Highly Controlled Synthesis and Super-Radiant Photoluminescence of Plasmonic Cube-in-Cube Nanoparticles

The plasmonic properties of metal nanostructures have been heavily utilized for surface-enhanced Raman scattering (SERS) and metal-enhanced fluorescence (MEF), but the direct photoluminescence (PL) from plasmonic metal nanostructures, especially with plasmonic coupling, has not been widely used as much as SERS and MEF due to the lack of understanding of the PL mechanism, relatively weak signals, and the poor availability of the synthetic methods for the nanostructures with strong PL signals. The direct PL from metal nanostructures is beneficial if these issues can be addressed because it does not exhibit photoblinking or photobleaching, does not require dye-labeling, and can be employed as a highly reliable optical signal that directly depends on nanostructure morphology. Herein, we designed and synthesized plasmonic cube-in-cube (CiC) nanoparticles (NPs) with a controllable interior nanogap in a high yield from Au nanocubes (AuNCs). In synthesizing the CiC NPs, we developed a galvanic void formation (GVF) process, composed of replacement/reduction and void formation steps. We unraveled the super-radiant character of the plasmonic coupling-induced plasmon mode which can result in highly enhanced PL intensity and long-lasting PL, and the PL mechanisms of these structures were analyzed and matched with the plasmon hybridization model. Importantly, the PL intensity and quantum yield (QY) of CiC NPs are 31 times and 16 times higher than those of AuNCs, respectively, which have shown the highest PL intensity and QY reported for metallic nanostructures. Finally, we confirmed the long-term photostability of the PL signal, and the signal remained stable for at least 1 h under continuous illumination

Discovery of Bioactive Metabolites by Acidic Stress to a Geldanamycin Producer, <i>Streptomyces samsunensis</i>

In an effort to activate silent biosynthetic gene clusters, Streptomyces samsunensis DSM42010, a producer of geldanamycin, was cultured at four different pHs (4.5, 5.4, 6.6, and 7.4). An acidic culture condition (pH 5.4) was selected for a chemical investigation since S. samsunensis showed a different metabolic profile compared to when it was cultured under other conditions. Seven new (1–7) and four known (8–11) compounds were isolated from these cultures. The structures of the isolated compounds were determined by spectroscopic techniques and chemical derivatization. Relative and absolute configurations of the new compounds (1–5) were established using JBCA, PGME method, advanced Marfey’s method, modified Mosher’s method, and comparison of observed and calculated ECD data. Interestingly, compounds 1–3 were truncated versions of geldanamycin, and compound 4 was also deduced to originate from geldanamycin. Compound 5 was composed of 3-methyltyrosine and 6-hydroxy-2,4-hexadienoic acid connected through an amide bond. Compounds 6 and 7 were dihydrogenated forms of geldanamycin with a hydroxy substitution. It is possible that culturing this strain under acidic conditions interfered to some degree with the geldanamycin polyketide synthase, leading to production of truncated versions as well as analogues of geldanamycin. Compounds 1, 8, and 9 showed significant antivirulence activity, inhibiting production of α-toxin by methicillin-resistant Staphylococcus aureus without growth attenuation and global regulatory inhibition; compounds 1, 8, and 9 may become promising α-toxin-specific antivirulence leads with less risk of resistance development

Dynamics and Mechanism of Flame Retardants in Polymer Matrixes: Experiment and Simulation

We investigate the dynamics and the mechanism of flame retardants in polycarbonate matrixes to explore for a way of designing efficient and environment-friendly flame retardants. The high phosphorus content of organic phosphates has been considered as a requirement for efficient flame retardants. We show, however, that one can enhance the efficiency of flame retardants even with a relatively low phosphorus content by tuning the dynamics and the intermolecular interactions of flame retardants. This would enable one to design bulkier flame retardants that should be less volatile and less harmful in indoor environments. UL94 flammability tests indicate that even though the phosphorus content of 2,4-di-<i>tert</i>-butylphenyl diphenyl phosphate (DDP) is much smaller with two bulky tertiary butyl groups than that of triphenyl phosphate (TPP), DDP should be as efficient of a flame retardant as TPP, which is a widely used flame retardant. On the other hand, the 2-<i>tert</i>-butylphenyl diphenyl phosphate (2-<i>t</i>BuDP), with a lower phosphorus content than TPP but with a greater phosphorus content than DDP, is less efficient as a flame retardant than both DDP and TPP. Dynamic secondary ion mass spectrometry and molecular dynamics simulations reveal that the diffusion of DDP is slower by an order of magnitude at low temperature than that of TPP but becomes comparable to that of TPP at the ignition temperature. This implies that DDP should be much less volatile than TPP at low temperature, which is confirmed by thermogravimetric analysis. We also find from Fourier transform infrared spectroscopy that Fries rearrangement and char formation are suppressed more by DDP than by TPP. The low volatility and the suppressed char formation of DDP suggest that the enhanced flame retardancy of DDP should be attributed to its slow diffusivity at room temperature and yet sufficiently high diffusivity at high temperature