Metal-enhanced luminescent probes

Chan YH, Chen J, Wark SE et al (2009) Using patterned arrays of metal nanoparticles to probe plasmon enhanced luminescence of CdSe quantum dots. ACS Nano 3 1735-1744... 

In a representative work, Chen and Rosenzweig34 were able to alter the selectivity of CdS nanoparticles to respond either to Zn2+ or Cu2+ simply by changing the capping layer. They showed that, while polyphosphate-capped CdS QDs responded to almost all mono- and divalent metal cations (thus showing no ion selectivity), luminescence emission from thioglycerol-capped CdS QDs was quenched only by Cu2+ and Fe3+, but was not affected by other ions at similar concentrations. On the other hand, the luminescence emission of L-cysteine-capped CdS quantum dots was enhanced in the presence of zinc ions, but was not affected by cations like Cu2+, Ca2+, and Mg2+. Using this set of QD probes, the authors described the selective detection of zinc and copper in physiological buffered samples, with detection limits of 0.8 pM and 0.1 p,M for Zn2+ and Cu2+, respectively. This was claimed to be the first use of semiconductor nanocrystals as ion probes in aqueous samples. 

Luminescence Sensing Some luminescent compounds can serve as probes of analytes such as metal ions. Upon coordination with a metal ion, the luminescence of a ligand may be enhanced, quenched, or shifted in color (the wavelength of maximum intensity). In addition, changes in the solution conditions (pH, ionic strength, temperature or the presence of macromolecular hosts capable of forming organized structures) often affect luminescence properties. 

Furthermore, lanthanides form stable complexes with polydentate chelators like DTPA, which exhibit a noncyclic structure. Two structures are depicted in Scheme 2. The following examples are only representatives for the variety of analyte molecules that can be determined by these kind of lanthanide complexes. Structure 9 employs a quinolyl ligand both as chromophore and acceptor for Zn ". The emission of the europium ion is strongly enhanced upon binding of Zn " and showing distinct selectivity over other biologically relevant metal cations in aqueous solution at neutral pH [29]. The luminescence of the chelate 10 is efficiently quenched by Cu " ions in aqueous medium [30]. The presence of Fe ", Co ", Ni ", Cr ", and Mn " interferes with the determination of Cu, although to a relatively small extent, whereas the ions Zn ", Cd ", Hg, and Pb do not interact with probe 10. 

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