Sébastien Noël
email: sebastien.noel@univ-artois.fr

) • Bastien Léger
email: bastien.leger@univ-artois.fr

Anne Ponchel
email: anne.ponchel@univ-artois.fr

) • Samahe

Eric Monflier
email: eric.monflier@univ-artois.fr

Cyclodextrins as multitask agents for metal nanoheterogeneous catalysis: a review

Keywords: metal nanoparticles, cyclodextrin, nanoheterogeneous catalysis

Metal nanoparticles, with high surface area to volume ratios and heterogeneous catalytic properties, are a very challenging research area. In the last decades, many efforts have been devoted to the design of supramolecular metal nanoparticles in order to be used in aqueous medium. In catalysis, water is a highly desirable solvent for developing chemical reactions, in accordance with the principles of green chemistry. In this context, the utilization of macrocyclic compounds, in association with transition metal nanoparticles, appears to be a very appealing approach for developing water-dispersible catalytic systems, with high levels of performances and selective molecular recognition abilities. The purpose of this review is to summarize the research done in this field, focusing attention on cyclodextrins by taking advantage of their unique topologies such as their rigid cyclic structure, their ability to form inclusion complexes with molecules of appropriate size and shape, the presence of hydroxyl groups enabling coordination with metal ions.

In this review, we discuss the most commonly used methods, which have been reported over the last forty years, for stabilizing catalytically active metal nanoparticles derived from various cyclodextrin-based systems. The most significant and representative examples devoted to the development of such systems are described, with the purpose of increasing stability, recyclability and catalytic activity, using supramolecular chemistry as a remarkable tool. Some pivotal aspects of their synthesis, characterization and catalytic behaviors are presented, with special attention to (i) the development of metal nanoparticles in solvent-dispersed form or immobilized onto supports in the presence of cyclodextrins or their derivatives, (ii) to the multifunctional role of cyclodextrins capable of acting as reducing agents, stabilizing/dispersing agents or mass transfer promoters, even possibly the three and (iii) to the design of more sophisticated catalytic systems, in which the cyclodextrin plays a central role of supramolecular host with dynamic equilibrium.

Introduction

Nanomaterials, including metal-based nanoparticles, are gradually emerging in the scientific community, due to their special optical, electronic and catalytic properties, originating from quantum size effects and low-coordination numbers of surface atoms [START_REF] Zhang | Nanomaterials for energy conversion and storage[END_REF][START_REF] Sharma | Preparation and catalytic applications of nanomaterials: a review[END_REF][START_REF] Baig | Nanomaterials: a review of synthesis methods, properties, recent progress, and challenges[END_REF]). In the field of metal catalysis, which plays a key role in our everyday life and economy by impacting approximately 90 % of all existing chemical processes in the world, the scientific research on nanocatalysts is certainly one of the most dynamic fields of current research [START_REF] Hu | Synthesis and stabilization of metal nanocatalysts for reduction reactions -a review[END_REF]. Indeed, metal-based nanoparticles composed of few atoms offer potential advantages over their bulk counterparts, allowing large surface area-to-volume ratios and precise control in terms of size (1-20 nm), shape, composition, structure and crystallinity [START_REF] Haruta | Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature far Below 0 °C[END_REF]. Such specificities at the nanoscale can be beneficial for inducing unusual chemical reactivity and selectivity. Additionally, this can result in the decrease of the reaction temperatures, naturally associated with energy and cost savings as well as environmental benefits. Commonly, there are two main strategies for synthesizing nanomaterials: top down and bottom-up. The latter can be described as dynamic self-organizing processes, in which individual components arrange to gradually form more complex assemblies through nucleation and growth mechanisms. The synthesis of metallic nanoparticles by chemical reduction is an example of the bottom-up approach. However, their applications can be hindered by their tendency to aggregate and lose their specificities especially in prolonged reactions. Utilization of soluble capping agents, such as surfactants, ligands or polymers, and immobilization on an organic or inorganic solid support are effective ways to circumvent these problems [START_REF] Niu | Acid-tolerant cyclodextrin-based ruthenium nanoparticles for the hydrogenation of unsaturated compounds in water[END_REF][START_REF] Ndolomingo | Review of supported metal nanoparticles: synthesis methodologies, advantages and application as catalysts[END_REF]. Given the need for sustainable development and reducing environmental footprint with metal-based nanoparticles, there is a considerable interest in developing cleaner synthesis protocols and chemical transformations, without harmful solvents, by favoring the use of water-soluble capping agents [START_REF] Shukla | Metallic nanoparticles: green synthesis and spectroscopic characterization[END_REF]. For instance, metal nanoparticles stabilized by ammonium salts, phosphanes, polymers or oligosaccharides have found a number of applications in the field of nanocatalysis, exhibiting interesting results in terms of activity, selectivity, stability and reusability [START_REF] Schulz | Aqueous Rhodium Colloidal Suspension in Reduction of Arene Derivatives in Biphasic System: a Significant Physico-chemical Role of Surfactant Concentration on Catalytic Activity[END_REF][START_REF] Debouttière | An organometallic approach for the synthesis of water-soluble ruthenium and platinum nanoparticles[END_REF][START_REF] Koczkur | Polyvinylpyrrolidone (PVP) in Nanoparticle Synthesis[END_REF][START_REF] Kurniasih | Dendritic nanocarriers based on hyperbranched polymers[END_REF]. Among these capping agents, cyclodextrins and their derivatives are considered as particularly interesting candidates. Indeed, cyclodextrins are naturally occurring cyclic oligosaccharides formed of n glucopyranose units, with an inner hydrophobic cavity and an outer hydrophilic surface [START_REF] Crini | Review: A History of Cyclodextrins[END_REF][START_REF] Morin-Crini | 130 years of cyclodextrin discovery for health, food, agriculture, and the industry: a review[END_REF]. This singular structure is not only known for allowing the stabilization of host-guest complexes, moleculeion adducts, but also metallic nanoparticles [START_REF] Wankar | Recent Advances in Host-Guest Self-Assembled Cyclodextrin Carriers: Implications for Responsive Drug Delivery and Biomedical Engineering[END_REF][START_REF] Molnár | Synthetic Application of Cyclodextrins in Combination with Metal Ions, Complexes, and Metal Particles[END_REF]). In addition, these polyhydroxylated compounds can also act as reducing agents of metal salt precursors to produce zerovalent metal nanoparticles under controlled conditions of pH and temperature [START_REF] Dong | Polyol synthesis of nanoparticles: status and options regarding metals, oxides, chalcogenides, and non-metal elements[END_REF]. In catalysis, the molecular recognition properties can help the substrate to be brought into close proximity with the metal surface sites and induce cooperative catalytic interactions, which was already described with water soluble organometallic complexes in aqueous-organic two-phase systems [START_REF] Hapiot | Recent breakthroughs in aqueous cyclodextrinassisted supramolecular catalysis[END_REF]). Accordingly, it has become to us obvious that the combination effects of cyclodextrin supramolecular structures and metal nanoparticles could contributed to the development of innovative catalysts and should be addressed in a dedicated review.

The present review, based on a bibliographic survey up to 2020, highlights the broad spectrum of reported metal nano-heterogeneous systems, including cyclodextrins in their formulations that have been investigated in catalysis. This implies that all papers describing the study of catalytically active metal oxide nanoparticles in presence of cyclodextrins will not be considered in this review [START_REF] Nikam | Wet chemical synthesis of metal oxide nanoparticles: a review[END_REF]. It also implies that the cyclodextrins (i) will not act as the proper catalyst, (ii) could be used as stabilizing and/or dispersing and/or reducing agents of metal nanoparticles and (iii) could behave as supramolecular hosts for enhancing the reaction rates or altering chemo or regio-selectivity of chemical reactions. This means that all works dealing with cyclodextrin-assisted syntheses of supported metal nanoparticles, involving thermal treatments such as calcination or carbonization, but also washing steps for the cyclodextrin removal are out of the scope of this work (Bai et al. 2013a;[START_REF] Tomer | Facile preparation of Ni / Al 2 O 3 catalytic formulations with the aid of cyclodextrin complexes : Towards highly active and robust catalysts for the direct amination of alcohols[END_REF][START_REF] Lannoy | Cyclodextrin-Directed Synthesis of Gold-Modified TiO2 Materials and Evaluation of Their Photocatalytic Activity in the Removal of a Pesticide from Water: Effect of Porosity and Particle Size[END_REF]). This review is organized in two sections depending on the nature of the metal nanoparticles and the conditions of the reaction process. In the first section, we will aim at providing a good overview of existing examples of cyclodextrin-based group metal nanocatalysts in solventdispersed form. Indeed, the solution-phase synthesis, which generally consists in the reduction and decomposition of the metal salt precursor in the presence of cyclodextrins in aqueous diluted media, allow the achievement of catalytically active metal nanoparticles of controlled size. The possibility of generating such water-dispersible nanoparticles through the use of different cyclodextrin-based systems, ranging from molecular cyclodextrins to cyclodextrin polymers will be described. We will also address that metal nanosystems with higher degrees of sophistication could be achieved by using host-guest complexes or supramolecular assemblies derived from cyclodextrins. Such approaches could lead to enhanced dynamic behaviors during catalysis, but also to new supramolecular catalytic media such as cyclodextrinbased hydrogels. The second section will be devoted to the review of the preparation and catalytic applications of materials, where metal nanoparticles are immobilized onto or into a support matrix in the presence of cyclodextrins. Three approaches will be discussed, relying on the anchorage of nanoparticles on different kinds of solid supports, e.g. inorganic supports, porous carbon materials or insoluble polymers, in the presence of cyclodextrins. This will be illustrated with examples on the stabilization of metal nanoparticles prior to their immobilization on the support, by more direct methods involving the reduction of the metal precursor in presence of cyclodextrin-functionalized support. Studies of simultaneous addition of cyclodextrins, metal precursor and support to give metal supported catalysts will also be discussed. Finally, cyclodextrin-assisted catalytic processes where cyclodextrins ared added just before the reaction catalyzed by supported nanoparticles are the last approach. Once again, it should be reminded that the structure of the cyclodextrin remain intact whatever the strategy.

All these studies, in solvent dispersed form or in immobilized form, will emphasize the beneficial impacts of the cyclodextrins on metal nano-heteregeneous systems applied in catalysis. The diversity of such systems mainly results from the multifunctional character of the cyclodextrin scaffold. Efforts will be directed towards a review of the roles of the cyclodextrins and structure-performance relationships of the catalysts, while taking into consideration the aspects of their eventual recovery and recycling. A number of catalytic reactions examples will be given including unsaturated compounds hydrogenation, nitroaromatics reduction, dyes photodegradation, electrooxidation and carbon-carbon bond formation. This article is an abridged version of the chapter published by Noël et al. (2020b) in the series Environmental Chemistry for a Sustainable World (10.1007/978-3-030-49308-0_5).

Unsupported metal nanoparticles in the presence of cyclodextrins

2.1. Metal nanoparticles stabilized by native cyclodextrins [START_REF] Komiyama | Colloidal Rhodium Dispersions Protected by Cyclodextrins[END_REF] synthesized rhodium nanoparticles using native cyclodextrins, i.e.

--and-cyclodextrin, for the catalytic hydrogenation of water-soluble α, β-conjugated carbonyls. The best Rh nanoparticles dispersion was obtained with -cyclodextrin and corresponded also to the best catalytic activity. No activity was observed for the hydrogenation of 3-buten-2-one in the presence of cyclohexanol as competing guest, hightlighting the beneficial effect of the -cyclodextrin.

During metal nanoparticles synthesis, the metallic precursor can be reduced in alkaline medium thanks to the hydroxyl groups of the cyclodextrin. This way of synthesis, known as the polyol process, has often been used with native cylodextrins [START_REF] Dong | Polyol synthesis of nanoparticles: status and options regarding metals, oxides, chalcogenides, and non-metal elements[END_REF]. [START_REF] Willner | Characterization of palladium-.beta.-cyclodextrin colloids as catalysts in the photosensitized reduction of bicarbonate to formate[END_REF] proceeded by this way for the synthesis of -cyclodextrin-stabilized palladium nanoparticles used for the efficient photosensitized reduction of sodium bicarbonate to sodium formate.

The development of cyclodextrin-based gold nanoparticles has grown up since two decades [START_REF] Wang | Cyclodextrin capped gold nanoparticles (AuNP@CDs): from synthesis to applications[END_REF]), but their use in catalysis is still limited. Qi reported, in 2009, the synthesis of -cyclodextrin-capped gold nanoparticles by reduction of HAuCl4 in alkaline solution at 60 °C [START_REF] Huang | Facile Synthesis and One-Dimensional Assembly of Cyclodextrin-Capped Gold Nanoparticles and Their Applications in Catalysis and Surface-Enhanced Raman Scattering[END_REF]. The concentrations of both -cyclodextrin and sodium hydroxide played important roles. The smallest diameter size was obtained using the highest cyclodextrin concentration. For pH values lower than 10.5, -cyclodextrin failed to reduce the gold precursor, while too high pH (pH =12) led to the particle aggregation. These colloidal suspensions were efficient hydrogen activators for the reduction of 4-nitrophenol using sodium borohydride. The kinetic reaction rate constants were shown to be inversely proportional to the particle size. [START_REF] Zhao | Three-in-One: Sensing, Self-Assembly, and Cascade Catalysis of Cyclodextrin Modified Gold Nanoparticles[END_REF] synthesized Au nanoparticles at 100 °C in a phosphate buffer solution using native cyclodextrins. The reduction of the gold precursor by the hydroxyl functions of cyclodextrins was unambiguously confirmed by X-ray photoelectron spectroscopy. The resulting oxidation of some hydroxyl groups of cyclodextrin led to the formation of carboxylated groups, which could further enhance the stabilization of the gold nanoparticles. These latter exhibited mimicking properties of both glucose oxidase and horseradish peroxidase. Noël et al. (2020) synthesized, in alkaline medium, gold nanoparticles stabilized by a mixture of native large-ring cyclodextrins, ranging from 8 to 38 glucopyranose units with different proportions, corresponding to 17 ± 6 glucopyranose units in average [START_REF] Ueda | Large-ring Cyclodextrins[END_REF].

Transmission electron microscopy images showed spherical particles from 10.1 to 13.2 nm. These gold nanoparticles were stable in time and their catalytic activity was evaluated in the reduction of 4-nitrophenol. This large-ring cyclodextrins mixture would have the advantage of impeding the formation of an inclusion complex with the aromatic substrate as compared to traditional cyclodextrins, such as -cyclodextrin. The lower activity of -cyclodextrinstabilized Au nanoparticles was indeed correlated to a too stable complex.

Silver nanoparticles could also be synthesized via the polyol process strategy with native cyclodextrins and the progress of reduction could be followed by the appearance of a surface plasmon resonance band by UV-visible spectrophotometry [START_REF] Pandiarajan | Properties, synthesis and toxicity of silver nanoparticles[END_REF].

In particular, γ-cyclodextrin was successfully used as reducing and stabilizing agent for the synthesis of Ag nanoparticles for the enhancement of the antibacterial efficiency of chloramphenicol [START_REF] Gannimani | γ-Cyclodextrin capped silver nanoparticles for molecular recognition and enhancement of antibacterial activity of chloramphenicol[END_REF]. This effect was probably due to supramolecular interactions occuring between cyclodextrin and chloramphenicol as evidenced by 1 H nucleus magnetic resonance spectroscopy (NMR) measurements. [START_REF] Li | Green synthesis of β-CD-functionalized monodispersed silver nanoparticles with ehanced catalytic activity[END_REF] studied the synthesis of silver nanoparticles in alkaline medium in the presence of native -cyclodextrin at room temperature. According to the authors, the formation and stabilization of the silver nanoparticles were not only due to cyclodextrin deprotonation, but also to the insoluble silver oxide formation, which allowed a better control of the growth of the silver nanoparticles with a 10-25 nm size range. These nanoparticles improved the rate of the reduction of 4-nitrophenol thanks to beneficial interactions between the oligosaccharide and the nitroaromatic, which was already observed by [START_REF] Gannimani | γ-Cyclodextrin capped silver nanoparticles for molecular recognition and enhancement of antibacterial activity of chloramphenicol[END_REF].

Macrocyclic compounds, such as native cyclodextrins and cucurbiturils, were used as stabilizers of pre-formed gold nanoparticles synthesized by chemical reduction of gold precursor with NaBH4 (del Pozo et al. 2018). Transmission electron micrographs showed spherical gold nanoparticles, which were organized into non-ordered structures with mean diameters ranging from 5.8 nm to 10.7 nm. The different stabilizers were compared by evaluating the catalytic activities with normalized rate constants depending on the gold amount or depending on the total gold surface. According to all these experiments, -cyclodextrin showed the best efficiency.

All previous examples described the metal nanoparticles synthesis in pure aqueous medium but others solvents can also be used. As an example, the synthesis of iron-platinum core-shell nanoparticles was investigated in organic media [START_REF] Mori | Catalytically active, magnetically separable, and water-soluble FePt nanoparticles modified with cyclodextrin for aqueous hydrogenation reactions[END_REF]). After thermal decomposition of Fe(CO)5 in dibenzylether, followed by the chemical reduction of Pt(acac)2 in an oleic acid/oleyamine mixture, the resulting core-shell nanoparticles were precipitated in ethanol and redispersed in hexane. In order to perform hydrogenation reactions in aqueous medium, -cyclodextrin was used as shuttle for bringing metallic particles from the non-polar solvent into water. These nanoparticles were more efficient for the hydrogenation of allylic alcohol in water than those in organic solvent without cyclodextrin, which could be explained by host-guest complexation between the substrate and the cyclodextrin. [START_REF] Zhao | β-cyclodextrin-capped palladium nanoparticle-catalyzed ligandfree Suzuki and Heck couplings in low-melting β-cyclodextrin/NMU mixtures[END_REF] investigated the palladium-catalyzed Suzuki carbon-carbon coupling in different carbohydrate-urea-organic salt mixtures as solvents, called low melting mixtures.These types of solvents can be simply prepared by mixing high melting-point organic materials, which easily associated each other through hydrogen-bond interactions by forming an eutectic mixture [START_REF] Zhang | Deep eutectic solvents: syntheses, properties and applications[END_REF][START_REF] Achkar | Basics and properties of deep eutectic solvents: a review[END_REF]. In particularly, the cyclodextrin/N-methylurea mixture was successfully used as solvent for Heck and Suzuki carbon-carbon coupling reactions at 85 °C using -cyclodextrin-prestabilized Pd nanoparticles.

The recyclability was studied and the catalytic system preserved its activity and stability after four runs.

Another type of non-conventional media has been developed and consisted into performing the reaction without any solvent. In this case, where the catalyst and the substrate are in the solid state, mechanochemistry appeared as an interesting alternative strategy towards greener chemical process [START_REF] Do | Mechanochemistry: A Force of Synthesis[END_REF]. It was reported that gold nanoparticles could be mechano-synthesized in the presence of cyclodextrins and were applied for the solid-state reduction of substituted nitrobenzene derivatives with NaBH4 [START_REF] Menuel | Cyclodextrins as e ff ective additives in AuNP-catalyzed reduction of nitrobenzene derivatives[END_REF]. Several cyclodextrins and saccharide additives were tested to afford well-dispersed gold nanocatalysts.

Except in the case of methylated saccharides, all additives improved the catalytic activity of Au nanoparticles, especially in the case of native -cyclodextrin, which resulted in the smallest average particles size. Recycling experiments were performed and no loss of the activity was noticed after three successive runs.

Metal Nanoparticles stabilized by functionalized cyclodextrins

Thiolated cyclodextrins

The first examples of metallic nanoparticles stabilized by thiolated cyclodextrins were reported by the Kaifer's group at the beginning of the 21 st century. Thus, [START_REF] Alvarez | Water-soluble platinum and palladium nanoparticles modified with thiolated β-cyclodextrin[END_REF] studied the stabilization of platinum and palladium nanoparticles by heptakis-(6-mercapto-6-deoxy)-cyclodextrin (per-6-thio--CD) for catalytic hydrogenation in water. The disappearance of the S-H stretching peak at 2560 cm -1 by Fourier-transform infrared spectroscopy (FTIR) proved the interaction of the thiolated group with the metal nanoparticle. After six hours of reaction, they showed that allylamine was totally converted in propylamine irrespective of the metal chosen. The decrease in the particle size in the case of palladium was investigated by optimizing the amount of per-6-thio--CD. These new metal nanoheterogeneous systems were tested in the Suzuki coupling [START_REF] Strimbu | Cyclodextrin-Capped Palladium Nanoparticles as Catalysts for the Suzuki Reaction[END_REF]) and hydrogenation reactions [START_REF] Liu | Tuning the Catalytic Activity of Cyclodextrin-Modified Palladium Nanoparticles through Host-Guest Binding Interactions[END_REF]). In the latter case, the authors were able to tune the catalytic activity of per-6-thio--CD-stabilized Pd nanoparticles by ordering host-guest interactions between the receptors and chosen guests in the solution.

Per-6-thio--CD (heptakis-(6-mercapto-6-deoxy)--cyclodextrin) was also employed to synthesize catalytically active palladium nanoparticles for the hydrogenation of isophorone [START_REF] Mhadgut | A metal nanoparticle-based supramolecular approach for aqueous biphasic reactions[END_REF]. Interestingly, the known property of phase transfer of the cyclodextrin was proved by introducing adamantane as a competitive substrate during the catalytic test.

Shortly after, these authors extended the use of these cyclodextrin-based nanocatalysts in the copper and phosphine-free Sonogashira reaction in water [START_REF] Xue | Sonogashira reactions catalyzed by watersoluble, $β$-cyclodextrin-capped palladium nanoparticles[END_REF]). Good to excellent isolated yields were obtained, highlighting the potential of these particles in catalysis.

More recently, aqueous copper nanoparticles with an average size of 2 nm were synthesized using mono-(6-mercapto-6-deoxy)--cyclodextrin as stabilizer and hydrazine as reducing agent (Zhong et al. 2016). Several shifts were observed in the FTIR spectrum of the synthesized nanoparticles, indicating the interactions between the cyclodextrin and the nanoparticle surface.

These Cu nanoparticles showed an enzyme-like behavior with a beneficial effect of the cyclodextrin on the reaction rate.

The development of hybrid catalysts based on the combination of metal nanoparticles with homogeneous complexes is possible but still rare. [START_REF] Li | An Artificial Supramolecular Nanozyme Based on $β$-Cyclodextrin-Modified Gold Nanoparticles[END_REF] synthesized gold nanoparticles stabilized by per-6-thio--CD (heptakis-(6-mercapto-6-deoxy)--cyclodextrin) and used them as support for triethylenetetramine-adamantane based copper complexes via supramolecular assembly for the catalytic cleavage of 4, 4'-dinitrophenylcarbonate. The kinetic analyses showed a typical Michaelis-Menten profile. A synergistic action between the catalytic centers and the three-dimensional structure of gold nanoparticles was emphasized. A similar approach was conducted in the presence of per-6-thio--CD with platinum nanoparticles and an iridium complex for the photocatalytic production of hydrogen from water [START_REF] Carballada | Variation of the Viologen Electron Relay in Cyclodextrin-Based Self-Assembled Systems for Photoinduced Hydrogen Evolution from Water[END_REF]) (Fig. 1).

Fig. 1 Self-assembled three components catalytic system for the photo-induced electron transfer. This catalytic system is including an iridium complex as photosensitizer, methyl viologen as electron relay and platinum nanoparticles coated by per-6-thio--CD (heptakis-(6-mercapto-6-deoxy) -cyclodextrin) as catalyst (adapted from [START_REF] Carballada | Variation of the Viologen Electron Relay in Cyclodextrin-Based Self-Assembled Systems for Photoinduced Hydrogen Evolution from Water[END_REF]).

Alkylated cyclodextrins

Alkylated cyclodextrins such as randomly methylated cyclodextrins [START_REF] Fenyvesi | Randomly methylated β-cyclodextrin derivatives enhance taxol permeability through human intestinal epithelial Caco-2 cell monolayer[END_REF]) and hydroxypropylated cyclodextrins [START_REF] Pitha | Hydroxypropyl-β-cyclodextrin: preparation and characterization; effects on solubility of drugs[END_REF]) have commonly been used for the stabilization of metal nanoparticles. The first examples of metal nanoparticles stabilized by randomly methylated cyclodextrins were reported by Monflier and Roucoux (Nowicki et al. 2006;[START_REF] Denicourt-Nowicki | Methylated cyclodextrins: an efficient protective agent in water for zerovalent ruthenium nanoparticles and a supramolecular shuttle in alkene and arene hydrogenation reactions[END_REF]. According to the transmission electron microscopy images, ruthenium nanoparticles were dispersed into non-ordered superstructures. They were evaluated in the catalytic hydrogenation of various hydrophobic unsaturated substrates under mild experimental conditions, such as room temperature and 1 bar of H2. Interestingly, the hydrogenation of aromatic rings was shown to depend on both the size of the cyclodextrin and its substitution degree. These results can be explained by the more or less important interactions between the cyclodextrin and the substrate. Some modifications in the synthesis procedure of the randomly methylated -cyclodextrinstabilized Ru nanoparticles were also performed (Chau et al. 2013b). The reducing agent was changed as well as the stabilizing agent introduction moment. NaBH4 was replaced by H2 and randomly methylated -cyclodextrin was added just after the reduction step contrary to others syntheses where the reducing step is performed in the presence of the stabilizing agent. As These methodologies were extended to ruthenium nanoparticles stabilized by randomly methylated -cyclodextrin grafted with chiral amino-acid moieties to be evaluated for the asymmetric hydrogenation of prochiral model substrates (Chau et al. 2013a). Despite the interesting obtained catalytic activities, modest enantiomeric excesses were measured which could be explained by a too weak cooperation between the substrate and the amino acid-grafted cyclodextrin-stabilized Ru nanoparticle during the hydrogenation reaction.

Concerning hydroxypropylated cyclodextrins, the pioneer work of Malta was devoted to the aqueous synthesis of palladium nanoparticles stabilized by 2-hydroxypropyl -cyclodextrin, without using other reducing agent, at room temperature [START_REF] Senra | Hydroxypropyl-α-Cyclodextrin-Capped Palladium Nanoparticles: Active Scaffolds for Efficient Carbon-Carbon Bond Forming Cross-Couplings in Water[END_REF]. Interestingly, the authors reported that this cyclodextrin could play several roles, including capping and reducing agent. The spherical particles in the size range of 2-7 nm were evaluated by means of palladocatalyzed carbon-carbon coupling reactions in water and showed in all cases good catalytic activities.

Deeper studies, such as the influence of the 2-hydroxypropyl cyclodextrin size and the Pd-tocyclodextrin ratio were performed in order to observe eventual changes in the size of the palladium particles tested in the Suzuki-Miyaura reaction [START_REF] Senra | Selectivity Studies Towards the Synthesis of Novel Biaryl Ureas by (Hetero)Nanocatalysis: Size Control and Support Effects[END_REF]. The best performances were obtained using 2-hydroxypropyl -cyclodextrin, and this was attributed to its strongest complexation capacity.

2-hydroxypropyl cyclodextrin was also used for the synthesis of silver nanoparticles for applications in the reduction of 4-nitrophenol [START_REF] Devi | Self-assembly of Ag nanoparticles using hydroxypropyl cyclodextrin: Synthesis, characterisation and application for the catalytic reduction of p-nitrophenol[END_REF]. The catalytic results were correlated with the size and morphology of the metallic particles. [START_REF] Bhoi | Cyclodextrin encapsulated monometallic and inverted core-shell bimetallic nanoparticles as efficient free radical scavengers[END_REF] prepared spherical monodispersed silver and gold nanoparticles dispersed in alkaline medium by chemical reduction of AgNO3 and HAuCl4 respectively, and by using native cyclodextrins or 2-hydroxypropyl -cyclodextrin as both stabilizing and reducing agent. Whatever the cyclodextrin which was used, no significant difference of the size has been noticed. All these colloidal suspensions showed radical scavenging behavior from the quenching of 2, 2'diphenyl-1-picrylhydrazyl light adsorption.

The combination of different kinds of stabilizing effects has also been studied to improve the stability and recyclability of metal nanoparticles. For instance, cyclodextrins functionalized with a long alkyl chain carboxylate group (oleic succinyl β-cyclodextrin) was proved to be an efficient stabilizer of ruthenium nanoparticles in aqueous medium [START_REF] Cocq | Anionic Amphiphilic Cyclodextrins Bearing Oleic Grafts for the Stabilization of Ruthenium Nanoparticles Efficient in Aqueous Catalytic Hydrogenation[END_REF]). The resulting Ru nanoparticles were evaluated in the hydrogenation of petrosourced and biosourced unsaturated compounds under mild experimental conditions and, notably, could be easily recovered and reused up to nine runs without any loss of activity and selectivity, thus demonstrating their high robustness.

Metal nanoparticles stabilized by polymers with cyclodextrins

Physical mixtures of polymer and cyclodextrin

Polymers are known to efficiently stabilize metal nanoparticles in water [START_REF] Li | Size Effects of PVP-Pd Nanoparticles on the Catalytic Suzuki Reactions in Aqueous Solution[END_REF][START_REF] Gacem | Effect of solvent polarity on the assembly behavior of PVP coated rhodium nanoparticles[END_REF][START_REF] Bhol | Polymer-matrix stabilized metal nanoparticles: Synthesis, characterizations and insight into molecular interactions between metal ions, atoms and polymer moieties[END_REF]. The possibility of using cyclodextrins as additives in the synthesis of ruthenium nanoparticles stabilized by poly(vinylpyrrolidone) was carefully studied [START_REF] Herbois | Cyclodextrins as growth controlling agents for enhancing the catalytic activity of PVP-stabilized Ru(0) nanoparticles[END_REF]. A decrease in the mean particle size was clearly noticed, especially with the methylated cyclodextrins. In line with the results on the hydrogenation of furfural, only a very small amount of cyclodextrin was required to highlight enhancement of catalytic activity.

It appeared that each part of the catalytic system had a precise role: the cyclodextrin could act as a particle size controller and poly(vinylpyrrolidone) as a long-term stabilizer.

In another study, [START_REF] Kuklin | New approach for highly selective hydrogenation of phenol to cyclohexanone: Combination of rhodium nanoparticles and cyclodextrins[END_REF] investigated the role of cyclodextrin on the selective hydrogenation of phenol to cyclohexanone in two different solvents (water and an ammoniumbased ionic liquid), using rhodium nanoparticles stabilized by poly(acrylic acid). In the presence of cyclodextrin, the selectivity improvement was attributed to the fact that the cyclohexenol, which was obtained as an intermediate, could form inclusion complexes with it (Fig. 2). If the reaction occurred in water, cyclodextrin could not be immobilized in the nearsurface layer around the Rh nanoparticles and the major product which was obtained was cyclohexanol. In N-hexyltriethylammonium bromide ionic liquid, cyclodextrin stayed on the metallic surface and could desorb the cyclohexanol via stable inclusion complex. In the path (a), phenol was firstly reduced to cyclohexenol. Just a small proportion of cyclohexenol can desorb from the catalyst and equilibrate itself to cyclohexanone which do not adsorb on catalyst and can not be reduced to cyclohexanol. In the path (b), with cyclodextrin, more amount of cyclohexenol can desorb from the catalyst by making an inclusion complex and avoiding its hydrogenation towards cyclohexanol (adapted from [START_REF] Kuklin | New approach for highly selective hydrogenation of phenol to cyclohexanone: Combination of rhodium nanoparticles and cyclodextrins[END_REF] The aerobic oxidative kinetic resolution of racemic secondary alcohols catalyzed by poly(vinylpyrrolidone)-stabilized gold nanoparticles in presence of aminated cyclodextrins with different cavity sizes was studied [START_REF] Hirano | Asymmetric Aerobic Oxidation of Secondary Alcohols Catalyzed by Poly( N -vinyl-2-pyrrolidone)-stabilized Gold Clusters Modified with Cyclodextrin Derivatives[END_REF]. Amino -cyclodextrin gave the best results both in activity and enantiodifferenciation. The authors suggested that the enantioselective oxidation could be explained by an eventual inclusion of the alcohol into the chiral cavity of the cyclodextrin.

In another interesting study, Au/Ag bimetallic core-shell nanoparticles with different core diameters were synthesized by [START_REF] Haldar | Core-Size-Dependent Catalytic Properties of Bimetallic Au/Ag Core-Shell Nanoparticles[END_REF] through a βcyclodextrin/poly(vinylpyrrolidone) assisted synthetic procedure. It was found that the catalytic activity during the catalytic reduction of 4-nitrophenol was strongly influenced by the size of the core. Noteworthy, the core-shell system with a core size of about 100 nm was 12 times more efficient than that of monometallic gold nanocatalyst of the same size, clearly revealing the impact of this core-shell structure on the catalytic activity.

More recently, a novel hybrid nanocomposite prepared from 2-hydroxypropyl -cyclodextrin and alginate polymer was used as stabilizer to synthesize silver nanoparticles with aqueous extract of Jasminum subtriplinerve leaves as reducing agent [START_REF] Nguyen | Biosynthesized AgNP capped on novel nanocomposite 2-hydroxypropyl-β -cyclodextrin/alginate as a catalyst for degradation of pollutants[END_REF]. The spherical Ag nanoparticles with an average particle size of 13.5 nm were found to be active in the catalytic degradation of methyl orange, rhodamine and 4-nitrophenol. A thermosensitive catalytic system based on silver nanoparticles immobilized into a double network hydrogels based on poly(N-isopropylacrylamide) and 2-acrylamido-2-methylpropane sulfonic acid was successfully prepared [START_REF] Wang | Synergetic catalytic effect of α-cyclodextrin on silver nanoparticles loaded in thermosensitive hydrogel[END_REF]. The lower critical solution temperature (LCST) of this hydrogel was determined by swelling investigation and its value is 308.65 K. The catalytic performances of these supported silver nanoparticles were evaluated in the 4-nitrophenol reduction. The post-addition of -cyclodextrin improved the catalytic activity by forming a complex with the 4-nitrophenol. Interestingly, the rate constant increased as the temperature increased from 298.15 to the LCST where a dramatic decrease was observed, which could be caused by the thermosensitive shrink-age of the co-polymer hydrogels.

Cyclodextrin-grafted polymers

The first example of metal nanoparticles stabilized by a cyclodextrin-grafted polymer was referenced by [START_REF] Shiraishi | Preparation and Catalysis of Poly(βcyclodextrin)-Stabilized Palladium Nanoparticles[END_REF]. Palladium nanoparticles, synthesized by refluxing an ethanol/water solution of palladium acetate, were evaluated in the hydrogenation of unsaturated aliphatic and aromatic carboxylic acids. The host-guest properties of the grafted-cyclodextrin polymer were defined as key factor for the enhanced catalytic activity of the Pd nanoparticles.

Platinum colloids (1-6 nm), synthesized by H2PtCl6 photoreduction in the presence of the same cyclodextrin-grafted polymers with different cavity sizes, were tested in the superoxide anion quenching reaction [START_REF] Taylor | Syntheses of poly ( cyclodextrin ) -stabilised metal nanoparticles and their quenching abilities of active oxygen species[END_REF]). These Pt nanoparticles were more active than the platinum nanoparticles stabilized by polyacrylic acid alone. [START_REF] Noël | Carboxylated polymers functionalized by cyclodextrins for the stabilization of highly efficient rhodium(0) nanoparticles in aqueous phase catalytic hydrogenation[END_REF] studied the aqueous phase synthesis of rhodium nanoparticles in the presence of poly(mono(-cyclodextrin-2-yl)-maleate-alt-maleate-alt-methylvinylether), a polymer containing carboxylate and -cyclodextrin moieties. Their catalytic properties were evaluated in the hydrogenation of various alkene and arene derivatives. The beneficial effect of the cyclodextrin-grafted polymer was correlated with both the average particle size and the dispersion of the Rh nanoparticles. The recyclability was studied during five runs with no loss of stability and activity. The robustness of the Rh catalyst was further confirmed by the absence of metal leaching in the organic phase using inductively coupled plasma spectrometry and also by the conservation of the size and morphology of the particles using transmission electron microscopy.

A deeper study on the stability and catalytic activity of the polycarboxylate-stabilized rhodium nanoparticles was performed by modifying the pH value of the solution before the reduction step and by using different ratios of grafted cyclodextrin [START_REF] Noël | Effective catalytic hydrogenation of fatty acids methyl esters by aqueous rhodium(0) nanoparticles stabilized by cyclodextrin-based polymers[END_REF]. According to transmission electron micrographs, a homogeneous dispersion of the metal nanoparticles was observed for the lowest initial pH value and, on the contrary, unstable colloidable suspensions were obtained for the highest pH value. The catalytic performances were evaluated in the hydrogenation of methyl linoleate under 10 bar of hydrogen at 30 °C. The rhodium nanoparticles stabilized by the cyclodextrin-grafted-polymer showed better activities than the colloidal system without cyclodextrins. Moreover, the grafted polymer allowed to get stable metal particles after catalysis, contrary to the physical mixture, which led to stable emulsions that were very difficult to separate and completely prevented the metal nanoparticles recovery. [START_REF] Herbois | Ruthenium-containing β-cyclodextrin polymer globules for the catalytic hydrogenation of biomass-derived furanic compounds[END_REF] reported another work, which consisted in the encapsulation of Ru nanoparticles (1-2 nm) into a water soluble crosslinked-β-cyclodextrin polymer derived from the polycondensation of β-cyclodextrin with citric acid. Due to electrostatic repulsions, the presence of the carboxylate moieties in the polymer backbone helped to stabilize microglobules, in which the metallic nanoparticles were confined. The catalytic efficiency of this hybrid system for the aqueous phase hydrogenation of furfural and 3-(2-furyl) acrolein was confirmed at 10 bar pressure and 30 °C. It was suggested that this structure with a microglobular character played a key role in the superior catalytic activity, acting as microreactors.

Another type of -cyclodextrin-polymer was used to stabilize ruthenium nanoparticles in aqueous medium. This polymer was synthesized by reacting -cyclodextrin with epichlorohydrin and glycidyl trimethyl ammonium chloride in alkaline medium (Noël et al. 2017). The main advantage of this crosslinked--cyclodextrin-polymer was its ability to stabilize metal nanoparticles in a wide range of pH (from 3 to 10). The resulting Ru nanoparticles were tested in the aqueous phase hydrogenation of different petro and biosourced substrates and showed catalytic activities close to the ruthenium nanoparticles stabilized by the citric acid-based β-cyclodextrin-polymer [START_REF] Herbois | Ruthenium-containing β-cyclodextrin polymer globules for the catalytic hydrogenation of biomass-derived furanic compounds[END_REF]. But more importantly, thanks to their stability in acidic aqueous medium, these Ru nanoparticles could also be employed for the hydrogenation of unsaturated carboxylic acids, especially linoleic and oleic acids. [START_REF] Vasconcelos | Preparation of Aun quantum clusters with catalytic activity in β-cyclodextrin polyurethane nanosponges[END_REF] reported the synthesis of size-controlled gold clusters using a polyurethane nanosponge as a template, followed by the core-etching of glutathione-capped Au nanoparticles. It was established that depending on the gold concentration and the core etching time, clusters with only 11-15 atoms were formed. The catalytic performance of these gold clusters for the reduction of 4-nitrophenol was studied. It was found that the Langmuir-Hinshelwood kinetic model satisfactorily fitted the reaction rates and that the reaction proceeded with no induction time.

Very recently, Silva et al. ( 2020) reported the use of a star-shaped amino-functionalized poly(glycerol methacrylate)-based -cyclodextrin, both as reducing and stabilizing agent for the gold nanoparticles synthesis in water. Well-dispersed spherical nanoparticles were obtained by optimizing the molar ratio of the tertiary amine to gold metal. Tested in the catalytic reduction of 4-nitrophenol, these star-shaped polymer-stabilized gold nanoparticles showed a higher kinetic constant than for control Au materials, although no enhancement was measured as compared to cetyltrimethyl ammonium bromide-stabilized Au nanoparticles system.

Metal nanoparticles stabilized by cyclodextrin-based inclusion complexes

The combination of cyclodextrins with other stabilizing agents, e.g. alkyl ammonium salts, phosphanes, star-shaped ferrocenyl-triazolyl-arenes, was also reported for the synthesis of metallic nanoparticles dispersed into the aqueous phase. Moreover, this kind of combinations could lead to the development of cyclodextrin-based hydrogels.

Quaternary ammonium-based salts

Quaternary ammonium salts are reported to be good metallic nanoparticles stabilizers [START_REF] Schulz | Aqueous Rhodium Colloidal Suspension in Reduction of Arene Derivatives in Biphasic System: a Significant Physico-chemical Role of Surfactant Concentration on Catalytic Activity[END_REF]. Moreover, this family of surfactants is also well-known to strongly interact with -cyclodextrin derivatives by forming inclusion complexes. Thus, randomly methylated cyclodextrin, whose cavity is occupied by an ammonium salt bearing a lipophilic chain, the chloride salt of N, N-dimethyl, N-hexadecyl, N-(2-hydroxyethyl) ammonium (HEA16Cl), was successfully used for the synthesis of aqueous Ru nanoparticles [START_REF] Hubert | Catalytically active nanoparticles stabilized by host-guest inclusion complexes in water[END_REF]). Its catalytic activity was examined in the hydrogenation of several aromatic compounds. Whatever the substrate tested, randomly methylated -cyclodextrin/HEA16Cl-stabilized ruthenium nanoparticles were much more active than those stabilized by the surfactant alone. The difference in activity was attributed to the difference in the dynamic organization of the supramolecular stabilizer around the metal surface during the catalytic reaction. This concept was extended to Rh nanoparticles protected by different combinations of methylated cyclodextrins derivatives and ammonium surfactants [START_REF] Chau | Active hydrogenation Rh nanocatalysts protected by new self-assembled supramolecular complexes of cyclodextrins and surfactants in water[END_REF]. It was proved that these systems could act as efficient catalysts for promoting biphasic hydrogenation of various substrates under mild experimental conditions, including ketones, olefins and aromatics, with high catalytic activities.

In another study, in the presence of a photoswitchable chemical compound, gold nanoparticles (3.6 nm) were pre-stabilized by a per-6-thio--CD (hexakis-(6-mercapto-6-deoxy) cyclodextrin) and studied in aqueous biphasic medium (water/toluene) [START_REF] Peng | Reversible Phase Transfer of Nanoparticles Based on Photoswitchable Host-Guest Chemistry[END_REF]). The addition of a trans-azobenzene-based surfactant was shown to lead to a spontaneous phase transfer of the Au nanoparticles from toluene to water under UV irradiation. Indeed, a trans-tocis photoisomerization occurred and resulted in the dissociation of the per-6-thio--CD/azobenzene complex (Fig. 3). These gold nanoparticles were tested in the 4-nitrophenol reduction. A judicious recovery and recycling can easily be achieved under a visible irradiation which bring the nanoparticles back in the organic phase.

Phosphanes

Water soluble phosphanes or diphosphanes have been used as ligands for metal nanoparticles in water [START_REF] Debouttière | An organometallic approach for the synthesis of water-soluble ruthenium and platinum nanoparticles[END_REF][START_REF] Guerrero | Alkyl sulfonated diphosphinesstabilized ruthenium nanoparticles as efficient nanocatalysts in hydrogenation reactions in biphasic media[END_REF]. Moreover, cyclodextrins have been traditionnaly used as mass-transfer promoters in aqueous biphasic catalysis using water-soluble organometallic complexes with sulfonated phosphines [START_REF] Monflier | First evidence of molecular recognition between cyclodextrins and a water-soluble ligand used in aqueous phase organometallic catalysis[END_REF].The idea was to combine the advantages of a sulfonated diphosphine as water soluble stabilizer of nanoparticles with a cyclodextrin for its supramolecular control effects and shuttle property in aqueous biphasic phase catalysis.

In line with this context, ruthenium nanoparticles were synthesized in THF by hydrogen reduction of the complex (1,5-cyclooctadiene)(1,3,5-cyclooctatriene)ruthenium(0), then stabilized by the association of 1,4-bis[(di-m-sulfonatophenyl)phosphine]butane and randomly methylated -cyclodextrin [START_REF] Guerrero | Efficient Ruthenium Nanocatalysts in Liquid-Liquid Biphasic Hydrogenation Catalysis: Towards a Supramolecular Control through a Sulfonated Diphosphine-Cyclodextrin Smart Combination[END_REF][START_REF] Guerrero | Organometallic synthesis of water-soluble ruthenium nanoparticles in the presence of sulfonated diphosphines and cyclodextrins[END_REF]. After synthesis, the ruthenium nanoparticles were isolated by precipitation and re-dispersed in water. Transmission electron images showed well-dispersed particles with an average diameter between 1.2 and 1.5 nm.

Resonance shifts in 1 H and 31 P NMR in D2O confirmed the presence of a weak interaction between the dppbs and randomly methylated -cyclodextrin. The catalytic performances of the diphosphine-stabilized ruthenium nanoparticles were evaluated in the hydrogenation of model compounds. Based on turn over frequency values, an activity improvement was clearly observed with increasing the initial amount of cyclodextrin.

2.4.3. Star-shaped ferrocenyl-triazolyl-arenes [START_REF] Astruc | Click Dendrimers and Triazole-Related Aspects: Catalysts, Mechanism, Synthesis, and Functions. A Bridge between Dendritic Architectures and Nanomaterials[END_REF] exploited mono-, bis-, and tris(4-ferrocenyl-1,2,3-triazolylmethyl)arene-βcyclodextrin adducts for stabilizing palladium nanoparticles (Fig. 4). The ferrocene moieties, which could readily form inclusion complexes with native -cyclodextrin, facilitated the dispersion of the catalytic system in water, while the triazolyl groups could serve as ligands for stabilization of the Pd nanoparticles. The transmission electron images showed small nanoparticles with 5-6 nm particle size. The comparison of the catalytic performances of these three catalysts established that the bolaamphiphile-like bis-and tris-ferrocenyl--cyclodextrins systems led to much better pre-catalysts, indicating the efficiency of encapsulation of a hydrophobic catalytic system with two or three peripheral water-solubilizing cyclodextrin caps.

A series of ligands containing bifunctional 1,2,3-triazole was also developed for the stabilization of water-solubilized gold nanoparticles of 3.0-11.2 nm (Li et al. 2014a). The catalytic activity of the gold nanoparticles stabilized by triazole groups were superior to that of the thiolate Au nanoparticles. This observation was attributed to the easier removal of the triazole ligands by the substrate from the fine gold nanoparticles. In literature, more and more sophisticated catalytic systems were developed in order to furnish active and selective catalysts, by playing on the stability of the metal nanoparticles as well as their confinement with the substrate. Among the reported systems, hydrogel media have become very popular for a wide range of applications including medicine, materials and catalysis. The embedment of metal nanoparticles into supramolecular-structured hydrogels based on host-guest interactions in the presence of cyclodextrins has been addressed in several research works.

For instance, Léger et al. ( 2012) succeeded in developing a thermoresponsive hydrogel for the stabilization of catalytically active ruthenium nanoparticles for hydrogenation reactions. More precisely, once metal nanoparticles were embedded into the supramolecular matrix, the system could be heated above the sol-gel transition temperature (39 °C) to give a sol phase, where the catalytic reaction could take place. The Ru nanoparticles stabilized in the hydrogel demonstrated a pronounced catalytic activity towards hydrogenation of terminal alkenes carried out at 50 °C. After cooling to ambient temperature, the hydrogel spontaneously returned to the gel state and consequently the hydrogenated products could be easily recovered as well as the catalyst. It is worth noting that transmission electon microscopy analysis showed that the dispersion of Ru nanoparticles in the hydrogel network remained intact after catalysis.

The complexation of guest molecules in the cyclodextrin cavity had also been studied with polymers. The research group of Harada had put forward the formation of inclusion complexes, called polypseudorotaxanes, between poly(ethyleneglycol) and native -cyclodextrin [START_REF] Li | Sol-Gel Transition during Inclusion Complex Formation between α-Cyclodextrin and High Molecular Weight Poly(ethylene glycol)s in Aqueous Solution[END_REF].

In order to immobilize the cyclodextrin along the polymer chain, a large number of polymers with stoppers had been reported. Thus, [START_REF] Wu | β -Cyclodextrin-Capped Polyrotaxanes : One-Pot Facile Synthesis via Click Chemistry and Use as Templates for Platinum Nanowires[END_REF] described the stabilization of platinum nanoparticles in a water/ethylene glycol mixture, in the presence of a polyrotaxane, in which native -cyclodextrins were threated onto poly(ethyleneglycol) chains terminated by two triazole--cyclodextrin units (Fig. 5). The platinum nanoparticles were tested in the reduction of 4-nitrophenol. The authors assumed that the high catalytic activity came from the high density and surface area of the platinum nanoparticles that had been adsorbed on the surface of the template. It was reported that silver nanoparticles could be embedded, through a two-step approach, in a supramolecular hydrogel made from the association of -cyclodextrin with a block copolymer Pluronic®F-68, composed of hydrophilic poly(oxyethylene) and hydrophobic poly(oxypropylene) units [START_REF] Ma | A novel route to in-situ incorporation of silver nanoparticles into supramolecular hydrogel networks[END_REF]). The first step consisted in the reduction of the silver ion in an aqueous solution of F-68. The second step involved the addition of -cyclodextrin to the colloidal suspension, yielding the formation of a gel phase due to the supramolecular selfassembly process. The beneficial effect of -cyclodextrin was evidenced in the catalytic reduction of methylene blue. [START_REF] Jia | Cyclodextrin modified microgels as "nanoreactor" for the generation of Au nanoparticles with enhanced catalytic activity[END_REF] reported a simple method for the immobilization of catalytically active gold nanoparticles in acrylate -cyclodextrin modified poly(N-vinylcaprolactam) microgel.

According to transmission electron images, monodispersed gold nanoparticles with an average diameter of 5-6 nm were obtained without addition of any reducing agent. The gold particles embedded within this microgel showed efficient catalytic activity for the reduction of aromatic nitro compounds.

More recently, Chevry et al. ( 2019) reported a similar strategy allowing the synthesis of gold nanoparticles embedded into a thermoresponsive hydrogel prepared by mixing the amphiphilic block copolymer Tetronic®90R4 and -cyclodextrin and their use in the catalytic hydrogenation of unsaturated compounds. No significant influence of the gold concentration was noticed on the mean diameters of Au nanoparticles which were always centered at about 7 nm. The recyclability study was validated in the styrene hydrogenation, showing that Au nanoparticles could be reused during five successive runs without any loss of activity.

Metal nanoparticles immobilization on supports in the presence of cyclodextrin

Cyclodextrins can also be applied for the stabilization and immobilization of metallic nanoparticles on support materials in order to furnish heterogeneous catalysts with uniform active sites and host-guest interactions. The different strategies can be classified into three categories (Fig. 6). The two-step methods, which are the most developed, imply that two consecutive stages occurred, where the intermediate material is fully characterized and/or isolated. This intermediate material can take different forms, such as pre-synthesized metal nanoparticles in presence or absence of cyclodextrins or a cyclodextrin-functionalized support where metal nanoparticles can be immobilized after the metal precursor reduction.

Some studies show, in the presence of cyclodextrins and support, the simultaneous reduction of the metallic salt, dispersion and immobilization of the resulting metal nanoparticles, wich are called one-step methods. The use of cyclodextrins as additive during the catalytic process in presence of supported metal nanoparticles has also be developed and called post-addition methods. In all these protocols, cyclodextrins can behave as multi-task agents, capable of inducing stabilizing, dispersing and molecular recognition effects. In another instance, [START_REF] Martina | Cyclodextrin-Grafted Silica-Supported Pd Nanoparticles : An Efficient and Versatile Catalyst for Ligand-Free C À C Coupling and Hydrogenation[END_REF] have developed a novel catalyst for ligand-free microwave-assisted carbon-carbon Suzuki and Heck couplings and semi-hydrogenation of the phenyl acetylene. To prepare the catalyst, silica was successively reacted with 3glycidoxypropyltrimethoxysilane, 10-undecynil-1-amine and 6-mono-azido-6-deoxy-β-cyclodextrin and then palladated. It was confirmed that introduction of cyclodextrin in the structure of the catalyst could increase the content of Pd. The catalyst showed good catalytic activities on a broad range of substrates and high level of recyclability.

Clays have also been utilized as supports for cyclodextrin-assisted nanoparticle stabilization.

For example, Sadjadi et al. synthesized a new support through covalent functionalization of halloysite outer surface with a melamine-based polymer, derived from the reaction of melamine and terephthalaldehyde, followed by its modification with β-cyclodextrin using a hydrothermal method (Sadjadi et al. 2018a). The final composite was employed for the immobilization of Pd nanoparticles to give a catalyst for the hydrogenation of nitroarenes in aqueous media. The catalyst was selective towards hydrogenation of the nitro group and showed high activity and recyclability. It was believed that cyclodextrin served as a stabilizer, ensuring the formation of fine Pd nanoparticles with a high degree of dispersion. Moreover, the formation of an inclusion complex between the -cyclodextrin unit and the nitro compound was confirmed.

In another instance, β-cyclodextrin was covalently attached on the outer surface of halloysite nanoclay as depicted in Fig. 7 and the resulting composite was successfully applied as a support for the immobilization of Pd nanoparticles [START_REF] Sadjadi | Palladium nanoparticles immobilized on cyclodextrin-decorated halloysite nanotubes: Efficient heterogeneous catalyst for promoting copper-and ligand-free Sonogashira reaction in water-ethanol mixture[END_REF]. The resulting catalyst exhibited high catalytic activity and recyclability for Sonogashira coupling reaction carried out with various halides and terminal alkynes under mild reaction conditions (60 °C). Subsequently, the same group reported that cyclodextrin nanosponges and halloysite nanoclay could be covalently conjugated to form a support for the immobilization of Pd nanoparticles (Sadjadi et al. 2018b). The palladated composite was further combined with graphitic carbon nitride, g-C3N4, through a hydrothermal process. The palladium leaching in the as-prepared ternary composite was efficiently suppressed and the composite exhibited high catalytic performance for the Sonogashira and Heck coupling reactions. A synergistic effect between the cyclodextrin nanosponge and g-C3N4 was prove. Actually, cyclodextrin nanosponge could form inclusion complex with the reagents and act as a phase transfer agent.

Magnetic support

Other class of materials that have extensively been investigated for immobilizing metal nanoparticles are magnetic supports. As an example, [START_REF] Sadjadi | A green approach to the synthesis of Ag doped nano magnetic γ-Fe2O3@SiO2-CD core-shell hollow spheres as an efficient and heterogeneous catalyst for ultrasonic-assisted A3 and KA2 coupling reactions[END_REF] reported the preparation of decorated core-shell hollow spheres, for which the magnetic core in -Fe2O3 was surrounded by an organic-inorganic hybrid silica shell functionalized by cyclodextrin units. The resulting magnetic system was applied as a suitable support for the immobilization of silver nanoparticles, reduced by hollyhock flower extract as a bio-based reducing agent. The resulting catalyst showed high activity for ultrasonic-assisted A 3 and KA 2 coupling reactions of acetylene, amines, aldehydes or ketones. Magnetic recovery of the catalyst, high recyclability and low Ag leaching were the advantages of the developed catalyst. It can be further noted that this research group was able to extend this chemistry to the preparation of another magnetic catalytic system composed of Pd nanoparticles decorating a core-shell structure with Fe3O4 as a core and hybrid cyclodextrin nanosponges-silica as a shell, rich in cyclodextrin units [START_REF] Sadjadi | Magnetic hybrid of cyclodextrin nanosponge and polyhedral oligomeric silsesquioxane: Efficient catalytic support for immobilization of Pd nanoparticles[END_REF].

In another attempt, [START_REF] Azaroon | Silver Nanoparticles Engineered β-Cyclodextrin/γ-Fe2O3@ Hydroxyapatite Composite: Efficient, Green and Magnetically Retrievable Nanocatalyst for the Aqueous Reduction of Nitroarenes[END_REF] also developed a magnetic composite in the presence of cyclodextrin. The synthetic approach involved the preparation of the magnetic hydroxyapatite, followed by the covalent bonding of β-cyclodextrin and the subsequent immobilization of Ag nanoparticles on the surface. It was suggested that the cyclodextrin in the structure of the support prevented Ag nanoparticles from aggregation and led to the formation of particles with sizes of 12-14 nm. They also established that the catalytic composite could selectively catalyze reduction of nitro-containing compounds to amines. Notably, the catalyst benefited from facile recovery and high recyclability thanks to its magnetic character.

Insoluble Organic support

Gopalan et al. ( 2006) employed a cyclic electrochemical process for the incorporation of Au nanoparticles into poly(aminothiophenol) and fabrication of a new glassy carbon electrode. In more detail, aminothiophenol and β-cyclodextrin formed an inclusion complex to furnish the electroactive layer on the electrode surface. Subsequently, gold nanoparticles, with size of about 10 nm, were uniformly deposited on the cyclodextrin-poly(aminothiophenol) layer through the electrolysis of the gold salt solution. The resulting material was revealed to be an efficient electrocatalyst for the oxygen reduction reaction.

Cyclodextrin-assisted procedure has also been utilized for supporting metal nanoparticles on various types of carbon materials, such as multi-walled carbon nanotubes or graphene-based compounds [START_REF] Jilani | Graphene and its derivatives: synthesis, modifications, and applications in wastewater treatment[END_REF]. As an instance, [START_REF] Yi | Palladium-nickel nanoparticles loaded on multi-walled carbon nanotubes modified with β-cyclodextrin for electrooxidation of alcohols[END_REF] prepared a cyclodextrindecorated support by sonication of β-cyclodextrin solution in the presence of a titania-modified multi-walled carbon nanotubes. Then, the authors applied the aforementioned support for the electrodeposition of bimetallic Pd-Ni nanoparticles. The use of β-cyclodextrin with the carbon material led to an enhanced control of the properties of the bimetallic nanoparticles, in terms of sizes and dispersions. These features resulted in higher electroactivity performances than those on control samples, with respect to the electrooxidation of some alcohols.

Cyclodextrin polymers can also be used for the modification of carbon supports [START_REF] Salipira | Carbon nanotubes and cyclodextrin polymers for removing organic pollutants from water[END_REF]. As an example, cyclodextrin nanosponges, prepared from the reaction of β-cyclodextrin with diphenyl carbonate, was covalently incorporated on multi-walled carbon nanotubes, providing a suitable support for the stabilization of Pd nanoparticles (Sadjadi et al. 2018c). The resulting catalyst was successfully applied for promoting the Sonogashira and Heck coupling reactions in aqueous media. The results of the comparative study evidenced that the combination of multi-walled carbon nanotubes and cyclodextrin nanosponges was beneficial for catalysis.

In another attractive research, [START_REF] Li | Shape-controlled synthesis of Pt nanoparticles via integration of graphene and β-cyclodextrin and using as a noval electrocatalyst for methanol oxidation[END_REF] developed the facile synthesis of a graphene nanosheets/β-cyclodextrin hybrid used as an efficient support for depositing Pt nanoparticles with a particular monodentric hydrangea-like morphology. To synthesize the catalyst, a simple wet chemical method was implemented, which included two consecutive steps: (i) the preparation of graphene nanosheets/β-cyclodextrin via reduction of graphene oxide by hydrazine in the presence of β-cyclodextrin, (ii) addition of PtCl6 2-and subsequent reduction by sodium borohydride or formic acid. The authors believed that the combination of β-cyclodextrin and graphene nanosheets could help loading nanodentrites Pt cluster with a uniform dispersion while providing a support with high surface area, conductivity and recognition property. Noteworthy, the amount of cyclodextrin was estimated to be 32 wt%, which made the hybrid system more water-dispersible. This hybrid system was successfully used for the oxidation of methanol with superior catalytic activity and CO tolerance compared to those of conventional catalysts.

Shortly after, [START_REF] Putta | Palladium nanoparticles on β-cyclodextrin functionalised graphene nanosheets: a supramolecular based heterogeneous catalyst for C-C coupling reactions under green reaction conditions[END_REF] proposed a similar kind of hybrid system based on palladium nanoparticles immobilized on graphene nanosheets/β-cyclodextrin support and successfully applied it for the Suzuki-Miyaura and Heck-Mizoroki cross-coupling reactions in aqueous medium. The synthesis scheme was very similar to that described earlier for Pt immobilized on the same support. The main difference came from the fact that PtCl6 2-was substituted by PdCl2 and NaBH4 by ethanol as in-situ reducing agent. The authors believed that the presence of cyclodextrin units on the graphene nanosheets played a role in the stabilization and dispersion of Pd nanoparticles by preventing agglomeration. Moreover, the cyclodextrin cavities could serve as inclusion sites for the reagents. Notably, the catalyst was reusable and could promote four successive reaction runs with no loss of the catalytic activity.

In another example, [START_REF] Ran | Synthesis of well-dispersive 2.0 nm Pd-Pt bimetallic nanoclusters supported on β-cyclodextrin functionalized graphene with excellent electrocatalytic activity[END_REF] reported an easy, mild and rapid procedure for the synthesis of monodispersed and ultrafine Pd-Pt bimetallic nanoclusters (2 nm). In a first step, the authors prepared the β-cyclodextrin-functionalized reduced graphene oxide -CD/rGO) support by mixing graphene oxide and β-cyclodextrin under adjusted pH (12). Then they used it for the immobilization of Pd-Pt bimetallic nanoclusters, using respectively PdCl2 and H2PtCl6 as metal precursors, ethylenediamine tetraacetic acid (EDTA) as stabilizer and NaBH4 as reducing agent. The resulting nanocomposite showed high electroactivity for methanol and ethanol oxidation in alkaline medium, superior to that of controls. Characterization of the final catalyst by transmission and scanning electron microscopies confirmed that globular Pd nanoparticles with a high degree of uniformity (10 nm) have been formed on the support. It was believed that the presence of cyclodextrin in the structure of the catalyst was responsible for the enhanced Pd nanoparticles dispersion and stability in aqueous media and its beneficial impact during catalysis. The authors also suggested the formation of an inclusion complex of the aryl halide with -cyclodextrin by performing a series of inhibitive tests using amantadine as a competing guest.

Sub-nanometer size noble metal colloids such as palladium could also be synthesized using a cyclodextrin polymer-based network prepared by the reaction between heptakis-(6-azido-6deoxy)--cyclodextrin and diethynylbenzene through click chemistry [START_REF] Huang | Cyclodextrin polymer networks decorated with subnanometer metal nanoparticles for high-performance low-temperature catalysis[END_REF].

Transmission electron micrographs revealed the formation of ultrasmall Pd nanoparticles, with a high degree of dispersion within the polymeric matrix. The catalytic activity of these colloids was evaluated in carbon-carbon cross coupling and reduction reactions. In the case of the 4nitrophenol reduction, the authors clearly emphasized the key role of the cavity of the cyclodextrin during the catalytic process.

Immobilization of Pd nanoparticles on the composite of cyclodextrin-decorated chitosan and magnetic graphene quantum dots have also been reported (Sadjadi and Koohestani 2020b). The resulting composite exhibited high catalytic activity for the hydrogenation of nitroarenes in aqueous media. Noteworthy, the catalyst was selective towards hydrogenation of nitro group and could be magnetically separated and reused with insignificant Pd leaching.

Microstructured support

The development of nanofibers has received an increasing interest since several years. This kind of material could be considered for the development of heterogeneous catalysts by the deposition or the incorporation of metal nanoparticles onto or into the nanofibers. The group of Uyar is a pioneer in this field. They reported incorporation of Pd nanoparticles into cyclodextrin nanofibers to form a catalyst, which was used for the catalytic reduction of 4-nitrophenol (Celebioglu et al. 2019a). A series of different nanofibers were prepared by electrospinning 2hydroxypropyl -cyclodextrin solutions in water or in N, N-dimethylformamide containing palladium acetate as palladium precursor and fully characterized by means of FTIR, X-ray photoelectron spectrometry, transmission and scanning electron microscopies. Interestingly, Pd nanoparticles were homogeneously dispersed into and onto the nanofibers with mean sizes between 3.7 and 4.9 nm depending on the nature of the solvent or palladium loading. The authors clearly highlighted the multiple role played by 2-hydroxypropyl -cyclodextrin for the preparation of these catalytic nanofibers, from reducing and stabilizing agent of metal nanoparticles to handy nanofibrous carrier matrix.

In order to improve the robustness of the nanofibers, another strategy consisted in the production of cyclodextrin nanofibers by the electrospinning of an aqueous solution of 2hydroxypropyl -cyclodextrin in the presence of 1,2,3,4-butanetetracarboxylic acid as multifunctional cross-linker [START_REF] Topuz | Atomic layer deposition of palladium nanoparticles on a functional electrospun poly-cyclodextrin nanoweb as a flexible and reusable heterogeneous nanocatalyst for the reduction of nitroaromatic compounds[END_REF]. After the manufacturing of these polycyclodextrin nanowebs, Pd nanoparticles were incorporated to the nanofibers by atomic layer deposition. Transmission electron microscopy measurements showed that Pd nanoparticles were homogeneously dispersed onto the nanofiber mats with a mean diameter centered at about 4 nm. These nanofibers were shown to be very active in the catalytic reduction of 4-nitrophenol.

The reusability of these catalytic nanofibers was also studied and it was possible to reuse the Pd nanoparticles deposited onto electrospun nanofibers during five successive runs without any loss of activity.

A very appealing strategy was recently applied for the loading of palladium nanoparticles (3-4 nm) on a 3D-network structure based on a -cyclodextrin polymer monolith [START_REF] Mizuno | Palladium nanoparticle loaded β-cyclodextrin monolith as a flow reactor for concentration enrichment and conversion of pollutants based on molecular recognition[END_REF]. The resulting polymer monolith-supported Pd was used as a microflow reactor for the reduction of 4-nitrophenol towards 4-aminophenol. Under optimized continuous conditions and due to the occurrence of host-guest intractions, the starting substrate (4-nitrophenol) could preferentially be adsorbed by the Pd-cyclodextrin monolith, while the reduction product (4aminophenol) was easily removed by elution.

3.1.2. Cyclodextrin-metal nanoparticles pre-stabilization followed by immobilisation Ponchel and coworkers have demonstrated that methylated cylodextrins (-, and cyclodextrin) could be used for synthesizing and stabilizing heterogeneous carbon-supported ruthenium nanoparticles. The synthetic strategy was based on the strong affinity of the cyclodextrins towards the carbon support and involved the prestabilization in water of Ru nanoparticles by randomly methylated cyclodextrins with NaBH4, followed by their spontaneous deposition on the support (Denicourt- [START_REF] Denicourt-Nowicki | Carbon-supported ruthenium nanoparticles stabilized by methylated cyclodextrins: a new family of heterogeneous catalysts for the gas-phase hydrogenation of arenes[END_REF]. Proof of the adsorption of the cyclodextrin-capped metallic Ru nanoparticles was obtained by means of thermal and transmission electron micrographs, in particular the presence of numerous isolated spherical particles of about 2.4 nm in size, in agreement with that obtained in solution.Ru nanoparticles appeared active in the gas-phase hydrogenation reaction of o-, m-and p-xylene at 85 °C, tested alone or in two component mixtures [START_REF] Wyrwalski | Chemically modified cyclodextrins as supramolecular tools to generate carbon-supported ruthenium nanoparticles: An application towards gas phase hydrogenation[END_REF]). Among the different formulations, the catalyst prepared from randomly methylated -cyclodextrin in a molar ratio of Ru to randomly methylated -cyclodextrin of 3 gave rise to the greatest activity and selectivity towards trans-dimethylcyclohexane. This result was attributed to a good balance between the stabilization, metal dispersion and molecular recognition surface properties of Ru nanoparticles.

Using a multi-step reaction, [START_REF] Zhou | Construction of a Repairable Fixed Porous Catalytic Bed Loaded with Gold Nanoparticles via Multivalent Host-Guest Interactions[END_REF] designed a new photoresponsive catalytic process for the hydrogenation of 4-nitrophenol in aqueous media. A porous nickel coated on silica (PNi@SiO2), 3-isocyanatopropyltriethoxysilane coupled with azobenzene (IPTS-Azo)

and heptakis-(6-mercapto-6-deoxy)--cyclodextrin-stabilized Au nanoparticles were independently synthesized. Then, PNi@SiO2 was treated with IPTS-Azo, which subsequently reacted with gold nanoparticles to furnish the final catalyst. This latter benefited from a connected porous structure and a high specific surface area that resulted in high Au loading through multivalent host-guest interactions. UV irradiation could be applied for inducing photoisomerization of the azo compound, dissociating the host guest complex formed between the cyclodextrin and the dye, facilitating regeneration of the catalytic system. [START_REF] Qu | Host-Guest Interaction Induced Rapid Self-Assembled Fe 3 O 4 @Au Nanoparticles with High Catalytic Activity[END_REF] also reported the use of prestabilized thiolated Au nanoparticles to design a novel magnetic catalyst for the the same catalytic reaction. To prepare the catalyst, ferrocenemodified Fe3O4 and thiolated β-cyclodextrin coated Au nanoparticles were easily self-assembled via host-guest interaction. The activity of this catalyst was superior compared to control catalysts and exhibited high recyclability and storage stability.

The group of Uyar reported the incorporation of Ag nanoparticles into nanofibers made from the electrospinning of a solution containing 2-hydroxypropyl -cyclodextrin and Ag nanoparticles prestabilized by polyol process, in alkaline solution (Celebioglu et al. 2019b). Ag nanoparticles were homogeneously dispersed onto the nanofiber mats and their mean diameter increased when the nanofibers were prepared in dimethylformamide instead of water and when the amount of silver increased in the nanofiber mats. The same tendency was observed for the mean diameter of the electrospun nanofibers. The antibacterial properties of these 2hydroxypropyl -cyclodextrin/Ag nanoparticles composite nanofibers against Gram-negative and Gram-positive bacteria were finally evaluated.

A similar strategy has also been developed by [START_REF] Fadlallah | Synthesis of novel catalytic composite nanofibers containing ruthenium nanoparticles stabilized by a citric acid-β-cyclodextrin polymer[END_REF] starting from presynthesized Ru nanoparticles stabilized by a citric acid-based β-cyclodextrin-polymer. These colloidal suspensions were electrospun in presence of poly(vinylalcohol). It was shown that the ruthenium particles were homogeneously dispersed at the surface of the nanofibers with the conservation of the Ru nanoparticles size, according scanning electron microscopy with highangle annular dark-field imaging, transmission electron microscopy and energy dispersive Xray spectroscopy analyses. These immobilized ruthenium nanoparticles were found to be active for the catalytic styrene conversion into ethylbenzene, without any metal leaching.

3.1.3. Metal nanoparticles pre-stabilization followed by immobilization on cyclodextrin-functionalized supports Using a self-assembly approach, Chen et al. ( 2014) prepared an electrocatalyst for the oxidation of methanol. The synthesis of this electrocatalyst was achieved in three steps through the adsorption of an inclusion complex between poly(aminothiophenol) and a β-cyclodextrin polymer grafted on graphene oxide, followed by the reduction of this support and the immobilization of the Pt nanoworms. This composite material showed high electroactivity but also stability in comparison with the classical Pt/C. Interestingly, this research group similarly reported the possibility of forming a ternary catalytic composite, prepared from the selfassembly of presynthesized Au nanoparticles onto poly(aminothiophenol) and β-cyclodextrin polymer grafted on reduced graphene oxide [START_REF] Chen | β-Cyclodextrin polymer as a linker to fabricate ternary nanocomposites AuNPs / pATP-β -CDP / rGO and their electrochemical application[END_REF]. The nanocomposite showed high electrochemical response towards imidacloprid and could be applied as an efficient electrocatalyst for the oxygen reduction.

In attempt to improve the stability and recyclability of transition metals, [START_REF] Li | Hairy Hybrid Nanorattles of Platinum Nanoclusters with Dual-Responsive Polymer Shells for Confined Nanocatalysis[END_REF] designed a novel catalytic system using a template-assisted approach, in which size-controlled Pt nanoclusters were encaged within a hollow hairy pH-and temperature-responsive polymeric shell, decorated by surface inclusion complexes formed between an adamantyl-terminated poly(N-vinylcaprolactam) and -cyclodextrin. This synthetic approach combined the use of solgel synthesis via the Stöber process, precipitation/polymerisation reactions, click chemistry for the covalent attachment of -cyclodextrin to the surface of the nanoparticles, supramolecular assemnly and finally HF etching. The resultant catalyst exhibited high activity and recyclability for the reduction of 4-nitrophenol using NaBH4 in aqueous media. The responsive polymeric shell not only avoid from Pt nanoparticles coalescence, but also furnish a hallow environment facilitating mass transfer to the encapsulated Pt nanoparticles.

Metal nanoparticles immobilization considering one step methods

Various metallic nanoparticles have been synthesized by chemical reduction and directly immobilized on supports in the presence of cyclodextrins. As an example, The Montazer's group prepared Ag/TiO2 nanocomposites in the presence of native β-cyclodextrin and examined their activity for photo-degradation of an organic dye [START_REF] Attarchi | Ag/TiO2/β-CD nano composite: Preparation and photo catalytic properties for methylene blue degradation[END_REF]. It was found that the activity of Ag/TiO2/β-cyclodextrin was the best in comparison with that of control samples.

The authors believed that β-cyclodextrin played several significant roles in the photocatalytic reaction, such as decreasing the recombination rate of electron-hole pairs, improving the formation of Ag nanoparticles on TiO2 and suppressing the aggregation process. Notably, the results affirmed that the β-cyclodextrin amount could deeply influence the efficiency of the photocatalytic process, especially in the case of high concentration of β-cyclodextrin where the formation of a multiple -cyclodextrin layer around TiO2 reduced the interaction between the dye and TiO.

As an extension of this work, Ag/TiO2/-cyclodextrin could also be deposited on cotton fabrics with an excellent antibacterial and enhanced self-cleaning effect [START_REF] Attarchi | Novel cellulose fabric with multifunctional properties through diverse methods of Ag/TiO2/β-cyclodextrin nanocomposites synthesis[END_REF]. In another research, Wu et al. reported a facile microwave-assisted procedure for the one-pot loading -cyclodextrin-stabilized Ag nanoparticles on TiO2, deposited themselves on an activated carbon [START_REF] Chen | Microwave-assisted rapid synthesis of Ag-βcyclodextrin/TiO2/AC with exposed {001} facets for highly efficient naphthalene degradation under visible light[END_REF]). The resulting material was used as photocatalyst for the naphtalene degradation. In this procedure, the authors applied NaF as a morphology controlling agent. Comparison of the activity of the catalyst with some control catalysts approved that the catalyst exhibited superior photoactivity. This was explained by the fact that -cyclodextrin could encapsulate naphtalene within its cavity, enhance its interaction with TiO2 and decrease electron-hole recombination rates.

More recently, titanium dioxide materials doped by Au nanoparticles of controlled sizes (12 nm) were synthesized for the photodegradation of methyl orange and the hydrogen evolution reaction [START_REF] Machut | Fast Microwave Synthesis of Gold-Doped TiO2 Assisted by Modified Cyclodextrins for Photocatalytic Degradation of Dye and Hydrogen Production[END_REF]. A microwave-assisted reduction of the gold precursor (HAuCl4) by 2-hydroxypropyl -cyclodextrin and randomly methylated -cyclodextrin was developed using a one-step procedure at 150 °C carried out in alkaline conditions in the presence of the titania support. For both catalytic reactions, the materials synthesized in the presence of cyclodextrins and gave much higher catalytic activities than those measured on the control material, i.e. prepared without cyclodextrin.

In another attempt, [START_REF] Li | One-pot synthesis of PtRh/β-CD-CNTs for methanol oxidation[END_REF] developed an one-pot hydrothermal procedure for the stabilization of bimetallic PtRh nanoparticles on β-cyclodextrin functionalized carbon nanotubes. In that approach, well-dispersed suspensions of carbon nanotubes, β-cyclodextrin and metallic salts with different ratios were sonicated and then hydrothermally treated. It was believed that β-cyclodextrin could act as both dispersing and reducing agent. The authors investigated the electrochemical activity of the as-prepared materials for the methanol oxidation and showed that the catalyst with a molar Pt to Ru ratio of 1 exhibited high activity due to the formation of fine nanoparticles with a high degree of dispersion.

β-cyclodextrin was also utilized for the one-step stabilization of metallic nanoparticles on fullerenes. As an instance, the group of Bai and Yang reported the synthesis of very welldispersed Pd nanoparticles in aqueous solution using a facile method, in which the palladium salt was added to a mixture of 2-hydroxypropyl -cyclodextrin and fullerene C60 and then reduced with KBH4 (Bai et al. 2013b;[START_REF] Zhang | High-Efficiency Palladium Nanoparticles Supported on Hydroxypropyl-β-Cyclodextrin Modified Fullerene [60] for Ethanol Oxidation[END_REF]. The authors confirmed the high electroactivity of the as-prepared compound for the oxidation of formic acid and ethanol.

Moreover, the comparative study through the use of different control catalysts confirmed that the combination of 2-hydroxypropyl -cyclodextrin and C60 was highly beneficial for the catalysis. In fact, 2-hydroxypropyl -cyclodextrin could coordinate Pd 2+ ion and act as a stabilizer, which consequently led to the formation of Pd nanoparticles with a smaller average size and high dispersion. [START_REF] Patil | Recyclable Supramolecular Ruthenium Catalyst for the Selective Aerobic Oxidation of Alcohols on Water: Application to Total Synthesis of Brittonin A[END_REF] also designed a novel catalytic system for the aerobic oxidation of benzylic, propargylic and aliphatic alcohols in aqueous media. To fabricate the catalyst, RuCl3 was introduced to the basic mixture of graphene oxide and randomly methylated -cyclodextrin.

Then, both the ruthenium salt and the graphene oxide were simultaneously reduced by addition of NaBH4. The catalyst was found to be efficient for the selective formation of the corresponding carbonyl compounds. Moreover, it could be recycled up to five runs with slight leaching of ruthenium and randomly methylated -cyclodextrin. It was believed that randomly methylated -cyclodextrin could play several roles. Apart from serving as a capping agent for Ru nanoparticles and phase transfer agent, randomly methylated -cyclodextrin could intercalate between the graphene oxide layers and functionalize the surface of the reduced graphene oxide. It is worth mentioning that the ruthenium catalyst was versatile and could promote both oxidation and reduction reactions through altering the reaction atmosphere from O2 to H2. The authors utilized this catalyst for the synthesis of Brittonin A through the sequential oxidation of dehydrobrittonin, followed by Wittig olefination and olefin reduction in aqueous medium, confirming that their catalyst could be utilized for designing cascade reactions. et al. (2015) devised a novel sensing platform for Rhodamine B dye. In this regard, hollow carbon nanospheres was fabricated through carbonization of polydopamine-functionalized silica, followed by the removal of the template and served as support for the immobilization of Au nanoparticles. After addition of -cyclodextrin on the supported catalyst, this latter was immobilized on a glassy carbon electrode. Taking advantage of water solubility and host-guest recognition of cyclodextrin as well as high specific surface area and electrochemical features of hollow carbon nanospheres, but also to the high electrocatalytic activity of the gold nanoparticles, a synergistic effect of the material led to highly sensitive electrochemical dye determination, excellent stability and reproducibility. Moreover, the performance of the hybrid catalyst was superior to a series of control glassy carbon electrodes.

Metal nanoparticles immobilization considering post-addition methods

Yi

A cooperation of cyclodextrin with a heterogeneous metal nanocatalyst was reported by [START_REF] Tang | Mussel-Inspired Green Metallization of Silver Nanoparticles on Cellulose Nanocrystals and Their Enhanced Catalytic Reduction of 4-Nitrophenol in the Presence of β-Cyclodextrin[END_REF]. They synthesized silver nanoparticles (10 nm) immobilized on dopaminefunctionalized cellulose nanocrystals and studied the reduction of 4-nitrophenol using NaBH4 as reducing agent. The initial rate was improved in the presence of native -cyclodextrin, which can be explained by host-guest interactions between 4-nitrophenol and -cyclodextrin.

Conclusion

This review has highlighted the different possible roles of cyclodextrin in catalytic applications using water dispersed or immobilized metal nanoparticles. The stability, the activity and the recyclability of the metal nanoparticles have been improved by using more and more complex cyclodextrin-based systems. Since the first study of [START_REF] Komiyama | Colloidal Rhodium Dispersions Protected by Cyclodextrins[END_REF], a lot of work has been done. Most of the examples reported in this review clearly showed that metal nanoparticles average size decreases and their dispersion was improved in comparison to their free-cyclodextrin controls, thus leading to the enhancement of their catalytic activity.

According to several studies, cyclodextrin could also be used as reducing agent of metal precursor thanks to its polyhydroxylated structure.

Due to a dynamic organization at the surface of metal nanoparticles, the classical mass transfer property of cyclodextrins has also been exploited with solvent dispersed nanoparticles or nanoparticles immobilized on a support. The catalytic activity is generally improved thanks to the inclusion complex formed between the substrate and the cyclodextrin via hydrophobic interactions. This inclusion complex could also, in some cases, improve the selectivity of the reaction by inhibiting some side reactions.

Cyclodextrins could also serve as agents to get confined catalytic systems by forming supramolecular hydrogels, where metal nanoparticles can be embedded. This confinement could improve both the catalytic activity by enhancing the proximity between the substrate and the metal particles and facilitate recovery of the catalytic system.

According to the objectives of the 21 st century catalysis, the challenges are numerous concerning the development of metal catalysts at nanoscale in the presence of cyclodextrin.

One of these challenges is to get the highest selectivity of a given reaction, such as the partial hydrogenation of bio-based substrates (fatty acid alkyl esters, lignin-derived compounds) thanks to the efficient confinement of the catalytic system [START_REF] Liu | Selective Partial Hydrogenation of Methyl Linoleate Using Highly Active Palladium Nanoparticles in Polyethylene Glycol[END_REF][START_REF] Rungsi | Tuning the porosity of sulfurresistant Pd-Pt/MCM-41 bimetallic catalysts for partial hydrogenation of soybean oilderived biodiesel[END_REF]. The synthesis of bimetallic nanoparticles as alloy or core-shell is important for the controlled use of noble metal precursors. The mixture of a transition metal and noble metal precursors can lead to a synergistic effect between the two metal moieties with the aims to improve the catalytic activity or selectivity for example. The development of bifunctional catalysts, i.e. supported metal nanoparticles with tailored metal and/or support properties is also a challenging area. As an example, the performances of a catalyst containing a metal active phase with a basic support would be an interesting pathway [START_REF] Zhu | Synergistic interaction between Ru and MgAl-LDH support for efficient hydrogen transfer reduction of carbonyl compounds under visible light[END_REF]).

  acid per-6-thio--CD Ir evidenced by transmission electron microscopy images, both approaches led to well-dispersed nanoparticles with close average diameters. The catalytic activity of the Ru nanoparticles prepared by each strategy was evaluated in the hydrogenation of four model substrates under 20 bar of hydrogen and no notable catalytic activity difference was observed.

Fig. 2

 2 Fig. 2 Proposed mechanism for the phenol hydrogenation without cyclodextrin (a) and with cyclodextrin assistance (b) in N-hexyltriethylammonium bromide ionic liquid. In the path (a), phenol was firstly

  (a) No cyclodextrin assistance (b) With cyclodextrin assistance

Fig. 3

 3 Fig. 3 Photoreversible inclusion of an azo-ligand into hexakis-(6-mercapto-6-deoxy) -cyclodextrin (per-6-thio--CD) itself coated on gold nanoparticles. The photoisomerization of trans-azobenzene to cis-azobenzene by UV light irradiation prevents the inclusion of this latter into the cyclodextrin cavity and a phase transfer of the Au nanoparticles from the organic to the aqueous phase, occurred in which the nitrophenol reduction can take place. The catalyst recovery can be achieved by the reverse isomerization reaction under visible light. (adapted from[START_REF] Peng | Reversible Phase Transfer of Nanoparticles Based on Photoswitchable Host-Guest Chemistry[END_REF]

Fig. 4

 4 Fig. 4 Aqueous phase synthesis of Pd nanoparticle materials stabilized by tris(4-ferrocenyl-1,2,3triazolylmethyl) benzene in combination with -cyclodextrin (adapted from Astruc et al. 2012)

Fig. 5

 5 Fig. 5 Polyrotaxane synthesized from -cyclodextrin (-CD) and -cyclodextrin (-CD) for the deposition of platinum nanoparticles (adapted from Wu et al. 2010)

Fig. 6

 6 Fig. 6 Different ways of synthesis of metal nanoparticles immobilized on a support in the presence of cyclodextrin. The two-step methods involve (i) the immobilization of pre-formed metal particles, stabilized or not by cyclodextrin-based systems or (ii) the reduction of the metal precursor in the presence of a cyclodextrin-pre-functionalized support. The one-step methods consist of the simultaneous metal reduction/metal nanoparticles immobilization in the presence of cyclodextrin-based systems. The use of cyclodextrin as mass transfer agent for nanoheterogeneous systems has also been reported as post-addition methods. All cases emphasize the multi-functional properties of cyclodextrins such as metal nanoparticles stabilizing agents, active phase dispersing agents, mass transfer agents, reducing agents in some cases.

Fig. 7

 7 Fig. 7 Synthesis of Pd@Hal--CD-OTs. In the first step, halloysite nanoclay (Hal) is silanized by (3chloropropyl)trimethoxysilane before reacting successively with thiourea and tosylated -cyclodextrin (-CD-OTs). The support is applied for Pd nanoparticles immobilization. It is presumed that the presence of cyclodextrin in the structure of the catalyst can act both as a capping agent for Pd nanoparticles and phase transfer agent for the hydrophobic substrate in coupling reaction (adapted from Sadjadi 2018)

Fig. 8

 8 Fig. 8 Preparation of the Pd-Pt/-CD/rGO nanohybrid by immobilization of bimetallic Pd-Pt clusters on β-cyclodextrin-functionalized reduced graphene oxide -CD/rGO) of Pd-Pt bimetallic nanoclusters using ethylenediamine tetraacetic acid (EDTA) as stabilizer and NaBH4 as reducing agent. The presence

Abbreviations

-CD/rGO

β-cyclodextrin-functionalized reduced graphene oxide