# Mussel Inspired Polydopamine as Silica Fibers Coating for Solid-Phase Microextraction
## 1 Introduction  
The performances of analytical methods depend on the different steps of the process, such as sampling, sample preparation, instrumental analysis, and data processing [1]. Interference's removal and analyte's preconcentration are the main goals of sample preparation that is indeed a prerequisite for the attainment of reliable results and is often considered the bottleneck of analytical workflows. Chemical analysis could be greatly improved by technological advances of modern sample preparation techniques. Solid-phase extraction (SPE), with various solid-phase sorbents, has been widely used as sample preparation approach over the years, while other sorption-based techniques, such as solid-phase microextraction (SPME), stir-bar sorptive extraction (SBSE) and matrix solid-phase dispersion (MSPD) have been more recently introduced. Solid-phase microextraction (SPME) is an evolving solventless technique for the extraction of organic compounds through the equilibration of the analytes between the sample matrix and a polymeric phase, generally coated onto a fiber. Its main advantages are rapidity, low cost, ability to perform in situ measurements, ease of use, and automation. SPME is proposed in many variants, in particular in-tube [2], hollow fiber [3], thin-film [4], coated-tip [5], and magnetic nanoparticles [6]. However, the fiber geometry is still the most widespread feature, even if the number of commercially available sorbents remains surprisingly limited. Thus, a new multidisciplinary research trend, involving materials science, nanotechnology, polymer synthesis, and analytical chemistry, has been recently introduced through the development and characterization of new SPME extraction sorbents [7; 8; 9; 10; 11; 12], which should possess intrinsic characteristics such as long lifetime, selectivity, chemical, thermal, and mechanical stability, and good adsorptive properties. Consequently,many innovative sorbents have been developed, including carbon-based [13], metal organic frameworks [14; 15], molecular imprinted polymers [15], ionic liquids [16; 17; 18], immunosorbents, sol-gel-based compounds [19; 20], and nanomaterial-based [21], among others. Unfortunately, many coatings are produced using environmentally harmful substances or precursors (organic alkoxysilanes, radical organic species, acrylic monomers). The production of fibers for SPME applications prior involves the synthesis of the bulk material (silica, stainless steel) and then the usual sol-gel coating processing for synthesizing the extractive layer. The bulk fused silica, for instance, should first undergo exposition of silanol groups onto the surface for further functionalization for creating the primary sites on sol solution and ensuring a dense sol-gel coating through chemical bonding, requiring alkali or acid solutions, organic and/or inorganic precursors, active organic solvents, a catalyst, and water [22]. Very recently, polydopamine (PDA) has been proposed as an interesting new material in sorption-based extraction methods for its extraordinary chemical and environmental stability, convenience for further modification [23] and eco-friendliness. In fact, polydopamine coating does not need pretreatments of the silica bulk fibers for material processing. PDA is a bioinspired polymer which, in vitro, mimics the protein sub-structures exploited by mussels for adhesion. It is produced via oxidative polymerization of dopamine (DA), a biological catecholamine neurotransmitter [24]. DA is a water-soluble monomer, easily polymerizable under alkaline conditions at room temperature, avoiding the use of toxic solvents and precursors, and directly exploiting the molecular oxygen as an abundant, cheap, and ecofriendly catalyst for polymerization [25]. The resulting polydopamine coating is soft, adhesive, biocompatible, and suitable for further chemical functionalization [26].