2.1. Materials and Reagents

Chloroplatinic acid 8 wt.%, chitosan (low molecular weight, 75–85% deacetylated 20–300 cP), hydroquinone, hexaamineruthenium(III) chloride, potassium phosphate monobasic, and sodium phosphate dibasic were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). Polycrystalline diamond suspensions (1 µm and 3 µm) and alumina slurry (0.05 µm) were obtained from Buehler (Lake Bluff, IL, USA). Lead acetate (30% w/v) was purchased from Fisher Scientific (Pittsburgh, PA, USA). Potassium ferrocyanide trihydrate was purchased from Ward’s Science (Rochester, NY, USA). Potassium ferricyanide and cysteine hydrochloride monohydrate were obtained from J.T. Baker (Phillipsburg, NJ, USA). Platinum/iridium working electrodes (Pt/Ir, BASi MF-2013, 1.6 mm diameter, 7.5 cm length), reference electrode (Ag/AgCl) and Pt auxiliary electrode were purchased from BASinc. (West Lafayette, IN, USA).

Sulfo-SMCC (sulfosuccinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide HCl (EDC) were obtained from Thermo Fisher Scientific (Waltham, MA, USA). N-Hydroxysuccinimide (NHS), 2-(morpholino)ethanesulfonic acid (MES) buffer, Ellman’s reagent (5,5′-dithiobis-(2-nitrobenzoic acid)), and platinum wire (99.95% Pt, 1.5 mm dia.) were obtained from Alfa Aesar (Ward Hill, MA). DiaEasy™ dialysis tubing (3.5 kDa MWCO) was purchased from Biovision (Milpitas, CA, USA).

L. monocytogenes aptamer developed by Ohk et al. [20] (47 DNA bases, K_D = 10³ CFU/mL, Mw 15,008 g/mol) terminated at the 5′ end with a thiol group was purchased from Gene Link (Hawthorne, NY, USA). Commercially sterilized chicken broth was purchased from a local grocery store. Tryptose phosphate broth (TPB) was acquired from HiMedia (Mumbai, India). Tryptic soy broth (TSB), tryptic soy agar (TSA), yeast extract, and buffered peptone water (BPW) were purchased from Becton, Dickson, and Company (Sparks, MD, USA). Potassium nitrate (KNO₃) was purchased from British Drug Houses(BDH) (Mississauga, ON, Canada). Tris EDTA (TE) buffer pH 7.4 was purchased from Quality Biological (Gaithersburg, MD, USA). Sodium chloride (NaCl) and potassium chloride (KCl) were acquired from EMD Millipore Corporation (Burlington, MA, USA).