Article pubs.acs.org/est
Superabsorbent Cryogels Decorated with Silver Nanoparticles as a Novel Water Technology for Point-of-Use Disinfection Siew-Leng Loo,†,‡ Anthony G. Fane,†,‡ Teik-Thye Lim,*,†,‡ William B. Krantz,†,§ Yen-Nan Liang,∥ Xin Liu,†,‡ and Xiao Hu*,†,∥ †
Singapore Membrane Technology Centre, Nanyang Technological University, 1 Cleantech Loop, CleanTech One, #05-05, Singapore 637141, Singapore ‡ School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, 50 Nanyang Avenue, Singapore 639798, Singapore § Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0424, United States ∥ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore S Supporting Information *
ABSTRACT: This paper reports the preparation of poly(sodium acrylate) (PSA) cryogels decorated with silver nanoparticles (AgNPs) for point-of-use (POU) water disinfection. The PSA/Ag cryogels combine the high porosity, excellent mechanical and water absorption properties of cryogels, and uniform dispersion of fine AgNPs on the cryogel pore surface for rapid disinfection with minimal Ag release (30 min) or (ii) a significant level of Ag release into the treated water that compromise the potability of water and the reusability of the nanocomposites. Furthermore, systems operated in the column mode may present some operational challenges especially when Received: Revised: Accepted: Published: 9363
March 19, 2013 June 18, 2013 July 25, 2013 August 8, 2013 dx.doi.org/10.1021/es401219s | Environ. Sci. Technol. 2013, 47, 9363−9371
Environmental Science & Technology
Article
Ultra spectrometer with a monochromatic Kα excitation source (hν = 1486.71 eV). The binding energies were calibrated using a C 1s core level at 284.8 eV as a reference. Field emission scanning microscope (FESEM, JEOL JSM-7600F) was used to image the morphology of the cryogel nanocomposites. The specimens were coated with Pt for 30 s (20 mA) using an autofine coater (JEOL JFC-1600) prior to imaging. An energydispersive X-ray spectroscopy detector (EDX) attached to the FESEM was used to determine the elemental composition of the nanocomposites. The morphology and size of the AgNPs were studied using a transmission electron miscroscope (TEM, Carl Zeiss Libra 120) at an accelerating voltage of 120 kV. TEM images were analyzed using image analysis software (ImageJ) to determine the particle-size distribution of the AgNPs. High-resolution TEM images of the AgNPs were obtained at an accelerating voltage of 200 kV (JEOL JEM2010). TEM samples were prepared by grinding the nanocomposites into fine particles before they were dispersed in absolute ethanol. A few drops of the suspension were added to a C-coated Cu grid (300 mesh) and were left to dry in a fume hood prior to imaging. UV−vis absorption spectra of the nanocomposite solutions were obtained using a UV−vis absorption spectrophotometer (Shimadzu UV-1700). The ion-exchange capacity of the PSA cryogels was determined by the acid−base titration method described elsewhere.24 The mechanical properties of the fully swollen nanocomposites of 10 mm thickness were characterized using a computercontrolled mechanical testing system (Instron 5567) at room temperature. A 5 kN load cell at a ramp rate of 10 mm min−1 was used, and the sample was compressed up to 95% strain of its initial length. Five replicate experiments were conducted. Antibacterial Tests. Escherichia coli (E. coli, ATCC 25922) and Bacillus subtilis (B. subtilis, ATCC 6633) were selected as the model gram-negative and -positive bacteria for the antibacterial tests. E. coli and B. subtilis were cultivated in tryptic soy broth and nutrient broth at 37 and 30 °C, respectively, and were harvested after reaching their midexponential growth phase. The harvested cells were washed by centrifugation followed by resuspension in phosphate buffered saline (PBS, 0.01 M, pH 7.45). For the disinfection tests, a 0.02 g cryogel sample was added into a 10 mL bacterial suspension of cell density 108 colony forming units per mL (cfu mL−1); manual shaking was provided during cryogel swelling in bacterial suspension. Control experiments were conducted without adding any cryogel into the bacterial suspension. After 15 s of swelling in the bacterial suspension, the swollen cryogels were quickly removed and squeezed to recover the absorbed water; hereafter, this will be referred to as “squeezed water”, while the term “bulk water” will refer to the excess bacterial suspension outside the cryogel (Figure 3b and c). After appropriate dilution in PBS, 0.1 mL of the control, squeezed water, and bulk water were streaked on agar plates followed by 24 h of incubation to enumerate the number of viable bacteria. The kinetics of the bactericidal action was studied by using universal quenching agent (UQA, 0.1% peptone + 0.1% Na2S2O3 + 0.5% Tween 80 + 0.07% lecithin) to quench the disinfection reaction.25,26 Aliquots of squeezed water, which were contacted with the PSA/Ag cryogel at various contact times, were diluted 10 times in UQA. The resultant mixture was allowed to react for 60 min before it was diluted in PBS and spread on agar plates. At least six replicate experiments were conducted.
deployed for applications in difficult circumstances such as in the aftermath of disasters.15 In this paper, we report for the first time the preparation of AgNPs-decorated superabsorbent PSA cryogels and their application for water disinfection in a novel POU process that employs the ability of cryogels to absorb water, which can be subsequently released via external stimuli. Cryogels are formed by conducting a polymerization reaction in a semifrozen system in which the ice crystals (for aqueous systems) act as the porogens resulting in a highly interconnected porous network.16 Functionalized polymeric cryogels have been used as adsorbents for removal of toxic pollutants from water.17−20 The design of PSA/Ag cryogels combines the high porosity and excellent mechanical and water absorption properties of cryogels and uniform dispersion of fine AgNPs on the cryogel pore surface for rapid disinfection with minimal Ag release. PSA/Ag cryogels are lightweight and permit easy recovery of the absorbed water via the application of minimal pressure, e.g., by manual hand compression. Because of their simple operation and ease of deployment, they offer promise to provide potable water in emergencies where there is limited access to the infrastructure.15
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EXPERIMENTAL SECTION Preparation of PSA Cryogels. The design principles and synthesis of PSA cryogels were previously described.21 Briefly, ammonium persulfate (APS, 98% purity, Sigma-Aldrich) and N,N,N′,N′-tetramethylethylenediamine (TEMED, ≥ 99%, Sigma-Aldrich) were added to a reaction mixture, containing sodium acrylate (SA, 97%, Sigma-Aldrich) and N,N′methylenebis(acrylamide) (MBA, 99%, Sigma-Aldrich), that was degassed and chilled in an ice bath. The APS and TEMED concentrations in the final reaction mixture were 1.75 mM and 0.125% (v/v), respectively. The monomer concentration (SA + MBA) used was 8% at a cross-linker ratio of 0.05 (mol MBA/ mol SA). The resultant reaction mixture was transferred into several poly(propylene) syringes (3 mL and 9 mm ID) that were then placed into a bath fluid (−20 °C, 1:1 mixture of ethylene glycol/Milli-Q water (18.2 MΩ cm at 25 °C)) incubated in an ultralow temperature freezer (Eutra EDFU4100). After 24 h, the PSA cryogels were thoroughly washed in Milli-Q water and dehydrated in t-butanol followed by drying in a freeze dryer (Alpha 1−4LD, −45 °C) before they were fractured into smaller cylindrical disk samples. Preparation of AgNPs-Decorated Cryogels. PSA/Ag cryogels were prepared using the intermatrix synthesis method.22,23 Typically, 1 g of the dried PSA cryogels was allowed to swell in a 250 mL solution of 1, 5, or 10 mM of AgNO3 (≥98%, Merck). The suspension was shaken at 120 rpm on an orbital shaker for 24 h. The cryogels were immersed in a 250 mL solution of NaBH4 (Alfa Aesar, 10:1 molar ratio of NaBH4 to AgNO3) to form silver nanoparticles (AgNPs). The resultant nanocomposites were thoroughly washed by immersion in Milli-Q water followed by vacuum filtration. After three repetitions of the washing step, the nanocomposites were dried using the same procedure that was used for the PSA cryogels. Characterization of Cryogel Nanocomposites. X-ray diffraction (XRD) spectra were acquired using a powder X-ray diffractometer (Shimadzu 6000) with a monochromatic intensity Cu Kα radiation (λ = 1.5418 Å) in a 2θ range of 5−80° at a scan rate of 1.5° min−1. X-ray photoelectron spectroscopy (XPS) studies were conducted on a Kratos Axis 9364
dx.doi.org/10.1021/es401219s | Environ. Sci. Technol. 2013, 47, 9363−9371
Environmental Science & Technology
Article
Table 1. Nomenclature and Summary of PSA/Ag Cryogel Propertiesa. sample
precursor Ag+ (M)
Ag content (mg g−1)
PSA gel AgNC-20 AgNC-90 AgNC-170
N.A. 0.001 0.005 0.010
N.A. 21.3 (0.3) 88.7 (12.3) 166.7 (15.0)
water recovery efficiencyc (%) 84.8 85.6 84.3 84.5
(1.0) (0.8) (1.5) (2.3)
Young’s modulusd(kPa)
mean AgNPs sizef (nm)
mean Ag crystallite sizeg (nm)
total Ag loss after 24 hh (%)
total Ag in squeezed water (μg L−1)
2.6 (0.5)e 3.0e 3.2 (0.4)e 3.2 (0.4)e
N.A. 4.3 (1.3) 6.2 (1.7) 8.6 (3.8)
N.A. 3.8 5.5 7.8
N.A. 1.79 (0.02) 0.08 0.03
N.A. 76.6 (0.7) 59.6 (0.8) 36.4 (1.5)
a
Note: N.A. = not applicable; standard deviations are shown in parentheses. bThe equilibrium swelling degree was computed by taking the ratio of swollen mass to that of dried mass of cryogel. cThe water recovery efficiency of cryogel was computed by taking the ratio of the difference between swollen and deswollen cryogel masses to the difference between swollen and dried cryogel masses. dYoung’s modulus was determined from the initial linear slopes of the stress−strain curves. eNo failure was observed at the end of the uniaxial compression test. fDetermined from TEM images. g Estimated from X-ray diffractograms using Scherrer equation; full-width-at-half-maximum of (111) reflection and a shape factor of 0.9 were used for computation. hExpressed as a percentage of total Ag in the initial PSA/Ag cryogel.
Figure 1. (a) FESEM image of AgNC-170 under a low magnification; the inset shows a high-magnification FESEM image of AgNPs dispersed on the pore surface of AgNC-170. The AgNPs size distribution and TEM images of the AgNPs are in (b) AgNC-20, (c) AgNC-90, and (d) AgNC-170.
standard addition27 was employed to determine the Ag+ concentration in the squeezed water (before digestion) using a silver/sulfide solid state ion selective electrode (ISE, Orion) connected to an ion meter (Orion Dualstar) to detect Ag as Ag+; all standards and samples were buffered with ionic strength adjuster (Orion) before ISE measurements. All experiments were conducted in triplicate. Statistical Analyses. The statistical significance of the difference between the obtained results was determined using the Student’s t test at a 95% confidence level. All measurements are reported as the mean ± one standard deviation of at least three replicates.
Analytical Methods for Ag Determination. Total Ag loss (as Ag+ ions and AgNPs) after a 24 h immersion in 10 mL of Milli-Q water was determined by measuring the total Ag content in the resultant solution. Ag loss for each sample was expressed as a percentage of the total Ag in the fresh nanocomposite. All samples were digested in HNO3 (67%, Merck) at 170 °C for 2 h on a digestor unit (Hach DRB 200) prior to total Ag analyses. The total Ag concentration in the samples was determined by using either an inductively coupled plasma-optical emission spectrophotometer (ICP-OES, PerkinElmer Optima 2000DV) or an inductively coupled plasma-mass spectrometer (ICP-MS, Elan DRC-e) depending on the concentration range of the samples. In order to determine the total dissolved Ag, samples were filtered through a syringe filter (0.45 μm, Pall) before ICP analyses. The method of 9365
dx.doi.org/10.1021/es401219s | Environ. Sci. Technol. 2013, 47, 9363−9371
Environmental Science & Technology
Article
Figure 2. (a) XPS and (b) XRD spectra of PSA cryogels with different Ag loadings. (c) HRTEM image and SAED pattern of AgNC-170. (d) Dynamic swelling profiles of cryogels. The swelling degrees were normalized with respected to their respective equilibrium swelling degrees, which are shown in the inset.
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contents (20 to 170 mg g−1, Table 1) and a predictable linear increase of the Ag content with the precursor Ag + concentration used (Table 1, R2 > 0.99). Alonso et al. also employed the intermatrix synthesis method to prepare Ag nanocomposites with high Ag content (>200 mg g−1) from fibrous ion-exchange materials.28,29 Hereafter, the as-synthesized PSA/Ag cryogel nanocomposites were denoted as AgNCx, where x (x = 20, 90, and 170) represents the Ag content (mg g−1) in the nanocomposites. Upon borohydride reduction, the cryogels changed color from white to dark brown (inset of Figure S2, Supporting Information). The dark brown color was due to the surface plasmon resonance of AgNPs as evidenced by the characteristic absorption peak in the vicinity of 420−430 nm (Figure S2, Supporting Information).30 The FESEM images show that the pore size and interconnectivity of the cryogels was relatively unaffected after AgNPs decoration (Figure 1a and Figure S3, Supporting Information). This is because of the large difference in the scale of the cryogel pores (1−100 μm) and AgNPs (mostly
