Thiol-reactive Atto-643 dye was commercially obtained from ATTO-Tech GmbH. Ultrapure water (resistance >18.2 MΩ.cm) purified by an ELGA Purelab classic UVF system, was used for all experiments and making buffers. Synthesis of 13 nm Gold Nanoparticles. Freshly prepared aqueous solution of gold (III) chloride trihydrate (1 mM, 400 mL) was placed in a 500 mL three-necked round-bottomed flask and the solution was then heated to reflux in a 130 °C oil bath under stirring. When the solution began to reflux, trisodium citrate solution (38 mM, 40 mL) was quickly added. The solution colour quickly turned from yellow to wine red in ∼1 min, indicating the formation of GNPs. The reaction was further refluxed under magnetic stirring for another 1 hr to ensure the reaction was complete. The GNP solution was then removed from the oil bath and was allowed to cool down to RT naturally under stirring. Then, the GNP solution was transferred to a clean glass container and stored at RT till use. This produced citrate stablished 13 nm GNPs (G13) with a mean diameter of ∼13 nm according to the TEM images.