Introduction

Metal ions and anions both play a key role in daily life1,2,3,4. Borate, an essential element in the earth, is widely used in industry, agriculture and medicine. For example, borate has widespread use in a solid lubricant in industry, and it may also be applied in welding repair to refrigeration equipment. In medicine, borate could be used for the anti-corrosion of the skin and mucous membranes as well as in the treatment of cancer. In animal medicine, as a feed additive, the research on borate has been attracting increasing attention. Nevertheless, abusing borates not only damages the environment but also endangers human health. Hence, the development of a rapid and convenient detection method for B4O72−could be of interest.

Up until now, with the development of optical sensors for recognizing heavy and transition metal ions in living organisms5,6,7,8,9,10,11,12,13,14,15, intense efforts have been devoted to the design and synthesis of high sensitivity fluorescent sensors due to their low cost and rapid response as well as the easy operability of the fluorescent technique16,17,18,19,20,21,22. According to the relevant literature, the metal complexes of N2O2 salen-type ligands and corresponding analogues could be used in catalysis23,24, nonlinear optical materials and magnetic materials25,26,27,28,29,30,31,32,33,34, supramolecular architecture35,36, ion recognition37,38,39,40,41,42,43,44,45, biological fields and so forth46,47,48,49,50,51,52. Today, studies on the participation of salamo-type compounds in ion recognition have yet to be explored53,54,55,56,57,58,59,60,61,62,63. Notably, compared with most of the known fluorescent probes for Zn2+, Cu2+, and CN, there are relatively few reports on fluorescent probes for B4O72−.

Herein, we have designed and synthesized a bis(salamo)-type sensor H4L for the recognition of B4O72− in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7) solutions. The UV–vis absorption spectra and fluorescence titration experiments for sensor H4L were investigated and the results indicated that sensor H4L has a high selectivity for B4O72− over many other ions based on the change in color visible to the naked eye and the fluorescence intensity at a low concentration as well as a mild environment.

Results and Discussion

The selectivity of sensor H4L to B4O7 2−

A series of host-guest recognition experiments were carried out to investigate the B4O72− recognition ability of sensor H4L with various anions and some compounds, B4O72−, Br, CI, CN, CO32−, HCO3, H2PO4, HSO4, NO3, OAc, S2O3, SCN, SO42−, Hcy and H2O2 in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH 7) solutions. As shown in Fig. S2a, all of the examined anions show the same absorption peaks with sensor H4L, however, only the addition of B4O72− displayed the highest absorbance under the same reaction conditions. There are no isosbestic points due to the differences in binding abilities between sensor H4L and all of these anions.

The interaction of sensor H4L and B4O72− was evaluated by a UV–vis titration method. As shown in Fig. S2b, with increasing concentrations of B4O72− (0.001 M) from 0.0–39.0 equiv. in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7) solutions, the absorbance showed a linear increase when the ratio of [B4O72−]/[H4L] is below 39:1, and the absorbance no longer changes when the ratio reached 39:1.

Effect of the pH on sensor H4L

In order to remove the interference by protons during the detection of B4O72− and to find the optimal sensing conditions, further tested was performed in the pH range of 1 to 12. As shown in Fig. 1, the results obtained show no dramatic spectral changes of sensor H4L in the wide pH range of 1–12, suggesting that sensor H4L was very stable. The H4L-B4O72− displayed a strong fluorescence intensity in the pH range of 1–7. The results above clearly indicate that sensor H4L can be employed as a sensitive relay-sensor to recognize and distinguish B4O72− in a wide pH range.

Figure 1
figure 1

Changes in the fluorescence spectra of H4L-B4O72− at various pH values at room temperature.

Fluorescence detection of sensor H4L towards B4O7 2−

Selectivity is a very important parameter to evaluate the performance of a fluorescence chemosensor. The fluorescence emission spectral responses of sensor H4L to various anions and some compounds (B4O72−, Br, CI, CN, CO32−, HCO3, H2PO4, HSO4, NO3, OAc, S2O3, SCN, SO42−, Hcy and H2O2) were evaluated in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH 7) solutions. As shown in Fig. 2a, all of the examined anions did not display any obvious response to sensor H4L, and only after the addition of B4O72− did, sensor H4L produce a significant enhancement of the fluorescence intensity at 430 nm (λex = 323 nm). These results suggested that sensor H4L displayed an excellent selectivity for B4O72− over all of the other anions tested.

Figure 2
figure 2

(a) Fluorescence spectra and (b) fluorescent intensity at 323 nm of sensor H4L (0.01 mM) in the presence of various anions (40.0 equiv. of Br, CI, CN, CO32−, HCO3, H2PO4, HSO4, NO3, OAc, S2O3, SCN, SO42−, Hcy and H2O2) in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7).

To further explore the high selectivity of sensor H4L for B4O72− in practice, we also investigated the ability of sensor H4L to detect B4O72− in the presence of equivalent and excess amounts of other anions, to determine whether they would interfere with coordination between sensor H4L and B4O72−. As shown in Fig. 2b, when anions and some compounds, including Br, CI, CN, CO32−, HCO3, H2PO4, HSO4, NO3, OAc, S2O3, SCN and SO42−, Hcy and H2O2, were separately added into a mixed solution of sensor H4L and B4O72−, the fluorescence intensity had little or negligible change. Hence, fluorescence interference experiments of various anions revealed that other anions could not affect the sensing process of sensor H4L for B4O72−. In order to further understand the binding behavior of H4L with B4O72−, the 1H NMR spectra experiments of H4L and H4L-B4O72− were also performed in DMSO-d6. The phenolic O-H in H4L has completely disappeared upon the addition of B4O72−, and all protons of the aromatic ring and aldimine CH=N in H4L were shifted down-field (Fig. S3). These changes may be due to the destruction of intermolecular electrostatic and hydrogen-bond interactions after the addition of B4O72− to H4L.

The fluorescence enhancement of the sensor H4L response to B4O72− may be attributed to that borates are hydrolyzed to form boric acid:

$${[{{\rm{B}}}_{4}{{\rm{O}}}_{5}{({\rm{OH}})}_{4}]}^{2-}+5{{\rm{H}}}_{2}{\rm{O}}\rightleftharpoons 4{{\rm{H}}}_{3}{{\rm{BO}}}_{3}+2{{\rm{OH}}}^{-}\rightleftharpoons 2{{\rm{H}}}_{3}{{\rm{BO}}}_{3}+2{\rm{B}}{({\rm{OH}})}_{4}^{-}$$

Four coordinated organoboron compounds based on N,O-chelation are constructed mainly by structures 1, 2 and 3 as the ligand backbone (Fig. S4a). The weak fluorescence of sensor H4L was attributed to the lone pairs of electrons on the nitrogen atoms, which lead to intra-molecular photoinduced electron transfer (PET). Due to the lack of electronic properties, the Lewis bases such as the N atoms of the salamo moieties from the H4L unit coordinate to the B atoms, resulting in a unique electronic structure and optical properties after B atoms are incorporated into the conjugated system. Four coordinated organoboron compounds can produce strong fluorescence with the excitation of light64. On the other hand, sensor H4L exhibited a very weak fluorescence intensity due to the photoinduced electron transfer process from the hydroxy oxygen atom to amino groups. However, when sensor H4L was coordinated with a B4O72− ion, the chelation-enhanced fluorescence process would be started, and the photoinduced electron transfer process would be inhibited at the same time (Fig. S4b). Hence, an obvious enhancement of the fluorescence intensity was observed.

Fluorescent titration was carried out to gain more insight into the recognition properties of sensor H4L as a B4O72− probe. As shown in Fig. 3, without B4O72−, sensor H4L had nearly no fluorescence. However, with increasing concentrations of B4O72−, the fluorescence intensity was remarkably increased at 430 nm. Significantly, a good linear relationship between the fluorescence intensity and the B4O72− concentration could be obtained (R2 = 0.95873), which is based on the fluorescence titration experiment. It can be seen that the fluorescence intensity change was nearly linear with the increase of concentration of B4O72− (Fig. S5). For many practical applications, it is very meaningful to detect the analytes at low concentrations. Meanwhile, based on the corrected Benesi-Hildebrand formula, the binding constant for the binding of B4O72− to sensor H4L was calculated as 4.72 × 103 M−165,66. The detection limit (LOD) could be calculated to be 8.61 × 10−7 M and the limit of quantitation (LOQ = 2.87 × 10−6 M) of sensor H4L for B4O72− anions was also obtained67. The LOD and LOQ were calculated based on the following equations:

$${\rm{LOD}}=3\times {\rm{\delta }}/{\rm{S}};\,{\rm{LOQ}}=10\times {\rm{\delta }}/{\rm{S}}.$$

Where δ (δ = 3.9 × 10−5) represents the standard deviation of the blank measurements, and S is the slope of the intensity versus sample concentration curve68,69.

Figure 3
figure 3

Fluorescence emission spectra of sensor H4L (0.01 mM) upon the subsequent addition of B4O72− (0–39 equiv. λex = 323 nm) in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7) solutions.

We investigated the binding stoichiometry and binding affinities of sensor H4L and B4O72−. A Job’s plot analysis for the fluorescence intensity was also measured by keeping the sum of the initial concentrations of sensor H4L and B4O72− constant at 10 µM (Fig. 4). The experiment was performed in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7) solutions at an excitation wavelengths of 323 nm. The results indicated that the binding stoichiometry between sensor H4L and B4O72− is 1:1.

Figure 4
figure 4

Job’s plot for determining the stoichiometry of sensor H4L and B4O72− in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7). Excitation wavelength: 323 nm.

The realization of a quick response to B4O72− is very meaningful for sensor H4L in its practical application in portable sensing devices. To facilitate the use of sensor H4L for the detection of B4O72−, test strips were made by soaking filter papers in a Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7) solution of sensor H4L followed by exposure to air until complete drying. Intriguingly, the obvious fluorescence color changes were observed immediately from gray to light blue in visible light when B4O72− anions were added. Therefore, sensor H4L exhibited excellent fluorescence sensing performance, which would be very useful for the fabrication of sensing devices with fast and convenient detection of B4O72− (Fig. 5).

Figure 5
figure 5

Photographs of the colorimetric test kit with H4L for detecting B4O72− under irradiation at 365 nm.

In order to be applied in real life and to find the optimal sensing conditions, the fluorescence intensity of sensor H4L over a period of time in the presence of B4O72− was determined in Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7) solutions. As shown in Fig. S6a, it was found that there were nearly no changes in the fluorescence intensity of H4L-B4O72− over a period of time, suggesting that H4L-B4O72− was very stable. Additionally, the fluorescence intensities at different temperatures were also determined. As shown in Fig. S6b, H4L exhibited satisfactory B4O72− sensing abilities when the temperature was in the range of 0–90 °C. Therefore, it was demonstrated that sensor H4L could work in a short time and at room temperature, and it can be applied in real life.

Prior to the imaging experiments, the cytotoxicity of H4L at different concentrations (0–100 µM) was evaluated through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays in BHK-21 cells. The results after 48 h revealed that H4L exhibited almost no toxicity or low toxicity (Fig. 6).The ability of sensor H4L to detect B4O72− in living cells was further studied by confocal luminescence imaging. As seen in Fig. 7, the BHK-21 cells incubated with sensor H4L (30 μM) alone for 30 min at 37 °C maintained a good shape and were viable, the solvent for the H4L concentrate is DMSO, and they also showed very good intracellular fluorescence. Interestingly, an enhanced intracellular fluorescence was detected in cells containing sensor H4L incubated with B4O72− for 3 h. From confocal fluorescence images of the BHK-21 cells, it was revealed that sensor H4L displayed good cell permeability and could be used to detect B4O72− ions in living cells.

Figure 6
figure 6

Cytotoxicity assays of H4L at different concentrations for BHK-21 cells.

Figure 7
figure 7

Confocal luminescence images of BHK-21 cells. (a) BHK-21 cells were incubated with sensor H4L (30 μM) for 30 min at 37 °C and (b) then further incubated with B4O72− (100 μM) for 30 min.

In conclusion, we designed and synthesized a bis(salamo)-type sensor H4L, which showed excellent recognition of B4O72−with different fluorescence changes and changes in color. In addition, the detection limit of the fluorescence response of sensor H4L to B4O72− is as low as 8.61 × 10−7 M. This sensing system shows many advantages. The test strips could conveniently, low cytotoxicity, efficiently and simply detect B4O72− in solutions. In addition, the free sensor H4L was achieved through regeneration by using EDTA and was able to further sense B4O72−. We believe that this study provides a potential application for constructing a fluorescent sensor for the highly sensitive and rapidly recognition of B4O72− ions based on different fluorescence intensities and changes in color in practical life.

Materials and General Methods

2-Hydroxy-3-methoxybenzaldehyde (99%), methyl trioctyl ammonium chloride (90%), pyridiniumchlorochromate (98%) and borontribromide (99.9%) were purchased from Alfa Aesar. Hydrobromic acid 33 wt% solution in acetic acid was purchased from J&K Scientific Ltd. The other reagents and solvents were analytical grade reagents from the Tianjin Chemical Reagent Factory and were used as received. Melting points were obtained by the use of a microscopic melting point apparatus made by the Beijing Taike Instrument Limited Company and were uncorrected. 1H NMR spectra was determined by a German Bruker AVANCE DRX-400 spectrophotometer. All of the UV–vis and fluorescence spectroscopy experiments were recorded on Shimadzu UV-2550 and Perkin-Elmer LS-55 spectrometers, respectively.

Synthesis of sensor H4L

The bis(salamo)-type sensor H4L was synthesized according to the previously reported procedure70,71,72,73,74,75,76,77,78. The IR, 1H NMR and UV-vis spectra of H4L are nearly consistent with the literature data (Fig. S1). The major reaction steps of sensor H4L are demonstrated in Fig. 8.

Figure 8
figure 8

Synthetic route to sensor H4L.

Statistical analysis

Statistical methods used are detailed at each experiment individually.