Fe₃O₄/ZIF-8 nanoparticles were synthesized through a room-temperature reaction between 2-methylimidazolate and zinc nitrate in the presence of Fe₃O₄ nanocrystals. The particle size, surface charge, and magnetic loading can be conveniently controlled by the dosage of Zn(NO₃)₂ and Fe₃O₄ nanocrystals. The as-prepared particles show both good thermal stability (stable to 550 °C) and large surface area (1174 m²g⁻¹). The nanoparticles also have a superparamagnetic response, so that they can strongly respond to an external field during magnetic separation and disperse back into the solution after withdrawal of the magnetic field. For the Knoevenagel reaction, which is catalyzed by alkaline active sites on external surface of catalyst, small Fe₃O₄/ZIF-8 nanoparticles show a higher catalytic activity. At the same time, the nanocatalysts can be continuously used in multiple catalytic reactions through magnetic separation, activation, and redispersion with little loss of activity.

Photonic crystal (PC) films are prepared by precipitation of colloidal crystal seeds in supersaturated solution of particles, followed by crystal growth and structure fixing with photo-polymerization. As the liquid monomer becomes a solid matrix, the highly concentrated particles are forced to precipitate into colloidal microcrystals in short time, and ‘polymerization-induced colloidal assembly’ (PICA) is shown to be the major driving force to form colloidal crystals. PICA is intrinsically different from evaporation-induced colloidal assembly, because the seed formation and crystal growth are separated into two independent steps, which makes the synthesis more flexible, controllable, and efficient. The PICA process is capable of quickly producing PC films with an ultra-narrow bandgap, tunable thickness, and large size. Based on these characteristics and the blocking effect of the outer PC layer to the reflection signal of inner layer, a coding–decoding system is developed in which the film's composition and stacking sequence can be identified by its distinctive reflection spectrum.

An efficient and straightforward method is developed to prepare a mechanochromic photonic gel by fixing the metastable SiO₂ colloidal crystalline array (CCA) in the mixture of ethylene glycol (EG) and poly(ethylene glycol) methacrylate (PEGMA) through photopolymerization. Thanks to the recent fabrication of solvent-wrapped, metastable CCA, a high volume fraction of EG (46%) is introduced to the photonic gel before particle assembly, but not by swelling after polymerization, which leads to a more deformable composite than most reported opal gels. Compared to traditional photonic gels, this opal gel not only has improved mechanochromic sensitivity to weak external force and extended color tuning range from red to blue (Δλ = 150 nm), but also possesses fast and reversible response in millisecond level (20–200 ms), repeatable reflection signals in cycling and fatigue tests, and good resolution in response to localized deformation, which renders it an ideal deformation-based photonic display screen. A new trigger system is designed to solve the large deformation causing color fading in conventional mechanochromic gels and brilliant red, green, and blue (RGB) pixels can be conveniently manipulated by ‘pushing’ operations.

Invisible photonic prints shown by deformation are prepared by soaking the mechanochromic photonic paper with crosslinker (PEGDA) and subsequently crosslinking part of the paper through a photo lithography process. The key point of this new technique is creating patterns and background with very close photonic structures but different mechanochromic capabilities, so that the invisible photonic patterns in relaxed state can be revealed under deformation due to the nonuniform change in photonic structure. Based on the relationship between crosslinking level and the reflection changes during deformation, one can conclude that a low crosslinking level favors the hiding of invisible patterns and a high crosslinking level favors the showing of patterns. The as-prepared samples can instantly and reversibly show the patterns by deformation and hide them by relaxation for many times, and the encapsulation by PDMS rubber prolongs its life time and enhances its durability in practical usages. The current printing technique is capable of creating invisible photonic prints in both macroscale and microscale range, which makes them potentially useful for security and antifraud applications in daily life.