Electrokinetic Assembly of Nanoparticles for Surface-Enhanced Raman Scattering-Based (Bio)Chemical Sensing
electrokinetics , microelectrodes , sensors , nanotechnology
Surface-enhanced Raman scattering (SERS) enables sensitive detection of (bio)chemical analytes. However, the need for nanostructured noble metal substrates poses a major obstacle to its application. In this thesis, we present a novel metallic nanostructured SERS substrate that is also produced in a nanofabrication-free manner. The substrates are formed through the electrokinetic assembly of silver nanoparticles, which results in extended dendritic structures. A unique feature of these nanostructures is their ability to function as concentration amplification devices, thereby enhancing their analyte detection efficiency. Moreover, the used SERS-active dendrites can be removed and replaced in a few minutes. In this work, we demonstrate the detection of illicit drugs in both aqueous samples and saliva using our SERS substrates. Identification and quantification of illicit drugs is accomplished via Principal Component Analysis coupled with a Support Vector Machine. We demonstrate 100% accuracy in the detection of four illicit drugs (cocaine, heroin, THC, and oxycodone), and 98.3% accuracy in the quantification of cocaine across four orders of magnitude. Finally, we demonstrate a method for the detection of cocaine in saliva to a limit of detection of 100 ppb. We also establish a microfabrication-free “SERS-from-scratch” technique to produce our SERS substrates from a wide range of materials, including commercially available glass slides pre-coated with indium tin oxide. Our SERS substrates also have applications in the detection of toxic contaminants in food products. We apply our chips for: (1) the detection of thiram, a toxic pesticide, in apple juice, to a limit of detection of 115 ppb, with no sample preprocessing; and (2) the detection of melamine, a toxic food additive, to a limit of detection of 1.5 ppm in milk and 100 ppb in infant formula. Finally, we extend our method to enable SERS-active gold nanostructures via the electrokinetic assembly of gold nanoparticles. We explore the effects of conductivity on the growth of these nanostructures, and demonstrate the sensing capabilities of these gold nanostructures via: (1) the chemical detection of rhodamine 6G and thiram; and (2) a specific bioassay to detect streptavidin on biotin-modified gold nanostructures.