13 research outputs found
Freezing Directed Construction of Bio/Nano Interfaces: Reagentless Conjugation, Denser Spherical Nucleic Acids, and Better Nanoflares
While nanoparticle
solutions cannot freeze in general, they may
remain stable in the presence of polymer stabilizers. We herein communicate
that gold nanoparticles (AuNPs) are stable in the presence of thiolated
DNA after a freeze–thaw cycle. The DNA is conjugated to AuNPs
during freezing without additional reagents and the conjugation can
be completed in a few minutes. More importantly, the DNA density is
20–30% higher than that prepared by the typical salt-aging
method. By lowering temperature, DNA hybridization is also promoted,
allowing the construction of better nanoflares with doubled probe
density and signaling sensitivity. This freezing method works for
AuNPs from 5 to 100 nm and all tested DNA sequences. The mechanism
was studied by separating the effect of temperature, freezing and
thawing, where the exclusion of salt and AuNPs by the growing ice
crystals is deemed critical. In addition to developing a simple method,
this study articulates unique physical processes during freezing with
important fundamental surface science implications, and it could be
extended to other systems
Parallel Polyadenine Duplex Formation at Low pH Facilitates DNA Conjugation onto Gold Nanoparticles
DNA-functionalized
gold nanoparticles (AuNPs) have been extensively
used in sensing, drug delivery, and materials science. A key step
is to attach DNA to AuNPs, forming a stable and functional conjugate.
Although the traditional salt-aging method takes a full day or longer,
a recent low-pH method allows DNA conjugation in a few minutes. The
effect of low pH was attributed to the protonation of adenine (A)
and cytosine (C), resulting in an overall lower negative charge density
on DNA. In this work, the effect of DNA conformation at low pH is
studied. Using circular dichroism (CD) spectroscopy, the parallel
poly-A duplex (A-motif) is detected when a poly-A segment is linked
to a random DNA, a design typically used for DNA conjugation. A DNA
staining dye, thiazole orange, is identified for detecting such A-motifs.
The A-motif structure is ideal for DNA conjugation because it exposes
the thiol group to directly react with the gold surface while minimizing
nonspecific DNA base adsorption. For nonthiolated DNA, the optimal
procedure is to incubate DNA and AuNPs followed by lowering the pH.
The i-motif formed by poly-C DNA at low pH is less favorable to the
conjugation reaction because of its unique way of folding. The stability
of poly-A and poly-G DNA at low pH is examined. An excellent stability
of poly-A DNA is confirmed, but poly-G has lower stability. This study
provides new fundamental insights into a practically useful technique
of conjugating DNA to AuNPs
Cation-Size-Dependent DNA Adsorption Kinetics and Packing Density on Gold Nanoparticles: An Opposite Trend
The
property of DNA is strongly influenced by counterions. Packing
a dense layer of DNA onto a gold nanoparticle (AuNP) generates an
interesting colloidal system with many novel physical properties such
as a sharp melting transition, protection of DNA against nucleases,
and enhanced complementary DNA binding affinity. In this work, the
effect of monovalent cation size is studied. First, for free AuNPs
without DNA, larger group 1A cations are more efficient in inducing
their aggregation. The same trend is observed with group 2A metals
using AuNPs capped by various self-assembled monolayers. After establishing
the salt range to maintain AuNP stability, the DNA adsorption kinetics
is also found to be faster with the larger Cs<sup>+</sup> compared
to the smaller Li<sup>+</sup>. This is attributed to the easier dehydration
of Cs<sup>+</sup>, and dehydrated Cs<sup>+</sup> might condense on
the AuNP surface to reduce the electrostatic repulsion effectively.
However, after a long incubation time with a high salt concentration,
Li<sup>+</sup> allows ∼30% more DNA packing compared to Cs<sup>+</sup>. Therefore, Li<sup>+</sup> is more effective in reducing
the charge repulsion among DNA, and Cs<sup>+</sup> is more effective
in screening the AuNP surface charge. This work suggests that physicochemical
information at the bio/nanointerface can be obtained by using counterions
as probes
Molecular Imprinting on Inorganic Nanozymes for Hundred-fold Enzyme Specificity
Enzyme-mimicking
nanomaterials (nanozymes) are more cost-effective
and robust than protein enzymes, but they lack specificity. Herein,
molecularly imprinted polymers were grown on Fe<sub>3</sub>O<sub>4</sub> nanozymes with peroxidase-like activity to create substrate binding
pockets. Electron microscopy confirmed a shell of nanogel. By imprinting
with an adsorbed substrate, moderate specificity was achieved with
neutral monomers. Further introducing charged monomers led to nearly
100-fold specificity for the imprinted substrate over the nonimprinted
compared to that of bare Fe<sub>3</sub>O<sub>4</sub>. Selective substrate
binding was further confirmed by isothermal titration calorimetry.
The same method was also successfully applied for imprinting on gold
nanoparticles (peroxidase mimics) and nanoceria (oxidase mimics).
Molecular imprinting furthers the functional enzyme mimicking aspect
of nanozymes, and such hybrid materials will find applications in
biosensor development, separation, environmental remediation, and
drug delivery
Magnetic Iron Oxide Nanoparticle Seeded Growth of Nucleotide Coordinated Polymers
The introduction of functional molecules
to the surface of magnetic
iron oxide nanoparticles (NPs) is of critical importance. Most previously
reported methods were focused on surface ligand attachment either
by physisorption or covalent conjugation, resulting in limited ligand
loading capacity. In this work, we report the seeded growth of a nucleotide
coordinated polymer shell, which can be considered as a special form
of adsorption by forming a complete shell. Among all of the tested
metal ions, Fe<sup>3+</sup> is the most efficient for this seeded
growth. A diverse range of guest molecules, including small organic
dyes, proteins, DNA, and gold NPs, can be encapsulated in the shell.
All of these molecules were loaded at a much higher capacity compared
to that on the naked iron oxide NP core, confirming the advantage
of the coordination polymer (CP) shell. In addition, the CP shell
provides better guest protein stability compared to that of simple
physisorption while retaining guest activity as confirmed by the entrapped
glucose oxidase assay. Use of this system as a peroxidase nanozyme
and glucose biosensor was demonstrated, detecting glucose as low as
1.4 μM with excellent stability. This work describes a new way
to functionalize inorganic materials with a biocompatible shell
Dissecting Colloidal Stabilization Factors in Crowded Polymer Solutions by Forming Self-Assembled Monolayers on Gold Nanoparticles
An
ideal colloidal system should be highly stable in a diverse
range of buffer conditions while still retaining its surface accessibility.
We recently reported that dispersing citrate-capped gold nanoparticles
(AuNPs) in polymers, such as polyethylene glycol (PEG), can achieve
such a goal because of contributions from depletion stabilization.
Because AuNPs can weakly adsorb PEG to exert steric stabilization
and the remaining citrate can impart charge stabilization, the extent
of the contribution of depletion stabilization is unclear. In this
work, we aim to dissect the contribution of each stabilizing factor.
This is achieved by coating AuNPs with a layer of thiolated compound,
which inhibits the adsorption of PEG and also allows for the control
of surface charge. We found that depletion stabilization alone was
insufficient to stabilize AuNPs at room temperature. However, when
working together with other stabilization mechanisms, ultrahigh stability
can be achieved. The size of both AuNPs and PEG was systematically
varied, and the trends were compared to theoretical calculations.
Finally, we report the importance of the surface chemistry of commercial
AuNPs
Rationally Designed Nucleobase and Nucleotide Coordinated Nanoparticles for Selective DNA Adsorption and Detection
Nanomaterials for DNA adsorption
are useful for sequence-specific DNA detection. Current materials
for DNA adsorption employ electrostatic attraction, hydrophobic interaction,
or π–π stacking, none of which can achieve sequence
specificity. Specificity might be improved by involving hydrogen bonding
and metal coordination. In this work, a diverse range of nucleobase/nucleotide
(adenine, adenosine, adenosine 5′-triphosphate (ATP), adenosine
5′-monophosphate (AMP), and guanosine 5′-triphosphate
(GTP)) coordinated materials containing various metal ions (AuÂ(III),
AgÂ(I), CeÂ(III), GdÂ(III), and TbÂ(III)) are prepared. In most cases,
nanoparticles are formed. These materials have different surface charges,
and positively charged particles only show nonspecific DNA adsorption.
Negatively charged materials give different adsorption kinetics for
different DNA sequences, where complementary DNA homopolymers are
adsorbed faster than other sequences. Therefore, the bases in the
coordinated materials can still form base pairs with the DNA. The
adsorption strength is mainly controlled by the metal ions, where
Au shows the strongest adsorption while lanthanides are weaker. These
materials can be used as sensors for DNA detection and can also deliver
DNA into cells with no detectable toxicity. By tuning the nanoparticle
formulation, enhanced detection can be achieved. This study is an
important step toward rational design of materials to achieve specific
interactions between biomolecules and synthetic nanoparticle surfaces
Hydrogen Peroxide Displacing DNA from Nanoceria: Mechanism and Detection of Glucose in Serum
Hydrogen
peroxide (H<sub>2</sub>O<sub>2</sub>) is a key molecule
in biology. As a byproduct of many enzymatic reactions, H<sub>2</sub>O<sub>2</sub> is also a popular biosensor target. Recently, interfacing
H<sub>2</sub>O<sub>2</sub> with inorganic nanoparticles has produced
a number of nanozymes showing peroxidase or catalase activities. CeO<sub>2</sub> nanoparticle (nanoceria) is a classical nanozyme. Herein,
a fluorescently labeled DNA is used as a probe, and H<sub>2</sub>O<sub>2</sub> can readily displace adsorbed DNA from nanoceria, resulting
in over 20-fold fluorescence enhancement. The displacement mechanism
instead of oxidative DNA cleavage is confirmed by denaturing gel electrophoresis
and surface group p<i>K</i><sub>a</sub> measurement. This
system can sensitively detect H<sub>2</sub>O<sub>2</sub> down to 130
nM (4.4 parts-per-billion). When coupled with glucose oxidase, glucose
is detected down to 8.9 μM in buffer. Detection in serum is
also achieved with results comparable with that from a commercial
glucose meter. With an understanding of the ligand role of H<sub>2</sub>O<sub>2</sub>, new applications in rational materials design, sensor
development, and drug delivery can be further exploited
Instantaneous Attachment of an Ultrahigh Density of Nonthiolated DNA to Gold Nanoparticles and Its Applications
The last 16 years have witnessed the landmark development
of polyvalent
thiolated DNA-functionalized gold nanoparticles (AuNP's) possessing
striking properties within the emerging field of nanobiotechnology.
Many novel properties of this hybrid nanomaterial are attributed to
the dense DNA shell. However, the question of whether nonthiolated
polyvalent DNA–AuNP could be fabricated with a high DNA density
and properties similar to those of its thiolated counterpart has not
been explored in detail. Herein, we report that by simply tuning the
pH of the DNA–AuNP mixture an ultrahigh capacity of nonthiolated
DNA can be conjugated to AuNP's in a few minutes, resulting in polyvalent
DNA–AuNP conjugates with cooperative melting behavior, a typical
property of polyvalent thiolated DNA-functionalized AuNP's. With this
method, large AuNP's (e.g., 50 nm) can be functionalized to achieve
the colorimetric detection of sub-nanometer DNA. Furthermore, this
fast, stable DNA loading was employed to separate AuNP's of different
sizes. We propose that a large fraction of the attached DNAs are adsorbed
via one or a few terminal bases to afford the high loading capacity
and the ability to hybridize with the complementary DNA. This discovery
not only offers a time- and cost-effective way to functionalize AuNP's
with a high density of nonthiolated DNA but also provides new insights
into the fundamental understanding of how DNA strands with different
sequences interact with AuNP's
Robust Hydrogels from Lanthanide Nucleotide Coordination with Evolving Nanostructures for a Highly Stable Protein Encapsulation
Metal coordination
with organic ligands often produce crystalline
metal–organic frameworks and sometimes amorphous nanoparticles.
In this work, we explore a different type of material from the same
chemistry: hydrogels. Lanthanides are chosen as the metal component
because of their important technological applications and continuously
tunable properties. Adenosine monophosphate (AMP) and lanthanides
form two types of coordination materials: the lighter lanthanides
from La<sup>3+</sup> to Tb<sup>3+</sup> form nanoparticles, whereas
the rest heavier ones initially form nanoparticles but later spontaneously
transform to hydrogels. This slow sol-to-gel transition is accompanied
by heat release, as indicated by isothermal titration calorimetry.
The transition is also accompanied by a morphology change from nanoparticles
to nanofibers, as indicated by transmission electron microscopy. These
gels are insensitive to ionic strength or temperature with excellent
stability. Gelation is unique to AMP because other nucleotides or
other adenine derivatives only yield nanoparticles or soluble products.
Entrapment of guest molecules such as glucose oxidase is also explored,
where the hydrogels allow a better enzyme activity and stability compared
to nanoparticles. Further applications of lanthanide coordinated hydrogels
might include biosensors, imaging agents, and drug delivery