8 research outputs found
Quantitative Measurement of Ligand Exchange with Small-Molecule Ligands on Iron Oxide Nanoparticles via Radioanalytical Techniques
Ligand
exchange on the surface of hydrophobic iron oxide nanoparticles
is a common method for controlling surface chemistry for a desired
application. Furthermore, ligand exchange with small-molecule ligands
may be necessary to obtain particles with a specific size or functionality.
Understanding to what extent ligand exchange occurs and what factors
affect it is important for the optimization of this critical procedure.
However, quantifying the amount of exchange may be difficult because
of the limitations of commonly used characterization techniques. Therefore,
we utilized a radiotracer technique to track the exchange of a radiolabeled <sup>14</sup>C-oleic acid ligand with hydrophilic small-molecule ligands
on the surface of iron oxide nanoparticles. Iron oxide nanoparticles
functionalized with <sup>14</sup>C-oleic acid were modified with small-molecule
ligands with terminal functional groups including catechols, phosphonates,
sulfonates, thiols, carboxylic acids, and silanes. These moieties
were selected because they represent the most commonly used ligands
for this procedure. The effectiveness of these molecules was compared
using both procedures widely found in the literature and using a standardized
procedure. After ligand exchange, the nanoparticles were analyzed
using liquid scintillation counting (LSC) and inductively coupled
plasmaâmass spectrometry. The labeled and unlabeled particles
were further characterized by transmission electron microscopy (TEM)
and dynamic light scattering (DLS) to determine the particle size,
hydrodynamic diameter, and zeta potential. The unlabeled particles
were characterized via attenuated total reflectanceâFourier
transform infrared spectroscopy (ATRâFTIR) and vibrating sample
magnetometry (VSM) to confirm the presence of the small molecules
on the particles and verify the magnetic properties, respectively.
Radioanalytical determination of <sup>14</sup>C-oleic acid was used
to calculate the total amount of oleic acid remaining on the surface
of the particles after ligand exchange. The results revealed that
the ligand-exchange reactions performed using widely cited procedures
did not go to completion. Residual oleic acid remained on the particles
after these reactions and the reactions using a standardized protocol.
A comparison of the ligand-exchange procedures indicated that the
binding moiety, multidenticity, reaction time, temperature, and presence
of a catalyst impacted the extent of exchange. Quantification of the
oleic acid remaining after ligand exchange revealed a binding hierarchy
in which catechol-derived anchor groups displace the most oleic acid
on the surface of the nanoparticles and the thiol group displaces
the least amount of oleic acid. Thorough characterization of ligand
exchange is required to develop nanoparticles suitable for their intended
application
pH Triggered Recovery and Reuse of Thiolated Poly(acrylic acid) Functionalized Gold Nanoparticles with Applications in Colloidal Catalysis
Thiolated
polyÂ(acrylic acid) (PAA-SH) functionalized gold nanoparticles
were explored as a colloidal catalyst with potential application as
a recoverable catalyst where the PAA provides pH-responsive dispersibility
and phase transfer capability between aqueous and organic media. This
system demonstrates complete nanoparticle recovery and redispersion
over multiple reaction cycles without changes in nanoparticle morphology
or reduction in conversion. The catalytic activity (rate constant)
was reduced in subsequent reactions when recovery by aggregation was
employed, despite unobservable changes in morphology or dispersibility.
When colloidal catalyst recovery employed a pH induced phase transfer
between two immiscible solvents, the catalytic activity of the recovered
nanoparticles was unchanged over four cycles, maintaining the original
rate constant and 100% conversion. The ability to recover and reuse
colloidal catalysts by aggregation/redispersion and phase transfer
methods that occur at low and high pH, respectively, could be used
for different gold nanoparticle catalyzed reactions that occur at
different pH conditions
Influence of LigandâPrecursor Molar Ratio on the Size Evolution of Modifiable Iron Oxide Nanoparticles
During the synthesis of nanoparticles
via thermal decomposition
of iron precursors, the capping ligandâprecursor ratio influences
the resulting size of the iron oxide nanoparticles. As the molar ratio
of aliphatic amines to iron precursor is increased, the average diameter
of the synthesized iron oxide nanoparticles decreases. This trend
is opposite to previously reported results. We investigated this phenomenon
by independently varying the ligand chain length, the ligandâprecursor
molar ratio, and the degree of saturation of the aliphatic chain.
Nuclear magnetic resonance spectra of the precursor illustrated the
presence of a primary amine peak before heating and the peak absence
after heating, potentially indicating that the primary amine acts
as reducing agent to promote the decomposition of the iron precursor.
We hypothesize that the amine groups play a dominant role in the nucleation
of the particles, while the chain length and degree of aliphatic saturation
have only a minor effect on particle size. The nanoparticlesâ
size and crystallinity were characterized with high-resolution transmission
electron microscopy, dynamic light scattering, and X-ray diffraction,
and the magnetic properties were characterized by magnetometry
Quantitative Measurement of Ligand Exchange on Iron Oxides via Radiolabeled Oleic Acid
Ligand exchange of hydrophilic molecules
on the surface of hydrophobic
iron oxide nanoparticles produced via thermal decomposition of chelated
iron precursors is a common method for producing aqueous suspensions
of particles for biomedical applications. Despite the wide use, relatively
little is understood about the efficiency of ligand exchange on the
surface of iron oxide nanoparticles and how much of the hydrophobic
ligand is removed. To address this issue, we utilized a radiotracer
technique to track the exchange of a radiolabeled <sup>14</sup>C-oleic
acid ligand with hydrophilic ligands on the surface of magnetite nanoparticles.
Iron oxide nanoparticles functionalized with <sup>14</sup>C-oleic
acid were modified with polyÂ(ethylene glycol) with terminal functional
groups including, l-3,4-dihydroxyphenylalanine, a nitrated l-3,4-dihydroxyphenylalanine, carboxylic acid, a phosphonate,
and an amine. Following ligand exchange, the nanoparticles and byproducts
were analyzed using liquid scintillation counting and inductively
coupled plasma mass spectroscopy. The labeled and unlabeled particles
were further characterized by transmission electron microscopy and
dynamic light scattering to determine particle size, hydrodynamic
diameter, and zeta potential. The unlabeled particles were characterized
via thermogravimetric analysis and vibrating sample magnetometry.
Radioanalytical determination of the <sup>14</sup>C from <sup>14</sup>C-oleic acid was used to calculate the amount of oleic acid remaining
on the surface of the particles after purification and ligand exchange.
There was a significant loss of oleic acid on the surface of the particles
after ligand exchange with amounts varying for the different functional
binding groups on the polyÂ(ethylene glycol). Nonetheless, all samples
demonstrated some residual oleic acid associated with the particles.
Quantification of the oleic acid remaining after ligand exchange reveals
a binding hierarchy in which catechol derived anchor groups displace
oleic acid on the surface of the nanoparticles better than the phosphonate,
followed by the amine and carboxylic acid groups. Furthermore, the
results show that these ligand exchange reactions do not necessarily
occur to completion as is often assumed, thus leaving a residual amount
of oleic acid on the surface of the particles. A thorough analysis
of ligand exchange is required to develop nanoparticles that are suitable
for their desired application
Effect of Postsynthesis Purifications on Gold and Silver Nanoparticle Ligand Coverage
Wet chemical nanoparticle synthesis
is commonly employed because
of the ability to tailor and control nanoparticle size, shape, polydispersity,
and surface chemistry; however, excess ligands or free surfactants
in the colloidal dispersion can be detrimental for many nanoparticle
applications. Postsynthesis purification using antisolvent precipitation
is a widely employed method to remove the excess ligands or precursors;
however, there has been little in-depth fundamental investigation
of the dynamics between the nanoparticle, ligand, and dispersing media
as well as the morphology and fate of the nanoparticle and ligands
during nanoparticle processing. In this paper, we investigate the
changes in ligand surface coverage for dodecanethiol-stabilized gold
and silver nanoparticles in response to repetitive antisolvent precipitation
and redispersion using gas chromatography, thermogravimetric analysis,
and small-angle neutron scattering. These techniques were each used
to determine percent surface coverage and equilibrium ligand partitioning
between the nanoparticle surface and bulk solution, which was then
modeled with the Langmuir isotherm to determine the binding free energy.
The binding free energy for dodecanethiol on 4.2 nm diameter gold
nanoparticles was found to be â23 kJ/mol by gas chromatography
(GC) and â34 kJ/mol by small-angle neutron scattering (SANS).
The binding free energy for dodecanethiol on 7.7 nm diameter silver
nanoparticles was found to be â21 kJ/mol by TGA and â29
kJ/mol by SANS. While these numbers demonstrate variability based
on the method, they are comparable to literature values. Other notable
results from this work demonstrate the optimization of the purification
process and avoidance of using excess antisolvent which can lead to
coprecipitation of the excess ligand with the nanoparticles, hindering
the purification. Finally, multiple techniques for determining ligand
binding free energy are demonstrated as well as evaluation of the
pros and cons of each method
Iron-Loaded Magnetic Nanocapsules for pH-Triggered Drug Release and MRI Imaging
Magnetic
nanocapsules were synthesized for controlled drug release,
magnetically assisted delivery, and MRI imaging. These magnetic nanocapsules,
consisting of a stable iron nanocore and a mesoporous silica shell,
were synthesized by controlled encapsulation of ellipsoidal hematite
in silica, partial etching of the hematite core in acid, and reduction
of the core by hydrogen. The iron core provided a high saturation
magnetization and was stable against oxidation for at least 6 months
in air and 1 month in aqueous solution. The hollow space between the
iron core and mesoporous silica shell was used to load anticancer
drug and a T<sub>1</sub>-weighted MRI contrast agent (Gd-DTPA). These
multifunctional monodispersed magnetic ânanoeyesâ were
coated by multiple polyelectrolyte layers of biocompatible poly-l-lysine and sodium alginate to control the drug release as
a function of pH. We studied pH-controlled release, magnetic hysteresis
curves, and T<sub>1</sub>/T<sub>2</sub> MRI contrast of the magnetic
nanoeyes. They also served as MRI contrast agents with relaxivities
of 8.6 mM<sup>â1</sup> s<sup>â1</sup> (<i>r</i><sub>1</sub>) and 285 mM<sup>â1</sup> s<sup>â1</sup> (<i>r</i><sub>2</sub>)
Monitoring pH-Triggered Drug Release from Radioluminescent Nanocapsules with Xâray Excited Optical Luminescence
One of the greatest challenges in cancer therapy is to develop methods to deliver chemotherapy agents to tumor cells while reducing systemic toxicity to noncancerous cells. A promising approach to localizing drug release is to employ drug-loaded nanoparticles with coatings that release the drugs only in the presence of specific triggers found in the target cells such as pH, enzymes, or light. However, many parameters affect the nanoparticle distribution and drug release rate, and it is difficult to quantify drug release <i>in situ</i>. In this work, we show proof-of-principle for a âsmartâ radioluminescent nanocapsule with an X-ray excited optical luminescence (XEOL) spectrum that changes during release of the optically absorbing chemotherapy drug, doxorubicin. XEOL provides an almost background-free luminescent signal for measuring drug release from particles irradiated by a narrow X-ray beam. We study <i>in vitro</i> pH-triggered release rates of doxorubicin from nanocapsules coated with a pH-responsive polyelectrolyte multilayer using HPLC and XEOL spectroscopy. The doxorubicin was loaded to over 5% by weight and released from the capsule with a time constant <i>in vitro</i> of âŒ36 days at pH 7.4 and 21 h at pH 5.0, respectively. The Gd<sub>2</sub>O<sub>2</sub>S:Eu nanocapsules are also paramagnetic at room temperature with similar magnetic susceptibility and similarly good MRI <i>T</i><sub>2</sub> relaxivities to Gd<sub>2</sub>O<sub>3</sub>, but the sulfur increases the radioluminescence intensity and shifts the spectrum. Empty nanocapsules did not affect cell viability up to concentrations of at least 250 ÎŒg/mL. These empty nanocapsules accumulated in a mouse liver and spleen following tail vein injection and could be observed <i>in vivo</i> using XEOL. The particles are synthesized with a versatile template synthesis technique which allows for control of particle size and shape. The XEOL analysis technique opens the door to noninvasive quantification of drug release as a function of nanoparticle size, shape, surface chemistry, and tissue type
Catalytic Reaction Triggered by Magnetic Induction Heating Mechanistically Distinguishes Itself from the Standard Thermal Reaction
As a recent advancement in reaction
engineering, magnetic
induction
heating (MIH) is utilized to initiate the intended reactions by enabling
the self-heating of the ferromagnetic catalyst particles. While MIH
can be energy-efficient and industrially scalable, its full potential
has been underappreciated in catalysis because of the perception that
MIH is merely an alternative heating approach. Unexpectedly, we show
that the MIH-triggered reaction could go beyond standard thermal catalysis.
Specifically, by probing the representative Pt/Fe3O4 catalysts with CO oxidation in both thermal and MIH modes
with consistent temperature profiles and catalyst structures, we found
that the MIH mode boosts the reactivity more than 25 times by modifying
PtâFeOx interfacial synergies and
promoting facile oxidation of the adsorbed carbonyl species by atomic
oxygen. As we preliminarily observed, this beneficial MIH catalysis
can be translational to other thermal reactions, potentially paving
the way to launch MIH catalysis as a distinct reaction category