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We recently introduced monoalkyl phosphinic acids as a ligand class for nanocrystal synthesis. Their metal salts have interesting reactivity differences with respect to metal carboxylates and phosphonates, and provide cleaner work-up compared to phosphonates. However, there is little known about the surface chemistry of nanocrystals with monoalkyl phosphinate ligands. Here, we probe the relative binding affinity of monoalkyl phosphinate ligands with respect to other X-type ligands. We perform competitive ligand exchange reactions with carboxylate and phosphonate ligands at the surface of HfO2, CdSe, and ZnS nanocrystals. We monitor the ligand shell composition by solution 1H and 31P NMR spectroscopy. Using a monoalkyl phosphinic acid with an ether functionality, we gain an additional NMR signature, apart from the typical alkene resonance in oleic acid and oleylphosphonic acid. We find that carboxylate ligands are easily exchanged upon exposure to monoalkyl phosphinic acids, whereas an equilibrium is reached between monoalkyl phosphinates and phosphonates, slightly in the favour of phosphonate (K = 2). Phosphinic acids have thus an intermediate binding affinity between carboxylic acids and phosphonic acids for all nanocrystals studied. These results enable the sophisticated use of monoalkyl phosphinic acids for nanocrystal synthesis and for post-synthetic surface engineering.

ACS nano 16(5), 7361 - 7372 (2022). doi:10.1021/acsnano.1c08966, Ligands play a crucial role in the synthesis of colloidal nanocrystals. Nevertheless, only a handful molecules are currently used, oleic acid being the most typical example. Here, we show that monoalkyl phosphinic acids are another interesting ligand class, forming metal complexes with a reactivity that is intermediate between the traditional carboxylates and phosphonates. We first present the synthesis of n-hexyl, 2-ethylhexyl, n-tetradecyl, n-octadecyl, and oleylphosphinic acid. These compounds are suitable ligands for high-temperature nanocrystal synthesis (240–300 °C) since, in contrast to phosphonic acids, they do not form anhydride oligomers. Consequently, CdSe quantum dots synthesized with octadecylphosphinic acid are conveniently purified, and their UV–vis spectrum is free from background scattering. The CdSe nanocrystals have a low polydispersity and a photoluminescence quantum yield up to 18% (without shell). Furthermore, we could synthesize CdSe and CdS nanorods using phosphinic acid ligands with high shape purity. We conclude that the reactivity toward TOP-S and TOP-Se precursors decreases in the following series: cadmium carboxylate > cadmium phosphinate > cadmium phosphonate. By introducing a third and intermediate class of surfactants, we enhance the versatility of surfactant-assisted syntheses., Published by Soc., Washington, DC

The synthesis and single-crystal X-ray structures of three

Surfactants play a crucial role in the synthesis of colloidal nanocrystals. Nevertheless, only a handful molecules are currently used, oleic acid being the most typical example. Here, we show that mono-alkyl phosphinic acids are an interesting surfactant class with a reactivity that is intermediate between carboxylic acids and phosphonic acids. We first present the synthesis of n-hexyl, 2-ethylhexyl, n-tetradecyl, n-octadecyl, and oleyl phosphinic acid. These compounds are suitable surfactants during high-temperature nanocrystal synthesis (240-300°C). In contrast to phosphonic acids, they do not form poly anhydride gels. Consequently, CdSe quantum dots synthesized with octadecylphosphinic acid are conveniently purified, and are free from background scattering in UV-Vis. The CdSe nanocrystals have a very low polydispersity and a photoluminescence quantum yield up to 18%, without additional shell. Furthermore, we could synthesize CdSe and CdS nanorods using phosphinic acid ligands and found a remarkable purity (i.e. without tetrapod impurities). We conclude that the reactivity towards TOP-S and TOP-Se precursors decreases in the series: cadmium carboxylate > cadmium phosphinate > cadmium phosphonate. By introducing a third and intermediate class of surfactants, we enhance the versatility of surfactant-assisted syntheses.

1-Octadecene is a widely used solvent for high temperature nanocrystal synthesis (120-320 degrees C). Here, we show that 1-octadecene spontaneously polymerizes under these conditions, and the resulting poly(1-octadecene) has a comparable solubility and size to nanocrystals stabilized by hydrophobic ligands. Typical purification procedures (precipitation/redispersion cycles or size exclusion chromatography) fail to separate the poly(1-octadecene) impurity from the nanocrystal product. To avoid formation of poly(1-octadecene), we replace 1-octadecene with saturated, aliphatic solvents. Alternatively, the nanocrystals' native ligands are exchanged for polar ligands, leading to significant solubility differences between nanocrystals and poly(1-octadecene), therefore allowing isolation of pure nanocrystals, free from polymer impurities. These results will help design superior syntheses and improve nanocrystal purity, an important factor in many applications.

1-octadecene is a widely used solvent for high temperature nanocrystal synthesis (120 – 320 °C). Here, we show that 1-octadecene spontaneously polymerizes under these conditions and the resulting poly(1-octadecene) has a comparable solubility and size to nanocrystals stabilized by hydrophobic ligands. Typical purification procedures (precipitation/redispersion cycles or size exclusion chromatography) fail to separate the poly(1-octadecene) impurity from the nanocrystal product. To avoid formation of poly(1-octadecene), we replaced 1-octadecene with saturated, aliphatic solvents. Alternatively, the native ligands are exchanged for polar ligands, leading to significant solubility differences between nanocrystals and poly(1-octadecene), therefore allowing isolation of pure nanocrystals, free from polymer impurities. These results will help design superior syntheses and improve nanocrystal purity, an important factor in many applications.

1-octadecene is a widely used solvent for high temperature nanocrystal synthesis (120 – 320 °C). Here, we show that 1-octadecene spontaneously polymerizes under these conditions and the resulting poly(1-octadecene) has a comparable solubility and size to nanocrystals stabilized by hydrophobic ligands. Typical purification procedures (precipitation/redispersion cycles or size exclusion chromatography) fail to separate the poly(1-octadecene) impurity from the nanocrystal product. To avoid formation of poly(1-octadecene), we replaced 1-octadecene with saturated, aliphatic solvents. Alternatively, the native ligands are exchanged for polar ligands, leading to significant solubility differences between nanocrystals and poly(1-octadecene), therefore allowing isolation of pure nanocrystals, free from polymer impurities. These results will help design superior syntheses and improve nanocrystal purity, an important factor in many applications.

Silica@zirconia core@shell oxide nanoparticles of approx. 50 nm diameter were developed in view of an application as a novel carrier with adjuvant activities for cancer vaccines. To this end, the aim was to cover the surface of the nanocarrier by potent single stranded DNA (ssDNA) type immune stimulators used in cancer vaccination. Prior to characterizing the binding of ssDNA to the nanocarrier surface, the adsorption of deoxynucleoside monophosphate building blocks (dNMPs) was studied. After optimization of conditions for synthesis, solvent exchange and surface modification, the effects and possible interference of several buffers, such as HEPES, PIPES, MOPS and MOPSO, were also investigated. Formation of a new compound was revealed by UV–VIS spectroscopy during the recording of zeta potential vs. pH titration curves in the presence of HEPES and PIPES buffer in the neutral-acidic pH region. We identified this new species as gold ions etched from the electrode of the zeta cuvette, stabilized by the buffers via chelation, which is then followed by their conversion into gold nanoparticles.