# Nanostructured Biosilica of Diatoms: From Water World to Biomedical Applications
## 4 Diatoms as Surface-Enhanced Raman Scattering (SERS) Platforms for Label-Free Diagnosis  
Surface-enhanced Raman scattering (SERS) is a powerful vibrational spectroscopy technique suitable in biomedical label-free sensing thanks to high sensitivity, low detection limit (\(10^{-8}\) M), and selective detection of analytes. Surface-enhanced Raman scattering analysis relies on the amplification of localized electromagnetic fields generated by the excitation of localized surface plasmons [58; 59; 60]. The choice of plasmonic material and its preparation are the main requirements for successful SERS analysis. The poor reproducibility and instability of conventional SERS substrates (e.g., silver, copper, gold) led scientists to study new classes of artificial optical structures known as \"photonic crystals\" [61]. The production of these materials requires highly specialized design, lithography techniques, advanced facilities, and experienced staff, resulting in expensive and time-consuming processes [62]. Surprisingly, diatoms are a cheaper and more accessible alternative material, whose ordered pore distribution bears a resemblance to a photonic crystal. A photonic crystal is a spatial, periodic distribution of refractive index, through which light cannot propagate in specific ranges [63]. The use of diatoms as plasmonic substrates and their combination with metal plasmonic structures resulted in an alternative to photolithography fabrication of cost-effective SERS substrates [1; 64; 65; 66; 61]. [PERSON] et al. [67] combined the photonic-crystal-like structure of _Pinularia_ sp. frustules with the plasmonic resonance properties of silver nanoparticles (AgNPs) to enhance the localized surface plasmon resonance. The synergy between the frustule photonic structure and the localized plasmon resonance of AgNPs led to a 4-fold improvement of nanoparticle sensitivity. [PERSON] et al. [68] proposed the electroless deposition of gold (Au) on diatom frustules for sensing applications. A three-step chemical method was adopted to obtain a uniform, continuous, thin layer of AuNPs on the surface of _Aulacoseria_ frustules. The gold coverage on diatom frustules provided an enhancement factor (EF) of \(1.0\times 10^{5}\) for p-mercaptoethanol(pMA) probe molecules adsorbed on frustules. Nonetheless, the intensity of single peaks changed from point to point, meaning that the SERS response was not completely homogeneous. The spectra enhancement reported by the authors demonstrated that diatom- based substrates could be considered good candidates for SERS analysis, but at the same time, their response is not completely reproducible in terms of intensity. A noteworthy strategy for the fabrication of a diatom-based SERS platform was proposed by [PERSON] et al. [69]. Diatomite nanoparticles were modified with amino-silanization and the amino-groups were covalently bound to the carboxyl groups of heterobifunctional PEG (PEG-DNPs). Positive charges of PEG-DNPs complex were involved in electrostatic interactions with AuCl\({}_{4}\) precursor ions resulting in an in-situ synthesis of AuNPs. Recently, [PERSON] et al. presented an ultrasensitive SERS immunoassay based on AuNPs and frustules from _Pseudostaurosira trainorii_ to detect interleukin-8 (IL-8) in human blood plasma [70]. The biosensor was a sandwich-type immunoassay composed of IL-8 antibodies covalently bound to the amino-silanized biosilica and anti-IL-8 antibodies conjugated to AuNPs and labeled with dinitrothiocyanobenzene (DNTB) as a Raman reporter. The SERS immunoassay developed on diatom frustules exhibited a limit of detection six orders of magnitude higher than a SERS immune assay on flat glass. [PERSON] et al. proposed an alternative method to analyze complex human plasma which employed thin layer chromatography (TLC) plates made from diatomic for SERS analysis for phenethylamine (PEA) detection [71]. The high molecular weight of diatomic plates offered separation performance outstanding standard TLC plates and a significant improvement of sensitivity.