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.
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.