Abstract:

The continuous developments in physical chemistry, improved methodology, and advanced techniques have spurred interest in chemical reaction at the microscopic scale. Experimental manipulation techniques at the microscopic level are demanded to enable in-depth studies regarding the regulation of chemical reactions, material structures, and properties. The development and application of microscopic research methods have become an emerging trend in physical chemistry. Techniques featuring the use of optical, magnetic, and acoustic tweezers have been developed to manipulate objects at the microscopic scale. Optical tweezers use momentum transfer between light and objects to manipulate objects and can stably trap and manipulate mesoscopic particles, even single molecules, by exerting pico-newton force. With advantages including non-invasiveness, non-damaging, and ultra-high sensitivity, optical tweezers are ideal for studying individual molecules, molecular aggregates, condensed matter, chemical bonds, and intermolecular interactions. This technique has the potential to revolutionize the fields of chemistry, physics, information technology, and life sciences. Arthur Ashkin was awarded the 2018 Nobel Prize in Physics for his contribution to the development of this technique. The trapping force of the conventional optical tweezers technique originates from the light intensity gradient. Because of the diffraction limit of light, the trapping and manipulation of micro-nano objects < 100 nm in size with traditional optical tweezers is difficult. However, simply increasing the optical power used for trapping induces serious thermal effects and photodamage. By developing unique materials and structures coupled with optical tweezers, researchers have broken the diffraction limit of light and achieved sub-nanometer single-molecule trapping. In this review, we summarize the recent advances in the application of various optical tweezers techniques in physical chemistry and demonstrate the technical principles of fiber, photonic crystal, and plasmonic optical tweezers, respectively. We focus on the development and application of plasmonic optical tweezers and single-molecule plasmonic optical trapping based on tunable nanogaps. Generally, optical tweezers can realize the trapping and manipulation of molecular-scale particles via two main technical routes. The first route is improving the laser focusing ability through unique optical path design and optical component fabrication. The second involves enhancing the trapping field through ingenious auxiliary structure design. Finally, we present the promising future developments and applications of optical tweezers technology.

Key words: Optical tweezers, Single-molecule, Trapping volume, Microscale manipulation, Plasmon, Solid-liquid interface