Elsevier

Analytica Chimica Acta

Volume 1133, 9 October 2020, Pages 180-198
Analytica Chimica Acta

Review
GUMBOS and nanoGUMBOS in chemical and biological analysis: A review

https://doi.org/10.1016/j.aca.2020.06.028Get rights and content

Highlights

  • The tunable features of GUMBOS allow for tailoring materials for specific uses.

  • NanoGUMBOS can exhibit different properties from GUMBOS.

  • Methods for synthesis of nanoGUMBOS with defined characteristics are described.

  • GUMBOS and nanoGUMBOS were designed for sensing and solid-phase extraction.

  • NanoGUMBOS are a viable alternative to overcome potential risks of other nanoparticles.

Abstract

GUMBOS (group of uniform materials based on organic salts) is a novel class of materials that exhibits similar features to those of ionic liquids, but have melting points between 25 and 250 °C. GUMBOS can be easily converted into nanomaterials (nanoGUMBOS), with advantages of working at nanoscale. Due to the huge number of possible cation-anion combinations, these materials can be multifunctional and designed for a specific task.

This review highlights the possibility of fine-tuning GUMBOS physical and chemical properties in view of changing their ionic counterparts. Their outstanding potential for analytical applications is shown through recent developments in areas such as sensing, and solid-phase extraction. Available methods for synthesis of nanoGUMBOS, and their different outcomes in shapes and optical properties are described, with pros and cons being outlined. Finally, an analysis is made of opportunities and challenges faced by this class of organic ionic materials.

Introduction

Although ionic liquids (ILs) have received increased attention recently, their history goes back to the mid-19th century when the formation of a ‘red oil’ was observed during the course of a Friedel-Crafts reaction [1]. ILs have been defined (perhaps vaguely) as organic salts composed entirely of ions that melt below the boiling point of water [2]. The choice of this maximum temperature limit is quite arbitrary, but serves the purpose of distinguishing ILs from molten salts [3]. It is also worth noting that there are many synonyms for materials that meet ILs definition, namely ‘room temperature molten salts’, ‘liquid organic salts’, and ‘ionic fluids’ [1].

In contrast to inorganic salts that have high melting temperatures because of strong interionic electrostatic interactions, ILs exhibit much lower melting points (m.p.< 100 °C) due to inefficient packing of the ions [4,5]. Symmetry seems to play an important role in crystal packing, with irregularly shaped cations leading to poor lattice matching and reduced melting temperatures [6,7]. For example, when Na+ of sodium chloride (NaCl) is replaced by the asymmetric and bulky 1-ethylimidazolium (C2im) cation, the melting point decreases from 800 to 58 °C (Fig. 1) [5,8]. The melting points of ILs are also influenced by the size and charge distribution of constituent ions as well as by the length of alkyl side chains [7,9]. Furthermore, it has been reported that polymorphism can lead to inhibition of crystallization, consequent depression of the melting point, and thus formation of ILs [10,11].

A growing interest in ILs as greener alternatives to conventional organic solvents is due to their peculiar features, namely unique solvation capability for a wide range of polar and nonpolar molecules, great chemical and thermal stability, high ionic conductivity, large electrochemical window, low flammability and vapor pressure. Moreover, ILs properties can readily be tuned by varying the cation, anion, and attached substituents [[12], [13], [14]]. For instance, the miscibility of ILs with water is primarily dictated by the hydrophilic/hydrophobic property of the anion. Hydrophilic anions such as halides and nitrate form ILs that are water miscible, whereas more hydrophobic anions, e.g., bis(trifluoromethylsulfonyl)imide and hexafluorophosphate usually hinder aqueous miscibility. Furthermore, increasing the alkyl chain length of an organic cation leads to enhanced hydrophobicity, and thus results in lower solubility [15,16]. In addition to water miscibility and hydrophobicity, the viscosity, density, and solvation behavior of ILs can also be modulated by changing their chemical structure [[17], [18], [19]]. Similar to what happens with the aforementioned properties, toxicity and biodegradability seem to be related to the structural elements of ILs, which allows the design of less toxic and more environmentally friendly compounds [20]. Ultimately, the cation and anion may have distinct and desirable functions, enabling the preparation of multifunctional materials in a simpler and more convenient way [21,22].

Over the last two decades, tremendous progress in the field of ILs has been observed, with applications extending from analytical chemistry [23,24] and electrochemistry [25,26] to biotechnology [27,28], engineering [29,30], drug delivery, and pharmaceutics [[31], [32], [33]]. In addition, ILs have been used at the industrial level, namely in the BASIL, Difasol, and Ionikylation processes developed by BASF, IFP (Institut Français du Pétrole), and PetroChina, respectively [34]. Applications of ILs have also been proposed for the food industry, including as surfactants, lubricants, and solvents for extraction procedures [35,36]. However, it is worth noting that the use of ILs can be restricted by their relatively low maximum temperature limit. Thus, the emergence of solid-state analogues of ILs has opened many new possibilities for applications [37].

Although much attention has been paid to use of ILs in liquid-state applications, much less reference has been made to solid-phase utilization. It was only a decade ago that the first reports emerged of so-called ‘GUMBOS’ (group of uniform materials based on organic salts) [38]. These materials share similar properties to those of ILs, but can be differentiated by the melting points (< 100 °C for ILs and 25–250 °C for GUMBOS), making them suitable for analytical, biomedical, and materials application [37]. In parallel, a number of studies have focused on controlled synthesis of nanomaterials derived from GUMBOS (nanoGUMBOS) as well as their characterization and potential uses [[39], [40], [41]].

This review updates the literature regarding GUMBOS-based materials, integrating studies conducted in this field since 2014. The versatility and tunable properties of these compounds are highlighted, with emphasis placed on applications that were not mentioned in Warner’s perspective article [37]. Here, we also describe alternative strategies for synthesis of nanoGUMBOS, and discuss their pros and cons. Finally, opportunities and challenges of this novel class of materials are outlined.

A list of abbreviations, names, and chemical structures of ions typically used to form ILs and GUMBOS mentioned in this review is provided in Table 1.

Section snippets

Ionic liquids

Within ILs, two broad categories can be distinguished. These categories are based on the melting point: room temperature ILs (RTILs, m.p. < 25 °C) and frozen ILs (FILs, 25 °C < m.p.< 100 °C) [37].

Group of uniform materials based on organic salts (GUMBOS)

GUMBOS have emerged primarily from interest of the Warner research group at Louisiana State University. The desire of this group is to manipulate properties of solid-phase organic ionic materials that do not fit the general definition of ILs. This group has proposed a melting point range between 25 and 250 °C for GUMBOS [37]. Therefore, many GUMBOS can also be categorized as FILs since some have melting points that fall below 100 °C (Fig. 3).

The development of solid-phase materials from organic

Nanomaterials derived from GUMBOS

Materials at the nanoscale (dimensions in the 1–100 nm range) often exhibit unique properties that distinguish them from bulk counterparts, namely quantum size effects and enhanced surface area [107,108]. Compared to larger particles of the same composition, NPs possess higher surface area to volume ratio, which results, among other things, in increased solubility and allows use for the delivery of hydrophobic drugs [[108], [109], [110]]. Additionally, the properties of NPs are dependent on

Other examples of solid-phase ILs chemistry

Besides the literature on GUMBOS and nanoGUMBOS, there are a few more references related to solid-phase ionic materials. Similar to what happens with ILs, there is no uniformity of terminology among these studies. For instance, ‘solid ionic liquids’ have been defined as salts with melting points below 120–140 °C and have been applied as sorbents for CO2 capture after immobilization on cellulose supports by a wet coating technique [132,133]. In addition, the terms ‘ion-based organic NPs’ and

Conclusions and future perspectives

GUMBOS share similar traits with ILs and provide extraordinary potential for extending their applications. As they are tailored, materials can be designed simply by judicious choice of the cations and anions, which has been shown to increase the prospects of these materials for specific uses, including analytical ones. This has been extended to the synthesis of nanoGUMBOS with a variety of shapes and spectral characteristics suitable for several purposes.

There are already nanoGUMBOS synthesized

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was supported by UID/QUI/50006/2019 with funding from FCT/MCTES through national funds. It was also received financial support from the European Union (FEDER funds through the Operational Competitiveness Program (COMPETE) POCI-01-0145-FEDER-030163– Project Tailored NanoGUMBOS: The green key to wound infections chemsensing).

Ana M.O. Azevedo was supported by a PhD grant from FCT, with a reference number SFRH/BD/118566/2016.

Ana M.O. Azevedo received her M.Sc. degree in Pharmaceutical Sciences from the Faculty of Pharmacy, University of Porto in 2013. She is currently in the 3rd year of her Ph.D. program in Pharmaceutical Sciences at LAQV, REQUIMTE, Faculty of Pharmacy, University of Porto, under the supervision of Prof. M. Lúcia Saraiva and Prof. João Santos. Her current research interests focus on the synthesis, and sensing applications of tunable solid-phase organic ionic materials (GUMBOS and nanoGUMBOS). She

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  • Cited by (0)

    Ana M.O. Azevedo received her M.Sc. degree in Pharmaceutical Sciences from the Faculty of Pharmacy, University of Porto in 2013. She is currently in the 3rd year of her Ph.D. program in Pharmaceutical Sciences at LAQV, REQUIMTE, Faculty of Pharmacy, University of Porto, under the supervision of Prof. M. Lúcia Saraiva and Prof. João Santos. Her current research interests focus on the synthesis, and sensing applications of tunable solid-phase organic ionic materials (GUMBOS and nanoGUMBOS). She has 8-refereed publications and 9 oral communications.

    João L.M. Santos is Assistant Professor at the Faculty of Pharmacy, University of Porto, and senior scientist at LAQV/REQUIMTE Associate Laboratory. He received his PhD in Analytical Chemistry in 2000, from Porto University. His research interests include nanotechnology and nanomaterials, namely the synthesis and characterization of binary and ternary quantum dots and metallic nanoparticles and their application in photocatalysis and for photoluminescent detection and nanodiagnostics.

    Isiah M. Warner is a Boyd Professor of the Louisiana State University System and an analytical/materials chemist whose research focuses on fluorescence spectroscopy, organized media, and ionic liquid chemistry, particularly as applied to solid phase ionic materials. He has more than 350-refereed publications. His educational philosophy, developed early in his career, maintains that students learn science if they function at higher levels of Bloom’s Taxonomy. Mentoring is a major focus of his educational role and his combined efforts in research, mentoring, and education have guided numerous students to successful STEM careers. He has earned many awards for these efforts.

    M. Lúcia M.F.S. Saraiva (Ph.D. in Analytical Chemistry, 2000) is Assistant Professor in the Faculty of Pharmacy, University of Porto and research scientist at REQUIMTE Associate Laboratory.

    Her main area is the development of sustainable automated (bio)analytical tools for application mainly in biomedical and environmental areas. Its current interests include ionic liquids, deep eutectic solvents, GUMBOS and nanoGUMBOS. Now, it is focused on studying its use mainly in nano diagnostics in wounds and its impact on the environment and humans by automatic enzymatic screening methodologies and whole cell toxicity assays.

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