Ultrasensitive, label-free detection of T4 ligase and T4 polynucleotide kinase based on target-triggered hyper-branched rolling circle amplification

•A simple but sensitive sensor is established for ligase and polynucleotide kinase (PNK) assay.•Exponential HRCA confers the sensor with high sensitivity and selectivity.•Label-free detection mode greatly simplifies the sensor design and reduces the cost.•The sensor was successfully used for ultrasensitive assay of T4 ligase and T4 PNK.•Such a sensing strategy can be easily extended to studies on enzyme inhibitors.