Ultrasensitive Upconversion Nanoparticle Immunoassay for Human Serum Cardiac Troponin I Detection Achieved with Resonant Waveguide Grating.

Selective detection of biomarkers at low concentrations in blood is crucial for the clinical diagnosis of many diseases but remains challenging. In this work, we aimed to develop an ultrasensitive immunoassay that can detect biomarkers in serum with an attomolar limit of detection (LOD). We proposed a sandwich-type heterogeneous immunosensor in a 3 × 3 well array format by integrating a resonant waveguide grating (RWG) substrate with upconversion nanoparticles (UCNPs). UCNPs were used to label a target biomarker captured by capture antibody molecules immobilized on the surface of the RWG substrate, and the RWG substrate was used to enhance the upconversion luminescence (UCL) of UCNPs through excitation resonance. The LOD of the immunosensor was greatly reduced due to the increased UCL of UCNPs and the reduction of nonspecific adsorption of detection antibody-conjugated UCNPs on the RWG substrate surface by coating the RWG substrate surface with a carboxymethyl dextran layer. The immunosensor exhibited an extremely low LOD [0.24 fg/mL (9.1 aM)] and wide detection range (1 fg/mL to 100 pg/mL) in the detection of cardiac troponin I (cTnI). The cTnI concentrations in human serum samples collected at different times during cyclophosphamide, epirubicin, and 5-fluorouracil (CEF) chemotherapy in a breast cancer patient were measured by an immunosensor, and the results showed that the CEF chemotherapy did cause cardiotoxicity in the patient. Having a higher number of wells in such an array-based biosensor, the sensor can be developed as a high-throughput diagnostic tool for clinically important biomarkers.

Focusing of surface plasmon polaritons propagating at the SiO2/Ag interface with 2-level and 4-level Fresnel phase zone pad structures.

We propose and demonstrate dielectric Fresnel phase zone pad (FPZP) structures for focusing surface plasmon polaritons (SPPs) propagating at the SiO2/Ag interfaces. We exploited up-conversion fluorescence microscopy to characterize the SPP focusing. We first report on the SPP focusing with 2-level FPZP structures that introduced a π-phase shift in the SPP wavefront between adjacent zones. We optimized the SPP focusing by fine-tuning the longitudinal width of the FPZP structure. This led to the enhancement of the peak intensity of the SPP focal spot and the reduction of the focal spot size in both the longitudinal and transverse directions. Such focusing was also demonstrated with different focal lengths. To further improve the SPP focusing, we developed a 4-level FPZP structure, which introduced a π/2-phase shift in the SPP wavefront between adjacent zones. With the optimized 4-level FPZP structure, the SPP focal spot peak intensity is further improved, and the spot size is reduced. To assist the design of the FPZP structures, we carried out theoretical analysis and numerical calculations to determine the SPP wavelengths at various oxide/Ag interfaces. We also carried out finite difference time domain (FDTD) calculations to simulate the SPP focusing with the FPZP structures. The results of the FDTD simulation agree with the experimental results qualitatively.

Surface plasmon polariton assisted near perfect light absorption from corrugated metal-insulator-metal structure exploiting lossy metal films.

We propose and demonstrate a lithography-free self-assembled corrugated Cr/TiO2/Cr metal-insulator-metal (Cr-cMIM) structure on silica opal substrates for broadband near perfect light absorption applications. Our optimal Cr-cMIM structure have reached a spectral average absorption rate above 98% over the visible wavelength range. We carried out numerical calculations to simulate the interaction between the incident light and the Cr-cMIM structure. The simulated absorption spectra qualitatively reproduced the experimental results. Detailed analysis of the simulation results indicates that the corrugation of the Cr layers successfully couples the incident light with the localized surface plasmon polariton. The incorporation of the surface plasmonic excitation and the intrinsic ohmic dissipation of the Cr layers results in the broadband near perfect light absorption over the visible wavelength range.

Free-Standing, Interwoven Tubular Graphene Mesh-Supported Binary AuPt Nanocatalysts: An Innovative and High-Performance Anode Methanol Oxidation Catalyst.

Pt-based alloy or bimetallic anode catalysts have been developed to reduce the carbon monoxide (CO) poisoning effect and the usage of Pt in direct methanol fuel cells (DMFCs), where the second metal plays a role as CO poisoning inhibitor on Pt. Furthermore, better performance in DMFCs can be achieved by improving the catalytic dispersion and using high-performance supporting materials. In this work, we introduced a free-standing, macroscopic, interwoven tubular graphene (TG) mesh as a supporting material because of its high surface area, favorable chemical inertness, and excellent conductivity. Particularly, binary AuPt nanoparticles (NPs) can be easily immobilized on both outer and inner walls of the TG mesh with a highly dispersive distribution by a simple and efficient chemical reduction method. The TG mesh, whose outer and inner walls were decorated with optimized loading of binary AuPt NPs, exhibited a remarkably catalytic performance in DMFCs. Its methanol oxidation reaction (MOR) activity was 10.09 and 2.20 times higher than those of the TG electrodes with only outer wall immobilized with pure Pt NPs and binary AuPt NPs, respectively. Furthermore, the catalyst also displayed a great stability in methanol oxidation after 200 scanning cycles, implying the excellent tolerance toward the CO poisoning effect.

Subwavelength-resolution imaging of surface plasmon polaritons with up-conversion fluorescence microscopy.

Imaging and characterization of surface plasmon polaritons (SPPs) are crucial for the research and development of the plasmonic devices and circuits. Here, we report on direct imaging of SPPs propagation on SiO2/metal interface with subwavelength spatial resolution using up-conversion fluorescence microscopy, that exploits rare-earth ions, such as Er3+, Yb3+, and Nd3+, doped nanoparticles as the fluorophores. We demonstrated that by further taking the intensity ratio of the image obtained with fluorescent emission at different wavelengths, we are able to substantially enhance the features associated to the SPP wavefronts in the image for quantitative analysis, such as the wavevector and propagation direction of the SPPs. Our results agree with the theoretic prediction of the SPP wavelengths quantitatively. We further demonstrate the evolution of the SPP wavefronts due to refraction SPPs, and reproduced the experiment with finite difference time domain (FDTD) method simulations. The relative refractive index of SPP estimated from the experiment also agrees quantitatively with those extracted from the theory and the simulation.

Edge-Rich Interconnected Graphene Mesh Electrode with High Electrochemical Reactivity Applicable for Glucose Detection.

The development of graphene structures with controlled edges is greatly desired for understanding heterogeneous electrochemical (EC) transfer and boosting EC applications of graphene-based electrodes. We herein report a facile, scalable, and robust method to produce graphene mesh (GM) electrodes with tailorable edge lengths. Specifically, the GMs were fabricated at 850 °C under a vacuum level of 0.6 Pa using catalytic nickel templates obtained based on a crack lithography. As the edge lengths of the GM electrodes increased from 5.48 to 24.04 m, their electron transfer rates linearly increased from 0.08 to 0.16 cm∙s-1, which are considerably greater than that (0.056 ± 0.007 cm∙s-1) of basal graphene structures (defined as zero edge length electrodes). To illustrate the EC sensing potentiality of the GM, a high-sensitivity glucose detection was conducted on the graphene/Ni hybrid mesh with the longest edge length. At a detection potential of 0.6 V, the edge-rich graphene/Ni hybrid mesh sensor exhibited a wide linear response range from 10.0 µM to 2.5 mM with a limit of detection of 1.8 µM and a high sensitivity of 1118.9 µA∙mM-1∙cm-2. Our findings suggest that edge-rich GMs can be valuable platforms in various graphene applications such as graphene-based EC sensors with controlled and improved performance.

Directional coupling of surface plasmon polaritons using single nano-slit with asymmetric sidewall structure and material composition.

We report on the performance of the asymmetric nano-slit that we design and fabricated with electron beam lithography (EBL) and glancing angle deposition techniques (GLAD) for directional coupling of surface plasmon polariton (SPP) on Ag surfaces. The slit structure includes asymmetric sidewalls in terms of material composition as well as structural morphology. The overall width of the slit was varied for optimization. We illuminated the slit with a focused 532nm laser beam and characterized the SPP signal on the Ag surface near the slit with nearfield scanning optical microscopy (NSOM). We demonstrate that optimal directional coupling of SPP toward either side of the slit can be achieved by selecting proper slit widths, with the best extinction ratio of 79000 ± 18000. We also carried out numerical calculations to simulate the interaction between the incident light and the slit structure. The results reproduced the experiment qualitatively. Detailed analysis of the distribution of the E-field and the time-averaged Poynting vector indicates that SPP excited on the Ag pad substructure in the slit plays an important role in the directional coupling of SPP.

Layer Control of Tubular Graphene for Corrosion Inhibition of Nickel Wires.

Corrosion protection of complex surface is an active area of research due to its importance to commercial applications such as electrochemical fabrication. However, conventional coatings exhibit limited conductivity, thermal stability, and durability and are thus not suitable. Recent work has shown the potential of graphene, a two-dimensional carbon allotrope, for corrosion protection. The studies, however, limited themselves to simple planar geometries that provide limited insight in the applicability to relevant morphologies such as mesh electrodes and roughened surfaces. We here study the corrosion protection ability of tubular graphene (TG) on Ni-wires as a model system for such complex geometries. TG-covered Ni wires of approximately 50 µm diameters were produced by the annealing of cellulose acetate (CA) on Ni. The high quality of the TG coating was confirmed by Raman spectroscopy, scanning electron microscopy, and electrical measurements. We show that the graphene layer number could be controlled by adjusting the CA membrane quantity. We found a direct relation between the degree of corrosion inhibition with the variation of graphene layer number. The increase of graphene layers on a Ni surface could enhance its corrosion inhibition in acidic, basic, and marine environments, which shows the potential of our approach for future applications.

Strong guided mode resonant local field enhanced visible harmonic generation in an azo-polymer resonant waveguide grating.

Guided mode resonance (GMR) enhanced second- and third-harmonic generation (SHG and THG) is demonstrated in an azo-polymer resonant waveguide grating (RWG), comprised of a poled azo-polymer layer on top of a textured SU8 substrate with a thin intervening layer of TiO2. Strong SHG and THG outputs are observed by matching either in-coming fundamental- or out-going harmonic-wavelength to the GMR wavelengths of the azo-polymer RWG. Without the azo-polymer coating, pure TiO2 RWGs, do not generate any detectable SHG using a fundamental beam peak intensity of 2 MW/cm(2). Without the textured TiO2 layer, a planar poled azo-polymer layer results in 3650 times less SHG than the full nonlinear RWG structure under identical excitation conditions. Rigorous coupled-wave analysis calculations confirm that this enhancement of the nonlinear conversion is due to strong local electric fields that are generated at the interfaces of the TiO2 and azo-polymer layers when the RWG is excited at resonant wavelengths associated with both SHG and THG conversion processes.

Directed Kinetic Self-Assembly of Mounds on Patterned GaAs (001): Tunable Arrangement, Pattern Amplification and Self-Limiting Growth.

We present results demonstrating directed self-assembly of nanometer-scale mounds during molecular beam epitaxial growth on patterned GaAs (001) surfaces. The mound arrangement is tunable via the growth temperature, with an inverse spacing or spatial frequency which can exceed that of the features of the template. We find that the range of film thickness over which particular mound arrangements persist is finite, due to an evolution of the shape of the mounds which causes their growth to self-limit. A difference in the film thickness at which mounds at different sites self-limit provides a means by which different arrangements can be produced.

Enhancing light extraction efficiency of polymer light-emitting diodes with a 12-fold photonic quasi crystal.

This work demonstrates the enhancement of light extraction of polymer light-emitting diodes (PLEDs) by incorporating a 12-fold photonic quasi crystal (PQC) in the device structure. Multi-exposure two-beam interference technique combined with inductively coupled plasma etching was employed to pattern a 12-fold PQC structure on the ITO film on a glass substrate of the diode. The air-hole coverage (AHC) and etching depth dependences of the light emitting performance of the 12-fold PQC patterned PLEDs were investigated. For AHC within the range between 6.4% and 32.3%, a nearly constant enhancement of the luminance efficiency of the PQC PLEDs was observed. On the other hand, the light emitting performance of the PQC PLEDs is very sensitive to the etching depth. The photoluminescence intensity of the PQC PLEDs increases monotonically with the etching depth. In contrast, the electro luminance efficiency shows a non-monotonic dependence on etching depth with a maximum occurring at 55 nm etching depth. The maximum improvement of luminance efficiency of the 12-fold PQC PLEDs reaches nearly 95% compared with an un-patterned PLED at an injection current of 110 mA.

Guided-mode resonance enhanced excitation and extraction of two-photon photoluminescence in a resonant waveguide grating.

Guided-mode resonances enhanced excitation and extraction of two-photon photoluminescence (TPP) is demonstrated with a one-dimensional resonant waveguide grating (RWG) with a layer of fluorescent polymer (polyfluorene, PFO) on top. In this work, we design and fabricate a PFO RWG, in which two dispersive resonant modes in TE-polarization were measured. By aligning the red-shifting resonant mode with excitation wavelength in the infrared range, and the blue-shifting resonant mode with TPP spectrum in the visible range, the intensity of TPP can be enhanced up to 300-fold compared with that from a flat film with the same thickness coated on a glass slide. Such high enhancement results from firstly the strong evanescent local field in the waveguide layer due to the resonance between the incident light and the waveguide structure according to the results of rigorous coupled-wave analysis calculation, and secondly the enhanced extraction of the emission light which also resonates with the waveguide structure.

Nonlinear excitation power dependence of surface enhanced fluorescence from a nanostructured Ag film.

We investigate the excitation power dependence of fluorescent emission from Cy3-tagged molecules separated from an Ag film prepatterned with arrays of nanostructures by a thin spacer. While the fluorescent intensities from both the patterned area and the flat Ag surfaces increase monotonically with the power of excitation light, the fluorescent contrast between them decreases with excitation power in a nonlinear fashion. We propose a simple theoretical model which includes basic properties of molecular fluorescence, the effect of near field enhancement from surface plasmon excited on the patterned structure, and the effect of enhancement of fluorescent emission rate and non-radiative decay rate. Our results agree qualitatively with the prediction of a model for which there is a larger enhancement of the excitation rate than that of the total decay rate of the excited molecule.

Pumping-power-dependent photoluminescence angular distribution from an opal photonic crystal composed of monodisperse Eu3+/SiO2 core/shell nanospheres.

High quality opal photonic crystals (PhCs) were successfully fabricated by self-assembling of monodisperse Eu(3+)/SiO(2) core/shell nanospheres. Angular resolved photoluminescence (PL) spectra of a PhC sample were measured with different pumping powers, and its PL emission strongly depended on spectroscopic position of the photonic stop band and the optical pumping power. Suppression of the PL occurred in the directions where the emission lines aligned with the center of the photonic stop band. Suppression and enhancement of the PL were observed at low- and high-pumping powers, respectively, in the directions where the emission lines were located at the edges of the photonic stop band. When pumping power exceeded 6 µJ/pulse, a super-linear dependence was found between the pumping power and PL intensity. The dramatic enhancement of PL was attributed to the amplification of spontaneous emission resulted from the creation of large population inversion and the slow group velocity of the emitted light inside the PhC. The opal PhC provided highly angular-selective quasi-monochromatic PL output, which can be useful for a variety of optical applications.

Optical modulation of guided mode resonance in the waveguide grating structure incorporated with azo-doped-poly(methylmethacrylate) cladding layer.

Optical modulation of guided mode resonance (GMR) is demonstrated in a waveguide grating structure (WGS) which contains a disperse-red1 (DR1)-doped poly(methylmethacrylate) (PMMA) cladding layer. The resonance wavelength of a GMR mode can be tuned by pumping the cladding layer with a 442 nm wavelength laser beam, because of photoinduced refractive index change in the layer. The resonance wavelength shifts to shorter wavelength side, and the shift increases with pumping power, up to a maximum shift of 5 nm. A detector was used to monitor the intensity of the light that was reflected from the WGS at the wavelengths of the GMR peak positions, and the WGS was found to exhibit optical modulation with a shortest switching time of less than 0.3s.

Doubly resonant surface-enhanced Raman scattering on gold nanorod decorated inverse opal photonic crystals.

We present a novel type of surface-enhanced Raman scattering (SERS) substrate constituted of a 3-dimensinal polymeric inverse opal (IO) photonic crystal frame with gold nanorods (Au-NRs) decorating on the top layer. This substrate employs resonant excitation as well as constructive backward scattering of Raman signals to produce large enhancement of SERS output. For the incoming excitation, Au-NRs with appropriate aspect ratio were adopted to align their longitudinal localized surface plasmon band with the excitation laser wavelength. For the outgoing SERS signal, the spectral position of the photonic band gap was tuned to reflect Raman-scattered light constructively. This SERS substrate produces not only strong but also uniform SERS output due to the well control of Au-NRs distribution by the periodic IO structure, readily suitable for sensing applications.

Fabrication of three-dimensional polymer quadratic nonlinear grating structures by layer-by-layer direct laser writing technique.

We demonstrate the fabrication of a three-dimensional (3D) polymer quadratic nonlinear (χ(2)) grating structure. By performing layer-by-layer direct laser writing (DLW) and spin-coating approaches, desired photobleached grating patterns were embedded in the guest-host dispersed-red-1/poly(methylmethacrylate) (DR1/PMMA) active layers of an active-passive alternative multilayer structure through photobleaching of DR1 molecules. Polyvinyl-alcohol and SU8 thin films were deposited between DR1/PMMA layers serving as a passive layer to separate DR1/PMMA active layers. After applying the corona electric field poling to the multilayer structure, nonbleached DR1 molecules in the active layers formed polar distribution, and a 3D χ(2) grating structure was obtained. The χ(2) grating structures at different DR1/PMMA nonlinear layers were mapped by laser scanning second harmonic (SH) microscopy, and no cross talk was observed between SH images obtained from neighboring nonlinear layers. The layer-by-layer DLW technique is favorable to fabricating hierarchical 3D polymer nonlinear structures for optoelectronic applications with flexible structural design.

Effect of gold nanopillar arrays on the absorption spectrum of a bulk heterojunction organic solar cell.

We report on the effect of arrays of Au nanopillars of controlled size and spacing on the spectral response of a P3HT: PCBM bulk heterojunction solar cell. Prototype nanopillar-patterned devices have nearly the same overall power conversion efficiency as those without nanopillars. The patterned devices do show higher external quantum efficiency and calculated absorption in the wavelength range from approximately 640 nm to 720 nm, where the active layer is not very absorbing. The peak enhancement was approximately 60% at 675 nm. We find evidence that the corresponding resonance involves both localized particle plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. We explore the role of the attenuation coefficient of the active layer on the optical absorption of such an organic photovoltaic device.

Hybrid surface-enhanced Raman scattering substrate from gold nanoparticle and photonic crystal: maneuverability and uniformity of Raman spectra.

A novel hybrid surface-enhanced Raman scattering (SERS) substrate based on Au nanoparticles decorated inverse opal (IO) photonic crystal (PhC) is presented. In addition to the enhancement contributed from Au nanoparticles, a desired Raman signal can be selectively further enhanced by appropriately overlapping the center of photonic bandgap of the IO PhC with the wavelength of the Raman signal. Furthermore, the lattice structure of the IO PhC provides excellent control of the distribution of Au nanoparticles to produce SERS spectra with high uniformity. The new design of SERS substrate provides extra maneuverability for ultra-high sensitivity sensor applications.