Continuous imaging of plasmon rulers in live cells reveals early-stage caspase-3 activation at the single-molecule level

Monocrystalline Labeling Enables Stable Plasmonic Enhancement for Isolation-Free Extracellular Vesicle Analysis

Sensitive detection of extracellular vesicles (EVs) as emerging biomarkers has shown great promises for disease diagnosis. Plasmonic metal nanostructures conjugated with molecules that bind specific biomarker targets are widely used for EVs sensing but involve tradeoffs between particle-size-dependent signal intensity and conjugation efficiency. One solution to this problem would be to induce nucleation on nanoparticles that have successfully bound a target biomarker to permit in situ nanoparticle growth for signal amplification, but approaches that are evaluated to date require harsh conditions or lack nucleation specificity, prohibiting their effective use with most biological specimens. This study describes a one-step in situ strategy to induce monocrystalline copper shell growth on gold nanorod probes without decreasing signal by disrupting probe-target interactions or lipid bilayer integrity to enable EV biomarker detections. This approach increases the detected nanoparticle signal about two orders of magnitude after a 10 min copper nanoshell growth reaction. This has significant implications for improved disease detection, as indicated by the ability of a novel immunoassay using this approach to detect low abundance EVs carrying a pathogen-derived biomarker, after their direct capture from serum, to facilitate the diagnosis of tuberculosis cases in a diagnostically challenging pediatric cohort.

Shedding Light on DNA-Based Nanoprobes for Live-Cell MicroRNA Imaging

DNA-based nanoprobes integrated with various imaging signals have been employed for fabricating versatile biosensor platforms for the study of intracellular biological process and biomarker detection. The nanoprobes developments also provide opportunities for endogenous microRNA (miRNA) in situ analysis. In this review, the authors are primarily interested in various DNA-based nanoprobes for miRNA biosensors and declare strategies to reveal how to customize the desired nanoplatforms. Initially, various delivery vehicles for nanoprobe architectures transmembrane transport are delineated, and their biosecurity and ability for resisting the complex cellular environment are evaluated. Then, the novel strategies for designing DNA sequences as target miRNA specific recognition and signal amplification modules for miRNA detection are presented. Afterward, recent advances in imaging technologies to accurately respond and produce significant signal output are summarized. Finally, the challenges and future directions in the field are discussed.

Advanced plasmonic technologies for multi-scale biomedical imaging

Plasmonic technologies are available for multi-scale biomedical imaging ranging from micrometre to angstrom level.

Core-satellite assembly of gold nanoshells on solid gold nanoparticles for a color coding plasmonic nanosensor

We present core-satellite assemblies comprising a solid gold nanoparticle as the core and hollow decahedral gold nanoshells as satellites for tuning the optical properties of the plasmonic structure for sensing. The core-satellite assemblies were fabricated on a substrate via the layer-by-layer assembly of nanoparticles linked by DNA. We used finite-difference time-domain simulations to help guide the geometrical design, and characterized the optical properties and morphology of the solid-shell nanoparticle assemblies using darkfield microscopy, single-nanostructure spectroscopy, and scanning electron microscopy. Plasmon coupling yielded resonant peaks at longer wavelengths in the red to near-infrared range for solid-shell assemblies compared with solid-solid nanoparticle assemblies. We examined sensing with the solid-shell assemblies using adenosine triphosphate (ATP) as a model target and ATP-aptamer as the linker. Binding of ATP induced disassembly and led to a decrease in the scattering intensity and a color change from red to green. The new morphology of the core-satellite assembly enabled plasmonic color-coding of multiplexed sensors. We demonstrate this potential by fabricating two types of assemblies using DNA linkers that target different molecules - ATP and a model nucleic acid. Our work expands the capability of chip-based plasmonic nanoparticle assemblies for the analysis of multiple, different types of biomolecules in small sample sizes including the microenvironment and single cells.

Highly stable measurement for nanoparticle extinction cross section by analyzing aperture-edge blurriness

In order to stabilize the extinction cross section measurement of a single nanoparticle, we propose to analyze the blurriness parameter of aperture edge images in real time, which provides a feedback to lock the sample position. Unlike the conventional spatial modulation spectroscopy (SMS) technique, a probe beam experiences both the spatial modulation by a piezo stage and the temporal modulation by a chopper. We experimentally demonstrate that the measurement uncertainty is one order magnitude less than that in the previous report. The proposed method can be readily implemented in conventional SMS systems and can help to achieve high stability for sensing based on light extinction by a single nanoparticle, which alleviate the impact from laboratory environment and increase the experimental sensitivity.

Individual Plasmonic Nanoprobes for Biosensing and Bioimaging: Recent Advances and Perspectives

With the advent of nanofabrication techniques, plasmonic nanoparticles (PNPs) have been widely applied in various research fields ranging from photocatalysis to chemical and bio-sensing. PNPs efficiently convert chemical or physical stimuli in their local environment into optical signals. PNPs also have excellent properties, including good biocompatibility, large surfaces for the attachment of biomolecules, tunable optical properties, strong and stable scattering light, and good conductivity. Thus, single optical biosensors with plasmonic properties enable a broad range of uses of optical imaging techniques in biological sensing and imaging with high spatial and temporal resolution. This work provides a comprehensive overview on the optical properties of single PNPs, the description of five types of commonly used optical imaging techniques, including surface plasmon resonance (SPR) microscopy, surface-enhanced Raman scattering (SERS) technique, differential interference contrast (DIC) microscopy, total internal reflection scattering (TIRS) microscopy, and dark-field microscopy (DFM) technique, with an emphasis on their single plasmonic nanoprobes and mechanisms for applications in biological imaging and sensing, as well as the challenges and future trends of these fields.

Effect of berberine nanoemulsion Photodynamic therapy on cervical carcinoma cell line

Cervical carcinoma is the most common gynecological cancer among young and adult women. There has been increasing interest in natural sources for cervical carcinoma treatment, especially for active compounds from plant extracts as antineoplastic agents. Berberine is an example of one these promising natural products. It is a natural isoquinoline alkaloid and comes from plants, such as Berbis, Coptis, and Hydrastis. It is widely used in Chinese medicine and has demonstrated activity against various cancer cell lines. This work aims to analyze the efficiency of berberine-containing nanoemulsions as photosensitizing agents in photodynamic therapy and their interaction with cervical carcinoma cells and immortalized human keratinocyte cell line. Among all groups tested, berberine nanoemulsions combined with photodynamic therapy induced the most statistically significant phototoxicity in the evaluated cell lines. Fluorescence microscopy demonstrated that the compound was present for up to 48 h when berberine nanoemulsions were used. The reactive oxygen species assay showed an increase in reactive oxygen species in the two studied cell lines after treatment of berberine-containing nanoemulsion combined with photodynamic therapy. The autophagy trial showed significant increases in cell death when berberine-containing nanoemulsion treatment was combined with photodynamic therapy when compared to trichostatin A treatment as a positive control. However, caspase-3 activity did not significantly increase in cervical carcinoma cells and immortalized human keratinocyte cell line. The results suggest that nanoemulsions with berberine have potential for use as photosensitizing agents in photodynamic therapy to treat cervical carcinoma.

Plasmonic Assemblies for Real-Time Single-Molecule Biosensing

Their tunable optical properties and versatile surface functionalization have sparked applications of plasmonic assemblies in the fields of biosensing, nonlinear optics, and photonics. Particularly, in the field of biosensing, rapid advances have occurred in the use of plasmonic assemblies for real-time single-molecule sensing. Compared to individual particles, the use of assemblies as sensors provides stronger signals, more control over the optical properties, and access to a broader range of timescales. In the past years, they have been used to directly reveal single-molecule interactions, mechanical properties, and conformational dynamics. This review summarizes the development of real-time single-molecule sensors built around plasmonic assemblies. First, a brief overview of their optical properties is given, and then recent applications are described. The current challenges in the field and suggestions to overcome those challenges are discussed in detail. Their stability, specificity, and sensitivity as sensors provide a complementary approach to other single-molecule techniques like force spectroscopy and single-molecule fluorescence. In future applications, the impact in real-time sensing on ultralong timescales (hours) and ultrashort timescales (sub-millisecond), time windows that are difficult to access using other techniques, is particularly foreseen.

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.

Rational Design of Functional Peptide-Gold Hybrid Nanomaterials for Molecular Interactions

Gold nanoparticles (AuNPs) have been extensively used for decades in biosensing-related development due to outstanding optical properties. Peptides, as newly realized functional biomolecules, are promising candidates of replacing antibodies, receptors, and substrates for specific molecular interactions. Both peptides and AuNPs are robust and easily synthesized at relatively low cost. Hence, peptide-AuNP-based bio-nano-technological approaches have drawn increasing interest, especially in the field of molecular targeting, cell imaging, drug delivery, and therapy. Many excellent works in these areas have been reported: demonstrating novel ideas, exploring new targets, and facilitating advanced diagnostic and therapeutic technologies. Importantly, some of them also have been employed to address real practical problems, especially in remote and less privileged areas. This contribution focuses on the application of peptide-gold hybrid nanomaterials for various molecular interactions, especially in biosensing/diagnostics and cell targeting/imaging, as well as for the development of highly active antimicrobial/antifouling coating strategies. Rationally designed peptide-gold nanomaterials with functional properties are discussed along with future challenges and opportunities.

Critical Review: digital resolution biomolecular sensing for diagnostics and life science research

One of the frontiers in the field of biosensors is the ability to quantify specific target molecules with enough precision to count individual units in a test sample, and to observe the characteristics of individual biomolecular interactions. Technologies that enable observation of molecules with "digital precision" have applications for in vitro diagnostics with ultra-sensitive limits of detection, characterization of biomolecular binding kinetics with a greater degree of precision, and gaining deeper insights into biological processes through quantification of molecules in complex specimens that would otherwise be unobservable. In this review, we seek to capture the current state-of-the-art in the field of digital resolution biosensing. We describe the capabilities of commercially available technology platforms, as well as capabilities that have been described in published literature. We highlight approaches that utilize enzymatic amplification, nanoparticle tags, chemical tags, as well as label-free biosensing methods.

Sensing Biomarkers with Plasmonics

The detection of biomarkers is critical for enabling early disease diagnosis, monitoring the progression, and tracking the effectiveness of therapeutic intervention. Plasmonic sensors exhibit a broad range of analytical capabilities, from the rapid generation of colorimetric readouts to single-molecule sensitivity in ultralow sample volumes, which have led to their increased exploration in bioanalysis and point-of-care applications. This perspective presents selected accounts of recent developments on the different types of plasmonic sensing platforms, the pervasive challenges, and outlook on the pathway to translation. We highlight the sensing of upcoming biomarkers, including microRNA, circulating tumor cells, exosomes, and cell-free DNA, and discuss the opportunity of utilizing plasmonic nanomaterials and tools for biomarker detection beyond biofluids, such as in tissues, organs, and disease sites. The integration of plasmonic biosensors with established and upcoming technologies of instrumentation, sample pretreatment, and data analysis will help realize their translation to clinical settings for improving healthcare and enhancing the quality of life.

Monitoring Proteolytic Activity in Real Time: A New World of Opportunities for Biosensors

Proteases play a pivotal role in several biological processes, from digestion, cell proliferation, and differentiation to fertility. Deregulation of protease metabolism can result in several pathological conditions (i.e., cancer, neurodegenerative disorders, and others). Therefore, monitoring proteolytic activity in real time could have a fundamental role in the early diagnosis of these diseases. Herein, the main approaches used to develop biosensors for monitoring proteolytic activity are reviewed. A comparison of the advantages and disadvantages of each approach is provided along with a discussion of their importance and promising opportunities for the early diagnosis of severe diseases. This new era of biosensors can be characterized by the ability to control and monitor biological processes, ultimately improving the potential of personalized medicine.

Discrimination between target and non-target interactions on the viral surface by merging fluorescence emission into Rayleigh scattering

Direct and quantitative determination of antibodies or cellular receptors dynamically binding to the surface of viral particles is the key issue for predicting the efficacy of therapeutic materials or host susceptibility to a new emerging pathogen. However, targeted visualization of infectious viruses is still highly challenging owing to their nanoscopic sizes and uncontrollable nonspecific interactions with loading molecules responsible for false signals. Here we present a multimodal single-molecule and single-particle (SMSP) visualization capable of simultaneously yet independently tracking Rayleigh scattering and fluorescence that, respectively, are generated from viruses (approximately 100 nm) and labeled interacting molecules. By analyzing real-time trajectories of fluorescent antibodies against a virus surface protein with reference to single virus-derived Rayleigh scattering, we determined heterogeneous binding stoichiometry of virus-antibody couplings irrespective of the nonspecific binder population. Therefore, our multimodal (or multi-level) SMSP assay visually identifies and selectively quantifies specific interactions between them with single binding event accuracy. As a 'specific-binding quantifier' to assess variable host susceptibility to a virus, it was further applied for distinguishing ratiometric bindings and spontaneous dissociation kinetics of synthesized isomeric receptors to influenza virus. The present framework could offer a solid analytical foundation for the development of a direct-acting antiviral agent inhibiting an integral viral enveloped protein and for nanobiological investigation for dissecting spatiotemporal nanoparticle-molecule interactions, which have been scarcely explored compared to those among plasmonic nanoparticles or among molecules only.

Metabolic mapping with plasmonic nanoparticle assemblies

A rapid and simple methodology for the biomolecular analysis of single cells and microenvironments via a stick-and-peel plasmonic sensing platform is reported. Substrate-bound assemblies of plasmonic gold nanoparticles linked by reconfigurable oligonucleotides undergo disassembly upon target binding. Changes in the light scattering intensity of thousands of discrete nanoparticle assemblies are extrapolated concomitantly to yield the mapping of local target concentrations. The methodology is completely free of labelling, purification and separation steps. We quantified the intracellular ATP levels for two ovarian cancer cell lines to elucidate the differences and cellular distribution, and demonstrated the potential of the stick-and-peel platform for mapping the microenvironment of a 2D heterogeneous surface. The portable and economical analytical platform may broaden the affordability and applicability of single-cell based analyses and enable new opportunities in clinical care such as on-site molecular pathology.

Metallic Nanoparticle-Based Optical Cell Chip for Nondestructive Monitoring of Intra/Extracellular Signals

The biosensing platform is noteworthy for high sensitivity and precise detection of target analytes, which are related to the status of cells or specific diseases. The modification of the transducers with metallic nanoparticles (MNPs) has attracted attention owing to excellent features such as improved sensitivity and selectivity. Moreover, the incorporation of MNPs into biosensing systems may increase the speed and the capability of the biosensors. In this review, we introduce the current progress of the developed cell-based biosensors, cell chip, based on the unique physiochemical features of MNPs. Mainly, we focus on optical intra/extracellular biosensing methods, including fluorescence, localized surface plasmon resonance (LSPR), and surface-enhanced Raman spectroscopy (SERS) based on the coupling of MNPs. We believe that the topics discussed here are useful and able to provide a guideline in the development of new MNP-based cell chip platforms for pharmaceutical applications such as drug screening and toxicological tests in the near future.

Plasmonic Nanoparticle-Interfaced Lipid Bilayer Membranes

Plasmonic nanoparticles are widely exploited in diverse bioapplications ranging from therapeutics to biosensing and biocomputing because of their strong and tunable light-matter interactions, facile and versatile chemical/biological ligand modifications, and biocompatibility. With the rapid growth of nanobiotechnology, understanding dynamic interactions between nanoparticles and biological systems at the molecular or single-particle level is becoming increasingly important for interrogating biological systems with functional nanostructures and for developing nanoparticle-based biosensors and therapeutic agents. Therefore, significant efforts have been devoted to precisely design and create nano-bio interfaces by manipulating the nanoparticles' size, shape, and surface ligand interactions with complex biological systems to maximize their performance and avoid unwanted responses, such as their agglomeration and cytotoxicity. However, investigating physicochemical interactions at the nano-bio interfaces in a quantitative and controllable manner remains challenging, as the interfaces involve highly complex networks between nanoparticles, biomolecules, and cells across multiple scales, each with a myriad of different chemical and biological interactions. A lipid bilayer is a membrane made of two layers of lipid molecules that forms a barrier around cells and plays structural and functional roles in diverse biological processes because they incorporate and present functional molecules (such as membrane proteins) with lateral fluidity. Plasmonic nanoparticles conjugated on lipid membranes provide reliable analytical labels and functional moieties that allow for studying and manipulating interactions between nanoparticles and molecules with single-particle resolution; they also serve as efficient tools for applying optical, mechanical, and thermal stimuli to biological systems, which stem from plasmonic properties. Recently, new opportunities have emerged by interfacing nanoparticle-modified lipid bilayers (NLBs) with complex systems such as molecular circuits and living systems. In this Account, we briefly review how plasmonic properties can be beneficially harnessed on lipid bilayer membranes to investigate the structures and functions of cellular membranes and to develop new platforms for biomedical applications. In particular, we discuss the versatility of supported lipid bilayers (SLBs), which are planar lipid bilayers on hydrophilic substrates, as dynamic biomaterials that provide lateral fluidity and cell membrane-like environments. We then summarize our efforts to create a quantitative analytical platform utilizing nanoparticles as active building blocks and SLBs as integrative substrates. Through this bottom-up approach, various functionalized nanoparticles have been introduced onto lipid bilayers to render nanoparticle-nanoparticle, nanoparticle-lipid bilayer, and biomolecule-lipid bilayer interfaces programmable. Our system provides a new class of tools for studying thermodynamics and kinetics in complex networks of nanostructures and for realizing unique applications in biosensing and biocomputing.

Tunable gap plasmons in gold nanospheres adsorbed into a pH-responsive polymer film

HYPOTHESIS

Plasmon nanorulers are exquisitely sensitive distance sensors that are based on the electromagnetic interaction between metal nanoparticles and surfaces. We hypothesize that nanorulers can act as quantitative probes of processes such as particle aggregation and adsorption, and deploy them to investigate particle adsorption onto stimulus-responsive polymer films. While such systems have previously been qualitatively investigated with plasmon nanorulers, our quantitative analysis should provide deeper insights.

EXPERIMENT

Gold nanospheres are adsorbed from solution onto pH-responsive, amine-rich polyelectrolyte multilayer (PEM) films that are either directly deposited on a gold substrate or onto an intermediate self-assembled monolayer (SAM) of charged thiols. Fitting the optical scattering spectrum to a full-wave calculation, we quantify the sphere-substrate gap distance with good accuracy.

FINDINGS

We find that the gold spheres partially embed into the PEMs rather than ride on top of them, and that although the amount of actuation of the spheres afforded by tuning the pH is well controlled, it is significantly smaller than the corresponding thickness changes in unstrained films. Further, the presence of a SAM below the PEM increases the amount of polymer in the PEM, except for the thickest and most highly charged films, where the SAM instead appears to displace from the area below the nanospheres.

All-optical tunable plasmonic nano-aggregations for surface-enhanced Raman scattering

Interparticle forces play a crucial role in nanoparticle-based nanoscience and nanoengineering for synthesizing new materials, manipulating nanoscale structures, understanding biological processes and ultrasensitive sensing. Complicated by the fluid-dynamical and chemical nature of the liquid environment of nanoparticles, previous attempts are limited to electromagnetic and chemical methods. Alternatively, optically induced forces provide a convenient and fabrication-free route to manipulate nanoparticles at the nanoscale. Here we demonstrate a new double laser trapping scheme for metallic nano-aggregation by inducing strong near-field optical interparticle forces without any chemical agents or complicated fabrication processes. These induced optical forces arising from strong localized plasmon resonance strongly depend on the interparticle separation well beyond the diffraction limit and the polarization of the incident laser field. We examine such sub-resolved interparticle separation in trapped nanoaggregates by measuring surface-enhanced Raman scattering, and further demonstrate the single-molecule sensitivity by implementing such nanostructures. This new technique opens a new avenue for all-optical manipulation of nanomaterials as well as ultra-sensitive bio-chemical sensing applications.

Real-Time in Situ Visualizing of the Sequential Activation of Caspase Cascade Using a Multicolor Gold-Selenium Bonding Fluorescent Nanoprobe

The caspase cascade is an ensemble of very important signaling molecules that plays a critical role in cell apoptosis. Real-time monitoring of the upstream and downstream activation relationships of the caspases in the signal pathway is of great significance for understanding the regulatory mechanisms of these signaling molecules in the development of various diseases. Herein, a multicolor fluorescent nanoprobe, GNP-Se-Casp, has been developed based on Au-Se bonding for real-time in situ monitoring caspase- (casp-) 3, 8, and 9 during cell apoptosis. In the real-time fluorescence imaging of apoptotic HeLa cells induced by staurosporine using GNP-Se-Casp, the fluorescence signals corresponding to casp-8 and casp-9 sequentially turn on, followed by the appearance of the fluorescence of casp-3, which visualizes the upstream and downstream relationships of casp-3, -8, and -9. Thus, GNP-Se-Casp is an effective tool for real-time in situ monitoring of caspase cascade activation in the apoptosis process of tumor cells. This design strategy is easily adaptable to in situ detection of other signal molecules, especially those with upstream and downstream activation relationships.

Development of an advanced nanoformulation for the intracellular delivery of a caspase-3 selective activity-based probe

The ability to label active caspase-3 represents a useful pharmacodynamic strategy to determine the efficacy of anti-tumour drugs. Activity-based probes (ABPs) provide a method for the labelling of activated caspases and the recent development of hybrid combinatorial substrate libraries (HyCoSuL) has allowed for the generation of highly selective ABPs to discriminately label these proteases. Here using this approach, a novel caspase-3 selective ABP (CS1) has been developed and validated in apoptotic cells to selectively bind caspase-3 over the closely related caspase-7. However, a critical bottleneck for ABPs is their cell penetrance and therefore this cell-impermeable CS1 probe was subsequently formulated into PLGA-based nanoparticles (CS1-NPs). We demonstrate the ability of these particles to be taken up by the cells and facilitate intracellular delivery of the ABP to effectively label caspase 3 in response to apoptotic stimuli. This work forms the foundation of a novel approach for the labelling of caspase 3 and may have downstream utility to measure real time apoptosis in tumours and other organs.

A graphene oxide nanosensor enables the co-delivery of aptamer and peptide probes for fluorescence imaging of a cascade reaction in apoptotic signaling

Cytochrome c (Cyt c) and caspase-3 are the key mediators in apoptotic signaling. As is known to all, the release of Cyt c from mitochondria is a vital caspase activation pathway and defines the point of no-return in cell apoptosis. However, it has not been reported that any fluorescence imaging tools could allow simultaneous visualization of Cyt c translocation and caspase-3 activation in apoptotic cells. Here, we develop a sensitive nanosensor that holds the capability of imaging of the released Cyt c from the mitochondria and a caspase-3 activation cascade reaction in apoptotic signaling. The nanosensor is constructed by the assembly of a fluorophore (Cy5)-tagged DNA aptamer on graphene nanosheets that have been covalently immobilized with a FAM-labeled peptide. After a spatially selective delivery into the cytoplasm, the Cy5-tagged DNA aptamer assembled on the nanosensor can bind with Cyt c released from the mitochondria to the cytoplasm and dissociate from graphene, triggering a red fluorescence signal. In addition, the caspase-3 activated by the Cyt c released to the cytoplasm can cleave the FAM-labeled peptide and result in a green fluorescence output. The nanosensor exhibits rapid response, high sensitivity and selectivity for in vitro assays, and high contrast imaging of Cyt c and caspase-3 in living cells. It also provides the method for the study of the kinetic relationship between the Cyt c translocation and caspase-3 activation through simultaneous imaging of Cyt c and caspase-3. The developed nanosensor described here will be an efficient and potential platform for apoptosis research.

Dynamic Single Molecular Rulers: Toward Quantitative Detection of MicroRNA-21 in Living Cells

Innovative techniques to measure microRNA (miRNA) in vivo could greatly improve the fundamental understanding of complex cellular processes. Herein, we report a novel method for real-time, quantitative miRNA detection inside living cells based on core-satellite plasmon rulers (PRs). This approach allows for the statistical analysis of single hybridization event caused by target miRNA. We investigated hundreds of satellite leaving events and found that the distribution of the time range for one strand displacement event is miRNA concentration-dependent, which obeyed Poisson statistics. Probing several such PRs under dark-field microscopy would provide precise determination of miRNA in vitro and in living cells, without photobleaching or blinking of the fluorophores. We believe the simple and practical approach on the basis of dynamic PRs with single-molecule sensitivity combined with statistical analysis hold promising potential to visualize native nucleic acids with short sequence and low-abundance.

Single-Molecule Kinetics in Living Cells

In the past decades, advances in microscopy have made it possible to study the dynamics of individual biomolecules in vitro and resolve intramolecular kinetics that would otherwise be hidden in ensemble averages. More recently, single-molecule methods have been used to image, localize, and track individually labeled macromolecules in the cytoplasm of living cells, allowing investigations of intermolecular kinetics under physiologically relevant conditions. In this review, we illuminate the particular advantages of single-molecule techniques when studying kinetics in living cells and discuss solutions to specific challenges associated with these methods.

Conformational Dynamics of a Single Protein Monitored for 24 h at Video Rate

We use plasmon rulers to follow the conformational dynamics of a single protein for up to 24 h at a video rate. The plasmon ruler consists of two gold nanospheres connected by a single protein linker. In our experiment, we follow the dynamics of the molecular chaperone heat shock protein 90 (Hsp90), which is known to show "open" and "closed" conformations. Our measurements confirm the previously known conformational dynamics with transition times in the second to minute time scale and reveals new dynamics on the time scale of minutes to hours. Plasmon rulers thus extend the observation bandwidth 3-4 orders of magnitude with respect to single-molecule fluorescence resonance energy transfer and enable the study of molecular dynamics with unprecedented precision.

Plasmon Coupling Effect-Enhanced Imaging of Metal Ions in Living Cells Using DNAzyme Assembled Core-Satellite Structures

We demonstrate a core-satellite plasmonic nanoprobe assembled via metal-ion-dependent DNA-cleaving DNAzyme linker for imaging intercellular metal ion based on plasmon coupling effect at a single-particle level. As metal ions are present in the system, the DNAzyme linker will be cleaved, and thus, disassembly of the core-satellite nanoprobes occurs, which results in distinct blue shift of the scattering spectra of Au core-satellite probes and naked color change of the scattering light. This change in scattering spectra has been supported by theoretical simulations. As a proof of concept, sensitive detection of Cu²⁺ with a limit of detection down to 67.2 pM has been demonstrated. The nanoprobes have been further utilized for intracellular Cu²⁺ imaging in living cells. The results demonstrate that the present strategy provides a promising platform for detection and imaging of metal ions in living cells and could be potentially applied to imaging other interesting target molecules simply by substituting the oligonucleotide sequence.

Advances in nanomaterial based optical biosensing and bioimaging of apoptosis via caspase-3 activity: a review

Caspase-3 plays a vital role in intrinsic and extrinsic pathways of programed cell death and in cell proliferation. Its detection is an important tool for early detection of some cancers and apoptosis-related diseases, and for monitoring the efficacy of pharmaceuticals and of chemo- and radiotherapy of cancers. This review (with 72 references) summarizes nanomaterial based methods for signal amplification in optical methods for the determination of caspase-3 activity. Following an introduction into the field, a first large section covers optical assays, with subsections on luminescent and chemiluminescence, fluorometric (including FRET based), and colorimetric assays. Further section summarize methods for bioimaging of caspase-3. A concluding section covers current challenges and future perspectives. Graphical Abstract ᅟ.

Electromagnetic Energy Redistribution in Coupled Chiral Particle Chain-Film System

Metal nanoparticle-film system has been proved that it has the ability of focusing light in the gap between particle and film, which is useful for surface-enhanced Raman scattering and plasmon catalysis. The rapid developed plasmonic chirality can also be realized in such system. Here, we investigated an electromagnetic energy focusing effect and chiral near-field enhancement in a coupled chiral particle chain on gold film. It shows large electric field enhancement in the gap between particle and film, as well as chiral near field. The enhancement properties at resonant peaks for the system excited by left circularly polarized light and right circularly polarized light are obviously different. This difference resulted from the interaction of circularly polarized light and the chiral particle-film system is analyzed with plasmon hybridization. The enhanced optical activity can provide promising applications for the enhancement of chiral molecule sensor for this chiral particle chain-film system.

Hybrid Plasmonic Microring Nano-Ruler

Surface plasmonic polariton (SPP) has attracted increasing interest for its ability of confining light in the subwavelength scale and breaking the diffraction limit. Recently, there have appeared several important developments of SPP applied in plasmon rulers, waveguides and resonators. By combing these concepts we present a novel hybrid plasmonic microring nano-ruler relying on the sensitive hybrid mode property and the microring resonator structure. The designed nano-ruler can measure distance in nanoscale resolution and offer adjustable sensitivity, which exceeds 14.8 as the distance is less than 5 nm by recording the transmission spectra and outstrips 200 dB/nm by observing the shift of output intensity. These demonstrations suggest that hybrid plasmonic microring nano-ruler could be a promising candidate enabling high-resoluation measurement.

Peptide-based nanoprobes for molecular imaging and disease diagnostics

Pathological changes in a diseased site are often accompanied by abnormal activities of various biomolecules in and around the involved cells. Identifying the location and expression levels of these biomolecules could enable early-stage diagnosis of the related disease, the design of an appropriate treatment strategy, and the accurate assessment of the treatment outcomes. Over the past two decades, a great diversity of peptide-based nanoprobes (PBNs) have been developed, aiming to improve the in vitro and in vivo performances of water-soluble molecular probes through engineering of their primary chemical structures as well as the physicochemical properties of their resultant assemblies. In this review, we introduce strategies and approaches adopted for the identification of functional peptides in the context of molecular imaging and disease diagnostics, and then focus our discussion on the design and construction of PBNs capable of navigating through physiological barriers for targeted delivery and improved specificity and sensitivity in recognizing target biomolecules. We highlight the biological and structural roles that low-molecular-weight peptides play in PBN design and provide our perspectives on the future development of PBNs for clinical translation.

Exploration of the Kinetics of Toehold-Mediated Strand Displacement via Plasmon Rulers

DNA/RNA strand displacement is one of the most fundamental reactions in DNA and RNA circuits and nanomachines. In this work, we reported an exploration of the dynamic process of the toehold-mediated strand displacement via core-satellite plasmon rulers at the single-molecule level. Applying plasmon rulers with unlimited lifetime, single-strand displacement triggered by the invader that resulted in stepwise leaving of satellite from the core was continuously monitored by changes of scattering signal for hours. The kinetics of strand displacement in vitro with three different toehold lengths have been investigated. Also, the study revealed the difference in the kinetics of strand displacement between DNA/RNA and DNA/DNA duplexes. For the kinetics study in vivo, influence from the surrounding medium has been evaluated using both phosphate buffer and cell lysate. Applying core-satellite plasmon rulers with high signal/noise ratio, kinetics study in living cells proceeded for the first time, which was not possible by conventional methods with a fluorescent reporter. The plasmon rulers, which are flexible, easily constructed, and robust, have proven to be effective tools in exploring the dynamical behaviors of biochemical reactions in vivo.

Hybrid Lithographic and DNA-Directed Assembly of a Configurable Plasmonic Metamaterial That Exhibits Electromagnetically Induced Transparency

Metamaterials are architectures that interact with light in novel ways by virtue of symmetry manipulation, and have opened a window into studying unprecedented light-matter interactions. However, they are commonly fabricated via lithographic methods, are usually static structures, and are limited in how they can react to external stimuli. Here we show that by combining lithographic techniques with DNA-based self-assembly methods, we can construct responsive plasmonic metamaterials that exhibit the plasmonic analog of an effect known as electromagnetically induced transparency (EIT), which can dramatically change their spectra upon motion of their constituent parts. Correlative scanning electron microscopy measurements, scattering dark-field microscopy, and computational simulations are performed on single assemblies to determine the relationship between their structures and spectral responses to a variety of external stimuli. The strength of the EIT-like effect in these assemblies can be tuned by precisely controlling the positioning of the plasmonic nanoparticles in these structures. For example, changing the ionic environment or dehydrating the sample will change the conformation of the DNA linkers and therefore the distance between the nanoparticles. Dark-field spectra of individual assemblies show peak shifts of up to many tens of nanometers upon DNA perturbations. This dynamic metamaterial represents a stepping stone toward state-of-the-art plasmonic sensing platforms and next-generation dynamic metamaterials.

Tuning the Sensing Performance of Multilayer Plasmonic Core-Satellite Assemblies for Rapid Detection of Targets from Lysed Cells

Optical sensors based on discrete plasmonic nanostructures are invaluable for probing biomolecular interactions when applied as plasmonic rulers or reconfigurable multinanoparticle assemblies. However, their adaptation as a versatile sensing platform is limited by the research-grade instrumentation required for single-nanostructure imaging and/or spectroscopy and complex data fitting and analysis. Additionally, the dynamic range is often too narrow for the quantitative analysis of targets of interest in biodiagnostics, food safety, or environmental monitoring. Herein we present plasmonic assembly comprising a core nanoparticle surrounded by multiple layers of satellite nanoparticles through aptamer linker. The layer-by-layer assembly of the satellite nanoparticles yields uniform discrete nanoparticle clusters on a substrate with enhanced optical properties. Binding of the model target (adenosine 5'-triphosphate, ATP) induces disassembly and leads to a dramatic decrease in the scattering intensity that can be analyzed readily from darkfield images. We demonstrate that the sensing performance, such as detection limit, dynamic range, and sensitivity, can be tuned by controlling the size of the assembly. The substrate-anchored nanoparticle assemblies are selective to only ATP, and not other adenine-containing compounds. By adapting the methodology to a flexible support, cellular ATP can be directly detected by lysing adherent cells in close contact with the plasmonic assemblies-a process that does not require any sample preparation or purification. Enhancing the optical detection signal via designing and engineering nanoparticle assemblies could enable their use with low-cost portable imaging systems and broaden their applicability beyond the study of biomolecular interaction.

In Situ Detection and Imaging of Telomerase Activity in Cancer Cell Lines via Disassembly of Plasmonic Core-Satellites Nanostructured Probe

The label-free localized surface plasmon resonance (LSPR) detection technique has been identified as a powerful means for in situ investigation of biological processes and localized chemical reactions at single particle level with high spatial and temporal resolution. Herein, a core-satellites assembled nanostructure of Au₅₀@Au₁₃ was designed for in situ detection and intracellular imaging of telomerase activity by combining plasmonic resonance Rayleigh scattering spectroscopy with dark-field microscope (DFM). The Au₅₀@Au₁₃ was fabricated by using 50 nm gold nanoparticles (Au₅₀) as core and 13 nm gold nanoparticles (Au₁₃) as satellites, both of them were functionalized with single chain DNA and gathered proximity through the highly specific DNA hybridization with a nicked hairpin DNA (O1) containing a telomerase substrate (TS) primer as linker. In the presence of telomerase, the telomeric repeated sequence of (TTAGGG)_n extended at the 3'-end of O1 would hybridized with its complementary sequences at 5'-ends. This led the telomerase extension product of O1 be folded to form a rigid hairpin structure. As a result, the Au₅₀@Au₁₃ was disassembled with the releasing of O1 and Au₁₃-S from Au₅₀-L, which dramatically decreased the plasmon coupling effect. The remarkable LSPR spectral shift was observed accompanied by a detectable color change from orange to green with the increase of telomerase activity at single particle level with a detection limit of 1.3 × 10^-13 IU. The ability of Au₅₀@Au₁₃ for in situ imaging intracellular telomerase activity, distinguishing cancer cells from normal cells, in situ monitoring the variation of cellular telomerase activity after treated with drugs were also demonstrated.

Metal-Substrate-Mediated Plasmon Hybridization in a Nanoparticle Dimer for Photoluminescence Line-Width Shrinking and Intensity Enhancement

Metal-film-coupled nanoparticles with subnanometer particle-film gaps possess an ultrasmall mode volume, responsible for a variety of intriguing phenomena in plasmonic nanophotonics. Due to the large radiative loss associated with dipolar coupling, however, the plasmonic-film-coupled nanocavities usually feature a low-quality factor, setting an ultimate limit of the increased light-matter interaction strength. Here, we demonstrate a plasmonic nanocavity composed of a metal-film-coupled nanoparticle dimer, exhibiting a significantly improved quality factor. Compared to a silica-supported dimer, the spectral line width of the nanocavity plasmon resonance is reduced by a factor of ∼4.6 and is even smaller than its monomer counterpart (∼30% reduction). Comprehensive theoretical analyses reveal that this pronounced resonance narrowing effect can be attributed to intense film-mediated plasmon hybridization between the bonding dipolar and quadrupolar gap modes in the dimer. More importantly, the invoking of the dark quadrupole resonance leads to a giant photoluminescence intensity enhancement (∼200 times) and dramatic emission line-width narrowing (∼4.6 times), compared to the silica-supported dimer. The similar spectral characteristics of the measured plasmonic scattering and photoluminescence emission indicate that the radiative decay of the coupled plasmons in the nanocavity is the origin of the observed photoluminescence, consistent with a proposed phenomenological model. Numerical calculations show that the intensity enhancement is mainly contributed by the dimer-film gap rather than the interparticle gap. These findings not only shed more light on the hybridized interaction between plasmon modes but also deepen the understanding of photoluminescence emission in coupled plasmonic nanostructures.

Planet-Satellite Micellar Superstructures Formed by ABCB Terpolymers in Solution

The occurrence and relative stability of planet-satellite nanostructures, composed of a host micelle (the planet) accompanied by a number of guest micelles (the satellites), in ABCB tetrablock terpolymer solutions are studied using the polymeric self-consistent field theory and dissipative particle dynamics simulations. The theoretical results demonstrate that the self-assembly of the ABCB tetrablock terpolymers with solvophobic A- and C-blocks and solvophilic B-blocks could lead to the formation of various planet-satellite superstructures, where the planet and satellites are composed of the A- and C-blocks, respectively. Furthermore, the number of satellites is controlled by the ratio of the two B-blocks. The arrangement of the satellites surrounding the planet resembles the solution of the well-known Thomson's problem concerning the optimum arrangement of a given number of electrons on a sphere. Besides providing a facile route to engineering novel multicompartment micelles with planet-satellite superstructures for potential advanced applications, the study strengthens the prospect that multiblock copolymers could become a useful platform for the fabrication of complex nanostructures.

Collagen-Gold Nanoparticle Conjugates for Versatile Biosensing

Integration of noble metal nanoparticles with proteins offers promising potential to create a wide variety of biosensors that possess both improved selectivity and versatility. The multitude of functionalities that proteins offer coupled with the unique optical properties of noble metal nanoparticles can allow for the realization of simple, colorimetric sensors for a significantly larger range of targets. Herein, we integrate the structural protein collagen with 10 nm gold nanoparticles to develop a protein-nanoparticle conjugate which possess the functionality of the protein with the desired colorimetric properties of the nanoparticles. Applying the many interactions that collagen undergoes in the extracellular matrix, we are able to selectively detect both glucose and heparin with the same collagen-nanoparticle conjugate. Glucose is directly detected through the cross-linking of the collagen fibrils, which brings the attached nanoparticles into closer proximity, leading to a red-shift in the LSPR frequency. Conversely, heparin is detected through a competition assay in which heparin-gold nanoparticles are added to solution and compete with heparin in the solution for the binding sites on the collagen fibrils. The collagen-nanoparticle conjugates are shown to detect both glucose and heparin in the physiological range. Lastly, glucose is selectively detected in 50% mouse serum with the collagen-nanoparticle devices possessing a linear range of 3-25 mM, which is also within the physiologically relevant range.

Single-molecule imaging of telomerase activity via linear plasmon rulers

A strategy for real-time monitoring of the extension of the telomerase primer based on plasmon rulers was demonstrated for the first time.

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.

Plasmon-enhanced Raman spectroscopic metrics for in situ quantitative and dynamic assays of cell apoptosis and necrosis

Plasmon-enhanced Raman spectroscopic metrics were developed for in situ quantitative and dynamic assays of viable, apoptotic and necrotic cells.

Apoptosis and necrosis are distinct cell death processes related to many cellular pathways. In situ, quantitatively and dynamically monitoring such processes may provide vitally important information for cell studies. However, such a method still remains elusive, even though current immunochemical methodologies have developed extremely valuable tools. Herein, we demonstrate Raman spectroscopic metrics for validating and quantifying apoptotic and necrotic cells based on their distinct molecular vibrational fingerprints. It not only allows us to quantify apoptotic and necrotic cell populations in situ in adherent cell samples, but also to be capable of continuously monitoring the dynamical processes of apoptosis and necrosis at the same time in one sample. This method provides comparable results with the “gold standard” of flow cytometry, moreover, with several incomparable advantages. Our work offers a powerful new tool for cell apoptosis and necrosis assays and is expected to become a benchmark technology in biological and medical studies.

Reduced graphene oxide decorated with gold nanoparticle as signal amplification element on ultra-sensitive electrochemiluminescence determination of caspase-3 activity and apoptosis using peptide based biosensor

Introduction: Growing demands for ultrasensitive biosensing have led to the development of numerous signal amplification strategies. In this report, a novel electrochemiluminescence (ECL) method was developed for the detection and determination of caspase-3 activity based on reduced graphene oxide sheets decorated by gold nanoparticles as signal amplification element and horseradish peroxidase enzyme (HRP) as ECL intensity enhancing agent. Methods: The ECL intensity of the luminol was improved by using the streptavidin coated magnetic beads and HRP in the presence of hydrogen peroxide. The cleavage behavior of caspase-3 was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques using biotinylated peptide (DEVD containing peptide) which was coated on reduced graphene oxide decorated with gold nanoparticle. The surface modification of graphene oxide was successfully confirmed by FTIR, UV-vis and x-ray spectroscopy. Results: ECL based biosensor showed that the linear dynamic range (LDR) and the lower limit of quantification (LLOQ) were 0.5-100 and 0.5 femtomolar (fM), respectively. Finally, the performance of the engineered peptide based biosensor was validated in the A549 cell line as real samples. Conclusion: The prepared peptide based biosensor could be considered as an excellent candidate for early detection of apoptosis, cell turnover, and cancer related diseases.

Nanoconjugation prolongs endosomal signaling of the epidermal growth factor receptor and enhances apoptosis

It is becoming increasingly clear that intracellular signaling can be subject to strict spatial control. As the covalent attachment of a signaling ligand to a nanoparticle (NP) impacts ligand-receptor binding, uptake, and trafficking, nanoconjugation provides new opportunities for manipulating intracellular signaling in a controlled fashion. To establish the effect of nanoconjugation on epidermal growth factor (EGF) mediated signaling, we investigate here the intracellular fate of nanoconjugated EGF (NP-EGF) and its bound receptor (EGFR) by quantitative correlated darkfield/fluorescence microscopy and density-based endosomal fractionation. We demonstrate that nanoconjugation prolongs the dwell time of phosphorylated receptors in the early endosomes and that the retention of activated EGFR in the early endosomes is accompanied by an EGF mediated apoptosis at effective concentrations that do not induce apoptosis in the case of free EGF. Overall, these findings indicate nanoconjugation as a rational strategy for modifying signaling that acts by modulating the temporo-spatial distribution of the activated EGF-EGFR ligand-receptor complex.

Oriented assembly of invisible probes: towards single mRNA imaging in living cells

Due to the complexity of biological systems and the ultralow concentration of analytes, improving the signal-to-noise ratio and lowering the limit of detection to allow highly sensitive detection is key to biomolecule analysis, especially intracellular analysis. Here, we present a method for highly sensitive imaging of mRNA in living cells by using novel invisible oriented probes to construct a turn-on signal generation mechanism from zero background. Two DNA probes (S1 and S2) are asymmetrically modified on two small gold nanoparticles (AuNPs) with a diameter of 20 nm. The hybridization of the two DNA probes with a single target mRNA leads to the formation of an AuNP dimer which shows a prominent plasmonic coupling effect. It generates a strong scattering signal from zero-background under a dark-field spectral analysis system. The unique design of the oriented assembly dimer has the ability to easily discriminate the target signal from the inherent cellular background noise in intracellular detection, thus making this approach a valuable technique for imaging single survivin mRNA and monitoring the distribution of survivin mRNA in tumor cells.

Plasmonic nanohalo optical probes for highly sensitive imaging of survivin mRNA in living cells

A novel strategy for sensitive detection of survivin mRNA based on Rayleigh light scattering spectroscopy of AuNP nanohalo probes.

Enabling High Efficiency Nanoplasmonics with Novel Nanoantenna Architectures

Surface plasmon polaritons (SPPs) are propagating excitations that arise from coupling of light with collective electron oscillations. Characterized by high field intensity and nanometric dimensions, SPPs fashion rapid expansion of interest from fundamental and applicative perspectives. However, high metallic losses at optical frequencies still make nanoplasmonics impractical when high absolute efficiency is paramount, with major challenge is efficient plasmon generation in deep nanoscale. Here we introduce the Plantenna, the first reported nanodevice with the potential of addressing these limitations utilizing novel plasmonic architecture. The Plantenna has simple 2D structure, ultracompact dimensions and is fabricated on Silicon chip for future CMOS integration. We design the Plantenna to feed channel (20 nm × 20 nm) nanoplasmonic waveguides, achieving 52% coupling efficiency with Plantenna dimensions of λ³/17,000. We theoretically and experimentally show that the Plantenna enormously outperforms dipole couplers, achieving 28 dB higher efficiency with broad polarization diversity and huge local field enhancement. Our findings confirm the Plantenna as enabling device for high efficiency plasmonic technologies such as quantum nanoplasmonics, molecular strong coupling and plasmon nanolasers.

The Spherical Nucleic Acids mRNA Detection Paradox

<p>From the 1950s onwards, our understanding of the formation and intracellular trafficking of membrane vesicles was informed by experiments in which cells were exposed to gold nanoparticles and their uptake and localisation, studied by electron microscopy.&nbsp; In the last decade, building on progress in the synthesis of gold nanoparticles and their controlled functionalisation with a large variety of biomolecules (DNA, peptides, polysaccharides), new applications have been proposed, including the imaging and sensing of intracellular events. Yet, as already demonstrated in the 1950s, uptake of nanoparticles results in confinement within an intracellular vesicle which in principle should preclude sensing of cytosolic events. To study this apparent paradox, we focus on a commercially available nanoparticle probe that detects mRNA through the release of a fluorescently-labelled oligonucleotide (unquenching the fluorescence) in the presence of the target mRNA. Using electron, fluorescence and photothermal microscopy, we show that the probes remain in endocytic compartments and that they do not report on mRNA level. We suggest that the validation of any nanoparticle-based probes for intracellular sensing should include a quantitative and thorough demonstration that the probes can reach the cytosolic compartment.</p>