Atmospheric Pressure Plasma Polymerisation of D-Limonene and Its Antimicrobial Activity

Abstract

Antibacterial coating is necessary to prevent biofilm-forming bacteria from colonising medical tools causing infection and sepsis in patients. The recent coating strategies such as immobilisation of antimicrobial materials and low-pressure plasma polymerisation may require multiple processing steps involving a high-vacuum system and time-consuming process. Some of those have limited efficacy and durability. Here, we report a rapid and one-step atmospheric pressure plasma polymerisation (APPP) of D-limonene to produce nano-thin films with hydrophobic-like properties for antibacterial applications. The influence of plasma polymerisation time on the thickness, surface characteristic, and chemical composition of the plasma-polymerised films was systematically investigated. Results showed that the nano-thin films deposited at 1 min on glass substrate are optically transparent and homogenous, with a thickness of 44.3 ± 4.8 nm, a smooth surface with an average roughness of 0.23 ± 0.02 nm. For its antimicrobial activity, the biofilm assay evaluation revealed a significant 94% decrease in the number of Escherichia coli (E. coli) compared to the control sample. More importantly, the resultant nano-thin films exhibited a potent bactericidal effect that can distort and rupture the membrane of the treated bacteria. These findings provide important insights into the development of bacteria-resistant and biocompatible coatings on the arbitrary substrate in a straightforward and cost-effective route at atmospheric pressure.

Keywords: ASTM E2149; D-limonene; E. coli bacteria; antimicrobial coating; atmospheric pressure; plasma polymerisation.

Figure 1

Schematic diagram of the APPP setup. The inset shows the photograph of plasma generated from the APPJ during the deposition process.

Figure 2

(a) Average static WCA and film thickness at different plasma polymerisation times. (b) Photographs of WCA of control and AP−PP−lim samples at different plasma polymerisation times. (c–g) Step height measurements of AP−PP−lim films at different plasma polymerisation times with their corresponding height profile plots. (h,i) 2D and 3D AFM profiles of the smooth AP−PP−lim nano-thin films deposited at 1 min.

Figure 3

ATR-FTIR spectra of D-limonene monomer and AP−PP−lim deposited on a glass substrate at plasma polymerisation times of 1, 3, 5, 7 and 9 min.

Figure 4

XPS of AP-PP-lim nano-thin films. (a) XPS survey spectrum and (b) core-level of C 1 s spectra.

Figure 5

Schematic representation of plasma polymer derived from D-limonene monomer using the APPJ system.

Figure 6

Antimicrobial performance of AP−PP−lim nano-thin films against Gram-negative (E. coli) bacteria. FESEM images of treated E. coli on the (a,c) control and (b,d) AP−PP−lim sample. The yellow square box indicates the rupture of E. coli cells. Fluorescent microscopic imaging of treated crystal violet-stained E. coli on the (e) control and (f) AP−PP−lim sample. (g) Count of treated E. coli per unit area (cm⁻²) on the control and AP−PP−lim sample. Illustrative fluorescence views in live-dead fluorescence assay of (h) E. coli attached on clean glass substrate and (i) E. coli attached on AP−PP−lim. Samples were incubated for 24 h. Live bacteria are represented by green, whereas dead bacteria are represented by red.