9 research outputs found
Rigid Single CarbonāCarbon Bond That Does Not Rotate in Water
Carbonācarbon bond is one
of the most ubiquitous molecular
building blocks for natural and man-made materials. Rotational isomerization
is fundamentally important for understanding the structure and reactivity
of chemical and biological molecules. Reported herein is the first
demonstration that a single CāC bond does not rotate in water.
The two distal CāS bonds in both 1,2-ethanedithiolate (<sup>ā</sup>SāCH<sub>2</sub>āCH<sub>2</sub>āS<sup>ā</sup>, 1,2-EDT<sup>2ā</sup>) and 2,3-butanedithiolate
(2,3-BuDT<sup>2ā</sup>) are exclusively in the <i>trans</i> conformer with reference to their respective center single CāC
bond. In contrast, both <i>trans</i> and <i>gauche</i> conformers are observed in neutral 1,2-ethanedithiol (1,2-EDT) and
2, 3-butanedithiol (2,3-BuDT). The insight from this work should be
important for understanding the charge effect on the molecular conformation
in aqueous solutions
Exposure to Bisphenol A, Bisphenol F, and Bisphenol S in U.S. Adults and Children: The National Health and Nutrition Examination Survey 2013ā2014
Bisphenol
F (BPF) and bisphenol S (BPS) are replacing bisphenol
A (BPA) in the manufacturing of products containing polycarbonates
and epoxy resins. Data on current human exposure levels of these substitutes
are needed to aid in the assessment of their human health risks. This
study analyzed urinary bisphenol levels in adults (<i>N</i> = 1808) and children (<i>N</i> = 868) participating in
the National Health and Nutrition Examination Survey (NHANES) 2013ā2014
and investigated demographic and lifestyle factors associated with
urinary levels of bisphenols. BPA, BPS, and BPF were detected in 95.7,
89.4, and 66.5% of randomly selected urine samples analyzed as part
of NHANES 2013ā2014, respectively. Median levels of BPA in
U.S. adult were higher (1.24 Ī¼g/L) than BPF and BPS levels (0.35
and 0.37 Ī¼g/L, respectively). For children, median BPA levels
were also higher (1.25 Ī¼g/L) than BPF and BPS levels (0.32 and
0.29 Ī¼g/L, respectively). The limits of detection for BPA, BPF,
and BPS were 0.2, 0.2, and 0.1 Ī¼g/L, respectively. Urinary levels
showed associations with gender, race/ethnicity, family income, physical
activity, smoking, and/or alcohol intake that depended on the specific
bisphenol. The results of this study indicate that exposure of the
general U.S. population to BPA substitutes is almost ubiquitous. Because
exposures differ across the U.S. population, further studies of environmental,
consumer, and lifestyle factors affecting BPF and BPS exposures are
warranted
Reactive Ag<sup>+</sup> Adsorption onto Gold
Proposed
mechanisms of monolayer silver formation on gold nanoparticle (AuNP)
include AuNP-facilitated under-potential reduction and antigalvanic
reduction in which the gold reduces Ag<sup>+</sup> into metallic atoms
Ag(0). Reported herein is the spontaneous reactive Ag<sup>+</sup> adsorption
onto gold substrates that include both as-obtained and butanethiol-functionalized
citrate- and NaBH<sub>4</sub>-reduced gold nanoparticles (AuNPs),
commercial high-purity gold foil, and gold film sputter-coated onto
silicon. The silver adsorption invariably leads to proton releasing
to the solution. The nominal saturation packing density of silver
on AuNPs varies from 2.8 Ā± 0.3 nmol/cm<sup>2</sup> for the AuNPs
preaggregated with KNO<sub>3</sub> to 4.3 Ā± 0.2 nmol/cm<sup>2</sup> for the AuNPs prefunctionalized with butanethiol (BuT). The apparent
Langmuir binding constant of the Ag<sup>+</sup> with the preaggregated
AuNPs and BuT-functionalized AuNPs are 4.0 Ć 10<sup>3</sup> M<sup>ā1</sup> and 2.1 Ć 10<sup>5</sup> M<sup>ā1</sup>, respectively. The silver adsorption has drastic effects on the
structure, conformation, and stability of the organothiols on the
AuNPs. It converts disordered BuT on AuNPs into highly ordered <i>trans</i> conformers, but induces near complete desorption of
sodium 2-mercaptoethanesulfonate and sodium 3-mercapto-1-propyl sulfonate
from AuNPs. Mechanically, the Ag<sup>+</sup> adsorption on AuNPs most
likely proceeds by reacting with molecules preadsorbed on the AuNP
surfaces or chemical species in the solutions, and the silver remains
as silver ion in these reaction products. This insight and methodology
presented in this work are important for studying interfacial interactions
of metallic species with gold and for postpreparation modulation of
the organothiol structure and conformation on AuNP surfaces
Comparative Study of the Self-Assembly of Gold and Silver Nanoparticles onto Thiophene Oil
Nanoparticle self-assembly is fundamentally
important for bottom-up
functional device fabrication. Currently, most nanoparticle self-assembly
has been achieved with gold nanoparticles (AuNPs) functionalized with
surfactants, polymeric materials, or cross-linkers. Reported herein
is a facile synthesis of gold and silver nanoparticle (AgNP) films
assembled onto thiophene oil by simply vortex mixing neat thiophene
with colloidal AuNPs or AgNPs for ā¼1 min. The AuNP film can
be made using every type of colloidal AuNPs we have explored, including
sodium borohydride-reduced AuNPs with a diameter of ā¼5 nm,
tannic acid-reduced AuNPs of ā¼10 nm diameter, and citrate-reduced
AuNPs with particle sizes of ā¼13 and ā¼30 nm diameter.
The AuNP film has excellent stability and it is extremely flexible.
It can be stretched, shrunken, and deformed accordingly by changing
the volume or shape of the enclosed thiophene oil. However, the AgNP
film is unstable, and it can be rapidly discolored and disintegrated
into small flakes that float on the thiophene surface. The AuNP and
AgNP films prepared in the glass vials can be readily transferred
to glass slides and metal substrates for surface-enhanced Raman spectral
acquisition
Comparative Study of the Self-Assembly of Gold and Silver Nanoparticles onto Thiophene Oil
Nanoparticle self-assembly is fundamentally
important for bottom-up
functional device fabrication. Currently, most nanoparticle self-assembly
has been achieved with gold nanoparticles (AuNPs) functionalized with
surfactants, polymeric materials, or cross-linkers. Reported herein
is a facile synthesis of gold and silver nanoparticle (AgNP) films
assembled onto thiophene oil by simply vortex mixing neat thiophene
with colloidal AuNPs or AgNPs for ā¼1 min. The AuNP film can
be made using every type of colloidal AuNPs we have explored, including
sodium borohydride-reduced AuNPs with a diameter of ā¼5 nm,
tannic acid-reduced AuNPs of ā¼10 nm diameter, and citrate-reduced
AuNPs with particle sizes of ā¼13 and ā¼30 nm diameter.
The AuNP film has excellent stability and it is extremely flexible.
It can be stretched, shrunken, and deformed accordingly by changing
the volume or shape of the enclosed thiophene oil. However, the AgNP
film is unstable, and it can be rapidly discolored and disintegrated
into small flakes that float on the thiophene surface. The AuNP and
AgNP films prepared in the glass vials can be readily transferred
to glass slides and metal substrates for surface-enhanced Raman spectral
acquisition
Studying the Effects of Cysteine Residues on Protein Interactions with Silver Nanoparticles
Studies of protein and organothiol
interactions with silver nanoparticles
(AgNPs) are important for understanding AgNP nanotoxicity, antimicrobial
activity, and material fabrications. Reported herein is a systematic
investigation of the effects of both reduced and oxidized protein
cysteine residues on protein interactions with AgNPs. The model proteins
included wild-type and mutated protein GB3 variants that contain 0,
1, or 2 reduced cysteine residues, respectively. Bovine serum albumin
(BSA) that contains a total of 34 oxidized (disulfide-linked) cysteine
residues and one reduced cysteine residue was also included. Protein
cysteine content has no detectable effect on the kinetics of protein/AgNP
binding. However, only proteins that contain reduced cysteine residues
induce significant AgNP dissolution. Proteins can slow down, but do
not prevent the AgNP dissolution induced by subsequently added organothiols.
The insights provided in this work are important to the mechanistic
understanding of AgNP stability in biofluids that are rich in proteins
and amino acid thiols
Ion Pairing as the Main Pathway for Reducing Electrostatic Repulsion among Organothiolate Self-assembled on Gold Nanoparticles in Water
Organothiol
binding to gold nanoparticles (AuNPs) in water proceeds
through a deprotonation pathway in which the sulfur-bound hydrogen
(RS-H) atoms are released to solution as protons and the organothiol
attach to AuNPs as negatively charged thiolate. The missing puzzle
pieces in this mechanism are (i) the significance of electrostatic
repulsion among the likely charged thiolates packed on AuNP surfaces,
and (ii) the pathways for the ligand binding system to cope with such
electrostatic repulsion. Presented herein are a series of experimental
and theoretical evidence that ion pairing, the coadsorption of negatively
charged thiolate and positively charged cations, is a main mechanism
for the system to reduce the electrostatic repulsion among the thiolate
self-assembled onto AuNP surfaces. This work represents a significant
step forward in the comprehensive understanding of organothiol binding
to AuNPs
Structures and Conformations of Alkanedithiols on Gold and Silver Nanoparticles in Water
Organodithiols with two distal thiols
have been used extensively
in gold and silver nanoparticle (AuNP and AgNP) applications. However,
understanding the structures and conformations of organodithiols on
these nanoparticles is challenging. Reported in this work is a combined
surface enhanced Raman spectroscopy (SERS), transmission electron
microscope (TEM), inductively coupled plasma mass-spectrometry (ICP-MS),
and localized surface plasmonic resonance (LSPR) study of alkyldithiol
(ADT, (HS-(CH<sub>2</sub>)<sub><i>n</i></sub>-SH, <i>n</i> = 2, 4, and 6) interactions with AuNPs and AgNPs in water.
These complementary techniques revealed a series of new insights that
would not be possible using individual methods. A large-fraction of
ADTs lies flat on AuNP surfaces. The upright ADTs are dimerized horizontally
through disulfide-bond, or remain as monothiolates on the AuNP surfaces.
The possibility of a significant amount of vertically disulfide-linked
organodithiol on the surface is excluded on the basis of ICP-MS and
AuNP LSPR experiments. ADTs induced significant AgNP disintegrations
in which ADTs are predominantly in dithiolate forms. This work highlights
the extraordinary complexity of organodithiol interactions with plasmonic
nanoparticles. The insights provided in this work will be important
for enhancing fundamental understanding of the structure and properties
of organothiol-functionalized AgNPs and AuNPs
Ligand Adsorption and Exchange on Pegylated Gold Nanoparticles
Previous
researchers proposed that thiolated polyĀ(ethylene glycol) (PEG-SH)
adopts a āmushroom-likeā conformation on gold nanoparticles
(AuNPs) in water. However, information regarding the size and permeability
of the PEG-SH mushroom caps and surface area passivated by the PEG-SH
mushroom stems are unavailable. Reported herein is our finding that
AuNPs that are covered by saturation packed PEG-SHs all have large
fractions of AuNP surface area available for ligand adsorption and
exchange. The model ligands adenine and 2-mercaptobenzimidazole (2-MBI)
can rapidly penetrate the PEG-SH overlayer and adsorb onto the AuNP
surface. Most of the ligand adsorption and exchange occurs within
the first minutes of the ligand addition. The fraction of AuNP surface
area passivated by saturation packed model PEG-SHs are ā¼25%,
ā¼20%, and ā¼9% for PEG-SHs with molecular weights of
2000, 5000, and 30ā000 g/mol, respectively. Localized surface
plasmonic resonance and dynamic light scattering show that the PEG-SH
overlayer is drastically more loosely packed than the protein bovine
serum albumin on AuNPs. Studies investigating the effect of aging
the AuNP/PEG-SH mixtures on subsequent adenine adsorption onto the
pegylated AuNPs revealed that PEG-SHs reach approximately a steady-state
binding on AuNPs within 3 h of sample incubation. This work sheds
new insights into the kinetics, structures, and conformations of PEG-SHs
on AuNPs and demonstrates that pegylated AuNPs can be used as an important
platform for studying ligand interaction with AuNPs. In addition,
it also opens a new avenue for fabrication of multicomponent functionalized
nanoparticles