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Highly sensitive naked-eye and ?uorescence ‘‘turn-on’’ detection of Cu2+ using Fenton reaction assisted signal ampli?cationw
Yi-Bin Ruan,a Chun Li,ab Jie Tangb and Juan Xie*a
Received 13th September 2010, Accepted 5th October 2010 DOI: 10.1039/c0cc03825c Increase of pH induced by Cu2+-catalyzed Fenton reaction promoted ring-opening of triazole-linked ?uorescein lactone, which enabled selective ‘‘turn-on’’ ?uorescent detection of Cu2+, along with ultralow naked-eye detection limit down to 200 nM. Copper as one of the most important trace essential elements is involved in electron transfer processes of a number of biological reactions.1 Its intracellular level is directly associated with functions of proteins and various neurodegenerative diseases such as Alzheimer’s, Menke’s and Wilson’s diseases.2 Owing to its biological importance, using optical techniques to monitor the tra?cking and location of Cu2+ in living cells has attracted much attention and resulted in fruitful work in recent years. Among the reported methods, ?uorescent chemosensors show distinct advantages in sensitivity and biological imaging. However, due to the intrinsic paramagnetic property of Cu2+, many probes show a ‘‘turn-o?’’ response via an electron/ energy transfer process.3 ‘‘Turn-on’’ ?uorescent probes reported for Cu2+ are mainly based on chelation-enhanced ?uorescence or chemodosimeters.4,5 Transformation of rhodamine-B derivatives from the non-?uorescent spirolactam to the ?uorescent ring-opened amide form usually results in a color change and ?uorescence enhancement, making it a viable sensory platform.5a–f The colorimetric method has also been extensively utilized due to its extreme simplicity as it can be read out by the naked eye without the aid of sophisticated instruments, thus facilitating the realization of point-of-use application.6 Jiang et al. reported visual detection of Cu2+ by azide- and terminal alkyne-functionalized gold nanoparticles (Au NPs) using click chemistry.6a However, up to now most probes available for naked-eye detection of Cu2+ still su?er from limited sensitivity and/or interference from other metal ions. To overcome the limitations, new elements such as catalysis and conjugated polymers could be incorporated to amplify the sensing processes. The Fenton reaction, discovered in 1894, demonstrates that some metals (Fe2+, Cu2+) can be powerful catalysts to generate highly reactive hydroxyl radicals ( OH),7 which have been recognized to be sources of the aging process and a variety of diseases, while the reaction has been successfully applied in food chemistry and environmental engineering.7 In spite of its great importance, the Fenton reaction has been
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rarely used in sensory processes.6b,8 Yang et al. found that DNAzyme could be cleaved into ssDNA in the presence of ascorbic acid and Cu2+, which enhanced stabilization of Au NPs against salt-induced aggregation. They successfully utilized this protocol for colorimetric detection of Cu2+.6b However, their system su?ers from low sensitivity and indistinct color change. So far most reports on Fenton reactions have paid much more attention to free radical species; however, here we found that the pH change induced by Fenton reaction could be another novel approach for highly sensitive naked-eye and ‘‘turn-on’’ ?uorescent detection of Cu2+ when ?uorescein derivative 1 was used as an indicator. As depicted in Scheme 1, under aerobic conditions, ascorbate (AscH?) not only is involved in the reduction of Cu2+ (a), but also reacts with O2 to produce H2O2 (b). Hydroxide and  OH are then yielded in the next Fenton reaction (c). Increase of pH by hydroxide promotes the ring-opening of 1 (d), which results in ?uorescence enhancement and remarkable color change. It is noteworthy that recycling of Cu2+ (c) may dramatically amplify signal transduction. Compounds 1 and 2 could be easily synthesized using click chemistry.9 The nature of ?uorescein derivatives in solution depends on pH, hydrogen-bonding interaction and polarity of their environment.10 For 1 it may exist in four di?erent forms including lactone, quinoid, cation and monoanion. Its photophysical properties are then closely associated with the changes of environment. We ?rst investigated the spectral properties of 1 under di?erent pH conditions. As shown in Fig. S1 (ESIw), it shows weak absorption or emission bands in the visible region when the pH is below 6.0. With increase of the pH, it exhibits an absorption band centered at 454 nm and emission band at 517 nm, respectively. Meanwhile a color change from colorless to yellow was observed and a plateau

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PPSM, Institut d’Alembert, ENS Cachan, CNRS, 61 av. President Wilson, F-94230 Cachan, France. E-mail: joanne.xie@ppsm.ens-cachan.fr b Institute of Medicinal Chemistry, Department of Chemistry, East China Normal University, Shanghai 200062, China w Electronic supplementary information (ESI) available: Experimental and spectroscopic data. See DOI: 10.1039/c0cc03825c

Scheme 1 Proposed mechanism for detection of Cu2+ based on the Fenton reaction.

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Fig. 2 Change in color (top) and ?uorescence (bottom) of 1 in the presence of various metal ions in H2O: [1] = 10.0 mM, [Cu+] = [Cu2+] = 0.5 mM and other metal ions 40 mM, [AscH?] = 1.0 mM, illuminated under 365 nm, reaction time 7 min. Fig. 1 Kinetics pro?les of 1 in the presence of di?erent concentrations of Cu2+ in H2O: [1] = 10.0 mM, [AscH?] = 1.0 mM; lex = 454 nm.

was reached when the pH is above 8.0, and the observed pKa of 1 in the ground and excited states are both ca. 7.0. As the pH of Mill-Q water used in our work is ca. 5.5, the form of 1 in pure water should be the lactone which could be a potential ‘‘turn-on’’ ?uorescent chemosensor. For the sake of high sensitivity, several signal ampli?cation approaches such as ?uorescent conjugated polymers, Au NPs, DNAzyme and metal-ion catalyzed reactions have been incorporated into metal-ion sensing processes.5i,p,11 Hydrolysis of non-?uorescent probes catalyzed by Cu2+ leads to ampli?ed ?uorescence signal transduction.5i,p We investigated the spectral response of 1 to Cu2+ in the absence of AscH? in H2O. No ?uorescence change was observed, which demonstrated that no binding event occurred between Cu2+ and 1. However, the introduction of 1 mM AscH? could slightly induce ring-opening of 1 because of slight increase of pH (Fig. S2, ESIw). Further addition of 0.5 mM Cu2+ led to a color change from colorless to light yellow and dramatically enhanced ?uorescence (Fig. S2, ESIw). Fig. 1 shows the kinetics pro?les of 1 in the presence of varying concentrations of Cu2+. When [Cu2+] is 500 nM, it reaches equilibrium in 5 min and remains constant over 10 min. With decreasing concentration of Cu2+, the reaction time is lengthened. When [Cu2+] is reduced to 200 nM, equilibrium is obtained in 25 min with ca. 6-fold ?uorescence enhancement (ca. 52% hydrolysis of 1, estimated from the ?uorescence intensity) (Fig. S3, ESIw). Consequently, under this condition 25 turnovers can be obtained, which demonstrates the e?cient catalysis assisted signal ampli?cation for the detection of Cu2+. However, when [Cu2+] is 100 nM, only a small increase of ?uorescence could be observed in 20 min and prolonging the reaction time did not lead to remarkably enhanced ?uorescence. Selectivity of this protocol was evaluated by screening various metal ions such as Co2+, Fe2+, Pb2+, Hg2+, Cd2+, Ni2+, Ag+, Mn2+, Zn2+, Ba2+, Ca2+ and Mg2+ at 40 mM level. As displayed in Fig. S4 (ESIw), only Cu2+ and Cu+ induce ?uorescence enhancement. Pb2+, Hg2+ and Ag+ show a slight quenching e?ect while others almost produce no changes. Fig. 2 shows that except for Cu2+ and Cu+ other metal ions cause no observable color changes, demonstrating its high selectivity for visual detection of copper ions. Its selectivity was further con?rmed by competition experiments, as shown in Fig. S5 (ESIw). Even in the presence of 8-fold
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concentration excess compared to [Cu2+], most ions produce almost no e?ect on detection of Cu2+, except for Pb2+ and Hg2+. Moreover, the interference may be attributed to acid species contained in the sources of metal ions. Fast and sensitive visual detection may facilitate on-site application. Fig. 3 shows photographs of 1 in the presence of di?erent [Cu2+]. It is found that no color change could be observed when [Cu2+] is 100 nM. However, as [Cu2+] increases, the color changes from colorless to light yellow. At 200 nM Cu2+ could already be clearly detected by the naked eye, i.e. a naked-eye detection limit of our protocol for Cu2+ is as low as 200 nM in 10 min response time. This result shows great advantages in the response time, sensitivity, simplicity and cost when compared with other reported methods.6 Introduction of other stronger binding sites with analyte is usually necessary for the reversibility of a chemosensor. However, for our sensing system this seems to not be required. As shown in Fig. S6 (ESIw), the ?uorescence intensity decreases to the initial level when the reaction time is prolonged to 80 min. We suggest that the self-recovery is due to neutralisation of the hydroxide by H+ yielded from deprotonation of the ascorbate radical (AscH ), as shown in Scheme 1(e). Taki et al. developed highly sensitive ?uorescein derivatives (FluTPA1) for the detection of intracellular Cu+ based on the cleavage of the C–O bond of benzyl ether.5n They suggested a copper-activated oxygen species as the reactive intermediate; however, the mechanism was still not clear. In our assay, we found that in the absence of AscH? addition of Cu+ led to no change of the spectral properties of 1 (Fig. S7, ESIw). Therefore AscH? and Cu+ both contribute to the change of spectra together, by initiating the Fenton reaction to yield  OH and hydroxide under aerobic conditions (Scheme 1). Fig. 4 shows the pro?le of pH change of AscH? solution after addition of Cu2+ which is consistent with the production of hydroxide by the Fenton reaction, causing the increase of pH, followed by

Fig. 3 Change in color (left) and ?uorescence (right) of 1 in the presence of varying concentrations of Cu2+ in H2O: [1] = 10.0 mM, [Cu2+] = 0, 100, 200, 300, 400, 500 nM; [AscH?] = 1.0 mM, illuminated under 365 nm, reaction time 10 min.

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Fig. 4 Kinetics pro?les of pH change of AscH? + Cu2+ in the absence and presence of 1 in H2O: [1] = 10.0 mM, [Cu2+] = 0.5 mM, [AscH?] = 1.0 mM.

the neutralisation with H+ produced from deprotonation of ascorbate radical (AscH ) ((e) in Scheme 1), leading to the subsequent decrease of pH. In the presence of 1, the pH change matches perfectly with that of AscH? and Cu2+. Experiments in bu?ered solutions at pH 7.2 (HEPES) and 6.0 (HEPES and PB) demonstrates that compound 1 existed essentially in the opened form and its ?uorescence was slightly inhibited after addition of Cu2+ (Fig S8, ESIw). Moreover, the auto-reversibility of the ?uorescence spectra also demonstrated that hydroxyl radical did not react with 1. We have thus veri?ed that it is the pH increase induced by Fenton reaction that causes the spectral change of 1 and that reactive hydroxyl radical do not promote the C–O ether bond cleavage in our case. Actually, hydroxyl radical could be reduced into H2O in the presence of a large excess of AscH?.7b Since it is a pH dependent sensing process, ligand 2 also shows a similar behavior upon addition of Cu2+ under the same conditions (Fig. S9, ESIw). In conclusion, using the Cu2+-catalyzed Fenton reaction we have developed a novel approach for highly sensitive nakedeye and ?uorescence ‘‘turn-on’’ detection of Cu2+. In our assay Cu2+ could selectively participate in the Fenton reaction to yield hydroxide which promotes hydrolysis of 1 and consequently results in the color change and ?uorescence enhancement. The naked-eye detection limit of this protocol could be as low as 200 nM, which is better than other reported systems. In view of its simplicity, short response time, low cost, highly selective and ultralow naked-eye detection, this gives leads on the design of chemosensors based on mimicking e?cient catalytic reactions. Furthermore, it may provide a new and direct approach to understand the mechanism of copper-catalyzed azide-alkyne cycloaddition (CuAAC).

Notes and references
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