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. 2022 Dec 23;150:108358. doi: 10.1016/j.bioelechem.2022.108358

CdTe QDs-sensitized TiO2 nanocomposite for magnetic-assisted photoelectrochemical immunoassay of SARS-CoV-2 nucleocapsid protein

Aijiao Guo a,b, Fubin Pei a,b, Wei Hu b, Mingzhu Xia a, Xihui Mu b, Zhaoyang Tong b, Fengyun Wang a,⁎, Bing Liu b,⁎
PMCID: PMC9783190  PMID: 36580690

Graphical abstract

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Keywords: Photoelectrochemical immunosensor, Magnetic capture probe, Z-scheme heterojunction, SiO2@TiO2@CdTe quantum dots, SARS-CoV-2 nucleocapsid protein

Abstract

A sensitive, reliable, and cost-effective detection for SARS-CoV-2 was urgently needed due to the rapid spread of COVID-19. Here, a “signal-on” magnetic-assisted PEC immunosensor was constructed for the quantitative detection of SARS-CoV-2 nucleocapsid (N) protein based on Z-scheme heterojunction. Fe3O4@SiO2@Au was used to connect the capture antibody to act as a capture probe (Fe3O4@SiO2@Au/Ab1). It can extract target analytes selectively in complex samples and multiple electrode rinsing and assembly steps were avoided effectively. CdTe QDs sensitized TiO2 coated on the surface of SiO2 spheres to form Z-scheme heterojunction (SiO2@TiO2@CdTe QDs), which broadened the optical absorption range and inhibited the quick recombination of photogenerated electron/hole of the composite. With fascinating photoelectric conversion performance, SiO2@TiO2@CdTe QDs were utilized as a signal label, thus further realizing signal amplification. The migration mechanism of photogenerated electrons was further deduced by active material quenching experiment and electron spin resonance (ESR) measurement. The elaborated immunosensor can detect SARS-CoV-2 N protein in the linear range of 0.005–50 ng mL−1 with a low detection limit of 1.8 pg mL−1 (S/N = 3). The immunosensor displays extraordinary sensitivity, strong anti-interference, and high reproducibility in detecting SARS-CoV-2 N protein, which envisages its potential application in the clinical diagnosis of COVID-19.

1. Introduction

Up to November 2022, coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a severe impact on the health of all mankind [1], [2]. Timely and reliable diagnosis of SARS-CoV-2 is of great significance to control the epidemic. The nucleocapsid (N) protein is a favorable biorecognition target for detecting SARS-CoV-2 due to its indispensable role in genome packaging [3]. Unfortunately, as a common method of detecting SARS-CoV-2 N protein currently, lateral flow immunoassays [4] are difficult to perform quantitative analysis, enzyme-linked immunosorbent assay [5], fluorescence immunosensor [6], and mass spectrometry [7] have the disadvantages of complex operation procedures and expensive equipment. Consequently, developing an economical, easy-operated, and sensitive detection method for the SARS-CoV-2 N protein is critical.

Photoelectrochemical (PEC) immunoassay, which owns the advantages of high sensitivity, easy maneuverability, and reasonable specificity, has been booming in recent years [8], [9], [10]. PEC immunosensor can be divided into two types: “signal on” and “signal off”. Generally speaking, the “signal on” PEC immunosensor generally perform better in terms of sensitivity ascribe to its low background signal [11]. However, common PEC immunosensor still have the following shortcomings: multi-step electrode assembly leads to large electrode processing amount, cumbersome process, poor reproducibility, and the interference in complex matrix leads to a low probability of practical application [12], [13]. Integrating PEC immune sensing and magnetic capture probe is an alternative to circumvent the above issues.

In this context, with strong magnetic affinity, excellent biocompatibility, and high stability, Fe3O4@SiO2@Au was a suitable choice introduced into the sensor [14]. On the one hand, Fe3O4@SiO2@Au bound with capture antibody can be used for capturing targets in complex samples selectively and the immunocomplex formed can be efficiently separated. Simultaneously, tediously electrode rinsing and assembly steps were greatly simplified was avoided, which can develop the reproducibility and stability of the sensor [15]. On the other hand, the SiO2 layer can overcome the disadvantage of agglomeration of the magnetic core, and gold nanoparticles (Au NPs) loaded on the surface of Fe3O4@SiO2 act as a bridge to connect the antibody [16]. Additionally, as another indispensable component of PEC immunosensor, photoactive materials greatly influence the analytical performances [17]. Therefore, it is necessary to exploit appropriate photoactive materials to act as a signal label.

Semiconductor is the most common photoactive material, among which titanium dioxide (TiO2) has been widely used in photoelectroimmune sensing due to its excellent photoelectric performance and good biocompatibility [18]. However, the powder TiO2 or bulk TiO2 alone is not thermal enough. Therefore, many reports have used SiO2 spheres with low cost, simple preparation and good stability as the template to synthesize SiO2@TIO2 core–shell structure with uniform particle size, large specific surface area, multiple active sites and strong light response[19], [20], [21], [22], [23]. In addition, possessing a wide bandgap, single TiO2 active component has the disadvantage of rapid recombination of electron/hole (e-/h+) and low visible light utilization [24]. Recombination with semiconductors to form heterojunctions can effectively ameliorate the drawbacks. Semiconductor quantum dots (QDs) are also attractive and excellent photoactive materials [25]. CdTe QDs have a narrow band gap that matches well with TiO2, making it an ideal sensitizer for amplifying photocurrent signals of TiO2 (1.54 eV. vs NHE) [26]. Furthermore, photogenerated electrons follow different transfer paths for different types of heterostructures. Thus, it is essential to explore the types of heterojunctions to interpret the significant increase in photocurrent of the photoactive material. The active species quenching experiment and the electron spin resonance (ESR) test are commonly used to study the types of active groups generated under light conditions and its roles to determine the type of heterojunction.

Here in, we constructed a “signal-on” magnetic-assisted PEC immunosensor based on Z-scheme heterojunction for quantitative detection of SARS-CoV-2 N protein. With strong magnetic responsiveness, Fe3O4@SiO2@Au was implemented to ligate the capture antibody to capture analytes selectively and experimental procedure was simplified significantly. The Z-scheme heterojunction (SiO2@TiO2@CdTe QDs) with fascinating performance in photoelectric conversion and optical absorption was used as signal label. Fe3O4@SiO2@Au/Ab1, SARS-CoV-2 N protein, and SiO2@TiO2@CdTe QDs-Ab2 bind to form a sandwich immunecomplex. Within certain limits, the greater the concentration of SARS-CoV-2 N protein, the more the label material SiO2@TiO2@CdTe QDs with excellent photoelectric response is combined, the more the photoelectric response of the complex will increase. The mechanism of photogenerated e-/h+ transfer was deduced was determined by active species quenching experiment and ESR measurement. Coupling the magnetic capture probe with the marvelous photoactive matrix, the proposed PEC immunosensor shows high-performance for detecting SARS-CoV-2 N protein, indicating a promising prospect in practical application.

2. Experimental section

Details of reagents, apparatus, and synthesis of SiO2@TiO2@CdTe QDs, Fe3O4@SiO2@Au, photodegradation measurementsand, and PEC detection conditions were presented in Supplementary material.

2.1. Fabrication of PEC immunosensor

Preparation of Fe3O4@SiO2@Au/Ab1: In the first, Fe3O4@SiO2@Au (5 mL) was washed with phosphate buffered saline (PBS, PH = 7.2–7.4, 0.01 M) by magnetic separation. After magnetic separation, 5 mL of Ab1 (2D3, 50 μg mL−1) was added and shaken at 4 ℃ for 12 h. The generated Fe3O4@SiO2@Au/Ab1 was washed and blocked by BSA (1 % w/v) at 4 ℃ for 1 h. Subsequently, the mixture was washed and Fe3O4@SiO2@Au/Ab1 was dispersed in 5 mL of PBS for storage.

Preparation of SiO2@TiO2@CdTe QDs-Ab2: SiO2@TiO2@CdTe QDs (5 mL) was washed with PBS, discard the supernatant. 5 mL of 1 mg mL−1 EDC and NHS (prepared by 2-(N-morpholino) ethanesulfonic (MES) buffer, containing 0.5 M NaCl, MES, pH = 4.7).were added to react at 25℃ shaken for 0.5 h, followed by washing with PBS. Then, 5 mL of Ab2 (3F2, 50 μg mL−1) was added and shaken for 4 h at 4 ℃. After centrifugation and washing, it was blocked by BSA (1 % w/v) at 4 ℃ for 1 h. SiO2@TiO2@CdTe QDs-Ab2 conjugates were dispersed in 5 mL PBS after centrifugation and washing.

Determination of SARS-CoV-2 N protein: First, Fe3O4@SiO2@Au/Ab1 (500 μL) was taken to magnetic separation to discard the supernatant, SARS-CoV-2 N protein (500 μL) with various concentrations was added to incubate at 37 ℃ for 1 h, and PBS was used as the blank sample. Afterward, Fe3O4@SiO2@Au/Ab1/SARS-CoV-2 N protein was washed by magnetic separation, 500 μL of SiO2@TiO2@CdTe QDs-Ab2 was added and incubated at 37 ℃ for 1 h. After washed, the conjugation was dispersed in 250 μL PBS. Finally, 15 μL of the product was cast on the indium tin oxide (ITO) electrode and followed by a photocurrent test after dried.

2.2. Simulated sample analysis

To investigate the applicability of the immunosensor, artificial saliva was used as a real sample. First, the samples were prepared containing different concentrations of SARS-CoV-2 N protein concentrations of 1, 10, 50 ng mL−1 by standard addition method [27]. Subsequently, 500 μL of Fe3O4@SiO2@Au/Ab1 suspension was taken to discard the supernatant after magnetic separation, followed by the addition of 500 μL of prepared SARS-CoV-2 N protein for 1 h. Then, SiO2@TiO2@CdTe QDs-Ab2 (500 μL) was added for 1 h after washed. Finally, after washing with PBS and dispersing in 250 μL PBS, 15 μL of the immunocomplex drops were taken and dried on the surface of ITO electrode for the photocurrent test.

Scheme 1 showed the schematic of preparation of Fe3O4@SiO2@Au/Ab1 (A), SiO2@TiO2@CdTe QDs-Ab2 (B) and fabrication of the PEC immunosensor.

Scheme 1.

Scheme 1

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The schematic of preparation of Fe3O4@SiO2@Au/Ab1 (A), SiO2@TiO2@CdTe QDs-Ab2 (B) and fabrication of the PEC immunosensor.

3. Results and discussion

3.1. Characterization of SiO2@TiO2@CdTe QDs and Fe3O4@SiO2@Au.

The morphological features of the composites were characterized by transmission electron microscopy (TEM). From Fig. S1 A, the average diameter of SiO2 sphere was about 225 nm. After TiO2 layer was coated on the smooth surface of SiO2 sphere, the average sizes of SiO2@TiO2 were increased to 355 nm (Fig. 1A). The mean sizes of SiO2@TiO2 shrink to 300 nm after the composite was calcined at 550 ℃ for 2 h (Fig. 1B). In Fig. S1B, after coating the charged polyethyleneimine (PEI) layer by self-assembly method, the Zeta potential of the material increased from + 2 mV to + 35.5 mV, which was attributed to the strong electropositivity of PEI [28] and negatively charged of carboxylated CdTe QDs (-40.5 mV). The zeta potential of the SiO2@TiO2@CdTe QDs was −19.2 mV, indicating the assembly of CdTe QDs on the SiO2@TiO2 surface was successfully. Fig. 1C provided further evidence of it. As shown in Fig. 1D, the prepared Fe3O4 spheres were about 190 nm in diameter. Fig. 1E exhibited that SiO2 was coated on the surface of Fe3O4 with a thickness of about 15 nm. Gold nanoparticles (Au NPs) with the size of 10 ∼ 20 nm scattered on the surface of Fe3O4@SiO2 (Fig. 1F). The elemental mapping of SiO2@TiO2@CdTe QDs (Fig. 1G) showed that SiO2@TiO2@CdTe QDs contained Si, Ti, Cd, and Te elements. Similarly, Fe3O4@SiO2@Au contained Fe, Si, and Au elements (Fig. S1C). Therefore, the above images indicated that SiO2@TiO2@CdTe QDs and Fe3O4@SiO2@Au had been synthesized successfully.

Fig. 1.

Fig. 1

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The TEM image of SiO2@TiO2 before calcining (A), SiO2@TiO2 after calcining (B), SiO2@TiO2@CdTe QDs (C), Fe3O4 (D), Fe3O4@SiO2 (E), Fe3O4@SiO2@Au (F); Elemental mapping images of SiO2@TiO2@CdTe QDs (G).

To further test the element composition of SiO2@TiO2@CdTe QDs, X-ray photoelectron spectra (XPS) test was performed. From Fig. 2 A, the full spectrum showed peaks of Si 2p, Ti 2p, O1s, Cd 3d, and Te 3d. The peak at 101.72 eV was ascribed to Si 2p (Fig. 2B) [29]. Fig. 2C showed the peaks at 457.58 eV and 463.30 eV, which were attributed to the Ti 2p3/2 and Ti 2p1/2 [30]. The O 1 s spectrum discreted peaks at 528.82 and 530.70 eV, which corresponded to the Ti − O bond in TiO2 and OH/H2O (Fig. 2D) [31]. The Cd 3d spectrum at 404.29 eV and 411.04 eV belonged to Cd 3d5/2 and Cd 3d3/2 (Fig. 2E) [32]. In Fig. 2F, the peaks at 571.34 eV and 581.68 eV in the Te 3d spectrum associated with the Te 3d5/2 and Te 3d3/2. The results proved that SiO2@TiO2@CdTe QDs had been prepared.

Fig. 2.

Fig. 2

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XPS spectra for SiO2@TiO2@CdTe QDs (A), High-resolution of Si 2p (B), Ti 2p (C), O1s (D), Cd 3d (E), and Te 3d (F).

The magnetic properties of the Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2@Au were investigated by magnetic hysteresis loops. Fig. 3 A revealed that the magnetization saturation (MS) values of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2@Au were 61, 44, and 41 emu g−1, respectively. The value of Ms decreased after coating of inert SiO2 layer and Au NPs, but Fe3O4@SiO2@Au retained strong magnetic responsiveness.

Fig. 3.

Fig. 3

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Magnetic hysteresis loops (A) of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2@Au (c): XRD patterns (B), UV–vis DRS spectra (C) and Photocurrent response (D) of SiO2@TiO2 and SiO2@TiO2@CdTe QDs.

The crystal structure of the SiO2@TiO2@CdTe QDs was determined by The X-ray diffraction (XRD) pattern. The diffraction peaks of SiO2@TiO2 corresponded to the anatase phase of TiO2 (JCPDS 21–1272). The diffraction peaks at 25.3°, 36.9°, 37.8°, 48°, 53.9°, 62.7°, 70.3°, and 75.0° can be indexed into the (1 0 1), (1 0 3), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (2 2 0), and (2 1 5) planes (Fig. 3B). The XRD patterns of the complex coated with CdTe QDs were not significantly different from that of SiO2@TiO2, which may be attributed to the low content of CdTe QDs in the composite.

3.2. Photochemical and photoelectrochemical behaviors

UV–vis diffuse reflectance spectra (DRS) shown in Fig. 3C demonstrated that SiO2@TiO2 displayed an ultraviolet light absorption band below 400 nm. After CdTe QDs coated on the surface of SiO2@TiO2, the absorption wavelength range of the material shifted from ultraviolet light to visible light region. Meanwhile, the photocurrents of SiO2@TiO2 and SiO2@TiO2@CdTe QDs were tested to further explore the photoelectrochemical properties of the composites. From Fig. 3D, the photocurrent of SiO2@TiO2 was about 1210nA, and it was significantly increased to 2481nA after the combination with CdTe QDs. This can be attributed to the efficient separation of photogenerated carriers and the significant broadening of the light absorption range [33]. Based on the above results, the combination with CdTe QDs significantly improved the ability of the visible light absorption and photoelectric conversion of the material.

3.3. Possible photogenerated e−/h+ transfer mechanism

The photocatalytic degradation methylene blue (MB) over SiO2@TiO2@CdTe QDs with various trapping scavengers was adopted to determine the types of active substances produced by materials and their roles under light radiation (Fig. 4 A). 1,4-benzoquinone (BQ, 1 mM), isopropanol (IPA, 5 mM), and disodium ethylenediaminetetraacetate (Na2-EDTA, 1 mM) were used to quench superoxide radicals (•O2–), and hydroxyl radicals (•OH) and h+, respectively [34]. After adding the quencher, the degradation efficiency by SiO2@TiO2@CdTe QDs decreased to varying degrees compared with that without the addition of quench agent, indicating that these three active substances were produced and played an essential role in the degradation process. Among them, the degradation efficiency decreased significantly after adding BQ, which indicated that •O2– was the primary active substance.

Fig. 4.

Fig. 4

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Photocatalytic degradation curves of MB over SiO2@TiO2@CdTe QDs with various trapping scavengers (A); ESR spectra of DMPO − OH• (B) and DMPO−•O2− (C) for SiO2@TiO2 @CdTe QDs; Schematic illustration of the migration mechanism of photogenic electron and hole in the immunosensor (D).

The ESR analysis of SiO2@TiO2@CdTe QDs was performed to further verify the existence of •O2 – and •OH and infer the photogenerated electrons transfer path (Fig. 4B, C). In the ESR measurement, 5,5-dimethyl-1-1pyrroline N-oxide (DMPO) as a spin trapping agent to detect DMPO-•O2 – and DMPO-·OH [35]. Under light radiation, obvious characteristic peaks of •O2 – and •OH can be observed while no characteristic peaks in the dark, indicating that SiO2@TiO2@CdTe QDs can produce the •OH and •O2 – in this process. Combined with the results of photocatalytic degradation experiment and ESR test, it can be concluded that SiO2@TiO2@CdTe QDs produces •O2 –, •OH, and h+ under light radiation, which significantly improved the photocatalytic and photoelectric conversion performance.

The standard potential the O2/•O2 – pair (-0.33 eV vs NHE) [36] was less negative than that the conduction band (CB) of CdTe QDs (-1.0 eV vs NHE) and more negative than the CB position of TiO2 (-0.29 eV vs NHE). We can conclude that O2 can be reduced to •O2 – by electrons on the valence band (VB) of CdTe QDs (+0.54 eV vs NHE). Similarly, the standard potential of the H2O/•OH pair (+1.99 eV vs NHE) [37] was more positive than the VB of CdTe QDs and less positive than the VB of TiO2 (+2.91 eV vs NHE) and thus holes on the VB oxidize –OH to •OH. Therefore, we propose a Z-scheme charge transfer mechanism.

Based on above result, the probable photogenerated e−/h+ transfer path of the immunosensor was speculated (Fig. 4D). Under the excitation of light, photogenerated electrons transferred from the VB of TiO2 and CdTe QDs to the corresponding CB, leaving holes in the respective VB. The electrons in the CB of TiO2 recombined with the holes in the VB of CdTe QDs, thus inhibiting the recombination of some electrons and holes in TiO2 and CdTe QDs. Therefore, there was an accumulation of electrons in CB of CdTe QDs and enrichment of holes in VB of TiO2. Strongly reducing electrons reduce O2 to •O2 – and strongly oxidizing holes oxidize H2O to •OH. Subsequently, •O2 –, •OH, and h+ reacted with ascorbic acid (AA) to produce AA+. In this immunosensor, electrons generated on the surface of the ITO electrode have transferred to the electrochemical workstation through the electrolyte and Pt electrode, and presented in the form of photocurrent visually.

3.4. Characterization of PEC immunosensor

Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) was utilized to characterize the interface characteristics of the electrodes (Fig. 5 A). Generally, the electron transfer resistance (Ret) was equal to the diameter of the high frequency of the semicircle. When the pure Fe3O4@SiO2@Au was coated, semicircular diameter with large diameter semicircles was exhibited. When Ab1, BSA, and SARS-CoV-2 N proteins were assembled sequentially, the value of Ret decreased gradually. Moreover, the value of Ret significantly reduced after incubation with SiO2@TiO2@CdTe QDs-Ab2. As shown in Fig S3, with the assembly of Ab1, BSA, and SARS-CoV-2 N proteins and SiO2@TiO2@CdTe QDs-Ab2, the redox peak current decreased gradually, and the difference of peak potential increased, indicating that the irreversibility of the electrode process increased. The above results suggested that Ab1, BSA, SARS-CoV-2 N protein, and SiO2@TiO2@CdTe QDs-Ab2 were assembled successively.

Fig. 5.

Fig. 5

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EIS spectrum (A) and Photocurrent response (B) of Fe3O4@SiO2@Au (a), Fe3O4@SiO2@Au/Ab1 (b), Fe3O4@SiO2@Au/Ab1/BSA (c) and Fe3O4@SiO2@Au /Ab1/BSA/ SARS-CoV-2 N protein (d) Fe3O4@SiO2@Au /Ab1/BSA/ SARS-CoV-2 N protein/ SiO2@TiO2@CdTe QDs-Ab2 (e); modified on GCE (A)/ITO (B).

The constructed PEC immunosensor was monitored by photocurrent-time curve (Fig. 5B). A weak current signal was generated, when the ITO electrode was modified with Fe3O4@SiO2@Au. When Ab1, BSA, and SARS-CoV-2 N proteins were sequentially attached to the Fe3O4@SiO2@Au, the photocurrents decreased proportionally. The decrease in photocurrent can be attributed to the fact that proteins block the transfer of photogenerated electrons. After SiO2@TiO2@CdTe QDs-Ab2 was conjugated, the photocurrent increased significantly. The results suggested that the PEC immunosensor was fabricated as expected.

3.5. Optimization of experimental conditions

To obtain a sensor with better performance, the amount of CdTe QDs added and the concentration of AA were optimized. The photocurrent of the composites first increased and then decreased with the increase of the volume of CdTe QDs added, reaching the maximum when the volume of CdTe QDs was 2 mL. Thus, 2 mL was the optimal volume (Fig. S4A). In Fig. S4B, when the concentration of AA increased from 0.05 M to 0.1 M, the photocurrent increased rapidly and reached a relatively large value. The photocurrent increased slowly when the concentration was greater than 0.1 M. Therefore, 0.1 M was chosen as the best concentration for AA. Consequently, 2 mL of CdTe QDs and 0.1 M of AA were employed as the optimal conditions for further studies.

3.6. PEC detection for SARS-CoV-2 N protein

The proposed PEC immunosensor was used to detect SARS-CoV-2 N proteins with various concentrations. The photocurrent of the sensor increased gradually with the increase of SARS-CoV-2 N protein concentration (Fig. 6 A), and the photocurrent response was positively correlated with the logarithm of SARS-CoV-2 N protein concentration. As shown in Fig. 6B (inset), in the range of 5 pg mL−1-50 ng mL−1, the regression equation was I (nA) = 310.1 + 93.2 lg c (R2 = 0.977). The detection limit was 1.8 pg mL−1 (S/N = 3), which was lower than those of previous reports (Table S1).

Fig. 6.

Fig. 6

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PEC responses of SARS-CoV-2 N protein (A) with different concentrations at 0, 0.005, 0.01,0.05, 0.1, 0.5, 1.0, 5.0, 10, and 50 ng mL−1 (from a to j) and their calibration curve (B), Stability survey of the immunosensor toward 50 ng mL−1 of SARS-CoV-2 N protein. (C) and Selectivity (D) of PEC immunosensor. Error bar = SD, n = 5.

3.7. Stability, selectivity, and reproducibility

Stability was also a vital characteristic of the immunosensor. After 11 cycles of light/dark, the intensity of the immunosensor with 50 ng mL−1 SARS-CoV-2 N protein did not change significantly (Fig. 6C). Thus, the immunosensor showed acceptable stability.

Specificity was an important parameter of PEC immunosensor, the specificity measure at SARS-CoV-2 N protein concentration of 5 ng mL−1 was used to evaluate the specificity of the sensor. In this process, in Middle East respiratory syndrome coronavirus (MERS) N protein, influenza A (FluA) N protein, influenza B (FluB) hemagglutinin, and BSA with 50 ng mL-1as the interfering substances. From Fig. 6D, the photocurrent response of the interferer was little, and was significantly smaller than the response to SARS-CoV-2 N protein. These results demonstrated the immunosensor possesses a satisfactory selectivity.

The photocurrent responses were 277.5, 273.1, 288.4, 298.0, and 294.4nA under the same conditions for detections of SARS-CoV-2 N protein at the concentration of 1 ng mL−1 five times. The RSD was 3.74 %, indicating that the sensor with good reproducibility.

3.8. Simulated simple analysis

To evaluate the reliability of the proposed sensor, SARS-CoV-2 N protein in artificial saliva samples was detected. The samples were prepared containing different concentrations of SARS-CoV-2 N protein concentrations of 1, 10, 50 ng mL−1. In Table S2, the RSD was between 1.4 % and 3.4 % and the recovery was in the range 84.2–116.7 %, indicating that the designed immunosensor was suitable for practical determination.

4. Conclusion

In this work, a “signal on” PEC immunosensor was constructed for SARS-CoV-2 N protein determination depending upon Z-scheme heterojunction (SiO2@TiO2@CdTe QDs) with excellent photoelectric conversion performance as signal label and Fe3O4@SiO2@Au with strong magnetic responsiveness as a solid support for the capture antibody. The magnetic capture probe can extract targets from complex substrates selectively and the operation procedure was simplified greatly. The heterojunction effectively separated photogenerated electrons and holes and widened the optical absorption range. Active material quenching experiment and ESR measurement were carried out to determine the migration mechanism of photogenerated e−/h+. The established PEC immunosensor owned high performance in sensitivity, specificity, reproducibility and stability, indicating its potential application in clinical diagnosis. Furthermore, the “signal on” PEC immunosensor based on this magnetic-assisted strategy has admirable magnetic enrichment and separation ability, which is expected to be used for the detection of large volume samples to reduce the detection limit of analytes.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was supported by Foundation of State Key Laboratory of NBC Protection for Civilian (SKLNBC2022-06) and the National Natural Science Foundation of China (52072180).

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.bioelechem.2022.108358.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Supplementary data 1
mmc1.docx (1,016.2KB, docx)

Data availability

Data will be made available on request.

References

  • 1.Xu M., Zhou J., Cheng Y., Jin Z., Clark A.E., He T., Yim W., Li Y., Chang Y.-C., Wu Z., Fajtová P., O’Donoghue A.J., Carlin A.F., Todd M.D., Jokerst J.V. A self-immolative fluorescent probe for selective detection of SARS-CoV-2 main protease. Anal Chem. 2022;94:11728–11733. doi: 10.1021/acs.analchem.2c02381. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Liu N., Liu R., Zhang J. CRISPR-Cas12a-mediated label-free electrochemical aptamer-based sensor for SARS-CoV-2 antigen detection. Bioelectrochemistry. 2022;146 doi: 10.1016/j.bioelechem.2022.108105. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Anderson G.P., Liu J.L., Esparza T.J., Voelker B.T., Hofmann E.R., Goldman E.R. Single-Domain antibodies for the detection of SARS-CoV-2 Nucleocapsid protein. Anal. Chem. 2021;93:7283–7291. doi: 10.1021/acs.analchem.1c00677. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 4.Xie Z., Feng S., Pei F., Xia M., Hao Q., Liu B., Tong Z., Wang J., Lei W., Mu X. Magnetic/fluorescent dual-modal lateral flow immunoassay based on multifunctional nanobeads for rapid and accurate SARS-CoV-2 nucleocapsid protein detection. Anal Chim Acta. 2022;1233 doi: 10.1016/j.aca.2022.340486. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Khan W.H., Khan N., Mishra A., Gupta S., Bansode V., Mehta D., Bhambure R., Ansari M.A., Das S., Rathore A.S. Dimerization of SARS-CoV-2 nucleocapsid protein affects sensitivity of ELISA based diagnostics of COVID-19. Int J Biol Macromol. 2022;200:428–437. doi: 10.1016/j.ijbiomac.2022.01.094. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Mao G., Yang Y., Cao S., Ye S., Li Y., Zhao W., An H., Liu Y., Dai J., Ma Y. Ratiometric fluorescence immunoassay of SARS-CoV-2 nucleocapsid protein via Si-FITC nanoprobe-based inner filter effect. Nano Res. 2022:1–8. doi: 10.1007/s12274-022-4740-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 7.Nikolaev E.N., Indeykina M.I., Brzhozovskiy A.G., Bugrova A.E., Kononikhin A.S., Starodubtseva N.L., Petrotchenko E.V., Kovalev G.I., Borchers C.H., Sukhikh G.T. Mass-Spectrometric detection of SARS-CoV-2 virus in scrapings of the epithelium of the nasopharynx of infected patients via nucleocapsid protein. J. Proteome Res. 2020;19:4393–4397. doi: 10.1021/acs.jproteome.0c00412. [DOI] [PubMed] [Google Scholar]
  • 8.Guo A., Pei F., Feng S., Hu W., Zhang P., Xia M., Mu X., Tong Z., Wang F., Liu B. A photoelectrochemical immunosensor based on magnetic all-solid-state Z-scheme heterojunction for SARS-CoV-2 nucleocapsid protein detection. Sens Actuators B Chem. 2023;374 doi: 10.1016/j.snb.2022.132800. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Zhu Q., Li C., Chang H., Jiang M., Sun X., Jing W., Huang H., Huang D., Kong L., Chen Z., Sang F., Zhang X. A label-free photoelectrochemical immunosensor for prostate specific antigen detection based on Ag2S sensitized Ag/AgBr/BiOBr heterojunction by in-situ growth method. Bioelectrochemistry. 2021;142 doi: 10.1016/j.bioelechem.2021.107928. [DOI] [PubMed] [Google Scholar]
  • 10.Guo J., Liu D., Yang Z., Weng W., Chan E.W.C., Zeng Z., Wong K.Y., Lin P., Chen S. A photoelectrochemical biosensor for rapid and ultrasensitive norovirus detection. Bioelectrochemistry. 2020;136 doi: 10.1016/j.bioelechem.2020.107591. [DOI] [PubMed] [Google Scholar]
  • 11.He X., Zhao X., Deng W., Tan Y., Xie Q. CdSe quantum dots-decorated ZnIn2S4 nanosheets for “signal-on” photoelectrochemical aptasensing of ATP by integrating exciton energy transfer with exciton-plasmon coupling. Sens. Actuators B Chem. 2021;348 [Google Scholar]
  • 12.Mo F., Han M., Weng X., Zhang Y., Li J., Li H. Magnetic-Assisted Methylene Blue-Intercalated Amplified dsDNA for Polarity-Switching-Mode Photoelectrochemical Aptasensing. Anal Chem. 2021;93:1764–1770. doi: 10.1021/acs.analchem.0c04521. [DOI] [PubMed] [Google Scholar]
  • 13.Li J., Weng X., Mo F., Han M., Li H. Superparamagnetic Nanostructures Coupled with an Entropy-Driven DNA Circuit for Elegant and Robust Photoelectrochemical Biosensing. Anal Chem. 2020;92:15145–15151. doi: 10.1021/acs.analchem.0c03580. [DOI] [PubMed] [Google Scholar]
  • 14.Deng X., Li W., Ding G., Chen X. Enantioselective separation of RS-mandelic acid using beta-cyclodextrin modified Fe3O4@SiO2/Au microspheres. Analyst. 2018;143:2665–2673. doi: 10.1039/c8an00427g. [DOI] [PubMed] [Google Scholar]
  • 15.Bai X.R., Wang L.H., Ren J.Q., Bai X.W., Zeng L.W., Shen A.G., Hu J.M. Accurate Clinical Diagnosis of Liver Cancer Based on Simultaneous Detection of Ternary Specific Antigens by Magnetic Induced Mixing Surface-Enhanced Raman Scattering Emissions. Anal Chem. 2019;91:2955–2963. doi: 10.1021/acs.analchem.8b05153. [DOI] [PubMed] [Google Scholar]
  • 16.Jabbar Z.H., Ebrahim S.E. Synthesis, characterization, and photocatalytic degradation activity of core/shell magnetic nanocomposites (Fe3O4@SiO2@Ag2WO4@Ag2S) under visible light irradiation. Opt. Mater. 2021;122 [Google Scholar]
  • 17.Zang Y., Cao R., Zhang C., Xu Q., Yang Z., Xue H., Shen Y. TiO2-sensitized double-shell ZnCdS hollow nanospheres for photoelectrochemical immunoassay of carcinoembryonic antigen coupled with hybridization chain reaction-dependent Cu(2+) quenching. Biosens Bioelectron. 2021;185 doi: 10.1016/j.bios.2021.113251. [DOI] [PubMed] [Google Scholar]
  • 18.Zhao J., Fu C., Huang C., Zhang S., Wang F., Zhang Y., Zhang L., Ge S., Yu J. Co3O4-Au polyhedron mimic peroxidase- and cascade enzyme-assisted cycling process-based photoelectrochemical biosensor for monitoring of miRNA-141. Chem. Eng. J. 2021;406 [Google Scholar]
  • 19.Fu X.a., Syed Q. Synthesis of titania-coated silica nanoparticles using ono-ionic water-in-oil. Colloids Surf. A. 2001;178:151–156. [Google Scholar]
  • 20.Holgado M., Cintas A., Ibisate M., Serna C.J., Lopez C., Meseguer F. Three-dimensional arrays formed by monodisperse TiO2 coated on SiO2 spheres. J. Colloid Interface Sci. 2000;229:6–11. doi: 10.1006/jcis.2000.6973. [DOI] [PubMed] [Google Scholar]
  • 21.Kim K.D., Bae H.J., Kim H.T. Synthesis and characterization of titania-coated silica fine particles by semi-batch process. Colloids Surf A Physicochem Eng Asp. 2003;224:119–126. [Google Scholar]
  • 22.Lee J.W., Othman M.R., Eom Y., Lee T.G., Kim W.S., Kim J. The effects of sonification and TiO2 deposition on the micro-characteristics of the thermally treated SiO2/TiO2 spherical core–shell particles for photo-catalysis of methyl orange. Microporous Mesoporous Mater. 2008;116:561–568. [Google Scholar]
  • 23.Joo J.B., Zhang Q., Lee I., Dahl M., Zaera F., Yin Y. Mesoporous Anatase Titania Hollow Nanostructures though Silica-Protected Calcination. Adv. Funct. Mater. 2012;22:166–174. [Google Scholar]
  • 24.Jo W.-K., Sivakumar Natarajan T. Facile Synthesis of Novel Redox-Mediator-free Direct Z-Scheme CaIn2S4 Marigold-Flower-like/TiO2 Photocatalysts with Superior Photocatalytic Efficiency. ACS Appl. Mater. Interfaces. 2015;7:17138–17154. doi: 10.1021/acsami.5b03935. [DOI] [PubMed] [Google Scholar]
  • 25.Sun C., Shen Y., Zhang Y., Du Y., Peng Y., Xie Q. Sandwich photoelectrochemical biosensing of concanavalin A based on CdS/AuNPs/NiO Z-scheme heterojunction and lectin-sugar binding. Talanta. 2022;253 doi: 10.1016/j.talanta.2022.123882. [DOI] [PubMed] [Google Scholar]
  • 26.Bang J., Kamat P. Quantum Dot Sensitized Solar Cells. A Tale of Two Semiconductor Nanocrystals: CdSe and CdTe. ACS Nano. 2009;3 doi: 10.1021/nn900324q. [DOI] [PubMed] [Google Scholar]
  • 27.Pei F., Feng S., Zhang Y., Wu Y., Chen C., Sun Y., Xie Z., Hao Q., Cao Y., Tong Z., Lei W. A photoelectrochemical immunosensor based on Z-scheme CdS composite heterojunction for aflatoxin B1. Biosens. Bioelectron. 2022;114500 doi: 10.1016/j.bios.2022.114500. [DOI] [PubMed] [Google Scholar]
  • 28.Wang C., Shen W., Rong Z., Liu X., Gu B., Xiao R., Wang S. Layer-by-layer assembly of magnetic-core dual quantum dot-shell nanocomposites for fluorescence lateral flow detection of bacteria. Nanoscale. 2020;12:795–807. doi: 10.1039/c9nr08509b. [DOI] [PubMed] [Google Scholar]
  • 29.Zhu L., Opulencia M.J.C., Bokov D.O., Krasnyuk I.I., Su C.-H., Nguyen H.C., Mohamed A., Zare M.H., Zwawi M., Algarni M. Synthesis of Ag-coated on a wrinkled SiO2@TiO2 architectural photocatalyst: New method of wrinkled shell for use of semiconductors in the visible light range and penicillin antibiotic degradation. Alex. Eng. J. 2022;61:9315–9334. [Google Scholar]
  • 30.Wang Y., Mino L., Pellegrino F., Homs N., Ramírez de la Piscina P. Engineered MoxC/TiO2 interfaces for efficient noble metal-free photocatalytic hydrogen production. Appl Catal B. 2022;318 [Google Scholar]
  • 31.Chen Z., Xu Y.J. Ultrathin TiO2 layer coated-CdS spheres core-shell nanocomposite with enhanced visible-light photoactivity. ACS Appl Mater Interfaces. 2013;5:13353–13363. doi: 10.1021/am4043068. [DOI] [PubMed] [Google Scholar]
  • 32.Liang Q., Liu L., Wu Z., Nie H., Shi H., Li Z., Kang Z. Polytriptycene@CdS double shell hollow spheres with enhanced interfacial charge transfer for highly efficient photocatalytic hydrogen evolution. J. Mater. Chem. A. 2021;9:9105–9112. [Google Scholar]
  • 33.Wang X.-T., Wei Q.-Y., Zhang L., Sun H.-F., Li H., Zhang Q.-X. CdTe/TiO2 nanocomposite material for photogenerated cathodic protection of 304 stainless steel. Mater. Sci. Eng. B. 2016;208:22–28. [Google Scholar]
  • 34.Liu X., Zhang J., Dong Y., Li H., Xia Y., Wang H. A facile approach for the synthesis of Z-scheme photocatalyst ZIF-8/g-C3N4 with highly enhanced photocatalytic activity under simulated sunlight. New J. Chem. 2018;42:12180–12187. [Google Scholar]
  • 35.Chen X., Guo R.-T., Pan W.-G., Yuan Y., Hu X., Bi Z.-X., Wang J. A novel double S-scheme photocatalyst Bi7O9I3/Cd0.5Zn0.5S QDs/WO3−x with efficient full-spectrum-induced phenol photodegradation. Appl Catal B. 2022;318 [Google Scholar]
  • 36.Yang Y., Zhao S., Bi F., Chen J., Wang Y., Cui L., Xu J., Zhang X. Highly efficient photothermal catalysis of toluene over Co3O4/TiO2 p-n heterojunction: The crucial roles of interface defects and band structure. Appl Catal B. 2022;315 [Google Scholar]
  • 37.Gong Y., Wu Y., Xu Y., Li L., Li C., Liu X., Niu L. All-solid-state Z-scheme CdTe/TiO2 heterostructure photocatalysts with enhanced visible-light photocatalytic degradation of antibiotic waste water. Chem. Eng. J. 2018;350:257–267. [Google Scholar]

Associated Data

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Supplementary Materials

Supplementary data 1
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Data Availability Statement

Data will be made available on request.

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