Article pubs.acs.org/Langmuir
Ligand Shell Composition-Dependent Effects on the Apparent Hydrophobicity and Film Behavior of Gold Nanoparticles at the Air− Water Interface Stephen M. Bradford, Elizabeth A. Fisher, and M.-Vicki Meli* Department of Chemistry and Biochemistry, Mount Allison University, 63C York Street, Sackville, New Brunswick E4L 1E2, Canada S Supporting Information *
ABSTRACT: Nanoparticles with well-defined interfacial energy and wetting properties are needed for a broad range of applications involving nanoparticle self-assembly including the formation of superlattices, stability of Pickering emulsions, and for the control of nanoparticle interactions with biological membranes. Theoretical, simulated, and recent experimental studies have found nanometer-scale chemical heterogeneity to have important effects on hydrophobic interactions. Here we report the study of 4 nm gold nanoparticles with compositionally well-defined mixed ligand shells of hydroxyl−(OH) and methyl−(CH3) terminated alkylthiols as Langmuir films. Compositions ranging from 0−25% hydroxyl were examined and reveal nonmonotonic changes in particle hydrophobicity at the air−water interface. Unlike nanoparticles capped exclusively with a methyl-terminated alkylthiol, extensive particle aggregation is found for ligand shells containing 98.5%), chloroform (HPLC-grade), 11-mercapto-1-undecanol (97%), dodecanethiol (>98%), 4-(dimethylamino)pyridine (DMAP, 99%), hexanes (reagent grade), acetic acid (99.7%), and Sephadex LH20 was obtained from Sigma-Aldrich Canada. Toluene (99.5%) was obtained from Caledon Laboratory Chemicals. Ethanol (95% and anhydrous) was obtained from Commercial Alcohols Canada. Chloroform-d and ethanol-d6 were obtained from Cambridge Isotope Laboratories, Inc. All chemicals were used as received. Deionized water with a resistivity of 18.2 MΩ cm was obtained from an Elga Purelab UHQ filtration system. Carbon and Formvar-coated 400 mesh copper grids for transmission electron microscopy (TEM) were obtained from Ladd Research Industries, Inc. Gold Nanoparticle Synthesis. In order to synthesize 4−6 nm particles capped with varying amounts of OH-terminated thiol, the method developed by Rucareanu et al. was used.37 First, DMAPprotected gold nanoparticles were prepared by creating a 0.03 M solution of HAuCl4 in 40 mL of deionized water and adding this solution to a 0.05 M solution of TOAB in 100 mL of toluene.38 The two-phases were stirred until the gold transferred to the organic phase. 9791
DOI: 10.1021/acs.langmuir.6b02238 Langmuir 2016, 32, 9790−9796
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Langmuir
Figure 1. Representative TEM images of the AuNPs during compression at 5 mN/m (left) and 30 mN/m (middle), and at 5 mN/m upon expansion (right). (A−C) CH3−AuNP, (D−F) OH-AuNPI, (G−I) OH-AuNPII, and (J−L) OH-AuNPIII. Scale bar represents 50 nm. (A, J insets) Lower magnification view of film morphology and island domains prior to compression, scale bar 200 nm. (H, K insets) Lower magnification view of film morphology and monolayer collapse at 35 mN/m, scale bars 100 and 200 nm, respectively. mg/mL nanoparticles in chloroform were prepared, with the exception of the OH-AuNPIII samples which required initial dissolution in 200 μL of anhydrous ethanol before diluting to 2.0 mL with chloroform. Fresh solutions were deposited onto the air−water interface using ∼3 μL drops every 10 s. After approximately 200 μL was spread, the films were allowed to equilibrate for 20 min. The interface was reduced at a rate of 15 cm2/min until a collapse in the films was observed. To test the hysteresis of the compression−expansion process, films were compressed to 35 mN/m and then re-expanded at the same rate after a 5 s pause. All results presented have been reproduced in triplicate at a minimum. Transmission Electron Microscopy (TEM). Samples of the Langmuir films were collected during both the compression and expansion isotherms at various surface pressures (i.e., 5, 15, and 35 mN/m) via horizontal lift-off onto TEM grids. To obtain samples for measuring the nanoparticle size distribution, a (
