Ultrasensitive visual and colorimetric determination of dopamine based on the prevention of etching of silver nanoprisms by chloride

The authors describe a selective and sensitive colorimetric method for determination of dopamine (DA) in serum. It is based on the protection of silver nanoprisms (AgNPRs) from being etched by chloride ion in the presence of DA. In the absence of DA, the shape of the AgNPRs is changed from triangle nanoplates to round nanodisk due to etching by chloride. This is accompanied by a stepwise color change from blue via purple and red to yellow. It is found that DA is strongly adsorbed on the surface of AgNPRs and thereby acts as a protective agent. As a result, etching by chloride is prevented and the color changes do not occur. This finding ease exploited to design a method for optical quantification of DA with either bare eyes or UV-vis spectrophotometry. The wavelength shift of the in-plane dipole surface plasmon band of the AgNPRs is linearly related to the DA concentration in the range from 0.5 to 100 nM. The detection limit is 0.16 nM (at an S/N ratio of 3) which is lower than that of most existing methods. Uric acid, ascorbic acid and other coexisting species do not interfere. The sensor is reproducible and stable and was applied to the determination of DA in spiked serum samples where it gave recoveries that ranged from 97.4 to 104.3%.