| Silver nanoparticles (AgNPs) have been widely used as antibacterial materials, catalytic materials and biosensor materials based on the unique physical and chemistry characteristics which depend on their structure, shape, size and size distribution. In this work, nearly monodispersed AgNPs were synthesized by the oil-water interface method. Compared with other methods, it had many advantages, such as, easy operation, low cost, high efficiency, mild reaction conditions and available reactants. Effects of organic solvents, surfactant concentration, pH, the concentration ratio of different reactants, reaction temperature, time and system had been discussed. Based on the results above, the formation mechanism was developed by exploring the effects of silver precursors, ratio of oil-water phase, reaction rate and role of surfactant in the oil water interface method. Furthermore, hydrophobic AgNPs, modified by cetyl trimethyl ammonium bromide (CTAB), were successfully transferred into water. Then, antibacterial activity was tested to obtain the minimal inhibitory concentration (MIC) of hydrophilic AgNPs and the modified AgNPs by CTAB.The main research results of this paper can be briefly described as follows:(1) AgNPs with face centered cubic crystalline were successfully synthesized by the oil-water interface method. The obtained products were well-dispersed in size and size distribution according transmission electron microscopy (TEM) images, and the mean size of the products were about 8.0-10.0 nm. Moreover, ultraviolet-visible spectroscopy (UV-Vis) spectrum displayed that the strong and sharp absorption peak was at 413 nm, certifying the nearly monodispersity of particles. By the analysis of the data from X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Thermo gravimetric analyzer (TGA), the products were characterized to be pure crystalline silver with a face-centred cubic (fee) structure and capped with oleic acid. Thermal stability results showed that the AgNPs capped with oleic acid were stable even at 160?.The optimum reaction conditions of synthesizing AgNPs were concluded. With cyclohexane as organic phase,3.0 mL oleic acid as surfactant, the reaction happened at 80?, the injection rate of sodium borohydride was 160 mL/h, and the reaction time was two hours. Besides, the volume ratio between oil and water phases was 1.0:1.0 while the ratio of [Ag+]:[BH4-] was 1.0:3.0.(2) The formation mechanism of AgNPs with the oil-water interface method was proposed: Once the oil phase and the water phase are mixed, the oleic acid molecules tend to occupy the interface to decrease the interface tension. Meanwhile, Ag oleate and NaBH4 diffuse to the interface to contact with each other, and then the reaction occurs to produce Ag monomers. While the produced Ag monomers are enough, the nucleation takes place. Once the nuclei of Ag particles are produced, the adsorption of oleic acid on the nuclei occurs to enhance their stability, along with the competitive deposition of Ag monomers on their surfaces. The growing particles with the proper adsorption will move from the oil-water interface to the oil phase in order to access the lowest Gibbs free energy and the most stable state, and the further growth is stopped. Therefore, nearly monisipersed AgNPs are formed in cylcohexane, and at the same time new nuclei are generated constantly.(3) The role of surfactant in the formation progress of monodispersed particles by the oil-water interface is shown in the following.The deposition rate of solute monomer on the surface of nuclei is fixed for a given system. So the surfactant adsorption rate and capacity on surfaces of NPs affect the size and size-distribution. When oleic acid was the surfactant, as a quick or high capacity adsorption happened, the preformed AgNPs in the oil-water interface could adsorb oleic acid molecules timely and completely. Once the proper or enough adsorption was achieved, the growing particles would get into the oil phase spontaneously without aggregation and the further growth of particles was inhibited, resulting in nearly monodispersed nanoparticles. Therefore, the proper surfactant concentration and reaction conditions were essential to produce nearly monodispersed AgNPs.(4) Hydrophobic AgNPs were transferred into water successfully by CTAB. The results showed that by this method AgNPs dispersed in cyclohexane could be transferred completely into water in 1 h. And after modification by CTAB, UV-Vis spectrum showed that the absorption peak of AgNPs appeared with a red-shift. TEM image displayed the well dispersed AgNPs after phase transferring. Meanwhile, effects of the reaction time, concentration of CTAB, pH of system were investigated. The results showed that the transfer efficiency achieved maximum value when the reaction time was 1 h, concentration of CTAB was 20 mM, and pH was 10.0. With the help of CTAB, the hydrophilic products can be applied in biological marker, biological monitoring, antibacterial, and etc.(5) Antibacterial activity was tested to obtain the minimal inhibitory concentration (MIC) of hydrophilic AgNPs and the CTAB modified AgNPs. AgNPs performed stronger antibacterial ability than silver nitrate to Gram-negative bacteria. Moreover, the AgNPs modified by CTAB showed an excellent antibacterial activity than the hydrophilic AgNPs produced by the oil-water interface method. |
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