Facile synthesis of intrinsically photoluminescent hyperbranched polyethylenimine and its specific detection for copper ion
Graphical abstract
Introduction
Photoluminescent polymers have attracted great attention during the past decades, because the functionalities and processability of polymers were integrated with light-emitting property together, which made them more conveniently used in various applications, such as bioimaging [1,2], fluorescent sensors [3], organic electronics and photonics [4], and so on.
Photoluminescent polymers generally contain aromatic building blocks acting as emitting entities. However, in 2004 two popular dendrimers, poly(amido amine) (PAMAM) and poly(propylene imine) (PPI), without any type of proverbial emitting units were addressed to show strong intrinsic photoluminescence (PL) under suitable conditions [5,6]. Since then, the intrinsically photoluminescent polymers (IPPs) without any conventional chromophores have aroused increasing attention and been extensively investigated. The most studied IPPs are those amino-rich ones carrying dendritic architectures, such as poly(amido amine) [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16]], poly(propylene imine) [13], polyethylenimine [[17], [18], [19]], poly(amino ester) [20,21], polyurea [22], poly(amido acid) [23], polysiloxane [24], phenolic formaldehyde amine [25] and poly(propyl ether imine) [26]. Since strong PL is also observed from linear amino-rich polymers including polyethylenimine [17], and poly(N-vinylimidazole) [27], dendritic architecture is only a favorable factor for light-emitting, but not the light-emitting origin. For amino-rich IPPs, the real luminogens have been ever assumed to be related with tertiary amine [21], oxidized tertiary amine [13,[28], [29], [30]], secondary amine oxide [31,32], or oxime [33]. Recently, Zhu group reported that both linear and hyperbranched aliphatic poly(amido amine)s exhibited aggregation-induced emission (AIE) phenomenon and their fluorescence could be dramatically enhanced by simple aggregation of polymer chains [34]. The emission mechanism was ascribed to the formation of a variety of intra- and interchain clusters consisting of amide and amine groups with shared lone-pair electrons, while the AIE mechanism is attributed to the structural rigidification of these “cluster chromophores” resulting from the restriction of intramolecular motions. Besides these amino-rich IPPs, others without amine groups were also found to illuminate under certain condition, which included linear poly(N-vinylpyrrolidone) [32], hyperbranched polyether [35], hyperbranched polycarbonate [36], hyperbranched poly(ether amide) [37], hyperbranched poly(ester-amide-ether) [38], hyperbranched polysiloxane [39,40], hyperbranched polyester [41], polyacrylonitrile [42], dithiol/amino-succinimides [43], sulfonated acetone-formaldehyde condensate [44], sulfonated ethylenediamine-acetone-formaldehyde condensate [45], polyisobutene succinic anhydrides and imides [46], and poly[(maleic anhydride)-alt-(vinyl acetate)] [47]. Based on the totally non-conjugated molecular structure of these polymers, cluster induced emission was recently proposed as the luminous mechanism in many systems.
Hyperbranched polyethylenimine (HPEI) is a water soluble cationic functional polymer containing numerous amino groups and its application fields are very wide. Commercial available HPEI is almost non-fluorescent, however, intrinsically photoluminescent HPEI (IP-HPEI) could be obtained upon modification with non-fluorescent units, such as methylation [17], grafting with cyclohexane carboxylic amide groups [19], Michael addition with divinylsulfone [48], formaldehyde crosslinking [49,50], pegylation [51], grafting with hyperbranched polyglycerol [12]. The above modifications were all carried out in solvents and organic modifiers were employed. Moreover, subsequent purification was normally required. In this work, we introduced a simpler and greener method to prepare IP-HPEI, i.e. heating the neat HPEI under air for 24 h, where none of solvents, organic modifiers and purification was required. Moreover, the obtained IP-HPEI could be used as fluorescence sensor to selectively detect copper ion.
Section snippets
Materials
Hyperbranched polyethylenimines, HPEI-1.2K (Alfa Aesar, Mn = 1200 g/mol, Mw/Mn = 1.04), HPEI-1.8K (Alfa Aesar, Mn = 1800 g/mol, Mw/Mn = 1.04) and HPEI-10K (Aldrich, Mn = 1.0 × 104 g/mol, Mw/Mn = 2.5) were used as received. Glycidol (99.9%, Alfa Aesar) was purified by distillation from CaH2 directly prior to use. Isobutyric anhydride (97%) was purchased from Alfa Aesar. All the inorganic salts were analytical grade and used as received.
Syntheses of IP-HPEIs
A certain mass of HPEIs in an open vessel was violently
Preparation and fluorescence properties of IP-HPEIs
The commercial-available and viscous HPEI polymers with Mn = 1.0 × 104, 1.8 × 103 and 1.2 × 103 g/mol are abbreviated as HPEI-10K, HPEI-1.8K and HPEI-1.2K, respectively. They are non-fluorescent in any state. However, when neat HPEIs are directly exposed under air and heated above 80 °C, they become fluorescent under UV irradiation. PL measurement of the aqueous solution of IP-HPEI (1 mg/mL) shows that mono-modal excitation and emission peak appear at around 360 and 460 nm, respectively (Fig. 1
Conclusions
IP-HPEIs can be prepared through directly heating the viscous neat HPEIs under air at 180 °C for 24 h. This is a simpler and greener method for the syntheses of IPPs in comparison with other reports. IP-HPEIs show stronger emission in more acidic condition, with the addition of nonsolvent or using lower molecular weight of HPEI as precursor. The dilute aqueous solution of IP-HPEI emits blue light. Raising the IP-HPEI concentration can lead to the emission light change from blue to green. The
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (21875159) and Tianjin Natural Science Foundation (18JCYBJC86800).
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