Upconversion photoluminescent metal ion sensors via two photon absorption in graphene oxide quantum dots

In-depth study on upconversion photoluminescence (UCPL) of graphene oxide quantum dots (GOQDs) was presented. Uniform GOQDs with a single-layer and a circular shape within 4 nm diameter were fabricated by acidic exfoliation from the graphite nanoparticles. The green UCPL of the GOQDs at ∼563 nm was observed by a femto-second (fs) Ti:sapphire pulsed laser (λex: 800 nm), and the mechanism of the observed UCPL was systemically characterized. We further utilized the UCPL of GOQD as an optical biosensor for detecting aptamer-based metal ions with high sensitivity and selectivity, and verified that the fluorescence quenching of the UCPL GOQDs was mediated by the electron transfer between the GOQD and the captured metal ion rather than fluorescence energy transfer. The understanding of the UCPL of graphene quantum dots would enhance their applicability in the biosensing and bioimaging research fields.