Hydrogen peroxide (H₂O₂) is an oxidant widely used in food and pharmaceutical proceedings as a sterilizing agent and released from several industrial processes. In addition, H₂O₂ is the product of the reactions catalysed by many oxidases, which is fundamental in food, pharmaceuticals, and environmental analysis. H₂O₂ can be detected electrochemically, and its concentration can be readily monitored and used to measure the amount of a particular biological species (Ammam and Easton, 2012). To date, a significant number of enzyme-based electrochemical sensors have been developed to detect H₂O₂. However, the enzymatic biosensors are limited by the poor stability, high cost, complicated immobilization procedure, and critical operational conditions inherent to the nature of enzymes. Nevertheless, enzyme-free electrochemical sensors for H₂O₂ have gained special attention and have become a current trend. POM-based electrochemical sensors for H₂O₂ have been reported as appealing non-enzymatic alternatives due to their fast response and high sensitivities, achieved by exploring the synergetic effect of hybrid materials enhancing electrocatalytic activity and stability. Table 1 summarizes the electrochemical sensors for H₂O₂ found in the literature for the last 2 decades, using electrodes modified with POMs. Table 1 provides information about the POM-hybrid material used to modify the electrode (POM-hybrid@electrode), the archetypal of POM anions, the working pH, the limit of detection, the stability of the sensor and if the sensor was applied to real samples (more details can be found in Supplementary Table S1 in Supplementary Material). In Table 1, notice the work of Guo et al. (2015a), who reported a good example of a POM-modified electrode for H₂O₂ consisting of a nanocomposite film containing the Dawson K₆P₂W₁₈O₆₂ (P₂W₁₈), carbon nanotubes (CNTs), and Au nanoparticles (AuNPs), abbreviated as P₂W₁₈/CNTs/AuNPs, applied on an indium tin oxide (ITO) electrode (listed in Table 1 as P₂W₁₈/CNTs/AuNPs@ITO). The sensor showed a good linear range (1–98 mM), an excellent detection limit (52 nM), and a response time to H₂O₂ of less than 1s. Another good example of a non-enzymatic H₂O₂ sensor was reported by Berbéc et al. (Berbeć et al., 2018), and this one was based on a Keggin PMo₁₂/AuNPs/reduced graphene oxide (rGO) over a glassy carbon electrode (GCE) and can be found in Table 1 under the designation of PMo₁₂/AuNPs/rGO@GCE. 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 rGO layer on the AuNPs/GC electrode, which significantly improved the sensitivity of AuNPs to H₂O₂. Nevertheless, both modified electrodes showed that the combination of carbon materials and AuNPs was responsible for significant enhancement of the sensor’s performance. Despite those figures of merit, those electrodes have a limitation related to the usual POMs instability at neutral pH, which constitutes a major drawback as they are restricted to work in acidic media, and therefore cannot be used in physiological systems.

Salimi et al. (2009) reported another type of POM-based hybrid composite that showed not only better performance but extended the pH working range a bit further. The new material was a combination of the Keggin SiMO₁₂, single-walled carbon nanotubes (SWCNTs), and a cationic copper complex resulted in a three-component hybrid composite SWCNTs/SiMo₁₂/[Cu(bpy)₂]²⁺. Due to electrostatic attraction between anions and cations, the stability of adsorbed heteropolyanion and Cu-complex increased at somewhat higher pH values (pH < 7). Even so, authors performed the amperometric detection of H₂O₂ at pH 1, where the system (as represented in Table 1 SWCNTs/SiMo₁₂/[Cu(bpy)₂]²⁺@GCE) was able to detect nanomolar concentrations (1 nM), with values comparable or even better than other electrodes modified with Cu²⁺⁻complexes reported in the literature (Salimi et al., 2009). In addition, this system showed a linear range of 10 nM–18 mM, stability for 30 days, and proved to be successful towards bromate reduction (reported ahead in Table 3). Although less sensible (at μM level), other authors (Ammam and Easton, 2012; Zhang et al., 2015; Li et al., 2016; Zhang et al., 2019a; Liu et al., 2020a; Zhu et al., 2021; Cui et al., 2020; Wang et al., 2018a) achieved to improve electrochemical properties and stability at high pH (physiological level) by pairing POM anions (Keggin, Dawson, crown-shape and Preyssler) with organic or organometallic compounds (Ammam and Easton, 2012; Zhu et al., 2021), with carbon materials decorated with metal nanoparticles (Zhang et al., 2015; Li et al., 2016; Zhang et al., 2019a) or nitrogen (Liu et al., 2020a), and with metal-organic frameworks (MOFs) (Cui et al., 2020; Wang et al., 2018a), that enhanced POM stability at less acidic pH. It is worth mentioning that for the application of POM complexes in an aqueous solution, a thorough insight into the solution chemistry is essential in order to understand the reaction mechanism. The recent work from Gumerova and Rompel (Gumerova and Rompel, 2020) summarizes the species that are present in isopoly- and heteropolyvanadates, -niobates, -molybdates and -tungstates aqueous solutions and covers their stability and transformations, presenting ion distribution diagrams over a wide pH range. These diagrams showed the POM species that are in equilibrium at the given pH value and could help researchers to design the POM according to desired target. Nevertheless, among the sensors for H₂O₂ represented in Table 1, working at physiological levels (between pH 7.0 and 7.4), the one exhibiting the best performance was a modified glassy carbon rotating disk electrode (GCRDE) with a POM combined with a Nitrogen-doped mesoporous carbon (N-MPC) (Liu et al., 2020a). This [Mo-oxo]_n/N-MPC@GCRDE (as listed in Table 1) modified electrode showed a good sensitivity of 2.2 mA mM⁻¹ cm⁻², a detection limit of 0.23 μM, a wide linear range from 50 μM to 5 mM, a response time of 2s, and excellent stability along 2 months. In addition, it was unaffected by many common contaminants. Besides, it is worth noting that among these sensors for H₂O₂ operating at physiological levels, the sensor based on K₅ [Ru (bpy)₃]-PW₁₈@GCE, a combination of a Dawson type POM and an organometallic moiety, allowed the determination of H₂O₂ either by reduction or oxidation, displaying an attractive and rare bifunctional catalytic property.

Nitrite is commonly used as a food preservative and a fertilizing agent. It is widely present in the soil, water, food, and physiological systems and plays a role in the global nitrogen cycle. However, it is highly toxic because it interacts with amines to form carcinogenic nitrosamines in the stomach, prone to cause gastric cancer. Excessive nitrite in food products, including vegetables, drinking water, and beverages, is a severe threat to human health and has become a global issue, with the European Community stipulating guideline limits of 0.1 mg L⁻¹ (∼2.2 mM) for drinking water (The European Parliament and the Council of the European Union, 2020). Analytical techniques used to detect nitrites were often spectrophotometry or spectrofluorimetry, and sometimes a chromatographic or electrophoresis separation is necessary. These techniques require somewhat bulky instrumentation, tedious and time-consuming sample pre-treatment, expensive reagents, or organic and toxic solvents. POM-based electrochemical sensors have been successfully used for nitrite detection due to their low cost, easy operation, and high sensitivity. Table 2 summarizes the POMs modified electrodes for NO₂ ⁻ found in the literature. Table 2 reveals detailed information about the POM-hybrid material used to modify the electrode (POM-hybrid@electrode), the POM archetype, the working pH, the limit of detection, the sensor lifetime, and the matrix where the sensor was tested (more details can be found in Supplementary Table S2 in Supplementary Material). It is noteworthy that the best performance was reported by Zuo et al. (2016), using an electrochemical sensor for nitrite based on a Keggin-type POM, H₆ [PMo₉V₃O₄₀] (PMo₉V₃), a poly (3,4-ethylenedioxythiophene) (PEDOT) and gold nanoparticles (AuNPs) fabricated by a combination of electrodeposition with self-assembly approach. Due to the synergistic contributions of POMs and PEDOT/AuNPs, the composite film electrode exhibited increased electrocatalytic activity towards the oxidation of nitrite and a faster transfer rate than the single-component film. The PEI/PMo₉V₃/PEDOT/AuNPs@GCE sensor (as listed in Table 2) showed a wide linear range (2.5 × 10⁻⁹–1.43 × 10⁻³ M) and a low detection limit of 1 nM, which are much better than most of the reported nitrite sensors (Zuo et al., 2016). Besides, the sensor presented a fast response time of 0.6 s and very good stability for long-term applications. Acceptable recoveries were obtained in a variety of samples (tap water, mineral water, apple juice, milk, and yoghurt) when spiking them with nitrite standards. Although many of the reported sensors were not tested in real samples, most of the nitrite Keggin or Dawson type POM-based sensors in contact with standards presented good stability and showed sensitivity and selectivity adequate to quantify nitrite at the established guideline values for drinking water.

Bromate is generally found in drinking water as a by-product of ozone disinfection, and it is widely used as a food additive for the maturation of flour and the production of fish paste and fermented beverages. Yet, bromate is a carcinogen. Iodine is an essential micronutrient, which is a crucial part of the thyroid hormones that play an essential role in the development of brain function and cell growth. Potassium iodate has been extensively used for the iodination of commercial table salts as a source of iodine. Deficiency or excess of iodine can cause serious health problems. These are examples of the importance of some of the ionic species (non-metallic oxides) listed in Table 3, and for which several electrochemical sensors based on POMs have been designed. Table 3 also includes information about the POM-hybrid materials used to modify the electrodes (POM-hybrid@electrode), the POM archetype and the figures of merit obtained with those electrodes when analysing standard solutions (more details can be found in Supplementary Table S3 in Supplementary Material). A small number of the reported studies include analysis of real samples. Again, the synergetic effect of the different combinations of POMs, carbon materials, organic compounds, ionic liquids, and metals allowed to improve the limits of detection of the CMEs.

The bifunctional sensor previously reported for H₂O₂ by Salimi et al. (2009), that used a combination of a Keggin SiMo₁₂, SWCNTs, and a cationic copper complex (listed as SWCNTs/SiMo₁₂/[Cu(bpy)₂]²⁺@GCE in Table 3) did also present an excellent electrocatalytic activity towards the reduction of BrO₃ ⁻ at lower over-potential, due to the copper-complex that catalyses the reduction of bromate. Linear range, detection limit, and stability were 10–200 nM, 1.1 nM, and 30 days, respectively. Even so, no interference or validation studies with real samples were reported.

Sharifi et al. (2021) reported a modified GCE with a tetra-component nanocomposite consisting of a [1,10-(1,4-butanediyl) dipyridinium] ionic liquid (bdpy), the Keggin-type SiW₁₁O₃₉Ni(H₂O) (abbreviated as SiW₁₁Ni), and Phosphorus-doped electrochemically reduced graphene oxide (P-rGO), by electrodeposition technique for iodate determination. The presence of bdpy provided an additional advantage for increasing the loading of POM, improving the stability of the nanocomposite due to the strong electrostatic attraction between SiW₁₁Ni and positively charged bdpy. The (bdpy)SiW₁₁Ni/P-rGO@GCE sensor, as listed in Table 3, showed very good stability (1 month), good repeatability (200 cycles), and reproducibility. Furthermore, the modified electrode showed improved analytical figures of merit, such as low limit of detection (0.47 nM), high sensitivity (28.1 μA mM⁻¹), good selectivity, and a wide linear range (10–1,600 μM⁻¹), compared to other CMEs. It was validated by measuring IO₃ ⁻ in mineral and tap water and in a commercial edible iodized salt, proving that it could be efficiently applied to quantifying trace-level IO₃ ⁻ in real samples.

It is noteworthy to highlight another POM-based sensor listed in Table 3: a plastic electrode (PE) decorated with a tetrabutylammonium (TBA) derivative salt of octamolybdate, [N(C₄H₉)₄]₄Mo₈O₂₆, reported as MO₈@PE and used for PO₄ ⁻ detection (Figueredo et al., 2021). This POM-based sensor, specially designed with TBA to be only soluble in organic solvents, achieved a remarkably LOD of 6.1 nM, better than the conventional analytical approaches reported by the authors. Plus, the standard spectrophotometric method used for PO₄ ^3- detection takes 1 h for colour to develop. In contrast, this single-use MO₈@PE sensor takes less than 5 min to show a result, and it was successfully applied in saline and seawater samples, being an affordable alternative for phosphate determination.

The excellent biocompatibility of POMs makes them extremely valuable in electrochemical biosensors, and POM-composite materials have been reported as highly successful electrocatalysts for the oxidation of biomolecules and bio-related species. Table 4 shows a list of POM-based electrodes used for a series of biomolecules, including the respective information about the POM-hybrid materials used to modify the electrodes (POM-hybrid@electrode), the POM archetype, and the figures of merit obtained with those electrodes when analysing standard solutions (more details can be found in Supplementary Table S4 in Supplementary Material). Among those biomolecules reported in Table 4, dopamine (DA) is a naturally occurring catecholamine that plays a very important role as a neurotransmitter in the mammalian central nervous system and plays a central role in Parkinson’s disease. For a healthy individual, the DA level lies in the range of 0.01–10 μM (Thakur et al., 2018). The easy electro-oxidation of DA turns electrochemical methods attractive. However, the coexistence of uric acid (UA) and a high concentration of ascorbic acid (AA) in the extracellular fluids of the central nervous system can cause significant interference because of its oxidation potentials which are close to that of DA on bare electrodes, resulting in poor selectivity. Accordingly, surface modification of electrodes with suitable electrocatalysts had been used to improve both the sensitivity and the selectivity of DA detection over AA and UA. Thakur et al. (2018) developed a novel POM-based sensor for highly selective and ultra-sensitive detection of DA, using a sandwich POM Na₁₂ [WCo₃(H₂O)₂(CoW₉O₃₄)₂] (abbreviated as Co₅POM) and poly (vinylimidazolium) cation [PVIM⁺] in combination with nitrogen-doped carbon nanotubes (N-CNTs), listed in Table 4 as PVIM−Co₅POM/N-CNTs@GE. This combination provides the synergy between PVIM−POM catalyst and N-CNTs as conductive support, which enhances the electron transport at the electrode/electrolyte interface. Besides, it eliminates the interference of AA at physiological pH (7.4). The novel PVIM−Co₅POM/N-CNTs composite achieved high selectivity and sensitivity, low detection limit (500 pM), and a wide linear working range of 0.0005–600 μM, even in the presence of a higher concentration of AA (500 μM), being one of the best catalysts reported so far for the selective electrochemical detection of DA. In an attempt to explore this sensor for practical applications, the PVIM−Co5POM/N-CNTs composite was analysed for the detection of DA in real sample using commercially available DA hydrochloride injections (40 mg ml⁻¹) by standard addition method, and the recovery of the sample was in the range of 95–102%, which demonstrated the applicability of the composite for real-time analysis. The stability of the PVIM−Co₅POM/N-CNTs composite was demonstrated during 100 cycles. Additionally, Zhou et al. (2013) reported a Dawson-type POM [P₂W₁₆V₂O₆₂]^8- decorated by Au-Pd alloy NPs, applied onto ITO electrodes, by LbL self-assembly technique, used to simultaneously determine AA and DA at pH 7 (listed in Table 4 as P₂W₁₆V₂/Au-PdNPs@ITO). AA is also an important biomolecule present in the mammalian brain, and it is a vital component in the human diet, used for the prevention and treatment of some diseases (common cold, mental illness, infertility, cancer, and AIDS). The sensing composite film exhibited high electrocatalytic activity towards the oxidation of AA and DA by decreasing the oxidation over-potentials and remarkably increasing the peak currents, attributed to the combining effect of P₂W₁₆V₂ and Au-Pd in the composite film. DA and AA detection limits were 0.83 and 0.43 μM, respectively, and the sensor showed high selectivity and sensitivity. The sensor could be used to determine in real samples. However, the highest sensibility for AA was achieved by Ammam and Easton (Ammam and Easton, 2011b) with a hybrid material based on a 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid [(BMIM)(BF₄)] and the Dawson-type ion [P₂Mo₁₈O₆₂]₆ ⁻, immobilized on glassy carbon electrode (GCE). The resulting AA sensor, [BMIM]₆-P₂Mo₁₈@GCE as listed in Table 4, presented a significant sensitivity of ∼63 nA/μM to AA, a fast response time (<9 s), low detection limit (<0.1 μM), high selectivity towards endogenous interferences such as uric acid, acetaminophen and DA, a linear range from 0.1 μM to at least 22 mM, and was stable for at least 2 weeks. In addition, this AA sensor operated in a pH range from 0 to at least 7, which was attributed first to the presence of the ionic liquid cation in the hybrid material, and second to the porous morphology of the deposited film, which allowed a facile charge equilibrium within the film.

Glucose amperometric biosensors, based on the immobilization of the glucose oxidase enzyme (GOx) on POM-hybrid composites, have been reported in the literature (Turdean et al., 2002; Turdean and Popescu, 2012; Yang et al., 2016a; Boussema et al., 2018; Xu et al., 2017), and are listed in Table 4. However, the best sensor performance for glucose detection was achieved by Ayranci et al. (2019) with a non-enzymatic electrochemical sensor. The Keggin-type K₇ [Co^IIICo^II(H₂O)W₁₁O₃₉].15H₂O, abbreviated as Co₂W₁₁, composed of unique mixed-valence Co(III) and Co(II) structures, was confined in a matrix of multi-walled carbon nanotubes (MWCNTs) on graphite electrodes (GEs) (listed in Table 4 as Co₂W₁₁/MWCNTs@GE). The proposed non-enzymatic sensor showed a wide linear range, from 0.1 to 10.0 mM of glucose. Besides, it exhibited a low detection limit of 1.21 μM, a fast response time of 6 s, high sensitivity (256.4 μA mM⁻¹ cm⁻²), and good stability (5 weeks). These good results were explained by the authors with the improvement of electroactive surface area and the synergistic electrocatalytic activity resulting from the combination of Co-POM and MWCNTs.

Another successful non-enzymatic electrochemical sensor was reported by Thakur et al. (2019) for cholesterol. Cholesterol is an essential lipid of the human body and remains one of the most frequently analysed in clinical practice due to its association with various cardiovascular and brain disorders. The normal level of total cholesterol in blood serum is approx. 200 mg dl⁻¹ and values higher than 240 mg dl⁻¹ are responsible for damage of the arteries and diseases such as arteriosclerosis, heart diseases, hypertension, and cerebral thrombosis. Thakur (Thakur et al., 2019) proposed a non-enzymatic electrochemical sensor for cholesterol based on a sandwich POM [WCo₃(H₂O)₂-(CoW₉O₃₄)₂]^12- (Co₅POM) combined with poly (vinylbutylimidazolium) [PVIM⁺], which acted as a conductive matrix, simultaneously balancing the high negative charge (−12) of the POM. After, the PVIM–Co₅POM conjugate was supported on nitrogen-rich mesoporous carbon (N-MPC) materials to enhance the activity. The modified graphite electrode (GE), listed in Table 4 as PVIM-Co₅POM/N-MPC@GE, demonstrated high selectivity and sensitivity for cholesterol, with a wide detection range from 1 fM to 5 mM and a response time around 5 s. The linear response for cholesterol ranged from 1 fM to 200 nM, and sensitivity was 210 μA mM⁻¹ cm⁻². Moreover, interferent species, such as glucose, UA, and AA, showed no significant effect on cholesterol sensing. The sensor was applied for the quantitative analysis of cholesterol in human blood serum at physiological pH.

All these successful examples of non-enzymatic sensors for important biological species proved that the drawback of enzyme-based sensors, such as restricted immobilization and easy inactivation, can be overcome, and POM-hybrid materials may be precursors for producing non-enzymatic electrode materials in the coming years.

Furthermore, POM-based aptasensors, meaning biosensors that use aptamers as recognition elements, have been developed. Jia et al. (2020) reported a novel nanohybrid of polyoxometalate-derived MoS₂ nanosheets (PMO₁₂-MoS₂ NSs) tightly and vertically grown over ß-FeOOH nanorods (NRs) that were exploited as platforms to immobilize the complementary DNA (cDNA) strands of microRNA-21 (miRNA) for further detection. Compared with other sensing systems referred by Jia et al. (2020), the PMO₁₂-MoS₂/β-FeOOH@Au modified electrode (as listed in Table 4) had superior sensing performance toward miRNA-21 with an incredible detection limit of 0.11 fM, a broad linear range from 1 fM to 5 nM, high selectivity, good stability (15 days), excellent reproducibility, and acceptable feasibility. Bao et al. (2020a) also reported a photoelectrochemical sensor based on the matrix FeOOH/Bi₂S₃/AuNPs and using the hedgehog-shape{Mo₃₆₈} cluster as an electron donor for the ultrasensitive detection of cardiac troponin I (cTnI). Combined {Mo₃₆₈}/FeOOH/Bi₂S₃/AuNPs with the specific recognition of antigen and antibody, a novel sensor based on a modified ITO, and listed in Table 4 as {Mo₃₆₈}/FeOOH/Bi₂S₃/AuNPs@ITO, was constructed, showing a wide detection range of 1.00 pg ml⁻¹–100 ng ml⁻¹ and a low detection limit (0.76 pg ml⁻¹). In general, the content of cTnI in normal human serum is below 0.2 ng ml⁻¹, but direct damage to the myocardium occurs when the concentration of cTnI is higher than 2.0 ng ml⁻¹. Therefore, the new sensor was able to detect cTnI at the early stages of cardiovascular disease. Despite the complex preparation procedures, these new strategies can open new routes for biosensing in clinical diagnosis by detecting other targets for which suitable probes (biomarkers) need to be anchored. Table 4 list POM-based electrochemical sensors for other biomolecules such as creatinine, bilirubin, xanthine, L-tyrosine, and L-tryptophan, among others. These sensors have in common to operate in the nM or pM range, and most of them were evaluated for stability and interferents and applied to real sample analysis, with reliable results, showing to be valuable alternatives to more costly and sophisticated analytical techniques.

Table 5 summarises the POM-based composite electrochemical sensors developed for medicines, pesticides, and toxic contaminants, listing the details about the POM-hybrid material used to modify the electrode (POM-hybrid@electrode), the POM archetype, working pH, the limit of detection, the stability of the sensor and information about tests with real samples (more details can be found in Supplementary Table S5 in Supplementary Material). Highlights go to an ultrasensitive electrochemical sensor for the selective measurement of trace ceftizoxime (CFX), proposed by Rouhani and Soleymanpour (Rouhani and Soleymanpour, 2021), using a thin film of Preyssler nanocapsules (PNCs) on pencil graphite electrode (PGE) surface modified with reduced graphene oxide (rGO). Under the optimized conditions, the PNCs/rGO@PGE sensor, as listed in Table 5, presents a wide linear concentration range, from 1.0 × 10⁻¹¹ to 3.0 × 10⁻⁸ M, and an excellent detection limit of 1.8 pM. The outstanding electrochemical performance of the PNCs/rGO@PGE sensor was related to the synergistic influence of PNCs/r-GO/PGE thin film, which enhanced efficiency in drug encapsulation, stability, and effective surface area of the electrode for the CFX oxidation. The novel sensor showed better sensitivity than the earlier reported techniques for the CFX measurement, and it was successfully used to determine the trace amounts of CFX in pharmaceutical formulations and blood serum with suitable recoveries.

POM-based electrochemical sensors have been reported using molecularly imprinted polymers (MIPs) to attain the selectivity required to determine hazardous compounds. Besides the MIP, they have been combined with carbon materials and metal nanoparticles to overcome the restrictions in the conductivity of POMs. A MIP is a polymer that has been synthesized using a molecular imprinting technique with a mould molecule, leaving cavities in the polymer matrix with an affinity for that chosen mould molecule. POM hybrid materials combined with MIPs, mostly Keggin-type, have been reported for sildenafil (Rouhani and Soleymanpour, 2020), simazine (Ertan et al., 2016), ochratoxin A (Yola et al., 2016), and γ-lindane (Pelin Böke et al., 2020), and are listed in Table 5, all presenting excellent detection limits (10⁻¹¹ M) and wide linear ranges. The sensors have been tested for common interferents and validated by testing their application in real sample analysis. Emphasis goes to POM/MIP-based electrochemical sensors for diazinon and citrinin, which proved to be able to determine these targets in food samples. Diazinon (DIA) is an organophosphorus pesticide and is considered very risky and harmful because of its noxious nature. Their recognition at ultra-trace levels in environmental samples and foodstuff is a serious analytical challenge. Medetalibeyoğlu et al. (Medetalibeyoğlu et al., 2020) reported the use of gold nanoparticles (AuNPs) incorporated Keggin-type POM/two-dimensional hexagonal boron nitride (2D-hBN) nanosheets and molecularly imprinted polymer (MIP) for the electrochemical detection of DIA molecule in fruit juice samples. The modified GCE electrode, listed as MIP/AuNPs/PW₁₂/2D-hBN@GCE in Table 5, showed high selectivity and stability, and reliability for DIA detection. The linearity range and detection limit were 1.00 × 10⁻¹¹ -1.00 × 10⁻⁸ M and 3.0 × 10⁻¹² M, respectively, and sensitivity was better than other reported sensors (Medetalibeyoğlu et al., 2020). Another ultra-sensitive POM/MIP-based electrochemical sensor was reported for Citrinin (CIT). CIT is a toxic mutagenic and carcinogenic secondary metabolite of fungi, resistant to decomposition, and it is found in diverse food samples such as cheese, barley, red yeast rice, and apples. Atar et al. (2016) reported a molecular imprinted voltammetric sensor for CIT based on GCE modified with platinum nanoparticles (PtNPs), involving the Keggin polyoxometalate H₃PW₁₂O₄₀ functionalized with reduced graphene oxide (rGO). The developed system, listed in Table 5 as MIP/PtNPs/PW₁₂/rGO@GCE, showed a performance comparable to other CIT-imprinted sensors and conventional analytical methods. It is ultra-sensitive, with a detection limit of 2.0 × 10⁻¹³ M, rapid, easy, shows very good stability (45 days), and it might be preferred to other published methods.

Heavy metals have been early identified as primary environmental contaminants due to their non-biodegradability, bioaccumulation, and toxicity. Because of their harmful effects above the permissible limits, it is necessary to measure the concentration of these heavy metals to preserve the environment and health of individuals. Traditional methods for metal analysis include UV-Vis absorption spectrometry, surface-enhanced Raman spectrometry (SERS), atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), ion chromatography (IC), inductively coupled plasma mass spectrometry (ICP-MS), and inductively coupled plasma optical emission spectrometry (ICP-OES). However, these techniques require expensive instruments, costly operations, and well-skilled operators to perform the multi-step sample preparation and complex analytical procedures, which are unsuitable for on-site and timely measurements necessary to monitor transient phenomena.

Hexavalent chromium (VI) is one of the most toxic heavy metal ions with high solubility in water. It has gained wide attention due to its high poisonousness and mutagenic-carcinogenic effects on human health. Therefore, the Cr(VI)-containing compounds were listed as human carcinogens by International Agency for Research on Cancer (IARC), and the World Health Organization (WHO) has established a maximum permissible concentration of total Cr(VI) in drinking water and industrial water of 0.05 and 0.5 ppm, respectively. POM-based electrochemical sensors were reported as alternatives for metal analysis, especially for chromium (VI), and are summarized in Table 6. Table 6 includes the respective information about the POM-hybrid materials used to modify the electrodes (POM-hybrid@electrode), the POM archetype, and the figures of merit obtained with those electrodes when analysing standard solutions (more details can be found in Supplementary Table S6 in Supplementary Material). Recently, Niu et al. (2021) reported the synthesis of two hourglass-type phosphomolybdate hybrids with different heterometallic centres, achieving an efficient electrochemical detection of ultra-trace Cr(VI) in wide pH ranges of 0–5. The reported sensors, {P₄Mo₆}/Cu/Mn/BBTZ@GCE (1) and {P₄Mo₆}/Na/Mn/BBTZ@GCE (2), at pH 0 displayed prominent sensitivities of 111.08 μA μM^{− 1} and 119.87 μA M⁻¹, along with ultra-low detection limits towards Cr(VI) of 1.59 nM (0.17 ppb) and 2.91 nM (0.30 ppb), respectively, which fully satisfy the WHO standards for drinking water. The activity origin of both hybrids for impressive electrochemical behaviours was originated from the synergistic effect between reduced {P₄Mo₆} cluster and heterometallic centres at the molecular level. In the pH range of 1–5, good sensitivities and low detection limits (<25 nM) were also achieved by both sensors. Moreover, both were insensitive to common interferences and very stabile.

Cadmium (Cd) and lead (Pb) are used extensively in industry and are carcinogenic agents. Their accumulation in the human body can cause serious harm to internal organs such as lungs, kidneys, liver, bones, and central nervous system. Dianat et al. (Dianat et al., 2019) reported a novel sensitive L-cysteine Keggin tungstophosphate-modified polycrystalline gold electrode, listed in Table 6 as PW₁₂/Cys@Au, developed for the electrochemical detection of Cd²⁺ and Pb²⁺ in trace amounts. The (Cys)PW₁₂ hybrid compound was selected to fabricate inorganic self-assembled monolayers (SAMs) on the Au surface, which benefited from high stability due to the strong Au–S interaction. The modified electrode showed a wide linear range (0.01–0.2 μM) for both analytes, excellent reproducibility, high sensitivity, and reasonable detection limit (9.0 and 4.0 nM for Cd²⁺ and Pb²⁺, respectively), comparable to the other electrochemical techniques or modified gold electrodes (Dianat et al., 2019). The PW₁₂/Cys@Au sensor was employed for Cd²⁺ or Pb²⁺ determination in industrial wastewater samples.

Table 8 summarizes the Lanthanide (Ln)-based materials (Ln-POMs-based fluorescence sensors) reported so far in the literature. Table 8 includes the POM archetype, the detection limit, the operation mode of the sensors and the matrix where sensors were tested (more details can be found in Supplementary Table S8 in Supplementary Material). Among Ln-POMs reported probes in Table 8, the Lindqvist europium decatungstate Na₉ [EuW₁₀O₃₆].32H₂O (EuW₁₀) was the most prevalent due to Eu fascinating property of changing colour and fluorescence dependence on its valency and coordination environment. However, as pure EuW₁₀ exhibited only weak photoluminescence in water, due to luminescence quenching of water molecules, quenching in solution must be prevented by self-assembly of Eu-POM with organic materials, such as polymers, metal-organic frameworks (MOFs) and surfactants.

Self-assembly of Eu-POM with polymers was reported by Wang et al. (2010). A highly transparent flexible self-supporting decatungsteuropate thin film, listed in Table 8 as EuW₁₀/agarose, was fabricated by a facile hydrogel casting technique. The strong interactions between agarose and EuW₁₀ by hydrogen bonds at the hydroxyl sites and the densely packed 3D network structure of agarose in the gel contributed to the homogenous distribution of EuW₁₀ and to the good mechanical properties of the nanocomposite films. When excited with UV-light, the thin-films of EuW₁₀/agarose displayed a strong red emission of Eu³⁺ that can be reversibly modulated, quenched by HCl gas, and recovered by NH₃ gas, behaving as a luminescent switch. Also a TbW₁₀/agarose composite thin film reported more recently by Bin et al. (Wang et al., 2019a), showed the same behaviour in the presence of HCl and NH₃ gases, with the green luminescent thin-film sensor presenting a detection limit of 0.2731 mmol L⁻¹ for HCl, showing that other Ln-POM (Dutta and Sarkar, 2016), besides Eu-POMs, could be used for sensing.

The self-assembly of Eu-POMs with 3D coordination networks (MOFs) also appears as a promising approach due to their crystalline nature, permanent porosity, chemical tunability, and robustness, offering an advantageous unique platform for the development of solid-state luminescent materials. Thus, POMs incorporated in the cavities of a metal-organic framework (POM/MOFs) have been used to prevent luminescence quenching. Salomon et al. (2018) reported the introduction of the luminescent EuW₁₀ into the cavities of highly porous zirconium luminescent MOF UiO-67, combining dual-luminescent properties of EuW₁₀/UiO-67, as listed in Table 8. The hybrid material proved to be a solid-state luminescent sensor for amino acids. Enhancement of the EuW₁₀/UiO-67 luminescence is observed in the presence of amino acids globally following the increase of the amino-acid pKa. Due to the strong quenching effect of Fe³⁺ (K_SV 2667 M⁻¹), the EuW₁₀/UiO-67 proved to be also a reusable sensor for Fe³⁺ in an aqueous solution, with an estimated LOD of 37 µM. Recently, the same group reported the encapsulation of the EuW₁₀ into a mesoporous MOF, a Tb-TATB, built of terbium tetranuclear units connected by TATB ligands (H₃TATB = triazine-1,3,5-tribenzoic acid). The dual-luminescent EuW₁₀/Tb-TATB composite (as listed in Table 8) behaved like a highly sensitive luminescent thermometer in the physiological domain and gave rise to a new family of hybrid dual-emitting LnPOM/LnMOF materials (Viravaux et al., 2021).

An example of surfactant-encapsulated polyoxometalates was reported by Hui et al. (Zhang et al., 2006), where the luminescent polyoxometaloeuropate EuW₁₀ was connected through electrostatic interaction with the multi-functional surfactant, trans-10-[4-(4′-pyridylvinylene)-phenyl] oxydecyldodecyldimethyl-ammonium bromide (PyC₁₀C₁₂N). The combined composite, listed as EuW₁₀/PyC₁₀C₁₂N in Table 8, worked as a luminescent logic gate with dual output, operated by light and zinc ion as inputs. Another example was reported by Panpan et al. (Sun et al., 2019), where the polyoxometaloeuropate was used to develop a sensitive, selective off-luminescence chemical sensor, listed as EuW₁₀/[C₁₄-2-C₁₄im] Br₂ in Table 8, for the label-free detection of Cr³⁺ and MnO₄ ⁻ in aqueous solution, with low detection limits of 0.926 and 1.70 μM, respectively, and a wide pH application range. The introduction of [C₁₄-2-C₁₄im]Br₂ did increase the luminescence effect, and the strongest luminescence was observed for EuW₁₀/[C₁₄-2-C₁₄im]Br₂, which was 32 times that of pure EuW10.

Although the luminescent EuW₁₀ dominate in the Ln-POM based fluorescence optical sensors group, other luminescent Eu-POMs have been reported (Table 8). Two systems, a penta-Eu^III sandwiched Dawson-type selenotungstate (Zhang et al., 2020a) [H₂N(CH₃)2]₁₀H₃{SeO₄Eu₅(H₂O)₈ [Se₂W₁₄O₅₂]₂}·40H₂O, represented in Table 8 as EuSe₃W₁₄, and a polyhydroxycarboxylic acid ligand bridged multi-Eu^III-incorporating tellurotungstate (Zhang et al., 2020b) K₁₄H₁₀ [Eu₄(H₂O)₄W₆(H₂glu)₄O₁₂(B-α-TeW₉O₃₃)₄]·60H₂O (H₆glu = D-gluconic acid), represented in Table 8 as EuTeW₉, were reported as sensors to detect Cu²⁺ ions in aqueous solution. Both systems exhibited high fluorescence signals and good selectivity for detecting Cu²⁺ ions in an aqueous solution (Zhang et al., 2020a; Zhang et al., 2020b). The Eu-tellurotungstate showed the best performance due to the hexagonal packing of the tetrameric polyoxoanions, providing excellent porous channels, which greatly increased the specific surface area of the whole framework and fluorescence sensing. This is the most sensitive POM-based fluorescence sensor for detecting Cu²⁺ ions in an aqueous solution reported so far (LOD 8.82 × 10⁻⁶ mM). Furthermore, the same sensors could be used in Cu²⁺⁻quenching systems. These “off-on” fluorescence sensors were used to detect Cysteine (Cys) in an aqueous solution with similar LODs (2.17 × 10⁻⁴ and 1.75 × 10⁻⁴ mM, with EuSe₃W₁₄ and EuTeW₉, respectively).

Besides Ln-substituted POMs, other luminescent hybrid POMs have been reported by binding chromophore species to POMs, which are summarized in Table 9, along with the POM archetype, their target analyte and the matrix where they were tested, the operation mode of the sensor and the detection limit (LOD) (more details can be found in Supplementary Table S9 in Supplementary Material).

Carraro et al. (2012) reported a bis-lacunary Keggin polyoxotungstate [γ-SiW₁₀O₃₆]^8– as a molecular nanosurface where the dansyl chromophore was anchored with a tweezer-type arrangement, which acted as a selective fluorescence sensor for Cu²⁺ and Pb²⁺ ions, in quenching and enhancing mode, respectively. Hong et al. (Liu et al., 2015) reported the use of Norfloxacin, a known fluorescence medicine, to produce a Norfloxacin-derivative functionalized octamolybdate, (dNF)₂ [γ-Mo₈O₂₆(dNF)₂].10H₂O, where dNF stands for decarboxylated norfloxacin. The combination, listed as SiW₁₀/dansyl in Table 9, showed to be an acid-base switch system, as both the addition of acid or base modulated its fluorescence. Another simple ionic association of a photoluminescent compound, the [Ir^III(ppy)₂ (bpy)]⁺ complex incorporating 2-phenyl- pyridine (ppy) and 2,20-bipyridine diimine (bpy), with an octamolybdate result in a strong modulation of its emission wavelength in the solid-state, varying from green to yellow, orange, orange-red, and red, by changing the nature of the POM and the design of the frameworks. The resulting hybrid materials, listed as Mo₈/[Ir^III(ppy)₂ (bpy)]⁺ in Table 9, turned to be an efficient selective chemosensor for VOC detection (Bolle et al., 2016).

Furthermore, Tian et al. (Tian et al., 2019) reported the synergetic combination of organic species such as benzoic acid (BA), thiamine (TH), and 3-(4-hydroxyphenyl)propionic acid (HPPA), with the Keggin-type polyoxotungstate intrinsic peroxidase Na₁₀ [α-SiW₉O₃₄] decomposing H₂O₂ into ▪OH radicals, which converted weakly fluorescent substrates to strongly fluorescent substrates, under basic pH conditions. Recently, S-/N-containing ligands, such as 2,2′-dimethyl-4,4′-bithiazole (Ying et al., 2019; Wang et al., 2020b) and 2,5-bis(4-pyridyl)thiazolo [5,4-d] thiazole (Mou et al., 2020), and N-containing ligands, such as 2,2′-bipyridyl (bpy) and 4-(3-imidazol-1-yl-ethyl)-4H-[1,3,4]triazole (MET) (Zhang et al., 2021a) were used to modify a series of different archetype POM compounds, in order to build fluorescence sensors for Hg²⁺. All compounds showed selective response to Hg²⁺, explained by the preferred interaction between the soft acidic Hg²⁺ ions and sulphur (soft base). In addition, all hybrid POMs proved to be multi-functional materials, showing not only photocatalytic activity for degradation of dyes (Ying et al., 2019; Wang et al., 2020b; Mou et al., 2020; Zhang et al., 2021a) but also redox properties, which makes them probe to act as electrochemical sensors for NO₂ ⁻ (Ying et al., 2019; Mou et al., 2020; Wang et al., 2020b; Zhang et al., 2021a), for H₂O₂ (Zhang et al., 2021), and Cr(VI) (Wang et al., 2020b).

Hybrid metal-POMs showing fluorescence properties and applied to sensing have also been reported. The terminal and bridging oxygen atoms on the surface of POMs not only can act as versatile proton acceptors and donors but can also coordinate with other metal ions. Mukul et al. (Raizada et al., 2017) synthesized a decavanadate hybrid material with 4-picoline N-oxide (Pyno) and triethylamine (NEt₃), the {Cu(Pyno)₄}{NEt₃H}₂ [H₂V₁₀O₂₈] cluster, denoted in Table 9 as V₁₀O₂₈-Cu-pyno-NEt, in which the metal ion linkers belong to distinct coordination complexes with peripheral organic ligands. This water-soluble inorganic-hybrid compound was investigated as the first aqueous-phase sensor for picric acid and Pd²⁺, with a low detection limit of 0.18 and 0.80 ppb, respectively, within WHO/US EPA prescribed limit for palladium. Another example was recently reported by Qian et al. (Li et al., 2021), where the Anderson type (NH₄)₃ [H₆Fe(III)Mo₆O₂₄] (FeMo₆), working as an oxidase-mimic nanoenzyme, exhibited the ability to catalytic oxidase of o-phenylenediamine (OPD), 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTs), and 3,3′,5,5′-tetramethylbenzidine (TMB). The proposed sensor based on two consecutive “turn on” fluorescence was developed for DA by employing the FeMo₆-OPD system, and the linear range was from 1 to 100 μM with the detection limit 0.0227 μM. In addition, by loading the FeMo₆-OPD system with 10% of reduced graphene oxide (rGO), listed in Table 9 as FeMo₆/rGO, the authors increased the oxidase-mimic activity of FeMo₆, with an enhancement of the detection limit to 0.012 μM.