# 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
Their practical usefulness is further enhanced by their greater thermal and  
Figure 4: The basic building blocks of carbon nanomaterials. Printed with permission from [104]. Copyright 2020, Elsevier. physical stability as compared to traditional SPME coatings [108]. As stated above, CNTs have two structural forms, namely single-walled (SWCNTs) and multi-walled (MWCNTs). Single-walled carbon nanotubes (SWNTs) are nanoscale tubes formed from a single rolled graphite layer, with diameters in the nanometer range and lengths reaching several micrometers, making them a prime example of one-dimensional materials. Because of the lack of mass and charge, photons are a great tool for studying and characterizing their unique structure [111; 112]. Multi-walled carbon nanotubes (MWCNTs) are cylindrical nanostructures composed of sp\({}^{2}\) carbon. They have aspect ratios ranging from 10 to 10 million with diameters ranging from 3 to 30 nm and lengths up to several centimeters. MWCNTs are more rigid than single-walled nanotubes because of their multi-layered structure. They are also different from carbon nanofibers in that they have a hollow center, smaller exterior diameters, and a different wall structure [113]. [PERSON] et al. described a novel method for assessing aroma compounds in water that employs a recently developed SPME coating based on multi-walled carbon nanotubes (MWCNTs). They emphasized the use of helical MWCNTs as an excellent adsorbent because of their high extraction efficiency. The study used a desirability functions method to optimize extraction time and temperature, which improved the process's performance. The validation results show that this approach is very dependable for detecting analytes at ultra-trace levels, with detection limits ranging from 0.2 to 13 ng/L, good precision (relative standard deviations \(<\) 20%), and high recovery rates (80 \(\pm\) 12% to 111 \(\pm\) 11%) [114]. [PERSON] et al. investigated the viability of using single-walled carbon nanotubes (SWCNTs) as an SPME adsorbent to detect monobutyltin, dibutyltin, and tributyltin in seawater. They stated that they adjusted key elements determining extraction efficiency, such as extraction and desorption time, temperature, and salinity. The approach showed linear calibration curves (r \(>\) 0.992) for concentrations ranging from 12 to 2000 ng/L, with detection limits \(<\) 5 ng/L, matching International Conference on Harmonization (ICH) criteria. They observed that, in comparison to commercial carboxen/polydimethylsiloxane fibers, the SWCNT fiber had greater heat stability (over 350 \({}^{\circ}\)C) and endurance, with a lifespan of more than 150 uses. They also stated that the procedure was successfully applied to Persian Gulf harbor seawater samples, where butyl tins were discovered, confirming SWCNTs' potential as a reliable SPME adsorbent for environmental analysis of butyltins in seawater [115]. [PERSON] et al. developed a novel SPME method for analyzing polybrominated diphenyl ethers (PBDEs) in environmental samples that uses multi-walled carbon nanotubes (MWCNTs) as the fiber coating for gas chromatography with electron capture detection (GC-ECD). They found that the MWCNT coating had much larger enhancement factors (616-1756) than poly (5% dibenzene-95% dimethylsiloxane) (139-384) and activated carbon coatings (193-423). A 30 min extraction of a 10 mL sample yielded detection limits of 3.6-8.6 ng/L, with high linearity (r\({}^{2}\) \(>\) 0.995). They found great precision in peak area and retention duration at the 500 ng/L level, with relative standard deviations (RSDs) of 6.9-8.8% and 0.6-0.9%, respectively. The method was successfully applied to real samples such as local river water, wastewater, and milk. Their research revealed PBDE recoveries ranging from 90% to 119% for boosted samples. No PBDEs were found in river water or skimmed milk samples; however, they found PBDE levels of 134-215 ng/L in wastewater, 13-484 ng/L in whole fat milk, and 21 ng/L of BDE-47 in a semi-skimmed milk sample [116]. A study by [PERSON] et al.