Co no3 2 6h2o

Reagents, such as Cu(NO₃)₂3H₂O, Co(NO₃)₂6H₂O, Al(NO₃)₃9H₂O, Na₂CO₃ and NaOH, were bought from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China), and used as received without further purification. Deionized water (Millipore Milli-Q system, resistivity ≥ 18.2 MΩ·cm) was used for the preparation of all solutions. Acetylene black, polyvinylidene fluoride (PVDF) and N–methylpyrrolidone (NMP) solution were purchased from KEJING Star Technology Co. Ltd. (Shenzhen, China). Nafion^® 117 membrane was obtained from Saosi Chemical Instrument Co., Ltd. (Hangzhou, China). Copper foil was bought from Amamda Hardware Trading Co., Ltd. (Shenzhen, China). After receipt, it was washed by ethanol for the removal of oil and grease, polished with 3000 mesh sandpaper and finally washed with distilled water. The pre-treated copper foil was dried in the shade for future use.

All the reagents are of analytical grade. Co(NO₃)₂·6H₂O, Cu(NO₃)₂·3H₂O, Na₂S and urea were supplied by Sinopharm Chemical Reagent Co. Ltd (Beijing, China).
The as-pressed nickel foam substrates with 2 × 2 cm² in size were cleaned in HCl solution and deionized water for each 15 min under ultrasonication. Typically, 1 mmol of Cu(NO₃)₂·3H₂O, 2 mmol of Co(NO₃)₂·6H₂O and 9 mmol of urea were dissolved in 35 mL of deionized water under magnetic stirring for 30 min. The solution was poured in 50 mL Teflon-lined stainless steel autoclave with a pretreated and weighted Ni foam, and then maintained at 120 °C for 4 h. Subsequently, the as-synthesized Cu–Co precursor loaded on Ni foams were ultrasonically cleaned with water for 1 min. The Cu–Co precursor was annealed at 350 °C in air for 2 h in order to fabricate Cu–Co oxide. The Cu–Co sulfides were prepared by the sulfuration of Cu–Co precursor or Cu–Co oxide with 10 mL 0.1 M sodium sulfide at 120 °C for 6 h. The Cu–Co sulfide obtained from precursor was recorded as P–Cu–Co sulfide, while the sulfide from Cu–Co oxide was marked as O–Cu–Co sulfide.

All the chemicals were of analytical grade and were used without further purification. Firstly, 1.8 g of glucose (Sinopharm Chemical Reagent Co., LTD. AR), 0.582 g of Co(NO₃)₂.6H₂O(Sinopharm Chemical Reagent Co., LTD. AR) and 15 g of NaCl (Tianjin Guangfu Technology Development Co., LTD. AR)were dissolved in 50 ml of deionized water under magnetic stirring until a transparent blue solution was obtained. Then, the solution was transfered into a drying oven and dried at 80 ^oC for 24 h. The dry block of crystal was ground to fine powder in an agatemortar. The composite powder was heated in a tubular vacuum furnace at 750 ^oC for 2 h under flowing N₂. Finally, it was annealed again at 300 ^oC for 4 h under air. Once cooled to room temperature, the as-synthesized black powder was washed by deionized water several times to remove NaCl, and then Co₃O₄@C@PGC nanosheets were obtained. For comparison, Co₃O₄/C composites were also synthesized by carbonizing the mixture of Co(NO₃)₂·6H₂O and glucose without adding NaCl at the same conditions. Meanwhile, Co₃O₄ NPs were also synthesized by sintering Co₃O₄@C@PGC nanosheets at 600 ^oC for 2 h in air. And PGC nanosheets were obtained by immersing Co₃O₄@C@PGC nanosheets in plenty of nitric acid for 4 h at 70 ^oC with magnetic stirring to remove the Co₃O₄ NPs.

All chemicals were used as-received without further processing. HAuCl₄·3H₂O, AgNO₃ and K₂PdCl₄ were obtained from Shanghai Reagent Co. (Shanghai, China) and used as received. Cetyltrimethylammonium bromide (CTAB), glutaraldehyde (GA), ascorbic acid (AA), Fe(NO₃)₃·9H₂O, Co(NO₃)₂·6H₂O, NH₄OH and NaOH were provided by Sinopharm Chemical Reagent Co. (China). BSA (96–99%) was purchased from Sigma (USA) and used as received. Estradiol antigen and anti-EST were purchased from Wanger Biotechnology Co., Ltd. (Beijing, China). All other reagents were of analytical grade. PBS (0.1 mol·L⁻¹ containing 0.1 mol·L⁻¹ NaCl) was used as electrolyte for all electrochemistry measurements. Ultra-pure water was prepared by a Millipore Milli-Q system and used throughout.