Controlling Molecular Dye Encapsulation in the Hydrophobic Core of Core-Shell Nanoparticles for In Vivo Imaging.

Polymeric nanoparticles with a hydrophobic core are valuable biomedical materials with potential applications in in vivo imaging and drug delivery. These materials are effective at protecting vulnerable molecules, enabling them to serve their functions in hydrophilic physiological environments; however, strategies that allow the chemical composition and molecular weight of polymers to be tuned, forming nanoparticles to control the functional molecules, are lacking. In this article, we review strategies for designing core-shell nanoparticles that enable the effective and stable encapsulation of functional molecules for biomedical applications. IR-1061, which changes its optical properties in response to the microenvironment are useful for in vitro screening of the in vivo stability of polymeric nanoparticles. An in vitro screening test can be performed by dispersing IR-1061-encapsulated polymer nanoparticles in water, saline, buffer solution, aqueous protein solution, etc., and measuring the absorption spectral changes. Through the screening, the effects of the polarity, molecular weight, and the chiral structure of polymers consisting of polymer nanoparticles on their stability have been revealed. Based on the findings presented here, more methodologies for the effective application of various biomolecules and macromolecules with complex high-dimensional structures are expected to be developed.

Effect of the enantiomeric structure of hydrophobic polymers on the encapsulation properties of a second near infrared (NIR-II) fluorescent dye for in vivo deep imaging.

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.

Effects of hydrophilic/hydrophobic blocks ratio of PEG-b-PLGA on emission intensity and stability of over-1000 nm near-infrared (NIR-II) fluorescence dye-loaded polymeric micellar nanoparticles.

Polymeric micellar nanoparticles (PNPs) composed of an amphiphilic block copolymer formed from hydrophilic and hydrophobic blocks and over-thousand-nanometer (OTN) near-infrared (NIR) fluorescent dye are promising fluorophores for the dynamic imaging of deep tissue. In this study, we examined the effect of the ratio of hydrophilic/hydrophobic blocks of a block copolymer, poly(ethylene glycol) (PEG)-b-poly(lactide-co-glycolide) (PLGA), on the properties of OTN-PNPs encapsulating IR-1061. OTN-PNPs with a higher molecular weight of PLGA cores showed higher emission and stabilities under physiological conditions. The PEG ratio to PLGA in the block copolymer decreased the stability of OTN-PNPs probably due to the invasion of water molecules into the polymer core. The results show that the in vivo stability and fluorescence properties can be tuned by adjusting the chain lengths of block copolymers and estimated using in vitro assays, which evaluates the brightness retention rate of the OTN-PNPs under physiological conditions.

Heat Treatment Effects for Controlling Dye Molecular States in the Hydrophobic Core of Over-1000 nm Near-Infrared (NIR-II) Fluorescent Micellar Nanoparticles.

Organic molecules that emit near-infrared (NIR) fluorescence at wavelengths above 1000 nm, also known as the second NIR (NIR-II) biological window, are expected to be applied to optical in vivo imaging of deep tissues. The study of molecular states of NIR-II dye and its optical properties are important to yield well-controlled fluorescent probes; however, no such study has been conducted yet. Among the two major absorption peaks of the NIR-II dye, IR-1061, the ratio of the shorter wavelength (900 nm) to the longer one (1060 nm) increased with an increase in the dye concentration in tetrahydrofuran, suggesting that the 900 nm peak is due to the dimer formation of IR-1061. Both absorption peaks are also observed when IR-1061 is encapsulated in the hydrophobic (stearyl) core of micellar nanoparticles (MNPs) of a phospholipid-poly(ethylene glycol). The dimers in the MNP cores decreased via dimer dissociation by enhancing the mobility of the hydrophobic stearyl chains by heat treatment of the dye-encapsulating MNPs at 50-70 °C. The MNPs maintained the dissociated IR-1061 monomers in the core after recooling to 25 °C and showed a higher NIR-II fluorescence intensity than those before heat treatment. This concept will provide better protocols for the preparation of NIR-II fluorescent probes with well-controlled fluorescence properties.

Design of Over-1000 nm Near-Infrared Fluorescent Polymeric Micellar Nanoparticles by Matching the Solubility Parameter of the Core Polymer and Dye.

Polymeric micellar nanoparticles (PNPs) encapsulating over-thousand-nanometer (OTN) near-infrared (NIR) fluorescent dye molecules in block polymers having hydrophobic and hydrophilic chains are promising agents for the dynamic imaging of deep tissue. To achieve OTN-NIR fluorescent PNPs (OTN-PNPs) having high brightness, it is crucial to increase the affinity between the core polymer and dye molecules by matching their polarities; thus, criteria and methods to evaluate the affinity are required. In this study, we used the Hansen solubility parameter (HSP), including the polarity term, to evaluate the affinity between the two substances. HSP values of the OTN-NIR fluorescent dye IR-1061 and four core polymers, poly(lactic-co-glycolic acid) (PLGA), poly(lactic acid) (PLA), poly(ε-caprolactone) (PCL), and polystyrene (PSt), were calculated using the Hansen solubility sphere method and molecular group contribution method, respectively. The relative energy density between IR-1061 and each core polymer calculated using their HSP values revealed that the affinities of PLGA and PLA for IR-1061 are higher than those of PCL and PSt. Therefore, OTN-PNPs composed of PLGA, PLA, and PCL core polymers were prepared and compared. The OTN-PNPs having PLGA and PLA cores could be loaded with larger amounts of IR-1061, had higher photoluminescence intensities, and showed higher stability in phosphate buffered saline than those having PCL cores. Moreover, the OTN-PNPs having PLGA or PLA cores were used for the dynamic imaging of live mice. Thus, matching the solubility parameters of the core polymer and dye molecule is a useful approach for designing high-performance OTN-NIR fluorescent probes.