High-sensitivity near-IR absorption measurements of nanoliter samples in a cavity enhanced fiber sensor

Abstract

A compact fiber optic sensor is described using Incoherent Broad-Band Cavity Enhanced Absorption Spectroscopy for sensitive detection of nanoliter samples of aqueous chemicals and microorganisms in capillaries. Absorption was measured in a 70 µm gap, comparable to the inside diameter of a capillary used for electrophoresis, between the ends of two short segments of multimode fiber. The other ends of the fibers were optically contacted to dielectric mirrors to form an 11-cm cavity resonator. Light from a superluminescent diode (λ=1054 nm, BW=35 nm FWHM) was coupled into one end of the cavity, and transmission through the cavity was measured using a silicon photodiode. Dilute aqueous solutions of near infrared dye were used to determine the minimum detectable absorption change of 4x10 -6 for 10 second integration and unity signal-to-noise ratio, which is approximately two orders of magnitude more sensitive than previously published results for systems with comparable sample path lengths.

Broadband cavity enhanced UV-VIS absorption spectroscopy for picolitre liquid samples

“…Optical fibres have been employed to act as a waveguide between the cavity mirrors and the sample to generate a CEAS with an enhancement factor of 9× in volumes ∼200 pl. 9,10 Using a plano-convex cavity configuration with volumes down to 50 fl an increased cavity enhancement factor of 266× was achieved. 4 However, the plano-convex geometry is tuned for a single resonant wavelength at ∼640 nm and measurements over a broad spectral range are difficult to achieve as they require the adjustment of the cavity geometry via a piezo actuator.…”

“…2 Measuring small volumes is particularly important for applications such as nanoparticle synthesis and characterisation, drug development, and single cell omics. 4,[9][10][11][12] The sensitivity required for measuring small volumes or low concentrations with traditional absorption spectroscopy has been beyond capabilities. 13 The limiting factor for a sample of concentration C is the pathlength, l, of light through the sample, which is directly related to the sample volume.…”