American scientists have created a polymer nanostructured material that can bend like a plant toward a light source. Due to the temperature difference in the rod of the new material, caused by local heating of the photosensitive nanoparticles, it bends until it is directed towards the incident beam. The new material can quadruple the efficiency of light energy conversion devices, according to the study.

Many living organisms are able to move under the influence of external environmental factors: cells and bacteria, for example, can migrate in the direction of increasing the concentration of nutrients, and plants orient their organs perpendicular to the incident light for more efficient energy production.

Scientists have already artificially created light-sensitive materials, but changes in shape were determined by the difference in the orientation of molecules in the material, rather than by the location of the source Light. The deformations were highly dependent on temperature, light intensity and other external factors, making it difficult to control the movement of the material.

Xiaoshi Qian and colleagues at the University of California, Los Angeles have created artificial phototropic rods from a variety of photosensitive polymers. Similar to the mechanism used by plants (cell elongation in darkened areas of the stem), researchers used materials that contracted when light entered and the temperature increased so that the stem was asymmetrically deformed.

To demonstrate the principle, scientists studied the behaviour of four materials: a thermosensitive hydrogel with gold nanoparticles or reduced graphene oxide as photoreceptors; a copolymer of this hydrogel with polyacrylamide and photosensitive polypyrrole; another hydrogel with polyaniline as a light absorber; as well as an indigo liquid crystal elastomer.

Artificial phototropism occurred due to several physical processes: the conversion of photon energy into thermal energy, the diffusion of substances under the action of heat, and mechanical deformation. The light-evoked plasmon resonance and spot heating of the illuminated hydrogel sections in the nano photo absorbers, as a result of which they contracted and the rod, turned until the temperature difference disappeared, that is, until it turned out to be in the direction of light incidence. When the lighting ceased, the rod returned to its original undeformed position, and when a new ray fell, it again quickly (by one degree in three-hundredths of a second) reacted to a change in its angle.

Several of these rods located on the same surface could deviate from the axis in the direction of light incidence by 84 degrees, providing four times greater possibility of converting light energy into thermal energy than static photo converters.

The authors of the study demonstrated the principle of four materials. According to them, other hydrogels, liquid crystal elastomers and polymers based on azobenzene or spiropyran can behave in a similar way.

It’s not just polymers that can be set in motion under the influence of light. For example, a group of Hong Kong chemists created light-controlled micro-swimmers from silicon, platinum and titanium oxide. Their movement was provided by the interaction of charges in them with the field, which arose as a result of photochemical reaction.