Conductive polymers (CPs) are a class of organic polymers that have π-conjugated electrons in their backbone structure, exhibiting electronic properties that resemble both metals and semiconductors while retaining the mechanical properties typical of organic macromolecules. Interest in CPs began with the preparation of highly conducting polyacetylene in 1977, which led to Shirakawa, MacDiarmid, and Heeger being awarded a Nobel Prize in chemistry [1]. The π-conjugated electronic system gives CPs unique electrical and optical properties, such as high conductivity, tunable energy levels, and strong absorption in the visible region of the electromagnetic spectrum. These properties make CPs attractive for use in various applications including electronics, sensors, batteries, and extraction [2]. The most common conductive polymers that have been widely investigated in the preparation of microextraction sorbents, in pure or composite form, include polyaniline (PANI) [3], polypyrrole (PPY) [4], and polythiophene (PTH) [5]. The widespread use of these CPs is due to their facile synthesis, good redox properties, high chemical and physical stability, and enhanced conductivity [6]. The common procedures for coating microextraction substrates include using glue [7], dipping into polymer solutions [8], sol-gel [9], chemical bonding [10], and in-situ electropolymerization [11].

In-situ electropolymerization is an attractive technique for microextraction purposes as it leads to more uniform and physiochemically stable coatings with tuned thickness. This technique is preferred due to its low cost, simple setup, and ability to tailor the coatings' features [12]. Aniline, pyrrole, and thiophene can be electropolymerized onto a substrate in appropriate electrolytes through anodic oxidation. By controlling electropolymerization conditions such as applied potential, polymerization time, counter ion type, monomer ion concentration, and doped ion type, desired coatings can be produced [13], [14], [15].

PPY is one of the most common SPME coatings due to its unique properties, including electropolymerization in aqueous or organic solutions in one step, stability in air and aqueous solutions, biocompatibility due to its hydrophilic nature, various interactions with analytes (π-π and dipole interactions, acid-base chemistry, hydrogen bonding, and ion exchange), and commercial accessibility [16], [17], [18], [19].

Most reports on SPME applications of PPY have utilized constant potential (CP) electropolymerization method. However, pulse potential (like constant potential pulse, CPP) and ramp potential methods (like cyclic voltammetry, CV) offer more variables to control when tailoring coating properties [6,[20], [21], [22], [23]]. To the best of our knowledge, no reports have been published on the preparation of SPME fiber coatings using CPP and CV techniques. However, a single report exists on the use of CV electropolymerization for PPY in stir-bar sorbent extraction applications [24].

Plant hormones, a group of special chemical substances produced by plants, are major internal factors that control physiological processes in plant material at very low concentrations [25]. Due to their trace amounts and coexistence with various interferents, determining phytohormones in plant tissues can be challenging. Therefore, simple and reliable procedures such as SPME are necessary for the separation and preconcentrating of phytohormones [26]. Several analytical techniques have been used to determine plant hormones including GC–MS [27] and LC-MS [28]. However, in GC–MS methods not only prior derivatization is required to increase volatility but also high temperatures in the GC injector and column may decompose unstable phytohormones. LC-MS is an expensive instrument that may not be available in many general chemical analysis laboratories. As a result, reversed-phase HPLC with UV detection (RP-HPLC-UV) has been demonstrated as the most desirable method for determining plant hormones when coupled with a separation strategy due to its greater availability, simplicity, lower cost, and adequate sensitivity [29].

In this study, a thin layer of pyrrole-dopamine copolymers (PPY/PDA) was coated on a stainless-steel narrow wire through an in-situ electropolymerization method to prepare a novel SPME fiber. The fiber was applied for the direct-immersion SPME (DI-SPME) sampling of plant hormones in fruit juice samples. The extracted hormones were then measured using RP-HPLC-UV. The influential experimental variables on the extraction performance were investigated and optimized using response surface methodology, including a Box-Behnken design. Different electropolymerization strategies, including CP, CPP, and CV, were optimized and compared for the preparation of the PPY/PDA-coated fibers. To the best of our knowledge, this is the first investigation to use PPY/PDA as an SPME fiber and the first study to compare different electropolymerization strategies for preparing SPME fibers using conductive polymers.