Wide-field interferometric phase imaging of plasmonic nanoparticles at the subcellular level

We present a new wide-field quantitative photothermal (PT) imaging method of gold nanoparticles (AuNPs), which is suitable for obtaining wide-field holographic molecular specificity in biological samples. To obtain this goal, we built a wide-field interferometric phase microscope and modified it for the excitation of plasmonic resonance in AuNPs, while recording their resultant phase signatures. To check the potential of the AuNPs as interferometric cellular labels, they were conjugated to a glass coverslip and excited with a laser at a wavelength corresponding to their absorption spectral peak. We then acquired an image sequence of the sample phase profile in time without the need for lateral scanning, and analyzed the entire field of view using a Fourier analysis, creating a map of the locations of the AuNPs. We obtained a strong PT signal at AuNPs central locations, exponentially dependent on the distance from their centers. This enabled identification of the central locations of the AuNPs in the chosen field of view. Moreover, these PT signals had shown a linear relation to the illumination intensity, distinguishing them from background noise and out-of-focus particles. To the best of our knowledge, we are the first to record wide-field interferometric PT signals at the subcellular level without the need of total-internal-reflection prisms or scanning.