Publisher Correction: Self-assembly of highly symmetrical, ultrasmall inorganic cages directed by surfactant micelles.

Change history: In Fig. 3b of this Letter, the labels for the outer (11.8 nm) and inner (7.4 nm) diameters of the structure were inadvertently omitted. Fig. 3 has been corrected online.

Self-assembly of highly symmetrical, ultrasmall inorganic cages directed by surfactant micelles.

Nanometre-sized objects with highly symmetrical, cage-like polyhedral shapes, often with icosahedral symmetry, have recently been assembled from DNA1-3, RNA 4 or proteins5,6 for applications in biology and medicine. These achievements relied on advances in the development of programmable self-assembling biological materials7-10, and on rapidly developing techniques for generating three-dimensional (3D) reconstructions from cryo-electron microscopy images of single particles, which provide high-resolution structural characterization of biological complexes11-13. Such single-particle 3D reconstruction approaches have not yet been successfully applied to the identification of synthetic inorganic nanomaterials with highly symmetrical cage-like shapes. Here, however, using a combination of cryo-electron microscopy and single-particle 3D reconstruction, we suggest the existence of isolated ultrasmall (less than 10 nm) silica cages ('silicages') with dodecahedral structure. We propose that such highly symmetrical, self-assembled cages form through the arrangement of primary silica clusters in aqueous solutions on the surface of oppositely charged surfactant micelles. This discovery paves the way for nanoscale cages made from silica and other inorganic materials to be used as building blocks for a wide range of advanced functional-materials applications.

General Expression for the Size-Dependent Optical Properties of Quantum Dots.

While initial theories on quantum confinement in colloidal quantum dots (QDs) led to analytical band gap/size relations or sizing functions, numerical methods describe size quantization more accurately. However, because of the lack of reliable sizing functions, researchers fit experimental band gap/size data sets using models with redundant, physically meaningless parameters that break down upon extrapolation. Here, we propose a new sizing function based on a proportional correction for nonparabolic bands. Using known bulk parameters, we predict size quantization for groups IV, III-V, II-VI, and IV-VI and metal-halide perovskite semiconductors, including straightforward adaptations for negative-gap semiconductors and nonspherical QDs. Refinement with respect to experimental data is possible using the Bohr diameter as a fitting parameter, by which we show a statistically relevant difference in the band gap/size relation for wurtzite and zinc blende CdSe. The general sizing function proposed here unifies the QD size calibration and enables researchers to assess bulk semiconductor parameters and predict the size quantization in unexplored materials.

Fluorescent Silica Nanoparticles with Well-Separated Intensity Distributions from Batch Reactions.

Silica chemistry provides pathways to uniquely tunable nanoparticle platforms for biological imaging. It has been a long-standing problem to synthesize fluorescent silica nanoparticles (SNPs) in batch reactions with high and low fluorescence intensity levels for reliable use as an intensity barcode, which would greatly increase the number of molecular species that could be tagged intracellularly and simultaneously observed in conventional fluorescence microscopy. Here, employing an amino-acid catalyzed growth, highly fluorescent SNP probes were synthesized with sizes <40 nm and well-separated intensity distributions, as mapped by single-particle imaging techniques. A seeded growth approach was used to minimize the rate of secondary particle formation. Organic fluorescent dye affinity for the SNP during shell growth was tuned using specifics of the organosilane linker chemistry. This work highlights design considerations in the development of fluorescent probes with well-separated intensity distributions synthesized in batch reactions for single-particle imaging and sensing applications, where heterogeneities across the nanoparticle ensemble are critical factors in probe performance.

Early Formation Pathways of Surfactant Micelle Directed Ultrasmall Silica Ring and Cage Structures.

By combining a surfactant, an organic pore expander, a silane, and poly(ethylene glycol) (PEG), we have observed the formation of a previously unknown set of ultrasmall silica structures in aqueous solutions. At appropriate concentrations of reagents, ∼2 nm primary silica clusters arrange around surfactant micelles to form ultrasmall silica rings, which can further evolve into cage-like structures. With increasing concentration, these rings line up into segmented worm-like one-dimensional (1D) structures, an effect that can be dramatically enhanced by PEG addition. PEG adsorbed 1D striped cylinders further arrange into higher order assemblies in the form of two-dimensional (2D) sheets or three-dimensional (3D) helical structures. Results provide insights into synergies between deformable noncovalent organic molecule assemblies and covalent inorganic network formation as well as early transformation pathways from spherical soft materials into 1D, 2D, and 3D silica solution structures, hallmarks of mesoporous silica materials formation. The ultrasmall silica ring and cage structures may prove useful in nanomedicine and other nanotechnology based applications.

Interface formation during silica encapsulation of colloidal CdSe/CdS quantum dots observed by in situ Raman spectroscopy.

We investigate the encapsulation of CdSe/CdS quantum dots (QDs) in a silica shell by in situ Raman spectroscopy and find a distinct shift of the CdS Raman signal during the first hours of the synthesis. This shift does not depend on the final silica shell thickness but on the properties of the initial core-shell QD. We find a correlation between the Raman shift rate and the speed of the silica formation and attribute this to the changing configuration of the outermost layers of the QD shell, where an interface to the newly formed silica is created. This dependence of Raman shift rate on the speed of silica formation process will give rise to many possible studies concerning the growth mechanism in the water-in-oil microemulsion, rendering in situ Raman a valuable instrument in monitoring this type of reaction.

Fabrication and characterization of on-chip silicon nitride microdisk integrated with colloidal quantum dots.

We designed and fabricated free-standing, waveguide-coupled silicon nitride microdisks hybridly integrated with embedded colloidal quantum dots. An efficient coupling of quantum dot emission to resonant disk modes and eventually to the access waveguides is demonstrated. The amount of light coupled out to the access waveguide can be tuned by controlling its dimensions and offset with the disk edge. These devices open up new opportunities for both on-chip silicon nitride integrated photonics and novel optoelectronic devices with quantum dots.

Nanoscale and Single-Dot Patterning of Colloidal Quantum Dots.

Using an optimized lift-off process we develop a technique for both nanoscale and single-dot patterning of colloidal quantum dot films, demonstrating feature sizes down to ~30 nm for uniform films and a yield of 40% for single-dot positioning, which is in good agreement with a newly developed theoretical model. While first of all presenting a unique tool for studying physics of single quantum dots, the process also provides a pathway toward practical quantum dot-based optoelectronic devices.

Low-loss silicon nitride waveguide hybridly integrated with colloidal quantum dots.

Silicon nitride waveguides with a monolayer of colloidal quantum dots embedded inside were fabricated using a low-temperature deposition process and an optimized dry etching step for the composite layers. We experimentally demonstrated the luminescence of the embedded quantum dots is preserved and the loss of these hybrid waveguide wires is as low as 2.69dB/cm at 900nm wavelength. This hybrid integration of low loss silicon nitride photonics with active emitters offers opportunities for optical sources operating over a very broad wavelength range.

Active liquid crystal tuning of metallic nanoantenna enhanced light emission from colloidal quantum dots.

A system comprising an aluminum nanoantenna array on top of a luminescent colloidal quantum dot waveguide and covered by a thermotropic liquid crystal (LC) is introduced. By heating the LC above its critical temperature, we demonstrate that the concomitant refractive index change modifies the hybrid plasmonic-photonic resonances in the system. This enables active control of the spectrum and directionality of the narrow-band (∼6 nm) enhancement of quantum dot photoluminescence by the metallic nanoantennas.

Cytotoxicity of cadmium-free quantum dots and their use in cell bioimaging.

The use of quantum dots (QDots) as bright and photostable probes for long-term fluorescence imaging is gaining more interest. Thus far, (pre)clinical use of QDots remains limited, which is primarily caused by the potential toxicity of QDots. Most QDots consist of Cd2+ ions, which are known to cause high levels of toxicity. In order to overcome this problem, several strategies have been tested, such as the generation of cadmium-free QDots. In the present study, two types of cadmium-free QDots, composed of ZnSe/ZnS (QDotZnSe) and InP/ZnS (QDotInP), were studied with respect to their cytotoxicity and cellular uptake in a variety of cell types. A multiparametric cytotoxicity approach is used, where the QDots are studied with respect to cell viability, oxidative stress, cell morphology, stem cell differentiation, and neurite outgrowth. The data reveal slight differences in uptake levels for both types of QDots (maximal for QDotZnSe), but clear differences in cytotoxicity and cell functionality effects exist, with highest toxicity for QDotZnSe. Differences between cell types and between both types of QDots can be explained by the intrinsic sensitivity of certain cell types and chemical composition of the QDots. At concentrations at which no toxic effects can be observed, the functionality of the QDots for fluorescence cell visualization is evaluated, revealing that the higher brightness of QDotZnSe overcomes most of the toxicity issues compared to that of QDotInP. Comparing the results obtained with common Cd2+-containing QDots tested under identical conditions, the importance of particle functionality is demonstrated, revealing that cadmium-free QDots tested in this study are not significantly better than Cd2+-containing QDots for long-term cell imaging and that more work needs to be performed in optimizing the brightness and surface chemistry of cadmium-free QDots for them to replace currently used Cd2+-containing QDots.

Hydrophilic, bright CuInS2 quantum dots as Cd-free fluorescent labels in quantitative immunoassay.

We report on the synthesis of core-shell CuInS2/ZnS quantum dots (QDs) in organic solution, their encapsulation with a PEG-containing amphiphilic polymer, and the application of the resulting water-soluble QDs as fluorescent label in quantitative immunoassay. By optimizing the methods for core synthesis and shell growth, CuInS2/ZnS QDs were obtained with a quantum yield of 50% on average after hydrophilization. After conjugation with an aflatoxin B1-protein derivative, the obtained QDs were used as fluorescent labels in microplate immunoassay for the quantitative determination of the mycotoxin aflatoxin B1. QDs-based immunoassay showed higher sensitivity compared to enzyme-based immunoassay.

The micropatterning of layers of colloidal quantum dots with inorganic ligands using selective wet etching.

The micropatterning of layers of colloidal quantum dots (QDs) stabilized by inorganic ligands is demonstrated using PbS core and CdSe/CdS core/shell QDs. A layer-by-layer approach is used to assemble the QD films, where each cycle involves the deposition of a QD layer by dip-coating, and the replacement of the native organic ligands by inorganic moieties, such as OH(-) and S(2-), followed by a thorough cleaning of the resulting film. This results in a smooth and crack-free QD film on which a photoresist can be spun. The micropatterns are defined by a positive photoresist, followed by the removal of uncovered QDs by selective wet etching with an HCl/H3PO4 mixture. The resulting patterns can have submicron feature dimensions, limited by the resolution of the lithographic process, and can be formed on planar and 3D substrates. It is shown that the photolithography and wet etching steps have little effect on the photoluminescence quantum yield of CdSe/CdS QDs. Compared with the unpatterned CdSe/CdS QD film, only a 10% degradation in the quantum yield is observed. These results demonstrate the feasibility of the proposed micropatterning method to implement the large-scale device integration of colloidal quantum dots.

Overcoming Barriers Associated with Oral Delivery of Differently Sized Fluorescent Core-Shell Silica Nanoparticles.

Oral delivery, while a highly desirable form of nanoparticle-drug administration, is limited by challenges associated with overcoming several biological barriers. Here, the authors study how fluorescent and poly(ethylene glycol)-coated (PEGylated) core-shell silica nanoparticles sized 5 to 50 nm interact with major barriers including intestinal mucus, intestinal epithelium, and stomach acid. From imaging fluorescence correlation spectroscopy studies using quasi-total internal reflection fluorescence microscopy, diffusion of nanoparticles through highly scattering mucus is progressively hindered above a critical hydrodynamic size around 20 nm. By studying Caco-2 cell monolayers mimicking the intestinal epithelia, it is observed that ultrasmall nanoparticles below 10 nm diameter (Cornell prime dots, [C' dots]) show permeabilities correlated with high absorption in humans from primarily enhanced passive passage through tight junctions. Particles above 20 nm diameter exclusively show active transport through cells. After establishing C' dot stability in artificial gastric juice, in vivo oral gavage experiments in mice demonstrate successful passage through the body followed by renal clearance without protein corona formation. Results suggest C' dots as viable candidates for oral administration to patients with a proven pathway towards clinical translation and may generate renewed interest in examining silica as a food additive and its effects on nutrition and health.

Quantum Dot Patterning and Encapsulation by Maskless Lithography for Display Technologies.

For their unique optical properties, quantum dots (QDs) have been extensively used as light emitters in a number of photonic and optoelectronic applications. They even met commercialization success through their implementation in high-end displays with unmatched brightness and color rendering. For such applications, however, QDs must be shielded from oxygen and water vapor, which are known to degrade their optical properties over time. Even with highly qualitative QDs, this can only be achieved through their encapsulation between barrier layers. With the emergence of mini- and microLED for higher contrast and miniaturized displays, new strategies must be found for the concomitant patterning and encapsulation of QDs, with sub-millimeter resolution. To this end, we developed a new approach for the direct patterning of QDs through maskless lithography. By combining QDs in photopolymerizable resins with digital light processing (DLP) projectors, we developed a versatile and massively parallel fabrication process for the additive manufacturing of functional structures that we refer to as QD pockets. These 3D heterostructures are designed to provide isotropic encapsulation of the QDs, and hence prevent edge ingress from the lateral sides of QD films, which remains a shortcoming of the current technologies.

Quantum dot lasing from a waterproof and stretchable polymer film.

Colloidal quantum dots (QDs) are excellent optical gain materials that combine high material gain, a strong absorption of pump light, stability under strong light exposure and a suitability for solution-based processing. The integration of QDs in laser cavities that fully exploit the potential of these emerging optical materials remains, however, a challenge. In this work, we report on a vertical cavity surface emitting laser, which consists of a thin film of QDs embedded between two layers of polymerized chiral liquid crystal. Forward directed, circularly polarized defect mode lasing under nanosecond-pulsed excitation is demonstrated within the photonic band gap of the chiral liquid crystal. Stable and long-term narrow-linewidth lasing of an exfoliated free-standing, flexible film under water is obtained at room temperature. Moreover, we show that the lasing wavelength of this flexible cavity shifts under influence of pressure, strain or temperature. As such, the combination of solution processable and stable inorganic QDs with high chiral liquid crystal reflectivity and effective polymer encapsulation leads to a flexible device with long operational lifetime, that can be immersed in different protic solvents to act as a sensor.

A Comparative Study of the Band-Edge Exciton Fine Structure in Zinc Blende and Wurtzite CdSe Nanocrystals.

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright-dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

Synthesis and characterization of A4[Re6Q8L6]@SiO2 red-emitting silica nanoparticles based on Re6 metal atom clusters (A = Cs or K, Q = S or Se, and L = OH or CN).

Metal atom clusters constitute very promising candidates as luminophores for applications in biotechnology because they are nanosized entities offering robust luminescence in the near-infrared field (NIR). However, they cannot be used as prepared for biological applications because of potential toxic effects and quenching of the clusters' luminescence in aqueous media, and they therefore need to be dispersed in a biocompatible matrix. We describe herein the encapsulation of octahedral rhenium clusters, denoted as A(4)[Re(6)Q(8)L(6)] (A = Cs or K, Q = S or Se, and L = OH or CN), in silica nanoparticles by a water-in-oil microemulsion process, paying particular attention to the clusters' stability. The obtained A(4)[Re(6)Q(8)L(6)]@SiO(2) nanoparticles are 30 nm in size with good monodispersity and a perfectly spherical shape, as shown by scanning electron microscopy (SEM). The presence of cluster units inside the silica matrix was evidenced by scanning transmission electron microscopy in annular dark-field mode (ADF-STEM). From the point of view of their optical properties, the A(4)[Re(6)Q(8)L(6)]@SiO(2) nanoparticles show red and NIR emission under UV excitation, even when dispersed in water. The evolution of the structural and luminescence properties of clusters before and after encapsulation was followed by Raman and photoluminescence spectroscopy.

Fine tuning of emission through the engineering of colloidal crystals.

We describe the preparation and characterization of photonic colloidal crystals from silica spheres with incorporated luminescent [Mo(6)Br(14)](2-) cluster units. These structures exhibit strong angle-dependent luminescent properties. The incorporation of one or several planar defects in the periodic structures gives rise to the creation of a passband in the stopband. In the energy range of this passband, an increase of the emission intensity has been found.

Synthesis and characterization of Eu3+, Ti4+ @ ZnO organosols and nanocrystalline c-ZnTiO3 thin films aiming at high transparency and luminescence.

By exploiting colloidal properties, such as transparency, rheology and versatile chemistry, we propose to synthesize new photonic nanomaterials based on colloidal solutions and thin films. This contribution highlights our efforts to elaborate and to characterize nanostructures based on the ZnO-TiO2 system. Using a recently developed sol-gel route to synthesize new Ti4+@ZnO organosols, we were able to prepare, at relatively low temperature (400 °C) and short annealing time (15 min), highly transparent, luminescent, nanocrystalline Eu3+ doped c-ZnTiO3 thin films. The organosols and thin films were characterized with UV-visible-near infrared absorption, ellipsometry, photoluminescence spectroscopy, dynamic light scattering, x-ray diffraction and scanning electron microscopy.