Development of Iron-Doped Silicon Nanoparticles As Bimodal Imaging Agents

We demonstrate the synthesis of water-soluble allylamine-terminated Fe-doped Si (Si_<i>x</i>Fe) nanoparticles as bimodal agents for optical and magnetic imaging. The preparation involves the synthesis of a single-source iron-containing precursor, Na₄Si₄ with <i>x</i>% Fe (<i>x</i> = 1, 5, 10), and its subsequent reaction with NH₄Br to produce hydrogen-terminated Si_<i>x</i>Fe nanoparticles. The hydrogen-capped nanoparticles are further terminated with allylamine <i>via</i> thermal hydrosilylation. Transmission electron microscopy indicates that the average particle diameter is ∼3.0 ± 1.0 nm. The Si_5Fe nanoparticles show strong photoluminescence quantum yield in water (∼10%) with significant <i>T</i>₂ contrast (<i>r</i>₂<i>/r</i>₁ value of 4.31). Electron paramagnetic resonance and Mössbauer spectroscopies indicate that iron in the nanoparticles is in the +3 oxidation state. Analysis of cytotoxicity using the resazurin assay on HepG2 liver cells indicates that the particles have minimal toxicity

EPR and Structural Characterization of Water-Soluble Mn²⁺-Doped Si Nanoparticles

Water-soluble poly­(allylamine) Mn²⁺-doped Si (Si_Mn) nanoparticles (NPs) were prepared and show promise for biologically related applications. The nanoparticles show both strong photoluminescence and good magnetic resonance contrast imaging. The morphology and average diameter were obtained through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM); spherical crystalline Si NPs with an average diameter of 4.2 ± 0.7 nm were observed. The doping maximum obtained through this process was an average concentration of 0.4 ± 0.3% Mn per mole of Si. The water-soluble Si_Mn NPs showed a strong photoluminescence with a quantum yield up to 13%. The Si_Mn NPs had significant <i>T</i>₁ contrast with an <i>r</i>₁ relaxivity of 11.1 ± 1.5 mM^–1 s^–1 and <i>r</i>₂ relaxivity of 32.7 ± 4.7 mM^–1 s^–1 where the concentration is in mM of Mn²⁺. Dextran-coated poly­(allylamine) Si_Mn NPs produced NPs with <i>T</i>₁ and <i>T</i>₂ contrast with a <i>r</i>₁ relaxivity of 27.1 ± 2.8 mM^–1 s^–1 and <i>r</i>₂ relaxivity of 1078.5 ± 1.9 mM^–1 s^–1. X-band electron paramagnetic resonance spectra are fit with a two-site model demonstrating that there are two types of Mn²⁺ in these NP’s. The fits yield hyperfine splittings (<i>A</i>) of 265 and 238 MHz with significant zero field splitting (<i>D</i> and <i>E</i> terms). This is consistent with Mn in sites of symmetry lower than tetrahedral due to the small size of the NP’s

Chemical Insight into the Origin of Red and Blue Photoluminescence Arising from Freestanding Silicon Nanocrystals

Silicon nanocrystals (Si NCs) are attractive functional materials. They are compatible with standard electronics and communications platforms and are biocompatible. Numerous methods have been developed to realize size-controlled Si NC synthesis. While these procedures produce Si NCs that appear identical, their optical responses can differ dramatically. Si NCs prepared using high-temperature methods routinely exhibit photoluminescence agreeing with the effective mass approximation (EMA), while those prepared <i>via</i> solution methods exhibit blue emission that is somewhat independent of particle size. Despite many proposals, a definitive explanation for this difference has been elusive for no less than a decade. This apparent dichotomy brings into question our understanding of Si NC properties and potentially limits the scope of their application. The present contribution takes a substantial step forward toward identifying the origin of the blue emission that is not expected based upon EMA predictions. It describes a detailed comparison of Si NCs obtained from three of the most widely cited procedures as well as the conversion of red-emitting Si NCs to blue emitters upon exposure to nitrogen-containing reagents. Analysis of the evidence is consistent with the hypothesis that the presence of trace nitrogen and oxygen even at the parts per million level in Si NCs gives rise to the blue emission