Oxygen Sensing Difluoroboron Dinaphthoylmethane Polylactide

Abstract

Dual emissive luminescence properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdk-PLA) materials have been utilized as biological oxygen sensors. Dyes with red-shifted absorption and emission are important for multiplexing and in vivo imaging, thus hydroxyl-functionalized dinaphthoylmethane initiators and dye-PLA conjugates BF2dnm(X)PLA (X = H, Br, I) with extended conjugation were synthesized. The luminescent materials show red-shifted absorbance (~435 nm) and fluorescence tunability by molecular weight. Fluorescence colors range from yellow (~530 nm) in 10 – 12 kDa polymers to green (~490 nm) in 20 – 30 kDa polymers. Room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) are present under a nitrogen atmosphere. For the iodine-substituted derivative, BF2dnm(I)PLA, clearly distinguishable fluorescence (green) and phosphorescence (orange) peaks are present, making it ideal for ratiometric oxygen-sensing and imaging. Bromide and hydrogen analogues with weaker relative phosphorescence intensities and longer phosphorescence lifetimes can be used as highly sensitive, concentration independent, lifetime-based oxygen sensors or for gated emission detection. BF2dnm(I)PLA nanoparticles were taken up by T41 mouse mammary cells and successfully demonstrated differences in vitro ratiometric measurement of oxygen.

Tailoring Oxygen Sensitivity with Halide Substitution in Difluoroboron Dibenzoylmethane Polylactide Materials

“…23 Compared to less soluble dinaphthyl initiators, 23 6–10 readily dissolved even in low loadings of molten lactide (i.e. 50 equivalents).…”

“…19 With increased conjugation in naphthyl-naphthyl based dyes (BF 2 dnmPLA), which corresponded to lower solubility and increased propensity for dyes to aggregate, a maximal blue-shifted fluorescence was reached with only 0.2% dye loading in PLA blends. 23 …”

“…22,23 This is linked to the singlet-triplet energy gap; as the gap becomes larger, the rate of intersystem crossing decreases, lowering the phosphorescence intensity. For short polymers, Br 14a and I 15a , the fluorescence and phosphorescence peaks overlap (Figure 4).…”

“…Instead, heavy atom substituted dyes that exhibit strong phosphorescence at low dye loading are desirable. 20,23 …”

“…Fluorescence serves as an internal standard while phosphorescence, which is susceptible to oxygen quenching, serves as the sensor. 21–23 As the dyes are exposed to oxygen, the fluorescence to phosphorescence (F/P) intensity ratio is measured and compared to an oxygen calibration curve to determine the concentration of oxygen present. In addition to fostering dual emissive properties of BF 2 bdk dyes—fluorescence and phosphorescence, rather than just fluorescence alone—PLA is a good choice because it is a biocompatible and degradable polymer.…”