Surface enhanced Raman spectroscopy SERS is
Surface-enhanced Raman spectroscopy (SERS) is one of the most powerful and ultra-sensitive analytical tools, which has been widely applied to food security analysis and many other fields (Craig, Franca, & Irudayaraj, 2013; Gukowsky, Xie, Gao, Qu, & He, 2018; Qi et al., 2013; Zhang et al., 2015). For example, SERS has been reported to successfully determine artificial histamine-spiked fish samples with various types substrates (Gao et al., 2015; Janči et al., 2017) Real-world food samples, however, are complex matrices that generally contain large molecules such as fat and proteins, which may exist strong signal interference or even block the access of the target molecules to the metallic nanoparticles (NPs) surface. Accordingly, some separation techniques have been combined with SERS to address efficient separation, such as liquid chromatography (LC) (Cowcher, Jarvis, & Goodacre, 2014) capillary electrochromatography (CE) (Karaballi, Nel, Krishnan, Blackburn, & Brosseau, 2015), electrostatic separation (ES) (Li, Li, Fossey, & Long, 2010), and thin layer chromatography (TLC) (Freye, Crane, Kirchner, & Sepaniak, 2013; Radu et al., 2016; Zhu, Cao, Cao, Chai, & Lu, 2014), Among these, SERS in tandem with TLC is the most attractive method due to its exclusive advantages such as low cost, simple pretreatment, high throughput, and capability for on-site detection when using portable Raman spectrometers. So far, TLC-SERS has been successfully applied to the separation and identification of various analytes from complex ingredients, such as tobacco-related biomarkers in urine samples (Huang, Han, & Li, 2013), aromatic pollutants in water (Li et al., 2011), natural dyes on works of art (Brosseau et al., 2009), apomorphine in human plasma (Lucotti et al., 2012), ephedrine in dietary supplements (Lv et al., 2015) and so on (Zhang, Liu, Liu, Sun, & Wei, 2014). Recently, Xie, Z. developed a PF-03084014 screening method by using Ag NPs and NaCl to obtain SERS spectra of fluram-derivatized histamine on TLC plates (Xie et al., 2017). Gao, F. presented remarkable success in determination of Sudan I in paprika powder using a molecularly imprinted polymers (MIP)–TLC–SERS biosensor (Gao et al., 2015). Yu, W. employed inkjet-printed paper substrates for TLC-SERS to detect melamine in food product (Yu & White, 2013). Nevertheless, in most TLC-SERS methods, the TLC plates are commercially available plates such as silica gel or cellulose, which are usually not SERS-active substrates. The sensitivity and resolution of these reported TLC-SERS was limited. Another challenge is that the low target concentration in complex samples and the multi-step treatment in the TLC-SERS procedure will result in nonlinear relationship between the spectral and the target concentration, which will induce difficulty in quantitative analysis. To overcome this challenge, chemometrics methods such as principal component analysis (PCA) and partial least squares regression (PLSR) have been applied to the TLC-SERS spectral analysis for qualification and quantification (Gao et al., 2015; Liu & Lu, 2017; Lv et al., 2015). However, there have been very few reports using nonlinear multivariate calibration methods, which can account for more measurement variations and provide better quantification accuracy. Diatomaceous earth is a kind of natural photonic crystal biosilica consisting of fossilized remains of diatoms, which are marine organisms that possess skeletal shells of hydrated amorphous silica, called frustules, with two dimensional periodic pores of hierarchical micro-and nanoscale features (Losic, Mitchell, & Voelcker, 2009; Losic, Rosengarten, Mitchell, & Voelcker, 2006). Diatoms have a variety of eminent properties in optics, physics, and chemistry such as their photonic-crystal nature and high surface-to-volume ratio (nearly 200 m2/g). Hybrid diatom-plasmonic nanoparticle structures have been proved to be excellent SERS substrates for ultra-high performance biosensors (Kong et al., 2016, Kong, Li et al., 2017, Kong, Squire, Chong, & Wang, 2017, Kong, Xi et al., 2017; Xu et al., 2013). In the previous work of our group, we have demonstrated that diatomaceous earth can function simultaneously as thin layer chromatography to separate toxic molecules from complex food samples and as ultrasensitive SERS substrates to probe the signature Raman peaks using a regular Raman microscope (Kong, Chong, Squire, & Wang, 2018; Kong, Li et al., 2017, Kong, Squire et al., 2017, Kong, Xi et al., 2017). Nowadays, commercial portable Raman spectrometers have been widely available at affordable cost and can achieve similar level of sensitivity compared to regular benchtop Raman microscope, which makes TLC-SERS a feasible method for on-site detection.