Amino Acid-Based Stabilization of Oxide Nanocrystals in Polar Media: From Insight in Ligand Exchange to Solution ¹H NMR Probing of Short-Chained Adsorbates

Abstract

1 AbstractLigand exchange is a crucial step between nanocrystal synthesis and nanocrystal application. Although colloidal stability and ligand exchange in nonpolar media is readily established, the exchange of native, hydrophobic ligands with polar ligands is less systematic. In this paper, we present a versatile ligand exchange strategy for the phase transfer of carboxylic acid capped HfO 2 and ZrO 2 nanocrystals to various polar solvents, based on small amino acids as the incoming ligand. To gain insight in the fundamental mechanism of the exchange, we study this system with a combination of FTIR, zeta potential measurements and solution 1 H NMR techniques. The detection of surface-associated, small ligands with solution NMR proves challenging in this respect. Tightly bound amino acids are undetectable but their existence can be proven through displacement with other ligands in titration experiments. Alternatively, we find that methyl moieties belonging to bound species can circumvent these limitations because of their more favorable relaxation properties as a result of internal mobility.

Insights into the Ligand Shell, Coordination Mode, and Reactivity of Carboxylic Acid Capped Metal Oxide Nanocrystals

“…This finding is not unexpected, because tightly bound surfactants have a significantly reduced mobility, inducing line broadening in NMR spectra, and the closer the protons are to the surface, the more predominant this effect becomes. Therefore, the first CH 2 units next to a functional group (e.g., carboxylic acid) are broadened beyond detection, and small ligands can even be completely invisible in solution 1 H NMR spectroscopy …”

“…Unfortunately, the surface chemistry was often inferred indirectly, because proton or carbon nuclei close to or on the surface are not detectable through solution 1 H NMR spectroscopy . Although derivatization and ligand exchange are commonly used to characterize ligands, they are invasive, ligand‐specific processes, which may have uncontrolled effects such as surface oxidation or residual water adsorption .…”

Mechanistic Aspects in the Formation, Growth and Surface Functionalization of Metal Oxide Nanoparticles in Organic Solvents

“…De Roo et al studied the mechanism of amino acid based ligand exchange reactionsf or the phase transfer of carboxylic acid capped HfO 2 and ZrO 2 nanocrystalst ov arious polar solvents using ac ombination of FTIR, zeta potentialm easurements, and 1 HNMR techniques. [278] They found that as trong acid was necessary to protonate the original carboxylic acidl igand at the nanocrystal surface, after which ap ositivelyc harged amino acid bound to the surface, stabilizing the dispersion electrostatically.H owever,as mall amount of remaining carboxylic acid provided an additional steric contribution to the colloidalstabilization.…”

Superconducting YBa2Cu3O7–δ Nanocomposites Using Preformed ZrO2 Nanocrystals: Growth Mechanisms and Vortex Pinning Properties

“…For this, additional studies should be extended to lower temperatures (20-30 K or even 4.2 K) as this will determine the YBCO nanocomposites' possibilities for high fi eld magnet applications. [ 11,12 ] Yet, we do believe in the potential of this new approach, using PNCs, as it offers the control over the pinning size and morphology, which will ultimately control the J C . Reactivity should be avoided to obtain complete control over the addition of the PNCs, as otherwise competition between growth kinetics of the PNCs and YBCO can occur.…”

“…Bretos et al described the introduction of amorphous BaZrO 3 nanoparticles in YBCO. [ 11 ] The ZrO 2 PNCs can be stabilized in high concentration, up to 50 mol%, for at least 2.5 months in a trifl uoroacetic acid (TFA) YBCO precursor solution, by using the combination of a strong organic acid and an amino acid, [ 12 ] confi rmed by dynamic light scattering analysis. More recently, we described the introduction of preformed CeO 2 and ZrO 2 NCs to the YBCO fi lm.…”