Biogenic amines (BAs) are a family of organic bases that may be found in foods such as fish and meat. The presence of BAs is the result of the decarboxylation of certain amino acids by microorganisms in these foods. The presence of microorganisms in food products not only poses a potential harm to human health, research has also found specific BAs (e.g. histamine) to cause headaches, diarrhea, edema, and other adversities. Thus, the simple, sensitive, and low-cost detection of BAs is of great importance in food safety. Methods such as HPLC and GC-MS have been employed in the sensing of BAs. However, tedious pretreatments, long reaction times, and high equipment costs limit the practicability and applicability of these techniques. Recently, localized surface plasmon resonance (LSPR) of metal nanoparticles (NPs) has been utilized for chemical and biological sensing. In particular, the collective oscillation of electrons within the NPs upon irradiation of incident light is sensitive to changes in environmental refractive index, making NPs suitable for developing plasmonic sensors.
Herein, we have developed paper-based plasmonic refractometric sensors through the embedding of metal NPs onto flexible papers using reversal nanoimprint lithography. The NP-embedded papers can serve as gas sensors for the detection of volatile BAs released from spoiled food. Commercial inkjet papers were employed as sensor substrates—their high reflectance (>80%) and smooth surfaces (roughness: ca. 4.9 nm) providing significant optical signals for reflection-mode plasmonic refractometric sensing and high particle transfer efficiency, respectively; in addition, because inkjet papers have light weight and are burnable and flexible, they are especially suitable for developing portable, disposable, cost-effective, eco-friendly sensing platforms. Solid silver NPs (SNPs), solid gold NPs (GNPs), and hollow Au–Ag alloyed NPs (HGNs) were immobilized on a solid mold and then transferred directly onto the softened paper surfaces. The particle number density and exposure height of the embedded NPs were dependent on two imprinting parameters: applied pressure and temperature. The optimal samples exhibited high particle transfer efficiency (ca. 85%), a sufficient exposure surface area (ca. 50% of particle surface area) presented to the target molecules, and a strong resonance reflectance dip for detection. Moreover, the HGN-embedded paper displayed a significant wavelength dip shift upon the spontaneous adsorption of BA vapors (e.g., Δλ = 33 nm for putrescine; Δλ = 24 nm for spermidine), indicating high refractometric sensitivity; in contrast, no visible spectroscopic responses were observed with respect to other possibly co-existing gases (e.g., air, N2, CO2, water vapor) during the food storage process, indicating high selectivity. Further in situ analyses for real samples (e.g., salmon) are in progress and will be presented at the conference.