A dual-emissive-materials design concept enables tumour hypoxia imaging

Abstract

Luminescent materials are widely used for imaging and sensing owing to their high sensitivity, rapid response and facile detection by many optical technologies. Typically materials must be chemically tailored to achieve intense, photostable fluorescence, oxygen-sensitive phosphorescence or dual emission for ratiometric sensing, often by blending two dyes in a matrix. Dual-emissive materials combining all of these features in one easily tunable molecular platform are desirable, but when fluorescence and phosphorescence originate from the same dye, it can be challenging to vary relative fluorescence/phosphorescence intensities for practical sensing applications. Heavy-atom substitution alone increases phosphorescence by a given, not variable amount. Here, we report a strategy for modulating fluorescence/phosphorescence for a single-component, dual-emissive, iodide-substituted difluoroboron dibenzoylmethane-poly(lactic acid) (BF(2)dbm(I)PLA) solid-state sensor material. This is accomplished through systematic variation of the PLA chain length in controlled solvent-free lactide polymerization combined with heavy-atom substitution. We demonstrate the versatility of this approach by showing that films made from low-molecular-weight BF(2)dbm(I)PLA with weak fluorescence and strong phosphorescence are promising as 'turn on' sensors for aerodynamics applications, and that nanoparticles fabricated from a higher-molecular-weight polymer with balanced fluorescence and phosphorescence intensities serve as ratiometric tumour hypoxia imaging agents.

Figure 1. Synthesis and solid-state emission of BF₂dbm(I)PLA (P1-P3)

a, Ring-opening polymerization of BF₂dbm(I)PLA (P1-P3). b-e, Steady-state emission spectra of polymers P1-P3 as powders (b,c) and spin-cast films (d,e) under air (b,d) and under N₂ (c,e). f,g, Simple-cast P1-P3 (right to left) films in vials under air (f) and N₂ (g) (ultraviolet excitation: λ_ex = 365 nm).

Figure 2. Oxygen sensitivity for P1 BF₂dbm(I)PLA film

a, Emission spectra of the spin-cast film (P1) under increasing oxygen levels (indicated by the arrow, 0-1%) normalized to the fluorescence band. b, Image showing yellow phosphorescence emission under a N₂ gas stream for a spin-cast P1 film under ultraviolet excitation. (Yellow phosphorescence turns on immediately on gas contact. Blue green background: weak P1 fluorescence.) c, Linear relationship between oxygen level and the fluorescence/phosphorescence intensity ratio at two fixed wavelengths (450 nm and 525 nm).

Figure 3. Tumour hypoxia imaging with P2 BF₂dbm(I)PLA nanoparticles

a-c, In vivo imaging of the breast cancer 4 T1 mammary carcinoma tumour region in a mouse window chamber model showing the bright-field (a) and BNP fluorescence/phosphorescence ratio while breathing carbogen—95% O₂ (b), room air—21% O₂ (c) and nitrogen—0% O₂ (d). Emission intensity was averaged from 430 to 480 nm (fluorescence) and 530 to 600 nm (phosphorescence). Several blood vessels run vertically on the left side of the images (dark lines in the bright-field image; more oxygenated yellow-red regions in the fluorescence/phosphorescence images), with the tumour comprising the region to the right of the vessels (less-oxygenated blue regions in the fluorescence/phosphorescence images).