Highly selective detection of histidine using o-phthaldialdehyde derivatization after the removal of aminothiols through Tween 20-capped gold nanoparticles

In this paper, we describe a simple and sensitive method for the selective detection of histidine by combining Tween 20-capped gold nanoparticles (Tween 20-AuNPs) as aminothiol removers and o-phthaldialdehyde (OPA) as the derivatization reagent. We have shown that Tween 20-AuNPs are capable of removing homocysteine (HCys), glutathione (GSH), and gamma-glutamylcysteine (Glu-Cys) at low pH conditions, but they are ineffective in the case of removal of histidine. In contrast, at high pH, Tween 20-AuNPs have strong hydrophobic interactions with the unprotonated imidazole group of histidine. It is observed that 48.0 nM Tween 20-AuNPs can remove 95.7% of HCys, 99.7% of GSH, and 99.5% of Glu-Cys from 40 mM phosphate solution at pH 2.0 in the presence of 0.1 mM cetyltrimethylammonium bromide, whereas only 2.1% of histidine was removed under identical conditions. In addition, OPA is a highly selective fluorogenic reagent for GSH, HCys, Glu-Cys, and histidine. Thus, after the centrifugation of a solution containing Tween 20-AuNPs, histidine, HCys GSH, Glu-Cys, and other amino acids, the selectivity of this method is remarkably high for histidine through OPA derivatization. Under optimum derivatization conditions, the lowest detectable concentration of histidine detected with this method was 5.2 nM. This method has been successfully applied to detect the presence of histidine in urine and serum samples.