Abstract: The invention relates to methods for producing gold nanoparticles functionalised with a telomerase substrate, or a linker nucleic acid having a region complementary to a nucleic acid comprising a telomerase substrate, for use in detecting telomerase activity. Functionalised gold nanoparticle solutions and functionalised gold nanoparticles obtained using the method are also provided for. The invention further relates to a functionalised gold nanoparticle complex, comprising gold nanoparticles coupled to a linker nucleic acid which is hybridised to a nucleic acid comprising a telomerase substrate. Telomerase assays using the functionalised gold nanoparticles or functionalised gold nanoparticle complexes of the invention and kits for detecting telomerase activity in a cell, comprising the functionalised gold nanoparticles or functionalised gold nanoparticle complexes of the invention are also provided for.