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pH-Mediated Fluorescent Polymer Particles and Gel from Hyperbranched Polyethyleneimine and the Mechanism of Intrinsic Fluorescence Shi Gang Liu, Na Li, Yu Ling, Kang Bei Hua, Shuo Geng, Nian Bing Li, and Hong Qun Luo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00201 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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pH-Mediated Fluorescent Polymer Particles and Gel from

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Hyperbranched Polyethyleneimine and the Mechanism of

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Intrinsic Fluorescence

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Shi Gang Liu, Na Li, Yu Ling, Bei Hua Kang, Shuo Geng, Nian Bing Li*, and Hong

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Qun Luo*

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Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education),

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School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR

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China

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*

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*

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*

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23 68253237; E-mail address: [email protected]

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*

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*

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23 68253237; E-mail address: [email protected]

Corresponding Authors

Nian Bing Li 2, Tiansheng Road, BeiBei District, Chongqing, 400715, China. Tel: +86 23 68253237; fax: +86

Hong Qun Luo 2, Tiansheng Road, BeiBei District, Chongqing, 400715, China. Tel: +86 23 68253237; fax: +86

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ACS Paragon Plus Environment

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ABSTRACT

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We report that fluorescence properties and morphology of hyperbranched polyethyleneimine

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(hPEI) crosslinked with formaldehyde are highly dependent on the pH values of the crosslinking

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reaction. Under acidic and neutral conditions, water-soluble fluorescent copolymer particles (CPs)

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were produced. However, under basic conditions, white gels with weak fluorescence emission

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would be obtained. The water-soluble hPEI-formaldehyde (hPEI-F) CPs show strong intrinsic

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fluorescence without the conjugation to any classical fluorescent agents. By the combination of

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spectroscopy and microscopy techniques, the mechanism of fluorescence emission was discussed.

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We propose that the intrinsic fluorescence originates from the formation of a Schiff base in the

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crosslinking process between hPEI

and formaldehyde. Schiff base bonds are

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fluorescence-emitting moieties and the compact structure of hPEI-F CPs plays an important role in

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their strong fluorescence emission. The exploration on fluorescence mechanism may provide a

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new strategy to prepare fluorescent polymer particles. In addition, the investigation shows that the

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hPEI-F CPs hold potential as a fluorescent probe for the detection of copper ions in aqueous

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media.

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the

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INTRODUCTION

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In recent years, fluorescent nano-materials such as semiconductor quantum dots (QDs), noble

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metal nanoclusters (NCs), and carbon dots (CDs), have created new avenues for sensors,

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bioimaging, drug and gene delivery, and other biomedical applications.1 In particular, fluorescent

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polymer nanoparticles have attracted a great deal of attention for their good biocompatibility, easy

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functionalization, and metal-free property.2,3 Therefore, many researchers focus on the

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development of fluorescent polymer nanoparticles, which are labeled with fluorochromes due to

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the lack of fluorescent chromophore for most polymers. Various encapsulation techniques have

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been developed for embedding fluorochromes in a polymer matrix.4-6 Unfortunately, these

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methods are generally complicated and time-consuming. In addition, the fluorescent dyes may

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leak out from the shell of nanoparticles in practical applications if the nanoparticles are fabricated

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via physical entrapment of non-covalent procedure. Ideal fluorescent polymer nanoparticles

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should be autofluorescent without doping an external fluorochrome. In this regard, conjugated

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polymer nanoparticles (CPNs), which contain large π-conjugated backbones and delocalized

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electronic structure, are a class of competitive materials because of their autofluorescence and

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other prominent properties including favorable water solubility, high quantum yield, good

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photostability, and low toxicity. The applications of CPNs have been extensively explored in many

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fields such as bioimaging, sensors, and material science.7-9 Recently, polydopamine nanoparticles

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have been reported that they can emit inherent fluorescence and their applications based on the

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property also were well studied.10,11 Additionally, a few reports have described the intrinsic

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fluorescence emission from dendritic polymers.12-15 The commercial poly(amidoamine) (PAMAM)

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dendrimers can exhibit strong blue fluorescence after a simple treatment such as oxidation or 3


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acidification.

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Hyperbranched polymers, one of dendritic polymers, have highly branched architecture and

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numerous terminal functional groups, and they can be produced by a simple one-pot reaction.

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Thus, they were considered as ideal candidates for applications in viscosity modifiers, coating,

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catalysis, and drug delivery vehicles.16 Hyperbranched polyethyleneimine (hPEI) is a

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water-soluble cationic polymer containing numerous amino groups and has been proposed for a

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wide range of biological applications,17,18 for example, the utilization in gene delivery.19 Over the

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past years, hPEI has been widely utilized as precursors or protective agents to synthesize

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fluorescent materials because it has excellent water solubility and high branched structure. For

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example, hPEI-templated metal nanoclusters,20,21 hPEI-functionalized carbon dots,22,23 and

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hPEI-coated semiconductor quantum dots24 have been successfully prepared for fluorescent

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bioanalysis, bioimaging, and chemical sensor. Recent work has reported dye-doped polymer

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nanoparticles formed by hPEI and aggregation induced emission (AIE) dyes.25 Interestingly,

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Pastor-Pérez and coworkers discovered that methylated hPEI and their linear analogues also

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emitted strong blue fluorescence without addition of any classical fluorescent ageants.26 However,

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the fluorescence of commercial hPEI is hardly able to be observed.

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The origin of fluorescence emission from dye-doped polymer nanoparticles and conjugated

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polymer nanoparticles are well comprehended because of their fluorescent agents and

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chromophores. The fluorescence of polydopamine nanoparticles can be attributed to their aromatic

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groups that can serve as the fluorescent chromophores, although the structure of polydopamine is

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still not completely indentified.27 However, for dendritic polymers such as PAMAM and hPEI,

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their inherent fluorescence is unexpected and bewildering because of the lack of classical 4


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chromophores or embedded fluorochromes, and therefore, much attention has been focused on

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it.12-15,28-30 Although the fluorescence emission from PAMAM dendrimers with different

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terminated groups including hydroxyl, carboxyl, and amine, was systematically investigated under

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various conditions, the origin of fluorescence is still unclear and is usually supposed to have a

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close relation with their terminal groups or macromolecular backbone. As for the inherent

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fluorescence of methylated hPEI, Pastor-Pérez and coworkers concluded that it was probably due

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to the creation of amine rich nanocluster and electron-hole recombination processes.26 Anyway, to

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the best of our knowledge, up to now the mechanism of inherent fluorescence emission from these

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dendritic polymers is still not clearly understood. But it is noticed that a simple treatment

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(oxidation, acidification, or methylation) for these dendritic polymers is important for their

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intrinsic strong fluorescence. In our recent work, we demonstrated that autofluorescent polymer

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nanoparticles can be formed by hPEI and aldehydes in pure water via hydrothermal treatment (95

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o

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fluorescence is not well investigated.

C) and were also utilized as a fluorescent probe for bioimaging.31 However, the origin of the

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In this study, we have found that fluorescence properties and morphology of hPEI crosslinked

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with formaldehyde were highly dependent on the pH values of the reaction (Scheme 1).

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Water-soluble fluorescent hPEI-formaldehyde copolymer particles (hPEI-F CPs) were fabricated

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under acidic or neutral reaction conditions, while white gels with weak fluorescence were obtained

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under basic conditions. Furthermore, by the combination of spectroscopy and microscopy

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techniques, the origin of intrinsic fluorescence of hPEI-F CPs and possible mechanism were

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proposed and studied, which may facilitate the preparation of more fluorescent polymer

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nanoparticles. 5


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Scheme 1. Scheme for the reaction between hPEI and formaldehyde at different pH values.

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EXPERIMENTAL SECTION

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Materials. Hyperbranched polyethyleneimine (hPEI, Mw = 10 000, 99%) and formaldehyde (35

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wt %) were purchased from Aladdin Ltd., Shanghai, China. Glutaraldehyde (50 wt %), acetone

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(99.5 wt %), ascorbic acid, D-glucose, and all other chemicals not mentioned here were of

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analytical reagent grade and used as received. Ultrapure water (18.2 MΩ cm) was used throughout

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the experiment. The Britton-Robinson (BR) buffer solutions of different pH values were prepared

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with 0.04 M mixed acid solution (H3PO4, H3BO3 and CH3COOH) and a sodium hydroxide

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solution (0.2 M). Other buffer solutions were prepared according to standard protocols.

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Preparation of hPEI-F Copolymer Particles and Gels. In this work, several hPEI-F

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CPs and gels were synthesized under different pH conditions. For a typically synthesis of hPEI-F

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CPs at pH 5.14, 50 µL of 0.1 g mL-1 hPEI was first dissolved in 445 µL of Britton-Robinson (BR)

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buffer solution (pH 5.14) by stirring for about 1 min, and then 5 µL of formaldehyde (35 wt %)

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was added. Subsequently, after vigorous stirring for 1 min, the mixture was heated at 50 oC for 30

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min via hydrothermal treatment. Then the as-synthesized hPEI-F CPs solutions were dialyzed

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against ultrapure water for 24 h through porous bag (molecular weight cut off 3000 Da). The

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products inside the dialysis bag were collected to further study. For the synthesis conditions of 6


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other hPEI-F CPs and gels, only the pH value of BR buffer solution was changed (other pH: 2.21,

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3.27, 4.10, 4.93, 5.77, 6.75, 7.40, 8.36, 9.32, and 9.75), and the other procedures were the same as

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described above.

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Preparation of hPEI-Glutaraldehyde, hPEI-Ascorbic Acid, hPEI-D-Glucose, and

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hPEI-Acetone. Similar to the synthesis of hPEI-F CPs, taking hPEI-glutaraldehyde as an

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example, 50 µL of hPEI (0.1 g mL-1) was first dissolved in 448 µL of BR buffer（pH 5.14）by

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stirring for about 1 min, and then 2 µL of glutaraldehyde (50 wt %) was added. Subsequently,

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after vigorous stirring for 1 min again, the mixture was heated at 50 oC for 30 min with water bath

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kettle. Other synthetic conditions of fluorescent materials were depicted simply as follows:

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The synthetic method of hPEI-ascorbic acid (or hPEI-D-glucose): 50 µL of hPEI (0.1 g mL-1),

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350 µL of BR buffer solution (pH 5.14), and 100 µL of ascorbic acid solution (0.1 M) (or

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D-glucose solution, 0.1 M) were mixed together and the mixture was heated at 90 oC for 30 min.

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The synthetic condition of hPEI-acetone: 50 µL of PEI (0.1 g mL-1), 445 µL of BR buffer

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solution (pH 5.14), and 5 µL of acetone (99.5 wt %) were mixed together and the mixture was

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heated at 90 oC for 24 h.

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Measurements. A Hitachi F-2700 spectrofluorophotometer (Hitachi, Japan) equipped with a

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150-W xenon lamp was used for recording the fluorescence spectra. UV-Vis absorption spectra

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were recorded using a UV-Vis 2450 spectrophotometer (Shimadzu, Japan). Scanning electron

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microscope (SEM) measurements were performed with a JEOL JSM-6510 LV scanning electron

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microscope (JEOL, Japan). To obtain SEM image, the sample was pasted on holders with

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conductive adhesive tape, and then it was sputtered with a layer of platinum to enhance the

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conductivity before being transferred to the SEM. Transmission electron microscopy (TEM) 7


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measurements were preformed with a JEM 1200EX transmission electron microscope (JEOL,

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Japan) operating at 200 kV. The sample was deposited onto a carbon-coated copper grid and then

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air-dried before TEM measurement. The Fourier transform infrared (FT-IR) spectra were obtained

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using a Bruker IFS 113v spectrometer (Bruker, Germany). For the preparation of FT-IR samples,

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the as-prepared hPEI-F CPs solution and gel were lyophilized to collect dry products, and then the

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fine powder was pelleted with KBr. The nuclear magnetic resonance (NMR) spectra were

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collected on a Bruker AVANCE III 600 (600 MHz) (Bruker, Germany) with the freeze dried

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product dissolved in CDCl3. A rapid mixing device (Ronghua Instrument Plant, Jiangsu, China)

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was utilized to mix solutions completely. The pH values of solutions were measured with a pH

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meter (PHS-3C, Shanghai Leici Instrument Company, Ltd., China).

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RESULTS AND DISCUSSION

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The hPEI was crosslinked by formaldehyde under a mild reaction condition (50 oC, 30 min)

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and the reaction pH played a crucial role in the process. hPEI-F CPs and gels were prepared in

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Britton-Robinson buffer solutions with different pH values. As shown in Figure 1a, when the

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solution is alkaline, a white gel with weak fluorescence can be obtained, but under neutral and

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acidic conditions, the solutions of hPEI-F CPs are faint yellow and exhibit strong yellowish-green

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fluorescence under ultraviolet lamp (365 nm). Figure 1b displays that the fluorescence excitation

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and emission spectra of the hPEI-F CPs solution and the maximum excitation and emission

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wavelengths are 365 and 508 nm, respectively. Figure 1c reveals the relationship between the

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reaction pH and the fluorescence intensities of the hPEI-F CPs solutions, and the maximum

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fluorescence intensity is obtained when the pH value is about 5. As exhibited in Figure 1d, the

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UV-Vis absorption spectra show that the hPEI-F CPs solution has a new absorption peak at 369

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nm, whereas hPEI and formaldehyde have nearly no absorption at above 250 nm. It is noted that

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sole hPEI or formaldehyde cannot emit fluorescence and has not any classical fluorescent

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chromophores. The intrinsic fluorescence emission very resembles those of PAMAM and

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methylated hPEI,13,14,26 and even a similar new absorption peak (380 nm) was also observed in

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fluorescent PAMAM that was a commercial PAMAM oxidized by NH4S2O8.14 Next, we

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investigated the micro-morphology and basic fluorescent properties of the hPEI-F CPs which were

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prepared with hPEI (0.01 g mL-1) and formaldehyde (0.35 wt %) at pH 5.14 by heating at 50 oC

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for 30 min. Scanning electron microscope (SEM) and transmission electron microscopy (TEM)

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were used to directly monitor the size and shape. Figure 2a is the SEM image and reveals that the

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hPEI-F CPs are monodisperse and exhibit rough spheres with a diameter of about 200 nm. A

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similar result is obtained from TEM image (Figure 2b). For excitation wavelengths from 335 to

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385 nm, there is a slight red shift (∼10 nm) of fluorescence emission spectra (Supporting

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Information, Figure S1), which may result from the non-uniform diameter of the particles.32 The

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quantum yield was found to be 42% by use of quinine sulphate as a standard, and it is superior to

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that of other fluorescent materials obtained from hPEI.21,22 The fluorescence intensity of the

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hPEI-F CPs could keep stable when stored at 4 oC and had a little increase when stored at room

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temperature (Supporting Information, Figure S2), suggesting that they own satisfied storage

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stability and could be stored for more than three months. In addition, the reaction pH was also

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controlled by different buffers, such as acetate buffer, borax buffer, carbonate buffer, and so on.

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Figure S3 in the Supporting Information shows that mixtures of hPEI and formaldehyde are

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fluorescent in different acidic buffers and in contrast, they turn to white gels in all alkaline buffers. 9


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However, the as-prepared fluorescent hPEI-F CPs solution and gel cannot transform to each other

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when their pH values were changed, suggesting that it is not a pH-induced reversible process.

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These results reveal that the fluorescence and morphology of the products formed by hPEI and

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formaldehyde greatly rely on the pH values of the reaction. With the help of adjusting reaction pH,

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the prepared condition of the hPEI-F CPs is milder than that of our previous report (95 oC).31

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Some published papers on the synthesis of fluorescent carbon dots using hPEI as a precursor

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needed harsh conditions such as high temperature (above 180 oC) or microwave.22,23 Compared to

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those fluorescent materials, the as-prepared hPEI-F CPs in this study possess obviously great

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advantages in the process of synthesis.

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Figure 1. (a) Photographs of hPEI-F CPs and gels synthesized at various pH (from left to right:

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pH 2.21, 3.27, 4.10, 4.93, 5.14, 5.77, 6.75, 7.40, 8.36, 9.32, and 9.75, BR buffer) under visible

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light (top) and UV light (365 nm, bottom). Concentration: hPEI (0.01 g mL-1), formaldehyde (0.35

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wt %). (b) Fluorescence excitation and emission spectra of the hPEI-F CPs synthesized at pH 5.14.

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(c) Fluorescence intensities of the hPEI-F CPs synthesized at various pH at the emission

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wavelength of 508 nm. (d) UV-Vis absorption spectra of hPEI (1 mg mL-1), formaldehyde (0.35

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wt %), and hPEI-F CPs synthesized at pH 5.14 (20% v/v). 10


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Figure 2. SEM (a) and TEM (b) images of hPEI-F CPs. Inset of (a) shows a single magnified

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particle. Preparation conditions of the hPEI-F CPs: hPEI (0.01 g mL-1), formaldehyde (0.35 wt %),

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pH 5.14 (BR buffer).

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The pH-mediated fluorescence and morphology of hPEI crosslinked with formaldehyde

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inspired us to explore the origin of its intrinsic fluorescence emission. As known, dendritic

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polymers lack fluorescence-emitting moieties, and published literature just found that their

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terminated groups or macromolecular backbone were crucial for their inherent fluorescence.13,14

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However, it is well known that the existence of fluorescent chromophores makes the fluorescence

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emission possible for fluorescent organic materials such as small-molecule dyes and conjugated

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polymers. On the other hand, amino resins like melamine-formaldehyde resins (MFR) and

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urea-formaldehyde resins (UFR), which are prepared based on crosslinking reaction between 11


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amino compounds and formaldehyde, have been widely applications in industry. But their

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chemical process for synthesis contains a series of reactions and has not been ultimately

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established due to its complexity.33-37 Generally, methylol (-CH2OH), Schiff base bond (C=N),

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methylene (-CH2-), and methylene ether (-CH2-O-CH2-) bridges could be formed in crosslinking

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processes. Considering these factors, we speculated that hPEI was crosslinked by formaldehyde

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and fluorescent chromophores (Schiff base bonds) were produced in the crosslinking process,

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even though there is rarely literature having described fluorescence emission from amino resins.

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As a result, the new absorption band around 369 nm and fluorescence emission of the hPEI-F CPs

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can be ascribed to n→π* and n←π* transitions of C=N bonds, respectively.38 With respect to the

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diversity of fluorescence intensity under different reaction pH values, it relates to the formation of

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the Schiff base bond, in which the carbonyl oxygen is attacked by a proton firstly and then

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nucleophilic attack on the carbonyl carbon by the primary amine takes place.39 However, highly

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acidic condition would lead to protonation of most amine groups. The protonated amine groups

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are lack of nucleophilicity, which is not beneficial to the Schiff base reaction. The maximum

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fluorescence intensity was obtained in this work at about pH 5, which was consistent with those of

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previous reports that utilized weak acidic reaction conditions (pH 5, and 6.5) to fabricate Schiff

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base.40,41 The weak fluorescence of gels was discovered under alkaline condition, indicating that

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the Schiff base reaction was hard to occur in crosslinking process under this condition and only a

19

few of Schiff base bonds were formed. Hence, a suitable pH is necessary for the formation of

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Schiff base bonds. We noticed that formaldehyde and formic acid were used in the preparation of

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methylated hPEI in Pastor-Pérez and coworkers’ report,26 and thereby fluorescence emission from

22

methylated hPEI may also derive from the formation of a Schiff base. Furthermore, under the pH 12


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5.14, the effect of other experimental conditions such as temperature, heating time and the reactant

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ratio of hPEI to formaldehyde on fluorescence intensity is shown in Figure S4 in the Supporting

3

Information. These reaction conditions apparently influence the intensity of fluorescence and the

4

optimal temperature, heating time, and the concentration of formaldehyde for a strong

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fluorescence emission are obtained as 50-60 oC, 30 min, and 0.35 wt % (5 µL, 35 wt %),

6

respectively. Besides, the molecular weight of hPEI had an insignificant impact on fluorescence

7

emission. When hPEI of different molecular weights was used (Mw = 600, 1800 and 10 000), the

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fluorescence intensity increased slightly, and the maximum emission peaks were constant

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(Supporting Information, Figure S5). These results demonstrate that the fluorescence emission of

10

hPEI-F CPs has a close relation to the formation of a fluorescence-emitting moiety. However,

11

when we used formaldehyde to react with small-molecule amine compounds including urea,

12

ethylenediamine, diethylenetriamine, and triethylamine, no significant fluorescence was found

13

from their products, indicating that the ample local amine groups are vital for the mass production

14

of Schiff base bonds. Another reason may be that the macromolecular structure of hPEI can

15

protect most Schiff base bonds from further crosslinking, whereas the Schiff base bonds formed

16

by small-molecule amines and formaldehyde have to be broken for further reaction.

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To further verify the formation of a Schiff base, we utilized Fourier transform infrared (FT-IR)

18

spectroscopy and 1HNMR spectroscopy to investigate the hPEI-F copolymers prepared at different

19

pH values. The complex macromolecular architectures for dendritic polymers lead to the difficulty

20

to determine their newly formed functional groups, which partly causes that it is not easy to

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identify the fluorescent chemical species. But obvious new signals of FT-IR and 1HNMR were

22

found in this work. As shown in Figure 3, curves a, b, and c represent FT-IR spectra of hPEI, 13


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hPEI-F CPs obtained at pH 5.14, and hPEI-F gel prepared at pH 9.75, respectively. The raw hPEI

2

has absorption peaks at 2950, 2840, and 1472 cm−1 corresponding to the stretching vibration and

3

bending vibration of CH2 bonds, and characteristic absorptions at 3420 and 1573 cm−1 belong to

4

N-H bond. Compared to the spectrum of hPEI, an obvious new peak at 1644 cm−1 was observed in

5

the hPEI-F CPs spectrum, which can be attributed to the C=N bond.42,43 In addition, the absorption

6

bands at 3220 and 1401 cm−1 are associated with the stretching vibration and bending vibration of

7

O-H, respectively, and the stretching vibration of C-O is located at 1100 cm−1, which reveals the

8

presence of C-OH. Moreover, the fact that curves b and c are similar indicates that the hPEI-F CPs

9

and gel maybe have similar function groups. But the different amounts of these function groups

10

result in the differences of fluorescence intensity and morphology. In addition, the 1HNMR spectra

11

of hPEI and hPEI-F CPs obtained at pH 5.14 are shown in Figure S6 in the Supporting

12

Information. It is apparent that the 1HNMR spectrum of hPEI-F CPs possesses a new peak at 8.00

13

ppm belonging to H2C=N protons,42,44 while hPEI is of no signal at this location. Additionally,

14

new chemical shifts located at 3.72, 2.93 and 2.85 ppm in the 1HNMR spectrum of hPEI-F CPs

15

can be attributed to the protons of N-CH2-N, CH2OH, and CH2-O- groups, respectively. These

16

results confirm the formation of a Schiff base and the crosslinking between hPEI and

17

formaldehyde. Consequently, the possible reaction mechanism including several reactions is put

18

forward in Scheme 2 on the basis of the discussion mentioned above.

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Figure 3. FT-IR spectra of hPEI (a), hPEI-F CPs synthesized at pH 5.14 (b), and hPEI-F gel

3

prepared at pH 9.75 (c).

4 5

Scheme 2. Possible reaction mechanism for the formation of fluorescent hPEI-F CPs.

6

According to a common principle, Schiff base bonds can be easily reduced by sodium

7

borohydride (NaBH4) to secondary amines via the following reaction:45

8 9

To acquire another evidence of the fluorescence deriving from the Schiff base bonds, we 15


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1

conducted an experiment in which 0.1 M NaBH4 was added to the hPEI-F CPs for 4 h at room

2

temperature. As shown in Figure 4A and 4B, after reduction, the characterized absorption of the

3

Schiff base bonds at 369 nm disappeared and the strong fluorescence of the hPEI-F CPs cannot be

4

observed. The results further demonstrated that the fluorescence-emitting moieties were mainly

5

the Schiff base bonds rather than other species. Anyway, the electron-donating groups in the

6

hPEI-F CPs, such as amine groups, may serve as assistants of chromophores to increase

7

fluorescence intensity.

8

9

Figure 4. (A) UV-Vis absorption spectra of hPEI-CPs (curve a) and hPEI-F CPs reduced by

10

NaBH4 (curve b). Inset is photographs of hPEI-F CPs (a) and hPEI-F CPs reduced by NaBH4 (b)

11

under visible light. (B) Fluorescence emission spectra (excitation: 365 nm) of hPEI-F CPs (curve a)

12

and hPEI-F CPs reduced by NaBH4 (curve b). Inset is photographs of hPEI-F CPs (a) and hPEI-F

13

CPs reduced by NaBH4 (b) under UV light (365 nm). Preparation conditions of the hPEI-F CPs:

14

hPEI (0.01 g mL-1), formaldehyde (0.35 wt %), and pH 5.14 (BR buffer).

15

We have ascribed the fluorescence emission of hPEI-F CPs to the formation of a Schiff base

16

as mentioned above. However, according to most of previous literature,46-48 usually, small

17

molecular Schiff base compounds are poorly water-soluble and slightly fluorescent, and only

18

when interacting with metal ions, could they emit strong fluorescence through the suppression of 16


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C=N isomerization. In this work, the hPEI-F CPs are excellently water-soluble and exhibit strong

2

fluorescence emission without adding any metal ions. The phenomena may result from the

3

following factors: (1) a number of unreacted amine groups of hPEI and hydroxyl groups formed

4

through the reaction are hydrophilic, while imine groups and the backbone of polymer are

5

hydrophobic, which make the hPEI-F copolymer easier to form nano-sized near-spherical particles

6

in aqueous media; (2) the compact and crosslinking structure of hPEI-F CPs restricts the

7

intramolecular rotations of Schiff base bonds, which confines the nonradiative channel and leads

8

to the radiative decay, thereby, making the polymers fluorescent. A similar principle has been used

9

to explain the fluorescence emission from aggregation induced emission dyes.49 Hyungmin and

10

co-workers demonstrated that the fluorescent properties of amphiphilic fluorescent polymer

11

dissolved in water were appreciably different from that in organic solvents.50 Consequently, the

12

hPEI-F CPs were dispersed in various solvents to observe the change of their fluorescence. But the

13

hPEI-F CPs are not freely soluble in most of organic solvents, which conforms to the trait of

14

polymeric Schiff bases.51-53 In polar solvents, the hPEI-F CPs are slightly soluble for the existence

15

of hydrophilic groups. Figure 5a shows that the fluorescence emission spectra of hPEI-F CPs in

16

water and 8 kinds of organic polar solvents. It can be seen that the fluorescence intensities of

17

hPEI-F CPs in organic solvents are higher than that in water, and meanwhile the emission peaks

18

exhibit a slight blue shift. The reason for these phenomena is that the hydrophilic groups on the

19

surface of hPEI-F CPs trend to fold and shrink with the decline of solvent polarity, which results

20

in changes of their surface state and further inhibition of the intramolecular rotations of Schiff

21

base bonds. Thus, the loss of energy by nonradiative channel is much less and more energy trends

22

to radiative decay. In addition, a shoulder peak at around 416 nm can be observed in the 17


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1

fluorescence spectra (Figure 5a). The spectra were obtained at a relatively low concentration of

2

hPEI-F CPs and with an excitation of 365 nm. According to a published literature about Raman

3

scatter in spectrofluorimetry,54 the Raman peak would appear at around 416 nm under excitation at

4

365 nm. Thus the shoulder peak in Figure 5a may be attributed to Raman scatter. Figure S7 in the

5

Supporting Information demonstrates the gradual changes of fluorescence intensity and emission

6

peak with increasing the proportion of ethanol in an ethanol-water mixed solvent. Figure 5b

7

displays the relationship between fluorescence intensities of hPEI-CPs and their concentrations in

8

ethanol and water, respectively. We can observe that the fluorescence intensities in ethanol are

9

higher than that in water in the range of 1%-12% (v/v). However, when the concentration of

10

ethanol increases to 20% (v/v), the fluorescence intensity in ethanol decreases and is below that in

11

water. This is because hPEI-F CPs cannot disperse well in ethanol at this concentration.

12

Nevertheless, the unique structure of hPEI-F CPs implies that their fluorescence can keep stable in

13

complicated aqueous media. As the hydrophilic groups on the surface can form a protective layer,

14

the fluorescent groups (C=N) situated in the inner of the particles can effectively avoid the

15

interference of other factors in aqueous solutions. As presented in Figure 6a, there is no drastic

16

change of fluorescence intensity when the hPEI-F CPs are dispersed in solutions with pH values

17

ranging from pH 2 to 10. Meanwhile, the fluorescence intensities of the hPEI-F CPs in solutions

18

with different NaCl concentrations are constant, even in 1 M NaCl solution (Figure 6b). These

19

results demonstrate that the hPEI-F CPs possess good stability toward a wide range of pH and

20

high ionic strengths. To further prove the structure of the hPEI-F CPs, we carried out experiments

21

in which a variety of metal ions were introduced to the hPEI-F CPs solution. Figure 6c reveals the

22

fluorescence response of hPEI-F CPs to 19 kinds of metal ions. It can be seen that the fluorescence 18


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intensity kept stable after the addition of most metal ions and Ni2+, Fe3+, and Hg2+ can cause a

2

slight decrease, while Cu2+ remarkably quenched the fluorescence. None of these metal ions can

3

obviously enhance the fluorescence intensity, suggesting that the metal ions cannot directly

4

interact with the Schiff base bond. But Cu2+ (or Ni2+, Fe3+, Hg2+) can form complexes with amine

5

groups on the surface of hPEI-F CPs, leading to the fluorescence quenching via energy transfer or

6

inner filter effect.21,23

7

8

Figure 5. (a) Fluorescence emission spectra of hPEI-F CPs (2% v/v) in different solvents

9

including water, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), tetrahydrofuran (THF),

10

ethanol, ethylene glycol, isopropanol, n-butanol, and methanol with an excitation of 365 nm. (b)

11

Relationship between fluorescence intensity and concentration of hPEI-F CPs in ethanol and water,

12

respectively.

19


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1 2

Figure 6. Effects of pH (a), NaCl concentrations (b), and metal ions (c) on the fluorescence

3

intensity of hPEI-F CPs (2% v/v). The concentration of the all metal ions in Figure 5(c) is 100

4

µM. Conditions: BR buffer, pH 7.0 (b) and (c); excitation: 365 nm.

5

The Cu2+-induced obvious fluorescence quenching indicates that the hPEI-F CPs hold

6

potential to serve as a fluorescent probe for the detection of Cu2+ in aqueous media. The changes

7

of emission spectrum and fluorescence intensity with increasing concentration of Cu2+ are shown

8

in Figure S8. A good linear relationship (R2 = 0.996) of (F0 – F)/F (where F0 and F denote the

9

fluorescence intensity of hPEI-F CPs before and after the addition of Cu2+, respectively) versus

10

the concentration of Cu2+ ranging from 1 to 120 µM was found and a corresponding linear

11

equation (F0 – F)/F = 0.0464C - 0.1024 can be obtained, where C (µM) is the concentration of

12

Cu2+. The limit of detection (LOD) was estimated to be 0.86 µM based on 3Sb/slope (here Sb is the

13

standard deviation of the blank signals of the hPEI-F CPs), which is much lower than the maximal

14

allowable level of copper in drinking water (~20 µM) set by U.S. Environmental Protection

20


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Agency. To evaluate the practicality, we applied the probe to the determination of Cu2+ in tap

2

water samples. The samples were tested and the results are displayed in Table S1 (Supporting

3

Information). The recoveries of Cu2+ in the samples ranged from 101.44% to 109.20%. In addition,

4

the relative standard deviation (RSD) was obtained by repeating the experiment 5 times under the

5

same conditions, which were in the range of 1.06% to 4.86%. The data of the recovery value and

6

the RSD are satisfied, indicating that the probe is reliable and applicable for Cu2+ detection in

7

aqueous media.

8

We can conclude that the formations of a Schiff base and polymer particles are the most

9

important factors for the intrinsic fluorescence emission of hPEI-F CPs. The identification of

10

fluorescence mechanism may remarkably promote the preparation of more fluorescent polymer

11

particles although the detailed chemical process for producing polymer particles and gels still need

12

further study. So we tried to prepare other hPEI-based fluorescent polymer materials for simplicity.

13

Since Schiff base can be produced when compounds containing amino groups reacted with

14

aldehydes (or ketones),38 other four compounds containing aldehyde or ketone structure, including

15

D-glucose, ascorbic acid, glutaraldehyde, and acetone, were utilized to react with hPEI for the

16

preparation of water-soluble fluorescent polymer particles. As expected, all of the prepared

17

products were fluorescent (Figure 7 shows the fluorescence emission spectra). Figure S9 in the

18

Supporting Information displays their SEM images, illustrating that they are also monodisperse

19

submicron particles. The results indicate that fluorescent polymer particles with various surface

20

groups would be available by this synthesis strategy. Even so, although the pH value of the

21

reaction was fixed, reaction time and temperature to obtain these fluorescent materials were not

22

same. Just as mentioned, the condition for fabricating fluorescent hPEI-F CPs was mild. However, 21


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1

the hPEI-acetone copolymer exhibiting strong fluorescence could be obtained only under

2

relatively higher temperature and longer heating time (90 oC, 24 hours). The differences also

3

involve the mechanism about the formation of a Schiff base, in which a carbonyl carbon with low

4

charge density is easy to be attacked by primary amine. The charge density of carbonyl carbon

5

from aldehydes is lower than that of ketones, and the charge density of carbonyl carbon for

6

formaldehyde is the lowest in all aldehydes. As a consequence, aldehydes are easier to react to

7

hPEI than ketones despite under the same reaction pH. Overall, these one-pot methods for the

8

preparation of these fluorescent polymer particles based on the crosslinking of commercial hPEI

9

with aldehydes (or ketones) are relatively simple.

10 11

Figure 7. Fluorescence emission spectra of hPEI-based fluorescent polymer particles, including (a)

12

hPEI-formaldehyde (2% v/v), (b) hPEI-ascorbic acid (10% v/v), (c) hPEI-glutaraldehyde (10%

13

v/v)，(d) hPEI-D-glucose (10% v/v), and (e) hPEI-acetone (10% v/v) which were synthesized in

14

pH 5.14 BR buffer. The inset is normalized fluorescence emission spectra of these materials.

15

Excitation: 365 nm (a), 420 nm (b), 358 nm (c), 428 nm (d), and 420 nm (e).

22


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1

CONCLUSIONS

2

In summary, we discovered that fluorescent polymer particles and gels can be fabricated by

3

using hPEI and formaldehyde at different pH values. The products and their fluorescent properties

4

are highly dependent on the reaction pH. Under acidic and neutral conditions, water-soluble

5

fluorescent hPEI-F CPs were produced, while white gels with weak fluorescence emission were

6

obtained under basic conditions. The as-prepared water-soluble hPEI-F CPs shows strong intrinsic

7

fluorescence emission without the conjugation to any classical fluorescent agents or metal ions

8

and they possess good photostability in aqueous media. Furthermore, the origin of fluorescence

9

and possible mechanism were proposed and investigated by the combination of spectroscopy and

10

microscopy techniques. The fluorescence-emitting moiety is ascribed to Schiff base bond and the

11

compact structure of hPEI-F CPs plays a significant role in their intrinsic strong fluorescence. The

12

exploration on fluorescence mechanism provides a new perspective for the study of the origin of

13

intrinsic fluorescence emission from dendritic polymers. Also, it can facilitate the production of

14

more fluorescent polymer particles. Both fluorescent polymer particles and gels have been

15

proposed for extensive use in biological and medical fields over the past decades. The developed

16

method for producing polymer particles and gels was facile and inexpensive. Thus, it is very

17

promising to apply them to biological tracing, bioimaging, fluorescent sensor and drug-delivery.

18

ASSOCIATED CONTENT

19

Supporting Information

20

Additional figures and tables including excitation-dependent emission, storage stability, photos of

21

hPEI-F CPs and gels, optimization of experiments, effect of molecular weight, 1HNMR, solvent

23


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Page 24 of 32

1

polarity-responsive emission, fluorescence response to Cu2+, data of assessing Cu2+ detection, and

2

SEM images. This information is available free of charge via the Internet at http://pubs.acs.org.

3

AUTHOR INFORMATION

4

Corresponding Authors

5

*

6

Notes

7

The authors declare no competing financial interest.

8

ACKNOWLEDGMENT

9

E-mail address: [email protected], [email protected].

This work was financially supported by the National Natural Science Foundation of China

10

(No. 21273174)

and

11

CSTC–2013jjB00002).

the

Municipal

Science

Foundation of Chongqing City (No.

12

24


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pH-Mediated Fluorescent Polymer Particles and Gel from

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