Absorption of ADPA on the surface of conjugated polymer nanoparticles and its effect in the determination of ΦΔ

SPADA, RAMIRO MARTIN; CHESTA, CARLOS ALBERTO; LORENTE, CAROLINA; PONZIO, RODRIGO ANDRÉS; PALACIOS, RODRIGO EMILIANO

Carlos Paz

Congreso; XIII Encuentro Latinoamericano de Fotoquímica y Fotobiología (XIII ELAFOT); 2017

The determination of singlet oxygen O2(1Δg) quantum yields (ΦΔ) is of great importance when characterizing new photosensitizers for photodynamic cancer therapy (PDT) [1,2], water photodecontamination [3], etc. There are direct and indirect methods for the determination of ΦΔ. Direct methods estimate ΦΔ by quantifying the characteristic O2(1Δg) phosphorescence in the NIR region (1270 nm). Indirect methods are based on the evaluation of the rate of oxidation of O2(1Δg) chemical traps. Among the most commonly used traps are dimethylanthracene (DMA) and its hydrophilic derivative, i.e. sodium 3,3- (anthracene-9,10-diyl) dipropionate (ADPA). These traps have characteristic absorption bands at ∼ 400 nm and react efficiently with O2(1Δg) to form the corresponding endoperoxides, which are transparent at that wavelenght. Thus, the O2(1Δg) production kinetics and consumption can be easily monitored by UV-vis spectroscopy [4]. We report herein on the syntesis of nanoparticles (NP) of poly (9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) undoped and doped with platinum octaethylporphyrin (PtOEP) and their capacity for generating O2(1Δg). NPs were characterized by UV-vis absorption and stationary and time-resolved emission spectroscopies. The morphological characterization was carried-out by atomic force miscroscopy (AFM). ΦΔ were determined by direct and indirect method using eosin-Y as reference. Results obtained from O2(1Δg) phosphorescence measurements show that NPs doped with 10% w / w PtOEP have a ΦΔ = 0.24, which is ∼3 times greater than that of undoped F8BT NPs, i.e. ΦΔ = 0.09. However, ΦΔ obtained from the indirect method, ca. from the oxidation rates of ADPA, are substantially higher, i.e. 0.69 and 0.22 for the doped and undoped NPs, respectively. The discrepancy between the results obtained by the methods motivated us to carry-out a more detailed study of the oxidation processes of ADPA sensitized by the F8BT NPs. From these studies it was concluded that the indirect method overestimates the valores values due to the ADPA adsorption on the NPs surface. The concentration of ADPA adsorbed on the NPs was estimated by comparing the NPs excitation spectra in absence and presence of different concentrations of ADPA. Applying the Langmuir adsorption model, the number of bond sites per particle (~14000) and the equilibrium constant associated to the adsorption process (~ 8000 M-1) were determined. Since O2(1Δg) is generated on the NP surface where a fraction of ADPA resides, the oxidation efficiency of the chemichal trap increases significantly, resulting in overestimated ΦΔ values. These results caution about the use of ADPA as a probe to evaluate ΦΔ in these systems.