Recent Developments in Near-Infrared-II Luminescence Imaging Using Inorganic Nanoparticles: Semiconductor Quantum Dots and Lanthanide Nanoparticles

Fluorescence imaging finds extensive application in cellular and small animal studies due to its superior temporal and spatial resolution. However, fluorescence imaging using visible light faces limitations such as shallow tissue penetration, phototoxicity from excitation sources, and compromised detection sensitivity owing to background autofluorescence interference. To address these issues, researchers have explored longer wavelength light, particularly near-infrared-I (NIR-I) in the 700–900 nm range. Moreover, there is growing interest in exploiting NIR-II light, which spans the 1000–1700 nm range, to enhance the detection sensitivity, resolution, and tissue-penetration depth. In the NIR-II region, light scattering is minimized, thus enabling deeper tissue penetration of up to ~ 10 mm, along with reduced tissue autofluorescence. This facilitates high-sensitivity and high-resolution fluorescence imaging. The present review highlights inorganic nanoparticle-based imaging probes characterized by exceptional photostability and easily tunable emission wavelengths, including quantum dots and lanthanide nanoparticles. Specifically, recent advancements in improving the luminescence efficiency of NIR-II quantum dots and lanthanide nanoparticles, tuning the emission wavelengths to longer ranges, and designing stimuli-responsive mechanisms for precise targeted imaging are discussed.