Novel Ag-coated nanofibers prepared by electrospraying as a SERS platform for ultrasensitive and selective detection of nitrite in food

Highlights

• An Ag-coated nanofiber SERS platform was prepared for rapid nitrite detection.

• Electrospraying technique was firstly used to assemble SERS platform.

• The Ag-coated nanofiber SERS platform has good homogeneity and reproducibility.

• The EF and the LOD was 2.635 × 10⁸, 2.216 × 10⁻¹² M, respectively.

• The SERS platform shows good compatibility in nitrite detection for real samples.

Abstract

Nitrite is commonly used as a preservative and color fixative in the meat industry. However, the risk of it transforming into N-nitrosamine restricts its intake. Herein, a novel sensitive Ag-coated nanofiber surface-enhanced Raman scattering (SERS) platform was developed for rapid nitrite detection. The electrospraying technique was firstly used to assemble Ag nanoparticles (NPs) on the nanofibers to obtaine SERS platform. The homogeneity and long-term stability of the SERS platform were evaluated. The limit of detection (LOD) of the SERS platform was estimated to be 2.216 × 10⁻¹² mol/L, corresponding to 15.29 ng·L⁻¹ and good linearity was shown between the relative SERS intensity and nitrite concentration range of 10–1 to 10⁻⁴ mol/L. The Ag-coated nanofiber SERS platform was utilized to assay-five common nitrite foods, and the results provided valid evidence for the compatibility of SERS platform in quantitative nitrite detection.

Introduction

Nitrite, as a kind of preservative and color fixative, has been widely used in the food industry due to its effective antimicrobial activities against Clostridium botulinum and its spores in a vacuum-packaged product as well as its superior fixation ability for cured meat color (Flores & Toldrá, 2021). Nevertheless, the acceptable daily intake (ADI) of nitrite has been severely restricted due to the risk of nitrite transforming to N-nitrosamine, which increases the risk of cancer (in particular, gastric cancer) and methemoglobinemia (Ma et al., 2016, Weitzberg and Lundberg, 2013). Therefore, a highly sensitive, selective, speedy and accurate method for the quantitative detection of nitrite is desirable for food safety. Traditional nitrite detection methods (Griess assay) are based on the diazotization reaction of nitrite with specific detection reagents, but the time-consuming pretreatment of food samples limits the need for extensive detection (Rycenga et al., 2011). In addition, modern analytical techniques for nitrite detection in foods, including high-performance liquid chromatography, gas chromatography, ion chromatography, mass spectrometry, and potentiometry, require large instruments, good receptor stability, and skilled personnel or expertise, limiting accessibility to the general public. Hence, various novel techniques for nitrite detection based on electrochemistry, fluorometry, and colorimetry have been developed in recent years, but the shortcomings of these techniques, e.g., susceptibility to interferents, poor sensor stability and reproducibility, need to be further overcome.

Surface-enhanced Raman scattering (SERS) is an efficient vibrational spectroscopic technique for molecular structure characterization and chemical sensing that exhibits great potential in hazardous molecule detection due to its minor susceptibility to environmental interference and portable instruments for in situ detection and is a highly sensitive, timely, accurate, and noninvasive analytical approach (Arabi et al., 2021). Based on the “hot spots” formed by the homogenized noble-metal nanoparticles on the surface of a substrate, the detection sensitivity of SERS can be dramatically enhanced by 10² to 10¹⁴ times compared to that of conventional Raman scattering, resulting in high sensitivity and low LOD (Li et al., 2021). Therefore, nitrite detection sensors based on SERS have been developed in recent years. Chen et al. (2016) prepared a 4-aminothiophenol (4-ATP) molecule-capped Fe₃O₄@SiO₂/Au magnetic nanoparticle-based SERS sensor for nitrite detection, which exhibited new bands at approximately 1141, 1392, and 1437 cm⁻¹ (named b₂ bands) when nitrite was exposed. (Chen et al., 2016). Liu et al. experimentally verified the relationship between the b₂ bands of 4-ATP and nitrite and found that nitrite triggered the conversion of 4-ATP to 4,4’-dimercaptoazobenzene (DMAB) in catalysis by a laser, leading to the formation of b₂ bands in SERS spectra (Liu et al., 2015).

Film-based SERS platforms (e.g., large contact area, flexibility and portability), with unique advantages over rigid substrate-based SERS sensors, have attracted increasing research interest in recent years (Jia et al., 2020, Liu et al., 2021b). Therefore, various attempts to develop film-based SERS platforms have been reported recently (Xu et al., 2019). As natural polymers, proteins with low-intensity interfering Raman signals are good candidates for the substrate of flexible film-based SERS platforms (López-Lorente, 2021). Gluten and zein, as coproducts obtained from processing, are cost-effective, biodegradable and eco-friendly and have been extensively processed into films (Bhargava et al., 2020). Therefore, gluten and zein exhibit a great potential to construct an eco-friendly, cheap, and flexible SERS platform with excellent SERS sensitivity for practical application.

Nanoparticles are arranged on a film-based SERS platform by the loading approach, which affects the formation of “hot spots” and the performance of SERS (McLamore et al., 2021). Generally, loading approaches mainly include physical vapor deposition, electrodeposition, chemical adsorption, ion beam sputtering and in situ synthesis. However, the drawbacks of these loading approaches, e.g., the limited nanoparticle shape, particle aggregation, poor uniformity of the clad layer and specific requirement for the film substrate, restrict the design and detection capacity of the SERS sensor. Electrospraying is an electrostatically driven technique used to prepare nanoparticles with simplicity, high efficiency, and ease of control (Lee et al., 2020). By using electrospraying, synthesized metal nanoparticles can be efficiently deposited on a film surface to form a SERS layer, which provides a novel strategy for the preparation of Ag-coated nanofiber SERS platforms with a controllable nanoparticle shape and coating density of the SERS response layer.

In this study, a novel, sensitive and specific gluten/zein film-based Ag-coated nanofiber SERS platform is prepared by electrospinning-assisted electrospraying for rapid nitrite detection. The main purpose of this study is to design a new nano-sensor for rapid, sensitive, and selective detection of nitrite. Unlike to the traditional self-adsorption prepared film SERS sensor, the electrospraying technique was first used to assemble Ag nanoparticles (Ag NPs) on the film surface for the preparation of the SERS sensor. Moreover, the difference of SERS performance between casted film substrate and nanofiber film substrate were also evaluated. The morphology and composition of the prepared film were characterized by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The SERS intensity homogeneity and long-term stability of the prepared film were also measured. To evaluate the nitrite selectivity of the film, other common ions, e.g., SO₃²⁻, Br−, H₂PO₄⁻, CO₃²⁻, Cl⁻, HCO₃⁻, and NO₃⁻, were chosen as interference factors. Moreover, the SERS response of food additives commonly accompanied by nitrite (e.g., d-sodium erythorbate, monosodium glutamate, sodium benzoate, and potassium sorbate) was also detected to evaluate the interference of the food environment. Finally, the film was successfully applied in nitrite detection for chicken sausage, canned pork, cured meat, ham and pickled vegetables.