10 Recent Patents on Nanotechnology 2009, 3, 10-20

1872-2105/09 $100.00+.00 © 2009 Bentham Science Publishers Ltd.

Recent Progress in Fabrication of Anisotropic Nanostructures for Surface-Enhanced Raman Spectroscopy

Teng Qiu1,2,

*, Wenjun Zhang2, Paul K. Chu

2,*

1Department of Physics, Southeast University, Nanjing 211189, P. R. China,

2Department of Physics and Materials

Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, P. R. China

Received: September 24, 2008; Accepted: October 7, 2008; Revised: October 9, 2008

Abstract: Surface-enhanced Raman scattering (SERS) is recognized as one of the most sensitive spectroscopic tools

offering highly sensitive chemical and biological detection. The fact that particle plasmon allows direct coupling of light

to resonant electron plasmon oscillation has spurred tremendous efforts in the design and fabrication of highly SERS-

active substrates in nanostructured films and metallic nanoparticles. Theoretical studies have shown that symmetry

breaking allows for more complex plasmon propagation, potentially leading to more intense electromagnetic field

generation along the structure and in gaps formed between these materials. Anisotropic metallic nanostructures have all of

the characteristics that make them excellent candidates as SERS substrates. Thus, SERS is expected from anisotropic

materials. This review focuses on the progress and advances in the design and fabrication of anisotropic nanostructures for

SERS, with an emphasis on future challenges.

Keywords: Surface-enhanced Raman scattering, surface plasmon resonance, anisotropic nanostructures, nanorods, nanowires, fractals, nanoprisms, noble metals.

INTRODUCTION

Surface-enhanced Raman scattering (SERS) has drawn substantial attention since its discovery in 1974 [1]. The discovery of SERS has opened up a promising way to overcome the low sensitivity problem plaguing traditional Raman spectroscopy. SERS not only improves the surface sensitivity which makes Raman spectroscopy more applicable but also generates a stimulus for the study of the interfacial processes involving enhanced optical scattering from adsorbates on metal surfaces [2]. The advent of SERS has spurred a worldwide effort to explore its origins, optimize it, and harness its potential in fields ranging from plasmonics to diagnostics [3-6].

The SERS effect has predominantly an electromagnetic origin that arises from an increase in the local optical field exciting the molecule and multiplicative amplification of the re-radiated Raman scattered light [2, 7]. This optical enhan-cement is commonly associated with the excitation of surface plasmon oscillations in most SERS systems [8]. Nanostructured surfaces often lead to surface plasma reso-nance formation. When the nanostructured metal surface is irradiated by a laser beam, coupling between the metal surface and electromagnetic radiation of the light occurs and the coherent oscillation of the free electrons confined on the nanometer-sized metal surface is called localized surface plasma resonance [9]. The oscillation usually gives rise to intense absorption in the near-IR and visible-near UV region, which can be experimentally observed and recorded by UV-visible spectroscopy. According to Mie’s theory [10], the surface plasma resonance normal modes of oscillation are resonant with both the excitation and scattered photon for

*Address correspondence to these authors at the Department of Physics, Southeast University, Nanjing 211189, P. R. China; Tel: +86 25 52090600

8305; Fax: +86 25 52090600 8204; E-mails: tqiu@seu.edu.cn; paul.chu@cityu.edu.hk

surface features smaller than the incident optical wavelength. The frequency of the surface plasma resonance depends on the dielectric constant of the metal which is responsible for the SERS effects observed from only a few metals such as Cu, Ag, Au, and alkali metals.

Accompanied by improved understanding of SERS mechanism, it is interesting to note that SERS is one of the few truly nanoscale effects [11]. SERS is strongly linked to nano-science. The SERS intensity decreases significantly as one constructs structures that are either much larger than ~100 nm or much smaller than ~10 nm such as nano-particles, nanorods and/or nanowires [12]. However, the SERS enhancement mainly depends on the geometrical configuration of nanoparticles. That is, the enhancement is induced by the aggregation state which can generate the corresponding plasmon resonance [13].

The fact that particle plasmon allows direct coupling of light to resonant electron plasmon oscillation has spurred tremendous efforts in the design and fabrication of highly SERS active substrates in nanostructured films and metallic nanoparticles [14, 15]. The most established substrates are ones sprayed with Ag or Au colloids that yield intense SERS signals at certain local junctions. Junctions between aggregated nanoparticles are believed to be SERS “hot spots” where large field enhancements down to the single molecule have been observed [16]. This is the result of localized surface plasma resonance coupling between nanoparticles and enhanced electromagnetic field intensity localized at nanoparticle junctions [17]. Though spraying Au or Ag colloids on a substrate leads to an extremely high SERS signal at some local “hot spots”, it is not easy to achieve a reliable, stable, and uniform SERS signal spanning a wide dynamic range using this method to date. This is because isolated particles and close-packed crystal-like structures that exhibit weak SERS enhancement are also components of the colloids. Currently, the most studied

SERS-Active Anisotropic Nanostructures Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 11

approach is bulk solution-based biochemical detection using colloidal noble nanoparticles [18]. However, such substrates suffer from limited stability and reproducibility and in general, are not amenable to large-scale production of SERS-based sensors. Theoretical studies have shown that symmetry breaking allows for more complex plasmon propagation, potentially leading to more intense electromagnetic field generation along the structure and in gaps formed between these materials [19-20]. Hence, SERS is expected from anisotropic materials.

Anisotropic metallic nanostructures have all of the characteristics that make them excellent candidates as SERS substrates. Some of key issues are listed below:

(1) Plasmon absorption bands can be tuned by adjusting the nanocrystal aspect ratio to be in resonance with common laser radiation sources to optimize the electromagnetic enhancement mechanism.

(2) Symmetry breaking allows for more complex plasmon propagation, potentially leading to more intense electromagnetic field generation from the structure and in gaps formed between these materials.

(3) Assembled or aggregated nanorods and nanowires can be designed such that analyte molecules adsorb in the fractal space between nanoparticles or in SERS “hot spots”. This gives rise to large field enhancement.

(4) Anisotropic nano-objects such as nanorods and nanoprisms have been shown to possess interesting size- and shape-dependent properties, thus motivating interest in the controlled assembly of them into functional architectures for SERS.

(5) Anisotropic nanostructures have highly curved, sharp surface features with dimensions of less than 100 nm. This increases the localized electromagnetic field up to a hundredfold and it is referred to as the “lightning rod” effect [21].

NANORODS AND NANOWIRES

The most obvious example of anisotropic metallic nano-structures can be found in nanorods and nanowires con-taining a long axis that induces multipolar plasmon activity [22]. Researchers have fabricated nanorods and nanowires by many synthesis techniques [23-28]. However, most one-dimensional nanostructures are not ordered. Although amplified Raman spectra can be obtained, it is not easy to gain deeper insight into the SERS mechanism and to conduct theoretical work to design SERS-active substrates.

In order to fabricate ideal SERS-active substrates with uniform nanorod or nanowire arrays, nanoimprint litho-graphy (NIL), a new developed technique from conventional contact printing technique, has been applied to synthesis SERS-active substrate [29]. Figure. (1) shows typical five large-area homogeneously patterned SERS-active substrates fabricated by combined NIL and physical vapor deposition (PVD). In this technique, the size and shape of the Ag nanostructures are controlled and 2-naphthalenethiol is applied to detect the substrate SERS response using both 532 nm and 780 nm laser excitation. Using 532 nm excitation, the Ag nanostructures immobilized on the parallel line

Fig. (1). Scanning electron microscope (SEM) and atomic force

microscope (AFM) images of silver nanorods grown on flat (a),

grated (b), and pillared (c-e) substrates. The SEM scale bars are in

nm, and the AFM scans are 1 1 μm2 [29].

structure show the greatest enhancement compared to the plain Ag standard film. The optimal nanostructure is also observed during 780 nm excitation. The results indicate that NIL may be a powerful method for the fabrication of repro-ducible, large-scale SERS active substrates with optical tunability.

Some other scanning-probe-based lithographic techni-ques such as dip-pen nanolithography are also used to fabricate one-dimensional nanostructures [30]. However, SERS research in this area has been relatively rare. The possible reason is that these lithographic techniques, also called point-by-point approaches, need a long time to fabricate regular nanostructures. Another shortcoming is that

12 Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 Qiu et al.

it is not easy to fabricate high aspect ratio structures and obtain large area substrates.

Another important nanofabrication technique is template synthesis. Anodic aluminum oxide (AAO) templates synthesized by electrodeposition have been found to be stable at high temperature and in organic solvents. It has been shown to be a low cost, high yield, and high throughput technique to produce large arrays of nanorods and nanowires [31]. The primary advantage of this method is ease of materials handling. The aluminum substrate provides both mechanical support and electrical contact. The thickness of the oxide layer can be varied freely from being very thin to very thick. Different pore size of AAO templates can be achieved by controlling the reaction time and the density of pores can be controlled by the applied voltage.

The AAO template was firstly used to fabricate Ag substrate for SERS by electrochemical deposition and partial removal of the oxide layer in 1995 [32]. Although the obtained benzoic acid SERS signals were weaker than those from the as-synthesized Ag colloid surfaces, AAO template synthesis opens the door to the fabrication of ordered one-dimensional nanostructured SERS substrates. Recently, Wang and Co-workers presented a SERS-active substrate made of an array of Ag rod-like nanoparticles partially embedded in AAO with nanochannels [33]. In the experiments, arrays of nanochannels are obtained with a wall thickness of 5 ± 2 nm by carefully controlling the etching time. This etching process allows fine tuning of the gap between the nanostructures in an array, as the walls separate the deposited Ag deposited in the nanochannels during the subsequent electrodeposition process. Figure. (2) depicts a collection of SEM and transmission electron microscopy (TEM) images of the Ag nanostructures. The results indicate that it is possible to fabricate Ag rod-like nanoparticle arrays with a mean interparticle gap as small as 5 + 2 nm. Theoretical and experimental studies have indicated that precise control of the gaps between nanostructures on a SERS-active substrate in the sub-10 nm regime, which is extremely difficult to obtain by current lithographic techniques, is likely to be critical for the fabrication of substrates with uniformly high enhancement factors and for better understanding of the collective surface plasmons existing inside the gaps. The authors have found that the substrate exhibits an ultrahigh Raman signal enhancement factor due to the unprecedented narrow gaps between the Ag nanoparticles. This experiment represents the first quan-titative observation of the collective SERS effect on a substrate with precisely controlled ‘hot junctions’ in the sub-10 nm regime and confirms the theoretical prediction of interparticle- coupling- induced Raman enhancement.

The same strategy has also been shown to fabricate Ag nanowires for SERS-active substrate [34, 35]. Figure (3) shows a collection of SEM images of benzenethiol-modified Ag-AAO templates obtained by partial dissolution of the alumina matrix. The “add-analyte-then-etch” and “etch-then-add-analyte” protocols are utilized to detect benzenethiol as the Raman probe. The results show that the analyte molecules pre-adsorbed on the nanowire tip clefts and gaps contribute mostly to the amplification of the Raman spectra

Fig. (2). Top-view SEM image of an AAO substrate with D

(particle diameter) = 25 nm and W (interparticle gap) = 5 nm, a)

before and b) after growth of Ag nanostructuress. c,d) Histograms

of D and W, respectively. e) TEM images of the Ag nanostructures.

f) SEM image of a substrate with D= 25 nm and W= 15 nm [33].

signals because of the benefit from the giant electrochemical fields between the nanowire junctions. The strategy of preparing SERS activated substrates from the AAO template is a promising protocol to fabricate reproducible substrates. The “add-analyte-then-etch” way can avoid contamination because the analyte is trapped in the nanowire junctions. Moskovits and co-workers have focused on Ag nanowires while Tian’s group has synthesized different noble and transition metal nanowires of different widths and lengths by the AAO template to develop SERS-active substrates. Even though Ni and Co are typically considered SERS-inactive, a slight response was observed [36-37]. The experimental and theoretical results show the ordered transition metal nanowires with suitable dimensions can serve as high SERS active substrates.

Recently, Zhang et al. reported a new and facile method to prepare large-area silver-coated silicon nanowire arrays for SERS-based sensing [38]. Silicon is an important semiconducting material and its nanostructures are pro-mising candidates for future applications in integrated optoelectronic nanodevices. The synthesis of uniform silicon nanostructures with advanced characteristics has become

SERS-Active Anisotropic Nanostructures Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 13

quite accessible and well controlled by using well-established silicon fabrication technology. Therefore, it is important that SERS substrate fabrication techniques are compatible with silicon fabrication technology, since SERS performance may be improved by designing the advanced characteristics of silicon nanostructures as desired. The procedures for the fabrication of the silver-coated silicon nanowire arrays involve three main steps as illustrated in Figure (4). Firstly, high-quality silicon nanowire arrays are prepared using simple chemical etching of a silicon wafer in an aqueous HF solution containing Ag

+ ions. It is a process

based upon silver seed-induced excessive local oxidation, dissolution of the silicon wafer [39], and subsequent modification by 3-aminopropyltrimethoxysilane (APTMS). After drying in air, the APTMS-modified silicon nanowire arrays are dipped in an aqueous solution containing gold nanoparticles with an average diameter of 3 nm for 10 h. The small gold nanoparticles are immobilized on the surface of the silicon nanowires via the amine group. Finally, the resulting gold-decorated silicon nanowire arrays are immersed in a plating solution to deposit silver. Using this technique, it is possible to control silver deposition, consequently ensuring homogeneous growth of the silver film on the silicon nanowire arrays. The optimized silver-coated silicon nanowire arrays have good potential in ultra-sensitive molecular sensing offereing high SERS signal enhancement ability, good stability, and reproducibility.

FRACTAL NETWORKS

There are many phenomena in which symmetries, if existing at all, are hidden, for instance, growth through accretion of clusters such as soot particles and algal colonies, growth of thin film by surface etching and metal sputtering, structure of the porous media, globular polymers and proteins, randomly branched objectives, and so on. Fractal substrates are among such phenomena. They are attractive

Fig. (4). Scheme of the fabrication process of the silver-coated

silicon nanowire arrays [38].

but quite complex when it comes to deeper understanding of the mechanisms, structures, and properties. The so-called “disordered” systems would result in novel or/and better performance of a material paving the way for the more plentiful and powerful material resources.

Since the concept and dimension of fractal were first presented by Mandelbrot [40], the study on fractal has been extended to both theoretical and application aspects. In particular, fractal structures of noble metals, including metal-polymer composites, have attracted much attention in the

Fig. (3). SEM images of benzenethiol -modified Ag-AAO templates obtained by partial dissolution of the alumina matrix in 0.1 mol/L

aqueous NaOH for (a) 0 s, (b) 210 s, (c) 270 s, and (d) 450 s [34,35].

14 Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 Qiu et al.

past two decades [41-43]. Metal fractal nanomaterials permit local-field enhancement in a broad spectral range [44]. This property is crucial to the design of optical materials with broadband amplification of nonlinear responses and various types of spectroscopy. The optics of disordered nano-materials displays a large variety of effects some of which are hardly intuitive. Field localization of various sorts occur and recur in a wide gamut of disordered systems, most strikingly in those possessing dilation symmetry, leading to the enhancement of many optical phenomena especially nonlinear processes [45]. The enhanced local responses of random nanocomposites can be used by various types of spectroscopy, including SERS of single molecules [46-47].

The diffusion-limited aggregation model [48] and cluster-cluster aggregation model [49-50] are used to explain and analyze these fractal phenomena. Of the various methods available to prepare nanoscale materials, the template method in which the desired materials are encapsulated into the channels or pores of a host has a number of interesting and useful features, since the size and shape of the desired materials can be easily altered using a well-defined template matrix [51-53]. Many templates have emerged, such as carbon nanotubes [54], porous anodic alumina [55], track-etch polycarbonate membranes [56], micelles [57,58], block copolymers [59], hybrid organic-inorganic mesoporous materials [60], and so do electrodeposition methods [61].

Fig. (5) shows a typical SEM image of large silver fractal networks using the method of self-selective electroless plating [39, 62]. The growth of silver fractal networks under nonequilibrium conditions should be considered within the framework of a diffusion-limited aggregation process [48] which involves cluster formation by the adhesion of a particle with random path to a selected seed upon contact and allows the particle to diffuse and stick to the growing structure. The silver fractal networks have a high SERS enhancement factor and large dynamic range as demonstrated in Fig. (6). The observed SERS efficiency can be explained in terms of strongly localized plasmon modes relative to the single particle plasmon resonance.

Kucheyev and co-workers reported the development of nanoporous Au (np-Au) as a highly active, stable, tunable, biocompatible, reusable, and affordable (particularly when used as a thin nanoporous Au film on a low-cost substrate) SERS substrate [63]. Other attractive features of np-Au are that it is compatible with well-studied self-assembled monolayers of thiols and can be used as linking layers in advanced sensor applications. The dependence of the average pore width obtained by statistical analysis of the SEM images is illustrated in Fig. (7). It shows that the largest SERS enhancement factors, with crystal violet as a test molecule and 632.8 nm laser excitation, are observed from np-Au with an average pore width of ~ 250 nm. This shows that the fractal, nanoscale morphology of the np-Au surface can result in complex patterns with electromagnetic field enhancement.

Silver nanodendrites which are natural, large fractal aggregates are often fabricated for SERS. Fig. (8) shows a collection of TEM images of different Ag nanodendrites obtained by a simple surfactant-free method using a

Fig. (5). (a) SEM image of the silver fractal networks. (The

growing time is 20 min.) (b) The growing-time dependent

morphologies of the silver structures, demonstrating the diffusion-

limited aggregation process. (The growing time is 1 min, 5 min, 10

min, and 60 min) [62]

Fig. (6). (a) SERS spectra of 10–6

M R6G adsorbed on the silver

fractal networks (top) and that adsorbed on the silver nanocrystals

(bottom). (b) SERS spectra of R6G adsorbed silver fractal networks

with different molar concentrations, demonstrating the intensity

variation of the SERS signals. (c) SERS signal at 1364 cm-1

as the

function of the molar concentration on a logarithmic scale. The

solid line is a guide to the eye [62].

suspension of zinc microparticles as a heterogeneous reducing agent [64]. In Figs. (8a) and (8b), the nascent small Ag nanoparticles may be active enough to undergo oriented attachment. In addition, the concentration gradient of the Ag nanocrystal precursors sets up a uniform diffusion front,

SERS-Active Anisotropic Nanostructures Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 15

Fig. (7). Typical SEM images (primary electron energy is 5 keV)

illustrating the surface morphology of as-dealloyed np-Au (a) and

np-Au annealed for 2 h at 300 °C (b), 450 °C (c), and 550 °C (d).

The scale bar is 1 μm in all four images [63].

Fig. (8). TEM images of different Ag nanostructures obtained from

the reaction: (a) sparse dendritic structure; (b) dense, symmetric

dendritic structure; (c) dense asymmetric dendritic structure; (d) Ag

nanocrystals sprecursors of the dendritic structures [64].

leading to the formation of the symmetrical dendrites with preferential crystal orientations dominating the reaction products. In Fig. (8c), diffusion plays a dominant role and the Ag nanocrystals are less active so that they are locked in the position once they are in contact with each other. This makes the angles of the stem/branch and branch/subbranch vary widely between 15 and 90° instead of 55° observed in the symmetrical dendrite. In this case, there are perhaps large concentration differences in different growth sites and as a result, asymmetrical dendrites are formed. SERS studies show that the Ag nanodentrites provide intense and enhanced

Raman scattering when pyridine is used as a probing molecule.

Ag flowerlike fractal networks have recently been prepared to serve as SERS-active substrates [65]. A theo-retical examination of the local electromagnetic properties by the finite difference time domain method has been used to assess the relative contributions of different geometries to the experimentally observed SERS intensities. It should be noted that the finite difference time domain approach has recently been shown to be highly useful in the study of the electromagnetic properties of metallic nanostructures for almost arbitrary complexity and this is the first time that the method has been used to examine the local electromagnetic properties of noble metal fractals. The authors suggest that strong SERS enhancement from the flowerlike pattern can be attributed to the fact that the flowers are assembled on the Si substrate with a very high density and with many horns. Therefore, there are hot spots galore. A typical flower with additional nanoparticles is shown in Fig. (9a). Fig. (9b) is a magnified model pattern for the finite difference time domain calculation. By careful observation of the local electromagnetic field distribution of around the leaves of the flower, one can find that a lot of hot spots exist near the interstitial areas of the leaves, e.g., spot 2 (<10 nm) and spot 3 (~10 nm) in Fig. (9c). In the interstitial areas between nanoparticle and leaf, the local electromagnetic filed still presents a weak enhancement even though they are relative close, e.g., spot 1 (~10 nm) in Fig. (9c). For a longer distance, e.g., spot 4 (~40 nm) in Fig. (9d), there is weaker interaction. However, the interstitial areas between nanoparticles, e.g., spot 5 (~30 nm) in Fig. (9d) show a very weak interaction. The interaction between leaves can contribute to a stronger local electromagnetic field enhancement than that between particles. This may be one of the reasons why the flowerlike patterns possess better SERS properties according to the finite difference time domain calculation and experimental verification.

Fig. (9). (a) A magnified Ag flower-like pattern. (b) A magnified

model pattern. (c) and (d) A magnified leaves area from (b) [65].

16 Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 Qiu et al.

More complex gold flowerlike nanoarchitectures have been fabricated on indium tin oxide coated glass using a low-cost electrochemical method by using a proper current density and polyvinylpyrrolidone (PVP) concentration in electrolyte [66]. The flowerlike nanoarchitectures are com-posed of blocks of two-dimensional flakelike nanostructures as shown in Fig. (10). Such gold flowerlike nanoarchitec-tures show strong SERS effects that are attributable to their geometries and are stronger than those from other particle films.

Fig. (10). SEM images of the as-prepared gold flowerlike nano-

architectures, electrodeposited at the PVP concentration of 20 g/l

and cathodic current density of 0.25 mA/cm2, after deposition for

(a) 72 min and (c) 36 min. (b) A local enlarged image of (a) [66].

NANOPRISMS

Two-dimensional assemblies of nanometer-sized building blocks with ordered structures have been used in a variety of important applications because their collective electronic, optical, and magnetic properties are distinctly different from those of the corresponding individual nano-particles or the bulk solid [67-68]. However, most studies have concentrated on the assembly of isotropic spherical structures. Recently, anisotropic assembly of anisotropic nano-objects into controlled structures have attracted considerable attention because they show new and/or improved properties depending on the direction of assembly, spatial arrangement, and the degree of order among the individual building blocks [69,70]. For instance, Bae et al. have reported the assembly of Ag nanoprisms with aniso-tropic orientation and the orientation-dependent properties of these assemblies [71]. The almost Ag nanoprisms are truncated and their average edge length is 30 ± 7 nm. Although the edge lengths of the nanoprisms are not homogeneous, their thicknesses (4.5 nm) are remarkably similar (Fig. (11a) and (b)). Using these Ag nanoprisms, two kinds of assembled structures are fabricated. One tends to

stack face-to-face in rows with their edges perpendicular to the substrate (Fig. (11a)). Alignment of the nanoprisms is not affected by the type of substrate and they always assemble with the upright orientation on different substrates, indicating that the attractive force between nanoprism faces should be stronger than that between the face and substrate. The other is fabricated by using the interfacial entrapment method directing the self-assembly of nanoprisms at the liquid/liquid interface in face-down fashion (Fig. (11b)). The nanoprisms assembled on glass substrates exhibit the characteristic plasmon absorption as the direction of the prisms in the assemblies (Fig. (11c)). Assembly B composed of horizontally oriented Ag nanoprisms exhibits only one peak at 670 nm whereas Assembly A comprising vertically oriented Ag nanoprisms shows one weak broadband centered at 504 nm with a shoulder at ~ 360 nm. The different observed optical properties of the two assemblies can be understood by the assumption that incident light should excite mostly in-plane and out-of-plane plasmon modes for Assemblies B and A, respectively due to the specific orientations of nanoprisms in the assemblies. Assemblies A and B also have different (X-ray powder diffraction) XRD patterns due to the directional alignment of the nanoprisms, thus demonstrating that the exposed crystallographic planes as well as optical properties can be controlled by anisotropic assembly of Ag nanoprisms.

Fig. (11). TEM images of Assembly A (a) and B (b). (c) UV-vis

spectra of Assembly A and B, and Ag nanoprisms in solution. (d)

XRD patterns of Assembly A and B [71].

It is interesting that the SERS intensities of the probes on Assembly A and B depend on the excitation wavelength by using 4-aminobenzenethiol (4-ABT) as a model compound. Assembly A exhibits stronger enhancement at 514 nm excitation than at 633 nm, whereas Assembly B shows an opposite trend as indicated by Fig. (12). It is also found that the extent of charge transfer is different between assemblies and result in different relative enhancement of b2 modes of 4-ABT. It is well known that the symmetric b2 modes of 4-ABT adsorbed on Ag are selectively enhanced in SERS via the charge transfer from Ag to 4-ABT. Thus, the Ag nano-

SERS-Active Anisotropic Nanostructures Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 17

prisms with controlled arrangements show distinct optical, crystallographic, and SERS properties depending on their orientation in the assemblies.

Although chemical approaches are capable of producing nanoprisms with sharp corners, they lead to disordered systems that cannot be easily used or manipulated for reproducible and predictable SERS enhancement. Cui and co-workers have reported an alternative technique to fabricate periodic nanoprism arrays over a large area by nanoimprint lithography which can produce non-equilateral nanoprisms that offer an additional degree for tuning and boosting the plasmonic properties [72]. The SEM image of the mould bearing an inverse-pyramid-shaped hole array is shown in Fig. (13a). Fig. (13b) shows an array of 20 nm Ag nanoprisms (with 2.5 nm Cr as adhesion layer) with an edge length of about 100 nm and a period of 200 nm. Three-dimensional nanopyramids can be fabricated by prolonged deposition as the holes become gradually closed by the evaporated materials. Fig. (13c) shows an array of nanopyramids after deposition and liftoff of 5 nm Cr as an adhesion layer and 100 nm Ag. The authors have used the discrete dipole approximation with a dipole grid length of 3 nm to calculate the near field and the absorption efficiency of the nanoprisms. As shown in Fig. (14a), the near field for a freestanding Ag nanoprism with an edge length of 100 nm and a thickness of 20 nm is greatly enhanced at the sharp corners of the nanoprism. However, the near field drops significantly with the addition of 5 nm of Cr (Fig. (14b)).

COMPLEX NANOSTRUCTURES

In addition to metal nanoprisms, more complex structures have been fabricated. Lee and co-workers have developed three-dimensional crescent-shaped nanoparticles that contain a sub-10 nm gap that eliminates the need for multiple particles to form a dimer or other plasmonic superstructure in order to generate strong SERS effects (Fig. 15a)) [73]. The materials and multilayer thickness are intentionally designed with the assistance of finite-element simulation in order to tune the plasmon-resonance wavelength of the composite nanocrescent matched with the excitation wave-length. The multilayer structure is not clearly distinguishable in the TEM image (Fig. (15b)) due to the low contrast between the different metallic materials of Au, Ag, and Fe. The nanocrescents suspended in the fluids are then controlled by the magnetic field during SERS imaging (Fig. (15c)). The Raman enhancement factor of a single nanocrescent is as high as those reported for nanoparticle clusters and it is suitable for high-resolution biomolecular sensing in living cells. The orientation modulation of nanocrescents by magnetic fields can further increase the signal-to-noise ratio in dynamic SERS detection.

CURRENT & FUTURE DEVELOPMENTS

Although substantial progress has been made with regard to the technical innovation in the fabrication of anisotropic nanostructures for SERS, there are challenges in the road-

Fig. (12). SERS spectra of 4-ABT adsorbed on Assembly A (a) and B (b) with 514 and 633 nm excitations. (c) The relative intensities of the

b2 modes of 4-ABT adsorbed on Assembly A and B [71].

18 Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 Qiu et al.

Fig. (13). SEM image of the nanosphere lithography mould with

200 nm period inverse-pyramid-shaped hole array; (b) SEM image

of a 200 nm period nanoprism array of 20 nm Ag (with 2.5 nm Cr

adhesion layer); (c) SEM image of a nanopyramid array resulting

from the evaporation of 100 nm Ag (with 5 nm Cr adhesion layer)

and subsequent liftoff. Sample tilted 75º during imaging [72].

Fig. (14). Near field of a freestanding 20 nm-thick Ag nanoprism

without (a) and with (b) a 5 nm Cr adhesion layer calculated by

discrete dipole approximation at = 775 nm. The arrow indicates

the polarization of the incident light [72].

Fig. (15). Composite material magnetic nanocrescent SERS probes.

(a) Schematic diagram of SERS detection on a single composite

nanocrescent. (b) TEM image of a single magnetic nanocrescent

SERS probe. The scale bar represents 100 nm. (c) Schematic

diagram of a SERS imaging system and the magnetic manipulation

system for intracellular biomolecular imaging (in fluids) using

standalone magnetic nanocrescent SERS probes [73].

map before these techniques can be used more extensively in practical and successful SERS-based systems. The key issue is that anisotropic assembly of anisotropic nano-objects into ordered structures is still an important but hard-to-control

SERS-Active Anisotropic Nanostructures Recent Patents on Nanotechnology 2009, Vol. 3, No. 1 19

task because they show new and/or improved properties depending on the direction of assembly, spatial arrangement, and the degree of order among the individual building blocks. Recent results also suggest that the controlled assembly of anisotropic nanostructures can be used as a powerful tool in the study of their physicochemical pro-perties and creation of new classes of functional materials.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 50801013), the Scientific Research Fund of Southeast University for New Teacher (Grant No. 9207022416), and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 112307.

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