Surface Enhanced Raman Scattering Spectroscopy for Pharmaceutical Determination

The rapid growth of pharmaceutical industries worldwide demands continuous development of effi cient analytical techniques that help not only to detect the presence of the molecules at extremely low concentration levels, but also to detect the structure. Optical spectroscopic techniques are widely used in pharmaceutical development and manufacturing because of their speed and versatility. However, IR and Raman are relatively insensitive. Surface enhanced Raman scattering (SERS) enhances the weak Raman signal, thus, extending the range of available applications. This allows fast, sensitive detection of trace levels of key pharma molecules. However, the use of SERS for analysis requires substrates like silver nanoparticles. In this review, the applications of nano-substrates for SERS will be discussed. The synthesis and fabrication of nanocomposites; such as gold and silver, and nanocomposites will be highlighted. The characterization of the fabricated nanomaterials provide information on structures and properties that could help to improve and control their activity in SERS. Review Article


Introduction
One of the main challenges in analytical science and technology is to develop methods that provide unambiguously the chemical nature of the material of interest with the lowest detection limits, no interferences and the shortest acquisition time. Among the promising methods for such purpose is the optical spectroscopy. The most appropriate one is Raman spectroscopy to determine the amount of substances. The Raman Effect occurs when a beam of monochromatic exciting radiation interacts with a sample and scattering occurs. Large portion of this scattered radiation has either the same energy as the incident photons (elastic scattering and known as Rayleigh scattering). A small portion of this scattered radiation ( Figure 1) has either higher or lower energy than the incident photons (inelastic scattering) and is known as Raman scattering. Due to light-matter interactions, energy is either gained or lost by the molecule during Raman scattering. This method of characterization can yield narrow, well-resolved vibrational bands which, in essence, provide a "fi ngerprint" of a given analyte and involve surface processes and interfacial reactions [1,2]. Several methods have been reported for various pharmaceutical compounds analysis [3][4][5][6][7][8][9], such as ofl axacin (antibiotic), amlodipine (antihypertensive), chlorpheniramine (antihistamine) and promethazine (antihistamine). However, optical spectroscopic methods attracted more attention over others.
Optical spectroscopy is considered selective technique, both because of the molecularly specifi c nature of the pattern of peaks obtained and because of the wide variation in the Raman cross section of different analytes. Water gives very weak Raman scattering and organic molecules usually have much larger scattering cross sections thereby enabling Raman scattering to be recorded from organic molecules in aqueous solution and allowing analytes to be identifi ed without the required pretreatment as in some other techniques.

Surface-enhanced Raman scattering (SERS)
SERS is a technique that enhances Raman scattering by the silver electrode by oxidation-reduction cycles [10]. The electrode surface was altered to allow for the examination of charge transfer between analyte molecules and substrate metal surface as well as orientation of the molecules.
The amplifi ed signal was explained by increased surface area that enables more pyridine molecules adsorbed on the electrode surface, from the roughened silver electrode [11].
Later, it was reported that the enormously strong surface Raman signal could be caused by another enhancement of Raman scattering effi ciency itself in addition to the surface area [12].

Conditions
SERS involves adsorption of molecules onto the substrate surface, Figure 2. Substrates are of a variety of metals, like silver, gold, or copper with differing morphologies [13]. Generally, gold and silver is most often used as SERS substrates because they are air stable materials. All three metals have the excitation of (1) An analyte is adsorbed on a surface patterned or roughened so that the chosen excitation frequency will excite a plasmon and create scattering.
(2) Energy from the plasmon is transferred to the adsorbed molecules and the Raman process occurs on the molecule.
(3) Energy is transferred back to the plasmon less the   between 5-20 nm [19]. SERS substrate of colloidal silver or gold nanoparticles can consistently yield large signal enhancement [20]. Colloids are generally produced by reducing metal salts, often silver nitrate with sodium citrate, which can be done in manner to create cubes, rods, triangles, and other structures [21]. The aggregated clusters of metal colloid can possess hot spot within the aggregate itself that achieve extremely high enhancement [20][21][22][23][24][25]. There is debate as to the exact reason for the areas of high enhancement and whether it is the aggregates specifi cally or if a "hot" particle becomes substrates [27][28][29], but certain limits related to reproducibility have prevented rapid development in these areas. Over other techniques, SERS has many advantages for use in a variety of areas. In its simplest form, SERS is comparable to Raman spectroscopy with better sensitivity. As such, SERS still provides specifi c detail about the "fi ngerprint" of a given molecule or process. However, since conventional Raman has weak signal intensity, the useful technique has not been applied as universally as other methods. SERS has the ability to not only improve the sensitivity for those applications already used by Raman while also expanding the potential uses of the method to those that would not be possible without the added sensitivity and limits of detection. SERS has the potential to impact the areas of analytical chemistry, biochemistry, forensics, environmental analysis, trace analysis, and many others.
SERS has been reported as a promising technique for quantitative and qualitative identifi cations of organic and inorganic targets [30][31][32][33][34][35]. Due to its ultra-sensitivity, SERS was used to detect trace organic and inorganic analytes in different media. For example, some organophosphorus compounds, such as methylparathiol and dimethoate, that exist in pesticides were identifi ed at the nanogram level [36]. Due to the fact that water molecules scatter weakly in Raman experiments made SERS technique an attractive choice to conduct useful characterizations of samples [37]. For example, SERS detection of organic and inorganic compounds in ground water was evaluated and proven to be effective [38]. Moreover, highly active polyhedral Ag nanocrystals SERS substrates have performed very well in low-level arsenate and arsenite sensing in aqueous solutions. Detection at 1 ppb of order of magnitude was achieved [39]. In addition, selected polycyclic aromatic hydrocarbon (PAH) compounds in artifi cial seawater were detected using SERS [42], and a limit of detection of 10 ppb for naphthalene and pyrene were recorded. For the PAH characterization, gold colloidal monolayer substrates were used [43] and have been shown to enhance Raman signals of PAH very dramatically. Other studies reported the detection of thiacyanine [44] and folic acid [45] in water and human serum using SERS technique. Moreover, trace analysis using SERS has been implemented to detect biomolecular systems prepared in aqueous solutions at low concentrations. For instance, SERS was successfully utilized to detect as low as 10-5 M dipeptides on a surface of colloidal silver [46].
One major advantage of SERS is the relative easiness of preparing its samples which are obtained from variety of sources with direct analysis without the need for pretreatment as in some other techniques. Currently, this technique has been implemented successfully for detecting trace amounts of pharmaceuticals [47,48]. It has been used in biochemical fi elds to help analyze electron transfer reactions in proteins [49] and provide quantitative DNA information [50,51]. It has been implemented in a variety of scientifi c areas and rivals fl uorescence spectroscopy in many ways [52]. SERS technique has used to compare relative intensity shifts and to investigate the adsorption geometry of protoberberine alkaloids on Ag nanoparticles [53]. It has been employed to study the interaction between protoberberine alkaloids and human serum albumin [54]. SERS with gold surface has been used for ultra-trace analysis of latent drug materials [55].

SERS has advantages of
• producing spectra which have sharp peaks whereas for example fl uorescence spectra are broad and overlapping and less specifi c for a particular molecule. This enables much higher numbers of analytes to be discriminated.
• It is used for the analysis of materials in different phases • There is no need to prepare the sample • SERS is a non-destructive and non-invasive method In the context of quality assurance: It is benefi cial to analyze pharmaceutical samples to determine both the overall composition of the sample and the actual distribution of the components within the tablet.
The ideal analytical tool for analyzing pharmaceutical samples should be fast, non-destructive, and record chemically specifi c data to differentiate between the multiple components within a pharmaceutical tablet.
The analytical method should provide specifi c data or signals that should be a fi ngerprint of the molecule of interest.
SERS has gained attention in the investigation of various pharmaceutical compounds [12][13][14][15][16][17][18], such as ofl axacin However, the achieved detection limits were not satisfactory. As a continuation of our research, we will target these compounds in the project since studying wider range of pharmaceuticals requires several years.
However, the development of SERS is being holdback because of some obstacles. For example, it is still a big challenge to prepare the appropriate SERS-active substrates to meet the requirements such as large enhancement factor, good stability and reproducibility.