Back Scatter Interferometry (BSI)

Back scatter interferometry (BS) is a universal detection method based on refractive index changes.

Compact, Inexpensive Refractive Index Detection in Femtoliter Volumes Using Commercial Optical Pickup Technology

(Analytical Methods 2019, 11(17), 2303-2310.)

Refractive index (RI) sensing in microfluidics has the advantage of universal detection, capable of sensing all species from simple monoatomic ions to complex proteins without external labels or additional contrast agents. Various forms of interferometry have been developed for RI sensing in microfluidics. In particular, backscatter interferometry (BSI) is easily implemented and well-suited for miniaturization. This is important for future applications in point-of-care or point-of-interest measurements, where the total analysis system needs to be easily deployed. The optical arrangement in BSI is similar to that used in optical pickup heads (OPHs), found in CD and DVD drives. This offers intriguing possibilities for repurposing OPHs for miniaturized RI detection in microfluidics. To explore the feasibility of this approach, commercially available OPHs are modified for RI detection in 75 μm i.d. (363 μm o.d.) fused silica capillaries. BSI interference patterns measured using a modified OPH positioned near the capillary are compared with simulations as a function of wavelength. Once characterized, the modified OPH is used to measure refractive index changes as sucrose solutions are injected through the 75 μm i.d. capillary. Signal level changes were recorded following the introduction of solutions ranging in concentration from 67 μM to 19.3 mM and the resulting calibration plot (67 μM to 4.8 mM) exhibited good linearity (R2 = 0.9993). Finally, a modified OPH was used to detect the electrophoretic separation of Na+ and Li+ using RI detection. While the measurements reported here used modified OPHs that bypassed the built-in photodiode detector, eventually all on-board components could be utilized for a completely self-contained, inexpensive, universal detector for field deployable microfluidic applications.

Optical Pickup Head for BSI

(A) CD and (B) CD/DVD OPHs adapted for RI sensing. (C) Simplified schematic of the optical path used in OPHs. Light from a laser diode (LD) is reflected from a beamsplitter and focused onto a disc (or fluidic channel) through an aspheric lens. Scattered light is collected by the same lens, passes through the beamsplitter, and is detected on a segmented photodiode detector.

Simulated BSI patterns

Measured (top) and simulated (bottom) BSI patterns collected using a modified KSS-213 OPH. The laser diode normally used in the OPH was replaced with light from a tunable diode laser and the segmented photodiode was removed to detect light exiting the OPH with a CCD camera. For these measurements, the OPH was positioned at its focus above a fused silica separation capillary (75 μm i.d., 363 μm o.d.) filled with ultra-pure water. Backscattered light from the capillary was collected with the objective of the OPH and detected on the CCD camera. The images compare measured and simulated intensity patterns as a function of wavelength.

Wavelength Modulated Back-Scatter Interferometry for Universal, On-Column Refractive Index Detection in Picoliter Volumes

(Analytical Chemistry 2018, 90 (11), 6789-6795.)

Wavelength modulated back scatter interferometry (M-BSI) is shown to improve the detection metrics for refractive index (RI) sensing in micro-separations. In M-BSI, the output of a tunable diode laser is focused into the detection zone of a separation channel as the excitation wavelength is rapidly modulated. This spatially modulates the observed interference pattern, which is measured in the back-scattered direction. Using a split photodiode detector aligned on one fringe of the of interference pattern, phase-sensitive detection is used to monitor RI changes as analytes are separated. Using sucrose standards, we report a detection limit of 700 μg/L in a 75 μm i.d. capillary at the 3σ level, corresponding to a detection volume of 90 pL. To validate the approach for electrophoretic separations, Na+ and Li+ were separated and detected with M-BSI and indirect-UV absorbance on the same capillary. A 4 mg/L NaCl and LiCl mixture leads to comparable separation efficiencies in the two detection schemes, with better signal-to-noise in the M-BSI detection, but less baseline stability. The latter arises in part from Joule heating, which influences RI measurements through the thermo-optic properties of the run buffer. To reduce this effect, a 25 μm i.d. capillary combined with active temperature control was used to detect the separation of sucrose, glucose, and lactose with M-BSI. The lack of suitable UV chromophores makes these analytes challenging to detect directly in ultra-small volumes. Using a 55 mM NaOH run buffer, M-BSI is shown to detect the separation of a mixture of 174 mg/L sucrose, 97 mg/L glucose, and 172 mg/L lactose in a 15 pL detection volume. The universal on-column detection in ultra-small volumes adds new capabilities for micro-analysis platforms, while potentially reducing the footprint and costs of these systems.

Wavelength Modulation for Enhanced BSI

(A) Schematic of BSI where an unfocused laser beam directed onto a CE capillary generates an interference pattern detected in the back-scattered direction. Fringes in the BSI pattern shift in response to changes in the refractive index of the solution filling the capillary. (B). Schematic of M-BSI where the output of a fiber coupled tunable diode laser is focused onto the CE capillary. The reference signal from a lock-in modulates the laser wavelength, which shifts the interference pattern as indicated in panel A. A fringe in the retroreflected interference pattern is detected on a split photodiode, amplified, and sent to a lock-in amplifier to measure changes in refractive index.

Calibration Plot using Modulated BSI

M-BSI response to increasing concentrations of sucrose solutions. Sucrose standards at the indicated concentrations were passed through a 75 μm i.d. (363 μm o.d.) using pressure. Following each standard, ultra pure water was introduced and the signal returned to baseline as shown. The inset shows the calibration plot generated from the measured signal level changes.