Figure 1 : The absorbance at 450 nm increases as we increase the concentration of HCl; the solution also turns yellow. As we can see from the image on the left, the colour of the solution turns from blue to yellow, meaning that as the concentration of HCl keeps increasing, the charge transfer complex dissociates according to scheme 2.

2.2 Principle: Colorimetric TMAO detection

As reported in the literature (Marquez & Dunford, 1997), PAHCl@MnO2 catalyses the formation of the charge transfer complex of TMB, which is blue and absorbs at 370 nmand 652 nm . In the presence of HCl, TMB gives yellow colour due to the formation of 2(Scheme 1).
Scheme 1 : Reaction of TMB with PAHCl@MnO2 to give off a blue colour. At a pH pf 2 – 2.5 the reaction between the blue charge transfer complex and 2is driven towards the 1 because of TMAO, which is where the detection happens.
Calibration plot : To trimethylamine-containing buffer, NH4OH (5 mM) and IACN (5 mM) were added and incubated for 15 minutes, then HCl (10 mM), followed by PAHCl@MnO2and TMB. The absorbance of the solutions was recorded, increasing the concentration of TMAO along with control (figure 2). The lower limit of quantification was determined to be 10 µM.