Compact, inexpensive refractive index detection in femtoliter volumes using commercial optical pickup technology
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.