**4. Conclusions**

A new water-soluble polythiophene-based fluorescent probe **L** with boronic acid groups on its side chains was designed and synthesized for the detection of biologically important ATP. **L** exhibited effective and selective recognition ability for ATP over other structurally similar nucleotides such

as ADP and AMP in aqueous solution at physiological pH 7.4. A remarkable fluorescent quenching took place upon addition of ATP into the buffer solution of **L** due to the formation of chiral aggregates induced by ATP via multi-sites binding between **L** and ATP. The change in fluorescent intensities for ATP is much larger than that for ADP and AMP, which is large enough to discriminate ATP from ADP and AMP. Based on the highly selective recognition of **L** for ATP, **L** was successfully utilized as a fluorescent probe for continuous and facile assays of ADK activity. In addition, **L** has been demonstrated to be a useful probe for monitoring the ATP levels in living cells by fluorescence imaging. With the advantages of good water solubility, high selectivity to ATP, and low cytotoxicity, further work to inspect the practical applicability in the real-time detection of enzyme activity in living cells is being carried out in our group.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/11/7/1139/s1. Scheme S1. Synthesis of the control polymer **PTB**; Figure S1. FTIR spectra of **L**; Figure S2. Effect of pH on absorption spectra of **L** and **L**/ATP complex; Figure S3. Job's plot; Figure S4. Determination of the binding constant between **PTB** and ATP; Figure S5. Determination of detection limit of **L**; Figure S6. Study on interaction of 3-pyridineboronic acid with ATP by 1H NMR. Figure S7. Cytotoxicity of **L**; Figure S8. Photostability of **L**; Table S1. Comparison of cation or boronic acid-contained polythiophene-based probes for the detection of ATP.

**Author Contributions:** Y.L. designed the study; D.C. synthetized the probe **L**; L.L., L.Z. and X.Y. performed the measurements; all authors discussed the results and Y.L. wrote the manuscript. All authors read and approved the final version of the manuscript.

**Funding:** We are grateful for the National Natural Science Foundation of China (51373122, 51703163), the Natural Science Foundation of Tianjin (Grant No.: 18JCZDJC34600, 18JCYBJC86700) and the Program for Prominent Young College Teachers of Tianjin Educational Committee.

**Conflicts of Interest:** The authors declare no conflict of interest.
