Читать книгу Handbook of Aggregation-Induced Emission, Volume 3 - Группа авторов - Страница 53

Conjugated polymers based on metal complexes or organic moieties have attracted remarkable interest in the field of fluorescent and electrochemiluminescent materials for their excellent photostability, brightness, and fast emission rate [4759–63].

Their nanoaggregated form has been, nowadays, widely exploited for different aims, like biosensing [64], bioimaging [65], controllable drug [66] and it was found to be an excellent ECL emitter [59, 67, 68]. Compared to quantum dots (QDs) [5, 21, 69], they are less toxic but they suffer from low ECL intensity due to their size and the slow diffusion in solution [70]. Therefore, much effort has been employed to prepare small polymeric nanoparticles (PNPs) with improved luminescence, in particular, choosing highly luminescent moieties and to improve it in aggregated form (AIE‐gens).

In order to achieve the high ECL intensity, Quan and coworkers have designed a three‐component polymer containing 9‐(diphenylmethylene)‐9H‐fluorene (DPF) that is known to be an important AIE‐gen [71]. They not only succeeded in preparing 10 nm nanoparticles in water with high AIE intensity at 543 nm but also demonstrated that such Pdots could give high ECL emission onto electrode surface upon addition of TPrA as coreactant [72]. The ECL spectrum showed the same AIE emission at 543 nm of DPF, clearly different from the PL peak at 415 nm of the other moiety 9,9‐dioctyl‐9H‐fluorene (DOF). The excited state of DPF is generated by Förster resonance energy transfer (FRET) mechanism from DOF moieties to DPF, since the latter has electrochemical inactivity at anodic scan. Both FRET and AIE properties improved their overall AI‐ECL emission.

The same group designed Pdots with donor‐acceptor AIE‐moieties in a three‐component structure obtained through Suzuki reaction (Figure 4.6) [31]. The performance of both designed polymers (P1 and P2 in Figure 4.6a) was assured by the different functions of the moieties. The D–A electronic components were represented by fluorene (P‐1)/carbazole (P‐2) as electron donor and the BODIPY as electron acceptor. The first intramolecular FRET pair was composed by the fluorene (P‐1)/carbazole (P‐2) unit, which was acting as a FRET donor, while the tetraphenylethene (TPE) AIE‐moiety as a FRET acceptor. The second intramolecular FRET pair was characterized by TPE as FRET donor and BODIPY as acceptor. TPE was essential for the enhancement of luminescence intensity after aggregation. Both annihilation and coreactant mechanisms were chosen to detect their ECL light in phosphate‐buffered saline (PBS) solution once immobilized onto the electrode surface. Results showed that P‐2 is more difficult to be oxidized in these conditions than P‐1 and their ECL emission happens at anodic potentials suggesting more stable radical anions than the cations. P‐2 ECL happens at a lower potential than P‐1 of 553 mV with a significant enhancement (Figure 4.6c). Their full photophysical study suggests that P‐2 contains a stronger electron‐donating unit such as the carbazole with a subsequent lower lowest unoccupied molecular orbital (LUMO) level that shifts the emission (25 nm) and makes weaker AIE intensity due to intramolecular charge transfer (ICT) phenomena. However, the lower LUMO facilitates the electron injection into the polymer, therefore, P‐2 ECL results higher in intensity than the P‐1 one. The coreactant ECL obtained by adding TPrA in PBS solution clearly enhanced both emissions, displaying always a higher emission for P‐2 than P‐1 for the aforementioned reason. The proposed mechanisms follow the annihilation pathway and the coreactant pathway initiated by the oxidation of TPrA presented by Bard and Miao [29]. The coreactant experiment was then allowed to record the ECL spectra, centered at 578 nm (P‐1) and 598 nm (P‐2).

Figure 4.6 (a) Chemical structures of polymers P1 and P2; (b) CVs (a and b) and ECL (a′ and b′) of P‐1 Pdots (a and a′) and P‐2 Pdots (b and b′) modified GCE in 0.1 M pH 7.4 nitrogen saturated PBS. Scan rate: 100 mV/s for CV and 500 mV/s for ECL. PMT: 850 V; (c) Coreactant ECL; and (d) corresponding CVs of P‐1 Pdots (blue trace), P‐2 Pdots (red trace) modified GCE, and bare GCE (black trace) in 0.1 M pH 7.4 PBS in the presence of 0.1 M TPrA as anodic coreactant. Scan rate: 100 mV/s. PMT: 700 V.

Source: Reproduced from Ref. [31].

Compared to organic moieties, metal complexes result still the highest efficient luminophores in ECL and its application. Then, they have been also integrated into polymers as pendants of a polyvinylpyridine/polystyrene (PVP : PS) backbone for ECL applications since the end of the twentieth century [47, 61, 62, 73, 74]. But it is only since two years ago that it was developed from our group the nanoaggregation strategy to enhance their emission, especially in aqueous systems through the formation of PNPs [27]. These metallopolymers, based on iridium(III) centers, have been easily synthesized and the formation of the respective PNPs was done by nanoprecipitation in water starting from a bulk tetrahydrofuran (THF) solution. They have shown to enhance their φ_PL and lifetime without modifying the emission wavelength and the oxidation potential. Therefore, an enhancement in ECL was obtained with a signal 12× higher than the polymeric thin film onto glassy carbon (GC). This is attributable to the formation of a soft structure in which the emitting centers are preserved from external quenching agents like oxygen and water.

Pdots and iridium‐based PNPs found, therefore, employment in the fabrication of disposable aptasensors where the enhanced ECL emission through aggregation could be significantly quenched or enhanced again from a secondary agent in affinity with the aptamer, leading to an “on–off” switch correlated to the concentration of the analyte [72, 75].

The same concept of PNP has been employed to prepare a copolymer with two different iridium centers in order to have a functional material, which could sensitize through energy transfer of the lower‐energy emission with a further enhancement by up of five times compared with metallopolymers containing only one type of iridium center [39]. The energy transfer occurs following electrochemical excitation (ECL‐ET) of PNPs obtaining a material that shows strong emission in water thanks to the aggregated form.

Also, Ruthenium was incorporated in polymeric structures to generate novel ECL active nanostructured materials with AI‐ECL properties. This is the case of a study on active block copolymers using the ring‐opening methatesis polymerization of two or three monomers among a Ru(MON), a C4(MON) (butyl‐MON), and a PEG(MON) [76], where MON is a monomer (4S,7S)‐6‐(1,3‐dioxo‐1,3,3a,4,7,7a‐hexahydro‐2H‐4,7‐epoxyisoindol‐2‐yl) hexanoic acid synthesized by the same group and reported elsewhere [77]. The resulting diblock/triblock copolymers, Ru‐C4, Ru‐PEG, and Ru‐C4‐PEG, presented the same electrochemical characteristic of [Ru(bpy)₃]²⁺ and Ru(MON). Self‐assembly of the polymers was done in acetonitrile by adding water. The aggregates were found by SEM to be on the order of hundreds of nanometers, possessing a strong lipophilic character. The presence of PEG sidechains provides a larger corona that increases the stability. The ECL activity of such aggregates was evaluated in acetonitrile, and water showing an emission intensity way higher after undergoing self‐assembly. The more restricted nanosphere morphology increases ECL emission by reducing the nonradiative vibrational routes and therefore favoring the radiative emission. This type of emission enhancement demonstrated by the nanospheres and the associated conformational change supports the rationale for an AI‐ECL‐based mechanism.