Direct Fabrication of Monodisperse Silica Nanorings from HollowSpheres − A Template for Core−Shell NanoringsKuo Zhong,† Jiaqi Li,‡,§ Liwang Liu,⊥ Ward Brullot,† Maarten Bloemen,† Alexander Volodin,§
Kai Song,*,∥ Pol Van Dorpe,‡,§ Niels Verellen,*,‡,§ and Koen Clays*,†
†Department of Chemistry, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium‡IMEC, Kapeldreef 75, B-3001 Leuven, Belgium§Laboratory of Solid-State Physics and Magnetism, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D,B-3001 Leuven, Belgium⊥Laboratory of Soft Matter and Biophysics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001Leuven, Belgium∥Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,Beijing 100190, China
*S Supporting Information
ABSTRACT: We report a new type of nanosphere colloidallithography to directly fabricate monodisperse silica (SiO2)nanorings by means of reactive ion etching of hollow SiO2 spheres.Detailed TEM, SEM, and AFM structural analysis is complementedby a model describing the geometrical transition from hollow sphereto ring during the etching process. The resulting silica nanorings canbe readily redispersed in solution and subsequently serve asuniversal templates for the synthesis of ring-shaped core−shellnanostructures. As an example we used silica nanorings (with diameter of ∼200 nm) to create a novel plasmonic nanoparticletopology, a silica-Au core−shell nanoring, by self-assembly of Au nanoparticles (<20 nm) on the ring’s surface. Spectroscopicmeasurements and finite difference time domain simulations reveal high quality factor multipolar and antibonding surfaceplasmon resonances in the near-infrared. By loading different types of nanoparticles on the silica core, hybrid and multifunctionalcomposite nanoring structures could be realized for applications such as MRI contrast enhancement, catalysis, drug delivery,plasmonic and magnetic hyperthermia, photoacoustic imaging, and biochemical sensing.
■ INTRODUCTIONNowadays, nanoparticles (NPs) come in a broad variety ofshapes and compositions, fabricated typically using lithographicand thin film deposition techniques, wet chemistry synthesis, oringenious combinations of both. Nanoparticle properties aredetermined by their material composition and nanoscopicgeometry which defines the confinement potential forelectrons, excitons, photons, surface plasmons, superconductingCooper pairs, magnetic spins, etc., enabling novel function-alities and applications. A nanoparticle’s shape and size definesits physical as well as its chemical and biological properties,such as cellular uptake and transport and biodistribution amongdifferent organs.1,2 New nanotopologies therefore have animportant potential impact in various scientific fields. In thispaper, we develop a method that enables the synthesis of a trulynew and unique nanotopology: the silica core−shell nanoring.The functionalized shell can consist of a combination ofmolecular layers and nanoparticle coatings.The ring shape in itself is a particularly interesting geometry.
Noble metallic nanorings, for example, exhibit strong electric
field enhancement at their localized surface plasmon resonances(LSPR) with applications in refractive index, bio- and chemicalsensing, and surface enhanced Raman scattering (SERS).3,4
Nanorings consisting of semiconductor materials such as CdSe,GaN, or InAs exhibit unique electronic states, useful foroptoelectronic devices.5−7 In addition, polymers, rare-earth-doped nanocrystals, and other materials such as TiO2 and DNAhave been made into ring-shaped structures to studymorphology-dependent properties.8−12
Dielectric core−shell nanoparticles, on the other hand, havefound various biomedical applications. In hyperthermia cancertherapy, Au NP-coated dielectric nanospheres (Au nanoshells)exhibit strong plasmonic absorption in the near-infrared (NIR),where human tissue is highly transmissive, inducing localheating and consequent cell death upon NIR irradiation.13,14
Magnetic nanoshells are being investigated for their use as
Received: January 19, 2016Accepted: March 31, 2016
magnetic resonance imaging (MRI) contrast agents15 and forlocalized drug delivery.16 When the shell consists of a ligand-functionalized lipid membrane, the core−shell nanoparticle caneven specifically bind to cancer cells and deliver drugcocktails.17 Many more application examples are situated inthe nanophotonics field and range from the enhancement ofnear-infrared fluorescence,18 surface enhanced infrared absorp-tion (SEIRA),19 and optical activity20 to the enhancement ofphotocurrents in 2D materials.21
Ring-shaped nanostructures that serve as the template (thecore) for the assembly of NPs (the shell) have not beenreported so far. The reason being that, in contrast tonanospheres, a simple and facile method to fabricatemonodisperse and dispersible nanorings has yet to bedeveloped. Indeed, template-assisted self-assembly of plasmonicNPs has mostly been limited to the use of nano- andmicrospheres or DNA as the template.22−25
A popular and cost-effective way to fabricate ring-shapednanostructures is nanosphere colloidal lithography.26,27 It usescolloidal spheres either as an etch or thin film depositionmask,28−32 as a template for capillary force assembly,4,33−39 orto imprint polymer membrane templates.40 The existingnanosphere lithography processes, however, still involvemultiple sophisticated fabrication steps. Additionally, andmore importantly, they leave the nanorings anchored to thesubstrate, preventing them from being dispersed directly in anaqueous medium. This strongly limits the application scope aswell as the possibilities of subsequent surface functionalization.Here, we present a new nanosphere lithography-based
method that produces monodisperse and dispersible silicananorings directly from hollow silica spheres using reactive ionetching (RIE).41,42 We first describe the fabrication process andcharacterize the morphological changes during the etching step.Next, a model describing these changes is put forward. Finally,we demonstrate the assembly of silica-Au core−shell nanoringstructures using redispersed silica nanorings as templates. Thisunique and new type of plasmonic nanoparticle topology isinvestigated by means of visible-NIR spectroscopy and finitedifference time domain (FDTD) simulations. It is shown thatthe obtained core−shell plasmonic nanorings display sharpmultipolar and antibonding resonances in the near-infraredfrequency range.
■ EXPERIMENTAL SECTIONSynthesis of Hollow Silica Spheres. The polystyrene (PS) cores
(∼214 ± 10 nm) were synthesized by emulsifier-free emulsionpolymerization. Because 2,2′-azobis(2-methylpropionamidine) dihy-drochloride (AIBA) was employed as initiator, the obtained PSspheres are positively charged. Therefore, uniform silica shells weredirectly coated on the surface of PS particles via the Stobermethod.43,44 Note that this would not be possible with a negativelycharged surface.45 Briefly, the obtained PS spheres were dispersed in50 mL of ethanol with a concentration of 0.6 wt %. Then 1.5 mL ofTEOS and 4 mL of ammonia were added to the diluted PS suspensionand stirred for 4 h at 50 °C. The PS@SiO2 colloids were purified byrepeated centrifugation and redispersion in ethanol (5 cycles). Thehollow silica spheres were obtained after removing the PS cores bycalcination at 450 °C for 3 h in air and then cooled down to roomtemperature. Calcination is a well-known technique to produce hollowspheres.44 It has been shown that a refractive index of 1.19 (implyingcomplete removal of the organic material for the specific case of a SiO2shell thickness of ∼23 nm and the core diameter of ∼214 nm) for thehollow sphere results in a good agreement between theory andexperiment in modeling the bandgap properties for photonic crystalswith hollow particles.46 The resulting hollow spheres with a diameter
of ∼270 nm and a shell thickness of ∼23 nm were redispersed inethanol for further usage.
Preparation of a Monolayer of Hollow Silica Spheres Arrays.Typically, 12 μL of a hollow silica sphere suspension (4 wt %) wasdropped on a hydrophilic glass slide and then added to the air/waterinterface within a dish filled with water (Ø = 6 cm) by tilting the slideby an angle of approximately 45 deg. After spreading, patches of thecolloidal monolayer were floating at the interface. Two μL of sodiumdodecyl sulfate (SDS, 2 wt %) solution was added to the water surfaceto compress the particles closer together, hence serving as a soft barrierto help monolayer crystallization. Next, a substrate was immersed intowater and gently lifted under a small angle to transfer the monolayeronto the substrate.
Reactive Ion Etching Process. The prepared monolayer ofhollow spheres was put into a reactive ion etcher chamber (OxfordPlasmalab 100) at a pressure of ∼20 mTorr. SF6 gas with a flow of 25standard cubic centimeters per minute (SCCM) was used to generatereactive ions. A forward power and ICP power of 5W and 150W wasapplied, respectively.
Synthesis of Au NPs. Au colloids were synthesized based on anaqueous citrate reduction.47 After synthesis, a deep red coloreddispersion was obtained. This dispersion was diluted 5× with Milli-Qwater before use to obtain a final Au concentration of 0.2 mM.
Formation of Au NPs Covered Silica Nanoring Composite.First, the obtained ordered arrays of silica nanorings on the substratewere immersed into 5 mL of a 1 M 3-aminopropyltrimethoxysilane(APTMS)/methanol solution for functionalization and subsequentlyrinsed with methanol and water. Please note that the use of theAPTMS linker (a small molecule with only short saturated singlebonds) does not absorb in the visible window and is not affecting theoptical properties of the particles. To optimize the nanoring yield, thesurface modification (immersion) was performed directly on thesubstrate with the nanoring monolayer. After the etching, thenanorings are no longer in direct contact with the substrate butremain in close contact. This provides complete surface modificationwhile still allowing large yield (no centrifugation step required torecover the nanorings from solution). The modified nanorings werethen redispersed into water by ultrasonication for 10 min.Subsequently, 200 μL of Au NPs (0.2 mM) was added while shakingfor self-assembly. The coverage of the Au NPs on the silica nanorings’surface was controlled by the length of the assembly time, typically, 15min for partial coverage and 1 h for complete coverage, as verified byTEM. (See Results and Discussion.)
Characterization. The morphological changes of hollow spheresfor different reactive ion etching times were characterized by scanningelectron microscopy (SEM), atomic force microscopy (AFM), andtransmission electron microscopy (TEM). SEM observations wereconducted on a Philips XL 30 ESEM FEG electron microscope. AFMmeasurements were performed using an Agilent 5500 scanning probemicroscope. The height and phase images were recorded simulta-neously while operating the instrument in tapping mode underambient conditions. The TEM sample of the hollow spheres withdifferent etching times and assembled composite of Au coated silicananorings with different reactive times were prepared by depositing adrop (2 μL) of the sample solution onto a carbon-coated copper grid.A piece of filter paper was employed to remove the excess solution onthe grid, and then the grid was allowed to dry under ambientconditions. TEM measurements were performed in a JEOL 1011electron microscope at an accelerating voltage of 100 kV. Theextinction (absorption + scattering) spectra of the isolated Au NPs andthe assembled composite Au-silica core−shell nanostructures weremeasured with a UV−vis-NIR spectrometer (PerkinElmer Lambda900).
■ RESULTS AND DISCUSSION
The silica nanoring, and subsequent silica-Au core−shellnanoring, fabrication process is illustrated in Figure 1. First, acolloidal crystal monolayer of hollow silica spheres is preparedon a cleaned substrate by assembly at the air/water interface.46
ACS Applied Materials & Interfaces Research Article
The spheres have a narrow size distribution with a diameter of∼270 nm and a shell thickness of ∼27 nm, as derived fromtransmission electron microscopy (TEM) images (see FigureS1 in the Supporting Information). After that, a sulfurhexafluoride (SF6) RIE etch (Oxford Plasmalab 100) with aflow of 25 sccm at a pressure of ∼20 mTorr and forward andICP power of 5W and 150W, respectively, was applied. Thisetching step gradually transforms the silica spheres into rings.Subsequently, the obtained nanorings are functionalized withan APTMS molecular linker layer. To optimize the nanoringyield, this surface modification was performed directly on thenanorings collected on the substrate. After the etching, thenanorings are no longer in direct contact with the substrate butremain in close contact. This provides complete surfacemodification of the rings while still allowing a large yield byeliminating the necessity of an additional centrifugation step.Because of the weak substrate contact, the nanorings are easilyredispersed into water using ultrasonication for 10 min. Finally,the silica rings act as a template for the self-assembly of Au NPs,and a composite of silica nanorings coated with Au NPs iscreated.
Figure 2A shows SEM images and Figure 2B AFM scans ofseveral stages of the morphological change of the hollow silicaspheres during etching. The SEM images in panel A show bothzooms of a single particle (top) and array views (bottom) takenafter etching times t of 0, 2, 4, 5, 6, and 8 min. Thecorresponding AFM scans are shown in Figure 2B. For theinitial hollow spheres (panel indicated by 0′) the zoom-inshows a TEM image. More TEM images can be found inSupporting Information, Figure S2. The monodisperse spheresreadily assemble into hexagonal arrays via assembly at the air/water interface as can be seen in the SEM image at t = 0′. After2 min of etching, a small hole appears in the top surface of thehollow spheres. The opening size (hole diameter) here is ∼90nm. Upon further etching, the hollow spheres obtain a bowl-shape with growing opening size. When the etching timeincreases to 5 min, also a hole appears at the bottom surface ofthe hollow spheres. The opening size at the top is larger than atthe bottom, which is clearly seen in the inset at t = 5′. Theseresults are further confirmed by AFM measurement as shown inFigure 2B, 5′. This indicates that the top surface of the sphere ispreferentially etched. Further increasing the etching timegradually increases the opening sizes (t = 6′). Finally, themorphology of the hollow spheres is transformed into ring-shaped structures after 8 min of etching.To quantify the etching process, we extracted the opening
size d and the height H of the nanostructures from the SEMand AFM data, respectively. The results, plotted in function ofthe etching time t, are shown in Figure 3B (H: blue dots, leftaxis; d: red squares, right axis). Notice that the height of thehollow spheres decreased from ∼245 nm after 2 min of etchingto ∼70 nm after 8 min. It also is worth noting that the innerdiameter of the obtained nanorings is ∼225 nm, which is largerthan the diameter of the cores, and that the outer diameter is∼256 nm, which is smaller than the original one. This is due tothe reduction of the shell thickness by plasma etching (from 27to 16 nm).
Figure 1. Schematic illustration of the process steps for the fabricationof monodisperse silica nanoring templates and core−shell plasmonicnanorings. Left: Fabrication of silica nanorings from silica hollowspheres by reactive ion etching (RIE). After surface modification witha molecular linker layer, the nanorings are redispersed in water andfunctionalized with Au NPs to create a self-assembled core−shellplasmonic nanoring composite.
Figure 2. SEM images (panel A) and AFM scans (panel B) showing the morphological changes from a hollow silica sphere to a ring during theetching process. Intermediate etching times of t = 2, 4, 5, 6, and 8 min are displayed in panel A. Scale bars: 500 nm. The top panels show highmagnification SEM images (except for the initial hollow silica spheres, indicated by 0′, a TEM image is takensphere diameter ∼270 nm and shellthickness ∼27 nm). Corresponding times in panel B are indicated. AFM line-scan profiles are taken through the center of the nanoring shown in theinsets.
ACS Applied Materials & Interfaces Research Article
Next, we construct a simple model, illustrated in Figure 3A,in order to explain the formation mechanism of the nanorings.The model is placed in a Cartesian coordinate system with thecenter of the hollow sphere as the origin and the y-axis as theetch direction. In the plasma environment, the SF6 reactivespecies are accelerated perpendicularly to the sample in aunidirectional electric field.48 As a result, for the initial etchingstage, the top surface of the hollow spheres is preferentiallyetched, leading to the formation of a hole in the top surface.The diameter of the hole, defined as the opening size d, is afunction of the etching time t and can be described by thefollowing equation
ν
νν ν
=< <
− − + ≤ <
⎧⎨⎪⎪
⎩⎪⎪
d tt
L
r r t LL
tR
( )0, 0
2 ( ) ,2 2
(1)
with r being the sphere’s inner radius, v the etch rate, and L theshell thickness. When the etching time t < L/v, the top of thesphere is still closed, so d equals zero. Once the etching time t =
L/v, the sphere’s shell has been penetrated. While the etchingtime t > L/v, the hole on the top surface increases in diameterd, and the bottom surface starts to be etched from the inside ofthe hollow sphere because the reactive species can penetratethrough the hole in the top. It also takes t = L/v time topenetrate through the bottom part of the shell, so t = 2L/v is acritical value for a completely etched bottom surface. Oncethere is a hole on the bottom surface, the rate at which theheight H changes is doubled (equal to 2v) since the etchingoccurs at both the top and bottom surfaces simultaneously.This is also observed in the experimental data in Figure 3B as achange in the slope of H, indicated by two arrows. Based onthis discussion the evolution of the height of the hollow spherescan be expressed as
νν
νν ν
=− < <
+ − ≤ <
⎧⎨⎪⎪
⎩⎪⎪
H tR t t
L
R L tL
tR
( )2 , 0
2
2 2 2 ,2
(2)
where R is the outer radius of the hollow spheres (R = r + L).Fits of eqs 1 and 2 to the experimental data points in Figure 3Bshow an excellent agreement (red and blue curves). Moreover,these two fitting curves now allow us to determine the etch ratev, which is found to be approximately 16 nm/min. It is evidentthat the change of H presents two different slopes. Thisindicates that the rate at which the height decreases doublesonce the bottom surface completely opened. This inflectionpoint is found at t = 3 min.As mentioned in the Introduction, one of the main benefits
of our method is that the obtained nanorings can be readilyredispersed in solution for further functionalization. Toillustrate the possibilities this offers, we assemble Au nano-particles (NPs) onto the silica template using an APTMSmolecular linker layer.47 This way, we obtain a uniquecomposite nanostructure consisting of a ring-shaped dielectriccore with a metallic shell which also supports strong collectivenear-infrared plasmonic resonances.To investigate the morphology-dependent properties of the
silica nanoring template-assisted assembled Au NPs, three typesof samples were prepared (details can be found in theExperimental Section) and characterized by TEM and visible-NIR spectroscopy. Sample A contains only the isolated Au NPsdispersed in an aqueous solution, shown in Figure 4A (left).The TEM image shows their narrow size distribution withdiameter determined ∼10 nm. The typical extinction bandaround 533 nm for Au NPs dispersed in water is shown inpanel A (right).47 The other two samples consist of silicananorings coated with Au NPs on their surface (self-assembly).Sample B has a partially coated surface, while for sample C thesurface is completely covered. The TEM images in Figures 4Band 4C compare the surface morphology of the obtainedcomposites. The corresponding optical spectra show that a lowsurface coverage broadens the single Au nanoparticle extinctionband of panel A and induces a red-shift. This is a consequenceof small particle aggregates forming on the ring’s surface.23−25
An important additional observation is that no distinct spectralfeatures are induced by the bare silica nanorings. For the densecoverage of sample C, the TEM data show that a continuousAu coating is reached. This now has a profound impact on theoptical properties: three pronounced extinction bandsindicated by colored dotsappear in the near-infrared spectralwindow (panel C). Note that for the lowest energy band (pink
Figure 3. A) Schematic of the geometrical parameters used to describethe changes of the hollow spheres during the etching process. Theequator section of the sphere model highlighted in gray represents theresulting ring-shaped structures. B) Diagram showing the evolution ofthe height H (blue dots) and opening size d (red squares) ofnanosphere/nanoring structures for increasing etch time. Data pointsare determined from the SEM and AFM analysis shown in Figure 2.Blue and red curves are fits for H and d according to the eqs 1 and 2,respectively.
ACS Applied Materials & Interfaces Research Article
dot), only the onset is experimentally obtained due to thestrong absorption of water in this spectral region.In order to gain more insight into the nature of the observed
spectral features, we performed FDTD simulations of theassembled silica-Au core−shell nanorings.49 The ring dimen-sions used for these simulations are based on the TEM results.The dense layer of adsorbed Au NPs is modeled as acontinuous uniform Au coating on the silica core (see the insetof Figure 4D). The background index for water was taken as1.33, and the permittivity for Au and SiO2 was taken from theliterature.50,51 Since the nanorings are randomly oriented in theaqueous solution, the experimental spectrum will be a weightedaverage of different directions of the incident light (k-vector)and its polarization (E-vector) relative to the nanoring. Figure4D shows the configuration for which light is incident from theside of the ring with polarization parallel to the ring. Threeprominent spectral resonances, indicated by colored dots,appear. From the simulations, we can extract the associatedelectric field and charge density distributions, as shown inFigure 4D. The field distributions clearly show the differentlobes of strong field enhancement expected for a dipolar (pink),quadrupolar (blue), and octupolar (red) localized surfaceplasmon mode in a nanoparticle with cylindrical symmetry.52
The associated charge density distributions further corroboratethe plasmonic nature of these modes.A comparison of the experimental results in panel C with the
simulation in panel D shows that this specific simulationconfiguration successfully captures the main experimentalspectral response. We therefore conclude that the three
experimentally observed extinction bands can indeed beassigned to collective surface plasmon resonances of theassembled silica-Au core−shell nanorings. Small spectraldifferences between the simulation and measurement areattributed to deviations of the idealized geometry in thesimulation compared to the fabricated nanorings and variationsin the exact material properties (i.e., permittivity) of the AuNPs and SiO2.The nanoring’s quadrupolar and octupolar modes have no, or
only very weak, net dipole moments. To observe them, anelectromagnetic field gradient (phase retardation) of theincident light along the structure is required. This is indeedthe case for many of the randomly oriented nanorings insolution and motivates the choice of the particular simulationconfiguration in panel D. Moreover, the absence of a strongdipole moment of theseoften referred to as dark modesresults in low radiative losses and consequently very narrow linewidths.52,53 Remarkably, the resulting high quality (Q) factorsurvives in the nanoparticle-synthesized core−shell nanorings.For the quadrupolar (blue dot) and octupolar (red dot) modesan experimental value of Q ≈ 30 (fwhm = 0.035 eV) and Q ≈20 (fwhm = 0.054 eV) at wavelengths of 1170 and 980 nm isreached, respectively. These values are strikingly highercompared to previously reported values for plasmonicresonances in this spectral range, which are typically below Q= 10.54 The high Q-factors in combination with the strongplasmonic near-field enhancements make the core−shellnanorings interesting candidates for applications such asrefractive index sensing, surface enhanced Raman spectroscopy(SERS), liquid-phase catalysis,55 photoacoustic imaging,56 andhyperthermia treatments.Next, we have a look at the simulation result for another E-k
orientation of the incident light relative to the nanoring. Theblack curve in Figure 5 shows the extinction spectrum for thecase of normal incident light. As expected, the dipole-inactivequadrupolar and octupolar modes are not excited. However,interestingly, in addition to the dipole mode (pink dot) a newmode appears (gray dot). As the corresponding field and chargedistributions reveal, this is a pronounced dipole−dipoleantibonding mode resulting from the hybridization of a dipolemode on the outer ring surface (a disk plasmon) with a dipoleon the inner surface (a cavity plasmon).57 This mode nowhappens to spectrally overlap with the nanoring’s octupolemode seen in Figure 4D (red dot). Both modes will thereforecontribute to the experimental resonance at 980 nm (red dot).For solid Au nanorings, the antibonding mode is typically
situated at shorter wavelengths and can experimentally not beobserved due to strong damping by the Au interbandtransitions around 500 nm.58 The unique topology of thesilica-Au core−shell nanoring, however, induces a considerablespectral red-shift, making this mode experimentally accessible,even in Au. To illustrate this effect, Figure 5 includes thesimulation result for a solid nanoringa copy of the core−shellnanoring but with the silica core replaced by Au(red curveand green dot), which does not exhibit the antiboding dipolarresonance.An important consequence of the antibonding mode-
hybridization is the strong boost in field enhancement insidethe silica core due to opposite charges on the outer and innerring surfaces (see corresponding field distribution). It is exactlythis strong enhancement that is also observed for the octupolarmode in Figure 4D. This is an interesting property that can beexploited for fluorescence enhancement, and even surface
Figure 4. TEM images of various aggregation types of the Au NPs andthe corresponding extinction spectra (scale bars are 100 nm.): A)Isolated (monodispersed) Au NPs; B) Low coverage of Au NPs onsilica nanorings; C) Densely covered Au NPs-capped silica nanorings.D) Simulated extinction spectra and corresponding near-field (log|E|2)and charge density distributions of the silica-Au core−shell nanoringcomposite.
ACS Applied Materials & Interfaces Research Article
plasmon assisted lasing, of molecules dispersed within thedielectric core, especially in combination with the high Q-factorof the resonance.59−61
■ CONCLUSIONSIn conclusion, we have developed a novel colloidal nanospherelithography method that allows creating monodispersenanorings by reactive ion etching of hollow SiO2 spheres.Although the current work describes only one size of nanorings,the structural parameters of the nanorings can be tailored bycontrolling the structural parameters of the hollow spheres.Compared to conventional nanosphere lithography, ourmethod does not involve any intermediate steps to generatean etch mask. Additionally, the resulting nanorings can bereadily redispersed in water and used as templates for furtherfunctionalization. As an example, Au NPs were assembled onthe silica core (nanoring) surface. By controlling the degree ofAu NP coverage, the plasmonic resonance properties of thefinal composite nanorings were tuned. For a dense surfacecoating, a novel silica-Au core−shell ring-shaped nanostructureis created. This unique nanoparticle topology was found toexhibit pronounced localized surface plasmon resonance modesthat are collectively supported by the full nanoring structure.The remarkably high quality factor resonances in the near-infrared make the silica-Au core−shell nanorings particularly
interesting as a platform for biosensing, liquid-phase catalysis,lasing applications, and hyperthermia. When using other shellmaterials, such as superparamagnetic iron oxide nanoparticles(SPIONs), applications in magnetic hyperthermia14 and MRIimaging could be found. In future work, we will introducemulticomponent hybrid nanoparticles to functionalize the silicacore and as such realize multifunctional core−shell nanoringstructures with novel properties.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b00733.
TEM image showing monodisperse hollow silica spheresand TEM images showing the morphological transitionfrom silica sphere to ring induced by the RIE etch (PDF)
■ ACKNOWLEDGMENTSK. Zhong thanks the support provided by the ChinaScholarship Council (CSC) of the Ministry of Education, P.R. China. N. Verellen acknowledges financial support from theFWO Flanders.
■ REFERENCES(1) Herd, H.; Daum, N.; Jones, A. T.; Huwer, H.; Ghandehari, H.;Lehr, C. M. Nanoparticle Geometry and Surface Orientation InfluenceMode of Cellular Uptake. ACS Nano 2013, 7, 1961−1973.(2) Blanco, E.; Shen, H.; Ferrari, M. Principles of NanoparticleDesign for Overcoming Biological Barriers to Drug Delivery. Nat.Biotechnol. 2015, 33, 941−951.(3) Kim, S.; Jung, J. M.; Choi, D. G.; Jung, H. T.; Yang, S. M.Patterned Arrays of Au Rings for Localized Surface PlasmonResonance. Langmuir 2006, 22, 7109−7112.(4) Larsson, E. M.; Alegret, J.; Kall, M.; Sutherland, D. S. SensingCharacteristics of NIR Localized Surface Plasmon Resonances in GoldNanorings for Application as Ultrasensitive Biosensors. Nano Lett.2007, 7, 1256−1263.(5) Chen, J. X.; Liao, W. S.; Chen, X.; Yang, T. L.; Wark, S. E.; Son,D. H.; Batteas, J. D.; Cremer, P. S. Evaporation-Induced Assembly ofQuantum Dots into Nanorings. ACS Nano 2009, 3, 173−180.(6) Li, K. H.; Ma, Z. T.; Choi, H. W. High-Q Whispering-GalleryMode Lasing from Nanosphere-Patterned Gan Nanoring Arrays. Appl.Phys. Lett. 2011, 98, 071106.(7) Lorke, A.; Luyken, R. J.; Govorov, A. O.; Kotthaus, J. P.; Garcia, J.M.; Petroff, P. M. Spectroscopy of Nanoscopic Semiconductor Rings.Phys. Rev. Lett. 2000, 84, 2223−2226.(8) Zinchenko, A. A.; Yoshikawa, K.; Baigl, D. DNA-TemplatedSilver Nanorings. Adv. Mater. (Weinheim, Ger.) 2005, 17, 2820−2823.(9) Sun, F. Q.; Yu, J. C.; Wang, X. C. Construction of Size-Controllable Hierarchical Nanoporous TiO2 Ring Arrays and TheirModifications. Chem. Mater. 2006, 18, 3774−3779.(10) Lewicka, Z. A.; Bahloul, A.; Yu, W. W.; Colvin, V. L. A FacileFabrication Process for Polystyrene Nanoring Arrays. Nanoscale 2013,5, 11071−11078.(11) Zhang, K. K.; Miao, H.; Chen, D. Y. Water-SolubleMonodisperse Core-Shell Nanorings: Their Tailorable Preparation
Figure 5. Comparison of a silica-Au core−shell and a solid Aunanoring for normal incidence illumination revealing the dipole−dipole antibonding mode in the NIR. Simulated extinction spectra areshown for a core−shell nanoring (black curve) with the samedimensions as in Figure 4, and a solid nanoring for which the silicacore has been replaced by Au. Electric field (log|E|2) and chargedensity distributions corresponding to the resonances indicated withthe colored dots are shown below. Gray and pink: core−shell nanoringdipole−dipole antibonding and dipole mode, respectively; green: solidnanoring dipole mode.
ACS Applied Materials & Interfaces Research Article
and Interactions with Oppositely Charged Spheres of a SimilarDiameter. J. Am. Chem. Soc. 2014, 136, 15933−15941.(12) Mullen, T. J.; Zhang, M.; Feng, W.; El-khouri, R. J.; Sun, L. D.;Yan, C. H.; Patten, T. E.; Liu, G. Y. Fabrication and Characterizationof Rare-Earth-Doped Nanostructures on Surfaces. ACS Nano 2011, 5,6539−6545.(13) Loo, C.; Lin, A.; Hirsch, L.; Lee, M. H.; Barton, J.; Halas, N.;West, J.; Drezek, R. Nanoshell-Enabled Photonics-Based Imaging andTherapy of Cancer. Technol. Cancer Res. Treat. 2004, 3, 33−40.(14) Chatterjee, D. K.; Diagaradjane, P.; Krishnan, S. Nanoparticle-Mediated Hyperthermia in Cancer Therapy. Ther. Delivery 2011, 2,1001−1014.(15) Su, C. H.; Sheu, H. S.; Lin, C. Y.; Huang, C. C.; Lo, Y. W.; Pu,Y. C.; Weng, J. C.; Shieh, D. B.; Chen, J. H.; Yeh, C. S. NanoshellMagnetic Resonance Imaging Contrast Agents. J. Am. Chem. Soc. 2007,129, 2139−2146.(16) Fuchigami, T.; Kitamoto, Y.; Namiki, Y. Size-Tunable Drug-Delivery Capsules Composed of a Magnetic Nanoshell. Biomatter2012, 2, 313−320.(17) Irvine, D. J. Drug Delivery: One Nanoparticle, One Kill. Nat.Mater. 2011, 10, 342−343.(18) Bardhan, R.; Grady, N. K.; Halas, N. J. Nanoscale Control ofnear-Infrared Fluorescence Enhancement Using Au Nanoshells. Small2008, 4, 1716−1722.(19) Kundu, J.; Le, F.; Nordlander, P.; Halas, N. J. Surface EnhancedInfrared Absorption (Seira) Spectroscopy on Nanoshell AggregateSubstrates. Chem. Phys. Lett. 2008, 452, 115−119.(20) Acevedo, R.; Lombardini, R.; Halas, N. J.; Johnson, B. R.Plasmonic Enhancement of Raman Optical Activity in Molecules nearMetal Nanoshells. J. Phys. Chem. A 2009, 113, 13173−13183.(21) Sobhani, A.; Lauchner, A.; Najmaei, S.; Ayala-Orozco, C.; Wen,F.; Lou, J.; Halas, N. J. Enhancing the Photocurrent and Photo-luminescence of Single Crystal Monolayer MoS2 with ResonantPlasmonic Nanoshells. Appl. Phys. Lett. 2014, 104, 031112.(22) Tan, S. J.; Campolongo, M. J.; Luo, D.; Cheng, W. L. BuildingPlasmonic Nanostructures with DNA. Nat. Nanotechnol. 2011, 6, 268−276.(23) Qian, Z. X.; Hastings, S. P.; Li, C.; Edward, B.; McGinn, C. K.;Engheta, N.; Fakhraai, Z.; Park, S. J. Raspberry-Like MetamoleculesExhibiting Strong Magnetic Resonances. ACS Nano 2015, 9, 1263−1270.(24) Muhlig, S.; Cunningham, A.; Scheeler, S.; Pacholski, C.; Burgi,T.; Rockstuhl, C.; Lederer, F. Self-Assembled Plasmonic Core-ShellClusters with an Isotropic Magnetic Dipole Response in the VisibleRange. ACS Nano 2011, 5, 6586−6592.(25) Lu, Z. D.; Goebl, J.; Ge, J. P.; Yin, Y. D. Self-Assembly andTunable Plasmonic Property of Gold Nanoparticles on Mercapto-Silica Microspheres. J. Mater. Chem. 2009, 19, 4597−4602.(26) Yang, S. M.; Jang, S. G.; Choi, D. G.; Kim, S.; Yu, H. K.Nanomachining by Colloidal Lithography. Small 2006, 2, 458−475.(27) Burmeister, F.; Schafle, C.; Keilhofer, B.; Bechinger, C.;Boneberg, J.; Leiderer, P. From Mesoscopic to Nanoscopic SurfaceStructures: Lithography with Colloid Monolayers. Adv. Mater.(Weinheim, Ger.) 1998, 10, 495−497.(28) Wang, Y.; Han, S.; Briseno, A. L.; Sanedrin, R. J. G.; Zhou, F. AModified Nanosphere Lithography for the Fabrication of Amino-silane/Polystyrene Nanoring Arrays and the Subsequent Attachmentof Gold or DNA-Capped Gold Nanoparticles. J. Mater. Chem. 2004,14, 3488−3494.(29) Marczewski, D.; Goedel, W. A. The Preparation ofSubmicrometer-Sized Rings by Embedding and Selective Etching ofSpherical Silica Particles. Nano Lett. 2005, 5, 295−299.(30) Ren, Z. Y.; Zhang, X. M.; Zhang, J. J.; Li, X.; Yang, B. BuildingCavities in Microspheres and Nanospheres. Nanotechnology 2009, 20,065305.(31) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Kall, M.; Bryant, G.W.; de Abajo, F. J. G. Optical Properties of Gold Nanorings. Phys. Rev.Lett. 2003, 90, 057401−4.
(32) Ye, J.; Shioi, M.; Lodewijks, K.; Lagae, L.; Kawamura, T.; VanDorpe, P. Tuning Plasmonic Interaction between Gold Nanorings anda Gold Film for Surface Enhanced Raman Scattering. Appl. Phys. Lett.2010, 97, 163106.(33) Yabu, H. Bottom-up Approach to Creating Three-DimensionalNanoring Arrays Composed of Au Nanoparticles. Langmuir 2013, 29,1005−1009.(34) McLellan, J. M.; Geissler, M.; Xia, Y. N. Edge SpreadingLithography and Its Application to the Fabrication of MesoscopicGold and Silver Rings. J. Am. Chem. Soc. 2004, 126, 10830−10831.(35) Li, J. R.; Lusker, K. L.; Yu, J. J.; Garno, J. C. Engineering theSpatial Selectivity of Surfaces at the Nanoscale Using ParticleLithography Combined with Vapor Deposition of Organosilanes.ACS Nano 2009, 3, 2023−2035.(36) Li, J. R.; Garno, J. C. Elucidating the Role of Surface Hydrolysisin Preparing Organosilane Nanostructures Via Particle Lithography.Nano Lett. 2008, 8, 1916−1922.(37) Cai, Y. J.; Li, Y.; Nordlander, P.; Cremer, P. S. Fabrication ofElliptical Nanorings with Highly Tunable and Multiple PlasmonicResonances. Nano Lett. 2012, 12, 4881−4888.(38) Liao, W. S.; Chen, X.; Chen, J.; Cremer, P. S. Templating WaterStains for Nanolithography. Nano Lett. 2007, 7, 2452−2458.(39) Geissler, M.; McLellan, J. M.; Chen, J. Y.; Xia, Y. N. Side-by-SidePatterning of Multiple Alkanethiolate Monolayers on Gold by Edge-Spreading Lithography. Angew. Chem., Int. Ed. 2005, 44, 3596−3600.(40) Yan, F.; Goedel, W. A. Preparation of Mesoscopic Gold RingsUsing Particle Imprinted Templates. Nano Lett. 2004, 4, 1193−1196.(41) Tan, B. J. Y.; Sow, C. H.; Lim, K. Y.; Cheong, F. C.; Chong, G.L.; Wee, A. T. S.; Ong, C. K. Fabrication of a Two-DimensionalPeriodic Non-Close-Packed Array of Polystyrene Particles. J. Phys.Chem. B 2004, 108, 18575−18579.(42) Choi, D. G.; Yu, H. K.; Jang, S. G.; Yang, S. M. ColloidalLithographic Nanopatterning Via Reactive Ion Etching. J. Am. Chem.Soc. 2004, 126, 7019−7025.(43) Liu, Z. F.; Ding, T.; Zhang, G.; Song, K.; Clays, K.; Tung, C. H.Ternary Inverse Opal System for Convenient and Reversible PhotonicBandgap Tuning. Langmuir 2008, 24, 10519−10523.(44) Lu, Y.; McLellan, J.; Xia, Y. N. Synthesis and Crystallization ofHybrid Spherical Colloids Composed of Polystyrene Cores and SilicaShells. Langmuir 2004, 20, 3464−3470.(45) Ge, C.; Zhang, D. Z.; Wang, A. L.; Yin, H. B.; Ren, M.; Liu, Y.M.; Jiang, T. S.; Yu, L. B. Synthesis of Porous Hollow Silica SpheresUsing Polystyrene-Methyl Acrylic Acid Latex Template at DifferentTemperatures. J. Phys. Chem. Solids 2009, 70, 1432−1437.(46) Zhong, K.; Demeyer, P. J.; Zhou, X.; Kruglova, O.; Verellen, N.;Moshchalkov, V. V.; Song, K.; Clays, K. A Facile Way to IntroducePlanar Defects into Colloidal Photonic Crystals for PronouncedPassbands. J. Mater. Chem. C 2014, 2, 8829−8836.(47) Brullot, W.; Strobbe, R.; Bynens, M.; Bloemen, M.; Demeyer, P.J.; Vanderlinden, W.; De Feyter, S.; Valev, V. K.; Verbiest, T. Layer-by-Layer Synthesis and Tunable Optical Properties of Hybrid Magnetic−Plasmonic Nanocomposites Using Short Bifunctional MolecularLinkers. Mater. Lett. 2014, 118, 99−102.(48) Deng, T.; Cournoyer, J. R.; Schermerhorn, J. H.; Balch, J.; Du,Y.; Blohm, M. L. Generation and Assembly of Spheroid-Like Particles.J. Am. Chem. Soc. 2008, 130, 14396−14397.(49) Lumerical Solutions, Inc. Http://Www.Lumerical.Com/Tcad-Products/Fdtd/ (accessed Apr 1, 2016).(50) Johnson, P. B.; Christy, R. W. Optical Constants of the NobleMetals. Phys. Rev. B 1972, 6, 4370−4379.(51) Palik, E. Handbook of Optical Properties of Solids; 1985.(52) Hao, F.; Larsson, E. M.; Ali, T. A.; Sutherland, D. S.;Nordlander, P. Shedding Light on Dark Plasmons in Gold Nanorings.Chem. Phys. Lett. 2008, 458, 262−266.(53) Nordlander, P. The Ring: A Leitmotif in Plasmonics. ACS Nano2009, 3, 488−492.(54) Verellen, N.; Van Dorpe, P.; Huang, C.; Lodewijks, K.;Vandenbosch, G. A.; Lagae, L.; Moshchalkov, V. V. Plasmon Line
ACS Applied Materials & Interfaces Research Article
Shaping Using Nanocrosses for High Sensitivity Localized SurfacePlasmon Resonance Sensing. Nano Lett. 2011, 11, 391−397.(55) You, L.; Mao, Y.; Ge, J. Synthesis of Stable SiO2@Au-NanoringColloids as Recyclable Catalysts: Galvanic Replacement Taking Placeon the Surface. J. Phys. Chem. C 2012, 116, 10753−10759.(56) Wilson, K.; Homan, K.; Emelianov, S. Biomedical Photo-acoustics Beyond Thermal Expansion Using Triggered NanodropletVaporization for Contrast-Enhanced Imaging. Nat. Commun. 2012, 3,618.(57) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. AHybridization Model for the Plasmon Response of ComplexNanostructures. Science 2003, 302, 419−422.(58) Ye, J.; Van Dorpe, P.; Lagae, L.; Maes, G.; Borghs, G.Observation of Plasmonic Dipolar Anti-Bonding Mode in SilverNanoring Structures. Nanotechnology 2009, 20, 465203.(59) Ayala-Orozco, C.; Liu, J. G.; Knight, M. W.; Wang, Y.; Day, J.K.; Nordlander, P.; Halas, N. J. Fluorescence Enhancement ofMolecules inside a Gold Nanomatryoshka. Nano Lett. 2014, 14,2926−2933.(60) Teperik, T.; Degiron, A. Numerical Analysis of an OpticalToroidal Antenna Coupled to a Dipolar Emitter. Phys. Rev. B: Condens.Matter Mater. Phys. 2011, 83, 245408.(61) Zhang, T.; Lu, G.; Li, W.; Liu, J.; Hou, L.; Perriat, P.; Martini,M.; Tillement, O.; Gong, Q. Optimally Designed Nanoshell andMatryoshka-Nanoshell as a Plasmonic-Enhanced Fluorescence Probe.J. Phys. Chem. C 2012, 116, 8804−8812.
ACS Applied Materials & Interfaces Research Article
