Fluorescent Probes for Near-Infrared Bioimaging

Fluorescent probes operating in the near-infrared (NIR) region have emerged as indispensable tools for visualising biological processes in vitro and in vivo. By exploiting wavelengths beyond 700 nm, these probes minimise background autofluorescence and enable deeper tissue penetration, allowing dynamic studies of molecular events in organs, tumours and vasculature. Key design considerations include maximising quantum yield and photostability while maintaining biocompatibility and cell permeability. Structural strategies range from π-conjugation extension in xanthene and coumarin scaffolds to incorporation of heavy atoms or heteroatoms for redshifted emission. Activatable or “turn-on” probes further enhance contrast by responding to pH changes, enzymatic activity or metal ions. Recent developments have introduced compact, low-molecular-weight dyes with rapid renal clearance, multifunctional sensing platforms for optoacoustic imaging and approaches that leverage ground-state antiaromaticity to achieve ultrafast excited-state dynamics. Together, these advances are broadening the scope of NIR imaging in diagnostics, guided surgery and precision therapy, and underpin ongoing efforts to tailor probe chemistry for specific biological targets.

Research from Nature Portfolio

Recent studies have reported a novel series of cationic aminofluorene dyes with molecular weights under 500 Da that cover emission wavelengths from 700 to 1,600 nm. By applying a ground-state antiaromatic design principle, researchers achieved remarkable photostability and rapid renal clearance through simple carboxylation, enabling accelerated optical diagnostics of organ injury and versatile multispectral fluorescence and optoacoustic imaging.

Another advance involves the development of hydroxymethyl germanium-rhodamine (HMGeR) probes that are pH-activatable in the NIR window. Optimisation of pKa, quantum efficiency and photostability has produced a leading candidate capable of conjugation to biomolecules without loss of function. This probe design has been successfully used to visualise endocytic pH changes in cells and to image tumour microenvironments in vivo, illustrating its potential for clinical translation.

Fluorescent Probes for Near-Infrared Bioimaging publication trend

The graph below shows the total number of articles in fluorescent probes for near-infrared bioimaging across all publications each year (not limited to Nature Index journals).

Technical terms

Near-Infrared (NIR): Electromagnetic radiation in the 700–1700 nm range, offering deep tissue penetration and low autofluorescence.

Stokes Shift: The wavelength difference between absorption and emission maxima of a fluorophore, critical for signal separation.

Quantum Yield: The efficiency of photon emission relative to photon absorption, indicating the brightness of a probe.

Photostability: The resistance of a dye to photobleaching under continuous illumination, determining imaging duration.

Fluorogenic Probe: A non-fluorescent or weakly fluorescent molecule that becomes strongly emissive upon interaction with a specific analyte or environment.

References

1. Ultra-photostable small-molecule dyes facilitate near-infrared biophotonics. Nature Communications (2024).
2. THQ–Xanthene: An Emerging Strategy to Create Next‐Generation NIR‐I/II Fluorophores. Advanced Science (2023).
3. Core remodeling leads to long wavelength fluoro-coumarins. Chemical Science (2020).
4. Design strategy for germanium-rhodamine based pH-activatable near-infrared fluorescence probes suitable for biological applications. Communications Chemistry (2019).
5. Late-stage functionalisation of alkyne-modified phospha-xanthene dyes: lysosomal imaging using an off–on–off type of pH probe. Chemical Science (2021).