Nanocolloidal Hydrogel for Heavy Metal Scavenging - ACS Nano

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Nanocolloidal Hydrogel for Heavy Metal Scavenging Moien Alizadehgiashi, Nancy Khuu, Amir Khabibullin, Andria Henry, Moritz Tebbe, Toyoko Suzuki, and Eugenia Kumacheva ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03202 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Nanocolloidal Hydrogel for Heavy Metal Scavenging Moien Alizadehgiashi†, Nancy Khuu†, Amir Khabibullin†, Andria Henry†, Moritz Tebbe†, Toyoko Suzuki†,∏, Eugenia Kumacheva*†‡§

†Department of Chemistry, University of Toronto, 80 Saint George street, Toronto, Ontario M5S 3H6, Canada ‡ Institute of Biomaterials and Biomedical Engineering, University of Toronto, 4 Taddle Creek Road, Toronto, Ontario M5S 3G9, Canada ∏ Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan §Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada

* E-mail: [email protected] KEYWORDS: nanoparticles, cellulose nanocrystals, graphene quantum dots, heavy metal scavenging, hydrogel.

ABSTRACT. We report a nanocolloidal hydrogel that combines the advantages of molecular hydrogels and NP-based scavengers of heavy metal ions. The hydrogel was formed by the chemical crosslinking of cellulose nanocrystals and graphene quantum dots. Over a range of hydrogel compositions, its structure was changed from lamellar to nanofibrillar, thus enabling control of hydrogel permeability. Using a microfluidic approach, we generated nanocolloidal microgels and explored their scavenging capacity for Hg2+, Cu2+, Ni2+ and Ag+ ions. Due to the large surface area and abundance of ion-coordinating sites on the surface of nanoparticle building blocks, the microgels exhibited a high ion-sequestration capacity. The microgels were recyclable and were used in several ion scavenging cycles. These features, in addition to the

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sustainable nature of the nanoparticles make this nanocolloidal hydrogel a promising ionscavenging material.

Contamination of water with heavy metals is a growing concern, as it limits access to potable water in developing countries.1 For example, mercury used in artisanal and small-scale gold mining2 is released into tailing ponds, thereby polluting water used for irrigation of crops and drinking.3, 4 Intake of mercury through consumption of contaminated water causes kidney, nervous system, and brain damage.1 Non-regulated recycling of electronic waste results in contamination of groundwater with copper and nickel.5,6 Large doses of copper affect liver, heart and kidney, and lead to cognitive decline.7, 8 Effective, cost-efficient, and sustainable methods to recycle polluted water would enable its use for irrigation, landscaping, and industrial needs and would conserve freshwater resources. Hydrogels have promising applications in scavenging of heavy metals from polluted water, as they possess a large surface area, can be designed to have a high density of metal ioncoordinating groups, and can be recycled.9,

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Both supramolecular and covalently crosslinked

hydrogels have been used for heavy metal remediation. In supramolecular hydrogels,11 metal ions act as a gelator for organic molecules.12,13 The limitation of these hydrogels is their insufficient mechanical integrity and the inability of recycling. Chemically crosslinked hydrogels carrying carboxylic or amine groups have been used for heavy metal ion sequestration,14,

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however molecular hydrogels have small pore size, which affects the transport of water and efficient metal ion adsorption. In a different approach, graphene,16 noble metal nanoparticles (NPs),10 and semiconductor quantum dots17 have been utilized for uptake of heavy metal ions. Nanoparticle 2 ACS Paragon Plus Environment

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ion scavengers offer a large surface area and a high density of surface active sites, as well as ionsensing capability,18, 19 however, their utilization may be limited by high NP cost, the difficulty in their separation after ion uptake, and potential NP toxicity. Nanocolloidal hydrogels formed by NP crosslinking combine the advantages of “molecular” hydrogels and NP-based heavy metal scavengers, in addition to offering a large pore size for mass transport of water and ions, stabilization and immobilization of NPs in the hydrogel network and the capability of recycling. Furthermore, to increase the rate of ion uptake, nanocolloidal hydrogels can be prepared as micrometer-size particles (microgels). Additionally, hydrogel cost can be reduced by synthesizing them from sustainable natural resources. Cellulose nanocrystals (CNCs) are mechanically strong and biocompatible NPs derived from natural resources.20,21 They have been utilized to remove metal ions from water by ion coordination to the negatively charged half-ester sulfate groups on the CNC surface22-24 and have been used as building blocks of nanofibrillar colloidal hydrogels.25-27 Recently, another type of NPs, namely, graphene quantum dots (GQDs) have attracted significant attention due to their fluorescence,28 low cytotoxicity,29 dispersibility in water,30 facile surface functionalization,31 and biocompatibility. Due to the presence of carboxylic acid groups on the GQD edges, they can act as scavengers of metal ions,32 however because of the small GQD size, a challenge exists in their separation after ion uptake and thus their applications are mainly limited to ion sensing. The immobilization of GQDs in a hydrogel would facilitate their separation from water. Recently, the preparation of supramolecular CNC-GQD hydrogels have been reported,30 however their low mechanical integrity and dissociation in water precluded their use for ionscavenging purposes.


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Here we report chemically crosslinked hydrogels formed from CNCs and GQDs. In these hydrogels, both CNCs and GQDs provide large surface area and large number of active sites which makes them efficient for scavenging of metal ions from aqueous solutions. By immobilizing the nanocolloidal building blocks through chemical crosslinking, hydrogels with tunable pore size and structure, sufficiently high mechanical strength, and good permeability are formed. Nanocolloidal microgels based on these two building blocks were prepared by the microfluidic means to improve the transport of water through the hydrogel and facilitate their applications in scaled-up systems such as packed beds. These microgel scavengers showed high adsorption capacity for all metal ions, especially, Hg2 and Ag+ ions and were recyclable for multiple scavenging steps.

RESULTS AND DISCUSSION The dimensions of disk-like pristine GQDs and amino-functionalized GQDs (aGQDs) were characterized using TEM and AFM imaging (Figure 1a and b). Based on the analysis of the TEM and AFM images, the mean diameter and height of aGQDs were determined to be 5.7±2.4 and 0.47±0.1 nm, respectively (the latter corresponded to a single-layer of graphene). The crosssectional profiles of aGQDs is shown in Figure S1. For pristine GQDs, the corresponding dimensions were 5.6±1.8 nm (n=43, Standard Deviation (SD)) and 1.3±0.3 nm (n=56, SD) (Figure S2, Supporting information). Thus their height corresponded to 2-3 layers of single graphene sheets33 and the hydrothermal treatment of GQDs resulted in exfoliation of GQDs. Figure 1c shows the absorption and photoluminescence (PL) spectra of GQDs prior to and after the modification with amino groups (the concentration of amino groups on the GQD surface was found to be to be 3.49 µmol/g (Figure S3, Supporting Information). The absorption 4 ACS Paragon Plus Environment

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and emission peaks of pristine GQDs were centered at 350 and 470 nm, respectively. For the aGQDs, both peaks were blue-shifted suggesting that the decrease in the height of the GQDs caused a stronger quantum confinement.32 The aGQDs showed PL intensity ∼9-fold higher than pristine GQDs, due to the increase in the fraction of sp2-hybridized carbon domains after amination.32 The details of XPS and X-ray diffraction characterization of the GQDs and aGQDs are shown in Figures S4 and S5, Supporting Information. The dimensions of pristine CNCs and aldehyde-modified CNCs (aCNCs) were characterized by analyzing their TEM and AFM images (Figure 1d and e, respectively). Both types of CNCs had a characteristic rod-like shape. The mean length of pristine CNCs were 180±45 (SD, n=45) and the length of aCNCs were 168 ±39 nm (SD, n=54) respectively (Figure S6, Supporting Information). Thus no significant change in the CNC shape occurred after their modification with aldehyde groups.34 The crosslinking reaction between aCNCs and aGQDs was characterized by acquiring the ATR-FTIR spectra of the individual NPs and of the aCNC/aGQD hydrogel (Figure 1f). For the aGQDs, the peaks at 1590, 3000-3500 and 1350 cm-1 were assigned to the vibrational bend of N-H of the amide groups, the stretching bend of N-H group of primary amines, and the stretching bend of the aromatic C-N group, respectively.32,35,36 For the aCNCs, the peak at 1720 cm-1 corresponded to the aldehyde carbonyl stretch34 and the peak at 1058 cm-1 corresponded to the C-O stretch,34 indicating that aldehyde groups were introduced to the CNC surface. The ATRFTIR spectra of the aCNC/aGQD hydrogel exhibited a peak at 1680 cm-1, which corresponded to the imine (formed via the Schiff base reaction).37


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In the hydrogel, the characteristic carbonyl peak of the aCNC and the peaks of amide or amine groups of aGQDs overlapped, thus making it challenging to quantify the fraction of unreacted functional groups.

Figure 1. (a) TEM image of the aGQDs. (b) AFM image of aGQDs. (c) Absorbance spectra of 1 mg/mL solutions of GQDs (―) and aGQDs (―). Photoluminescence spectra of 1 mg/mL solutions of GQDs (---) and aGQDs (---) at λexc=350 nm. (d) TEM image of the aCNCs. (e) AFM image of aCNCs (f) FTIR spectra of aCNCs (―), aGQDCs (―) and the aCNCs/aGQDs gel (―).

Hydrogel preparation. Figure 2a illustrates the preparation of the chemically crosslinked aCNC/aGQD hydrogel due to the reaction between electrophilic carbon atoms of the aldehyde groups on the aCNC surface and nucleophilic amino groups on the surface of aGQDs, with the


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formation of imine. Later in the text, we refer to the concentration of aCNC, as to CaCNC, to the concentration of aGQD, as to CaGQD, and to the ratio of CaCNC-to-CaGQD as R. Hydrogel formation was examined qualitatively in an inversion test (Figure 2b and c) and by examining the rheological properties of the aCNC/aGQD mixture (Figure 2d). Figure 2b shows the aCNC/aGQD hydrogel under ambient light illumination (left) and ultraviolet illumination at λexc=365 nm (right), indicating fluorescence properties of the hydrogel. In general, the hydrogel color varied from light-yellow to dark-brown with increasing CaGQD, caused by GQD absorption in the visible spectral range.30, 38 Figure 2c shows the state diagram for the aCNC/aGQD mixture. The hydrogels were formed at CaCNC ≥10 mg/mL and CaGQD ≥10 mg/mL.


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Figure 2. (a) Schematic of the preparation of the chemically crosslinked aCNC/aGQD hydrogel due to the Schiff base formation. (b) Photographs of the hydrogel under ambient light illumination (left) and at λexc= 365 nm (right). CaGQD = 40 mg/mL, CaCNC = 40 mg/mL. (c) State diagram outlining the concentration regimes of aCNCs and aGQDs for the formation of a nanocolloidal hydrogel. The diagram was obtained using inversion test 6 h after mixing the aCNC and aGQD suspensions. (d) Gelation time determined by measuring the cross-over point between the storage modulus Gꞌ and the loss modulus Gꞌꞌ for Ctot=80 mg/mL at different values of R at room temperature. The error bars show the SD obtained in three independent experiments.


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Hydrogel formation from the aCNCs/aGQDs suspensions was further confirmed in oscillatory shear rheology experiments. (Figure S7, Supporting Information). The cross-over point of the storage modulus, Gꞌ and loss modulus, Gꞌꞌ measured as a function of time after mixing of the aCNCs and aGQDs suspensions was taken as the gelation time.39 At a total solid content, Ctot, of 80 mg/mL the average gelation time was 50 min, 1 min, and 3 min at R of 0.33, 1.0 and 3.0, respectively (Figure 3d), that is, an increase in CaCNC resulted in shorter gelation times.40 A significantly longer gelation time at R = 0.33 than at R of 1.0 and 3.0 was attributed to the difference in gelation mechanisms (see below). A small increase in gelation time at R=3.0, in comparison with that at R=1.0, was caused by the higher viscosity of the former system (Figure S8, Supporting Information), which slowed down the diffusion of the nanoparticles toward crosslinking sites.41 The Young’s modulus of the hydrogel measured in compression stress-strain experiments 24 h after gel formation at Ctot=80 mg/mL. varied from 11.25 to 38.08 kPa for R of 0.33 to 3.0, respectively (Figure S9, Supporting Information). We note that in addition to chemical crosslinking between aCNCs and aGQDs, upon their mixing gelation could initially occur due to hydrogen bonding between amino groups of the aGQDs and hydroxyl groups of aCNCs,30 ionic interactions of sulfate half-ester groups of aCNCs with protonated amines,26 and hydrophobic interactions between the basal plane of the aGQDs and the hydrophobic facets of aCNCs.30 Hydrogel properties. Figure 3 shows representative SEM images of the aCNC/aGQD hydrogels. With R increasing from 0.33 to 3.0 (that is, with an increasing aCNC content) the structure of the hydrogel underwent a significant change. At R=0.33, large pores were formed by thick lamellar-like walls, however at R=3.0 the hydrogel had a nanofibrillar structure with small 9 ACS Paragon Plus Environment

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pores (Figure 3a and c, respectively). At R=1.0, the hydrogel exhibited a transitional morphology from the lamellar to the nanofibrillar structure. A marked difference in the hydrogel structure with increasing R suggested different mechanisms of hydrogel formation and explained the variation in gelation time with varying R (Figure 2d). The larger pores and the lamellar structure of the walls in the hydrogel at R=0.33, that is, at a low aCNC content (or high aGQD content) facilitated aCNC crosslinking at multiple crosslinking points in a side-by-side manner, that is, by bridging them with aGQDs, thus yielding thick walls and large pores. At high R, that is, at a high aCNC content, the crosslinking of aGQDs and aCNCs occurred at fewer points on aCNC surface, leading to a nanofibrillar structure. Additional SEM images are shown in Figure S10, Supporting Information.

Figure 3. (a-c) SEM images of the aCNC/aGQD hydrogels with (a) R=0.33, CaCNC = 20 mg/mL, 10 ACS Paragon Plus Environment

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CaGQD = 60 mg/mL. (b) R=1.0, CaCNC = 40 mg/mL, CaGQD = 40 mg/mL, and (c) R=3.0, CaCNC = 60 mg/mL and C aGQD = 20 mg/mL. Scale bar in a-c is 10 µm. (d) Darcy permeability of the corresponding hydrogels. In (a-d) Ctot=80 mg/mL Error bars show the SD obtained in three independent experiments. Student’s t-test shows that for R=0.33 and R=1.0; and R=0.33 and R=0.3 statistically significant values Ks were obtained (P99.8%), ethylene glycol, rhodamine B isothiocyanate (RBITC), N,N-Dimethylformamide (DMF), sodium periodate, AgNO3, CuCl2, NiCl2, and HgCl2 were purchased from SigmaAldrich Canada. Ammonium hydroxide (29.4% assay NH3) was purchased from Caledon Laboratory Chemicals. Acetic acid was purchased from Fisher Scientific. Nitric acid was purchased from ACP Chemicals Inc. Polydimethylsiloxane (PDMS, Sylgard ® 184) was purchased from Dow Corning. Fluorinated oil (3-ethoxy-1,1,1,2,3,4,4,5,5,6,6,6-dodecafluoro-2(trifluoromethyl)-hexane, HFE-7500 3M Novec) was purchased from 3M. Later in the text it is referred to as F-oil. An aqueous 12 wt% suspension of CNCs was purchased from the University of Maine Process Development Center. Milli-Q deionised (DI) water with a resistance of 18.2 MΩ cm (Milli-Q, Merck KGaA, Germany) was used in all experiments. Preparation of GQDs. The GQDs were prepared using a procedure reported elsewhere (Supporting Information, S1).47 Briefly, 8.4 g of citric acid was mixed with 0.8 g L-cysteine and heated to 200 oC. Upon melting, within 5 min the reaction mixture turned into a light brown liquid. The mixture was then cooled down to room temperature and the product was dispersed in 18 ACS Paragon Plus Environment

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10 mL of water, followed by centrifugation (10000 g, 30 min, 22 oC). The supernatant collected after centrifugation was purified by dialysis against deionized water using a cellulose ester membrane (Spectra/Por Biotech Cellulose Ester (CE) Dialysis MWCO: 100-500 Da) for 24 h, and then freeze-dried to obtain the free GQDs powder. The resulting particles showed an average zeta potential of -33 mV at pH~7. Functionalization of GQDs with amino groups. To prepare GQDs functionalized with amino groups (aGQDs), 6 mL of the 60 mg/mL GQD suspension was mixed with an equal volume of ammonium hydroxide and heated to 200 oC in an autoclave for 12 h. The product was isolated in a rotary evaporator and redispersed in deionized water. In order to ensure the removal of unreacted ammonium hydroxide, this step was repeated 3 times. The dry product was dispersed in 10 mL of deionized water and dialyzed for 12 h against deionized water using a cellulose ester membrane (MWCO: 100-500 Da). The collected sample was dried in a vacuum oven at 50 oC overnight to yield a powder of aGQDs. The aGQDs were covalently linked with rhodamine-B isothiocyanate (RBITC) via the click reaction of the isothiocyanate group of RBITC with primary amino groups on the aGQD surface.55 A 0.175 mg/µL solution of RBITC in DMF (35 mg in 200 µL) was added to 2 mL of 0.4 wt% solution of aGQDs in 0.1 M, pH 9 NaHCO3 buffer. The mixture was stirred overnight in the dark at room temperature. The solution of aGQDs modified with RBITC was dialysed for 20 days using a cellulose ester membrane (Spectra/Por Biotech Cellulose Ester (CE) Dialysis MWCO: 100-500 Da) to ensure the removal of unreacted RBITC. The concentration of amino groups was determined by using a calibration graph (Figure S6, Supporting Information) and was found to be 3.49 µmol/g of the GQDs. The electrokinetic potential (ζ-potential) of aGQDs was

-6.7 mV at pH≈7.


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Surface functionalization of CNCs with aldehyde groups. The aldehyde-modified CNCs (aCNCs) were prepared using a protocol reported elsewhere (Figure S5, Supporting Information).34 Briefly, 43 mL of 7 wt% CNC suspension was diluted with 100 mL of deionized water and added to a 500 mL flask. 12 g NaIO4 was dissolved in 157 mL water and added to this flask to achieve the total CNC concentration of 1 wt%. The flask was covered with aluminum foil. The pH of the suspension was adjusted to 3.5 using a 15 wt% acetic acid solution and stirred for 3.5 h at 40 oC. Then, ethylene glycol (2 g) was added to the CNC suspension and the suspension was stirred at room temperature for 2 h to quench the oxidation reaction. The product was then filtered and dialyzed against deionized water for one week using cellulose membrane (Sigma, MWCO 14000 Da). The density of aldehyde groups on the aCNCs surface was determined by converting the aldehyde groups to carboxylic groups using 0.1 M NaOH and subsequently, titrating the hydroxide ions consumed with sulfuric acid. The concentration of aldehyde groups was found to be 5200 µmol/g of CNCs. Preparation of aCNC/aGQD Hydrogel In a typical experiment, a suspension of aCNCs with the concentration, CaCNC, from 10 to 60 mg/mL and aGQDs with the concentration, CaGQD, from 2.5 to 60 mg/mL was prepared by mixing aCNCs and aGQDs suspensions with different concentrations in different volumetric ratios to reach the total volume of 2 mL. Characterization of Nanoparticles Atomic Force Microscopy (AFM). The height of GQDs and the dimensions of CNCs were determined by AFM (Figure S1, S2 and S5, Supporting Information). 0.25 mL of 0.001 mg/mL 20 ACS Paragon Plus Environment

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suspension of GQDs and aGQDs and 0.05 mg/mL suspension of CNCs and aCNCs were spuncoated (Laurell Technologies Corporations, USA) on a freshly cleaved mica substrate (Grade V1 Mica, TedPella Inc, USA) at 2000-3000 rpm for 1 min. The AFM images were acquired using a Dimension Icon (Bruker Corporation). High-resolution probes from MikroMasch (Hi’ResC19/Cr-Au-5) with a nominal spring constant of 0.5 N/m, a resonance frequency of 65 kHz, and a tip radius of 1 nm were used. Imaging was carried out in an intermittent contact mode (ac) under soft tapping conditions within the repulsive interaction force regime. The AFM images were analyzed using the NanoScope Analysis software by Bruker and open source software Gwyddion. Images were leveled by third-order polynomial planefit and image flattening (excluding large features by selective masking), followed by applying a low-pass filter to suppress high spatial frequency components. Data analysis was performed by applying a linear cross-section to individual GQDs and aGQDs. Transmission Electron Microscopy (TEM). The dimensions of nanoparticles were also determined using TEM. A small droplet of 0.001 mg/mL suspension of GQDs or aGQDs was placed on a copper grid (Holey Carbon 400 mesh, Electron Microscopy Sciences) and allowed to dry. For CNCs and aCNCs, 3 droplets of 0.0001 mg/mL of the stock solution was deposited on the grid and allowed to dry for 30 min. The images were analyzed using an ImageJ software. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). Characterization of the modification of GQDs with amino groups was performed on the powder of GQDs or aGQDs using ATR-FTIR spectroscopy (Bruker Vertex 70 spectrometer with a 1.85 mm diameter diamond crystal). Characterization of the modification of CNCs with aldehyde groups was performed on a powder of CNCs or aCNCs using the same instrument.


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X-ray Photoelectron Spectroscopy (XPS). To characterize the composition of GQDs before and after modification with amino groups, suspensions of GQDs and aGQDs were casted on a glass slide and dried to form a homogenous film. Thermo Fisher Scientific Kα spectrometer with a monochromatic Al Kα X-ray radiation source generating X-ray photons of 1486.7 eV in energy was used in an ultrahigh-vacuum chamber with base pressure of 10−9 Torr for XPS data (Figure S3 and S16, Supporting Information). Both survey and regional spectra were acquired from samples and the data analysis was carried out using the Avantage software. X-ray Diffraction. X-ray Diffraction (XRD) spectra of GQDs and aGQDs were recorded on a Rigaku MiniFlex Benchtop X-ray diffractometer (equipped with Cu Kα X-ray tube) operating at 600 watts (tube voltage 40 kV and 15 mA) with a 0.03 degree per step and 0.7 deg/min. The samples were prepared by drying the GQD and aGQD dispersions in a vacuum oven. The spectra are shown in Figure S4, Supporting Information. Characterization of aCNC/aGQD hydrogels Permeability test. To determine Darcy permeability, hydrogel samples were prepared by mixing 50, 75, or 100 µL of 80 mg/mL aCNC suspension with 100, 75, or 50 µL of 80 mg/mL aGQD suspension. The mixed suspension was immediately transferred to a sealed chamber with dimensions of 13.7 mm× 3 mm× 3 mm (length x width x height), which was fabricated in polydimethylsiloxane (PDMS) and equilibrated overnight. The experimental setup for permeability measurements is depicted in Figure S11, Supporting Information. The pressure difference across the hydrogel was changed by varying the height of the inlet reservoir with water relative to that of the outlet reservoir. In a typical experiment, a pressure difference, ∆P, of 1842 Pa was applied across the hydrogel. By measuring the volumetric flow rate, QP, of the flow of water across the aCNC/aGQD hydrogel, the Darcy permeability constant, KS, was 22 ACS Paragon Plus Environment

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calculated as

!"# $ ∆&

, where ( is the viscosity of water at room temperature ((=1.002 cP56, L

is the length of the hydrogel (13.7 mm) and A is the cross-sectional area of the hydrogel, 9 mm2). To ensure that the flow of the liquid does not disrupt the hydrogel structure, the value of QP was measured for the range of ∆P from 862 to 2332 Pa. The values of KS and QP plotted as a function of ∆P followed a linear relationship over this pressure range, suggesting that the hydrogel structure was not changed by forced water flow (Figure S11, Supporting Information). Rheological tests. The mechanical properties of the aCNC/aGQD hydrogels were characterized using an AR-1000 TA Instruments cone and plate rheometer with a cone diameter of 40 mm. To control hydrogel temperature, an integrated Peltier plate was used. Solvent evaporation was suppressed by using a solvent trap. The hydrogels were equilibrated for 50 min at 22 °C. Prior to the measurements, strain and frequency sweep experiments were performed. Strain sweep experiments were conducted at a strain varying from 0.1 to 50% and a frequency of 1 Hz. Frequency sweep experiments were conducted at a frequency varying from 1 to 100 Hz and a strain of 1%. At 1% strain and frequency of 1 Hz, the hydrogels exhibited a linear viscoelastic response. These conditions were used in time sweep experiments to determine the storage modulus, G′, and the loss modulus, G′′, of the nanocolloidal hydrogels at 22 °C (Figure S7, Supporting Information). Mechanical Characterization. The compression Young’s modulus of the aCNC/aGQD hydrogels was determined using a Mach-1 Mechanical tester (Biomomentum Inc., QC, Canada). The hydrogel were prepared in disk shapes for mechanical testing. The disks were 4 mm in height and 10 mm in diameter. The hydrogels were equilibrated for 24 h. The compression test was performed at 20% strain in the z-direction at a rate of 0.1 mm/s. The Young’s modulus was determined from the linear portion of the stress−strain. Figure S9 (Supporting Information) 23 ACS Paragon Plus Environment

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shows the representative stress-strain curves and the variation in Young’s modulus as a function of composition of the hydrogel. Scanning Electron Microscopy. Scanning electron microscopy (SEM) imaging of aCNC/aGQD hydrogels was carried out on the Quanta FEI Scanning Electron Microscope (FEG 250). The hydrogels were frozen using liquid propane to suppress water crystallization.25 This method of sample preparation was used instead of the supercritical point drying, since methanol and ethanol used in the latter method make GQDs colloidally unstable and because a reverse Schiff base reaction is favored,30 thus covalent crosslinks between the aCNCs and aGQDs would break. After freezing the samples in liquid propane, the water was removed by freeze-drying for 24 h at -80˚C and 100 mTorr. The resulting aCNC/aGQD aerogels were coated with gold using a SC7640 High Resolution Sputter Coater (Quorum Technologies) for 30 s at 2.0 kV. Preparation of microgels. The 3D printed master with bas-relief features of the microfluidic (MF) flow-focusing droplet generator57 was designed and printed using Projet 3510 HD. The design of the MF device and the details of its fabrication are provided in Figure S12, Supporting Information. The device was fabricated by casting PDMS (SYLGARD® 184, Dow Corning, USA) onto the master and bonding it to a planar PDMS sheet. Droplets were generated from the mixed aCNCs/aGQD suspension in the fluorinated oil phase (F-oil) containing 1.0 wt% of triblock copolymer perfluoropolyether and

poly(ethylene oxide-co-propylene oxide)

surfactant.58 The droplets were collected in a glass vial and equilibrated for 1 day to ensure complete gelation. The resulting microgels were centrifuged with the consecutive exchange of Foil with F-oil containing 10 wt% perfluorooctanol, hexane with 0.5 wt% Span 80, and pure hexane. The microgels were dispersed in deionized water for ion-scavenging experiments.


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Metal ion absorption experiments. The absorption capacity of the microgels was evaluated by submerging them into solutions of 0.01 M of AgNO3, CuCl2, NiCl2, and HgCl2 for 24 h at room temperature and pH=7. The concentration of metal ions Hg2+, Cu2+, Ni2+ and Ag+ that were absorbed by the microgels or remained in the supernatant was quantified by using ICP-OES (Optima 7300 DV, PerkinElmer, USA). After the ion-scavenging experiment, the microgels were separated using centrifugation, removed from the solution, and dried at 40˚C. The dried microgels were then weighed and immersed in 1 mL of 50 wt% HNO3 overnight. After dilution of the resulting suspension, it was filtered using 0.2 µm syringe filter and the concentration of ions in the solution was determined using ICP-OES. Statistical Analysis. In the present work, all error bars represent standard deviation (SD) of the data. Each SD was obtained by examining at least, 3 samples, or conducting at least, 3 experiments.

ASSOCIATED CONTENT Supporting Information. Supporting Information includes the characterization of GQDs by X-ray photoelectron spectroscopy (XPS), TEM and AFM of GQDs and CNCs, X-ray diffraction (XRD) pattern of the GQDs and aGQDs, Fourier transform infrared spectroscopy (FTIR) spectrum of CNCs and aCNCs, time-dependent variation of the storage and loss moduli of aCNC/aGQD hydrogels, Darcy permeability test setup, details of MF device design and fabrication and histogram of microgel size distribution. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Eugenia Kumacheva, Email: [email protected] ACKNOWLEDGEMENT The authors are grateful for financial support of this work by NSERC Canada (Strategic and Discovery grants). E.K. thanks Canada Research Chair program. M.A. thanks NSERC Vanier Canada Graduate Scholarship. M.T. thanks the Alexander von Humboldt-Foundation for a Feodor Lynen Research Fellowship. The authors thank Ilya Gourevich (Centre for Nanostructure Imaging) for assistance in electron microscopy imaging and Dario Bogojevic (St. Michael’s Hospital), for 3D printing of the microfluidic mold. M.A. thanks Dr. Mahshid Chekini for fruitful discussions. REFERENCES 1.

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Table of Contents Figure


Nanocolloidal Hydrogel for Heavy Metal Scavenging - ACS Nano

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