Colorimetric and visual dopamine assay based on the use of gold nanorods

The authors describe an optical method for the determination of dopamine (DA). It is based on the use of gold nanorods (AuNRs) with an aspect ratio of 3.2 and an absorption maximum at 700 nm. The AuNRs were synthesized via a seed-mediated growth route. The addition of various DA concentrations to the aqueous AuNRs results in a color change bluish-purple to colorless. The addition of DA is accompanied by a linear decrease in absorbance, a blue-shift of the peak to 687 nm, and the formation of a new band peaking at 720 nm on increasing in the DA concentration to 100 mu M. It is found that DA causes an aggregation of the AuNRs through the formation of interparticle plasmon-linkages between DA molecules and AuNRs. This was confirmed by transmission electron microscopy. This new colorimetric probe for DA shows good selectivity for DA even in the presence of potentially interfering species. Several linear ranges are observed between the changes in absorption at around 687 and at around 720 nm and the DA concentration in the range between 100 nM and 10 mM. The limit of detection is 30 nM. The method was successfully employed to the determination of DA in spiked human urine samples.