Selective and sensitive colorimetric detection of the neurotransmitter serotonin based on the aggregation of bifunctionalised gold nanoparticles

We report a simple, sensitive and selective method for the colorimetric detection of serotonin (5-HT) in aqueous media using bifunctionalized gold nanoparticles (AuNPs). The probe (1) consisted of AuNPs functionalised with dithiobis(succinimidylpropionate) (DSP) and N-acetyl-L-cysteine (NALC). DSP was chosen to react with the amino group of 5-HT, whereas NALC was chosen to bind the hydroxyl group in 5-HT through hydrogen bonding and electrostatic interactions. A double interaction between nanoparticles and the hydroxyl and the amino group of serotonin led to interparticle-crosslinking aggregation. This, resulted in a colour change from red to blue that can be observed by the naked eye. The probe was selective to 5-HT and no colour modulation was observed in the presence of other neurotransmitters (i.e. dopamine, epinephrine, norepinephrine), selected biomolecules (i.e. L-tyrosine, gamma-aminobutyric acid, L-cysteine, uric acid, oxalic acid, aspartic acid and glutamic acid) and common inorganic species. A limit of detection as low as 0.1 μM was determined in buffered water at pH 7 by UV–vis titrations. Similar response of the probe to 5-HT was observed in simulated blood serum, with a limit of detection of 0.12 μM, and a linear response within the 0–3 μM concentration range, which is within the range of the 5-HT concentrations of clinical interest. Finally, the performance of probe (1) in real human blood samples was evaluated, and showed a remarkable ability to distinguish between normal 5-HT levels and 5-HT levels indicative of disease.