Scientists havediscovered new ways to eliminate bacteriaDo you ever feel as though you are compulsivelychecking your Smartphone? Do you mindlessly scroll through Facebook? Have you everconsidered how much bacteria is being transferred from the touch screen to yourfingers, to your face, to your home, to your family…? Most of us do not thinkto clean our devices but recent studies have shown mobile phones can have tentimes more bacteria than your toilet seat. 1 It is not just yourmobile phone to worry about, either.
Use and sharing of touch screen devices inhospitals is becoming increasingly widespread, making them a perfect carrier ofnosocomial pathogens.2 Worryingly, this could increase the chanceof catching Healthcare Associated Infections (HCAIs), which the World HealthOrganisation has found are already occurring in 7.6% of patients, and in 30% ofpatients in intensive care units.3 Numerous tests have beencarried out to determine the most effective alcohol wipe to use to disinfectthese devices, however cleaning must take place every 6 hours for it to beeffective.4 It would clearly be better for the materials used indevices to intrinsically self-clean, bypassing the need for manual cleaning, whichis easily forgotten in a overburdened healthcare system.
Itmay seem a tall order to make our devices from materials which intrinsicallykill bacteria such as Escherichia coli and Staphylococcus aureus but teams ofscientists are discovering materials which can do just this. Light-activated antibacterial reactionsAphotoreaction is one which is activated by the absorption of photons of light.Photocatalysts are materials which enhance these reactions. 5 Photonsof a specific wavelength (proportional to the band gap of the material) areabsorbed by electrons (e-) in the conduction band. These electrons arethen promoted to the valence band, leaving behind positive holes in theconduction band (h+). In photocatalysts the h+ and e-quickly move apart, preventing their recombination. 6 Therefore theycan interact with oxygen and water molecules at the material surface, mostnotably to produce reactive oxygen species (ROS) such as the hydroxyl radical (·OH),hydrogen peroxide (H2O2) and the superoxide anion (·O2-).These highly reactive species can then go on to attack the cell membranes ofboth gram positive and gram negative bacteria.
7 Figure 1 showsthe equations followed by the electron/hole pairs generated and Figure 2illustrates the photocatalytic mechanism.Figure 2: Diagram of the photocatalytic process. 8What photocatalytic materials arethere? Titanium oxide, TiO2Oneof the best known photocatalytic materials is TiO2, consisting of Ti4+cations and O22- anions in either the rutile or anatasestructure.
5 Figure 3 shows these two polymorphs. Figure 3: (a) Rutile structureof TiO2,(b) Anatase structureof TiO2. 5 Theband gap of the anatase structure is 3.0 electron volts (eV), which is theenergy between the valence band and the conduction band in the material,equating to the absorption of photons of wavelength, ?, = 385nm.9 This is in the Ultra Violet region of the light spectrum.
The anatase polymorph is most commonly used because the recombination ofhole/electron pairs is less likely and so more ROS can be generated. Theseproperties make anatase TiO2 perfect for use in touch screens devicesbecause the metal oxide is transparent so the screen can still be seen. However, more often than not TiO2 isdoped with other transition metals or light activated antimicrobial agents(LAAAs) to decrease the band gap of the material and allow for the absorptionof visible light to catalyse the photoreaction. This is important because UVlight makes up less than 5% of the solar spectrum. 10 TiO2 nanotubes doped withsilver nanoparticlesAg+ions have long been used as an antimicrobial agent. They work by bonding tothiol groups (those containing sulfur-hydrogen bonds) in the enzymes ofbacterial cells, and thus deactivating the respiratory chain and bringing abouttheir death. 11 One research team found that the addition ofsilver ions in TiO2 nanotubes at a concentration of 20 parts permillion eliminated 99.
99% of gram positive Staphylococcus aureus bacteria afterjust one hour after sample irradiation.12 Graphene doped TiO2TiO2doped with graphene could be used in the search to find a solution to safelysanitise drinking water. It is estimated that ? ofpeople currently do not have access to clean, safe drinking water 8,which will only be exacerbated as global population continues to riseexponentially. Graphene is a single sheet of hexagonal latticed carbon, whichmakes up the structure of the graphite allotrope.
5 One studyfound that TiO2 doped with graphene along with the presence of silverions inactivated E coli after 180 minutes of exposure. 13 This wasmore effective than just TiO2 on its own, as graphene was found toincrease the hole/electron pair combination time, and thereby increase the timeavailable for generation of ROS. Cuprous oxide, Cu2OFindingvisible light-activated antibacterial materials is vital if the naturalbacklight from touch screen devices is to be used as the irradiation light. Inone study, poly(ethylene terephthalate) (PET) was combined with 500nm cubic Cu2Onanoparticles. This material was shown to produce almost a 91% reduction in bacteriapresence when a concentration of 2mg/mL was used. 14 With a bandgap of 1.
92eV, cubic cuprous oxide is activated by visible light, unlike commonZnO and TiO2 photocatalysts. Copper is also a cheaper alternative tothese photocatalysts as copper ore is relatively more abundant, with the addedbenefit of being non-toxic. 14 This is a significant considerationgoing forward if these materials are to be used on a large, global scale. Quantum DotsAtthe University of Colorado Boulder, cadmium telluride quantum dot nanoparticleshave been engineered to destroy multiple drug resistant bacteria, includingSalmonella enteric and E coli. 15 By fine tuning the size of thequantum dots, it was possible to ensure that only the superoxide anion wasgenerated from molecular oxygen at the material surface. This ROS has thelongest lifetime and can travel the furthest, which means the potential toinvade bacteria cells is greatly extended. Excitingly, adding these quantum dotparticles to antibiotics could be the way forward in combating highly drug-resistantpathogens. 15 Photosensitising DyesPhotosensitisingdyes are highly coloured light activated antimicrobial agents; methylene blueand crystal violet are currently at the forefront of research, the structuresof which are shown in Figure 4.
Figure 4: The structures ofmethylene blue (A) and crystal violet (B). 16 Extended conjugationthroughout the molecules is responsible for both the colour and absorption ofphotons in the visible region. Thesemolecules work by absorbing a photon and being excited to their singlet state,followed by an “intersystem crossing” to their triplet state.16 Itis this triplet state of the photosensitiser which interacts with molecularoxygen to form high energy singlet oxygen, O2 (1?g),a strong oxidising agent capable of inactivating bacteria.17 Agroup of chemists at University College London used this knowledge to treatcurrent mobile phone screen protectors with both methylene blue and crystalviolet.
16 Under visible light conditions akin to hospitals, thescreen protectors showed significant antimicrobial activity, demonstrating asimple and cost effective way to tackle the prevalence of HCAIs. Mostimportantly, addition of these dyes to the surface of the screen protectors didnot affect transparency and so the devices could still fulfil their purpose. Fascinatingly,it has also been discovered that materials containing methylene blue andcrystal violet can continue to kill bacteria in the dark.18Another team at UCL incorporated methylene blue and crystal violet alongside2nm sized gold nanoparticles into a silicone polymer.
The presence of the goldnanoparticles was thought to enhance the excitation of the photosensitisers,which absorb photons in the visible region of light (Figure 5). Testing foundthat numbers of S. epidermidis and E. coli bacteria were reduced to non-harmfullevels in a time frame of 3 hours and 6 hours respectively. Further testing isnow being carried out to find whether these “multidye nanogold incorporatedpolymers” can be effective in a medical environment. Figure 5: UV-Vis spectra showingthe photon absorption by MB/Au (blue), CV/Au (violet), and MB/CV/Au (purple).It is clear from this the photoreaction is activated using visible light. 18 Meso-tetraphenylporphyrin(TPP) is another photosensitising dye under investigation, which has beenincorporated into poly(methylmethacrylate) (PMMA) nanofibres alongside silvernanoparticles.
19 TPP has an absorption peak at 405nm, withphotons of corresponding wavelength activating the dye to allow generation ofROS and kill pathogenic bacteria. Using other light-activated antibacterialpolymer composites such as polyurethane may also have applications for the futurein hospitals, including use in bedding, sterile wound dressings and cathetertubing, thereby reducing the need for antibiotics. 17 Forexample, one research group investigated the antibacterial activity seen on theMRSA super bug using polyurethane sheet composites. 20 Magnesiumdoped zinc oxide and crystal violet were integrated into the polymer, whichdisplayed the greatest antibacterial activity using ZnMgO nanoparticles ofbetween 2 and 4 nanometres in size. 20 The Zn2+ ion andMg2+ ion have similar ionic radii as well as identical charge, makingdoping into ZnO a simple process. This success points again to promising futureuse in healthcare. urn approach to antibacterial materials uses photoinacti-vation of bacteria16–19that is based on photosensitized gen-eration of singlet oxygen O2(1Dg). The antibacterial mecha-nism includes photoexcitation of the photosensitizer andformation of its triplet states followed by energy transfer totriplet oxygen O2(3Rg) leading to O2(1Dg) formation.
20,21Singlet oxygen, O2(1Dg), is a powerful oxidant of biologicaltargets, such as proteins22or cell membranesDark-activated TiO2Dark-activatedantibacterial activity has not been limited to only silicone polymers. One teamfound TiO2 nanosheets to alsodisplay “memory” of photocatalytic antibacterial processes.7 Thisworks by the storage of excited electrons in the material after UV irradiation,which are later released in the dark to generate superoxide anions and otherROS, and thus continue bacteria deactivation. This deactivation was shown tocontinue unchanged for 40 hours in the dark,7 showing theeffectiveness of this material against disease causing pathogens long afterirradiation. Are these materials marketable?Manyof these materials are still in the research and testing stages, especially inthe medical field.
11 However, commercialisation of light-activatedantibacterial materials for the mass market may be closer than you think. Infact, many companies are jumping at the chance to fill this gap in the market.One such example is Corning®, which is best known for its scratch and impactresistant Gorilla® Glass for touch screen devices. In 2014 an Antimicrobialversion of the glass became available, which incorporates Ag+ ionsinto the structure to deactivate 99.9% of bacteria. 21 Mostimportantly, addition of Ag+ ions to the glass does not affect theoptical transmission. This means the screens remain transparent and so can fulfiltheir functionality without impediment. In the coming years it is likely this materialwill become more commonplace in new touch screen devices; it is even beingtrialled in the United States for new ATM machines.
22 Biocoteis also developing light-activated antimicrobial technology, a company whichhas created its own antimicrobial coating using copper and silver nanoparticles.23This can be used to cover all sorts of existing products, from textiles topaints to paper to plastics, and most notably to coat the new Dyson Airbladehand-drier. Looking to the futureThisexciting new class of materials is still at the forefront of ongoing research buttheir importance for the future is clear, confirmed by the existence of clear InternationalOrganisation for Standardisation standards to test for photocatalytic materialquality.8 PersonallyI feel as though these materials are a perfect example of clever chemistry atwork; the engineering of materials with specific functionality. They have theability to reduce HCAIs (although bacteria mutation still needs to beconsidered), to sanitise our drinking water and to clean our touch screendevices; their functions are not limited.
If the spreading of bacteria can bereduced this can only be a good thing for our health. Inmy opinion, the focus now should be on scaling up the production of visible light-activatedantibacterial materials, to see whether they can be manufactured on anindustrial level rather than just in the laboratory. If we can do this, andproduce functional materials which are safe, stable and non-toxic, we are on toa winner. And if you are anything like me, we might even be able to soon putaway the antibacterial hand gel.