## 4 Conclusions  
In this work, we have shown that the colorimetric reaction product of Au anions with the cationic surfactant CTAB can be quenched by thiols proportionally to the thiol concentration. Based on this observation, low molecular weight biothiols were determined in simulated and physiological biological fluids. The method afforded satisfactory recoveries, low detection limits in relation to biothiol levels in biofluids, and good reproducibility. The assay is simple to perform since the end-user just mixes the necessary reagents with the sample and uses a flatbed scanner as a simple and inexpensive microtiter photometric  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Measured (μM)** & **Spiked (μM)** & **Found (μM) a** ** & **RSD (\%, \(n\) = 3)** \\ \hline Simulated urine c  & 0 & 150.0 & 101.0 & 67.3 & 8.4 \\  & 0 & 250.0 & 190.5 & 76.2 & 6.3 \\ Simulated blood plasma c  & 0 & 150.0 & 142.0 & 94.6 & 5.9 \\  & 0 & 250.0 & 307.5 & 123.0 & 7.0 \\ Urine d  & 108 & 100.0 & 212.1 & 103.8 & 6.3 \\ Blood plasma d  & 201 & 100.0 & 331.1 & 115.0 & 5.8 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Application of the method to the analysis of real samples.  
detector. Importantly, all reagents are commercially available at low cost and are stable under normal conditions. These features render the method suitable for applications in low-resource settings, such as decentralized or remote healthcare units (e.g., rural), and for in-clinic analysis (e.g., nurse's bench) where there is access to basic equipment and infrastructure capabilities. The experimental procedure can be further simplified by using the appropriate filters to remove red blood cells and plasma proteins, alleviating the need for centrifugation. In this manner, the method can be applied even in non-laboratory conditions, i.e., where there is a lack benchtop instrumentation (such as centrifuges) and trained operators.  
The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/6/2705/sl, Figure S1: Absorbance spectra of Au-CTAB complex (black line) and Au-CTAB in the presence of 50 uM of glutathione. No peaks above 500 nm are observed suggesting that gold has not been reduced to its respective gold nanoparticle species under the optimum experimental conditions (0.25 mM AuCl4, 10 mM CTAB, 50 uM GSH, sodium acetate/acetic acid buffer pH 6, 15 min incubation time at room temperature); Figure S2: Response of the colorimetric assay in various artificial biofluids. The linear curves are the result of linear regression while error bar represent the standard error calculated for triplicate samples. AUS: Artificial urine solution, ABP: artificial blood plasma, DW: distilled water.  
Conceptualization, D.L.G.; methodology, E.A.A. and D.L.G.; formal analysis, D.L.G. and N.K.; investigation, E.A.A.; data curation, A.G.V.; visualization, E.A.A. and A.G.V.; writing--original draft preparation, D.L.G.; writing--review and editing, N.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
Data Availability Statement: Data are available from the corresponding authors upon reasonable request.  
The authors declare no conflict of interest.  
References