Abstract
Over-thousand-nanometer (OTN) near-infrared (NIR) fluorophores are useful for biological deep imaging because of the reduced absorption and scattering of OTN-NIR light in biological tissues. IR-1061, an OTN-NIR fluorescent dye, has a hydrophobic and cationic backbone in its molecular structure, and a non-polar counter ion, BF4−. Because of its hydrophobicity, IR-1061 needs to be encapsulated in a hydrophobic microenvironment, such as a hydrophobic core of polymer micelles, shielded with a hydrophilic shell for bioimaging applications. Previous studies have shown that the affinity of dyes with hydrophobic core polymers is dependent on the polarity of the core polymer, and that this characteristic is important for designing dye-encapsulated micelles to be used in bioimaging. In this study, the dye–polymer affinity was investigated using hydrophobic polymer films with different chiral structures of poly(lactic acid). IR-1061 showed higher affinity for l- and d-lactic acid copolymers (i.e., poly(dl-lactic acid) (PDLLA)) than to poly(l-lactic acid) (PLLA), as IR-1061 shows less dimerization in PDLLA than in PLLA. In contrast, the stability of IR-1061 in PDLLA was less than that in PLLA due to the influence of hydroxyl groups. Choosing hydrophobic core polymers for their robustness and dye affinity is an effective strategy to prepare OTN-NIR fluorescent probes for in vivo deep imaging.
Highlights
Fluorescence bioimaging enables highly sensitive and dynamic observation of biomolecules and microstructures without ionizing radiation.[1]
We examined the difference in the molecular state of the dye depending on the enantiomeric structure of Poly(lactic acid) (PLA) when IR-1061loaded micellar nanoparticles were synthesized using two types of poly(ethylene glycol)-block-(PEG-b-) PLA, PEG-b-poly(L-lactic acid) (PLLA) and PEG-b-PDLLA
The results showed that IR-1061 formed less dimers in PDLLA than in PLLA (Fig. 2), suggesting that IR-1061 showed higher affinity for PDLLA than PLLA
Summary
Fluorescence bioimaging enables highly sensitive and dynamic observation of biomolecules and microstructures without ionizing radiation.[1]. There is a wide variety of OTN-NIR uorescent probes[6] including quantum dots,[7,8,9,10] single-walled carbon nanotubes,[11,12,13,14,15,16,17,18,19,20] lanthanide-doped ceramic nanoparticles,[3,21,22,23] One such probe, designed using IR-1061, consists of polymer micelles encapsulating this dye in their hydrophobic core.[24,28] Previous studies have shown that the molecular state of IR-1061 varies between monomer and dimer depending on the polymer and solvent being used, as well as its concentration.[37] In short, IR-1061 is a low polarity molecule that, while it can exist as monomers in environments with high affinity even at locally high concentration, tends to form dimers that show low uorescence intensity in non-polar and hydrophobic environments.[37] Previous studies have shown that the aggregate formation like dimers of organic dye molecules alters their optical properties such as emission intensity and wavelength.[37,38,39] It is important to clarify the in uence of the polymer. We examined the difference in the molecular state of the dye depending on the enantiomeric structure of PLA when IR-1061loaded micellar nanoparticles were synthesized using two types of poly(ethylene glycol)-block-(PEG-b-) PLA, PEG-b-PLLA and PEG-b-PDLLA
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