The transition from locally excited states to twisted intramolecular charge transfer states for fluorescence methylene blue labeled in biodegradable silica particles

Abstract

Abstract Fluoresce signal is widely used in fields of solid state lasers, optoelectronic devices and medical diagnostic devices. Its brightness and photostability are related to the aggregation state of organic fluorescence dye, such as methylene blue (MB). Herein, silica-MB-ABS powders were synthesized by a modified Stober method following a treatment with acetate buffer solutions (ABS, pH\u202f=\u202f3.60) and characterized by scanning electron microscopy, Fourier transform infrared spectra and Raman spectra. An effective strategy was employed in these fluorescent powders to modulate the aggregation state and fluorescent properties of MB through the hindering effects of Si-O-Si network and acetate ions. The entrancement of acetate ions destroys the π-π stacking of MB monomers, resulting in an increased release of MB monomers from silica-MB-ABS. In comparison with that of free MB particles, fluorescent intensity of silica-MB increases significantly with a blue-shifted emission, indicating that luminogenic MB molecules are assigned to locally excited states during aggregation-induced emission. With the presence of acetate ions, silica-MB-ABS exhibits a red-shifted emission with decreased intensity because MB molecules are aggregated in a twisted intramolecular charge transfer state. This observation is confirmed by calculated values of large twisted angles of MB released from silica-MB-ABS, resulting in a fast rate of charge transfer from excited states though a non-emission decay. Meanwhile, the kinetic release of MB is well described by a Korsmeyer-Peppas equation and controlled by a combination of diffusion and matrix degradation mechanisms. The simulated release exponents of n decrease for silica-MB-ABS synthesized with increased concentration of acetate ions, indicating the reduction of limitation of matrix degradation on outward diffusion of MB from high-degraded particles. This study could shed light on tiny modification of aggregation state of organic fluorescence dyes and the modulation of fluorescent properties used in solid state lasers, fluorescence diagnostic devices and photochemical sensors.