Handbook of Aggregation-Induced Emission, Volume 3. Группа авторов

of ECL applications, dubbed the ion annihilation (Figure 4.2) and the coreactant approaches (Figure 4.3). The latter can be further classified into oxidative‐reductive ECL or reductive‐oxidative ECL.

Figure 4.2 Schematic diagram describing the electron transfer reactions responsible for emission during annihilation ECL.

Figure 4.3 Schematic diagram describing the electron transfer reactions responsible for emission during a coreactant ECL reaction: on the left the oxidative‐reductive pathway, on the right side the reductive–oxidative pathway.

In the ECL timeline, the ion annihilation approach for the generation of excited states was the first one explored due to the relative simplicity of the system involved. One single species, A, is both reduced and oxidized (to A⁺ and A⁻, respectively) at an electrode, by alternate pulsing of the electrode potential, according to Equations 4.1 and 4.2 [3, 30]. These species react in the Nernst diffusion layer over the electrode according to Equation 4.3, to yield an excited state A* along with a molecule in its ground state A.

(4.1)

(4.2)

(4.3)

      (4.4)

      Most annihilation ECL materials are organometallic complexes, and only one report has shown an AI‐ECL generated by annihilation pathway, which consists of a donor‐acceptor conjugated polymer dot (Pdot) and it will be elucidated in Section 4.3.2 [31].

      All the other reports on AI‐ECL, involve the coreactant mechanisms, which not only are easier to operate but also cover the major research direction of possible applications in biosensing. Indeed, the ECL coreactant mechanism is the basis of all commercially available instruments [32].

      Considering one potential step generation, coreactant ECL shows several advantages over annihilation ECL. First, there is no need for a wide potential window so other solvents with a narrow potential window and aqueous solution can be also used. Further, there is no need of rigorously purified and deoxygenated solvents because oxygen and water quenching are less efficient. Thus, a reaction can be carried out in the air. Finally, the use of coreactant makes ECL possible even for some fluorophores that have only a reversible electrochemical oxidation or reduction, while annihilation ECL, in general, requires both of them.

      A coreactant is a species that upon electrochemical oxidation or reduction undergoes fast chemical decomposition to form a high‐energy reducing or oxidizing intermediate. The latter can react with an oxidized or reduced luminophore to generate excited states (Figure 4.3).

The oxidative‐reductive coreactant mechanism finds one of the best candidates in oxalates [33], as discovered by Bard and co‐workers, and extensively studied by many research groups [34–38]. AI‐ECL can also be generated by using oxalate in aqueous solution, as explained during the first discovery by our group and in subsequent reports [27, 28, 39]. Oxidation of oxalate would produce oxalate anion radicals (C₂O₄^∙−), which is then followed by a chemical decomposition to form a highly reducing intermediate (CO₂^∙−, E°= 1.9 V vs. NHE) which react with the oxidized AS at the electrode to generate ECL emission. The corresponding mechanism is described in Equations 4.5–4.11, and shown in on the left side of Figure 4.3.

(4.5)

      (4.6)

      (4.7)

      (4.8)

(4.9)

      This is a typical case of an electron transfer chemical reaction (EC′) reaction [40], which has been extensively discussed by Bard et al. [33]. By applying the anodic potential, the AS is first oxidized to AS⁺ at the electrode surface. This cation is then capable of oxidizing C₂O₄²⁻ in the diffusion layer close to the electrode surface to form an oxalate radical anion C₂O₄^•−, which decomposes to a highly reducing anion radical CO₂^•− and carbon dioxide. The excited state AS* can be obtained by direct reaction between CO₂^•− and the oxidized AS⁺.

Another important example of oxidative‐reductive ECL is based on the use of alkylamines. [Ru(bpy)₃]²⁺ or its derivatives with tri‐n‐propylamine (TPrA) as coreactant, exhibits the highest ECL efficiency and represents the most common luminophore/coreactant couple, which forms the basis of commercial immunoassays and DNA analyses [3, 4], and it can be considered as an ECL standard. Noffsinger and Danielson have first reported the [Ru(bpy)₃]²⁺ ECL reaction with alkylamines [41], and in 1990 Leland and Powell first reported the use of TPrA with Ru(bpy)₃²⁺ to produce highly intense ECL [12]. The experiment was carried out on a gold electrode in a buffer solution of TPrA and Ru(bpy)₃²⁺.

      Also, to generate AI‐ECL the employment of trialkylamines was chosen many times, and examples will be clarified in Section 4.3 [42–44].) Coreactant ECL using trialkylamines can proceed through several possible parallel routes. One pathway for AS‐TPrA coreactant ECL is represented by the following reactions [12, 45, 46]:

      (4.10)

(4.11)