# Nanostructured Biosilica of Diatoms: From Water World to Biomedical Applications
## 4 Diatoms as Surface-Enhanced Raman Scattering (SERS) Platforms for Label-Free Diagnosis
The same group integrated AuNPs on diatom-based TLC for detecting trace levels of tetrahydrorocannabinol (THC) in saliva and urine samples via a portable Raman spectrometer [72]. Rapid and sensitive detection of THC analyte was performed in concentrations ranging from 1000 to 10 ppm in complex samples. The group also developed a microfluidic diatomic analytical device (\(\upmu\)DAD) for cocaine sensing [73]. In this work, the nanoporous photonic channels showed high-resolution separation of complex biofluids and high-sensitivity detection of cocaine down to parts-per-billion. The use of TLC-SERS plates made fromdiatom was also investigated by the group for the detection and quantification of seafood allergen in situ to enhance food safety [74]. A detailed investigation of the utilization of diatom frustules as substrates for the detection of different analytes has been recently shown by [PERSON] et al. [75]. The group investigated the growth of silver nanoparticles (AgNPs) on the porous surface of _Pinnularia_ sp. frustules to develop ultrathin layer chromatography (UTLC)-SERS plates. Frustules were deposited as a monolayer on a glass film through polyelectrolyte layer-by-layer dip-coating technique, using four bilayers of positively charged poly (allylamine hydrochloride) (PAH) and negatively charged poly (sodium 4-styrene sulfonate) (PSS) (Figure 4A). The SEM images revealed that coverage of diatom frustules on the thin film was approximately 10 \(\upmu\)g\(\cdot\)cm\({}^{-2}\) and that most of the frustules adhered face down to the surface valve, creating an ideal layer of 5 \(\upmu\)m for UTLC. AgNPs with a size of 43 nm were deposited by in situ Ag\({}^{+}\) nucleation process to form a single layer on the outer surface and within the pores. The non-polar Nile red and the weakly cationic malachite green were examined as model analytes to evaluate the separation performance of the diatom-based UTLC plate by micro-Raman spectroscopy (Figure 4B,C). When the excitation light source (532 nm Raman laser) coupled by the guided mode resonances within the diatom frustule pore arrays, the electromagnetic field generated by AgNPs improved the local enhancement of SERS signals associated to the analyte molecule on the frustule. Flexible, gold-coated frustules guarantee favorable chemical detection levels towards single-molecule detection. Flexibility has been considered a crucial parameter for the development of SERS substrates, since it allows the frustules to trap the molecules in the hot spot sites generating a strong enhancement. Figure 4: **(A)** Fabrication of the AgNPs–diatom frustule biosilica monolayer on glass. **(B)** Configuration of micro-Raman laser for scanning ultra-thin layer chromatography film embedded with diatom for surface enhancement Raman scattering (SERS). **(C)** Schematic representation of SERS signal intensity of separated analyte plume around AgNPs–diatom frustule monolayer versus AgNPs on the glass, illustrating the guided-mode resonance effect. Reproduced and adapted from Reference [75], copyright (2020), with permission from John Wiley and Sons. [PERSON] et al. used strips filled with diatom frustules to design simple, inexpensive, and flexible adhesive tapes [76]. A transparent adhesive tape was applied to the template strip and peeled off to obtain a tape strip embedding diatoms. Tape strips embedded with frustules and decorated with AgNPs showed an enhancement factor nine times higher than the control. In this way, the great potentiality of diatom-based SERS strips for global health applications was outlined. Diatom biosilica and diatom frustules have not only the unique property of enhancing SERS sensitivity thanks to the 2D periodic pores, but they also display a high Langmuir adsorption capacity. This adsorption isotherm model is commonly used for molecular adsorption at interfaces, providing an insight into the surface coverage via physisorption or chemisorption phenomenon [77].