High-sensitivity online detection for microfluidics via cavity ringdown spectroscopy

Abstract

We report the coupling of cavity ringdown spectroscopy (CRDS) with a microfluidic chip fabricated using a rapid prototyping method, in order to demonstrate high-sensitivity, non-contact online detection in microfluidics. Conventional UV-vis absorption techniques are largely ineffective for microfluidic detection due to the small sample volumes and short path lengths. The multipass absorption achieved in cavity ringdown spectroscopy increases the effective absorption pathlength by several orders of magnitude, and hence enhances the detection sensitivity. A cavity ringdown spectrometer, operating at a single wavelength of 532 nm for the purposes of the proof-of-concept measurements presented here, has been developed for online detection on a polymer/glass microchip fabricated by frontal photopolymerisation. High sensitivity absorption measurements on liquid samples with volumes of tens to hundreds of nanolitres and absorption pathlengths ranging from tens to hundreds of microns are demonstrated. A series of proof-of-concept experiments show that the technique has the ability to monitor both static and time-varying analyte concentrations. Firstly, the detection limit of the system is estimated from a three-standard-deviation error analysis of absorption measurements made on dilute aqueous solutions of potassium permanganate (natural absorption coefficient (4805 ± 10) M⁻¹ cm⁻¹ at 532 nm). The detection limit was found to be ∼210 nM for a 466 μm pathlength, corresponding to an absorption of 1.0 × 10⁻³ cm⁻¹. Online pH measurements on a 20 nL sample are performed by monitoring the absorption of phenolphthalein indicator present at millimolar concentrations. Finally, CRDS has been applied, for the first time, to monitoring chemical reaction kinetics on a microfluidic chip, tracking the oscillation period of the well-known Belousov–Zhabotinsky reaction.