# Applications of Nanomaterial Coatings in Solid-Phase Microextraction (SPME)
## 4 Advances in the Adsorbent Type Used in the Extraction Phase of the SPME Fiber
### Titanium Oxide Nanoparticles as Sorbents for SPME
#### 4.1.3 Carbon Nanoparticles in SPME  
There are several different allotropic forms of carbon; the spherical C60 fullerene was the first to be discovered in 1985. Since then, other allotropes have been created or found, frequently by accident. Because of their exceptional electrical, mechanical, thermal, and adsorptive qualities, carbon nanotubes are the most often utilized of these for analytical reasons. Advanced materials such as fullerenes, carbon nanotubes, nanocones, and nanodiamonds have been used in the field of solid-phase extraction to provide analytical procedures that offer notable advantages over traditional approaches [19; 101; 102]. Carbon nanomaterials are popular nano-adsorbents in wastewater management due to their abundance, strong adsorption capabilities, cost-effectiveness, and high chemical and thermal stability. Activated carbon (AC), known for its high porosity and large surface area, has been a widely used adsorbent, though its cost drives the need for more economical options. As a result, alternatives such as graphene, carbon nanotubes (CNTs), carbon nitride (g-C\({}_{3}\)N\({}_{4}\)), and carbon dots have been explored for adsorption applications. The excellent regeneration potential of these nanomaterials further supports their use in contaminant removal from wastewater. Carbon-based nanomaterials like fullerenes and graphene, recognized by Nobel Prizes in 1996 and 2010, offer unique properties for a range of applications. Additionally, diverse carbon nanomaterials, including carbon nanoparticles, graphitic carbon nitride, ordered mesoporous carbon, and microporous carbon nanospheres, as shown in Figure 4 below, have been adapted as SPME fiber coatings [11; 103; 104]. The structural conformation and hybridization state of carbon-based nanomaterials are intimately related to their electrical, chemical, and physical characteristics. The ground-state arrangement of the six electrons in carbon is 1s\({}^{2}\), 2s\({}^{2}\), and 2p\({}^{2}\). Carbon can hybridize into sp, sp\({}^{2}\), or sp\({}^{3}\) configurations depending on its bonding with nearby atoms because of the promotion of an electron from the s orbital to an empty p orbital made possible by the tiny energy gap between the 2s and 2p shells. The higher energy of the promoted electron is offset by the energy obtained via covalent connections with nearby atoms. After taking into account the out-of-plane bonding from \(\pi\) bonds created by unhybridized p orbitals, this compensation is almost identical for both sp\({}^{2}\) and sp\({}^{3}\) hybridization [105; 106]. The unique characteristics of these materials, including their high surface areas, large aspect ratios, and exceptional mechanical strength, along with excellent electrical and thermal conductivities, have led to a wide array of applications [107]. Carbon nanoparticles have been widely used as sorbent materials in SPME. Carbon Nanotubes (CNTs) as Sorbent Materials in SPME  
Considered tiny tubes made from rolled graphite sheets, carbon nanotubes (CNTs) come in two structural varieties: single-walled (SWCNTs) and multi-walled (MWCNTs). SWCNTs may range in length from hundreds of nanometers to several microns, with diameters usually between 1 and 10 nm. However, because MWCNTs are made up of numerous concentric cylinders held together by van der Waals forces, they have larger diameters, ranging from 5 nm to several hundred nanometers [108; 109]. [PERSON] reported synthesizing a new carbon structure with needle-like tubes (carbon nanotubes) in 1991. These tubes were created using an arc-discharge evaporation procedure, which is similar to the synthesis of fullerenes [110]. Carbon nanotubes' extraordinary qualities open the door to new developments in engineering and research. In wastewater treatment and environmental monitoring, these materials offer a variety of uses for identifying and eliminating pollutants. CNTs are interesting options for SPME coatings because of their large adsorption area and high porosity.