Advances of stimuli-responsive rare earth-based smart luminescent materials in optical encoding and bio-applications

As an important branch of smart materials, stimuli-responsive materials have become a significant part in a wide range of fields with the development of science and technology. Among them, stimuli-responsive smart luminescent materials refer to a type of materials whose optical properties could respond to the physical or chemical stimuli of the external environment, including photo-, thermo-, pH-, magnetic-, electric-, mechanical- and molecule-responsive. These stimuli-responsive smart materials could be utilized to design sensitive sensors, detectors and fluorescent probes for wide applications in optical encoding, biosensing, medical diagnosis and treatment. Featuring unique optical properties, including narrow emission band, long lifetime, large Stokes shift and excellent photostability, rare-earth ions have acted as ideal luminescence centers for fabricating smart luminescent materials and thus showing great application potential in chemistry, biology and logic. In recent years, through rational design of structures and compositions of organic molecules and nanomaterials, a variety of stimuli-responsive rare-earth doped smart luminescent materials could be obtained and have attracted extensive attention in optical encoding, biosensing, vivo imaging and drug delivery. In this review, the advances of stimuli-responsive rare-earth based smart luminescent materials in optical encoding and biomedicine are summarized according to the types of external stimuli. Conventional optical information storage is generally constrained by limited pathways, such as excitation wavelength, emission color and intensity. However, there are certain risks that these parameters would be reproduced easily. Based on this, studies on stimuli-responsive optical properties contribute to introduce new parameters for multi-modes optical encoding. This strategy has greatly improved information storage capacity and security. Moreover, researches on rare-earth luminescence lifetime provides a new dimension for optical encoding. Stimuli-responsive luminescent lifetime t could be exploited to code information. Development of time-gated (TG) luminescence imaging technology further facilitated the lifetime-encoding study. TG luminescence imaging could differentiate compositions with different lifetimes at the same wavelength and greatly broadens the scope for optical multiplexing encoding. In the field of biology, systems can greatly improve the signal-to-noise ratio of imaging and the accuracy of treatment for precise nanomedicine, which facilitates reducing side effects. With the rapid development of rare earth nanomaterials, their unique optical and magnetic properties have also been used in bioimaging. In addition to being used as imaging agents, they are also widely used as therapeutic agents. Functional modification further expands the application of rare earth nanomaterials in nanomedicine. Combined with the stimuli-responsive strategy, rare-earth based smart nanomaterials have greater potential in theranostics. In the review, we mainly introduced the cancer diagnosis and treatment application of rare-earth nanomaterials stimulated by tumor microenvironment. The advances of stimuli-responsive rare-earth based smart luminescent materials in optical encoding and biomedicine are summarized and the future of this smart material is highlighted. Moreover, the future of this kind of smart materials is highlighted.