The use of molecularlymodified electrodes in catalysis heraldsa new paradigm in designing chemical transformations by allowing controlof catalytic activity. Herein, we provide an overview of reportedmethods to develop electrodes functionalized with organometallic complexesand a summary of commonly used techniques for characterizing the electrodesurface after immobilization. In addition, we highlight the implicationsof surface functionalization in catalysis to emphasize the key aspectsthat should be considered during the development and optimizationof functionalized electrodes. Particularly, surface–moleculeelectronic coupling and electrostatic interactions within a hybridsystem are discussed to present effective handles in tuning catalyticactivity. We envision that this emerging type of hybrid catalyticsystem has the potential to combine the advantages of homogeneouscatalysis and heterogeneous supports and could be applied to an expandedrange of transformations beyond energy conversion. It has been shown thatimmobilizing molecular catalysts onto an electrically addressablesurface may help circumvent solvent incompatibility, in addition topromoting electron transfer, simplifying the separation process ofproducts from the catalyst, and preventing the formation of aggregates.13 Thus, a surface-immobilized molecular catalyst is expected to bemore efficient and recyclable in electrochemical synthesis than itsnonimmobilized counterpart. An array of studies has demonstrated thatchemically modified electrodes prepared by immobilizing organometalliccomplexes onto an electrically addressable surface provide an excellentplatform for obtaining potent catalysts for key transformations relatedto energy generation.21The immobilizationof molecular catalysts on an electrode surfacemay be a solution to bridge the gap between freely dispersed molecularcompounds and heterogeneous catalysts 1. A molevia noncovalent and covalent interactions will be elaborated in detailfor carbon-based electrodes given their accessibility, availability,and feasibility. Modifications of the molecular moiety may be requiredto accommodate the anchoring handles on the electrode surface in orderto implement a specific linkage. We demonstrate the possibility toharness electrode-supported organometallic catalysts in electrochemicaltransformations via different immobilization methodsand their disparate effects on catalytic activity. To provide a clearstarting point for those who are new to the field, we also includetechniques commonly employed in characterizing modified surfaces thatplay an indispensable role in interpreting the properties of the functionalizedmaterial. Aspects regarding the linkage, local reaction environment,and local structure of the designed system after immobilization willbe discussed next along with applications of the functionalized electrodein electrocatalytic transformations related to energy conversion.Particular attention is devoted to the electrochemical control andtunability of the functionalized catalysts. Electronic and electrostaticinteractions between the molecule–surface interface will bediscussed in detail, showcasing necessary mechanistic investigationsof the hybrid system. Local electric fields induced by charged functionalgroups of a supporting ligand represent another unique handle to modulatereactivity through electrostatic effects. Moreover, some compellingexamples discussing the effect of externally applied electric fieldsin a hybrid system on catalytic turnover will be included to showcasethe diversity of future applications. As this review will elaborateon the difference and connections between electrocatalysis and catalysispowered by electrostatic interactions, we hope to encourage effortson broadening the scope of applications harnessing molecularly modifiedelectrodes as potent catalysts. Overall, we aim to inspire researchersto interface organometallic chemistry, surface chemistry, and electrochemistry.This review aims to stimulateideas and critical thinking on therational design of chemically modified electrodes through immobilizinga molecular functionality, primarily an organometallic complex, ontothe electrode surface. Applications of the hybrid catalytic materialfocus on maximizing energy efficiency, improving catalyst lifetime,optimizing catalytic activity, and expanding application scope. Byillustrating the unique reactivity and comparing the catalytic behaviorof different hybrid systems, we strive to provide a systematic viewof the field. First, we will offer a technical perspective on thepossible approaches to immobilize a molecular catalyst based on thetype of electrically addressable surface. Metal, metal oxide, andcarbon-based electrode materials are discussed due to their broadapplication in the field of electrochemistry. Furthermore, strategiesfor surface immobilization of metal complexes 2hybrid catalyticmaterial based on the type of electrode surface. Depending on availableanchoring sites and immobilization methods, modifications of electricallyaddressable surfaces with anchoring handles may be necessary. A representative selectionof examples concentrating on immobilizing organometallic complexeson each type of electrode surface will be discussed.In this section, we will introducecommon strategies to constructa molecularly modified electrode or the 2.122 The intrinsicelectronic structure of the metal electrode determines the electrocatalyticactivity of the system, while the desorption energy of active specieson the surface can be modulated by polarizing the electrode surface.23 In light of that, surface modification of metalelectrodes was envisaged to modulate the catalytic activity of theanchored species. For example, Deng and co-workers triggered an electropolymerizationof molecules to generate organic frameworks on the surface of a goldelectrode that created a compact interface enriched by amino groups.24 This noncovalent modification of the metal electrodesurface resulted in higher CO production and selectivity than a baregold electrode under the same conditions. To increase the availableanchoring sites on metal surfaces, premodifications of the metal arecommonly required. Sulfur-based self-assembled monolayers on goldare pervasive.25 For example, a gold surfacemodified with alkanethiols featuring terminal azides was functionalizedby a biomimetic metal complex equipped with terminal alkynes via a “click reaction” show great stabilityin binding to metal electrodes, especially gold electrodes, with versatilityfor further functionalization.31 Adjustable polymer brushes bound to ruthenium were successfullyattached to a gold surface through an NHC–Au interaction , making them great candidates as electrode materials.47Metal oxides represent another promising platformfor immobilizationbecause of their accessibility, thermodynamic stability, and potentialto be prepared in various nanometer-sized structures.via intervening ligands.48 Direct adsorption onto metal oxide surfacesand physical entrapment of molecules inside porous layers are commonstrategies to assemble such heterogenized catalysts for transformationssuch as water oxidation.50 It is important tonote that the metal/ligand can react with the oxide support in variousways. Surface hydroxyl groups were found to serve as both Brønstedacid and base sites.48 However, the focusof this review is not the coordination chemistry or reaction mechanismsof supported metal complexes on metal oxide surfaces; instead, wehighlight the possibilities to leverage the interaction with metaloxide surface hydroxyl groups.50Organometallic complexes can be introduced onto metal oxidesupportseither by direct bonding or 52 Moreover, various types of anchoring handles weredeveloped to create diverse and stable linkages between the surfaceand metal complex.56 Metal complexes can be immobilized covalently onto metal oxide surfacesby implementing a designed anchoring handle in the ligand structure.6157 On the other hand, metal oxide surfaces can be modifiedwith those anchoring groups which serve as the surface ligand to coordinatewith the metal center.62via metal coordination.62 Otherexamples of anchoring handles include π–π interactionsbetween compounds bearing aromatic functionalities and pyrene-modifiedmetal oxide surfaces,47 as well as covalent graftingof metal complexes containing vinyl functionalities to surface hydroxylsby electro- or light-initiated polymerizations.66Axial coordinationof metal centers to the abundant hydroxyl groupson metal oxide surfaces is readily available.57 The presence of various typesof binding modes to the surface hydroxyl groups and the possible coexistenceof multiple binding modes differentiates metal oxide supports frommetal and carbon-based surfaces, as will be discussed later in thereview.Common anchoring groups exemplified by oxyacids  and silatranes displaydifferent binding modes when attached to the metal oxide 4. The ty2.269 Various immobilization methodson carbon-based electrodes  havebeen reported.13 Molecular modificationsof carbon-based electrodes combine the stability and conductivityof carbon electrodes with the high activity and selectivity of metalcomplexes.71 Common strategies for functionalizing carbon-based electrodes includepolymer coating, adsorption, and noncovalent and covalent methods are commonly used as the heterogeneoussubstrate; however, metal oxides are also suitable for this method.47 Increased catalyst lifetime, enhanced activity, and/or improvedproduct selectivity of the immobilized catalyst are commonly observedcompared to the parent metal complex in electrocatalytic reactions,likely due to the efficient electron transfer between the electrodesurface and the immobilized metal complex via π–πstacking.105 For example, the iridium complex immobilized on CNT displayed highturnover numbers  and turnover frequencies (TOF = 15s–1) in the selective electrocatalytic reductionof CO2 to formate.102Carbon-based electrodes represent idealcandidates for noncovalentimmobilization of metal complexes because of their high surface area,stability, and, most importantly, availability to form π–πinteractions with an aryl group of the ligand framework.via π–π stacking.105 Axial coordination of the metal center tothe heteroatoms doped on the carbon electrode offers additional promotingeffects after immobilization 12]− 12]− 8b.113 Re3.2.3via amidebond formation and diamine condensation.114 The presence of carboxylicacids and o-quinones on the edge plane of graphiticelectrodes provides available sites for forming amide bonds and fullyconjugated pyrazine linkages. Re(CO)3Cl was fully conjugated to graphite and achieved superioractivity in CO2 reduction  displayed remarkably high turnover numbers, greaterthan 12,000, with CO as the only product. A graphite-conjugated cobalttetraphenylporphyrin (GCC-CoTPP) and a nonconjugated cobalt porphyrincomplex (Amide-CoTPP) were investigated in comparison , a long alkene linker (II), π–πinteractions (III), and simple adsorption (IV) of cobalt complexes demonstrated that different types of immobilizationmethods may be advantageous and preferable under specific conditions, homogeneous compounds displayeda differenttrend (Co-iii > Co-ii > Co-i). Overall, all three immobilization methods had the potential toafford a catalytically active material, which was found to have animproved lifetime and minimal catalyst aggregation. Thus, an evaluationof the advantages and disadvantages of different immobilization strategiesand a comparison study of the resulted systems is necessary for reactionoptimization.It is important to keep in mind that one type ofimmobilizationmay not be necessarily superior to another method. A study comparingfunctionalized electrodes through a short-conjugated linker , electrostatic interfacialinteractions in the electrolyte, effects of functional groups on theligand, and possible metal–metal interactions upon surfacedepositing. In Lastly, surface immobilization may induce molecule–surfaceinteractions and interactions between molecular entities that maylead to different implications. Catalyst loading and aggregation,solvent and electrolytes, and many other factors have to be considered,in addition to the choice of electrode and linkage for a hybrid catalyticmaterial in real life applications. We include the aspects to considerfor a rational design of an electrocatalytic system enabled by a molecularlymodified electrode in 451A comprehensive characterization of the molecularly modified surfacepaves the way to apply the functionalized material in catalysis. Mostorganometallic complexes with redox-active metal centers or redoxnoninnocent functionalities can deliver electrochemical responsesin cyclic voltammetry to display unique features. Besides electrochemicalcharacterization, heavy elements and representative binding modeswith the electrode surface can also be analyzed by spectroscopic techniquessuch as UV–vis, IR, and X-ray photoelectron spectroscopy, whichcould provide complementary information to cyclic voltammetry undersome circumstances.4.1116 In such a system,the working electrode will host the redox reaction of interest andthe counter electrode can balance the current flowing from the Faradaicreaction on the working electrode surface. The reference electrodewill allow accurate measurements of the working electrode potential.In the three-electrode system, the Gouy–Chapman–Sternmodel at the interface between the working electrode and electrolyteis often used to describe the interfacial effect (117 The double layer on an electrode surface includes a compact doublelayer within the outer Helmholtz plane (OHP) and a diffusion layerconnected to ion redistribution.118 Toensure a negligible contribution from the migration to the mass transportof the charged active species during a Faradaic reaction, an excessof supporting electrolyte is usually used to simplify the mathematicaltreatment of the Faradaic current and establish a uniform ionic strengththroughout the solution even under steady-state Faradaic conditions.119The experimentof collecting the reversible behavior between the flowing currentand a linearly changed potential with time is generally known as cyclicvoltammetry (CV). Cyclic voltammetry is commonly carried out in athree-electrode system 12a.116 Il effect 12b.117 T114 Unlike their solubilized counterparts that are freeto move around in an electrolyte solution, surface-anchored compoundshave a limited diffusion range depending on the linkage length andsolvent, and usually do not diffuse through the electrical doublelayer. They gain or lose electrons through electron transfer due tothe directional potential difference with the electrode. Therefore,the peak current of surface anchored compounds in typical cyclic voltammogramswould be linearly proportional to the scan rate rather than the squareroot of the scan rate as for their free soluble analogues,120 a feature that could be employed to validatethat the compounds are anchored can reflect the system’s chemical stability  and electrochemicalreversibility .Cyclic voltammetry is a powerful tool to characterize theelectrodesurface modified with molecular functionalities.anchored 12c.40 Fuipa/ipc), and the other isthe separation of the two peak potentials . For a chemicallystable system, the ratio of peak currents is generally equal to 1.120 However, one should be cautious in calculatingit because the anodic peak current (if the cathodic scan is startedfirst) must be corrected to a zero current baseline to mitigate thedisturbance of the capacitance effect at the interface.120 For a Nernstian wave with a one-electron transfer,the difference between the two peak potentials should be 57–60mV at room temperature.119Two parameters may beextracted from the analysis. One is the ratiobetween the cathodic and anodic peak current  additionalfeatures from the cyclic voltammogram, such as the peak width andits dependence on different scan rates should be evaluated and thewhole electroanalysis should include results at different experimentalconditions; (2) the data interpretation from cyclic voltammetry shouldbe corroborated with other surface-sensitive techniques, discussedin the following section.Cyclicvoltammetry may not be helpful when characterizing somenoncovalently anchored systems with strong electronic interactionsbetween the electrode and the molecular compound.4.2122 IR, and Raman,126 X-ray photoelectron spectroscopy (XPS),38 and X-ray absorption spectroscopy (XAS),127 are commonly engaged. For example, a triazole formed from azide–alkynecoupling has a diagnostic band in IR spectroscopy, offering a uniquefeature to confirm the success of the immobilization.17 XPS allows for the identification of elements that arepresent on the surface.96 UV–vis spectroscopyis also cogent and is often combined with XAS to provide complementaryinformation on the anchored site.114 X-ray absorption near edge structure (XANES) andextended X-ray absorption fine structure analysis (EXAFS) are oftenemployed to provide complementary structural information.114To providesupporting information to cyclic voltammetry, spectroscopic techniques,including UV–vis,in situ monitoring of the surface attached species and/or near surface intermediatesduring a reaction and provide insight into the reaction mechanism,i.e., change in ν(CO) in CO2 reduction80 or changes in the oxidation state of a catalyticallyactive metal center.114 Finally, recentefforts on characterizing interfacial electric fields at electrode/electrolytesurfaces using vibrational sum frequency generation (VSFG) providean understanding of the electrified interface on a molecular level.A recent comprehensive review emphasized the potential of VSFG inexploring fingerprint bands of surface-adsorbed molecular speciescompared with other spectroscopic techniques.128 For example, VSFG was applied to study the strength of the interfacialelectric field generated in CO2 reduction by a rhenium-functionalizedgold electrode.129 Investigations on thebinding site of the catalyst by sum frequency generation spectroscopy(SFG) and IR spectroscopy provided insight into the strength of theinterfacial electric field generated near the charged electrode surfaceby monitoring the shifts of the carbonyl stretching frequencies andamplitude strongly dependent on the applied potential. The magnitudeof the interfacial electric field was calculated to be 108–109 V/m based on experimental data. Remarkably,the order of magnitude of the electric field strength determined inthis work resembles the results determined inside active sites ina biological system that bridges the gap between enzymatic, homogeneous,and heterogeneous catalysis.131Additionally, many techniques allow an in situ surface characterization.132 To aid the interpretation of the surface-functionalizedcompound’s performance, comparisons made between the immobilizedand the homogeneous analogue using multiple techniques are highlyvaluable.Additionally,surface-enhanced Raman spectroscopy (SERS) representsanother powerful tool for 5In 5.1121 A heterogenized molecular catalyst on the electrode decreases thedistance for electrons to travel and circumvents the limitation onmass transportation.15 Understanding the electron transferpathway helps with the interpretation of the reaction mechanism andultimately benefits the optimization of the hybrid catalyst. Homogenouselectrocatalysts usually undergo an outer-sphere interfacial electrontransfer, meaning electrons traverse the double layer between theelectrode and redox intermediates dissolved in the solution.21 Such a process is commonly carried out in two steps: electron transferand substrate activation. An outer-sphere stepwise electron transferpathway is also widely adopted by surface-anchored catalysts predominantlydepending on the type and length of the linkage. The electrode inthis scenario serves as the source or sink of electrons, and the redoxbehavior of the resulting system is anticipated to be similar to thehomogeneous analogue.Efficient electron transferfrom the electrode to the reactant or redox mediator is a key factorin determining the reaction rate of an electrocatalytic transformation.21However, when the attached catalyst getsclose enough to the surface, for example, through a strongly conjugatedlinkage 13, the e114 Graphite conjugationthrough amine-quinone condensation formed a conductive pyrazine linkagefeaturing a strong electronic coupling between the functionalizedmoieties and the electrode  of RhIII/I is more positive than the thermodynamic potential forhydrogen evolution. Overall, GCC-Rh operates at extended pH rangesand displays reactivity that is not dependent on E1/2 (RhIII/I)  15b.o-phenylenediamines with o-quinones on the electrodesuffers from the low population of surface quinone groups on carbonelectrodes as well as applicability toward different types of molecularfunctionalities.In summary, the graphite-conjugatedsystem enabled a concertedelectron transfer and substrate active pathway that is commonly observedfor metal electrodes. Meanwhile, a molecular-level control of theactive sites can be achieved by modifying the conjugated moiety withmolecular precision. However, the condensation of 5.2o-quinoneson the electrode surface, the formation of phenazine or pyrazine maybe restricted. Under such circumstances, whether a strong electroniccoupling between the surface attachment and the electrode throughvarious types of linkage can be achieved regardless of conjugationbecomes an intriguing frontier to explore. The use of a solvent inwhich the molecular moiety is poorly soluble may enforce a concertedmechanism for a weakly conjugated amide-linked cobalt complex.114 Chemically modified electrodes derived froma covalent anchoring strategy through an amide linkage were proposedto adopt a stepwise and redox-mediated reaction pathway during electrocatalysis.120 The metal centers reach their active statesby gaining or losing electrons through an electrostatic potentialgradient within the electrical double layer. The electron transferprocess is then followed by chemical reactions of the substrates atthese active metal sites in the solution. In the comparative studyof graphite-conjugated cobalt tetraphenylporphyrin (GCC-CoTPP) andnonconjugated cobalt porphyrin complex (Amide-CoTPP) shown in 114In the previous examples ofgraphite-conjugated pyrazines and rhodium complexes, a fully conjugatedlinkage between the molecular components and the electrode, namely,a conductive phenazine, was indispensable for the strong electroniccoupling between the electrode and appended metal complex. However,due to the lack of naturally available II/I redox event was observed, consistent withthe analogous homogeneous system. As anticipated, it was found tocatalyze hydrogen evolution via a stepwise pathway,akin to molecular catalysts. In contrast, no cobalt-based redox eventwas observed by cyclic voltammetry in aqueous media at various pHvalues. The key difference between the molecularly modified electrodein the two systems is that, in acetonitrile, the homogeneous analogueand the anchored catalyst remained soluble and thus are consideredfreely solvated and stay outside the double layer. In contrast, themolecular cobalt complex and the anchored compound were insolublein the aqueous solution and were forced to stay close to the electrodeand likely within the double layer  or preferential surfaceinteractions (electrostatic coupling). The strong electronic couplingdescribed in the graphite-conjugated systems represented by GCC-CoTPPonly allows for the concerted pathway by the full conjugation betweenthe surface and appended molecule. To achieve an alternation betweendifferent mechanisms, a long and flexible linkage allowing both surface-adsorbedand freely solvated forms of the catalyst is needed. Furthermore,the molecular–surface interaction is profoundly influencedby the solvent. A solvent in which the appended metal complex is insolubleleads to preferential surface interactions, such as axial coordinationand π–π interactions, alters the reaction mechanismof the cobalt catalyst immobilized with a flexible linkage from “molecular-like”to “metal-like” causing starkly disparate catalyticactivity. Therefore, solvent-induced electrostatic interactions betweenthe anchored molecular catalyst and the electrode surface, as demonstratedin this example, are another important parameter to consider in thedesign of electrodes modified with molecular active sites.5.3134 Due to the local electric fields generated by the charged electrodesurface, substrates bearing charges tend to diffuse across the solutionand accumulate outside the Helmholtz plane.67 In heterogeneous CO2 reduction reactions, alkali metalcations in the electrolyte, for example, are found near the activesite and contribute to the electrostatic stabilization of reactionintermediates. Besides ions in the electrolyte, charged substituentsof the appended organometallic complex have the potential to stabilizekey intermediates during catalysis as well.135 However, only a few investigations on the electrostatic effectsinduced by the ligand substituents of the surface-immobilized systemwere reported.138 In this section, we will discuss how ligand substituents could contributeto stabilizing reaction intermediates, in addition to providing electronicinductive effects and reducing catalyst aggregation on the surface.The surface-attached cobalt complex with a flexibleamide linkage depicted in 146 Additionally, the local structure of the ligand was found to assertits influence on catalysis through electrostatic interactions withsubstrates and intermediates.147 Recentstudies on the effect of ligand substituents on molecular electrocatalysisemphasize the importance of considering through-space effects causedby the installed substituents in addition to the inductive effects(through-structure) that are commonly invoked at the molecular level.148 In other words, the implementation of charged substituents on amolecular functionality being immobilized not only alters the electronicproperties of that functionality by donating or withdrawing electronsbutalso may induce electrostatic stabilization of key intermediates.In homogeneous catalysis, when no external electric field is applied,local electric fields induced by charged groups play an importantrole in controlling catalytic activity.150 A series of homogeneous substituted iron porphyrins was synthesizedto investigate inductive and electrostatic effects in CO2 reduction. Installing an electron-withdrawing substituent was foundto lower the overpotential by reducing the electron density aroundthe metal center. In contrast, electron-donating substituents wereobserved to behave the opposite.151 A linear correlation between log(turnover frequency)and the standard potential was found when the electronic inductiveeffect of the substituents was taken into account. However, incorporatingcharged substituents was found to stabilize key intermediates throughelectrostatic attraction or repulsion, causing a significant deviationof those catalysts from the linear correlation.148 In other words, the presence of electron-donating and electron-withdrawingsubstituents in iron porphyrin complexes resulted in well-understoodinductive effects, whereas the presence of positively and negativelycharged substituents on the ligand resulted in significant through-spaceelectrostatic effects  and Hammett constants (σ) were foundfor neutral and cationic substituents.138 Neutral substituents, including both electron-withdrawing and electron-donatingsubstituents, were found to affect inductively the electronic propertiesof the cobalt complex, while the cationic substituents deviated, indicatingadditional effects. Cationic substituents were again postulated toprovide an electrostatic stabilization of the intermediate in therate-determining step.The major challenge in interpreting the implications of ligandsubstitutions remains in disentangling inductive (through-structure)and electrostatic (through-space) effects. effects 17a.133,1137 Gotz et al. presented a study on the influence of mesityl and thiopheneperipheral substituents on surface-appended iron porphyrin complexes than for the less stericallyhindered Co(II) phthalocyanine (CCG/CoPc) under the same conditionscatalyzed COeduction 18.137 Innditions18a. In anditions18b.In this section, wediscussed important aspects of investigatingthe electronic and electrostatic coupling between the electrode surfaceand the attached catalyst to maximize the combined advantages of theheterogeneous support and homogeneous metal complex. A comprehensiveunderstanding of electron communication and other possible staticinteractions within the system helps the design, development, andoptimization of functionalized electrode materials. By presentingthe inductive, steric, and static effects of ligand substituents,we emphasized that the local structure of the immobilized complex,along with the local environment of the system, is an effective handleto modulate catalytic activity. Although efforts are still neededto interpret the effect of ligand substituents beyond electronic induction,correlating the structure of the attached entity to its catalyticactivity introduces the possibility of precise manipulation of thecatalytic active site on the modified electrode surface.6131 Catalysisenabled by an externally applied electric potential has just startedto emerge as a viable method in the synthetic chemistry toolbox.155 Furthermore, a patterned electrode provides a platform to achievespatiotemporal control over catalytic processes by changing the currentand the voltage.132In the previoussection, we focused on presenting potential applicationsof molecularly modified electrodes in electrocatalysis concentratingon redox reactions related to energy conversions with a brief discussionon possible electrostatic interaction between the surface and theattached molecule. In this section, we will primarily focus on theapplications beyond energy conversion. In addition to current-initiatedelectrocatalytic processes, externally applied electric fields mayalso facilitate catalytic transformations mimicking the powerful electricfields in enzyme catalysis.6.1134 Aligning an electric field along a specificbond axis can either elongate or shorten the bond.156 By precisely controlling the orientation of an externalelectric field along a reaction axis (the direction of bond breakingand bond-forming), a reaction can be accelerated or slowed down.156 In nature, many enzymatic reactions rely on a preoriented localelectric field for electrostatic catalysis.131 Efforts have been made to mimic the electrostatic interactions inenzymes and apply them to synthetic chemistry. Detailed discussionson implementing external electric fields, oriented external electricfields, and electrostatic interactions in wide-ranging applicationshave been included in recent reviews.161 However, common strategies employing scanning tunneling microscopy(STM) to harness oriented external electric fields in catalysis, forexample, suffer from scalability. As an alternative, a design of electriccells with “parallel plates” has been developed to demonstratethe possible application of electrostatic field in catalysis.An external electric field refers tothe uniform electric field generated by a voltage bias.134 Recently, naphthalenediimide was functionalizedon a conductive ITO surface to expand the range of electric field-assistedcatalysis. The anion-π catalysis of the addition of a malonicacid half thioester to enolates was explored , interfacial electricfields exist near the surface of charged electrodes where ions gatheraround and create a double layer in response to an electric field.explored 19.162 Ap165 As a consequence,a parallel plate cell that allows the application of a large electricfield to the system was developed. Adapting this setup, an electricfield-induced change in selectivity in an Al2O3-catalyzed epoxide rearrangement was observed ,and 10 Å of Al2O3 atop 40 Å of TiO2 (“Al2O3/TiO2/Si”)21a. The2O3 coated surfacescaused a decrease in the ratio of E/F, whereas on TiO2 surfaces, an increase in the ratio was found (3), decrease in the ratiowas observed for both Al2O3 and TiO2. A similar observation was found when a blockinglayer of alkylphosphonate RhI on TiO2 was used, whereas a decrease was found when(TPP)RhI(PPh3) on TiO2 was employed.similar selectivity with covalentlyattached and surface-adsorbed rhodium complexes with or without aphosphate ester handle. However, the presence of an extra phosphineligand or a layer of phosphonate can block the necessary interactionsbetween the surface and the metal center leading to the inversionof activity. For instance, an 3.selectivity by increasingthe ionicstrength of the surface by codepositing decorating layers of phosphonate-linkedmethylimidazolium bis(trifluoromethanesulfonyl)imide and/or polystyrenesulfonatecompared to metal oxide surfaces without those decorating layersThis study demonstratesan incredible control of otherwise challengingselectivity of a Rh-catalyzed intramolecular carbene reaction, achievingthe following:6.2167 However, it is undeniable thatmost applications of molecularly modified electrode catalysts arelimited to the energy sector, and breakthroughs are rarely reportedin other areas.6Amolecularly modified electrode has the potential to play dual rolesin electrochemical processes: providing ready access to electronsand/or enforcing a catalytic process by an external/local electricfield. Being able to adjust the applied potential over time createsan opportunity to achieve temporal control in catalysis.168 However, each substrate/catalyst bearing a unique functionalgroup imposing a specific inductive effect requires laboratory synthesisand may not be easily accessible. Computational studies of functionalgroup derivatives can assist but still require experimental validations.Due to the limitations on the number of experimentally accessiblefunctional groups and their associated electronic properties, a discontinuityin the inductive effects is inevitable and cannot be alleviated usingthe traditional organic synthesis approach. In a typical electrochemistrysetup, however, the voltage applied can be tuned incrementally andcontinuously. An unprecedented example took advantage of the electricpotential to replace the functional groups that impose different inductiveeffects. The electronic properties of gold-bound substrates were manipulatedby applying different potentials. The modified surface was then investigatedin cross-coupling and amidation reactions  have been recognized as powerful externalstimuli to control multistep transformations.183 cationic polymerizations,184 and ring-opening polymerizations.186 The selective application of an electric stimulus to trigger redoxevents at a redox-active metal complex provides an alternative wayto chemically controlled redox events. In particular, electrochemicalcontrol in redox-switchable polymerization reactions alleviates theuse of a stoichiometric amount of redox reagents in these processes.It further enables multiple switches by discrete manipulation of theapplied potential. Switching between reductive and oxidative potentialsallows for on/off control of the polymerization process, control overmolecular weights and dispersity, as well as over the stereochemistryand composition of the polymer chain.187 However, homogeneous catalysts involved in electrochemically controlledswitchable polymerizations are limited by mass transport since thecatalysts have to diffuse closer to the electrode surface to be activated.Localizing the catalyst near the electrode surface could circumventthis issue. In that context, the combination of electrode supportsand molecular metal complexes may address both spatial and temporalcontrol in polymerization reactions. An electrochemically controlledsurface-initiated polymerization was possible by using a bis(imino)pyridineiron(II) complex anchored onto titanium oxide nanoparticles grown from the surface that were comparable to those of drop-castedpolymers on the surface ,articles 23a.40 Th surface 23b. The  surface 23c. The  surface 23d. ApplInspired by the selective behavior of electrochemically controllediron(II)/(III) catalyzed polymerization, a substrate containing twoelectrically isolated domains was used to demonstrate the spatialcontrol of polymer composition. Exposing one domain to an oxidizingpotential while the other domain remained untouched resulted in aniron(III) and iron(II) cofunctionalized electrode surface, where thechemoselectivity toward different monomers remained undisrupted: thegrowth of polylactide was observed on the untouched domain containingiron(II), while polycyclohexene oxide was obtained on the oxidizeddomain of iron(III). Overall, a spatially controlled pattern of polymerswas achieved on the electrically defined surface.via a molecular functionalizedelectrode surface in which an electrical potential altered the redoxstate of the immobilized catalyst, affecting the catalyst’sreactivity and enabling a selective polymerization of two monomers.As a result, a spatially controlled surface polymerization patternwas achieved by switching the oxidation state of the iron complexeson two isolated domains in one system.This studydemonstrates a successful case of electrochemicallycontrolled polymerization 7Throughout this review, we tried to convey theadvantages of usinga functionalized electrode as a hybrid material for electrochemicalcatalysis. We started by demonstrating strategies for surface immobilizationof molecular catalysts categorized by the type of electrode supportand viable linkages based on anchoring handles. We discussed severalcommonly employed conductive materials such as metal and metal oxides.We highlighted carbon-based electrodes due to their conductivity,stability, and availability. Common strategies being applied to modifyeach type of electrode material were surveyed, along with techniquesfor characterizing the material after fabrication.To accommodatethe available anchor sites on the electrode surface,complementary modifications of molecular components with necessaryfunctional groups are required. The selection of an appropriate immobilizationmethod depends on the type of reactions and interactions that mayoccur between the surface and the molecular functionality to be appended.The resulting linkage and/or molecular–surface interactionplays an important role in manipulating electron transfer and determiningthe reaction mechanism. With an overview of some recent reports onelectrochemical transformations related to energy conversion, we comparedthe effect on the catalytic activity of different linkages/interactionsbetween the anchored metal complex and the electrode. Modificationsat the molecular level may result in inductive electronic effects,steric effects, as well as electrostatic effects. Overall effectson the catalytic activity of a hybrid system need to be evaluatedin comparison to the homogeneous analogue to get a full picture ofthe newly established system.2 reduction, hydrogen evolution, etc.), potential applicationsin electric field assisted catalysis, in situ modificationof electronic properties of the surface attachment, and spatiotemporallycontrolled electrode surface-initiated polymerizations along withshowcasing the unique advantages of using a hybrid catalytic material.Although the primary field of application for functionalized electrodesremains to be reactions related to energy conversion, molecularlymodified electrodes have a great potential in the electrocatalyticvalorization of organic molecules188 andelectrochemical synthesis beyond the scope of this review.198In (1)Select the molecularcomponent andthe electrode materialA homogeneous metal complex with optimalcatalytic behavior is often a good starting point for a well-knownreaction. Similarly, there is sometimes a previously employed combinationwith a specific electrode material. However, there are several thingsto keep in mind: (i) implementing a molecular handle for immobilizationon the selected molecular catalyst may affect the catalytic behaviorof the resulting system compared to the parent catalyst and (ii) successfulimmobilization does not necessarily result in an invariable translationof the reactivity of the homogeneous catalyst to the hybrid system.(2)Evaluate and implementdifferent immobilizationstrategiesLearning from the examples discussed in (3)Utilize different characterizationmethods to understand the hybrid systemCyclic voltammetry isa convenient tool to characterize redox-active surface-anchored molecularcatalysts. However, a strong electrostatic interaction between themetal center and the electrode will limit the utility of cyclic voltammetry.Instead, cyclic voltammetry could produce confusing results, suchas the lack of clear redox peaks. To address this issue, other characterizationtools should be employed to assist surface analysis for the hybridsystem, such as X-ray techniques that can help examine the existenceof metal centers, their oxidation state, and their coordination environment.4)Investigate catalytic activitiesinapplications related to energy conversion199It is common to observemore than one type of interaction in a hybrid system. Electronic and/orelectrostatic interactions between the surface and the appended moleculesmay impact the catalytic activity. Furthermore, the catalytic behaviorof the molecular catalyst does not always translate to the immobilizedstate. The catalytic activity of a hybrid system is a result of thecombination of the parent molecular compound, the linkage used, andthe reaction macroenvironment, including the solvent and the electrolyte.A comprehensive study of the molecule–surface interactions,electrode/electrolyte interface, electron transfer kinetics, and fundamentalunderstanding of the reaction mechanism are necessary.5)Expand the reaction scopeWe hope to encourage further discussion on combining the meritsof molecular catalysts and electrode supports to achieve the optimalactivity, selectivity, and durability for a wide scope of chemicaltransformations. To provide a reference point for beginners in thefield, we summarize the following learning outcomes from this review.198 New electrode materials, represented by single-atom catalysts, molecularlymodified electrodes, and other types of interfacial catalysts, arediscussed only in a small number of reviews.196 Most importantly, the application of theelectrode-immobilized catalysts is limited to products with only afew carbon atoms, in most cases, one or two; new electrode-supportedcatalysts should help produce more complicated structures than thosecurrently produced, and even polymers.Although thefield of surface immobilization and electrode-supportedmolecular catalysis is growing, the majority of studies are concentratedon electrochemical transformations related to energy conversion. Electroorganicsynthesis is an emerging field taking advantage of electricity toreplace fossil fuel and/or toxic redox reagents; however, homogeneouselectrocatalysts are almost exclusively employed.2 reduction and electrochemical organic synthesisof valuable products with higher complexity, we suggest the followingapproaches:(I)Design and develop a functionalizedelectrodeThroughout this piece, we hope we demonstrated that,by immobilizing homogeneous catalysts, represented by metal complexes,the catalytic activity and selectivity of the tunable molecular structurecan be integrated with recyclable and readily available electrodesupport. The molecularly modified electrode, which represents an emergingtype of electrocatalyst, is worth investigating for future applications.The rational design and development of a hybrid catalyst could beoptimized based on structure–property or structure–activityrelationships of the active site, providing an extra degree of controlover the catalytic activity.(II)Investigate the potential of couplingredox events in both half cells203 For example, a value-added process in the counter chamber can be coupled with a targeted reaction toreduce the energy loss during electrolysis Integrate multistep transformationswith multifunctional or interdisciplinary catalysts2 reduction via a single-stepprocess remains challenging in many cases.205 However, the use of multifunctional or several catalysts  to conduct multistep transformationspresents an attractive alternative.211 Compatibility, unfortunately, remains a major obstacle toward integratingmultiple catalytic events in one system.212 Nonetheless, a polymerization catalyzed by a palladium catalystwas integrated with electrochemical CO2 reduction to CO,demonstrating the construction of polymeric material from a simplebuilding block like CO2.213 Nonalternatingpolyketones with tunable CO content were obtained based on the appliedcurrent density.Upcyclingof activated small molecules from electrochemical transformationssuch as CO(IV)Engineerthe reaction setupTo bridge the gap betweenelectrochemical small molecule activationsuch as CO220To implement the above-mentioned ideas and possibly advancesomeof the applications to industrial practice, a reaction setup thataccommodates compatible, practical, and large-scale electrochemistryis needed. Limitations of the traditional design of electrochemicalcells such as mass transportation impede its applications. High-throughputelectrochemical cells, parallel plate cells, and flow reactors aredesigned to suit different needs and potentially address the issuesin large-scale electrochemical synthesis.via a handful of immobilization methods. Effects on electron transferrate, catalyst lifetime, activity, and product selectivity are discussed.Implications of functional groups, external electric fields, and solventsare emphasized for catalyst optimization. By presenting an array ofapplications implementing the organometallic complex modified electrodeas the catalytically active material, we hope to convey the advantagesof combining a homogeneous compound and a heterogeneous support inelectrochemistry. We envision that the integration of present electrochemicalactivation of simple building blocks such as CO2 with subsequentprocesses represents a future direction of accomplishing complicatedsyntheses. Overall, the surface functionalization of molecular compoundson an electrically addressable surface heralds a new paradigm in designingand conducting chemical transformations.In summary, we highlight the potential of immobilizing molecularcatalysts, especially organometallic catalysts, onto electrodes toafford a potent catalytic material to interconvert electrical andchemical energy. Such a hybrid system is constructed from a modifiedelectrically addressable surface and metal complexes


Every cancer genome is unique, resulting in potentially near infinite cancer cell phenotypes and an inability to predict clinical outcomes in most cases. Despite this profound genomic heterogeneity, many cancer types and subtypes display a non-random distribution of metastasis to distant organs, a phenomenon known as organotropism. Proposed factors in metastatic organotropism include hematogenous versus lymphatic dissemination, the circulation pattern of the tissue of origin, tumor-intrinsic factors, compatibility with established organ-specific niches, long-range induction of premetastatic niche formation, and so-called “prometastatic niches” that facilitate successful colonization of the secondary site following extravasation. To successfully complete the steps required for distant metastasis, cancer cells must evade immunosurveillance and survive in multiple new and hostile environments. Despite substantial advances in our understanding of the biology underlying malignancy, many of the mechanisms used by cancer cells to survive the metastatic journey remain a mystery. This review synthesizes the rapidly growing body of literature demonstrating the relevance of an unusual cell type known as “fusion hybrid” cells to many of the hallmarks of cancer, including tumor heterogeneity, metastatic conversion, survival in circulation, and metastatic organotropism. Whereas the concept of fusion between tumor cells and blood cells was initially proposed over a century ago, only recently have technological advancements allowed for detection of cells containing components of both immune and neoplastic cells within primary and metastatic lesions as well as among circulating malignant cells. Specifically, heterotypic fusion of cancer cells with monocytes and macrophages results in a highly heterogeneous population of hybrid daughter cells with enhanced malignant potential. Proposed mechanisms behind these findings include rapid, massive genome rearrangement during nuclear fusion and/or acquisition of monocyte/macrophage features such as migratory and invasive capability, immune privilege, immune cell trafficking and homing, and others. Rapid acquisition of these cellular traits may increase the likelihood of both escape from the primary tumor site and extravasation of hybrid cells at a secondary location that is amenable to colonization by that particular hybrid phenotype, providing a partial explanation for the patterns observed in some cancers with regard to sites of distant metastases. Despite our best efforts as clinicians and cancer researchers, this biological complexity has impeded the identification of new cancer therapies that consistently retain efficacy across multiple cancer types and subtypes. Intratumoral heterogeneity, accumulation of new molecular aberrations, clonal evolution with development of therapeutic resistance, and interaction with each individual’s unique immunophenotype further obfuscate the predictive and/or prognostic potential of currently available biomarkers, resulting in the potential for very different treatment outcomes even in two patients with the same subtype of cancer.Transformative work by Hannahan, Weinberg, and others has identified specific “hallmarks” that together define cancer as a disease entity –3. Of th1.2Although propensity for metastasis and primary sites of metastatic lesion development vary across different cancer types, cancers arising from the same tissue-of-origin may exhibit a non-random distribution of spread, a phenomenon known as ‘‘organotropism’’ or organ-specific metastasis. For example, breast cancers tend to metastasize to the liver, bones, lungs/pleura, and brain, while prostate cancers predominantly show metastatic spread to bone, with a considerably lower frequency of metastasis to other anatomical sites  ; and interactions between tumor cells and the host secondary tumor microenvironment that promote colonization after extravasation  , 16. An While circulatory pattern may largely explain the difference in major site of metastatic lesion development for proximal versus distal rectum, in other cases the reasons for metastasis site pattern are less straightforward. Importantly, the point in tumor progression and evolution at which tropism of a given cancer cell is determined remains unknown. It is possible that a subset of primary tumor cells acquires specific properties at their initial site that allow them to reach specific secondary sites that are hospitable only to cancer cells with those specific properties. Alternatively, tumor cells may simply shed from the primary site indiscriminately, leaving the circulation and secondary tissue microenvironment as the major determinants of site of eventual lesion development. It is likely that all of the factors listed above in some way influence the pattern of metastatic distribution for a given cancer type.2in vivo epithelial-immune cell fusion as a driver of tumorigenesis and tumor heterogeneity. Subsequent sections will address tumor cell-immune cell fusion in acquisition of malignant potential and metastatic organotropism.Increasing evidence supports homotypic or heterotypic cellular fusion as an important process in cancer biology. Indeed, the century-old cancer cell-leukocyte fusion theory of metastasis views the acquisition of a metastatic phenotype as a secondary feature imposed on a primary tumor cell via fusion with a healthy migratory leukocyte such as a macrophage . There iCell-cell fusion is a well-established physiological process involved in mesenchymal cell differentiation as well as fertilization, placentation, myogenesis, osteogenesis, wound healing and tissue regeneration , 23, 24.fusion hybrid cells) have been detected in breast  include either daughter cells with a euploid/diploid karyotype or aneuploid daughter cells that are genomically unstable, the latter of which may resemble the instability seen in transformed, malignant cells. Indeed, cell fusion generates aneuploidy, chromosomal instability, and DNA damage that result in phenotypic heterogeneity, transformation, and the capacity to form tumors –23.chromothripsis may occur   . Chromotpieces”) . Similarpieces”) .Given the staggering number of possible genome combinations generated via HST/ploidy reduction, it is likely that many of the resulting fusion hybrids do not survive. A post-hybrid selection process (PHSP) after cell-cell fusion has been proposed, wherein selective pressures within the microenvironment ensure survival of only those hybrid daughter cells that are able to overcome the resulting genomic instability and re-establish cellular and metabolic functionality , 39. ImpDespite the potential for ongoing chromosomal instability in the process of tumor formation, karyotypic analysis showed retention of the original hybrid clonal karyotypes in tumors grown within immunodeficient hosts . Interes2.2While homotypic cell fusion does appear to result in acquisition of new cellular phenotypes, aneuploid cells generated by homotypic fusion cannot be distinguished from cells in which aneuploidy has arisen via other processes . Additioin vitro and in vivo have varied across a number of elegant studies, and include size and flow cytometric scatter properties, co-expression of nuclear and/or cytoplasmic fluorescent proteins, and karyotype analyses following fusion of cells bearing different sex chromosomes   , 22. Het is rare , 28, 29.2.2.1Aguirre et al. conducted fusion assays using human lung cancer cell lines and different human leukocyte lineages isolated from buffy coats of peripheral blood samples . FollowiH2BmRFP colon cancer cells and GFP-expressing macrophages were also able to functionally divide into viable daughter cells expressing both reporter proteins and bearing a transcriptional profile characteristic of both parental predecessors yet also exhibiting new, unique characteristics  and in human subjects with known cancers. Gast et al. injected RFP-labeled B16F10 melanoma cell lines into actin-GFP and R26R-YFP Cre reporter mice to determine whether dual-positive fusion hybrids could be formed in vivo  and disseminated tumor cells (DTCs), often identified as cells expressing epithelial markers yet found in the blood or bone marrow, respectively, of patients with carcinoma . CTCs arand not expressing the pan-leukocyte antigen CD45  gen CD45 –52. Thisgen CD45 . Though Despite their low abundance within primary tumors , CHCs ap3.2The consistent finding that circulating fusion hybrids outnumber conventional CTCs also begs the question of whether fusion hybrids interact differently with the immune system than unfused cancer cells. In cytokine exposure assays, transforming growth factor (TGF-beta 1-3) induced dose-dependent suppression of MC38 proliferation, but showed no effect on hybrids . More st4The term “premetastatic niche” was coined to describe a microenvironment in a secondary organ site that has been rendered permissive to metastatic outgrowth in advance of cancer cell entry through the activity of circulating factors released in a variety of forms by the primary tumor . These t4.1Extravasation and adaptation of newly arrived cancer cells to the tissue microenvironment of distal organs are stringent rate-limiting steps in metastasis. Although we are gaining a better understanding of how premetastatic niches are prepared, it is still unclear how or why they are the sites selected. This section will discuss the available evidence for acquisition of specific leukocyte chemokine receptor and/or cellular adhesion molecule expression by tumor cells as a possible contributor to metastatic organotropism.Leukocyte homing and trafficking are essential functions of both innate and adaptive immune responses. The basic molecular processes involved in trafficking are chemotaxis and cellular adhesion. While chemotaxis is mediated mainly by chemokines, cellular adhesion is mediated by selectins and integrins and their interactions with their respective receptors. Chemokine signaling through chemokine receptors is also a powerful physiologic activator of integrins via “inside-out” signaling, which regulates the ligand-binding affinity of the cell surface receptors in response to changes in the environment . ExpressIn chemotaxis assays, fusion hybrids migrated toward colony-stimulating factor 1 (CSF1) or stromal cell-derived factor-1 (SDF1) at multiple concentrations, whereas unfused MC38 cancer cells showed no response to either chemoattractant . IncreasInterestingly, experimental metastasis studies in mice inoculated with non-small cell lung cancer stem cells versus monocyte-lung cancer cell hybrids found hybrid cells, but not unfused parental lung cancer stem cells, in lymph nodes and spleens up to 28 weeks after tail vein injection . Similar4.2As extravasation at two of the major sites for metastases – liver and lung – appears to be primarily selectin-independent , this fiITGB2) (Itgb2), and at levels similar to that of unfused monocytes (Itgax), albeit to a lesser degree than unfused monocytes. While Itgax was also expressed at a very low level by the parent lung cancer cell line, Itgb2 was not detected, demonstrating acquired expression of a leukocyte-specific gene by hybrids  . Macrophonocytes . Interes hybrids .Itga4 was upregulated in macrophage-colon cancer hybrids by nearly 3-fold above that of the parental MC38 cell line  is the primary ligand for vascular cell adhesion molecule -1 (VCAM-1), a leukocyte adhesion molecule whose normal expression is restricted to endothelial cells and subpopulations of bone marrow cells . VLA-4 iell line . Thus, fThere are myriad roles for cell adhesion molecules in metastasis, and several ligand-integrin combinations undoubtedly contribute to organotropism. It is also certainly theoretically possible for tumor cells to gain expression of leukocyte-specific integrins through processes such as genomic instability or uptake of extracellular vesicles bearing β2 integrins or their transcripts . However5Modern medicine has come a long way in developing therapies to successfully treat cancers that are localized or exhibit only regional spread, yet survival rates remain considerably lower in patients with distant metastases Table 1., resulting in fusion hybrid cells with retained features of both parent cell types as well as new, unique expression patterns and cellular behaviors. Importantly, fusion hybrids show increased migratory and invasive behavior and appear to greatly outnumber conventionally-defined CTCs in circulation, further supporting fusion as a possible mechanism for acquisition of metastatic capability of tumor cells. Moreover, expression of leukocyte-restricted cell adhesion molecules by fusion hybrids could augment affinity for sites of inflammation, such as premetastatic niches, or even expand the number of locations at which cancer cells can extravasate. In light of the studies discussed in this review, fusion hybrids appear to be an important variable to consider in metastatic spread in general and in the etiology of metastatic organotropism observed in prevalent cancers.Theories abound as to how a normal epithelial cell can undergo the genetic changes required for malignancy, many of which point to transformation through gradual and sequential accrual of individual driver mutations resulting in increasing genomic instability, tumor heterogeneity, and tumor evolution . HoweverMultiple studies defining macrophage-cancer cell fusion hybrids as cancer cells expressing the monocyte/macrophage lineage marker hemoglobin-haptoglobin complex scavenger receptor, CD163, have demonstrated a significant decrease in overall survival and distant recurrence-free survival in patients with greater numbers of CD163-positive tumor cells in their primary tumor , 67. IntP = 0.0029, hazard ratio of 8.31), while number of CD45-/pan-CK+ cells did not correlate with either stage or survival. Similar to other studies discussed in previous sections, CD45-/pan-CK+ cells in this study were detected at quantities an order of magnitude lower than that of hybrid cells in circulation (CHCs have also been shown to correlate with survival. Gast et al. conducted survival analysis on patients with pancreatic ductal adenocarcinoma to determine whether number of circulating tumor-immune hybrids cells and/or unfused circulating tumor cells correlated with disease stage, risk of death, or time-to-death in this population . In thisculation . In lighculation , this obin vitro and in vivo analyses of macrophage activation have demonstrated a staggering number of macrophage activation phenotypes based on factors including the current cytokine milieu, oxygen tension, and proximity to tumor cells, among others (Interestingly, the proportion of fusion hybrid cells in human tumors varies greatly between different types of cancer, suggesting that certain types of cancer may be more or less amenable to cell fusion , 40. Actg others , 69. GivProfound heterogeneity among hybrid cells as well as inconsistencies in results across fusion studies further underscore the complexity of this research question. Multiple studies have demonstrated that macrophage-cancer cell fusion can also give rise to tumor hybrid cells exhibiting lesser or no metastatic capacity when compared to the parental cancer cell line , 70. GivDespite acknowledged inconsistencies and limitations, analyses of possible correlation between fusion hybrids and cancer outcomes support the importance of tumor cell-myeloid cell nuclear fusion in cancer morbidity and mortality. Indeed, further research into the fascinating phenomenon of heterotypic cell fusion and the roles of fusion hybrids in human cancer biology is sorely needed. We look forward to future analyses revealing how a fusion partner is selected, which of the two cell types initiates fusion, and what the cues may be that begin the fusion process. Should fusion of cancer cells with macrophages underlie the phenomenon of metastatic organotropism, these studies may provide critical insight into therapeutic options to more effectively treat patients at risk for development of metastatic cancers.AJC was the primary contributor for writing, reviewing, editing, and visualization. SDH and MFC additionally contributed to reviewing, editing, and visualization. All authors contributed to the article and approved the submitted version.


The COVID-19 pandemic resulted in an unprecedented number of patients hospitalized in intensive care units (ICUs) because of severe SARS-CoV-2 infection (1). Respiratory failure in ICU patients, whether due to respiratory infection or other causes, may necessitate mechanical ventilation if oxygen levels cannot be restored with less invasive devices, such as continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP) ventilators. Although ventilators can be life-saving, their use is associated with some risks, including secondary bacterial infections that cause pneumonia. Prior studies have established that patients with SARS-CoV-2 infection on a mechanical ventilator are more likely to have ventilator-associated pneumonia (VAP) compared with other ICU patients, including those with influenza (2–4). This increased risk of VAP might occur because SARS-CoV-2 infection induces such profound lung injury compared with other infectious insults.JCI, Gao, Markov, Stoeger, and colleagues developed a new approach to assess features that associate with VAP and mortality in a cohort of ICU patients with severe pneumonia and respiratory failure (5). The research team used electronic health records from 585 mechanically ventilated patients hospitalized at Northwestern Memorial Hospital, including 190 patients with COVID-19, 252 patients with bacterial pneumonia, 50 patients with other viral respiratory infections, and 93 control patients who had respiratory failure without pneumonia. The patients with COVID-19 spent more time in the ICU and were mechanically ventilated longer than other patient groups in the study. In order to compare the trajectory of the distinct patient groups in the study with varying lengths of stay in the ICU, the authors developed a machine-learning algorithm called CarpeDiem that used 44 different clinical parameters from electronic health records to cluster days with similar patient features. This approach was further validated utilizing electronic health records from the Beth Israel Deaconess Medical Center that are publicly available in the MIMIC-IV database.In this issue of the This study revealed that unsuccessful treatment of VAP is associated with mortality across all patient groups. While it may be intuitive that unresolving secondary bacterial infection associates with increased mortality risk, the authors provide clear data definitively illustrating the occurrence of subsequent bacterial pneumonia in the different patient groups. Furthermore, their findings suggest that patients with COVID-19 are at higher risk of VAP associated with the prolonged respiratory failure triggered by SARS-CoV-2 infection relative to the other ICU patient groups in this study. An important implication of the work is that better approaches to detect and treat, or prevent, VAP could potentially improve outcomes. Further studies are needed to disentangle the many comorbidities associated with VAP, such as the selection of antibiotics, other drug exposures, and individual risk factors that make patients susceptible. Last, the machine-learning tool developed by the research team may have broader clinical applications in other areas of critical care medicine and could provide an innovative approach to examine other complications and treatments in the ICU.


Epilepsy is a prevalent condition characterized by recurrent, unpredictable seizures. Monitoring with surface electroencephalography (EEG) is the gold standard for diagnosing epilepsy, but a time-consuming, uncomfortable and sometimes ineffective process for patients. Further, using EEG over a brief monitoring period has variable success, dependent on patient tolerance and seizure frequency. The availability of hospital resources and hardware and software specifications inherently restrict the options for comfortable, long-term data collection, resulting in limited data for training machine-learning models. This mini-review examines the current patient journey, providing an overview of the current state of EEG monitoring with reduced electrodes and automated channel reduction methods. Opportunities for improving data reliability through multi-modal data fusion are suggested. We assert the need for further research in electrode reduction to advance brain monitoring solutions towards portable, reliable devices that simultaneously offer patient comfort, perform ultra-long-term monitoring and expedite the diagnosis process. The main symptoms are recurrent seizures which can be a traumatic experience for the individual . EpilepsSurface or scalp electroencephalography (herein termed ‘EEG’), recorded from the head surface rather than inside the scalp, has been a requisite initial diagnostic test in epilepsy since the 1940s . ClinicaThis mini-review focuses on surface EEG rather than alternative EEG monitoring such as intracranial or stereotactic EEG (sEEG). Briefly, sEEG involves the placement of electrodes in the deep brain structures, where the electrical activity cannot be effectively monitored from the head surface. A more extensive explanation of intracranial, microelectrode-based recordings in epilepsy is made elsewhere . This re. 2In scalp EEG, electrodes serve as a crucial hardware component for recording electrical activity in the brain . The con. 3The electrical activity recorded from the scalp generally represents the summation of the postsynaptic potentials from the electrical dipoles between the soma and apical dendrites of large populations of pyramidal neurons. Therefore, EEG recordings continuously monitor neural activity and capture spontaneous or induced changes. Visually, scalp EEG enables the analysis of the spectral characteristics of the electrical signals, usually ranging in frequency from 1 to 30 Hz. Also captured in the spectral frequencies are evoked potentials—electrical signals that vary due to external or internal stimuli. Changes in EEG signals could represent spontaneous electrical brain activities that exhibit dynamic, stochastic, nonlinear, non-stationary and complex behaviour with high temporal resolution. Given the electrode position on the scalp, the spectral power can be used as a marker of local synchronization. Such markers can then be altered with computations to infer synchronization between two distant regions in the brain.. 4The lengthy epilepsy diagnosis process involves multiple tests and continuous monitoring of EEG and video (vEEG). The diagnostic process can occur in a hospital ward or via an ‘ambulatory’ service where the EEG device is worn at home. The possibility of delayed diagnosis or misdiagnosis arises through several aspects of the diagnosis process. First, if seizures are not evident on the EEG or vEEG, there is a high likelihood of inconclusive EEG results, where epilepsy is neither confirmed nor excluded. Thus the risk of misdiagnosis must be weighed against that of a false positive diagnosis, a complex decision for clinicians with severe outcomes for patients . The natA valuable extension of the relatively short EEG monitoring periods could be sending the individual home with a wearable device suitable for long-term daily EEG recordings, which a reduced electrode set would make feasible. Yet this path has its challenges. From an engineering perspective, technological advances have made automated seizure detection and device portability a reality. The longest scalp EEG recording using wet or dry electrodes is approximately 10 days. From a patient's perspective, mobile EEG systems must be comfortable and appealing to wear, and current designs are lacking. Since patients' self-record of their seizures can be inaccurate, achieving a balance between technical features and patient comfort in a portable EEG system would further evolve home EEG monitoring.. 5. 5.1Currently, EEG is the gold standard to detect interictal, ictal and subclinical epileptic activity, including critical conditions like status epilepticus ,12. TypiElectrode positioning and a typical ictal event are illustrated in . 5.2Computer-automated techniques, such as machine learning (ML) algorithms, can detect and label seizures. Channel selection involves reducing the number of electrodes to be analysed or utilizing a range of feature selection and classification techniques, or both –18. The In automated seizure detection, digital signal processing allows extracting typical EEG signal features, which can be raw or engineered. Prior human studies examined engineered features such as relative average amplitude, relative scale energy, average cross-correlation function, relative power, bounded variation , phase-aConventional ML methods can then be trained to classify the EEG signals based on the selected EEG features –40. DuriThe reliance of automated seizure detection models on engineered features highlights their vulnerability to changes in the brain state, electrode count, recording region, EEG artefacts and signal noise ,49. Subs. 5.3Channel selection involves applying a computational or statistical methodology such as machine learning to extract the channels with the most importance at any given moment. If machine learning techniques are used, then sensitivity and specificity scores are applied to the result, indicating whether the model successfully extracted the most important channels. Sensitivity is taken to measure the true positive rate (TPR), whereas specificity measures the true negative rate (TNR). Sensitivity and specificity act as widely accepted, essential measures to evaluate the machine learning model's performance. The important channels can be selected across a group of patients, or they can be patient specific , inferriet al. used the so-called ‘greedy backward elimination’ algorithm to select a subset of seven features that produce the lowest false positive rate from each channel [−1, yet sensitivity and detection delay worsened, from 99% to 97% and 7.8 s to 11.2 s, respectively. More recently, Moctezuma and Molinas compared the popular greedy backward elimination algorithm with two versions of the non-dominated sorting genetic algorithm (NSGA), NSGA-II and NSGA-III [A foundational study by Shih  channel . The redNSGA-III . They acet al. reduced channels by selecting the largest variance in channels, achieving a detection performance on three channels near equivalent to a clinical neuro-physiologist's review on the same dataset EEG [et al. used a random forest algorithm to determine which channels contributed the most variation to discrimination of seizure versus non-seizure events [The role of variance contributed by individual channels has also been widely explored. Duun-Henriksen aset EEG . Birjande events . Howevere events . The oneet al. focused on domain knowledge to inform the channel selection [et al. investigated automated seizure detection in a paediatric population of six patients aged 7–12 years [A unique approach by Shah election . They ex12 years . Using t. 6Despite the clear benefits of using fewer electrodes, the recording of less data, or poor spatial resolution compromising the quality and reliability of the diagnostic method remains a concern. Thus, reducing electrodes requires a trade-off between electrode count and signal quantity and quality.et al. [et al. [et al. compared human expert seizure annotations of intensive care unit (ICU) EEG recordings with an automated electrode reduction model [The impact of electrode reduction on human and computer seizure detection performance is problematic. Deterioration in performance is attributed to the reduced availability and accuracy of the EEG data and technological challenges prevail in data collection and hardware design. Stevenson et al.  found el [et al.  had two on model . A step-Indeed, since much data is discarded during pre-processing steps, beginning with the maximum amount of data may be advantageous. Further, a complete electrode set-up enables the EEG recording to capture the seizure activity's complete spatial and temporal evolution compared with the sparse spatial coverage by a few electrodes. This substantially helps to differentiate between seizure activity, ‘normal’ EEG and artefacts, as artefacts often contaminate EEG due to muscle activity or movements. A higher number of electrodes used in monitoring provides a higher chance that some of the electrodes capture mostly brain activity (rather than artefacts) which can be clearly assigned to a seizure.For instance, if an individual experiences an unexpected seizure type not captured in their ward stay , a channel or electrode in only one or two regions may miss valuable information. Additionally, though channel selection studies highlight the wealth of possibilities in channel selection, the model combinations and interchangeably used terms (electrode and channel) make delineation, comparison and reproducibility of best-in-class models challenging. These impositions illustrate the need for further research.. 7Achieving the balance between data quality and fewer electrodes in a portable EEG system would greatly evolve home EEG monitoring. In particular, a low channel device could provide reliable information about seizure frequency to monitor treatment and get a better ground truth for drug trials, thus improving patient comfort while maintaining diagnostic reliability.Accuracy of patient-reported seizures is highly dependent on seizure type, and patient ability . Such deNon-invasive mobile EEG systems offer solutions to such challenges and have been reported by patients as valuable in seizure detection and management . ReducinIn the second scenario, the channel selection methods can augment other neuroimaging data, such as MRI, to corroborate imaging findings and aid the determination of treatment pathways. In the third scenario, channel selection methods would be valuable for highlighting the optimal brain regions for placement of invasive or ambulatory devices for ultra-long-term monitoring. In such cases, the onset zone is likely to be established using gold standard clinician assessment.In the clinical setting of the ICU, the use of a reduced number of electrodes for continuous EEG (cEEG) without compromising seizure detection accuracy is a common practice –76. The Several studies in paediatric and adult ICUs demonstrate that electrode reduction does not significantly compromise the sensitivity and specificity of seizure detection. One recent study exploring the impact of electrode reduction in a paediatric epilepsy monitoring unit showed the sensitivity and specificity of human detection were only marginally lower for the reduced montage (0.65) compared with the full montage (0.76) . AnotherAll the studies noted that including a human assessor could greatly enhance automated seizure detection; thus, electrode reduction may be less problematic with human assessors involved. Perhaps it is only in the non-human evaluation of EEG recordings, such as in computer-based automated seizure detection, where electrode reduction poses a problem. Therefore opportunities to boost reduced electrode EEG recordings and channel selection methods may be through incorporating data from diffusion magnetic resonance imaging (dMRI) and wearables.. 7.1Advances in neuroimaging of brain structural connectivity may offer a unique solution whereby a patient's MRI scan guides electrode placement. The role of structural connectivity in focal seizures, as observed in the brain's white matter, is largely established . Therefoa) and (b) were imaging pipelines explored in our previous work [c) and (d) illustrate a schematic workflow for the addition of EEG data, resulting in (e), identifying the optimal implant site for a long-term sub-scalp electrode or device.Although the previous works focused on concurrently recorded dMRI and EEG using the standard full set of electrodes, the advantages of including dMRI in an analysis pipeline are evident. Individualized dMRI data combined with the automated ranking of EEG signals can support a patient-specific pipeline for seizure onset zone selection by indicating the optimal electrode placement and the number of electrodes required to optimize automated seizure detection. ous work  while  recording to guide automated channel selection; however, this requires interictal spikes to be present in the recording and also align with clinical information.To realize the vision of combining multiple data modalities in real time, hospitals must be equipped with high-throughput computer systems and hardware, an unlikely position for most publicly funded hospitals, especially those in low-resource settings. Therefore the combination of dMRI and EEG, though valuable, remains feasible primarily for the pre-monitoring stage where clinicians might use dMRI metrics to guide the placement of EEG electrodes before monitoring or electrode reduction in home monitoring. Fusing EEG with data from wearables may be another practical solution.. 7.2Non-invasive wearable devices can also augment data recorded from fewer EEG channels. A recent systematic review and meta-analysis evaluated the utility of non-invasive wearable devices in identifying epileptic seizures . The revElectrocardiogram (ECG) data has been commonly evaluated as an added seizure marker, with some studies finding it improved seizure detection when combined with a limited channel EEG . Other bGiven the promise of these devices and EEG channel selection methods, researchers should combine the two modalities to determine their suitability for augmenting the EEG data so that seizures can be accurately and reliably detected. Research shows patients with epilepsy prefer wearables with higher sensitivity over those with lower false alarm rates . Therefo. 8Advances in EEG wearables primarily seek to reduce the number of electrodes and fuse innovative techniques with the optimal locations for securing the electrodes to the scalp. An extensive review of modern EEG wearables with reduced electrodes is available elsewhere . HoweverThe ‘Insight’ device by EMOTIV (EMOTIV) offers five channels, while the Neurotrail by Neuro-Pro AG starts at eight electrodes and can be reduced to one; both utilize dry electrodes. The Neurotrail exemplifies the multi-modal physiological signal fusion discussed in §7.2, as it contains a gyroscope and accelerometer to support its algorithms in real-time artefact detection and removal. The Advanced Brain Monitoring device (Advanced Brain Monitoring Inc.) also offers multi-modal signal fusion, using three frontopolar channels to record EEG, electrooculography and electromyography signals. Ceribell Inc. offers an eight-channel seizure monitoring device with an inbuilt seizure alert system, making it advantageous for low-resource or intensive care settings where experienced clinicians may not be available.The REMI device (Epitel) attaches to the skin using a disposable sticker containing a hydrogel that acts as both the adhesive and the conductive gel . The attIndeed, aside from the REMI device, most of these devices are hardly discreet, deterring their use beyond a clinical or research setting. Behind-the-ear or in-ear EEG addresses such concerns through discreet placement, and recent studies have shown it can achieve a reasonable level of accuracy in seizure detection ,102.At the forefront of the EEG wearable innovation are liquid metal-based wearables . Gallium. 9Since EEG is an established and reliable method for capturing and identifying seizure activity, evolving beyond the current scalp electrode requirements would amplify its utility. Given the challenges in automated seizure detection and logging, a portable, reduced electrode device presents an invaluable solution to improve diagnosis and the patient journey. A portable device with only a few electrodes would enable long-term recordings, providing rich neurophysiological data for patient-specific data processing.The analysis of existing works highlights the inspiration researchers can take from settings like the ICU to improve electrode reduction study designs and methodologies. In epilepsy, automated seizure detection models could be refined and enhanced by adding selective neuroimaging biomarkers and metrics from dMRI. The opportunity to improve the biometric and physiological data capture while maintaining reliable seizure assessment may lie in the augmentation of EEG with additional data from non-invasive wearables.A resurgence of research into electrode reduction in the clinical setting will support the development of portable, reliable devices, thereby enabling long-term monitoring and enhancing patient quality of life. Future research should prospectively explore precisely how much long-term data with a full electrode set is required to produce an optimal channel selection outcome. Similarly, researchers should look to quantify the number of reduced electrodes, that are sufficient to produce a diagnosis or act as an effective long-term monitoring device when combined with wearable device data. Personalized models for such multi-modal data analysis would also be beneficial.Transforming the existing solutions from a diagnosis-only state to a seizure logging and management aid for clinicians and patients will enhance the patient journey. Neurobehavioural and psychiatric comorbidities of epilepsy might be better understood ,109. The