Chemical detection, photoluminescence

An attractive alternative to organic fluorophores in formulated polymeric sensor films for chemical detection is to use inorganic luminescent materials. Semiconductor nanocrystals have a dramatically improved photostability and are attractive as luminescent labels.1415 However, some nanocrystals also exhibit photoluminescence (PL) that is sensitive to the local environment. For example, it was observed that PL of CdSe nanocrystals incorporated into polymer thin films responded reversibly to different gases.16 Because in sensing applications nanomaterials bring previously unavailable capabilities1719 and unexpected results,20 23 we explored the environmental sensitivity of semiconductor nanocrystals upon their incorporation in different rationally selected polymeric matrices.24 26... 

Other approaches to chemical detection systems concentrate on the transduction of the binding event, highlighting the sensitivity. In this case, the function of the material is to report the interaction of the sensor with the target. Photoluminescence, for example, is a desirable property because changes in the emitted light can be manipulated by materials development to indicate the interaction of the target with the recognition site. 

FigureS.7 Near-field two-photon excitation images of single gold nanorods detected by two-photon induced photoluminescence. Nanorod dimensions (length, diameter) are 540 nm, 20 nm for (a) and 565 nm, 21 nm in (b). Scale bars lOOnm. (Reproduced with permission from The Chemical Society of Japan [11]).

Although there are numerous techniques which can be used to detect chemical perturbations, they often lack sensitivity. Photoluminescence spectroscopy can be considered ideal because of its high sensitivity and nondestructive nature (14, 33, 34). 

Unlike photoluminescent techniques, where some degree of selectivity is often derived from the intrinsic excitation and emission wavelengths of the analyte, the inherent selectivity of chemiluminescence detection arises from the limited number of chemical reactions that produce significant amounts of light. Furthermore, wavelength discrimination usually offers no advantage to the chemiluminescence detection, as different analytes often lead to the same emitting species, and should be avoided due to the detrimental effect on sensitivity. 

In this section we deal with quantitative steady-state luminescence intensity determination in solution, at a fixed emission wavelength, using a commercial spectrofluorimeter with right-angle excitation (perpendicular geometry). These kind of measurements are particularly important in analyte detection, titrations, quenching and sensitization experiments, photoreaction and photoluminescence quantum yield determination, and whenever a luminescence signal is used to monitor a chemical process. 

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