The presence of soluble TnT causes competition between this form and the surface-immobilized form, which leads to a signal decrease. If the signal measured with a predefined concentration of soluble TnT is called 𝑎 and the signal of the same concentration of phages in the absence of TnT is 𝑎 0 , the fraction of occupied active sites (saturation) may be found (Eq.7). This approach (firstly proposed by Friguet ) enables the comparison of a signal from total phage active sites and partially saturated phages. Theoretically, a correlation between the concentration of phageexposed active sites and a measured signal may be approximated in at least two ways.