Research Article pubs.acs.org/journal/ascecg
Superfast Adsorption−Disinfection Cryogels Decorated with Cellulose Nanocrystal/Zinc Oxide Nanorod Clusters for WaterPurifying Microdevices Duan-Chao Wang,† Hou-Yong Yu,*,†,‡ Mei-Li Song,† Ren-Tong Yang,† and Ju-Ming Yao†,‡ †
The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education, College of Materials and Textile, Zhejiang Sci-Tech University, Xiasha Higher Education Park 2 Avenue-5, Hangzhou 310018, China ‡ National Engineering Lab for Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech University, Xiasha Higher Education Park 2 Avenue-5, Hangzhou 310018, China S Supporting Information *
ABSTRACT: In outdoor activities and disaster environments, a portable microdevice is very important for the rapid purification of field drinking water. Smart cryogels were fabricated by one-pot copolymerization among acrylamide (AM) monomers, 2-(dimethylamino) ethyl methacrylate (DMAEMA) monomers, and cellulose nanocrystal/zinc oxide (CNC/ZnO) nanorod clusters through a simple ice-template method. The effect of nanohybrid contents on microstructural, thermal, swelling, and antibacterial properties of cryogels-ZnO was investigated. With the incorporation of 1 wt % CNC/ZnO, the cryogels showed macroporous-networked structures with a high porosity of 93% and homogeneous nanorod clusters on the macroporous wall. Cryogels-ZnO showed high mechanical strength in both dry and wet states, an adsorption capacity of 30.8 g/g, superfast adsorption time (2.5 s), and a stable swelling−deswelling ability after 10 cycles. Additionally, the adsorption capacity of cryogels-ZnO presented a dual temperature/pH response. Upon treatment of the simulated field water with cryogelsZnO for 45 min, the cfu amounts of simulated field water were decreased from 1862 to 6 cfu/mL. This cfu amount for treated water was much lower than the 100 cfu/mL of China’s national drinking water standard. In addition, cryogels-ZnO can effectively handle 14.3−16.1 g/g of disinfected drinking water by manual compression. Such cryogels-ZnO exhibited a great potential for field drinking-water-purifying microdevices. KEYWORDS: Cryogels, Cellulose nanocrystals, ZnO, Adsorption property, Antibacterial activity
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INTRODUCTION Microbiological contamination of potable water sources has been considered one of the important threats to public health. Additionally, some sport enthusiasts or soldiers need more purified water in the field. Indeed, there are about 1.8 million people who die annually from diarrheal diseases due to microbiologically low-quality water.1 Thus, main methods including ultraviolet lamp irradiation, activated carbon adsorption, and ultrafiltration membrane treatment are used for the purification of drinking water, but these treatments suffer from poor portable performance, limited adsorption, and easily blocked pores.2−4Cryogels based on organic and inorganic nanoparticles can be used as effective microdevices for the purification of field drinking water because of their high porosity, low density, and antimicrobial properties.5,6 Recently, the incorporation of silver nanoparticles (Ag NPs)1,7,8 and gold nanoparticles (Au NPs)3 with excellent antimicrobial properties into polymer cryogels has received more and more interest in water-purifying fields. Loo et al.1,7 © 2017 American Chemical Society
optimized the preparation of poly(sodium acrylate) (PSA) cryogels with more interconnected pores and a higher swelling rate and adsorption capacity by tailoring reaction conditions. Then, the resultant PSA cryogels can be served as adsorption carriers/templates for the deposition of a uniform dispersion of Ag NPs for the fabrication of smart cryogels. PSA−Ag cryogels show fast swelling, high adsorption capacity, good recovery, and a disinfection effect for absorbed water via manual compression. Ag NPs can render the cryogels with a strong antimicrobial performance, but the mechanical strength of the cryogels was not improved. Subsequently, Nata et al.8 have reported a multifunctional poly(vinyl alcohol) cryogel by incorporation of rigid chitin nanofibrils (CNFs) and Ag NPs. Compared to that of neat PVA cryogel, the storage modulus of as-produced cryogels is improved by adding CNF−Ag. Nevertheless, the Received: April 5, 2017 Revised: June 22, 2017 Published: July 11, 2017 6776
DOI: 10.1021/acssuschemeng.7b01029 ACS Sustainable Chem. Eng. 2017, 5, 6776−6785
Research Article
ACS Sustainable Chemistry & Engineering swelling−adsorption behavior of the macroporous cryogels in field water is not evaluated. Also, the cost of Ag NPs is too high, and the release of Ag in water has strict requirements. Therefore, the development of functional cryogels without Ag NPs has received more interest. In this study, we provide a method for the fabrication of a low-cost and intelligent cryogel with dual pH/temperature sensitivity by one-pot polymerization among acrylamide (AM) monomers, 2-(dimethylamino) ethyl methacrylate (DMAEMA) monomers, and flowerlike cellulose nanocrystal/zinc oxide (CNC/ZnO) nanohybrids. Indeed, the CNC/ZnO nanohybrids with modulated morphology, low cost, and an antibacterial property show great potential as bifunctional agents in matrix cryogels, compared to Ag NPs.9−11 Moreover, the smart cryogels exhibited a dual pH/temperature responsiveness for modulation of the water swelling ability, because different field water has some differences in environmental pH and temperature, which will affect the swelling ability of the resulting cryogels. Further, the pore morphological, microstructural, mechanical, water-adsorption, and antibacterial properties of smart cryogels were optimized by different AM/ DMAEMA molar ratios and CNC/ZnO nanohybrid contents, separately. Finally, the use of smart cryogels was evaluated in potential water-purifying microdevices for simulated field water.
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Scheme 1. Schematic Preparation Process and Image of Smart Cryogels-ZnO (Possible Reaction among AM Monomers, DMAEMA Monomers, and CNC/ZnO Nanohybrids)
EXPERIMENTAL SECTION
Materials. Zinc nitrate hexahydrate, ammonium hydroxide, hydrochloric acid, and sodium hydroxide were purchased from Guoyao Group Chemical Reagent Co., Ltd. (Shanghai, China). Acrylamide (AM), ammonium persulfate (APS) and potassium bromide (KBr) were purchased from Aladdin Industrial Corporation (Shanghai, China). 2-(Dimethylamino)ethyl methacrylate (DMAEMA), N,N,N′,N′-tetramethylethylenediamine (TEMED), and N,N′methylene bis(acrylamide) (MBA) were provided by Molbase Chemical Industry (Shanghai, China). All the materials and reagents were used as received without further purication. Preparation of Spherical CNCs and Flowerlike CNC/ZnO Nanohybrids. The preparation method of spherical CNCs and flowerlike CNC/ZnO nanohybrids has been described in our previous reports.9,12 Briefly, the CNC suspension was mixed with zinc nitrate aqueous solution (0.1 g/mL), and the pH value of the mixtures was adjusted to 11 with ammonium hydroxide. The reaction temperature and time are 110 °C and 2 h, respectively. The resulting products were separated by centrifugation with deionized water. Finally, the precipitates of CNC/ZnO nanohybrids were dried for 24 h at 60 °C. Preparation of Dual-Responsive Cryogels and Smart Cryogels. First, 0.15 g of APS was added to 20 mL of deionized water for preparation of an initiator solution; 0.5 mL of TEMED was added to 20 mL of deionized water for the formation of an initiator active agent solution. A 0.1 g sample of MBA was added to 10 mL of deionized water for the preparation of a cross-link solution. The reaction mixtures of AM, DMAEMA, MBA, and flowerlike CNC/ZnO nanohybrids were adequately ultrasonicated and degassed for chilling in an ice bath at 0 °C. Subsequently, 0.15 g of APS and 0.5 mL of TEMED were slowly added into reaction mixtures, and then transferred into a poly(propylene) centrifugation tube. Then, they were placed into the liquid cooling chamber of a chiller for 20 h at −50 °C. The resultant dual-responsive cryogels were first washed with deionized water, then dehydrated in a series of aqueous ethanol solutions, and subsequently dried in a freeze-dryer13,14 at a temperature of −50 °C and a degree of vacuum of 1.3−13 Pa for 48 h. For ensurance of the reproducibility of the freezing patterns, reaction mixtures with the same volume and centrifugation tube with the same dimensions were used for cryogel preparation. Scheme 1 shows the preparation process of cryogels. For the neat cryogels, there is no addition of ZnO, and the total amount of monomers (AM/ DMAEMA) is 0.5 g. According to the molar ratio of AM/DMAEMA
(5/5, 6/4, and 8/2), the resulting cryogels were designated as cryogels5/5, cryogels-6/4, and cryogels-8/2. The hybrid cryogels were denoted as cryogels-ZnO-1% and cryogels-ZnO-5% according to the mass of added CNC/ZnO hybrids based on cryogels-6/4.
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CHARACTERIZATION
The morphology of dried cryogels was characterized by field-emission scanning electron microscopy (FE-SEM, JSM-5610; JEOL, Japan), with the accelerating voltage parameter as 15 kV at room temperature. The potassium bromide (KBr) disc method is an effective way to measure chemical structure of the samples via a Fourier-transform infrared (FT-IR) spectrometer (Nicolet 5700, Thermo Electron Corp.) at ambient temperature. The wavenumber of FT-IR was scanned at the range 4000−400 cm−1. The optical properties of the cryogels were characterized in the wavelength interval between 300 and 700 nm using a solid-phase ultraviolet spectrometry (solid UV− vis) device (Lambda 35; PerkinElmer Corp.). The X-ray powder diffraction (XRD) analysis was performed for investigation of the crystal phase of CNC/ZnO nanohybrids and the cryogels. The parameters of the X-ray powder diffractometer (ARL X’RA, ThermoElectron Corp.) are monochromatic Cu Kα radiation at λ = 1.540 56 Å in the 2θ range 15−80° at a scan rate of 2 deg/min. The X-ray generator tension and current were 40 kV and 30 mA, respectively. The thermostability of cryogels was observed with thermogravimetric analysis (TGA) using an analyzer (Pyris Diamond I, PerkinElmer Corp.). The samples of cryogels (about 3−8 mg) were heated from 30 to 600 °C at the rate of 20 K/min under dynamic nitrogen atmosphere with a flow rate of 30 mL/min. A uniaxial compression measurement was used for the mechanical properties of the cryogels. Cylindrically shaped cryogel samples with 12 mm thickness were tested under an unconfined compression condition using a custom computer-controlled mechanical testing system. The samples were compressed at room temperature with a 5 kN load cell at a ramp rate of 10 mm/min. The Young’s modulus was determined from initial linear slopes of the stress−strain curves according to the following equation. Δh F =E 0 h πr 2 6777
(1) DOI: 10.1021/acssuschemeng.7b01029 ACS Sustainable Chem. Eng. 2017, 5, 6776−6785
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ACS Sustainable Chemistry & Engineering
Figure 1. (a) FE-SEM images, (b) pore diameter, (c) FTIR spectra, (d) TGA curves, and (e) swelling−deswelling cycles of cryogels-5/5, cryogels-6/ 4, and cryogels-8/2. Here, the terms are defined as follows: F is the force applied to cryogels with a cross-sectional area of πr2, h is the height of the cryogel during compression, and Δh0 is the change in height of the sample during compression. The swelling properties of cryogels are one of the most important properties in a field drinking-water-purifying moicrodevice,1,11,15 which can be determined by gravimetric analysis. For the determination of the swelling profile, a dried cryogel sample with a 14 mm diameter and a 12 mm height was swollen in excess deionized water. Excess surface water was gently wiped off using filter paper before the measurement of the mass of the swollen cryogel. Water uptake of the cryogel sample was determined by recording the increase of cumulative mass at a predetermined time interval. The time t of full swelling was measured by the above method several times, and the value was recorded when the mass of the fully swollen cryogel did not increase. Furthermore, at a different pH and temperature, the absorbent capacity of smart cryogels was obtained by the above-mentioned methods. The pH included values of 2, 4, 5, 6, 7, 8, 9, 10, and 12, and the temperature covered 25−70 °C. Finally, the degree curve of swelling at time t was calculated as follows:
degree of swelling =
mt m0
(2)
where mt and m0 are the mass of the fully swollen cryogels at time t and the mass of dried cryogels, respectively. For the deswelling studies, the fully swollen cryogels were pressed by hand until their mass did not decrease. Subsequently, the pressed cryogels would recover by immersion into deionized water. Water recovery from the swollen cryogel was calculated according to the following equation: water recovery =
mswollen − mdeswollen × 100% mswollen − mdried
(3)
where mdried, mswollen, and mdeswollen are the masses of dried, swollen, and deswollen cryogels, respectively. The swelling−deswelling cycles of cryogels were conducted. The porosity of cryogels can be tested by the water volume method because the cryogel volume of samples did not change and clearly swell after absorbing water.16 Columnar cryogel was put into an appropriate measuring cylinder with water for a short time, and the sample with water was removed for the acquisition of the volume of 6778
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Figure 2. FE-SEM images of (a) cryogels-ZnO-1% and (b) cryogels-ZnO-5%, (c) FTIR spectra, (d) UV−vis absorbance of cryogels-6/4 and cryogels-ZnO-1%, (e) XRD patterns of cryogels-ZnO-1% and cryogels-ZnO-5%, and (f) TGA and DTG curves of cryogels-ZnO-1% and cryogelsZnO-5%. Each sample was tested 5 times. In the formula, N0 is the mean number of bacteria on the pure liquid medium with full cells, and N is the mean number of bacteria on the cryogel samples (cfu/cryogel). To obtain the antibacterial result of simulated field water, our groups made simulated water: 10 μL of liquid medium with full bacteria was added into water (100 mL), and 100 μL of the mixture liquid was spread on plates before incubation for 12 h at 37 °C. Subsequently, the cryogels with a diameter of 14 mm and a height of 12 mm were added into the bottle and then set in the incubator for 15 min. Finally, 100 μL of treated mixed water was spread on plates, and the Petri dish was set in the incubator for 12 h at 37 °C. With this process being repeated three times, the antibacterial results were obtained for the cryogels at different times (15, 30, and 45 min). The antibacterial ratio of simulated water was determined by eq 5 above. The release concentration of Zn2+ (Zn leaching) was determined using atomic absorption spectroscopy (AAS, Agilent 240 AA, Agilent Technologies). A 1 g sample of metallic zinc was dissolved in 20 mL of 6 M HCl, and then 980 mL of 2% HCl was added. Standard solutions (1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 ppm) were prepared using the solution from above (1000 mg/L), and the AAS test data were used for a plot of the standard curve. The AAS analysis was set to be detected at a wavelength of 213.9 nm, with a slit width of 0.5 nm and a lamp current
the remaining water. The porosity of the sample was computed by the following equation:
porosity =
Vwater × 100% Vsample
(4)
where Vwater is the volume of the cryogel with absorbed water and Vsample is the volume of dried cryogel. The antibacterial property of the samples was assessed by using Escherichia coli (E. coli, ATCC 25922) and Staphylococcus aureus (S. aureus, ATCC 27217) as model Gram-negative and Gram-positive bacteria for the antibacterial tests. The qualitative antibacterial method was the comparison of the transparency of the culture liquid medium with that of the cryogels. The quantitative mean colony-forming unit method was used for characterization of the antibacterial ratio of the cryogels against S. aureus and E. coli according to the previous literature.10,11,17,18 The antibacterial ratio was determined by counting the microorganism colonies or mean colony-forming units (cfu’s) according to the following equation:
⎛N − N ⎞ antibacterial ratio (%) = ⎜ 0 ⎟ × 100 ⎝ N0 ⎠
(5) 6779
DOI: 10.1021/acssuschemeng.7b01029 ACS Sustainable Chem. Eng. 2017, 5, 6776−6785
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Figure 3. (a) Equilibrium swelling degree; (b) dynamic swelling profiles; (c) swelling degree evaluated after up to 10 rinsing−adsorption cycles (the swelling degrees are normalized with respected to their respective equilibrium swelling degrees, insets are the digital photos of swelling−deswelling cycles); (d) porosity of cryogels-5/5, cryogels-6/4, cryogels-ZnO-5%, and cryogels-ZnO-1%; and (e) photos of the removal of methylene blue (200 mg/L) below silicone oil with the cryogels-ZnO-1%. of 4.0 mA using an air−acetylene flame. The minimum detection limit for the device is 0.02 ppm.
DMAEMA molar ratio, average pore size was increased from 161 to 479 μm, and the pore interconnectivity became poor while the cryogels-6/4 exhibited a narrower pore-size distribution with homogeneous porosity (Figure 1b). This hints that the AM/DMAEMA molar ratio played an important role in the cryogel morphology. The peaks at 3250, 1650, and 1722 cm−1 belonged to NH stretching, CO bending, and CO stretching, respectively, from AM and DMAEMA units in the cryogels. Also, intensities in the NH and CO bending peaks of the cryogels increased with AM/DMAEMA molar ratio, whereas the CO stretching peak intensity was reduced gradually (Figure 1c), indicating that the cryogel polymerization reaction between the monomers of AM and DMAEMA was successfully carried out. For a study into the
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RESULTS AND DISCUSSION Functional cryogels as water-purifying microdevices generally need a suitable porous structure, high mechanical strength, quick swelling−deswelling ability, and antibacterial activity.14,15,19−21 Thus, it is important to optimize ideal cryogels as templates to fabricate such cryogels. FE-SEM as a suitable technique was used for measurement of structure and pore variations of the resulting cryogels with an increasing AM/ DMAEMA molar ratio. Clearly, the cryogels showed a porous three-dimensional (3D) structure composed of thin interconnected sheet walls (Figure 1a). With increasing AM/ 6780
DOI: 10.1021/acssuschemeng.7b01029 ACS Sustainable Chem. Eng. 2017, 5, 6776−6785
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Figure 4. Swelling ratio as functions of (a) temperature and (b) pH values for neat cryogel and smart cryogels-ZnO.
impurities or byproducts. TGA and DTG (derivative thermogravimetry) results (Figure 2f) could support the idea that flowerlike CNC/ZnO nanohybrids did not change the microstructure of the cryogels, and thus similar thermal degradation behavior was observed for neat cryogels and cryogels-ZnO, although the cyrogels-ZnO showed relatively higher T50% values. The T50% values of cyrogels-ZnO-1% and cyrogels-ZnO-5% were about 391.7 and 395.2 °C, respectively. The stronger interactions between nanohybrids and polymer cryogels might provide a thermal barrier for the cryogel chain decomposition for the improvement of their thermal stability. In addition, the higher contents of thermally stable ZnO can be used as a flame retardant for the prevention of the thermal degradation of the cryogels-5, inducing a higher degradation temperature and weight residue (Figure 2f). High adsorption capacity and quick adsorption equilibrium or swelling−deswelling properties are important parameters for water-purifying microdevices, and thus the dynamic adsorption behavior of neat cryogel and cryogels-ZnO was studied in distilled water at 37 °C. Generally, people can treat water by squeezing the smart cryogels-ZnO by hand several times, which will bring its temperature closer to body temperature (37 °C). So, we finally chose 37 °C for the test temperature, instead of the previous room-temperature-based factor, for the stable swelling performance of smart cryogels-ZnO at this temperature. In Figure 3a, cryogels-ZnO (except cryogels-ZnO-5%) show a higher equilibrium swelling capacity than cryogels-5/5 and cryogels-6/4. The enhanced swelling capacity was achieved at low nanohybrid content because of the increased permeability of the cryogel network. Indeed, the interaction between electron-dense ZnO nanorod clusters caused freevolume expansion for the absorbance of more water, but higher nanohybrid contents would induce a low swelling capacity, resulting from the reduced pore size (supported by Figure 2b). In Figure 3b, both neat cryogel and cryogels-ZnO took 2.5 s to absorb about 80% of the equilibrium swelling capacity, and reached the equilibrium swollen state after 10 s. An increase in the nanohybrid loading generally led to a slight decrease in the swelling speed of polymer cryogels, because increased nonswellable components (ZnO) blocked some “pipes” of polymer cryogels. The cryogels-ZnO showed excellent water recovery efficiencies, and 99% of the absorbed water could be recovered by manual hand squeezing with 2.5 s (inset in Figure 3b). The cryogels-ZnO in this work exhibited the higher adsorption speed and similar swelling capacity value, as compared with polyacrylamide/chitosan nanofiber cryogels26 and PDMAEMAbased interpenetrating polymer network (IPN) cryogels.27
effect of AM/DMAEMA molar ratio on the thermal stability and swelling−deswelling ability of the cryogels, TGA and adsorption capacity tests of the cryogels were preformed. All of the cryogels exhibited three similar degradation stages, which were assigned to the evaporation of water associated with the copolymer, the loss of the NH2 groups, and the degradation of the polymer chains in the cryogels.22 In addition, the decomposition temperatures (T50%’s) of cryogels-5/5, cryogels-6/4, and cryogels-8/2 were about 379.5, 389.7, and 393.6 °C, respectively (Figure 1d), and meanwhile, constant adsorption capacity values were around 19.8, 30.3, and 30.8 g/g after each cycle (Figure 1e). These results demonstrated that the pore structure and thermal and swelling properties of copolymer cryogels can be modulated by the AM/DMAEMA molar ratio. In addition, the thermal stability and reusability of the cryogels were improved with increasing AM/DMAEMA molar ratio. From above, the cryogels-6/4 could be determined as an ideal template for the preparation of functional cryogels based on consideration of homogeneous pore-size distribution, medium thermal stability, and reusability. For confirmation of the successful anchoring of flowerlike ZnO nanorod clusters on the resulting cryogels, FE-SEM, FTIR, soild UV−vis, and XRD measurements were used for analysis of the morphology and microstructure of smart cryogels-ZnO. Compared to those of neat cryogel, the porous structure and interconnectivity for the cryogels-ZnO were almost unchanged (Figure 2a,b) with different nanohybrid contents, and obvious flowerlike ZnO clusters were found on pore walls of cryogel under a higher magnification (inset in Figure 2a,b), showing that the nanohybrids were very well dispersed into the cryogel network. These results proved the cross-linking reaction or electrosteric stabilization between residual carboxyl groups of the nanohybrids and amide groups of the cryogels (as shown in Scheme 1). However, high nanohybrid contents resulted in a smaller pore size for cryogelsZnO-5%. Moreover, FT-IR adsorption peaks at 460 cm−1 (Figure 2c) and a UV−vis adsorption peak at 360 nm in smart cryogels-ZnO (cryogels-ZnO-1% as a model sample, Figure 2d) were characteristic adsorption peaks for ZnO nanoparticles from nanohybrids,5 which could further confirm the deposition of nanohybrids onto the cryogel surface. Figure 2e gives characteristic ZnO XRD patterns at 31.7°, 34.3°, 36.2°, 47.5°, 56.4°, 62.8°, 67.7°, and 69.1° assigned to (100), (002), (101), (102), (110), (103), (112), and (201), respectively.23−25 The absence of extra peaks in the XRD spectra was found for cryogels-ZnO-1% and cryogels-ZnO-5%, indicating the successful incorporation of CNC/ZnO nanohybrids without any 6781
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Figure 5. Compressive stress−strain curves compressed to 75% and 80% strain of cryogels-6/4 and cryogels-ZnO-1% in (a) a dry environment and (b) a wet environment, and (c) an overview of compressive stress vs density of various cellulose-based cryogels reported in the literature (hexagons: 1, MFC cryogels,32 2015; 2, NFC cryogels,33 2014; 3, CNC cryogels,34 2014; 4, CNC cryogels,35 2015; 5, CNC cryogels,36 2016; 6, CNC cryogels,37 2016; T, this work).
Figure 3c shows the oscillatory swelling−deswelling behavior of neat cryogel and cryogels-ZnO. Most of the smart cryogels exhibited superfast and stable oscillatory swelling−deswelling up to 10 cycles without any significant loss in the swelling degree and recovery of water, demonstrating the improved robustness and mechanical strength for cryogels in an emergency. Constant adsorption capacity values of around 30.3, 30.8, 22.5, and 21.3 g/g were found for cryogels-6/4, cryogels-ZnO-1%, cryogels-ZnO-5%, and cryogels-5/5 after each cycle, while the shapes of the four samples were not significantly changed, suggesting the reusability of cryogelsZnO for water-purifying microdevices. The remarkable degree of adsorption and adsorption speed of cryogels-ZnO were ascribed to hydrophilic ability, effective cross-linking density, high porosity of more than 90% (Figure 3d), and pore interconnectivity (Figure 2a).1,7,14,28−30 Moreover, macroporous cryogels-ZnO-1% can selectively remove methylene blue with high contents from hydrophobic solvent media (silicone oil as a model hydrophobic solvent, Figure 3e). In this work, copolymerization of multi-stimuli-responsive DMAEMA monomers and AM absorbent monomers on CNC/ ZnO nanorod clusters can lead to pH/temperature responsiveness and smart macroporous cryogels. The adsorption capacity values of neat cryogel and cryogels-ZnO were plotted as a function of temperature and pH values in Figure 4. Clearly, the equilibrium adsorption capacity of neat cryogel and copolymer cryogels decreased with increasing temperature (Figure 4a), while the adsorption capacity exhibited, first, a decrease to a minimum value at pH = 7, and then a gradual increase with
increasing pH values (Figure 4b). Moreover, the incorporation of CNC/ZnO nanohybrids did not affect pH- and temperatureresponsiveness of copolymer cryogels, and the DMAEMA monomers played an important role in the temperature responsiveness of the smart cryogels. The reduced adsorption capacity with rising temperature might be attributed to the electrostatic interaction between the positive charges of the DMAEMA/AM copolymer cryogels and residual carboxyl groups (COO−) of the nanohybrids, resulting in a lower equilibrium swelling ratio. The changed trend in adsorption capacity values of neat cryogel and cryogels-ZnO was attributed to the competitive equilibrium between the protonation degree of tertiary amine groups from DMAEMA (weak polycation) at a low pH value, and carboxyl (COO−) groups from AM or nanohybrids at a high pH value. The increases of the protonation degree of tertiary amine groups and carboxyl (COO−) groups were helpful for the improvement of the adsorption degree of cryogels-ZnO.28,30,31 For potential field drinking-water-purifying microdevices, the cryogels-ZnO should remain intact and have the ability to swell and deswell reversibly without losing mechanical integrity when they are compressed in both dry (air) and wet (water) environments (Figure 5). Therefore, the compressive stress− strain curves of cryogels-6/4 and cryogels-ZnO-1% (as a model sample) in dry and wet environments were investigated. It is found that the cryogels-6/4 showed relatively high strains in both dry and wet environments, while the cryogels-ZnO-1% gave higher stress in both environments at the same compressive strain (Figure 5a,b). Furthermore, compared to 6782
DOI: 10.1021/acssuschemeng.7b01029 ACS Sustainable Chem. Eng. 2017, 5, 6776−6785
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Figure 6. (a) Bacterial reduction of cryogels-ZnO-1% and a blank sample; (b) antibacterial ratio against two bacteria for cryogels-ZnO-1% (insets are the digital images of the water disinfection effect); (c) cfu amounts of simulated water after being treated with cryogels-ZnO-1% at different times (insets are digital images of the quantitative mean colony-forming units); and (d) treated water amounts after antibacterial treatment with cryogelsZnO-1% at cycle times.
cellulose-based cryogels and some smart cryogels (Figure 5c),32−37 the cryogels-ZnO-1% displayed unexpectedly higher dry and wet strengths. Cryogels-ZnO-1% showed the dry compressive stress (at 70% strain) of 125 kPa and wet compressive stress (at 80% strain) of 133 kPa, which were larger than 20.5 and 3.7 kPa (at 80% strain) for chemically cross-linked MFC (microfibrillated cellulose) cryogels,32 8.5 and 12.1 kPa (at 80% strain) for NFC (nanofibrillated cellulose) cryogels,33 76 kPa (wet, at 70% strain) for macroporous polyacrylamide (PAAm)/poly(N-isopropylacrylamide) (PNIPA) double-network cryogels,20 and 22−62 kPa (dry, at 95% strain) for polyacrylamide/chitosan nanofiber cryogels at different loadings. In addition, the cryogels-ZnO-1% had good shape-recovery ability after 10 cycles in air or water. The simultaneous improvements of compressive stress in both dry and wet environments for smart cryogels-ZnO-1% were attributed to good deformation tolerance of the macropores, homogeneously embedded CNC/ZnO nanohybrids (high rigidity of CNC38 and ZnO), and covalent bonds between DMAEMA/AM copolymer chains and the nanohybrids by chemical cross-links. These factors can prevent deformation and collapse of macropores, ZnO nanorod cluster slippage, and water disruption of some hydrogen bonds.39 Field water usually contains many bacteria and suspended particles, before water-purifying microdevices with cryogelsZnO are used, and their antibacterial activity should be evaluated. Thus, an antibacterial test of cryogels-ZnO-1% as a model sample was performed on simulated field water with S. aureus and E. coli by using qualitative and quantitative methods. In Figure 6a, no significant bacterial reduction was observed for
the blank sample (neat cryogel), while a significant decrease in bacterial amounts for smart cryogels-ZnO was found after 1 h. This suggests strong antibacterial activity of cryogels-ZnO due to the incorporation of the excellent antibacterial ZnO nanorod cluster from the nanohybrids. Indeed, ZnO would generate hydrogen peroxide (H2O2) which would contact the cell membrane surface and thus perturb the permeability and respiration functions of the bacteria, resulting in strong antibacterial activity.11,17 Moreover, the smart cryogels-ZnO exhibited high antibacterial ratios of 99.71% and 99.83% killing S. aureus and E. coli, respectively (Figure 6b). In addition, with the addition of cryogels-ZnO into simulated field water for 12 h, the water became transparent, and the amounts of bacterial colonies in the solid medium were decreased greatly (inset in Figure 6b), indicating that most of the bacteria in the simulated field water were killed. Moreover, in practical applications, realtime monitoring was a useful method for the evaluation of the disinfection efficiency for cryogels-ZnO in simulated field water. With the increase of contacting time from 0 to 45 min, the cfu amounts were reduced from 1862 to 6 cfu/mL (Figure 6c), which was much lower than 100 cfu/mL for China’s national drinking water standard.40 This result was supported by the inset images in Figure 6c. The reason for high water disinfection may be due to the macroporous structure and quick adsorption ability of smart cryogels-ZnO. Therefore, the cryogels-ZnO can afford enough space and time for ZnO nanorod clusters to kill more bacteria. Furthermore, Figure 6d shows that 1 g of cryogels-ZnO-1% was sufficient for the rapid production of 16.1 g of disinfected drinking water in 1 cycle, and gave 14.3 g after 5 cycles via manual compression. These 6783
DOI: 10.1021/acssuschemeng.7b01029 ACS Sustainable Chem. Eng. 2017, 5, 6776−6785
Research Article
ACS Sustainable Chemistry & Engineering experimental data were obtained by repeatedly treating the water containing the bacteria, which contained some impurities for blocking of the pores of cryogels-ZnO-1%, resulting in a slight decrease in the treated water amounts. The “treated water amount” was the amount of water extruded from cryogels-ZnO1%. This suggests that cryogels-ZnO-1% had good reusability in the disinfection of drinking water due to the high stability of flowerlike CNC/ZnO nanorod clusters in cryogel pores. Moreover, smart cryogels-ZnO with a modulated pore structure and temperature/pH dual-responsiveness will exhibit a significant disinfection effect on simulated field water in the future. To ensure the safety of the material for drinking water, we studied the Zn leaching by AAS. The AAS results show that cryogels-ZnO-1% and cryogels-ZnO-5% did not release Zn2+ (Zn leaching) during swelling−deswelling cycles. The release behavior of CNC/ZnO nanohybrids without cryogels in the aqueous solution was also studied; a 100 ppm concentration of the solution released only a small amount of Zn leaching (8.6166 ppm) within 12 h. The results were calculated from the equation of the standard curve in Figure S1 and Table S1 (Supporting Information). The release studies showed no Zn leaching from the smart cryogels-ZnO probably because of the strong connection or interaction between ZnO and cryogels.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86 571 86843618. Fax: 86 571 86843619. E-mail:
[email protected]. ORCID
Hou-Yong Yu: 0000-0002-6543-5924 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work is funded by the National Natural Science Foundation of China (51403187), “521” Talent Project of Zhejiang Sci-Tech University, and Open fund in Top Priority Discipline of Zhejiang Province in Zhejiang Sci-Tech University (2016YXQN07).
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REFERENCES
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CONCLUSIONS Smart cryogels were successfully fabricated by one-pot copolymerization among DMAEMA monomers, AM monomers, and flowerlike CNC/ZnO nanorod clusters with the classic ice-template method. This study first demonstrates that pore structure and thermal and swelling properties of copolymer cryogels can be modulated by the increase of the AM/DMAEMA molar ratio. According to thermal and swelling results, the cryogels-6/4 can be selected as an ideal template for the preparation of smart cryogels with CNC/ZnO nanohybrids. Moreover, the influences of nanohybrid contents in the microstructure and the thermal, swelling, and antibacterial properties of cryogels-ZnO were evaluated. Compared to cryogels-ZnO-5% (with high nanofiller contents), the cryogelsZnO-1% showed more homogeneous CNC/ZnO nanorod clusters deposited on pore walls, and thus a better absorption ability with 30.8 g/g water, temperature/pH dual-responsiveness, and a superfast swelling−deswelling ability (2.5 s) even with a treatment of 10 cycles (without any mechanical loss). Moreover, the smart cryogels-ZnO exhibited a high disinfection effect on simulated field water. With the treatment of simulated field water with cryogels-ZnO, cfu amounts were decreased from 1862 to 6 cfu/mL under the contact time of 45 min, and this cfu amount was much lower than the 100 cfu/mL of China’s national drinking water standard. In addition, cryogelsZnO can quickly produce 14.3−16.1 g/g disinfected drinking water by manual compression. This study provides a new type of cryogels-ZnO as a portable water-purifying microdevice for field drinking-water disinfection in outdoor-activity and disaster-relief applications.
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Standard curve of zinc standard solutions from atomic absorption spectroscopy, and concentrations of Zn leaching (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01029. 6784
DOI: 10.1021/acssuschemeng.7b01029 ACS Sustainable Chem. Eng. 2017, 5, 6776−6785
Research Article
ACS Sustainable Chemistry & Engineering
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DOI: 10.1021/acssuschemeng.7b01029 ACS Sustainable Chem. Eng. 2017, 5, 6776−6785
