## 3 Results and Discussion  
### Mechanism of Biothi Detection  
The sensing mechanism of the biothiols was investigated by studying the color and spectral transitions of the AuCl4\({}^{-}\) and Au-CTAB complex solutions in the presence and absence of cysteine, which is the most representative species of biothiols in biological fluids. The UV-Vis spectra of CTAB showed no absorption band while the AuCl4\({}^{-}\) solutions exhibit two absorption bands at 220 and 290 nm due to the ligand-to-metal charge transfer (LMCT) band and the ligand field (LF) band of AuCl4\({}^{-}\), respectively [15]. When Cys was added into the solution, it formed a metal complex with the Au ions and the absorption of the Au-SH complex solution increased and red-shifted to 235 and 300 nm, respectively. On both occasions the colors were barely conceivable by the bare eye and the scanning imaging device (Figure 1b). When CTAB was mixed with AuCl4\({}^{-}\) ions, the color of the solution turned orange and the absorption red-shifted (as compared to Au solutions) to 260 and 405 nm (Figure 1c). These absorption bands can be explained on the basis of two phenomena: The first, which explains the appearance of an orange coloration in the Au-CTAB solution, is the formation of AuBr4\({}^{-}\), due to the fast, multi-step, ligand substitution reaction of Cl\({}^{-}\) from Br\({}^{-}\) for anions, released from the dissociation of CTAB [15; 17]. In an aqueous solution, Au species may be distributed among various hydroxyl-containing gold complexes as a function of the pH, in the general form of \((\text{AuCl}_{\text{x}}(\text{OH})_{\text{y}})^{-}\) (where x + y = 4). As the Br\({}^{-}\) anions released from the dissociation of CTAB are in large excess compared to AuCl4\({}^{-}\), we assume that all gold chloride complexes are transformed to gold bromide complexes so that the Au species  
Figure 1: Color and UV-Vis absorbance spectra of **(a)** the CTAB solution (10 mM); **(b)** 0.25 mM AuCl4\({}^{-}\) solutions in the absence and presence of 50 uM Cys; **(c)** 0.25 mM AuCl4\({}^{-}\) and 10 mM KBr solutions in the absence and presence of 50 uM Cys; **(d)** 0.25 mM AuCl4\({}^{-}\) and 10 mM CTAB solutions in the absence and presence of 50 uM Cys. will actually be present in various bromide/hydroxyl complexes in the general form of (AuBr\({}_{x}\)(OH)\({}_{y}\))\({}^{-}\) (x + y = 4). The UV-vis spectrum of the (AuBr\({}_{x}\)(OH)\({}_{y}\))\({}^{-}\) species, however, show a peak at 385 nm (Figure 1d), while the spectrum of the Au-CTAB complex is red-shifted to 405 nm. In addition, the color of the Au-CTAB solution is darker than that of the Au-KBr solution at equimolar concentration levels, possibly due to the high local concentration of (AuBr\({}_{x}\)(OH)\({}_{y}\))\({}^{-}\) species on the micelle surface. This observation is a strong indication that the (AuBr\({}_{x}\)(OH)\({}_{y}\))\({}^{-}\) anions are not free in the solution but incorporated into the Stern layer of the CTAB micelles, forming an ion-pair complex with the positive head groups of the CTAB. The formation of an orange precipitate in the Au-CTAB mixture after a few hours also supports the above reaction combined with the fact that no precipitate was formed in the Au-KBr solutions even after prolonged incubation (1 week). In the presence of Cys, the UV-Vis spectra of the Au-CTAB-cysteine solution blue-shifts to 395-400 nm, the absorbance intensity decreases, and the color of the solution turns yellow. This colorimetric and spectral change can be attributed to the formation of Au-SH complexes that disrupt the (AuBr\({}_{x}\)(OH)\({}_{y}\))\({}^{-}\)-CTA\({}^{+}\) complexes and release gold ions from the micelle surface. Other phenomena, such as the hydrolysis of Au, shall not have any effect since all experiments were performed in an acetate/acetic acid buffer (0.1 M, pH 6). In addition, no new peaks at 500-540 nm were observed, which excludes the possibility of Au reduction neither by CTAB nor by thiols and the formation of gold nanoparticles, (Figure S1).