Tygon tubing

The droplet capillary reactor was made from a 100 cm long section of Tygon tubing (inner diameter = 0.51 mm, outer diameter = 1.52 mm, Sigma-Aldrich, USA) into which a through-hole was punched 15 cm from the end of the tubing using a small syringe needle. Two 5 cm long pieces of fused silica capillary (inner diameter = 100 µm, outer diameter = 360 µm, Supelco, USA) were inserted into the hole and arranged so that the two ends formed a 90° angle in the middle of the Tygon tubing. The holes were sealed with small amount of hot-melt adhesive. The remaining tubing after the junction was submerged in a water bath and the end was placed into a vial filled with water for sample collection. For droplet experiments the formation of droplets was recorded using a bright-field microscope (IX37, Olympus, Japan). Droplet diameters were manually measured using Image J (National Institute of Health, USA). For volume calculations, droplets were assumed to be of spherical shape if the droplet diameter was smaller than the diameter of the tubing, and of cylindrical shape for larger droplet diameters.

For the droplet capillary reactor, a 190 cm long piece of Tygon tubing (0.51 nm inner diameter, 1.52 nm outer diameter, Sigma-Aldrich, USA) was used into which several holes were punched using syringe needles. Into the first two holes, 15 cm from the beginning of the tubing, two fused silica capillaries (100 µm inner diameter, 360 µm outer diameter, Supelco, USA) were inserted that they formed a 90° angle in the center of the Tygon tubing. Furthermore, single capillaries were inserted at 100, 130, and 160 cm perpendicular to the center of the Tygon tubing for the addition of gold precursor. After insertion of the fused silica capillaries, the holes were sealed using a hot melt adhesive. At the outlet of the capillary droplet reactor, the optical transmission was measured using a light emitting diode (LED, 585 nm main emission wavelength), a photoconductor, and a pair of lenses to focus on the center of the tubing (Supplemental Materials S1, 2), all housed in a custom three-dimensional (3D) printed housing. A custom written Python script was used for transmission measurements, running of the optimization algorithm, and manipulation of the flowrate (Supplemental Material S3). For temperature control of the reactor, the entire assembly was submerged in a temperature controlled water bath at 70 °C.