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REVIEW Synthesis and biomedical applications of hollow
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Table 1 Synthetic strategies for the hollow nanomaterials. Applied methods Hollow materials Initial materials templates Shape Ref. Co3 S4 Co9 S8 CoSe Co Sphere 21 33,Pt CoO Pt Co Yolk shell 21. Fe3 O4 Fe Fe3 O4 Sphere 34,Fe2 O3 Fe Sphere 35 44,ZnAl2 O4 ZnO Al2 O3 Tube 36 37. CoSe2 Co3 S4 CoTe Co Necklace 38,Ni2 P Co2 P Ni Co Sphere 39 40. CeO2 ZrO2 CeO2 ZrO2 Sphere box 41,FePt CoS2 FePt Co Yolk shell 98. FePt Fe2 O3 FePt Fe2 O3 Yolk shell 99,Nanoscale Kirkendall effect.
Au Fe2 O3 Au Fe2 O3 Yolk shell 42,Pt Cu Pt Cu Core shell 45. Cu2 x Se Cu2 O Sphere 46,CuO Cu Tube 47,Ag Ag2 Se Ag2 Se Ag Sphere tube 48 49. PbS Pb PbS Pb Ag Pb Sphere 50,CdS Cd Sphere 43,Co3 S4 Co CO3 0 35 Cl0 20 OH 1 10 Tube 51. ZnS ZnO Sphere 52,ZnO Zn Sphere 53,Fe Fe Box frame 54. Fe phosphide Fe3 O4 Fe2 O3 Sphere box 22,Mn phosphide MnO Sphere multi pods 22.
Mn Fe phosphide MnFe2 O4 Sphere 22,Chemical etching. ZnO Au ZnO Pt ZnO Au Pt ZnO ZnO Sphere 57,Co Co Box frame 55. Pd Pd Box frame 56,Cu2 O Cu2 O Dodecahedral frame 58. Au Ag Box cage triangular ring prism shaped box tubes 3 4 62 68. multiple walled shell or tube,Galvanic replacement Pd Ag Pt Ag Ag Box frame 69. Au Ag Pt AuPt CoPt Co Sphere 70 75,AuPt Co Necklace 71.
Au Pt Pd Co Necklace 76,Fe2 O3 Fe3 O4 FeOOH Capsule 23. K An T Hyeon,Fe2 O3 Silica Sphere 77,Nanotemplate Silica Fe3 O4 Sphere 78. FeOOH Organics Tube 80,Co CoO Parallelepiped 81,Applications of hollow nanostructures 361. tructures 1 Usually the hollow structures obtained via a sulfur dissolved in organic solution resulted in the forma. template mediated approach using silica or polymer parti tion of hollow cobalt sul de nanocrystals of either Co3 S4 or. cles as the templates are often larger than 200 nm because Co9 S8 depending on the molar ratio of sulfur and cobalt. it is hard to make smaller sized template particles In addi 21 33 During the transformation cobalt nanocrystals are. tion to this size limitation the template mediated method rst covered with a cobalt sul de shell Next the diffusion. has other disadvantages the post treatment necessary to of cobalt and sulfur atoms in opposite directions takes place. remove the templates adds complexity to the whole syn at the surface of cobalt and cobalt sul de As the reac. thetic process and increases the chance of the structural tion proceeds voids form in the cobalt side of the interface. deformation as well as the introduction of impurities In because the outward diffusion of cobalt atoms is faster than. order to overcome these limitations simple and novel the inward diffusion of sulfur atoms Similar intermediate. strategies are highly desired In this review we will focus. on the colloidal synthesis of high quality hollow nanostruc. tures with their sizes smaller than 200 nm The structures. of our interest encompass nanoframes and nanocages with. porous walls as well as those with solid walls The synthetic. approaches are categorized into four main types according. to how the hollow nanostructures are formed the Kirkendall. effect 2 21 chemical etching 22 galvanic replacement. 3 4 and template mediated approach see Table 1 All. of these approaches exploit nanoparticles as starting mate. rials except for the nanotemplate mediated approaches. where these starting nanomaterials play the role of the. sacri cial templates that are dissolved or transformed to. generate hollow nanostructures Hollow nanostructures can. nd various biomedical applications including simultaneous. diagnosis and therapy The large pore volume inside the hol. low nanostructures can be used to incorporate various drugs. and biomolecules and release them in a controlled manner. At the same time the surface of the hollow nanostructures. can be readily functionalized with optical labels and tar. geting agents 3 4 This review covers the recent progress. on the synthesis and biomedical applications of various hol. low nanostructures 23 30 The rst four sections describe. the synthesis of hollow nanostructures using four different. synthetic approaches followed by their various biomedical. applications,Nanoscale Kirkendall effect, In 1942 Kirkendall reported that net mass transport. occurred across the interface between two different metal. species upon annealing and vacancy assisted hopping was. proposed as the main mode of atomic transport 31 At. the interface two metal species in contact have differ. ent diffusion rates resulting in the net ux of atoms from. one side of the interface to the other As atoms diffuse. through the interface vacancies move in the opposite direc. tion and accumulate to form voids The stress originated Figure 1 a f TEM images showing the shape evolution of. from an increase in the void volume during diffusion induces Fe Fe3 O4 core shell nanoparticle a via Kirkendall effect. the porosity in the solid This phenomenon is now known Core shell void intermediates obtained by the reaction for 1 h. as the Kirkendall Effect In 1947 Smigelkas and Kirkendall b and 2 h at 130 C c and 40 min d and 80 min at 210 C. demonstrated that at an elevated temperature the differ e Hollow Fe3 O4 nanoparticles from the reaction for 120 min. ence between the diffusion rates of copper and zinc in brass at 210 C f g Schematic illustration showing the shape evo. led to the formation of pores in the species 32 In 2004 lution from the core shell to the hollow structure reproduced. Alivisatos and his co workers reported exploiting the Kirk with permission from 34 copyright 2007 Wiley VCH h m. endall effect at the nanometer scale for the fabrication of TEM images showing the shape evolution of hollow iron iron. hollow nanostructures 21 In their report cobalt nanocrys oxide nanoparticles exposed to dry 20 oxygen for 1 min at room. tals were transformed to hollow chalcogenide nanocrystals temperature h 1 h i and 12 h at 80 C j 5 min k and 1 h at. by introducing either oxygen sulfur or selenium into the 150 C l Fully oxidized iron oxide nanocrystals were obtained. hot dispersion of cobalt nanocrystals For example the sul by oxidation for 1 h at 350 C m reproduced with permission. dation of cobalt nanocrystals by the addition of elemental from 35 copyright 2007 American Chemical Society. 362 K An T Hyeon, structures were observed in many other hollow nanomateri by the controlled oxidation under O2 atmosphere adopting.
als Peng and Sun observed a core shell void intermediate the nanoscale Kirkendall effect 35 The reaction temper. structure during the synthesis of hollow Fe3 O4 nanocrystals ature and oxidation time allowed for precise tuning of the. 34 In their report they synthesized amorphous Fe Fe3 O4 thickness of the oxide shell as shown in Fig 1h m. nanoparticles Fig 1a via thermal decomposition of Fe CO 5 The Kirkendall effect can also be applied to fabricate. in a hot organic solution and subsequent air oxidation 1 dimensional 1 D hollow nanomaterials Fan and their. The controlled oxidation of Fe Fe3 O4 nanoparticles in the co workers fabricated single crystalline ZnAl2 O4 spinel nan. presence of an oxygen transfer reagent trimethylamine otubes Fig 2a and b with a diameter of 40 nm and a wall. N oxide yielded monodisperse hollow Fe3 O4 nanoparticles thickness of 10 nm using ZnO Al2 O3 core shell nanowires. with controlled sizes via the nanoscale Kirkendall effect It as the starting material 36 37 They proposed a model. was observed that when the reaction was quenched by low explaining the formation mechanism of their 1 D hollow. ering the temperature in the middle of the reaction process structures the so called Kirkendall effect surface diffu. multiple voids were formed in the particle By adjusting the sion process 37 According to the proposed model the. reaction temperature and time it was possible to obtain a surface diffusion process is a dominant mass ow mode. series of intermediate structures from the solid particle to responsible for the enlargement of interior pores after their. the core shell void the yolk shell and nally the hol formation induced by the Kirkendall effect Fig 2c illus. low structures which is a good evidence for the Kirkendall trates the different diffusion processes in the growth of. effect Fig 1a f show that the shape of the nanoparti hollow nanostructures At the early stage small Kirkendall. cles evolved from the core shell to the hollow ones as the voids are generated via bulk diffusion at the interface When. reaction time was increased and as the temperature was large numbers of voids contact the inner surface of the. raised which is also illustrated in Fig 1g The Alivisatos shell the surface diffusion of the atoms of the core material. group also synthesized a series of monodisperse iron iron becomes dominant along the skeletal bridges Through the. oxide core void shell and hollow iron oxide nanoparticles channels of the shell layer the material exchange proceeds. Figure 2 TEM images of a and b the ZnAl2 O4 spinel nanotubes reproduced with permission from 36 Copyright 2006 Nature. Publishing Group and b Schematic illustration showing the different diffusion processes in the growth of the hollow nanostructure. reproduced with permission from 2 copyright 2007 Wiley VCH. Applications of hollow nanostructures 363, Figure 3 a TEM HRTEM inset and b SEM image of the wires of hollow CoSe2 nanocrystals c Schematic illustration showing. the formation of hollow CoSe2 nanocrystals from cobalt nanoparticles in the 1 absence and 2 presence of an alternating magnetic. eld reproduced with permission from 38 copyright 2006 Wiley VCH. via direct dissolution in the solution phase or evaporation partial hollow structures in which the unreacted Cd core. in the gas phase generating the hollow nanostructure and the coalesced vacancies were separated into two dis. Xu et al applied the nanoscale Kirkendall process to tinct spherical caps It was attributed to the faster diffusion. magnetically assembled 1 D hollow nanostructures of cobalt of cadmium atoms through the polycrystalline shells com. chalcogenides 38 First they assembled cobalt nanocrys pared to sulfur atoms 43 They further extended the. tals with the size of 20 nm into necklace like structures synthetic method of the hollow core shell void nanopar. by the magnetic dipolar interactions Then the cobalt ticles to generate various quasi ternary superlattices 44. nanocrystal assemblies were transformed to CoSe2 hollow Recently Fan and co workers reviewed the fabrication of. nanostructures retaining the chain like shape via the Kirk nanotubes and hollow nanoparticles based on the Kirkendall. endall process Fig 3 During the synthesis by exerting effect and summarized various kinds of hollow nano and. an alternating magnetic eld a small number of individ micro materials 2. ual hollow CoSe2 nanocrystals were obtained which were. fractured from the chain because the applied vibrational. magnetic torque disrupted the dipolar interactions between Chemical etching. the particles, Several hollow nanostructures of oxides sul des Etching or partial dissolution of the interior of nanopar. selenides and phosphides have been synthesized by many ticles is another approach to synthesize hollow or. researchers via the nanoscale Kirkendall effect 39 53 porous nanomaterials Several kinds of hollow nanomate. For example the Schaak and Chiang groups indepen rials synthesized via the selective etching process have. dently reported hollow Ni2 P nanoparticles synthesized from been reported The Hyeon group synthesized hollow iron. oleylamine stabilized Ni nanoparticles in the presence of nanoframes by thermal decomposition of a Fe II stearate. trioctylphosphine TOP 39 40 The Li group fabricated complex in the presence of sodium oleate and oleic acid at. CeO2 ZrO2 nanocages by reacting ceria nanospheres with 380 C Fig 4a 54 In this process solid iron nanocubes. zirconium IV in a glycol medium 41 The Alivisatos group are initially generated from the thermal decomposition of. synthesized gold iron oxide core hollow shell nanopar Fe II stearate complex and subsequent reduction by the. ticles in which the formation of the hollow oxide shell byproducts from the decomposition of oleic acid These solid. resulted from the oxidation of the iron shell by oxygen nanocubes were subsequently transformed into nanoframes. 42 The same group also reported asymmetric Cd CdS with a hollow core This transformation was attributed to. 364 K An T Hyeon, Figure 4 a TEM and HRTEM inset images of the Fe nanoframes and the overall shape evolution of the Fe nanoparticles in the. lower panel reproduced with permission from 54 copyright 2007 American Chemical Society b and c TEM images of ZnO hollow. nanoparticles and the corresponding diameter distribution and electron diffraction pattern in the insets Schematic illustration of. the selective etching strategy for ZnO hollow nanoparticles using a sacri cial template is shown in the lower reproduced with. permission from 57 copyright 2008 American Chemical Society. sodium molten salt derived from sodium oleate in the solu cobalt nanocages and skeletons through NaF assisted etch. tion during the aging process at 380 C The molten salt ing of cobalt nanocube aggregates 55 Xiong et al reported. corrosion is a well known etching process of metal and corrosion based synthesis of palladium nanoboxes 56 They. it was suggested that in situ generated sodium salts in reported that the local corrosion of the surface of the. the solution corrode 1 1 0 facets of the preformed iron nanocubes capped with poly vinyl pyrrolidone PVP took. nanocubes selectively to transform them into nanoframes place at the initial stage of the oxidative etching of pal. Fig 4a lower panel Similarly Wang et al synthesized ladium nanocubes by O2 dissolved in the solvent Further. Applications of hollow nanostructures 365, Figure 5 a d TEM images of Fe2 O3 nanocubes a and dumbbell shaped solid MnO nanocrystals c and the corresponding. hollow oxide nanoparticles obtained via the etching process b and d e HRTEM image of the core shell void intermediate. sampled during the etching of Fe2 O3 nanocubes in the left panel The elemental mapping images for oxygen red upper and. iron and phosphorus green and blue lower from electron energy loss spectroscopy EELS analysis are shown superimposed on the. same TEM image in the right panels f The plots for the relative abundance of each element as functions of position which were. measured by energy dispersive X ray spectroscopy EDX on the core shell void intermediate reproduced with permission from. 22 copyright 2008 American Chemical Society For interpretation of the references to color in this gure legend the reader is. referred to the web version of the article, etching dissolved away the core of the nanocubes while the of hollow nanoparticles retaining the size and shape of the.
shell was left intact resulting in the formation of palladium original nanocrystals Speci cally when Fe2 O3 nanocubes. nanoboxes Fig 5a and dumbbell shaped MnO nanoparticles Fig 5c. Recently Zeng and co workers developed a weak acid were used as the starting materials for the etching pro. selective etching strategy to fabricate oxide based hollow cess hollow nanoboxes and nanodumbbells were produced. nanoparticles using metal oxide core shell nanostructures while their morphologies were preserved Fig 5b and d. Fig 4b and c 57 In their method Zn ZnO core shell The control experiments revealed that impurities such as. nanoparticles were fabricated via laser ablation of a alkylphosphonic acids in technical grade TOPO were respon. zinc plate in water and then etched with weak acids sible for the etching process Elemental mapping analysis. such as tartaric acid C4 H6 O6 chloroauric acid HAuCl4 showed that the shell is phosphorus rich while the core. and chloroplatinic acid H2 PtCl6 in aqueous solutions to retains the composition of metal oxide Fig 5e and f This. form ZnO hollow nanoparticles Interestingly gold and or result revealed that there was both the inward diffusion of. platinum were successfully inserted into the ZnO hollow phosphorus atoms which might come from phosphonic acid. nanoparticles using HAuCl4 and H2 PtCl6 as precursors dur coordinated onto the surface of the nanocrystals and the. ing the etching process yielding Au ZnO Pt ZnO and outward diffusion of metal atoms during the etching pro. Au Pt ZnO hollow core shell nanoparticles Kuo and Huang cess which is reminiscent of the Kirkendall effect However. fabricated Cu2 O nanocages and nanoframes by particle the phosphonic acid etching process is different from the. aggregation and a selective acid etching process 58 In Kirkendall effect in that the evolution of the hollow nanos. this report Cu OH 2 was initially formed from the reaction tructure is not only the result of the difference between. of CuCl2 and NaOH then reacted with NH2 OH to produce the diffusion rate of phosphorus and metal atoms in solid. Cu2 O nanoframes and nanocages By selective etching of the but also the dissolution of metal atoms into the solution. 1 1 0 facets by HCl Cu2 O nanoparticles were transformed resulting in vacancies in the nanoparticles. to the nanoframes,Very recently the Hyeon group developed a novel. nanoscale etching process for the synthesis of various hol Galvanic replacement. low oxide nanoparticles 22 Simple heating metal oxide. nanocrystals dispersed in technical grade trioctylphosphine Novel metal nanoparticles have attracted a lot of attention. oxide TOPO at 300 C for hours resulted in the formation for their unique optical properties induced by surface plas. 366 K An T Hyeon, mon resonance SPR and their many potential applications tion initiated locally on the sites of steps point defects. 59 61 Upon the irradiation of light conduction electrons or stacking faults with high surface energy As the reaction. con ned in noble metal nanoparticles oscillate collectively proceeds the small hole serves as an anode where silver. at a certain resonance frequency which is determined by atoms are oxidized The released electrons from silver atoms. the nature of the metal the dielectric constant of media migrate to the surface of the nanocube and reduce AuCl4. and the size and shape of the nanoparticles 26 Plasmon ions to Au atoms which adsorb and grow into an Au shell. resonance results in strong light absorption and scattering on the nanocubes epitaxially During the reaction the holes. at and near the resonance frequency and induces strong on the faces of the nanocubes serve as the dissolution site. eld enhancement near the surface of the nanoparticles for the silver and nally the silver nanocubes are converted. This effect has been utilized especially for surface enhanced into Au Ag alloy nanoboxes with empty interiors Fig 6c. Raman spectroscopy SERS and other non linear optics The When there is a suf cient amount of HAuCl4 the surface. plasmon resonance frequency of noble metal nanoparticles holes are gradually closed via the mass transport or direct. can be tuned in the range of visible to near infrared near deposition of Au atoms generating perfect Au nanoboxes. IR by controlling their dimensions and morphologies Among with uniform wall thickness In the presence of even larger. the noble metal nanoparticles with SPR effect the hollow. nanostructures have a very high scattering coef cient and. its plasmon resonance frequency can be easily controlled by. changing the dimension of the hollow core and the thickness. of the shell Due to these features as well as their strong. biocompatibility various hollow noble metal nanostructures. have attracted a lot of attention for their applications such. as bioassay sensing and drug delivery system 3 4 For. the synthesis of hollow novel metal nanostructures the. galvanic replacement reaction provides a simple and ver. satile route The Xia group has developed various hollow. nanostructures of Ag Au Pt and Pd with controlled pore. volume and wall thickness via the galvanic replacement. reaction 62 63 This reaction takes place when the metal. nanoparticles are in contact with other metal ions of higher. reduction potential For example the standard reduction. potential for AgCl Ag is 0 22 V Compared to other noble. metals this value is quite low 0 99 V for AuCl4 Au 0 76 V. for PtCl4 2 Pt and 0 59 V for PdCl4 2 Pd 3 Then when. silver nanoparticles co exist with AuCl4 ions in the solu. tion these ions take electrons from Ag atoms and replace. them relieving Ag ions At the same time the dissolving. silver nanoparticles act as the sacri cial template for the. formation of gold nanoparticles The morphologies of the. nanoparticles formed via the galvanic replacement of the. sacri cial template are usually hollow structures Depend. ing on the size and shape of the initial Ag nanoparticles. various hollow noble metal nanostructures were generated. including cubic nanoboxes nanocages triangular nanor. ings prism shaped nanoboxes single walled nanotubes and. multiple walled nanoshells or nanotubes 3 4 62 68 Gen. erally by controlling the size and morphology of the metal. template and the degree of replacement of metal atoms in. the template the shell thickness porosity and composition. of the hollow structure can be tailored, In the synthetic process for hollow Au nanoboxes or. frames HAuCl4 is added to the suspension of Ag nanocubes in Figure 6 a d SEM images of Ag nanocubes a Au Ag alloy. a controlled manner The silver template mediated galvanic nanocubes with small holes b Au Ag nanoboxes c and Au. replacement reaction is described as follows nanocages d In the insets the corresponding TEM and elec. tron diffraction images are shown The lower panel shows the. schematic for the shape evolution during the galvanic replace. 3Ag s AuCl4 aq Au s 3Ag aq 4Cl aq, ment reaction reproduced with permission from 3 copyright. 2008 American Chemical Society e and f TEM image of Au Ag. When PVP capped Ag nanocubes with sharp corners alloy nanorattles e and SEM image of multi walled nanoshells. shown in Fig 6a were reacted with a small amount of f Schematics for the synthetic processes are shown below. HAuCl4 small holes were formed on the faces of the in which Au Ag alloy and Ag are colored in orange and blue. nanocubes in the early stage of the galvanic replacement respectively reproduced with permission from 62 copyright. reaction Fig 6b This indicates that the replacement reac 2008 Professional Engineering Publishing. Applications of hollow nanostructures 367, amounts of HAuCl4 dealloying process takes place and sil thermally treated at 300 C for 5 hr in air During the.
ver atoms in the Au Ag nanoboxes are selectively removed thermal treatment spindle shaped FeOOH nanoparticles. Fig 6d As the dealloying proceeds further voids form in were transformed into hollow hematite nanocapsules via the. the nanoboxes and Au nanoframes with truncated corners thermal dehydroxylation of FeOOH Fig 7 The silica. are generated minimizing the total energy of the structure coated nanocapsules were further treated under H2 ow. 3 62 In contrast when Ag nanocubes with rounded cor at 500 C to transform hematite Fe2 O3 into magnetite. ners are used as the sacri cial template the replacement Fe3 O4 to change their magnetic property During this pro. reaction occurs at all corners where PVP binds weaker than cess the silica shell coated on the surface of FeOOH. the other faces resulting in the formation of Au nanocages nanoparticles played two critical roles rst it acted as. with pores on every corner of the cube 3 62 Furthermore the template for generation of the hollow structures and. by coating Au Ag alloy nanoparticles with silver interest second it prevented the aggregation of the core nanopar. ing hollow structures such as nanorattles and multi walled ticles at high temperatures The silica shell was removed. nanoshells or nanotubes can be obtained 3 64 68 When by dispersing iron oxide silica composite in an aqueous. silver coated Au Ag alloy nanoparticles are used as the solution of sodium hydroxide with sonication It seems. starting material for galvanic replacement nanorattles are that the formation of the nanocapsule in the silica shell. obtained Fig 6e Similarly repeating the silver shell coat via thermal treatment is related to the volume reduc. ing and galvanic replacement with AuCl4 ions leads to the tion resulting from the dehydroxylation Similarly Yang et. formation of multiple concentric shells 67 68 Following al synthesized novel hollow silica nanoparticles via the. the same strategy by using 1 D silver nanoparticles for the so called wrap melt bake process 78 They used magnet. replacement reaction double walled nanotubes of Au Ag ically assembled Fe3 O4 nanoparticles as the template for the. alloy can be prepared by repeating the replacement reaction large cavity inside the silica nanoparticles During the syn. Fig 6f Pt Ag and Pd Ag hollow structures have also been thesis clusters of Fe3 O4 nanoparticles bound together with. prepared via the galvanic replacement reaction because PVP were wrapped with silica shell using the St ber method. both PtCl4 2 and PdCl4 2 ions have higher standard reduc 79 After removing the Fe3 O4 core with hydrochloric acid. tion potential than the AgCl Ag pair as mentioned above and calcinating the shell at high temperatures hollow silica. 69 nanoparticles with a diameter of 80 nm and a large pore. Cobalt nanoparticles have also been used as a template size of 45 nm were obtained. for the preparation of noble metal hollow nanostructures Organic micelles have been also utilized as the template. via the galvanic replacement reaction 70 76 Liang et The Hyeon group reported uniform goethite FeOOH nan. al have synthesized Pt Au and AuPt hollow nanoparti otubes having a parallelogram cross section when reacting. cles by using cobalt nanoparticles as the template 70 72 hydrazine with Fe oleate complex in reverse micelles 80. Interestingly they synthesized AuPt bimetallic hollow tubu In this reaction the reverse micelles were formed by mix. lar nanomaterials using chain like templates composed of ing oleic acid xylene and water and functioned as the. cobalt nanoparticles assembled by the magnetic dipole soft template for the formation of the nanotubes In the. interaction 71 Similarly Zeng et al prepared necklace three approaches for the synthesis of hollow nanostructures. like Au Pt and Pd hollow nanostructures by using cobalt that we discussed so far the role of the surfactants has not. nanoparticles as the template which assembled into 1 D been clearly addressed It has been known that the surfac. structure by the external magnetic eld 76 tants bound to the surface of nanoparticles contribute to. controlling the shape of the nanoparticles Although only. a few studies on the role of surfactants on hollow nanos. Nanotemplate mediated approach tructure formation have been published so far there are. some suggestive results The Park group recently reported. Monodisperse nanoparticles can be utilized as templates the synthesis of parallelepiped cobalt hollow nanoparticles. for uniform hollow nanomaterials The Suslick group syn using CoO nanocrystals as the starting material 81 From. thesized hollow hematite Fe2 O3 nanocrystals by the the mechanism study they found that fcc CoO was reduced. sonochemical method using carbon nanoparticles as the to fcc cobalt by oleylamine bound to the cobalt cation. template 77 In this procedure Fe CO 5 in the solu on the surface of CoO nanocrystals at high temperatures. tion was sonicated in the presence of spherical carbon Also various hollow or tubular metal oxide nanostructures. nanoparticles with a diameter of 12 nm resulting Fe carbon were synthesized under hydrothermal solvothermal condi. nanocomposite Upon exposure to air the Fe shell on tions using strong binding surfactants onto the speci c facets. the carbon template was oxidized into Fe2 O3 At the of the nanoparticles 82 83. same time auto ignition of carbon took place remov. ing the carbon template and hollow nanostructures of. Fe2 O3 were obtained Nano sized silica has also success Biomedical applications of hollow. fully used as the inner or outer templates to generate nanostructures. hollow nanostructures The Hyeon group reported a novel. wrap bake peel WBP process involving the steps of sil In these days considerable progress has been made to. ica coating heat treatment and removal of the silica deliver biomolecules and drugs to target organs by using. layer In this process the phase morphology and struc various functional nanocarriers that realize simultaneous. ture of the nanomaterials can be transformed 23 For diagnosis and therapy 84 93 Hollow silica materials have. example FeOOH nanoparticles were wrapped with sil been used as drug delivery vehicles for their many advan. ica shell and then the FeOOH silica composites were tages such as biocompatibility excellent chemical stability. 368 K An T Hyeon, Figure 7 a Schematic illustration of the wrap bake peel process for the synthesis of uniform sized iron oxide nanocapsules. b TEM images of FeOOH nanoparticles silica coated FeOOH nanoparticles silica coated iron oxide nanocapsules after the. thermal treatment and iron oxide nanocapsules after removing silica shell in order from left to right The inset is a picture of the. water dispersion of iron oxide nanocapsules reproduced with permission from 23 Copyright 2008 Nature Publishing Group. and ease of bioconjugation via silane chemistry 29 30 92 magnetic resonance imaging MRI 94 95 Since super. One or more speci c nanomaterials can be integrated into paramagnetic iron oxide SPIO nanoparticles were rst. the silica nanomaterials to form multifunctional systems used as the contrast agent for the liver magnetic iron. However the large sizes of silica particles which often oxide nanoparticles have been used extensively as T2 MRI. exceed 200 nm have limited their utility in biological appli contrast agents due to their high magnetic moment com. cations As mentioned previously Yang et al synthesized pared to other contrast agents such as magnetic metal. hollow silica nanoparticles with large pores for drug delivery complexes 29 30 95 The magnetite hollow nanocapsules. 78 Doxorubicin DOX a potent chemotherapeutic drug developed by the Hyeon group were utilized as multifunc. was easily loaded into the pore of the hollow nanoparticles tional nanocarriers to deliver drugs and provide contrast for. by mixing the hollow nanomaterials with drugs The drug MRI 23 Fig 8a shows the T2 contrast effect of the nanocap. uptake seems to be proceeded by the concentration gradient sules When the DOX loaded nanocapsules were incubated. between a buffer solution and pores in the nanomaterials with the breast carcinoma cell line SKBR3 increased cyto. The nanoscale wall thickness and high porosity of the hollow toxicity was observed compared to free DOX demonstrating. nanomaterials enable drugs and uorescent molecules to the ability of these nanocapsules to act as a drug deliv. penetrate into the pores To impart controlled release abil ery vehicle Fig 8b Similarly Wu et al developed porous. ity the hydroxyl surface of hollow silica nanoparticles were iron oxide nanocapsules as drug delivery vehicles 96 Flu. modi ed with amine to induce a positive surface charge orescent molecules were incorporated into the porous iron. and then covered with poly ethylene glycol PEG a pro oxide nanorods in order to investigate the release behavior. cess often called PEGylation 29 30 Notable sustained and intracellular delivery in Hela cells The surface of the. drug release was observed from the amine modi ed hollow nanorods was functionalized with layers of polyelectrolytes. nanoparticles due to the stronger ionic interaction of the such as polyacrylic acid PAA and polyethylenimine PEI. DOX carboxylic group with the amine group of the hollow The release of the entrapped uorescein isothiocyanate. nanoparticles compared to the hydroxyl terminated hollow FITC from the nanocapsules exhibited either controlled. nanoparticles demonstrating their potential application as or sustained release trends depending on the compactness. a controlled drug delivery vehicle of the polyelectrolyte shells on the surface of the nanocap. Recently magnetic nanoparticles have attracted much sules Hu et al also reported stimuli responsive drug. interest for their applications as contrast agents for release behavior of PVP modi ed silica Fe3 O4 core shell. Applications of hollow nanostructures 369, Figure 8 a T2 weighted magnetic resonance images of the magnetite nanocapsules b In vitro cytotoxicity of free DOX and. DOX loaded magnetite nanocapsules against SKBR3 cells reproduced with permission from 23 Copyright 2008 Nature Publishing. nanoparticles 97 These core shell nanoparticles showed were released due to rupturing of the shell under the alter. surprisingly good controlled release and non release of uo nating magnetic eld. rescent molecules encapsulated inside of the silica core The In addition to being useful as nanocarriers for drug deliv. dense single crystalline Fe3 O4 shell ef ciently prevented ery hollow nanoparticles have been utilized for diagnostics. the undesired spontaneous release of the uorescent dye and therapeutics in many ways The Xu group synthesized. during the course of delivery When the nanoparticles FePt CoS2 yolk shell nanoparticles via the nanoscale Kirk. reached the disease site a high frequency magnetic eld endall effect using FePt Co core shell nanoparticles as. was exerted and the molecules encapsulated in the core the starting material and evaluated the cytotoxicity of. Figure 9 a Schematic illustration of the possible mechanism accounts for FePt CoS2 yolk shell nanocrystals killing HeLa cells. b and c HRTEM images of FePt CoS2 yolk shell nanoparticles in mitochondria of HeLa cells b and hollow CoS2 nanoparticles after. the FePt cores were diffused out to mitochondria c reproduced with permission from 98 copyright 2007 American Chemical. Society d The optical image of HeLa cells incubated with 5 g mL of FePt CoS2 nanoparticles for 3 days reproduced with. permission from 26 copyright 2008 Elsevier,370 K An T Hyeon. FePt CoS2 yolk shell nanoparticles 26 98 After cellular with disease speci c molecules the yolk shell nanopar. uptake Pt atoms were released from the FePt core and ticles could target tumor cells or tissues and detect the. diffused through the CoS2 shell It was shown that these transformation of the tumor noninvasively using MRI. Pt atoms had about 7 times higher cytotoxicity compared The Xia group developed hollow Au nanocages as cancer. to cisplatin in terms of the amount of Pt According to targeting nanoprobes for bioimaging and photothermal. the data HeLa cells incubated with 5 g mL of FePt CoS2 therapy agents Au nanoparticles are especially advanta. yolk shell nanoparticles were killed after uptake of the geous for biomedical applications because of their compact. nanoparticles Fig 9a shows the plausible mechanism of the size biocompatibility chemical stability and excellent bio. cytotoxicity of FePt CoS2 yolk shell nanoparticles against conjugation ability via Au thiolate chemistry 3 4 The Au. the HeLa cells After cellular uptake through the endocytic nanocages with a dimension of 40 nm have SPR peaks. pathway FePt cores were oxidized and destroyed to yield around 800 nm a common range used for optical coherence. Fe3 and Pt2 species The permeability of CoS2 shells allows tomography OCT imaging 4 100 Since the image contrast. these Pt2 species to diffuse out of shells easily Next the in OCT mainly comes from the scattering and absorption. Pt2 species enter the nucleus and mitochondria bind to of light by tissues both the sensitivity and speci city of. DNA and lead to apoptosis of the cell Comparing Fig 9b OCT depend strongly on the intrinsic optical properties of. and c it can be seen that the FePt core disintegrated the biological sample In OCT measurements Au nanocages. after cellular uptake They also fabricated bifunctional strongly absorbed light in the near IR region with an absorp. FePt Fe2 O3 yolk shell nanoparticles that exhibited high tion cross section 5 orders of magnitude higher than the. cytotoxicity from the FePt yolks and a strong contrast effect most commonly used contrast agent such as indocyanine. for MRI from the Fe2 O3 shells 99 Surface functionalized green ICG This result suggests their potential use as. Figure 10 a and b PAT images of a rat s cerebral cortex before a and about 2 h after the nal injection of PEGylated Au. nanocages b which is the peak enhancement point c A differential PAT image d An open skull photograph of the rat s. cerebral cortex revealing features of the vasculature reproduced with permission from 101 copyright 2007 American Chemical. Applications of hollow nanostructures 371, an ef cient contrast agent for optical diagnostics of can extended drug release Hollow nanoparticles composed. cers Also the in vivo use of Au nanocages as contrast of magnetic metal oxide have the contrast enhancement. enhancement agents for photoacoustic tomography PAT effect necessary for MRI Furthermore it was demonstrated. was demonstrated 3 101 102 PAT combines the merits of that magnetic hollow nanoparticles can be used for drug. both optical and ultrasonic imaging by measuring the ultra release by magnetic stimulation Other than drug delivery. sonic waves originated from the thermoelastic expansion of yolk shell nanoparticles with a cytotoxic core were used for. tissue upon the absorption of light It provides higher spa therapeutics inducing apoptosis of cancer cells Noble metal. tial resolution than purely optical imaging in deep tissues hollow nanoparticles have interesting optical properties due. and can overcome the disadvantages of ultrasonic imaging to the SPR effect Using Au nanocages with high scattering. such as low biochemical contrast and the speckle artifact and absorption coef cients contrast enhancement in opti. 101 It was shown that Au nanocages can be used in PAT cal imaging and the photothermal effect for therapeutics. to enhance the contrast between blood and the surrounding were achieved. tissues As shown in Fig 10a and b after three successive We anticipate that by manipulating the physical chem. injections of PEGylated Au nanocages a gradual enhance ical and biological properties of hollow nanomaterials it. ment of the optical absorption in the cerebral cortex of will be possible to design and fabricate hollow nanomaterials. a rat by up to 81 was observed A differential opti to utilize them for the biomedical applications enabling to. cal absorption image Fig 10c and a photograph of the carry out diagnosis and therapy simultaneously For this pur. open skull Fig 10d reveal that the anatomical features pose we need further studies on many issues including the. of the vasculature match well with those of PAT images biocompatibility in vivo targeting ef ciency and long term. which shows the effectiveness of Au nanocages for the con stability of hollow nanomaterials. trast enhancement in PAT Compared with Au nanoshells. Au nanocages appear to be more effective for PAT due to Acknowledgements. their larger absorption cross section The large absorption. cross section of Au nanocages leads to a strong photother. We acknowledge nancial support by the Korean Ministry. mal effect by which absorbed photons are converted into. of Education Science and Technology through the National. heat It has been demonstrated that in vitro photothermal. Creative Research Initiative Program and the World Class. destruction of breast cancer cells is possible with antibody. University WCU program of the Korea Science and Engi. functionalized Au nanocages 100 103 105,neering Foundation KOSEF.
Conclusions and outlook References, In the past few years there have been considerable 1 X W Lou L A Archer Z Yang Adv Mater 20 2008 3987. advancements concerning the synthesis of hollow materials 2 H J Fan U G sele M Zacharias Small 3 2007 1660. Several synthetic strategies capable of synthesizing uniform 3 S E Skarabalak J Chen Y Sun X Lu L Au C M Cobley et. hollow nanomaterials have been developed so far We cate al Acc Chem Res 41 2008 1587. 4 J Chen F Saeki B J Wiley H Cang M J Cobb Z Y Li et. gorize these strategies into four main classes 1 Kirkendall. al Nano Lett 5 2005 473, effect 2 chemical etching 3 galvanic replacement 5 F Caruso R A Caruso H Mohwald Science 282 1998 1111. and 4 template mediated approaches Each approach has 6 R A Caruso A Susha F Caruso Chem Mater 13 2001 400. its own advantages The Kirkendall effect based on mass 7 F Caruso M Spasova A Susha M Giersig R A Caruso. transport through the interface between different solid Chem Mater 13 2001 109. phases is the most general and well understood process 8 A Imhof Langmuir 17 2001 3579. This approach has been applied for synthesizing hollow 9 C Graf D L J Vossen A Imhof A van Blaaderen Langmuir. nanostructures of various metal oxides chalcogenides and 19 2003 6693. some phosphides There is a diversity of chemical etch 10 S W Kim M Kim W Y Lee T Hyeon J Am Chem Soc 124. ing reaction routes used to form hollow nanostructures 2002 7642. 11 X M Sun Y D Li Angew Chem Int Ed 43 2004 3827, including molten salt corrosion oxidative etching and acid. 12 M Yang J Ma C L Zhang Z Z Yang Y F Lu Angew Chem. etching The galvanic replacement is a simple method espe Int Ed 44 2005 6727. cially for the synthesis of noble metal hollow nanoparticles 13 S B Yoon K Sohn J Y Kim C H Shin J S Yu T Hyeon Adv. Using silver nanoparticles as the sacri cial template hollow Mater 14 2002 19. nanostructures of various morphologies have been fabri 14 J Hwang B D Min J S Lee K Keem K Cho M Y Sung. cated Template mediated approaches are effective and M S Lee S Kim Adv Mater 16 2004 422. easily controllable methods and have been applied for the 15 D H M Buchold C Feldmann Nano Lett 7 2007 3489. synthesis of hollow silica and iron oxide nanoparticles 16 Y S Li J L Shi Z L Hua H R Chen M L Ruan D S Yan. The large pore volume as well as the small size and their Nano Lett 3 2003 609. other useful properties such as magnetism and plasmon res 17 Q Peng Y J Dong Y D Li Angew Chem Int Ed 42 2003. onance make hollow nanoparticles promising candidates for. 18 C Z Wu Y Xie L Y Lei S Q Hu C Z OuYang Adv Mater 18. use as multimodal nanoprobe carriers for both drug deliv 2006 1727. ery and bioimaging Until now there have been several 19 S S Kim W Z Zhang T J Pinnavaia Science 282 1998 1302. reports on the utility of hollow nanoparticles for diagnostics 20 H L Xu W Z Wang Angew Chem Int Ed 46 2007 1489. and therapeutics Hollow silica and metal oxide nanoparti 21 Y Yin R Rioux C K Erdonmez S Hughes G A Somorjai. cles have been utilized as drug delivery systems exhibiting A P Alivisatos Science 304 2004 711. 372 K An T Hyeon, 22 K An S G Kwon M Park H B Na S I Baik J H Yu et al 63 S E Skrabalak L Au X Li Y Xia Nat Protoc 2 2007 2182. Nano Lett 8 2008 4252 64 Y Sun Y Xia Adv Mater 15 2003 695. 23 Y Piao J Kim H B Na D Kim J S Baek M K Ko et al 65 Y Khalavka J Becker C S nnichsen J Am Chem Soc 131. Nat Mater 7 2008 242 2009 1871, 24 M De P S Ghosh V M Rotello Adv Mater 20 2008 4225 66 Y Sun Y Xia J Am Chem Soc 126 2004 3892.
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