Ultra-sensitive SERS detection of L-tyrosine on the microstructurally tuned DC sputtered plasmonic Bi thin films

This work aims on developing plasmonic bismuth (Bi) thin films for ultra-sensitive detection of L-tyrosine (Tyr) using label-free Surface Enhanced Raman Spectroscopy (SERS) technique. Tyr is a crucial precursor in the formation of several neurotransmitters, melanin, and thyroid hormones. Sensing the qualitative and quantitative imbalances in Tyr opens up the possibility of diagnosing a variety of autoimmune, neurodegenerative, and physiological illnesses. Bi thin films were fabricated through DC magnetron sputtering employing both in-situ deposition and ex-situ vacuum annealing processes. Varied annealing temperatures of 100 °C and 200 °C were used in each case. Among all, the ex-situ films at 200 °C exhibited enhanced microstructural, morphological, and optical characteristics. This film showed an optimum surface morphology aiding in intense Localized Surface Plasmon Resonance (LSPR) absorption. The HRSTEM results observed are supported by Raman modes and GI-XRD pattern. A preferential orientation change is observed in the tailored Bi film. Multiple SPRs arising from interband transitions and XPS etching studies confirm the metallic nature of Bi films. FESEM with EDS demonstrates film homogeneity and compositional purity. All the Bi films were tested for a preliminary SERS limit of detection (LoD) study using Rhodamine 6G (RH6G), for selecting a better substrate, supported by the hydrophobicity and Raman mapping studies done. The optimized SERS platform exhibited an unprecedented sensitivity, reaching upto 1 pM level LoD for Tyr with an ultra-sensitive SERS Enhancement Factor (EF) of 1.69 × 10¹². The detection of Tyr suggests the potential of Bi thin films as a promising alternative for ultra-sensitive SERS biosensing platforms.