Scientists a perfect carrier of nosocomial pathogens.[2] Worryingly,

 

Scientists have
discovered new ways to eliminate bacteria

Do you ever feel as though you are compulsively
checking your Smartphone? Do you mindlessly scroll through Facebook? Have you ever
considered how much bacteria is being transferred from the touch screen to your
fingers, to your face, to your home, to your family…? Most of us do not think
to clean our devices but recent studies have shown mobile phones can have ten
times more bacteria than your toilet seat. 1 It is not just your
mobile phone to worry about, either. Use and sharing of touch screen devices in
hospitals is becoming increasingly widespread, making them a perfect carrier of
nosocomial pathogens.2 Worryingly, this could increase the chance
of catching Healthcare Associated Infections (HCAIs), which the World Health
Organisation has found are already occurring in 7.6% of patients, and in 30% of
patients in intensive care units.3 Numerous tests have been
carried out to determine the most effective alcohol wipe to use to disinfect
these devices, however cleaning must take place every 6 hours for it to be
effective.4 It would clearly be better for the materials used in
devices to intrinsically self-clean, bypassing the need for manual cleaning, which
is easily forgotten in a overburdened healthcare system.

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It
may seem a tall order to make our devices from materials which intrinsically
kill bacteria such as Escherichia coli and Staphylococcus aureus but teams of
scientists are discovering materials which can do just this.

 

Light-activated antibacterial reactions

A
photoreaction is one which is activated by the absorption of photons of light.
Photocatalysts are materials which enhance these reactions. 5 Photons
of a specific wavelength (proportional to the band gap of the material) are
absorbed by electrons (e-) in the conduction band. These electrons are
then promoted to the valence band, leaving behind positive holes in the
conduction band (h+). In photocatalysts the h+ and e-
quickly move apart, preventing their recombination. 6 Therefore they
can interact with oxygen and water molecules at the material surface, most
notably 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 of
both gram positive and gram negative bacteria. 7 Figure 1 shows
the equations followed by the electron/hole pairs generated and Figure 2
illustrates the photocatalytic mechanism.

Figure 2: Diagram of the photocatalytic process. 8

What photocatalytic materials are
there?

 

Titanium oxide, TiO2

One
of the best known photocatalytic materials is TiO2, consisting of Ti4+
cations and O22- anions in either the rutile or anatase
structure. 5 Figure 3 shows these two polymorphs.

 

Figure 3: (a) Rutile structure
of TiO2,

(b) Anatase structure
of TiO2. 5

 

The
band gap of the anatase structure is 3.0 electron volts (eV), which is the
energy 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 of
hole/electron pairs is less likely and so more ROS can be generated. These
properties make anatase TiO2 perfect for use in touch screens devices
because the metal oxide is transparent so the screen can still be seen.

 

However, more often than not TiO2 is
doped with other transition metals or light activated antimicrobial agents
(LAAAs) to decrease the band gap of the material and allow for the absorption
of visible light to catalyse the photoreaction. This is important because UV
light makes up less than 5% of the solar spectrum. 10

 

TiO2 nanotubes doped with
silver nanoparticles

Ag+
ions have long been used as an antimicrobial agent. They work by bonding to
thiol groups (those containing sulfur-hydrogen bonds) in the enzymes of
bacterial cells, and thus deactivating the respiratory chain and bringing about
their death. 11 One research team found that the addition of
silver ions in TiO2 nanotubes at a concentration of 20 parts per
million eliminated 99.99% of gram positive Staphylococcus aureus bacteria after
just one hour after sample irradiation.12

 

Graphene doped TiO2

TiO2
doped with graphene could be used in the search to find a solution to safely
sanitise drinking water. It is estimated that ? of
people currently do not have access to clean, safe drinking water 8,
which will only be exacerbated as global population continues to rise
exponentially. Graphene is a single sheet of hexagonal latticed carbon, which
makes up the structure of the graphite allotrope. 5 One study
found that TiO2 doped with graphene along with the presence of silver
ions inactivated E coli after 180 minutes of exposure. 13 This was
more effective than just TiO2 on its own, as graphene was found to
increase the hole/electron pair combination time, and thereby increase the time
available for generation of ROS.

 

Cuprous oxide, Cu2O

Finding
visible light-activated antibacterial materials is vital if the natural
backlight from touch screen devices is to be used as the irradiation light. In
one study, poly(ethylene terephthalate) (PET) was combined with 500nm cubic Cu2O
nanoparticles. This material was shown to produce almost a 91% reduction in bacteria
presence when a concentration of 2mg/mL was used. 14 With a band
gap of 1.92eV, cubic cuprous oxide is activated by visible light, unlike common
ZnO and TiO2 photocatalysts. Copper is also a cheaper alternative to
these photocatalysts as copper ore is relatively more abundant, with the added
benefit of being non-toxic. 14 This is a significant consideration
going forward if these materials are to be used on a large, global scale.

 

Quantum Dots

At
the University of Colorado Boulder, cadmium telluride quantum dot nanoparticles
have been engineered to destroy multiple drug resistant bacteria, including
Salmonella enteric and E coli. 15 By fine tuning the size of the
quantum dots, it was possible to ensure that only the superoxide anion was
generated from molecular oxygen at the material surface. This ROS has the
longest lifetime and can travel the furthest, which means the potential to
invade bacteria cells is greatly extended. Excitingly, adding these quantum dot
particles to antibiotics could be the way forward in combating highly drug-resistant
pathogens. 15

 

Photosensitising Dyes

Photosensitising
dyes are highly coloured light activated antimicrobial agents; methylene blue
and crystal violet are currently at the forefront of research, the structures
of which are shown in Figure 4.

 

Figure 4: The structures of
methylene blue (A) and crystal violet (B). 16 Extended conjugation
throughout the molecules is responsible for both the colour and absorption of
photons in the visible region.

 

These
molecules work by absorbing a photon and being excited to their singlet state,
followed by an “intersystem crossing” to their triplet state.16 It
is this triplet state of the photosensitiser which interacts with molecular
oxygen to form high energy singlet oxygen, O2 (1?g),
a strong oxidising agent capable of inactivating bacteria.17 A
group of chemists at University College London used this knowledge to treat
current mobile phone screen protectors with both methylene blue and crystal
violet.16 Under visible light conditions akin to hospitals, the
screen protectors showed significant antimicrobial activity, demonstrating a
simple and cost effective way to tackle the prevalence of HCAIs. Most
importantly, addition of these dyes to the surface of the screen protectors did
not affect transparency and so the devices could still fulfil their purpose.

 

Fascinatingly,
it has also been discovered that materials containing methylene blue and
crystal violet can continue to kill bacteria in the dark.18
Another team at UCL incorporated methylene blue and crystal violet alongside
2nm sized gold nanoparticles into a silicone polymer. The presence of the gold
nanoparticles was thought to enhance the excitation of the photosensitisers,
which absorb photons in the visible region of light (Figure 5). Testing found
that numbers of S. epidermidis and E. coli bacteria were reduced to non-harmful
levels in a time frame of 3 hours and 6 hours respectively. Further testing is
now being carried out to find whether these “multidye nanogold incorporated
polymers” can be effective in a medical environment.

 

Figure 5: UV-Vis spectra showing
the 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 been
incorporated into poly(methylmethacrylate) (PMMA) nanofibres alongside silver
nanoparticles. 19 TPP has an absorption peak at 405nm, with
photons of corresponding wavelength activating the dye to allow generation of
ROS and kill pathogenic bacteria. Using other light-activated antibacterial
polymer composites such as polyurethane may also have applications for the future
in hospitals, including use in bedding, sterile wound dressings and catheter
tubing, thereby reducing the need for antibiotics. 17

 

For
example, one research group investigated the antibacterial activity seen on the
MRSA super bug using polyurethane sheet composites. 20 Magnesium
doped zinc oxide and crystal violet were integrated into the polymer, which
displayed the greatest antibacterial activity using ZnMgO nanoparticles of
between 2 and 4 nanometres in size. 20 The Zn2+ ion and
Mg2+ ion have similar ionic radii as well as identical charge, making
doping into ZnO a simple process. This success points again to promising future
use in healthcare.

 

urn approach to antibacterial materials uses photoinacti-

vation of bacteria

16–19

that is based on photosensitized gen-

eration of singlet oxygen O

2

(

1

D

g

). The antibacterial mecha-

nism includes photoexcitation of the photosensitizer and

formation of its triplet states followed by energy transfer to

triplet oxygen O

2

(

3

R

g

) leading to O

2

(

1

D

g

) formation.

20,21

Singlet oxygen, O

2

(

1

D

g

), is a powerful oxidant of biological

targets, such as proteins

22

or cell membranes

Dark-activated TiO2

Dark-activated
antibacterial activity has not been limited to only silicone polymers. One team
found TiO2  nanosheets to also
display “memory” of photocatalytic antibacterial processes.7 This
works by the storage of excited electrons in the material after UV irradiation,
which are later released in the dark to generate superoxide anions and other
ROS, and thus continue bacteria deactivation. This deactivation was shown to
continue unchanged for 40 hours in the dark,7 showing the
effectiveness of this material against disease causing pathogens long after
irradiation.

 

 

 

 

Are these materials marketable?

Many
of these materials are still in the research and testing stages, especially in
the medical field.11 However, commercialisation of light-activated
antibacterial materials for the mass market may be closer than you think. In
fact, 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 impact
resistant Gorilla® Glass for touch screen devices. In 2014 an Antimicrobial
version of the glass became available, which incorporates Ag+ ions
into the structure to deactivate 99.9% of bacteria. 21 Most
importantly, addition of Ag+ ions to the glass does not affect the
optical transmission. This means the screens remain transparent and so can fulfil
their functionality without impediment. In the coming years it is likely this material
will become more commonplace in new touch screen devices; it is even being
trialled in the United States for new ATM machines.22

 

Biocote
is also developing light-activated antimicrobial technology, a company which
has created its own antimicrobial coating using copper and silver nanoparticles.23
This can be used to cover all sorts of existing products, from textiles to
paints to paper to plastics, and most notably to coat the new Dyson Airblade
hand-drier.

 

Looking to the future

This
exciting new class of materials is still at the forefront of ongoing research but
their importance for the future is clear, confirmed by the existence of clear International
Organisation for Standardisation standards to test for photocatalytic material
quality.8

 

Personally
I feel as though these materials are a perfect example of clever chemistry at
work; the engineering of materials with specific functionality. They have the
ability to reduce HCAIs (although bacteria mutation still needs to be
considered), to sanitise our drinking water and to clean our touch screen
devices; their functions are not limited. If the spreading of bacteria can be
reduced this can only be a good thing for our health.

 

In
my opinion, the focus now should be on scaling up the production of visible light-activated
antibacterial materials, to see whether they can be manufactured on an
industrial level rather than just in the laboratory. If we can do this, and
produce functional materials which are safe, stable and non-toxic, we are on to
a winner. And if you are anything like me, we might even be able to soon put
away the antibacterial hand gel.