Broad-Spectrum Antimicrobial Effects of Photocatalysis Using Titanium Dioxide Nanoparticles Are Strongly Potentiated by Addition of Potassium Iodide

Abstract

f Photocatalysis describes the excitation of titanium dioxide nanoparticles (a wide-band gap semiconductor) by UVA light to produce reactive oxygen species (ROS) that can destroy many organic molecules. This photocatalysis process is used for environmental remediation, while antimicrobial photocatalysis can kill many classes of microorganisms and can be used to sterilize water and surfaces and possibly to treat infections. Here we show that addition of the nontoxic inorganic salt potassium iodide to TiO 2 (P25) excited by UVA potentiated the killing of Gram-positive bacteria, Gram-negative bacteria, and fungi by up to 6 logs. The microbial killing depended on the concentration of TiO 2 , the fluence of UVA light, and the concentration of KI (the best effect was at 100 mM). There was formation of long-lived antimicrobial species (probably hypoiodite and iodine) in the reaction mixture (detected by adding bacteria after light), but short-lived antibacterial reactive species (bacteria present during light) produced more killing. Fluorescent probes for ROS (hydroxyl radical and singlet oxygen) were quenched by iodide. Tri-iodide (which has a peak at 350 nm and a blue product with starch) was produced by TiO 2 -UVA-KI but was much reduced when methicillin-resistant Staphylococcus aureus (MRSA) cells were also present. The model tyrosine substrate N-acetyl tyrosine ethyl ester was iodinated in a light dose-dependent manner. We conclude that UVA-excited TiO 2 in the presence of iodide produces reactive iodine intermediates during illumination that kill microbial cells and long-lived oxidized iodine products that kill after light has ended. Heterogeneous photocatalysis is the use of photoactivated titanium dioxide to produce reactive oxygen species in order to destroy organic pollutants and to kill different classes of microorganisms (1). TiO 2 is a wide-band gap semiconductor, and when a photon of sufficient energy is absorbed, an electron is excited from the valence band to the conduction band, generating a positive hole in the valence band (2). Since energy levels are not available to promote ready recombination of the electron and the hole (as they are in metallic conductors), the electrons and holes survive long enough to carry out reactions. The electrons can reduce oxygen to superoxide, while at the same time the holes can oxidize water to hydroxyl radicals (3). Hydrogen peroxide and singlet oxygen are also produced (4).Although TiO 2 can occur in several different crystalline forms (anatase, rutile, and brookite), the anatase form is usually employed for applications in photocatalysis (5). Because the process is heterogeneous, TiO 2 nanoparticles that have the maximum surface area/mass ratio are optimum for efficient catalytic activity. The maximum absorption wavelength of the TiO 2 nanoparticle is about 360 nm (6), equivalent to the semiconductor band gap of 3.35 eV (7). The preparation of TiO 2 known as Degussa or Aeroxide (Evonik) P25 is composed of 21-nm-diameter nanoparticles, which are about 75% anatase, 1...

Potassium Iodide Potentiates Broad-Spectrum Antimicrobial Photodynamic Inactivation Using Photofrin

“…We reported that the addition of KI led to strong potentiation of killing. 17 When the concentration of KI was kept at 10 mM, then hypoiodite was formed as a product, which could eradicate bacteria that were added immediately after light, but not when bacteria were added after a 2 h interval. However, when the KI concentration was increased to 100 mM, then the bactericidal activity in the solution remained after the light was switched off until 24 h later.…”

“…We generally used concentrations of KI up to 10 mM 15 but recently used concentrations as high as 100 mM in studies with TiO 2 /UVA. 17 …”

“…In the case of TiO 2 /UVA/KI the mechanism of potentiation was shown to include an additional two-electron oxidation of I − to give hypoiodite as well as a one-electron oxidation to give iodine. 17 …”

Potentiation of antimicrobial photodynamic inactivation by inorganic salts

“…We studied the potentiation of TiO 2 antimicrobial photocatalysis by addition of KI to the suspension of titania nanoparticles and microbial cells [20]. The killing of gram-positive bacteria, gram-negative bacteria, and fungi was increased by up to 6 logs by addition of KI.…”

Advances in antimicrobial photodynamic inactivation at the nanoscale

“…Huang et al [192] wanted to know whether antimicrobial photocatalysis mediated by TiO 2 could be potentiated by addition of KI and whether the mechanism was via photocatalytic production of iodine/tri-iodide or hypoiodite. Their results showed that addition of the nontoxic inorganic salt KI to TiO 2 (P25) excited by UVA potentiated the killing of Gram-positive bacteria (MRSA), Gram-negative bacteria ( E. coli ), and fungi ( C. albicans ) by up to 6 logs.…”