Bioconjugated silicon quantum dots from one-step green synthesis

Ultrasmall silicon nanoparticles as a promising platform for multimodal imaging

Bimodal systems for nuclear and optical imaging are currently being intensively investigated due to their comparable detection sensitivity and the complementary information they provide. In this perspective, we have implemented both modalities on biocompatible ultrasmall silicon nanoparticles (Si NPs). Such nanoparticles are particularly interesting since they are highly biocompatible, have covalent surface functionalization and demonstrate very fast body clearance. We prepared monodisperse citrate-stabilized Si NPs (2.4 ± 0.5 nm) with more than 40 accessible terminal amino groups per particle and, for the first time, simultaneously, a near-infrared dye (IR800-CW) and a radiolabel (64Cu-NOTA = 1,4,7-triazacyclononane-1,4,7-triacetic acid) have been covalently linked to the surface of such Si NPs. The obtained nanomaterials have been fully characterized using HR-TEM, XPS, UV-Vis and FT-IR spectroscopy. These dual-labelled particles do not exhibit any cytotoxicity in vitro. In vivo studies employing both positron emission tomography (PET) and optical imaging (OI) techniques revealed rapid renal clearance of dual-labelled Si NPs from mice.

Functional Bio-inorganic Hybrids from Silicon Quantum Dots and Biological Molecules

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.

Tailoring Photoluminescence from Si-Based Nanocrystals Prepared by Pulsed Laser Ablation in He-N2 Gas Mixtures

Using methods of pulsed laser ablation from a silicon target in helium (He)-nitrogen (N2) gas mixtures maintained at reduced pressures (0.5-5 Torr), we fabricated substrate-supported silicon (Si) nanocrystal-based films exhibiting a strong photoluminescence (PL) emission, which depended on the He/N2 ratio. We show that, in the case of ablation in pure He gas, Si nanocrystals exhibit PL bands centered in the "red - near infrared" (maximum at 760 nm) and "green" (centered at 550 nm) spectral regions, which can be attributed to quantum-confined excitonic states in small Si nanocrystals and to local electronic states in amorphous silicon suboxide (a-SiOx) coating, respectively, while the addition of N2 leads to the generation of an intense "green-yellow" PL band centered at 580 nm. The origin of the latter band is attributed to a radiative recombination in amorphous oxynitride (a-SiNxOy) coating of Si nanocrystals. PL transients of Si nanocrystals with SiOx and a-SiNxOy coatings demonstrate nonexponential decays in the micro- and submicrosecond time scales with rates depending on nitrogen content in the mixture. After milling by ultrasound and dispersing in water, Si nanocrystals can be used as efficient non-toxic markers for bioimaging, while the observed spectral tailoring effect makes possible an adjustment of the PL emission of such markers to a concrete bioimaging task.

Spectral tuning of colloidal Si nanocrystal luminescence by post-laser irradiation in liquid

We report a simple technique to tune the luminescence spectra of blue-emitting colloidal silicon nanocrystals (Si-ncs) to the ultraviolet region via post-laser irradiation.

Laser-Processed Nanosilicon: A Multifunctional Nanomaterial for Energy and Healthcare

This review describes promising laser-based approaches to produce silicon nanostructures, including laser ablation of solid Si targets in residual gases and liquids and laser pyrolysis of silane. These methods are different from, and complementary to, widely used porous silicon technology and alternative synthesis routes. One can use these methods to make stable colloidal dispersions of silicon nanoparticles in both organic and aqueous media, which are suitable for a multitude of applications across the important fields of energy and healthcare. Size tailoring allows production of Si quantum dots with efficient photoluminescence that can be tuned across a broad spectral range from the visible to near-IR by varying particle size and surface functionalization. These nanoparticles can also be integrated with other nanomaterials to make multifunctional composites incorporating magnetic and/or plasmonic components. In the energy domain, this review highlights applications to photovoltaics and photodetectors, nanostructured silicon anodes for lithium ion batteries, and hydrogen generation from water. Application to nanobiophotonics and nanomedicine profits from the excellent biocompatibility and biodegradability of nanosilicon. These applications encompass several types of bioimaging and various therapies, including photodynamic therapy, RF thermal therapy, and radiotherapy. The review concludes with a discussion of challenges and opportunities in the applications of laser-processed nanosilicon.

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials, thereby enabling the fabrication of bioconjugates and heterogeneous catalysts. Accurate size control of LSPC-synthesized materials ranging from quantum dots to submicrometer spheres and recent upscaling advancement toward the multiple-gram scale are helpful for extending the applicability of LSPC-synthesized nanomaterials to various fields. By discussing key reports on both the fundamentals and the applications related to laser ablation, fragmentation, and melting in liquids, this Article presents a timely and critical review of this emerging topic.

Facile fabrication of bioactive ultra-small protein–hydroxyapatite nanoconjugates via liquid-phase laser ablation and their enhanced osteogenic differentiation activity

Ultra-small protein–hydroxyapatite nanoconjugates promote the osteogenic differentiation of mesenchymal stem cells.

Nanoparticle functionalized laser patterned substrate: an innovative route towards low cost biomimetic platforms

Integration of nanotechnology and advanced manufacturing processes presents an attractive route to produce devices for adaptive biomedical device technologies.

Origin of blue photoluminescence from colloidal silicon nanocrystals fabricated by femtosecond laser ablation in solution

We present a detailed investigation into the origin of blue emission from colloidal silicon (Si) nanocrystals (NCs) fabricated by femtosecond laser ablation of Si powder in 1-hexene. High resolution transmission electron microscopy and Raman spectroscopy observations confirm that Si NCs with average size 2.7 nm are produced and well dispersed in 1-hexene. Fourier transform infrared spectrum and x-ray photoelectron spectra have been employed to reveal the passivation of Si NCs surfaces with organic molecules. On the basis of the structural characterization, UV-visible absorption, temperature-dependent photoluminescence (PL), time-resolved PL, and PL excitation spectra investigations, we deduce that room-temperature blue luminescence from colloidal Si NCs originates from the following two processes: (i) under illumination, excitons first form within colloidal Si NCs by direct transition at the X or Γ (Γ25 → Γ'2) point; (ii) and then some trapped excitons migrate to the surfaces of colloidal Si NCs and further recombine via the surface states associated with the Si-C or Si-C-H2 bonds.

Ultrasmall inorganic nanoparticles: State-of-the-art and perspectives for biomedical applications

Ultrasmall nanoparticulate materials with core sizes in the 1-3nm range bridge the gap between single molecules and classical, larger-sized nanomaterials, not only in terms of spatial dimension, but also as regards physicochemical and pharmacokinetic properties. Due to these unique properties, ultrasmall nanoparticles appear to be promising materials for nanomedicinal applications. This review overviews the different synthetic methods of inorganic ultrasmall nanoparticles as well as their properties, characterization, surface modification and toxicity. We moreover summarize the current state of knowledge regarding pharmacokinetics, biodistribution and targeting of nanoscale materials. Aside from addressing the issue of biomolecular corona formation and elaborating on the interactions of ultrasmall nanoparticles with individual cells, we discuss the potential diagnostic, therapeutic and theranostic applications of ultrasmall nanoparticles in the emerging field of nanomedicine in the final part of this review.

DNA assembly of silicon quantum dots/gold nanoparticle nanocomposites

We report the formation of a nanocomposite composed of silicon quantum dots (Si-QDs) and a gold nanoparticle (Au-NP) by DNA hybridization. The composites are dispersible in water and exhibit bright near infrared luminescence.

Facile production of stable silicon nanoparticles: laser chemistry coupled to in situ stabilization via room temperature hydrosilylation

Stable, alkyl-terminated, light-emitting silicon nanoparticles have been synthesized in a continuous process by laser pyrolysis of a liquid trialkyl-silane precursor selected as a safer alternative to gas silane (SiH4). Stabilization was achieved by in situ reaction using a liquid collection system instead of the usual solid state filtration. The alkene contained in the collection liquid (1-dodecene) reacted with the newly formed silicon nanoparticles in an unusual room-temperature hydrosilylation process. It was achieved by the presence of fluoride species, also produced during laser pyrolysis from the decomposition of sulfur hexafluoride (SF6) selected as a laser sensitizer. This process directly rendered alkyl-passivated silicon nanoparticles with consistent morphology and size (<3 nm), avoiding the use of costly post-synthetic treatments.

Fast and cost-effective fabrication of large-area plasmonic transparent biosensor array

Surface enhanced Raman-based sensors are widely used for chemical and biological species analysis; but to date the high cost, long production time, hazardous, and toxic content as well as small sensing area and opacity are limiting their capabilities for widespread applications in the medical and environmental fields. We present a novel cost-effective method for fast laser-based fabrication of affordable large-area and transparent periodic arrays of ligand-free metallic nanoparticles, offering a maximum possibility for the adsorption/immobilization of molecules and labeling. Further, we demonstrate a remarkable detection limit in the picomolar range by means of Raman scattering, thus evidencing a superior signal-to-noise ratio compared to other sensor substrates. The high sensitivity performance along with a fast and cheap fabrication procedure of reusable large-area transparent plasmonic devices opens the route for direct, in situ multimodal optical analysis with broad applications in the biomedical/analytical fields.

Nanodiamonds and silicon quantum dots: ultrastable and biocompatible luminescent nanoprobes for long-term bioimaging

Ultra-stability and low-toxicity of silicon quantum dots and fluorescent nanodiamonds for long-termin vitroandin vivobioimaging are demonstrated.

A facile high-speed vibration milling method to mass production of water-dispersible silicon quantum dots for long-term cell imaging

A facile HSVM method has been developed for mass production of water-dispersible SiQDs for long-term cell imaging.

Mild two-step method to construct DNA-conjugated silicon nanoparticles: scaffolds for the detection of microRNA-21

We describe a novel two-step method, starting from bulk silicon wafers, to construct DNA conjugated silicon nanoparticles (SiNPs). This method first utilizes reactive high-energy ball milling (RHEBM) to obtain alkene grafted SiNPs. The alkene moieties are subsequently reacted with commercially available thiol-functionalized DNA via thiol-ene click chemistry to produce SiNP DNA conjugates wherein the DNA is attached through a covalent thioether bond. Further, to show the utility of this synthetic strategy, we illustrate how these SiNP ODN conjugates can detect cancer-associated miR-21 via a fluorescence ON strategy. Given that an array of biological molecules can be prepared with thiol termini and that SiNPs are biocompatible and biodegradable, we envision that this synthetic protocol will find utility in salient SiNP systems for potential therapeutic and diagnostic applications.

Study on the productivity of silicon nanoparticles by picosecond laser ablation in water: towards gram per hour yield

An investigation on the productivity of silicon nanoparticles by picosecond laser ablation in water is presented. A systematic experimental study is performed as function of the laser wavelength, fluence and ablation time. In case of ablation at 1064 nm silicon nanoparticles with a mean diameter of 40 nm are produced. Instead, ablation at 355 nm results in nanoparticles with a mean diameter of 9 nm for short ablation time while the mean diameter decreases to 3 nm at longer ablation time. An original model based on the in-situ ablation/photo-fragmentation physical process is developed, and it very well explains the experimental productivity findings. The reported phenomenological model has a general validity, and it can be applied to analyze pulsed laser ablation in liquid in order to optimize the process parameters for higher productivity. Finally, an outlook is given towards gram per hour yield of ultra-small silicon nanoparticles.

Silicon nanocrystals and nanodiamonds in live cells: photoluminescence characteristics, cytotoxicity and interaction with cell cytoskeleton

Our study on biological interaction of silicon nanocrystals (a) and nanodiamonds (b) with cells encourages their use in human medicine.

Silicon quantum dots for biological applications

Semiconductor nanoparticles (or quantum dots, QDs) exhibit unique optical and electronic properties such as size-controlled fluorescence, high quantum yields, and stability against photobleaching. These properties allow QDs to be used as optical labels for multiplexed imaging and in drug delivery detection systems. Luminescent silicon QDs and surface-modified silicon QDs have also been developed as potential minimally toxic fluorescent probes for bioapplications. Silicon, a well-known power electronic semiconductor material, is considered an extremely biocompatible material, in particular with respect to blood. This review article summarizes existing knowledge related to and recent research progress made in the methods for synthesizing silicon QDs, as well as their optical properties and surface-modification processes. In addition, drug delivery systems and in vitro and in vivo imaging applications that use silicon QDs are also discussed.