Ketocyanine Dyes:  H⁺-Selective Ionophores for Use in Integrated Waveguides Absorbance Optodes

Abstract

The optical and analytical characteristics of a series of neutral H+-selective chromoionophores in PVC membranes are described. Such indicators have been synthesized so that they can operate in bulk optode membranes as the chemically active region of integrated waveguide absorbance optodes (IWAOs), lambdamax near 780 nm. Their spectral characteristics, acid-base properties, chemical stability, and solubility in the membrane phase are given and discussed. The response characteristics are first tested in a conventional absorbance/transmittance flow cell. They offer a wide range of pKa's in PVC membranes, good sensitivity as a result of their high molar absortivities, excellent solubility in the plasticizer, and quick response times. They present good chemical stability in common laboratory conditions when stored in the dark, and the absence of leaching guarantees a long lifetime. Membranes have been finally applied as the sensing region of an integrated waveguide optode, demonstrating the extraordinary sensitivity improvement while preserving the remaining analytical features. Calibration curve slopes are multiplied by 3-28, and response times lower than 2 min are obtained.

Full-range optical pH sensor based on imaging techniques

“…We studied the balance of error in the pH ranges [0,7) and (7,14] in the validation data set. All the 70 data in the validation data were separated into two different sets with 35 experiments for basic pH and 35 experiments for acid pH.…”

“…Thus, calibration functions in pH sensors come from mass-action law relationships between pH and radiation intensity, referring here only to absorption-based sensors, or experimental functions obtained by complete calibration in the case of pH-dependent interactions between indicator and matrix [4], and using in all cases: (a) single-intensity measurements both absolute [5], relative [6] or normalized signals [7] or (b) multiple intensity measurements as the intensity ratio at two or more wavelengths [8].…”

“…Different types of immobilization techniques for pH indicating dyes have been used [9], including (a) dye entrapment in different materials such as cellulose acetate [10,11], sol-gel [12], PVC [7], methacrylic-acrylic copolymers [13] and different composites like SiO 2 /ZrO 2 -organic polymer (styrene/methyl methacrylate copolymer or Nafion) [14], (b) retention of dye by ion-exchange materials such as Amberlite XAD-2 resin [15] or Dowex l-X10 resin [16], in some instances including the ionexchanger containing dyes in polymeric encapsulated membranes using PVC [17], (c) adsorption of the dye on materials such as non-ionic styrene/divinylbenzene copolymer [18], polyester/lycra blends textile [19], cellulose [20], cellulose acetate [21], or polymer track membranes combining retention in surface and bulk [22], (d) covalent binding of dye by different synthetic strategies to form microparticles or membranes, in some cases formed on glass fibre, using different polymers such as polyacrylamide [8,23], triacetylcellulose [24], cellulose acetate [25,26], agarose [27] or polyamide [28], among others, (e) polymerization of monomers to prepare both membrane and dye, as with aniline [29] or pyrrole [30].…”

Ionophore-based ion-selective potentiometric and optical sensors

“…For a compound to act as a chromoionophore in an optode membrane, it must be sufficiently lipophilic to remain in the organic membrane phase and must demonstrate fairly strong ion-induced changes in UV-Vis absorption or fluorescence. While a number of different compounds have been proposed as chromoionophores [69,70,71,72,73], those that have been most widely employed in such a capacity are H + -sensitive Nile Blue and fluorescein derivatives, like those in Fig. 4 [4].…”

Characterisation of new norcyanine dyes and their application as pH chromoionophores in optical sensors

“…Of the available light sources, diode lasers emitting at 670 and 780 nm are preferred due to their high intensity, low cost and suitable performance when waveguides are to be used. However, the lack of commercial chromoionophores, which display absorption bands in the far viseNIR region of the electromagnetic spectrum, is a significant hindrance to the exploitation of integrated optical sensors [16,17] and, only very few types of dyes with such characteristics have been successfully applied in bulk optodes and integrated sensors [18]. Therefore, some promising new types of chromoionophores have been synthesised in our research group.…”