Development of Fluorescent Turn-On Sensor for Selective Detection of Cu2+in Aqueous Solution

Abstract

Cu is one of the most essential metal ions in biological systems, since it plays an important role in numerous biological processes and can induce various disorders, including some neurodegenerative diseases such as Alzhemer’s, Prion, and Parkinson’s diseases. Thus, quantifying the amount of Cu in a systems crucial for studying the pathological roles of Cu in living systems. The development of fluorescent sensors for Cu has received much attention in last decade because of their simplicity and high sensitivity. In addition, fluorescent sensors can offer the potential application in real time detection in living systems with high spatial resolution that is difficult or impossible to obtain using conventional techniques such as atomic absorption spectroscopy and inductively coupled plasma mass spectroscopy. In particular, the development of a fluorescent turn-on sensor is critical to enhance detection sensitivities. Although there have been reports on fluorescent Cu sensors, fluorescent turn-on sensors are still relatively rare because the paramagnetic nature of Cu (3d) tends to quench the fluorescence. Recently, several groups have proposed copper promoting reactions including hydrolysis, oxidative cyclizations of thiosemicarbazone to develop fluorescent turn-on sensor. Rhodamine derivatives are one of the most widely used fluorescent dyes in the design of fluorescent sensors due to their great photo-stability, high quantum yield, emission at long wavelengths, and applicability in biological systems. Recently, Li et al. reported a rhodamine sensor containing a thiourea group immobilized on mesoporous silica for the detection of Cu. However the sensor showed a lack of selectivity for Cu versus Hg, because a rhodamine B derivative containing a thiourea group involves the ring opening of spirolactam followed by cyclization in the presence of Hg. Herein, we report on the development of rhodamineimmobilized mesoporous silica (RhMS) and the characterization of its metal recognition properties. To discriminate the Cu from Hg, we changed the thiourea group to a urea group to diminish the rhodamine derivative reactivity toward Hg. We found that the fluorescence turnon chemodosimeter RhMS exhibited a rapid response and good selectivity toward Cu over other metal ions. Moreover, the RhMS exhibited simultaneous colormetric and fluorescence enhancements upon the addition of Cu. The RhMS was synthesized as shown in Scheme 1. Figure 1 shows a TEM image of the synthesized mesoporous silica (MS) (a) and RhMS (b). As shown in Figure 1, the synthesized RhSi and RhMS were well-ordered. The immobilization process did not give rise to significant changes in the ordered structure. To confirm the successful grafting of Rh-Si onto the mesoporous silica, FT-IR spectroscopy, TGA analysis, and nitrogen adsorption-desorption measurements were carried out. Figure 2 illustrates the IR spectra of the mesoporous silica, RhMS, and Rh-Si in solid form. The absorption bands of the