In this article, we report a simple approach for the selective sensing of homocysteine thiolactone (HTL) using fluorosurfactant (FSN)-capped gold nanoparticles (AuNPs) as aminothiol removers and as sensors. We have shown that HTL did not bind to the surface of the FSN-AuNPs in the pH range of 4.0-10.0. In contrast, under these pH conditions, the FSN-AuNPs are aggregated upon the addition of homocysteine (HCys) and cysteine (Cys). On the basis of this feature, we have demonstrated that FSN-AuNPs are effective sorbent materials for HCys and Cys, but not for HTL. It is found that the FSN-AuNPs can remove of >98% of HCys and >99% of Cys from an aqueous solution. Thus, after the centrifugation of a solution containing AuNPs, HTL, and other aminothiols, only HTL remains in the supernatant. When NaOH is added to the supernatant, HTL is hydrolyzed to HCys, leading to the aggregation of the FSN-AuNPs. As a result, the selectivity of the probe is significantly higher for HTL in aqueous solutions than for other aminthiols. The sensitivity of FSN-AuNPs toward HTL can be further improved by optimizing the AuNP concentrations. Under optimum conditions, the lowest detectable concentration of HTL through this approach is 100 nM. We have validated the applicability of our method through the analyses of HTL in urine samples.
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.
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