Urea ch4n2o

Feedstock solution was prepared by mixing 0.4 M aqueous tin chloride pentahydrate (SnCl₄·5H₂O; J. T. Baker, Mexico City, Mexico and urea (CH₄N₂O; Sigma Aldrich, Mexico City, Mexico in 1:2 ratio. Then the mixed solution was vigorously stirred and heated until the solution temperature reaches around 93 ± 5 °C. Unlike the former case (urea), for ammonia (NH₄OH; Sigma Aldrich) as precipitation agent it was added dropwise to 0.4 M aqueous SnCl₄·5H₂O until the pH of the solution reached 12. The resultant precipitates in both cases were centrifuged at 400 rpm for 1 h using a ROTINA-420R centrifuge (Hettich, Toluca, Mexico) and the obtained pastes were dried at 100 °C for 24 h to eliminate any remaining solvent. Finally, the dried powders were calcined in a furnace at 800 °C for 2 h to obtain pure SnO₂ powders. All the synthesis conditions were studied, optimized and reported in our previous works [33 ,34 ].

Ammonium heptamolybdate ((NH₄)₆Mo₇O₂₄), sodium tungstate (Na₂WO₄), and urea (CH₄N₂O) were all purchased from Sigma-Aldrich with a purity of more than 99%. The crystal structure of the material was investigated through X-ray diffraction analysis (XRD, PANalytical X’Pert Pro MRD device), and the surface morphology of the material was done with scanning electron microscopy (SEM, TESCAN VEGA3). Electrochemical tests including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Chronoamperometry and Galvanostatic charge/discharge (GCD) were performed with a Metrohm potentiostat/galvanostat instrument (302N).

Human hair was collected from local barber shops. PCL (Mn 45,000), urea (CH₄N₂O), sodium dodecyl sulfate (SDS), and 2-mercaptoethanol were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sodium hydroxide and formic acid were purchased from Samchun Chemicals (Seoul, Korea).

Ethanol (Ethyl alcohol 99.8%) was purchased from Merck & Co. Deionized water was purchased from RCI Labscan Limited (Bangkok, Thailand). Sodium chloride (NaCl), potassium chloride (KCl), lactic acid (C₃H₆O₃), and urea (CH₄N₂O) were purchased from Sigma-Aldrich. Acetone (C₃H₆O) was purchased from GNS Chemical Co., Ltd (Songkhla, Thailand). All reagents were analytical grade and used without further purification.
The alcohol MOX gas sensor (MICS5524) was purchased from SGX Sensortech (Shanghai, China). The microcontroller (ESP32 WROOM-32 WiFi + Bluetooth development board) was purchased from Espressif Systems Co., Ltd. (Shanghai, China). Rechargeable lithium polymer battery 3.7 V 1000 mAh was purchased from Kayo Battery Co., Ltd. (Shenzhen, China). A 5 V 1 A booster and charger power bank module was purchased from Cytron Technologies (Bangkok, Thailand). A 3D printer (Anet A8 print) was purchased from Anet Technology Co., Ltd. (Shenzhen, China). Polyethylene terephthalate glycol-modified (PETG) filament was purchased from X3D Technology Palawatr Co., Ltd. (Nakhon Pathom, Thailand). Semi-permeable membrane (Opsite Flexifix©) was purchased from Smith & Nephew Ltd. (London, UK).

Zinc
nitrate hexahydrate
(Zn(NO₃)₂·6H₂O, Sigma-Aldrich),
Poly(ethylenimine) (PEI) solution (Sigma-Aldrich, MW ∼750,000
in H₂O), nitric acid (HNO₃, Sigma-Aldrich),
ammonium hydroxide solution (NH₄OH, Sigma-Aldrich), sodium
sulfide (Na₂S, Sigma-Aldrich), an urea (CH₄N₂O, Sigma-Aldrich) were purchased and used in the experiments.

Aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O), magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), urea (CH₄N₂O), sodium bicarbonate (NaHCO₃), and methyl orange (C₁₄H₁₄N₃NaO₃S) were purchased from Sigma-Aldrich Co., LLC. (St. Louis, MO, USA). Sodium hydroxide (NaOH) in pellet form was purchased from Daejung Chemicals & Metals Co., Ltd. (Siheung, Republic of Korea). All reagents were used without further purification.