The concentration of CuO nanosheets could also significantly
affect the DI of the CL system. At the optimum concentrations of
H2O2 (0.015 mol L1), NaOH (0.01 mol L1) and luminol
(500 lmol L1), the concentration of CuO nanosheets was investigated
in the range of 1–18 mg L1 (see Fig. 4d). The results indicated
that the DI of the CL system increased considerably by
increasing the concentration of CuO nanosheets up to 12 mg L1,
further increase of CuO nanosheets concentration resulted in a
gradual decrease in the DI of the CL system. Some researchers have
attributed this effect to the strong interactions among nanomaterials
when their concentration is too high [28,29]. It is believed that
in higher concentration of CuO nanosheets the CL energy is transferred
between the CuO nanosheets which are in small distances
from each other [41–43]. This effect reduces the catalyzing performance
of the nanosheets in the CL system. The other reason could
be related to the absorption of the emitted radiation by CuO nanosheets
at concentrations higher than 12 mg L1. These lead to the
decrease of the DI of the CL system and consequently sensitivity
of the CL system to the analyte is reduced. In order to obtain higher
sensitivity, 12 mg L1 CuO nanosheets was used as the most suitable
concentration.