Cucurbituril-Based Reusable Nanocomposites for Efficient Molecular

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Cucurbituril-based Reusable Nanocomposites for Efficient Molecular Encapsulation Huifang Liu, Yange Luan, Bonhan Koo, Eun Yeong Lee, Jinmyoung Joo, Thuy Nguyen Thi Dao, Fei Zhao, Linlin Zhong, KyuSik Yun, and Yong Shin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06506 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Sustainable Chemistry & Engineering

Cucurbituril-based Reusable Nanocomposites for Efficient Molecular Encapsulation

Huifang Liu1, Yange Luan1, Bonhan Koo1, Eun Yeong Lee1, Jinmyoung Joo1, Thuy Nguyen Thi Dao1, Fei Zhao1, Linlin Zhong2, Kyusik Yun2, Yong Shin1,*

1Department

of Convergence Medicine, Asan Medical Institute of Convergence Science and

Technology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Republic of Korea 2Department *To

whom

of Bionanotechnology, Gachon University, Gyeonggi-do 13120, Republic of Korea correspondence

should

be

addressed.

Tel:

+82-2-3010-4193;

Email:

[email protected]

Abstract Cucurbituril (CB) has recently been employed in many fields, including water purification, solar cells, energy conversion, and biomedical engineering. However, the poor solubility of CB poses a serious obstacle to the further development of CB applications. To enhance the solubility of members of the CB family (CB[5-8]) by preventing self-aggregation in aqueous solutions, the synthesis of highly stable, rapid, and water-dispersible particles is presented in this paper based on a simple process that employs a nanocomposite composed of CB and amine-modified diatomaceous earth (DA). CB can be coated onto the surface of the DA and stabilized to produce a novel material that is useful for various applications. The nanocomposite (CB-DA) exhibited strong host-guest interaction, exhibiting a more than 100-fold increase in efficiency and greater stability in dye and pathogen encapsulation as a result of host-guest interaction, electrostatic interaction, and covalent bonding. We applied CB-DA to a commercialized filter system and were able to purify the water within 2 min. We believe that CB-DA will open a new avenue for the efficient utilization of super-molecular materials in aqueous molecular encapsulation applications.

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Conflict of Interest The authors declare no conflict of interest.

Key Words Super-molecule,

Bio-silica,

Nanocomposite,

Rapid

absorbance,

encapsulation


Recycle,

Molecular

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Introduction Despite recent advances in water treatment technology, many developing countries still lack effective water purification techniques. Unpurified water can lead to waterborne diseases such as cholera, enteric fever, and hepatitis A due to microorganism contamination and the lack of chlorination1-2. Although purified water provides public health and economic benefits for developing countries, the high costs associated with water purification and the use of water treatment systems in remote areas are limitations that need to be overcome3. Motivated by the widespread development of nanotechnology, various nanomaterials and novel technologies have recently been employed in a diverse range of fields, including water purification, solar cells, energy conversion, and biomedical engineering4-7. In particular, the pumpkin-shaped cucurbituril (CB) family, which plays an important role in host-guest chemistry, has been employed in various applications involving molecular encapsulation, water treatment, surface adhesion, biomarker-targeted theranostics, and drug delivery8-15. CB consists of n glycoluril motifs (n=5-8, 10, 14) linked by methylene bridges to form two hydrophilic carbonylated portals and a hydrophobic cavity (Table S1)16-18. Driven by a diverse range of inter- and intramolecular interactions, CB has been identified as an ideal host for charged amphiphilic guests due to ion-dipole stabilization and possible hydrogen bonding inside the CB cavity14, 19. Although the great potential of CB has been highlighted in many studies, three main performance limitations have also been identified for the CB family: (1) unexplained poor solubility in aqueous solutions11, (2) unsuccessful functional group modification20, and (3) unclarified ion effects in CB applications21. In particular, the poor solubility of CB poses a serious obstacle for the development of CB applications. Thus, numerous research efforts have attempted to overcome these limitations by developing new water-soluble host/guest systems with a synergistic blend of supramolecular assemblies and nanomaterials15, 22-23. In the present study, CB was coated on diatomaceous earth (DE) to maximize the efficiency of CB in the removal of polluting dyes and pathogens from aqueous solutions. DE is a promising biosilica for use in synthetic nanomaterials due to its low environmental impact, high



biocompatibility, and significant advantages in terms of scalability, structural reproducibility, and low production costs. As such, it has been successfully employed in various fields, such as sensing, optoelectronics, energy conversion, and storage24-25. We modified the surface of DE with amine groups and then coated CB onto the modified DE (DA), producing a nanocomposite referred to here as CB-DA. We investigated the possible mechanisms behind the role of CB in improving the stability of DE by preventing self-aggregation. We also applied the CB-DA composite in the treatment of both tap water and water samples from the Han River in Seoul, Korea to test its practical utility. We found that the CB-DA nanocomposite had an efficiency that was 100 times greater than that of CB alone and it exhibited high stability in the encapsulation and removal of dye molecules and pathogens. When we applied the CB-DA nanocomposite to a filtration system, water was able to be purified within 2 min. Therefore, a CB-DA platform can be employed for molecular encapsulation in water treatment and in a broad range of biological systems that require rapid results and high stability.

Experimental Section Materials All reagents were of analytical grade and used without further purification. We ordered biocompatible diatomaceous earth DE powder, suitable for most filtration applications, from Sigma-Aldrich (St. Louis, MO, USA). (3-aminopropyl) triethoxysilane (APTES, 98%), cucurbit[5]uril hydrate (C30H30N20O10, 545198-100MG), cucurbit[6]uril hydrate (C36H36N24O12, 94544-1G-F), cucurbit[7]uril hydrate (C42H42N28O14, 545201-100MG) and cucurbit[8]uril hydrate (C48H48N32O16, 545228-100MG) were also obtained from Sigma-Aldrich. Trypan blue (TB; 0.4%) was ordered from Gibco, a division of Life Technologies Corporation (Grand Island NY 14072 USA). Methylene blue (C6H18ClN3S·3H2O) was obtained from Daejung Chemicals and Metal Co., Ltd (Gyeonggi-do, Korea) and rhodamine B (C28H31ClN2O3) was purchased as R6626-25G 95% (HPLC) from Sigma-Aldrich. Magnesium chloride (M4880-100G, 97%), calcium chloride (383147-100G, 96%), and sodium chloride (S3014-500G, 98%) were supplied


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by Sigma-Aldrich. M111-100 mL of sodium dodecyl sulfate (SDS; 10%) manufactured by VWR International LLC, Life Science was used. The other two silica materials used were silica gel (pore volume 0.43 cm2/g, 28-200 mesh) produced in Germany by Sigma-Aldrich and silica sand (Cat. No. 37549-01, Tokyo Japan). Milli-Q water with a resistance greater than 18 MΩ, 99% ethyl alcohol, phosphate-buffered saline (PBS, 10×, pH 7.4), and streptavidin-coupled magnetic beads (Thermo Fisher Scientific, Waltham, MA, USA) were used in all experiments.

Instruments The morphology of the samples was examined using scanning electron microscopy (SEM; JEOL JSM-7500F, Tokyo, Japan). Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700, Thermo Scientific) was used for the analysis of the chemical properties of the samples. Raman measurements were performed using a Renishaw inVia Raman microscope system (Renishaw, Wotton-under-Edge, UK). The spin-down device used for centrifugation (CF-5, 100−240 Vas, 50/60 Hz, 8 W), the vortex mixer (T5AL, 60 Hz, 30 W, 250 V), the LABOGENE 1730R (220 V, 60 Hz, 2.0 kVA), and the MSH-30d stirring heater were produced by Daihan Scientific Co., Ltd. (Wonju-Si, South Korea). Dynamic light scattering (DLS, DynaPro NanoStar, Wyatt) was used for surface charge and average dimension measurements. The UV (Alpha-1106) and Libra 22 UV/Visible spectrophotometers manufactured by Biochrom Ltd. conformed to the requirements of dye removal. The Brunauer-Emmett-Teller (BET) surface area was measured for DE using an accelerated surface area and porosimetry system (ASAP 2020 Micromeritics [USA] V3.04 H).

Material Functionalization Pure DE and functionalized DE (3-aminopropyl triethoxysilane modification (DA), DA and cucurbit[X]uril modification, CB[X]-DE and CB[X]-DA) were prepared for this study. Gravity-powered washing was used to wash out any fragments in the commercial DE and the uniformity of the DE was confirmed using DLS based on particle size patterns. This included three steps. First, 3 g of DE was dissolved in 150 mL of deionized water (DW) in a 250-mL



flask, followed by stirring at 500 rpm for 10 mins. After standing for 1 min, the precipitate was removed and the supernatant collected in a 50-mL tube. This was then centrifuged at 1400 rpm, after which the supernatant was removed and the precipitate collected. Secondly, 99% EtOH was used instead of DW and the above process was repeated to dissolve contaminating ions. Thirdly, following the stirring and washing of DE (500 rpm) with 100 mL DW, the supernatant was removed and the precipitate collected after 30 s of standing. The sizes of the particles were measured using DLS, until a relatively uniform pure DE solution was obtained. The DE was then dried in a drying oven and stored in a reagent bottle. APTES-functionalized diatomaceous earth (DA) was prepared following the process described in our previous study26. In short, 2 mL of APTES was added drop-wise into 100 mL of 95% ethanol solution while stirring at 400 rpm for 3 min at room temperature (RT). After this, 500 mg of DE was dispersed into this solution under stirring (600 rpm) for 4 h. The precipitate was washed twice with ethanol to remove any free silanol. The DA was collected via centrifugation and subsequently dried in a vacuum overnight at RT and stored in a reagent bottle. Finally, cucurbit[X]uril modified DE and DA were prepared. Due to the different solubilities of the four members of the CB family (CB[X]; X = 5, 6, 7, or 8), we used the same molar concentration for CB[X]. The DE or DA precipitate was dissolved in DW (100 mL), then a certain amount of CB[X] was added under stirring at 500 rpm for 40 mins. After several cycles of washing with DW, the precipitate was collected, dried in a drying oven, and stored in a reagent bottle.

Test Material Characterization Chemical characterization For the characterization of DE and modified DA, CB[X]-DE, and CB[X]-DA, SEM was used to assess the morphology of the materials. In addition, the elements present in the composite materials were analyzed using EDX, while FTIR measurements were used to determine the chemical bonding of the composite materials. Finally, the vibrational, rotational,


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and other low-frequency modes of the composite materials were observed using Raman spectroscopy. The specific surface area of the materials was quantified using BET. DE (1 g) was dissolved and placed in a detection chamber at 250 ℃ for 10 h. The total surface area (Stotal) and the specific surface area (SBET) are given by Stotal = (νm *N*s)／V; SBET = Stotal /a,

(1)

where νm is the monolayer volume of the adsorbate gas, N is Avogadro's number, s is the adsorption cross-section of the adsorbing species, V is the molar volume of the adsorbate gas, and a is the mass of the solid sample or adsorbent. The size of the materials was measured using DLS.

Dye removal (adsorption) A standard stock solution of TB dye was prepared via dilution with deionized water to a final concentration of 40 mg/L. The dye solution exhibited maximum absorbance at a wavelength of 598 nm. The concentrations were calculated using the Beer-Lambert equation: Absorbance = ε* Cs * l,

(2)

where ε is the molar absorptivity, Cs is the concentration of the sample, and l is the thickness of the absorbing medium (1 cm). The amount of absorbed dye was calculated from the difference between the initial and final concentrations at 20-min intervals. Adsorption levels were assessed using duplicated mini-quantity experiments: 40-100 μL (50 mg/mL) of DE materials (D, DA, CB[X]-DE, CB[X]-DA) were tested with 1 mL (40 mg/L) of TB. After the batch adsorption process, the absorbance of TB in the residual solution was estimated using a UV/visible spectrophotometer. Adsorption of the dye onto the materials was determined in terms of distribution coefficients (Kd) and percentage adsorption (%). The percentage adsorption and Kd were estimated using the equations Dye removal (%) = (Ci – Cr) / Ci * 100,

(3)

where Ci and Cr are the concentrations of the dye in the initial and real-time solutions, respectively21, 27, and



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Kd = (Adsorbent dye / residual dye) * (V / m) [mL/g],

(4)

where V is the volume of the dye solution (mL), and m is the weight of the adsorbent. The efficiency of CB is limited by its poor solubility. In particular, the poor solubility results in the coagulation shown in Fig. S4A. Solubility was estimated using the homogenous surface contact in SEM (a1: CB[6], 10-5 mol and a2: CB[6], 10-5 mol with TB dye). Therefore, a dose concentration of 2 x 10-4 mmol/L for CB[6] was selected for further testing. The fit of the results of the experimental data to the pseudo-first order, pseudo-second order, and intraparticle diffusion models for natural and modified DE was investigated. The qm and Kx rate constants from the slope and intercept were calculated. The Langmuir equation in linear form is expressed as C∞ / q∞ = C∞ / qm + 1 / (qm Kx),

(5)

where qm represents the maximum adsorption capacity of the nanocomposite in mg/g, and Kx is the Langmuir constant related to the affinity of the binding sites in L/mg. The plot of C∞ vs q∞ and the linear plot of C∞ / q∞ vs C∞ provide the values of qm and Kx from the slope and the intercept of the curve, respectively28.

Pathogen capture testing A standard stock solution of E. coli was prepared via dilution with deionized water to a final maximum absorbance of 0.99 (Abs) at a wavelength of 600 nm. Following this, 100 μL and 50 mg/mL of the nanomaterials (D, DA, D-CB[X] or DA-CB[n]) were added respectively to a cuvette (4 mL). After shaking for 2 min and then holding the cuvette still for 50 min, the absorbance of TB in the residual solution was estimated using a UV/Visible spectrophotometer. The baseline was recorded using water in a reference cuvette with glass slides treated with only n-hexyltriethoxysilane. No cuvette was used for glass slides measured in the air in the absence of a solvent. All measurements were performed at an ambient temperature.


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Results and Discussion CB-coated amine-modified DE (CB-DA) for dye and pathogen removal Fig. 1 presents an illustrative diagram of the cucurbituril-coated diatomaceous earth (CB-DA) nanocomposite for use in dye and pathogen removal that improves the solubility of CB in water. CB-DA was produced by coating CB onto DE which had undergone amine group modification of its surface. Amine groups surround the inner and outer surfaces of the DE skeleton after APTES treatment, enhancing its chemical stability, allowing it to be used for extended periods, and leading to a robust coating of saline due to covalent bond formation26. When the lack of electron rich sources of carbonyl groups of CB portals meets the positive amine-group, the intermediate form with the negative charge can be neutralized to form a complete structure13, 29. In addition, the C=O bonds on the edge of the CB molecule possibly react with the NH2, suggesting that the amine group of the APTES-modified diatom (DA) might be an anchoring site for CB30-31. In addition, the C-N bond (1089.9 cm−1) in Figure S6A indicates that the oxygen atoms, which are located along the edges of the CB molecule, also bond with the amine group from the DA (Figure 1D). The ion-dipole interaction between the hydrogen bonds and the alkyl chain might also influence the anchoring of CB to the DA32-33. Due to the increased surface area of DE after amine-group modification and the well-dispersed CB on the DA, the absorbency efficiency of the CB-DA conjugate in its interaction with other molecules is enhanced by covalent bonding, physical adsorption, electrostatic interaction, and heterogeneous surface binding.



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Figure 1. Illustrative diagram of CB-coated amine-modified DE (CB-DA) for dye and pathogen removal. CB-DA can capture dyes and pathogens via physical adsorption, electrostatic interaction, and heterogeneous surface binding.

The morphology of the materials To synthesize hydrophilic CB-DA with high absorbance, DE was first fragmented using gravity-powered washing34. The size distribution profile of the DE was then determined to be about 5 to 25 μm using SEM and DLS (Fig. S1). The specific surface area (1.4901 m2/g) of the DE was estimated using the simple and rapid Brunauer-Emmett-Teller (BET) method35. Subsequent

calculations

of

the

pore

volume

of

the

DE

were

conducted

using

Barret-Joyner-Harret (BJH adsorption/desorption) analysis (Fig. 2). The absorption/desorption curves reveal a slender hysteresis loop, which is characteristic of the high specific surface area of DE (Fig. 2A-B). We confirmed that the thickness of the DE was around 20 nm (Fig. 2C), and the diameter of its pores was less than 100 nm (Fig. 2D).


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Figure 2. Characterization of the surface of DE. Brunauer-Emmett-Teller (BET) analysis of the specific surface area (1.4901 m2/g) of DE. (A) Obtained absorption/desorption curves. (B) BET surface area plot. (C) t-plot of the quantity adsorbed (Harkins and Jura method). (D) Pore volume as a result of BJH desorption.

The zeta potential of the materials After the fragmentation of the DE, CB was then coated on the DA using simple stirring for 20 min. To assess the electrostatic properties of CB-DA in solution, the zeta potentials of the test materials were measured (Fig. 3A). DE alone, CB alone, and a CB-DE conjugation were confirmed to be poorly soluble with weak zeta potentials. On the other hand, DA exhibited good solubility due to the amine groups on its surface (Fig. S1C)36. Notably, the CB-DA conjugation exhibited a higher zeta potential than did the DA (Fig. 3A). These results indicate that the CB coated the surface of the DA well, with a subsequent increase in potential charge. Previous



research has reported that materials with a weak zeta potential in solution tend to easily coagulate, while those with a high zeta potential are electrically stable36-39. Hence, CB-DA is easily dispersed in solution due to its high zeta potential, and it can be distinguished from DA only by human eyes. Although we could not investigate why the zeta potential of CB-DA composites was increased, we confirmed that the CB-DA composites were stable and well dispersed in solution compared to other materials.

Dye removal testing To test the absorbance of CB-DA in the removal of TB dye from solution, the adsorption capability of the test materials was compared. The dye removal efficiency of CB-DA was 100 times higher than those of the other materials (Fig. 3B) at 0.115 Abs/mg. In Fig. S2, the absorbance curves indicate that CB-DA exhibited good aggregation with the dye within 4 h, but CB on its own in the dye solution produced a different characteristic peak due to self-aggregation or aggregation with the dye. Amplified SEM images of the testing materials before and after the dye removal test are presented in Fig. S3; a clear change in the surface can be observed. To optimize the synthesis of CB-DA, the optimal concentrations of CB and DA were determined. The curve in Fig. 3C indicates that the efficiency of CB on its own was limited due to its poor solubility, resulting in the coagulation presented in Fig. S4A. This low efficiency was confirmed by homogenous surface contact analysis using SEM (a1: CB, 10-5 mol and a2: CB, 10-5 mol with TB dye). Therefore, a CB dose concentration of 2 x 10-4 mmol/L was selected for further testing. Because solubility varies considerably between the members of the CB family, we examined CB[5-8] with DA using the optimized synthesis protocol. CB[8]-DA demonstrated the highest dye removal rate compared to the other CBs, with approximately 92% within 4 h (Fig. 3D).


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Figure 3. Cucurbituril (CB) coating of amine-modified diatomaceous earth (DE) in an aqueous medium. (A) Zeta potential of the prepared materials: pure DE (DE), pure cucurbituril (CB), amine-modified DE (DA), the complex of pure DE with CB (CB-DE), and CB coated on amine-modified DE (CB-DA). The error bars indicate the standard deviation from the mean based on at least three independent replications. (B) The adsorption capacity of the materials for dye removal. The error bars indicate the standard deviation from the mean based on at least three independent replications. (C) The optimum concentration ratio of DA to CB. (D) The absorbance of supernatant after treatment with different CBs (CB[5], CB[6], CB[7] and CB[8]) with DA for dye removal.

Dye removal mechanisms Heterogeneous surface binding analysis The chemical mechanisms and complex stability of host-guest CB-based materials are still under investigation25, 35-36. Chemical-optical spectrum analysis was performed to determine why



CB-DA improved TB dye removal in solution. Following Fourier transform infrared (FTIR) spectrum analysis of pure DE (Fig. 4A, black curve), the absorption peak at 1450 cm-1 can be ascribed to the asymmetric stretching vibrations of Si-O-Si, and the peak at 1410 cm-1 is a result of the Si-CH2 bond. In addition, the absorption peaks at 3295 and 1180 cm-1 can be attributed to Si-OH and C-N, respectively, on the surface of the pure DE. On the other hand, FTIR analysis of the pure CB found peaks at 1705 cm-1, which can be ascribed to C-H stretching vibrations and those at 1152 and 1680 cm-1 to C-N stretching vibrations due to the amine groups directly bonded to the DE (Fig. S4B). After the surface modification of DA and CB, the well-defined absorption bands at 1100 cm−1, 2250 cm−1, and 2720 cm−1 represent C-C-C bonding, C-H bonding, and O-H bonding, respectively. The stretching vibrations at 2850-3000 cm−1 (CH, CH2, and CH3) and aldehyde (C-H) at 2720 cm−1 in the CB-DA group (blue curve) verified the presence of a super-molecule, and the bending vibrations at 1680 cm−1 (C-N), 1450 cm−1 (S=O), and 1180 cm−1 (S=O) in CB-DA with the TB dye group (green curve) indicate bonding with the TB dye (Fig. 4A). Furthermore, the comparison of the optical spectra for pure DE, pure CB, DA, and CB-DA presented in Fig. 4B and Fig. S4C using surface-enhanced Raman scattering (SERS)40-42 found that the SERS intensity for all molecular vibration frequencies decreased when the coverage of silica increased. In the CB-DA group, the intensity decreased due to the covered super-molecule (CB) with vibrational mode assignments. The silanol group of the DE is very active and can react with many polar organic compounds and various functional groups35-36. Hence, after the TB dye is used as a hosting molecule in the CB-DA group, the TB dye reacts with the silanol groups spread over the surface of the DE. The missed silica vibration shift indicates that the molecule was subject to intense field enhancement on the dye due to the CB junctions.

Electrostatic interaction analysis It was confirmed that electrostatic interaction was the mechanism behind the removal of the TB dye by CB-DA (Figs. S5A-B). Dissociation assays were employed using DW, ethyl alcohol


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(EtOH), and sodium dodecyl sulfonate (SDS). The amphiphilic SDS dissociated all of the dye molecules from the CB-DA-TB dye complex, while the DW and EtOH did not. Next, we used SDS solution to test CB-DA binding in the presence of high-concentration salts. FTIR analysis found that there was no change in the surface spectra of the CB-DA before and after SDS treatment (Fig. S6A). The stretching vibrations at 2850-3000 cm−1 (CH, CH2, and CH3), the aldehyde (C-H) at 2918 cm−1, and the bending vibrations at 1089 cm−1 (C-N) in the CB-DA group (black and red curves) were observed both in the presence and absence of SDS treatment (Fig. S6A). In addition, the CB-DA spectra after the SDS treatment were completely different from that of the DA or DE alone. Thus, the CB-DA conjugation will be able to capture dye molecules as normal in the presence of a high concentration of salt. We also examined the function of the CB-DA composite in TB dye removal at different pH levels ranging from pH 3.0 to 9.0. The CB-DA composite was stable for all pH levels (Fig. S6b). As a result, the CB-DA composite was proven suitable for practical sewage treatment, with the reported pH levels of real wastewater being approximately 4.2-9.6. 43 Using SDS, we tested the reusability of CB-DA in dye removal. When we conducted reproducibility testing for CB-DA using SDS solution, the CB-DA conjugate still formed after SDS treatment. Although the efficiency of the dye removal was lower due to the loss of the conjugate during the washing step, the efficiency of the CB-DA conjugation in dye removal was higher than that for DE alone (Fig. S5B). We thus confirmed that the proposed CB-DA composite is a stable approach to the active encapsulation of dyes and capture of pathogens. Though the loss of some CB-DA is inevitable, the kinetic parameter plots in Fig. S5B indicate that CB-DA has suitable reusability, which is a significant advantage for large-scale industrial applications. The resoluble nature CB-DA in SDS resoluble also confirmed the electrostatic interaction between the CB-DA and dye. We examined which individual elements were associated with the proposed mechanisms at play in dye removal by CB-DA, including inorganic matter (principally magnesium) and other minor constituents such as Na+(NaCl), Ca2+(CaCl2) and Mg2+(MgCl2) in 10 μL (1 mmol/L) of



solution. The ion time-absorbance plots are presented in Fig. 4C, revealing that competing ion complexation disrupts dye removal. It was found that the sorption of divalent cations (Ca2+, Mg2+) was more effective than was that of monovalent cations (Na+). We observed that the free ions enhanced the efficiency of CB-DA in terms of dye removal (Fig. 4C). To determine whether DE was unique in its ability to enhance the solubility of the CB nanocomposite, two other silica materials (silica sand and silica gel) were modified using the same method as for CB. Fig. 4D indicates that CB-DA had a stronger effect than the other silica materials on TB dye removal. These results confirmed that high electrostatic interaction is an important factor in CB-DA dye removal and the natural diatom material supplies numerous ion elements that enhance its molecule-capturing properties (Fig. S7).

Dye and pathogen removal testing To test the CB-DA conjugation with various dyes and pathogens, we used a mixed solution containing two dyes, rhodamine B and methylene blue. The absorbance curves for the amount absorbed per unit weight of CB-DA (Abs/mg) for the dye mixture are presented in Fig. 5A. The CB-DA was effective at capturing the two types of dye molecule. Additionally, the CB-DA exhibited greater pathogen capturing ability compared to DE and DA only (Fig. 5B). In particular, CB[8]-DA had the highest and most stable pathogen capturing ability. This similarity to the dye removal results indicates that the bulk capturing ability of CB-DA is related to the portals and cavity size of the CB molecules.


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Figure 4. Characterization of CB-DA. (A) Fourier transform infrared (FTIR) spectrum analysis of the materials with dye. Pure DE (DE, black line), amine-modified DE (DA, red line), cucurbituril coated on amine-modified DE (CB-DA, blue line) and CB-DA with trypan blue dye (CB-DA-TB, green line). (B) Surface-enhanced Raman scattering (SERS) spectrum analysis of the materials with dye. (C) Ion time-absorbance plots to test the effect of ions on TB dye absorption. The error bars indicate the standard deviation from the mean based on at least three independent replications. (D) The absorbance of other silica materials with CB. Silica sand (SS), CB coated on SS (CB-SS), silica gel (SG), CB coated on SG (CB-SG), pure DE (D) and CB-DA. The error bars indicate the standard deviation from the mean based on at least three independent replications.

Real water sample testing Water samples from the Han River were used for practical application testing. According to an assessment of Han River water quality in 2016, the mean pH of the Han River is 7.7 (Min: 5.9, Max: 8.9), and the total coliform load is about 18,200 CFU/100 mL41. The FTIR results



presented in Figure S8A illustrate that the CB-DA composites are stable in the Han River water, which contains complex chemical impurities. In order to determine whether these impurities, especially ions, affect the dye absorbance and pathogen capture ability of the CB-DA, we examined Han River water mixed with TB and E.coli, respectively. The absorbance of TB dye and the removal of E.coli in the Han River water treated with the CB-DA composites is presented in Figs. S8B-C. The results indicate that the complex impurities in Han River water do not reduce the absorbance capability of the CB-DA composites, thus confirming that they are suitably stable for water treatment. Meanwhile, we found that the lifetime of CB-DA composites is about 30 days for stable dye absorbance capacity in the real water sample (Fig. S8D).

Time-saving loading design As described above, CB-DA can be utilized for dye and pathogen removal within 4 h in aqueous applications. However, reducing the reaction time is an important factor in real-world water purification. Therefore, we applied 2 mg of CB-DA composites to a commercialized filter (1-μm pore size) system in order to improve the purification time (Fig. 5C-D). We added CB-DA to the surface of the filter with a pipette. The dye solution was then added to the filter. Separation clearly occurred within 2 min (Fig. 5D). The absorbance rate and sedimentation time curves in Fig. 5D confirm that CB-DA removed the dye rapidly and efficiently. The filter-based CB-DA method thus exhibited more efficient dye and pathogen removal in solution compared to other studies (Table 1)44-46.


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Figure 5. Applications of CB-DA. (A) Absorbance curves of the amount absorbed per unit weight of CB-DA (Abs/mg) for a mixture of rhodamine B and methylene blue dye. (B) The absorbance of supernatant after treatment with different CBs (CB[5], CB[6], CB[7] and CB[8]) with DA for pathogen removal. The error bars indicate the standard deviation from the mean based on at least three independent replications. (C-D) CB-DA applied to a filter system. This platform can remove dyes and pathogens within 2 minutes. (D) Comparison of the CB-DA filter after 2 min and natural sedimentation after 4 h. The error bars indicate the standard deviation from the mean based on at least three independent replications.



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Table 1. Comparison with other researches: the KL , the absorbance constant, pH effect and reaction condition for the materials. KL, [b]

Abs(%) [c]

pH effect

RC[d]

125 to 106

19~50%

--

48 h

PIC PIN

6.1 to 3.6 2.7 to 3.0

39~80%

CB[7]

RhB

0.94 to 0.18

74~81%

CB[8] CB[6]

RB19

0.99 to 0.61

--

TB MB, RhB

0.99 to 0.07

Pathogen

--

Material[a]

Tested

DE

MB, RY

CB[6] CB[7]

CB-DA

RB,

at 80 °C

Sensitive

at 88 °C

7h

~92% （RT）

Stable

2 min at RT[e]

~97%

[a] Materials reference 9, 44-47 and our study CB-DA. [b] Langmuir constant (L/mg). [c] Absorbance constant of tested materials. [d] Reaction Condition. [e] Room Temperature. (MB: Methylene Blue, RB: Reactive Black, RY: Reactive Yellow, RhB: Rhodamine B, PIC: Pseudoisocyanine, PIN: Pinacyanol, TB: Trypan Blue)

Conclusion In summary, we developed a novel nanocomposite of highly stable, water-dispersible cucurbituril (CB) with amine-modified diatomaceous earth (DA) for use in molecular encapsulation in water purification. The CB coating the diatomaceous earth (DE) prevents the self-aggregation of CB due to its insolubility and enhances the ability of CB to capture molecules in solution. The key to the success of this approach is the presence of well-coated CB on the surface of the DA (CB-DA). This proposed approach leads to strong molecule host-guest interactions and an efficiency that is 100 times higher in terms of TB dye and pathogen removal,


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with excellent stability, recycling, and reusability. These properties of CB-DA, when applied to a filtration system, offer new avenues of development for water purification in terms of the rapid removal of unwanted molecules. Altogether, these useful properties suggest that the use of CB-DA-based filtration systems could be a promising platform for combining various CBs with DA of a controlled size. This proposed system could prove useful for many applications in aqueous solutions.

Supporting Information. Brief statement in nonsentence format listing the contents of the material supplied as Supporting Information.

Acknowledgements This study was supported by a grant from the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (HI16C-0272-010016), and also supported by the Ministry of Science, ICT, and Future Planning (MSIP) through the National Research Foundation of Korea (NRF) (2017R1A2B4005288), Republic of Korea.

Author information Affiliations Department of Convergence Medicine, Asan Medical Center, University of Ulsan College of Medicine and Biomedical Engineering Research Center, Asan Institute of Life Sciences, Asan Medical Center, 88 Olympicro-43gil, Songpa-gu, Seoul, Republic of Korea Yong Shin, Huifang Liu, Yange Luan, Bonhan Koo, Eun Yeong Lee, Jinmyoung Joo, Thuy Nguyen Thi Dao, Fei Zhao Department of Bionanotechnology, Gachon University, Gyeonggi-do 13120, Republic of Korea Kyusik Yun, Linlin Zhong



Contributions Y. Shin and H. Liu conceived the research. Y. Shin supervised the whole project. H. Liu designed the experiments. Y. Shin, H. Liu, Y. Luan, K. Yun and L. Zhong performed the analysis and made interpretations of data. B. Koo, and E. Lee provided chemicals and supported data analysis. J. Joo, T. Dao and F. Zhao made comments, suggested appropriate modifications. Y. Shin and H. Liu wrote and edited the manuscript. All authors read and approved the final manuscript.

Competing interests The authors declare no competing interests.

Corresponding author Correspondence to Yong Shin

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Abstract graphic

Synopsis: The ternary super-molecular nanocomposite (CB-DA) loaded-filter could be reused rapidly in the aqueous molecular encapsulation.


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Figure 1 for enhancing the quality of the figure. 338x196mm (150 x 150 DPI)





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Cucurbituril-Based Reusable Nanocomposites for Efficient Molecular

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