Cationic fluorophores are prepared from their respective radical species, after release of an electron, and are utilized for amyloid beta detection after being conjugated with citrate-stabilized gold nanoparticles. The turn-on fluorescence mechanism is used for detect amyloid beta monomer in nanomolar concentration from 10 to 130 nM.

Gold nanoparticle-based fluorogenic enhancement for detection of analytes requires a strategy to quench the fluorescence property of the fluorophore effectively before the treatment of analytes. The designing principle of the fluorophore for gold nanoparticle surface coverage is challenging in comparison with chemical modification of a small organic fluorophore for similar quenching. In this report, the ionic interactions between negatively charged gold nanoparticle and cationic fluorophores have been considered . The interaction between two ionic systems is much more effective than the interaction between charged gold nanoparticle and neutral fluorophore for fluorescence quenching. This simple and sensitive strategy for selective nanomolar detection of monomeric amyloid beta (Aβ) has been demonstrated using the combination of citrate-stabilized gold nanoparticles (AuNP) and cationic fluorophores (1 and 2). The cationic fluorophores have been generated in aqueous medium from their corresponding radicals by releasing electron. The prompt turn on fluorescence response of 1 has found to be superior to the similar response from 2. The response is suitable for the fluorogenic detection of Aβ monomer within the concentration range 10–130 nM. Further, both the cationic probes have further been utilized successfully to detect monomeric Aβ level in artificial cerebrospinal fluid and human serum albumin.