5-(N-Ethylcarbazol-3-yl)thiophene-2-carbaldehyde (ECTC): a novel fluorescent sensor for ferric ion

Introduction

The development of chemosensors for highly selective detection of metal ions has attracted considerable attention in recent years owing to their important roles in biological, environmental and chemical domains [1], [2], [3], [4]. As one of the most essential trace elements in living organism, Fe³⁺ plays a major role in a wide range of biological processes such as oxygen metabolism, electron transport, and DNA synthesis. The variation of Fe³⁺ concentrations can produce a significant influence on physiological function of organism [5]. Moreover, iron(III) is an important limiting element for phytoplankton in water environment [6]. Thus, it is very important to establish efficient detecting and monitoring methods with high sensitivity and selectivity for the detection of Fe³⁺ in biological and environmental systems. To date, a series of fluorescent sensors for the detection of Fe³⁺ have been developed [7], [8], [9], [10], [11]. Some of these sensors suffer from complicated structure, laborious synthesis, and poor selectivity, which restricts their applications in environmental monitoring and biological assays [12], [13], [14], [15]. Moreover, the detection limits of these sensors are mainly in the range of 10⁻³–10⁻⁶m [16], [17], [18], [19] and it is difficult to detect low concentration of Fe³⁺ in solution. Interestingly, I⁻ can be used as an assistance reagent to detect Fe³⁺ at 10⁻⁶m or even at lower concentration [20]. In this indirect approach, Fe³⁺ reacts with excess I⁻ to produce I₃⁻ that quenches the fluorescence of sensor molecules. Nevertheless, these systems containing the extra reagent are complicated. Therefore, it is a great challenge to find a fluorescent sensors for Fe³⁺ with simple structure and low detection limit.

Carbazole is the key structural moiety of many electroluminescent materials due to its excellent fluorescent properties [21], [22]. However, the reports about fluorescent sensors based on carbazole to detect and sense metal ions are relatively rare [23]. Herein, we designed and synthesized a novel fluorescent compound, 5-(N-ethylcarbazol-3-yl)thiophene-2-carbaldehyde (ECTC) via Suzuki coupling reaction. Upon addition of Fe³⁺ ions to the solution of ECTC, a significant fluorescence quenching is observed, whereas almost no quenching is observed in the presence of other tested metal ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Mn²⁺ and Co²⁺). Hence, ECTC could act as a fluorescent sensor to detect and recognize Fe³⁺ ion in solution.

Results and discussion

ECTC was synthesized according to the synthetic route outlined in Scheme 1. Bromination of 9-ethylcarbazole [24] with N-bromosuccinimide followed by the Suzuki reaction with 5-formyl-2-thiopheneboronic acid provided the desired compound ECTC with good yield. Its structure was characterized by ¹H NMR, ¹³C NMR and elemental analysis.

Scheme 1

First, the spectroscopic properties of compound ECTC were investigated. As shown in Figure 1, the maximum UV absorption peak is around 240 nm, which can be attributed to π-π* transitions of carbazole and thiophene systems. The maximum fluorescent emission appears at 526 nm.

Figure 1

The UV–vis absorption and fluorescence emission spectra of ECTC (10 μm) in dichloromethane.

Second, some common metal ions, Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Mn²⁺, Co²⁺ and Fe³⁺, were used to evaluate the metal ion binding properties of ECTC in ethanol. As shown in Figure 2, the fluorescence spectrum of ECTC shows a strong emission owing to the characteristic emission of the carbazole skeleton. Upon addition of Fe³⁺ ions to the solution of ECTC, a significant fluorescence quenching is observed. By contrast, changes in the fluorescence profile of ECTC in the presence of other tested metal ions are negligible. These results demonstrate that ECTC could be a potential fluorescent probe for the selective determination of Fe³⁺.

Figure 2

The effect of various metal ions on fluorescence of ECTC in ethanol.

Concentrations: [ECTC]=1×10⁻⁵m, [metal ion]=1×10⁻⁵m; λ_ex=240 nm.

In order to describe the quenching effect for each tested metal ion more accurately, the degrees of fluorescence quenching η were measured at two wavelengths and the results are shown in Figure 3. The quenching degree η is calculated from the formula: η=(I₀−I)/I₀. Therein, I₀ and I are the fluorescence intensities of ECTC in ethanol in the absence and presence of a particular metal ion, respectively. The η value for Fe³⁺ is 36.4% at 526 nm. By contrast, the η values for Cu²⁺ and Co²⁺ are 4.8% and 3.6% at 526 nm, respectively. The η values of other ions are below 2% at 526 nm. These quantitative results indicate that the presence of these metal ions in solution would hardly disturb the detection of Fe³⁺ in some complex system.

Figure 3

The quenching degrees of fluorescence of ECTC in ethanol at 368 nm and 526 nm in the presence of various metal ions.

Conditions: [ECTC]=1×10⁻⁵m, [metal ion]=1×10⁻⁵m, λ_ex=240 nm.

The effect of concentration of Fe³⁺ on quenching was then investigated and the results are shown in Figure 4. As can be seen, quenching increases with increase in concentration of Fe³⁺ and upon addition of 50 equiv of Fe³⁺ ion to 10 μm of ECTC, almost complete quenching is observed. The fluorescent intensity of each solution at 526 nm was chosen as the reference to investigate the quantitative relationship between fluorescent intensity and concentration of Fe³⁺ because the other fluorescence emission at 368 nm was lower. As shown in Figure 5, under optimized conditions, the fluorescence intensity of ECTC is linearly quenched (linearly dependent coefficient R-square: 0.99) by Fe³⁺ ions in a wide range of 0.1 μm–10 μm and the limit of detection could be below 0.1 μm.

Figure 4

The fluorescence emission spectra of ECTC as functions of different concentrations of Fe³⁺; [ECTC]=1×10⁻⁵m, λ_ex=240 nm.

Figure 5

The fluorescence titration of ECTC with Fe³⁺ ion: [ECTC]=1×10⁻⁵m, λ_ex=240 nm.

Finally, we wish to propose a possible quenching mechanism. It can be suggested that similarly to the reaction between Fe³⁺ and I⁻ [20], the aldehyde group of ECTC is oxidized by Fe³⁺ to a carboxylic acid group. Since the coordinating capability of the carboxylic group is better than that of the aldehyde group, coordination of the oxidized ECTC molecule with iron³⁺ ion is expected to be stronger that of ECTC, which would decrease planarity of the unfused biaromatic system which, in turn, would reduce fluorescence intensity, as observed [25].

Conclusion

A novel fluorescence sensor ECTC was synthesized and characterized. This compound exhibits strong fluorescence quenching in the presence of Fe³⁺ ion, whereas almost no quenching is observed in the presence of Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Mn²⁺ and Co²⁺. Under optimized conditions, the fluorescence intensity of ECTC is quenched by Fe³⁺ ion in a linear fashion in a wide range of 0.1 μm–10 μm. Compound ECTC could be an efficient fluorescent probe for the selective determination of Fe³⁺.

Experimental