Uptake and effect of highly fluorescent silver nanoclusters on Scenedesmus obliquus

Nanoparticles and plants: A focus on analytical characterization techniques

Nanotechnology has brought about significant progress through the use of goods based on nanomaterials. However, concerns remain about the accumulation of these materials in the environment and their potential toxicity to living organisms. Plants have the ability to take in nanomaterials (NMs), which can cause changes in their physiology and morphology. On the other hand, nanoparticles (NPs) have been used to increase plant development and control pests in agriculture by including them into agrochemicals. The challenges of the interaction, internalization, and accumulation of NMs within plant tissues are enormous, mainly because of the various characteristics of NMs and the absence of reliable analytical tools. As our knowledge of the interactions between NMs and plant cells expands, we are able to create novel NMs that are tailored, targeted, and designed to be safe, thus minimizing the environmental consequences of nanomaterials. This review provides a thorough examination and comparison of the main microscopy techniques, spectroscopic methods, and far-field super-resolution methodologies used to examine nanomaterials within the cell walls of plants.

Silver nanoparticles (AgNPs) internalization and passage through the Lactuca sativa (Asteraceae) outer cell wall

Silver nanoparticle (AgNPs) toxicity is related to nanoparticle interaction with the cell wall of microorganisms and plants. This interaction alters cell wall conformation with increased reactive oxygen species (ROS) in the cell. With the increase of ROS in the cell, the dissolution of zero silver (Ag0) to ionic silver (Ag+) occurs, which is a strong oxidant agent to the cellular wall. AgNP interaction was evaluated by transmission electron microscopy (TEM) on Lactuca sativa roots, and the mechanism of passage through the outer cell wall (OCW) was also proposed. The results suggest that Ag+ binds to the hydroxyls (OH) present in the cellulose structure, thus causing the breakdown of the hydrogen bonds. Changes in cell wall structure facilitate the passage of AgNPs, reaching the plasma membrane. According to the literature, silver nanoparticles with an average diameter of 15nm are transported across the membrane into the cells by caveolines. This work describes the interaction between AgNPs and the cell wall and proposes a transport model through the outer cell wall.

Algal cells harvesting using cost-effective magnetic nano-particles

Innovative iron-based nanoparticles were synthesized, characterized and tested for the first time for harvesting single and mixed algal culture from real wastewater. The tailor-made magnetic nanoparticles (MNPs; Fe-MNP-I and Fe-MNP-II) achieved a percentage algae harvesting efficiency (%AHE) higher than 95% using a concentration of MNPs (C_MNP) of 25 ± 0.3 (std. dev = 0.08) mg.L^-1, mixing speed (M_speed) of 120 ± 2 (std. dev = 0.10) rpm, short contact time (C_t) of 7 ± 0.1 (std. dev = 0.05) min and separation time (SP_t) of 3 ± 0.1 (std. dev = 0.09) min. The optimum operational conditions for harvesting of Chlorella vulgaris (C.v) were determined at (C_MNP = 40 ± 0.4 (std. dev = 0.5) g_MNPs.L^-1, SP_t = 2.5 ± 0.4 (std. dev = 0.1) min, M_speed = 145 ± 3 (std. dev = 1.50) rpm and C_t = 5 ± 0.3 (std. dev = 0.10) min using surface response methodology. Langmuir model describes better the adsorption behavior of algae-Fe-MNP-I system, while both Langmuir and Freundlich fit well the adsorption behavior of algae-Fe-MNP-II. The maximum adsorption capacity of Spirulina platensis (SP.PL) (18.27 ± 0.07 (std. dev = 0.19) mg_DWC.mg_particles^-1) was higher than that for Chlorella vulgaris (C.v) (11.52 ± 0.01 (std. dev = 0.34) mg_DWC.mg_particles^-1) and mixed algal culture (M.X) (17.20 ± 0.07 (std. dev = 0.54) mg_DWC.mg_particles^-1) over Fe-MNP-I. Zeta potential measurements revealed that the adsorption mechanism between MNPs and algal strains is controlled by electrostatic interaction. The synthesized MNPs were recycled 10 times using alkaline-ultrasonic regeneration procedure.

Effect of Chlamydomonas reinhardtii on the fate of CuO nanoparticles in aquatic environment

In this study, the effect of Chlamydomonas reinhardtii on the fate of CuO nanoparticles (CuO-NPs) in aquatic environment were investigated in terms of the colloidal stability, the free Cu²⁺ releasing, extracellular adsorption Cu (Cu_ex) and intracellular assimilation Cu (Cu_in). The results showed that, with the increasing microalgal density, the absolute value of zeta potential of CuO-NPs decreased and the mean hydrodynamic diameter (MHD) became larger, leading to a better aggregation and settling behavior of CuO-NPs. The microalgae also promoted the free Cu²⁺ releasing, however, inhibited adsorption and assimilation of metal nanoparticles (MNPs) into microalgal cells, resulting in the reduction of the Cu_ex and Cu_in per microalgal cell. The phenomenon was probably due to the reduced chance of contact between microalgae and MNPs. The internalization of CuO-NPs was also observed in microalgal cells by high resolution transmission electron microscope (HRTEM). Furthermore, the results of fast fourier transform (FFT)/inversed FFT (IFFT) analysis indicated that the CuO-NPs was reduced to Cu₂O-NPs in the microalgae cells. The above results suggested that the microalgae can significantly affect the fate of MNPs, and subsequently, influencing the bioavailability and toxicity of MNPs in the aquatic environment.

The use of magnetic iron oxide based nanoparticles to improve microalgae harvesting in real wastewater

A novel approach for harvesting Scenedesmus sp. microalgae from real wastewater by using adsorbents of magnetite-based nanoparticles (Fe₃O₄ NPs) was tested in this study for the first time for this microalgae. Using these NPs, the harvesting efficiency was even higher than 95%. The optimal conditions (0.14 gNPs/L, a short magnetic separation time of only 8 min and 27 min of contact time) were found using the response surface methodology. The best fitting of the adsorption equilibrium results was achieved by the Langmuir isotherm model, and the maximum adsorption capacity for Scenedesmus sp. reached 3.49 g dry cell weight (DCW)/g Fe₃O₄ NPs. Zeta potential measurements and the Dubinin-Radushkevich isotherm model analysis pointed out that the main adsorption mechanism between Scenedesmus sp. cells and Fe₃O₄ NPs was electrostatic interaction. Finally, Fe₃O₄ NPs were six times successfully reused by combining an alkaline treatment with an ultrasonication process, which implies microalgae lysis. The results herein obtained highlight the potential for magnetic separation of microalgae from wastewater, which is capable of reaching a high harvesting efficiency in a very short time.

Metabolites change of Scenedesmus obliquus exerted by AgNPs

With increasing emission of silver nanoparticles (AgNPs) into the environment, it is important to understand the effects of ambient concentration of AgNPs. The biological effects of AgNPs on Scenedesmus obliquus, a ubiquitous freshwater microalgae, was evaluated. AgNPs exerted a minor inhibitory effect at low doses. Non-targeted metabolomic studies were conducted to understand and analyze the effect of AgNPs on algal cells from a molecular perspective. During the 48 hr of exposure to AgNPs, 30 metabolites were identified, of which nine had significant changes compared to the control group. These include d-galactose, sucrose, and d-fructose. These carbohydrates are involved in the synthesis and repair of cell walls. Glycine, an important constituent amino acid of glutathione, increased with AgNP exposure concentration increasing, likely to counteract an increased intracellular oxidative stress. These results provide a new understanding of the toxicity effects and mechanism of AgNPs. These metabolites could be useful biomarkers for future research, employed in the early detection of environmental risk from AgNPs.

Synthesis of biogenic silver nanoparticles using Althaea officinalis as reducing agent: evaluation of toxicity and ecotoxicity

Silver nanoparticles (AgNPs) are known mainly because of their bactericidal properties. Among the different types of synthesis, there is the biogenic synthesis, which allows the synergy between the nanocomposites and substances from the organism employed for the synthesis. This study describes the synthesis of AgNPs using infusion of roots (AgNpR) and extract (AgNpE) of the plant Althaea officinalis. After the synthesis through reduction of silver nitrate with compounds of A. officinalis, physico-chemical analyzes were performed by UV-Vis spectroscopy, nanoparticles tracking analysis (NTA), dynamic light scattering (DLS) and scanning electron microscopy (SEM). Toxicity was evaluated through Allium cepa assay, comet test with cell lines, cell viability by mitochondrial activity and image cytometry and minimal inhibitory concentration on pathogenic microorganisms. Biochemical analyzes (CAT - catalase, GPx - glutathione peroxidase e GST - glutationa S-transferase) and genotoxicity evaluation in vivo on Zebrafish were also performed. AgNpE and AgNpR showed size of 157 ± 11 nm and 293 ± 12 nm, polydispersity of 0.47 ± 0.08 and 0.25 ± 0.01, and zeta potential of 20.4 ± 1.4 and 26.5 ± 1.2 mV, respectively. With regard to toxicity, the AgNpE were the most toxic when compared with AgNpR. Biochemical analyzes on fish showed increase of CAT activity in most of the organs, whereas GPx showed few changes and the activity of GST decreased. Also regarding to bactericidal activity, both nanoparticles were effective, however AgNpR showed greater activity. Althaea officinalis can be employed as reducing agent for the synthesis of silver nanoparticles, although it is necessary to consider its potential toxicity and ecotoxicity.

Green Algae as Carriers Enhance the Bioavailability of 14C-Labeled Few-Layer Graphene to Freshwater Snails

The waterborne exposure of graphene to ecological receptors has received much attention; however, little is known about the contribution of food to the bioaccumulation potential of graphene. We investigated the effect of algal food on the uptake and distribution of ¹⁴C-labeled few-layer graphene (FLG) in freshwater snails, a favorite food for Asian people. In a water-only system, FLG (∼158 μg/L) was ingested by and accumulated in the snails. Adding algae to the water significantly enhanced FLG accumulation in the snails, with a bioaccumulation factor of 2.7 (48 h exposure). Approximately 92.5% of the accumulated FLG was retained in the intestine; in particular, the accumulated FLG in the intestine was able to pass through the intestinal wall and enter the intestinal epithelial cells. Of them, 1.3% was subsequently transferred/internalized to the liver/hepatocytes, a process that was not observed in the absence of the algae. Characterizations data further suggested that both of the extra- and intracellular FLG in the algae (the algae-bound fraction was 30.2%) significantly contributed to the bioaccumulation. Our results provide the first evidence that algae as carriers enhanced FLG bioavailability to the snails, as well as the potential of FLG exposure to human beings through consuming the contaminated snails.

Unraveling the molecular mechanism of photosynthetic toxicity of highly fluorescent silver nanoclusters to Scenedesmus obliquus

While the discovery of numerous attractive properties of silver at the nanoscale has increased their demand in many sectors including medicine, optics, sensing, painting and cosmetics, it has also raised wide public concerns about their effect on living organisms in aquatic environment. Despite the continuous effort to understand the various aspects of the toxicity of silver nanomaterials, the molecular level understanding on their cytotoxicity mechanism to biological organisms has remained unclear. Herein, we demonstrated the underlying mechanism of the photosynthetic toxicity against green algae namely, Scenedesmus obliquus by using an emerging silver nanomaterial, called silver nanoclusters (defined as r-Ag NCs). By exploiting the unique fluorescence properties of r-Ag NCs along with various other analytical/biological tools, we proposed that the photosynthetic toxicity of r-Ag NCs was largely attributed to the "joint-toxicity" effect of particulate form of r-Ag NCs and its released Ag+, which resulted in the disruption of the electron transport chain of light reaction and affected the content of key enzymes (RuBP carboxylase/ oxygenase) of Calvin cycle of algae cells. We believe that the present study can also be applied to the assessment of the ecological risk derived from other metal nanoparticles.

Metal Nanoclusters with Synergistically Engineered Optical and Buffering Activity of Intracellular Reactive Oxygen Species by Compositional and Supramolecular Design

Metal nanoclusters featuring tunable luminescence and high biocompatibility are receiving attention as fluorescent markers for cellular imaging. The recently discovered ability of gold clusters to scavenge cytotoxic reactive oxygen species (ROS) from the intracellular environment extends their applicability to biomedical theranostics and provides a novel platform for realizing multifunctional luminescent probes with engineered anti-cytotoxic activity for applications in bio-diagnostics and conceivably cellular therapy. This goal could be achieved by using clusters of strongly reactive metals such as silver, provided that strategies are found to enhance their luminescence while simultaneously enabling direct interaction between the metal atoms and the chemical surroundings. In this work, we demonstrate a synergic approach for realizing multifunctional metal clusters combining enhanced luminescence with strong and lasting ROS scavenging activity, based on the fabrication and in situ protection of Ag nanoclusters with a supramolecular mantle of thiolated-Au atoms (Ag/Au-t). Confocal imaging and viability measurements highlight the biocompatibility of Ag/Au-t and their suitability as fluorescent bio-markers. ROS concentration tests reveal the remarkable scavenging activity of Ag-based clusters. Proliferation tests of cells in artificially stressed culture conditions point out their prolonged anti-cytotoxic effect with respect to gold systems, ensuring positive cell proliferation rates even for long incubation time.

Solution-processed Au–Ag core–shell nanoparticle-decorated yarns for human motion monitoring

Wearable strain sensors based on Au–Ag core–shell nanoparticle decorated yarns were fabricated by a solution-based approach.

Fluorometric “Turn-On” glucose sensing through the in situ generation of silver nanoclusters

A fluorometric “turn-on” glucose detection is performed based on the Fenton reaction which can trigger the generation of Ag nanoclusters.