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Liquid−liquid extraction based on surface nanodroplets can be a green and sustainable technique to extract and concentrate analytes from a sample flow. However, because of the extremely small volume of each droplet (<10 fL, tens of micrometers in base radius and a few or less than 1 μm in height), only a few in situ analytical techniques, such as surfaceenhanced Raman spectroscopy, were applicable for the online detection and analysis based on nanodroplet extraction. To demonstrate the versatility of surface nanodroplet-based extraction, in this work, the formation of octanol surface nanodroplets and extraction were performed inside a 3 m Teflon capillary tube. After extraction, surface nanodroplets were collected by injecting air into the tube, by which the contact line of surface droplets was collected by the capillary force. As the capillary allows for the formation of ∼10 12 surface nanodroplets on the capillary wall, ≥2 mL of octanol can be collected after extraction. The volume of the collected octanol was enough for the analysis of offline analytical techniques such as UV−vis, GC-MS, and others. Coupled with UV−vis, reliable extraction and detection of two common water pollutants, triclosan and chlorpyrifos, was shown by a linear relationship between the analyte concentration in the sample solution and UV−vis absorbance. Moreover, the limit of detection (LOD) as low as 2 × 10 −9 M for triclosan (∼0.58 μg/L) and 3 × 10 −9 M for chlorpyrifos (∼1.05 μg/L) could be achieved. The collected surface droplets were also analyzed via gas chromatography (GC) and fluorescence microscopy. Our work shows that surface nanodroplet extraction may potentially streamline the process in sample pretreatment for sensitive chemical detection and quantification by using common analytic tools.

Liquid-liquid extraction based on surface nanodroplets, namely nanoextraction, can continuously extract and enrich target analytes from the flow of a sample solution. This sample preconcentration technique is easy to operate in a continuous flow system with a low consumption of organic solvent and a high enrichment factor. In this review, the evolution from single drop microextraction to advanced nanoextraction will be briefly introduced. Moreover, the formation principle and key features of surface nanodroplets will be summarized. Further, the major findings of nanoextraction combined with in-droplet chemistry toward sensitive and quantitative detection will be discussed. Finally, we will give our perspectives for the future trend of nanoextraction.

The droplet-based biphasic reaction is an efficient strategy for the fabrication of surface-bound nanostructures. Here, we developed a process of fabricating ordered micro-ring arrays of silver (Ag) nanostructures from surface nanodroplet reaction on a micro-patterned hydrophobic substrate for reproducible detection by surface-enhanced Raman spectroscopy (SERS). Our process consisted of the generation of surface nanodroplet arrays, followed by a biphasic chemical reaction between droplets and the continuous flow of silver nitrate (AgNO3) precursor solution. The parameters in the formation and reaction of the droplet array were well controlled to maintain the uniformity of Ag nanostructures throughout the substrate. By scaling up the process parameters and the size of the microchamber, we were able to produce a SERS substrate with a surface area of > 60 cm^2 in a single run. Such a large area could be sufficient for analyzing more than a thousand samples. We demonstrated the repeatability of SERS measurements using Ag nanostructures by analyzing three environmental (rhodamine 6G, chlorpyrifos, triclosan), a biological (indoxyl sulfate), and a psychoactive drug (tetrahydrocannabinol) compounds. 2D mapping of SERS intensities was also performed for both small and large-scale substrates by collecting data from more than 100 locations on the substrate. Our work demonstrated droplet-based biphasic reaction as a simple approach for the fabrication of SERS substrate with a large area. The technique may help to eliminate the requirement for sophisticated equipment for the fabrication of SERS active substrate.

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