Synergistic interactions of cadmium-free quantum dots embedded in a photosensitised polymer surface: efficient killing of multidrug-resistant strains at low ambient light levels

Abstract

Cadmium-free quantum dots (QD) were combined with crystal violet photosensitising dye and incorporated into medical grade polyurethane via a non-covalent dipping process known as 'swell-encapsulation-shrink'. The antibacterial efficacy of the prepared quantum dot-crystal violet polyurethane substrates (QD + CV PU) was investigated under low power visible light illumination at similar intensities (500 lux) to those present in clinical settings. The antibacterial performance of QD + CV PU was superior to the constituent polymer substrates, eliminating ∼99.9% of an environmental P. aeruginosa strain, a clinical P. aeruginosa strain from a cystic fibrosis patient and a clinical E. coli strain. The nature of the reactive oxygen species (ROS) involved in antibacterial activity of the QD + CV PU surface was investigated using ROS inhibitors and time-resolved optical spectroscopy. The photo-physical interactions of the green-emitting QDs with CV lead to a combination of Type I and II electron transfer and energy transfer processes, with the highly potent ROS singlet oxygen playing a dominant role. This study is the first to demonstrate highly efficient synergistic killing of clinical and environmental strains of intrinsically resistant and multi-drug resistant Gram-negative bacteria using light-activated surfaces containing biocompatible cadmium-free QDs and crystal violet dye at ambient light levels.

Fig. 1. (A) Normalised absorption (purple trace) and emission (black trace) spectra of QDs. (B) High resolution TEM image of the green-emitting QD nanoparticles. (C) Size distribution of QD nanoparticles from TEM.

Scheme 1. Preparation of antibacterial polyurethane substrates containing cadmium-free quantum dots and crystal violet.

Fig. 2. Viable counts of (A) environmental P. aeruginosa P12 on modified polyurethane surfaces after 24 h irradiation at 500 lux, (B) clinical P. aeruginosa P1068 on modified polyurethane surfaces after 24 h irradiation at 500 lux (C) multi-drug resistant E. coli 1030 on modified polyurethane surfaces after 18 h irradiation at 500 lux. * indicates that QD + CV PU has P ≤ 0.05 compared to CV PU.

Fig. 3. (A) Spectral overlap of QD emission and crystal violet absorbance. PL excitation at 400 nm. (B) Emission of QD with increasing CV concentration in solution at 400 nm excitation. Inset: S–V relationship of QD quenching vs. CV concentration in solution at 500 nm. (C) Emission of QD alone or in combination with increasing concentrations of CV into polyurethane (excitation at 400 nm). Inset: Stern–Volmer plot of QD fluorescence quenching ratio vs. CV concentration in polyurethane at 500 nm. (D) Time-resolved emission of QD alone or in combination with increasing concentrations of CV in polyurethane.

Fig. 4. Time-resolved singlet oxygen phosphorescence recorded at 1270 nm following pulsed laser irradiation of (A) QD solution (black line) and QD + CV combination (red line). (B) Modified polyurethane containing QD only (black line), CV only (blue line), and a combination of QD and CV (red line). The traces have been subtracted from the control (untreated polyurethane).

Fig. 5. Photo-bactericidal activity of control PU, CV PU and QD + CV PU against EMRSA in the absence of ROS scavengers (‘no scavenger’) and in the presence of catalase (H₂O₂ scavenger), mannitol (hydroxyl scavenger) and l-histidine (¹O₂ scavenger). ** indicates that QD + CV PU has P ≤ 0.01 compared to CV PU.