Multistep Phase Transition and Molecular Reaction of Plasmonic Nanoparticles at the Three-Phase Contact Line of an Evaporating Sessile Droplet

Understanding the events of depositions of photochemically reactive nanoscale particles in evaporating sessile droplets holds important consequences in liquid-based printing at a high speed and resolution. Herein, we report that depending on the pH of the starting dispersion, the process could involve the juxtaposition of phase transition and reaction in single or multiple steps. We used aqueous droplet dispersions of 4-aminothiophenol (4-ATP)-functionalized Ag nanoparticles (AgNPs) and probed the consequences of evaporation at the three-phase contact line (TPCL) using surface-enhanced Raman spectroscopy (SERS) and atomic force microscopy (AFM). The results indicated that at pH 7.0, a single phase transition-led deposition followed by a photochemical reaction occurred at the outer, middle, and inner boundaries of the deposited ring. On the other hand, at pH 9.4, the dominant observation was the product formation of 4,4′-dimercaptoazobenzene (DMAB) preceding phase transition-led depositions at all three positions of the ring. Importantly, at pH 8.4, several steps of simultaneous product formation and depositions were the most prominent at the center of the deposited ring, while a smaller number of steps or single-step similar events were observed to have occurred at the inner and outer peripheries of the ring. AFM images evidenced the stepwise depositions leading to changes in the height and width of the ring with time.