Nitric acid

The used chemicals were hydrochloric acid, nitric acid, and lactic acid from Scharlau (Scharlab S.L., Spain), and iron nitrate, ascorbic acid, and 70% ethanol (Sigma Aldrich, Stockholm, Sweden). The deionized water was generated by Millipore Milli‐Q plus ultrapure water system (Millipore, Solna, Sweden). The four yeast extracts (YE1 to YE4) used for the baking studies were as follows: YE1 (Acros organics from Fisher Scientific, UK), YE2 (Bacto yeast extract from Difco Laboratories, UK), YE3 (Oxoid yeast extract from Oxoid Ltd, UK), and YE4 (yeast extract from Sigma Aldrich, Stockholm, Sweden).

All chemicals used were reagent grade unless otherwise stated. Aqueous nickel solutions were prepared from a commercial standard solution of 1000 mg·L⁻¹ from Merck (Darmstadt, Germany). Nitric acid (65%), sodium chloride, sulfuric acid (95–97%), sodium hydroxide and toluene were obtained from Scharlab (Barcelona, Spain). Buffer solutions were prepared using N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES) from Biochemical (Barcelona, Spain). N,N-Dimethylformamide (DMF), ethanol (96%) and perchloric acid (60%) were obtained from Merck (Darmstadt, Germany). Hydrochloric acid was purchased from JT Baker (Phillipsburg, NJ, USA). Humic acid sodium salt was obtained from Aldrich (Steinheim, Germany). The active component of the liquid membrane, 1,2-cyclohexanedione bis-benzoylhydrazone (1,2-CHBBH), was synthesised as previously described [33 (link)]. All solutions were prepared using deionised water (18 MΩ·cm⁻¹) from a Milli-Q analytical reagent grade water purification system (Millipore, Bedford, MA, USA).
Solutions used as a source phase in optimization experiments consisted of 1 mg·L⁻¹ Ni(II) and 35 g·L⁻¹ NaCl. These solutions were adjusted to pH 8 using HEPES and sodium hydroxide. Appropriate amounts of sodium chloride and Humic acid sodium salt were added to source solutions, to investigate the effects of organic and inorganic ligands.

Copper(II) nitrate (Sigma-Aldrich, Madrid, Spain), ethanol 99.5% (Quimipur, Madrid, Spain), acetic acid glacial (Panreac, Barcelona, Spain), nitric acid (Scharlau, Barcelona, Spain), dichloromethane (Labkem, Barcelona, Spain), sodium nitrate (Sigma-Aldrich, Madrid, Spain), MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) dye (Sigma Cat. No. M5655, Sigma-Aldrich, Madrid, Spain), and filter membranes (Magna, Nylon 47 mm 0.45 µm membrane disk, Fisher Scientific, Waltham, MA, USA) were used throughout the study. Low density polyethylene (LDPE, Riblene MP30R, d25 °C = 0.925 g/cm³, melting point of 112 °C, Versalis, Milan, Italy) and polylactic acid (PLA, IngeoTM Biopolymer 2003D NatureWorks, Blair, NE, USA) were kindly gifted by EMSUR (Spain). S. enterica (ATCC 35664) was obtained from the American Type Culture Collection (ATCC, Manassas, VA, USA) and L. monocytogenes was obtained from a sample recovered at the Hospital Universitario Príncipe de Asturias (Alcalá de Henares, Spain).