Abstract
A successful matching of a PEG group size with the EPR effect for an off-to-on responsive NIR-fluorophore conjugate has been accomplished which allows two distinct in vivo tumor imaging periods, the first being the switch on during the initial tumor uptake via enhanced permeability into the ROI (as background is suppressed) and a second, later, due to enhanced retention within the tumor. Software simulation (https://mihaitodor.github.io/particle_simulation/index.html), synthetic chemistry, with in vitro and in vivo imaging have been synergistically employed to identify the optimal PEG-fluorophore conjugate for in vivo tumor imaging.