Gold nanoparticles are of great interest for researchers in many fields [1]. The size distribution of the particles is an important physical characteristic that influences particle behavior [2]. Thus, the development of techniques for the controlled synthesis of gold nanoparticles of well-defined size is a big challenge [3]. In this work, Au NPs were synthesized using sodium borohydride as reducing agent. The effect of five parameters including the concentration of gold salt (HAuCl4) and reducing agent (NaBH4), the molar ratio of gold (in form of gold chloride) to reducing agent, the rate of addition of the reducing agent with aid of syringe pump and pH was studied on the size of generated gold nanoparticles calculated by using theory and UV-Vis spectrum of gold nanoparticles. Using the central composite design and response surface methodology (RSM), the quantitative relationship between the variables and their impacts on the response was investigated. Regression analysis based on R2 (adj) values for evaluating the optimum models was used. The best fitted empirical models achieved with R2 = 0.97. Our results imply moderate correlation between experimental and predicted responses. Furthermore, the analysis of the results has shown that all parameters have important effects on the size of nanoparticles. A decrease in size was obtainable with decreasing of the rate of addition of the reducing agent, the molar ratio of gold to reducing agent and increasing of pH, concentration of gold salt and reducing agent.

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