Introduction

In recent years, layered materials with particular structure and properties have attracted tremendous interests in fields of electrochemical and photochemical catalysis [1,2,3,4,5,6,7]. As a typical layered metal oxide semiconductor firstly discovered in 1979 [8], layered CsTi2NbO7 has been widely investigated as electrocatalysts due to its ability of ion exchanging [9], swelling [10], and delaminating to nanosheets [11], favorable charge transfer, and easy modification characters [12,13,14]. The corresponding crystal structure of CsTi2NbO7 is shown in Fig. 1; cesium ions (Cs+) lie between MO6 (M=Ti, Nb) octahedral layers, which are joined by sharing corner and edge. One of the most remarkable aspects of the exfoliated nanosheets is that they can be used as building blocks for fabricating various nanostructures [15]. However, the drawbacks of pure CsTi2NbO7 are (i) low electrochemical activity and (ii) small specific surface area. Therefore, it is necessary to take some efficient modifications to enhance electrochemical activity of CsTi2NbO7.

Fig. 1
figure 1

The crystal structure of CsTi2NbO7

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As we know, water-soluble porphyrins and their derivatives show excellent activities in redox, optics, and electricity [16], which are sensitive to pH and temperature. In our group, metalloporphyrin, such as MnTMPyP and CoTMPyP, has been successfully intercalated into the interlayer spacing of K2Ti4O9 [17], K4Nb6O17 [18, 19], KLaNb2O7 [20], and KNb3O8 [21], showing enhanced electrochemical catalytic activities towards O2, NaNO2, and H2O2. It is noteworthy that layered inorganic matrices with large surface area are really needed for the synthesis of solventless catalysts and have been chosen as a class of preferable material on account of their better stability during the process of electrochemical detection. Therefore, it is feasible to construct a novel nanocomposite by combining metalloporphyrin with Ti2NbO7− nanosheets to solve the abovementioned problems.

Ascorbic acid (AA) is a water-soluble intracellular antioxidant that can directly participate in various biological reactions, and plays an important role in the regulation of the activity of cellular immunity including synthesizing collagen, remedying scurvy, and preventing arteriosclerosis and cancer [22, 23]. Therefore, the detection of AA has aroused great interests in recent decades. Up to now, various methods including colorimetric method [24], flow injection [25], fluorometric method [26], and electrochemistry [27, 28] have been used to detect it. Among those detection methods, the electrochemical technique has been widely used due to short time and low cost. Recently, there are various porphyrin-based nanocomposites that have been utilized in the electrochemical detection of AA, such as KLaNb2O7 [20], graphene [29], and carbon nanotubes [30].

In this paper, we have developed a novel sandwich-structured nanocomposite based on Ti2NbO7− nanosheets and cobalt porphyrin (CoTMPyP). The resultant Ti2NbO7/CoTMPyP composite was detailed and characterized by X-ray powder diffraction (XRD), scanning electron micrograph (SEM), transmission electron microscope (TEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared (FTIR), and UV-vis. Furthermore, the electrocatalytic oxidation of AA on Ti2NbO7/CoTMPyP-modified electrode was studied via cyclic voltammetry (CV) and differential pulse voltammetry (DPV) analysis technologies.

Experimental Details

Exfoliation of Ti2NbO7 − Nanosheets

The detailed preparation process for Ti2NbO7/CoTMPyP composite was shown in Fig. 2. Layered CsTi2NbO7 was synthesized by calcinating the mixture of Cs2CO3, TiO2, and Nb2O5 with the molar ratios of 1.1:4:1 and continuously heated at 750, 950, and 1050 °C for 12 h at each temperature [10]. The protonated form of HTi2NbO7 was obtained by treating CsTi2NbO7 with 2 mol L−1 HCl for 3 days at room temperature, and the acid was exchanged each day to ensure proton reaction. Then, 0.05 g HTi2NbO7 was dispersed in 20 mL distilled water, and then 430 μL TBAOH aqueous solution (10 wt%) was added under vigorous stirring for 4 days. The resultant subtransparent colloidal solution was centrifuged at 4000 rpm for 10 min to remove unexfoliated particles.

Fig. 2
figure 2

Schematic illustration of the self-assembly process of Ti2NbO7/CoTMPyP nanocomposite

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Fabrication of Ti2NbO7/CoTMPyP Composite

To prepare Ti2NbO7/CoTMPyP composite, 1 mmol L−1 CoTMPyP aqueous solution was added dropwise into Ti2NbO7− nanosheet colloidal solution with an appropriate volume ratio (CoTMPyP: Ti2NbO7− = 1:1). After hours, the flocculated precipitate was centrifuged (8000 rpm) and washed with distilled water for three times. The product was dried at 50 °C in a vacuum oven overnight.

Apparatus

X-ray diffraction patterns of the resulted samples were carried out with a RINT 2000 diffractometer (Rigaku) using Cu Kα radiation (λ = 0.154 nm) with 2θ ranging from 1.5° to 60° at a scan rate of 1°/min. The scanning electron micrograph (SEM) images were collected by a JSM-5600 apparatus (JEOL) operating at 15 kV for the Au-coated samples. The TEM images were obtained by a Philips Tecnai 12 transmission electron microscope operating at 120 kV. The EDS analysis was performed on a FEI Tecnai G2F30S-TWIN high-resolution transmission electron microscope. The atomic force microscope (AFM) images were taken with a Bruker dimension edge SPM apparatus. FTIR spectra were measured on a Shimadzu FTIR-8400S apparatus spectrometer with KBr pellets. UV-vis absorption spectra were recorded using a UV-vis spectrometer (UV-2550). Zeta potential of Ti2NbO7− colloidal solution was collected by a Malvern Zetasizer Nano instrument. Electrochemical measurements were performed with a three-electrode electrochemical cell in a CHI660E electrochemical workstation, with a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode, and a glass carbon electrode (GCE) coated by the Ti2NbO7/CoTMPyP hybrid as the working electrode.

Results and Discussion

Characterization of Ti2NbO7/CoTMPyP Nanocomposite

The XRD pattern of CsTi2NbO7 has a characteristic peak at 9.62° (Fig. 3a), corresponding to (020) of JCPDS Card No. 73-0680, matching well with the diffraction peaks as previously reported [12]. After the proton exchange reaction, the (020) peak shifts from 9.62° to 8.36° due to the exchange of Cs+ to H3O+. By combination of Ti2NbO7− nanosheets with CoTMPyP, (020) peak of the resulted Ti2NbO7/CoTMPyP was changed to 4.06°, demonstrating the successful intercalation of metalloporphyrin molecule into Ti2NbO7 layers. In addition, it was confirmed that different volume ratios of Ti2NbO7 and CoTMPyP have no effect on the arrangement of Ti2NbO7/CoTMPyP nanocomposite (Fig. S1). The basal spacing and Δd values of products are listed in Table 1. Since the basal spacing of host CsTi2NbO7 is 0.92 nm, the thickness of the single Ti2NbO7− layer can be calculated as 0.58 nm by subtracting the diameter of Cs+ (about 0.338 nm) [31], and the net interlayer height of Ti2NbO7/CoTMPyP can be calculated as 1.59 nm. In consideration of the molecular dimension of CoTMPyP (1.8 nm × 1.8 nm × 0.75 nm, estimated by MM2 calculation) [20], the arrangement of CoTMPyP molecular between the interlayers of Ti2NbO7− nanosheets can be inferred as a tilted monolayer and its inserting angle is approximately 63° (Fig. 4).

Fig. 3
figure 3

XRD patterns of (a) CsTi2NbO7, (b) HTi2NbO7, and (c) Ti2NbO7/CoTMPyP

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Table 1 XRD data of the different samples
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Fig. 4
figure 4

Proposed possible arrangement of CoTMPyP molecules between the Ti2NbO7 nanosheets

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The SEM images of the resulted samples were shown in Fig. 5. The SEM image of the host CsTi2NbO7 is compact, with a typical layered structure (Fig. 5a). After proton reaction, the obtained HTi2NbO7 remains the two-dimension laminar appearance (Fig. 5b), and then through exfoliation/restacking process, the resultant Ti2NbO7/CoTMPyP was arranged with stratified structure (Fig. 5c). In an effort to assess the size and thickness of Ti2NbO7− nanosheets, the diluted exfoliated solution sample was dripped on the mica plate where AFM imaging was performed at multiple locations across the sample. As shown in Fig. 5d, e, the Ti2NbO7− nanosheet possesses an unilaminar morphology with its surface distance of approximately hundreds of nanometers and vertical distance of 1.08 nm, in line with the results reported in previous literature [15]. In comparison with the thickness calculated by XRD data, the thicker nanosheets may be owing to the existence of H3O+ molecule on the surface of nanosheets.

Fig. 5
figure 5

SEM images of a CsTi2NbO7 and b HTi2NbO7. c TEM image of Ti2NbO7/CoTMPyP. d AFM images of Ti2NbO7 nanosheets. e Nanoscope analysis of Ti2NbO7 nanosheets

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The FT-IR spectra of CsTi2NbO7, CoTMPyP, and Ti2NbO7/CoTMPyP were shown in Fig. 6a. For CoTMPyP, a peak at 1640 cm−1 can be ascribed to the stretching vibration of C=N in the pyridine substituent group, while those at 1510, 1410, and 1090 cm−1 are assigned to the stretching vibration of C=N, C=C, and C-N in porphyrin rings, respectively. The bands at region from 1000 to 400 cm−1 are characteristic peaks of Ti-O and Nb-O stretching vibration in CsTi2NbO7. After combining CoTMPyP with Ti2NbO7 nanosheets, the characteristic peaks of CoTMPyP were also observed with a shift in the resulted Ti2NbO7/CoTMPyP composite. It indicates that there is a strong interaction between CoTMPyP with Ti2NbO7 nanosheets.

Fig. 6
figure 6

a FT-IR spectra of (a) CsTi2NbO7, (b) CoTMPyP, and (c) Ti2NbO7/CoTMPyP. b UV-vis absorption spectra of (a) CoTMPyP aqueous solution and (b) Ti2NbO7/CoTMPyP

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UV-vis optical spectra of CoTMPyP and Ti2NbO7/CoTMPyP nanocomposite were shown in Fig. 6b. The CoTMPyP aqueous solution (Fig. 6b (a)) has a Soret band and a Q band in 437 and 550 nm, respectively. There are 18 and 13 nm red shifts in Soret and Q band after intercalation. Besides, the two broaden absorption bands in Ti2NbO7/CoTMPyP are probably caused by the effect of steric hindrance of immobilizing CoTMPyP molecule between Ti2NbO7 layers. There are numerous similar situations reported in the recent years [32, 33]; some of them speculate that a certain degree of superposing and reuniting of metalloporphyrin molecules may result in these phenomena [34]. These results confirmed a strong intercalation of CoTMPyP molecules into Ti2NbO7− nanosheets.

The zeta potential analysis was carried out to study the driving force of the exfoliation/restacking process. As is shown in Fig. S2, tyndall phenomenon can be observed in the exfoliated colloidal suspension with a zeta potential of − 41.8 mV, which expresses the colloid is in a stable and well-disseminated condition. When CoTMPyP aqueous solution was added into Ti2NbO7− nanosheets, the potential of Ti2NbO7− nanosheets increased gradually, accompanied with a large number of flocculent precipitates (Fig. 7). It shows that the exfoliation/restacking approach is unique as it not only exhibits excellent time-saving advantage but also makes the best use of high specific surface area of Ti2NbO7− nanosheets. When the volume ratio of CoTMPyP aqueous solution and Ti2NbO7− nanosheets was 0.65, the potential value tends to be zero. Furthermore, the potential value changed towards positive by a continuous addition of CoTMPyP into Ti2NbO7− nanosheets. It can be inferred that the recombination process of Ti2NbO7/CoTMPyP nanocomposite is based on electrostatic force.

Fig. 7
figure 7

The relationship between zeta potential and CoTMPyP/Ti2NbO7− volume ratio

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In order to certify the combination of Ti2NbO7− nanosheets and CoTMPyP, EDS analysis was performed via dropping the dispersion liquid of Ti2NbO7/CoTMPyP nanocomposite on copper wire mesh. Figure S3 indicates the presence of C, N, O, Ti, Nb, and Co elements in the nanocomposite. Furthermore, Ca signal comes from the ultrapure water.

Electrochemical Behavior of Ti2NbO7/CoTMPyP Nanocomposite Thin Film

Based on the above combined results, it can be concluded that Ti2NbO7/CoTMPyP nanocomposite was successfully fabricated through electrostatic self-assembly method. Then, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) analysis technologies were measured to further study the electrocatalytic performance of this hybrid film. Firstly, it is necessary to understand the role of Ti2NbO7− matrix during a redox process. Figure 8a gives a comparison of electrochemical behaviors between CoTMPyP aqueous solution and Ti2NbO7/CoTMPyP-modified electrode in N2-saturated 0.2 mol L−1 and pH 7.0 PBS at a scan rate of 100 mV s−1. There are two pairs of redox peaks for CoTMPyP aqueous solution; the reduction potentials are located at − 0.751 and − 0.953 V, corresponding to CoII/ITMPyP and CoIII/IITMPyP redox couple, respectively [35]. While the redox peaks for Ti2NbO7/CoTMPyP-modified electrode appear at − 0.745 and − 0.969 V, it is obvious that the potential of CoIII/IITMPyP redox couple turns negative accompanied with a clear increase in current, indicating that the Ti2NbO7 inorganic matrix in the nanocomposite can promote the movement of charge in the electrochemical process [36]. The reasons for the superior electrochemical performances of Ti2NbO7/CoTMPyP nanocomposite may derive from the truth that the layered Ti2NbO7 provides a two-dimensional channel and acts as an electron transporter during the process.

Fig. 8
figure 8

a CV curves of (a) CoTMPyP aqueous solution and (b) Ti2NbO7/CoTMPyP-modified electrode in N2-saturated 0.2 mol L−1 and pH 7.0 PBS at a scan rate of 100 mV s−1. b CV curves of (a) Ti2NbO7/CoTMPyP-modified electrode (solid line) and (b) bare electrode (dash line) in N2-saturated 0.2 mol L−1 and pH 7.0 PBS containing 8 mmol L−1 AA at a scan rate of 50 mV s−1

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Secondly, in this paper, ascorbic acid was chosen for testing the electrochemical oxidation abilities towards chemical species on the surface of Ti2NbO7/CoTMPyP-modified electrode. To begin with, a comparison between bare electrode and modified electrode has been implemented. The CV curves were conducted in N2-saturated 0.2 mol L−1 and pH 7.0 PBS containing 8 mmol L−1 AA at a scan rate of 50 mV s−1. As is shown in Fig. 8b, there is a strong oxidation process of AA; however, the coupled cathodic signal is absent in the reverse scan. This phenomenon is caused by the irreversible electron transfer process. In previous studies, it has been proved that when the oxidation process of ascorbic acid proceeds in pH < 8 condition, the existence of two successive one electron oxidation steps accompanied by rapid dehydration leads to irreversibility [37,38,39]. Obviously, the oxidation peak potential for bare electrode is around 0.28 V, while for Ti2NbO7/CoTMPyP-modified electrode, the oxidation potential shifts negatively towards 0.041 V and the oxidation peak current increases remarkably. It really indicates that the Ti2NbO7/CoTMPyP nanocomposite has promoted the electron transfer reaction of AA. For this reason, there is no doubt that Ti2NbO7/CoTMPyP nanocomposite is an excellent catalyst for electrochemical oxidation towards AA.

In addition, the influence of scan rates was examined. In Fig. 9a, with the improvement of scan rates, the oxidation peak current increases gradually and the oxidation peak potential turns positively; this may be also attributed to the irreversibility of AA oxidation [36]. Furthermore, as we can see in Fig. 9b, there is a good linear relationship between square root of scan rate (v1/2) and oxidation peak current (Ipa). The linear equation can be expressed as Ipa = − 47.65 − 15.65 v1/2, |R| = 0.9993, so it can be speculated that the electrochemical oxidation process towards AA on the modified electrode is diffusion-controlled. What’s more, according to related literatures [40,41,42], the mechanism of electrocatalytic oxidation of ascorbic acid on the modified electrode can be proposed as follows:

$$ {\mathsf{H}}_{\mathsf{2}}\mathsf{A}\to {\mathsf{H}\mathsf{A}}^{-}+{\mathsf{H}}^{+} $$
(1)
$$ {\mathsf{Co}}^{\mathsf{II}\mathsf{I}}\mathsf{TMPyP}+{\mathsf{H}\mathsf{A}}^{-}\to {\mathsf{Co}}^{\mathsf{II}}\mathsf{TMPyP}\bullet \bullet \bullet {\mathsf{A}}^{\bullet -}+{\mathsf{H}}^{+} $$
(2)
$$ {\mathsf{Co}}^{\mathsf{II}}\mathsf{TMPyP}\bullet \bullet \bullet {\mathsf{A}}^{\bullet -}\to {\mathsf{Co}}^{\mathsf{II}}\mathsf{TMPyP}+\mathsf{A}+{\mathsf{e}}^{-} $$
(3)
$$ {\mathsf{Co}}^{\mathsf{II}}\mathsf{TMPyP}\to {\mathsf{Co}}^{\mathsf{II}\mathsf{I}}\mathsf{TMPyP}+{\mathsf{e}}^{-} $$
(4)
$$ \mathsf{The}\mathsf{total}\mathsf{reaction}\mathsf{is}\ {\mathsf{H}}_{\mathsf{2}}\mathsf{A}\to \mathsf{A}+{\mathsf{2}\mathsf{H}}^{+}+{\mathsf{2}\mathsf{e}}^{-} $$
(5)
Fig. 9
figure 9

a CV curves of Ti2NbO7/CoTMPyP-modified electrode in N2-saturated 0.2 mol L−1 and pH 7.0 PBS containing 8 mmol L−1 AA at a scan rate of 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, and 300 mV s−1, respectively. b The relationship curve between the oxidation peak current (Ipa) and the square root of scan rate (v 1/2). c DPV curves of Ti2NbO7/CoTMPyP-modified electrode in N2-saturated 0.2 mol L−1 and pH 7.0 PBS with different concentrations of AA. d The relationship between AA concentration (C) and peak current (Ipa)

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The controlling step of this process is the migration of HA− towards the modified electrode surface, namely rate-determining step.

Thirdly, differential pulse voltammetry (DPV) analysis technology was utilized for accurate electrochemical determination of AA (Fig. 9c). The relationship between AA concentration (C) and peak current (Ipa) was given in Fig. 9d, and the linear equation can be expressed as Ipa = − 0.093 − 4.36 C (mmol L−1) (|R| = 0.9995) with a detection range of 4.99 × 10−5 to 9.95 × 10−4 mol L−1. A detection limit of 3.1 × 10−5 mol L−1 can be calculated at a signal-to-noise ratio of 3.0. These results confirm that the sensitivity of Ti2NbO7/CoTMPyP towards ascorbic acid detection is comparable with other reported modified electrodes (Table S1).

Last but not least, in order to investigate the stability of Ti2NbO7/CoTMPyP-modified electrode, the CVs and DPVs for 0.25 mmol L−1 ascorbic acid in 0.2 mol L−1 and pH 7.0 PBS were recorded for every 2 min (Figs. S4 and S5). It was found that the oxidation peak current and potential remained almost the same with the relative standard deviation (RSD) of 0.38% for CVs and 0.25% for DPVs. In addition, to ascertain the reproducibility of the results, Ti2NbO7/CoTMPyP hybrid was coated on two different electrodes and stored at room temperature for a month; the RSD was only 0.78% (Fig. S6). All these observations indicate that Ti2NbO7/CoTMPyP nanocomposite has satisfactory stability, reproducibility, and repeatability.

Conclusion

A facile and rapid electrostatic self-assembly technique was used for the combination of Ti2NbO7− nanosheets and CoTMPyP cations for the first time. The size and thickness of Ti2NbO7− nanosheets were confirmed by AFM. Besides, the zeta potential of exfoliated nanosheets was − 41.8 mV, suggesting the colloidal solution was in a stable and well-dispersed condition. In addition, the Ti2NbO7/CoTMPyP nanocomposite with outstanding stability and reproducibility exhibits excellent electrochemical catalytic ability towards ascorbic acid in pH 7.0 PBS, and a detection limit was calculated as 3.1 × 10−5 mol L−1 by DPV analysis. In summary, this research offers a convenient method for the development of CsTi2NbO7-based lamellar nanocomposites, and it can be pointed out that the Ti2NbO7/CoTMPyP nanocomposite is a promising material in constructing ascorbic acid biosensors.