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Amplification of Refractometric Biosensor Response through
Biomineralization of Metal–Organic Framework Nanocrystals
Jingyi Luan, Rong Hu, Sirimuvva Tadepalli, Jeremiah J.
Morrissey, Evan D. Kharasch, and Srikanth Singamaneni*
DOI: 10.1002/admt.201700023
considered to be highly promising for the development of simple,
portable, sensitive, on-chip biodiagnostics for resource-limi ted
settings such as at-home care, rural clinics, developing countries
with low and moderate incomes and the battlefield.
While there has been tremendous pro-gress in the rational design
of nanotrans-ducers with high sensitivity and the devel-opment of
handheld read-out devices, the translation of these biosensors to
resource-limited settings is hindered by the poor thermal,
chemical, and environmental sta-bility of the natural antibodies,
which are the most commonly employed biorecogni-tion elements. We
have previously demon-strated that artificial antibodies, achieved
through molecular imprinting on plas-monic nanostructures, are a
viable alter-native to natural antibodies.[4,5] Although the
artificial antibodies exhibit excellent temperature, chemical and
environmental
stability, the sensitivity of plasmonic biosensors based on
artifi-cial antibodies is generally inferior compared to those
based on natural antibodies due to the relatively low binding
affinity of artificial antibodies compared to natural
antibodies.
Methods to overcome the limited sensitivity of the plasmonic
biosensors based on artificial antibodies are extremely impor-tant
to make this class of biosensors relevant to biodiagnostic
applications in the real world. Amplification of the sensor
response to a biomolecular binding event through an enzymatic
reaction is a powerful technique that is employed in enzyme-linked
immunosorbent assay (ELISA) for achieving high sen-sitivity and low
detection limit. A number of amplification
Plasmonic biosensors based on the refractive index sensitivity
of localized surface plasmon resonance (LSPR) are highly promising
for on-chip and point-of-care diagnostics. In particular, plasmonic
biosensors that rely on artificial antibodies are highly attractive
for applications in resource-limited settings due to the excellent
thermal, chemical, and environmental stability of these
biorecognition elements. In this work, a universal LSPR response
amplification strategy based on the biomineralization of a
metal–organic framework (MOF) on the captured analyte proteins is
demonstrated. The amplification relies on the differential ability
of abiotic recognition elements and captured biomolecules to induce
biomineralization of a MOF. The rapid amplification process (less
than 10 min) demonstrated here results in nearly 100% higher
sensitivity and three times lower limit of detection compared to
the innate sensor. The amplification approach can be broadly
applied to a wide variety of bioanalytes and can be rapidly
implemented in real-world con-ditions without compromising the
assay time or reusability of the plasmonic biochip.
J. Luan, Dr. R. Hu, S. Tadepalli, Prof. S. SingamaneniDepartment
of Mechanical Engineering and Materials ScienceInstitute of
Materials Science and EngineeringWashington University in St.
LouisSt Louis, MO 63130, USAE-mail: [email protected]. J.
J. Morrissey, Prof. E. D. KharaschDepartment of
AnesthesiologyDivision of Clinical and Translational
ResearchWashington University in St. LouisSt. Louis, MO 63110,
USA
Prof. J. J. Morrissey, Prof. E. D. Kharasch, Prof. S.
SingamaneniSiteman Cancer CenterSt. Louis, MO 63110, USAProf. E. D.
KharaschDepartment of Biochemistry and Molecular
BiophysicsWashington University in St. LouisSt. Louis, MO 63110,
USAProf. E. D. KharaschThe Center for Clinical PharmacologySt.
Louis College of Pharmacy and Washington University School of
MedicineSt. Louis, MO 63110, USA
Artificial Antibodies
Localized surface plasmon resonance (LSPR) of metal
nano-structures involves the collective oscillation of
dielectrically confined conduction electrons, which results in a
number of unique optical properties such as large absorption and
scat-tering cross sections and large enhancement of electromagnetic
field surrounding metal nanostructures.[1] LSPR wavelength of metal
nanostructures is highly sensitive to composition, size, shape,
dielectric constant of the surrounding medium, and proximity to
other metal nanostructures (plasmon coupling).[2] The sensitivity
of LSPR wavelength to the changes in the refrac-tive index of the
surrounding medium lends itself to a powerful class of
refractometric biosensors.[3] LSPR biosensors are
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strategies have been investigated in the context of plasmonic
biosensors. The spectral shift of LSPR biosensor depends on the
number of analyte molecules bound on the plasmonic nanotransducer,
the size of the analyte species (which deter-mines the thickness of
the adsorbate layer), and the refractive index difference between
the analyte and the prior medium.[6] For the same number of
molecules bound on the surface, a larger analyte molecule generally
results in a thicker absorbate layer, thus inducing a larger
spectral shift. This particular phe-nomenon has been exploited to
amplify the LSPR shift. Fol-lowing the selective binding of the
target analyte to the cap-ture antibodies immobilized on
nanotransducers, exposure to a nonoverlapping primary antibody
results in the binding of the primary antibody to the target
analyte. The binding of the primary antibody to the analyte
essentially increases the size of the adsorbate layer, thus
increasing the LSPR shift.[7] Van Duyne and co-workers have
demonstrated plasmon-coupling-based signal amplification using
primary antibody conjugated to gold nanoparticles.[8] While these
strategies are attractive, they require a specific nonoverlapping
antibody and in some cases tedious labeling procedures that are
expensive, time-consuming, and impractical in resource-limited
settings. These considerations highlight the need for a rapid,
inexpensive, and “universal” amplification strategy.
Owing to their large surface area, tunable porosity, organic
functionality, and high thermal stability, metal–organic
frame-works (MOFs), consisting of metal ions or clusters linked by
organic ligands,[9] have received increased scientific and
tech-nological interest.[10] In a recent study, Falcaro and
co-workers demonstrated the encapsulation of a wide range of
biomole-cules within MOFs by growing them in the presence of the
biomolecules under mild and biocompatible conditions (e.g., aqueous
solution at room temperature).[11] They have shown that a wide
range of biomolecules (e.g., proteins and DNA) can efficiently
localize MOF precursors, resulting in prenuclea-tion and rapid
biomineralization of MOF crystals around the biomolecules. MOF
growth exhibited an excellent spatial selec-tivity with crystal
growth confined to the regions with immobi-lized biomolecules on
solid substrates.[12]
Here, we demonstrate that biomineralization of MOF can be
employed as a universal amplification strategy for LSPR biosen-sors
based on abiotic recognition elements (e.g., artificial
anti-bodies), where the captured target biomolecules serve as
nucle-ation sites for the formation of MOF crystals on the
nanotrans-ducer. The MOF crystals formed on the biomolecules
increase the change in the refractive index and amplify the LSPR
shift, effectively lowering the limit of detection of the
biosensor. Owing to the widely available precursors and rapid
growth, we have employed zeolitic imidazolate framework-8 (ZIF-8)
as the MOF for LSPR amplification in this work. The MOF
biomin-eralization-based amplification approach is universal and
does not require any nonoverlapping antibodies or tedious labeling
procedures. We demonstrate the generality of the amplification
process using three different model analytes, namely, human serum
albumin (HSA), lysozyme, and hemoglobin (Hb).
Prior to implementing the biomineralization-based
ampli-fication, the preferential nucleation and growth of ZIF-8 on
and around the biomolecules was investigated using atomic force
microscopy (AFM). Owing to its atomically flat surface,
a freshly cleaved highly ordered pyrolytic graphite (HOPG) was
employed as a substrate for the AFM investigation. HSA (molecular
weight = 66.5 kDa) is one of the most abundant pro-teins in human
blood, and has a heart-shaped spheroid struc-ture and dimensions of
≈9.5 × 5 × 5 nm.[13] HSA was adsorbed onto the HOPG surface from a
dilute solution (200 ng mL−1). AFM images revealed uniform and
sparse distribution of HSA on HOPG surface (Figure 1A) with an
average height of ≈1.9 ± 0.7 nm (Figure 1C). The height of HSA
globules adsorbed on HOPG was found to be significantly lower
com-pared to the reported dimensions of HSA (9.5 × 5 × 5 nm)
possibly due to the spreading of proteins upon adsorption on the
surface. Subsequently, HSA immobilized HOPG substrate was immersed
into MOF precursor solution, followed by thor-ough rinsing and
drying under a stream of dry nitrogen. AFM images revealed globules
with a significantly higher thickness (≈16.9 ± 4.5 nm) compared to
HSA (1.9 ± 0.7 nm) (Figure 1B and the height histogram shown as
Figure 1D). Conversely, the density and distribution of the
globules were found to be similar to that of the HSA before
exposure to MOF precursor, indicating the preferential growth of
MOF around the sparsely adsorbed HSA (Figure 1A,B).
To further confirm the biomineralization of MOF, we have
employed HSA-coated Au nanorods (AuNR). The HSA-coated AuNRs
adsorbed on a glass substrate were exposed to MOF precursor
solution. LSPR wavelength of HSA-coated AuNR exhibi ted a red shift
of 15.1 nm after exposure to MOF precursor solution for 10 min
indicating the formation of MOF crystals on the AuNR. As a negative
control, pristine AuNR did not exhibit a red shift after being
exposed to MOF precursor solution of identical concentration, which
further confirms the preferential growth of MOF around the HSA
biomolecule (Figure 1E). We have employed Raman spectroscopy and
powder X-ray diffrac-tion (XRD) to confirm the chemical composition
of the MOF formed around HSA. We observed strong Raman bands at
1146 and 1458 cm−1 corresponding to C5N stretching and methyl
bending, respectively, which confirm the formation of ZIF-8
nanocrystals (Figure 1F, and Figure S1, Supporting
Informa-tion).[14] XRD and Fourier transform infrared (FTIR)
spectro-scopy further confirmed the formation of ZIF-8 nanocrystals
on HSA (Figure 1G, and Figures S2 and S3, Supporting Informa-tion).
The peak positions in the XRD pattern are in agreement with the
expected structure of ZIF-8 except for the absence of (011) and
(112) plane, implying the possible orientation of ZIF-8 formed on
immobilized HSA.[11] The XRD pattern also shows a strong peak at
10.88°, indicating the partial orientation of the crystals in (001)
direction. It has been previously reported that the surface
properties can significantly influence the nucle-ation and crystal
growth of ZIF-8.[15]
Now we turn our attention to the MOF biomineralization-based
amplification of LSPR biosensor response. In this study, we have
employed AuNR as the nanotransducers due to the large tunability
and high refractive index sensitivity of the lon-gitudinal LSPR
wavelength, and the electromagnetic hot spots at the edges. The
AuNR, synthesized through a seed-mediated approach,[16] exhibited a
narrow size distribution with a length of 49.5 ± 3.8 nm and a
diameter of 12.7 ± 0.8 nm (Figure 2A, inset of Figure 2B). The
vis–NIR extinction spectrum of the as-synthesized aqueous AuNR
suspension is characterized
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by two peaks: one at higher wavelength and the other one at
lower wavelength, corresponding to the longitudinal and transverse
surface plasmons, respectively (Figure 2B). The longitudinal
plasmon band, which exhibits a higher refrac-tive index sensitivity
compared to the transverse band, was employed for monitoring the
molecular imprinting process and implementing the artificial
antibody-based biosensor. Various steps involved in the fabrication
and implemen tation of the biosensor are illustrated in Figure 2C.
The AuNRs, immo-bilized on a clean glass substrate, are modified
with p-ami-nothiophenol (pATP) and glutaraldehyde (GA) (step 1),
which serve as linkers to immobilize HSA (model target protein) on
the surface of AuNR by forming reversible imine bonds (step 2). Two
monomers, namely, (3-aminopropyl) trimethox-ysilane (APTMS) and
trimethoxysilane (TMPS), are then copo-lymerized around the
immobilized template protein (step 3). The methoxy group of two
functional monomers undergoes rapid hydrolysis and subsequent
condensation to form an amorphous polymer network, leaving the
functional groups (NH3+, OH, CH3) interacting with the template
protein through electrostatic, hydrogen bonding, and hydrophobic
interactions. Subsequently, the template protein is released from
the polymer matrix and AuNR surface by a mixture of oxalic acid and
sodium dodecyl sulfate (SDS) solution (step 4), which breaks the
imine bond and overcomes the noncovalent
interactions, respectively. The removal of the template pro-tein
leaves cavities in the siloxane copolymer that are com-plementary
in size, shape, and chemical functionality to the template protein.
These cavities serve as artificial antibodies, preferentially
capturing template (like) species in the analyte solution (step 5).
Following the capture of the target proteins, the biochip is
exposed to MOF precursor solution resulting in the
biomineralization of MOF around the captured biomol-ecules (step
6).[11] To evaluate the HSA binding and the MOF formation (steps
4–6), we performed AFM imaging of the plasmonic biochip at each
stage. After the template removal, the HSA-imprinted AuNR exhibited
a smooth surface and the AuNR were found to be uniformly
distributed on the sub-strate with no signs of aggregation (Figure
2D). Exposure to HSA solution resulted in the capture of the
biomolecules (as evidenced by the LSPR shift discussed below),
however, no significant change in the morphology of the AuNR was
noted (Figure 2E). In stark contrast, after the exposure of the HSA
captured biochip to MOF precursors, the AFM images revealed small
globular features, presumably MOF nanocrys-tals, around the AuNR
(Figure 2F). Additionally, AFM investi-gation also revealed the
preferential growth of the small glob-ules at the two ends of AuNR
(inset AFM image in Figure 2F showing an individual AuNR with MOF
nanocrystals). In fact, we have demonstrated that during the
imprinting process,
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Figure 1. AFM height images of HSA absorbed on HOPG substrate A)
before (height scale: 15 nm) and B) after incubation in ZIF-8
precursors (height scale: 30 nm) (insets show the schematic
illustration of MOF growth around immobilized HSA). C) Height
histogram of HSA adsorbed on HOPG obtained from AFM image in Figure
1 A. D) Height histogram of globules formed after incubation in
ZIF-8 precursors obtained from AFM image in Figure 1 B. E) LSPR
shift versus MOF growth time on HSA modified AuNR and bare AuNR,
respectively. F) Raman spectra of HSA before and after exposure to
ZIF-8 precursors. G) XRD pattern of HSA coated with ZIF-8
nanocrystals.
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Figure 2. A) TEM image of AuNRs employed as nanotransducers for
artificial antibody-based LSPR biosensor. B) Vis–NIR extinction
spectrum of aqueous suspension of AuNR (inset shows the histogram
of the length of AuNR obtained from TEM image, revealing the
average length of AuNR ≈49.5 nm). C) Schematic illustration showing
the MOF-amplified plasmonic biosensor based on artificial
antibodies. Schematic illustration depicts the various steps
involved in fabrication of MIP-based plasmonic biosensor, target
protein capture, followed by the mineralization of MOF around the
captured protein to enhance the LSPR signal. Representative AFM
images of AuNR after D) template protein removal, E) recapture of
the target protein, and F) MOF biomineralization (insets show
individual AuNR, Z scale is 40 nm for all the images).
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the molecular linkers (pATP and GA) tend to bind at the ends of
AuNR due to the relatively low density of surfactant at the ends
compared to the side walls.[4,5] This phenomenon results in
site-specific formation of artificial antibody and subsequent
capture of the antigen. The captured antigen present at the ends of
AuNR in turn serves as the nucleation point for the MOF
nanocrystals.
We monitored the LSPR wavelength shift corresponding to each
step in the imprinting process, protein capture, and MOF
amplification. Cumulative red shift of ≈11 nm was observed after
the first three steps of the imprinting process (i.e., formation of
cross-linker layer (pATP+GA), protein (HSA) immobilization,
polymerization) followed by a blue shift of ≈4.6 nm after the
extraction of the template protein (steps 1–4 in Figure 3A, and the
corresponding vis–NIR spectra are shown in Figure S4, Supporting
Information). Following the molecular imprinting process using HSA
as template protein, the plasmonic biochip was exposed to a
relatively high concen-tration of HSA (25 µg mL−1), resulting in
the binding of HSA to artificial antibodies on AuNR and a
consequent LSPR red shift of 3.9 nm (step 5 in Figure 3A).
Subsequently, the biochip with the captured analyte was exposed to
ZIF-8 precursor solu-tion for 10 min resulting in the growth of MOF
nanocrystals on the AuNR. The MOF crystal growth around HSA
resulted in an additional red shift of 7.9 nm (step 6 in Figure
3A). The LSPR wavelength shifts after the capture of HSA and after
the MOF-based amplification are also represented as steps 4–5 and
steps 5–6 in Figure 3B, respectively. The MOF growth resulted in a
nearly 200% enhancement (7.9 nm) in the LSPR shift com-pared to the
shift from the target protein binding (3.9 nm). It is known that
ZIF-8 dissociates under acidic conditions because of the loss of
the coordination between the zinc ions and imida-zole.[11,17] As
such, one should expect a blue shift in the LSPR wavelength after
exposure to acidic buffer. After step 6, the plasmonic chip was
exposed to buffer solution at pH 5, which resulted in a blue shift
of ≈5.9 nm. Finally, we demonstrate the regrowth (exposure to ZIF-8
precursor solution) and redis-solution (exposure to acidic buffer)
of ZIF-8 on the plasmonic biochip as depicted in steps 8 and 9,
respectively. In the past, we have demonstrated that one of the
significant advantages of the plasmonic biosensors based on
artificial antibodies is
their reusability after desorbing the bound target protein using
a mixture of sodium dodecyl sulfate (SDS) and oxalic acid.[4]
Considering the fact that the MOF could be dissociated under acidic
conditions, the amplification process introduced here does not
compromise the reusability of this class of biosensors. These
results demonstrate the robust and repeatable amplifica-tion of the
LSPR shift in a plasmonic biosensor with a simple and rapid MOF
growth process.
Next, we attempted to quantify the improvement in sen-sitivity
and limit of detection of the artificial antibody-based plasmonic
biosensor using MOF-based amplification. The plasmonic biosensor
with artificial antibodies specific to HSA was exposed to phosphate
buffer (at pH 8) spiked with various concentrations of HSA. The
LSPR shift exhibited a monotonic increase with increase in the
concentration of HSA. The LSPR shift at the highest concentration
tested here (25 µg mL−1) was found to be ≈3.9 nm (Figure 4A).
Following exposure to the analyte solution at each concentration,
the LSPR shift was amplified by exposing the plasmonic biochip to
ZIF-8 precursor solution. The LSPR wavelength exhibited a much
larger shift corresponding to each concentration after
ampli-fication process (Figure 4A). Figure 4B showed the innate and
amplified response of the plasmonic biosensor to low concentrations
of HSA. Over this small concentration range, where the plasmonic
biosensor exhibited a linear response, the innate sensitivity was
calculated to be 0.09 nm nm−1. The sen-sitivity after amplification
process (0.16 nm nm−1) was found to be nearly twice compared to the
innate sensitivity. Further-more, defining the minimum detectable
LSPR shift as 0.5 nm (3σ noise level), the limit of detection of
the pristine plasmonic biosensor was found to be ≈400 ng mL−1
whereas the same after amplification process was found to be nearly
three times lower (130 ng mL−1).
For efficient deployment and widespread utility in point-of-care
and resource-limited settings, the amplification process would need
to be simple and rapid. In most previous cases, the amplification
process to enhance the LSPR response signifi-cantly prolongs the
testing period. We investigated the optimal time for MOF
growth-based amplification reaction. The LSPR shift upon exposure
to ZIF-8 precursors showed a rapid increase within the first 60 s
and essentially plateaued within
Adv. Mater. Technol. 2017, 2, 1700023
Figure 3. A) LSPR shift of AuNR after each step along the
fabrication of MIP, target protein capture, MOF growth, MOF
dissolution, and subsequent regrowth and dissolution of MOF. B)
Extinction spectra exhibiting a red shift in the LSPR wavelength of
MIP-AuNR after the capture of HSA and a significant enhancement of
the shift after biomineralization.
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10 min (Figure 4C). In order to verify whether the saturation in
the LSPR shift was due to the depletion of precursors, we immersed
the MOF-amplified plasmonic biochip (after 10 min amplification
reaction) to fresh precursor solution for an extra 30 min and no
significant red shift was observed (Figure S5, Supporting
Information). As a negative control, pristine plas-monic biochip
(i.e., plasmonic nanostructures with artificial antibodies) was
exposed to ZIF-8 precursor solution. The pris-tine biochip did not
exhibit significant LSPR shift indicating the absence of MOF
crystal growth (Figure 4C). These results indicate the artificial
antibodies themselves do not possess suf-ficient affinity to the
ZIF-8 precursors to induce prenucleation and growth of ZIF-8
nanocrystals.[18] It is important to note that the absence of MOF
growth on artificial antibodies is in stark contrast to natural
antibodies, which induces biominer-alization of ZIF-8. In fact,
ZIF-8 has been employed as a pro-tective coating to render thermal
stability to natural antibodies immobilized on plasmonic
nanostructures.[19] Overall, we introduced an ultrafast process to
enhance the LSPR response, which outperforms most of the existing
LSPR enhancement methods.
Finally, we demonstrate the generalizability of the
amplifi-cation process for the detection of various proteins. We
have employed lysozyme and hemoglobin as two other model
bio-markers. The LSPR shift corresponding to each step along
the
imprinting process is shown in Figure S6 (Supporting
Infor-mation). The biochips imprinted with lysozyme and hemo-globin
were exposed to 20 µg mL−1 of the corresponding pro-tein solutions
to capture the target biomolecules, followed by the immersion in
ZIF-8 precursors. Both sensors exhibited enhancement in the LSPR
shift, although with a small differ-ence in the amplification
efficiency, compared to the corre-sponding innate responses (Figure
4D). The difference in the amplification efficiency is probably due
to the difference in amino acid composition of the proteins,
resulting in a differ-ence in the affinity to ZIF-8 precursors and
ability to induce biomineralization of ZIF-8 crystals.
To summarize, we introduced a biomineralization-based signal
amplification of refractometric biosensors based on abi-otic
recognition elements. To the best of our knowledge, this is the
first demonstration of biomolecule-free and label-free LSPR
enhancement of a plasmonic biosensor. At the heart of the
amplification process lies the abiotic nature of the
biorecog-nition elements employed in this class of sensors. The
differen-tial affinity of MOF precursors to the artificial
antibodies and the captured protein biomarkers and their
differential ability to induce biomineralization have been
exploited to realize the amplification of LSPR response. The highly
specific and ultrafast growth of MOF crystals enhance the LSPR
response of the plasmonic biosensors by ≈200% and significantly
lower
Figure 4. A) LSPR shift versus concentration of the target
protein biomarker (HSA) before and after MOF amplification. B)
Concentration-dependent LSPR shift of innate and amplified
biosensor at low target protein concentration. C) MOF growth
reaction time-dependent LSPR shift on molecularly imprinted AuNR
before and after target protein capture. D) MOF amplification of
MIP-based plasmonic for three different model protein analytes
showing the generality of the signal amplification strategy
(results are the mean ± SD, n ≥ 3).
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Adv. Mater. Technol. 2017, 2, 1700023
the limit of detection without significantly increasing the
assay time. We also demonstrate that this ultrafast and label-free
method introduced here could be broadly applied to various protein
biomarkers, albeit with small variations in the enhance-ment
efficiency.
Supporting InformationSupporting Information is available from
the Wiley Online Library or from the author.
AcknowledgementsThe authors acknowledge support from National
Science Foundation (CBET1254399 and CBET1512043) and National
Institutes of Health (R21DK100759 and R01 CA141521). The authors
thank Nano Research Facility (NRF) at Washington University for
providing access to electron microscopy facilities.
Conflict of InterestThe authors declare no conflict of
interest.
Keywordsartificial antibodies, localized surface plasmon
resonance (LSPR), metal–organic framework, molecular imprinting,
plasmonic biosensors
Received: January 31, 2017Revised: March 22, 2017
Published online: May 18, 2017
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