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Quantum dots (QDs) are semiconductor nanoparticles that exhibit photoluminescent properties useful for applications in the field of diagnostics and medicine. Successful implementation of these QDs for bio-imaging and bio/chemical sensing typically involves conjugation to biologically active molecules for recognition and signal generation. Unfortunately, traditional and widely studied QDs are based upon heavy metals and other toxic elements ( e.g. , Cd- and Pb-based QDs), which precludes their safe use in actual biological systems. Silicon quantum dots (SiQDs) offer the same advantages as these heavy-metal-based QDs with the added benefits of nontoxicity and abundance. The preparation of functional bio-inorganic hybrids from SiQDs and biomolecules has lagged significantly compared to their traditional toxic counterparts because of the challenges associated with the synthesis of water-soluble SiQDs and their relative instability in aqueous environments. Advances in SiQD synthesis and surface functionalization, however, have made possible the preparation of functional bio-inorganic hybrids from SiQDs and biological molecules through different bioconjugation reactions. In this contribution, we review the various bioconjugate reactions by which SiQDs have been linked to biomolecules and implemented as platforms for bio-imaging and bio/chemical sensing. We also highlight the challenges that need to be addressed and overcome for these materials to reach their full potential. Lastly, we give prospective applications where this unique class of nontoxic and biocompatible materials can be of great utility in the future.

Ratiometric photoluminescent detection of the toxicologically potent organophosphate ester nerve agents paraoxon (PX) and parathion (PT) using the complementary optical and chemical properties of the long Stokes shift green fluorescent protein variant, mAmetrine1.2 (mAm), and red-emitting silicon-based quantum dots (SiQDs) is reported. PX and PT selectively quench SiQD photoluminescence (PL) through a dynamic quenching mechanism, thereby, facilitating the development of a ratiometric sensor platform that shows micromolar limits of detection for PX and PT and that is unaffected by the presence of common inorganic and organic interferents. As a part of the present study, we also demonstrate that the paper-based sensors derived from mAm and SiQDs detect PX and PT at concentrations as low as 5 μM using a readily available commercial color analysis smartphone "app". The ratiometric sensor reported herein can potentially be used for the convenient and rapid on-site detection and quantification of PX and PT in real-world samples.

This study reports the preparation of functional bioinorganic hybrids, through application of the thiol-ene reaction, that exhibit catalytic activity and photoluminescent properties from enzymes and freestanding silicon nanocrystals. Thermal hydrosilylation of 1,7-octadiene and alkene-terminated poly(ethylene oxide)methyl ether with hydride-terminated silicon nanocrystals afforded nanocrystals functionalized with alkene residues and poly(ethylene oxide) moieties. These silicon nanocrystals were conjugated with representative enzymes through the photochemical thiol-ene reaction to afford bioinorganic hybrids that are dispersible and photostable in buffer, and that exhibit photoluminescence (λ max = 630 nm) and catalytic activity. They were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dynamic light scattering analysis (DLS), absorption spectroscopy, steady-state and time-resolved photoluminescence spectroscopy, and pertinent enzyme activity assays. The general derivatization approach presented for interfacing enzymes with biocompatible silicon nanocrystals has far reaching implications for many applications ranging from sensors to therapeutic agents. The bioinorganic hybrids presented herein have potential applications in the chemical detection of nitrophenyl esters and urea. They can also be employed in enzyme-based theranostics as they combine long-lived silicon nanocrystal photoluminescence with substrate-specific enzymatic activity.

This study reports the preparation of functional bioinorganic hybrid materials exhibiting catalytic activity and photoluminescent properties arising from the combination of enzymes and freestanding silicon-based nanoparticles. The hybrid materials reported herein have potential applications in biological sensing/imaging and theranostics, as they combine long-lived silicon-based nanoparticle photoluminescence with substrate-specific enzymatic activity. Thermal hydrosilylation of undecenoic acid and alkene-terminated poly(ethylene oxide) with hydride-terminated silicon nanocrystals afforded nanoparticles functionalized with a mixed surface made up of carboxylic acid and poly(ethylene oxide) moieties. These silicon-based nanoparticles were subsequently conjugated with prototypical enzymes through the carbodiimide-mediated amide coupling reaction in order to form bioinorganic hybrids that display solubility and photostability in phosphate buffer, photoluminescence (λ max = 630 nm), and enzymatic activity. They were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dynamic light scattering analysis (DLS), photoluminescence spectroscopy, and pertinent enzyme activity assays.

We explore the dynamics of blue emission from dodecylamine and ammonia functionalized silicon nanocrystals (Si NCs) with average diameters of ∼3 and ∼6 nm using time-resolved photoluminescence (TRPL) spectroscopy. The Si NCs exhibit nanosecond PL decay dynamics that is independent of NC size and uniform across the emission spectrum. The TRPL measurements reveal complete quenching of core state emission by a charge transfer state that is responsible for the blue PL with a radiative recombination rate of ∼5 × 10(7) s(-1). A detailed picture of the charge transfer state emission dynamics in these functionalized Si NCs is proposed.

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.

Luminescent colloidal silicon quantum dots (SiQDs) are explored as alternatives to metal‐based quantum dots for light‐emitting diodes (LEDs) because of the abundance and biocompatibility of silicon. To date, the broad electroluminescence (EL) bandwidth (>100 nm) and blueshifting of EL at high applied voltages of SiQD‐LEDs are outstanding challenges that limit a competitive spectral purity and device stability. Herein, the fabrication and testing of SiQD‐LEDs that incorporate a Fabry–Pérot cavity are reported which exhibit a narrow spectral linewidth as low as ≈23 nm. The presented devices also provide wavelength and visual stability from +4 to +8 V, as well as spectral tunability. [ABSTRACT FROM AUTHOR]

In this work the temperature‐dependent photoluminescence of alkyl‐capped silicon nanocrystals with mean diameters of between 3 and 9 nm has been investigated. The nanocrystals were characterized extensively by FTIR, TEM, powder XRD, and X‐ray photoelectron spectroscopy prior to low‐temperature and time‐resolved photoluminescence spectroscopy experiments. The photoluminescence (PL) properties were evaluated in the temperature range of 41–300 K. We found that the well‐known temperature‐dependent blueshift of the PL maximum decreases with increasing nanocrystal diameter and eventually becomes a redshift for nanocrystal diameters larger than 6 nm. This implies that the observed shifts cannot be explained solely by band‐gap widening, as is commonly assumed. We propose that the luminescence of drop‐cast silicon nanocrystals is affected by particle ensemble effects, which can explain the otherwise surprising temperature dependence of the luminescence peak. Nanocrystal luminescence: Low‐temperature, monochromatic, and time‐resolved photoluminescence spectroscopy experiments (see figure) have revealed the influence of ensemble effects on the temperature‐dependent photoluminescence behavior of alkyl‐capped silicon nanocrystals of different diameters. The observed trends have been attributed to a combination of band‐gap widening, saturation effects, and energy transfer within the ensembles. [ABSTRACT FROM AUTHOR]

Silicon nanocrystals exhibit optoelectronic properties that are favorable for applications such as sensing and biological imaging. Synthesizing a stimuli responsive material with controlled hydrophilicity can benefit these applications. One of the most promising surface species for inducing reversible control of hydrophilicity contains amidine functional groups; unfortunately, this functionality will also bind to the nanocrystal surface and influence the photoluminescence of the material. To combat these limitations, a Lewis acid protection method has been developed to prevent the nitrogen atoms from attaching to the silicon surface for the functionalization of amidine terminated silicon nanocrystals. [ABSTRACT FROM AUTHOR]

Various Lewis acids are found to efficiently catalyze hydrosilylation reactions of hydride-terminated 2D silicon nanosheets at room temperature. The hydride-terminated nanosheets can be functionalized with a variety of unsaturated functional substrates and still possess their unique characteristic (opto)electronic properties (e.g., photoluminescence). This is demonstrated by successfully implementing the readily functionalized materials into new silicon/semiconducting polymer-based field-effect transistors (FETs). Surface modification of the freestanding silicon nanosheets opens new possibilities to form highly homogeneous blends with the already broadly used conventional polymers poly(3-hexylthiophene-2,5-diyl). The consequential combination of the different properties of the materials enables the enhancement of the sensitivity of the solution-gated FETs and increases the transconductance of the operating device. [ABSTRACT FROM AUTHOR]

Remarkable advances in surface hydrosilylation reactions of C=C and C=O bonds on hydride-terminated silicon have revolutionized silicon surface functionalization. However, existing methods for functionalizing hydride-terminated Si nanocrystals (H-SiNCs) require long reaction times and elevated temperatures. Herein, we report a room-temperature method for functionalizing H-SiNC surfaces within seconds by stripping outermost atoms on H-SiNC surfaces with xenon difluoride (XeF2). Detailed analysis of the reaction byproducts by in situ NMR spectroscopy and GC-MS provided unprecedented insight into NC surface composition and reactivity as well as the complex reaction mechanism of XeF2 activated hydrosilylation. [ABSTRACT FROM AUTHOR]

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3-hexylthiophene) (SiNC@P3HT) by applying surface-initiated Kumada catalyst transfer polycondensation (SI-KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor-acceptor interfaces. Systematic characterization provides evidence of a core-shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type-II heterostructure. [ABSTRACT FROM AUTHOR]

Hydride-terminated photoluminescent silicon nanocrystals (SiNCs) were functionalized with organolithium compounds. The reaction is proposed to proceed through cleavage of SiSi bonds and formation of a SiLi surface species. The method yields colloidally stabilized SiNCs at room temperature with short reaction times. SiNCs with mixed surface functionalities can be prepared in an easy two-step reaction by this method by quenching of the SiLi group with electrophiles or by addressing free SiH groups on the surface with a hydrosilylation reaction. [ABSTRACT FROM AUTHOR]

We present a method to combine the functional features of poly(diethyl vinylphosphonate) (PDEVP) and photoluminescent silicon nanocrystals. The polymer-particle hybrids were synthesized in three steps through surfaceinitiated group transfer polymerization using Cp2YCH2TMS- (thf) as a catalyst. This pathway of particle modification renders the nanoparticle surface stable against oxidation. Although SiNC properties are known to be sensitive toward transition metals, the hybrid particles exhibit red photoluminescence in water. The temperature-dependent coiling of PDEVP results in a change of the hydrodynamic radius of the hybrid particles in water. To the best of our knowledge, this is the first example of controlled catalytic polymerization reactions on a silicon nanocrystal surface. [ABSTRACT FROM AUTHOR]

This article describes the synthesis of silicon nanocrystals (Si-NCs) and their functionalization with three different surface groups. We also report the effects of various surface groups on the optical properties of Si-NCs with similar particle size distribution. The Si-NCs were characterized using various methods such as XRD, FT-IR, transmission electron microscopy, and fluorescence spectroscopy. Butylamine (blue), trioctylphosphine oxide (yellow), and dodecyl (red) functionalized Si-NCs dispersed in toluene under UV-illumination. [ABSTRACT FROM AUTHOR]

The reactivity of diazonium salts towards freestanding, photoluminescent silicon nanocrystals (SiNCs) is reported. It was found that SiNCs can be functionalized with aryl groups by direct reductive grafting of the diazonium salts. Furthermore, diazonium salts are efficient radical initiators for SiNC hydrosilylation. For this purpose, novel electron-deficient diazonium salts, highly soluble in nonpolar solvents were synthesized. The SiNCs were functionalized with a variety of alkenes and alkynes at room temperature with short reaction times. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) nanomaterials have been prepared via the solid-state metathesis reaction of various silica sources, magnesium and carbon. This approach enables synthesis of crystalline β-SiC nanomaterials of varied morphologies at 600 °C – the lowest temperature reported to date. The resulting materials were characterized using XRD, FTIR, XPS, TEM and SEM techniques. [ABSTRACT FROM AUTHOR]

In article number 1606764, the modification of twodimensional silicon nanosheets catalyzed by different Lewis acids is described by Jonathan G. C. Veinot, Bernhard Rieger, and co‐workers. A derived functional hybrid film of silicon nanosheet/poly(3‐hexylthiophene) is applied to fabricate field effect transistors with enhanced sensitivity and increased transconductances. The authors thank Johannes Richers for designing the artwork and TUM GS for financial support. [ABSTRACT FROM AUTHOR]

Phosphorus pentachloride (PCl5) has long been used to chlorinate hydrocarbons. It has also been applied in silicon surface chemistry to facilitate alkylation via a two-step halogenation/Grignard route. Here we report a study of the reaction of PCl5 with hydride-terminated silicon nanocrystals (H-SiNCs). An examination of the reaction mechanism has allowed us to establish a functionalization protocol that uses PCl5 as a surface radical initiator to introduce alkyl and alkenyl moieties to the surface of H-SiNCs. The reaction proceeds quickly in a single step, at room temperature and the functionalized silicon nanocrystals retained their morphology and crystallinity. The resulting materials exhibited size-dependent photoluminescence that was approximately 3x as bright as that observed for thermally hydrosilylated SiNCs. Furthermore, the absolute PL quantum yield (AQY) was more than double. The high AQY is expected to enable SiNCs to compete with chalcogenide-based quantum dots in various applications. [ABSTRACT FROM AUTHOR]

The photoluminescence of freestandingsilicon quantum dots (QDs)responds rapidly to various gases in the atmosphere. Under short wavelengthirradiation, the luminescence can change over a time frame of secondsto minutes and can feature orders-of-magnitude shifts in the luminescenceintensity and rapid “jumps” in the peak wavelength.Starting with hydride-terminated Si QDs, the luminescence can be reversiblyquenched or strongly activated, leading to a partly reversible, three-stateluminescence condition consisting of regular, quenched, and activatedstates. These changes were characterized using photoluminescence spectroscopy,Fourier transform infrared spectroscopy, and electron spin resonance.The response is a multistep process involving several stages, includingenergy transfer, abstraction of hydrogen, and rapid optical-irradiation-assistedoxidation. These properties make freestanding Si QDs a potentiallyattractive material for optical sensor applications. [ABSTRACT FROM AUTHOR]

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