New smart materials could emerge from the development of photochromic active colloids by researchers.

In nature, the skin of cephalopods (animals with tentacles attached to the head) exhibits unparalleled camouflage ability. Their skin contains pigment groups that can sense changes in environmental light conditions, and they modify their appearance through the action of pigment cells. Although complex in nature, this ability to change color relies primarily on a mechanical mechanism in which pigment molecules are bent or unfolded under the control of radial muscles.

Inspired by this natural process, a research team led by Dr Jinyao Tang of the Department of Chemistry at the University of Hong Kong (HKU) developed a wavelength-selective smart colloidal system to achieve light-controlled multidimensional phase separation in collaboration with scientists from the Hong Kong University of Science and Technology and Xiamen University. .

The team forms dynamic photonic nanoclusters by mixing cyan, magenta and yellow microbeads, achieving large-scale photochromicity. This photochromic colorimetry is based on light-induced vertical phase stratification of the mixture of active microbeads, which results in enrichment of colored microbeads corresponding to the incident spectrum.

In contrast to existing color-changing materials, this new photochromic colloidal swarm relies on rearranging existing pigments rather than generating new chromophores in situ, and is therefore more reliable and programmable. The team’s findings provide a simple method for applications such as electronic ink, displays, and active optical camouflage, marking a major breakthrough in the field of active matter. The results of their research were recently published in the journal nature.

Self-acting active particles are micro/nanoparticles that mimic the directional swimming of microorganisms in a liquid. Recently, they have attracted significant interest in nanoscience and non-equilibrium physics and are being developed for potential biomedical applications. One of the main research goals of active particles is to develop medical micro/nanobots based on these particles for drug delivery and non-invasive surgery.

However, the structure of active particles is very simple, and the driving mechanism and environment perception are very limited. In particular, the relatively simple size and structure of individual micro/nanoactive particles restrict the complexity of executing functions on their bodies. The challenge and key to realizing the future application is how to make energetic particles with smart properties despite their simple structure.

Light-powered microswimmers, a type of self-powered active particle, have been developed with the purpose of creating a controllable nanorobot, which offers the potential for biomedical applications and functional new materials such as swimmer activity, alignment direction and particle interaction. It can be easily adjusted with the incident light. On the other hand, light not only induces photosensitive motility in microspheres, but also alters the effective interaction between particles. For example, catalytic interactions can alter the local chemical gradient field, which in turn affects the motion path of neighboring particles through a diffuse swimming effect, resulting in long-range attraction or repulsion.

In this work, Tang’s team designed a simple wavelength-selective TiO₂ active microbeads system based on their previous research on light-powered microswimmers. Upon photoexcitation, a redox reaction on TiO2₂ The particles generate a chemical gradient that adjusts the effective interaction between the particles. That is, the interaction between particles can be controlled by combining incident light of different wavelengths and intensities.

TiO₂ Microbeads with different photosensitizing activities can be formed by selecting dye-sensitizing codons with different spectral characteristics. by mixing several otherwise identical TiO₂ Types of microbeads loaded with dyes with different absorption spectra and modulation of incident light spectra, particle separation is achieved on demand.

The purpose of realizing particle phase separation is to control the accumulation and dispersion of particles in the liquid at both the micro and macro levels. Effectively, this resulted in a new photo-responsive ink by mixing microbeads with different photo-sensitivities that would possibly be applied to electronic paper. The principle is similar to pigment groups in the skin of cephalopods that can sense the light condition in the environment and alter the appearance of surrounding pigment cells through their corresponding actions.

“The results of the research have greatly advanced our knowledge of swarm intelligence in synthetic active materials and paved the way for the design of innovative active smart materials. With this achievement, we expect to develop a programmable photochromic ink that can be used in various applications such as electronic ink, display ink, and even active optical camouflage ink.” , Dr. Jinyao Tang said.