Enhanced photophysical properties of silica-methylene blue@amorphous carbon as fluorochromes with modulated Raman-active vibration modes

Abstract

As a kind of near-infrared dye, methylene blue (MB) molecule is proneness to form excimers in solid powder or solution at high concentration, which results in weak photoluminescence intensity during clinic diagnostic and cell labeling applications. Aiming to improve the quantum yield of MB, silica-MB particles are herein prepared and treated hydrothermally (HT) to synthesize silica-MB-HT and silica-MB@amorphous carbon with glucose as carbon source (silica-MB@AmC). Significantly, hydrothermal treatment causes destruction of Si-O-Si network, modulation of aggregation state, variations of energy gap and release behavior of MB. In comparison with that of silica-MB, emission intensities of both silica-MB-HT and silica-MB@AmC increase greatly and their photoluminescence peaks are blue-shifted, which are closely related with Raman-active vibration modes of MB in a locally-excited state. Enhanced emissions of both silica-MB-HT120 and silica-MB@AmC120 are observed with increased intensities of Raman peaks assigned to in-plane bending vibrations of CH bond during skeletal deformation of CC in ring. In contrast, the in-plane bending vibration for CH bonds at end group of MB is considered as a way of radiation decay, and strong vibration of this Raman-active mode causes weak emission intensities of silica-MB-HT180 and silica-MB@AmC160. The stabilities of photophysical properties are compared after 72 h of incubation in buffered mediums or 5 h of illumination under ultraviolet light. The photoluminescence intensity of silica-MB@AmC120 fluorophore increase remarkably after illumination, meeting the requirements of reduced photobleaching, minized solvatochromic shift and increased fluorescent efficiency during versatile applications.