Alternative Controlling Agent of Theobroma grandiflorum Pests: Nanoscale Surface and Fractal Analysis of Gelatin/PCL Loaded Particles Containing Lippia origanoides Essential Oil

Abstract

A new systematic structural study was performed using the Atomic Force Microscopy (AFM) reporting statistical parameters of polymeric particles based on gelatin and poly-ε-caprolactone (PCL) containing essential oil from Lippia origanoides. The developed biocides are efficient alternative controlling agents of Conotrachelus humeropictus and Moniliophtora perniciosa, the main pests of Theobroma grandiflorum. Our results showed that the particles morphology can be successfully controlled by advanced stereometric parameters, pointing to an appropriate concentration of encapsulated essential oil according to the particle surface characteristics. For this reason, the absolute concentration of 1000 µg·mL^-1 (P₁₀₀₀ system) was encapsulated, resulting in the most suitable surface microtexture, allowing a faster and more efficient essential oil release. Loaded particles presented zeta potential around (-54.3 ± 2.3) mV at pH = 8, and particle size distribution ranging from 113 to 442 nm. The hydrodynamic diameter of 90% of the particle population was found to be up to (405 ± 31) nm in the P₁₀₀₀ system. The essential oil release was evaluated up to 80 h, with maximum release concentrations of 63% and 95% for P₅₀₀ and P₁₀₀₀, respectively. The best fit for the release profiles was obtained using the Korsmeyer-Peppas mathematical model. Loaded particles resulted in 100% mortality of C. humeropictus up to 48 h. The antifungal tests against M. perniciosa resulted in a minimum inhibitory concentration of 250 µg·mL^-1, and the P₁₀₀₀ system produced growth inhibition up to 7 days. The developed system has potential as alternative controlling agent, due to its physical stability, particle surface microtexture, as well as pronounced bioactivity of the encapsulated essential oil.

Keywords: Conotrachelus humeropictus; Lippia origanoides; Moniliophtora perniciosa; Theobroma grandiflorum; controlled release; controlling agent; fractal analysis; nanoscale surface.

Figure 1

Two-dimensional and three-dimensional AFM micrographs: (a) unloaded particles (P₀), (b) loaded particles using 500 µg·mL⁻¹ of essential oil (P₅₀₀), and (c) loaded particles using 1000 µg·mL⁻¹ of essential oil (P₁₀₀₀).

Figure 2

Sk values and volume parameters concerning the height distribution of the particle surface. (a–c) Particle surface of all systems (P₀, P₅₀₀, and P₁₀₀₀) presenting a heavy-tailed distribution (Leptokurtic) with great tapering of the height distribution; (d–f) thickness of material on the particles surface, evaluated by the height distribution according to the Sk parameter family; (e,f) displacements of the Sk curve; and (g–i) graphic behaviors considering the volume parameters of the particles surface.

Figure 3

Renderings of the particle surface microtexture. Particles presented similar shapes in (a) P₀, while (b) P₅₀₀ and (c) P₁₀₀₀ acquired smaller and more randomized sizes.

Figure 4

Surface texture directions for (a) P₀, (b) P₅₀₀, and (c) P₁₀₀₀. All systems presented a similar microtexture, as the direct texture parameters did not show any statistically significant differences (p-value > 0.05).

Figure 5

NTA particle size distribution analysis of P₀ and P₁₀₀₀ systems. Representative histograms of the average size distribution (black line) from three measurements of a single sample. Red areas specify the standard deviation (SD) between measurements, and blue numbers indicate the maxima of individual peaks.

Figure 6

Confocal microscopy images of the particles from loaded system (P₁₀₀₀).

Figure 7

Fluorescence measurements of the loaded (regions 1 and 2) and unloaded particles.

Figure 8

Controlled release curves of the P₅₀₀ and P₁₀₀₀ systems: (a) concentration of released essential oil (µg·mL⁻¹), and (b) kinetic essential oil release (µg·mL⁻¹ h⁻¹).