Engineered nanoparticle adhesion and removal from tomato surfaces

Nonadditivities of the Particle Sizes Hidden in Model Pair Potentials and Their Effects on Physical Adsorptions

It is important to understand the mechanism of colloidal particle assembly near a substrate for development of drug delivery systems, micro-/nanorobots, batteries, heterogeneous catalysts, paints, and cosmetics. Understanding the mechanism is also important for crystallization of the colloidal particles and proteins. In this study, we calculated the physical adsorption of colloidal particles on a flat wall mainly using the integral equation theory, wherein small and large colloidal particles were employed. In the calculation system, like-charged electric double-layer potentials were used as pair potentials. In some cases, it was found that the small particles are more easily adsorbed. This result is unusual from the viewpoint of the Asakura-Oosawa theory, and we call it a "reversal phenomenon". Theoretical analysis revealed that the reversal phenomenon originates from the nonadditivities of the particle sizes. Using the knowledge obtained from this study, we invented a method to analyze the size nonadditivity hidden in model pair potentials. The method will be useful for confirmation of various simulation results regarding the adsorption and development of force fields for colloidal particles, proteins, and solutes.

Stomata facilitate foliar sorption of silver nanoparticles by Arabidopsis thaliana

Application of nanopesticides may substantially increase surface attachment and internalization of engineered nanoparticles (ENPs) in food crops. This study investigated the role of stomata in the internalization of silver nanoparticles (Ag NPs) using abscisic acid (ABA)-responsive ecotypes (Ler and Col-7) and ABA-insensitive mutants (ost1-2 and scord7) of Arabidopsis thaliana in batch sorption experiments, in combination with microscopic visualization. Compared with those of the ABA-free control, stomatal apertures were significantly smaller for the Ler and Col-7 ecotypes (p ˂ 0.05) but remained unchanged for the ost1-2 and scord7 mutants, after exposure to 10 μM ABA for 1 h. Generally Ag NP sorption to the leaves of the Ler and Col-7 ecotypes treated with 10 μM ABA was lower than that in the ABA-free control, mainly due to ABA-induced stomatal closure. The difference in Ag NP sorption with and without ABA was less pronounced for Col-7 than for Ler, suggesting different sorption behaviors between these two ecotypes. In contrast, there was no significant difference in foliar sorption of Ag NPs by the ost1-2 and scord7 mutants with and without ABA treatment. Ag NPs were widely attached to the Arabidopsis leaf surface, and found at cell membrane, cytoplasm, and plasmodesmata, as revealed by scanning electron microscopy and transmission electron microscopy, respectively. These results highlight the important role of stomata in the internationalization of ENPs in plants and may have broad implications in foliar application of nanopesticides and minimizing contamination of food crops by ENPs.

Particle Adsorption on Hydrogel Surfaces in Aqueous Media due to van der Waals Attraction

Particle adhesion onto hydrogels has recently attracted considerable attention because of the potential biomedical applications of the resultant materials. A variety of interactions have been taken advantage of for adsorption, including electrostatic forces, hydrophobic interactions and hydrogen bonding. In this study, we report significant adsorption of submicron-sized silica particles onto hydrogel surfaces in water, purely by van der Waals (vdW) attraction. The vdW forces enabled strong adhesions between dielectric materials in air. However, because the Hamaker constant decreases in water typically by a factor of approximately 1/100, it is not clear whether vdW attraction is the major driving force in aqueous settings. We investigated the adsorption of silica particles (diameter = 25-600 nm) on poly(acrylamide) and poly(dimethylacrylamide) gels using optical microscopy, under conditions where chemical and electrostatic adsorption is negligible. The quantity of adsorbed particles decreased on decreasing the Hamaker constant by varying the refractive indices of the particles and medium (ethyleneglycol/water), indicating that the adsorption is because of the vdW forces. The adsorption isotherm was discussed based on the adhesive contact model in consideration of the deformation of the gel surface. The present findings will advance the elucidation and development of adsorption in various types of soft materials.

Isoelectric points and points of zero charge of metal (hydr)oxides: 50years after Parks' review

The pH-dependent surface charging of metal (hydr)oxides is reviewed on the occasion of the 50th anniversary of the publication by G.A. Parks: "Isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems" in Chemical Reviews. The point of zero charge (PZC) and isoelectric point (IEP) became standard parameters to characterize metal oxides in aqueous dispersions, and they define adsorption (surface excess) of ions, stability against coagulation, rheological properties of dispersions, etc. They are commonly used in many branches of science including mineral processing, soil science, materials science, geochemistry, environmental engineering, and corrosion science. Parks established standard procedures and experimental conditions which are required to obtain reliable and reproducible values of PZC and IEP. The field is very active, and the number of related papers exceeds 300 a year, and the standards established by Parks remain still valid. Relevant experimental techniques improved over the years, especially the measurements of electrophoretic mobility became easier and more reliable, are the numerical values of PZC and IEP compiled by Parks were confirmed by contemporary publications with a few exceptions. The present paper is an up-to-date compilation of the values of PZC and IEP of metal oxides. Unlike in former reviews by the same author, which were more comprehensive, only limited number of selected results are presented and discussed here. On top of the results obtained by means of classical methods (titration and electrokinetic methods), new methods and correlations found over the recent 50years are presented.

Evaluation of Postharvest Washing on Removal of Silver Nanoparticles (AgNPs) from Spinach Leaves

There is increasing use of silver nanoparticles (AgNPs) as pesticides for fruits and vegetables due to the particles' unique antimicrobial and insecticidal properties. However, residual AgNPs in harvested produce may transfer through the food chain and pose a potential risk to public health. The objective of this study is to determine whether postharvest washing can effectively remove AgNPs that had accumulated on fresh produce. Ten microliters of commercially available 40 nm citrate coated AgNPs (0.4 mg/L) was dropped to a (1 × 1 cm(2)) spot on spinach leaves, followed by washing with deionized water (DI water), Tsunami 100 (80 mg/L), or Clorox bleach (200 mg/L). Then, the AgNP removal efficiency of the three treatments was evaluated by surface-enhanced Raman spectroscopy (SERS), scanning electron microscopy (SEM)-energy dispersive spectrometry (EDS), and inductively coupled plasma mass spectrometry (ICP-MS). ICP-MS results showed that deionized water removed statistically insignificant amounts of total Ag (5%), whereas Tsunami 100 and Clorox bleach yielded 21 and 10% decreases in total Ag, respectively (P < 0.05). The increased removal efficiency resulted from AgNP dissolution and Ag(+) release upon contact with the oxidizing agents in Tsunami 100 (peroxyacetic acid, hydrogen peroxide) and Clorox bleach (sodium hypochlorite). According to the SERS results, the deionized water and Tsunami 100 treatments removed nonsignificant amounts of AgNPs. Clorox bleach decreased Ag NPs by >90% (P < 0.05); however, SEM-EDS images revealed the formation of large silver chloride (AgCl) crystals (162 ± 51 nm) on the leaf, which explained the low total Ag removal from ICP-MS. This study indicates current factory washing methods for fresh produce may not be effective in reducing AgNPs (by water and Tsunami 100) and total Ag (by all three means). This highlights the necessity to develop an efficient washing method for NP removal from food surfaces in the future.

Nanomaterials in consumer products: a challenging analytical problem

Many products used in everyday life are made with the assistance of nanotechnologies. Cosmetic, pharmaceuticals, sunscreen, powdered food are only few examples of end products containing nano-sized particles (NPs), generally added to improve the product quality. To evaluate correctly benefits vs. risks of engineered nanomaterials and consequently to legislate in favor of consumer's protection, it is necessary to know the hazards connected with the exposure levels. This information implies transversal studies and a number of different competences. On analytical point of view the identification, quantification and characterization of NPs in food matrices and in cosmetic or personal care products pose significant challenges, because NPs are usually present at low concentration levels and the matrices, in which they are dispersed, are complexes and often incompatible with analytical instruments that would be required for their detection and characterization. This paper focused on some analytical techniques suitable for the detection, characterization and quantification of NPs in food and cosmetics products, reports their recent application in characterizing specific metal and metal-oxide NPs in these two important industrial and market sectors. The need of a characterization of the NPs as much as possible complete, matching complementary information about different metrics, possible achieved through validate procedures, is what clearly emerges from this research. More work should be done to produce standardized materials and to set-up methodologies to determine number-based size distributions and to get quantitative date about the NPs in such a complex matrices.

Efficient anaerobic digestion of whole microalgae and lipid-extracted microalgae residues for methane energy production

The primary aim of this study was to completely investigate extensive biological methane potential (BMP) on both whole microalgae and its lipid-extracted biomass residues with various degrees of biomass pretreatment. Specific methane productivities (SMP) under batch conditions for non-lipid extracted biomass were better than lipid-extracted biomass residues and exhibited no signs of ammonia or carbon/nitrogen (C/N) ratio inhibition when digested at high I/S ratio (I/S ratio⩾1.0). SMP for suitably extracted biomass ranged from 0.30 to 0.38LCH4/gVS (volatile solids). For both whole and lipid-extracted biomass, overall organic conversion ranged from 59.33 to 78.50 as a measure of %VS reduction with greater percentage biodegradability in general found within the lipid-extracted biomass. Higher production levels correlated to lipid content with a linear relationship between SMP and ash-free lipid content being developed at a R(2) of 0.814.