Luminescence-based sensing

The study of emission 1,2-enedithiolates now represents a formidable body of literature, even though the chemistiy, photochemistry, and photophysical properties of metallo-l,2-enedithiolate complexes are not yet as well understood or developed as those of the group metallo-Vlll-diimine and metalloporphyrin complexes. However, recent developments in the synthesis of 1,2-enedithiolates have led to the discovery of room temperature emitters and complexes with useful properties. As new methods allow for the synthesis of yet unknown complexes in this family, the unique and useful properties of these complexes will become even more evident. Many of the heterocyclic-substituted 1,2-enedithiolates now available are dual emitters with a short-lived and analyte-quenchable long-lived excited states. Clearly, these dual emitters will have a unique place in the detection of quenching analytes since selective quenching of Ae long-lived excited state eliminates several problems encountered with luminescence-based sensing. 

The commercialization of inexpensive robust LED and laser diode sources down to the uv region (370 nm) and cheaper fast electronics has boosted the application of luminescence lifetime-based sensors, using both the pump-and-probe and phase-sensitive techniques. The latter has found wider application in marketed optosensors since cheaper and more simple acquisition and data processing electronics are required due to the limited bandwidth of the sinusoidal tone(s) used for the luminophore excitation. Advantages of luminescence lifetime sensing also include the linearity of the Stem-Volmer plot, regardless the static or dynamic nature of the quenching mechanism (equation 10) ... 

Fig. 37 (a) QD-based sensing of cocaine by the formation of a cocaine-aptamer supramolecular structure that triggers FRET and (b) time-dependent luminescence spectra of the system in the presence of cocaine. The inset shows a calibration curve for variable concentrations of cocaine and a fixed so observation time of 15 min. (c) Schematic of the FRET-based TNT sensor and (d) increase of the QD luminescence upon addition of TNT in the competitive assay format. (Reprinted with permission from [220, 221], Copyright 2009 Royal Society of Chemistry and 2005 American Chemical Society)... 

Continuous wave techniques do not offer the optimum use of luminescence for sensing applications. CW methods, also known as intensity-based techniques, have many inherent limitations. These limitations will be discussed later in the chapter. Many of the limitations of intensity-based methods can be overcome by using steady-state modulated excitation of the form... 

In order to implement frequency domain based sensing systems capable of monitoring the temporal luminescence of sensors, in few seconds, data must be collected at multiple frequencies simultaneously. Single-frequency techniques have been used to make frequency domain measurements of luminescent decays. 14, 23 28) This approach is unsuitable for real-time applications since data must be acquired at several frequencies in order to precisely and accurately determine the temporal variables of luminescent systems. 1 Each frequency requires a separate measurement, which makes the single frequency approach too slow to monitor the evolution... 

Metal-based receptors provide excellent frameworks for anion host organisation and can act as reporter groups in electrochemical and luminescent anions sensing applications. 

The majority of Os(II) complexes exhibit weak luminescence and might not appear to be attractive candidates for use in various sensing schemes based on luminescence. Systems serving as oxygen sensors, including Os(II) complexes, have recently been reviewed [105]. Nonetheless, there are a few recent examples of applications of this sort and one recent application of a Ru(II)/Os(II) energy transfer based sensing system. 

Current gold nanopaiticle solution based methods of biosensing are not limited strictly to this one type of nanopaiticle but can incorporate other particles as well. Peptide linked gold nanoparticle - quantum dot biosensors have been created by Chang et al., that rely on the ability of the gold nanoparticles to quench the photoluminescence of the quantum dots when in their close proximity bound state [26]. The method of sensing is also considered an on sensor since the default state of the particles is off (no luminescence), and it is converted to on (luminescence) once sensing takes place. 

This section has demonstrated that significant developments have taken place in recent times in luminescent anion sensing using organic structures. The examples presented have been based on both charged and charge neutral receptors. However, the discussion was not extended to the use of metal ion complexes, as the next section reviews some of the achievements made in that field in recent times. 

The combinatorial possibility to synthesize a large number of different receptors based on DNA structure has been exploited for the development of luminescent-based sensor arrays [21]. In this case DNA is particularly attractive, because it offers the stability and the versatility as a biopolymer to allow the preparation of a wide range of different structures, which can also be tailored to the particular application. The DNA structure should be functionalized with a fluorescent dye, which acts as the unit signalling the interaction with the analyte. Initially both single and double stranded DNAs have been tested for sensing application, but only the single-stranded DNAs showed different responses to the VOCs tested, which can be related to the DNA sequences exploited. In these preliminary studies DNA oligomers were first stained in solution with two different dyes, both of them... 

In the truest sense, a multiplex sensor would use a single probe to detect each analyte. However, in practice, finding a single material capable of meeting this requirement is challenging. Multi-sensors (predominantly luminescence-based) which contain several analyte-specific probes in a single device are therefore more common [52]. However, determination of the correct probe combination is by no means trivial and requires similar considerations to those mentioned previously for... 

The versatility of luminescence goes beyond intensity-, wavelength- and kinetic-based measurements. Fluorescence polarization (or anisotropy) is an additional parameter still largely unexplored for optical sensing yet widely used in Biochemistry to study the interaction of proteins, the microfluidity of cell membranes and in fluorescence immunoassays. Although only a few optosensors based on luminescence polarization measurements can be found in the literature, elegant devices have recently been reported to measure chemical parameters such as pFI or O2 even with the bare eye41. 

Another approach, developed in our laboratory, consists of the compartmentalization of the sensing layer25"27. This concept, only applicable for multi-enzyme based sensors, consist in immobilizing the luminescence enzymes and the auxiliary enzymes on different membranes and then in stacking these membranes at the sensing tip of the optical fibre sensor. This configuration results in an enhancement of the sensor response, compared with the case where all the enzymes are co-immobilized on the same membrane. This was due to an hyperconcentration of the common intermediate, i.e. the final product of the auxiliary enzymatic system, which is also the substrate of the luminescence reaction, in the microcompartment existing between the two stacked membranes. 

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