Polypyrrole‑Coated FePt Nanoparticles: A Multifunctional Platform for Photothermal Therapy and Photoacoustic Imaging

Abstract

A novel iron–platinum (FePt) nanoparticle platform, encapsulated with polypyrrole (PPy), has been engineered to combine photothermal therapy (PTT) and photoacoustic imaging (PAI) for cancer treatment. The PPy‑coated FePt (FePt@PPy) nanoparticles exhibit excellent biocompatibility, photothermal stability, and strong near‑infrared (NIR) absorption, enabling efficient tumor ablation under 808‑nm laser irradiation and robust PA signal generation. In vitro studies confirm potent cancer‑cell killing and clear PA contrast, positioning FePt@PPy as a promising multifunctional theranostic agent.

Background

Photothermal therapy has emerged as a highly selective, minimally invasive approach for cancer ablation, owing to its precise spatial–temporal control and limited systemic toxicity [1,2]. By converting NIR light into heat, FePt nanoparticles offer superior photothermal conversion efficiency compared to gold counterparts and provide dual CT/MRI contrast [5,6,7]. However, surface functionalization remains essential to improve aqueous stability and biocompatibility. Polypyrrole, a conductive polymer with robust NIR absorption, photothermal stability, and biocompatibility, has been successfully employed in various biomedical contexts [11,12,13]. This study reports the synthesis of PPy‑coated FePt (FePt@PPy) nanoparticles and evaluates their performance for combined PTT and PAI.

Methods

Materials

All reagents, including Pt(acac)₂, Fe(CO)₅, 3‑mercaptopropionic acid (3‑MPA), pyrrole, and others, were purchased from Sigma‑Aldrich and used without further purification. Cell culture reagents and media were sourced from HyClone. Distilled water was used for all aqueous preparations.

Synthesis of FePt@PPy Nanoparticles

The fabrication involved three consecutive steps:

1. Synthesis of hydrophobic FePt NPs: Pt(acac)₂, Fe(CO)₅, 1,2‑hexadecandiol, oleyl amine, and oleic acid were refluxed at 240 °C under Argon. The resulting FePt NPs were collected by centrifugation and washed with hexane.
2. Ligand exchange to obtain hydrophilic FePt NPs: 3‑MPA was introduced to replace oleyl amine/oleic acid, rendering the particles water‑soluble. The hydrophilic NPs were isolated by centrifugation and washed with cyclohexanone, ethanol, and acetone.
3. Polypyrrole coating: Hydrophilic FePt NPs were dispersed in water, stabilized with PVA, and polymerized with pyrrole in the presence of ammonium persulfate. After 2 h of polymerization, the FePt@PPy NPs were centrifuged, washed with hot water, and resuspended in PBS.

Characterization

Transmission electron microscopy (TEM) revealed a core diameter of 8.3 nm and a PPy shell thickness of ~10 nm, yielding an overall size of 42 nm. Energy‑dispersive X‑ray spectroscopy confirmed an Fe:Pt ratio of 20:80. Fourier‑transform infrared spectroscopy (FTIR) confirmed the presence of PPy functional groups. UV‑Vis‑NIR spectra displayed a pronounced absorption band across 700–900 nm, indicating strong NIR activity.

Photothermal and Photostability Testing

FePt@PPy solutions (20–120 µg mL⁻¹) were irradiated with an 808‑nm laser (1 W cm⁻², 6 min). Temperature rises ranged from 39 °C (20 µg mL⁻¹) to 71 °C (120 µg mL⁻¹). Heating/cooling cycles (six repeats) showed negligible temperature deviation and unchanged UV‑Vis spectra, confirming excellent photothermal stability.

Long‑Term Storage

Nanoparticles stored at 4 °C in PBS or cell‑culture media retained size (< 45 nm) and absorption characteristics over 30 days, indicating robust colloidal stability.

Cellular Cytotoxicity and Uptake

MTT assays on MDA‑MB‑231 breast cancer cells revealed >95 % viability even at 120 µg mL⁻¹ FePt@PPy, whereas uncoated FePt exhibited ~80 % viability. Prussian blue staining confirmed efficient cellular internalization.

In Vitro Photothermal Therapy

After 24 h incubation with FePt@PPy (10–100 µg mL⁻¹) followed by 808‑nm irradiation (1 W cm⁻², 4–6 min), cell viability decreased in a dose‑ and time‑dependent manner. At 100 µg mL⁻¹ and 6 min irradiation, ~70 % of cells were killed, surpassing many reported systems at comparable laser powers.

In Vivo Heating Experiment

Subcutaneous injection of 100 µL FePt@PPy (100 µg mL⁻¹) in a nude mouse followed by 808‑nm irradiation (1 W cm⁻², 6 min) increased the skin temperature by ~19 °C, localizing the thermal effect to the injection site.

In Vitro Photoacoustic Imaging

Phantom studies with 50–200 µg mL⁻¹ FePt@PPy demonstrated clear PA contrast that increased with concentration, underscoring the potential for image‑guided phototherapy.

Conclusions

Polypyrrole‑coated FePt nanoparticles combine high NIR absorption, robust photothermal conversion, excellent biocompatibility, and strong PA signal generation. Their performance in vitro and in vivo underscores their promise as multifunctional agents for combined photothermal therapy and photoacoustic imaging.