Deep eutectic solvents. The new generation of green solvents in analytical chemistry

Highlights

• DESs characteristics make them ideal as green materials in analytical chemistry.

• Different DESs (NADESs, THEDESs and polyDESs) have been used in analytical chemistry.

• DESs have been used in sorbent-based and solvent-based sample preparation procedures.

• DESs have been used in chromatographic and electrophoretic techniques.

• Further studies are needed to understand synthesis and action mechanisms of DESs.

Abstract

The incessant effort to perform a more sustainable chemistry has brought about the development of new materials that accomplish the principles of the green chemistry. In this context, deep eutectic solvents (DESs) have surged as one of the most promising alternatives to the use of toxic organic solvents. Their unique properties have led to a huge development of these materials and a sharp increase of their applications in the recent years in analytical chemistry. They have been applied in sample preparation and analytical techniques. The quantity of new information generated from studies associated with DESs is enormous and needs to be continuously revised. In this review, the most relevant aspects related to the application of DESs in the last five years (2016–2020) have been compiled and critically discussed in order to provide a global view about the advantages and limitations of these new materials in the area of analytical chemistry.

Introduction

First introduced in 2001 [1] and defined as “deep eutectic solvent” in 2003 [2], the discovery of Abbott et al. has burst into the science world with great force. In fact, it is considered as one of the most important discoveries of the 21st century, even going so far as to revolutionise the industry, medicine, and science in general; and their potential does not seem to be over yet. These neoteric solvents were initially defined as mixtures of at least two substances, a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), with a freezing point much lower than that of either of each component. The interactions established between both elements give rise with new solvents characterised by having dipolar nature, non-flammability and thermal stability, among others. Trying to respond to the 12 principles of green chemistry established by Anastas and Warner [3], it appears that DESs are promising sustainable materials candidates in the attempt to reduce or eliminate conventional solvents, and even other renown green alternatives such as ionic liquids (ILs). Generally, the most widely used DESs are considered green substances due to their high biocompatibility and biodegradability, easy synthesis, as well as the low cost and high availability of raw materials, giving rise to more than 1 million of possible combinations [[4], [5], [6]]. However, the fact that the components of DES present low or non-existent toxicity does not imply that the mixture of these results as such. Therefore, the study of the toxicity of DES is necessary to be able to describe them as “green materials”.

The interactions between HBA and HBD are, mainly, hydrogen bonds, however, van der Waals interactions or even occasional electrostatic forces are also present. Furthermore, it has been noticed that most of these are intramolecular bonds between the halide anion usually used as HBA and the HBD. However, depending on the components, the establishment of HBD-HBD hydrogen bonds is also possible, generating even more complex structural networks [7]. These strong interactions lead to mixtures with a hydrophilic character because most DESs contain an ionic component. To date, most DESs have been classified into four different types: type I – quaternary ammonium salt (QAS) and metal chloride; type II – QAS and metal chloride hydrate; type III – QAS and HBD; type IV – metal chloride hydrate and HBD.

In recent years, DESs have had an exponential evolution and the number of publications per year has tripled in the last five years. A quick search carried out at the end of July 2020 of the term “deep eutectic solvent” yields 3886 results in Scopus database (article title, abstract and keywords). Although it is not striking compared to other terms, we must consider that in the period 2004–2015, only 705 articles had been published related with this concept. Among them, most of the applications focused on synthesis, electrochemistry, the study of the properties and separation processes, and less than 1% of them intended for analysis [8]. This highlights the rapid growth and the great expansion that DESs are having in multitude of fields, changing the trends in their application towards other areas, including analytical chemistry, which today could reach more than 30% of published articles [9]. This fact indicates the need to continuously update the information to have accurate data available in the literature. In this field, the main applications of DESs have been not only in extraction processes for both solid and liquid samples or gas matrices, but also publications regarding synthesis and modification of sorbents and applications in separation techniques can be found in the literature [10].

Several review articles have described specific approaches based on DESs application in analytical chemistry. Li and Row [11] reviewed the use of DESs in dispersive liquid-liquid microextraction (DLLME) procedures. van Osch et al. [12], Lee et al. [13], Makoś et al. [14], and Dwamena [15], covered reported information related to hydrophobic DES (HDESs), including properties, types, synthesis, and their main applications in extraction procedures. Cai and Qiu [16], in 2019, published a review about the use of DES in chromatography. However, the authors themselves concluded that the information collected until then was insufficient to understand the behaviour of DESs and their advantages, as well as the mechanisms of interaction, and its influence on separation techniques. Another interesting review is the one developed by Cunha and Fernandes, about the use of DESs in extraction techniques (liquid-phase microextraction (LPME), ultrasound-assisted (UA) extraction, as solvents in liquid-liquid extraction (LLE), and as sorbents in solid-phase extraction (SPE)). They also revised their application as dissolution solvents, in electrode approaches, as molecularly imprinted polymers (MIPs) modifiers, and their use in analytical techniques. However, with the rapid advance in this field, the update of the new information obtained is needful by those researchers currently working in this or related areas. More recently, Shishov et al. [10] published a very interesting review, entitled “Deep eutectic solvents are not only effective extractants” that reviews the application of DESs in other areas of analytical chemistry such as electrochemical analysis or the modification of new sorbents, among others. For these reasons, the objective of this review is to critically complement the information from previously reported revisions and update the information of the last 5 years (2016–2020). The latest advances in analytical chemistry in solvent- and sorbent-based extraction techniques in food, environmental and biological samples will be discussed, as well as the most recent applications in separation techniques, including chromatography and electrophoresis. Besides, special emphasis will be placed on some controversial issues and the latest trends towards using natural DESs (NADESs), HDESs and polymeric-DES (poly-DESs), among other types of DESs.