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

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.

      4.2.3 Organic Molecules

      Over the past decade, polycyclic aromatic hydrocarbons (PAHs) have been considered a fantastic class of emitters thanks to their low cost of production, structural tailorability, and excellent optoelectronic properties [78, 79]. Also, ECL was evaluated for PAHs with not satisfying results for their possible application, in particular, in biosensing [18, 70, 80]. These PAHs emitters were investigated usually in organic phase in nonaggregated form. The effect of an aggregation has been, therefore, studied with the advent of the AI‐ECL phenomenon.

      The first organic molecule investigated for AI‐ECL was made from coumarin derivatives in an aqueous solution [81]. A donor‐acceptor structure based on 6‐[4‐(N,N‐diphenylamino)phenyl]‐3‐ethoxycarbonyl coumarin (DPA‐CM) was reprecipitated in water from a THF solution in order to form nanoparticles of 5.82 nm. The resulting particles revealed a red‐shifted absorption and a blue‐shift in the emission spectra in water compared with the one in THF with an enhancement of the φPL in water of 6 rather than 0.8% in THF. The low φPL in organic solvent is ascribed to the structural flexibility of DPA‐CM, which is lost as nanoparticle [82] and to the intramolecular photoinduced electron transfer from DPA to CM [83]. By coating a GC electrode with DPA‐CM nanoparticles, the corresponding ECL emission during anodic scan was significantly enhanced with an outstanding reproducibility and lower standard deviation. The system was then employed for the detection of ascorbic acid (AA), uric acid (UA), and dopamine (DA) in an aqueous solution, which will be discussed in the applications and outlooks (Section 4.4).

The first observation of AI‐ECL of hexagonal tetraphenylethylene microcrystals (MCs) in water was done by Yuan and coworkers recently (Figure 4.7) [42]. This enhancement was explained by the restriction of intramolecular motions limited by the crystal formation. In particular, it shows anodic ECL behavior with a great enhancement comparing the dispersed molecule in organic solution (Figure 4.7b). From the study, it is also clear that there is a notable redshift by almost 235 nm between the PL and ECL emission spectra, typical of aggregation events in solution. The AI‐ECL of these microcrystals then was employed to build an ultrasensitive «off‐on» ECL biosensor for Mucin1 (MUC1), which is a transmembrane glycoprotein expressed in human malignancies and therefore used as a biomarker for cancer.

Figure 4.7 (a) TPE molecular structure; (b) ECL‐potential profile for bare GCE in 0.1 M TBAPF₆ THF solution containing 1 mg/ml TPE monomers and 20 mM TEA (curve a, blue trace), and in 0.1 M PBS containing 1 mg/ml TPE MCs and 20 mM TEA (curve b, red trace). The inset of panel (b) shows a schematic diagram of the relationship between emission intensity and molecular states for the TPE; (c) ECL transients obtained for bare GCE in 0.1 M TBAPF₆ THF solution containing 1 mg/ml TPE monomers and 20 mM TEA (blue trace), and 0.1 M PBS containing 1 mg/ml TPE MCs and 20 mM TEA by stepping the potential between 0 and 1.6 V (red trace); (d) Schematic illustration of the preparation of TPE NCs by self‐assembly and the NIR aggregation‐induced enhanced ECL properties; (e) TEM images of TPE NCs.

      Source: Reproduced from Refs. [42, 43] The Royal Society of Chemistry.

      TPE nanocrystals (NCs) were also obtained by the same group, showing NIR enhanced ECL emission upon aggregation in aqueous solution, while it is invisible as a molecular entity in organic solvents (Figure 4.7d) [43]. The synthesis involved desolvation method for obtaining crystals of 450 nm (Figure 4.7e). For both MCs and NCs, triethylamine (TEA) was employed as a coreactant. From the spectral characterization and ECL studies, they give two possible options to explain the ECL enhancement: the first one follows the idea of a possible reduced energy gap when TPE molecules aggregate to form NCs, which allow highly efficient electron–hole recombination to obtain the excited state; the second possibility is that the restriction of intramolecular free rotation of phenyl ring reduces the nonradiative relaxations to obtain further ECL enhancement. Interestingly,