Visual detection of arginine based on the unique guanidino group-induced aggregation of gold nanoparticles

A simple, cost-effective and rapid method for visual detection of arginine based on the citrate-capped gold nanoparticles (AuNPs) aggregation has been developed in this paper. Arginine is the only amino acid with guanidino group, and has the highest isoelectric point (pI) at about 10.8. At pH 9.62, negatively charged citrate-capped AuNPs are well dispersed because of strong electrostatic repulsion. However, positively charged arginine (pH < pI) easily induces negatively charged citrate-capped AuNPs aggregation through electrostatic and hydrogen-bonding interactions, resulting in a red to blue color change of the solution. Using a UV–vis spectrophotometer, the method enables the detection of arginine in the range of 0.08–13.2 μM with a detection limit (3σ/slope) of 16 nM. Particularly, as low as 0.4 μM arginine can be easily detected by the naked eye without using any complicated or expensive instruments. Furthermore, this method can provide satisfactory results for the determination of arginine in arginine injection and compound amino acid injection samples.