ABSTRACT
Fluorescent dye-doped nanoparticles represent an important class of nanomaterials for optical bioimaging [37-39]. Prominent among these materials are silica and organically modified silica (ORMOSIL), which have several favorable properties such as optical transparency, nonantigenicity, and rich surface chemistry for facile bioconjugation [42-44]. Here, a number of fluorescent dye molecules can be contained within each silica/ORMOSIL particle, which protects them from photobleaching and prevents their interaction with the biological environment. Fluorescent nanoparticles can permeate across cell membranes, which make them suitable for subcellular imaging applications. The absorption and emission of these nanoparticle-based probes are determined by the properties of encapsulated fluorophores. An increase of the signal output from individual nanoparticles can be accomplished by increasing the loading of the dye, which leads to an increase in the absorption cross-section per nanoparticle-based probe. 4.2.4 Gold NanoparticlesNanometer-sized metallic particles such as gold have emerged as a new class of materials for applications ranging from physics to
biology [42-44]. Gold colloids are well-known for their localized surface plasmon resonance (LSPR) properties, which originate from the collective oscillation of their electrons in response to optical excitation. The LSPR frequency of a particular metal colloid has been shown to depend on particle shape, composition, and refractive index of the surrounding medium, among many other factors [45, 46]. Metallic nanoparticles can be functionalized with biomolecules (e.g., small-molecule drugs, aptamers, peptides, and antibodies) for specific targeting of tumor cells and early detection of cancer [47-49]. GNPs are also extensively investigated in the area of in vitro diagnosis by virtue of their unique and tunable plasmonic signatures, which change color upon aggregation and deaggregation. In addition, as GNPs are known to be safe for biomedical use, their use as targeted in vivo diagnostic agents is fast gaining popularity. Giordano et al. have reported the targeted delivery of bioconjugated GNPs in the lung vasculature using a phage display approach, which constitutes a promising diagnostic platform for imaging cancer, emphysema, and other lung diseases [49]. 4.2.5 Iron Oxide Nanoparticles Owing to their high magnetic susceptibility, biocompatibility, and ease of surface biofunctionalization, iron oxide nanoparticles serve as excellent contrast agents for magnetic resonance imaging (MRI) [50-52]. These nanoparticles are already approved for use in clinical MRI (ferridex) [53]. In addition to in vivo MRI, magnetic biosensors based on these nanoparticles have been designed to detect a wide range of targets, including DNA/mRNA, proteins, enzymes, drugs, pathogens, and tumor cells [54-56]. 4.2.6 Carbon Nanotubes CNTs have several unique physical and catalytic properties, which include high electrical conductivity, chemical stability, and mechanical strength [57-58]. These make CNTs ideal for use in electrochemical sensors, which has already found applications in the detection of neurotransmitters, other proteins and peptides, small molecules such as glucose, and DNA. Different types of electrochemical methods are used in these sensors, including direct electrochemical detection with amperometry or voltammetry, indirect detection
of an oxidation product using enzyme sensors, and detection of conductivity changes using CNT-field effect transistors (FETs) [59].
