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Preparation and characterization of multi stimuliresponsive photoluminescent nanocomposites of graphene quantum dots with hyperbranched polyethylenimine derivatives† Xing Liu, Hua-Ji Liu, Fa Cheng and Yu Chen* Oxidized graphene sheets (OGS) were treated with a hyperbranched polyethylenimine (PEI) under hydrothermal conditions to generate nanocomposites of graphene quantum dots (GQDs) functionalized with PEI (GQD–PEIs). The influence of the reaction temperature and the PEI/OGS feed ratio on the photoluminescence properties of the GQD–PEIs was studied. The obtained GQD–PEIs were characterized by TEM, dynamic light scattering, elemental analysis, FTIR, zeta potential measurements and 1H NMR spectroscopy, from which their structural information was inferred. Subsequently, isobutyric amide (IBAm) groups were attached to the GQD–PEIs through the amidation reaction of isobutyric anhydride with the PEI moieties, which resulted in GQD–PEI–IBAm nanocomposites. GQD–PEI–IBAm was not only thermoresponsive, but also responded to other stimuli, including inorganic salts, pH, and loaded organic guests. The cloud point temperature (Tcp) of aqueous solutions of GQD–PEI–IBAm could be modulated through changing the number of IBAm units in GQD–PEI–IBAm, by varying the type and concentration of the inorganic salts and loaded organic guests, or by varying the pH. All the obtained GQD–PEI–IBAm nanocomposites were photoluminescent, and their maximum emission wavelengths were not influenced by outside stimuli. Their emission intensities were influenced a little or negligibly by pH, traditional salting-out anions (Cl and SO42), and the relatively polar aspirin guest. However, the traditional salting-in I anion and the more hydrophobic 1-pyrenebutyric acid (PBA) guest could

Received 10th February 2014 Accepted 10th April 2014

effectively quench their fluorescence. 2D NOESY


H NMR spectra verified that GQD–PEI–IBAm

accommodated the relatively polar aspirin guest using the PEI–IBAm shell, but adsorbed the relatively DOI: 10.1039/c4nr00739e

hydrophobic PBA guest through the nanographene core. The release rate of the guest encapsulated by

the thermoresponsive GQD is different below and above Tcp.

Introduction In recent years, zero-dimensional graphene quantum dots (GQDs) have attracted much interest as uorescent nanocarbons.1–3 GQDs are semiconducting materials because of the presence of strong quantum connement and edge effects.4–9 The band gaps of GQDs can be tailored, leading to the emission of light of different colours.10–12 Furthermore, GQDs also possess other distinct merits, such as a simple fabrication process, low production cost, chemical inertness, large optical absorptivity, and ne biocompatibility, as well as low toxicity. These particular characteristics endow GQDs with many

Department of Chemistry, School of Sciences, Tianjin University and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Elemental analysis data; typical FTIR spectra; typical photographs of the GQD solution before and aer phase transition; typical luminescence photographs, and typical photoluminescence spectra. See DOI: 10.1039/c4nr00739e

This journal is © The Royal Society of Chemistry 2014

potential applications, such as bioimaging,13–15 medical diagnosis,3 catalysis,16 and photovoltaic devices.17–20 During the past decade, polymers with stimulus-responsive properties, such as fast and reversible conformational or phase changes in response to variations in temperature and/or pH, have gained much interest in the elds of biology and medicine.21 One of the most appealing stimuli-responsive species are thermoresponsive hydrophilic polymers with lower critical solution temperatures (LCSTs) in aqueous solution, which means that their solubility in water dramatically decreases above a specic temperature.22 Such thermoresponsive polymers are frequently utilized to endow thermoresponsive properties to luminescent materials. Apart from the vast numbers of thermoresponsive photoluminescent materials, a few materials with dual stimuli-responsive properties have been developed.23–26 Nonetheless, multifunctional photoluminescent materials that respond to more than two stimuli have seldom been prepared.27 To date, only a few polymeric reagents, such as polyethylene glycol (PEG) star polymers13 and diamine

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terminated PEG,10 have ever been integrated with luminescent GQDs. The main function of these polymers is to impart aqueous stability to GQDs in buffer solutions and other biological media. To the best of our knowledge, GQDs with superior biocompatibility have never been integrated with thermoresponsive polymers. Herein, we report multi stimuliresponsive GQD nanocomposites integrated with thermoresponsive hyperbranched polymers. Furthermore, such GQDs are not only photoluminescent, but are also able to host guest molecules as nanocarriers. Hence, it is possible for such GQDs to be used as a drug-delivery system with a self-bioimaging function.

Experimental Materials Synthetic graphite powder with a size of Cl > I. This sequence is in accordance with the well-known Hofmeister series for biopolymers and synthetic water-soluble polymers.29–31 The sensitivity of T-GQD3 to salts is similar to that of the dendritic thermoresponsive polymer PEI– IBAm,32,33 but much higher than that of the linear thermoresponsive polymer PNIPAm.34 For instance, concentrations of ca. 0.3 M Na2SO4 lowered the LCST of linear PNIPAm to approximately 10  C, while NaI (ca. 0.3 M) elevated its LCST by about 2  C. Whereas, from Fig. 7A it can be observed that 0.2 M Na2SO4 can lower the Tcp of T-GQD3 to about 11  C, while 0.2 M NaI can enhance the Tcp of T-GQD3 by ca. 10  C. The thermoresponsive properties of the obtained T-GQD3 are also pH-sensitive (Fig. 7B). The pH of T-GQD3 in water is close to 7. Increasing the acidity signicantly increases Tcp. Conversely, adjusting the pH to approximately 7.5 clearly lowers the Tcp. The decrease in Tcp becomes insignicant aer further

Fig. 7 The Tcp values of T-GQD3 influenced by (A) different salts: (-) NaI, (C) NaCl, (:) Na2SO4, and (B) pH (T-GQD3 is the sample, its concentration is 12 mg mL1).

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Fig. 8 A typical 2D NOESY 1H NMR spectrum of the supramolecular complex of T-GQD3 with (A) aspirin, and (B) PBA.

increasing the pH. The pH response of T-GQD3 is interpreted as follows: as the acidity of an aqueous solution of T-GQD3 increases, more amino groups of T-GQD3 are protonated to form more polar N+ groups. This signicant increase in polarity increases Tcp. In contrast, increasing the basicity gradually turns the more polar protonated N+ groups into less polar amines, decreasing Tcp. PEI–IBAm can encapsulate organic molecules with carboxylic acid groups through acid–base neutralization.35,36 Thus, we studied whether T-GQDs could also accommodate organic acid guests, and how these organic guests affected the thermoresponsive properties of the T-GQDs. Aspirin and PBA were utilized as the guest models. First, 2D NOESY 1H NMR spectroscopy, which is a powerful technique for investigating the interactions between two different components in close proximity (

Preparation and characterization of multi stimuli-responsive photoluminescent nanocomposites of graphene quantum dots with hyperbranched polyethylenimine derivatives.

Oxidized graphene sheets (OGS) were treated with a hyperbranched polyethylenimine (PEI) under hydrothermal conditions to generate nanocomposites of gr...
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