Plasmonic nanochains, derived from the one-dimensional assembly of individual plasmonic nanoparticles (NPs), remain infrequently explored in biological investigations due to their limited colloidal stability, ineffective cellular uptake, and susceptibility to intracellular disassembly. We report the synthesis of polydopamine (PDA)-coated plasmonic “nanoworms” (NWs) by sonicating citrate-capped gold (Cit-Au) NPs in a concentrated dopamine (DA) solution under alkaline conditions. DA mediates the assembly of Cit-Au NPs into Au NWs within 1 min, and subsequent self-polymerization of DA for 60 min enables the growth of an outer conformal PDA shell that imparts stability to the inner Au NW structure in solution, yielding “core–shell” Au@PDA NWs with predominantly 4–5 Au cores per worm. Our method supports the preparation of monometallic Au@PDA NWs with different core sizes and bimetallic PDA-coated NWs with Au and silver cores. The protonated primary amine and catechol groups of DA, with their ability to interact with Cit anions via hydrogen bonding and electrostatic attraction, are critical to assembly. When compared to unassembled PDA-coated Au NPs, our Au@PDA NWs scatter visible light and absorb near-infrared light more intensely and enter HeLa cancer cells more abundantly. Au@PDA NWs cross the cell membrane as intact entities primarily via macropinocytosis, mostly retain their inner NW structure and outer PDA shell inside the cell for 24 h, and do not induce noticeable cytotoxicity. We showcase three intracellular applications of Au@PDA NWs, including label-free dark-field scattering cell imaging, delivery of water-insoluble cargos without pronounced localization in acidic compartments, and photothermal killing of cancer cells.

Understanding gas sorption by water in the atmosphere is an active research area because the gases can significantly alter the radiation and chemical properties of the atmosphere. We attempt to elucidate the molecular details of the gas sorption of water and three common atmospheric gases (NO, SO, and O) by water droplets/slabs in molecular dynamics simulations. The system size effects are investigated, and we show that the calculated solvation free energy decreases linearly as a function of the reciprocal of the number of water molecules from 1/215 to 1/1000 in both the slab and the droplet systems. By analyzing the infinitely large system size limit by extrapolation, we find that all our droplet results are more accurate than the slab results when compared to the experimental values. We also show how the choice of restraints in umbrella sampling can affect the sampling efficiency for the droplet systems. The free energy changes were decomposed into the energetic ΔU and entropic -TΔS contributions to reveal the molecular details of the gas sorption processes. By further decomposing ΔU into Lennard-Jones and Coulombic interactions, we observe that the ΔU trends are primarily determined by local effects due to the size of the gas molecule, charge distribution, and solvation structure around the gas molecule. Moreover, we find that there is a strong correlation between the change in the entropic contribution and the mean residence time of water, which is spatially nonlocal and related to the mobility of water.

Droplet coalescence is a critical issue in atmospheric sciences. In this work, the coalescence of water nanodroplets was studied by performing equilibrium and nonequilibrium molecular dynamics simulations. To understand the intrinsic nature of the process, we obtained the free energy change as a function of droplet size and droplet–droplet distance. We decomposed the free energy change ΔF into energetic ΔU and entropic TΔS contributions to understand the molecular details. ΔU was dominated by the change in Coulomb interactions, which strongly correlated with the change in the number of hydrogen bonds. We found a strong positive correlation between the mobility of water molecules and TΔS. To analyze the dynamics, two colliding water droplets of the same size were given different initial speeds, impact parameters, and collision angles. We found when the collision is head-on, the time for thorough mixing between interfacial and bulk molecules decreases when the initial speed increases, whereas when the collision is off-center, the induced torque significantly increases the mixing time, which can last up to hundreds of picoseconds. The initial impact of a collision can push the interfacial molecules away from the center of mass and provide an evaporation mechanism of the interfacial/adsorbed molecules on the droplet.

Organic matter is prevalent in the atmosphere. How such organic matter affects droplet coalescence is a critical issue in atmospheric sciences. In this paper, we investigated the effects of three different organic coatings formed by benzoic acid, heptanoic acid, and pimelic acid on coalescence. We studied the thermodynamics of coalescence by calculating the free energy change and decomposed it into energetic ΔU and entropic TΔS contributions to understand the molecular details. ΔU was primarily determined by the flexibility of the organic compound, its solubility in water, and hydrogen bond networks. We found a strong positive correlation between TΔS and the mobility of water and organic molecules. To understand the dynamics of coalescence, we studied collisions between two organic-coated water droplets of the same size at various initial speeds, impact parameters, and collision angles and reported the critical speeds at which coalescence transitions to separation/shattering. We showed that when a collision of the droplets leads to shattering, an organic coating can influence the size distribution of particulate matter that has implications for air pollution. We also discovered that collisions of organic-coated droplets can often induce rotations.

Organic matter is ubiquitous in the atmosphere. It can form coatings that directly influence both the physical and chemical properties of the atmospheric water droplets and can have important but nontrivial consequences of climate changes. To understand the effects of organic coatings on gas uptake by atmospheric water droplets, we studied the uptake of three common atmospheric gases (H2O, O3, and SO2) by benzoic acid (BA)- or lauric acid (LA)-coated water droplets in both equilibrium and nonequilibrium molecular dynamics simulations. Free energy ΔF profiles for gas uptake by these coated systems were compared with the uncoated results in our previous work. We found that the changes in Coulomb energy dominate the changes in potential energy ΔU in the BA-coated systems, whereas the changes in Lennard-Jones energy play an important role in the LA-coated systems due to the long alkyl chains. We also found a clear positive correlation between the entropic contribution TΔS and the mobility of water and organic molecules. To study the dynamics and kinetics, we investigated how the organic coatings affect the mass accommodation coefficient α in nonequilibrium gas impinging runs. While the effects of the organic coating on α can be different for different gases, we found a general explanation for the change in α in terms of the relative positions of the air–organic interface and ΔF minimum.