3D SERS and Raman imaging of protective microcapsules containing bio‐active terpenoids

Terpenoids play a major role in agriculture, given their fungicidal and herbicidal actions, as well as their favorable toxicological, ecotoxicological, and environmental profiles. Despite all these advantages, terpenoids are reported to be unstable in direct sunlight and atmospheric conditions, so both commercial suppliers and scientific literature foresee their protection by encapsulation. The so-called microcapsules (mu Caps) are therefore of high relevance as drug-delivery vectors, but very few techniques focus on their surface as well as on their morphological characterization. Indeed, these aspects are of great importance, given that their surface chemistry governs both their colloidal stability and mechanism of action. Common analysis techniques, such as chromatographic and mass-spectrometric ones, are destructive, require sample preparation, and do not result in the complete morphological characterization of the microcapsules. Micro-Raman spectroscopy, in conjunction with the surface-enhanced Raman spectroscopy (SERS) effect, offers a valuable alternative method of investigation capable of achieving a complete and non-destructive morphological characterization of the terpenoid-encapsulating systems, the dimensions of which fall within the micrometric range. In addition, the SERS effect can be exploited by fabricating the microcapsules with gold nanostars (AuNSs) modified with chitosan and a SERS reporter (Nile Blue A). Thanks to the high contrast provided by the SERS signals of this tag, it was possible to localize and confirm the chitosan in the morphology of the microcapsules. The results of this study shed new light on the possibility of analyzing terpenoid-encapsulating microcapsules and possibly other kinds of encapsulates brought by using Raman spectroscopy and by exploiting the SERS effect.