Article pubs.acs.org/ac
Turn-On Fluorescence Sensor for Intracellular Imaging of Glutathione Using g‑C3N4 Nanosheet−MnO2 Sandwich Nanocomposite Xiao-Long Zhang, Cheng Zheng, Shan-Shan Guo, Juan Li,* Huang-Hao Yang,* and Guonan Chen The Key Lab of Analysis and Detection Technology for Food Safety of the MOE, Fujian Provincial Key Laboratory of Analysis and Detection Technology for Food Safety, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, People’s Republic of China S Supporting Information *
ABSTRACT: Herein, a novel fluorescence sensor based on gC3N4 nanosheet−MnO2 sandwich nanocomposite has been developed for rapid and selective sensing of glutathione (GSH) in aqueous solutions, as well as living cells. The graphitic-phase C3N4 (g-C3N4) nanosheet used here is a new type of carbon-based nanomaterial with high fluorescence quantum yield and high specific surface area. We demonstrate a facile one-step approach for the synthesis of a g-C3N4 nanosheet−MnO2 sandwich nanocomposite for the first time. The fluorescence of g-C3N4 nanosheet in this nanocomposite is quenched, which attributing to fluorescence resonance energy transfer (FRET) from a g-C3N4 nanosheet to the deposited MnO2. Upon the addition of GSH, MnO2 is reduced to Mn2+, which leads to the elimination of FRET. As a result, the fluorescence of g-C3N4 nanosheet is restored. Importantly, the chemical response of the g-C3N4−MnO2 nanocomposite exhibits great selectivity toward GSH relative to other electrolytes and biomolecules. Under the optimal conditions, the detection limit of 0.2 μM for GSH in aqueous solutions can be reached. Furthermore, the g-C3N4−MnO2 nanocomposite is confirmed to be membrane-permeable and have low cytotoxicity. Moreover, we successfully apply this sensor for visualizing and monitoring change of the intracellular GSH in living cells. Moreover, the proposed sensor shows satisfying performance, such as low cost, easy preparation, rapid detection, good biocompatibility, and turn-on fluorescence response. organic fluorophores are rich in various colors and reactive forms, their potential for bioimaging and tracking of fluorescently labeled substances are limited by their susceptibility to photobleaching, poor photostability, and sensitivity to the local environment.11c,13a,15 Chalcogenide QDs have been accepted as promising substitutes for conventional organic fluorophores with the advantages of bright, stable, and sizedependent tunable photoluminescence, but one of the main shortcomings is that most of these chalcogenide QDs compose of toxic heavy-metal elements, for instance, cadmium.16−18 So researchers have focused efforts on the development of fluorescent nanomaterials with lower or without toxic elements. Recently, Tian et al. proposed a fluorescent sensor based on nontoxic gold nanoclusters for highly sensitive sensing of GSH in living cells.12 However, the toxic heavy metal Hg2+ ion was used for quenching the fluorescence of gold nanoclusters. UCPs represent another class of low-toxic luminescent nanomaterials. There has been increasing interest in exploiting UCPs as fluorescent sensors for signal-on luminescent detection of biothiols with high signal-to-noise ratios.13 However, the generality of UCPs-based fluorescent bioimaging remains a problem. There are no commercial, ready-to-use
G
lutathione (GSH) is the predominant nonprotein thiol in mammalian and eukaryotic cells, which is synthesized endogenously from the precursor amino acids L-cysteine, Lglutamic acid, and glycine. It serves several vital functions in biological systems, for example, as an important antioxidant in defense against toxins and free radicals, maintenance of thiol status, and modulation of cell proliferation.1 The level of GSH has been reported to associate with various human diseases, such as cancer, liver damage, AIDS, aging, and diabetes.2 Therefore, the detection and quantification of GSH are of sustained interest, because of its biological and clinical significance. Nowadays, a variety of methods have been proposed for sensing of GSH or monitoring changes of the intracellular GSH, including high-performance liquid chromatography (HPLC),3 electrochemistry,4 electrogenerated chemiluminescence,5 surface-enhanced Raman scattering (SERS),6 mass spectrometry,7 and fluorescence spectroscopy.8 Compared with other techniques, fluorescence spectroscopy holds significant advantages for its high sensitivity, simplicity, and nondestructive properties.9 Furthermore, with the progress of fluorescence microscope techniques, mapping the spatial and temporal distribution of the intracellular GSH can be obtained. Currently, great efforts have been made to design fluorescent probes for the detection of GSH, e.g., organic fluorophores,8,10 quantum dots (QDs),11 gold nanoclusters,12 upconversion nanoparticles (UCPs),13 and carbon nanomaterials.14 Although © 2014 American Chemical Society
Received: December 1, 2013 Accepted: March 11, 2014 Published: March 11, 2014 3426
dx.doi.org/10.1021/ac500336f | Anal. Chem. 2014, 86, 3426−3434
Analytical Chemistry
Article
Figure 1. Schematic representation of g-C3N4−MnO2 nanocomposite for sensing of GSH.
imaging instruments in the market for UCPs, since most instruments were designed according to traditional downconversion probes.19c Furthermore, a relatively high power density (102−103 mW cm−2) NIR laser is required for upconversion fluorescence imaging, which may lead to the presence of overheating effect.19 The fluorescent carbon-based nanomaterials, emerging as a new class of photostable luminescent nanomaterials, have received much attention in bioimaging and drug delivery with improved properties in terms of relatively high biocompatibility, chemical stability, low cost, and low toxicity.20,21 Graphene oxide, one of exciting carbon nanomaterials, has become extremely popular in biosensing and biomedical fields because of its extraordinary physical and chemical properties.22 However, its relatively low fluorescence quantum yields20b,22f may hinder the straightforward application in biological imaging. On the other hand, carbon dots, and graphene QDs exhibit stronger photoluminescent properties because of the quantum confinement and edge effects.14,20,21,23 Nevertheless, because of their small size (
