A microscale-molecular weight sensor: probing molecular diffusion between adjacent laminar flows by refractive index gradient detection.

Abstract

A detection scheme that measures the refractive index gradient (RIG) between adjacent laminar flows in a microfluidic device has been used to develop a microscale-molecular weight sensor. The behavior of low Reynolds number flows has been well documented and shows that molecular transport (mixing) between adjacent laminar flows occurs by molecular diffusion between flow boundaries. A diode laser beam, incident upon and illuminating the entire width of a microchannel, measured the transverse concentration gradient at two different positions along a microchannel. The concentration gradient is impacted by the transverse diffusion from a flow with analyte into a flow initially without analyte. The RIG that forms as analyte diffuses from one adjacent flow to the other causes the laser beam, impinging orthogonal to the RIG through the microchannel, to be deflected. The angle of deflection is then monitored on a position-sensitive detector (PSD) at two different positions along the axis of flow to provide a measurement of analyte diffusion. The two positions are just after the flow initially without analyte merges with the flow initially containing all of the analyte (upstream) and then after the two streams have had more time to diffuse together (downstream). The ratio of the PSD signals obtained at the two positions along the flow, downstream signal divided by the upstream signal, is readily correlated to the analyte diffusion coefficient and, thus, the analyte molecular weight for a given class of compounds. The device was evaluated as a molecular weight sensor for poly(ethylene glycol) (PEG) solutions over a molar mass range from 106 to 22,800 g/mol. The ratio signal was found to be both independent of PEG concentration and sensitive to molecular weight changes for samples ranging from 960 to 22,800 g/mol. Independence of concentration is important for obtaining a reliable molecular weight measurement. The limit of detection for 11,840 g/mol PEG measured at the upstream detection position was determined to be 56 ppm, equivalent to 4.5 x 10(-6) RI (3sigma). This technique provides a much needed universal detection method, without requiring analyte derivatization chemistry (e.g., fluorescence), for microfluidic analyses that are becoming increasingly useful in monitoring chemical systems such as continuous-flow reactors or batch polymerization processes. Thus, the molecular weight determination capability is potentially applicable to other compound classes, such as DNA or proteins.