Synthesis and Application of Silica-Coated Quantum Dots in Biomedicine.

Quantum dots (QDs) are semiconductor nanoparticles with outstanding optoelectronic properties. More specifically, QDs are highly bright and exhibit wide absorption spectra, narrow light bands, and excellent photovoltaic stability, which make them useful in bioscience and medicine, particularly for sensing, optical imaging, cell separation, and diagnosis. In general, QDs are stabilized using a hydrophobic ligand during synthesis, and thus their hydrophobic surfaces must undergo hydrophilic modification if the QDs are to be used in bioapplications. Silica-coating is one of the most effective methods for overcoming the disadvantages of QDs, owing to silica's physicochemical stability, nontoxicity, and excellent bioavailability. This review highlights recent progress in the design, preparation, and application of silica-coated QDs and presents an overview of the major challenges and prospects of their application.

Rapid and sensitive SERS detection of the cytokine tumor necrosis factor alpha (tnf-α) in a magnetic bead pull-down assay with purified and highly Raman-active gold nanoparticle clusters.

Tumor necrosis factor alpha (TNF-α) is a cytokine with significance in early diagnosis of cardiovascular diseases, obesity and insulin resistance. We demonstrate the proof of concept for a rapid and sensitive detection of TNF-α using a magnetic bead pull-down assay in combination with surface-enhanced Raman scattering (SERS). The use of purified and highly SERS-active small clusters of gold nanoparticles (AuNP) provides the high sensitivity of the assay with a limit of detection of ca. 1 pg/mL. Continuous density gradient centrifugation was employed for separating the very bright silica-encapsulated AuNP dimers and trimers from the significantly weaker AuNP monomers. Negative control experiments with other cytokines (IL-6, IL-8) and bovine serum albumin (BSA) confirm the high specificity of the assay, but indicate also space for future improvements by further reducing non-specific binding between proteins and the SERS nanotags. The multiplexing potential of this SERS-based detection scheme is exemplarily demonstrated by using a set of three spectrally distinct and highly SERS-active AuNP clusters with unique spectral barcodes. Graphical abstract ᅟ.

Bright and Fast Emission from Robust Supramolecular J-Aggregate Nanostructures through Silica-Encapsulation.

We introduce a two-step silica-encapsulation procedure to optimize both the optical efficiency and structural robustness of 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), a two-dimensional sheet-like J-aggregate. We report a fluorescence quantum yield of ∼98%, the highest quantum yield recorded for any J-aggregate structure at room temperature, and a fast, emissive lifetime of 234 ps. Silica, as an encapsulating matrix, provides optical transparency, chemical inertness, and robustness to dilution, while rigidifying the J-aggregate structure. Our in situ encapsulation process preserves the excitonic structure in TDBC J-aggregates, maintaining their light absorption and emission properties. The homogeneous silica coating has an average thickness of 0.5-1 nm around J-aggregate sheets. Silica encapsulation permits extensive dilutions of J-aggregates without significant disintegration into monomers. The narrow absorbance and emission line widths exhibit further narrowing upon cooling to 79 K, which is consistent with J-type coupling in the encapsulated aggregates. This silica TDBC J-aggregate construct signifies (1) a bright, fast, and robust fluorophore system, (2) a platform for further manipulation of J-aggregates as building blocks for integration with other optical materials and structures, and (3) a system for fundamental studies of exciton delocalization, transport, and emission dynamics within a rigid matrix.

Silica encapsulation of thiol-stabilized lead selenide (PbSe) quantum dots in aqueous solution.

Silica encapsulation of lead selenide quantum dots (PbSe QDs) in aqueous solution is reported. Thioglycolic acid (TGA) stabilized PbSe QDs were modified with 3-mercaptopropyl trimethoxysilane (MPS) through vigorous stirring in water for 18-24 h in alkaline solution (pH 10.4-10.6). Silica shell was developed by controlled deposition and precipitation of silicates from sodium silicate solution onto MPS modified QDs surfaces. TEM images showed multiple PbSe QDs encapsulated in silica shell. The size of PbSe-SiO2 core-shell nanocrystals was estimated to be 25-30 nm by TEM. Elemental compositions (Pb, Se and Si) were investigated by EDX analysis. The purified colloids of PbSe-SiO2 QDs were stable for months when kept at 4 °C.

Silica Encapsulation of Hydrophobic Optical NP-Embedded Silica Particles with Trimethoxy(2-Phenylethyl)silane.

Nanoparticles (NP) with optical properties embedded silica particles have been widely used in various fields because of their unique properties. The surfaces of optical NPs have been modified with various organic ligands to maintain their unique optical properties and colloidal stability. Among the surface modification methods, silica encapsulation of optical NPs is widely used to enhance their biocompatibility and stability. However, in the case of NPs with hydrophobic ligands on the surface, the ligands that determine the optical properties of the NPs may detach from the NPs, thereby changing the optical properties during silica encapsulation. Herein, we report a generally applicable silica encapsulation method using trimethoxy(2-phenylethyl)silane (TMPS) for non-hydrophilic optical NPs, such as quantum dots (QDs) and gold NPs. This silica encapsulation method was applied to fabricate multiple silica-encapsulated QD-embedded silica NPs (SiO2@QD@SiO2 NPs; QD2) and multiple silica-encapsulated gold NP-embedded silica NPs labeled with 2-naphthalene thiol (SiO2@Au2-NT@SiO2). The fabricated silica-encapsulated NPs exhibited optical properties without significant changes in the quantum yield or Raman signal intensity.

Method to Isolate Dormant Cancer Cells from Heterogeneous Populations.

Cancer recurrence is responsible for a high percentage of cancer-related deaths. Primary tumor removal, chemotherapy, and radiotherapy often leave behind cancer cells that are clinically undetectable. Recent evidence has shown that subpopulations of these residual cancer cells enter into a prolonged dormant state, remaining quiescent for months to years, and eventually lead to metastases and relapse (Sosa et al. Nat Rev Cancer 14:611-622, 2014). Identifying the presence of and isolating these dormancy-capable cells (DCCs) from resected tumors or bodily fluids may therefore provide an opportunity to understand their biology and develop personalized treatments for patients at risk for relapse. Physical confinement in a stiff and porous 3D matrix, which inhibits proliferation, migration, and growth of the immobilized cells, has been shown to isolate DCC populations (Preciado et al. Technology 05:1-10, 2017; Reátegui et al. J Mater Chem B 2:7440-7448, 2014). Isolated DCCs can then be recovered from the gel and analyzed. Here we describe this immobilization method that can be used to isolate DCCs from heterogeneous cell populations that may also include dormancy-incapable cancer cells and host cells.

Corrigendum: Signal Disruption Leads to Changes in Bacterial Community Population.

[This corrects the article DOI: 10.3389/fmicb.2019.00611.].

Preparation and In Vitro Cytotoxicity Assessments of Spherical Silica-Encapsulated Liposome Particles for Highly Efficient Drug Carriers.

This work reports the formation of spherical solid silica-encapsulated liposome particles (SLPs) as functions of the concentration of silica precursor, reaction time, temperature, and volume ratios of solvent, respectively. The solid SLPs are more robust and have better drug-loading-efficiency liquid liposomes in carrier formulations. The liquid-state liposomes are hard to handle and have a lower drug-loading efficiency because they are fragile to external stimuli and have narrow hydrophobic phospholipid bilayers. The SLPs were obtained by silicification with tetraethyl orthosilicate (TEOS) in the hydrophilic region of phospholipid bilayers by a sol-gel process. These SLPs were characterized by scanning electron microscopy (SEM), focused ion beam (FIB)-SEM, confocal laser scanning microscopy (CLSM), thermogravimetric analysis (TGA), ζ-potential, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and UV-vis spectroscopy. The obtained SLPs were spherical with an average size of 2-3 µm. The hydrophilic region of the SLP phospholipid bilayers was confirmed using CLSM with green fluorescent fluorescein isothiocyanate (FITC) labeling and FIB-SEM. Furthermore, the drug-loading capacity and in vitro cytotoxicity assessments were performed using several drug compounds and L929 cells. The drug-loading capacity of the SLPs was >95%, and in particular, the hydrophobic drug-loading capacity was 2.3 times higher than that of general liposomes. Moreover, the result of an in vitro cytotoxicity assessment of the SLPs was good, about 99% of cell viability.

Removal of Diclofenac, Paracetamol, and Carbamazepine from Model Aqueous Solutions by Magnetic Sol-Gel Encapsulated Horseradish Peroxidase and Lignin Peroxidase Composites.

Sustainable and green synthesis of nanocomposites for degradation of pharmaceuticals was developed via immobilization and stabilization of the biological strong oxidizing agents, peroxidase enzymes, on a solid support. Sol-gel encapsulated enzyme composites were characterized using electron microscopy (TEM, SEM), atomic force microscopy, FTIR spectroscopy, and thermogravimetric analysis. Horseradish peroxidase (HRP) and lignin peroxidase (LiP) were adsorbed onto magnetite nanoparticles and sol-gel encapsulated in a surface silica layer. Encapsulation enhanced the stability of the biocatalysts over time and thermal stability. The biocatalysts showed appreciable selectivity in oxidation of the organic drinking water pollutants diclofenac, carbamazepine, and paracetamol with improved activity being pharmaceutical specific for each enzyme. In particular, sol-gel encapsulated LiP- and HRP-based nanocomposites were active over 20 consecutive cycles for 20 days at 55 °C (24 h/cycle). The stability of the sol-gel encapsulated catalysts in acidic medium was also improved compared to native enzymes. Carbamazepine and diclofenac were degraded to 68% and 64% by sol-gel LiP composites respectively at pH 5 under elevated temperature. Total destruction of carbamazepine and diclofenac was achieved at pH 3 (55 °C) within 3 days, in the case of both immobilized HRP and LiP. Using NMR spectroscopy, characterization of the drug decomposition products, and decomposition pathways by the peroxidase enzymes suggested.

Signal Disruption Leads to Changes in Bacterial Community Population.

The disruption of bacterial signaling (quorum quenching) has been proven to be an innovative approach to influence the behavior of bacteria. In particular, lactonase enzymes that are capable of hydrolyzing the N-acyl homoserine lactone (AHL) molecules used by numerous bacteria, were reported to inhibit biofilm formation, including those of freshwater microbial communities. However, insights and tools are currently lacking to characterize, understand and explain the effects of signal disruption on complex microbial communities. Here, we produced silica capsules containing an engineered lactonase that exhibits quorum quenching activity. Capsules were used to design a filtration cartridge to selectively degrade AHLs from a recirculating bioreactor. The growth of a complex microbial community in the bioreactor, in the presence or absence of lactonase, was monitored over a 3-week period. Dynamic population analysis revealed that signal disruption using a quorum quenching lactonase can effectively reduce biofilm formation in the recirculating bioreactor system and that biofilm inhibition is concomitant to drastic changes in the composition, diversity and abundance of soil bacterial communities within these biofilms. Effects of the quorum quenching lactonase on the suspension community also affected the microbial composition, suggesting that effects of signal disruption are not limited to biofilm populations. This unexpected finding is evidence for the importance of signaling in the competition between bacteria within communities. This study provides foundational tools and data for the investigation of the importance of AHL-based signaling in the context of complex microbial communities.

Bright Surface-Enhanced Raman Scattering with Fluorescence Quenching from Silica Encapsulated J-Aggregate Coated Gold Nanoparticles.

Plexitonic nanoparticles offer variable optical properties through tunable excitations, in addition to electric field enhancements that far exceed molecular resonators. This study demonstrates a way to design an ultrabright surface-enhanced Raman spectroscopy (SERS) signal while simultaneously quenching the fluorescence background through silica encapsulation of the semiconductor-metal composite nanoparticles. Using a multistep approach, a J-aggregate-forming organic dye is assembled on the surface of gold nanoparticles using a cationic linker. Excitonic resonance of the J-aggregate-metal system shows an enhanced SERS signal at an appropriate excitation wavelength. Further encapsulation of the decorated particles in silica shows a significant reduction in the fluorescence signal of the Raman spectra (5× reduction) and an increase in Raman scattering (7× enhancement) when compared to phospholipid encapsulation. This reduction in fluorescence is important for maximizing the useful SERS enhancement from the particle, which shows a signal increase on the order of 104 times greater than J-aggregated dye in solution and 24 times greater than Oxonica S421 SERS tag. The silica layer also serves to promote colloidal stability. The combination of reduced fluorescence background, enhanced SERS intensity, and temporal stability makes these particles highly distinguishable with potential to enable high-throughput applications such as SERS flow cytometry.

Large-Area Heterostructures from Graphene and Encapsulated Colloidal Quantum Dots via the Langmuir-Blodgett Method.

This work explores the assembly of large-area heterostructures comprised of a film of silica-encapsulated, semiconducting colloidal quantum dots, deposited via the Langmuir-Blodgett method, sandwiched between two graphene sheets. The luminescent, electrically insulating film served as a dielectric, with the top graphene sheet patterned into an electrode and successfully used as a top gate for an underlying graphene field-effect transistor. This heterostructure paves the way for developing novel hybrid optoelectronic devices through the integration of 2D and 0D materials.

A SERS nano-tag-based fiber-optic strategy for in situ immunoassay in unprocessed whole blood.

Assay technologies capable of detecting biomarker concentrations in unprocessed whole blood samples are fundamental for applications in medical diagnostics. SERS nano-tags integrated fiber-optic biosensor (FOB) was realized for the first time for in situ immunoassay in whole blood. The reliability and sensitivity of this method rely, in a large extent, on the quality and properties of the SERS nano-tags. The constructed silica-coated Ag SERS nano-tags as labels were used in a rapid and specific in situ FOB immune sensor to detect alpha fetoprotein (AFP) in unprocessed blood samples. Preliminary results of in vivo and in situ dynamic observation of AFP of whole blood in wistar rat highlight the power of this new method.

Highly sensitive SERS-based immunoassay of aflatoxin B1 using silica-encapsulated hollow gold nanoparticles.

Aflatoxin B1 (AFB1) is a well-known carcinogenic contaminant in foods. It is classified as an extremely hazardous compound because of its potential toxicity to the human nervous system. AFB1 has also been extensively used as a biochemical marker to evaluate the degree of food spoilage. In this study, a novel surface-enhanced Raman scattering (SERS)-based immunoassay platform using silica-encapsulated hollow gold nanoparticles (SEHGNs) and magnetic beads was developed for highly sensitive detection of AFB1. SEHGNs were used as highly stable SERS-encoding nano tags, and magnetic beads were used as supporting substrates for the high-density loading of immunocomplexes. Quantitative analysis of AFB1 was performed by monitoring the intensity change of the characteristic peaks of Raman reporter molecules. The limit of detection (LOD) of AFB1, determined by this SERS-based immunoassay, was determined to be 0.1 ng/mL. This method has some advantages over other analytical methods with respect to rapid analysis (less than 30 min), good selectivity, and reproducibility. The proposed method is expected to be a new analytical tool for the trace analysis of various mycotoxins.

Protective ligand shells for luminescent SiO2-coated alloyed semiconductor nanocrystals.

SiO2 encapsulation of alloyed CdSeZnS nanocrystals (NCs) shows differences in terms of optical properties and luminescence quantum yield, depending on the surface composition, size, and ligand content. In this work, emphasis has been placed on the fine control required to obtain luminescent SiO2 encapsulated NCs by studying the role of oleic acid (OA), stearic acid (SA), and dodecanethiol (DDT) ligands on the alloyed NCs. While the use of anchored DDT molecules is essential to preserve the optical properties, intercalated OA and SA play a critical role for SiO2 nucleation, as stated by (1)H NMR (including DOSY and NOESY) spectroscopy. These results emphasize the importance of surface chemistry in NCs; it is crucial to control their reactivity, and therefore their impact, in different applications, from optics to biomedicine.

Direct silica encapsulation of self-assembled-monolayer-based surface-enhanced Raman scattering labels with complete surface coverage of Raman reporters by noncovalently bound silane precursors.

Silica-coated surface-enhanced Raman scattering (SERS) labels with a self-assembled monolayer (SAM) on the entire surface of the metal colloid combine high chemical and mechanical stability with bright and reproducible Raman signals due to the complete surface coverage and uniform molecular orientation within the SAM. Currently available chemical syntheses are either based on the direct encapsulation of covalently bound silane precursors or comprise several steps, such as the sequential addition of noncovalently bound polyelectrolytes to render the surface vitreophilic. Here, a generic approach for the direct and fast silica encapsulation of commercially available Raman reporter molecules with polar head groups by noncovalently bound silane precursors is reported. The formation of highly SERS-active silica-coated clusters during silica encapsulation is determined by several parameters, in particular the type of Raman reporter molecule, the solvent, and the type and amount of the silane precursor.

Inhibitation of cellular toxicity of gold nanoparticles by surface encapsulation of silica shell for hepatocarcinoma cell application.

Nanotechnology, as a double-edged sword, endows gold nanoparticles (GNPs) more "power" in bioimaging and theragnostics, whereas an outstanding issue associated with the biocompatibility of GNPs should also be addressed. Especially for the silica-coated gold nanospheres (GNSs) and gold nanorods (GNRs), there is increasing attention to explore the application, because the surface silica encapsulation has been proved to be an alternative strategy for other organic surface coatings. However, among those reports there are very limited publications to focus on the toxicity of silica-coated GNSs and GNRs. Besides, the existing detoxification methods via surface chemistry on GNPs greatly improve the biocompatibility but still undergo challenges for high dose (>100 pM) demand and long-term stability. Here, we demonstrated a straightforward, low-cost, universal strategy for the surface chemistry on GNPs via silica encapsulating. Different size, shape, dose, and surface capping of GNPs for the nanotoxicity test have been carefully discussed. After silica encapsulating, the detoxification for all GNPs presents significantly from HepG2 cell proliferation results, especially for the GNRs. This new straightforward strategy will definitely rationalize the biocompatibility issue of GNPs and also provide potential for other surface chemistry methodology in biomedical fields.