Room-temperature phosphorescence by Mn-doped ZnS quantum dots hybrid with Fenton system for the selective detection of Fe²⁺

Abstract

The phosphorescent 3-mercaptopropionic acid (MPA) capped Mn-doped ZnS quantum dots (MPA–Mn:ZnS QDs)–Fenton hybrid system was developed for a highly sensitive detection of Fe²⁺ in environmental samples and biological fluids. The phosphorescence of the MPA–Mn:ZnS QDs can be effectively quenched by hydroxyl radicals produced from the Fenton reaction between Fe²⁺ and H₂O₂ at a low concentration level. However, the phosphorescence of the MPA–Mn:ZnS QDs cannot be quenched by either Fe²⁺ or H₂O₂ individually at the same concentration level. Thus, Fe²⁺ can be indirectly detected using the phosphorescence quenching caused by hydroxyl radicals based on the Fenton reaction. A possible mechanism for the quenching effect of Fe²⁺ was elucidated as electron transfer from the conduction band of the MPA–Mn:ZnS QDs to the unoccupied band of hydroxyl radicals. The phosphorescent hybrid system allowed a highly sensitive detection of Fe²⁺ in an aqueous solution with a wide linear range of 0.01–10 μM and a detection limit of 3 nM, and the precision for 11 replicate detections of 0.1 μM Fe²⁺ was 1.5% (relative standard deviation, RSD). The developed method was applied to determine Fe²⁺ in environmental samples and biological fluids with quantitative spike recoveries from 95% to 104%.

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Introduction

Many metals are considered essential trace elements and must be present in low concentrations in the human body for normal cellular function.^1–7 Iron is one of the most important elements and plays a central role in environmental and biological systems owing to the easy redox reaction between Fe²⁺ and Fe³⁺.^7–12 In environmental systems, Fe²⁺ is the most common form of iron in ground water and mineral water, and drinking mineral water is beneficial to the health of the human body.⁹ In biological systems, Fe²⁺ is predominantly high spin in aqueous biological environments, for example, ferrous heme can interact with molecular oxygen.^11–14 Thus, the discrimination and selective detection of Fe²⁺ has potential applications in the environment as well as in biological systems.

However, to date, many methods for the quantitative detecting of Fe³⁺ have been developed by colorimetric methods,¹³ atomic absorption spectrometry,¹⁵ voltammetry,¹⁶ and fluorescent probes,^10,17–21 little attention is paid to the detection of Fe²⁺ due to its instability. Recently, a colorimetric approach for selectively sensing Fe²⁺ ions was reported using CTAB-stabilized Au–Ag nanorods (CTAB–Au–Ag NRs) in the presence of poly(sodium 4-styrenesulfonate) (PSS).²² This method can be utilized without complicated pretreatment; however, most of the colorimetric probes suffer drawbacks due to poor sensitivity. Moreover, a fluorescent nanoprobe method was reported for the discrimination and sensitive detection of Fe²⁺ based on the fluorescence quenching of GSH–CdTe QDs.²³ Fluorescent probes have a high photoluminescence efficiency, but the CdTe QDs suffer drawbacks, such as potential toxicity from the toxic raw material Cd²⁺, which hinder their application as promising optical labels for sensing in biological fluids.^24,25 Thus, it is necessary to develop some environmentally friendly methods for the discrimination of different iron species and sensitive detection of Fe²⁺ in virtue of sensitivity and convenience.

Recently, room-temperature phosphorescence (RTP) detection has attracted considerable attention due to its many advantages, such as the wider gap between excitation and emission spectra, the longer emission lifetime, the anti-interference from autofluorescence and the scattering light of the matrix.^26,27 The selectivity is also enhanced because phosphorescence is less common than fluorescence.²⁸ It is widely employed for developing sensors with great success; thus, it has become a hot topic.^27,29–34 A series of RTP methods based on the phosphorescence properties of Mn-doped ZnS QDs has been reported for the detection of enoxacin in biological fluids,²⁷ glucose in serum samples,³⁰ acetone in natural water,³¹ heparin in human serum³³ and for the selective determination of catechol from organic isomers.²⁹ However, to our knowledge, QDs based RTP probes for the discrimination of different iron species and sensitive detection of Fe²⁺ at trace levels in environmental samples and biological fluids have not been reported before.

Herein, MPA–Mn:ZnS QDs–Fenton hybrid system is explored to develop a RTP method for the facile, sensitive, and selective detection of Fe²⁺ in environmental samples and biological fluids. First, different metal species (Fe²⁺ and Fe³⁺) were discriminated by the different quenching kinetics of MPA–Mn:ZnS QDs. Moreover, Fe²⁺ can be sensitively and indirectly detected using the phosphorescence quenching effect of enlargement caused by hydroxyl radicals, which are produced from the Fenton reaction between Fe²⁺ and H₂O₂ at low concentrations.^35,36 Likewise, the hydroxyl radicals lead to the phosphorescence quenching of the MPA–Mn:ZnS QDs due to electron transfer from the conduction band of the QDs to the unoccupied band of the hydroxyl radicals, allowing a highly selective and sensitive detection of Fe²⁺ in an aqueous solution with a detection limit of 3 nM.

Experimental section

Reagents

All the reagents used were at least of analytical grade. ZnSO₄·7H₂O, Mn(CH₃COO)₂·4H₂O, Na₂S·9H₂O, ascorbic acid, Tris–HCl buffer solution (10 mM Tris, pH 7.4) were purchased from Tianjin Guangfu Fine Chemical Research Institute. 3-Mercaptopropionic acid (MPA, 99%) was obtained from Beijing J&K Chemical Co., Ltd. H₂O₂ (30%) was obtained from Tianjin Benchmark Chemical Reagent Co., Ltd. Ultrapure water was obtained from the Wahaha company. Aqueous solutions of Fe²⁺, Fe³⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺, Cr³⁺ and Zn²⁺ were prepared from FeSO₄·7H₂O, FeCl₃·6H₂O, KCl, NaCl, CaCl₂·4H₂O, MgCl₂·6H₂O, AlCl₃·9H₂O, MnCl₂·4H₂O, CoCl₂·6H₂O, NiCl₂·6H₂O, CuCl₂·2H₂O, HgCl₂·6H₂O, Cr(NO₃)₃·9H₂O and ZnCl₂·7H₂O, respectively. These reagents were purchased from Tianjin Kewei Co., Ltd.

Apparatus

RTP measurements were performed on a Cary Eclipse Fluorescence spectrophotometer (Agilent Technologies) equipped with a plotter unit and a quartz cell (1 × 1 cm²) in the phosphorescence mode. The phosphorescent emission spectra were recorded in the wavelength range of 500–700 nm upon excitation at 316 nm. The slit widths for excitation and emission were 10 and 20 nm, respectively. The photomultiplier tube (PMT) voltage was set at 700 V. Absorption spectra were recorded on an Ocean Optics DH-2000-BAL UV-VIS-NIR LIGHTSOURCE. Fourier transform infrared (FT-IR) spectra (4000–400 cm⁻¹) were recorded using a NICOLET 6700 FT-IR spectroscope with KBr pellets. Transmission electron microscopy (TEM) was carried out using a Tecnai G2 F20 (FEI), which was operated at an accelerating voltage of 200 kV. The samples for TEM were obtained by drying sample droplets from a Tris–HCl (pH 7.4, 10 mM) dispersion onto a 300-mesh Cu grid coated with a carbon film. X-ray diffraction (XRD) spectra were collected on a Bruker D8 diffractometer at a scanning rate of 1° min⁻¹ in the 2θ range from 5° to 80°. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos Axis Ultra DLD spectrometer fitted with a monochromated Al KR X-ray source (hν = 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multichannel plate and delay line detector. The Zeta potential was measured using a Malvern Zetasizer Nano ZS (red badge) with a 633 nm He–Ne laser. Quantitative composition analysis of Fe²⁺ for seven samples was carried out on an X7 Series Inductively coupled plasma mass spectrometer (ICP-MS) (Thermo Electron Corporation).

Synthesis of Mn-doped ZnS QDs

Mn-doped ZnS QDs were synthesized in an aqueous solution using MPA as a stabilizer on the basis of a published procedure with minor modifications.²⁷ 50 mL of 0.04 M MPA, 5 mL of 0.1 M ZnSO₄, and 5 mL of 0.01 M Mn(CH₃COO)₂·4H₂O were added to a three-necked flask. The mixture was adjusted to pH 11 with 1 M NaOH and stirred under nitrogen at room temperature for 30 min. Then, 5 mL of 0.1 M Na₂S was quickly injected into the solution. After stirring for 20 min, the solution was aged at 50 °C under air for 2 h to form MPA–Mn:ZnS QDs. These QDs were precipitated with ethanol, separated by centrifuging, and dried in vacuum. The highly soluble MPA–Mn:ZnS QDs were obtained.

Phosphorescence experiments

For discrimination between Fe²⁺ and Fe³⁺, 150 μL of 200 mg L⁻¹ MPA–Mn:ZnS QDs, 1 mL of 0.1 M Tris–HCl buffer solution (pH 7.4), and 2 mL of 5 μM Fe²⁺ or Fe³⁺ standard solution were added to a 10 mL calibrated test tube. An Fe²⁺ solution was prepared from FeSO₄·7H₂O in ultrapure water just prior to use. The mixture was diluted to the desired volume with ultrapure water, mixed thoroughly, and immediately scanned using a Cary Eclipse Fluorescence spectrophotometer. The phosphorescence intensity was recorded every 1 min for a total time of 20 min to observe the quenching kinetics.

For the determination of Fe²⁺, 150 μL of 200 mg L⁻¹ MPA–Mn:ZnS QDs, 1 mL of 0.1 M Tris–HCl buffer solution (pH 7.4), 500 μL of 10 μM H₂O₂, and 2 mL of 0.5 μM Fe²⁺ standard solution or 2 mL of real samples (water samples or biological fluid samples) were added to a 10 mL calibrated test tube. An Fe²⁺ solution was prepared from FeSO₄·7H₂O in ultrapure water just prior to use. The mixture was diluted to the desired volume with ultrapure water, mixed thoroughly, and 10 minutes later scanned using a Cary Eclipse Fluorescence spectrophotometer. The phosphorescence spectra of the MPA–Mn:ZnS QDs were recorded upon excitation at 316 nm. The phosphorescence intensity at the maximum phosphorescence wavelength was used for quantification.

Environmental samples and biological fluid samples

Two tap water, three river water and two mineral water samples were collected locally. All the water samples were filtered through 0.22 μm filters, and analyzed immediately after sampling. For analysis, all the water samples were subjected to 5-fold dilution.

Fresh human urine and serum samples were collected from healthy young volunteers treated in the local hospital. Each urine sample was centrifuged at 10 000 rpm for 10 min to remove particulate matter and the supernatants were used and diluted 100 times with Tris–HCl buffer solution (pH 7.4) before analysis. Each serum sample was centrifuged at 14 000 rpm for 10 min to remove deposition to eliminate the possible interference of proteins in human serum and the supernatants were used and diluted 100 times with Tris–HCl buffer solution (pH 7.4) before analysis.

Results and discussion

Characterization of MPA–Mn:ZnS QDs

MPA–Mn:ZnS QDs were prepared based on a previously published method with minor modifications.²⁷ Fig. 1A shows that the phosphorescence intensity of the MPA–Mn:ZnS QDs gradually increased as its concentration increased, and these variables had a good linear relationship in a large concentration range of 1–13 mg L⁻¹ (Fig. S1†). This also indicated the efficient solvent dispersibility of MPA–Mn:ZnS QDs in an aqueous solution. The TEM image (Fig. 1B) of the prepared MPA–Mn:ZnS QDs showed that they were quasi-spherical shaped and had an almost uniform size with a diameter of about 3.5 nm. The crystal lattice is clearly visible in the inset of Fig. 1B. Further information on the structure of the as-prepared MPA–Mn:ZnS QDs was obtained from the selected area electron diffraction pattern (inset of Fig. 1C). The selected area electron diffraction pattern exhibits broad diffuse rings, which are typical of particles. The (111), (220) and (311) planes were indexed, confirming the cubic phase. This result agreed well with the powder XRD pattern (Fig. 1C). The presence of three characteristic diffraction peaks in the XRD pattern also demonstrated that the lattice structure of the MPA–Mn:ZnS QDs was close to a cubic zinc blende structure. The FT-IR spectra of the MPA–Mn:ZnS QDs and pure MPA were compared to examine the capping of MPA on the surface of Mn:ZnS QDs (Fig. 1D). It was observed that the sulfhydryl band at 2570 cm⁻¹ disappeared, the carboxyl group stretching band shifted from 1710 cm⁻¹ to 1545 cm⁻¹, and the free hydroxyl groups band of MPA at 3000 cm⁻¹ shifted to 3400 cm⁻¹. These results indicated that MPA had been successfully capped onto the surface of Mn:ZnS QDs by the sulfhydryl functionality.

Fig. 1 (A) The concentration-dependent phosphorescence spectra of MPA–Mn:ZnS QDs; (B) TEM image of MPA–Mn:ZnS QDs; (C) XRD pattern and selected area electron diffraction pattern of MPA–Mn:ZnS QDs; (D) the FT-IR spectra of MPA–Mn:ZnS QDs and pure MPA.

The phosphorescence characteristics of the as-prepared MPA–Mn:ZnS QDs are illustrated in Fig. 2. An absorption peak at about 300 nm appeared in the absorption spectrum of the MPA–Mn:ZnS QDs (Fig. 2, curve a). No phosphorescence was emitted from the MPA–Mn:ZnS QDs before aging (Fig. 2, curve c), but a strong phosphorescence emission was observed at 588 nm when the QDs were excited at 316 nm after aging at 50 °C for 2 h (Fig. 2, curve b). In addition, without Mn doping, the ZnS QDs did not show any phosphorescence emission.²⁷ The phosphorescence lifetime of the prepared MPA–Mn:ZnS QDs was evaluated to be 2 ms from the decay curve of the phosphorescence emission (inset of Fig. 2). The observed phosphorescence was attributed to the transition of Mn²⁺ from the triplet state (⁴T₁) to the ground state (⁶A₁).³⁷

Fig. 2 Absorption spectrum of MPA–Mn:ZnS QDs after aging at 50 °C (curve a) and RTP spectra of MPA–Mn:ZnS QDs after and before aging at 50 °C (curves b and c). The inset shows the decay curve of the phosphorescence lifetime of MPA–Mn:ZnS QDs aged at 50 °C.

The strategy for the RTP detection of Fe²⁺ based on the MPA–Mn:ZnS QDs–Fenton reaction hybrid system

It is well-known that Fe²⁺ has been found to react with H₂O₂ to produce extremely reactive hydroxyl radicals, the so-called Fenton reaction.^35,36,38 In this study, we have indirectly detected Fe²⁺ by investigating the quenching effect of enlargement caused by hydroxyl radicals on the phosphorescence of the MPA–Mn:ZnS QDs. To investigate the Fe²⁺-based Fenton reaction on the MPA–Mn:ZnS QDs, the concentrations of Fe²⁺ and H₂O₂ were set at very low levels to ensure that individual Fe²⁺ or H₂O₂ had a negligible influence on the phosphorescence intensity of the MPA–Mn:ZnS QDs (Fig. 3A, curve b and c). The generated hydroxyl radicals from the mixture of Fe²⁺ and H₂O₂ were observed to quench the phosphorescence of the MPA–Mn:ZnS QDs to a much larger extent than individual Fe²⁺ or H₂O₂ (Fig. 3A, curve d). However, Fe³⁺ and H₂O₂ could not produce the same phenomenon by the same operation (Fig. S2†).

Fig. 3 (A) Phosphorescence spectra of the MPA–Mn:ZnS QDs (3 mg L⁻¹): (a) in the absence of Fe²⁺ and H₂O₂; (b) 10 min after the addition of 0.1 μM Fe²⁺; (c) 10 min after the addition of 0.5 μM H₂O₂; (d) 10 min after the addition of 0.1 μM Fe²⁺ and 0.5 μM H₂O₂. (B) The phosphorescence quenching efficiencies of the MPA–Mn:ZnS logic gate under eight input-conditions. The concentrations of Fe²⁺ and Fe³⁺ were 0.1 μM and the concentration of H₂O₂ was 0.5 μM. All the measurements were carried out in Tris–HCl buffer solution (pH 7.4, 10 mM).

It was necessary to reduce the interference of Fe³⁺ before the detection of Fe²⁺ was possible. Interestingly, the phosphorescence quenching of the MPA–Mn:ZnS QDs exhibited different responses to Fe²⁺ and Fe³⁺. The time course of the phosphorescence of the MPA–Mn:ZnS QDs at high concentrations of Fe³⁺ or Fe²⁺ is illustrated in Fig. S3.† The phosphorescence was quenched by about 6.5% in 10 min by 1 μM Fe²⁺, and remained unchanged with further increasing reaction time, indicating that a quenching equilibrium had been reached. In contrast, the phosphorescence intensity did not decrease for 20 min after the addition of 1 μM Fe³⁺. Moreover, we detected the effect of capping agents for the Mn:ZnS QDs on the quenching behavior of Fe²⁺ and Fe³⁺ using thioglycolic acid (Fig. S4†). The results showed that the quenching behavior of Fe²⁺ and Fe³⁺ were not affected by different capping agents. It was also observed that Fe²⁺ was stable for at least 10 min. Thus, the interference from Fe³⁺ can be reduced as far as possible in the selective detection of Fe²⁺.

Molecular logic gates from Fe²⁺-based Fenton reaction on MPA–Mn:ZnS QDs

We designed a molecular logic gate, which used the concept of logic gates to validate the strategy for the selective detection of Fe²⁺ based on the MPA–Mn:ZnS QDs–Fenton reaction hybrid system. Molecular logic gates (AND, OR, and NAND) distinguish analytes through different signal outputs (particularly, optical responses) resulting from changing inputs.^39,40 The logic gate employed Fe³⁺, Fe²⁺ and H₂O₂ as the three inputs, and the phosphorescence quenching efficiency of the MPA–Mn:ZnS QDs was defined as the output. With respect to the inputs, the presence and absence of Fe³⁺, Fe²⁺ and H₂O₂ were defined as “1” and “0”, respectively. In addition, we defined the phosphorescence quenching and unchanged phosphorescence as outputs “1” and “0”, respectively. Fig. 3B shows the characteristics of the MPA–Mn:ZnS QDs logic gate for the phosphorescence responses under eight possible input conditions. It was found that there were only two input conditions, namely, (011) and (111), which would induce the phosphorescence quenching (output 1). Thus, the MPA–Mn:ZnS QDs–Fenton reaction hybrid system could be used to selectively detect Fe²⁺ because the phosphorescence quenching of the MPA–Mn:ZnS QDs exhibited different responses to Fe²⁺ and Fe³⁺.

Factors affecting the sensitivity of the MPA–Mn:ZnS QDs–Fenton reaction hybrid system for the RTP detection of Fe²⁺

Fig. 4A shows that the enhancement of the phosphorescence intensity of the MPA–Mn:ZnS QDs was proportional to the increase in the concentration of the MPA–Mn:ZnS QDs. Simultaneously, the quenching efficiency of the MPA–Mn:ZnS QDs in the presence of 0.1 μM Fe²⁺ and 0.5 μM H₂O₂ decreased rapidly when the concentration of the MPA–Mn:ZnS QDs was increased. Considering the phosphorescence intensity and the quenching efficiency, 3 mg L⁻¹ of MPA–Mn:ZnS QDs were used in the subsequent detection process.

Fig. 4 (A) Quenching efficiency in the presence of 0.1 μM Fe²⁺ and 0.5 μM H₂O₂ against the concentration of MPA–Mn:ZnS QDs, and the phosphorescence evolution with the concentration of MPA–Mn:ZnS QDs in the absence of Fe²⁺ and H₂O₂. (B) pH-dependent phosphorescence quenching efficiency of 3 mg L⁻¹ MPA–Mn:ZnS QDs in the presence of 0.1 μM Fe²⁺ and 0.5 μM H₂O₂. (C) Fe²⁺-concentration dependent phosphorescence quenching efficiency of 3 mg L⁻¹ MPA–Mn:ZnS QDs in the absence of H₂O₂. (D) H₂O₂-concentration dependent phosphorescence quenching efficiency of 3 mg L⁻¹ MPA–Mn:ZnS QDs in the presence of 0.1 μM Fe²⁺. All the measurements were carried out in Tris–HCl buffer solution (pH 7.4, 10 mM).

The effect of pH on the phosphorescence quenching efficiency of 3 mg L⁻¹ MPA–Mn:ZnS QDs was tested with 0.1 μM Fe²⁺ and 0.5 μM H₂O₂. Because the MPA–Mn:ZnS QDs were unstable in acidic media, pH values lower than 6 were not considered. In the studied pH range of 6.0–7.4, the quenching efficiency of the MPA–Mn:ZnS QDs gradually increased in the presence of Fe²⁺ and H₂O₂. In contrast, it gradually decreased in the studied pH range of 7.4–9.5 (Fig. 4B). To keep the MPA–Mn:ZnS QDs as stable as possible and to ensure a highly sensitive detection of Fe²⁺, Tris–HCl buffer solution (pH 7.4, 10 mM) was used.

The effect of Fe²⁺-concentration on the phosphorescence quenching efficiency of 3 mg L⁻¹ MPA–Mn:ZnS QDs was tested without H₂O₂ in Tris–HCl (pH 7.4, 10 mM, Fig. 4C). The concentration of Fe²⁺ was set at very low levels to ensure that individual Fe²⁺ had a negligible influence on the phosphorescence intensity of the MPA–Mn:ZnS QDs. Therefore, 0.1 μM Fe²⁺ solution was determined as the optimum detecting concentration.

The effect of H₂O₂-concentration on the phosphorescence quenching efficiency of 3 mg L⁻¹ MPA–Mn:ZnS QDs was tested with 0.1 μM Fe²⁺ in Tris–HCl (pH 7.4, 10 mM). There was no obvious change in the phosphorescence quenching efficiency of the MPA–Mn:ZnS QDs in the studied H₂O₂ concentration range of 0.1–10 μM (Fig. 4D). Concentrations of H₂O₂ higher than 10 μM were not tested because H₂O₂ can individually cause the quenching of MPA–Mn:ZnS QDs. Therefore, 0.5 μM H₂O₂ solution was employed in the detection process.

Selectivity of the MPA–Mn:ZnS QDs–Fenton reaction hybrid system for the RTP detection of Fe²⁺

To show the potential application of the MPA–Mn:ZnS QDs–Fenton hybrid system for detecting Fe²⁺, the phosphorescence responses of the MPA–Mn:ZnS QDs toward K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺, Zn²⁺, Ni²⁺, Mn²⁺, Co²⁺, Fe³⁺, Cu²⁺, Hg²⁺, Cr³⁺ and Fe²⁺ in the presence of 0.5 μM H₂O₂ were studied (Fig. S5†). The results showed that only Fe²⁺ had a significant phosphorescence quenching effect on the MPA–Mn:ZnS QDs, indicating the high selectivity of the MPA–Mn:ZnS QDs–Fenton hybrid system for the detection and specific recognition of Fe²⁺ in an aqueous solution.

Further experiments on the effect of various co-existing metal cations and some anions on the phosphorescence quenching of MPA–Mn:ZnS QDs (3 mg L⁻¹) by 0.1 μM Fe²⁺ in the presence of 0.5 μM H₂O₂ were carried out to show high anti-interference from other co-existing ions for detecting Fe²⁺ (Table 1). The phosphorescence quenching of MPA–Mn:ZnS QDs (3 mg L⁻¹) by 0.1 μM Fe²⁺ was unaffected (an error of ±3.0% in the relative phosphorescence intensity was considered tolerable) by 1 mM K⁺ and Na⁺, 400 μM Ca²⁺ and Mg²⁺, 5 μM Al³⁺, 1 μM Fe³⁺and Zn²⁺, 0.2 μM Mn²⁺and Cu²⁺, 0.1 μM Co²⁺, 0.05 μM Ni²⁺, 0.05 μM Hg²⁺ and Cr³⁺, 600 μM SO₄²⁻, 1 mM Cl⁻. The tolerant concentrations for transition metal ions were considerably lower than those for alkali and alkali-earth metal ions. For example, transition metal ions, such as Mn²⁺, Ni²⁺, and Co²⁺, can also participate in “Fenton-like” reactions, but their ability to induce the generation of hydroxyl radicals was considerably lower than that of Fe²⁺.⁴¹ As the average concentrations of these co-existing metal ions in a river water matrix did not exceed the tolerant concentrations (Table S1†),⁴² most potential interferences from these co-existing ions in river water samples can be eliminated by simple dilution for the detection of Fe²⁺.

Table 1 Effect of co-existing ions on the detection of 0.1 μM Fe²⁺ by the proposed QDs–Fenton hybrid system based phosphorescence probe

Metal ion	Concentration/μM	Quenched phosphorescence change/%
K^+	1000	+1.1
Na^+	1000	−2.4
Ca^2+	400	−0.7
Mg^2+	400	−0.4
Al^3+	5	−1.8
Zn^2+	1	+2.5
Ni^2+	0.05	+1.7
Mn^2+	0.2	+1.2
Co^2+	0.1	+1.9
Fe^3+	1	+2.3
Hg^2+	0.05	+2.3
Cr^3+	0.05	+1.6
Cu^2+	0.2	−2.4

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Figures of merit for the MPA–Mn:ZnS QDs–Fenton reaction hybrid system for the RTP detection of Fe²⁺

The phosphorescence quenching process can be described by the Stern–Volmer equation (Fig. 5):⁴³

P₀/P = 1 + K_SVC

where P₀ and P are the phosphorescence intensities of the MPA–Mn:ZnS QDs in the absence and presence of analyte, respectively, K_SV is the Stern–Volmer quenching constant, which is related to the quenching efficiency, and C is the concentration of analyte. Fig. 5A shows that the phosphorescence intensity of the MPA–Mn:ZnS QDs was gradually quenched as the concentration of Fe²⁺ increased, and Fig. 5B displays the Stern–Volmer plots for Fe²⁺. The P₀/P for MPA–Mn:ZnS QDs had a good linear relationship with the concentration of Fe²⁺ (R² = 0.991) in the concentration range of 0.01–10 μM Fe²⁺ (Fig. 5B). Likewise, the linear regression equation was P₀/P = 0.553C + 1.15 (where C is the concentration of Fe²⁺ in μM). The relative standard deviation (RSD) for 11 replicate detections of 0.1 μM Fe²⁺ was 1.5%, showing the high precision for the detection of Fe²⁺. Moreover, the MPA–Mn:ZnS QDs–Fenton hybrid system also displayed a low detection limit (3σ) of 3 nM for Fe²⁺, which was comparable to or better than those obtained by other measures for different iron species (Table 2).

Fig. 5 (A) Effect of the concentration of Fe²⁺ on the phosphorescence spectra of the MPA–Mn:ZnS QDs (3 mg L⁻¹) in the presence of 0.5 μM H₂O₂. (B) The Stern–Volmer plot for the phosphorescence quenching of the QDs by Fe²⁺ in the presence of 0.5 μM H₂O₂.

Table 2 Comparison of the analytical performance of the sensing systems for the detection of Fe²⁺

Materials	Linear range (μM)	Detection limits (nM)	Reference
CTAB–Au–Ag NRs	1–15	1000	22
CdTe QDs	0.01–1	5	23
CePO4:Tb^3+	0.003–2	2	44
MOF-253	5–100	500	45
DNA–GO	0.01–1	2.4	46
MPA–Mn:ZnS QDs	0.01–10	3	This work

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Possible mechanism of the MPA–Mn:ZnS QDs–Fenton reaction hybrid system for the RTP detection of Fe²⁺

It is well-known that Fe²⁺ has been found to react with H₂O₂ to produce extremely reactive hydroxyl radicals, the so-called Fenton reaction.^35,36,38

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ˙OH

The hydroxyl radical (˙OH) is a type of reactive oxygen species that has an extremely strong ability to capture electrons, and ˙OH is also an efficient optics quencher. To further confirm that the quenching effect of enlargement was caused by ˙OH, ascorbic acid (Vc) (a kind of free radical scavenger) was added before and after the interaction between the Fenton hybrid system and the MPA–Mn:ZnS QDs (Fig. S6†). First, the concentration of Vc was set at very low levels (0.5 μM) to ensure that Vc alone had a negligible influence on the phosphorescence intensity of the MPA–Mn:ZnS QDs (Fig. S6,† curve a). Then, the effect of the phosphorescence quenching of the MPA–Mn:ZnS QDs was observed to weaken gradually (Fig. S6,† curve b versus d). Therefore, it was inferred that the quenching effect for Fenton hybrid system on MPA–Mn:ZnS QDs came from ˙OH.

Moreover, ˙OH is an important active oxygen-containing species with a redox potential of 2.8 V (vs. standard hydrogen electrode, SHE) and has an extremely strong ability to capture electrons (Fig. S7†).⁴⁷ The redox potential of the MPA–Mn:ZnS QDs measured by zeta was −1.68 V. Thus, there may be an electron transfer from the conduction band of MPA–Mn:ZnS QDs to the unoccupied band of ˙OH.

To understand the quenching of the ˙OH mechanism better, two samples were prepared for analysis by XPS (Fig. S8†), curve a and curve b related to MPA–Mn:ZnS QDs in the absence and presence of Fe²⁺ (5 μM) and H₂O₂ (25 μM), respectively. The inspection of the S 2p spectral region indicated that after the addition of Fe²⁺ and H₂O₂, a new valence state of sulfur appeared on the surface (Fig. S8,† curve b) with S 2p binding energy of about 168 eV. The new peak was attributed to the S(VI) in sulfite.⁴⁸ Therefore, this may indicate that S on the surface of the MPA–Mn:ZnS QDs was partly oxidized to S(VI) because of the electron transfer from the conduction band of the MPA–Mn:ZnS QDs to the unoccupied band of ˙OH.

Application of the MPA–Mn:ZnS QDs–Fenton reaction hybrid system for the determination of Fe²⁺ in environmental samples and biological fluids

The high selectivity and sensitivity made the MPA–Mn:ZnS QDs–Fenton hybrid system promising for detecting Fe²⁺ in environmental samples and biological fluids. In environmental samples, two samples from the tap, three samples from local rivers and two bottled mineral water samples were collected for the detection. As shown in Table 3, the quantitative spike-recoveries for detecting Fe²⁺ in real water samples ranged from 95% to 104%. For biological fluids, two samples from fresh human urine and three samples from human serum were collected for the detection. As shown in Table 4, the quantitative spike-recoveries for detecting Fe²⁺ in biological fluids ranged from 95% to 102%. In addition, the analytical results for Fe²⁺ in environmental samples and biological fluids obtained by the proposed method were in good agreement with those obtained by a flow injection inductively coupled plasma mass spectrometry (ICP-MS) method.⁴⁹ The operating conditions for flow injection ICP-MS are shown in Table S2.† The abovementioned results demonstrate the accuracy of the QDs–Fenton hybrid system for the selective detection of Fe²⁺ in environmental samples and biological fluids.

Table 3 Analytical results for the determination of Fe²⁺ in environmental samples

Sample	Found by this method (mean ± σ, n = 3)/μM	Recovery^b (mean ± σ, n = 3)/%	Found by ICP-MS^49 (mean ± σ, n = 3)/μM
a nd: not detected.b For 0.20 μM Fe^2+ spiked in environmental samples.
Tap water 1	0.119 ± 0.013	97 ± 5	0.136 ± 0.028
Tap water 2	0.143 ± 0.040	102 ± 3	0.154 ± 0.033
River water 1	nd^a	99 ± 4	nd
River water 2	nd	95 ± 5	nd
River water 3	nd	104 ± 3	nd
Mineral water 1	nd	98 ± 3	nd
Mineral water 2	nd	96 ± 4	nd

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Table 4 Analytical results (mean ± σ, n = 3) for the determination of Fe²⁺ in biological fluids

Sample	Fe^2+ in samples/μM	Fe^2+ spiked/μM	Fe^2+ found/μM	Recovery/%
a nd: not detected.
Urine-1	nd^a	0.1	0.094 ± 0.004	95 ± 3
Urine-2	nd	0.5	0.496 ± 0.009	97 ± 4
Human serum-1	nd	0.1	0.095 ± 0.005	96 ± 4
Human serum-2	nd	0.5	0.509 ± 0.005	102 ± 5
Human serum-3	nd	1	0.985 ± 0.003	99 ± 2

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Conclusions

In this study, an MPA–Mn:ZnS QDs–Fenton hybrid system was developed for the highly sensitive determination of Fe²⁺ based on the phosphorescence quenching effect of enlargement caused by hydroxyl radicals, which were produced by the Fenton reaction. The phosphorescence quenching of the MPA–Mn:ZnS QDs exhibited different responses to Fe²⁺ and Fe³⁺, which were applied for discrimination of different iron species and the selective detection of Fe²⁺ at the nanomolar level. The Mn:ZnS QDs were nontoxic in comparison with some traditional QDs. Moreover, the hybrid system of Mn:ZnS QDs combined with the Fenton reaction presented a simple and feasible strategy to solve the discrimination and detection of different metal species.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (21375095, 20975054), the Tianjin Natural Science Foundation (12JCZDJC21700), the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (FANEDD-201023), the Program for Innovative Research Team in University of Tianjin (TD12-5038) and Program for young backbone talents in Tianjin (ZX110GG015).

Notes and references

1. D. J. Williams, C. Critchley, S. Pun, M. Chaliha and T. J. O'Hare, J. Agric. Food Chem., 2010, 58, 8512–8521.
2. H. H. Yin, H. Kuang, L. Q. Liu, L. G. Xu, W. Ma, L. B. Wang and C. L. Xu, ACS Appl. Mater. Interfaces, 2014, 6, 4752–4757.
3. S. P. Jia, M. M. Liang and L. H. Guo, J. Phys. Chem. B, 2008, 112, 4461–4464.
4. Y. M. Zhang, Y. Chen, Z. Q. Li, N. Li and Y. Liu, Bioorg. Med. Chem., 2010, 18, 1415–1420.
5. Y. H. Zhang, Y. M. Zhang, Y. Chen, Y. Yang and Y. Liu, Org. Chem. Front., 2014, 1, 355–360.
6. H. Kim, Y. J. Na, E. J. Song, K. B. Kim, J. M. Bae and C. Kim, RSC Adv., 2014, 4, 22463–22469.
7. E. L. Que, D. W. Domaille and C. J. Chang, Chem. Rev., 2008, 108, 1517–1549.
8. D. C. Crans, K. A. Woll, K. Prusinskas, M. D. Johnson and E. Norkus, Inorg. Chem., 2013, 52, 12262–12275.
9. S. T. Moin, T. S. Hofer, A. B. Pribil, B. R. Randolf and B. M. Rode, Inorg. Chem., 2010, 49, 5101–5106.
10. Z. Yang, M. Y. She, B. Yin, J. H. Cui, Y. Z. Zhang, W. Sun, J. L. Li and Z. Shi, J. Org. Chem., 2012, 77, 1143–1147.
11. D. J. Goss and E. C. Theil, Acc. Chem. Res., 2011, 44, 1320–1328.
12. T. L. Foster and J. P. Caradonna, J. Am. Chem. Soc., 2003, 125, 3678–3679.
13. S. P. Wu, Y. P. Chen and Y. M. Sung, Analyst, 2011, 136, 1887–1891.
14. D. J. R. Lane and D. R. Richardson, Free Radical Biol. Med., 2014, 75, 69–83.
15. J. E. T. Andersen, Analyst, 2005, 130, 385–390.
16. C. M. G. van den Berg, Anal. Chem., 2006, 78, 156–163.
17. R. K. Pathak, J. Dessingou, V. K. Hinge, A. G. Thawari, S. K. Basu and C. P. Rao, Anal. Chem., 2013, 85, 3707–3714.
18. C. X. Yang, H. B. Ren and X. P. Yan, Anal. Chem., 2013, 85, 7441–7446.
19. M. Wang, J. G. Wang, W. J. Xue and A. X. Wu, Dyes Pigm., 2013, 97, 475–480.
20. K. G. Qu, J. S. Wang, J. S. Ren and X. G. Qu, Chem.–Eur. J., 2013, 19, 7243–7249.
21. C. M. Wansapura, C. J. Seliskar and W. R. Heineman, Anal. Chem., 2007, 79, 5594–5600.
22. Y. F. Huang, Y. W. Lin and H. T. Chang, Langmuir, 2007, 23, 12777–12781.
23. P. Wu, Y. Li and X. P. Yan, Anal. Chem., 2009, 81, 6252–6257.
24. H. B. Ren, B. Y. Wu, J. T. Chen and X. P. Yan, Anal. Chem., 2011, 83, 8239–8244.
25. K. Ding, L. H. Jing, C. Y. Liu, Y. Hou and M. Y. Gao, Biomaterials, 2014, 35, 1608–1617.
26. I. Sánchez-Barragán, J. M. Costa-Fernández, M. Valledor, J. C. Campo and A. Sanz-Medel, Trends Anal. Chem., 2006, 25, 958–967.
27. Y. He, H. F. Wang and X. P. Yan, Anal. Chem., 2008, 80, 3832–3837.
28. J. M. Traviesa-Alvarez, I. Sánchez-Barragán, J. M. Costa-Fernández, R. Pereiro and A. Sanz-Medel, Analyst, 2007, 132, 218–223.
29. H. F. Wang, Y. Y. Wu and X. P. Yan, Anal. Chem., 2013, 85, 1920–1925.
30. P. Wu, Y. He, H. F. Wang and X. P. Yan, Anal. Chem., 2010, 82, 1427–1433.
31. E. Sotelo-Gonzalez, M. T. Fernandez-Argüelles, J. M. Costa-Fernandez and A. Sanz-Medel, Anal. Chim. Acta, 2012, 712, 120–126.
32. W. S. Zou, D. Sheng, X. Ge, J. Q. Qiao and H. Z. Lian, Anal. Chem., 2011, 83, 30–37.
33. H. Yan and H. F. Wang, Anal. Chem., 2011, 83, 8589–8595.
34. P. Wu, T. Zhao, S. L. Wang and X. D. Hou, Nanoscale, 2014, 6, 43–64.
35. X. Zeng and A. T. Lemley, J. Agric. Food Chem., 2009, 57, 3689–3694.
36. M. M. Liang, S. P. Jia, S. C. Zhu and L. H. Guo, Environ. Sci. Technol., 2008, 42, 635–639.
37. J. H. Chung, C. S. Ah and D. J. Jang, J. Phys. Chem. B, 2001, 105, 4128–4132.
38. H. J. H. Fenton, J. Chem. Soc., Trans., 1894, 65, 899–911.
39. Z. H. Qi, P. M. de Molina, W. Jiang, Q. Wang, K. Nowosinski, A. Schulz, M. Gradzielski and C. A. Schalley, Chem. Sci., 2012, 3, 2073–2082.
40. M. Shahid, T. McCarthy-Ward, J. Labram, S. Rossbauer, E. B. Domingo, S. E. Watkins, N. Stingelin, T. D. Anthopoulos and M. Heeney, Chem. Sci., 2012, 3, 181–185.
41. K. S. Kasprzak, Free Radical Biol. Med., 2002, 32, 958–967.
42. S. R. Taylor and S. M. McLennan, in The continental crust, its composition and evolution, Blackwell Scientific Publications, New York, 1999, pp. 15–16.
43. K. Sauer, H. Scheer and P. Sauer, Photochem. Photobiol., 1987, 46, 427–440.
44. H. Q. Chen and J. C. Ren, Analyst, 2012, 137, 1899–1903.
45. Y. Lu, B. Yan and J. L. Liu, Chem. Commun., 2014, 50, 9969–9972.
46. W. T. Huang, W. Y. Xie, Y. Shi, H. Q. Luo and N. B. Li, J. Mater. Chem., 2012, 22, 1477–1481.
47. S. K. Poznyak, D. V. Talapin, E. V. Shevchenko and H. Weller, Nano Lett., 2004, 4, 693–698.
48. Y. Zhang, Y. Li and X. P. Yan, Anal. Chem., 2009, 81, 5001–5007.
49. X. P. Yan, M. J. Hendry and R. Kerrich, Anal. Chem., 2000, 72, 1879–1884.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04026d

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