Stimuli-Responsive Extraction and Ambidextrous Redispersion of

Jun 22, 2016 - The lamellar supramolecules incorporated the AgNPs into their 3D network and extracted them from water. C16CA supramolecules dissolved ...
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Stimuli-Responsive Extraction and Ambidextrous Redispersion of Zwitterionic Amphiphile-Capped Silver Nanoparticles Clara Morita-Imura,*,† Katsuya Zama,† Yoshiro Imura,‡ Takeshi Kawai,‡ and Hitoshi Shindo*,† †

Department of Applied Chemistry, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8614, Japan

‡

S Supporting Information *

ABSTRACT: Citrate-stabilized silver nanoparticles (AgNPs) were functionalized with a pH-responsive amphiphile, 3-[(2carboxy-ethyl)-hexadecyl-amino]-propionic acid (C16CA). At pH ∼ 4, the zwitterionic C16CA assembled into lamellar structures due to the protonation of the amine groups of the amphiphile that neutralized the anionic charge of the carboxylate groups. The lamellar supramolecules incorporated the AgNPs into their 3D network and extracted them from water. C16CA supramolecules dissolved into water (at pH > 6) and organic solvents; consequently, the recovered C16CAAgNPs were redispersed not only to water but also to chloroform and tetrahydrofuran without any additional functionalization. C16CA acted as a pH-responsive stabilizer of AgNPs and formed a solvent-switchable molecular layer such as a bilayered structure in water and densely packed monolayer in chloroform and tetrahydrofuran. Redispersion of the AgNPs was achieved in different solvents by changing the solvent affinity of the adsorbed C16CA molecular layer based on the protonation of the amine groups of the pH-responsive amphiphile. The morphology of redispersed AgNPs did not change during the recovery and redispersion procedure, due to the high steric effect of the network structure of C16CA supramolecules. These observations can lead to a novel solvent-exchange method for nanocrystals without aggregation and loss of nanocrystals, and they enable effective preparations of stimuli-responsive plasmonic nanomaterials.

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INTRODUCTION Noble metal nanomaterials, including Au, Ag, and Pt, attract a great deal of interest due to their optical and catalytic properties.1−5 In particular, Ag nanocrystals6−9 are important materials in nanoplasmonics due to their ability to generate strong plasmons.10−12 Over the past decade, these materials have found applications in sensing and detection13−17 via surface-enhanced Raman scattering (SERS), near-field optical microscopy, and surface plasmon resonance. Ag nanocrystals are also useful in biosensing18−20 because of their biocompatibility. Generally, postpreparative processing is a necessary step for using Ag nanocrystals as plasmonic sensing materials. These processing methods include ligand exchange, coating, or bioconjugation,5,21−23 and are required for the solvent exchange of the nanocrystals into suitable dispersion media. For biological applications, water-dispersed nanocrystals are required, while, for optoelectronic applications, the nanocrystals should be compatible with organic solvents.24 However, the asprepared nanocrystals are often dispersed in either aqueous or organic media but not both. Extraction of the as-prepared nanocrystals to remove the initial dispersion solvent, such as the common centrifuging procedure, is a useful step before solvent replacement for further surface modification. Recently, supramolecules have been used in an inclusion complexation with nanocrystals.25−27 © XXXX American Chemical Society

Amphiphilic ligands, which can form highly ordered selfassemblies such as lamellar supramolecules, have great potential in the incorporation of nanomaterials from dispersion by utilizing adsorption and inclusion phenomena.28,29 Ionic amphiphilic ligands are often adsorbed on the surfaces of nanomaterials by noncovalent interactions. However, these ligands can be easily displaced by covalent bonding ligands such as functional thiol derivatives30,31 with the appropriate functionality after the recovery procedure. Previously, we have reported that the pH-responsive amphiphile 3-[(2-carboxy-ethyl)-hexadecyl-amino]-propionic acid (C16CA, Figure 1) transforms from its assembled structure such as spherical micelles (anionic state, Figure 1a) to wormlike micelles (cationic state, Figure 1c), and also forms precipitates with a lamellar structure (zwitterionic state, Figure 1b) in aqueous solutions due to protonation of the hydrophilic portion of the amine and carboxylate groups.28,29 The lamellar supramolecules of C16CA have been found to incorporate gold nanocrystals with a spherical or rod-like morphology due to the adsorption of the amphiphile on the metal surface. Carboxyl and amine groups also adsorb onto silver surfaces32,33 such as Received: May 7, 2016 Revised: June 21, 2016

A

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NaOH (18 mmol) were added to 18 mL of methanol, and the mixture was stirred for 1 day at room temperature. The solvent was evaporated off to yield 3-[(2-carboxy-ethyl)-hexadecyl-amino]-propionic acid (C16CA) as a white solid, which was washed with hexane and methanol (yield: 90%). C16CA: 1H NMR (D2O, 400 MHz): δ 0.88 (t, 3H, CH3), 1.25 (br, 28H, CH2), 1.45 (br, 4H, CH2CH3, CH2CH2CH2N), 2.36 (t, 4H, CH2CH2CO), 2.42 (t, 2H, CH2N), 2.73 (t, 4H, NCH2CH2CO). Critical micelle concentration (cmc) values at 27 °C were 0.10 mM in aqueous solution of anionic C16CA (Wilhelmy plate method) and 0.09 mM in chloroform solution of zwitterionic C16CA (dye solubilization method), respectively. In methanol solution of zwitterionic C16CA, the cmc value could not be determined. Preparation of the AgNPs. Nine mg (0.053 mmol) of AgNO3 was dissolved in 50 mL of deionized water. Four mL of 77 mM aq. sodium citrate was added to the solution, and the mixture was stirred for 5 min. Then, 0.2 mL of ice-cold 0.26 M aq. NaBH4 was slowly added to the solution and vigorously stirred for 1 day at room temperature. Recovery and Redispersion of the AgNPs. To obtain the initial C16CA-capped AgNPs dispersion (C16CA-AgNPs) with excess C16CA ([C16CA]/[Ag] = 100), 1.5 mL of 100 mM aq. C16CA was added to 1.5 mL of the as-prepared AgNPs dispersion. The mixture was stirred for 1 h to exchange the ligands from citrate to C16CA. The initial dispersion was titrated with 0.1 M HCl until the C16CA lamellar assembly precipitated at pH 4. The precipitates were removed from the solution by filtration, and 3 mL of different solvents (0.1 M aq. NaOH, chloroform, tetrahydrofuran (THF), and methanol, respectively) was added to resolute the dried C16CA-AgNP precipitates. The AgNPs was redispersed into the solvents through gentle shaking of the vials. The redispersed NPs were analyzed by transmission electron microscopy (TEM) and UV−vis spectroscopy. Measurements. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns of the AgNPs were obtained using a JEOL 2100 instrument operated at 200 kV. The samples for TEM were prepared by dipping carbon-coated copper grids into the dispersions and drying them in air at room temperature. UV−vis spectra were obtained with a JASCO J-630 spectrometer using 1 cm path-length quartz cuvettes. Fourier transform infrared (FT-IR) spectra with a resolution of 2 cm−1 were obtained with a Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific) equipped with an MCT detector. For the FT-IR measurements, a demountable liquid cell with a CaF2 window was used with D2O, CH3OD, and CDCl3 solutions. For each spectrum, 1000 scans were recorded and averaged. To study the protonated states of the carboxyl groups, D2O was used instead of H2O for avoiding the overlapping of the OH bending mode of water around 1640 cm−1. To determine the adsorbed amount of C16CA on Ag, a quartz crystal microbalance (QCM) equipped with QCA 922 (SEIKOEG&G) was used.40 Here, the adsorbed mass of C16CA was estimated from the frequency change (ΔF) for a QCM quartz resonator before and after adsorption using the Sauerbrey equation,41 which gives the linear relationship between the adsorbed mass Δm and ΔF

Figure 1. Molecular structure of C16CA in the (a) anionic state at pH > 6, (b) zwitterionic state at 6 > pH > 2, and (c) cationic state at pH < 2.

amino acid or DNA.34−36 It is expected that recovery systems using C16CA assemblies have great potential for use in the recovery of Ag nanocrystals. Furthermore, from an application perspective, it would be useful if the recovered nanocrystals could be redispersed into a solvent of choice. The dispersion of nanocrystals is affected by the nature of the ligand layer on the metal surface. The most important factor for a good dispersion is the adsorption of the ligand molecules on the metal surface. Moreover, the high affinity with the dispersion media is also necessary for the molecules. A limited number of the versatile amphiphilic ligands have been reported, which change the solvent affinity using external stimuli,37,38 or molecular flexibility,39 allowing the dispersion of nanoparticles either in water or in organic solvents. In our previous reports on the recovery of Au nanocrystals using C16CA, the redispersion media was limited to water.28,29 We expected C16CA to show a transition in solvent affinity based on a change of molecular charge from the anionic to the zwitterionic state (Figure 1). It would be a challenge to extend this method to the recovery of plasmonic Ag nanocrystals, and to redisperse them both into water and into organic solvents for the further application. Herein, we expect that the anionic form of C16CA would help to disperse the nanocrystals in aqueous media, while the zwitterionic form would disperse them in organic media due to the neutralization of the electrostatic charge on the amphiphile molecules by ion-pair interactions between the amine and carboxyl groups. In this work, we attempt to change the nature of the adsorbed molecular layer by protonation of C16CA based on pH changes, and we try to recover and redisperse Ag nanoparticles (AgNPs) into water and organic solvents using this switch in solvent affinities without the risk of NPs coagulation or emulsification of solvents.

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EXPERIMENTAL SECTION

Materials and Synthesis of C16CA.28,29 Hexadecylamine (Aldrich Chemicals) was recrystallized from hexane. Methyl acrylate (Kanto Chemicals) was purified by distillation under reduced pressure. All other reagents were used as received. Cetyltrimethylammonium bromide (CTAB) was purchased from Aldrich Chemicals. Sodium borohydride (NaBH4), sodium salicylate (NaSal), ascorbic acid, and silver nitrate (AgNO3) were purchased from Kanto Chemicals. The 1 M hydrochloric acid (HCl) and 1 M sodium hydroxide (NaOH) solutions were obtained from Wako Pure Chemical Industries. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O) was obtained from Nacalai Tesque. Methyl acrylate (14.30 g, 0.17 mol) was added to hexadecylamine (2.0 g, 8.30 mmol) in 15 mL of methanol. The solution was stirred at 40 °C for 3 days, followed by the removal of the solvents and excess methyl acrylate by rotary evaporation to yield 3-[(2-methoxycarbonylethyl)-hexadecyl-amino]-propionic acid methyl ester (C16ME) as a viscous liquid. C16ME (3.0 g, 7.23 mmol) and 18 mL of aq. 1 M

Δf = − 2f0 2

Δm A μq ρq

(1)

where f 0 is the fundamental resonance frequency of crystal, μq is the shear modulus, ρq is the density of quartz, and A is the area of the Ag electrode. The relation is valid when the adsorbed mass is distributed evenly over the crystal and Δm is much smaller (2%) of the crystal and the adsorbed material is rigidly and thin. For the experiments, Ag coated AT-cut quartz crystals with a resonant frequency of 9 MHz and an area of 0.196 cm2 was used. The Sauerbrey equation gives a mass sensitivity of 1.07 ng/Hz.

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RESULTS AND DISCUSSION We prepared the citrate-stabilized AgNPs with a diameter of ∼11.3 nm (Figure 2a). The as-prepared dispersion showed a B

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Figure 2. TEM images of the (a) as-prepared Ag NPs and (b) initial dispersion of C16CA-AgNPs.

brownish yellow color, which is attributed to the surface plasmon (SP) band of the AgNPs. Generally, the SP band also reflects the morphology of the nanocrystals including the shape, size, and their dispersion state.1,42−46 We used the SP band to determine the structure and dispersibility of the AgNPs. The UV−vis spectra (Figure 3a) show a strong absorption peak at

Figure 4. Photographs of the (a) initial C16CA-AgNPs dispersion, (b) phase-separated solution, (c) C16CA-AgNP precipitate, and (d) filtrate after removal of the C16CA-AgNP precipitates.

To redisperse the recovered C16CA-AgNPs (Figure 4c), the C16CA lamellar precipitates should be dissolved again in a solvent. In the zwitterionic state of C16CA at pH 4 (Figure 1b), the amine groups are protonated and tend to neutralize the anionic charge of the carboxylate groups by ion complexation.28 Thus, the hydrophilicity of C16CA decreased around the isoelectric point of pH ∼ 3.5. It was also noted that the strong molecular packing of the long alkyl chains (C16H33) of C16CA induced lamellar structure formation in water. It is assumed that the breaking of these interactions leads to the resolution of the C16CA lamellar precipitates, which affects the redispersion of the C16CA-AgNPs. Initially, we tried the redispersion of AgNPs in water by adjusting the pH of the solution. Upon the addition of 0.1 M aq. NaOH to the recovered C16CA-AgNP precipitates, the lamellar precipitates dissolved in water due to the breaking up of the ion complexation by deprotonation of the amine groups. The AgNPs redispersed in water at pH 10 similar to our previous work on the recovery and redispersion of Au nanocrystals.28,29 TEM observation revealed that the redispersed AgNPs have a diameter of 11.7 nm (Figure 5a). SAED patterns show a typical polycrystalline fcc structure of Ag,7,8,47,48 which did not change throughout the recovery− redispersion procedure (Figure 5b and c). The UV−vis spectra show the SP band of the AgNPs at 400 nm, which is the same as in the case of the initial dispersion before the recovery procedure (Figure 5). These results indicate that the morphology of the AgNPs did not change throughout the recovery−redispersion procedure, and the AgNPs rarely coagulated. The SP band area of the redispersed AgNPs was 87% of that of the initial dispersion. This phenomenon resembled the case of AuNPs in our previous work, though any shoulder peaks which imply the presence of NPs aggregation were not observed in AgNPs spectra. Thus, C16CA was thought to be a better stabilizer on Ag, and almost all of the AgNPs were recovered and redispersed in water by a simple pH regulation. Next, the resolution of the C16CA lamellar precipitates in an organic solvent was attempted, prior to the redispersion of the recovered C16CA-AgNPs (Figure 4c) in organic solvents. The solubility of anionic and zwitterionic C16CA in several organic solvents was investigated, and the results are given in Table 1. The C16CA lamellar precipitates dissolved in methanol, chloroform, and THF. However, they did not dissolve in

Figure 3. UV−vis spectra of the (a) as-prepared AgNPs dispersion, (b) initial C16CA-AgNPs dispersion, and (c) filtrate after removal of the C16CA-AgNP precipitate.

400 nm, indicating a typical SP band for monodispersed AgNPs. Upon the addition of aq. C16CA to obtain the C16CAcapped AgNPs (C16CA-AgNPs) dispersion, the SP band did not change from ∼400 nm (Figure 3b). TEM images also indicated that the diameter of ∼10.7 nm did not change upon functionalization of the AgNPs with C16CA (Figure 2b). The C16CA-AgNPs were well dispersed without any morphological change. The dimers of AgNPs were formed via a drying process of the TEM mesh grid and did not generate in the dispersion. This was in good agreement with the SP band results in the UV−vis spectra (Figure 3). Recovery and redispersion was carried out for the C16CAAgNPs. The light yellow dispersion (Figure 4a) initially showed a pH value of 8. On adjusting the pH value to 4 by the addition of 0.1 M HCl, lamellar assemblies precipitated immediately from the solution (Figure 4b) due to the protonation of the C16CA amine groups (Figure 1).28,29 The filtration afforded a colorless upper solution (Figures 3c and 4d), while the lamellar precipitates were yellowish in color (Figure 4c). These results indicated that almost all of the AgNPs were incorporated into the lamellar assembly. Thus, the recovery procedure using C16CA is also effective in the case of the Ag nanocrystals due to the interactions between the carboxyl and amine groups of the C16CA amphiphiles and the Ag surface. C

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Figure 6. FT-IR spectra of the C16CA methylene chain in (a) water, (b) chloroform, and (c) methanol. Narrow lines show anionic C16CA, and bold lines show zwitterionic C16CA.

In accordance with the solubility results, the recovered C16CA-AgNP precipitates dissolved in chloroform, THF, and methanol. The C16CA-AgNPs redispersed easily in chloroform, as shown in Figure 7a. The SP band of redispersion at

Figure 5. (a) UV−vis spectra and (inset) photographs and TEM images of the redispersed C16CA-AgNPs in aqueous media at pH 10. SAED patterns of the (b) as-prepared and (c) redispersed C16CAAgNPs.

Table 1. Solubility of Anionic and Zwitterionic C16CA for Various Solventsa solvent water chloroform THF methanol ethanol acetone toluene n-hexane n-octane a

anionic C16CA sol (pH insol insol partially partially insol partially insol insol

11)

insol insol insol

zwitterionic C16CA insol (pH 4) sol sol sol insol insol insol insol insol

Figure 7. Photographs of the redispersion of AgNPs in (a) chloroform, (b) THF, and (c) methanol.

sol, soluble; insol, insoluble.

toluene, acetone, and ethanol. The FT-IR spectra for the methylene chain band of the anionic and zwitterionic C16CA in each solvent are shown in Figure 6. In the lamellar assembly state of the zwitterionic C16CA in water (Figure 6a), typical absorption peaks at ∼2917 and ∼2850 cm−1 originating from the antisymmetric and symmetric stretching of the CH2 methylene chain were found, indicating that the hydrocarbon chain was in an all-trans state and was well-packed in the lamellar structure.28,49,50 In the sol state of the zwitterionic C16CA in organic solvents (Figure 6b and c), these bands were shifted to high wavenumbers (∼2927 and ∼2855 cm−1) similar to the aqueous solution at pH 10 (Figure 6a). It is to be noted that the melting of the hydrocarbon chain could be monitored by these methylene chain bands. The bands in methanol were slightly shifted to 2928 and 2858 cm−1 in synchronization with micellar deformation, indicating that the zwitterionic C16CA melted more in methanol compared to chloroform.

Figure 8. UV−vis spectra of the redispersion of AgNPs in (a) chloroform, (b) THF, and (c) methanol.

∼400 nm (Figure 8a) was slightly shifted from that of the initial C16CA-AgNPs dispersion due to solvent change. However, the SP band area was 80% of that of the initial dispersion. TEM observations revealed the spherical morphology of the particles with a diameter of 11.5 nm (Figure 9a), and fcc structure for Ag was confirmed by the SAED patterns of the redispersed AgNPs in chloroform (Figure 10a). These results indicate that the D

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Figure 9. TEM images of redispersed AgNPs in (a) chloroform and (b) THF. (c) Optical microscopic image of the aggregates of AgNPs in methanol.

Figure 10. SAED patterns of redispersed AgNPs in (a) chloroform and (b) THF.

AgNPs did not change from that of initial AgNPs without aggregation. In the case of THF, the C16CA-AgNPs also redispersed well retaining their morphological characteristics with a spherical diameter of 10.8 nm (Figures 7b, 9b, and 10b). However, there was a decrease in the SP band area to 50% (Figure 8b). Further, for methanol, the C16CA lamellae dissolved but the recovered AgNPs did not redisperse (Figures 7c and 9c), and the UV−vis spectra of the methanol solution (Figure 8c) did not show the SP band of AgNPs. This depression of the SP band implied the deposition of the aggregated AgNPs. If the AgNPs were well-capped with the C16CA molecular layer, the hydrophobic alkyl chains would allow the redispersion of the AgNPs in methanol without aggregation. In general, the aggregation of nanocrystals is likely to occur upon the removal of the capping agent.40,51 The variations in the redispersibility of the nanoparticles in the different organic solvents indicate that the adsorption property of C16CA depends on the solvent. Thus, the adsorption property of C16CA on the Ag surface was detected in alkaline aqueous media, chloroform, THF, and methanol. The adsorption of C16CA on Ag substrates in various solvents was analyzed using a quartz crystal microbalance (QCM). The adsorbed amounts were examined for anionic C16CA (Figure 1a) in alkaline water and for zwitterionic C16CA (Figure 1b) in chloroform, THF, and methanol, respectively. Figure 11 shows the adsorption isotherms for C16CA on a Ag surface. Each isotherm reached a plateau region where the adsorption saturated, around the cmc of the amphiphile.52,53 Table 2 shows the adsorbed mass and adsorbed mole at the saturated adsorption point for each solvent. The adsorbed mole in chloroform and THF solution was around 5−8 μmol/m2, which is similar to the case of a well-adsorbed monolayer of the zwitterionic surfactant at the air−water or solid−water

Figure 11. Adsorption isotherms on the Ag substrate of anionic C16CA in water and of zwitterionic C16CA in chloroform, THF, and methanol solution at 27 °C.

interface.54−57 This indicates that the C16CA molecules were densely packed on the Ag surface. This could be due to the strong interactions of the carboxyl groups with the Ag surface, and the electrostatic interactions in zwitterionic structure.54,55 The adsorbed mass in water at pH 11 was almost twice that in the chloroform solution. It was also found that the amount of C16CA ligand on the water-redispersed AgNPs was twice that on the chloroform-redispersed AgNPs (see the Supporting Information). This indicates that a bilayer structure58,59 was formed on the AgNPs surface. On the other hand, in the methanol solutions, the adsorption was lower and an almost flat isotherm was obtained. These results indicate that poor adsorption layers were formed in these solvents compared to the chloroform and water solutions. These differences in adsorption properties are related to the solvent affinity of C16CA. As shown in Figure 6, the zwitterionic C16CA molecule melted well in methanol, and therefore, the molecule tends to desorb from the Ag surface in methanol solutions. It was observed that the stability and dispersibility of the C16CA-AgNPs in solvents could be correlated to the adsorption property on Ag. The bilayered adsorption of the anionic C16CA in water made the AgNP surface hydrophilic (Scheme 1b) and allowed their redispersion in water. However, the monolayered adsorption of the zwitterionic C16CA in organic solvents exposed the hydrophobic alkyl chains (Scheme 1c) that help to redisperse the AgNPs into chloroform and THF. The poor adsorption (Scheme 1d) of C16CA inhibited E

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Table 2. Adsorbed Amount of C16CA on the Ag Surface in Water, Chloroform, THF, and Methanol Solution at 27 °C surfactant

solvent

ΔF (Hz)

adsorbed mass (ng·cm−2)

adsorbed mole (μmol·m−2)

anionic C16CA zwitterionic C16CA

water (pH 11) chloroform THF methanol

−93 −52 −32 −5

507 280 180 27

13.2 7.8 4.6 0.7

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Scheme 1. Illustration of the Proposed Mechanism for the Redispersion of AgNPs from C16CA Lamellar Precipitates

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01753. The differential scanning calorimetry of the redispersed C16CA-AgNPs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. the redispersion of the AgNPs and promoted aggregation due to the exposure of the Ag surface through dissolution of the lamellae. These results indicate that the C16CA adsorption layer on the Ag surface switches according to the protonation of amine groups and the solubility of the surfactant. These variations in solubility have been effectively applied to the multiple solvation of AgNPs. Thus, C16CA has a superior ability for promoting the recovery and redispersion of the nanocrystals in both aqueous and organic solvents.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (Nos. 24510144 and 25886013) and the Institute of Science and Engineering of Chuo University.

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REFERENCES

(1) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics? Angew. Chem., Int. Ed. 2009, 48, 60−103. (2) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586. (3) Jain, P.; Huang, X.; El-Sayed, I.; El-Sayed, M. Review of Some Interesting Surface Plasmon Resonance-enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems. Plasmonics 2007, 2, 107−118. (4) Hu, M.; Chen, J.; Li, Z.-Y.; Au, L.; Hartland, G. V.; Li, X.; Marquez, M.; Xia, Y. Gold nanostructures: engineering their plasmonic properties for biomedical applications. Chem. Soc. Rev. 2006, 35, 1084−1094. (5) Cobley, C. M.; Chen, J.; Cho, E. C.; Wang, L. V.; Xia, Y. Gold nanostructures: a class of multifunctional materials for biomedical applications. Chem. Soc. Rev. 2011, 40, 44−56. (6) Tsuji, M.; Maeda, Y.; Hikino, S.; Kumagae, H.; Matsunaga, M.; Tang, X.-L.; Matsuo, R.; Ogino, M.; Jiang, P. Shape Evolution of Octahedral and Triangular Platelike Silver Nanocrystals from Cubic and Right Bipyramidal Seeds in DMF. Cryst. Growth Des. 2009, 9, 4700−4705. (7) Zeng, J.; Zheng, Y.; Rycenga, M.; Tao, J.; Li, Z.-Y.; Zhang, Q.; Zhu, Y.; Xia, Y. Controlling the Shapes of Silver Nanocrystals with Different Capping Agents. J. Am. Chem. Soc. 2010, 132, 8552−8553. (8) Wiley, B.; Sun, Y. G.; Mayers, B.; Xia, Y. N. Shape-controlled synthesis of metal nanostructures: The case of silver. Chem. - Eur. J. 2005, 11, 454−463.

CONCLUSION

In this work, the recovery−redispersion procedure using C16CA lamellar assemblies was applied to AgNPs via strong adsorption of the carboxyl groups. Almost all of the AgNPs were recovered into the lamellar assembly of zwitterionic C16CA at pH 4, and easily redispersed in water at pH 10 due to the dissolution of the C16CA lamellae. The recovered AgNPs in C16CA lamellae also redispersed in organic solvents such as chloroform and THF. This redispersion of the AgNPs depended on the adsorption property of C16CA on the Ag surface, which depended on the molecular charge and was closely related to the solvent affinity. Monolayered adsorption leads to a hydrophobic surface allowing the redispersion of AgNPs in organic solvents. On the other hand, bilayered adsorption leads to a hydrophilic surface allowing the redispersion of the particles in aqueous media. Thus, C16CA has great potential not only in extending the recovery system of Ag nanocrystals but also in facilitating solvent exchange to both aqueous and organic solvents. An additional benefit of C16CA is that the noncovalent adsorption on the NP surface allows further ligand exchange in a suitable solvent. Our results would contribute to the development of methods for further functionalization and novel applications of nanocrystals. F

DOI: 10.1021/acs.langmuir.6b01753 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b01753 Langmuir XXXX, XXX, XXX−XXX