Introduction

L-lysine (Lys) is an essential amino acid for animal and human nutrition. A Lys misbalanced diet entails severe diseases and so it is often added as a dietary supplement to foods and drugs. Lys content in foods or drugs can be used as an index of nutritional quality or for the evaluation of processing techniques [1]. Because Lys is easily damaged by thermal treatment and storage conditions, the development of the detection to Lys in food products or drugs is important. Despite some several methods have been described for lysine analysis or detection [2–5], these methods do not satisfy the requirements for an obvious and fast response. As it well known, rare earth ions have excellent luminescence [6, 7], long luminescence lifetime, and strong binding with biological molecules [8–10]. In view of the expensive rare earth elements compared with the common elements, Eu-doped Mg-Al LDH and Eu-doped Zn-Al LDH have been paid high attention [11, 12]. Moreover, the use of Eu-doped LDH as a fluorescent probe to detect amino acids with phenyl groups has been attempted [13, 14]. Although there are limited studies on Tb–complex intercalated into interlayer of MgAl-LDHs [15, 16], the investigation on Tb(III) doped onto layers of LDHs should be paid more attention because the Tb doped onto the layers of LDHs is more favorable for touching outer biological molecules compared with that of the Tb(III)–complex intercalated into interlayer of LDHs. For this reason, the Tb(III) was incorporated onto layers of MgAl-LDH and the green emission of Tb-doped Mg-Al LDH response to L-lysine was investigated in the present work. Meanwhile, the mechanism of the fluorescent response was probed.

Experimental

Materials Synthesis

Tb(NO3)3 solution (0.05 mol·L−1) was prepared by Tb2O3 solid dissolved in mixed solution of concentrated HNO3 and H2O2. The Tb-doped MgAl-LDH, with the molar ratios of Mg/(Al + Tb) of 2.0 and Tb/(Al + Tb) of 0.06, was synthesized by co-precipitation method [11, 12]. Namely the mixed solution of Mg2+, Al3+, and Tb3+ ions with certain molar ratio was prepared by Mg(NO3)2·6H2O and Al(NO3)3·9H2O solid dissolved in HNO3 and mixed with Tb(NO3)3 solution. After the addition of 3.5 mol·L−1 NH3 solution to the mixed solution, a precipitate formed (pH =8 ~ 9). Then the precipitant suspension was aged at 40 °C for two hours. After being filtrated, washed, and dried at 70 °C for 12 h, the Tb-doped MgAl-LDH was obtained (labeled as Tb-LDH).

Five Tb-doped MgAl-LDH aliquots ~1.0 g samples were added to 0.01, 0.05, 0.1, 0.25, and 0.5 mol·L−1 alkaline lysine aqueous solution, respectively, and kept on stirring at room temperature for 12 h. After the slurries were filtered, water-washed, and dried at 40 °C for 12 h, the corresponding products were labeled as Tb-LDH/Lys-1, Tb-LDH/Lys-2, Tb-LDH/Lys-3, Tb-LDH/Lys-4, and Tb-LDH/Lys-5. The physical mixture of Tb-doped LDH/Lys contained 0.5 g Tb-LDH and 0.5 g L-lysine solid powders. Ultra pure water was used in the whole experiment.

Characterization

X-ray diffraction (XRD) patterns were measured with X-ray diffractometer (XD-3, Beijing Puxi Tongyong Yiqi Ltd. China, CuK α radiation). All the samples were scanned in the 2θ range of 3–70° at a scan rate of 2° /min. Chemical contents of Mg, Al, and Tb were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES Optima 5300DV American Pe Company, America) as well as scanning electron microscope equipped with energy dispersive X-ray analysis (SEM-EDX, JEOL JSM-6701F). The C, H, and N contents of the Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) composites were determined by Element Analyzer (Elementar Vario EL II, Germany). Fourier transform infrared (FT-IR) spectra of the samples were surveyed by the KBr method using a FT-IR spectrometer (Himadzu IR Prestige-21). The fluorescent study was performed using a Spectrophotometer (F-7000 FL).

Result and Discussion

Compositional and Structural Analyses

The chemical compositions of Tb-LDH and Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) were determined by ICP, SEM-EDX, and CHN elemental analyses (seen in Table 1 and Fig. 1). According to elemental analyses and charge balance principle [17], the Tb-LDH/Lys-1, Tb-LDH/Lys-2, Tb-LDH/Lys-3, Tb-LDH/Lys-4, and Tb-LDH/Lys-5 contained 0.02, 0.03, 0.05, 0.07, and 0.09 mol lysine per 1 mol Tb-LDH, respectively, which is in accordance with the initial concentration of lysine solution. In addition, the molar ratio of Mg2+/(Al3++Tb3+) to be 2.0 was found in the Tb-LDH sample, which is in agreement with the initial ratio of raw reactants. Moreover, the Mg2+/(Al3++Tb3+) molar ratio of 2.0 kept in all the Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) composites, indicating the layers of Tb-LDH preservation during the reaction of Tb-LDH and lysine.

Table 1 Chemical compositions of Tb-LDH and Tb-LDH/Lys –n(n = 1, 2, 3, 4, 5)
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Fig. 1
figure 1

EDX of Tb-LDH, Tb-LDH/Lys-1, Tb-LDH/Lys-3, and Tb-LDH/Lys-5

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Figure 2 displays XRD patterns of Lys, Tb-LDH, mixture of Tb-LDH/Lys, and Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5). The XRD pattern of Lys was in accordance with the literature [18], and the XRD pattern of Tb-LDH is similar to those of previous LDHs [17, 19–26]. The {003} basal plane space of 8.70 Å for the Tb-LDH is obviously larger than that of previous MgAl-LDHs [22–24]. This may due to the doped Tb, different Mg2+/(Al3++Tb3+) molar ratio, and interlayer anions, etc. After the Tb-LDH was exposed to 0.01, 0.05, 0.1, 0.25, and 0.5 mol·L−1 Lys solution, respectively, the basal spacing of the product Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) tended to decrease with the increase in the concentration of Lys solution. This result suggested the Lys not intercalated into interlayer spacing of Tb-LDH, but possibly adsorbed on surface of the Tb-LDH. However, the basal spacing is subject to various factors, including interlayer guest type, amount, and state, etc. It is very complicated to define the exact factor. The XRD pattern of the mixture of Tb-LDH/Lys appeared the reflections attributed to Tb-LDH and lysine. No reflections traceable to the Lys represented in the Tb-LDH/Lys-n(n = 1, 2, 3, 4, 5) composites. These results illustrated that the Lys is possibly amorphous or highly dispersed in the Tb-LDH/Lys-n composites. In addition, SEM images showed the morphology of Tb-LDH did not obviously change after the Tb-LDH reacted with different concentration of Lys solution (seen in Fig. 3), indicating the Tb-LDH not damaged by the alkaline L-lysine solution.

Fig. 2
figure 2

XRD patterns of Lys, Tb-LDH, Mixture of Tb-LDH/Lys, Tb-LDH/Lys-1, Tb-LDH/Lys-2, Tb-LDH/Lys-3, Tb-LDH/Lys-4, and Tb-LDH/Lys-5

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Fig. 3
figure 3

SEM images of a Tb-LDH, b Tb-LDH/Lys-1, c Tb-LDH/Lys-3, and d Tb-LDH/Lys-5

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IR Spectral Analysis

FT-IR spectra of Tb-LDH, Lys, mixture of Tb-LDH/Lys, and Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) present in Fig. 4. For the lysine (Lys) solid powder, the weak band around at 3402 is due to O–H group stretching vibration of water molecules [27]. Stretching vibrations of N-H bonds in the IR spectra are revealed as bands at 3046 and 2939 cm−1, which is overlapped with strong band in the region of stretching vibrations of C-H bonds [27, 28]. In the region of asymmetric stretching vibration of carboxylate group and asymmetric deformation vibration of NH3 + groups there are peaks at 1635 and 1586 cm−1. The band at 1504 cm−1 may be owing to symmetric deformation vibration of NH3 + groups, while the band at 1409 cm−1 is ascribed to symmetric stretching vibration of carboxylate group [27–30]. The stretching vibration of the C-C group is observed at 1137 cm−1, and C-C-N symmetric stretching appeared at 864 cm−1 [31]. With regard to Tb-LDH, the bands around at 3445 and 1636 cm-1 were assigned to O–H group stretching and bending vibrations of the hydroxide basal layer or interlayer water molecules [32]. The band at 1385 cm−1 may be due to deformation mode of carbonate ions absorbed [33]. The lattice vibration mode of the LDH sheets was observed at 658 cm−1 [34]. As expected, the IR spectrum of the mixture of Tb-LDH/Lys contained some bands attributed to Tb-LDH and Lys. In contrast to the mixture of Tb-LDH and Lys, the IR spectra of Tb-LDH/Lys-n were very different. Moreover, with the initial concentration of lysine solution varying from 0.01, 0.05, 0.1, 0.25, to 0.5 mol·L−1, the IR spectra of the Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) gradually changed. Two group of IR spectra can be found for the Tb-LDH/Lys-n(n = 1,2,3,4,5). One group consisted of Tb-LDH/Lys-1 and Tb-LDH/Lys-2, and their spectra are similar to that of the Tb-LDH; the other group contained Tb-LDH/Lys-3, Tb-LDH/Lys-4, and Tb-LDH/Lys-5, and their IR spectra are very different from that of the Tb-LDH. The bands attributed to lysine obviously present in the Tb-LDH/Lys-4 and Tb-LDH/Lys-5, which may be due to more lysine adsorbed by the Tb-LDH with the increase in the concentration of lysine solution.

Fig. 4
figure 4

IR spectra of Tb-LDH, Lys, Mixture of Tb-LDH/Lys, Tb-LDH/Lys-1, Tb-LDH/Lys-2,Tb-LDH/Lys-3, Tb-LDH/Lys-4, and Tb-LDH/Lys-5

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Green Emission of Tb-LDH Response to lysine

The excitation spectrum was monitored for the 546 nm emissions of the Tb-LDH (Fig. 5), and four bands at 353, 370, 377, and 488 nm occurred, which is similar to the literature [35]. The bands at 353, 370, and 377 nm were attributed to 7F6 → 5G4, 7F6 → 5 L10, and 7F6 → 5G6 transitions, respectively [36, 37], while the band at 488 nm may be due to 5D4 ← 7F6 transition [38, 39]. Under the excitation of 370 nm wavelength, the Tb-LDH, Lys solid powders, mixture of Tb-LDH/Lys, and Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) were subject to measurement for the emission spectra. As shown in Fig. 6, the emissions attributed to 5D4–7FJ (J = 3, 4, 5, 6) transitions of Tb3+ were found in the Tb-LDH [40–42]. The most intense green emission attributed to 5D4-7F5 transition of Tb3+ stands at 546 nm which is as strong as that of organic Tb-complexes [43–47]. The emission due to 5D4-7F5 transition is hypersensitive to surroundings of Tb3+, and the 5D4-7F6 transition however has a magnetic dipole character and its intensity is almost independent of the environment of Tb3+. For the Lys, a broad peak appeared at about 490 nm, which is similar to the literatures [48, 49]. As expected, the emissions attributed to Tb-LDH and Lys were observed in the mixture of Tb-LDH/Lys. As for the Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) composites, the emission owing to 5D4-7F5 transition of Tb3+ markedly decreased and almost quenched, which indicated the green emission of Tb-LDH was sensitive to Lys (lysine). However, the emission attributed to Lys took on different changes when the Tb-LDH was exposed to different concentration of lysine solution. As the Tb-LDH was exposed to 0.01 and 0.05 mol·L−1 lysine solution, respectively, the emission attributed to 5D4-7F5 transition of Tb3+ almost quenched, while the emission ascribed to Lys greatly increased, and obviously shifted to high energy (from 490 to 463 nm). With the increase in the concentration of lysine solution from 0.05, 0.1, 0.25, to 0.5 mol·L−1, the increment in the emission ascribed to Lys gradually decreased, and the emission belonged to 5D4-7F5 transition of Tb3+ still quenched. The result is against to most reports related to intramolecular energy transfer from ligand to metal ions, which can increase the luminescence of Tb3+ ions [50–54]. Some researchers think that fluorescence of Tb3+ (III) quenched by organic ligand is due to energy loss via the intersystem crossing channel from T1 to singlet S0 [39] or intramolecular back energy transfer from the 5D4 (Tb3+) state to the lowest triplets of the ligands [55]. In order to understand the fluorescent quenching, UV-visible reflectance spectra of samples represented in Fig. 7. The band-edge of the Tb-LDH exhibited in 320 ~ 375 nm in the light of the reflectance spectrum, which is in agreement with the result of excitation spectrum. The band-edge of the Lys was hardly defined from its reflectance spectrum due to poor crystallinity. As expected, the reflectance spectrum of the mixture of Tb-LDH/Lys combined the features of Lys and Tb-LDH. However, the reflectance spectra of Tb-LDH/Lys-n(n = 1,2,3,4,5) composites are different from those of Lys, Tb-LDH, and mixture of Tb-LDH/Lys. The band-edge of the Tb-LDH/Lys-1 is close to that of the Tb-LDH. With increasing Lys adsorbed in the Tb-LDH, the band-edge of Tb-LDH/Lys-n shifted to low energy, and the reflectance spectrum profile is more similar to that of the Lys. Moreover, the reflectance spectra of Tb-LDH/Lys-n are obviously different from that physical mixture of Tb-LDH/Lys, indicating interaction between Tb-LDH and Lys present in the Tb-LDH/Lys-n. The present fluorescent quenching may be due to the interaction. In view of the difficult coordination of Tb3+ and -NH2 or -COOH group of lysine due to the blocking of OH groups in the layers, there may be no direct coordination between the lysine and Tb-LDH. Therefore, the fluorescent change is possibly relevant to other interaction, such as the hydrogen-bonding interaction or electrostatic interaction between NH2 (or COOH) groups of Lys (lysine) and the OH groups on the layers of LDH. This interaction lead to energy transfer from the 5D4 (Tb3+) state to the lowest triplets of the lysine, accordingly lead to quenching of Tb3+ green emission.

Fig. 5
figure 5

Excitation spectrum of Tb-LDH (λem = 546 nm)

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Fig. 6
figure 6

Emission spectra of Tb-LDH, Lys, mixture of Tb-LDH/Lys, and Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5) (λex = 370 nm)

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Fig. 7
figure 7

UV-vis reflectance spectra of Tb-LDH, Lys, mixture of Tb-LDH/Lys, and Tb-LDH/Lys-n (n = 1, 2, 3, 4, 5

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Conclusions

The green emission of Tb-doped LDH response to lysine has been investigated. The green emissions due to the 5D4–7FJ (J = 3, 4, 5, 6) transitions of Tb-LDH markedly decreased after the Tb-doped LDH was exposed to lysine solution. Compositional analyses suggested that the content of lysine present in the Tb-LDH/Lys composites gradually increased with increasing concentration of lysine solution. The green emission of Tb-LDH optimal response to lysine happened at 0.05 mol·L−1 of lysine. This fluorescence of Tb-LDH sensitive to low concentration of lysine would be potential application in biological fluorescent prober.