Bulk optodes

The reagents are dissolved in an immiscible phase that is held up to an inert polymeric matrix in bulk optodes. The whole constitutes the sensing membrane, and the signal relies on concentration changes within the bulk. 

They are often plasticized PVC matrices, which occlude an ionophore as the key selective element, a chromoionophore or a fluoroionophore as the chemical-optical transducer and, sometimes, ionic additives to maintain electroneutrality. Such optodes follow ion-exchange mechanisms between the membrane and the aqueous solution and the analytical response originates from the ratio of the concentration of ions in the solution or from their product (Fig. 3). Moreover, selectivity is ruled by the ion distribution coefficients between both phases and by the formation constants of complexes within the membrane. 

The main advantages of using this approach are that response is not affected by the ionic strength of the solution and that the development of specific ion-selective optodes is easily adapted from their analogous well-studied ion-selective electrodes [32 - 34], because the same selective reagents and membrane constituents are employed. Most of the ion-selective optodes 

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Seiler K. and Simon W., Theoretical aspects of bulk optode membranes, Anal. Chim. Acta 1992, 266 73. 

Koncki R., Mohr G., Wolfbeis O.S., Enzyme sensor for urea based on novel pH bulk optode membrane, Biosens. Bioelectr. 1995 10 653-659. 

Ion-selective electrodes are now well understood in terms of the underlying theory, and this has made it possible for new sensing principles to emerge that make use of the thousands of chemical receptors originally developed for ion-selective electrodes. One is the field of optical sensors, which has not been discussed here because it is outside the focus of this chapter. Such so-called bulk optodes do not require electrical connectivity between the sensing and detection unit and are therefore more easily brought into various shapes and sizes, including particle formats, which suit the need of modem chemical analysis. 

E. Bakker, P. Buhlmann, and E. Pretsch, Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics. Chem. Rev. 97, 3083-3132 (1997). 

E. Bakker and W. Simon, Selectivity of ion-sensitive bulk optodes, Anal. Chem., 64 (1992) 1805-1812. 

As in the case of bulk optodes, plasticizers can be added to modify the properties of polymers (e.g., gas permeability). Plasticizers are mainly used to design ion-sensitive nanobeads. 

Wygladacz K, Radu A, Xu C, Qin Y, Bakker E (2005) Fiber-optic microsensor array based on fluorescent bulk optode microspheres for the trace analysis of silver ions. Anal Chem 77 4706 1712... 

M. Lerchi, E. Bakker, B. Rusterholz, and W. Simon, Lead-selective bulk optodes based on neutral ionophores with subnanomolar detection limits, Anal. Chem. 64, 1534-1540 (1992). 

Fig. 2 Schematic representation of optochemical sensors depending on the arrangement of the optically active reagents, a Sin-face optodes reagents are directly immobilized over a solid support by (1) covalent bonding, (2) adsorption or (3) electrostatic interactions or a waveguide or are trapped in a porous matrix (4). b Bulk optodes reagents are dissolved in a plasticizer (5)...

Fig. 3 Basic composition of ion-selective bulk optodes and extraction mechanisms. This type of sensor follows an extraction equilibrium between the aqueous solution and the membrane and the signal is related to the analyte activity in the aqueous solution...

Table 1 Near-IR chromoionophores applied in bulk optodes. The absorption maxima, the type of matrix employed, the sensor configuration, the optical properties measured, the analyte and year of publication are listed... 

The latter system has been called an IWAO and the results obtained have demonstrated its suitability to be employed as a sensing platform, which confers versatility, robustness and mass-production capabilities besides high sensitivity on conventional bulk optodes. For these reasons it is going to be described in more detail next. 

IWAOs have been designed to address some of the weak points concerning absorbance optochemical sensors based on bulk optodes. In order to reach a practical application level, new research is needed to overcome some signifi-... 

The basic structure is composed by a rib ARROW waveguide (as the input waveguide), an open region, where the bulk optode is deposited (as the... 

As is well known, the response of a bulk optode follows the Beer-Lambert law, which linearly relates the absorbance to the concentration of an absorbent species (C), its molar absorptivity (e) and the optical path length (d) ... 

Different optical membranes have been applied to IWAOs to validate their properties. They consist of the well-studied bulk optode membranes. As stated in the Introduction, they contain a selective ionophore, a lipophilic ionic salt to maintain the electroneutrality and a second ionophore with spectroscopic characteristics, denominated chromoionophore. Such optodes rely on concentration changes within the bulk of the sensing membrane and fol-... 

The membrane used to activate this potassium-selective IWAO [134] consists of a potassium bulk optode based on 0.5 wt % chromoionophore ETH 5294, 1.0 wt% ionophore valinomycin, 0.5 wt% ionic additive potassium tetrakis(4-chlorophenyl)borate (KtpClPB), 31.0 wt % polymer PVC, 67.5 wt % organic solvent and plasticizer bis(2-ethylhexyl)sebacate (DOS) [142], This commercially available optode not only acts as an example of the development of an enhanced ion-selective IWAO, but also serves to validate the previously remarked features, because results can be compared with the ones obtained with membranes of the same composition and thickness in a con-... 

The IWAO proves to be a very promising alternative to the conventional optical sensors, as well as those based on optical fibers such as the conventional flow cell configuration. Fully reversible, reproducible, fast and sensitive bulk optodes are obtained. 

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