Kuo Zhong

Wonders of colloidal assembly

In nature the spontaneous formation of ordered structures from molecules, called self-assembly, is a very common process occurring in inorganic matter and living organisms. It is driven by atoms, molecules, particles, granular matter, etc. trying to reach the lowest possible energy state while interacting with each other. Deeper understanding of the subtleties of such interactions will allow mimicking of this kind of behaviour to build custom structures from synthetic molecules. This review attempts to cover the existing techniques for directed self-assembly that are currently used for colloidal crystal growth with a brief explanation of the interactions involved in each technique. It provides examples of the fundamental phenomena occurring in photonic crystals that, in the future, can be exploited in various applications.

A facile way to introduce planar defects into colloidal photonic crystals for pronounced passbands

We demonstrate a facile method for introducing planar defects into colloidal photonic crystals. Firstly, a 2D monolayer of SiO2 microspheres (guest spheres) was fabricated at the air/water interface by compressing the individual microspheres with a surfactant into long-range hexagonal arrays. The floating monolayer, which served as our defect layer, was then transferred onto a pre-deposited colloidal crystal slab consisting of PS@SiO2 microspheres (host spheres). Subsequently, a second colloidal crystal slab of host spheres was deposited on the surface of the defect layer. In comparison to previous methods to introduce planar defects into colloidal photonic crystals, this fabrication results in pronounced passbands in the band gaps of the colloidal photonic crystals. More importantly, the FWHM of the passband in our experiment is just 16 nm, which is narrower than the previously reported results to the best of our knowledge. Furthermore, the defect modes can be engineered by changing the diameter of the guest spheres and/or transforming the host spheres from PS@SiO2 spheres to hollow SiO2 spheres by calcination. The measured defect modes in the spectra match well with the simulated results.

Modifying the symmetry of colloidal photonic crystals: a way towards complete photonic bandgap

Colloidal photonic crystals (CPCs) are a type of photonic crystals that are made of periodically arranged submicron spheres. Because of the unique advantages of cost-effectiveness, easiness and relatively large scalability for their fabrication, they have attracted a great deal of research interest for a wide range of applications. However, most of the CPCs are made of spherical building blocks with face-centred-cubic lattice, which bears only a pseudo photonic bandgap between the second and third bands. Theoretical simulation has suggested that lowering the symmetry of the building blocks or the dielectrics of the materials can potentially open a full bandgap, namely, the complete photonic bandgap. In this paper, recent efforts towards this end were thoroughly reviewed and summarised from three aspects: the symmetries of the building blocks, the crystalline lattices and the dielectrics of materials. In the end, a conclusion was given to the recent research in this field and related challenges were outlined. A promising outlook was proposed for the future direction along with its impact to the scientific community.

Real-Time Fluorescence Detection in Aqueous Systems by Combined and Enhanced Photonic and Surface Effects in Patterned Hollow Sphere Colloidal Photonic Crystals

Hollow sphere colloidal photonic crystals (HSCPCs) exhibit the ability to maintain a high refractive index contrast after infiltration of water, leading to extremely high-quality photonic band gap effects, even in an aqueous (physiological) environment. Superhydrophilic pinning centers in a superhydrophobic environment can be used to strongly confine and concentrate water-soluble analytes. We report a strategy to realize real-time ultrasensitive fluorescence detection in patterned HSCPCs based on strongly enhanced fluorescence due to the photonic band-edge effect combined with wettability differentiation in the superhydrophobic/superhydrophilic pattern. The orthogonal nature of the two strategies allows for a multiplicative effect, resulting in an increase of two orders of magnitude in fluorescence.

Fabrication of optomicrofluidics for real-time bioassays based on hollow sphere colloidal photonic crystals with wettability patterns

An optomicrofluidic device was developed by introducing 3D wettability patterns into hollow SiO2 sphere colloidal photonic crystals. Aqueous liquids flow through the superhydrophilic channel due to the surface tension confinement effect. Based on the significant fluorescence enhancement from photonic band gap (PBG) effects in these channels, real-time specific bioassays with high sensitivity were realized. To demonstrate this strategy, with two complementary single stranded DNA molecules acting as a target (fluorophore labeled) and a probe respectively, a 150-fold enhancement of fluorescence was observed compared with a similar device on a standard glass plate. This enhancement results from the strong PBG effect in an aqueous environment for these structures. While the PBG effect diminishes from refractive index matching in conventional solid sphere colloidal photonic crystals with water infiltrated, it is effectively enhanced in hollow sphere colloidal photonic crystals. This is because the dense shell of the hollow spheres prevents water from infiltrating into the inner air cavity of the hollow spheres, while water fills the voids between spheres. This creates a larger refractive index contrast, resulting in a pronounced PBG effect and strong fluorescence enhancement.

Direct Fabrication of Monodisperse Silica Nanorings from Hollow Spheres - A Template for Core-Shell Nanorings.

We report a new type of nanosphere colloidal lithography to directly fabricate monodisperse silica (SiO2) nanorings by means of reactive ion etching of hollow SiO2 spheres. Detailed TEM, SEM, and AFM structural analysis is complemented by a model describing the geometrical transition from hollow sphere to ring during the etching process. The resulting silica nanorings can be readily redispersed in solution and subsequently serve as universal templates for the synthesis of ring-shaped core-shell nanostructures. As an example we used silica nanorings (with diameter of ∼200 nm) to create a novel plasmonic nanoparticle topology, a silica-Au core-shell nanoring, by self-assembly of Au nanoparticles (<20 nm) on the rings surface. Spectroscopic measurements and finite difference time domain simulations reveal high quality factor multipolar and antibonding surface plasmon resonances in the near-infrared. By loading different types of nanoparticles on the silica core, hybrid and multifunctional composite nanoring structures could be realized for applications such as MRI contrast enhancement, catalysis, drug delivery, plasmonic and magnetic hyperthermia, photoacoustic imaging, and biochemical sensing.

Tunability of Size and Magnetic Moment of Iron Oxide Nanoparticles Synthesized by Forced Hydrolysis

To utilize iron oxide nanoparticles in biomedical applications, a sufficient magnetic moment is crucial. Since this magnetic moment is directly proportional to the size of the superparamagnetic nanoparticles, synthesis methods of superparamagnetic iron oxide nanoparticles with tunable size are desirable. However, most existing protocols are plagued by several drawbacks. Presented here is a one-pot synthesis method resulting in monodisperse superparamagnetic iron oxide nanoparticles with a controllable size and magnetic moment using cost-effective reagents. The obtained nanoparticles were thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) measurements. Furthermore, the influence of the size on the magnetic moment of the nanoparticles is analyzed by superconducting quantum interference device (SQUID) magnetometry. To emphasize the potential use in biomedical applications, magnetic heating experiments were performed.

Instantaneous, Simple, and Reversible Revealing of Invisible Patterns Encrypted in Robust Hollow Sphere Colloidal Photonic Crystals

The colors of photonic crystals are based on their periodic crystalline structure. They show clear advantages over conventional chromophores for many applications, mainly due to their anti-photobleaching and responsiveness to stimuli. More specifically, combining colloidal photonic crystals and invisible patterns is important in steganography and watermarking for anticounterfeiting applications. Here a convenient way to imprint robust invisible patterns in colloidal crystals of hollow silica spheres is presented. While these patterns remain invisible under static environmental humidity, even up to near 100% relative humidity, they are unveiled immediately (≈100 ms) and fully reversibly by dynamic humid flow, e.g., human breath. They reveal themselves due to the extreme wettability of the patterned (etched) regions, as confirmed by contact angle measurements. The liquid surface tension threshold to induce wetting (revealing the imprinted invisible images) is evaluated by thermodynamic predictions and subsequently verified by exposure to various vapors with different surface tension. The color of the patterned regions is furthermore independently tuned by vapors with different refractive indices. Such a system can play a key role in applications such as anticounterfeiting, identification, and vapor sensing.

Epitaxial growth of bulky calcite inverse opal induced by a single crystalline calcite substrate

Centimetre-sized single crystalline calcite inverse opals (IOs) were fabricated by the combination of vapour diffusion and epitaxial growth methods in the template of colloidal crystals. The crystallinity of the IOs is closely related to the surface chemistry of the templates.

Ultrafast revealing of invisible patterns encrypted in colloidal photonic crystals

We describe the necessary steps towards the realization of ultrafast revealing of invisible patterns encrypted in colloidal photonic crystals. These include the development of hollow air-core – dense-silica-shell core-shell monodisperse and spherical nanoparticles; introducing of a pattern of hydrophilic regions in a hydrophobic surrounding; and the combination of these two approaches by selective oxygen plasma etching of hollow core-shell nanospheres. The pattern imprinted by the difference in only surface property remains invisible in normal conditions of static environmental humidity. The hydrophilic regions in the patterns are reversible and immediately unveiled by dynamic humid flow. The specific properties of a human breath in terms of relative humidity and vapor flow are ideal for optimal revealing in terms of the spectral shift of the photonic bandgap of the colloidal crystal. The revealing of the pattern is determined by the surface tension of the vapor, while the color of the imprinted pattern is independently determined by its refractive index.