Review sobre SAMs

Review sobre SAMs

(Parte 1 de 3)

nature materials | VOL 8 | OCTOBER 2009 | 781 review article Published online: 6 sePtember 2009 | doi: 10.1038/nmat2496

Nanoscale objects are the focus of much attention not only as critical components in the emergence of cellular life1, but as small-scale materials with advanced functions and properties that can be isolated or assimilated into numerous applications, such as in bioelectronics, sensing, drug delivery, catalysis and nanocomposites2–6. In this review, we focus on hybrid nano-objects that are constructed by using organic components to coordinate the nucleation, growth, organization and transformation of inorganic nanophases to produce discrete integrated objects or higher-order structures under equilibrium or non-equilibrium conditions. Using illustrative examples, we systematize the concepts that underpin these assembly processes, and discuss theoretical considerations.

integrative self-assembly We begin by surveying present strategies used to coordinate the structure and organization of discrete hybrid nano-objects under equilibrium conditions5,7. These materials consist of inorganic nano- components, such as metals (Au, Ag), metal-ion salts (Ca3(PO4)2, CaCO3), quantum dots (CdS), magnetic (Fe3O4) and photo active

(TiO2) oxides, and glassy solids (SiO2), which are chemically integrated with discrete organic nano structures. We classify the construction processes leading to single nano-objects as ‘inte grative self-assembly’, and identify three systematic approaches: nano scale incarceration, supramolecular wrapping and nano structure templating (Fig. 1a–d).

Nanoscale incarceration. In general, hybrid nano-objects comprising entrapped inorganic components necessitate the preorganization of persistent self-assembled organic architectures with hollow accessible interiors. An important archetype of this approach is the use and application of the capsid-like protein, apoferritin, for the controlled nucleation and confinement of inorganic nanoparticles8. Addition of various inorganic reaction mixtures under appropriate conditions (such as low concentrations, slow reaction rates, substoichiometric ratios and so on) has led to the synthetic construction of a wide range of novel protein-encapsulated core– shell hybrid nano-objects; most recently, these include ferritins with incarcerated nanoparticles of CaCO3 (ref. 9), PbS (ref. 10), Ag self-assembly and transformation of hybrid nano-objects and nanostructures under equilibrium and non-equilibrium conditions stephen mann

Understanding how chemically derived processes control the construction and organization of matter across extended and multiple length scales is of growing interest in many areas of materials research. Here we review present equilibrium and nonequilibrium self-assembly approaches to the synthetic construction of discrete hybrid (inorganic–organic) nano-objects and higher-level nanostructured networks. We examine a range of synthetic modalities under equilibrium conditions that give rise to integrative self-assembly (supramolecular wrapping, nanoscale incarceration and nanostructure templating) or higherorder self-assembly (programmed/directed aggregation). We contrast these strategies with processes of transformative selfassembly that use self-organizing media, reaction–diffusion systems and coupled mesophases to produce higher-level hybrid structures under non-equilibrium conditions. Key elements of the constructional codes associated with these processes are identified with regard to existing theoretical knowledge, and presented as a heuristic guideline for the rational design of hybrid nano-objects and nanomaterials.

(ref. 1) and In (ref. 12), as well as Pd (ref. 13) or CoPt (ref. 14) for use in catalytic hydrogenation and magnetic storage, respectively. Similar approaches have been developed using viral capsids15–17. In both systems, inorganic reactants permeate the polypeptide shell through molecular channels within the self-assembled architecture, and nucleation occurs specifically within the internal cavity. Although molecular diffusion into the ferritin cage is passive, pH- induced swelling of the shell of cowpea chlorotic mottle virus can be used to access the internal environment16. Interestingly, under certain conditions, the above protocols can be run in reverse such that preformed inorganic nanoparticles of appropriate size, shape and surface functionality can be used as platforms for the self-assembly of viral coat proteins15 or apoferritin subunits10. For example, coat proteins assemble spontaneously around gold nanoparticles functionalized with anionic moieties, specific nucleic-acid packaging signals, or a monolayer of carboxy-terminated polyethylene glycol18–20 (Fig. 2a).

Nanoscale incarceration of inorganic components within integrated hybrid objects can also occur under special conditions by confinement within lyotropic organic mesophases that are constrained in particle size. Although less common than cage-mediated entrapment, this process is intriguing as the hybrid nano-objects do not adopt core–shell architectures but instead have unusual mesostructured interiors. For example, quenching of synthesis mixtures of cationic amphiphiles and silicon alkoxides by rapid aerosol drying gives rise to the spontaneous co-assembly of spherical silica– surfactant nanoparticles with ‘onion-like’ internal architecture, in which the inorganic phase is incarcerated within a concentrically arranged lamellar mesostructure21 (Fig. 2b). In contrast, rapid dilution and neutralization of analogous reaction mixtures produces oblate ellipsoidal nanoparticles of the hexagonally ordered silica– surfactant mesophase MCM-41, which have a highly unusual modulated mesostructure owing to structure-induced shape transformations during nucleation22 (Fig. 2c).

Supramolecular wrapping. In this approach, supramolecular assemblies or macromolecules with well-defined persistent

Centre for Organized Matter Chemistry, School of Chemistry, University of Bristol, Bristol BS8 1TS, UK. e‑mail:

782 nature materials | VOL 8 | OCTOBER 2009 | three-dimensional architectures are enveloped in a continuous inorganic coating under near-equilibrium conditions to produce discrete core–shell hybrid nano particles23. This approach is related to templating strategies (see below), but is differentiated by the continuous nature of the inorganic component and general absence of surface patterning. Sol–gel reactions, particularly those involving the hydrolysis and condensation of silica precursors, seem to be very compatible with high-fidelity wrapping. For example, organogel nanostructures have been successfully transcribed into silicified hybrids with cylindrical24 or helical morphology25, and similar procedures have been used to prepare silica-coated porphyrin-based nanotapes26,27 and collagen fibrils28.

Increasing attention is being placed on maintaining the functionality of the organic architectures after silica-shell wrapping to produce core–shell hybrids with integrated properties. Ideally, the organic functionality should be retained after assimilation of the inorganic component, and remain accessible to external stimuli such as changes in pH or optical excitation27. In practice, these triggers are transmitted to the embedded organic nanostructure through nanopores in the ultrathin silica envelope, thereby enabling collective functions to operate within a single hybrid nano-object. Significantly, these experimental protocols have been extended to the silica/organoclay wrapping of single molecules of polysaccharides29,30, proteins30–32, enzymes31,3 and DNA32 (Fig. 2d). In each case, the wrapped biomolecules remain structurally intact and maintain their functionality even under adverse conditions.

Nanostructure templating. A wide range of self-assembled organic architectures have been used as supramolecular templates for the construction of original hybrid nano-objects under equilibrium conditions. In general, slow reaction rates — aided by, for example, low levels of supersaturation and reactant concentrations — are used to facilitate favourable inter actions specifically at the organic surface so that nanoscale inorganic deposition occurs preferentially along the accessible surfaces of the template (Fig. 2e). This site selectivity is improved in many cases by sequential exposure of the preorganized organic nanostructure to the individual inorganic reactants34. In practice, this often involves the substoichiometric binding of metal cations to the template surface, followed by adding ions/molecules that trigger inorganic deposition or crystallization. These procedures are particularly effective for the templating of metal or semiconductor nano particles within spherical objects such as dendrimer nano particles35, or on the surface of highly anisotropic biological nanostructures such as DNA36 and self-assembled microtubules37. Similarly, arrays of Au, Ag or Pt nanoparticles have been prepared by in situ deposition on the external or internal surface of rod-shaped tobacco mosaic virus particles34,38,39. As these hybrids are uniform in length and width, mechanically robust and accessible to physical manipulation, they may have important technological uses as components of digital memory devices39 or as electrically conducting nanowires40.

A diverse range of synthetic organic molecules has been used to prepare anisotropic nanostructures (such as filaments, tubes, helicoids and so on) that promote the template-directed assembly of integrated hybrid nanoscale objects under equilibrium conditions. Some representative examples include chiral lipids41,42, peptide-based surfactants43,4, block copolymers45,46, dendron rod–coil triblocks47,48 and T-shaped dendro-calixarene amphiphiles49 (Fig. 3). Self-assembling peptides with sequences programmed to have appropriate polar or charged surface amino acid residues50,51 that induce β-sheet (amyloid) formation52,53, or initiate coiled-coil intermolecular interactions54, have also been investigated. In many cases, these molecules self-assemble in water into nanostructured objects by enthalpic and entropic processes, and adopt highly anisotropic architectures because of the intricacies of molecular shape and size, and specificity of the intermolecular interactions.

In general, the above amphiphiles and peptides show certain key characteristics that are designed into the molecular structure to facilitate their use as effective templates for nanoscale inorganic ba c ed 50 nm

80 nm30 nm

20 nm ab cd ef

Figure 1 | Present approaches to the construction and organization of discrete hybrid nano-objects under equilibrium conditions. a–d, Integrative assembly. Nanoscale incarceration by confinement of inorganic reactions within preformed supramolecular organic containers (a), or by self‑assembly of organic subunits around preformed inorganic nanoparticles (b). Wrapping of supramolecular organic nano‑objects with ultrathin inorganic shells (c). Site‑directed templating of inorganic components on organic nanostructures (d). e,f, Higher‑order assembly of unitary nano‑object constructs by programmed aggregation (e), and extended nanostructures by multicomponent reconstitution (f).

Figure 2 | transmission electron microscopy (tem) images of hybrid nano-objects produced by integrative self-assembly. a, Nanoscale incarceration of a single preformed gold particle by self‑assembly of viral coat proteins. b,c, Incarceration of SiO within nanoparticles of ordered lyotropic surfactant mesophases showing interiors with concentric lamellae (b), and a modulated hexagonal structure (c), viewed side‑on. d, Supramolecular wrapping of a single‑plasmid DNA molecule in a continuous ultrathin shell of condensed organoclay oligomers. e, Template‑directed deposition of gold nanoparticles on the surface of a nanostructured tobacco mosaic virus rod‑like particle to produce metallized biostructures with high shape anisotropy. Figures reproduced with permission: a, © 2006 ACS; b, © 1999 NPG; c, © 2002 Wiley‑VCH; d, © 2007 ACS; e, © 2008 RSC.

nature materials | VOL 8 | OCTOBER 2009 | 783 deposition. For example, although phospholipid (1) (Fig. 3a) has a molecular structure closely related to that of conventional lipids, there are three features that make this amphiphile particularly useful as a template with programmed structure and embedded functionality. First, the zwitterionic headgroup provides flexibility in interfacial binding, such that anions (silicate)41 or metal cations42 can be sequestered preferentially at the headgroup surface. Second, the chiral centre positioned at the junction between the phosphocholine headgroup and acyl chains drives the molecular packing into a highly anisotropic twisted tubular architecture55, which is retained in the hybrid counterparts (Fig. 3f). And third, position- ing of the diacetylenic groups between C8 and C9 atoms promotes strong interchain interactions that structurally stabilize the bilayer motif, as well as facilitate UV-induced ene–yne chain polymerization to produce hybrid nano-objects with solvatochromic, thermochromic and mechanochromic properties41.

Similar considerations have been applied in the design of the dendron rod–coil molecule (2) (Fig. 3b), which assembles into helical nanotapes by hydrogen bonding between the hydroxyl and carbonyl groups in the dendron segment and π-π stacking interactions of the aromatic rod domain47. Incubation of the twisted rib- bons with Cd(NO3)2 in tetrahydrofuran, followed by reaction with

H2S, promotes nucleation of CdS nanocrystals specifically around the organic nanostructure to produce helically shaped nano-objects

(Fig. 3g). Significantly, detailed investigations indicate that the CdS component is located on only one side of the twisted template (Fig. 3h), possibly because helicoids with a slightly coiled axis would have one face more exposed to the reactant molecules present in the solvent48. In other studies, nanoscale hybrid objects with desired biocompatibility are being developed by using peptide-based nanostructured templates in combination with inorganic components — such as Ca3(PO4)2 and CaCO3 — that mimic natural biominerals. For example, the peptide-alkyl-chain surfactant (3) (Fig. 3c) con- sists of three key engineered features in the headgroup region44: (i) four consecutive cysteine amino acids are included in the peptide sequence to generate disulphide bonds between adjacent molecules when self-assembled to produce robust nanofibres; (i) a phosphoserine residue is included to promote binding of Ca2+ and subsequent site-directed nucleation of calcium phosphate; and (i) the cell-binding motif Arg-Gly-Asp (RGD) is sited at the end of the headgroup to facilitate cell adhesion.

Complementary strategies based on block copolymer self- assembly are being developed for the template-directed construction of hybrid nano-objects with high shape anisotropy. For instance, cylindrical micelles prepared from poly(acrylic acid)-containing block copolymers have been used as organic templates for the formation of constructs comprising linear arrays of metallic, metal oxide or metal sulphide nanoparticles56,57. In a recent development, hf g n+m=9 n-Bu

Me Me Me

NPh Ph

C x ij k 100 nm

5 25 nm500 nm

Figure 3 | Nanostructure templating of hybrid nano-objects. a–e, Examples of template molecules. a, Chiral phospholipid (1). b, Dendron rod‑coil (2). c, Peptide‑alkyl‑chain surfactant (3). d, Diblock copolymer (4). e, T‑shaped dendro‑calix[4]arene (5) (see text for details). f, SiO helicoid based on (1). g,h, CdS helicoid (g) based on (2), and corresponding graphic (h) showing templating of CdS on one surface of helicoid (2). i,j, Co‑micelle of (4) with quaternized P2VP central region and unfunctionalized P2VP ends (i), and corresponding TEM image (j) showing patterned deposition of TiO in the middle domain. k, Graphic showing cut‑away structure of a persistent cylindrical micelle formed from (5) with ordered surface corrugations of high negative charge. Figures reproduced with permission: a,b,g,h, © 2002 Wiley‑VCH; d,j, © 2009 Wiley‑VCH; e, © 2007 Wiley‑VCH.

784 nature materials | VOL 8 | OCTOBER 2009 | the diblock copolymer, poly(ferrocenyldimethylsilane)-b-poly

(2-vinylpyridine) (PFS17-b‑P2VP170, (4) (Fig. 3d) has been used to prepare discrete cylindrical co-micelles that consist of a posi- tively charged central region with a quaternized P2VP corona, along with un-functionalized end segments (Fig. 3i). The spatially defined coronal chemistries have been exploited for the localized nucleation of titania specifically across the central segment of each co-micelle46 (Fig. 3j).

Although the above studies clearly demonstrate the success of nanostructure templating for the integrative self-assembly of hybrid nano-objects, the fidelity of transcription is usually low when viewed at sub-10-nm dimensions. This is because the surface charge distribution associated with packing of the amphiphilic molecules is generally homogeneous at this length scale, so there are no distinctive regions for site-specific inorganic nucleation or spatial confinement of the primary growth clusters. As a consequence, the inorganic components are nucleated randomly across the organic surface, and subsequently grow along the interface until they come into contact with each other. In principle, these limitations can be circumvented by using a sterically constrained bulky amphiphile with a sufficiently rigid molecular geometry to produce packing arrangements that impose structurally persistent binding domains across the surface of the template. A possible archetype of this system is the self-assembly of the T-shaped dendro-calix[4]arene amphiphile (5) (Fig. 3e) into structurally rigid micelles with regularly arranged 2.5-nm-wide surface corrugations lined with several carboxylate ligands58 (Fig. 3k). In this case, the high negative charge density within these surface domains is sufficient to restrict the nucleation and growth of CdS clusters to the corrugated regions, such that quantum dots are organized as discrete components along the cylindrical micelle49.

higher-order equilibrium self-assembly The considerable difficulties associated with achieving high- resolution spatial separation and organization of individual components within single nano-objects prepared by in situ deposition (integrative assembly) can be alleviated to some extent by using preformed nanostructured components that undergo spontaneous higher-order assembly under equilibrium conditions to produce unitary nano-objects or extended nanostructutures (Fig. 1e,f). In each case, the extent of constructional ordering and component placement is dependent on the level of interparticle specificity encoded into the individual building blocks.

Unitary nano-objects. Co-organization of two or more types of particles within the same nanoscale object is attracting much attention as a route to preparing dispersed nano-objects with multifunctionality. Typically, this involves conjugation of spherically shaped particles of different size and composition, with the larger component acting as a central platform to control the coupling of the smaller constituents to produce a unitary nano-object with satellite-like organization59 (Fig. 4a). Interparticle specificity is often achieved by ligation of complementary organic molecules to the different nanoparticle surfaces, such that the components are assembled into persistent arrangements by molecular recognition. This strategy of ‘programmed aggregation’ often involves biomolecular-induced interparticle coupling, and is related to pioneering work on the DNA-induced assembly of gold nanoparticles into disordered networks60. Similar approaches using high-affinity non-covalent recognition between streptavidin/biotin or antibody/ antigen conjugates have been developed to construct a range of multicomponent, biologically active nanoscale hybrid objects61. As the coupling interactions are reversible and highly selective, the encoded nano-objects can be assembled and disassembled by changes in temperature and ligand concentrations.

The programmed assembly of nanoscale objects from components of different size and shape seems to be a promising approach to a range of new multifunctional nanostructures with ordered constituents. Several approaches have been investigated based on the use of biomolecular, covalent or electrostatic binding. For example, oligonucleotide-capped gold nanoparticles and protein-coated iron oxide nanoparticles (ferritin) have been assembled on the surface of individual carbon nanotubes (CNT) to produce constructs with optical, superparamagnetic and metallic/semi-conductive properties62. As the coupling interactions between the gold and protein molecules are dependent on biotin/streptavidin linkages and duplexation, addition of excess biotin or thermal treatment results in disassembly specifically of the gold nano particles (Fig. 4b), as well as subsequent changes in function that might be exploited in bioelectrochemical and biosensing applications. An intriguing example of reversible assembly triggered by conformational changes, rather than by biomolecular decoupling, is the stabilization and release of a single CdS guest nanoparticle from the open-ended cylindrical cavity of a barrel-shaped chaperonin protein complex63 (Fig. 4c).

Other studies have used covalent coupling to addressable amino acids to prepare biologically derived hybrid nano-objects from nanostructured components of different size and shape. For example, gold nanoparticles have been assembled onto amyloid-like fibres genetically engineered with surface-accessible cysteine residues64, tagged with surface Ni-NTA groups (NTA = nitrilo-triacetic acid) and positioned specifically within the 1 nm ring of the heatshock protein complex, SP1, by high-affinity binding to six localized histidines65, or functionalized with hydroxyl–succimido ligands for assembly onto collagen-like peptide fibres containing a designed sequence of lysine residues66. Significantly, high levels of definition in the placement of gold nanoparticles can be achieved by exploiting the underlying spatial arrangement of the coat proteins of viral capsids. For example, preformed gold nanoparticles are assembled at specific locations around the external surfaces of cowpea mosaic viral particles by genetically engineering cysteine residues at symmetry-related positions in the icosa hedral cage67. Although this level of fidelity is difficult to emulate in non-biological nanoscale objects, recent studies have indicated that synthetically derived nano-objects can be patterned by electrostatic interactions during co-assembly of the constituent units. For example, the addition of carboxylate-functionalized gold nano particles to poly(ferrocenyldimethylsilane)-bpoly(2-vinylpyridine) (4) (Fig. 3d) co-micelles, which consist of a central positively charged segment, results in spatially patterned hybrid nanocylinders due to charge matching between the nanoscale components45 (Fig. 4d).

Finally in this section, we note that unitary nano-objects can be produced spontaneously by the self-assembly of a single nanoscale component with a composite hybrid substructure. For example, 2-nm-sized gold nanoparticles functionalized with reactive maleimido groups have been coupled specifically to an exposed cysteine residue of the engineered β-subunit of the chaperonin HSP60, and the hybrid construct used for the spontaneous self-assembly of a hollow double-ring nanostructure with spatially ordered gold nanoparticles68 (Fig. 4e). This approach is facilitated by strong subunit–subunit interactions that drive the system enthalpically and entropically towards the higher-order structure. Clearly, the nanoparticles have to be small enough to be physically accommodated within the integrated architecture, so consideration of the size ratio of the components is an important criterion. A similar strategy has been demonstrated by attaching gold nanoparticles to F-actin subunits before self-assembly of spatially patterned Au/actin nanofibres69.

Extended nanostructures. The formation of extended networks by interparticle conjugation between fluid-dispersed hybrid nanoparticle building blocks is facilitated by electrostatic, steric, van der Waals, hydrophobic and dipole–dipole interactions70. The simplest situation involves the spontaneous assembly of weakly

nature materials | VOL 8 | OCTOBER 2009 | 785 or non-interacting spherical nanoparticles, which produces close-packed, high-symmetry arrangements, typical of many colloidal nano crystals prepared by evaporation-induced assembly from organic solvents containing hydrophobically functionalized inorganic nanoparticles2,3. The hydrophobic and near-contact van der Waals attractive energies are proportional to e–d/λ and 1/d respectively (d = distance between surfaces, λ = 1–2 nm)70. Moreover, high perfection in lattice ordering demands stringent levels of uniformity in particle size and shape, which are often difficult to achieve synthetically. Interestingly, incarceration of inorganic nanoparticles in spherical capsid-like architectures (for example, ferritin, cowpea brome mosaic virus) can circumvent problems associated with polydispersity of the inorganic phase, as well-defined molecular interactions between the protein shells override the imperfections to produce very accurate placement of the hybrid nanoparticles71.

Deviations from close packing of spherical nanoparticles can be accomplished under certain conditions — for example, low-density diamond-like lattices have been assembled from strongly electrostatically interacting spherical nanoparticles in the presence of counterion screening effects72, where the attractive/repulsive energies scale proportionally with eκ/r (r = distance between ions, κ–1 = screening length)70. However, it is generally true that increased levels of organization in nanoparticle-based higher-level structures require interparticle specificity and directionality. Specificity is increased considerably in systems comprising spherical nano-objects by programmed aggregation using DNA-based coupling with appropriate design of biomolecular linkers and thermal pathways73,74.

a bd c

Au 100 nm

ATP Release

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Figure 4 | Higher-order assembly of nanoscale hybrid objects. a–e, Self‑assembly of unitary nano‑objects. a, Programmed aggregation based on DNA‑directed attachment of gold‑nanoparticle satellites to single nanoparticles of mesoporous silica. b, Coupling of oligonuceotide‑functionalized gold nanoparticles to ferritin molecules adsorbed onto the surface of a CNT through a streptavidin linker. The TEM image shows the specific disassembly of the gold nanoparticles (imaged as larger dark dots) by heat‑induced dehybridization of the DNA linkages. The iron oxide cores of ferritin (smaller, less‑ dense spots) remain firmly attached to the CNT platform. c, Scheme showing triggered release of a single semiconductor nanoparticle from a chaperonin– CdS nanoconstruct. d, Patterned electrostatic assembly of gold nanoparticles on individual block‑copolymer cylindrical co‑micelles of (4). e, Scheme showing self‑assembly of a gold/chaperonin ring‑shaped architecture from gold‑nanoparticle‑tagged protein subunits. f,g, Higher‑order extended structures. f, TEM image showing spontaneous assembly of linear chain of spherical gold nanoparticles with branching bifurcations. The graphic illustrates dipolar interactions associated with ligand partitioning on the gold nanoparticle surfaces. g, Mesocrystal of BaCO showing helical stacking of iso‑oriented crystals produced by selective adsorption of a phosphonated double hydrophilic poly(ethylene oxide‑oxabutylacrylate ester) block copolymer onto the (110) face of the nanocrystals during aqueous precipitation. Figures reproduced with permission: a, © 2005 Wiley‑VCH; b, © 2005 RSC; c, © 2003 NPG; d, © 2007 ACS; e, © 2002 NPG; g, © 2005 NPG.

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Alternatively, higher-order arrays of hybrid nano-objects with considerable spatial directionality can be constructed by exploiting interparticle dipole–dipole interactions. Significantly, when these interactions are dominant over competing interparticle forces — for example, for particles with considerable intrinsic magnetic75 or electric dipoles76 — then one-dimensional linear chains of isometric nanoparticles self-assemble spontaneously in the dispersed medium. Chain assembly can be facilitated also by modifications in the hybrid nature of the nanoparticles. For example, reducing the screening effect of the organic stabilization layer that surrounds spherical CdTe semiconductor nanoparticles promotes linear chain aggregation owing to increases in the interparticle electric dipole–dipole interactions76. Unlike semiconductor quantum dots, face-centred-cubic metallic nanoparticles show no intrinsic electric dipole, yet gold nano particles spontaneously assemble into linear arrays when conjugated with several types of surface-attached ligands77,78 (Fig. 4f). Clustering of the organic molecules into discrete domains on the inorganic surface produces an extrinsic electric dipole that is sufficient to align the nanoparticles through cumulative dipolar interactions to produce linear chains and networks showing plasmonic coupling6,7.

(Parte 1 de 3)