Yes, SENSITIZER is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, N 1, S 1, I 1, T 1, I 1, Z 10, E 1, R 1.

Is it a valid word?

SENSITIZER is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(chemistry) a substance other than a catalyst that facilitates the start of a catalytic reaction

Not playable in Scrabble US: SENSITISER.