A. Evaporation-induced self-assembly

In general, uniform size nanoparticles can form close-packed arrays on different substrates through slow evaporation of the solvent from the particle dispersion. This self-assembly process is driven by the balance of van der Waals interaction forces between the particles and the steric repulsion of the attached ligands. At room temperature, magnetic interaction between superparamagnetic particles is isotropic, and thus superparamagnetic particles behave essentially similar to nonmagnetic nanoparticles. The resulting films are composed of a self-assembled nanoparticle superlattice with a long-range ordered structure, such as one-dimensional lines,Reference Nakata, Hu, Uzun, Bakr and Stellacci¹⁰ two-dimensional monolayers,Reference Siffalovic, Majkova, Chitu, Jergel, Luby, Capek, Satka, Timmann and Roth^11, Reference Hou, Xu and Sun¹² and three-dimensional films and superstructures.Reference Zeng, Li, Liu, Wang and Sun^13, Reference Lacroix, Lachaize, Falqui, Respaud and Chaudret¹⁴

Crystals and superlattices of nanocrystal assemblies exhibit a host of unusual physical properties.Reference Shechtman, Blech, Gratias and Cahn^15, Reference Levine and Steinhardt¹⁶ These intermediate structures between amorphous solids and regular crystalline materials can be made to self-assemble from binary colloidal nanoparticle systems through evaporation-induced self-assembly. As seen in Fig. 1, by adjusting the particle size ratio, nanoparticles with two different sizes can rearrange themselves into quasicrystals with highly ordered periodic structures [Fig. 1(a)].Reference Talapin, Shevchenko, Bodnarchuk, Ye, Chen and Murray¹⁷ With selected size ratios, other binary AB, AB₂, AB₅, and AB₁₃ colloidal crystals have been prepared using a similar method. For example, an ordered monolayer of a CoPt₃ nanoparticle array can display the AB, AB₂, and AB₅ structures [Fig. 1(b)].Reference Shevchenko, Talapin, Rogach, Kornowski, Haase and Weller¹⁸ Mixing two sizes of 11 nm Fe₂O₃ and 6 nm PbSe nanoparticles results in binary superlattices with AB₁₃ and AB₂ structure [Fig. 1(c)].Reference Redl, Cho, Murray and O’Brien¹⁹ The evaporation-induced self-assembly method can be combined with the application of an external magnetic field to assemble nanoparticle superlattices. Further, combination with a physical patterning method can produce patterned nanoparticle arrays. As seen in Fig. 1(d), with the use of a modulated magnetic field, long-range order of magnetic nanoparticle arrays with macroscopic dimensions can be obtained.Reference Ahniyaz, Sakamoto and Bergstrom²⁰ Inducing dipolar attraction during the initial stage of the drying-mediated self-assembly process results in assembling the superparamagnetic nanocubes into large and defect-free superstructures with both translational and orientational order. The characteristic dimensions of the superlattice are controlled both by the particle concentration as well as the duration of the externally applied magnetic field. Micrometer-sized three-dimensional mesocrystals have also been obtained when the drying dispersion is subjected to an external magnetic field of moderate strength for the entire duration of the self-assembly process.Reference Ahniyaz, Sakamoto and Bergstrom²⁰ As compared to a stable crystal structure in which the individual elemental constituents are strongly chemically bonded, these nanoparticle assemblies are usually weakly interacting since the nanoparticle subunits are linked by weak hydrogen bond, van der Waals, or electric/magnetic dipole interactions.

FIG. 1.

(a) Binary colloidal quasicrystal, reprinted by permission from Macmillan Publishers Ltd: Nature, Talapin et al., Reference Talapin, Shevchenko, Bodnarchuk, Ye, Chen and Murray¹⁷ copyright 2009. (b) Binary CoPt₃ superlattices with AB₅ structures, reprinted with permission from Shevchenko et al.,Reference Shevchenko, Talapin, Rogach, Kornowski, Haase and Weller¹⁸ copyright 2002 ACS. (c) Binary Fe₂O₃–PbSe superlattices with AB₁₃ structures, reprinted by permission from Macmillan Publishers Ltd: Nature, Redl et al.,Reference Redl, Cho, Murray and O’Brien¹⁹ copyright 2003. (d) Magnetic field-induced self-assembly of oriented superlattices from maghemite nanocubes, reprinted by permission from PNAS, Ahniyaz et al., Reference Ahniyaz, Sakamoto and Bergstrom²⁰ copyright 2007.

B. Template-assisted self-assembly

Template-assisted self-assembly approaches, including organic molecule-assisted self-assembly, porous templates, and nanoparticle templates, have been developed to prepare SP NP assemblies. As compared to the nanoparticle assemblies prepared by the evaporation-induced process, templated nanoparticle assemblies are much more stable since the template acts as a support for stabilizing the nanoparticle assembly. Moreover, the template serves as a structure-directing agent, and depending upon the shape and structure of the template, the shape and structure of the resulting nanoparticle assembly is tunable.

1. Organic molecule templates

Organic molecule-assisted self-assembly methods have been widely used to prepare nanoparticle assemblies.Reference Medintz, Uyeda, Goldman and Mattoussi^21, Reference Shenhar, Norsten and Rotello²² The primary approaches include: (i) assembling nanoparticles into clusters and (ii) inducing ordering and orientation of the nanoparticles. Organic molecule templates can be charged surfactant molecules, amphiphilic block copolymers, and so on.Reference Nikolic, Olsson, Salcher, Kornowski, Rank, Schubert, Frömsdorf, Weller and Förster^23, Reference Soler-Illia, Sanchez, Lebeau and Patarin²⁴ Ionic surfactants are composed of a hydrophilic head group and a long hydrophobic tail group within the same molecular moiety. Protonation of amphiphilic block copolymers depends upon the solution pH. In an organic molecule templating self-assembly process, both organic molecule and inorganic soluble species direct the synthesis of nanoparticle assemblies, which are strongly dependent on the interaction between the organic molecule and its inorganic precursors.Reference Soler-Illia, Sanchez, Lebeau and Patarin²⁴ As for the case of a single-molecule chain of organic molecule template in the case of ionic surfactants, the formation of the nanoparticle assemblies is mostly governed by electrostatic interactions. Other assembly routes rely on nonionic surfactants, where the primary interaction between the template and the inorganic species is H-bonding or dipolar.

The shape and structure of the nanoparticle assemblies can be further tuned by controlling the structure and molecular weight of the organic molecule micelles in the solvent. Upon progressive increase of surfactant concentration in an aqueous solution, micellar aggregates organize to form different shapes, but always follow the order: direct spheres, direct cylinders, lamellae, inverse cylinders, and inverse spheres.Reference Alexandridis, Holzwarth and Hatton^25, Reference Evans and Wennerström²⁶ Other arrangements, such as sponge-like and sheet-like structures, are also observed in a number of reported organic molecule-nanoparticle assemblies.Reference Li, Li, He, Zhu and Zhao²⁷

A series of anionic polyamidoamine dendrimers have been used to control the interparticle spacing of SP NP assemblies.Reference Frankamp, Boal, Tuominen and Rotello²⁸ As seen in Fig. 2(a), the resulting assemblies display a systematic increase in the average interparticle spacing over a range of 2.4 nm. The particles can be completely coated by careful control of the ratio and concentration of the nanoparticles and the dendrimer. Further, the amount of dendrimer embedded between adjacent particles enables control of the interparticle spacing. The increase in the spacing between the nanoparticles modulates the collective magnetic behavior by effectively lowering the dipolar coupling between the particles. With the use of polymer micelles as templates, clusters of assembled iron oxide nanoparticles can be encapsulated in the polymer micelles, as seen in Figs. 2(b)–2(d).Reference Euliss, Grancharov, O’Brien, Deming, Stucky, Murray and Held^29, Reference Kim, Qiu, Wang and Taton³⁰ The size of uniform composite nanospheres is also tunable, ranging from 10–100 nm.Reference Niu, Li, Ma, Diao, Gu, Chen, Zhao, Ruan, Zhang and Shi³¹ The magnetization of the assemblies is determined by the number of SP NPs embedded in a single assembly. Although the number of nanoparticles formed in one micelle can be tuned by simply changing the concentration of nanoparticles, the maximum number of nanoparticles is still relatively small, thus resulting in a low magnetization of the final assembly.

FIG. 2.

(a) Schematic depiction of dendrimer-mediated nanoparticle assembly and control of interparticle spacing, reprinted with permission from Frankamp et al.,Reference Frankamp, Boal, Tuominen and Rotello²⁸ copyright 2005 ACS. (b) Schematic illustration. (c and d) TEM images of the nanoparticle-micellar assemblies, reprinted with permission from (c) Euliss et al.,Reference Euliss, Grancharov, O’Brien, Deming, Stucky, Murray and Held²⁹ copyright 2003 ACS, and (d) Kim et al.,Reference Kim, Qiu, Wang and Taton³⁰ copyright 2005 ACS.

2. Nanopore templates

In contrast to organic molecules that are soft templates, porous materials, such as mesoporous materials,Reference Soler-Illia, Sanchez, Lebeau and Patarin²⁴ porous anodic aluminum oxide,Reference Teng, Liang, Rahman and Yang^32, Reference Hulteen and Martin³³ and so on, can serve as hard templates to grow ordered nanoparticles within the confined space of the nanopore channels. Mesoporous materials provide some advantages, especially in their specific topological stability, highly ordered porous structures, and controllability.Reference Davis³⁴ In the hard template route, the voids of a preformed mesoporous solid are utilized and are impregnated with the desired precursors. The subsequent mineralization/solidification of these precursors and removal of the solid template leads to ordered nanoparticle assemblies with different compositions. Since the inorganic nanoporous templates are stable at relatively high temperatures, post-annealing can be carried out to improve the crystallinity and to form the desired magnetic phase, while maintaining the structure of the nanoparticle assembly. The synthetic procedure is schematically illustrated in Fig. 3(a). It should be noted that in many reported cases there is only partial filling of the pore channels. To prepare highly ordered nanoparticle assemblies by the hard template route, the following three characteristics are generally necessary: (i) the templates should possess a high degree of order and robust thermal stability; (ii) a full impregnation of the desired precursors within the voids of templates is necessary; (iii) the original template should be completely removable to achieve a faithful replica.

FIG. 3.

(a) Schematic illustration of nanopore-templated assembly and control of nanoparticle assemblies. (b) Assemblies of SP NPs inside mesoporous silica. Reprinted with permission from MacLachlan et al.,Reference MacLachlan, Ginzburg, Coombs, Raju, Greedan, Ozin and Manners³⁵ copyright 2000 ACS. (c) Ordered mesoporous iron oxide with crystalline walls. Reprinted with permission from Jiao et al.,Reference Jiao, Harrison, Jumas, Chadwick, Kockelmann and Bruce⁴⁰ copyright 2006 ACS.

MacLachlan et al. reported on the synthesis of superparamagnetic ceramic nanocomposites.Reference MacLachlan, Ginzburg, Coombs, Raju, Greedan, Ozin and Manners³⁵ With the use of silaferrocenophanes as the iron precursor, SP NPs are uniformly formed in the pore channels of mesopoeous silica [Fig. 3(b)]. Since the organometallic complexes used in the synthesis are hydrophobic, attachment of product nuclei to the pore walls is induced only after the formation of the Si-O bridges through hydrolysis of the silacyclobutane ring. With the use of less hydrophilic precursors, the nanopore filling ratio is low since the pore surface is hydrophilic. Several papers have also reported the synthesis of magnetic mesoporous silica nanocomposites.Reference Zhao, Gu, Zhang, Chen and Shi^36–Reference Giri, Trewyn, Stellmaker and Lin³⁹ However, these materials display poor magnetic response due to the difficulty in increasing the Fe₃O₄ mass fraction.Reference Kim, Lee, Lee, Yu, Kim, An, Hwang, Shin, Park, Kim and Hyeon³⁷ Moreover, they possess irregular core-shell structuresReference Lin, Wu, Hung, Chou, Chang, Lin, Tsai and Mou³⁸ and poor dispersity in water, which limits their applicability. To completely fill the pore channels, new synthetic strategies of using inorganic salts as precursors have been developed for the preparation of a variety of magnetic nanoparticle/nanowire arrays.Reference Jiao, Harrison, Jumas, Chadwick, Kockelmann and Bruce^40–Reference Imperor-Clerc, Bazin, Appay, Beaunier and Davidson⁴² As seen in Fig. 3(c), highly ordered iron oxide nanoparticle arrays with very large surface area of 210 m²/g have been prepared using iron nitride as the precursor and mesoporous silica as the template. After removing the template, stable and ordered SP NP arrays are obtained.Reference Jiao, Harrison, Jumas, Chadwick, Kockelmann and Bruce⁴⁰

3. Nanoparticle templates

Forming core-shell composite assemblies by directly growing SP NPs on the surface of presynthesized nanoparticle templates, or by converting the nanoparticle templates to superparamagnetic phase, is an alternate strategy to prepare SP NP assemblies. The nanoparticle templates can be metallic, semiconducting, or even insulating (e.g., silica).

Sun’s group at Brown University has reported the synthesis of unusual core-shell-void Fe-Fe₃O₄ nanoparticles and hollow Fe₃O₄ nanoparticles by controlled oxidation of amorphous core-shell Fe–Fe₃O₄ nanoparticles.Reference Peng and Sun⁴³ Fig. 4(a) provides a schematic illustration of the synthetic procedure. By proper choice of the heating condition, the Fe₃O₄ shell [Fig. 4(b)] is controlled to be either amorphous or polycrystalline. Isolated core-shell-void Fe–Fe₃O₄ nanoparticles [Fig. 4(c)] are also obtained under similar reaction conditions. Additional oxidation of core-shell-void Fe–Fe₃O₄ nanoparticles results in the formation of hollow Fe₃O₄ nanoparticles and their self-assembled monodisperse superlattice arrays [Fig. 4(d)]. Controlled surface oxidation of metal nanoparticles based on the nanoscale Kirkendall effect provides a simple and elegant approach for fabricating SP NPs with unique structures. The synthetic strategy can be extended to other types of hollow or core-shell-void transition-metal oxide nanoparticles. Moreover, metallic nanoparticles, such as Fe,Reference Peng, Wang, Xie and Sun⁴⁴ and Co,Reference Chen, Liu, Li and Zeng⁴⁵ among others, have also been used to prepare monodisperse superparamagnetic core-shell composite nanoparticles and hollow nanostructures based on the Kirkendall effect.Reference Yin, Rioux, Erdonmez, Hughes, Somorjai and Alivisatos⁴⁶

FIG. 4.

(a) Schematic illustration of the synthesis of core-shell-void Fe–Fe₃O₄ and hollow Fe₃O₄ nanoparticles from Fe–Fe₃O₄ nanoparticle seeds. TEM images of (b) Fe–Fe₃O₄ nanoparticle seeds, (c) core-shell-void Fe–Fe₃O₄, and (d) hollow Fe₃O₄ nanoparticles. Reprinted by permission from Wiley, Peng and Sun,Reference Peng and Sun⁴³ copyright 2007.

Semiconducting CdSe, PbSe, and PbS nanoparticles and insulating silica nanoparticles have also been utilized as templates to grow SP NP assemblies in the form of core-shell structures.Reference Tartaj and Serna⁴⁷ Nucleation of SP NPs usually occurs on the surface of the nanoparticle templates. As compared to metallic nanoparticles that can serve as sacrificial reactants for growing superparamagnetic oxide nanoparticle shells, chalcogenide and silica nanoparticles are usually retained in the final product. Thus, the most commonly formed assemblies are core-shell bicomponent nanostructures. Considering the complexity of the assembly procedure, it is not surprising that there are only a few reports in the literature on the synthesis of tricomponent multi-shell core-shell superparamagnetic nanostructures. Yin’s group has reported the synthesis of interesting superparamagnetic composite colloids containing three components.Reference Ge, Hu, Zhang and Yin⁴⁸ The composite colloids are produced through one-step emulsion polymerization of styrene using Fe₃O₄/SiO₂ as the seed. As seen in Fig. 5, the composite colloids have anisotropic structures such as ellipsoids formed by one-step emulsion polymerization [Fig. 5(c)] and doublets produced with distinct steps [Fig. 5(d)]. Spherical colloids can also be produced in one-step emulsion polymerization, both without [Fig. 5(a)] and with [Fig. 5(b)] using the crosslinker divinylbenzene. All types of colloid particles have superparamagnetic cores that are concentrically or eccentrically located in the polystyrene shells. The formation of such novel shapes and structures is due to the interfacial tension between the hydrophilic seed particle and the hydrophobic monomer.

FIG. 5.

Synthesis scheme for Fe₃O₄/SiO₂/PS composite colloids through seeded emulsion polymerization. The morphologies of the final products are controlled by: (a) interfacial tension, (b) cross-linking, and (c) single and (d) separated steps of swelling and phase separation. All scale bars represent 400 nm. Reprinted with permission from Ge et al.,Reference Ge, Hu, Zhang and Yin⁴⁸ copyright 2007 ACS.

C. Free-Template, Direct Self-assembly

Many applications require SP NP assemblies with small size, large magnetization, and special geometrical architectures, such as hollow structures, to achieve facile manipulation of the particles, improved sensitivity for magnetic detection, increased porosity and inner surface area for drug delivery, and so on. However, to directly prepare such SP NP assemblies in a controlled manner is challenging since the uniformity of nucleation, growth, and subsequent assembly of nanoparticles is difficult to achieve simultaneously in a single reaction.

Long et al. have developed a crystalline superparamagnetic nanoarchitecture of iron oxide while still retaining the desirable bicontinuous pore-solid network, significant porosity, and large surface area expected from an aerogel [Fig. 6(a)].Reference Long, Logan, Rhodes, Carpenter, Stroud and Rolison⁴⁹ The iron oxide aerogels are prepared by minor modification of a previously reported procedure using epoxide-based proton scavengers to initiate the sol-gel reactions for hydrolysis and polycondensation.Reference Gash, Tillotson, Satcher, Poco, Hrubesh and Simpson⁵⁰ The pore size has a wide distribution, ranging from 7 to 80 nm. The iron oxidation state and related vacancy content of the Fe₃O₄-Fe₂O₃ phases can be readily tuned while preserving the nanoscale particle size and pore structure of the aerogel.

FIG. 6.

TEM images of (a) aerogel-templated mesoporous magnetic architectures, reprinted with permission from Long et al.,Reference Long, Logan, Rhodes, Carpenter, Stroud and Rolison⁴⁹ copyright 2004 ACS, (b) superparamagnetic ferrite hollow submicrospheres, reprinted with permission from Li et al.,Reference Li, Zhang and Chen⁵¹ copyright 2006 ACS, (c) superparamagnetic Fe₃O₄/ZnS hollow nanospheres, reprinted with permission from Wang et al.,Reference Wang, Wu, Chen and Zhou⁵² copyright 2009 ACS, (d) superparamagnetic magnetite colloidal Fe₃O₄ nanocrystal clusters, reprinted with permission from Wiley, Ge et al.,Reference Ge, Hu and Yin⁵³ copyright 2007, and (e–f) self-supported superparamagnetic nanostructures of spherical and rodlike CoFe₂O₄ nanocrystals, reprinted with permission from Bao et al.,Reference Bao, Shen, Wang, Ma, Mazumdar and Gupta⁵⁴ copyright 2009 ACS.

The synthesis of hollow structures usually requires sacrificial templates through a successive multistep process involving preparation of a template, coating of the template with desired materials, and removal of the template to leave a hollow center. Li et al. reported a direct self-assembly strategy for preparation of hollow spheres [Fig. 6(b)].Reference Li, Zhang and Chen⁵¹ The approach involves the formation of ferrite nanoparticles and the simultaneous self-assembly of nanoparticles and block copolymer into hollow spheres under solvothermal conditions. Furthermore, this approach is general for the preparation of a variety of different ferrite hollow spheres with a composition of M_1–xFe_2+xO₄ (M = Fe, Co, and Mn; 0 < x < 1). Unlike conventional hollow spheres, which are either hydrophobic or hydrophilic, the products prepared using this method exhibit excellent dispersibility in both polar and nonpolar solvents, and are promising for a broad range of applications.

In addition to single phase structures, bicomponent Fe₃O₄/ZnS superparamagnetic hollow nanospheres [Fig. 6(c)] have been synthesized through reaction of monodisperse FeS particles in a mixture containing zinc acetylacetonate, poly(vinylpyrrolidone), ammonium nitrate, glycol, and water at 150 °C for 10 h.Reference Wang, Wu, Chen and Zhou⁵² The formation of such hollow structures does not obey the conventional Kirkendall effect since the initial formation of a layer of Fe₃O₄ and ZnS on the surface of the FeS particles completely hinders any further direct chemical reaction between FeS and zinc acetylacetonate. The authors consider that the FeS and ZnS particles are positively charged, while Fe₃O₄ is negatively charged. Thus, the position of the Fe₃O₄ and ZnS particles being determined by the electrostatic interaction, with the Fe₃O₄ layer located between the positively charged FeS core and the outer ZnS layer. With a decreasing concentration of reactants, Ostwald ripening sets in. The small Fe₃O₄ and ZnS particles formed around the surface of the FeS continually grow into larger particles and then coalesce to form a shell. Such reactions continue until the FeS core disappears, finally leaving behind the bicomponent Fe₃O₄/ZnS hollow structures.

SP NPs with dimensions in the range of 10 nm have a low magnetization per particle, so it is difficult to effectively manipulate them using moderate magnetic fields. The strategy of forming clusters of magnetite nanocrystals has the advantage of controllably increasing the magnetization while retaining the superparamagnetic characteristics. Recently, Yin’s group developed a high-temperature hydrolysis process for the preparation of polyacrylate-capped superparamagnetic magnetite (Fe₃O₄) colloidal nanocrystal clusters with tunable sizes from 30 to 180 nm [Fig. 6(d)].Reference Ge, Hu and Yin⁵³ The clusters are composed of a number of small primary crystals of size 6–8 nm. Since each cluster is composed of many magnetite crystallites that are magnetically decoupled, they retain their superparamagnetic characteristics.

Recently, our group also reported on novel shape- and structure-controlled superparamagnetic nanostructures composed of self-supported spherical and rodlike CoFe₂O₄ colloidal nanocrystals.Reference Bao, Shen, Wang, Ma, Mazumdar and Gupta⁵⁴ They are prepared by thermolysis of a stoichiometric Co²⁺Fe₂³⁺-oleate complex in organic solution with periodic injection of hexane to control the nucleation, assembly, and growth of the nuclei. These large self-supported superparamagnetic nanocrystal structures have the advantage of increasing the magnetic moment while retaining the superparamagnetic characteristics. By merely changing the heating rate, the product shape and size can be varied from being spherical [40.5 ± 4.5 nm, Fig. 6(e)] to cubic [36.5 ± 2.5 nm, Fig. 6(f)]. These nanostructures are composed of individual spherical nanocrystals of <5 nm in size, or nanorods with dimensions of ~3.6 nm in diameter and <25 nm in length, respectively. They can form highly ordered two-dimensional assemblies over a large area. The formation of such superparamagnetic nanocrystal assemblies is likely because the burst boiling accompanied by a temperature drop slows down the growth of the nuclei and promotes their assembly to form clusters by particle coalescence (from interaction between the nuclei) or oriented attachment to decrease the surface energy.Reference Zheng, Smith, Jun, Kisielowski, Dahmen and Alivisatos⁵⁵

A. Magnetically responsive colloidal photonic crystals

Functional photonic materials and devices can be created by assembling monodisperse colloidal particles into periodically ordered structures. The optical band gap of photonic crystals is determined by the lattice constants and symmetry of the colloidal arrays. An external magnetic field provides an effective means for controlling the lattice spacing of magnetic colloidal photonic crystals and thus the photonic properties. Calderon et al. initially observed that emulsion droplets containing γ-Fe₂O₃ nanoparticles can diffract light in the visible range by applying an external magnetic field.Reference Calderon, Stora, Monval, Poulin and Bibette⁵⁶ In later work, Xu et al. developed monodisperse polymer-SP NP colloidal particles for photonic applications. They embedded superparamagnetic iron oxide nanoparticles in a polymer matrix through an emulsion polymerization process.Reference Xu, Friedman, Humfeld, Majetich and Asher^57, Reference Xu, Friedman, Humfeld, Majetich and Asher⁵⁸ By introducing high density charges to the colloid surface, they successfully constructed three-dimensional photonic crystals by taking advantage of the strong repulsive electrostatic interactions. However, because of the small loading of magnetic materials in the polymer, the field-responsive tuning range of these photonic crystals is limited, and the response time is rather slow.

Photonic crystals fabricated using pure colloidal magnetic particles are expected to offer a faster response to an external field. If the colloids possess uniform shape, controllable size, and narrow size distribution, then the lattice constant of photonic crystals can also be precisely tuned. Further, with particle sizes in the range of 100–200 nm, the assembled structures will have periodicities comparable to the wavelength in the visible or near infrared range. Yin’s group has reported the synthesis of superparamagnetic clusters that satisfy all these conditions quite well.Reference Yin, Rioux, Erdonmez, Hughes, Somorjai and Alivisatos⁴⁶ They successfully prepared polyacrylate-capped superparamagnetic clusters of Fe₃O₄ nanocrystal assemblies with tunable sizes from 30 to 180 nm, and a narrow size distribution.Reference Ge, Hu and Yin^53, Reference Ge, Hu, Biasini, Beyermann and Yin⁵⁹ Colloidal photonic crystals fabricated using these superparamagnetic clusters [Fig. 6(d)] exhibit highly tunable stop bands across the entire visible spectral region [Fig. 7] because of highly charged polyacrylate-capped surfaces and strong interaction of the magnetite clusters with an external magnetic field. The optical response to changes in the magnetic field strength is quite fast and fully reversible. The superparamagnetic clusters have been used to fabricate field-responsive polymer composite films for potential applications, such as color displays and sensors.

FIG. 7.

(a) Photograph of colloidal crystals formed in response to an external magnetic field; the magnet–sample distance decreases gradually from right to left. (b) Dependence of the reflection spectra at normal incidence of the colloidal crystals on the distance of the sample from the magnet. Diffraction peaks blue-shift as the distance decreases from 3.7 to 2.0 cm with step sizes of 0.1 cm. Reprinted with permission from Wiley, Ge et al.,Reference Ge, Hu and Yin⁵³ copyright 2007.

B. Biomedical and biological applications

Gold nanoparticles are important reagents for many highly sensitive and selective DNA and protein detection schemes.Reference Rosi and Mirkin⁶⁰ Magnetic particles are also attractive and have been widely exploited in biology for diagnostic and therapeutic applications. There is a need in the field of biomedicine for the development of composite materials that combin gold for its stability, surface chemistry, and optical properties and superparamagnetic particles for easy manipulation. Stoeva et al. have developed three-layer composite magnetic nanoparticles for detecting DNA.Reference Stoeva, Huo, Lee and Mirkin⁶¹ The composite particles are composed of a gold surface, silica core, and superparamagnetic inner layer. The DNA-modified three layer magnetic nanoparticles can be repeatedly centrifuged and redispersed in a buffer solution without aggregation. Moreover, the DNA-modified three-layer magnetic nanoparticles assemble into macroscopic aggregates upon hybridization with complementary linking oligonucleotides.

Superparamagnetic colloids are also ideal systems for bioseparation of specific biological molecules (e.g., DNAs, proteins, and cells) from solution because the external control of magnetization enables the ready separation of adsorbed biomolecules with a magnetic field. In a typical separation procedure, the surface of magnetic particles is first modified with biocompatible molecules, providing conjugation between the magnetic particles and the targeted biological molecules. The biological molecules labeled by superparamagnetic colloids are then separated from the solution by application of an external magnetic field. Magnetic microspheres consisting of an iron oxide core and silica shell have recently attracted attention for bioseparation.Reference Xu, Deng, Gao, Yu, Yang and Zhang⁶² Zhao’s group has reported on novel sandwich-structured mesoporous silica microspheres composed of a silica-coated magnetite core and ordered mesoporous silica shell with perpendicularly oriented channels.Reference Deng, Qi, Deng, Zhang and Zhao⁶³ Three steps are involved in the synthesis procedure: (i) uniform magnetite microspheres are first coated with a thin silica layer using a sol-gel approach; (ii) a mesostructured silica composite is then deposited on the Fe₃O₄/nSiO₂ microspheres through a surfactant-templating approach using cetyltrimethylammonium bromide as a template; (iii) final removal of surfactant templates by acetone extraction to form a mesoporous SiO₂ shell results in well-dispersed tricomponent Fe₃O₄-nonporous SiO₂-mesoporous SiO₂ microspheres. The resulting microspheres are superparamagnetic, with a high magnetization of 53.3 emu/g, uniform accessible 2.3 nm mesochannels, high surface area of 365 m²/g, and a large pore volume of 0.29 cm³/g. They exhibit enhanced adsorption efficiency for fast removal of large microcystins (1000 Da) from solution.

Magnetic resonance imaging (MRI) is routinely used for clinical diagnostic imaging. The working principle of MRI is to measure the change in magnetization of protons in a magnetic field after being exposed to a radio frequency pulse. MRI contrast agents are often used to decrease the relaxation time so that the magnetic moment of a proton relaxes back faster to its initial value, thus producing better contrast.Reference Browne and Semelka⁶⁴ SP NPs are magnetically saturated at the magnetic field strength used in an MRI scanner so they generate local variations in the magnetic susceptibility of the system under a magnetic field. For example, Berret et al. reported on the structural and magnetic properties of such complexes made from 6.3 nm diameter superparamagnetic iron oxide nanoparticles and cationic-neutral copolymers.Reference Berret, Schonbeck, Gazeau, El Kharrat, Sandre, Vacher and Airiau⁶⁵ The copolymers investigated include poly(trimethylammonium ethylacrylate methyl sulfate)-b-poly-(acrylamide), with molecular weights 5000-b-30,000 g/mol and 110,000-b-30,000 g/mol. The mixed copolymer-nanoparticle aggregates have been characterized using a combination of light scattering and cryotransmission electron microscopy. Their hydrodynamic diameters have been determined to be in the range of 70–150 nm, and the number of nanoparticles per aggregate varies from tens to the hundreds. In addition, magnetic resonance spin-echo measurements show that the complexes have a better contrast in MRI than single nanoparticles and that these complexes are suitable for biomedical applications.