Non-Traditional Intrinsic Luminescence (NTIL): Dynamic Quenching Demonstrates the Presence of Two Distinct Fluorophore Types Associated with NTIL Behavior in Pyrrolidone-Terminated PAMAM Dendrimers

Historically, poly(amidoamine) (PAMAM) dendrimers were the first macromolecular structures reported to exhibit “non-traditional intrinsic luminescence” (NTIL) properties. Initially, this unique intrinsic luminescent property suggested the possibility of dendrimer-based biological imaging without the need for conjugating external labels. Unfortunately, low NTIL intensity levels exhibited by most simple surface-modified PAMAMs presented a serious barrier to progress in that area. Unexpectedly, a simple surface modification of amine-terminated PAMAM dendrimers with N-(4-carbomethoxy)pyrrolidone groups (4-CMP) was found to dramatically increase NTIL fluorescence intensity (i.e., >50-fold) while substantially enhancing biocompatibility and reducing cytotoxicity/complement activation properties. This current study focuses on the use of conventional and time-resolved spectroscopic measurements to characterize the NTIL behavior of 4-CMP PAMAM dendrimers over four generation levels (i.e., G2–G5). We describe and discuss the impact of polymer size and composition on NTIL intensity levels and quantum yields. Fluorescence lifetime measurements revealed two discrete major lifetime components, which were similar for all dendrimer generations and remained unaffected by changes in pH. Time-resolved fluorescence quenching studies involving a collisional quencher (methyl red) and a dynamic proximity quencher (nitrobenzoxadiazole dipeptide derivative) provided evidence for two spatially separated NTIL-type emission sites within this 4-CMP PAMAM dendrimer series. In summary, these results provide important insights into the molecular-level NTIL mechanism and demonstrate the critical role of pyrrolidone surface modification as well as separate contributions made by interior dendrimer components to the observed enhancement of NTIL fluorescence intensity.