Introduction

Magnetic nanocomposites, integrating unique magnetic properties/functions and desired functions from other components, have drawn increasing attractions and have been applied in various fields [1,2,3,4,5]. Plenty of preparation methods have been developed to meet various requirements. Due to abundant surface properties of magnetic materials, it is readily to introduce/modify other components onto the surface of magnetic core and form functional shell(s) [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]. The exposed shell makes itself sensitive and reactive to the surroundings, thus realizes applications such as sensing [6, 7, 10, 11, 16], adsorption [12, 13], catalysis [14, 15], therapy [9], and so on. On the other hand, the shell cooperates with the magnetic core, even generates synergistic effect [9, 12, 13]. Currently, the construction of single (functional) layer of shell is well established [6,7,8,9,10]. On the other hand, multiple layers of shells have to be elaborated for multiple extraneous functions [22,23,24,25]. The introduction of multiple shells, however, makes the preparation complicated and time consuming. It also brings about other deficiencies such as the unstable linking between layers and the spatial separation of magnetic core from the outer shell. Therefore, methods that can introduce layer of shell with multiple components/functions should flourish the branches of magnetic nanocomposites and lead to new applications.

Owning to the abundant properties of ferrous elements and their compounds, plenty of strategies have been developed for the modification of outer layers (shells) to magnetic nanomaterials, mainly including physical and chemical ways [1, 3,4,5]. Recently, a sacrificing method was also explored with characteristics of supplying reactants of shell from the sacrificing of magnetic nanomaterials themselves through chemical/electrochemical ways [7, 9, 10]. Briefly, ferrous components in magnetic nanomaterials release ferrous ions in acidic solution, followed by the formation of new shell via the reaction between the released ferrous ions with other co-existing reactants. For example, recently our group developed a new electrochemical method to converse magnetic nanoparticles (MNPs) to Prussian blue (PB) [7]. By applying a high-potential on the gold electrode, water can be split into gaseous oxygen and hydrogen ions, which reacted with MNPs (Fe3O4) to release ferrous ions. Then in a low-potential process, ferrous ions reacted with co-existing Fe(CN)64− ions to form PB. The sacrificing methods presented unique proximity effect, namely, confining the reactions occurred on the surface of magnetic nanomaterials. Moreover, the formed shell shares same ferrous components with magnetic nanomaterials, gifting high stability of the core@shell structure. However, current researches exploited the sacrificing method for the generation of sole-component shell, leaving the methods for the preparation of multiple-component shell rarely exploited [7, 9, 10]. Furthermore, the shell generated from the sacrificing method is PB analogues. As well known, PB analogues are with high catalysis ability and even the peroxidase-like activity, however, the lack of chemical groups on the surface inhibits further functionalization [26], so complicated procedures are required to introduce other species onto PB surface [27,28,29]. This significantly limits potential of applications. Therefore, by integrating the unique advantages of sacrificing method with other properties/functions of magnetic nanomaterials, it remains vast space and impetus to exploit new method to extend the modification/functionalization of magnetic nanomaterials to fulfill diversified requirements of applications.

A self-sacrificing-catalyzing method is proposed to prepare MNP@Au-PB core@dual-functional-shell nanocomposites for sensing applications. Owning to the wide presence of H2O2 in natural environmental and applications in industrial fields and biological study, efficient detection of H2O2 is of high importance [30,31,32,33]. Therefore, electrochemical and colorimetric sensors for H2O2 were developed using the new nanocomposites. Briefly as shown in Scheme 1, HCl, K4Fe(CN)6, HAuCl4 and NH2OH are added to the suspension of Fe3O4, in which there are two paths of chemical reactions occurred. The first one is that HCl reacts to Fe3O4 to release ferrous ions (Fe3+ and Fe2+), which then react to coexisting K4Fe(CN)6 to generate PB and Prussian white (PW, analogue of PB, classified as PB in convenience below) in the proximity of MNPs. The second one is the reduction of HAuCl4 by NH2OH to generate Au nanoparticles triggered by the catalysis effect of MNPs. Finally, the nanocomposites with MNPs as the core and with Au-PB as the dual-functional-shell are yielded. This method takes advantages of the chemical reactivity and catalysis effect of MNPs, generates the dual-functional-shell composites in a one-pot and one-step way. Three state-of-art sensing applications were further studied by exploring the multi-function deriving from the shell. Digital inspection, electrochemical methods, scanning electron microscopy (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were employed to characterize the morphology, constituent, electrochemical and catalytic properties of the composites. The yielded dual-functional-shell nanocomposites presented higher catalysis activity to H2O2 and higher immobilization amount of choline oxidase (ChOx) comparing with the analogues with single-shell. Electrochemical and colorimetric sensing and biosensing platforms were further developed based on these performances. The method might bring new insight for the preparation of multi-functional magnetic composites for diversified applications.

Scheme 1
scheme 1

Illustration of the preparation of MNP@Au-PB nanocomposites

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Experimental section

Materials and apparatus

Fe3O4 MNPs were purchased from Aladdin Bio-Chem Technology Co., LTD (Shanghai, China, www.aladdin-e.com). Hydrogen peroxide (H2O2, 30%), sulfuric acid (H2SO4, assay NLT 98.0%), hydrochloric acid (HCl, assay NLT 36.0%), potassium ferricyanide (K3Fe(CN)6), potassium ferrocyanide (K4Fe(CN)6), ferrous sulfate (FeSO4), hydroxylamine hydrochloride (NH2OH·HCl), chloroauric acid (HAuCl4) and sodium citrate (Na3C6H5O7) were purchased from Shanghai Chemical Reagent Co. (Shanghai, China, www.reagent.com.cn). ChOx and choline chloride were purchased from Sigma-Aldrich (St. Louis, MO, USA, www.sigmaaldrich.com). N-(Sulfopropyl)-3,3′,5,5-tetramethylbenzidine, sodium salt (TMB-PS), L-Cysteine, L-Aspartic acid, L-Glycine and L-Proline were purchased from Sangon Biotech. (Shanghai, China, www.sangon.com). Phosphate buffer (0.1 M, pH = 6.0, 0.1 M K2HPO4/KH2PO4 + 0.1 M K2SO4) was adopted for sensing. All chemicals were of analytical grade or better quality. Milli-Q (Millipore, ≥18 MΩ cm) ultra-pure water was used throughout. All experiments were conducted at room temperature (25 °C).

All electrochemical experiments were conducted on a CHI660E electrochemical workstation (CH Instrument Co., China, www.chinstr.com) with a conventional three-electrode electrolytic cell. The Au electrode with 2.0-mm diameter served as the working electrode, a KCl-saturated calomel electrode (SCE) as the reference electrode, and a carbon rod as the counter electrode. All potentials reported here are cited versus SCE. The FT-IR spectrophotometry, SEM images, TEM images and UV-vis spectrophotometry were collected in a JASCO-4100 Fourier transform infrared spectrometer (Nicolet Instrument Co., Madison, www.thermofisher.com), SU-8010 (Hitachi, Japan, www.hitachi.com) scanning electronic microscope, JEOL JEM-1200EX transmission electron microscope (Hitachi, Japan, www.hitachi.com) and Agilent 8453 UV-vis spectrophotometer (Agilent, USA, www.agilent.com), respectively.

Preparation of MNP@Au-PB, MNP@PB and PB

For the typical preparation of MNP@Au-PB, 4 μL of HCl (1 M), 50 μL of HAuCl4 (1 mM), 50 μL of NH2OH·HCl (0.1 M) and 50 μL of K4Fe(CN)6 (10 mM) were added to 150 μL of MNPs suspension (0.5 mg mL−1) in sequence. After 10 min, the yielded black-blue suspension was washed with dilute hydrochloric acid (pH = 6.0) for three times assisted by magnetic separation and was finally dispersed in 0.1 M phosphate buffer (pH = 6.0).

The preparation of MNP@PB was similar to that for the MNP@Au-PB except the addition of NaAuCl4 and NH2OH·HCl. For the preparation of PB thin film, a typical electrochemical deposition method was adopted according to literature [34].

Electrochemical detection of H2O2

Firstly, 2 μL of MNP@Au-PB suspension was dropped on the surface of Au electrode. By placing a magnet under the electrode, the MNP@Au-PB composites were anchored on the electrode for further characterization and detection. The characterization of the presence of PB and Au was conducted using CV in a mixture solution containing 0.1 M H2SO4 and 0.1 M K2SO4 and a solution of 0.05 M H2SO4, respectively. Potentiostatic measurements at −0.05 V were adopted for the electrochemical detection of H2O2 in 0.1 M phosphate buffer (pH 6.0). Briefly, H2O2 stock solutions of different concentrations were successively added to the stirred phosphate buffer, and the differences between currents before and after each addition of H2O2 were adopted as response signals.

Optical detection of H2O2

Firstly, 30 μL of TMB (10 mM) and 30 μL of H2O2 of different concentrations were added into 30 μL of MNP@Au-PB suspensions and kept for 10 min at room temperature for reaction, followed by the addition of 10 μL of 6 M H2SO4 to stop the reaction. Second, the suspension was magnetically separated and 60 μL of the supernatant was used for quantification in UV-vis measurements at 460 nm. For the specificity evaluation, H2O2 was replaced by ascorbic acid, L-cysteine, L-aspartic acid, L-proline, L-glycine and urea, respectively.

Optical detection of glucose in wine samples

The white grape wine, J.J.Muller wine (Jerry Moselle, Germany), was bought from a local supermarket (Hangzhou, China). For glucose assay, the calibration plot was obtained first. The detection procedures are similar to that for H2O2 except that the addition of 30 μL of H2O2 was replaced by the addition of the mixture of 15 μL of 0.6 mg·mL−1 glucose oxidase and 15 μL of glucose of different concentrations (n = 3). For the detection of glucose in wine samples, the procedures were the same to the assay of glucose except replacing the addition of 15 μL glucose solutions by 15 μL of wine samples (n = 3).

Immobilization of ChOx

MNP@Au-PB suspensions were diluted 10-fold with 0.1 M phosphate buffer (pH = 6.0, 50 μL), which were then mixed with 50 μL of ChOx (1 mg mL−1) and kept at 4 °C for 1 h. Afterwards, the mixtures were magnetically separated and washed with 0.1 M phosphate buffer (pH = 6.0, 50 μL) for three times, and the supernatants were collected for the quantification using UV-vis spectroscopy at 280 nm. The mixtures of 50 μL of 0.1 M phosphate buffer (pH = 6.0) and 50 μL of ChOx (1 mg mL−1) were adopted as the control and treated similarly, followed by the quantification of ChOx in supernatants using the same method. The immobilization rate was calculated as the rate of the absorbance of ChOx in the MNP@Au-PB case to that of the control. For comparison, the immobilization rates of ChOx in MNPs, MNP@PB and MNP@Au cases were also obtained using the same method.

Results and discussion

Characterization of MNP@Au-PB

We inspected the preparation of the MNP@Au-PB nanocomposites through digital imaging, as shown in Table S1 (please see detailed description in ESM). Because the preparation involved a series of precursors, we thus mixed some precursors to investigate the mechanism. The mixture of NaAuCl4 and NH2OH (sample 2) presented colorless, because NH2OH was not able to reduce NaAuCl4 solely [6]. In contrast, the presence of MNPs led to the production of MNP@Au (sample 3) [6]. The reason was that the reduction of NaAuCl4 by NH2OH was catalyzed by the nucleating effect from MNPs [6]. The mixing of MNPs, HCl and K4Fe(CN)6 (sample 4) led to rapid appearance of plenty of blue PB products, as illustrated as the Path 1 in Scheme 1. The mixture of NaAuCl4, NH2OH, K4Fe(CN)6 and HCl (sample 6) slowly turned to light blue due to the formation of PB-like KFex[Au(CN)2]y compounds [35]. Finally, the mixture of MNPs, NaAuCl4, NH2OH, K4Fe(CN)6 and HCl (sample 7) rapidly showed blue-light-green color, indicating the co-existing of PB and PB-like KFex[Au(CN)2]y compounds. The PB-like KFex[Au(CN)2]y were generated in solution while Au-PB was on the surface of MNPs (proximity effect). Hence, one can readily purify the MNP@Au-PB nanocomposites by magnetic separation (sample 9). In a magnetic field, MNP@Au-PB nanocomposites were rapidly collected leaving the supernatant totally clear, indicating the MNPs core and the magnetic property were well retained even after the self-sacrificing. It should guarantee the magnetic operability for applications.

SEM and TEM were further used to characterize the morphology of MNP@Au-PB, as shown in Fig. 1. The bare MNPs presented as nano-spheres (images a and c), and the MNP@Au-PB showed sphere-like clusters of cubes with whole size of 258 ± 56 nm (n = 25) (images b and d). According to previous report, these cubes were PB crystals [36]. Imaginably, the core surrounded by PB crystals should be the residual MNPs. Moreover, FTIR and XPS were adopted to characterize MNP@Au-PB in order to verify the composition of MNP@Au-PB (Fig. S1). As shown in Fig. S1a for FTIR, the bands of CN group, M-CN-M′ and Fe-O bond indicated the contents of PB and Fe3O4 [10]. As shown in Fig. S1b for the XPS results, the peaks of Fe (725.1 eV and 710.1 eV for Fe 2p1/2 and Fe 2p3/2, respectively), C (284 eV), N (397.8 eV), and O (530.1 eV) represented the presence of Fe3O4 and PB in both MNP@Au-PB and MNP@PB [37, 38]. Specially for MNP@Au-PB, two doublet peaks are observed with maxima at 84.1 eV and 88.1 eV, which are assigned to the 4f7/2 and 4f5/2 core levels of Au0, indicating the presence of Au [39, 40].

Fig. 1
figure 1

SEM and TEM images of MNPs (a and c) and MNP@Au-PB (b and d)

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To further determine the components of the nanocomposites, electrochemical tests using CV were conducted as shown in Fig. 2. MNP@Au-PB nanocomposites were modified onto the Au electrodes through casting-coat method, and then CV was collected in 0.1 M pH 6.0 phosphate buffer. For comparison, PB modified Au electrode prepared through conventional electrochemical deposition method was also tested similarly. As shown in Fig. 2a, the classic redox peaks of PB modified electrode are located at 0.19 V/0.13 V (PB and PW) and ca. 0.95 V/0.81 V (Berlin green/PB) [34]. For the MNP@Au-PB modified electrode, similar redox peaks at 0.19 V/0.13 V are found, as well as a oxidation peak of much higher intensity starting from 0.8 V and two reduction peaks at 0.81 V and 0.6 V. Here, these peaks appear on account of two pairs of redox reactions namely, the pairs of Berlin green/PB (ca. 0.95 V/0.81 V) and Au oxide/Au (ca. 1.0 V/0.6 V), which share the oxidation peak zoom, demonstrating the presence of PB and Au again. Moreover, a new oxidation peak also arises at 0.47 V, which is ascribed to the oxidation of Fe2+ to Fe3+ in Fe3O4, proving the presence of MNPs again. Furthermore, to more solidly prove the presence of Au, MNP@Au-PB modified glass carbon electrodes (GCE) were prepared similarly and then treated with NaOH to remove PB but retain Au. The mechanism is that PB can be readily degraded in alkaline solution. Then, CV of the electrode in a solution containing 0.1 M H2SO4 was collected (Fig. 2b). Clearly, in the absence of PB, no related peaks are observed, and the pair of redox peaks of Au oxide/Au are found starting at around 1.2 V and 0.9 V, respectively, proving the existence of Au component again.

Fig. 2
figure 2

(a) CV curves of PB and MNP@Au-PB modified Au electrodes in pH 6.0 phosphate buffer; (b) CV curves of the bare GCE and alkaline-treated MNP@Au-PB/GCE electrode in solution containing 0.1 M H2SO4 and 0.1 M K2SO4. Scan rate: 0.05 V s−1

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Electrochemical detection of H2O2

To investigate the catalysis performance of MNP@Au-PB, the nanocomposites were modified on Au electrodes and then the electrochemical catalysis ability to H2O2 was examined using potentiostatic method, as shown in Fig. 3a. For comparison, MNPs, MNP@Au and MNP@PB modified Au electrodes were also prepared (with the same concentration of MNPs) and tested. The MNPs and MNP@Au modified electrodes gave current responses of 0.20 ± 0.02 μA (n = 3, same as below) and 0.41 ± 0.03 μA, respectively. In contrast, the MNP@PB modified electrodes presented significantly enhanced responses up to 12.9 ± 0.9 μA, due to the high catalysis ability of PB component. Surprisingly, the MNP@Au-PB modified electrodes, furthermore, promoted the responses to 21.6 ± 1.2 μA, which is 1.7 folds of that of MNP@PB. The reason should be the incorporation of Au that can enhance the conductivity of the nanocomposites. This result also highlights the advantages of the dual-function shell structure.

Fig. 3
figure 3

(a) Chronoamperometric responses of the MNPs (1), MNP@Au (2), MNP@PB (3) and MNP@Au-PB (4) modified Au electrodes to 1 mM H2O2 at −0.05 V in pH 6.0 phosphate buffer. (b) Chronoamperometric responses of MNP@Au-PB modified Au electrodes to successive additions of H2O2 at −0.05 V in pH 6.0 phosphate buffer and the corresponding calibration plot

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Based on the high catalysis ability of MNP@Au-PB, we further used the modified electrodes for the electrochemical detection of H2O2 by potentiostatic method at −0.05 V in 0.1 M phosphate buffer (pH 6.0). As shown in Fig. 3b, the current increases along with the addition of H2O2 and stabilizes within 5 s. The sensor exhibits a sensitivity of 0.738 μA cm-2 μM-1, a linear detection range (LDR) from 4.0 μM to 22.12 mM and a limit of detection (LOD) of 1.1 μM (S/N = 3). Compared with some H2O2 sensors reported (Table S2), performance of the sensor is comparable to or better than most of analogues. The high performance should be the contribution of strong catalysis ability of PB and the synergetic effect from Au.

Colorimetric detection of H2O2

To further explore the application of the MNP@Au-PB nanocomposites, an optical colorimetric sensing strategy was developed. Briefly, MNP@Au-PB was mixed with TMB and H2O2 of given concentration, then the mixture was kept undisturbed for 10 min. After the reaction was stopped using H2SO4, UV-vis spectra were collected finally. During the detection, PB catalyzed H2O2 and simultaneously oxidized TMB to its oxide (blue in color), which turned to yellow in the strong acidic solution. Therefore, the response signal was collected using UV-vis spectra at 460 nm. As shown in Fig. 4a, along with the increase of the concentration of H2O2, the final solutions gradually turned deepened yellow (the inserted image), and the absorption intensities at 460 nm also gradually increased. The phenomena indicate the catalysis of TMB to its oxide by the MNP@Au-PB in the presence of H2O2. The colorimetric sensor exhibits LDR from 5.0 μM to 5.0 mM with LOD at 1.27 μM (S/N = 3), which is equal to that of electrochemical sensor. These results highlight again the high performance of MNP@Au-PB and the potential for sensing applications.

Fig. 4
figure 4

(a) UV-vis spectra of the final supernatants of the mixture of TMB and 1 mM H2O2 (1), as well as the supernatants for the detection of different concentrations of H2O2: 0 (2), 5 μM (3), 10 μM (4), 50 μM (5), 100 μM (6), 500 μM (7), 1 mM (8) and 5 mM (9) (the insert shows the digital picture of MNP@Au-PB suspensions after the detections, samples from left to right are presented corresponding to above concentrations). (b) The absorption peaks at 460 nm were adopted for calculation to obtain the calibration plot (n = 3)

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Reversibility and selectivity

During the catalysis process, PB played a role mimic to HRP. Briefly, PB catalyzed the reduction of H2O2 and turned themselves to the oxidative form. While the co-existing TMB turned the oxidative form of PB to reductive one that was capable to catalyze H2O2 again. Therefore, the catalysis ability of MNP@Au-PB was reversible. Moreover, the reusability of the nanocomposites was examined. Briefly, after one detection, the nanocomposites were collected using magnet and then used for another round of detection. After five rounds of detections, the responses (n = 3) were still (95 ± 1) % of the initials. This should prove the nanocomposites are capable for repeated detections.

To investigate the selectivity of MNP@Au-PB-based sensor, some interferences, including ascorbic acid, urea, L-Cysteine, L-Aspartic acid, L-Glycine and L-Proline, were chosen for comparison. The results are shown in Fig. S2. Briefly, the intensity of the absorption peak at 460 nm of 500 μM H2O2 was compared with the those of ultrapure water (control experiment), 1 mM ascorbic acid, 1 mM urea, 1 mM L-Cysteine, 1 mM L-Aspartic acid, 1 mM L-Glycine and 1 mM L-Proline. All above interferences exhibited responses lower than 6.0% of that of H2O2, indicating good selectivity. Because a key procedure of the colorimetric detection is the oxidation of TMB to its oxide, general interferences lack of oxidation ability have minor influence on the detection.

Detection of glucose in wine samples

The feasibility of the sensor for real application was examined by applying for the detection of glucose in wine. Glucose was catalyzed by glucose oxidase to generate H2O2, which then was detected using the sensor. The value of adsorption peak at 460 nm were collected for quantification. Firstly, we obtained the calibration plot for the detection of glucose in 0.1 M phosphate buffer (pH 6.0). As shown in Fig. S3, the linear detection range was from 10.0 μM to 500.0 μM. Then the white grape wine, whose glucose content is 28.58 mM as determined by a standard high performance liquid chromatography method [41], was diluted for the real sample detection. The concentration detected by the sensor was 29.89 ± 2.9 mM. The relative standard deviation is 104.6%. Above results should prove that the sensor is feasible for real sample detection.

Immobilization of ChOx

The construction of dual-functional shell has gifted the advantage of synergetic catalysis to enhance the sensing performance. Here, another merit of this structure was exploited for biosensing. As mentioned before, PB is powerful for catalysis, however, its surficial property for immobilization of other species is poor [26]. Therefore, the incorporation of Au, which is a type of commonly adopted matrix material for immobilization, should gift the enhanced immobilization ability. To evaluate the speculation, MNPs, MNP@PB, MNP@Au and MNP@Au-PB were exposed to enzyme ChOx and then the immobilization amounts were obtained using UV-vis spectroscopy. Briefly, MNPs or MNP composites were incubated with ChOx and then magnetically separated, while the supernatants were collected for UV-vis spectroscopy measurements at 280 nm. As a control, ChOx solution with the same concentration as the added for immobilization was treated with the same procedures, and then the UV-vis spectra were also collected. Therefore, the decrease of the peak intensity comparing to original ChOx solution was approximately transferred to be the amount immobilized. Finally, the immobilization capability was calculated as the ratio of immobilized ChOx in mg to the MNPs or MNP composites in g. As shown in Fig. 5a, the adsorption of ChOx on MNP@PB (162 ± 15 mg g−1) decreased to be 38% of that on MNPs (429 ± 38 mg g−1), highlighting the poor immobilization ability of PB. On the other hand, MNP@Au nanocomposites exhibited adsorbability of 813 ± 95 mg g−1, indicating the significant enhancement of adsorption due to the Au shell. The incorporation of Au (MNP@Au-PB) thus made the adsorption amount (593 ± 61 mg g−1) 1.38 fold of that on MNPs. Therefore, MNP@Au-PB nanocomposites integrate the catalysis ability of PB and immobilization ability of Au, thus should contribute as multi-functional biosensing platform.

Fig. 5
figure 5

(a) UV-vis spectra of the supernatants of the mixtures (1 h) of MNP/ChOx, MNP@PB/ChOx, MNP@Au-PB/ChOx, MNP@Au/ChOx and ChOx. (b) UV-vis spectra of supernatants of ChOx (1), MNP/ChOx (2), MNP@PB/ChOx (3) and MNP@Au-PB/ChOx (4) after the incubation (10 min) with 10 mM TMB and 100 mM choline chloride in pH 6.0 phosphate buffer

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For a state-of-concept application, we further examined and compared the biosensing abilities of MNPs, MNP@PB and MNP@Au-PB to choline using the colorimetric method. After the modification of ChOx on above three matrices, TMB and choline were added to trigger the catalysis cascade. Namely, ChOx catalyzed choline to generate H2O2, which was catalyzed by peroxidase-like PB to make TMB turn to its oxide (blue in color). Finally, the supernatants after the magnetic separation were collected for UV-vis spectroscopy measurements at 650 nm, as shown in Fig. 5b. Clearly, the efficiency of the enzyme catalysis cascade should be determined by the amount of ChOx and the catalysis ability of the peroxidase-like analogues. As the control, the ChOx solution itself can not finish the catalysis cascade in the absence of PB, thus gave no absorption peak. Similarly, although the MNP/ChOx have good ChOx immobilized and MNPs themselves play a little role as peroxidase, the absence of PB led to very small absorption peak. In contrast, although MNP@PB/ChOx had less ChOx than MNP@ChOx, the strong catalysis ability of PB shell gifted a complete enzyme catalysis cascade, giving a strong peak. Imaginably, the MNP@Au-PB contributed the strongest adsorption peak, which is 1.62 fold of that of MNP@PB, due to the increased ChOx immobilization. Again, this result highlights the superiority of the dual-function shell and promising prospect of MNP@Au-PB for biosensing applications.

Conclusions

In summary, a self-sacrificing-catalyzing method has been developed to prepare MNP@Au-PB core@dual-functional-shell nanocomposites. The nanocomposites exhibit inherent magnetic property from the core for magnetic operations, and integrate the peroxidase-like catalysis ability of PB and enhanced conductivity from Au, exhibiting sensing performances comparable with or better than most of analogues. Furthermore, the incorporation of Au also gifts the nanocomposites with the superiority to immobilize ChOx for biosensing. The MNP@Au-PB should be promising in catalysis and sensing/biosensing fields, and the self-sacrificing-catalyzing method might open new avenue for preparation of multi-functional nanocomposites. However, because PB is inherently susceptive to alkali, so alkaline solutions should be avoided. Moreover, appropriate block of the surface of the nanocomposites should be conducted to avoid interferences prior to the application in complex samples. Therefore, further efforts are needed to make the nanocomposites more robust.