Chromogenic and Fluorogenic Reactands: New Indicator Dyes for Monitoring Amines, Alcohols and Aldehydes
In the past few years a wide range of optical sensors for ions has been presented. Sensors for pH are based on the protonation/deprotonation of pH indicator dyes [1] and sensors for cations and anions use a combination of pH indicator dyes with selective
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Chromogenic and Fluorogenic Reactands: New Indicator Dyes for Monitoring Amines, Alcohols and Aldehydes GERHARD
J. MOHR
1 Introduction
In the past few years a wide range of optical sensors for ions has been presented. Sensors for pH are based on the protonation/deprotonation of pH indicator dyes [ 1] and sensors for cations and anions use a combination of pH indicator dyes with selective ionophores (via the mechanisms of coextraction or ionexchange) [2]. In the case of coextraction and ion-exchange, the uncoloured ionophore recognizes the analyte while the pH indicator dye changes its colour [2) . This approach is highly cross-sensitive to pH and has not found practical application so far. A more sophisticated and synthetically challenging approach is to use selective ftuoro- and chromoionophores [1,3,4). They are advantageous because the dyes both selectively recognize the analyte and simultaneously
Ligands for ionic analytes (analyte recognition via complexation)
~
Dye
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~ __-
Dye -
K•
I I\
Reactands for neutral analytes (analyte recognition via formation of a covalent bond)
Dye
+
" ' \ I / __ analyte
non-fluorescent
~
Dye-analyte
---;I I \ ""ltigllly-fluorescent
Fig.l Principle of analyte recognition using ftuorogenic ligands and reactands. Apart from an increase in fluorescence upon interaction with the analyte, changes in absorbance can be observed as well
R. Narayanaswamy et al., Optical Sensors © Springer-Verlag Berlin Heidelberg 2004
52
Gerhard J. Mohr
change their colour. Fluoroionophores for sodium and potassium with very low cross-sensitivity to pH can be found in the AVL OPTI devices [5]. The situation is more complex when electrically neutral analytes have to be detected. First, the interaction between indicator dyes and neutral analytes usually is rather weak (Van der Waals interactions, hydrogen bonding, hydrophobic interactions) [6] . Secondly, complexation of a neutral analyte has a much weaker effect on the electron delocalisation of an indicator dye than the complexation of an ion, thus causing only small changes in colour. In order to provide sufficient signal changes upon exposure to neutral analytes, chemical reactions have been introduced into optical sensing (Fig. 1). Narayanaswamy et al. have used pararosaniline immobilized on ion-exchange resin for the detection of formaldehyde and acrolein in aqueous solution [7]. A sensor for the detection of hydrazine made use of the reaction of hydrazine with p-dimethylaminobenzaldehyde to form a coloured benzalazine in sol-gel glass [8]. A sensor for glucose has been developed using 3-aminophenyl boronic acid copolymerised with aniline to form a polyaniline layer with analyte-dependent absorbance changes in the near infrared spectral range [9]. Turner et al. investigated the ability of a hemithioacetal-based polymer to react with primary amines and to form a fluorescent isoindole complex [10]. Pretsch et al. used the bisulphite addition to a lipophilic aldehyde for optical detection of sulphur dioxide [11]. A method t
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