ECL Luminophores

Abstract

Finding new luminophores with higher ECL efficiencies and modifying a moiety of the emitter to use it for the labeling of biomolecules are the two driving forces that lead to the synthesis of a number of new ECL-emitting species in the past several years. Three categories of luminophores are discussed in this chapter, including (a) inorganic systems, which mainly contain organometallic complexes; (b) organic systems, which cover polycyclic aromatic hydrocarbons (PAHs); and (c) semiconductor nanoparticle systems.Keywords Luminophores Á Organic systems Á Inorganic systems Á Dendrimers Á Nanoparticle systems Á Nanoclusters Á Carbon nanoparticles Á Semiconductor nanocrystals Á Quantum dots Since the discovery of ECL process, there has been considerable advancement in ECL system. Different types of ECL systems have been introduced, such as organic, inorganic, and nanoparticle systems. Organic SystemsThe first exhaustive study on ECL involved organic systems [1][2][3]. In the early ages of ECL, PAHs, rubrene, and DPA have been studied extensively owing to their high fluorescence quantum yields and stable radical cations and anions in aprotic media. Some other organic systems, such as luminol [4], acridinium esters [5], polymers [6][7][8], and siloles [9], have also been widely explored. The reader is referred to the tremendous reviews [10][11][12] and ECL monograph [13] for more thorough study about organic systems. Acridinium esters, taking example of lucigenin (N,N'-dimethyl-9,9-bisacridinium) is reported ECL active [14] in the presence of hydrogen peroxide [15] and has been applied for the detection of riboflavin [16], human chorionic gonadotropin [17], and hemin [18]. The ECL reaction of methyl-9-(p-formylphenyl)acridinium carboxylate fluorosulfonate (MFPA) gives intense ECL signal and is employed as a label in bioassays [17]. Lucigenin ECL has also been extended to surfaces [19]. Self-assembled monolayers (SAMs) modified electrodes and solutions containing Triton X-100 surfactant molecules caused the enhancement in ECL of lucigenin.Fluorene-substituted PAHs have been investigated with enhanced ECL efficiency and stability [20]. Derivatives of DPA, pyrene, and anthracene were produced by using fluorene as a capping agent. These molecules have high PL quantum yields and can generate stable radical ions. On introducing the fluorene groups, steric hindrance is imparted, preventing interchromophore interactions and blocking the active position of PAH cores subject to electrochemical decomposition. Due to the electrochemical instability of its cationic radical, pyrene molecule displays poor ECL properties. Hence, a strategy for ECL enhancement and radical stability by peripheral multidonors on pyrene derivatives has been adopted in which the ECL efficiencies of pyrene derivatives increase in proportion to the number of peripheral N, N-dimethyl aniline donors [21].An ECL laser offers numerous advantages over traditional techniques, including tunability, lack of additional source and range of available wavelength...