Ligand displacement exposes binding site heterogeneity on CdSe nanocrystal surfaces

Nanocrystal ligand interactions and ligand exchange processes are usually described by a uniform distribution of equal binding sites. Here, we analyze this assumption by a quantitative study of the displacement of Z-type cadmium oleate ligands from CdSe nanocrystals by addition of L-type ligands. First, we determined the stoichiometry of the displacement reaction by analyzing the equilibrium upon dilution using solution nuclear magnetic resonance spectroscopy. We found that 1 equiv of tetramethylethylene-I,2-diamine (TMEDA) or 2 equiv of n-butylamine or benzylamine bind to the displaced cadmium oleate. We only reached a comprehensive description of the displacement isotherm by including two types of binding sites with a different equilibrium constant. We corroborated this finding by density functional theory calculations on a CdSe model nanocrystal, which show that even single facets contain a broad variety of binding sites. Finally, we analyzed the thermodynamics of the displacement equilibrium for the weaker binding sites by constructing van't Hoff plots for the different displacers. Whereas displacement with TMEDA appears to be enthalpically neutral, it is entropically favorable. In contrast, displacement with the primary amines is entropically unfavorable but is associated with a negative change in enthalpy. Since the distribution of binding energy emanates from the large fraction of edge and vertex sites on a nanocrystal facet, these findings are most likely inherent to nanocrystals in general and should be considered when analyzing surface reactions on such materials

Probing solvent-ligand interactions in colloidal nanocrystals by the NMR line broadening

Although solvent-ligand interactions play a major role in nanocrystal synthesis, dispersion formulation, and assembly, there is currently no direct method to study this. Here we examine the broadening of H-1 NMR resonances associated with bound ligands and turn this poorly understood descriptor into a tool to assess solvent-ligand interactions. We show that the line broadening has both a homogeneous and a heterogeneous component. The former is nanocrystal-size dependent, and the latter results from solvent-ligand interactions. Our model is supported by experimental and theoretical evidence that correlates broad NMR lines with poor ligand solvation. This correlation is found across a wide range of solvents, extending from water to hexane, for both hydrophobic and hydrophilic ligand types, and for a multitude of oxide, sulfide, and selenide nanocrystals. Our findings thus put forward NMR line-shape analysis as an indispensable tool to form, investigate, and manipulate nanocolloids

The temperature dependence of L-type promoted Z-type ligand displacement on oleate-terminated semiconductor nanocrystals observed through NMR

Ligand exchange is central in the processing of metal oxides and semiconductor nanocrystals (NCs) and requires understanding of surface chemistry. In this respect, the Covalent Bond Classification provides an apt framework, see Figure 1. This framework is most useful in order to describe recently developed ligand displacement schemes where association reactions are used to promote ligand desorption. Examples include the displacement of X2 carboxylic acids from HfO2 NCs through ion-pair formation1 and of Z-type metal carboxylates form CdSe or PbS NCs through complexation.2 Such reactions have raised extensive interest since they allow for a fine tuning of the ligand shell composition and could also occur during nanocrystal synthesis. Here, we analyse the thermodynamics of such association driven ligand displacement reactions, i.e. either through ion-pair or complex formation, by temperature dependent NMR. Most importantly, we find that these association reactions can render the overall ligand displacement reaction exothermic or endothermic, implying that the exchange equilibrium shifts to adsorption with increasing or decreasing temperature depending on the nature of the NC. By quantitative 1D 1H NMR, the thermodynamic equilibrium between adsorbed and displaced ligands can be mapped as a function of temperature. This provides us with key thermodynamic quantities such as the reaction enthalpy, which we compare for different materials and different association reactions. These results are most relevant for NC syntheses as they typically proceed at high temperature, where the adsorption of amine can prove negligible or substantial over the binding of metal carboxylates, but also to get NCs with ligand shells that have a finely tuned ligand density

The surprising thermodynamics of L-type promoted Z-type ligand displacement

Ligand exchange is central to the processing of semiconductor nanocrystals (NCs) and requires understanding of their surface chemistry. In this respect, the Covalent Bond Classification provides an apt framework, see Figure 1. This framework is most useful in order to describe recently developed ligand displacement schemes where association reactions are used to promote ligand desorption. For example, the displacement of Z-type metal carboxylates form CdSe or PbS NCs through complexation with an amine.1 Such reactions have raised extensive interest since they allow for a fine tuning of the ligand shell composition and could also occur during nanocrystal synthesis. Here, we analyse the thermodynamics of such association driven ligand displacement reactions, i.e. complex formation, by temperature dependent NMR. Most importantly, we find that depending on the NC, these association reactions can render the overall ligand displacement exothermic or keep it endothermic. The exchange equilibrium thus shifts to adsorption with increasing or decreasing temperature depending on the nature of the NC. By quantitative 1D 1H NMR, the thermodynamic equilibrium between adsorbed and displaced ligands can be mapped as a function of temperature. This provides us with key thermodynamic quantities such as the reaction enthalpy, which we compare on different systems, such as CdSe and PbS NCs. These results are most relevant for NC syntheses as they typically proceed at high temperature, where the understanding of the surface chemistry of NCs is still limited, but also to obtain NCs with ligand shells that have a finely tuned ligand density. Figure 1. The Covalent Bond Classification divides ligands in three groups: L-type (Lewis bases), Z-type (Lewis acids) and X-type (radical). Next to common exchange reaction, L-ligands, such as amines, can displace Z-type ligands from CdSe and PbS NCs

The temperature dependence of L-type promoted Z-type ligand displacement on oleate-terminated semiconductor nanocrystals observed through NMR

Ligand exchange is central in the processing semiconductor nanocrystals (NCs) and requires understanding of surface chemistry. In this respect, the Covalent Bond Classification provides an apt framework. This framework is most useful in order to describe recently developed ligand displacement schemes where association reactions are used to promote ligand desorption. For example, the displacement of Z-type metal carboxylates form CdSe or PbS NCs through complexation.1 Such reactions have raised extensive interest since they allow for a fine tuning of the ligand shell composition and could also occur during nanocrystal synthesis. Here, we analyse the thermodynamics of such association driven ligand displacement reactions, i.e. complex formation, by temperature dependent NMR. Most importantly, we find that these association reactions can render the overall ligand displacement reaction exothermic or endothermic, implying that the exchange equilibrium shifts to adsorption with increasing or decreasing temperature depending on the nature of the NC. By quantitative 1D 1H NMR, the thermodynamic equilibrium between adsorbed and displaced ligands can be mapped as a function of temperature. This provides us with key thermodynamic quantities such as the reaction enthalpy, which we compare for different materials and different association reactions. These results are most relevant for NC syntheses as they typically proceed at high temperature, where the adsorption of amine can prove negligible or substantial over the binding of metal carboxylates, but also to get NCs with ligand shells that have a finely tuned ligand density. References 1. N.C. Anderson, M.P. Hendricks, J.J. Choi, J.S. Owen J. Am. Chem. Soc. 2013, 135 (49), 18536–18548

Oral intake of added titanium dioxide and its nanofraction from food products, food supplements and toothpaste by the Dutch population

Titanium dioxide (TiO2) is commonly applied to enhance the white colour and brightness of food products. TiO2 is also used as white pigment in other products such as toothpaste. A small fraction of the pigment is known to be present as nanoparticles (NPs). Recent studies with TiO2 NPs indicate that these particles can have toxic effects. In this paper, we aimed to estimate the oral intake of TiO2 and its NPs from food, food supplements and toothpaste in the Dutch population aged 2 to over 70 years by combining data on food consumption and supplement intake with concentrations of Ti and TiO2 NPs in food products and supplements. For children aged 2–6 years, additional intake via ingestion of toothpaste was estimated. The mean long-term intake to TiO2 ranges from 0.06 mg/kg bw/day in elderly (70+), 0.17 mg/kg bw/day for 7–69-year-old people, to 0.67 mg/kg bw/day in children (2–6 year old). The estimated mean intake of TiO2 NPs ranges from 0.19 μg/kg bw/day in elderly, 0.55 μg/kg bw/day for 7–69-year-old people, to 2.16 μg/kg bw/day in young children. Ninety-fifth percentile (P95) values are 0.74, 1.61 and 4.16 μg/kg bw/day, respectively. The products contributing most to the TiO2 intake are toothpaste (in young children only), candy, coffee creamer, fine bakery wares and sauces. In a separate publication, the results are used to evaluate whether the presence of TiO2 NPs in these products can pose a human health risk

Oral intake of added titanium dioxide and its nanofraction from food products, food supplements and toothpaste by the Dutch population

Titanium dioxide (TiO2) is commonly applied to enhance the white colour and brightness of food products. TiO2 is also used as white pigment in other products such as toothpaste. A small fraction of the pigment is known to be present as nanoparticles (NPs). Recent studies with TiO2 NPs indicate that these particles can have toxic effects. In this paper, we aimed to estimate the oral intake of TiO2 and its NPs from food, food supplements and toothpaste in the Dutch population aged 2 to over 70 years by combining data on food consumption and supplement intake with concentrations of Ti and TiO2 NPs in food products and supplements. For children aged 2–6 years, additional intake via ingestion of toothpaste was estimated. The mean long-term intake to TiO2 ranges from 0.06 mg/kg bw/day in elderly (70+), 0.17 mg/kg bw/day for 7–69-year-old people, to 0.67 mg/kg bw/day in children (2–6 year old). The estimated mean intake of TiO2 NPs ranges from 0.19 μg/kg bw/day in elderly, 0.55 μg/kg bw/day for 7–69-year-old people, to 2.16 μg/kg bw/day in young children. Ninety-fifth percentile (P95) values are 0.74, 1.61 and 4.16 μg/kg bw/day, respectively. The products contributing most to the TiO2 intake are toothpaste (in young children only), candy, coffee creamer, fine bakery wares and sauces. In a separate publication, the results are used to evaluate whether the presence of TiO2 NPs in these products can pose a human health risk

Oral intake of added titanium dioxide and its nanofraction from food products, food supplements and toothpaste by the Dutch population

Titanium dioxide (TiO2) is commonly applied to enhance the white colour and brightness of food products. TiO2 is also used as white pigment in other products such as toothpaste. A small fraction of the pigment is known to be present as nanoparticles (NPs). Recent studies with TiO2 NPs indicate that these particles can have toxic effects. In this paper, we aimed to estimate the oral intake of TiO2 and its NPs from food, food supplements and toothpaste in the Dutch population aged 2 to over 70 years by combining data on food consumption and supplement intake with concentrations of Ti and TiO2 NPs in food products and supplements. For children aged 2–6 years, additional intake via ingestion of toothpaste was estimated. The mean long-term intake to TiO2 ranges from 0.06 mg/kg bw/day in elderly (70+), 0.17 mg/kg bw/day for 7–69-year-old people, to 0.67 mg/kg bw/day in children (2–6 year old). The estimated mean intake of TiO2 NPs ranges from 0.19 μg/kg bw/day in elderly, 0.55 μg/kg bw/day for 7–69-year-old people, to 2.16 μg/kg bw/day in young children. Ninety-fifth percentile (P95) values are 0.74, 1.61 and 4.16 μg/kg bw/day, respectively. The products contributing most to the TiO2 intake are toothpaste (in young children only), candy, coffee creamer, fine bakery wares and sauces. In a separate publication, the results are used to evaluate whether the presence of TiO2 NPs in these products can pose a human health risk.</p