Plasmonic Nanostructures for Color-Sensitive Biosensors

Abstract

Plasmonic sensors are highly sensitive to the electrical characteristics of their surrounding dielectric and recently received great attention as they can be applied to spectrometer-free biosensing. Many biomolecules and biomaterials are transparent or colorless, so that it is hard to distinguish among them using the optical microscopic system. However, they induce the difference in the optoelectrical properties such as a refractive index or a dielectric constant and then the changes cause the spectral shift of plasmonic nanohole arrays. In particular, the sensor with color factor analysis is considered as a powerful tool due to its simplified sensing process and various applications. To develop the color-sensitive plasmonic sensor, nanostructures have been designed, fabricated, and evaluated to understand the sensing parameters and the mechanisms precisely. Based on the study of spectral sensing system, spectrometer-free and label-free biosensing platform has been suggested for simplicity and cost-efficiency. Several plasmonic structures, subwavelength slits, rods, and holes are fabricated to observe the extraordinary transmission and absorption. It is studied which one is the most appropriate structure to be applied for the detection of refractive index and transparent material. For this, their optical properties are measured and evaluated. Geometric effects of nanohole arrays are investigated after the selection. The plasmonic structures are designed to have a resonant wavelength in the visible light region. Contribution of geometric parameters such as the diameter of each hole and the spacing between adjacent nanoholes is evaluated to maximize the spectral change among different biomolecules. A larger diameter and spacing enabled the biomolecules to be easily distinguished. Spectral shift is relatively larger by 1 and 3 nm as the spacing and the diameter are increased by 80 and 40 nm, respectively. In addition to the analysis with spectral shift, it is confirmed that color changes could be utilized the sensing factors as the nucleus and the cytosol of human embryonic kidney-293 cells are distinguished by Hue and CIELab color spaces. A color-sensitive and spectrometer-free plasmonic sensor using nanohole arrays and the color components of the CIELab color space is developed for the analysis of optically transparent materials in the visible light region. Typical detection method based on spectral shifts and changes of plasmonic structures can be applied to real-time bio-detection, complex optical instrumentations and low spatial resolution limit the sensing ability. Color imaging process instead of spectral analysis provides advantages for prediction of the refractive index by collecting and calculating distinctive color information of each pixel in transmission images with transparent materials. To establish the sensing system, only a lens, an image sensor, and a JAVA program are needed. Study on the correlation between the spacing of plasmonic structures and the color sensitivity to the refractive index suggests optimal geometric conditions of the structures. The weighted mean calculation offers an improved prediction results for color-sensitive detection. As a result, a color sensitivity up to 156.94 RIU-1 and a minimum mean absolute error of 1.298×10-4 RIU are achieved. Improvement in the color sensitivity is demonstrated as employing a nanohole array-silicon dioxide-nanohole array structure. The coupling of surface plasmon and Fabry-Perot resonances in nanohole arrays in top and bottom aluminum layers and silicon dioxide cavity induce the synergistic and attenuated changes in transmission spectra and characteristic colors. The Y-factor of CIEXYZ is only common variable in L*, a*, and b* of the CIELab color space. As optimizing the coupling of both resonances to increase the change in the Y-factor, it is possible to enhance the color sensitivity and then obtain more accurate prediction results of the refractive index. An aluminum-silicon dioxide-aluminum structure shows the color sensitivity up to 122.29 (L*), 169.58 (a*), and 174.72 (b*) RIU-1 with imaging process and statistical analysis. Moreover, the sensor can distinguish the difference of bovine serum albumin concentration with a limit of detection of 9×10-4 RIU. Finally, human embryonic kidney-293 cells are analyzed by the sensor structure and the imaging process. The nucleus, nucleolus, and endoplasmic reticulum of cells could be observed and defined. In this Theses, the study of details in plasmonic sensor system confirms great potential of color-sensitive plasmonic structures as superior biosensors. This unique sensing platform is expected to be utilized as a spectrometer-free, label-free, and functionalization-free detection of the refractive index, concentration, type of various materials.