Flow-through reactor for photomediated synthesis of silver nanoprisms

Abstract

Silver nanoprisms (AgNPrs) have been widely used due to their outstanding optical properties which is one of an important phenomenon that depends on their size, shape, and compositions with characteristic localized surface plasmon resonance (LSPR). The changes in LSPR of AgNPrs can be simply observed by the color changes of the AgNPrs colloidal solution, therefore, the AgNPrs have been mostly used as chemical sensors as they provide high sensitivity and capable to be functionalized to improve the detection selectivity. In this study, we report a photochemical technique to induce the shape conversion of the spherical silver nanoparticle to AgNPrs by light-emitting LED at several wavelengths. The synthesis reaction was simply started with the reduction of a silver ion and sodium borohydride (NaBH4) as a reducing agent to generate silver nano-seeds stabilized by citrate. The reaction was controlled under a nitrogen atmosphere to avoid the oxidation reaction from oxygen. The reaction was then illuminated with narrowband Light Emitting Diode (LEDs) of 464 nm (blue), 513 nm (green), and 630 nm (red). It was found that the shape evolution was occurred when only LEDs source with 514 nm (> 2.59 eV) was used. The synthesized AgNPrs with out-of-plane quadrupole LSPR and in-plane dipole LSPR were obtained from the green LEDs. Effects of the pH and the mole ratio of the starting reagent (including Ag+: TSC: BH4-) were investigated. The shape conversion of AgNPs using light irradiation occurred when the system contains adequate amount of Ag+ (low mole ratio of reducting agent). By monitoring their LSPR, the synthesized AgNPrs show good stability after storing at 4°C in closed vials to prevent the effect of external light for 45 days. Finally, the flow-through reactor for synthesis AgNPrs was designed and operated to synthesize the AgNPrs. It was found that the product (AgNPrs) from bach synthesis and flow reactor are similar by monitoring their LSPR.