Uptake of Silver from Polyvinylpyrrolidine Coated Silver Nanoparticles in a Terrestrial System
Abstract
The widespread use of silver nanoparticles (Ag NPs) h as facilitated their uninterrupted entry into various ecosystems. Nanoparticles are stabilized using a variety of approaches for various applications. The present study has investigated the uptake of polyvinylpyrrolidine (PVP) coated Ag NPs in a terrestrial system. Two insect (Acheta domesticus and Tenebrio molitor) and two plant species (Sorghum vulgare and Helianthus annuus) were used in the study. The effect of concentration and size of PVP -coated Ag NPs was investigated. The test species were maintained in soil spiked with 0, 1, 5, 25, 125, and 625 mg/kg PVP -coated 30 -50 nm Ag NPs to test the effect of concentration of Ag NPs on uptake. Similarly, the test species were maintained in soil spiked with 25 mg/kg of 20, 30 -50, and 50 -80 nm PVP -coated Ag NPs t o study the effect of size of nanoparticles on uptake. The PVP -coated Ag NPs were characterized using transmission electron microscopy, dynamic light scattering and powder X -ray diffraction techniques. The levels of silver in test samples were measured using inductively coupled plasma -optical emission spectroscopy. A concentration dependent increase in the levels of Ag in both the insect species was observed as a function of increasing concentrations of coated Ag NPs in soil. An increase in the levels of Ag as a function of increasing size of coated Ag NPs was observed with Acheta domesticus. No apparent trend was observed with Tenebrio molitor species. A concentration dependent increase in the levels of Ag in the roots of both the plants was observed as a f unction of increasing concentrations of coated Ag NPs in soil. Additionally, the translocation of Ag to other plant tissues was observed in Helianthus annuus, a dicot plant.
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Introduction
The widespread use of silver nanoparticles (Ag NPs) for a variety of consumer, industrial and medical applications has resulted in an increase in the anthropogenic release of silver in to the environment [1,2]. Silver from many Ag NP containing products is predicted to enter into the wastewater streams and eventually wastewater treatment plants [3]. Much of the silver at the treatment plants is partitioned in to sewage sludge [2,4,5]. Ag NPs eventually find their way in to the terrestrial ecosystems through the application of sewage sludge to agricultural lands [4].
The colloidal stability of nanoparticles significantly affects their mobility, uptake/bioavailability, and toxicity in a given ecosystem [6]. Environmental conditions present in an ecosystem such as pH, ionic strength, background electrolyte composition, etc. affect the colloidal stability of nanoparticles. Additionally, the presence of capping agents/coatings on nanoparticles also influences their colloidal stability [7,8]. Ag NPs are highly reactive due to their high surface area -to- volume ratio. This leads to phenomenon such as particle aggregation and settling in Ag NPs, unless stabilized/protected by suitable coatings [9,10]. Coatings prevent the aggregation of nanoparticles through electrostatic repulsions, steric repulsions, and a combination of both [9,11]. Coatings that are commonly used during the synthesis of Ag NPs include chemicals such as citrate, sodium borohydride (NaBH 4), and polyvinylpyrrolidine (PVP) [9,11].
PVP coatings are known to sterically stabilize Ag NPs. The mechanism of steric stabilization of nanoparticles involves the adsorption of uncharged polymer on the surface of nanoparticles [6,12,13]. Another mechanism involves the formation of weak coordinative bonds between PVP and Ag+ ions [14,15]. The effect of surface coatings on the stability, transformation, uptake, and toxicity of nanoparticles has been discussed in literature [13, 16 -21]. The present study investigates the uptake of PVP-coated Ag NPs (henceforth referred to as coated Ag NPs) in a terrestrial system by two insect ( Acheta domesticus and Tenebrio molitor ) and plant species ( Sorghum vulgare and Helianthus annuus ). PVP -coated Ag NPs were chosen as they demonstrated excellent shape and size stability characteristics in an ecotoxicological medium compared to other charge stabilized (citrate coated) Ag NPs [13]. The insect and plant species selected for the study were found to be native to the region where the soil was collected. Therefore, investigating the uptake of Ag NPs by these species would help us understand the role of these species in the transport of metal contaminants along the food chain of native insectivorous and granivorous species.
Conclusion
The uptake of PVP -coated Ag NPs in a terrestrial system by insect and plant species was investigated. The phenomenon of aggregation of Ag NPs in the presence of a PVP coating was found to be minimal. Also, the size of PVP -coated Ag NPs was found to be larger than pristine Ag NPs. The composition of PVP -coated Ag NPs was confirmed by powder X -ray diffraction. A concentration dependent increase in the uptake of Ag from PVP -coated Ag NPs in soil was observed. No effect of size of PVP-coated Ag NPs on their uptake by the test species was observed. Considering the increasing use of Ag NPs for a variety of applications, the results from this study would be helpful in elucidating the role of coatings on Ag NPs in determining their uptake by plant and insect species. Eventually, the results from this study would also help understand the role of coatings in affecting the bioaccumulation and biomagnification of Ag NPs along the food webs.