Wafer-Scale Nanopillars Derived from Block Copolymer Lithography for Surface-Enhanced Raman Spectroscopy

Flexible SERS strip based on HKUST-1(cu)/biomimetic antibodies composite multilayer for trace determination of ethephon

A surface-enhanced Raman scattering (SERS) sensor based on the folding and assembly characteristics of the three-dimensional structure of paper fibers, the skeleton controllability of metal-organic framework materials (MOFs), and the morphology designability of plasmonic noble metal materials has been established for rapid on-site determination of ethephon in food. HKUST-1(Cu) was assembled onto a carbon-treated chromatographic paper matrix by electrodeposition, and its skeleton respiration and sponge effect were used to overcome the bottleneck problem of poor affinity of SERS substrate for target molecules. Further coupled with the targeted recognition specificity of biomimetic antibodies, a paper-based interface with high specificity of molecular sensitivity was constructed. A sandwich multi-stage progressive enhancement structure was designed to couple plasma pine branch-shaped silver material in situ at the interface to realize superposition and collaborative amplification of SERS signals. When the paper-based strip sensor was used to monitor ethephon, it demonstrated a linear range of 10^-3 to 10 mg kg^-1 and a detection limit of around 1.39 × 10^-4 mg kg^-1. The construction and application of the paper-based HKUST-1(Cu)/biomimetic antibodies/pine branch-shaped silver material sensor will provide technical means and theoretical support for the rapid and efficient identification of biological ripening agent residues in food with multi-level signal enhancement.

Au-Based Thin-Film Metallic Glasses for Propagating Surface Plasmon Resonance-Based Sensor Applications

We deposited Au-Cu-Si, an Au-based thin-film metallic glass (TFMG) of ∼50 nm thickness, as the activation layer for propagating surface plasmon resonance (PSPR)-based sensors on a BK7 glass substrate to substitute the commonly used gold layer. The film composition was tuned to yield the maximum Au content (∼65 at %), while the structure remained amorphous. The results showed that the Au-based TFMG could support surface plasmon resonance and gave rise to the extinction in the angle-resolved reflection spectrum. Using deionized water and ethyl alcohol with the refractive index difference of ∼0.03 as the analytes, the angle shift given by Au-based TFMG was 4° compared to 5° given by the Au film. Hence, Au-based TFMG is feasible to be used as the activation layer in PSPR-based sensors. Compared to the Au film, Au-based TFMG has the advantages of being less expensive, lacking grain boundary scattering, better adhesion to the substrate, and higher resistance to scratch and corrosion because of its amorphous structure with excellent mechanical properties.

Dimensional Design for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.

Thin film block copolymer self-assembly for nanophotonics

The nanophotonic engineering of light-matter interactions has profoundly changed research behind the design and fabrication of optical materials and devices. Metasurfaces-arrays of subwavelength nanostructures that interact resonantly with electromagnetic radiation-have emerged as an integral nanophotonic platform for a new generation of ultrathin lenses, displays, polarizers and other devices. Their success hinges on advances in lithography and nanofabrication in recent decades. While existing nanolithography techniques are suitable for basic research and prototyping, issues of cost, throughput, scalability, and substrate compatibility may preclude their use for many metasurface applications. Patterning via spontaneous self-assembly of block copolymer thin films offers an enticing alternative for nanophotonic manufacturing that is rapid, inexpensive, and applicable to large areas and diverse substrates. This review discusses the advantages and disadvantages of block copolymer-based nanopatterning and highlights recent progress in their use for broadband antireflection, surface enhanced Raman spectroscopy, and other nanophotonic applications. Recent advances in diversification of self-assembled block copolymer nanopatterns and improved processes for enhanced scalability of self-assembled nanopatterning using block copolymers are also discussed, with a spotlight on directions for future research that would enable a wider array of nanophotonic applications.

Wafer-scale nanocracks enable single-molecule detection and on-site analysis

Large-area surface-enhanced Raman spectroscopy (SERS) sensing platforms displaying ultrahigh sensitivity and signal uniformity have potentially enormous sensing applicability, but they are still challenging to prepare in a scalable manner. In this study, silver nanopaste (AgNPA) was employed to prepare a wafer-scale, ultrasensitive SERS substrate. The self-generated, high-density Ag nanocracks (NCKs) with small gaps could be created on Si wafers via a spin-coating process, and provided extremely abundant hotspots for SERS analyses with ultrahigh sensitivity-down to the level of single molecules (enhancement factor: ca. 10¹⁰; detection limit: ca. 10^-18 M)-and great signal reproducibility (variation: ca. 3.6%). Moreover, the Ag NCK arrays demonstrated broad applicability and practicability for on-site detection when combined with a portable 785 Raman spectrometer. This method allowed the highly sensitive detection of a diverse range of analytes (benzo[a]pyrene, di-2-ethylhexyl phthalate, aflatoxins B1, zearalenone, ractopamine, salbutamol, sildenafil, thiram, dimethoate, and methamidophos). In particular, pesticides are used extensively in agricultural production. Unfortunately, they can affect the environment and human health as a result of acute toxicity. Therefore, the simultaneous label-free detection of three different pesticides was demonstrated. Finally, the SERS substrates are fabricated through a simple, efficient, and scalable process that offers new opportunities for mass production.

Surface-enhanced Raman scattering with gold-coated silicon nanopillars arrays: The influence of size and spatial order

Nanopillars have been extensively explored as promising substrates for surface-enhanced Raman scattering (SERS) owing to their high sensitivity and excellent reproducibility. Most of the researches have been focused on the fabrication methods of nanopillars, and the dependences of SERS effects on geometrical size and spatial order are rarely investigated. In this work, SERS properties of nanopillars with different sizes (115-185 nm) and spatial orders (square and rhombus orders) have been studied. The work has shown that the nanopillars not only have high enhancement capability and high signal reproducibility, but also the enhancement is insensitive to the size and spatial orders. The measured enhancement factors (EFs) are 2.3-4.0 × 10⁶ and signal reproducibility (relative standard deviation, RSD) are ∼ 5.2%-6.9%, which are among the best of the similar SERS substrates reported. The variation of SERS intensity was as low as approximately 4.8% with the variation of pillar size from 115, 135, 145, to 160 nm. The insensitiveness and high reproducibility have been ascribed to the combined excitation of localized surface plasmon resonance (LSPR) and propagating surface plasmons (SPPs) of the nanopillars. Optical properties of the nanopillars are studied both experimentally and numerically to understand the physics behind the SERS performance.

A route to engineered high aspect-ratio silicon nanostructures through regenerative secondary mask lithography

Silicon nanostructuring imparts unique material properties including antireflectivity, antifogging, anti-icing, self-cleaning, and/or antimicrobial activity. To tune these properties however, a good control over features' size and shape is essential. Here, a versatile fabrication process is presented to achieve tailored silicon nanostructures (thin/thick pillars, sharp/truncated/re-entrant cones), of pitch down to ∼50 nm, and high-aspect ratio (>10). The approach relies on pre-assembled block copolymer (BCP) micelles and their direct transfer into a glass hard mask of an arbitrary thickness, now enabled by our recently reported regenerative secondary mask lithography. During this pattern transfer, not only can the mask diameter be decreased but also uniquely increased, constituting the first method to achieve such tunability without necessitating a different molecular weight BCP. Consequently, the hard mask modulation (height, diameter) advances the flexibility in attainable inter-pillar spacing, aspect ratios, and re-entrant profiles (= glass on silicon). Combined with adjusted silicon etch conditions, the morphology of nanopatterns can be highly customized. The process control and scalability enable uniform patterning of a 6-inch wafer which is verified through cross-wafer excellent antireflectivity (<5%) and water-repellency (advancing contact angle 158°; hysteresis 1°). The implementation of this approach to silicon nanostructuring is envisioned to be far-reaching, facilitating fundamental studies and targeting applications spanning solar panels, antifogging/antibacterial surfaces, sensing, amongst many others.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Biosensors for Studies on Adhesion-Mediated Cellular Responses to Their Microenvironment

Cells interact with their microenvironment by constantly sensing mechanical and chemical cues converting them into biochemical signals. These processes allow cells to respond and adapt to changes in their environment, and are crucial for most cellular functions. Understanding the mechanism underlying this complex interplay at the cell-matrix interface is of fundamental value to decipher key biochemical and mechanical factors regulating cell fate. The combination of material science and surface chemistry aided in the creation of controllable environments to study cell mechanosensing and mechanotransduction. Biologically inspired materials tailored with specific bioactive molecules, desired physical properties and tunable topography have emerged as suitable tools to study cell behavior. Among these materials, synthetic cell interfaces with built-in sensing capabilities are highly advantageous to measure biophysical and biochemical interaction between cells and their environment. In this review, we discuss the design of micro and nanostructured biomaterials engineered not only to mimic the structure, properties, and function of the cellular microenvironment, but also to obtain quantitative information on how cells sense and probe specific adhesive cues from the extracellular domain. This type of responsive biointerfaces provides a readout of mechanics, biochemistry, and electrical activity in real time allowing observation of cellular processes with molecular specificity. Specifically designed sensors based on advanced optical and electrochemical readout are discussed. We further provide an insight into the emerging role of multifunctional micro and nanosensors to control and monitor cell functions by means of material design.

Block Copolymer Derived Vertically Coupled Plasmonic Arrays for Surface-Enhanced Raman Spectroscopy

A surface-enhanced Raman spectroscopy sensing template consisting of gold-covered nanopillars is developed. The plasmonic slab consists of a perforated gold film at the base of the nanopillars and a Babinet complementary dot array on top of the pillars. The nanopillars were fabricated by the incorporation of an iron salt precursor into a self-assembled block copolymer thin film and subsequent reactive ion etching. The preparation is easy, scalable, and cost-effective. We report on the increase in surface-enhanced Raman scattering efficiency for smaller pillar heights and stronger coupling between the dot array and perforated gold film with average enhancement factors as high as 10⁷. In addition, the block copolymer-derived templates show an excellent relative standard deviation of 8% in the measurement of the Raman intensity. Finite difference time domain simulations were performed to investigate the nature of the electromagnetic near-field enhancement and to identify plasmonic hot spots.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Toward Flexible Surface-Enhanced Raman Scattering (SERS) Sensors for Point-of-Care Diagnostics

Surface-enhanced Raman scattering (SERS) spectroscopy provides a noninvasive and highly sensitive route for fingerprint and label-free detection of a wide range of molecules. Recently, flexible SERS has attracted increasingly tremendous research interest due to its unique advantages compared to rigid substrate-based SERS. Here, the latest advances in flexible substrate-based SERS diagnostic devices are investigated in-depth. First, the intriguing prospect of point-of-care diagnostics is briefly described, followed by an introduction to the cutting-edge SERS technique. Then, the focus is moved from conventional rigid substrate-based SERS to the emerging flexible SERS technique. The main part of this report highlights the recent three categories of flexible SERS substrates, including actively tunable SERS, swab-sampling strategy, and the in situ SERS detection approach. Furthermore, other promising means of flexible SERS are also introduced. The flexible SERS substrates with low-cost, batch-fabrication, and easy-to-operate characteristics can be integrated into portable Raman spectroscopes for point-of-care diagnostics, which are conceivable to penetrate global markets and households as next-generation wearable sensors in the near future.

Automated Chemical Sensing Unit Integration for Parallel Optical Interrogation

We report the integration of an automated chemical optical sensing unit for the parallel interrogation of 12 BICELLs in a sensing chip. The work was accomplished under the European Project Enviguard (FP7-OCEAN-2013-614057) with the aim of demonstrating an optical nano-biosensing unit for the in-situ detection of various chemical pollutants simultaneously in oceanic waters. In this context, we designed an optical sensing chip based on resonant nanopillars (R-NPs) transducers organized in a layout of twelve biophotonic sensing cells (BICELLs). The sensing chip is interrogated in reflection with a 12-channels optical spectrometer equipped with an embedded computer-on-chip performing image processing for the simultaneous acquisition and analysis (resonant mode fitting) of the 12 spectra. A microfluidic chip and an automated flow control system composed of four pumps and a multi-path micro-valve makes it possible to drive different complex protocols. A rack was designed ad-hoc for the integration of all the modules. As a proof of concept, fluids of different refractive index (RI) were flowed in the system in order to measure the time response (sensogram) of the R-NPs under optical reflectance, and assess the sensors’ bulk sensitivity (285.9 ± 16.4 nm/RIU) and Limit of Detection (LoD) (2.95 × 10⁻⁶ RIUS). The real-time response under continuous flow of a sensor chip based on R-NP is showed for the first time, obtaining 12 sensograms simultaneously, featuring the unit as a potential excellent multiplexed detection system. These results indicate the high potential of the developed chemical sensing unit to be used for in-situ, multiplex and automatic optical biosensing.

Ambient electrospray deposition Raman spectroscopy (AESD RS) using soft landed preformed silver nanoparticles for rapid and sensitive analysis

Ambient electrospray deposition Raman spectroscopy (AESD RS) using soft landed preformed silver nanoparticles for rapid and sensitive SERS analysis.

Ultralarge Area Sub-10 nm Plasmonic Nanogap Array by Block Copolymer Self-Assembly for Reliable High-Sensitivity SERS

Effective surface enhancement of Raman scattering (SERS) requires strong near-field enhancement as well as effective light collection of plasmonic structures. To this end, plasmonic nanoparticle (NP) arrays with narrow gaps or sharp tips have been suggested as desirable structures. We present a highly dense and uniform Au nanoscale gap array enabled by the customized design of NP shape and arrangement employing block copolymer self-assembly. Block copolymer self-assembly in thin films offers uniform hexagonally packed nanopost template arrays over the entire surface of a 2 in. wafer. Conventional evaporative metal deposition over the nanotemplate surface allows precise geometric control and positional arrangement of metal NPs, constituting tunable, strong plasmonic near-field enhancement particularly at the "hot spots" near interparticular nanoscale gaps. Underlying field distribution has been investigated by a finite-difference time-domain simulation. In the detection of thiophenol, our Au nanogap array shows a remarkable enhancement of Raman intensity greater than ∼10⁴, a standard deviation as small as 12.3% compared to that of the planar Au thin film. In addition, adenine biomolecules can be detected with a detection limit as low as 100 nM. Our approach proposes highly sensitive and reliable SERS on the basis of a scalable, low-cost bottom-up strategy.

Influence of the long-range ordering of gold-coated Si nanowires on SERS

Controlling the location and the distribution of hot spots is a crucial aspect in the fabrication of surface-enhanced Raman spectroscopy (SERS) substrates for bio-analytical applications. The choice of a suitable method to tailor the dimensions and the position of plasmonic nanostructures becomes fundamental to provide SERS substrates with significant signal enhancement, homogeneity and reproducibility. In the present work, we studied the influence of the long-range ordering of different flexible gold-coated Si nanowires arrays on the SERS activity. The substrates are made by nanosphere lithography and metal-assisted chemical etching. The degree of order is quantitatively evaluated through the correlation length (ξ) as a function of the nanosphere spin-coating speed. Our findings showed a linear increase of the SERS signal for increasing values of ξ, coherently with a more ordered and dense distribution of hot spots on the surface. The substrate with the largest ξ of 1100 nm showed an enhancement factor of 2.6 · 10³ and remarkable homogeneity over square-millimetres area. The variability of the signal across the substrate was also investigated by means of a 2D chemical imaging approach and a standard methodology for its practical calculation is proposed for a coherent comparison among the data reported in literature.

Nanotextured Si surfaces derived from block-copolymer self-assembly with superhydrophobic, superhydrophilic, or superamphiphobic properties

We demonstrate the use of wafer-scale nanolithography based on block-copolymer (BCP) self-assembly for the fabrication of surfaces with enhanced wetting properties.

Sensitive and uniform detection using Surface-Enhanced Raman Scattering: Influence of colloidal-droplets evaporation based on Au-Ag alloy nanourchins

Surface Enhanced Raman Scattering (SERS) has been developed into a powerful vibrational spectroscopy technique for chemical detection. However, the fabrication of colloidal droplets-based SERS substrates with well reproducibility and uniformity still remains challenging. In this paper, colloidal suspensions of hollow Au-Ag alloy nanourchins (HAAA-NUs) and Au nanoparticles (Au NPs) with different morphologies were employed as SERS-active substrates. After evaporation of colloidal suspensions, we evaluated the SERS performance based on the following features: "Coffee Ring Effects", adsorption processes of probe molecule and colloidal NPs, spin coating and morphologies of suspended NPs. The results demonstrated that SERS signals could be enhanced enormously in the marginal region of Coffee Ring patterns. The limit of detection (LOD) for amaranth molecule would be reached 10^-8 M. Moreover, by combining the droplets evaporation of HAAA-NUs suspensions with spin coating, the relative standard deviation (RSD) could be down to 3.5%, showing excellent reproducibility. The investigation here would provide a simple, practical and portable SERS detection method with excellent signal uniformity.

Wafer Scale Fabrication of Dense and High Aspect Ratio Sub-50 nm Nanopillars from Phase Separation of Cross-Linkable Polysiloxane/Polystyrene Blend

We demonstrated a simple and effective approach to fabricate dense and high aspect ratio sub-50 nm pillars based on phase separation of a polymer blend composed of a cross-linkable polysiloxane and polystyrene (PS). In order to obtain the phase-separated domains with nanoscale size, a liquid prepolymer of cross-linkable polysiloxane was employed as one moiety for increasing the miscibility of the polymer blend. After phase separation via spin-coating, the dispersed domains of liquid polysiloxane with sub-50 nm size could be solidified by UV exposure. The solidified polysiloxane domains took the role of etching mask for formation of high aspect ratio nanopillars by O₂ reactive ion etching (RIE). The aspect ratio of the nanopillars could be further amplified by introduction of a polymer transfer layer underneath the polymer blend film. The effects of spin speeds, the weight ratio of the polysiloxane/PS blend, and the concentration of polysiloxane/PS blend in toluene on the characters of the nanopillars were investigated. The gold-coated nanopillar arrays exhibited a high Raman scattering enhancement factor in the range of 10⁸-10⁹ with high uniformity across over the wafer scale sample. A superhydrophobic surface could be realized by coating a self-assembled monolayers (SAM) of fluoroalkyltrichlorosilane on the nanopillar arrays. Sub-50 nm silicon nanowires (SiNWs) with high aspect ratio of about 1000 were achieved by using the nanopillars as etching mask through a metal-assisted chemical etching process. They showed an ultralow reflectance of approximately 0.1% for wavelengths ranging from 200 to 800 nm.

A primary SERS-active interconnected Si-nanocore network for biomolecule detection with plasmonic nanosatellites as a secondary boosting mechanism

We report in this study, the development of a polymorphic biosensitive Si nanocore superstructure as a SERS biosensing platform.