The final spectrum is compared to a target AgNP absorbance spectrum by cosine similarity. The absorbance spectra of each droplet are then combined into an absorbance map containing the temporal evolution of the absorbance spectra during the whole AgNP synthesis. The time evolution of the absorbance peak is extracted to estimate the reaction rate, here defined as the ratio between the amplitude of the absorbance peak and the residence time of the reacting droplet (see Fig. , bottom).