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Nanoparticle induced formation of self-assembled
zwitterionic surfactant microdomains which mimic
microemulsions for the in situ fabrication anddispersion of silver nanoparticles†
Taichi Nakagawa,a Akihide Hibara,bWillie L. Hinzec and Yoshitaka Takagai*aThe eﬀective synthesis of highly concentrated and stable spherical silver nanoparticles (Ag-NPs) enclosed
within microdomains formed by the Ag-NP induced self-assembly of zwitterionic surfactants is described.
In this paper, the rapid and eﬃcient synthesis of dispersed and stable Ag-NPs in zwitterionic surfactants,
such as 3-(nonyldimethylammonio)- or 3-(decyldimethyl-ammonio)-propyl sulfate (C9– or C10–APSO4)
that self-assemble in the presence of the Ag-NPs to form microdomains akin to microemulsions (ME)
Received 7th August 2020
Accepted 10th September 2020

without the need of any organic solvent, co-surfactant or polymer is presented. Essentially all (>99.9%)of the synthesized Ag-NPs (TEM diameter 16.8  4.3 nm) were incorporated and dispersed within the ME

DOI: 10.1039/d0ra06824a

microdomains (213.5  48.0 nm). A mechanism for formation of ME and its enclosing of the Ag-NPs in

rsc.li/rsc-advances

the ME is proposed.

1. Introduction
Self-assembling nanoparticles (NPs) on/in conned microdomains, such as microparticles, microdroplets and emulsions have been intensely researched1 owing to their attractive
chemical properties which improve the eﬀectiveness of theresultant materials to function as catalysts,2 sensors,3 andmedical agents.4 The synthesis of NPs on surfaces or inside
micro-domains can have a positive inuence on properties
such as the reaction speed,2 eﬃciency,5 sensitivity,6 andenhance phenomenon such as emission.7 In particular,
numerous reports focus on applications of silver (Ag)-NPs
synthesized using microemulsion technology in various
elds8,9 such as catalysis, nanoreactors, biology, medicine,
optics, and electronics among others. Although it is diﬃcult tocontrol the size and homogeneity of Ag-NPs prepared bychemical reduction methods,10 utilization of surfactant oil-inwater (o/w) or water-in-oil (w/o) emulsion systems have provena

Faculty of Symbiotic Systems Science, Cluster of Science and Technology, Fukushima
University, 1 Kanayagawa, Fukushima 960-1296, Japan. E-mail: s015@ipc.
fukushima-u.ac.jpbInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, 21-1 Katahira, Aoba-ku, Sendai 980-8577, Japanc
Department of Chemistry, Wake Forest University, P. O. Box 7486, Winston-Salem,
North Carolina 27109, USA. E-mail: hinze@wfu.edu† Electronic supplementary information (ESI) available: The elemental mapping;
the impact of citric acid concentration; absorption spectra; the relationshipbetween the surfactant concentration and the volume of second phase;
relationship between the conductivity and the concentration of C10–APSO4
surfactant; the relationship between the C10–APSO4 surfactant and conductivity.
See DOI: 10.1039/d0ra06824a

This journal is © The Royal Society of Chemistry 2020benecial in the synthesis of small and uniform Ag-NPs whichwere formed within the emulsion microdroplets.8,9 Bulavchenko et al. reported that the fabrication eﬃciency of Ag-NPs
was enhanced by utilizing microemulsions relative to larger
emulsions.11 Thus far, the microemulsion systems employed
have been composed of water, water–immiscible organic
solvent (oil), surfactant and in some cases, a co-surfactant or
polymer.8,9Herein, we report the use of simple aqueous solutions ofzwitterionic surfactants, such as 3-(nonyldimethylammonio)or 3-(decyldimethyl-ammonio)-propyl sulfate (C9– or C10–
APSO4), for the eﬃcient synthesis of Ag-NPs. At concentrations greater than their critical micelle concentration (CMC),
surfactants of this class are thermo-responsive and exist asa homogeneous aqueous micellar solution at temperatures
above their upper critical solution temperature (UCST),
whereas they phase separate at temperatures below the UCST
to form a small surfactant-rich phase and a larger aqueous
surfactant-poor phase.12–14 In this study, the surfactant
concentrations were below the CMC values and at temperatures such that homogeneous solutions were formed inwhich the surfactant was initially present in its monomeric(molecular) form. During the course of the synthetic reaction
at elevated temperature, these surfactants undergo induced
self-assembly to form micro-domains (ME) in which the AgNPs are sequestered and dispersed in a manner akin to thatobserved in o/w microemulsions or swollen micelles exceptthat no added organic solvent, co-surfactant or polymer isrequired.

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Experimental

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2.1. Instrumentation
Ultraviolet-visible spectra were obtained using a JASCO V-650
double beam spectrophotometer equipped with quartz cuvette(1 cm  1 cm). The sample solution was measured without
dilution. Dynamic light-scattering (DLS) measurements were
performed using a HORIBA SZ-100 nanoparticle analyzer witha uorescence analytical grade quartz cuvette (1 cm  1 cm) at
room temperature (25.0  C) and green laser output at 532 nm
and the measurement scattering angle set to 90 . DLS
measurement of particle size is strongly inuenced by theviscosity of sample solution. Thus, a sine-wave vibro viscometer
SV-A (A&D Co. Ltd, Tokyo, Japan) was used to measure theviscosity of the sample solutions prior to the DLS measurement.
The viscosity was then imputed as the parameter of DLS
measurement. The hydrodynamic diameters reported are themean diameters. Transmission electron microscopy (TEM)
images were obtained using a JEOL JEM-1400plus transmission
electron microscope (TEM; Tokyo, Japan) operating at 120 kV.
The HD-2700 (Hitachi, Ltd) microscope was employed for thescanning TEM (STEM) analysis. The acceleration voltage was
200 kV at magnication between 30 000 and 100 000. The
Octane T Ultra (AMETEK Co., Ltd) energy-dispersive X-ray (EDX)
analyzer was used, which was connected to the STEM apparatus.
TEM samples were prepared by placing 5.0 mL aliquots of theaqueous sample onto a carbon-lmed 200-mesh Cu grid (Nisshin EM Co. Ltd, Tokyo, Japan). Aer sample loading, the grids
were dried at room temperature prior to the TEM measurements. The microemulsion size distribution was typicallydetermined by measuring 20 such grids. Nanoparticles size
distribution and concentration measurements were made usinga ViewSizer3000 (HORIBA Ltd, Japan). The dimension of the cellwas 10  10  45 mm with four-plane transmission (Hella
Analytics, Germany). The measurement conditions were set
temperature 22  C, 3 lasers (450, 520, 650 nm) were used withthe scattering angle set to 90 . The zeta potential measurements
were made employing a Nano Z ZEN3600 Zetasizer (Malvern
Instruments Ltd, Worcestershire, UK). The pH measurements
were made using a HORIBA handy D-50 pH meter equippedwith a long ToupH combination 9680-10D electrode. This
electrode probe was employed to directly measure solution pH
in the conical-shaped centrifuge tube. The conductivity
measurements were made using a HORIBA handy D-74T water
quality meter with a Conductivity Cell (Submersible Type) 938210D electrode. This electrode was employed to directly measuresolution conductivity in the centrifuge tube.

2.2. Materials
All chemicals and reagents were used without any further
purication, unless otherwise noted. N,N-Dimethyldecylamine(>95.0%) was purchased from Tokyo Chemical Industry Co., Ltd(Japan). Trimethylene sulphate (98%) was obtained from
Combi-Blocks, Inc. (USA). Acetone ($99.5%), trisodium citrate
dihydrate ($99.0%), sodium hydroxide ($97.0%) and diethyl
ether ($99.5%) were acquired from FULIFILM Wako Chemicals

34162 | RSC Adv., 2020, 10, 34161–34166

Paper
Co. (Japan). Silver nitrate ($99.0%) was obtained from SigmaAldrich (USA). Distilled water was obtained using a water purication apparatus RFD240NA (ADVANTEC, Japan).
The zwitterionic surfactants, C10–APSO4, C9–APSO4 andC8–APSO4 were synthesized using published literatureprocedures.12,14
2.3. Procedures
2.3.1. Determination of the zeta potential. The zetapotential of the surfactants was determined by an extrapolation method. Namely, the surfactants were diluted with a water–
methanol mixed solvent system containing diﬀerent amountsof methanol and for each, the zeta potential was measured. A
plot of the apparent zeta potential versus the methanol content
was constructed and found to be linear. The zeta potential of thesurfactant was taken to be its extrapolated value for 0%
methanol.
2.3.2. Determination of the CMC. The CMC of the surfactant was determined by measuring the electrical conductivity ofa series of aqueous surfactant solutions which contained
diﬀerent concentrations (C) of the surfactant. A graph of molar
pﬃﬃﬃﬃ
conductivity versus C was made and the CMC was determinedfrom the inection point of the curve.
2.3.3. Ag-NP synthesis. The recipe for synthesis of Ag-NPs
in MEs was as follows: typically, 1.00 mL of an aqueous 0.50
w/w% zwitterionic surfactant C9– or C10–APSO4, 0.50 mL of
5.0 mM trisodium citrate aqueous solution were mixed together
in a conical centrifuge tube (tube) and heated in an oil bath (108C) for 2 min to prepare a homogeneous solution. The resultant
solution was adjusted to pH 11 by the addition of 0.25 mL of
0.10 M NaOH solution with continued heating in an oil bath(108  C) for 2 min. Next, 0.50 mL of 0.50 mM AgNO3 solution
was added to the tube and the resultant solution kept static at
108  C for 8 min (for synthesis of Ag-NPs). This solution wasthen cooled for 5 min down to room temperature (25  C). The
solution was characterized by UV-visible absorption spectrophotometer, DLS, TEM, and STEM with EDX. To count andmeasure the NPs, a NP tracking analyzer (NTA) was employed.

3.

Results and discussion

3.1. Synthesis and characterization
The Ag-NPs prepared in the ME appeared as a yellow homogeneous solution (see inset to Fig. 1A) and exhibited a surface
plasmon resonance (SPR) absorption band at 401.5 nm (Fig. 1A
solid red line) with the peak width at half maximum (PWHM) of
95 nm. The TEM images indicate that the Ag-NPs are monodisperse and roughly spherical-shaped in the ME, which iscomposed of the surfactant C10–APSO4 (Fig. 1B). Furthermore,
STEM-EDX mapping (Fig. S1 in ESI†) conrmed that the Ag-NPs
were dispersed in the C10–APSO4 surfactant ME microdomains.
The mean diameter of the Ag-NPs in the ME was 16.8  4.3 nm(n ¼ 50) as determined from the TEM images, whereas theaverage hydrodynamic diameter obtained from DLS was 55.3 
46.3 nm (Fig. 1C). Essentially all (>99.9%) of the synthesized AgNPs were incorporated and dispersed within the ME. The size ofThis journal is © The Royal Society of Chemistry 2020

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RSC AdvancesFig. 1 Characterization of the Ag-NPs in ME. Panel [A] shows a photograph of the solution containing Ag-NPs and the UV-visible absorption
spectra of Ag-NPs prepared by this method in the presence (solid red line) and absence of the surfactant (red dotted line). The black dotted line isspectra of Ag-NPs obtained using a typical citric acid reductant method.17 Panel [B] shows transmission electron microscopy (TEM) image of AgNPs in the (ME). Panel [C] shows the dynamic light scattering (DLS) proﬁle of the Ag-NPs prepared by this method (red line) and a typical citric acid
reductant method17 (black dotted line). Panel [D] shows the NP tracking analyzer (NTA) proﬁle which indicates the presence of both the Ag-NPs(mode diameter 28 nm) and the ME (mode diameter 110 nm). Experimental conditions: [C10–APSO4]T ¼ 0.20% (w/w); [AgNO3]T ¼ 0.11 mM;[trisodium citrate]T ¼ 1.10 mM; [NaOH]T ¼ 0.010 mM; total volume ¼ 2.25 mL; pH ¼ 11; reaction time: 8 min at 108  C. TEM voltage 120 kV (30k to
50k magniﬁcation).

the ME microdomains was determined as 213.5  48.0 nm (n ¼25; dry state) from the TEM image. On conducting the synthetic
protocol in the absence of C10–APSO4, occulation (aggregation)of the Ag-NPs occurred (Fig. 1A dotted red line). We propose thatthe surfactant adsorbs onto the surface of Ag-NPs providing
a capping layer, which stabilizes the Ag-NPs and mediates their
growth.8 In the absence of Cn–APSO4 surfactant, no suchconnement of growth occurs and the resultant NPs are notuniform and unstable. A typical Ag-NP synthesis by the citricacid method (without surfactant) was reported as yielding
a range of Ag-NP sizes (10–200 nm) with their SPR bands (lmax)
ranging from 397 to 427 nm.12 For comparison, the previously
PWHM (near 130 nm) and size distribution (89.6  94.6 nm)15
which are larger than those of the Ag-NPs synthesized in thiswork (Fig. 1A and C dotted black lines). The smaller number oflmax and PWHM represents a smaller size and more uniformed
size distribution for the Ag-NPs.15,16
The NTA prole (Fig. 1D) indicates the presence of two major
size distributions (37  12 and 162  87 nm). The two peaks
correspond to the values obtained from DLS (33 nm, modal AgNP diameter) and TEM (213.5  48.0 nm as dried ME). As ME

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liquid droplets were dried for TEM observation, the apparent
diameter (aer drying) in the TEM image is larger than that ofthe dispersive state in NTA. This method of synthesizing Ag-NPs
in an ME serves to regulate the size (57.7 to 72.1 nm; describedin the Section 3.2) and shape (spherical) of the NPs. NTA indicated that the number of Ag-NPs and ME clusters were 6.2  109
and 8.5  107 particle per mL, respectively. The resultant
concentration for Ag-NPs in ME was 73 particle/ME, whereas themean apparent concentration of the Ag-NPs in ME was 15  10
particle/ME (n ¼ 50) as determined from TEM images (max.
frequency).
3.2. Eﬀect of the concentration of Ag+ ion
Fig. 2 shows the relationships between the concentration of Ag+
and the diameter of the Ag-NPs and their concentration in the
ME. Although the formed Ag-NPs (average size) only slightly
decreased (from 72.1 to 57.7 nm) with increases in the silver ion
concentration (Fig. 2 blue line), the uncertainties associatedwith these values were ca. 25%. Thus, there was no appreciably
diﬀerence in the size of Ag-NPs with variations in the concentration of Ag+ ion. Thus, this method can synthesize a specic

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Ag-NPs in the ME was only observed at pH 11. Dong et al. reported that the fabrication rate of Ag-NPs at pH 11 was fasterthan other pH values (citric acid reduction method) owing to
the reduction activity.17 Perhaps the rate of the surfactant
binding onto the precursor of Ag-NPs is faster than that of citricacid (as capping agent). At pH values > 11 Ag-NP, synthesis isdiﬃcult due to formation of AgOH (Ksp ¼ 2  108).18

3.4. Eﬀect of surfactant and impact on ME production

Fig. 2 Inﬂuence of the variation of Ag+ concentration upon the Ag-NP
particle size (blue line) and surfactant ME diameter (green line) as wellas the Ag-NP concentration in the ME (red line). Experimental conditions: [C10–APSO4]T ¼ 0.20% (w/w); [trisodium citrate]T ¼ 1.10 mM;[NaOH]T ¼ 0.010 mM; total volume ¼ 2.25 mL; pH ¼ 11; reaction time:
8 min at 108  C. Diameters and concentrations determined from
replicate (n ¼ 10) DLS and TEM measurements, respectively.

sized Ag-NP in the C10–APSO4 ME. This is in contrast to a literature report in which the Ag-NPs size typically increased with anincrease in [Ag+].8 The concentration of Ag-NPs in the ME
increases with concentration of Ag+ (Fig. 2 red line). At
concentrations > 0.22 mM AgNO3, formation of agglomerated or
aggregated Ag-NPs resulted owing to saturation. The ME size isalso impacted by the silver ion concentration (Fig. 2, green line).
Over the concentration range examined, the size of ME
increases from ca. 100 to 200 nm with increases in [Ag+]. This
result suggests that the presence of Ag+ is an important factor in
the formation of the ME. No ME formation was observed ina mixture of Ag+ and C10–APSO4 surfactant without citric acid(reducing agent). This indicates that the formation of thesurfactant ME microdomains is related to the presence of Ag-NP
precursor. The precursor functions to induce assembly of
surfactant monomers initially present in solution to form
aggregates with microdomains which mimic the behavior ofa microemulsion (ME). It is important to note that no such ME
formed if pre-prepared citrate coated Ag-NPs were used insteadof Ag+ (i.e. AgNO3) in the described procedure as the Ag-NPs
existed in an as-dispersed state in solution. It is consideredthat rapid exchange of Cn–APSO4 surfactant with the pre-coatedcitric acid Ag-NPs is diﬃcult resulting in the absence ofa surfactant capping layer on the Ag-NPs.

The preparation of dispersed Ag-NPs in the ME was successful
using both C9– and C10–APSO4 as surfactants (Fig. 3). The
number of Ag-NPs in the ME was essentially the same (15  4 for
C9–APSO4 and 15  10 for C10–APSO4). The diameter of the AgNPs in the C9–APSO4 and C10–APSO4 MEs were also very similar.
The ME size was 100  31 nm (from TEM image) for C9–ASPO4
which is smaller than that of C10–APSO4. However, use of theshorter carbon chain surfactant, (C8–APSO4), only resulted in
the formation of Ag-NP aggregates (Fig. 3C). It should be notedthat the diameters of the C9– and C10–APSO4 ME aggregates
containing the dispersed Ag-NPs are much greater relative tonormal micelles formed from similar zwitterionic sulfobetaine
surfactants.19,20 For instance, the surfactant SB3-10 (sameformula as C10–APSO4 except the terminal anionic group issulfonate, –SO3, rather than sulfate) has a diameter of only 2–
4 nm.19,20
Although a Cn–APSO4 surfactant is neutral overall, the zetapotential of the solution including this surfactant at pH 3
(without Ag-NPs) was determined to be +23.2, +10.2 and20.0 mV for C8–, C9–, and C10–APSO4 respectively. These
values are in the range of reported values for other zwitterionicsurfactant systems which can range from slightly positive to70 mV depending upon the type and concentration of added
salts and pH.20–23 The bipolar ionic moiety of the zwitterionicsurfactants can interact with the uncapped or citrate capped AgNPs and the values become more negative, i.e., 89  12 mV for
C10–APSO4 at pH 11. A similar phenomenon was reported forthe potential of a NP-surfactant bilayer composite, in which thepotential was gradually decreased by the surfactant-coating on
metallic NPs from 0 mv to 57.9 mV.24,25 Taking into accountthe fact that the zeta potentials for zwitterionic surfactant
systems (as well as citrate coated Ag-NPs themselves) decrease
with increases in solution pH, the zeta potential obtained for
the C10–APSO4/Ag-NP ME in this work is reasonable and in thegeneral range reported for Au- and Ag-NPs dispersed or cappedwith other zwitterionic species.13,26,27

3.3. Impact of the concentration of citrate ions and pH
The eﬀect of the citrate ion concentration upon the size andconcentration of the Ag-NPs in the ME was briey examined.
The results (Fig. S2 in ESI†) indicate that at citrate concentrations > ca. 1.0 mM, the concentration of Ag-NPs in the ME
noticeably decreased and their size increased (due to aggregation). The results suggest that citrate functions as a reductant;however, at higher concentrations, it is not only a reductant but
also a dispersing agent. The pH impact on the fabrication ofboth Ag-NP and ME is shown in Fig. S3 in ESI.† The formation of

34164 | RSC Adv., 2020, 10, 34161–34166

TEM images of the Ag-NPs formed using three diﬀerent zwitterionic surfactants in the synthetic procedure: [A] C10–APSO4. [B] C9–
APSO4. [C] C8–APSO4.
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The C10–APSO4 concentration range that allows formation of
the ME for the synthesis of Ag-NPs as reported is rather limited.
The upper concentration is dictated by the phase separation
behavior (UCST) exhibited by this surfactant. As shown (Fig. S4
in ESI†), at concentrations $ 0.6 w/w% (18.5 mM), a heterogeneous solution results which phase separates (the volume of thesurfactant-rich phase formed is indicated in the gure in thepresence and absence of Ag-NPs). In addition, the concentrationof the surfactant needs to be smaller than the CMC at the nal
temperature in the procedure (25  C). The CMC was 0.24w/w%(7.4 mM) at 25  C (Fig. S5; green line in ESI†). In addition, theCMC value modestly increased with the formation of Ag-NPs as
shown in Fig. 4, whereas the presence of Ag+ (at any concentration) did not aﬀect the CMC values (Fig. 4, supported by
Fig. S6 in ESI†). The CMCs of a homologous series of zwitterionic surfactants in water are temperature dependent; namely,as the temperature is increased, an initial slight decrease in the
CMC value is observed until a minimum value is reached aer
which the CMCs start to increase with temperature.28–30 The
CMC of C10–APSO4 increased with temperature (i.e. 0.24 w/w%,
0.38 w/w%, and 0.44w/w% for 25  C, 35  C, and 42  C, respectively; Fig. S5 in ESI†).

RSC Advances

Illustration of the possible mechanism of fabrication ofdispersive ME enclosed Ag-NPs, prepared by following the synthetic
procedure (see Section 2.3.3).

Fig. 53.5. Mechanism
Fig. 5 illustrates a potential mechanism for fabrication of the
Ag-NPs in the C9– or C10–APSO4. Initially, the surfactants exist in
solution in monomeric form along with the Ag+ ion reactant(stage I). As citrate ion is added, silver ion is converted to Ag-NPs
to which the bipolar zwitterionic surfactant head group bind ata faster rate relative to the citrate ions (stage II). Several Ag-NPs
can then interact with each other via hydrophobic interactions;
i.e., the alkyl chain of one surfactant coated Ag-NP interacts withthat of another (or several) other such coated Ag-NPs (stage III).
Lastly, the exterior of such self-assembled zwitterionic surfactant microdomains or clusters (i.e., the alkyl chain of the coated
Ag-NPs) can interact with the alkyl chain of the monomericsurfactants such that the self-assembled microdomains thenhas the bipolar zwitterionic group in contact with the bulk water
solvent (stage IV). The formation of such zwitterionic surfactant

coated (monolayer or bilayer) individual metal NPs or citratecoated NPs has literature precedence.21–23,31 In this work, it
appears that the entire exterior of the C10–APSO4–Ag-NP
microdomains (clusters) are thus coated as well, mimicking
an oil-in-water microemulsion system that houses the dispersed
Ag-NPs.

4. Conclusions
In summary, the rapid and eﬃcient synthesis of dispersed andstable Ag-NPs in zwitterionic Cn–APSO4 surfactants that selfassemble in the presence of the Ag-NPs to form microdomains akin to microemulsions without the need of any
organic solvent or co-surfactant was developed. Essentially all
the Ag-NPs were present within the ME. The eﬀects of silver ion
concentration and pH upon the size and concentration of the
Ag-NPs were assessed. In addition, the concentration of thesurfactant, its zeta-potential, and the variation of CMC wasinvestigated. The mechanism for the formation of ME and theenclosing of Ag-NPs in the ME was presented. It is thought that
the Ag-NPs in the surfactant ME could potentially be employed
as a suitable surfactant-based dispersion solution for diﬀerent
applications.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
Fig. 4 The transition of critical micelle concentration (CMC) values in
the presence of Ag+ (blue line) and Ag nanoparticles (orange line).

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The authors gratefully acknowledge funding from the Japan
Atomic Energy Agency (JAEA) Nuclear Energy S&T and Human

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Resource Development Project through concentrating wisdom
Grant Number JPJA19H19210081 and Japan Society for the
Promotion of Science (JSPS) Grant-in-Aid for ScienticResearch(B) JP20H04352.

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