||This thesis consists of two parts. Part Ⅰ covers Chapters 1 to 4, and focuses on the electrochemical synthesis of well-defined Ag nano- and microstructures and their application as substrates in single particle SERS. Part Ⅱ includes Chapters 5 to 7, and is devoted to the applications of Ag nanoparticle-nanocavity dual-structured network film in SERS-based biosensing. In Part Ⅰ, after a concise overview of the relevant work done by others in Chapter 1, Chapters 2 to 4 describe several electrochemical methods for the synthesis of homogeneous Ag structures. Chapter 2 presents a cyclic voltammetric method for the synthesis of homogenous Ag fractal, nest, cube, cubic cage, flower and rod by tuning the concentration of precursor without employing any seeds and surface capping agent (concentration-tuning method). Chapter 3 demonstrates an amperometric method for the synthesis of Ag cube, cubic cage, truncated cube, nest, flower and fractal by changing deposition time only without using any seeds and capping agent (time-tuning method). Chapter 4 reports the electrochemical methods for the synthesis of 2-dimensional Ag fractal matrices on template arrays, with variable size and density. To the best of our knowledge, this is the first report for the synthesis of 2-dimensional noble metal fractal arrays. The surfaces of the Ag structures synthesized in Chapters 2 to 4 are clean, allowing the chemical modification by desired molecules. The as-prepared Ag structures were employed as the substrates to demonstrate real time, far-field single particle SERS in the study of molecules without (4-mercaptobenzoic acid or 4-MBA) and with electronic resonance (Rhodamine 6G), and with biological relevance (cytochrome-c). The as-prepared Ag structures can be easily visible and tracked under conventional optical microscope, making it very convenient to conduct single particle SERS. Regular Ag structures display high SERS activity attributable to their edges and vertexes. The surface enhancement factor of several types Ag structures reaches as high as ~108. The as-synthesized Ag fractals display very high activity in SERS with estimated enhancement factors higher than 109. In Part Ⅱ, following an introduction of the relevant work done by others in Chapter 5, Chapters 6 and 7 report nonelectrochemically fabricated nanostructured Ag network films, originally developed in our lab, for the detection of single strand DNA (ssDNA) and microRNA (miRNA) in Chapter 6, and a SERS-based aptameric biosensor for ATP detection in Chapter 7. The concentration of the target ssDNA and miRNA at 1 nmol can be easily detected. This platform can differentiate three mismatched bases in the middle of a 26-mer miRNA. On the SERS-based aptameric biosensor, 0.5 μm of ATP can be easily and selectivity detected in the presence of GTP, cAMP and cGMP. These two examples demonstrated that 2-dimensional nanoparticle-nanocavity dual-structured network films can be employed as a good SERS-active substrate for applications in SERS-based biosensing. At the end, a brief summary and future perspective are given on the basis of the studies carried out in this thesis.