Silver nanoparticles: Synthesis, investigation techniques, and properties

Are Smaller Nanoparticles Always Better? Understanding the Biological Effect of Size-Dependent Silver Nanoparticle Aggregation Under Biorelevant Conditions

Purpose

Silver nanoparticles (AgNPs) are one of the most commonly investigated nanomaterials, especially due to their biomedical applications. However, their excellent cytotoxic and antimicrobial activity is often compromised in biological media due to nanoparticle aggregation. In this work, the aggregation behavior and the related biological activity of three different samples of citrate capped silver nanoparticles, with mean diameters of 10, 20, and 50 nm, respectively, were examined.

Methods

Following nanoparticle synthesis and characterization with transmission electron microscopy, their aggregation behavior under various pH values, NaCl, glucose, and glutamine concentrations, furthermore in cell culture medium components such as Dulbecco’s Modified Eagle’s Medium and fetal bovine serum, was assessed through dynamic light scattering and ultraviolet-visible spectroscopy.

Results

The results indicated that acidic pH and physiological electrolyte content universally induce micron-scale aggregation, which can be mediated by biomolecular corona formation. Remarkably, larger particles demonstrated higher resistance against external influences than smaller counterparts. In vitro cytotoxicity and antimicrobial assays were performed by treating cells with nanoparticulate aggregates in differing stages of aggregation.

Conclusion

Our results revealed a profound association between colloidal stability and toxicity of AgNPs, as extreme aggregation led to the complete loss of biological activity. The higher degree of aggregation resistance observed for larger particles had a significant impact on the in vitro toxicity, since such samples retained more of their activity against microbes and mammalian cells. These findings lead to the conclusion that aiming for the smallest possible nanoparticles might not be the best course of action, despite the general standpoint of the relevant literature.

Antimicrobial and In Vitro Cytotoxic Efficacy of Biogenic Silver Nanoparticles (Ag-NPs) Fabricated by Callus Extract of Solanum incanum L

The in vitro callus induction of Solanum incanum L. was executed on MS medium supplemented with different concentrations of auxin and cytokinin utilizing petioles and explants of leaves. The highest significant fresh weights from petioles and leaf explants were 4.68 and 5.13 g/jar for the medium supplemented with1.0 mg L-1 BA and 1.0 mg L-1 2,4-D. The callus extract of the leaves was used for the green synthesis of silver nanoparticles (Ag-NPs). Analytical methods used for Ag-NPs characterization were UV-vis spectroscopy, Fourier Transform Infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM). Spherical, crystallographic Ag-NPs with sizes ranging from 15 to 60nm were successfully formed. The FT-IR spectra exhibited the role of the metabolites involved in callus extract in reducing and capping Ag-NPs. The biological activities of Ag-NPs were dose-dependent. The MIC value for Staphylococcus aureus, Bacillus subtilis, and Escherichia coli was 12.5 µg mL-1, while it was 6.25 µg mL-1 for Klebsiella pneumoniae, Pseudomonas aeruginosa, and Candida albicans. The highest inhibition of phytopathogenic fungi Alternaria alternata, Fusarium oxysporum, Aspergillus niger, and Pythium ultimum was 76.3 ± 3.7, 88.9 ± 4.1, 67.8 ± 2.1, and 76.4 ± 1.0%, respectively at 200 µg mL-1. Moreover, green synthesized Ag-NPs showed cytotoxic efficacy against cancerous cell lines HepG2, MCF-7 and normal Vero cell line with IC50 values of 21.76 ± 0.56, 50.19 ± 1.71, and 129.9 ± 0.94 µg mL-1, respectively.

Bactericidal and In-Vitro Cytotoxic Efficacy of Silver Nanoparticles (Ag-NPs) Fabricated by Endophytic Actinomycetes and Their Use as Coating for the Textile Fabrics

An endophytic strain of Streptomyces antimycoticus L-1 was isolated from healthy medicinal plant leaves of Mentha longifolia L. and used for the green synthesis of silver nanoparticles (Ag-NPs), through the use of secreted enzymes and proteins. UV-vis spectroscopy, Fourier-transform infrared (FT-IR), transmission electron microscopy (TEM), X-ray diffraction (XRD), and dynamic light scattering (DLS) analyses of the Ag-NPs were carried out. The XRD, TEM, and FT-IR analysis results demonstrated the successful biosynthesis of crystalline, spherical Ag-NPs with a particle size of 13-40 nm. Further, the stability of the Ag-NPs was assessed by detecting the surface Plasmon resonance (SPR) at 415 nm for one month or by measuring the NPs surface charge (-19.2 mV) by zeta potential analysis (ζ). The green-synthesized Ag-NPs exhibited broad-spectrum antibacterial activity at different concentrations (6.25-100 ppm) against the pathogens Staphylococcus aureus, Bacillus subtilis Pseudomonas aeruginosa, Escherichia coli, and Salmonella typhimurium with a clear inhibition zone ranging from (9.5 ± 0.4) nm to (21.7 ± 1.0) mm. Furthermore, the green-synthesized Ag-NPs displayed high efficacy against the Caco-2 cancerous cell line (the half maximal inhibitory concentration (IC₅₀) = 5.7 ± 0.2 ppm). With respect to antibacterial and in-vitro cytotoxicity analyses, the Ag-NPs concentration of 100 ppm was selected as a safe dose for loading onto cotton fabrics. The scanning electron microscopy connected with energy-dispersive X-ray spectroscopy (SEM-EDX) for the nano-finished fabrics showed the distribution of Ag-NPs as 2% of the total fabric elements. Moreover, the nano-finished fabrics exhibited more activity against pathogenic Gram-positive and Gram-negative bacteria, even after 10 washing cycles, indicating the stability of the treated fabrics.

Nanocomposite Membranes for Liquid and Gas Separations from the Perspective of Nanostructure Dimensions

One of the critical aspects in the design of nanocomposite membrane is the selection of a well-matched pair of nanomaterials and a polymer matrix that suits their intended application. By making use of the fascinating flexibility of nanoscale materials, the functionalities of the resultant nanocomposite membranes can be tailored. The unique features demonstrated by nanomaterials are closely related to their dimensions, hence a greater attention is deserved for this critical aspect. Recognizing the impressive research efforts devoted to fine-tuning the nanocomposite membranes for a broad range of applications including gas and liquid separation, this review intends to discuss the selection criteria of nanostructured materials from the perspective of their dimensions for the production of high-performing nanocomposite membranes. Based on their dimension classifications, an overview of the characteristics of nanomaterials used for the development of nanocomposite membranes is presented. The advantages and roles of these nanomaterials in advancing the performance of the resultant nanocomposite membranes for gas and liquid separation are reviewed. By highlighting the importance of dimensions of nanomaterials that account for their intriguing structural and physical properties, the potential of these nanomaterials in the development of nanocomposite membranes can be fully harnessed.