ACS Applied Materials & Interfaces This document is confidential and is proprietary to the American Chemical Society and its authors. Do not copy or disclose without written permission. If you have received this item in error, notify the sender and delete all copies. Graphene Quantum Dot-Aerogel: from Nanoscopic to Macroscopic Fluorescent Materials. Sensing Polyaromatic Compounds in Water. Journal: ACS Applied Materials & Interfaces Manuscript ID am-2018-02162m.R2 Manuscript Type: Article Date Submitted by the Author: 04-May-2018 Complete List of Authors: Martin Pacheco, Ana; Universidad de Castilla-La Mancha, Organic Chemistry Del Rio Castillo, Antonio Esau; Istituto Italiano di Tecnologia, Graphene Labs Martín, Cristina; Universidad de Castilla-La Mancha, Herrero, Maria; Universidad de Castilla La Mancha, Departamento de quimica Organica Merino, Sonia; Universidad de Castilla-La Mancha, Area de Quimica Organica Fierro, Jose; CSIC, ITO. CATAL. PETROL. Díez Barra, Enrique; University of Castilla-La Mancha, Organic Chemistry Vázquez, Ester; Universidad de Castilla-la Mancha, ACS Paragon Plus Environment Page 1 of 38 ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 Graphene Quantum Dot-Aerogel: from Nanoscopic 8 9 10 11 to Macroscopic Fluorescent Materials. Sensing 12 13 14 15 16 Polyaromatic Compounds in Water. 17 18 19 20 Ana Martín-Pacheco,† ‡ Antonio Esaú Del Río Castillo,† ‡ Cristina Martín,† María Antonia 21 22 †ʃ †ʃ § †ʃ 23 Herrero, Sonia Merino, José Luis García Fierro, Enrique Díez-Barra * and Ester 24 25 Vázquez†ʃ* 26 27 28 † Instituto Regional de Investigación Científica Aplicada (IRICA), 13071 Ciudad Real, Spain. 29 30 31 ʃ 32 Departamento de Química Orgánica, Facultad de Ciencias y Tecnologías Químicas, 33 34 Universidad de Castilla-La Mancha (UCLM), 13071 Ciudad Real, Spain. 35 36 37 § Instituto de Catálisis y Petroleoquímica, CSIC, 28049 Madrid, Spain. 38 39 40 41 KEYWORDS: graphene quantum dots, aerogel, nanocomposite, sensor, polyaromatic 42 43 compounds 44 45 46 47 ABSTRACT: Fluorescence based on quantum confinement is a property restricted to the 48 49 nanoscopic range. The incorporation of nanoparticles in a three-dimensional polymeric network 50 51 could afford macroscopic scaffolds that show nanoscopic properties. Moreover, if these scaffolds 52 53 54 are based on strong bonds, the stability of the resulting materials can be preserved, thus 55 56 enhancing their final applications. We report for the first time the preparation of a graphene 57 58 59 60 ACS Paragon Plus Environment 1 ACS Applied Materials & Interfaces Page 2 of 38 1 2 3 quantum dot (GQD)-composite based on a cationic covalent network. This new material has 4 5 6 unusual features: (i) the final composite remains stable after several swelling-deswelling cycles, 7 8 thus demonstrating the strong interactions between GQDs and the polymeric material, and 9 10 therefore it could be used as a portable system. (ii) Fluorescence emission in the composite and 11 12 in solution is quasi-independent to the excitation wavelength. (iii) However, and in contrast to 13 14 15 the behavior observed in GQD solutions, the fluorescence of the composite remains unaltered 16 17 over a wide pH range and in the presence of the different ions commonly found in tap water. (iv) 18 19 Fluorescence quenching is only observed as a consequence of molecules that bear aromatic 20 21 22 systems and this could be applied to the preparation of in situ water sensors. 23 24 25 INTRODUCTION 26 27 1 28 Quantum confinement is the origin of the fluorescence of quantum dots. Among the quantum 29 30 dots reported to date, carbon (CQDs)2–4 and graphene (GQDs)5,6 quantum dots have received a 31 32 great deal of attention due to their chemical inertness, low toxicity, hydrophilicity and 33 34 35 photostability. In recent studies, the fluorescence properties of these nanoparticles have been 36 37 exploited, e.g., materials for solar cells,7 analytical science8–10 and a biosensing toolkit,11 and 38 39 biomedical applications.12,13 However, the tendency for aggregation of these fluorescent 40 41 42 nanoparticles in dry states is considered to be a limiting factor that can affect the 43 14,15 44 photoluminescence behavior. In an effort to prevent this agglomeration, nanoparticles can be 45 46 incorporated into appropriate matrices. 47 48 Soft matrices such as hydrogels have been extensively used and have shown great potential for 49 50 16,17 51 a wide range of relevant applications. In particular, the incorporation of nanoparticles within 52 53 three-dimensional hydrogel structures to form nanocomposites is a useful strategy to enhance the 54 55 mechanical properties or the response to certain stimuli. Taking advantage of this synergistic 56 57 58 59 60 ACS Paragon Plus Environment 2 Page 3 of 38 ACS Applied Materials & Interfaces 1 2 3 effect, hydrogel nanocomposites have found applications in diverse fields and, in this respect, it 4 5 18,19 6 is worth highlighting the biomedical applications. 7 8 Several research groups have incorporated QDs into hydrogels to preserve the nanoparticle and 9 10 impart photoluminescence on the resulting nanocomposite.20,21 However, there are fewer 11 12 examples of hydrogels in which carbon or graphene quantum dots are embedded. Recently, low 13 14 15 molecular weight gelators (LMWG) have been used to prepare hybrid carbon dot-hydrogels. 16 17 These nanocomposites are fluorescent probes for mono-22 and divalent cations.23 Silane 18 19 derivatives of carbon dots have also been entrapped in organically modified silicate thin film 20 21 24 22 xerogels and the intrinsic fluorescence of C-dots is maintained. In addition, carbon-dot-loaded 23 24 alginate gels have been described as a copper(II) probe.25 In a very recent example, a GQD- 25 26 amino acid supramolecular hydrogel composite has been described that shows electron transfer 27 28 26 29 and self-healing properties. All of these approaches lead to hydrogels and organogels based on 30 31 electrostatic interactions, hydrogen bonds, van der Waals forces or π−π interactions, amongst 32 33 other interactions (physical gels). Furthermore, chemically crosslinked hydrogels based on 34 35 27,28 36 carbon dots have been described in the literature. For instance, a chemically crosslinked 37 38 hydrogel based on acrylamide and acrylic acid incorporating graphene quantum dots has been 39 40 recently prepared as a white-light-emitting hydrogel.29 This material has been used as a sensor of 41 42 43 pH, vapors or temperature. 44 45 In general physical gels are prepared using reversible interactions while covalently bonded 46 47 hydrogels are more robust materials and can afford reversible swelling-deswelling cycles without 48 49 compromising the mechanical properties. This property can be very useful, for example in the 50 51 30 52 preparation of 3D aerogels starting from polymer hydrogels. 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 3 ACS Applied Materials & Interfaces Page 4 of 38 1 2 3 The preparation of a macroscopic, three-dimensional and stable material that has fluorescence 4 5 6 properties associated with the quantum confinement of graphene quantum dots remains a 7 8 challenge. We envisioned these materials as easily handled sensors, in the form of dry portable 9 10 ‘stones’ that could be transported outside the laboratory and swelled in situ for the detection of 11 12 water contaminants. 13 14 15 We report here the preparation of macroscopic fluorescent scaffolds consisting of GQD-based 16 17 hydrogels that can be easily transformed into aerogels. GQDs were prepared by a completely 18 19 green protocol. The hydrogel is a cationic network prepared by radical polymerization of [2- 20 21 22 (acryloyloxy)ethyl]trimethyl-ammonium chloride (AETA) and N,N'-methylenebisacrylamide 23 24 (MBA) as a cross-linker. The positively charged nitrogen was used as an anchor for the GQDs. 25 26 The fluorescence properties of the materials were studied. The macroscopic structure retains the 27 28 29 quasi-independent emission (λem) from the excitation wavelength (λex) observed for the GQDs. 30 31 The possibility of using these nanocomposites as sensors for the detection of polyaromatic 32 33 molecules in water was also studied. 34 35 RESULTS AND DISCUSSION 36 37 38 Preparation and characterization of GQDs and their photoluminescence and sensing 39 40 properties 41 42 The application of the Principles of Green Chemistry31 to nanotechnology has been a frequent 43 44 32 45 theme in recent years. Efforts are currently focused on the assessment of environmental and 46 47 safety risks associated with the materials themselves, rather than the risks related to their 48 49 synthesis and/or modification. Synthetic routes are generally not optimized to satisfy green 50 51 52 nanoscience objectives, such as energy saving or waste minimization, and this approach is 53 54 transferred to industrial processes. This situation encouraged us to develop a new, easy and green 55 56 57 58 59 60 ACS Paragon Plus Environment 4 Page 5 of 38 ACS Applied Materials & Interfaces 1 2 3 method for the preparation of these materials based on mechanochemistry.33–37 GQDs were 4 5 6 prepared by ball-milling graphite in the presence of sodium percarbonate (SPC) as a solid 7 38 8 oxidant. Sodium percarbonate is a cheap, non-toxic, stable and easily handled oxidant. This 9 10 oxidant is a green source of hydrogen peroxide and carbon dioxide is the only by-product.39 In 11 12 addition, mechanically induced oxidation preserves the crystallinity of natural graphite40 and 13 14 15 guarantees a large number of edge-located oxygenated functional groups, which provide good 16 17 water solubility and can also be used for further functionalization. 18 19 This approach is related to the mechanochemical method reported previously by our group for 20 21 41 42 22 the preparation of few-layered graphene from graphite and from carbon fibers.