Table The best results were obtained in the presence of MOPS with 5 mM MgCl2. Consistent with our previous observations regarding gold nanoparticle activity, we achieved substantially better conversion at pH 9 compared to pH 8, with butyric acid (1b) yields of 84 % and 79 % respectively, after 24 h at 60 °C (Figures ). This aligns with the fact that this type of catalyst performs best at more basic pH levels. At higher pH levels, using TRIS-HCl and CAPS buffers, the results were less satisfactory, with no butyric acid (1b) formation in TRIS, and yields of 36 % and 56 % for CAPS at pH 10 and pH 11 respectively (Figures ). This demonstrates the negative influence of these two buffers, especially TRIS, which completely inhibits the activity of gold nanoparticles as previously observed. In CAPS buffer, we still observed a positive effect of increasing pH, although the yields are lower compared to those obtained in MOPS buffer. Despite the best yield appearing slightly lower compared to the conversion observed in our previous study using the same buffer without Mg, we were able to achieve a 94 % yield in 24 h by replicating these same conditions with a freshly prepared Au/CaO (Table , Figure ). This shows that the presence of Mg cation has little or no effect on catalyst activity, and that the catalyst is sensitive and requires careful storage conditions. Consequently, all subsequent experiments were carried out with freshly prepared catalyst. Regarding the influence of the metal cation, we observed a decrease in yield at 24 h in the presence of MnCl2, with 48 % and 50 % of butyric acid (1b) formed in the presence of 5 mM of MgCl2, and a combination of 5 mM of both MnCl2 and MgCl2 respectively, compared to 79 % when only MgCl2 was added (Figures ). This suggests that the manganese ion inhibits partially the activity of the gold nanoparticles, although the exact mechanism remains unknown. The nature of the metal cation thus appears to be a critical parameter for the implementation of tandem catalysis. Finally, we also observed a noticeable degradation of CAPS and MOPS buffers, with their concentrations decreasing during the reaction according to HPLC monitoring, coupled to the appearance of new, unknown peaks. For TRIS-HCl buffer this effect was much less pronounced. Overall, our findings revealed that the reaction conditions of the enzymatic reaction, i.d. MOPS buffer pH 8 at 60 °C, are suitable for the oxidation step catalyzed by gold nanoparticles. The Mg cation was retained as its presence as a negligible effect on carboxylic acid yield, which is not the case with Mn cation. Even if the yield in carboxylic acid was higher at pH 9 than at pH 8, the difference was not significant, with very high yields within 24 h in both cases.