Dr Raphaël Lamare

Dr Romain Ruppert

Corinne Boudon

Laurent Ruhlmann
email: lruhlmann@unistra

Dr Jean Weiss
email: jweiss@unistra.fr

Prof. François Diederich

Prof C Boudon

Prof L Ruhlmann

Porphyrins and Polyoxometalate Scaffolds

Keywords: Porphyrins, Polyoxometalates, Self-Organization, Electrochemistry, Photochemistry

la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Context

Demographic growth, especially in developing countries, as well as the rise of the living standards, are the most obvious reasons for the actual increase of energy consumption. 1,2 Nowadays, the planetary energy consumption uses fossil fuels for 80%, nuclear fuels for 5% and renewable energy for 14%, 3 which generates environmental issues at the local, national and international scale. Indeed, energy production and consumption are responsible for 80% of the carbon dioxide and two thirds of the emission of greenhouse gases. 3 In this regard, diversifying energy sources worldwide is a necessary requirement and the most sustainable option is to increase the contribution of renewable energy. Among all sources, the total amount of solar energy reaching Earth largely exceeds the needs of the living population and providing clean and sustainable electricity to the world will have ecologic and economic consequences on top of solving the global energetic crisis. Whereas in the past, solar cells were mainly used in small electronic devices, nowadays the solar solution is one that is most seriously foreseen. In this context, new materials that utilize the principles of photo conversion and catalysis (fuel-cell storage) known for decades, based upon optical and physical phenomena, are needed. For this reason, great efforts have been devoted both at the European and worldwide levels to develop alternatives to Si based devices. Even if the Si based modules still dominate in terms of cost/performances (both for market and research), new non Si-based materials and technologies are considered and expected to grow in the future. 4 Among the more than 20 different types of photovoltaic cells that have been reported, 5 aside from the dominating single crystalline or amorphous silicon derived cells, organic, dye sensitized 6,7 and perovskite cells 8 are the current objects of interest but the global challenge of the field is the balance of production costs vs performances and lifetime. In the search for new molecular materials for photoconversion, polyoxometalates have been

Lindqvist Anderson-Evans Keggin

Wells-Dawson considered. Polyoxometalates (POMs) are anionic structures obtained by acidic condensation 9 of metal oxides MO4 n-and consist of metal oxides connected by µ-oxo bridges. In general, the metal ions are Mo, W or V in their highest oxidation state, thus respectively (+VI), (+VI) and (+V) and two types of oxygen atoms, terminal and bridging, are present. There is a large structural variety of polyoxometalates, differing by their shapes, their size and their global charge. The most common types of POMs are schematically represented by the generic structures in Figure 1 and the center of the structure can be occupied by a heteroatom such as phosphorus or silicon (X in green) for example. 10 POMs offer a large variety of charged states and their solubility can be controlled by the nature of the counter cations. As a result, POMs have found applications in various fields such as catalysis, 11,12,13,14 energy, 15 , 16 materials 17 or therapy. 18 POMs have been mostly used as photocatalysts for the formation of H2 19 , 20 , 21 or the reduction of CO2 22,23 due to their ability to undergo multiple (up to 6) and successive electron transfers. Unfortunately, their efficiency suffers from a quasi-exclusive absorption of energy in the UV region of the spectrum.

In order to enhance the energy collection, POMs can be associated to chromophores absorbing in the visible region of the solar spectrum. Porphyrin analogues such as chlorophylls and bacteriochlorophylls, the pigments of life, are involved in natural light harvesting antennae such as LH1 and LH2. 24,25,26,27 Typical absorption bands of synthetic porphyrins are extremely intense in the 400-450 nm region of the spectrum ( = 300 000) and very intense in the 500-800 nm ( = 30 000). 28 Thus, these tetrapyrrolic macrocycles are good candidates for the sensitization of POMs.

Scope of the review

Whereas single porphyrin-POM hybrids are of interest to study the photochemical properties of dyads, the major purpose of POM-porphyrin combinations is to associate several porphyrins with one POM to take advantage of an antennae effect. In general, covalent methods leading to single porphyrin-POM hybrids are also suitable for associating more than one porphyrin to a POM but several methods leading to multiple-porphyrin-POM scaffolds have been designed specifically. In the following sections, scaffolds comprising porphyrins and POMs will be presented. These examples will be classified by the mode of attachment connecting the POMs and the porphyrins. The scaffolds described will be discussed in terms of properties and, when possible, the structure/properties relationship will be established.

Possible approaches for the preparation of structurally controlled POM-porphyrin scaffolds will be proposed.

POM-porphyrin hybrids built from coordination

In the case of polyoxometalates and porphyrins, two strategies based on coordination have been pursued. The first consists in the axial coordination of a POM on the metal core of a metalloporphyrin. An example has been reported in which a sandwich comprising two molybdenum porphyrins derivatives wrapped around a Keggin type POM was obtained. The two porphyrins are highly distorted dodecaphenyl-porphyrins which adopt a saddle-ruffled shape in the presence of the matching shape of a Keggin type POM (Figure 2). 29 The concave shape thus favors "C-H … O" interactions between the protons of some phenyl substituents on the porphyrin periphery and the oxygen atoms of the POM which complement the strong biding interaction of opposite terminal oxygen of the POM with the Mo located in the porphyrin core. The synthetic methodology has been extended to P containing Keggin POMs and the complexes have displayed very interesting features in their crystalline packing, forming layers of tubular shaped alignments. The individual sandwich motif is maintained in solution and withstand multiple electron transfer. 30 POMs can be chemically modified via controlled degradation in alkali medium, removing one or more M=O and leaving lacunary spaces surrounded by highly nucleophilic oxygen atoms. These lacunary POMs (Figure 3) can be used as ligands for metals and this has led to the development of oxidation catalysts. 31 Utilizing the sandwich approach, lacunary POMs containing ruthenium as catalyst have been inserted between concave porphyrins and engaged in the oxidation of substrates such as benzyl alcohol into aldehydes. Interestingly, individual components of the sandwich have no effect on the substrates whereas the sandwich combination of the ruthenated POM and the Mo distorted porphyrins is efficient. The effect has been assigned to the significant anodic shift of the Ru IV/III and Ru V/IV potentials when bound to the porphyrins. 32 The binding properties of lacunary POMs has also been used to form mixed species in which a metal ion is coordinated by both a porphyrin and a lacunary POM (Figure 4). 33 These complexes, isolated in good yields, have been used as models for the interactions of Hf IV and Zr IV porphyrin derivatives with surfaces such as ITO and the application of porphyrins in the sensitization of ITO based photovoltaic devices. The second approach involves the presence of coordinating heteroatoms in the porphyrin structure, which can bind to a metal complex derived from a lacunary POM. The preparation of such metal complexes has been largely exploited in the Lindqvist series and the imido-POMs are the most commonly prepared by treatment of the [Mo6O19] 2-cluster with iminophosphoranes 34 or isocyanides. 35 One of the first examples of coordination complex on a POM in which one terminal Mo=O had been replaced by a transition metal was demonstrated by 1 H NMR, 36 providing evidence for interaction between pyridine and imidazole with α-[CoSiW11O39] 6-or α-[NiSiW11O39] 6-. More recently, several examples of organic-inorganic hybrids using this approach have been reported. In particular, pyridine appended porphyrins have been combined with Co II or Ni II inserted in lacunary Keggin type POMs (Figure 5). 37 For the complexes in Figure 5, association as high as 10 6 M have been measured by monitoring the extinction of luminescence for the porphyrin in the presence of the POM. More recently, several nitrogen ligands such as pyridine, 4,4'bipyridine and meso-tetrakis-(4-pyridyl)-porphyrin (Figure 4) have been used in the competitive formation of self-assembled face-toface lacunary Keggin type phosphomolybdates and face-to-face bis porphyrins. 38 Of course, the formation of coordination hybrids always depends on the strength of the coordination bond and the use of covalent bond formation has also attracted the attention of synthetic chemists.

Covalent POM-porphyrin hybrids

The replacement of terminal hydroxy ligands by alkoxy substituents on POM provides a simple access to attaching molecular sub-units to the inorganic structure. Similarly bridging oxygen atoms can also be turned into bridging oxygen atoms from alkoxy groups. Although at the time the connection has been classified as coordination, the first replacement of a hydroxo on a polyoxovanadate has been reported as early as 1992. In the resulting structure, the stabilization of the hexavanadate core by the alkoxy ligands has been established and assigned to the reduction of the total (negative) charge of the cluster. 39 Similar effects have been observed in a Keggin type phosphotungstate in which a bridging oxo has been alkylated. Interestingly the latter method leaves the structure unaltered. 40 Most of the functionalization methods employed for the preparation of the covalent POM-porphyrin hybrids presented hereafter have been covered in several reviews and will not be detailed. 41 In 2009, two porphyrins have been introduced on a Wells-Dawson POM using linkers which disrupt any electronic coupling between the two porphyrins. 42 The synthesis utilized a generic functionalization of a POM precursor reported at the turn of the century which opened the way to many chromophore decorated POMs, 43 using classical clickchemistry conditions. 44 A few years later, the same approach has been employed to attach an antennae network of porphyrins to a Wells-Dawson POM (Figure 6). 45 Remarkably, an energy cascade was introduced in the covalent scaffold in which zinc porphyrins collected energy to be transferred (ET) to a free base (metal free) porphyrin for a final electron transfer (eT) to the POM. Several other covalent POM-porphyrin scaffolds have been reported using a tris-alkoxy substitution of three hydroxyl group that can be applied to all types of POMs. 46 Among these, the hybrids described in (Figure 6) combined Anderson-Evans POMs α-[MnMo6O24] 4 and Wells-Dawson [P2V3W15O62] 9-types were used to observe photo-induced electron transfers from the porphyrins to the POM in the Wells-Dawson case only. 47 In this derivative, photophysical studies yielded a transfer rate of k S1 et = 1,6 x 10 10 s -1 which is consistent with literature values for dyads using Zn porphyrins. 48,49,50,51 The Wells-Dawson hybrid depicted in Figure 7. has been deposited on ITO plates and the instantaneous generation of photocurrent under irradiation in the visible region has been observed using I3 -/ I -as redox relay. 52 Recently, the synthesis and structure of a Lindqvist type hexavanadate covalently linked to two zinc porphyrins has been reported. For the first time, the hybrid structure was characterized by X-ray diffraction (Figure 8. Linear porphyrin dimer connected by a Lindqvist type POM (generated with Mercury™ from CCDC file 961099).Figure 8). The scaffold has been utilized for the photocatalytic degradation of pollutants in aqueous media. 53 Similar structures bearing one, two (cis and trans) or three Wells-Dawson type polytungstates [V3P2W15O62] 9-have been reported. When non-centrosymmetric, the porphyrin/POM hybrids were used in third order non-linear optic applications 54 , 55 but more generally, this class of hybrids is well suited for catalysis in general and more particularly acetylation reactions 56 or alkene epoxidation with NaIO4 57 when manganese or tin tetrakis(paminophenylporphyrin) linked to four Lindqvist hexamolybdate are used. Very recently, two new looped covalently bonded Anderson-Evans-type polyoxometalate-porphyrin hybrids composed of two Anderson-Evans-type POM moieties and two porphyrin moieties were synthesized. 58 The preparation of a hybrid material in which tetrakis(paminophenylporphyrin) is covalently linked to a Lindqvist structure of polyoxometalate, is also reported. The direct coupling of the hexamolybdate polyoxoanion, [Mo6O19] 2-and tetrakis(4aminophenyl)porphyrin, was successful using dicyclohexylcarbodiimide (DCC) as a coupling reagent. 56,57

Electrostatic Hybrids

Due to their polyanionic nature, POMs are able to interact with positively charged molecular components. Cationic porphyrins that have been mostly developed to enhance their solubility in aqueous media are thus well suited to form assemblies with POMs based on the development of electrostatic interactions. In 2000, the first supramolecular complexes resulting from the combination of species depicted in Figure 9 Figure 1have been reported in the literature. 59 Many cationic porphyrins obtained by simple alkylation of porphyrins bearing pyridine substituents have been reported but similar species obtained by protonation can also be combined with Keggin type POMs. The two species represented in Figure 10 illustrate this easy access to hybrid scaffolds with Keggin POMs which can be used for photocatalytic H2 production 60 or in

M8 4+

non-linear optics. 61 The hybrids have shown electrocatalytic activity for the oxygen reduction reaction (ORR). Other catalytic studies performed with similar hybrids such as the oxidation of (cis)-enes, geraniol and terpenes have involved POM-porphyrin hybrids assembled by electrostatic interactions. 62 Ideally, in order to maximize the interactions, neutrality should be reached.

The combination represented in Figure 11 Erreur ! Source du renvoi introuvable.matches 16 negative charges on a Wells-Dawson type POM sandwich with four tetracationic porphyrins to form a neutral assembly for which an ideal structure is proposed. 63 Similar systems were obtained with other tetracationic species derived from octaethylporphyrin and the activity of these scaffolds has been demonstrated by the photoreduction of Ag + in the case of Keggin type POMs. 64 However, all the structures represented here are idealized and a rational analysis of the performances in the light of structural data remains difficult. Tweezers shaped porphyrin dimers having positive charges in precise locations have been prepared and are represented in Figure 12. 65 Preliminary studies show that depending on the positioning of the charges, association constants with a Lindqvist type POM vary from logKa = 2.6 to 4.2 and that the fluorescence quenching of the porphyrin results from the formation of a host guest complex and interactions in the ground state.

Layer by layer Assembling

Pioneering works have described the formation of layer-by-layer (LbL) assemblies on glassy carbon electrodes. Using polytungstates containing either P or Si as heteroatoms, the better stability of Si containing multilayered films was demonstrated 66 and the resulting modified electrode were used for electroinduced hydrogen evolution reaction and two electron oxygen reduction reaction with cobalt porphyrins. 67,68 After deposition, the stability of LbL films obtained in the case of tetrapyridinium porphyrin (free base and zinc complex) and a europium containing polyoxometalate EuPW11O39K4 has been demonstrated by AFM studies. The structure of the films on glass, mica and ITO has been investigated and have shown that the thickness of the deposited layers might be different from one substrate to another. For example, the thicknesses of films on glass and ITO are approximately 3 times thinner when compared to films on mica. 69 Although linear UV absorption evolution is generally used as proof of LbL deposition, models 70 suggest that AFM experiments are the ultimate proof of the homogeneous deposition of a hybrid on a substrate. The pronounced chromophoric activity of the porphyrin constituent in the LbL hybrid materials has consequently involved the use of transparent ITO electrodes in photovoltaic applications. Efficient photocurrent generation can be obtained by combining a Fe substituted POM [α2Fe III P2W17O61] 7-and a tetracationic porphyrin 5,10,15,20tetrakis-(4-trimethylammonium)-phenyl)-porphyrin. 71 A mixture of a cationic cobalt 5,10,15,20-tetrakis(1-methyl-4pyridinio)porphyrin and an anionic metatungstate (POM) supported on graphite carbon was developed as a high performance novel cathode catalyst using a rotating ring-disk electrode for the oxygen reduction reaction (ORR) with high current density and low hydrogen peroxide production in a fuel cell. 72

R= p-CH3(C6H4)-

Porphyrin metal organic frameworks (PMOFs)

Inspired by the fast growing research on metal organic frameworks (MOFs) which give access to multidimensional porous structures, 73,74 coordination based approaches of POM-porphyrin hybrid materials have been developed, leading to POM containing MOFs. These self-organized structures contain tetrakis-(4-pyridyl)-porphyrin linkers connecting ferrous or cuprous ions, forming a 3D cationic porous network. Within the pores hexamolybdates can be accommodated and fill the pore regularly to reach charge neutrality for the resulting scaffold. 75 Porphyrin based MOFs are attractive due to their bio-relevance regarding light harvesting and catalytic processes. 76 Among these, zirconium based porphyrin MOFs like MOF-545 77 (porphyrin tetracarboxylate) are extremely porous and provide space to accommodate large species such as [(PW9O34)2Co4(H2O)2] 10-for noble metal-free photocatalytic water oxidation. 78 , 79 Porphyrins bearing carboxylate functions can also be directly associated with Keggin type POMs to form porous networks with catalytic properties such as the selective oxidation of olefins, 80 the oxidation of alkylaryls with Mn porphyrins 81 and the reduction of CO2. 82

Hybrid copolymers

As large aromatic macrocyles, porphyrins can undergo electron transfers to generate rather stable species which reactivity can be controlled. For more than four decades, it is known that the radical cation of octaethylporphyrins can react with a nucleophile to generate a cationic adduct such as that described in Figure 13Figure 13. 83 Since the initial observation, electrochemical alternative methods have been developed to form the radical cations at the surface of electrodes. Similar reactivity has also been found for tetraphenyl porphyrins 84 and the use of linearly bridging nucleophiles, like 4,4'-bipyridine, has allowed to form films of polymers at the surface of electrodes. 85,86 When performed in the presence of POMs, the later could be incorporated in the well-organized (n sweeps) E= 0 to 1.9 V vs SCE polymer films. 87 To strengthen the connection between the POM species and the porphyrins in the electrogenerated porphyrinbipy-POM polymers, the bridging nucleophile can be built on the POM itself as represented in Figure 14. The films produced using this method generate stable photocurrents under visible irradiation. The intensity of the current depends on the type of bis-pyridine spacer, the film thickness and finally of the applied potential on the ITO electrode. 88 Using similar method, four hybrid polyoxometalate-porphyrin copolymer films were also obtained by the electrooxidation of zinc-β-octaethylporphyrin or free base 5,15-ditolylporphyrin in the presence of organosilyl functionalized Keggin-type POMs TBA3[PW11Si2O40C26H16N2] and Wells-Dawson-type POMs TBA6[P2W17Si2O62C26H16N2] bearing two remote pyridyl groups. Their impedance properties were and their photovoltaic performances were investigated showing a correlation between impedance and photovoltaic performances, the films based on Wells-Dawson type POMs and Zn porphyrins giving the best results. This last system displayed one of the best photocurrent efficiency for a reported POM photosensitized hybrid (-7,2 µA.cm - 2 ). 89 The highest current density value measured in an optimized film was -7,2 µA.cm -2 which is at least four times the value observed for polymers sensitized by ruthenium complexes 90 or phthalocyanine sensitized polymers 91,92 and are close to those observed in hemicyanines sensitized POMs. 93 Interestingly, the performance of the polymers is best when the connection between the POM and the porphyrin is well-defined than when POMs are combined with porphyrin oligomers by electrostatic interactions. Finally, recently, the introduction of nanoparticles (MNPs) at the surface of cationic poly-porphyrin films, obtained by electrostatic interaction between the bis-porphyrin copolymer and the Preyssler type polyoxometalate P5W30@MNPs, enhances the photocurrent (up to 2.5-3 times greater as a function of the used nanoparticle). 94

Conclusion

The combination of porphyrins with POMs has greatly enlarged its field of applications over the last decades. While hybrid materials obtained at the mesoscale by oligomerization and selfassembly at the surface of electrodes, their intimate structure is usually more idealized than precisely determined. In general, the performances of molecular species described in this mini-review are better than those of bulk POM-porphyrins combinations. Thus, the performances of the hybrid materials could benefit from detailed structural information obtained from models assemblies utilizing the same interactions to associate POMs and porphyrins. For example, systematic studies of the charge localization on the porphyrins in electrostatic assemblies may help to obtain more efficient hybrid materials through a rational analysis of their structure-properties relationships and such investigations require significant efforts in supramolecular chemistry and porphyrin synthesis, two emblematic fields which have been explored indepth by our mentor and friend François Diederich during his brilliant career.

((max. width: 5.5 cm; max. height: 5.0 cm))

Please delete this box prior to submission ((max. width: 11.5 cm; max. height: 2.5 cm))

Please delete this box prior to submission

Figure 1 .

 1 Figure 1. General formula of polyoxometalates and their various shapes.

Raphaël

  Lamare received his PhD in 2020 under the co-supervision of Prof. Laurent Ruhlmann and Dr. Jean Weiss after obtaining a Master degree in Chemistry and Supramolecular Chemistry from the University of Strasbourg and a research internship in synthetic Organic Chemistry. He is currently post-doctoral fellow of the Institut des Sciences Chimiques de Rennes 1. Romain Ruppert completed his PhD in 1985 with J. P. Sauvage at the Université Louis Pasteur (ULP). He then worked as Alexander von Humboldt fellow with E. Steckhan in Bonn and held a second post-doctoral position in the group of W. Armstrong at Berkeley. CNRS researcher since 1988, he worked successively with F. Petit, M. W. Hosseini and H. J. Callot before joining Jean Weiss' group in 2007. Corinne Boudon received her PhD in physical chemistry in 1987 from the Université Louis Pasteur working on the electrochemistry of cryptates. She has played a pivotal role in the long time collaboration between the Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide and Prof. Diederich (> 100 joint publications) and she is now Emeritus Professor at the University of Strasbourg since 2019. Laurent Ruhlmann completed his PhD at the University of Strasbourg in 1997. After a post-doctoral position at the Freie Universität Berlin in Germany in 1998 he became associate professor at the University Paris-Sud 11. He is now full professor in chemistry since 2011 at the University of Strasbourg where he is the team leader of the Laboratory of Electrochemistry and of Chemical-Physic of the Solid State. He is expert in electrochemistry and develops hybrid materials based on (iso)porphyrin and polyoxometalate for photo (electro)catalyzed applications. Jean Weiss received his PhD in 1986 with J.-P. Sauvage (ULP). After two post-doctoral positions with D. J. Cram at UCLA and H. A. Staab at the MPI in Heidelberg as an Alexander von Humboldt fellow, he joined the group of M. Gross at the CNRS in Strasbourg. Since 2007, the group's research topics include porphyrinoïds (extended porphyrins, corroles, chlorins, phlorins), hemoprotein models, redox molecular switches, self-assembly, and molecular electronics.

Figure 4 .

 4 Figure 4. A Keggin POM coordinated to two distorted MoPorphyrins. Si: yellow, W: blue, O: red, Mo: aqua blue, C: grey, N: dark blue (generated from CCDC-622714.

Figure 5 .

 5 Figure 5. Schematic proposed structure of H2T3P-4-Py or H2T3P-3-Py and MSiW11 (M = Co II or Ni II ).

Figure 3 .

 3 Figure 3. A lacunary POM in which a terminal MO has been replaced by [Ru III (DMSO)]. generated with Mercury™ from CCDC 606353.

Figure 2 .

 2 Figure 2. Schematic structure of POM-porphyrin scaffolds linked by Hf or Zr coordination and X-ray structure of the Hf IV derivative generated with Mercury™ from the CCDC file 717412. Solvents removed for clarity.

Figure 8 .

 8 Figure 8. Photosynthetic mimic comprising a porphyrin antennae and a Wells-Dawson POM for charge storage.

Figure 6 .

 6 Figure 6. Linear porphyrin dimer connected by a Lindqvist type POM (generated with Mercury™ from CCDC file 961099).

Figure 7 .

 7 Figure 7. Tris-alkoxy substitution leading to (left): Anderson-Evans POM based bis-porphyrin hybrid and (righ): Wells-Dawson POM porphyrin hybrid.

Figure 10 .

 10 Figure 10. Protonated porphyrins which were combined with Keggin type POMs for hydrogen production (left) and non-linear optics (right).

Figure 9 .

 9 Figure 9. Components of the first electrostatically driven POM-porphyrin assemblies.

Figure 13 .

 13 Figure 13. Two substitutions of an octaethylporphyrin radical cation.

Figure 12 .

 12 Figure 12. Series of dicationic tweezers shaped porphyrin dimers obtained by stepwise quaternization.

Figure 11 .

 11 Figure 11. Idealized structure of a neutral electrostatic assembly formed meso-tetrakis(N-methyl-4-pyridium)-Zn porphyrin and a Keggin POM sandwich.

Figure 14 .

 14 Figure 14. Porphyrin/POM copolymers with bridging bis-pyridine attached to the POM unit.

Acknowledgements

All authors are thankful to the University and the CNRS for constantly supporting their work. RL acknowledge the Fondation Recherche en Chimie ic-FRC of the University of Strasbourg and the Region Grand-Est for a PhD. Fellowship.


Wilfried Uhring
email: wilfried.uhring@unistra.fr

Laurent Millet

Bertrand Misischi

Fatah Rarbi

Fabrice Guellec

Daniel Dzahini

Octavian Maciu

Jean-Baptiste Kammerer

Gilles Sicard

A Multi Millions Frames per Second CMOS Sensor With Digital Storage

Keywords: CMOS video sensor, ultra fast burst imaging, digital storage, CMOS image sensor, 3D stacking

This paper describes a 3D Integrated Circuit (3D-IC) architecture of a burst image sensor (BIS) with embedded digitization and digital storage. This architecture also proposes a new technique to increase both the frame rate and the stored image capacity at the cost of a spatial resolution reduction. A 2D monolithic demonstrator that takes into account the constraints of a future 3D-IC imager has been fabricated. Experimental results are presented showing that a frame rate from 5 up to 45 Mega frames per second can be achieved. This fully functional approach paves the way to the very first digital BIS.

I. INTRODUCTION

Conventional video systems extract data from the image sensor at each frame and store it into an external memory for post processing or display. The constrained number of Input/Output (I/O) pads of the sensors limits the pixel readout rate to about 25 Gpixel/s for the fastest currently available video camera [1]. The Burst Image Sensor (BIS) approach circumvent the I/O issue by storing the video stream in the sensor at a very high data rate during the acquisition and readout them afterward at a relaxed speed. At this time, published BIS achieved a pixel rate of about 1 Tera pixel/s by using analog CMOS capacitances [START_REF] Zlatanski | Sub-500 ps temporal resolution streak-mode optical sensor[END_REF][3] or CCD channel memories [START_REF] Crooks | Kirana: a solid-state megapixel uCMOS image sensor for ultrahigh speed imaging[END_REF] and are designed in 2D monolithic integrated circuit (2D IC). Whereas analog BIS can hardly store more than 200 frames in in-focal-plane storage topologies, a digital BIS could easily record respectively more than 1000 or 4000 frames by using a SRAM or a DRAM based memory bank [START_REF] Bonnard | New 3Dintegrated burst image sensor architectures with in-situ A/D conversion[END_REF]. On the other hand, the surface occupied by the ADC within the pixel will reduce the available area to the memory in a single 2D IC sensor. A 3D stacked IC approach overcomes this limitation by allowing a distribution of electronic functions on dedicated tiers.

The paper describes a 3D-IC architecture of a scalable design for BIS with on the fly digitization and embedded digital storage applied to a 3D stack of 3 integrated circuits. In order to demonstrate the benefits of this approach, a 2D monolithic circuit integrating the different electronic parts of the sensor side by side has been designed in a 65 nm CMOS technology.

II. OVERALL SENSOR ARCHITECTURE

The targeted cross-section of the sensor is presented on fig. 1. A first IC is dedicated to the analog pixel array. A second one is devoted to the ADC and the last one is dedicated to the embedded digital memory. For optimal implementation of the memory and the high performance ADC, a cluster approach has been adopted for the BIS. The detailed architecture of the sensor according to the different tiers is shown on fig. 2. A cluster of pixels is composed of P=10 frontend pixels connected to one ADC. A bloc of L clusters, i.e. LxP pixels, shares the same digital memory for an optimal floorplan configuration that allows embedding the maximal amount of memory.

Furthermore this cluster of pixels approach sharing the same memory allows to digitize an adaptable number of pixels. In "full frame" mode an ADC operating at a sampling rate Fs and reading all the P pixels of the cluster gives an achievable frame rate of Fs/P. At the cost of a spatial resolution reduction, the frame rate can be increases as the number PRC[1;P] of read pixels in the cluster is reduced and is given by Fs/PRC. Meanwhile, the movie length is also increased by a factor P/PRC. While using this feature, the ADC and memory efforts are kept constant whereas the pixel front-end operates at a higher speed. Thus the front-end bandwidth and sensitivity has to be high enough to operate at the highest achievable frame rate. Therefore, a 27fF conversion gain CTIA has been implemented in the pixel. A buffered analog multiplexer allows to select one of the pixel output to the shared ADC input. A sample and hold circuitry with a ping pong structure to store the even and odd frames makes possible a pipeline integration. The 10 pixels cluster layout is shown in Fig. 3.

In the case of a digital BIS the main bottleneck is the ADC. As the cluster approach allows to use a single ADC shared for several pixels, it can occupy the corresponding silicon surface minus the space required by some Trough Silicon Via (TSV) mandatory to convey signals from the top tier. In order to offers both a high signal to noise ratio and a high conversion rate, while maintaining the power and the silicon area consumption to a reasonable value, a SAR ADC described in detail in [START_REF] Rarbi | An 8-Bit, 100-MSPS Fully Dynamic SAR ADC for Ultra-High Speed Image Sensor[END_REF] has been implemented. The core's size is about 210100 μm 2 so that two ADCs fit the area of two clusters, i.e. 500100 µm², and a space of more than 6000 µm² is available for the TSV required to the 3D stacked final version of the sensor. The effective number of bit (ENOB) is 7.5-b and the total power consumption is 1.55 mW per ADC at 100 MS/s. In the final 3D stacked sensor, the digital memory should be designed with an advanced CMOS process such as a 28 nm technology. In our monolithic demonstrator realized in a 65 nm process, a not optimized SRAM memory bank has been used. The memory receives the single bit stream provided by the ADCs and stores each pixel value into a 16384 bits large memory bank for each cluster. By assuming an 8 bits dynamic, the storable movie length is then 16384/8/10 = 204 frames. The sensor operation is controlled with some embedded state machines (SM) that allow to precisely control the fast different unit. These SMs are clocked by a 100 MHz to 1 GHz master clock provided by an external PLL and almost all the timing of these SM are configurable by writing to their specific register.

III. MONOLITHIC DEMONSTRATOR AND RESULTS

The photography of the designed monolithic demonstrator is depicted in Fig. 5 (a). It is composed of 12 clusters of 10 pixels with their 12 associated ADC. Some additional test structure are also included. The memory block merged the 12 clusters data with a total of 1216384 bits. Two clusters (orange) and their two associated ADCs (blue) are highlighted to show their shape factor and especially the area consumption. A SPI port allows to configure the sensor and extract the acquired data. The response of the sensor has been tested under a pulsed illumination of a 635 nm laser diode emitting a 15 pJ, 100 ps FWHM long pulse of light that illuminates the whole sensor sensitive area [START_REF] Uhring | A low-cost high-repetition-rate picosecond laser diode pulse generator[END_REF]. The acquisition mode is "full frame" and the ADC dynamic is set to 8 bits with a 5 Mfps frame rate. The pulse of light is generated during the frame 4 exposure time. For clarity reason, only a single cluster is represented over the 6 first frames (Fig. 6). We clearly see that the light pulse is detected only on the frame 4. The pixel response non uniformity is meanly due to the speckles pattern of the laser diode.

IV. CONCLUSION

A 3D-IC architecture of BIS with a digital storage has been presented. A 2D monolithic demonstrator integrating the electronic of the future 3 different tiers has been designed, realized and characterized. The results show that the proposed architecture is fully functional and can operate at the frame rate of 5 Mfps. This work paves the way to the very first 3D stacked BIS with digital storage.

Fig. 2 .

 2 Fig. 2. IC repartition of a 3D CMOS stacked BIS. The first tier is for the pixel frontend, the second tier shares one ADC for one pixel clusters and the third tier is for the deigital memory shared for several ADC clusters.

Fig. 1 .

 1 Fig. 1. Cross section of the targeted 3D CMOS stacked

Fig. 3 .Fig. 5 .

 35 Fig. 3. tier 1 (top) and tier 2 (bottom) floorplan partitionning (100µm×500µm double cluster)

ACKNOWLEDGMENT

We thank the Agence Nationale de la Recherche for financial support under grant ANR-14-CE26-0024-01


R S Borysiewicz

B J T Morgan

V Hénaux

T Bregnballe

J.-D Lebreton

O Gimenez

Multi-Site Integrated Population Modelling

Keywords: integrated analysis, Kalman filter, mark-recapture-recovery data, multistate models, Phalcrocorax carbo sinensis, recruitment, state-space models

HAL is a

Introduction

Mark-Recapture-Recovery Models

The development of models for mark-recapture-recovery (MRR) data began in the 1960s with the Cormack-Jolly-Seber (CJS) model [START_REF] Cormack | Estimates of survival from sightings of marked animals[END_REF][START_REF] Jolly | Explicit estimates from capture-recapture data with both death and immigration-stochastic models[END_REF][START_REF] Seber | A note on the multiple-recapture census[END_REF], which estimates survival and capture rates from recapture data of an animal population collected at a single site. This model initiated a surge of interest into constructing models for this type of data and numerous extensions have since been developed. These include incorporation of categorical variables for characterising individuals and analysis of multiple data sets using group effects [START_REF] Lebreton | Modeling survival and testing biological hypotheses using marked animals: a unified approach with case studies[END_REF]. Also, models that allow the integrated analysis of both recaptures of live animals and recoveries of dead animals have been developed [START_REF] Burnham | A theory for combined analysis of ring-recovery and recapture data[END_REF][START_REF] Lebreton | A simultaneous analysis of dead recovery and live recapture data[END_REF][START_REF] Barker | Joint modeling of live-recapture, tag-resight, and tag-recovery data[END_REF][START_REF] Catchpole | Integrated recovery/recapture data analysis[END_REF].

A further development which has enhanced the potential of MRR data analysis has been the extension of the CJS model to the multi-site framework. First established by [START_REF] Arnason | Parameters estimates from mark-recapture-recovery experiments on two populations subject to migration and death[END_REF][START_REF] Arnason | The estimation of population size, migration rates, and survival in a stratified population[END_REF] and later developed by [START_REF] Schwarz | Estimating migration rates using tagrecovery data[END_REF] and [START_REF] Brownie | Capture-recapture studies for multiple strata including non-markovian transitions[END_REF], this extension allows estimation of survival and transition probabilities as well as recapture parameters and parameter redundancy of these multi-site models has been assessed in [START_REF] Gimenez | Parameter redundancy in multistate capturerecapture models[END_REF]. The Arnason-Schwarz model has been further generalised to multi-state models [START_REF] Lebreton | Competing events, mixtures of information and multistratum recapture models[END_REF] and multi-event models [START_REF] Pradel | Multievent: An extension of multistate capture-recapture models to uncertain states[END_REF]. A number of computer software packages for fitting such models are available, including program MARK [START_REF] White | Program Mark: survival estimation from populations of marked animals[END_REF] and M-Surge [START_REF] Choquet | M-Surge: new software specifically designed for multistate capture-recapture models[END_REF].

Integrated Population Analysis

Integrated population analysis combines data from a variety of sources. Integrating MRR and census data was first proposed by [START_REF] Besbeas | Integrating mark-recapturerecovery and census data to estimate animal abundance and demographic parameters[END_REF] as a method for estimating productivity, otherwise not estimable from either type of data alone. It is possible to form the likelihood for the MRR data as outlined in section (2.1). A state space model of the census data comprises two parts -the observation equation and the underlying state equation. Gaussian assumptions provide the key to the accessibility of these models and makes it possible to form the likelihood, using a recursive procedure known as the Kalman filter [START_REF] Harvey | Forecasting, structural time series models and the Kalman filter[END_REF][START_REF] Durbin | Time series analysis by state space methods[END_REF]. Assuming independence, it is possible to combine the MRR and census likelihoods to provide one global likelihood which can be optimised to provide maximum likelihood estimates of all parameters.

A multivariate normal approximation to the exact mark-recapture-recovery likelihood [START_REF] Besbeas | The efficient integration of abundance and demographic data[END_REF] more efficiently integrates both data sets. Further, [START_REF] Brooks | A Bayesian approach to combining animal abundance and demographic data[END_REF] introduced a Bayesian approach, while [START_REF] Besbeas | The potential of integrated population modelling[END_REF] provides a discussion on further possible advances. [START_REF] Besbeas | Completing the ecological jigsaw[END_REF] gives details of the Kalman filter methodology, including initialisation procedures in ecological applications and a discussion of the breakdown of independence assumptions and the effects of introducing overdispersion in the state space model.

The integrated analysis performed in this paper is on a simulated multi-site data set which contains both breeding and non-breeding birds. Recruitment, defined as the progression from a non-breeding state to a breeding state, is a parameter of biological interest, and it is this type of transition, along with dispersals between study areas which will be estimated from the contribution of MRR multi-site/state and multi-site census simulated data.

Motivation for the Study

This investigative simulation study was motivated by data on cormorants, Phalacrocorax carbo sinensis, collected by the National Environmental Research Institute in Denmark. P. carbo is the most widely distributed of all cormorants -known to breed in North America, Europe, Asia, Africa and Australasia. The studied cormorants belong to the Eurasian subspecies P. carbo sinensis. This subspecies is smaller than the North Atlantic subspecies P. carbo carbo, and often breeds and winters inland [START_REF] Hatch | Great cormorant (Phalacrocorax carbo). In: The Birds of North America[END_REF].

The data were collected as part of a larger ringing programme started in Denmark in 1977, which continues to the current day. The data correspond to a period of population expansion (1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993) in 6 colonies located 32-234km apart. Recapture and recovery data from this period were analysed in detail by [START_REF] Henaux | Dispersal and recruitment during population growth in a colonial bird, the great cormorant[END_REF] who estimated dispersal and recruitment.

The oldest of the six colonies, Vorsø, was established in 1944 and along with Ormø (OR, est. 1972) andBraendegård Sø (BR, est. 1973) 14,018 cormorant chicks were marked between 1981 and 1991 with a standard metal ring on one leg and a coloured plastic band, engraved with a unique combination of 3 alphanumeric characters, on the other leg. Resightings of the ringed cormorants took place from 1983 to 1993. Resightings were of breeding cormorants only and these breeders were identified using strict biological criteria identified in [START_REF] Henaux | Dispersal and recruitment during population growth in a colonial bird, the great cormorant[END_REF]. Recoveries spanned a large geographical area, ranging from northern United Kingdom to Southern Algeria and from western Spain to eastern Romania. Recoveries of birds for which only one ring was found were excluded to avoid negative bias due to ring loss.

Each of the six colonies were censused in early May. Data consisted of a count of all occupied nests. The location of nests varies between colonies: in MA nests were built on the ground, in BR nests were found in trees and on the ground while in the other colonies all nests were in trees.

Based on the parameters of this real-life investigation, this simulation study demonstrates the statistical gains of performing integrated population modelling on multi-site data and also the ease with which even complex models, such as the recruitment structure, can be incorporated into an integrated population modelling framework.

Methods

Formation of the Mark-Recapture-Recovery Likelihood

The closed-form likelihood for Arnason-Schwarz models is derived in [START_REF] King | Closed-form likelihoods for Arnason-Schwarz models[END_REF]. Suppose captures or recaptures occur for animals age j ∈ J = {0, . . . , J} and the study site is split into R regions. The set of model parameters includes: φ j (r) is the probability that an animal in location r at age j survives until age j + 1; λ j (r) is the probability that an animal in location r at age j dies and is recovered dead before age j + 1; p j+1 (r) is the probability that an animal in location r at age j + 1 is recaptured; and ψ j (r, s) is the probability that an animal in location r at age j moves to location s by age j + 1 given that it is alive at age j + 1.

The encounter history of each animal can be broken down into three partial histories. These are: last live encounters and beyond, consecutive live sightings and dead recoveries. The likelihood can similarly be deconstructed into these elements and full details of the likelihood construction can be found in the Appendix.

Formation of the Census Likelihood

The State Space Model and the Kalman Filter

The Kalman filter is a recursive procedure for computing the optimal estimator of a state vector at time t, based on the information available at time t [START_REF] Harvey | Forecasting, structural time series models and the Kalman filter[END_REF]. By imposing Gaussian assumptions it is possible to calculate the maximum likelihood estimates of unknown model parameters. The general linear Gaussian state space model is:

y t = Z t α t + t (1) α t+1 = T t α t + η t (2) with t ∼ N (0, H t ) and η t ∼ N (0, Q t ).
Equation ( 1) is the observation equation and ( 2) is the state space equation. The state vector, α t , is unobserved and y t is a vector of observations. The matrices Z t , T t , H t and Q t are assumed to be serially independent and independent of each other at all times.

The initial state vector α 1 is assumed to be N (a 1 , P 1 ) independently of 1 , . . . , n and η 1 , . . . , η n . In practice, some or all of the matrices will depend on elements of an unknown model parameter vector.

The aim is to obtain a conditional distribution of α t+1 given Y t = {y 1 , . . . , y t }. Since all distributions are normal, conditional distributions of subsets of variables given other subsets are also normal; the required distribution is therefore determined by a knowledge of a t+1 = E(α t+1 |Y t ) and P t+1 = V ar(α t+1 |Y t ).

The Kalman Filtering derives the filtering equations and may also compute the smoothed estimates of the error vectors t and η t , given all the observations y 1 , . . . , y n . Denoting the parameter vector by θ and using our previous assumptions of normality, the likelihood is

L(θ | y) = p(y 1 , . . . , y n | θ) = p(y 1 | θ) n t=2 p(y t |Y t-1 , θ)
and the log-likelihood is given by

log L(θ | y) = n t=1 log p(y t |Y t-1 , θ)
where p(y 1 |Y 0 , θ) = p(y 1 ). Following substitution of appropriate parameters we obtain

log L(θ | y) = - np 2 log 2π - 1 2 n t=1 log(|F t | + v t F -1 t v t ) (3)
This likelihood is known as the prediction error decomposition form of the likelihood since v t can be interpreted as a vector of prediction errors, y t -E(y t ). F t is the covariance matrix of the conditional distribution of the observations and both vectors v t and matrix F t are calculated directly from the Kalman filter.

Ecological application of the Kalman filter

Define N i x t to be the number of animals in state x = {N, B} (where N denotes non-breeder and B denotes breeder) in site i = 1, 2, 3 at time t. State vector α t is

α t = N N 1 N N 2 N N 3 N B 1 N B 2 N B 3 T t ,
then following the notation of equation ( 1), since only breeders are observed

Z t =        0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1        , ∀t.
Vector y t is the observed census data and the observation error, σ, is assumed to be constant over time and site; we discuss relaxing this assumption later. The underlying state space equation is governed by a Leslie matrix [START_REF] Caswell | Matrix Population Models: Construction, Analysis and Interpretation[END_REF], and the explicit form of the Leslie matrix for this application is given in section (4.4).

The Kalman filter is initialised by specifying values for a 1 and P 1 , the mean and covariance matrix of the initial state vector, respectively. Following [START_REF] Besbeas | Kalman filter initialization for modelling population dynamics[END_REF] we initialised the filter using the stable age distribution of the population. Other initialisation procedures were implemented on the same simulated data set and the model structure was robust to the assumption of the stable age distribution start.

Simulation of multisite recruitment data

150 simulated non-breeding animals were marked at each site and mark-recapture-recovery data were simulated on this cohort for seven years. All ringing was carried out on nonbreeders and recaptures were of breeders only. Recapture and survival probabilities are site-and state-dependent and the recovery probability is independent of recovery site. Three types of transition were considered, which reflect the model structure used by Henaux et al.

(2007):

-natal dispersal: the movement of a non-breeder from one geographical location to another whilst remaining a non-breeder.

-recruitment: the accession from non-breeder to breeder.

-breeding dispersal: the movement of a breeder from one geographical location to another.

Cormorants can only start breeding from age two and remain capable of breeding in every subsequent year [START_REF] Henaux | Dispersal and recruitment during population growth in a colonial bird, the great cormorant[END_REF], thus, within this simulation study we assume that animals start recuiting at age 2 and that all animals have recruited by age 5 and once a breeder, they remain a breeder until death. Only breeding birds are ever recaptured and hence constraints applied to natal dispersal and recruitment need to be enforced in order to ensure identifiability of all transitions, and so it is assumed that natal dispersal occurs only in the first year of life and subsequently recruitment then occurs at a single site and no further dispersals occur between sites until the animal has reached the breeding state.

The assumptions, defined above, are in addition to the traditional Arnason-Schwarz model assumptions, and in order to proceed with integrated analysis the MRR data and census data must be independent. Violation of this assumption can lead to biased estimates, as shown in [START_REF] Besbeas | Completing the ecological jigsaw[END_REF].

The simulated census data were site-specific with structure governed by a Leslie matrix. Twenty years of site-dependent census information were simulated which, like the cormorant data set, was of breeding birds only. Parameter values were chosen to reflect biologically reasonable values. Fecundity (defined to be the number of offspring multiplied by the probability of reproduction in a particular year) was fixed at a constant value of 1.2.

Fecundity however could be adapted to allow for time, site or density dependence. Transition, survival, capture and recovery probabilities were set to values which were biologically reasonable, with non-breeder survival (0.6) assumed lower than breeder survival (0.8) and recovery rates (0.4) lower than recapture rates (0.7). Dispersal rates varied between 0.05 and 0.3 between sites.

Specification of the Leslie Matrix

Suppose we denote the three non-breeding states as 4, 5 and 6 and the three breeding states as 1, 2 and 3 for the respective three sites in each case. The parameters involved in the MRR likelihood, following the notation of section (4.1), are:

Survival Probability: φ j (r) = φ(r) for all j, for r ∈ {1, 2, 3, 4, 5, 6}, where r ∈ {1, 2, 3} denotes breeder survival and r ∈ {4, 5, 6} denotes non-breeder survival;

Recovery Probability: λ j (r) = λ for all j and r;

Recapture Probabilities: p j+1 (r) = p(r) for all j and r ∈ {1, 2, 3};

Natal Dispersal: ψ 1 (r, s) for r, s ∈ {4, 5, 6}

Breeding Dispersal: ψ(r, s) for r, s ∈ {1, 2, 3}

Recruitment: ψ 2+ (r, r -3) for r ∈ {4, 5, 6}

Non-Maturation: ψ 2+ (r, r) for r ∈ {4, 5, 6}

The structure of the Leslie matrix for the state space model, using the notation above is then:

               φ(4)ψ 2+ (4, 4) 0 0 f φ(4)ψ 1 (4, 4) f φ(5)ψ 1 (5, 4) f φ(6)ψ 1 (6, 4) 0 φ(5)ψ 2+ (5, 5) 0 f φ(4)ψ 1 (4, 5) f φ(5)ψ 1 (5, 5) f φ(6)ψ 1 (6, 5) 0 0 φ(6)ψ 2+ (6, 6) f φ(4)ψ 1 (4, 6) f φ(5)ψ 1 (5, 6) f φ(6)ψ 1 (6, 6) φ(4)ψ 2+ (4, 1) 0 0 φ(1)ψ(1, 1) φ(2)ψ(2, 1) φ(3)ψ(3, 1) 0 φ(5)ψ 2+ (5, 2) 0 φ(1)ψ(1, 2) φ(2)ψ(2, 2) φ(3)ψ(3, 2) 0 0 φ(6)ψ 2+ (6, 3) φ(1)ψ(1, 3) φ(2)ψ(2, 3) φ(3)ψ(3, 3)               
where f is the fecundity parameter.

Computational Implementation

Matlab was used to code the MRR likelihood and the Kalman filter which constructed the census likelihood. The global likelihood, formed by multipliying the two likelihoods, assuming independence, was then optimised using a built-in optimisation method within the matlab software. The logistic link was used to constrain recapture, recovery and survival probabilities between 0 and 1, whilst the generalised logit link, which is an extended logit function was used to ensure that as well as transition probabilities being between 0 and 1, appropriate combinations of the transitions added to 1. Further details of the use of the generalised logit link can be found in [START_REF] Choquet | M-Surge 1-7 user's manual[END_REF]. Figure 1 demonstrates the formation of the appropriate likelihoods and also the common parameters used to model each type of data.

Conclusions

These procedures are extensions of the single-site integrated population analysis. MRR is now a widely used type of data, with even advanced MRR models in common use.

Frequently, however, census information is collected concurrently with MRR data. Until now, census data have been analysed separate from the MRR data, thereby sacrificing their shared information. The methodologies described in this paper are simple to implement and can be completed 'post-hoc' by using a simple multivariate normal approximation to the MRR likelihood; if only MLEs and associated standard errors are available, they can be used to construct an approximate diagonal variance-covariance matrix to facilitate analysis and complicated population transitions can be incorporated into the state space model.

The potential of single-site integrated population analysis has been assessed in numerous studies as a method for estimating previously inestimable parameters. This simulation study has shown that not only does multi-site/state integrated population analysis estimate these parameters, but also greatly improves the precision of some parameters. This is obviously desirable in complex models with a large number of parameters that are frequently estimated with low precision.

Single site integrated population modeling with one dimensional census information allows the estimation of additional parameters, however little change is made to the precision of common parameters. By incorporating multi-dimensional census data, in terms of site or state-specific census data, we have shown that this extra information has greatly improved the precision of parameters within the model. live sightings, dead recoveries. The likelihood can similarly be deconstructed into these elements. The following derived probabilities facilitate the likelihood construction.

Let χ (j,k) (r) denote the probability that an animal is seen for the last time at age j ∈ J in location r ∈ R, and would be age k at the end of the study, with j ≤ k ≤ J . Then,

χ (j,k) (r) =              1 (j = k) 1-φ (j) (r) 1 -s∈R ψ j (r, s){1 -p j+1 (s)}χ (j+1,k) (s) -{1 -φ j (r)}λ j (r) (j < k)
O (k,j) (r, s) denotes the probability that an animal in location r ∈ R at age k ∈ J remains unobserved until it is subsequently resighted in location s ∈ R at age j + 1, for 0 ≤ k ≤ j ≤ J -1. Then, O (k,j) (r, s) = p j+1 (s)Q (k,j) (r, s),

where Q (k,j) (r, s) denotes the probability that an animal migrates from region r ∈ R at age k ∈ J to location s ∈ R at age j + 1, for 0 ≤ k ≤ j ≤ J -1, and is unobserved between these ages, and is given by

Q (k,j) (r, s) =      φ k (r)ψ k (r, s) (k = j)
φ k (r) R l=1 {1 -p k+1 (l)}ψ k (r, l)Q k+1,j (l, s) (k < j) D (k,j) (r) denotes the probability that an animal is recovered dead in the interval (j, j + 1)

given that it is last seen at age k ≤ j in r ∈ R and is given by

D kj (r) =     
{1 -φ j (r)}λ k (r) (k = j) R l=1 {1 -φ j (l)}λ j (l){1 -p j (l)}Q k,j-1 (r, l) (k < j)

The following sufficient statistics which are obtained from the encounter history data are then formed: v (j,k) (r) denotes the number of animals that are recaptured for the last time in location r ∈ R aged j and would be aged j ≤ k ≤ J at the end of the study; n (k,j) (r, s) denotes the number of animals that are observed in location r ∈ R at age k and next observed alive in location s ∈ R at age j + 1; and d (k,j) (r) denotes the number of animals recovered dead between ages j and j + 1 that are last observed alive at age k ≤ j in location r ∈ R.

The likelihood function has the form given below: where θ comprises the model parameters {Φ, Λ, P, Ψ} and we denote our MRR likelihood by L M RR (Φ, Λ, P, Ψ).

  comprised the only colonies present in Denmark at the start of the ringing study. Colonies Toft Sø (TO), Dyrefod (DY) and Mågeøerne (MA) established during the study in 1982, 1984 and 1985 respectively.

Figure 1 :

 1 Figure 1: Directional acyclic graph showing how two types of data (MRR data and census data) are combined to obtain global estimates of parameters: Φ -survival parameters; Λrecovery parameters; P -recapture parameters; Ψ -transition parameters, f -fecundity, σ -observation error. The Arnason-Schwarz model is used to form the MRR likelihood whilst the Kalman filter is applied to a state space model to form the census likelihood

  j) (r, s)} n (k,j) (r,s)  

Table 1 :

 1 The maximum likelihood estimates and associated standard errors calculated by a finite difference method, of the MRR data used alone and the integrated MRR and census data.

	Parameter	True Value MRR Estimate	MRR SE Integrated Estimate	Integrated SE
	φ(1, 2, 3)	0.80	0.826	0.0312	0.820	0.0279
	φ(4, 5, 6)	0.60	0.587	0.0211	0.586	0.0211
	p	0.70	0.714	0.0396	0.712	0.0379
	ψ(4, 5) = ψ(4, 6)	0.10	0.078	0.0395	0.073	0.0350
	ψ(5, 4) = ψ(5, 6)	0.20	0.189	0.0441	0.193	0.0418
	ψ(6, 4) = ψ(6, 5)	0.05	0.024	0.0227	0.025	0.0236
	ψ(1, 2) = ψ(1, 3)	0.20	0.248	0.0367	0.209	0.0213
	ψ(2, 1) = ψ(2, 3)	0.10	0.083	0.0283	0.106	0.0194
	ψ(3, 1) = ψ(3, 2)	0.30	0.332	0.0351	0.325	0.0279
	ψ(4, 1) = ψ(5, 2) = ψ(6, 3)	0.60	0.593	0.0593	0.601	0.0559
	λ	0.40	0.406	0.0258	0.405	0.0258
	f	1.2	-	-	1.167	0.1420
	Parameter	MSE (MRR Data Alone)	MSE (Integrated Data)
	φ(1, 2, 3)	0.0752		0.0525
	φ(4, 5, 6)	0.0106		0.0110
	p		0.0422		0.0375
	ψ(4, 5) = ψ(4, 6)	0.4436		0.4621
	ψ(5, 4) = ψ(5, 6)	0.1504		0.1273
	ψ(6, 4) = ψ(6, 5)	1.6180		1.5683
	ψ(1, 2) = ψ(1, 3)	0.2402		0.0362
	ψ(2, 1) = ψ(2, 3)	0.2184		0.0594
	ψ(3, 1) = ψ(3, 2)	0.1733		0.1056
	ψ(4, 1) = ψ(5, 2) = ψ(6, 3)	0.0612		0.0544
	λ		0.0120		0.0119
	f		-		0.0156
	σ		-		0.0232

Table 2 :

 2 The mean square errors of the parameters for the MRR data used alone and the integrated MRR and census data.

Acknowledgements

We wish to thank Jens Gregersen and Lars Abrahamsen for their great effort in ringing and recording breeding attempts of colour-ringed cormorants.

Results

The maximum likelihood estimates (MLEs) are shown in Table 1. There were no issues of either intrinsic or extrinsic parameter redundancy in the integrated model structure, so the MRR data analysed alone is able to estimate all survival, transition and capture/recovery probabilities. Once the MRR data were combined with census information we were also able to estimate the fecundity of the population. The addition of census data improves the precision of capture and recovery probabilities slightly (Table 2), however the change is small as the only information added by the census data is through the correlation structure of these parameters. The largest improvement in precision is for the breeding dispersal parameters, with no improvement in the natal dispersal parameter. This is most likely due to the fact that natal dispersal only appears in the Leslie matrix as a product with the unknown parameter fecundity. Thus, precision is not improved without compromising the precision of the fecundity parameter. Attempting to estimate parameters from the census data alone results in parameter estimates with low precision due to the census data producing a flat likelihood (the fecundity estimate from census data alone is 1.77 with standard error 14.854), however once the census data are combined with MRR data, the fecundity parameter estimate was precise (MSE = 0.0156). Adult survival gains some precision whilst juvenile survival does not.

The generalised variances of the common parameters of the MRR analysis and the integrated analysis are 3.64 × 10 -13 and 2.80 × 10 -17 respectively. Thus, combining the additional census data has considerably improved the overall precision of common parameters and allowed fecundity to be estimated with good precision.

The combined analysis has also accurately estimated the observation error of the census data. Though it was assumed constant for this simulation, it is interesting to allow observation error to vary proportionally to population size, e.g. t ∼ N (0, y t-1 σ 2 ) [START_REF] Tavecchia | Estimating population size and hidden demographic parameters with state-space modelling[END_REF]. Similarly, complex fecundity structures, such as density dependence could also be considered.

Appendix

The construction of the closed-form Arnason-Schwarz likelihood This appendix gives an outline of the construction of the explicit MRR likelihood first derived in [START_REF] King | Closed-form likelihoods for Arnason-Schwarz models[END_REF]. Recall that the encounter history of an animal can be broken down into three partial histories: last live encounters and beyond, consecutive


Gerardo Salinas

Seyyed Mohsen Beladi-Mousavi

Alexander Kuhn
email: kuhn@enscbp.fr

Recent Progress in Enzyme-Driven Micro/Nanoswimmers: from Fundamentals to Potential Applications

Keywords: microswimmers, enzymes, biocompatibility, biomimetic systems

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Smart biological systems in living organisms can transform chemical energy into mechanical action, and hence carry out precise tasks in confined spaces [1,[START_REF] Schwarz | Hybrid biomicromotors[END_REF]. Inspired by such natural motors, artificial biomimetic systems that can transform different types of energy into mechanical motion, have been developed. These so-called micro/nanoswimmers have gained considerable attention due to their numerous potential applications, e.g., for sensing, cargo delivery, and environmental remediation. However, these artificial devices often suffer from the toxicity of their components (e.g. Al, Ga, Cu) or the employed fuels (e.g. acidic or alkaline solutions, H 2 O 2 ), thus partially preventing their use for biomedical applications, such as biorecognition, biosensing, imaging and nano-surgery. In order to circumvent these limitations, more recent studies suggested miniaturized engines based on enzymatic catalysts, providing unique characteristics such as biocompatibility of the swimmers and fuels, as well as adaptability to different environments. Such self-propelled enzymatic micro/nanoswimmers, powered by different enzymes, have been proposed for exciting potential applications e.g. active targeted drug delivery [START_REF] Tang | Enzyme-powered Janus platelet cell robots for active and targeted drug delivery[END_REF] or blood-brain barrier crossing [START_REF] Joseph | Chemotactic synthetic vesicles: Design and applications in blood-brain barrier crossing[END_REF], which might be important for transporting pharmaceutical compounds toward localized areas inside organs. The rational design of these swimmers, incorporating various enzymes, is a critical factor concerning their performance, in particular the efficiency of their motion in complex environments. Hence, swimmers with different shapes (spherical or tubular) and sizes (from 200 nm to 5 mm), decorated with single or multiple enzymes, have been developed [5•]. Among the different enzymes used to power motion of micro-and nano swimmers, two different enzymatic processes are mostly used. The first category is based on redox reactions like it is the case for glucose oxidase, catalase, bilirubin oxidase or peroxidase, whereas a second category employs hydrolysis reactions e.g. urease and lipase (Table 1). The present review aims to discuss the recent advances concerning the mechanisms of propulsion and the potential applications of single-and multiple-enzyme powered micro/nanoswimmers. In addition, this report especially focuses on devices where motion is achieved by enzymatic process based on redox reactions or fully triggered by electrochemical processes. [START_REF] Tang | Enzyme-powered Janus platelet cell robots for active and targeted drug delivery[END_REF][START_REF] Feng | Self-adaptive enzyme-powered micromotors with switchable propulsion mechanism and motion directionality[END_REF][START_REF] Arque | Ionic species affect the self-propulsion of urease-powered micromotors[END_REF][START_REF] Hortelao | Enzyme-powered nanobots enhance anticancer drug delivery[END_REF], acetylcholinesterase, glucose oxidase, aldolase [START_REF] Arque | Intrinsic enzymatic properties modulate the self-propulsion of micromotors[END_REF], catalase [START_REF] Somasundar | Positive and negative chemotaxis of enzyme-coated liposome motors[END_REF][START_REF] Sun | Enzyme-powered nanomotors with controlled size for biomedical applications[END_REF][START_REF] Chen | Motion of enzyme powered microshell motors[END_REF], and lipase [START_REF] Hu | Autonomous motion of immobilized enzyme on Janus particles significantly facilitates enzymatic reactions[END_REF][START_REF] Wang | Enzyme conformation influences the performance of lipasepowered nanomotors[END_REF] L structure [START_REF] Gao | Biocatalytic self-propelled submarine-like metal-organic framework microparticles with pH-triggered buoyancy control for directional vertical motion[END_REF]. At neutral pH, the oxygen generated by the enzymatic reaction binds to the hydrophobic PDPA, leading to an ascending motion. In contrast, at slightly acidic pH, PDPA becomes hydrophilic, causing the release of oxygen, and consequently a sinking of the device. In a recent work, protein micro-tubes, functionalized with urease, exhibit selfpropulsion in the presence of urea, due to a concentration gradient established by the enzymatic reaction in the inner part of the tube [START_REF] Sugai | Nonbubble-propelled biodegradable microtube motors consisting only of protein[END_REF] Motion of dissymmetric magnetic microparticles, asymmetrically modified with multilayers of biotinylated urease, has also been reported [START_REF] Ming | Enhanced propulsion of ureasepowered micromotors by multilayered assembly of ureases on Janus magnetic microparticles[END_REF]. Most importantly, these devices move at physiological urea concentrations (10 mM) and in liquids with viscosities four times higher than the viscosity of water, which is important for future in vivo applications. Ma et al. synthesized hollow mesoporous SiO 2 microspheres functionalized with urease at the sphere surface [START_REF] Xu | Enzymatic micromotors as a mobile photosensitizer platform for highly efficient on-chip targeted antibacteria photodynamic therapy[END_REF]. The resulting propulsion is due to the intrinsic asymmetry of the SiO 2 spheres, generating ionic self-diffusiophoresis caused by the enzymatic decomposition of urea. An interesting alternative is the design of systems that can be triggered by different stimuli, thus facilitating motion in complex fluids. For example, lipase-modified dendritic silica/carbon/Pt dissymmetric nanoparticles can move either via self-diffusiophoresis or bubble propulsion, as well as self-thermophoresis (Figure 1c) [START_REF] Xing | Janus dendritic silica/carbon@Pt nanomotors with multiengines for H 2 O 2 , near-infrared light and lipase powered propulsion[END_REF]. In these devices, the Pt reducing microcathode at the other end (Figure 2a). In the presence of glucose, the electron flow Glucose → GOx → RPI → fiber → RPII → BOD → O 2 is accompanied by an ion flow along the carbon fiber, resulting in its propulsion at the water/oxygen interface [29 2b-c) [START_REF] Toebes | Spatial control over catalyst positioning on biodegradable polymeric nanomotors[END_REF]. In another example, Ma et al.

assembled a core-shell nanoswimmer utilizing metal-organic frameworks to promote photodynamic therapy (PDT) and starvation therapy (ST) (Figure 2d). In the involved domino reaction, GOx initially catalyzes the decomposition of intracellular glucose to promote cell starvation and H 2 O 2 generation, whereas catalase decomposes the generated H 2 O 2 , thus triggering motion and oxygen production, which promotes PDT [START_REF] You | Self-propelled enzymatic nanomotors for enhancing synergetic photodynamic and starvation therapy by self-accelerated cascade reactions[END_REF]. An analog en y atic syste as e pl yed by t dler et al., based on Janus microswimmers decorated by enzymes only on one hemisphere (Figure 2e). The diffusion of these biocompatible swimmers is enhanced in the presence of glucose [START_REF] Schattling | Enhanced diffusion of glucose-fueled Janus particles[END_REF]. In a more recent study, He et al. elaborated a nanoswimmer powered by GOx and catalase, trapped in a carbonaceous nanoflask (CNF), moving in a glucose solution via a domino reaction mechanism [35], similar to previous studies (Figure 2f) [START_REF] Pantarotto | Autonomous propulsion of carbon nanotubes powered by a multienzyme ensemble[END_REF][START_REF] Abdelmohsen | Dynamic loading and unloading of proteins in polymeric stomatocytes: Formation of an enzyme-loaded supramolecular nanomotor[END_REF][START_REF] Toebes | Spatial control over catalyst positioning on biodegradable polymeric nanomotors[END_REF][START_REF] You | Self-propelled enzymatic nanomotors for enhancing synergetic photodynamic and starvation therapy by self-accelerated cascade reactions[END_REF][START_REF] Schattling | Enhanced diffusion of glucose-fueled Janus particles[END_REF]. In this work, it was demonstrated that changes in surface wettability (hydrophobicity or hydrophilicity) of the nanoswimmer can dictate the direction of motion [35]. In another study, a dissymmetric microparticle was proposed where one side included two enzymes, GOx and trypsin, which were working as independent engines using different fuels, namely glucose and bis(benzyloxycarbonyl-L-arginine amide) (Figure 2g). Both enzymes contribute to the enhanced diffusion of swimmers using biocompatible fuels [36]. Finally, Van Hest et al.

reported swimmers with a shape similar to the one of Figure 2b, containing multiple enzymes, promoting several feedforward loops using various substrates to produce kinetic energy as the output of the enzymatic network (Figure 2h) [37••]. Adapted from reference [START_REF] You | Self-propelled enzymatic nanomotors for enhancing synergetic photodynamic and starvation therapy by self-accelerated cascade reactions[END_REF]. e) Janus microswimmer with one hemisphere decorated with GOx and catalase. Adapted from reference [START_REF] Schattling | Enhanced diffusion of glucose-fueled Janus particles[END_REF] catalyze the oxidation of the azo-dye compounds. An interesting alternative is a design of "2 in 1" syste s, here the en y atic reacti n enables the propulsion mechanism and catalyzes the decomposition of organic pollutants. For example, lipase-modified mesoporous silica nanoparticles present random-walk in triglyceride/PBS solutions [START_REF] Lei | Lipase-powered mesoporous silica nanomotors for triglyceride degradation[END_REF].

As motion is triggered by the catalytic decomposition of triglycerides, causing a concentration gradient, these devices can act as active cleaners of fully and partially soluble oils. In a recent study, PEDOT-Au/peroxidase microswimmers were used as dynamic catalytic systems for the removal of phenolic compounds, such as phenol, bisphenol A, guaiacol, pyrogallol and catechol [39•]. In these devices the immobilized peroxidase not only catalyzes the decomposition of H 2 O 2 to form oxygen (bubble propulsion), but also degrades the phenolic compounds (Figure 3a). Different urease-powered pH-responsive swimmers have been designed for the delivery of chemotherapeutic agents and bioimaging probes [START_REF] Tang | Enzyme-powered Janus platelet cell robots for active and targeted drug delivery[END_REF][START_REF] Chen | Enzyme-powered Janus nanomotors launched from intratumoral depots to address drug delivery barriers[END_REF][START_REF] Llopis-Lorente | Enzyme-powered gated mesoporous silica nanomotors for on-command intracellular payload delivery[END_REF]. Drug-loaded catalase-powered nanoswimmers have been used for local drug delivery against inflammations [START_REF] Wang | Self-propelled PLGA micromotor with chemotactic response to inflammation[END_REF]. These devices present positive chemotaxis towards H 2 O 2 concentration gradients, produced by phorbol ester-stimulated macrophages.

Recently, catalase-powered Au-mesoporous silica nanoparticles, functionalized with disulfide-linked oligo-(ethylene glycol) chains, acting as Janus gatekeepers have been reported [START_REF] Llopis-Lorente | Stimulus-responsive nanomotors based on gated enzyme-powered Janus Au-mesoporous silica nanoparticles for enhanced cargo delivery[END_REF]. These devices exhibit motion via the catalytic decomposition of H 2 O 2 and deliver their cargo upon recognition of a reducing agent (glutathione). Finally, enzymepowered swimmers, equipped with different transducers for pH, DNA, and biomarkers, have been used for sensing [START_REF] Russell | Multifunctional motion-to-color Janus transducers for the rapid detection of sepsis biomarkers in whole blood[END_REF][START_REF] Zhang | Bubble-propelled Jellyfish-like micromotors for DNA sensing[END_REF][START_REF] Patino | Self-sensing enzyme-powered micromotors equipped with pH responsive DNA nanoswitches[END_REF]. An interesting alternative is to take advantage of the on-off mode of light emission in the presence of a fluorescent molecule. For example, catalase-powered dissymmetric rods, functionalized with tetraphenylethene (TPE) derivatives and fluorescein isothiocyanate (FITC) as fluorophores, exhibit self-propulsion due to the enzymatic decomposition of H 2 O 2 , and fluorescence changes from blue to green after the capture of tumor cells [START_REF] Zhao | Janus micromotors for motioncapture-ratiometric fluorescence detection of circulating tumor cells[END_REF]. Another approach is the use of the changes in the swimming behavior, i.e. trajectory or speed, to detect species of interest. For example, PEDOT/Au-catalase micro-tubes exhibit biocatalytic inhibition of the enzyme in the presence of chemical stress (e.g., heavy metals, pesticides and herbicides), which results in a decrease of bubble production, directly detectable via their swimming behavior (Figure 3b)

Conclusions and perspectives

In this review, we have examined recent work in the field of enzyme-driven micro/nanoswimmers based on different propulsion mechanisms. In the case of singleenzyme-powered swimmers, the enzymatic reaction triggers either the asymmetric release of bubbles or the self-generation of chemical or electric gradients, causing the propulsion of the devices. In contrast to this, in multiple-enzyme swimmers, a cascade of enzymatic reactions can generate motion based on bubble propulsion or self-electrophoresis. In addition, different applications of these devices are presented, where the enzymatic reaction is used either for the degradation of organic molecules or as analytical tools. These devices allow biomimetic self-propulsion, associated with improved biocompatibility, and efficient energy conversion. Their size and controlled trajectories might enable their use in different organs and cells. Nevertheless, there are still many issues to address, e.g. the toxicity of some fuels, the easy and direct visualization, the efficient biodegradability and the limited life-time of the devices (recyclability). However, different approaches have been proposed

to solve these problems, like modifying the shape, increasing the catalytic performance of the enzymes or using cascade reactions, and employing materials able to be digested completely by enzymes (e.g., proteases) or in biological media. Finally, the enzyme-based nanoswimmers, in particular those using redox reactions, have shown promises for biomedical applications. However, in order to develop drug delivery systems for "real rld" in vivo applications, tiny robotic systems inspired by biological reactions in which robots and fuels are entirely biocompatible are mandatory and deserve additional research efforts. Furthermore, many other features of the robots, most importantly stability, biocompatibility, selectivity, directionality and the velocity in different biological environments need to be significantly improved prior to their commercialization.

Acknowledgements

The • Enzymatic degradation of azo-dye pollutants by using laccase-based swimmers

  photothermal effect of the carbon part causes a thermal gradient when irradiated with near-

  Figure 1. Schematic illustrations of different single-enzyme-powered

Figure 2 .

 2 Figure 2. Schematic illustrations of dual/multiple-enzyme-powered micro/nanoswimmers.

  [50•]. Recently, the directional control of self-electrophoretic BOD/Ppy/oligo-BT 2 T 4 swimmers has been demonstrated [22••]. Enantioselective clockwise or anti-clockwise motion is caused in this case by a site-specific proton flux associated with the oxidation of an enantiomer present in the solution (Figure3c). In addition, the curvature of the track can be used for the direct visualization of the degree of enantiomeric excess in the solution.

Figure 3 .

 3 Figure 3. a) Scheme f a "2 in 1" per xide-driven micromotor for bioremediation of

  25. Zhao X, Gentile K, Mohajerani F, Sen A: Powering motion with enzymes. Acc Chem Res 2018, 51:2373-2381. •C prehensive revie c vering the field of enzyme powered swimmers. 26. Patiño T, Arqué X, Mestre R, Palacios L, Sánchez S: Fundamental aspects of enzyme-powered micro-and nanoswimmers. Acc Chem Res 2018, 51:2662-2671. 27. Ou J, Liu K, Jiang J, Wilson DA, Liu L, Wang F, Wang S, Tu Y, Peng F: Micro-/nanomotors toward biomedical applications: The recent progress in biocompatibility. Small 2020, 16:1906184. 35. Gao C, Zhou C, Lin Z, Yang M, He Q: Surface wettability-directed propulsion of glucose-powered nanoflask motors. ACS Nano 2019, 13:12758-12766. 36. Schattling PS, Ramos-Docampo MA, Salgueiriño V, Städler B: Double-fueled Janus swimmers with magnetotactic behavior. ACS Nano 2017, 11:3973-3983. 37. Nijemeisland M, Abdelmohsen LKEA, Huck WTS, Wilson DA, van Hest JCM: A compartmentalized out-of-equilibrium enzymatic reaction network for sustained autonomous movement. ACS Cent Sci 2016, 2:843-849. •• First exa ple f multiple-enzyme-driven swimmers promoting several feedforward loops. 38. Choi H, Cho SH, Hahn SK: Urease-powered polydopamine nanomotors for intravesical therapy of bladder diseases. ACS Nano 2020, 14:6683-6692. 39. Bayraktaroglu M, Jurado-Sanchez B, Uygun M: Peroxidase driven micromotors for dynamic bioremediation. J Hazard Mater 2021, 418:126268. •"2 in 1" syste here the en y atic reacti n pr duces ti n and simultaneously degradation of organic pollutants. 40. Uygun M, Asuncion-Nadal V, Evli S, Uygun DA, Jurado-Sanchez B, Escarpa A: Dye removal by laccase-functionalized micromotors. Appl Mater Today 2021, 23:101045.

Table 1 .

 1 Representative enzymatic reactions used for the propulsion of micro and nanoswimmers.

	Enzymatic processes using redox reactions
	Enzyme	Reaction
	Glucose oxidase (GOx)	-D-Glucose	D-glucono-1,5-lactone
	Catalase

  Such domino reactions avoid the addition of H 2 O 2 and hence lower the biotoxicity[START_REF] Pantarotto | Autonomous propulsion of carbon nanotubes powered by a multienzyme ensemble[END_REF]. Following a similar elegant principle, Wilson et al. proposed bowl-

	shaped polymeric systems with a narrow opening containing GOx and catalase enzymes
	(Figure 2b) [31•]. These systems not only prevent the hydrolysis of the protected enzymes,
	but also allows generating motion of the biocompatible swimmers in the presence of
	physiological amounts of glucose (Figures

••]. Feringa, et al. designed swimmers based on multi-wall carbon nanotubes (MWCNTs), decorated with GOx and catalase to transform glucose into H 2 O 2 and then to water and oxygen, respectively.

  work has been funded by the European Research Council (ERC) under the European

	22. Arnaboldi S, Salinas G, Karajic A, Garrigue P, Benincori T, Bonetti G, Cirilli R,
	Bichon S, Gounel S, Mano N, Kuhn A: Direct dynamic readout of molecular chirality
	with	autonomous	enzyme	driven	swimmers.	Nat	Chem	2021,
	https://doi.org/10.1038/s41557-021-00798-9.
	•• First exa ple of enantioselective trajectory control.
	23. Ma X, Hortelão AC, Patiño T, Sánchez S: Enzyme catalysis to power
	micro/nanomachines. ACS Nano 2016, 10:9111-9122.
	Uni n's H ri n 2020 research and inn vati n pr gra (grant agree ent n° 741251, ERC
	Advanced grant ELECTRA).
	7. References and recommended reading
	•Paper f special interest.
	••Paper f utstanding interest.
	1. Gao C, Wang Y, Ye Z, Lin Z, Ma X, He Q: Biomedical micro-/nanomotors: From
	overcoming biological barriers to in vivo imaging. Adv Mater 2020, 2000512.

•Different enzyme-powered micro/nanoswimmers are discussed in detail.

24. Mathesh M, Sun J, Wilson DA: Enzyme catalysis powered micro/nanomotors for biomedical applications. J Mater Chem B 2020, 8:7319-7334.

Graphical abstract

399