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. 2026 Sep 25;16(45):57702–57715. doi: 10.1039/d6ra08255f

Design and computational investigation of a polyaniline–pyrrole copolymer and its antimicrobial and cytotoxic properties

Chetna Kumari a, Sapana Jadoun b, Gulshan Kumar a, Ved Prakash Meena c, Sweta Singh d, Nirmala Kumari Jangid a,✉, Jaya Dwivedi a
PMCID: PMC13614485  PMID: 42799231

Abstract

The present study focused on the single-pot synthesis of polyaniline (PANI) and polyaniline-pyrrole (PANI-Py) copolymer via the chemical oxidative polymerization method (COPM). The structural features of synthesised PANI and PANI-Py were characterised using UV-visible and FTIR spectroscopy. The thermal stability of synthesised PANI and PANI-Py copolymer was examined by TGA and DSC over the temperature range of 33–1000 °C. BET and viscosity measurements analysed surface area and molecular weight. Conductivity of PANI and PANI-Py was measured by a two-probe method. DFT calculations confirmed the stability and reactivity of PANI and PANI-Py. Additionally, the antifungal activity against A. niger and P. chrysogenum was determined by IC50 and minimum inhibitory concentration (MIC), and the antibacterial activity of PANI and PANI-Py was tested against E. coli, P. aeruginosa, S. aureus, and B. subtilis. Furthermore, cytotoxicity of PANI and PANI-Py was assessed using the MTT assay. The bonding interactions between the Py ring and the surrounding residues of 1IX1 and 1UKC were examined by molecular docking. Consequently, PANI-Py exhibits strong antifungal and antibacterial activity and higher binding affinity for bacterial and fungal strains than pure PANI.

The present study focused on the single-pot synthesis of polyaniline (PANI) and the polyaniline-pyrrole (PANI-Py) copolymer via chemical oxidative polymerisation, their characterization and biological applicationsgraphic file with name d6ra08255f-ga.webp

1. Introduction

The development of potent antimicrobial drugs is urgently needed to eradicate antibiotic-resistant bacterial species. Antimicrobial drugs should ideally fulfil several conditions, including a wide antimicrobial spectrum at short contact times, affordability of synthesis, high stability under intended use and storage conditions, and regenerating activity after loss. Because of their residual toxicity, low-molecular-weight antimicrobial agents are not recommended for use in soil sterilisation, food preservation, or water disinfection, even at appropriate concentrations.1,2 To address many issues with antibiotics, synthetic polymers with various molecular weights and biocidal moieties have been proposed as a novel class of antimicrobial drugs. Initially, the adsorption of negatively charged polycations onto the bacterial cell surface creates an additional barrier that disrupts normal bacterial activity. The polymer's molecular weight is also important if the killing effect is primarily determined by its permeability through the bacterial cell wall. Due to their ease of manufacture, low cost, low toxicity, and biocompatibility, two well-known conducting polymers with different molecular weights, PANI and polypyrrole (Ppy), have attracted considerable attention in this area.3,4 Additionally, antimicrobial polymers or polymeric biocides have low cytotoxicity because they are non-volatile, stable, and do not penetrate the skin. However, because PANI is insoluble in most organic solvents and only partially soluble, the use of conducting polymers (CPs) is restricted.5 Additionally, there is weak processability and infusibility at various cycle periods.6,7 Additionally, CPs' MICs (minimum inhibitory concentrations) are far greater than those of antibiotics, and they are generally weaker or even useless against fungi. Functionalized carbon dots (CDs) (f-CDs), copolymers, and polymer nanocomposites containing metal oxides or small molecules can improve antibacterial qualities. To mitigate these drawbacks, other approaches have been followed, including doping with various organic functional groups, copolymerization with other monomers, and the development of metal oxide nanocomposites with conducting polymers.8–11 Because silver and copper are so toxic to bacteria at very low concentrations, there is a growing trend to produce these metal nanoparticles and incorporate them into polymer networks. When combined with metal oxide nanoparticles, conducting polymers form nanocomposites with superior mechanical, conductive, antibacterial, and antifungal properties.12–14

Several bacterial strains have been used to assess PANI's antibacterial properties for use in food packaging and medical equipment.15 PANI has demonstrated strong antibacterial activity against S. aureus and E. coli in both visible and dim light.16 A variety of Gram-positive and Gram-negative bacterial strains were susceptible to the antibacterial activity of PANI dissolved in dimethyl sulfoxide (DMSO) at different concentrations.17 A major advantage over previously suggested antibacterial agents is that PANI's insolubility in aqueous solutions prolongs its antibacterial activity and enables the reuse of coatings based on it.18 Furthermore, PANI's aromatic ring would become more soluble in organic solvents upon the addition of functional groups, facilitating the formation of thin films from the polymer.18,19 Thus, a superior coating with exceptional utility would result from the combination of antibacterial activity and high solubility in organic solvents.

Santos et al. examined the effects of usnic acid doping on undoped PANI, which offers important advantages, including increased polyaniline bactericidal activity against S. aureus and E. coli, as well as the lichen derivative's anti-biofilm properties. The synergistic combination of the very porous polyurethane surface, the potent bactericidal activity of PANI, and the action of usnic acid results in a low-cost material with good antibiofilm performance.20 The one-pot synthesis of PANI and PANI-imidazole copolymer by COPM was studied by Kumari et al. Furthermore, the MIC, IC50, and MBC of PANI and PANI-imidazole against S. aureus and E. coli were used to assess their antibacterial efficacy. When compared to pure PANI, the PANI-imidazole copolymer exhibits potent antibacterial action and a greater binding affinity towards the bacterial enzyme.21 The present study successfully synthesized and characterized pure PANI, PANI-Au nanocomposites, and PANI-Au-Pt nanocomposites using a chemical approach. Gram-positive (B. subtilis and S. aureus) and Gram-negative (E. coli and V. cholerae) bacterial pathogens were used to assess the antibacterial activity of pristine PANI, PANI-Au, and PANI-Au-Pt nanocomposites.22 Kumari et al. synthesized PANI, polyaniline-co-pyrazine (PANI-co-Pyr), and PANI-pyrazole in a single pot using COPM. The antifungal activity of PANI-co-Pyr and PANI-pyrazole against P. chrysogenum and A. niger was measured using MIC and IC50, while their antibacterial properties were evaluated against S. aureus, E. coli, B. subtilis, and P. aeruginosa. According to the results, PANI-co-Pyr exhibits higher binding affinity for bacterial and fungal strains and potent antibacterial and antifungal activity.23,24 The chemically produced PANI/ZrO2-Ag nanohybrids by COPM were investigated by Sampurnam et al. The antibacterial efficacy of the PANI/ZrO2-Ag nanohybrid was evaluated against Gram-positive and Gram-negative bacterial pathogens, including P. vulgaris, E. coli, and M. luteus, using the agar well diffusion assay.25 Using an extract from Manilkara zapota leaves, Manzoor et al. examined the green synthesis of magnesium oxide (MgO) and cobalt oxide (Co3O4) nanoparticles, their surface modification by PANI, and their antifungal properties against A. niger.26 Polyaniline and its modified derivatives are summarized in Table 1.21,23,24,27–30

Table 1. Polyaniline and its derivatives synthesized by chemical oxidative polymerization method for biological activity.

S. No. Composites Method of synthesis Biological activity Reference
1 PANI (dodecylbenzene sulphonic acid (DBSA)-phthalic acid) Chemical oxidative polymerization Antibacterial 27
2 PANI (DBSA-oxalic acid) Chemical oxidative polymerization Antibacterial 27
3 PANI (DBSA-C. procera latex) Chemical oxidative polymerization Antibacterial 27
4 PANI-3,5-dinitrobenzoate (DN) Chemical oxidative polymerization Antibacterial, antifungal 28
5 PANI-imidazole Chemical oxidative polymerization Antibacterial 21
6 PANI-co-Pyr Chemical oxidative polymerization Antibacterial, antifungal 23
7 PANI-pyrazole Chemical oxidative polymerization Antibacterial, antifungal, antituberculosis 24 and 29
8 PANI-tetrazole Chemical oxidative polymerization Antibacterial, antifungal, antituberculosis 29
9 PANI-indole Chemical oxidative polymerization Antibacterial, antifungal, antituberculosis, antimalarial 30
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Pyrrole has a variety of biological actions, including antibacterial, antifungal, anticancer, antimalarial, and antioxidant properties. Its aromatic, nitrogen-containing heterocyclic framework, which enables strong interactions with biomolecules such as proteins, DNA, and microbial cell membranes, is responsible for its biological activity. Antimicrobial and antifungal efficacy can be increased by substitution of the pyrrole ring on the PANI backbone, which can also improve lipophilicity, binding affinity, and selectivity. The growing number of antibiotic-resistant microbes necessitates the development of novel antimicrobial agents. The unique structural and electrical properties of conductive polymers, such as PANI, combined with Py units, make them effective antifungal and antibacterial agents.31

In this study, PANI and PANI-Py copolymer were synthesised via COPM. Several techniques were used to characterise PANI and PANI-Py, including viscosity, conductivity measurements, UV-visible spectroscopy, FTIR spectroscopy, TGA, DSC, and BET. PANI's antibacterial and antifungal qualities were enhanced by the heteroring Py. PANI and PANI-Py's antifungal properties were evaluated against P. chrysogenum and A. niger, while their antibacterial properties were evaluated against E. coli, P. aeruginosa, S. aureus, and B. subtilis. The synthesised copolymers exhibited strong antibacterial and antifungal activity, highlighting their potential for biological applications.

2. Experimental

2.1. Materials

The following chemicals were used in the synthesis of PANI and its copolymer: ethanol (99.9% pure), ammonium persulfate (APS, (NH4)2S2O8, 98% pure), nitric acid (HNO3, 99.5% pure), hydrochloric acid (HCl, 98% pure), aniline (C6H5NH2, 99% pure), and heterocyclic moiety pyrrole (C4H5N, 99% pure). We used all the chemicals from SRL for the synthesis exactly as provided. Gram-negative and Gram-positive bacteria (E. coli MTCC1652, P. aeruginosa MTCC741, S. aureus MTCC3160, B. subtilis MTCC441) and fungal strains (P. chrysogenum MTCC2552, A. niger MTCC282) were obtained from MTCC, Chandigarh.

2.2. Synthesis of PANI

To make a homogeneous solution, 20 mL of nitric acid (1.0 M) in distilled water as a dopant was mixed with 6 mL of aniline. The mixture was then stirred for 1 h at 800 rpm. After that, the solution was co-doped with 20 mL of hydrochloric acid (0.5 M) and stirred at 800 rpm for at least 1 h at 0 °C in an ice bath. As an oxidant, 100 mL of ammonium persulfate (1.0 M) was added dropwise to the polymerization mixture, which was stirred continuously for 4–5 h at 0 °C. Following polymerization, the filtrate was separated and thoroughly washed using ethanol and distilled water. To synthesize a free-flowing powder of emeraldine salt of PANI, the residue was oven-dried for 24 h at 50–60 °C. After drying, the resulting pure PANI was dark green. In order to synthesize PANI emeraldine base, the synthesized emeraldine salt of PANI was left in an ammonium hydroxide solution overnight. The synthesized emeraldine base of PANI is depicted in Scheme 1. PANI was synthesized using the COPM.21

Scheme 1. Synthesis of polyaniline.

Scheme 1

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2.3. Synthesis of polyaniline–pyrrole copolymer

By employing an aniline-to-pyrrole ratio (i.e., 1 : 1), PANI-Py copolymer was synthesized via the COPM. In this experiment, 5 mL of aniline (0.05 mol) and 3.60 g of pyrrole (0.05 mol) were added to an aqueous acidic medium containing 20 mL of nitric acid (1.0 M) and 20 mL of hydrochloric acid (0.5 M). In an ice bath, the solution was stirred for 2 h at 0 °C at 800 rpm. The polymerization method involved adding 90 mL of 1.0 M ammonium persulfate dropwise to the solution at 0 °C, with continuous stirring at 800 rpm for 4 h. A dark green precipitate formed after 7 h. The filtrate was separated by filtration and then washed with distilled water and ethanol to remove impurities. A free-flowing powder of PANI-Py was obtained by oven-drying the residue at 50–60 °C for 24 h. After drying, the synthesized PANI-Py copolymer was dark green. The PANI-Py ratios of 1 : 2 and 2 : 1 were also synthesized under similar conditions.23 The synthesis of the PANI-Py copolymer is depicted in Scheme 2. The COPM that changes PANI into PANI-Py are shown in Scheme S1.

Scheme 2. One-pot synthesis of PANI-Py copolymer.

Scheme 2

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2.4. Characterization of synthesized PANI and PANI-Py

The synthesised PANI and PANI-Py were characterised using various analytical techniques, including FTIR, UV-visible, TGA, DSC, BET, viscosity, and conductance measurements. FTIR spectra in the 4000–600 cm−1 range were analyzed using the Bruker ALPHA FTIR spectrometer. A UV-visible spectrophotometer, SHIMADZU UV-1900, was used to obtain the absorption spectra of the synthesized PANI and PANI-Py between 200-800 nm with a spectral resolution of 1 nm. TGA and DSC thermal analysis were used to examine the thermal characteristics of the synthesized PANI and PANI-Py copolymers. TGA and DSC were used to assess the thermal stability of PANI and PANI-Py copolymers in the range of 33–1000 °C under a nitrogen atmosphere at a heating rate of 30 °C min−1 using the STD 650 instrument. Using a Quantachrome Autosorb automated gas sorption device at 77 K, the surface area, pore size, and total pore volume were determined by the Brunauer–Emmett–Teller (BET) method. The Ostwald viscometer was used to measure the intrinsic viscosity [η] and viscosity-average molar mass (Mv) of PANI and PANI-Py at room temperature (34 °C). DMSO solutions containing 1 mg mL−1 were prepared for the viscosity studies of PANI and PANI-Py. After that, the mixture was sonicated for 1 h at 40 °C. The Ostwald viscometer was then filled with 13 mL of the PANI and PANI-Py solutions. A Gay-Lussac-type pycnometer was used to measure the solution densities.

The Gaussian 16 software was used to theoretically analyse the adsorbents' energetic and geometrical molecular properties, including bond lengths, dipole moments, total energies, bond angles, and dihedral angles. The TZVP basis set was used to compute various functionals. The local minima of the optimization were validated by the absence of any imaginary frequencies. Further, to correlate with and identify the cause of the experimental absorption spectra, electronic spectra were also calculated using different functionals. Important electronic characteristics, such as the energy gap (ΔE), the highest occupied molecular orbital energy (EH), and the lowest unoccupied molecular orbital energy (EL), were further examined. Additionally, HOMO–LUMO data have been used to calculate reactivity descriptors, including the electrophilicity index, energy gap, chemical hardness, and softness. Frontier molecular orbital (FMO) theory is used to understand the molecule's interaction and reactivity. The energy gap (ΔE), which has fundamental chemical implications, explains the stability and reactivity of the complexes' structures. The HOMO and LUMO energy values were used to predict different physicochemical properties. The chemical hardness of a molecule, which is directly connected to the HOMO–LUMO energy gap, can be used to explain its reactivity. A molecule is less reactive if its E gap or chemical hardness is greater.32,33 Temperature-dependent conductivity measurements were performed between 30–75 °C. The conductivities of different PANI and PANI-Py copolymers were measured using the two-probe method with a Keithley electrometer 6517-A (India). Electrical connections were made using silver paste. It has two spring-loaded contact probes. These probes travel in a pipe and are insulated by Teflon washers. The proper platform on which this probe configuration is put also holds the sample plate and resistance temperature detector (RTD) sensor in place. The stand further functions as the proportional-integral-derivative (PID) control oven lid. Teflon-coated leads connect the high-voltage power supply, EHT-11, to the Digital Picoammeter, DPM-111. The setup's maximum current was 100 × 10−12 A, and its highest voltage was 1000 V.

The antibacterial properties of PANI, PANI-Py, and amoxicillin were tested against S. aureus, B. subtilis, E. coli, and P. aeruginosa using the microbroth dilution method. The minimal inhibitory concentration (MIC) and inhibitory concentration 50 (IC50) were evaluated following a 24 h incubation period. Using the same approach, the antifungal activity of PANI, PANI-Py, and ketoconazole was evaluated against A. niger and P. chrysogenum. Following 48–72 h of incubation, the MIC and IC50 were determined. The optical density was measured at 600 nm. To ensure accuracy, the trials were repeated three times. The L929 cell line was procured from the National Centre for Cell Science (NCCS), Pune. Cells were maintained as a subconfluent monolayer in Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1 × penicillin/streptomycin, under standard culture conditions (37 °C, 5% CO2, humidified incubator). To assess the cytotoxicity of individual hydrogel components at varying concentrations, the MTT assay was employed. Briefly, 1 × 104 cells were seeded into 96-well plates and allowed to adhere and proliferate for 24 h. Subsequently, cells were exposed to different copolymer concentrations (1–400 µg mL−1) in 100 µL of medium and incubated under the same conditions. After 24 h of treatment, cell viability was determined by adding 10 µL of MTT reagent to each well and incubating for 10–60 minutes at 37 °C. Then, absorbance was recorded at 590 nm using a Thermo Scientific UV-Visible spectrophotometer. PANI and PANI-Pyrrole were molecularly docked into the microenvironments of 1IX1 and 1UKC to identify their energetically and geometrically stable conformations. The Protein Data Bank was used to obtain protein crystal structures. Molecular docking was performed using the DFT-optimized three-dimensional (3D) structures of PANI and PANI-Pyrrole. A total of 11 binding sites in 1IX1 and 18 binding sites in 1UKC were found for both ligands using the site finder method, which identified the appropriate binding sites for placing the ligands in the active regions of proteins before docking. The best-fitting domain was selected for the docking study after the ligands were docked to the advantageous protein site identified by MOE's site finder, utilizing the Triangle Matcher placement technique.

3. Results and discussion

3.1. FTIR analysis

The FTIR spectra of the synthesised PANI and PANI-Py copolymers were obtained in the 4000–600 cm−1 region. Many shifted bands are visible in the PANI-Py FTIR spectra, suggesting that pure PANI has been doped. The FTIR spectra, which show the distinct peaks of PANI and PANI-Py as shown in Fig. S1, confirmed the effective substitution of Py on the backbone of PANI. The benzene ring substitution from PANI to PANI-Py was confirmed by the para substitution at 744, 798, 786, and 750 cm−1 for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively.23 The C Created by potrace 1.16, written by Peter Selinger 2001-2019 C stretching vibrations of the aromatic ring were observed at 1496, 1486, 1485, and 1490 cm−1 for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively. The C Created by potrace 1.16, written by Peter Selinger 2001-2019 C stretching vibrations of the benzenoid ring were observed at 1582, 1564, 1550, and 1553 cm−1 for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively. Py shifted the aromatic and benzenoid peaks, indicating altered conjugation (a minor shift due to the substitution effect). The stretching vibrations of the C–N band showed strong peaks at 1292, 1293, 1207, and 1285 cm−1 for PANI and PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively. The stretching vibrations of the C–H band were observed at 2878, 2897, 2953, and 2963 cm−1 for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively. The stretching vibrations of the N–H bands were observed at 3223, 3208, 3475, and 3223 cm−1 for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively. The protonation band was observed at 1158, 1114, 1101, and 1099 cm−1 for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively. Hydrogen bonding is influenced by the Py nitrogen atoms, which cause this peak to shift slightly or widen. In addition, the observed N–H peak shows that, for PANI-Py ratios of 1 : 1, 1 : 2, and 2 : 1, Py polymerizes through C with the N of PANI. PANI and PANI-Py FTIR data are displayed in Table 2 and Fig. S1.

Table 2. FTIR data of PANI and PANI-Py.

S. No. Vibrational modes IR frequency (cm−1)
PANI PANI-Py (1 : 1) PANI-Py (1 : 2) PANI-Py (2 : 1)
1 Para substitution 744 798 786 750
2 C Created by potrace 1.16, written by Peter Selinger 2001-2019 C stretching (aromatic ring) 1496 1486 1485 1490
3 C Created by potrace 1.16, written by Peter Selinger 2001-2019 C stretching (quinoid ring) 1582 1564 1550 1553
4 C–H stretching (aromatic ring) 2878 2897 2953 2963
5 N–H stretching (PANI) 3223 — — —
6 N–H stretching (pyrrole) — 3208 3475 3223
7 C–N stretching 1292 1293 1207 1285
8 Protonation band 1158 1114 1101 1099
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3.2. UV-visible spectroscopic analysis

A UV-visible spectrophotometer was used to analyze the synthesized copolymers in the wavelength range of 200–800 nm. Two prominent peaks were detected in the UV-visible spectra of PANI in DMSO, one at 326 nm and the other at 625 nm. The π–π* transitions in benzenoid rings were identified in the UV region at 326 nm, whereas the polaronic transitions were linked to the visible region at 625 nm.23 Similarly, the π-π* transitions were associated with a peak in the UV region at 326 nm in the UV-visible spectra of PANI-Py (1 : 1). The visible region peak at 625 nm exhibited prolonged conjugation or polaronic transitions, as shown in Fig. 1(a). Additionally, there are noticeable variations in intensity in the PANI and PANI-Py UV-Visible spectra. There is a small hyperchromic shift in the polaronic transition band for the PANI-Py (1 : 1), showing increased conjugation as a result of Py's insertion into the PANI backbone, as shown in Fig. 1. The graph plotted (αhν)1/2vs. energy (hν) shows band gaps of 2.57, 2.23, 2.32, and 2.44 eV for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively, as shown in Fig. 1(b).

Fig. 1. (a) UV spectra of PANI, and PANI-Py, (b) band gap of PANI, and PANI-Py copolymer.

Fig. 1

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3.3. Thermogravimetric analysis (TGA)

The TGA method was used to examine the thermal stabilities of PANI and PANI-Py. The weight loss with temperature varies between the synthesized PANI and PANI-Py. Four phases of weight loss were identified in a TGA study as a function of temperature. The first weight losses of 5.5%, 7.1%, 4.7%, and 3.5% for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1) were observed near ∼100 °C, due to the vaporisation of H2O molecules and other volatile components. The second weight losses of 3.7%, 29.1%, 27.8%, and 37.4% were observed between 85–160 °C, 147–468 °C, 137–406 °C, and 130–470 °C, respectively, and are related to the loss of dopants. The third weight losses of 42.9%, 14.9%, 14.7%, and 14.7% occurred between 160–637 °C, 468–788 °C, 406–667 °C, and 470–804 °C due to the thermal breakdown of PANI-Py. The fourth weight losses of 10.8%, 5.1%, 10.4%, and 3.5% were observed at 637–979 °C, 788–983 °C, 667–981 °C, and 804–977 °C due to the continuous disintegration of the PANI-Py. Thermal behaviour supports the presence of Py in the copolymer.21,23 PANI and PANI-Py have 37, and 43% remaining weight at 1000 °C. Thus, PANI-Py is thermally more stable than pure PANI, as shown in Table 3 and Fig. S2.

Table 3. TGA data of PANI and PANI-Py.

S. No. Synthesized copolymers Losses in weight (%) at various temperatures (°C)
50 100 200 300 400 500 600 700 800 900 1000
1 PANI 98 92 89 82 73 64 50 45 41 38 37
2 PANI-Py (1 : 1) 97 94 91 80 69 61 56 52 48 45 43
3 PANI-Py (1 : 2) 98 95 92 80 68 60 55 50 47 44 42
4 PANI-Py (2 : 1) 98 96 91 77 64 56 51 47 44 42 40
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3.4. Differential scanning calorimetry (DSC)

The thermal stability of PANI and PANI-Py copolymers was also measured by DSC analysis. The DSC thermogram of PANI, and PANI-Py displays two endothermic and one exothermic peak, used to track phase changes related to a polymeric material, as shown in Fig. 2. The first endothermic peak, or glass transition temperature (Tg) of PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), was observed at 98.6, 96.6, 96.6, and 94.6 °C, respectively. This was explained by the removal of weakly attached water molecules from the polymer matrix. The structural rearrangement of polymer chains in the PANI and PANI-Py copolymers may be the cause of the second small endothermic peak observed at 258.9, 276.1, 271.9, and 265.4 °C for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively. There was a noticeable exothermic peak was observed at approximately 607.2, 609.6, 609.6, and 628.9 °C for PANI, PANI-Py (1 : 1), PANI-Py (1 : 2), and PANI-Py (2 : 1), respectively, indicating the degradation and decomposition of the polymeric backbone. PANI-Py is more thermally stable than PANI because it has a larger area under the degradation endothermic peak.34–36 The DSC graph of PANI and its copolymers is shown in Fig. 2.

Fig. 2. DSC graph of (a) polyaniline, (b) polyaniline-pyrrole (1 : 1), (c) polyaniline-pyrrole (1 : 2), and (d) polyaniline-pyrrole (2 : 1).

Fig. 2

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3.5. Brunauer–Emmett–Teller (BET)

The PANI and PANI-Py copolymers' specific surface area, pore size, and total pore volume were determined using the nitrogen adsorption–desorption methodology.23,37 The PANI-Py (1 : 1) and PANI-Py (1 : 2) surface areas were determined to be 48.64 and 34.32 m2 g−1, respectively. However, the PANI surface area was found to be smaller (22.39 m2 g−1). The micropores in the PANI-Py increased the surface area. The nitrogen adsorption–desorption isotherms for polyaniline and polyaniline-pyrrole are shown in Fig. 3. When pyrrole was incorporated into the PANI backbone, the surface area increased from 22.39 m2 g−1 to 48.64 m2 g−1, as shown in Table 4. The copolymer's high surface area indicates its superior sorption capacity.

Fig. 3. Nitrogen adsorption–desorption isotherms of (a) polyaniline, (b) polyaniline-pyrrole (1 : 1), (c) polyaniline-pyrrole (1 : 2), and (d) polyaniline-pyrrole (2 : 1).

Fig. 3

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Table 4. Pore size, surface area, and pore volume of synthesized polyaniline and polyaniline–pyrrole copolymers.

S. No. Synthesized copolymers Pore size (Å) Surface area (m2 g−1) Total pore volume (cm3 g−1)
1 PANI 1.74 22.39 1.95
2 PANI-Py (1 : 1) 1.31 48.64 3.19
3 PANI-Py (1 : 2) 8.08 34.32 1.38
4 PANI-Py (2 : 1) 4.16 26.35 5.49
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3.6. Viscosity measurement

The intrinsic viscosities were calculated using the Mark–Houwink equation [η] = K Mva. In this equation, η represents intrinsic viscosity ([η] = 1.95 × 10−6Mv1 (ref. 36)), Mv is the average molar mass, while K and a are Mark–Houwink constants. The inherent viscosity of PANI and PANI-Py was used to determine the viscosity-average molar mass.23 The average molar mass and intrinsic viscosities of PANI and PANI-Py are shown in Table 5. Because of the degree of conjugation and polymerization, PANI-Py has a greater viscosity-average molar mass than pure PANI. Adding Py to the PANI backbone often increases the viscosity-average molar mass, indicating increased conjugation and polymer chain growth due to copolymerization. The PANI-Py (1 : 1) copolymer exhibited the highest intrinsic viscosity (2.10 dL g−1) and molar mass (2.8 × 104 g mol−1). Remarkably, the molar mass of the PANI-Py (2 : 1) copolymer was somewhat smaller than that of the (1 : 1) and (1 : 2) copolymers. This decrease might be explained by a less ideal ratio for chain extension, which could lead to fewer oligomer units joining efficiently and producing a lower overall molar mass.

Table 5. Intrinsic viscosities and viscosity-average molar mass of PANI and PANI-Py.

S. No. Synthesized copolymers Intrinsic viscosity [η] (dL g−1) Viscosity-average molar mass (Mv) (g mol−1)
1 PANI 1.70 2.1 × 104
2 PANI-Py (1 : 1) 2.10 2.8 × 104
3 PANI-Py (1 : 2) 2.08 2.7 × 104
4 PANI-Py (2 : 1) 1.76 2.2 × 104
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3.7. Density functional theory (DFT)

The PANI, and PANI-Py copolymer were first optimized at the ground state, and corresponding electronic spectra were calculated at the B3LYP/TZVP level of theory. The optimized geometries of PANI and PANI-Py are shown in Fig. 4. Notably, it was discovered that PANI-Py was 11.37 keV more stable than PANI. The HOMO and LUMO energies are also commonly used as reactivity indicators to assess a molecule's tendency to donate and accept electrons under favourable conditions, thereby establishing its antioxidant and related characteristics. The HOMO and LUMO of the compounds have been examined and are shown in Fig. 4(a).

Fig. 4. (a) Optimized geometry of PANI and PANI-pyrrole, along with HOMO–LUMO pictorial representation depicting the PANI and PANI-pyrrole energy gap (b) graphical illustration of FMO, ΔE, IP, and ω.

Fig. 4

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The stability of polyaniline and polyaniline-pyrrole was supported by the negative eigenvalues of HOMO and LUMO and the positive values of ΔE, as shown in Table S1. The frontier molecular orbitals, HOMO/LUMO gap, and TDDFT gaps are shown in Fig. 4(a), indicating that PANI and PANI-Py have HOMO/LUMO gaps of 2.13 eV and 2.01 eV, respectively. A similar observation was made for the TDDFT gaps, which were 1.93 eV and 1.79 eV, respectively, and are near the experimental absorption spectra, demonstrating that PANI-Py has high polarizability and reactivity. According to the FMO topology, LUMO is mostly distributed within the centred aniline unit of polyaniline and polyaniline-pyrrole, while HOMO is distributed over the entire molecular unit. Additionally, lower values of the energy gap (ΔE) and ionisation potential (IP) enhance the electron-donating capacity of PANI-Pyrrole (Fig. 4(b)), resulting in active participation in biological activities.

Additionally, the compounds' ability to accept electrons from the environment and subsequently reduce their energy was demonstrated by their positive electronegativity (χ) and negative chemical potentials (µ). Furthermore, the compound's stability and resistance to decomposition into their constituent elements are supported by the negative chemical potentials (−3.63 eV for PANI and −3.6 eV for PANI-Py). Furthermore, PANI-Py's low chemical hardness (1.01) and comparatively high chemical softness (0.5) make it a viable option for biological applications. The global electrophilicity index (ω), which gauges the molecule's energy stabilization, was calculated using η and χ.

Additionally, the compounds' negative chemical potentials (µ) and positive electronegativity (χ) demonstrated that they might take in electrons from their surroundings, lowering their energy. Furthermore, the compound's stability and resistance to decomposition into elemental forms are supported by the negative chemical potentials (−3.63 eV for PANI and −3.6 eV for PANI-Py). Furthermore, PANI-Py is a promising option for biological applications due to its comparatively high chemical softness (0.5) and low chemical hardness (1.01). The global electrophilicity index (ω), which quantifies the molecule's energy stabilisation, was calculated using η and χ. Parr et al. show that the electrophilic power of a molecule can be computed as its hardness divided by its chemical potential squared.38 PANI-Py was shown to have a much higher electrophilicity index (6.42) than PANI (6.16), suggesting that PANI-Py has more biological activity. PANI-Py's stability in 1IX1 and 1UKC was thus supported by the calculated electrophilicity index value, as has been covered in the molecular docking section.

3.8. Conductivity measurement

Pyrrole rings were substituted onto the PANI backbone to form Py-substituted PANI. Structural investigations show that interactions between the PANI and pyrrole heteroring occur at the N atoms of imine groups on the polymeric backbone, thereby increasing the delocalisation of conjugated electrons.23 Conductivity of PANI found to be 3.39 × 10−3 S cm−1, and 5.52 × 10−3 S cm−1 for PANI-Py (1 : 1), respectively. The Py heteroring is incorporated into the PANI backbone to increase conductivity. Adding pyrrole to the polyaniline backbone increased electrical conductivity from 3.39 × 10−3 S cm−1 to 5.52 × 10−3 S cm−1, indicating that the structural and electronic changes induced by copolymerisation favour charge transport. Fig. 5 displays the plotted graphs of (a) σ vs. T and (b) log10 σ vs. 1000/T for PANI and PANI-Py. The conductivities of PANI and PANI-Py are shown in Table 6.

Fig. 5. Plot of PANI and PANI-Py copolymers: (a) σ vs. T, and (b) log10 σ vs. 1000/T.

Fig. 5

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Table 6. Conductivity of PANI and PANI-Py copolymers.

S. No. Synthesized copolymers Conductivity (S cm−1)
1 PANI 3.39 × 10−3
2 PANI-Py (1 : 1) 5.43 × 10−3
3 PANI-Py (1 : 2) 4.77 × 10−3
4 PANI-Py (2 : 1) 4.47 × 10−3
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3.9. Antibacterial activity

The antibacterial activity of PANI and PANI-Py copolymers was investigated against B. subtilis, S. aureus, E. coli, and P. aeruginosa, at various concentrations (200, 150, 130, 100, 80, 50, 30, 10, 1, 0.50, and 0.25 µg mL−1). Activity as a percentage was calculated. As a result, MIC and IC50 values were calculated. PANI, PANI-Py, and amoxicillin's antibacterial qualities were evaluated using MIC and IC50 calculations. The MICs of PANI were 1 µg mL−1 against E. coli and S. aureus, and 0.5 µg mL−1 against B. subtilis and 0.25 µg mL−1 against P. aeruginosa. The MICs of PANI-Py (1 : 1) and amoxicillin were found to be 0.25 µg mL−1 against E. coli, P. aeruginosa, B. subtilis, and S. aureus, respectively, as shown in Fig. 6. In addition, PANI and PANI-Py (1 : 1) exhibit significant activity against E. coli, with IC50 values of 227.87 ± 2.26 µg mL−1 and 136.34 ± 2.65 µg mL−1, respectively. The standard drug amoxicillin exhibits excellent activity against B. subtilis, with an IC50 of 24.50 ± 2.55 µg mL−1. PANI-Py (1 : 1) indicates a stronger activity against Gram-negative bacterial strains than pure PANI. The results of the antibacterial properties of PANI and PANI-Py against E. coli, P. aeruginosa, B. subtilis, and S. aureus are illustrated in Table 7 and Fig. 6, respectively. The antibacterial activity of PANI and PANI-Py is strongly influenced by their surface area. A larger BET surface area provides more active sites and increased surface interaction between the copolymer and microbial cell walls. This improved interaction enables stronger binding, membrane penetration, or membrane rupture, thereby increasing antibacterial and antifungal activity. Therefore, PANI-Py greater BET surface area as compared to pure PANI can be directly linked to the observed improvement in antibacterial effectiveness. The antibacterial activity of PANI-Py copolymers is remarkably higher than that of PANI.

Fig. 6. The graphical representation of (a) MIC and (b) IC50 values of polyaniline and polyaniline–pyrrole copolymers, and amoxicillin against E. coli, P. aeruginosa, B. subtilis, and S. aureus.

Fig. 6

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Table 7. Antibacterial activity of PANI, PANI-Pyrrole copolymers, and standard drug amoxicillin against E. coli, P. aeruginosa, B. subtilis, and S. aureus.

S. No. Synthesized copolymers E. coli P. aeruginosa B. subtilis S. aureus
MIC (µg mL−1) IC50 (µg mL−1) MIC (µg mL−1) IC50 (µg mL−1) MIC (µg mL−1) IC50 (µg mL−1) MIC (µg mL−1) IC50 (µg mL−1)
1 PANI 1 227.87 ± 2.26 0.25 303.65 ± 97.87 0.5 311.94 ± 12.54 1 303.26 ± 3.31
2 PANI-Py (1 : 1) 0.25 136.34 ± 2.65 0.25 222.71 ± 19.54 0.25 243.26 ± 0.20 0.25 241.72 ± 1.49
3 PANI-Py (1 : 2) 0.5 161.17 ± 4.05 0.25 293.81 ± 115.14 0.25 266.67 ± 30.57 0.25 262.95 ± 20.80
4 PANI-Py (2 : 1) 0.25 182.87 ± 1.79 0.25 260.16 ± 21.67 0.25 250.57 ± 29.07 0.25 271.39 ± 48.93
5 Amoxicillin 0.25 69.10 ± 0.28 0.25 93.77 ± 2.28 0.25 24.50 ± 2.55 0.25 58.25 ± 0.28
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3.10. Antifungal activity

The antifungal activity of polyaniline and polyaniline–pyrrole copolymers was investigated against A. niger, and P. chrysogenum at various concentrations (200, 150, 130, 100, 80, 50, 30, 10, 1, 0.50, and 0.25 µg mL−1). The antifungal activity of polyaniline, polyaniline-pyrrole, and ketoconazole was determined as MIC and IC50 values. The MICs of PANI were found to be 0.25 and 0.50 µg mL−1 against A. niger and P. chrysogenum, respectively. The MICs of PANI-Py and ketoconazole were found to be 0.25 µg mL−1 against A. niger and P. chrysogenum, respectively, as shown in Fig. 7(a). In addition, the IC50 for PANI against A. niger was found as 363.22 ± 10.44 µg mL−1, 215.10 ± 1.14 µg mL−1 for PANI-Py (1 : 1), and 91.04 ± 2.84 µg mL−1 for ketoconazole, respectively, whereas the IC50 for polyaniline against P. chrysogenum was 334.83 ± 75.37 µg mL−1, 221.17 ± 16.84 µg mL−1 for PANI-Py (1 : 1), and 121.64 ± 5.92 µg mL−1 for ketoconazole, respectively. PANI-Py exhibited more significant antifungal activity than PANI due to the substitution of Py on the backbone of PANI. Notably, PANI-Py copolymers show greater antifungal activity than polyaniline, especially against A. niger. As a result, the antifungal activity of PANI and PANI-Py against A. niger and P. chrysogenum is shown in Table 8 and Fig. 7, respectively. Additionally, statistical analysis of the data using ONE-WAY ANOVA revealed that the MIC and IC50 values for antibacterial and antifungal activities varied significantly, as indicated in Tables S2 and S3.

Fig. 7. The graphical representation of (a) MIC and (b) IC50 values of polyaniline and polyaniline–pyrrole copolymers, and ketoconazole against A. niger, and P. chrysogenum.

Fig. 7

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Table 8. Antifungal activity of PANI, PANI-Py copolymers, and ketoconazole against A. niger and P. chrysogenum.

S. No. Synthesized copolymers A. niger P. chrysogenum
MIC (µg mL−1) IC50 (µg mL−1) MIC (µg mL−1) IC50 (µg mL−1)
1 PANI 0.25 363.22 ± 10.44 0.50 334.83 ± 75.37
2 PANI-Py (1 : 1) 0.25 215.10 ± 1.14 0.25 221.17 ± 16.84
3 PANI-Py (1 : 2) 0.25 229.90 ± 29.87 0.25 237.77 ± 2.74
4 PANI-Py (2 : 1) 0.25 216.54 ± 1.84 0.25 249.84 ± 15.31
5 Ketoconazole 0.25 91.04 ± 2.84 0.25 121.64 ± 5.92
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3.11. Cytotoxicity analysis

The MTT assay revealed clear differences in how PANI and PANI-Py (1 : 1) affected cell viability across the tested concentration range (1 µg mL−1 to 400 µg mL−1). At the highest doses (400 µg mL−1 and 250 µg mL−1), both PANI and PANI-Py (1 : 1) reduced metabolic activity compared with the untreated control, indicating cytotoxicity. However, PANI-Py (1 : 1) consistently showed stronger inhibition than PANI, suggesting greater potency. At intermediate concentrations (100 µg mL−1 and 50 µg mL−1), PANI-Py (1 : 1) maintained this trend, while PANI produced only modest changes. A striking observation was made at 25 µg mL−1, where PANI-Py (1 : 1) showed its peak response, markedly higher than that of PANI. This concentration appeared to represent an optimal point for PANI-Py (1 : 1) activity, highlighting a non-linear, biphasic dose–response pattern. At lower concentrations (10 µg mL−1 and 1 µg mL−1), the effect of PANI-Py (1 : 1) declined but remained above that of PANI, as shown in Fig. 8. In contrast, PANI displayed a relatively flat profile across all doses, with limited variation in cytotoxicity. Taken together, these results indicate that PANI-Py (1 : 1) exerts stronger, more concentration-dependent cytotoxicity than PANI. The pronounced peak at moderate concentration suggests a unique mechanism of action, positioning PANI-Py (1 : 1) as a more promising agent for further evaluation.

Fig. 8. Cytotoxicity analysis of PANI and PAN-Py (1 : 1).

Fig. 8

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3.12. Molecular docking

A molecular docking analysis of PANI and PANI-Py with 1IX1 and 1UKC was performed to determine the most likely binding site. Significantly, PANI and PANI-Py are available in the linear form for optimisation. Nevertheless, they exhibited a notable conformational shift from a linear to an angular shape upon docking in 1IX1 and 1UKC. The PANI-1IX1 complex has a free energy of −7.39 kcal mol−1, while the PANI-1UKC complex has a free energy of 7.08 kcal mol−1.23 PANI-Py in 1IX1 reveals that ligand is surrounded by residues such as Leu93, Glu135, Gly91, His134, Cys92, Tyr88, Leu127, Lys67, Asp66, Tyr40, Glu65, Tyr99, Ser64, Pro43, Gly44, Asp62, Ile45, and Gly46, as shown in Fig. 9(a). The PANI-Py 1IX1 complex's free energy is −7.71 kcal mol−1.

Fig. 9. (a) The docked position of PANI-Py in a 1IX1 cavity displays the surface of the ligand. The representation of amino acid residue proximity in 2D and 3D (left to right) (b) the docked position of PANI-Py in a 1UKC cavity displays the surface of the ligand. The representation of amino acid residue proximity in 2D and 3D (left to right).

Fig. 9

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On the other hand, PANI-Py in 1UKC reveal that the ligand is surrounded by residues, Pro376, Val374, Asp525, Lys379, Gly378, Leu381, Glu338, Asp337, Ser340, Thr336, Ser392, Gly339, Ala393, Gly389, Asp397, Ala390, Tyr375, as shown in Fig. 9(b). The PANI-Py 1UKC complex has a free energy of −7.28 kcal mol−1. Additionally, the hydrogen bonding interactions shown in the table below stabilise the PANI-Py complex. The binding energy, surrounding residues, and hydrogen bonding interactions of PANI and PANI-Py copolymers for proteins 1IX1 and 1UKC are displayed in Table 9. In both 1IX1 and 1UKC, PANI-Py demonstrated higher stability than PANI. The PANI-Py complexes docked to 1IX1 and 1UKC formed more hydrogen bonds, which is the main cause of the increased stability. The pyrrole unit of PANI-Py was noteworthy for its involvement in intermolecular hydrogen bonding, which was crucial for molecular stabilisation and further enhanced biological activity.

Table 9. PANI-Py for protein 1IX1 and 1UKC.

Protein ID Synthesized copolymers Binding energy (Kcal mol−1)/RMSD Surrounding residues (in the proximity of 4.5 Å) Hydrogen bonding interaction
1IX1 PANI-Py −7.71/4.33 Leu93, Glu135, Gly91, His134, Cys92, Tyr88, Leu127, Lys67, Asp66, Tyr40, Glu65, Tyr99, Ser64, Pro43, Gly44, Asp62, Ile45, Gly46 6π⋯Gly44 [3.91 Å; −1.4 kcal mol−1] 6π⋯Ile45 [4.36 Å; −0.7 kcal mol−1]
1UKC PANI-Py −7.28/3.40 Pro376, Val374, Asp525, Lys379, Gly378, Leu381, Glu338, Asp337, Ser340, Thr336, Ser392, Gly339, Ala393, Gly389, Asp397, Ala390, Tyr375 6π⋯Ser392 [3.09 Å; −0.9 kcal mol−1] 6π⋯Asp337 [3.11 Å; −4.5 kcal mol−1] 6π⋯Lys379 [3.39 Å; −1.8 kcal mol−1]
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4. Conclusion

The COPM was used to synthesize PANI and PANI-Py copolymers co-doped with dual acids in a single pot. The structural alterations in the synthesised PANI and PANI-Py were confirmed using FTIR and UV-visible spectroscopy. TGA confirms the four phases of weight loss for synthesising PANI and PANI-Py copolymers, whereas the DSC thermogram shows two endothermic and one exothermic peak used to track phase changes in a polymeric material. TGA and DSC confirmed that PANI-Py is more thermally stable than PANI because it has a larger area under the degradation endothermic peak. Surface area increases from 22.39 m2 g−1 to 48.64 m2 g−1 when pyrrole was added to the PANI backbone, according to BET analysis. By measuring viscosity, it was found that PANI-Py (1 : 1) had a larger average molar mass (2.8 × 104 g mol−1) than pure PANI (2.1 × 104 g mol−1). According to DFT, the negative eigenvalues of the HOMO and LUMO and the positive values of ΔE supported the stability of PANI and PANI-Py. PANI and PANI-Py were found to have theoretical band gaps of 2.13 and 2.01 eV, as demonstrated by DFT. The two-probe method was used to evaluate conductivity, and it was found that PANI-Py (1 : 1) (5.43 × 10−3 S cm−1) has a higher conductivity than pure PANI (3.39 × 10−3 S cm−1). Both PANI and PANI-Py (1 : 1) showed substantial activity against E. coli, with MIC and IC50 values of 1 µg mL−1 and 227.87 ± 2.26 µg mL−1, and 0.25 µg mL−1 and 136.34 ± 2.65 µg mL−1, respectively. Both PANI and PANI-Py (1 : 1) demonstrated substantial activity against A. niger, with MIC and IC50 values of 0.25 and 363.22 ± 10.44 µg mL−1 and 0.25 and 215.10 ± 1.14 µg mL−1, respectively. Further, cytotoxicity analysis also revealed that PANI-Py (1 : 1) exerts stronger, more concentration-dependent cytotoxicity than PANI. According to the findings, the synthesized PANI-Py copolymer has stronger antibacterial and antifungal properties than pure PANI. According to molecular docking, the PANI-Py copolymer had a greater affinity for binding (−7.28 Kcal mo−−1) to a protein and showed superior antibacterial and antifungal activity compared to PANI (−7.08 Kcal mol−1). The PANI-Py's one-pot synthesis highlights its antifungal and antibacterial properties.

Author contributions

Chetna Kumari: writing – original draft, investigation, Sapana Jadoun: writing – review & editing, Gulshan Kumar: analysis, Ved Prakash Meena: analysis, Sweta Singh: analysis Nirmala Kumari Jangid: writing – review & editing, supervision, and conceptualization, Jaya Dwivedi: investigation.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary Material

RA-016-D6RA08255F-s001

Acknowledgments

The authors thank Prof. Ina Aditya Shastri, Vice Chancellor, Banasthali Vidyapith, Rajasthan, India, and DST-FIST India (Order Number SR/FST/2022/252) for providing the necessary facilities for the successful completion of this work.

Data availability

The data supporting this article are included in manuscript as well as in the supplementary information (SI). Supplementary information: the mechanism of polymerisation, FTIR and TGA figures, a table of DFT data, and antimicrobial and antifungal activity. See DOI: https://doi.org/10.1039/d6ra08255f.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA08255F-s001
RA-016-D6RA08255F-s001.pdf (508KB, pdf)

Data Availability Statement

The data supporting this article are included in manuscript as well as in the supplementary information (SI). Supplementary information: the mechanism of polymerisation, FTIR and TGA figures, a table of DFT data, and antimicrobial and antifungal activity. See DOI: https://doi.org/10.1039/d6ra08255f.

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