English

• 1. Introduction

  Owing to natural aspects and the increasing human activities, a large amount of nutrients, including nitrogenous compounds and phosphate, have been released into natural water [1,2]. Particularly in lakes with low fluidity, the input rate outperforms the consumption rate of nutrients. The resultant accumulation of N- and P-containing components, integrated with the additional proper temperature and light irradiation consequently cause the regional harmful algae bloom [3,4]. In China, Taihu Lake located in Jiangsu and Dianchi Lake in Yunnan are typical locations suffering from long-term algae bloom [5,6]. Besides, Lake Erie in America and Lough Neagh in Britain have also occurred serious cyanobacteria bloom in recent years [7,8]. Other regional sites in France, Japan, Australia, Brazil witnessing the issue [9-12]. Drew from the above cases, it is confirmed that harmful algae bloom has become one of the most serious biological pollutions worldwide in 21^st century.

  In principle, cyanobacteria are dominantly responsible for algae bloom in freshwater. Cyanobacteria, classified into Chroococcales, Oscillatoriales, Nostocales and Stigonematales, are procaryotic organisms that are capable of photosynthesis [13]. Therein, the most serious algae bloom is primarily governed by Microcystis and Anabaena affiliated to Chroococcales and Oscillatoriales, respectively. Microcystis aeruginosa (M. aeruginosa) is the representative species in Microcystis and often becomes the dominant culprit in actual freshwater, which is mainly attributed to the followed-up aspects: (ⅰ) Compared to other algae cells, M. aeruginosa has higher utilization of light with low intensity [14]. As a result, the growth rate of M. aeruginosa is likely to maintained in cloudy weather. (ⅱ) M. aeruginosa adapts to wider pH ranges, especially under alkaline conditions [15,16]. Coincidently, high basicity of freshwater in extensive region of China is beneficial for rapid growth of M. aeruginosa. (ⅲ) M. aeruginosa possess better tolerance to high temperature [17]. (ⅳ) Thanks to the accumulation of carotenoid, M. aeruginosa exhibits strong resistance to ultraviolet (UV) from intense sunlight and can utilize its partial photon energy [18-20]. (ⅴ) To our best knowledge, the intensity of incident light decays with water depth [21]. The airbag-like structure (gas vesicle) in cells of M. aeruginosa is conductive to enable M. aeruginosa floating onto water surface, which helps M. aeruginosa compete more incident sunlight against other water-born organisms [22]. Integrated with the aforementioned reasons, M. aeruginosa generally becomes dominant species in freshwater in summer, which contributes to impose hazards to human being and ecology (Fig. 1): (ⅰ) The disgust in terms of sense. M. aeruginosa may cause decrease of water transparency, forming oil-like green layer and generating pungent smelling [23]. (ⅱ) The imbalance of ecosystem. As mentioned above, M. aeruginosa competes sunlight with other aquatic plants (especially submerged plants) and depletes dissolved oxygen (DO) in water, significantly inhibiting the growth of other water-born organisms [24]. On the other hand, M. aeruginosa can release toxic microcystins (MCs, including three isomers named MC-LR, MC-RR and MC-YR). For instance, MC-LR has been reported to notably affect the biodiversity, destroying the nervous system of zebrafish [25-27]. (ⅲ) The threat to human health. MCs are chemically stable and difficult to be degraded in actual water. Therefore, MCs are likely to be enriched via food chains or directly ingested by people via drinking water, which might exert potential effects to human health [28]. Especially, MC-LR demonstrates the highest acute toxicity to organisms among three MCs. It has been reported that MC-LR can result in intensive diseases like liver cancer by inhibiting the activity of protein phosphatases in vivo, even contributing to death [29,30]. As an example, up to 52 dialysis patients in Caruaru, Brazil died out owing to the MC-LR-polluted hemodialysis solution [31]. (ⅳ) The damage to artificial infrastructure. The accumulation of M. aeruginosa is prone to block the water conservancy engineering and affect propulsion system of vessels, causing huge economy loss [32,33]. Accordingly, it is imperative to mitigate the harmful algae bloom governed by M. aeruginosa.

  Figure 1

  

  Figure 1.  Possible adverse effects of M. aeruginosa to human and ecology.

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  Up to now, the strategies for M. aeruginosa removal can be classified into physical, chemical, and biological treatment processes [34]. Physical salvage is one of the most commonly used method. Specifically, the superficial algae and substrate can be removed by artificial equipment, which is conductive to reduce a large amount of hypopus (akinetes) of M. aeruginosa [35]. However, this method is regarded as an emergency handling, which is energy-costing and constrained on removing apparent algae bloom. Ultimately, the recurrence of M. aeruginosa and the leaching of MCs can not be fundamentally solved. Adding chemical flocculants into water is capable of forming algae flocculation rapidly. Nevertheless, the secondary pollutants might be introduced [36]. As for biological treatment, despite cultivating submerged plants is eco-friendly, the cultivation cost remains high, and the governance cycle is also prolonged [37]. Thus, the desirable auxiliary methods and materials should be exploited to realize long-term and thorough control toward M. aeruginosa bloom.

  Metal-organic frameworks (MOFs) have been hot-spot functional materials nowadays [38]. MOFs are the periodic frameworks built up from metal ions and organic linkers [39]. Over the past decades, MOFs have demonstrated significant performances in application to adsorption [40-42], catalysis [43-46], drug delivery [47,48], sensing [49,50], energy [51,52], antimicrobial [53,54], and supercapacitor [55]. Encouragingly, for M. aeruginosa inhibition, MOFs possess superiority that lies in (Table 1): (ⅰ) The structure of MOFs can be characterized and determined facilely, which is accessible to accomplish bottom-up functionalization and modification, designing specific functional materials with controllable toxicity and stability [56,57]. (ⅱ) MOFs can be activated to simultaneously generate holes (h⁺) and other reactive oxidizing species (ROSs) while M. aeruginosa grows rapidly under the light irradiation. (ⅲ) The optimal growth temperature of M. aeruginosa can increase the catalytic activity of MOFs. (ⅳ) Compared to physical treatment, MOFs can synchronously degrade MCs released from M. aeruginosa [58]. (ⅴ) MOFs can exhibit multiple inhibition mechanisms toward M. aeruginosa, including but not limited to cell rapture (via slow-released metal ions and ROSs), physical puncture, co-flocculation/aggregation, and shading effects by MOFs nanoparticles [59]. The aforementioned effects often occur simultaneously. Therefore, MOFs tend to exert thorough M. aeruginosa inhibition theoretically. (ⅵ) Compared to other homogeneous chemicals like flocculants (e.g., alums and polyaluminium chloride) and quaternary ammonium salts, MOFs can be easy to immobilize onto macroscopic substrates by various interactions for repeated use in water [60].

  Table 1

  

  Table 1.  The advantages of MOFs for M. aeruginosa control compared with other homogeneous agents.

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  Materials	MOFs	Other homogeneous agents
  Structure	Easy-tailored and explicit	Unknown
  Inhibition mechanism	Multiple	Generally single
  Stability	Controllable	Disposable
  Toxicity	Controllable	Unknown
  Function	Multiple (e.g., adsorption, catalysis, flocculation)	Generally single
  Separation accessibility from water	Easy to be recovered after immobilization	Difficult to be reused

  Since Gu et al. [61] used Zn-Fe-LDH for M. aeruginosa inhibition by photocatalysis in 2016, MOFs started to attract increasing attention as anti-algae candidates (Fig. 2). Until the end of 2023, there have been 21 relevant papers regarding algae removal by MOFs-based materials (excluding toxic assessment of MOFs toward algae). Therein, more than 40% papers selected M. aeruginosa as representative species, indicating the importance of M. aeruginosa control (Fig. 3a). In 2023, the published papers concerning M. aeruginosa removal by MOFs-based materials increased notably, implying that MOFs for M. aeruginosa removal has great potential to be hotspot topic (Fig. 3b). Although the existing reviews have introduced recent progresses of heterogeneous materials (including some MOFs-based materials) for algae inhibition [62,63], no publications specifically discussed the interaction between MOFs and M. aeruginosa. To this end, it is necessary to systematically discuss the related studies to provide insight into the relationship between MOFs-based materials and M. aeruginosa, and pave the way for future direction.

  Figure 2

  

  Figure 2.  Recent progresses of M. aeruginosa removal by MOFs-based materials.

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  Figure 3

  

  Figure 3.  (a) Frequency percentage of different algae species involved in relevant studies. (b) Number of published research papers concerning M. aeruginosa removal by MOFs-based materials with years (data was collected from Web of Science between January 1, 2018 to December 31, 2023).

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  In this critical review, we narrowed the discussion scope, summarizing the state-of-the-art progresses of M. aeruginosa control by MOFs-based materials, introducing and comparing the different M. aeruginosa inhibition mechanisms by diverse MOFs-based materials. This is the first attempt using statistics to analyze the research trend of MOFs-based materials for M. aeruginosa control and the environmental-related factors that are easily neglected but critical. Taking potential water environmental implications into account, we discussed the existing controversial aspects including insufficient or improper characterization methods for estimating anti-algae performances by statistic, providing corresponding guidance in terms of evaluating M. aeruginosa inhibition performances of MOFs-based materials. Finally, we proposed the future perspectives concerning rational design of MOFs-based materials and application scenarios based on the research margins and limitations discussed above.

  2. Overview

  M. aeruginosa is prokaryotic organism. Similar to other bacteria, the normal M. aeruginosa cell is negatively charged [64]. The cell structure of M. aeruginosa is illustrated in Fig. 4a. Apart from ribosome, M. aeruginosa bears no other organelles. Besides, M. Aeruginosa has cell wall, phycobiliprotein (PB), thylakoid, chlorophyll, nucleoid, and gas vesicle. In detail, PB, thylakoid, and chlorophyll are critical components being responsible for photosynthesis. Gas vesicle is a kind of gas bag encapsulated by a protein layer, which is conductive to help M. aeruginosa float onto the upper level of water [65].

  Figure 4

  

  Figure 4.  Schematic illustrations representing (a) cell structure of M. Aeruginosa, and (b) inhibition mechanism of MOFs toward M. aeruginosa.

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  MOFs are organic-inorganic hybrid materials with periodic framework structure consisting of organic linkers and metal ions. On the one hand, some MOFs can be served as the potential reservoir of metal ions because of their moderate water stability [66]. On the other hand, a large number of MOFs can act as semiconductor-like materials, especially under light irradiation. When proton energy in light radiation excess or being identical to band gap of MOFs, MOFs can be excited, generating light-induced electrons (e⁻) and holes. Specifically, the light-induced electrons would undergo metal-to-ligand or ligand-to-metal charge transfer processes to lowest unoccupied molecular orbital (LUMO), leaving h⁺ into highest occupied molecular orbital (HOMO) [67]. Then, h⁺ and e⁻ would further transfer onto the surface of MOFs to participate oxidative or reductive reactions [68]. Meanwhile, some molecules in water (like DO) can be transformed into other oxidative ROSs (HO^•, O₂^•−, and ¹O₂) with the aid of h⁺ and e⁻ [69-72]. Taking the above two aspects into consideration, the M. Aeruginosa inhibition mechanisms by MOFs can be primarily divided into: (ⅰ) The toxicity caused by the released metal ions and the organic ligands from MOFs [73], (ⅱ) the oxidative stress and damage governed by ROSs derived from photocatalysis when MOF is used as a catalyst (Fig. 4b) [59,74]. Moreover, the sweep flocculation or charge neutralization effects, and the physical puncture by nanoscale MOFs particles also play the important role in M. aeruginosa inhibition in some cases [75].

  For the first inhibition mechanism mentioned above, the intrinsic toxicity of MOFs to M. aeruginosa is dominantly ascribed to the metal ions released by MOFs, such as Cu²⁺, Zn²⁺, and Ag⁺ because MOFs inevitably undergo slow decomposition in water [76]. They possess high bioactivity and prone to bind with biological macromolecules (especially thiol groups). Consequently, metal ions would interact with lipid bi-layers, resulting in destroying membrane permeability and integrity [53]. Then, some metal ions can enter into M. aeruginosa cells to interact with intracellular enzymes and genetic materials and cause oxidation stress, generating more intracellular ROSs on cells' own. To alleviate the oxidation stress, M. aeruginosa would also produce antioxidases, whereas the metal ions can inactivate the critical enzymes. During lipid peroxidation, malondialdehyde (MDA) would be formed simultaneously as a characteristic by-product [77]. As a result, the excess ROSs and metal ions lead to ultimate death of algae cells.

  Correspondingly, organic linkers have also been suggested to affect the toxicity of MOFs to some extent. Nevertheless, the intrinsic connections between the toxicity of MOFs and those of their ligands are not clear. For instance, different MOFs constructed by same metal ions and different ligands possess similar toxicity under the same conditions in some cases [78]. Moreover, it has been reported that the toxicity of MOFs did not positively correlate to the toxicity of their organic parts [79]. Accordingly, it is still generally accepted that the toxicity of MOFs primarily depends on the types and the forms of metal ions.

  Besides the type of material itself, the toxicity of MOFs is also in accordance with the dosage. The content and release rate of metal ions and organic linkers in water would be governed by the stability of MOFs. In this regard, researchers have focused on the design of robust MOFs to realize slow release of metal ions. According to Pearson's hard/soft acid/base (HSAB) theory, the carboxylate-based ligands can be regarded as hard bases and they are prone to form stable MOFs with high-valent metal ions like Ti⁴⁺, Zr⁴⁺, Al³⁺, Fe³⁺, and Cr³⁺. Meanwhile, soft azolate linkers (e.g., imidazolates, pyrazolates, triazolates, tetrazolates) and soft divalent metal ions (e.g., Zn²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Ag⁺) can also construct stable MOFs. In addition, the particle size and shape have been also proved to influence the bioactivity of MOFs toward M. aeruginosa. Generally, MOFs particles with smaller size are easier to enter the cell by endocytosis effect, interrupting the intracellular metabolic system [76].

  Turning to photocatalysis, the ectogenic ROSs directly produced by MOFs under light illumination can attack cell membrane by oxidation [62]. After cell rupture, ROSs would further attack other intracellular matters including photosynthetic system (e.g., chlorophyll and PB), metabolic system and other genetic matters, causing entire collapse of alga cells. The leaching intracellular organic matters can be further degraded by continuous-generated ROSs. Besides, other physiology effects of M. aeruginosa under oxidation stress are similar to those treated by metal ions.

  Up to now, the MOFs-based materials for M. aeruginosa can be classified into three categories: (ⅰ) pristine MOFs, (ⅱ) MOFs-based composites, and (ⅲ) MOFs-based floating monoliths (Table 2).

  Table 2

  

  Table 2.  Different MOFs-based materials for algae removal.

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  Material type	MOFs-based materials	Target algae	Dominant inhibition mechanism	Ref.
  Pristine MOFs	Cu-MOF-1, Cu-MOF-2, Cu-MOF-3, Cu-MOF-4	M. aeruginosa	Photocatalytic ROSs oxidation, hetero-aggregation, metal ion release	[85]
  Pristine MOFs	UiO-66-NH2	M. aeruginosa	Nanotoxicity by cell internalization	[82]
  Pristine MOFs	Cu-MOF-74, MIL-125(Ti), ZIF-8, Zn-MOF-74	M. aeruginosa	Metal ion release, ligand release,	[81]
  Pristine MOFs	Cu-MOF-74	M. aeruginosa	Metal ion release, hetero-aggregation, ROSs generation	[73]
  Pristine MOFs	BUC-16	M. aeruginosa, Fragilariaceae, Pediastrum boryanum, Merismopedia, Spirogyra, Anabeana, Scenedesmus dimorphus, Pediastrum duplex, Chlorella vulgaris, Cmbella, Cocconeis, Euglena	Metal ion release	[80]
  Pristine MOFs	Co-SIM1, Zn-SIM1, Ag-TAZ	Anabaena, Synechococcus, Chlamydomonas reinhardtii	Metal ion release	[78]
  Pristine MOFs	MIL-101(Cr)-NH2	M. aeruginosa	Hetero-aggregation	[83]
  Pristine MOFs	MIL-101(Cr)-NH2	Chlamydomonas reinhardtii	Hetero-aggregation	[123]
  Functionalized MOFs	Zn-MOF-FA	M. aeruginosa	Allelopathic effect, metal ion release	[87]
  MOFs-based heterojunctions	Ag/AgCl@ZIF-8	M. aeruginosa	Photocatalytic ROSs oxidation	[95]
  MOFs-based heterojunctions	Ag/AgCl@g-C3N4@UiO-66(NH2)	M. aeruginosa	Photocatalytic ROSs oxidation	[97]
  MOFs-based heterojunctions	Bi2O3@Cu-MOF	Karenia mikimotoi	Photocatalytic ROSs oxidation	[94]
  MOFs-based heterojunctions	NZVI@PCN-224	Pseudomonas aeruginosa, Chlorella vulgaris	Photocatalytic ROSs oxidation	[118]
  MOFs-based heterojunctions	AgBr/NH2-MIL-125(Ti)	M. aeruginosa	ROSs generation	[96]
  MOFs-based composites	g-C3N4/Cu-MOF	M. aeruginosa	Photocatalytic ROSs oxidation, hetero-aggregation	[98]
  MOFs-based composites	SNP-TiO2@Cu-MOF	Karenia mikimotoi	Photocatalytic ROSs oxidation, hetero-aggregation	[119]
  MOFs-based monoliths	Fe3O4-BC@Cu-MOF-74	M. aeruginosa	Photocatalytic ROSs oxidation, adsorption, metal ion release	[114]
  MOFs-based monoliths	Ag/AgCl@ZIF-8 floating coating	M. aeruginosa	Photocatalytic ROSs oxidation	[108]
  MOFs-based monoliths	(Ag/AgCl@g-C3N4@UiO-66(NH2)-coated) foam	M. aeruginosa	Photocatalytic ROSs oxidation, hetero-aggregation	[107]
  MOFs-based monoliths	MS/Bi2O3@Cu-MOF	M. aeruginosa	Photocatalytic ROSs oxidation, adsorption, metal ion release	[110]
  MOFs-based monoliths	Co-MOF/PVA polymeric network	Navicula	Unknown	[103]
  MOFs-based monoliths	Cu-BDC MOF chips	M. aeruginosa	Metal ion release, ROSs generation from Cu2O	[104]
  MOFs-based monoliths	ZIF-8/chitosan/melamine sponge	Nitzschia	Membrane permeability change by chitosan	[109]

  3. MOFs-based materials for M. aeruginosa removal

  3.1 Pristine MOFs

  From the initial works, MOFs were added directly into water for M. aeruginosa control. Liu et al. [80] first applied a silver-based MOF (BUC-16) to M. aeruginosa suspension to evaluate its anti-alga performances. It was reported that BUC-16 could notably reduce the cell density. The cell rapture observed by scanning electron microscope (SEM) also demonstrated the damage of cell membrane. The antimicrobial mechanism may because of the slow-released Ag⁺ ions leading to the change of cell permeability, as mentioned above. Fan et al. [81] also use different MOFs like Cu-MOF-74, ZIF-8, Zn-MOF-74, and MIL-125(Ti) to inhibit the growth of M. aeruginosa. Results revealed that optimal M. aeruginosa inhibition effect toward M. aeruginosa occurred in the presence of Cu-MOF-74. Subsequently, researchers further invested the anti-algae mechanisms of Cu-MOF-74, finding that the slow release of Cu²⁺ was applicable to induce the self-generation of intracellular ROSs, which might be the primary effect. When the particle size of MOFs reaches nanoscale, MOFs particles might also enter into algae cells directly. Li et al. [82] observed the distribution of UiO-66-NH₂ particles (with particle size ranging from 40 nm to 380 nm) in M. aeruginosa cells via the fluorescence imaging. The intracellular UiO-66-NH₂ particles with high dosage (> 20 mg/L) were found to boost the release of organic matter. However, the aforementioned MOFs might induce the release of MCs into water when cells are damaged. It is also worth mentioning that Cu²⁺, Zn²⁺, and Ag⁺ could also influence the growth of other microorganisms in water matrices without selectivity. Additionally, some metal ions like Cd²⁺ would pose severe adverse effects to environment. Within this context, the selection of metal salts and the particle size control during MOFs synthesis should be considered thoughtfully.

  Different from the above mechanisms, forming flocculation has been regarded as another feasible strategy to separate M. aeruginosa cells from water, even without cell rapture. Li et al. [83] recognized that NH₂-MIL-101(Cr) facilitated hetero-aggregation of M. aeruginosa cells, and the co-setting rate was considerably higher than that of other commercial flocculants like ferric chloride and chitosan. Since M. aeruginosa tended to form colony via production of extracellular polymers in tough conditions, which poses high potential to resurrection, this study also discussed the long-term inhibition effect over NH₂-MIL-101(Cr). Results indicated that NH₂-MIL-101(Cr) could effectively hamper the re-growth of M. aeruginosa. During the antimicrobial mechanism discussion, authors proposed that the —NH₂ functional groups in NH₂-MIL-101(Cr) was capable of bridging and capture of algae cells rather than electrostatic interactions. More interesting, reducing initial pH value leads to protonation of -NH₂, inducing charge neutralization mediated flocculation. It is also worthwhile mentioning that this hetero-aggregation process did not cause cell rapture, which prevented MC-LR from leaching. This work provides a feasible alternative for efficient M. aeruginosa separation from water matrices by functionalized MOFs. Nevertheless, the toxicity of chromium ions should be taken into account. Fe³⁺ ion is the alternative because it is easy to undergo hydrolysis under basic conditions to form aggregation, and the toxicity of Fe ions to microorganisms is relatively low [84]. Future studies can also focus on the design of more amino-functionalized MOFs for cyanobacteria separation with Fe³⁺ ions as the metal center. Additionally, it is worth discussing how to efficiently separate the co-precipitation consisting of MOFs and M. aeruginosa without secondary pollutants generation. Whether the precipitation would release MCs and heavy metal ions should also be discussed in-depth.

  In other cases, pristine MOFs has been served as photocatalysts for M. aeruginosa elimination under light irradiation. As an example, Yue et al. [85] fabricated Cu-MOF-1~4 photocatalysts with different particle sizes by controlling reaction temperature, time, and rinse methods. Different from the common results in other studies, the removal efficiencies over Cu-MOF-1~4 against M. aeruginosa did not increase with the decrease of MOFs' particle size. Besides nanoeffects, the interfacial properties and water stability also played inevitable role in regulating the photocatalytic activity. Compared to smaller MOF particles, the larger particles possessed relatively higher water stability, and optimal hydrophobicity and surface charge for M. aeruginosa enrichment. In addition, the growth rate of M. aeruginosa was reported to be accelerated slightly under low concentration of Cu-MOFs. Finally, this study investigated MC-LR removal after cell collapse, finding that Cu-MOFs could effectively adsorb MC-LR, followed by its degradation via attacking the methoxy groups and the conjugated double bonds in MC-LR by ROSs. The aforementioned study indicates that some photo-responsive MOFs can also be adopted as photocatalysts with high oxidative stress to simultaneously degrade cyanobacteria cells and harmful MCs in-situ.

  Based on the advantage that MOFs can be functionalized and modified owing to various coordination modes, some related research has modified MOFs with some allelochemicals. Ferulic acid (FA) is a kind of polyphenol allelochemical with high biocompatibility and it can generate some oxidative products like ROSs to inhibit algae growth by self-oxidation [86]. For a typical example, Wang et al. [87] synthesized a FA-grafted MOF (Zn-MOF-FA) solvothermally by using FA as an organic linker. Then, Zn-MOF-FA was firstly employed in M. aeruginosa control under visible light. Rapid light curves (RLCs), the maximum quantum yield of PSII (F_v/F_m), the photosynthetic efficiency (α), and the maximal electron transport rates (rETR_max) were chosen as the representative parameters to test the MOF's impact on photosynthetic system. Furthermore, the concentration of intracellular antioxidases were also detected. According to the control experiment, Zn²⁺ was not the only anti-algae agent but the synergy of Zn²⁺ and FA. The cyanobacteria inhibition mechanism was proposed to be the blocking of electron transfer and photon energy capture in photosynthetic system, along with the loss of oxidation resistance by excessive ROSs generated from FA.

  3.2 MOFs-based composites

  Pristine MOFs catalysts usually suffer from the limitation of light absorption and easy recombination of h⁺ and e⁻ during the photocatalytic processes [88]. To address the above issues, heterojunction and composite fabrication are regarded as desirable strategy. Endowing MOFs-based materials with efficient visible light utilization is one of the most important directions for potential applications in actual water bodies [89]. Particularly, when MOFs are integrated with semiconductors with narrow band gap, the band structure of resultant composites would be optimized, which is favorable for widening the light adsorption and elevating the redox potential [90,91]. Moreover, heterojunctions can provide the shuttle of charge carriers, which is helpful for rapid e⁻ separation and transfer from the conduction band (CB) [92,93]. Accordingly, the utilization efficiency of h⁺ and e⁻ would be enhanced, driving the generation of more high-oxidative ROSs participating degradation of algae cells and MCs [94].

  For instance, Fan et al. [94] reported Ag/AgCl@ZIF-8 heterogeneous composites for M. aeruginosa elimination. Compared to other MOFs-based photocatalysts like ZIF-8, MOF-235, Bi₂WO₆/MIL-100(Fe), and BiOBr/MOF-5, Ag/AgCl@ZIF-8 exhibited superior photocatalytic performances. Taking the residual chlorophyll a concentration as a representative indicator, Ag/AgCl@ZIF-8 heterojunction (10 mg/L) could reduce 93.1% M. aeruginosa (initial density was 4.78 × 10⁶ cells/mL) within 6 h. Results implies that Ag/AgCl@ZIF-8 could not only destroys the cell wall and anti-oxidative system, but also degrade the leaching organic matters. The high algae degradation rate may be attributed to the localized surface plasmon resonance (LSPR) effect caused by Ag⁺ nanoparticles from the heterojunction. As well, Ag⁺ also served as a shuttle for e⁻ transfer, accelerating the charge carrier separation [94]. In the follow-up research, Fan et al. [97] fabricated an Ag/AgCl@g-C₃N₄@UiO-66(NH₂) (A-GUN) ternary heterojunction via in-situ deposition process. This photocatalyst can realize the simultaneous charge transfer between g-C₃N₄/AgCl and UiO-66(NH₂) on the relevant CBs. Based on the characterization toward cell morphology, protein content and organic matters, this ternary heterojunction was proved to result in changing the cell permeability, affecting cell's physiological activity and cellular organic matters. Furthermore, this study also discussed the effects of co-existing dissolved organic matter (DOM, using humic acid (HA) as a representative). Results reveal that low HA content tends to promote the photocatalytic reaction via producing a series of active substances by self-excitation of photons, but excess HA would consume ROSs and compete the active sites of A-GUN against photons. Wang et al. [98] proposed a composite containing g-C₃N₄ and a Cu-MOF by in-situ growth of Cu-MOF onto the surface of as-prepared g-C₃N₄ sheet. The excellent photocatalytic inhibition performances toward M. aeruginosa was primarily due to the elevation of hydrophobicity and reduced surface native charge, which is conductive to enhance the affinity between photocatalyst and algae cells. Although this study did not confirm the formation of heterojunction, the sink and intermediates of MC-LR released from the damaged M. aeruginosa cells was taken into consideration. The composite denoted as g-C₃N₄/Cu-MOF was demonstrated to accomplish MC-LR adsorption and subsequent degradation.

  3.3 MOFs-based floating monoliths

  Despite MOFs and their heterogeneous composites have posed significant photocatalytic inhibitory effects toward M. aeruginosa, the aggregation and recover difficulty of MOFs' powders hamper their practical applications in actual water matrices [99]. Since the recoverability of photocatalysts is of growing environmental concern, increasing number of studies have immobilized MOFs onto macroscopic substrates as supports [100,101]. Consequently, the rapid loss of MOFs has been mitigated and the MOFs dispersion are optimized via the size confinement effect by substrates [102-104]. More specifically, the fabrication of MOFs-based floating monoliths have attracted extensive interest primarily because of the following reasons: (ⅰ) MOFs-based floating monoliths enhanced the contact with M. aeruginosa. This is because most vigorous M. aeruginosa also tends to float onto the water surface with the aid of intracellular gas vesicles. Therefore, the floating photocatalysts are more favorable for realizing regional and selective water treatment vertically, which can preserve other aquatic organisms from being exposed by high concentration of ROSs. (ⅱ) As mentioned above, floating monoliths are easy to recycle and separate from water without complicated processes and apparatus like air flotation or ex-situ filtration. (ⅲ) Floating photocatalysts are beneficial to utilize sunlight. To our best knowledge, the intensity of incident light in water decreases sharply with the water depth. Thereby, the superficial MOFs-based floating monoliths can absorb sunlight with maximum intensity, giving full play to their photocatalytic advantage. So far, some substrates like polymeric sponges, polymeric membranes or films, perlites, and biochars have been used to fabricate MOFs-based monoliths [105]. The comparison of floating supports candidates for MOFs immobilization is shown in Table 3.

  Table 3

  

  Table 3.  The advantages and limitations of different floating supports candidates for MOFs immobilization.

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  Floating supports	Advantages	Limitations
  Polymeric sponges	Large specific surface area, large pores, high elasticity and flexibility	Microplastics generation, light scattering/shading effect, potential pollutant enrichment by adsorption
  Biochar	High chemical inertness, large specific surface area	Fragile, light scattering/shading effect, potential pollutant enrichment by adsorption
  Perlites	High chemical inertness, large specific surface area	Fragile, light scattering/shading effect, potential pollutant enrichment by adsorption
  Polymeric film/membrane	Transparent, high elasticity and flexibility, large contact area, high integrability	Membrane fouling, microplastics generation, potential pollutant enrichment by adsorption

  Among various floating supports, polymeric sponges are macroporous materials. The representative polymeric sponges are built up from melamine and polyurethane [60]. Polymeric sponges possess high mechanical strength and they can bear repeated squeezing and stretching. With the MOFs introduction, the resultant composites integrate the advantages of both MOFs and sponges, forming hierarchically porous structures, which is welcome for rapid enrichment and transfer of target pollutants [106]. To date, the universally-accepted immobilization method is dip-coating method, which takes the advantage of the ultrahigh liquid adsorption capacity of polymeric sponge [107,108]. Briefly, the sponge is immersed into organic solvent suspension containing MOFs or MOFs' precursors, followed by ex-situ or in-situ MOF deposition into the porous channels of sponge [109]. Finally, the organic solvents are removed from the MOF-loaded sponge via evaporation or rinsing (Fig. 5). Note that melamine sponges are soluble into some organic solution like N,N-dimethylformamide (DMF) solution. Therefore, in-situ growth of MOFs on melamine sponges in the presence of DMF is not applicable.

  Figure 5

  

  Figure 5.  Preparation processes of MOFs-based floating sponges by dip-coating method (taking sponge as an example).

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  Based on the former research regarding MOFs-based heterostructure photocatalysts, Fan et al. [107,108] immobilized the as-obtained Ag/AgCl@ZIF-8 and A-GUN onto the sodium dodecyl sulfate (SDS) treated sponges, respectively. SDS was served as a surfactant for enhancing the electrostatic interactions between MOFs and sponge support. The resulting firm MOF adhesion enables maintained high removal efficiency toward M. aeruginosa during cycling utilization. Fan et al. also investigated the impact of Ag/AgCl@ZIF-8 coated sponge on density, diversity and biomass of other phytoplankton in real water matrices, realizing that the floating composites also exerted notable inhibition effects on other aquatic microorganisms. This result suggests that MOFs-based materials may pose considerable adverse effects to biodiversity and community structures. In this regard, the comprehensive assessments should be conducted to discuss the long-term impact of MOFs-based materials on actual waters in the future. In terms of A-GUN loaded sponge (A-GUN/SMF), authors found that the photocatalytic activity under real sunlight was higher than that under the simulated visible light. And the removal efficiency toward M. aeruginosa increased with the elevation of initial cell density. The two results confirm that the MOFs-based floating monoliths pose great potential for cyanobacteria-polluted water remediation under sunlight. In another case, Wang et al. [110] adopted a modified strategy for immobilization of MOFs-based materials onto sponges, proposing a Bi₂O₃@Cu-MOF coated floating photocatalyst. During the fabrication process, sponges were dipped into suspension containing the as-prepared Bi₂O₃@Cu-MOF composite. Then, sodium alginate was added to the mixture. Subsequently, Ca²⁺ was supplemented to induce crosslinking of alginate after the composite filled into sponge pores. Results demonstrated that this strategy significantly reduced the leaching of copper ions from Cu-MOF because of the chemical binding of poly-alginate. The porous structure from sponge is helpful for pre-enrichment of M. aeruginosa before photocatalytic degradation. And the slight Cu²⁺ release was also suggested to contribute oxidation stress of cyanobacteria cells. Compared to sponges that are easy to occur light scattering, floating polymeric membranes or films are more favorable for light adsorption with large contact area and transparent properties [111]. Although there are no publications reporting MOFs-based membranes for M. aeruginosa removal, the high integrability of membranes or films is helpful for their future scale-up applications. Nevertheless, both polymeric sponges and membranes/films are likely to release microplastics in water, which should be paid attention to in depth [112].

  Other than polymeric sponges, biochar (BC) has also been the widely concerned floating material. The raw materials for BC production are easy to attain ranging from all kinds of bio-wastes generated from plants and foods. More encouraging, the solid waste can be reused [113]. BC itself often bears high porosity with certain adsorption performances. Within this context, BC is applicable to be the desirable support of MOFs-based materials for M. aeruginosa removal via the synergy of adsorption and degradation. Liu et al. [114] deposited magnetite onto the magnetized BC, after which, Cu-MOF-74 was grew onto the treated BC in-situ via solvothermal process. Similar to other MOFs-based floating photocatalysts, the prepared Fe₃O₄-BC@Cu-MOF-74 composite could cause oxidation stress to M. aeruginosa thanks to the MOFs' introduction. Simultaneously, benefited from the porous structure of BC, the composite exerts high affinity to components from algae cells like chlorophyll a as a result, the synergistic effects of Fe₃O₄-BC@Cu-MOF-74 increased removal efficiency against M. aeruginosa. However, the light shading effect of BC remains unknown, which needs further investigation to optimize the incorporation ratio of MOFs to BC.

  Apart from the published substrates for MOFs' loading, perlites are also promising candidates and pose great potential to future practical applications. Perlites primarily composed of silicon dioxide ubiquitously exists in nature with high chemical stability. For commercial usage, perlites have been widely employed as soil amendments with extremely low cost [115]. On the other hand, perlites are porous material, which is welcome for MOFs loading owing to the large specific surface area [116]. Unfortunately, perlites loaded with MOFs have not been applied for M. aeruginosa removal yet. As iron-based MOF MIL-88A(Fe) has been successfully immobilized onto perlites by Liu et al. for antibiotics degradation via photo-Fenton reaction in recent published work, we have great confidence that the above assumption can be realized in the future [116].

  4. Characterizations for evaluating anti-algae performance

  Drew from the bibliographic statistics, there has been 17 indicators used for examining MOFs' anti-algae performances, involving chlorophyll a content, cellular morphology, optical density, soluble proteins content, zeta potential, antioxidases content, hetero-aggregation experiment, shading effect, membrane integrity and permeability, PB content, intra- and extra-cellular organic compounds (IOMs and EOMs), lipid peroxidation (MDA), the ratio of variable to maximal fluorescence (F_v/F_m), extracellular total organic carbon (TOC) content, MCs content, esterase activity, and odor content. And the mentioned indicators can be further classified into (ⅰ) morphology and cell integrity, (ⅱ) photosynthetic activity, (ⅲ) antioxidant system activity, (ⅳ) metabolic activity, and (ⅴ) other indicators.

  4.1 Morphology and cell integrity

  Firstly, to visually characterize the cell integrity after MOFs treatment, it is necessary to observe the macroscopic and microscopic morphology, as depicted in Fig. 6. Optical microscope is the commonly-used device to attain macroscopic morphology of M. aeruginosa cells. Briefly, algae suspension can be added dropwise onto the hemocytometer before directly observed by optical microscope [80]. At the same time, SEM is usually adopted for microscopic observation. During the sample pretreatment, glutaraldehyde has been regarded as the desirable agent for cell immobilization, while ethanol is served as the dehydration solvent [117].

  Figure 6

  

  Figure 6.  M. Aeruginosa cell density characterization methods: (a) Direct observation by optical microscope; (b) Optical density measured by OD₆₈₀.

  DownLoad: Full-Size Img PowerPoint

  The cell integrity can also be evaluated by staining method by using propidium iodide (PI). PI can generate red fluorescence after reaction with double-bond nucleic acid under blue light illumination. However, PI cannot through membranes of viable cells. Only algae cells are damaged by MOFs-based materials does the red fluorescence occur [95]. Therefore, whether there exists dead M. Aeruginosa cells after MOFs' introduction can be detected by flow cytometry according to the fluorescent intensity, as illustrated in Fig. 7a.

  Figure 7

  

  Figure 7.  Different methods for (a) membrane integrity characterization, (b) intracellular ROSs detection, and (c) esterase activity estimation.

  DownLoad: Full-Size Img PowerPoint

  Membrane integrity and permeability are also the important indicators to reflect the anti-algae performances of MOFs-based materials. Normal M. aeruginosa cells possess permselectivity, whereas dead cells would leach electrolyte and non-electrolyte components out of the membrane. Thus, OD₂₆₄ can be employed in measuring the concentration of leached non-electrolyte components [118]. Meanwhile, the electrolyte components are commonly evaluated by measuring the concentration of free K⁺, Ca²⁺ and Mg²⁺, because these ions play critical roles in maintaining osmotic pressure of algae cells [97]. In addition, OD₂₈₀ and OD₂₆₀ are applicable to reflect the concentration of intracellular soluble proteins and nucleic acid, respectively [110].

  The TOC content can describe the degradation of M. aeruginosa cells by MOFs-based materials to some extent. Generally, TOC concentration tends to be increased initially and finally decreased with MOFs' introduction, especially in photocatalytic system. This is because cell rapture causes a large number of intracellular compounds releasing to the aqueous solution at the very early stage, followed by being further degraded by MOFs gradually [107]. Moreover, it is of significance to discuss MOFs' impact on types and content of compounds. Therein, the EOMs and IOMs can be characterized by three-dimensional excitation-emission matrix (EEM) spectra. Judging from the EOM and IOM proportion, the algae cell destruction extent can be inferred [118].

  4.2 Photosynthetic activity

  Chlorophyll a is the critical photosynthetic pigment in M. aeruginosa cells. Thus, the relative chlorophyll a content represents the situation of growth and photosynthetic system over M. aeruginosa. Before chlorophyll a extraction, MgCO₃ suspension should be added to prevent pigment from decomposition. Then, acetone is applied to selectively extract chlorophyll a under low temperature. Finally, the purified sample is measured by spectrophotometer and the chlorophyll a concentration is further calculated by Eq. 1 [107].

  Chlorophylla(mg/L)=[11.85(OD664−OD750)−1.54(OD647−OD750)−0.08(OD630−OD750)]V1V2L

  (1)

  where OD_x (x = 664, 647, 630, or 750) is the absorbance when light wavelength is identical to x, V₁ is the acetone volume (mL), V₂ is the sample volume (L), L is the optical path of quartz cuvette (cm).

  Additionally, PBs, including phycocyanin (PC), phycoerythrin (PE) and allophycocyanin (APC), are responsible for capture and transfer of incident photon energy [110]. Therefore, the impact of MOFs-based materials on photosynthetic system of algae can also be examined by the relative concentration of PBs.

  Furthermore, the ratio of variable to maximum fluorescence (F_v/F_m) represents the maximum quantum yield of photosynthetic system Ⅱ of M. aeruginosa. Some studies also utilized F_v/F_m to elucidate the photosynthetic activity of algae cells [87].

  4.3 Antioxidant system activity

  When M. aeruginosa is stimulated by adverse conditions (particularly ROSs induced by MOFs-based materials), the antioxidant system in algae cells would be activated, generating antioxidases to mitigate the imposed oxidation stress to cells themselves. Among various antioxidases, superoxide dismutase (SOD), hydrogen peroxidase (CAT), and peroxidase (POD) are the representative compounds [119]. Accordingly, the relative content variation of SOD, CAT, and POD in algae cells can indirectly demonstrate the oxidation stress caused by MOFs-based materials. In other cases, the content of lactate dehydrogenase (LDH) and dehydrogenase (DHA) in algae cells have also been taken into consideration [120]. Typically, the level of intracellular antioxidases tends to increase and decrease in turn. This phenomenon is because M. aeruginosa is capable of control the ROSs level at the early stage. However, the continuously generated ROSs from MOFs-based materials overwhelm the self-remediation capacity of M. aeruginosa, resulting in the ultimate damage of antioxidant system. Also, the excess ROSs accumulation would cause lipid peroxidation effect, forming MDA as a peroxidation product. As such, some studies discussed the extent of lipid peroxidation in M. aeruginosa cells via measuring the MDA concentration [121].

  Normally, the generation and elimination of intracellular ROSs maintain dynamic balance. By comparison, the imbalance of intracellular ROSs level occurs with MOFs' introduction, as mentioned above. In this regard, the intracellular ROSs content is able to indirectly reflect the MOFs' impact in algae cells. Generally, the 2′7′-dichlorodihydrofluorescein diacetate (DCFH-DA) has been commonly used as the probe. DCFH-DA itself is lack of luminescence. Fortunately, DCFH-DA can pass through cell membrane and tend to be hydrolyzed by intracellular esterase, forming DCFH that cannot pass through cell membrane [85]. At last, the resultant DCF is oxidated by ROSs, generating fluorescence that can be further measured, as shown in Fig. 7b.

  4.4 Metabolic activity

  In terms of evaluating metabolic activity of M. aeruginosa, the esterase activity of algae cells is the important aspect to reflect cell viability. During characterization processes, fluorescein diacetate (FDA) is often adopted as a probe. FDA can penetrate into algae cell. Subsequently, FDA would be catalyzed by esterase, generating green fluorescence [83]. The fluorescent intensity can be further detected by flow cytometry to infer the metabolic activity of algae cells, as illustrated in Fig. 7c.

  The soluble protein content in algae cells is another indicator representing the status of metabolism. In brief, the soluble proteins are extracted after decomposition of algae cells by freezing. In the follow-up procedure, the extracted organic matters are measured by kit of Bradford protein assay [61].

  4.5 Other indicators

  In addition to the aforementioned indicators, there are some characterizations without being utilized universally in the published literature. Since M. aeruginosa often adsorb anionic DOM from actual waters, the surface of M. aeruginosa tends to be negatively-charged, keeping colloidal stability of algae cells [122]. When cells are damaged, the superficial charge would become imbalanced. Based on this characteristic, the adverse effects imposed by MOFs-based materials can be indicated by measuring the variation of zeta potential on algae suspension. Moreover, M. aeruginosa would attach to some positively charged MOFs via electrostatic interaction, contributing to hetero-aggregation and co-precipitation containing MOFs and algae cells. At this time, the flocculation formation can be observed. In some cases, the co- and sum-settling curves have been adopted to evaluate the flocculation ability of MOF to cyanobacteria [123].

  It is also worthwhile mentioning that the secondary pollutant MCs holds great potential to release into water when algae cells are entirely decomposed, which exerts intensive risk to ecology and human health. Consequently, the monitoring of MCs must be taken into consideration in practical applications. Enzyme-linked immunosorbent assay (ELISA) has been regarded as an effective detection method for MCs with high sensitivity [124]. However, ELISA can not identify isomers of MCs. Therefore, ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) should be simultaneously applied to confirm the precise structure of MCs [125].

  Furthermore, the live M. aeruginosa cells are prone to produce odors like geosmin (GSM), 2-methylisoborneol (2-MIB) and β-cyclocitral [80,126]. Correspondingly, the odor concentration variation measured by gas chromatography combined with mass spectrometry (GC–MS) is accessible to reflect the algae inhibition extent over MOFs-based materials [23].

  4.6 Outstanding issues

  In order to analyze the importance and recognition extent of different indicators for evaluating M. aeruginosa inhibition performances over MOFs-based materials, we provide the statistic graph, as shown in Fig. 8. Upon different characterization indicators, the top 4 most recognized indicators include chlorophyll (especially chlorophyll a), microscopic (SEM/TEM, TEM represents transmission electron microscope) morphology, macroscopic (optical) morphology and optical density. Nevertheless, there remains several characterization limitations that should be taken into account.

  Figure 8

  

  Figure 8.  Occurrence frequency of different indicators characterizing algal removal performance of MOFs-based materials in relevant studies.

  DownLoad: Full-Size Img PowerPoint

  Taking OD₆₈₀ as an example, the MOFs nanoparticles are likely to affect the light absorbance. Despite the background value of MOFs particles has been subtracted, the linear relationship between cell density and OD₆₈₀ is still difficult to be established. Another alternative is separating MOFs with algae cells by filter papers with pore size between the particular size of MOFs and dimension of algae cells. However, partial MOFs particles would still be rejected onto membrane to some extent due to the size exclusion of formed filter cakes. Similarly, during the chlorophyll a extraction, there also exists residue MOFs onto fibrous diacetate papers, which would affect the following detection by absorption spectroscopy. Additionally, in the case of optical microscopy, MOF particles agglomerating into micrometer size would be misleading for algae cell counting if their shape are similar to those of algae cells. To reduce the above identification error, specific staining methods can be integrated to selectively color algae cells or MOF particles. For instance, Fan et al. used Lugol's iodine solution for coloration of algae cell nuclei before observation by optical microscope. On the contrary, Li et al. [123] grafted MOF (MIL-101(Cr)-NH₂) with molecule rhodamine B isothiocyanate (RITC) to endow MOF with specific luminescence before adding MOF into M. aeruginosa suspension. In this way, the algae cells and MIL-101(Cr)-NH₂ can be facilely differentiated by confocal laser scanning microscope based on different fluorescent channels. And the MOF internalization by algae cells can be observed as well.

  The content of antioxidases in M. aeruginosa is also widely accepted indicator. Most studies have investigated the variation of antioxidases in the presence of MOFs-based materials. However, the long-term transformation of antioxidases has not been well recognized, especially monitoring antioxidases activity when MOFs are removed from the water. Since the algae cell can eliminate the excess ROSs generated from MOFs and recover the original viability, it is worthwhile confirming whether MOFs cause irreversible damage to of antioxidant system of M. aeruginosa. Within this context, the optimal MOFs addition dose can also be determined to ensure the ultimate inactivation and degradation of relevant antioxidases.

  At the same time, the hetero-aggregation test between MOFs and M. aeruginosa cells has been adopted in some related cases, whereas little attention has been paid to the transfer and transformation of formed flocculation. Simultaneously, the feasible treatment technology for flocculation removal from actual waters is lack of further discussions. In this regard, future studies should focus on the flocculation separation method and attempt to recycle or reuse the inner value-added components like alginate and extracellular polymeric substances. Otherwise, the MCs, and heavy metal ions (derived from MOFs) accumulated in precipitation would be "non-time bomb" threatening the environment and human health.

  Additionally, the release and sink of MCs in water is intensively important indicator after algae cell degradation by MOFs-based materials. Disappointingly, judging from the statistics (Fig. 8), most works have not taken this aspect into consideration until 2023. Particularly in photocatalytic process, algae cells are prone to undergo notable decomposition, leaching a large number of MCs into water matrices. Odour released from live M. aeruginosa is another indicator to be monitored, but up to now, only one work has been attempted to elucidate the relationship between odour concentration and algae activity or MOFs' antimicrobial performance. Liu et al. [80] selected β-cyclocitral as the representative odour. By introduction of a silver MOF (BUC-16), the β-cyclocitral level from M. aeruginosa decreased with the day. After a week, there were no observed live cyanobacteria cells as well as no detected β-cyclocitral, implying that the presence of β-cyclocitral is a feasible indicator representing the activity of M. aeruginosa. For better elucidate the environmental implications over MOFs-based materials, we recommend concerning these environmental-related indicators in further studies.

  Furthermore, it is inevitable that MOFs itself would undergo slow decomposition in aqueous solutions. As a result, the leaching metal ions and organic linkers from MOFs pose great potential to elevate the TOC value and interfere the 3D EEM spectrum of algae suspension. Thus, we suggest discussing the robustness of MOFs-based materials in water matrices before conducting algae removal experiments.

  5. Conclusion and perspectives

  This review summarized the recent progresses of MOFs-based materials for M. aeruginosa control. The methods and indicators evaluating M. aeruginosa inhibition performances were subsequently critically discussed. Drew from the state-of-the-art studies, two promising routes can be chosen to realize M. aeruginosa removal: (ⅰ) Separating M. aeruginosa cells by using MOFs with moderate bioactivity as flocculants to form aggregation without cell rapture. (ⅱ) Using MOFs-based materials as catalyst to accomplish simultaneous oxidative degradation toward cells and MCs in-situ. It is also worth highlighting that increasing interest is focusing on fabrication of MOFs-based floating monoliths as heterogeneous visible light driven photocatalysts. Although the preparation and characterization methods of MOFs-based materials tend to be mature, there still remains some limitations and challenges urging great efforts to be addressed as the following: (ⅰ) As for design of MOFs-based floating macroscopic materials, the selection of supports and substrates for MOF immobilization is limited. (ⅱ) The stability, fate, and the environmental risks of MOFs-based materials are lack of in-depth discussion. (ⅲ) For estimating the M. aeruginosa inhibition performances, some methods need to be further modified in the presence of MOFs micro- or nanoparticles. (ⅳ) Some important environmental-related indicators, such as the release of secondary microplastics from polymeric monoliths and the release of MCs from destroyed cells have not been well-recognized. (ⅴ) There are no MOFs-based derivatives used for M. aeruginosa control up to now. (ⅵ) HA released from the damaged M. aeruginosa by MOFs-based catalysts might become a key challenge against MCs degradation in actual water, since HA can deplete ROSs. (ⅶ) The pilot-scale applications with MOFs-based materials have not been conducted.

  Based on the above aspects, we provide the fit-for-purpose suggestions as the following:

  (ⅰ) For fabricating MOFs-based floating monoliths, alternative supports other than polymeric sponges can be used to lower the invested cost and maintenance of microplastic, including but not limited to perlites, low-density biodegradable resins, and biochar.

  (ⅱ) For selecting MOFs-based materials, MOFs-based derivatives can be considered as a novel candidate.

  (ⅲ) The concentration effect of MOFs-based materials should be taken into account. Since MOFs with low concentration might accelerate the growth of M. aeruginosa via providing nutrients like metal ions, the minimum effective dose is necessary to be confirmed.

  (ⅳ) Studies should not just end up to reveal M. aeruginosa inhibition mechanism over MOFs-based materials. Based on the revealing fundamental mechanism of powder MOFs-based materials, pilot experiments and scale-up applications of MOFs-based monoliths are recommended to be conducted. The impact of co-existing compounds and microorganisms in actual waters on inhibition performances should be discussed as well.

  (ⅴ) The variation of water quality (especially MCs concentration) and biological community should be monitored after MOF introduction. Whether M. aeruginosa bloom would recur should not be neglected.

  (ⅵ) Cutting off the nutrient supply to algae is the fundamental strategy to restrain algae growth in water. As MOFs often bear porous structure and large specific surface area, the nutrient (especially for N and P) adsorption performances of different MOFs-based materials are suggested to be investigated, which is accessible to design muti-functional materials realizing removal of both M. aeruginosa and nutrients synchronously.

  (ⅶ) The porosity and large specific surface area of MOFs are also beneficial as carriers of some allelochemicals. Allelopathic substances with algal inhibition properties could be adsorbed or encapsulated in the pores of MOFs. Thus, MOFs can accomplish slow release of effective constituents for cyanobacteria control, which is similar to drug delivery.

  (ⅷ) Constrained by high fabrication cost, interference of co-existing matters and uncertain gross environmental impact, the application of MOFs-based materials for M. aeruginosa removal is still in its infancy. It is not wise to expect that MOFs can achieve effective scalable algae removal in the short term, and MOFs itself should not be regarded as an emergency means of algae removal. We propose that MOFs-based materials should be employed in small ponds after pretreatment.

  In conclusion, we hope that this review can offer constructive protocols in the design and performance evaluation of novel MOFs-based material for M. aeruginosa removal to reduce or eliminate the algal bloom.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

  CRediT authorship contribution statement

  Hong-Yu Chu: Writing – original draft, Visualization. Guang-Chi Liu: Methodology. Fu-Xue Wang: Methodology. Lian-Sheng Cui: Project administration, Conceptualization. Chong-Chen Wang: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization.

  Acknowledgments

  This work was supported by National Natural Science Foundation of China (Nos. 22176012, 52370025), the Pyramid Talent Training Project of Beijing University of Civil Engineering and Architecture (No. JDLJ20220802), the Doctor Graduate Scientific Research Ability Improvement Project of Beijing University of Civil Engineering and Architecture (No. DG2023014), and Guangxi Key Laboratory of Urban Water Environment.

  
  
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  Figure 1  Possible adverse effects of M. aeruginosa to human and ecology.

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  Figure 2  Recent progresses of M. aeruginosa removal by MOFs-based materials.

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  Figure 3  (a) Frequency percentage of different algae species involved in relevant studies. (b) Number of published research papers concerning M. aeruginosa removal by MOFs-based materials with years (data was collected from Web of Science between January 1, 2018 to December 31, 2023).

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  Figure 4  Schematic illustrations representing (a) cell structure of M. Aeruginosa, and (b) inhibition mechanism of MOFs toward M. aeruginosa.

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  Figure 5  Preparation processes of MOFs-based floating sponges by dip-coating method (taking sponge as an example).

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  Figure 6  M. Aeruginosa cell density characterization methods: (a) Direct observation by optical microscope; (b) Optical density measured by OD₆₈₀.

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  Figure 7  Different methods for (a) membrane integrity characterization, (b) intracellular ROSs detection, and (c) esterase activity estimation.

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  Figure 8  Occurrence frequency of different indicators characterizing algal removal performance of MOFs-based materials in relevant studies.

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  Table 1.  The advantages of MOFs for M. aeruginosa control compared with other homogeneous agents.

  Materials	MOFs	Other homogeneous agents
  Structure	Easy-tailored and explicit	Unknown
  Inhibition mechanism	Multiple	Generally single
  Stability	Controllable	Disposable
  Toxicity	Controllable	Unknown
  Function	Multiple (e.g., adsorption, catalysis, flocculation)	Generally single
  Separation accessibility from water	Easy to be recovered after immobilization	Difficult to be reused

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  Table 2.  Different MOFs-based materials for algae removal.

  Material type	MOFs-based materials	Target algae	Dominant inhibition mechanism	Ref.
  Pristine MOFs	Cu-MOF-1, Cu-MOF-2, Cu-MOF-3, Cu-MOF-4	M. aeruginosa	Photocatalytic ROSs oxidation, hetero-aggregation, metal ion release	[85]
  Pristine MOFs	UiO-66-NH2	M. aeruginosa	Nanotoxicity by cell internalization	[82]
  Pristine MOFs	Cu-MOF-74, MIL-125(Ti), ZIF-8, Zn-MOF-74	M. aeruginosa	Metal ion release, ligand release,	[81]
  Pristine MOFs	Cu-MOF-74	M. aeruginosa	Metal ion release, hetero-aggregation, ROSs generation	[73]
  Pristine MOFs	BUC-16	M. aeruginosa, Fragilariaceae, Pediastrum boryanum, Merismopedia, Spirogyra, Anabeana, Scenedesmus dimorphus, Pediastrum duplex, Chlorella vulgaris, Cmbella, Cocconeis, Euglena	Metal ion release	[80]
  Pristine MOFs	Co-SIM1, Zn-SIM1, Ag-TAZ	Anabaena, Synechococcus, Chlamydomonas reinhardtii	Metal ion release	[78]
  Pristine MOFs	MIL-101(Cr)-NH2	M. aeruginosa	Hetero-aggregation	[83]
  Pristine MOFs	MIL-101(Cr)-NH2	Chlamydomonas reinhardtii	Hetero-aggregation	[123]
  Functionalized MOFs	Zn-MOF-FA	M. aeruginosa	Allelopathic effect, metal ion release	[87]
  MOFs-based heterojunctions	Ag/AgCl@ZIF-8	M. aeruginosa	Photocatalytic ROSs oxidation	[95]
  MOFs-based heterojunctions	Ag/AgCl@g-C3N4@UiO-66(NH2)	M. aeruginosa	Photocatalytic ROSs oxidation	[97]
  MOFs-based heterojunctions	Bi2O3@Cu-MOF	Karenia mikimotoi	Photocatalytic ROSs oxidation	[94]
  MOFs-based heterojunctions	NZVI@PCN-224	Pseudomonas aeruginosa, Chlorella vulgaris	Photocatalytic ROSs oxidation	[118]
  MOFs-based heterojunctions	AgBr/NH2-MIL-125(Ti)	M. aeruginosa	ROSs generation	[96]
  MOFs-based composites	g-C3N4/Cu-MOF	M. aeruginosa	Photocatalytic ROSs oxidation, hetero-aggregation	[98]
  MOFs-based composites	SNP-TiO2@Cu-MOF	Karenia mikimotoi	Photocatalytic ROSs oxidation, hetero-aggregation	[119]
  MOFs-based monoliths	Fe3O4-BC@Cu-MOF-74	M. aeruginosa	Photocatalytic ROSs oxidation, adsorption, metal ion release	[114]
  MOFs-based monoliths	Ag/AgCl@ZIF-8 floating coating	M. aeruginosa	Photocatalytic ROSs oxidation	[108]
  MOFs-based monoliths	(Ag/AgCl@g-C3N4@UiO-66(NH2)-coated) foam	M. aeruginosa	Photocatalytic ROSs oxidation, hetero-aggregation	[107]
  MOFs-based monoliths	MS/Bi2O3@Cu-MOF	M. aeruginosa	Photocatalytic ROSs oxidation, adsorption, metal ion release	[110]
  MOFs-based monoliths	Co-MOF/PVA polymeric network	Navicula	Unknown	[103]
  MOFs-based monoliths	Cu-BDC MOF chips	M. aeruginosa	Metal ion release, ROSs generation from Cu2O	[104]
  MOFs-based monoliths	ZIF-8/chitosan/melamine sponge	Nitzschia	Membrane permeability change by chitosan	[109]

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  Table 3.  The advantages and limitations of different floating supports candidates for MOFs immobilization.

  Floating supports	Advantages	Limitations
  Polymeric sponges	Large specific surface area, large pores, high elasticity and flexibility	Microplastics generation, light scattering/shading effect, potential pollutant enrichment by adsorption
  Biochar	High chemical inertness, large specific surface area	Fragile, light scattering/shading effect, potential pollutant enrichment by adsorption
  Perlites	High chemical inertness, large specific surface area	Fragile, light scattering/shading effect, potential pollutant enrichment by adsorption
  Polymeric film/membrane	Transparent, high elasticity and flexibility, large contact area, high integrability	Membrane fouling, microplastics generation, potential pollutant enrichment by adsorption

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