Abstract
We have recently discovered that the photodynamic action of many different
photosensitizers (PSs) can be dramatically potentiated by addition of a solution containing a range
of different inorganic salts. Most of these studies have centered around antimicrobial photodynamic
inactivation that kills Gram-negative and Gram-positive bacteria in suspension. Addition of nontoxic
water-soluble salts during illumination can kill up to six additional logs of bacterial cells (one
million-fold improvement). The PSs investigated range from those that undergo mainly Type I
photochemical mechanisms (electron transfer to produce superoxide, hydrogen peroxide, and
hydroxyl radicals), such as phenothiazinium dyes, fullerenes, and titanium dioxide, to those that
are mainly Type II (energy transfer to produce singlet oxygen), such as porphyrins, and Rose Bengal.
At one extreme of the salts is sodium azide, that quenches singlet oxygen but can produce azide
radicals (presumed to be highly reactive) via electron transfer from photoexcited phenothiazinium
dyes. Potassium iodide is oxidized to molecular iodine by both Type I and Type II PSs, but may also
form reactive iodine species. Potassium bromide is oxidized to hypobromite, but only by titanium
dioxide photocatalysis (Type I). Potassium thiocyanate appears to require a mixture of Type I and
Type II photochemistry to first produce sulfite, that can then form the sulfur trioxide radical anion.
Potassium selenocyanate can react with either Type I or Type II (or indeed with other oxidizing
agents) to produce the semi-stable selenocyanogen (SCN)2. Finally, sodium nitrite may react with
either Type I or Type II PSs to produce peroxynitrate (again, semi-stable) that can kill bacteria and
nitrate tyrosine. Many of these salts (except azide) are non-toxic, and may be clinically applicable.