In turn, the recently proposed achromatic metalens utilizing transparent, high-index materials such as titanium dioxide has been restricted by the small thickness and showed relatively low focusing efficiency at longer wavelengths. Even though significant progress has been made, the efficiency of technologically relevant silicon metalenses is limited by the intrinsic material loss above the bandgap. In this review, we have focused on recent advances in the chemical design and synthesis of NIR-II fluorophores from small organic molecules to organic and inorganic nanoparticles, and we have further discussed recent advances and key operational differences in reported NIR-II imaging systems and biomedical applications based on NIR-II imaging, such as multimodal imaging, photothermal and photodynamic therapy, guidance for intraoperative surgery, and drug delivery.Over the past years, broadband achromatic metalenses have been intensively studied due to their great potential for applications in consumer and industry products. The presence of simultaneous diagnostic and therapeutic capabilities in a single probe can be used for precise treatment. NIR-II fluorophores may convert NIR-II photons into heat for photothermal therapy and be excited by NIR-II light to produce singlet oxygen for photodynamic therapy. The chemical design and synthesis of NIR-II fluorophores suitable for multimodal imaging is thus emerging as a new and powerful strategy for obtaining high-definition images. Among various state-of-the-art bioimaging modalities, one of the greatest challenges in developing novel probes is to achieve both high resolution and sensitivity. Because of reduced scattering, minimal absorption and negligible autofluorescence, NIR-II imaging provides high resolution, a high signal-to-noise ratio, and deep tissue penetration capability.
The development of fluorophores and molecular probes for the second near-infrared biological window (NIR-II, 1000-1700 nm) represents an important, newly emerging and dynamic field in molecular imaging, chemical biology and materials chemistry.
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