Synthesis and spectroscopic characterisation of heptamethincyanine NIR dyes for their use in optochemical sensors Cristina Encinas a , Serguei Miltsov a , Elena Otazo a , Laia Rivera a , Mar Puyol b , Julian Alonso a, * a Sensors & Biosensors Group, Analytical Chemistry, Autonomous University of Barcelona, 08193 Bellaterra, Spain b Applied Physics Department, Universidad de Zaragoza, 50009 Zaragoza, Spain Received 19 April 2005; accepted 25 May 2005 Available online 21 September 2005 Abstract A set of near-infrared absorbing tricarbocyanine dyes has been synthesised and characterised for their future application as chromoionophores in optochemical sensors. Their absorption maxima are localised in the NIR region, where matrix interferences are minimal and the use of suitable and inexpensive optical communication components gives great advantages. The acid form of the synthesised dyes in pure ethanol spans the region from 675 to 815 nm and the molar absorptivities are up to 3.3 ! 10 5 l/mol cm. The calculated pK a values in ethanol lie between 11.4 and more than 13. The effect of the introduction of substituents on the pK a values and on the spectroscopic characteristics of the dyes is also discussed. Moreover, pH-sensitive aggregation processes have been observed in aqueous solution. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Heptamethincyanine dyes; NIR; LED laser; pH-indicators 1. Introduction Few years ago, a type of plasticized PVC optodes based on the well known ion-selective electrodes was proposed for optical sensors [1,2]. These selective membranes should contain a chromoionophore, which could provide the optical transduction of the signal generated by the chemical interaction. Since direct and selective analyteechromoionophore interactions were usually difficult to attain or were irreversible, a combi- nation of a certain selective ionophore and a chromo- phore was proposed to be added to the membrane composition. The analyteeionophore interaction alters in some way the spectroscopic characteristics of the chromophore in order to maintain the electroneutrality of the membrane. In the case of a pH indicator, the chromophoric structure is modified when a proton is released or added when the ionic analyte penetrates into the membrane. Classic acidebase indicators, with absorption maxima in the visible region of the electromagnetic spectrum, are widely used in aqueous acidebase titrations. They would be potential chromoionophores in the visible region but their solubility in the commonly used plasticizers for PVC membranes is limited. On the other hand, the recent outgrowth of the telecommunication industry has provided the techno- logical support needed to accelerate the development of optochemical sensors since new and cheap, active and * Corresponding author. Tel.: C34 935812149; fax: C34 935812379. E-mail address: julian.alonso@uab.es (J. Alonso). 0143-7208/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2005.05.013 Dyes and Pigments 71 (2006) 28e36 www.elsevier.com/locate/dyepig