Efficiency of lignin nanocapsules for delivering neem oil and capsaicin against pest insects: insights into the system Eruca sativa – Plutella xylostella

In this paper, we report on the design, production and in depth characterization of nanoformulations based on kraft lignin for delivering neem oil and capsaicin as insect repellents. The procedure followed was aimed at establishing a protocol for scalable preparations, which can also ensure that the obtained dispersions are stable in water media, from where they can be administered by safe and easy routes (e.g. foliar spray). Lignin was initially dispersed in alkaline solution to obtain a concentration of 5% w/w. After oil addition in comparable proportion (4.5% v/v), the resulting dispersed aggregates were downsized by sonication. To increase the insect repellency effect, capsaicin was added to half of the samples by dissolution in the oil phase. Extensive structural characterization by DLS, electron microscopy and SAXS showed that all formulations contained well-defined particles with moderate polydispersity and globular shape, which tended to be more elongated in the case of lower starting pH and consequent lower surface charge of the particles. In all the samples, negative zeta potential values were measured, thus ensuring good stability by electrostatic repulsion. These findings represent a favourable premise for applications, since one possible drawback in the production of dispersed systems from natural sources is the ill-defined nature of the ensuing formulation, often showing thread-like interconnected structures coexisting with a small fraction of discrete objects, which can impart poor stability. The potentiality of the present formulations as insect repellents was tested on Eruca sativa plantlets against larvae of Plutella xylostella with encouraging results.