Naked eye colorimetric multifunctional sensing of nitrobenzene, Cr(VI) and Fe(III) with a new green emission Ag6S6 multi-metal-cluster

  • Rui-Sha Zhou
  • Zhi-Zhu Lin
  • Li-Dong Xin
  • Jiang-Feng Song
  • Hu Liu
  • Zhanhu Guo
Original Research


With the increasing need of detecting chemical pollutants, modern analytical instruments show many advantages but also have some common drawbacks such as high operational cost and tedious pretreatment procedures. Thus, developing a fast, simple, and convenient colorimetric sensing system is a challenge. Here, a new Ag6S6 cluster-based coordination compound formulated as Ag6(dmpymt)6 (1) (Hdmpymt = 4,6-dimethylpyrimidine-2-thione) was synthesized under solvothermal condition and displayed apparently green luminescence emission and good stabilities in water and some organic solvents. Fluorescence experiments illustrated that 1 displayed efficiently naked eye colorimetric sensing for nitrobenzene (NB), Cr(VI), and Fe(III) ions by fluorescence quenching in the emulsions. The possible sensing mechanisms are attributed to the competitive absorption of excitation wavelength energy between the analytes and the compound 1. Furthermore, the fluorescent test papers of 1 were prepared as well and showed efficient, convenient, and easily recycled characteristics, presenting a potential sensing application for the environmental concerns.

Graphical abstract

A new Ag6S6 multi-metal-cluster-based luminescent test papers display efficiently naked eye colorimetric sensing of nitrobenzene, Cr(VI) and Fe(III).


Organic sulfur Luminescence Colorimetric sensing Quenching mechanisms 

1 Introduction

With the rapid development of industrialization and urbanization, environmental pollution with hazardous materials such as nitrobenzene (NB) and heavy-metal ions has increased dramatically. NB is an important raw material and widely used in the products of pesticides, explosives, dyes, and pharmaceuticals [1, 2, 3, 4, 5]. Even at very low concentration, NB can cause nausea, shortness of breath, and vomiting problem as well as its carcinogenic and mutagenic natures, thus NB has been listed as a priority pollutant by the US Environmental Protection Agency (EPA) [2, 3]. Chromium, with three thermodynamically stable forms, i.e., Cr, Cr(III), and Cr(VI), is widely used in industry fields such as dye production, leather tanning, wood preservation, and chrome plating. Notably, only Cr(VI), rather than Cr or Cr(III), is known to be toxic and carcinogenic, causing health problems such as liver damage, pulmonary congestions, vomiting, and severe diarrhea [4, 5, 6]. Furthermore, Cr(VI) is water soluble in the full pH range and the most probable species are Cr2O72− and CrO42−. Typically, Fe(III) plays an important role in numerous biological systems such as oxygen transport, electron transfer processes, and regulation of body temperature [7]. However, excessive Fe(III) easily harms the nucleic acids and proteins [8, 9]. Therefore, it is necessary to develop a highly efficient multi-functional detection method regarding hazardous pollutants for the human health and environmental safety.

So far, numerous methods have been developed for the determination of hazardous pollutants such as chromatographic [10], electrochemical [11], microbial [12], chemical, and spectrophotometric methods [13]. All of which depend on modern analytical instruments. However, these instruments have some common drawbacks such as high operational cost, tedious pretreatment procedures, and cumbersome portability issues during the in-field usage. Thus, developing an efficient, fast-response, convenient, and low-cost approach to detect hazardous pollutants is a challenging work. Recently, fluorescent sensors based on coordination compounds for the detection of hazardous pollutants have attracted more attention owing to their not only fascinating and tunable structures but also high sensitivity, low cost, simplicity, portability, and short response time. Some pioneering works regarding luminescent MOF-based sensors mainly focused on the mono-functional or dual-functional detection [14, 15, 16, 17], however, few examples engaged in multifunctional sensing of NB, Cr(VI), and Fe(III). Recently, we have successfully accomplished the colorimetric sensing of NB and Fe(III) through fluorescence quenching of the solvent-emulsions [18, 19, 20, 21], however, the tedious preparation procedures of the solvent-emulsions and the recyclability of the fluorescent samples prohibit the real applications.

In this paper, a simple and convenient colorimetric sensing system is reported with a new Ag6S6 multi-metal-clusters possessing green emission, Ag6(dmpymt)6 (1) (Hdmpymt = 4,6-dimethylpyrimidine-2-thione). Compound 1 was fully characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, thermal analysis, and powder X-ray diffraction. The luminescent properties of 1 were investigated in the solid state and solvent-emulsions, displaying naked eye colorimetric multifunctional detection of NB, Fe(III), and Cr(VI) through fluorescence quenching, and the possible quenching mechanisms were proposed. The fluorescent test papers were prepared based on 1 with efficient, convenient, and easily recycled chemosensors, presenting a promising approach for the detection of NB, Fe(III), and Cr(VI) ions.

2 Experimental

2.1 Materials

Hdmpymt was obtained from Alfa Aesar Chemical Company. Metal salts and organic reagents were all purchased from Sinopharm Chemical Reagent Co., Shanghai, China. All reagents and solvents for the synthesis were commercially available and used as received without any further purification.

2.2 Preparation of Ag6(dmpymt)6 (1)

A mixture of AgNO3 (3.4 mg, 0.02 mmol), KCl (1.5 mg, 0.02 mmol), and Hdmpymt (2.8 mg, 0.02 mmol) was dissolved in 6 mL mixed solvents DMF/CH3CN/CH3OH (v:v:v = 1:2:3). The abovementioned mixture, to which ethylenediamine (0.1 mL, 1 mol L−1) was added, was transferred into a Teflon-lined autoclave and kept under autogenous pressure at 80 °C for 3 days. After slowly cooling down to room temperature, brown crystals of 1 were obtained (yield 41%, based on Hdmpymt). Elemental analysis. Calcd C36H42Ag6N12S6 (1482.46): C, 29.14; H, 2.83; and N, 11.33. Found: C, 29.19; H, 2.87; and N, 11.28. IR data (KBr, cm−1): 1575 (s); 1519 (s); 1423 (s); 1379 (m); 1334 (m); 1242 (s); 1093 (w); 1047 (w); 883 (m); 833 (m); and 761 (m).

2.3 Physical measurements

Elemental analysis (C, H, and N) were performed on a Perkin Elmer 240C elemental analyzer. Infrared (IR) spectra were obtained with KBr pellets on a Perkin Elmer Spectrum One FT-IR spectrometer in the range of 400–4000 cm−1. Powder X-ray diffraction (PXRD) patterns of the samples were recorded using a RIGAKU-DMAX2500 X-ray diffractometer with Cu-Kα radiation (λ = 1.542 Å) with a scanning rate of 10°/min and a step size of 0.02°. Thermal gravimetric analysis (TGA) was performed on a Perkin-Elmer TGA-7000 thermogravimetric analyzer with a heating rate of 10 °C min−1.

The solvent emulsion and solid fluorescence spectra of the compound were obtained on a HITACHI F-2700 fluorescence spectrophotometer at room temperature.

2.4 Single-crystal structure determination

Crystal structures were determined by single-crystal X-ray diffraction. Reflection data were collected on a Bruker SMART CCD area-detector diffractometer (Mo-Kα radiation, graphite monochromator) at room temperature with ω-scan mode. Empirical adsorption correction was applied to all data using SADABS. The structure was solved by direct methods and refined by full-matrix least squares on F2 using SHELXTL 97 software. Non-hydrogen atoms were refined anisotropically. All C-bound H atoms were refined using a riding model with Uiso(H) = 1.2. All calculations were carried out using SHELXTL 97. The crystallographic data and pertinent information are given in Table 1; the selected bond lengths and angles in Table S1. The CCDC number of compound 1 is 1,568,549.
Table 1

Crystal data and structure refinement information for 1



Empirical formula


Formula weight


Crystal system


Space group




















Absorption coef.


Reflns collected


Unique reflns (Rint)






R1, wR2 [I > 2σ(I)]a

0.0314, 0.0746

R1, wR2 (all data)

0.0364, 0.0774

aR1 = Σ||Fo| – |Fc||/Σ|Fo|; wR2 = [Σw(Fo2 – Fc2)2/Σw(Fo2)2]1/2

3 Results and discussion

3.1 Crystal structure of compound 1

Compound 1 crystallizing in the trigonal space group, R-3c, is a distorted octahedral core of six silver atoms (Fig. 1a). The asymmetric unit consists of one Ag(I) ion and one dmpymt anion. The Ag(I) ion with a triangular planar geometry is coordinated by two thiolate sulfur atoms and one pyrimidine nitrogen from three independent dmpymt anions with Ag–S distances of 2.229(2)–2.269(2) Å and Ag–N distances of 2.013(7)–2.049(7) Å. The sulfur atom of dmpymt bridges two silver atoms and each Ag(I) is bound by two sulfur atoms, resulting in the formation of a staggered Ag3S3 ring. Two nearly parallel Ag3S3 rings are interconnected into a distorted Ag6S6 cluster with an octahedral geometry by Ag–S bonds and the shortest Ag–Ag distance is 2.83 Å (Table S1), shorter than the sum of the van der Waals radii (3.4 Å) of Ag(I) centers [22, 23, 24, 25], indicating strong Ag–Ag interactions within the Ag6S6 cluster. Six Ag ions and six pyrimidine units are interconnected by Ag–N and Ag–S bonds into a neutral AgI6(dmpymt)6 molecule.
Fig. 1

a The Ag6(dmpymt)6 unit in compound 1. b Powder XRD of compound 1. Simulated and the samples after dispersing in H2O, DMF, NB, K2CrO4, K2Cr2O7, and Fe(NO3)3 aqueous solution, respectively

3.2 Characterization

The experimental and simulated PXRD patterns of 1 are shown in Fig. 1b. The simulated PXRD patterns from the single-crystal X-ray diffraction data are in good agreement with the observed ones, indicating the phase purity of these crystalline products. Different intensities between the simulated and experimental patterns may be caused by the preferred orientation of the powder samples. In order to evaluate the stability of compound 1 for the solvents and water, the samples of compound 1 (30 mg) were immersed in water and N, N-dimethylformamide (DMF), respectively, then treated by ultrasonication for 30 min and kept for 24 h. Samples of 1 were collected by filtration, washed with EtOH and dried at room temperature, and then characterized by PXRD. The PXRD of compound 1 in the DMF and H2O matched well with the simulated patterns, demonstrating high stability of 1 under these conditions (Fig. 1b).

Figure S1 shows the FT-IR spectra of 1 and Hdmpymt. In compound 1, characteristic peaks at 1575 and 1519 cm−1 are attributed to the symmetric stretching of C=C and C=N bonds, and the characteristic peak around 1334 cm−1 corresponds to the C=C and C=N asymmetric stretching. The relatively intense band at 1093 and 1047 cm−1 is in the typical region for C–S bonds in compound 1 [22, 23]. Additionally, to investigate the thermal stability of compound 1, its thermal behavior was studied from 30 to 750 °C under nitrogen atmosphere. The TGA curve displayed compound 1 was stable until 268 °C (Fig. S2), the weight loss corresponded to the combustion of pyrimidine-2-thione ranging from 268 to 750 °C.

3.3 Solid state fluorescence property

The Solid state photoluminescence of compound 1 was performed at room temperature (Fig. 2). The luminescence spectrum of compound 1 exhibits green light emission with the maximum at about 515 nm when excited at 369 nm. The emission band of 1 may be attributed to ligand-to-metal charge transfer (LMCT) and d-s transitions by Ag–Ag interactions within Ag6 clusters [26, 27, 28]. Interestingly, the green light emission of compound 1 is apparently different from the red light emission of Ag6(bmt)6·6THF (Hbmt = 2-benzimidazolethiol, THF = tetrahydrofuran) [22]. The difference may be due to different Ag–Ag distances and the organic sulfur ligands (the shortest Ag–Ag distance is 2.83 Å in compound 1, however, the shortest distance is 3.15 Å in Ag6(bmt)6·6THF). Furthermore, the fluorescence decay of compound 1 in the solid state at ambient temperature is fitted into double-exponential decay laws with the following formula: I = A1 exp.(t/τ1) + A2 exp.(t/τ2), where τ1 and τ2 are defined as the fast and slow components of the luminescence lifetimes, while A1 and A2 denote the pre-exponential factors. The fitted fluorescence lifetimes τ1 and τ2 are 6.97 μs (43.49%) and 2.15 μs (56.51%) for compound 1. As a result, the average decay times (τ*) may be determined by the equation as follows: τ* = (A1τ12 + A2τ22)/(A1τ1 + A2τ2), giving the corresponding average lifetimes of 5.59 μs (Fig. 2b), and the absolute quantum yields of compound 1 was 49.98%. The solid samples of compound 1 were irradiated for 30 min by ultraviolet lamp (369 nm), and the corresponding fluorescence intensities with time were almost unchanged (Fig. S3), indicating the high fluorescence stability of Ag6S6 multi-metal-clusters.
Fig. 2

a The solid excitation (black) and emission (red) spectra of compound 1. Insets are the photoimages of compound 1 under daylight (left) and UV illumination (right) at 365 nm. b Luminescence decay curves of compound 1

3.4 Sensing of organic solvent molecules

The apparent green luminescence of compound 1 inspired us to explore the application for sensing organic solvent molecules. Solvent emulsions of compound 1 were prepared as follows: 0.5 mg samples were dispersed into 4.00 mL acetone, acetonitrile, benzene, methanal, H2O, methanol, ethanol, n-butyl alcohol, glycol, N, N-dimethylformamide (DMF), dimethylacetamide (DMA), tetrahydrofuran (THF), trichloromethane (CHCl3), dichloromethane (CH2Cl2), and nitrobenzene (NB), respectively, and treated by ultrasonication for approximately 30 min; then, their fluorescence spectra were measured.

Figure 3 shows the fluorescence spectra of compound 1 (λex = 369 nm) dispersed in different solvents. The maximum emission peak positions and shapes for all the 1-solvent emulsions are consistent with the solid-state luminescence spectrum; however, the intensities are largely dependent on the solvent molecules, which are gradually decreased in the order: benzene > DMF > H2O > THF > n-butylalcohol > DMA > Glycol > ethanol > methanal > CHCl3 > CH2Cl2 > acetonitrile > methanol > acetone > NB (Fig. 3a and Fig. S4). Obviously, 1-benzene emulsion exhibits the strongest fluorescence emission; however, NB is the most effective quenchers for compound 1, resulting in a nearly 100% fluorescence quenching. The emissive visible green light from 1-NB emulsions is darker than that from the other emulsions under UV light (Fig. 3b). Such solvent-dependent luminescence quenching properties of compound 1 might be a promising luminescent probe for detecting NB molecules.
Fig. 3

a Emission spectra. b The luminescence photographs of compound 1 in different solvents under UV light. c The fluorescence titration dispersed in DMF by gradual addition of NB. d The fluorescence intensity vs NB concentration in compound 1

To explore the ability of 1 to sense NB molecules, fluorescence-quenching gradient tests were performed with an incremental addition of NB (0.1 M) to 1-DMF emulsion. The experiment was as follows: 0.5 mg samples of compound 1 were dispersed in 4 mL DMF as a standard emulsion, then NB was gradually added to the standard emulsion, and the corresponding changes in the fluorescence intensity of 1-DMF emulsion were monitored. As shown in Fig. 3c, sensitive and high fluorescence quenching was observed upon increasing the NB concentration. The fluorescence quenching percentages (QP) can be calculated using Eq. (1) [29]:
$$ {Q}_{\mathrm{P}}=\left(1-I/{I}_0\right)\times 100\% $$
where I0 is the initial fluorescence intensity of the emulsion without NB molecules, and I is the fluorescence intensity of 1-DMF emulsion with NB molecules. When the concentration of NB increased to 0.50 mM, the QP value was 85.14%, and the fluorescence intensity was almost completely quenched with a QP of 99.24% at NB content of 8.26 mM. Noticeably, the fluorescence decrease of 1-DMF emulsion was nearly proportional to the NB concentration, which was well fitted with a first-order exponential decay (Fig. 3d), indicating that the fluorescence quenching behavior for NB molecules was a diffusion-controlled process [30]. According to 3δ/K equation (where δ is the standard deviation and K is the slope of the fitting curve of the luminescence intensity of 1 at different analyte concentration) [31], the detection limit of NB was calculated to be 1.66 × 10−6 M (Fig. S5), which is comparable to the sensitivity of [Tb2(H2L)3(H2O)2]·21H2O (4.2 × 10−7 M) [32]. The abovementioned result revealed that compound 1 could be a promising luminescent probe for sensing NB molecules.

To analyze the mechanism of luminescent quenching by NB, the PXRD pattern of 1 after immersion in NB for 24 h was measured (Fig. 1b) and was in good agreement with the simulated pattern obtained from single-crystal structure analysis, indicating that the framework of 1 was maintained intact in NB, so the quenching was not caused by the framework collapse. In order to further investigate the quenching mechanism, the UV-Vis spectra of NB and other solvents were measured (Fig. S6). The results reveal that only NB has a strong absorption ranging from 200 to 440 nm, while other solvents have no significant absorption in this range. Notably, the excitation wavelength of the compound 1 is 369 nm, which is completely overlapped by the absorbing band of NB. So, the efficient quenching of NB in this system might be ascribed to a competition for the excitation energy between the emission band of the fluorophore and the absorption band of the analyte [33]. Moreover, the luminescence quenching behavior for NB may be due to the electron transfer from the electron-donating ligands to the electron-deficient NB molecules [34, 35].

3.5 Anion detection

In general, water pollution is a global environmental issue, and considerable researches have been paid to the sensing and removal of pollutants from waste water. The high stabilities in various solvents prompted us to investigate the sensing of toxic anions in water such as Cr2O72− and CrO42−. Compound 1 was applied as luminescent probes as follows: 0.5-mg samples of compound 1 was dispersed in 4-mL aqueous solutions containing the same concentration of various anionic species as their corresponding potassium salts (KmX, 1 × 10−2 mol∙L−1, X = F, Cl, Br, I, SCN, IO3, BF4, Ac, NO3, SO42−, C2O42−, S2O82−, CrO42−, Cr2O72−, PO43−), after treatment by ultrasonication for approximately 30 min, the corresponding fluorescence spectra were measured (Fig. 4). Apparently, the luminescence intensities of the emulsions are associated with the anion types, i.e., C2O42−, SCN, and IO3-, have fluorescence enhancement effect, whereas others show varying degrees of luminescence quenching compared with the control groups without any anions. Significantly, CrO42− or Cr2O72− ions were the most effective quenchers for 1-H2O emulsion and resulted in about 100% fluorescence quenching (Fig. 4a; Fig. S7). The remarkable quenching behavior may be used to easily detect CrO42− and Cr2O72− anions by naked eye colorimetric observation, since the emissive visible green light from the 1-H2O solution containing CrO42− or Cr2O72− is obviously darker than that from the emulsions in the presence of other tested anions (Fig. 4b). The results reveal that compound 1 might be promising luminescent probes for detecting CrO42− and Cr2O72−.
Fig. 4

a Emission spectra. b The luminescence photographs of 1-H2O emulsions containing different anions under UV light. c The fluorescence titration of 1-H2O emulsions with the addition of different concentrations of Cr2O72−. d Stern–Volmer plot for the luminescence intensity of 1 upon addition of Cr2O72− solution in water. The insert is Stern–Volmer plot at low Cr2O72− concentrations

To further investigate the sensing sensitivity towards CrO42− and Cr2O72− ions, the titration experiments of CrO42− and Cr2O72− ions have been carried out. Compound 1 was dispersed in aqueous solution with gradually increasing the CrO42− or Cr2O72− concentration. The luminescence intensity of 1-H2O emulsion was observed to heavily depend on the CrO42− or Cr2O72− concentrations (Fig. 4c; Fig. S8a). The fluorescence quenching efficiency was further analyzed using the Stern–Volmer (SV) equation as shown in Eq. (2) [36]:
$$ \left({I}_0/I\right)={\mathrm{K}}_{\mathrm{SV}}\ \left[\mathrm{M}\right]+1 $$
where I0 is the initial fluorescence intensity before the addition of CrO42− or Cr2O72−, I is the fluorescence intensity in the presence of CrO42− or Cr2O72−, [M] is the molar concentration of CrO42− or Cr2O72−, and KSV is the quenching constant (M−1). The SV plot for CrO42− or Cr2O72− was nearly linear at low concentrations and subsequently deviated from linearity, bending upwards at higher concentrations (Fig. 4d; Fig. S8b). The KSV values of compound 1 for CrO42− and Cr2O72− are 2.16 × 104 M−1 and 1.65 × 104 M−1, respectively, higher than that of the reported fluorescent MOF sensors for the detection of Cr(VI) ions in the aqueous phase (Table 2), indicating an higher quenching efficiency to the fluorescent compound 1. The detection limits of CrO42− and Cr2O72− are 1.25 × 10−6 M (Cr content 0.065 mg/L) and 1.33 × 10−6 M (Cr content 0.138 mg/L), respectively (Fig. S9 and Fig. S10), which is comparable to or even better than the results of other fluorescence sensors for the detection of Cr(VI) ions (Table 2). Moreover, detection limit of CrO42− reaches the permissible limits in drinking water (0.1 mg/L) by the USA and is close to the drinking water limits in China or the EU (0.05 mg/L) [46].
Table 2

Comparison among various MOF-based sensors for detection of Cr(VI) ions

Material (LMOF/LCP)


Media (aqueous/organic)

Quenching constant (KSV)

Limit of detection



{[Zn2(TPOM) (NH2-BDC)2]·4H2O}n



4.45 × 103 M−1/7.59 × 103 M−1

4.8 μM/3.9 μM





3.19 × 103 M−1/4.23 × 103 M−1

10 μM/2 μM





2.35 × 103 M−1/2.19 × 103 M−1

20 μM/2 μM





1.00 × 103 M−1/1.37 × 103 M−1

18.33 μM/12.02 μM





1.30 × 103 M−1/2.91 × 103 M−1

2.52 μM/2.26 μM





6.4 × 103 M−1 (Cr2O72−)

37.6 μM (Cr2O72−)





4.97 × 103 M−1 (Cr2O72−)

48.6 μM (Cr2O72−)





1.97 × 104 M−1

0.19 μM





4.34 × 103 M−1

0.054 μM





1.18 × 103 M−1/4.5 × 103 M−1

0.03 μM/0.04 μM





1526 M−1






1.13 × 104 M−1 (Cr2O72−)

0.1 μM


{[Tb4Mn(BPDC)33-OH)4 (HCOO)1.5(H2O)4]·2.5OH·8H2O}n



0.5 × 104 M−1 (Cr2O72−)

0.1 μM





6630 M−1



Compound 1



2.67 × 104 M−1/1.65 × 104 M−1

1.25 μM/1.33 μM


Generally, waste water contains more than one type of pollutant anions, and therefore, it is essential to investigate the influence of mixed anions on the selectivity of luminescence sensing. Thus, the selectivity towards CrO42− or Cr2O72− in the presence of other anions was investigated. CrO42− or Cr2O72− aqueous solution (2 mL, 3 mM) was added into 1-H2O emulsions with different metal ions (2 mL, 10 mM), and the changes of the emission intensities were recorded. As shown in Fig. 5, the quenching efficiency of CrO42− or Cr2O72− with other anions remained nearly unchanged. The abovementioned results demonstrate that compound 1 exhibits highly selective and sensitive sensing of Cr(VI) ions over other anions with high quenching efficiency.
Fig. 5

Competitive experiments of 1 toward CrO42− or Cr2O72− in the presence of other anions

The fine particles which were centrifuged and washed from the aqueous solution containing Cr(VI) ions still exhibit strong green light emission under UV light, indicating that the framework structure of compound 1 remains intact, and the corresponding PXRD further verifies the crystallinity of compound 1 (Fig. 1b). Hence, a quenching mechanism by the collapse of framework can be ruled out. To better understand the mechanism of luminescent quenching by CrO42− or Cr2O72−, UV-vis spectra of aqueous solutions of the corresponding potassium salts (KmX) with various anionic species were measured (Fig. S11). Apparently, the UV-vis spectra of aqueous solutions of K2CrO4 and K2Cr2O7 showed a broad absorption ranging from 200 to 550 nm, however, other potassium salts with various anionic species have no significant absorption beyond 300 nm. Notably, the excitation wavelength of compound 1 is 369 nm, which is completely overlaid by the absorbing band of K2CrO4 and K2Cr2O7. Upon excitation at 369 nm, there is a competition for the absorption of the light source energy between as-synthesized 1 and K2CrO4 and K2Cr2O7. So, the possible sensing mechanism for Cr(VI) ions can be attributed to a competition for the excitation energy between the analytes and compound 1 [38, 39, 44, 47, 48, 49, 50, 51, 52].

3.6 Metal-ion detection

In the reality, the detection of metal ions in water is also significant. Similar to the process of anion detection, 0.5 mg samples of compound 1 was dispersed in different aqueous solutions (4 mL) including 10−2 M of M(NO3)x (M = Na+, Ag+, Co2+, Cu2+, Zn2+, Cd2+, Mg2+, Pb2+, Ni2+, Al3+, and Fe3+), and the corresponding luminescent spectra were measured after ultrasonication for approximately 30 min. The Al3+ ion is helpful to enhance the emission intensities, however, other metal ions result in varying degrees quenching effect compared with the control group, particularly, Fe3+ has nearly 100% fluorescence quenching (Fig. 6a and Fig. S12). Apparently, the emissive visible green light from 1-H2O emulsion with Fe3+ ions is darker than that from the other emulsions under UV light (Fig. 6b). To further study the sensing sensitivity towards Fe3+ ion, the luminescence intensities of 1-H2O were measured with gradually increasing the Fe3+ concentration (Fig. 6c). Notably, with the increase of Fe3+ concentration, the luminescence intensity gradually decreased, when the Fe3+ concentration reached 0.29 mM, the corresponding emission intensity was attenuated by approximately 93.64%. According to the Stern–Volmer equation (I0/I = 1 + KSV[M]), a nearly linear plot of (I0/I–1) vs Fe3+ concentration was observed at low concentrations and the corresponding quenching constant (KSV) was 4.99 × 104 M−1(Fig. 6d). The detection limits of Fe3+ reaches 7.26 × 10−7 M (Fig. S13), which is comparable to the sensitivity of NTU-9-NS (4.5 × 10−7 M) [53].
Fig. 6

a Emission spectra. b The luminescence photographs of compound 1 in aqueous solution containing different metal ions under UV light. c The fluorescence titration of 1 dispersed in aqueous solution with the addition of different concentrations of Fe3+. d Stern–Volmer plot of 1 quenched by Fe3+. The insert is SV plot at low Fe3+ concentrations

In addition, a series of competitive studies in the presence of interfering metal ions were carried out. For example, Fe3+ aqueous solution (3 mM, 2 mL) was added into the aqueous solution containing 1 with different metal ions (10 mM, 2 mL), and the changes of the emission intensities were recorded (Fig. S14). The results indicated that fluorescence quenching effect remained unchanged, indicating that other metal ions have little effects on the quenching effect by Fe3+. The luminescence explorations demonstrated that 1 exhibited highly selective and sensitive sensing of Fe3+ over metal ions with high quenching efficiency.

The fine particles which were centrifuged and washed from the aqueous solution containing Fe3+ still exhibit strong green light emission under UV light, indicating that the framework structure of the compound remains intact. The corresponding PXRD pattern of 1 after centrifuged in Fe(NO3)3 aqueous solution was measured (Fig. 1b), which were in good agreement with the simulated pattern obtained from single-crystal structure analysis, indicating the crystallinity of compound 1. Therefore, the luminescence quenching was not caused by the framework collapse. As shown in Fig. S15, the UV-vis spectra of Fe(NO3)3 and other M(NO3)x aqueous solutions indicate that only Fe(NO3)3 showed broad absorption bands, which were completely overlaid excitation wavelength of the compound. Thus, the luminescence quenching effect of compound 1 can be attributed to a competition for the excitation energy between Fe(NO3)3 and compound 1 [40].

4 Fluorescent test papers

To make a simple and convenient, naked-eye colorimetric sensor, visible test papers for practical detection application of 1 towards NB, Fe(III) ion and Cr(VI) ions were prepared as follows: 1-methanol suspension (2 mg/mL) was poured on a clean filter paper and then dried in air at room temperature and prepared for use. As shown in Fig. 7, when the ethanol solution of NB (0.1 M), the aqueous solutions of Fe(III) or Cr(VI) ions (0.01 M) were dropped to the surface of test papers, the fluorescent color of the papers changed from green to black under the irradiation of UV light at 365 nm, and the color difference could be easily distinguished by naked eyes. Whereas, no significant color changes were observed when other different kinds of organic solvent molecules, the aqueous solutions of potassium salts (KmX) with different anions or M(NO3)x with different cations were dropped onto the surface. Thus, the test papers displayed a sensitive and high selectivity to detect NB, Fe(III), and Cr(VI) ions by colorimetric method. At the same time, we investigated the fluorescence quenching of test papers with the concentration of K2Cr2O7 (Fig. S16). When 0.01 mM K2Cr2O7 is dropped on test paper, the fluorescence is slightly weakened compared with that without K2Cr2O7, but it is difficult to be quenched even if it is immersed in K2Cr2O7 aqueous solution for a long time. With the increase of the concentration of K2Cr2O7, the fluorescence gradually decreases. When 1-mM K2Cr2O7 is added to the test paper, the fluorescence is almost completely quenched and the color of test paper is apparently darker than that of other anions with a concentration of 1 M (Fig. S17), indicating that quenching ability of Cr2O72− is far stronger than the other anions. Similarly, 10-mM NB or 1 mM Fe(III) can also cause the fluorescence quenching of the test papers. Such colorimetric sensing has a potentially practical application in an environmental pollution field.
Fig. 7

Colorimetric photographs of the visual luminescent test paper for the detection of NB (0.1 M), Fe(III), and Cr(VI) ions with concentration of 0.01 M under UV-vis light (365 nm)

The recycling performance of the test paper was important in serving as luminescent sensors. The quenching and recovery experiments of the test papers were investigated. Interestingly, after treated with 2 mM K2CrO7 aqueous solution and with excessive pure water, the test papers still display a significant green emission after five runs tests (Fig. 8), indicating the reusability of the test paper. Similarly, the test papers also show the sensitivity and reusage performance in the sensing of NB, Fe3+, and CrO42−.
Fig. 8

The luminescence curves of the test paper and that found after five recycles; the inserted image indicates the luminescence intensity of the test paper can be recovered from Cr2O72−–1

5 Conclusions

In summary, a new luminescent Ag6S6 cluster-based coordination compound based on Hdmpymt ligand has been solvothermally synthesized and characterized. The powder X-ray diffraction patterns confirmed that compound 1 could maintain the crystalline structure after titration experiments. The luminescent properties of compound 1 have been exploited as multifunctional detection fluorosensors for NB, Fe(III), and Cr(VI) ions through fluorescence quenching. The possible sensing mechanisms are attributed to the competitive absorption of excitation wavelength energy between the analytes and the compound 1. The carry-on fluorescent test papers of 1 were prepared and showed efficient, convenient, and easily recycled characteristics, presenting a potential sensing application for the environmental concerns.



This work was supported by the National Natural Science Foundation of China (no: 21201155 and 51272239), the Natural Science Young Scholars Foundation of Shanxi Province (no: 2012021007-5 and 2013021008-6), Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi, and 131 Talent Plan of Higher Learning Institutions of Shanxi.

Compliance with ethical standards

Conflict of interests

The authors declare that they have no conflict of interests.

Supplementary material

42114_2018_61_MOESM1_ESM.doc (6.9 mb)
ESM 1 (DOC 7102 kb)


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Copyright information

© Springer Nature Switzerland AG 2018

Authors and Affiliations

  1. 1.Department of ChemistryNorth University of ChinaTaiyuanPeople’s Republic of China
  2. 2.Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular EngineeringUniversity of TennesseeKnoxvilleUSA
  3. 3.National Engineering Research Center for Advanced Polymer Processing TechnologyZhengzhou UniversityZhengzhouPeople’s Republic of China

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