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480 D. PATRA AND R. EL KURDI obtained in moderate yield when adding curcumin alone but with the presence of some nanospheres. Thus, the addition of silver nitrate has enhanced the activity of curcumin and 100% nanorods were obtained. Thus, Moussawi et al. were able to form Au nanorods at pH 9.2, where El Kurdi et al. have obtained spherical shapes. These different results emphasize the role of curcumin as a reducing agent in the production of different shapes of gold nanoparticles at the same pH. Meaning that, upon the reaction conditions, curcumin acts in a different way to produce several shape and size of gold nanoparticles. Finally, a study was published by Patra et al. using curcumin and potassium carbonate (67). The prep- aration of Au NPs was occurred using curcumin as a reducing agent, where Au3+ were easily reduced to Au0. Patra et al. have established the kinetic reaction of the Au NPs synthesis in order to follow up the reduction process. In fact, when adding curcumin, an amount of Au3+ was reduced to Au0. This was confirmed by the UV-Visible spectrophotometer where a minor peak was obtained at 425 nm (curcumin peak) and a major peak was obtained at 525 nm (Au NPs). However, within the time, the absorbance at 425 nm decreased dramatically and the absorbance at 525 nm increased gradually after 4 h. This change in the absor- bance is due to the fact that curcumin is being reacted in the solution and thereby achieving the reduction of Au3+ to Au0. To end up, based on the formed Au NPs, several sensing nanoprobes were produced in order to detect different analytes. For example, El-Kurdi et al. were able to detect uric acid (64), adenosine triphosphate (62,63), glucose (54), and alpha-tocopherol (65,68). Al Shehab et al. (21) and Patra et al. (67) were able to detect melamine and nucleic acid, respectively. However, each type of Au NPs prepared was selective for specific analytes, in different ranges of concentration. Besides that, Sreelakshmi et al. (57) have evaluated the cytotoxicity of the prepared Au NPs against three different cell lines, where Au NPs have shown an effective role in the inhibition of the proliferation. Fur- thermore, Sindhu et al. (59) have verified the biocompat- ibility of the Au NPs, where the studies have shown that the prepared Au NPs are biocompatible when treating with normal human blood cells. Finally, Singh et al. (58) and Shaabani et al (60) have established the role of Au NPs as an antioxidant. Hence, whatever is the pro- cedure followed to prepare the nanoparticles, the use of curcumin enhances the activity of the Au NPs, where they have shown promising features based on their bio- medical applications. 2.2. Synthesis of silver nanoparticles Kundu et al. were the first to establish the role of curcu- min as a reducing and stabilizing agent for silver nano- particle synthesis. Ag+ ions were immediately reduced to Au0 under continuous stirring at 80°C. According to Kundu et al. (69) when heating the curcumin in an aqueous solution at high temperature, the orange color started to disappear. After almost 20 min, the color changes from orange to faint light yellow. This change in color is due to the nucleation of Ag particles. Later on, after one hour, the color changes to dark yellow/yellowish green. This color indicates the matu- ration of Ag clusters and thereby the formation of Ag NPs. The mechanism of Ag NPs can be explained as below (see Figure 7). Another study by Verma et al. (70) was conducted using curcumin as a reducing agent in the presence of NaOH. Verma et al. have verified the use of basic media as an effective condition to accelerate the reac- tion of curcumin. Curcumin in basic media is easily deprotonated. Thus, the reduction of Ag+ to Ag0 easily occurs. In this case, the color changes from yellowish red to greenish yellow in five minutes under ultrasonication. To evaluate the difference between Ag NPs prepared using curcumin and other reducing agents, Selvan et al. have prepared silver nanoparticles using garlic, tea extract, and curcumin (71). In the three cases, the redu- cing agent was dissolved in NaOH, added in a second step to Ag NO3 solution, and kept under stirring over- night. Despite that curcumin reacted immediately in basic media, the solution was kept for overnight to have the same condition for all reducing agents. The reduction of Ag+ to Ag0, according to Selvan et al., is due to the flavonoids present in the garlic, tea extract, and curcumin. However, these flavonoids consist of hydroxyl groups that have a strong ability to bind with Ag+ ions. Hence, once the interaction occurred, Ag+ is reduced to Ag0 and therefore induces the formation of Ag NPs. However, Ag NPs prepared using curcumin were smaller in size compared to the ones prepared using garlic and tea extract. Hence, no major difference was observed during the preparation and characteriz- ation analysis. Thus, based on Selvan et al.’s results, the most relevant difference was that the Ag NPs prepared using curcumin had the most biological effect. In addition, Alsammarraie et al. (72) and Sathishkumar et al. (73) were in agreement with the statement pro- posed by Selvan et al. Hence, when the curcumin sol- ution changes in color, this is mainly due to the surface excitation of the plasmon resonance phenom- enon of silver metal. Yet, curcumin contains a highPDF Image | Curcumin as a novel agent for metallic nanoparticles
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