4584

Langmuir 2002, 18, 4584-4591

Sizing of Colloidal Nanoparticles by Electrospray andDifferential Mobility Analyzer Methods
I. Wuled Lenggoro, Bin Xia, and Kikuo Okuyama*
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University,
Higashi-Hiroshima 739-8527, Japan

Juan Fernandez de la Mora
Department of Mechanical Engineering, Yale University, New Haven, Connecticut 06520
Received November 7, 2001. In Final Form: March 31, 2002
The goal of this work was to develop a simple technique for sizing colloidal particles by means of electrospray
and aerosol techniques. Size distribution of different types of colloids (oxides, metals, and polymers) suchas silica, gold, palladium, and polystyrene latex particles, with different nominal sizes below 100 nm wasdetermined online. Nanometer-sized particles were dispersed into the gas phase as an aerosol via
electrosprays operating in the cone-jet mode of a colloidal solution followed by a charge reduction of thesprayed droplets to unity and subsequent evaporation of the solvent. The size distribution of the generated
aerosol particles was then determined by a differential mobility analyzer combined with a condensation
nucleus/particle counter. For comparison, particle sizes were determined by electron microscopy (EM)using the samples which were obtained by (i) naturally dried sols and (ii) on-line deposited on a substrateduring electrospraying. The proposed technique is capable of detecting the degree of dispersity of all colloid
samples, and the measured values were comparable to results obtained by EM and dynamic light scattering.
The results clearly show that the method described here constitutes a convenient, reliable, and rapid tool
for the size determination of colloidal nanoparticles.

1. Introduction
The extensive applications of nanoparticles ranging insize from about 1 nm to as large as 100 nm require highresolution sizing and classification techniques. It is wellknown that a rich variety of liquid-phase methods, via
chemical approaches such as microemulsion, sol-gel,reverse micelles, etc., are available for producing colloidal
nanoparticles. They are also frequently used in studies
related to materials science, chemistry, and biology,
including macromolecules, sols, nanocrystallites, dendrimers, polymer latexes, and proteins or living cells.
Complete knowledge of nanoparticle size and their dispersity is highly desirable, since many of the properties
associated with nanosized particles are strongly size
dependent.
To analyze the size of colloidal nanoparticles, microscopy
techniques such as transmission or scanning electron
microscopy (TEM/SEM), scanning near-field optical microscopy,1 scanning probe microscopy, and atomic force
microscopy have been used conventionally.2 In thesemethods, good sampling techniques and skilled operation
as well as time-consuming procedures are required.
Schmid and Chi3 reported that for a smaller order, theimaging or sizing of metallic Au clusters by high-resolution
transmission electron microscopy is usually accompanied
by a coalescence process between the cluster molecules,
leading to colloidal particles of a larger size in the timeorder of seconds.
It is important to characterize the size of isolated
colloidal nanoparticles without the particles being imbedded within an oxide layer or substrates. A number(1) Zenobi, R.; Deckert, V. Angew. Chem., Int. Ed. 2000, 39, 1746.
(2) Lacava, L. M.; Lacava, B. M.; Azevedo, R. B.; Lacava, Z. G. M.;
Buske, N.; Tronconi, A. L.; Morais, P. C. J. Magn. Magn. Mater. 2001,
225, 79.
(3) Schmid, G.; Chi, L. F. Adv. Mater. 1998, 10, 515.

instruments involve the use of various principles that havebeen developed to measure and classify colloidal particles,
such as small-angle X-ray scattering,4,5 dynamic lightscattering (DLS), which is also known as “quasi-elastic
light scattering” and “photon correlation spectroscopy(PCS)”,6 field-flow fractionation,7 and analytical ultracentrifugation.8 However, the current situation remains
particularly challenging and a great need continues toexist for the development of new types of reliable onlinesizing techniques with high resolution but which involve
sufficient user-friendly facilities.
One of the authors9 has proposed an on-line techniquefor measuring the size distribution of colloidal particles.
The procedure involves (i) transferring the colloidal
particles from the liquid into gas the phase (as an aerosol)using electrospray and (ii) particle sizing in the gas phaseusing an inertial impactor or a differential mobility
analyzer (DMA). The need to use two instruments in series
follows from the fact that the charge on the particles ishigh and unknown. Very large particle losses in the flowlines between the electrospray and the detector lead
therefore to considerable deformation of the size distribution. The capabilities of this technique have been assessedby analyzing colloids of known sizes with standard
polystyerene latex (PSL) spheres having nominal diam(4) de Moor, P. P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys.
Chem. B 1999, 103, 1639.
(5) Rieker, T.; Hanprasopwattana, A.; Datye, A.; Hubbard, P.
Langmuir 1999, 15, 638.
(6) Luckham, P. F.; Ukeje, M. A. J. Colloid Interface Sci. 1999, 220,347.
(7) Lee, S.; Rao, S. P.; Moon, M. H.; Giddings, J. C. Anal. Chem. 1996,
68, 1545.
(8) Borger, L.; Colfen, H.; Antonietti, M. Colloids Surf., A 2000, 163,29.
(9) de Juan, L.; de la Mora, J. F. In Nanotechnology, Molecularly
Designed Materials; Chow, G. M., Gonsalves, K., Eds.; ACS Symposium
Series 622; American Chemical Society: Washington, DC, 1996; p 20.

10.1021/la015667t CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/17/2002

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Langmuir, Vol. 18, No. 12, 2002 4585

Figure 1. Schematic of the electrospray of colloidal solutions and an online sizing using aerosol techniques.

and a condensation particles counter (also referred to asa condensation nucleus counter, CNC). In this procedurethe degree of contamination can be reduced compared toother sizing techniques. As test colloids, we used polystyrene latex, commercial silica, and gold and palladiumsols with different nominal diameters ranging below 100
nm in size. Prior to measurement of the particle size
distributions, the regime at which the liquid cone and thespray operates was distinguished and controlled. Conditions wherein one particle is captured per droplet wereconfirmed by controlling the initial sol concentration andthe size of sprayed droplet.

eters ranging from 74 to 21 nm. However the original
study did not provide comparative data relative to othermethods, e.g., microscopic analysis. In addition, a needstill exists for a measurement method that can be applied
to the analysis colloids with “unknown” sizes as well as
for smaller particles.
The electrospray technique is one of the few known
schemes which is capable of atomizing a liquid into
ultrafine droplets. In this method, a meniscus of aconducting solution supported at the end of a capillary
tube becomes conical when charged to a voltage of several
kilovolts with respect to a counter electrode. Droplets arethen generated by the continuous breakup of a steady jet
which is naturally formed at the apex of this cone (Taylorcone). The full structure is often referred to as a conejet.10 Electrospray can transfer colloidal nanoparticles fromthe liquid to the gas phase where the generated droplets
are sufficiently small (down to tens of nanometers)11 to
capture one particle in one droplet (Figure 1). In the caseof other atomization techniques with larger droplets, it is,in practice, difficult to obtain a condition suitable forsizing,12 which is extremely important in this technique.
For example, if the most commonly used atomization
techniques, i.e., ultrasonic nebulizers or twin-fluid types
that generate droplets in the range of several to tens of
micrometers, are used to generate the droplets as low as
0.0008% (in volume), the initial colloidal solution must be
satisfied in order to obtain particles 100 nm in diameterfrom typical initial droplets of 5 µm. In practice, these low
solution concentrations lead to difficulties in maintaininga high purity of the colloid. Moreover, a nonvolatile
impurity can influence the size of the dried particles aswell as the size resolution, in the case of a large droplet.
In the present study, electrospray was used for transferring the colloidal particles to the gas phase followed bysizing the on-the-flight nanoparticles by using a DMA

As shown in Figure 2, the system used for the online-sizing
measurement, which was similar to that reported in our previous
study,13 consisted of (i) an electrospray source, (ii) a neutralizer,and (iii) a size-analysis section, including a differential mobility
analyzer (DMA, TSI model 3071 or 3085) and a particle counter(a condensation particle/nucleus counter, TSI model CNC 3025)
operated under atmospheric pressure. The purpose of the DMA
is to extract particles that fall within a narrow range of electricalmobility from a steady stream of charged particles suspended ina gas. The narrow range of electrical mobilities in the aerosolthat is classified by the DMA directly translates to a narrow
range of particle size. This feature has been widely exploited inaerosol research involving particles and ions ranging from around
1 µm down to the subnanometer range.11,14
It is also known that the droplets generated in an electrospray
are always highly charged. Hence, the large electrical mobility
associated with them leads to considerable losses, as a result of
deposition onto the walls, which drastically decreases the overallthroughput efficiency of the aerosol/particle. In present study,a radioactive bipolar ion source (241Am) was used to neutralize
the electrosprayed drops. Charge reduction of the sprayed
droplets to unity is essential for the sizing method.15 In anelectrical-mobility detection technique (DMA), the aerosol passes
through a bipolar ion neutralizer, which generates a high level(10) Cloupeau, M.; Prunet-Foch, B. J. Aerosol Sci. 1994, 25, 1021.
(11) de la Mora, J. F.; de Juan, L.; Eichler, T.; Rosell, J. Trends Anal.
Chem. 1998, 17, 328.
(12) Kousaka, Y.; Okuyama, K.; Shimada, M.; Ohshima, K.; Hase,
T. J. Aerosol Res. Jpn. 1989, 4, 294.

(13) Lenggoro, I. W.; Okuyama, K.; de la Mora, J. F.; Tohge, N. J.
Aerosol Sci. 2000, 31, 121.
(14) Adachi, M.; Okuyama, K.; Kousaka, Y.; Moon, S. W.; Seinfeld,
J. H. Aerosol Sci. Technol. 1990, 12, 225.
(15) Kaufman, S. L. J. Aerosol Sci. 1998, 29, 537.

2. Experimental Section

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Figure 2. Experimental setup for measuring the size distribution of colloidal particles.
of positive and negative ions. The neutralizer brings the aerosol
charge level to a Fuch’s equilibrium charge distribution.14 Nearly
all droplets or particles acquire either a single positive, single
negative, or zero charge. The particles then enter the differential
mobility analyzer and are separated according to their electricalmobility. The mobility distribution obtained is converted to areal size distribution by considering the equilibrium charge
distribution.
Several silica sols with different nominal diameters below 100
nm (types ZL, XL, 50, C, XS) fabricated via an ion-exchange
process by Nissan Chemical Industries Ltd., Tokyo, were used.
Colloidal gold particles with two nominal diameters (referred to
as Au-A and Au-B) provided by Professor Takahagi’s group,
Hiroshima University, were prepared using a procedure reported
by Slot and Geuze.16 Additional gold (Au-T) and palladium (Pd)colloids prepared by a precipitation method (Japanese Patent,
05-155629) were provided by Toda Kogyo Corp., Hiroshima, withthe surfactants octadecyltrimethylammonium chloride (C18H37N(CH3)3Cl) and poly(vinyl alcohol). A polystyrene latex (PSL)
particle with a certified mean diameter of 21 nm and an
uncertainty of 1.5 nm (Nanosphere size standards, Duke Scientific
Corp., California) was also used in the current study. The certified
diameter of this particle size standard was obtained by photon
correlation spectroscopy (PCS/DLS). However the manufacturer
has noted that the 20-40 nm particle standards are notrecommended for aerosol generation, since such suspensions
contain a large amount of surfactant which generates a residue
when the particles are dried or dispersed as an aerosol.
In this study, colloidal particles were dispersed in a mixtureof water/methanol (50:50 vol %) and methanol (water/methanol
ratio 0.01:99.9vol %) in a few cases. Particularly for the colloids
with a small size (below 40 nm), to increase the electricalconductivity of the solution in the order of 0.01-0.1 S/m,ammonium acetate at a concentration of between 0.2 and 20 mM
was added. The mixture ratio 50:50 of water/methanol17 and the
addition of ammonium acetate18 have been widely used inelectrospray studies. The electrical conductivity of a solution
was measured by means of a conductivity meter (Toko Chemical
Inc., TX-90) at 25 °C.
It is well-known that high conductivites and a low flow rateof a spray solution allow the generation of small droplets withdiameter of a few tens or hundreds of nanometer. Scaling laws
can be used to estimate the size of the droplets generated in the(16) Slot, J. W.; Geuze, H. J. Eur. J. Cell Biol. 1985, 38, 87.
(17) Nohmi, T.; Fenn, J. B. J. Am. Chem. Soc. 1992, 114, 3241.
(18) Kaufman, S. L.; Skogen, J. W.; Dorman, F. D.; Zarrin, F.; Lewis
K. C. Anal. Chem. 1996, 68, 1895.

cone-jet mode. Rossel-Llompart and Fernández de la Mora19 foundthat droplets are formed mainly in a single size, whose initial
diameter, Dd, scales with the parameters described in eq 1.
Equations can be given for the case of a polar liquid such as
methanol and water with electrical permittivity  > 6 andelectrical conductivity K typically larger than 10-5 S/m20

Dd ) G(Qo/K)1/3(1)

where Q is the flow rate through the jet (from the Taylor cone)and o is the electrical permittivity of a vacuum. Available dataon G were compiled in the literature.13,21 G mainly is dependenton  but is also influenced by the viscosity variable.19,22
The proper initial concentration, at which one electrosprayed
droplet captures one colloidal particle, can be predicted byestimating the droplet size using eq 1, e.g., by controlling theliquid flow rate Q and controlling the electrical conductivities Kof the spraying liquid. To use a colloidal solution which is
sufficiently dilute to lead no more than one particle per droplet,the colloidal particle concentration of the starting solution Cs(particles/solution in volume fraction) must be

Cs < (Dp3)/(Dd3)(2)

Cs < Dp3/[G3(Qo/K)]

(3)

or

where Dp is the diameter of colloidal particle. If the densities of
the colloidal particles and that of the solvent are known values,
the initial concentration of the solution in terms of weight fraction
can be calculated from that in the volume fraction.
For example, in the case of silica type C with K ) 0.02 S/m(when 20 mM ammonium acetate was added) and Q ) 0.018
mL/h, the scaling law predicted the generation of initial droplet
diameters in the order of 378 nm. As depicted later in Figure 3e,silica C has an average diameter of 18.5 nm (measured by
microscopy). It can be predicted that one would need to prepare(19) Rossel-Llompart, J.; de la Mora, J. F. J. Aerosol Sci. 1994, 25,
1093.
(20) de Juan, L.; de la Mora, J. F. J. Colloid Interface Sci. 1997, 186,
280.
(21) Chen, D. R.; Pui, D. Y. H. Aerosol Sci. Technol. 1997, 27, 367.
(22) Ku, B. K.; Kim, S. S.; Kim, Y. D.; Lee, S. Y. J. Aerosol Sci. 2001,
32, 1459.

Sizing of Colloidal Particles
a C solution of concentration below 0.012 or 0.022 wt % in orderto capture one particle in one droplet.
The liquid was sprayed inside a six-way cross stainless steel
chamber (Figure 2). Two of the opposing sides were used forsupplying the spraying liquid and sampling the generated
aerosols. A positive dc high voltage source was connected to thestainless steel capillary having inner/outer diameters of 0.1/0.3
mm. The capillary tip was tapered conically down to nearly zerothickness. The liquid was supplied to the capillary through ahigh-resolution syringe pump (model PHD 2000, Harvard
Apparatus, Holliston). A stainless steel tube with an innerdiameter of 5 mm was used to sample the aerosol. Alternatively,
to obtain particles sampled after electrospraying for microscopy
measurement, a 150-mesh brass grid or a brass plate was placedat the tip of another 5 mm tube (Sampling-2/Figure 2). Several
holes were made along the side of the tube for the aerosol to flow
through. This sampling tube was grounded through an electrometer (model 6512, Keithley, Cleveland) in order to measure
the electric current delivered by the droplets that were depositedon it. This allowed the monitoring of the stability of theelectrospray. The spray capillary electrode and sampling tube
were set horizontally to facilitate the connection of the spraychamber to a DMA-CNC system. CO2 was used as a carrier gas
because of its relatively high electrical breakdown thresholdcompared with that of air or N2. The influence of ambient
temperature on the aerosols (nanoparticles) was also investigated.
Following this heated zone, the resulting dry silica or gold
particles were sized with a DMA-CNC system. The sheath air
in the DMA is set to 20 L/min and the flow ratio of aerosol (CO2)to sheath air is maintained at 0.1. The size and morphology ofthe naturally dried sols (before spraying, Sampling-1/Figure 2)
and particles sampled during electrospraying were observed using
a Hitachi S-5000 field emission scanning electron microscope(FE-SEM) and a field emission transmission electron microscope(FE-TEM, HF-2100, Hitachi Ltd., Tokyo) to verify and support
the data obtained from the electrospray-DMA (ES-DMA) system.

3. Results and Discussion
Determination of the Stability Domain of Electrospray. Different spray modes may lead to quite
different drop size distributions for a given solution. For
each sample, we measured typical curves of the electric
current versus the applied voltage of the spraying solutions
with different concentrations using the apparatus shownin Figure 2. The applied voltage was first increased
gradually and then reduced after the liquid meniscus at
the capillary tip passed some spray modes. When theapplied voltage increased from a small initial absolutevalue, the liquid on the capillary tip was changed from ameniscus to a dripping mode and then a pulsating mode.
Increasing the voltage further increases the pulsating
frequency, until, at a critical voltage, the liquid meniscus
forms a stable Taylor cone and the current jumps
drastically.13 For a given solution concentration Cs, a stablecone-jet may be formed only within a certain domain offlow rates Q and applied voltage V, Qmin(V) < Q < Qmax(V),Qmin and Qmax are the corresponding minimum andmaximum flow rates.10,13,23 This stable domain is affectedby Cs and electrical conductivity K of the spraying
solutions. It is clear that before measuring the particle
size distributions, it is necessary to determine and control
the regime at which the liquid cone and the spray operate.
Since it has been previously established that lower flow
rates of spray liquids generate smaller and more narrowly
dispersed electrospray droplets,19 an attempt was madein this study to maintain Q near the minimum value Qmin.
At a certain applied voltage, V(Q), we found that the cone
was most easily controlled near Qmin, where the fluctuationof the current is also smallest.
(23) de la Mora, J. F.; Loscertales, I. G. J. Fluid Mech. 1994, 260,
155.

Langmuir, Vol. 18, No. 12, 2002 4587
Table 1. Average Particle Diameter (Dp, nm) andGeometrical Standard Deviation (GSD) of Naturally
Dried Colloids Measured by Electron Microscopy (EM)and Those Obtained by the Electrospray-DMA (ES-DMA)
and a Commercial Dynamic Light Scattering (DLS)
Techniquea
sampleDp (GSD) EM

Dp (GSD) ES-DMA

Dp DLS

PSL
ZL
XL
50
C
XS
Au-A
Au-B

21.0e
109.2 (1.16)
57.6 (1.16)
29.1 (1.10)
18.5 (1.25)
f
7.1 (1.14)
14.9 (1.10)

21.0e
136.6
74.0
57.0
N/A
N/A
N/A
N/A

Au-T

10.9 (1.45)

Pd

4.3 (1.51)

24.0 (1.36)
111.1 (1.15)
71.6 (1.14)
32.6 (1.09)
19.5 (1.30)
13.0 (1.17)
11.5 (1.46)
19.0 (1.19)
17.9 (1.24)b
14.6 (1.32)c
10.3 (1.16)c
14.2 (1.26)d
9.5 (1.17)c
8.2 (1.16)b,c

N/A
N/A

a All ES-DMA experiments were run using a mixture of watermethanol as the solvent at a temperature of 25 °C except for thefollowing: b for the run at 100 °C; c for the use of methanol (99.9
vol %); d for the use of water (the original/raw solution). PSL particle
was measured using DLS method (by the manufacturer Duke
Scientific Corp.). e The DLS measurement of all samples (except
ZL, XL, and 50) have no good accurate mean diameter, i.e., no good
repeatability (N/A). f Below 10 nm.

Photographs and Size Distribution of Naturally
Dried Colloids. Size distributions of raw colloidal nanoparticles were obtained by naturally drying the samples
prior to SEM/TEM analysis. Figure 3 shows photographsof the naturally dried colloidal particles of silica (Figure3a-e), gold (Figure 3f-h), and palladium (Figure 3i). Morethan 600 particles were counted to obtain a size distribution. From these results, the average diameters (number
based) of the raw colloidal particles were found to be in
the range from 109.2 nm for silica ZL to 4.3 nm for
palladium (Pd) as shown in Table 1. Unfortunately, the
XS silica sol was not easily analyzed since its naturally
dried particles have a sintering-like morphology, as shownin Figure 3e. From their TEM photographs, it is clearthat XS is constructed from particles having a size below
10 nm. It is also clear that gold samples Au-A and Au-B
have a relatively high monodispersity; i.e., geometricalstandard deviations are in the order of 1.1.
TEM observation of the palladium samples (Figure 3i)
demonstrates that the Pd particles are insulated fromeach other by some spaces (white area), which suggeststhat the particles are encapsulated by a large amount of
the surfactants (octadecyltrimethylammonium chloride
and poly(vinyl alcohol)). As discussed later, the surfactants
generate a residue surrounding the particles during
solvent evaporation when the particles are dried (Figure3i) or dispersed as an aerosol (Figure 7e,f).
Particle Size Distributions of Electrosprayed
Colloids. The particle size distributions of the colloidal
particles were measured using the ES-DMA system. In
each case, the ambient was maintained at a temperatureof 25 °C and the liquid flow rate Q was near or at theminimum value Qmin. The influence of initial concentrationof the spraying solution on final particle size distribution
was investigated as shown in Figure 4. Using an XL silica
colloid, four solutions with different concentrations (Cs),
0.33, 0.165, 0.083, and 0.033 wt %, were electrosprayedwithout added ammonium acetate at room temperature.
From the size distributions, it is clear that Cs ) 0.33 and0.165 wt % solutions generated dried agglomerated
particles with size peaks around 150 and 130 nm,

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Figure 3. Electron microscope images of naturally dried colloids of silica (a, ZL; b, XL; c, 50; d, C; e, XS), gold (f, Au-A; g, Au-B;
h, Au-T), and palladium (i).

respectively. This difference indicates that the initialsolution of 0.33 or 0.165 wt % is too high to achieve thecondition of one particle per one droplet. Without adding
ammonium acetate, decreasing Cs changed the electricalconductivity of the solution substantially from the orderof 10-3 to 10-4 S/m. Meanwhile, another parameter which
can also control drop size, i.e., the liquid flow rate, wasmaintained at a constant value of 0.1 mL/h. Therefore, it
is clear that the lower the concentration, the smaller the(agglomerated) particle size. To confirm the generation of
an isolated nanoparticle in the gas phase, the XL sol
concentration was decreased to 0.083 and 0.033 wt %.
The electrosprays of these solutions yielded dried silica
particles having sizes of around 72 nm. The size did not
become smaller at a lower concentration (0.033 wt %) due
to the existence of colloidal particles inside the droplets.
By lowering the concentration of the spraying liquid, it ispossible to define the condition for achieving one particle
per droplet. Electron microscope photographs of theelectrosprayed four XL colloids confirm the formation of
agglomerated particles and isolated nanoparticles as afunction of their initial solution concentrations. In fact,
for some colloids prepared in this study, up to five different
solution concentrations Cs (10-5, 10-4, 10-3, 10-2, and 10-1
wt %) were electrosprayed to confirm the generation ofisolated single nanoparticles in the gas phase.
Figure 5 shows the size distributions of particles
generated from positive charged electrosprays at room
temperature for polystyrene latex (Figure 5a) and various
silica samples (Figure 5b-f). By use of the ES-DMA
system, the PSL particle (with a certified mean diameterof 21 nm obtained by the PCS method) was analyzed andthe measured average diameter was 24 nm with ageometrical standard deviation of 1.36. It is known that
this PSL contains a large amount of surfactant, which
will generate a residue when the particles are dispersed
as an aerosol. Therefore the size of the PSL aerosol
measured by ES-DMA is somewhat larger than that fora hydrosol measured by the PCS method. A discussion on
the influence of the surfactant to the measured particle
size is also included in the next section.
As also shown in the right row of Table 1, the average
diameter and the geometrical standard deviation (GSD),
as measured by the ES-DMA system, were in the range
from 111 nm for silica ZL to 13 nm for XS. It should be
noted that it was difficult to determine the size distribution
of XS by electron microscopy. All of the cases (except Figure5f) show that the measured samples have unimodal
particle size distributions. Figure 5f shows the spectrum
obtained when the silica C and XS were mixed at thesame solution concentration. Both peaks are interpreted
as C and XS, respectively. It was very difficult to obtainsuch a bimodal size distribution result as shown in Figure
5f, probably due to complex interactions involving repulsive forces and van der Waals attractive forces on thesurface between two or more colloids having different sizes
and properties. None of the runs required any additional
dispersion technique or dispersant to decrease the agglomeration of the particles such as ultrasonic andmechanical homogenizer. Although a discrepancy between
sizing with ES-DMA and the electron microscope analysis

Sizing of Colloidal Particles

Langmuir, Vol. 18, No. 12, 2002 4589

Figure 4. Measured particle size distribution of a silica XL
colloid at different initial concentrations of the spraying solution.
The numbers indicate the initial concentration of XL in wt %.
Spraying flow rate Q: 0.1 mL/h.

(a few percent up to 20%) still exists, we conclude that
their measured particle sizes are comparable.
The present results showed that isolated single particles
could be generated during the electrospraying of a colloidal
solution. Figure 6 indicates that the electron microscope
analysis also confirmed the generation of isolated single
nanoparticles. In this figure, two of the silica samples anda gold sample (C and Au-B) showed single colloidal
particles dispersed on the substrate (TEM grid). Althougha sufficient number of electrosprayed particles is notavailable for a size distribution analysis, the sizes ofdispersed particles are in good agreement with thoseobtained by the ES-DMA system and are shown in Figure
3.
Only a few studies on the discrepancy between size
distributions of the same sample obtained with different
measurement techniques for colloids having size below
100 nm have been reported.5 Lee et al.7 reported that withlatex particles having diameters ranging from 50 nm to
1 µm, SEM yielded an average diameter about 20% smaller
than those obtained from the PCS and the flow-field
fractionation method. For larger particles having a size
above 100 nm, Luckham and Ukeje6 reported a discrepancy
between the average size of the same sample, as obtainedwith PCS and with TEM. The measured sizes of theirnarrow distributed polystyrene latex particles were 400
nm for PCS and 380 nm for TEM. Similar to most casesin our study, a shift to smaller diameter from a sizing
found by electron microscopy (EM) is likely due to theshrinkage of sample colloidal particles during drying in
the high vacuum ambient used in the EM measurement
system.7
In the case of the silica XS colloid, the naturally driedparticles have a sintering-like morphology and were linked
with each other. We attempted to analyze small silica(XS, C) and gold (Au-A, Au-B, Au-T) colloids by asking
instrument manufacturers to measure their particle size
distribution with various techniques based on the transmission of light such as the DLS. However, all samples

Figure 5. Measured particle size distributions of PSL (a) andvarious silica colloids, (b) ZL, (c) 50, (d) C, (e) XS, and (f) mixtureof C and XS, at ambient temperature of 25 °C. The flow rates
were set near the minimum value for Qmin at each run.

have a strong tendency to aggregate, and those techniques
resulted in a measured diameter that is quite large (a few
hundred nanometers for some samples). Thus, thesecommon techniques are not easily able to detect the size
distribution of colloids used in this study, especially thosethat have a sintering-like morphology. Although somediscrepancies still exist between the results measured by
the ES-DMA system and those by electron microscope
analysis, the present study could easily determine thedispersity of an XS colloid with a measured average
diameter of 12.96 nm and GSD of 1.17.
Furthermore, on size measurement using the DLS
technique, we have compared five apparatus from different
manufacturers. After reviewing the particle sizes obtained
from these apparatus, we mainly used the fiber-optics
particle analyzer (FPAR-1000) of Photal, Otsuka Electronics Co. Ltd. (Japan). The average particle size (number
based) measured by DLS is also shown in Table 1 and allof them show larger values compared with those obtainedby EM or ES-DMA. In the DLS, measurements had to berepeated at several runs (e.g., changing the scattering
angle) to obtain an accurate mean diameter. In fact, mostof our samples have no good accurate mean diameter, i.e.,
no good repeatability.
Influence of Solvent and Operation Temperatureon the Measured Particle Size Distribution. As shownin Table 1, a large discrepancy (21-38%) exists in themeasurement of Au-A and Au-B gold colloids, when awater-methanol mixture (50:50) was used as solvent.

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Figure 6. Electron microscope photographs of isolated nanoparticles of silica-C (a) and gold Au-B (b) dispersed on the substrateduring their electrospraying at the same condition as those depicted in Figure 5d (silica-C) and Figure 7c (Au-B).

Figure 7. Measured particle size distributions of three samplesof gold and a palladium colloid at different conditions. The flow
rates were set near the minimum value for Qmin in each run.
Experimental conditions (colloid, solvent, temperature) are as
follows: (a) Au-A, 50:50 vol % (water-methanol), 25 °C; (b)Au-B, methanol (99.9 vol %), 25 °C; (c) Au-B, 50:50 vol %, 25°C; (d) Au-B, 50:50 vol %, 100 °C; (e) Au-T, methanol, 25 °C;(f) Pd, methanol, 25 °C.

Table 1 and Figure 7a,c show that the average diameters,
as measured by EM and ES-DMA, are 7.1 and 11.5 nm
for Au-A and 14.9 and 19.0 nm for Au-B, respectively.
These differences might be the result of the existence ofan outer layer surrounding the core particles, since metal
colloids require a protecting ligand shell or layer to bestabilized in solution.3 Au-A and Au-B colloids are likely
composed of a gold core24 surrounded by a negative ionicdouble layer. The stability of these gold colloids decreases
in the sequence of water, ethanol, chloroform, and benzene
solutions.25 It follows, therefore, that the thickness of theouter layer decreases in the above sequence. Using TEM,
Huang et al.25 reported that gold nanoparticles (nanodots)
were insulated from each other by uniform spaces, whichsuggest that gold colloidal particles were encapsulated bya layer. We also observed the similar tendency in the caseof a TEM image of the palladium (Pd) sample. Among theraw samples with pure water as the solvent, we could
form a cone-jet (stable) electrospray in the case of a raw
gold colloid Au-T (Toda Kogyo Corp.) without using alcoholor adding ammonium acetate. This is due to the highelectrical conductivity of this gold colloid (0.022 S/m). The
result, also shown in Table 1, indicates that the technique
we introduce (ES-DMA) is not limited to certain solvents
such as water/methanol mixtures.
By use of the ES-DMA system, the influences of ambient
temperature on the drying process (solvent evaporation)
as well as the solvent type on the final particle size were
investigated. The temperature was varied between 25 and
100 °C. On the other hand, a mainly methanol-based
solvent (methanol-water mixture ratio in a volume of
99.9:0.1) replaced the methanol-water mixture (50:50).
Note that initially the raw gold colloids (Au-A and Au-B)
were dispersed in pure water. Au-B, which has an average
diameter measured by EM of 14.9 nm with the GSD of
1.10, was initially investigated. When the ambient temperature of a path in the ES-DMA system was elevatedfrom 25 to 100 °C, the diameter of Au-B changed from
19.0 nm (GSD, 1.19) to 17.9 nm (GSD, 1.24). This decreasein size was further detected when methanol (99.9 vol %)was substituted for a water-methanol mixture at temperature of 25 °C. The size of the Au-B particles obtainedby ES-DMA became 14.6 nm (GSD, 1.32). The discrepancy
in the size measured by the ES-DMA and those by the EM
is only 2%.
The Pd shows a large discrepancy even though methanol
was used as solvent and the operating temperature waselevated to 100 °C. As mentioned previously, this is dueto the large amount of surfactants used which form aresidue during drying as shown in its TEM images as awhite area (Figure 3i). On the other hand, the average
diameter of the gold colloid provided by Toda Kogyo Corp.
(24) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. Langmuir 2000, 16,9912.
(25) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi,
T. Jpn. J. Appl. Phys. Lett. 1999, 38, L437.

Sizing of Colloidal Particles(Au-T), as determined by the ES-DMA, is in good agreement with that obtained using EM (10.3 and 10.9 nm).
This is probably due to the use of methanol as the solventin which no “detectable” shell or layer is formed on thesurface of the gold particles during the measurement of
EM or ES-DMA.
From these results, we conclude that the thickness ofa “layer” surrounding a gold particle (Au-A, -B) can be
reduced by changing the solvent from water-methanol
to methanol or by heating the aerosol. In the case of a“wet” gold particle, a surface layer about 2 nm in thickness
was observed. The bulk structure (chemical compositionand substructure) and surface morphology of colloidal
particles may also play important roles, although theeffects of these parameters are usually neglected in most
discussions. The present study shows that the outer layer
which is present on gold particles can be partiallyeliminated by changing the solvent from water or watermethanol to methanol as well as by heating the aerosolpath.
4. Conclusions
The potential of the electrospray technique in the sizingof nanosized colloidal particles in the range below 100 nm
in diameter has been demonstrated by an on-line size
measurement using a differential mobility analyzer
system and has been confirmed by the electron microscope(TEM and FE-SEM) images of (i) naturally dried colloidsand (ii) samples collected during electropraying. Electro-

Langmuir, Vol. 18, No. 12, 2002 4591

spray is able to finely atomize solutions of colloids tonanometer order-sized droplets and allows each colloidal
particle to be captured in a different sprayed droplet, i.e,
one droplet in one particle. This study also showed thatthe outer layer, which is formed on the metal particles,
could be partially removed by changing the solvent orelevating the ambient temperature. Moreover, the approach described here offers the potential of the rapid
online sizing of nanoparticles in solutions.
Acknowledgment. The authors thank Kyoko Iimori
for her assistance in these experiments; The Takahagi
group at the Graduate School of Advanced Sciences ofMatter, Hiroshima University and Dr. T. Iwaki (Toda
Kogyo Corp.), for providing the gold and palladium colloids;
Nissan Chemical Industries Ltd. for silica samples; andthe Research Center for Nanodevices and Systems,
Hiroshima University, for the TEM instruments. This
work was funded by Grant-in-Aid for Exploratory Research(O.K., I.W.L.), Grant-in-Aid for Encouragement of Young
Scientists (I.W.L.), and a Postdoctoral Fellowship (B.X.)
sponsored by the Japan Society for the Promotion of
Science and the Ministry of Education, Culture, Sports,
Science and Technology of Japan and also supported by
the New Energy and Industrial Technology Development
Organization’s “Nanotechnology Particle Project” basedon funds provided by the Ministry of Economy, Trade andIndustry, Japan.
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