Enabling local deposition and controlled synthesis of Au-nanoparticles using a femtopipette

We demonstrate local synthesis and deposition of gold nanoparticles (AuNPs) using a femtopipette, which is a device similar to an atomic force microscope (AFM) sensor but featuring integrated fluidic channels. Two femtopipettes were used, one filled with chloroauric-acid (HAuCl4), and the second with sodium-azide (NaN3). We dispensed two equal droplets (HAuCl4 and NaN3) adjacent to each other, allowing them to coalesce and mix. In this manner, cross-contamination between the tips was prevented. The deposited volumes (per reagent) were 2.4fL, 1.2fL, 0.5fL and 0.25fL. Upon mixture, a chemical reduction process was triggered, which led to the local formation of AuNPs. The performed analysis indicate that the size of the nanoparticles is dominated by droplet volume. For the biggest droplet-mixture, the dominant particle size was 14nm. Whereas for the smallest droplets, the dominant particle size was 4nm. Similarly, the size distribution decreased with smaller droplet volumes. To the best of our knowledge, the combination of deposition and local chemistry using femtoliter-droplets, has not been reported before and represents a new level of compartmentalized nanoparticle synthesis and positioning.