Introduction

Selective recognition and detection of cations by receptors have attracted a significant amount of attention in chemistry, biology, and environmental science1,2,3,4. Among various detection receptors, fluorescent probe, based ion-induced changes in absorption or fluorescence spectra, appears to be particularly attractive due to their simplicity, high sensitivity, excellent selectivity, and instantaneous response5,6,7. However, properties of a fluorescent probe depend strongly on the binding properties of the target cation and the recognizing site in probe molecular8,9. Therefore, the binding properties and the geometrical configuration of the coordination sites in the corresponding probes are the most important parameters to consider during design and synthesis fluorescent probes. A commonly strategy is to attach a ligand in fluorophore to form the target recognizing site in the design of fluorescent probes10,11,12. High sensitivity and selectivity are fundamental goals and core design principles for an excellent probe. Though a large amount of researches had reported in the past decades, these researches did not mention electronic and steric effects that can regulate the performance in the recognition process13,14,15,16.

Copper, as the third most abundant essential trace element in human body, plays a crucial role for a broad range of biological processes17,18,19,20. However, copper excess cause toxicity to most living organisms, and copper release can eventually lead to serious environmental contamination21,22,23. Therefore, a convenient and rapid method for the detection of copper became increasingly important in biological and environmental concerns. Though chelating of Cu2+ with chemo sensors is well known to induce intrinsic fluorescence quenching due to the paramagnetic nature of Cu2+24, hundreds of fluorescent probes have been developed for varying purposes within the last few decades25,26,27. Systematic studies of these effects thus have expected to get great significance for the further development of Cu2+ fluorescent probes. Evermore, selection of an adequate probe for CLSM studies in bio-, geo-, and environmental sciences became more and more challenging28.

In order to investigate the effect of structure on performance for a probe, we synthesized and established a series of Cu2+ probe which contains 9 probes1a-1i based on our previous work (Fig. 1)29. Each of them is synthesized using rhodamine B as the fluorophore, conjugated to various differently substituted cinnamyl aldehyde with C=N Schiff base structural motif as its core segment. These probes show superior stable optical properties for the detection of Cu2+ which are not affect by fluorescence quenching. All these probes are synthesized easily under chemically mild conditions with a high yield (>80%) through a two-step reaction via Rhodamine B with substituted cinnamyl aldehyde. To our surprise, all probes showed different absorption and fluorescence responsive intensity to Cu2+.

Figure 1
figure 1

Synthesis route and molecular structure of probes 1a-1i.

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In this work, we focused on the structure-property relationships of this probe model, investigated the change of optical properties caused by different electronic effect and steric effect of the recognition group, and also employed DFT calculation simulate the Ring-Close and Ring-Open form of all the probes (Fig. 2). Comprehensive analyses on the regulatory effects of substituent group about this probe model have been elaborated using frontier molecular orbital theory, NBO charge population and Fukui functional analysis.

Figure 2
figure 2

Schematic of the strategy for studying structure-property relationships of 9 modular Cu2+ fluorescent probes.

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Results and Discussion

UV-vis properties

Binding affinities of each probe toward varies metal ions, namely the chloride salts of Li+, Na+, K+, Ba2+, Ca2+, Cd2+, Mg2+, Co2+, Mn2+, Zn2+, Pb2+, Ni2+, Fe2+, Hg2+, Fe3+, Al3+, Cr3+, Cu2+ and the nitrate salt of Ag+ were evaluated by UV-vis spectroscopy. Upon addition of the respective metal ions, the absorption spectrum of the individual probes changed in different manners as shown in Figure S9. However, all tested metal ions but Cu2+ showed only minor spectral or color changes. Similar responses were observed when Cu2+ ions were added to the probe solutions in the presence of other metal ions. The absorption spectrum of probe 1a-1i exhibited a broad band at 556 nm at room temperature in the PBS buffer solution (pH = 7.4, 50% (v/v) ethanol). Since the closed form of rhodamine did not show an intense absorption band in the visible region30, this result suggested that probe Cu2+ complexes were not present in the form of closed spirolactone, but in a opened quinoid configuration at neutral pH. Furthermore, the absorption response all individual probes toward Cu2+ showed linear relationships in the range 0–50.0 μM of Cu2+ (all the graphs are showed in Supplementary Information).

More importantly, the UV absorbance of these probes were shown to be adjusted by the different substituent groups. As shown in (Fig. 3a), the comparison of probe 1a, 1b, 1c indicates that with a volume increases of R1, the strength of UV absorbance decreases; the reasons for this phenomenon may be caused by the increase of steric hindrance effects that obstruct the combination of probe with Cu2+. In other words, R1 plays a negative, volume-dependent role in the detection of Cu2+. When other heavy elements used as substituent at position R1 instead, the UV absorption spectrum in presence of Cu2+ showed a strong absorption band. In this study, we used Br as a substituent for instance. Moreover, we recognized visually that electronegative groups in the specific recognition unit of probe had a positive effect on the UV absorbance whereas electron-donating groups have a negative effect (probe 1e: R1=NO2, probe 1f: R1=OCH3). In addition, the absorbance of the probe + Cu2+ complexes were reduced gradually (1g > 1h > 1i), when methyl was introduced into different positions of benzene ring, whereas in ortho-, meta-, para-position it had scarcely any effect on the UV absorbance response in this series of probes.

Figure 3
figure 3

Absorbance and fluorescence intensities of probes 1a-1i (10 μM) + Cu2+ (50 μM) in ethanol-PBS (5/5, v/v, pH 7.4), (a) Absorbance: maximum absorption at 556 nm; (b) fluorescence intensities: λex = 550 nm, λem = 576 nm.

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Fluorescence properties

The fluorescence properties of the probes and their respective responses to Cu2+ was evaluated in ethanol-PBS (5/5, v/v, pH 7.4) solution. The remarkable orange fluorescence emission of our probes increase that is induced by Cu2+ can interpreted as the opening of the spiral ring at appropriate pH; the emission band of this probe series appeared at 576 nm. The changes of 1a-1i (10 μM) upon addition of Cu2+ (0–5.0 equiv.) have been studied (Figure S9). Fluorescence titration spectra of 1a-1i with Cu2+ from 0 μM to 50 μM showed that these spectroscopic responses increased with increasing concentration of Cu2+ for each probe in this series. We also performed competition experiments with metal ions other than Cu2+ under the same conditions as for the absorbance study. The result revealed that this series of probes has significant anti-jamming capabilities in the presence of 5.0 equiv. of other ions (Figure S5). Furthermore, concentrations of Cu2+ and fluorescence intensities have a good linear relationship as shown in Figure S6. The coordination also shown to be a reversible process. When excess ethylenediamine was added to the colored solution of the complex, the fluorescence disappeared and the pink solution turned back to colorless owing to the de-coordination of Cu2+. Thus, 1a-1i can be classified as a conditional reversible probe for Cu2+.

As show in (Fig. 3b), fluorescence intensity of probe 1a-1c was reduced by increasing steric hindrance of substituent group R1. In addition, heavy elements substituting for R1 (e.g. Br) enhanced the fluorescence response significantly. For comparison, we have synthetized a congeneric probe 1j which is substituted by Cl at the same position of Br and exhibited a lower fluorescence intensity than that of 1d. Similarly, we introduced -NO2 and -OCH3 substituents to the recognition unit of the molecule to analyze the relationship between fluorescence intensity and electronic effects. The result suggested that electron-drawing groups promote fluorescence enhancement to a higher extent as compared to electron-donating groups. Substitutions in the ortho-position of the benzene ring increased the fluorescence intensity, whereas this effect was not observed when the substitution was inserted in meta- or para-position.

Theoretical calculation analysis

To better understand the structure-property relationships of these 9 probes, the mode of ring-close and ring-open were studied by DFT31 calculations, all the calculations were performed with B3LYP functional with a combination of basis of double-ζ quality consisting of 6-31G** for C, H elements and 6-31+G* for N, O, Br elements on Gaussian 09 Program32. (Full citations are given in supporting information). The optimized structures were confirmed to be local minimums due to the non-existence of imaginary frequency. The environmental effect was included via PCM model with ethanol as the solvent molecule.

As show in Table 1, all the Ring-Open forms possesses higher energy than that of Ring-Close form over 12 Kcal/mol. These results indicated that these probes prefer to present in a lactam form without environmental disturbance. Especially, probe 1a-1c show gradualness energy increase, which may be attributed to the block transition process by the growing steric hindrance. Extraordinary for probe 1d, the exist of heavy atom Br could promote the equalization of electronic cloud distribution and reduce energy accordingly, and the energy required for the formation of Ring-Open structure is less than any other probe, which indicated that probe 1d may show a better optical property because of its easy changed structure. The molecule energy seems to be insensitive to the Push-pull electronic effect and spatial effect on the benzene ring of recognition group.

Table 1 The calculated energy(E) of all structures.
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The HOMO-LUMO orbital energies and the spatial distributions for these probes were determined; the spatial distributions HOMO and LUMO were displayed in Fig. 4. In the Ring-Close form, all HOMOs were distributed to xanthene moiety and LUMOs were located on the recognition part. The distribution was exactly the opposite in the Ring-Open form. It’s obvious that all the Ring-Open structures have higher HOMO energy and lower LUMO energy except probe 1e. The LUMO energy of probe 1e showed an abnormal reduction from other probes, which may due to the evenly distributed electron density caused by the strong electrophilic effect of NO2.

Figure 4
figure 4

The HOMO-LUMO Gaps of probe 1a-1i.

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The results of NBO analysis33 demonstrate the polarization of Charge distribution (Fig. 5), the electron was flowing from xanthene moiety to the recognized moiety during the process of transform, the charge is which was consistent with reported literature. The larger density of electron cloud represents the higher electron donating ability.

Figure 5
figure 5

NBO charge, 3D representation of the Fukui function f−(r) of the iso-value of 0.003 a.u. (positive in red color and negative in green color) and the D-value condensed Fukui function f−(r) of O, N1 and N2.

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On the other hand, f−(r) has been successfully used to describe the reactivity concerning electrophilic attack34, CuCl2 attack in this model of probe. The condensed Fukui function of individual atom is obtained from NBO analysis. As show in Fig. 5 the 3D representation of the f−(r) clearly demonstrates that the region around carbonyl O atom and hydrazide N atom possesses higher reactivity than other parts of the Ring-Open form and there is no obvious electrophilic site in the recognized moiety of Ring-Close form. These data illustrate that all the probes have the tendency to be opened rather than closed under the entrainment and coordination of Cu2+.

CLSM bioimaging applications

These probes were selected for Cu2+-localization studies in microbial cell-extracellular polymeric substances (EPS)-mineral aggregates. The photosynthetic Fe(II)-oxidizing Rhodobacter sp. strain SW2 was selected as an model organism because SW2 is known to form such aggregates35 and for being highly resistant to heavy metals during its Fe(II) oxidization36. It was incubated under permanent illumination in fresh water mineral medium for 1 week. The resulting suspension of aggregates that consisted of microbial cells, extracellular polymers and Fe(III)-minerals was equilibrated with 50 μM of CuCl2 for 30 min. Probe 1a-1i (50 μM), SYTO 9 green fluorescent nucleic acid stain (1:500) and polysaccharide-specific Wheat germ agglutinin, ALEXA FLUOR 633 conjugate (1:200)28,29 were applied to visualize the distribution of Cu2+, bacterial cells and EPS respectively. The CLSM images show that probe 1a-1i was able to detect Cu2+ in the aqueous microbial cell-EPS-mineral aggregates. The results furthermore indicate that Cu2+ is collocated with the bacterial cells and with the free, cloud-like EPS, suggesting that the organic surfaces provide binding sites for Cu2+, even though the detailed spatial distribution of Cu2+ on subcellular level and microbial sorption mechanism still remain unclear and need to be analyzed by higher resolution analytical microscopy approaches (Fig. 6). In summary, probe 1a-1i can be used to label Cu2+-biosorption in microbial cell-EPS-mineral aggregates and biofilms. Since their fluorescence spectra are compatible for the combination with commercial available DNA- and polysaccharide-specific fluorescent dyes, they further provide the possibility to explore the correlation between Cu2+ and organic components in this system on the single cell level under natural and hydrated conditions. Our approach also provides a successful case wherein it is shown how to design, synthesize and improve other metal fluorescent probes with high sensitivity and selectivity. Finally, our attempt also revealed that the library-based strategy37 for imaging-based fluorescence screening is a powerful tool to select the most appropriate fluorescent metal probes for CLSM studies.

Figure 6: Single cell scale maps showing the sorption of Cu2+ to cell-EPS-mineral aggregates formed by Fe(II)-oxidizing Rhodobacter sp. strain SW2 incubated with 50 μM CuCl2 for 1 h at 25 °C.
figure 6

The aggregate was simultaneously incubated with 50 μM of probes 1a-1i (2) and 20% C2H5OH, fluorescent nucleic acid stain (1) and lectin conjugate (4) for 1 h at 25 °C, minerals of biogenic origin were visualized employing their reflection signal (5) (λex = 488 nm, 561 nm, 635 nm). The overlay image of (1) (2) and (4) is shown in (3); the overlay image of all four signals is shown in (6). Brighter colour indicates a higher metal concentration.

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Living cells bioimaging applications

Next, we proceeded to investigate the utility of probe 1a-1i in intracellular imaging with L929 to examine whether it can work in biological systems, the distribution of the probe within the cells was observed by laser scanning confocal microscopy following excitation at 543 nm. As show in Fig. 7, all the probes can permeate cellular membrane due to the molecule as well hydrophobicity, following addition of exogenous Cu2+, show relatively obvious intracellular fluorescence with excitation. Bright-field transmission measurements after the probe incubation confirm that the cells are viable. The overlay of fluorescence and bright-field images revealed that the fluorescence signals were localized in the perinuclear region of the cytosol, indicating the subcellular distribution of Cu2+ which was internalized into the living cells from the growth medium. We anticipate that these probes will be of great benefit for biomedical researchers investigating the effects of Cu2+ in biological systems.

Figure 7
figure 7

Fluorescent images of L929 cells incubated with 10 μM probes for 30 min (1) and then further incubated with 50 μM Cu2+ for 30 min (2). (3) Bright field image of cells. The overlay image of (2) and (3) is shown in (4) (λex = 543 nm).

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Methods

Materials and Methods

All the reagent grade chemicals consumed in this work were procured from commercial sources and used as received. Cu2+ stock solution is prepared to be 200 μmol/L with ethanol using CuCl2 as the source, and probe 1a-1i stock solution (50 ml) were prepared to be 200 μmol/L with ethanol and solubilized by little acetone (0.1 ml). The solutions of metal ions were prepared in ethanol for the selectivity and competitive study with chloride salts of Li+, Na+, K+, Ba2+, Ca2+, Cd2+, Mg2+, Co2+, Mn2+, Zn2+, Pb2+, Ni2+, Fe2+, Hg2+, Fe3+, Al3+, Cr3+, Cu2+ and the nitrate salt of Ag+.

To a 10 mL volumetric tube, different concentration of Cu2+, 5.0 mL PBS and 0.50 mL of 200 μM probes were added. The mixtures were diluted to 10 mL with ethanol. Then, 3.0 mL of each solution were transferred to a 1 cm cuvette. The absorbance was recorded at 556 nm and the fluorescence intensity was recorded at 576 nm. The excitation and emission wavelength bandpasses were both set as 5.0 nm and the excitation wavelength was set at 550 nm.

A Beijing TAIKE XT-4 microscopy melting point apparatus was used to measure the melting points of the compounds. Mass spectrometry was performed on a BRUKER micrOTOF-Q II ESI-Q-TOF LC/MS/MS Spectroscopy. NMR spectra were recorded on a VARIAN INOVA-400 spectrometer, using TMS (TMS trimethylsilyl) as an internal standard. The absorbance spectra were recorded on a SHIMADZU UV-1700 spectrophotometer. A HITACHI F-4500 FL spectrophotometer employed to record the fluorescence spectra. A VARIO EL III analyzer was used for performing the elemental analyses. Samples for mapping Cu2+ sorption to cell-mineral aggregates of Fe(II)-oxidizing bacteria were prepared under anoxic conditions in a glovebox under N2 atmosphere. The samples were analyzed using an upright confocal laser scanning microscope (LEICA SPE, Wetzlar, Germany), equipped with 4 lasers (405 nm, 488 nm, 561 nm, 635 nm) and a 63x water immersion objective (ACS APO 63x, NA = 1.15).

Conclusions

In summary, we have synthesized and evaluated a series of Cu2+ probe which contains 9 probes, all the probes are based on the core segment of C=N Schiff base structural motif. This study revealed the relation between the structure and the optical properties of nine probes focusing on the influence of various substituents and provided a significant strategy for studying structure-property relationships which were illuminated, summarized and confirmed by DFT calculation simulation on the Ring-Close and Ring-Open form of this modular Schiff base Cu2+ fluorescent probes. We expect that these results of our research would help to guide the synthesis of high performance metal fluorescence probes with various properties. On the other hand, all the probes could permeate cellular membrane to investigating the effects of Cu2+ in living cells. Furthermore, CLSM experiments suggested that all the probes would be a powerful tool for sensitive and selective detection and mapping of Cu2+ adsorbed in environmental microbial systems.

Additional Information

How to cite this article: She, M. et al. A novel approach to study the structure-property relationships and applications in living systems of modular Cu2+ fluorescent probes. Sci. Rep. 6, 28972; doi: 10.1038/srep28972 (2016).