Abstract
We present a fast, wide-field holography system for detecting photothermally excited gold nanospheres with combined quantitative phase imaging. An interferometric photothermal optical lock-in approach (POLI) is shown to improve SNR for detecting nanoparticles (NPs) on multiple substrates, including a monolayer of NPs on a silanized coverslip, and NPs bound to live cells. Furthermore, the set up allowed for co-registered quantitative phase imaging (QPI) to be acquired in an off-axis holographic set-up. An SNR of 103 was obtained for NP-tagging of epidermal growth factor receptor (EGFR) in live cells with a 3 second acquisition, while an SNR of 47 was seen for 20 ms acquisition. An analysis of improvements in SNR due to averaging multiple frames is presented, which suggest that residual photothermal signal can be a limiting factor. The combination of techniques allows for high resolution imaging of cell structure via QPI with the ability to identify receptor expression via POLI.
Highlights
Due to their unique optical properties, plasmonic nanoparticles (NPs) have been utilized as labels for molecularly specific cell imaging
An optical lockin photothermal approach was combined with optical coherence microscopy (OCM) [5], producing 34dB signal to noise ratio (SNR) but requiring up to 5 seconds for image acquisition
OCM operates in a backscattering configuration where the signal scales as the refractive index change (n) while in transmission, the approach used here, the signal scales as the refractive index relative to the background (n-1)
Summary
Due to their unique optical properties, plasmonic nanoparticles (NPs) have been utilized as labels for molecularly specific cell imaging. Multiple optical techniques have been established to detect NPs on multiple substrates [2] These include, among others, dark-field microscopy [3], which takes advantage of enhanced NP scattering at their plasmonic resonance wavelength, and photothermal microscopy, which visualizes refractive index changes in the sample media due to localized NP heating [4,5,6,7,8,9]. One limitation of these approaches is that cellular structures are typically not visible.
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