ReviewMagnetic ionic liquids in analytical chemistry: A review
Graphical abstract
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.
Section snippets
Magnetic susceptibility
The ability to precisely control the motion of MILs by application of a magnetic field represents an important advantage of these materials over conventional ILs. Fig. 1 shows an example of the manipulation of a MIL using a neodymium magnet. By incorporating different paramagnetic metals in either the cation or anion of the IL structure, the magnetic susceptibility of MILs can be tuned for a specific application. Higher magnetic moments may be advantageous for MIL solvents when employed in
MIL-based extractions in non-aqueous samples
Liquid-liquid extraction (LLE) is a technique that relies on the differential partitioning of solutes between two immiscible liquid phases to extract analytes, typically from a complex sample matrix. Although conventional ILs can be viewed as alternatives to the flammable or toxic organic solvents employed in classical LLE approaches, IL-based LLE typically requires centrifugation or evaporation to isolate the analyte-enriched extraction phase prior to analysis. In addition to the desirable
Conclusions and outlook
The application of MILs in analytical chemistry is rapidly expanding. By combining magnetic properties and structural tunability within the MIL solvent, new magnet-based technologies are being investigated. From analytical extractions to chemical sensors, the ease with which MIL solvents can be magnetically manipulated represents a key advantage over conventional methods. Furthermore, the physicochemical and magnetic properties of MILs can be readily tailored to meet the requirements of a
Kevin D. Clark obtained his BA degree in chemistry at Gustavus Adolphus College (Minnesota) in 2012. He is currently a PhD candidate in analytical chemistry at Iowa State University under the supervision of Dr. Jared L. Anderson. His research involves the application of magnetic ionic liquids in sample preparation and bioanalytical chemistry.
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Kevin D. Clark obtained his BA degree in chemistry at Gustavus Adolphus College (Minnesota) in 2012. He is currently a PhD candidate in analytical chemistry at Iowa State University under the supervision of Dr. Jared L. Anderson. His research involves the application of magnetic ionic liquids in sample preparation and bioanalytical chemistry.
Omprakash Nacham received his Bachelor of Science degree in Pharmacy from Osmania University (Hyderabad, India) in 2009. He worked as a senior executive in quality assurance department in Dr. Reddy's Laboratories (Hyderabad, India) from 2009 to 2011. He is currently a PhD student at Iowa State University under the supervision of Prof. Jared Anderson. His research involves the synthesis and design of different classes of ionic liquids and their application in nucleic acid analysis.
Jeffrey A. Purslow received his Associate of Science degree and Associate of Arts degree from Williston State College in 2013 and his Bachelor of Science degree in chemistry from Worcester State University in 2015. He is currently a Ph.D student at Iowa State University under the supervision of Prof. Jared Anderson. His research explores the use of magnetic ionic liquids as extraction solvents for the extraction of DNA.
Stephen A. Pierson received his BS degree from Worcester State University (Massachusetts) in 2015. He currently attends Iowa State University where he is a graduate student studying analytical chemistry under the supervision of Dr. Jared L. Anderson. His research involves the synthesis of new classes of magnetic ionic liquids.
Jared L. Anderson is a Professor in the Department of Chemistry at Iowa State University. His research focuses on the development of stationary phases for multidimensional gas chromatography, alternative approaches in sample preparation, particularly in nucleic acid extraction, and developing analytical tools for trace analysis within active pharmaceutical ingredients.