Larvicidal potential of silver nanoparticles synthesized from Leucas aspera leaf extracts against dengue vector Aedes aegypti.

Vector-borne diseases caused by mosquitoes are one of the major economic and health problems in many countries. Aedes aegypti mosquito is a vector of several diseases in humans like yellow fever and dengue. Vector control methods involving use of chemical insecticides are becoming less effective due to the development of insecticides resistance, biological magnification of toxic substances through the food chain, and adverse effects on environmental quality and nontarget organisms including human health. Application of active toxic agents from plant extracts as an alternative mosquito control strategy was available from ancient times. These are nontoxic, easily available at affordable prices, biodegradable, and show broad-spectrum target-specific activities against different species of vector mosquitoes. Today, nanotechnology is a promising research domain which has a wide ranging application in vector control programs. The present study investigates the larvicidal potential of solvent leaf extracts of Leucas aspera and synthesized silver nanoparticles using aqueous leaf extract against fourth instar larvae of A. aegypti. Larvae were exposed to varying concentrations of plant extracts and synthesized AgNPs for 24 h. The results were recorded from UV-Vis spectra, X-ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM), and were used to characterize and support the biosynthesis of silver nanoparticles. The formation of the synthesized AgNPs from the XRD spectrum compared with Bragg reflections can be indexed to the (111) orientations, respectively, confirmed the presence of AgNPs. The FTIR spectra of AgNPs exhibited prominent peaks at 3,447.77, 2,923.30, and 1,618.66 cm(-1). The spectra showed sharp and strong absorption band at 1,618.66 cm(-1) assigned to the stretching vibration of (NH) C═O group. The band 1,383 developed for C═C and C═N stretching, respectively, and was commonly found in the proteins. SEM analysis of the synthesized AgNPs clearly showed the clustered and irregular shapes, mostly aggregated and having the size of 25-80 nm. Energy-dispersive X-ray spectroscopy (EDX) showed the complete chemical composition of the synthesized AgNPs. In larvicidal activity, the results showed that the maximum efficacy was observed in synthesized AgNP from leaf extracts against the fourth instar larvae of A. aegypti with LC₅₀ values of 8.5632, 10.0361, 14.4689, 13.4579, 17.4108, and 27.4936 mg/l and LC₉₀ values of 21.5685, 93.03928, 39.6485, 42.2029, 31.3009, and 53.2576 mg/l respectively. These results suggest that the synthesized AgNP from leaf extracts have a higher larvicidal potential as compared to crude solvent extracts thus making them an effective combination for controlling A. aegypti.