Ligand adsorption and exchange on pegylated gold nanoparticles

Authors: ZHANG, DONGMAO - Mississippi State University; SIRIWARDANA, KUMUDU - Mississippi State University; GADOGBE, MANUEL - Mississippi State University; ANSAR, SIYAM - Mississippi State University; ZOU, SHENGLI - Mississippi State University; WALTERS, KEISHA - Mississippi State University

Submitted to: Journal of Physical Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2014
Publication Date: 4/29/2014
Citation: Zhang, D., Siriwardana, K., Gadogbe, M., Ansar, S.M., Zou, S., Walters, K.B. 2014. Ligand adsorption and exchange on pegylated gold nanoparticles. Journal of Physical Chemistry. 118:11111-11119.

Interpretive Summary: In summary, three ligand interactions with AgNPs and AuNPs were studied using a combination of physical and chemical means including surface enhanced Raman spectroscopy. No PADT (a ligand)-induced AuNP or AgNP cross-linking was observed under any of the investigated experimental conditions. The SERS measurements revealed that PADTs on AuNPs and AgNPs exist predominantly as monothiolates but not dithiolates as believed in the literature. Appreciable dithiolation of BDMT and EDT (two other ligands) only appeared on AuNPs when the dithiol concentrations are low. Even under such conditions, the possibility for these dithiols to cross-link two AuNPs as single-molecular dithiolate spacers is likely low. This is because the two BDMT and EDT thiol groups are more likely attached to the same AuNP, instead of bridging two different AuNPs. The latter process is likely kinetically less favorable than the former because of the low AuNP concentration (~nano molar concentrations) used in typical AuNP binding experiments. The findings and insights provided in this work should be important for a wide range of plasmonic nanoparticles studies where organodithiol and SERS measurements are commonly involved. The work should provide the foundation for the development of new technology for the detection of food pathogens using gold nanoparticles.

Technical Abstract: Previous researchers proposed that thiolated poly(ethylene glycol) (PEG-SH) adopts a “mushroom-like” conformation on gold nanoparticles (AuNPs) in water. However, information regarding the size and permeability of the PEG-SH mushroom caps and surface area passivated by the PEG-SH mushroom stems are unavailable. Reported herein is our finding that AuNPs that are covered by saturation packed PEG-SHs all have large fractions of AuNP surface area available for ligand adsorption and exchange. The model ligands adenine and 2-mercaptobenzimidazole (2-MBI) can rapidly penetrate the PEG-SH overlayer and adsorb onto the AuNP surface. Most of the ligand adsorption and exchange occurs within the first minutes of the ligand addition. The fraction of AuNP surface area passivated by saturation packed model PEG-SHs are ~25%, ~20%, and ~9% for PEG-SHs with molecular weights of 2000, 5000, and 30'000 g/mol, respectively. Localized surface plasmonic resonance and dynamic light scattering show that the PEG-SH overlayer is drastically more loosely packed than the protein bovine serum albumin on AuNPs. Studies investigating the effect of aging the AuNP/PEG-SH mixtures on subsequent adenine adsorption onto the pegylated AuNPs revealed that PEG-SHs reach approximately a steady-state binding on AuNPs within 3 h of sample incubation. This work sheds new insights into the kinetics, structures, and conformations of PEG-SHs on AuNPs and demonstrates that pegylated AuNPs can be used as an important platform for studying ligand interaction with AuNPs. In addition, it also opens a new avenue for fabrication of multicomponent functionalized nanoparticles.