Modulating physical, chemical, and biological properties of silver nanoparticles obtained by green synthesis using different parts of the tree Handroanthus heptaphyllus (Vell.) Mattos
Graphical abstract
Introduction
Nanotechnology is a branch of the knowledge that contributes with innovative solutions to many areas, such as medicine, agriculture, and food supply, several industrial sectors, and others by developing outstanding nanomaterials such as metal nanoparticles (MNPs) [[1], [2], [3]]. A nanomaterial has some properties which occur only in nanoscale and may benefit its application. Reducing the size and proportionally increasing the surface contact area can facilitate the sustained delivery of active compounds [4], allow the quantification of pesticides used in farming [5], and increase the antimicrobial potential [6].
One of the most used applications of MNPs, like silver nanoparticles (AgNPs), is based on a broad spectrum of antimicrobial activities at low concentrations and facile synthesis routes. However, the full mechanisms of action of AgNPs are still not very well-established, and at least one mechanism widely accepted is that released silver ions interact with cellular components of a microorganism leading to cell death [7]. Generally, researchers use the chemical approach for the synthesis of AgNPs. Nonetheless, this strategy generates potentially harmful residues and requires the use of a chemical reagent for reduction and another for the stabilization of the AgNPs. On the other hand, a green synthesis is an eco-friendly approach, feasible as a one-step method, and also it is necessary only a biological material and silver ions (Ag+) [8]. Among the many biological resources possible to perform the green synthesis, plant parts are the most used. In fact, green synthesis of AgNPs using Coffea arabica seed extract showed that the hydrodynamic diameter of nanoparticles can be altered with AgNO3 concentration variation, as well as its antimicrobial activity [9]. In another study by using green tea it was also possible to obtain antimicrobial AgNPs, but they were not biocompatible and the authors suggested that the nanoparticles could be further coated with polyethylene glycol (PEG) [10]. Still another study using aqueous extract of Calotropis procera flowers demonstrated the formation of cubic and rectangular nanoparticles [11]. Despite the fact that several plant parts can be used for green synthesis of AgNPs, the most commonly used parts of the plants are still are the leaves, due to the ease of collection of this botanical material and the amount available [12]. Among hundreds of others, a recent study [13] showed the use of Combretum erythrophyllum leaf extract as a silver nitrate reducing agent demonstrated the formation of AgNPs with antimicrobial activity.
The mechanism of antimicrobial action of AgNPs is not yet well-understood. There are some hypotheses that suggest their routes of action. The main theory is that AgNPs may interact with bacterial membrane proteins that contain exposed sulfur moiety causing it to leak intracellular content causing damage to the bacterial respiratory chain, accumulation of reactive oxygen species (ROS), and even direct DNA molecules damage [14]. An advantage of using green synthesis is that phytochemicals can form a stabilizing layer around AgNPs and they can enhance the antimicrobial activity [15].
The reproducibility of the physicochemical characteristics of nanomaterials produced by green synthesis routes is commonly very difficult since the metabolites responsible for the reduction of metal ions are very variable in plants [16]. Additionally, green synthesis makes it difficult to modulate the final characteristics of the AgNPs. Therefore, we hypothesized that by using several different parts to make the synthesis of AgNPs using the same plant species and specimen, can lead to the understanding of more specific characteristics and even achieve the physicochemical and also biological modulation, as it could be expected different formation yields, variable sizes/shapes, and a specific range of antimicrobial activities of particles. Therefore, the present study aimed to use different parts, both reproductive and vegetative, of the pink trumpet tree Handroanthus heptaphyllus, a deciduous tree native to South America, to synthesize AgNPs by a green synthesis route.
Section snippets
Preparation of parts and extracts from H. heptaphyllus organs
Botanical materials were collected in an urban area of Brasília-DF (Brazil) according to the license numbers (CGEN: 02001.007580/2014–95 and SISBIO: 57671–1). Fig. 1 illustrates the plant parts separated into flowers (L), floral buds (B), petioles (P), and five leaflets (F1, F2, F3, F4, and F5). All plant parts were washed using 0.1% Extran® for 2 min. The parts were cut about 5 mm2 and boiling water was added until the concentration of 100 mg/mL was reached following incubation during 2 min
UV–V is spectral analysis
Monitoring of the reactions by spectrophotometer readings at 450 nm showed almost stabilization phase of the formation curve of AgNPs with all types of botanical materials (vegetative-V and reproductive-R parts), as shown in Fig. 2a. The signals (absorbance) were relatively more intense for AgNP-R when compared to AgNP-V samples. This could happen because reproductive parts contain abundant antioxidant molecules which are responsible for attracting pollinators offering nutritive value to the
Conclusion
AgNPs were obtained by green synthesis routes from aqueous extracts of both vegetative (petiole and leaflets) as well as reproductive (flower and flower bud) parts of H. heptaphyllus. It was possible to find some specific characteristics of the plant that led to the modulation of physicochemical properties of AgNPs such as diameter and ZP, and even to the alteration of the antimicrobial potential. So, if the intention of the green synthesis route is to produce larger amounts of AgNPs with
Acknowledgements
This work was supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Apoio à Pesquisa do Distrito Federal (FAP-DF), Empresa Brasileira de Pesquisa Agropecuária (Embrapa), and Universidade de Brasília (UnB). The authors are grateful to Dijalma Barbosa da Silva for the identification of the plant species.
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