  Label-free observation of nanoparticles by far-field optical microscopy is
challenging because their ability to scatter or absorb light dramatically
diminishes with decreasing size. Surface plasmon resonance (SPR) and localized
surface plasmon resonance (LSPR) imaging have shown promises and respective
limitations. For instance, it is challenging to achieve diffraction-limited
resolution in SPR imaging; its refractive index sensitivity is not as localized
as desired. LSPR imaging is typically employed with dark-field microscopy on
sparse noble metal nanostructures, leading to low light throughput and
incomplete imaging. Here we demonstrate ultra near-field index modulated
PlAsmonic NanO-apeRture lAbel-free iMAging (PANORAMA) that addresses existing
issues for both SPR and LSPR imaging. PANORAMA produces diffraction-limited
lateral resolution with higher surface sensitivity compared to SPR. Its system
configuration is identical to a standard bright-field microscope using a
trans-illumination tungsten-halogen lamp instead of a laser or other
high-intensity light sources. Additionally, PANORAMA addresses the sparse
sampling issue in LSPR imaging by achieving dense sampling with a large imaging
fill factor. The bright-field approach provides much higher light throughput
compared to dark-field microscopy. Overall, our technique can provide a
panoramic view both laterally and longitudinally - overcoming the lack of
imaging depth for both SPR and LSPR imaging and the insufficient lateral
sampling for LSPR imaging. We have demonstrated that PANORAMA can size single
nanoparticle down to 25 nm, count individual nanoparticles in a cluster, and
dynamically monitor single nanoparticle approaching the plasmonic surface down
to the millisecond timescale. PANORAMA is potentially useful in single
biological nanoparticle analysis of exosomes and viruses.
