# Returning to Nature for the Design of Sportive Phases in Solid-Phase Microextraction
## 3 Cotton
Figure 1 shows the macroscopic appearance of raw cotton (Figure 1a) and CFs (Figure 1b), which maintain the fibrous structure (Figure 1c) of the original substrate. Denoted as carbon fibers aerogels (CFAs) in some cases, these materials have been employed in various fields including energy storage, adsorption, thermal insulation, and flame retardancy [52]. In the microextraction context, CFAs are usually modified following the carbonization process in order to enhance the selectivity of the sorbent material. For this reason, CFAs coated with chitosan and dopamine-polyethylenimine complex were used to remove nanopollutants such as citrate-capped gold and silver nanoparticles from water samples [53]. In some cases, activation agents such as ZnCl\({}_{2}\)[54], H\({}_{3}\)PO\({}_{4}\)[55], KOH or NaOH [56], K\({}_{2}\)CO\({}_{3}\), AlCl\({}_{3}\), and Na\({}_{2}\)HPO\({}_{4}\)[57] are used for the obtention of fibers with better strength, yield, and adsorption capacities. ACFs were used for the isolation of dyes (methylene blue, crystal violet, or Alizarin Red S) and nitrobenzene from aqueous samples [58; 59; 60; 61], in addition to the determination of chlorophenols from urine samples [62]. In the last case, the material was immobilized in a syringe barrel allowing the dispersion of the sorbent in the sample matrix, resulting in good accuracy, sensitivity, and precision values in a simple extraction procedure. Moreover, magnetic CFs have been employed for the determination of bisphenol compounds from environmental samples [63]. On the other hand, a relation between the pyrolysis degree (concerning time and temperature) with the polarity of the resulting CFs has been recently accomplished. Thereby, the pyrolysis parameters may be tailored according to the analytical problem, increasing in this way the selectivity of the extraction process [64]. In recent years, cotton waste has been employed as a novel source in the development of cellulosic-based sorbents in order to obtain a high-added-value product at low cost. A million tons of cotton cloths are produced annually, increasing at the same time the generation of their waste. Nowadays, environmental concern has gained great importance, arising the consciousness about recycling and resource utilization. For this reason, microextraction applications using waste products, such as cotton cloths waste, are increasingly appearing in scientific publications [65; 66; 67]. Cotton cloths waste has been used for the production of nanocellulose materials [68], including activated and porous carbon [69; 70]. There are two main types of nanocellulose materials arising from cotton fibers, viz. cellulose nanofibers (CNF) and cellulose nanocrystals (CNC), which differ in their dimensions, functions, and preparation methods. Generally, the production of CNF consists of mechanical treatments, while acid hydrolysis is the primary chemical process used to produce CNC [71]. These nanomaterials reveal a host of advantages such as high surface area-to-volume ratio, large aspect ratio (typically 20-70), high strength, and thermal stability, due to their nanometric dimensions together with a high degree of molecular order [72]. For this reason, CNC and CNF have been used in the microextraction context, among other applications, for isolating fluoroquinolones from egg samples as well as crystal violet dyes and magnetic nanoparticles from water samples, respectively [73; 74]. The main applications of cotton in the microextraction context are summarized in Table 2.  
Figure 1: (**a**) Raw cotton; (**b**) Carbon fibers; (**c**) Scanning electron microscopy picture of carbon fibers where the fibrous structure is observable. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Technique** & **Material** & **Analytes** & **Matrix** & **LOD** & **Instrumentation** & **Ref.