Magnetic ionic liquids in analytical chemistry: A review

Highlights

• Review of magnetic ionic liquid applications in analytical chemistry.

• MILs can be designed for extractions, chromatography, and chemical sensing.

• Relationships between MIL structure and physicochemical properties examined.

• Advantages of MILs in magnet-based technologies are discussed.

Abstract

Magnetic ionic liquids (MILs) have recently generated a cascade of innovative applications in numerous areas of analytical chemistry. By incorporating a paramagnetic component within the cation or anion, MILs exhibit a strong response toward external magnetic fields. Careful design of the MIL structure has yielded magnetoactive compounds with unique physicochemical properties including high magnetic moments, enhanced hydrophobicity, and the ability to solvate a broad range of molecules. The structural tunability and paramagnetic properties of MILs have enabled magnet-based technologies that can easily be added to the analytical method workflow, complement needed extraction requirements, or target specific analytes. This review highlights the application of MILs in analytical chemistry and examines the important structural features of MILs that largely influence their physicochemical and magnetic properties.

Introduction

Following their discovery over a century ago, ionic liquids (ILs) have contributed to significant advances in the chemical sciences. ILs are a class of molten salts exhibiting melting points at or below 100 °C. Commonly composed of organic cations and organic/inorganic anions, ILs possess a number of advantageous physicochemical properties including negligible vapor pressure at ambient temperatures, high thermal stability, wide electrochemical window, and tunable solvation properties [1]. Owing to readily interchangeable and customizable cations and anions, an estimated 10¹⁸ possible combinations of ILs can be generated [2]. Although initially applied largely in electrochemistry [3], catalysis [4], and organic synthesis [5], rational design of the chemical structure of ILs has fueled their rapid expansion in the field of analytical chemistry [6], [7].

Recently, a new subclass of ILs known as magnetic ionic liquids (MILs) has been the subject of intensified interest in numerous analytical applications. These magnetic solvents are produced by the incorporation of a paramagnetic component in either the cation or anion of the IL structure [8], [9], [10]. Often comprised of transition metal or lanthanide metal ions, MILs possess similar physicochemical properties to conventional ILs while also exhibiting a strong response to external magnetic fields. Although the compound had been previously reported [11], the paramagnetic behavior of the 1-butyl-3-methylimidazolium tetrachloroferrate(III) ([BMIM⁺][FeCl₄⁻]) MIL was first demonstrated by Hayashi et al. using superconducting quantum interference device (SQUID) magnetometry [8]. This discovery was significant because it had been previously assumed that metal-based ILs lacked the long-range interactions responsible for paramagnetic phenomena [12]. Subsequently, MILs based on other transition metals including manganese [13] and cobalt [14] were prepared and similarly shown to be paramagnetic liquids. Rare earth metals including neodymium [15], gadolinium [13], and dysprosium [16] were also incorporated into MILs resulting in luminescent materials with considerably higher magnetic moments.

Although early research in MILs was oriented toward synthesis and fundamental studies, the paramagnetic properties and structural tunability of MILs have been an inspiration for new magnet-based technologies. Unlike ferrofluids that require suspended magnetic particles to convey magnetic properties to the bulk material, MILs are transparent and exist as neat magnetic solvents [13], as thoroughly discussed in a recent review of MILs [17]. MILs also exhibit low volatility, circumventing the need for flammable dispersants or stabilizing organic solvents that are often employed in ferrofluids to prevent particle agglomeration [18]. By modifying the chemical structure of the cation or anion, MILs can be designed to possess physicochemical properties that are desirable for specific applications. For example, functionalizing MILs with long aliphatic groups to impart hydrophobic character enables their use in aqueous extraction systems. This Review will focus on the physicochemical and magnetic properties of MILs that influence their performance in analytical applications, as well as provide an overview of the applications of MILs in analytical chemistry.