Small-molecule Fluorescent Chemosensors for Hg2+  Ion

Abstract

1271 introductionMercury pollution is a global problem. 1,2 Owning to the grisly immunotoxic, genotoxic, and neurotoxic effects, mercury is considered to be a highly dangerous element by the United States Environmental Protection Agency. 3,4 For the purposes of detection and quantitative determination of mercury ions, much effort has been devoted to the development of appropriate methods that are low-cost, rapid, deft and applicable to natural environmental and biological milieus. 5,6 In contrast with many of the traditional quantitative approaches to Hg 2+ analysis that require intricate, multistep sample preparation and/or sophisticated instrumentation, fluorometric detections following the changes in the emission of light are more convenient and suitable for monitoring Hg 2+ in either natural or biological environments. 6There are multiple advantages in using fluorescence: (i) its powerful visual effects; (ii) its greater sensitivity than that of absorption spectroscopy; (iii) its simple equipment, commonly available in many scientific laboratories. 7,8 As a consequence, for fluorometric determination of Hg 2+ , a large number of fluorescent sensors including two components, an ionophore (receptor) for selective binding of the substrate and a fluorophore (transducer) for providing the means of signaling this binding, have recently been designed and synthesized. 8 This review focuses mainly on the small-molecular fluorescent chemosensors for Hg 2+ ion in organic or aqueous solution that were developed by our research laboratories and other groups in recent years. Fluorescent chemosensors in this review are divided into two sections based on the bearing fluorophore. The first section focuses on the development of rhodamine-derived Hg 2+ ion sensors, while the other section gave a brief introduction about Hg 2+ ion sensors bearing other functional fluorophores. Mechanisms generally used in explaining the response of a fluorophore to Hg 2+ binding include photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), internal charge transfer (ICT), photoinduced charge transfer (PCT) and excimer/exciplex formation or extinction. 8,92 Rhodamine-derived hg 2+ ion SensorsNoelting and Dziewonski first reported the preparation of the rhodamine in 1905. 10 However, it was only from the 21th century that the rhodamine-B derivatives and its ring-opening reaction received a great deal of attention and have been applied as fluorescent metal ion sensors in organic and biological researches. 11As schematically shown in Fig. 1

A new rhodamine B based fluorometric chemodosimeter for Cu2+ ion in aqueous and cellular media

“…But upon the addition of Cu 2 þ to RBPH in the pH range 1-6 the solution turns pink and becomes highly fluorescent due to the transformation of spiro closed form to open form [21][22][23][24][25][26][27]. The above transformation was achieved by coordination of Cu 2 þ to the 'O' atom of the carbonyl group and 'N' atom of the imido goup of phenyl hydrazide moiety of the probe leads to the formation of the complex and subsequent hydrolysis of this complex leads to the formation of delocalized xanthene conformation of the rhodamine moiety [21,22] (Fig.…”

“…Xanthene derivatives possess very attractive spectroscopic properties such as large molar extinction coefficient, high fluorescence quantum yield, and visible light excitation as well as long wavelength emission [21][22][23][24][25][26][27]. These derivatives are colorless and non fluorescent, in spiro ring closed form whereas ring-opened forms are colored and have strong fluorescence emissions.…”

Acridine-derivated receptor for selective mercury binding based on chelation-enhanced fluorescence effect

“…To date, a number of small-molecule Hg 2 þ ion detection methods have been examined [11]. Most of these systems have limitations, which include interference from other metal ions [12], delayed response to Hg 2 þ [13], and/or a lack of water solubility, requiring the use of organic solvent [14].…”

Hg(II) ion specific dual mode signalling in a thiophene derivatized rhodamine based probe and their complexation cooperativity

“…Further, chromogenic signalling module offers a visual detection while fluorogenic module mostly addresses the sensitivity issue, hence, it should be advantageous to design chemosensors that facilitate both chromogenic and fluorogenic modes of signalling. Rhodamine based probes [17,[25][26][27] are ideal in this regard as the dye (a) exhibits excellent spectroscopic characteristics such as large molar extinction coefficient, high emission quantum yield, absorption and emission at longer wavelengths, (b) modulates the optical signal in aqueous media that facilitates in vitro and/or in vivo monitoring of biologically relevant species and (c) renders a contrasting structure-function correlation through its lactonization-delactonization process, i.e. its commonly existing spirolactam form which is colourless and non-fluorescent in protic solvents structurally equilibrates to the coloured and highly fluorescent ring-opened amide form upon complexation.…”

“…its commonly existing spirolactam form which is colourless and non-fluorescent in protic solvents structurally equilibrates to the coloured and highly fluorescent ring-opened amide form upon complexation. Following this protocol, many rhodamine based probes for Hg(II) ion detection have been reported [25][26][27][28][29][30][31][32] where the signal transduction pathway involves delactonization of its spiro-ring through either metal ion coordination or ion-induced chemical reactions. The signalling action of many of these probes is limited in terms of practical utility as they have either exhibited irreversible signalling with Hg(II) ion that defy the reusability or pose a restricted detection limit and often are cross sensitivity towards other metal ions.…”