Tetracycline

All chemical materials were used devoid of additional purification. Ferric nitrate hydrate (Fe(NO₃)₃·9H₂O, ≥99.0%, Tianjin Tianli chemical reagent Co., Ltd., Tianjin, China), aquae hydrogenii dioxidi (H₂O₂, 30%, Tianjin Tianli chemical reagent Co., Ltd., Tianjin, China), ethylene glycol ((HOCH₂)₂, EG, Tianjin Beifang chemical reagent Co., Ltd., Tianjin, China), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, ≥99.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), silver nitrate (AgNO_3, ≥99.0%, Tianjin Hengxing Chemical reagent Co., Ltd., Tianjin, China), potassium iodide (KI, ≥99.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), urea (CO(NH₂)₂, AR, Tianjin Zhiyuan chemical reagent Co., Ltd., Tianjin, China), and tetracycline (TC, ≥99.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). The water utilized in all experiments was deionized water, and the whole experiment process was carried out at room temperature.

Cabbage waste was derived from the vegetable market in Hefei, China. ZnCl₂, methylene blue, acid magenta, tetracycline, potassium dichromate, and MgCl₂ were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

Oligonucleotides were purchased from Sangon Biotech Co., Ltd (Shanghai, China). All the sequences (Table S1†) were dissolved in Tris-HAc buffer (50.0 mmol L⁻¹, pH 7.9, 50.0 mmol L⁻¹ KAc). CAP, cefixime, ciprofloxacin, tetracycline, amoxicillin, and spermine were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). NMM was purchased from J&K Scientific Ltd (Beijing, China). A CAP enzyme-linked immunosorbent assay (ELISA) kit was obtained from Beacon Analytical Systems, Inc. (Saco, MA, USA). Milk samples were obtained from a supermarket in Zhanjiang city (Yili Corporation, Inner Mongolia, China). Ultrapure water (18.1 MΩ cm, 25 °C) was obtained using a 350 Nanopure water system (Guangzhou, China) and was utilized in all experiments. All reagents were of analytical grade.
The two hairpin DNA probes (H1 and H2) and the initiator probes were heated at 90 °C for 10 min and then slowly cooled to room temperature to form satisfactory hairpin structures. A solution of Apt and Apt-C was heated at 90 °C for 10 min and then slowly cooled to 25 °C to form dsDNA (Apt·Apt-C).