Abstract
Near infrared (NIR) light offers high transparency in biological tissue. Recent advances in NIR fluorophores including organic dyes and lanthanide-doped inorganic nanoparticles have realized the effective use of the NIR optical window for in vivo bioimaging and photodynamic therapy. The narrow energy level intervals used for electronic transition that involves NIR light, however, give rise to a need for guidelines for reducing heat emission in luminescence systems, especially in the development of organic/inorganic hybrid structures. This review presents an approach for employing the polarity and vibrational energy of ions and molecules that surround the luminescence centers for the development of such hybrid nanostructures. Multiphonon relaxation theory, formulated for dealing with heat release in ionic solids, is applied to describe the vibrational energy in organic or molecular systems, referred to as phonon in this review, and we conclude that surrounding the luminescence centers either with ions with low vibrational energy or molecules with small chemical polarity is the key to bright luminescence. NIR photoexcited phosphors and nanostructures in organic/inorganic mixed systems, designed based on the guidelines, for photodynamic therapy are reviewed.
Highlights
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When a micellar nanoparticle is composed of a polymer core having a chemical structure with a small energy difference from the dye molecule IR-1061, based on the three factors of dispersion, polarity, and hydrogen bonding, the nanoparticle encapsulates the dye with high efficiency and provides a probe with high fluorescence performance and encapsulation stability [57]
Heat emission in a photonic system that involves near infrared (NIR) light is intense because the energy level intervals used for electronic transitions are narrow, as compared to those of visible light
Summary
Light in the near infrared (NIR) wavelength region exhibits high optical transmittance to living organisms and has been attracting increasing attention for applications in biomedical photonics. A simple way, supported by experimental results, is to describe the surrounding atoms, ions, and molecules as a sum of electric dipole moments. During the 1980s, a vast number of organic fluorescent dyes that emit NIR fluorescence were developed for the purpose of optical amplification in fiber optical communication [13,14,15,16,17,18,19,20,21] To control heat absorption and emission, regardless of whether it is organic or inorganic, we should pay attention to the energy of vibration, }ω, and the polarity of the space created by the atoms, ions, and molecules surrounding the luminescence center. We discuss the application of nanostructures in which ions and molecules are hybridized and designed for NIR biomedical photonics based on these concepts
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