Effect of Postsynthesis Purifications on Gold and Silver Nanoparticle Ligand Coverage

Abstract

Wet chemical nanoparticle synthesis is commonly employed because of the ability to tailor and control nanoparticle size, shape, polydispersity, and surface chemistry; however, excess ligands or free surfactants in the colloidal dispersion can be detrimental for many nanoparticle applications. Postsynthesis purification using antisolvent precipitation is a widely employed method to remove the excess ligands or precursors; however, there has been little in-depth fundamental investigation of the dynamics between the nanoparticle, ligand, and dispersing media as well as the morphology and fate of the nanoparticle and ligands during nanoparticle processing. In this paper, we investigate the changes in ligand surface coverage for dodecanethiol-stabilized gold and silver nanoparticles in response to repetitive antisolvent precipitation and redispersion using gas chromatography, thermogravimetric analysis, and small-angle neutron scattering. These techniques were each used to determine percent surface coverage and equilibrium ligand partitioning between the nanoparticle surface and bulk solution, which was then modeled with the Langmuir isotherm to determine the binding free energy. The binding free energy for dodecanethiol on 4.2 nm diameter gold nanoparticles was found to be −23 kJ/mol by gas chromatography (GC) and −34 kJ/mol by small-angle neutron scattering (SANS). The binding free energy for dodecanethiol on 7.7 nm diameter silver nanoparticles was found to be −21 kJ/mol by TGA and −29 kJ/mol by SANS. While these numbers demonstrate variability based on the method, they are comparable to literature values. Other notable results from this work demonstrate the optimization of the purification process and avoidance of using excess antisolvent which can lead to coprecipitation of the excess ligand with the nanoparticles, hindering the purification. Finally, multiple techniques for determining ligand binding free energy are demonstrated as well as evaluation of the pros and cons of each method.

Selected Standard Protocols for the Synthesis, Phase Transfer, and Characterization of Inorganic Colloidal Nanoparticles

“…This size selective precipitation allows for collecting NPs with similar sizes and discarding extremely small NPs (Figure 1.2.9) 17 . It is worth to note here that the use of excess methanol should be avoided, since this will coprecipitate excess ligand molecules in solution along with the collected NPs, and thus hinder the purification 18 . Synthesis of Au NPs in presence of decanoic acid: Decanoic acid-stabilized Au NPs ( 7 nm core diameter): For this purpose, a solution of 43 mg (250 µmol) decanoic acid (Sigma Aldrich, #C1875) dissolved in 2.5 mL toluene, and a solution of 25 mg (97.2 mmol) TBAB dissolved in 1 mL DDAB stock solution (100 mM in toluene) were added in a 25 mL flask.…”

“…Purification on the other hand also may change the properties of the NPs, e.g. modify their ligand shell 18 .…”

Extremely robust and post-functionalizable gold nanoparticles coated with calix[4]arenes via metal–carbon bonds

“…Reported strategies for the controlled formation of monolayers are mainly based on the introduction of bulky protecting groups. 8 We have developed a different strategy that relies on the grafting of calix [4]arenes bearing diazonium groups at their large rim. 9 Thanks to the well pre-organized and 3D conic structure of the calix [4]arene core, densely packed monolayers and mixed binary monolayers were obtained on flat surfaces, with a good control of the surface composition and a high homogeneity of the deposit.…”

“…8 We have developed a different strategy that relies on the grafting of calix [4]arenes bearing diazonium groups at their large rim. 9 Thanks to the well pre-organized and 3D conic structure of the calix [4]arene core, densely packed monolayers and mixed binary monolayers were obtained on flat surfaces, with a good control of the surface composition and a high homogeneity of the deposit. 10 Provided that the small rim of the calix [4]arene is decorated with appending reactive arms, like carboxylic groups, the immobilized calix [4]arene moieties can be easily post-functionalized with various molecules.…”

Size Matters: Influence of Gold‐to‐Ligand Ratio and Sulfur‐Sulfur Distance of Linear Thioether Heptamers on the Size of Gold Nanoparticles

“…They showed that by altering the molar ratio between the ligand (dodecanethiols) and the gold salt (HAuCl 4 ), and even the temperature or the rate at which the reduction is conducted, AuNPs between 1.5 and 5.2 nm can be synthesized. Since then, many different modifications of the two‐phase Brust–Schiffrin method focusing on different sizes by altering ligand/Au ratio were published as well as by altering the pH‐value . In addition, the work of Hussain et al .…”