to L929
⁎
Corresponding authors. E-mail addresses: [email protected] (G. Fu), [email protected] (Z. Zhang).
https://doi.org/10.1016/j.cej.2019.121994 Received 13 March 2019; Received in revised form 5 June 2019; Accepted 16 June 2019 Available online 17 June 2019 1385-8947/ © 2019 Published by Elsevier B.V.
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Mouse Fibroblast cells. Since its excellent photochromic and biological properties, the hydrogel has potential applications in the field of smart materials such as photochromic contact lenses, sensors and artificial intelligent devices or bionic skin.
1. Introduction
crystals in air and the mobility of the halogens, reverse reaction in soft material matrix is difficult [26,27]. Photochromic materials based on silver halides have been studied in soft materials such as films [28–30] and fibres [31], however, the study on photochromic hydrogel based on silver halides yet reported. Furthermore, limiting the side reaction of silver and halogen atoms and applying the photochromic silver halide in hydrogels based on the above-mentioned extensive application of hydrogels remain challenging. To solve the above-mentioned problem, polyvinyl pyrrolidone (PVP) and glycerol were introduced in the hydrogel system. PVP, which is a kind of nontoxic and non-ionic polymer, with C]O, C–N, and –CH2 functional groups has been widely used for nanoparticle synthesis [32], as it can be used in water or many non-aqueous liquids due to the highly polar amide group within the pyrrolidone ring and polar methylene and methine groups in the ring, as well as its backbone [33]. Meanwhile, glycerol could be attached to Cu+ between its two hydroxyl oxygen molecules, as determined by Laurence Boutreau et al. [34] Simultaneously, glycerol has a viscosity coefficient of 56.0 mPa·s (25 °C), and can form a hydrogen bond with water [35]; thus, can hinder the spill of bromine in the system. Therefore, inspired by the photochromic glass, a photochromic hydrogel system was designed on the basis of silver bromide (AgBr) with cuprous bromide (CuBr) as catalyst. The fabrication of the photochromic hydrogel was based on AgBr and included two steps (Scheme 1). Firstly, PVP was selected as a surfactant for nanoparticle preparation in glycerol solution. Secondly, hydrogel based on poly(hydroxyethyl acrylate) (pHEA) and polyacrylamide (pAM), which possess excellent transparency detected at ≥88%, was employed as the matrix for the photochromism. In general, the light yellow AgBr after photolysis can be regenerated from silver (Ag0) and free bromine molecule (Br2) without irradiation under cuprous catalysis to achieve optical reversible colour conversion [10,36], as shown in Scheme 1. With the existence of cuprous ions, the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel exhibited reversible and considerably fast colour switching response on the basis of the reaction of AgBr to UV and visible light. This hydrogel might be used in the field of photochromic contact lenses, ornaments and smart devices.
With the development of soft robots and sensors, smart materials, which respond to external stimuli such as heat, light, electric, magnetism, force, humidity, and chemical inputs, have attracted greater attentions [1–7]. Among these stimuli, light, especially solar light, have the unique advantages including universality, renewability, and availability, in addition to the advantages of easy-to-control parameters, such as precise location, different wavelengths, intensity, and illumination duration [8,9]. Photochromic materials, which change colour reversibly by light stimulus, have been extensively developed [10–12]. Among them, photochromic hydrogels have been studied and has potential applications in various fields such as smart devices, sensors, tissue engineering and in soft robotics [13–15]. Researchers have extensively studied organic molecule-based photochromic materials [15–19], but inorganic molecule-based photochromic hydrogels are few. For example, Gao et al. [1,20]. developed an ammonium molybdate-based photochromic hydrogel into polyacrylamide (pAM) for display, storage, and ink-free printing, while Geng et al. [21] fabricated a tungstate nanosheet-based flexible photochromic hydrogel; and Cong Wang et al. [22] fabricated an amorphous tungsten oxide-based photochromic hydrogel to release H2O2 for cancer inhibition. Even inorganic photochromic materials have a considerably better stability under repeated/prolonged light illumination than organic systems [8], with the production process are not as complex as organic photochromic molecules synthesis procedures. However, the repeatability of these inorganic-based photochromic hydrogels is not ideal. Therefore, fabricating photochromic materials with simple steps and stable photochromic properties remains a challenge. Among inorganic photochromic materials, silver halides are widely used in commercial borosilicate or aluminoborosilicate glasses, making the glass to undergo reversible colour switching when exposed to sunlight and with excellent photochromic repeatability [10]. This inspired the design of a photochromic hydrogel on the basis of silver halide with excellent repeatability. Within the glass, silver halides absorb photons and electron-hole pairs liberated under light irradiation; after which, the photo-generated electrons become easily trapped by the Ag+, resulting in the formation of silver atoms (Ag0). Thereafter, shielding the light or in the dark, Ag0 and halides could be reversed back to silver halides with the help of cuprous salts or copper [23–25]. This process allows silver halide-doped glasses to effectively control the transmittance of light through glasses, which have been applied to ophthalmic lenses. However, given the irradiation activity of the silver
2. Experimental section 2.1. Chemical Chemicals. Glycerol (purity, ~99.99%) and AgNO3 (purity,
Scheme 1. Schematic illustration of a fabricated photochromic hydrogel based on AgBr nanoparticles. 2
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~99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd, while NaBr (purity, ~99.99%), polyvinyl pyrrolidone (PVP) (K30) (purity, ~99.99%), hydroxyethyl acrylate (HEA), acrylamide (AM) were all purchased from Shanghai Macklin biochemical Co. Ltd. All the reagents were used as received without further purification. Milli-Q filtered water with resistivity > 18 MΩ·cm was used throughout.
3. Results and discussion 3.1. Optimization of reaction system and photochromic solution preparation Prior to fabrication of photochromic hydrogel, photochromic solutions based on AgBr with CuBr as a catalyst were prepared and the system was optimized. The stability of the photochromic solutions based on AgBr of different solvents with CuBr (glycerol, water, and ethylene glycol) are depicted in Fig. 1. Here, using the molar ratio of silver to cuprous (10:4) as a model, because more CuBr cannot be completely dissolved, while the systems has no obvious photochromism with less CuBr. Among the three solutions, glycerol and ethylene glycol solutions were photochromic-active, but only the glycerol solution was reversibly stable, with the macro-change being photographed (Fig. 1[a]). As shown in Fig. 1(b), the glycerol solution could remain stable for at least one week, whereas water solution or ethylene glycol started to change their coloration only after few hours. Interestingly, the glycerol solution still maintained its good photochromic and stability properties, even after six months of storage (Movie 1), unlike ethylene glycol solution, which could only be maintained for several days.
2.2. Fabrication of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel The preparation of AgBr-glycerol solution was discussed in the Supporting Information. P(HEA-AM)-glycerol-AgBr/CuBr hydrogel was obtained by adding 1 mL AgBr-glycerol solution in a system containing 1.4 g of AM, 0.6 mL of HEA, 0.5 mL of MBAA (0.1%, wt% aqueous solution) in 5 mL water. After complete mixing, 0.5 mL KPS aqueous solution (0.2 M). TEMED was thereafter added to initiate the polymerization process and the photochromic hydrogel was obtained. 2.3. Photochromic behaviour of the hydrogel The reversible colour-change of the hydrogel was detected by UV2450 UV–Vis spectrophotometer (Japan), with the wavenumber ranging from 800 nm to 200 nm and at a resolution of 1 nm. The specimens were irradiated with a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.) at 10.0 cm distance on a sample stage. 2.4. In vitro antibacterial tests and cytotoxicity Gram-positive staphylococcus aureus (S. aureus, ATCC 29213) and gram-negative Escherichia coli (E. coli, ATCC 25922) were used to evaluate the antibacterial activity of the hydrogels colony forming unit (CFU) through method, according to the reference [37]. Additionally, MTT assays were employed to evaluate in vitro cytotoxicity against L929 Mouse Fibroblast cells of the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel. Details are described in the Supporting Information.
Movie 1.
To explain light-responsible phenomenon of the various solution, the powders collected from various solutions were characterized by Powder X-ray Diffraction (PXRD) to confirm the content of crystals. The PXRD patterns of powders collected from glycerol, water, and ethylene glycol are provided in Fig. 2. The results showed that no peak from
Fig. 1. (a) Photos of photochromism in various solvents and (b) of changes for different solvents.
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Fig. 2. (a) XRD spectra of nanoparticles prepared in three solvents. (I) glycerol, (II) water and (III) ethylene glycol, and (IV) AgBr/CuBr nanoparticles prepared in glycerol without PVP. (b–d) SEM of samples (I), (II) and (III) corresponding to (a), respectively.
CuBr existed in the reaction system, with glycerol as the solvent and PVP as the surfactant. Diagnostic peaks at 26.8°, 30.9°, 44.3°, 55.2°, 64.5°, and 73.2° were attributed to AgBr (PDF#06-0438), without a single peak belonging to CuBr crystals collected from glycerol and ethylene glycol. Such result has two plausible reasons. Firstly, the PVP molecule as a kind of surfactant, could promote the CuBr solubility in the glycerol solution [38]; and secondly, glycerol complexes with cuprous ions further promote the solubility of CuBr [34]. However, in water solution, the peak at 14.7° was indexed to ( −1 0 1) of cupric bromine (CuBr2) (PDF#45-1063), whereas the peaks at 26.6°, 34.15°, and 44.0° belonged to the (1 1 3), (0 4 0), and (4 4 0) crystal lattices of Ag2O3 (PDF#40-0909), respectively, as shown in Fig. 2(a)(II). Thus, the nanoparticle fabricated in aqueous solution was easier to be oxidized than in the other two media, resulting in the instability of the solution that could not be maintained for a day (Fig. 1). Meanwhile, glycerol as an example of green solvent, has been drawing considerable attention for several reasons: (i) it has a unique combination of several properties, including high polarity, low toxicity and high boiling point, and can also dissolve both organic and inorganic compounds [39,40]; (ii) it is relatively cheap and abundantly sourced as it is a value-added byproduct in biodiesel industry and in the new conversion of cellulose and lignin process [41,42]; (iii) in some cases, it can enhance effectiveness and selectivity [43–45]; and (iv) the glycerolic phase containing the catalysts could be re-used in catalyst recycling [46], or the catalyst ability of cuprous compounds could be improved [45]. Based on the above analyses, glycerol was selected to form a protective environment
for AgBr nanoparticles to maintain the photochromic stability in the following study. Without PVP in the glycerol system, a recognizable CuBr peak existed in the XRD peak because CuBr was not easily dissolved but precipitated out without a surfactant. Simultaneously, given the existence of the amide group, PVP could be an anchoring agent with Ag+ or Ag atoms, possibly preventing silver from precipitating [47,48]. The other reason to selecting PVP is that materials containing silver halide crystals at approximately 50–800 Å (5–80 nm) diameter can achieve excellent photochromic property [29], with Zhang et al. [49] analysing the possible mode of PVP in protecting the particles with diameters of approximately 50 nm (see Formulae (1) and (2)). As shown in Fig. 2(b), the nanoparticles demonstrated polygons at ≤ 80 nm, which is suitable for photochromism. Hence, PVP was chosen as the surfactant in our study system. The main reason of PVP protecting AgBr nanoparticles is that the N in PVP coordinated with silver to form the protection layer, while it easily polarized the functional group ‘C]O’ in its repeated unit. ‘O’, with a negative charge, prefers to interact with positively charged ‘Ag’ to compensate the local surface charge imbalance, thereby stabilizing the crystal surfaces [50]. The reactions are expressed through the Formulae (3) and (4). Therefore, the PVP with glycerol were selected to form a protective environment for AgBr nanoparticles in order to maintain the photochromic stability in the following study. Additionally, the morphologies of the prepared nanoparticles in water and ethylene glycol are shown in Fig. 2(c) and (d), respectively. As shown in Fig. 2(c), the particles exhibited stacked, polygonal and triangular-like plates, with uneven sizes, while the nanoparticles obtained from
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ethylene glycol (Fig. 2(d)) have similar morphology with that of glycerol, but with the appearance some bigger triangular plates. Since morphology can affect the properties of materials, hence, different morphologies resulting to various photochromic properties are shown in Fig. 1.
matrix due to its excellent visible light transparency (Fig. 4) [52–54].The hydrogels were prepared using MBA as a crosslinking agent, with the monomers HEA and AM copolymerized through freeradical polymerization and initiated by KPS. The formulae for hydro-
(1)
(2) gels are shown in Table S1. As shown in Fig. 4. The T% of P(HEA-AM) obtained in this study was greater than 88% in the wave number range of 329–800 nm, but decreased slightly when the glycerol was added. The T% of P(HEA-AM)-glycerol-AgBr hydrogel was still above 83% in the range of 438–800 nm, but decreased in the range of 329–438 nm. Before 380 nm, the T% of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel was less than 74%, which was caused by the UV absorption of AgBr in the system [50,55].
(3)
(4) Additionally, to verify the influence of CuBr in the photochromic process, CuBr-free AgBr-glycerol solution was prepared as a blank experiment. Both AgBr-glycerol solutions with and without CuBr were irradiated using a solar light simulator for 5 min, respectively, after which the light was removed for 3 min, and the optical properties tracktested by UV–vis spectroscopy, with the result shown in Fig. 3. After 5 min of irradiation, the prepared glycerol solution without CuBr only slightly changed after a further 3 min of removing irradiation and did not fade reversibly. However, after 5 min of irradiation, the AgBr-glycerol solution with CuBr exhibited an obvious absorption at around 425 nm, with the peak almost disappearing after 3 min fading, indicating that the coloration timing was delayed without CuBr, which is consistent with the previous research [51]. Thus, a hypothesis is then proposed that photochromic AgBr-glycerol solution containing CuBr could endow the hydrogel with photochromic behaviour, hence, AgBrglycerol with CuBr was used to fabricate the hydrogel.
3.3. Mechanical properties of P(HEA-AM)-based hydrogels Furthermore, the mechanical properties of the hydrogels were measured at room temperature via tensile and compression tests. Fig. 5(a) and (b) show the tensile and compression stress–strain curves of the P(HEA-AM)-based hydrogels. After the addition of glycerol, the stretching variable increased, as well as that of the compression strain, which might be due to the strong hydrogen bonding exhibited by glycerol and water [35]. After the introduction of AgBr and CuBr into the hydrogel system, the tensile stress increased progressively. This could be ascribed to the chelation cross-linking between the glycerol and Ag+ or Cu+ [34]. Furthermore, setting the maximum compression strain at 95%, the compress stress data of the P(HEA-AM)-glycerol, P(HEA-AM)glycerol-AgBr and P(HEA-AM)-glycerol-AgBr/CuBr hydrogels were successively increased, which could be attributed to introducing nanoparticles into the soft system accompanied by the chelation crosslinking between glycerol and cations. Therefore, both compression resistance and tensile strength were improved by introducing inorganic
3.2. Synthesis and transparency of P(HEA-AM)-based hydrogels In this current study, hydrogel P(HEA-AM) was employed as the
Fig. 3. UV–vis spectra for irradiation and fade process of glycerol solution without (c) and with (d) CuBr.
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which can be assigned to Br at the monovalent oxidation state [55,58]. The results further confirmed that there were dynamic equilibrium reactions in the system, which made the photochromism feasible. 3.5. Photochromic properties of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel To study the photochromic behaviour, the sample was irradiated by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.). The sample was placed under the collimated light, with the distance between the sample and the light source being set at 10 cm, while the optical density was 60 mW cm−2 (Fig. S1). The P(HEA-AM)-glycerolAgBr/CuBr hydrogel was obtained by integrating photochromic AgBrglycerol solution into the P(HEA-AM) hydrogel matrix. Consistent with our assumptions, when the hydrogel was irradiated under a natural sunlight, a reversible colour change process was observed. As shown in Fig. 7, after 10 s exposure, the colour of the hydrogel became light brown and visible to the naked eye, and the colour became deepened as the irradiation time increased. After 5 min of irradiation, the sample was moved indoor, and the colour faded over time, becoming almost reversibly transparent after 120 min. The photochromic process of the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel was tracked using the UV–vis absorption spectra (Fig. 8[a and b]). As shown in Fig. 8(a), a gradually enhanced absorption peak appeared within 340–560 nm after the irradiation with a solar simulator, with the absorption peak at 408 nm being the highest, suggesting that Ag0 increased continually with increasing irradiation time. The point diagram Fig. 8(c) was obtained, corresponding to the peak values of 408 nm in Fig. 8(a). In Fig. 8(c), the peak values increased linearly with the irradiation time. The relationship between the absorption peak and the irradiation time can be obtained by the fitting A = A0 + 0.0017ti , (R2 = 0.9969), where A0 is the absorption of the hydrogel before light irradiation, and A is the absorption of hydrogel after the irradiation of ti s. In addition, Fig. 8(b) showed the fading process of the hydrogel after 10 min irradiation by the solar light simulator. After 8 min of fading, the colour faded more than a half; hence, the half-life period (t1/2) was less than 8 min. The point diagram in Fig. 8(d) was obtained, corresponding to the peak values of 408 nm in Fig. 8(b). As shown in Fig. 8(d), the peak values decreased exponentially with the fading time. Thus, the fading speed was inconsistent with the speed of colouring, [51] and the relationship between the absorption peak and irradiation time was calculated as A' = A 0' exp( t /584.5) + 0.1167 , (R2 = 0.9984), where A0′ is the absorption of the hydrogel after light irradiation (1.13 a.u.), A′ is the absorption of hydrogel after fading, and t denotes time
Fig. 4. Transparency of the hydrogels based on P(HEA-AM).
materials in the soft system. 3.4. Valence states of the metal ions in hydrogel P(HEA-AM)-glycerolAgBr/CuBr The valence states of the metal ions in hydrogels were characterized by XPS. Fig. 6 shows the typical XPS spectra of Ag 3d, Cu 2p and Br 3d, respectively. In Fig. 6(a), two bands at ca. 367.7 and 373.7 eV are ascribed to ag 3d5/2 and Ag 3d3/2 binding energies, respectively. These bands can be further de-convoluted into four peaks at 367.6 and 368.0 eV, and 373.5 and 373.9 eV, respectively, where the bands at 367.6 and 373.5 eV were ascribed to the Ag+, whereas those at 368.0 and 373.9 eV were attributed to the metallic Ag0 [56]. The Cu 2p3/2 core level was employed to investigate the Cu valence states. Fig. 6(b) demonstrates the main and satellite peaks of Cu 2p3/2 and Cu 2p1/2 of the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel. Copper ions and cuprous ions can co-exist in the system in the form of CuBr2 and CuBr. The broad Cu 2p3/2 peak is de-convoluted into two peaks at 932.3 and 934.3 eV, which are related to Cu/Cu+ and Cu2+, respectively [57]. Additionally, the spectra of Br 3d in Fig. 6(c) shows that the binding energies of Br 3d3/2 and Br 3d5/2 are approximately 69.3 and 68.3 eV, respectively,
Fig. 5. (a) Tensile and (b) compress stress-strain curves of P(HEA-AM)-based hydrogels, respectively. Curves (1), (2), (3) and (4) are P(HEA-AM), P(HEA-AM)glycerol, P(HEA-AM)-glycerol-AgBr and P(HEA-AM)-glycerol-AgBr/CuBr hydrogels, respectively. 6
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Fig. 6. XPS spectra of (a) Ag 3d, (b) Cu 2p, and (c) Br 3d of the hydrogel P(HEA-AM)-glycerol-AgBr/CuBr.
Fig. 7. Reversible colour-changing process of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel exposure under a solar light simulator. 7
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Fig. 8. Reversible colour-changing process of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel shown in (a) and (b); (c) and (d) show the absorption intensity at 425 nm corresponding to (c) and (d), respectively. Error bars denote the standard deviation for at least three measurements.
cycle is carried out. The result is shown in Fig. 9, with the light-controlled reversible absorbance being realized in at least 15 cycles without apparent degradation in intensity. In addition, the hydrogel irradiation (2 min) and colour fading (3 min) process was uninterruptedly repeated and shown in Movie 2 (the video with 4x speed playback). As shown in Fig. S2(a), the cumulative colouring and fading speed were observed. Given that the Ag0 species were not completely converted into Ag+ after a shorter fading time, the colour was cumulated. As displayed in Fig. S2(b), the stronger the absorption intensity under light irradiation, the faster of the fading speed, contributing to the more Ag0 accelerating the reverse reaction of Eq. (1) [51].
Fig. 9. UV–vis absorption value according to the photochromic repeatability of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel.
Movie 2.
(s). t1/2 could be calculated as 407 s (6.78 min). To further study the photochromic repeatability, the hydrogel was repeatedly irradiated (2 min) and faded (3 min) for several cycles. Herein, the hydrogel was allowed to fade completely before another
Ag1 + Cu1
8
hv dark
Ag 0 + CuII
(1)
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Fig. 10. Antibacterial activity of the hydrogels. a) S. aureus and E. coli suspensions that were cultured with different specimens (P(HEA-AM), P(HEA-AM)-glycerol, P (HEA-AM)-glycerol-AgBr and P(HEA-AM)-glycerol-AgBr/CuBr hydrogels) for 24 h. (c) Optical density measurement of S. aureus and E. coli growth during the cultured process. (d) Cytotoxicity assay of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel on L929 Mouse Fibroblast cells.
3.6. In vitro antibacterial test and cytotoxicity
Even after 24 h, the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel exhibited antibacterial activity of 100% against both the E. coli and S. aureus, respectively. These results demonstrated that P(HEA-AM)-glycerol-AgBr/CuBr hydrogel possess good antimicrobial activity against both E. coli (Gram-negative) and S. aureus (Gram-positive) after 24 h. Compared with the antibacterial activity of four different kinds of hydrogels, it indicated that silver play a crucial role in the antibacterial activity. As earlier discussed stated, there was an equilibrium between Ag(I) and Ag, as well as Cu(I) and Cu(II) in the photochromic hydrogel system to induce the colouration reversibility. The broth diffusing into the network of the hybrid hydrogel contacted the surface of AgBr/CuBr nanoparticle, after which the released Ag(I) and Cu (II) pass through the hydrogel network into the liquid phase, driven by the concentration gradient. Then as reported, both the silver and copper ions exhibited broad-spectrum antimicrobial activity [37,59,60]. Free Ag(I) attaches to the cell surface and disturbs the biochemical processes of the cellular enzymes and DNA by coordinating with electron-donating groups. [61] Simultaneously, copper have been known to be able to inactivate microorganism cells, not only by changing the charge of the cell surface, but also in the production of oxidative stress. [60] Hence, silver alongside with copper endowed the fabricated P(HEA-AM)-glycerolAgBr/CuBr hydrogel effective and significant antibacterial activity towards both Gram-negative and Gram-positive bacterium.
The P(HEA-AM)-glycerol-AgBr/CuBr hydrogel displayed strong antibacterial activities because of the Ag ions. Antibacterial activity of the hydrogel was evaluated against Staphylococcus aureus (S. aureus) (Gram-positive bacterium) and Escherichia coli (E. coli) (Gram-negative bacterium). Images of bacterial suspensions that were co-cultured 24 h with hydrogels are presented in Fig. 10(a and b). The suspensions of the P(HEA-AM) and P(HEA-AM)-glycerol were almost same as the blank, while the suspensions of the P(HEA-AM)-glycerol-AgBr and P(HEAAM)-glycerol-AgBr/CuBr hydrogels were clear, indicating the almost complete death of bacteria. The percentage reduction of E. coli and S. aureus on different samples with various incubation time were summarized in Fig. S3. The results showed that both the P(HEA-AM) and P (HEA-AM)-glycerol exhibited negligible antibacterial activities. Integrating AgBr in the hydrogel systems endowed P(HEA-AM)-glycerolAgBr and P(HEA-AM)-glycerol-AgBr/CuBr hydrogels excellent antibacterial activity (> 99.9%). Moreover, the percentage reduction of E. coli and S. aureus on the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel at 6 h, 12 h, and 24 h were shown in Fig. 10c, respectively, with the corresponding photographs shown in Fig. S4. After 6 h, it can be seen that the percentage reduction of E. coli and S. aureus were 100% and 99.92%, respectively, while it reached 100% for S. aureus after 12 h. 9
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Furthermore, the potential toxicity of P(HEA-AM)-glycerol-AgBr/ CuBr hydrogel was studied. As shown in Fig. 10(d), insignificant cytotoxicity of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel nanoparticles was observed towards L929 Mouse Fibroblast cells using the solution extracts (2 g of each as-prepared hydrogel immersed in culture for 24 hr) added to the 96-well plates with a concentration of 8000 cell per well. The cell viabilities at 91.6%, 91.1%, 92.0%, and 110.7% for adding 0 mL, 0.1 mL, 0.25 mL, 0.5 mL, 1 mL AgBr-glycerol solution to the hydrogel systems, respectively, indicated the excellent biological safety of the photochromic antibacterial hydrogel. After 5 days, it could also be observed that the hydrogel with 0.1 mL, 0.25 mL, 0.5 mL AgBr-glycerol solution could facilitate cell growth. As shown in Fig. 10(d), the cell viability of the samples increased with the increasing content of AgBr/ CuBr glycerol solution, which could be attributed to the trace of copper in the system. Actually, Cu metal possesses physiological security and is a type of trace element and also an essential component of many metalloenzymes in the human body. [62] The intercellular uptake behaviour of L929, 1 × 105 cells/well was seeded in a laser confocal culture dish and allowed to culture with extracts of the hydrogels for 12 h and 24 h, respectively, at 37 °C and 5% CO2 (Fig. S5).
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4. Conclusions In summary, a novel photochromic hydrogel has been developed on the basis of silver bromide, responding to simulated sunshine light. Polyvinyl pyrrolidone and glycerol were selected to hinder ions from being oxidized and separated out. The reversible photochromic hydrogel could be repeated for least 15 cycles without clear degradation in intensity, with the half-life period of the photochromic hydrogel being 6.78 min. Importantly, the photochromic hydrogel exhibited a remarkable antibacterial ability to both Gram-negative and Gram-positive bacterium, in addition to possessing an excellent non-cytotoxicity. The successful integration of photo-response to the mimetic interaction of photochromic glass based on silver bromide in polymer hydrogel suggested the possibility of developing various photochromicfunctional materials. This current study provides a basis for the application of silver bromide in hydrogel materials to expand its applications such as photochromic contact lenses, and artificial intelligent devices. Acknowledgment This work was supported by National Natural Science Foundation of China under the Grant 326 9Z07040007D6 and 9Z07040055D6. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.121994. References [1] Y. Yang, L. Guan, G. Gao, Low-cost rapidly responsive, controllable, and reversible photochromic hydrogel for display and storage, ACS Appl. Mater. Interfaces 10 (2018) 13975–13984. [2] E. Acome, S.K. Mitchell, T.G. Morrissey, M.B. Emmett, C. Benjamin, M. King, M. Radakovitz, C. Keplinger, Hydraulically amplified self-healing electrostatic actuators with muscle-like performance, Science 359 (2018) 61. [3] S. Li, D.M. Vogt, D. Rus, R.J. Wood, Fluid-driven origami-inspired artificial muscles, Proc. Natl. Acad. Sci. 114 (2017) 13132. [4] W. Chen, Z. Songshan, W. Zhaofeng, W. Fu, Z. Hui, Z. Jiachi, C. Zhipeng, S. Luyi, Efficient mechanoluminescent elastomers for dual-responsive anticounterfeiting device and stretching/strain sensor with multimode sensibility, Adv. Funct. Mater. (2018) 0 1803168. [5] N.A. Carter, T.Z. Grove, Protein self-assemblies that can generate, hold, and discharge electric potential in response to changes in relative humidity, J. Am. Chem. Soc. 140 (2018) 7144–7151. [6] W. Wang, X. Fan, F. Li, J. Qiu, M.M. Umair, W. Ren, B. Ju, S. Zhang, B. Tang, Magnetochromic photonic hydrogel for an alternating magnetic field-responsive color display, advanced, Opt. Mater. (2017) 1701093. [7] Z. Jiang, J. Chen, L. Cui, X. Zhuang, J. Ding, X. Chen, Advances in stimuli-
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Sunlight-driven photochromic hydrogel based on silver bromide with antibacterial property and non-cytotoxicity
T
Mengmeng Kanga, Yaoyao Denga, Olayinka Oderindea, Fangfang Sub, Wenjing Maa, Fang Yaoa, ⁎ ⁎ Guodong Fua, , Zhihong Zhangb, a
School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing, Jiangsu Province 211189, China Henan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou University of Light Industry, No. 166, Science Avenue, Zhengzhou, Henan Province 450002, China
b
HIGHLIGHTS
silver bromide-based photochromic • Ahydrogel was designed and fabricated. and PVP were used to main• Glycerol tain photochromism of the hydrogel. photochromic process of the fab• The ricated hydrogel was studied and
GRAPHIC ABSTRACT
Mimicking sunlight-driven glass, a novel photochromic hydrogel system was fabricated based on silver bromide. PVP and glycerol were employed to achieve repeatability and fast photochromism. The photochromic hydrogel displayed remarkable antibacterial ability and non-cytotoxicity, which can be as photochromic contact lenses, ornaments and artificial intelligent devices. Hence, photochromic hydrogel is a new direction in the field of silver bromine-based materials.
concluded.
hydrogel shows excellent anti• The bacterial property and non-cytotoxicity.
ARTICLE INFO
ABSTRACT
Keywords: Photochromic hydrogel Antibacterial Non-cytotoxic Silver bromide Glycerol
A novel photochromic hydrogel system was designed and fabricated based on silver bromide and cuprous bromide in poly(hydroxyethyl acrylate-co-polyacrylamide) hydrogel matrix. The first time we overcome the atoms side reaction and migration of halides in opening hydrogel system to realize photochromism. Here, polyvinyl pyrrolidone (PVP) and glycerol were employed as a surfactant and sustainable co-solvent, respectively, whose synergistic effect make AgBr endows the hydrogel with photochromic stability and repeatability. The photochromic hydrogel can be changed reversibly between light-blue and brown color for least 15 times in air under a simulated solar light irradiation. Both UV–vis absorption (approximately 408 nm) and the valence states of the metal ions were studied and verified the conversion between Ag+ and Ag0. The process of color changing was studied and the fabricated hydrogel with a half-life fading period (t1/2) was 6.78 min under a simulated solar light irradiation. Reasonably, the as-fabricated photochromic hydrogel displayed remarkable antibacterial ability to both Gram-positive- and Gram-negative bacterium with the Ag ions in the system. Interestingly, due to the little dosage of AgBr and CuBr in the hydrogel, it also possessed excellent non-cytotoxicity property to L929
⁎
Corresponding authors. E-mail addresses: [email protected] (G. Fu), [email protected] (Z. Zhang).
https://doi.org/10.1016/j.cej.2019.121994 Received 13 March 2019; Received in revised form 5 June 2019; Accepted 16 June 2019 Available online 17 June 2019 1385-8947/ © 2019 Published by Elsevier B.V.
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Mouse Fibroblast cells. Since its excellent photochromic and biological properties, the hydrogel has potential applications in the field of smart materials such as photochromic contact lenses, sensors and artificial intelligent devices or bionic skin.
1. Introduction
crystals in air and the mobility of the halogens, reverse reaction in soft material matrix is difficult [26,27]. Photochromic materials based on silver halides have been studied in soft materials such as films [28–30] and fibres [31], however, the study on photochromic hydrogel based on silver halides yet reported. Furthermore, limiting the side reaction of silver and halogen atoms and applying the photochromic silver halide in hydrogels based on the above-mentioned extensive application of hydrogels remain challenging. To solve the above-mentioned problem, polyvinyl pyrrolidone (PVP) and glycerol were introduced in the hydrogel system. PVP, which is a kind of nontoxic and non-ionic polymer, with C]O, C–N, and –CH2 functional groups has been widely used for nanoparticle synthesis [32], as it can be used in water or many non-aqueous liquids due to the highly polar amide group within the pyrrolidone ring and polar methylene and methine groups in the ring, as well as its backbone [33]. Meanwhile, glycerol could be attached to Cu+ between its two hydroxyl oxygen molecules, as determined by Laurence Boutreau et al. [34] Simultaneously, glycerol has a viscosity coefficient of 56.0 mPa·s (25 °C), and can form a hydrogen bond with water [35]; thus, can hinder the spill of bromine in the system. Therefore, inspired by the photochromic glass, a photochromic hydrogel system was designed on the basis of silver bromide (AgBr) with cuprous bromide (CuBr) as catalyst. The fabrication of the photochromic hydrogel was based on AgBr and included two steps (Scheme 1). Firstly, PVP was selected as a surfactant for nanoparticle preparation in glycerol solution. Secondly, hydrogel based on poly(hydroxyethyl acrylate) (pHEA) and polyacrylamide (pAM), which possess excellent transparency detected at ≥88%, was employed as the matrix for the photochromism. In general, the light yellow AgBr after photolysis can be regenerated from silver (Ag0) and free bromine molecule (Br2) without irradiation under cuprous catalysis to achieve optical reversible colour conversion [10,36], as shown in Scheme 1. With the existence of cuprous ions, the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel exhibited reversible and considerably fast colour switching response on the basis of the reaction of AgBr to UV and visible light. This hydrogel might be used in the field of photochromic contact lenses, ornaments and smart devices.
With the development of soft robots and sensors, smart materials, which respond to external stimuli such as heat, light, electric, magnetism, force, humidity, and chemical inputs, have attracted greater attentions [1–7]. Among these stimuli, light, especially solar light, have the unique advantages including universality, renewability, and availability, in addition to the advantages of easy-to-control parameters, such as precise location, different wavelengths, intensity, and illumination duration [8,9]. Photochromic materials, which change colour reversibly by light stimulus, have been extensively developed [10–12]. Among them, photochromic hydrogels have been studied and has potential applications in various fields such as smart devices, sensors, tissue engineering and in soft robotics [13–15]. Researchers have extensively studied organic molecule-based photochromic materials [15–19], but inorganic molecule-based photochromic hydrogels are few. For example, Gao et al. [1,20]. developed an ammonium molybdate-based photochromic hydrogel into polyacrylamide (pAM) for display, storage, and ink-free printing, while Geng et al. [21] fabricated a tungstate nanosheet-based flexible photochromic hydrogel; and Cong Wang et al. [22] fabricated an amorphous tungsten oxide-based photochromic hydrogel to release H2O2 for cancer inhibition. Even inorganic photochromic materials have a considerably better stability under repeated/prolonged light illumination than organic systems [8], with the production process are not as complex as organic photochromic molecules synthesis procedures. However, the repeatability of these inorganic-based photochromic hydrogels is not ideal. Therefore, fabricating photochromic materials with simple steps and stable photochromic properties remains a challenge. Among inorganic photochromic materials, silver halides are widely used in commercial borosilicate or aluminoborosilicate glasses, making the glass to undergo reversible colour switching when exposed to sunlight and with excellent photochromic repeatability [10]. This inspired the design of a photochromic hydrogel on the basis of silver halide with excellent repeatability. Within the glass, silver halides absorb photons and electron-hole pairs liberated under light irradiation; after which, the photo-generated electrons become easily trapped by the Ag+, resulting in the formation of silver atoms (Ag0). Thereafter, shielding the light or in the dark, Ag0 and halides could be reversed back to silver halides with the help of cuprous salts or copper [23–25]. This process allows silver halide-doped glasses to effectively control the transmittance of light through glasses, which have been applied to ophthalmic lenses. However, given the irradiation activity of the silver
2. Experimental section 2.1. Chemical Chemicals. Glycerol (purity, ~99.99%) and AgNO3 (purity,
Scheme 1. Schematic illustration of a fabricated photochromic hydrogel based on AgBr nanoparticles. 2
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~99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd, while NaBr (purity, ~99.99%), polyvinyl pyrrolidone (PVP) (K30) (purity, ~99.99%), hydroxyethyl acrylate (HEA), acrylamide (AM) were all purchased from Shanghai Macklin biochemical Co. Ltd. All the reagents were used as received without further purification. Milli-Q filtered water with resistivity > 18 MΩ·cm was used throughout.
3. Results and discussion 3.1. Optimization of reaction system and photochromic solution preparation Prior to fabrication of photochromic hydrogel, photochromic solutions based on AgBr with CuBr as a catalyst were prepared and the system was optimized. The stability of the photochromic solutions based on AgBr of different solvents with CuBr (glycerol, water, and ethylene glycol) are depicted in Fig. 1. Here, using the molar ratio of silver to cuprous (10:4) as a model, because more CuBr cannot be completely dissolved, while the systems has no obvious photochromism with less CuBr. Among the three solutions, glycerol and ethylene glycol solutions were photochromic-active, but only the glycerol solution was reversibly stable, with the macro-change being photographed (Fig. 1[a]). As shown in Fig. 1(b), the glycerol solution could remain stable for at least one week, whereas water solution or ethylene glycol started to change their coloration only after few hours. Interestingly, the glycerol solution still maintained its good photochromic and stability properties, even after six months of storage (Movie 1), unlike ethylene glycol solution, which could only be maintained for several days.
2.2. Fabrication of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel The preparation of AgBr-glycerol solution was discussed in the Supporting Information. P(HEA-AM)-glycerol-AgBr/CuBr hydrogel was obtained by adding 1 mL AgBr-glycerol solution in a system containing 1.4 g of AM, 0.6 mL of HEA, 0.5 mL of MBAA (0.1%, wt% aqueous solution) in 5 mL water. After complete mixing, 0.5 mL KPS aqueous solution (0.2 M). TEMED was thereafter added to initiate the polymerization process and the photochromic hydrogel was obtained. 2.3. Photochromic behaviour of the hydrogel The reversible colour-change of the hydrogel was detected by UV2450 UV–Vis spectrophotometer (Japan), with the wavenumber ranging from 800 nm to 200 nm and at a resolution of 1 nm. The specimens were irradiated with a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.) at 10.0 cm distance on a sample stage. 2.4. In vitro antibacterial tests and cytotoxicity Gram-positive staphylococcus aureus (S. aureus, ATCC 29213) and gram-negative Escherichia coli (E. coli, ATCC 25922) were used to evaluate the antibacterial activity of the hydrogels colony forming unit (CFU) through method, according to the reference [37]. Additionally, MTT assays were employed to evaluate in vitro cytotoxicity against L929 Mouse Fibroblast cells of the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel. Details are described in the Supporting Information.
Movie 1.
To explain light-responsible phenomenon of the various solution, the powders collected from various solutions were characterized by Powder X-ray Diffraction (PXRD) to confirm the content of crystals. The PXRD patterns of powders collected from glycerol, water, and ethylene glycol are provided in Fig. 2. The results showed that no peak from
Fig. 1. (a) Photos of photochromism in various solvents and (b) of changes for different solvents.
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Fig. 2. (a) XRD spectra of nanoparticles prepared in three solvents. (I) glycerol, (II) water and (III) ethylene glycol, and (IV) AgBr/CuBr nanoparticles prepared in glycerol without PVP. (b–d) SEM of samples (I), (II) and (III) corresponding to (a), respectively.
CuBr existed in the reaction system, with glycerol as the solvent and PVP as the surfactant. Diagnostic peaks at 26.8°, 30.9°, 44.3°, 55.2°, 64.5°, and 73.2° were attributed to AgBr (PDF#06-0438), without a single peak belonging to CuBr crystals collected from glycerol and ethylene glycol. Such result has two plausible reasons. Firstly, the PVP molecule as a kind of surfactant, could promote the CuBr solubility in the glycerol solution [38]; and secondly, glycerol complexes with cuprous ions further promote the solubility of CuBr [34]. However, in water solution, the peak at 14.7° was indexed to ( −1 0 1) of cupric bromine (CuBr2) (PDF#45-1063), whereas the peaks at 26.6°, 34.15°, and 44.0° belonged to the (1 1 3), (0 4 0), and (4 4 0) crystal lattices of Ag2O3 (PDF#40-0909), respectively, as shown in Fig. 2(a)(II). Thus, the nanoparticle fabricated in aqueous solution was easier to be oxidized than in the other two media, resulting in the instability of the solution that could not be maintained for a day (Fig. 1). Meanwhile, glycerol as an example of green solvent, has been drawing considerable attention for several reasons: (i) it has a unique combination of several properties, including high polarity, low toxicity and high boiling point, and can also dissolve both organic and inorganic compounds [39,40]; (ii) it is relatively cheap and abundantly sourced as it is a value-added byproduct in biodiesel industry and in the new conversion of cellulose and lignin process [41,42]; (iii) in some cases, it can enhance effectiveness and selectivity [43–45]; and (iv) the glycerolic phase containing the catalysts could be re-used in catalyst recycling [46], or the catalyst ability of cuprous compounds could be improved [45]. Based on the above analyses, glycerol was selected to form a protective environment
for AgBr nanoparticles to maintain the photochromic stability in the following study. Without PVP in the glycerol system, a recognizable CuBr peak existed in the XRD peak because CuBr was not easily dissolved but precipitated out without a surfactant. Simultaneously, given the existence of the amide group, PVP could be an anchoring agent with Ag+ or Ag atoms, possibly preventing silver from precipitating [47,48]. The other reason to selecting PVP is that materials containing silver halide crystals at approximately 50–800 Å (5–80 nm) diameter can achieve excellent photochromic property [29], with Zhang et al. [49] analysing the possible mode of PVP in protecting the particles with diameters of approximately 50 nm (see Formulae (1) and (2)). As shown in Fig. 2(b), the nanoparticles demonstrated polygons at ≤ 80 nm, which is suitable for photochromism. Hence, PVP was chosen as the surfactant in our study system. The main reason of PVP protecting AgBr nanoparticles is that the N in PVP coordinated with silver to form the protection layer, while it easily polarized the functional group ‘C]O’ in its repeated unit. ‘O’, with a negative charge, prefers to interact with positively charged ‘Ag’ to compensate the local surface charge imbalance, thereby stabilizing the crystal surfaces [50]. The reactions are expressed through the Formulae (3) and (4). Therefore, the PVP with glycerol were selected to form a protective environment for AgBr nanoparticles in order to maintain the photochromic stability in the following study. Additionally, the morphologies of the prepared nanoparticles in water and ethylene glycol are shown in Fig. 2(c) and (d), respectively. As shown in Fig. 2(c), the particles exhibited stacked, polygonal and triangular-like plates, with uneven sizes, while the nanoparticles obtained from
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ethylene glycol (Fig. 2(d)) have similar morphology with that of glycerol, but with the appearance some bigger triangular plates. Since morphology can affect the properties of materials, hence, different morphologies resulting to various photochromic properties are shown in Fig. 1.
matrix due to its excellent visible light transparency (Fig. 4) [52–54].The hydrogels were prepared using MBA as a crosslinking agent, with the monomers HEA and AM copolymerized through freeradical polymerization and initiated by KPS. The formulae for hydro-
(1)
(2) gels are shown in Table S1. As shown in Fig. 4. The T% of P(HEA-AM) obtained in this study was greater than 88% in the wave number range of 329–800 nm, but decreased slightly when the glycerol was added. The T% of P(HEA-AM)-glycerol-AgBr hydrogel was still above 83% in the range of 438–800 nm, but decreased in the range of 329–438 nm. Before 380 nm, the T% of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel was less than 74%, which was caused by the UV absorption of AgBr in the system [50,55].
(3)
(4) Additionally, to verify the influence of CuBr in the photochromic process, CuBr-free AgBr-glycerol solution was prepared as a blank experiment. Both AgBr-glycerol solutions with and without CuBr were irradiated using a solar light simulator for 5 min, respectively, after which the light was removed for 3 min, and the optical properties tracktested by UV–vis spectroscopy, with the result shown in Fig. 3. After 5 min of irradiation, the prepared glycerol solution without CuBr only slightly changed after a further 3 min of removing irradiation and did not fade reversibly. However, after 5 min of irradiation, the AgBr-glycerol solution with CuBr exhibited an obvious absorption at around 425 nm, with the peak almost disappearing after 3 min fading, indicating that the coloration timing was delayed without CuBr, which is consistent with the previous research [51]. Thus, a hypothesis is then proposed that photochromic AgBr-glycerol solution containing CuBr could endow the hydrogel with photochromic behaviour, hence, AgBrglycerol with CuBr was used to fabricate the hydrogel.
3.3. Mechanical properties of P(HEA-AM)-based hydrogels Furthermore, the mechanical properties of the hydrogels were measured at room temperature via tensile and compression tests. Fig. 5(a) and (b) show the tensile and compression stress–strain curves of the P(HEA-AM)-based hydrogels. After the addition of glycerol, the stretching variable increased, as well as that of the compression strain, which might be due to the strong hydrogen bonding exhibited by glycerol and water [35]. After the introduction of AgBr and CuBr into the hydrogel system, the tensile stress increased progressively. This could be ascribed to the chelation cross-linking between the glycerol and Ag+ or Cu+ [34]. Furthermore, setting the maximum compression strain at 95%, the compress stress data of the P(HEA-AM)-glycerol, P(HEA-AM)glycerol-AgBr and P(HEA-AM)-glycerol-AgBr/CuBr hydrogels were successively increased, which could be attributed to introducing nanoparticles into the soft system accompanied by the chelation crosslinking between glycerol and cations. Therefore, both compression resistance and tensile strength were improved by introducing inorganic
3.2. Synthesis and transparency of P(HEA-AM)-based hydrogels In this current study, hydrogel P(HEA-AM) was employed as the
Fig. 3. UV–vis spectra for irradiation and fade process of glycerol solution without (c) and with (d) CuBr.
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which can be assigned to Br at the monovalent oxidation state [55,58]. The results further confirmed that there were dynamic equilibrium reactions in the system, which made the photochromism feasible. 3.5. Photochromic properties of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel To study the photochromic behaviour, the sample was irradiated by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.). The sample was placed under the collimated light, with the distance between the sample and the light source being set at 10 cm, while the optical density was 60 mW cm−2 (Fig. S1). The P(HEA-AM)-glycerolAgBr/CuBr hydrogel was obtained by integrating photochromic AgBrglycerol solution into the P(HEA-AM) hydrogel matrix. Consistent with our assumptions, when the hydrogel was irradiated under a natural sunlight, a reversible colour change process was observed. As shown in Fig. 7, after 10 s exposure, the colour of the hydrogel became light brown and visible to the naked eye, and the colour became deepened as the irradiation time increased. After 5 min of irradiation, the sample was moved indoor, and the colour faded over time, becoming almost reversibly transparent after 120 min. The photochromic process of the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel was tracked using the UV–vis absorption spectra (Fig. 8[a and b]). As shown in Fig. 8(a), a gradually enhanced absorption peak appeared within 340–560 nm after the irradiation with a solar simulator, with the absorption peak at 408 nm being the highest, suggesting that Ag0 increased continually with increasing irradiation time. The point diagram Fig. 8(c) was obtained, corresponding to the peak values of 408 nm in Fig. 8(a). In Fig. 8(c), the peak values increased linearly with the irradiation time. The relationship between the absorption peak and the irradiation time can be obtained by the fitting A = A0 + 0.0017ti , (R2 = 0.9969), where A0 is the absorption of the hydrogel before light irradiation, and A is the absorption of hydrogel after the irradiation of ti s. In addition, Fig. 8(b) showed the fading process of the hydrogel after 10 min irradiation by the solar light simulator. After 8 min of fading, the colour faded more than a half; hence, the half-life period (t1/2) was less than 8 min. The point diagram in Fig. 8(d) was obtained, corresponding to the peak values of 408 nm in Fig. 8(b). As shown in Fig. 8(d), the peak values decreased exponentially with the fading time. Thus, the fading speed was inconsistent with the speed of colouring, [51] and the relationship between the absorption peak and irradiation time was calculated as A' = A 0' exp( t /584.5) + 0.1167 , (R2 = 0.9984), where A0′ is the absorption of the hydrogel after light irradiation (1.13 a.u.), A′ is the absorption of hydrogel after fading, and t denotes time
Fig. 4. Transparency of the hydrogels based on P(HEA-AM).
materials in the soft system. 3.4. Valence states of the metal ions in hydrogel P(HEA-AM)-glycerolAgBr/CuBr The valence states of the metal ions in hydrogels were characterized by XPS. Fig. 6 shows the typical XPS spectra of Ag 3d, Cu 2p and Br 3d, respectively. In Fig. 6(a), two bands at ca. 367.7 and 373.7 eV are ascribed to ag 3d5/2 and Ag 3d3/2 binding energies, respectively. These bands can be further de-convoluted into four peaks at 367.6 and 368.0 eV, and 373.5 and 373.9 eV, respectively, where the bands at 367.6 and 373.5 eV were ascribed to the Ag+, whereas those at 368.0 and 373.9 eV were attributed to the metallic Ag0 [56]. The Cu 2p3/2 core level was employed to investigate the Cu valence states. Fig. 6(b) demonstrates the main and satellite peaks of Cu 2p3/2 and Cu 2p1/2 of the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel. Copper ions and cuprous ions can co-exist in the system in the form of CuBr2 and CuBr. The broad Cu 2p3/2 peak is de-convoluted into two peaks at 932.3 and 934.3 eV, which are related to Cu/Cu+ and Cu2+, respectively [57]. Additionally, the spectra of Br 3d in Fig. 6(c) shows that the binding energies of Br 3d3/2 and Br 3d5/2 are approximately 69.3 and 68.3 eV, respectively,
Fig. 5. (a) Tensile and (b) compress stress-strain curves of P(HEA-AM)-based hydrogels, respectively. Curves (1), (2), (3) and (4) are P(HEA-AM), P(HEA-AM)glycerol, P(HEA-AM)-glycerol-AgBr and P(HEA-AM)-glycerol-AgBr/CuBr hydrogels, respectively. 6
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Fig. 6. XPS spectra of (a) Ag 3d, (b) Cu 2p, and (c) Br 3d of the hydrogel P(HEA-AM)-glycerol-AgBr/CuBr.
Fig. 7. Reversible colour-changing process of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel exposure under a solar light simulator. 7
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Fig. 8. Reversible colour-changing process of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel shown in (a) and (b); (c) and (d) show the absorption intensity at 425 nm corresponding to (c) and (d), respectively. Error bars denote the standard deviation for at least three measurements.
cycle is carried out. The result is shown in Fig. 9, with the light-controlled reversible absorbance being realized in at least 15 cycles without apparent degradation in intensity. In addition, the hydrogel irradiation (2 min) and colour fading (3 min) process was uninterruptedly repeated and shown in Movie 2 (the video with 4x speed playback). As shown in Fig. S2(a), the cumulative colouring and fading speed were observed. Given that the Ag0 species were not completely converted into Ag+ after a shorter fading time, the colour was cumulated. As displayed in Fig. S2(b), the stronger the absorption intensity under light irradiation, the faster of the fading speed, contributing to the more Ag0 accelerating the reverse reaction of Eq. (1) [51].
Fig. 9. UV–vis absorption value according to the photochromic repeatability of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel.
Movie 2.
(s). t1/2 could be calculated as 407 s (6.78 min). To further study the photochromic repeatability, the hydrogel was repeatedly irradiated (2 min) and faded (3 min) for several cycles. Herein, the hydrogel was allowed to fade completely before another
Ag1 + Cu1
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hv dark
Ag 0 + CuII
(1)
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Fig. 10. Antibacterial activity of the hydrogels. a) S. aureus and E. coli suspensions that were cultured with different specimens (P(HEA-AM), P(HEA-AM)-glycerol, P (HEA-AM)-glycerol-AgBr and P(HEA-AM)-glycerol-AgBr/CuBr hydrogels) for 24 h. (c) Optical density measurement of S. aureus and E. coli growth during the cultured process. (d) Cytotoxicity assay of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel on L929 Mouse Fibroblast cells.
3.6. In vitro antibacterial test and cytotoxicity
Even after 24 h, the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel exhibited antibacterial activity of 100% against both the E. coli and S. aureus, respectively. These results demonstrated that P(HEA-AM)-glycerol-AgBr/CuBr hydrogel possess good antimicrobial activity against both E. coli (Gram-negative) and S. aureus (Gram-positive) after 24 h. Compared with the antibacterial activity of four different kinds of hydrogels, it indicated that silver play a crucial role in the antibacterial activity. As earlier discussed stated, there was an equilibrium between Ag(I) and Ag, as well as Cu(I) and Cu(II) in the photochromic hydrogel system to induce the colouration reversibility. The broth diffusing into the network of the hybrid hydrogel contacted the surface of AgBr/CuBr nanoparticle, after which the released Ag(I) and Cu (II) pass through the hydrogel network into the liquid phase, driven by the concentration gradient. Then as reported, both the silver and copper ions exhibited broad-spectrum antimicrobial activity [37,59,60]. Free Ag(I) attaches to the cell surface and disturbs the biochemical processes of the cellular enzymes and DNA by coordinating with electron-donating groups. [61] Simultaneously, copper have been known to be able to inactivate microorganism cells, not only by changing the charge of the cell surface, but also in the production of oxidative stress. [60] Hence, silver alongside with copper endowed the fabricated P(HEA-AM)-glycerolAgBr/CuBr hydrogel effective and significant antibacterial activity towards both Gram-negative and Gram-positive bacterium.
The P(HEA-AM)-glycerol-AgBr/CuBr hydrogel displayed strong antibacterial activities because of the Ag ions. Antibacterial activity of the hydrogel was evaluated against Staphylococcus aureus (S. aureus) (Gram-positive bacterium) and Escherichia coli (E. coli) (Gram-negative bacterium). Images of bacterial suspensions that were co-cultured 24 h with hydrogels are presented in Fig. 10(a and b). The suspensions of the P(HEA-AM) and P(HEA-AM)-glycerol were almost same as the blank, while the suspensions of the P(HEA-AM)-glycerol-AgBr and P(HEAAM)-glycerol-AgBr/CuBr hydrogels were clear, indicating the almost complete death of bacteria. The percentage reduction of E. coli and S. aureus on different samples with various incubation time were summarized in Fig. S3. The results showed that both the P(HEA-AM) and P (HEA-AM)-glycerol exhibited negligible antibacterial activities. Integrating AgBr in the hydrogel systems endowed P(HEA-AM)-glycerolAgBr and P(HEA-AM)-glycerol-AgBr/CuBr hydrogels excellent antibacterial activity (> 99.9%). Moreover, the percentage reduction of E. coli and S. aureus on the P(HEA-AM)-glycerol-AgBr/CuBr hydrogel at 6 h, 12 h, and 24 h were shown in Fig. 10c, respectively, with the corresponding photographs shown in Fig. S4. After 6 h, it can be seen that the percentage reduction of E. coli and S. aureus were 100% and 99.92%, respectively, while it reached 100% for S. aureus after 12 h. 9
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Furthermore, the potential toxicity of P(HEA-AM)-glycerol-AgBr/ CuBr hydrogel was studied. As shown in Fig. 10(d), insignificant cytotoxicity of P(HEA-AM)-glycerol-AgBr/CuBr hydrogel nanoparticles was observed towards L929 Mouse Fibroblast cells using the solution extracts (2 g of each as-prepared hydrogel immersed in culture for 24 hr) added to the 96-well plates with a concentration of 8000 cell per well. The cell viabilities at 91.6%, 91.1%, 92.0%, and 110.7% for adding 0 mL, 0.1 mL, 0.25 mL, 0.5 mL, 1 mL AgBr-glycerol solution to the hydrogel systems, respectively, indicated the excellent biological safety of the photochromic antibacterial hydrogel. After 5 days, it could also be observed that the hydrogel with 0.1 mL, 0.25 mL, 0.5 mL AgBr-glycerol solution could facilitate cell growth. As shown in Fig. 10(d), the cell viability of the samples increased with the increasing content of AgBr/ CuBr glycerol solution, which could be attributed to the trace of copper in the system. Actually, Cu metal possesses physiological security and is a type of trace element and also an essential component of many metalloenzymes in the human body. [62] The intercellular uptake behaviour of L929, 1 × 105 cells/well was seeded in a laser confocal culture dish and allowed to culture with extracts of the hydrogels for 12 h and 24 h, respectively, at 37 °C and 5% CO2 (Fig. S5).
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4. Conclusions In summary, a novel photochromic hydrogel has been developed on the basis of silver bromide, responding to simulated sunshine light. Polyvinyl pyrrolidone and glycerol were selected to hinder ions from being oxidized and separated out. The reversible photochromic hydrogel could be repeated for least 15 cycles without clear degradation in intensity, with the half-life period of the photochromic hydrogel being 6.78 min. Importantly, the photochromic hydrogel exhibited a remarkable antibacterial ability to both Gram-negative and Gram-positive bacterium, in addition to possessing an excellent non-cytotoxicity. The successful integration of photo-response to the mimetic interaction of photochromic glass based on silver bromide in polymer hydrogel suggested the possibility of developing various photochromicfunctional materials. This current study provides a basis for the application of silver bromide in hydrogel materials to expand its applications such as photochromic contact lenses, and artificial intelligent devices. Acknowledgment This work was supported by National Natural Science Foundation of China under the Grant 326 9Z07040007D6 and 9Z07040055D6. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.121994. References [1] Y. Yang, L. Guan, G. Gao, Low-cost rapidly responsive, controllable, and reversible photochromic hydrogel for display and storage, ACS Appl. Mater. Interfaces 10 (2018) 13975–13984. [2] E. Acome, S.K. Mitchell, T.G. Morrissey, M.B. Emmett, C. Benjamin, M. King, M. Radakovitz, C. Keplinger, Hydraulically amplified self-healing electrostatic actuators with muscle-like performance, Science 359 (2018) 61. [3] S. Li, D.M. Vogt, D. Rus, R.J. Wood, Fluid-driven origami-inspired artificial muscles, Proc. Natl. Acad. Sci. 114 (2017) 13132. [4] W. Chen, Z. Songshan, W. Zhaofeng, W. Fu, Z. Hui, Z. Jiachi, C. Zhipeng, S. Luyi, Efficient mechanoluminescent elastomers for dual-responsive anticounterfeiting device and stretching/strain sensor with multimode sensibility, Adv. Funct. Mater. (2018) 0 1803168. [5] N.A. Carter, T.Z. Grove, Protein self-assemblies that can generate, hold, and discharge electric potential in response to changes in relative humidity, J. Am. Chem. Soc. 140 (2018) 7144–7151. [6] W. Wang, X. Fan, F. Li, J. Qiu, M.M. Umair, W. Ren, B. Ju, S. Zhang, B. Tang, Magnetochromic photonic hydrogel for an alternating magnetic field-responsive color display, advanced, Opt. Mater. (2017) 1701093. [7] Z. Jiang, J. Chen, L. Cui, X. Zhuang, J. Ding, X. Chen, Advances in stimuli-
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