Excitation and emission enhancement by using the plasmon mode formed on a plasmonic chip was studied with a microscope and micro-spectroscope. Surface plasmon resonance wavelengths were observed on one-dimensional (1D) and two-dimensional (2D) plasmonic chips by measuring reflection and transmission spectra, and they were assigned to the plasmon modes predicted by the theoretical resonance wavelengths. The excitation and emission enhancements were evaluated using the fluorescence intensity of yellow–green fluorescence particles. The 2D grating had plasmon modes of kgx45(2) (diagonal direction with m = 2) in addition to the fundamental mode of kgx(1) (direction of a square one side) in the visible range. In epifluorescence detection, the excitation enhancement factors of kgx(2) on the 1D and 2D chips were found to be 1.3–1.4, and the emission enhancement factor of kgx45(2) on the 2D chip was 1.5–1.8, although the emission enhancement was not found on the 1D chip. Moreover, enhancement factors for the other fluorophores were also studied. The emission enhancement factor of kgx(1) was shown to depend on the fluorescence quantum yield. The emission enhancement of 2D was 1.3-fold larger than that of 1D considering all azimuth components, and the 2D pattern was shown to be advantageous for bright fluorescence microscopic observation.
Breast cancer cells of MDA-MB-231 express various types of membrane proteins in the cell membrane. In this study, two types of membrane proteins in MDA-MB-231 cells were observed using a plasmonic chip with an epifluorescence microscope. The targeted membrane proteins were epithelial cell adhesion molecules (EpCAMs) and epidermal growth factor receptor (EGFR), and Alexa®488-EGFR antibody and allophycocyanin (APC)-labeled EpCAM antibody were applied to the fluorescent detection. The plasmonic chip used in this study is composed of a two-dimensional hole-array structure, which is expected to enhance the fluorescence at different resonance wavelengths due to two kinds of grating pitches in a square side and a diagonal direction. As a result of multi-color imaging, the enhancement factor of Alexa®488-EGFR and APC-EpCAM was 13 ± 2 and 12 ± 2 times greater on the plasmonic chip, respectively. The excited wavelength or emission wavelength of each fluorescent agent is due to consistency with plasmon resonance wavelength in the hole-arrayed chip. The multi-color fluorescence images of breast cancer cells were improved by the hole-arrayed plasmonic chip.
Modification of the photoluminescence (PL) behavior of a single colloidal quantum dot (QD) using plasmonic nanostructures has attracted considerable attention. Here, we attempted to control the PL behavior of a single CdSe/ZnS QD by using a one-dimensional plasmonic chip (1D-PC) with the simplest Ag grating pattern. By optimizing the distance between the single QD and 1D-PC, the PL intensity of the single QD was successfully enhanced 5 times with maintaining single-photon emission via enhancement of the excitation rate due to the electric field of the surface plasmon on the 1D-PC. On the other hand, control of the emission photon statistics, that is, single-photon emission and multiphoton emission, from the single QD was difficult because enhancement of the radiative rate of the single QD was not achieved by the 1D-PC. These results indicate that the 1D-PC is an attractive plasmonic nanostructure to obtain efficient single-photon emission which is indispensable for next-generation quantum information technologies.
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