# Applications of Nanomaterial Coatings in Solid-Phase Microextraction (SPME)
## 4 Advances in the Adsorbent Type Used in the Extraction Phase of the SPME Fiber
### SPME Based on Nanoparticles
#### 4.1.1 Metal Nanoparticles in SPME
They explained that a layer-by-layer assembly method was used to apply the nanoparticles to SPME fibers, producing consistent coatings that were roughly 3 \(\upmu\)m thick. Gallic acid or H\({}_{2}\)O\({}_{2}\) was used as a reducing agent in the synthesis of the nanoparticles. Despite the thinner coating, the researchers observed that these fibers had detection limits as low as 0.13 \(\upmu\)g/L and partition coefficients twice as high as those of commercial polyacrylate fibers. They also noted that after more than 100 extractions, the fibers showed good durability and repeatability, with relative standard deviation values ranging from 8.6% to 21.3% [55]. In conclusion, gold nanoparticles (AuNPs) are extremely versatile substances with proven physical and chemical characteristics that are available using established synthesis techniques. Because of their readily adjustable surface chemistry, they are very appealing for incorporation into a range of applications. AuNPs are now a popular option in academic research and an essential part of many industrial and point-of-care medical devices because of their qualities. They are perfect candidates for improving fiber coatings in the context of SPME because of their high stability and adjustable surface characteristics, which will improve analyte adsorption and increase the technique's application in a variety of analytical problems [52]. Silver Nanoparticles as Sorbent Material in SPME  
It is well known that silver ions can chemically combine with double-bonded molecules to produce strong interactions. In this interaction, electrons from the compound's \(\pi\) orbital are donated to the silver ion's empty s orbital, forming \(\sigma\)-bonds, and electrons from the silver ion's d orbital are formed from the compound's \(\pi^{4}\) antibonding orbital. These chemical bonds are reversible, breakable at high temperatures, and stronger than van der Waals forces [56]. Silver chemistry has been studied in the creation of assisted transport membranes and stationary phases in chromatography (such as argentation chromatography), which are used to separate olefin/paraffin combinations [57; 58]. Despite being more affordable than gold nanoparticles (AuNPs), there has been less research on the application of silver nanoparticles (AgNPs) as SPME coatings. Graphene oxide and polymers are combined as hybrid composites for the majority of AgNP applications, while the other components mostly drive extraction [59]. For SPME, other silver-based structures have also been investigated. These consist of electrodeposited silver dendrites, silver nanoplates or dendrites on copper wire supports, silver wires functionalized by sol-gel processes or conditioned by etching, and silver mirror coatings on stainless steel. To improve performance, these structures frequently need extra changes such as the use of polymer sheets [60; 61; 62; 63; 64; 65]. Silver nanoparticles (AgNPs) made with glucose or gallic acid as reducing agents were used to develop a unique SPME coating, according to [PERSON] et al. They stated that a layer-by-layer technique was used to deposit the AgNPs onto stainless steel wires that had already been coated with a silver mirror, allowing for exact control over coating thickness. In close agreement with theoretical estimations, they observed that the coatings measured 3.2 \(\upmu\)m thick for fibers prepared with gallic acid and 3.5 \(\upmu\)m thick for fibers prepared with glucose. These fibers were used for the direct-immersion SPME-GC-FID analysis of sixteen polycyclic aromatic hydrocarbons (PAHs) in water, according to the scientists. They stressed that the AgNP fibers, especially those made with gallic acid, achieved remarkable analytical performance, including detection limits as low as 0.6 ng\(\cdot\)mL\({}^{-1}\), RSDs below 26%, and durability for roughly 150 extraction/desorption cycles, despite the thinner coatings when compared to standard PDMS fibers.