Label-free nanofluidic scattering microscopy of size and mass of single diffusing molecules and nanoparticles.

Label-free characterization of single biomolecules aims to complement fluorescence microscopy in situations where labeling compromises data interpretation, is technically challenging or even impossible. However, existing methods require the investigated species to bind to a surface to be visible, thereby leaving a large fraction of analytes undetected. Here, we present nanofluidic scattering microscopy (NSM), which overcomes these limitations by enabling label-free, real-time imaging of single biomolecules diffusing inside a nanofluidic channel. NSM facilitates accurate determination of molecular weight from the measured optical contrast and of the hydrodynamic radius from the measured diffusivity, from which information about the conformational state can be inferred. Furthermore, we demonstrate its applicability to the analysis of a complex biofluid, using conditioned cell culture medium containing extracellular vesicles as an example. We foresee the application of NSM to monitor conformational changes, aggregation and interactions of single biomolecules, and to analyze single-cell secretomes.

Single Particle Nanoplasmonic Sensing in Individual Nanofluidic Channels.

Nanoplasmonics allows label-free optical sensing and spectroscopy at the single nanoparticle level by exploiting plasmonic excitations in metal nanoparticles. Nanofluidics offers exclusive possibilities for applying and controlling fluid flow and mass transport at the nanoscale and toward nanosized objects. Here, we combine these two concepts in a single device, by integrating single particle nanoplasmonic sensing with nanofluidics using advanced nanofabrication. The developed devices enable on-chip referenced parallel single particle nanoplasmonic sensing inside multiple individual nanofluidic channels with dimensions down to the 100 nm range. Beyond detailed discussion of the nanofabrication, general device characterization, and parallelized single particle plasmonic readout concepts, we demonstrate device function on two examples: (i) in situ measurements of local buffer concentrations inside a nanofluidic channel; (ii) real time binding kinetics of alkanethiol molecules to a single plasmonic nanonatenna sensor in a single nanochannel. Our concept thus provides a powerful solution for controlling mass transport to and from individual (plasmonic) nanoparticles, which in a long-term perspective offers unique opportunities for label-free detection of analyte molecules at low concentrations and for fundamental studies of fluids in extreme confinement.

The Role of Grain Boundary Sites for the Oxidation of Copper Catalysts during the CO Oxidation Reaction.

The oxidation of transition metal surfaces is a process that takes place readily at ambient conditions and that, depending on the specific catalytic reaction at hand, can either boost or hamper activity and selectivity. Cu catalysts are no exception in this respect since they exhibit different oxidation states for which contradicting activities have been reported, as, for example, in the catalytic oxidation of CO. Here, we investigate the impact of low-coordination sites on nanofabricated Cu nanoparticles with engineered grain boundaries on the oxidation of the Cu surface under CO oxidation reaction conditions. Combining multiplexed in situ single particle plasmonic nanoimaging, ex situ transmission electron microscopy imaging, and density functional theory calculations reveals a distinct dependence of particle oxidation rate on grain boundary density. Additionally, we found that the oxide predominantly nucleates at grain boundary-surface intersections, which leads to nonuniform oxide growth that suppresses Kirkendall-void formation. The oxide nucleation rate on Cu metal catalysts was revealed to be an interplay of surface coordination and CO oxidation behavior, with low coordination favoring Cu oxidation and high coordination favoring CO oxidation. These findings explain the observed single particle-specific onset of Cu oxidation as being the consequence of the individual particle grain structure and provide an explanation for widely distributed activity states of particles in catalyst bed ensembles.

Shedding Light on CO Oxidation Surface Chemistry on Single Pt Catalyst Nanoparticles Inside a Nanofluidic Model Pore.

Investigating a catalyst under relevant application conditions is experimentally challenging and parameters like reaction conditions in terms of temperature, pressure, and reactant mixing ratios, as well as catalyst design, may significantly impact the obtained experimental results. For Pt catalysts widely used for the oxidation of carbon monoxide, there is keen debate on the oxidation state of the surface at high temperatures and at/above atmospheric pressure, as well as on the most active surface state under these conditions. Here, we employ a nanoreactor in combination with single-particle plasmonic nanospectroscopy to investigate individual Pt catalyst nanoparticles localized inside a nanofluidic model pore during carbon monoxide oxidation at 2 bar in the 450-550 K temperature range. As a main finding, we demonstrate that our single-particle measurements effectively resolve a kinetic phase transition during the reaction and that each individual particle has a unique response. Based on spatially resolved measurements, we furthermore observe how reactant concentration gradients formed due to conversion inside the model pore give rise to position-dependent kinetic phase transitions of the individual particles. Finally, employing extensive electrodynamics simulations, we unravel the surface chemistry of the individual Pt nanoparticles as a function of reactant composition and find strongly temperature-dependent Pt-oxide formation and oxygen spillover to the SiO2 support as the main processes. These results therefore support the existence of a Pt surface oxide in the regime of high catalyst activity and demonstrate the possibility to use plasmonic nanospectroscopy in combination with nanofluidics as a tool for in situ studies of individual catalyst particles.

Copper catalysis at operando conditions-bridging the gap between single nanoparticle probing and catalyst-bed-averaging.

In catalysis, nanoparticles enable chemical transformations and their structural and chemical fingerprints control activity. To develop understanding of such fingerprints, methods studying catalysts at realistic conditions have proven instrumental. Normally, these methods either probe the catalyst bed with low spatial resolution, thereby averaging out single particle characteristics, or probe an extremely small fraction only, thereby effectively ignoring most of the catalyst. Here, we bridge the gap between these two extremes by introducing highly multiplexed single particle plasmonic nanoimaging of model catalyst beds comprising 1000 nanoparticles, which are integrated in a nanoreactor platform that enables online mass spectroscopy activity measurements. Using the example of CO oxidation over Cu, we reveal how highly local spatial variations in catalyst state dynamics are responsible for contradicting information about catalyst active phase found in the literature, and identify that both surface and bulk oxidation state of a Cu nanoparticle catalyst dynamically mediate its activity.

Operando detection of single nanoparticle activity dynamics inside a model pore catalyst material.

Nanoconfinement in porous catalysts may induce reactant concentration gradients inside the pores due to local conversion. This leads to inefficient active material use since parts of the catalyst may be trapped in an inactive state. Experimentally, these effects remain unstudied due to material complexity and required high spatial resolution. Here, we have nanofabricated quasi-two-dimensional mimics of porous catalysts, which combine the traits of nanofluidics with single particle plasmonics and online mass spectrometry readout. Enabled by single particle resolution at operando conditions during CO oxidation over a Cu model catalyst, we directly visualize reactant concentration gradient formation due to conversion on single Cu nanoparticles inside the "model pore" and how it dynamically controls oxidation state-and, thus, activity-of particles downstream. Our results provide a general framework for single particle catalysis in the gas phase and highlight the importance of single particle approaches for the understanding of complex catalyst materials.

Plasmonic Metasurface for Spatially Resolved Optical Sensing in Three Dimensions.

The highly localized sensitivity of metallic nanoparticles sustaining localized surface plasmon resonance (LSPR) enables detection of minute events occurring close to the particle surface and forms the basis for nanoplasmonic sensing. To date, nanoplasmonic sensors typically consist of two-dimensional (2D) nanoparticle arrays and can therefore only probe processes that occur within the array plane, leaving unaddressed the potential of sensing in three dimensions (3D). Here, we present a plasmonic metasurface comprising arrays of stacked Ag nanodisks separated by a thick SiO2 dielectric layer, which, through rational design, exhibit two distinct and spectrally separated LSPR sensing peaks and corresponding spatially separated sensing locations in the axial direction. This arrangement thus enables real-time plasmonic sensing in 3D. As a proof-of-principle, we successfully determine in a single experiment the layer-specific glass transition temperatures of a bilayer polymer thin film of poly(methyl methacrylate), PMMA, and poly(methyl methacrylate)/poly(methacrylic acid), P(MMA-MAA). Our work thus demonstrates a strategy for nanoplasmonic sensor design and utilization to simultaneously probe local chemical or physical processes at spatially different locations. In a wider perspective, it stimulates further development of sensors that employ multiple detection elements to generate distinct and spectrally individually addressable LSPR modes.

Resolving single Cu nanoparticle oxidation and Kirkendall void formation with in situ plasmonic nanospectroscopy and electrodynamic simulations.

Copper nanostructures are ubiquitous in microelectronics and heterogeneous catalysis and their oxidation is a topic of high current interest and broad relevance. It relates to important questions, such as catalyst active phase, activity and selectivity, as well as fatal failure of microelectronic devices. Despite the obvious importance of understanding the mechanism of Cu nanostructure oxidation, numerous open questions remain, including under what conditions homogeneous oxide layer growth occurs and when the nanoscale Kirkendall void forms. Experimentally, this is not trivial to investigate because when a large number of nanoparticles are simultaneously probed, ensemble averaging makes rigorous conclusions difficult. On the other hand, when (in situ) electron-microscopy approaches with single nanoparticle resolution are applied, concerns about beam effects that may both reduce the oxide or prevent oxidation via the deposition and cross-linking of carbonaceous species cannot be neglected. In response we present how single particle plasmonic nanospectroscopy can be used for the in situ real time characterization of multiple individual Cu nanoparticles during oxidation. Our analysis of their optical response combined with post mortem electron microscopy imaging and detailed Finite-Difference Time-Domain electrodynamics simulations enables in situ identification of the oxidation mechanism both in the initial oxide shell growth phase and during Kirkendall void formation, as well as the transition between them. In a wider perspective, this work presents the foundation for the application of single particle plasmonic nanospectroscopy in investigations of the impact of parameters like particle size, shape and grain structure with respect to defects and grain boundaries on the oxidation of metal nanoparticles.

Heterodimers for in Situ Plasmonic Spectroscopy: Cu Nanoparticle Oxidation Kinetics, Kirkendall Effect, and Compensation in the Arrhenius Parameters.

The ability to study oxidation, reduction, and other chemical transformations of nanoparticles in real time and under realistic conditions is a nontrivial task due to their small dimensions and the often challenging environment in terms of temperature and pressure. For scrutinizing oxidation of metal nanoparticles, visible light optical spectroscopy based on the plasmonic properties of the metal has been established as a suitable method. However, directly relying on the plasmonic resonance of metal nanoparticles as a built-in probe to track oxidation has a number of drawbacks, including the loss of optical contrast in the late oxidation stages. To address these intrinsic limitations, we present a plasmonic heterodimer-based nanospectroscopy approach, which enables continuous self-referencing by using polarized light to eliminate parasitic signals and provides large optical contrast all the way to complete oxidation. Using Au-Cu heterodimers and combining experiments with finite-difference time-domain simulations, we quantitatively analyze the oxidation kinetics of ca. 30 nm sized Cu nanoparticles up to complete oxidation. Taking the Kirkendall effect into account, we extract the corresponding apparent Arrhenius parameters at various extents of oxidation and find that they exhibit a significant compensation effect, implying that changes in the oxidation mechanism occur as oxidation progresses and the structure of the formed oxide evolves. In a wider perspective, our work promotes the use of model-system-type in situ optical plasmonic spectroscopy experiments in combination with electrodynamics simulations to quantitatively analyze and mechanistically interpret oxidation of metal nanoparticles and the corresponding kinetics in demanding chemical environments, such as in heterogeneous catalysis.

In Situ Plasmonic Nanospectroscopy of the CO Oxidation Reaction over Single Pt Nanoparticles.

The ongoing quest to develop single-particle methods for the in situ study of heterogeneous catalysts is driven by the fact that heterogeneity in terms of size, shape, grain structure, and composition is a general feature among nanoparticles in an ensemble. This heterogeneity hampers the generation of a deeper understanding for how these parameters affect catalytic properties. Here we present a solution that in a single benchtop experimental setup combines single-particle plasmonic nanospectroscopy with mass spectrometry for gas phase catalysis under reaction conditions at high temperature. We measure changes in the surface state of polycrystalline platinum model catalyst particles in the 70 nm size range and the corresponding bistable kinetics during the carbon monoxide oxidation reaction via the peak shift of the dark-field scattering spectrum of a closely adjacent plasmonic nanoantenna sensor and compare these changes with the total reaction rate measured by the mass spectrometer from an ensemble of nominally identical particles. We find that the reaction kinetics of simultaneously measured individual Pt model catalysts are dictated by the grain structure and that the superposition of the individual nanoparticle response can account for the significant broadening observed in the corresponding nanoparticle ensemble data. In a wider perspective our work enables in situ plasmonic nanospectroscopy in controlled gas environments at high temperature to investigate the role of the surface state on transition metal catalysts during reaction and of processes such as alloying or surface segregation in situ at the single-nanoparticle level for model catalysts in the few tens to hundreds of nanometer size range.