Single-particle Studies Research Articles

Accurate representations of the physicochemical properties of atmospheric aerosols: when are laboratory measurements of value?

Laboratory studies can provide important insights into the processes that occur at the scale of individual particles in ambient aerosol. We examine the accuracies of measurements of core physicochemical properties of aerosols that can be made in single particle studies and explore the impact of these properties on the microscopic processes that occur in ambient aerosol. Presenting new measurements, we examine here the refinements in our understanding of aerosol hygroscopicity, surface tension, viscosity and optical properties that can be gained from detailed laboratory measurements for complex mixtures through to surrogates for secondary organic atmospheric aerosols.

Metal Adsorbate-Induced Plasmon Damping in Gold Nanorods: The Difference Between Metals

We presented a single particle study on the metal adsorbate-induced plasmon damping in Au nanorods (AuNRs) through adsorbing clusters of different metals including Pt, Au and Ag. AuNRs with different longitudinal surface plasmon resonance (LSPR) wavelength were measured and investigated individually. Linewidth broadening, plasmon shift and reduction of plasmonic resonance of single AuNRs were studied and compared between Pt, Au and Ag adsorbates. The measured linewidths perfectly match the theoretical predictions of the billiard model with increased scattering coefficients resulted from the metal adsorbates. The results indicate that the plasmon damping in case of Ag is significantly weaker than Pt and Au, which can be attributed to longer relaxation time of free electrons in Ag and therefore less loss of the oscillating plasmon electrons. In contrast to the red shift observed from Au and Pt, blue shift of the LSPR is observed in case of Ag. It suggests that plasmonic properties brought by the metal adsorbates can exert dramatic influence on the nanoparticle that is adsorbed with. We believe that our study not only provides important understanding on plasmon damping but pave the road for the fabrication of complex nanostructures with two or more metal elements.

Nanodiamond-based nanostructures for coupling nitrogen-vacancy centres to metal nanoparticles and semiconductor quantum dots.

The ability to control the interaction between nitrogen-vacancy centres in diamond and photonic and/or broadband plasmonic nanostructures is crucial for the development of solid-state quantum devices with optimum performance. However, existing methods typically employ top-down fabrication, which restrict scalable and feasible manipulation of nitrogen-vacancy centres. Here, we develop a general bottom-up approach to fabricate an emerging class of freestanding nanodiamond-based hybrid nanostructures with external functional units of either plasmonic nanoparticles or excitonic quantum dots. Precise control of the structural parameters (including size, composition, coverage and spacing of the external functional units) is achieved, representing a pre-requisite for exploring the underlying physics. Fine tuning of the emission characteristics through structural regulation is demonstrated by performing single-particle optical studies. This study opens a rich toolbox to tailor properties of quantum emitters, which can facilitate design guidelines for devices based on nitrogen-vacancy centres that use these freestanding hybrid nanostructures as building blocks.

Dispersive Plasmon Damping in Single Gold Nanorods by Platinum Adsorbates.

Surface modifications of plasmonic nanoparticles with metal adsorbates are essential in applications such as plasmonic sensing, plasmon-enhanced photocatalysis, etc., where spectral broadening is usually observed. A single particle study is presented on plasmon damping by adsorption of platinum (Pt) clusters. Single particle dark-field spectroscopy is employed to measure exactly the same gold nanorod before and after the Pt adsorption. The Pt-induced plasmon damping in terms of linewidth increase is found dependent on the resonance wavelength of the measured nanorod, which is dispersive in nature. The measured dispersion generally matches the theoretical prediction, and it basically exhibits a gradual increase with decreasing resonance energy. This increase can be attributed to the fact that the nanorod as a better resonator is more susceptible to the Pt adsorption than the spherical particles. Moreover, simulated results based on discrete dipole approximation method further indicate that the damping is mainly contributed from the adsorbates on the ends of the nanorod and independent on the type of the metal adsorbed. Knowledge and insights gained in this study can be very important for the design and fabrication of plasmonic heterostructures as functional nanomaterials.

Nanoscale aerovirology: An efficient yet simple method to analyze the viral distribution of single bioaerosols

ABSTRACTThe aerosolization mechanisms of viruses are poorly known, because of the challenges related to their sampling and observation. For example, single particle studies are needed to improve our understanding of bioaerosol enrichment processes. Such studies would help to develop models of airborne disease propagation. We propose a novel approach to study viral aerosols in single particles using a combination of fluorescence and transmission electron microscopy (TEM). This method allows for rapid analysis of labeled bacteriophages aerosolized and captured on a black membrane filter. It also requires performing image analyses on fluorescent spots. TEM is necessary to determine a single bacteriophage dimensions. Thus, the clustering of bacteriophage PP01 in a single aerosol particle was investigated and found to give a comparable number of virions to what was observed with TEM. The impact of the GFP (green fluorescent protein) in the head of PP01 virion compared to wild type (WT) PP01 was also tested by ...

Quantification and key factors in delamination of (Mg1 − yNiy)1 − xAlx(OH)2(NO3)x·mH2O

Dispersions of delaminated layered double hydroxides (LDH) contain nanosheets of interest for single particle studies and for preparation of different types of clay mineral-clay mineral or clay mineral–polymer nanocomposites and coatings. Reproducible methods were currently benchmarked, key parameters influencing the degree of delamination were identified, and prerequisites for achieving high yield delamination routes evaluated. Inert synthesis, screening by turbidity measurements and gravimetric quantification allowed identification of how various parameters influence delamination. For (Mg1−yNiy)1−xAlx(OH)2(NO3)x·mH2O preparative parameters as hydrothermal treatment, drying conditions and pH during synthesis have significant impact on delamination abilities. Poorly delaminating Ni0.75Al0.25(OH)2(NO3)0.25·mH2O LDH is by adequate hydrothermal treatment converted to a material that undergoes almost complete delamination. The adopted procedures allow systematic studies on how preparative parameters as well as more chemical parameters such as interlayer chemistry and cationic composition in the brucite like layers interacts with solvent properties. Improved insight into parameters controlling delamination properties open up for routes on large scale manufacturing of clay mineral-clay mineral nanocomposites requested for novel types of catalysts. Particles, dispersions and nanosheets were characterized by ICP-OES, CN-analysis, turbidity, XRD, TEM, SAXS, AFM and XPS measurements.

Size-dependent catalytic kinetics and dynamics of Pd nanocubes: a single-particle study.

Due to the well-known significant effect of the size on the catalytic activity of nanocatalysts, here we use single-molecule fluorescence microscopy to study the size-dependent catalytic kinetics and dynamics of individual Pd nanocubes. A series of size-dependent catalytic properties were revealed in both product formation and product desorption processes. It was found that, due to the different adsorption mechanisms of substrate molecules on Pd nanocubes, H2 adsorption is independent of the size of Pd nanocubes, while the large flat resazurin molecules show stronger adsorption on larger sized Pd nanocubes. Apparently, the Pd nanocubes can be divided into three types: when the size of the Pd nanocube is small, substrate binding can prohibit product desorption and product desorption prefers the direct pathway; when the size is in an appropriate range, the product desorption process could be independent of substrate binding and shows no selectivity between two parallel desorption pathways; if the size is large enough, substrate binding can promote product desorption and product desorption prefers the indirect pathway. We also observed the surface-restructuring-induced dynamic heterogeneity of individual Pd nanocubes in both product formation and desorption processes with timescales of about tens to one hundred seconds. The activity fluctuation of individual Pd nanocubes was found to be mainly due to the spontaneous surface-restructuring rather than the catalysis. Furthermore, we estimated the size-dependent activation energies and time scales of spontaneous dynamic surface restructuring, which are fundamental to heterogeneous catalysis. The work presented here reveals new insight into nanocatalysis and exemplifies the advantages of the single-molecule approach in probing the catalytic properties of nanocatalysts.

Photo-spectroscopy of mixtures of catalyst particles reveals their age and type.

Within a fluid catalytic cracking (FCC) unit, a mixture of catalyst particles that consist of either zeolite Y (FCC-Y) or ZSM-5 (FCC-ZSM-5) is used in order to boost the propylene yield when processing crude oil fractions. Mixtures of differently aged FCC-Y and FCC-ZSM-5 particles circulating in the FCC unit, the so-called equilibrium catalyst (Ecat), are routinely studied to monitor the overall efficiency of the FCC process. In this study, the age of individual catalyst particles is evaluated based upon photographs after selective staining with substituted styrene molecules. The observed color changes are linked to physical properties, such as the micropore volume and catalytic cracking activity data. Furthermore, it has been possible to determine the relative amount of FCC-Y and FCC-ZSM-5 in an artificial series of physical mixtures as well as in an Ecat sample with unknown composition. As a result, a new practical tool is introduced in the field of zeolite catalysis to evaluate FCC catalyst performances on the basis of photo-spectroscopic measurements with an off-the-shelf digital single lens reflex (DSLR) photo-camera with a macro lens. The results also demonstrate that there is an interesting time and cost trade-off between single catalyst particle studies, as performed with e.g. UV-vis, synchrotron-based IR and fluorescence micro-spectroscopy, and many catalyst particle photo-spectroscopy studies, making use of a relatively simple DSLR photo-camera. The latter approach offers clear prospects for the quality control of e.g. FCC catalyst manufacturing plants.

Characterizing the optical properties of single palladium-coated core-shell gold nanorods as multifunctional orientation probes.

Bimetallic core-shell gold nanorods are promising multifunctional orientation probes due to the synergetic effect of two different metals, as examined under an optical microscope. Herein, we performed a single particle study to characterize the optical properties of palladium-coated core-shell gold nanorods (Pd-AuNRs) using scattering-based dark-field (DF) and differential interference contrast (DIC) microscopy, and to examine their use as multifunctional orientation probes in biological and physical studies. The longitudinal surface plasmon resonance (SPR) was strongly damped with the increased SPR linewidth for single Pd-AuNRs due to Pd metals coated on the AuNR surface under single particle scattering spectroscopy. Despite the strong plasmon damping with much reduced scattering intensity, DIC microscopy allowed us to detect single Pd-AuNRs with much higher sensitivity. We found polarization-dependent DIC images and intensities of single Pd-AuNRs, which enabled the determination of their orientation angle under DIC microscopy. Furthermore, we found that DIC polarization anisotropy can be obtained from the orthogonally polarized bright and dark intensities for more accurate and reliable angle measurements of the Pd-AuNR probes. The results support the idea that bimetallic Pd-AuNRs can be used to develop multifunctional orientation probes.

Structure of the Sec61 channel opened by a signal sequence.

Secreted and integral membrane proteins compose up to one-third of the biological proteome. These proteins contain hydrophobic signals that direct their translocation across or insertion into the lipid bilayer by the Sec61 protein-conducting channel. The molecular basis of how hydrophobic signals within a nascent polypeptide trigger channel opening is not understood. Here, we used cryo-electron microscopy to determine the structure of an active Sec61 channel that has been opened by a signal sequence. The signal supplants helix 2 of Sec61α, which triggers a rotation that opens the central pore both axially across the membrane and laterally toward the lipid bilayer. Comparisons with structures of Sec61 in other states suggest a pathway for how hydrophobic signals engage the channel to gain access to the lipid bilayer.

Super-Resolution Optical Fluctuation Bio-Imaging with Dual-Color Carbon Nanodots.

Success in super-resolution imaging relies on a proper choice of fluorescent probes. Here, we suggest novel easily produced and biocompatible nanoparticles-carbon nanodots-for super-resolution optical fluctuation bioimaging (SOFI). The particles revealed an intrinsic dual-color fluorescence, which corresponds to two subpopulations of particles of different electric charges. The neutral nanoparticles localize to cellular nuclei suggesting their potential use as an inexpensive, easily produced nucleus-specific label. The single particle study revealed that the carbon nanodots possess a unique hybrid combination of fluorescence properties exhibiting characteristics of both dye molecules and semiconductor nanocrystals. The results suggest that charge trapping and redistribution on the surface of the particles triggers their transitions between emissive and dark states. These findings open up new possibilities for the utilization of carbon nanodots in the various super-resolution microscopy methods based on stochastic optical switching.

Single-Particle Time-of-Flight Mass Spectrometry Utilizing a Femtosecond Desorption and Ionization Laser.

Single-particle time-of-flight mass spectrometry has now been used since the 1990s to determine particle-to-particle variability and internal mixing state. Instruments commonly use 193 nm excimer or 266 nm frequency-quadrupled Nd:YAG lasers to ablate and ionize particles in a single step. We describe the use of a femtosecond laser system (800 nm wavelength, 100 fs pulse duration) in combination with an existing single-particle time-of-flight mass spectrometer. The goal of this project was to determine the suitability of a femtosecond laser for single-particle studies via direct comparison to the excimer laser (193 nm wavelength, ∼10 ns pulse duration) usually used with the instrument. Laser power, frequency, and polarization were varied to determine the effect on mass spectra. Atmospherically relevant materials that are often used in laboratory studies, ammonium nitrate and sodium chloride, were used for the aerosol. Detection of trace amounts of a heavy metal, lead, in an ammonium nitrate matrix was also investigated. The femtosecond ionization had a large air background not present with the 193 nm excimer and produced more multiply charged ions. Overall, we find that femtosecond laser ablation and ionization of aerosol particles is not radically different than that provided by a 193 nm excimer.

Absorption Spectroscopy of Single Optically Trapped Gold Nanorods.

Extinction spectra of single gold nanorods optically trapped in water were measured by spatial modulation spectroscopy. Comparison of the extinction cross sections and resonance frequencies to finite element calculations allows us to determine the dimensions of the nanorod and estimate the contribution of radiation damping to the LSPR line width. Subtracting the radiation damping and bulk contributions from the measured line widths yields the electron-surface scattering contribution. The results show that the surfactant coating for the nanorods causes large electron-surface scattering effects with significant particle-to-particle variations. These effects are more pronounced than those seen for substrate-supported particles in previous single particle studies. Indeed, the measured line widths are only slightly narrower than that of the ensemble spectrum. These results show the importance of removing surfactant for sensing applications of these materials.

Segmented GFP-like aptamer probes for functional imaging of viral genome trafficking

Recently developed GFP-like RNA aptamers harbor a few unique potential benefits for in vivo RNA imaging applications, including co-packaging of viral genomes. Here we examine them in the context of co-packaging of RNA strands during virion assembly and trafficking. The approach is applicable both in vitro and in vivo, thus bridging an existing methodological gap. We have found that splitting the aptamer sequence in the loop region into two separate parts allows for subsequent self-assembly into a functional unit, which preserves the dye-binding pocket. In presence of the dye, virus-like particles encapsulating segmented GFP-like aptamers provided bright fluorescence emission and showed negligible bleaching due to continuous chromophore exchange: two desirable characteristics for real-time in vivo single particle studies requiring a broader dynamic range than currently available. Proof-of-principle in vivo imaging experiments confirmed detectability of aptamer-loaded virus-like particles in barley root cells even in presence of significant autofluorescence background.

Exploiting Conjugated Polyelectrolyte Photophysics toward Monitoring Real-Time Lipid Membrane-Surface Interaction Dynamics at the Single-Particle Level.

Herein we report the real-time observation of the interaction dynamics between cationic liposomes flowing in solution and a surface-immobilized charged scaffolding formed by the deposition of conjugated polyanion poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene (MPS-PPV) onto 100-nm-diameter SiO2 nanoparticles (NPs). Contact of the freely floating liposomes with the polymer-coated surfaces led to the formation of supported lipid bilayers (SLBs). The interaction of the incoming liposomes with MPS-PPV adsorbed on individual SiO2 nanoparticles promoted the deaggregation of the polymer conformation and led to large emission intensity enhancements. Single-particle total internal reflection fluorescence microscopy studies exploited this phenomenon as a way to monitor the deformation dynamics of liposomes on surface-immobilized NPs. The MPS-PPV emission enhancement (up to 25-fold) reflected on the extent of membrane contact with the surface of the NP and was correlated with the size of the incoming liposome. The time required for the MPS-PPV emission to reach a maximum (ranging from 400 to 1000 ms) revealed the dynamics of membrane deformation and was also correlated with the liposome size. Cryo-TEM experiments complemented these results by yielding a structural view of the process. Immediately following the mixing of liposomes and NPs the majority of NPs had one or more adsorbed liposomes, yet the presence of a fully formed SLB was rare. Prolonged incubation of liposomes and NPs showed completely formed SLBs on all of the NPs, confirming that the liposomes eventually ruptured to form SLBs. We foresee that the single-particle studies we report herein may be readily extended to study membrane dynamics of other lipids including cellular membranes in live cell studies and to monitor the formation of polymer-cushioned SLBs.

(Invited) Variance Spectroscopy: A New Bridge Between Ensemble and Single-Particle Studies

We have developed a new experimental technique that probes variations in the spectra from small regions of heterogeneous bulk samples resulting from statistical variations in composition. The method is demonstrated with suspensions of single-walled carbon nanotubes (SWCNTs) containing mixtures of distinct structural species. Using dilute SWCNT samples, focused excitation beams, multichannel detection, and quick data collection, we capture several thousand emission spectra sampling different spatial regions of the sample. The data are analyzed to find both the mean emission intensity and the variance of emission intensity as a function of wavelength. Those spectra are then combined to extract information that is not available from conventional experiments, including the abundances of different (n,m) species and their relative emission efficiencies. The variance data are further analyzed for correlations between intensity fluctuations at different wavelengths. This gives novel two-dimensional maps that reveal the spectra of homogeneous sub-populations within the heterogeneously broadened bulk spectra. The off-diagonal features in these maps show spatially correlated concentration variations for nanotubes of different types, which expose the earliest stages of SWCNT aggregation. Variance spectroscopy should prove a powerful new experimental tool that can extract sub-ensemble level detail from measurements on bulk nanoparticle samples.

Guidelines for the fitting of anomalous diffusion mean square displacement graphs from single particle tracking experiments.

Single particle tracking is an essential tool in the study of complex systems and biophysics and it is commonly analyzed by the time-averaged mean square displacement (MSD) of the diffusive trajectories. However, past work has shown that MSDs are susceptible to significant errors and biases, preventing the comparison and assessment of experimental studies. Here, we attempt to extract practical guidelines for the estimation of anomalous time averaged MSDs through the simulation of multiple scenarios with fractional Brownian motion as a representative of a large class of fractional ergodic processes. We extract the precision and accuracy of the fitted MSD for various anomalous exponents and measurement errors with respect to measurement length and maximum time lags. Based on the calculated precision maps, we present guidelines to improve accuracy in single particle studies. Importantly, we find that in some experimental conditions, the time averaged MSD should not be used as an estimator.

Hydrodynamic focusing for micro-rheological single-particle study

Hydrodynamic focusing is a widely used tool in microfluidics world. Typical working models are treating this phenomenon as two-dimensional, however the requirements of high throughput analysis show the urgent need to operate in higher Reynolds number range Re > 10, where the three dimensional effects are no longer negligible. Current work, based on both experimental results of confocal laser scanning microscopy (CLSM) and computational fluid dynamics (CFD) modeling describes the problematic range of flow parameters and presents the model describing the shape of the focused stream. In specific, the discussion is aimed at possible use of the proposed solution in single particle rheology, where such a model gives novel control possibilities over the experimental parameters.

Single-particle cryo-EM of the ryanodine receptor channel in an aqueous environment.

Ryanodine receptors (RyRs) are tetrameric ligand-gated Ca2+ release channels that are responsible for the increase of cytosolic Ca2+ concentration leading to muscle contraction. Our current understanding of RyR channel gating and regulation is greatly limited due to the lack of a high-resolution structure of the channel protein. The enormous size and unwieldy shape of Ca2+ release channels make X-ray or NMR methods difficult to apply for high-resolution structural analysis of the full-length functional channel. Single-particle electron cryo-microscopy (cryo-EM) is one of the only effective techniques for the study of such a large integral membrane protein and its molecular interactions. Despite recent developments in cryo-EM technologies and break-through single-particle cryo-EM studies of ion channels, cryospecimen preparation, particularly the presence of detergent in the buffer, remains the main impediment to obtaining atomic-resolution structures of ion channels and a multitude of other integral membrane protein complexes. In this review we will discuss properties of several detergents that have been successfully utilized in cryo-EM studies of ion channels and the emergence of the detergent alternative amphipol to stabilize ion channels for structure-function characterization. Future structural studies of challenging specimen like ion channels are likely to be facilitated by cryo-EM amenable detergents or alternative surfactants.

The Use of Connected Masks for Reconstructing the Single Particle Image from X-Ray Diffraction Data

The task of reconstructing the image of a single macromolecular object from X-ray diffraction data can be formulated as a task of the reconstruction of the 3D electron density distribution from the magnitudes of its Fourier transform. This task can be reduced to a series of standard X-ray crystallography tasks, namely, the recovery of a periodic function from its Fourier transform magnitudes (structure factors), which are determined in an X-ray diffraction experiment. In this work, a new approach to the solution of these tasks is suggested which is based on the use of connected binary masks as an approximation of the required electron density distribution. The approach includes the random generation of a great number of connected masks, the selection of the masks that are in agreement with an experimental and a prior information about the object, and the alignment and the averaging of the phase sets of the structure factors that correspond to the selected masks. The averaged phase values together with the experimentally determined magnitudes are used for the calculation of the Fourier synthesis, which is applied for the visualization of the object under study. The approach can be used in studies of both single particles and crystalline species; however, it holds the greatest promise for investigations of single objects. The results of testing the approach are presented.