Dual-modal SERS/fluorescence AuNP probe for mitochondrial imaging

A novel SERS/fluorescent multimodal imaging probe for mitochondria has been synthesised using 12 nm diameter gold nanoparticles (AuNP) surface functionalised with a rhodamine thiol derivative ligand. The normal pH dependant acidic fluorescence of the rhodamine based ligand is inversed when conjugated with the AuNP and higher emission intensity is observed at basic pH. This switch correlates to a pKa at pH 6.62, which makes it an ideal candidate for a pH sensitive imaging probe in the biological range (6.5-7.4). The observed pH sensitivity when attached to the AuNP is thought to be due to the formation of a spirolactam ring on the ligand, going from positively charged (+18 mV) to negatively charged (-60 mV) as the pH is changed from acidic to basic. Additionally, conjugation of the ligand to the AuNP serves to enhance the Raman signal of the rhodamine ligand through Surface Enhanced Raman Scattering (SERS). Confocal microscopy has shown that the probe enters HEK293 (kidney), A2780 (ovarian cancer) and Min6 (pancreatic beta) cells within an hour and a half incubation time. The probe was shown to localise in the mitochondria, thus providing a novel pH dependent SERS/fluorescent multimodal imaging probe for mitochondria