Electrochemiluminesence of ruthenium complex and its application in biosensors

Jing Li, Xiaofang Jia and Erkang Wang

DOI: 10.1039/9781849737791-00001

Electrochemiluminesence (ECL), also called electrogenerated chemiluminescence, is optical emission that arises from the high-energy electron-transfer reaction between electrogenerated species. ECL is an approach of converting electrical energy into radiative energy. Different from photoluminescence, ECL does not require the use of external light sources and therefore problems derived by light scattering can be avoided.

1 Introduction

Electrochemiluminesence (ECL), also called electrogenerated chemiluminescence, is optical emission that arises from the high-energy electron-transfer reaction between electrogenerated species. ECL is an approach of converting electrical energy into radiative energy. Different from photo-luminescence, ECL does not require the use of external light sources and therefore problems derived by light scattering can be avoided. As an important analytical method, ECL, a marriage of chemiluminescence (CL) and electrochemistry, exhibits potential advantages over CL: (1) ECL allows the time and position of the light-emitting reactions to be accurately controlled by applying a suitable potential on an electrode surface. By controlling time, ECL can be obtained until some reactions have taken place. The better control over the emission position by confining light emission to a region that is precisely located with respect to the detector can be beneficial for sensitivity by dramatically improving the signal-to-noise ratio. In addition, control over position enables the determination of multi-analytes by interrogating each electrode in an array with the development of ECL microscopy, (2) ECL can be initiated selectively by switching the electrode potentials, (3) Some ECL emitters can be regenerated during the ECL process, which greatly enhances the sensitivity of the technique, saves reactants and simplifies the set-up, (4) During the ECL process, the current signal and light signal are obtained simultaneously, which facilitate the investigation of light emission mechanism by electrochemical methods.

Since the first detailed investigations about the ECL emission were reported in the mid-1960s, a variety of ECL emitters have been explored including organic (e.g., luminol), inorganic (e.g., metal complexes) and nanomaterials based ECL systems (e.g., QDs). Among them, ECL of ruthenium complex and its co-reactants as a sensitive detection method have been extensively investigated, especially the application of Ru(bpy)32+.

Up to now, ECL assays based on the ruthenium complex have been elaborately designed and widely used in the areas of clinical diagnostics, food and water testing, environmental monitoring, biowarfare agent detection, and scientific research. Progress in the field were summarized in several excellent review articles. And certain types of ECL instruments are now commercially available, such as cobas e 411 analyzer from Roche Diagnostics Corp., Sector PR Reader 400 by Meso Scale Discovery Corporation and capillary electrophoresis (CE)-ECL system developed by the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and manufactured by Xi'an Remax Electronic Co. LTD. The basic components of an ECL instrument include an electrical energy supply for initiating the ECL reaction at an electrode within an electrochemical cell and an optical detection system for measurement of either the emitted light intensity(for quantitative analysis) or its spectroscopic response (for qualitative analysis). The light detection system can be integrated by a photo- multiplier tube (PMT) biased at a high voltage with a high-voltage power supply, a charged coupled device (CCD) or a photodiode. The utilization of a PMT can provide the most sensitive way to detect light with single photons. The use of CCD camera has received more attention in ECL imaging and high throughput analysis owing to instant image manipulation, high spatial resolution, and multi-channel detection ability.

In this chapter, we summarize advances in the development of ruthenium complex based ECL biosensors mainly focusing on the principle of ECL and their applications in the fabrication of various biosensors.

2 Principle of ECL

There are two dominant pathways through which ECL can be obtained: annihilation pathway and co-reactant pathway. Although most of the ECL applications are based on the co-reactant pathway, the early ECL emission originated from annihilation ECL.

2.1 Annihilation ECL

In the annihilation pathway, the reduced and oxidized species are both generated in the vicinity of the electrode surface by alternate pulsing of the electrode potential. The corresponding process is outlined in the Eqn (1–4). The annihilation ECL can also be achieved in the "mixed systems". The ECL is achieved via "cross-reactions" between the radical ions of the different species.

R - e- -> R•+ (Oxidation at electrode) (1)

R + e- -> R•+ (Reduction at electrode) (2)

R•+ + R•- -> R* + R (Excited state formation) (3)

R* -> R + hv (Light emission) (4)

The excited state R* can represent the lowest singlet state species (1R*) or triplet state species (3R*) according to the energy available during the annihilation reaction. If the enthalpy of the ion annihilation (ΔH) exceeds the energy required to produce the lowest excited states from the ground state (Es, which can be obtained spectroscopically), the light emission process follows the singlet route "S-route" (Eqn (3a)) and the system is called the energy-sufficient system. ΔH can be calculated based on Eqn (5), where TΔS is estimated to be about 0.1 eV at 25 °C, E0 is reversible standard potentials of the redox couples. Most ECL systems based on aromatic compounds are in accordance with this mechanism, such as the ECL of rubrene (ΔH obtained from the electron-transfer reaction is 2.32 eV, and Es is 2.30 eV). Another example of an energy sufficient system is the inorganic species Ru(bpy)32+ (ΔH is ca. 2.6 eV, and Es lies at 2.10 eV with λmax = 620 nm).

[MATHEMATICAL EXPRESSION OMITTED] (5)

(1) S-route

R•+ + R•- ->1R* + R (Excited singlet formation) (3a)

In contrast, if ΔH is smaller...