Gold nanocubes - Direct comparison of synthesis approaches reveals the need for a microfluidic synthesis setup for a high reproducibility

Abstract

The production of non-spherical gold nanoparticles is a rather complex process and requires a multistep synthesis including surface blocking detergents such as CTAC and CTAB (cetyltrimethylammonium chloride; -bromide). Especially gold nanocubes are difficult to realize since they have six close packed 100-planes and they need at least three separate, highly controlled production steps: the synthesis step, where the seed particles will be formed, followed by two independent growing steps. One main challenge is to find the optimal conditions for the different steps, since the incubation times differs in several magnitudes for each step (seeds: ms, growing solution 1: sec, growing solution 2: min). Based on this discrepancy we present a study and comparison of different synthetic methods for each step. Starting with a classical batch approach we also transferred the synthesis into microfluidics and therefore compare continuous (one-phase) as well as segmented flow techniques, and discusses the benefits of each method. We present the additional advantage of CTAC in microfluidics since it passivates the channel surfaces and thereby inhibits clogging. A further detailed kinetic study of the two growing steps identifies the incubation time as critical parameter for a defined (cubed-like) geometry and facilitates the use of microfluidic methods for the first growing step, since it eliminates subjective decisions. Due to the kinetic study in combination with electron-microscopic characterization we propose a Figure Of Merit (FOM) for the second growth step that simplifies the evaluation of the point in time when to terminate the reaction to obtain perfectly shaped Au nanocubes. We also demonstrate the sensitivity (comparison of Au nanocube and Au nanosphere) and the possibility of tuning the cube edge length (and so the plasmon peak position) and highlight their correlation as a fundament for an automated microfluidic synthesis and versatile applications, like biosensing.