Synthesis of polyaniline and polydopamine copolymers, doping with PbI2 and spectroscopic investigation

Polydopamine (PDA) is recognized for its compatibility with biological systems and draws inspiration from nature. As a material inspired by natural processes, PDA features functional groups like dihydroxy (catechol), quinone (o-quinone), amine, and imine, which are central to its distinctive characteristics, as highlighted in various studies (Batul, et al., 2017; Mrówczyński, et al., 2016; Ho and Ding, 2014). It mimics the adhesive capabilities of mussel foot proteins by incorporating high levels of catechol, alongside primary and secondary amines, thanks to components like 3,4-dihydroxy-Lphenylalanine, lysine, and histidine (Ryu, et al., 2018; Kord Forooshani and Lee, 2017; Hemmatpour, et al., 2023). The versatility of PDA is underscored by its properties, including significant adhesion strength, stability, biocompatibility, and photothermal activity (Zhou, et al., 2022; Niezni, et al., 2022). Its applications are wide-ranging, from antibacterial treatments and sensory devices to biomedical imaging, drug delivery, tissue engineering, catalytic processes, and metal ion chelation (Huang, et al., 2020; Massoumi, et al., 2020; Shen, et al., 2018; Tan, et al., 2018; Ghorbani, et al., 2019; Fu, et al., 2021; Yang, et al., 2021, Wang, et al., 2020; Ball, et al., 2018; Zahidova, et al., 2023). The synthesis of PDA is straightforward, achieved through self-polymerization in alkaline aqueous solutions (pH > 7.5) with atmospheric oxygen (Deng, et al., 2018; Liu, et al., 2014), or using various oxidants ((NH4)2S2O8, CuSO4+H2O2, NaIO4, and KClO3) in a range of pH settings to produce polydopamine not only in alkaline but also in acidic and neutral aqueous environments (Deng, et al., 2018; Feng, et al., 2012). Methods such as plasma treatment, electrochemical, and enzymatic oxidation approaches have also been utilized. The pioneering work of Lee et al. in 2007 introduced the oxidative polymerization of dopamine to produce PDA, which can be coated on numerous surface types thanks to molecular interactions like covalent bonds, metal coordination, and more (Lee, et al., 2007). It shows molecular interactions which consist of covalent bonding, metal coordination, H-bonding, π-π and cation--π interactions (Lee, et al., 2020; Liebscher, 2019). In 2007, the Messersmith group demonstrated that dopamine is easily polymerized by oxidative polymerization and can be easily coated on any surface. The most important advantage of the polymerized polydopamine is its ability to deposit on any organic or inorganic surface, including superhydrophobic surfaces, with controllable film thickness and durability. Thus, polydopamine has revolutionized the modification of many different surfaces and has enabled extensive research (Liu, et al., 2014; Yang, et al., 2015). This has significantly expanded surface modification research and applications. PDA presents as a dark brown-black insoluble biopolymer, produced by the in-situ polymerization of dopamine (Batul, et al., 2017). Polyaniline (PANI) is distinguished as a conductive polymer, celebrated for its noteworthy features such as electrical conductivity, compatibility with biological tissues, affinity for water, low toxicity, substantial stability against environmental factors, and simple production techniques. These advantages are evidenced by multiple studies (Bhadra, et al., 2009; Xiao, et al., 2023, Zare, et al., 2019). Additionally, PANI's fabrication is straightforward, and it showcases a wide range of electrical conductivities as well as exceptional resistance to environmental, thermal, and chemical adversaries. However, the applicability of PANI faces hurdles due to its poor processability, low degradability, and insolubility in many common solvents, as pointed out by Zare et al. (2019) and Samadi et al. (2021). Current researchs aim to mitigate these drawbacks by exploring PANI in composite forms, blends, or as copolymers, attracting interest for their distinct electrical traits and other attributes such as ease of synthesis, cost-effectiveness, superior environmental stability, and diverse functionalities like reactive NH groups, electrochemical, and optoelectronic features (Samadi, et al., 2021). Typically, PANI is synthesized via chemical or electrochemical oxidation methods in acidic conditions, using or not using doping agents. In the chemical oxidation, a variety of oxidizing agents and doping reagents like ammonium persulfate, hydrogen peroxide, ferric chloride, and others have been employed (Beygisangchin, et al., 2003; Ayad, et al., 2003; Bernasik, et al., 2005; Krishna, et al., 2005; Wang and Tan, 2006). PANI is most commonly encountered as a green protonated emeraldine salt with an average conductivity of 1 S/cm, marking a conductivity higher than that of many polymers but lower than metals (Stejskal and Gilbert, 2002). In the synthesis outcomes of PANI, both fully reduced amine and fully oxidized imine forms exhibit insulating properties (Trychova, et al., 2006). Copolymerization is employed in general to improve the solubility and processing of polymers. Copolymers embody the characteristics that are intermediate to those of homopolymers drawn from individual monomers (Zareh, et al., 2021). Various PANI copolymers have been synthesized and characterized in literature. For instance, Sankar, et al. (2023) produced a copolymer named poly(aniline-co-indole) (PANI-co-PIN) along with its nanocomposites featuring Cu-Al2O3, highlighting significant optical, conductivity, and dielectric properties that render them ideal for optoelectronic applications and charge storage devices (Sankar, et al., 2023). Similarly, the poly(aniline-co-pyrrole) (PANI-co-PPy) copolymer was synthesized, forming composites with reduced band gap energies (Sankar, et al., 2022). Such copolymers, including PANI-co-PIN and PANI-co-PPy, have also been utilized for NH3 gas sensing (Sankar, et al., 2022). Moreover, Tan, et al. (2018). crafted dopamine-modified polyaniline through a one-step chemical oxidation process, mixing aniline monomer with dopamine hydrochloride and ammonium persulfate in an acidic medium, examining various characteristics like FTIR, SEM, and their electrical conductance and cytotoxicity (Tan, et al. 2018). Moosaipour and colleagues (2022) created a random copolymer, polyaniline-co-polydopamine (PANI-co-PDA), blending it with poly(vinyl alcohol) (PVA) to fabricate nanofibers and investigate their electrical conductance, mechanical strength, and biodegradability (Moosaipour, et al., 2022).. Chen, et al. (2019) explored polydopamine-modified polyaniline/nanodiamond ternary hybrids, examining their electromagnetic absorption characteristics (Chen, et al., 2019). Poly(dopamine-co-aniline) (poly(DA-co-ANI)) has been noted for its application in blends or composites with materials such as graphene oxide/poly(DA-co-ANI) and poly(DA-co-ANI)/calcium phosphate, indicating a trend towards innovative material composites (Yang, et al., 2019; Huang, et al., 2022; Paknia, et al., 2022). In our research, we synthesized a novel copolymer, poly(dopamine-co-aniline) (poly(DA-co-ANI)), through the chemical oxidation polymerization process where aniline and dopamine monomers were polymerized using ammonium persulfate in a hydrochloric acid solution. This copolymer underwent a series of detailed analyses, such as UV–visible spectrophotometry, fluorescence (FL) emission spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), field-emission scanning electron microscopy - energy dispersion x-ray spectroscopy (FE-SEM-EDS), x-ray diffraction (XRD), and tests for electrical conductivity to elucidate its characteristics. Our investigations revealed that poly(DA-co-ANI) copolymer is readily dissolved in N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) solvents. Additionally, we calculated the homo polymers' and copolymer's optical band gap energies using the data from UV–visible absorption analysis. The copolymer was compared with PANI and PDA homopolymers, which were synthesized under similar conditions, to highlight that copolymerization significantly enhances polymer solubility and processability Specifically, the optical band gap energy for direct transitions of the poly(DA-co-ANI) polymer film was measured as 3.40 eV, while the indirect transition was measured as 1.00 eV. Significantly, doping the copolymer with PbI2 lowered the direct transition optical band gap energy to 2.40 eV and the indirect transition to 0.40 eV. The FL emission bands of the poly(DA-co-ANI) copolymer were observed at 390 and 533 nm as the maximum wavelengths, while the PANI showed FL amission at about 400-410 nm, the PDA showed FL amission at about 410-430 nm. The PbI2 binding equilibrium constants of P(DA-co-ANI) 50:50 copolymer were calculated according to Stern-Volmer and Modified Stern-Volmer using FL emission quenching data. The Stern-Volmer constant KSV value of the PbI2 binding was found to be 1.57 x 104 M-1. The binding equilibrium constant constant Ka was calculated as 3.02 x 107 M-1 and the number of binding sites n was calculated as 1.4211. In the measurements made according to the FL emission quenching method, PbI2 has a high binding equilibrium constant to the P(DA-co-ANI) 50:50 copolymer. SEM analysis showed that the copolymer forms crystalline rectangular particles ranging in size from 0 to 1500 nm under acidic conditions, and amorphous particles ranging from 0 to 600 nm under basic conditions. The PbI2 doped copolymer has very different particle sizes. The XRD pattern of the copolymer doped with PbI2 showed no amorphous phases, only the crystalline peaks of PbI2. The copolymer's electrical conductivity was found to be 2.25 x 10-5 S/cm in its acidic form and 8.50 x 10-5 S/cm in its basic form. Additionally, the average molecular weight of the copolymer was determined to be 2628 Da, extending up to 5500 Da through MALDI-TOF-MS measurements. Given its solubility in NMP and DMF, along with its low optical band gap energy, poly(DA-co-ANI) doped with PbI2 is identified as a prospective material for applications in light absorption, fluorescence emission, and semi-conductivity.