Microscopic Origin of the Hofmeister Effect in Gelation Kinetics of Colloidal Silica.

The gelation kinetics of silica nanoparticles is a central process in physical chemistry, yet it is not fully understood. Gelation times are measured to increase by over 4 orders of magnitude, simply changing the monovalent salt species from CsCl to LiCl. This striking effect has no microscopic explanation within current paradigms. The trend is consistent with the Hofmeister series, pointing to short-ranged solvation effects not included in the standard colloidal (DLVO) interaction potential. By implementing a simple form for short-range repulsion within a model that relates the gelation timescale to the colloidal interaction forces, we are able to explain the many orders of magnitude difference in the gelation times at fixed salt concentration. The model allows us to estimate the magnitude of the non-DLVO hydration forces, which dominate the interparticle interactions on the length scale of the hydrated ion diameter. This opens the possibility of finely tuning the gelation time scale of nanoparticles by just adjusting the background electrolyte species.We acknowledge financial support from: Unilever Plc (E.S.); the Ernest Oppenheimer Fellowship at Cambridge (to 1st June 2014), and by the Technische Universität München Institute for Advanced Study, funded by the German Excellence Initiative and the European Union Seventh Framework Programme under grant agreement 291763 (A.Z.); the Winton Programme for the Physics of Sustainability (B.O.C.).This is the accepted manuscript. The final version is available at http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.5b01300

Luminescent nanoclusters of silver and gold

Gold and silver clusters with sizes up to ∼100 atoms have unique properties, such as luminescence, that are not found in bulk materials or in larger nanoparticles. Clusters can be prepared with atomic monodispersity rather than a size distribution, as is common for larger nanoparticles. This allows for correlation between size and properties on an atomic level. This thesis describes the synthesis and characterisation of gold and silver clusters in aqueous solutions. The clusters are capped with lipoic acid, a bidentate ligand which binds to Ag or Au with two sulfur atoms and provides an efficient barrier against aggregation. The silver clusters are luminescent and atomically monodisperse, with 29 Ag atoms and 12 ligands. Over time, the clusters degrade and lose their luminescence, although this process is reversible, which is possibly unique for such Ag clusters. Further investigation into the synthesis procedure of Ag29 found that reduction of silver ions in the presence of lipoic acid initially results in the formation of larger Ag clusters with around 100 atoms. These are then etched by excess ligands to give Ag29. The luminescence efficiency and stability of Ag29 can be enhanced by doping with gold. A singly doped cluster is formed, Au1Ag28. With X-ray spectroscopy, we find that the Au atom preferentially occupies the central position in the cluster. Doping of Ag29 with more than a few percent of Au does not lead to the formation of stable clusters. Pure Au clusters with lipoic acid can be prepared, but these are polydisperse. However, the average size and optical properties of the clusters can be tuned by varying the NaOH concentration during synthesis. A lower NaOH concentration corresponds to a larger average cluster size. The synthesis proceeds in two steps. We find that in the first step of the synthesis, the Au precursor HAuCl4 reacts with lipoic acid resulting in the formation of large, unstable Au nanoparticles. This reaction is inhibited at high NaOH concentrations, where it must compete with hydrolysis of HAuCl4. The resulting AuCl4-xOHx- species (x =3–4) may be less reactive towards LA. The second step in the synthesis of Au clusters involves the reduction of the synthesis intermediates to form clusters. Many of the conclusions presented in this thesis are based on X-ray spectroscopy studies. This technique was used to demonstrate that the synthesis of Ag29 clusters proceeds via larger particles, that the Au atom in Au1Ag28 is preferentially located in the centre of the cluster, and that Au cluster samples with high NaOH concentration contain AuCl4-xOHx- species. A great advantage of X-ray spectroscopy is that it is element-selective. It enables us to study for instance only the Au dopant in bimetallic clusters, or all Ag species during the synthesis of Ag29. Solids, solutions, mixtures and disordered species can all be measured, and no purification is necessary to remove excess salts or ligands as these are invisible in X-ray spectroscopy

Luminescent nanoclusters of silver and gold

Gold and silver clusters with sizes up to ∼100 atoms have unique properties, such as luminescence, that are not found in bulk materials or in larger nanoparticles. Clusters can be prepared with atomic monodispersity rather than a size distribution, as is common for larger nanoparticles. This allows for correlation between size and properties on an atomic level. This thesis describes the synthesis and characterisation of gold and silver clusters in aqueous solutions. The clusters are capped with lipoic acid, a bidentate ligand which binds to Ag or Au with two sulfur atoms and provides an efficient barrier against aggregation. The silver clusters are luminescent and atomically monodisperse, with 29 Ag atoms and 12 ligands. Over time, the clusters degrade and lose their luminescence, although this process is reversible, which is possibly unique for such Ag clusters. Further investigation into the synthesis procedure of Ag29 found that reduction of silver ions in the presence of lipoic acid initially results in the formation of larger Ag clusters with around 100 atoms. These are then etched by excess ligands to give Ag29. The luminescence efficiency and stability of Ag29 can be enhanced by doping with gold. A singly doped cluster is formed, Au1Ag28. With X-ray spectroscopy, we find that the Au atom preferentially occupies the central position in the cluster. Doping of Ag29 with more than a few percent of Au does not lead to the formation of stable clusters. Pure Au clusters with lipoic acid can be prepared, but these are polydisperse. However, the average size and optical properties of the clusters can be tuned by varying the NaOH concentration during synthesis. A lower NaOH concentration corresponds to a larger average cluster size. The synthesis proceeds in two steps. We find that in the first step of the synthesis, the Au precursor HAuCl4 reacts with lipoic acid resulting in the formation of large, unstable Au nanoparticles. This reaction is inhibited at high NaOH concentrations, where it must compete with hydrolysis of HAuCl4. The resulting AuCl4-xOHx- species (x =3–4) may be less reactive towards LA. The second step in the synthesis of Au clusters involves the reduction of the synthesis intermediates to form clusters. Many of the conclusions presented in this thesis are based on X-ray spectroscopy studies. This technique was used to demonstrate that the synthesis of Ag29 clusters proceeds via larger particles, that the Au atom in Au1Ag28 is preferentially located in the centre of the cluster, and that Au cluster samples with high NaOH concentration contain AuCl4-xOHx- species. A great advantage of X-ray spectroscopy is that it is element-selective. It enables us to study for instance only the Au dopant in bimetallic clusters, or all Ag species during the synthesis of Ag29. Solids, solutions, mixtures and disordered species can all be measured, and no purification is necessary to remove excess salts or ligands as these are invisible in X-ray spectroscopy

Spatial distribution of nanocrystals imaged at the liquid-air interface

The 3D distribution of nanocrystals at the liquid-air interface is imaged for the first time on a single-particle level by cryogenic electron tomography, revealing the equilibrium concentration profile from the interface to the bulk of the liquid. When the surface tension of the liquid is decreased, the interaction of the nanocrystals with the liquid-air interface shifts from adsorption to desorption. Macroscopic surface tension measurements do not detect this transition, due to the presence of surface-active molecular species

Single Au Atom Doping of Silver Nanoclusters

Ag29 nanoclusters capped with lipoic acid (LA) can be doped with Au. The doped clusters show enhanced stability and increased luminescence efficiency. We attribute the higher quantum yield to an increase in the rate of radiative decay. With mass spectrometry, the Au-doped clusters were found to consist predominantly of Au1Ag28(LA)12 3-. The clusters were characterized using X-ray absorption spectroscopy at the Au L3-edge. Both the extended absorption fine structure (EXAFS) and the near edge structure (XANES) in combination with electronic structure calculations confirm that the Au dopant is preferentially located in the center of the cluster. A useful XANES spectrum can be recorded for lower concentrations, or in shorter time, than the more commonly used EXAFS. This makes XANES a valuable tool for structural characterization

Single Au Atom Doping of Silver Nanoclusters

Ag29 nanoclusters capped with lipoic acid (LA) can be doped with Au. The doped clusters show enhanced stability and increased luminescence efficiency. We attribute the higher quantum yield to an increase in the rate of radiative decay. With mass spectrometry, the Au-doped clusters were found to consist predominantly of Au1Ag28(LA)12 3-. The clusters were characterized using X-ray absorption spectroscopy at the Au L3-edge. Both the extended absorption fine structure (EXAFS) and the near edge structure (XANES) in combination with electronic structure calculations confirm that the Au dopant is preferentially located in the center of the cluster. A useful XANES spectrum can be recorded for lower concentrations, or in shorter time, than the more commonly used EXAFS. This makes XANES a valuable tool for structural characterization

Challenges of implementing nano-specific safety and safe-by-design principles in academia

Safe-by-design is an essential component for creating awareness of the potential novel risks associated with the introduction of sophisticated nanomaterials (NMs) with novel properties. SbD is also a useful tool for meeting EU policy ambitions such as the European Green Deal which includes circular economy and moving towards a zero pollution (pollution-free) environment. Unidentified risks are a growing concern with the rapid and exponential advances of nanotechnology innovation, and the increase in fundamental research on NMs and their potential applications. Therefore, addressing nano-specific safety issues early in the innovation process is vital for reducing the uncertainties of novel NMs. The challenge is that many innovators and material scientists are not toxicologist and are not aware on how to assess the safety of their innovations and novel materials. Safe-by-design is a concept that aims at reducing uncertainties and risks for humans and the environment, starting at an early phase of the innovation process and covering the whole innovation value chain, including research. This perspective tries to get a better understanding on the role of safe-by-design within engineered nanomaterial research to create awareness on the importance on assessing the safety early in research. A method was developed that integrates SbD with a set of questions to aid material scientists assess the safety of their materials (nano-specific safety aspects) and Risk Analysis and Technology Assessment (RATA). Here we present the results of a workshop for material scientists (PhD students) with limited toxicology knowledge at the Debye Institute for Nanomaterials Science (Utrecht University, The Netherlands) with the main goals to create awareness with regard to basic NM safety and to explore the possibilities for applying safe-by-design principles in academia. The approach presented here can be applied by researchers and innovators to assess the safety of NMs at an early stage of the innovation process, and this work is framed in the context of Responsible Research and Innovation using RATA

Characterisation, degradation and regeneration of luminescent Ag29 clusters in solution

Luminescent Ag clusters are prepared with lipoic acid (LA) as the ligand. Using a combination of mass spectrometry, optical spectroscopy and analytical ultracentrifugation, the clusters are found to be highly monodisperse with mass 5.6 kDa. We assign the chemical composition [Ag29(LA)12](3-) to the clusters, where LA likely binds in a bidentate fashion. The Ag29 clusters show slow degradation, retaining their deep red emission for at least 18 months if stored in the dark. Purification or exposure to light results in faster degradation. No other cluster species are observed during the degradation process. Once degraded, the clusters could easily be regenerated using NaBH4, which is not usually observed for thiolate-capped Ag clusters