Current and Emerging Technologies for the Characterization of
Nanomaterials
O. A. Sadik,*,†
N. Du,‡
V. Kariuki,†
V. Okello,†
and V. Bushlyar†
†
Center for Advanced Sensors & Environmental Systems (CASE), Department of Chemistry, State University of New York −
Binghamton, P.O. Box 6000, Binghamton, New York 13902-6000, United States
‡
Laboratories and Scientific Services Directorate, Springfield Laboratory, U.S. Customs and Border Protection, U.S. Department of
Homeland Security, 7501 Boston Blvd., Suite 113, Springfield, Virginia 22153, United States
ABSTRACT: This paper provides a survey of conventional
and emerging techniques that are available for characterizing
engineered nanoparticles in complex matrices. Techniques that
were considered include microscopy (TEM, SEM, HRTEM,
DLS, SNOM), chromatography (HDC, FFF), mass spectroscopy
(ICP-MS, SEC-ICP/MS, MALDI, FFF-ICP-MS), spICP-MS,
and electrochemical techniques. A case study is
presented from the authors’ laboratories for the design of a
portable nanoparticle analyzer based on tangential flow
filtration and electrochemical detection (EC-TFF). EC-TFF is equipped with poly(amic) acid membrane filter electrodes
(PMFE) arrays that perform multiple functions to capture, isolate, and detect (CID) engineered nanoparticles. The application
of EC-TFF is presented for the characterization of engineered nanosilver in real-world samples. A size-dependent isolation of
AgNPs was achieved at varying particle sizes with over 98.5% removal efficiency. PEC-TFF AA showed an excellent performance
not only for isolation at subnanometer-sized ranges but also as a platform for detection of engineered nanoparticles at low ppb
levels.
KEYWORDS: Nanotools, Engineered nanoparticles, Electrochemistry, Tangential flow filtration
■ INTRODUCTION
Engineered nanoparticles (ENPs) are ultrafine particles with
one dimension between 1−100 nm1
. Due to their small size,
ENPs have very large surface areas relative to their weight, so
they react quite differently from a bulk solid or dissolved
material of the same composition. Consequently, their
mechanical, magnetic, catalytic, electronic, optical, and biological
features can easily be manipulated. ENPs are becoming
valuable materials for applications in a broad range of fields,
including food additives, cosmetics, biological samples, and
pharmaceuticals, as well as in biocidal packaging, fuel cell
technology, and electronics. However, the increase in the use of
ENPs in consumer products has raised the concern over their
environmental fate, potential toxicity, and overall risks to both
environmental and human health.2−4
For this reason, there is
an urgent need to develop appropriate analytical tools and
methods that are particularly useful for the evaluation of ENPs
in complex matrices, consumer products, and personal
electronics. The desirable tools are expected to be applicable
or tunable to the specific properties of ENPs such as size
distribution, particle number concentration, and mass concen-
tration.5,6
These methods must allow not only the determination
of the NPs mass concentration but also the evaluation of
their size, size distribution, and/or toxicity.
Extensive reviews of nanocharacterization tools have been
published.7−11
Numerous analytical techniques have been
reported for the characterization of ENPs. In particular, the
most commonly used techniques for obtaining the size
distribution of ENPs and structural information include
transmission electron microscopy (TEM), scanning electron
microscopy (SEM), dynamic light scattering (DLS), atomic
force microscopy to study morphology and topography, and
NP tracking analysis and separation techniques. Others include
capillary electrophoresis, hydrodynamic chromatography
(HDC), ion mobility spectrometry, and field flow fractionation
(FFF).12
The investigation of the shape of ENPs (a relevant
parameter in many applications) is readily achieved using TEM,
SEM, DLS, atomic force microscopy, and FFF.13
In addition,
the chemical identification of major elements is possible using
X-ray-based techniques, X-ray photoelectron spectroscopy
(XPS), and energy dispersive X-ray spectroscopy. Thus, a
rough elemental distribution of NPs is usually achieved by
integrating these X-ray-based techniques with TEM/SEM or
DLS information.14
Despite the existence of these characterization tools, there is
no quantitative analytical tool for the measurement of ENPs in
complex matrices. As discussed in this review, mass
spectrometry has been coupled with chromatography, and the
Special Issue: Sustainable Nanotechnology 2013
Received: March 10, 2014
Revised: May 12, 2014
Published: May 20, 2014
Research Article
pubs.acs.org/journal/ascecg
© 2014 American Chemical Society 1707 dx.doi.org/10.1021/sc500175v | ACS Sustainable Chem. Eng. 2014, 2, 1707−1716
resulting hyphenated techniques have been reported. These
hyphenated techniques have enabled the prior removal of major
interferences, and their performance characteristics are hereby
evaluated on a case-by-case basis.7
■ MICROSCOPY TECHNIQUES
Microscopy is one of the most powerful techniques that can be
used to determine the size, shape, and morphology of
nanomaterials. Examples include electron and proton microscopy,
atomic force microscopy (AFM), scanning probe
microscopy, and scanning tunneling microscopy (STM), and
each has unique advantages.7
These techniques, when used,
give little or no chemical information but provide excellent
spatial resolution down to the atomic scale. However, some
exceptions exist; for example, derivatized AFM tips can be used
for chemical recognition.15
In TEM, when electrons are accelerated up to high energy
levels and focused on a sample, changes to the electrons either
through back scattering elastically or inelastically yields X-rays,
auger electrons, or light. This interaction between sample and
electron beams then gives rise to images. With a spatial
resolution of 10 nm−1
, modern TEM is a very powerful
technique for the characterization of nanoparticles. AFM,
backed by its ability to provide real topographical images of
surfaces, is used to measure surface properties such as frictional
force on nanoscale, hardness of surfaces, surface charge
distribution, surface magnetization, and yield stress.10
STM is
used for viewing the atomic surfaces. It uses a tunneling current
to probe the density of the states of a material. It can achieve up
to ∼0.1 nm lateral resolution and 0.01 nm depth resolution.10
Dynamic light scattering (DLS) and scanning electron
microscopy (SEM) or transmission electron microscopy
(TEM) are commonly used in in vitro cytotoxicity of silver
nanoparticles (AgNPs). These generally enable nanoparticle
size characterization once they have been dispersed in a buffer
solution16
or in a culture medium.17
A DLS measurement of
the hydrodynamic size of the particles in these complex media
can be used to monitor the formation of nanoparticle
agglomeration. Hassellöv et al.18
and Calzolai et al.19
have
previously reviewed the limitations of DLS. Challenges have
been largely attributed to the interpretation of data especially
for polydispersed systems given the strong particle size
dependence of the scattering intensity. SEM and TEM analyses
allow us to study the changes in the shape of the nanoparticles
along with the size distribution, although an accurate
quantitative measurement requires counting and measuring a
high number of particles,18
which is time-consuming.
Cellular (in vitro) uptake of nanoparticles has also been
examined in a number of studies16,17,20,21
using two main
techniques: Confocal laser scanning microscopy (CLS)16,21
and
TEM.17,20
CLS microscopy measures the reflectance of the
signal from silver nanoparticles or the fluorescence signal from
the labeled nanoparticles using laser as an excitation source.16
HRTEM can be the best technique to identify crystalline
structures as reported by Lan et al.22
for CdS nanowires or for
palladium nanorods as reported by Huang et al.23
and graphene
edges by Jia et al.24
This technique does have some challenges.
It is subject to artifacts caused by sample preparation. It also
requires high vacuum and thin sample sections or particles of
limited diameter to allow the electron beam to penetrate
through the sample.25
SEM is one of the most versatile instruments available for the
examination and analysis of the morphology and determination
of chemical composition. SEM characterization usually occurs
in vacuum, and sample preparation is achieved in solid state.26
Besides yielding three-dimensional images, SEM has the ability
to image large areas of samples and bulk materials such as thin
films.10
Toxicity of ZnO and Ag nanoparticles on bacterial cells
has been examined using SEM by Sinha et al. This study
validated the fact that the toxicity of the NPs depends primarily
on the nature of the membrane and the interaction of the
nanoparticles with the membrane. AFM, which has been used
for imaging of samples in their native environment and to
measure surface area of nanoparticles, is critical in nanotoxicity
studies.27
Scanning near-field optical microscopy (SNOM), an
“optical relative” of AFM and STM, enables the study of a
sample’s optical properties with resolution far beyond the
diffraction limit. SNOM combines excellent spectroscopic and
temporal selectivity of classical optical microscopy with a lateral
resolution reaching well into the sub-100 nm regime.28
Sample
fluorescence, light emission, transmission, scattering, etc. can be
mapped with the spatial resolution down to tens of nanometers.
Two main approaches to the near-field microscopy exist: (i)
aperture type SNOM and (ii) apertureless techniques.29
SNOM operates by illuminating a monochromatic plane wave
from a laser into an optical fiber with a sharp tip such that only
a very small aperture (diameter ≈ 50 nm) passes laser light. If
brought close to the sample (within a few nm or into the
optical near-field), this nanometer light source can be used to
illuminate a spot on a sample that is considerably smaller than
the optical diffraction limit. SNOM has a major limitation in its
optical throughput when using very small apertures. This limits
both the spatial resolution and the optical brightness of SNOM
nano light sources. This limitation can be overcome by using
aperture less near-field methods, such as tip-enhanced Raman
scattering (TERS). A comprehensive review on SNOM and its
applications has been written by Hecht et al.28
In summary, electron microscopy methods have been shown
to provide powerful information,30
but artifacts due to sample
preparation, the high particle count required, and the lack of
Figure 1. Survey of conventional and emerging tools for charactering
engineered nanoparticles. SP-ICP-MS = single particle inductively
coupled mass spectrometer. FFF-ICP-MS = fluid flow fractionation
inductively coupled mass spectrometer. EC-TFF = electrochemical
tangential fluid flow. DOX-EC = dissolved oxygen sensor coupled with
electrochemical technique. DLS = dynamic light scattering.
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suitability for high-throughput applications limit the application
of these techniques in complex matrices.
Mass Spectrometry. There is a growing use of the mass
spectrometry technique for the characterization of ENPs. This
is because this technique offers invaluable elemental and
molecular information on the composition, structure, and
chemical state of NPs. In addition, the compatibility of MS with
any type of sample can provide novel insights into the nature of
NPs, while providing real time information due to its extremely
high sensitivity and ease of coupling with other separation
techniques.31
ICP-MS is an example of a coupled or hyphenated technique
that is gaining importance as a tool for characterization of
ENPs. In the ICP source, most elements are efficiently
vaporized, atomized, and ionized (even from NPs) to provide
a rapid and robust elemental chemical analysis. The outstanding
features of ICP-MS have led to its broad application in
nanoparticle characterization. These include robustness, high
sensitivity (subparts-per-trillion level), wide dynamic range (up
to nine of orders of magnitude), high selectivity, virtual matrix
independence, etc. Also, ICP-MS requires simple calibration
standards, and the use of isotope dilution techniques has
proven very useful for an accurate elemental quantification.32
Some illustrative applications for quantification and elemental
analysis reported in the literature include the direct
determination of metal oxide NPs such as Al2O3, TiO2,
CeO2, and ZnO or metallic Ag and AuNPs in environmental
samples.33
Determination of metallic impurities in NPs has also
been reported using ICP-MS. For example, a simultaneous
determination of multielemental trace impurities in high-purity
CeO2, ZnO, SiO2, and TiO2 NPs after a microwave-assisted
acid digestion has been conducted using high-resolution ICP-
MS.34
Satoshi et al. used ICP-MS after microwave sample digestion
in their investigation of the possibility of using AgNPs as a
preservative in cosmetics.35
They concluded that AgNP is safe
to use as a preservative in cosmetic products because it does
not penetrate into the skin except when there is a wound on the
skin. Recently, when coupled with other techniques, ICP-MS
has been used to probe real environmental samples (Figure 2).
Hydrodynamic chromatography-inductively coupled mass
spectrometry (HDC-ICP-MS) has been used for detecting
and characterizing ENPs in aqueous matrices. Using this
technique, Karen et al.36
have investigated the nature and fate
of AgNPs in an activated sludge. The particle-size distribution
of the silver ENPs in the sludge was determined using TEM
and HDC-ICP-MS. Size separation was achieved using a
hydrodynamic chromatography column.
ENPs may consist of a core or a core/shell that is coated by
ligands of different natures to confer more stability to the NP as
a whole or to provide it with a specific functionality as shown in
Figure 3. Such protective ligands may play a critical role in the
surface chemistry that governs any further functionalization
processes used for subsequent bionanotechnological applica-
tions.37
Molecular techniques such as electron spray ionization
(ESI) and matrix-assisted laser desorption/ionization (MALDI)
can provide such information. Molecular MS allows the core
size to be estimated, the ligand layer to be characterized, and
the molecular formula to be obtained. The analysis of thiolateprotected
Au MPCs using MALDI has been well docu-
mented.40
It is worth noting that the use of NPs to enhance MS
tools has been proposed. NP-based substrates such as QDs,
SiO2, and TiO2 NPs can be used in laser desorption/ionization
MS techniques. Because NPs are excellent laser energy
absorbing materials, they seem to be better than the
Figure 2. ICP-MS hyphenated techniques. FFF = field flow fractionation. HDC = chromatography. ICP-MS = inductively coupled plasma mass
spectrometry. SEC = size exclusion chromatography. AES = auger electron spectroscopy.
Figure 3. (a) Core−shell Fe3O4 and Au nanoparticle with an outmost
organic shell encapsulation (R) CO2H or −NH2.38
(b) Schematic
representation of a functionalized core−shell quantum dot bound to a
biomolecule.39
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conventional organic matrices. This is significant as it facilitates
analyte absorption and desorption processes, thus resulting in
an improvement on spectra acquisition especially in the low
mass range.40
In hyphenated techniques, the use of ICP-MS as a detector
confers significant advantages. Specifically, it provides elemental
stoichiometry and qualitative elemental information to control
the recoveries and efficiencies in the synthesis of NPs. Size
exclusion chromatography inductively coupled mass spectrometry
(SEC-ICP-MS) is one such technique that has been used
to obtain size distribution of AuNPs. HDC, an emerging
alternative to SEC, has a short analysis time and requires
minimal sample treatment. HDC-ICP-MS has been used for
the characterization of Au and AgNPs.36
Another hyphenated technique is FFF-ICP-MS, which is
considered highly versatile for the separation, detection, and
quantification of NPs in real environmental matrices. FFF
separates nanoparticles according to their hydrodynamic
diameter in the absence of a stationary phase. FFF is the
only technique that can fractionate NPs within the range of 1 to
100 nm. The technique can be coupled to different online
detectors and can be operated under different pH and ionic
strength conditions. Recently FFF-ICP-MS has been used for
size characterization and mass determination of Ag and Au NP
in aqueous media as illustrated by Bednar et al.41
Mitrano et al.
has compared two MS-based hyphenated approaches for the
characterization of size distribution.42
Presently, the only technique currently capable of determining
particle size distribution in addition to number and mass
concentration for ENP-containing samples at or below 10 μg/L
is SP-ICP-MS.43
This interesting has been developed by
Degueldre et al.,43
and further method refinement has
continued.42,44−46
If samples containing NPs are introduced
at a low flow rate and the number of particles in the solution is
sufficiently low, analysis using ICP-MS in time-resolved mode
makes it possible to collect the intensity for a single particle as
it is vaporized and atomized in the plasma. Then, each
measured data point can be correlated to the size and mass
fraction of a unique NP. This method of NP characterization is
called single particle ICP-MS (SP-ICP-MS) analysis. The key
feature of this technique lies in the capacity of the ICP-MS to
distinguish the data collected for each individual NP. Also, the
different behaviors of the dissolved metal and metal NPs such
as Ag and Au under single particle detection conditions have
been used to differentiate directly between both forms of silver/
gold in aqueous samples.44
For this reason, care must be taken
in the sample dilution (particle number in solution) and in the
selection of the integration time.41,42,44,47
Although the
integration times used in the literature vary (typically around
5 ms), a maximum integration time of 10 ms is
recommended42,44,45,47
to avoid the double-counting highlighted
above.
Jani et al. have investigated the particle size and
concentration of silver nanoparticles within the range of
about 20−80 using sp-ICP-MS.43
Also, Laborda et al. have
demonstrated that SP-ICP-MS can be used to differentiate
between dissolved silver and silver nanoparticles in a sample.
They achieved number concentration limits of detection of 1 ×
104
L−144
. Gray et al. extracted Ag and Au nanoparticles from
biological matrices and used sp-ICP-MS to determine size
distributions and particle number concentrations.41
The main drawback of the SP-ICP-MS technique is the size
limit of ENP identification, currently approximately 20 nm for
Au and Ag ENPs, which depends on the sensitivity of the
instrument and the ionic background for the metal of
interest.44,46,48
Another challenge for SP-ICP-MS analysis is
the application of the method to real samples and the use of
this method to measure nanoparticles containing elements such
as titanium, iron, or silicon due to the potential for spectral
interferences.49
Moreover, SP-ICP-MS is not suitable for
determining the size of NPs (unless their elemental
composition is previously known) because it is not able to
distinguish between different particles with the same elemental
composition. To overcome this limitation, a previous HDC
separation was recently coupled online with SP-ICP-MS and
applied to the simultaneous determination of the size, number
concentration, and metal content of Au NPs.50
In summary, MS-based techniques have been shown to have
a greater analytical application for the characterization of NPs
as they may overcome drawbacks of better established
techniques such as TEM/SEM, DLS, XPS, and UV−vis
spectroscopy. In the future, MS-based techniques will be
widely used for the characterization of water-soluble NPs and
NP bioconjugate architectures.51
Electrochemical Characterization Using Nanostructured
Poly(amic) Acid Membranes: A Case Study. The
author’s group has discovered that a new class of amphiphilic
conducting poly(amic) acid (PAA) could be used to fabricate
nanoporous membranes with potential applications in the
filtration and detection of ENPs.52−57
PAA is a conducting
electroactive polymer, and its properties can be tuned by
manipulating the delocalized π electron system for chemical
and electrocatalytic applications. Its synthesis and molecular
structure are given in Scheme 1. The carbonyl and amide
functionalities in PAA act as anchors that have been previously
employed in the fabrication of flexible nanostructured PAA−
silica (PSG) films.53
We have reported the preparation of different classes of PAA
membranes by keeping the processing temperature at or below
75 °C in order to avoid the imidization process in Scheme
1.52,53,55−57
The low temperature prevented the cyclization of
the PAA to polyimide, while retaining the intrinsic structural
integrity and stability of the PAA. Depending on the
composition, solvent used, and processing conditions, we
have also discovered that PAA can be prepared as transparent
stand-alone or translucent membranes, electrodeposited films
on solid electrodes, and as permeable membranes for
nanofiltration (Scheme 1).
The focus of this work is to utilize the specific transport and
electronic conductivity of PAA membranes for filtration and
detection of ENPs. When prepared using a phase-inverted
process, PAA membranes have similar morphology as the
typical phase-inverted membranes, which are thin, smooth,
dense, and supported by a porous sublayer.52,53
The phase-inverted PAA derivatives were also found to have
a number of unusual but practically favorable properties, in
particular, a near-zero shrinkage or volumetric expansion upon
curing (polymerization) as well as low water absorption. In
addition, the polymers had high glass-transition temperatures as
a result of their high cross-link densities. Explanations for these
properties have been hypothesized in terms of favorable
hydrogen-bonding effects, a sensitive ring current associated
with aromatic moieties, and the ability of the PAA functional
units to self-assemble into reversible polymeric networks. The
smooth hydrophobic surfaces of the membranes lead to
frictionless flow of water across the surface, thus enabling the
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transport that is an order of magnitude higher than the
transport in conventional pores. Figure 4 shows the SEM data
obtained for PAA membranes using different concentrations of
PAA solution. This synthetic approach affords well-controlled
membranes, leading to various desired sizes (from less than 5
nm to greater than 100 nm).
Previous work using the PAA membranes for filtration of
quantum dots, TiO2, Fe2O3, and gold nanoparticles showed
greater than 98% filtration efficiency. The present work
demonstrates that the membranes could be retrofitted into a
conventional tangential flow filtration mechanism for nanofiltration
and electrochemical detection of silver nanoparticles.
Electrochemical Tangential Flow Filtration (EC-TFF).
The design of a portable EC-TFF is shown in Figure 5. The
portable analyzer contains nanostructured poly(amic) acid filter
membrane electrode (PMFE) arrays incorporated within
cassette filter, TFF analyzer, potentiostat, and reader.
In EC-TFF, the sample solution is propelled by pressure
initiated by a pump. As the feed enters the channel equipped
with the PMFE membrane, molecules (e.g., ions) that are small
enough to fit into the pores or macro voids of the membrane
pass on the film as filtrate (permeate). All particles (e.g.,
bacterial, viruses, natural organic matter (NOM)) too large to
fit through the pores are recycled back to the sample reservoir
(retentate), and the cycle repeats. The application of electrical
potential to the PMFE provides the added advantage of being
able to detect the permeate downfield. The abundant carbonyl
and amine group on the membrane act as “molecular anchors”
to bind ENPs and isolate the particles. The porosity of the
PMFE determines the molecular weight cutoff for both the
permeate and the retentate. We ensured good retention by
designing the PMFE that is 3−5 times size of the target
molecule. Different classes of PAA membranes have been
designed. One class was designed to retain particles in the range
from 0.2−2 μm in width or diameter and up to 1−10 μm in
length. These should be adequate for the nonspherical species
including bacteria, viruses, and natural organic matter (NOM)
varying from 200−60,000 amu/Da. Another class of PMFE
membranes was designed for ENPs such as TiO2, AgNP, and
AuNP. The design of the PMFE array is shown in Figure 6.
Electrode arrays were fabricated directly onto the PAA
membranes (with the reference and counter electrodes
fabricated onto the PAA membrane cassette).
Due to the widespread use of gold working electrodes, it was
sputtered with gold film onto the lower side of the PAA
membrane. The film thickness was tailored (by controlling the
sputtering conditions) to ensure a favorable electrochemical
response without blocking the pores of the PAA. Eight working
electrodes were fabricated on one side of the paper substrate,
Scheme 1. Synthesis and Molecular Structure of Poly(Amic)
Acid and Inhibition of Imidization Steps at Low
Temperature for Electrodeposition, Stand-Alone Membrane
Formation, and Nanoporous Membranes for Filtration and
Sensinga
a
PMDA = pyromellitic dianhydride, oxy-dianiline.
Figure 4. SEM of PAA membranes of varying pore sizes (left, cross section), middle (10−100), right (5−15 nm).
Figure 5. Design of the portable electrochemical tangential flow
filtration system.
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Figure 6. (a) Design of on-membrane electrode/filter combination. (b) Design of combined cassette/PAA membrane filter electrode for fluidic
control.
Figure 7. Sample PAA on-membrane electrodes: (a) Gold working electrodes on paper substrates, (b) gold counter and silver/silver chloride
electrodes, (c) working electrodes coated with PAA membranes, and (d) carbon working electrodes. Right: Gold array electrodes fabricated onto
paper substrates, with subsequent coating of PAA membranes (notice the shiny PAA).
Figure 8. Design of integrated PMFE and prototype cassette for EC-TFF: (a) Cassette design and (b) production version of the cassette.
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while the gold counter electrode and Ag/AgCl reference
electrode were fabricated on the other side of the substrate.
The surface of the working electrodes was covered by the PAA
membrane to control the size of the filtered samples. Both sides
of the filter paper outside the electrodes area (circled area in the
center of filter paper) were insulated with polyimide. Figure 6b
shows the design of the cassette to hold the PMFE. One piece
(left) has the outlets for permeates and probes for counter and
reference electrodes. The PMFE was placed into this piece,
facing the other piece of the cassette as indicated by dashed
rectangle lines. Then the sealing ring was placed on top of the
PMFE as shown with the dashed circle ring. The other piece of
the cassette (middle) has the outlets for feed and retentate and
eight probes for working electrodes. The polymer mesh was
placed in this piece to provide flow path. The right side of
Figure 6b shows the flow direction for both main flow and
cross-flow.
Figure 7 shows the actual PMFE electrode fabricated onto
paper substrate. The electrochemical character of the cassette
and PAA membrane were preserved simultaneously by
controlling the electrochemical properties of the material
surrounding the cartridge both on the diameter and on the
ends. In addition, the cassette design provided sufficient
structural support to accommodate routine handling. Figure 8
shows the design of the PAA electrode/membrane holder with
electrodes. Eight working electrodes were fabricated on one
side of the filter, while the gold counter electrode and Ag/AgCl
reference electrode were placed on the other side of the filter.
The surface of the working electrodes was modified with PAA
membranes to control the size of the filtered samples. Both
sides of the circled area in the center of filter paper were
insulated with polyimide.
Figure 8 shows the design of the integrated PMFE and TFF
process. One piece (left) has the outlets for permeates and
probes for counter and reference electrodes. The PMFE was
placed into this piece by facing the other piece of the cassette as
indicated by dashed rectangle line. Then O-ring seal was placed
on the membrane as indicated by the dashed circle. The other
piece of the cassette (middle) has the outlets for feed and
retentate. Eight probes were designed for working electrodes.
The polymer mesh was placed in this piece to provide flow
path. The right side of Figure 8 shows the direction of flow for
both the main flow and cross-flow. A production version of this
device was injection-molded and shown in Figure 8b. The
electrochemical properties of the PMFE fitted onto the
cartridge were tested using electrochemical techniques.
Electrochemical Characterization. In order to confirm that
the gold electrodes fabricated onto filter paper are electrochemically
active, we carried out cyclic voltammetry of
ferricyanide. Cyclic voltammetry is often used to characterize
Figure 9. EC-TFF data for simultaneous electrochemical detection of nanosilver. (a) 10 mM K3FeCN6 in 0.1 M phosphate buffer solution as the
supporting electrolyte,various scan rate on one of gold electrodes. (b) Silver NPs were detected on five gold electrodes in a parallel testing in 10 mM
phosphate buffer with 0.25 M NaCl adjusted to pH 7.0 at a scan rate of 10 mV/s. (c) Silver NPs were detected on one of the gold electrodes at
various scan rate in same buffer solution as b. (d) DPV data recorded for nanosilver in food supplements samples after filtration. DPV spectrum of
standard 40 nm silver NPs at various concentrations. Experimental condition for DPV: 20 mV/s scan rate, 17 ms sample width, 50 ms pulse width,
200 ms pulse period, and 1 mA/V sensitivity. Experimental condition for CV: 50 mV/s scan rate and 100 μA/V sensitivity.
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conducting polymer films for studying the reversibility of
electron transfer because the oxidation and reduction can be
monitored in the form of a current−potential diagram. The
electrochemical characterization of K3[Fe(CN)6] of the bare
gold array electrode surfaces was achieved by cycling the
potential repeatedly between −600 and 600 mV at a varying
scan rate between 10 and 100 mV/s. PMFE was then
characterized in 0.1 M KCl at different scan rates (0.01−0.1
V/s). The oxidation and reduction peaks increased with the
scan rates, which indicates a diffusion controlled reaction. In
both tests using bare electrodes (carbon or gold) and a PAAcoated
electrode, the redox peaks of K3[Fe(CN)6] were
observed (Figure 9). However, the bare carbon electrode
gave a higher redox potential at 220 mV, while the PAAcovered
electrode gave a redox potential at 190 mV.
When characterized in a supporting electrolyte, the PMFE
showed two major peaks. The first redox couple (Epa1 and
Epc1) referred to as E0
1, and the second redox couple (Epa2
and Epc2) referred to as E0
2. The PAA film was considered to
have good diffusion-controlled electron mobility as evidenced
by the increase in current response at both couples (E0
1 and
E0
2) as the scan rate was increased. Randles Sevcik plots of
current (A) vs scan rate (mV/s) confirmed a direct
proportionality with correlation coefficients (R2
) of ±0.99.
The peak where peak potential remains relatively constant for
PAA indicates that the polymer film is attached onto the gold
surface. The diffusion coefficient (De), which is a measure of
how fast charge can be transported through the polymer layer,
was calculated using Randles Sevcik equation
= ×i n D v(2.68 10 )( ) (A) (C)( )p
5 3/2
e
1/2 1/2
(1)
The De was found to be 7.10 × 10−6
cm2
/s for E0
1 and 9.11
× 10−6
cm2
/s for E0
2, in good agreement with the De of PAA
previously reported.51
After several hours, the top contacting
pad area was still dry, which meant the PI insulation layer
worked successfully.
EC-TFF for Characterization of Nanosilver. The EC-TFF
designed was tested for the characterization of engineered
nanosilver in food samples. Silver nanoparticles food supplement
samples were purchased from various sources including
mesosilver (>20 ppm) from Purest Colloids, Inc. (U.S.A.),
colloidal silver (35−45 ppm) from Golden Touch Mfg./Ultra
Pure (U.S.A.), and sovereign silver (10 ppm) from Natural
Immunogenics Corp. (U.S.A.). The 40 nm AgNPs standard
aqueous solution (Ted Pella, Inc.) was diluted with nanopure
water in various concentrations. These solutions were used as
AgNPs standards for electrochemical measurements. The
efficiency of the PAA membrane for silver NPs was calibrated
using the mesosilver sample. Mesosilver was mixed with
deionized water into a series of diluted samples with different
percentage concentrations (from 5% to 80%) compared to its
original concentration (100%). The filtrate from the PAA
membrane was measured, and concentration was compared
with its original concentration.
The filtration or capture efficiency is defined as the
percentage of NPs captured on the filter. Because the volume
of sample is not changed before and after filtration,
concentration was used instead of direct NPs number. Equation
2 shows the filtration efficiency (or capture efficiency) based on
the change of NPs number and their concentration. Where η is
filtration efficiency, N is number of NPs, and C is
concentration.
η = ×
= ×
=
−
×
N
N
C
C
C C
C
(captured)
(total)
100%
(captured)
(total)
100%
(total (filtrate)
(total)
100%
(2)
Following the capture of the nanoparticles, simultaneous
detection was achieved using differential pulse voltammetry.
Nanosilver metal particles were thoroughly oxidized to metal
ions using the applied electrical potential. Results show that
there was no electrochemical signal recorded either for the
blank electrode or the neat PAA membranes. However, upon
coating the membranes with the nanoparticles, the redox peak
was observed at ∼100 mV and was linearly dependent on the
concentration of the silver nanoparticles (Figure 9d).
The differential pulse voltammetry (DPV) technique was
also used to monitor the electrochemical oxidation of silver
NPs following their isolation from food supplements samples.
DPV has higher sensitivity than CV and could provide a better
resolution as well as quantitative information related to trace
amounts of silver NPs concentrations. Results showed that food
supplement samples produced oxidation peaks at approximately
the same potential (Figure 9d), while the control surfaces
consisting of the blank gold and blank PAA membrane did not
produce any peak in the range from −200 mV to 400 mV. In
addition, the DPV of these food supplements not only
produced the expected sharp oxidation peaks for the silver
NPs but also showed some tailing after each peak. This can be
attributed to transition of different oxidation states of silver
oxides. Because the heights of these DPV peaks represented the
amount of silver NPs in each samples, the concentrations of
these food supplements samples were calculated from the
calibration plots.
■ CONCLUSIONS
We have provided a short review of conventional and emerging
techniques that are currently being used to characterize
engineered nanoparticles. The challenges of microscopy
(TEM, SEM, HRTEM, DLS, SNOM), chromatography
(HDC, FFF), mass spectroscopy (ICP-MS, SEC-ICP/MS,
MALDI, FFF-ICP-MS), sp-ICP-MS, and electrochemical
techniques have been discussed. We have provided a case
study for the design and fabrication of a new tool for ENP
characterization based on EC-TFF. The EC-TFF method using
PAA membranes was successfully used to capture and detect
silver NPs quantitatively. The PAA membranes provided a
simple approach to concentrate and isolate the NPs sample by
varying filtration volume with minimal sample preparation, and
the electrochemical detection was fast, requiring only few
minutes. A conventional optical method can provide
quantitative measurement only for nonprotected silver NPs,
while an electrochemical method without PAA membranes
cannot eliminate the effect of silver ions.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: osadik@binghamton.edu. Fax: 607:777-4478.
Notes
The authors declare no competing financial interest.
ACS Sustainable Chemistry & Engineering Research Article
dx.doi.org/10.1021/sc500175v | ACS Sustainable Chem. Eng. 2014, 2, 1707−17161714
■ ACKNOWLEDGMENTS
The authors acknowledge the National Science Foundation,
CBET 1230189, for funding.
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