Asymmetric nanoparticle may go “active” at room temperature

Abstract

Using molecular dynamics simulations, we show that an asymmetrically shaped nanoparticle in dilute solution possesses a spontaneously curved trajectory within finite time interval, instead of the generally expected random walk. This unexpected dynamic behavior has a similarity to that of active matters, such as swimming bacteria, cells or even fishes, but is of a different physical origin. The key to the curved trajectory lies in the non-zero resultant force originated from the imbalance of the collision forces acted by surrounding solvent molecules on the shaped nanoparticle during its orientation regulation. Theoretical formulae based on the microscopic observation have been derived to describe this non-zero force and the resulted motion of the nanoparticle.PACS number: 66.10.cg, 83.10.Rs, 87.90.+yThe motion of molecules caused by thermal fluctuations plays an essential role in determining the probability for them to meet targets upon functioning [1-4], as found in many physical processes [3,5], chemical reactions [6,7] and biological functioning [2,3]. In conventional theories, the molecules/particles have been treated as perfect spheres with their trajectories described as random walks, following the original work of Einstein [8][9][10][11]. However, it has been realized by Einstein himself that this picture will break down if we can inspect the motion of the particles at much smaller time and length scales [9]. Significant progresses have been made on investigating the motion of particles at micrometers and timescales from microseconds to seconds, showing unconventional behaviors within relatively short time interval [12][13][14][15][16][17][18]. Han et al. experimentally observed a crossover from short-time anisotropic to long-time isotropic diffusion behavior of ellipsoidal particles along different axial directions [12]. Huang et al. experimentally measured the mean-square displacement of a 1-µm-diameter silica sphere in water using optical trapping technique and found that the particle motion cannot be described by conventional theory until sufficiently long time [14]. We note that, a majority of kinetic and dynamic processes related to molecules take place in nanoscale space [1][2][3][19][20][21][22] and accomplish in just several picoseconds [23,24], such as self-assembling [25][26][27][28][29], triggering chemical reaction [7,30], intercellular signal transduction [31], and neurotransmission [32]. Unfortunately, there is rare report on the unconventional behavior in the free motion of the molecules/nanoparticles solely under thermal fluctuations within short time at the nanoscale.On the other hand, molecular dynamics (MD) simulation has been widely accepted as a powerful tool for studying the dynamics of molecules at nanoscales [33][34][35][36][37][38][39][40][41][42]. Our recent atomistic MD simulations showed interesting anisotropic motion of small asymmetric solute molecules, such as methanol and glycine, in water solely due to thermal fluctuations, which indicates the existence of rich dynamic be...

Disorder in Aqueous Solutions and Peak Broadening in X-ray Photoelectron Spectroscopy

“…In aqueous solutions, thermal fluctuations are ubiquitous and fundamentally important in almost all physical/chemical/biological processes, such as molecular diffusion, , chemical reaction, ice formation, , water evaporation, aggregation of solute molecules, motion of molecular machines, and transportation through nanochannels. − The key to the fluctuations in liquids is the motions and collisions at the molecular level, such as breaking/forming of hydrogen bonds and the structural change of the ions’ solvation shells. , Thermal fluctuations must provide an additional contribution to the broadening of the XPS peaks; however, few detailed studies on the response of photoelectron spectroscopy to thermal fluctuations have been carried out.…”