Silver nanoparticles modified with sulfanilic acid for one-step colorimetric and visual determination of histidine in serum

The authors demonstrate a single-step histidine assay that is based on the use of silver nanoparticles modified with sulfanilic acid (SAA-AgNPs). The presence of histidine leads to a visually detectable gradual color change from bright yellow via orange to purple. The effect is assumed to be mainly due to strong π-π stacking, electrostatic interaction, and hydrogen bonding between SAA and histidine. The assay has a 52.7 nM detection limit and works in the 0 to 3.5 μM concentration range. It is selective over other compounds when using appropriate masking agents. The method has been successfully applied to the colorimetric determination of histidine in (spiked) serum samples. Recoveries ranged between 97 % and 107 %, and relative standard deviations are <0.92 % (for n = 3). The method was also applied to detect polyhistidine-tagged cysteine (His-His-His-His-His-His-Cys) which can be quantified in concentrations down to 5.57 nM. This finding links the method to His-tag technology. Graphical Abstract A single-step colorimetric assay for histidine is developed using sulfanilic acid modified silver nanoparticles (SAA-AgNPs) via π stacking, electrostatic interaction, and hydrogen bonding resulting in the aggregation of the AgNPs and a color change from yellow to orange.