Читать книгу Handbook of Aggregation-Induced Emission, Volume 3 - Группа авторов - Страница 50

There is not a particular class of mechanisms for AI‐ECL, therefore, ECL classic mechanisms can be used to explain this phenomenon [1, 29]. In ECL, the excited state responsible for the emission of the photon, is generated from the reaction of intermediates generated electrochemically. Indeed, ECL shares feature with both chemiluminescence and electroluminescence, since light emission is ultimately initiated and controlled by application of a potential at an electrode.

In a basic ECL experiment, a potential scan from anodic to cathodic part is performed to a solution of [Ru(bpy)₃]²⁺. A red luminescence develops at the working electrode at the oxidation and reduction potentials of the complex. The same red emission can be obtained when an amine is added to the solution. Upon anodic scan, it is possible to collect light right in front of the electrode surface.

The two cases outlined represent the two main approaches used in the development 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)

(4.12)

(4.13)

(4.14)

(4.15)

followed by Equation 4.9.

When the concentration of TPrA is high and the concentration of AS is low, as is generally the case when the luminophore is being used as a probe in a bioanalytical context, an alternative pathway emerges [29].

(4.16)

(4.17)

Here, TPrA^• generates the reduced form of the luminophore via homogeneous electron transfer (Equation 4.11), following which, the excited state may be formed via reaction, according to Equation 4.3 (annihilation), or by reaction with the radical cation TPrA^•+ (Equation 4.17).

In general, there is a strong dependence of ECL intensities on pH according to analytes. Oxalates show virtual independence of ECL intensities on sample pH but are mostly detected at pH 6.0 [47]. Alkylamines produce maximum intensities between pH 4.5 and 6.0 [48, 49].

The reductive‐oxidative mostly employed mechanism concerning K₂S₂O₈, which has been firstly explored by White and Bard in 1982 [50], and used in AI‐ECL of siloles [51]. K₂S₂O₈ offers some advantages with respect to amines. Because S₂O₈²⁻ does not react appreciably with water or oxygen, it appears particularly promising for aqueous ECL, where the decomposition of water generally prohibits the direct production by electrolysis of the reactant that generates the excited state. The pathways are summarized by the Equations 4.18–4.21:

(4.18)

(4.19)

(4.20)

(4.21)

At first, AS gets reduced to generate the radical anion AS^•− (Equation 4.18). Then, the reaction between this radical anion and S₂O₈²⁻ produces the strong oxidant species SO₄^•− (Equation 4.19). Following the electron transfer between the AS^•− and SO₄^•− that generates the excited state (Equation 4.20) that finally emits light (Equation 4.21).