The density of aptamer packing on the electrode surface is an important parameter to optimize signal. Depending on the size and nature of target molecule, different aptamer packing densities favor signal gain. Studies have shown that small target molecules enable a greater signal gain for low density aptamer packing, while larger proteins as a target generate the greatest signal at intermediate probe packing densities. Signal gain decreases as packing density increase above the range of optimal signal gain due to steric hindrance. When the probe surface neighboring an aptamer is blocked by an adjacent aptamer, the redox tag on the target-bound aptamer will not have room to come into contact with the electrode, therefore failing to report target binding. The concentration of aptamer in solution that incubates a clean probe is found to be proportional to the density of aptamers that are immobilized on the probe. Studies have reported suggesting that small targets such as cocaine E-AB sensors generate the most signal with the lowest probe packing density. Conversely, larger protein targets such as the protein Thrombin generate the most signal at intermediate probe packing densities.