The therapeutic efficacy of CdTe and CdSe quantum dots for photothermal cancer therapy

Abstract

Fluorescent quantum dots (QDs) used for biomedical imaging and diagnostics have attracted considerable attention over the past decade. Here, we report our finding regarding the therapeutic efficacy of the popularly used red/brown, brown or close to black CdTe and CdSe QDs. Upon 671-nm laser irradiation, these QDs can rapidly convert light energy into heat, both in vitro and in vivo. In the present study, the growth of mouse melanoma tumors injected with CdTe(710) QDs coated with a silica shell (SiO₂) was significantly inhibited after laser irradiation, with eventual disappearance of the tumor. In contrast, tumors injected with the silica-coated QDs without subsequent irradiation continued to grow over time. They had a growth rate close to that of tumors injected with SiO₂ or phosphate-buffered saline, with or without laser irradiation. In conclusion, our data suggest that the popularly used CdTe and CdSe QDs have great potential in the treatment of cancer using photothermal therapy.

Introduction

Recently, quantum dots (QDs) have drawn a great deal of attention owing to their interesting photophysical properties that include high fluorescence quantum yield (QY), superior stability against photobleaching, a narrow and symmetric emission spectrum and a broad and continuous excitation spectrum [1], [2], [3]. These properties make QDs superior fluorescent probes to organic fluorescent dyes and proteins for in vitro and in vivo biomedical imaging [4], [5]. In addition, QDs have potential in cancer photodynamic therapy as photosensitizers, since ultraviolet (UV) light irradiation can induce them to generate reactive oxygen species (ROS) such as ·OH and $\cdot {\text{O}}_2^{\mathrm{-}}$, cytotoxic single oxygen (¹O₂) and toxic heavy metal ions [6], [7], [8]. However, for well-protected QDs the shells (such as ZnS) on the QD surface create higher energy barriers between oxidizing molecules and the QD cores, preventing the effective production of ROS [9], [10]. It has been demonstrated that the level of ROS produced by the QDs is lower than that produced by classic photosensitizers [6], [11]. Therefore, although the use of QDs as cancer therapy agents has attracted attention, most of the work concerning QD biomedical applications as of yet has been focused on fluorescence imaging.

In recent years, CdTe and CdSe nanocrystals have been the most prominent QDs used for life science applications [5], because they exhibit higher fluorescent QYs and can be synthesized using a simpler strategy (e.g. CdTe QDs with >70% QY can be conveniently prepared in aqueous solution [12]) than III/V group or ternary QDs such as InP and InGaP. In the present study, we report on a unique property of the popularly used CdTe and CdSe QD aqueous solutions with red/brown, brown or close to black colors in bright fields (especially the latter two deep colors), namely that they can rapidly convert light energy into heat after laser irradiation. This property is potentially useful for cancer photothermal therapy, and was evaluated in the present study using a mouse melanoma tumor model.