Optical Characteristics of Plasmonic Nanoparticles and Its Application to Colorimetric Imaging of Histone in Senescence Cell

Plasmonic nanoparticles (e.g., silver and gold nanoparticles) exhibit unique optical characteristics. When these plasmonic nanoparticles are placed close enough, the plasmonic coupling effect occurs, which makes the scattering spectrum shift to a longer wavelength. In addition, the spectral shift of scattering light differs according to the arrangements, inter-particle distances, and materials of the plasmonic nanoparticles. Due to their unique optical characteristics, they are utilized in various applications, from sensors to imaging methods. For example, these plasmonic nanoparticles can be used as a ruler by utilizing their distance-dependent scattering spectra.<br/>Histone is a protein at chromatin in a nuclear of cell. The histone has a tail and various histone modifications are at the tail. Its numerous combinations of modifications are called histone code. This histone code is determined by reader and eraser enzymes. The determination of enzymes is affected by various causes including senescence of a cell or diseases and it affects the topology of the chromatin. For example, Heterochromatin Protein 1 (HP1), a reader protein, identifies and combines with the H3K9me3 histone modifications. H3, K9, and me3 represent histone 3, 9th lysine, and trimethylation, respectively. HP1 concentrates together, making chromatin compact and it is called heterochromatin. The interaction between HP1 and another protein, lamin A, makes the heterochromatin to be located near the nuclear membrane. However, as a cell becomes senescence, H3K9me3 tends to concentrate inside the nuclear and create sites where the histones are closely packed. It is called senescence-associated heterochromatin foci (SAHF). To date, various histone modifications are found and it is revealed that some of them are closely related to diseases like cancer. Thus, understanding the relationship between histone modifications and topological change of chromatin during a cell’s senescence becomes significant because of the relation between the distribution of histone modifications and the diseases.<br/>The conventional method for imaging the distribution of the histone modifications, using organic fluorescence dye for example, has encountered limits including poor resolution and blurred images. Furthermore, the distribution of the histone modifications cannot be distinguished when organic dyes are at a close distance. Therefore, plasmonic nanoparticles are implemented in this study by utilizing their unique optical characteristics. Plasmonic nanoparticles are used as a probe for imaging the distribution of the histone modifications for the first time to overcome the drawbacks of the conventional methods.<br/>In this work, the scattering spectra are calculated with various arrangements and inter-particle distances of plasmonic nanoparticles. Some arrangements showed unique scattering spectra at certain inter-particle distances. These spectra were distinctive since they exhibited additional dominant scattering peaks (i.e., double peaks) while other spectra have only a single peak. Furthermore, the unique spectra are more often found when silver and gold nanoparticles are placed together. Dominant peaks are found even at the shorter wavelength owing to the silver nanoparticles. The combinatory peaks of scattering spectra make each scattering spectra distinguishable. Not only the distinctive scattering spectra but also a broader redshift of scattering spectra exhibited when silver and gold nanoparticles are placed together. Based on the simulation results, the distributions of the histone modifications in a cell were predicted. The distributions predicted were ranging from 2D to 3D arrangements with inter-particle distances of from 1 nm to 7 nm. We believe that this novel visualization method would be useful for monitoring the progress of diseases and also further understanding of the optical characteristics of plasmonic nanoparticles would lead us to advanced optical applications.