1.4.1. The Metal Ion Probes
Sensing toxic metal ions is essential in monitoring of the environment, the control of chemical processes, and medical applications. Of particular importance is the selective detection of the mercury ion (II), which is biologically highly toxic and is known to cause a variety of phenomena in humans, such as damage to the central nervous system and resulting neuropsychiatry disorders. Currently existing sensors for the detection of Hg2+, such as thin-films device of gold,101 organic probes,102,103 polymeric material104 and bio-composites,105 are subject to certain inferiorities. These include high operation temperatures and complexity,101 latively low sensitivity, non-aqueous environments,102,106 and slow response due to the long-time equilibrium.105 Ever since Pederson et al. discovered numerous metal ions could be
detected by cyclic polyethers (crown ethers) in 1967.107 Numerous strategies of metal ion-sensors have been designed. Among these, organic fluorophores linked with ion recognition sites turn out to be the most popular metal ion probes. For example, our co-workers, P. T. Chou et al., developed a highly sensitive Ca2+ and Na+ probe by anchoring organic fluorophore with a aza-crown that is suitable for the ion sizes, the design has achieved an excellent result (see Figure 1.34).108 However, most of organic dyes acting as fluorophore tend to have narrow excitation spectra, and often exhibit broad emission bands with red tailing, which, due to spectral overlap, makes simultaneous quantitative evaluation of relative amounts of different probes present in the same sample difficult. To overcome such a barrier, Kim and co-workers have successfully prepared gold nanoparticles with the capability of sensing heavy metal ions.81 In their work, a simple colorimetric technique for the detection of small concentrations of aqueous heavy metal ions, including toxic metals such as lead, cadmium, and mercury was described. As depicted in Figure 1.35, they used 13.6 ± 0.4 nm diameter gold nanoparticles capped with 11-mercaptoundecanoic acid (MUA) as a probing system. Aqueous suspensions of the functionalized gold nanoparticles displayed intense plasmon absorptions that rendered the suspensions red. Aggregation of the particles (e.g. by addition of a metal ion salt solution) yielded both a shift in plasmon band energy and a substantial increase in long-wavelength Rayleigh scattering, resulting in a red-to-blue color shift (Figure 1.35).
Although this case successfully utilized gold nanoparticles as the metal ion probe, highly luminescent semiconductor QDs with broad excitation spectra and narrow, symmetric and tunable emission spectra could be superior to colorimetric metal ion sensor using gold nanoparticles. In 1996, Sooklal et al. demonstrated that manganese ion could be probed by ZnS QDs.109 Similar works were performed by many groups employing quenching the fluorescence of semiconductor nanoparticles, CdS110-112 and
CdTe.113 More recently, the group of Pang published two papers about the use of CdSe QDs and CdSe@ZnS QDs modified with bovine
serum albumin (BSA)114,115 for Ag+ and
Cu2+ detections,
respectively. In Chapter 2, the design and the
properties of the high sensitivity metal ion probes with type I core/shell QDs will be discussed.
1.4.2. Intermolecular Energy Transfer System
Furthermore, in order to extend the applications of type-I QDs, continuing the idea of metal ions probe by aza-crown,108 (see Figure 1.34) we developed an energy transfer system with crown-dithio QDs. Herein, the Intermolecular Energy Transfer are briefly introduced as follows.
A. Aromatic Hydrocarbons
Among the hydrocarbons anthracene is probably one of the most thoroughly studied molecules. If anthrancene solutopns are irradiated in the absence of oxygen, the product is
Figure 1.34 Metal ion probe by the organic fluorophore anchored with aza-crown.108
Figure 1.35 Heavy-metal ion recognition and binding of the functionalized gold nanoparticle.81
Figure 1.36 Colormetric responses for metal ion recognition.109
dianthrancene. If oxygen is present, however, the product is the bridged peroxide.
Furthermore, the photophysical properties of the novel hexapyropheophorbide a – fullerene hexaadduct (FHP6) compound (Figure 1.37) were studied by Röder et al. using both steady-state and time-resolved spectroscopic methods116. It was found that neighboring pyropheophorbide a (pyroPheo) molecules covalently linked to one fullerene moiety due to the length and high flexibility of carbon chains could stack with each other.
B. Carbonyls
Another important type of molecule participating in photochemistry is that containing the carbonyl group. Regioregular silylene-spaced copolymers
Figure 1.37 Structural formula of FHP6 and the reference compound FHP1.116
composed of an energy gradient with three different chromophores recently have been achieved by Yen-Ju Cheng and Tien-Yau Luh.117 Upon excitation of the donor chromophore (D1), only emission from the acceptor (A) was observed.
Efficient and sequential energy transfer across three different chromophores along the polymeric backbone may proceed smoothly in these silylene-spaced copolymers. (Figure 1.38)
C. Protein–protein interactions
A twofold increase in quenching constant was noted for KI quenching of AEDANS fluorescence emission in the presence of apoE CT domain, indicative of alterations in Ab conformation upon interaction with apoE CT domain. (See Figure 1.39) Narayanaswami, V. and his group118 propose intermolecular FRET (Fluorescence Resonance Energy Transfer) analysis as a discriminating approach to examine apoE/Ab interaction, a potentially critical factor in early events involved in amyloid formation.
Figure 1.38 Concentration-dependent fluorescence spectra of an equal molar mixture of two compounds in CHCl3.117
The development of molecule-based ion sensors has been a pivotal issue currently receiving considerable attention. A prototypical detection scheme lies in two requirements, namely a sensing moiety with satisfactory ionic selectivity and the sensitivity, i.e. the measurable changes in response to the recognition.
Among numerous methodologies, the exploitation of ionophores incorporated with optical transduction as a reporter seems to be a very promising one.119 In this approach, ionophores are usually coupled with a molecular chromophore, of which the associated spectral properties in either absorption or emission are sensitive to the environmental stimuli. Up to this stage, a large portion of chromophores being developed are hydrophobic, which consequently limits their applications in aqueous media. Recently, metal nanoparticles have been emerged as an important colorimetric reporter due to their extremely high extinction coefficient (i.e. absorptivity) that is also very sensitive to the transition from mono-dispersion to the aggregation, resulting in a distinct color change.52,81,120-124 For the case of gold nanoparticle, this phenomenon of which is well known as the surface plasmon absorption, the color change upon aggregation is due to the coupling of the surface plasmon resonance as a result of the proximity between two Au nanoparticles.125,126 Recently, based on a
Figure 1.39 Fluorescence Resonance Energy Transfer Analysis of Apolipoprotein E C-Terminal.118
sandwich complex of 2:1 between 15-crown-5 and K+ [12−19], Chen and coworkers127 reported on capping the Au nanoparticles with the 15-crown-5 functionality and the resulting 15-crown-5 functionalized Au nanoparticles were successfully exploited as a novel prototype to probe K+, in which the transduction is signified by subtle changes of the surface plasma resonance, i.e. a colormetry type of sensing mechanism. On this basis, other crown ether-modified metal nanoparticles, especially those of II-VI and III-V semiconductor composites, may serve as an alternative, taking advantages of their superior fluorescence properties such as high quantum efficiency and narrow bandwidth (good contrast luminescence color), etc. In Chapter 2, we report the design and synthesis of 15-crown-5 functionalized, water soluble CdSe/ZnS quantum dots (QDs) as well as their exploitation as a sensing unit toward K+ in aqueous solution. The sensing mechanism utilizing either aggregation property in single size QDs as well as the Förster type of energy transfer in dual color QDs system renders a great versatility and flexibility in view of future applications.
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