5 research outputs found
Multimode Surface Functional Group Determination: Combining Steady-State and Time-Resolved Fluorescence with X‑ray Photoelectron Spectroscopy and Absorption Measurements for Absolute Quantification
The
quantitative determination of surface functional groups is
approached in a straightforward laboratory-based method with high
reliability. The application of a multimode BODIPY-type fluorescence,
photometry, and X-ray photoelectron spectroscopy (XPS) label allows
estimation of the labeling ratio, i.e., the ratio of functional groups
carrying a label after reaction, from the elemental ratios of nitrogen
and fluorine. The amount of label on the surface is quantified with
UV/vis spectrophotometry based on the molar absorption coefficient
as molecular property. The investigated surfaces with varying density
are prepared by codeposition of 3-(aminopropyl)Âtriethoxysilane (APTES)
and cyanoethyltriethoxysilane (CETES) from vapor. These surfaces show
high functional group densities that result in significant fluorescence
quenching of surface-bound labels. Since alternative quantification
of the label on the surface is available through XPS and photometry,
a novel method to quantitatively account for fluorescence quenching
based on fluorescence lifetime (Ï„) measurements is shown. Due
to the complex distribution of Ï„ on high-density surfaces, the
stretched exponential (or Kohlrausch) function is required to determine
representative mean lifetimes. The approach is extended to a commercial
Rhodamine B isothiocyanate (RITC) label, clearly revealing the problems
that arise from such charged labels used in conjunction with silane
surfaces
Role of Metal Cations in Plasmon-Catalyzed Oxidation: A Case Study of <i>p</i>‑Aminothiophenol Dimerization
The
mechanism of the plasmon-catalyzed reaction of <i>p</i>-aminothiophenol
(PATP) to 4,4′-dimercaptoazobenzene (DMAB)
on the surface of metal nanoparticles has been discussed using data
from surface-enhanced Raman scattering of DMAB. Oxides and hydroxides
formed in a plasmon-catalyzed process were proposed to play a central
role in the reaction. Here, we report DMAB formation on gold nanoparticles
occurring in the presence of the metal cations Ag<sup>+</sup>, Au<sup>3+</sup>, Pt<sup>4+</sup>, and Hg<sup>2+</sup>. The experiments were
carried out under conditions where formation of gold oxide or hydroxide
from the nanoparticles can be excluded and at high pH where the formation
of the corresponding oxidic species from the metal ions is favored.
On the basis of our results, we conclude that, under these conditions,
the selective oxidation of PATP to DMAB takes place via formation
of a metal oxide from the ionic species in a plasmon-catalyzed process.
By evidencing the necessity of the presence of the metal cations,
the reported results underpin the importance of metal oxides in the
reaction
Intermixed Terpyridine-Functionalized Monolayers on Gold: Nonlinear Relationship between Terpyridyl Density and Metal Ion Coordination Properties
Aiming at the functionalization of surfaces with terpyridine
anchors
for the coordinative deposition of additional layers, mixed self-assembled
monolayers (SAMs) were prepared from binary solutions of 12-(2,2′:6′,2″-terpyridine-4′-yl)Âdodecane-1-thiol
(TDT) and 1-decanethiol (DT). The SAMs and the order of the constituting
molecules were analyzed by X-ray photoelectron spectroscopy (XPS),
near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and
time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The composition
of the (TDT/DT)-SAMs and with it the surface density of terpyridyl
groups correlates linearly with the relative concentrations of the
two compounds in the solution used for depositing them. In marked
contrast, the amount of terpyridine-coordinated Pd<sup>II</sup> ions
significantly deviates from this trend with an optimum at a 1:3 ratio
of TDT/DT. This indicates a major fraction of the terpyridines in
TDT-rich SAMs not to be accessible for Pd<sup>II</sup> ion coordination.
In agreement, NEXAFS spectroscopy reveals the alkyl backbones in TDT-rich
SAMs not to be ordered, while they are preferentially upright oriented
in the optimal 1:3-(TDT/DT)-SAMs. We interpret this in terms of terpyridine
backfolding in TDT-rich SAMs, while they are located in accessible
positions on top of the SAM in the 1:3-(TDT/DT)-SAM. While the alkyl
backbones in the 1:3-(TDT/DT)-SAM are ordered, NEXAFS spectroscopy
shows the terpyridyl groups not to have a preferential orientation
in this SAM and thus retain enough flexibility to adjust to molecules
that are deposited on top of the mixed SAM. In conclusion, the novel
SAM does not undergo phase separation and consists predominantly of
intermixed phases with adjustable surface density of quite flexible
terpyridine anchor groups. The terpyridine–Pd<sup>II</sup> anchors
are not only available for a future deposition of the next layer,
but the metal ions also represent a sensitive probe for the accessibility
of the terpyridyl groups
Deposition of Ordered Layers of Tetralactam Macrocycles and Ether Rotaxanes on Pyridine-Terminated Self-Assembled Monolayers on Gold
The deposition of tetralactam macrocycles and the corresponding
benzyl ether rotaxanes on gold substrates is investigated for the
first time exploiting metallo-supramolecular chemistry. Two pyridine-terminated
self-assembled monolayers (SAMs) are developed that are used as well-ordered
template layers. The two SAMs differ with respect to the rigidity
of the terminal pyridines as shown by angle-resolved near-edge X-ray
absorption fine structure (NEXAFS) spectroscopy. The template layers
are then used for the metal-mediated self-assembly of macrocylces
and rotaxanes on solid supports. The SAM with the more rigid terminal
pyridine shows a higher coverage with the macrocycles and is therefore
preferable. Angle-resolved NEXAFS spectroscopy also shows the deposited
supramolecules to be oriented preferentially upright. This order is
only achieved for the macrocycles through the deposition on the more
rigid SAM template, whereas rotaxanes form oriented layers on both
SAMs. Time-of-flight secondary-ion mass spectrometry analysis was
used to determine the deposition time required for the self-assembly
process
Sequence-Programmable Multicomponent Multilayers of Nanometer-Sized Tetralactam Macrocycles on Gold Surfaces
Multicomponent multilayers have been
deposited on gold surfaces
by metal-ion-mediated layer-by-layer self-assembly of differently
functionalized tetralactam macrocycles. The layer stack can be programmed
with respect to the sequences of metal ions and macrocycles by the
deposition sequence