Introduction

In recent years, two-dimension layered materials have drawn considerable interest in numerous fields including photochemistry, electrochemistry, and catalysis mainly due to their particular structure and characteristics [1–5]. As a kind of artificial layered materials firstly discovered by Clearfield and Stynes in 1964 [6], α-Zr(HPO4)2·H2O (abbreviated as α-ZrP) owns several characteristics such as larger surface charge density and higher aspect ratio and ion-exchange capacity [7–10] except for commonalities of layered nanomaterials. Hence, some related articles about preparation of the intercalation compounds of α-ZrP/MB (methylene blue) [11], α-ZrP/porphyrin [12], α-ZrP/hemoglobin [13–15], and α-ZrP/insulin [16] were reported. In addition, α-ZrP has been modified to obtain various hybrid materials with diverse applications by replacing hydroxyl groups located on the surface of α-ZrP with organic functional groups [17, 18]. Concerning the structure of α-ZrP, three oxygen atoms belonging to one phosphate group are bonded to three different Zr atoms forming the laminate, while OH connecting to P atoms points into the interlayer region or on the surface [19] (Fig. 1).

Fig. 1
figure 1

Schematic structure of α-Zr(HPO4)2·H2O

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As a well-known class of compounds extensively existed in biological systems, porphyrins and their derivatives play an important role in several aspects of oxygen transfer (Hemoglobin), storage (Myohemoglobin), activation (Cytochrome), and photosynthesis (Chlorophyll). The multifunctional composites constructed by the immobilization of porphyrin derivatives in various inorganic layered materials have become the research focus [20–24].

Many efforts have been devoted to the preparation of nanocomposites through the ion-exchange process previously which usually takes a longer period [25–27]. Nevertheless, time-saving exfoliation/restacking route has attracted considerable interests recently due to opening access to explore the inner surface of laminate. At present, much attention was paid to delamination of transition metal dichalcogenides (MoS2) [28], layered double hydroxides (LDHs) [29–32], and metal oxides (Ca2Nb3O10 −, TiNbO5 −) [33–35]; meanwhile, the exfoliation of α-ZrP nanosheets has also become the research focus [36–39]. Therefore, it is promising to introduce metalloporphyrin into the interlayer of α-ZrP through the exfoliation/restacking method (Fig. 2).

Fig. 2
figure 2

Schematic illustration of the reassembly process between α-ZrP nanosheets and metalloporphyrin

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As a kind of common inorganic pollutant in environment and food industry, nitrite can be converted to N-nitrous compounds which are carcinogenic to humans. The quantitative analysis of nitrite can be achieved by electrochemical method [40]. Herein, we made an attempt to fabricate α-ZrP/MnTMPyP lamellar nanocomposite via electrostatic interaction between manganese porphyrin aqueous solution and colloidal dispersion of α-ZrP nanosheets, so that we could make further research on electrochemical determination of nitrite.

Experimental

Preparation of exfoliated α-ZrP nanosheets

α-ZrP was prepared by a slightly modified HF approach reported in previous literature [41]. 3.0 g ZrOCl2·8H2O powder dispersed in 30 ml distilled water beforehand was mixed with 9 ml phosphoric acid and 3 ml hydrofluoric acid at 80 °C in a plastic flask, and the resulting white precipitate was collected by centrifugation, washed with distilled water several times, and dried at 50 °C. Typically, 0.05 g α-ZrP material was then dispersed into thirty milliliters distilled water with stoichiometric amounts of 10 (wt%) TBAOH aqueous solution and agitated uniformly for 3 days in a single-necked flask; the resulting translucent colloidal suspension was centrifuged at 6000 rpm to remove the unexfoliated particles to avoid affecting the later experiments.

Fabrication of α-ZrP/MnTMPyP intercalation hybrids

Regarding the fabrication of α-ZrP/MnTMPyP intercalation hybrids, 1 mM MnTMPyP aqueous solution was added into the colloidal dispersion of α-ZrP mentioned above. The precipitate was centrifuged under 8000 rpm, washed with distilled water several times, and dried at 50 °C for further characterization.

Characterization

X-ray diffraction patterns were collected with a RINT 2000 diffractometer (Rigaku) using Cu Kα radiation (λ = 0.154 nm) with 2θ from 2° to 40°. Zeta potential of α-ZrP colloidal suspension was monitored using a Malvern Zetasizer Nano instrument, and water at 25 °C was selected as the dispersion solvent. Infrared spectra were measured on a Shimadzu FTIR-8400S spectrometer with the use of KBr pellets. UV–Vis absorption spectra were recorded on a UV–vis spectrometer (UV-2550). The morphology of the samples was investigated by a scanning electron microscopic apparatus (JEOL, JSM-6390), and the specimens should be treated by spray-gold firstly.

Electrochemical characterization and property

The electrochemical experiments were carried out in a conventional three-electrode cell at room temperature, with a platinum wire electrode as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the α-ZrP/MnTMPyP hybrid thin film-modified glass carbon electrode (GCE) as the working electrode. The acting electrolyte was 0.2 mol L−1 phosphate buffer solution (PBS) which should be purged with N2 for 20 min before examination to avoid the influence of oxygen. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) scans were all carried out on a CHI660c electrochemical workstation.

Results and discussion

XRD analysis

Figure 3 shows the XRD patterns of α-ZrP host material and α-ZrP/MnTMPyP intercalation compound. It can be clearly seen that α-ZrP sample with high crystallinity exhibited intense diffraction peaks, and the interlayer distance was calculated as 0.75 nm according to 2θ angle (11.75°) of the 002 characteristic peak, while the interlayer distance of the restacking product increases to 1.64 nm with the lower 2θ angle of 5.4°. With respect to the fabrication of α-ZrP colloidal dispersion and MnTMPyP aqueous solution, it should be attributed to the electrostatic interaction between the negatively charged α-ZrP nanosheets and metalloporphyrin cations with ESD mechanism [42]. In order to investigate the angle of MnTMPyP located in the galley of the hybrid, the rough calculation was made as follows. Considering the host layer thickness of 0.63 nm [38], a conclusion can be drawn that MnTMPyP molecules were placed into the interlayer by a monolayer inclined angle of 34° according to the known size of MnTMPyP ring (18.0 × 18.0 × 7.5 Å, estimated by MM2 method).

Fig. 3
figure 3

XRD patterns of a α-ZrP host material and b α-ZrP/MnTMPyP hybrid

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Zeta potential analysis

To further confirm the restacking process, the detection of the surface potential was achieved by a Zetasizer Nano instrument. The zeta potential of α-ZrP colloidal dispersion is −40.1 mV, which indicated that the dispersion was stable and well dispersed. In order to acquire zero potential point of reaction, a few volume ratios of α-ZrP colloidal dispersion and MnTMPyP aqueous solution were tested as shown in Fig. S1; the zeta potential increases with the addition of MnTMPyP aqueous solution, and it turned out that the zeta potential of the 1:0.58 ratio approaches nearly zero. Flocculation phenomenon about coassembly of α-ZrP nanosheets and MnTMPyP aqueous solution at this ratio was displayed in Fig. 4b, and the reaction is extremely fast and can be finished within 20 min which is superior to the traditional ion-exchange method. As observed in Fig. 4a, clear Tyndall light scattering was found in the colloidal solution of α-ZrP nanosheets, suggesting the occurrence of exfoliation [29].

Fig. 4
figure 4

a Tyndall phenomenon of colloidal dispersion of exfoliated α-ZrP nanosheets, b photographs of colloidal suspensions of α-ZrP nanosheets in TBAOH aqueous solution (left), MnTMPyP aqueous solution (middle), and the corresponding mixture of α-ZrP and MnTMPyP (right)

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IR spectra analysis

The characteristic peaks of MnTMPyP at 1642, 1560, 1506, and 1458 cm−1 correspond to stretching vibration of C=N or C=C in the pyridine substituent and porphyrin rings (Fig. 5). Besides, the absorption bands at 1106 cm−1 should be assigned to the single bond of C–N. The infrared spectrum of α-ZrP/MnTMPyP hybrid exhibited similar absorption in the range of 1700–1000 cm−1 with a slight shift merely, possibly attributing to interaction of intercalated guest molecules and the host layer [43]. As noted by line (a), these bands ascribed to the phosphate groups [36] in the host layer were also present in IR spectrum of α-ZrP/MnTMPyP intercalation compound.

Fig. 5
figure 5

FT-IR spectra of a α-ZrP, b MnTMPyP, and c α-ZrP/MnTMPyP

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UV–Visible absorption spectra analysis

Figure 6 gives UV–Vis spectra of MnTMPyP aqueous solution and α-ZrP/MnTMPyP nanocomposite. Compared with MnTMPyP aqueous solution, the hybrid exhibits a 9-nm red shift in the Soret band caused by restricted manganese porphyrin molecules located in the galley consistent with several analogous reports [44, 45]. The existence of a broadening phenomenon in the hybrid was due to some degree of aggregation and stacking of the metalloporphyrin molecules [46].

Fig. 6
figure 6

UV–visible absorption spectra of a MnTMPyP aqueous solution and b α-ZrP/MnTMPyP hybrid film

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SEM analysis

As observed in Fig. 7, the origin material of α-ZrP with high crystallinity reveals a large average size of about 1–3 μm. While, the introduction of MnTMPyP led to the formation of intercalation hybrid with irregular shape and rough surface. However, two-dimensional layered structure of the final composite was reconstructed by the reassembly of α-ZrP nanosheets and MnTMPyP molecules.

Fig. 7
figure 7

SEM images of a α-ZrP and b α-ZrP/MnTMPyP

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Electrochemical characterization

The CV curves of MnTMPyP aqueous solution and α-ZrP/MnTMPyP hybrid film in pH 7.0 PBS at 50 mV s−1 scan rate are shown in Fig. 8. A couple of well-defined oxidation/reduction peaks with redox potentials were at −0.257 and −0.183 V for MnTMPyP aqueous solution, with the midpoint potential E m = (E pa + E pc)/2 = −0.22 V and the peak separation ΔE p = 74 mV. A couple of similar electrochemical characteristic oxidation/reduction peaks appear in α-ZrP/MnTMPyP hybrid film-modified GCE at −0.256 and −0.197 V, respectively, and the peak separation ΔE p was reduced to 59 mV.

Fig. 8
figure 8

CV curves of a MnTMPyP aqueous solution (dash line) and b α-ZrP/MnTMPyP-modified GCE (solid line) in N2-saturated pH 7.0 PBS solution at 50 mV s−1

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As shown in Fig. 9, the CV experiments of the modified electrode in 0.2 M PBS at different scan rates from 50 to 400 mV s−1 were conducted. It is observed that the anodic peak shifted positively and the cathodic peak shifted negatively with the increase of the scan rate. Meanwhile, the peak separation ΔE p went from 59 mV to 123 mV as well owing to the steric hindrance effect of the host layer [47]. The linear relationship of peak current (I) and square root of the scan rate (v 1/2) was expressed in the attached drawing. The calibration equations are I pa (μA) = 9.19 − 43.32 v 1/2 (V1/2 s−1/2) (r = 0.9985) and I pc (μA) = −7.87 + 37.57 v 1/2 (V1/2 s−1/2) (r = 0.9991).

Fig. 9
figure 9

CV curves of α-ZrP/MnTMPyP-modified GCE in N2-saturated pH 7.0 PBS solution at 50, 100, 150, 200, 300, and 400 mV s−1; the inset is the relationship curve between I and v 1/2

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Electrocatalytic activities of α-ZrP/MnTMPyP hybrid film toward oxidation of nitrite

As can be seen from Fig. 10, α-ZrP/MnTMPyP hybrid film-modified GCE shows good electrocatalytic activities toward oxidation of nitrite in pH 7.0 PBS. The oxidation peak potential at 0.997 V should be assigned to bare GCE, which is corresponding to the conversion of NO2 − to NO3 − through a two-electron oxidation process, while the oxidation peak potential of the modified electrode shifts negatively toward 0.897 V with an increase in peak current at a certain degree indicating that α-ZrP/MnTMPyP nanocomposite can efficiently promote the oxidation of NO2 − [42].

Fig. 10
figure 10

CV curves of a α-ZrP/MnTMPyP-modified GCE (solid line) and b bare GCE (dash line) in N2-saturated pH 7.0 PBS solution containing 4 mM NaNO2 at 100 mV s−1

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According to related literatures [48], the mechanism of electrocatalytic oxidation on nitrite of the modified electrode can be illustrated using the following equations:

$$ {\text{Mn}}^{\text{III}} {\text{TMPyP}}^{ 5+ } + {\text{NO}}_{ 2}^{ - } \rightleftharpoons \left[ {{\text{Mn}}^{\text{II}} {\text{TMPyP }}\left( {{\text{NO}}_{ 2} } \right)} \right]^{ 4+ } $$
(1)
$$ \left[ {{\text{Mn}}^{\text{II}} {\text{TMPyP }}\left( {{\text{NO}}_{ 2} } \right)} \right]^{ 4+ } \rightleftharpoons {\text{Mn}}^{\text{III}} {\text{TMPyP}}^{ 5+ } + {\text{NO}}_{ 2} + {\text{e}}^{ - } $$
(2)
$$ 2 {\text{NO}}_{ 2} + {\text{H}}_{ 2} {\text{O}} \rightleftharpoons 2 {\text{H}}^{ + } + {\text{NO}}_{ 3}^{ - } + {\text{NO}}_{ 2}^{ - } $$
(3)
$$ {\text{NO}}_{ 2}^{ - } + {\text{H}}_{ 2} {\text{O}} \to 2 {\text{H}}^{ + } + {\text{NO}}_{ 3}^{ - } + 2 {\text{e}}^{ - } $$
(4)

With the purpose of investigating the influence of the scan rate toward peak current, the CV curves of the modified electrode at different scan rates from 30 to 200 mV s−1 were made. It can be seen that the anodic peak potential shifts positively with the increase of scan rate observed in Fig. 11. The inset shows linear relationship between peak current (Ipa) and square root of the scan rate (v 1/2), in the light of the calibration equation: I (μA) = 5.30 + 229.46 v 1/2 (V1/2 s−1/2) (r = 0.9988), electrochemical oxidation of nitrite on the surface of the modified electrode should be defined as an irreversible diffusion-controlled process [49].

Fig. 11
figure 11

CV curves of α-ZrP/MnTMPyP-modified GCE in N2-saturated pH 7.0 PBS solution containing 4 mM NaNO2 at 30, 40, 50, 60, 70, 100, 150, and 200 mV s−1; the inset is the relationship curve between I pa and v 1/2

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In order to realize the quantitative analysis of the modified electrode toward nitrite oxidation, differential pulse voltammetry experiments were conducted with the NO2 − concentration ranging from 0.15 to 4.76 mM shown in Fig. 12. The linear relationship between the peak current (I pa) and the concentration of nitrite was observed in the inset whose calibration equation can be expressed as I (μA) = −0.45 + 6.16c (mmol L−1) (r = 0.9995). According to a signal-to-noise ratio of 3.0, the detection limit of 5.3 × 10−5 M was estimated.

Fig. 12
figure 12

DPV curves of α-ZrP/MnTMPyP-modified GCE in N2-saturated pH 7.0 PBS solution with the NaNO2 concentration ranging from 0.15 to 4.76 mM; the inset is the relationship curve between I pa and c

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Conclusions

A convenient method called the exfoliation/restacking route was adopted to prepare α-ZrP/MnTMPyP laminar nanocomposite. The α-ZrP colloidal suspension obtained in the delamination process was traced by a Zetasizer Nano instrument. In addition, the arrangement of MnTMPyP molecule in the galley of the hybrid has been proposed. The CV measurements of α-ZrP/MnTMPyP film-modified GCE indicated that the as-obtained nanocomposite has exhibited excellent electrocatalytic activities on the oxidation of nitrite in pH 7.0 PBS. A detection limit of 5.3 × 10−5 M was estimated at a signal-to-noise ratio of 3.0 indicated by DPV results.