You can run the reaction with an immobilized enzyme if you can keep the nanoparticles fully suspended during the reaction, then pellet them quickly (either by centrifugation or with a magnet) and measure product formation in the supernatant. One way to do this is to perform the reactions in microcentrifuge tubes that fit into an Eppendorf Thermomixer (or similar instrument). If the nanoparticles are small enough, they may even stay in suspension well enough without mixing, as long as the reaction time is not too long.

Thank you Dr. Adam, your illuminating answer is really appreciated.

I measured in two ways. 1. Tiny magnets were lowered into the cuvette, the cuvette stood on a magnetic stirrer, and there was a slow constant stirring. 5 seconds before the measurement, the cuvette was moved to the spectrophotometer. 2. The cuvette was constantly in the spectrophotometer. 7-10 seconds before taking readings of the device, carefully mixed the contents of the cuvette with a stick.

Thanks Dr. Olga N. Solovjeva