Imaging three-dimensional rotational diffusion of plasmon resonant gold nanorods using polarization-sensitive optical coherence tomography

We demonstrate depth-resolved viscosity measurements within a single object using polarized optical scattering from ensembles of freely tumbling plasmon resonant gold nanorods (GNRs) monitored with polarization-sensitive optical coherence tomography. The rotational diffusion coefficient of the GNRs is shown to correlate with viscosity in molecular fluids according to the Stokes-Einstein relation. The plasmon resonant and highly anisotropic properties of GNRs are favorable for microrheological studies of nanoscale properties.

Figure 1

(a) PS-OCT interferometer setup. (b) Example $M$-mode images [using an absolute value of $\mathrm{ã}(z,t)$] in the $\mathit{HV}$ configuration showing an increasing rate of intensity fluctuations for samples with decreasing viscosities. (Note: Intensity fluctuations only up to 40 ms shown.)

Figure 2

Representative $g_{\mathit{HV}}^{(1)}(\tau )$ for samples with varying viscosity. Decay time is observed to decrease as the viscosity decreases. The inset shows a representative inverse exponential fitting to $g_{\mathit{HV}}^{(1)}(\tau )$ (dotted line), based on Eq. (1).

Figure 3

Comparison of experimental $D_R$ with theoretical predictions assuming GNRs as smooth cylinders [14] (solid line: for the actual sizes of the GNRs; dotted line: considering an average CTAB layer of 2 nm).

Figure 4

(a) Double-chamber design. (b) $M$-mode images [using an absolute value of $\mathrm{ã}(z,t)$] showing samples with different viscosities separated by a cover glass. (Note: Intensity fluctuations only up to 40 ms shown.) (c) $g_{\mathit{HV}}^{(1)}(\tau )$ of the samples showing two different decay time scales. (d) $D_R$ as a function of depth in the double chamber.