Probing subdiffraction limit separations with plasmon coupling microscopy: concepts and applications

Two-color interferometric scattering (iSCAT) microscopy reveals structural dynamics in discrete plasmonic molecules

Plasmonic molecules are discrete assemblies of noble metal nanoparticles (NPs) that are of interest as transducers in optical nanosensors. So far, NPs with diameters of ∼40 nm have been the preferred building blocks for plasmonic molecules intended as optical single molecule sensors due to difficulties associated with detecting smaller NPs through elastic scattering in conventional darkfield microscopy. Here, we apply 405 nm, 445 nm two-color interferometric scattering (iSCAT) microscopy to characterize polyethylene glycol (PEG) tethered dimers of 10 nm and 20 nm Ag NPs and their monomers. Dimers of both NP sizes can be discerned from their respective monomers through changes in the average iSCAT contrast. In the case of 20 nm Ag NPs, dimer formation induces a change in the sign of the iSCAT contrast, providing a characteristic signal for detecting binding events. 20 nm Ag NP dimers with 0.4 kDa and 3.4 kDa polyethylene glycol (PEG) spacers show iSCAT contrast distributions with significantly different averages on both wavelength channels. The iSCAT contrast measured for individual PEG-tethered 10 nm or 20 nm NP dimers as a function of time shows contrast fluctuations indicative of a rich structural dynamics in the assembled plasmonic molecules, which provides an additional metric to discern dimers from monomers and paves the path to a new class of interferometric plasmon rulers.

Roadmap on optical sensors

Optical sensors and sensing technologies are playing a more and more important role in our modern world. From micro-probes to large devices used in such diverse areas like medical diagnosis, defence, monitoring of industrial and environmental conditions, optics can be used in a variety of ways to achieve compact, low cost, stand-off sensing with extreme sensitivity and selectivity. Actually, the challenges to the design and functioning of an optical sensor for a particular application requires intimate knowledge of the optical, material, and environmental properties that can affect its performance. This roadmap on optical sensors addresses different technologies and application areas. It is constituted by twelve contributions authored by world-leading experts, providing insight into the current state-of-the-art and the challenges their respective fields face. Two articles address the area of optical fibre sensors, encompassing both conventional and specialty optical fibres. Several other articles are dedicated to laser-based sensors, micro- and nano-engineered sensors, whispering-gallery mode and plasmonic sensors. The use of optical sensors in chemical, biological and biomedical areas is discussed in some other papers. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed.

Investigation of Plasmonic-Enhanced Solar Photothermal Effect of Au NR@PVDF Micro-/Nanofilms

Gold nanospheres (Au NSs) and gold nanorods (Au NRs) are traditional noble metal plasmonic nanomaterials. Particularly, Au NRs with tunable longitudinal plasmon resonance from the visible to the near-infrared (NIR) range were suitable for highly efficient photothermal applications due to the extended light-receiving range. In this work, we synthesized Au NRs and Au NSs of similar volumes and subsequently developed them into Au NR/poly(vinylidene fluoride) (PVDF) and Au NS/PVDF nanofilms, both of which exhibited excellent solar photothermal performance evaluated by solar photothermal experiments. We found that the Au NR/PVDF nanofilm showed a higher solar photothermal performance than the Au NS/PVDF nanofilm. Through detailed analysis, such as morphological characterization, optical measurement, and finite element method (FEM) modeling, we found that the plasmonic coupling effects inside the aggregated Au NR nanoclusters contributed to the spectral blue shifts and intensified the photothermal performance. As compared to Au NS/PVDF nanofilms, the Au NR/PVDF nanofilm exhibited a higher efficient light-to-heat conversion rate because of the extended light-receiving range and high absorbance, as a result of the strong plasmonic interactions inside nanoclusters, which was further validated by monochromatic laser photothermal experiments and FEM simulations. Our work proved that the Au NRs have huge potential for plasmonic solar photothermal applications and are envisioned for novel plasmonic applications.

Plasmonic nano-antimicrobials: properties, mechanisms and applications in microbe inactivation and sensing

Bacterial, viral and fungal infections pose serious threats to human health and well-being. The continuous emergence of acute infectious diseases caused by pathogenic microbes and the rapid development of resistances against conventional antimicrobial drugs necessitates the development of new and effective strategies for the safe elimination of microbes in water, food or on surfaces, as well as for the inactivation of pathogenic microbes in human hosts. The need for new antimicrobials has triggered the development of plasmonic nano-antimicrobials that facilitate both light-dependent and -independent microbe inactivation mechanisms. This review introduces the relevant photophysical mechanisms underlying these plasmonic nano-antimicrobials, and provides an overview of how the photoresponses and materials properties of plasmonic nanostructures can be applied in microbial pathogen inactivation and sensing applications. Through a systematic analysis of the inactivation efficacies of different plasmonic nanostructures, this review outlines the current state-of-the-art in plasmonic nano-antimicrobials and defines the application space for different microbial inactivation strategies. The advantageous optical properties of plasmonic nano-antimicrobials also enhance microbial detection and sensing modalities and thus help to avoid exposure to microbial pathogens. Sensitive and fast plasmonic microbial sensing modalities and their theranostic and targeted therapeutic applications are discussed.

Roadmap for metal nanoparticles in radiation therapy: current status, translational challenges, and future directions

This roadmap outlines the potential roles of metallic nanoparticles (MNPs) in the field of radiation therapy. MNPs made up of a wide range of materials (from Titanium, Z = 22, to Bismuth, Z = 83) and a similarly wide spectrum of potential clinical applications, including diagnostic, therapeutic (radiation dose enhancers, hyperthermia inducers, drug delivery vehicles, vaccine adjuvants, photosensitizers, enhancers of immunotherapy) and theranostic (combining both diagnostic and therapeutic), are being fabricated and evaluated. This roadmap covers contributions from experts in these topics summarizing their view of the current status and challenges, as well as expected advancements in technology to address these challenges.

Ultrahigh-Sensitivity Sandwiched Plasmon Ruler for Label-Free Clinical Diagnosis

Optical biosensors, especially those based on plasmonic structures, have emerged recently as a potential tool for disease diagnostics. Plasmonic biosensors have demonstrated impressive benefits for the label-free detection of trace biomarkers in human serum. However, widespread applications of these technologies are hindered because of their insufficient sensitivity, their relatively complex chemical immobilization processes, and the use of prism couplers. Accordingly, a sandwiched plasmon ruler (SW-PR) based on a Au nanohole array with ultrahigh sensitivity arising from the plasmonic coupling effect is developed. Highly confined surface charges caused by Bloch wave surface plasmon polarizations substantially increase the coupling efficiency. This platform exhibits thickness sensitivity as high as 61 nm nm^-1 and can detect at least 200 000-fold lower analyte concentrations than a nanowell sensing platform with the same wavelength shift. Additionally, the sandwiched plasmonic biosensor allows precise and label-free testing of clinical biomarkers, namely C-reactive protein and procalcitonin, in patient serum samples without requiring a sophisticated prism coupler, extra antibodies, or a chemical immobilization technique. This study yields new insight into the structural design of plasmon rulers and will open exciting avenues for disease diagnosis and therapy follow-up at the point-of-care.

Characterizing Large-Scale Receptor Clustering on the Single Cell Level: A Comparative Plasmon Coupling and Fluorescence Superresolution Microscopy Study

Spatial clustering of cell membrane receptors has been indicated to play a regulatory role in signal initiation, and the distribution of receptors on the cell surface may represent a potential biomarker. To realize its potential for diagnostic purposes, scalable assays capable of mapping spatial receptor heterogeneity with high throughput are needed. In this work, we use gold nanoparticle (NP) labels with an average diameter of 72.17 ± 2.16 nm as bright markers for large-scale epidermal growth factor receptor (EGFR) clustering in hyperspectral plasmon coupling microscopy and compare the obtained clustering maps with those obtained through fluorescence superresolution microscopy (direct stochastic optical reconstruction microscopy, dSTORM). Our dSTORM experiments reveal average EGFR cluster sizes of 172 ± 99 and 150 ± 90 nm for MDA-MB-468 and HeLa, respectively. The cluster sizes decrease after EGFR activation. Hyperspectral imaging of the NP labels shows that differences in the EGFR cluster sizes are accompanied by differences in the average separations between electromagnetically coupled NPs. Because of the distance dependence of plasmon coupling, changes in the average interparticle separation result in significant spectral shifts. For the experimental conditions investigated in this work, hyperspectral plasmon coupling microscopy of NP labels identified the same trends in large-scale EGFR clustering as dSTORM, but the NP imaging approach provided the information in a fraction of the time. Both dSTORM and hyperspectral plasmon coupling microscopy confirm the cortical actin network as one structural component that determines the average size of EGFR clusters.

Ligand Density and Nanoparticle Clustering Cooperate in the Multivalent Amplification of Epidermal Growth Factor Receptor Activation

Multivalent presentation of ligands on nanoparticles (NPs) is considered a general strategy for enhancing receptor binding and activation through amplification of ligand-receptor interactions within the footprint of the individual NPs. The spatial clustering of ligand-functionalized NPs represents an additional, less well understood mechanism for increasing local ligand-receptor interactions, especially for receptors that form higher-order assemblies, such as the epidermal growth factor (EGF) receptor (EGFR). To shed light on the interplay between ligand density ( i.e., multivalency) and NP clustering in signal amplification, we apply EGF-functionalized 72 ± 1 nm gold nanoparticles (NP-EGF) with known ligand loading (10-200 EGF/NP) as quantifiable and experimentally tractable units of EGFR activation and characterize the NP-mediated amplification of EGFR phosphorylation as a function of both EGF surface density and NP-EGF clustering for two cancer cell lines (HeLa and MDA-MB-468). The measurements confirm a strong multivalent amplification of EGFR phosphorylation through NP-EGF on the cellular level that results in EGF-loading-dependent maximum EGFR phosphorylation levels. A microscopic analysis of NP-EGF-induced EGFR phosphorylation reveals a heterogeneous spatial distribution of EGFR activation across the cell surface. Clustering of multivalent NP-EGF on sub-diffraction-limited length scales is found to result in a local enhancement of EGFR phosphorylation in signaling "hot spots" from where the signal can spread laterally in an EGF-independent fashion. Increasing EGF loadings of the NP enhances NP-EGF clustering and intensifies EGFR phosphorylation. These observations suggest that NP-EGF clustering and the associated local enhancement of ligand-receptor interactions are intrinsic components of the multivalent amplification of phosphorylation for the heterogeneously distributed EGFR through NP-EGF.

Membrane-wrapped nanoparticles probe divergent roles of GM3 and phosphatidylserine in lipid-mediated viral entry pathways

Gold nanoparticles (NPs) wrapped in a membrane can be utilized as artificial virus nanoparticles (AVNs) that combine the large nonblinking or bleaching optical cross-section of the NP core with the biological surface properties and functionalities provided by a self-assembled lipid membrane. We used these hybrid nanomaterials to test the roles of monosialodihexosylganglioside (GM3) and phosphatidylserine (PS) for a lipid-mediated targeting of virus-containing compartments (VCCs) in macrophages. GM3-presenting AVNs bind to CD169 (Siglec-1)-expressing macrophages, but inclusion of PS in the GM3-containing AVN membrane decreases binding. Molecular dynamics simulations of the AVN membrane and experimental binding studies of CD169 to GM3-presenting AVNs reveal Na+-mediated interactions between GM3 and PS as a potential cause of the antagonistic action on binding by the two negatively charged lipids. GM3-functionalized AVNs with no or low PS content localize to tetherin+, CD9+ VCC in a membrane composition-depending fashion, but increasing amounts of PS in the AVN membrane redirect the NP to lysosomal compartments. Interestingly, this compartmentalization is highly GM3 specific. Even AVNs presenting the related monosialotetrahexosylganglioside (GM1) fail to achieve an accumulation in VCC. AVN localization to VCC was observed for AVN with gold NP core but not for liposomes, suggesting that NP sequestration into VCC has additional requirements beyond ligand (GM3)-receptor (CD169) recognition that are related to the physical properties of the NP core. Our results confirm AVN as a scalable platform for elucidating the mechanisms of lipid-mediated viral entry pathways and for selective intracellular targeting.

Localized Surface Plasmon Coupling between Mid-IR-Resonant ITO Nanocrystals

Sn-doped indium oxide (ITO) nanocrystals (NC) provide tunable localized surface plasmon resonance in the mid-infrared. To evaluate the applicability of these n-doped plasmonic semiconductors in field-enhanced spectroscopies, it is necessary to assess how the low, free-electron density affects the E-field localization and plasmon coupling in NC films when compared to metal nanoparticles (NP). In this article, we investigate plasmon coupling between approximate 6 nm diameter ITO NC on the collective resonance and quantify the effect of the electromagnetic field enhancement on the absorbance signal of surface-attached ligands in NC films and monolayers with different ratios of doped and undoped indium oxide NC.

Two-color dark-field (TCDF) microscopy for metal nanoparticle imaging inside cells

Noble metal nanoparticles (NPs) supporting localized surface plasmon resonances are widely used in the context of biotechnology as optical and absorption contrast agents with great potential applicability to both diagnostics and less invasive therapies. In this framework, it is crucial to have access to simple and reliable microscopy techniques to monitor the NPs that have internalized into cells. While dark field (DF) microscopy takes advantage of the enhanced NP scattering at their plasmon resonance, its use in cells is limited by the large scattering background from the internal cell compartments. Here, we report on a novel two-color dark field microscopy that addresses these limitations by significantly reducing the cell scattering contribution. We first present the technique and demonstrate its enhanced contrast, specificity and reliability for NP detection compared to a standard optical dark field. We then demonstrate its potential suitability in two different settings, namely wide-field parallel screening of circulating cells in microfluidic chips and high-resolution tracking of internalized NPs in cells. These proof of principle experiments show a promising capability of this approach with possible extension to other kinds of targeted systems like bacteria and vesicles.

Macroscopic Aerogels with Retained Nanoscopic Plasmonic Properties

Aerogels can bridge the nanoscopic to the macroscopic world. One physical phenomenon typically limited to the nanoscopic world is the occurrence of localized surface plasmon resonances (LSPRs), which are observed in conductive nanoparticles. Once brought into close contact, assemblies or superstructures of these nanoparticles often lose their plasmonic properties in the transition stage towards the bulk material. Therefore, LSPRs are typically not observed in macroscopic objects. The present work aims at voluminous nanoparticle-based aerogels with optical properties close to that of the initial colloidal solution and the possibility to manipulate the final plasmonic properties by bringing the particles into defined distances. In detail, Ag nanocrystals with silica shells ranging from 0 to 12 nm are employed as building blocks, which are assembled from their solution into macroscopic three-dimensional superstructures by freezing and subsequent lyophilization. These cryogelated aerogels are synthesized as monoliths and thin films in which the Ag nanocrystals are arranged in defined distances according to their silica shell. The resulting aerogels exhibit plasmonic properties ranging from a behavior similar to that of the building blocks for the thickest shell to a heavily distorted behavior for bare Ag nanocrystals.

Roadmap on optical sensors

Sensors are devices or systems able to detect, measure and convert magnitudes from any domain to an electrical one. Using light as a probe for optical sensing is one of the most efficient approaches for this purpose. The history of optical sensing using some methods based on absorbance, emissive and florescence properties date back to the 16th century. The field of optical sensors evolved during the following centuries, but it did not achieve maturity until the demonstration of the first laser in 1960. The unique properties of laser light become particularly important in the case of laser-based sensors, whose operation is entirely based upon the direct detection of laser light itself, without relying on any additional mediating device. However, compared with freely propagating light beams, artificially engineered optical fields are in increasing demand for probing samples with very small sizes and/or weak light-matter interaction. Optical fiber sensors constitute a subarea of optical sensors in which fiber technologies are employed. Different types of specialty and photonic crystal fibers provide improved performance and novel sensing concepts. Actually, structurization with wavelength or subwavelength feature size appears as the most efficient way to enhance sensor sensitivity and its detection limit. This leads to the area of micro- and nano-engineered optical sensors. It is expected that the combination of better fabrication techniques and new physical effects may open new and fascinating opportunities in this area. This roadmap on optical sensors addresses different technologies and application areas of the field. Fourteen contributions authored by experts from both industry and academia provide insights into the current state-of-the-art and the challenges faced by researchers currently. Two sections of this paper provide an overview of laser-based and frequency comb-based sensors. Three sections address the area of optical fiber sensors, encompassing both conventional, specialty and photonic crystal fibers. Several other sections are dedicated to micro- and nano-engineered sensors, including whispering-gallery mode and plasmonic sensors. The uses of optical sensors in chemical, biological and biomedical areas are described in other sections. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed. Advances in science and technology required to meet challenges faced in each of these areas are addressed, together with suggestions on how the field could evolve in the near future.

Super-Resolution Imaging and Plasmonics

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

Monitoring transient nanoparticle interactions with liposome-confined plasmonic transducers

The encapsulation of individual pairs of plasmonic nanoparticles (NPs) in liposomes is introduced as a new strategy for utilizing plasmon coupling to monitor interactions between co-confined NPs in a nanoconfinement that ensures high local NP concentrations. We apply the approach to monitor transient binding contacts between noncovalently tethered 55 nm diameter gold NPs, which were functionalized with cytosine (C)-rich DNAs, in acidic and mildly basic buffer conditions. At pH = 8, a rich spectral dynamics indicates DNA-mediated transient binding and unbinding of co-confined NPs due to weak attractive interparticle interactions. A decrease in pH from 8 to 4 is observed to favor the associated state for some co-confined NPs, presumably due to a stabilization of the bound dimer configuration through noncanonical C-C⁺ bonds between the DNA-functionalized NPs. Plasmonic nanoemitters whose spectral response switches in response to chemical cues (in this work pH) represent optical transducers with a rich application space in chemical sensing, cell analysis and nanophotonics.

Molecular Plasmonics

In this review, we survey recent advances in the field of molecular plasmonics beyond the traditional sensing modality. Molecular plasmonics is explored in the context of the complex interaction between plasmon resonances and molecules and the ability of molecules to support plasmons self-consistently. First, spectroscopic changes induced by the interaction between molecular and plasmonic resonances are discussed, followed by examples of how tuning molecular properties leads to active molecular plasmonic systems. Next, the role of the position and polarizability of a molecular adsorbate on surface-enhanced Raman scattering signals is examined experimentally and theoretically. Finally, we introduce recent research focused on using molecules as plasmonic materials. Each of these examples is intended to highlight the role of molecules as integral components in coupled molecule-plasmon systems, as well as to show the diversity of applications in molecular plasmonics.

Assembling Color on the Nanoscale: Multichromatic Switchable Pixels from Plasmonic Atoms and Molecules

Nanoparticles of different materials, shapes, and sizes are integrated into plasmonic atoms and molecules of defined shape and location through sequential directed self-assembly following a single patterning step. A rational tuning of the emitted color across the visible range of the electromagnetic spectrum and switchable polarization properties are demonstrated. Self-assembled plasmonic pixels provide tunable, stable, and switchable optical responses.

Optical Clearing Delivers Ultrasensitive Hyperspectral Dark-Field Imaging for Single-Cell Evaluation

A single-cell optical clearing methodology is developed and demonstrated in hyperspectral dark-field microscopy (HSDFM) and imaging of plasmonic nanoprobes. Our strategy relies on a combination of delipidation and refractive index (RI) matching with highly biocompatible and affordable agents. Before applying the RI-matching solution, the delipidation step by using a mild solvent effectively eliminates those high-density, lipid-enriched granular structures which emit strong scattering. Upon treatment, the background scattering from cellular organelles could be repressed to a negligible level while the scattering signals from plasmonic nanomaterials increase, leading to a significant improvement of the signal-to-noise ratio (SNR). With this method established, the versatility and applicability of HSDFM are greatly enhanced. In our demonstration, quantitative mapping of the dimerization-activated receptor kinase HER2 is achieved in a single cancer cell by a nonfluorescent approach. High-resolution imaging for oncogenic mRNAs, namely ER, PR, and HER2, is performed with single labeling. More importantly, in situ multiplex detection of mRNA and protein is made possible by HSDFM since it overcomes the difficulties of complex staining and signal imbalance suffered by the conventional optical imaging. Last, we show that with optical clearing, characterization of intracellularly grown gold particulates is accomplished at an unprecedented spatiotemporal resolution. Taken together, the uniqueness of optical clearing and HSDFM is expected to open ample avenues for single-cell studies and biomedical engineering.

Aluminum Nanocrystals as a Plasmonic Photocatalyst for Hydrogen Dissociation

Hydrogen dissociation is a critical step in many hydrogenation reactions central to industrial chemical production and pollutant removal. This step typically utilizes the favorable band structure of precious metal catalysts like platinum and palladium to achieve high efficiency under mild conditions. Here we demonstrate that aluminum nanocrystals (Al NCs), when illuminated, can be used as a photocatalyst for hydrogen dissociation at room temperature and atmospheric pressure, despite the high activation barrier toward hydrogen adsorption and dissociation. We show that hot electron transfer from Al NCs to the antibonding orbitals of hydrogen molecules facilitates their dissociation. Hot electrons generated from surface plasmon decay and from direct photoexcitation of the interband transitions of Al both contribute to this process. Our results pave the way for the use of aluminum, an earth-abundant, nonprecious metal, for photocatalysis.

Lipid-Mediated Targeting with Membrane-Wrapped Nanoparticles in the Presence of Corona Formation

Membrane-wrapped nanoparticles represent a versatile platform for utilizing specific lipid-receptor interactions, such as siallyllactose-mediated binding of the ganglioside GM3 to Siglec1 (CD169), for targeting purposes. The membrane wrap around the nanoparticles not only serves as a matrix to incorporate GM3 as targeting moiety for antigen-presenting cells but also offers unique opportunities for constructing a biomimetic surface from lipids with potentially protein-repellent properties. We characterize nonspecific protein adsorption (corona formation) to membrane-wrapped nanoparticles with core diameters of approximately 35 and 80 nm and its effect on the GM3-mediated targeting efficacy as a function of surface charge through combined in vitro and in vivo studies. The stability and fate of the membrane wrap around the nanoparticles in a simulated biological fluid and after uptake in CD169-expressing antigen-presenting cells is experimentally tested. Finally, we demonstrate in hock immunization studies in mice that GM3-decorated membrane-wrapped nanoparticles achieve a selective enrichment in the peripheral regions of popliteal lymph nodes that contain high concentrations of CD169-expressing antigen-presenting cells.

Single-Cell Quantification of Cytosine Modifications by Hyperspectral Dark-Field Imaging

Epigenetic modifications on DNA, especially on cytosine, play a critical role in regulating gene expression and genome stability. It is known that the levels of different cytosine derivatives are highly dynamic and are regulated by a variety of factors that act on the chromatin. Here we report an optical methodology based on hyperspectral dark-field imaging (HSDFI) using plasmonic nanoprobes to quantify the recently identified cytosine modifications on DNA in single cells. Gold (Au) and silver (Ag) nanoparticles (NPs) functionalized with specific antibodies were used as contrast-generating agents due to their strong local surface plasmon resonance (LSPR) properties. With this powerful platform we have revealed the spatial distribution and quantity of 5-carboxylcytosine (5caC) at the different stages in cell cycle and demonstrated that 5caC was a stably inherited epigenetic mark. We have also shown that the regional density of 5caC on a single chromosome can be mapped due to the spectral sensitivity of the nanoprobes in relation to the interparticle distance. Notably, HSDFI enables an efficient removal of the scattering noises from nonspecifically aggregated nanoprobes, to improve accuracy in the quantification of different cytosine modifications in single cells. Further, by separating the LSPR fingerprints of AuNPs and AgNPs, multiplex detection of two cytosine modifications was also performed. Our results demonstrate HSDFI as a versatile platform for spatial and spectroscopic characterization of plasmonic nanoprobe-labeled nuclear targets at the single-cell level for quantitative epigenetic screening.

Increasing the Morphological Stability of DNA-Templated Nanostructures with Surface Hydrophobicity

DNA has been extensively used as a versatile template to assemble inorganic nanoparticles into complex architectures; thanks to its programmability, stability, and long persistence length. But the geometry of self-assembled nanostructures depends on a complex combination of attractive and repulsive forces that can override the shape of a molecular scaffold. In this report, an approach to increase the morphological stability of DNA-templated gold nanoparticle (AuNP) groupings against electrostatic interactions is demonstrated by introducing hydrophobicity on the particle surface. Using single nanostructure spectroscopy, the nanometer-scale distortions of 40 nm diameter AuNP dimers are compared with different hydrophilic, amphiphilic, neutral, and negatively charged surface chemistries, when modifying the local ionic strength. It is observed that, with most ligands, a majority of studied nanostructures deform freely from a stretched geometry to touching particles when increasing the salt concentration while hydrophobicity strongly limits the dimer distortions. Furthermore, an amphiphilic surface chemistry provides DNA-linked AuNP dimers with a high long-term stability against internal aggregation.

Probing DNA Stiffness through Optical Fluctuation Analysis of Plasmon Rulers

The distance-dependent plasmon coupling between biopolymer tethered gold or silver nanoparticles forms the foundation for the so-called plasmon rulers. While conventional plasmon ruler applications focus on the detection of singular events in the far-field spectrum, we perform in this Letter a ratiometric analysis of the continuous spectral fluctuations arising from thermal interparticle separation variations in plasmon rulers confined to fluid lipid membranes. We characterized plasmon rulers with different DNA tethers and demonstrate the ability to detect and quantify differences in the plasmon ruler potential and tether stiffness. The influence of the nature of the tether (single-stranded versus double-stranded DNA) and the length of the tether is analyzed. The characterization of the continuous variation of the interparticle separation in individual plasmon rulers through optical fluctuation analysis provides additional information about the conformational flexibility of the tether molecule(s) located in the confinement of the deeply subdiffraction limit interparticle gap and enhances the versatility of plasmon rulers as a tool in Biophysics and Nanotechnology.

Super-resolution of fluorescence-free plasmonic nanoparticles using enhanced dark-field illumination based on wavelength-modulation

Super-resolution imaging of fluorescence-free plasmonic nanoparticles (NPs) was achieved using enhanced dark-field (EDF) illumination based on wavelength-modulation. Indistinguishable adjacent EDF images of 103-nm gold nanoparticles (GNPs), 40-nm gold nanorods (GNRs), and 80-nm silver nanoparticles (SNPs) were modulated at their wavelengths of specific localized surface plasmon scattering. The coordinates (x, y) of each NP were resolved by fitting their point spread functions with a two-dimensional Gaussian. The measured localization precisions of GNPs, GNRs, and SNPs were 2.5 nm, 5.0 nm, and 2.9 nm, respectively. From the resolved coordinates of NPs and the corresponding localization precisions, super-resolution images were reconstructed. Depending on the spontaneous polarization of GNR scattering, the orientation angle of GNRs in two-dimensions was resolved and provided more elaborate localization information. This novel fluorescence-free super-resolution method was applied to live HeLa cells to resolve NPs and provided remarkable sub-diffraction limit images.

Quantifying lipid contents in enveloped virus particles with plasmonic nanoparticles

Phosphatidylserine (PS) and monosialotetrahexosylganglioside (GM1 ) are examples of two host-derived lipids in the membrane of enveloped virus particles that are known to contribute to virus attachment, uptake, and ultimately dissemination. A quantitative characterization of their contribution to the functionality of the virus requires information about their relative concentrations in the viral membrane. Here, a gold nanoparticle (NP) binding assay for probing relative PS and GM1 lipid concentrations in the outer leaflet of different HIV-1 and Ebola virus-like particles (VLPs) using sample sizes of less than 3 × 10(6) particles is introduced. The assay evaluates both scattering intensity and resonance wavelength, and determines relative NP densities through plasmon coupling as a measure for the target lipid concentrations in the NP-labeled VLP membrane. A correlation of the optical observables with absolute lipid contents is achieved by calibration of the plasmon coupling-based methodology with unilamellar liposomes of known PS or GM1 concentration. The performed studies reveal significant differences in the membrane of VLPs that assemble at different intracellular sites and pave the way to an optical quantification of lipid concentration in virus particles at physiological titers.

Widefield spectral monitoring of nanometer distance changes in DNA-templated plasmon rulers

The nanometer-scale sensitivity of electromagnetic plasmon coupling allows the translation of minute morphological changes in nanostructures into macroscopic optical signals. We demonstrate here a widefield spectral analysis of 40 nm diameter gold nanoparticle (AuNP) dimers, linked by a short DNA double strand, using a low-cost color CCD camera and allowing a quantitative estimation of interparticle distances in a 3-20 nm range. This analysis can be extended to lower spacings and a parallel monitoring of dimer orientations by performing a simple polarization analysis. Our measurement approach is calibrated against confocal scattering spectroscopy using AuNP dimers that are distorted from a stretched geometry at low ionic strength to touching particles at high salt concentrations. We then apply it to identify dimers featuring two different conformations of the same DNA template and discuss the parallel colorimetric sensing of short sequence-specific DNA single strands using dynamic plasmon rulers.

Dressing up Nanoparticles: A Membrane Wrap to Induce Formation of the Virological Synapse

Next-generation nanoparticle-based drug delivery systems require the ability to target specific organelles or subcellular regions in selected target cells. Human immunodeficiency virus type I (HIV-1) particles are evolutionarily optimized nanocarriers that have evolved to avoid intracellular degradation and achieve enrichment at the synapse between mature dendritic cells (mDCs) and T cells by subverting cellular trafficking mechanisms. This study demonstrates that integration of the glycosphingolipid, GM3, in a membrane around a solid nanoparticle (NP) core is sufficient to recapitulate key aspects of the virus particle trafficking in mDCs. GM3-presenting artificial virus NPs (GM3-AVNs) accumulate in CD169(+) and CD81(+) nonlysosomal compartments in an actin-dependent process that mimics the sequestration of HIV-1. Live-cell optical tracking studies reveal a preferential recruitment and arrest of surface scanning CD4(+) T cells in direct vicinity to the AVN-enriched compartments. The formed mDC-T cell conjugates exhibit strong morphological similarities between the GM3-AVN-containing mDC-T cell synapse and the HIV-1 virological synapse, indicating that GM3-CD169 interactions alone are sufficient for establishing the mDC-T cell virological synapse. These results emphasize the potential of the GM3-AVN approach for providing therapeutic access to a key step of the host immune response--formation of the synaptic junction between an antigen-presenting cell (mDC) and T cells--for modulating and controlling immune responses.

Plasmonic biosensors

The unique optical properties of plasmon resonant nanostructures enable exploration of nanoscale environments using relatively simple optical characterization techniques. For this reason, the field of plasmonics continues to garner the attention of the biosensing community. Biosensors based on propagating surface plasmon resonances (SPRs) in films are the most well-recognized plasmonic biosensors, but there is great potential for the new, developing technologies to surpass the robustness and popularity of film-based SPR sensing. This review surveys the current plasmonic biosensor landscape with emphasis on the basic operating principles of each plasmonic sensing technique and the practical considerations when developing a sensing platform with the various techniques. The 'gold standard' film SPR technique is reviewed briefly, but special emphasis is devoted to the up-and-coming localized surface plasmon resonance and plasmonically coupled sensor technology.

Nanoparticle-Film Plasmon Ruler Interrogated with Transmission Visible Spectroscopy

The widespread use of plasmonic nanorulers (PNRs) in sensing platforms has been plagued by technical challenges associated with the development of methods to fabricate precisely controlled nanostructures with high yield and characterize them with high throughput. We have previously shown that creating PNRs in a nanoparticle-film (NP-film) format enables the fabrication of an extremely large population of uniform PNRs with 100% yield using a self-assembly approach, which facilitates high-throughput PNR characterization using ensemble spectroscopic measurements and eliminates the need for expensive microscopy systems required by many other PNR platforms. We expand upon this prior work herein, showing that the NP-film PNR can be made compatible with aqueous sensing studies by adapting it for use in a transmission localized surface plasmon resonance spectroscopy format, where the coupled NP-film resonance responsible for the PNR signal is directly probed using an extinction measurement from a standard spectrophotometer. We designed slide holders that fit inside standard spectrophotometer cuvettes and position NP-film samples so that the coupled NP-film resonance can be detected in a collinear optical configuration. Once the NP-film PNR samples are cuvette-compatible, it is straightforward to calibrate the PNR in aqueous solution and use it to characterize dynamic, angstrom-scale distance changes resulting from pH-induced swelling of polyelectrolyte (PE) spacer layers as thin as 1 PE layer and also of a self-assembled monolayer of an amine-terminated alkanethiol. This development is an important step toward making PNR sensors more user-friendly and encouraging their widespread use in various sensing schemes.

Glycosphingolipid-functionalized nanoparticles recapitulate CD169-dependent HIV-1 uptake and trafficking in dendritic cells

Ganglioside GM3, a host-derived glycosphingolipid incorporated in the membrane of human immunodeficiency virus-1 (HIV-1) viral particles, mediates interactions between HIV-1 and Siglec1/CD169, a protein expressed on dendritic cells (DCs). Such interactions, which seem to be independent of viral envelope glycoprotein gp120, are poorly understood. Here we develop a model system consisting of self-assembled artificial virus nanoparticles (AVNs) that are free of viral glycoproteins or other host-derived glycolipids and glycoproteins. These plasmonic AVNs contain a membrane of defined composition wrapped around a solid metal core. GM3-containing AVNs are captured by CD169-expressing HeLa cells or mature DCs, and are sequestered within non-lysosomal tetraspanin-positive compartments. This distribution is reminiscent of CD169-dependent HIV-1 sequestration in mature DCs. Our results highlight GM3-CD169 binding as a gp120-independent signal for sequestration and preservation of HIV-1 infectivity. They also indicate that plasmonic AVNs offer improved features over liposome-based systems and represent a versatile tool for probing specific virus-cell interactions.