optical probe for fluorescenceturn-ondetection of DNA and label-freefluorescencenuclease assay. The fluorescence of 1 increases largely upon mixing with DNA, in particular for long DNA, indicating that 1 can be used for fluorescenceturn-ondetection of DNA. More interestingly, 1 can be employed to follow the DNA cleavage process by nuclease. Therefore, a label-freefluorescencenuclease assay method