Solvent-free magnetic-tip microextraction into a single drop for fluorescence sensing

In this work, a simple fluorescence sensing device based on an approach of solvent-free magnetic-tip microextraction into a single drop was developed. As proof-of-principle, the strategy was used to extract chloramphenicol (CAP). CAP was selectively recognized by a DNA-aptamer and replaced by a trigger in the assay. By combining DNA signal amplification, DNA walkers constructed DNA chains with G-quadruplex/protoporphyrin IX (PPIX) as the branches on the surface of the added Fe3O4 nanosheets (NTs). A pipette tip-based magnetic bar was utilized to separate the NTs from the sample solution in the form of a single drop (4 μL) containing the analyte, at the tip. In this way, the fluorescent signal of G-quadruplex/PPIX, which served as the proxy of the CAP, was extremely enhanced and could be directly detected via a nanodrop spectrophotometric platform. This novel method was applied to the fluorescence detection of CAP in genuine milk and urine samples. It achieved a low limit of detection of 26.5 aM (0.00856 fg/mL), and a linear dynamic range from 0.1 fM to 1 nM with good linearity (r = 0.998). [Display omitted] •Solvent-free magnetic-tip microextraction into a single-drop was developed.•A pipette tip-based magnetic bar was utilized as a fluorescence sensing device.•By combining DNA signal amplification, chloramphenicol was detected as proof-of-principle.