Fluorophores optical sensors

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

Thus pH responsive dyes or fluorophores can be used to design optical sensors for C02. To prevent pH interference, a hydrophobic membrane with high C02 permeability is used to selectively allow CO2... 

Perhaps the most well-recognized fluorescent dye for detection of DNA hybridization is ethidium bromide (EtBr). EtBr is a cationic phenanthridinium compound that can bind to DNA by intercalation. This dye has an excitation maxima at 518 nm when bound to double-stranded DNA (dsDNA). Excitation of EtBr is often done by use of an argon ion laser, making this fluorophore a viable choice for applications in optical sensors as well as confocal scanning laser microscopy and fluorometry [41]. The structure of ethidium bromide is shown in Fig. 6. 

Barone PW, Parker RS, Strano MS. In vivo fluorescence detection of glucose using a single-walled carbon nanotube optical sensor design, fluorophore properties, advantages, and disadvantages. Analytical Chemistry 2005, 77, 7556-7562. 

Another field of work is directed to DNA analysis with NIR fluorophores. Pilevar et al. have recently described a fiber optic sensor for DNA hybridisation, in which the dye IRD 41 (of LI-COR Inc., Lincoln, NE, USA) was used for the real-time hybridisation of RNA of Helicobacter Pylori at picomolar concentrations [150]. Recently optical nanosensing in which nanometer-scale probes are used for intra-cel-lular measurements has also been pioneered [151]. 

The first intravascular sensor for simultaneous and continuous monitoring of the pH, pC>2, and pCC>2 was developed by CDI-3M Health Care (Tustin CA)14 based on a system designed and tested by Gehrich et al.15. Three optical fibres (core diameter = 125 pm) are encapsulated in a polymer enclosure, along with a thermocouple embedded for temperature monitoring (Figure 3). pH measurement is carried out by means of a fluorophore, hydroxypyrene trisulfonic acid (HTPS), covalently bonded to a matrix of cellulose, attached to the fibre tip. Both the acidic ( eXc=410 nm) and alkaline ( exc=460 nm) excitation bands of the fluorophore are used, since their emission bands are centred on the same wavelength (/-cm 520 nm). The ratio of the fluorescence intensity for the two excitations is measured, to render the sensor relatively insensitive to fluctuations of optical intensity. 

The intrinsic sensors are based on the direct recognition of the chemicals by its intrinsic optical activity, such as absorption or fluorescence in the UV/Vis/IR region. In these cases, no extra chemical is needed to generate the analytical signal. The detection can be a traditional spectrometer or coupled with fiber optics in those regions. Sensors have been developed for the detection of CO, C02 NOx, S02, H2S, NH3, non-saturated hydrocarbons, as well as solvent vapors in air using IR or NIR absorptions, or for the detection of indicator concentrations in the UV/ Vis region and fluorophores such as quinine, fluorescein, etc. 

The fourth type of mediator-based cation optical sensing is using potential sensitive dye and a cation selective ionophore doped in polymer membrane. Strong fluorophores, e.g. Rhodamine-B C-18 ester exhibits differences in fluorescence intensity because of the concentration redistribution in membranes. PVC membranes doped with a potassium ionophore, can selectively extract potassium into the membrane, and therefore produce a potential at the membrane/solu-tion interface. This potential will cause the fluorescent dye to redistribute within the membrane and therefore changes its fluorescence intensity. Here, the ionophore and the fluorescence have no interaction, therefore it can be applied to develop other cation sensors with a selective neutral ionophore. 

In fluorescent molecular sensors, the fluorophore is the signaling species, i.e. it acts as a signal transducer that converts the information (presence of an analyte) into an optical signal expressed as the changes in the photophysical characteristics of the fluorophore. In contrast, in an electrochemical sensor, the information is converted into an electrical signal. 

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