Microlens-Assisted Light-Scattering Imaging of Plasmonic Nanoparticles at the Single Particle Level

Abstract

We present a microlens-assisted imaging approach to record the scattering light of plasmonic nanoparticles at the single particle level. The microlens can significantly enhance the backscattering of visible light from individual plasmonic nanoparticles by several dozen folds, and single gold nanoparticles with a diameter as low as 60 nm can be imaged under a conventional optical microscope. This can benefit from a significant increase in the scattering intensity afforded by the microlens, meaning that the imaging of gold nanoparticles at a high temporal resolution (up to 5000 Hz) can be achieved, which is fast enough to record single particle adhesion events on the substrate. This research presents a fast and efficient means of acquiring scattering light from plasmonic nanoparticles, which has great potential to develop plasmonic nanoparticle-based biosensors and investigate a wide range of plasmonic nanoparticle-based fast interaction processes.

Keywords: dielectric microsphere; high temporal resolution; light-scattering imaging; microlens; plasmonic nanoparticle; single-nanoparticle imaging.

Figure 1

(a) Schematic illustration of the optical configuration. The observation was conducted through an inverted microscope under a reflected optical path. The microlenses were embedded in the PDMS layer and positioned above the objective. The AuNP solution was dropped on the microlenses. (b) Illustration of the formation of the virtual image using the microlens. The microlens act as an auxiliary lens that forms a magnified virtual image above the specimen’s surface. Upon illumination, the scattered light of AuNPs is collected by the microlens and finally recorded and imaged by the optical microscope.

Figure 2

(a) Scanning electron microscope (SEM) image of the individual microlens. The inset image shows the embedded microlens. The top surface of the microlens is exposed out of the PDMS layer. Scale bar = 5 µm. (b) The optical image of the microlens array in the solution under the inverted microscope with a reflected optical path. The focus is on the contour of microlenses. Scale bar = 10 µm. (c) Gray level virtual image of 100 nm AuNPs captured under a microlens. The dashed circle indicates the position of a microsphere lens. The bright spots in the image indicate the individual AuNPs. The focus is on the virtual image plane of the microlens. Scale bar = 10 µm. (d) Colored image (captured by a CCD camera) of 100 nm AuNPs under a microlens with white light illumination. Scale bar = 10 µm. The color exhibited by the AuNPs was caused by the surface plasmon resonance phenomenon. AuNPs imaged under a microsphere lens (image (e)) and corresponding AuNPs imaged with SEM (image (f)). The white arrows indicate their positions for comparison. Scale bar = 5 µm in image e and scale bar = 500 nm in image (f).

Figure 3

(a) Photoelectron images of 150 nm diameter AuNPs imaged with (top image) and without (bottom image) the microlens. Photoelectron scale = photoelectrons/10⁴. The images were acquired at an exposure time of 10 ms with white light illumination (power intensity = 78 mW/cm²). Scale bar = 5 µm. Note that for the convenience of visual identification, the scale bars of the photoelectrons are different. (b) Relative photoelectrons from individual AuNPs indicated by dashed lines in image a. The green and orange lines denote the AuNPs imaged with and without the microlens, respectively. Their photoelectrons were normalized by the maximum value of AuNPs imaged without the microlens. Insert: the enlarged view of AuNPs imaged without the microlens. (c), Finite element method simulation of the electric field (|E|²) distribution generated by the microlens. The diameter of the microlens was set at 22 μm, and the wavelength of the illumination light was set at 600 nm. The simulation shows that the incident light was converged by the microlens at its shadow side. (d) Gray-level images of AuNPs of different diameters (150 nm, 100 nm, 80 nm, and 60 nm, respectively) imaged under a microlens. Images were acquired at an exposure time of 1 ms with white light illumination (power intensity = 1 W/cm²). (e), Cross-section profile of the corresponding AuNPs (indicated by the white dashed line) from image (d).

Figure 4

Single particle adhesion event monitored by the microlens. (a) Real-time images of an adhesion event at the single particle level from a video recorded at 5000 Hz (corresponding to an exposure time of 200 μs). Video was acquired at an exposure time of 200 μs with white light illumination. The red dashed circle indicates the area where a single AuNP (100 nm) appeared (image ii) and became resident on the microlens surface (image iii). (b, i–iii) Schematic illustration of the corresponding adhesion event. (c) Intensity of the adhesion area (as indicated in image (a) with a red dashed circle (i)) recorded at 5000 Hz under the microlens. The relative intensity value represents the normalized mean intensity of the adhesion area. At time ~70 ms, the AuNP appeared, and the intensity started to increase. At time ~110 ms, the AuNP adhered, became resident on the microlens surface, and the intensity was maintained as stable.