The Molecular Footprint of Peptides on the Surface of Ultrasmall Gold Nanoparticles (2 nm) Is Governed by Steric Demand

Abstract

Ultrasmall gold nanoparticles were functionalized with peptides of two to seven amino acids that contained one cysteine molecule as anchor via a thiol−gold bond and a number of alanine residues as nonbinding amino acid. The cysteine was located either in the center of the molecule or at the end (C-terminus). For comparison, gold nanoparticles were also functionalized with cysteine alone. The particles were characterized by UV spectroscopy, differential centrifugal sedimentation (DCS), highresolution transmission electron microscopy (HRTEM), and small-angle X-ray scattering (SAXS). This confirmed the uniform metal core (2 nm diameter). The hydrodynamic diameter was probed by 1 H-DOSY NMR spectroscopy and showed an increase in thickness of the hydrated peptide layer with increasing peptide size (up to 1.4 nm for heptapeptides; 0.20 nm per amino acid in the peptide). 1 H NMR spectroscopy of water-dispersed nanoparticles showed the integrity of the peptides and the effect of the metal core on the peptide. Notably, the NMR signals were very broad near the metal surface and became increasingly narrow in a distance. In particular, the methyl groups of alanine can be used as probe for the resolution of the NMR spectra. The number of peptide ligands on each nanoparticle was determined using quantitative 1 H NMR spectroscopy. It decreased with increasing peptide length from about 100 for a dipeptide to about 12 for a heptapeptide, resulting in an increase of the molecular footprint from about 0.1 to 1.1 nm 2 .

Simulated HRTEM images of nanoparticles to train a neural network to classify nanoparticles for crystallinity

“…It should be emphasized that gold nanoparticles represent a particularly good system for this approach because they have a high electron contrast and do not tend to be oxidized. 38 Therefore, this analysis was possible even for the challenging case of ultrasmall nanoparticles (1–3 nm). This approach will become easier for larger particles (like plasmonic particles), but more difficult for metal particles of lighter elements like silver or the light platinum metals.…”

Possibilities and limitations of solution-state NMR spectroscopy to analyze the ligand shell of ultrasmall metal nanoparticles

“…With longer distance from the surface, the mobility of the molecules increases and structural inhomogeneities are averaged out. 52,65–67 In addition, transient magnetic fields causing spin–spin relaxation ( T 2 relaxation) must be taken into account as another relaxation pathway resulting in larger 1 H and 13 C linewidths for molecules on metallic particles larger than 3 nm. 68 Therefore, the chances to get direct insight into the surface structure are diminishing at larger particle sizes, but focusing on ultrasmall nanoparticles potentially allows to reach this goal.…”

“…the area covered (or needed) by each ligand on the nanoparticle surface. The molecular footprint is often surprisingly small with 0.1 nm 2 per ligand or less, 37,50,67,114 indicating a high density of ligands on the particle surface facilitated by the high surface curvature of the ultrasmall nanoparticles. 53 Wu et al have quantified the ligand shell consisting of (16-mercaptohexadecyl)trimethylammonium bromide on gold nanoparticles from 1.2 to 25 nm by quantitative 1 H NMR spectroscopy both in dispersion and after reductive detachment of the ligands.…”