Silica-coated Quantum Dots Research Articles

Deterministic positioning of few aqueous colloidal quantum dots.

Emerging quantum technologies that critically require the integration of quantum emitters on photonic platforms are hindered by the control over their position, quantity, and scalability. Herein, we describe a facile strategy to deposit aqueous silica-coated quantum dots (QDs) in a template of polymethyl methacrylate (PMMA) nanoholes that leverages saturated ethanol vapor drop-casting and subsequent lift-off of the template. Ethanol vapor incorporation into water droplets during the drying process reduces the meniscus contact angle, which increases capillary forces and enhances particle confinement within the pinning contact region. Furthermore, induced Marangoni flow controls the particle transport dynamics inside the droplets, making large-scale deposition possible. Controlling the hole diameter of the template demonstrates changes in the number of QDs per hole, which is consistent with the Poissonian distribution with the best results of ∼40% single-particle yield from an ∼80% total site occupancy. This method employs a simple setup, eliminating the need for intricate optimization, yet offers the potential for deterministic patterning within complex photonic platforms.

Functional nanoparticles from chemically coupled precipitation processes

Coupled chemical reactions are known to produce ordered patterns under certain conditions far from thermodynamic equilibrium. Such phenomena can be exploited to obtain complex mineral structures, as reported for self-assembled composites of crystalline carbonates and amorphous silica. In the present work, we show that the dynamic pH-based coupling underlying the formation of these inorganic composites can be extended to produce core-shell nanostructures of functional materials. Specifically, the concept was applied to synthesize fluorescent silica-coated quantum dots in a simple one-pot procedure at ambient conditions based on the coupled co-precipitation of metal chalcogenides and silicate. This straightforward approach can likely be generalized to produce a broad variety of functional nanoparticles with possible applications in the areas of optics, energy storage, catalysis, and beyond.

Drastic photostability improvement of silica particles impregnated with multiple emitting CdSe quantum dots prepared through efficient surface silanization

Numerous studies have been performed on silica coated quantum dots (QDs) for the retention of photostability of QDs over long period of time. However, the photostability of silica coated QDs has been found to be inferior to that of polymer coated QDs under some circumstances, and the quantitative data is missing in prior arts. Here, we report the drastic improvement of photostability of CdSe QDs encapsulated into silica particles by optimizing the preparation conditions. We have prepared silica particles (ca. 60 nm in size) encapsulated with multiple, over dozens of emitting CdSe QDs. The key step is a surface silanization of QDs, where the original ligands are replaced by a partially hydrolyzed alkoxide in hydrophobic solution. The evaluation using single particle detection methods shows that the photostability increases up to 8 times as compared with the polymer coated counterparts by optimizing the molar ratio of alkoxide against QDs upon the silanization. When the ratio is optimum, the packing density of QDs in the silica particle becomes the maximum accompanied by the most obvious red shift of ensemble photoluminescence peak. This indicates a developed silica network in close proximity to the QD surface on the particle is formed. The knowledge acquired from this work is universal, and can be extended to other emitting QDs such as InP and perovskites.

Superior hydrophobic silica-coated quantum dot for stable optical performance in humid environments

Quantum dot (QD) features many exceptional optical performances but is also vulnerable to moisture which results in structural damage and luminescent decrease. This work provided and fabricated a novel superior hydrophobic methylated core/shell silica-coated QD (MSQ) for high water stability. QD was coated with a silica shell and then surface-methylated by trimethyl silane. Mercaptopropyl trimethoxy silane, tetraethyl orthosilicate, and ethoxy trimethyl silane were utilized as the ligand exchanger, the raw material of silica, and the surface modification, respectively. Characterization results illustrated the core/shell structure of MSQ. In addition, its water contact angle was up to 159.6°. QD-, silica-coated QD(SQ)-, and MSQ-silicone were made and displayed similar absorption, emission, and excitation spectra but different water stabilities. The photoluminescence intensity and photoluminescence quantum yield of MSQ-silicone hardly changed during 15 d of water immersion, in contrast to the dramatical decrease of other two kinds of composite silicone. Specifically, the photoluminescence quantum yield decreases of MSQ-, SQ-, and QD-silicone were 1%, 40%, and 43%, respectively. Therefore, MSQ had a much better water stability. The superior hydrophobic methylated silica-coated QD has a great potential to realize the long-term working stability in a humid environment and the wider application in diverse fields.

The Study on the Catalytic Effect of HBr on the Hydrolyzed Silica-Coated Quantum Dots of CsPbBr3 with Improved Stability

The Study on the Catalytic Effect of HBr on the Hydrolyzed Silica-Coated Quantum Dots of CsPbBr3 with Improved Stability

Highly stable fluorescent probe based on mesoporous silica coated quantum dots for sensitive and selective detection of Cd2+

Cadmium ions have been of crucial concern due to the high biological toxicity and serious environmental risks. Various fluorescent Cd-sensitive probes have been reported with improved sensing properties, but still severely suffer from poor stability and insufficient selectivity. In this work, a stable fluorescent probe based on silica encapsulated quantum dots (QDs) have been developed for rapid, sensitive and selective detection of cadmium ion. To improve fluorescence stability, the strategy of mesoporous silica encapsulation was adopted, in which the mesoporous silica shell offers numerous channels for Cd2+. Further, the Forster Resonance Energy Transfer (FRET) system, where QDs@mSiO2 and rhodamine B (RB) are used as donors and acceptors respectively, has been constructed, in which the mesoporous silica shell also serves as spacers with tunable thickness for controlling the QD-RB distance. Under optimal conditions, the probes possess a sensitive fluorescence response with a detection limit of 1.25 μM. Visual detection can be realized by the obvious fluorescence changes of the FRET system. In addition, the FRET system shows promising sensing performances both in tap water samples and rice-washed water samples, confirming a great potential for practical application.

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.

Fluorescent color analysis of ascorbic acid by ratiometric fluorescent paper utilizing hybrid carbon dots-silica coated quantum dots

Monitoring the concentration of ascorbic acid (AA) in biological samples is essential to food quality and healthcare. Various fluorescence methods widely reported for the detection of AA are usually based on “turn-on” or “turn-off” principle. However, few ratiometric fluorescent sensors were reported for AA determination. Additionally, the application of ratiometric fluorescence probe is usually limited by the narrow range of color changes and insensitivity color change of the AA concentration. Herein, a novel ratiometric fluorescent test paper to achieve the continuous color changes from red to blue was developed for the fluorometric qualitative and quantitative determination of AA. The nanoprobe consists of blue-emitting carbon dots (C-dots) and red-emitting silica coated quantum dots (QDs@SiO2) with certain ratiometric fluorescence intensity of blue to red. The fluorescence of blue-emitting CDs (peaking at 450 nm) is quenched by Fe3+, then the blue fluorescence is restored consecutively with the gradual increasing of AA concentration due to the reduction from Fe3+ to Fe2+ by using AA. While the fluorescence of red-emitting QD@SiO2 (peaking at 630 nm) works as a stable internal standard. Therefore, a distinguish color transition from red to blue can be observed by naked eyes intuitively with the increasing of AA concentration. By calculating the ratiometric fluorescent intensity at 450 nm and 630 nm or observing the color of the mixture solution by naked eyes, the concentration of AA can be detected easily and accurately. Under optimized conditions, the fluorescent intensity ratio (I450/I630) shows linear relationship with the concentration of AA from 0 to 70 μM with the detection limit as low as 3.17 μM. Finally, a ratiometric fluorescent sensor paper was further developed for AA detection with satisfactory results in fruit juice. The method proposed here gives the wide applications of fluorescent test paper in food and biological assays.

Ratiometric fluorescent nanosensor for dosage-sensitive visual discrimination of glucose based on electron transfer mechanism

Owing to the great role of glucose in biological systems and food industry, it is significant to maintain the dynamic balance of organisms. Various methods widely reported are focused on the glucose detection with high sensitivity and low detection limit. However, the sensor with ultra-low concentration sensitivity is not suitable for the direct detection of high concentration of glucose in human serum and food samples. Additionally, the application of ratiometric fluorescence probe is usually limited by the narrow range of color changes and insensitivity color change of the target. Here, a novel dual-emission ratiometric fluorescence probe with green to red fluorescence intensity of 6:1 was first constructed by fluorescein isothiocyanate (FITC) with green fluorescence as signal unit and silica coated quantum dot (QD@SiO2) with red fluorescence as the stable internal standard. The glucose was oxidized to generate H2O2 by the catalysis of glucose oxidase, which can oxidize Fe2+ to form Fe3+ ion. The formation of Fe3+ ion will cause the quenching of FITC fluorescence, which can realize the continuous fluorescence color change from green to yellow to orange to red for the qualitative and quantitative detection of blood glucose and fruit juice. The proposed method is cheap, simple, intuitive and has a wide detection range for glucose detection, which is of significance in designing the dual-emission ratiometric fluorescence sensor with wide range of color change.

A phenobarbital containing polymer/ silica coated quantum dot composite for the selective recognition of mercury species in fish samples using a room temperature phosphorescence quenching assay

An analytical procedure using low-cost instrumentation (fluorescence/phosphorescence spectrophotometer) has been developed to assess total mercury in fishery products. Determinations were based on the room temperature phosphorescence (RTP) quenching of a composite Ph-QDs consisting of phenobarbital-containing polymer/silica coated Mn-doped ZnS quantum dots. Under optimum conditions (fish extract pH of 8.0, Ph-QDs concentration of 20 mg L−1, and an interaction time of 12 min), the material offers high selectivity for inorganic mercury and methyl-mercury over other common ions present in the fish matrix. Moreover, good linearity was obtained for mercury concentrations within the 0–100 μg L−1 range, and the obtained limit of detection (68.2 μg kg−1) is low enough for a reliable assessment of total mercury in fish and seafood samples. The developed method was found to be free of matrix effects, and offers the advantage that the fish extracts can be directly analysed even at a 1:10 dilution. The method was found to be accurate after analysing a fish certified reference material, and after comparing total mercury levels in a set of fish samples analysed by the proposed chemosensor probe and by inductively coupled plasma mass spectrometry after an acid decomposition sample pre-treatment.

Photon generator combined with silica-coated quantum dot and metamaterial fabricated by the scanning probe microscope lithography

We prototyped the solid-state photon generator composed of silica-coated quantum dot (QD) and metamaterial element using the scanning probe microscope lithography. The metamaterial element acts as an optical resonator and controls the emission of QD. For prototyping the device, QDs were trapped in a nanohole formed, and then, a metamaterial element was set around it. We succeeded in observing fine photon emission peak from the prototyped devices by the micro photoluminescence measurement. The temperature-independent wavelength of the emission peak suggested the resonance of the QD emission and the induced current in the metamaterial element.

A fluorescence microplate assay based on molecularly imprinted silica coated quantum dot optosensing materials for the separation and detection of okadaic acid in shellfish.

Molecularly imprinted polymers (MIPs) are attracting substantial interest as artificial plastic antibodies because of their biometric capability for targeting small molecules. In this study, molecularly imprinted silica material-coated quantum dots (MIS-QDs) with selective recognition capability to okadaic acid (OA) were developed and characterized. The synthesized MIS-QDs with specific imprinting cavities exhibited excellent recognition capability similar to those of biological antibodies and high fluorescence (FL) quenching selectivity for OA. Furthermore, the MIS-QDs with unsaturated bonds were immobilized onto the surface of 96-well microplates by cold plasma-induced grafting. A novel direct competitive microplate assay strategy was then proposed. The FL quenching properties of the developed microplate assay showed an excellent linear relationship with OA in the range of 10.0-100.0μg/kg with a correlation coefficient of 0.9961. The limit of detection for OA was 0.25μg/kg in the shellfish samples. The mean quantitative recoveries were 92.5%-101.0% and 92.9%-101.3%, with relative standard deviations of <7.7% and 7.6% for pure solvents and purified shellfish samples, respectively. The established microplate assay strategy can be used as a rapid and high-throughput method for analyzing OA marine toxins in biological samples.

Study on the luminescence stability of CdSe/CdxZn1-xS quantum dots during the silication process

CdSe/CdxZn1-xS core-shell quantum dots capped with oleic acid have been encapsulated into SiO2 nanospheres by a modified reverse microemulsion method. As an effective approach to convert quantum dots with nonpolar capping agent into hydrophilic and biocompatible ones, silica coating technology is limited by the inevitable luminescence degradation during the silication process. CdxZn1-xS shell involved to protect CdSe core quantum dots from luminescence quenching was found to be quite helpful to restrain the luminescence degradation during the silica coating process. The luminescent intensity has been highly enhanced by controlling the thickness of CdxZn1-xS shells. Luminescence quenching has also been observed when the as-prepared silica coated quantum dots were transferred from the reaction solution into different solvent. According to the time-resolved luminescence and stability examination, the micro channels in the SiO2 shells should be responsible for the degradation. Polyethylene glycol has thus been introduced to block the channel, resulting in a product with much higher emission efficiency, which performed well as a fluorescent labeling probe for MCF-7 cells.

Silanization of quantum dots: Challenges and perspectives

Here we report the facile strategies for synthesis of silica-coated quantum dots (QDs). Due to their small size, chemical and optical stability and high luminescence quantum yield these particles can be further applicable in chemical and biomedical analysis, and in vivo imaging. The article gives a structured and detailed overview of the silanization strategies and potential pitfalls and common technical issues in function of the stability of the obtaining QDs. Different types of QDs, In- and Cd-based, were compared in terms of their behaviour during the silica-encapsulation. To achieve that each silica nanoparticle contains only one single QD the silanization reaction was performed via a water-in-oil emulsion formation. We investigated a number of parameters which have an impact on the silica-coating process and on characteristics of the obtained nanoparticles, such as a nature of initial ligands on the QD surface, a silanization reagent, surfactant, ZnS shelling, etc. It was found that DDT is an optimal ligand for the silanization of InP-based quantum dots, but not for CdZnSeS-nanocomposites. GOPTES and CEST are the best silanization agents for covering InP-QDs, and the AOT/isooctane microemulsion is preferable for the silanization of CdZnSeS-composites.

Surface modification toward luminescent and stable silica-coated quantum dots color filter

A highly pixelated and luminescent silica-coated quantum dot color filter (QDCF) was achieved by surface conjugation with epoxy functional group. Epoxy-functionalized silica-coated quantum dots (QDs) can be thoroughly mixed with SU-8 photoresist up to 25 wt.% without aggregation. The quantum yield (QY) of the silica-coated QDCF can be significantly improved from 19.3% to 36.5% after epoxy treatment. The pristine QDCF experienced a 40% QY decrease, while the epoxied silica-coated QDCF maintained its luminescence even after irradiation (300 mW cm−2 @450 nm) for over 25 days. The well-controlled epoxy cap plays a critical role in attaining the ideal optical properties of the QDCF.

Bioimprinting for multiplex luminescent detection of deoxynivalenol and zearalenone

A sensitive tool for simultaneous quantitative determination of two analytes in a single spot with the use of a bioimprinted protein is presented for the first time. BSA is chosen as a scaffold for generation of binding sites specific towards two compounds. A multiplex immunosorbent assay for screening of two cereal-born mycotoxins, deoxynivalenol and zearalenone, in wheat and maize is realized with the use of fluorescent silica coated quantum dots as labels. Water-soluble fluorescent nanostructures consist of core/shell Cd-QDs enrobed in silica shells to ensure their solubility. The mycotoxins are simultaneously detected by scanning the assay outcome at two different wavelengths, since two QD@SiO2 labelled conjugates fluorescent in different parts of the spectrum. The assay is combined with a rapid extraction technique. The limits of detection for the simultaneous determination were 100 and 700 µg kg−1 in both matrices for zearalenone and deoxynivalenol, respectively. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was used to confirm the obtained results.

Ultrasensitive electrochemiluminescent salbutamol immunoassay with dual-signal amplification using CdSe@SiO2 as label and gold nanoparticles as substrate

The authors describe a dual signal amplification electrochemiluminescent (ECL) immunoassay for ultrasensitive determination of the β-adrenergic agonist salbutamol (SAL). It is based on the use of gold nanoparticles (AuNPs) and silica-coated quantum dots of the type CdSe@SiO2. The CdSe@SiO2 nanoparticles (NPs) were synthesized by water-in-oil reversed-phase microemulsion. Amino-modified NPs were then prepared and used as an ECL signal probe for detection of SAL in the presence of K2S2O8. The gold NPs with their large specific surface carry the antigen, facilitate electron transfer and improve the electrochemical reaction efficiency of QDs and K2S2O8. The ECL immunosensor was fabricated by coating the AuNPs with antigen, and then SAL competed with SAL antigen for the binding sites of the antibody on the CdSe@SiO2 NPs. Under optimized conditions, the ECL intensity is linearly related to the logarithm of the SAL concentration in the range from 0.001 to 1000 ng mL−1, with a lower detection limit of 0.17 pg mL−1 (at an S/N ratio of 3). The assay has good operational stability, is highly sensitive and selective. It is judged to possess a wider scope in that it may be applied to many other small molecule analytes for which adequate antibodies are available.

An easy and sensitive sandwich assay for detection of Mycobacterium tuberculosis Ag85B antigen using quantum dots and gold nanorods

Mycobacterium tuberculosis is a serious global infectious pathogen causing tuberculosis (TB). The development of an easy and sensitive method for the detection of M. tuberculosis is in urgent need due to complex and low specificity of the current assays. Herein, we present a novel method for M. tuberculosis detection based on a sandwich assay via antigen-antibody interaction using silica-coated quantum dots (SiQDs) and gold nanorods (AuNRs). A genetically engineered recombinant antibody (GBP-50B14 and SiBP-8B3) was bound to surfaces of AuNRs and SiQDs respectively, without any surface modification. The antigen-antibody interaction was revealed using M. tuberculosis-specific secretory antigen, Ag85B. Two biocomplexes showed a quenching effect in the presence of the target antigen through a sandwich assay. The assay response was in the range of 1×10−3–1×10−10μgmL−1 (R=0.969) and the limit of detection for Ag85B was 13.0pgmL−1. The Ag85B was selectively detected using three different proteins (CFP10, and BSA), and further specifically confirmed by the use of spiked samples. Compared with existing methods, a highly sensitive and selective method for Ag85B-expressing M. tuberculosis detection has been developed for better diagnosis of TB.

Sensitive QD@SiO2-based immunoassay for triplex determination of cereal-borne mycotoxins

A sensitive tool for simultaneous quantitative determination of three analytes in one single well of a microtiter plate is shown for the first time. The developed technique is based on use of colloidal quantum dot enrobed into a silica shell (QD@SiO2) derivatives as a highly responsive label. Silica-coated quantum dots were prepared and subsequently modified via the co-hydrolysis with tetraethylorthosilicate (TEOS) and various organosilane reagents. Different surface modification schemes were compared in terms of applicability of the obtained particles for the multiplex immunoassay, e.g. stability and simplicity of their conjugation with biomolecules. As model system a multiplex immunosorbent assay for screening of three mycotoxins (deoxynivalenol, zearalenone and aflatoxin B1) in cereal-based products was realized via a co-immobilization of three different specific antibodies (anti- deoxynivalenol, anti-zearalenone and anti-aflatoxin B1) in one single well of a microtiter plate. Mycotoxins were simultaneously determined by labelling their conjugates with QD@SiO2 emitting in different parts of the visible spectrum. The limits of detection for the simultaneous determination were 6.1 and 5.3, 5.4 and 4.1, and 2.6 and 1.9µgkg−1 for deoxynivalenol, zearalenone and aflatoxin B1 in maize and wheat, respectively. As confirmatory method, liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was used.

Optical Trapping and Two-Photon Excitation of Colloidal Quantum Dots Using Bowtie Apertures

We demonstrate bowtie apertures that were designed and fabricated by a lift-off process to optically trap individual, 30 nm, silica-coated quantum dots (scQD). Simulations and experiments confirm the trapping capability of the system with a relatively low continuous wave trapping flux of 1.56 MW/cm2 at 1064 nm. Additionally, the scQD emits upon trapping via two-photon excitation from the trapping laser due to strong field enhancement inside the aperture. This system is an exciting platform for studying light–matter interactions and mulitphoton processes in single emitters.