Metallic nanostructures as SERS substrates for chemical and biological sensing

Surface-enhanced Raman scattering (SERS) has emerged as a transformative analytical technique at the intersection of nanotechnology, biomedical sciences, and forensic applications. Despite its ability to provide sensitive detection by amplifying Raman signals, SERS lacks in areas such as low concentration protein detection, or countering interference from nonspecific binding. Detecting (bio)molecules in complex environments is essential for disease diagnosis and environmental monitoring but maintaining specificity while improving sensitivity is challenging. Here we discuss our contributions to the development of electrochemically fabricated metallic nanodendritic structures for ultrasensitive detection of protein molecules in physiologically relevant samples. We use micron-scale electric fields for achieving accelerated and deterministic capture of target analytes on the sensor's surface, followed by electric field-guided superimposition of silver nanoparticles to create a sandwich assay. This label-free, electric field-assisted, sandwich-based SERS assay (LESS) enabled us to detect low concentrations of proteins and viruses. Recently, we uniquely integrated the plasmonic activity of our SERS assay with molecular recognition to achieve selective detection of target proteins. Specifically, our silver dendrites were coated with a thin gold shell, resulting in bimetallic silver-gold core-shell nanostructures. Subsequently, we immobilized antibodies on the plasmonic structures using a thiol-based self-assembled monolayer (SAM). The sensitivity and specificity of the detection were assessed using green fluorescent protein (GFP) as the model analyte. The SERS signal of the captured protein was then amplified by superimposing silver nanoparticles with the aid of an electric field, forming a "sandwich" structure that preferentially enhances the SERS signal of the top molecular layer (captured protein). Our method is highly specific, reproducible, and capable of reliable detection of target proteins in physiologically relevant complex media. Our work paves the way for the incorporation of hybrid detection methods, such as SERS-based immunoassays, into rapid, reliable, user-friendly, point-of-need diagnostic devices.