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Tunichrome-inspired gold-enrichment dispersion matrix and its application in water treatment: A proof of concept investigation Amarendra Dhar Dwivedi, Rega Permana, Jitendra Pal Singh, Hakwon Yoon, Keun Hwa Chae, Yoon-Seok Chang, and Dong Soo Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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ACS Applied Materials & Interfaces

Tunichrome-Inspired Gold-Enrichment Dispersion Matrix and Its Application in Water Treatment: A Proof of Concept Investigation

Amarendra Dhar Dwivedi # a,b, Rega Permana # a, Jitendra Pal Singh c, Hakwon Yoon a, Keun Hwa Chae c, Yoon-Seok Chang a, Dong Soo Hwang * a,b a

Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Pohang 37673, Republic of Korea

b

Division of Integrative Bioscience and Biotechnology, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

c

Advanced Analysis Centre, Korea Institute of Science and Technology, Seoul-02792, Republic of Korea

Corresponding author: *Dong Soo Hwang (E-mail: [email protected]) #

Author contributions:

A.D.D. and R.P. contributed equally.

ACS Paragon Plus Environment


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Abstract Tunicate, a filter-feeder in seawater, is able to accumulate high amount of metals using intracellular polymer matrices. The woven pyrogallol structures of tunichrome, a small peptide contained in tunicate’s blood cells, is believed to be responsible for selective metal sequestration in tunicates from seawater. However, the intriguing tunichrome matrix is difficult to harvest from the tunicate nor to synthesize massively due to the extreme oxidation sensitivity of the pyrogallol moiety which limits the study scope. Here, we succeeded to mimic tunichrome by conjugating two cheap and naturally occurring components- pyrogallol-5-carboxylic acid (gallic acid) and chitin nanofiber. A tunicate mimetic infiltration matrix of surface tailored chitin nanofibers with pyrogallol moieties (CGa) demonstrated the versatility of strategy in generation of ingenious filtration material, especially for unprecedented fine and clean gold recovery inside of the tunicate mimetic infiltration matrix (>99%, 533 mg gold per gram weight), which exceeds that of the present most popular materials. Complexation between pyrogallol on the nanofiber and gold was similar to a tunichrome’s metal sequestration. Extended X-ray absorption fine structure (EXAFS) spectroscopy and data-fitting elucidated the decreased coordination numbers for Au-Au nearest neighbors, demonstrating gold coordinated to pyrogallol units, followed by an intramolecular association of Au0. A catalytic reduction of 4-nitrophenol mediated by the tunicate mimetic matrix with harvested gold revealed an excellent recyclability up to 30 cycles (~95% reduction), together with methylene blue reduction and antimicrobial performances, indicating a versatile characteristic of sustainable processes by the tunichrome-mimetics. This strategy opens door for fast-developing new biomimetic alternatives for precious metal recovery which is not only restricted to gold but also it can offer a tool for multifaceted soft/hard nanomaterials.


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Keywords: Tunichrome, Gold recovery, Gallic acid, Chitin nanofiber, Water treatment



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1. Introduction Even despite sharp advances have been made in recent years, recovery aiming at high affinity to concentrate gold is an exploratory research due to their ever increasing demand and huge economic values1-3. Sodium cyanide and mercury (toxic chemicals) are used in gold extraction for over a century, and ion exchange4, solvent exchange5, sorption6, precipitation7 methods are also explored as the alternative methods. However, the search is still on for environmentally friendly and sustainable alternative recovery methods with superior capacity, selectivity and regenerability, aiming at performance improvement, efficiency, and inexpensive processing3, 8-9. In addition, further researches are being carried out to utilize the sequestered gold or gold nanoparticles (Au NPs) to green catalytic materials. In this regard, metallic nanoparticles within the nanostructured network are potential candidates for a variety of applications, however, the main drawbacks; such as, NPs without carrier may easily aggregate in solution due to colloidal instability, which decreases the catalytic efficiency and slower reaction kinetics10-11. These aggregations of NPs reduce the surface active sites and interfacial free energy, thus diminishing their reactivity profile12. So far, strategies to address adorned NPs on carriers, such as active carbon13, graphene10, metallic oxide14, protein15, and cellulose nanocrystals16 have been shown some success. We now turn our attention to the importance of a tunichrome-mimetic nano-fibrous interface, as a green and highly effective matrix to sequester metal that can be translated into a biomimetic material for separating and anchoring Au NPs for the aforementioned green catalytic materials. Metal sequestration using intracellular polymer matrix is a natural strategy to remove and trap metal ions by the marine organisms17. As an example, tunicate, a sessile filter feeder is able to sequester high amounts of metal ions into blood cells. Now it is confirmed that the bright yellow-


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green blood cells containing a small peptide called tunichrome play an important key role on the metal sequestration18-19. Tunichrome contains unique 3,4,5-trihydroxyphenylalanine (TOPA) or pyrogallol functionalities, which are reported as the main repository for metals. For example, trace concentrations (~3x10-4 to 3x10-3 mg/L) of vanadium are found in seawater, but some tunicates have been recorded to accumulate vanadium up to 3000 mg/L concentration in blood cells20. This can be understood as the noncovalent bonding interaction and chelation potential which facilitates the solid-liquid interfacial attachments21. The chemical synergy between pyrogallol and crystalline cellulose nanostructured fiber (tunicin) of the tunicate bodies inspired us to generate the ingenious filtration material extracellularly for water treatment applications as occurred in tunicate. However, TOPA containing compounds are extremely oxygen sensitive and aqueous purification gives poor yield, and they are difficult to synthesize sufficient enough for the water purification. To overcome the TOPA limitation issues, a combination of naturally abundant and recyclable chitin and pyrogallol-5-carboxylic acid (gallic acid), which mimicked a tunichrome unit, was investigated here. Gallic acid (phenols with pyrogallol group), which is economically harvested from wood bark, was used as a prototype precursor to overcome TOPA limitations, and, by analogy, in place of cellulose component, chitin nanofiber (CNF) was chosen because

of

the

physicochemical

similarity,

biodegradability,

and

ease

in

surface

functionalization24. CNF matrix is chemically and mechanically more stable compared to other nanofiber platforms (e.g. electrospun nanofiber and self-assembled peptide)22 due to its insolubility in aqueous solutions nor common organic solvents and added advantages of excellent stiffness (~40 GPa) and insolubility of the single CNF contributes to improved mechanical properties in wet



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conditions23-25. Therefore, the intriguing merit of the tunicate-inspired network can provide a vital platform for both sequestering and immobilization of Au species. So, in this study, together with gold sequestration strategy, potential catalytic removal of organic contaminants and microbial inhibitory effects by tunichrome-mimetic matrix with sequestered gold were tested. We synthesized tunichrome-mimetic ingenious filtration matrix for separation and green catalytic performances, and investigate the interaction forces in gold-laden waters, and explore the impacts using a variety of conditions for recovery of gold and implications for real waters (Figure 1). Evidence on mechanisms underlying the high gold capacity of the tunicate mimetic filtration matrix was revealed with synchrotron-based EXAFS (extended X-ray absorption fine structure) technique describing complexation by probing interactions at the atomic levels. The judicially designed analog of tunichrome matrix resulted in examples of unprecedented fine gold recovery. Additionally, the tunicate mimetic infiltration matrix with sequestered Au NPs successfully applied in removal and detoxification of 4-nitrophenol and methylene blue as a demonstration in its practical water treatment application.

2. Materials and methods All of the chemicals were used as received without further purification. Descriptions of the procedure including materials used, experiments, data analyses, catalytic, and antimicrobial tests are provided in the Supporting Information (SI), text S1-S3. 2.1 Material syntheses The essence of the approach to synthesize the product is depicted graphically in Figure 1. Material synthesis occurred in a two-step reaction; first partial deacetylation produced the amine-


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functional chitin nanofiber from the native chitin, which underwent peptide bond formation with gallic acid yielding the final product. Amine-functional chitin nanofiber (CNF) CNF was prepared by a previous protocol with little modifications24. Briefly, the native chitin sample (20 g) was suspended in aq. NaOH (20 wt.%) and refluxed at 150 oC for 6 h. Washed thoroughly several times using deionized (DI) water to remove any remaining base. Then the chitin was dispersed in DI water to be 1 wt.%. Few drops of acetic acid added to adjust the pH to 4. For further nanofibrillization, the wet sample was homogenized using a high-performance grinder (MKCA6-3; Masuko Sangyo Co., Ltd.) with a rotation speed of 15,000 rpm and clearance up to -1.5, corresponding to 0.15 mm shift. Grinding was performed for several cycles until it formed a clear hydrogel. Hydrophilic amine groups of chitin were easily hydrated in wet conditions, yielding white soft gel of chitin nanofiber that had ca. 0.57 wt.% chitin concentration and lower degree of acetylation (DA; ca. 88.8%) than pure chitin (~98-99%). Chitin nanofiber-gallic acid conjugate (CGa) Chitin nanofiber-gallic acid conjugate was synthesized by a simple EDC (N-Ethyl-N’-(3(dimethylamino)propyl)carbodiimide) chemistry, enabling easy functionalization of pyrogallol groups on the nanofiber surface. EDC-mediated surface reaction activated the carboxyl groups of gallic acid, which induced the formation of a strong covalent peptide bond between the CNF and gallic acid26. In brief, gallic acid (2 eq. per acetylglucosamine unit of CNF) and EDC (1.1 eq. per gallic acid) were dissolved in absolute ethanol under cooling in an ice bath. Waited 15 min, NHS (N-hydroxysuccinimide) (1.1 eq. per EDC) was added and allowed to mixed together. Then CNF-dispersed hydrogel (0.57 wt.%) added to the gallic acid mixture and were left at 4 oC under



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stirring for 24 h. The gallic acid-conjugated CNF was purified by dialysis against DI water (24 h) and obtained material (CGa) stored at 4 oC.

2.2 Material characterization techniques 2.2.1 Surface characterization For transmission electron microscopy (TEM), a droplet of diluted CGa sample was mounted on formvar/carbon-coated copper grid. One drop of 2% uranyl acetate was added on the sample before drying for negative staining, and the sample was dried at room temperature, analyzed by using a transmission electron microscope (TEM, JEM-1011, JEOL, Tokyo, Japan), equipped with energy dispersive spectrometer (EDS) was used to visualize the surface morphology and structure of materials. Surface functional groups were analyzed by Fourier transform infrared spectroscopy (FTIR; Nicolet iS50, Thermo Scientific, Waltham, MA, USA). For surface charge analysis, ζ-potential of 0.57 wt% CGa solution was measured by a Zetasizer Nano S (Malvern Instruments, Malvern, UK) in which measurement was repeated three times and an average of them presented in the plot. 2.2.2 Crystallinity and thermal behavior Crystalline structures were analyzed by X-ray diffraction (XRD, D/MAX-2500/PC) with Cu-Kα radiation (40 kV, 100 mA). Powder diffraction data were recorded for 2θ angles between 50 and 800 with a scanning speed of 40/min. Thermogravimetric analyses (TGA) were performed to detect the thermal changes and stability of samples in a temperature range of 33 to 800 0C (Q200 instrument, TA Instruments). 2.2.3 X-ray absorption spectroscopy


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Extended X-ray absorption fine structure (EXAFS) measurements were employed to probe the interactions of gold with the CGa at hard X-ray 1D XRS KIST beamline, Pohang Accelerator Laboratory (PAL-South Korea). This beamline operates at 3 GeV with a maximum storage beam current of 320 mA and equipped with a bending magnet as the X-ray source to give monochromatic energies (4-16 keV). Higher harmonics were reduced by detuning of the incident beam to 60% of maximum intensity. The program Athena was used to sum the data, determine the beginning of the absorption edge (E0), fit pre- and post-edge backgrounds, thus obtained the normalized absorbance χ as a function of the modulus of the photoelectron wave vector k. The EXAFS data were Fourier transformed to R-space to investigate the atomic structure and relative bond-lengths with respect to absorbing atoms. The normalized spectra were simulated using ARTEMIS27. Reference compounds used for the X-ray absorption investigations were HAuCl4 and KAu(CN)2 for Au(III) and Au(I), respectively.

3. Results and discussions Strides in green chemistry have propelled to the advances of cleaner process via newer concepts as an alternative to the conventional methods28-30. We observed that tunicates prudently use pyrogallol moieties of tunichrome blood cells for the efficient enrichment of metals underwater19. This fact inspired us to mimic tunichrome matrix from the recyclable and earth-abundant raw materials. The naturally abundant gallic acid (rich in pyrogallols) and chitin nanostructured fiber; which has not yet received much attention in the research literature, are used as the precursors (CGa, Figure 1). This sustainable tool could be designed for the excellent gold recovery and enrichment encompassing separation and catalytic methods without any significant losses which obliquely contributed to aspects of greener features in water treatment applications.



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3.1 Physicochemical characterization FTIR characteristic vibration bands identified the peptide bond (∼1656 cm-1 and 1552 cm-1) and aromatic ring vibrations (∼1500 to ∼1450 cm-1) due to conjugated gallic acid onto CNF in CGa (SI, Figure S1). EDC/NHS formed a peptide bond, however, an ester linkage (∼1750 cm-1) with the surface hydroxyl CNF could not be ignored31. As in SI, Figure S2, the DS (degree of substitution) of gallic acid group (ca. 12.18%; w/w) on CNF backbone was estimated by Arnow’s colorimetric assay32 (a detailed test procedure can be seen in SI). The thermal behavior of CGa was performed by TGA in a nitrogen atmosphere (SI, Figure S3). TGA data indicated 9.3 % weight loss due to desorption of absorbed water molecules before 245 °C. A further 61.5 % weight loss noticed possibly due to phenolic hydroxyl functional groups in the sample in temperature from 245 to 380 °C. 16.8 % weight loss occurred due to decomposition or combustion of material on a further increase in temperature up to 800 °C. As in Figures 2a-c, lamellar networks of individualized fibrils (long-length fibers (>1µm), 97%) in acidic thiourea [(NH4)2CS (0.5M) in HCl (1M)]. Metallic gold deposited on the material surface formed a strong complex with thiourea ligand in acidic conditions. The dissolution of gold in acidic thiourea solutions is represented as38: + 2 ( ) → [ ( ) ]

( ) +


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3.3 Material performance under a varied water chemistry conditions Minimal interferences on gold recovery were noticed either at acidic or basic aqueous solutions (SI, Figure S7). Almost 100% recovery was obtained at ⩾6 mg (dry matter) dosing after 24 h at initial 100 mg/L concentration (SI, Figure S8). Therefore an optimum pH (3.36±0.2) was naturally selected for the unadjusted 100 mg/L Au(III) solution in all subsequent experiments. Notably, initial pH values (between 2.4-10.3) led to final equilibrium pH values (2.4 to 4) soon after the gold uptake. Active functional groups on CGa composition was believed to possess buffering stabilizer which resisted against pH changes in solution. From SI, Figure S9, ζpotential (varying between +43.8 and -8.9 mv) showed that surface charge became (+) to (-) due to amphoteric surface behavior (between 2.6 and 10). The point of zero charge (pHpzc) (net surface charge = 0) occurred at 8.2, thus the surface was positively charged at pH < pHpzc; conversely, the surface was negatively charged at pH > pHpzc. Considering electronic environment, surface charge distributions, and as we anticipated, CGa was dominated by a higher positive charged form, leading strong surface complexation with soft metal ions mainly via pyrogallol chemical moieties. The competition effect of industrially important metals [Cd(II), Co(II) and, Ni(II) ions] over gold recovery was tested at 100 mg/L Au(III) concentration with 10 mg/L metals mixed (Figure 3d). We could not see much difference in recovery (~99%) indicating minimal interferences by the presence of these metals in mixed-contaminated waters. Differences in redox potentials of metals could ease the sorption equilibrium and reduction reactions of gold36, indicating selectivity of gold for CGa binding sites. Nevertheless, in contrast to Cd(II) (70% of the Earth's surface), and laboratory tap water, showing performance improvement compared to that of the popular traditional materials (e.g. powdered activated carbon; PAC and granular activated carbon; GAC). To evaluate its application in practice, seawater sample was collected from Yongildae beach, Pohang, Gyeongsangbuk province (Republic of Korea) and synthetic wastewater was made up using OECD guidelines, adapted from the method described in Zhao et al40 and provided in SI, Table S2. Gold capture from tap water (~100%; CGa, PAC and 75% GAC), wastewater (>90%; CGa, PAC and ~75% GAC), and seawater (~90% CGa, PAC and ~40% GAC) revealed that, among all tested waters, performances of CGa composite were as excellent as PAC and distinct improvement was noticed compared to GAC even at a typically high Au(III) ions concentration (spiked 100 mg/L). This, therefore, showed that the tunichrome-mimetic Au recovery and dispersions could exhibit superior performance profile combined with environmental friendliness.


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3.4 Catalytic and antimicrobial performance Encouraged by superior gold capacity by CGa; the material catalyst was deployed for the reduction of nitrophenols and dye (experimental details are provided in SI, text S3). Among the mono-nitrophenol group, particularly, 4-nitrophenol (PNP) is the most important intermediate in industrial processes in terms of the quantities41-42, however, owing to high toxicity, the US environmental protection agency (USEPA) has classified PNP in priority pollutant list43. Reduction of the refractory PNP to lesser toxic 4-aminophenol (PAP) has significance in the points of pollution control and resource regeneration. Also, methylene blue (MB), a highly colored dye, is widely used in science and dyeing industries, which is aesthetic pollution source impacting the water quality at significant levels. It is challenging to prohibit their use in important industrial purposes, consequently, their release cannot be ignored in the environment44, and thus reasonably, removal and detoxification of these contaminants are essential in today’s world10, 45-47. No change in the absorbance of PNP was observed (at 400 nm) even after one week in the NaBH4 (as an additive solution) indicating that no reduction occurred in the absence of material. Unmodified CGa did not work for the catalytic reaction and CGa/Au without an additive did not work either. However, reduction performance employing NaBH4 as a hydrogen source demonstrated that CGa-supported gold nanoparticles greatly accelerated the hydrolysis reaction of NaBH4 in ambient conditions which expedited electrons transfer from a donor (BH4-) to acceptor molecules (PNP). Tang et al. have described the electron transfer reactions by diffusion of PNP to the metal surface, interfacial transfer, and diffusion of PAP from surface16:

O2N

OH

NaBH4

O-Na+

O2N

CGa/Au

NH2

HO

H2O

PNP

4-nitrophenolate


PAP


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PNP absorbance peak (at 317 nm) shifted in the presence of NaBH4 (at 400 nm) due to the deprotonation of PNP (pKa 7.6)48. On addition of catalyst, absorbance peak gradually decreased with reaction time at 400 nm, simultaneously accompanied by the evolution of PAP, the reduced product, as a new peak at 300 nm (Figure 4a). Calibration curve and the regression equation for the catalytic reduction of PNP by CGa/Au are provided in SI, text S3 and Figures S10 and S11. Clearly, the bright yellow color of PNP was disappeared to the colorless PAP, as presented in the inset of Figure 4a. This greener protocol further facilitated an easy and excellent recyclability of the stable catalyst tested up to thirty times without any significant loss of the catalytic efficiency (see Figure 4b). Notably, reduction reaction time varied in the range of 25 min (1-10 cycles), 45 min (11-18 cycles), 60 min (19-24 cycles), and 75 min (25-30 cycles). Furthermore, catalytic reduction of MB solution by NaBH4 with CGa/Au was investigated. One hour stirring ensured the equilibrium between CGa/Au and MB so that the adsorption effect could be eliminated. Figure 4c shows the absorption spectra of an aqueous solution of MB in the presence of CGa/Au and NaBH4. The density of the peak at 665 nm sharply decreased upon addition of NaBH4 within seconds, showing MB reduction, due to catalytic activity rather than adsorption. The deep blue color of MB disappeared to the colorless leuco MB compound (inset of Figure 4c). It became clear that the reduction was minimal using only NaBH4 and NaBH4/CGa. In fact, the CGa metallized with embedded Au NP acted as electron relays and thus accelerated the reduction reaction: H N

N H3C

CH3 N

H3C

S

MB

N CH3

+ H+ + e-

H3C

CH3 N

S

H3C


Leuco MB

N CH3

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A detailed experimental investigation on leaching test was carried out to validate the heterogeneous nature of the reaction. No formation of the corresponding product was observed even after several days without the gold-supported CGa disproving homogeneous catalysis under reaction conditions. ICP-OES analysis revealed that the reaction proceeded via heterogeneous catalysis as we could not identify the soluble Au species in the filtrate. Additionally, fluorescein diacetate (FDA) used to estimate the viability of bacterial cells49 (Escherichia coli (E. coli), a traditional indicator of biological contamination). FDA passively diffuses through the cell membrane and hydrolyzes to yield a green fluorescent compound, fluorescein, by esterase enzyme which measures its fluorescence. Gold, chitin nanofiber, and gallic acid have antimicrobial activities, but to which extent when they mixed together, was remained unexplored. Herein, among the fabricated materials tested in experiment, it ranked as: CGA/Au (~30%) > CGa (~75%) > CNF (~87%), compared to 100% relative viable growth of E. coli (DH5-α) cells (106 CFU/mL) (as in Figure 4d). Notably, 23 wt.% gold (ICP-OES analysis) impregnation on chitin nanofiber-gallic acid conjugate showed an improved disinfection advantage over the chitin-only and/or chitin-gallic acid compositions due to the embedded and dispersed Au NP into the material. Since CGa itself, is combined with antimicrobial properties on account of the partially deacetylated chitin and gallic acid, thus this strategy can drive to construct antifouling catalytic materials that prevent deterioration of high-value product lifetime.

3.5 X-ray absorption spectroscopic investigation Spectra were examined by the energy calibration, background subtraction and normalization with respect to the incident photon flux and converted to k-space (Å-1) and k3 weighted. The x(k)k3 spectrum was Fourier-transformed over 3 to 9 Å-1 (SI, Figure S12). The peaks were individually



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isolated, back transformed, and analyzed by relating unknown to reference compound qualitatively. It is evident in Figure 5a that Au L3-edge spectra (in the range of 11870-12000 eV) of treated CGa and model control (Ga) (a dominant gold nucleation center), displayed similarity in the edge peaks as well as the oscillations with reference (Au foil). Au0 showed two characteristic peaks at ~11947 and ~11970 eV, on a comparison, spectral features clearly indicated no resemblance with the Au(I) and Au(III) compounds (as could be seen in reference spectra), signifying efficient conversion of initial Au(III) into Au0 which formed gold nanoclusters embedded in the material’s matrix. Gallic acid is a polyphenolic compound that has three hydroxyl groups attach to its phenol ring. As the gold solution mixed with the composite material (CGa), the hydroxyl group possibly oxidized to form quinone and accordingly the gold solution, which was in its trivalent state, reduced to its zerovalent state forming a nanoparticle in the fibrous matrix. These results were consistent with that deduced from the simulated gold radial distribution functions of EXAFS spectra of gold foil, treated Ga model, and treated CGa (Figure 5b). Four parameters-coordination number (CN), interatomic distance (R), DebyeWaller value (σ2), and correction to main edge energy (∆E) were evaluated in the analysis of each sample. The goodness of data fit was evaluated by the R-factor. For the treated samples of GGa and Ga (control model), the Au0 or Au-Au bond length occurred at 2.85-2.86 Å revealing the metallic gold generation, as can be seen in Table 3. Nevertheless, a change in the coordination number for the CGa/Au (CN ~8) indicated a distorted local structure when compared with the bulk metal (CN ~12) and treated control model (CN ~12). It was possibly due to the fact that chitin nanofiber unit of CGa could show gold capture due to the glucosamine-gold interactions50, although a low value of ~50% uptake by ICP-OES analysis, which intriguingly improved with gallic acid conjugation on a CNF network (>99% capture). A decrease in


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coordination numbers for noble metal particle at nanometer level has currently proposed in the literature51-52. The interface interaction of support (CNF-gallic acid) with gold waters involved a change in the electronic/atomic environments, which were induced by solvated electron transfers of the support to gold particles, suggesting that adsorption was the first step in the formation of Au NP, as previous reports6, 9. Thus pyrogallol groups, coupling mediators of the tunichrome matrix, could provide significant interfacial attachment to inorganic metal (gold) surfaces following intramolecular associations. These results were reasonably coherent with the tunichrome-metal complexation of tunicates showing metal-chelating potential through pyrogallol moieties of TOPA, such as tunichrome-vanadium chemistry demonstrated the accumulation and reduction of pentavalent vanadium (V5+) into vanadium trivalent (V3+) in tunicate’s blood cells through intramolecular coordination forming the tris-complexes17-18. However, specific coordination chemistry of tunichrome-metal conjugation is still unclear. The data presented here improves understanding on the metallo-(bio)organic complexation, particularly for strategic approaches in tunicate’s water research.

Conclusions In summary, pursuing the bioinspired synthetic strategy in green and sustainable development, extracellular material design for both the separation and catalytic applications was demonstrated by mimicking blood cells termed tunichrome of the tunicates. This study developed a high performance system from readily available starting materials (chitin nanofiber conjugated gallic acid) aiming at excellent gold recovery, nitrophenol (recycled thirty times without any significant loss in catalytic activity), cationic dye reduction reactions, and a high-value natural antimicrobial product. The molecular evidence by EXAFS at the biointerface with inorganic



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metal proved the intramolecular association of Au-Au nearest neighbors by complexation modes between gallic acid and gold. Such studies can improve our understanding of metal deposition and mobilization in the environment, however, the formation of metal complexes by natural materials deserves more research to address the long-term fate of metals.

Supplementary Information Detailed descriptions of procedure including materials used, experiments, data analyses, catalytic, and antimicrobial tests, modified Arnow’s assay, material characterizations (FTIR, TGA, XRD graphics, elemental mapping, and zeta potential), material performances (impacts of pH, doses, and isotherm data), k3-weighted EXAFS spectra of samples, and synthetic wastewater composition details

Acknowledgements This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016M3D3A1A01913258, C1 Gas Refinery Program; NRF-2015K1A3A1A59074243: KONNECT, and NRF-2016M1A5A1027594. This work was also supported by the Marine Biotechnology program (Marine BioMaterials Research Center),

which

was

funded

by

the

Ministry

of

Oceans

and

Fisheries,

Korea

(D11013214H480000110). This work was also supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20144030200460).


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TOC/Abstract art ‘Pyrogallol chemistry and structured nanofiber provide novel functionality, scaffold and biointerface for gold separation and green catalytic methods by mimicking a molecule in the tunicates’ blood cells called tunichrome’



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Figure 1: Synthesis scheme of tunichrome-mimetic extracellular matrix (CGa) and goldenrichment dispersion matrix (CGa/Au)


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Figure 2: Bright-field TEM micrographs of (a) CNF; (b) CGa; (c) CGa/Au matrix; (d) gold nanoparticles; (e) dark-filed imaging of in situ produced single gold nanoparticle; (f) corresponding SAED patterns for the CGa/Au



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Figure 3: (a) Intraparticle diffusion plot (time varied from 0.05 to 48 h); (b) isotherm plots ([Au(III)]0 concentration varied from 5-925 mg/L; (c) desorption efficacy (CGa dose = 1 g/L and eluent volume = 0.02 L); (d) impact of metals mixed together (Au(III)]0 = 100 mg/L and Cd(II)0 = Co(II)0 = Ni(II)0 = 10 mg/L); (e) impact of aging periods (in days) on gold recovery; (f) performance comparison with standard materials (PAC and GAC) in a varied water conditions (tap water (TW), wastewater (WW), and seawater (SW)) (for all experiments, [Au(III)] 0 = 100 mg/L except isotherm plots, CGa dosing = 0.3 g/L except desorption, time = 24 h except kinetic plot, pH[Au(III)]0 = 3.36 (uncontrolled pH), ambient temperature)


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Figure 4: (a) Catalytic reduction of 4-nitrophenol (PNP) by CGa/Au (inset: disappearance of bright yellow color of PNP) ([CGa/Au]0 = 2 g/L, [NaBH4]0 = 30 mM, [PNP]0 = 5 mM); (b) reusability of CGa/Au as a catalyst for reduction reactions; (c) catalytic removal of methylene blue (MB) (inset: disappearance of bright blue color of MB) ([CGa/Au]0 = 0.3 g/L, [NaBH4]0 = 30 mM, [MB]0 = 50 mg/L); (d) antibacterial performance (viability of bacterial cells was estimated using fluorescein diacetate) (reaction time = 1 h, [E. coli]0 = 106 CFU/ml, [material doses]0 = 0.3 g/L, NS; Not significant [p>0.05], *Significant [p

Tunichrome-Inspired Gold-Enrichment Dispersion ... - ACS Publications

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