Hollow Structures in Micro/nanomotors: Design Principles, Propulsion Mechanisms, and Functional Applications

Conspectus The advent of self-propelled micro/nanomotors has captivated both researchers and the broader public since the early days of nanotechnology. Over the past 15 years, the field has witnessed remarkable expansion, with research groups worldwide driving innovations that have unlocked diverse and impactful applications, including targeted drug delivery, in vivo biosensing, environmental remediation, and intelligent manufacturing. Advances in nanomaterial synthesis have profoundly shaped the evolution of micro/nanomotors as their functional performance is intricately tied to their topological design. Among the various architectures explored, hollow-structured micro/nanomotors have emerged as a particularly promising class, owing to their confined interior space, high surface area, and multifunctional potential. Compared to their solid counterparts, hollow micro/nanomotors offer several distinct advantages: (1) their cavities provide well-defined spaces for physicochemical reactions, essential for propulsion; (2) the interior can be functionalized with catalysts, therapeutic agents, imaging agents, or enzymes to support specific tasks; and (3) the cavities can be equipped with gated materials for payload protection and on-demand release, a critical feature for the in vivo delivery of sensitive molecules such as drugs, antibodies, and enzymes. Despite substantial progress in this area, an up-to-date review dedicated specifically to hollow micro/nanomotors is still lacking. In this Account, a critical overview of recent developments in hollow micro/nanomotors is presented with emphasis on synthetic methodologies, propulsion mechanisms, and emerging applications. Four representative fabrication strategies, the hard-templating method, the soft-templating method, the rolled-up technology, and the osmosis-induced method, are first outlined, and their respective advantages and limitations are described in detail. The role of hollow architectures in enhancing propulsion is subsequently examined, with particular attention given to the interrelationships among structure, mechanism, and motion dynamics. Key applications exploiting hollow structures are then highlighted, including biomedical applications, environmental remediation, advanced motion-manipulation techniques, and catalytic applications. Finally, the prevailing challenges and future opportunities in this rapidly evolving field are discussed. It is anticipated that the unique functional advantages and sophisticated motion control offered by hollow architectures will play a pivotal role in advancing micro/nanomotor systems from a conceptual demonstration to practical application.