Recent advancements in metal – organic frameworks integrating quantum dots ( QDs@MOF ) and their potential applications

: Design and development of new materials and their hybrids are key to addressing current energy issues. Thanks to their tunable textural and physiochemical properties, metal – organic frameworks ( MOFs ) show great potential toward gas sorption, catalysis, sensing, and electrochemical energy applications. Nevertheless, prac - tical applications of MOFs have been hampered because of their limited electrical conductivity, micropore size, and poor stability. However, smart integration of zero - dimensional quantum dots ( QDs ) into an MOF template, where the host structure o ﬀ ers suitable interactions for enhancing the stability and synergic properties, may be a solution. The objective of this review is to summarize recent advances in the ﬁ eld of QD@MOFs, highlighting fresh approaches to synthesis strategies and progress made in their application to optoelectronic devices, sen - sing, biomedical, catalysis, and energy storage. The current challenges and future directions of QDs@MOFs hybrids toward advancing energy and environmental applications are also addressed. We anticipate that this review will inspire researchers to develop novel MOF hybrids for energy, optoelectronics, and biomedical applications.

Formally, zero-dimensional (0D) semiconductor nanocrystal materials comprising groups II-VI, III-V, or IV elements with a diameter of 2-10 nm are denoted as quantum dots (QDs) and have acquired considerable interest due to their excellent size-dependent tunable electronic properties and budding applications in the areas of sensing, catalysis, nano-medicine, and bio-imaging [61][62][63][64][65][66][67].In most cases, this 0D material consists of core (e.g., InP, TiO 2 , GaAs, CdS)-shell (e.g., CdS ZnS, PbS, ZnO) structures encompassing a combination of a large number of atoms comprising mainly groups 12-16 (like ZnSe, ZnO, CdSe, etc.) or 13-15 (InP, InAs, etc.) [68][69][70][71][72][73].Despite its several benefits, this form of QDs has a cytotoxic effect on live cells and tissues.To solve the shortcomings of traditional QDs, a new generation of QDs was designed, such as Si QDs, Ag 2 Se QDs, carbon dots (CDs), graphene QDs (GQDs), and perovskite.Recently, perovskite QDs have gained popularity in the field of electric and optoelectronics due to their adjustable bandgap, high light-absorption efficiency, high photoluminescence (PL) quantum yield, etc. [74][75][76][77].Furthermore, these 0D materials can be simply modified by the surface modification method.Importantly, most of QDs are sustainable in aqueous systems and have coatings that comprise various functional groups, such as alcohols, amines, thiols, and carboxylic acids.Interestingly, a wide range of covalently conjugated molecules has been developed utilizing these functional groups.Moreover, because of the enormous surface area and quantum detention upshot, these materials have some advantages juxtaposed with traditional chromophores, such as extensive absorption bands, low photobleaching, thin and even emission bands, extended lifetimes, and high quantum yields [78][79][80][81][82].Although QDs have excellent properties, the easy agglomeration of QDs leads directly to fluorescence quenching, which restricts their utilization in various fields [83][84][85][86].Considerable effort has been invested in overcoming this hurdle, for example, passivation with an additional semiconductor film with a suitable bandgap, embedment of QDs with a variety of material (e.g., polymers or micelles), casing QDs with silica shell to harvest QDs@SiO 2 composites, etc. [87][88][89][90][91][92][93][94][95].However, all these methods are time-consuming and could lead to the formation of unwanted products or undesired behavior in the system.To provide QDs with multifunctional functionality, attempts to encapsulate these semiconductor nanoparticles and adjustable composite architectures must be investigated.Surprisingly, MOFs with high porosity and specific surface area provide an exciting platform, creating an ideal environment for loading QDs and preventing them from aggregation.At the same time, QDs enhance the physicochemical properties of MOFs.As a result, the amalgamation of QDs and MOFs results in good dispersion and great stability.In this context, MOF-derived QDs (QDs@MOF) have piqued curiosity and opened up a new avenue for a variety of applications [75,77,[96][97][98][99][100].These hybrid materials have captivating properties, such as outstanding PL, excellent biocompatibility, good mechanical/thermal stability, and relative simplicity of functionalization [101][102][103][104][105][106].As displayed in Figure 1a, the number of papers on QDs@MOF has increased dramatically in recent years, particularly since 2016.A large variety of QDs@MOF materials have evolved, and their characteristics and intriguing applications have been studied ever since (Figure 1b).Although the research on QDs@MOFs is increasing (Table 1), there have been only a few review articles in this research field so far.In view of this flourishing research area, we have summarized the QDs@MOF fabrication strategy, its unique properties, and the wide-ranging applications, and we believe that this review will unfold the path toward newer innovative research and diverse applications.

Fabrication strategy for
QDs@MOF Various fabrication strategies have been adopted for the synthesis of QDs@MOF over the last decade.Normally, the strategies comprise two customized methods, "shipin-a-bottle" (ship-bottle) and "bottle-around-the-ship" (bottle-ship), and the additional two approaches are "photo deposition" and "direct surface functionalization" (Figure 2), which are deliberated in the following section.

Ship-bottle
The ship-bottle approach (Figure 2) involves the immobilization of small molecules or QDs precursors in the pore windows of MOFs followed by further treatment to attain the desired structure.Various methodologies, such as vapor deposition, solution infiltration, and solid-state grinding, have been employed to introduce QD precursors into MOFs, although precisely controlling the location, content, structure, and morphology of the incorporated guests is sometimes quite challenging.Based on the synthesis condition, the ship-bottle technique is further categorized into three types including "solution infiltration," "chemical vapor infiltration," and "double solution method."In this context, Gao et al.   passed into the hydrophilic cavities of MOFs via capillary action and hydrophilic contact, thus reducing the amount of QDs deposited on the exterior part of MOF.In this context, Meng and colleagues [110] used a twofold solution approach whereby a small amount of glucose (G) and CdS QD precursor solution were co-infiltrated into the cavity of MIL-101, which resulted in the formation of G/CdS@MIL-101, further subjected to calcination at 200°C to obtain the ultimate product carbon nanodots (CDs)/CdS@MIL-101.
It is worth noting that ship-bottle preparation procedures frequently rely on very extreme reaction conditions, such as elevated temperature and redox state, which might result in local network deterioration.This could also reduce the surface area of the MOF matrix, thus having a significant influence on applications that need porosity.However, the most significant advantage of this technology is to enable the creation of conformal MOF layers around QDs, which is a unique and demanding operation.

Bottle-ship
The bottle-ship strategy (Figure 2) is commonly known as the model synthesis methodology for QDs@MOF preparation.Following this method, QDs are initially produced and spread in a solvent-based stabilizer, such as a surfactant, to avoid agglomeration.Following that, MOF precursors are added to the solvent, which initiates MOF development around the QDs.During this process, organic linkages form divalent connections with capping moieties on the surface of the QDs.In this context, to develop a CdTe/Eu-MOF composite, Kaur et al. [111] employed CdTe QDs capped with cysteamine that were introduced to the Eu-MOF precursor solution, resulting in an ordered distribution of CdTe QDs in the Eu-MOF environment due to the interaction between the -COOH moieties of Eu-MOF and the -NH 2 groups on the exterior section of the CdTe QDs.Similarly, Mo et al. [112] followed the same methodology and prepared Fe(III)-MIL-88B-NH 2 @ZnSeQDs for antigen detection where the solution of a MOF precursor and ZnSe QDs were heated at 100°C (for 20 h) in a Teflon-lined reactor.The ultimate desired compound was obtained by subsequent cooling followed by centrifugation (Figure 3c).Furthermore, Wang et al. [113] employed a capped polyvinyl pyrrolidone agent, which not only maintained the firmness and distribution of the particles but also stimulated the formation of ZIF-8 on the surface, thus establishing an intimate heterogeneous assembly between them.Importantly, this method successfully lowers the quantity of the QDs accumulated on the exterior part of the MOFs by avoiding the diffusion impedance of the nanoparticles infiltrating into the environment of the MOFs.Furthermore, because nanoparticles may be agglomerated prior to the assembly of frameworks, the shape and dimension of QDs can be tailored for specific applications.Unlike the ship-bottle and bottle-ship techniques, in which QDs were embedded in MOFs, the "photochemical-deposition" technique (Figure 2) involves depositing QDs on the exterior part of MOFs.

Photochemical deposition
The in situ synthesis and admission of QD particles into the exterior part of a MOF is aided by light in this method; photo-reduction of metallic precursors with a sufficient redox potential induces the production of QDs on the MOF surface.Direct binding with an appropriate linking group can be used to modify the surface area of MOFs with QDs.Utilizing this approach, Wang's group [114] used UV light to create hybrid materials of MIL-125(Ti) accumulated by CdS, CuS, and Ag 2 S QDs.The mechanism of the photodeposition of Me x S y on MIL-125(Ti) is shown in Figure 3d.Similarly, Lin et al. [115] prepared a three-component hybrid material denoted as UiO-66/CdS/1% reduced graphene oxide, following the above-described method, and acquired significant results toward photocatalytic applications.The key drawbacks of this technology include two critical procedures to synthesize QD-MOF composites: first, it is the in situ synthesis and distribution of QDs on the interface of MOFs and, second, the adequate photoreduction potential to convert QD precursors to QDs, which might be tough at times.

Direct surface functionalization
Direct surface functionalization (Figure 2) is another tactic to suspend QDs on the interface of MOFs.The surface ligands of QDs are sequentially replaced with an appropriate capping group, which can establish direct interaction with MOF particles either by coordinative interaction or by some nonspecific contacts.The primary distinction between this strategy and the three preceding ones is that both MOFs and QDs are pre-designed prior to being assembled.Moreover, the key benefit of this type of synthesis includes the easy regulation of the form and size of QDs, as well as the construction of MOFs.To prepare porphyrin-based MOFs, Jin et al. [116] followed the abovementioned method and developed porphyrin-based MOFs with CdSe/ZnS QDs where the amino-functionalized QDs firmly adhered to the surface of the MOFs through zinc metal (Figure 3e).Similarly, employing the same strategies, Mondal et al. [117] prepared an MOF-functionalized cysteine-capped CdTe QDs, which functioned as a proficient white light-emitting phosphor material for display applications.In contrast to the previous three approaches, in this method, MOFs and QDs are pre-formed before being assembled.This approach has the advantage of enabling to control the shape and size of QDs as well as the morphology of MOFs.

Other synthesis methodologies
In addition, some other methods, such as intercalation [118], physical fusion [74,119] drop casting [120], and electrochemical depositions [121,122], have been developed for the preparation of QDs@MOF composites.The "physical mixing" approach is more simple and may be divided into two categories.The first one involves the use of physical force binding to combine QDs with MOFs, and the second one employs ultrasonic fusion.Utilizing the above-cited methods, Wang et al. [123] fabricated white light-emitting phosphor materials using carbon dots (CDs) and a Zr(IV)-MOF by physical fusion with a binding agent.Another extensively used technique is electrochemical deposition, which involves dispersing QDs in an electrolyte and depositing them on the interface of MOFs using an electric current.Recently, Chen et al. [106] adopted a slightly different tactic for the synthesis of UiO-66-NH 2 /black phosphorus QDs (MOF/BPQDs).The in situ synthesis of the composite was carried out on the carboxyl cellulose nanofiber (CNF) surface, which served as nucleation centers due to the presence of abundant carboxyl groups.The CNF aerogel shows high structural adaptability and little MOF erosion in BP@CNF-MOF, which demonstrates the reciprocal physical contact and involvement of CNFs, along with excellent binding affinities among MOF crystals and the CNF aerogel.According to the aforementioned methodologies, the techniques of embedding QDs within MOF matrixes are more effective than seeding MOF crystals with QDs.Not only does the encapsulation of QDs inside MOFs prevent QDs from covering MOFs, but it also inhibits QDs from clustering.In addition, after being encapsulated by MOFs, the robustness of QDs increases.

Application of QDs@MOFs
As a new functional hybrid material, QDs@MOFs hold greater stability, robust adsorption capacity, and unique intriguing properties, which make them ideal candidates for numerous applications.The applicability of QDs@MOFs in different fields (Figure 4), such as sensing, bio-imaging, energy generation, and energy storage, is detailed in this section and the applications of these new fields of interest are presented in Table 1.

Sensing
Sensors can help to improve the quality of life by aiding in medical diagnosis, increasing the efficiency of renewable resources, such as fuel cells and batteries, photovoltaics, pollution management, enhanced health, welfare, and security for people.Recently, QDs@MOFs have been considered attractive materials for fabricating different fluorescence sensors due to their high quantum yields, extended lives, outstanding photo-stability, and sizedependent emission wavelengths.

Metal ion detection
With the industrial growth, a large number of metal ions, for example, Pb 2+ , Fe 3+ , Cr 2+ , Cu 2+ , and Hg 2+ , have been discharged into water, destroying the water environment and posing a threat to human safety.As a result, the advancement of an effective metal ion detection technology is a precondition for heavy metal pollution prevention and management.Because of its high level of sensitivity, low detection limit, strong selectivity, broad detection array, quick response, decent anti-jamming capability, and simplicity of maneuver, metal ion detection using fluorescence-based QD biological sensors has recently drawn a lot of attention.For instance, Chen et al. [114] employed UiO-66-NH 2 /black phosphorus QDs (MOF/BPQDs) adorned on the CNF aerogel for uranium extraction from seawater (Figure 5a).The creation of a heterojunction between BPQDs and UiO-66-NH 2 displays outstanding photocatalytic activity (Figure 5b), which efficiently kills marine bacteria by releasing a huge amount of reactive oxygen species.Similarly, to design a ratiometric fluorescence device for the recognition of Cr, Wang et al. [124] rationally developed CDs@Eu-MOFs (Figure 5c).Surprisingly, the synthesized CDs@Eu-MOFs outperform Cr(VI) in terms of excellent selectivity (Figure 5d(i)) in the presence of a wide range of metal ions (Na + , K + , Zn 2+ , Pb 2+ , NH 4 ) with potentiometric detection of Cr(VI) under optimal circumstances, with a linear range of 2-100 µM and a low detection limit (LOD) of 0.21 µM (Figure 5d(ii)).

Detection of biomolecules
Direct identification of biological systems, for example, an enzyme or an antigen, by means of a QDs@MOF probe should be more inventive for biological intuiting applications.Several studies have been conducted so far using QDs@MOFs for the detection of biomolecules.For example, Wang et al. [125] developed a CdTe QDs@ZIF-365 as a bi-functional ratiometric probe for highly subtle recognition of L-histidine and Cu 2+ by adopting the post-synthesis strategy (Figure 6a).The experimental findings revealed that the CdTe QDs@ZIF-365 can be employed as an outstanding photo-luminescent probe for L-histidine and Cu 2+ with a steep K sv (6.0507 × 10 8 [M −1 ] and 2.7417 × 10 7 [M −1 ]) value and low detection (Figure 6a).Similarly, Xie et al. [126] recently implemented LMOFs (luminous metal-organic frameworks) -CDs@ZIF-8 by incorporating blue-emitting CDs into ZIF-8 and employed it as a fluorescent sensor for highly sensitive and discerning recognition of dopamine (DA) (Figure 6b).The sustained pores in ZIF-8 not only provide free space for analytes but they may also selectively collect and intensify DA molecules through interactions between the analyte DA and the functional site of the framework.Furthermore, CDs can be used as signal probes to convert chemical signals from CD-analyte interactions into fluorescent signals.When compared to CDs, the CDs@ZIF-8 creates a novel sensing platform.As a result, the CDs@ZIF-8-based recognition technique for DA was shown to have a large concentration dynamic (0.1-200 M) and an LOD of 16.64 nM (Figure 6b and c).According to the findings, the produced QDs@MOF might be ideal probes for detecting cell biological properties and could be utilized as cell strength monitors and bio-probes.

Recognition of other entities
QDs@MOFs have been investigated for the recognition of other materials.For example, Zhou et al. [127] developed a robust ultra-sensitive electrochemiluminescence sensor (CDs@HKUST-1) for the recognition of catechol.The results revealed that the definite surface area of HKUST-1 on CDs might significantly increase the sensor's sensitivity.The as-synthesized sensor showed a varied linear range of 5.0109-2.5105mol L −1 under ideal circumstances, with an LOD of 3.8109 mol L −1 (S/N = 3).Employing a post-synthetic modification method, Yang et al. [128] discovered an amine-CQDs@UiO-66 fluorescence probe by using amine-functionalized carbon QDs (amine-CQDs) in combination with UiO-66.In this investigation, UiO-66 was employed as an adsorbent to selectively collect and augment the target compounds.Here, the amine-CQDs were used as a template molecule to evaluate the connection between UiO-66 and the target compounds in a specific fashion and to subsequently convert these chemical reactions into recognizable fluorescence signals.As a result, QDs@MOFs provide a novel approach to creating hybrids with synergistic characteristics, fluorescence, and excellent durability for various sensing applications.

Biomedical
The unusual features (excellent biocompatibility, bioavailability, and renewability) of QDs@MOFs have attracted a lot of interest in the biomedical profession in recent years because, among other applications, they can be used for real-time tissue imaging (bioimaging), diagnostics, singlemolecule probes, and medication administration.Herein, we have concentrated on two major biomedical applications: bioimaging and photothermal therapy.

Bioimaging
Bioimaging is a useful research strategy in contemporary biology and medicine that may quickly and easily offer clear and understandable biological information.Several investigations have shown that QDs@MOFs have great bioimaging capabilities due to their PL characteristics.For the first time, He et al. [129] used a straightforward two-step technique to create CDs and ZIF-8-based nanocomposites.The coordination contacts between Zn 2+ ions and functional groups (-COOH/-N) on the CDs were reinforced to encase the CDs on the ZIF-8.The resulting CDs@ZIF-8 showed green fluorescence as well as being an excellent pH-receptive anti-cancer drug carrier and cell imaging (Figure 7a and b).According to in vitro cell studies, it has been corroborated that the nanocomposites exhibited excellent cyto-compatibility and could be endocytosed through cells for cell imaging and drug administration (Figure 7c).Furthermore, Qin and coworkers [130] recently developed a biodegradable nano-platform of molecularly imprinted polymer (MIP)-alleviated fluorescent ZIF-8 loaded with doxorubicin (DOX), (FZIF-8/DOX-MIPs) for drug delivery and imaging in a glutathione (GSH)/pH multi-stimulation system (Figure 7d and e).It is worth mentioning that CDs generate bright red fluorescence, allowing more precise tumor cell imaging.With time, the fluorescent gesture of FZIF-8/DOX-MIPs grew in the tumor location of mice.Furthermore, in an acidic tumor environment, the biological degradation of ZIF-8 and MIPs was favorable for drug release.

Photothermal therapy
Photothermal treatment (PTT) using near-infrared (NIR) light for tumor hyperthermia ablation has been intensively explored in recent years and has sparked a lot of interest.PTT has fewer adverse effects than standard tumor treatment techniques because local heat may be properly regulated in temporal and spatial lobes.MOFs and other two-dimensional (2D) materials have recently been investigated as photodynamic agents (PTAs) for PTT applications in vitro and in vivo.When three-dimensional (3D) MOFs are converted into 2D sheets, the resulting MOF sheets may absorb a huge quantity of guest molecules via a noncovalent contact.Nevertheless, the poor photothermal renovation efficacy (PTCE) of 2D materials, as well as their considerable dimensional size, limits their practical application in PTT.As a result, there is a significant need for ultra-small PTAs with immense PTCE to attain a remarkable competence in photothermal tumor therapy.QDs@MOF materials, on the other hand, provide a suitable space for loading QDs and avoiding QD aggregation due to their large specific surface area and ordered pores.Furthermore, QDs help MOFs to acquire better physicochemical properties.The close heterojunction or interfacial contact between QDs and MOFs speeds up the transfer of electrons and efficiently prevents the recombination of photo-generated charges.In addition, the developed QDs@MOFs might be suitable PTAs because of their outstanding NIR adsorption and biocompatibility characteristics.For example, Liu et al. [131] employed an MOF hybridized with black phosphorus QDs (BPQDS) as a tandem catalyst to improve the treatment of hypoxic tumor cells (Figure 8a).The integrated MOF system was able to alter H 2 O 2 to O 2 in the MOF-alleviated catalase superficial layer, and then, O 2 was introduced unswervingly into the MOF-sensitized BQ central, resulting in an excellent quantum yield of singlet oxygen.Remarkably, without catalase, the MOF system's photodynamic treatment efficacy was 8.7 times higher after internalization, indicating an improved therapeutic impact besides hypoxic tumor cells (Figure 8b).
These findings imply that QDs@MOFs will usher in a new era of tumor PTT.Tian et al. [132] employed a very simple one-pot technique to formulate a versatile manifesto for a symbiotic chemo-and photothermal therapy.They utilized ZIF-8 as drug nanocarriers where the implanted GQDs functioned as indigenous photothermal kernels (Figure 8c).When DOX was exploited, a prototypical anticancer drug, the findings revealed that the monodisperse ZIF-8/GQDs (size 500-1,000 Å) were able to capture DOX throughout the manufacturing phase and activate DOX discharge under acidic circumstances.The DOX-loaded ZIF-8/GQDs were able to readily transform NIR illumination into heat and, therefore, raise the temperature.When breast cancer 4T1 cells were used as a prototype biological system, the findings confirmed that combining chemo-thermal treatment and PTT with DOX-ZIF-8/GQDs had a substantial harmonious impact, leading to greater performance in killing cancer cells than the photothermal therapy and chemotherapy alone (Figure 8d).As a result, ZIF-8/GQDs might be useful as adaptable nanosystems in cancer treatment.These investigations revealed that QDs@MOFs have an extensive range of applications in biological and medical fields that are both benign and proficient.

Catalysis
Recently, tremendous progress has been achieved in the improvement of MOF-based QD materials as high competence catalysts/co-catalysts in catalysis systems, including GQDs, CDs, Se QDs, and Mxene QDs.In this context, QDs@MOFs are considered a promising catalyst/co-catalyst due to their tenability nature and robustness.

Electrocatalysis
Electrocatalysis is a highly advanced oxidation process (AOP) that has been extensively investigated in energy and conservational applications, such as the hydrogen reduction reaction, nitrogen reduction reaction (NRR), hydrogen evolution reaction (HER), methanol oxidation reaction, oxygen reduction reaction (ORR), and oxygen evolution reaction (OER).Among renewable-energy technologies, electrocatalytic applications are becoming highly indispensable.As a result of their outstanding characteristics, QDs@MOFs might play a significant part in the electrocatalytic processes.Despite the fact that many investigations have focused so far on the use of QDs in electrocatalysis, exploration of QDs@MOFs in electrocatalysis has just freshly come to the forefront.Zhou et al. [180] tested a unique MOF catalyst, CdS@PCN-224(Ni), and utilized it for HER in an acidic environment.The findings revealed a prodigious electrocatalytic performance with a Tafel slope of ∼91 mV dec −1 , an overpotential of 120 mV, and a current density of 10 mA cm −2 , which is nearly identical to the Pt/C (∼43 mV dec −1 ).CdS@PCN-224(Ni) has a doublelayer capacitance (Cdl) of 9.75 mF cm −2 , which is significantly higher than PCN-224 (2.33 mF cm −2 ) (Figure 9a and b).Similarly, fuel cells have been actively investigated among various energy conversion technologies because of their lower pollution levels, superior energy transformation proficiency, and fuel diversity.The oxygen reduction process, however, is significantly limiting the overall reaction efficiency of the fuel cells due to their essentially slow kinetics.In this context, Ye et al. [192] reported a new and simple approach for manufacturing ZIF-derived Co-N-C ORR catalysts by carefully regulating the rate of crystallization of ZIFs.The experimental evidence showed that in an alkaline medium, the Co-N-C catalyst has a high ORR activity (E 1/2 of 0.9 V), which can compete with commercial Pt/C (E 1/2 of 0.83 V) (Figure 9c  and d).Similarly, a bifunctional electrocatalyst was prepared by Ye et al. [59] using a simple hydrothermal technique.Such highly permeable cuboids of Pt QDs@Fe-MOF material demonstrated excellent electrocatalytic activity toward HER, OER, and overall water splitting.Interestingly, in 1 M KOH, the electrocatalyst with exceptionally low Pt QD content (1.85 µg cm −2 ) only required an overpotential of 191 and 33 mV, respectively, to achieve a current density of 100 and 10 mA cm −2 .Furthermore, the Pt QDs@Fe-MOF/NF (Ni foam) electrodes had exceptional potency, delivering a current density of 10 mA cm −2 at 1.47 V during at least 100 h of water splitting.These findings suggest that QDs@MOFs show promising potential in the realm of electrocatalysis; however, additional studies are warranted.

Photocatalysis
Another proficient AOP, photocatalysis, has been extensively investigated and seems extremely promising for dealing with global energy and environmental concerns.In this context, QDs@MOFs are viewed as potential visiblelight catalysts for assorted systems, namely photocatalytic CO 2 reduction, H 2 production, H 2 reduction, pollutant degradation, and other applications in this domain.As an example, Liu et al. [186] illustrated the integration of GQDs on MIL-101(Fe) to create GQD/MIL-101(Fe)(G/M101) by employing a one-step solvothermal technique.With the use of MIL-101(Fe) and GQD sensitization, the photocatalytic reduction efficiency of CO 2 to generate CO could be considerably improved.Experimental evidence revealed that the rate of CO generation over G/M101-5% (224.71μmol h −1 g −1 ) is five times greater in comparison with MIL-101(Fe) (46.2 μmol h −1 g −1 ) (Figure 10a and b).Furthermore, photocatalytic nitrogen fixation is regarded as a potential strategy for obtaining high NH 3 production, which is critical for human growth and industrial advancement.Nevertheless, because of the inert nature of nitrogen, it is important to investigate superior competence catalysts for nitrogen reduction.In this context, Qin et al. [191] used MXene QDs (Ti 3 C 2 -QDs) and a 2D nickel metalorganic framework (Ni-MOF), following a self-assembly approach, to increase the photocatalytic proficiency of the N 2 reduction process.The optimum Ti 3 C 2 -QDs/Ni-MOF heterostructure produced a significant amount of ammonia (88.79 μmol g cat 1 − h −1 ) (Figure 10c).These findings provide space for further applications of QDs@MOF in photocatalysis.

Photoelectrocatalysis (PEC)
PEC is a potent technology that combines heterogeneous photocatalysis with electrochemical methods.Extensive research has been conducted so far on the use of QDs@MOFs in PEC for water splitting.As a result of their unique characteristics, QDs@MOFs may have great application potential in PEC.Shi et al. [221] suggested that MOFderived TiO 2 can be used to boost the productivity of a TiO 2 -QDs established PEC system for hydrogen evolution.When compared to conventional TiO 2 films, an MOFimpregnated TiO 2 film stimulated by core-shell CdSe@CdS QDs demonstrated a +42.1% increase in the PEC device stability and a +47.6% increase in the PEC performance.The inclusion of mixed rutile/anatase phases enhances the performance by creating a promising band energy arrangement for the dissociation of photogenerated charges.Even though there are only a few studies on PEC with QDs@MOFbased materials, this approach is intriguing and should be given greater attention.These findings pave the way for QDs@MOFs being used in photocatalysis in the future.

Energy storage
In this twenty-first decade, increasing energy consumption, the depletion of fossil fuels, and growing concerns about industrial pollution have stimulated the improvement of eco-friendly technologies to create new alternative and renewable energy resources.In this context, QDs@MOFs are regarded as viable catalytic systems for energy storage devices because of their unique chemical, physical, and electrical features.

Batteries
With the advancement of science and technology, a great deal of focus has been placed on creating next-generation electrochemical energy storage technologies (a few examples are batteries, supercapacitors, solar cells, etc.).Because of their environmental friendliness and great energy density, batteries are the most frequently investigated electrochemical energy storage devices.
QDs@MOF-based materials have shown considerable promise for battery applications in recent years due to their better theoretical Li storage capacity, advantageous electrical conductivity, truncated functional voltage range, low dispersal fences for Li mobility, and outstanding mechanical characteristics.For instance, Saroha et al. [104] recently prepared multilayer porous N-doped C nanofibers encompassing vanadium nitride QDs and MOF-based hollow N-doped C nanocages for improved lithium-sulfur batteries as functional interlayers.The experimental findings revealed that because of the high sulfur concentration (80 wt%) and loading (ca. 4 mg cm −2 ) in the sulfur electrodes, the Li-S cell utilizing the novel nanostructured self-supporting interlayer displayed better rate proficiency and steady cycling recital (decay rate of 0.02%/cycle at 0.5 C).Interestingly, after 100 cycles of charging and discharging at 0.05 C, the Li-S cell provided a steady areal capacity of 5.0 mA h cm −2 despite an ultra-high sulfur loading of 11.0 mg cm −2 (Figure 11a-c).Zhang et al. [222] prepared ZIF-8/graphene oxide hybrids as anode materials for sodium-ion batteries.The capacity of the synthesized material was quite stable at 539 mA h g −1 at 100 mA g −1 , 512 mA h g −1 at 200 mA g −1 , and 456 mA h g −1 at 500 mA g −1 after 100 cycles.After 300 cycles, upon raising the current density to 1 A g −1 , the capacity still attained 362 mA g −1 (Figure 11d).This investigation revealed that QDs@MOFbased materials have the potential for developing highperformance electrode material in batteries.

Supercapacitors
A supercapacitor, like batteries, is an imperative energy storage equipment that has the potential to be used in electric vehicles and other portable devices because of its extraordinary power density, firm charging/discharging capabilities, and extended life cycle.Due to their structural flexibility, excellent electrical conductivity, hydrophilic surface, and high surface area, QDs@MOFs have been extensively researched.These intriguing features may result in ultra-high volumetric capacitance as they provide quick electron transfer pathways and a huge electrochemically energetic surface for a quick and reversible faradaic reaction.Yang et al. [118] reported in situ formations of Co 9 S 8 QDs in the interlayer of MOF-derived layered double hydroxide (LDH) nanoarrays for supercharged amalgamated supercapacitors.Remarkably, the selectively produced Co 9 S 8 -QDs displayed numerous active sites that enhanced the electrochemical characteristics, such as cyclic stability, capacitive performance, and electrical conductivity.Because of the mutually beneficial partnership, the composite material distributed an extremely high electrochemical capacity of 350.6 mA h g −1 (2,504 F g −1 ) at 1 A g −1 .Moreover, blended supercapacitors produced with CF@NiCoZn-LDH/Co 9 S 8 -QDs and carbon nanosheets, enhanced by single-walled carbon nanotubes, had a remarkable energy density of 56.4 W h kg −1 at a power density of 875 W kg −1 , with a capacity retention of 95.3% after 8,000 charging and discharging cycles.(Figure 12a and b).Similarly, Liu et al. [210] prepared a hybrid material, NQD-NC, made up of Nb 2 O 5 QDs implemented on nitrogendoped porous carbon imitative from ZIF-8 dodecahedrons, which showed excellent electrochemical enactment including ultrahigh energy and power density (76.9 W h kg −1 and 11,250 W kg −1 , respectively) and longer cyclic firmness after 4,500 cycles with the retaining capacity of ∼85% at 5 A g −1 in a voltage range of 0.5-3.0V (Figure 12c and d).In this investigation, QDs@MOFs were shown to be potential options for developing extraordinary recital supercapacitor devices.

Optoelectronic devices
The core and most fundamental component of optoelectronic technology is optoelectronic devices, and as the technology advances, a wide range of optoelectronics, such as optical switches, white light-emitting diodes (W-LEDs), solar cells, and lasers, are developed.In the limited range of visible light, MOFs are recognized as excellent luminous materials, whereas QDs are considered a suitable candidate for the preparation of white light-emitting devices due to their broad absorption range, high extinction coefficient, and high quantum yield.Many MOF-imitative QDs, such as GQDs, CQDs, perovskite QDs, and Mxene QDs, have recently been demonstrated to have exceptional electron donors and acceptors in their photoexcited states, making them interesting for deployment in optoelectronics.For example, Wang et al. [123] developed, by combining CDs with Zr(IV)-MOFs, a novel rare-earth free material that emits white light when excited at 365 nm, with a PL quantum yield of 37% in the solid state (Figure 13a).A CIE chromaticity coordinate of (0.31, 0.34) (Figure 13b and c), a luminous efficiency of 1.7 lm W −1 , and a high color-rendering index (CRI) of 82 were achieved by dropping the CDs/Zr-MOF on a marketable UV LED chip.
Ren et al.

Other applications
In addition to the aforementioned uses, QDs@MOFs have shown high potential for other promising applications thanks to their exceptional characteristics.Biodegradable drug delivery transporters with long-term drug release properties are useful in cancer therapy because they help to reduce some of the adverse effects.In this context, Pooresmaeil et al. [170] developed a simple technique for fabricating MOFs inside a carboxymethylcellulose (CMC)/ GQD matrix, which is utilized for anticancer drugs.The findings revealed that the MIL-53@CMC/GQDs may be offered as a viable drug delivery vehicle.Furthermore, it was observed that MIL-53@CMC/GQDs have greater DOXloading capacity than MIL-53, as demonstrated by the pH-dependent DOX release behavior in drug release tests, showing meticulous release actions in vitro, which is in good agreement with the first-order kinetic model and the non-Fickian mechanism.The cytocompatibility of MIL-53@CMC/GQDs against the human cancerous cell lines was confirmed using a cytotoxic test (MDA-MB 231).Furthermore, by following a step-by-step bisacrificial template scheme, Zhu et al. [220] were the first to report a bimetallic sulfide QDs, Cu 2 SnS 3 (CTS)-involved MOF nanosheets, CuBDC (BDC = 1,4-benzenedicarboxylate).According to a Z-scan investigation conducted under the illumination of a 532 nm laser, the CTS@CuBDC film exhibited significant optical limiting (OL) performance and precise truncated OL thresholds (0.92 J cm 2 ), along with high third-order nonlinear susceptibility (1.9 × 10 −6 esu).This research shows that the new methodical approach (bisacrificial templates) to producing metal sulfide QD-distributed MOF hybrid composites is interesting and practical and might be a good option for nonlinear optical applications.

Conclusions and prospects
In this article, we have presented a comprehensive overview of advancements in QDs@MOF hybrids, covering synthetic techniques, structures, and applications.The produced QDs@MOF hybrid demonstrated improved stability as well as novel characteristics and application potential.Recently, QDs@MOFs have gained popularity because of their remarkable physicochemical and opticalelectrical characteristics and are therefore considered a cutting-edge branch of materials.Various methodologies have been adopted by which QDs@MOFs endowed with a diversity of exclusive properties, such as exceptional PL characteristics, high selectivity to target analytes, or biocompatibility, can now be prepared.Furthermore, QDs@MOFs may be employed in a broader range of applications, including catalysis, sensing, bioimaging, optoelectronics, and batteries.Despite these amazing results, a number of obstacles illustrated in Figure 14 must be addressed to encourage further progress in this field.
1) The most straightforward way to adjust the characteristics of QDs@MOFs to specific applications is using the appropriate synthesis process.Many investigations have revealed that QDs@MOFs can be synthesized via a variety of methods.The ship-bottle/bottle-ship techniques are the most frequent in QDs@MOF synthesis, whereas photochemical decomposition/direct surface functionalization has received less attention.As a result, it is worthwhile to put more effort into QDs@MOF synthesis.2) Controlling the morphology and surface properties of QDs@MOFs remains a daunting task.As a result, various experimental parameters, such as temperature, reaction duration, solvent impact, and reaction device, should be used more thoroughly to identify the growth mode.This can result in an MOF-based hybrid QD material with better morphologies and surface functionalization.3) This field is still in its infancy and may not be appropriate for industrial manufacturing on a large scale.As a result, novel synthetic methodologies must be developed that will be not only cost-effective in terms of laboratory research but also suitable for massive commercial applications.4) Many studies have shown that QDs@MOFs exhibit outstanding fluorescence behavior, photonic, photothermal transformation, and photoelectronic characteristics in tests.Even though these features of QDs@MOFs have been already demonstrated, there are still challenges to address.Right away, attention should be focused more on improving their current qualities and expanding these properties to other study domains.In this context, the combination of theoretical models and actual experiments will lead to successfully investigating novel QDs@MOFs with new and better properties.5) Based on the categories of QDs@MOFs, it has been observed that most works were conducted employing CdS, CdSe, CdTe, GQDs, etc., meaning that other QD materials should be paid greater attention to.MXene QDs, for example, have attractive features and have shown to be useful in energy production, catalysis, and sensing owing to extremely sensitive surface terminations.As a result, MXene QDs@MOF advancement is extremely desirable.6) The majority of MOF hybrid QD materials addressed thus far were made of MOFs and single-QD materials.As a result, numerous different materials or MOFs Recent advancements in QDs@MOF  1967 coupled with some other MOFs deserve further investigation.7) Although a great deal of research has been done on QDs@MOF materials due to their versatile nature, their application is still a long way off.Solar cells, for example, are an important part of the solution to the worldwide energy crisis.Up to date, many resources have been investigated in the field of solar cells; nonetheless, certain drawbacks, such as short service life, low energy efficiency, or higher cost, have severely hampered their practical applicability.In the meantime, numerous studies have shown that QD-based materials are promising prospects for solar cell applications.Therefore, it is worth putting extra effort into investigating the use of QDs@MOFs in solar cells.8) Furthermore, diverse hybridized QDs@MOFs enable a wide range of electrochemical applications, whereas studies related to NRR are rarely conducted.9) Importantly, more research into the applicability of QDs@MOF-based immunosensing is required.Since there has been some research on bio(sensing)-based QDs@MOFs, more methodologies and unique protocols for the identification of cell cultures and cancer biomarkers should be developed.10) The relevance of hybrid materials for applications requiring NLO characteristics, upconversion, and lasing has already been emphasized by the available results.Nevertheless, this is still a relatively new and developing field of study, and factors impacting, e.g., the higher-order nonlinear optical features of QDs@MOF materials should be analyzed in an unswerving manner.11) Currently, the preparation of perovskite-based MOF hybrid materials is a hot topic due to their various intriguing applications [223,224], but the eco-toxicity of heavy metal ions is a key problem for lead perovskites.As a result, additional effort must be invested in the future in investigating and developing novel nontoxic and environmentally acceptable materials for the perovskite@MOF composite.
In general, we have presented some of the most recent research findings of QDs@MOF.This study aims to provide in-depth knowledge about the variety of synthesis, characteristics, and uses of QDs@MOF, alongside stimulating future research into these new and fascinating domains.Despite the significant accomplishments, there are still some basic and technological gaps and obstacles in this area and, therefore, considerable effort should be put into the exploration of novel preparation techniques, physicochemical characteristics, and prospective applications.
The abovementioned concerns must be addressed to stimulate continued progress in the synthesis and implementation of new QDs@MOF hybrids, which will have a substantial influence on chemistry, material science, and a broad range of applications.

Figure 1 :
Figure 1: (a) The number of journal articles published on QDs@MOF (source: ISI Web of Knowledge, 2010s to 2021); (b) the outline depicts recent advances in the creation of several QDs@MOFs.

Figure 2 :
Figure 2: Schematic representation of various approaches deployed for the preparation of QDs@MOF.

Figure 4 :
Figure 4: A schematic representation of QDs@MOF for various captivating applications.

Figure 5 :
Figure 5: (a) Under light irradiation, uranium adsorption capacity and elution efficiency in six consecutive cycles.(b) A schematic representation of the photocatalytic reduction of U(VI) utilizing MOF/BPQDs hybrid under solar-light irradiation [106].(c) A schematic representation of the photocatalytic reduction of U(VI) utilizing MOF/BPQDs hybrid under solar-light irradiation.(d) The CDs@Eu-MOFs fluorescence selectivity spectra in the absence and presence of different metal ions [124].

Figure 6 :
Figure 6: (a) The illustrative representation of fabrication of a CdTe QDs@ZIF-365 ratiometric fluorescence probe and its use for very delicate recognition of L-histidine and Cu 2+ [125]; (b) diagrammatic representation of detection of DA through a possible mechanism.(c) FL emission bands of CDs@ZIF-8, with different DA concentrations [126].

Figure 8 :
Figure 8: (a) Stepwise construction of BQ-hybridized MOF catalyst and action toward hypoxic tumor cell treatment.(b) After being inoculated with BQ-MIL@cat-fMIL, time-dependent in vivo fluorescence images of a mouse carrying a subcutaneous HeLa tumor and after injection of BQ-MIL@cat-fMIL or BQ-MIL@fMIL in vivo fluorescence imaging evaluated the treatment upshot on mice malignant cells [131].(c) Schematic representation of the development of ZIF-8/GQDs with the recapitulation of DOX.(d) After 8 h of incubation, cell viability study of 4T1 cells with and without free DOX, ZIF-8/GQD, and DOX-ZIF-8/GQD suspensions and deprived of 3 min NIR radiation [132].

[ 76 ]
employed a novel technique for solving the stability issues of CsPbX 3 perovskite QDs by implanting CsPbX 3 perovskite QDs into mesoporous MOF-5 crystals (Figure 13d).It has been observed that the CsPbX 3 /MOF composites have enhanced stability while keeping their excellent optical characteristics intact.The experimental findings revealed that, under 200 mA, CsPbX 3 /MOF-5 W-LED produced hot white light and the PL maximum was in agreement with the PL bands of the corresponding CsPbX 3 /MOF-5 (Figure 13e).From the CRI value (83) and luminous efficiency (21.6 lm W −1 ), it has demonstrated the excellent efficacy of CsPbX 3 /MOF-5 W-LED whereas the CIE color coordinate triangle of the CsPbX 3 /MOF5 W-LED comprehends 124% of the National Television System Committee standard (Figure 13f).

Table 1 :
List of documented QDs@MOF, including the type of QDs and MOF, utilized their preparative methods and their potential applications in various fields