When photons meet electrons...

















Read the following review paper if you want to know more about these optical imaging techniques:
Wei Wang*, Imaging the chemical activity of single nanoparticles with optical microscopy,  Chem. Soc. Rev . , 2018, 47, 2485-2508. [
PDF ] [ URL ]


Surface Plasmon Resonance Microscopy

Planar SPR effect is based on the propagation of surface plasmon polaritons (SPPs) that are generated at a planar metal-dielectric interface under particular light excitation. The presence of individual nanoparticles on the surface interacts with the SPPs and creats a special parabolic pattern in the SPRM image. Because the image contrast is dependent on its dielectric constant, a physical property that applies to any material, SPRM is thus suitable for studying all kinds of nanomaterials including plasmonic, dielectric and soft matter nanomaterials. Mechanisms fo applications rely on the sensitive dependence of SPRM intensity on the dielectric constant and size of nanoparticle, as well as the vertical distance between nanoparticle and gold film.

Single Molecule/Particle Fluorescence Microscopy

Fluorescence (FL) is the light emission accompanied with the radiative relaxation when an excited state substance returns to its ground state. Typically, FL emission is with longer wavelength compared with the excitation light. Because many types of nanomaterials and molecules exhibit rather high FL quantum yield, and the background scattering can be effectively filtered out by using optical filters, FLM has become the earliest and the most commonly used optical microscopes for imaging single nanoparticles with excellent signal-background ratio. FLM is capable of resolving the intensity, spectrum, lifetime and polarization of nanomaterials that are either intrinsically fluorescent or labeled with fluorescent tags.

Photo Thermal Microscopy

When a substance at the excited state returns back to the ground state, it releases the energy through both non-radiative thermal relaxation (dissipation of heat) and radiative relaxation (emission of photons). For most common nanomaterials, the thermal relaxation is a dominate pathway, which leads to an increased local temperature via heat dissipation from nanoparticle to surrounding medium. Photothermal microscopy (PTM) is one kind of optical microscopy that maps the photo-thermal efficiency of single nanoparticles by measuring the local temperature with optical approaches. PTM is suitable for studying light-absorbing materials.

Dark Field Microscopy

The image contrast in DFM is from the Rayleigh scattering of light by nanoparticles as shown in Figure. Oblique illumination keeps the incident light from entering the objective, thus causing a dark background. Rayleigh scattering by nanoparticles on the substrate re-emits photons towards all angles. Part of them is collected by the objective to generate a bright dot in the DFM image. DFM is most powerful for studying plasmonic nanomaterials because of their large Rayleigh scattering cross section as well as the sensitive dependence of plasmonic band on the size, geometry, surface adsorbates and the surrounding dielectric environments.

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