Luminescent Zr(iv) metal–organic frameworks as sensors of waterborne persistent organic pollutants: reproducibility and transferability

Abstract

Metal–organic frameworks (MOFs) have shown great potentiality as chemical sensors of waterborne persistent organic pollutants (POPs). To emphasise the relevance, in this applicative context, of statistically grounded protocols ensuring results reproducibility, reliability and comparability, we investigated the two polymorphic MOFs NU-1000 and NU-901 as luminescent sensors of four exemplary waterborne pharmaceuticals: diclofenac sodium (DCF), fluoxetine hydrochloride, paracetamol (PAR) and acetylsalicylic acid. In POP distilled water solutions 0.3–9.5 µM, NU-1000 exhibited a reproducible behaviour irrespective of the pollutant. Nonetheless, its response was distinguishable from that due to its simple dilution, i.e. it was genuinely due to the POP, only with DCF. Toward the latter, NU-1000 showed a reproducible performance even across aqueous matrices of higher complexity – bottled and tap water – and under two different experimental protocols, with binding constants and limits of detection, respectively, on the order of 10⁵ M⁻¹ (namely: 2.4(1) × 10⁵ M⁻¹ in distilled water, and 2.1(7) × 10⁵ M⁻¹ averaged over the three matrices) and 10⁻¹ µM (0.7(2) µM averaged over the three matrices). In similar experimental conditions, NU-901 showed reproducibility only with DCF and PAR, with a non-monotonic trend due to the alternative prevalence, on the sensor signal, of MOF/POP interactions or MOF dilution. Overall, this case study successfully emphasises the importance of crucial features – statistically validated reproducibility of sensor performance and operative conditions, such as sensor dilution, complexity of the working matrix – thus concurring to advance best practices in the field and providing a basis upon which improved porous material-based sensing systems can be engineered and tested.