Biogenic silver nanoparticles from Trachyspermum ammi (Ajwain) seeds extract for catalytic reduction of p-nitrophenol to p-aminophenol in excess of NaBH4

Abstract

Plasmonic silver nanoparticles (AgNPs, size = 3–50 nm) were synthesized by biogenic reduction of aqueous AgNO3 using Trachyspermum ammi (TA, Ajwain) seeds extract. Increase in concentration of TA, accelerated the reduction rate of Ag+ and affected the AgNPs particle size. Pronounce effect of the AgNP's aging (24, 48 and 72 h) on particle concentration/size and their corresponding catalytic activity, was exhibited by the systems. Surface plasmonic resonance band centered between 420 and 430 nm wavelength, recognized as first exitonic peak of UV–vis absorption spectra of Ag NPs, was used to estimate the particle size (10–30 nm) of Ag NPs, which was consistent with the particle size observed with the FESEM (5–20 nm) and XRD observations (12.74 nm). XRD analysis also indicated that the silver nanoparticles were crystallized in the cubic crystal pattern. However, some cubic/rod like patterns grown along the 111 plane, was also observed by FESEM and HRTEM. ATR (Attenuated total reflectance) profile of aging of TA supported Ag NPs, exhibited the decrease in intensity of 3394, 1716 and 1618 cm− 1 peaks with respect to the pristine TA extract sample. That reflects the increase in particle concentration with time (24–48 h). These peaks correspond to the OH group, carbonyl group and C C stretching along with COC and CN stretching in TA-AgNPs aggregates. ATR and IR results suggested the presence of the reducing agent/phytochemicals (nicotinic acid, thymol, sugars, proteins, saponin, etc) incorporated NPs. Impedance study revealed that the rate of charge transfer in TA-Ag NPs aggregates is inversely proportional to the concentration of TA that confirms the stability of the Ag NPs in water. These biogenic Ag NPs are also characterized using transmission electron microscopy (TEM) and their corresponding energy dispersive X-ray analysis (EDX), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) studies, etc. On the basis of the above observations and optoelectronic characteristics, the most probable mechanism of biogenesis of the stable Ag NPs, is suggested. As-synthesized 24, 48 and 72 h aged Ag NPs, were tested for their catalytic reduction activity towards the conversion of p-nitro phenol to p-aminophenol in excess of NaBH4. 48 h-aged Ag NPs show highest catalytic activity for conversion of p-nitrophenol to p-aminophenol in excess of NaBH4 in terms of rate (r = 0.34539 min− 1) with complete reduction time 12 min.