Brownian Fluctuations and Heating of an Optically Aligned Gold Nanorod

Abstract

We present the first quantitative measurements of the torque exerted on a single gold nanorod in a polarized three-dimensional optical trap. We determined the torque both by observing the time-averaged orientation distribution and by measuring the dynamics of the rotational brownian fluctuations. The measurements are in good agreement with calculations, where the temperature profile around the hot nanorod gives rise to a reduced, effective viscosity. The maximum torque on a 60 nm×25 nm nanorod was 100 pN·nm, large enough to address single-molecule processes in soft and biological matter.

Transverse spinning of a sphere in a plasmonic field

“…Moreover in air, the exponential decay is twice slower when the sphere moves away from the plane. We finally emphasize that from an ex-perimental point of view, these frequencies perfectly fall within the bandwidths of standard displacement detection systems, based on power spectral analysis or rotational Doppler shifts, for instance [5,42,43]. …”

Thermophoretic torque in colloidal particles with mass asymmetry

“…4) τ/ ∼ 3 will scale linearly with the characteristic energy scale of the system under investigation. Using an energy of 1 kJ/mol, corresponding to typical dispersion interactions, we get ∼5 pN nm, to be compared with torques achieved experimentally using metallic nanoparticles, 100 pN nm [14].…”

“…The thermodiffusive behavior of * j.olarte@imperial.ac.uk † mrubi@ub.edu ‡ f.bresme@imperial.ac.uk colloidal rods (fd viruses) was studied recently [13], and it has been shown that the heating of metallic nanorods in a polarized optical trap induces a thermal gradient around the rod and a torque that can be significant (∼10 2 pN nm) [14]. Theoretical and experimental studies of anisotropic particles under thermal gradients have uncovered fascinating behaviors that could open new routes to manipulate small colloids using thermal fields.…”

Metal Nanoparticles for Microscopy and Spectroscopy

“…Au and Ag nanorods therefore align with the trapping laser polarization. The optical torque that can be exerted on a single Au nanorod was recently quantified to be *100 pN nm for 80 mW of trapping laser power [19], enough to twist biomolecules such as DNA. Although the absorption cross-section of the NP at the trap laser wavelength is small, it is non-negligible, and causes the NP to heat and eventually melt if the trapping power is sufficiently high.…”

“…Although the absorption cross-section of the NP at the trap laser wavelength is small, it is non-negligible, and causes the NP to heat and eventually melt if the trapping power is sufficiently high. Typical heating rates of metal NPs in an optical trap are 0.1-1 K/mW, depending on the shape and size of the NP [19][20][21].…”