Analytica Chimica Acta 527 (2004) 13–20 Holographic sensors for the determination of ionic strength Alexander J. Marshall 1 , Duncan S. Young, Satyamoorthy Kabilan, Abid Hussain, Jeff Blyth, Christopher R. Lowe ∗ Institute of Biotechnology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QT, UK Received 16 June 2004; received in revised form 11 August 2004; accepted 11 August 2004 Available online 5 October 2004 Abstract Holographic sensors for monitoring ionic strength have been fabricated from charged sulphonate and quaternary ammonium monomers, incorporated into thin, polymeric hydrogel films which were transformed into volume holograms. The diffraction wavelength or reflected colour of the holograms was used to characterise their swelling or de-swelling behaviour as a function of ionic strength in various media. The effects of co-monomer structure, buffer composition, ion composition, pH and temperature were evaluated, whilst the reversibility and reproducibility of the sensor was also assessed. An acrylamide-based hologram containing equal molar amounts of negatively and positively charged monomers was shown to be able to quantify ionic strength independent of the identity of the ionic species present in the test solution. The sensor was fully reversible, free of hysteresis and exhibited little response to pH between 3 and 9 and temperature within the range 20–45 ◦ C. The system was successfully used to quantify the ionic strength of milk solutions, which contain a complex mixture of ions and biological components. © 2004 Elsevier B.V. All rights reserved. Keywords: Hologram; Sensor; Hydrogel; Ionic strength 1. Introduction The measurement of electrolyte content and ionic strength is important in the environmental, agricultural, food, bever- age, biotechnology and biomedical industries. For example, in the healthy body, electrolytes are subject to tight regu- lation by the kidneys due to balancing absorption and ex- cretion processes. Disorders of electrolyte homeostasis may result from metabolic disturbances associated with coronary heart disease, angina pectoris, acute myocardial infarction, diabetes mellitus, dehydration, renal failure and chronic al- cohol abuse [1]. Currently, there is an urgent requirement for inexpensive, mass-producible point-of-care diagnostics for the measurement of electrolytes in biological fluids such as ∗ Corresponding author. Tel.: +44 1223 334160; fax: +441223334162. E-mail addresses: ajm205@cam.ac.uk (A.J. Marshall), crl1@biotech.cam.ac.uk (C.R. Lowe). 1 Tel.: +44 1233 334152. blood, urine, saliva, sweat and tears to aid diagnosis and mon- itor the progress of electrolyte therapies. Furthermore, in the environmental sector, the monitoring of water quality, in par- ticular dissolved salt is a crucial aspect of the maintenance of public health. The monitoring of water quality and salinity is also of major importance in the food and beverage industry as well as in many industrial processes. Whilst it is important to be able to determine the concen- tration of individual electrolytes, it is often desirable to obtain a general measure of electrolyte content in many cases. In such situations, sensors that can determine ionic strength are of interest [2]. In this context, there is much interest in the use of stimuli-sensitive hydrogels as ionic strength sensors since the hydration of these polymers is known to vary with the ionic strength of the bathing solution [3–5]. Hydrogels can be prepared to undergo large, reversible changes in solvent- swollen volume in response to specific stimuli including pH [6 and Refs. therein], ions [4,7], temperature [8,9], electric fields [10] solvents [11] and specific chemical [12,13] or 0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.08.029