Graphene-Based Analytical Platforms for Fluorescence and Raman Spectrometry

Abstract

Chapter 1 describes the general introduction of the thesis. The synthesis methods and properties of graphene are briefly introduced. One of the fascinating applications of graphene is Graphene-Enhanced Raman Spectroscopy (GERS), which is the phenomenon that the Raman signal of a molecule can be considerably enhanced on graphene, while background fluorescence is quenched due to ultrafast charge carrier mobility of graphene. However, the underlying mechanism of GERS and fluorescence quenching has not been fully understood yet. In Chapter 2.1, the Raman scattering properties of sandwich structures are investigated, revealing that the enhancement depends on whether the molecular geometries are planar or not. The planar dye molecules between two graphene layers follow the conventional Raman enhancement mechanism induced by the overlap of molecular absorbance and excitation laser wavelengths. On the other hand, abnormal enhancement of Raman signals was observed from graphene-sandwiched nonplanar dye molecules, which is believed to be originated from the synergistic resonance with new excited energy states created by quantum void space surrounding dye molecules. These results not only provide deeper understanding of the mechanism of GERS but also suggest the graphene-sandwiched resonance Raman Spectroscopy as a more sensitive and robust analytical platform for various chemical and biological studies. In Chapter 2.2, the single layer graphene by itself is capable of catalyzing the photoreduction of dye molecules, which has been revealed by graphene-enhanced Raman spectroscopy studies. The proposed mechanism involves the electron transfer from graphene to temporarily empty HOMO states of photoexcited dye molecules, which can be interpreted as ultrafast hole transfer from dyes to graphene. It is also confirmed that graphene-encapsulated nitrobenzene dyes show less photoreduction, implying that the ambient hydrogen molecules are the importance source of photoreduction into aniline dyes. The photocatalytic reactivity of graphene would find numerous energy and environmental applications in the future. In Chapter 3, enhancing the sensitivity and selectivity of graphene-based DNA sensor by surface passivation of graphene will be discussed. Graphene oxide (GO) efficiently quenches the florescence from fluorophore-labeled DNAs, which has been utilized for highly selective and sensitive sensing of DNA hybridization, because the single and double helix DNAs show different binding affinity to GOs. As a result, the introduction of complementary DNAs (c-DNAs) turns on the fluorescence signals as the probe-DNA/c-DNA double helix is detached from the GO surface, while addition of mismatched DNAs (m-DNAs) shows a slight change in the fluorescence signals. However, in actual experiments, it is often observed that the surface of GO is not fully covered with probe-DNAs, and the introduced c-DNAs or m-DNAs bind to the uncovered GO surface rather than interacting with probe-DNAs. This undesired binding considerably degrades the sensitivity and the selectivity of DNA sensing because the fluorescence intensity change is small with increasing DNA concentration. Thus, the passivation of uncovered GO surface is important to ensure the higher performance of the GO-based DNA sensors.