The Effects of Photodynamic Therapy in Oral Biofilms - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 442

ground state is important for photochemical processes due to its high chemical
potential and unique characteristics of reactivity (Turro 1991 ). According to
Molecular Orbital Theory, the electron configuration of oxygen in the ground
state has two unpaired electrons in molecular orbital degenerates

y.
These electrons tend to have the same maximum spin multiplicity and produce a
state of low energy. This is why the ground state of oxygen is a triplet. The lifetime
of singlet oxygen in solution is deeply influenced by the nature of the solvent: in
water, for example, the lifetime of singlet oxygen is approximately 4.0

ˀ x and ˀ

s (Ochsner
1997b ; Dougherty et al. 1998 ). In biological systems, singlet oxygen lifetimes are
extremely low, less than 0.04 ʼ s. Therefore, their range is very low, < 0.02 ʼ m
(Malik et al. 2010 ).

ʼ

3.2 Photosensitiser

Initial preparations of photosensitisers for photodynamic therapy were based on a
complex mixture of porphyrins, referred to as haematoporphyrin derivatives.
Extensive chemical and biological studies have been performed in the past
20 years to identify new photosensitisers that belong to different classes of com-
pounds (Brown et al. 2004 ; Tardivo et al. 2005 ).
A photosensitiser is capable of absorbing light of a specific wavelength and
transforming it into useful energy. In PDT, the photosensitiser is responsible for the
production of cytotoxic agents, their main function being to induce a desired
biological effect (Sharman et al. 1999 ; Lee et al. 2012 ). The ideal photosensitiser
should not be toxic by itself but should exert toxicity only after activation by
irradiation, not cause allergic reactions or hypotension, be water soluble, exhibit
rapid excretion and be biologically stable. Moreover, such a photosensitiser should
be photochemically effective and selective (Meisel and Kocher 2005 ), have a strong
binding affinity for microorganisms, have a low affinity for mammalian cells
(avoiding photodestruction of host tissues), have a minimal risk of promoting
mutagenic processes and have low chemical toxicity (Jori et al. 2006 ; Soukos and
Goodson 2011 ).
Over 400 compounds have known photosensitising properties, including dyes,
medicines, cosmetics, chemicals and many natural substances (Malik et al. 2010 ).
Antimicrobial photosensitisers such as porphyrins, phthalocyanines and phenothi-
azines, such as toluidine blue O and methylene blue, have a positive charge and
may act directly on both Gram-negative and Gram-positive bacteria (Merchat
et al. 1996a ; Minnock et al. 1996 ; Wilson et al. 1995 ). The positive charge appears
to promote binding of the photosensitiser to the bacterial outer membrane, inducing
localised damage and favouring penetration (Merchat et al. 1996b ). Toluidine blue
and methylene blue are commonly used in oral antimicrobial photodynamic ther-
apy. Toluidine blue is vital for staining the mucus abnormalities in the cervix
and oral cavity, and, furthermore, defines the extent of injury before excision