**2. Historical background and gradual development of SERS**

Raman spectroscopy is a spectroscopic technique based on molecular vibration and is dependent on the inelastic scattering of monochromatic light, usually from a laser in the visible, near-infrared, or near-ultraviolet range of electromagnetic spectra. This effect was discovered by famous Indian physicist Professor C.V. Raman in 1928 [10].

**Figure 1** shows the schematic diagram to explain the principle of Raman scattering. The weak Raman signal observed in conventional Raman spectroscopy can be explained by the low scattering cross section (~10−30 molecule cm−2). Therefore, Raman spectroscopy will provide low sensitivity in terms of signal and is the main reason for its inapplicability in practical fields for a long period [5–7]. For an extremely low scattering cross section in the range 1012–1014 molecule cm−2, usually present in a monolayer, it is very difficult to detect signal obtained from a Raman probe molecule even by the most effective modern and sophisticated Raman spectrometer. Raman spectroscopy is competent enough to obtain fingerprint information of species by detecting the vibrational bands. In Raman spectroscopy, the sample containing the Raman probe molecule is illuminated with a laser beam of appropriate wavelength. Wavelengths close to the laser line arises as a result the Rayleigh scattering is filtered out, while the rest of the unfiltered light is dispersed onto a detector. Spontaneous Raman scattering is very weak as compare to the Rayleigh scattering and identification and partition of these two signals are important to obtain a high quality Raman spectrum. Traditionally, holographic diffraction gratings are employed in Raman spectrometers to yield a high degree of laser rejection. On the other hand, modern instrumentation unanimously employs notch filters for laser rejection. Development of modern instrumentation along with the introduction of fast-Fourier transform (FFT)-based spectrometers, confocal microscopes, and charge coupled device (CCD) detectors has brought a new dimension in Raman instruments, providing very high sensitivity. **Figure 2** demonstrates the development of Raman instrumentation from the Raman spectrometer of C. V. Raman to the latest sophisticated Horiba-Jobin model of micro-Raman system.

which are ~ 12–14 orders of magnitude lower than the fluorescence cross section for various biological and organic molecules, which are highly fluorescent in nature [1–7]. Therefore, the discovery of Fleischmann and coworkers from the University of Southamption, UK in 1974 [8], which demonstrated the unexpected high Raman signals obtained from pyridine molecules adsorbed on a rough silver electrode, has attracted considerable attention of researchers from various fields such as physics, chemistry, biology, mathematics, and engineering. In a published paper, Fleischmann et al. reported an extraordinary million-fold enhancement of Raman signal from pyridine molecules adsorbed onto electrochemically roughened silver electrode compared to that from free molecules in a liquid environment [8]. Surface-enhanced Raman scattering (SERS) effect deals with the gigantic amplification of the weak Raman scattering intensity by molecules in the presence of a nanostructured metallic surface [5–8]. The SERS enhancement factor can be defined as the ratio between the Raman signals obtained from a given number of molecules in the presence and in the absence of the metal nanostructure and this factor is dependent largely on the size and morphology of the nanostructures.

Since its discovery in the year 1974, surface-enhanced Raman scattering (SERS) has attracted significant interest of researchers [1–7]. The discovery of SERS has opened up a promising way to overcome the low-sensitivity problem associated with conventional Raman spectroscopy. Introduction of the SERS technique not only improves the overall surface sensitivity making Raman spectroscopy more applicable but also stimulates the study of the interfacial processes involving enhanced optical scattering from adsorbates on metal

This review article covers the current development in SERS research along with brief discussion on the fabrication of various SERS active substrates, the various theoretical explanations of the mechanism of SERS effect and its various diverse applications in sensing, diagnostics, and catalysis. The article first deals with a short historical assessment of the SERS effect, followed by an overview on the preparation of various SERS active substrates. The article concludes with the citations of some recent applications of SERS from the literature. Due to insufficiency in space, a comprehensive review of all current work based on SERS is impossible. However, we have summarized a few representative examples including our own

Raman spectroscopy is a spectroscopic technique based on molecular vibration and is dependent on the inelastic scattering of monochromatic light, usually from a laser in the visible, near-infrared, or near-ultraviolet range of electromagnetic spectra. This effect was discovered

**Figure 1** shows the schematic diagram to explain the principle of Raman scattering. The weak Raman signal observed in conventional Raman spectroscopy can be explained by the low scat-

, but it may reach value as high as 1010 at

In general, SERS enhancement value is around 106

surfaces [9].

294 Raman Spectroscopy and Applications

definite highly effective subwavelength regions of the surface [5–8].

results to demonstrate the recent advancement in the SERS research.

by famous Indian physicist Professor C.V. Raman in 1928 [10].

**2. Historical background and gradual development of SERS**

However, the intensity of Raman signal obtained from most of the systems is very weak and is only about 10−10 times the intensity of the incident laser. Fleischmann and his group from the University of Southampton, UK, carried out Raman spectroscopic study with expected high intensity of signal by increasing the number of adsorbed molecules on a roughened metal electrode surface. In 1974, they reported very high quality Raman spectra of pyridine molecule (Raman probe molecule with high scattering cross section) adsorbed on electrochemically roughened Ag electrodes [8]. The authors attributed the enhancement in the Raman intensity to an increase in the surface area of the Ag electrode by the electrochemical

**Figure 1.** Schematic diagram to explain the principle of Raman scattering and Rayleigh scattering.

**Figure 2.** The development of Raman instrumentation from the Raman spectrometer of C. V. Raman to the latest sophisticated Horiba-Jobin model of the micro-Raman system.

roughening method. **Figure 3** illustrates the schematic diagram to explain the principle of SERS. The technique is so sensitive that even single molecule can be detected.

**Figure 4** shows the photograph of Fleischmann, who invented SERS. Fleischmann, Hendra, and Mcquillan of University of Southampton, UK, discovered surface-enhanced Raman scattering (SERS) spectroscopy by chance when they tried to carry out Raman study with pyridine (Py) molecule having very high Raman cross section on the roughened silver (Ag) electrode [8]. The spectra were found to be dependent on the applied electrode potential.

**Figure 3.** Schematic diagram to explain the principle of SERS.

roughening method. **Figure 3** illustrates the schematic diagram to explain the principle of

**Figure 2.** The development of Raman instrumentation from the Raman spectrometer of C. V. Raman to the latest

**Figure 4** shows the photograph of Fleischmann, who invented SERS. Fleischmann, Hendra, and Mcquillan of University of Southampton, UK, discovered surface-enhanced Raman scattering (SERS) spectroscopy by chance when they tried to carry out Raman study with pyridine (Py) molecule having very high Raman cross section on the roughened silver (Ag) electrode [8]. The spectra were found to be dependent on the applied electrode potential.

SERS. The technique is so sensitive that even single molecule can be detected.

sophisticated Horiba-Jobin model of the micro-Raman system.

296 Raman Spectroscopy and Applications

Initially, it was thought that an increase of surface area to be responsible for the enhancement of Raman signal. Afterward, in 1977 Jeanmaire and Van Duyne [11], from Northwestern University, USA, first realized that the surface area is not the key point in the above phenomenon. Albrecht and Creighton [12] of University of Kent, UK, reported a similar result in the same year. Both the groups independently supported enough proofs to exhibit that the strong surface Raman signal must be created by an authentic augmentation of the Raman scattering efficiency (10<sup>5</sup> to 106 enhancement). Later, this effect was referred as surface-enhanced Raman scattering and now, it is a unanimously acknowledged surface analytical technique. In spite of the fact, the first SERS spectra were obtained employing an electrochemical system (Py + roughened Ag electrode), all significant reactions occurring on various surfaces like metal and semiconductors can be investigated by the SERS technique. The technique is so sensitive that even single molecule can be detected in addition to various electrochemical processes.

The precise mechanism responsible for the enhancement effect observed in SERS is still highly controversial as found from the literature. There are two major mechanisms that are responsible for the large enhancement of weak Raman signal obtained from pyridine molecules adsorbed on electrochemically roughened Ag surface. Jeanmaire and Van Duyne first proposed a theory based on the electromagnetic effect responsible for the enhancement of Raman signal [11]. This is known as the electromagnetic theory of SERS effect and is based on the excitation of localized surface plasmons (LSP). Albrecht and Creighton first proposed a theory based on the charge transfer effect of the adsorbed molecule on the enhancement of Raman signal [12]. This is known as the chemical enhancement. This chemical enhancement theory depends on the charge transfer complex formation of the adsorbed molecule by absorption of photon of the suitable wavelength. Nevertheless, it is extremely complicated to separate these two effects experimentally and understand the overall mechanism of SERS.

**Figure 4.** The photograph of Fleischmann, the inventor of SERS.

In the mid-1980s, the spotlight on SERS research diverted to the exploitation of SERS effect for new and novel analytical and biological applications from the basic understanding of the mechanism responsible for the SERS phenomenon. However, it was extremely difficult to record high-quality, highly reproducible and stable SERS spectra as demonstrated by some of the investigations carried out in the mid-1980s as well as the early 1990s. The SERS spectra obtained were highly irreproducible, which can be explained by the small variations in the fabrication of SERS substrates. This shortcoming has prevented the development of SERS as a quantitative tool for long period. For that reason, fabrication of an SERS active substrate is very important in SERS research so that highly uniform, stable, and highly reproducible SERS signals can be obtained.

In the mid-1990s, VIIIB transition metals were employed as SERS active substrates to carry out electrochemical surface-enhanced Raman spectroscopy (EC-SERS) and these SERS substrates were further utilized for electrochemistry and catalysis [13]. Professor Tian and his coworkers at the Xiamen University, China, first introduced quite a few surface roughening procedures and demonstrated that the SERS effect can be directly obtained from transition metals such as pure Pt, Ru, Rh, Pd, Fe, Co, and Ni electrodes with a surface enhancement factor in the range between 1–3 orders of magnitude [14]. Since the early-2000s, the randomly roughened surfaces were replaced by the well-controlled nanostructures of both coinage (e.g., Au, Ag, and Cu) and transition metals due to the gradual and rapid development of nanoscience and nanotechnology. These nanostructures were considered as a very promising class of excellent SERS-active substrate. Up to now, molecular-level investigations by Raman spectroscopy on diverse adsorbates at various electrodes had been carried out.

The next major landmark in the field of SERS research was the observation of SERS spectra from single molecules (SM-SERS) by two independent research groups in the year 1997 [15, 16]. The detection of single molecules using the SERS technique and attainment of ultimate limit of detection in any analytical detection was possible by combining other techniques, for instance, fluorescence spectroscopy and scanning tunneling methods along with SERS technique.

Under suitable conditions, SERS enhancements in the order of 1014 can be obtained. It is important to mention here that special sites, sometimes referred to as "hot spots," are responsible for the observed enhancement in the SERS effect to a large extent. On the basis of these considerations, a great deal of the current research work in SERS is focused on the controlled and reproducible fabrication of metallic nanostructures which can create hot geometries like "hot spots" where the Raman probe molecules are correctly and inevitably located for gigantic Raman enhancement. This will provide new information in novel research areas like plasmonics.
