Furan Donor for NIR-II Molecular Fluorophores with Enhanced Bioimaging Performance

Abstract

The second near-infrared (NIR-II, 1,000 to 1,700 nm) molecular fluorophores containing donor-acceptor-donor conjugated backbone have attracted substantial attention due to their outstanding advantages, such as stable emission and facilely tuned photophysical properties. However, it is still challenging for them to simultaneously achieve high brightness and red-shifted absorption and emission. Herein, furan is adopted as the D unit to construct NIR-II fluorophores, demonstrating red shift of absorption, enhanced absorption coefficient, and fluorescent quantum yield when compared with the generally used thiophene counterparts. The high brightness and desirable pharmacokinetics of the optimized fluorophore, IR-FFCHP, endows improved performance for angiography and tumor-targeting imaging. Furthermore, dual-NIR-II imaging of tumor and sentinel lymph nodes (LNs) has been achieved with IR-FFCHP and PbS/CdS quantum dots, enabling the in vivo imaging navigated LN surgery in tumor-bearing mice. This work demonstrates the potential of furan for constructing bright NIR-II fluorophores for biological imaging.

Scheme. 1.

Structural illustration of S-D-A-D-S fluorophores and structural regulation of molecular fluorophores in this work.

Fig. 1.

Optical properties of the synthesized fluorophores. Absorption (A) and emission (B) properties of the un-PEGylated fluorophores in toluene. Absorption (C) and emission (D) properties of the PEGylated fluorophores in deionized water. Measured concentration: 4.0 × 10⁻⁵ M, excitation: 808-nm laser. Photoluminescence (PL) intensity at varied long-pass (LP) wavelength (900 to 1,400 nm) for IR-FFCHP with a low concentration in phosphate buffer saline (0.5 μM) under irradiation of a 65 mW/cm² laser for 5 ms (E) and increasing exposure time (F). a.u., arbitrary units.

Fig. 2.

Molecular dynamic simulations of the fluorophores. (A) Schematic diagram of interaction between H₂O and fluorophores in aqueous circumstances through molecular dynamic simulations. Brown, red, yellow, blue, and thin line parts with gray and red represent the C, O, S, N, and PEG chain, respectively. (B) RDF of O atoms from water around the BBTD of fluorophores with the radius (angstrom) around BBTD core. (C) Counted H₂O number surrounding BBTD.

Fig. 3.

Imaging performance of the fluorophores. Vessels imaging for the whole-body (A) and magnified hindlimb (B). (C) The signal-to-background ratio (SBR) of the specified position of imaged hindlimb at different post-injection time points. (D) Brain vessel imaging in mice treated with indocyanine green (ICG) and IR-FFCHP. The PL intensity and SBR changes of marked position on brain vascular for ICG (E) and IR-FFCHP (F) with different post-injection time points. Intravenous injection dose: 100 μl, 1 mg/ml; LP filter: 1,200 nm; exposure time: 200 ms; excitation: 808-nm laser (80 mW/cm²). The scale bars represent 1 cm.

Fig. 4.

Pharmacokinetics and targeted imaging of 4T1 tumor model with IR-FFCHP. (A) In vivo circulation of IR-FFCHP. (B) Imaging of a mouse with 4T1 tumor after intravenous injection of IR-FFCHP; the high EPR effect on 4T1 tumor is observed because of the long circulation of fluorophores. The corresponding fluorescence signal intensity of liver (C) and tumor (D) as a function of time. (E) Ex vivo imaging of main organs of IR-FFCHP-treated mice after 7 d post-injection. (F) The fluorescence intensity values of main organs of IR-FFCHP treated mice. Imaging condition: 1,200-nm LP, 100-ms exposure time, 80 mW/cm² laser power. The scale bars represent 1 cm. In, intestine; Li, liver; Sp, spleen; He, heart; Lu, lung; Ki, kidney; Bl, bladder; Tu, tumor; St, stomach; Br, brain.

Fig. 5.

NIR-II imaging guided surgery with IR-FFCHP. (A) The schematic illustration of dual-colored in vivo NIR-II imaging guided 4T1 tumor-sentinel LN surgery. (B) Tumor imaging with IR-FFCHP in 1,200- to 1,300-nm window and sentinel LN imaging with QDs in >1,500-nm window. (C) PL intensity of tumor and LN at different time points after sequential injection of IR-FFCHP and QDs. (D) H&E stain of the resected tumor and LNs with/without tumor metastasis. The scale bar represents 1 cm.