Coupling a novel spiro-rhodamine B lactam derivative to Fe₃O₄ nanoparticles for visual detection of free copper ions with high sensitivity and specificity

Abstract

We herein reported a novel spiro-rhodamine B lactam derivative (RhBLA), which can be coupled to Fe₃O₄ nanoparticles (NPs) and act as a Cu²⁺-selective visual sensor. We demonstrated that the RhBLA-functionalized Fe₃O₄ NPs can specifically chelate with Cu²⁺ to create a colour change from colourless to pink, which provides a sensing platform for the simple, rapid and field-portable visual detection of trace Cu²⁺. By using this sensing platform, a sensitive and specific visual sensor for the rapid and on-site detection of trace Cu²⁺ was developed. The coupling of RhBLA to Fe₃O₄ NPs facilitates the magnetic separation of Cu²⁺ from a bulk sample solution, which leads to the selective detection of Cu²⁺ at low concentration and the robust resistibility of the sensor to the matrix of the sample. The proposed visual sensor can be used to directly detect as little as 50 nM of Cu²⁺, which is much lower than the maximum allowable level of Cu²⁺ (∼20 μM) in drinking water defined by the USA EPA, in 100 mL of river or tap water by only naked-eye observation within 15 min. In addition, the method proposed in this study is simple, cost effective, rapid, and does not need any sophisticated instruments. The above features make our sensor a promising approach for the rapid and on-site visual detection of ultra-trace free copper ions in aqueous environments by only naked-eye observation.

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Introduction

The highly selective and sensitive detection of metal ions in aqueous solution is of great importance in tracking and studying the physiological functions of these ions in living organisms.¹ Copper is an essential trace element for the life activities of organisms. It serves as a catalytic component for many important enzymes and binding proteins in organic oxidation–reduction reaction processes.^2–6 However, free copper ions (Cu²⁺) are toxic to cells.^7,8 Indeed, the total copper ion concentration in healthy cells is generally around 100 mM, but most of this is present in complex form within compounds. The free copper ion concentration in cells is on average less than one free ion per cell.⁹ Many researchers have reported that excessive intake of copper will lead to physiological function disorders and cause numerous diseases such as Alzheimer’s disease, cardiovascular disease and Wilson’s disease.^10–13 Accordingly, the US Environmental Protection Agency (EPA) has established a safe limit of 1.3 ppm (∼20 μM) for free copper ions in drinking water.¹⁴ For the reasons mentioned above, it is thus crucial to develop a simple and sensitive method for the on-site determination of trace Cu²⁺ in living cells and in the aqueous environment.

The traditional analytical techniques for Cu²⁺ detection including atomic absorption spectrometry (AAS), atomic emission spectrometry (AES) and inductively coupled plasma mass spectrometry (ICP-MS) have high sensitivity and excellent selectivity. However, all of them require sophisticated and expensive equipment, which makes on-site and real time detection impossible.¹⁵ The limitations of traditional analytical techniques have pushed researchers to develop novel methods for the rapid, sensitive and specific detection of Cu²⁺ in the field. In conjunction with this goal, some colorimetric methods including chemosensors and biosensors have been developed for the visual and on-site detection of Cu²⁺ due to the outstanding analytical advantages of a visual method, such as portability, low cost, short detection time and lack of requirement for sophisticated equipment.^16–26 Unfortunately, most colorimetric chemosensors and biosensors previously reported have lower sensitivity and poorer resistibility to the matrix, which means these methods can not be used for the visual detection of trace Cu²⁺ in cell and water samples by only naked-eye observation. So, improving the sensitivity of the visual methods is a key factor for the on-site detection of Cu²⁺.

Rhodamine B (RhB) and its derivatives are highly favorable fluorophores owing to its excellent photophysical properties.²⁷ Most importantly, the colourless spirocyclic structures of rhodamine B derivatives can convert into ring-open forms with visible colour differences during sensing events. Such a strong colour difference facilitates “naked-eye” detection, and provides convenient and fast sensing platforms for the development of visual chemosensors. Therefore, rhodamine B and its derivatives have been widely used in the development of visual sensors for detecting various metallic ions including Cu²⁺, Hg²⁺, Fe³⁺, Pb²⁺ and Ag⁺.^28–37 However, all visual sensors previously reported, which were based on rhodamine B and its derivatives, can not provide the necessary sensitivity and selectivity for the on-site detection of ultra-trace Cu²⁺ in aqueous environments by only naked-eye observation. Mono-dispersed magnetic Fe₃O₄ nanoparticles (Fe₃O₄ NPs) are always of great interest to researchers due to their unique properties, such as being highly soluble, easy to functionalize and easy to collect. The presence of Fe₃O₄ NPs in Cu²⁺-selective sensors should facilitate the magnetic separation of Cu²⁺ from the bulk sample solution, which would lead to the selective detection of Cu²⁺ at low concentration. Not long ago, Tan et al. reported an ultra sensitive method for the visual detection of Cu²⁺ based on Fe₃O₄ NP-based solid phase extraction together with functionalized gold nanoparticles.³⁸ The method reported by them has high sensitivity, however, it needs a relatively complex operation process including the pre-chelation of Cu²⁺ with ammonium pyrrolidinedithiocarbamate, Fe₃O₄ NP-based solid phase extraction and chromogenic reaction. In addition, their method is a “turn-off” sensor (colour change from red to colourless upon the addition of Cu²⁺), which is susceptible to various interferences. In an effort to overcome the limitations of previous Cu²⁺-selective visual sensors, in this research we developed a novel rhodamine B derivative and further coupled the rhodamine B derivative to Fe₃O₄ NPs to act as a Cu²⁺-selective visual sensor for the rapid detection of trace Cu²⁺ by only naked-eye observation, with the object of providing a convenient method for the on-site detection of trace free copper ions in aqueous environments. The coupling of Fe₃O₄ NPs facilitates the magnetic separation of Cu²⁺ from the bulk sample solution, which leads to a higher sensitivity and more robust resistibility to the matrix of the sample for the sensor. In addition, the method proposed in this study is simple, cost effective, rapid, and does not need any sophisticated instruments.

Experimental section

Synthesis of the spiro-rhodamine B lactam derivative (RhBLA)

The chemical structure of RhBLA and its synthetic route are depicted in Scheme 1. Firstly, compound 1 and compound 2 were synthesized from rhodamine B according to the reported process (the detailed process and electrospray ionization mass spectrometry analysis are summarized in the ESI†).^39,40

Scheme 1 Schematic illustration of the synthesis of spiro-rhodamine B lactam derivative (RhBLA)-functionalized magnetic Fe₃O₄ NPs.

To synthesize RhBLA from compound 2, in a 100 mL flask, about 0.511 g of compound 2 (1 mmol) and 0.20 g of 3,4-dihydroxybenzhydrazide (1.2 mmol) were refluxed in 30 mL of absolute ethanol for 5 hours under a flow of nitrogen by heating the whole in an oil bath. Then, the resulting mixture was added into 60 mL of saturated NaCl solution, and a purple flocculent precipitate was observed immediately. The resulting purple precipitate was filtered and washed 3 times with cold ethanol (about 15 mL), and then was dried in a vacuum oven to afford the crude product of RhBLA. The crude product was purified by column chromatography (silica gel), which used MeOH–CH₂Cl₂ (v/v = 1/50) as the solvent, to obtain pure RhBLA as a purple powder (0.45 g, 70%).

The ¹H-NMR analysis (see Fig. S1 in ESI†), ¹³C-NMR analysis (see Fig. S2 in ESI†) and ESI-MS (electrospray ionization mass spectrometry) data (see Fig. 1) are provided as proof of purity of RhBLA. Briefly as follows: ¹H-NMR (400 MHz, C₂D₆OS, δ ppm): 11.65 (s, 1H, HNC=O), 9.7 (s, 1H, Ar-OH), 9.3 (s, 1H, Ar-OH), 7.94 (s, 1H, N=CH), 7.92 (s, 1H, N=CH), 7.84–7.86 (d, 1H, Ar-H), 7.53–7.64 (m, 3H, Ar-H), 7.26 (s, 1H, Ar-H), 7.16–7.21 (d, 1H, Ar-H), 7.03–7.07 (d, 1H, Ar-H), 6.78–6.82 (d, 1H, Ar-H), 6.46 (s, 1H, Ar-H), 6.42 (s, 1H, Ar-H), 6.35–6.37 (d, 2H, Ar-H), 5.76 (s, 1H, Ar-H), 3.29–3.37 (m, 8H, CH₂), 1.06–1.12 (t, 12H, CH₃). ¹³C-NMR (400 MHz, C₂D₆OS, δ ppm): 165.73 (C=O); 153.54, 152.34 (ArC); 148.65 (C=N); 132.83, 130.12, 128.57, 128.14, 123.95, 122.62, 108.33, 106.00, 97.99 (ArC); 65.26, 44.16 (CH₂); 12.93 (CH₃). ESI-MS (m/z): M⁺ calculated for C₃₇H₃₈N₆O₅ is 646.30, found: 647.27 (M + H)⁺.

Fig. 1 The ESI-MS (electrospray ionization mass spectrometry) analysis of RhBLA.

Preparation of the RhBLA-functionalized magnetic Fe₃O₄ NPs

The RhBLA-functionalized Fe₃O₄ NPs used for detecting Cu²⁺ were prepared by coupling RhBLA to the surface of Fe₃O₄ NPs via Fe–O bonds (see Scheme 1) by referring to the reported process.⁴¹ Firstly, the monodispersed oleylamine-coated Fe₃O₄ NPs (∼9 nm) were synthesized according to a published procedure.⁴² About 2 mmol of Fe(acac)₃ was dissolved in a mixture of 10 mL benzyl ether and 10 mL oleylamine. The mixture was dehydrated at 110 °C for 1 h under the protection of nitrogen gas, and quickly heated to 300 °C and kept at this temperature for 2 h under the protection of nitrogen gas. Then, 40 mL of ethanol was added into the mixture after it was cooled to room temperature. The precipitate (oleylamine-coated Fe₃O₄ NPs) was collected by centrifugation at 8000 rpm and was washed 3 times with ethanol. Finally, the Fe₃O₄ NPs (150 mg) were re-dispersed in hexane. Secondly, 10 mg of the previously synthesized RhBLA (0.015 mmol) was dissolved in a mixture of 500 μL CH₂Cl₂ and 200 μL DMSO, then 0.5 mg of the above prepared Fe₃O₄ NPs were added into the mixture. The whole was incubated overnight at room temperature under protection from light and with full stirring to couple RhBLA to the surface of the Fe₃O₄ NPs. The resulting RhBLA-modified Fe₃O₄ NPs were precipitated by adding sufficient n-hexane. The RhBLA-modified Fe₃O₄ NP precipitate was collected by centrifugation at 10 000 rpm and was washed with CH₂Cl₂–n-hexane (1/5, v/v) three times. Finally, the RhBLA-modified Fe₃O₄ NP precipitate was re-dispersed in the mixture of 1 mL DMSO and 200 μL CH₂Cl₂, and the final solution was used for detecting Cu²⁺.

TEM (transmission electron microscopy) and FT-IR (fourier transform infrared spectrometry) measurements of the RhBLA-functionalized Fe₃O₄ NPs were performed to confirm whether the RhBLA was successfully conjugated on the surface of the Fe₃O₄ NPs. As the TEM images show in Fig. 2, the oleylamine-coated Fe₃O₄ NPs have a homogeneous spherical shape, with a 7–8 nm particle size. After modification with RhBLA, there was a 2 nanometer thick RhBLA self-assembly monolayer on the Fe₃O₄ NP surface. In the IR spectra shown in Fig. S3 (in ESI†), the Fe₃O₄ NPs (curve a) exhibited a typical characteristic peak of Fe–O at 592 cm⁻¹, and RhBLA (curve b) showed a typical characteristic peak of lactam C=O at 1703 cm⁻¹. In comparison with RhBLA and the Fe₃O₄ NPs, the RhBLA-functionalized Fe₃O₄ NPs (curve c) showed a similar IR spectrum to that of pure RhBLA with a typical characteristic peak of lactam C=O at 1695 cm⁻¹. At the same time, the RhBLA-functionalized Fe₃O₄ NPs (curve c) showed a typical characteristic peak of Fe–O at 600 cm⁻¹ too. The above IR experimental results suggest that RhBLA has been successfully coupled to the Fe₃O₄ NPs.

Fig. 2 The TEM images of the Fe₃O₄ NPs (A) and RhBLA-functionalized Fe₃O₄ NPs (B).

Visual detection of Cu²⁺ with the RhBLA-functionalized Fe₃O₄ NPs

For the detection of trace Cu²⁺, 100 μL of the above prepared RhBLA-functionalized Fe₃O₄ NP solution was added into 100 mL of a water sample (or more than 100 mL, according to the concentration of Cu²⁺ in the sample). The whole was heated to 80 °C and kept at this temperature for 15 min together with agitation. After the whole was cooled to room temperature, the RhBLA-functionalized Fe₃O₄ NPs were separated from the solution with a magnet. Then, the RhBLA-functionalized Fe₃O₄ NPs were re-dispersed in a mixture of 300 μL HAc–NaAc (50 mM, pH 6.00) buffer solution and 300 μL methanol, and the colour was recorded with a digital camera. The concentration of Cu²⁺ was quantified based on naked-eye observation.

Results and discussion

Detection principle for the RhBLA-functionalized Fe₃O₄ NPs and their optical responses to Cu²⁺

In this study, the RhBLA coupled on the surface of Fe₃O₄ NPs was employed for Cu²⁺ recognition and signal generation, and the Fe₃O₄ NPs were used for signal amplification. As shown in Fig. 3, RhBLA modified onto the surface of Fe₃O₄ NPs can specifically chelate with Cu²⁺ at pH 6.0 to induce the ring-opening of the rhodamine B spirolactam, which leads to the colour change from colourless to pink and a strong adsorption peak at 562 nm. After chelating with Cu²⁺, RhBLA-functionalized Fe₃O₄ NPs can be rapidly and easily collected from the bulk sample solution with a magnet. By re-dispersing RhBLA-functionalized Fe₃O₄ NPs in a mixture of 300 μL HAc–NaAc (50 mM, pH 6.00) buffer and 300 μL methanol (since the RhBLA–Cu²⁺ complex has a maximal absorption at pH 6.0 and the addition of methanol can improve the dispersion of the RhBLA-functionalized Fe₃O₄ NPs in the HAc–NaAc buffer), the colour change of the RhBLA-functionalized Fe₃O₄ NPs can be much more sensitively and easily observed by the naked eye, which provides a ultra sensitive and visual platform for the on-site determination of ultra-trace Cu²⁺.

Fig. 3 Schematic illustration of the principle for the visual detection of Cu²⁺ using RhBLA-functionalized magnetic Fe₃O₄ NPs.

In order to verify whether the sensor worked as expected, the optical responses of RhBLA to Cu²⁺ were investigated before and after it was conjugated on the surface of the Fe₃O₄ NPs, respectively. As shown in Fig. 4, the pure RhBLA solution (in HAc–NaAc–methanol buffer, v/v = 1/1, 50 mM, pH = 6.0) showed no colour and no adsorption peak at 562 nm. Whereas, upon the addition of Cu²⁺, the RhBLA solution became pink and a strong absorption band around 562 nm appeared, indicating that RhBLA can chelate with Cu²⁺ to induce the ring-opening of the rhodamine B spirolactam at pH 6.0. For the RhBLA-functionalized Fe₃O₄ NP solution (in HAc–NaAc–methanol buffer, v/v = 1/1, 50 mM, pH = 6.0), upon the addition of Cu²⁺, the solution also showed a colour change from colourless to pink, just like the pure RhBLA solution. This indicated that RhBLA had no change in its optical responses to Cu²⁺ after it was conjugated on the surface of the Fe₃O₄ NPs.

Fig. 4 The optical responses to Cu²⁺. (a): reagent blank; (b): RhBLA-functionalized Fe₃O₄ NPs; (c): pure RhBLA solution.

Optimization of the conditions for detecting Cu²⁺ with the RhBLA-functionalized Fe₃O₄ NPs

The reaction pH, temperature, time etc. will affect the chelation of RhBLA and Cu²⁺, and thus finally influence the sensitivity and selectivity of the proposed sensor. In this study, the reaction pH for the reaction between RhBLA and Cu²⁺ was controlled using HAc–NaAc buffer solution (50 mM). The effect of pH on the chelation of RhBLA and Cu²⁺ was investigated in the range from 3.6 to 6.5. The experimental results (see Fig. S4 in ESI†) showed that the maximal absorption peak of the RhBLA–Cu²⁺ complex has a slight blue-shift with the increase of pH. At the same time, the absorption of the RhBLA–Cu²⁺ complex gradually increased when the pH increased from 3.6 to 6.0, then, the absorption began to decrease when the pH was higher than 6.0. The experimental results indicate that the optimum pH for the chelation of RhBLA and Cu²⁺ is pH 6.0.

The temperature will affect the chelation rate of RhBLA and Cu²⁺, and eventually influence the analysis time, which is a key factor for a rapid and field-portable visual method. The effect of the temperature on the chelation rate between RhBLA and Cu²⁺ was investigated in detail by varying the temperature from room temperature to 90 °C. The experimental results indicate that the reaction rate increased with the increase of the temperature when the temperature was in the range of room temperature to 80 °C, then, the reaction rate began to keep a constant value when the temperature was higher than 80 °C. Therefore, 80 °C was selected as the optimum temperature in this study. At 80 °C, the chelation of RhBLA and Cu²⁺ was complete within 15 min (see Fig. S5 in ESI†).

The selectivity of the RhBLA-functionalized Fe₃O₄ NPs

As is well known, there are various metallic ions existing in river water or tap water samples, and it is possible that the existence of these metallic ions will interfere with the detection of Cu²⁺. In order to investigate whether other metallic ions would disturb the detection of Cu²⁺, various metallic ions including Fe³⁺, Fe²⁺, Pb²⁺, Ni²⁺, Ca²⁺, Mn²⁺, Cr³⁺, Al³⁺, Zn²⁺, Ba²⁺, Mg²⁺ and Cu²⁺ were used to test the specificity of the RhBLA-functionalized Fe₃O₄ NPs and pure RhBLA, respectively. As the results show in Fig. 5 (RhBLA-functionalized Fe₃O₄ NPs) and Fig. S6 (pure RhBLA, in ESI†), only Cu²⁺ can chelate with the RhBLA-functionalized Fe₃O₄ NPs or pure RhBLA to induce the ring-opening of the rhodamine B spirolactam, which gives rise to a change in solution colour from colourless to pink and a strong adsorption peak at 562 nm. No colour change and only a negligible absorption at 562 nm were observed for the other metallic ions, even if their concentrations were higher than that of Cu²⁺, indicating that other metallic ions do not chelate with the RhBLA-functionalized Fe₃O₄ NPs or pure RhBLA and thus do not interfere with the detection of Cu²⁺. The experimental results above reveal that the proposed sensor, which is based on RhBLA-functionalized Fe₃O₄ NPs, has excellent specificity to Cu²⁺.

Fig. 5 The photographs of the re-dispersed RhBLA-functionalized Fe₃O₄ NP solutions in the presence of 200 nM Cu²⁺ or 40 μM of other metal ions.

The sensitivity of the proposed sensor and visual detection of Cu²⁺ in river water

The colour response of the RhBLA-functionalized Fe₃O₄ NPs to different concentrations of Cu²⁺ in 100 mL of sample solution has been investigated to test the sensitivity of the proposed sensor. As the results show in Fig. 6, upon the addition of Cu²⁺ from 0 to 500 nM, the colour of the re-dispersed RhBLA-functionalized Fe₃O₄ NP solution changed from colourless to deep pink step by step. When the concentration of Cu²⁺ was 500 nM, the colour of the re-dispersed RhBLA-functionalized Fe₃O₄ NP solution became deep brown, and we guess the reason is that excess Cu²⁺ not only chelated with RhBLA but also induced the slight aggregation of the RhBLA-functionalized Fe₃O₄ NPs. As the photographs in Fig. 6 show, the colour change could be clearly observed with the naked eye when the concentration of Cu²⁺ was 50 nM, i.e., the visual detection limit of Cu²⁺ was about 50 nM when 100 mL of sample was used for determination. The visual detection limit of 50 nM is much lower than that of pure RhBLA (see Fig. S7 in ESI†), indicating that the coupling of RhBLA to Fe₃O₄ NPs facilitates the magnetic separation of Cu²⁺ from the bulk sample solution and finally results in a higher sensitivity. The visual detection limit of our method is also much lower than the maximum allowable level of Cu²⁺ (∼20 μM) in drinking water defined by the USA EPA,¹⁴ indicating that our method meets the requirement of rapid and on-site detection of Cu²⁺ in the aqueous environment by only naked-eye observation. It must be mentioned that the visual detection limit of our method can be further reduced by using a greater volume of water sample.

Fig. 6 The photographs of the re-dispersed RhBLA-functionalized Fe₃O₄ NP solutions in the presence of different concentrations of Cu²⁺ and a river water sample.

To further confirm the applicability and reliability of the developed sensor, the Cu²⁺ ions in the river water samples were analyzed with our method. The river water was collected from Minjiang river of Fujian, China. The river water was first filtered through a 0.22 micrometer membrane filter to remove any sediment, and then the amount of Cu²⁺ in it was directly detected with our method by naked-eye observation. The results obtained were compared with the results obtained by ICP-MS, a standard method for Cu²⁺ determination. From the results shown in Fig. 6, the Cu²⁺ in the river water can be easily detected with our method by only naked-eye observation, with a concentration of about 50 nM. The concentration of Cu²⁺ in the river water detected with ICP-MS was 3.11 ppb (49.2 nM). The result obtained with our method was consistent with that of ICP-MS, indicating that the proposed sensor is reliable.

As we mentioned above, so far, several visual methods have been developed for the on-site detection of Cu²⁺.^16–26,28 However, most of these methods have lower sensitivity and poor resistibility to the matrix of the sample, which means these methods can not be used to detect trace Cu²⁺ in aqueous environments by only naked-eye observation. In comparison with these visual methods (see Table S1 in ESI†), the proposed method has a much higher sensitivity, much lower visual detection limit and stronger resistibility to the matrix of the sample. Not long ago, an ultrasensitive method, which was based on Fe₃O₄ NP-based solid phase extraction and functionalized gold nanoparticles, was developed for the visual detection of Cu²⁺.³⁸ In comparison with this method, our method has a similar sensitivity, more simple operation process and shorter analysis time. Especially, the method proposed in this study is a “turn-on” sensor, whereas the previous one is a “turn-off” sensor.³⁸ The success of this study provides a promising approach for the rapid and on-site detection of ultra-trace free copper ions in aqueous environments by only naked-eye observation.

Conclusions

In summary, we herein reported a novel spiro-rhodamine B lactam derivative (RhBLA), and further coupled RhBLA to Fe₃O₄ NPs to develop a sensitive and specific visual sensor for the rapid and on-site detection of ultra-trace Cu²⁺. We demonstrated that the RhBLA-functionalized Fe₃O₄ NPs can specifically chelate with Cu²⁺ at pH 6.0 to create a colour change from colourless to pink, which provides a sensing platform for the simple, rapid and field-portable visual detection of trace Cu²⁺. The proposed visual sensor, which is based on RhBLA-functionalized Fe₃O₄ NPs, has high sensitivity, excellent specificity and robust resistibility to the matrix of the sample. It can be used to directly detect as little as 50 nM of Cu²⁺ in river water by using only naked-eye observation when 100 mL of the water sample was used for detection. The visual detection limit of 50 nM is much lower than the maximum allowable level of Cu²⁺ (∼20 μM) in drinking water defined by the USA EPA. In addition, the method proposed in this study is simple, cost effective, rapid, and does not need any sophisticated instruments. The above features make our sensor a promising approach for the rapid and on-site detection of ultra-trace free copper ions in aqueous environments by only naked-eye observation.

Acknowledgements

The authors gratefully acknowledge the NSFC (21377025), Ministry of Education, Science and Technology Development Center (20123514110001), Fujian Provincial Department of Science and Technology (2014Y0042), Program for Changjiang Scholars and Innovative Research Team in University (no. IRT1116), and National Key Technologies R & D Program of China during the 12th Five-Year Plan Period (no. 2012BAD29B06) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04272k

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