Microfluidic chip-based liquid–liquid extraction and preconcentration using a subnanoliter-droplet trapping technique

Abstract

A robust and simple approach for microfluidic liquid–liquid (L–L) extraction at the subnanoliter-scale was developed for on-chip sample pretreatment. Organic solvent droplets of a few hundred pL were trapped within micro recesses fabricated in the channel walls of a microfabricated glass chip. L–L extraction was performed by delivering aqueous samples through the channel, with the sample stream continuously flowing adjacent to the droplets. The analytes in aqueous streams were enriched within the droplet with high preconcentration factors owing to both phase transfer and dissolution of organic solvent into the bypassing aqueous sample. An aqueous solution of butyl rhodamine B (BRB) and 1-hexanol were used, respectively, as sample and extractant to demonstrate the performance of the system. The fluorescence intensity of the dye extracted into the droplet was monitored in situ by LIF. The system proved to be an efficient means for achieving high enrichment factors of over 1000, with sample consumption of a few µL. Quantitative measurement of the extracted analyte was achieved with a linear response in the range 1 × 10⁻⁹–8 × 10⁻⁷ M BRB. The precision of the measured fluorescence values for a 10⁻⁷ M BRB standard with a 12.5 min preconcentration period was 6.6% RSD (n = 5).