||The thesis is focused on the applications of the layer-by-layer (LbL) electrostatically assembled nanostructured thin films in immunoassay and electrocatalytic analysis of biospecies. Chapter 1 introduces the state-of-the-art technique of the LbL assembly, including the assembly principle, characterization methods and specific applications. Chapter 2 presents the development of a novel class of biofunctional fluorescent microparticles and their applications in immunodetection. LbL deposition of fluorescence-labeled polyelectrolytes onto charged latex microparticles results in a linear increase of fluorescent out-put without quenching. Fluorescent microscopy image (FMI) analysis provided direct measurement of the emission intensity of single microparticles, which overcome the problem of inaccuracy of the conventional steady-state fluorescence measurement. Chapter 3 demonstrates a new approach to encapsulate uncharged organic microcrystals via LbL assembly. Preadsorption of amphiphile (ionic surfactant, phospholipid and polyelectrolyte) molecules introduced charges onto the organic crystals, which facilitated the following electrostatic assembly of polyelectrolyte multilayers. The encapsulated core particles could be removed by mild organic solvent, while hollow polymer capsule remained. This method extends the applicable area of LbL technique and has potential application in controlled drug release. Chapter 4 describes the development of another novel class of particulate fluorescent immunolabel via LbL approach. Fluorescein diacetate microcrystals were encapsulated by polyelectrolyte multilayers and a layer of antibody molecules and then used as labels in fluorescent immunoassays. Extremely high fluorescent dye/protein ratio was obtained and high amplification of fluorescent signal in immunotests was successfully achieved. Chapter 5 reports the strategy and technique to prepare a new type of bifunctional films by LbL deposition and photochemical reduction. Cationic metalloporphyrin and anionic gold precursor were deposited alternatively on ITO electrodes. Ultraviolet irradiation of the resulting multilayer film yielded monodisperse gold nanoparticles embedded in metalloporphyrin matrix, which gave good response in simultaneous electrocatalytic detection of nitric oxide and oxygen. This achievement is promising in in situ detection of multicomponent in a single cell.