Boric acid

The cocaine·HCl standard was purchased from Lipomed (Arlesheim, Switzerland). Standards of phenacetin, diltiazem, lidocaine, procaine, hydroxyzine, benzocaine, paracetamol and myo-inositol were purchased from Sigma-Aldrich (Diegem, Belgium). Standards of benzoic acid and levamisole were purchased from Acros Organics (Geel, Belgium). Standards of caffeine, boric acid, glucose, maltose and starch were purchased from VWR Chemicals (Leuven, Belgium) and a standard of d-sorbitol was purchased from Merck Chemicals KGaA (Overijse, Belgium). Authentic cocaine street samples were obtained from the National Institute for Criminalistics and Criminology (Brussels, Belgium). Gelatine gel B was supplied by PB gelatins (United Kingdom). Carbon ItalSens IS-C Screen Printed Electrodes (SPE) were purchased from PalmSens (Utrecht, The Netherlands) and were used during all electrochemical measurements. The electrode surface area is 7.07 mm². All laboratorium-based electrochemical measurements were performed using a Metrohm μAutolab III Potentiostat and NOVA 1.11 software.

The chemicals [2,2-diphenyl-1-picrylhydrazyl; 2′-azino-bis (3-ethylbenzothiazoline- 6-sulfonic acid); 2,4,6-Tris(2-pyridyl)-s-triazine], reagents (Folin–Ciocalteu reagent), standards: ferulic acid, ellagic acid, proline, tyrosine, glycine, lysine, histidine, leucine, aspartic acid, valine were obtained from Sigma Aldrich (St., MO, USA), Elisa kits for testosterone and estradiol from Abbexa (Cambridge, UK) and buffer components (chloroform, ethyl acetate, formic acid, ethanol, 1-butanol, 2-propanol, boric acid) were purchased from Avantor Performance Materials Poland SA (APM, Gliwice, Poland).

Phosphatidylcholine (DPPC) and phosphatidylethanolamine (DPPE) of 99% purity were acquired from Sigma-Aldrich (Darmstadt, Germany). With the use of a 20 mg/mL lipid in a n-decane/butanol mixture (3:1) solution, the membrane was created using the Mueller and Rudin method [33 (link)].
As the electrolyte solution, the buffer was used in the pH range of 2–12 prepared according to Britton and Robinson [34 (link)]. The buffer was prepared by adding 0.2 M sodium hydroxide to 100 mL of the solution (initial pH was 1.81) with the following composition: 0.04 M 80% acetic acid, 0.04 M phosphoric acid, and 0.04 M boric acid (all chemicals from Avantor Performance Materials Poland, Gliwice, Poland).
The proper pH of the buffer was determined depending on the amount of sodium hydroxide added. Buffer pH was changed to, e.g., 3.29 after the addition of 20 cm³ NaOH (Avantor Performance Materials Poland, Gliwice, Poland) or to 6.80 if 50 cm³ was added. The buffer, as mentioned previously, was used in the experiments because it is widely used in biological studies as a standard buffer due to the wide pH range (2–12) and a composition that does not affect the biological membrane.

A basic standard solution of boron in the form of boric acid (1 g/dm³) and a basic standard solution of lanthanum in the form of lanthanum nitrate (1 g/dm³) were supplied by Merck KGaA, Darmstadt, Germany. Chitosan (molecular weight of 600,000–800,000 g/mol) was purchased from Acros Organics, Geel, Belgium; the degree of deacetylation of CTS, as determined by means of a H¹ NMR method, was equal to 97%. lanthanum nitrate, La(NO₃)₃·6H₂O, was supplied by Reachim Ltd., Moscow, Russian Federation. boric acid, sodium hydroxide, hydrochloric acid, and glacial acetic acid were purchased from Avantor Performance Materials Poland S.A., Gliwice, Poland. All the reagents employed in the study were analytical reagent grade.

Chitosan (molecular weight 600,000–800,000) was purchased from Acros Organics (Geel, Belgium); its degree of deacetylation, determined by a ¹H NMR method, was 97%. Manganese chloride, MnCl₂·4H₂O, and other reagents, namely potassium nitrate, sodium hydroxide, and hydrochloric acid, were supplied by Avantor Performance Materials Poland S.A., Gliwice, Poland. A working boron solution of a concentration of 5 g/L was prepared using boric acid provided by Avantor Performance Materials Poland S.A., Gliwice, Poland. A basic standard solution of boron in the form of boric acid (1 g/L) and a manganese standard solution of 1 g/L were supplied by Merck.