3.1.

Nanospheres

Chemically synthesized gold and silver nanoparticles of spherical geometry are the one of the oldest and most popular substrate in SERS studies.^{25,26,28,41–45} Their preparation is simple, cost-effective, and have been adapted for various SERS-based applications. The most common strategy to prepare them is to reduce metal salt with a relevant reducing agent and a capping agent. There are two methods of preparation of plasmonic nanoparticles that are extensively employed in SERS: 1. the Lee and Meisel method,⁴² which is based on citrate reduction of either ${\mathrm{AgNO}}_3$ or ${\mathrm{HAuCl}}_4$ and the resultant silver or gold nanoparticle has an average size of around 60 nm; or 2. the Creighton method,⁴³ which uses ice-cold sodium borohydride to reduce ${\mathrm{AgNO}}_3$ or ${\mathrm{HAuCl}}_4$ and leads to nanoparticles of average size around 20 nm. Both these methods give excellent enhancement in Raman signature and have been employed in various SERS experiments.

Although individual nanoparticle themselves facilitate Raman enhancement, it is the junction of these nanoparticles^46–48 which provide enhanced optical fields that makes single molecule SERS^40,41 based detection a possibility. This has motivated various researchers to systematically study the dimer^{32,34,37,46–52} and multiple junction nano-lens structures⁵³ of plasmonic nanoparticles. The gap junction between the nanoparticles is essentially an SERS hot-spot.^54–57 Any molecule in this junction is subjected to an enormous electric field, and hence their Raman scattering signals are enhanced by many orders of magnitude. In recent years, many innovative methods⁵⁸ have been employed to produce plasmonic nano-dimers. For example, Li et al.,⁴⁶ have introduced a simple chemical synthesis method that can generate dimers of silver nanospheres [Fig. 1(a)]. Their design yielded dimers of single crystal silver nanosphere ($\sim 30\mathrm{nm}$ in diameter) and with a gap of around 2 nm. Such controlled and repeatable procedures provide well-defined systems to study SERS hot spots. The authors recorded SERS spectra of isolated nanoparticles, well-separated nanoparticles, and closely interacting dimer structures, as shown in Fig. 1(b). They observed that the molecular signature was evident only in the dimer configuration indicating that the contribution of the gap junction facilitates enhanced optical field leading to SERS signature of the molecule.

Fig. 1

(a) TEM images of silver nanosphere dimers with the inset showing a schematic illustration of nanocrystals. (b) SERS spectra of 4-methyl-benzenethiol taken from (top) a dimer of silver nanospheres, (middle) two silver nanospheres separated by $\sim 600\mathrm{nm}$, and (bottom) a single silver nanosphere. The scale bars in the insets correspond to 50 nm. Reproduced with permission from Ref. 46.

To facilitate SERS hot spots on individual nanoparticles, sharp metallic features can be incorporated on the surface of nanospheres. Recently, such nanostructure, usually called as spiked nanoparticle or nanostars,^59–63 have been utilized to obtain large electric field enhancements.

Another important category of nanoparticles are the core-shell nanostructures,⁶⁴ or the nanoshells.^65–67 One of the major advantages of the shell-based nanoparticle is the ability to tune the plasmon resonance by tuning the physical and chemical parameters of both the shell and the core substance.^13,68,69 Such nanoshell geometries have now found applications as tunable SERS substrates.¹³ An added advantage of this configuration is that two different kinds of nanomaterials can be incorporated as core and shell. For example, one can synthesize a nanoparticle with magnetic core and a plasmonic shell,⁷⁰ thereby making the nanoparticle not only sensitive to light but also an external magnetic field. Such magnetic core metallic nanostructures can be utilized as probes for magnetic resonance imaging (MRI), which is an important diagnostic tool. Due to their versatile optical properties, nanoshells also have good potential as probes for photothermal treatment of cancer, as revealed recently.⁶⁸

3.2.

Nanocubes

Precisely shaped plasmonic nanocubes have been recently synthesized^71–78 and studied in the context of SERS. One of the advantages of nanocubes is their equidistant sharp edges that facilitate highly localized optical fields, which can be harnessed to enhance Raman scattering signatures. In order to test the effect of edge sharpness, Xia and co-workers prepared isolated Ag nanocubes with sharp and truncated edges⁷¹ as shown in SEM images of Fig. 2. A thin layer of 1,4-benzenedithiol (1,4 BDT) was adsorbed on these nanocubes and a linearly polarized Raman excitation laser of 514 nm was incident on these individual nanocubes and the polarization was varied as shown in the Fig. 2. The Raman spectra in Fig. 2(a)–2(c) represent the polarization dependent SERS signatures from sharply-edged Ag nanocubes corresponding to the directions indicated on the SEM images. The data indicated that when the polarization is aligned along one of the edges of the nanocube, the Raman signal intensity is enhanced. In contrast, Fig. 2(d)–2(f) shows that when the edges are truncated, the SERS signature is independent of the polarization of Raman excitation. This observation was also supported by polarization dependent discrete dipole approximation calculation of near-fields, which indicated that field enhancement along the diagonal of the sharp-edged nanocubes was greater for the aligned electric field. Camargo et al.⁷⁴ have further extrapolated this work to show that SERS hot-spots can be isolated and probed by bringing two silver nanocubes close to each other. By employing a technique based on plasma etching of molecules, they isolated the molecules from all parts of the nanocube dimer except for the gap between them, thereby exclusively measuring the SERS signal between the coupled nanocubes. Such innovation can be further harnessed and applied to other plasmonic dimer nanostructures, where the SERS contribution from the gap junction is to be probed exclusively. Recently, Rycenga et al.,⁷⁸ have also shown that single silver nanocubes placed on metallic substrates can facilitate single molecule SERS sensitivity. Their strategy was to use coupling between the sharp edges of the nanocube with metallic layer underneath it. This coupling effectively provides a reproducible hot-spot with large field enhancement, thereby leading to ultrasensitive SERS effect on a consistent basis. Plasmon coupling effects have also been observed⁷⁵ in two-dimensionally (2-D) ordered gold nanocubes on indium tin oxide substrates indicating that nanocube geometries are good substrates to study certain fundamental aspects of plasmonics and SERS.

Fig. 2

Normalized SERS spectra of 1,4-BDT adsorbed on a Ag nanocube with sharp corners [left panel, (a)–(c)] and a highly truncated Ag nanocube [right panel, (d–(f)], at various angles relative to.the polarization of the excitation laser. Each SEM image shows the nanocube used and the arrows indicate the polarization directions of the incident laser corresponding to the spectra. The scale bar applies to both images. Reproduced with permission from Ref. 

3.3.

Nanotriangle

Another important category of plasmonic nanostructure that has been studied in the context of SERS is nano-triangular patterns.^79–81 Nanotriangles have sharp edges at their vertex which facilitate localized optical fields, and hence can be harnessed for SERS applications. One of the effective ways to produce patterns of nanotriangle pioneered by Van Dyune and co-workers^82,83,84 is to employ nanosphere lithography (NSL), which is an inexpensive, versatile, and inherently parallel lithography method. Briefly, the procedure of NSL is as follows. First, size-monodisperse nanospheres are self assembled on a substrate such as glass to form a 2-D colloidal crystal deposition mask [see Fig. 3(a)]. Second, upon solvent evaporation, the desired metal is deposited either by thermal evaporation or electron beam deposition, or pulsed laser deposition through the nanosphere mask. Next, the nanosphere mask is removed by sonication of the sample in a solvent, thereby leaving behind the metal deposited in the interstitials of the nanosphere mask. A typical sample prepared using NSL is shown in Fig. 3(b). One of the major advantages of this technique is that it can be modified to vary the size, shape and arrangement of the plasmonic nanostructures.^85–89 Schmidt et al.⁹⁰ employed NSL to study SERS properties of triangular shaped silver nanocluster arrays. They made direct comparison of rhodamine 6G SERS signals between ordered nanocluster region and amorphous Ag regions in the same sample [Fig. 3(c)]. The authors found that for the case of nanotriangles, the SERS signal intensity was enhanced by a factor of 3 compared to amorphous Ag regions, indicating large electromagnetic fields facilitated by the edges of the nanotriangles.

Fig. 3

Schematic illustration of a colloidal crystal mask (a) and representative AFM image (b) of Ag nanotriangular stuructures. Reproduced with permission from Ref. 82. (c) SERS enhancement achieved from R6G adsorbed on ordered 200 nm Ag nanocluster regions (gray trace) in comparison with amorphous Ag “film” regions (black trace) within the same sample. The signal increase is $\sim 3\times$. Reproduced with permission from Ref. 90.

Another category similar to triangular geometry is the plasmonic nanopyramids.^91–94 Stoerzinger et al.⁹¹ have recently screened individual gold nanopyramid shells and their assembly in a systematic way. The authors employed fabrication methods involving phase-shifting photolithography, etching, E-beam, and lift off (PEEL) procedures to create plasmonic nanopyramids, and by controlling the evaporation rates of the solvent of nanopyriamids, they could assemble them in three different categories: isolated single pyramids, lower order assembly, and higher order assembly (Fig. 4). They observed that as the order of assembly increased, the Raman signal intensity increased. They supported their experimental data with numerical simulations that indicated enhanced electromagnetic field between adjacent particle faces in the higher order structure. Such hierarchical nanostructures are useful to produce few-particle localized electromagnetic fields in a controlled fashion.

Fig. 4

(a) (top) SEM and (bottom) corresponding Raman images of the $1624{\mathrm{cm}}^{-1}\mathrm{MB}$ vibrational mode intensity from a single tip-down pyramid (TD), a low-order dimer of pyramidal nanoshells (L2), and a high-order dimer of pyramidal nanoshells (H2). The scale bar applies to all images. (b) Dark-field scattering spectra of TD, L2, and H2. (c) Raman spectra corresponding to the most intense point of the Raman image in (a). Reproduced with permission from Ref. 91.

3.4.

Nano-Rods

Nanorods are one of the extensively studied anisotropic plasmonic geometries.^69,95–99 Various research groups have studied optical properties of gold nanorods^69,98–102 and have utilized them for a variety of applications.⁹⁷ Owing to their anisotropic geometry, nanorods exhibit two plasmon modes: transverse and longitudinal.⁹⁶ The transverse plasmon mode is at lower wavelength and the longitudinal mode is at higher wavelength. The longitudinal plasmon mode is sensitive to the aspect ratio of the nanorod, and hence can be utilized in wavelength dependent studies. One of the important issues in plasmonics is to understand how plasmons in individual nanostructures can be coupled to obtain enhanced electromagnetic interactions. In this context, gold nanorods have been employed to study the plasmon coupling in anisotropic nanostructures.^{69,98,101,103,104} Kumar and Thomas¹⁰⁴ recently investigated the SERS properties of individual nanorods and their assembly. The authors chemically couple the 111 plane at the edges of Au nanorods with linker molecules and show that the SERS signature is enhanced when the rods are in dimer geometry (Fig. 5). They further show that end-to-end coupling can be extended to form a linear chain of Au nanorods leading to an enhanced signature in Raman intensity. Such linear chains of anisotropic nanorods are good candidates to study sub-diffraction plasmonic coupling and electromagnetic transport.

Fig. 5

TEM images of gold nanorods (a)–(c) in isolated, dimer, and chain configurations; and Raman spectrum of molecule bipy-DT (d)–(f) at various stages of plasmon coupling. Reproduced with permission from Ref. 104.

Since Au nanorods are anisotropic in their geometry, they are sensitive to linear polarization of incident radiation. El-Sayed and co-workers¹⁰⁵ have utilized this polarization dependence to show that Au nanorods conjugated with antibodies can be aligned and assembled on the surface of cancer cells. They further show that such aligned assembly of nanorods can be used as cancer diagnostic markers. Such innovations have opened up avenues for biological applications of nanorod based SERS.

3.5.

Nano-Wires

Plasmonic nanowires are unique substrates as they support propagating plasmon polaritons.^12,106–111 They sustain plasmon modes with wavelengths shorter than the incident light. Interestingly, chemically synthesized plasmonic nanowires,^112,113 such as Ag nanowires, show minimal plasmon damping due to well-defined crystal structure, and hence have been employed in studies of plasmon polariton based waveguiding.^{111,114–118} In the context of SERS, coupling of an individual nanowire to another plasmonic nanostructure^119–121 creates a spatially confined SERS hot spot at the junctions. One of the recent innovations in nanowire-based SERS is the remote excitation of Raman scattering.^120,122 The authors have shown that the propagating plasmon property of the plasmonic nanowires can be used to perform Raman scattering at a remote location (see Fig. 6). By exciting the plasmon polariton at one end of the nanowire, a source of light can be created at the distal end of the nanowire, which further can be utilized as a Raman excitation source. The advantage of this technique is that the Raman excitation source is confined to the apex of the nanowire, and is devoid of background radiation of incident laser excitation (Fig. 6). Thus by separating the channels of excitation source and Raman scattering source, molecules in the vicinity of the distal end of the nanowire can be probed at high spatial resolution. Nanowires have also been utilized to create SERS hot-spots.^122–124 Recently, Chikkaraddy et al.¹²⁴ have shown that by coupling two plasmonic nanowires in an end-to-end configuration, SERS hot-spots can be created at the nanowire junction. The advantage of this method is that the end-to-end coupling configuration not only facilitates SERS hot-spot, but also provides SPP based light propagation capability from one end of the nanowire pair to other distal end.

Fig. 6

Transmission (a), wide-field illumination (b), and focused laser excitation (c) images of a NPs-pATP-nanowire exhibiting seven SERS hot-spots. Focused excitation was at the left end of the wire in panel (c). (d) 3-D view of the image in panel (c). Reproduced with permission from Ref. 122.

3.6.

Nanotips

Although SERS microscopy facilitates large scale enhancement factors, it is constrained by the diffraction limit, and hence cannot spatially resolve entities smaller than the Raman excitation wavelength. Tip enhanced Raman scattering (TERS)^125–129 has recently emerged as method to perform vibrational spectroscopy beyond diffraction limit. The technique employed in TERS is essentially borrowed from apertureless near-field optical microscopy, where a sharp metallic tip is brought into close proximity of the sample deposited on the surface ^130–132 The tip facilitates the localized optical field which interacts with molecules beneath it, thus enhancing the Raman signal intensity. One of the important components in TERS is the morphology and composition of the tip.^{125,133–140} The radius of curvature of the apex of the tip determines the spatial resolution,¹³⁴ and hence it is necessary to fabricate high quality plasmonic nano-tip. The most popular configuration of the tip used in TERS is a gold or silver nano-tip.¹³⁴ This is usually prepared by chemical etching of wire.¹³⁸ Another method to peform TERS is to employ a plasmonic nanoparticle attached to dielectric tip.^141,142

In recent times, an alternative method based on adiabatic focusing of plasmons at the apex of the tip has been introduced to perform TERS by Raschke and co-workers.¹⁴³ The advantage of this method is the non-local generation of nanoscale Raman excitation source which facilitates intrinsic background suppression of Raman pump beam. This was achieved by cutting a grating on a chemically etched plasmon nano-tip (Fig. 7) and exciting it with Raman pump beam. Propagating plasmons are generated due to this coupling, and further propagate towards the apex of the tip, thus creating a nanoscale light source at the discontinuity. This light source can be harnessed to perform Raman spectroscopy as shown in Fig. 7(c). Adiabatic nano-focusing is an efficient method of converting far-field optical signal into near-field localized light source,¹⁴⁴ and has immense potential to perform background-free nanoscale chemical spectroscopy and imaging.^145–149

Fig. 7

(a) Schematic of the adiabatic nanofocus based the Raman scattering experiment. Electrochemically etched Au tips are mounted onto the quartz tuning fork of a shear-force AFM, and a grating is cut using FIB. Incident light is focused onto the grating, and the Raman scattered light excited by the nanofocused SPP at the apex is detected at a 90-deg angle. (b) SEM micrograph of an electrochemically etched Au tip superimposed with optical image of grating illumination and apex emission. (c) Grating-coupled TERS distance dependence for $\lambda =632.8\mathrm{nm}$ of Malachite Green on a Au surface. Reproduced with permission from Ref. 143.

Recently, Capasso and co-workers¹⁵⁰ have also shown that optical antenna arrays can be fabricated on an optical fiber and can be utilized as SERS micro-probes, thus expanding the prospects of in situ SERS detection.