Probing the intracellular refractive index and molecular interaction of gold nanoparticles in HeLa cells using single particle spectroscopy.

Background: We have introduced a novel method to quantify the intracellular refractive index (RI) of living cells and determine the molecular interaction of two interacting molecules using single particle spectroscopy. The advantages of this proposed technique over fluorescence-based imaging techniques is that it does not require any contrasting agent and it does not blink and bleach. Instead, our technique provides a non-destructive, non-invasive, high-resolution imaging of live cells.

Methods: To verify our technique, we initially tested our approach for a dielectric medium where gold nanoparticles (AuNPs) were embedded in a polyvinyl alcohol (PVA) matrix, which was then extended to the cellular environment. In the dielectric medium, we identified the single particle and dimer and determined the interparticle distance of AuNPs using confocal laser scattering microscopy. We also determined the single particle RI from dark-field scattering microscopy images, which was confirmed with Mie theory and finite-difference time-domain (FDTD) simulated results. The single particle spectroscopy and microscopy technique was then extended to determine the intracellular RI and biomolecular interaction inside living cells using hyperspectral imaging and dark-field scattering microscopy.

Results: The novelty of the paper lies in the demonstration of a direct and accurate method to probe the intracellular RI and molecular interaction focused on single particle analysis whereas previous demonstrations were based on AuNP ensembles. Optically acquired single particle and dimer images was verified by correlated SEM images also optical spectrum with analytical models and FDTD simulations for both the dielectric and cellular environment. We reported the interparticle distance of AuNPs inside HeLa cells and intracellular refractive index, which was also confirmed with Mie Theory and extensive FDTD simulations.

Conclusion: Moreover, we believe that our in-depth plasmonic NP-based alternate imaging technique will provide a new insight in monitoring cellular dynamics and tracking the targeted NPs within live cells, enabling us to use plasmonic NPs as an intracellular biosensor.