Subdiffraction-limited plasmonic imaging with anisotropic metal nanoparticles.

We have developed a high-resolution nonfluorescent imaging method based on superlocalization of gold nanorods (AuNRs). By taking advantage of their anisotropic optical property of the plasmonic scattering of AuNRs, selective imaging of only a fraction of AuNRs can be achieved by rotating the sample relative to the linear polarized illumination under cross-polarization microscopy with a high NA objective. The AuNR positions obtained from a series of images could then be used to reconstruct the overall image. Two AuNRs with center-to-center distances of 80 nm were successfully resolved. This simple but deterministic super-resolution imaging technique can potentially be used to fingerprint optically anisotropic metal nanoparticles and their assemblies for labeling, sensing, and encryption applications.

Color difference amplification between gold nanoparticles in colorimetric analysis with actively controlled multiband illumination.

Spectral chemical sensing with digital color analysis by using consumer imaging devices could potentially revolutionize personalized healthcare. However, samples with small spectral variations often cannot be differentiated in color due to the nonlinearity of color appearance. In this study, we address this problem by exploiting the color image formation mechanism in digital photography. A close examination of the color image processing pipeline emphasizes that although the color can be represented digitally, it is still a reproducible subjective perception rather than a measurable physical property. That makes it possible to physically manage the color appearance of a nonradiative specimen through engineered illumination. By using scattering light imaging of gold nanoparticles (GNPs) as a model system, we demonstrated via simulation that enlarged color difference between spectrally close samples could be achieved with actively controlled illumination of multiple narrow-band light sources. Experimentally, darkfield imaging results indicate that color separation of single GNPs with various sizes can be significantly improved and the detection limit of GNP aggregation-based colorimetric assays can be much reduced when the conventional spectrally continuous white light was replaced with three independently intensity-controlled laser beams, even though the laser lines were uncorrelated with the LSPR maxima of the GNPs. With low-cost narrow-band light sources widely available today, this actively controlled illumination strategy could be utilized to replace the spectrometer in many spectral sensing applications.