The eco‐friendly biosynthesis of silver nanoparticles (AgNPs), using starch as reducing agent, was addressed in this work. Starch solutions with three different concentrations (0.5, 1.0, and 1.5% w/v), heated at 90°C for 12 h and stabilized with 100 µL of NaOH 0.1 M were used during the synthesis process. AgNPs were characterized by UV–visible (UV–Vis) spectrophotometry, SEM equipped with energy dispersive spectrometer and TEM. The antibacterial activity of the AgNPs was evaluated on Staphylococcus aureus using the disc diffusion technique (Kirby–Bauer method) relative to enrofloxacin antibiotic. Solutions containing AgNPs developed a yellow color and presented UV–Vis spectra characteristic for AgNPs, with surface plasmon resonance peaks between 411 and 414 nm. Spherical‐shape AgNPs were observed by TEM with sizes between 10 and 30 nm. These silver nanoparticles displayed an antimicrobial activity against S. aureus in colloidal state and films.
The thermal equilibrium of the formation of negatively charged excitons is studied in this work, by measuring the current and quantum-well photoluminescence in biased resonant tunneling double barrier diodes. We observe that the intensity ratio of negatively charged and neutral excitons depends linearly on the current for fixed temperature and illumination conditions. We propose that the results can be interpreted in terms of a mass-action law governing the concentrations of neutral and charged excitons and free electrons. Measurements at different temperatures and bias yield an electron concentration and a dwell time in the well that are in good agreement with the values previously reported in the literature. We also analyze the dependence of the luminescence on excitation intensity.
The development of new materials from biodegradable constituents is enticing for applications in bioelectronics and biomedicine, due to their considerable durability, versatility and biodegradability. The present work aims to determine the chemical and physical properties of chitosan membranes after neutralization of the polycation solution. This work was divided into two stages: solubilization and neutralization. In the first stage, the chitosan was solubilized with three different concentrations of acetic acid (1%, 0.8% and 0.5%) in order to evaluate whether the acid affected the physical-chemical properties of the films. In the second stage, the chitosan polycation was neutralized with two strong bases (NaOH and KOH) using three different molar concentrations, by the slow drip method for obtaining films with pH values close to neutrality. The characterization of the membranes obtained from casting method showed that the different concentrations of acetic acid did not affect the physical-chemical properties of the films. In contrast, the neutralization process did affect their properties, and in particular, different behaviors were observed depending on the type of base used for neutralization. More ductile and plasticity were obtained in the films when the polycation was neutralized with KOH, whereas the films neutralized with NaOH exhibited a more fragile behavior.
In this work we study the effect of different reducing agent (chitosan, starch and sugars) on to synthesis of silver (AgNPs) and gold (AuNPs) nanoparticles by reduction of silver nitrate (AgNO3) and chloroauric acid (HAuCl4), respectively. The plasmon absorption peak between 419 - 434 nm, measured using a UV-Vis spectrophotometer indicates the existence of AgNPs, this reaction was favored by the increase of temperature with optimal results at 90°C. Synthesis of AuNPs was only obtained using chitosan as reducing agent, the use of solutions of starch and sugars allows the reduction of the gold salt present in the chloroauric acid, however the low pH of the solutions creates thermodynamic instabilities for the AuNP synthesis, due to low repulsive force for colloid stability.
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