Synthesis and antibacterial activities of bromelain stabilized fluorescent copper nanoclusters against E. coli and S. aureus

Abstract

Bromelain, a bioactive proteolytic enzyme is composed of a mixture of cysteine protease. It is obtained from fruits and stems of pineapple. Cu in bulk as well as in nano form and bromelain individually demonstrate antibacterial activities. With the aim to make bromelain and Cu act synergistically against bacteria, in this work, bromelain-capped copper nanoclusters (Bro-CuNCs) were synthesized for the first time by adhering to the principles of green chemistry. Synthesized nanoclusters exhibit a quantum yield (QY) of 4.47% in an aqueous medium. These nanoclusters were characterized by several spectroscopy and microscopy techniques such as UV–VIS spectroscopy, Fluorescence spectroscopy, FT-IR spectroscopy, and HR-TEM. The findings reveal that Bro-CuNCs interact more effectively with the Gram-negative strain of E. coli than the Gram-positive strain of S. aureus. Additionally, the Bro-CuNCs exhibit an enhancement of fluorescence intensity when exposed to E. coli, while demonstrating a diminution in the presence of S. aureus. Surprisingly, they also demonstrate efficacy against Gram-negative bacteria, P. aeruginosa. This strain is inherently resistant to conventional antibiotics such as penicillin.