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Antibiotics 2018, 7, 76 2 of 12 Silver, which has long been used as a disinfectant for pathogenic microorganisms [2], has become one of the best exponents of this transition. Silver nanoparticles (AgNPs), which feature antibacterial, antifungal and antitumor activities [3–5], are one of the most popular active ingredients employed to enhance the efficacy of plant protection products. Furthermore, they can be prepared through green synthesis procedures with the aid of plant extracts [6], which act as reducing and stabilizing agents. Polyol components, polysaccharides, and water-soluble heterocycles (such as those from Stevia rebaudiana [7], Curcuma longa [8], Pongamia pinnata [9], Gliricidia sepium [10], Eucalyptus hybrida [11], or Quercus brantii [12]) have been reported to lead to a synergistic effect in the resulting phytonanocomposites [13–15]. Nonetheless, bioactive compounds from plants (that include phenolic acids, flavonoids, curcuminoids, coumarins, quinones, tannins and lignans), in spite of having a wide range of activities, generally suffer from a number of drawbacks derived from their inherent physicochemical characteristics (poor water solubility, low bioavailability, chemical instability, photodegradation, rapid metabolism and short half-life) [16], which limit their applications. To stabilize them and improve their bioavailability, one well-known approach is to use biopolymers, such as chitin, chitosan, starch, and cellulose, or other macromolecular systems [17]. Binary composites based on chitosan with polyphenols (e.g., gallic acid or curcumin [18–20]) and ternary composites that also include AgNPs [21,22] with a broadened bio-activity have been reported in the literature. Other approaches to improve solubility are based on forming inclusion compounds with terpene glycosides (such as rubusoside, stevioside, rebaudioside, or steviol monoside) or cyclodextrins, which result in an enhancement of the solubility of polyphenols [23,24], or on using deep eutectic solvents (DES). DES are an excellent extraction medium for phenolic compounds [25] and may be used in combination with inclusion compounds [26] or with chitosan [27,28]. Phytophthora cinnamomi is a pathogen with over 1000 host species, transmitted by the soil and which causes rotting of the roots of many horticultural and forestry crops [29]. P. cinnamomi can collapse, which cause sudden death of plants and a decrease in fruit yield and size. The infection by P. cinnamomi can also occur together with other species of Phytophthora, mainly P. cambivora, P. cryptogea, P. citricola and P. cactorum. Its eradication by means of fungicides is expensive and causes damage to the environment, and fumigation is not always effective for deeper roots [30,31]. Consequently, the European Union is promoting the development of new natural bioactive compounds to replace conventional systemic fungicides, such as the organophosphorus compound fosetyl-Al or acylalanines such as metalaxyl. It has been shown, in vitro, that AgNPs synthesized using aqueous plant extracts have had antifungal effect on Phytophthora pathogens [32], and so do chitosan [33], the binary combinations of the two [34] and their ternary combinations with propolis [35]. Nonetheless, to the best of the authors’ knowledge, no studies have explored the use of composites of AgNPs with polyphenol inclusion compounds, combined either with chitosan oligomers (COS) or with DES, for the control of P. cinnamomi or any other oomycetes. In the present study, four polyphenols (gallic acid, silymarin, ferulic acid and curcumin) were assessed for the microwave-assisted formation of the new inclusion compounds with stevioside. AgNPs were subsequently incorporated into the composites [36]. Two types of host matrices were tested, namely COS in a hydroalcoholic solution, and a DES based on a choline chloride and urea solution (1:2 v/v) in glycerol, evaluating in vitro their response against P. cinnamomi at different concentrations. 2. Results The antifungal activity of the different treatments in aforementioned two preparation media (COS in hydroalcoholic solution and DES) against P. cinnamomi (isolate MYC43) was studied in vitro by monitoring the radial growth of the mycelium (Figures 1 and 2).PDF Image | Silver Nano Polyphenol Inclusion Compounds Composites
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