Mapping the l-tryptophan capped copper nanocluster mediated binding and targeted pH-responsive release of doxorubicin via fluorescence resonance energy transfer (FRET)†

Abstract

Targeted delivery and controlled release of anticancer drugs pose significant challenges in effective cancer therapy. In this study, we developed a tryptophan-capped copper nanocluster (Trp-Cu NC) as a nano-drug carrier for the selective and pH-responsive release of the anticancer drug doxorubicin (Dox). The Trp-Cu NC exhibits substantial spectral overlap with Dox, forming an efficient fluorescence resonance energy transfer (FRET) pair that enables precise monitoring of drug binding interactions through both steady-state and time-resolved fluorescence measurements. Upon increasing the Dox concentration (∼160 μM), the photoluminescence (PL) intensity and the lifetime of the Trp-Cu NC (donor) decreased significantly, indicating enhanced FRET efficiency (E_FRET) and reduced donor–acceptor distance (r_DA). The interaction between Trp-Cu NC and Dox under neutral pH resulted in the formation of a Trp-Cu NC–Dox nanoconjugate of diameter ∼24.7 ± 1.1 nm meeting the size criterion suitable for good drug delivery performance. Under acidic conditions (pH 5.5), mimicking the tumor microenvironment, the Trp-Cu NC–Dox nanoconjugate dissociated back to the nanocluster (diameter ∼2.7 ± 0.1 nm) releasing the drug, which translated into a remarkable increase in the Trp-Cu NC (donor) lifetime followed by a decreased E_FRET. Such a phenomenon was absent under physiological pH 7.4, making the Trp-Cu NC a suitable nano-carrier for targeted drug release in cancer cells. The cytotoxicity studies further corroborate that Trp-Cu NC can selectively release Dox to the cancer cells enhancing the therapeutic efficacy of the drug by ∼3.6-fold, concurrently decreasing its toxicity appreciably towards normal cells. Overall, these findings substantiate an easy and economical strategy to develop a novel nano-drug carrier that offers selectivity and improved drug-release performance, potentially overcoming the systemic toxicity associated with conventional chemotherapy.