¤@¡BRole of 5-thio-(2-nitrobenzoic acid)-capped gold nanoparticles in the sensing of chromium(VI): remover and sensor This study describes a simple, rapid method for sensing Cr(VI) using 5-thio-(2-nitrobenzoic acid) modified gold nanoparticles (TNBA-AuNPs) as a remover for Cr(III) and as a sensor for Cr(VI). We discovered that TNBA-AuNPs were dispersed in the presence of Cr(VI), whereas Cr(III) induced the aggregation of TNBA-AuNPs. Due to this phenomenon, TNBA-AuNPs can be used as a sorbent material for the removal of > 90% Cr(III), without removing Cr(VI). After centrifuging a solution containing Cr(III), Cr(VI), and TNBA-AuNPs, Cr(III) and Cr(VI) were separately present in the precipitate and supernatant. In other words, TNBA-AuNPs are capable of separating a mixture of Cr(III) and Cr(VI). The addition of ascorbic acid to the supernatant resulted in a reduction of Cr(VI) to Cr(III), driving the aggregation of TNBA-AuNPs. The selectivity of this approach is more than 1000-fold for Cr(VI) over other metal ions. The minimum detectable concentration of Cr(VI) was 1 £gM using this approach. Inductively coupled plasma mass spectrometry provided an alternative for the quantification of Cr(III) and Cr(VI) after a mixture of Cr(III) and Cr(VI) had been separated by TNBA-AuNPs. The applicability of this approach was validated through the analysis of Cr(VI) in drinking and tap water. ¤G¡BFluorescent Sensing of Total, Protein-bound, Free, and Oxidized Homocysteine in Plasma through the Combination of Tris(2-carboxyethyl)Phosphine Reduction, Fluorosurfactant-Capped Gold Nanoparticles Extraction, and o-Phthaldialdehyde Derivatization This study reports a simple, selective, and sensitive method for fluorescent detection of total, protein-bound, free, and oxidized homocysteine (HCys) using tris(2-carboxyethyl)phosphine (TCEP) as a reducing agent, fluorosurfactant-capped gold nanoparticles (FSN-AuNP) as a preconcentrating probe, and o-Phthaldialdehyde (OPA) as a derivatizing agent. TCEP was used to reduce the disulfide bonds of protein-bound and oxidized HCys. FSN-AuNPs were capable of extracting HCys from a complicated complex because the FSN capping layer can stabilize the AuNPs in a high-salt solution and inhibit non-specific adsorption. HCys was selectively derivatized with OPA in the absence of a nucleophile. By taking advantage of these features, the selectivity of the proposed system is greater than 100-fold for HCys and homocystine (HCys-HCys disulfide; diHCys) compared to any aminothiols. The limits of detection (LODs) for HCys and diHCys were 4.4 and 4.6 nM, respectively. Compared to other sensors, the proposed system provides an approximately 3-300-fold improvement in the detection of HCys. Different forms of plasma HCys were determined by varying the order of disulfide reduction with TCEP. The proposed system was successfully applied to determine the total, protein-bound, free, and oxidized HCys in plasma. To the best of our knowledge, the proposed system not only provides the first method for detecting various forms of plasma HCys, but also has the lowest LOD value for HCys when compared to other sensors.