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Smart Materials: Made on Measure Reagents
Francesc A. Esteve-Turrillas and Miguel de la Guardia
Department of Analytical Chemistry, University of Valencia, Burjassot, Spain
1.1 Role of Smart Materials in Analytical Chemistry
Analytical chemistry can be considered, from an applied pragmatic point of view, as the development and application of chemical methods to find an appropriate answer to social and research and development challenges by solving underlying analytical problems [1]. Thus, analytical chemistry is a multidisciplinary science in continuous evolution that must be adapted to face new problems and limits. Modern analytical chemistry must be focused to provide validated methods and tools to fulfill solutions to present and future issues, in a rapid and efficient way without any reduction of the main figures of merit of available methods while reducing human and economic consumed resources, without forgetting to be environmentally conscientious. In this sense, the conception of Green Analytical Chemistry considers in its 12 principles aspects such as: (i) direct analytical techniques instead of sample treatment, (ii) sample and residue reduction, (iii) automatization and miniaturization, and (iv) multianalyte determination methods [2]. Improvements in current analytical instrumentation have allowed achievement of many of these milestones, but their use in combination with smart materials has allowed us to go a further step.
A specific definition of smart materials is that they have some properties that can be modulated significantly in a controlled way through external stimuli such as stress, temperature, pH, moisture, electric, or magnetic fields [3, 4]. However, in this book we have focused on the analytical process and define as smart materials those tailored, task-specific, or designed materials that provide tremendous enhancements of practical properties, at any level of sample preparation and analytical determination, such as their selectivity, sensitivity, easy automation, or speediness. Consequently, their use can incorporate added value to well-established analytical methods. Discoveries of novel functional materials have played very important roles in improving conventional analytical methods and in developing novel technologies and procedures, giving huge improvements in terms of sensitivity, selectivity, ease of use, rapidity, and miniaturization of modern analytical methods.
In recent years, the application of smart materials has attracted the attention of researchers, as shown by the high increase in published papers related to analytical determination using smart materials (Figure 1.1). Nanoparticles, carbon-based materials, ionic liquids, enzymes, antibodies, aptamers, molecularly imprinted polymers (s), restricted access materials (s), or metal-organic frameworks (s) are among these new tools suitable for modifying the characteristics of analytical methods. These smart materials have been applied in different steps of an analytical process, affording high efficacy sorbents in sample treatment, improved stationary phases in chromatography, main molecular recognizing components of electrochemical sensors and portable systems, among other functions. In this chapter and throughout both volumes of the Handbook of Smart Materials in Analytical Chemistry the main advantages and uses of these special reagents will be analyzed in detail.
Figure 1.1 Evolution of the number of articles published in per-reviewed journals related to analytical determination using smart materials, such as nanoparticles, carbon nanotubes (CNTs), graphene, ionic liquids (s), quantum dots (QDs), antibodies, immunomaterials, aptamers, metal-organic frameworks (MOFs), and molecularly imprinted polymers (MIPs).
Source: Scopus (Elsevier B.V., Amsterdam, Netherlands).
1.2 Smart Materials for Sample Treatment
Usually, an analytical procedure has been considered as a succession of steps systematically organized, like a chain made up of several links, with the treatment of samples being the most crucial step, and also the weakest, link (see Figure 1.2). Moreover, it has been quantified that sampling and sample treatment steps involve 67% of the analysis time, but most importantly they give rise to 60% of error sources [5]. Sample preparation generally involves the clean-up of the sample matrix and the enrichment of target analytes to provide an interference-free signal enhancement. Consequently, both the sensitivity and selectivity enhancement of the method are the main challenges. In particular, sample preparation is the most critical step in the analysis of biological matrices, due to the complexity of the matrix and the presence of multiple interferents at diverse concentrations, such as protein, polypeptides, lipids, fatty acids, sugars, etc., together with analyte related species such as metabolites [6]. In this sense, the development and use of novel smart materials with improved properties for sample treatment is considered one of the most promising strategies to improve practical aspects and, in particular, to decrease analysis time and labor, together with an increase in the efficacy, selectivity, simplicity, and speed of the treatment. Obviously, the final analytical properties of the method not only depend on the sample treatment, they are strongly related to the employed separation method (liquid and gas chromatography, or capillary electrophoresis) and the detection technique. Thus, chromatography techniques coupled to mass spectrometry provide high selectivity and sample treatment is based on a simple clean-up of extracts or sample matrix directly to remove macromolecules and proteins using inexpensive and low selective sorbents; while using detection systems with relatively low selectivity, such as UV-visible, fluorescence, or ion mobility spectrometry, the use of sorbents with high selectivity toward target analytes is required. Thus, the application of smart materials for sample treatment can be summarized as: (i) increased selectivity in the target analyte retention and pre-concentration, (ii) high adsorption capacity due to the improved surface area to volume ratio, (iii) extension of novel chemical analyte-sorbent interactions with high extraction efficacy, and (iv) easy handling of materials and speed of processes related to the use of magnetic materials. On the other hand, the aforementioned advances provided by smart materials in the separation and determination steps focus on the improvement of selectivity including specificity in chiral analysis or the separation of strongly related chemical forms. In this sense, the use of smart materials for building column or capillary materials together with their use as mobile phases have been exciting possibilities in clinical, environmental, and food analysis.
Figure 1.2 Steps of the analytical process, their objectives and challenges, and the main advantages provided by the use of smart materials.
Figure 1.3 shows the most promising smart materials employed as sorbents for selective and non-specific sample treatments. Smart materials employed for the selective extraction of target analytes include antibodies and aptamers, from biological sources, but also synthetic materials like MIPs, MOFs, and RAMs. In the case of non-specific sorbents, many sample treatment approaches have been developed using materials like graphene, carbon nanotubes (s), silica nanomaterials and monoliths, surfactant-based compounds, or ionic liquids, which offer high extraction efficacies and could be also improved by the incorporation of modified surface activities for the selective extraction of target analytes. In fact, all the aforementioned smart materials have gained the attention of researchers to be employed as sorbent in different extraction techniques [7].
Figure 1.3 Selective (a) and non-specific (b) smart materials employed as sorbents for sample treatment.
1.2.1 Solid-Phase Extraction
Worldwide, one of the most frequently used sample treatment techniques in laboratories is solid-phase extraction (), where the target analytes are transferred to a solid sorbent from a liquid or dissolved sample; the analytes are released in a later step using elution solvents. SPE provides as main advantages simplicity, versatility, efficacy, low-cost, and high recoveries. Traditional SPE sorbents are based on adsorption, reversed phase, normal phase, and ion exchange interactions, using silica gels with chemically bonded stationary phases or porous polymers. The development of novel sorbents for SPE has played an important role in recent decades, in order to improve extraction efficiency and selectivity [8].
The use of carbon-based materials as SPE sorbents was introduced following the discovery of fullerene (C60) in 1985, with the use of materials like single- and multi-walled CNTs, nanohorns, nanocones, nanofibers, graphene oxide, or graphene [9]. CNTs have been widely employed in recent years because of their p-p interactions with aromatic compounds, as well as their interesting properties like high surface area, easy functionalization, wide accessibility, and relatively low price [10]....
