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Sep 18, 2018 - One-step assembly of iron(III)-tannic acid (Fe3+-TA) complex forms nanothin (~10 nm) films on various substrates within minutes...
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Salt-Induced, Continuous Deposition of Supramolecular Iron(III)-Tannic Acid Complex Taegyun Park, Won Il Kim, Beom Jin Kim, Hojae Lee, Insung S Choi, Ji Hun Park, and Woo Kyung Cho Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02686 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 23, 2018

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Salt-Induced, Continuous Deposition of Supramolecular Iron(III)-Tannic Acid Complex Taegyun Park,† Won Il Kim,† Beom Jin Kim,† Hojae Lee,† Insung S. Choi†,* Ji Hun Park,‡,* and Woo Kyung Cho,§,* †

Center for Cell-Encapsulation Research, Department of Chemistry, KAIST, Daejeon 34141,

Korea ‡

Department of Science Education, Ewha Womans University, Seoul 03760, Korea

§

Department of Chemistry, Chungnam National University, Daejeon 34134, Korea

Abstract

One-step assembly of iron(III)-tannic acid (Fe3+-TA) complex forms nanothin (~10 nm) films on various substrates within minutes. In this deposition scheme, however, the film does not grow continuously over time even though Fe3+-TA complex is still abundant in the coating solution. In this paper, we report that the salt addition dramatically changes the one-off coating characteristic to continuous one, and each salt has its optimum concentration (CMFT) that produces maximum film thickness. For detailed investigation of the salt effects, we employed various salts including LiCl, NaCl, KCl, CaCl2, SrCl2, BaCl2, NaBr, and NaNO3, and found that only cations played an

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important role in the continuous deposition of the Fe3+-TA complex, with smaller CMFT values for the cations of higher valency and larger size. Based on the results, we suggested that the positively charged cations screened the negative surface charges of Fe3+-TA complex particles, leading to coagulation and continuous deposition, further supported by the ζ-potential measurement and time-resolved dynamic light scattering analysis.

Introduction Surface chemical-modification has been regarded as a useful molecular-level strategy for modulating the interactions at the material-environment interface.1,2 It allows for control over the physicochemical properties of material surfaces and also enables introduction of chemical functionalities that are not innate to the materials. Most surface-modification methods have been substrate-specific, relying on the unique and specific interactions between substrates and coating compounds.3-6 To widen the scope of substrates to coat and generalize coating strategies, substrate-independent-coating materials, including polydopamine,7 plant-derived polyphenolic compounds,8 turmeric,9 and urushiol,10 have recently received a great deal of attention. For example, tannic acid (TA), composed of five digalloyl ester groups attached to a central glucose molecule, shows the characteristics of both universal adsorption and cross-linking by multivalent metal ions (e.g., Fe3+).11 Simple mixing of Fe3+ and TA in aqueous solution leads to the conformal-film formation of supramolecular Fe3+-TA coordination complex on almost any kind of substrates regardless of topology and surface chemical-compositions.11 However, this one-step assembly forms the films with thickness of only ~10 nm, meaning that a small proportion of the Fe3+-TA complexes generated in the coating solution are deposited limitedly onto a substrate of

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interest. In other words, most of the formed Fe3+-TA complexes are wasted in the solution phase. Therefore, a multi-step process has been essentially preferred for producing films with thickness of tens-of-nanometers. The layer-by-layer deposition of Fe3+ and TA was attempted, but it generates ultrathin (~2 nm) films per cycle.12,13 An intriguing approach, utilizing rusted nails, has been developed for continuous film deposition, but it requires extensive stirring during the whole coating time and is difficult to scale up.14 There has very recently been a report on formation of thick Fe3+-TA films by using filtered Fe3+-TA complex,15 but it is still demanding to develop simple, one-step, continuous deposition methods for facile preparation of thick Fe3+-TA films. Our group has utilized the Fe3+-TA complex for cell nanoencapsulation, by taking advantage of its biocompatibility and mild coating conditions.16-19 During the studies, we unexpectedly observed that the thickness of Fe3+-TA films was 3.4 times thicker in isotonic saline (0.145 M NaCl) than in deionized water.18 Caruso has also observed that the film became thicker and rougher as the salt concentration increased for the 10-s coating process.20 Intrigued, we, in this work, further investigated the detailed, long-term salt effects with varied valencies and sizes of the cations. We found that the presence of salts made the deposition continuous with no significant contribution of anions, and that the critical cation-concentration for maximum film thickness depended on valency and size of the cations. Further analyses indicated that the cations would reduce the negative surface-charges of Fe3+-TA complex particles and cause their coagulation, which led to continuous film deposition and particle growth in the solution phase.

Experimental Section

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Materials. Tannic acid (TA, Sigma), iron(III) chloride hexahydrate (FeCl3·6H2O, Sigma), potassium chloride (KCl, Junsei), sodium chloride (NaCl, Daejung), lithium chloride (LiCl, Sigma), barium chloride dihydrate (BaCl2·2H2O, Merck), strontium chloride hexahydrate (SrCl2·6H2O, Sigma), calcium chloride dihydrate (CaCl2·2H2O, Sigma), sodium bromide (NaBr, Sigma), sodium nitrate (NaNO3, Sigma), acetone (Samchun), and ethanol (Samchun) were used as received. Ultrapure water (18.3 MΩ·cm) from the Direct-Q® Water Purification System (Millipore) was used. Effects of NaCl on Precipitation of Fe3+-TA Particles. To a 10-mL vial was added 7.84 mL of DI water or 75-mM NaCl solution, followed by sequential addition of 80 µL of FeCl3·6H2O stock solution (10 mM) and 80 µL of TA stock solution (10 mM) with 10-sec mixing after each addition (final concentration: [Fe3+] = 0.1 mM, [TA] = 0.1 mM). The samples were placed under static conditions (i.e., without stirring) at room temperature, and optical images were taken after 6 days with a digital camera. Effects of NaCl on Film Deposition of Fe3+-TA Complex. For the film deposition, silicon wafers were cut into 1 × 1 cm2 slides, cleaned with acetone and ethanol under sonication, and dried under a stream of argon (Ar) gas. The silicon substrate was immersed in 3.92 mL of DI water or 75 mM NaCl solution prepared, and 40 µL of the FeCl3·6H2O stock solution and 40 µL of TA stock solution were added sequentially. After the predetermined time of incubation (10 sec, 15 min, 1 h, 2 h, 4 h, 6 h, 9 h, 12 h, 18 h, 24 h, and 48 h), the substrate was taken out from the solution, washed with DI water, and dried under a stream of Ar gas. The film thickness was measured with an L116s ellipsometer (Gaertner Scientific Corporation) equipped with a He-Ne laser (632.8 nm) at a 70° angle of incidence. For each sample, more than five different points

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were measured, and the averaged value is reported in this paper. For the film deposition on an inverted substrate, the silicon substrate was floated upside down on DI water or 75 mM NaCl solution. Salt Effects. The 3.92-mL solutions of various salts (KCl, NaCl, NaBr, NaNO3, LiCl, BaCl2, SrCl2, and CaCl2) were prepared in a 12-well plate. The salt concentrations were varied: 25, 37.5, 50, 75, 100, 150, 200, 500, and 1000 mM for KCl, NaCl, and LiCl; 25, 50, 75, 100, 150, 200, 500, and 1000 mM for NaBr and NaNO3; 1, 2.3, 4.75, 6.25, 10, 20, 50, and 100 mM for BaCl2, SrCl2, and CaCl2. The silicon slides were immersed in the salt solutions, followed by the addition of the FeCl3·6H2O and TA stock solutions. After 24 h, the substrates were taken out from the salt solutions, washed with DI water, and dried under a stream of Ar gas. As a control, only TA stock solution (40 µL) was added to DI water, 75-mM NaCl, or 6.25-mM CaCl2 solution (3.96 mL) containing a silicon substrate. Characterizations of Fe3+-TA Particles and Films. The hydrodynamic diameter (Dh) of Fe3+TA complex particles was measured at regular intervals by dynamic light scattering (DLS) analysis. ζ-potentials of the Fe3+-TA particles were measured right after their formation in order to investigate the charge screening effect of salts. Time-resolved DLS analysis was conducted for each salt with three different concentrations around its CMFT value: 25, 37.5, and 50 mM for KCl; 50, 75, and 100 mM for NaCl, NaBr, and NaNO3; 100, 150, and 200 mM for LiCl; 1, 2.3, and 4.75 mM for BaCl2; 2.3, 4.75, and 6.25 mM for SrCl2; 4.75, 6.25, and 10 mM for CaCl2. ζpotential measurement was conducted at the CMFT value with a Zetasizer Nano ZS90 (Malvern). The surface chemical composition and surface morphology of the Fe3+-TA films were characterized by X-ray photoelectron spectroscopy (XPS) and field-emission scanning electron

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microscopy (FE-SEM). XPS spectra were obtained with a K-alpha X-ray photoelectron spectrometer (Thermo VG Scientific), and FE-SEM imaging was performed with a Philips XN30FEG microscope (FEI-Philips Co.) with an accelerating voltage of 10 kV, after sputtercoating the samples with platinum.

Results and Discussion We first investigated the salt effects on the film deposition of Fe3+-TA complex, with silicon (Si/SiO2) substrates, in deionized (DI) water (without salts) and the NaCl (75 mM) solution (final concentrations: [Fe3+] = 0.1 mM, [TA] = 0.1 mM). The NaCl solution of Fe3+ and TA, initially bluish purple, became transparent gradually over time, and the Fe3+-TA complex precipitated out at the bottom of the reaction vial within 24 h, while the suspension in DI water kept stable even after 6 d (Figure 1a). The ellipsometric measurements indicated that the film grew continuously over time only in the NaCl solution, forming ca. 23 nm-thick films after 24 h (Figure 1b). Of interest was that no film was formed in DI water even after 48 h. We characterized the substrates, after 24 h of incubation, by field-emission scanning electron microscopy (FE-SEM). The substrate in the NaCl solution was coated with the agglomerated Fe3+-TA complex particles, while that in DI water had no or little particles (Figure 1c).

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Figure 1. Characterizations of the Fe3+-TA complex solutions and films (DI water and 75-mM NaCl solution). (a) An optical image taken at 6 d after mixing. (b) A graph of film thickness vs. incubation time. (c) FE-SEM micrographs of the deposited films.

X-ray photoelectron spectroscopy (XPS) spectrum showed the peaks at 285 (C 1s), 712/725 (Fe 2p3/2/Fe 2p1/2), and 1072 eV (Na 1s) for the NaCl-induced Fe3+-TA film (Figure 2). No peaks for Fe3+ and Na+ ions were detected from the substrate in DI water. These results clearly indicated that NaCl not only assisted in the initial deposition of Fe3+-TA complex but also induced the continuous deposition. Furthermore, we did not observe the Cl 2s peak for both DI water- or NaCl-induced Fe3+-TA films, implying that the anions were not involved in the deposition process. The FE-SEM images additionally indicated that the salt-induced continuous deposition of Fe3+-TA complex might be related with its coagulation and precipitation. To clarify whether gravity affected the film deposition or not in our system, we floated a silicon substrate upside down in the coating solution for 24 h. The ellipsometric measurement showed that ~20-

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nm-thick films were formed in the NaCl solution, but no films in DI water (Figure S1). Therefore, we thought that the film deposition process proceeded by Brownian motion-involved coagulation rather than gravitational precipitation.

Figure 2. XPS spectra of the silicon substrates after 24 h of incubation. (left) DI water and (right) 75-mM NaCl solution.

We used various salts with different cation sizes and valencies for detailed studies on salt effects. Specifically, we chose monovalent cations (KCl, NaCl, and LiCl) and divalent cations

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(BaCl2, SrCl2, and CaCl2). For each salt, the concentrations were varied, and the film thickness was measured after 24 h of incubation (see the Experimental Section for the details). As seen in Figure 3a and b, each salt had its own concentration for producing the maximum film thickness (CMFT). We found that the CMFT value was extremely critical in the continuous deposition of the Fe3+-TA complex; a little deviation from CMFT made the film far much thinner. All three divalent cations (Ba2+, Sr2+, and Ca2+) had much lower CMFT values (2.3 mM for Ba2+, 4.75 mM for Sr2+, and 6.25 mM for Ca2+) than monovalent cations (37.5 mM for K+, 75 mM for Na+, and 150 mM for Li+) (Figure 3c). In the set of the same valency, larger cations had lower CMFT values (ion size: K+ > Na+ > Li+, Ba2+ > Sr2+ > Ca2+). The variation of anions (NaCl, NaBr, and NaNO3) did not change the CMFT values (Figure S2a and b). The XPS analyses showed that all the cations were present in the films (Figure S3).

Figure 3. (a and b) Graphs of ellipsometric film thickness vs. salt concentration: (a) monovalent cations and (b) divalent cations. (c) A CMFT-value table for various salts.

The requirement of Fe3+ for the film formation (Figure S4) suggested that the salt-induced deposition should result from the coagulation of Fe3+-TA complexes, not TA alone. The ζpotential of Fe3+-TA complex particles was measured to be -31.4 mV in the absence of salts,

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negative enough to stabilize the colloidal suspension by electrostatic repulsion.21 The value also explained why the Fe3+-TA complex particles did not coagulate for at least 6 days in DI water and why the deposition did not occur on a negatively charged silicon surface. Taken together, we thought that the positively charged cations screened the negative charge of Fe3+-TA complex and reduced electrostatic repulsion among the complexes, leading to the coagulation and continuous surface-deposition (Figure S5). This hypothesis was supported strongly by the ζ-potential measurements with the salt of CMFT (Figure 4). The addition of the salts greatly increased ζpotentials, and, unexpectedly, the values converged to the range of -11.2 and -10.0 mV.

Figure 4. ζ-Potential values of Fe3+-TA complex particles in DI water and various salt solutions at CMFT.

It is well known that the larger the cation valency is, the less concentration is needed for the coagulation of the negatively charged colloid particles.22,23 In our system, the higher electrostatically attractive force and bridging capability of the divalent cations (Ba2+, Sr2+, and Ca2+) led to the lower CMFT values than the monovalent cations (K+, Na+, and Li+). In the same

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valency group, higher polarizability and weaker hydration of larger cations would make the adsorption onto Fe3+-TA complex particles more facile and ion exchange efficiency higher,24 resulting in the lower CMFT values. Time-resolved dynamic light scattering (DLS) analysis further supported our hypotheses (Figure 5a and Figure S6). Any noticeable changes in the hydrodynamic diameter (Dh) were neither found in DI water nor in the 50 mM NaCl solution, which also reconfirmed the sensitivity of CMFT. However, at CMFT (75 mM NaCl), the Dh value increased slowly over time. With 100 mM of NaCl, it increased rapidly over time, indicating uncontrollable, greater coagulation in the solution phase. We calculated the coagulation rate from the slope (Dh/s) in Figure 5a and found that the rate abruptly changed at CMFT (Figure 5b).

Figure 5. (a) Time-resolved DLS analysis of Fe3+-TA complex particles. (b) A graph of coagulation rate as a function of NaCl concentration. The inset is a magnification of the first three points.

The rate was nearly zero below CMFT and far fast above CMFT. Accordingly, the FE-SEM analyses showed that films formed less in the 50- and 100-mM NaCl solutions than in the 75mM NaCl solution (Figure S7). Another observation from the FE-SEM images was that the sizes of deposited particles were not changed noticeably with NaCl concentrations, although much bigger Fe3+-TA complex particles were generated in the 100-mM NaCl solution phase. These

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results arguably suggested that big Fe3+-TA complex particles could not be deposited onto the substrates. In other words, at the concentrations higher than CMFT, free (i.e. non-complexed), surface-adherable TA and its complex were available less in the solution, because nondepositable, big Fe3+-TA complex particles were generated rapidly under the conditions. Similar phenomena have been reported for the polydopamine coating: when a silicon substrate was immersed in the polydopamine particle solution, no film deposition occurred, suggesting that unoxidized dopamine or small oligomers were required in the film-deposition process.25 We also pre-mixed Fe3+ and TA in the 75-mM NaCl solution to make Fe3+-TA complex particles several seconds before substrate immersion, and found that the film thickness significantly decreased to 9 nm from 23 nm.

Conclusions In summary, we demonstrated the one-step, salt-induced continuous deposition of Fe3+-TA complex onto a silicon substrate. Detailed mechanistic studies showed that the cation played a critical role in the coagulation of small Fe3+-TA particles by charge screening and subsequent deposition. The salt concentration for maximum film thickness (CMFT) depended on the valency and size of the cation, which could be determined indirectly by the ζ-potential value of -11.2 ~ 10.0 mV. Considering that this salt-induced film deposition of Fe3+-TA complex is simple and extremely cost-effective, we believe that our work would have a great deal of potential in largescale surface chemical-modification for various fields, such as chemistry, materials science, and bioengineering, and also be applied to other metal-polyphenol complexes for functional film formation.

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Additional experimental data and the graphic for the proposed mechanism (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (W.K.C.), [email protected] (J.H.P.), [email protected] (I.S.C.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT) (NRF2016R1A1A1A05921718 to W.K.C., and NRF-2018R1C1B5045778 to J.H.P.). ABBREVIATIONS

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TA, tannic acid; CMFT, the optimum concentration of the salt that produces maximum film thickness; DLS, dynamic light scattering; Dh, hydrodynamic diameter; XPS, X-ray photoelectron spectroscopy; FE-SEM, field-emission scanning electron microscopy REFERENCES (1) Ejima, H.; Richardson, J. J.; Caruso, F. Metal-phenolic networks as a versatile platform to engineer nanomaterials and biointerfaces. Nano Today 2017, 12, 136-148. (2) Richardson, J. J.; Cui, J.; Björnmalm, M.; Braunger, J. A.; Ejima, H.; Caruso, F. Innovation in Layer-by-Layer Assembly. Chem. Rev. 2016, 116, 14828-14867. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 11031170. (4) Haensch, C.; Hoeppener, S.; Schubert, U. S. Chemical modification of self-assembled silane based monolayers by surface reactions. Chem. Soc. Rev. 2010, 39, 2323–2334. (5) Queffélec, C.; Petit, M.; Janvier, P.; Knight, D. A.; Bujoli, B. Surface Modification Using Phosphonic Acids and Esters. Chem. Rev. 2012, 112, 3777–3807. (6) Hammond, P. T. Recent explorations in electrostatic multilayer thin film assembly. Curr. Opin. Coll. Interface Sci. 2000, 4, 430-442. (7) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426-430.

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Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table of Contents (TOC)

ACS Paragon Plus Environment

18