As far as nature is concerned, the skin of cephalopods (animals with tentacles attached to their heads) exhibits extraordinary camouflage abilities.
Their skin is composed of clusters of pigments which can sense variations in environmental light conditions and adjust their appearance through the action of pigment cells. Although complex, this ability to change color depends on a mechanical mechanism whereby the pigment particles have been folded or unfolded under the control of the radial muscles.
Motivated by this natural process, a research group chaired by Dr. Tang Jinyao from the Department of Chemistry at Hong Kong University (HKU) came up with a new wavelength-selective intelligent colloid system to obtain light-controlled multi-dimensional phase separation in partnership with researchers from Hong Kong University of Science and Technology and Xiamen University.
The group developed dynamic photochromic nanoclusters by mixing cyan, magenta, and yellow microbeads, thus obtaining photochromics at the macro scale.
This macroscopic photochromism relies on light-induced vertical phase stratification in a mixture of active microbeads, leading to an increase in colored microbeads similar to the incident spectrum.
In contrast to today’s color-changing materials, this new batch of photochromic colloids relies on rearranging existing pigments instead of generating new chromophores. in place and, thus, highly reliable and programmable.
Their study results offer easy techniques for applications such as displays, electronic inks, and active optical camouflage. This is a significant discovery in the field of active materials. The results of the study were recently reported in a well-known academic journal Natural.
Self-propelled active particles known as micro- or nano-particles copy the swimming direction of microorganisms in liquids. Recently, they have received great attention in nanoscience and non-equilibrium physics and are being developed for possible biomedical applications.
One of the main research goals of active particles is to produce medical micro- or nano-robots based on them for non-invasive surgery and drug delivery.
However, the structure of the active particle is very simple, and the driving mechanism and perception of the environment are very limited.
In particular, the relatively simple size and structure of individual micro- or nano-active particles constrains the complexity of implementing functions in their bodies. The difficulty and key to identifying future applications is how to create active particles with intelligent characteristics despite their simple structure.
Light-powered micro-swimmers, a kind of self-propelled active particles, have recently emerged to create controllable nanorobots, which provide possibilities for functional new materials and biomedical applications as the swimmer’s activity alignment direction, and interactions between particles can be easily modified by incident light.
At the same time, light induces not only photosensitive motion in microswimmers, but also variations in the efficient interactions between particles.
For example, photocatalytic reactions can alter local chemical gradient fields, which in turn affect the trajectories of neighboring particles through the diffusion-swimming effect, leading to long-distance attractions or repulsions.
In this study, Tang’s team developed a simple wavelength-selective TiO2 the active microbeads system depends on their previous studies on light-powered microswimmers. When subjected to photoexcitation, the redox reaction on TiO22 particles create a chemical gradient, which sets up efficient particle-particle interactions.
That is, particle-particle interactions can be regulated by integrating incident light of different wavelengths and intensities. TiO2 Microbeads with various photosensitive activities can be developed by selecting dye sensitization codes with various spectral characteristics.
Through the mixing of different identical TiO2 Microbead species loaded with dyes of various absorption spectra and adjusting to the spectrum of incident light, on-demand particle segregation are identified.
The motivation for identifying particle phase segregation is to regulate the aggregation and dispersion of particles in liquids at both the micro and macro levels. Efficiently, it produces new photo-responsive inks by blending microbeads with a wide range of photo sensitivities that can be used in e-paper.
The principle is identical to the pigment clusters in the skin of cephalopods which can sense the light conditions around them and change the appearance of the surrounding pigment cells through appropriate actions.
Research findings have contributed significantly to advancing our knowledge of swarm intelligence in artificial active ingredients and have paved the way for designing innovative smart active ingredients.
Dr. Jinyao Tang, Department of Chemistry, University of Hong Kong
Tang added, “With this breakthrough, we anticipate the development of programmable photochromic inks that could be used in a variety of applications such as e-inks, display inks and even active optical camouflage inks..”
Zheng, J., et al. (2023) Photochromism of wavelength-selective colloid phase segregation. Natural. doi.org/10.1038/s41586-023-05873-4.