The principle of bioluminescence resonance energy transfer (BRET) is employed for NAD+ concentration detection. When the NAD+ response protein binds to an NAD+ molecule, it changes from an 'open' to a 'closed' state. This change facilitates a closer proximity between the energy transfer donor and receptor, leading to a more efficient resonance energy transfer. The change in resonance energy transfer efficiency, caused by the NAD+ molecule, is manifested as a shift in the emission intensity ratio of the probe's donor and receptor, indicating the NAD+ concentration in the system.