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Superfast adsorption-disinfection cryogels decorated with cellulose nanocrystal/zinc oxide nanorod clusters for water-purifying microdevice Duan-Chao Wang, Hou-Yong Yu, Mei-Li Song, Ren-Tong Yang, and Juming Yao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01029 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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ACS Sustainable Chemistry & Engineering

Superfast adsorption-disinfection cryogels decorated with cellulose nanocrystal/zinc oxide nanorod clusters for water-purifying microdevice Duan-Chao Wang†, Hou-Yong Yu*†‡, Mei-Li Song†, Ren-Tong Yang†, 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

*Corresponding author. Tel.: 86 571 86843618; fax: 86 571 86843619. E-mail addresses: [email protected] (Hou-Yong Yu)

ABSTRACT: In the outdoor activities and disaster environments, a portable microdevice is very important to 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 microstructure, thermal, swelling and antibacterial properties of cryogels-ZnO were investigated. With the incorporation of 1wt.% CNC/ZnO, the cryogels showed macroporous networked structures with high porosity 93% and homogeneously nanorod clusters on the macroporous wall. Cryogels-ZnO showed high mechanical strength in both dry and wet states,

1

ACS Paragon Plus Environment


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adsorption

capacity

of

30.8

g/g,

superfast

adsorption

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time

(2.5s)

and

stable

swelling–deswelling ability after 10 cycles. Besides, the adsorption capacity of cryogels-ZnO presented dual temperature/pH responsive. By treating simulated field water with cryogels-ZnO for 45 minutes, the CFU amounts of simulated field water were decreased from 1862 CFU/mL to 6 CFU/mL. This CFU amount of treated water was very lower than 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 microdevice. KEYWORDS: Cryogels; Cellulose nanocrystals; ZnO; Adsorption property; Antibacterial activity



INTRODUCTION Microbiological contamination of potable water sources has been considered as one of

important threats to public health. Besides, some sports enthusiasts or soldiers need more purified water in the field. Indeed, there are about 1.8 million people died annually from diarrheal diseases due to low microbiological quality water 1. Thus, main methods including ultraviolet lamp irradiation, activated carbon adsorption and ultrafiltration membrane treatment are used to purify the drinking water, but these treatments suffer from poor portable performance, limited adsorption, and easily blocked pores

2-4

.The cryogels based on organic

and inorganic nanoparticles can be used as effective microdevices to purify field drinking water due to 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 interests in water-purifying fields. Loo et al

1,7

2


optimized the preparation of

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poly(sodium acrylate) (PSA) cryogels with more interconnected pores, higher swelling rate and adsorption capacity by tailoring reaction conditions. Then the resultant PSA cryogels can be served as adsorption carrier/templates to deposit uniform dispersion of Ag NPs to fabricate smart cryogels. PSA/Ag cryogels show fast swelling, high adsorption capacity, good recovery and disinfection effect of the absorbed water via manual compression. However, Ag NPs can render the cryogels with strong antimicrobial performance, but the mechanical strength of the cryogels was not improved. Subsequently, Nata et al

8

have reported multifunctional

poly(vinyl alcohol) cryogel by incorporation of rigid chitin nanofibrils (CNFs) and Ag NPs. Compared to neat PVA cryogel, the storage modulus of as-produced cryogels is improved by adding CNF–Ag. Nevertheless, the swelling adsorption behavior of the macroporous cryogels in field water is not evaluated. Also, the cost of Ag NPs is too high and the Ag release has a strict requirement in water. Therefore, the development of the functional cryogels without Ag NPs has received more interest. In this study, we provide a method to fabricate low cost and intelligent cryogel with dual pH/temperature sensitivity by one-pot polymerization among AM monomers, DMAEMA monomers and flower-like CNC/ZnO nanohybrids. Indeed, the CNC/ZnO nanohybrids with modulated morphology, low cost and antibacterial property show great potential as bifunctional agents into matrix cryogels, compared to Ag NPs9-11. Moreover, Then smart cryogels exhibited dual pH/temperature responsiveness to modulate 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 morphology, microstructure, mechanical, water adsorption and antibacterial properties of smart cryogels were optimized by different AM/DMAEMA molar ratios and CNC/ZnO nanohybrid contents, respectively. Finally, the use of smart cryogels was evaluated in potential water-purifying microdevices for stimulated field water.

3



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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 persulphate(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 bisacrylamide(MBA) were provided by Molbase Chemical Industry(Shanghai,

4


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China).All the materials and reagents were used as received without further purication. Preparation of spherical CNCs and flower-like CNC/ZnO nanohybrids. The preparation method of spherical CNC and flower-like CNC/ZnO nanohybrids has been described in our previous reports

. Briefly, the CNC suspension was mixed with zinc nitrate aqueous

9,12

solution (0.1 g/mL), and the pH value of mixtures was adjusted to 11 by using ammonium hydroxide. The reaction temperature and time are 110 °C and 2 hour, respectively. The resulting products were separated by centrifugation with deionized water. Finally, the precipitates of CNC/ZnO nanohybrids were dried for 24h at 60 °C. Preparation of dual-responsive cryogels and smart cryogels. Firstly, 0.15 g of APS was added to 20 mL of deionized water to prepare an initiator solution; 0.5 mL of TEMED was added to 20 mL of deionized water to form an initiator active agent solution; And 0.1 g of MBA were added to 10 mL of deionized water to prepare a crosslink solution. The reaction mixtures

of

AM,

DMAEMA,

MBA

and

flower-like

CNC/ZnO

nanohybrids

were ultra-sonicated adequately and degassed for chilling into an ice bath at 0 oC. Subsequently, 0.15 g APS and 0.5 mL 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 oC. The resultant dual-responsive cryogels were firstly washed with deionized water and then dehydrated in a series of aqueous ethanol solutions followed by drying in a freeze-dryer 13,14, and freeze-dried at a temperature of -50 oC and a degree of vacuum of 1.3-13 Pa for 48 hours. To ensure 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, 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 cryogels-5/5, cryogels-6/4 and 5



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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.



CHARACTERIZATION The morphology of dried cryogels was characterized by the Field-emission scanning

Electron Microscopy (FE-SEM, JSM-5610; JEOL, Japan), and the parameter was shown that the accelerating voltage is 15 kV at room temperature. The potassium bromide (KBr) disc method is an effective way to measure chemical structure of the samples via Fourier-transform infrared (FT-IR) spectrometer (Nicolet 5700, Thermo Electron Corp, USA) at ambient temperature. The wavenumber of FT-IR has been scanned at the range of 4,000–400 cm-1. The optical properties of the cryogels were characterized in the wavelength interval between 300 and 700 nm by using solid-phase ultraviolet spectrometry (Solid UV–vis) spectrophotometer

(Lambda

35;

PerkinElmer

Corporation,

USA).

X-ray

powder

diffractometer (XRD) analysis was performed to investigate the crystal phase of CNC/ZnO nanohybrids and the cryogels. The parameters of X-ray powder diffractometer (ARL X’RA, ThermoElectron Corp) are monochromatic Cu Kα radiation at λ = 1.54056 Å in the 2θ range of 15–80° at scan rate of 2° min-1. The X-ray generator tension and current were 40 kV and 30 mA, respectively. The thermostability of cryogels was observed on TGA experiment using a thermogravimetric analyzer (TGA, Pyris Diamond I; PerkinElmer Corp). The samples of cryogels (about 3-8 mg) were heated from 30 °C to 600 °C at the rate of 20 K/min under dynamic nitrogen atmosphere with flow rate of 30 mL/min. Uniaxial compression measurement was used to measure mechanical properties of the cryogels. Cylindrically shaped cryogels sample with 12 mm thickness was tested under unconfined compression condition using a custom computer-controlled mechanical testing system. The samples were compressed at room temperature with 5kN load cell at a ramp rate

6


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of 10 mm min-1. The Young’s modulus was determined from initial linear slopes of the stress-strain curves according to the following equation: h F E 0 2  r h

(1)

where F is the force applied to cryogels with a cross-sectional area of π r2, h is the height of the cryogel during compression, and ∆ho is the change in height of the sample during compression. The swelling properties of cryogels are one of the most important properties in the field drinking water-purifying moicrodevice1,11,15, which can be determined by gravimetric analysis. For the determination of the swelling profile, a dried cryogel sample with 14 mm of diameter and 12 mm of height was swollen in excess deionized water. Excess surface water was gently wiped off using a filter parer before measuring mass of the swollen cryogel. Water uptake of the cryogel sample was determined by recording the increase of cumulative mass at a pre-determined time interval. The time t of fully swelling was measured by the above method several times and recorded the value when the mass of the fully swollen cryogel did not increase. Furthermore, at the different pH and temperature, the absorbent capacity of smart cryogels was obtained by above mentioned methods. The pH included 2, 4, 5, 6, 7, 8, 9, 10, 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 mo are the masses of the fully swollen cryogels at time t and dried cryogels, respectively. For the deswelling studies, the fully swollen cryogels was pressed by hand until the mass of cryogels did not decrease. Subsequently, the pressed cryogels would recover by immersing into deionized water. Water recovery from the swollen cryogel was calculated according to 7



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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 swell clearly after absorbing water

. Columned

16

cryogel was put into an appropriate measuring cylinder with water for a short time, and remove the sample with water to obtain the volume of rest water. The porosity of 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. 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 comparing the transparency of the culture liquid medium with the cryogels. The quantitative mean colony-forming unit method was used to characterize antibacterial ratio of the cryogels against S. aureus and E. coli according to previous literatures

10,11,17,18

. The antibacterial ratio

was determined by counting the microorganism colonies or mean colony-forming units (CFU) according to the following equation:

Antibacterial ratio (%)  (

NO  N )*100 NO

(5)

Each sample was tested five 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 cryogels 8


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samples (CFU/cryogels). In order to obtain antibacterial result of simulated field water, our groups have made simulated water: 10 μL of liquid medium with full bacteria was added into water (100mL), and 100 μL mixture liquids have been spread on the plates before incubation of 12 h at 37 °C. Subsequently, the cryogels with the size of 14 mm diameter and 12 mm height were added into the bottle and then set in incubator for 15min. Finally, 100 μL of treated mixed water was spreaded on plates and the petri dish was set in incubator for 12 h at 37 °C. Repeating this process three times, the antibacterial results were obtained for the cryogels at different time (15, 30 and 45min). The antibacterial ratio of simulated water was determined by the above equation (5). The release concentration of Zn2+ (Zn leaching) was determined using atomic absorption spectroscopy (AAS, Agilent 240 AA, Agilent Technologies, USA). 1 g of metallic zinc was dissolved in 20 mL of 6 M HCl , then 980 mL of 2% HCl was added. Standard solutions (1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ppm) were prepared using above solution (1000 mg / L) and the AAS test data were used to plot the standard curve. The AAS was set to be detected at a wavelength of 213.9 nm, a slit width of 0.5 nm and a lamp current of 4.0 mA using an air-acetylene flame. The minimum detection limit for the device is 0.02 ppm.



RESULTS AND DISCUSSION The functional cryogels as water-purifying microdevices generally need suitable porous

structure, high mechanical strength, quick swelling–deswelling ability and antibacterial activity

. Thus it is important to optimize ideal cryogels as templates to fabricate

14,15,19-21

above cryogels. FE-SEM as a suitable technique was used to measure structure and pore variations of the resulting cryogels with increasing AM/DMAEMA molar ratio. Clearly, the cryogels showed a porous three-dimensional (3D) structure composed of thin interconnected

9



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sheet walls (Fig.1a). With increasing AM/DMAEMA molar ratio, average pore size was increased from 161 μm to 479 μm, and the pore interconnectivity became poor while the cryogels-6/4 exhibited narrower pore-size distribution with homogenous porosity (Fig.1b). It hints that the AM/DMAEMA molar ratio played an important role in the cryogel morphology. The peaks at 3250, 1650 and 1722 cm-1 were belonged to the N-H stretching, C=O bending and C=O stretching from AM and DMAEMA units in the cryogels, respectively. Also, intensities in the N-H and C=O bending peak of the cryogels were increased with AM/DMAEMA molar ratio, whereas C=O stretching peak intensity was reduced gradually (Fig.1c), indicating cryogel polymerization reaction between the monomers of AM and DMAEMA was successfully carried out. In order to study the 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 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 degradation of the polymer chains in the cryogels, respectively

. Besides, the decomposition temperatures (T50%) of cryogels-5/5,

22

cryogels-6/4 and cryogels-8/2 were about 379.5, 389.7 and 393.6 °C, respectively(Fig.1d), meanwhile constant adsorption capacity values were around 19.8, 30.3 and 30.8g/g after each cycle(Fig.1e). These results demonstrated that the pore structure, thermal and swelling properties of co-polymer cryogels can be modulated by with AM/DMAEMA molar ratio. Besides, 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 preparing functional cryogels based on consideration of homogenous pore-size distribution, medium thermal stability and reusability.

10


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Fig.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, cryogels-8/2.

To confirm the successful anchoring of flower-like ZnO nanorod clusters on the resulting cryogels, FE-SEM, FT-IR, soild-UV-vis and XRD measurements were used to analyze the morphlogy and microstructure of smart cryogels-ZnO. Compared to neat cryogel, porous structure and interconnectivity for the cryogel-ZnO were almost unchanged (Fig.2a-b) with different nanohybrid contents, and obvious flower ZnO clusters on pore walls of cryogel were found under a higher magnification (insert in Fig.2a-b), showing that the nanohybrids were 11



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dispersed very well into the cryogel network. These results proved cross-linking reaction or electrosteric stabilization between residual carboxyl groups of nanohybrids and amide groups of the cryogels (as shown in Scheme 1). However, high nanohybrid contents resulted in smaller pore size for cryogels-ZnO-5%. Moreover, FT-IR adsorption peaks at 460 cm-1 (Fig.2c) and UV-vis adsorption peak at 360 nm in smart cryogels-ZnO (cryogels-ZnO-1% as model sample, Fig.2d) were characteristic adsorption of ZnO nanoparticles from nanohybrids , which could further confirm the depositing of nanohybrids onto the cryogel surface. Fig.2e

5

gives characteristic ZnO XRD patterns at 31.7º, 34.3º, 36.2º, 47.5º, 56.4º, 62.8º, 67.7º and 69.1º assigned to the (100), (002), (101), (102), (110), (103), (112) and (201), respectively . The absence of extra peaks in the XRD spectra was found for cryogel-ZnO-1% and

23-25

cryogel-ZnO-5%, indicating the successful incorporation of CNC/ZnO nanohybrids without any impurities or byproducts. TGA and DTG results (Fig.2f) could support that flower-like CNC/ZnO nanohybrids did not change the microstructure of the cryogels, and thus similar thermal degradation behavior were observe for neat cryogels and cryogel-ZnO, although the cyrogel-ZnO showed relatively higher T50% values. The T50% values of cyrogel-ZnO-1% and cyrogel-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 to improve their thermal stability. In addition, the higher contents of thermally stable ZnO can be used as a flame retardant to prevent the thermal degradation of the cryogels-5, inducing a higher degradation temperature and weight residue(Fig.2f).

12


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Fig.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 pattern of cryogels-ZnO-1% and cryogels-ZnO-5%; (f) TGA and DTG curves of cryogels-ZnO-1% and cryogels-ZnO-5% .

High adsorption capacity and quick adsorption equilibrium or swelling-deswelling property are important parameters for water purification microdevice, 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 hand several times, which will bring the temperature of smart cryogels-ZnO closer to body temperature (37 oC). So, we finally chose 37 oC for the test temperature instead of room temperature based above factor and stable swelling performance of smart cryogels-ZnO at this temperature. In Fig.3a, cryogels-ZnO (except cryogels-ZnO-5%) shows higher equilibrium swelling capacity than cryogels-5/5 and cryogels-6/4. The enhanced swelling capacity was achieved at low nanohybrid content due to 13



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increased permeability of the cryogel network. Indeed, the interaction between electron-dense ZnO nanorod clusters caused free volume expansion for absorbing more water, but more nanohybrid contents would induce low swelling capacity, resulting from the reduced pore size (supported by Fig.2b). In Fig.3b, both neat cryogel and cryogels-ZnO took 2.5 seconds to absorb about 80% of equilibrium swelling capacity, and reached equilibrium swollen state after 10 seconds. An increase in the nanohybrid loading generally led to a slight decrease in the swelling speed of polymer cryogels, because increased no-swellable 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 2.5s (inset in Fig.3b). The cryogels-ZnO in this work exhibited the higher adsorption

speed

and

similar

swelling

polyacrylamide/chitosan nanofibers cryogels polymer network (IPN) cryogels

capacity 26

value,

as

compared

with

and PDMAEMA based interpenetrating

. Fig.3c shows the oscillatory swelling–deswelling

27

behavior of neat cryogel and cryogels-ZnO. Most of 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 improved robustness and mechanical strength for cryogels in 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 four samples were not significantly changed, suggesting the reusability of cryogels-ZnO for water purification microdevice. The remarkably adsorption degree and adsorption speed of cryogels-ZnO were ascribed to hydrophilic ability, effective cross-linking density and high porosity of more than 90% (Fig.3d)

and

pore-interconnectivity

(Fig.2a)

.

1,7,14,28-30

Moreover,

macroporous

cryogels-ZnO-1% can selectively remove methylene blue with high contents from hydrophobic solvent medium (silicone oil as a model hydrophobic solvent) (Fig.3e).

14


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Fig.3 (a)Equilibrium swelling degree, (b)dynamic swelling profiles, (c)swelling degree was 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%, cryogels-ZnO-1% and (e)removal of methylene blue (200mg/L) below silicone oil with the cryogels-ZnO-1%. In this work, copolymerization of multi-stimuli-responsive DMAEMA monomers and AM absorbent monomers on the CNC/ZnO nanorod clusters can give pH/temperature 15



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responsive 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 Fig. 4. Clearly, the equilibrium adsorption capacity of neat cryogel and copolymer cryogels was decreased with increasing temperature (Fig.4a), while the adsorption capacity exhibited first decrease to minimum value at pH=7 and then gradual increase with increasing pH values (Fig.4b). Moreover, the incorporation of CNC/ZnO nanohybrids did not affect pH- and temperature-responsiveness of copolymer cryogels, and the DMAEMA monomers played an important role in the temperature responsiveness of the smart cryogels. The reduced adsorption capacity at 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 lower equilibrium swelling ratio. The change trend in adsorption capacity values of neat cryogel and cryogels-ZnO was attributed to competitive equilibrium between protonation degree of tertiary amine groups from DMAEMA (weak polycation) at low pH value and carboxyl (COO-) amounts of AM or nanohybrids at high pH value. The increases of the protonation degree of tertiary amine groups and carboxyl (COO-) groups were helpful to improve adsorption degree of cryogels-ZnO 28,30,31.

Fig.4 Swelling ratio as functions of temperature (a) and pH values (b) for neat cryogel and smart cryogels-ZnO. For potential field drinking water-purifying microdevice, the cryogels-ZnO should remain 16


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intact and have ability to reversibly swelling and deswelling without losing mechanical integrity when they were compressed in both dry (air) and wet (water) environments (Fig.5). Therefore, the compressive stress-strain curves of cryogels-6/4 and cryogels-ZnO-1% (as a model sample) in dry and wet environment 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 two environments at the same compressive strain (Fig.5a and b). Furthermore, compared to cellulose-based cryogels and some smart cryogels (Fig.5c) 32-37, the cryogels-ZnO-1% displayed unexpected 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 cryogels 32, 8.5 and 12.1 kPa (at 80% strain) for NFC cryogels ,

33

76

kPa

(wet,

at

70%

strain)

for

macroporous

(PAAm)/poly(N-isopropylacrylamide) (PNIPA) double-network cryogels

polyacrylamide 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 CNC

38

and ZnO) and

covalent bonds between DMAEMA/AM copolymer chains and the nanohybrids by chemical cross-links. These factors can prevent deformation and collapsing of macropores, ZnO nanorod cluster slippage, and water disruption of some hydrogen bonds 39.

17



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Fig.5 Compressive stress-strain curves compressed to 75% and 80% strain of cryogels-6/4 and cryogels-ZnO-1% in (a) dry environment, (b) wet environment, and (c) overview of compressive stress vs density of various cellulose-based cryogels reported in literatures, hexagon indicate literatures(1: MFC cryogels, ref.32, 2015; 2: NFC cryogels, ref.33, 2014; 3: CNC cryogels, ref.34, 2014; 4: CNC cryogels, ref.35, 2015; 5: CNC cryogels, ref.36, 2016; 6: CNC cryogels, ref.37, 2016; T: this work).

Field water usually contains many bacteria and suspended particles, before the cryogels-ZnO was used for water-purifying microdevice, and their antibacterial activity should be evaluated. Thus 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 method. In Fig. 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 hour. This suggests strong antibacterial activity of 18


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cryogels-ZnO due to incorporation of excellent antibacterial ZnO nanorod cluster from the nanohybrids. Indeed, ZnO would generate hydrogen peroxide (H2O2) to contact cell membrane surface and thus perturb the permeability and respiration functions of the bacteria, resulting in strong antibacterial activity

. Moreover, the smart cryogels-ZnO exhibited

11,17

high antibacterial ratio of 99.71 % and 99.83 % killing S. aureus and E. coli, respectively (Fig. 6b). Besides, by adding cryogels-ZnO into simulated field water for 12h, the water became transparent and the amounts of bacterial colony in solid medium were decreased greatly (inset in Fig. 6b), indicating that most of bacteria in simulated field water were killed. Moreover, in practical application, real-time monitoring was a useful method to evaluate 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 CFU/mL to 6 CFU/mL (Fig. 6c), which was very lower than 100 CFU/mL for China's National Drinking Water Standard40. This result was supported by inset images in Fig. 6c. The reason for high water disinfection may be due to 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, Fig. 6d shows that 1 g of cryogels-ZnO-1% was sufficient to rapidly produce 16.1 g of disinfected drinking water in 1 cycle, and give 14.3 g after 5 cycles via manual compression. These experimental data were obtained by repeatedly treating the water containing the bacteria, which contained some impurities to block the pores of cryogels-ZnO-1%, resulting in a slight decrease in the treated water amounts. The “ treated water amount ” was the water amounts extruded from cryogels-ZnO-1%. It suggests that cryogels-ZnO-1% had good reusability of disinfecting drinking water due to high stability of flower-like CNC/ZnO nanorod clusters in cryogel pores. Moreover, smart cryogels-ZnO with modulated pore structure and temperature/pH dual responsive will exhibit a significant disinfection effect on simulated field water in future. In order to ensure the safety of the 19



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Page 20 of 28

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, 100 ppm of the solution released only a small amount of Zn leaching (8.6166ppm) within 12 hours. The results were calculated from equation of standard curve in Figure S1 and Table S1 (Supporting Information) . The release studies showed no Zn leaching from the smart cryogels-ZnO probably due to strong connection or interaction between ZnO and cryogels .

Fig.6 (a) Bacterial reduction of cryogels-ZnO-1% and blank sample, (b)antibacterial ratio against two bacteria for cryogels-ZnO-1% (insets are the digital images of water disinfection effect), (c) CFU amounts of simulated water after treating with cryogels-ZnO-1% at different time (insets are digital images of quantitative mean colony-forming units), (d) treated water amounts after antibacterial treatment with cryogels-ZnO-1% at cycle times. 20


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

CONCLUSIONS Smart cryogels were successfully fabricated by one-pot co-polymerization among

DMAEMA monomers, AM monomers and flower-like CNC/ZnO nanorod clusters with classic ice-template method. This study firstly demonstrates that pore structure, thermal and swelling properties of copolymer cryogels can be modulated by increasing AM/DMAEMA molar ratio. According to thermal and swelling results, the cryogels-6/4 can be selected as an ideal template for preparing smart cryogels with CNC/ZnO nanohybrids. Moreover, the influence of nanohybrid contents in microstructure, thermal, swelling and antibacterial properties of cryogels-ZnO were evaluated. Compared to cryogels-ZnO-5% (with high nanofiller contents), the cryogels-ZnO-1% showed more homogeneous CNC/ZnO nanorod clusters deposited on pore walls, and thus better absorption ability with 30.8 g/g water, temperature/pH dual responsive and super-fast swelling–deswelling ability (2.5s) even treatment of 10 cycles (without any mechanical loss). Moreover, the smart cryogels-ZnO exhibited high disinfection effect on simulated field water. By treating simulated field water with cryogels-ZnO, CFU amounts were decreased from 1862 CFU/mL to 6 CFU/mL under the contact time of 45 min, and this CFU amount was very lower than 100 CFU/mL of China National Drinking Water Standard. Besides, cryogels-ZnO can quickly produce 14.3-16.1 g/g of disinfectant drinking water by manual compression. This study provides a new type of cryogels-ZnO as a portable water purification microdevice for field drinking water disinfection in outdoor activities and disaster relief applications.



ASSOCIATED CONTENT

*Supporting Information Standard curve of zinc standard solutions from atomic absorption spectroscopy (Figure S1 ) and concentration of Zn leaching from synthesized CNC/ZnO, cryogel-ZnO-1% and

21



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cryogel-ZnO-5% after a series of swelling-deswelling cycles in aqueous solution(Table S1). This material is available free of charge via the Internet at http:// pubs.acs.org. 

AUTHOR INFORMATION

Corresponding Author * Hou-Yong Yu (H.Y. Yu); Tel.: 86 571 86843618; E–mail addresses: [email protected]. Notes The authors declare no competing financial interest.



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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For Table of Contents Use Only

Novel

method

was

presented

to

fabricate

smart

cryogels-ZnO

adsorption-disinfection for field drinking water-purifying microdevice.

28


with

superfast

Superfast AdsorptionâDisinfection Cryogels ... - ACS Publications

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