Mana Toma

Plasmon-Enhanced Fluorescence Biosensors: a Review

Surfaces of metallic films and metallic nanoparticles can strongly confine electromagnetic field through its coupling to propagating or localized surface plasmons. This interaction is associated with large enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield, and control far field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. In conjunction with interfacial architectures for the specific capture of target analyte on a metallic surface, plasmon-enhanced fluorescence (PEF) that is also referred to as metal-enhanced fluorescence (MEF) represents an attractive method for shortening detection times and increasing sensitivity of various fluorescence-based analytical technologies. This review provides an introduction to fundamentals of PEF, illustrates current developments in design of metallic nanostructures for efficient fluorescence signal amplification that utilizes propagating and localized surface plasmons, and summarizes current implementations to biosensors for detection of trace amounts of biomarkers, toxins, and pathogens that are relevant to medical diagnostics and food control.

Fabrication of Broadband Antireflective Plasmonic Gold Nanocone Arrays on Flexible Polymer Films

Flexible broadband antireflective and light-absorbing nanostructured gold thin films are fabricated by gold vapor deposition onto Teflon films modified with nanocone arrays. The nanostructures are created by the oxygen plasma etching of polystyrene bead monolayers on Teflon surfaces. The periodicity and height of the nanocone arrays are controlled by the bead diameter and the overall etching time. The gold nanocone arrays exhibit a reflectivity of less than 1% over a wide spectral range (450-900 nm) and a wide range of incident angles (0-70°); this unique optical response is attributed to a combination of diffractive scattering loss and localized plasmonic absorption. In addition to nanocones, periodic nanostructures of nanocups, nanopyramids, and nanocavities can be created by the plasma etching of colloidal bilayers. This fabrication method can be used to create flexible nanocone-structured gold thin films over large surface areas (cm(2)) and should be rapidly incorporated into new technological applications that require wide-angle and broadband antireflective coatings.

Bloch surface wave-enhanced fluorescence biosensor

A new approach to signal amplification in fluorescence-based assays for sensitive detection of molecular analytes is reported. It relies on a sensor chip carrying a one-dimensional photonic crystal (1DPC) composed of two piled up segments which are designed to increase simultaneously the excitation rate and the collection efficiency of fluorescence light. The top segment supports Bloch surface waves (BSWs) at the excitation wavelength and the bottom segment serves as a Bragg mirror for the emission wavelength of used fluorophore labels. The enhancement of the excitation rate on the sensor surface is achieved through the resonant coupling to BSWs that is associated with strong increase of the field intensity. The increasing of collection efficiency of fluorescence light emitted from the sensor surface is pursued by using the Bragg mirror that minimizes its leakage into a substrate and provides its beaming toward a detector. In order to exploit the whole evanescent field of BSW, extended three-dimensional hydrogel-based binding matrix that is functionalized with catcher molecules is attached to 1DPC for capturing of target analyte from a sample. Simulations supported by experiments are presented to illustrate the design and determined the performance characteristics of BSW-enhanced fluorescence spectroscopy. A model immunoassay experiment demonstrates that the reported approach enables increasing signal to noise ratio, resulting in about one order of magnitude improved limit of detection (LOD) with respect to regular total internal reflection fluorescence (TIRF) configuration.

Active Control of SPR by Thermoresponsive Hydrogels for Biosensor Applications.

The use of thermoresponsive poly(N-isopropylacrylamide)-based hydrogel (pNIPAAm) for rapid tuning of surface plasmon resonance (SPR) is reported. This approach is implemented by using an SPR layer architecture with an embedded indium tin oxide microheater and pNIPAAm film on its top. It takes advantage of rapid thermally induced swelling and collapse of pNIPAAm that is accompanied by large refractive index changes and leads to high thermo-optical coefficient of dn/dT = 2 × 10–2 RIU/K. We show that this material is excellently suited for efficient control of refractive index-sensitive SPR and that it can serve simultaneously as a 3D binding matrix in biosensor applications (if modified with biomolecular recognition elements for a specific capture of target analyte). We demonstrate that this approach enables modulating of the output signal in surface plasmon-enhanced fluorescence spectroscopy biosensors and holds potential for simple time-multiplexing of sensing channels for parallelized readout of fluorescence assays.

Surface plasmon-coupled emission on plasmonic Bragg gratings

Surface plasmon-coupled emission (SPCE) from emitters in a close proximity to a plasmonic Bragg grating is investigated. In this study, the directional fluorescence emission mediated by Bragg-scattered surface plasmons and surface plasmons diffraction cross-coupled through a thin metallic film is observed by using the reverse Kretschmann configuration. We show that controlling of dispersion relation of these surface plasmon modes by tuning the refractive index at upper and lower interfaces of a dense sub-wavelength metallic grating enables selective reducing or increasing the intensity of the light emitted to certain directions. These observations may provide important leads for design of advanced plasmonic structures in applications areas of plasmon-enhanced fluorescence spectroscopy and nanoscale optical sources.

Flexible Gold Nanocone Array Surfaces as a Tool for Regulating Neuronal Behavior

Accelerated neurite outgrowth of rat cortical neurons on a flexible and inexpensive substrate functionalized with gold nanocone arrays is reported. The gold nanocone arrays are fabricated on Teflon films by a bottom-up approach based on colloidal lithography followed by deposition of a thin gold layer. The geometry of nanocone arrays including height and pitch is controlled by the overall etching time and template polystyrene beads size. Fluorescence microscopy studies reveal high viability and significant morphological changes of the neurons on the structured surfaces. The elongation degree of neurite is maximized on the nanocone arrays created with 1 µm polystyrene beads by a factor of two with respect to the control. Furthermore, the interface between the neurons and the nanocones is investigated by scanning electron microscopy and focused ion beam cross-sectioning. The detailed observation of the neuron/nanocone interfaces reveals the morphological similarity between the nanocone tips and the neuronal processes, the existence of interspace at the interface between the cell body and the nanocones, and neurite bridging among the neighboring structures, which may induce the acceleration of neurite outgrowth. The flexible gold nanocone arrays can be a good supporting substrate of neuron culture with noble electrical and optical properties.

Enhanced fluorescence microscopy with the Bull’s eye-plasmonic chip

A Bulls eye-plasmonic chip composed of concentric circles was applied to enhanced fluorescence microscopy. Among one dimensional (1-D), 2-D, and Bulls eye periodic structures, the Bulls eye-plasmonic chip provided the most enhanced fluorescence intensity under the epi-fluorescence microscope, because incident light through the objective lens with all azimuthal angles can be effectively applied to the surface plasmon resonance- field (excitation field) and the plasmon-enhanced emission was also effectively collected. In the fluorescence observation of a single nanoparticle, the enhanced fluorescence images for a microsphere with ϕ 2 μm and a nanosphere with ϕ 200 nm were observed. For the nanospheres with ϕ 40 and 20 nm, the fluorescence image, which was undetectable on a glass slide, was observed in a spatial resolution of roughly diffraction limit on the Bulls eye-plasmonic chip. Furthermore, the use of an appropriate pinhole at the aperture stop in the incident optical system improved the fluorescence enhancement. The applicability of a Bulls eye-plasmonic chip to fluorescence imaging was demonstrated.

Polydopamine Thin Films as Protein Linker Layer for Sensitive Detection of Interleukin-6 by Surface Plasmon Enhanced Fluorescence Spectroscopy

Polydopamine (PDA) thin films are introduced to the surface modification of biosensor surfaces utilizing surface plasmon enhanced fluorescence spectroscopy (SPFS) as the linker layer of capture antibody on to the sensor surfaces. The capture antibody can be directly attached to the sensor surface without using any coupling agent by functionalizing the gold sensor surface with PDA thin films. The PDA coating is performed by a single-step preparation process by applying the dopamine solution on the sensor surface, which requires an extremely short incubation time (10 min). The real-time in situ measurement of the adsorption kinetics of the capture antibody onto the PDA-coated sensor surface is studied by surface plasmon resonance (SPR) spectroscopy. It reveals that the immobilization of capture antibody immediately occurs after introduction of a solution containing capture antibody, and the sensor surface is fully covered with the capture antibody. The sensitive detection of the cytokine marker interleukin-6 (IL-6) is performed by SPFS using a sandwich assay format with fluorescently labeled detection antibody. The sensor chips functionalized by PDA chemistry exhibited sensitive sensor responses with low nonspecific adsorption of the detection antibody onto the sensor surface. The detection limit of IL-6 with the developed SPFS biosensor is determined to be 2 pg/mL (100 fM), which is within the range of the diagnostic criteria. Our observation elucidates the remarkable utility of PDA coatings for chemical modification of the metallic sensor surfaces by a simple, brief, and inexpensive manner.

Dual-Color Fluorescence Imaging of EpCAM and EGFR in Breast Cancer Cells with a Bull’s Eye-Type Plasmonic Chip

Surface plasmon field-enhanced fluorescence microscopic observation of a live breast cancer cell was performed with a plasmonic chip. Two cell lines, MDA-MB-231 and Michigan Cancer Foundation-7 (MCF-7), were selected as breast cancer cells, with two kinds of membrane protein, epithelial cell adhesion molecule (EpCAM) and epidermal growth factor receptor (EGFR), observed in both cells. The membrane proteins are surface markers used to differentiate and classify breast cancer cells. EGFR and EpCAM were detected with Alexa Fluor® 488-labeled anti-EGFR antibody (488-EGFR) and allophycocyanin (APC)-labeled anti-EpCAM antibody (APC-EpCAM), respectively. In MDA-MB231 cells, three-fold plus or minus one and seven-fold plus or minus two brighter fluorescence of 488-EGFR were observed on the 480-nm pitch and the 400-nm pitch compared with that on a glass slide. Results show the 400-nm pitch is useful. Dual-color fluorescence of 488-EGFR and APC-EpCAM in MDA-MB231 was clearly observed with seven-fold plus or minus two and nine-fold plus or minus three, respectively, on the 400-nm pitch pattern of a plasmonic chip. Therefore, the 400-nm pitch contributed to the dual-color fluorescence enhancement for these wavelengths. An optimal grating pitch of a plasmonic chip improved a fluorescence image of membrane proteins with the help of the surface plasmon-enhanced field.

Study on the Mechanism of Diarylethene Crystal Growth by In Situ Microscopy and the Crystal Growth Controlled by an Aluminum Plasmonic Chip

The microcrystalline film of an open-ring isomer (1o) of diarylethene 1 was prepared on an Al plasmonic chip with a grating structure. Photoisomerization from 1o to the closed-ring isomer (1c) and growth of needle-shaped crystals in 1c were observed in situ under an upright-inverted microscope. In the center part of the film, crystal growth of needle-shaped-crystal of 1c was observed upon UV irradiation from the top side, but not upon UV irradiation from the bottom side. However, crystallization occurred at the edge of the film upon UV irradiation from the bottom side. It was suggested that crystal growth of 1c required a high mobility of 1c near the film surface. Furthermore, the existence of 1o platform is also found to be required for alignment of 1c molecules by the results under the irradiation from the bottom and top sides. With the Al plasmonic chip, the conversion rate from 1o to 1c was larger inside the grating by the plasmonic enhanced field. Therefore, when the attenuated UV light was irradiated to the film edge with high mobility of 1c from the bottom side, the conversion rate was more than 60%, and the needle-shaped crystals of 1c were observed only inside the grating area. Crystal growth was controlled by the conversion rate of 1c promoted inside the grating. From the above, the larger conversion rate of 1c more than 60%, a high mobility of 1c near the film surface or edge, and the existence of the 1o platform for alignment of 1c molecules, are considered to be required for crystal growth in 1c.