Sensors and Actuators B 169 (2012) 213–221 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Microfluidic device for compositional analysis of solvent systems at microwave frequencies David J. Rowe a,b,∗ , Adrian Porch a,1 , David A. Barrow a,1 , Christopher J. Allender b,2 a Institute for Green Electronic Systems, Cardiff School of Engineering, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK b Cardiff School of Pharmacy and Pharmaceutical Sciences, Cardiff University, Redwood Building, King Edward VII Avenue, Cardiff CF10 3NB, UK a r t i c l e i n f o Article history: Received 31 December 2011 Received in revised form 9 March 2012 Accepted 23 April 2012 Available online 3 May 2012 Keywords: Dielectric measurements Microwave sensors Microfluidics a b s t r a c t A device for analysing the chemical composition of single- and multi-phase solvent flows using microfluidic dielectric spectroscopy is demonstrated in this work. An open-circuited, half-wavelength, coaxial resonator operating at microwave frequencies (i.e. harmonics of 600 MHz) was embedded in a compression-sealed polytetrafluoroethylene microfluidic chip for in situ characterization of solvent–solvent and solute–solvent mixtures of varying concentration, and an aqueous–organic seg- mented flow. Results are shown for a solvent mixture of acetonitrile in toluene as a test system, exhibiting a sensitivity limit of 400 nM. In addition to being highly sensitive, the measurement system is fast, robust and non-invasive, and can be readily miniaturized. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The ability to characterize liquid composition is of fundamental importance throughout chemistry and the life sciences. Dielec- tric spectroscopy allows for such interrogation in a label-free environment through electromagnetic measurements of complex permittivity spectra, as demonstrated for a variety of applications [1–4]. This work focuses on the development of a flexible platform for performing microfluidic experiments with an embedded spec- troscopic sensor, with a view to enabling scientific investigation by in situ, non-contact characterization of chemical, biological and pharmaceutical processes in real time. We demonstrate such capa- bilities through various measurements of single- and multi-phase microfluidic systems. We have previously reported the development of a coupling structure for obtaining transmission measurements from one end of a microwave-frequency open-circuited half-wavelength coax- ial resonator [5]. Measurements were performed by perturbing the evanescent field at the other (i.e. uncoupled) end of the probe with a dielectric sample. An empirical inversion was used to infer com- plex permittivity values based upon measurements of solvents of ∗ Corresponding author at: Institute for Green Electronic Systems, Cardiff School of Engineering, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK. Tel.: +44 029 2087 0533/5824; fax: +44 0 29 2087 4939/4149. E-mail addresses: rowedj@cf.ac.uk (D.J. Rowe), porcha@cf.ac.uk (A. Porch), barrow@cf.ac.uk (D.A. Barrow). 1 Tel.: +44 029 2087 0533; fax: +44 029 2087 4939. 2 Tel.: +44 029 2087 5824; fax: +44 029 2087 4149. known permittivity. Multi-frequency data were obtained through the exploitation of higher order harmonic transverse electromag- netic (TEM) modes of the resonator. This coupling structure was used to perform resonator mea- surements of individual solvents in bulk [5], of binary mixtures of varying concentration in a capillary flow system [6], and pre- liminary measurements of individual solvents [7] and multi-phase systems [8] in an embedded microfluidic device. In this work, we present detailed design and analysis of single- and multi-phase measurements to demonstrate the suitability of a compression- sealed polytetrafluoroethylene (PTFE) microfluidic device with an embedded coaxial resonator for general flow system characteriza- tion. There are several novel enabling aspects for such a device. From a spectroscopic perspective, it allows a non-invasive, non-contact, non-destructive, label-free and high-throughput measurement that can be used for simultaneous heating and measurement at high power levels. The measurement requires no additional sam- ple preparation and therefore allows direct in situ characterization. In this work, microwave heating is not considered in detail, other than to state that the power dissipated as a result of the electric field interactions used to characterize liquid samples is negligibly small (of the order of nW) for the microwave input power levels here (typically 1 mW). The use of a compression-sealed PTFE substrate (which can be micromachined with standard circuit board fabrication equipment) allows for rapid prototyping of robust microfluidic devices. PTFE offers excellent chemical resistance and is preferable to glass or curable resin substrates from a microwave measurement perspec- tive because of its extremely low dielectric loss. It also has a small 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.04.069