Ultra-Fast Fluorescence Imaging in Vivo with Conjugated Polymer Fluorophores in the Second Near-Infrared Window (1406.6452v1)

Abstract: In vivo fluorescence imaging in the second near-infrared window (1.0-1.7 microns) can afford deep tissue penetration and high spatial resolution, owing to the reduced scattering of long-wavelength photons. Here, we synthesize a series of low-bandgap donor/acceptor copolymers with tunable emission wavelengths of 1050-1350 nm in this window. Non-covalent functionalization with phospholipid-polyethylene glycol results in water-soluble and biocompatible polymeric nanoparticles, allowing for live cell molecular imaging at > 1000 nm with polymer fluorophores for the first time. Importantly, the high quantum yield of the polymer allows for in vivo, deep-tissue and ultrafast imaging of mouse arterial blood flow with an unprecedented frame rate of > 25 frames per second. The high time resolution results in spatially and time resolved imaging of the blood flow pattern in cardiogram waveform over a single cardiac cycle (~ 200 ms) of a mouse, which has not been observed with fluorescence imaging in this window before.

• The paper presents a breakthrough by synthesizing donor-acceptor copolymers with a high quantum yield of 1.7%, significantly surpassing traditional fluorophores.
• It achieves unprecedented in vivo imaging speeds over 25 fps, enabling real-time visualization of arterial blood flow and deep tissue structures.
• The study further demonstrates targeted molecular imaging, as polymers conjugated with cetuximab effectively highlight EGFR-positive tumors for diagnostic applications.

Evaluation of Novel Conjugated Polymers for Enhanced NIR-II Imaging

This paper presents a compelling advancement in the domain of in vivo fluorescence imaging utilizing novel conjugated polymers. The paper diverges from traditional approaches by focusing on imaging within the second near-infrared window (NIR-II, 1.0-1.7 µm), delivering enhanced tissue penetration and resolution. The team has synthesized a series of low-bandgap donor/acceptor copolymers capable of emission in the 1050-1350 nm range, effectively addressing some of the limitations posed by previously utilized fluorophores like quantum dots and single-walled carbon nanotubes.

Key Findings:

1. Polymer Synthesis and Functionalization: The synthesized polymers exhibit a high quantum yield of approximately 1.7%, which is notably superior to the 0.4% yield commonly associated with carbon nanotubes. The high yield is achieved through strategic donor-acceptor copolymerization, with benzo[1,2-b:3,4-b']difuran and fluorothieno-[3,4-b]thiophene being the key monomers. Non-covalent functionalization using phospholipid-polyethylene glycol renders the polymers water-soluble and biocompatible, facilitating their application in live cell imaging.
2. Imaging Capabilities: The polymers offer a breakthrough in real-time imaging capabilities. For the first time, in vivo deep-tissue imaging was achieved with an unprecedented rate exceeding 25 frames per second, enabling visualization of mouse arterial blood flow across a cardiac cycle. This capability is attributed to the combination of high fluorescence efficiency and significant tissue penetration inherent to NIR-II imaging.
3. Practical Applications and Implications: By functionalizing these polymers with targeting ligands, the research demonstrates successful molecular imaging of cancer cell surfaces, as evidenced by the conjugation with cetuximab to target EGFR-positive breast tumor cells. This sets the stage for their potential application in disease diagnostics and therapeutic monitoring.
4. Future Directions: The paper opens avenues for further research into the modular design of conjugated polymers, allowing for customized optical properties through alterations in donor-acceptor configurations and polymer length. The enhancement in the temporal and spatial resolution (reaching a frame rate of up to 50 fps) sets a new benchmark for imaging fast physiological processes without necessitating cardiac gating.

Theoretical and Practical Implications:

These findings hold notable implications for the field of biomedical imaging. The development of these polymer-based fluorophores can considerably extend the horizon for diagnostic applications, particularly in situations requiring deep-tissue imaging and high temporal resolution, such as in cardiovascular research. Moreover, the synthesis methodology allows facile tuning of properties, making it highly versatile for multifaceted biological applications.

Notably, the absence of heavy metals in the conjugated polymer-based fluorophores potentially minimizes toxicity, a critical consideration for future human clinical applications. However, it is pertinent that comprehensive studies be carried out to fully delineate the toxicological profile over the longer term.

In conclusion, this research provides a substantial contribution to the NIR-II imaging field by surpassing the limitations of conventional agents and offering a practical and versatile approach to biological imaging. The implications extend beyond imaging, suggesting possibilities in personalized medicine through targeted imaging agents, with particular advantages in rapid and deep tissue imaging. Future explorations could refine these polymers further, achieving even greater fluorescence efficiency and extended capabilities tailored to specific clinical needs.