Jin-wu Yana,
Jia-ying Zhua,
Qi-feng Tana,
Lin-fu Zhoua,
Pei-fen Yaob,
Yu-ting Lub,
Jia-heng Tanb and
Lei Zhang*a
aSchool of Bioscience and Bioengineering, South China University of Technology, Guangzhou, P. R. China. E-mail: lzhangce@scut.edu.cn; Tel: +86 20 39380678
bSchool of Pharmaceutical Science, Sun Yat-sen University, Guangzhou 510006, China
First published on 4th July 2016
Carbon monoxide (CO) has captivated great attention in part due to the discovery of the therapeutic and cell-signalling role that CO plays in biological systems. The ever-increasing interest in CO has resulted in the development of rapid and facile approaches for the sensitive and selective detection of CO, which still remains challenging. Herein, the first example of a colorimetric and near-infrared fluorescent dual probe for carbon monoxide (CO) has been developed by assembling an allyl chloroformate moiety with a naphthofluorescein fluorophore, which enables the label-free and visual detection of CO. The living cell imaging results indicate that the probe shows great potential in sensing intracellular CO.
Recently, fluorescence-based imaging techniques have emerged as powerful tools to monitor and sense various biomolecules in biological systems with high spatial and temporal resolution.9–12 A variety of fluorescent probes for endogenous gasotransmitter, such as NO and H2S, have been reported over the past decade.13,14 However the development of fluorescent probes for CO is still at a very early stage. A very limited number of fluorescent probes for CO have been developed to date,15–23 including our newly reported probe.24 However, almost all of these probes display emissions in the visible region, which greatly restricts their applications in biological imaging, due to the strong autofluorescence and limited tissue penetration. By contrast, near-infrared (NIR) fluorescent probes are extremely favourable for bioimaging because of deep tissue penetration, minimum autofluorescence and photo-damage.25 To this end, development of NIR fluorescent probes for selective and sensitive detection of intracellular CO is highly desired and urgent.
On this basis, we prepared a first example of colorimetric and NIR fluorescent dual probe (NF-APC). This dual probe consists of a naphthofluorescein fluorophore condensed with allyl chloroformate moieties and possesses the desirable properties described above (Scheme 1). Our overall strategy for this probe relies on CO-induced in situ generation of Pd0, which consequently mediate the Tsuji–Trost reaction.26 Naphthofluorescein was chosen as a signaling fluorophore due to its long excitation and emission wavelengths (>600 nm), relatively large Stokes shift and easy availability.27–29 Based on the literature and our previous work, allyl chloroformate was linked to the hydroxyl groups of naphthofluorescein as the response site to trap Pd0 according to the Tsuji–Trost reaction. We anticipated that the derivatization of the hydroxy groups of naphthofluorescein in position 4 and 9 with allyl chloroformates would lead to a colorless and non-fluorescent closed lactone form, and the free naphthofluorescein moiety would be released in the presence of CO and Pd2+, leading to the color change and the generation of strong NIR fluorescence emission.
To investigate the response of NF-APC to CO, UV-vis and fluorescence spectroscopic studies were conducted to monitor the time-dependent changes of the absorption and fluorescence spectra upon addition of CO into the Pd2+-containing buffer. In this study, the water-soluble complex [Ru(CO)3Cl(glycinate)] (CORM-3) was used as the CO source. As shown in Fig. 1, NF-APC exhibits extremely weak absorption and emission in the visible region. After the addition of 10 equiv. of CORM-3, a new absorption peak appeared at around 620 nm, which increased gradually with incubation time and reached its maximum value after about 45 min of incubation (Fig. 1A and S7A†). The significant absorbance enhancement (15-fold) was accompanied by a marked and vivid color change from achromatic to blue in ambient light (insert of Fig. 1A), demonstrating that NF-APC can serve as an on-site and visual indicator for CO. In the fluorescence spectra, a new strong emission peak appeared at approximately 670 nm and its intensity increased gradually with incubation time, reaching a plateau in 45 min (Fig. 1B and S7B†). The turn-on fluorescence at 670 nm (35-fold increase) may be attributed to the elimination of the allyl chloroformate in NF-APC, and to the concomitant formation of the free naphthofluorescein moiety, which was confirmed by almost the same absorbance and emissive peak with naphthofluorescein from commercial source (Fig. S8†), and the HRMS result (Fig. S9†). Besides, the turn-on red fluorescence could also be seen with the naked eye under UV light (insertion of Fig. 1B). The effect of pH on the probe response and stability were also studied and the results suggested that NF-APC was stable in the pH range of 5–8.5, and displayed remarkable response from 7 to 8.5 (Fig. S10†). These results indicated that NF-APC could be used as an efficient naked-eye tool for colorimetric and NIR fluorescent turn-on probe for CO.
Further UV-vis and fluorescence titration experiments were carried out to explore the sensing ability of NF-APC to CO. As shown in Fig. 2 and S11,† the new absorption band at 620 nm and the emission at 670 nm increased gradually with the increasing of CORM-3 concentrations. The plot of absorbance and fluorescence intensity versus CORM-3 concentration showed linear correlation in the range of 0–40 μM (Fig. S12†), and the detection limit for CO was deduced to be as low as 0.255 μM and 0.127 μM (3σ/k), respectively, indicating that our probe was highly sensitive to CO. Additionally, NF-APC displayed excellent photostability, which is very important for its application (Fig. S13†).
We next assessed the selectivity of NF-APC towards CO and various biologically relevant chemical species. As shown in Fig. 3, only the addition of CORM-3 caused such strong signal changes in the absorption and fluorescence spectra of NF-APC. In contrast, nearly negligible signal interference was observed in the presence of other reactive biological species, such as ClO−, H2O2, H2S, O2−, NO, HSO3−, SO32−, Cys, Hcy and GSH (Fig. 3), indicating that NF-APC exhibited excellent specificity for CO over other biologically relevant species. Taking all these results together, NF-APC displayed great potential for the sensitive and selective detection of CO with both colorimetric and fluorescent outputs.
Encouraged by the remarkable properties of this probe for the detection of CO in solution, we further investigated the intracellular applications of NF-APC to image CO in living cells by using confocal laser scanning microscopy. In this regard, a standard MTT assay was performed to evaluate the cytotoxicity of NF-APC, CORM-3 and Na2PdCl4 to HeLa and 293T cells. The results indicated that NF-APC, CORM-3 and Na2PdCl4 exhibited very low cytotoxicity to living cells for 48 h (Fig. S14†). As shown in Fig. 4A–H, co-staining living HeLa cells with Hochest 33342 nuclear stain and NF-APC revealed that NF-APC alone or in the presence of Pd2+ exhibited nearly no fluorescence emission, consistently with the results obtained in solution. In sharp contrast, after incubation with CORM-3 for 45 min, a distinct strong fluorescence emission was detected (Fig. 4I–L). These observations demonstrated that NF-APC is both cell-permeable and capable of sensing CO in living cells.
Footnote |
† Electronic supplementary information (ESI) available: Synthesis and characterization of NF-APC, experimental procedures, and supplemental spectra and graphs. See DOI: 10.1039/c6ra14409h |
This journal is © The Royal Society of Chemistry 2016 |