Fluorescent enhancement of polyethyleneimine nano-polymers and the application in cellar imaging

Nano-polymers (NPs) with non-conjugated moiety emit bright fluorescence when irradiated by ultraviolet lights, which arouses extensive interests since they do not have traditional fluorophores or conjugated moieties. Although the mechanism was put forward in some reports and entitled as “crosslink enhanced emission” (CEE) the oxidation of starting material is also contributable for the enhanced fluorescence according to our researches. However this has been neglected until now. Herein, when aqueous solution of pure polyethyleneimine (PEI) was suffered from heating at 180° in air for 60 min crosslink of PEI molecules took place and produced PEI NPs. It emitted bright blue fluorescence and the quantum yield reached as high as 18.24% while PEI itself possessed none of fluorescence. Infrared, 1H nuclear magnetic resonance and X-ray photoelectron spectra disclosed the oxidation of PEI during the lasting heating. And the image of transmission electron microscope confirmed the production of PEI NPs. When PEI NPs were prepared in the presence of hydrogen peroxide they performed a successive enhancement of fluorescent intensity, which demonstrated the oxidation of PEI contributed a lot for the fluorescent enhancement of PEI NPs. Experimental results also demonstrated that the PEI NPs have a potential application in cellar imaging on account of the lower cytotoxicity and the reactivity to the activated folic acid.