Surface plasmon-coupled emission imaging for biological applications

Regulation of laser-induced nanogratings by tuning the Marangoni-plasmon-coupled effect

Laser-induced subwavelength nanogratings on films find widespread applications in enhancing a spectrum through surface plasmon excitation. It is challenging to achieve high uniformity, diversity, and controllability due to the intricate interplay between two basic mechanisms in laser nanostructuring: the Marangoni effect and surface plasmon polaritons (SPPs). We tune the coupled effect on Ge2Sb2Te5 films by adjusting the laser polarization, whose component controls the two effects' strength ratio. The Marangoni effect dominates when the SPPs' direction mismatches with the growing direction of nanogratings. Tuning this competition relationship helps to create nanogratings with tunable duty cycle and distribution, which are significant for light modulation applications. A highly efficient direct writing method with a line-shaped laser beam is employed to create large-area regular nanogratings by enhancing the effect tuning. We demonstrate diverse Au nanogratings with the aid of a lift-off operation and apply them in surface plasmon-coupled emission (SPCE), showcasing exceptional enhancement and narrowing performance.

Review of Gold Nanoparticles in Surface Plasmon-Coupled Emission Technology: Effect of Shape, Hollow Nanostructures, Nano-Assembly, Metal-Dielectric and Heterometallic Nanohybrids

Point-of-care (POC) diagnostic platforms are globally employed in modern smart technologies to detect events or changes in the analyte concentration and provide qualitative and quantitative information in biosensing. Surface plasmon-coupled emission (SPCE) technology has emerged as an effective POC diagnostic tool for developing robust biosensing frameworks. The simplicity, robustness and relevance of the technology has attracted researchers in physical, chemical and biological milieu on account of its unique attributes such as high specificity, sensitivity, low background noise, highly polarized, sharply directional, excellent spectral resolution capabilities. In the past decade, numerous nano-fabrication methods have been developed for augmenting the performance of the conventional SPCE technology. Among them the utility of plasmonic gold nanoparticles (AuNPs) has enabled the demonstration of plethora of reliable biosensing platforms. Here, we review the nano-engineering and biosensing applications of AuNPs based on the shape, hollow morphology, metal-dielectric, nano-assembly and heterometallic nanohybrids under optical as well as biosensing competencies. The current review emphasizes the recent past and evaluates the latest advancements in the field to comprehend the futuristic scope and perspectives of exploiting Au nano-antennas for plasmonic hotspot generation in SPCE technology.

Fluorescence enhancement of surface plasmon coupled emission by Au nanobipyramids and its modulation effect on multi-wavelength radiation

Surface plasmon coupled emission (SPCE), a novel surface-enhanced fluorescence technique, can generate directional and amplified radiation by the intense interaction between fluorophores and surface plasmons (SPs) of metallic nanofilms. For plasmon-based optical systems, the strong interaction between localized and propagating SPs and "hot spot" structures show great potential to significantly improve the electromagnetic (EM) field and modulate optical properties. Au nanobipyramids (NBPs) with two sharp apexes to enhance and restrict the EM field were introduced through electrostatic adsorption to achieve a mediated fluorescence system, and the emission signal enhancement was realized by factors over 60 compared with the normal SPCE. It has been demonstrated that the intense EM field produced by the NBPs assembly is what triggered the unique enhancement of SPCE by Au NBPs, which effectively overcomes the inherent signal quenching of SPCE for ultrathin sample detection. This remarkable enhanced strategy offers the chance to improve the detection sensitivity for plasmon-based biosensing and detection systems, and expand the range of applications for SPCE in bioimaging with more comprehensive and detailed information acquisition. The enhancement efficiency for various emission wavelengths was investigated in light of the wavelength resolution of SPCE, and it was discovered that enhanced emission for multi-wavelength could be successfully detected through the different emission angles due to the angular displacement caused by wavelength change. Benefit from this, the Au NBP modulated SPCE system was employed for multi-wavelength simultaneous enhancement detection under a single collection angle, which could broaden the application of SPCE in simultaneous sensing and imaging for multi-analytes, and expected to be used for high throughput detection of multi-component analysis.

Smartphone-Based Attomolar Cyanide Ion Sensing Using Au-Graphene Oxide Cryosoret Nanoassembly and Benzoxazolium-Based Fluorophore in a Surface Plasmon-Coupled Enhanced Fluorescence Interface

Photoplasmonic platforms are being demonstrated as excellent means for bridging nanochemistry and biosensing approaches at advanced interfaces, thereby augmenting the sensitivity and quantification of the desired analytes. Although resonantly coupled electromagnetic waves at the surface plasmon-coupled emission (SPCE) interface are investigated with myriad nanomaterials in order to boost the detection limits, rhodamine moieties are ubiquitously used as SPCE reporter molecules in spite of their well-known limitations. In order to overcome this constraint, in this work, a benzoxazolium-based fluorescent molecule, (E)-2-(4-(dimethylamino)styryl)-3-methylbenzo[d]oxazol-3-ium iodide (DSBO), was synthesized to selectively detect the cyanide (CN^-) ions in water samples. To this end, the sensitivity of the fabricated SPCE substrates is tested in spacer, cavity, and extended cavity nanointerfaces to rationalize the configurational robustness. The performance of the sensor is further improved with the careful engineering of gold (Au)-graphene oxide (GO) cryosoret nanoassemblies fabricated via an adiabatic cooling technology. The unique dequenching (turn-on) of the quenched (turn-off) fluorescent signal is demonstrated with the hybridized metal-π plasmon synergistic coupling in the nanovoids and nanocavities assisting delocalized Bragg and localized Mie plasmons. The spectro-plasmonic analysis yielded highly directional, polarized (>95%), and enhanced emission attributes with an attomolar limit of detection of 10 aM of CN^- ions with high linearity (R² = 0.996) and excellent reliability, in addition to an exceptional correlation with the theoretically obtained TFclac simulations. The CN^- ion sensing is experimentally validated with the smartphone-based cost-effective SPCE detection technology to render the device amenable to resource-limited settings. We believe that the unique fluorophore-cryosoret nanoassemblage presented here encourages development of frugal, unconventional, and highly desirable strategies for the selective quantitation of environmentally and physiologically relevant analytes at trace concentrations for use in point-of-care diagnostics.

Biosensing Technologies: A Focus Review on Recent Advancements in Surface Plasmon Coupled Emission

In the past decade, novel nano-engineering protocols have been actively synergized with fluorescence spectroscopic techniques to yield higher intensity from radiating dipoles, through the process termed plasmon-enhanced fluorescence (PEF). Consequently, the limit of detection of analytes of interest has been dramatically improvised on account of higher sensitivity rendered by augmented fluorescence signals. Recently, metallic thin films sustaining surface plasmon polaritons (SPPs) have been creatively hybridized with such PEF platforms to realize a substantial upsurge in the global collection efficiency in a judicious technology termed surface plasmon-coupled emission (SPCE). While the process parameters and conditions to realize optimum coupling efficiency between the radiating dipoles and the plasmon polaritons in SPCE framework have been extensively discussed, the utility of disruptive nano-engineering over the SPCE platform and analogous interfaces such as 'ferroplasmon-on-mirror (FPoM)' as well as an alternative technology termed 'photonic crystal-coupled emission (PCCE)' have been seldom reviewed. In light of these observations, in this focus review, the myriad nano-engineering protocols developed over the SPCE, FPoM and PCCE platform are succinctly captured, presenting an emphasis on the recently developed cryosoret nano-assembly technology for photo-plasmonic hotspot generation (first to fourth). These technologies and associated sensing platforms are expected to ameliorate the current biosensing modalities with better understanding of the biophysicochemical processes and related outcomes at advanced micro-nano-interfaces. This review is hence envisaged to present a broad overview of the latest developments in SPCE substrate design and development for interdisciplinary applications that are of relevance in environmental as well as biological heath monitoring.

Au-Ag Alloy Nanoshuttle Mediated Surface Plasmon Coupling for Enhanced Fluorescence Imaging

Surface plasmon-coupled emission (SPCE), a novel signal enhancement technology generated by the interactions between surface plasmons and excited fluorophores in close vicinity to metallic film, has shown excellent performance in bioimaging. Variable-angle nanoplasmonic fluorescence microscopy (VANFM), based on an SPCE imaging system, can selectively modulate the imaging depth by controlling the excitation angles. In order to further improve the imaging performance, Au-Ag alloy nanoshuttles were introduced into an Au substrate to mediate the plasmonic properties. Benefiting from the strong localized plasmon effect of the modified SPCE chip, better imaging brightness, signal-to-background ratio and axial resolution for imaging of the cell membrane region were obtained, which fully displays the imaging advantages of SPCE system. Meanwhile, the imaging signal obtained from the critical angle excitation mode was also amplified, which helps to acquire a more visible image of the cell both from near- and far-field in order to comprehensively investigate the cellular interactions.

Three-dimensional imaging of biological cells using surface plasmon coupled emission

SIGNIFICANCE

Biological cell imaging has become one of the most crucial research interests because of its applications in biomedical and microbiology studies. However, three-dimensional (3D) imaging of biological cells is critically challenging and often involves prohibitively expensive and complex equipment. Therefore, a low-cost imaging technique with a simpler optical arrangement is immensely needed.

AIM

The proposed approach will provide an accurate cell image at a low cost without needing any microscope or extensive processing of the collected data, often used in conventional imaging techniques.

APPROACH

We propose that patterns of surface plasmon coupled emission (SPCE) features from a fluorescently labeled biological cell can be used to image the cell. An imaging methodology has been developed and theoretically demonstrated to create 3D images of cells from the detected SPCE patterns. The 3D images created from the different SPCE properties at the far-field closely match the actual cell structures.

RESULTS

The developed technique has been applied to different regular and irregular cell shapes. In each case, the calculated root-mean-square error (RMSE) of the created images from the cell structures remains within a few percentages. Our work recreates the base of a circular-shaped cell with an RMSE of ≲1.4 % . In addition, the images of irregular-shaped cell bases have an RMSE of ≲2.8 % . Finally, we obtained a 3D image with an RMSE of ≲6.5 % for a random cellular structure.

CONCLUSIONS

Despite being in its initial stage of development, the proposed technique shows promising results considering its simplicity and the nominal cost it would require.

Cylindrical Al Nano-Dimer Induced Polarization in Deep UV Region

The polarization properties of asymmetric plasmonic nanostructures originating from optical anisotropy show great application prospects in many fields, such as display, sensing, filtering, and detection. Here, we report the realization of polarization control in the deep ultraviolet (UV) region using Al nano-dimer structures. The simulation results indicated that the polarization effect was generated by the modulation of inter-coupling between the quadrupole plasmon resonances of the asymmetric dimer. By further optimizing the size and gap of the dimer, the extinction in the 200-nm deep UV region obtained a polarization ratio of 18%. This research is helpful for understanding the resonance hybridization of high-order surface plasmons in UV region and is of great significance to the emerging polarized micro-nano photonics fields, such as spin optoelectronics and deep UV optoelectronic devices.

Tailoring Resonant Energy Transfer Processes for Sustainable and Bio-Inspired Sensing

Dipole–Dipole interactions (DDI) constitute an effective mechanism by which two physical entities can interact with each other. DDI processes can occur in a resonance framework if the energies of the two dipoles are very close. In this case, an energy transfer can occur without the need to emit a photon, taking the name of Förster Resonance Energy Transfer (FRET). Given their large dependence on the distance and orientation between the two dipoles, as well as on the electromagnetic properties of the surrounding environment, DDIs are exceptional for sensing applications. There are two main ways to carry out FRET-based sensing: (i) enhancing or (ii) inhibiting it. Interaction with resonant environments such as plasmonic, optical cavities, and/or metamaterials promotes the former while acting on the distance between the FRET molecules favors the latter. In this review, we browse both the two ways, pointing the spotlight to the intrinsic interdisciplinarity these two sensing routes imply. We showcase FRET-based sensing mechanisms in a variety of contexts, from pH sensors to molecular structure measurements on a nano-metrical scale, with a particular accent on the central and still mostly overlooked role played between a nano-photonically structured environment and photoluminescent molecules.

Titanium nitride as an alternative and reusable plasmonic substrate for fluorescence coupling

The development of alternative plasmonic materials that can replace gold and silver is of long-standing interest in materials research. In this study, we have prepared and characterized thin films of TiN, an emerging plasmonic material, and examined its effectiveness for fluorescence coupling in metal-dielectric structures having TiN as the plasmonically active component. We have used a combination of experiment and reflectivity calculations to determine the nature and dispersion of the optical modes sustained by the metal-dielectric structures, which furthermore are adjustable by varying the thickness of the dielectric layer. Our results reveal that fluorophores placed on the TiN substrates can couple with the surface-plasmon mode and/or the waveguide modes supported by these structures, to provide polarized and directional emission over narrow angular ranges. The performance of TiN substrates for surface plasmon-coupled emission (SPCE) and waveguide-coupled emission (WGCE) is found to be comparable with conventional Au substrates. Importantly, the TiN thin films are reusable, which is certainly advantageous for their use in SPCE or WGCE-based fluorescence sensing applications.

Single-Molecule Blinking Fluorescence Enhancement by Surface Plasmon-Coupled Emission-Based Substrates for Single-Molecule Localization Imaging

Surface plasmon-coupled emission (SPCE) substrates to enhance the blinking fluorescence of spontaneously blinking fluorophores in single-molecule localization microscopy (SMLM) were fabricated to reduce the excitation power density requirement and reveal the distribution of fluorophore-labeled proteins on a plasma membrane with nanoscale-level resolution. The systemic investigation of the contribution of local field enhancement, modified quantum yield, and emission coupling yield through glass coverslip substrates coated with metal layers of different thicknesses revealed that the silver-layer substrate with a thickness of 44 nm produces the highest SPCE fluorescence in spontaneously blinking fluorophores, and it has a highly directional SPCE fluorescence, which helps improve the detection efficiency. Moreover, the uniform and surface-enhanced field created on the substrate surface is beneficial for fluorescence background reduction in single fluorophore detection and localization, as well as for revealing the real position of fluorophores. Consequently, compared with a glass coverslip substrate, the presented SPCE substrate demonstrated a fluorescence enhancement of 480% and an increase in blinking events from a single spontaneously blinking fluorophore; moreover, the required excitation power density for SMLM imaging was significantly reduced to 23 W cm^-2 for visualizing the distribution of epidermal growth factor receptors (EGFRs) on the basal plasma membrane of A549 lung cancer cells with a localization precision of 19 ± 7 nm. Finally, the fluorophore-labeled EGFRs on the basal plasma membrane in the presence of PIKfyve-specific inhibitor treatment were explored using SPCE-SMLM imaging; the results revealed a distinct reduction in the density of localization events because of a decrease in EGFR abundance at the plasma membranes of the cells.

Diattenuation and retardance signature of plasmonic gold nanorods in turbid media revealed by Mueller matrix polarimetry

Plasmonic gold nanorods (GNRs) are finding increasing use in biomedicine due to their unique electromagnetic properties, optical contrast enhancement and biocompatibility; they also show promise as polarization contrast agents. However, quantification of their polarization-enhancing properties within heterogeneous turbid media remains challenging. We report on polarization response in controlled tissue phantoms consisting of dielectric microsphere scatterers with varying admixtures of GRNs. Experimental Mueller matrix measurements and polarization sensitive Monte-Carlo simulations show excellent agreement. Despite the GNRs’ 3D random orientation and distribution in the strong multiply scattering background, significant linear diattenuation and retardance were observed. These exclusive measurable characteristics of GNRs suggest their potential uses as contrast enhancers for polarimetric assessment of turbid biological tissue.

Plasmon Coupling Enhanced Micro-Spectroscopy and Imaging for Sensitive Discrimination of Membrane Domains of a Single Cell

Different cell membrane domains play different roles in many cell processes, and the discrimination of these domains is of considerable importance for the elucidation of cellular functions. However, the strategies available for distinguishing these cell membrane domains are limited. A novel technique called plasmon coupling enhanced micro-spectroscopy and imaging to discriminate basal and lateral membrane domains of a single cell combines the application of an additional plasmonic silver film for surface plasmon (SP) excitation to selectively excite and enhance the basal membranes in the near-field with directional enhanced microscopic imaging and spectroscopy. The SP and critical evanescent fields are induced upon excitation through a silver-coated semitransparent coverslip at the surface plasmon resonance and critical angles, respectively. The basal and lateral membrane domains located within the SP and critical evanescent fields can be selectively excited and distinguished by adjusting the incident angle of laser irradiation. Moreover, the brighter images and more intense spectra of membrane-targeting fluorescence-Raman probes under directional excitation than in conventional EPI mode allow clear identification of the membrane domains.

Surface Plasmon-Assisted Fluorescence Enhancing and Quenching: From Theory to Application

The integration of surface plasmon resonance and fluorescence yields a multiaspect improvement in surface fluorescence sensing and imaging, leading to a paradigm shift of surface plasmon-assisted fluorescence techniques, for example, surface plasmon enhanced field fluorescence spectroscopy, surface plasmon coupled emission (SPCE), and SPCE imaging. This Review aims to characterize the unique optical property with a common physical interpretation and diverse surface architecture-based measurements. The fundamental electromagnetic theory is employed to comprehensively unveil the fluorophore-surface plasmon interaction, and the associated surface-modification design is liberally highlighted to balance the surface plasmon-induced fluorescence-enhancement efforts and the surface plasmon-caused fluorescence-quenching effects. In particular, all types of surface structures, for example, silicon, carbon, protein, DNA, polymer, and multilayer, are systematically interrogated in terms of component, thickness, stiffness, and functionality. As a highly interdisciplinary and expanding field in physics, optics, chemistry, and surface chemistry, this Review could be of great interest to a broad readership, in particular, among physical chemists, analytical chemists, and in surface-based sensing and imaging studies.

Fluorescence Coupling to Internal Modes of 1D Photonic Crystals Characterized by Back Focal Plane Imaging

The coupling of fluorescence with surface electromagnetic modes, such as surface plasmons on thin metal films or Bloch surface waves (BSW) on truncated one-dimensional photonic crystals (1DPC), are presently utilized for many fluorescence-based applications. In addition to the surface wave, 1DPCs also support other electromagnetic modes that are confined within the 1DPC structure. These internal modes (IMs) have not received much attention for fluorescence coupling due to lack of spatial overlap of their electric fields with the surface bound fluorophores. However, our recent studies have indicated that the fluorescence coupling with IMs occurs quite efficiently. This observed internal mode-coupled emission (IMCE) is (similar to BSW-coupled emission) indeed wavelength dependent, directional and S-polarized. In this paper, we have carried out back-focal plane (BFP) imaging to reveal that the IMs of 1DPCs can couple with surface bound excited dye molecules, with or without a BSW mode presence. Depending on the emission wavelength, the coupling is observed with BSW and IMs or only IMs of the 1DPC structure. The experimental results are well matching with numerical simulations. The occurrence of IMCE regardless of the availability of BSWs removes the dependence on just the surface mode for obtaining coupled emission from 1DPCs. The observation of IMCE is expected to widen the scope of 1DPCs for surface-based fluorescence sensing and assays.