Bioenabled
Plasmon-Resonant Metal
Nanoparticles
Nanophotonics
Yeechi Chen,* Keiko Munechika,*
and David S. Ginger
Abstract
Biological molecules such as oligonucleotides, proteins, or peptides can be used for
the synthesis, recognition, and assembly of materials with nanoscale dimensions.
Of particular interest are the fields of near-field optics and plasmonics. Many potential
optical applications depend on the ability to control the relative positioning of organic
dyes, plasmon-resonant metal nanoparticles, and semiconductor quantum dots with
nanoscale precision. In this article, we describe some recent achievements in biological
assembly and nanophotonics, and discuss potential uses of biological materials for
assembling optically functional nanostructures. We emphasize the use of biological
materials to build well-defined nanostructures for near-field plasmon-enhanced
fluorescence.
Introduction
Biological materials assembly is gaining
popularity in the field of materials science.
Methods using biological systems offer
researchers a number of potentially attractive features: the ability to catalyze the
growth of inorganic materials under mild
conditions,1,2 the means to control the
microstructure and nanostructure of the
assembled materials,3 the ability to recognize molecules and surfaces with good
sensitivity and selectivity,4–6 and the flexibility to build nearly arbitrary supramolecular architectures with nanometer
precision.7 All of this is accomplished with
the flexible chemistry of biomolecules
such as DNA, peptides, and proteins, and
relies heavily on leveraging the tools of
modern biochemistry.
Historically, much effort has been
devoted to characterizing the micro- and
nanoscale structure of biologically grown
materials in comparison to their bulk inorganic counterparts, largely because of the
remarkable difference in materials properties. For example, the interlinking proteins
in abalone nacre make the composite material 3,000 times more fracture-resistant than
the inorganic component alone.8 More
recently, there has been a virtual explosion
of research investigating potential uses of
biological materials in nanoelectronics9–11
and nanophotonics.12–14 As shown in
Figure 1, many structural motifs in biology seem size-matched to interesting
physical and materials length scales.
Simultaneously, an increasing number of
labeling and sensing applications in biology are exploiting the unique optical,
electrical, and chemical properties of
nanostructures.14–24 Rather than trying to
cover all possible permutations of “bionano” research in a short overview, this
article focuses on the intersection of the
fields of near-field optics and plasmonics
with biological materials assembly. Broadly,
near-field optics concerns the behavior of
light at sub-wavelength distances, while
plasmonics seeks to harness collective
oscillations of free electrons in metals
for a variety of applications. Illuminating
metal nanoparticles can excite plasmons,
which in turn, produce areas of confined,
intense electric fields near the nanoparticle. We focus on the use of biological
materials to position fluorophores (components that fluoresce) near metal nanostructures, with the goal of producing more
intense and stable fluorescence for a variety
of applications.17–21 In this rapidly expanding area, we cannot possibly reference all
of the interesting articles that have been
published.
A localized surface plasmon resonance
(LSPR) is a collective excitation of conduction electrons in a metal nanostructure.
These plasmon resonances can be excited
by light and are responsible for the sizeand shape-dependent optical properties
of metal nanoparticles, including the
strong absorption, scattering, and large
local-field enhancements exhibited by
these materials. Figure 2 shows a series of
colloidal silver nanoparticle solutions,
made by following standard literature
methods,25,26 which differ only in the size
and shape of the nanoparticles. These
solutions also highlight the degree of synthetic control over the LSPRs of metal
nanoparticles that can now be achieved.
The synthesis, optical properties, and
applications of plasmon-resonant metal
nanoparticles have been the subjects of
several recent reviews.27–34
In the context of this article, one of the
most interesting properties of these metal
nanoparticles is the large local-field
enhancement that occurs near the particle
surface when the metal is illuminated by
light that excites plasmon resonance. For
example, Figure 3 shows the local-field
intensities around both single and pairs of
silver nanoparticles as calculated by
Schatz and co-workers. The intensity
(⎪E⎪2) near the particle surface can be over
500 times larger than that which would
occur in the absence of the particle.27,35
Because of their ability to concentrate incident radiation in local hot spots, metal
nanostructures are also referred to as
nanoscopic antennae. Local-field calculations have been performed for a range of
particle sizes and shapes using a variety
of computational tools such as finitedifference time domain (FDTD),36 discrete
dipole approximation (DDA),27 or multiple multipole (MMP)37 calculations. As
seen in Figure 3, the magnitude of the
field enhancement depends strongly on
the particle size and shape, the proximity
to sharp points and narrow gaps, and the
frequency of the light relative to the plasmon resonance.27,35,38 These highly confined electric fields have been used in a
variety of near-field enhanced spectroscopies and imaging modes including
near-field scanning optical microscopy39–41
and surface-enhanced Raman spectroscopy (SERS).33,42,43
SERS and other surface-enhanced
spectroscopies, including one- and twophoton fluorescence, experienced a period
of intense research roughly 25 years
*Chen and Munechika contributed equally to this article.
536
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Bioenabled Nanophotonics
Figure 1. Size comparison of chemically, biologically, and lithographically produced structures with relevant length scales.
Plasmon-Enhanced Fluorescence
Figure 2. Tunability of plasmon resonance in colloidal solutions. A series of colloidal silver
nanoparticle solutions show a variety of colors due to the different sizes and shapes of the
nanoparticles within each solution. Photograph courtesy of Keiko Munechika, Ginger Lab.
ago.33,42,43 Since then, advances in instrumentation (the development of scanning
probe microscopy and single-molecule
spectroscopy) along with advances in
sample preparation (the development of
new synthetic methods for preparing narrow distributions of size- and shapecontrolled metal nanoparticles) have led
to a recent upsurge in interest and rapid
growth in the field.
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While surface-enhanced Raman spectroscopy (SERS) has motivated much of the
research into surface-enhanced spectroscopy in recent years, the widespread
use of fluorescence-based detection in biomedicine and the importance of radiative
decay near metal electrodes in thin-film
optoelectronic devices44–46 have also led to a
great deal of interest in the study of simple
fluorescence near metal nanostructures.
Planar metal films have long been known
to quench emission from fluorophores at
nanometer distances,47,48 but the effects of
metal nanostructures are more complex.
Depending on the details of the system
under investigation, fluorescence quenching,49–53 enhancement,44,54–62 or both63–65
have been observed in experimental studies of fluorescent dyes and quantum dots
placed near nanostructured metals. While
the increased surface area (and hence the
increased amount of adsorbed dye) of a
nanostructured metal surface compared
with a planar substrate might account for
some of the reports of enhancement, the
537
Bioenabled Nanophotonics
a
ond contribution is enhanced emission:
the nanoparticle’s antenna effect can also
enhance the fluorophore’s radiative
decay rate, potentially improving both
the quantum yield and photostability of
the fluorophore. We can summarize the
various effects of a nanoparticle on the
apparent fluorescence intensity, YAPP, of a
nearby fluorophore as
E
YAPP = γex(ωex)QEM(ωem)ηcoll(ωem)σ
b
c
Figure 3. (a) Local electric field enhancement around a silver nanoprism (100 nm sides)
calculated for polarized incident light (770 or 460 nm wavelength) at the resonance
frequency (left) and the off-resonance frequency (right) using discrete dipole approximation
(DDA) calculations. At its resonance frequency, the nanoparticle concentrates the incident
field strength (E ) ~20-fold. (Reprinted from Reference 27.) (b) Electric field enhancement in
between two spherical silver nanoparticles (36 nm diameter, 2 nm apart) illuminated with
520 nm light have a calculated electric-field intensity (E *E ) enhancement of ~104. (Adapted
from Reference 35.) (c) Electric field enhancement between triangular prisms (~60 nm side
lengths, 12 nm thick) showing a hot spot of more than 50,000 times the incident electric
field intensity. (Adapted from Reference 35.)
observation of enhancement in singlemolecule experiments52,53,58,62,64,65 and
planar dye layers with adsorbed nanoparticles59 indicates that nontrivial enhancements of fluorescence using near-field
effects are achievable.
Figure 4 shows a simple variation of an
experiment59 that enhances fluorescence
with single metal nanoparticles. A monolayer of an organic dye (Rhodamine
Red) is bound to 2-mercaptopropyltrimethoxysilane-treated indium tin
oxide (ITO)-coated glass slide. Metal
nanoprisms are then sparsely adsorbed on
top of the dye. The optical dark-field
image and corresponding scanning electron microscopy (SEM) images show the
538
nanoprisms to be single, optically isolated
particles. Although the dye layer is uniform, the corresponding fluorescence
image is not uniform. Locations of
increased fluorescence correlate with the
positions of many of the nanoparticles. If
planar metal films quench fluorescence,
how does a metal nanoparticle lead to
enhanced fluorescence?
Qualitatively, fluorescence enhancement near a metal nanoparticle can be
understood as arising from two possible
contributions. The first contribution is
an enhanced excitation rate: the light
intensity is higher near a nanoparticle
antenna, so a fluorophore at such a hot
spot will be excited more often. The sec-
(1)
where γex(ωex) is the excitation rate of the
fluorophores in the particle near field at
the excitation frequency, ωex; QEM(ωem)
is the quantum yield for far-field emission
at the emission frequency, ωem; ηcoll(ωem)
is the collection efficiency per unit area of
the far-field light in the experimental
geometry (accounting for any modification of the free-space spatial emission profile and the fixed acceptance of the
detector); and σ is a normalization factor
accounting for attachment density and
total area excited. Although straightforward, the σ and ηcoll(ωem) factors in
Equation 1 are often neglected. Since
metal nanoparticles can help increase the
quantum yield of a fluorophore, it is often
easier to demonstrate fluorescence
enhancement for fluorophores with low
intrinsic quantum yields. However, many
groups have also reported fluorescence
enhancement using both organic dyes and
semiconductor quantum dots with high
intrinsic quantum yields.55,56,58,59,61,63
The γex(ωex) and QEM(ωem) terms are
extremely sensitive to the excitation and
emission frequencies of the fluorophore,
the distance between the fluorophore and
nanoparticle, and the orientation of the
fluorophore relative to the nanoparticle.
Generally, γex(ωex) depends on both the
absorption coefficient of the dye and the
local (nanoparticle-enhanced) field intensity. Since the field intensity increases
closer to the nanoparticle surface, γex(ωex)
should be maximized closest to the particle surface. The behavior of QEM(ωem) is
more complicated, as the quantum yield of
the dye is a ratio of the radiative-decay rate
to the sum of all possible decay rates. Not
only can the metal-altered local photonic
mode density lead to changes in the
radiative-decay rate of the fluorophore, but
the presence of the metal opens up new
nonradiative-decay pathways via energy
transfer back to the metal.48,66–69 In addition,
energy transferred to the metal as excited
plasmon modes can be re-scattered back
into the far field by nanoparticles or periodic structures,48,70 or the energy of the
excited dye can be quenched by loss to
nonradiative-decay pathways in the metal.
Thus, a metal nanostructure can lead to
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Bioenabled Nanophotonics
either an increase or a decrease in the fluorescence quantum efficiency of a nearby
fluorophore, depending on the relative
contributions of the enhancement and
quenching terms.
The relative importance of fluorescence
enhancement and quenching effects is also
expected to be sensitive to the shape of
the metal particle, the orientation of the
fluorophore, and the distance between
the fluorophore and the metal,67–69,71,72 as
is the case for dyes attached to planar
metal films.47,48 Many groups have
observed variations in fluorescence intensity as a function of the distance between a
layer of fluorophores and a number of
nanostructured
metal
surfaces,54,57
adsorbed colloidal particles,55,63 or suspended colloidal particles.50,51 Singlemolecule experiments have even provided
strong evidence for the existence of a local
maximum in the fluorescence intensity
versus distance curve.58,62,64,65 By using a
scanning probe microscope to control the
distance between a single dye molecule
and a single spherical metal nanoparticle,
Novotny and co-workers were able to
measure the distance-dependent fluorescence enhancement effects shown in
Figure 5.64 This distance dependence is
similar to that reported in a number of
other experiments involving either dyes or
quantum dots, and films of metal nanoparticles.63 From a qualitative standpoint, the
existence of an optimal distance can be
understood as arising from the competing
effects and different distance dependencies of the excitation enhancement, emission enhancement, and quenching terms.
In addition to the distance dependence,
the local-field enhancements surrounding
metal nanostructures are strongly wavelength-dependent.27,35 As a result, the observation of fluorescence enhancement can
depend on the spectral properties of both
the metal nanoparticles and fluorophores. For instance, our group has
used single-particle spectroscopy of DNAfunctionalized silver nanoparticles to correlate the fluorescence intensity from organic
dyes with the scattering spectra of the silver
nanoparticles to which they are attached.73
Figure 6 shows the results from one such
experiment, as well as a series of plots
showing the overall trends for three common fluorescent dyes. In all cases, there is a
strong dependence of the apparent brightness on the spectral overlap between
nanoparticle LSPR and dye excitation and
emission, with the most fluorescence being
observed when the dye emission peak is
slightly red-shifted from the scattering
peak. These results are in general agreement with several other experimental and
theoretical studies.58,62,64,65,69,72
a
b
c
d
Figure 4. (a) Illustration of the fluorescence enhancement experiment: silver nanoprisms
are adsorbed onto a monolayer of Rhodamine Red dye covalently bound to
3-mercaptopropyltrimethoxysilane (MPTMS) on indium tin oxide (ITO). (b) High-resolution
scanning electron microscopy (SEM) images of labeled nanoparticles in both (c) and (d).
(c) Dark-field optical scattering image of an area of nanoparticles on the dye. Each colored
spot indicates a single silver nanoparticle. (d) Fluorescence image of the same area shown
in the dark-field image (c). The fluorescence image of the same area shows spots of
brighter fluorescence, which correlate to the locations of the silver nanoparticles.
a
b
200nm
Figure 5. (a) Illustration of the distance-dependent fluorescence rate experiment:
a vertically oriented dye molecule is excited by a radially polarized laser beam near a single
gold nanoparticle (d = 80 nm) affixed to the end of a pointed optical fiber. Inset: SEM image
of a gold nanoparticle at the end of an optical fiber. (b) Plot of fluorescence rate of the dye
molecule as a function of the spacing, z, between the dye and the nanoparticle (black dots:
experimental data; solid red line: theory). The dashed line indicates the background
fluorescence rate. γem is the modified emission rate of the molecule near the nanoparticle.
0 is the unmodified emission rate of the molecule, far from metal nanoparticles. The
γ em
emission rate changes near a metal nanoparticle. The ratio of the rates gives the relative
enhancement. (Reprinted from Reference 64.)
Finally, for one-photon fluorescence,
the excitation enhancement term in
Equation 1 is linear in intensity (or ⎪E⎪2).
Nonlinear processes scale with higher
powers of the electric field and could
exhibit even larger enhancements than
simple one-photon fluorescence. For exam-
MRS BULLETIN • VOLUME 33 • MAY 2008 • www.mrs.org/bulletin
ple, in two-photon fluorescence, the excitation rate scales with the square of the intensity (or ⎪E⎪4). Metal nanoparticles could be
used to produce significant enhancements
in two-photon fluorescence, and have been
reported to achieve enhancements of up to
105 for two-photon fluorescence.74
539
Bioenabled Nanophotonics
a
b
c
Alexa Fluor 488
e
d
Rhodamine Red
f
g
Figure 6. DNA-directed assembly of organic dyes on single silver nanoprisms. (far left) Silver nanoprisms (NP) are fixed to a surface and
functionalized with a monolayer of single-stranded DNA (ssDNA). Dyes coupled to complementary ssDNA hybridize to the NP, fixing dyes at a
finite distance from the NP. (a) Dark-field image of NP hybridized with a 1:1 molar mix of two dyes. (b, c) Images of fluorescence emission from
each of two different dyes of the same area in (a). (d) Scattering spectra for particles in (a). (e−g) Summary plots of average fluorescence
intensity versus particles’ localized surface plasmon resonance (LSPR) peaks for three different dyes. The excitation (dotted) and emission
(dashed) spectra for each dye are shown. Y-error bars are the standard deviation of the mean fluorescence intensity observed from particles
with LSPR peaks within each 20 nm bin. N represents the sample size. (Adapted from Reference 73.)
Linking Fluorophores and Metal
Nanoparticles Using Biomolecules
As discussed earlier, near-field effects
are exquisitely sensitive to distance with
characteristic length scales spanning from
a few nanometers to a few 10s of nanometers. These distances are too large to
easily access using conventional organic
chemistry but are too small to reliably
engineer with standard top-down microand nanofabrication tools (Figure 1).
On the other hand, nature has evolved
several motifs for organizing materials on
the necessary 1–100 nm length scale. Thus,
biomolecules become attractive from a
synthetic perspective, in addition to their
obvious importance as receptors in sensing and diagnostic applications.
In a now classic paper, Mirkin’s group
functionalized colloidal gold nanoparticles
with thiolated, single-stranded oligonucleotides and showed that introducing
complementary DNA linker sequences
could cause the nanoparticles to assemble
into large clusters.75 As the interparticle
links were formed by double-stranded
DNA, they showed that the nearest-neighbor spacing between the gold nanoparticles could be controlled by varying the
number of base pairs in the DNA linking
540
sequences.76 In addition to developing a
range of bioassays, which are experiments
that test the effect of the nanoparticles on
biological agents, they showed that DNA
could be patterned on surfaces and used to
template the assembly of optically and
electronically active nanostructures.10,77
In our group, we have used DNA
to attach fluorescent dyes to silver
nanoprisms for the study of near-field fluorescence enhancement and quenching
(Figure 6).73 This biological attachment
strategy has been useful to us for several
reasons. First, the DNA serves as a spacer
of finite length to help maintain a uniform
distance between the dyes and nanoparticles. Second, the specific attachment gives
us very little fluorescence background
from nonspecific binding to the substrate
or from noncomplementary sequences,
allowing us to be certain that the fluorescence we observed comes from dyes
attached to the prisms. Finally, we have
the flexibility to attach multiple types of
dyes to each nanoparticle, by simply mixing together differently labeled oligonucleotides into the hybridization solution
(as shown in Figure 6a−6c).
A number of other groups have used
DNA to study near-field interactions
between fluorescent dyes and metal
nanoparticles. Several groups have examined the changes in lifetimes of quantum
dots and fluorescent dyes as a function of
distance from a metal surface.49,50,53
Strouse and co-workers have used DNA
spacers to control the distance between
fluorescent donors and very small (~1.5
nm diameter) gold nanoparticle energy
acceptors.78 They concluded that the distance dependence of the fluorescence lifetime for the dye/nanoparticle pair scaled
as 1/R4 rather than the traditional 1/R6
that would be expected for simple Förster
resonance energy transfer (FRET)
processes. However, these results were in
good agreement with other predictions79
for nanoscale surface energy transfer
(NSET). The results suggest that small
nanoparticle energy acceptors could be
used to extend the range of traditional
FRET experiments.80
Although extremely useful, DNA is not
the only biomolecule that can be used to
link fluorophores with metal nanoparticles. Mattoussi and co-workers studied the
distance-dependent fluorescence quenching of CdSe quantum dots with small Au
nanoparticles linked by rigid variablelength β-sheet peptides and also found
MRS BULLETIN • VOLUME 33 • MAY 2008 • www.mrs.org/bulletin
Bioenabled Nanophotonics
that the distance dependence was closer to
1/R4 than 1/R6.81 Larger proteins are also
promising candidates for assembling
nanostructures with new optical properties. Several groups have used antibodies
and streptavidin/biotin linkages between
metal nanoparticles and quantum dots to
observe enhancement rather than quenching: Kotov’s group has been able to
enhance the luminescence from CdTe
nanowires by creating assemblies of CdTe
wires with spherical Au or Ag nanoparticles using biotin/streptavidin82,83 as well
as streptavidin/anti-streptavidin linkers.84
All of the examples cited so far have
used biomolecules as purely structural
materials, with the bio-inorganic coupling
being performed by grafting reactive
chemical groups onto biological molecules (such as a covalent modification of a
protein or the use of thiol-functionalized
oligonucleotides). As biology is adaptable,
an exciting possibility may be to tailor biomolecules to perform the binding as well
as linking steps of the supramolecular
assembly process. Our colleagues have
generated a number of promising peptide and protein structures with specific
affinities for various inorganic materials.5,6
As a step toward controlling fluorescence with these engineered peptides, Zin
et al. demonstrated that bifunctional linkers consisting of short combinatorially
selected polypeptides could be used to
anchor fluorescent CdSe quantum dots to
gold surfaces with different spacings and
densities.85 The peptides were selected for
their binding affinity for gold surfaces and
then functionalized at the N-terminus
with a biotin label. After the peptides were
bound to the gold surface, they were used
to capture streptavidin-coated quantum
dots. Although this proof-of-concept
demonstration only used the peptides as
anchors, it may be possible to use constrained peptides6 or engineered proteins
to control distance or even orientation.86
Conclusions
Local-field enhancements around metal
nanostructures can be useful in applications ranging from SERS to one- and twophoton fluorescence. Harnessing these
effects to the fullest extent requires the ability to precisely control the spacing between
metal nanoparticles and organic dyes or
semiconductor quantum dots. The critical
length scale for fluorescence enhancement
seems to be on the order of ~5−10 nm.
At much shorter distances, quenching
begins to play a significant role; at longer
distances, the local-field enhancement
effects are too weak to make a large impact.
Peptides and modified DNA have been
successfully used to assemble individual
metal nanoparticles and fluorophores
into discrete supramolecular structures
with controlled dimensions. Ultimately,
designer proteins and peptides could
be used as biomolecular building blocks
to enable the self-assembly of geometrically well-defined clusters that position multiple components (e.g., organic
fluorophores, metal nanoparticles, semiconductor quantum dots) with subnanometer precision. The vision is that
each building block can be customized to
optimize the optical properties of a cluster
(local-field enhancement, radiative rate,
photostability, effective cross section, and
brightness) for a specific application.
However, before such exciting nanostructures can be realized in applications, we
must first refine our methods of coupling
fluorophores and plasmon-resonant metal
nanoparticles together with biomolecules
into programmable structures. In addition, we must improve our understanding
of the properties of fluorophores placed in
these extreme optical environments.
Nevertheless, as demonstrated throughout this issue, rapid progress is being made
on each of these fronts. We believe that, as
the relationship between biology and photonics grows and matures, both fields will
benefit. Not only will optical phenomena
continue to serve as important probes of
biological structure, but engineered biological structures could facilitate the assembly
and study of discrete nanostructures with
remarkable optical properties.
Acknowledgments
The authors acknowledge the Air Force
Office of Scientific Research, the National
Science Foundation Materials Research
Science and Engineering Center (MRSEC)
program through the Genetically Engineered Materials Science & Engineering
Center (GEMSEC) (DMR 0520567), and
the American Chemical Society Petroleum
research fund for directly supporting the
authors’ work described in this article.
D.S.G. also thanks the Camille Dreyfus
Teacher-Scholar Awards Program for support. D.S.G. is a Cottrell Scholar of the
Research Corporation and an Alfred
P. Sloan Foundation Research Fellow.
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