# Recent Advances in Spectroscopy Technology for Trace Analysis of Persistent Organic Pollutants
## 2 Applications of Spectroscopy Technology in Detection of POPs
### Sers  
SERS is based on the enhancement of the Raman scattering signals of targeted molecules, which are able to interact with nanostructured metallic surfaces under the right conditions [51]. The SERS phenomenon was originally reported in the 1970s by [PERSON] and coworkers [52]. They first noticed the enhanced performance of Raman scattering from a pyridine monolayer adsorbed on an electrochemically roughened silver electrode. However, the first report introducing of the SERS technique for the detection of PAHs was in 1990s by [PERSON] et al. [53]. They fabricated silver SERS-active substrates using sol-gel methods, and investigated PAHs in artificial seawater with a detection limit up to nanomolar concentrations. Subsequently, two different strategies were used in the improvement of substrates to further enhance the sensitivity of SERS detection. One of them is the construction of substrates with specific geometries, which possess abundant nanogaps (\"hot spots\") and contribute to effectively enriching targeted molecules by physical adsorption [54; 55], and the other is modification of the substrate surface with functional molecules like antibodies and aptamers to facilitate targeted molecules with low affinity to approach localized electromagnetic fields [56].  
Sensitivity of SERS detection mainly depends on the nanostructures of the substrates [57]. In other words, these metallic plasmonic nanostructures, referred to as SERS-active substrates, are of importance for observing the SERS phenomenon, as well as the detection sensitivity. The high sensitivity of SERS detection can be achieved by preparing elaborate substrates with large amounts of hot spots and huge surface areas [58]. For example, highly sensitive SERS substrates can be constructed by Fe\({}_{3}\)O\({}_{4}\) microspheres and AuNPs (Fe\({}_{3}\)O\({}_{4}\)@Au nanoparticles), for which Fe\({}_{3}\)O\({}_{4}\) microparticles could render an inhomogeneous distribution of local plasmon resonance and lead to a local increase in the electromagnetic field density. A significant SERS enhancement of the substrate was attributed to the interparticle gap between AuNPs grafted on Fe\({}_{3}\)O\({}_{4}\) (Figure 2A) [31] and the detection limits of PAHs, such as benzo [_a_] anthracen and benzo (_g.li_) perylene, and were up to 5 nmol-L\({}^{-1}\) (Table 1). In addition, AuNPs and silver nanoparticles (AgNPs) can be coated on a highly roughened morphology instead of a smooth surface to fabricate an ultrasensitive substrate [59; 60]. Figure 2B shows AgNPs sputtered in bi-nanorings to acquire optimal substrates, which exhibited excellent analytical performance for PCBs with a detection limit of 1 nmol-L\({}^{-1}\)[41]. In addition to these, nanosensors fabricated with an aggregation of AgNPs and AuNPs functionalized by dithiols could be used to identify and quantify OPs (including aldrin, \(\alpha\)-endosulfan, dieldrin and lindane) (Figure 2C), with the lowest detection limit of aldrin being down to 45 ug\(\cdot\)L\({}^{-1}\)[48]. Figure 2D shows the SERS spectra of analyzed OPs. Limits of detection for analyzed OPs were different, and probably resulted from their showing different affinities to the metal surfaces of the substrates. The maximum detection efficiency was obtained for substrates possessing a maximum number of binding sites just at the formed hot spots localized in the gaps between nanoparticles. These achievements have significantly improved the sensitivity of SERS detection for POPs, and SERS has become a powerful spectroscopic technique for analyzing low-concentration contaminants; however, continuous efforts will be devoted to extending novel fabrication of SERS substrates in the future.  
Figure 2: _Cont._