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Nanomaterials (NMs) in Analytical Sciences

1.1 Introduction

Analytical science is the branch of science that finds solutions and improvement of classical methods by fulfilling the demands for analytical information modeled by modern economic and scientific society [1]. Analytical information is about describing chemical systems and determination of their components. Technically, analytical information consists of two major components, i.e. analytical capabilities and analytical properties of the system to be analyzed. The measurement of upgrading of an analytical method, characterization, and its results is called analytical capabilities, while analytical properties can be of three different types, i.e. basic, principal, and socio-environmental. In the past, most of the developments are generally focused on techniques and solving instrumental problems, and there was no scope of socio-environmental aspects.

The rapid progresses of the microelectronic industry since 1960s established semiconductor preparation techniques to enhance the density of transistors in integrated circuits. In 1980s, these techniques led to the first design and fabrication of the microelectromechanical systems (MEMSs) [2]. With the introduction of microelectronics and computer technologies, analytical science has undergone remarkable expansions in terms of automation, miniaturization, higher sensitivities, and greater precision and complete analytical effectiveness.

Nanotechnology is defined as the technology that appreciates and controls the matter at dimensions between 1 and 100?nm, called nanomaterials (NMs). NMs have received much attention in the past decade due to the novel physical and chemical properties associated with their size and shape [3-12]. Widespread applications and outstanding performance of NMs not only have accelerated the development of materials science but also offer many opportunities in other related disciplines and have a significant impact on many fields of science, including chemistry, electronics, optics, medicine, and bioanalysis. NMs possess different properties compared to the same material in their coarser or bulk form. NM structures can be in the form of particles, pores, wires, or tubes or combination of these. Unusual properties such as high conductivity, greater heat transfer, higher melting temperature, exceptional optical and magnetic properties, and super adsorbent, etc., provide NMs for a wide range of novel applications [13-20]. The main objective of nanotechnology is to use these exceptional properties of NMs and develop new products, tools, and methods. Nanotechnology can play an important role in analytical sciences. Principal design and fabrication of NMs with the incorporation of interfacial elements would be of paramount importance for the whole process of molecular analysis at present times and near future. The objective of this chapter is to focus on recent developments with different types of NMs in analytical sciences, i.e. sample preparation, separation, extraction, and identification techniques.

NMs can be highly selective materials when it comes to purification techniques. During last few years, these have been intensively researched in all types of chromatographic techniques such as gas chromatography [21], liquid chromatography [22], capillary chromatography [23], and membrane technology [24]. Overall, NMs are proven extremely important and exhibited effective improvements over present systems. However, obstacles to overcome them in separation sciences are the aggregations, stability, safety, and economic issues.

This chapter provides an overview of NMs in analytical sciences. The chapter starts with the description of the types of NMs including carbon nanotubes (CNTs), fullerenes (FULs), graphene, inorganic nanoparticles, and magnetic nanoparticles. Then, various examples on the application of NMs are demonstrated.

1.2 Types of NMs

Unique properties of NMs and new methods for the analysis of NMs are strong areas for research these days. NMs have initiated the development of new ways of performing target concentration and detection and new analytical methods and instrumentation for measuring the properties of NMs. The optical and electronic properties of NMs are often dominated by their surface chemistry, and this makes the task of analyzing NMs immensely more challenging than bulk materials or homogeneous solutions. A particularly intriguing incentive for developing new approaches to NMs fabrication is the potential for creating new constructs with nanoscale order and unique functional properties. NMs for analysis can be constructed with the manipulation of one or more components.

1.2.1 Graphene

Graphene was discovered in 2004 by Professor Andre Geim and Professor Kostya Novoselov from the University of Manchester [25]. Since then, many efforts were put into the design and development of new graphene-based functional materials. In addition, the Nobel Prize for Physics in 2010 was awarded to Geim and Novoselov in the University of Manchester for their research on graphene [26]. The excellent features of graphene make it the starting point of new technologies in many application areas. Although graphene has a very low thickness, it is more robust than the diamond and 300 times stronger than steel. It conducts electricity better than copper, passes light, and bends without any deformation and can be put into any desired shape. It also exhibits great features such as large surface area, high stability, and layered structure. These superior features of graphene make it as an excellent NM in many applications such as separation processes, sensors, bioimaging, and drug delivery [27-30].

Due to the large requirement for research and applications, there has been much investment put into the development of methods to prepare the high-quality graphene in bulk. The preparation methods can be classified into two categories, one is "top-down" and the other is "bottom-up." The former one is to break graphite into graphene through approaches like mechanical cleavage, liquid exfoliation, thermal expansion, and electrochemical exfoliation. The latter one is to synthesize graphene by the techniques such as chemical vapor deposition (CVD), arc discharge, epitaxial growth on SiC, and unzipping CNTs [31-34].

1.2.2 Carbon Nanotubes (CNTs)

CNTs were first discovered by Ijima [35] in 1991 and were successfully employed for different purposes in analytical sciences due to their mechanical, electric, optical, and magnetic properties as well as their extremely large surface area [36, 37]. CNTs are hollow graphitic materials composed of one or multiple layers of graphene sheets: single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs), respectively. The schematic depiction of the formation of SWCNTs and MWCNTs by rolling of graphite layer is shown in Figure 1.1.

The synthesis of CNTs can be carried out by means of three main techniques: CVD [39], laser ablation (LA) [40], and catalytic arc discharge (CAD) [41]. CVD seems to be the most efficient approach for the synthesis of CNTs for analytical applications due to high purity and desirable tuning at low temperature. However, for all the synthesis methods, the presence of different undesired by-products (such as carbonaceous residues, amorphous carbon, metal impurities, and others) makes it necessary to purify CNTs. To purify the synthesized nanotubes, different strategies including chemical oxidation, physical separation, or combination of both chemical and physical techniques have been employed so far. Chemical oxidation is a purification system based on the fact that carbonaceous impurity residues are oxidized sooner than CNTs. The main advantage is its easy use, but it should be noted that the oxidation process affects the structure of the nanotube introducing functional groups (hydroxyl, carbonyl, and carboxyl) and defects in the side walls. Physical purification procedures are based on the different physical properties (such as size, ratio, weight, electrical and magnetic characteristics, etc.) between impurities and CNTs. Filtration, centrifugation, chromatography, and electrophoresis are the commonly employed techniques. The disadvantages of these procedures are: first, the elimination of certain impurities is inefficient; second, a high dispersion of CNTs is required; and third, only a low quantity of CNT can be purified.

Figure 1.1 The schematic depiction of the formation of SWCNTs and MWCNTs by rolling of graphite layer.

Source: Khan et al. [38]. © 2017, Elsevier.

The adsorption sites on CNTs are on the wall and in the interstitial spaces between tubes. These sites are easily accessed for both adsorption and rapid desorption. The impurity coverage on the CNT reduces their availability because the sorbate has to diffuse through these impurities to reach the CNT. Moreover, the porous structure of impurities introduces mass transfer limitations, slowing both adsorption and desorption. Understanding these characteristics is important for their application separation media. The excellent features of CNTs, along with their nanoscale features, make them ideal...