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This report presents nanoparticles composed of a liquid gallium core with a reduced graphene oxide (RGO) shell (Ga@RGO) of tunable thickness. The particles are produced by a simple, one-pot nanoprobe sonication method. The high near-infrared absorption of RGO results in a photothermal energy conversion of light to heat of 42.4%. This efficient photothermal conversion, combined with the large intrinsic thermal expansion coefficient of liquid gallium, allows the particles to be used for photoacoustic imaging, that is, conversion of light into vibrations that are useful for imaging. The Ga@RGO results in fivefold and twofold enhancement in photoacoustic signals compared with bare gallium nanoparticles and gold nanorods (a commonly used photoacoustic contrast agent), respectively. A theoretical model further reveals the intrinsic factors that affect the photothermal and photoacoustic performance of Ga@RGO. These core–shell Ga@RGO nanoparticles not only can serve as photoacoustic imaging contrast agents but also pave a new way to rationally design liquid metal-based nanomaterials with specific multi-functionality for biomedical applications.

Noncontact human-machine interaction provides a hygienic and intelligent approach for the communication between human and robots. Current noncontact human-machine interactions are generally limited by the interaction distance or conditions, such as in the dark. Here we explore the utilization of hand as an infrared light source for noncontact human-machine interaction. Metallic gratings are used as the human-machine interface to respond to infrared radiation from hand and the generated signals are visualized as different infrared structural colors. We demonstrate the applications of the infrared structural color-based human-machine interaction for user-interactive touchless display and real-time control of a robot vehicle. The interaction is flexible to the hand-interface distance ranging from a few centimeters to tens of centimeters and can be used in low lighting condition or in the dark. The findings in this work provide an alternative and complementary approach to traditional noncontact human-machine interactions, which may further broaden the potential applications of human-machine interaction.

Highly-oriented, interconnected graphene frameworks have been considered as promising candidates to realize high-performance thermal management in microelectronics.

Radiative cooling, which normally requires relatively high infrared (IR) emissivity, is one of the insects’ effective thermoregulatory strategies to maintain their appropriate body temperature. Recently, the physical correlation between the delicate biological microstructures and IR emissivity for thermal radiation draws increased attention. Here, a scent patch region on the hindwing of Rapala dioetas butterfly is found to exhibit enhanced IR emissivity compared with the non-scent patch regions. A series of optical simulations are conducted to differentiate the effect of biological structures and material composition on the high IR emissivity. Besides the intrinsic IR absorption (emission) of chitin (the main composition of butterfly wings), the hierarchical microstructures of the scent patch scale further improve the IR absorption (emission) through the increased inner surface area and multi-scattering effect. This enhancement of IR emissivity enables the butterfly to efficiently radiate heat from the scent patch region to the environment with a limited volume of chitin. This study of the correlation between IR emissivity and microstructural designs may offer additional pathways to engineer bioinspired materials and systems for radiative cooling applications.