Size-Dependent Reactivity in Hydrosilylation of Silicon Nanocrystals

Electrochemiluminescence of Ultrasmall Silica Nanoparticles from Size Modulation and Multipath Surface State Adjustment for Ultrasensitive HIV-DNA Fragment Detection

Here, ultrasmall SiO2 nanoparticles (u-SiO2 NPs, <5 nm) with obvious electrochemiluminescence (ECL) phenomenon, which was absent for conventional silica nanoparticles (c-SiO2 NPs), were reported. In a finite ultrasmall volume, the u-SiO2 NPs exhibited increasing ground state energy and higher optical absorption strength due to the electron-hole confinement model and favored catalyzing the reaction through the rapid diffusion of bulk charge, resulting in apparent ECL emission. Then, Zn2+-induced u-SiO2 nanoaggregates (Zn/u-SiO2-Ov nAGG) were synthesized and exhibited improved ECL performance via multipath surface state adjustment of u-SiO2 from several aspects, including aggregation-induced ECL, the generation of oxygen vacancy (Ov), and more positive surface charge. In addition, an ECL biosensor was constructed for ultrasensitive human immunodeficiency virus-related deoxyribonucleic acid detection from 100 aM to 1 nM with a low limit of 50.48 aM, combining the ECL luminescence of Zn/u-SiO2-Ov nAGG with three-dimensional DNA nanomachine-mediated multioutput amplification for enhanced accuracy and sensitivity compared to the single-output method. Therefore, exploring the ECL of ultrasmall nanoparticles via the adjustment of size and surface state provided a valuable indication to a wider investigation and application of novel ECL materials for clinical diagnostic.

Si-H Surface Groups Inhibit Methacrylic Polymerization: Thermal Hydrosilylation of Allyl Methacrylate with Silicon Nanoparticles

Hydrogen-terminated silicon nanoparticles (H-SiNPs) inhibit anerobic thermal autopolymerization of methacrylates. When heated to 100 °C under an inert atmosphere, allyl methacrylate (AMA) was stable for at least 95 h in the presence of 1.2 wt % H-SiNPs, exhibiting less than 0.15% conversion, whereas the neat monomer solidified within 24 h (over 10% conversion after 34 h). A mechanism is proposed that is based on H-transfer from SiNPs to the thermally activated methacrylic dimer biradical, quenching autopolymerization. An analysis of SiNPs isolated after heating in AMA reveals the grafting of ester groups. Thermal hydrosilylation offers a facile way to attach an allyl group to the surface of SiNPs.

Fast room-temperature functionalization of silicon nanoparticles using alkyl silanols

We report a fast, easy and efficient method for the functionalization of hydrogen-terminated silicon nanoparticles (H-Si NPs). Using silanol compounds, a range of functionalized Si NPs could be produced in only 1 h reaction time at room temperature.

Synthesis of PEG-grafted boron doped Si nanocrystals

Silicon nanocrystals are intriguing materials for biomedical imaging applications because of their unique optical properties and biological compatibility. We report a new surface functionalization route to synthesize biological buffer soluble and colloidally stable silicon nanocrystals, which is enabled by surface boron doping. Harnessing the distinctive Lewis acidic boron surface sites, postsynthetic modifications of plasma synthesized boron doped nanocrystals were carried out with polyethylene glycol (PEG-OH) ligands in dimethyl sulfoxide under photochemical conditions. The influence of PEG concentration, PEG molecular weight, and boron doping percentage on the nanocrystal solubility in a biological buffer has been investigated. The boron doping facilitates the surface functionalization via two probable pathways, by providing excellent initial dispersiblity in polar solvents and providing available acidic boron surface sites for bonding. These boron doped silicon nanocrystals have nearly identical absorption features as intrinsic silicon nanocrystals, indicating that they are promising candidates for biological imaging applications.

Superhydrophobic Silicon Nanocrystal-Silica Aerogel Hybrid Materials: Synthesis, Properties, and Sensing Application

Silicon nanocrystals (SiNCs) are abundant and exhibit exquisitely tailorable optoelectronic properties. The incorporation of SiNCs into highly porous and lightweight substrates such as aerogels leads to hybrid materials possessing the attractive features of both materials. This study describes the covalent deposition of SiNCs on and intercalation into silica aerogels, explores the properties, and demonstrates a prototype sensing application of the composite material. SiNCs of different sizes were functionalized with triethoxyvinylsilane (TEVS) via a radical grafting approach and subsequently used for the synthesis of photoluminescent silica hybrids. The resulting SiNC-containing aerogels possess high porosities, SiNC-based size-dependent photoluminescence, transparency, and a superhydrophobic macroscopic surface. The materials were used to examine the photoluminescence response toward low concentrations of 3-nitrotoluene (270 μM), demonstrating their potential as a sensing platform for high-energy materials.

Protease sensing using nontoxic silicon quantum dots

Herein is presented a proof-of-concept study of protease sensing that combines nontoxic silicon quantum dots (SiQDs) with Förster resonance energy transfer (FRET). The SiQDs serve as the donor and an organic dye as the acceptor. The dye is covalently attached to the SiQDs using a peptide linker. Enzymatic cleavage of the peptide leads to changes in FRET efficiency. The combination of interfacial design and optical imaging presented in this work opens opportunities for use of nontoxic SiQDs relevant to intracellular sensing and imaging.

In situ IR-spectroscopy as a tool for monitoring the radical hydrosilylation process on silicon nanocrystal surfaces

Among a variety of SiNC functionalization methods, radical initiated grafting is very promising due to its straightforward nature and low propensity to form surface oligomers. In the present study, we employed in situ IR spectroscopy in combination with visible light transmittance measurements to investigate the radical induced grafting process on the well-defined SiNCs. Our findings support the proposed model: unfunctionalized hydride-terminated SiNCs form agglomerates in organic solvents, which break up during the grafting process. However, clearing of the dispersion is not a valid indicator for complete surface functionalization. Furthermore, radical-initiated grafting reactions in which azobisisobutyronitrile (AIBN) is the initiator are strongly influenced by external factors including initiator concentration, grafting temperature, as well as substrate steric demand. The monomer concentration was proven to have a low impact on the grafting process. Based on these new insights an underlying mechanism could be discussed, offering an unprecedented view on the functionalization of SiNC surfaces via radical initiated hydrosilylation.

Synthetic Developments of Nontoxic Quantum Dots

Semiconductor nanocrystals, or quantum dots (QDs), are candidates for biological sensing, photovoltaics, and catalysis due to their unique photophysical properties. The most studied QDs are composed of heavy metals like cadmium and lead. However, this engenders concerns over heavy metal toxicity. To address this issue, numerous studies have explored the development of nontoxic (or more accurately less toxic) quantum dots. In this Review, we select three major classes of nontoxic quantum dots composed of carbon, silicon and Group I-III-VI elements and discuss the myriad of synthetic strategies and surface modification methods to synthesize quantum dots composed of these material systems.

Radical Initiated Hydrosilylation on Silicon Nanocrystal Surfaces: An Evaluation of Functional Group Tolerance and Mechanistic Study

Hydrosilylation is among the most common methods used for modifying silicon surface chemistry. It provides a wide range of surface functionalities and effective passivation of surface sites. Herein, we report a systematic study of radical initiated hydrosilylation of silicon nanocrystal (SiNC) surfaces using two common radical initiators (i.e., 2,2'-azobis(2-methylpropionitrile) and benzoyl peroxide). Compared to other widely applied hydrosilylation methods (e.g., thermal, photochemical, and catalytic), the radical initiator based approach is particle size independent, requires comparatively low reaction temperatures, and yields monolayer surface passivation after short reaction times. The effects of differing functional groups (i.e., alkene, alkyne, carboxylic acid, and ester) on the radical initiated hydrosilylation are also explored. The results indicate functionalization occurs and results in the formation of monolayer passivated surfaces.

Near-Unity Internal Quantum Efficiency of Luminescent Silicon Nanocrystals with Ligand Passivation

Spectrally resolved photoluminescence (PL) decays were measured for samples of colloidal, ligand-passivated silicon nanocrystals. These samples have PL emission energies with peak positions in the range ∼1.4-1.8 eV and quantum yields of ∼30-70%. Their ensemble PL decays are characterized by a stretched-exponential decay with a dispersion factor of ∼0.8, which changes to an almost monoexponential character at fixed detection energies. The dispersion factors and decay rates for various detection energies were extracted from spectrally resolved curves using a mathematical approach that excluded the effect of homogeneous line width broadening. Since nonradiative recombination would introduce a random lifetime variation, leading to a stretched-exponential decay for an ensemble, we conclude that the observed monoexponential decay in size-selected ensembles signifies negligible nonradiative transitions of a similar strength to the radiative one. This conjecture is further supported as extracted decay rates agree with radiative rates reported in the literature, suggesting 100% internal quantum efficiency over a broad range of emission wavelengths. The apparent differences in the quantum yields can then be explained by a varying fraction of "dark" or blinking nanocrystals.

Tunability Limit of Photoluminescence in Colloidal Silicon Nanocrystals

Luminescent silicon nanocrystals (Si NCs) have attracted tremendous research interest. Their size dependent photoluminescence (PL) shows great promise in various optoelectronic and biomedical applications and devices. However, it remains unclear why the exciton emission is limited to energy below 2.1 eV, no matter how small the nanocrystal is. Here we interpret a nanosecond transient yellow emission band at 590 nm (2.1 eV) as a critical limit of the wavelength tunability in colloidal silicon nanocrystals. In the “large size” regime (d > ~3 nm), quantum confinement dominantly determines the PL wavelength and thus the PL peak blue shifts upon decreasing the Si NC size. In the “small size” regime (d < ~2 nm) the effect of the yellow band overwhelms the effect of quantum confinement with distinctly increased nonradiative trapping. As a consequence, the photoluminescence peak does not exhibit any additional blue shift and the quantum yield drops abruptly with further decreasing the size of the Si NCs. This finding confirms that the PL originating from the quantum confined core states can only exist in the red/near infrared with energy below 2.1 eV; while the blue/green PL originates from surface related states and exhibits nanosecond transition.

Synthesis and Ligand Exchange of Thiol-Capped Silicon Nanocrystals

Hydride-terminated silicon (Si) nanocrystals were capped with dodecanethiol by a thermally promoted thiolation reaction. Under an inert atmosphere, the thiol-capped nanocrystals exhibit photoluminescence (PL) properties similar to those of alkene-capped Si nanocrystals, including size-tunable emission wavelength, relatively high quantum yields (>10%), and long radiative lifetimes (26-280 μs). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy confirmed that the ligands attach to the nanocrystal surface via covalent Si-S bonds. The thiol-capping layer, however, readily undergoes hydrolysis and severe degradation in the presence of moisture. Dodecanethiol could be exchanged with dodecene by hydrosilylation for enhanced stability.

Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization with Conjugated Organic Groups

The quantum confinement and enhanced optical properties of silicon quantum dots (SiQDs) make them attractive as an inexpensive and nontoxic material for a variety of applications such as light emitting technologies (lighting, displays, sensors) and photovoltaics. However, experimental demonstration of these properties and practical application into optoelectronic devices have been limited as SiQDs are generally passivated with covalently bound insulating alkyl chains that limit charge transport. In this work, we show that strategically designed triphenylamine-based surface ligands covalently bonded to the SiQD surface using conjugated vinyl connectivity results in a 70 nm red-shifted photoluminescence relative to their decyl-capped control counterparts. This suggests that electron density from the SiQD is delocalized into the surface ligands to effectively create a larger hybrid QD with possible macroscopic charge transport properties.

Size-independent organosilane functionalization of silicon nanocrystals using Wilkinson’s catalyst

A low-temperature size-independent method for modifying the surface chemistry of the silicon nanocrystal (SiNC) surface is reported. Wilkinson’s catalyst has been applied to accelerate the dehydrogenative coupling reaction between organosilane molecules and hydride-terminated SiNCs. Two strategies and multiple organosilanes were evaluated to study surface modification efficiency. During the investigations, it was determined that surface functionalization efficiency showed some dependence upon the organic modifier in question. The comparatively low reactivity of octadecyldimethyl silanes may result from the formation of “Si–Rh–Si” intermediates hindered by the steric bulk of the silanes when reacted with activated SiNCs. Quenching of the SiNC-based photoluminescence is observed and the origin of this phenomenon has been attributed to the present of trace rhodium on SiNCs detected using X-ray photoelectron spectroscopy.

Versatile "click chemistry" approach to functionalizing silicon quantum dots: applications toward fluorescent cellular imaging

In this study, we describe a solution procedure for the preparation and surface modification of photostable colloidal silicon quantum dots (SiQDs) for imaging of cancer cells. Photoluminescent SiQDs were synthesized by reduction of halogenated silane precursors using a microemulsion process. It was shown that 1,8-nonadiyne molecules could be grafted onto the surface of hydrogen-terminated SiQDs via ultraviolet (UV)-promoted hydrosilylation, demonstrated by Fourier transform infrared spectroscopy (FTIR) measurements. In addition, various azide molecules were coupled onto nonadiyne-functionalized particles, rendering particles dispersible in selected polar and nonpolar solvents. The photoluminescence of functionalized SiQDs was stable against photobleaching and did not vary appreciably within biologically applicable pH and temperature ranges. To demonstrate compatibility with biological systems, water-soluble SiQDs were used for fluorescent imaging of HeLa cells. In addition, the SiQDs were shown to be non-cytotoxic at concentrations up to 240 μg/mL. The results presented herein provide good evidence for the versatility of functionalized SiQDs for fluorescent bioimaging application.

Highly photoluminescent silicon nanocrystals for rapid, label-free and recyclable detection of mercuric ions

Hydrothermal treatment of 3-aminopropyltrimethoxysilane (APTMS) in the presence of sodium citrate generates a suspension of highly fluorescent silicon nanocrystals that fluoresces blue under UV irradiation. The photoluminescent quantum yield of the as-prepared silicon nanocrystals was calculated to be 21.6%, with quinine sulfate as the standard reference. Only mercuric ions (Hg(2+)) can readily prevent the fluorescence of the silicon nanocrystals, indicating a remarkably high selectivity towards Hg(2+) over other metal ions. The optimized sensor system shows a sensitive detection range from 50 nM to 1 μM and a detection limit of 50 nM. The quenching mechanism was explained in terms of optical absorption spectra and time-resolved fluorescence decay spectra. Due to the strong interaction of Hg(2+) with the thiol group, the fluorescence can be fully recovered by biothiols such as cysteine and glutathione, therefore, a regenerative strategy has been proposed and successfully applied to detect Hg(2+) by the same sensor for at least five cycles. Endowed with relatively high sensitivity and selectivity, the present sensor holds the potential to be applied for mercuric assay in water.

Colloidal silicon quantum dots: synthesis and luminescence tuning from the near-UV to the near-IR range

This review describes a series of representative synthesis processes, which have been developed in the last two decades to prepare silicon quantum dots (QDs). The methods include both top-down and bottom-up approaches, and their methodological advantages and disadvantages are presented. Considerable efforts in surface functionalization of QDs have categorized it into (i) a two-step process and (ii) in situ surface derivatization. Photophysical properties of QDs are summarized to highlight the continuous tuning of photoluminescence color from the near-UV through visible to the near-IR range. The emission features strongly depend on the silicon nanostructures including QD surface configurations. Possible mechanisms of photoluminescence have been summarized to ascertain the future challenges toward industrial use of silicon-based light emitters.

Colloidal silicon quantum dots: from preparation to the modification of self-assembled monolayers (SAMs) for bio-applications

Summarizes recent advances in the preparation, surface modification and bio-applications of silicon quantum dots.

Water-soluble photoluminescent d-mannose and l-alanine functionalized silicon nanocrystals and their application to cancer cell imaging

Mannose and alanine functionalized SiNPs have been synthesized, and these particles were evaluated as luminescent agents for imaging MCF-7 cells.

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 via 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.

Photoluminescent silicon nanocrystal-based multifunctional carrier for pH-regulated drug delivery

A core-shell structured multifunctional carrier with nanocrystalline silicon (ncSi) as the core and a water-soluble block copolymer as the shell based on a poly(methacrylic acid) (PMAA) inner shell and polyethylene glycol (MPEG) outer shell (ncSi-MPM) was synthesized for drug delivery. The morphology, composition, and properties of the resulting ncSi-MPM were determined by comprehensive multianalytical characterization, including (1)H NMR spectroscopy, FTIR spectroscopy, XPS spectroscopy, TEM, DLS, and fluorescence spectroscopy analyses. The size of the resulting ncSi-MPM nanocarriers ranged from 40 to 110 nm under a simulated physiological environment. The loading efficiency of model drug doxorubicin (DOX) was approximately 6.1-7.4 wt % for ncSi-MPM and the drug release was pH controlled. Cytotoxicity studies demonstrated that DOX-loaded ncSi-MPM showed high anticancer activity against Hela cells. Hemolysis percentages (<2%) of ncSi-MPM were within the scope of safe values. Fluorescent imaging studies showed that the nanocarriers could be used as a tracker at the cellular level. Integration of the above functional components may result in ncSi-MPM becoming a promising multifunctional carrier for drug delivery and biomedical applications.

Highly bright tunable blue-violet photoluminescence in SiC nanocrystal-sodium dodecyl sulfonate crosslinked network

We report strong photoluminescence in an ultra-small surface oxidized SiC quantum dot-sodium dodecyl sulfonate crosslinked network. The peak emission wavelength is tunable spanning a wide blue-violet spectral region showing clear quantum confinement effects. The photoluminescence decay exhibits triple recombination dynamics with an average lifetime of 13.65 ns.

Stabilization of mid-sized silicon nanoparticles by functionalization with acrylic acid

We present an enhanced method to form stable dispersions of medium-sized silicon nanoparticles for solar cell applications by thermally induced grafting of acrylic acid to the nanoparticle surface. In order to confirm their covalent attachment on the silicon nanoparticles and to assess the quality of the functionalization, X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier spectroscopy measurements were carried out. The stability of the dispersion was elucidated by dynamic light scattering and Zeta-potential measurements, showing no sign of degradation for months.

Enhanced open-circuit voltage in visible quantum dot photovoltaics by engineering of carrier-collecting electrodes

Colloidal quantum dots (CQDs) enable multijunction solar cells using a single material programmed using the quantum size effect. Here we report the systematic engineering of 1.6 eV PbS CQD solar cells, optimal as the front cell responsible for visible-wavelength harvesting in tandem photovoltaics. We rationally optimize each of the device's collecting electrodes-the heterointerface with electron-accepting TiO(2) and the deep-work-function hole-collecting MoO(3) for ohmic contact-for maximum efficiency. We report an open-circuit voltage of 0.70 V, the highest observed in a colloidal quantum dot solar cell operating at room temperature. We report an AM1.5 solar power conversion efficiency of 3.5%, the highest observed in >1.5 eV bandgap CQD PV device.