Hypericin: Single molecule spectroscopy of an active natural ingredient

TL;DR

This study presents the first single molecule spectroscopy analysis of hypericin, revealing its photophysical properties and how embedding in a PVA matrix affects its fluorescence and stability, crucial for medical applications like photodynamic therapy.

Contribution

It provides novel single molecule spectroscopic data on hypericin, including fluorescence, SERS, and blinking dynamics, advancing understanding of its photophysical behavior for therapeutic use.

Findings

Hypericin exhibits significant photostability suitable for single molecule studies.

Embedding in PVA alters blinking dynamics and increases photostability.

Single molecule SERS detects both neutral and deprotonated forms of hypericin.

Abstract

Hypericin can be found in nature in Hypericum perforatum (St. John's Wort) and has become subject of intense biochemical research. Studies report of antidepressive, antineoplastic, antitumor and antiviral activity of hypericin. Among the variety of potential applications hypericin can be used as photosensitizer in photodynamic therapy (PDT), where it is brought into cancer cells and produces singlet oxygen upon irradiation with a suitable light source. Therefore, the photophysical properties of hypericin are crucial for a successful application in a medical treatment. Here, we present the first single molecule optical spectroscopy study of hypericin. Its photostability is large enough to obtain single molecule fluorescence, surface enhanced Raman spectra (SERS), fluorescence lifetime, antibunching and blinking dynamics. Embedding hypericin in a PVA matrix changes the blinking dynamics, reduces the fluorescence lifetime and increases the photostability. Single molecule SERS spectra show both the neutral and deprotonated form of hypericin and exhibit sudden spectral changes, which can be associated with a reorientation of the single molecule with respect to the surface.