Dendrimer-Encapsulated Nanoparticles: New Synthetic and Characterization Methods and Catalytic Applications

In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized using a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently reduced chemically to yield nearly size-monodisperse particles having diameters in the 1-2 nm range. Monometallic, bimetallic (alloy and core@shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica, but also as a stabilizer for the nanoparticle. In this perspective, we report on progress in the synthesis, characterization, and applications of these materials since our last review in 2005. Significant advances in the synthesis of core@shell DENs, characterization, and applications to homogeneous and heterogeneous catalysis (including electrocatalysis) are emphasized.U.S. Department of Energy, Office of Basic Energy Sciences DE-FG02-09ER16090U.S. National Science Foundation 0847957Robert A. Welch Foundation F-0032Chemistr

Underpotential deposition as a synthetic and characterization tool for core@shell dendrimer-encapsulated nanoparticles

textThe synthesis and characterization of Pt core/ Cu shell (Pt@Cu) dendrimer-encapsulated nanoparticles (DENs) having full and partial Cu shells deposited via electrochemical underpotential deposition (UPD) is described. Pt DENs containing averages of 55, 147, and 225 Pt atoms immobilized on glassy carbon electrodes served as the substrate for the UPD of a Cu monolayer. This results in formation of Pt@Cu DENs. Evidence for this conclusion is based on results from the analysis of cyclic voltammograms (CVs) for the UPD and stripping of Cu on Pt DENs, and from experiments showing that the Pt core DENs catalyze the hydrogen evolution reaction before Cu UPD, but that after Cu UPD this reaction is inhibited. Results obtained by in-situ electrochemical X-ray absorption spectroscopy (XAS) confirm the core@shell structure. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N₂-saturated solution, and no detectable oxidation when the DENs are in contact with H₂. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd²⁺-complexing ligands such as chloride. For the air- and N₂-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N2-saturated solution, and no detectable oxidation when the DENs are in contact with H2. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd2+-complexing ligands such as chloride. For the air- and N2-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy.Chemistr

Phosphonium-Based Binary and Ternary Super-Concentrated Liquid Electrolytes for Magnesium Batteries

Here we describe the use of organo ionic phosphonium salts to access a broad range of useful binary and ternary (super)-concentrated liquid electrolytes for rechargeable magnesium batteries

Dendrimerencapsulated nanoparticles: New synthetic and characterization methods and catalytic applications. Chem Sci 2011

In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized using a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently reduced chemically to yield nearly size-monodisperse particles having diameters in the 1-2 nm range. Monometallic, bimetallic (alloy and core@shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica, but also as a stabilizer for the nanoparticle. In this perspective, we report on progress in the synthesis, characterization, and applications of these materials since our last review in 2005. Significant advances in the synthesis of core@shell DENs, characterization, and applications to homogeneous and heterogeneous catalysis (including electrocatalysis) are emphasized

Site-Selective Cu Deposition on Pt Dendrimer-Encapsulated Nanoparticles: Correlation of Theory and Experiment

The voltammetry of Cu underpotential deposition (UPD) onto Pt dendrimer-encapsulated nanoparticles (DENs) containing an average of 147 Pt atoms (Pt₁₄₇) is correlated to density functional theory (DFT) calculations. Specifically, the voltammetric peak positions are in good agreement with the calculated energies for Cu deposition and stripping on the Pt(100) and Pt(111) facets of the DENs. Partial Cu shells on Pt₁₄₇ are more stable on the Pt(100) facets, compared to the Pt(111) facets, and therefore, Cu UPD occurs on the 4-fold hollow sites of Pt(100) first. Finally, the structures of Pt DENs having full and partial monolayers of Cu were characterized in situ by X-ray absorption spectroscopy (XAS). The results of XAS studies are also in good agreement with the DFT-optimized models

Early Stage Anodic Instability of Glassy Carbon Electrodes in Propylene Carbonate Solvent Containing Lithium Hexafluorophosphate

Irreversible changes to the morphology of glassy carbon (GC) electrodes at potentials between 3.5 and 4.5 V vs Li/Li⁺ in propylene carbonate (PC) solvent containing lithium hexafluorophosphate (LiPF₆) are reported. Analysis of cyclic voltammetry (CV) experiments in the range of 3.0 to 6.0 V shows that the capacitance of the electrochemical double-layer increased irreversibly beginning at potentials as low as 3.5 V. These changes resulted from nonfaradaic interactions, and were not due to oxidative electrochemical decomposition of the electrode and electrolyte, anion intercalation, nor caused by the presence of water, a common impurity in organic electrolyte solutions. Atomic force microscopy (AFM) images revealed that increasing the potential of a bare GC surface from 3.0 to 4.5 V resulted in a 6× increase in roughness, in good agreement with the changes in double-layer capacitance. Treating the GC surface via exposure to trichloromethylsilane vapors resulted in a stable double-layer capacitance between 3.0 and 4.5 V, and this treatment also correlated with less roughening. These results inform future efforts aimed at controlling surface composition and morphology of carbon electrodes

Tuning the Stability of Organic Active Materials for Nonaqueous Redox Flow Batteries via Reversible, Electrochemically Mediated Li⁺ Coordination

We describe an electrochemically mediated interaction between Li⁺ and a promising active material for nonaqueous redox flow batteries (RFBs), 1,2,3,4-tetrahydro-6,7-dimethoxy-1,1,4,4-tetramethylnaphthalene (TDT), and the impact of this structural interaction on material stability during voltammetric cycling. TDT could be an advantageous organic positive electrolyte material for nonaqueous RFBs due to its high oxidation potential, 4.21 V vs Li/Li⁺, and solubility of at least 1.0 M in select electrolytes. Although results from voltammetry suggest TDT displays Nernstian reversibility in many nonaqueous electrolyte solutions, bulk electrolysis reveals significant degradation in all electrolytes studied, the extent of which depends on the electrolyte solution composition. Results of subtractively normalized in situ Fourier transform infrared spectroscopy (SNIFTIRS) confirm that TDT undergoes reversible structural changes during cyclic voltammetry in propylene carbonate and 1,2-dimethoxyethane solutions containing Li⁺ electrolytes, but irreversible degradation occurs when tetrabutylammonium (TBA⁺) replaces Li⁺ as the electrolyte cation in these solutions. By combining the results from SNIFTIRS experiments with calculations from density functional theory, solution-phase active species structure and potential-dependent interactions can be determined. We find that Li⁺ coordinates to the Lewis basic methoxy groups of neutral TDT and, upon electrochemical oxidation, this complex dissociates into the radical cation TDT^•+ and Li⁺. The improved cycling stability in the presence of Li⁺ relative to TBA⁺ suggests that the structural interaction reported herein may be advantageous to the design of energy storage materials based on organic molecules

Pancreatic surgery outcomes: multicentre prospective snapshot study in 67 countries

Background: Pancreatic surgery remains associated with high morbidity rates. Although postoperative mortality appears to have improved with specialization, the outcomes reported in the literature reflect the activity of highly specialized centres. The aim of this study was to evaluate the outcomes following pancreatic surgery worldwide.Methods: This was an international, prospective, multicentre, cross-sectional snapshot study of consecutive patients undergoing pancreatic operations worldwide in a 3-month interval in 2021. The primary outcome was postoperative mortality within 90 days of surgery. Multivariable logistic regression was used to explore relationships with Human Development Index (HDI) and other parameters.Results: A total of 4223 patients from 67 countries were analysed. A complication of any severity was detected in 68.7 percent of patients (2901 of 4223). Major complication rates (Clavien-Dindo grade at least IIIa) were 24, 18, and 27 percent, and mortality rates were 10, 5, and 5 per cent in low-to-middle-, high-, and very high-HDI countries respectively. The 90-day postoperative mortality rate was 5.4 per cent (229 of 4223) overall, but was significantly higher in the low-to-middle-HDI group (adjusted OR 2.88, 95 per cent c.i. 1.80 to 4.48). The overall failure-to-rescue rate was 21 percent; however, it was 41 per cent in low-to-middle-compared with 19 per cent in very high-HDI countries.Conclusion: Excess mortality in low-to-middle-HDI countries could be attributable to failure to rescue of patients from severe complications. The authors call for a collaborative response from international and regional associations of pancreatic surgeons to address management related to death from postoperative complications to tackle the global disparities in the outcomes of pancreatic surgery (NCT04652271; ISRCTN95140761)