This paper, written by researchers from Qingdao University of Science and Technology and others, discusses High-activity Fe3O4 nanozyme as signal amplifier: a simple, low-cost but efficient strategy for ultrasensitive photoelectrochemical immunoassay. The paper is published in an important journal < Biosensors and Bioelectronics >. IF：8.173.

In recent years, the research work of microwave chemical instrument used in the synthesis of materials has become a hot direction of scientific research, which has been paid great attention to by many scholars!

Sensitive but with simple, inexpensive detection of disease-related biomarkers in real biological samples is of quite necessity for early diagnosis and disease surveillance. We herein first introduced high-activity Fe3O4 nanozyme as signal amplifier to develop an ultrasensitive photoelectrochemical (PEC) immunoassay, which meanwhile has the distinct merits of both simplicity and low cost compared with previously reported enzyme-labeling PEC immunoassays. In the proposal, to illustrate and describe the PEC platform, prostate-specific antigen (PSA, Ag) was used as a target model. Specifically, ZnO nanorods (ZnO-NRs) grown vertically on a bare indium–tin oxide (ITO) electrode was deposited with ZnIn2S4 nanocrystals, producing ZnIn2S4/ZnO-NRs/ITO photoelectrode as the PEC matrix to modify capture PSA antibody (Ab1). Histidine-modified Fe3O4 (his-Fe3O4) nanozyme as signal amplifier was linked with signal PSA antibody (Ab2) to form his-Fe3O4@Ab2 conjugate, and was anchored through specific sandwich immunoreaction. The labeling his-Fe3O4 nanozyme acted as a peroxidase to induce the generation of the insoluble and insulating precipitation, resulting in an evident decrease in the photocurrent signal. On account of combined effects of high catalytic efficiency of the his-Fe3O4 nanozyme and excellent PEC properties of the ZnIn2S4/ZnO-NRs/ITO photoelectrode, ultralow detection limit of 18 fg/mL for target Ag detection was achieved. Besides, as high-activity his-Fe3O4 nanozyme has substituted natural enzyme as signal amplifier, simplicity and low cost of the PEC immunoassay was realized.

In summary, high-activity nanozyme instead of natural enzyme as signal amplifier was first introduced in PEC immunoassay for ultrasensitive detection of target biomarker. As signal amplifier, the his-Fe3O4 nanozyme exhibited obviously higher catalytic activity than natural enzyme HRP, and it also had the characteristics of simple preparation, low cost of production, and easy modification. The ZnIn2S4/ZnO-NRs/ITO photoelectrode as low-toxic PEC matrix had excellent properties of evident photocurrent output and good stability. Benefiting from combined effects of high activity of the his-Fe3O4 nanozyme and excellent PEC properties of the ZnIn2S4/ZnO-NRs/ITO photoelectrode, the pleased sensitivity of this PEC immunoassay was realized. Although the activities of the most other nanozymes need further improvement, they are ideal alternatives for corresponding natural enzyme toward the labeling PEC immunoassay in the near future.

The microwave synthesis was performed with a XH-800S Microwave parallel synthesis system (Beijing Xianghu Science and Technology Development Co. Ltd., China). Field-emission scanning electron microscopy (FE-SEM) was carried out on a Hitachi S-4800 scanning electron microscope (Hitachi Co., Japan). Transmission electron microscopy (TEM) was performed with a JEOL-2100 transmission electron microscope (JEOL, Japan). Powder X-ray diffraction (XRD) pattern was obtained from a Philips X’pert Pro X-ray diffractometer (Cu Kα radiation, λ=0.15418 nm, Netherlands). The UV-vis diffuse reflectance spectra were recorded on a spectrophotometer (Hitachi U-3010, Japan) with fine BaSO4 powder as reference. Dynamic light scattering (DLS) characterization was performed by a ZETASIZER nanoseries (Nano-ZS, Malvern, U.K.). The UV-visible (UV-vis) absorption spectra were tested on a UV-3600 UV-visible spectrophotometer (Shimadzu, Japan). Electrochemical impedance spectroscopy (EIS) was performed on an Autolab potentiostat/galvanostat (PGSTAT 30, Eco Chemie B.V., Utrecht, Netherlands) with a three-electrode system in 0.1 M KCl solution containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture as a redox probe, and recorded in the frequency range of 0.01 Hz-100 kHz with an amplitude of 50 mV. Photoelectrochemical (PEC) measurements were performed with a homebuilt PEC system. A 8 150 W xenon lamp was utilized as the irradiation source with the light intensity of 300 mW∙cm-2 estimated by a radiometer (Photoelectric Instrument of Beijing Saifan Co., LTD., China). Photocurrent output was recorded on a CHI 760D electrochemical workstation (Shanghai Chenhua Apparatus Corporation, China) with a three-electrode system: a 0.25 cm2 modified ITO as working electrode, a Pt wire as counter electrode, and a saturated Ag/AgCl electrode as reference electrode.