5688 wileyonlinelibrary.com emission. [ 5 ] The emission from nanocrystalline silicon is much more effi cient than the corresponding process in bulk silicon due to a combination of two effects: 1) electron and hole wave functions overlap more effectively in quantum-confi ned silicon, [ 6 ] and 2) nonradiative impurities and lattice defects are not as accessible when the bulk silicon material is divided into nanocrystalline domains. [ 5,7 ] It is widely accepted that the green to near infrared PL from PSi originates from band-toband recombination of quantum-confi ned excitons, [ 8 ] and that the emissive centers responsible for PL are strongly infl uenced by oxides and other species at the Si surface. [ 9 ] This also appears to be the case for nanoparticles derived from PSi.Micron-scale and nanoscale particles of luminescent PSi have been employed for biological applications due to their biocompatibility, biodegradability and large specifi c capacity for therapeutic reagents. [10][11][12][13][14][15][16][17] For in vivo imaging, luminescent PSi nanoparticles (LPSiNPs) are an attractive alternative to conventional heavy-metal-containing quantum dots, which have been shown to be toxic in biological environments. [18][19][20][21] In addition, the long-lived excited state of LPSiNPs allows high fi delity, low background imaging when employed in time gated experiments. [ 22 ] A limitation of LPSiNPs has been that the quantum yield is typically <10%, [ 10 ] signifi cantly lower than direct band-gap semiconductor quantum dots or many of the common organic imaging fl uorophores. For instance, with the proper surface passivation, CdSe and CdS quantum dots can achieve PL quantum yields of ≈80-90%. [23][24][25][26] A number of reports have demonstrated quantum yields for individual silicon nanocrystals as large as 60%, [27][28][29][30][31][32] however these are dense Si nanocrystals and not porous nanostructures. The relatively low quantum yield of LPSiNPs is assumed to be due to the existence of nonradiative defects at the surface of the high surface area silicon skeleton. A method for overcoming this specifi c limitation is needed.Here we present a systematic study of the activation of photoluminescence in PSi derived nanoparticles by controlled chemical oxidation of the surface. A number of reports have demonstrated LPSiNPs as biological imaging agents, [ 4,10,22,33,34 ] and the material used in all of these can be considered to be a core-shell nanoparticle in which a shell of SiO 2 encases the active porous Si skeleton. However, the growth of oxide used to activate PL in these LPSiNPs has not been investigated in Photoluminescent Porous Si/SiO 2 Core/Shell Nanoparticles Prepared by Borate OxidationJinmyoung Joo , Jose F. Cruz , Sanahan Vijayakumar , Joel Grondek , and Michael J. Sailor * A systematic study on the activation of photoluminescence from luminescent porous silicon nanoparticles (LPSiNPs) by oxidation in aqueous media containing sodium tetraborate (borax) is presented. The treatment promotes surface oxidation ...

A composite material consisting of Fe3O4 nanoparticles embedded in a 200 nm‐diameter porous Si (pSi) nanoparticle “superstructure” is prepared as a potential magnetic resonance imaging contrast agent. Dipolar magnetic coupling between Fe3O4 nanoparticles is enhanced due to their proximity in the pSi host matrix, resulting in an increase in the saturation magnetization and coercivity of the composite.

Studies of Escherichia coli 30S ribosomal subunit assembly have revealed a hierarchical and cooperative association of ribosomal proteins with 16S ribosomal RNA; these results have been used to compile an in vitro 30S subunit assembly map. In single protein addition and omission studies, ribosomal protein S13 was shown to be dependent on the prior association of ribosomal protein S20 for binding to the ribonucleoprotein particle. While the overwhelming majority of interactions revealed in the assembly map are consistent with additional data, the dependency of S13 on S20 is not. Structural studies position S13 in the head of the 30S subunit > 100 Å away from S20, which resides near the bottom of the body of the 30S subunit. All of the proteins that reside in the head of the 30S subunit, except S13, have been shown to be part of the S7 assembly branch, that is, they all depend on S7 for association with the assembling 30S subunit. Given these observations, the assembly requirements for S13 were investigated using base-specific chemical footprinting and primer extension analysis. These studies reveal that S13 can bind to 16S rRNA in the presence of S7, but not S20. Additionally, interaction between S13 and other members of the S7 assembly branch have been observed. These results link S13 to the 3 major domain family of proteins, and the S7 assembly branch, placing S13 in a new location in the 30S subunit assembly map where its position is in accordance with much biochemical and structural data.

Ribonucleoprotein particles (RNPs) are important components of all living systems, and the assembly of these particles is an intricate, often multistep, process. The 30 S ribosomal subunit is composed of one large RNA (16 S rRNA) and 21 ribosomal proteins (r-proteins). In vitro studies have revealed that assembly of the 30 S subunit is a temperature-dependent process involving sequential binding of r-proteins and conformational changes of 16 S rRNA. Additionally, a temperature-dependent conformational rearrangement was reported for a complex of primary r-protein S4 and 16 S rRNA. Given these observations, a systematic study of the temperature-dependence of 16 S rRNA architecture in individual complexes with the other five primary binding proteins (S7, S8, S15, S17, and S20) was performed. While all primary binding r-proteins bind 16 S rRNA at low temperature, not all r-proteins/16 S rRNA complexes undergo temperature-dependent conformational rearrangements. Some RNPs achieve the same conformation regardless of temperature, others show minor adjustments in 16 S rRNA conformation upon heating and, finally, others undergo significant temperature-dependent changes. Some of the architectures achieved in these rearrangements are consistent with subsequent downstream assembly events such as assembly of the secondary and tertiary binding r-proteins. The differential interaction of 16 S rRNA with r-proteins illustrates a means for controlling the sequential assembly pathway for complex RNPs and may offer insights into aspects of RNP assembly in general.

In this report, we prepared a porous Si nanoparticle with a pore morphology that facilitates the proximal loading and alignment of magnetite nanoparticles. We characterized the composite materials using superconducting quantum interference device magnetometry, dynamic light scattering, transmission electron microscopy, and MRI. The in vitro cytotoxicity of the composite materials was tested using cell viability assays on human liver cancer cells and rat hepatocytes. An in vivo analysis using a hepatocellular carcinoma (HCC) Sprague Dawley rat model was used to determine the biodistribution properties of the material, while naïve Sprague Dawley rats were used to determine the pharmocokinetic properties of the nanomaterials. The composite material reported here demonstrates an injectable nanomaterial that exploits the dipolar coupling of superparamagnetic nanoparticles trapped within a secondary inorganic matrix to yield significantly enhanced MRI contrast. This preparation successfully avoids agglomeration issues that plague larger ferromagnetic systems. A Fe 3 O 4 :pSi composite formulation consisting of 25% by mass Fe 3 O 4 yields an maximal T2* value of 556 mM Fe −1 s −1 . No cellular (HepG2 or rat hepatocyte cells) or in vivo (rat) toxicity was observed with the formulation, which degrades and is eliminated after 4-8 h in vivo. The ability to tailor the magnetic properties of such materials may be useful for in vivo imaging, magnetic hyperthermia, or drug-delivery applications.