Light-induced fluorescence

J. E. Dickens, M. Ponstingl, S. Christian, C. Dixon, J. Schroeder and S. Horvath, A novel light induced fluorescent (LIE) sensor for process and field applications, IFPAC Conference, Baltimore MD, 2009. 

Luminescence is a well-established class of analytical spectroscopic techniques where a species emits light after excitation. Emission is an elecnonic nansition from an excited state as opposed to the ground state as is the case in most other spectroscopies. Photoluminescence, or light-induced fluorescence (LIE), is the most common route to induce emission where sufficient incident photons of a particular energy excite the target species via absorption. Although less common, nomadiative excitation can also occur via a chemical reaction termed chemiluminescence. Unless otherwise stated, the terms luminescence and fluorescence within this review infers excitation by light induction. 

In recent years luminescence nomenclature has become confusing within the literature and in practice. Luminescence involves both phosphorescence and fluorescence phenomena. While luminescence is the appropriate term when the specific photochemical mechanism is unknown, fluorescence is far more prevalent in practice. Moreover, the acronym LIE has historically inferred laser -induced fluorescence however, in recent years it has evolved to the more general term light -induced fluorescence due to the various light sources found within laboratory and real-time instruments. Within this chapter fluorescence and LIE are interchangeable terms. 

C.K. Lai, A. Zahari, B. Miller, W.E. Katstra, M.J. Cimna and C.L. Cooney, Nondestructive and on-line monitoring of tablets using light-induced fluorescence technology, AAPS PharmSciTech, 5 (1), Article 3 (http //www.aapsp-harmscitech.org/) (2004). 

It is to be expected that by means of light-induced fluorescence diagnostics, further improvements in the differentiation of morphological findings can be achieved. 

Figure 4 (left panels) shows light-induced fluorescence induction kinetics, the variable part of which reflects the photoreduction of QA in the thylakoid membranes of control and the Chi Mess mutant [Melis and Duysens 1979]. The fluorescence induction kinetics of the control strain were faster than that of the Chi Mess mutant, suggesting a larger PSII Chi antenna size for the former. 

Fig. 2. (A) Fluorescence yield (in arbitrary units) for isolated, dark-adapted chloroplasts as a function of time in the absence (a) and presence (b) of DCMU. (B) Light-induced fluorescence-yield change in lyophilized chloroplasts (control), in hexane-methanol extracted chloroplasts, and in extracted chloroplasts reconstituted with PQ-9. Small upward arrows indicate weak, modulated green monitoring light turned on larger arrows for intense actinic light on (upward arrow) and off (downward arrow). (A) adapted from Zankel and Kok (1972) Estimation of pool size and kinetic constants. In A San Pietro (ed) Methods in Enzymology 24 222 (B) from Liu, Hoff, Gu, Li and Zhou (1991) The relationship between the structure of plastoquinone derivatives and their biological activity in photosystem II of spinach. Photosynthesis Res 30 100.

As described earlier, the Chl-a fluorescence yield ofchloroplasts is influenced by the redox state ofthe electron acceptor Q, and thus the amplitude of fluorescence yield may be used to monitor the midpoint potential ofQ by means of a redox titration. Such a redox titration was first carried out by Cramer and Butler in 1969 with spinach chloroplasts at pH 7. Two transitions, presumably representative of two quenchers of fluorescence or electron acceptors, were observed in the titration process, with one of value between -20 and -35 mV, and another with a much more negative value, between -270 and -320 mV. The quencher Q, which had been held for the previously described, light-induced fluorescence yield changes, was ascribed to tbe more positive quenching transition. 

D Mauzeraii (1972) Light-induced fluorescence changes in Chlorella and the primary photoreduction for the production of oxygen. Proc Nat Acad Sci, USA 69 1358-1362... 

L. Thiberville, G. Sutedja, P. Vermijlen (1999). A multicenter European study using the light induced fluorescence endoscopy system to detect pre-cancerous lesions in high-risk individuals. Eur. Resp. J., 14, 2475. 

FLD = fluorescence detection, LIF = light induced fluorescence, LOD = limit of detection, LC = liquid chromatography, ECD = electrochemical detection, PFET = photoluminescence following electron transfer, MS/MS = tandem mass spectrometry, CE = capillary electrophoresis, FSCV = fast-scan cyclic voltammetry. 

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