Surface Chemistry Characterization of Conjugated Gold Nanoparticles

This work examines the surface chemistry of conjugated gold nanoparticles, where the molecular modifiers are linked to the gold NP via a gold-sulfur bond, including obtaining information on the bonding environment, the surface coverage of conjugate molecules and the stability of the Au-S bond.

Intended Impact

This research provides surface chemistry property data on engineered gold nanoparticle materials for use in clinical diagnostic and therapeutic applications. Surface chemistry data, including chemical composition, chemical bonding, and susceptibility to oxidation will aid in the correct interpretation of in vitro and in vivo assay results and will lead to safe and efficacious engineered nanomaterial products.

Objectives

• To develop metrology tools to evaluate the surface chemical composition of nano-particle materials intended for clinical therapeutic or diagnostic applications.
  
• To disseminate both measurement method protocols and data on study materials to facilitate the translation of engineered nanoparticles into the clinical realm.

Technical approach

	Scheme 1

Surface analysis techniques, proven in the characterization of self-assembled monolayers of thiolated molecules on planar gold surfaces, were adapted to the evaluation of conjugated gold nanoparticles. This approach included optimizing sampling techniques to probe the chemical bonding environment of the conjugated molecules to the nanoparticle surface using X-ray photoelectron spectroscopy (XPS). This techniques was applied to gold nanoparticle study materials with simple linear molecular modifiers and extended to more complex branched dendron modifiers (Scheme 1).

XPS spectroscopy was used to probe the chemical bonding environment between the sulfur atom of the molecular modifier and the gold atom surface. Photoelectrons, originating in the S 2p_3/2 and S 2p_1/2 orbitals were examined. Photoelectrons with binding energies of 161 eV / 162 eV are associated with sulfur atoms bound to the gold surface. Photoelectrons shifted to higher binding energies are associated with unbound sulfur atoms (163 eV / 164 eV) and with oxidized sulfur atoms (168 eV / 169 eV).

Gold nanoparticle samples analyzed within 24 hours of preparation showed mainly bound sulfur-gold photoelectrons. Aged samples showed evidence of oxidation of the sulfur atom and a decrease in the amount of sulfur bound to the gold surface. The relative amount of unbound sulfur atoms was found to be higher for the G2 dendron samples. The intensity of the bound sulfur 2p_3/2 peak was compared to the intensity of the gold 4f_7/2 peak to determine the relative surface coverage of modifier on the Au surface. We compared sulfur/gold ratios measured on the nanoparticle samples to those measured on reference samples composed of alkanethiol monolayers (HS-(CH₂)_n-CH₃, where n = 6,8,10,12,14,16,18) on planar gold. Relative percentage coverages determined for the dendron modified NP samples are listed in Table 1.

This data indicates that the G1 dendron modifier is able to pack more tightly on the curved NP surface as compared to a planar surface and that the G2 dendron modifier is unable to pack as tightly, possibly due to repulsion between dendron moieties.

Recent Publications

Gas-phase ion-mobility characterization of SAM-functionalized Au nanoparticles

• Developed sampling techniques for XPS of Au NPs.
• Characterized chemical bonding environment of conjugate molecules to Au NP surface.
• Evaluated surface coverage and stability of molecular bond to Au NP surface.
• Demonstrated that XPS can be used to quantitatively characterize gold nanoparticle surface coverage