Preaggregated Ag nanoparticles in dry swellable gel films for off-the-shelf surface-enhanced Raman spectroscopy

Large, thin (50 μm) dry polymer sheets containing numerous surface-enhanced Raman spectroscopy (SERS) active Ag nanoparticle aggregates have been prepared by drying aqueous mixtures of hydroxyethylcelloulose (HEC) and preaggregated Ag colloid in 10 × 10 cm molds. In these dry films, the particle aggregates are protected from the environment during storage and are easy to handle; for example, they can be cut to size with scissors. When in use, the highly swellable HEC polymer allowed the films to rapidly absorb aqueous analyte solutions while simultaneously releasing the Ag nanoparticle aggregates to interact with the analyte and generate large SERS signals. Either the films could be immersed in the analyte solution or 5 μL droplets were applied to the surface; in the latter method, the local swelling caused the active area to dome upward, but the swollen film remained physically robust and could be handled as required. Importantly, encapsulation and release did not significantly compromise the SERS performance of the colloid; the signals given by the swollen films were similar to the very high signals obtained from the parent citrate-reduced colloid and were an order of magnitude larger than a commercially available nanoparticle substrate. These "Poly-SERS" films retained 70% of their SERS activity after being stored for 1 year in air. The films were sufficiently homogeneous to give a standard deviation of 3.2% in the absolute signal levels obtained from a test analyte, primarily due to the films' ability to suppress "coffee ring" drying marks, which meant that quantitative analysis without an internal standard was possible. The majority of the work used aqueous thiophenol as the test analyte; however, preliminary studies showed that the Poly-SERS films could also be used with nonaqueous solvents and for a range of other analytes including theophylline, a therapeutic drug, at a concentration as low as 1.0 × 10(-5) mol dm(-3) (1.8 mg/dm(3)), well below the sensitivity required for theophylline monitoring where the target range is 10-20 mg/dm(3).