Plasmon-enhanced fluorescence for biophotonics and bio-analytical applications

Fluorescence spectroscopy serves as an ultrasensitive sophisticated tool where background noises which serve as a major impediment to the detection of the desired signals can be safely avoided for detections down to the single-molecule levels. One such way of bypassing background noise is plasmon-enhanced fluorescence (PEF), where the interactions of fluorophores at the surface of metals or plasmonic nanoparticles are probed. The underlying condition is a significant spectral overlap between the localized surface plasmon resonance (LSPR) of the nanoparticle and the absorption or emission spectra of the fluorophore. The rationale being the coupling of the excited state of the fluorophore with the localized surface plasmon leads to an augmented emission, owing to local field enhancement. It is manifested in enhanced quantum yields concurrent with a decrease in fluorescence lifetimes, owing to an increase in radiative rate constants. This improvement in detection provided by PEF allows a significant scope of expansion in the domain of weakly emitting fluorophores which otherwise would have remained unperceivable. The concept of coupling of weak emitters with plasmons can bypass the problems of photobleaching, opening up avenues of imaging with significantly higher sensitivity and improved resolution. Furthermore, amplification of the emission signal by the coupling of free electrons of the metal nanoparticles with the electrons of the fluorophore provides ample opportunities for achieving lower detection limits that are involved in biological imaging and molecular sensing. One avenue that has attracted significant attraction in the last few years is the fast, label-free detection of bio-analytes under physiological conditions using plasmonic nanoparticles for point-of-care analysis. This review focusses on the applications of plasmonic nanomaterials in the field of biosensing, imaging with a brief introduction on the different aspects of LSPR and fabrication techniques.

Multiscale modeling of surface enhanced fluorescence

The fluorescence response of a chromophore in the proximity of a plasmonic nanostructure can be enhanced by several orders of magnitude, yielding the so-called surface-enhanced fluorescence (SEF). An in-depth understanding of SEF mechanisms benefits from fully atomistic theoretical models because SEF signals can be non-trivially affected by the atomistic profile of the nanostructure's surface. This work presents the first fully atomistic multiscale approach to SEF, capable of describing realistic structures. The method is based on coupling density functional theory (DFT) with state-of-the-art atomistic electromagnetic approaches, allowing for reliable physically-based modeling of molecule-nanostructure interactions. Computed results remarkably demonstrate the key role of the NP morphology and atomistic features in quenching/enhancing the fluorescence signal.

An optofluidic antenna for enhancing the sensitivity of single-emitter measurements

Many single-molecule investigations are performed in fluidic environments, for example, to avoid unwanted consequences of contact with surfaces. Diffusion of molecules in this arrangement limits the observation time and the number of collected photons, thus, compromising studies of processes with fast or slow dynamics. Here, we introduce a planar optofluidic antenna (OFA), which enhances the fluorescence signal from molecules by about 5 times per passage, leads to about 7-fold more frequent returns to the observation volume, and significantly lengthens the diffusion time within one passage. We use single-molecule multi-parameter fluorescence detection (sm-MFD), fluorescence correlation spectroscopy (FCS) and Förster resonance energy transfer (FRET) measurements to characterize our OFAs. The antenna advantages are showcased by examining both the slow (ms) and fast (50 μs) dynamics of DNA four-way (Holliday) junctions with real-time resolution. The FRET trajectories provide evidence for the absence of an intermediate conformational state and introduce an upper bound for its lifetime. The ease of implementation and compatibility with various microscopy modalities make OFAs broadly applicable to a diverse range of studies.

Ultraviolet Resonant Nanogap Antennas with Rhodium Nanocube Dimers for Enhancing Protein Intrinsic Autofluorescence

Plasmonic optical nanoantennas offer compelling solutions for enhancing light-matter interactions at the nanoscale. However, until now, their focus has been mainly limited to the visible and near-infrared regions, overlooking the immense potential of the ultraviolet (UV) range, where molecules exhibit their strongest absorption. Here, we present the realization of UV resonant nanogap antennas constructed from paired rhodium nanocubes. Rhodium emerges as a robust alternative to aluminum, offering enhanced stability in wet environments and ensuring reliable performance in the UV range. Our results showcase the nanoantenna's ability to enhance the UV autofluorescence of label-free streptavidin and hemoglobin proteins. We achieve significant enhancements of the autofluorescence brightness per protein by up to 120-fold and reach zeptoliter detection volumes, enabling UV autofluorescence correlation spectroscopy (UV-FCS) at high concentrations of several tens of micromolar. We investigate the modulation of fluorescence photokinetic rates and report excellent agreement between the experimental results and numerical simulations. This work expands the applicability of plasmonic nanoantennas to the deep UV range, unlocking the investigation of label-free proteins at physiological concentrations.

Molecularly imprinted polymers-isolated AuNP-enhanced CdTe QD fluorescence sensor for selective and sensitive oxytetracycline detection in real water samples

A molecularly imprinted polymers (MIPs)-isolated AuNP-enhanced fluorescence sensor, AuNP@MIPs-CdTe QDs, was developed for highly sensitive and selective detection of oxytetracycline (OTC) in aqueous medium. The developed sensor combined the advantages of strong fluorescence signal of metal-enhanced fluorescence (MEF), high selectivity of MIPs, and stability of CdTe QDs. The MIPs shell with specific recognition served as an isolation layer to adjust the distance between AuNP and CdTe QDs to optimize the MEF system. The sensor demonstrated the detection limit as low as 5.22 nM (2.40 μg/L) for a concentration range of 0.1-3.0 μM OTC and good recovery rates of 96.0-103.0% in real water samples. In addition, high specificity recognition for OTC over its analogs was achieved with an imprinting factor of 6.10. Molecular dynamics (MD) simulation was utilized to simulate the polymerization process of MIPs and revealed H-bond formation as the mainly binding sites of APTES and OTC, and finite-difference time-domain (FDTD) analysis was employed to obtain the distribution of electromagnetic field (EM) for AuNP@MIPs-CdTe QDs. The experimental results combined with theoretical analyses not only provided a novel MIP-isolated MEF sensor with excellent detection performance for OTC but also established a theoretical basis for the development of a new generation of sensors.

Plasmon-enhanced fluorescence for ellagic acid detection based on surface structure of gold nanoparticles

Ellagic acid (EA), as a natural polyphenolic acid, is considered a naturally occurring inhibitor of carcinogenesis. Herein, we developed a plasmon-enhanced fluorescence (PEF) probe for EA detection based on silica-coated gold nanoparticles (Au NPs). A silica shell was designed to control the distance between silica quantum dots (Si QDs) and Au NPs. The experimental results indicated that an 8.8-fold fluorescence enhancement was obtained compared with the original Si QDs. Three-dimensional finite-difference time-domain (3D-FDTD) simulations further demonstrated that the local electric field enhancement around Au NPs led to the fluorescence enhancement. In addition, the fluorescent sensor was applied for the sensitive detection of EA with a detection limit of 0.14 μM. It can be used to detect EA in pomegranate rind with a recovery rate of 100.26-107.93%. It can also be applied to the analysis of other substances by changing the identification substances. These experimental results indicated that the probe provides a good option for clinical analysis and food safety.

Plasmon-Enhanced Fluorescence of Single Quantum Dots Immobilized in Optically Coupled Aluminum Nanoholes

Fluorescence-based optical sensing techniques have continually been explored for single-molecule detection targeting myriad biomedical applications. Improving signal-to-noise ratio remains a prioritized effort to enable unambiguous detection at single-molecule level. Here, we report a systematic simulation-assisted optimization of plasmon-enhanced fluorescence of single quantum dots based on nanohole arrays in ultrathin aluminum films. The simulation is first calibrated by referring to the measured transmittance in nanohole arrays and subsequently used for guiding their design. With an optimized combination of nanohole diameter and depth, the variation of the square of simulated average volumetric electric field enhancement agrees excellently with that of experimental photoluminescence enhancement over a large range of nanohole periods. A maximum 5-fold photoluminescence enhancement is statistically achieved experimentally for the single quantum dots immobilized at the bottom of simulation-optimized nanoholes in comparison to those cast-deposited on bare glass substrate. Hence, boosting photoluminescence with optimized nanohole arrays holds promises for single-fluorophore-based biosensing.

Recent advances in plasmon-enhanced luminescence for biosensing and bioimaging

Plasmon-enhanced luminescence (PEL) is a unique photophysical phenomenon in which the interaction between luminescent moieties and metal nanostructures results in a marked luminescence enhancement. PEL offers several advantages and has been extensively used to design robust biosensing platforms for luminescence-based detection and diagnostics applications, as well as for the development of many efficient bioimaging platforms, enabling high-contrast non-invasive real-time optical imaging of biological tissues, cells, and organelles with high spatial and temporal resolution. This review summarizes recent progress in the development of various PEL-based biosensors and bioimaging platforms for diverse biological and biomedical applications. Specifically, we comprehensively assessed rationally designed PEL-based biosensors that can efficiently detect biomarkers (proteins and nucleic acids) in point-of-care tests, highlighting significant improvements in the sensing performance upon the integration of PEL. In addition to discussing the merits and demerits of recently developed PEL-based biosensors on substrates or in solutions, we include a brief discussion on integrating PEL-based biosensing platforms into microfluidic devices as a promising multi-responsive detection method. The review also presents comprehensive details about the recent advances in the development of various PEL-based multi-functional (passive targeting, active targeting, and stimuli-responsive) bioimaging probes, highlighting the scope of future improvements in devising robust PEL-based nanosystems to achieve more effective diagnostic and therapeutic insights by enabling imaging-guided therapy.

Single-Molecule Optical Biosensing: Recent Advances and Future Challenges

In recent years, the sensitivity and specificity of optical sensors has improved tremendously due to improvements in biochemical functionalization protocols and optical detection systems. As a result, single-molecule sensitivity has been reported in a range of biosensing assay formats. In this Perspective, we summarize optical sensors that achieve single-molecule sensitivity in direct label-free assays, sandwich assays, and competitive assays. We describe the advantages and disadvantages of single-molecule assays and summarize future challenges in the field including their optical miniaturization and integration, multimodal sensing capabilities, accessible time scales, and compatibility with real-life matrices such as biological fluids. We conclude by highlighting the possible application areas of optical single-molecule sensors that include not only healthcare but also the monitoring of the environment and industrial processes.

Surface Plasmon-Enhanced NIR-II Fluorescence in a Multilayer Nanoprobe for Through-Skull Mouse Brain Imaging

Fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) holds great potential for the accurate visualization of deeply located biological structures in vivo. However, the weak fluorescence of current NIR-II fluorophores remains a long-standing challenge for the ever-growing imaging demand. Here, we describe a surface plasmon-enhanced NIR-II fluorescence strategy by incorporating silica-coated gold nanorods (GNRs) and polymer dots (Pdots) into a multilayer nanostructure. Precise manipulation of the silica spacing layer thickness signifies an optimum distance of 8.6 nm, where an enhancement factor of up to 6.4 is achieved in the NIR-II imaging window. The surface plasmon enhancement approach is successfully extended to several types of Pdots fluorophores with NIR-II emission. We finally perform outer-layer encapsulation and PEGylation for the multilayer probes and demonstrate surface plasmon-enhanced NIR-II fluorescence for mouse brain imaging through the skull, which exhibits a refined signal-to-background ratio and penetration depth as compared to the clinically approved ICG dye.

Ultrasensitive detection of local acoustic vibrations at room temperature by plasmon-enhanced single-molecule fluorescence

Sensitive detection of local acoustic vibrations at the nanometer scale has promising potential applications involving miniaturized devices in many areas, such as geological exploration, military reconnaissance, and ultrasound imaging. However, sensitive detection of weak acoustic signals with high spatial resolution at room temperature has become a major challenge. Here, we report a nanometer-scale system for acoustic detection with a single molecule as a probe based on minute variations of its distance to the surface of a plasmonic gold nanorod. This system can extract the frequency and amplitude of acoustic vibrations with experimental and theoretical sensitivities of 10 pm Hz^−1/2 and 10 fm Hz^−1/2, respectively. This approach provides a strategy for the optical detection of acoustic waves based on molecular spectroscopy without electromagnetic interference. Moreover, such a small nano-acoustic detector with 40-nm size can be employed to monitor acoustic vibrations or read out the quantum states of nanomechanical devices.

.Sensitive detection of weak acoustic signals at nanometer scale is challenging. Here, the authors present an acoustic detection system based on a single molecule as a probe, where frequency and amplitude of acoustic vibrations can be extracted from its minute variations in distance to the surface of a plasmonic gold nanorod.

Can super-resolution microscopy become a standard characterization technique for materials chemistry?

The characterization of newly synthesized materials is a cornerstone of all chemistry and nanotechnology laboratories. For this purpose, a wide array of analytical techniques have been standardized and are used routinely by laboratories across the globe. With these methods we can understand the structure, dynamics and function of novel molecular architectures and their relations with the desired performance, guiding the development of the next generation of materials. Moreover, one of the challenges in materials chemistry is the lack of reproducibility due to improper publishing of the sample preparation protocol. In this context, the recent adoption of the reporting standard MIRIBEL (Minimum Information Reporting in Bio-Nano Experimental Literature) for material characterization and details of experimental protocols aims to provide complete, reproducible and reliable sample preparation for the scientific community. Thus, MIRIBEL should be immediately adopted in publications by scientific journals to overcome this challenge. Besides current standard spectroscopy and microscopy techniques, there is a constant development of novel technologies that aim to help chemists unveil the structure of complex materials. Among them super-resolution microscopy (SRM), an optical technique that bypasses the diffraction limit of light, has facilitated the study of synthetic materials with multicolor ability and minimal invasiveness at nanometric resolution. Although still in its infancy, the potential of SRM to unveil the structure, dynamics and function of complex synthetic architectures has been highlighted in pioneering reports during the last few years. Currently, SRM is a sophisticated technique with many challenges in sample preparation, data analysis, environmental control and automation, and moreover the instrumentation is still expensive. Therefore, SRM is currently limited to expert users and is not implemented in characterization routines. This perspective discusses the potential of SRM to transition from a niche technique to a standard routine method for material characterization. We propose a roadmap for the necessary developments required for this purpose based on a collaborative effort from scientists and engineers across disciplines.

Highly Stable Polymer Coating on Silver Nanoparticles for Efficient Plasmonic Enhancement of Fluorescence

Surface coating of plasmonic nanoparticles is of huge importance to suppress fluorescence quenching in plasmon-enhanced fluorescence sensing. Herein, a one-pot method for synthesizing polymer-coated silver nanoparticles was developed using a functional polymer conjugated with disulfide-containing anchoring groups. The disulfides played a crucial role in covalently bonding polymers to the surface of the silver nanoparticles. The covalent bond enabled the polymer layer to form a long-term stable coating on the silver nanoparticles. The polymer layer coated was adequately thin to efficiently achieve plasmonic enhancement of fluorescence and also thick enough to effectively suppress quenching of fluorescence, achieving a huge net enhancement of fluorescence. The polymer-coated plasmonic nanoparticles are a promising platform for demonstrating highly sensitive biosensing for medical diagnostics.

Plasmonic Enhancement of Two-Photon Excited Luminescence of Gold Nanoclusters

Plasmonic-enhanced luminescence of single molecules enables imaging and detection of low quantities of fluorophores, down to individual molecules. In this work, we present two-photon excited luminescence of single gold nanoclusters, Au₁₈(SG)₁₄, in close proximity to bare gold nanorods (AuNRs). We observed 25-times enhanced emission of gold nanoclusters (AuNCs) in near infrared region, which was mainly attributed to the resonant excitation of localized surface plasmon resonance (LSPR) of AuNRs and spectral overlap of LSPR band with photoluminescence of AuNCs. This work is an initial step in application of combined nanoparticles: gold nanorods and ultrasmall nanoclusters in a wide range of multiphoton imaging and biosensing applications.

Near-Infrared-Fluorescent Probe for Turn-On Lipopolysaccharide Analysis Based on PEG-Modified Gold Nanorods with Plasmon-Enhanced Fluorescence

Lipopolysaccharide (LPS), as the major component of the outer membrane of Gram-negative bacteria, can trigger a variety of biological effects such as sepsis, septic shock, and even multiorgan failure. Herein, we developed a near-infrared-fluorescent probe for fluorescent turn-on analysis of LPS based on plasmon-enhanced fluorescence (PEF). Gold nanorods (Au NRs) modified polyethylene glycol (PEG) was used as PEF materials. Au NRs were prepared with different longitudinal surface plasmon resonance (LSPR), and their fluorescence enhancement was investigated. Three kinds of molecular weights (1000, 5000, and 10000) of polyethylene glycol (PEG) were employed to control the distance between the Au NRs and the fluorescence substances of cyanine 7 (Cy7). Experimental analysis showed that the enhancement was related to the spectral overlap between the plasmon resonance of Au NRs and the extinction/emission of fluorophore. The three-dimensional finite-difference time-domain (3D-FDTD) simulation further revealed that the enhancement was caused by local electric field enhancement. Furthermore, the probe was used for the ultrasensitive analysis of LPS with a detection limit of 3.85 ng/mL and could quickly distinguish the Gram-negative bacterium-Escherichia coli (E. coli) (with LPS in the membrane) from Gram-positive bacterium-Staphylococcus aureus (S. aureus) (without LPS), as well as quantitative determination of E. coli with a detection limit of 1.0 × 10⁶ cfu/mL. These results suggested that the prepared probe has great potential for biomedical diagnosis and selective detection of LPS from different bacterial strains.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Correlative nanophotonic approaches to enlighten the nanoscale dynamics of living cell membranes

Dynamic compartmentalization is a prevailing principle regulating the spatiotemporal organization of the living cell membrane from the nano- up to the mesoscale. This non-arbitrary organization is intricately linked to cell function. On living cell membranes, dynamic domains or 'membrane rafts' enriched with cholesterol, sphingolipids and other certain proteins exist at the nanoscale serving as signaling and sorting platforms. Moreover, it has been postulated that other local organizers of the cell membrane such as intrinsic protein interactions, the extracellular matrix and/or the actin cytoskeleton synergize with rafts to provide spatiotemporal hierarchy to the membrane. Elucidating the intricate coupling of multiple spatial and temporal scales requires the application of correlative techniques, with a particular need for simultaneous nanometer spatial precision and microsecond temporal resolution. Here, we review novel fluorescence-based techniques that readily allow to decode nanoscale membrane dynamics with unprecedented spatiotemporal resolution and single-molecule sensitivity. We particularly focus on correlative approaches from the field of nanophotonics. Notably, we introduce a versatile planar nanoantenna platform combined with fluorescence correlation spectroscopy to study spatiotemporal heterogeneities on living cell membranes at the nano- up to the mesoscale. Finally, we outline remaining future technological challenges and comment on potential directions to advance our understanding of cell membrane dynamics under the influence of the actin cytoskeleton and extracellular matrix in uttermost detail.

Thermoresponsive Fluorescence Switches Based on Au@pNIPAM Nanoparticles

Despite numerous studies emphasizing the plasmonic impact on fluorescence, the design of a dynamic system allowing on-demand fluorescence switching in a single nanostructure remains challenging. The reversibility of fluorescence switching and the versatility of the approach, in particular its compatibility with a wide range of nanoparticles and fluorophores, are among the main experimental difficulties. In this work, we achieve reversible fluorescence switching by coupling metal nanoparticles with fluorophores through stimuli-responsive organic linkers. As a proof of concept, we link gold nanoparticles with fluorescein through thermoresponsive poly(N-isopropylacrylamide) at a tunable grafting density and characterize their size and optical response by dynamic light scattering, absorption, and fluorescence spectroscopies. We show that the fluorescence emission of these hybrid nanostructures can be switched on-demand using the thermoresponsive properties of poly(N-isopropylacrylamide). The described system presents a general strategy for the design of nanointerfaces, exhibiting reversible fluorescence switching via external control of metal nanoparticle/fluorophore distance.

DNA-Engineerable Ultraflat-Faceted Core-Shell Nanocuboids with Strong, Quantitative Plasmon-Enhanced Fluorescence Signals for Sensitive, Reliable MicroRNA Detection

There has been enormous interest in understanding and utilizing plasmon-enhanced fluorescence (PEF) with metal nanostructures, but maximizing the enhancement in a reproducible, quantitative manner while reliably controlling the distance between dyes and metal particle surface for practical applications is highly challenging. Here, we designed and synthesized fluorescence-amplified nanocuboids (FANCs) with highly enhanced and controlled PEF signals, and fluorescent silica shell-coated FANCs (FS-FANCs) were then formed to fixate the dye position and increase particle stability and fluorescence signal intensity for biosensing applications. By uniformly modifying fluorescently labeled DNA on Au nanorods and forming ultraflat Ag shells on them, we were able to reliably control the distance between fluorophores and Ag surface and obtained an ∼186 fluorescence enhancement factor with these FANCs. Importantly, FS-FANCs were utilized as fluorescent nanoparticle tags for microarray-based miRNA detection, and we achieved >10³-fold higher sensitivity than commercially available chemical fluorophores with 100 aM to 1 pM dynamic range.

In situ modulation of gold nanorod's surface charge drives the growth of end-to-end assemblies from dimers to large networks that enhance single-molecule fluorescence by 10 000-fold

End-to-end assemblies of anisotropic plasmonic nanostructures with small nanogaps are of great interest as they create strong hot spots for enhancing weak fluorescence and/or scattering of molecules. Here we report the growth of dithiol-linked end-to-end assemblies of gold nanorods from dimers to large networks containing thousands of individual nanorods, directed by in situ tuning of nanorod's surface charge. Surface charge was lowered to initiate the aggregation process but was subsequently increased to achieve slow tip-specific growth over seven days to form end-to-end networks of nanorods, which were stable in solution for over one month. Furthermore, we showed that these assemblies contained strong plasmonic hot spots which enhanced the fluorescence signal of a weak emitter by 10⁴-fold. This enhancement is approximately 10-fold larger than that obtained using a single gold nanorod and is comparable to the largest enhancement obtained using more expensive lithographically made in-plane antenna arrays.

Imaging of Nanoscale Light Confinement in Plasmonic Nanoantennas by Brownian Optical Microscopy

The strongly enhanced and confined subwavelength optical fields near plasmonic nanoantennas have been extensively studied not only for the fundamental understanding of light-matter interactions at the nanoscale but also for their emerging practical application in enhanced second harmonic generation, improved inelastic electron tunneling, harvesting solar energy, and photocatalysis. However, owing to the deep subwavelength nature of plasmonic field confinement, conventional optical imaging techniques are incapable of characterizing the optical performance of these plasmonic nanoantennas. Here, we demonstrate super-resolution imaging of ∼20 nm optical field confinement by monitoring randomly moving dye molecules near plasmonic nanoantennas. This Brownian optical microscopy is especially suitable for plasmonic field characterization because of its capabilities for polarization sensitive wide-field super-resolution imaging.

Control of Vibronic Transition Rates by Resonant Single-Molecule-Nanoantenna Coupling

Plasmonic nanostructures dramatically alter the radiative and nonradiative properties of single molecules in their vicinity. This coupling-induced change in decay channels selectively enhances specific vibronic transitions, which can enable plasmonic control of molecular reactivity. Here, we report coupling-dependent spectral emission shaping of single Rhodamine 800 molecules in the vicinity of plasmonic gold nanorods. We show that the relative vibronic transition rates of the first two vibronic transitions of the spontaneous emission spectrum can be tuned in the weak coupling regime, by approximately 25-fold, on the single molecule level.

Aggregation-Induced Plasmon Coupling-Enhanced One- and Two-Photon Excitation Fluorescence by Silver Nanoparticles

Plasmon coupling-induced intense local electrical field in the gap of closely packed metal nanoparticles (NPs) has been known capable of significantly enhancing optical properties of chromophores. Here, we have investigated aggregation-induced plasmon coupling-enhanced one-photon excitation (1PE) and two-photon excitation (2PE) fluorescence of dyes using Ag NPs of three different sizes (20, 36, and 48 nm). The fluorescence of a model dye, Rhodamine B isothiocyanate (RiTC), was prequenched by attaching to Ag NPs and subsequently enhanced upon forming aggregates of Ag NPs. It was found that aggregates of larger sized Ag NPs gave larger 1PE and 2PE fluorescence enhancement on the basis of free dyes, while aggregates of smaller counterparts displayed larger enhancement on the basis of the corresponding prequenched ones. 1PE and 2PE fluorescence were enhanced by 2.5- and 10.2-fold by aggregated 48 nm Ag NPs compared to free dyes and by 8.0- and 22.5-fold by aggregated 20 nm Ag NPs compared to the quenched ones, respectively. This scheme achieved fluorescence enhancement significantly beyond the level of fluorescence recovery, much larger than conventional turn-on fluorescence probes, which is attractive for developing sensitive fluorescence turn-on-based detection with reduced background.

Target-Triggered Assembly of Nanogap Antennas to Enhance the Fluorescence of Single Molecules and Their Application in MicroRNA Detection

Nanogap antennas are plasmonic nanostructures with a strong electromagnetic field generated at the gap region of two neighboring particles owing to the coupling of the collective surface plasmon resonance. They have great potential for improving the optical properties of fluorophores. Herein, nanogap antennas are constructed using an aqueous solution-based method to overcome the defects of weak fluorescence and photobleaching associated with traditional organic dyes, and a highly sensitive nanogap antenna-based sensing strategy is presented for the detection of low-abundance nucleic acid biomarkers via a target-triggered strand displacement amplification (SDA) reaction between two DNA hairpins that are tagged to the tips of gold nanorods (Au NRs). In the presence of targets, end-to-end Au NR dimers gradually form, and the fluorophores quenched by the Au NRs exhibit a dramatic fluorescence enhancement due to the plasmon-enhanced fluorescence effect of nanogap antennas. Meanwhile, the SDA reaction results in secondary amplification of fluorescence signals. Combined with single-molecule counting, this method applied in miRNA-21 detection can achieve a low detection limit of 97.2 × 10^-18 m. Moreover, accurate discrimination between different cells through miRNA-21 imaging demonstrates the potential of this method in monitoring the expression level of low-abundance nucleic acid biomarkers.

Ultrabright fluorescent nanoscale labels for the femtomolar detection of analytes with standard bioassays

The detection and quantification of low-abundance molecular biomarkers in biological samples is challenging. Here, we show that a plasmonic nanoscale construct serving as an ‘add-on’ label for a broad range of bioassays improves their signal-to-noise ratio and dynamic range without altering their workflow and read-out devices. The plasmonic construct consists of a bovine-serum-albumin scaffold with approximately 210 IRDye 800CW fluorophores (with fluorescence intensity approximately 6700-fold that of a single 800CW fluorophore), a polymer-coated gold nanorod acting as a plasmonic antenna, and biotin as a high-affinity biorecognition element. Its emission wavelength can be tuned over the visible and near-infrared spectral regions by modifying its size, shape and composition. It is compatible with multiplexed bead-based immunoassays (it improves the limit of detection by up to 4,750-fold in fluorescence-linked immunosorbent assays), immuno-microarrays, flow-cytometry and immunocytochemistry methods, and it shortens overall assay times and lowers sample volumes, as shown for the detection of a pro-inflammatory cytokine in mouse interstitial fluid and of urinary biomarkers in patient samples.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Interaction of atomic systems with quantum vacuum beyond electric dipole approximation

The photonic environment can significantly influence emission properties and interactions among atomic systems. In such scenarios, frequently the electric dipole approximation is assumed that is justified as long as the spatial extent of the atomic system is negligible compared to the spatial variations of the field. While this holds true for many canonical systems, it ceases to be applicable for more contemporary nanophotonic structures. To go beyond the electric dipole approximation, we propose and develop in this article an analytical framework to describe the impact of the photonic environment on emission and interaction properties of atomic systems beyond the electric dipole approximation. Particularly, we retain explicitly magnetic dipolar and electric quadrupolar contributions to the light-matter interactions. We exploit a field quantization scheme based on electromagnetic Green’s tensors, suited for dispersive materials. We obtain expressions for spontaneous emission rate, Lamb shift, multipole-multipole shift and superradiance rate, all being modified with dispersive environment. The considered influence could be substantial for suitably tailored nanostructured photonic environments, as demonstrated exemplarily.

Parametric optimization of visible wavelength gold lattice geometries for improved plasmon-enhanced fluorescence spectroscopy

We report the exploitation of spectroplasmonics for innovations in optical transducer development, specifically in the well-established application of labeled fluorescent analytes known as fluorescence spectroscopy. Presented herein are comprehensive analyses of nanoscale plasmonic lattice feature geometries using finite-difference time-domain software to determine the largest surface electric ($E$E) field enhancement resulting from localized surface plasmon resonance for reducing the limit of detection of plasmon-enhanced fluorescence. This parametric optimization of the critical dimensions of the plasmon resonance of noble metal nanostructures will enable improved excitation and emission enhancement of fluorophores used in visible wavelength fluorescence spectroscopy.

Strong Plasmon Enhancement of the Saturation Photon Count Rate of Single Molecules

Plasmon resonances have appeared as a promising method to boost the fluorescence intensity of single emitters. However, because research has focused on the enhancement at low excitation intensity, little is known about plasmon-fluorophore coupling near the point where the dye saturates. Here we study plasmon-enhanced fluorescence at a broad range of excitation intensities up to saturation. We adopt a novel DNA-mediated approach wherein dynamic single-molecule binding provides a controlled particle-fluorophore spacing, and dynamic rebinding circumvents artifacts due to photobleaching. We find that near saturation the maximum photon count rate is enhanced by more than 2 orders of magnitude at the optimal particle-fluorophore spacing, even for a dye with a high intrinsic quantum yield. We compare our results to a numerical model taking into account dye saturation. These experiments provide design rules to maximize the photon output of single emitters, which will open the door to studying fast dynamics in real time using single-molecule fluorescence.

Erythrocyte membrane-biointerfaced spherical nucleic acids: Robust performance for microRNA quantification

Direct and absolute analysis of microRNAs (miRNAs) in complex media (e.g., human serum) is still a big challenge due to the issues with off-analyte absorption, low sensitivity and specificity. In this work, we have fabricated the erythrocyte membrane-biointerfaced spherical nucleic acids (EMSNAs) for miRNA assay, which not only enables tailor-engineered signal amplification but also exhibits anti-interference property. As a consequence, it is possible to achieve a single-step quantification of miRNAs in complex media without the process of enzymatic amplification, which can vastly simplify the detection procedure. Experimental results reveal that the assay permits ultrasensitive quantification of miR-141, with a limit of detection down to 33.9 aM, and show a high selectivity for discriminating miR-200 family members. More importantly, the assay enables robust miRNA analysis in human serum and can accurately differentiate lung cancer patients and prostate cancer patients from healthy donors. Its performance may satisfy the requirements for direct, rapid, sensitive and specific early diagnosis of cancer, signifying its great potential in clinical diagnostics.

A Robust and General Approach to Quantitatively Conjugate Enzymes to Plasmonic Nanoparticles

Bioconjugates of plasmonic nanoparticles have received considerable attention due to their potential biomedical applications. Successful bioconjugation requires control over the number and activity of the conjugated proteins and the colloidal stability of the particles. In practice, this requires reoptimization of the conjugation protocol for each combination of protein and nanoparticle. Here, we report a robust and general protocol that allows for the conjugation of a range of proteins to different types of nanoparticles using very short polyethylene-glycol(PEG) linkers, while simultaneously preserving protein activity and colloidal stability. The use of short linkers ensures that the protein is located close to the particle surface, where the refractive index sensitivity and near-field enhancement are maximal. We demonstrate that the use of a Tween20 containing stabilizing buffer is critical in maintaining colloidal stability and protein function throughout the protocol. We obtain quantitative control over the average number of enzymes per particle by either varying the number of functional groups on the particle or the enzyme concentration during incubation. This new route of preparing quantitative protein-nanoparticle bioconjugates paves the way to develop rational and quantitative strategies to functionalize nanoparticles for applications in sensing, medical diagnostics, and drug delivery.

Gold nanoworms: Optical properties and simultaneous SERS and fluorescence enhancement

Fabrication of platforms for efficient Raman and fluorescence enhancement is very attractive for plasmon-based molecular sensing. For superior sensitivity, the plasmonic constituents of such platforms should be effective electromagnetic field enhancers. Furthermore, nanoparticles having plasmon peak in the spectral range of therapeutic window are superior for biomedical application. Herein, we show that worm-shaped Au nanoparticles can be used for simultaneous surface enhanced Raman scattering (SERS) and metal enhanced fluorescence (MEF). Gold nanoworms (Au-NWs) with an intense plasmon absorption in the therapeutic window were synthesized using a facile single-stepped recipe. Electron microscopy imaging revealed that Au-NWs have a non-uniform surface. Owing to their special morphology, three distinct plasmon bands were seen in the experimental spectrum. The presence of three plasmon peaks was also verified by finite element based simulations. The simulation results further show that Au-NWs can provide intense near field enhancement for multiple excitation wavelengths. As a proof of concept, we have used Au-NWs based platforms for simultaneous enhancement of fluorescence and Raman signal of rhodamine 6G (R6G) dye molecule.

Rotation of Single-Molecule Emission Polarization by Plasmonic Nanorods

The strong light-matter interactions between dyes and plasmonic nanoantennas enable the study of fundamental molecular-optical processes. Here, we overcome conventional limitations with high-throughput single-molecule polarization-resolved microscopy to measure dye emission polarization modifications upon near-field coupling to a gold nanorod. We determine that the emission polarization distribution is not only rotated toward the nanorod's dominant localized surface plasmon mode as expected, but it is also unintuitively broadened. With a reduced-order analytical model, we elucidate how this distribution broadening depends upon both far-field interference and off-resonant coupling between the molecular dipole and the nanorod transverse plasmon mode. Experiments and modeling reveal that a nearby plasmonic nanoantenna affects dye emission polarization through a multicolor process, even when the orthogonal plasmon modes are separated by approximately 3 times the dye emission line width. Beyond advancing our understanding of plasmon-coupled emission modifications, this work promises to improve high-sensitivity single-molecule fluorescence imaging, biosensing, and spectral engineering.

Construction and Evaluation of a Self-Calibrating Multiresponse and Multifunctional Graphene Biosensor

Recently, many studies have been focused on the development of graphene-based biosensors. However, they rely on one type of signal and need to be calibrated by other techniques. In this study, a nonenzymatic graphene-based biosensor has been designed and constructed. Its ability to detect glucose and Escherichia coli by three different types of signals has been investigated. For its preparation, dopamine-functionalized polyethylene glycol and 2,5-thiophenediylbisboronic acid were conjugated onto the surface of graphene sheets by nitrene [2 + 1] cycloaddition and condensation reactions, respectively. Multivalent interactions between boronic acid segments and biosystems consequently increased the quantifiable fluorescence emission and UV absorption of dopamine segments. Additionally, changing the electrochemical behavior of the functionalized graphene sheets was possible and resulted in a measurable output signal. Conjugation of mannose onto the surface of the biosensor improved its magnitude of signals and specificity for sensing E. coli in a complex medium. The efficiency and accuracy of each signal was monitored by others, which resulted in a real-time self-calibrating biosensor. Taking advantage of the versatility of the three different indicators, including florescence, UV, and electrochemistry, the functionalized graphene sheets have been used as self-regulating biosensors to detect a variety of biosystems with a high accuracy and specificity in a short time.

DNA-Assisted Assembly of Gold Nanostructures and Their Induced Optical Properties

Gold nanocrystals have attracted considerable attention due to their excellent physical and chemical properties and their extensive applications in plasmonics, spectroscopy, biological detection, and nanoelectronics. Gold nanoparticles are able to be readily modified and arranged with DNA materials and protein molecules, as well as viruses. Particularly DNA materials with the advantages endowed by programmability, stability, specificity, and the capability to adapt to functionalization, have become the most promising candidates that are widely utilized for building plenty of discrete gold nanoarchitectures. This review highlights recent advances on the DNA-based assembly of gold nanostructures and especially emphasizes their resulted superior optical properties and principles, including plasmonic extinction, plasmonic chirality, surface enhanced fluorescence (SEF), and surface-enhanced Raman scattering (SERS).

Plasmon-Enhanced Single-Molecule Enzymology

We present a numerical study on plasmon-enhanced single-molecule enzymology. We combine Brownian dynamics and electromagnetic simulations to calculate the enhancement of fluorescence signals of fluorogenic substrate converted by an enzyme conjugated to a plasmonic particle. We simulate the Brownian motion of a fluorescent product away from the active site of the enzyme, and calculate the photon detection rate taking into account modifications of the excitation and emission processes by coupling to the plasmon. We show that plasmon enhancement can boost the signal-to-noise ratio (SNR) of single turnovers by up to 100 fold compared to confocal microscopy. This enhancement factor is a trade-off between the reduced residence time in the near-field of the particle, and the enhanced emission intensity due to coupling to the plasmon. The enhancement depends on the size, shape and material of the particle and the photophysical properties of the fluorescent product. Our study provides guidelines on how to enhance the SNR of single-molecule enzyme studies and may aid in further understanding and quantifying static and dynamic heterogeneity.

Plasmonic Enhancement of Two-Photon-Excited Luminescence of Single Quantum Dots by Individual Gold Nanorods

Plasmonic enhancement of two-photon-excited fluorescence is not only of fundamental interest but also appealing for many bioimaging and photonic applications. The high peak intensity required for two-photon excitation may cause shape changes in plasmonic nanostructures, as well as transient plasmon broadening. Yet, in this work, we report on strong enhancement of the two-photon-excited photoluminescence of single colloidal quantum dots close to isolated chemically synthesized gold nanorods. Upon resonant excitation of the localized surface plasmon resonance, a gold nanorod can enhance the photoluminescence of a single quantum dot more than 10 000-fold. This strong enhancement arises from the combined effect of local field amplification and the competition between radiative and nonradiative decay rate enhancements, as is confirmed by time-resolved fluorescence measurements and numerical simulations.

Add-on plasmonic patch as a universal fluorescence enhancer

Fluorescence-based techniques are the cornerstone of modern biomedical optics, with applications ranging from bioimaging at various scales (organelle to organism) to detection and quantification of a wide variety of biological species of interest. However, the weakness of the fluorescence signal remains a persistent challenge in meeting the ever-increasing demand to image, detect, and quantify biological species with low abundance. Here, we report a simple and universal method based on a flexible and conformal elastomeric film with adsorbed plasmonic nanostructures, which we term a "plasmonic patch," that provides large (up to 100-fold) and uniform fluorescence enhancement on a variety of surfaces through simple transfer of the plasmonic patch to the surface. We demonstrate the applications of the plasmonic patch in improving the sensitivity and limit of detection (by more than 100 times) of fluorescence-based immunoassays implemented in microtiter plates and in microarray format. The novel fluorescence enhancement approach presented here represents a disease, biomarker, and application agnostic ubiquitously applicable fundamental and enabling technology to immediately improve the sensitivity of existing analytical methodologies in an easy-to-handle and cost-effective manner, without changing the original procedures of the existing techniques.

Suppression of photo-oxidation of organic chromophores by strong coupling to plasmonic nanoantennas

Intermixed light-matter quasi-particles-polaritons-have unique optical properties owing to their compositional nature. These intriguing hybrid states have been extensively studied over the past decades in a wide range of realizations aiming at both basic science and emerging applications. However, recently, it has been demonstrated that not only optical but also material-related properties, such as chemical reactivity and charge transport, may be significantly altered in the strong coupling regime of light-matter interactions. We show that a nanoscale system, composed of a plasmonic nanoprism strongly coupled to excitons in a J-aggregated form of organic chromophores, experiences modified excited-state dynamics and, therefore, modified photochemical reactivity. Our experimental results reveal that photobleaching, one of the most fundamental photochemical reactions, can be effectively controlled and suppressed by the degree of plasmon-exciton coupling and detuning. In particular, we observe a 100-fold stabilization of organic dyes for the red-detuned nanoparticles. Our findings contribute to understanding of photochemical properties in the strong coupling regime and may find important implications for the performance and improved stability of optical devices incorporating organic dyes.

Light-Driven Nano-oscillators for Label-Free Single-Molecule Monitoring of MicroRNA

Here, we present a mapping tool based on individual light-driven nano-oscillators for label-free single-molecule monitoring of microRNA. This design uses microRNA as a single-molecule damper for nano-oscillators by forming a rigid dual-strand structure in the gap between nano-oscillators and the immobilized surface. The ultrasensitive detection is attributed to comparable dimensions of the gap and microRNA. A developed surface plasmon-coupled scattering imaging technology enables us to directly measure the real-time gap distance vibration of multiple nano-oscillators with high accuracy and fast dynamics. High-level and low-level states of the oscillation amplitude indicate melting and hybridization statuses of microRNA. Lifetimes of two states reveal that the hybridization rate of microRNA is determined by the three-dimensional diffusion. This imaging technique contributes application potentials in a single-molecule detection and nanomechanics study.

Combining gold nanoparticle antennas with single-molecule fluorescence resonance energy transfer (smFRET) to study DNA hairpin dynamics

The association of a plasmonic nano-antenna with single-molecule FRET technique presents new prospects to investigate the dynamics of biological molecules. However, the presence of a plasmonic nano-antenna significantly modifies the FRET rate and efficiency; this makes its applicability to the prevalent single-molecule FRET experiments unclear. Herein, using gold nanoparticle antennas of different sizes and DNA hairpins labelled with FRET pairs (Cy3 and Cy5) as the model system, we performed experiments to study the folding dynamics of single DNA hairpins at various salt concentrations. Our results indicate that gold nanoparticle antennas can enhance single-molecule fluorescence of Cy3 and Cy5 up to 3-5 folds, substantially reduce the FRET efficiency, and alter the obtained FRET efficiency histograms. However, the folding dynamics of DNA hairpins remains unaffected, and the correct kinetic and dynamic information can still be extracted from the seriously modified FRET efficiencies. Therefore, our experiments demonstrate the feasibility and compatibility for applying plasmonic nano-antennas to the mostly used single-molecule FRET assays, which provide a broad range of possibilities for the future applications of these nano-antennas in studying various essential biological processes.

Plasmon-enhanced fluorescence spectroscopy

Fluorescence spectroscopy with strong emitters is a remarkable tool with ultra-high sensitivity for detection and imaging down to the single-molecule level. Plasmon-enhanced fluorescence (PEF) not only offers enhanced emissions and decreased lifetimes, but also allows an expansion of the field of fluorescence by incorporating weak quantum emitters, avoiding photobleaching and providing the opportunity of imaging with resolutions significantly better than the diffraction limit. It also opens the window to a new class of photostable probes by combining metal nanostructures and quantum emitters. In particular, the shell-isolated nanostructure-enhanced fluorescence, an innovative new mode for plasmon-enhanced surface analysis, is included. These new developments are based on the coupling of the fluorophores in their excited states with localized surface plasmons in nanoparticles, where local field enhancement leads to improved brightness of molecular emission and higher detection sensitivity. Here, we review the recent progress in PEF with an emphasis on the mechanism of plasmon enhancement, substrate preparation, and some advanced applications, including an outlook on PEF with high time- and spatially resolved properties.

Fast recognition of single quantum dots from high multi-exciton emission and clustering effects

Recognition of single quantum dots (QDs) from high multi-exciton emission and clustering effects is challenging using the conventional second-order correlation function method. Here we demonstrate a method for fast recognizing single QDs based on the probabilities of detecting single- and two-photon events. The time-tagged, time-resolved and time-correlated single-photon counting technique is applied to effectively remove multi-exciton emission and low-counting background. By this way, single QDs can be fastly recognized by the spatial coincidence-counting model. In addition, the fast recognition of single QDs by using the collected photons during the confocal scanning imaging process has been achieved synchronously.

Study of Fluorescence Quenching Ability of Graphene Oxide with a Layer of Rigid and Tunable Silica Spacer

The fluorescence quenching property of graphene oxide (GO) has been newly demonstrated and applied for fluorescence imaging and biosensing. In this work, a new nanostructure was designed for effectively studying the quenching ability of GO. The key element in this design is the fabrication of a layer of rigid and thickness adjustable silica spacer for manipulating the distance between the GO and fluorophores. First, a silica core modified with organic dye molecules was prepared, followed by the formation of a silica shell with a tunable thickness. Afterward, the GO was wrapped around silica nanoparticles based on the electrostatic interaction between the negatively charged GO and positively charged silica. The quenching efficiency of GO to different dye molecules was studied at various spacer thicknesses and varying concentrations of GO. Fluorescence lifetime of fluorophores was measured to determine the quenching mechanism. We found that the quenching efficiency of GO was still around 30% when the distance between dyes and GO was increased to more than 30 nm, which indicated the long-distance quenching ability of GO and confirmed the previous theoretical calculation. The quenching mechanisms were proposed schematically based on our experimental results. We expected that the proposed nanostructure could act as a feasible model for studying GO quenching property and shed light on designing GO-based fluorescence sensing systems.