pH-Mediated Fluorescent Polymer Particles and Gel from Hyperbranched Polyethylenimine and the Mechanism of Intrinsic Fluorescence

Abstract

We report that fluorescence properties and morphology of hyperbranched polyethylenimine (hPEI) cross-linked with formaldehyde are highly dependent on the pH values of the cross-linking reaction. Under acidic and neutral conditions, water-soluble fluorescent copolymer particles (CPs) were produced. However, under basic conditions, white gels with weak fluorescence emission would be obtained. The water-soluble hPEI-formaldehyde (hPEI-F) CPs show strong intrinsic fluorescence without the conjugation to any classical fluorescent agents. By the combination of spectroscopy and microscopy techniques, the mechanism of fluorescence emission was discussed. We propose that the intrinsic fluorescence originates from the formation of a Schiff base in the cross-linking process between hPEI and formaldehyde. Schiff base bonds are the fluorescence-emitting moieties, and the compact structure of hPEI-F CPs plays an important role in their strong fluorescence emission. The exploration on fluorescence mechanism may provide a new strategy to prepare fluorescent polymer particles. In addition, the investigation shows that the hPEI-F CPs hold potential as a fluorescent probe for the detection of copper ions in aqueous media.

High‐Shear‐Imparted Tunable Fluorescence in Polyethylenimines

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In general, fluorescent polyethylenimine (PEI) nanoparticles absorb primarily UV light, with the fluorophores typically having extended conjugated structures. [4] Even though simple oxidation, acidification or methylation can further enhancet he intrinsic fluorescence, [5] the mechanism of fluorescencef rom these dendritic polymers is not well understood. Tunability of the fluorescence was achieved by varying the flow rate of liquid entering the rapidly rotating tube in av ortex fluidic device (VFD), without the need for additional reagents.

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“…Tunability of the fluorescence was achieved by varying the flow rate of liquid entering the rapidly rotating tube in av ortex fluidic device (VFD), without the need for additional reagents. [7] These fluorescent polymers can contain Schiff base moieties, [5] tertiary amine or carbonyl groups in an dendrimer interior with terminal groups such as monohydroxyl, [8] air [6] or hydrogen peroxide [9] oxidized amines, amine-rich nanoclusters, [5] and carbamato anion. [4] Even though simple oxidation, acidification or methylation can further enhancet he intrinsic fluorescence, [5] the mechanism of fluorescencef rom these dendritic polymers is not well understood.…”

“…[4] Even though simple oxidation, acidification or methylation can further enhancet he intrinsic fluorescence, [5] the mechanism of fluorescencef rom these dendritic polymers is not well understood. [1,5] Polyethylenimine (PEI) is aw ater-solublec ationic polyelectrolytew hich contains al arge number of amino groups, and has been used to prepare various fluorescent materials. [7] These fluorescent polymers can contain Schiff base moieties, [5] tertiary amine or carbonyl groups in an dendrimer interior with terminal groups such as monohydroxyl, [8] air [6] or hydrogen peroxide [9] oxidized amines, amine-rich nanoclusters, [5] and carbamato anion.…”

“…[7] These fluorescent polymers can contain Schiff base moieties, [5] tertiary amine or carbonyl groups in an dendrimer interior with terminal groups such as monohydroxyl, [8] air [6] or hydrogen peroxide [9] oxidized amines, amine-rich nanoclusters, [5] and carbamato anion. [5] Refluxing 25 kDa PEIi nn itric acid at 120 8Cf or 12 ha ffords photoluminescent nanoparticles (l ex = 360 nm, l em = 450 nm) bearing amide linkages (NHCO). [1,5] Polyethylenimine (PEI) is aw ater-solublec ationic polyelectrolytew hich contains al arge number of amino groups, and has been used to prepare various fluorescent materials.…”

“…[5] Refluxing 25 kDa PEIi nn itric acid at 120 8Cf or 12 ha ffords photoluminescent nanoparticles (l ex = 360 nm, l em = 450 nm) bearing amide linkages (NHCO). [3,5] Hydrothermal treatmento fh PEI with aldehydes at 95 8Cg enerates fluorescent polymer nanoparticles, [3] depending on the pH which can complicate the processing. [13] Adding formaldehyde at 90 8Cr esultsi nf luorescent polymericn anoparticles (l ex = 365 nm, l em = 508 nm) or gels (l ex = 350 nm, l em = 476 nm).…”