The Evaluation of Ester Functionalised TCF‐based Fluorescent Probes for the Detection of Bacterial Species

The ester functionality is commonly seen in the areas of chemical biology and medicinal chemistry for the design of cell-permeable active molecules. Ester-based pro-drug/pro-sensor strategies are employed to mask polar functional groups (i. e. carboxylic acids) and improve the overall cell permeability of these functional molecules. However, their use as reactive units for sensing applications, including bacterial detection, has not been fully explored. Herein, we synthesised two TCF-based fluorescent probes, TCF-OAc and TCF-OBu. As expected, both TCF-OAc and TCF-OBu demonstrated a significant fluorescence (22- and 43-fold, respectively) and colorimetric response (yellow to purple) towards porcine liver esterase (PLE) with a limit of detection of 1.18 mU/mL and 0.45 mU/mL, respectively. With these results in hand, the ability of these probes to detect planktonic suspensions of gram-positive Staphylococcus aureus (S. aureus) and gram-negative Pseudomonas aeruginosa (P. aeruginosa), and Escherichia coli (E. coli) were evaluated. Different fluorescence responses for gram-positive and gram-negative bacteria were observed between TCF-OAc and TCF-OBu. After 1 h incubation, TCF-OAc proved more sensitive towards S. aureus, demonstrating a significant fluorescence “turn on” response (16-fold); whereas, TCF-OBu was more selective towards P. aeruginosa, with a 22-fold increase in the fluorescence response observed. These results demonstrate the influence of the ester chain length on the selectivity for bacterial species.