Editorial Board

Food additives and pesticides are widely used in the agricultural sector and are packed or ready-to-serve food items. The primary purposes of such adulterants are protection, preservation, texture improvement, and enhancing appearance. However, this often results in the excessive use of these chemical components, leading to various adverse effects on human health with prolonged consumption.

Therefore, it is essential to measure the contents of these materials, and SERS can be employed for this purpose. In particular, packaged and edible drinks are often laced with such products to increase their shelf life. Numerous studies have been conducted to simulate real samples by adding contaminants to food products and using SERS substrates for analysis. Thiabendazole (TBZ) food preservatives are frequently used in juices and drinks, and high doses or prolonged consumption can lead to thyroid hormone imbalances and liver damage. Xuan et al. [24] showed that the metal-organic frameworks (MOF) composed of terephthalic acid (PTA) and Fe³⁺ with gold and silver on the surface of MOF can be used to identify thiabendazole concentrations as low as 50 ppb in juice. Where the experimental procedure of this work showcased SERS spectra results indicating TBZ identification in the juice sample containing the TBZ contamination. The silver (Ag) NPs and MOF of Material of Institute Lavoisier 101 with Iron (MIL-101(Fe)) can be used for determining sodium thiocyanate (NaSCN) preservative in water and milk as low as 18.5 μg/L and 96.3 μg/L respectively. NaSCN long term intake could be resulting in low absorption of iodine in human body, and abnormal thyroid hormone disorder as suggested by Wang et al. [25].

Several other MOFs containing silver (Ag) and gold (Au) NPs have been utilized for pesticide detection. Among the common pesticides and insecticides are parathion-methyl (PTM) and methenamine. Research conducted on the MOF of Au/Cys-Fe₃O₄/MIL-101 by Zhu et al. [26] successfully detected PTM at 5 ppb in juice, and that on the MOF of Au@MIL-101(Fe) by Cai et al. [27] effectively determined 1.0 x 10⁻⁹ M methenamine in vermicelli. New Coccine (NC) and Orange II (OII) in contaminated soft drinks and paprika were studied and identified with a low limit of detection (LOD) of 0.4015 mg/L for NC and 0.0546 mg/L for OII. The MOF used here was UiO-66(NH₂)@Au as reported by Wu et al. [28].

Besides MOF, SERS substrates containing AgNPs were also employed by Mandrekar et al. for thiram pesticide analysis on the fruit's surface, detecting concentrations as low as 1 µM [29], from the apple surface contaminated from concentration ranging 1 µM of lowest up to 1 mM of thiram. as illustrated in Fig. 1. This study involved fabricating a flexible tattoo-paper-based AgNPs substrate capable of identifying pesticides on curved surfaces of fruits and vegetables. Fig. 1(a) displays a photograph of the fabricated tattoo-paper-based SERS substrate with AgNPs, along with scanning electron microscopy (SEM) images. Fig. 1(b) showcases a sample collected from the fruit's surface using the tattoo-paper SERS substrate. Raman spectra of various concentrations of thiram solution on the fruit specimen's surface are presented in Fig. 1(c). Similar chemical measurement of explosive molecules of trinitrotoluene (TNT) down to 10^-10 M was carried out by Kong et al. [30]. The preparation of the AgNPs SERS substrates was carried out by in-situ growth method from electroless deposited seeds within the photonic nano porous diatom for the 3-dimentional (3-D) hybrid plasmonic–photonic were fabricated on crystal biosilica. Such SERS substrates showing impressive chemical identification qualities are very useful in terms of future applications for sensors based on chemical molecules.

Fig. 1. 
(a)(i) Image of the flexible tattoo-paper SERS substrate with printed AgNPs, (a)(ii,iii,iv) SEM images of the silver AgNPs; (b) Sample collection on the apple surface contaminated with thiram; (c) SERS intensity spectra of the apple specimens showing peak intensities at 560 cm⁻¹, 933 cm⁻¹, 1144 cm⁻¹ and 1382 cm⁻¹ for the different concentration of ranging from 1 μM to 1 mM of thiram solutions. Reprinted with permission from Ref. [29]. Copyright (2023) MDPI.

The emissions of harmful and toxic gases into the atmosphere have increased in recent years. The major reasons for this could be increasing industrialization and vehicle transportation. Toxic gas emissions from such sources and occupational exposure can lead to poor health quality [31]. Hence, sensitive sensing techniques are of great importance to prevent and detect accidental leakage of such gases. SERS, with its excellent molecular identification qualities, can be utilized for such purposes. Ethanol and acetone vapors, classified as VOCs, are extensively used in industries and household applications, and they can be found in the air, posing harm to human health through long-term inhalation. Exposure to these VOCs can cause several health concerns, including headaches, drowsiness, eye irritation, and breathing difficulties [32,33]. Additionally, VOCs like acetone can serve as essential biomarkers for diabetes when detected in a patient's exhaled breath [34]. Multiplex SERS-based detection of VOCs, including acetone and ethanol vapors, has been achieved using SERS substrates of silicon nanopillars coated with Au carried out by Wong et al. [35]. The work exhibited detection of mixture of acetone and ethanol vapors with a low detection range of concentrations of 0.0017 ng and 0.0037 ng respectively. The results also proved to be great example to show the use of SERS sensing technique for multiple molecular analyses simultaneously.

There are various possibilities for the generation of other explosive and flammable gases and vapors in the air due to different reasons. SERS allows for the sensitive detection of such gaseous components. Among the numerous studies, one involved the detection of the explosive molecule 4-aminothiophenol (4-ATP) down to 10⁻⁸ M, utilizing gold nanowires (AuNWs) on two-dimensional graphitic carbon nitride nanosheets (g-C₃N₄/AuNWs) with reusability up to six cycles by Xu et al. [36]. Warfare agents, known for their explosive and destructive nature, can be highly dangerous. Study carried out by Lafuente et al. [37] identified dimethyl methyl phosphonate (DMMP) with a detection limit as low as 130 ppb in the gas phase using citrate-capped AuNP monolayers. Additionally, 2,4-dinitrotoluene (DNT) vapor was deposited on a polyethylene terephthalate (PET) sheet using a flexible AgNP ink SERS substrate and analyzed by Emamian et al. [38] successfully using Raman spectroscopy.

SERS substrates also have advantages in gas sensing for the food sector, where identifying spoiled eatables is crucial to ensure food safety. Spoiled food often releases specific odors and gases [39]. Chen et al. [40] suggested gold nano-bipyramids (Au NBPs) encapsulated by zeolitic imidazolate framework-8 (ZIF-8) (Au NBPs@ZIF-8) were successful in identifying 0.2 nM to 20 mM hydrogen sulfide (H₂S) gas released from spoiled fish meat. Sample collection and analysis process was carried out by storage of different fish samples of C. saira, H. olidus, and L. polyactis at different storage time durations of 0 h, 12 h, 24 h and 48 h such that the fish samples spoiled and the released H₂S gas which was then measured using handheld Raman spectrometer. The fabricated SERS had detection range of 0.2 nM to 20 mM with a LOD of 0.17 nM.

Work involving another zeolite framework, Au@ zeolite imidazolate-8 (ZIF-8) SERS, was able to recognize important volatile indicators for evaluating food spoilage, namely putrescine and cadaverine, with the lowest concentrations of 79.99 ppb and 115.88 ppb, respectively, in spoiled salmon, chicken, beef, and pork samples by Kim et al. [41]. The work done by Chen et al. [42] employed poly acrylic acid (PAA), poly methyl methacrylate (PMMA) and polydimethylsiloxane (PDMS) multiple SERS gas sensor matrix fabrication along with AgNP. Three target gases involved were phenethyl alcohol, acetophenone and anethole. Fig. 2(a) highlights the gas detection process in schematical form. Fig. 2(b)(i,ii,iii) are the resultant Raman spectra of phenethyl alcohol, acetophenone and anethole gases respectively. These studies shed light on the capabilities of SERS technology in gas sensing, demanding highly sensitive sensors with low-concentration range applications, making SERS a preferable option in gas-sensing applications.

Fig. 2. 
(a) Schematic representation of gas generation and detection process; (b) Raman spectra of (i) phenethyl alcohol gas, (ii) acetophenone gas, (iii) anethole gas. Reprinted with permission from Ref. [42]. Copyright (2021) MDPI.

The human body tends to produce various abnormal biomolecular signals, which can be advantageous for identifying the early stages of various diseases or other health abnormalities in an individual [43-45]. These biomolecules can serve as key identification markers in SERS technology, and liquid biopsy, which involves analyzing biological liquid samples, can aid in biomarker recognition. Cao et al. [46] suggested that SERS substrate of zinc oxide (ZnO) tips decorated with AuNP can be advantageous for trace identification of nicotine in human saliva as low as 1 × 10⁻¹⁰ mol/L. The visual schematic representation of hydrothermal synthesis of the substrate is shown in Fig. 3(a). Raman spectra of various concentrations of nicotine in saliva samples is exhibited in Fig. 3(b). Peak at 1589 cm⁻¹ was chosen for the nicotine detection due to easy observation without any spectral interference.

Fig. 3. 
(a) Schematic representation of hydrothermal fabrication of SERS substrates containing ZnO tips decorated with AuNP; (b) Raman spectra of different concentrations of nicotine in saliva. Reprinted with permission from Ref. [46]. Copyright (2021) MDPI.

A peroxidase-mimicking nanozyme was developed by decorating magnetic ring-like iron oxide (Fe₃O₄) with AuNPs (R–Fe₃O₄/Au) by Huang et al. [47] for colorimetric and SERS dual-mode detection of biomolecules in human serum. This nanozyme facilitates detecting spiked biomolecules such as glutathione (GSH) and cholesterol in human blood serum. The substrate allows for the lowest sensing of glutathione (GSH) up to 0.10 μM and cholesterol up to a concentration of 0.08 μM, with substrate stability and reusability of up to 5 times.

Regarding the forensic use of SERS substrates, the work done by Atta et al. [48] using synthesized bimetallic gold nanostars covered with AgNPs (BGNS-Ag) SERS platforms provided excellent sensing capacity of cocaine and heroin in spiked water and human urine samples having the detection LOD as low as 10 pg/mL for cocaine and 100 pg/mL for heroin.

The SERS technique with an annealed AgNP/porous silicon Bragg mirror composite substrate was used for biomolecular analysis of breast cancer patient serum and comparison with healthy human serum samples as reported by Cheng et al. [49]. These results revealed that valine and collagen could be used as diagnostic biomarkers. Additionally, differences in the peak intensities of proteins, bases, carotenoids, and lipids are useful for comparison between normal and cancerous serum samples. The enhancement of such molecular intensities in cancerous samples compared to normal samples can be an indicator, considering the abnormal secretion in contrast to healthy individuals' samples.

Apart from liquid biopsy, SERS can be used for molecular reporter identification in cancer detection in cancerous and tumor tissues. Several studies have been conducted to detect breast cancer tissues and their biomarkers. Choi et al. [50] highlighted from his work of study use of polyethylene glycol (PEGylated) Ag-Au hollow nanospheres functional SERS tags in breast cancer cells Sk-Br3 and MDA-MB-231. The cell lines were analyzed for three cancer biomarkers: anti-epithelial cell adhesion molecule (EpCAM), anti-erythroblastic oncogene B2 (ErbB2), and anticluster of differentiation (CD44), involving Raman reporters for each biomarker, namely malachite green (MGITC), rhodamine B isothiocyanate (RBITC), and 3,3'-diethyithiatri carbocyanine iodide (DTDC), respectively. This study involved Raman mapping identification of cell lines. Also, Qiu et al. [51] carried out study involving Au@Ag core–shell nanoparticles, decorated with a double layer of Raman reporter on the surface of the Au core and Ag shell, functionalized with polyethylene glycol (HS-PEG-NHS), were fabricated for mapping tumor cell markers, specifically epidermal growth factor receptors (EGFR and ErbB2), and insulin-like growth factor 1 (IGF1). The Raman reporters used were similar to those reported in a previous study. Malachite green isothiocyanate (MGITC) for EGFR, rhodamine B 5-isothiocyanate (RBITC) for ErbB2, and 3,3′-diethyithiatri carbocyanine iodide (DTDC) for IGF1. Human normal mammary epithelial cell line (MCF-10A) and human breast cancer cell lines (KPL-4, SK-BR-3, and MDA-MB-468) were used for work. Therapeutic efficacy after chemotherapy and surgical treatment was also evaluated through SERS imaging in live mouse specimens where different organs of the mice were studied using Raman mapping for evaluating the effect of antitumor drug therapy and surgery treatment. Similarly, the breast cancer cell lines MDA-MB-231 and MCF-7 were subjected to SERS sensing of urokinase plasminogen activation receptor (uPAR) and EGFR as target peptides using GNS with Raman-active molecules 4-nitrothiophenol (NTP) and Diamino-1,3,5-triazine-2-thiol (DATT) by Li et al. [52].