Steric protection of near-infrared fluorescent dyes for enhanced bioimaging

Abstract

Near-fluorescent (NIR) dyes that absorb and emit light in the wavelength range of 650-1700 nm are well-suited for bioimaging due to the improved image contrast and increased penetration of the long-wavelength light through biological tissue. However, the imaging performance of NIR fluorescent dyes is limited by several inherent photophysical and physicochemical properties including, low fluorescence quantum yield, high chemical and photochemical reactivity, propensity to self-aggregate in water, non-specific association with off-target biological sites, and non-optimal pharmacokinetic profiles in living subjects. In principle, all these drawbacks can be alleviated by steric protection which is a structural process that surrounds the fluorophore with bulky groups that block undesired intermolecular interactions. The literature methods to sterically protect a long-wavelength dye can be separated into two general strategies, non-covalent dye encapsulation and covalent steric appendage. Illustrative examples of each method show how steric protection improves bioimaging performance by providing: (a) increased fluorescence brightness, (b) higher fluorophore ground state stability, (c) decreased photobleaching, and (d) superior pharmacokinetic profile. Some sterically protected dyes are commercially available and further success with future systems will require experts in chemistry, microscopy, cell biology, medical imaging, and clinical medicine to work closely as interdisciplinary teams.

Scheme 1. Representative fluorophores that can absorb/emit in the NIR wavelength range.

Scheme 2. Limitations of long-wavelength fluorophores.

Scheme 3. Two general methods to sterically protect a NIR fluorophore are: (top) covalent steric appendage, and (bottom) non-covalent encapsulation.

Scheme 4. Fluorophore encapsulation and protection by nanoparticles.

Scheme 5. Fluorophore protection by non-covalent encapsulation. The chemical structures within each panel are, (left) fluorophore/host complex, (middle) encapsulated fluorophore component, (right) surrounding macrocyclic host component.

Scheme 6. Fluorophore protected by covalently appended, (a) hydrophilic dendritic polymer, (b) two projecting hydrophilic arms, (c) four projecting hydrophilic arms.

Fig. 1. Mouse hind leg vasculature images acquired using fluorescent molecular probes that enable, (a) conventional NIR imaging (800 nm fluorescence), or (b) NIR-II imaging (1100–1700 nm fluorescence). Reprinted with permission from Wiley. Copyright©2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 7. Fluorophore protected by covalent double straps or loops.

From left to right: Sai Shradha Reddy Kommidi, Kirk M. Atkinson and Bradley D. Smith