Phuoc Hoang Ho

Fabrice Salles

Francesco Di Renzo

Philippe Trens
email: philippe.trens@enscm.fr

One-pot synthesis of 5-FU@ZIF-8 and ibuprofen@ZIF-8 nanoparticles

Keywords: Zinc imidazolate, Synthesis, Ibuprofen, Fluorouracil, 5-FU, Encapsulation

In the present study, the synthesis of microporous zinc imidazolate nanoparticles was assessed with and without the presence of ibuprofen or 5-fluorouracil (5-FU) anticancer drug. The drugs loading was ensured by thermogravimetric analysis, nitrogen adsorption/desorption. The crystalline structure of the different prepared materials was monitored by using X-ray powder diffraction. Up to 5% loading (molar ratio of drug/Zn 2+ ), there was no evidence of crystalline drugs in the XRD patterns, suggesting an optimized occupation of the ZIF-8 cells. The pattern of the samples remained identical with and without drug. Particle size ranging between 30 and 95 nm could be deduced according to the Scherrer equation. It was found to depend on the dilution of the reagents, but also on the presence of drugs in the synthesis medium. The morphology of the samples was presented by using scanning and transmission electron microscopies, confirming the size obtained using XRD. The presence of drugs induced a significant increase in the particle size of the synthesized materials; however, those sizes are still in range for drug delivery application (< 200 nm). The drugs@ZIF-8 nanoparticles were found to be hexagonal or cubic shaped, depending on the reaction duration.

Introduction

MOFs are a new type of hybrid inorganic -organic materials composed of clusters (metal ions) linked by organic ligands, forming extended network structures. [START_REF] Furukawa | The chemistry and applications of metal-organic frameworks[END_REF] Their intrinsic properties such as remarkable porosity, high thermal/chemical stability and structure tailorability offer a wide range of applications, e.g. gas capture/separation, catalysis, sensors, and healthcare. [START_REF] Horcajada | Metal-Organic Frameworks in Biomedicine[END_REF] With such special properties, MOFs are excellent nanomaterials for drug-delivery system in terms of storage, transport, and release of active ingredients or imaging agents. Since Férey and co-workers investigated the first study of iron-based MOFs for biomedical applications, [START_REF] Horcajada | Metal-organic frameworks as efficient materials for drug delivery[END_REF] numerous works have been developed for significant extension of MOFs in healthcare. [START_REF] Horcajada | Metal-Organic Frameworks in Biomedicine[END_REF][START_REF] Lan | Nanoscale metal-organic frameworks for phototherapy of cancer[END_REF][START_REF] Hidalgo | Nanostructured metal -organic frameworks and their bio-related applications[END_REF][START_REF] Doonan | Metal-Organic Frameworks at the biointerface: Synthetic Strategies and Applications[END_REF][START_REF] Lismont | Metal-Organic Framework Nanoparticles in Photodynamic Therapy: Current Status and Perspectives[END_REF] Due to a mandatory requirement of biocompatible composition, the most common MOFs for applications in biomedicine are based on iron, [START_REF] Horcajada | Metal-organic frameworks as efficient materials for drug delivery[END_REF][START_REF] Horcajada | Porous metal-organic-framework nanoscale carriers as a potential platform for drug deliveryand imaging[END_REF][START_REF] Rojas | Metal organic frameworks based on bioactive components[END_REF][START_REF] Wu | Biocompatible and biodegradable zeolitic imidazolate framework/polydopamine nanocarriers for dual stimulus triggered tumor thermo-chemotherapy[END_REF][START_REF] Hoop | Biocompatibility characteristics of the metal organic framework ZIF-8 for therapeutical applications[END_REF][START_REF] Zhang | Postsynthetic Modification of ZIF-90 for Potential Targeted Codelivery of Two Anticancer Drugs[END_REF][START_REF] Chen | Zeolitic imidazolate framework materials: Recent progress in synthesis and applications[END_REF] or recently Zr. [START_REF] Lázaro | Application of zirconium MOFs in drug delivery and biomedicine[END_REF] ZIFs, especially ZIF-8 is a special member (based on Zn) with specific thermal and chemical stability, [START_REF] James | Kinetics of ZIF-8 Thermal Decomposition in Inert, Oxidizing, and Reducing Environments[END_REF] and used in various studies for drug delivery [START_REF] Tiwari | Curcumin encapsulated zeolitic imidazolate frameworks as stimuli responsive drug delivery system and their interaction with biomimetic environment[END_REF][START_REF] Zheng | One-pot Synthesis of Metal-Organic Frameworks with Encapsulated Target Molecules and Their Applications for Controlled Drug Delivery[END_REF][START_REF] Wang | Near-infrared light-induced dissociation of zeolitic imidazole framework-8 (ZIF-8) with encapsulated CuS nanoparticles and their application as a therapeutic nanoplatform[END_REF]. For biomedical applications, apart from the aforementioned properties, MOFs must adapt intravenous administration which requires particle size smaller than 200 nm to avoid embolism. [START_REF] Horcajada | Metal-Organic Frameworks in Biomedicine[END_REF] In particular, particle size in range of 1 -100 nm is more preferable due to their unique physical characteristics making them advantageous as drug carriers, therapeutic and deep tissue contrast agents and sensitive probes. [START_REF] Raliya | Perspective on Nanoparticle Technology for Biomedical Use[END_REF] Several methods have been used for MOFs preparation including hydro/solvothermal, conventional heating, microwave-assisted, electrochemical, mecanochemical, ionothermal, dry-gel conversion, and microfluidic syntheses. [START_REF] Chen | Zeolitic imidazolate framework materials: Recent progress in synthesis and applications[END_REF][START_REF] Stock | Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites[END_REF] Venna et al. studied the crystallinity of ZIF-8 as a function of time in conventional synthesis under magnetic bar stirring at room temperature (RT). [START_REF] Venna | Structural Evolution of Zeolitic Imidazolate Framework-8 Surendar[END_REF] They found that the relative crystallinity was achieved after 24 h, and particle size at 1 h was around 250 nm while the size was doubled after 12 h. The surface area was around 800 m 2 g -1 at 12 h. Zhuang et al. reported that uniform 70 nm particles with single-crystalline structure of fluorescein-encapsulated ZIF-8 can be obtained after 5 min of solvothermal synthesis in methanol at RT under stirring of magnetic bar. [START_REF] Zhuang | Optimized metal-organic-framework nanospheres for drug delivery: Evaluation of small-molecule encapsulation[END_REF] In another work, Cravillon and coworkers controlled the nano-and microcrystal formation of ZIF-8 by modulators such as sodium formate, 1-methylimidazole, and n-butylamine using conventional stirring at room temperature for 24 h. [START_REF] Cravillon | Controlling zeolitic imidazolate framework nano-and microcrystal formation: Insight into crystal growth by time-resolved in situ static light scattering[END_REF] A particle size of around 65 nm was observed in absence modulator, while it could go down to 10 nm in presence of n-butylamine. They also reported that stirring resulted in a wider range of particle size distribution compared to static conditions.

Other synthesis methods are important in order to control particle size, size distribution as well as morphology, all of which are crucial features of porous materials for catalytic reactions, adsorption and separation of molecules. Furthermore, nanocrystalline MOFs with high adsorption capacities are envisioned for biomedical applications. For these targets sonochemical and microwave-assisted (MW) methods have been applied. [START_REF] Stock | Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites[END_REF] MW-assisted method allows a direct interaction of the radiation with the solution/reactants and hence results in a very energy efficient method of heating. It is widely used for MOFs syntheses mainly focusing on the acceleration of the crystallization, the formation of nanoscale products, as well as improvement of product selectivity and purity.

In sonochemical syntheses, the wavelength of ultrasound is much larger than molecular dimensions and thus there is no direct interaction between ultrasound and molecules responding for chemical reactions. However, high-energy ultrasound interacts with liquid resulting in the cyclic formation of tiny bubbles, growth, and collapse, which is so-called cavitation. The formation of cavitation leads to rapid release of energy with heating and cooling rate of > 10 10 K s -1 , temperature of ~ 5000 K, and pressure of ~ 1000 bar. In homogeneous liquid, chemical reactions can take place within cavity (extreme conditions) at the interface (intermediate temperature and pressure) or in the bulk media, where intense shear forces are present. [START_REF] Stock | Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites[END_REF] Sonochemistry was applied to the investigation of MOFs starting in 2008 but it is still largely unexplored. Compared to conventional heating, sonochemical synthesis not only can promote the formation of nucleation but also can help dispersing the nuclei in a homogeneous fashion. [START_REF] Stock | Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites[END_REF] In this study, we focused on the optimization of physicochemical parameters allowing for the synthesis of crystalline ZIF-8 nanoparticles. We varied the solution concentration, the molar ligand/metal ratio, the synthesis time and temperature as well as type of solvent. In a second step, we demonstrated that ibuprofen or 5-FU could be included in the cavities of ZIF-8 via one-pot synthesis.

Experimental

2. 1. Preparation of ZIF-8 and encapsulation of drugs in ZIF-8 nanoparticles ZIF-8 materials were mostly prepared by sonochemical method, in which four parameters were studied including synthesis time, temperature, concentration of precursors, and type of solvents. In a typical synthesis, each precursor, 0.94 g of zinc nitrate hexahydrate (Zn(NO3)2.6H2O, 98%, Sigma-Aldrich) and 3.3 g of 2-methylimidazole (99%, Sigma-Aldrich), was dissolved separately in a beaker containing 20 mL of methanol (MeOH) and stirred for 15 min until it was completely soluble. The two beakers were then placed in water bath (30 o C) of an ultrasound instrument (FB15047, Fisher Scientific, ultrasound frequency 37 kHz, power consumption total 90 W) for 5 min. The 2-methylimidazole ligand solution was then quickly added to the zinc nitrate solution and the ultrasound was turned on for 60 min. Noted that the molar ratio of MeOH/Zn after mixing the two solutions was 333. The milky solution was centrifugated at 10,000 rpm (Centrifuge 5804, Eppendorf) and washed thrice with 40 mL of methanol (100%, VWR Chemicals) for each. After washing the ZIF-8 product was dried at 80 o C for 12 h. For comparison purpose, one sample was prepared by the same condition but using conventional stirring with a magnetic bar for 2 h at 30 o C,. Using methanol solvent, the synthesis time was varied for 5, 10, 15, 25, 60, and 120 min while the volume of methanol used was also changed to 40, 60, and 80 mL resulted in eventual MeOH/Zn = 666, 999, and 1332, respectively. In addition, the syntheses were performed in other solvents including water, ethanol, 1propanol, and DMF. Volume of 40 mL solvent was used to dissolve each precursor, Zinc nitrate or 2-methyl imidazole and the synthesis was carried out at 60 min.

Encapsulation of Ibuprofen (IBU) or 5-Fluorouracil (5-FU) in ZIF-8 structure was performed by one-pot synthesis in which a given amount of drug (Ibuprofen 98%, Sigma-Aldrich) or 5-Fluorouracil, 99%, Sigma-Aldrich) was added together with 2methylimidazole when preparing the solution of organic ligand. Molar ratio of drug per Zn 2+ were 2.5 or 5% for IBU and 5% for 5-FU and the samples were denoted 2.5% IBU@ZIF-8, 5% IBU@ZIF-8, and 5% 5-FU@ZIF-8, respectively. (Eq. 1)

Characterization of the materials

𝑎 = 𝑑 √ℎ + 𝑘 + 𝑙 (Cubic structure) (Eq. 2) 𝑑 = (Eq. 3)
Where 𝜏 is mean size, λ is X-ray wave-length, β is the full width half maximum intensity (FWHM), and θ is the Bragg angle, hkl are Miller indices of planes.

Morphology of ZIF-8 was analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM) using a Hitachi S2600N microscope (Hitachi, Japan) JEOL 1200 EX2 microscope (Hitachi, Japan), respectively. For TEM analysis, fine powder of materials was suspended in ethanol and the suspension was then dropped wise on a Cu grid and dried at 100 o C before doing the analyses. In both cases of SEM and TEM, the particle size distributions were determined by ImageJ software with data measured over 150 particles.

Textural properties of ZIF-8 materials were determined by nitrogen adsorption and desorption at -196 o C using Micromeritics Tristar instrument. Prior to analysis, 50 mg of sample was degassed at 100 o C for 8 h. The specific surface area was calculated using BET model in relative pressure range of 0.01 -0.1 p/p° while the pore volume and the external surface area were assessed using t-plot method.

TGA Thermal gravimetric measurements were analyzed using a TA Instruments SDT Q600 Thermal Gravimetric Analyzer. Approximately 10 mg of the sample was used for each analysis. The experiment was performed in temperature range from 40 to 900 o C with a ramping rate 10 o C min -1 under 60 mL min -1 of N2.

Results and Discussion

The synthesis conditions of ZIF-8 were optimized based on the literature as there are still issues on the best conditions required producing high yields of ZIF-8 with optimum textural and morphological properties. [START_REF] Lee | ZIF-8: A comparison of synthesis methods[END_REF][START_REF] Jang | ZIF-8: A comparison of synthesis methods[END_REF] Even though the synthesis conditions could be modified upon the addition of drugs in the synthesis medium, we first focused on the synthesis of pure ZIF-8. We first assessed the influence of the experimental conditions, more precisely, stirring, reagents concentration, and reaction medium and time. In Figure 1, the XRD patterns of synthesized ZIF-8 are compared in the case of magnetic and ultrasound stirring for two hours of reaction. The diffractograms look very similar, with a good accordance with former published XRD patterns for ZIF-8. [START_REF] Park | ZIFs -first synthesis[END_REF] The morphology of the obtained nanoparticles is also very similar, as seen in Figure 2. The particle size is narrow in both cases, which suggest homogeneous reaction conditions. It can be deduced that, when possible, ultrasound method should be preferred, due to a shorter time of reaction. We therefore decided to only focus on ultrasound synthesis reactions optimization. Additionally, it must be noted that ZIF-8 can also be synthesized by mechanical synthesis. [START_REF] Bennett | Ballmilling-induced amorphization of zeolitic imidazolate frameworks (ZIFs) for the irreversible trapping of iodine[END_REF] However, ball milling induced irreversible trapping of iodine species which is not the topic of our study. Lee et al. prepared ZIF-8 by six different methods including solvothermal, microwave-assisted, sonochemical, mechanochemical, dry-gel and microfluidic method. [START_REF] Lee | ZIF-8: A comparison of synthesis methods[END_REF] When comparing the catalytic activity of the ZIF-8 for the Knoevengel condensation reaction, they found that the catalysts prepared by dry-gel and sonochemical routes exhibited superior performance due to their significantly smaller particle sizes than those of other methods. However, the catalyst prepared by the sonochemical route in that study showed a particle size of 300 -500 nm, which was probably not suitable for medicine applications.

Synthesis time for ultrasound method

Ultrasound syntheses were performed using different reaction duration, up to two hours.

The obtained materials were characterized by powder X-ray diffraction. The results are presented in Figure 3. It can be seen that after 5 minutes, ZIF-8 has already formed, as the usual XRD pattern of ZIF-8 is displayed. [START_REF] Park | ZIFs -first synthesis[END_REF] Increasing the reaction duration leads to more intensive diffraction peaks but narrower full width half maximum (FMHW), suggesting larger particle size. However, we focused on a reaction time of 60 minutes due to a plateau in yield at this duration (~30%). Increasing the reaction time allows for the production of larger nanoparticles which are less suitable for drug release applications. Indeed, in Figure 4, the SEM pictures of the materials prepared with increasing reaction times are shown.

Whereas the particle size averages 240 nm when the reaction time is set to 10 minutes, it increases up to 330 nm for 120 minutes of reaction duration (Figure 2d). However, these sizes are still too large as compared to the optimal particle size for in vivo drug release, typically around 150 nm. We therefore studied the influence of the reagents dilution on the particle size distribution. In Figure 5a, we reported the XRD of the ZIF-8 nanoparticles obtained using different dilutions ratios.

Influence of dilution on the ZIF-8 particle size.

Increasing the dilution, through the methanol/Zn molar ratio, leads to a decrease of the height of the diffraction peaks. Indeed, at low dilution (methanol/Zn = 333), the obtained diffractogram shows intense peaks, compared to higher dilutions. It would seem wise to choose this particular dilution; however, the yield increases when the dilution factor increases as shown in Figure 5b. Electron microscopies reveal the occurrence of hexagonal or cubic-shaped nanoparticles differing in size, according to the dilution factor (Figure 6). This trend was confirmed by plotting the average particle size obtained using the Sherrer equation on the XRD patterns of Figure 5 versus the dilution ratio (Figure 7). These consistent results confirm the influence of dilution to provide small ZIF-8 nanoparticles. Figure 7. Average particle size obtained using the Scherrer equation applied on Figure 5 versus the dilution factor.

Additionally, we performed nitrogen adsorption on these materials to reveal the differences in terms of textural properties. As the particle size is modified, it can be expected to notice differences in terms of external surface extent and interparticular porosity. The corresponding sorption isotherms are presented in Figure 8. They exhibit a similar shape, typically, a type I sorption isotherm, attributed to microporous materials, as usually found for ZIF-8 materials. The specific surface areas, derived using the t-plot method yielded similar results confirming that the size of the nanoparticles had little influence on the specific surface area (between 1680 and 1780 m 2 .g -1 ). It can be deduced that most the surface is located in the microporosity of the materials, rather than developed in the external surface, despite the fact that the particles are in the nanoscale range.

A difference can be noticed between these sorption isotherms, which concerns the high relative pressure range. As the dilution factor increases, a hysteresis loop develops and become very large for the highest dilution factor used (methanol/Zn ratio = 1332).

Solvent nature influence

In our study, we first decided to use pure methanol as solvent as some authors proved it appropriate. [START_REF] Bustamante | Influence of the solvent in the synthesis of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals at room temperature[END_REF] Even being investigated longer than one decade, the synthesis of ZIF-8 The nitrogen amount adsorbed at saturation is rather poor which suggests that the porosity of the material prepared using DMF as solvent is disrupted. In contrast, when water is used under the same synthesis conditions, the ZIF-8 phase is not obtained at all. In addition, they also reported the presence of unknown impurity phase identified from XRD patterns.

Zheng et al. used a one-pot synthesis to encapsulate the anticancer drug doxorubicin (DOX) using the conventional stirring method with H2O as solvent which is questioning, according to our findings on the difficulty to prepare ZIF-8 using water as solvent. [START_REF] Zheng | One-pot Synthesis of Metal-Organic Frameworks with Encapsulated Target Molecules and Their Applications for Controlled Drug Delivery[END_REF] Although they did not mention about any impurity, the XRD patterns were close to the one reported by Kaur et al. [START_REF] Kaur | Synthesis and characterization of ZIF-8 nanoparticles for controlled release of 6mercaptopurine drug[END_REF] They also found that the presence of drug DOX led to an increase of the particle size compared to pristine ZIF-8 (100 ±30  230 ±150 nm). Other recent works related the use of ZIF-8 as carrier for drug delivery. For instance, Liedana successfully encapsulated caffeine in ZIF-8 nanoparticles, with loadings as high as 35 wt %. [START_REF] Liédana | CAF@ZIF-8: One-step encapsulation of caffeine in MOF[END_REF] It can be thought that a part of such a high amount is located on the external surface of ZIF-8 nanoparticles as crystalline clusters of drug. In our case, we focused on small amounts loaded, up to 5 mol %, to avoid any side crystalline formation of drugs in the synthesis medium. In Figure 10, the thermal analyses of the different materials are presented. ZIF-8 ZIF-8@5% IBU ZIF-8@5% 5-FU included in ZIF-8. 5-Fluorouracil being hydrophilic, a water loss can be observed up to 150°C. This can be better observed in Figure 10 right, where an inset of Figure 10 left is displayed. This weight loss does not exist in the case of pure ZIF-8, known to be hydrophobic, but also in the case of ibuprofen@ZIF-8, as ibuprofen is also known to be hydrophobic. The presence of drugs in the ZIF-8 cavities can also be deduced from the weight losses observed at higher temperatures. In the case of 5-FU, a clear sub-step can be distinguished between 400 and 45°C. Its extent is ~3.5 wt %, which is consistent with the nominal insertion of 5 mol% of 5-FU in the material. Comparing pure 5-FU with the encapsulated 5-FU in terms of stability, it can be noticed that the encapsulation plays a stabilizing role towards the decomposition of the encapsulated species. Indeed, the decomposition commences at 100°C higher in the case of 5-FU@ZIF-8 nanoparticles.

Focusing on ibuprofen@ZIF-8 nanoparticles, it can be noted that the trace is very close to that of pure ZIF-8 at low temperature (below 200°C). At higher temperature, ibuprofen@ZIF-8 is more stable, likely due to the reciprocal stabilization between ibuprofen and ZIF-8 which could originate from the pi-stacking between host and guest species. It has been shown in other systems. [START_REF] Planchais | Adsorption of benzene in the cation-containing MOFs MIL-141[END_REF] The ibuprofen weight loss is more complex to define. By comparison with the trace obtained with pure ZIF-8, it can be deduced that it happens between 250°C and 300°C, that is where the traces are not parallel, suggesting the occurrence of different decomposition processes.

In Figure 11, the XRD patterns of the materials prepared including ibuprofen and 5-FU drugs in the synthesis medium are shown. When comparing with the XRD pattern obtained with pure ZIF-8, similar patterns were obtained in the case of drugs@ZIF-8. The same number of peaks were obtained and the relative peaks intensities are also identical.

The only difference is the cell parameter which has increased when drugs were included in the ZIF-8 structure. More precisely, the cell parameter varied from a = 16.981 Å up to a = 16.992 Å when 2.5 mol % ibuprofen was included in the structure of ZIF-8. This suggests that to accommodate guests in its cavity, ZIF-8 suffered a slight enlargement of its cell dimensions. Increasing the proportion of drugs, up to 5 mol %, the cell parameters are further increased up to 16.998 Å which is consistent with our former findings. Additionally, no extra peaks can be evidenced in the XRD patterns, confirming that the drugs were introduced as molecular entities, rather than agglomerated in the cavities but also on the external surface of ZIF-8. [START_REF] Boudjema | Confinement-Induced Electronic Excitation Limitation of Anthracene: The Restriction of Intramolecular Vibrations[END_REF] The influence of the encapsulation of drugs in ZIF-6 can be also seen by electron microscopies. In Figure 12 Another evidence of the inclusion of drugs in ZIF-8 nanoparticles can be discussed, based on the nitrogen sorption isotherms. They have been plotted in Figure 13. As the amount of drugs encapsulated is moderate, their influence is rather on the sorption isotherms. It can be noted that these sorption isotherms belong to the type I, which indicates that the microporosity of the original ZIF-8 material is preserved to a large extent. The specific surface area of the different materials are reported in Table 1. A clear decrease can be noticed and it can be related to the amount of drug encapsulated in the materials. Indeed, the specific surface areas are reported as per gram of material which includes the weight of drug. The difference between the specific surface area of pure ZIF-8 and 5% ibuprofen@ZIF-8 is close to 5% which corresponds to the weight of ibuprofen introduced. It can therefore be concluded that ibuprofen and 5-FU could be encapsulated in ZIF-8 nanoparticles, while preserving the microporosity, accessible to other processes, such as solvents, or simulated body fluids. Equilibrium relative pressure / p/p o ZIF-8 5% 5-FU@ZIF-8 2.5% IBU@ZIF-8 5% IBU@ZIF-8

Figure 13. Nitrogen sorption isotherms performed on the drugs@ZIF-8 nanoparticles.

Table 1. Textural properties of the drugs@ZIF-8 materials Material SBET / m 2 g -1 Vmicro / cm 3 g -1 Vtotal / cm In this study, we first assessed the synthesis of ZIF-8 nanoparticles by evaluating the influence of key parameters, such as stirring, reaction time, solvent or dilution. It was shown that using ultrasound, the reaction time was highly reduced while preserving the structural and textural properties of ZIF-8 nanoparticles. It was also shown that adjusting the dilution ratio could be a simple way of reducing the mean particle size, which is essential for in vivo applications. Using the optimized parameters defined in this paper, we achieved the one-pot synthesis of ibuprofen@ZIF-8 and 5-FU@ZIF-8 nanoparticles.

We ensured that adding the drugs to the reaction medium did not deny our former conclusions. Even though the particle size was enlarged when including the drugs, the nanoparticles thus prepared are in the nanosize domain. We are currently performing drug release in water and simulated body fluids with the same materials, before moving to other pharmaceuticals of interest.

X

  -ray diffraction (XRD) measurement was conducted using D8 Advance Diffractometer (Bruker AXS, Germany, CuKα radiation and a Ni filter). The patterns were recorded in 2θ range of 2 -40 o with an angular step size of 0.0105 o and counting time per step was 1 s.Particle size and cell parameter were average values calculated for the first five peaks of XRD pattern, using equation (1) and (2), respectively. 𝜏 = .

Figure 1 .

 1 Figure 1. XRD patterns of ZIF-8 nanoparticles prepared by (top) magnetic stirring, (bottom) ultrasound medium.

Figure 2 .

 2 Figure 2. SEM images of the materials prepared using (top) magnetic stirring, (bottom) ultrasound medium. (Right) Higher magnification of Figures 2a and 2c.

Figure 3 .

 3 Figure 3. XRD patterns of ZIF-8 nanoparticles prepared using different reaction times.

Figure 4 .

 4 Figure 4. Scanning Electron Microscopy of ZIF-8 nanoparticles prepared with different synthesis duration at increasing magnification from left to right.

Figure 5 .

 5 Figure 5. (a) XRD patterns for ZIF-8 nanoparticles using different dilution ratios; (b) Yield versus reagents dilution factor.

Figure 6 .

 6 Figure 6. (left) Scanning and (right) transmission electron microscopy of ZIF-8 nanoparticles for different dilution ratios. (a) Dilution ratio = 333; (b) Dilution ratio = 666; (c) Dilution ratio = 999; (d) Dilution ratio = 1332.

Figure 8 .

 8 Figure 8. Sorption isotherms of nitrogen performed at 77K on the ZIF-8 materials prepared with different dilution factors.

  is still interesting due to the influence of various parameters on the properties of the final products. Recently, Malekmohammadi et al. studied the impact of the solvent and ligand/metal molar ratio on the final characteristics of the synthesized ZIF-8 structures using conventional synthesis protocols at either room temperature or 130 o C.[START_REF] Malekmohammadi | A comparative study on ZIF-8 synthesis in aqueous and methanolic solutions: Effect of temperature and ligand content[END_REF] They stated that methanol solvent easily resulted in pure phase of ZIF-8 while mixture of MeOH/water led to pure structure of ZIF-8 only at room temperature and ligand/metal molar ratio of 4. Zhang et al. followed four protocols reported from recent literature to obtained four samples of ZIF-8 with different crystallinity and studied their stability in water at room temperature.[START_REF] Zhang | Stability of ZIF-8 in water under ambient conditions[END_REF] They found that all samples were not stable, got dissolved in water to form zinc and imidazolate ions. The residual solid collected by filtration remained as ZIF-8 but the ones recovered by evaporation at 40°C forms a new phase of a crystalline structure with a dense, leaf-like morphology.Using our optimized parameters, we varied the nature of solvent to confirm these results. Our results are presented in Figure9. They are in line with the results published by Bustamante et al.[START_REF] Bustamante | Influence of the solvent in the synthesis of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals at room temperature[END_REF] Aliphatic alcohols allows the formation of ZIF-8 nanoparticles as can be seen by XRD, SEM and further confirmed by nitrogen adsorption. Indeed, the XRD patterns exhibit the peaks usually observed for this material whereas type I sorption isotherms are found in these three cases. In the case of DMF, the peaks observed in the XRD pattern are broadened and they are not very intense.

3. 5 .

 5 Drugs encapsulation in ZIF-8 nanoparticles. Kaur et al. used one-pot synthesis to trap 6mercaptopurine into cages of ZIF-8 prepared by conventional stirring for 5 min in the presence of DMSO.[31] They found that the presence of drug 6mercaptopurine resulted in an increase of particle size compared to the bare ZIF-8 (200 nm vs 80 nm).

Figure 9 .

 9 Figure 9. (a) XRD patterns of ZIF-8 prepared in different solvents, (b) Nitrogen sorption isotherms, (c) SEM images of ZIF-8 prepared in a) water, b) Ethanol, c) 1-Propanol, and d) DMF.

Figure10.

  Figure10. Thermal analyses of the different materials performed under nitrogen; (left) full TGA curves, (right) inset at low temperature.

Figure 11 .

 11 Figure 11. XRD patterns of ZIF-8 and drugs@ZIF-8 materials.

  , we gathered the obtained pictures corresponding to the different materials. It can be noted that the inclusions of drugs lead to very homogeneous hexagonalshaped nano-particles. Interestingly, the mean particle size increased, compared to pure ZIF-8 nanoparticles. This is in good accordance with former studies. [17,31] Additionally, increasing the amount of ibuprofen in ZIF-8 nanoparticles increased the particle size of the corresponding material.

Figure 12 .

 12 Figure 12. Electron microscopies for the

Acknowledgments. We wish to thank T. Cacciaguerra and C. Biolley for performing Electron microscopy and TGA measurements.


Christine Tran

Guillaume Lefèvre

Mansour Haddad

Virginie Ratovelomanana-Vidal
email: virginie.vidal@chimie-paristech.fr

Unexpected formation of 2

Keywords:

come

Introduction

2-Alkynylpyridines exhibit interesting biological properties as second mitochondria-derived activator of caspases (SMAC) mimetics. These molecules could be applied for the treatment and prevention of abnormal cell proliferation in cancer diseases [1,2]. Additionally, 2-alkynylpyridines represent synthetically relevant intermediates in organic synthesis, because their triple bonds could readily undergo diverse transformations, such as annulation [3], cyclization [4], or 1,3-dipolar cycloaddition [5]. In this context, these compounds contribute to the construction of a large array of heterocyclic frameworks, including azaindoles [6,7], furo [3,4-b]pyridines [8], indolizines [9], isoquinolines [10], aryl-substituted pyridines [11], and N-metallacycles [12]. As a result, the construction of functionalized 2-alkynylpyridines constitutes an attractive challenge, considering the synthetic and biological aspects of this scaffold.

We have recently developed a practical route to access iodo-selenopyridine derivatives [13]. We envisaged the functionalization of these selenopyridines, through a Sonogashira cross-coupling reaction. First, a reaction was performed with diarylselenium 1a and 1-hexyne (2a). Surprisingly, the desired product 4 was not obtained, but 2-alkynylpyridine 3a was isolated with 91% yield (Scheme 1).

In view of this unexpected result (Scheme 1), we focused our studies around the construction of 2-alkynylpyridines derivatives [14][15][16][17][18][19][20][21][22][23][24][25]. In addition, some DFT calculations were performed to enlighten the origin of this result, thus providing mechanistic insights into this access to the alkynylpyridine pattern.

Results and discussion

In light of the preliminary result, we thus investigated the scope of this reaction in the presence of diverse diarylselenium compounds. As depicted in Table 1, this method allows the access to a wide range of 2-alkynylpyridines from a large array of selenopyridines and diverse aryl/alkyl terminal alkynes. From selenopyridine 1a, alkynylpyridines 3a-e were obtained in good to excellent yields (71-91%, Table 1, entries 1 to 5). The reaction was also well-tolerated in the presence of 2-ethynylpyridine, with 75% yield (entry 6). Cross-coupling reactions [START_REF]Iodo-selenopyridine derivatives 1 (Table 1) were also evaluated in additional cross-coupling reactions. The Suzuki coupling was attempted with 1 (X = indene-1,3-dione) with phenyl boronic acid (2.0 equiv), Pd(OAc) 2 (5 mol%), K 2 CO 3 (3.0 equiv), CataCXium A (0.1 equiv) in DMF (0.3 M) at 90 °C for 48 h, affording the recovered starting material. The Heck reaction was performed with 1 (X = 2,2-dimethyl-1,3-dioxane) in standard conditions (tert-butyl acrylate (3.5 equiv), Pd(OAc) 2 (20 mol%), K 2 CO 3 (1.0 equiv), P t Bu 3 (2.0 equiv)[END_REF], performed in the presence of 1b [START_REF]For the synthesis of 1b-1f[END_REF], provided the desired compounds in 50 to 86% yields (Table 1, entries 7 to 11). The structure of 3i was confirmed by 2D NMR (NOESY experiment, cf. Supporting Information). It is noticeable that the reaction involving phenylacetylene led to pyridine derivatives with lower yields, probably due to steric effects (Table 1, entry 10). A good tolerance was also noted with the selenopyridines 1c and 1d [START_REF]For the synthesis of 1b-1f[END_REF], bearing an oxygen atom or a diketone group, affording the expected alkynylpyridines 3l-o with high yields (68 to 94%). Moreover, the preparation of unsymmetrical alkynylpyridines was envisaged with selenium derivatives 1e and 1f [START_REF]For the synthesis of 1b-1f[END_REF], furnishing the corresponding products 3p and 3q in 97 and 54% yield, respectively. In the course of understanding the mechanism, the reaction between selenopyridine 1a and 1-hexyne (2a) was conducted without copper iodide and palladium catalyst (Table 2, entry 1). No cross-coupling occurred and the starting materials (1a and 2a) were recovered. Moreover, no reaction occurred in the presence of either CuI or PdCl 2 (PPh 3 ) 2 alone (Table 2, entries 2 and 3). With solely CuI, the Glaser homo-coupling product was formed, suggesting that the presence of the homo-coupling diyne product arose from this off-cycle pathway [START_REF] Glaser | [END_REF]29]. An amine base was also required for the reaction, because the desired product 3a was not observed in the absence of triethylamine (Table 2, entry 4).

Replacing the -SeArI functional group by -SeMes delivered 3a in 42% yield under the same reaction conditions demonstrating that the iodo group is not mandatory for the transformation.

To explain the preferred alkynylation of the C Se bond of the 2-selenopyridyl moiety compared to the classic alkynylation of the C I bond in the methodology described herein, DFT calculations were performed. Pd(0)(PMe 3 ) 2 , 2-iodophenyl-2-pyridylselenide A and the preformed copper acetylide MeC CCu(THF) B were used as computational models. Three different pathways were investigated (Scheme 2) and are discussed thereafter.

The first pathway relies on a Cu(I)/Cu(III) cross-coupling catalytic cycle involving a key oxidative addition step of the copper catalyst into the C Se bond, excluding any role of the palladium in the catalytic process (Scheme 2a) [START_REF] Font | For recent examples of Cu(I)/Cu(III) catalytic cycles involving the formation of C Se bonds[END_REF]. A high 32.7 kcal•mol -1 barrier has been computed for this pathway, which would be challenging to overcome under the reaction conditions. This explains why no reactivity is observed when the copper catalyst is used alone (Table 2, entry 2). . Taking into account of these mechanistic investigations, a plausible catalytic cycle for the transformation reported in this work is thus suggested in Scheme 4. This catalytic cycle displays two interconnected cycles, the first one relying on palladium catalysis, leading to the activation of the C Se bond of the substrate, the second one relying on the formation of the copper acetylide involved in a key copper-to-palladium transmetalation step.

Conclusion

In summary, the unexpected formation of 2-alkynylpyridines obtained from iodo-selenopyridines through a palladium/copper co-catalytic system was reported. This transformation provides a broad array of substrates in good to excellent yields. Mechanistic studies demonstrated that the key step of this transformation is a selective activation of the C Se bond of the starting iodo-selenopyridine, which undergoes oxidative addition of the palladium(0) catalyst, whereas the C I bond remains unreactive. DFT calculations suggest that this selectivity originates from the existence of a low-energy Meisenheimer intermediate obtained in the course of the C Se bond activation. This intermediate evolves toward a pyridylpalladium(II) species. A copper-to-palla dium transmetalation between the latter and an acetylide copper(I) species followed by a reductive elimination leads to the 2-alkynylpyridine product.

U N C O R R E C T E D P R O O F C.

Experimental

General procedure for the synthesis of 2-alkynylpyridines

In a round-bottom flask, PdCl 2 (PPh 3 ) 2 (20 mol%) and CuI (10 mol%) were solubilized in THF (1.5 mL) and NEt 3 (1.5 mL). The yellow solution was stirred for 10 min. A mixture of selenopyridine 1 (1.0 eq) and alkyne 2 (4.0 eq) in THF (1.5 mL) and NEt 3 (1.5 mL) was then added. The reaction was stirred at 50 °C for 8 h. The crude mixture was then quenched with a saturated aqueous solution of ammo nium chloride. The organic layer was extracted with ethyl acetate (x3), washed with water, dried over MgSO 4 , filtered and concentrated under reduced pressure. The residue was purified by flash chromatography over silica gel to afford the corresponding alkynylpyridines.

Scheme 1 .

 1 Scheme 1. Synthesis of 2-alkynylpyridine 3a.

1 ,

 1 Subsequently, we sought to compare the respective barriers of the C I (Scheme 2b) and C Se (Scheme 2c) bond activation by oxidative addition at the Pd(0) catalyst. Computation of the oxidative addition of Pd(0)(PMe 3 ) 2 onto the C I bond of A occurs through a classic three-center oxidative addition transition state (TS 2 , Scheme 2b) with a modest 14.3 kcal•mol -1 barrier, leading to the stable palladium(II) adduct D (exothermicity of 24.1 kcal•mol-1 with respect to the reactants). Interestingly, the analysis of the potential energy surface for the oxidative addition occurring at the C Se bond (Scheme 2c) revealed that a pre-complex involving the diarylselenide A as a σ-acceptor ligand to the Pd(0) center could be formed in an endothermic step (formation of E, with a computed 11.3 kcal•mol -1 barrier). Complex E then evolves in a barrierless process toward the formation of a Meisenheimer-type intermediate F, stabilized by 1.4 kcal•mol -1 with respect to E, which involves a palladium(II) center obtained by quaternization of the C 2 atom of the pyridyl ring (Scheme 2c). Accordingly, an analysis of the Mulliken charge borne by the palladium atom and by the pyridyl moiety in reactants A and Pd(0)(PMe 3 ) 2 on one hand and on the stable Meisen heimer adduct F on the other hand supports the hypothesis of a charge transfer from the palladium center of Pd(0)(PMe 3 ) 2 onto the pyridyl ring in the course of formation of F. Charge borne by palladium indeed evolves from +0.04 |e| in Pd(0)(PMe 3 ) 2 to +0.48 |e| in F, whereas the charge borne by the pyridyl group evolves from +0.09 |e| in A to -0.30 |e| in F. Adduct F prefigures the breaking of the C Se bond, which increases from 1.93 Å in E to 2.31 Å in F. Evolution of F finally provides the diarylpalladium(II) species G, which formally represents the oxidative addition product of Pd(0)(PMe 3 ) 2 into the C Se bond of A. It is interesting to note that G is obtained with a small exothermicity with respect to the reactants (overall exothermicity of 2.3 kcal•mol -Scheme 2c): this echoes the similar trend for the activation of C S bonds by oxidative addition of nickel(0) complexes, which proved to be easily reversible [31]. Comparison of the three pathways computed in Scheme 2 shows that the activation of the C Se bond of A is preferred over the activation of the C I bond by ca. 3 kcal•mol -1 (which corresponds to the free energy difference between the rate-determining steps in Scheme 2b and c, respectively leading to the formation of TS 2 and E). This can explain the selective functionalization of the C Se bond observed in this report. These results in hand, the following pathway has been computed for the formation of the observed 2-alkynylpyridine from species G (Scheme 3). B can undergo a transmetalation reaction with G, affording the palladium(II) pyridyl-acetylide H, as well as copper(I) selenide (2-C 6 H 4 ISe)Cu(THF). The latter can then enter a new copper catalytic cycle to later afford copper(I) acetylide B. H finally affords the observed 2-alkynylpyridine I via TS 3 (with a computed 14.3 kcal•mol -1 barrier), with a global exothermicity of 26.0 kcal•mol -1

U

  N C O R R E C T E D P R O O F C.Tran et al. Catalysis Communications xxx (xxxx) xxx-xxx

3 nr 2 .Scheme 3 .

 323 Scheme 2. Competitive pathways investigated by DFT computation.

4. 2 .

 2 DFT calculations DFT calculations were performed using the Gaussian 09 code. See Gaussian 09, Revision D.01, M. J. Frisch et al., Gaussian, Inc., Wallingford CT, 2013, and see SI for additional details.

UScheme 4 .

 4 Scheme 4. Postulated reaction mechanism.

Table 1

 1 Tran et al. Catalysis Communications xxx (xxxx) xxx-xxx Synthesis of 2-alkynylpyridines via palladium-catalyzed cross-coupling.a

	Entry	Product	Yield (%) b	Entry	Product	Yield (%) b
	1	3a	91	10	3j	50
	2	3b	80	11	3k	72
	3	3c	82	12	3l	68
	4	3d	81	13	3m	89
	5	3e	71	14	3n	88
	6	3f	75	15	3o	94
	7	3g	72	16	3p	97
	8	3h	80	17	3q	54
	9	3i	86

a Reaction conditions: 1 (0.1-0.3 mmol), 2 (4.0 equiv), PdCl 2 (PPh 3 ) 2 (20 mol%), CuI (10 mol%), NEt 3 (3 mL), THF (3 mL), 50 °C, 8 h. b Isolated yield.

Table 2

 2 Coupling reaction for mechanistic insights with selenopyridine 1a.

	Entry	Reaction conditions

Acknowledgements

We thank the Ministère de l'Enseignement Supérieur de la Recherche et de l'Innovation, Chimie ParisTech (C.T.) and the CNRS for financial support. We are grateful to Dr. M.-N. Rager (Chimie ParisTech) for 2D NMR experiments.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.105757.


Keywords: Antenna arrays, beamforming, multibeam antenna, reconfigurable antenna, array synthesis

The design and realization of a reconfigurable antenna array operating in the 3.4-3.8 GHz band is presented. Array synthesis methods are developed to reduce the power consumption and are experimentally validated by measurements. The array is composed of 16 dual-polarized dipoles whose excitations are provided by a baseband arbitrary waveform generator. The array is able to radiate simultaneously two or more focused beams in one plane over a scanning range of ±45°, these beams can be reconfigured at will. To achieve these radiation pattern requirements while reducing as much as possible the array power consumption, specific antenna array synthesis tools are developed. In particular, the way to synthesize two sparse arrays at two frequencies having common switched-off antennas is proposed and described. This strategy, together with the synthesis of uniform amplitude excitations, enables to harness the consumption characteristics of power amplifiers. This energyefficient synthesis means that two mobile operators can share the same antenna array. Several representative measurement results of the array prototype validate the radiation performances and experimentally demonstrate the achieved reduction of power consumption.

I. INTRODUCTION

Phased arrays or active antenna arrays provide beam scanning capabilities that make them attractive for a wide range of applications such as RADAR, tracking and direction finding to name just a few [START_REF] Mailloux | Phased array theory and technology[END_REF], [START_REF] Brookner | Phased array radars-past, present and future[END_REF]. This technology was first mainly restricted to military applications because of its high cost. Active arrays have more recently been successfully applied to automotive radars [START_REF] Hasch | Millimeter-wave technology for automotive radar sensors in the 77 GHz frequency band[END_REF], [START_REF] Xu | An array antenna for both long-and medium-range 77 GHz automotive radar applications[END_REF] and they are now considered as an interesting candidate for 5G communication systems [START_REF] Schulpen | Design of a Phased-Array Antenna for 5G Base Station Applications in the 3[END_REF], [START_REF] Sanchez-Olivares | Circular Conformal Array Antenna with Omnidirectional and Beamsteering Capabilities for 5G Communications in the 3.5-GHz Range[END_REF].

Active antenna array associated with beamforming has been identified as one of the potential enabling technology for future terrestrial communications [START_REF] Mohammad Razavizadeh | Three-Dimensional Beamforming: A new enabling technology for 5G wireless networks[END_REF]. The flexibility brought by smartly combining several antennas allows indeed to focus the beam in a given direction and achieve high data rate and reliable communications, as expected by the future mobile networks [START_REF] Mohammad Razavizadeh | Three-Dimensional Beamforming: A new enabling technology for 5G wireless networks[END_REF], [START_REF] Dahlman | 5G wireless access: requirements and realization[END_REF]. Such a solution is complementary to the increase of operating frequency [START_REF] Schulpen | Design of a Phased-Array Antenna for 5G Base Station Applications in the 3[END_REF], [START_REF] Roh | Millimeter-wave Beamforming as an Enabling Technology for 5G Cellular Communications: Theoretical Feasibility and Prototype Results[END_REF] that enables to enhance the bandwidth and achieve even higher data rates.

This paper presents the design of a fully reconfigurable antenna array working in the 3.4 -3.8 GHz band. This frequency range is considered as a potential candidate for the first 5G services in Europe [START_REF]Radio spectrum policy group strategic roadmap towards 5G for Europe[END_REF] - [START_REF]Federal Ministry of Transport and Digital Infrastructure, "5G strategy for Germany[END_REF]. Consequently, this array can be seen as a proof of concept for the future base stations of the 5G terrestrial communications operating in S-band. Indeed, the proposed active array is able to generate simultaneously two or more focused beams at different frequencies, which means that several mobile network operators could share the same antenna. These multiple beams have a scanning range of ±45° and can be reconfigured as desired.

In addition, a special focus is brought on reducing the power consumption of the active array. Because of the inevitable use of several power amplifiers (PA), this point is crucial for the design of base station with beamforming capabilities [START_REF] Alsharif | Survey of Green Radio Communications Networks: Techniques and Recent[END_REF], [START_REF] Bhondge | Strategy for Power Consumption Management at Base Transceiver Station[END_REF]. For that purpose, a collection of antenna array synthesis tools is proposed and applied not only to comply with predefined radiation pattern templates but also to harness the consumption characteristics of power amplifiers. The array synthesis approaches are formulated as convex optimization problems [START_REF] Boyd | Convex Optimization[END_REF]. Such a formulation has proven to be a good choice to solve array synthesis problem [START_REF] Lebret | Antenna pattern synthesis via convex optimization[END_REF]. This is indeed a trade-off between the efficiency of analytical array synthesis techniques and the versatility of global optimization strategies that can get stuck in local minima, as explained in [START_REF] Isernia | A hybrid Approach for the Optimal Synthesis of Pencil Beams Through Array Antennas[END_REF][18] [START_REF] D'urso | An effective hybrid approach for the optimal synthesis of monopulse antennas[END_REF]. The characterization of a fully reconfigurable 16 crossed-dipoles antenna array validates experimentally the radiation performances of the active array prototype and demonstrate the power consumption reduction.

The paper is organized as follows. The antenna array synthesis methods to achieve the radiation performances while reducing the power consumption are detailed in Section II. The prototype of the active and reconfigurable antenna array is described in Section III and experimentally validated in Section IV. Conclusions and perspectives are drawn in Section V. The goal of the antenna array synthesis is to find the excitations feeding the array elements in order to generate a focused beam that respects predefined constraints. After introducing the main notations, the synthesis of focused beam patterns is described. Several synthesis strategies are then proposed to use efficiently the amplifiers and thereby reduce the array power consumption. In addition, the synthesis of robust antenna arrays is addressed in order to design arrays that fit the pattern template despite the inevitable uncertainties encountered when implementing the excitations. Finally, the resolution of these array synthesis problems, that are formulated as convex optimization ones, is discussed.

A. Array Synthesis Formulation

Let us consider an array composed of N antennas located at the positions 𝑟 " with n=1,…,N. The problem is described for a 1-D pattern synthesis, i.e. over the azimuthal angle φ in a fixed elevation plane θ=θ 0 that is omitted for conciseness. Each antenna radiates a far field pattern 𝑔 " 𝜑 that is simulated or measured in presence of the other antennas. These so-called embedded element patterns enable to take into account the coupling between antennas [START_REF] Kelley | Array antenna pattern modeling methods that include mutual coupling effects[END_REF][21] [START_REF] Pozar | A relation between the active input impedance and the active element pattern of a phased array[END_REF]. The electrical far field pattern 𝐸 𝜑 radiated by the antenna array is:

𝐸 𝜑 = 𝐚 𝜑 ( 𝐱 (1)
with the steering vector 𝐚 𝜑 = 𝑔 * 𝜑 𝑒 ,-. / .. 1 … 𝑔 4 𝜑 𝑒 ,-. 5 .. 1 (

where 𝐱 is the complex excitation vector of size N, 𝑟 𝜑 is the unit vector in the direction 𝜑, the operator . ( stands for the Hermitian transposition. Note that the steering vector is computed at a frequency f corresponding to the wavenumber k.

The synthesis problem is to find, at the frequency 𝑓 7 , the excitation vector 𝐱 7 in order to radiate a focused beam in the direction 𝜑 7 . This problem can be written as follows:

find 𝐱 7 subject to 𝐶 7 (𝐱 7 )

with the constraints

𝐶
where 𝜑 I JKL , 𝜑 7 XYZ[ and 𝜑 7 `aa are the direction of the maximum radiation, the half power beamwidth (HPBW) region and the sidelobe level region at the frequency 𝑓 7 , respectively. The sidelobe level is denoted 𝜌 `aa . Note that 𝐚 7 is the steering vector (2) computed at the frequency 𝑓 7 . In the first constraint of (3), the real part of the radiated field is set to be greater than a given level to keep the convexity of the problem. This is equivalent to setting a lower bound on the absolute value of the field since the excitations 𝐱 7 are determined up to a global phase shift, see the demonstration in [START_REF] Mutapcic | Beamforming with uncertain weights[END_REF]. In addition, the parameter 𝛼 is the field level radiated by the array at a distance d. This level 𝛼 is linked to the equivalent isotropic radiated power (EIRP) according to the following formula [START_REF] Milligan | Modern Antennas Design[END_REF]:

α = de * ghiY j (4)
In our study, we want to achieve an EIRP of 48 dBm. The radiation pattern constraints are represented in Fig. 1.

A more general synthesis problem is to generate two (or more) different focused beam patterns, each one radiating at a given frequency. In the case of two frequencies 𝑓 7 and 𝑓 k , the synthesis problem is: find 𝐱 7 , 𝐱 k subject to 𝐶 7 𝐱 7 and 𝐶 k 𝐱 k .

(5)

B. Array Synthesis for Reduced Power Consumption

The array architecture involves the use of amplifiers (see details in Section III-A). The power consumption of these amplifiers is the main degree of freedom that can be leveraged in order to synthesize an energy-efficient antenna array. For that purpose, three complementary synthesis approaches are proposed.

1) Use of Amplifiers in Linear Regime

The output power of the amplifiers must be below a given level in order to work in their linear regime and avoid distorting the signal. To comply with this requirement, extra constraints can be added to the synthesis problem (4). The maximum power at the output of each amplifier n is bounded as follows:

x ",7 𝟐 + x ",k 𝟐 ≤ P " JKL [START_REF] Sanchez-Olivares | Circular Conformal Array Antenna with Omnidirectional and Beamsteering Capabilities for 5G Communications in the 3.5-GHz Range[END_REF] where P " JKL is the 1 dB compression point (P *jZ ). The constraint ( 6) is added to the ones in [START_REF] Hasch | Millimeter-wave technology for automotive radar sensors in the 77 GHz frequency band[END_REF]. It is mandatory in order to synthesize an array that can be implemented.

2) Group-Sparse Arrays

The consumption of the whole active antenna array is logically reduced when some amplifiers are switched-off. The synthesis procedure that minimizes the number of antennas . while complying with a predefined pattern template is known as sparse array synthesis [START_REF] Prisco | Maximally sparse arrays via sequential convex optimizations[END_REF], [START_REF] Fuchs | Synthesis of sparse arrays with focused or shaped beampattern via sequential convex optimizations[END_REF].

In our case of interest, the excitation feeding the antenna n should be equal to zero for both frequencies 𝑓 7 and 𝑓 k in order to be able to switch-off the amplifier n. Therefore, we want to foster the situation where x ",7 + x ",k = 0 , so as to achieve a reduction of power consumption. For that purpose, the mixed 𝑙 * /𝑙 U -norm that enables to promote the sparsity on predefined groups of excitations is used. Such a structured sparsity is thoroughly described in [START_REF] Bach | Optimization with sparsity-inducing penalties[END_REF] and has been recently applied to array synthesis problems in [START_REF] Mehanna | Joint multicast beamforming and antenna selection[END_REF], [START_REF] Fuchs | Array pattern synthesis with excitation control via norm minimization[END_REF].

To achieve group-sparsity, we build N groups of two excitations 𝐠 " = x ",7 , x ",k for n=1,…,N where x ",w is the complex excitation feeding the antenna n at the frequency 𝑓 w with 𝐾 = 𝐴, 𝐵 . The mixed 𝑙 * /𝑙 U -norm reads:

𝐠 * 𝐠 U … 𝐠 4 *,U = 𝐠 " U 4 "{* = x ",7 U + x ",k U 4 "{* (7)
The synthesis problem (5) becomes:

min 𝐱 | ,𝐱 } 𝐠 * 𝐠 U … 𝐠 4 *,U subject to 𝐶 7 𝐱 7 and 𝐶 k (𝐱 k ) (8)

3) Uniform Amplitude Excitations

There are several benefits in the design of antenna arrays having excitations of the same magnitude. It is indeed easier to tune amplifiers for an optimal efficiency when they all have to deliver the same output power. This is why the synthesis of socalled isophoric array has drawn a lot of attention lately [START_REF] Bucci | Deterministic synthesis of uniform amplitude sparse arrays via new density taper techniques[END_REF].

The minimization of the 𝑙 ~-norm of a vector x limits the range of its components x " which then tends to be equal to the same limit value, i.e. x " = 𝐱 ~, as explained in [START_REF] Fuchs | Array pattern synthesis with excitation control via norm minimization[END_REF]. We recall that the 𝑙 ~-norm of a complex vector x of size N reads:

𝐱 ~= max "{{*,..,4} x " (9)
Concretely, after applying the group-sparsity (8) described in the previous section, we solve the following synthesis problem for the remaining non-zero excitations denoted 𝐱 • : min 𝐱 𝑨 ,𝐱 𝑩 𝐱 𝑨 ~+ 𝐱 𝑩 ~ subject to 𝐶 7 𝐱 𝑨 and 𝐶 k (𝐱 𝑩 ) (10)

C. Robust Array Synthesis

The radiation performances of an array may be very sensitive to the implementation of its excitations. The synthesis of robust arrays is of uppermost practical interest since it consists in determining the excitation vector 𝐱 such that the array performs well despite potential uncertainties in the realization of the excitations. These uncertainties between the intended excitations 𝐱 and the actual implemented ones 𝐱 can be modeled as follows

x " = x " 1 + 𝛿 " with 𝛿 " ≤ 𝜌 " , for 𝑛 = 1, … , 𝑁 [START_REF]Considerations for the 3.5 GHz IMT range: Getting ready for use[END_REF] where the complex numbers 𝛿 " are the relative errors, as proposed in [START_REF] Mutapcic | Beamforming with uncertain weights[END_REF]. The positive number 𝜌 " is the maximum relative error in implementing x " . The uncertainty set is a disk of radius 𝜌 " in the complex plane. As a representative example, setting the value 𝜌 " = 0.1 corresponds approximately to a maximum magnitude variation of ±0.9 dB and a maximum phase variation of ±6°.

To synthesize robust arrays, we consider the worst case approach introduced in [START_REF] Mutapcic | Beamforming with uncertain weights[END_REF]. It amounts to apply the radiation constraints ( 5) for all excitation vectors 𝐱 [START_REF]Considerations for the 3.5 GHz IMT range: Getting ready for use[END_REF]. We consider the case where all excitations have the same uncertainty (𝜌 " = 𝜌, ∀𝑛). After some manipulations (mostly triangular inequalities), the robust synthesis problem can be written:

find 𝐱 subject to 𝐶(𝐱)

where the constraints 𝐶(𝐱) are

Re 𝒂 𝜑 JKL ( 𝐱 ≥ 𝛼 + 𝜌 𝐚 𝜑 JKL ( . 𝐱 max 𝐚 𝜑 (

D. Array Synthesis Resolution

All previously described synthesis problems are convex optimization ones [START_REF] Boyd | Convex Optimization[END_REF]. It means that they can be solved very reliably and efficiently for problems having up to a few thousands of unknowns, which is our case of interest. Moreover, a convex formulation implies that the best solution of the problem can be found with a guaranteed convergence at a given accuracy. In case no solution is reached, it means that the synthesis problem does not admit any solution and some constraints must be relaxed. Many convex optimization solvers are readily and freely available. We use the package CVX [START_REF]CVX: Matlab Software for Disciplined Convex Programming, Version 2.0 Beta[END_REF] to specify and solve convex programs.

III. RECONFIGURABLE ANTENNA ARRAY

A reconfigurable array composed of 16 antennas has been designed and manufactured. The array architecture and antenna design are now described. .

A. Array Architecture

The architecture of the active antenna array is provided in Fig. 2 and the prototype is shown in Fig. 3. We use the arbitrary waveform generator DN6.662-16 of SPECTRUM Instrumentation company [START_REF]Spectrum, 16 bit Arbitrary Waveform Generator[END_REF] to generate 16 signals whose amplitude and phase can be accurately controlled. The generator of sampling rate 625 MS/s is piloted by a computer and can generate 16 signals in baseband from 0 to 200 MHz between ±2.5 V.

These baseband signals are transposed in S-band by the use of 16 mixers [START_REF]Double Balanced Mixer: RF/LO Freq 2300 -7600 MHz[END_REF]. Each mixer requires a signal of frequency 3.57 GHz that is brought by splitting the local oscillator signal (LO) into 16 channels (see the power dividers in Fig. 3).

To generate simultaneously two frequencies, e.g. 𝑓 7 = 3.58 GHz and 𝑓 k = 3.62 GHz, with their own excitations 𝐱 7 and 𝐱 k , the baseband signal at the output of the waveform generator is the sum of 10 and 50 MHz signals, respectively. The bandwidth for each frequency can be set to 20 MHz. A larger bandwidth would enable a more important data transmission capacity but the radiation constraints would be more difficult to respect.

Once in S-band, the signal must be amplified to reach an EIRP of 48 dBm with the antenna array. For that purpose, 16 power amplifiers (denoted PA) are used. We have chosen PA by Berex BMT352 [START_REF] Berex | High Power Amplifier[END_REF] because their characteristics match our expectation: a working frequency between 3.4 and 3.8 GHz and an output power of P 1dB = 30 dBm. The power consumption and efficiency (η=Pout/P DC ) are measured as a function of the output power, as shown in Fig. 4. The best efficiency is logically achieved for an output power close to P 1dB . The bias of each PA has two states (on or off) that are controlled by computer via switches. As seen in Section II, we want to switch-off as many PA as possible to reduce the power consumption. Moreover, for the PA that are left on, the best efficiency will be reached when they all have the same P out , this is the reason why we perform the uniform amplitude excitation synthesis [START_REF]Radio spectrum policy group strategic roadmap towards 5G for Europe[END_REF]. Finally, the outputs of the 16 PA are connected with cables of the same length to the 16 antennas.

B. Antenna Design

The array is made up of 16 antennas. Each antenna is composed of two ±45° crossed dipoles, we consider only one polarization in our investigation. This choice is typical for an array dedicated to base stations in a radio-communication scenario where the antenna must be low cost, easy to manufacture, robust and dual-polarized [START_REF] Chen | Antennas for base stations in wireless communications[END_REF].

The crossed dipoles and the full array have been designed with CST Microwave Studio. As can be seen in the exploded side views of the antenna on Fig. 5, the dipole and its balun are each printed on one side of the substrate. The substrate used is Neltec with a thickness of 0.8 mm and a relative permittivity ε r = 3. The metallic parts on the substrate are printed in copper with a thickness of 0.35 µm. The dimensions of the two dipoles are identical except for the baluns and the slot on each of them, enabling them to be crossed. The dimensions are summarized in Table I. The antennas are placed in a U-shaped reflector and spaced by 0.5λ at 3.6 GHz. The U-shaped reflector has a length of 745 mm, a width of 83.33 mm and a height of 9.66 mm.

. When embedded in the array, as shown in Fig. 6, each dipole must exhibit the following electrical performances: a matching and a mutual coupling below dB over 3.4 -3.8 GHz band. The Fig. 7 show the electrical performances achieved by a dipole placed in the middle of the array. This result is representative of all dipoles except the one on the edges. We denote: S ii the return loss of the dipole i, S ij the mutual coupling between the dipole i and its neighbor j and 𝑆 ,-the coupling between the two crossed dipoles denoted i and 𝚤.

The antenna array has been manufactured in the IETR workshop and the whole architecture has also been assembled at IETR. The return loss of the dipole i (Si i), the coupling between the two crossed dipoles (𝑆 ,-) and the mutual coupling between the dipole i and its neighbor j (Si j).

C. Antenna Array Coupling

The coupling between antennas must be carefully taken into account despite the low values of the coupling coefficients between adjacent antennas (|S ij | ≤ -15 dB) shown in Fig. 7. The embedded element patterns have been simulated and measured. They are included in the array synthesis procedure. Moreover, the active reflection coefficient of each antenna Γ š K›oe•žŸ depends on the set of excitations and therefore changes as the radiated beam is steered [START_REF] Pozar | A relation between the active input impedance and the active element pattern of a phased array[END_REF]. In order to ensure that each antenna remains matched whatever the array radiation configuration, we limit the active reflection coefficient of each antenna by adding the following constraint Γ š K›oe•žŸ ≤ -10 dB in the synthesis procedure.

IV. EXPERIMENTAL VALIDATIONS

The experimental validation of the antenna array has been carried out in the anechoic chamber of TDF. Two scenarios are considered:

-a single beam with 𝜑 JKL = -15° at 3.62 GHz -two simultaneous beams: 𝜑 7 JKL = -15° at f A =3.58 GHz and 𝜑 k JKL = 30° at f B =3.62 GHz.

For each beam, the requirements represented in Fig. 1 are: an HPBW 𝜑 XYZ[ lower than 10° and sidelobe levels lower than -15 dB.

The RF channels of our active antenna array have been calibrated using a vector network analyzer to measure the 16 transfer functions and ensure that the desired excitations are feeding the antennas. This calibration step is necessary to compensate for the inevitable differences in amplitude and phase between the 16 outputs induced by the power splitters, power amplifiers, mixers and RF cables.

A. Radiation Performances 1) Single Beam Configuration

We solve the synthesis problem (3) while minimizing the 𝑙 * --norm of the excitations in order to have a sparse array. Two antennas are muted. Then, we solve again the problem (3) by minimizing the 𝑙 ~-norm (9) so as to achieve excitations with uniform amplitudes. The synthesized excitations are plotted in Fig. 8a. The phase distribution is not exactly linear in order to . comply with the pattern template despite the uniform amplitude excitations and the two zero excitations.

At this stage, it is important to point out that the coupling between antennas plays a major role. For that reason, the embedded antenna patterns and the output impedances of the PA in state on or off are taken into account in order to model at best the array coupling phenomena. The achieved agreement between the simulated and measured patterns is fairly good for both co-and cross-polarizations, as shown in Fig. 8b. Moreover, the measured cross-polarization level is 17 dB lower than the co-polarization in the main beam.

2) Dual Beam Configuration

We solve here the synthesis problem [START_REF] Dahlman | 5G wireless access: requirements and realization[END_REF]. For both frequencies, the patterns must be respected: a main beam pointing in a predefined direction, an half power beamwidth of at most 10° and sidelobes below -15 dB in the proper angular regions.

Moreover, we minimize the group sparsity of the excitations [START_REF] Mohammad Razavizadeh | Three-Dimensional Beamforming: A new enabling technology for 5G wireless networks[END_REF] in order to enforce amplifiers to be switched-off for both frequencies. This step leads to switch-off the amplifier number 2, as shown in Fig. 9a. We then solve the problem [START_REF]Radio spectrum policy group strategic roadmap towards 5G for Europe[END_REF] foster the non-zero excitations to have the same magnitude. This goal is almost reached at 3.58 GHz. At 3.62 GHz, a small amplitude taper is required to respect the desired HPBW and steer the beam at the direction +30°.

3) Summary and Discussion

The simulated and measured radiation performances of both configurations are summarized in Table II. The achieved EIRP values are always lower than the expected 48 dBm because the output impedances of the PA were not considered in the synthesis procedure when optimizing the array excitations. Not including the active element patterns has led to an overestimation of the EIRP. The values of the simulated EIRP in Table II are computed considering the active element patterns with the implemented excitations. This phenomenon is all the more true in presence of switched-off PA that exhibit different output impedance. For the cases, not shown in the paper, where all PA are switched-on, the agreement between simulation and measurements is excellent. Besides, the realized excitation distribution does not exactly match the synthesized ones because of inevitable circuitry and manufacturing uncertainties. This also yields differences between simulation and measurements. To overcome the latter issue and render the radiated field less sensitive to the synthesized excitations, a robust synthesis approach has been proposed in Section II-C.

.

B. Reduction of Power Consumption

We compare the power distribution, i.e. the power P out at the 16 outputs of the PA, for two types of antenna arrays. The first type, called sparse and uniform amplitude, is achieved by solving (3) with a 𝑙 * -norm minimization of the excitations and (3) with a 𝑙 ~-norm minimization [START_REF] Roh | Millimeter-wave Beamforming as an Enabling Technology for 5G Cellular Communications: Theoretical Feasibility and Prototype Results[END_REF]. The second array type, denoted is synthesized solving simply (3).

The output power distribution for both array types are plotted in Fig. 10a and10b, when the beam is steered in -15° and -45°, respectively. The measured power consumptions are compared in Table III for various configurations. The beam configurations 𝜑 JKL = -15° and 𝜑 JKL = -45° correspond to the pattern shown in Fig. 10a and10b, respectively. As expected, when more PA are switched-off, the power consumption reduction is greater. The scenario 𝜑 JKL = 0° has 3 switched-off PA and yields a 10 % reduction of the power consumption with respect to a tapered array type. The case of a large beam steering angle (𝜑 JKL = -45°) is also considered. We compare the uniform amplitude with the tapered approach. We notice that only a very small reduction of the power consumption is achieved thanks to the uniform excitation. However, the uniform excitation could be further leveraged with variable bias tensions. In that case, the bias could be tuned to make all PA work in the region where their efficiency is better.

Furthermore, it is important to point out that a lower EIRP leads to more switched-off PA and therefore a greater reduction of the power consumption, as shown in Table IV. Consequently, the choice of the PA, and specifically their 1 dB compression point P 1dB , is crucial in order to achieve a desired EIRP while reducing the power consumption. Indeed, the value P 1dB must be high enough to be able to switch-off some PA but not too high in order to use the PA in their linear and efficient regime.

To sum up, the reduction of power consumption is logically tightly linked to both the radiation specifications and the characteristics of the PAs used in the array. It means that larger reductions of the power consumption could be achieved using the proposed array synthesis methodology.

V. CONCLUSION

The prototype of a reconfigurable antenna array operating in S-band (3.4-3.8 GHz) has been presented and experimentally validated. The active array is composed of 16 dual-polarized dipoles that can radiate a focused beam-pattern in one plane for angles ranging between ±45°. Moreover, thanks to the use of an arbitrary waveform generator, two focused beams pointing in two directions at two different frequencies can be generated simultaneously and steered independently at will. The design of this active array has been driven by the reduction of the power consumption. Antenna array synthesis tools have been developed to leverage at best the power consumption characteristics of the amplifiers. These tools have been experimentally validated, demonstrating their practical . relevance and ability to handle with the coupling between antennas. Moreover, the reduction of power consumption has been measured in a number of representative configurations.

This fully reconfigurable array calls for further promising extensions that are left for future works. Indeed, without any change of the active array architecture, several features of practical relevance could be achieved. For instance, other types of radiation patterns, e.g. sectorial or cosecant patterns, could be generated. More than two frequency bands together with their associated beams could be simultaneously generated. Finally, the transmission of a modulated signal instead of a single frequency would also be of interest.

Fig. 1 .

 1 Fig. 1. Radiation pattern constraints with the associated notations and the angular regions: 𝜑 JKL , 𝜑 lmm and 𝜑 (nko .

  𝐱 + 𝜌 max 𝐚 𝜑 ( . 𝐱 ≤ T U , for 𝜑 ∉ 𝜑 XYZ[ max 𝐚 𝜑 ( 𝐱 + 𝜌 max 𝐚 𝜑 ( . 𝐱 ≤ T \ ]^^,for 𝜑 ∈ 𝜑 `aa[START_REF]Federal Ministry of Transport and Digital Infrastructure, "5G strategy for Germany[END_REF]

Fig. 2 .

 2 Fig. 2. Active antenna array architecture composed of a baseband arbitrary waveform generator with 16 channels. Each channel is associated to a mixer with a local oscillator (LO) signal, a power amplifier (PA) and an antenna.

Fig. 3 .

 3 Fig. 3. Picture of the active antenna array composed of 16 crossed dipoles, power amplifiers and mixers. The five power splitters divide the local oscillator signal. The pattern synthesis is achieved in the azimuthal plane for an angle φ varying from ±90°.

Fig. 4 .

 4 Fig. 4. Measured DC power consumption PDC and efficiency η (=Pout/PDC) of the BMT352 power amplifier as a function of the output power Pout.

Fig. 5 .

 5 Fig. 5. Geometry of the ±45° crossed dipoles. The dipole and the balun are printed on opposite sides of the substrate, face 1 and face 2 respectively. The baluns allow to match the feeding port of the dipoles to 50 Ω.

Fig. 6 .

 6 Fig. 6. Close view of the antennas composed of a two crossed dipoles having a ±45° orientation. Each dipole is fed by a connector and fixed to the reflector.

Fig. 7 .

 7 Fig. 7. S-parameters associated to the dipole i located at the middle of the array.

Fig. 8 .

 8 Fig. 8. Synthesis and measurement results of the single beam scenario. (a) Synthesized amplitude and phase distribution. (b) Measured and simulated coand cross-polarization of the far field radiated by the array at 3.62 GHz.

Fig. 9 .

 9 Fig. 9. Synthesis and measurement results of the dual beam scenario. (a) Synthesized excitations with one common switched off amplifier. (b) Measured radiation patterns with simultaneously two beams, one at 3.58 GHz and one at 3.62 GHz. These patterns are normalized with respect to the maximum amplitude achieved at 3.62 GHz.

Fig. 10 .

 10 Fig. 10. Output power distribution Pout at 3.62 GHz for two arrays (sparse uniform and tapered) for a main beam pointing at (a) -15° and (b) -45°.

TABLE I

 I GEOMETRIC PARAMETERS OF THE PRINTED DIPOLES

	Dimensions	(mm)	Dimensions	(mm)
	h1	29	w2	20
	h2	11.9	w3	2.5
	h3	13.9	w4	0.8
	h4	13.7	w5	2
	h5	4.5	w6	14.2
	h6	17.1	w7	1.1
	h7	15.6	w8	2.9
	h8	3	r1	3
	w1	39	r2	4.2

TABLE III POWER

 III CONSUMPTION OF THE AMPLIFIERS FOR VARIOUS CONFIGURATIONS

	Freq.	Array type	𝜑 JKL PDC	Switched	Power
	(GHz)			(W)	-off PA	consumption
						reduction
	3.58	Sparse uniform	0°	27.3	3	10 %
		Tapered		30.2	0
	3.62	Sparse uniform	-15° 27.1	2	6 %
		Tapered		28.8	0
	3.62	Sparse uniform	+30° 28.3	2	4 %
		Tapered		29.5	0
	3.62	Uniform	-45° 30.4	0	1 %
		Tapered		30.7	0

TABLE IV RELATION

 IV BETWEEN EIRP AND POWER CONSUMPTION (𝜑 ¡¢£ = -15 ° AT 3.62 GHZ, WITH SPARSE UNIFORM ARRAY TYPE)

	Desired EIRP (dBm)	Switched-off PA	Measured PDC (W)
	42	5	19
	48	2	27.1

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TAP.2020.3026888, IEEE Transactions on Antennas and Propagation

ACKNOWLEDGMENT

The authors would like to thank Frédéric Boutet and Christophe Guitton for their important help to build and assemble the antenna prototype.

Manuscript received XX. This work was supported by TDF in Liffré, France, and in part by the European Union through the European Regional Development Fund (ERDF), and by the french region of Brittany, Ministry of Higher Education and Research, Rennes Métropole and Conseil Départemental 35, through the CPER Project SOPHIE / STIC& Ondes.

S. Fofana is with the IETR and TDF, Rennes, France.