Spectroscopic, microscopic and size dependent antibacterial efficiency of Ag nanoparticles (NPs) synthesized by green approach were studied. Five different samples of Ag NPs having average sizes in the range of ∼14 to ∼21 nm were synthesized using Psidium guajava (Guava) leaf extract (0.25 ml, 0.5 ml, 1 ml, 2 ml, 4 ml, respectively) in 50 ml aqueous AgNO₃ solution of molar concentration of 1 mM. The sizes of the NPs were found to increase with increase in concentration of leaf extract. Such increase in NP size is mainly due to the increase in biomolecules, in the solution, that transforms the Ag ions to Ag NPs. Spectroscopic and microscopic properties of as-synthesized Ag NPs were obtained by characterizing the prepared samples using suitable and affordable methodologies. These Ag NPs showed significant size dependent antibacterial effect. The minimum inhibitory concentration and minimum lethal concentration of the sample showing highest zone of inhibition against Escherichia coli (E. coli) was determined as 40 µg/ml and 80 µg/ml, respectively. Percentage of survivability was also measured through viable plate count. The smallest Ag NPs (average size ∼14 nm) considered here produced the best antibacterial activity against the tested E. coli compared to Ag NPs having larger sizes at identical bacterial concentration. The enhanced antibacterial efficiency for smaller Ag NPs is mainly due to larger surface area-to-volume ratio of smaller NPs. The probable mechanism of bio-reduction of silver ions and formation of Ag NPs has also been well explained, which justifies the result obtained in this work.

研究了通过绿色方法合成的银纳米粒子 (NPs) 的光谱、微观和尺寸依赖性抗菌效率。使用Psidium guajava (Guava) 叶提取物（分别为 0.25 ml、0.5 ml、1 ml、2 ml、4 ml）在 50 ml中合成了平均大小在 14 到 21 nm 范围内的五种不同的 Ag NP 样品AgNO ₃水溶液摩尔浓度为 1 mM 的溶液。发现纳米颗粒的大小随着叶提取物浓度的增加而增加。NP 尺寸的这种增加主要是由于溶液中生物分子的增加，将 Ag 离子转化为 Ag NP。通过使用合适且负担得起的方法表征制备的样品，获得合成的 Ag NPs 的光谱和微观特性。这些银纳米颗粒显示出显着的尺寸依赖性抗菌作用。样品的最低抑菌浓度和最低致死浓度显示对大肠杆菌（E.coli) 分别测定为 40 µg/ml 和 80 µg/ml。存活率的百分比也通过活菌落计数来测量。与在相同细菌浓度下具有较大尺寸的 Ag NPs 相比，此处考虑的最小的 Ag NPs（平均尺寸约为 14 nm）对测试的大肠杆菌产生了最佳的抗菌活性。较小的 Ag NPs 增强的抗菌效率主要是由于较小的 NPs 具有较大的表面积与体积比。银离子的生物还原和 Ag NPs 形成的可能机制也得到了很好的解释，这证明了这项工作中获得的结果。