Dynamic Covalent Nanoparticle Building Blocks

Abstract Rational and generalisable methods for engineering surface functionality will be crucial to realising the technological potential of nanomaterials. Nanoparticle‐bound dynamic covalent exchange combines the error‐correcting and environment‐responsive features of equilibrium processes with the stability, structural precision, and vast diversity of covalent chemistry, defining a new and powerful approach for manipulating structure, function and properties at nanomaterial surfaces. Dynamic covalent nanoparticle (DCNP) building blocks thus present a whole host of possibilities for constructing adaptive systems, devices and materials that incorporate both nanoscale and molecular functional components. At the same time, DCNPs have the potential to reveal fundamental insights regarding dynamic and complex chemical systems confined to nanoscale interfaces.

Abstract: Rational and generalisable methods for engineering surface functionality will be crucial to realising the technological potential of nanomaterials. Nanoparticlebound dynamic covalent exchange combines the errorcorrecting and environment-responsivef eatures of equilibrium processes with the stability, structuralp recision, and vast diversity of covalentc hemistry,d efining an ew and powerful approachf or manipulatings tructure, function and properties at nanomaterial surfaces. Dynamic covalent nanoparticle (DCNP) building blockst hus present aw hole host of possibilities for constructing adaptive systems, devices andm aterials that incorporateb oth nanoscale and molecular functional components.A tt he same time, DCNPs have the potential to reveal fundamental insights regarding dynamic and complex chemical systems confined to nanoscale interfaces.

The Importance of Nanoparticle Surface Functionality
Tremendous synthetic and analytical advances over more than two decades have openedu panew region of chemical space on the nanoscale, leadingt oe xcitement on account of the extraordinary properties observed for myriadt ypes of nanoparticles (NPs) and other materials in this size regime. [1] Virtually any potentiala pplication of nanomaterials will require careful control over aw ide range of features, as well as integration with any number of other components. [1d,e, 2] However,s trategies for functionalising and assembling these new chemical entitiesh ave not kept pace with advances in their synthesis and there is now ap ressing need to bridget his capability gap if the technological potentialo fn anomaterials is to be realised.
The chemical nature of nanoscale surfaces is inherently critical to the behaviour of all nanomaterials. The canonical example is am onolayer-stabilised nanoparticle (NP), in which an inorganic core is stabilisedb yasurface-bound layer of molecular ligands. The properties of these multicomponent systems are defined both by the material, size and shape of the inorganic core and by the characteristics of the surface-bound species (Figure 1a). [3] Furthermore, the ligand shell provides ah andle for attaching other components-be they molecules, surfaces or other nanomaterials. Despite al ong history, [4] the controlled synthesis of NPs of anys ort remains challenging and the range of compatible surface functionalities restricted. [5] 'Ligand ex-change',w hereby temporary surface-bound molecules are replaced in their entirety (Figure 1b,I )h as been widely exploited for some systems, [6] yet still presentss everal practical challenges, hasl imited scope for linking to complex functionalities or non-molecular components, and is not applicable to all core materials. Consequently,r obusta nd predictable methods for modifying NP-bound surfaces peciesi napostsynthetic fashion will be criticalf or manipulatingN Ps, tuning their properties, and assembling devicesa nd materials from these nanoscale buildingb locks. [7] Ideally these methodologies should be independentoft he underlying nanomaterial and therefore generalisable across aw ide array of nanostructures that can have moleculesa ttached to the surface.
The currents trategies for in situ modification of NP-bound molecules (Figure 1b,I I-IV) have considerable drawbacks that prevent them offering universal solutions.E xploitingn oncovalent interactions between biomolecules-and, in particular,h ybridisation to single-stranded oligonucleotides-has proven particularly successful, leading to myriadN P-based devicesa nd materials. [9] However,t he structurala nd chemical stabilityo f biomolecules is only maintained within tightly defined conditions;c omplex high molecular weighta rchitecturesl imit the scope for structural and chemical variation;a nd molecularlevel characterisation can be extremely challenging. Nonbiomolecular approaches have the potential to draw upon the full diversity of synthetic chemistry for optimisings tructure, function andp roperties. [7] Innovative designst hat exploit noncovalent interactions for NP functionalisation, [10] aggregation, [11] and surfacei mmobilisation, [12] have recently been explored, but still cannotm atch the stability, specificity,a nd selectivity of DNA. Stimuli-responsive molecular switchesi ncorporatedw ithin NP ligand shellsh ave recently produced an umber of impressive dynamic NP systems, in particularf or control of NP aggregation. [13] The challenges for this relativelyu nexplored strategy include avoiding degradation processes that lead to switch fatigue, and extending the switching phenomena to properties beyondt he aggregation state. [14] Covalent modification of NP surface functionality is an obvious attractive alternative and has indeed been intensively ex- Figure 1. a) Calculated structure of am onolayer-stabilised NP (5 nm PbS core capped with oleicacid ligands)and the crucial features that are determined by the ligand shell; b) cartoonrepresentation of strategies for postsynthetic NP surface engineering:I.Ligand exchange;II. host-guestcomplexation;I II. stimuli-responsive molecularswitches; IV.kinetically controlled covalent reactions; V. dynamic covalent reactions( highlighted). Panel (a) adapted from ref. [8] with permission from AAAS. plored.H owever,t ypical examples rely on kinetically controlled reactions, often inspired by mild, robust and high-yielding protocols developed for bioconjugation applications. [15] These only offer one-shot transformations and fail to match the programmability of oligonucleotide approaches. Dynamic covalent bonds-which, under appropriate conditions, can form and break many times over,w hile under alternative conditions can be kinetically inert-offer au nique solution to these issues (Figure 1b,V ), and the concept of dynamic covalent nanoparticle (DCNP) buildingb locks opens up ah ost of possibilities for repeatedly switching-and subtly tuning-NP functionalisation and properties, and for controlling NP self-assembly.

Dynamic Covalent Chemistry:T owardsAdaptive Chemical Systems, Materials and Surfaces
Processes in whichc ovalentb onds are formed reversibly under conditions of thermodynamic controlh ave for decades been recognised as important in certainb ranches of chemistry,i ncluding carbohydrates tereochemistry [16] and polymer synthesis. [17] Inspired by the prospecto fe xtending the newly established principles of supramolecular chemistry into the molecular world-and enabled by advances in analyticalt echnology that made feasible the characterisation of mixtures-int he late 1990s, pioneering groups developed dynamic covalent chemistry as am eans of combining the strength,d irectionality and potential for kinetic stability of covalentb onds, with the benefits of error-correcting self-assembly,p roduct stabilityc ontrol, and stimuli-responsiveness. [18] Creating equilibrating mixtures of molecular species that could adapt in response to molecular recognition interactions or environmental changes afforded an ew strategy for template-induced selection of optimised supramolecular hosts, guestsa nd catalysts, or forh ighyield assembly of complex molecular architectures. [18e, 19] The intervening years have seen rapid progress, leading to severalremarkable achievements, including selection of unforeseen macrocyclic receptors from dynamic combinatorial libraries, [20] construction of three-dimensional molecular cages and capsules, [21] preparation of hitherto inaccessible interlockedm olecular architectures, [22] crystallisation of infinite covalento rganic frameworks, [23] self-replication from ad ynamic mixture of competing molecules, [24] ando peration of sophisticated artificial molecular nanomachines. [25] Logicale xtensions of these concepts have led to the self-assembly of responsive and adaptive materials, [26] including dynamic covalentp olymers [27] and selfselectings mall molecule 'dynablocks'. [28] Characterisation of inherently dynamic and multicomponent chemicals ystems is af ormidable challenge in any setting, [29] even more so when movinga way from the familiar solutionstate environmenti nto condensed phases or on to surfaces. [19b] Dynamicc ovalent processes occurring at interfaces have been correspondingly demanding to establish.P ioneering studies exploitede merging characterisation techniques including atomic force microscopy and confocal fluorescence microscopy to visualise erasable molecular patterns [30] and directed surface diffusiono fc ovalently attachedm acromolecules, [31] operating by reversible condensation and hydrolysiso fi mines on self-as-sembled monolayers (SAMs). More recently,p H-controlled patterningofi mine SAMs was achieved through dynamic covalent selection from am ixture of small-molecule amines or aminecontaining proteins. [32] Multiple orthogonal dynamicc ovalent processes have since been combined to construct an impressive series of chemically responsive multicomponent surface architectures in which several coaxially aligned photoconductive channels can be grown normal to the surface with control over patterning parallel to the substrate. [33] Enabled by developments in scanning probe microscopy, potentially reversible covalentr eactions have also been investigated for the constructiono fe xtended noncovalent and covalent atomically thin 2D networks at solid surfaces under vacuum, [34] at the solid/liquid interface [35] and at the liquid/air interface. [36] Recently,d ynamic covalente xchange was demonstratedi no ne such system, where the free energy of surface physisorption can act as ad riving force for constituent selection and amplification from adynamic library of imines. [37] The emergence of surface-confined dynamic covalent systems now suggests several compellingp ossibilities, including spatiotemporalc ontrol over the exchange process, leading to the prospect of creatings urface patterns or achieving compartmentalised chemical behaviour spanning severall ength scales. These studies have helped to reveal the often considerable influenceo ft he unique surfacem icroenvironment when chemicalprocesses are confined to interfaces.

Dynamic Covalent Nanoparticle Building Blocks
These contemporaneous developments for both nanomaterials and dynamic molecular systemsn ow set the stage for extending dynamic covalent reactions onto nanosurfaces. Intermediate in size between macromolecules and extended surfaces, applyingr eversiblec ovalentr eactions to nanosurfaces allows the principles of equilibrium control to be appliedt oa chieve responsive and adaptive behaviour on the nanoscale, while exploiting the precisiona nd diversity of synthetic molecular structures. Working with nanosized chemical entitiesi ntroduces several new challenges, but also opens up excitingp ossibilitiesf or elucidating fundamental features of chemical processest aking place at interfaces that are just not availablef or extended substrates.
In ah andful of cases,c argoes have been attached to and then released from nanomaterials through formation and cleavage of the same covalent bond. For example, inspired by stationaryp hases developed for affinity chromatography,b oronic acids have been associated with magnetic NPs and used to isolate and enrich polyhydroxylated biomolecules from biological samples followed by release for analysis by mass spectrometry. [38] Boronic acid functionalised gold nanoparticles (AuNPs) have also been used to reversibly trap molecular cargoes in the pores of silica solids that display polyhydroxyl surface functionality;c argo release being achieved by lowering pH, consistent with am echanism involving pH-switchedb oronate ester condensation/hydrolysis. [39] As imilara pproachh as been used fort he reversible gating of mesoporous silica nano-particles (MSNPs), where the silica surface was functionalised with boronic acids and the capping unit was the polyhydroxylated glycoprotein insulin. [40] Moreover,i mine linkages have been used to attach and detach hydrophobic dendrons on the surfaceo fs ilica-coated superparamagnetic microspheres [41] and both hydrazones [42] and oximes [43] have been used for loading and releaseo fs imple carbonyl-containing units on MSNPs.
Ta king place on sparsely-functionalised irregular surfaces, direct characterisation of the molecular processes in such systems is af ormidable challenge. These examples all exploit efficient interconversion of functional groups through high-yielding condensation and hydrolysis reactions. Undere ach set of conditions, formation or cleavage of ac ovalent bond is essentially irreversible and is therefore conceptually similar to other covalents urfacem odifications. [15] However,d ynamic covalent exchange processes would allow subtle and responsive tuning of the NP surfacef unctionality,c ontrolled by thermodynamic differences between an umber of exchanging species, and exhibiting constitutional adaptation in response to myriad parameters and interactions. Dynamic covalentn anoparticle (DCNP) buildingb locks therefore introduce ap owerful new approach to nanomaterial surface engineering, and the construction of responsive NP-based devices, assemblies and materials.

Dynamic covalent exchange on phospholipidnanosurfaces
Myriad dynamic biomolecular reaction and recognition processes take place on phospholipid bilayers;f or example, disulfide exchange with exofacial thiols on membrane-associated proteins can mediate signalling pathways, viral entry and nonendosomal cellular uptake processes. [44] In addition to the considerable analytical challenges, achieving dynamic covalent reactions on artificial nanoscale membranes must also overcome the often limited stabilityo fs elf-assembled amphiphile structures. Nevertheless, Otto and co-workers successfully demonstrated dynamic covalent thioester exchange on the surfaceo f large (d % 200 nm) unilamellar vesiclesa ssembled from egg phosphatidyl choline and 10 mol %o famembrane-anchored thioester derivative 1 (Figure 2). [45] Surfacec onfinementw as found to retard the dynamic covalentt hioester exchange reactions only to am odest extent and equilibrium control was maintained. Significantly,t hioester libraries equilibratedo n these liposome surfaces exhibited markedly differentc ompositions compared to bulk solution.F or example, when amphiphilic bis-thioester 1 was equilibrated with dithiol 2,ahigh proportion of linear compounds (formed by cross-linking several membrane-anchored units)w as observed, in contrastt o the small monomeric and dimeric products formed in solution ( Figure 2b). This can be attributed to the higherl ocal concentration of 1 at the interface, compared to when at the same overall concentration in bulk solution.

Dynamiccovalent exchange on monolayer-stabilised inorganicnanosurfaces
Our group first explored the DCNP concept by using AuNPs (d % 3.0 nm) bearing ah ydrazone-terminated surface monolayer (AuNP-3;F igure 3). [46] On introducing an aldehyde exchange unit, such as 4,t ogether with an acid catalyst, the monolayer composition couldb er eliably and reversibly modified through dynamic covalent hydrazone exchange. The single-component monolayer achievese xtremelyh igh surfaced ensities of exchangeable units, which couldb eq uantitativelym odified;o r alternatively,m ixed monolayers could be accessed with compositions defined by the thermodynamic stabilityo fe ach hydrazone and the reaction conditions. Simply removing the acid catalysta fforded kinetically stable products that could be isolated, purified, characterised, and stored without further changes to the monolayer.
In contrast to larger and sparsely functionalised nanosurfaces-which behave in many respects as curved analogueso f 2D extendedS AMs-these small colloidal NPs, stabilisedb y ad ense, single-component monolayer,a re perhaps better considered as au nique category of 3D SAMs, with many features akin to large macromolecules. [47] This pseudomolecular nature allowsa nalytical tools such as NMR spectroscopy to be exploited for in situ characterisation of surface-bound structures and processes;o pportunities that are not applicable to analogous 2D systemso rs parselyf unctionalised larger NPs. Monitoring DCNP hydrazone exchange by 19 FNMR spectroscopy allowed both NP-bound and unbound speciest ob eq uantified in real time (Figure 3b,c). As might be intuitively expected, the surface-confined process proceeded more slowly than analogous solution-phase reactions. [46] However,t he retardation was relatively modesta nd dynamic covalente xchange was found to be significantly faster than analogous ligand exchange processes at the AuÀSb ond, which of course also first require each Figure 2. a) Schematic representation of abilayer-templated dynamic combinatorial library formedb yt hioester exchange between surface-activeb isthioester 1 and dithiol 2 in the presenceo fl ipid bilayer vesicles;b )HPLC tracesc omparing library compositions produced in bulk aqueouss olution (top trace)a nd at the lipid bilayer interface (bottom trace, 10 mm egg phosphatidylc holine, 1.0 mm 1 and 1.0 mm 2). [45] Adaptedf rom ref. [45] under the termsofC C-BY 2.0. new ligand to be synthetically prepared. The ability to characterise molecular-level details promises aq uantitative understanding of NP-bound dynamic covalent reactions that will underpin rational syntheticm ethods for manipulating DCNPs with predictabilitya nd precision.
One of the advantages of the DCNP concept is the ability to exploit the vast toolboxo fs ynthetic organicc hemistry to tune virtually any aspect of the surface-bound molecular structure, properties or reactivity.W hereas hydrazone-exchange reactions tend to reach equilibrium on at imescale of minutes to hours under acid-or nucleophile-catalysed conditions, the reaction between ab oronic acid and various dihydroxy compounds to yield boronatee sters occurs extremelyr apidlyi nt he presence of Lewis bases. Hydrazones and boronate esters therefore represent two chemically orthogonal dynamic covalent functional groups with an attractive contrasti nk inetic characteristics. We recently developed AuNP-7 (d % 3.4 nm;F igure 4), stabilised by ah omogenous monolayer of structurally simple boronic acids, which reacts with 1,2-diols in the presence of aL ewis base to provide NP-bound boronate esters. [48] In situ analysisb y 19 FNMR spectroscopy revealed that boronate ester formation and dynamic covalent exchange (when more than one diolcontaining exchange unit is introduced) occurs rapidly and reversibly,a chieving high surface-saturation concentrations. Mixed boronate ester monolayers display pathway-independent compositions that adapt to changes in the mixtureo fe xo-genouslyi ntroduced diols, confirming that dynamic covalent exchange on the NP surfacer emains an equilibrium process under thermodynamic control ( Figure 4). [48] Independently,Otto and co-workers have exploited kinetically facile imine exchange to achievet emplate-driven dynamic covalentN Pf unctionalisation ( Figure 5). [49] On treating AuNP-12/13 (d % 11.7 nm), which is stabilised by am ixed monolayer incorporating approximately 10 mol %a ldehyde ligand 12,w ith mixtures of simple primary amines, negligible NP-bound imine formation was observed. However,o ni ntroducing short (16mer) oligonucleotides, selective uptake of amines from solution was observed. This could be explained by thermodynamic stabilisation of NP-bound imines as ar esult of multivalent interactions between the imine-functionalised NP surface and the oligonucleotide template. Significantly,t he degree of functionalisation,a nd the composition of NP-bound imine libraries, depended on the oligonucleotide sequence employed (Figure 5b): the dynamic covalent ligand shell can adaptt oo ptimise binding to each specific DNA template. [49] Very recently, the same approach was extendedt ot he formation of NPbound hydrazones,w hereby mixtures of two different NPbound hydrazones were obtainedd epending on the DNA template sequence. [50] This second-generation system also has the advantage of employing an eutral surface monolayer,w hich minimises nonspecific binding to the anionic oligonucleotide templates.
Chem.E ur.J. 2016, 22,10706 -10716 www.chemeurj.org lenges-associated with characterising dynamic covalentp rocesses in this unconventional environment. The diversity of abiotic synthetic chemistry may now be exploited to develop aw hole series of DCNP building blockst hat would constitute au niversal set of environment-responsive 'nanoparticle synthons' with optimised reactivity and properties for aw ide range of applications.

Reversible Control of DCNP Properties
Mild methodsf or postsynthetic NP property control are highly desirable and would have significant benefits for nanomaterial handling and processing. We reportedh ow hydrazone exchange can be used to achieve reversible and stimuli-responsive control over DCNP physicochemical properties by appropriate choiceo fm olecular exchange unit ( Figure 6). [46] AuNP-3 was colloidally stable only in polar aprotic solvents, yet hydrazone exchange with hydrophobic aldehyde 14 produced AuNP-15,w hich exhibited colloidal stability in less polar organic solvents. Likewise,e xchange with charged aldehyde 16 produced AuNP-17,w hich was colloidally stable in water.I nc ontrast to alternative approaches such as ligand exchange or noncovalent encapsulation, DCNPsc ombine excellent stability in each state with complete reversibility;a ny one state can be accessed from any of the others,a nd each state corresponds to as ingle, covalently bound entity,w ith no requirement for weakly associated stabilisers or surfactants.
In ar ecent study,T akahara, Otsuka and co-workers prepared large silica NPs (d % 100 nm) functionalised with polymer brushesb earing approximately 10 mol %a lkoxyamine side chains( NP-18,F igure 7). [51] The dynamic covalentr adicalc rossover reaction of alkoxyamines was then exploited to graft on appropriately functionalised polymere xchange units, such as poly(4-vinylpyridine) derivative P19.A lthought he extent of exchange was very limited (ca. 2.2 %a sq uantified by XPS analysis), subsequentq uaternisation of the pyridine units rendered silica NP-18ÀP19Me + + dispersible in water.R eaction with the small-molecule alkoxyamine 20 recovered both the XPS signature and solvent compatibility properties of the initial sample, consistentw ith af ully reversible dynamic covalent exchange process. [51] Molecule-Directed Assembly of DCNP Building Blocks Structuralc ontrol over nanoparticle assemblies is important for defining an umber of emergenta nd collective properties when severalN Ps are broughtt ogether,a nd is therefore of crucial importance to many innovativeN Pa pplications, from sensing to catalysis;o ptoelectronics to thermoelectrics. [1d,e] Furthermore, integrating nanostructures with components from existing technologies, such as microelectronics or optics, requires the controlled assembly and patterning of NPs across several size scales. [2] Although NP 'superlattices' may be assembled, driven by nonspecific dispersion forces on controlled solvent evaporation, these tend to form close-packed structures with arrangements that are specified by the size and shape of the NP building blocks. [52] Independent control over NP building block characteristics and assembly structure through chemistry-ledd esign of interparticle linkersh as been exemplified by Figure 5. a) DNA-templateddynamiccovalent AuNP-bound imine monolayers;negligibleimine formation occursint he absence of DNA;b)monolayer composition depends on the specific base pair sequence of the doublestranded DNA template. [49] Panel (b) adapted with permission from ref. [49], copyright W iley-VCH Verlag GmbH &C o. KGa, Weinheim. Figure 6. Reversible control of DCNP solvent compatibility by dynamicc ovalent hydrazonee xchange;s olvents:A= hexane, B = chloroform,C= tetrahydrofuran, D = methanol, E = N,N-dimethylformamide, F = water. [46] Adapted with permission from ref. [46],copyrightWiley-VCHV erlag GmbH &C o. KGa, Weinheim. the emergence of ordered, non-close-packed NP arrays connected by oligonucleotide hybridisation. [9g] However,a ll of the attendant issues with regards to DNA stabilitya nd customisability remain, and assembly at surfaces is still very much in its infancy. [53] Despite severaln otable advances exploiting nonbiomolecular linkers, [11,54] all-covalent strategies tend to encounter kinetic traps, whereas noncovalenta pproaches can require very specific conditions or complex molecular designs to produce well-defined kinetically stable structures.
DCNP buildingb locks have the exciting potentialt os elf-assemble under error-correcting, thermodynamically controlled conditions, yet be connected by structurally unambiguous, stable and diverse covalentl inkers. Exploiting dynamic covalent exchange of bilayer-confined thioesters, Ravoo and coworkers reported the reversiblea ssembly of liposome aggregates. [55] Liposomes (d % 100 nm) constructedf rom soy bean lecithin and2 5mol %t hioester 21 were treated with dithiol 22, resultingi nc ovalent cross-links between the vesicles (Figure 8). Although the assembly morphology has not been determined, dynamic light scattering indicated the formation of polydisperse aggregates over ap eriod of several hours, which could subsequently be disruptedb ya ddition of an excess of monofunctional thiol 23 to drive the dynamic covalent exchange process back towards the starting thioester.
An opticals ensor for assessing enantiomeric excess of chiral diols has been demonstrated, based on enantioselective disruption of colloidally stable aggregates produced from saccharide-functionalised AuNPs in the presence of inorganicb orate anions. [56] Aggregation,which produces an easily detectible optical signature on account of changes to the AuNP-localised surfacep lasmon resonance, wasa ttributed to the formation of dynamic covalent spiroborate cross-links between NP-bound vicinal diols. These linkages are subsequently disrupted on introduction of the small-molecule diol analytes.
Combining organic molecule linkers with monolayer-stabilised DCNPs,w er ecently demonstratedt he dynamic covalent assembly of extended andr esponsive NP aggregates by using boronic acid-functionalised AuNP-7 (Figure 4a nd 9) in combination with ditopic linkerst hat can themselves be varied to tune the aggregate morphologyi namodularf ashion. [48] In the presence of ab ifunctionald iol linker,d ynamic covalent boronate ester cross-links are formed between the DCNPs.T he re- Figure 8. Reversible liposomecross-linkingb ydynamicc ovalent thioester exchange. [55] Adaptedf rom ref. [55] with permission from The Royal Society of Chemistry. Figure 7. Dynamic covalent polymer brush-functionalised silica NPs. Alkoxyamine exchange can be used to reversiblyattach macromolecular poly(4-vinylpyridine) side chains P19.Subsequentquaternisation of pyridinenitrogen atoms results in ahydrophilic NP coating that can subsequently be removed by dynamic covalent exchange with small molecule 20. [51] sulting extended aggregates eventually precipitate from solution, producing low-density open networks that are consistent with adiffusion-limited aggregation process. Assembly is quantitative and switchable:t he DNCP building block and molecular linker do not interactuntil addition of aLewis base to stabilise the NP-bound boronate esters, thus initiating assembly.R emarkably,d espite being linked by covalent bonds, the resulting solid-state aggregates can then be completely disassembled by introducing ac ompetitive monofunctional diol (Figure 9). [48] Interestingly,D CNP aggregate morphologyw as found to vary depending on the nature of the bifunctional linker,s uggesting that morphology can be tuned through structural modification of as mall-molecule component. With the ability to characterise NP-bound dynamic covalent processes in situ (vide supra), this offersastraightforward and rational approach to varying morphological parameters over aw ider range than is possible using biomolecular systems. This modular strategy might also be extended to incorporate additional chemical, physicalo rs tructural features within the molecular linker design.T he diversity of complementary and orthogonal dynamic covalente xchange processes points the way to yetmore sophisticated assemblies and devices that incorporate severald ifferentD CNP building blocks, designed to be selectively reactive with each other and/or with specific species. The DCNP building block concept thus suggests severali ntriguing avenues towards responsive, reconfigurable and truly multifunctionalh ybrid nanomaterials in which both molecular and nanoscale components are precisely arranged, and combine to define emergent system properties.

Conclusion and Outlook
One only has to look to biology to understand the potential for producing remarkablef unctional materials or complex chemicaln etworks by nanoscale confinement of dynamic molecular systems on interfaces or within compartments. Advan-ces in both synthetic and analytical technology are now allowing chemists to consider emulating some of these extraordinary systems, even if only at ac omparatively rudimentary level. Dynamic covalent reactions offer unique advantages in their combination of specific and selectivet hermodynamically controlled reactivity,w ithin structurally robusta nd unambiguous small-molecule covalent structures that are synthetically and analytically tractable.C onferringd ynamic covalent reactivity on the vast array of nanomaterials that can now be generated with increasingly precise control over chemical composition, size, shape and dispersity,w ill therefore establish av ersatile new category of nanochemical synthon:t he DCNP building block.
Whether on soft or hard nanosurfaces, combining the errorcorrecting and environment-responsive features of equilibrium processes with the stability and structurald iversity of covalent chemistry amounts to a' best of both worlds' solution to the problemo fe ngineering surface functionality.T he basic concept is largely independent of the nanomaterial itself, and so can ultimately be generalised across ar ange of materials, shapes and sizes. This is in starkc ontrastt ol igand-exchange strategies, which by definition must be specific for each different nanomaterial-molecule interaction;d ynamic covalent exchange of simple units on the periphery of as tabilising monolayer can occur significantly faster under milder conditions and avoids the necessity for multistep synthesis of each ligand from scratch. The advantages of an onbiomoleculara pproach include the ability to readily vary structural and chemical parameters, and operate under aw ide range of conditions. The ever-increasing number of well-characterised dynamic covalent reactions [19c, 57] affords ad iversity of complementary ando rthogonal reactivities, spanning ah uge range of kinetic behaviours and operating conditions. Furthermore, with the full gamuto fs ynthetic small-molecule chemistry at our disposal, the possibilities for augmenting both structurea nd properties through the supporting molecular scaffold are almost limitless. For all thesereasons, DCNPs can provide at oolbox of universal Figure 9. Boronate ester-driven DCNPassembly and disassembly on sequential addition of abifunctional linker (blue), followed by am onofunctional capping unit (pink). [48] Adapted from ref. [48] with permission from The Royal Society of Chemistry. nanoscale buildingb locks that can be predictably modified, combined, and assembled to suit numerous of applications.
The pseudomolecular nature of colloidallys table NPs-at least at the lower end of the size-scale-means that although the analyticalc hallenges are significant, they are not insurmountable. Severals tudies discussed herein have demonstrated that these systems can reveal direct insighti nto chemical processeso ccurring at interfaces, something that hasp roven extremelyc hallenging in other settings.C onsequently,D CNPs constitute au nique platform on which to study surface-bound molecular behaviour.U nderstanding the subtle and complex network of interactions governing reactivity within NP-bound monolayers will be crucial to arriving at rational andp redictable synthetic methodologies for working with DCNPs, for example by quantifying and understanding the influence of surface confinement on reactionk inetics. [46] At the same time, such fundamentalq uestions are equallyr elevant to better understanding av ariety of other surface-confined molecular processes, from heterogeneousc atalysis to cell-surface recognition. The profound influence of the surface-bound microenvironmento nt he thermodynamics of equilibrating dynamic covalent systemsh as already been demonstrated, [45,49] and points the way towards templated nanosurfaces that array numerous copies of severalf unctionalities across an anoscale surface area, thus providing synthetic analogues form odelling or perturbing large-area multivalenti nteractions that are at the heart of numerousnatural processes. [58] The essential role that surface-bound species play in defining ar ange of nanomaterial properties meanst hat dynamic covalentm odification heralds ap owerful, environment-responsive route to controlling system-level behaviours, including physicochemical properties or microscopic and mesoscopic assembly structures. We have shown that this approach can be used to reversibly switch and tune NP solvent compatibility using very simple molecular exchange units, [46] or else to assemble covalently linked NP aggregates that are responsive to specific chemical stimuli. [48] There is rich potential for creating adaptivea nd reconfigurable devicesa nd materialsi nw hich structural features and physicochemical properties can be tuned in am odular fashion by rational selection of molecular and NP components that are assembled in thermodynamically controlled error-correcting processes and mayb ep erturbed by externalstimuli.
Mastering complex systems that arise throught he interaction of dynamic chemical processes, reactionn etworks and structures is one of the next grand challenges facing synthetic, analytical and theoretical chemists in the coming decades; [59] dynamic covalent chemistry and nanomaterial buildingb locks will undoubtedly play significant roles in realising this. Althought he concept of ar ational 'heterosupramolecular chemistry' whereby supramolecular chemistry principles could be applied to achieve molecular-level control over nanoparticles was proposed at least two decades ago, [60] it is only now that synthetic and analytical capabilities have reached al evel of maturity that allows such aspirations to be met. Dynamic covalentr eactions are being exploited in all sorts of original ways, [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] including in systemst hat are coupled to external energy sources, such as out-of-equilibrium chemical reactions, [24a] transmembrane concentration gradients [61] or repeatedly appliede xternals timuli. [62] We can expect that cross-fertilisation between each of these themes will serve as inspiration for ever more innovative designsa nd complexb ehaviours on the nanoscale. With the ability to revealf undamental insights,a nd transformative potential as an enabling technology,D CNPs will contributet ot his future chemical synthetic science, whereby molecular,n anoscale and macroscopic building blocks may be combined with equalp recision,t oc reate complex, responsive and adaptive systems operating across several size scales.