The newly developed photochromic active colloid describes the development of new intelligent active ingredients

May 18, 2023

(Nanowerk News) In nature, the skin of cephalopods (animals with tentacles attached to their heads) exhibits unsurpassed camouflage abilities. Their skin contains groups of pigments which can sense changing light conditions of the environment and adjust their appearance through the action of pigment cells. Despite its complex nature, this color-changing ability is fundamentally based on a mechanical mechanism in which pigment particles are folded or unfolded under the control of the radial muscles.

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

The team formed dynamic photochromic nanoclusters by mixing cyan, magenta and yellow microbeads, achieving photochromism on a macro scale. This macroscopic photochromism relies on light-induced vertical phase stratification in a mixture of active microbeads, resulting in an enrichment of colored microbeads that match the incident spectrum. The novel ink composed of multi-colored micro-beads adapts to the appearance of received light by light-driven separation. (Image: University of Hong Kong)

Unlike existing color-changing agents, this new batch of photochromic colloids relies on rearranging existing pigments rather than generating new chromophores. in place and, therefore, more reliable and programmable. Their findings provide a simple method for applications such as electronic inks, displays, and active optical camouflage, which are major breakthroughs in the field of active materials.

The results of the study were recently published in the journal Natural(“Photochromism of wavelength-selective colloid phase segregation”).

Self-propelled active particles are micro/nano particles that mimic the swimming direction of microorganisms in liquids. Recently, they have attracted significant attention 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 and nano robots based on these particles for drug delivery and non-invasive surgery. 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 an individual micro/nano active particle limits the complexity of applying the function to its body. The challenge and key to realizing future applications is how to make active particles with intelligent characteristics despite their simple structure.

Light-powered micro-swimmers, a type of self-propelled active particle, were recently developed for the purpose of creating controllable nanorobots, which offer potential for biomedical applications and functional novel materials because the swimmer’s activity, alignment directions, and interactions between the particles can be carried out by easy. modulated with incident light.

On the other hand, light not only induces photosensitive movements in microswimmers but also changes the effective 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, resulting in long-distance attractions or repulsions.

In this work, Tang’s team designed a simple wavelength-selective TiO22 active microbeads system based on their previous research on light-powered micro-swimmers. After photoexcitation, the redox reaction on TiO22 particles create a chemical gradient, which sets up the effective particle-particle interactions. That is, the particle-particle interaction can be controlled by combining incident light with different wavelengths and intensities. TiO2 microbeads with different photosensitive activities can be formed by selecting dye sensitization codes with different spectral characteristics. By mixing several identical TiO22 species of microbeads loaded with dyes of different absorption spectra and adjusting the spectrum of incident light, on-demand separation of particles is realized.


The aim of realizing particle phase segregation is to control the aggregation and dispersion of particles in liquids at the micro and macro levels. Effectively, it produces new photoresponsive inks by mixing microbeads with different photo sensitivities that might be applied to e-paper. The principle is similar to the pigment clusters in the skin of cephalopods which can sense light conditions in the environment and change the appearance of the surrounding pigment cells through appropriate actions.

‘The 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. With this breakthrough, we anticipate the development of programmable photochromic inks that can be used in a variety of applications such as e-inks, display inks and even active optical camouflage inks,” concludes Dr Jinyao Tang.

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