Fluorescence Resonance Energy Transfer (FRET) sensor

The FRET has been widely used as a spectroscopic technique in all applications of fluorescence including medical diagnostics, DNA analysis, optical imaging [1] and for various sensing properties [2-11]. Generally, fluorescence-based sensors adopt three different strategies: (a) fluorescence quenching (turn-off), (b) fluorescence enhancement (turn-on) and (c) ratiometric FRET. FRET sensors became popular tools for studying intracellular processes [12-16]. FRET between two molecules is an important physical phenomenon with considerable interest for the understanding of some biological systems and with potential applications in optoelectronic and thin film device development [17, 18]. In this account, we review the application of FRET for the designing of various sensors, focusing primarily on ion sensor, hard water sensor, pH sensor and bio-sensor. The technique of FRET, when applied to optical microscopy, permits to determine the approach between two molecules within several nanometers. FRET was first described over 50 years ago, that is being used more and more in biomedical research and drug discovery today. FRET is an electrodynamic phenomenon that occurs through non-radiative process where an excited state donor D (usually a fluorophore) transfers energy to a proximal ground state acceptor A through longrange dipole−dipole interactions [19]. The acceptor must absorb energy at the emission wavelength(s) of the donor. The rate of energy transfer depends on a number of factors including the fluorescence quantum yield of the donor in the absence of acceptor, the refractive index of the solution, the dipole angular orientation of each molecule. FRET can be an accurate measurement of molecular proximity at angstrom order distances (10-100Å). Due to its sensitivity to distance, FRET has been used to investigate molecular level interactions [20-27]. Though the physics and chemistry behind the FRET have been well studied theoretically for years but only with recent technical advances it has become feasible to apply FRET to sensing research [2-11]. Recent advances in the technique have led to qualitative and quantitative improvements, including increased spatial resolution, distance range and sensitivity. FRET mechanisms are also important to other phenomena, such as photosynthesis, chemical reactions and Brownian dynamics [28,29]. Recently, FRET phenomenon has been employed for the study of conformation and the structure of protein [30], detection of spatial distribution and assembly of proteins [31], designing of biosensor [10], nucleic acid hybridization [32], distribution and transport of lipids [33]. One of the important methods for sensing of different chemical and biological materials is fluorescent sensors. In recent years, fluorescence spectroscopy has become a powerful tool for the detection of transition and heavy metal ions with high sensitivity and simplicity [34-37]. However, many of the chemosensors have only one signal for detecting, i.e., the fluorescence intensity, and could be easily perturbed by the environmental and instrumental conditions [38, 39]. Introduction of ratiometric chemosensors can eliminate or reduce the effect of these factors by the self-calibration of the two emission bands [40-51]. In this regard FRET can be an interesting candidate to design ratiometric sensors [49, 50, 5256]. The design of ratiometric sensors can be done by two methods (i) ICT (intermolecular charge transfer) and (ii) FRET. For many ICT based ion sensors it is difficult to determine the ratio between two relatively broad signal emissions. The advantage of FRET over ICT is that the ratio between two fluorescence intensities is independent of the external factors such as fluctuation of excitation source and sensor concentration. FRET observes the changes in the ratio of donor acceptor emission intensities, resulting in an increase in the signal selectivity. A significant advantage of FRETbased sensing is that it simplifies the design of the fluorophore. Recently, FRET based sensing has become most effective method for the detection of ions in environment. FRET based sensors have been widely used in metal ion Abstract