A range of fluorescein derivatives were synthesised via Pd0-mediated cross-coupling chemistry of the mono triflate of fluorescein with a variety of boronic acids and the optical properties of each dye was studied. Among these derivatives, a new multicolour pH–dependant anthofluorescein which was highly sensitive to pH changes and viscosity changes was identified. Work was carried out to explore intracellular catalysis based on the immobilisation of Pd0 nanoparticles on microspheres. The entrapped Pd0 nanoparticles were rapidly taken up by cells, stay harmlessly within the cytoplasm for days and were shown to carry out novel cell-based chemistry. This included an allylcarbamate cleavage and a Suzuki-Miyaura cross-coupling reaction for the in situ generation of a mitochondria-localized “switch-on” fluorophore.