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Self-assembled monolayers (SAMs) on plasmonic substrates play a significant role applications of surface-enhanced Raman scattering (SERS). At the same time, localized surface plasmon resonances (LSPRs) can be employed for a broad range of plasmon-supported chemical modifications. Here, micropatterning using the derivatization of SAMs on gold nanosubstrates for rewritable SERS-based security labels or as the basis for sensing arrays functionalized with biomolecules is demonstrated using different plasmon-catalyzed reactions. The formation of 4,4′-dimercaptoazobenzene (DMAB) from p-aminothiophenol (PATP) as well as from p-nitrothiophenol (PNTP) and the reduction of PNTP to PATP are used to change the functionality of the substrate in specified positions. Employing LSPR, the reactions are started by illumination using visible laser light at a high intensity in a focal spot of a microscope objective and yield microscopic patterns of the reaction product. The obtained molecular patterns can be erased by other reactions, enabling different strategies for rewriting, encryption, or stepwise functionalization.
The discussion of the surface-enhanced Raman scattering (SERS) spectra of p-aminothiophenol (PATP) and of ist photocatalytic reaction product 4,4′-dimercaptoazobenzene (DMAB) is important for understanding plasmon-supported spectroscopy and catalysis. Here, SERS spectra indicate that DMAB forms also in a nonphotocatalytic reaction on silver nanoparticles. Spectra measured at low pH, in the presence of the acids HCl, H2SO4, HNO3, and H3PO4, show that DMAB is reduced to PATP when both protons and chloride ions are present. Moreover, the successful reduction of DMAB in the presence of other, halide and nonhalide, ligands suggests a central role of these species in the reduction. As discussed, the ligands increase the efficiency of hot-electron harvesting. The pH-associated reversibility of the SERS spectrum of PATP is established as an Observation of the DMAB dimer at high pH and of PATP as a product of its hot-electron reduction at low pH, in the presence of the appropriate ligand.
Plant tissues are complex composite structures of organic and inorganic components whose function relies on molecular heterogeneity at the nanometer scale. Scattering-type near-field optical microscopy (s-SNOM) in the mid-infrared (IR) region is used here to collect IR nanospectra from both fixed and native plant samples. We compared structures of chemically extracted silica bodies (phytoliths) to silicified and nonsilicified cell walls prepared as a flat block of epoxy-embedded awns of wheat (Triticum turgidum), thin sections of native epidermis cells from sorghum (Sorghum bicolor) comprising silica phytoliths, and isolated cells from awns of oats (Avena sterilis). The correlation of the scanning-probe IR images and the mechanical phase image enables a combined probing of mechanical material properties together with the chemical composition and structure of both the cell walls and the phytolith structures. The data reveal a structural heterogeneity of the different silica bodies in situ, as well as different compositions and crystallinities of cell wall components. In conclusion, IR nanospectroscopy is suggested as an ideal tool for studies of native plant materials of varied origins and preparations and could be applied to other inorganic–organic hybrid materials.
Hot carriers (electrons and holes) generated from the decay of localized surface plasmon resonances can take a major role in catalytic reactions on metal nanoparticles. By obtaining surface enhanced Raman scattering (SERS) spectra of p-aminothiophenol as product of the reduction of p-nitrothiophenol by hot electrons, different catalytic activity is revealed here for nanoparticles of silver, gold, and copper. As a main finding, a series of different ligands, comprising halide and non-halide species, are found to enhance product formation in the reduction reaction on nanoparticles of all three metals. A comparison with the standard electrode potentials of the metals with and without the ligands and SERS data obtained at different electrode potential indicate that the higher catalytic activity can be associated with a higher Fermi level, thereby resulting in an improved efficiency of hot carrier generation. The concept of such a ligand-enhanced hot electron reduction provides a way to make light-to-chemical energy conversion more efficient due to improved electron harvesting.
The spectroscopic characterization by surface-enhanced Raman scattering (SERS) has shown great potential in studies of heterogeneous catalysis. We describe a plug-in multifunctional optofluidic platform that can be tailored to serve both as a variable catalyst material and for sensitive optical characterization of the respective reactions using SERS in microfluidic systems. The platform enables the characterization of reactions under a controlled gas atmosphere and does not present with limitations due to nanoparticle adsorption or memory effects. Spectra of the gold-catalyzed reduction of p-nitrothiophenol by sodium borohydride using the plug-in probe provide evidence that the borohydride is the direct source of hydrogen on the gold surface, and that a radical anion is formed as an intermediate. The in situ monitoring of the photoinduced dimerization of p-aminothiophenol indicates that the activation of oxygen is essential for the plasmon-catalyzed oxidation on gold nanoparticles and strongly supports the central role of metal oxide species.
Multivariate analysis of MALDI imaging mass spectrometry data of mixtures of single pollen grains
(2018)
Mixtures of pollen grains of three different species (Corylus avellana, Alnus cordata, and Pinus sylvestris) were investigated by matrixassisted laser desorption/ionization time-of-flight imaging mass spectrometry (MALDI-TOF imaging MS). The amount of pollen grains was reduced stepwise from > 10 to single pollen grains. For sample pretreatment, we modified a previously applied approach, where any additional extraction steps were omitted.
Our results show that characteristic pollen MALDI mass spectra can be obtained from a single pollen grain, which is the prerequisite for a reliable pollen classification in practical applications. MALDI imaging of laterally resolved pollen grains provides additional information by reducing the complexity of the MS spectra of mixtures, where frequently peak discrimination is observed. Combined with multivariate statistical analyses, such as principal component analysis (PCA), our approach offers the chance for a fast and reliable identification of individual pollen grains by mass spectrometry.
This study demonstrates the bottom-up synthesis of
silver nanolenses.Arobust coating protocol enabled the
functionalization of differently sized silver nanoparticles with
DNAsingle strands of orthogonal sequence.Coated particles
10 nm, 20 nm, and 60 nm in diameter were self-assembled by
DNAorigami scaffolds to form silver nanolenses.Single
molecules of the protein streptavidin were selectively placed in
the gap of highest electric field enhancement. Streptavidin
labelled with alkyne groups served as model analyte in surface-
enhanced Raman scattering (SERS) experiments.Bycorre-
lated Raman mapping and atomic force microscopy, SERS
signals of the alkyne labels of asingle streptavidin molecule,
from asingle silver nanolens,were detected. The discrete,self-
similar aggregates of solid silver nanoparticles are promising
for plasmonic applications.
Diese Studie demonstriert die Bottom-up-Synthese
von Silbernanolinsen. Ein robustes Beschichtungsprotokoll
ermçglichte die Funktionalisierung unterschiedlich großer
Silbernanopartikel mit einzelstr-ngiger DNAunterschiedlicher
Sequenz. Derartig beschichtete Partikel mit Durchmessern von
10 nm, 20 nm und 60 nm wurden mithilfe von DNA-Origami-
Gergsten zu Silbernanolinsen angeordnet. Ein einzelnes Mo-
lekgldes Proteins Streptavidin ist in demjenigen Spalt zwi-
schenden Partikeln immobilisiert worden, der die hçchste
Feldverst-rkung zur Verfggung stellt. Streptavidin war dabei
mit Alkin-Gruppen modifiziert und fungierte als Modellanalyt
in Experimenten zur oberfl-chenverst-rkten Raman-Streuung
(SERS). Korrelierte Raman- und Rasterkraftmikroskopie-
Messungen ermçglichten die Aufnahme von SERS-Signalen
der Alkingruppen eines einzelnen Streptavidins,von einer
einzelnen Silbernanolinse.Diese diskreten, selbst-hnlichen
Silbernanopartikelaggregate versprechen vielf-ltige Anwen-
dungen im Feld der Plasmonik
The characterization of a catalyst often occurs by averaging over large areas of the catalyst material. On the other hand, optical probing is easily achieved at a resolution at the micrometer scale, specifically in microspectroscopy. Here, using surface-enhanced Raman scattering (SERS) mapping of larger areas with micrometer-sized spots that contain tens to hundreds of supported gold nanoparticles each, the photoinduced dimerization of p-aminothiophenol (PATP) to 4,4′-dimercaptoazobenzene (DMAB) was monitored. The mapping data reveal an inhomogeneous distribution of catalytic activity in the plasmon-catalyzed reaction in spite of a very homogeneous plasmonic enhancement of the optical signals in SERS. The results lead to the conclusion that only a fraction of the nanostructures may be responsible for a high catalytic activity. The high spot-to-spot variation in catalytic activity is also demonstrated for DMAB formation by the plasmon-catalyzed reduction from p-nitrothiophenol (PNTP) and confirms that an improvement of the accuracy and reproducibility in the characterization of catalytic reactions can be achieved by microspectroscopic probing of many positions. Using SERS micromapping during the incubation of PATP, we demonstrate that the reaction occurs during the incubation process and is influenced by different parameters, leading to the conclusion of dimerization in a gold-catalyzed, nonphotochemical reaction as an alternative to the plasmon-catalyzed process. The results have implications for the future characterization of new catalyst materials as well as for optical sensing using plasmonic materials.
Role of metal cations in plasmon-catalyzed oxidation: A case study of p-aminothiophenol dimerization
(2017)
The mechanism of the plasmon-catalyzed reaction of p-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+, Au3+, Pt4+, and Hg2+. 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.