Facile preparation of Gd3+ doped carbon quantum dots: Photoluminescence materials with magnetic resonance response as magnetic resonance/fluorescence bimodal probes
Graphical abstract
Introduction
Modern clinical cancer treatments require precise positional information. Where is the tumor located? How large is it? Is it confined, or has it spread [1]? However, until now, the diagnosis of diseased tissue stay in CT, magnetic resonance (MR) and other imaging studies [2]. Conventional CT and MR can not found the cancer cells until they show abnormal morphological changes. Whereas the molecular imaging technology overcome the limitations of conventional imaging technology, which can detect the cancer cells in the early stage at the molecular level (i.e. before showing abnormal morphological changes) and is expected to enhance our ability to fight with cancer greatly. Furthermore, the MR/fluorescent bimodal molecular imaging technology provides both high-resolution histological structure information and high-sensitivity functional imaging and becomes a hot topic in recent years [3]. We are optimistic that the MR/fluorescent bimodal probe will make a great contribution in improving the care of cancer patients.
Comparing with semiconductor quantum dots which have been employed mostly to constructe MR/fluorescent bimodal probes combining with chelated gadolinium (3+) ions (Gd3+), carbon quantum dots (CQDs) are just coming to play the same role [4]. Semiconductor quantum dots containing cadmium or other heavy metals are difficult to go through the kidneys and their toxicity prevents the further clinical application [5]. CQDs always prepared through simple procedure express excellent photoluminescence properties similar to that of the semiconductor quantum dots, which have attracted more and more interesting in heavy metals ions detection [6], [7], [8], Photocatalys [9], photoelectric conversion [10]. Besides, owing to the biocompatibility, CQDs have big potential to instead of semiconductor quantum dots especially for clinical use [11], [12].
To date, there are only a few bimodal molecular imaging probes composed of Gd3+ and CQDs, furthermore, performances of the existed bimodal molecular imaging probes need to be improved urgently. Bourlinos et al. separately used tris (hydroxymethyl)-aminomethane as a carbon source and gadopentetic acid as a Gd3+ source to synthesize Gd3+-doped CQDs for MR/fluorescence bimodal imaging [13]. But neither the quantitative properties nor the doping form of the resultant Gd3+-doped CQDs were reported. In addition, the carbon source and Gd3+ source are provided by two kinds of materials, it is difficult to ensure the integrity of the Gd3+ and CQDs in Gd3+-doped CQDs. In our early study, gadopentetate monomeglumine (GdPM) was employed firstly as a source to provide carbon source and Gd3+ source simultaneously, and CQDs doped with Gd3+ chelates in molecular level were obtained [14]. However, both the photoluminescence and relaxation properties of the prepared CQDs were ordinary and hardly improved simultaneously. Moreover, Gong N., et al., prepared Gd3+-doped CQDs using sucrose solution, concentrated H2SO4, GdCl3 and diethylene glycol as precursors which must undergo complex pretreatment procedures before pyrolysis [15].
In this paper, Gd3+-codoped CQDs (Gd3+-CQDs) showing good photoluminescence as well as particularly excellent relaxation properties were prepared by hydrothermal method and employing gadopentetate monomeglumine as a precusor. The usual contradiction between the fluorescence intensity and the longitudinal relaxivity was solved by adjusting the hydrothermal conditions.
Section snippets
Hydrothermal synthesis of Gd3+-CQDs
As a precursor, gadopentetate monomeglumine (GdPM, Shandong Hongfuda Pharmchem Co., Ltd) was pyrolyzed in a micro hydrothermal synthesis reactor (SPK-100ML, Binhai Puxin Instrument Co., Ltd) to prepare the Gd3+-CQDs through a process as follows: GdPM (0.24 g) was completely dissolved in deionized water (60 mL) and a homogeneous solution was obtained, followed by measuring its pH value (pH1). Then this solution was transferred into the reactor and pyrolyzed at a certain temperature for a period
Hydrothermal synthesis of Gd3+-CQDs
Based on previous research of our team, either low temperature or short time is not enough to pyrolyze and carbonize the GdPM, there is not favorable for the luminescence ability of the prepared CQDs. However, too high pyrolysis temperature or too long time will promote the decomposition of Gd3+-chelates in GdPM, and the loss of Gd3+ ions will inevitably reduce the relaxation rate of the Gd3+-doped CQDs. GdPM is composed of meglumine and gadopentetic acid units. During the pyrolysis of GdPM,
Conclusions
In this paper, Gd3+-CQDs were prepared employing GdPM as a precursor and by hydrothermal method which did prepare CQDs with high longitudinal relaxation rate and high quantum yield (7.1%) at low temperature facilely. The surface of Gd3+-CQDs was shown to contain the Gd3+ chelates, Gd2O3, and hydrophilic functional groups (-OH, -COO-) stabilizing the particles in water solution. The size of Gd3+-CQDs, as determined by TEM, was approximately 1.6 nm. The CCK-8 assay disclosed that the Gd3+-CQDs
Acknowledgments
This work is supported by the National Natural Science Foundation of China (no. 81171318) and Shaanxi Health Department Foundation, China (no. 2010E07).
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