Typically, the reaction was carried out in a quartz cuvette with an optical path length of 1 cm and monitored using UV-vis spectroscopy (Jasco V-750, Japan) at 25°C in a scanning range of 250 to 500 nm. An aqueous solution of 4-NP (2.5 ml; Merck, Germany) (0.13 mM) was mixed with 0.5 ml of freshly prepared aqueous solution of sodium borohydride (0.1 M), and a yellow mixture was obtained. To the above solution, an appropriate amount of catalyst was added to start reduction, and the solution was quickly subjected to UV-vis measurements. During the reaction, the mixture was continuously stirred and the progress of the reaction was recorded in situ with a time interval of 60 s. The initially obtained data are assigned to the reaction start time, t = 0. The reaction was considered complete when the solution became colorless. The rate constant of the reduction process was determined by measuring the change in absorbance at 400 nm as a function of time.

In the case of reduction tests in simulated sea water, the solution was prepared by the dissolution of 3.6 g of simulated sea water (Merck, Germany) in 100 ml of deionized water. In this stock solution, 3.6 mg of 4-NP was dissolved and a yellow solution was obtained. The reduction process was carried out at a temperature of 5°C. The methodology of reduction was the same as described above.

To study the reusability of the catalyst, the reduction reaction was repeated 25 times. After each cycle, the catalyst was recovered by filtration, washed several times with water, and dried in air. For comparison, the catalytic activity of a carbon-based scaffold with the same quantity of catalyst was measured in the same experimental conditions as those described above.