Catechol

All reagents their highest available purity were purchased through Sigma-Aldrich (Gillingham, UK) and Lancaster Synthesis (Lancaster, UK) and were used as received without any further purification steps; catechol (99%, Aldrich, Gillingham, UK), glutathione (98%, Sigma-Aldrich), D,L-homocysteine (≥95%, Sigma), and D,L-cysteine (97%, Lancaster Synthesis). All solutions were prepared with deionized water at a resistivity of no less than 18.2 MΩ·cm⁻¹ at 25 °C (Millipore, Watford, UK). The buffer solutions, 0.15 M, were prepared using potassium monohydrogen phosphate (K₂HPO₄) (≥98%, Sigma-Aldrich), potassium dihydrogen phosphate (KH₂PO₄) (≥99%, Sigma-Aldrich), and potassium hydroxide (KOH) (≥85%, Sigma-Aldrich) accordingly to the required pH range. All buffer solutions were freshly made prior to experiments with supporting electrolyte of 0.10 M potassium chloride (KCl) (99%, Sigma-Aldrich) added to each solution.

The neurotransmitters and metabolites tested were acetamidophenol, catechol, L-DOPA, dopamine, vanillylamine, homovanillic acid, norepinephrine, resorcinol, vanillic acid (all from Aldrich Chemical, Milwaukee, WI, USA), and uric acid (Fisher Scientific). The working solution was prepared in universal buffer with a concentration of 1.12–2.04 ppm (approximately 1 × 10⁻⁵ M) for each of the 10 compounds listed above.
The second class of analytes was nucleic acids and heterocyclic bases including adenine, adenosine, cytidine, cytosine, guanine, guanosine, thymidine, and uridine (all from Aldrich Chemical, Milwaukee, WI, USA). The working solution was prepared in universal buffer with a concentration of 56–204 ppm (approximately 5.00 × 10⁻⁴ M) for each of the eight analytes in this group.
The capsaicinoids used were capsaicin, dihyrocapsaicin, and N-vanillylnonanamide (VANA) (all from Sigma Chemical, St. Louis, MO, USA). The working solution was prepared in acetonitrile with a concentration of 148–152 ppm (approximately 5.00 × 10⁻⁴ M) for each of the three analytes listed above.

All reagents were of an analytical grade and all solutions were prepared using deionized water from Milli-Q system (Millipore, Billerica, MA, USA). Samples for electrochemical measurements were prepared on phosphate buffer in the presence of KCl as a background electrolyte, reagents purchased from Merck KGaA (Darmstadt, Germany).
For the graphene synthesis, it was used as a graphite powder (50 µm, sulfuric acid (98%), sodium nitrate (99%), and potassium permanganate (99%), purchased from Merck KGaA (Darmstadt, Germany); hydrogen peroxide (30%) was purchased from Riedel-de Haën (Germany). Used ionic resins were strong cationic resin C100E and weak anionic resin A520E (Purolite) purchased from Merck KGaA. Ba(NO₃)₂ was purchased at Acros Organics (Geel, Belgium).
The material needed for the building of the electrode were: Resineco Epoxy Kit 125 resin supplied from Resineco green composites (Barcelona, Spain) and graphite powder (particle size < 50 μm) was received from BDH (BDH Laboratory Supplies, Poole, UK). For the biosensor, Laccase from Agaricus Bisporus, 7.2 U·mg⁻¹ (EC number: 420-150-4), 1-ethyl-3-(3-dimethylaminopropyl carbodiimide (EDAC), and N-hydroxysulfosuccinimide (sulfo-NHS) (purchased from Merck KGaA) were used.
Finally, catechol, acid benzoic, and EDTA were purchased from Merck KGaA.

Catechol, potassium chloride (KCl), sodium acetate (AcONa), sodium phosphate dibasic, sodium phosphate monobasic, glutaraldehyde 25% v/v, and laccase from Trametes versicolor (TvLac) were purchased from Merck Life Science (Milan, Italy). All solutions were prepared using Milli-Q water (R = 18.2 MΩ cm at 25 °C; TOC < 10 μg L⁻¹, Millipore, Molsheim, France). TvLac was solubilized in sodium acetate buffer (AcONa) 0.01 M, pH 5, stored at −20 °C, and Catechol standard solutions were freshly prepared in AcONa buffer 0.01 M, pH 5, for each experiment.

Phenolic standards, namely, catechin, catechol, syringic acid, chlorogenic acid, cinnamic acid, ellagic acid, kaempferol, ferulic acid, gallic acid, rutin, caffeic acid, naringenin, benzoic acid, o-coumaric acid, p-hydroxybenzoic acid, pyrogallol, p-coumaric acid, quercetin, quinol, rosmarinic acid, and vanillic acid were bought from Merck KGaA (Darmstadt, Germany). The purities of standards were up to 99%. The solvents used were of analytical grade, dimethylsulfoxide (DMSO; Alfa Aesar GmbH & Co KG, Massachusetts, United States), orthophosphoric acid (H₃PO₄), ethanol, methanol, and acetonitrile HPLC-grade (Fisher Scientific International, Inc., Hampton, New Hampshire, United States).

The following chemicals of analytical grade (Merck) and tri-distilled water were used for the preparation of photocatalysts and solutions: Phenol (C₆H₆O), Catechol (C₆H₆O₂) Hydroquinone (C₆H₆O₂), Zinc Acetate dihydrate (ZAC, Zn(CH₃COO)₂·2H₂O), Hydrochloric Acid (HCl) potassium dichromate (K₂Cr₂O₇), sulphuric acid (H₂SO₄), Ethanol (95% purity), Sodium Hydroxide (NaOH), Ammonium chloride (NH₄Cl), Ammonium Hydroxide (NH₄OH), Potassium ferricyanide (K₃Fe(CN)₆), and 4-Aminoantipyrine. Duranit inert balls 3–5 mm (80% SiO₂–20% Al₂O₃) were purchased from VEREINIGTE FÜLLKÖRPER-FABRIKEN-VFF (Baumbach, Germany).

Benzyl butyl phthalate (BBP), monobenzyl phthalate (MBzP), mono-n-butyl phthalate (MBP), benzyl alcohol, 1-butanol, benzaldehyde, butanal, benzoic acid (BA), butyric acid, phthalic acid (PA), protocatechuic acid (PCA), and other phthalate diesters were purchased from Sigma-Aldrich GmbH (Germany). Catechol was purchased from Merck (Germany). Methanol, chloroform, and ethyl acetate, both analytical and HPLC grade, were purchased from Merck (India). All other chemicals and reagents used in this study were of analytical grade and used without further purification.