We present here the development of an all-solid-state optical sensor based on phenyl-substituted diaza-18-crown-6 hydroxyquinoline (DCHQ-Ph) for the indirect selective detection of microcystin-LR (MC-LR), reaching a very low detection limit of 0.05 μg L-1, well below the World Health Organisation (WHO) guideline value (1 μg L-1) in potable water. We demonstrate the potential applicability of the developed method in fast and low-cost water toxicity estimation.