Facile fabrication of cupric ion-carbon quantum dots as tumor-specific nanotheranostics for cuprous ion-mediated chemodynamic therapy and real-time fluorescence imaging

Highlights

• Carbon quantum dots-based nanotheranostics were designed via a ‘one stone and two birds’ strategy.

• Its fluorescence was quenched by complexing with prolonged blood circulation.

• Cu(I) ions were generated and the fluorescence of carbon quantum dots was recovered in tumor cells.

Abstract

Carbon quantum dots (CDs) have attracted intense interest in biomedical imaging. However, their very small diameter and non-characteristic fluorescence limit the practical application. Herein, Cu(II)-complexed CDs (Cu(II)-CDs NPs) have been designed via facile complexing CDs with Cu(II) ions, for the tumor intracellular glutathione-activated cuprous ion-mediated chemodynamic therapy and real-time fluorescence imaging. They were quite stable in both neutral and weakly acidic solutions even with lower glutathione (GSH)-level and dilution, emitting very weak fluorescence, while as in the simulated intracellular microenvironment, slight or strong fluorescence was emitted in the normal or tumor cells, respectively. Especially, they could release CDs and Cu(I) ions due to GSH-triggered disintegration. Such features make them promising nanotheranostics in tumor treatment, and the tumor-specific cuprous ion-mediated chemodynamic therapy and real-time fluorescence imaging were revealed in the in vitro cellular tests.

Introduction

Carbon quantum dots (CDs) have attracted more and more interest in biomedical imaging, owing to their excellent stability, cytocompatibility, and fluorescence [1]. However, there are still two challenges restricting their practical application. First, their very small diameter makes them rapidly clear from the circulation through extravasation or renal clearance [2]. Therefore, they have been usually used in the form of nanocomposites [3]. Their fluorescence would be heavily diminished due to the static quenching when they were incorporated via covalent bonds from their surface amino or carboxyl groups. On the other hand, the CDs-based self-assemblies via non-covalent interactions were not so stable, especially upon dilution.

Second, the fluorescence of CDs is non-characteristic. That is to say, CDs could emit fluorescence in all biomedical cases. At the best, there is a bit of intensity difference in various biological conditions, such as blood circulation, intercellular or intracellular microenvironments of normal cells and tumor cells. The concentration of CDs also affects their fluorescence intensity; thus, the weak intensity difference could hardly be used as diagnostic evidence. Even with the pH-responsive CDs [4], it is impossible to differentiate the intracellular microenvironment of normal cells and tumor cells because of the similar acidity. The tumor-specific fluorescence is still desired for the tumor diagnosis.

Besides the biomedical imaging application, CDs have been widely used as sensors or probes for heavy metal ions because the fluorescence of CDs could be quenched with various quenching mechanism [5]. The complexation of CDs with heavy metal ions might provide a potential for the ‘turn-on’ fluorescence mode in tumor intracellular microenvironment, by relieving the complexation between the CDs and the variant valence metal ions, triggered by the high GSH-level therein.

Cu is one of the essential trace elements of the human body. As a divalent ion, Cu(II) ion could crosslink the CDs into nanoparticles. Meantime, the complexation could quench the fluorescence of CDs via static quenching mechanisms [6]. The standard reduction potential of Cu(II/I) is 0.153 V. The reduction of Cu(II) to Cu(I) could be induced by GSH, which is over-expressed in the tumor cells. The proposed Cu(II)-complexed carbon quantum dots (Cu(II)-CDs NPs) could GSH-triggered disintegrate in the tumor intracellular microenvironment to release CDs and Cu(I) ions. Because of the much lower quenching effect of Cu(I) than Cu(II) [7], the GSH-triggered disintegration of the Cu(II)-CDs NPs is expected to recover the fluorescence of CDs. Moreover, the released Cu(I) ion could induce OH generation via a Fenton-like reaction, which had been widely used in cancer chemodynamic therapy. Recently, cuprous ion-mediated chemodynamic therapy has been widely reported with the cupric ion-based nanomedicines [8], in the form of Cu(I) [9] or Cu(II) [10] complex, Cu²⁺-based metal–organic frameworks [11], Cu²⁺-crosslinked gel [12], Cu-based layered double hydroxide (CuFe-LDH) nanosheets [13], copper peroxide nanodots [14], Cu₂S quantum dots [15], copper peroxide nanoparticles [16], and so on. Comparatively speaking, the proposed Cu(II)-CDs NPs could be easily fabricated, killing two birds with one stone.

Based on the hypothesis, the Cu(II)-CDs NPs have been designed in the present work (Scheme 1A). The tumor-specific disintegration, fluorescence recovery, and Cu(I)-induced ROS generation were revealed. As expected, the tumor-selective chemodynamic therapy and fluorescence imaging were achieved in the in vitro cellular experiments (Scheme 1B).