Pushing complexometric titrations in the nanomolar range through SERS monitoring

Complexometric titrations have been developed in the 40's and 50's for virtually every metal cation. However, the colorimetric of spectrophotometric monitoring of their endpoints limit their sensitivity range to the µM to mM range. Hence, these methods were quickly replaced by atomic instrumental methods, which became the new benchmark, when the corresponding instruments became widely accessible. However, in the context of a global lack of water quality data, especially in low income or geographically isolated areas, these instrumental golden standards are out of reach. In this work, we revisit a popular complexometric titration of the heavy metal cation Cu 2+ through monitoring the endpoint in Surface Enhanced Raman Spectroscopy (SERS). We demonstrate a sensitivity at environmentally relevant concentrations, from 1 µM and down to 250 nM, thanks to SERS titration curves that mirror those predicted by thermodynamics. Remarkably, the method is cheap and frugal: it can be run with moderate chemical training, on a simple benchtop, using an instrument worth 10 keuros that can be battery-operated on field. Trueness of at least 75 ± 1% for coarse titrations and 96 ± 3% for a finer-resolution were obtained, in 2 hrs for 6 to 9 readouts (including duplicates and blanks), and for a consolidated cost of 28 euros, demonstrating the frugality of the overall method.