# 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  
The natural form of titanium, titanium dioxide (TiO\({}_{2}\)), comes in three main crystal forms: brookite (which has a rhombohedral structure), metastable anatase (both of which have tetragonal structures), and stable rutile. The electrical structure, crystal form, size, shape, and any doping present influence the distinctive physicochemical characteristics of TiO\({}_{2}\) crystals [75]. It has been reported that TiO\({}_{2}\) nanotubes (TDNTs) possess notable qualities such as inertness, chemical and thermal stability, non-toxicity, low cost, durability, and corrosion resistance, making them highly suitable for various applications. These include environmental purification, photocatalysis, self-cleaning coatings, hydrogen generation and storage, gas sensing, biomedical uses, and electrode materials for solar cells. The structure of TDNTs is typically hydrogen titanate, with TiO\({}_{2}\) sheets rolled and separated by H+ ions, giving them a multi-walled, scroll-like appearance with large internal and external surfaces. These features make TDNTs ideal for use as sorbents. TiO\({}_{2}\), in various forms like nanowires, nanoparticles, and nanotubes, is a versatile and cost-effective material with significant potential in photocatalysis, electronics, and electrochemical systems, especially in analytical chemistry due to its high surface area and adsorption capabilities [14; 76]. Anodized TiO\({}_{2}\) nanotube fibers were used by [PERSON] et al. for in-headspace SPME in conjunction with gas chromatography-mass spectrometry (GC-MS) to identify volatile organic chemicals like xylenes (BTEX), benzene, toluene, and ethylbenzene. The research examined how the anodizing duration and annealing temperature affected the formation of TiO\({}_{2}\) nanotubes while maximizing the conditions for adsorption (time, ionic strength, and temperature) and desorption (time and temperature) for BTEX measurement. The technique showed linearity between 0.5 and 15,000 \(\mathrm{\SIUnitSymbolMicro}\)/L (R2 \(>\) 0.9954) and obtained detection limits for individual BTEX chemicals between 0.1 and 2.0 \(\mathrm{\SIUnitSymbolMicro}\)/L. The strategy demonstrated the potential of TiO\({}_{2}\) nanotube fibers as an efficient method for BTEX analysis by producing recoveries ranging from 92% to 97% when applied to actual samples [77]. [PERSON] et al. anodized titanium wire in ethylene glycol using NH\({}_{4}\)F to create TiO\({}_{2}\)-nanosheet-coated fibers for SPME. After extracting polycyclic aromatic hydrocarbons, phthalates, and UV filters using TiO\({}_{2}\) nanosheets that were roughly 1 \(\mathrm{\SIUnitSymbolMicro}\)m long, 300 nm wide, and 80 nm thick, HPLC-UV detection was performed. With detection limits of 0.026-0.089 \(\mathrm{\SIUnitSymbolMicro}\)/L and calibration ranges of 0.1-400 \(\mathrm{\SIUnitSymbolMicro}\)/L, the fibers demonstrated excellent extraction efficiency and selectivity. With recoveries ranging from 88.8% to 107%, the technique reliably identified UV filters in environmental water samples and showed strong repetition (4.50-8.76%) and reproducibility (7.75-9.64%). The fibers were repeatable, robust, and stable [78]. Iron Oxide Nanoparticles as Sorbents for SPME  
The emergence of nanotechnology has revolutionized science by making it possible to create materials at the nanoscale (1-100 nm), which have special qualities because of their small size and high surface area-to-volume ratio. Iron oxide nanoparticles (IONPs) stand out among them due to their remarkable superparamagnetism, especially when they are between 2 and 20 nm in size. In the absence of an external magnetic field, this behavior, which is characterized by random variations in magnetization direction influenced by temperature and time (sometimes referred to as the Neel relaxation time), precludes net bulk magnetization. They are more magnetically susceptible than paramagnets, nevertheless, when exposed to a magnetic field.