Synthesis and application of two-photon induced photoluminescent graphene oxide quantum dots

Abstract

Recent researches on the graphene quantum dots (GQDs) and their derivatives have garnered an in-creasing attention due to their unique photoluminescent (PL) properties. The PL of the GQDs, however, is emitted upon irradiation by one-photon excitation using either UV or a blue light source with higher energy than the band-gap of them. Since the up-conversion or two-photon induced (TPI) PL with near-IR excitation has advantages in the biomolecular imaging and sensing due to minor autofluorescence background signals, deep tissue penetration depths, and low photodamage, it would be a quite interesting topic to investigate the two-photon induced PL phenomena of the GQDs and their derivatives. Although there are few reports showing that GQDs display up-conversion luminescence by monochromatic light using a xenon lamp as an excitation light source, it is still elusive and controversial as regards the reasons behind the observed luminescence from the GQDs. The aim of this thesis is to perform the in-depth characterization on the TPI PI graphene oxide quantum dots (GOQDs) by means of near-infrared two-photon excitation via a femto-second (fs) Ti:sapphire pulsed laser system, and to elucidate the phenomenon of two-photon induced green photoluminescent of GOQDs in comparsion with the one-photon induced PL. We further utilize the TPI PL GOQD as an optical biosensor for detecting DNAzyme-based metal ions with high sensitivity and selectivity, and verify that the fluorescence quenching of the TPI PL GOQDs was mediated by the electron transfer between the GOQD and the captured metal ion rather than fluorescence energy transfer. In Chapter 2, uniform graphene oxide quantum dots (GOQDs) were fabricated as a single-layer and circular shape within 4 nm diameter by acidic exfoliation from the graphite nanoparticles. The green lumines-cence of the GOQDs at ~563 nm was observed by a two-photon induced femto-second Ti:sapphire pulsed laser (λex: 800 nm), and the PL red shift (