Abstract
Polyoxometalates (POMs) are a class of metal oxide complexes with a large structural diversity. Effective control of the final chemical and physical properties of POMs could be provided by fine-tuning chemical modifications, such as the inclusion of other metals or non-metal ions. In addition, the nature and type of the counterion can also impact POM properties, like solubility. Besides, POMs may combine with carbon materials as graphene oxide, reduced graphene oxide or carbon nanotubes to enhance electronic conductivity, with noble metal nanoparticles to increase catalytic and functional sites, be introduced into metal-organic frameworks to increase surface area and expose more active sites, and embedded into conducting polymers. The possibility to design POMs to match properties adequate for specific sensing applications turns them into highly desirable chemicals for sensor sensitive layers. This review intends to provide an overview of POM structures used in sensors (electrochemical, optical, and piezoelectric), highlighting their main functional features. Furthermore, this review aims to summarize the reported applications of POMs in sensors for detecting and determining analytes in different matrices, many of them with biochemical and clinical relevance, along with analytical figures of merit and main virtues and problems of such devices. Special emphasis is given to the stability of POMs sensitive layers, detection limits, selectivity, the pH working range and throughput.
Highlights
Polyoxometalates (POMs) are negatively charged polyoxoanions of general formula [MmOy] n, where M represents the metal centre surrounded by oxygen atoms (O)
Comparing the figures of merit of H2O2 analytical determination in Supplementary Table S1 with the two electrodes, it could be concluded that linear range and detection limits were similar, but in what concerns to sensitivity (listed in Supplementary Table S1, the Berbéc electrode was superior (sensitivity of 596.1 μA mM−1 cm−2 and 740.8 μA mM−1 cm−2, for Guo (Guo et al, 2015a) and Berbéc (Berbeć et al, 2018) electrodes, respectively) due to the reduced graphene oxide (rGO) layer on the gold nanoparticles (AuNPs)/GC electrode, which significantly improved the sensitivity of AuNPs to H2O2
It is noteworthy that the best performance was reported by Zuo et al (2016), using an electrochemical sensor for nitrite based on a b) c) Anderson (Dawson) (Keggin)-type POM, H6 [PMo9V3O40] (PMo9V3), a poly (3,4ethylenedioxythiophene) (PEDOT) and gold nanoparticles (AuNPs) fabricated by a combination of electrodeposition with self-assembly approach
Summary
Polyoxometalates (POMs) are negatively charged polyoxoanions of general formula [MmOy] n–, where M represents the metal centre surrounded by oxygen atoms (O). They are typically composed of transition metal ions in their highest oxidation state (e.g. M = VV, MoVI, WVI, TaV, NbV), bridged by oxo ligands (O2–), to form closed 3-dimensional frameworks (Pope and Müller, 1991). Mainly heteroatoms, acting as coordination centres, usually labelled as X, can be part of the POM framework [XxMmOy]n− (Pope and Müller, 1991; Hutin et al, 2013). The growing interest in POMs is focused on two main features: 1) the structural diversity due to the coordination flexibility in their metal-oxo structures, and 2) the vast number of elements of the periodic table that can be incorporated inside POM clusters, leading to an overwhelming diversity of molecular structures, of various shapes and sizes, with a diverse range of physical and chemical properties
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