The luminescence inner filter effect of Mn²⁺-doped (ZnS)₂·octylamine inorganic/organic hybrid thin films and their sensor application for environmental contaminants

Abstract

The II–VI group/monoamine hybrid was a novel inorganic/organic (I/O) layered structure and it exhibited a strong quantum confinement effect and well-defined Mn²⁺-doped luminescence, which was imperative and valuable academically for its application. In this work, the pure and well-crystalline (ZnS:Mn)₂·oa (oa = octylamine) hybrid was prepared by solvothermal synthesis and its thin films were fabricated by the drop-casting method. The 5 at% Mn²⁺-doped hybrid exhibited a strong Mn²⁺ luminescence at 597 nm with a 300 nm excitation. The inner filter effect (IFE) for this luminescence was investigated and verified by detecting three selected environmental contaminant species: n-butyl xanthate (BX), crystal violet (CV), and reaction black 5 (RB5), which indicated that this hybrid thin film had a good luminescence IFE-based linear response for the μM species solution. Furthermore, the 597 nm luminescence remained intact for the solution with a pH from 4 to 10 and the hydrophobicity of the hybrid thin film was favorable for its rapid and multiple IFE-based detection. Therefore, this hybrid thin film is a competitive candidate for the new-generational fluorescence sensors to probe the occurrence of these environmental contaminants.

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1. Introduction

Nowadays, the development of novel fluorescence-based assay sensor devices has received intensive and continuing attention owing to the distinct advantages of inherent sensitivity, nondestruction, suitability for microscopic imaging, low-cost and structural diversity. There is an enormous demand for a luminescence-based sensing method in many fields such as environmental monitoring, industrial and food processing, biomedical technology, healthcare, and clinical analysis. The fluorescence inner filter effect (IFE) is a kind of fluorescence quench mechanism that originated from the absorption of the excitation and/or emission light of the fluorescent species by the absorbent species, which was usually treated as an annoying source of error in spectrofluorometry.^1,2 Oppositely, this effect can be designed as a fluorescence sensor method, in which an optical absorbent and a fluorophore are employed and the absorption band of the former, to some extent, overlaps with the excitation and/or emission bands of the latter. That is, the analytic absorption signal is converted to the fluorescence signal of the luminescent species within the fluorescence IFE. And the efficiency is related to the spectral overlap and optical absorption ability. Compared to the conventional fluorescence assays, including kinetic/static quench or nonradiative/resonance energy transfer (PET, EET, FRET) and charge transfer (TICT, ICT, MLCT), the IFE is simple and preferred, without needing the proximity at the nanoscale between an absorbent and a fluorophore. Since the pioneering work of Cary in 1984,³ IFE-based fluorescence sensors have witnessed a fast development to detect various chemical and biological analytes, ranging from metal ions/anions to proteases, and single nucleotide polymorphisms.^4–8

The inorganic/organic (I/O) hybrid compounds are a kind of composite with an inorganic and organic component combination at the atomic level, which have the advantages of inorganic rigidity, stability, a lightweight organic component, flexibility, processability and diversity.^9,10 Furthermore, they possess unique electronic/optical properties which are attributed to their individual constituents, and they have become new-generational functional materials and the research focus of materials science for more than 30 years.^11–14 Firstly reported in 2003, the II–VI group/amine hybrid compounds were a novel type of I/O hybrid layered structure, with a chemical formula that can be generalized as (ME)_p(L)_q (M = Zn, Cd, Mn; E = S, Se, or Te, p, q = 1 or 2, and L = aliphatic monoamines, diamines or hydrazine).¹⁵ Due to their structural uniformity and periodicity, the II–VI group/amine hybrid compounds are particularly attractive. In these hybrids, the valence electrons are quantum confined and so they perform the modified semiconductor properties in the inorganic II–VI group ME slabs, and this is linked to the organic amine spacing layers caused by the M–N bonding. And both layers are stacked alternately in an ordered periodic way to form the layered structure.¹⁵ Research of these hybrids has included studies on the intralayer structure,^14–18 thermal expansion behaviour,^19–21 pyrolysis properties,^22,23 energy band structure (by a DFT study),^24,25 photoelectric properties,^29–31 and applications have been found in white-light light emission diodes.^26–28 The room temperature UV optical absorption with a typical band edge exciton feature exhibited a large blueshift when compared with their II–VI semiconductor counterparts, which can be attributed to the strong quantum confinement effect (QCE)¹⁵ of the ME inorganic slabs, and the hybrid can be regarded as a periodic multiple quantum well (MQW) structure. Furthermore, the Mn²⁺ substitutional doping provided the hybrid with a strong Stokes luminescence (2.12 eV, 586 nm) at room temperature.¹⁶ It has been manifested that the luminescence intensity reached its maximum for the Mn²⁺ doped atomic percentage at 0.05. As for device application, the spin-coating method was reported for the fabrication of the inorganic/organic hybrid luminescent films.²⁹ The (ZnS:Mn)₂·monoamine I/O hybrid compounds have good Mn²⁺ fluorescence, which can be applied into novel analytic probes and fluorescence sensors. It is interesting and important to study the quench mechanism of Mn²⁺ luminescence by various species, which is not only vital for the optical properties of this kind of hybrid academically, but also fundamental for its sensor application.

In this paper, based on the luminescence IFE, three representative molecular species, n-butyl xanthate (BX), crystal violet (CV), and reactive black 5 (RB5) (see Scheme 1) were selected to test the IFE for the (Zn_0.95Mn_0.05S)₂·oa hybrid thin film with a strong and well-defined Mn²⁺ luminescence (λ_ex = 300 nm, λ_em = 597 nm), because BX, CV, and RB5, have a matched optical absorption for the excitation, emission or both maximums of the film’s fluorescence, respectively. BX is one of the important flotation reagents in metallurgy, and it has a pungent odour with a destructive impact on fish.³² CV is used as low-cost aquaculture fungicide in fisheries, which seriously affects food safety,³³ and RB5 is one of the typical azo dyes found in industrial waste water.³⁴ They are all water-soluble anions and are harmful to the water environment and the ecosystem. At present the most common detection methods of these three molecules are the ultraviolet absorption spectrophotometric method, the liquid chromatography method and the liquid chromatography–mass spectrometry method. All of these demand a lot of instrument use and there is an enormous demand for an efficient and simple analytical method to detect these three molecules.

Scheme 1 Three environmental contaminants for investigating the luminescence IFE of the hybrid thin film.

Firstly, the (Zn_0.95Mn_0.05S)₂·oa hybrid was prepared by solvothermal synthesis. Its fluorescence properties were characterized, which indicated a strong excitation peak at 300 nm and Mn²⁺ fluorescence at 597 nm, and the optimal fluorescence was obtained at the 5 at% Mn²⁺ doping content, which was employed as the luminescence probe for the three species. The linear relationship between the luminescence intensity and the concentration was established at the μM level.

2. Experimental

2.1 Reagents and materials

Octylamine (C₈H₁₉N) was purchased from Aladdin Chemical. Co. Ltd. Analytical grade Mn(Ac)₂·4H₂O, Zn(Ac)₂·2H₂O, S, C₂H₅OH and CH₃CO₂C₂H₅ were purchased from Beijing Chemical Co. Ltd. Potassium butyl xanthate (BX, C₅H₉OS₂K, 188.3527), Crystal violet (CV, C₂₅H₃₀N₃Cl·9H₂O, 407.98), and reactive black 5 (RB5, C₂₆H₂₁N₅O₁₉S₆Na₄, 991.8161), were purchased from J&K Chemical Co. Ltd. All of these reagents were used without further purification. Deionized water was used in all of the experimental processes.

2.2 Characterization

The X-ray diffraction patterns (XRD) of the films were recorded using a PANalytical X′ Pert PRO MPD under the following conditions: 40 kV, 40 mA, and Cu Kα radiation (λ = 1.541844 Å) with step scanning (0.0330°/2θ per step) in the range from 2 to 70° using a count time of 59.6900 s per step. The morphology and thickness of the thin films were investigated by using a scanning electron microscope (SEM, Hitachi S-4800) and the accelerating voltage applied was 20 kV. The UV-Vis absorption spectra were collected in the range from 260 to 700 nm on a Shimadzu UV-2450 spectrophotometer with the slit width of 1.0 nm. The steady state fluorescence excitation and emission spectra were performed on a Perkin-Elmer-LS55 fluorescence spectrophotometer, and both the excitation and emission slits were set to 6.0 nm, with the excitation at 300 nm. The fluorescence decay curves were recorded with the excitation at 300 nm from the Xe lamp.

2.3 Hybrid synthesis and thin film fabrication

2.3.1 Preparation of the hybrid compounds. (Zn_0.95Mn_0.05S)₂·oa was synthesized by the typical solvothermal method. Firstly, a solution of 35 mL octylamine (oa), 4.75 mmol Zn(Ac)₂·2H₂O, 0.25 mmol Mn(Ac)₂·2H₂O and 5 mmol sulfur powder was mixed and magnetically stirred, then it was transferred into a 50 mL Teflon-liner, which was put into the autoclave, sealed and heated at 170 °C for 5–7 days. After naturally cooling to room temperature, the off-white lamellar product was obtained by washing with deionized water and 90% absolute ethanol. The powdered product was dried at 60 °C in a vacuum for the next step.

2.3.2 Fabrication of the hybrid thin film. The quartz substrate for thin film fabrication was cleaned by several procedures, firstly it was soaked in aqua regia, then ultrasonically rinsed sequentially with deionized water, a H₂SO₄ : 30% H₂O₂ (3 : 1, v/v) mixture solution (40 min), and deionized water, respectively. Then the quartz plate was dipped and washed into boiling ethanol for 10 min, and dried in a vacuum oven for 3–10 h. The hybrid suspension (2 mg mL⁻¹) of the (Zn_0.95Mn_0.05S)₂·oa was obtained by ultrasonically dispersing in ethyl acetate for over 5 hours. The as-prepared hybrid suspension was dropped onto a neat treated quartz substrate with caution to ensure the suspension coated the whole substrate homogeneously, and this dropping was repeated at least three times, then the thin film was left to stand still in the air until the solvent evaporated completely.

2.4 Fluorescence detection assays of CV, RB5 and BX

Micromolar scale concentrations of the CV, RB5 and BX aqueous solution were prepared by dissolving them in deionized water, respectively. Then the hybrid thin films were inserted into a quartz cuvette with solutions of different concentrations. The fluorescence quenching of the thin film was recorded with fluorescence spectroscopy.

3. Results and discussion

3.1 Structure and morphology characterization

The XRD measurement was performed to check the periodic structure of the as-prepared hybrid powder and thin film. The XRD patterns of the as-prepared hybrid and its drop-casted films are shown in Fig. 1A, in which a typical layered structure could be revealed with the first three Bragg diffraction peaks at 2.84° (001), 5.68° (002) and 8.49° (003), respectively, corresponding to a periodic space of about 3.1 nm. This indicated that the hybrid powder and its thin film were pure and well crystalline. The 3.1 nm period implied that the double-layered octylamine alignment between the inorganic (ZnS)₂ layers had the nonpolar octyl groups point opposite each other, like a cell membrane, with oa having a molecular length of 1.19 nm, and this finding was similar to other hybrid homologs (such as, propylamine, butylamine and hexylamine).⁷ Moreover, the film fabrication process held this layered crystalline structure intact. The subtle difference at 26 to 32°/2θ in the XRD pattern between the powder and thin film (Fig. 1A inset) indicated that the thin film preferred an orientation with (001) crystalline plane packing in the substrate, and the other hk0 (h, k ≠ 0) Bragg peaks disappeared, which was typical for an inorganic layered structure like graphite. The top-view SEM image (Fig. 1B) showed that the hybrid powders had an irregular lump shape with a micrometre size. The top-view SEM image of the as-prepared thin film (Fig. 1C) showed that the thin film was continuous, homogenous with small hybrid nanoparticles, and the inset showed the film’s thickness was 5 μm. These results confirmed the hybrid thin film fabricated by the drop-casting method was structure-held, homogeneous and continuous. The as-prepared thin film on the quartz substrate exhibited a strong orange luminescence under UV illumination (Fig. 1D).

Fig. 1 The structure and morphology characterizations. (A) XRD of the hybrid powder (black profile) and the film (red profile), the inset is the enlarged section of 15–40° (2θ), (B) SEM images of the hybrid powder, (C) the micrograph shows the surface of the thin film and the inset is a cross section of the film (D) a photograph of the thin film under daylight and UV light.

3.2 Optical properties of the hybrid film and selective determined species

Fig. 2 shows that the hybrid film had a strong Mn²⁺ luminescence at 597 nm, with the maximum excitation at 300 nm. The three selective environmental contaminants BX, CV, and RB5 have a matched spectral overlap with the emission/excitation band of the hybrid thin film, meaning the luminescence IFE can be characterized. The absorption band of BX ranged from 275 nm to 325 nm with the maximum peak at 300 nm (ε₃₀₀ = 1.488 × 10⁴ M⁻¹ cm⁻¹), which was selected to absorb the excitation light of the film. In the CV absorption spectroscopy, the maximum absorbance at 582 nm (ε₅₈₂ = 5.689 × 10⁴ M⁻¹ cm⁻¹) absorbs the Mn²⁺ emission light and there is also relatively weak absorption at 300 nm (ε₃₀₀ = 1.524 × 10⁴ M⁻¹ cm⁻¹). As far as RB5 is concerned, it has two broad peaks with a similar intensity at 597 nm (ε₅₉₇ = 4.122 × 10⁴ M⁻¹ cm⁻¹) and 300 nm (ε₃₀₀ = 2.576 × 10⁴ M⁻¹ cm⁻¹), that absorbs both the emission and excitation light to a similar extent. Furthermore, it was found that the BX, CV, RB5 aqueous solution with a μM concentration absorbed the UV and visible light stably. Therefore, these three selected species represent the typical situation for investigating the luminescence IFE of the hybrid thin film.

Fig. 2 The photoluminescence excitation (dash black) and emission (solid black) spectra of the (Zn_0.95Mn_0.05)₂·oa hybrid film and the optical absorption spectra of the three selected environmental contaminants BX (red curve, 31.86 μM), CV (green curve, 7.35 μM) and RB5 (blue curve, 10.08 μM).

3.3 The luminescence IFE of the selected species

The relationship between the Mn²⁺ luminescence intensity of the hybrid thin film and the aqueous concentration of the three species was investigated for characterizing its IFE. Experimentally, this was realized by in situ detecting the luminescence intensity of the hybrid thin film when dipping it into the species solution at a certain concentration. Accordingly, the Mn²⁺ luminescence quench was observed to different extents, which could be attributed to the luminescence IFE of the three species to the (Zn_0.95Mn_0.05S)₂·oa hybrid thin film. In order to reveal the prominent luminescence quench, the Mn²⁺ luminescence intensity at 582 nm was used to detect CV, while 597 nm was selected to detect BX and RB5.

Fig. 3 shows the logarithm of the luminescence intensity (log I) vs. concentration (c) plots for the three selected species. The luminescence of the hybrid thin film fell gradually upon increasing the concentration of the BX, CV and RB5 aqueous solution. A good linear relationship of the logarithm of fluorescence intensity to concentration was found (R², 0.981 to 0.994, see Table 1). All of the linear fitting parameters for the three species are listed in Table 1. As shown in Table 1, the linear relationship held for different ranges for the three species, with the range of BX being the broadest (0–53.10 μM), RB5 in the middle (0–16.13 μM), and with CV (0–11.03 μM) having the narrowest range. This trend is consistent with the extinction coefficients in the order of BX (ε₃₀₀ = 1.488 × 10⁴ M⁻¹ cm⁻¹), RB5 (ε₅₉₇ = 4.122 × 10⁴ M⁻¹ cm⁻¹, ε₃₀₀ = 2.576 × 10⁴ M⁻¹ cm⁻¹), and CV (ε₅₈₂ = 5.689 × 10⁴ M⁻¹ cm⁻¹). That is, the luminescence quench is more efficient for the absorption of the emission light than that of the excitation light. The absorption for both resulted in a narrow linear range for RB5. Beyond the upper ranges, the luminescence intensity deviated from the linear relationship and could not be employed for the concentration detection of the three species. On the other hand, the absolute value of the slope (|a|) of the fitted straight lines also exhibited the same trends as that of the linear range (CV > RB5 > BX), which implied that this hybrid thin film has the best resolution for probing CV based on the luminescence IFE.

Fig. 3 The log I vs. concentration (c/μM, pH = 7) plot for the hybrid thin film.

Table 1 The linear fitting parameter of the log I vs. c plot in Fig. 3

	a/(×10^−2)^a	b^a	Linear range/μM	Molecular weight	R^2 ^a
a The fitting equation was log I = ac + b. R^2 is the correlation coefficient.
BX	−1.090	2.801	0–53.10	188.35	0.986
CV	−4.998	2.771	0–11.03	407.98	0.981
RB5	−3.570	2.858	0–16.13	991.82	0.994

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3.4 The Stern–Volmer plots of the Mn²⁺ luminescence of hybrid thin film

Luminescence quench is a typical character of the IFE. To further study the luminescence IFE of the hybrid thin film for the three species, the Stern–Volmer plots in their linear response ranges were illustrated. In Fig. 4, the (I₀/I − 1) was plotted against the concentration (c) and was fitted as a straight line. All of the fitting parameters are listed in Table 2. It could be found that for all of the three species, the linear fitting could be obtained in the identical linear response range of the log I − c plot. The K_SV values are calculated to be 52 320, 244 100 and 159 200 L mol⁻¹ for BX, CV and RB5, respectively, which indicate that they are good quenchers for the Mn²⁺ luminescence of the hybrid. The near zero value of b in the fitting also implied that the quench agreed with the theoretical S–V equation. The fitted K_SV of CV was the greatest, RB5 next and BX was the least, which is just the reverse to the linear response range of the log I − c plots. It is understandable that a smaller K_SV for the quencher gives a broader linear response range for detection.

Fig. 4 The Stern–Volmer plot of the Mn²⁺ luminescence in the hybrid thin film by the three species solution at pH = 7, I₀ and I are the initial and observed luminescence intensities, respectively.

Table 2 The Stern–Volmer plot parameters in Fig. 4

	KSV^a/(L mol^−1)	b^a	Linear range/μM	R^2 ^a
a The fitting equation was: I0/I − 1 = KSVc + b. R^2 is the correlation coefficient.
BX	5.232 × 10^4	−1.650 × 10^−1	0–53.10	0.980
CV	2.441 × 10^5	−7.341 × 10^−2	0–11.03	0.987
RB5	1.592 × 10^5	−2.002 × 10^−1	0–16.13	0.964

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3.5 The sensing repeatability of the hybrid thin film

As a fluorescence sensor, the repeatability was crucial for its application. Differing from the fluorescence probing method based on intermolecular interactions such as electron transfer or energy transfer, the IFE is a typical photophysical effect, free of the interaction between the fluorophore and the absorbents, and is favorable for its repeatable sensing implementation and device design. As shown in Fig. 5, this hybrid thin film showed good sensing repeatability for CV, the luminescence quench/recover cycle can be implemented up to 10 times without any obvious luminescence degradation. The quenched fluorescence will recover after being rinsed by deionized water. The hybrid thin film also exhibits the repeatable response to the BX and RB5 solutions (Fig. S1 and S2, in ESI†). In fact, another hybrid was also tried ((Zn_0.95Mn_0.05Se)₂·oa) and a thin film was fabricated for the IFE investigation, however, its unstable Mn²⁺ luminescence made it fail the repeatability measurement investigation and so it can not be used as an IFE-based sensor. As for the thermal stability of the luminescence, it can be found that the Mn²⁺ luminescence was stable below 80 °C, and can be used in the luminescence sensing of environmental pollutants (Fig. S3, in ESI†).

Fig. 5 The cycle repeatability of the Mn²⁺ luminescence in the hybrid thin film for probing the CV aqueous solution (4.43 μM).

Furthermore, this hybrid thin film had a hydrophobic surface due to the long octyl group of the oa molecules, and when dipped into the aqueous solution, the solution could not thoroughly wet the film surface. This surface hydrophobicity was favorable for the rapid luminescence recovery for the next detection based on the IFE, and this type of recovery is impossible for other fluorescence methods based on an intermolecular interaction mechanism (e.g. FRET). As far as the pH stability is concerned, the Mn²⁺ luminescence of the hybrid was unstable when the solution was too acidic or too alkaline. It was found that the 597 nm luminescence remains intact and pH-independent for the pH values that range from 4 to 10 (Fig. S4†). In an acidic solution (e.g. pH = 2), the hybrid will decompose to release ammonia, and in a strong alkaline solution (e.g. pH = 12), the intensity of the 597 nm luminescence will fall and an unknown peak will appear. Although there is this limit, it can be concluded that this hybrid thin film can probe various aqueous solution environments with a pH value ranging from 4 to 10.

4. Conclusions

In summary, the pure and well-crystalline (Zn_0.95Mn_0.05S)₂·oa (oa = octylamine) I/O hybrid was prepared by solvothermal synthesis at 170 °C for 5–7 days, grey hybrid sheets with a micrometer size were obtained, and its thin films were fabricated by the drop-casting method. The thin film was orientated and it was normal for all of the hybrid sheets to be smooth and continuous, as confirmed by the SEM observation. Its fluorescence properties were characterized, which indicated a strong absorption at 300 nm and Mn²⁺ fluorescence at 597 nm, and the optimal fluorescence was obtained for the 5 at% Mn²⁺ doping content. The luminescence inner filter effect (IFE) of the thin film was verified and investigated. The hybrid thin film was sensitive enough to monitor the concentration of three selected environmental contaminants. The log I − c and Stern–Volmer plots showed the linear relationship at the μM scale with the broadest range for BX detection (0–53.10 μM) and BX had the smallest K_SV (52 320 L mol⁻¹). Moreover, this hybrid thin film had a hydrophobic surface and this was favorable for the rapid luminescence recovery in multiple IFE-based detections. The Mn²⁺ luminescence at 597 nm for detection shows good cycle repeatability for up to 10 times and this luminescence was stable when detecting the aqueous solution with a pH ranging from 4 to 10, which is the case for ordinary environmental water samples.

This work indicated that this (Zn_0.95Mn_0.05S)₂·oa hybrid thin film can serve as a novel and feasible IFE fluorescence sensor for probing the occurrence of these selected environmental contaminant species at the μM scale, and it is also suitable for probing other species meeting the requirement of the IFE. The hybrid thin film has a low cost, good photo/chemical stability and is easy to prepare. The fluorescent probe based on the IFE provides the obvious advantages of simplicity, convenience, and rapid implementation and thus has potential application for detection in environmental analysis. However, there are still some problems that need further study including the luminescence quench mechanism, the IFE for other hybrids and for excitation and emission, and so on.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities of China.

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Footnote

† Electronic supplementary information (ESI) available: The cycle reversibility of the Mn²⁺ luminescence of the hybrid thin film responding to the BX (23.12 μM) and RB5 (8.739 μM) aqueous solution, and the luminescence spectra of the hybrid thin film dipping into the blank aqueous solution with pH = 4 to 10. See DOI: 10.1039/c5ra08518g

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