A reaction-based fluorescent turn-on probe for Cu²⁺ in complete aqueous solution

Abstract

Based on a Cu²⁺-promoted hydrolysis reaction of the picolinate moiety, a novel fluorescent turn-on probe (FP) was designed and synthesized. The probe displayed good water solubility, a rapid response time, high sensitivity, and high selectivity for Cu²⁺ over other metal ions.

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The development of fluorescent probes for detecting transition metal ions has attracted considerable interest due to the advantages of high selectivity and sensitivity, non-destructive analysis, rapid real-time detection and simple instrumentation.¹ Copper is the third (after iron and zinc) most abundant essential trace element found in the human body, and plays a vital role in many fundamental physiological processes.² However, copper has Janus-faced properties in organisms, when the levels of copper exceed cellular needs, copper exhibits toxicity probably by generating reactive oxygen species,³ which may lead to many neurodegenerative diseases such as Alzheimer's,⁴ Parkinson's,⁵ Menke's,⁶ and Wilson's disease.⁷ In addition, copper is identified as an environmental pollutant⁸ due to its extensive use. Owing to the significance of copper ions in biological and environmental systems, much effort has been devoted to developing selective and sensitive copper probes.⁹

Among the reported fluorescent probes for copper ions, they can be categorized to two types based on the sensing strategy: one based on metal coordination and the other based on a chemical reaction. Due to the paramagnetic characteristics of Cu²⁺,¹⁰ Cu²⁺ coordination usually results in fluorescence quenching, which is not as sensitive as fluorescence enhancement and sometimes gives false-positive results caused by other quenchers. Alternatively, reaction-based fluorescent probes can detect an analyte by a specific irreversible chemical reaction between the probe molecule and the target analyte, leading to the release of fluorescent products. Usually, the fluorescent products do not coordinate to Cu²⁺, which allow for highly selective and sensitive signaling. Thus, reaction-based fluorescent turn-on probes have received increasing attention. Excellent examples of fluorescent turn-on probes for Cu²⁺ via chemical reactions include Cu²⁺-induced the transformation of a thiosemicarbazide moiety to an isothiocyanate group,¹¹ Cu²⁺-promoted hydrolysis of cyclic peptide nucleic acid,¹² acetyl groups,¹³ hydroxylamide,¹⁴ amides,¹⁵ hydrazide,¹⁶ hydrazone,¹⁷ lactone,¹⁸ and picolinate,¹⁹ Cu²⁺-mediated oxidation of phenothiazine,²⁰ o-phenylenediamine,²¹ dihydrorosamin,²² and oxidative cyclization of aza-aromatics,²³ thiosemicarbazone,²⁴ 4-N,N-dimethylaminobenzaldehyde hydrazone,²⁵ N-acylhydrazones,²⁶ Cu²⁺-triggered dethioacetalization,²⁷ and Cu²⁺-catalyzed Heck reaction.²⁸

Nevertheless, many reaction-based fluorescent probes suffer from the shortcomings that they can act only in specific detection conditions such as need an organic solvent,^{20,23,25,27,28} high temperature,^12,18,28 long reaction time,^12,14,22,27 or non physiological pH,^14,17 which limit their practical application. Therefore, developing novel fluorescent turn-on probe for rapid response of Cu²⁺ under physiological conditions is still challenging and very attractive.

Recently, our group contributed to the development of reaction-based fluorescent probes for the selective detection of different types of analytes.²⁹ In a continuation of this effort, we report a novel fluorescent turn-on probe FP for Cu²⁺ based on Cu²⁺-promoted hydrolysis reaction of the picolinate moiety in this work. FP was composed of a latent methylfluorescein (FM) fluorophore and a reaction site of picolinate moiety (Scheme 1). We envisioned that the hydroxy group of methylfluorescein (FM) is protected by the picolinate moiety, the methylfluorescein moiety of FP exists in a nonfluorescent spirocyclic form. Cu²⁺-mediated hydrolysis of the picolinate moiety would release the ring-opening delocalized methylfluorescein (FA) and give “off–on” fluorescence response.

Scheme 1 Synthetic strategy for FP and proposed reaction mechanism for Cu²⁺.

To test this design hypothesis, FP was prepared by the straightforward coupling of 2-picolinic acid with methylfluorescein in 65% yield (Scheme 1). FP was characterized by ¹H and ¹³C NMR spectroscopy, and ESI mass spectrometry.

With FP in hand, we assessed the photophysical response of FP to Cu²⁺. The absorption and fluorescence spectra of FP before and after reaction with Cu²⁺ are shown in Fig. 1. The solution of FP alone exhibits a very weak absorption above 400 nm (Fig. 1a), which indicates that the spirocyclic form of FP predominates. Upon addition of Cu²⁺, the solution of FP showed a new maximum absorption wavelength at 454 nm, which can be ascribed to the ring-opening delocalized form (FA) of methylfluorescein. As expected, FP almost has no fluorescence (Φ_f ≈ 0.027) upon excitation at 460 nm. However, upon addition of Cu²⁺, the solution of FP showed significant fluorescent emission at 514 nm (Fig. 1b), with a quantum yield of Φ_f ≈ 0.193, which is similar to that of FM (Φ_f ≈ 0.194). A colour change from colorless to green can be visualized under illumination with a 254 nm UV lamp (inset of Fig. 1b). Interestingly, the absorption spectra, fluorescence spectra and fluorescence colour of the reaction system resemble those of FM in HEPES buffer (Fig. S1†), supporting the fact that the Cu²⁺-promoted hydrolysis reaction of the picolinate moiety causes the release of free FM and subsequently rearranges to form FA.

Fig. 1 (a) Absorption spectra of FP (1 μM) (black line) before and (red line) after reaction with Cu²⁺ (20 μM) in HEPES buffer (10 mM, pH 7.0, 25 °C). (b) Fluorescence spectra of FP (1 μM) (black line) before and (red line) after reaction with Cu²⁺ (20 μM) in HEPES buffer (10 mM, pH 7.0, 25 °C, λ_ex = 460 nm). The fluorescence colour changes of FP before and after the reaction under illumination with a 254 nm UV lamp are shown in the inset.

For further investigation, we examined the experimental conditions such as pH and solubility for optimization. At room temperature (25 °C), the fluorescence intensity at 514 nm of FP in the absence and presence of Cu²⁺ at different pH was studied (Fig. S2a†). It could be seen that the fluorescence intensity increased slightly in the pH ranging from 3.0 to 7.0 in the absence of Cu²⁺. However, the fluoresecence intensity drastically increased with pH increasing in the range of 7.0–10.5 in the absence of Cu²⁺. On the other hand, in the presence of Cu²⁺, the fluorescence intensity was increased in the region of pH 4.5–7.0. These findings indicated that alkaline condition (>7.0) could cause the hydrolysis of FP, which increased background fluorescence. The effect of pH on the fluorescence intensity at 514 nm of FM was also investigated (Fig. S2b†). FM exhibits a very weak fluorescence in the pH range between 2.5 and 3.5, indicating that FM exists in the spirocyclic form. However, the fluoresecence intensity obviously increased and reached a plateau with pH varying from 4.0 to 7.0, indicating that FM exists in the ring-opening delocalized form (FA) when pH is 7.0.³⁰ Thus, pH 7.0 is suitable for Cu²⁺ detection. In addition, both FP and FM exhibit good solubility in complete aqueous solution (10 mM HEPES, pH 7.0, 25 °C) according to the absorption and fluorescence spectra (Fig. S3†), respectively. Based on the above observations, buffer solution of HEPES (10 mM, pH 7.0, 25 °C) was selected as the optimized reaction conditions for further spectral investigation.

Next, we assessed the photophysical properties of FP under the optimized reaction (10 mM HEPES, pH 7.0, 25 °C). As shown in Fig. 2, FP (1 μM) exhibits weak fluorescence. Upon addition of Cu²⁺ (20 μM) for 2 min, the solution of FP showed significant fluorescent emission, and the fluorescence intensity at 514 nm reached a plateau within 1 min with a 10-fold enhancement. The fluorescent emission change clearly demonstrate that the hydrolysis of FP was triggered by Cu²⁺ and subsequently liberated FM. To gain an insight into the proposed reaction mechanism, the reaction product of FP was also checked by ESI analysis. The ESI spectrum of the reaction solution of FP with Cu²⁺ shows a major peak at m/z = 347.1 [M + H]⁺, which is characteristic of the mass of FA (Fig. S4†). All above results clearly demonstrate that reaction of FP with Cu²⁺ causes the hydrolysis of the picolinate moiety, the release of FM and subsequent rearrangement.

Fig. 2 (a) Fluorescence spectra of FP (1 μM) recorded every 12 s within 2 min after the addition of Cu²⁺ (20 μM) in HEPES buffer (10 mM, pH 7.0, 25 °C, λ_ex = 460 nm). (b) The fluorescence intensity at 514 nm changes as a function of time.

To test sensitivity, the time-dependent fluorescence spectra of FP in the presence of varied concentrations of Cu²⁺ were studied and the hydrolysis was monitored by measuring the fluorescence at 514 nm (Fig. 3a). It can be seen that the fluorescence intensity of FP (1 μM) at 514 nm increased gradually with the increasing of the concentration of Cu²⁺ from 0 μM to 20 μM and then reached a platform. The results demonstrate that higher concentrations of Cu²⁺ result in faster hydrolysis reaction and greater variation in the fluorescence intensity at 514 nm. In contrast, negligible changes in fluorescence were detected in the absence of Cu²⁺ for the same interval of time. Moreover, A linear equation of F_{514 nm} = 18.554 + 55.883 × [Cu²⁺] (R² = 0.995) was obtained in the concentration range of 0–1 μM Cu²⁺ (Fig. 3b and S5b†). The detection limit of FP for Cu²⁺ was thus calculated to be 55 nM based on 3σ/k (where σ is the standard deviation of blank measurement, k is the slope between the fluorescence intensity versus Cu²⁺ concentration.),³¹ which is comparable to those of some previously reported highly sensitive fluorescent probes¹⁷ (Table S1†) and is much lower than the limit of copper in drinking water (∼20 μM) set by the U. S. Environmental Protection Agency.³² In additon, the reaction of FP (1 μM) with Cu²⁺ (20 μM) was completed within 1 min under the optimized conditions, which is much faster than the previously reported fluorescent probes (Table S1†). Kinetic measurement of FP (1 μM) with Cu²⁺ (100 μM) under the pseudo-first-order conditions gave an observed rate constant of k_obs = 0.284 s⁻¹ (Fig. S6†). Therefore, this reaction time of 1 min was used for the following experiments.

Fig. 3 (a) Plot of fluorescence intensity at 514 nm vs. the reaction time in the presence of varied concentrations of Cu²⁺. Data were acquired in HEPES buffer (10 mM, pH 7.0, 25 °C, λ_ex = 460 nm). (b) The fluorescence intensity at 514 nm as a function of the concentrations of Cu²⁺ in the range of 0–1 μM. Each data point represents the average of three trials. Error bars were calculated as standard deviation.

We then proceeded to examine the selectivity of FP. As shown in Fig. 4, only Cu²⁺ resulted in a significant enhancement in the fluorescent intensity at 514 nm, whereas very weak fluorescence variations were observed for the other metal ions, such as alkali or alkaline-earth metal ions (Na⁺, K⁺, Ca²⁺, Mg²⁺ and Ba²⁺), heavy and transition metal ions (Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, Co²⁺, Fe²⁺, Pb²⁺, Mn²⁺, Ni²⁺, Al³⁺, Fe³⁺, Sn⁴⁺). High selectivity toward the analyte over the potentially competing species is a necessity for a probe. Thus, the competitive experiments were conducted under identical conditions. No significant variations in the fluorescent intensity at 514 nm were found (Fig. 4), even when Na⁺, K⁺, Ca²⁺, and Mg²⁺ were present at micromolar levels. Therefore, FP exhibits excellent selectivity for Cu²⁺ over other metal ions, which can be ascribed to the specific hydrolysis of the picolinate moiety by Cu²⁺.¹⁹

Fig. 4 Fluorescence intensity at 514 nm for FP determined in the absence and presence of different metal ions: 1, none; 2, Na⁺ (1 mM); 3, K⁺ (1 mM); 4, Ca²⁺ (1 mM); 5, Mg²⁺ (1 mM); 6, Ag⁺ (20 μM); 7, Zn²⁺ (20 μM); 8, Cd²⁺ (20 μM); 9, Hg²⁺ (20 μM); 10, Ba²⁺ (20 μM); 11, Co²⁺ (20 μM); 12, Fe²⁺ (20 μM); 13, Pb²⁺ (20 μM); 14, Mn²⁺ (20 μM); 15, Ni²⁺ (20 μM); 16, Al³⁺ (20 μM); 17, Fe³⁺ (20 μM); 18, Sn⁴⁺ (20 μM); 19, Cu²⁺ (20 μM). Gray bars: free probe and probe treated with the marked metal ions. Black bars: probe treated with the marked metal ions followed by Cu²⁺ (20 μM). Data were acquired in HEPES buffer (10 mM, pH 7.0, 25 °C, λ_ex = 460 nm). The results are the mean ± standard deviation of three separate measurements.

Conclusions

In this work, we designed and synthesized a novel fluorescent turn-on probe FP based on Cu²⁺-promoted hydrolysis reaction of the picolinate moiety. The probe displayed a rapid response time (within 1 min), very high sensitivity (detection limit of 55 nM), and high selectivity for Cu²⁺ over other metal ions. Importantly, FP can detect Cu²⁺ in complete aqueous solution, indicating that FP has potential biological applications.

Acknowledgements

We thank the Scientific Research and Technological Development Program of Guangxi (GKH14251003), the Middle-aged and Young Teachers' Basic Ability Promotion Project of Guangxi (KY2016YB452), the Doctor's scientific research foundation of Hezhou University (HZUBS201509) and Key Laboratory of Food and Agricultural Products Quality and Safety in Guangxi Colleges and Universities for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedure and characterization of FP, and additional spectroscopic data. See DOI: 10.1039/c6ra18669f

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