Structural characterisation of 1,10-phenanthroline–montmorillonite intercalation compounds and their application as low-cost electrochemical sensors for Pb(II) detection at the sub-nanomolar level

A low-cost electrochemical sensor for lead detection was designed and developed in the present study, using an organo-clay obtained by the intercalation of 1,10-phenanthroline (OP) within montmorillonite (MMT). Intercalates of 1,10-phenanthroline in montmorillonite were prepared from a 1/1 water–ethanol aqueous solutions at pHs 3, 5 and 8. Their structural characterisation was done by a variety of spectroscopic and analytical techniques: XRD, FTIR, 13C and 23Na NMR, ICP and thermal analysis. Under acidic conditions, OP was protonated and a cation exchange took place with the interlayer sodium ions, leading to a two-dimensional arrangement of protonated OP molecules laying parallel to the clay mineral interlayer surfaces. Under basic conditions, neutral OP was intercalated in the presence of hydrated Na+ ions forming two OP layers plausibly H-bonded to the water molecules coordinating the sodium ions. This latter OP-MMT intercalate was used as Pb(II) sensor via a carbon paste electrode (CPE) by adsorptive stripping voltammetry because it should lead to the exchange of hydrated sodium ions by Pb(II). The amount of accumulated Pb(II) increased with an increase of the accumulation time and remained constant after saturation. The optimal pH of the accumulation medium was 6 and the best electrolysis time and potential were 60 s and − 0.8 V respectively. The results obtained in the case of Pb(II) led to a limit of detection in the sub-nanomolar range (4 × 10− 10 M), two orders of magnitude lower than in the case of Cu(II) (4.0 × 10− 8 M). There were no interferences with Cu(II) when it was at concentrations 0.1 × [Pb(II)]. For equimolar concentrations, the Pb(II) signal was reduced by 50%. The interferences were minimal in the case of Ag(I). In contrast, the Pb(II) signal was reduced by 70% in the presence of Hg(II) at concentrations 0.1 × [Pb(II)], showing a strong Hg(II) interference.

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► 1,10-orthophenanthroline (OP) was intercalated in montmorillonite (MMT).
► OP is intercalated by displacement of the exchangeable cation (sodium) at acidic pHs.
► OP is co-intercalated with sodium at neutral and slightly basic pHs (OP–MMT).
► OP–MMT is a useful material for the highly sensitive determination of Pb(II).
► The limit of detection of Pb(II) is in the sub-nanomolar range (4 × 10− 10 M).