A method of synthesising a 3,7-bis(disubstituted-amino)-1,9-(optionally substituted)-phenothiazin-5-ium salt, 7, the method comprising the steps of, in order: nitrosylation, in which an N, N -disubstituted-3-optionally substituted aniline, 1, is 4-nitrosylated to give said N, N -disubstituted-3-optionally substituted-4-nitrosyl aniline, 2: nitrosyl reduction, in which an N,N -disubstited-3-optionally substituted-4-nitrosyl aniline. 2, is reduced to form said N, N -disubstituted-1,4-diamino-5-optionally substituted benzene. 3: thiosulfonic acid formation (TSAF), in which an N, N -disubstituted-1,4-diamino-5-optionally substituted benzene, 3, is oxidized in the presence of a thiosulfate to give said thiosulfuric acid S-{2-(amino)-3-(optionally substituted)-5-(disubstituted-amino)-phenyl} ester, 4: wherein during or prior to the thiosulfonic acid formation an activating agent is added, oxidative coupling, in which a thiosulfuric acid S-{2-(amino)-3-(optionally substituted)-5-(disubstituted amino)-phenyl} ester, 4, is oxidatively coupled to an N, N -disubstituted-3-optionally substituted-aniline, 5, using an oxidizing agent that is or comprises Cr, to give a [4-{2-(thiosulfate)-4-(disubstituted amino)-6-(optionally substituted)-phenyl-imino}-3-(optionally substituted)-cyclohexa-2,5-dienylidene]- N, N- disubstituted ammonium, 6: ring closure, in which said [4-{2-(thiosulfate)-4-(disubstituted amino)-6-(optionally substituted)-phenyl-imino}-3-(optionally substituted)-cyclohexa-2,5-dienylidene]- N, N -disubstituted ammonium, 6, is subjected to ring closure to give a 3,7-bis(disubstituted-amino)-1,9-(optionally substituted)-phenothiazin-5-ium salt, 7: wherein; each of R 1 and R 9 is independently selected from: -H; C 1-4 alkyl; C 2-4 alkenyl; and halogenated C 1-4 alkyl; each of R 3NA and R 3NB is independently selected from: C 1-4 alkyl; C 2-4 alkenyl; and halogenated C 1-4 alkyl; each of R 7NA and R 7NB is independently selected from: C 1-4 alkyl; C 2-4 alkenyl; and halogenated C 1-4 alkyl; and X - is a halide counter ion with the proviso that the compound 7 is not a ZnCl 2 mixed salt; and wherein the activating agent is or comprises Al and wherein the molar ratio of Al to the N, N -disubstituted-1,4-diamino-5-optionally substituted benzene, 3, is from 0.05 to 1.0. nitrosylation, in which an N, N -disubstituted-3-optionally substituted aniline, 1, is 4-nitrosylated to give said N, N -disubstituted-3-optionally substituted-4-nitrosyl aniline, 2: nitrosyl reduction, in which an N,N -disubstited-3-optionally substituted-4-nitrosyl aniline. 2, is reduced to form said N, N -disubstituted-1,4-diamino-5-optionally substituted benzene. 3: thiosulfonic acid formation (TSAF), in which an N, N -disubstituted-1,4-diamino-5-optionally substituted benzene, 3, is oxidized in the presence of a thiosulfate to give said thiosulfuric acid S-{2-(amino)-3-(optionally substituted)-5-(disubstituted-amino)-phenyl} ester, 4: wherein during or prior to the thiosulfonic acid formation an activating agent is added, oxidative coupling, in which a thiosulfuric acid S-{2-(amino)-3-(optionally substituted)-5-(disubstituted amino)-phenyl} ester, 4, is oxidatively coupled to an N, N -disubstituted-3-optionally substituted-aniline, 5, using an oxidizing agent that is or comprises Cr, to give a [4-{2-(thiosulfate)-4-(disubstituted amino)-6-(optionally substituted)-phenyl-imino}-3-(optionally substituted)-cyclohexa-2,5-dienylidene]- N, N- disubstituted ammonium, 6: ring closure, in which said [4-{2-(thiosulfate)-4-(disubstituted amino)-6-(optionally substituted)-phenyl-imino}-3-(optionally substituted)-cyclohexa-2,5-dienylidene]- N, N -disubstituted ammonium, 6, is subjected to ring closure to give a 3,7-bis(disubstituted-amino)-1,9-(optionally substituted)-phenothiazin-5-ium salt, 7: wherein; each of R 1 and R 9 is independently selected from: -H; C 1-4 alkyl; C 2-4 alkenyl; and halogenated C 1-4 alkyl; each of R 3NA and R 3NB is independently selected from: C 1-4 alkyl; C 2-4 alkenyl; and halogenated C 1-4 alkyl; each of R 7NA and R 7NB is independently selected from: C 1-4 alkyl; C 2-4 alkenyl; and halogenated C 1-4 alkyl; and X - is a halide counter ion