  Preconcentration of the target compound is a critical step that ensures the
accuracy of the subsequent chemical analysis. In this work, we present a
straightforward yet effective liquid-liquid extraction approach based on
surface nanodroplets (i.e., nanoextraction) for offline analysis of highly
diluted sample solutions. The extraction and sample collection were streamlined
in a 3-m microcapillary tube. The concentration of the target analyte in
surface nanodroplets was significantly increased compared to the concentration
in the sample solution, reaching several orders of magnitudes. A limit of
detection (LOD) was decreased by a factor of $\sim 10^3$ for an organic model
compound in Fourier-transform infrared spectroscopy (FTIR) measurements and
$\sim 10^5$ for a model fluorescent dye in fluorescence detection. The
quantitative analysis of the organic compound was also achieved in a wide
concentration region from $10^{-3}$ M to $10^{-4}$ M. The total volume of
surface nanodroplets can be manipulated to further enhance extraction
efficiency, according to the principle that governs droplet formation by
solvent exchange. Additionally, our method exhibited significantly improved
sensitivity compared to traditional dispersive liquid-liquid microextraction
(DLLME). The LOD of the fluorescent dye and the organic model compound obtained
with DLLME was 3 orders of magnitude and 20 times higher than the LOD achieved
through nanoextraction approach. The nanoextraction developed in this work can
be applied to preconcentrate multi-compounds from river water samples, without
clear interference from each other. This can further extend its applicability
for the detection and quantification of target analytes in complex aqueous
samples by common analytical instruments.
