Plasmon-driven chemical transformation of a secondary amide probed by surface enhanced Raman scattering.

Plasmon-driven chemical conversion is gaining burgeoning interest in the field of heterogeneous catalysis. Herein, we study the reactivity of N-methyl-4-sulfanylbenzamide (NMSB) at nanocavities of gold and silver nanoparticle aggregates under plasmonic excitation to gain understanding of the respective reaction mechanism. NMSB is a secondary amide, which is a frequent binding motive found in peptides and a common coupling product of organic molecules and biomolecules. Surface-enhanced Raman scattering (SERS) is used as a two-in-one in-situ spectroscopic tool to initiate the molecular transformation process and simultaneously monitor and analyze the reaction products. Supported by dissociative electron attachment (DEA) studies with the gas phase molecule, a hot electron-mediated conversion of NMSB to p-mercaptobenzamide and p-mercaptobenzonitrile is proposed at the plasmonic nanocavities. The reaction rate showed negligible dependence on the external temperature, ruling out the dominant role of heat in the chemical transformation at the plasmonic interface. This is reflected in the absence of a superlinear relationship between the reaction rate constant and the laser power density, and DEA and SERS studies indicate a hot-electron mediated pathway. We conclude that the overall reaction rate is limited by the availability of energetic hot electrons to the NMSB molecule.

Single-Molecule Surface-Enhanced Raman Scattering Measurements Enabled by Plasmonic DNA Origami Nanoantennas.

Surface-enhanced Raman scattering (SERS) has the capability to detect single molecules for which high field enhancement is required. Single-molecule (SM) SERS is able to provide molecule-specific spectroscopic information about individual molecules and therefore yields more detailed chemical information than other SM detection techniques. At the same time, there is the potential to unravel information from SM measurements that remain hidden in Raman measurements of bulk material. This protocol outlines the SM SERS measurements using a DNA origami nanoantenna (DONA) in combination with atomic force microscopy (AFM) and Raman spectroscopy. A DNA origami fork structure and two gold nanoparticles are combined to form the DONAs, with a 1.2-2.0 nm gap between them. This allows an up to 1011-fold SERS signal enhancement, enabling measurements of single molecules. The protocol further demonstrates the placement of a single analyte molecule in a SERS hot spot, the process of AFM imaging, and the subsequent overlaying of Raman imaging to measure an analyte in a single DONA.

Influence of Different Salts on the G-Quadruplex Structure Formed from the Reversed Human Telomeric DNA Sequence.

G-rich telomeric DNA plays a major role in the stabilization of chromosomes and can fold into a plethora of different G-quadruplex structures in the presence of mono- and divalent cations. The reversed human telomeric DNA sequence (5'-(GGG ATT)4; RevHumTel) was previously shown to have interesting properties that can be exploited for chemical sensing and as a chemical switch in DNA nanotechnology. Here, we analyze the specific G-quadruplex structures formed by RevHumTel in the presence of K+, Na+, Mg2+ and Ca2+ cations using circular dichroism spectroscopy (CDS) and Förster resonance energy transfer (FRET) based on fluorescence lifetimes. CDS is able to reveal strand and loop orientations, whereas FRET gives information about the distances between the 5'-end and the 3'-end, and also, the number of G-quadruplex species formed. Based on this combined information we derived specific G-quadruplex structures formed from RevHumTel, i.e., a chair-type and a hybrid-type G-quadruplex structure formed in presence of K+, whereas Na+ induces the formation of up to three different G-quadruplexes (a basket-type, a propeller-type and a hybrid-type structure). In the presence of Mg2+ and Ca2+ two different parallel G-quadruplexes are formed (one of which is a propeller-type structure). This study will support the fundamental understanding of the G-quadruplex formation in different environments and a rational design of G-quadruplex-based applications in sensing and nanotechnology.

Molecular states and spin crossover of hemin studied by DNA origami enabled single-molecule surface-enhanced Raman scattering.

The study of biologically relevant molecules and their interaction with external stimuli on a single molecular scale is of high importance due to the availability of distributed rather than averaged information. Surface enhanced Raman scattering (SERS) provides direct chemical information, but is rather challenging on the single molecule (SM) level, where it is often assumed to require a direct contact of analyte molecules with the metal surface. Here, we detect and investigate the molecular states of single hemin by SM-SERS. A DNA aptamer based G-quadruplex mediated recognition of hemin directs its placement in the SERS hot-spot of a DNA Origami Nanofork Antenna (DONA). The configuration of the DONA structure allows the molecule to be trapped at the plasmonic hot-spot preferentially in no-contact configuration with the metal surface. Owing to high field enhancement at the plasmonic hot spot, the detection of a single folded G-quadruplex becomes possible. For the first time, we present a systematic study by SM-SERS where most hemin molecule adopt a high spin and oxidation state (III) that showed state crossover to low spin upon strong-field-ligand binding. The present study therefore, provides a platform for studying biologically relevant molecules and their properties at SM sensitivity along with demonstrating a conceptual advancement towards successful monitoring of single molecular chemical interaction using DNA aptamers.

Plasmonic reactivity of halogen thiophenols on gold nanoparticles studied by SERS and XPS.

Localized surface plasmon resonances on noble metal nanoparticles (NPs) can efficiently drive reactions of adsorbed ligand molecules and provide versatile opportunities in chemical synthesis. The driving forces of these reactions are typically elevated temperatures, hot charge carriers, or enhanced electric fields. In the present work, dehalogenation of halogenated thiophenols on the surface of AuNPs has been studied by surface enhanced Raman scattering (SERS) as a function of the photon energy to track the kinetics and identify reaction products. Reaction rates are found to be surprisingly similar for different halothiophenols studied here, although the bond dissociation energies of the C-X bonds differ significantly. Complementary information about the electronic properties at the AuNP surface, namely, work-function and valence band states, has been determined by x-ray photoelectron spectroscopy of isolated AuNPs in the gas-phase. In this way, it is revealed how the electronic properties are altered by the adsorption of the ligand molecules, and we conclude that the reaction rates are mainly determined by the plasmonic properties of the AuNPs. SERS spectra reveal differences in the reaction product formation for different halogen species, and, on this basis, the possible reaction mechanisms are discussed to approach an understanding of opportunities and limitations in the design of catalytical systems with plasmonic NPs.

Spatial Separation of Plasmonic Hot-Electron Generation and a Hydrodehalogenation Reaction Center Using a DNA Wire.

Using hot charge carriers far from a plasmonic nanoparticle surface is very attractive for many applications in catalysis and nanomedicine and will lead to a better understanding of plasmon-induced processes, such as hot-charge-carrier- or heat-driven chemical reactions. Herein we show that DNA is able to transfer hot electrons generated by a silver nanoparticle over several nanometers to drive a chemical reaction in a molecule nonadsorbed on the surface. For this we use 8-bromo-adenosine introduced in different positions within a double-stranded DNA oligonucleotide. The DNA is also used to assemble the nanoparticles into nanoparticles ensembles enabling the use of surface-enhanced Raman scattering to track the decomposition reaction. To prove the DNA-mediated transfer, the probe molecule was insulated from the source of charge carriers, which hindered the reaction. The results indicate that DNA can be used to study the transfer of hot electrons and the mechanisms of advanced plasmonic catalysts.

Kinetics and Mechanism of Plasmon-Driven Dehalogenation Reaction of Brominated Purine Nucleobases on Ag and Au.

Plasmon-driven photocatalysis is an emerging and promising application of noble metal nanoparticles (NPs). An understanding of the fundamental aspects of plasmon interaction with molecules and factors controlling their reaction rate in a heterogeneous system is of high importance. Therefore, the dehalogenation kinetics of 8-bromoguanine (BrGua) and 8-bromoadenine (BrAde) on aggregated surfaces of silver (Ag) and gold (Au) NPs have been studied to understand the reaction kinetics and the underlying reaction mechanism prevalent in heterogeneous reaction systems induced by plasmons monitored by surface enhanced Raman scattering (SERS). We conclude that the time-average constant concentration of hot electrons and the time scale of dissociation of transient negative ions (TNI) are crucial in defining the reaction rate law based on a proposed kinetic model. An overall higher reaction rate of dehalogenation is observed on Ag compared with Au, which is explained by the favorable hot-hole scavenging by the reaction product and the byproduct. We therefore arrive at the conclusion that insufficient hole deactivation could retard the reaction rate significantly, marking itself as rate-determining step for the overall reaction. The wavelength dependency of the reaction rate normalized to absorbed optical power indicates the nonthermal nature of the plasmon-driven reaction. The study therefore lays a general approach toward understanding the kinetics and reaction mechanism of a plasmon-driven reaction in a heterogeneous system, and furthermore, it leads to a better understanding of the reactivity of brominated purine derivatives on Ag and Au, which could in the future be exploited, for example, in plasmon-assisted cancer therapy.

A Versatile DNA Origami-Based Plasmonic Nanoantenna for Label-Free Single-Molecule Surface-Enhanced Raman Spectroscopy.

DNA origami technology allows for the precise nanoscale assembly of chemical entities that give rise to sophisticated functional materials. We have created a versatile DNA origami nanofork antenna (DONA) by assembling Au or Ag nanoparticle dimers with different gap sizes down to 1.17 nm, enabling signal enhancements in surface-enhanced Raman scattering (SERS) of up to 1011. This allows for single-molecule SERS measurements, which can even be performed with larger gap sizes to accommodate differently sized molecules, at various excitation wavelengths. A general scheme is presented to place single analyte molecules into the SERS hot spots using the DNA origami structure exploiting covalent and noncovalent coupling schemes. By using Au and Ag dimers, single-molecule SERS measurements of three dyes and cytochrome c and horseradish peroxidase proteins are demonstrated even under nonresonant excitation conditions, thus providing long photostability during time-series measurement and enabling optical monitoring of single molecules.

Probing Cancer Cells through Intracellular Aggregation-Induced Emission Kinetic Rate of Copper Nanoclusters.

pH-responsive luminescent copper nanoclusters (Cu NCs) with aggregation-induced emission (AIE) characteristics have been synthesized. Upon internalization into living cells, the NCs displayed a cellular pH environment-dependent luminescence change with orange-red emission at pHi 4.5, whereas bright green emission was observed over time at pHi 7.4, through their AIE attributes. Furthermore, the intracellular AIE kinetics of the NC probe was measured in MCF-7 cells and compared to that of HEK-293 cells. Intriguingly, the intracellular rate constant value derived for AIE kinetics in MCF-7 cells was found to be 3-fold higher than that in HEK-293 cell lines, whereas the value was 2-fold higher than that observed in aqueous medium. This provided a new platform to study different cell lines based on intracellular AIE in living cells, with additional potential for future applications in cellular imaging, diagnostics, and disease detection.

Influence of Nb on the Microstructure and Fracture Toughness of (Zr0.76Fe0.24)100-xNbx Nano-Eutectic Composites.

The present study demonstrates the evolution of eutectic microstructure in arc-melted (Zr0.76Fe0.24)100-xNbx (0 ≤ x ≤ 10 atom %) composites containing α-Zr//FeZr2 nano-lamellae phases along with pro-eutectic Zr-rich intermetallic phase. The effects of Nb addition on the microstructural evolution and mechanical properties under compression, bulk hardness, elastic modulus, and indentation fracture toughness (IFT) were investigated. The Zr-Fe-(Nb) eutectic composites (ECs) exhibited excellent fracture strength up to ~1800 MPa. Microstructural characterization revealed that the addition of Nb promotes the formation of intermetallic Zr54Fe37Nb9. The IFT (KIC) increases from 3.0 ± 0.5 MPa√m (x = 0) to 4.7 ± 1.0 MPa√m (x = 2) at 49 N, which even further increases from 5.1 ± 0.5 MPa√m (x = 0) and up to 5.9 ± 1.0 MPa√m (x = 2) at higher loads. The results suggest that mutual interaction between nano-lamellar α-Zr//FeZr2 phases is responsible for enhanced fracture resistance and high fracture strength.

Transferrin-Copper Nanocluster-Doxorubicin Nanoparticles as Targeted Theranostic Cancer Nanodrug.

Transferrin (Tf)-templated luminescent blue copper nanoclusters (Tf-Cu NCs) are synthesized. They are further formulated into spherical Tf-Cu NC-doxorubicin nanoparticles (Tf-Cu NC-Dox NPs) based on electrostatic interaction with doxorubicin (Dox). The as-synthesized Tf-Cu NC-Dox NPs are explored for bioimaging and targeted drug delivery to delineate high therapeutic efficacy. Förster resonance energy transfer (FRET) within the Tf-Cu NC-Dox NPs exhibited striking red luminescence, wherein the blue luminescence of Tf-Cu NCs (donor) is quenched due to absorption by Dox (acceptor). Interestingly, blue luminescence of Tf-Cu NCs is restored in the cytoplasm of cancer cells upon internalization of the NPs through overexpressed transferrin receptor (TfR) present on the cell surface. Finally, gradual release of Dox from the NPs leads to the generation of its red luminescence inside the nucleus. The biocompatible Tf-Cu NC-Dox NPs displayed superior targeting efficiency on TfR overexpressed cells (HeLa and MCF-7) as compared to the cells expressing less TfR (HEK-293 and 3T3-L1). Combination index (CI) revealed synergistic activity of Tf-Cu NCs and Dox in Tf-Cu NC-Dox NPs. In vivo assessment of the NPs on TfR positive Daltons lymphoma ascites (DLA) bearing mice revealed significant inhibition of tumor growth rendering prolonged survival of the mice.

Biomimetically crystallized protease resistant zinc phosphate decorated with gold atomic clusters for bioimaging.

A luminescent probe synthesised via biomimetic crystallization of zinc phosphate in the presence of protein fragment stabilised gold (Au) nanoclusters is reported. The engineered probe - with Au nanoclusters assembled on the crystal - was protease resistant and offered efficient bioimaging and uptake studies.