Plasmon-enhanced spectroscopy on homogeneous nanostructures

Abstract

Plasmon-enhanced spectroscopy has been of great interest in many disciplines including physics, chemistry, biology, and materials science. Numerous experimental methods based on surface plasmon resonance (SPR) of metal nanoparticles have been developed for spectroscopic, biosensing, and imaging purposes, where large enhancements in the optical signals of Raman scattering, infrared absorption, and fluorescence are required. Strong local electric fields of metal nanoparticles induced by SPR excitation are inevitable in the large signal enhancements, which can be optimized in general by the shape, size, or composition of the metal nanoparticles. Fluorescence spectroscopy has been widely used in many research fields, where higher sensitivity is generally required to lower the detection limits to single molecule level, for example, and higher photostability is required for longer measurement times in biosensing and trace analysis of bioactive components in cells. However, it is considered challenging to find fluorophores with both high sensitivity and photostability. Alexa Fluor dyes widely used in biological imaging due to their high photostability and selective binding capability to biological active sites show relatively low quantum yields in aqueous solutions. Raman spectroscopy has also been adopted in numerous chemical and biological sensing as a non-invasive and non-destructive analysis technique. However, powerful structure-specific chemical information of Raman scattering in a wide vibrational frequency range is mostly limited by the infinitesimal Raman cross-sections which is about 106 times smaller than those of fluorescence signals. Thus, enormous signal enhancements by the SPRs of metal nanostructures may be inevitable for the applications of Raman spectroscopy with the low detection limits or biological samples of very low concentrations. Metal-enhanced fluorescence (MEF) and surface-enhanced Raman scattering (SERS) have been considered indispensable for the wide application of fluorescence and Raman spectroscopy, respectively, in overcoming the disadvantages of these spectroscopic methods including the signal sensitivity, photostability, etc. Fluorescence of chromophore is significantly enhanced when it is located near metal nanoparticles or nanosurfaces due to locally enlarged electric fields around the metal nanoparticles and plasmon-coupled emission accompanying the energy transfer between the SPR of metal nanoparticles and the excited states of chromophore. The photostability of chromophore increases with the reduced photobleaching or shortened emission lifetime. Similarly, Raman cross-sections of analytes can be enormously enhanced up to 1014–1015 times when located near metal nanoparticles. Locally enlarged electric fields of metal nanoparticles and charge transfer effects of surface adsorbates are generally considered as the origin of enormous signal enhancements. Raman measurements at single particle or molecular levels have been reported with highly optimized metal nanostructures. Moreover, vibrational probe of SERS measurements conveys local chemical information of site-specific surface adsorption of adsorbates on metal nanosurfaces, which is often considered indispensable in chemical and biological analysis of surface adsorbates. In this dissertation, the MEF and emission kinetics of dye molecules are studied with homogeneous metal nanosubstrates with well-controlled particle diameters and well-defined SPR bands by steady-state and time-resolved fluorescence spectroscopy such as time-correlated single photon counting (TCSPC). Optimal conditions for the MEF with the homogeneous metal nanosurfaces are investigated by changing the particle diameters of the metal nanoparticles or by mixing different-sized metal nanoparticles in various composition ratios to obtain insights into the development of more efficient fluorescent probes for bioimaging and sensing. The SERS spectra and surface adsorption geometry of aromatic amino acids and short peptides depending on the local environments and surface properties of metal nanoparticles are investigated to understand those of biological macromolecules such as proteins, DNA, and RNA composed of the basic units, which can be applied in biological applications of SERS. The theoretical basis of MEF and SERS, and synthesis methods for metal colloidal solutions and homogeneous and composite metal colloidal nanosurfaces, and details of TCSPC and micro-Raman setup for fluorescence and Raman measurements are introduced in Chapter 1. In Chapter 2 and 3, the MEF and emission kinetics changes of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), rhodamine 700 (Rh700), 4-dimethylamino-4′-nitrobiphenyl (DNBP), and coumarin 343 (C343) on homogeneous silver nanosurfaces composed of a specific diameter of 60–220 nm are discussed. An improved semi-empirical model based on the finite-difference time-domain (FDTD) simulation and emission kinetics measurement results is introduced to analyze the mechanism of the MEF by the dipole and quadrupole SPR of the homogeneous silver nanosurfaces. In Chapter 4, the fluorescence enhancement and emission kinetics of DNBP with the composite silver nanosurfaces showing the dipole SPR of small silver nanoparticles and the quadrupole SPR of large silver nanoparticles in similar wavelengths to the emission of DNBP are compared with those with the silver nanosurfaces composed of only small or large silver nanoparticles. Based on the results in the Chapter 2–4, the optimal conditions for strong MEF with the homogeneous metal nanosurfaces are discussed. In Chapter 5, the differences in the pH-dependent SERS spectra and surface adsorption geometry of L-alanyl-L-tryptophan (Ala-Trp) with citrate-reduced (CT) and borohydride-reduced (BH) gold colloidal nanoparticles are discussed. The difference in the surface geometry of Ala-Trp on the CT and BH gold nanosurfaces are explored in relation to the surface zeta potential change of the gold colloidal nanoparticles depending on pH. In Chapter 6, the surface modifications of CT gold nanoparticles by using cetyltrimethylammonium bromide (CTAB) surfactant as an aggregation agent are introduced to increase the SERS activity of several hydroxyanthraquinones (HAQs). In Chapter 7, the changes in the surface adsorption geometry of L-tyrosine (L-Tyr) and short peptides containing L-Tyr such as Gly-L-Tyr depending on pH are discussed based on the pH-dependent SERS spectra with BH and CT silver colloidal nanoparticles. Vibrational assignments of L-Tyr for major Raman bands are corrected based on the comparison of pH-dependent SERS spectra of L-Tyr with those of tyramine (TRM) and 3-(4-hydroxyphenyl)propionic acid (HPPA) and the density functional theory (DFT) calculation results. The pH-dependent SERS spectra and surface adsorption geometry of a short peptide, glycyl-L-tyrosine (Gly-L-Tyr) are compared with those of its local structure L-Tyr.