Article pubs.acs.org/JPCC
Synthesis of Au Nanorods through Prereduction with Curcumin: Preferential Enhancement of Au Nanorod Formation Prepared from CTAB-Capped over Citrate-Capped Au Seeds Rasha N. Moussawi and Digambara Patra* Department of Chemistry, American University of Beirut, Beirut 1107 2020, Lebanon S Supporting Information *
ABSTRACT: Despite seed-mediated growth approach being the state of art method for synthesis of gold nanorods, the mechanism of gold nanorod formation to control final aspect ratio of the nanorods is not yet well understood. In this work, formation of Au nanorods has been investigated using curcumin, a hydrophobic molecule, as secondary reducing agent, instead of popularly used hydrophilic ascorbic acid. It is found that the nature of the secondary reducing agent plays a crucial role in preferential enhancement of Au nanorod formation. Although in the seed-mediated growth approach the concentration of curcumin has similar effect on Au nanorod formation as observed for ascorbic acid earlier, the combination of the nature of the capping agent of seed particles as well as that of the secondary reducing agent determines the final aspect ratio of the nanorods. Hydrophobic (secondary) reducing agent, curcumin, prefers CTAB capped seed solution compared with citrate capped seed solution. The mechanism of nanorod formation has been explored. AgNO3 is known to enhance the aspect ratio of the nanorod formation when the ascorbic acid and citrate capped seed method is used; however, in contrast when curcumin is used, AgNO3 encourages nanorod formation for only CTAB capped seed solution but discourages nanorod formation for citrate capped seed solution. A mechanism has been suggested for this discrepancy. Present results help to gain more understanding of the formation process by bringing new insight in the mechanism of Au nanorod formation through seed-mediated growth approach, which may trigger better design of nanomaterials. solution.7 Among the various ways, the seed-mediated growth approach introduced by Murphy and co-workers6 is the state of art method for synthesis of gold nanorods. However, here choice of the reducing agent determines to a great extent the rate of formation of the nanorods and the final shape and anisotropy. Ascorbic acid (AA) is the secondary mild reducing agent usually used because it is too weak to reduce the additional gold salt in the growth step from Au3+ to Au0 alone. This allows for the growth to occur over a long time (minutes to hours), which aids in anisotropic growth. The structuredirecting agent used is cetyltrimethylammonium bromide (CTAB).6 Research on the seed mediated method has studied the effects of many parameters including the gold seed, concentration of reactants, temperature, duration of growth, additives, etc.23−27 The choice of the capping agent used for Au seed is also an important factor that affects the yield of nanorods since it affects the crystal structure of the seed itself. Another approach is to use a silver-assisted method.5 Low aspect ratio nanorods (∼1−6, up to ca. 90 nm in length) starting from 1.5 nm CTAB capped Au seeds28,29 can be synthesized with high yield (∼90%) after two rounds of
1. INTRODUCTION One of the aims of materials science is defining and understanding the mechanisms that eventually dictate crystal shape. Without a firm understanding of these formation processes, oriented production of crystals with the desired shape and crystalline structure will not be fulfilled. In particular, gold has attracted intense research because of its fascinating optical, electronic, and chemical properties as well as biocompatibility.1,2 These properties nominate gold as an ideal candidate for promising applications in nanoelectronics, biomedicine, sensing, and catalysis.3,4 Various wet chemical techniques have been established to produce gold nanoparticles with various shapes such as rods,5−7 wires,8,9 plates,10,11 prisms,12,13 cubes,14 polyhedra,15,16 and branched particles.17,18 Gold nanorods have received tremendous attention in recent years due to their exciting potential applications in chemical sensing, biological imaging, drug delivery, and phototherapeutics. A “nanorod” is defined to be a nanoscale particle with an aspect ratio (length/width ratio) that is between 1 and ∼20−25; higher aspect ratio materials are termed as “nanowires” (with diameter
