Luminescent passive-oxidized silicon quantum dots as biological staining labels and their cytotoxicity effects at high concentration

Luminescent Quantum Dots: Synthesis, Optical Properties, Bioimaging and Toxicity

Luminescent nanomaterials such as semiconductor nanocrystals (NCs) and quantum dots (QDs) attract much attention to optical detectors, LEDs, photovoltaics, displays, biosensing, and bioimaging. These materials include metal chalcogenide QDs and metal halide perovskite NCs. Since the introduction of cadmium chalcogenide QDs to biolabeling and bioimaging, various metal nanoparticles (NPs), atomically precise metal nanoclusters, carbon QDs, graphene QDs, silicon QDs, and other chalcogenide QDs have been infiltrating the nano-bio interface as imaging and therapeutic agents. Nanobioconjugates prepared from luminescent QDs form a new class of imaging probes for cellular and in vivo imaging with single-molecule, super-resolution, and 3D resolutions. Surface modified and bioconjugated core-only and core-shell QDs of metal chalcogenides (MX; M = Cd/Pb/Hg/Ag, and X = S/Se/Te,), binary metal chalcogenides (MInX₂; M = Cu/Ag, and X = S/Se/Te), indium compounds (InAs and InP), metal NPs (Ag, Au, and Pt), pure or mixed precision nanoclusters (Ag, Au, Pt), carbon nanomaterials (graphene QDs, graphene nanosheets, carbon NPs, and nanodiamond), silica NPs, silicon QDs, etc. have become prevalent in biosensing, bioimaging, and phototherapy. While heavy metal-based QDs are limited to in vitro bioanalysis or clinical testing due to their potential metal ion-induced toxicity, carbon (nanodiamond and graphene) and silicon QDs, gold and silica nanoparticles, and metal nanoclusters continue their in vivo voyage towards clinical imaging and therapeutic applications. This review summarizes the synthesis, chemical modifications, optical properties, and bioimaging applications of semiconductor QDs with particular references to metal chalcogenide QDs and bimetallic chalcogenide QDs. Also, this review highlights the toxicity and pharmacokinetics of QD bioconjugates.

Microfluidic synthesis of quantum dots and their applications in bio-sensing and bio-imaging

Bio-sensing and bio-imaging of organisms or molecules can provide key information for the study of physiological processes or the diagnosis of diseases. Quantum dots (QDs) stand out to be promising optical detectors because of their excellent optical properties such as high brightness, stability, and multiplexing ability. Diverse approaches have been developed to generate QDs, while microfluidic technology is one promising path for their industrial production. In fact, microfluidic devices provide a controllable, rapid and effective route to produce high-quality QDs, while serving as an effective in situ platform to understand the synthetic mechanism or optimize reaction parameters for QD production. In this review, the recent research progress in microfluidic synthesis and bio-detection applications of QDs is discussed. The definitions of different QDs are first introduced, and the advances in microfluidic-based fabrication of quantum dots are summarized with a focus on perovskite QDs and carbon QDs. In addition, QD-based bio-sensing and bio-imaging technologies for organisms of different scales are described in detail. Finally, perspectives for future development of microfluidic synthesis and applications of QDs are presented.

Optimizing plasmon enhanced luminescence in silicon nanocrystals by gold nanorods

The great application potential of photoluminescent silicon nanocrystals, especially in biomedicine, is significantly reduced due to their limited radiative rate. One of the possible ways to overcome this limitation is enhancing the luminescence by localized plasmons of metallic nanostructures. We report an optimized fabrication of gold nanorod - silicon nanocrystal core-shell nanoparticles with the silica shell as a tunable spacer. The unprecedented structural quality and homogeneity of our hybrid nanoparticles allows for detailed analysis of their luminescence. A strong correlation between dark field scattering and luminescence spectra is shown on a single particle level, indicating a dominant role of the longitudinal plasmonic band in luminescence enhancement. The spacer thickness dependence of photoluminescence intensity enhancement is investigated using a combination of experimental measurements and numerical simulations. An optimal separation distance of 5 nm is found, yielding a 7.2× enhancement of the luminescence intensity. This result is mainly attributed to an increased quantum yield resulting from the Purcell enhanced radiative rate in the nanocrystals. The ease of fabrication, low cost, long-term stability and great emission properties of the hybrid nanoparticles make them a great candidate for bio-imaging or even targeted cancer treatment.

Catalytic parameters and thermal stability of chondroitinase ABCI on red porous silicon nanoparticles

The bacterial enzyme chondroitinase ABC, which digests extracellular chondroitin sulfate proteoglycans, has been shown to enhance axonal regeneration. However, the utilization of this enzyme as therapeutics is notably restricted due to its thermal instability. Therefore, red luminescent porous silicon that hold promise for potential applications in biological/medical imaging was used as a carrying matrix for chondroitinase with the aim of enhancing its stability. Porous Si nanoparticles were prepared by electrochemical etching of silicon wafers in ethanolic HF solution. The size of nanoparticles (210 nm) and the mean pore diameter (8 -20 nm) were determined using dynamic light scattering and scanning electron microscopy. Purified chondroitinase was then incorporated into the silicon pores. Results revealed similar K_m and lower V_max value for the immobilized enzyme when compared with the free enzyme. The immobilized chondroitinase exhibited about a 4 fold increase in stability at 37 °C after 50 min. It is likely possible that, the enzyme was protected inside the pores resulted in higher stability. Moreover, porous silicon was seen to be capable of holding the chondroitinase for repeated cyclic tests for three times. The cell viability assay exhibited no significant cytotoxicity for Psi-chondroitinase up to 24 h.

Group IV nanodots: synthesis, surface engineering and application in bioimaging and biotherapy

Group IV nanodots (NDs) mainly including carbon (C), silicon (Si), germanium (Ge) have aroused much attention as one type of important nanomaterials that are widely studied in optoelectronics, semiconductors, sensors and biomedicine-related fields owing to the low cost of synthesis, good stability, excellent biocompatibility, and some attractive newly emerged properties. In this review, the synthesis, surface engineering and application in bioimaging and biotherapy of group IV NDs are summarized and discussed. The recent progress in the rational synthesis and functionalization, specific therapy-related properties, together with in vivo and in vitro bioimaging are highlighted. Their new applications in biotherapy such as photothermal therapy (PTT) and photodynamic therapy (PDT) are illustrated with respect to C, Si and Ge NDs. The current challenges and future applications of these emerging materials in bioimaging and biotherapy are presented. This review provides readers with a distinct perspective of the group IV NDs nanomaterials for synthesis and surface engineering, and newly emerging properties related to applications in biomedicine.

Alterations in Growth and Morphology of Ganoderma lucidum and Volvariella volvaceae in Response to Nanoparticle Supplementation

Use of nanoparticles (NPs) in several commercial products has led to emergence of novel contaminants of air, soil and water bodies. The NPs may exhibit greater ecotoxicity due to nano-scale dependent properties over their bulk counterparts. The present investigation explores the effect of in vitro supplementation of TiO₂, silica and silver NPs on radial growth and ultrastructural changes in the hyphae and spores of two mushroom genera, Ganoderma lucidum and Volvariella volvaceae. A concentration dependent decrease in radial growth on NP amended potato dextrose agar medium was recorded. However, in comparison to control, there was decrease in radial diameter on supplementation with TiO₂ NPs while an increase was recorded for silica and silver NPs amendments as compared to their bulk salts at same concentrations after 48 h of incubation. Optical microscopy studies showed decrease in the number of spores while increase in spore diameter and thinning of hyphal diameter on NPs supplementation. Scanning electron microscopy analysis of fungal growth showed presence of deflated and oblong spores in two fruiting strains of Ganoderma while Volvariella exhibited decreased sporulation. Further, hyphal thinning and branching was recorded in response to NP amendments in both the test mushrooms. Enhancement of protein content was observed on NP compared to bulk supplementation for all cultures, concentrations and hours of incubation except for TiO₂ NPs. Likewise, bulk and NP supplementations (at 100 mg L ^-1) resulted in enhanced laccase activity with occurrence of laccase specific protein bands on SDS-PAGE analysis.

A new strategy to improve the water solubility of an organic fluorescent probe using silicon nanodots and fabricate two-photon SiND-ANPA-N3 for visualizing hydrogen sulfide in living cells and onion tissues

A water-soluble fluorescent probe based on SiNDs for H₂S detection can be used in both fully aqueous media and living cells.

Introductory lecture: origins and applications of efficient visible photoluminescence from silicon-based nanostructures

This review highlights many spectroscopy-based studies and selected phenomenological studies of silicon-based nanostructures that provide insight into their likely PL mechanisms, and also covers six application areas.

Interfacing enzymes with silicon nanocrystals through the thiol-ene reaction

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.

Quick Microwave Assisted Synthesis and In Vitro Imaging Application of Oxygen Doped Fluorescent Carbon Dots

In this paper, a fast and simplest one-pot tactic was used to synthesis fluorescent oxygen doped carbon dots from Tween-20 (TTO-CDs) is reported. The TTO-CDs were microwavically synthesized by using Tween-20 as both the carbon precursor and the oxygen dopant as well. The surface morphology, crystalline and/or amorphous nature, composition and optical assets of synthesized TTO-CDs were studied by means of existing techniques. From the results, it was confirmed that the as-synthesized TTO-CDs are amorphous in nature, monodispersed, sphere-shaped and the typical particle size range is 5 ± 1.5 nm. The synthesized TTO-CDs emits strong blue fluorescence at 390 nm under excitation of 335 nm. Most interestingly, the excitation dependent emission property of synthesized TTO-CDs was exposed from fluorescence results. The synthesized TTO-CDs have quantum yield of about 14% against quinine sulfate as reference standard. The biotoxicity of synthesized TTO-CDs on HeLa cells was assessed through cytotoxicity assay. These results implied that the fluorescent TTO-CDs showed less biotoxicity, and further which was efficaciously applied as a multicolor staining and bioimaging probe for the confocal imaging of HeLa cells.

Functional Bioinorganic Hybrids from Enzymes and Luminescent Silicon-Based Nanoparticles

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.

Fluorescence quenching of MoS2 nanosheets/DNA/silicon dot nanoassembly: effective and rapid detection of Hg2+ ions in aqueous solution

Mercury (Hg) contamination of aquatic sites represents a serious risk for human health and the environment. Therefore, effective and rapid monitoring of Hg in aqueous samples is a challenge of timely importance nowadays. In the present study, a rapid and sensitive mercury sensor based on the fluorescence quenching of MoS₂ nanosheets/DNA/silicon dot nanoassembly has been developed for the efficient detection of mercury(II) in aquatic environments. In this process, silicon dots were synthesized through one-step high-temperature calcinations and thermomagnesium reduction method at 900 °C using rice husk as a silicon source, which demonstrates superior photophysical properties and excitation-dependent fluorescence behavior. The interaction between MoS₂ nanosheets/DNA/silicon dot nanoassembly and Hg²⁺ ions was studied using photoluminescence spectroscopy. The addition of Hg²⁺ ions to the assay solution induced the detachment of fluorescent probe from the surface of MoS₂ nanosheets. Thus, the fluorescent probes sustained its fluorescence intensity. The developed sensor was tested on various concentrations of Hg²⁺ ions ranging from 0 to 1000 nM as well as on various metal ions. In addition, MoS₂ nanosheets/DNA/silicon dot nanoassembly fluorescent Hg sensor efficiently detected the presence of Hg²⁺ ions in real-time water samples, which was comparably detected by the conventional atomic absorbance spectrometer (AAS). Overall, our results highlighted the high reliability of the present approach for environmental monitoring of Hg²⁺ ions, if compared to that of the customary method with a lowest detection limit of 0.86 nM.

Correlation between luminescence and structural evolution of colloidal silicon nanocrystals synthesized under different laser fluences

We present a detailed investigation of the structural evolution and photoluminescence (PL) properties of colloidal silicon (Si) nanocrystals (NCs) synthesized through femtosecond laser ablation at different laser fluences. It is shown that the mean size of colloidal Si NCs increases from ∼0.97-2.37 nm when increasing laser fluence from 1.0-2.5 mJ cm^-2. On the basis of structural characterization, temperature-dependent PL, time-resolved PL, and PL excitation spectra, we identify that the size-dependent spectral shift of violet emission is attributed to the quantum confinement effect. The localized excitons' radiative recombination via the oxygen-related surface states on the surface of the colloidal Si NCs is employed to explain the origin of the blue emission.

The interaction between human blood neutrophil granulocytes and quantum dots

For biomedical applications, it is important to know, which kinds of blood cells can capture quantum dots (QDs). The maximum accumulation of QDs was found for the monocyte fraction of leukocytes, the minimum binding of QDs was observed for lymphocytes. It was found that CdSe/ZnS-MPA QDs are actively absorbed by the cells and have more expressed toxicity. The classical mechanism of the phagocytosis of QDs was revealed for neutrophils, when the QDs are located in phagolysosomes. The capture of QDs by neutrophil granulocytes has resulted in a destruction of certain types of QDs. The interaction of the neutrophils with the QDs has resulted in the death of the cells by one of the following cell death mechanisms: necrosis, apoptosis, autophagy, NETos, or mummification. The aggregation of the QDs manifested as an increase of the hydrodynamic diameter of the QDs was found to occur under the influence of serum and under the influence of blood cells (lymphocytes and neutrophils) in a serum-free medium.

Effects of silica nanoparticle exposure on mitochondrial function during neuronal differentiation

BACKGROUND

Nanomedicine offers a promising tool for therapies of brain diseases, but potential effects on neuronal health and neuronal differentiation need to be investigated to assess potential risks. The aim of this study was to investigate effects of silica-indocyanine green/poly (ε-caprolactone) nanoparticles (PCL-NPs) engineered for laser tissue soldering in the brain before and during differentiation of SH-SY5Y cells. Considering adaptations in mitochondrial homeostasis during neuronal differentiation, metabolic effects of PCL-NP exposure before and during neuronal differentiation were studied. In addition, kinases of the PI3 kinase (PI3-K/Akt) and the MAP kinase (MAP-K/ERK) pathways related to neuronal differentiation and mitochondrial function were investigated.

RESULTS

Differentiation resulted in a decrease in the cellular respiration rate and the extracellular acidification rate (ECAR). PCL-NP exposure impaired mitochondrial function depending on the time of exposure. The cellular respiration rate was significantly reduced compared to differentiated controls when PCL-NPs were given before differentiation. The shift in ECAR was less pronounced in PCL-NP exposure during differentiation. Differentiation and PCL-NP exposure had no effect on expression levels and the enzymatic activity of respiratory chain complexes. The activity of the glycolytic enzyme phosphofructokinase was significantly reduced after differentiation with the effect being more pronounced after PCL-NP exposure before differentiation. The increase in mitochondrial membrane potential observed after differentiation was not found in SH-SY5Y cells exposed to PCL-NPs before differentiation. The cellular adenosine triphosphate (ATP) production significantly dropped during differentiation, and this effect was independent of the PCL-NP exposure. Differentiation and nanoparticle exposure had no effect on superoxide levels at the endpoint of the experiments. A slight decrease in the expression of the neuronal differentiation markers was found after PCL-NP exposure, but no morphological variation was observed.

CONCLUSIONS

PCL-NP exposure affects mitochondrial function depending on the time of exposure before and during neuronal differentiation. PCL-NP exposure during differentiation was associated with impaired mitochondrial function, which may affect differentiation. Considering the importance of adaptations in cellular respiration for neuronal differentiation and function, further studies are needed to unravel the underlying mechanisms and consequences to assess the possible risks including neurodegeneration.

Surface chemistry and density distribution influence on visible luminescence of silicon quantum dots: an experimental and theoretical approach

The impact of the surface reconstruction of the density distribution and photoluminescence of silicon quantum dots (QDs) embedded in a silicon oxide matrix (SiO_x) has been studied. Annealing treatments carried out on the as-deposited samples provoked the effusion of hydrogen species. Moreover, depending on the surrounding density and coalescence of QDs, they resulted in a change in the average size of the particles depending on the initial local environment. The shift in the luminescence spectra all over the visible region (blue, green and red) shows a strong dependence on the resultant change in the size and/or the passivation environment of QDs. Density functional theoretical (DFT) calculations support this fact and explain the possible electronic transitions (HOMO-LUMO gap) involved. Passivation in the presence of oxygen species lowers the band gap of Si₂₉ and Si₃₅ nanoclusters up to 1.7 eV, whereas, surface passivation in the environment of hydrogen species increases the band gap up to 4.4 eV. These results show a good agreement with the quantum confinement model described in this work and explain the shift in the luminescence all over the visible region. The results reported here offer vital insight into the mechanism of emission from silicon quantum dots which has been one of the most debated topics in the last two decades. QDs with multiple size distribution in different local environments (band gap) observed in this work could be used for the fabrication of light emission diodes (LEDs) or shift-conversion thin films in third generation efficient tandem solar cells for the maximum absorption of the solar spectrum in different wavelength regions.

Synthesis of Diagnostic Silicon Nanoparticles for Targeted Delivery of Thiourea to Epidermal Growth Factor Receptor-Expressing Cancer Cells

The novel thiourea-functionalized silicon nanoparticles (SiNPs) have been successfully synthesized using allylamine and sulforaphane, an important anticancer drug, followed by a hydrosilylation reaction on the surface of hydrogen terminated SiNPs. Their physiochemical properties have been investigated by photoluminescence emission, Fourier transform infrared spectroscopy (FTIR) and elemental analysis. The MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay has been employed to evaluate in vitro toxicity in human colorectal adenocarcinoma (Caco-2) cells and human normal colon epithelial (CCD) cells. The results show significant toxicity of thiourea SiNPs after 72 h of incubation in the cancer cell line, and the toxicity is concentration dependent and saturated for concentrations above 100 μg/mL. Confocal microscopy images have demonstrated the internalization of thiourea-functionalized SiNPs inside the cells. Flow cytometry data has confirmed receptor-mediated targeting in cancer cells. This nanocomposite takes advantage of the epidermal growth factor receptor (EGFR) active targeting of the ligand in addition to the photoluminescence properties of SiNPs for bioimaging purposes. The results suggest that this novel nanosystem can be extrapolated for active targeting of the receptors that are overexpressed in cancer cells such as EGFR using the targeting characteristics of thiourea-functionalized SiNPs and therefore encourage further investigation and development of anticancer agents specifically exploiting the EGFR inhibitory activity of such nanoparticles.

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.

Water-Soluble Silicon Quantum Dots with Quasi-Blue Emission

In this study, water-soluble silicon quantum dots have quasi-blue emission at 390 nm by being capped with 1-vinylimidazole in resese micelles. As-obtained silicon quantum dots have a diameter of 2~5 nm and high crystallinity. The quasi-blue emission of the silicon quantum dots is likely attributed to the polarity of the capping ligands. Moreover, the silicon quantum dots are water-soluble and have photoluminescence nanosecond decay time, suggesting their potential application in biological field.

Enhanced Luminescent Stability through Particle Interactions in Silicon Nanocrystal Aggregates

Close-packed assemblies of ligand-passivated colloidal nanocrystals can exhibit enhanced photoluminescent stability, but the origin of this effect is unclear. Here, we use experiment, simulation, and ab initio computation to examine the influence of interparticle interactions on the photoluminescent stability of silicon nanocrystal aggregates. The time-dependent photoluminescence emitted by structures ranging in size from a single quantum dot to agglomerates of more than a thousand is compared with Monte Carlo simulations of noninteracting ensembles using measured single-particle blinking data as input. In contrast to the behavior typically exhibited by the metal chalcogenides, the measured photoluminescent stability shows an enhancement with respect to the noninteracting scenario with increasing aggregate size. We model this behavior using time-dependent density functional theory calculations of energy transfer between neighboring nanocrystals as a function of nanocrystal size, separation, and the presence of charge and/or surface-passivation defects. Our results suggest that rapid exciton transfer from "bright" nanocrystals to surface trap states in nearest-neighbors can efficiently fill such traps and enhance the stability of emission by promoting the radiative recombination of slowly diffusing excited electrons.

Silicon-based quantum dots induce inflammation in human lung cells and disrupt extracellular matrix homeostasis

Quantum dots (QDs) are nanocrystalline semiconductor materials that have been tested for biological applications such as cancer therapy, cellular imaging and drug delivery, despite the serious lack of information of their effects on mammalian cells. The present study aimed to evaluate the potential of Si/SiO2 QDs to induce an inflammatory response in MRC-5 human lung fibroblasts. Cells were exposed to different concentrations of Si/SiO2 QDs (25-200 μg·mL(-1)) for 24, 48, 72 and 96 h. The results obtained showed that uptake of QDs was dependent on biocorona formation and the stability of nanoparticles in various biological media (minimum essential medium without or with 10% fetal bovine serum). The cell membrane damage indicated by the increase in lactate dehydrogenase release after exposure to QDs was dose- and time-dependent. The level of lysosomes increased proportionally with the concentration of QDs, whereas an accumulation of autophagosomes was also observed. Cellular morphology was affected, as shown by the disruption of actin filaments. The enhanced release of nitric oxide and the increase in interleukin-6 and interleukin-8 protein expression suggested that nanoparticles triggered an inflammatory response in MRC-5 cells. QDs decreased the protein expression and enzymatic activity of matrix metalloproteinase (MMP)-2 and MMP-9 and also MMP-1 caseinase activity, whereas the protein levels of MMP-1 and tissue inhibitor of metalloproteinase-1 increased. The present study reveals for the first time that silicon-based QDs are able to generate inflammation in lung cells and cause an imbalance in extracellular matrix turnover through a differential regulation of MMPs and tissue inhibitor of metalloproteinase-1 protein expression.

Fluorescent porous silicon biological probes with high quantum efficiency and stability

We demonstrate porous silicon biological probes as a stable and non-toxic alternative to organic dyes or cadmium-containing quantum dots for imaging and sensing applications. The fluorescent silicon quantum dots which are embedded on the porous silicon surface are passivated with carboxyl-terminated ligands through stable Si-C covalent bonds. The porous silicon bio-probes have shown photoluminescence quantum yield around 50% under near-UV excitation, with high photochemical and thermal stability. The bio-probes can be efficiently conjugated with antibodies, which is confirmed by a standard enzyme-linked immunosorbent assay (ELISA) method.

Solution synthesis, optical properties, and bioimaging applications of silicon nanocrystals

Understanding and unlocking the potential of semiconductor nanocrystals (NCs) is important for future applications ranging from biomedical imaging contrast agents to the next generation of solar cells and LEDs. Silicon NCs (Si NCs) have key advantages compared with other semiconductor NCs due to silicon's high natural abundance, low toxicity and strong biocompatibility, and unique size, and surface dependent optical properties. In this Account, we review and discuss the synthesis, surface modification, purification, optical properties, and applications of Si NCs. The synthetic methods used to make Si NCs have improved considerably in the last 5-10 years; highly monodisperse Si NCs can now be produced on the near gram scale. Scaled-up syntheses have allowed scientists to drive further toward the commercial utilization of Si NCs. The synthesis of doped Si NCs, through addition of a simple elemental precursor to a reaction mixture or by the production of a single source precursor, has shown great promise. Doped Si NCs have demonstrated unique or enhanced properties compared with pure Si NCs, for example, magnetism due to the presence of magnetic metals like Fe and Mn. Surface reactions have reached a new level of sophistication where organic (epoxidation and diol formation) and click (thiol based) chemical reactions can be carried out on attached surface molecules. This has led to a wide range of biocompatible functional groups as well as a degree of emission tuneability. The purification of Si NCs has been improved through the use of size separation columns and size selective precipitation. These purification approaches have yielded highly monodisperse and pure Si NCs previously unachieved. This has allowed scientists to study the size and surface dependent properties and toxicity and enabled the use of Si NCs in biomedical applications. The optical properties of Si NCs are complex. Using a combination of characterization techniques, researchers have explored the relation between the optical properties and the size, surface functionalization, and preparation method. This work has led to a greater fundamental understanding of the unique optical properties of Si NCs. Si NCs are being studied for a wide range of important applications, including LEDS with tunable electroluminescence ranging from NIR to yellow, the encapsulation of Si NCs within micelles terminated with proteins to allow targeted in vivo imaging of cells, Si NC-polymer hybrid solar cells, and the use of Si NCs in battery anodes with high theoretical capacity and good charge retention.

Effects of silica and titanium oxide particles on a human neural stem cell line: morphology, mitochondrial activity, and gene expression of differentiation markers. Int J Mol Sci. 2014;15:11742-11759. [PMID: 24992594 PMCID: PMC4139811 DOI: 10.3390/ijms150711742]

Several in vivo studies suggest that nanoparticles (smaller than 100 nm) have the ability to reach the brain tissue. Moreover, some nanoparticles can penetrate into the brains of murine fetuses through the placenta by intravenous administration to pregnant mice. However, it is not clear whether the penetrated nanoparticles affect neurogenesis or brain function. To evaluate its effects on neural stem cells, we assayed a human neural stem cell (hNSCs) line exposed in vitro to three types of silica particles (30 nm, 70 nm, and <44 μm) and two types of titanium oxide particles (80 nm and < 44 μm). Our results show that hNSCs aggregated and exhibited abnormal morphology when exposed to the particles at concentrations ≥ 0.1 mg/mL for 7 days. Moreover, all the particles affected the gene expression of Nestin (stem cell marker) and neurofilament heavy polypeptide (NF-H, neuron marker) at 0.1 mg/mL. In contrast, only 30-nm silica particles at 1.0 mg/mL significantly reduced mitochondrial activity. Notably, 30-nm silica particles exhibited acute membrane permeability at concentrations ≥62.5 μg/mL in 24 h. Although these concentrations are higher than the expected concentrations of nanoparticles in the brain from in vivo experiments in a short period, these thresholds may indicate the potential toxicity of accumulated particles for long-term usage or continuous exposure.

Silicon quantum dots: surface matters

Silicon quantum dots (SiQDs) hold great promise for many future technologies. Silicon is already at the core of photovoltaics and microelectronics, and SiQDs are capable of efficient light emission and amplification. This is crucial for the development of the next technological frontiers-silicon photonics and optoelectronics. Unlike any other quantum dots (QDs), SiQDs are made of non-toxic and abundant material, offering one of the spectrally broadest emission tunabilities accessible with semiconductor QDs and allowing for tailored radiative rates over many orders of magnitude. This extraordinary flexibility of optical properties is achieved via a combination of the spatial confinement of carriers and the strong influence of surface chemistry. The complex physics of this material, which is still being unraveled, leads to new effects, opening up new opportunities for applications. In this review we summarize the latest progress in this fascinating research field, with special attention given to surface-induced effects, such as the emergence of direct bandgap transitions, and collective effects in densely packed QDs, such as space separated quantum cutting.

Cell-based in vitro blood-brain barrier model can rapidly evaluate nanoparticles' brain permeability in association with particle size and surface modification

The possibility of nanoparticle (NP) uptake to the human central nervous system is a major concern. Recent reports showed that in animal models, nanoparticles (NPs) passed through the blood–brain barrier (BBB). For the safe use of NPs, it is imperative to evaluate the permeability of NPs through the BBB. Here we used a commercially available in vitro BBB model to evaluate the permeability of NPs for a rapid, easy and reproducible assay. The model is reconstructed by culturing both primary rat brain endothelial cells and pericytes to support the tight junctions of endothelial cells. We used the permeability coefficient (P_app) to determine the permeability of NPs. The size dependency results, using fluorescent silica NPs (30, 100, and 400 nm), revealed that the P_app for the 30 nm NPs was higher than those of the larger silica. The surface charge dependency results using Qdots^® (amino-, carboxyl-, and PEGylated-Qdots), showed that more amino-Qdots passed through the model than the other Qdots. Usage of serum-containing buffer in the model resulted in an overall reduction of permeability. In conclusion, although additional developments are desired to elucidate the NPs transportation, we showed that the BBB model could be useful as a tool to test the permeability of nanoparticles.

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.

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.

Assessing clinical prospects of silicon quantum dots: studies in mice and monkeys

Silicon nanocrystals can provide the outstanding imaging capabilities of toxic heavy-metal-based quantum dots without employing heavy metals and have potential for rapid progression to the clinic. Understanding the toxicity of silicon quantum dots (SiQDs) is essential to realizing this potential. However, existing studies of SiQD biocompatibility are limited, with no systematic progression from small-animal to large-animal studies that are more clinically relevant. Here, we test the response of both mice and monkeys to high intravenous doses of a nanoconstruct created using only SiQDs and FDA-approved materials. We show that (1) neither mice nor monkeys show overt signs of toxicity reflected in their behavior, body mass, or blood chemistry, even at a dose of 200 mg/kg. (2) This formulation did not biodegrade as expected. Elevated levels of silicon were present in the liver and spleen of mice three months post-treatment. (3) Histopathology three months after treatment showed adverse effects of the nanoformulation in the livers of mice, but showed no such effects in monkeys. This investigation reveals that the systemic reactions of the two animal models may have some differences and there are no signs of toxicity clearly attributable to silicon quantum dots.

Silicon and germanium nanoparticles with tailored surface chemistry as novel inorganic fiber brightening agents

Low-molecular-weight organic molecules, such as coumarins and stilbenes, are used commercially as fluorescent whitening agents (FWAs) to mask photoyellowing and to brighten colors in fabrics. FWAs achieve this by radiating extra blue light, thus changing the hue and also adding to the brightness. However, organic FWAs can rapidly photodegrade in the presence of ultraviolet (UV) radiation, exacerbating the yellowing process through a reaction involving singlet oxygen species. Inorganic nanoparticles, on the other hand, can provide a similar brightening effect with the added advantage of photostability. We report a targeted approach in designing new inorganic silicon- and germanium-based nanoparticles, functionalized with hydrophilic (amine) surface terminations as novel inorganic FWAs. When applied on wool, by incorporation in a sol-gel Si matrix, the inorganic FWAs improved brightness properties, demonstrated enhanced photostability toward UV radiation, especially the germanium nanoparticles, and also generated considerably lower levels of reactive oxygen species compared to a commercial stilbene-based organic FWA, Uvitex NFW.

Interaction of silicon-based quantum dots with gibel carp liver: oxidative and structural modifications

Quantum dots (QDs) interaction with living organisms is of central interest due to their various biological and medical applications. One of the most important mechanisms proposed for various silicon nanoparticle-mediated toxicity is oxidative stress. We investigated the basic processes of cellular damage by oxidative stress and tissue injury following QD accumulation in the gibel carp liver after intraperitoneal injection of a single dose of 2 mg/kg body weight Si/SiO2 QDs after 1, 3, and 7 days from their administration.QDs gradual accumulation was highlighted by fluorescence microscopy, and subsequent histological changes in the hepatic tissue were noted. After 1 and 3 days, QD-treated fish showed an increased number of macrophage clusters and fibrosis, while hepatocyte basophilia and isolated hepatolytic microlesions were observed only after substantial QDs accumulation in the liver parenchyma, at 7 days after IP injection.Induction of oxidative stress in fish liver was revealed by the formation of malondialdehyde and advanced oxidation protein products, as well as a decrease in protein thiol groups and reduced glutathione levels. The liver enzymatic antioxidant defense was modulated to maintain the redox status in response to the changes initiated by Si/SiO2 QDs. So, catalase and glutathione peroxidase activities were upregulated starting from the first day after injection, while the activity of superoxide dismutase increased only after 7 days. The oxidative damage that still occurred may impair the activity of more sensitive enzymes. A significant inhibition in glucose-6-phosphate dehydrogenase and glutathione-S-transferase activity was noted, while glutathione reductase remained unaltered.Taking into account that the reduced glutathione level had a deep decline and the level of lipid peroxidation products remained highly increased in the time interval we studied, it appears that the liver antioxidant defense of Carassius gibelio does not counteract the oxidative stress induced 7 days after silicon-based QDs exposure in an efficient manner.

Temperature dependent photoluminescence of size-purified silicon nanocrystals

The photoluminescence (PL) of size-purified silicon nanocrystals is measured as a function of temperature and nanoparticle size for pure nanocrystal films and polydimethylsiloxane (PDMS) nanocomposites. The temperature dependence of the bandgap is the same for both sample types, being measurably different from that of bulk silicon because of quantum confinement. Our results also suggest weaker interparticle and environmental coupling in the nanocomposites, with enhanced PL and an unexpected dependence of lifetime on size for the pure nanocrystal films at low temperatures. We interpret these results through differences in the low-temperature size dependence of the ensemble nonradiative equilibrium constants. The response of the PDMS nanocomposites provides a consistent measure of local temperature through intensity, lifetime, and wavelength in a polymer-dispersed morphology suitable for biomedical applications, and we exploit this to fabricate a small-footprint fiber-optic cryothermometer. A comparison of the two sample types offers fundamental insight into the photoluminescent behavior of silicon nanocrystal ensembles.

Chemically differentiating ascorbate-mediated dissolution of quantum dots in cell culture media

To investigate the dynamic dissolution of quantum dots (QDs) in cell culture media, in this study we constructed an online automatic analytical system comprising a sequential in-tube solid phase extraction (SPE) device and an inductively coupled plasma (ICP) mass spectrometer. By means of selectively extracting QDs and cadmium ions (Cd(2+)) onto the interior surface of the polytetrafluoroethylene (PTFE) tube, this novel SPE device could be used to determine the degree of QD dissolution through a simple adjustment of sample acidity. To the best of our knowledge, this study is the first to exploit PTFE tubing as a selective SPE adsorbent for the online chemical differentiation of QDs and Cd(2+) ions with the goal of monitoring the phenomenon of QD dissolution in complicated biological matrices. We confirmed the analytical reliability of this system through comparison of the measured Cd-to-QD ratios to the expected values. When analyzing QDs and Cd(2+) ions at picomolar levels, a temporal resolution of 8 min was required to load sufficient amounts of the analytes to meet the sensitivity requirements of the ICP mass spectrometer. To demonstrate the practicability of this developed method, we measured the dynamic variations in the Cd-to-QD705 ratio in the presence of ascorbate as a physiological stimulant to generate reactive oxygen species in cell culture media and trigger the dissolution of QDs; our results suggest that the ascorbate-induced QD dissolution was dependent on the time, treatment concentration, and nature of the biomolecule.

Evaluation of anti-inflammatory drug-conjugated silicon quantum dots: their cytotoxicity and biological effect

Silicon quantum dots (Si-QDs) have great potential for biomedical applications, including their use as biological fluorescent markers and carriers for drug delivery systems. Biologically inert Si-QDs are less toxic than conventional cadmium-based QDs, and can modify the surface of the Si-QD with covalent bond. We synthesized water-soluble alminoprofen-conjugated Si-QDs (Ap-Si). Alminoprofen is a non-steroid anti-inflammatory drug (NSAID) used as an analgesic for rheumatism. Our results showed that the "silicon drug" is less toxic than the control Si-QD and the original drug. These phenomena indicate that the condensed surface integration of ligand/receptor-type drugs might reduce the adverse interaction between the cells and drug molecules. In addition, the medicinal effect of the Si-QDs (i.e., the inhibition of COX-2 enzyme) was maintained compared to that of the original drug. The same drug effect is related to the integration ratio of original drugs, which might control the binding interaction between COX-2 and the silicon drug. We conclude that drug conjugation with biocompatible Si-QDs is a potential method for functional pharmaceutical drug development.

Bioconjugation of luminescent silicon quantum dots to gadolinium ions for bioimaging applications

Luminescent imaging agents and MRI contrast agents are desirable components in the rational design of multifunctional nanoconstructs for biological imaging applications. Luminescent biocompatible silicon quantum dots (SiQDs) and gadolinium chelates can be applied for fluorescence microscopy and MRI, respectively. Here, we report the first synthesis of a nanocomplex incorporating SiQDs and gadolinium ions (Gd³⁺) for biological applications. The nanoconstruct is composed of a PEGylated micelle, with hydrophobic SiQDs in its core, covalently bound to DOTA-chelated Gd³⁺. Dynamic light scattering reveals a radius of 85 nm for these nanoconstructs, which is consistent with the electron microscopy results depicting radii ranging from 25 to 60 nm. Cellular uptake of the probes verified that they maintain their optical properties within the intracellular environment. The magnetic resonance relaxivity of the nanoconstruct was 2.4 mM⁻¹ s⁻¹ (in terms of Gd³⁺ concentration), calculated to be around 6000 mM⁻¹ s⁻¹ per nanoconstruct. These desirable optical and relaxivity properties of the newly developed probe open the door for use of SiQDs in future multimodal applications such as tumour imaging.

Ensemble brightening and enhanced quantum yield in size-purified silicon nanocrystals

We report on the quantum yield, photoluminescence (PL) lifetime, and ensemble photoluminescent stability of highly monodisperse plasma-synthesized silicon nanocrystals (SiNCs) prepared though density-gradient ultracentrifugation in mixed organic solvents. Improved size uniformity leads to a reduction in PL line width and the emergence of entropic order in dry nanocrystal films. We find excellent agreement with the anticipated trends of quantum confinement in nanocrystalline silicon, with a solution quantum yield that is independent of nanocrystal size for the larger fractions but decreases dramatically with size for the smaller fractions. We also find a significant PL enhancement in films assembled from the fractions, and we use a combination of measurement, simulation, and modeling to link this "brightening" to a temporally enhanced quantum yield arising from SiNC interactions in ordered ensembles of monodisperse nanocrystals. Using an appropriate excitation scheme, we exploit this enhancement to achieve photostable emission.