Single atoms and small clusters of atoms may accompany Au and Pd dendrimer-encapsulated nanoparticles.

We report the presence of small clusters of atoms (<1 nm) (SCs) and single atoms (SAs) in solutions containing 1-2 nm dendrimer-encapsulated nanoparticles (DENs). Au and Pd DENs were imaged using aberration-corrected scanning transmission electron microscopy (ac-STEM), and energy dispersive spectroscopy (EDS) was used to identify and quantify the SAs/SCs. Two main findings have emerged from this work. First, the presence or absence of SAs/SCs depends on both the terminal functional group of the dendrimer (-NH2 or -OH) and the elemental composition of the DENs (Au or Pd). Second, dialysis can be used to remove the majority of SAs/SCs in cases where a high density of SAs/SCs are present. The foregoing conclusions provide insights into the mechanisms for Au and Pd DEN synthesis and stability. Ultimately, these results demonstrate the need for careful characterization of systems containing nanoparticles to ensure that SAs/SCs, which may be below the detection limit of most analytical methods, are taken into consideration (especially for catalysis experiments).

Ethanol Electrooxidation at 1-2 nm AuPd Nanoparticles.

We report a systematic study of the electrocatalytic properties and stability of a series of 1-2 nm Au, Pd, and AuPd alloy nanoparticles (NPs) for the ethanol oxidation reaction (EOR). Following EOR electrocatalysis, NP sizes and compositions were characterized using aberration-corrected scanning transmission electron microscopy (ac-STEM) and energy dispersive spectroscopy (EDS). Two main findings emerge from this study. First, alloyed AuPd NPs exhibit enhanced electrocatalytic EOR activity compared to either monometallic Au or Pd NPs. Specifically, NPs having a 3:1 ratio of Au:Pd exhibit an ~8-fold increase in peak current density compared to Pd NPs, with an onset potential shifted ~200 mV more to the negative compared to Au NPs. Second, the size and composition of AuPd alloy NPs do not (within experimental error) change following 1.0 or 2.0 h chronoamperometry experiments, while monometallic Au NPs increase in size from 2 to 5 nm under the same conditions. Notably, this report demonstrates the importance of post-catalytic ac-STEM/EDS characterization for fully evaluating NP activity and stability, especially for 1-2 nm NPs that may change in size or structure during electrocatalysis.

Paper Biosensor for the Detection of NT-proBNP Using Silver Nanodisks as Electrochemical Labels.

We report on the use of silver nanodisks (AgNDs), having a diameter of 50 ± 8 nm and a thickness of 8 ± 2 nm, as electrochemical labels for the detection of a model metalloimmunoassay for the heart failure biomarker NT-proBNP. The detection method is based on an electrochemically activated galvanic exchange (GE) followed by the detection of Ag using anodic stripping voltammetry (ASV). The AgNDs labels are superior to Ag nanocubes and Ag nanospheres in terms of the dynamic range for both the model and NT-proBNP metalloimmunoassays. The linear dynamic range for the model composite is 1.5 to 30.0 pM AgNDs. When AgND labels are used for the NT-proBNP assay, the dynamic range is 0.03-4.0 nM NT-proBNP. The latter range fully overlaps the risk stratification range for heart failure from 53 pM to 590 pM. The performance improvement of the AgNDs is a result of the specific GE mechanism for nanodisks. Specifically, GE is complete across the face of the AgNDs, leaving behind an incompletely exchanged ring structure composed of both Ag and Au.

Size-frequency distributions and physical properties of chondrules from x-ray computed microtomography and digital data extraction.

Within this work, we present the first true three-dimensional (3D) analysis of chondrule size. Knowledge about the physical properties of chondrules is important for validating astrophysical theories concerning chondrule formation and their aggregation into the chondritic meteorites (known as chondrites) that contain them. The classification of chondrites into chemical groups also relies on chondrule properties, including their dimensions. Within this work, we quantify the diameters of chondrules in five ordinary chondrites (OCs; comprised of the H, L, and LL chondrites) and one low-iron enstatite (EL) chondrite. To extract the chondrule size data, we use x-ray computed microtomography to image small (~1-2 cm3 ) chondrite samples followed by manual digital segmentation to isolate chondrules within the volumes or subvolumes. Our data yield true 3D results without stereographic corrections necessary for two-dimensional (2D) or petrographic thin section-based determinations of chondrule sizes. Our results are completely novel, but are consistent with previous surface analysis (2D) data for OCs. Within our OC chondrule diameter data, we find the trend of mean chondrule diameters increasing in the order H < L < LL. We also present the first detailed EL chondrite chondrule size-frequency distribution. Finally, we examine the shapes and collective orientations of the chondrules within the chondrites and show that chondrite petrofabrics can be explored with our methodology. Chondrule shape-preferred orientations are identical to the orientations of the metal and sulfide grains in the chondrites and this is likely due to impact-related compaction. HIGHLIGHTS: We present a first true three-dimensional analysis of chondrule size. Our ordinary chondrite chondrule diameter data demonstrate the trend of mean chondrule diameters increasing in the order H chondrites < L chondrites < LL chondrites. We also present the first detailed low-iron enstatite chondrite chondrule size-frequency distribution. We examine the shapes and collective orientations of the chondrules and show that chondrite petrofabrics can be explored with our methodology.