Effects of Processing pH on Emission Intensity of Over-1000 nm Near-Infrared Fluorescence of Dye-Loaded Polymer Micelle with Polystyrene Core.

Fluorescence imaging using the over-thousand-nanometer (OTN) near-infrared (NIR) light is an emerging method for an in vivo imaging analysis of deep tissues without physical sectioning. Polymer micelle nanoparticles (PNPs) composed of organic polymers encapsulating an OTN-NIR fluorescent dye, IR-1061, in their hydrophobic core are expected to be biocompatible probes. Because IR-1061 quickly quenches due to the vibration of polar hydroxyl bonding in its surroundings, the influence of hydroxyl ions should be minimized. Herein, we investigated the effect of the hydrogen ion concentration during the preparation process using IR-1061 and an organic polymer, poly(ethylene glycol)-block-polystyrene (PEG-b-PSt), on the emission properties of the obtained OTN-PNPs. The OTN-PNP has a hydrodynamic diameter of 20 - 30 nm and emits 1110-nm fluorescence that is applicable to angiography. The loading efficiency of IR-1061 in the OTN-PNPs increased when prepared in an aqueous solution with a low hydroxyl ion concentration. In this solution (pH 3.0), highly emissive OTN-PNPs was obtained with IR-1061 at lower nominal concentrations. Decreasing the hydroxyl ion concentration during the preparation process yields highly emissive OTN-PNPs, which may improve the in vivo imaging analysis of biological phenomena in deep tissues.