A single-layer, planar, optofluidic Mach-Zehnder interferometer for label-free detection

Abstract

We have developed a planar, optofluidic Mach-Zehnder interferometer for the label-free detection of liquid samples. In contrast to most on-chip interferometers which require complex fabrication, our design was realized via a simple, single-layer soft lithography fabrication process. In addition, a single-wavelength laser source and a silicon photodetector were the only optical equipment used for data collection. The device was calibrated using published data for the refractive index of calcium chloride (CaCl(2)) in solution, and the biosensing capabilities of the device were tested by detecting bovine serum albumin (BSA). Our design enables a refractometer with a low limit of detection (1.24 × 10(-4) refractive index units (RIU)), low variability (1 × 10(-4) RIU), and high sensitivity (927.88 oscillations per RIU). This performance is comparable to state-of-the-art optofluidic refractometers that involve complex fabrication processes and/or expensive, bulky optics. The advantages of our device (i.e. simple fabrication process, straightforward optical equipment, low cost, and high detection sensitivity) make it a promising candidate for future mass-producible, inexpensive, highly sensitive, label-free optical detection systems.

Fig. 1

(a) Schematic of the device with arrows depicting the familiar light path of a Mach–Zehnder interferometer. (b) An experimental image of the device, post-insertion of optical fibers.

Fig. 2

The dynamics of Solution 2 replacing Solution 1, using ink as a visible marker for characterization.

Fig. 3

Voltage output from the optical detector during a single experiment analyzing a 100 mM solution of CaCl₂ (solid line). Number of oscillations (ΔΦ₁/2π) in the experimental data calculated over the duration of the experiment (dashed line).

Fig. 4

Comparison between the augmented experimental data and a plot of the theoretical equation.

Fig. 5

Comparison of signals acquired for solutions with various concentrations of CaCl₂.

Fig. 6

The number of oscillations for given concentrations of CaCl₂ from experiments. The separation between horizontal bars indicates a spread of four standard deviations.

Fig. 7

Detection of BSA protein using our device. The separation between horizontal bars indicates a spread of four standard deviations.