Fluorescence and electrochemiluminescence of luminol-reduced gold nanoparticles: photostability and platform effect

Water-soluble gold nanoparticles (AuNPs) capped with both fluorescent (FL) 3-aminophthalate (APA) and electrochemiluminescent (ECL) luminol molecules were described in our previous work. The synthetic and characteristic efforts of these functionalized AuNPs (lumAuNPs) were subsequently followed by investigations of their FL and ECL properties, as reported in the present work. It was observed that the FL intensity of a single gold nanoparticle was 70 times brighter than that of one free APA molecule, even though 91% of the FL emission of APA molecules on the surfaces of AuNPs was inhibited by gold cores through both intra- and interparticle quenching effects. Moreover, the photobleaching of surface-bound APA molecules was found to be dramatically inhibited compared with that of free ones in carbonate buffer. The improvement of photostability was attributed to the reactive AuNPs which acted as radical scavengers to protect the surface-bound APA molecules from oxidation by carbonate radicals. Furthermore, as-prepared lumAuNPs could react with cysteine to produce strong electrochemiluminescence, which was enhanced by 20-fold compared with that in the absence of cysteine. The experimental results suggested that luminol and cysteine were coadsorbed on the gold nanoparticle platform via Au-N and Au-S interactions, respectively. The shorter distance between reactant molecules by overcoming the electrostatic repulsion, that is, platform effect, was proposed to be responsible for the ECL enhancement. Combined with the biocompatibility of gold cores, the brighter FL emission, enhanced photostability, and stronger ECL intensity may make as-prepared lumAuNPs promising FL and ECL biomarkers for their applications in biosensors and bioimaging.