Single molecule photobehavior of a chromophore interacting with silica-based nanomaterials

Single molecule studies of the free DY-630-MI and interacting with MCM-41 and (Al)MCM-41, show the conformational diversity of the molecule. The free dye is characterized by a single broad (fwhm = 0.7 ns) lifetime distribution histogram centered on 1.47 ns, which is also reflected in the broadness of the polarization value distribution histogram, covering almost the full range of values from -1 to 1. The fluorescence intensity traces of the free DY-630-MI show strong blinking behavior and weak photostability. Upon interaction with the mesoporous silica nanomaterials, MCM-41 and (Al)MCM-41, the dye molecule becomes more stable, with less blinking present in the fluorescence traces. The lifetime distribution histogram in the case of DY-630-MI/MCM-41 complexes is fitted by 3 Gaussians, indicating 3 distinct interaction sites. The Gaussian with the largest amplitude is centered on 2.19 ns, consistent with the confinement effect of MCM-41 and in agreement with the ensemble average studies. The polarization value distribution histogram becomes narrower in comparison with the free molecule and is more biased towards the positive limit. Replacing few Si(4+) ions with Al(3+) ones in the regular MCM-41 changes the local electrostatic field within the nanotube. This atomic substitution in the nanohosts results in a more selective orientation of the dye molecules, giving two populations with time constants 1.56 and 2.10 ns.