Graphite rod

The materials and chemicals used in this work are listed as follows: graphite rod (99.999%, St. Louis, MO, USA, Sigma-Aldrich), ultrapure water (18.2 MΩ cm⁻¹), ethanol (99.9%, Bangkok, Thailand, RCl Labscan Limited), sodium hydroxide (Lobachemie, Mumbai, India), copper(II) oxide nanopowder (Sigma-Aldrich, St. Louis, MO, USA), poly-(dimethyldiallylammonium chloride) (PDDA, MW = 200,000–350,000, 20 wt% in water, Sigma-Aldrich, St. Louis, MO, USA), D-glucose (Fisher scientific, New Hampshire, USA), sucrose (Ajax Finechem, Prospect, South Australia), lactose (Fluka, St. Louis, MO, USA), dopamine (Sigma-Aldrich, St. Louis, MO, USA), ascorbic acid (Ajax Finechem, Prospect, South Australia), and uric acid (Sigma-Aldrich, St. Louis, MO, USA). All the chemicals used were of analytical reagent grade.
Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were obtained using a transmission electron microscope (JEM 2010, Jeol, Japan) and a field emission scanning electron microscope (JSM 6335 F, Jeol, Japan), respectively. FTIR spectrum was recorded on a FTIR spectrophotometer (Thermo Scientific, Massachusetts, USA) from 400 to 4000 cm⁻¹. EmStat potentiostats (PalmSens, Houten, Netherlands) was employed for electrochemical detection.

Experiments were carried out in a H cell with a Nafion 117 proton exchange membrane (Ion Power, Inc.) to separate the cathodic and anodic compartments. An electrochemical workstation VSP-300, Bio Logic was used to perform electrochemical procedures. A piece of carbon felt (Alfa Aesar >99.0%, 3.2 mm thickness) infiltrated with 10 mg Pd/C connected to a graphite rod (Sigma Aldrich, 99.99%) was used as working electrode in the cathode compartment. A platinum wire (Alfa Aesar, 99.9%) was used as counter electrode in the anodic compartment. Ag/AgCl was used as a reference electrode. The cathode and anode compartments were filled with 60 mL 0.2 M H₃PO₄ solution as electrolyte solution. All reactions were performed at atmospheric pressure at constant potential referred to the reverse hydrogen electrode (RHE). The catalyst was first activated under a constant current of −40 mA for 10 min before adding benzyl alcohol (20 mM) into the cathode compartment. Then an electric potential was applied for reaction. Product analysis is the same as performed in the catalytic reaction under OCP.

The polycrystalline Cu powder (−625 mesh, APS 0.50–1.5 micron, 99% metal basis) is purchased from Alfa Aesar. Cu foil (0.1 mm thick, 99.9999% metal basis) is purchased from Alfa Aesar. Potassium carbonate (99.997% trace metals basis) is purchased from Alfa Aesar. Chelex 100 sodium form is purchased from Sigma-Aldrich. Isopropanol (99.999% trace metal basis) is purchased from Sigma-Aldrich. Dimethyl sulfoxide (≥99.9%) is purchased from Alfa Aesar. Deuterium oxide (99.9 atom% D) is purchased from Sigma-Aldrich. Nafion solution (5 wt%) is purchased from Sigma-Aldrich. Phosphoric acid (ACS reagent, ≥85 wt% in H₂O) is purchased from Sigma-Aldrich. Graphite rod (99.995% trace metals basis) is purchased from Sigma-Aldrich. Sigracet 39 BC carbon fiber paper is purchased from Fuel Cell Store. Carbon Dioxide (99.999%), oxygen (99.999%), and argon (99.999%) are purchased from Air Liquide. All electrolyte solutions are prepared using Milli-Q water (18.2 MΩ cm).

All chemical materials were of the highest purity and used as they were, with no further purification, unless otherwise mentioned. Acetaminophen (AMP, C₈H₉NO₂, ≥99.0%), graphite rods (6 mm diameter, 99.995% purity), potassium chloride (KCl, 99.98%), uric acid (UA, C₅H4N₄O₃, 99%), and L-(+)-Tartaric acid (TA, C₄H₆O₆, ≥99.5%) were obtained from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). Sodium dihydrogen phosphate (H₂NaO₄P, 100%) and disodium hydrogen phosphate anhydrous (HNa₂O₄P, 99.7%) were acquired from VWR BDH Chemicals (Leuven, Belgium). Sodium acetate anhydrous (CH₃COONa, ≥99.0%), sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O), and ammonium sulfate ((NH₄)₂SO₄, ≥99.0%) were purchased from ChimReactiv SRL (Bucuresti, Romania). Potassium hexacyanoferrate(III) (K₄[Fe(CN)₆, ≥99%), L(+)-ascorbic acid (AA, C₆H₈O₆, 99.7%) were acquired from Merck (Darmstadt, Germany). Citric acid (CA, C₆H₈O₇, 99+%) and L-cysteine (CYS, C₃H₇NO₂S, 98+%) were acquired from Alpha Aesar GmbH & Co KG (Karlsruhe, Germany) aqueous solutions were freshly prepared using ultrapure water (18.2 MΩ, Milli-Q water purification system), protected from light by refrigeration at 4 °C when not in use.

GQDs were synthesized according to the published electrochemical method [29 (link)]. Briefly, two graphite rods (> 99.99999%, Sigma) as anode and cathode were inserted in ultrapure MiliQ water and subjected to 30 V from a direct current power supply. After the electrochemical reaction for 120 h, a dark brown solution was obtained, from which GQDs were collected and purified via centrifugation at 10000 rpm for 1 h, followed by filtration and freeze-drying.