Fluorescence interferometry: principles and applications in biology

Versatile transmission/reflection tomographic diffractive microscopy approach

Tomographic diffractive microscopy (TDM) has gained interest in recent years due to its ability to deliver high-resolution, three-dimensional images of unlabeled samples. It has been applied to transparent samples in transmission mode, as well as to surface studies in reflection mode. Mudry et al. [Opt. Lett.35, 1857 (2010)OPLEDP0146-959210.1364/OL.35.001857] introduced the concept of mirror-assisted TDM (MA-TDM), an elegant approach for achieving quasi-isotropic-resolution microscopic imaging, but which is still to be experimentally applied. In this work, we show that a simplified version of MA-TDM allows for transforming a reflective TDM setup into a more versatile instrument, also capable of observing transparent samples in transmission mode if using specific sample holders made out of a mirror and coated with a low-thickness transparent spacer.

Detection and controlled depletion of cancer cells using photothermal phase microscopy

We present a dual-modality technique based on wide-field photothermal (PT) interferometric phase imaging and simultaneous PT ablation to selectively deplete specific cell populations labelled by plasmonic nanoparticles. This combined technique utilizes the plasmonic reaction of gold nanoparticles under optical excitation to produce PT imaging contrast by inducing local phase changes when the excitation power is weak, or ablation of selected cells when increasing the excitation power. Controlling the entire process is carried out by dynamic quantitative phase imaging of all cells (labelled and unlabelled). We demonstrate our ability to detect and specifically ablate in vitro cancer cells over-expressing epidermal growth factor receptors (EGFRs), labelled with plasmonic nanoparticles, in the presence of either EGFR under-expressing cancer cells or white blood cells. The latter demonstration establishes an initial model for depletion of circulating tumour cells in blood. The proposed system is able to image in wide field the label-free quantitative phase profile together with the PT phase profile of the sample, and provides the ability of both detection and selective cell ablation in a controlled environment. Quantitative phase imaging with molecular specificity and specific cell depletion. (a) Label-free quantitative phase profiles of mixed population of EGFR(+) /EGFR(-) cancer cells. (b) When weak modulated PT excitation is applied, selective phase contrast is generated in the modulation frequency only for the EGFR(+) cancer cells labelled with plasmonic nanoparticles. (c) When stronger modulated PT excitation is applied, selective ablation of the EGFR(+) cancer cells labelled with plasmonic nanoparticles occurs. White scalebars represent 10 µm upon sample.

Plasmonic Mach-Zehnder interferometer for ultrasensitive on-chip biosensing

We experimentally demonstrate a plasmonic Mach-Zehnder interferometer (MZI) integrated with a microfluidic chip for ultrasensitive optical biosensing. The MZI is formed by patterning two parallel nanoslits in a thin metal film, and the sensor monitors the phase difference, induced by surface biomolecular adsorptions, between surface plasmon waves propagating on top and bottom surfaces of the metal film. The combination of a nanoplasmonic architecture and sensitive interferometric techniques in this compact sensing platform yields enhanced refractive index sensitivities greater than 3500 nm/RIU and record high sensing figures of merit exceeding 200 in the visible region, greatly surpassing those of previous plasmonic sensors and still hold potential for further improvement through optimization of the device structure. We demonstrate real-time, label-free, quantitative monitoring of streptavidin-biotin specific binding with high signal-to-noise ratio in this simple, ultrasensitive, and miniaturized plasmonic biosensor.

Vertical plasmonic Mach-Zehnder interferometer for sensitive optical sensing

Vertical plasmonic Mach-Zehnder Interferometers are investigated theoretically and experimentally, and their potential for ultra-sensitive optical sensing is discussed. Plasmonic interferences arise from coherently coupled pairs of subwavelength slits, illuminated by a broadband optical source, and this interference modulates the intensity of the far-field scattering spectrum. Experimental results, obtained using a simple experimental setup, are presented to validate theoretically predicted interferences introduced by the surface plasmon modes on top and bottom surfaces of a metal film. By observing the wavelength shift of the peaks or valleys of the interference pattern, this highly compact device has the potential to achieve a very high sensitivity relative to other nanoplasmonic architectures reported.

Interferometric fluorescent super-resolution microscopy resolves 3D cellular ultrastructure

Understanding molecular-scale architecture of cells requires determination of 3D locations of specific proteins with accuracy matching their nanometer-length scale. Existing electron and light microscopy techniques are limited either in molecular specificity or resolution. Here, we introduce interferometric photoactivated localization microscopy (iPALM), the combination of photoactivated localization microscopy with single-photon, simultaneous multiphase interferometry that provides sub-20-nm 3D protein localization with optimal molecular specificity. We demonstrate measurement of the 25-nm microtubule diameter, resolve the dorsal and ventral plasma membranes, and visualize the arrangement of integrin receptors within endoplasmic reticulum and adhesion complexes, 3D protein organization previously resolved only by electron microscopy. iPALM thus closes the gap between electron tomography and light microscopy, enabling both molecular specification and resolution of cellular nanoarchitecture.