Tuning Dichroic Plasmon Resonance Modes of Gold Nanoparticles in Optical Thin Films

A simple method is presented to tune the gold surface plasmon resonance (SPR) modes by growing anisotropic nanoparticles into transparent SiO₂ thin films prepared by glancing angle deposition. In this type of composite film, the anisotropy of the gold nanoparticles, proved by gracing incidence small angle X‐ray scattering, is determined by the tilted nanocolumnar structure of the SiO₂ host and yields a strong film dichroism evidenced by a change from an intense colored to a nearly transparent aspect depending on light polarization and/or sample orientation. The formation in these films of lithographic non‐dichroic SPR patterns by nanosecond laser writing demonstrates the potentialities of this procedure to develop novel optical encryption or anti‐counterfeiting structures either at micrometer‐ or macroscales.     

透射式金属光栅耦合SPR传感器

用磁控溅射技术在双面抛光的蓝宝石衬底上沉积了20 nm Ti和100 nm Au的金属薄膜,通过标准光刻工艺制备出1.6和2.0μm两种周期结构的一维光栅表面等离子体共振(SPR)传感器。用时域有限差分算法(FDTD)模拟仿真并结合实验测试的透射光谱,研究分析了不同周期结构的金属薄膜光栅型SPR传感器的特性。基于金属光栅耦合,利用表面等离子体激元(SPP)的局域特性和光栅的选频特性,实现了SPR传感器的信号增强和滤波功能。研究结果表明,利用金属薄膜光栅表面介质的变化引起的光栅透射光谱中激发表面等离子体共振峰的位置变化,可以获得被测物体的物理、生物和化学等相关特征信息。     

Surface Plasmon Resonance Sensors Based on Polymer Optical Fiber

Surface Plasmon Resonance （SPR） is a powerful technique for directly sensing in biological studies, chemical detection and environmental pollution monitoring. In this paper, we present polymer optical fiber application in SPR sensors, including wavelength interrogation surface enhanced Raman scattering SPR sensor and surface enhanced Raman scattering （SERS） probe.
Long-period fiber gratings are fabricated on single mode polymer optical fiber （POF） with 120 μm period and 50% duty cycle. The polarization characteristic of this kind of birefringent grating is studied. Theoretical analysis shows it will be advantageous in SPR sensing applications.     

Designing Multicolor Micropatterns of Inverse Opals with Photonic Bandgap and Surface Plasmon Resonance

Structural coloration provides unique features over chemical coloration, such as nonfading, color tunability, and high color brightness, rendering it useful in various optical applications. To develop the structural colors, two different mechanisms of coloration–photonic bandgap (PBG) and surface plasmon resonance (SPR)–have been separately utilized. In this work, a new method is suggested to create structurally colored micropatterns by regioselectively employing SPR in a single film of inverse opal with PBG. The inverse opals are prepared by thermal embedding of opal into a negative photoresist and its subsequent removal. The inverse opals have a hexagonal array of open pores on the surface which serves as a template to make SPR‐active nanostructures through a directional deposition of gold, a perforated gold film and an array of curved gold disks are formed. With a shadow mask lithographically prepared, the gold is regioselectively deposited on the surface of the inverse opal, which results in two distinct regions of gold‐free inverse opal with PBG and gold nanostructure with SPR. As PBG and SPR develop their own structural colors respectively, the resultant micropatterns exhibit pronounced dual colors. More importantly, the micropatterns show the distinguished optical response for evaporation of volatile liquids that occupy the pores.     

Layer‐by‐Layer Fabrication and Characterization of Gold‐Nanoparticle/Myoglobin Nanocomposite Films

Multilayer thin films of ～ 7 nm diameter gold nanoparticles (GNPs) linked with horse heart myoglobin (Mb) are fabricated, for the first time, by layer‐by‐layer (LbL) assembly on glass slides, and silicon and plastic substrates. The GNP/Mb nanocomposite films show sharp surface plasmon resonance (SPR) absorption bands that are used to follow the LbL growth of the film and to determine the kinetics of GNP adsorption on the Mb‐modified surface. The GNP/Mb nanocomposite films are characterized using atomic force microscopy, transmission electron microscopy, polarized UV‐vis spectroscopy, and spectroscopic ellipsometry. The GNPs in the multilayer films are spatially separated from one another, and interparticle interactions remain in the film, making it optically anisotropic. The GNP/Mb nanocomposite films are stable in air at temperatures up to 100 °C, and can withstand successive immersions in strongly acidic and basic solutions. The SPR absorption band of the GNP/Mb nanocomposite film in air exhibits a red‐shift in the wavelength maximum and an increase in the maximum absorbance relative to that in water. This result, which is in contrast to that observed with a GNP monolayer on an aminosilane‐functionalized substrate, suggests the shrinkage in air and swelling in water of Mb molecules embedded in the nanocomposite film.     

两类典型光纤型SPR传感器的理论模型比较

针对基于SPR效应的两类典型光纤型传感器:传统型包层腐蚀的镀金属多模光纤传感器、新型镀金属单模光纤光栅传感器,指出其不同的SPR模型处理方法及传感特性。首先,根据传统型SPR光纤传感器的结构特点,利用平板SPR理论给出反射谱特性,同时指出并证明薄膜光学理论在平板SPR结构中与平板SPR理论的等价性。其次,针对新型SPR光纤光栅传感器结构特点,依据模式耦合思想,结合SPW模式特征,提出了光纤光栅SPR结构的理论处理方法,得到了镀金膜三包层LPFG结构的透射谱。最后,对基于SPR效应的两类典型光纤型传感器的传感特性进行了比较分析,结果表明,两类传感器对环境折射率均具有较高的分辨率,但新型光纤光栅SPR传感器的分辨率高出传统SPR光纤传感器3个数量级。     

Short‐Wavelength Lasing‐Spasing and Random Spasing with Deeply Subwavelength Thin‐Film Gain Media

Lasing‐spasers are subwavelength‐sized metal/dielectric structures that emit light via stimulated emission of surface plasmons. Here, it is demonstrated that silver nanoparticles combined with deeply subwavelength, blue‐emitting conjugated polymer thin films can function as room‐temperature lasing‐spasers and random spasers with quality factors up to 250. In contrast to other thin‐film‐based spaser and plasmonic random laser studies, which have used gain films ranging from ≈200 nm to 500 nm in thickness and which monitor emission guided to the sample edges, in this study, the thickness of the thin‐film gain medium ranges from 30 nm to 70 nm and emission is collected normal to the plane of the film. This eliminates effects that arise from optical trapping of scattered emission within the gain medium that is typically associated with plasmonic random lasing. The use of the conjugated polymer thin‐film gain medium allows higher chromophore densities compared to organic dye‐doped layers, which enables spasing using deeply subwavelength gain layers. Samples implementing gold nanoparticles and the conjugated polymer gain medium do not exhibit stimulated emission, demonstrating that it is the spectral overlap between the silver nanoparticle's surface plasmon resonance and the gain medium's emission that is necessary for observation of stimulated emission from this material system.     

Inside Front Cover: Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly (Adv. Funct. Mater. 7/2006)

The synthesis of three‐dimensionally ordered, transparent gold‐nanocrystal (NC)/silica superlattice thin films using the self‐assembly (by spin‐coating) of water‐soluble gold nanocrystal micelles and soluble silica is reported by Fan and co‐workers on p. 891. The robust, 3D NC/silica superlattice films are of interest for the development of collective optical and electronic phenomena, and, importantly, for the integration of NC arrays into device architectures. Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

Integrated optical SPR sensor based on mode conversion efficiency

A novel type of integrated optical surface plasmon resonance (SPR) sensor is proposed, for which the operational principle is based on the launching efficiency of eigenmodes in the sensor head. The sensor comprises an inverted-rib-type dielectric waveguide, a portion of which is covered with a thin gold layer. Eigenmodes in the sensor head are coupled modes of a surface plasmon polariton and a dielectric guided wave. The excitation efficiency of the coupled modes varies significantly depending on the refractive index of the analyte medium on the sensor head. Following this principle, the transmission coefficient of light through the sensor head can be used as a sensitive measure of the variation in the refractive index of the analyte medium.     

Optical Enhancement via Electrode Designs for High‐Performance Polymer Solar Cells

To capture the essence of the rapid progress in optical engineering exploited in high‐performance polymer solar cells (PSCs), a comprehensive overview focusing on recent developments and achievements in PSC electrode engineering is provided in this review. To date, various kinds of electrode materials and geometries are exploited to enhance light‐trapping in devices through distinct optical strategies. In addition to the widely used nanostructured electrodes that induce plasmonic‐enhanced light absorption, planar ultra‐thin metal films also have attracted significant attention due to their remarkably reflective transparent properties that beget efficient optical microcavities. These microcavities confine incident light with resonant frequencies between two reflective electrodes due to optically coherent interference, boosting the light absorption of thin‐film PSCs while maintaining efficient charge dissociation and extraction. After reviewing the challenges in developing high‐performance microcavity‐enhanced PSCs (MCPSCs), we discuss strategies to improve MCPSC performance further to showcase the potential of harnessing microcavity resonance effects in thin‐film PSCs.     

基于双D型光纤表面等离子共振折射率传感研究

以双D型光纤作为传输载体,研究了一种基于表面等离子共振技术的双D型光纤折射率传感器。利用时域有限差分法,分析了双D型光纤剩余包层厚度、金膜厚度、金膜表面粗糙度以及双通道传输对光纤SPR传感器性能的影响。仿真结果表明,当剩余包层厚度为300~500nm、覆盖的金膜厚度为50nm时,双D型光纤SPR传感器的性能得到优化;金膜表面粗糙度也是影响传感器性能的重要因素,当金膜表面粗糙度的均方根值低于2nm或其相关长度大于160nm时,金膜表面粗糙度对传感器性能的影响显著减小,且在折射率为1.33~1.36的传感环境下具有较好的线性度;在双D型光纤两侧覆不同的金属膜,可以实现信号的双通道测量。     

Impact of Substrate and Processing on Confinement of Nafion Thin Films

Thin films of ion‐conducting polymers are an important area of study due to their function in many electrochemical devices and as analogues for interfacial phenomena that occur in bulk films. In this paper, the properties of Nafion, a prototypical ionomer, are investigated as thin films (4 to 300 nm) on carbon, gold, and platinum substrates that are fabricated using different casting methods and thermal histories. Specifically, water uptake, swelling, and morphology are investigated by quartz‐crystal microbalance, ellipsometry, and grazing‐incidence X‐ray scattering to develop structure/property/processing relationships. For all substrates, as the films' thickness decreased, there is an initial decrease in swelling followed by a subsequent increase for film thicknesses below ≈20 nm due to a disordering of the film hydrophilic/hydrophobic structure. Decreased swelling and less structural order is observed on gold for spin‐cast films compared to self‐assembled films; the opposite effect is observed for films on carbon. The presented systematic data set and analyses represent a thorough study of the behavior of Nafion thin films on model substrates of interest in metal catalyst/carbon electrodes, and these insights help to elucidate the underlying polymer physics and confinement effects in these and related systems.     

In Situ Studies of Surface‐Plasmon‐Resonance‐Coupling Sensor Mediated by Stimuli‐Sensitive Polymer Linker

Hybrid plasmonic nanostructures comprising gold nanoparticle (AuNP) arrays separated from Au substrate through a temperature‐sensitive poly(N‐isopropylacrylamide) (PNIPAM) linker layer are constructed, and unique plasmonic‐coupling‐based surface plasmon resonance (SPR) sensing properties are investigated. The optical properties of the model system are investigated by in situ and scan‐mode SPR analysis. The swelling‐shrinking transitions in the polymer linker brush are studied by in situ contact‐mode atomic force microscopy at two different temperatures in water. It is revealed that the thickness of the PNIPAM layer is decreased from 30 to 14 nm by increasing the temperature from 20 to 32 °C. For the first time the dependence of the coupling behavior in AuNPs is investigated with controlled density on the temperature in a quantitative manner in terms of the change in SPR signals. The device containing AuNPs with optimized AuNP density shows 3.2‐times enhanced sensitivity compared with the control Au film‐PNIPAM sample. The refractive index sensing performance of the Au film‐PNIPAM‐AuNPs is greater than that of Au film‐PNIPAM by 19% when the PNIPAM chains have a collapsed conformation above lower critical solution temperature.     

Use of Reversal Nanoimprinting of Nanoparticles to Prepare Flexible Waveguide Sensors Exhibiting Enhanced Scattering of the Surface Plasmon Resonance

A flexible surface plasmon resonance (SPR)‐based scattering waveguide sensor is prepared by directly imprinting hollow gold nanoparticles (NPs) and solid gold NPs onto flexible polycarbonate (PC) plates—without any surface modification—using a modified reversal nanoimprint lithography technology. Controlling the imprinting conditions, including temperature and pressure, allows for the fine adjustment of the depths of the embedded metal NPs and their SPR properties. This patterning approach exhibits a resolution down to the submicrometer level. A 3D finite‐difference time domain simulation is used to examine the optical behavior of light propagating parallel to the air/substrate interface within the near‐field regime. Consistent with the simulations, almost an order of magnitude enhancement in the scattering signal after transferring the metal NPs from the glass mold to the PC substrate is obtained experimentally. The enhanced signal is attributed to the particles' strong scattering of the guiding‐mode waves (within the waveguide) and the evanescent wave (above the waveguide) simultaneously. Finally, the imprinting conditions are optimized to obtain a strongly scattering bio/chemical waveguide sensor.     

Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly

Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

Hydrogel‐Enabled Transfer‐Printing of Conducting Polymer Films for Soft Organic Bioelectronics

The use of conducting polymers such as poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) for the development of soft organic bioelectronic devices, such as organic electrochemical transistors (OECTs), is rapidly increasing. However, directly manipulating conducting polymer thin films on soft substrates remains challenging, which hinders the development of conformable organic bioelectronic devices. A facile transfer‐printing of conducting polymer thin films from conventional rigid substrates to flexible substrates offers an alternative solution. In this work, it is reported that PEDOT:PSS thin films on glass substrates, once mixed with surfactants, can be delaminated with hydrogels and thereafter be transferred to soft substrates without any further treatments. The proposed method allows easy, fast, and reliable transferring of patterned PEDOT:PSS thin films from glass substrates onto various soft substrates, facilitating their application in soft organic bioelectronics. By taking advantage of this method, skin‐attachable tattoo‐OECTs are demonstrated, relevant for conformable, imperceptible, and wearable organic biosensing.     

Phase Segregation in Thin Films of Conjugated Polyrotaxane– Poly(ethylene oxide) Blends: A Scanning Force Microscopy Study

Scanning force microscopy (SFM) is used to study the surface morphology of spin‐coated thin films of the ion‐transport polymer poly(ethylene oxide) (PEO) blended with either cyclodextrin (CD)‐threaded conjugated polyrotaxanes based on poly(4,4′‐diphenylene‐vinylene) (PDV), β‐CD–PDV, or their uninsulated PDV analogues. Both the polyrotaxanes and their blends with PEO are of interest as active materials in light‐emitting devices. The SFM analysis of the blended films supported on mica and on indium tin oxide (ITO) reveals in both cases a morphology that reflects the substrate topography on the (sub‐)micrometer scale and is characterized by an absence of the surface structure that is usually associated with phase segregation. This observation confirms a good miscibility of the two hydrophilic components, when deposited by using spin‐coating, as suggested by the luminescence data on devices and thin films. Clear evidence of phase segregation is instead found when blending PEO with a new organic‐soluble conjugated polymer such as a silylated poly(fluorene)‐alt‐poly(para‐phenylene) based polyrotaxane (THS–β‐CD–PF–PPP). The results obtained are relevant to the understanding of the factors influencing the interfacial and the intermolecular interactions with a view to optimizing the performance of light‐emitting diodes, and light‐emitting electrochemical cells based on supramolecularly engineered organic polymers.     

Biologically Enabled Syntheses of Freestanding Metallic Structures Possessing Subwavelength Pore Arrays for Extraordinary (Surface Plasmon‐Mediated) Infrared Transmission

A scalable wet chemical process has been used to convert the intricate silica microshells (frustules) of diatoms into gold structures that retained the three‐dimensional (3‐D) frustule shapes and fine patterned features. Combined use of an amine‐enriching surface functionalization protocol and electroless deposition yielded thin (<100 nm) conformal nanocrystalline gold coatings that, upon selective silica dissolution, were converted into freestanding gold structures with frustule‐derived 3‐D morphologies. By selecting a diatom frustule template with a quasi‐regular hexagonal pore pattern (Coscinodiscus asteromphalus, CA), gold replica structures possessing such pore patterns were produced that exhibited infrared transmission maxima/reflection minima that were not observed for the starting silica diatom frustules or for flat nonporous gold films; that is, such extraordinary optical transmission (EOT) resulted from the combined effects of the quasi‐periodic hexagonal hole structure (inherited from the CA diatom frustules) and the gold chemistry. Calculated and measured IR transmission spectra obtained from planar gold films with quasi‐periodic hexagonal CA‐derived hole patterns, or with short‐range periodic hexagonal hole patterns, indicated that the enhanced IR transmission exhibited by the gold CA frustule replicas was enabled by the generation and transmission of surface plasmons. This scalable bio‐enabled process provides a new and attractive capability for fabricating self‐supporting, responsive, 3‐D metallic structures for use as dispersible/harvestable microparticles tailored for EOT‐based applications.     

Surface‐Initiated Atom Transfer Radical Polymerization of Oligo(ethylene glycol) Methyl Methacrylate from a Mixed Self‐Assembled Monolayer on Gold

This paper describes the in‐situ synthesis of an oligo(ethylene glycol)‐functionalized polymer brush in which the oligo(ethylene glycol) chains are presented as side‐chains from a methacrylate backbone that is anchored to the surface. These polymer “bottlebrushes” have been synthesized by surface‐initiated atom transfer radical polymerization (SI‐ATRP) of oligo(ethylene glycol) methyl methacrylate (OEGMA) from a mixed self‐assembled monolayer (SAM) of an ATRP initiator‐functionalized alkanethiol and a diluent, methyl‐terminated thiol. The systematic control of the ATRP initiator surface density afforded by the mixed SAM on gold and the polymerization time enables the polymer chain length and surface density to be independently controlled. Surface plasmon resonance (SPR) spectroscopy of fibronectin (Fn) adsorption on poly(OEGMA) grown from the surface of the mixed SAMs on gold shows that above a threshold solution molar ratio of the ATRP‐initiator thiol to methyl‐terminated thiol of 0.2, and a dry film thickness of ～ 4 nm, Fn adsorption on the surface‐initiated poly(OEGMA) coatings was below the detection limit of SPR. The relatively low surface density of the ATRP initiator required to confer protein resistance to the surface suggests that SI‐ATRP may be a viable strategy to create protein resistant polymer brushes on real‐world materials.     

Effect of Immobilized Nerve Growth Factor on Conductive Polymers: Electrical Properties and Cellular Response

The use of biologically active dopants in conductive polymers allows the polymer to be tailored for specific applications. The incorporation of nerve growth factor (NGF) as a co‐dopant in the electrochemical deposition of conductive polymers is evaluated for its ability to elicit specific biological interactions with neurons. The electrochemical properties of the NGF‐modified conducting polymers are studied by impedance spectroscopy and cyclic voltammetry. Impedance measurements at the neurobiologically important frequency of 1 kHz reveal that the minimum impedance of the NGF‐modified polypyrrole (PPy) film, 15 kΩ, is lower than the minimum impedance of peptide‐modified PPy film (360 kΩ). Similar results are found with NGF‐modified poly(3,4‐ethylene dioxythiophene) (PEDOT). The microstructure of the conductive polymer films is characterized by optical microscopy and electron microscopy and indicates that the NGF‐functionalized polymer surface topology is similar to that of the unmodified polymer film. Optical and fluorescence microscopy reveal that PC‐12 (rat pheochromacytoma) cells adhered to the NGF‐modified substrate and extended neurites on both PPy and PEDOT, indicating that the NGF in the polymer film is biologically active. Taken together these data indicate that the incorporation of NGF can modify the biological interactions of the electrode without compromising the conductive properties or the morphology of the polymeric film.