Nanoextraction based on surface nanodroplets for chemical preconcentration and determination Hongyan Wu1Chiranjeevi Kanike1 2Arnab Atta2and Xuehua Zhang1 3 a

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Nanoextraction based on surface nanodroplets for chemical

preconcentration and determination

Hongyan Wu,1Chiranjeevi Kanike,1, 2 Arnab Atta,2and Xuehua Zhang1, 3, a)

1)Department of Chemical and Materials Engineering, University of Alberta, Alberta T6G 1H9,

Canada

2)Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302,

India

3)Physics of Fluids Group, Max Planck Center Twente for Complex Fluid Dynamics, JM Burgers Center for Fluid Dynamics,

Mesa+, Department of Science and Technology,University of Twente, Enschede 7522 NB,

The Netherlands

(*Electronic mail: xuehua.zhang@ualberta.ca)

(Dated: 7 October 2022)

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. Also, the formation

principle and key features of surface nanodroplets will be summarized. Further, the major ﬁndings of nanoextraction

combined with in–droplet chemistry towards sensitive and quantitative detection will be discussed. Finally, we will

give our perspectives for the future trend of nanoextraction.

I. INTRODUCTION

Chemical analysis at trace levels is crucial in various ﬁelds

such as food safety screening,1,2 environmental pollutant

monitoring,3,4 clinical forensics,5,6 and biological contami-

nants sensing.7,8 A preliminary step of analyte extraction and

enrichment prior to detection contributes to improve the limit

of detection (LoD) of analytical tools.9,10 Due to the large ac-

tive surface area, microscopic droplets are of great interest for

preconcentration of target analytes across the droplet-liquid

interface at extremely low concentrations.11–14 The enhanced

preconcentration depends on the partition of the compound

between droplet liquid and surrounding sample liquid.

Recently, dispersive liquid-liquid microextraction

(DLLME) has received great attention because of its

rapid and efﬁcient extraction.15–23 DLLME is a spontaneous

emulsiﬁcation technique based on a ternary mixture con-

taining dispersive solvent, extractant, and aqueous sample

solution.24,25 The formation of extractant microdroplets

increases the interfacial mass transfer of the analytes, leading

to increased extraction efﬁciency. The microdroplets are then

centrifuged and collected from the bulk for subsequent analy-

sis. The main disadvantages of DLLME are the requirement

of two individual steps for extraction and equipment-assisted

sample separation, as well as the consumption of relatively

large amount of disperser solvents.

Surface nanodroplets on immersed substrates provide an

alternative platform for efﬁcient extraction. The most-used

method to induce surface nanodroplets is the solvent ex-

change. In this process, surface nanodroplet nucleation and

subsequent growth occur due to the droplet liquid transient

oversaturation when a good solvent of the droplet liquid is

a)https://sites.google.com/view/soft-matter-interfaces/home

displaced by a poor solvent.26 Surface nanodroplet exhibits

huge surface-to-volume ratio and excellent long-term stabil-

ity against evaporation or dissolution.27,28 These features en-

able surface nanodrops to continuously extract and enrich

trace amounts of solutes from the ﬂow of an aqueous solu-

tion. The extraction efﬁciency of surface nanodroplets pro-

duced on a planar or a curved surface was investigated by Yu

et. al in 2016.29 The term ‘nanoextraction’ describing the liq-

uid–liquid extraction based on surface nanodroplets was pro-

posed by Li et al. in 2019.30

Nanoextraction can be integrated with surface reactions for

further application in chemical analysis. It has been demon-

strated that many chemical reactions in nanodroplets are faster

and more effective than their macroscopic counterparts in a

bulk medium.31,32 Reactive components could be introduced

to a single droplet to impart multi–functionalities under well-

controlled mixing conditions without the inﬂuence of uncon-

trolled collision, coalescence or Ostwald ripening for droplet

reaction in a bulk system. Nanoextraction coupled with in-situ

surface reactions provides an active platform for synthesis of

functionalized nanomaterials and combinative analysis.30,33

This review aims to summarize the latest progress and point

out the current research trends of nanoextractions. Herein,

we begin by brieﬂy introducing the historical development of

droplet-based extraction, followed by elaborating the forma-

tion principle and control parameter of surface nanodroplets.

Then, we discuss the current research progress of nanoextrac-

tion, as well as reaction-assisted nanoextraction. Finally, we

give our perspectives of what would be the future development

of nanoextraction in terms of analytical practices. Prior review

articles on surface nanodroplet mainly focus on its formation

and dissolution dynamics.34,35 To the best of our knowledge,

there has been no review article discussing the nanoextraction

based on surface nanodroplet. We hope this review could pro-

vide support for further development of nanoextraction in the

trace chemical component analysis.

arXiv:2210.02608v1 [physics.flu-dyn] 5 Oct 2022

FIG. 1. (A) Illustration of the classical single drop microextraction

(SDME) process. (B) Schematic illustration of a ternary phase dia-

gram of oil (O), water (W), and solute (S). Oil or water droplets can

be formed in the ouzo region or reverse ouzo region, respectively.

B.V.

II. DEVELOPMENT OF DROPLET-BASED EXTRACTION

Single drop microextraction (SDME), as a solvent minia-

turized extraction technique, was explored by Liu et al.38 and

Jeannot et al.39 in 1996. In a typical SDME process, an or-

ganic microdroplet (∼1–10 µL) is suspended at the tip of

a syringe needle which is surrounded by an aqueous sam-

ple solution (Fig. 1A). After extraction, the drop with en-

riched analytes is withdrawn and injected into an analyti-

cal instrument for detection and quantiﬁcation. The extrac-

tion of target compounds from aqueous phase into an or-

ganic solvent drop is based on passive diffusion.40,41 The

extraction efﬁciency is intrinsically determined by the an-

alyte partition coefﬁcient between the droplet liquid and

water. Different approaches to perform SDME, such as

continuous-ﬂow microextraction,42,43 headspace SDME,44,45

drop-to-drop microextraction,46,47 and direct immersion-

SDME,48,49 have been extensively explored for analytical ap-

plications. Jeannot et al.50 summarized the historical devel-

opment and pointed out advantage/disadvantages of various

modes of SDME technique. A recent review from 2021 fo-

cused on the applications of SDME combined with multi-

ple analytical tools (spectroscopy, chromatography, and mass

spectrometry).51 The popularity of SDME mainly lies in its

cost-effectiveness and low consumption of organic solvent.

Its drawbacks are the instability of the hanging droplet, nar-

row drop surface, and consequently slow diffusion kinetics

and limited sensitivity.

DLLME, as a modiﬁed solvent microextraction technique,

was introduced in 2006 by Rezaee et al. for the extraction

of organic analytes from aqueous samples.15 In this method,

multiple extractant microdroplets are formed and stably dis-

persed in an aqueous sample comprising of the target analytes.

The formation of small droplets is based on the spontaneous

emulsiﬁcation in a ternary system, which is well known as

the “Ouzo effect”.52 A typical ternary mixture consists of a

good solvent (e.g., ethanol), a poor solvent (e.g., water), and

a small ratio of extractant (e.g., oil). The three-phase diagram

of a representative ternary system is shown in Fig. 1B. The

nucleation and growth of the droplets spontaneously occur in

the ouzo regime, which is surrounded by the binodal and spin-

odal curves.53 This emulsiﬁcation without surfactant is kinet-

ically stable over hours or days.54 The dramatically increased

surface area leads to the enhanced diffusion kinetics and high

recovery efﬁciency.

The typical procedure of DLLME is demonstrated in Fig.

1C, microdroplets of extractant are spontaneously formed

when a mixture containing an extracting solvent and a dis-

perse solvent is rapidly injected into an aqueous sample so-

lution. The partition equilibrium at the droplet interface

is reached in a few seconds owing to the large surface

area. Consequently, the extraction is almost independent of

time.55 The cloudy emulsion is then centrifuged to collect

the droplets with concentrated target compound for analy-

sis. Over the years, DLLME technique has been developed

from its basic approach into many other advanced conﬁgu-

rations. Recent revolutions of DLLME have been made re-

garding to the selection of extracting and disperse solvent,56,57

combination with other extraction techniques,58,59 associa-

tion with derivatization reaction,60,61 mechanical agitation-

assisted emulsiﬁcation,62,63 etc. The review devoted specif-

ically to DLLME was published by Rezaee et al.37 in

2010 comprehensively summarizing the early development

of DLLME. Sajid et al.64 reviewed latest advancements of

DLLME with respect to its evolved design of devices, green

aspects, and application extensions. Compared to SDME,

DLLME greatly improves the enrichment efﬁciency. Some

limitations still exist, such as the consumption of toxic extrac-

tion solvents (e.g., carbon tetrachloride, chlorobenzene, and

cyclohexane).37

III. FORMATION OF SURFACE NANODROPLETS

The formation principle of surface nanodroplets is also

based upon the “ouzo effect”.26 The standard protocol of the

solvent exchange to form surface nanodroplets is shown in

Fig. 2A. Different from the generation of dispersive micro-

droplets in a DLLME process, surface nanodroplets are in-

duced by an oversaturation pulse at the interacting front of the

ouzo solution (solution A) and the poor solvent (solution B) in

a narrow chamber.36 As shown in Fig. 2B, the oversaturation

level is demonstrated by the integrated area between the bin-

odal curve and the dilution path through the ouzo region. The

nanodroplets pinned on the substrate are 5–500 nm in height

and 0.1–10 µm in lateral diameter.68

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NanoextractionbasedonsurfacenanodropletsforchemicalpreconcentrationanddeterminationHongyanWu,1ChiranjeeviKanike,1,2ArnabAtta,2andXuehuaZhang1,3,a)1)DepartmentofChemicalandMaterialsEngineering,UniversityofAlberta,AlbertaT6G1H9,Canada2)DepartmentofChemicalEngineering,IndianInstituteofTechnologyKharagp...

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Nanoextraction based on surface nanodroplets for chemical preconcentration and determination Hongyan Wu1Chiranjeevi Kanike1 2Arnab Atta2and Xuehua Zhang1 3 a

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