Nafion 117

To prepare the Ir/C ink, 11 mg of Ir/C (5 wt% of Ir, Premetek Co.) was dispersed in 1 ml of water and ethanol solution (1:4, v/v), followed by the addition of 45 μl of Nafion 117 solution (Sigma-Aldrich). The mixture was then magnetic stirred to form a homogenous ink. Meanwhile, the IrO₂ ink was also prepared, namely 5.14 mg of IrO₂ and 5 mg of Ketjen Black dispersed in 1 ml of water and ethanol solution (1:4, v/v), followed by the addition of 40 μl of Nafion 117 solution (Sigma-Aldrich). The mixture was then sonicated briefly to form a homogenous ink. The ink was drop-casted onto the Ni foam and Ni/Au_disc electrodes and left dried in air.

Chloroauric acid (HAuCl₄), d-glucose, copper(ii) chloride, sodium hydroxide (NaOH), uric acid (UA), l-glutamic acid, ascorbic acid (AA), dopamine (DA), sodium phosphate dibasic (Na₂HPO₄), sodium phosphate monobasic (NaH₂PO₄), and Nafion™ 117 (approximately 5% in a mixture of lower aliphatic alcohols and water) were purchased from Sigma-Aldrich Canada. Potassium chloride (KCl), sodium chloride (NaCl), nitric acid (HNO₃) and hydrochloric acid (HCl) were purchased from ACP Chemicals Canada. All these chemicals were of analytical grade and were used as received. A 0.1 M NaOH solution was used as the supporting electrolyte. Deionized (DI) water (with resistivity ≥ 18 MΩ cm) was used for preparing various aqueous solutions. A 100 mM glucose stock solution was prepared in DI water and stored in a refrigerator in a dark plastic bottle. Different concentrations of glucose were prepared by diluting the stock solution with 0.1 M NaOH. Suitable amounts of KCl, NaCl, Na₂HPO₄, and NaH₂PO₄ were dissolved in DI water to prepare 100 mM phosphate buffered saline (PBS) solutions of various pH, containing 2.7 mM KCl and 137 mM NaCl.

Ttc, thiodiacetic acid (tda), mercaptosuccinic acid (MSA), Nafion 117 (formulas are shown in Figure 1), titanium(IV) isopropoxide, graphite flakes, NaNO₃, KMnO₄, sodium acetate trihydrate, acetic acid, Hg(NO₃)₂, water and other chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) in ACS purity (chemicals meet the specifications of the American Chemical Society). MiRNA was also purchased from Sigma-Aldrich and its sequence was 5′ UAA GGC ACG CGG UGA AUG CCA AAA AAA AAA 3′. Standard nucleobases were adenine and cytosine (MP Biomedicals, Santa Ana, CA, USA), guanine and uracil (Sigma-Aldrich, St. Louis, MO, USA).