High-throughput in-focus differential interference contrast imaging of three-dimensional orientations of single gold nanorods coated with a mesoporous silica shell

Abstract

Plasmonic gold nanorods (AuNRs) have been widely applied as optical orientation probes in many biophysical studies. However, characterizing the various three-dimensional (3D) orientations of AuNRs in the same focal plane of the objective lens is a challenging task. To overcome this challenge, we fabricated single AuNRs (10 nm × 30 nm) coated with either an elliptical or spherical mesoporous silica shell (AuNRs@mSiO₂). Unlike bare AuNRs and elliptical AuNRs@mSiO₂, spherical AuNRs@mSiO₂ contained randomly oriented AuNR cores in 3D space, which could be observed on the same focal plane within a single frame by differential interference contrast (DIC) microscopy. The spherical AuNRs@mSiO₂ thus achieved high-throughput detection. The proposed approach can overcome the limitations of the current gel-matrix method, which requires vertical scanning of the embedded AuNRs to capture different focal planes.

Fig. 1. (A–C) TEM images of (A) bare AuNRs, (B) elliptical AuNRs@mSiO₂ (red curve), and (C) spherical AuNRs@mSiO₂ (blue curve). (D) Overlaid UV-vis extinction spectra of bare AuNRs (yellow curve), elliptical AuNRs@mSiO₂ (red curve), and spherical AuNRs@mSiO₂ (blue curve). The anisotropic AuNRs yielded two distinct LSPR peaks.

Fig. 2. (A) Schematic of bare AuNRs deposited on a glass slide. Their projected lengths are almost the same. (B) Schematic of elliptical AuNRs@mSiO₂ particles and their projections on a glass slide. (C) Schematic of spherical AuNRs@mSiO₂ particles with their AuNR cores randomly oriented in the shell. Their projected lengths depend on their spatial orientations, which are freely available in 3D space.

Fig. 3. (A) TEM image of spherical AuNRs@mSiO₂ with randomly oriented AuNR cores inside the silica shell. (B) SEM image of spherical AuNRs@mSiO₂ particles, and (C) DIC image of AuNRs@mSiO₂ particles, showing their different orientations on the same focal plane. The color image was generated in ImageJ for better demonstration of bright and dark DIC images of AuNRs@mSiO₂. Note that high-throughput detection is possible.

Fig. 4. (A) DIC image of spherical AuNRs@mSiO₂. (B) DIC images of spherical AuNR1@mSiO₂ captured at different rotational angles (interval = 30°). (C) Dark and bright intensities of spherical AuNR1@mSiO₂ as functions of rotation angle. (D) DIC polarization anisotropy computed from the dark and bright intensities of the spherical AuNR1@mSiO₂.

Fig. 5. Histograms showing the polar angle (θ) distributions of (A) elliptical AuNRs@mSiO₂ and (B) spherical AuNRs@mSiO₂, determined from the DIC measurements.