Mussel-inspired plasmonic nanohybrids for light harvesting

Article abstract of DOI:10.1002/adma.201305766

Core-shell plasmonic nanohybrids are synthesized through a simple solutionbased process utilizing mussel-inspired polydopamine (PDA). The multi-purpose PDA not only facilitates plasmonic metal formation, but also serves as a scaffold to incorporate photosensitizers around the metal cores, as well as an adhesive between the nanohybrids and the substrate. The resulting plasmonic assembly exhibits highly enhanced light absorption in photo catalytic systems to augment artificial photosynthesis.

Full text of DOI:10.1002/adma.201305766

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Mussel-Inspired Plasmonic Nanohybrids for Light
Harvesting
Minah Lee, Jong Uk Kim, Joon Seok Lee, Byung Il Lee, Jonghwa Shin,*
and Chan Beum Park*
Solar energy conversion begins with light absorption by photo-
sensitizers, whereby the electric excitation is transferred to a
specific electron acceptor. In green plants, for example, pro-
tein complexes tightly bound with highly concentrated chro-
mospheres in an elaborate architecture ensure strong light
absorption and effective energy transport to reaction centers
during photosynthesis.[1] Inspired by natural photosynthesis,
amplifying light absorption has been regarded as one of key
challenges in achieving maximal efficiency in artificial photo-
synthetic systems through solar energy harvesting.[2] Coupling
such photocatalytic systems with metallic nanostructures that
exhibit strong localized surface plasmon resonance (LSPR) is
considered to be a promising strategy.[3,4] The intense, local
electric fields induced by collective electron oscillation in
metals upon resonant excitation can enhance photoexcitation
of adjacent photoactive species, such as molecular chromo-
phores or semiconductors.[5,6] Recent studies have exploited
LSPR to efficiently harvest and utilize solar energy for applica-
tions in photovoltaics to generate electricity,[7–9] and in chemical
processes to obtain energetically useful or chemically valuable
products.[10–17]
The primary issue in achieving plasmon-enhanced light
harvesting lies in the precise architecture of metal nanostruc-
tures and photosensitizers, because different kinds of energy
transfer interactions can predominate depending on their
geometries.[18] Over the last few decades, there have been
significant advances in fabrication routes for metallic nano-
structures, which include focused ion beam milling, electron-
beam lithography, and wet chemical synthesis.[19–21] However,
obstacles such as low throughput and high cost due to serial
writing under high vacuum in case of top-down approaches
and the need for extra schemes in the arrangement and adhe-
sion of colloidal nanoparticles (NPs) on substrates limit their
applicability to artificial light harvesting systems. Further-
more, synthetic strategies for precise incorporation of pho-
tosensitizers to such fine metal nanostructures, which often
occurs in a random manner,[22,23] have yet to be established
for the development of complex plasmon-enhanced light har-
vesting assemblies.[24] Therefore, inexpensive, scalable, and
accurate synthetic strategies for assembling plasmonic metal
nanostructures, as well as strategies for hybridizing them
with targeted photocatalytic systems, are in high demand to
realize practical applications of plasmonics in solar-to-energy
conversion.
Here we present a simple and versatile approach for the con-
struction of plasmonic metal/photosensitizer core-shell nano-
hybrids for efficient light harvesting by adopting multi-purpose
polydopamine (PDA) nanolayers inspired by mussel adhesion.
As a mimicry of mussel adhesive proteins, PDA coating can be
applied to a wide variety of material surfaces[25] and allows facile
synthesis of various functional nanostructures.[26–28] In our
plasmonic core-shell assembly (Figure 1a), PDA coating plays
multiple roles: (1) a reducing agent for the synthesis of metal
NPs, (2) a scaffold for the encapsulation of photosensitizing dye
molecules, and (3) an adhesive layer between the nanohybrid
and the substrate. In contrast to nanolithography processes,
the entire synthetic procedure can be handled in an aqueous
solution under mild conditions and requires no intricate equip-
ment, which confers advantages in large-scale production. Also,
by virtue of the remarkable adhesive versatility of PDA coating,
this approach can be applied to the development of elaborate
core-shell nanostructures regardless of material type and mor-
phology of substrates. We found that the resulting plasmonic
nanohybrids exhibit strongly enhanced photocatalytic activity
during visible light-induced artificial photosynthesis as a result
of amplified light absorption by molecular photosensitizers
through LSPR from the plasmonic metal NPs. We expect that a
diverse range of metal core (e.g., gold and silver) and dye mol-
ecule combinations are possible through the use of our strategy
to facilitate the synthesis of assorted sets of nanohybrids with
desired optical properties, allowing design flexibility in solar
energy conversion applications.
The synthesis of plasmonic nanohybrids with metal cores
surrounded by photosensitizer-encapsulating shells is accom-
plished through a facile, three-step incubation process, as
depicted in Figure S1. The first step is to form the PDA layer
on a substrate through oxidative polymerization of dopamine
using a simple immersion process. Plasmonic metal NPs are
then deposited on the PDA-coated substrate in a metal pre-
cursor solution by utilizing the reducing capability of catecholic
moieties in PDA. Finally, the secondary PDA coating is applied
to encapsulate photosensitizing molecules around the plas-
monic metal cores. While both gold and silver NPs were tested
as plasmonic noble metals in this work, Au NPs were mainly
utilized for photocatalytic reactions because of their higher
thermal, chemical, and photochemical stabilities compared to
Ag NPs. While different types of photosensitizing molecules
M. Lee, J. U. Kim, Dr. J. S. Lee, B. I. Lee, Prof. J. Shin,
Prof. C. B. Park
Department of Materials Science and Engineering
KAIST Institute for NanoCentury
KAIST, Daejeon 305–701, Republic of Korea
E-mail: qubit@kaist.ac.kr; parkcb@kaist.ac.kr
DOI: 10.1002/adma.201305766
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Figure 1. a) Schematic illustration of mussel-inspired plasmonic nanohybrid for light harvesting. PDA coating enables formation of core-shell nanostruc-
tures integrated with metal NPs and photosensitizers, irrespective of the material type and morphology of substrates. b) Photographs of nanohybrid
films formed on flexible PET substrates. Eosin Y was coupled with Au NPs as a model system of plasmonic nanohybrid for artificial photosynthesis.
can be incorporated in the PDA ad-layer (see Figure S2), we
used eosin Y (EY) as a model photosensitizer for plasmon-
enhanced solar energy conversion. EY, one of the xanthene
dyes, has been identified as an excellent light harvester for
biocatalyzed artificial photosynthesis.[29,30] The photographs in
Figure 1b show red translucent films of nanohybrids formed
on a flexible polyester (PET) substrate, where PDA-EY indicates
a film consisting of a primary PDA layer and a secondary PDA
layer that encapsulates EY, and PDA-Au-EY corresponds to a
sample having the primary PDA layer, plasmonic Au NP cores,
and the secondary PDA layer with EY.
substantial decrease in the characteristic peaks of Au 4f after
the secondary PDA-coating in the PDA-Au-EY sample indicates
that most Au NPs are enclosed by the PDA shell. The XRD
pattern of PDA-Au matched well with the database of metallic
Au (JCPDS No. 65–8601), which further confirms the forma-
tion of crystalline Au NPs on the surface of the PDA-coated
substrate (Figure S5). The broad peaks in the range of 20–30°
in the PDA-only and PDA-Au films are due to the amorphous
nature of PDA. In addition, our approach enabled the synthesis
of plasmonic nanohybrids by employing Ag NPs as metal cores
(Figure S6).
We performed mussel-inspired nanohybrid synthesis on
three different substrates (slide glasses, flexible PET films, and
silica beads [900 nm in diameter]) to demonstrate the mate-
rial- and morphology-independency of our approach. The SEM
images in Figure 2a (left) show that plasmonic Au NPs were
successfully formed on each PDA-coated substrate through
incubation in a gold chloride solution. The Au NPs exhibited
a size distribution ranging from 30 to 100 nm with the average
size being ∼70 nm (Figures S3, S4). After the secondary PDA
coating accompanying EY encapsulation, the EY shell layer sur-
rounding Au NPs was observed. The clear contrast between
bright inner cores—corresponding to Au NPs—and pale PDA
sheaths of approximately 20 nm in thickness was easily observ-
able, indicating the existence of well-defined core/shell nano-
structures. This hybrid nanostructure of metal core/dye shell
ensures that a significant portion of dye molecules should be
located in the plasmonic enhancement region.
We analyzed PDA-only, PDA-Au, PDA-EY, and PDA-Au-EY
films assembled on glass substrates using XPS spectroscopy to
examine the composition of plasmonic nanohybrids (Figure 2b).
All of the films exhibited characteristic peaks at C 1s, N 1s,
and O 1s that originated from PDA. The PDA-Au film clearly
shows additional peaks located at the binding energy of 84.2
and 87.8 eV corresponding to Au 4f, which suggests successful
formation of Au NPs. In the PDA-EY and PDA-Au-EY spectra,
the characteristic peaks of Br 3p at 190 eV and Br 3d at 69.5 eV
confirm the existence of EY in the samples. Note that the
We investigated the LSPR of Au NPs and their interaction
with EY using UV-Vis absorption spectroscopy by comparing
the absorbance spectra of different hybrid samples (Figure 3a).
The characteristic absorption peak of EY at 538 nm is easily
identifiable in the PDA-EY film when compared to the PDA-
only film. With regards to PDA-Au-EY nanohybrids, two neigh-
boring peaks appear in the spectrum, which originate from the
presence of EY and Au NPs. Compared to the PDA-EY film, the
Au NP-incorporating sample (i.e., PDA-Au-EY) exhibits signifi-
cantly enhanced absorption intensity at 538 nm. The additional,
broad peak at near 600 nm is attributed to the LSPR peaks of
Au NPs that have been inhomogeneously broadened due to the
diameter and shape variations of Au NPs. Also, the peak posi-
tion is red-shifted compared to that of bare Au NPs formed on
a PDA layer (i.e., PDA-Au), which is an expected outcome of the
increased effective refractive index of the surrounding medium.
We further investigated the optical properties of plasmonic
nanohybrids using numerical electromagnetic simulation to
confirm electromagnetic field enhancement by Au NPs and to
examine their effect on dye absorption. Finite difference time
domain (FDTD) calculation governed by Maxwell’s equation
has been performed (see the Experimental Section for details),
which is a highly accurate tool in the study of electromag-
netic properties of complex nanostructures.[31] As shown in
Figure S7, the resulting spectra of calculated absorbance for dif-
ferent hybrid films (e.g., PDA-only, PDA-EY, PDA-Au, and PDA-
Au-EY films) were consistent with the experimental results in
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Figure 3a; the LSPR peak of Au NPs overlapped with the EY
absorption peak. In the numerical investigation of PDA-Au-
EY film, the absorption due to each component of layer could
be separately calculated using local electric field distributions.
According to the results shown in Figure 3b, the most intense
absorption occurred in the top EY layer and the absorption
intensity of EY in the PDA-Au-EY film increased markedly in
comparison with the absorption of EY in the PDA-EY film not
having Au NPs, although non-negligible absorption of Au NPs
did exist. Figure 3c illustrates electric field distribution at the
absorption band of EY, which demonstrates the strong field
enhancement induced by the Au NPs due to the LSPR. In the
PDA-Au-EY film, EY molecules in the PDA shell covering the
Au core are located in the plasmonic field enhancement region;
thus, the enhanced light absorption by EY is attributed to the
field distribution. The field enhancement by the LSPR of Au
NPs was also supported by an analysis using surface-enhanced
resonance Raman spectroscopy (SERS). Figure 3d shows the
SERS spectra of PDA, PDA-EY, and PDA-Au-EY films, which
were acquired using an excitation of 514 nm. Significant
enhancement in the EY’s Raman peaks at 1621, 1506, 1341,
1281, and 1179 cm−1 was observed in the sample with Au NPs
(i.e., PDA-Au-EY film) in comparison with the PDA-EY film,
which originate from typical vibrational modes in the xanthene
and benzene rings of the EY molecule.[32] The amplified SERS
signal clearly evidences the enhanced electromagnetic field by
Au NPs in the PDA-Au-EY film.
We employed the plasmonic nanohybrid assemblies for
biocatalyzed artificial photosynthesis, which facilitates vis-
ible light-induced recycling of nicotinamide cofactor (NADH)
and redox enzymatic chemical synthesis to mimic natural
photosynthesis.[33–35] In green plants, photosynthesis occurs
through photo-regeneration of nicotinamide cofactor and their
consumption in the Calvin cycle, converting solar energy into
chemical energy.[36–38] As illustrated in Figure 4a, during the
artificial photosynthetic reaction using PDA-Au-EY, photo-
excited electrons in EY are transferred to an electron mediator
(M = [Cp*Rh(bpy)H2O]2+, Cp* = C5Me5, bpy = 2,2′-bipyridine)
under visible light illumination (λ > 420 nm), which facilitates
the regeneration of NADH in an enzymatically-active form so
that NADH-dependent oxidoreductase (e.g., L-glutamate dehy-
drogenase, GDH) can consume NADH to produce L-gluta-
mate from α-ketoglutarate. As demonstrated earlier, EY in the
core-shell conformation of PDA-Au-EY film should experience
the enhanced local electromagnetic field near the Au cores
upon visible-light illumination, whereby light absorption and
photoelectron generation in EY can be boosted. Furthermore,
the secondary PDA-coating encapsulating EY can electroni-
cally and chemically insulate the metal core from direct con-
tact with the photocatalysts and redox mediators[39–41] to pre-
vent charge recombination and metal corrosion.[7,9] Figure 4b
shows photoregeneration yields of NADH from NAD+ under
visible light with different nanohybrid films (PDA-Au, PDA-
EY, and PDA-Au-EY). A detailed experimental set-up is
described in the Experimental Section. Significant enhance-
ment in the photocatalytic activity of EY due to the incorpo-
ration of plasmonic Au NPs was evident; PDA-Au-EY film
resulted in a NADH photoregeneration yield of 14.7% while
PDA-EY film regenerated only 2.1% of NAD+ to NADH. Note
Figure 2. a) SEM images of plasmonic nanohybrids formed on slide
glasses, PET films, and silica beads (900 nm), indicating general appli-
cability of our strategy. Au NPs formed on PDA coated substrates (left)
are confined by EY-incorporated shells after secondary PDA coating
(right). b) XPS survey spectra (top), Br 3d XPS spectra (bottom, left),
and Au 4f XPS spectra (bottom, right) of nanohybrids with different
layers.
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Note that the result of plasmonic enhance-
ment during NADH regeneration reflected
comprehensive interactions of all the dyes
in the 20 nm-thick PDA layers, where the
distances between Au NPs and dyes were
varied from 0 to 20 nm in addition to the
thickness of the spacing layers. The photo-
synthetic yields of the nanohybrids with the
spacing layers were always lower than that
of the PDA-Au-EY film without the spacing
layer, which gradually decreased as the
distance (i.e., the spacing layer thickness)
between Au and EY increased. The distance
dependence confirms that the enhancement
in the photosynthetic reaction is strongly
dependent on the LSPR produced by Au
NPs, because LSPR induces higher concen-
tration of light in closer proximity to metal
NPs.[42,43] We further validated the distance
dependence of the field enhancement by Au
NPs using FDTD calculation (Figure S10).
Additional predictable interactions between
dye molecules and plasmonic metal NPs
in the nanohybrid structure, such as non-
radiative energy transfer from dye to metal
that occurs at short distances,[18] or charge
Figure 3. a) Absorbance of nanohybrids with different layers. b) Simulated total absorption
of PDA-Au-EY and the individual absorption spectra of each material from FDTD methods. recombination due to direct contact of metal
and dye molecules,[9] appear to be insignifi-
cant compared to the absorption enhance-
c) Distributions of the electric field intensity around Au NPs (70 nm) with the EY-incorporated
PDA shell (20 nm). d) Comparison of the SERS spectra for PDA, PDA-EY, and PDA-Au-EY films.
ment since PDA-Au-EY without the spacing
that no regeneration of NADH was observed with PDA-Au
film under the same reaction conditions. We calculated the
plasmon-enhancement factor to be five using the NADH
photoregeneration yields of PDA-EY and PDA-Au-EY. Because
the surface areas for both samples differed depending on the
presence of Au NPs, the augmentation factor of 1.4 for EY cov-
erage in the PDA-Au-EY sample has been taken into account
for the calculation. The PDA-Au-EY film was completely recov-
ered without any dye leaching or plasmonic band change
for Au NPs after the photocatalytic reaction (Figure S8). The
plasmon-derived enhancement was also demonstrated in the
enzymatic photosynthesis of L-glutamate by GDH coupled
with the photochemical NADH regeneration. As shown in the
inset of Figure 4b, PDA-Au-EY produced 2 mM of L-glutamate
while PDA-EY produced less than 0.1 mM of L-glutamate from
10 mM of α-ketoglutarate.
The photocatalysis of NADH regeneration using plasmonic
nanohybrids was further studied by introducing a spacing
layer between the Au core and the EY layer to observe distance
dependence of the plasmon enhancement, providing addi-
tional evidence for LSPR from Au NPs. The placement of the
spacing layer was achieved by applying another PDA coating
through incubation of the PDA-Au film in a dopamine-only
solution prior to the formation of the EY layer. PDA spacing
layers with three different thicknesses (of 2.5, 11, and 22 nm)
were introduced by controlling the duration of the immer-
sion of PDA-Au film for 20 min, 3 hrs, and 6 hrs, respectively
(Figure S9). The ratio of NADH regeneration yields by PDA-
Au-EY is plotted against the spacing thickness in Figure 4c.
layer always resulted in higher regeneration yields than the
nanohybrids with PDA spacings. Therefore, our results show
that the core-shell structure of the PDA-Au-EY nanohybrids is
suitable for plasmon-enhanced artificial photosynthesis and
the field enhancement by the LSPR from Au NPs is critical
for the enhanced photocatalytic efficiency of EY during NADH
photoregeneration.
In summary, we created core-shell nanohybrid assemblies
for plasmon-enhanced light harvesting by employing multi-
functional mussel-inspired PDA nanolayers as a reducing
agent for plasmonic metal deposition, a scaffold for dye encap-
sulation, and an adhesive between nanohybrid and substrate.
Utilizing the remarkable adhesive versatility of PDA coating,
we successfully synthesized elaborate, core-shell nanostruc-
tures on different substrates to demonstrate general applica-
bility to materials irrespective of their chemical compositions
or morphologies. The resulting plasmonic nanohybrids, in
which dye molecules are precisely located in the field enhance-
ment region by the LSPR around the metal core, enabled
plasmon-enhanced light absorption and charge carrier genera-
tion in the photosensitizing molecules. Upon visible-light illu-
mination, the plasmonic metal NPs incorporated in the light
harvesting assemblies significantly enhanced the photocata-
lytic activity by the molecular photosensitizer during biocata-
lyzed artificial photosynthesis. The mussel-inspired approach
provides an effective and innovative platform to attain
nanoscale architectures integrated with plasmonic metals and
photocatalysts with desired optical properties for solar energy
harvesting.
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Figure 4. a) Energy level diagram of plasmon-enhanced biomimetic photosynthesis by PDA-Au-EY nanohybrids. b) Photoregeneration of NADH using
PDA-Au, PDA-EY, and PDA-Au-EY nanohybrid films under visible light. The inset shows photoenzymatic synthesis of L-glutamate by GDH coupled with
photoregeneration of NADH by nanohybrid films. c) Ratio of the NADH photoregeneration yield of PDA-Au-EY with PDA spacing to that of PDA-Au-EY
without PDA spacing plotted against the thickness of the spacing layers.
Characterization of Nanohybrids: The morphologies of nanohybrid
films were observed using an S-4800 field emission scanning electron
Experimental Section
Materials: Dopamine hydrochloride, gold(III) chloride hydrate,
silver nitrate, tris base, eosin Y (EY), phloxine B, tris(2,2′-bipyridyl)
microscope (Hitachi High-technologies CO., Japan). The analysis of
elements comprising the films was performed by X-Ray photoelectron
spectroscopy (XPS) using a sigma probe spectrometer (Thermo VG
Scientific Co., UK) equipped with a microfocusing monochromated
X-ray source (90 W) under an ultrahigh vacuum condition (∼10−10
Torr). Absorbance spectra of the samples were measured using a V/650
spectrophotometer (Jasco, Inc., Tokyo, Japan). The Raman spectra
were obtained with a LabRAM UV-vis-NIR high-resolution dispersive
Raman microscope (Horiba Jobin Yvon, France). The diffraction patterns
of metal nanoparticles were measured using a D/MAX-2500 X-ray
diffractometer (Rigaku Co., Japan).
dichlororuthenium(II)hexahydrate
(Ru[bpy]3),
triethanolamine
(TEOA), NAD+, α-ketoglutarate, ammonium sulfate, and glutamate
dehydrogenase (GDH) were purchased from Sigma-Aldrich (St. Louis,
MO). Cu(II) tetrakis(4-sulfonatophenyl)porphyrin (TPPS) was purchased
from Frontier Scientific (Logan, UT). Slide glasses and PET films were
washed with ethanol and deionized water prior to nanohybrid synthesis.
Silica beads were synthesized by the seeded growth method as described
in the literature.[44] [Cp*Rh(bpy)H2O]2+ was synthesized according to the
previous study.[29]
Mussel-Inspired Nanohybrid Synthesis: The synthesis of nanohybrid
films was achieved on the substrates of different materials and
morphologies through the same three-step incubation procedure. First,
PDA coating was created by immersing the substrate into a dopamine
solution (2 mg mL−1) in Tris buffer (10 mM, pH 8.5). Then, the PDA-
coated substrate was incubated in the metal precursor solution to induce
reduction of plasmonic metal nanoparticles by catecholic moieties in
PDA. 1 mM of gold chloride solution was used for Au NPs synthesis
and 50 mM of silver nitrate solution was used for Ag NPs synthesis.
Secondary PDA coating for dye encapsulation was followed by dipping
the resulting sample into a dye (1 mg mL−1)/dopamine (2 mg mL−1)
solution in Tris buffer. All of the incubation steps were performed in a
shaking incubator (JO Tech., Korea) at a shaking rate of 100 rpm for
16 hrs.
FDTD Simulation: In order to calculate the optical properties of
nanohybrid systems with FDTD simulations, complex permittivities of
PDA and EY were first modeled. For PDA, the real part of the permittivity
was exploited from the previous reported value.[45] The transfer-matrix
method was employed to the retrieve the imaginary part of PDA from
the experimental absorbance result. EY permittivity was calculated
εLorentz ⋅ωR2esonace
using a Lorentz model, εEY = εPDA
+
, where εPDA was
ωR2eonance − ω 2 − iΓω
a calculated value, εLorentz = 0.03, damping (Γ) = 3.5 × 1014 s−1, and
ωresonance = 3.5 × 1015 s−1 (λresonance = 540 nm). For Au, an empirical result
of Johnson and Christy was used.[46] With these material parameters,
numerical simulations were conducted using the finite-difference time-
domain method. A mean size (70 nm) of Au NPs from the experiment
was chosen to evaluate the local optical properties of nanohybrid films.
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In this simulation, simulation volume was meshed into 1 nm resolution
in the organic and metallic regions. Far removed from the organic and
metallic regions, a larger mesh size was used to reduce computational
load. Total simulation volume was 170 × 220 × 170 nm3 and a plane
wave source, propagating in y direction, was used. Periodic boundaries
were employed in x- and z-direction and perfectly matched layers were
adopted in y direction for termination. 3-dimensional and 2-dimensional
monitors were used to collect electric field data for calculation of the
field enhancement and absorption.
NADH Photoregeneration and Photoenzymatic Synthesis of L-Glutamate:
For the photoregeneration of NADH, nanohybrid films formed on slide
glasses were immersed in 3 mL of reaction medium in quartz cells and
were kept in the dark with vigorous stirring for 30 minutes to ensure
fully homogenized reactors before the photocatalytic reaction. The
reaction medium was composed of NAD+ (1 mM), [Cp*Rh(bpy)H2O]2+
(0.5 mM), and TEOA (15 w/v%) in a phosphate buffer (100 mM, pH
7.5). A xenon lamp (450 W) with a 420-nm cut-off filter was used as a
light source. During light irradiation, the absorbance of the reaction
medium at 340 nm was measured to estimate the concentration of
photoregenerated NADH. The photoenzymatic synthesis of L-glutamate
coupled with photoregeneration of NADH was performed using the
same experimental set-up with the exception of the reaction medium.
The reaction medium for L-glutamate conversion consisted of NAD+
(1 mM), [Cp*Rh(bpy)H2O]2+ (0.5 mM), TEOA (15 w/v%),
α-ketoglutarate (5 mM), (NH4)2SO4 (100 mM), and GDH (40 U) in a
phosphate buffer (100 mM, pH 7.5). The amount of L-glutamate during
the photoenzymatic reaction was analyzed using liquid chromatography
(LC-20A prominence, Shimadzu, Japan) equipped with an Inertsil C18
column (ODS-3V, length: 150 mm).
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DOI: 10.1002/adma.201305766