Synergism of Holmium Orthovanadate/Phosphorus-Doped Carbon Nitride Nanocomposite: Nonenzymatic Electrochemical Detection of Hydrogen Peroxide

Developing efficient and robust electrode materials for electrochemical sensors is critical for real-time analysis. In this paper, a hierarchical holmium vanadate/phosphorus-doped graphitic carbon nitride (HoVO4/P-CN) nanocomposite is synthesized and used as an electrode material for electrochemical detection of hydrogen peroxide (H2O2). The HoVO4/P-CN nanocomposite exhibits superior electrocatalytic activity at a peak potential of −0.412 V toward H2O2 reduction in alkaline electrolytes while compared with other reported electrocatalysts. The HoVO4/P-CN electrochemical platform operated under the optimized conditions shows excellent analytical performance for H2O2 detection with a linear concentration range of 0.009–77.4 μM, a high sensitivity of 0.72 μA μM–1 cm–2, and a low detection limit of 3.0 nΜ. Furthermore, the HoVO4/P-CN-modified electrode exhibits high selectivity, remarkable stability, good repeatability, and satisfactory reproducibility in detecting H2O2. Its superior performance can be attributed to a large specific surface area, high conductivity, more active surface sites, unique structure, and synergistic action of HoVO4 and P-CN to benefit enhanced electrochemical activity. The proposed HoVO4/P-CN electrochemical platform is effectively applied to ascertain the quantity of H2O2 in food and biological samples. This work outlines a promising and effectual strategy for the sensitive electrochemical detection of H2O2 in real-world samples.


Instrumentation
X-ray diffraction (XRD) analysis was carried out using a Bruker AXS D8 advance X-ray diffractometer with the Cu-Kα radiation of wavelength 1.540 Å. Fourier-transform infrared (FT-IR) spectroscopy was performed using a PerkinElmer Frontier FT-IR spectrometer.The X-ray photoelectron (XPS) spectra were acquired using a JEOL JPS-9030 spectrometer.Transmission electron microscopy (TEM) images were recorded by a JEOL JEM-2100F system equipped with a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping function, and energy-dispersive X-ray spectroscopy (EDS).Electrochemical impedance spectroscopy (EIS) was conducted on a Metrohm Autolab NOVA 2 system.A signal amplitude of 5 mV was applied in the frequency ranging from 0.01 Hz to 100 kHz.High performance liquid chromatography (HPLC) was analyzed by Vanquish Flex UHPLC.A conventional three-electrode electrochemical system used for the electrochemical measurement consists of the CHI electrochemical workstation, a glassy carbon electrode (GCE; 0.07 cm 2 ) participating as the working electrode, a platinum wire acting as the counter electrode, and Ag/AgCl (Sat.KCl) acting as the reference electrode.

Synthesis of HoVO 4 nanorices
The nanorice like HoVO 4 was synthesized by the hydrothermal method.In detail, 0.468 g of NH 4 VO 3 was dissolved in 40 mL of DI water at 60 °C under constant magnetic stirring.Teflon-lined stainless-steel autoclave and then heat-treated at 180 °C for 12 hr.After cooling, the obtained precipitate was centrifuged, washed with ethanol/DI water, and dried in an oven at 60 °C.The resultant yellowish product was calcined at 600 °C for 3 hr with a heating rate of 5 °C min -1 to form nanorice like HoVO 4 .

Synthesis of P-CN nanosheets
The P-CN nanosheets were prepared by the following process.A mixture of NH 4 H 2 PO 4 and C 3 H 6 N 6 was dispersed in 50 mL of DI water at room temperature by ultrasonication for 90 min to obtain a homogeneous suspension.The resultant precursor material was thoroughly washed with DI water and dried in an oven at 50 °C overnight.The prepared material was well-grounded in a mortar and then calcined in the muffle furnace at 550 °C for 3 hr.Finally, the obtained precipitate was centrifuged, washed, and dried in an oven at 60 °C.
After 15 min, 0.96 g of NaOH was added into the above solution at 60 °C under constant magnetic stirring to obtain solution A. 1.76 g of Ho(NO 3 ) 3 •5H 2 O was dissolved in 40 mL of DI water under constant magnetic stirring for 30 min to obtain solution B. Next, solution B was slowly poured into solution A under constant stirring for 30 min to yield a mixed homogeneous solution.This mixed homogeneous solution was transferred into a 100 mL S-3

Table S1 .
Recovery measurement result of H 2 O 2 in milk and human urine samples.