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Abstract

• Carbon nano-onions (CNOs) are highlighted as promising agents for bioimaging due to their low toxicity and ability to penetrate cells.

• This study details the functionalization of CNOs with a specific BODIPY dye, enabling fluorescent activity that can switch on/off in response to pH changes.

• The switching mechanism is attributed to the photoinduced electron transfer (PET) attributes of the BODIPY dye.

Introduction

• Diagnostic applications increasingly rely on nanomaterials that can sense physiological changes in cells.

• Fluorescent nanosensors are cost-effective and simple to operate, capable of generating optical responses to stimuli.

• Intracellular pH is critical to cell health, with fluctuations indicating various cellular processes.

Properties of CNOs

• CNOs are an attractive class of carbon nanomaterials for imaging due to:

  • Low density and high surface area to volume ratio

  • Spherical shape allowing versatile functionalization

• CNOs show low inflammatory response and cytotoxicity, with successful internalization into cancer cells.

Functionalization Process

• The procedure includes:

  • Oxidation of CNOs using nitric acid to create carboxylic groups.

  • Reaction of oxidized CNOs with BODIPY dye to create fluorescently labeled CNOs (fluo-CNOs).

Mechanism of Action

• The modulation of fluorescence in fluo-CNOs is linked to protonation states of the dimethylamino group in the BODIPY:

  • Non-protonated form (BODIPY 3) exhibits lower fluorescence at 737 nm when pH is neutral or basic.

  • Protonated form (BODIPY 4) leads to enhanced fluorescence at 637 nm in acidic conditions.

• The fast and reversible nature of the on/off fluorescence switching is ideal for pH-sensing applications.

Results and Discussion

Synthesis

• Synthetic Schemes:

  • Scheme 1: Details the BODIPY synthesis.

  • Scheme 2: Illustrates the preparation of oxidized CNOs.

Characterization

• Photophysical Properties:

  • Emission spectra confirm the switching between different BODIPY states upon changing pH.

• Dynamic Light Scattering (DLS) indicated effective hydrodynamic diameters of oxi-CNOs and fluo-CNOs.

• Confocal Microscopy studies demonstrate successful internalization and pH-triggered fluorescence in HeLa cells.

Cytotoxicity Studies

• Fluo-CNOs showed moderate toxicity, with cell viability above 80% across various concentrations.

  • Impacts on HeLa cells further emphasize the materials' suitability for use as intracellular sensors.

Conclusion

• BODIPY-CNO probes were synthesized and demonstrated rapid fluorescence modulation based on pH changes.

• The potential applications of these probes in biological sensing highlight their relevance in diagnostics.

Experimental Techniques

• Characterization Techniques: Include TGA, Raman spectroscopy, fluorescence spectroscopy, DLS, and NMR spectroscopy to assess the compounds.

• Cell Culture: HeLa cells were maintained and assessed for cytotoxicity using standard protocols.