Calcium nitrate

), sodium triphosphate (polyP), ATMP, ammonium phosphate dibasic [(NH 4 ) 2 HPO 4 ; purity  99.0%] and ammonium hydroxide solution 30% (NH 4 OH; purity: 28-30%) were purchased from Sigma-Aldrich. Calcium nitrate [Ca(NO 3 ) 2 ; purity  99.0%] was purchased from Panreac (Barcelona, Spain). Ethanol (C 2 H 5 OH; purity  99.5%) was obtained from Scharlab (Barcelona, Spain).

Sodium hydrogen phosphate and dihydrogen phosphate, aniline, tris(hydroxymethyl) aminomethane (BioPerformance Certified) were from Sigma Aldrich, ethanol, sodium hydroxide, hydrochloric acid, sulfuric acid, sodium nitrate, potassium nitrate and calcium nitrate were from Panreac, tetrahydrofuran (Chromasolv) was from Fisher. All reagents were p.a. (for analysis) unless stated otherwise. High molecular weight polyvinyl chloride (PVC), dibutyl phthalate (DBP), potassium tetrakis(4-chlorophenyl)borate (KTPB), tridodecylmethylammonium chloride (TDMACl) and ionophores were from Fluka. Screen-printed electrodes (SPE) with gold working and auxiliary electrodes and silver reference electrode were from DropSens (Spain). Ultrapure water produced by Merck Millipore Water System (18 MΩcm -1 ) was used for solution preparation and sensor washing.
Solutions of PSTs, namely saxitoxin (STX), decarbamoyl saxitoxin (dcSTX) and Nsulfocarbamoyl toxins gonyautoxin GTX5 and C1&C2, were certified reference material from the Institute for Marine Biosciences, National Research Council, Halifax, Canada. When working with PSTs, long sleeved lab coat and non-permeable nitrile or latex gloves should be used. Toxin containing waste should be decontaminated using a 10% solution of sodium hypochlorite during 30 minutes and disposed of down the drain with plenty of water.