Single Phase Transition Leads to the Nanoparticle Deposition in an Evaporating Sessile Droplet

Abstract

The ubiquitous ring stain formation, upon evaporation of a sessile droplet, is known to be driven by a capillary flow taking the dispersed particles toward the three-phase contact line (TPCL) for gradual deposition with a rapid acceleration of the flow in the final leg of drying. However, we report that the process is akin to phase transition, with particles depositing at the contact line in a short span of time and well before the complete drying of the droplet. We used surface-enhanced Raman spectroscopy (SERS) when aqueous droplets of sodium 4-marcaptobenoate-stabilized nanomolar plasmonic gold nanoparticles (NaMBA-AuNPs) were evaporated under ambient conditions. The Raman signals of the molecules were observed to have sharply increased in a short window of time that matched well with simultaneous digital photographing of the deposition. Higher initial AuNP concentrations led to faster phase transitions; however, the final concentration at the TPCL was about 13 μM for 25 nm AuNPs, irrespective of the initial concentrations studied. The time for phase transitions decreased with increasing particle size at a fixed initial AuNP concentration.

Differing Plasmonic Intra-nanoparticle and Inter-nanoparticle Molecular Reaction Rates at the Three-Phase Contact Line of an Evaporating Sessile Droplet

“…Change in the intensity of the appeared 1078 cm -1 Raman peak (combined C=C and C−S stretching vibration), that is the characteristic signal of 4-ATP, with time was used to monitor the transition from dispersion to deposit phase of ATP@AgNPs. 12 As evidenced from Figure 2c(i) and Figure 2c(iv), the phase transition was observed at 20.7 min. The highlighted peak at 1434 cm -1 in Figure 1a and Figure 1b was due to the N=N stretching vibration of the dimerization product DMAB.…”

“…The sudden jump in intensity was possibly due to the plasmonic 'hot-spot' generation during phase transition from dispersion to deposit state. 12 The peak at 950 cm -1 account for structures having intraparticle (Figure 2c) and interparticle (Figure 2d) 4-ATP molecules H-bonded to H2O molecules on the surface of AgNP, with the calculated peaks appearing 959 cm -1 for both (Figure 2b). Therefore, the experimentally observed Raman peak at 950 cm -1 was due to the combined vibration of 4-ATP and H-bonded water molecules in the plasmonic 'hot-spot' environment around AgNPs.…”

“…In the recent study 12 , we reported that in order to form coffee-ring, deposition of gold nanoparticles at the three-phase contact line (TPCL) follows phase transition behavior from dispersion to deposition at a certain concentration of particles, depending on diameter of nanoparticles. During evaporation of the droplet, nanoparticles bearing shells of water come close to each other at TPCL.…”

“…Phase transition also implies the formation of plasmonic 'hot-spot' at a certain time when molecule laden nanoparticles come to the closest proximity. 12 Now, a chemical reaction occurring at the TPCL, following deposition and in the presence of plasmonic hot spots and evaporating water, may reveal new information about the nature of reactions especially by distinguishing the rates of intra-nanoparticle and inter-nanoparticle reactions. In this respect, the photochemical oxidative dimerization of 4-aminothiophenol (4-ATP) 13 on nanoparticle surface is a versatile and well-studied reaction that can occur between two molecules attached to a single nanoparticle or two nanoparticles.…”

Distinction of Plasmonic Intrananoparticle and Internanoparticle Molecular Reaction Rates at the Three-Phase Contact Line of an Evaporating Sessile Droplet

“…Thus, sodium 4-marcaptobenzoate (NaMBA) stabilized Au nanoparticles in an aqueous dispersion, when evaporated as sessile droplets and probed by surface-enhanced Raman spectroscopy (SERS) at the three-phase contact line (TPCL), a phase transition signifying the depositions of the nanoparticles occur all at once. 13 Now, a chemical reaction occurring at the TPCL following deposition and in the presence of plasmonic hot spots and evaporating water may reveal new information about the nature of reactions, especially by distinguishing the rates of intrananoparticle and internanoparticle reactions. In this respect, the photochemical oxidative dimerization of 4aminothiophenol (4-ATP) on a nanoparticle surface is a versatile and well-studied reaction that can occur between two molecules attached to a single nanoparticle or two nanoparticles.…”