Synthesis, Characterisation, and In Vitro Evaluation of Biocompatibility, Antibacterial and Antitumor Activity of Imidazolium Ionic Liquids

Abstract

In recent decades, ionic liquids (ILs) have garnered research interest for their noteworthy properties, such as thermal stability, low or no flammability, and negligible vapour pressure. Moreover, their tunability offers limitless opportunities to design ILs with properties suitable for applications in many industrial fields. This study aims to synthetise two series of methylimidazolium ILs bearing long alkyl chain in their cations (C9, C10, C12, C14, C16, C18, C20) and with tetrafluoroborate (BF₄) and the 1,3-dimethyl-5-sulfoisophthalate (DMSIP) as counter ions. The ILs were characterised using ¹H-NMR and MALDI-TOF, and their thermal behaviour was investigated through DSC and TGA. Additionally, the antimicrobial, anticancer, and cytotoxic activities of the ILs were analysed. Moreover, the most promising ILs were incorporated at different concentrations (0.5, 1, 5 wt%) into polyvinyl chloride (PVC) by solvent casting to obtain antimicrobial blend films. The thermal properties and stability of the resulting PVC/IL films, along with their hydrophobicity/hydrophilicity, IL surface distribution, and release, were studied using DSC and TGA, contact angle (CA), SEM, and UV-vis spectrometry, respectively. Furthermore, the antimicrobial and cytotoxic properties of blends were analysed. The in vitro results demonstrated that the antimicrobial and antitumor activities of pure ILs against t Listeria monocytogenes, Escherichia coli, Pseudomonas fluorescens strains, and the breast cancer cell line (MCF7), respectively, were mainly dependent on their structure. These activities were higher in the series containing the BF₄ anion and increased with the increase in the methylimidazolium cation alkyl chain length. However, the elongation of the alkyl chain beyond C16 induced a decrease in antimicrobial activity, indicating a cut-off effect. A similar trend was also observed in terms of in vitro biocompatibility. The loading of both the series of ILs into the PVC matrix did not affect the thermal stability of PVC blend films. However, their T_onset decreased with increased IL concentration and alkyl chain length. Similarly, both the series of PVC/IL films became more hydrophilic with increasing IL concentration and alkyl chain. The loading of ILs at 5% concentration led to considerable IL accumulation on the blend film surfaces (as observed in SEM images) and, subsequently, their higher release. The biocompatibility assessment with healthy human dermal fibroblast (HDF) cells and the investigation of antitumoral properties unveiled promising pharmacological characteristics. These findings provide strong support for the potential utilisation of ILs in biomedical applications, especially in the context of cancer therapy and as antibacterial agents to address the challenge of antibiotic resistance. Furthermore, the unique properties of the PVC/IL films make them versatile materials for advancing healthcare technologies, from drug delivery to tissue engineering and antimicrobial coatings to diagnostic devices.

Keywords: PVC/IL films; antibacterial tests; antitumor activity; in vitro cytotoxicity; methylimidazolium-based ionic liquids.

Scheme 1

Schematic representation of IL synthesis.

Figure 1

DSC curves of C14mimBF₄ at different cooling and heating runs. (A) Cooling runs; (B) second heating runs. Curves are shifted for clarity. The legend refers to both graphics.

Figure 2

DSC curves of the series CnMimBF₄ at 10 °C/min cooling and heating runs. (Up) cooling cycle; (Down) second heating cycle. Curves are displaced for clarity. The legend refers to both graphics.

Figure 3

DSC curves of the series CnmimDMSIP at 10 °C/min cooling and heating runs. (Up) cooling cycle; (Down) second heating cycle. Curves are displaced for clarity. The legend refers to both graphics.

Figure 4

TGA curves of the synthetised ILs containing BF₄ as the counter ion. The DTG curves of the inset are displaced for clarity. The legend refers to both graphics.

Figure 5

TGA curves of the synthetised ILs, comprising DMSIP as the counter ion. The DTG curves of the inset are displaced for clarity. The legend refers to both graphics.

Figure 6

SEM images (magnification 1000×) of PVC blend films containing 5 wt% of (A) C12mimBF₄, (B) C14mimBF₄, (C) C16mimBF₄, (D) C12mimDMSIP, (E) C14mimDMSIP, (F) C16mimDMSIP. Inset magnification 10,000×.

Figure 7

Water contact angle of PVC/CnmimBF₄ blends as a function of IL weight percent.

Figure 8

Water contact angle of PVC/CnmimDMSIP blends as a function of IL weight percent.

Figure 9

IL release from PVC/CnmimBF₄ blends. Points are the mean of three independent replicates with n = 24 each. Bars represent ± SD.

Figure 10

IL release from PVC/CnmimDMSIP blends. Points are the mean of three independent replicates with n = 24 each. Bars represent ± SD.

Figure 11

Images of inhibition halos induced by PVC/C14mimBF₄ (A) and PVC/C14mimDMSIP (B) against Listeria monocytogenes, Escherichia coli, Pseudomonas fluorescens growth spread on NA plates at a concentration of 10⁶ CFU/mL.

Figure 12

Indirect cytotoxic test. Alamar blue assay to evaluate the cell viability of (A) CnmimBF₄-loaded PVC matrix (at different weight percentage, wt%) and (B) CnmimDMSIP-loaded PVC matrix (at different weight percentage, wt%) after 24 h of elution time and 24 h of in vitro cell culture * p < 0.01; ° p < 0.001; # p < 0.001 vs. HDF.

Figure 13

Direct cytotoxic test. Alamar blue assay to evaluate the cell viability of (A) CnmimBF₄-loaded PVC matrix (at different weight percentage, wt%) and (B) CnmimDMSIP-loaded PVC substrates at direct contact with HDF cells (at different weight percentage, wt%). * p < 0.01; ° p < 0.001; # p < 0.001 vs. HDF.

Figure 14

In vitro cell adhesion. Qualitative analyses of HDF cells stained with CellTracker™ Red CMTPX on CnmimBF₄/CnmimDMSIP-loaded PVC substrates after 24 h of cell culture. Scale bar = 500 µm.