Advances in Single-Chain Nanoparticles for Catalysis Applications

Review Advances in Single-Chain Nanoparticles for Catalysis Applications

Jon Rubio-Cervilla 1, Edurne González 1 and José A. Pomposo 1,2,3,* ID 1 Centro de Física de Materiales (CSIC, UPV/EHU)—MPC, Materials Physics Center, Paseo Manuel de Lardizabal 5, E-20018 San Sebastian, Spain; [email protected] (J.R.-C.); [email protected] (E.G.) 2 Departamento de Física de Materiales, Universidad del País Vasco (UPV/EHU), Apartado 1072, E-20080 San Sebastian, Spain 3 IKERBASQUE—Basque Foundation for Science, María Díaz de Haro 3, E-48013 Bilbao, Spain * Correspondence: [email protected]; Tel.: +34-943-018-801; Fax: +34-943-015-800

Received: 28 September 2017; Accepted: 19 October 2017; Published: 21 October 2017

Abstract: Enzymes are the most efficient catalysts known for working in an aqueous environment near room temperature. The folding of individual polymer chains to functional single-chain nanoparticles (SCNPs) offers many opportunities for the development of artificial enzyme-mimic catalysts showing both high catalytic activity and specificity. In this review, we highlight recent results obtained in the use of SCNPs as bioinspired, highly-efficient nanoreactors (3–30 nm) for the synthesis of a variety of nanomaterials (inorganic nanoparticles, quantum dots, carbon nanodots), polymers, and chemical compounds, as well as nanocontainers for CO2 capture and release.

Keywords: nanoparticles; nanocontainers; catalysts

1. Introduction Enzymes are the most efficient catalysts, known for biochemical reactions that take place in aqueous environment under crowding conditions near room temperature [1]. They have been a source of inspiration for the construction of a variety of artificial enzyme-mimic catalysts, such as macrocyclic compounds [2], star [3] and helical [4] polymers, dendrimers [5], micelles [6], vesicles [7], and polymersomes [8]. The specific, well-defined structure of enzymes accounts for its exceptional catalytic activity and specificity. Many enzymes exhibit a precision hydrophobic nano-cavity that is stabilized by hydrophobic interactions where catalysis takes place in a very efficient manner. Often, the globular structure of enzymes provides a unique local environment for the active (catalytic) site in which the biochemical reaction occurs. This folded structure is responsible for the specificity of the enzyme, and the formation and stabilization of the transition state during the transformation of reagent(s) to product(s). The folded globular structure of enzymes is often maintained via hydrogen bonding, hydrophobic interactions, and disulfide bonds. Hence, the structure of enzymes and thus its catalytic properties are highly sensitive to changes in pH, temperature, or redox potential. For efficient catalysts to occur, many enzymes also bind to the active site other small compounds (cofactors), such as metal ions or low-molecular-weight organic molecules, which work together with enzymes to enhance reaction rates [1]. Recently, single-chain nanoparticles (SCNPs) have emerged as valuable artificial soft nano-objects, which can be efficiently endowed with useful enzyme-mimic catalytic activity (Figure1)[ 9–12]. The folding of individual polymer chains to functional SCNPs is reminiscent of protein folding to its functional, native state, although current SCNPs lack the perfection in sequence, uniformity in size, and precise morphology that is found in enzymes [9]. Even so, the folding of a synthetic polymer to a collapsed state provides with one (or more) denser local packaging zone(s) where catalytic

Nanomaterials 2017, 7, 341; doi:10.3390/nano7100341 www.mdpi.com/journal/nanomaterials Nanomaterials 2017, 7, 341 2 of 20

Nanomaterials 2017, 7, 341 2 of 20 activity and selectivity can be enhanced. This should be promoted by a local polymer environment, forenvironment allowing stabilization, for allowing of the stabilization transition stateof the during transition catalysis. state In dur analogying catalysis. with enzymes, In analogy metal with ions, oren low-molecular-weightzymes, metal ions, or organiclow-molecular molecules-weight can organic work together molecules with can SCNPs work together to enhance with reaction SCNPs rates.to Veryenhance recently, reaction both rates. exceptional Very recently, catalytic both activity exceptional [13] and catalytic speciﬁcity activity [14 []13 have] and been specificity demonstrated [14] have by usingbeen metal-containing demonstrated by SCNPs.using metal-containing SCNPs.

Cu(OAc)2

P1 THF, r.t. NP1

MMA

AEMA

FigureFigure 1. 1.Schematic Schematic illustration illustration ofof thethe formationformation of of catalytic catalytic single single-chain-chain nanoparticle nanoparticle (SCNP) (SCNP) NP1NP1 fromfrom functional functional precursor precursor polymer polymer P1P1 viavia intra-chainintra-chain copper(II) copper(II)-complexation-complexation (reprinted (reprinted from from [14] [14 ] withwith permission, permission, Copyright Copyright American American Chemical Chemical Society, Society, 2014). 2014). THF THF = tetrahydrofuran; = tetrahydrofuran; MMA MMA = methyl = methacrylate;methyl methacrylate; AEMA = AEMA (2-acetoacetoxy)ethyl = (2-acetoacetoxy)ethyl methacrylate. methacrylate.

Concerning the morphology of SCNPs in solution, two limiting conformations can be obtained (typeConcerning I or intrinsically the morphology disordered of prSCNPsotein-like in solution, morphology two,limiting and type conformations II or globular can enzyme be obtained-like (typeconformation I or intrinsically) depending disordered on the synthesis protein-like conditions morphology, followed and, and type the II nature or globular of the enzyme-like precursor conformation)polymer employed depending and/or on external the synthesis cross-linker conditions used (Figure followed, 2) [9,12 and]. On the one nature hand, ofby theperforming precursor polymerthe folding/collapse employed and/or process external in good cross-linkersolvent conditions used (Figure, a sparse,2)[ non9,12-compact]. On one SCNP hand, morphology by performing is thegenerally folding/collapse obtained process as a consequence in good solvent of the conditions, self-avoiding a sparse, conformation non-compact of the SCNP chain morphology under such is generallycircumstances. obtained In as this a consequenceway, local compaction of the self-avoiding along the chain conformation (giving to of a the pearl chain-necklace under-like such circumstances.morphology) is In promoted this way, instead local of compaction compact globule along formation.the chain (givingThe sparse to aSCNP pearl-necklace-like morphology morphology)resembles that is promotedare shown by instead intrinsically of compact disordered globule proteins formation. (IDPs), Thein which sparse local SCNP compact morphology zones resemblesare connected that are by shown flexible by spacers intrinsically (Figure disordered 2a) [15]. Conversely, proteins (IDPs), when in the which folding/collapse local compact is carried zones are connectedout by self by-assembly ﬂexible spacers of neutral (Figure [16–218]a) [or15 charged]. Conversely, [19] amphiphilic when the folding/collapse random copolymers is carried or by outthe by self-assemblycombination of of neutral thiol- [yne16– 18 coupling] or charged reaction [19] amphiphilic and relatively random long copolymers crosslinkers or by[20], the a combination globular ofcore−shell thiol-yne couplingmorphology reaction is obtained and relatively (Figure long 2b) crosslinkers. The molecular [20], a weight globular of core-shell the SCNP morphology precursor is obtainedpolymer (Figure and its2 b).functionalization The molecular degree weight are of essential the SCNP parameters precursor that polymer control and SCNP its functionalizationsize. degree are essential parameters that control SCNP size. Another way to control the degree of compaction of SCNPs is through the nature of the interactions Type I: Type II: employedSparse to performSCNPs the folding/collapseIDPs of the precursorGlobular polymer SCNPs chains, whichEnzymes can be permanent covalent bonds or dynamic (reversible) interactions (e.g., hydrogen bonding, metal complexation, dynamic covalent bonds). Figure3 provides a schematic illustration of the interplay between the nature of the intra-chain interactions……..…… and solvent quality on the resulting……..…… SCNP morphology and size. A signiﬁcant effort has been recently performed to predict the size reduction upon the folding/collapse of a precursor polymer chain to a SCNP via dynamic interactions [21], or covalent bonds [22] as a function of precursor molar mass (M) and amount of functional groups (x). In fact, covalent-bonded SCNPs in solution with similar nature, and identical values of M and x than responsive SCNPs (b) do display, on average, a higher(a) level of chain compaction. Moreover, in both good and selective

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environment, for allowing stabilization of the transition state during catalysis. In analogy with enzymes, metal ions, or low-molecular-weight organic molecules can work together with SCNPs to enhance reaction rates. Very recently, both exceptional catalytic activity [13] and specificity [14] have been demonstrated by using metal-containing SCNPs.

Cu(OAc)2

P1 THF, r.t. NP1

MMA

AEMA

Figure 1. Schematic illustration of the formation of catalytic single-chain nanoparticle (SCNP) NP1 from functional precursor polymer P1 via intra-chain copper(II)-complexation (reprinted from [14] with permission, Copyright American Chemical Society, 2014). THF = tetrahydrofuran; MMA = methyl methacrylate; AEMA = (2-acetoacetoxy)ethyl methacrylate.

Concerning the morphology of SCNPs in solution, two limiting conformations can be obtained (type I or intrinsically disordered protein-like morphology, and type II or globular enzyme-like conformation) depending on the synthesis conditions followed, and the nature of the precursor polymer employed and/or external cross-linker used (Figure 2) [9,12]. On one hand, by performing the folding/collapse process in good solvent conditions, a sparse, non-compact SCNP morphology is generally obtained as a consequence of the self-avoiding conformation of the chain under such Nanomaterialscircumstances.2017, 7 , 341In this way, local compaction along the chain (giving to a pearl-necklace-like3 of 20 morphology) is promoted instead of compact globule formation. The sparse SCNP morphology resembles that are shown by intrinsically disordered proteins (IDPs), in which local compact zones solvents,Nanomaterials SCNPs 2017, 7 constructed, 341 from exactly the same precursor polymer via non-covalent interactions3 of 20 are connected by flexible spacers (Figure 2a) [15]. Conversely, when the folding/collapse is carried areout expected by self- toassembly show a of larger neutral size [16 than–18] SCNPs or charged prepared [19] amphiphilic through covalent random bonds copolymers [23]. The or currentlyby the Figure 2. Illustration of the two limiting conformations of SCNPs: (a) sparse morphology resembling availablecombination intramolecular of thiol-yne cross-linking coupling methodologies reaction and for relatively the synthesis long crosslinkers of SCNPs have [20], been a globularthe subject of a recentthat typical review of intrinsically [24]. Also, disordered recent advances proteins (IDPs); in the and synthesis, (b) globular characterization, morphology as often simulations found and core−shellin enzymes morphology (reprinted from is obtained [9] with permission,(Figure 2b) Copyright. The molecular Society of Chemical weight ofIndustry, the SCNP 2016). precursor applicationspolymer and of SCNPsits functionalization have been compiled degree are in essential a book [ 25parameters]. that control SCNP size. Another way to control the degree of compaction of SCNPs is through the nature of the interactions employed to perform the folding/collapse of the precursor polymer chains, which can be Type II: permanentType covalentI: bonds or dynamic (reversible) interactions (e.g., hydrogen bonding, metal Globular SCNPs Enzymes complexation,Sparse dynamicSCNPs covalent bonds).IDPs Figure 3 provides a schematic illustration of the interplay between the nature of the intra-chain interactions and solvent quality on the resulting SCNP morphology and size. A significant effort has been recently performed to predict the size reduction ……..…… ……..…… upon the folding/collapse of a precursor polymer chain to a SCNP via dynamic interactions [21], or covalent bonds [22] as a function of precursor molar mass (M) and amount of functional groups (x). In fact, covalent-bonded SCNPs in solution with similar nature, and identical values of M and x than responsive SCNPs do display, on average, a higher level of chain compaction. Moreover, in both good and selective solvents(a), SCNPs constructed from exactly the same( b precu) rsor polymer via non-covalent interactions are expected to show a larger size than SCNPs prepared through covalent bondsFigure [23] 2.. TheIllustration currently of theavailable two limiting intramolecular conformations cross of-linking SCNPs: methodologies (a) sparse morphology for the resemblingsynthesis of SCNPs that typical have been of intrinsically the subject disordered of a recent proteins review (IDPs); [24]. and Also (b) globular, recent morphology advances asin often the synthesis, found in characterization,enzymes (reprinted simulations from [9 ]and with applications permission, of Copyright SCNPs have Society been of Chemicalcompiled Industry, in a book 2016). [25].

Covalent bonds

Non-covalent bonds

Good Selective solvent solvent

FigureFigure 3. 3. InterplayInterplay between intra intra-chain-chain interactions interactions and and solvent solvent quality quality on on the the resulting resulting SCNP SCNP morphologymorphology and and size size (draws (draws taken taken with with permission permission from from [9], Copyright [9], Copyright Society Society of Chemical of Chemical Industry, 2016Industry, and [23 2016], Copyright and [23], Copyright American American Chemical Chemical Society, 2014).Society, 2014).

UpUp to to now, now, three three differentdifferent approachesapproaches have have been been followed followed to to endow endow single single-molecule-molecule nanogels nanogels andand SCNPs SCNPs withwith enzyme-mimeticenzyme-mimetic activity activity (Figure (Figure 4):4 ():i) (i)thethe “imprinted “imprinted particle particle”” route route,, in which in whichthe thecatalytic catalytic site site is imprinted is imprinted during during the the synthesis synthesis of the of the single single-molecule-molecule nanogel nanogel via viaa template a template that that is is subsequently removed from each particle [26,27]; (ii) the “hydrophobic cavity” approach, in which subsequently removed from each particle [26,27]; (ii) the “hydrophobic cavity” approach, in which an amphiphilic copolymer is self-folded in water and the catalytic sites are placed in the resulting an amphiphilic copolymer is self-folded in water and the catalytic sites are placed in the resulting hydrophobic nano-cavity [18,28,29], and (iii) the “concurrent binding/folding” strategy, which is hydrophobic nano-cavity [18,28,29], and (iii) the “concurrent binding/folding” strategy, which is based based on the use of folding/collapsing activators playing simultaneously the dual role of on the use of folding/collapsing activators playing simultaneously the dual role of cross-linkers and cross-linkers and (nanoparticle-entrapped) catalysts [14,30,31]. While the hydrophobic cavity (nanoparticle-entrapped)technique is restricted catalysts to water [14 as, 30a, 31solvent,]. While both the hydrophobic the imprinted cavity particle technique and is concurrent restricted to waterbinding/folding as a solvent, approaches both the imprinted can also be particle used for and enzyme concurrent-like catalysis binding/folding in organic approaches solvents. can also be used for enzyme-like catalysis in organic solvents.

Nanomaterials 2017, 7, 341 4 of 20 NanomaterialsNanomaterials 20172017,, 77,, 334141 44 ofof 2020

EnzymeEnzyme--mimicmimic SCNPsSCNPs

ConcurrentConcurrent binding/folding ImprintedImprinted binding/folding particleparticle

HydrophobicHydrophobic cavitycavity

FigureFigure 4. 4. 4. DifferentDifferentDifferent strategies strategies strategies to to to endow endow endow SCNPs SCNPs SCNPs with with with enzyme-mimetic enzyme enzyme--mimeticmimetic activity: activity: activity: imprinted imprinted imprinted particle particle particle route, routehydrophobicroute,, hydrophobhydrophob cavityicic cavity approach,cavity approachapproach and concurrent,, andand concurrentconcurrent binding/folding binding/foldingbinding/folding technique. technique.technique.

ThTheThee presentpresent reviewreview summarizessummarizes summarizes researchresearch research inin in thethe the pastpast past fewfew few yearsyears years thatthat that hashas has investigatedinvestigated investigated thethe the useuse use ofof SCNPsofSCNPs SCNPs as as as bioinspired, bioinspired, bioinspired, highly highly highly-efﬁcient--efficientefficient nanoreactors nanoreactors nanoreactors (3 (3– (3–30–3030 nm) nm) nm) for for for the the the synthesis synthesis synthesis of of of a a a variety variety of of nanomaterialsnanomaterials (inorganic (inorganic nanoparticles, nanoparticles, quantum quantum dots, dots, carbon carbon nanodots), nanodots), polymers polymers and and chemical chemical compounds, as well as nanocontainers for CO2 capture and release. The rapid evolution of this compoundscompounds,, as as well as nanocontainers for CO2 capture and release. The The rapid rapid evolution evolution of of this emergentemergent field ﬁeldfield is is illustrated illustrated inin FigureFigure5 5.5..

Number of papers of Number 2

00 20102010 20112011 20122012 20132013 20142014 20152015 20162016 20172017 YearYear Figure 5. Evolution of the field of SCNPs for catalysis applications over recent years (2011–2017). Figure 5. EvolutionEvolution of of the the field ﬁeld of of SCNPs SCNPs for for catalysis catalysis applications applications over over recent recent years years (2011 (2011–2017).–2017). Data for 2017 include only papers published until August. Data for 2017 include only papers published until August. 2. Single-Chain Nanoparticles as Bioinspired Nanoreactors/Nanocontainers 2. Single Single-Chain-Chain Nanoparticles Nanoparticles as Bioinspired Nanoreactors/Nanocontainers A variety of uses of SCNPs in bioinspired catalysis have been reported by taking advantage of A variety of uses of SCNPs in bioinspired cataly catalysissis have been reported b byy taking advantage of their folded/collapsed structure, reduced size and soft matter characteristics. Higher reactivity and thetheirir folded/collapsed structure, structure, reduced reduced size size and and soft soft matter matter characteristics characteristics.. Higher Higher reactivity and selectivity has been achieved when some chemical reactions are conducted in nanoreactors. selectivity has has been been achieved achieved when when some some chemical chemical reactions reactions are conducted are conducted in nanoreactors. in nanoreactors. Moreover, Moreover,Moreover, the the enca encapsulationpsulation of of some some nanomaterials nanomaterials generated generated in in situ situ by by using using SCNPs SCNPs as as nanoreactorsnanoreactors can can provide provide with with a a better better stability stability against against aggregation aggregation or or oxidation oxidation issues. issues. InIn next next

Nanomaterials 2017, 7, 341 5 of 20 Nanomaterials 2017, 7, 341 5 of 20

sections, the main use of catalytic SCNPs for the synthesis of nanomaterials, polymers and chemical the encapsulation of some nanomaterials generated in situ by using SCNPs as nanoreactors can provide compounds is reviewed. with a better stability against aggregation or oxidation issues. In next sections, the main use of catalytic SCNPs for the synthesis of nanomaterials, polymers and chemical compounds is reviewed. 2.1. Synthesis of Nanomaterials 2.1. SynthesisA diversity of Nanomaterials of nanomaterials have been synthesized involving SCNPs as highly-efficient nanoreactorsA diversity (Figure of nanomaterials 6). In a pionnering have work been by synthesized He et al. [32], involving gold nanoparticles SCNPs as highly-efﬁcient (Au-NPs) with nanoreactorssizes between (Figure 6 6and). In a9 pionnering nm were work synthesized by He et al.in [ 32 situ], gold by nanoparticles using SCNPs (Au-NPs) based with on sizesN,N- betweendimethylaminoethyl 6 and 9 nm were methacrylate synthesized (DMAEMA) in situ by usingas nanoreactors SCNPs based (Figure on N ,6a).N-dimethylaminoethyl Functional polymer methacrylateprecursors of (DMAEMA) high apparent as nanoreactors molecular weight (Figure (1046a).– Functional110 kDa) containing polymer precursors 7 or 13 mol of high% of apparentcoumarin molecularmoieties were weight employed. (104–110 As kDa) intramolecular containing cross 7 or 13-linking mol %technique of coumarin for the moieties synthesis were of the employed. SCNPs, Asthe intramolecular reversible photo cross-linking-induced coumarin technique cycloaddition for the synthesis was of used. the SCNPs, Dimerization the reversible of the photo-inducedcoumarin units coumarinwas carried cycloaddition out upon ultra was used.-violet Dimerization (UV) irradiation of the ( coumarinλ = 310 nm) units of wasa dilute carried solution out upon containing ultra-violet the (UV)polymer irradiation precursor (λ (1= mg/mL). 310 nm) The of a successful dilute solution formation containing of the SCNPs the polymer was confirmed precursor by (1 a mg/mL).variety of Thecharacterization successful formation techniques of the, including SCNPs was sizeconﬁrmed exclusion by chromatography a variety of characterization (SEC), viscosimetry techniques,, and includingdifferential size scanning exclusion cal chromatographyorimetry (DSC) (SEC),, as well viscosimetry, as transmission and differential electron scanning microscopy calorimetry (TEM) (DSC),measurements. as well as Interestingly, transmission partial electron photo microscopy-cleavage (TEM) of themeasurements. dimerized coumarin Interestingly, units partial was photo-cleavagedemonstrated ofupon the dimerizedirradiation coumarin of the SCNPs units wasat 254 demonstrated nm. The coordination upon irradiation ability of of the the SCNPs tertiary at 254amine nm. in The DMAEMA coordination with ability many ofmetals the tertiary was exploited amine in by DMAEMA these authors with to many synthesize metals Au was-NPs exploited in situ byby theseusing authorsthe SCNPs to synthesizeas nanoreactors, Au-NPs without in situ any by additional using the reductants SCNPs as (Figure nanoreactors, 6a). Remarkably, without any the additionalkinetics of reductants Au-NP (Figure formation6a). Remarkably, was found the to kineticsstrongly of Au-NPdepend formation on the was SCNP found intramolecular to strongly dependcross-linking on the degree, SCNP intramolecular which was attributed cross-linking to the degree, closer whichproximity was of attributed gold ions to in the the closer more proximity compact ofcross gold-linked ions in structures. the more compact cross-linked structures.

(b) (c) (a)

FigureFigure 6. 6.Examples Examples of of nanomaterials nanomaterials synthesized synthesized by using by using SCNPs SCNPs as individual as individual nanoreactors: nanoreactors: (a) gold (a) nanoparticlesgold nanoparticles (Au-NPs); (Au- (NPs);b) cadmium (b) cadmium sulﬁde sulfide quantum quantum dots (CdS-QDs), dots (CdS dash-QDs) circle, dash in circle the inset in the shows inset ashows single a CdS-QD; single CdS and,-QD (c);carbon and, (c nanodots) carbon (C-NDs)nanodots (reprinted (C-NDs) (reprintedwith permission with permission from [32], Copyright from [32], RoyalCopyright Society Royal of Chemistry, Society of 2011; Chemistry, [33], Copyright 2011; [33], Wiley-VCH, Copyright 2012; Wiley and,-VCH, [34 ],2012; Copyright and, [34], Royal Copyright Society ofRoyal Chemistry, Society 2013). of Chemistry, 2013).

QianQian et al. et [33 al.] reported [33] hydrophilic reported SCNPs hydrophilic synthesized SCNPs through synthesized Bergman cyclization-mediated through Bergman intramolecularcyclization-mediated folding/collapse, intramolecular followed folding/collapse by hydrogenolysis, followed on Pd/C by that hydrogenolysis were utilized as on size-tunable Pd/C that nanoreactorswere utilized to as fabricate size-tunable and encapsulate nanoreactors quantum to fabricate dots and (QDs) encapsulate in a one-pot quantum reaction. dots SCNPs (QDs) were in a preparedone-pot from reaction. linear SCNPs precursor were polymers prepared of different from linear size (number-average precursor polymers molecular of different weight ( M sizen): 30.3,(number 62.5,-average and 115.2 molecular kDa) and weight functionalization (Mn): 30.3, 62.5 (10, and and 115.2 25 molkDa) % and of functionalization reactive enediyne (10 groups). and 25 SECmol measurements% of reactive conﬁrmed enediyne groups) the successful. SEC measurements formation of SCNPs, confirmed whereas the successful the 3D morphology formation of of theSCNPs, hydrophilic whereas SCNPs the 3D on morphology a mica surface of wasthe visualizedhydrophilic by SCNPs atomic on force a mica microscopy surface (AFM).was visualized As a proof by ofatomic concept, force photoluminescent microscopy (AFM (PL)). zincAs a sulﬁde proof (ZnS)of concept, QDs were photoluminescent fabricated and encapsulated(PL) zinc sulfide in SCNPs (ZnS) QDs were fabricated and encapsulated in SCNPs of different size. Smaller nanoreactors produce a single QD, each (4.1 nm in size) giving to higher PL emission intensity (quantum yield (QY) = 17% at

Nanomaterials 2017, 7, 341 6 of 20 of different size. Smaller nanoreactors produce a single QD, each (4.1 nm in size) giving to higher PL emission intensity (quantum yield (QY) = 17% at the 310 nm excitation wavelength) while larger nanoreactors form multiple QDs each, resulting in fluorescence quenching (QY = 2%). To further investigateNanomaterials the generality2017, 7, 341 of the method, cadmium sulfide (CdS) QDs (4.7 nm in size, Figure66 ofb) 20 were synthesized and encapsulated in the smaller nanoreactors showing excellent values of quantum yield (QY =the 45%). 310 nm excitation wavelength) while larger nanoreactors form multiple QDs each, resulting in Moreover,fluorescence the quenching use of SCNPs (QY = as 2%). sacrificial To further nanoreactors investigate for the the generality synthesis of of the bright method, photoluminescent cadmium sulfide (CdS) QDs (4.7 nm in size, Figure 6b) were synthesized and encapsulated in the smaller carbon nanodots (C-NDs) was reported by the same group in 2013 (Figure6c) [ 34]. C-NDs, which nanoreactors showing excellent values of quantum yield (QY = 45%). show excellentMoreover, PL behavior, the use low of SCNPs toxicity, as and sacrificial environmental nanoreactors friendliness, for the are synthesis candidates of bright to replace otherphotoluminescent PL nanomaterials carbon for in nanodots vivo imaging (C-NDs) and was lightreported harvesting, by the same among group other in 2013 applications. (Figure 6c) [34]. Firstly, SCNPsC-NDs were, preparedwhich show via excellentBergman PL cyclization-mediated behavior, low toxicity intramolecular, and environmental folding/collapse friendliness from, are linear precursorcandidates polymers to replace of different other PL size nanomaterials (Mn: 11.4, for 35.7, in vivo and imaging 228.4 kDa) and andlight amount harvesting, of enediyneamong other groups (fromapplications. 10 to 26.7 mol Firstly, %). Secondly, SCNPs were the SCNPs prepared were via dispersed Bergman in cyclization a high molecular-mediated weight intramolecular poly(methyl acrylate)folding/collapse matrix and from carbonized linear precursor under polymers continuous of different air flow size (500 (Mn◦:C, 11.4, 30 35.7 min)., and Finally, 228.4 kDa the) and surface amount of enediyne groups (from 10 to 26.7 mol %). Secondly, the SCNPs were dispersed in a high of the resulting C-NDs was reduced with NaBH4 (room temperature (r.t.), 3 h) and decorated with molecular weight poly(methyl acrylate) matrix and carbonized under continuous air flow (500 °C, 30 diamine-terminated poly(ethylene glycol) (PEG) (120 ◦C, 72 h). Interestingly, the optimal emission min). Finally, the surface of the resulting C-NDs was reduced with NaBH4 (room temperature (r.t.), 3 wavelength of the C-NDs was found to red-shift upon decreasing the size of the C-ND, which was h) and decorated with diamine-terminated poly(ethylene glycol) (PEG) (120 °C, 72 h). Interestingly, attributedthe optimal to the emission presence wavelength of an amorphous of the C- core,NDs aswas supported found to red by-shift calculations upon decreasing for model the compounds size of basedthe on C time-ND, dependentwhich was attributed density functional to the presence theory of an (TD-DFT). amorphous For core C-NDs, as supported with a crystalline by calculations core, just the oppositefor model behavior compounds was based reported on time [35]. dependent Smaller C-NDs density showed functional a maximum theory (TD PL-DFT). emission For C-NDs intensity centeredwith at a 448crystalline nm, while core, largerjust the C-NDs opposite displayed behavior awas PL peakreported maximum [35]. Smaller at 401 C-NDs nm. Theshowed quantum a yieldsmaximum of the C-NDs PL emission were found intensity to be centered around at 17% 448 at nm the, while 340 nm larger excitation C-NDs wavelength. displayed a PL The peak surface chemistrymaximum of the at C-NDs 401 nm. was The foundquantum to affectyields theof the QY C through-NDs were aggregate-induced found to be around energy/charge 17% at the 340 nm transfer processes,excitation leading wavelength to a deteriorated. The surface QY. chemistry of the C-NDs was found to affect the QY through aggregate-induced energy/charge transfer processes, leading to a deteriorated QY. The above examples clearly illustrate the huge possibilities of SCNPs as efficient nanoreactors for The above examples clearly illustrate the huge possibilities of SCNPs as efficient nanoreactors the synthesis of other complex nanomaterials. for the synthesis of other complex nanomaterials. 2.2. Synthesis of Polymers 2.2. Synthesis of Polymers SCNPsSCNPs have have been been used used as as bioinspired bioinspired nanoreactorsnanoreactors for for the the synthesis synthesis of ofseveral several polymers polymers via via ring-openingring-opening polymerization polymerization as as well well as as controlled controlled radicalradical polymerization polymerization (see (see Figure Figure 7). 7).

= B(C6F5)3

= THF

= GPE =

= Poly(THF)

(a) (b)

FigureFigure 7. Schematic 7. Schematic illustration illustration of of the the use use of of SCNPsSCNPs as bioinspired bioinspired na nanoreactorsnoreactors for forthe thesynthesis synthesis of of polymerspolymers through: through: (a) ring-opening(a) ring-opening polymerization; polymerization; and and,, ( (bb) )living living radical radical polymerization polymerization (reprinted (reprinted with permission from [30] and [29], Copyright American Chemical Society, 2013 and 2017, with permission from [30] and [29], Copyright American Chemical Society, 2013 and 2017, respectively). respectively).

In a pionneringIn a pionnering work work by by Perez-Baena Perez-Baena etet al.al. [[3030], SCNPs were were endowed endowed with with enzyme enzyme-mimetic-mimetic activityactivity by means by means of a of new a new strategy strategy called called the the “concurrent“concurrent binding/folding binding/folding”” technique technique (see (seeFigure Figure 4). 4). The techniqueThe technique relies relies on on the the selection selection of of appropriate appropriate polymer polymer precursors precursors allowing allowing concurrent concurrent catalyst-assisted intramolecular cross-linking and binding of the catalyst to intramolecular SCNP sites. As a proof of concept, enzyme-mimetic SCNPs were generated from glycidylic polymers via

Nanomaterials 2017, 7, 341 7 of 20 catalyst-assisted intramolecular cross-linking and binding of the catalyst to intramolecular SCNP sites. As a proof of concept, enzyme-mimetic SCNPs were generated from glycidylic polymers via B(C6F5)3-assisted intramolecular ring-opening polymerization, and B(C6F5)3 binding to oxygen-containing functional groups (ether, carbonyl) of the SCNPs through B ... O interactions (Figure7a). Different SCNPs were generated with a range of hydrodynamic radii form 1.5 to 20 nm based on copolymers of benzyl methacrylate or cyclohexyl methacrylate and gycidyl methacrylate. The enzyme-mimetic activity resulted from the immobilization of the catalyst used to promote the folding/collapse in multiple, compartmentalized internal nanoparticle sites that were accessible to reagents. Tuning the global SCNP size, which presumably influences the size, composition, number, and placement of catalytic cavities, was found to have a significant effect on reaction kinetics during catalysis. Remarkably, these SCNPs showed polymerase-like activity toward tetrahydrofuran (THF) via THF ring opening polymerization activated exclusively by the presence of small amounts of glycidyl phenyl ether (GPE), a compound that played the role of cofactor. By working at low SCNP concentrations (0.3 to 2 mg/mL) and moderate reaction times (6 to 48 h), poly(THF-co-GPE) copolymers of high molecular weight (55 to 150 kDa) and very low content of GPE (<3%) but with relatively high polydispersity index (2.2 to 3.2) were synthesized. In a further work by the same group [18], the polymerase activity displayed by metalloenzymes [36–39] was taken as inspiration to prepare copper-containing SCNPs mimicking their globular morphology and living radical polymerization activity. These bioinspired water-borne single-chain globules were constructed through intramolecular copper complexation using amphiphilic random copolymers (47.1–113.6 kDa) that are decorated with different amounts of functional groups (12–33 mol %). Interestingly, these SCNPs allowed the controlled/living radical polymerization of different water soluble monomer to be carried out under reductive conditions. A comparison of the efficiency of these bioinspired SCNPs to that of Laccase enzyme for the synthesis of poly(oligoethylene glycol methyl ether methacrylate) (polyOEGMA) under exactly the same reaction conditions is illustrated in Table1 (see reference [37] for details).

Table 1. Comparison of Laccase enzyme and single-chain nanoparticles (SCNPs) as artiﬁcial metalloenzymes for the synthesis of high molecular weight polyOEGMA via controlled/living radical polymerization in water.

1 2 3 4 Catalyst Type c Mn (kDa) Ð Ieff (%) Reference Laccase 0.87 36.2 1.8 36 [37] SCNPs 0.97 60.9 1.1 24 [18] 1 Fractional conversion. 2 Number-average molecular weight. 3 Polydispersity index. 4 Initiator efﬁciency.

In addition to polyOEGMA, high molecular weight poly(acrylic acid) (polyAA) (Mn = 115.5 kDa, Ð = 2.8) and poly(N-isopropyl acrylamide-co-acrylic acid) (polyNIPAM-co-AA) (Mn = 50.7 kDa, Ð = 1.8) were synthesized using these SCNPs as artificial metalloenzymes. The efficiency of the SCNPs for the synthesis of polyNIPAM-co-AA is compared in Table2 with the efficiency of cysteine (Cys)-blocked bovine Hemoglobin for the synthesis of poly(N-isopropyl acrylamide) (polyNIPAM), showing the better control offered by the SCNPs over the polydispersity index of the resulting macromolecules, as well as the increased initiator efficiency.

Table 2. Comparison of cysteine (Cys)-blocked bovine Hemoglobin and SCNPs for the synthesis of high molecular weight poly(N-isopropyl acrylamide) (polyNIPAM) via controlled/living radical polymerization in water.

Catalyst Type c Mn (kDa) ÐIeff (%) Reference Hemoglobin 0.73 200 2.2 3.2 [39] SCNPs 0.63 50.7 1.8 6.4 [18] Nanomaterials 2017, 7, 341 8 of 20

Recently, iron-containing SCNPs of high activity, stability, and functionality tolerance (e.g., acid, hydroxyl, and air) have been prepared by Azuma et al. [29] and employed for the efficient living radicalNanomaterials polymerization 2017, 7, 341 of various alkyl methacrylate monomers (Figure8). SCNPs were prepared from8 of 20 an amphiphilic terpolymer (Mn = 71.9 kDa, Ð = 1.29) with 22 mol % of aniline reactive groups showing ashowing hydrodynamic a hydrodynamic radius of 6 radius nm in water.of 6 nm Firstly, in water 2,6-pyridinedicarboxaldehyde. Firstly, 2,6-pyridinedicarboxaldehyde was used as bifunctional was used cross-linkeras bifunctional of thecross self-folded-linker of terpolymerthe self-folded in water, terpolymer giving in rise water upon, giving intramolecular rise upon cross-linkingintramolecular to thecross generation-linking to of the bis(imino)pyridine generation of bis(imino)pyridine ligand cavities. Secondly, ligand cavities. coordination Secondly,of FeBr coordination2 into the available of FeBr2 SCNPinto the bis(imino)pyridine available SCNP bis(imino)pyridine ligand cavities was ligand carried cavit outies via was iron carried complexation. out via iron UV complexation. spectroscopy measurementsUV spectroscopy of the measurements SCNPs and model of the iron SCNPs catalysts and supported model iron that catalysts the bis(imino)pyridine supported that units the effectivelybis(imino)pyridine coordinate units FeBr effectively2 to form coordinate bis(imino)pyridine FeBr2 to form iron bis(imino)pyridine complexes. The iron low complexes. and reversible The redoxlow and potential reversible of theredox iron-containing potential of the SCNPs iron-containing was ideal forSCNPs their was use ideal as catalyst for their for use living as catalyst radical polymerization.for living radical Aspolymerization. a proof of concept, As a proof the SCNPsof concept, were the applied SCNPs as were catalyst applied tothe as catalyst living radical to the polymerizationliving radical polymerization of methyl methacrylate of methyl (MMA) methacrylate (Figure9 (MMA)a) in toluene (Figure with 9a) a bromidein toluene initiator with agiving bromide to well-controlledinitiator giving polyMMAto well-controlled of narrow polyMMA molecular of weightnarrow distribution molecular weight (conversion distribution = 90%, M(conversionn = 4.9 kDa, = Ð90=% 1.1,, Mn Br = end4.9 functionalitykDa, Ð = 1.1, = Br 94%). end Interestingly, functionality the = 94 iron-containing%). Interestingly, SCNPs the were iron effectively-containing removed SCNPs afterwere MMAeffectively polymerization removed after by MMA simply polymerization washing the reactionby simply medium washing with the reaction water to medium give bilayer with solutions.water to give The bilayer upper organic solutions phase. The containing upper organic polyMMA phase was containing foundto polyMMA be colorless. was Upon found solvent to be evaporation,colorless. Upon pure solvent polyMMA evaporation was obtained, pure polyMMA (Fe content was < 5 obtained ppm) (Figure (Fe content9b). < 5 ppm) (Figure 9b).

Figure 8. SchematicSchematic illustration illustration of of the the preparation preparation of ofiron iron-containing-containing SCNPs SCNPs of high of high activity activity for the for theefficient efﬁcient living living radical radical polymerization polymerization of of various various methacrylates methacrylates by by using using amphiphilic random random terpolymers containing aniline reactive groups.groups. Intramolecular cross-linkingcross-linking was promoted byby imine formation, which which was was followed followed by byiron iron complexation complexation to the to resulting the resulting bis(imino)pyridine bis(imino)pyridine ligand cavities ligand (reprintedcavities (reprinted from [29 from] with [29] permission, with permission, Copyright Copyright American American Chemical Chemical Society, 2017).Society, 2017).

Nanomaterials 20172017,, 77,, 341341 9 of 20

(b) (a)

Figure 9.9. Different alkyl methacrylatemethacrylate monomers employed for the synthesis of block and randomrandom copolymers using using iron-containing iron-containing SCNPs SCNPs as enzyme-mimeticas enzyme-mimetic catalysts catalysts (a); and (a); easyand recovery easy recovery of nearly of purenearly product pure product (Fe content (Fe < content 5 ppm) < after 5 ppm) polymerization after polymerization (b) (reprinted (b) from (reprinted [29] with from permission, [29] with Copyrightpermission, American Copyright Chemical American Society, Chemical 2017). Society, 2017).

When reused in a a second second catalystcatalyst cycle,cycle, thethe iron-containingiron-containing SCNPs SCNPs showedshowed similarsimilar catalytic catalytic propertiesproperties although although slightly slightly slower slower polymerization polymerization kinetics, kinetics which, which was attributed was attributed to partial to oxidation partial ofoxidation the iron of catalyst the iron during cataly thest first during cycle. the To first illustrate cycle. the To generalillustrate applicability the general of applicabilitythe iron-containing of the SCNPsiron-containing as catalyst SCNPs for controlled as catalyst radical for polymerization,controlled radical a variety polymerization, of methacrylate a variety monomers, of methacrylate including polyethylenemonomers, including glycol methyl polyethylene ether methacrylate glycol methyl (PEGMA), ether methacrylate 2-hydroxyethyl (PEGMA), methacrylate 2-hydrox (HEMA),yethyl andmethacrylate methacrylic (HEMA), acid (MAA) and methacrylic were successfully acid (MAA) polymerized. were successfully Also, random polymerized. and block Also, copolymer random architecturesand block copolymer were prepared architectures using these were SCNPs prepared as highly-efficient using these SCNPs catalyst as (Figure highly9a).-efficient catalyst (FigureGiven 9a). the good results obtained with SCNPs when used as enzyme-mimetic nano-objects for the synthesisGiven the of good well-defined results obtained polymers, with an SCNPs increasing when research used as activity enzyme in-mimetic this field nano is expected-objects for in thethe future,synthesis including of well- thedefined evaluation polymers, as catalyst an increasing of several research metallo-containing activity in this SCNPs field thatis expected were recently in the synthesizedfuture, including [40–44 the]. evaluation as catalyst of several metallo-containing SCNPs that were recently synthesized [40–44]. 2.3. Synthesis of Chemical Compounds 2.3. Synthesis of Chemical Compounds Many different chemical compounds have been synthesized using SCNPs as bioinspired nanoreactors.Many different In fact, chemical the possibility compounds to tune have the SCNP been size synthesized offers many using possibilities SCNPs as for bioinspired endowing SCNPsnanoreactors. with enzyme-mimetic In fact, the possibility activity to bothtune inthe organic SCNP andsize aqueousoffers many media. possibilities Size can for be controlledendowing through:SCNPs with (i) appropriate enzyme-mimetic selection activity of molecular both in weight organic and and number aqueous of reactive media. groupsSize can in be the controlled precursor polymer;through: (ii)(i) typeappropriate of intramolecular selection cross-linking of molecular chemistry weight and (covalent number bonds, of reactive dynamic groups interactions); in the and,precursor (iii) solvent polymer selection; (ii) type (good of solvent,intramolecular selective cross solvent).-linking chemistry (covalent bonds, dynamic interactions)The first; attemptand, (iii) tosolvent mimic selection the active (g (catalytic)ood solvent, site selective of enzymes solvent). with a single-molecule nanogel of 40The kDa first was attempt carried to out mimic by Wulff the active et al. [(catalytic)26] through site the of useenzymes of an “imprintedwith a single particle”-molecule technique. nanogel Theof 40 catalytic kDa was site carried was imprintedout by Wulff during et al. the[26] synthesis through the of cross-linked use of an “imprinted single-molecule particle” particles technique. via aThe diphenyl catalytic phosphate site was imprinted template during that was the subsequentlysynthesis of cross removed-linked from single each-molecule particle. particles These via soft a nanoparticlesdiphenyl phosphate with a size template similar to that those was of enzymessubsequently showed removed Michaelis–Menten from each kinetics particle. for These carbonate soft hydrolysisnanoparticles as observed with a in size its natural similar counterparts to those of although enzymes with showed a very Michaelislow value– ofMenten turnover kinetics frequency for (TOFcarbonate = number hydrolysis of moles as ofobserved substrate in thatits natural a mole of counterparts catalyst can although convert per with unit a timevery [45 low]) (Tablevalue 3 of). Theturnover “imprinted frequency particle” (TOF approach= number wasof moles subsequently of substrate adopted that a bymole Njikang of cataly et al.st can [27] convert to demonstrate per unit thetime superior [45]) (Table enantioselectivity 3). The “imprinted and binding particle” capacity approach against was chiral subsequently amino acid adopted derivatives by Njikang of imprinted et al. SCNPs[27] to demonstrate when compared the tosuperior conventional enantioselectivity imprinted micelles. and binding capacity against chiral amino acid derivatiTheves use of of imprinted the “hydrophobic SCNPs when cavity” compared approach to forconventional the development imprinted of catalytic micelles. SCNPs in water was pioneeredThe use of by the Terashima “hydrophobic et al. [cavity”28]. Catalytically approach activefor the SCNPs development were prepared of catalytic through SCNPs self-folding in water ofwas an pioneeredamphiphilic by random Terashima terpolymer et al. and [28]. post-loading Catalytically of activeRu(II) catalyst SCNPs via were ligand prepared exchange through with 2–3self- Rufolding atoms ofper an particle amphiphilic [46,47 random]. The resulting terpolymer SCNPs and were post- evaluatedloading of as Ru(II) catalysts catalyst for the via transfer ligand hydrogenationexchange with 2 of–3 ketones Ru atoms in per the particle presence [46,47 of sodium]. The resulting formate asSCNPs the hydrogen were evaluated source as showing catalysts TOF for valuesthe transfer around hydrogenation 20 h−1 (Table 3of). ketones The same in systems the presence were foundof sodium to be formate uniquely as active the hydrogen in the oxidation source showing TOF values around 20 h−1 (Table 3). The same systems were found to be uniquely active in

Nanomaterials 2017, 7, 341 10 of 20 of secondary alcohols in the presence of t-BuOOH as the oxidant, showing TOF values as high as 600 h−1 [48], which was ascribed to the signiﬁcant “concentrator effect” giving to a local increase of the concentration of substrates around the catalytic centers. Interestingly, high selectivity toward more hydrophobic secondary alcohol was observed, which was attributed to the hydrophobic interior of the compartmentalized SCNP catalyst system. This hydrophobic selectivity was also observed in the reverse reaction, the transfer hydrogenation [48]. When benzene-1,3,5-tricarboxamide (BTA) functional groups capable to produce helical stacked self-assemblies via multiple hydrogen bonds were incorporated into the SCNPs, they were found to not improve the catalytic properties of the SCNPs [46,47].

Table 3. Summary of reactions catalyzed by unimolecular particles as bioinspired nanoreactors organized by the different strategies followed to endow them with enzyme-mimetic activity.

Reaction type 1 Solvent 2 T (◦C) 3 t (h) c (%) 4 TOF (h−1) 5 Reference Imprinted particle −3 Carbonate hydrolysis H2O/MeCN 10 - - 4.4 × 10 [26] Hydrophobic cavity Hydrogenation of ketones H2O 40 50 98 20 [28] −2 Oxidation of secondary alcohols H2O r.t. 7 × 10 >99 600 [48] Aldol reaction H2O 25 24 99 8 [49] CuAAC PBS r.t. 0.2 >99 13 [50] Mono-depropargylation reaction PBS r.t. 5 >99 0.2 [50] Bis-depropargylation reaction PBS r.t. 25 >99 0.04 [50] Benzoin condensation reaction THF 80 24 65 0.3 [51] Enantioselective sulfoxidation H2O 25 1 99 198 [52] Concurrent binding/folding Reduction of α-diketones CH2Cl2 r.t. 0.1 96 5580 [30] Alkyne dimerization Bulk 60 8 >98 25 [14] Reduction of secondary amines THF r.t. 16 >99 0.6 [53] Allylation of benzophenone THF 35 24 97 8 [53] Biphenyl formation THF 80 16 >99 0.2 [53] Sonogashira coupling HN(C2H5)2 r.t. 24 45 21 [54] CuAAC H2O 50 24 >99 16,667 [13] Photoreduction of CO2 DMF 80 1.5 - 2529 [55] Amination of allyl alcohol C6D6 100 24 99 16 [56] Hydroxylation of phenol H2O 60 1 28 872 [31] 1 CuAAC = Cu(I)-catalyzed azide-alkyne cycloaddition. 2 MeCN = Acetonitrile; PBS = Phosphate buffer (0.01 M); THF = Tetrahydrofuran; DMF = Dimethyl formamide. 3 r.t. = Room temperature. 4 c = Conversion. 5 TOF (turnover frequency) = Amount of products (mol)/[Amount of catalyst active sites (mol)·time (h)].

On a further work by the same group, the same principle of self-folding of amphiphilic random copolymers was employed to produce water-soluble organocatalytic SCNPs containing L-proline catalytic centers (5 mol %) in the hydrophobic SCNP core [49]. These organocatalytic SCNPs were used as catalyst for the aldol reaction of cyclohexanone and p-nitrobenzaldehyde in water with a moderate TOF of 8 h−1 (Table3), a diastereomeric excess up to 94%, and a moderate enantiomeric excess up to 70%. When BTA functionalized L-proline was incorporated to a BTA-based SCNP via molecular recognition, a higher stereoselectivity was observed [57]. Structural and surface hydration properties resembling that of enzymes were found for these water-soluble organocatalytic SCNPs [58]. However, further research is needed to understand why a retarded surface water diffusivity of the organocatalytic SCNPs is critical for activity. Subsequently, Palmans, Meijer, and coworkers reported on a modular synthetic platform for the construction of functional SCNPs for catalysis applications [50]. BTA-based amphiphilic SCNPs were prepared using the post-polymerization functionalization of poly(pentaﬂuorophenyl acrylate). Ligand-containing SCNPs capable of coordinating to Cu(I) and Pd(II) were prepared, which accelerated azide-alkyne cycloaddition reactions and catalyzed depropargylation reactions, respectively (Table3). Interestingly, these reactions were found to proceed in phosphate buffer at physiological pH and at low Nanomaterials 2017, 7, 341 11 of 20

substrate concentrations which render this kind of SCNPs promising systems to function in complex media such as cellular environments [46]. Recently, the “hydrophobic cavity” approach has also been adopted by Lambert et al. [51] for the construction of imidazolium acetate-containing SCNPs, which generated SCNP-supported N-heterocyclic carbenes (NHCs) for organocatalyzed benzoin condensation. A statistical copolymer composed of four different comonomer units, including styrene, grafted poly(ethylene oxide) chains, and antagonist benzimidazol- and chlorobenzyl-based units was synthesized as SCNP precursor (Figure 10). The organocatalytic SCNPs (hydrodynamic diameter < 10 nm) were formed in two steps: (i) intramolecular quaternization in THF at 80 ◦C for 48 h; and (ii) anion metathesis in dry methanol at 40 ◦C for 24 h using potassium acetate. The SCNPs were tested for the organocatalyzed benzoin condensation, by generating NHCs in THF at 80 ◦C thanks to the ability of acetate anions to interact with protons in C2-position of benzimidazolium rings upon heating, giving rise to compartmentalized NHCs. With SCNPs with 10 mol % of supported NHCs (based on the benzimidazolium content), conversions in benzoin around 65% were achieved after 24 h of reaction time, demonstrating the in situ generation of polymer-supported NHC organocatalysts. However, the catalytic activity of these SCNPs was found to be lower than other polymer-supported NHCs, which was attributed to a non-optimized Nanomaterialscopolymer 2017 structure., 7, 341 12 of 20

FigureFigure 10. SchematicSchematic illustration illustration of of the the forma formationtion of of imidazolium imidazolium acetate acetate-containing-containing SCNPs SCNPs via via intramolecularintramolecular quaternization quaternization and and anion anion metathesis metathesis from from a a statistical statistical copolymer copolymer composed composed of of four four differentdifferent comonomer comonomer units units,, including including styrene, styrene, grafted grafted poly(ethylene poly(ethylene oxide) oxide) chains, chains, and and antagonist antagonist benzimidazolbenzimidazol-- and and chlorobenzyl chlorobenzyl-based-based units, units, and and the the use use of of these these SCNPs SCNPs for for the the organocatalyzed organocatalyzed benzoibenzoinn condensation (adapted(adapted from from [51 [51] with] with permission, permission, Copyright Copyright American American Chemical Chemical Society, Society, 2017). 2017). A further report of successful use of the “hydrophobic cavity” approach for bioinspired catalysis in waterA was further made report by Zhang of et successful al. [52]. Firstly, use of thermoresponsive the “hydrophobicN-isopropyl cavity” acrylamideapproach for (NIPAm)-based bioinspired catalysis in water was made by Zhang et al. [52]. Firstly, thermoresponsive N-isopropyl acrylamide (NIPAm)-based random copolymers decorated with hydrophobic Ti(salen) pendants were prepared showing chirality by CD measurements due to the structure-forming motif of the Ti(salen) moiety employed (Figure 11). The average hydrodynamic sizes were 15.8, 11.9, 8.8, and 6.5 nm for SCNPs containing 3.2, 5.6, 8.3, and 11.5 mol % of Ti(salen) pendants. Consequently, the size of the SCNPs gradually decreased upon increasing the content of the hydrophobic Ti(salen) monomer in the precursor copolymer. Secondly, these SCNPs were evaluated as enzyme-mimetic catalysts for the enantioselective oxidation of various sulfides in water to the corresponding sulfoxides. These enzyme-mimic SCNPs were capable of significantly accelerating the asymmetric sulfoxidation in the water of methyl aryl sulfides, as well as other alkyl phenyl sulfides when compared to a traditional chiral Ti(salen) complex. Moreover, only 0.5 mol % loading of catalyst was sufficient to give excellent conversion of methyl phenyl sulfide (85%–99%) in 1 h, with high chemo- (89%–95%) and enantio-selectivity (95%–98%). Conversely, under identical conditions, a traditional chiral Ti(salen) complex gave only 48% of conversion, 65% of chemoselectivity, and 76% of enantio excess (ee). In particular, the ee values observed for electron-rich substrates (>95% ee) represented the best results obtained so far in Ti(salen) complexes. When a batch of alkyl phenyl sulfides with various lengths of the alkyl chain was investigated, the catalytic efficiency and selectivity increased upon increasing the number of atoms of the alkyl chain. As an example, n-hexyl phenyl sulfide was almost quantitatively transformed to (R)-n-hexyl phenyl sulfoxide within 50 min, with 99% of chemoselectivity, and 99% of ee value. The enzyme-mimetic characteristics of these SCNPs were

Nanomaterials 2017, 7, 341 12 of 20 random copolymers decorated with hydrophobic Ti(salen) pendants were prepared showing chirality by CD measurements due to the structure-forming motif of the Ti(salen) moiety employed (Figure 11). The average hydrodynamic sizes were 15.8, 11.9, 8.8, and 6.5 nm for SCNPs containing 3.2, 5.6, 8.3, and 11.5 mol % of Ti(salen) pendants. Consequently, the size of the SCNPs gradually decreased upon increasing the content of the hydrophobic Ti(salen) monomer in the precursor copolymer. Secondly, these SCNPs were evaluated as enzyme-mimetic catalysts for the enantioselective oxidation of various sulfides in water to the corresponding sulfoxides. These enzyme-mimic SCNPs were capable of significantly accelerating the asymmetric sulfoxidation in the water of methyl aryl sulfides, as well as other alkyl phenyl sulfides when compared to a traditional chiral Ti(salen) complex. Moreover, only 0.5 mol % loading of catalyst was sufficient to give excellent conversion of methyl phenyl sulfide (85–99%) in 1 h, with high chemo- (89–95%) and enantio-selectivity (95–98%). Conversely, under identical conditions, a traditional chiral Ti(salen) complex gave only 48% of conversion, 65% of chemoselectivity, and 76% of enantio excess (ee). In particular, the ee values observed for electron-rich substrates (>95% ee) represented the best results obtained so far in Ti(salen) complexes. When a batch of alkyl phenyl sulfides with various lengths of the alkyl chain was investigated, the catalytic efficiency and selectivity increased upon increasing the number of atoms of the alkyl chain. As an example, n-hexyl phenyl sulfide was almost quantitatively transformed to (R)-n-hexyl phenyl sulfoxide within 50 min, with 99% of chemoselectivity, and 99% of ee value. The enzyme-mimetic characteristics of these SCNPs were attributed to: (i) shielding of the catalytic sites from the surrounding aqueous environment (i.e., “site isolation”); and, (ii) confined catalysis in the compartmentalized SCNP that effectively concentrates the substrates within the hydrophobic catalytic core enhancing the reaction rate and selectivity (i.e., “confinement”). Remarkably, due to their thermo-responsive properties these SCNPs were easily recovered after reaction by simple separation upon heating at 80 ◦C and reused up to seven times without significant loss in activity and selectivity. Concerning the “concurrent binding/folding” technique to endow SCNPs with enzyme-mimetic activity, it was first demonstrated in a pioneering work by Perez-Baena et al. [30]. Catalytic SCNPs were constructed from copolymer precursors containing glycidylic functional groups through B(C6F5)3-assisted intramolecular ring-opening polymerization and concurrent B(C6F5)3 binding to oxygen-containing functional groups (ether, carbonyl) of the SCNPs via B... O interactions (Figure7a). The observed enzyme-mimetic catalytic activity resulted from the immobilization of catalytic B(C6F5)3 molecules in multiple, compartmentalized internal nanoparticle sites that were accessible to reagents. After precipitation in hexane and further drying under vacuum, the isolated SCNPs displayed reductase enzyme-mimetic activity for reactions carried out in inert organic solvents, such as dried halogenated solvents, benzene or toluene. The catalytic activity, however, was absent when reactions were performed in solvents that form adducts with B(C6F5)3, such as acetonitrile, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), or alcohols. The reductase activity of these SCNPs was evaluated in a model reaction involving the reduction of α-diketones to silyl-protected 1,2-diols, carried out in dichlorometane at r.t. Remarkably, the smallest SCNPs (hydrodynamic radius = 1.5–2 nm) showed the maximum catalytic activity (TOF = 5880 h−1) with a typical 2-fold decrease in reaction time when compared to the largest ones (Table3). In a subsequent work by the same group [14], the concurrent binding/folding strategy was utilized to synthesize catalytic SCNPs based on metallo-folded polymer chains containing complexed Cu(II) ions, showing catalytic specificity during the oxidative coupling of mixtures of chemically related terminal acetylene substrates. In this work, the intrachain cross-linking of an appropriate poly(methyl methacrylate) (PMMA)-based precursor containing acetoacetoxy ligand units towards Cu ions was used to concurrently fold the individual chains to SCNPs and endow the resulting SCNPs with catalytic activity and substrate specificity through the binding of the Cu ions to the ligand units. The resulting SCNPs approached the size of natural enzymes displaying catalytic specificity a low concentration of Cu(II) ions during the oxidative coupling of mixtures of chemically related terminal alkyne substrates (Figure 12), which cannot be achieved with classical catalysts like CuCl2, Nanomaterials 2017, 7, 341 13 of 20

Nanomaterials 2017, 7, 341 13 of 20

Cu(OAc)2, or Cu(acac)2 [14]. The speciﬁcity of the SCNPs was attributed to the presence of multiple, compartmentalizedattributed to: (i) shielding local catalytic of the catalytic sites surrounded sites from bythe an surrounding environment aqueous of methyl environmen methacrylatet (i.e., units “site allowingisolation” an); and optimum, (ii) conf transitionined catalysis state stabilization in the compartmentalized for the preferred SCNP substrate. that A effectively solvent-based concentrates strategy forthe tuningsubstrates the spatialwithin distribution the hydrophobic of catalytic catalytic sites core in metallo-folded enhancing the SCNPs reaction has rate recently and selectivity been reported (i.e., by“confinement combining” results). Remarkably, from SEC, due small-angle to their thermo X-ray scattering-responsive (SAXS), properties and molecularthese SCNPs dynamics were easily (MD) simulationsrecovered after [59 ].reaction However, by simple further separation research is upon needed heating to investigate at 80 °C and how reused the internal up to structureseven times of metallo-foldedwithout significant SCNPs loss affects in activity both and catalytic selectivity. activity and selectivity.

FigureFigure 11. 11. FormationFormation via via selfself-folding-folding in water of of poly( poly(NN-isopropyl-isopropyl acrylamide)-based acrylamide)-based SCNPs SCNPs IV containingcontaining a a chiral chiral salen salen Ti TiIV complex in the core core for for asymmetric asymmetric sulfoxidation sulfoxidation in in water water of of alkyl alkyl phenylphenyl sulfides sulfides (re (reprintedprinted from from [52 [52] with] with permission, permission, Copyright Copyright Royal Royal Society Society of Chemistry, of Chemistry, 2017). 0 2017).CL1 CL1 = (R,R)-N-(3,5-di-tert-butylsalicylidene)-N -(3-tert-butyl-5-chloromethyl-salicylidene)-1,2- = IV cyclohexanediamine;(R,R)-N-(3,5-di-tert-butylsalicylideIL/Ti(salen) =ne) vinyl-N′- imidazolium(3-tert-butyl- ionic5-chloromethyl liquid-modified-salicylidene) chiral salen-1,2- Ticyclohexanecomplex; NIPAAMdiamine; IL = /Ti(salen)N-isopropylacrylamide; = vinyl imidazolium AIBN ionic = azobisisobutyro-nitrile. liquid-modified chiral salen TiIV complex; NIPAAM = Mono-N-isopropylacry and bi-metalliclamide; AIBN organometallic = azobisisobutyro SCNPs-nitrile. containing Rh(I), Ir(I), and Ni(0) atoms based on polycycloocta-1,5-diene precursor polymers were synthesized by means of the concurrent binding/foldingConcerning method the by“concurrent Mavila et al. binding/folding” [53]. Ir(I)-based SCNPs technique showed to a catalytic endow efficiency SCNPs during with reductionenzyme-mimetic of secondary activity, amines it was andfirst allylationdemonstrat ofed benzophenone in a pioneering similar work toby thatPerez of-Baena small-molecule et al. [30]. modelsCatalytic (Table SCNPs3). Unexpectedly, were constructed Rh(I)-based from SCNPs copolymer gave precur biphenylsors as containing a product during glycidylic cross-coupling functional reactionsgroups through between 4-nitrobenzaldehyde B(C6F5)3-assisted intramolecular and phenyl boronic ring-opening acid showing polymerization the first example and of concurrent selective homo-couplingB(C6F5)3 binding under to oxygen conditions-containing favorable functional to cross-coupling. groups (ether, Interestingly, carbonyl) a substantial of the SCNPs improvement via B…O ininteractions the formation (Figure of the 7a). cross-coupled The observed product enzyme was-mimetic observed catalytic by inducing activity SCNP resulted unfolding from upon the theimmobilization addition of a of NHC catalytic ligand. B(C6F5)3 molecules in multiple, compartmentalized internal nanoparticle sites Pd(II)-based that were accessible SCNPs were to synthesizedreagents. After by Willencacher precipitation et in al. hexane [54] via and metal further complexation drying underusing avacuum, poly(styrene) the isolated precursor SCNPs polymer displayed decorated reductase with enzyme triarylphosphine-mimetic activity ligand for moieties. reactions These carried SCNPs out werein inert used organic as catalysts solvents, in the such Sonogashira as dried coupling halogenated of 2-bromopyridine solvents, benzene and phenylacetylene, or toluene. The showing catalytic activity, however, was−1 absent when reactions were performed in solvents that form adducts with a value of TOF = 21 h (Table3), slightly lower to that of a Pd(PPh 3)2Cl2 complex used as a reference (TOFB(C6F =5)3 35, such h−1 ). as acetonitrile, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), or alcohols. The reductaseCu(I)-containing activity SCNPsof these havingSCNPs was low evaluated toxicity and in a lowmodel metal reacti loadingon involving were the synthesized reduction byof Baiα-diketones et al. [13] to through silyl-protected Cu(II)-mediated 1,2-diols intramolecular, carried out in cross-linking dichlorometane of aspartate-containing at r.t. Remarkably, polyofelfinsthe smallest inSCN waterPs (hydrodynamic followed by in situradius reduction = 1.5–2 withnm) sodiumshowed ascorbate.the maximum The resultingcatalytic activity Cu(I)-containing (TOF = 5880 SCNPs h−1) catalyzedwith a typical azide-alkyne 2-fold decre cycloadditionase in reaction reactions time when with compared high efficiency to the inlargest water ones showing (Table a 3). TOF value as In a subsequent work by the same group [14], the concurrent binding/folding strategy was utilized to synthesize catalytic SCNPs based on metallo-folded polymer chains containing complexed Cu(II) ions, showing catalytic specificity during the oxidative coupling of mixtures of

Nanomaterials 2017, 7, 341 14 of 20 chemically related terminal acetylene substrates. In this work, the intrachain cross-linking of an appropriate poly(methyl methacrylate) (PMMA)-based precursor containing acetoacetoxy ligand units towards Cu ions was used to concurrently fold the individual chains to SCNPs and endow the resulting SCNPs with catalytic activity and substrate specificity through the binding of the Cu ions to the ligand units. The resulting SCNPs approached the size of natural enzymes displaying catalytic specificity a low concentration of Cu(II) ions during the oxidative coupling of mixtures of chemically related terminal alkyne substrates (Figure 12), which cannot be achieved with classical catalysts like CuClNanomaterials2, Cu(OAc)2017, 72,, 341or Cu(acac)2 [14]. The specificity of the SCNPs was attributed to the presence14 of of 20 multiple, compartmentalized local catalytic sites surrounded by an environment of methyl methacrylate units allowing an optimum transition state stabilization for the preferred substrate. A high as 16,667 h−1 (Table3). Remarkably, these SCNPs catalyzed intracellular click reactions inside both solvent-based strategy for tuning the spatial distribution of catalytic sites in metallo-folded SCNPs bacteria and mammalian cells. In one illustrative example, a ﬂuorescent compound was synthesized has recently been reported by combining results from SEC, small-angle X-ray scattering (SAXS), and inside human cancer cells via azide-alkyne cycloaddition catalyzed by the Cu(I)-containing SCNPs. molecular dynamics (MD) simulations [59]. However, further research is needed to investigate how In another example, an antimicrobial agent inducing strong inhibition of bacterial cell growth was the internal structure of metallo-folded SCNPs affects both catalytic activity and selectivity. synthesized inside E. coli via azide-alkyne cycloaddition, catalyzed by these SCNPs.

Figure 12. 12.SCNPs SCNPs synthesized synthesized via the via “concurrent the “concurrent binding/folding” binding/folding” approach basedapproach on metallo-folded based on polymermetallo-folded chains polymercontaining chains complexed containing Cu(II) complexedions, showing Cu(II) catalytic ions, speciﬁcity showing during catalytic the specificity oxidative duringcoupling the of mixturesoxidative of coupling chemically of mixturesrelated terminal of chemically acetylene related substrates terminal (reprinted acetylene from substrates [14] with permission,(reprinted from Copyright [14] with American permission, Chemical Copyright Society, American 2014). Chemical Society, 2014).

Recently,Mono- and enzyme-mimetic bi-metallic organometallic Ni(II)-containing SCNPs SCNPs containing have been Rh(I), prepared Ir(I), and by Thanneeru Ni(0) atoms et al.based [55] viaon polycyclooctaintramolecular-1,5 Ni-thiolate-diene precursor coordination polymers utilizing a were PMMA-based synthesized precursor by meanspolymer of functionalized the concurrent with binding/foldinghydroxyl and thiol method groups. by The Mavila resulting et al. SCNPs [53]. Ir(I) were-based highly SCNPs active showed and selective a catalytic for the efficiency photoreduction during reductionof CO2 to CO of in secondary DMF using amines TiO2 as and a light allylation absorber of support benzophenone and triethanolamine similar to that as a of sacrificial small-molecule electron modelsdonor, showing (Table a 3). TOF Unexpectedly, value as high as Rh(I) 2529-based h−1 at SCNPs 80 ◦C. Similar gave catalyticbiphenyl activity as a was product observed during by crossreplacing-coupling DMF reactionsby CH3CN, between which is 4- notnitrobenzaldehyde a good solvent for and the phenyl PMMA boronic backbone, acid so showing the binding the firstand photoreductionexample of selective of CO homo2 was- notcoupling affected under to a largeconditions extent favorable by the solvation to cross of-coupling. the SCNPs. Interestingly, No detectable a substantialamounts of improvement H2 and formate/formic in the formation acid were of the observed cross-coupled under such product conditions. was observed When comparedby inducing to SCNPa metalloenzyme unfolding upon highly the active addition for various of a NHC conversion ligand. pathways in the biological metabolism of CO2, suchPd(II) as the- carbonbased SCNPs monoxide were dehydrogenase, synthesized by the Willencacher Ni(II)-containing et al. [54 SCNPs] via metal exhibited complexation far better stabilityusing a poly(styrene)at high temperatures precursor and polymer under aerobic decorated conditions. with triarylphosphine ligand moieties. These SCNPs wereRecyclable used as catalysts homogeneous in the catalysts Sonogashira based coupling on Pt(II)-containing of 2-bromopyridine SCNPs have and been phenylacetylene, reported by showingKnöfel et a al.value [56 ].of TOF A PS-based = 21 h−1 (Table precursor 3), slightly polymer lower containing to that of triarylphosphine a Pd(PPh3)2Cl2 complex ligand used moieties as a −1 referencealong the (TOF backbone = 35 h allowed). for SCNP formation via the addition of Pt(1,5-cyclooctadiene)Cl2 in dilute solution. The resulting Pt(II)-containing SCNPs were found to be active homogeneous catalysts for the amination of allyl alcohol showing a TOF value of 16 h−1 with only 0.24 mol % of catalyst. These SCNPs were as active and selective as the homogeneous reference catalyst (i.e., Pt(1,5-cyclooctadiene)Cl2). Moreover, the Pt(II)-containing SCNPs were recovered easily after reaction by applying dialysis in methanol. After recovery, the SCNPs retained its folded structure and were found to remain catalytically active. Very recently, Thanneeru et al. [31] have reported the synthesis of Cu-containing SCNPs as artificial metalloenzymes for hydroxylation reactions of phenols (Figure 13). A PMMA-based prec-ursor polymer functionalized with hydroxyl and imidazole groups was prepared, and nanoparticle formation Nanomaterials 2017, 7, 341 15 of 20 was triggered by Cu-imidazole binding at high dilution using the concurrent binding/folding technique. The SCNPs with an optimized Cu loading showed a high selectivity (>80%) and a value of TOF of 872 h−1 for the hydroxylation reaction of phenol to catechol (CAT) at 60 ◦C for 1 h in water, using H2O2 as an oxidant. Although the amount of Cu loading was found to not affect the catalytic activity (>50% conversion of phenol after 1.5 h for both SCNPs with 6.4 and 18.7 Cu atoms per particle), it affected the selectivity toward the CAT product to a large extent. Hence, a higher selectivity toward CAT (96–73%) was observed for the SCNPs with 6.4 Cu atoms per particle when compared to the SCNPs with 18.7 Cu atoms per particle (78–54%). This was attributed to the difference in flexibility of folded polymer chains that will be largely limited in the presence of large amounts of Cu-imidazole complexes. In this sense, it is worth of mention that the dynamics of the folded polymer backbone is aNanomaterials factor known 2017, to7, 3 be41 critical for the activity of enzymes [60]. 16 of 20

Figure 13.13. SchematicSchematic illustration of Cu-containingCu-containing SCNPs employed as artiﬁcialartificial metalloenzymes for hydroxylation reactionsreactions ofof phenolphenol toto catecholcatechol inin waterwater showingshowing highhigh selectivityselectivity (>80%)(>80%) and turnover frequencyfrequency (TOF) (TOF) (872(872 h h−−11)) (reprinted (reprinted from from [31 [31] with with permission, permission, Copyright Copyright American American Chemical Chemical Society, 2014).2014).

2.4.2.4. CO2 Capture and Release

Very recently,recently, the first ﬁrst example example of of CO CO2 2capturecapture and and release release by bySCNPs SCNPs has hasbeen been reported reported by Fan by Fanet al. et [6 al.1] [.61 Water]. Water-soluble-soluble SCNPs SCNPs with with a hydrodynamic a hydrodynamic size size of of 7.7 7.7 nm nm were were prepared prepared based on NN,N,N-dimethylaminoethyl-dimethylaminoethyl methacrylate methacrylate (DMAEMA) (DMAEMA) precursor precursor polymers polymers containing containing coumarin coumarin moieties viamoieties coumarin via coumarin photodimerization photodimerization [32]. The average[32]. The hydrodynamic average hydrodynamic size of the size SCNPs of the in water SCNPs was in foundwater towas increase found fromto increase 7.7 to 10from nm 7 after.7 topassing 10 nm after CO2 passingfor 10 min CO and2 for then 10 min to decrease and then to to the decrease initial size to afterthe initial subsequently size after bubbling subsequently N2 for bubbling 10 min (Figure N2 for 1014 a).min The (Figure CO2 capture14a). The and CO release2 capture by SCNPsand release was foundby SCNPs to be was a reversible found to be process, a reversible as illustrated process, in as Figure illustrated 14b. in Bubbling Figure 14b. CO 2Bubblingresulted CO in protonation2 resulted in ofprotonation the tertiary of amine the tertiarygroups of amine the DMAEMA-containing groups of the DMAEMA SCNPs,-containing making the SCNPs, nanoparticles making more the hydrophilicnanoparticles and more promoting hydrophilic SCNP and swelling promoting through SCNP water absorption.swelling through Conversely, water bubbling absorption. N2 removedConversely, CO 2bubbling, deprotonating N2 removed the tertiary CO2, aminesdeprotonating and shrinking the tertiary the SCNPs amines to and the initialshrinking hydration the SCNPs level. Theto the effect initial of hydration CO2 capture level. and The release effect onof CO the2 thermoresponsivecapture and release behavior on the thermoresponsive of the precursor behavior polymer andof the the precursor corresponding polym SCNPser and is the depicted corresponding in Figure SCNPs 14c. The is precursor depicted polymerin Figure shows 14c. The a cloud precursor point ◦ ◦ temperaturepolymer shows of arounda cloud 25 pointC, whichtemperature increases of toaround 36 C 25 upon °C, SCNPwhich formation.increases to After 36 °C CO upon2 bubbling, SCNP ◦ theformation. cloud point After of CO both2 bubbling, the precursor the cloud and point SCNPs of increasedboth the precursor to about 75 andC SCNPs as a result increased of protonation to about that75 °C enhances as a result their of water protonation solubility. that Upon enhances N2 bubbling, their water the cloud solubility. points Upon came backN2 bubbling, to the initial the values. cloud Thepoints use came of these back CO to2 thenanocontainers initial values. for The synthesizing use of these Au-NPs CO2 nanocontainers was also investigated for synthesizing by Fan et Au al.-NPs [61]. Remarkably,was also investigated by using theby DMAEMA-containingFan et al. [61]. Remarkabl SCNPsy, by as using gas-tunable the DMAEMA nanoreactors-containing the rate SCNPs of Au-NP as formationgas-tunable can nanoreactors be sped-up the or slown-down rate of Au-NP by formation bubbling CO can2 orbe Nsped2, respectively.-up or slow Then-down increased by bubbling rate of Au-NPCO2 or formationN2, respectively. by bubbling The increased CO2 was rate attributed of Au-NP to the formation swelling by of bubbling the nanoreactors CO2 was that attributed facilitates to − − accessthe swelling of AuCl of4 thespecies, nanoreactors their association that facilitates with protonated access amines,of AuCl and4 species in situ, reduction their association to zerovalent with goldprotonated by nonprotonated amines, and amines. in situ reduction to zerovalent gold by nonprotonated amines.

Figure 14. Changes in the hydrodynamic size distribution of N,N-dimethylaminoethyl methacrylate (DMAEMA)-containing SCNPs in water upon bubbling CO2 and N2 for 10 min (a); demonstration

that the CO2 capture and release by these SCNPs as nanocontainers is a reversible process (b); and

Nanomaterials 2017, 7, 341 16 of 20

Figure 13. Schematic illustration of Cu-containing SCNPs employed as artificial metalloenzymes for hydroxylation reactions of phenol to catechol in water showing high selectivity (>80%) and turnover frequency (TOF) (872 h−1) (reprinted from [31] with permission, Copyright American Chemical Society, 2014).

2.4. CO2 Capture and Release

Very recently, the first example of CO2 capture and release by SCNPs has been reported by Fan et al. [61]. Water-soluble SCNPs with a hydrodynamic size of 7.7 nm were prepared based on N,N-dimethylaminoethyl methacrylate (DMAEMA) precursor polymers containing coumarin moieties via coumarin photodimerization [32]. The average hydrodynamic size of the SCNPs in water was found to increase from 7.7 to 10 nm after passing CO2 for 10 min and then to decrease to the initial size after subsequently bubbling N2 for 10 min (Figure 14a). The CO2 capture and release by SCNPs was found to be a reversible process, as illustrated in Figure 14b. Bubbling CO2 resulted in protonation of the tertiary amine groups of the DMAEMA-containing SCNPs, making the nanoparticles more hydrophilic and promoting SCNP swelling through water absorption. Conversely, bubbling N2 removed CO2, deprotonating the tertiary amines and shrinking the SCNPs to the initial hydration level. The effect of CO2 capture and release on the thermoresponsive behavior of the precursor polymer and the corresponding SCNPs is depicted in Figure 14c. The precursor polymer shows a cloud point temperature of around 25 °C, which increases to 36 °C upon SCNP formation. After CO2 bubbling, the cloud point of both the precursor and SCNPs increased to about 75 °C as a result of protonation that enhances their water solubility. Upon N2 bubbling, the cloud points came back to the initial values. The use of these CO2 nanocontainers for synthesizing Au-NPs was also investigated by Fan et al. [61]. Remarkably, by using the DMAEMA-containing SCNPs as gas-tunable nanoreactors the rate of Au-NP formation can be sped-up or slown-down by bubbling CO2 or N2, respectively. The increased rate of Au-NP formation by bubbling CO2 was attributed to the swelling of the nanoreactors that facilitates access of AuCl4− species, their association with Nanomaterialsprotonated2017 amines,, 7, 341 and in situ reduction to zerovalent gold by nonprotonated amines. 16 of 20

Figure 14.14. Changes in thethe hydrodynamichydrodynamic sizesize distributiondistribution ofof NN,N,N-dimethylaminoethyl-dimethylaminoethyl methacrylatemethacrylate (DMAEMA)-containing(DMAEMA)-containing SCNPs in water upon bubblingbubbling COCO22 and N2 for 10 minmin ((aa);); demonstrationdemonstration

that the the CO CO22 capturecapture and and release release by by these these SCNPs SCNPs as asnanocontainers nanocontainers is a isreversible a reversible process process (b); and (b); and inﬂuence of CO2 capture and release on the thermoresponsive behavior of the precursor polymer

and the corresponding SCNPs (c) (reprinted from [61] with permission, Copyright American Chemical Society, 2014).

3. Conclusions Enzyme-mimetic catalysis with bioinspired SCNPs is a field of growing interest since enzymes are the most efficient catalysts known for biochemical reactions that take place in aqueous environment under crowding conditions near room temperature. The folding of individual polymer chains to functional SCNPs is reminiscent of protein folding to its functional, native state, even if current SCNPs lack the perfection in sequence, uniformity in size, and precise morphology found in enzymes. Even so, the folding of a synthetic polymer to a collapsed state provides with one (or more) denser local packaging zone(s) where catalytic activity and selectivity can be enhanced. This should be promoted by a local polymer environment allowing stabilization of the transition state during catalysis. In analogy with enzymes, metal ions or low-molecular-weight organic molecules can work together with SCNPs to enhance reaction rates. A variety of bioinspired catalysis have been reported by taking advantage of the folded/collapsed structure, reduced size, and soft matter characteristics of SCNPs. Concerning their morphology in solution, two limiting conformations can be obtained by current synthetic methods: a sparse morphology resembling that typical of intrinsically disordered proteins (IDPs) and a globular morphology as often found in enzymes. The molecular weight of the SCNP precursor polymer and its functionalization degree are essential parameters that control SCNP size, in addition to the nature of the interactions employed to perform the folding/collapse (covalent bonds or reversible interactions) and solvent quality (good solvent, selective solvent). Catalytic SCNPs have been prepared through three different methods: (i) the “imprinted particle” route, in which the catalytic site is imprinted via a template that is subsequently removed from each particle; (ii) the “hydrophobic cavity” approach, in which an amphiphilic copolymer is self-folded in water and the catalytic sites are placed in the resulting hydrophobic nano-cavity; and, (iii) the “concurrent binding/folding” strategy, that is based on the concurrent catalyst-assisted intramolecular cross-linking and binding of the catalyst to intramolecular SCNP sites. While the hydrophobic cavity technique is restricted to water as a solvent, both the imprinted particle and concurrent binding/folding approaches can also be used in organic solvents. In this review, recent results obtained in the use of SCNPs as bioinspired, highly-efficient nanoreactors (3–30 nm), for the synthesis of a variety of hard nanoparticles and soft materials of low and high molecular weight, as well as nanocontainers for CO2 capture and release have been discussed in detail. To conclude, driven by the promising results obtained with SCNPs when used as enzyme-mimetic nano-objects for the synthesis of nanomaterials, polymers, and fine chemicals, an increasing research Nanomaterials 2017, 7, 341 17 of 20 activity in this emergent field is expected in the near future. However, much work needs to be done to establish general rules (including type of solvents and temperature) to reach high catalytic activities, as well as to determine the added value of catalytic SCNPs against current competitors (e.g., engineered enzymes, dendrimers, metal-organic frameworks).

Acknowledgments: Financial support by the Spanish Ministry “Ministerio de Economia y Competitividad”, MAT2015-63704-P (MINECO/FEDER, UE), the Basque Government, IT-654-13, and the Gipuzkoako Foru Aldundia, Programa Red Gipuzkoana de Ciencia, Tecnología e Innovación 2017, is acknowledged. Jon Rubio-Cervilla is grateful to the Materials Physics Center-MPC for his predoctoral grant. Edurne González received funding from the “Fellows Gipuzkoa” fellowship of the Gipuzkoako Foru Aldundia. Author Contributions: Jon Rubio-Cervilla, Edurne González and José A. Pomposo jointly conceived, organized and wrote this review paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Copeland, R.A. Structural Components of Enzymes. In Enzymes: A Practical Introduction to Structure, Mechanism and Data Analysis, 2nd ed.; Wiley-VCH: New York, USA, 2000; pp. 42–75. 2. Breslow, R.; Dong, S.D. Biomimetic Reactions Catalyzed by Cyclodextrins and Their Derivatives. Chem. Rev. 1998, 98, 1997–2011. [CrossRef][PubMed] 3. Inoue, K. Functional dendrimers, hyperbranched and star polymers. Prog. Polym. Sci. 2000, 25, 453–571. [CrossRef] 4. Yamamoto, T.; Yamada, T.; Nagata, Y.; Suginome, M. High-Molecular-Weight Polyquinoxaline-Based Helically Chiral Phosphine (PQXphos) as Chirality-Switchable, Reusable, and Highly Enantioselective Monodentate Ligand in Catalytic Asymmetric Hydrosilylation of Styrenes. J. Am. Chem. Soc. 2010, 132, 7899–7901. [CrossRef][PubMed] 5. Liang, C.; Fréchet, J.M.J. Applying key concepts from nature: Transition state stabilization, pre-concentration and cooperativity effects in dendritic biomimetics. Prog. Polym. Sci. 2005, 30, 385–402. [CrossRef] 6. Wang, Y.; Xu, H.; Ma, N.; Wang, Z.; Zhang, X.; Liu, J.; Shen, J. Block Copolymer Micelles as Matrixes for Incorporating Diselenide Compounds: A Model System for a Water-Soluble Glutathione Peroxidase Mimic Fine-Tuned by Ionic Strength. Langmuir 2006, 22, 5552–5555. [CrossRef][PubMed] 7. Discher, D.E.; Eisenberg, A. Polymer vesicles. Science 2002, 297, 967–973. [CrossRef][PubMed] 8. Xu, H.; Meng, F.; Zhong, Z. Reversibly crosslinked temperature-responsive nano-sized polymersomes: Synthesis and triggered drug release. J. Mater. Chem. 2009, 19, 4183–4190. [CrossRef] 9. Latorre-Sanchez, A.; Pomposo, J.A. Recent bioinspired applications of single-chain nanoparticles. Polym. Int. 2016, 65, 855–860. [CrossRef] 10. Huo, M.; Wang, N.; Fang, T.; Sun, M.; Wei, Y.; Yuan, J. Single-chain polymer nanoparticles: Mimic the proteins. Polymer 2015, 66, A11–A21. [CrossRef] 11. Pomposo, J.A. Bioinspired single-chain polymer nanoparticles. Polym. Int. 2014, 63, 589–592. [CrossRef] 12. Gonzalez-Burgos, M.; Latorre-Sanchez, A.; Pomposo, J.A. Advances in Single Chain Technology. Chem. Soc. Rev. 2015, 44, 6122–6142. [CrossRef][PubMed] 13. Bai, Y.; Feng, X.; Xing, H.; Xu, Y.; Kim, B.K.; Baig, N.; Zhou, T.; Gewirth, A.A.; Lu, Y.; Oldfield, E.; et al. A Highly Efficient Single-Chain Metal-Organic Nanoparticle Catalyst for Alkyne-Azide “Click” Reactions in Water and in Cells. J. Am. Chem. Soc. 2016, 138, 11077–11080. [CrossRef][PubMed] 14. Sanchez-Sanchez, A.; Arbe, A.; Colmenero, J.; Pomposo, J.A. Metallo-Folded Single-Chain Nanoparticles with Catalytic Selectivity. ACS Macro Lett. 2014, 3, 439–443. [CrossRef] 15. Sanchez-Sanchez, A.; Akbari, S.; Moreno, A.J.; Lo Verso, F.; Arbe, A.; Colmenero, J.; Pomposo, J.A. Design and Preparation of Single-Chain Nanocarriers Mimicking Disordered Proteins for Combined Delivery of Dermal Bioactive Cargos. Macromol. Rapid Commun. 2013, 34, 1681–1686. [CrossRef][PubMed] 16. Akagi, T.; Piyapakorn, P.; Akashi, M. Formation of Unimer Nanoparticles by Controlling the Self-Association of Hydrophobically Modified Poly(amino acid)s. Langmuir 2012, 28, 5249–5256. [CrossRef][PubMed] 17. Terashima, T.; Sugita, T.; Fukae, K.; Sawamoto, M. Synthesis and Single-Chain Folding of Amphiphilic Random Copolymers in Water. Macromolecules 2014, 47, 589–600. [CrossRef] Nanomaterials 2017, 7, 341 18 of 20

18. Sanchez-Sanchez, A.; Arbe, A.; Kohlbrecher, J.; Colmenero, J.; Pomposo, J.A. Efficient Synthesis of Single-Chain Globules Mimicking the Morphology and Polymerase Activity of Metalloenzymes. Macromol. Rapid Commun. 2015, 36, 1592–1597. [CrossRef][PubMed] 19. Chen, M.; Riddles, C.J.; Van De Mark, M.R. Electroviscous Contribution to the Rheology of Colloidal Unimolecular Polymer (CUP) Particles in Water. Langmuir 2013, 29, 14034–14043. [CrossRef][PubMed] 20. Perez-Baena, I.; Asenjo-Sanz, I.; Arbe, A.; Moreno, A.J.; Lo Verso, F.; Colmenero, J.; Pomposo, J.A. Efficient Route to Compact Single-Chain Nanoparticles: Photoactivated Synthesis via Thiol–Yne Coupling Reaction. Macromolecules 2014, 47, 8270–8280. [CrossRef] 21. Pomposo, J.A.; Rubio-Cervilla, J.; Moreno, A.J.; Lo Verso, F.; Bacova, P.; Arbe, A.; Colmenero, J. Folding Single Chains to Single-Chain Nanoparticles via Reversible Interactions: What Size Reduction Can One Expect? Macromolecules 2017, 50, 1732–1739. [CrossRef] 22. De-La-Cuesta, J.; González, E.; Moreno, A.J.; Arbe, A.; Colmenero, J.; Pomposo, J.A. Size of Elastic Single-Chain Nanoparticles in Solution and on Surfaces. Macromolecules 2017, 50, 6323–6331. [CrossRef] 23. Gillissen, M.A.J.; Terashima, T.; Meijer, E.W.; Palmans, A.R.A.; Voets, I.K. Sticky Supramolecular Grafts Stretch Single Polymer Chains. Macromolecules 2013, 46, 4120–4125. [CrossRef] 24. Mavila, S.; Eivgi, O.; Berkovich, I.; Lemcoff, N.G. Intramolecular Cross-Linking Methodologies for the Synthesis of Polymer Nanoparticles. Chem. Rev. 2016, 116, 878–961. [CrossRef][PubMed] 25. Pomposo, J.A. Single-Chain Polymer Nanoparticles: Synthesis, Characterization, Simulations and Applications; Wiley-VCH: Weinheim, Germany, 2017. 26. Wulff, G.; Chong, B.-O.; Kolb, U. Soluble Single-Molecule Nanogels of Controlled Structure as a Matrix for Efficient Artificial Enzymes. Angew. Chem. Int. Ed. 2006, 45, 2955–2958. [CrossRef][PubMed] 27. Njikang, G.; Liu, G.; Hong, L. Chiral Imprinting of Diblock Copolymer Single-Chain Particles. Langmuir 2011, 27, 7176–7184. [CrossRef][PubMed] 28. Terashima, T.; Mes, T.; De Greef, T.F.A.; Gillissen, M.A.J.; Besenius, P.; Palmans, A.R.A.; Meijer, E.W. Single-Chain Folding of Polymers for Catalytic Systems in Water. J. Am. Chem. Soc. 2011, 133, 4742–4745. [CrossRef][PubMed] 29. Azuma, Y.; Terashima, T.; Sawamoto, M. Self-Folding Polymer Iron Catalysts for Living Radical Polymerization. ACS Macro Lett. 2017, 6, 830–835. [CrossRef] 30. Perez-Baena, I.; Barroso-Bujans, F.; Gasser, U.; Arbe, A.; Moreno, A.J.; Colmenero, J.; Pomposo, J.A. Endowing Single-Chain Polymer Nanoparticles with Enzyme-Mimetic Activity. ACS Macro Lett. 2013, 2, 775–779. [CrossRef] 31. Thanneeru, S.; Duay, S.S.; Jin, L.; Fu, Y.; Angeles-Boza, A.M.; He, J. Single Chain Polymeric Nanoparticles to Promote Selective Hydroxylation Reactions of Phenol Catalyzed by Copper. ACS Macro Lett. 2017, 6, 652–656. [CrossRef] 32. He, J.; Tremblay, L.; Lacelle, S.; Zhao, Y. Preparation of polymer single chain nanoparticles using intramolecular photodimerization of coumarin. Soft Matter 2011, 7, 2380–2386. [CrossRef] 33. Qian, G.; Zhu, B.; Wang, Y.; Deng, S.; Hu, A. Size-Tunable Polymeric Nanoreactors for One-Pot Synthesis and Encapsulation of Quantum Dots. Macromol. Rapid Commun. 2012, 33, 1393–1398. [CrossRef][PubMed] 34. Zhu, B.; Sun, S.; Wang, Y.; Deng, S.; Qian, G.; Wang, M.; Hu, A. Preparation of carbon nanodots from single chain polymeric nanoparticles and theoretical investigation of the photoluminescence mechanism. J. Mater. Chem. C 2013, 1, 580–586. [CrossRef] 35. Kwon, W.; Rhee, S.W. Facile synthesis of graphitic carbon quantum dots with size tunability and uniformity using reverse micelles. Chem. Commun. 2012, 48, 5256–5258. [CrossRef][PubMed] 36. Ng, Y.-H.; di Lena, F.; Chai, C.L.L. PolyPEGA with predetermined molecular weights from enzyme-mediated radical polymerization in water. Chem. Commun. 2011, 47, 6464–6466. [CrossRef][PubMed] 37. Ng, Y.-H.; di Lena, F.; Chai, C.L.L. Metalloenzymatic radical polymerization using alkyl halides as initiators. Polym. Chem. 2011, 2, 589–594. [CrossRef] 38. Sigg, S.J.; Seidi, F.; Renggli, K.; Silva, T.B.; Kali, G.; Bruns, N. Horseradish Peroxidase as a Catalyst for Atom Transfer Radical Polymerization. Macromol. Rapid Commun. 2011, 32, 1710–1715. [CrossRef][PubMed] 39. Silva, T.B.; Spulber, M.; Kocik, M.K.; Seidi, F.; Charan, H.; Rother, M.; Sigg, S.J.; Renggli, K.; Kali, G.; Bruns, N. Hemoglobin and Red Blood Cells Catalyze Atom Transfer Radical Polymerization. Biomacromolecules 2013, 14, 2703–2712. [CrossRef][PubMed] Nanomaterials 2017, 7, 341 19 of 20

40. Tooley, C.A.; Pazicni, S.; Berda, E.B. Toward a tunable synthetic [FeFe] hydrogenase mimic: Single-chain nanoparticles functionalized with a single diiron cluster. Polym. Chem. 2015, 6, 7646–7651. [CrossRef] 41. Greb, L.; Mutlu, H.; Barner-Kowollik, C.; Lehn, J.-M. Photo- and Metallo-responsive N-Alkyl α-Bisimines as Orthogonally Addressable Main-Chain Functional Groups in Metathesis Polymers. J. Am. Chem. Soc. 2016, 138, 1142–1145. [CrossRef][PubMed] 42. Berkovich, I.; Mavila, S.; Iliashevsky, O.; Kozucha, S.; Lemcoff, N.G. Single-chain polybutadiene organometallic nanoparticles: An experimental and theoretical study. Chem. Sci. 2016, 7, 1773–1778. [CrossRef][PubMed] 43. Wang, F.; Pu, H.; Jin, M.; Wan, D. Supramolecular Nanoparticles via Single-Chain Folding Driven by Ferrous Ions. Macromol. Rapid Commun. 2016, 37, 330–336. [CrossRef][PubMed] 44. Pavlov, G.M.; Perevyazko, I.; Happ, B.; Schubert, U.S. Intra- and inter-supramolecular complexation of poly(butyl methacrylate)-co-2-(1,2,3-triazol-4-yl)pyridine copolymers induced by CoII, FeII, and EuIII ions monitored by molecular hydrodynamics methods. J. Polym. Sci. Part A Polym. Chem. 2016, 54, 2632–2639. [CrossRef] 45. Kozuch, S.; Martin, J.M.L. “Turning Over” Deﬁnitions in Catalytic Cycles. ACS Catal. 2012, 2, 2787–2794. [CrossRef] 46. Artar, M.; Palmans, A.R.A. Catalysis Inside Folded Single Macromolecules in Water. In Effects of Nanoconﬁement on Catalysis; Poli, R., Ed.; Springer International Publishing: Cham, Switzerland, 2017; pp. 105–123. 47. Artar, M.; Terashima, T.; Sawamoto, M.; Meijer, E.W.; Palmans, A.R.A. Understanding the catalytic activity of single-chain polymeric nanoparticles in water. J. Polym. Sci. Part A Polym. Chem. 2014, 52, 12–20. [CrossRef] 48. Artar, M.; Souren, E.R.J.; Terashima, T.; Meijer, E.W.; Palmans, A.R.A. Single Chain Polymeric Nanoparticles as Selective Hydrophobic Reaction Spaces in Water. ACS Macro Lett. 2015, 4, 1099–1103. [CrossRef] 49. Huerta, E.; Stals, P.J.M.; Meijer, E.W.; Palmans, A.R.A. Consequences of Folding a Water-Souble Polymer Around an Organocatalyst. Angew. Chem. Int. Ed. 2013, 52, 2906–2910. [CrossRef][PubMed] 50. Liu, Y.; Pauloehrl, T.; Presolski, S.; Albertazzi, L.; Palmans, A.R.A.; Meijer, E.W. Modular Synthetic Platform for the Construction of Functional Single-Chain Polymeric Nanoparticles: From Aqueous Catalysis to Photsensitization. J. Am. Chem. Soc. 2015, 137, 13096–13405. [CrossRef][PubMed] 51. Lambert, R.; Wirotius, A.-L.; Taton, D. Intramolecular Quaternization as Folding Strategy for the Synthesis of Catalytically Active Imidazolium-Based Single Chain Nanoparticles. ACS Macro Lett. 2017, 6, 489–494. [CrossRef] 52. Zhang, Y.; Tan, R.; Gao, M.; Hao, P.; Yin, D. Bio-inspired single-chain polymeric nanoparticles containing a chiral salen TiIV complex for highly enantioselective sulfoxidation in water. Green Chem. 2017, 19, 1182–1193. [CrossRef] 53. Mavila, S.; Rozenberg, I.; Lemcoff, N.-G. A general approach to mono- and bimetallic organometallic nanoparticles. Chem. Sci. 2014, 5, 4196–4203. [CrossRef] 54. Willenbacher, J.; Altintas, O.; Trouillet, V.; Knöfel, N.; Monteiro, M.J.; Roesky, P.W.; Barner-Kowollik, C. Pd-complex driven formation of single-chain nanoparticles. Polym. Chem. 2015, 6, 4358–4365. [CrossRef] 55. Thanneeru, S.; Nganga, J.K.; Amin, A.S.; Liu, B.; Jin, L.; Angeles-Boza, A.M.; He, J. “Enzymatic” Photoreduction of Carbon Dioxide using Polymeric Metallofoldamers Containing Nickel-Thiolate Cofactors. ChemCatChem 2017, 9, 1157–1162. [CrossRef] 56. Knöfel, N.D.; Rothfuss, H.; Willenbacher, J.; Barner-Kowollik, C.; Roesky, P.W. Platinum(II)-Crosslinked Single-Chain Nanoparticles: An Approach towards Recyclable Homogeneous Catalysts. Angew. Chem. Int. Ed. 2017, 56, 4950–4954. [CrossRef][PubMed] 57. Huerta, E.; van Genabeek, B.; Stals, P.J.M.; Meijer, E.W.; Palmans, A.R.A. A Modular Approach to Introduce Function into Single-Chain Polymeric Nanoparticles. Macromol. Rapid Commun. 2014, 35, 1320–1325. [CrossRef][PubMed] 58. Stals, P.J.M.; Cheng, C.-Y.; van Beek, L.; Wauters, A.C.; Palmans, A.R.A.; Han, S.; Meijer, E.W. Surface water retardation around single-chain polymeric nanoparticles: Critical for catalytic function? Chem. Sci. 2016, 7, 2011–2015. [CrossRef] 59. Basasoro, S.; Gonzalez-Burgos, M.; Moreno, A.J.; Lo Verso, F.; Arbe, A.; Colmenero, J.; Pomposo, J.A. A Solvent-Based Strategy for Tuning the Internal Structure of Metallo-Folded Single-Chain Nanoparticles. Macromol. Rapid Commun. 2016, 37, 1060–1065. [CrossRef][PubMed] Nanomaterials 2017, 7, 341 20 of 20

60. Henzler-Wildman, K.; Kern, D. Dynamic personalities of proteins. Nature 2007, 450, 964–972. [CrossRef] [PubMed]

61. Fan, W.; Tong, X.; Farnia, F.; Yu, B.; Zhao, Y. CO2-Responsive Polymeric Single-Chain Nanoparticles and Self-Assembly for Gas-Tunable Nanoreactors. Chem. Mater. 2017, 29, 5693–5701. [CrossRef]

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