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Flexible Surface Enhanced Raman Scattering Substrates for Trace Analysis

 

With a growing human population and increasing demand on food production, food safety monitoring for food hygiene, additives and adulteration as well as pesticide and antibiotic residues will need to become more stringent. Current techniques based on MS are expensive, time consuming and restricted to laboratory setting. 

Consequently, new low-cost diagnostic tools that allow de-centralized monitoring, i.e., at point of use are becoming more essential. Surface-enhanced Raman spectroscopy (SERS) is the one technique that can address these challenges, particularly due to its increased sensitivity and minimum sample preparation requirements. Recent advances in SERS are highlighted with the detection of food contaminants and adulterants; concentrated specifically on antibiotics, drugs, melamine, and pesticides. However, a key challenge limiting the uptake of SERS for commercial sensor applications is the lack of reproducible and reliable approaches for substrate nano-patterning and fabrication. 

To address this challenge, a low-cost approach for fabrication of SERS substrates was developed by templating a flexible thermoplastic polymer against an aluminium soda can, to produce a rough nanostructured surface.  When coated with an evaporated silver over-layer, localised electromagnetic “hot-spots” are formed between the metal nano-gaps. Electron microscopy, FEM simulations and spectroscopic characterisation showed the SERS response arose at these “hot spot” regions of high electromagnetic field strength, following illumination.

These substrates are highly versatile and have been applied to a variety of different compound residues in water including crystal violet (antibiotic), imidacloprid (neonicotinoid pesticide) and benzocaine (pharmaceutical active ingredient),  with characteristic “fingerprint” molecular spectra obtained for each molecule, see Figure 1.  Low concentrations of melamine in milk and infant formula solutions (100 ppb) were detected in 10 minutes without the need for sample pre-treatment.  Finally, back excitation and collection through the SERS substrate is also demonstrated which permits coupling with photonic devices such as optical fibres required for future remote sensing applications.  
 

FLEXIBLE SURFACE ENHANCED RAMAN SCATTERING SUBSTRATES FOR TRACE ANALYSIS
SERS substrates fabricated using a soda can template providing molecular fingerprint spectra of key target molecules

 

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