Fabrication and characterization of flexible and tunable plasmonic nanostructures

Increasing gold nanostars SERS response with silver shells: a surface-based seed-growth approach

A straightforward method to prepare surface enhanced Raman spectroscopy (SERS) chips containing a monolayer of silver coated gold nanostars (GNS@Ag) grafted on a glass surface is introduced. The synthetic approach is based on a seed growth method performed directly on surface, using GNS as seeds, and involving a green pathway, which only uses silver nitate, ascorbic acid and water, to grow the silver shell. The preparation was optimized to maximize signals obtaining a SERS response of one order of magnitude greater than that from the original GNS based chips, offering in the meantime good homogeneity and acceptable reproducibility. The proposed GNS@Ag SERS chips are able to detect pesticide thiram down to 20 ppb.

Raman spectroscopy and its plasmon-enhanced counterparts: A toolbox to probe protein dynamics and aggregation

Protein unfolding and aggregation are often correlated with numerous diseases such as Alzheimer's, Parkinson's, Huntington's, and other debilitating neurological disorders. Such adverse events consist of a plethora of competing mechanisms, particularly interactions that control the stability and cooperativity of the process. However, it remains challenging to probe the molecular mechanism of protein dynamics such as aggregation, and monitor them in real-time under physiological conditions. Recently, Raman spectroscopy and its plasmon-enhanced counterparts, such as surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS), have emerged as sensitive analytical tools that have the potential to perform molecular studies of functional groups and are showing significant promise in probing events related to protein aggregation. We summarize the fundamental working principles of Raman, SERS, and TERS as nondestructive, easy-to-perform, and fast tools for probing protein dynamics and aggregation. Finally, we highlight the utility of these techniques for the analysis of vibrational spectra of aggregation of proteins from various sources such as tissues, pathogens, food, biopharmaceuticals, and lastly, biological fouling to retrieve precise chemical information, which can be potentially translated to practical applications and point-of-care (PoC) devices. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > Diagnostic Nanodevices Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Recent Development and Applications of Stretchable SERS Substrates

Surface-enhanced Raman scattering (SERS) is a cutting-edge technique for highly sensitive analysis of chemicals and molecules. Traditional SERS-active nanostructures are constructed on rigid substrates where the nanogaps providing hot-spots of Raman signals are fixed, and sample loading is unsatisfactory due to the unconformable attachment of substrates on irregular sample surfaces. A flexible SERS substrate enables conformable sample loading and, thus, highly sensitive Raman detection but still with limited detection capabilities. Stretchable SERS substrates with flexible sample loading structures and controllable hot-spot size provide a new strategy for improving the sample loading efficiency and SERS detection sensitivity. This review summarizes and discusses recent development and applications of the newly conceptual stretchable SERS substrates. A roadmap of the development of SERS substrates is reviewed, and fabrication techniques of stretchable SERS substrates are summarized, followed by an exhibition of the applications of these stretchable SERS substrates. Finally, challenges and perspectives of the stretchable SERS substrates are presented. This review provides an overview of the development of SERS substrates and sheds light on the design, fabrication, and application of stretchable SERS systems.

Layer-by-layer coating of natural diatomite with silver nanoparticles for identification of circulating cancer protein biomarkers using SERS

Surface-enhanced Raman scattering (SERS) is an emerging spectroscopy technique for detecting and characterizing chemical or biological structures in the vicinity of plasmonic nanostructures. Colloidal, solid, and flexible nanostructures are widely used in SERS experiments to enhance the Raman intensity. The nanostructure used in SERS is one of the main influencing parameters and a growing research area. Fabrication of simple and cheap SERS substrates with a high enhancement factor is desired. In this study, we fabricated a reproducible, cheap, and flexible SERS active strip by coating natural diatomite (biosilica) with silver nanoparticles (AgNPs) using the layer-by-layer assembly method and the fabricated strip is used for the label-free identification of circulating cancer protein biomarkers. SERS active strips were fabricated having different numbers of AgNP layers on natural diatomite and comprehensive characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV/vis absorption spectrophotometry were used. SERS activities of the strips depending on the number of layers were evaluated using 4-aminothiophenol (4-ATP) and rhodamine 6G (Rh6G) molecules. We found that the SERS intensity is strongly dependent on the number of AgNP layers, with the maximum SERS intensity obtained from the strip with 5 layers of AgNPs, having a 2.0 × 105 enhancement factor. The strip with the highest SERS activity was used for the label-free identification of circulating cancer protein biomarkers (HER2, CA15-3, PSA, MUC4, and CA27-29). The results demonstrate that the fabricated strip can help in the effective label-free identification of circulating protein biomarkers and open new directions for SERS-based label-free biosensing applications.

The Wavelength-Dependent SERS Template Based on a Nanopillar Array

Surface-enhanced Raman spectroscopy (SERS) can be regarded as a powerful tool for probing chemical molecules by effectively enhancing Raman signals. However, the enhancement factors depend on the SERS template, the probed molecular structures, and the excitation laser wavelength. Herein, we proposed a simple and easily fabricated nanostructured template for SERS and analyzed the wavelength-dependent factors. Three types of golden nanopillar arrays on silicon wafers were designed and manufactured. The SERS signals of the Rhodamine 6G (R6G) molecules were extracted. Three laser sources, a blue 17 mW 458 nm diode laser, a green 20 mW 532 nm laser, and a red 6 mW 633 nm laser, were employed as the excitation laser sources. The 458 nm laser was located far from the resonate spectrum of R6G. The optical intensity distributions for the different SERS templates excited by three laser beams were also simulated. The enhancement factors (EFs) of R6G on the three nanostructured templates were measured and compared. The photoluminescence spectrum of the nanostructured templates and SERS signals of R6G were also measured. In addition, the experimental results concerned optical simulations. The analysis tool that was used was a convolution profile of multiple Lorentzian line shapes with a Gaussian profile. It is helpful to understand the SERS signals when the excitation laser wavelength is located out of the resonance region of molecules. It can also provide a new design approach to fabricate an SERS Template with a nanopillar array for different excitation wavelengths.

SERS and EC-SERS detection of local anesthetic procaine using Pd loaded highly reduced graphene oxide nanocomposite substrate

The localized surface plasmon resonance (LSPR) excitations are critical towards achieving sizable spectral enhancements of the Raman scattered light. Herein, the synthesis of palladium-based highly reduced graphene oxide (Pd-HRG) with LSPR properties as an effective surface-enhanced Raman scattering (SERS) substrate and its utility in the highly sensitive detection of procaine are reported. The concentration detection of procaine samples was optimized by applying a set of pre-concentration parameters. The Pd-HRG nanocomposite showed a remarkable LSPR response with a Raman enhancement factor of 8.7 × 10². The Pd-HRG is employed to modify fluorine doped tin oxide electrode (Pd-HRG/FTO), resulted with an enhancement factor of 7.5 × 10⁴ corresponding to the EC-SERS technique. The electronic and surface properties of synthesized Pd-HRG and functionalized FTO electrode were evaluated using Raman, infrared, EIS, XRD, FESEM and EDX techniques. Quantum chemical calculations were carried out to elaborate on the nature of interaction of procaine molecules with a nanostructured surface model. Pd-HRG, with an efficient and cost-effective fabrication, can be considered as a promising EC-SERS substrate for the detection of organic therapeutic drugs.

A Flexible Plasmonic-Membrane-Enhanced Broadband Tin-Based Perovskite Photodetector

Lead-free perovskite quantum dots (QDs) have been widely investigated for optoelectronic devices because of their excellent electrical and optical properties. However, optoelectronic devices based on such lead-free perovskites still have much lower performance than those made of Pb-based counterparts. Herein, we developed a lead-free photodetector with an enhanced broadband spectral response ranging from 300 to 630 nm. By balancing plasmonic near-field enhancement and surface energy quenching through precisely controlling the thickness of Al₂O₃ spacer between the CsSnBr₃ QDs and silver nanoparticle membrane, the photodetector with 5 nm thick Al₂O₃ experiences a maximum photocurrent enhancement of 6.5-fold at 410 nm, with a responsivity of 62.3 mA/W and detectivity of 4.27 × 10¹¹ Jones. Moreover, its photocurrent shows a negligible decrease after 100 cycles of bending, which is ascribed to the tension-offset induced by the self-assembled nanoparticle membrane. The proposed plasmonic membrane enhancement provides a great potential for high-performance perovskite optoelectronic devices.

Shape and deposition angle control of silver film-over-nanosphere SERS substrates

Thin metallic films on dielectric nanospheres are demonstrated to have a high potential for the fabrication of cost-effective SERS substrates. In addition to the morphological advantages that nanospheres offer for attaining a high density of hot spots, possessing shape adjustability by uncomplicated thermal treatment makes them an attractive platform for tuneable SERS substrates. Furthermore, when combined with the oblique angle metal deposition technique, adjustable gaps at a high density and adjustable shape of metal films, such as Ag films, can be achieved on nanospheres. Applying small changes in deposition angle can provide means for fine adjustment of the Raman enhancement factor (EF), resulting in EF up to 10⁸measured using crystal violet dye molecule as a Raman analyte. This practice paves the way for the fabrication of high EF SERS substrates at a reasonable cost using a monolayer of self-organized nanosphere patterns. An ultra-thin Ag film coated at 5° tilt is shown to be an excellent substitute for a film deposited at 0° with double the thickness. There is a strong agreement between the experimental results and finite-elements-method-based Maxwell simulations exhibiting expected field enhancements up to 10⁹at a tilt angle of 5°.

Thermoplasmonic effect onto Toad physiology signals by plasmonic microchip structure

Cardiovascular diseases are considered as the leading cause of death and almost 80% of deaths from this disease are developed in poor and less developed countries where early detection facilities are less available, along with overlooking the importance of screening. In other words, real-time monitoring of the physiological signals using flexible and wearable biosensors plays an important role in human life style. Thus, the present study aims to propose two dimensional flexible and wearable gold covered plasmonic samples as a physiological signal recorder, in which chips with nano array of resonant nanowire patterns performing in an integrated platform of plasmonic devices. The produced surface plasmon waves in our main chip were paired with an electric wave from the heart pulse and it use for recording and detecting the heartbeat of a toad with high accuracy. This measurement was performed in normal state and under external laser heating process to check the ability of signal recording and also thermoplasmonic effect onto the toad's heart signal. Our results show that our sensor was enough sensitive for detection while raising the body temperature of the toad and changing its heart rate as flatting T and P waves by thermoplasmonic effect.

Identification of amyloid beta in small extracellular vesicles via Raman spectroscopy

One of the hallmarks of Alzheimer's disease (AD) pathogenesis is believed to be the production and deposition of amyloid-beta (Aβ) peptide into extracellular plaques. Existing research indicates that extracellular vesicles (EVs) can carry Aβ associated with AD. However, characterization of the EVs-associated Aβ and its conformational variants has yet to be realized. Raman spectroscopy is a label-free and non-destructive method that is able to assess the biochemical composition of EVs. This study reports for the first time the Raman spectroscopic fingerprint of the Aβ present in the molecular cargo of small extracellular vesicles (sEVs). Raman spectra were measured from sEVs isolated from Alzheimer's disease cell culture model, where secretion of Aβ is regulated by tetracycline promoter, and from midbrain organoids. The averaged spectra of each sEV group showed considerable variation as a reflection of the biochemical content of sEVs. Spectral analysis identified more intense Raman peaks at 1650 cm^-1 and 2930 cm^-1 attributable to the Aβ peptide incorporated in sEVs produced by the Alzheimer's cell culture model. Subsequent analysis of the spectra by principal component analysis differentiated the sEVs of the Alzheimer's disease cell culture model from the control groups of sEVs. Moreover, the results indicate that Aβ associated with secreted sEVs has a α-helical secondary structure and the size of a monomer or small oligomer. Furthermore, by analyzing the lipid content of sEVs we identified altered fatty acid chain lengths in sEVs that carry Aβ that may affect the fluidity of the EV membrane. Overall, our findings provide evidence supporting the use of Raman spectroscopy for the identification and characterization of sEVs associated with potential biomarkers of neurological disorders such as toxic proteins.

Space curvature-inspired nanoplasmonic sensor for breast cancer extracellular vesicle fingerprinting and machine learning classification

Extracellular vesicles (EVs) are micro and nanoscale lipid-enclosed packages that have shown potential as liquid biopsy targets for cancer because their structure and contents reflect their cell of origin. However, progress towards the clinical applications of EVs has been hindered due to the low abundance of disease-specific EVs compared to EVs from healthy cells; such applications thus require highly sensitive and adaptable characterization tools. To address this obstacle, we designed and fabricated a novel space curvature-inspired surfaced-enhanced Raman spectroscopy (SERS) substrate and tested its capabilities using bioreactor-produced and size exclusion chromatography-purified breast cancer EVs of three different subtypes. Our findings demonstrate the platform's ability to effectively fingerprint and efficiently classify, for the first time, three distinct subtypes of breast cancer EVs following the application of machine learning algorithms on the acquired spectra. This platform and characterization approach will enhance the viability of EVs and nanoplasmonic sensors towards clinical utility for breast cancer and many other applications to improve human health.

Biosensing Applications Using Nanostructure-Based Localized Surface Plasmon Resonance Sensors

Localized surface plasmon resonance (LSPR)-based biosensors have recently garnered increasing attention due to their potential to allow label-free, portable, low-cost, and real-time monitoring of diverse analytes. Recent developments in this technology have focused on biochemical markers in clinical and environmental settings coupled with advances in nanostructure technology. Therefore, this review focuses on the recent advances in LSPR-based biosensor technology for the detection of diverse chemicals and biomolecules. Moreover, we also provide recent examples of sensing strategies based on diverse nanostructure platforms, in addition to their advantages and limitations. Finally, this review discusses potential strategies for the development of biosensors with enhanced sensing performance.

Strain Sensor via Wood Anomalies in 2D Dielectric Array

Optical sensing is one of many promising applications for all-dielectric photonic materials. Herein, we present an analytical and numerical study on the strain-responsive spectral properties of a bioinspired sensor. The sensor structure contains a two-dimensional periodic array of dielectric nanodisks to mimic the optical behavior of grana lamellae inside chloroplasts. To accumulate a noticeable response, we exploit the collective optical mode in grana ensemble. In higher plants, such a mode appears as Wood's anomaly near the chlorophyll absorption line to control the photosynthesis rate. The resonance is shown persistent against moderate biological disorder and deformation. Under the stretching or compression of a symmetric structure, the mode splits into a couple of polarized modes. The frequency difference is accurately detected. It depends on the stretch coefficient almost linearly providing easy calibration of the strain-sensing device. The sensitivity of the considered structure remains at 5 nm/% in a wide range of strain. The influence of the stretching coefficient on the length of the reciprocal lattice vectors, as well as on the angle between them, is taken into account. This adaptive phenomenon is suggested for sensing applications in biomimetic optical nanomaterials.

Recent Advances in the Fabrication and Functionalization of Flexible Optical Biosensors: Toward Smart Life-Sciences Applications

Over the last 30 years, optical biosensors based on nanostructured materials have obtained increasing interest since they allow the screening of a wide variety of biomolecules with high specificity, low limits of detection, and great sensitivity. Among them, flexible optical platforms have the advantage of adapting to non-planar surfaces, suitable for in vivo and real-time monitoring of diseases and assessment of food safety. In this review, we summarize the newest and most advanced platforms coupling optically active materials (noble metal nanoparticles) and flexible substrates giving rise to hybrid nanomaterials and/or nanocomposites, whose performances are comparable to the ones obtained with hard substrates (e.g., glass and semiconductors). We focus on localized surface plasmon resonance (LSPR)-based and surface-enhanced Raman spectroscopy (SERS)-based biosensors. We show that large-scale, cost-effective plasmonic platforms can be realized with the currently available techniques and we emphasize the open issues associated with this topic.

Surface-enhanced Raman scattering effect in binary systems formed by graphene on aluminum plasmonic nanostructures

Binary systems (BS) formed by graphene (GR) deposited on top of aluminum (Al) nanoconcaves (Al-NC) and Al nanodomes (Al-ND) were synthesized by electrochemical anodization of Al. Using the plasmonic response of Al-NC and Al-ND and the distinctive physical and chemical properties of GR, these BS are proposed as Surface-Enhanced Raman Scattering (SERS) sensors using rhodamine 6G (R6G) as a proof molecule. As expected, the BS significantly enhances Raman signals of R6G molecules in comparison with substrates used as references, also suppressing the fluorescence background of R6G molecules.

Nanosphere Lithography for Structuring Polycrystalline Diamond Films

This paper deals with the structuring of polycrystalline diamond thin films using the technique of nanosphere lithography. The presented multistep approaches relied on a spin-coated self-assembled monolayer of polystyrene spheres, which served as a lithographic mask for the further custom nanofabrication steps. Various arrays of diamond nanostructures—close-packed and non-close-packed monolayers over substrates with various levels of surface roughness, noble metal films over nanosphere arrays, ordered arrays of holes, and unordered pores—were created using reactive ion etching, chemical vapour deposition, metallization, and/or lift-off processes. The size and shape of the lithographic mask was altered using oxygen plasma etching. The periodicity of the final structure was defined by the initial diameter of the spheres. The surface morphology of the samples was characterized using scanning electron microscopy. The advantages and limitations of the fabrication technique are discussed. Finally, the potential applications (e.g., photonics, plasmonics) of the obtained nanostructures are reviewed.

Are plasmonic optical biosensors ready for use in point-of-need applications?

Plasmonics has drawn significant attention in the area of biosensors for decades due to the unique optical properties of plasmonic resonant nanostructures. While the sensitivity and specificity of molecular detection relies significantly on the resonance conditions, significant attention has been dedicated to the design, fabrication, and optimization of plasmonic substrates. The adequate choice of materials, structures, and functionality goes hand in hand with a fundamental understanding of plasmonics to enable the development of practical biosensors that can be deployed in real life situations. Here we provide a brief review of plasmonic biosensors detailing most recent developments and applications. Besides metals, novel plasmonic materials such as graphene are highlighted. Sensors based on Surface Plasmon Resonance (SPR), Localized Surface Plasmon Resonance (LSPR), and Surface Enhanced Raman Spectroscopy (SERS) are presented and classified based on their materials and structure. In addition, most recent applications to environment monitoring, health diagnosis, and food safety are presented. Potential problems related to the implementation in such applications are discussed and an outlook is presented.

Effective plasmon coupling in conical cavities for sensitive surface enhanced Raman scattering with quantitative analysis ability

Conical silver nanocavity arrays are fabricated by directly depositing Ag on porous alumina templates with V-shaped nanopores. By controlling the thickness of deposited Ag, complete and cracked cavity arrays are constructed respectively. The cracked cavity arrays with the cavity wall consisting of Ag nanoparticles are demonstrated to exhibit higher surface enhanced Raman scattering (SERS) activity than the complete one. Numerical simulation reveals that an effective coupling of the cavity modes with the surface plasmons of Ag nanoparticles (NPs) generates a significantly enhanced local electric field on the cavity wall responsible for the high SERS activity. The optimized cavity array presents an enhancement factor (EF) of ∼7.4 × 106 and an excellent uniformity with a relative standard deviation (RSD) as small as ∼5% for rhodamine 6G (R6G) molecules. Moreover, a good linear correlation between the logarithmic Raman intensity and the molecular concentration endows the array with quantitative analysis ability. These cavity arrays therefore are of great potential for qualitative and quantitative chemical and biomedical analysis with high sensitivity and reproducibility.

Quantum plasmon excitations in gold-fullerene mixture films

Controllable access to the hybrid plasmonic nanostructures built of small metal nanoparticles and organic spacer offers a tempting set of electronic excitations, which proper handling promises valuable applications and bright fundamental prospect. Here, we report on remarkable plasmonic properties of the Au _x C₆₀ hybrid nanostructures formed through self-assembling the depositing mixture of metal and fullerene. Using optical absorption spectra, we demonstrate establishing of quantum plasmon (QP) excitations upon the controllable increase of spatial density and size of the Au clusters formed in the films. Detection of two plasmonic modes evidences the QP hybridization enabling by nm-scaled proximity of the neighboured Au clusters. Variation of the QP mode parameters with gradual decrease of the inter-cluster spacing ΔL to the sub-nanometre scale driven by the Au concentration in the film x allowed us to evidence the quantum tunnelling regime in the QP hybridization launching at ΔL ≈ 0.9 nm. The later result designates an important role of the C₆₀ molecules, separating the Au clusters, in design of plasmonic and transport properties of the hybrid films. The obtained results represent the self-assembled Au _x C₆₀ nanocomposites as the promising plasmonic materials with potential for application in nanoplasmonics, nanoelectronics, and nanomedicine.

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.

Raman Techniques: Fundamentals and Frontiers

Driven by applications in chemical sensing, biological imaging and material characterisation, Raman spectroscopies are attracting growing interest from a variety of scientific disciplines. The Raman effect originates from the inelastic scattering of light, and it can directly probe vibration/rotational-vibration states in molecules and materials. Despite numerous advantages over infrared spectroscopy, spontaneous Raman scattering is very weak, and consequently, a variety of enhanced Raman spectroscopic techniques have emerged. These techniques include stimulated Raman scattering and coherent anti-Stokes Raman scattering, as well as surface- and tip-enhanced Raman scattering spectroscopies. The present review provides the reader with an understanding of the fundamental physics that govern the Raman effect and its advantages, limitations and applications. The review also highlights the key experimental considerations for implementing the main experimental Raman spectroscopic techniques. The relevant data analysis methods and some of the most recent advances related to the Raman effect are finally presented. This review constitutes a practical introduction to the science of Raman spectroscopy; it also highlights recent and promising directions of future research developments.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Engineering surface lattice resonance of elliptical gold nanodisk array for enhanced strain sensing

We demonstrate an elliptical gold nanodisk array (GNA) for engineering the spectral profile of surface lattice resonance (SLR). The nanodisk's shape has a great impact on SLR. Small linewidth of 20 nm at an aspect ratio of 1.17, as well as large wavelength tuning of 64 nm within 4% strain via different orientations and polarizations, are achieved experimentally. The enhanced wavelength response of 6.93 nm per 1% strain variation for elliptical GNA is 2.4 times better than that for general circular GNA. Furthermore, the strain sensing for elliptical GNA approaches is 5.7 times greater than that for circular GNA.

Selective Electroless Metallization of Micro- and Nanopatterns via Poly(dopamine) Modification and Palladium Nanoparticle Catalysis for Flexible and Stretchable Electronic Applications

The authors report a new patterned electroless metallization process for creating micro- and nanoscale metallic structures on polymeric substrates, which are essential for emerging flexible and stretchable optical and electronic applications. This novel process features a selective adsorption of catalytic Pd nanoparticles (PdNPs) on a lithographically masked poly(dopamine) (PDA) interlayer in situ polymerized on the substrates. The moisture-resistant PDA layer has excellent stability under a harsh electroless plating bath, which enables electroless metallization on versatile substrate materials regardless of their hydrophobicity, and significantly strengthens the attachment of electroless plated metallic structures on the polymeric substrates. Prototype devices fabricated using this PDA-assisted electroless metallization patterning exhibit superior mechanical stability under high bending and stretching stress. The lithographic patterning of the PDA spatially confines the adsorption of PdNPs and reduces defects due to random adsorption of catalytic particles on the undesired area. The high resolution of the lithographic patterning enables the demonstration of a copper micrograting pattern with a linewidth down to 2 μm and a silver plasmonic nanodisk array with a 500 nm pitch. A copper mesh is also fabricated using our new patterned electroless metallization process and functions as flexible transparent electrodes with >80% visible transmittance and <1 Ω sq^-1 sheet resistance. Moreover, flexible and stretchable dynamic electroluminescent displays and functional flexible printed circuits are demonstrated to show the promising capability of our fabrication process in versatile flexible and stretchable electronic devices.

Continuous reversible tuning of the gap size and plasmonic coupling of bow tie nanoantennas on flexible substrates

As a multifunctional device for sensing experiments and fundamental research, tailor-made plasmonic nanostructures with continuously tunable resonances are created by preparing bow tie-shaped nanostructures on a flexible substrate. The bow ties are fabricated by electron beam lithography on a chromium sacrificial layer and transferred to a polydimethylsiloxane (PDMS) substrate. The structures on PDMS are analyzed by reflection dark-field spectroscopy and scanning electron microscopy. Dark-field spectra of individual nano-antennas are obtained while the substrate is relaxed, and while strain is applied and the substrate is elastically stretched. Depending on the alignment of the bow ties relative to the direction of the strain, the deformation of the substrates leads to an increase or decrease of the nanostructure gaps, and therefore to a fully reversible decrease or increase of the antenna coupling, respectively. The continuous change in coupling is visible as a blue-shift in the resonance of the coupling mode for increasing gap widths, and a red-shift for decreasing gap widths. This configuration offers interesting perspectives for molecular transport and sensing investigations under variable coupling conditions as well as for tunable SERS substrates and optical strain sensor applications. In particular, very narrow gaps are within reach in the transversal configuration.

Interface-induced nucleation and growth: a new route for fabricating ordered silver nanohole arrays

Metal nanohole arrays exhibit fascinating optical properties originating from the excitation of surface plasmons, and have been demonstrated to be of great potential in many applications. However, the fabrication of large-area ordered metal nanohole arrays with a tunable optical response is still highly desired. Herein, a novel interface-induced vapor phase growth method is developed to achieve hexagonally arranged silver nanohole arrays with a centimeter-scale area, in which an interface is introduced via an ordered template and used to induce Ag selective nucleation and growth. The adhesive force of the template with the substrate is found to be crucial in the determination of the nucleation sites and the resulting nanostructures. The plasmonic responses of the nanohole arrays are regulated by controlling their structural features, which are realized through simply changing the template parameters and the Ag deposition thickness. The Ag nanohole array exhibits more than 20-fold Raman enhancement compared to a rough Ag film when its localized surface plasmon resonance (LSPR) is tuned to an optimized range, which indicates its potential in biochemical sensing applications. The present method for the preparation of large-area metal nanohole arrays may open up a new avenue to fabricate novel metal nanostructures and develop high-performance plasmonic devices.

Strategies to synthesize various nanostructures of silver and their applications - a review

Due to their various beneficial application-based properties, such as behavior, structure, and size, the synthesis of silver nanoparticles (Ag-NPs) with different structures has become an interesting yet common task for researchers to produce nanostructures for applications in various fields. This is because silver nanoparticles have interesting and unique properties, such as optical and catalytic, resulting from their different structures and sizes. These properties extend the use of nanostructures in various fields of research, especially in medicine, pharmacy, electronics, etc. Also, variations in their parameters affect the structures and sizes of Ag-NPs. This review provides an overview/brief presentation of various methodologies used to synthesize different application-based silver nanoparticles and lists areas where these nanoparticles are suitable for use according to their specific structures and sizes.

Viable stretchable plasmonics based on unidirectional nanoprisms

Well-defined ordered arrays of plasmonic nanostructures were fabricated on stretchable substrates and tunable plasmon-coupling-based sensing properties were comprehensively demonstrated upon extension and contraction. Regular nanoprism patterns consisting of Ag, Au and Ag/Au bilayers were constructed on the stretchable polydimethylsiloxane substrate. The nanoprisms had the same orientation over the entire substrate (3 × 3 cm²) via metal deposition on a single-crystal microparticle monolayer assembly. The plasmonic sensor based on the Ag/Au bilayer showed a 6-fold enhanced surface enhanced Raman scattering signal under 20% uniaxial extension, whereas a 3-fold increase was observed upon 6% contraction, compared with the Au nanoprism arrays. The sensory behaviors were corroborated by finite-difference time-domain simulation, demonstrating the tunable electromagnetic field enhancement effect via the localized surface plasmon resonance coupling. The advanced flexible plasmonic-coupling-based devices with tunable and quantifiable performance herein suggested are expected to unlock promising potential in practical bio-sensing, biotechnological applications and optical devices.

Plasmonic resonances in hybrid systems of aluminum nanostructured arrays and few layer graphene within the UV-IR spectral range

The size-controllable and ordered Al nanocavities and nanodomes arrays were synthesized by electrochemical anodization of aluminum using phosphoric acid, citric acid and mixture both acids. Few layer graphene (FLG) was transferred directly on top of Al nanostructures and their morphology were evaluated by scanning electron microscopy. The interaction between FLG and the plasmonic properties of Al nanostructures arrays were investigated based on specular reflectivity in the ultraviolet-visible-infrared range and Raman spectroscopy. We found that their optical reflectivity was dramatically reduced as compared with unstructured Al. At the same time pronounced reflectivity dips were detectable in the 200-896 nm wavelength range, which were ascribed to plasmonic resonances. The plasmonic properties of these nanostructures do not exhibit evident changes by the presence of FLG in the UV-vis range of the electromagnetic spectrum. By contrast, the surface-enhanced Raman spectroscopy of FLG was observed in nanocavities and nanodomes structures that result in an intensity increase of the characteristic G and 2D bands of FLG induced by the plasmonic properties of Al nanostructures.

Flexible and Tunable 3D Gold Nanocups Platform as Plasmonic Biosensor for Specific Dual LSPR-SERS Immuno-Detection

Early medical diagnostic in nanomedicine requires the implementation of innovative nanosensors with highly sensitive, selective, and reliable biomarker detection abilities. In this paper, a dual Localized Surface Plasmon Resonance - Surface Enhanced Raman Scattering (LSPR- SERS) immunosensor based on a flexible three-dimensional (3D) gold (Au) nanocups platform has been implemented for the first time to operate as a relevant “proof-of-concept” for the specific detection of antigen-antibody binding events, using the human IgG - anti-human IgG recognition interaction as a model. Specifically, polydimethylsilane (PDMS) elastomer mold coated with a thin Au film employed for pattern replication of hexagonally close-packed monolayer of polystyrene nanospheres configuration has been employed as plasmonic nanoplatform to convey both SERS and LSPR readout signals, exhibiting both well-defined LSPR response and enhanced 3D electromagnetic field. Synergistic LSPR and SERS sensing use the same reproducible and large-area plasmonic nanoplatform providing complimentary information not only on the presence of anti-human IgG (by LSPR) but also to identify its specific molecular signature by SERS. The development of such smart flexible healthcare nanosensor platforms holds promise for mass production, opening thereby the doors for the next generation of portable point-of-care devices.

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists

Self-assembly of quasi-spherical colloidal particles in two-dimensional (2D) arrangements is essential for a wide range of applications from optoelectronics to surface engineering, from chemical and biological sensing to light harvesting and environmental remediation. Several self-assembly approaches have flourished throughout the years, with specific features in terms of complexity of the implementation, sensitivity to process parameters, characteristics of the final colloidal assembly. Selecting the proper method for a given application amidst the vast literature in this field can be a challenging task. In this review, we present an extensive classification and comparison of the different techniques adopted for 2D self-assembly in order to provide useful guidelines for scientists approaching this field. After an overview of the main applications of 2D colloidal assemblies, we describe the main mechanisms underlying their formation and introduce the mathematical tools commonly used to analyse their final morphology. Subsequently, we examine in detail each class of self-assembly techniques, with an explanation of the physical processes intervening in crystallization and a thorough investigation of the technical peculiarities of the different practical implementations. We point out the specific characteristics of the set-ups and apparatuses developed for self-assembly in terms of complexity, requirements, reproducibility, robustness, sensitivity to process parameters and morphology of the final colloidal pattern. Such an analysis will help the reader to individuate more easily the approach more suitable for a given application and will draw the attention towards the importance of the details of each implementation for the final results.

Patterned Plasmonic Surfaces-Theory, Fabrication, and Applications in Biosensing

Low-profile patterned plasmonic surfaces are synergized with a broad class of silicon microstructures to greatly enhance near-field nanoscale imaging, sensing, and energy harvesting coupled with far-field free-space detection. This concept has a clear impact on several key areas of interest for the MEMS community, including but not limited to ultra-compact microsystems for sensitive detection of small number of target molecules, and "surface" devices for optical data storage, micro-imaging and displaying. In this paper, we review the current state-of-the-art in plasmonic theory as well as derive design guidance for plasmonic integration with microsystems, fabrication techniques, and selected applications in biosensing, including refractive-index based label-free biosensing, plasmonic integrated lab-on-chip systems, plasmonic near-field scanning optical microscopy and plasmonics on-chip systems for cellular imaging. This paradigm enables low-profile conformal surfaces on microdevices, rather than bulk material or coatings, which provide clear advantages for physical, chemical and biological-related sensing, imaging, and light harvesting, in addition to easier realization, enhanced flexibility, and tunability.

Flexible Nanowire Cluster as a Wearable Colorimetric Humidity Sensor

Wearable plasmonic devices combine the advantages of high flexibility, ultrathinness, light weight, and excellent integration with the optical benefits mediated by plasmon-enhanced electric fields. However, two obstacles severely hinder further developments and applications of a wearable plasmonic device. One is the lack of efficient approach to obtaining devices with robust antimotion-interference property, i.e., the devices can work independently on the morphology changes of their working structures caused by arbitrary wearing conditions. The other issue is to seek a facile and high-throughput fabrication method to satisfy the financial requirement of industrialization. In order to overcome these two challenges, a functional flexible film of nanowire cluster is developed, which can be easily fabricated by taking the advantages of both conventional electrochemical and sputtering methods. Such flexible plasmonic films can be made into wearable devices that work independently on shape changes induced by various wearing conditions (such as bending, twisting and stretching). Furthermore, due to plasmonic advantages of color controlling and high sensitivity to environment changes, the flexible film of nanowire cluster can be used to fabricate wearable items (such as bracelet, clothes, bag, or even commercial markers), with the ability of wireless visualization for humidity sensing.

Review of SERS Substrates for Chemical Sensing

The SERS effect was initially discovered in the 1970s. Early research focused on understanding the phenomenon and increasing enhancement to achieve single molecule detection. From the mid-1980s to early 1990s, research started to move away from obtaining a fundamental understanding of the phenomenon to the exploration of analytical applications. At the same time, significant developments occurred in the field of photonics that led to the advent of inexpensive, robust, compact, field-deployable Raman systems. The 1990s also saw rapid development in nanoscience. This convergence of technologies (photonics and nanoscience) has led to accelerated development of SERS substrates to detect a wide range of chemical and biological analytes. It would be a monumental task to discuss all the different kinds of SERS substrates that have been explored. Likewise, it would be impossible to discuss the use of SERS for both chemical and biological detection. Instead, a review of the most common metallic (Ag, Cu, and Au) SERS substrates for chemical detection only is discussed, as well as SERS substrates that are commercially available. Other issues with SERS for chemical detection have been selectivity, reversibility, and reusability of the substrates. How these issues have been addressed is also discussed in this review.

Nanosphere lithography for optical fiber tip nanoprobes

This paper reports a simple and economical method for the fabrication of nanopatterned optical fiber nanotips. The proposed patterning approach relies on the use of the nanosphere lithography of the optical fiber end facet. Polystyrene (PS) nanospheres are initially self-assembled in a hexagonal array on the surface of water. The created pattern is then transferred onto an optical fiber tip (OFT). The PS monolayer colloidal crystal on the OFT is the basic building block that is used to obtain different periodic structures by applying further treatment to the fiber, such as metal coating, nanosphere size reduction and sphere removal. Ordered dielectric and metallo-dielectric sphere arrays, metallic nanoisland arrays and hole-patterned metallic films with feature sizes down to the submicron scale are achievable using this approach. Furthermore, the sizes and shapes of these periodic structures can be tailored by altering the fabrication conditions. The results indicate that the proposed self-assembly approach is a valuable route for the development of highly repeatable metallo-dielectric periodic patterns on OFTs with a high degree of order and low fabrication cost. The method can be easily extended to simultaneously produce multiple fibers, opening a new route to the development of fiber-optic nanoprobes. Finally, we demonstrate the effective application of the patterned OFTs as surface-enhanced Raman spectroscopy nanoprobes.

Computational Sensing Using Low-Cost and Mobile Plasmonic Readers Designed by Machine Learning

Plasmonic sensors have been used for a wide range of biological and chemical sensing applications. Emerging nanofabrication techniques have enabled these sensors to be cost-effectively mass manufactured onto various types of substrates. To accompany these advances, major improvements in sensor read-out devices must also be achieved to fully realize the broad impact of plasmonic nanosensors. Here, we propose a machine learning framework which can be used to design low-cost and mobile multispectral plasmonic readers that do not use traditionally employed bulky and expensive stabilized light sources or high-resolution spectrometers. By training a feature selection model over a large set of fabricated plasmonic nanosensors, we select the optimal set of illumination light-emitting diodes needed to create a minimum-error refractive index prediction model, which statistically takes into account the varied spectral responses and fabrication-induced variability of a given sensor design. This computational sensing approach was experimentally validated using a modular mobile plasmonic reader. We tested different plasmonic sensors with hexagonal and square periodicity nanohole arrays and revealed that the optimal illumination bands differ from those that are "intuitively" selected based on the spectral features of the sensor, e.g., transmission peaks or valleys. This framework provides a universal tool for the plasmonics community to design low-cost and mobile multispectral readers, helping the translation of nanosensing technologies to various emerging applications such as wearable sensing, personalized medicine, and point-of-care diagnostics. Beyond plasmonics, other types of sensors that operate based on spectral changes can broadly benefit from this approach, including e.g., aptamer-enabled nanoparticle assays and graphene-based sensors, among others.

Light-sheet based lithography technique for patterning an array of microfluidic channels

We propose a Light-sheet laser interference lithography technique for fabricating periodic microfluidic channels. This technique uses multiple light-sheet illumination pattern that is generated using a spatial filter at the back-aperture of the cylindrical lens. Specially designed spatial filter is used that give rise to a periodic pattern at the focal plane which is essentially a 1D Fourier transform of the spatial filter transfer function. One-dimensional focusing property of the cylindrical lens result in the generation of line shaped channel geometry. To design microfluidic channels, the illumination pattern is exposed to the glass substrate coated with a photopolymer sensitized to 532 nm and subsequently developed using standard chemical protocols. Experimentally, the 1D periodic channel structure has an approximate width and periodicity of approximately 11.25 microns. Light-sheets based lithography technique offer a fast and single-shot process to generate microfluidic channels.

Continuous fabrication of nanostructure arrays for flexible surface enhanced Raman scattering substrate

Surface-enhanced Raman spectroscopy (SERS) has been a powerful tool for applications including single molecule detection, analytical chemistry, electrochemistry, medical diagnostics and bio-sensing. Especially, flexible SERS substrates are highly desirable for daily-life applications, such as real-time and in situ Raman detection of chemical and biological targets, which can be used onto irregular surfaces. However, it is still a major challenge to fabricate the flexible SERS substrate on large-area substrates using a facile and cost-effective technique. The roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) technique provides a solution for the continuous fabrication of flexible SERS substrate due to its high-speed, large-area, high-resolution and high-throughput. In this paper, we presented a facile and cost-effective method to fabricate flexible SERS substrate including the fabrication of polymer nanostructure arrays and the metallization of the polymer nanostructure arrays. The polymer nanostructure arrays were obtained by using R2R UV-NIL technique and anodic aluminum oxide (AAO) mold. The functional SERS substrates were then obtained with Au sputtering on the surface of the polymer nanostructure arrays. The obtained SERS substrates exhibit excellent SERS and flexibility performance. This research can provide a beneficial direction for the continuous production of the flexible SERS substrates.

SERS detection of microRNA biomarkers for cancer diagnosis using gold-coated paramagnetic nanoparticles to capture SERS-active gold nanoparticles

A highly sensitive magnetic-capture SERS assay for detecting cancer-related microRNAs was developed by enhancing the formation of SERS “hot spots”.

Programmable and reversible plasmon mode engineering

Plasmonic nanostructures with enhanced localized optical fields as well as narrow linewidths have driven advances in numerous applications. However, the active engineering of ultranarrow resonances across the visible regime-and within a single system-has not yet been demonstrated. This paper describes how aluminum nanoparticle arrays embedded in an elastomeric slab may exhibit high-quality resonances with linewidths as narrow as 3 nm at wavelengths not accessible by conventional plasmonic materials. We exploited stretching to improve and tune simultaneously the optical response of as-fabricated nanoparticle arrays by shifting the diffraction mode relative to single-particle dipolar or quadrupolar resonances. This dynamic modulation of particle-particle spacing enabled either dipolar or quadrupolar lattice modes to be selectively accessed and individually optimized. Programmable plasmon modes offer a robust way to achieve real-time tunable materials for plasmon-enhanced molecular sensing and plasmonic nanolasers and opens new possibilities for integrating with flexible electronics.

Sub-100 nm gold nanohole-enhanced Raman scattering on flexible PDMS sheets

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive vibrational spectroscopy technique enabling detection of multiple analytes at the molecular level in a nondestructive and rapid manner. In this work, we introduce a new approach to fabricate deep subwavelength-scaled (sub-100 nm) metallic nanohole arrays (quasi-3D metallic nanoholes) on flexible and highly efficient SERS substrates. Target structures have been fabricated using a two-step process consisting of (i) direct pattern transfer of spin-coated polymer films onto polydimethylsiloxane (PDMS) substrates by plasma etching with transferred anodic aluminum oxide masks, and (ii) producing SERS-active substrates by functionalization of the etched polymeric films followed by Au deposition. Such an all-dry, top-down lithographic approach enables on-demand patterning of SERS-active metallic nanoholes with high structural fidelity even onto flexible and stretchable substrates, thus making possible multiple sensing modes in a versatile fashion. For example, metallic nanoholes on flexible PDMS substrates are highly amenable to their integration with curved glass sticks, which can be used in optical fiber-integrated SERS systems. Au surfaces immobilized by probe DNA molecules show a selective enhancement of Raman scattering with Cy5-labeled complementary DNA (as compared to flat Au surfaces), demonstrating the potential of using the quasi-3D Au nanohole arrays for bio-sensing applications.

Flexible Plasmonic Sensors

Mechanical flexibility and the advent of scalable, low-cost, and high-throughput fabrication techniques have enabled numerous potential applications for plasmonic sensors. Sensitive and sophisticated biochemical measurements can now be performed through the use of flexible plasmonic sensors integrated into existing medical and industrial devices or sample collection units. More robust sensing schemes and practical techniques must be further investigated to fully realize the potentials of flexible plasmonics as a framework for designing low-cost, embedded and integrated sensors for medical, environmental, and industrial applications.

SERS of Individual Nanoparticles on a Mirror: Size Does Matter, but so Does Shape

Coupling noble metal nanoparticles by a 1 nm gap to an underlying gold mirror confines light to extremely small volumes, useful for sensing on the nanoscale. Individually measuring 10 000 of such gold nanoparticles of increasing size dramatically shows the different scaling of their optical scattering (far-field) and surface-enhanced Raman emission (SERS, near-field). Linear red-shifts of the coupled plasmon modes are seen with increasing size, matching theory. The total SERS from the few hundred molecules under each nanoparticle dramatically increases with increasing size. This scaling shows that maximum SERS emission is always produced from the largest nanoparticles, irrespective of tuning to any plasmonic resonances. Changes of particle facet with nanoparticle size result in vastly weaker scaling of the near-field SERS, without much modifying the far-field, and allows simple approaches for optimizing practical sensing.

Magnetically-responsive silica-gold nanobowls for targeted delivery and SERS-based sensing

Composite colloidal structures with multi-functional properties have wide applications in targeted delivery of therapeutics and imaging contrast molecules and high-throughput molecular bio-sensing. We have constructed a multifunctional composite magnetic nanobowl using the bottom-up approach on an asymmetric silica/polystyrene Janus template consisting of a silica shell around a partially exposed polystyrene core. The nanobowl consists of a silica bowl and a gold exterior shell with iron oxide magnetic nanoparticles sandwiched between the silica and gold shells. The nanobowls were characterized by electron microscopy, atomic force microscopy, magnetometry, vis-NIR and FTIR spectroscopy. Magnetically vectored transport of these nanobowls was ascertained by time-lapsed imaging of their flow in fluid through a porous hydrogel under a defined magnetic field. These magnetically-responsive nanobowls show distinct surface enhanced Raman spectroscopy (SERS) imaging capability. The PEGylated magnetically-responsive nanobowls show size-dependent cellular uptake in vitro.

Plasmonic Nanogels for Unclonable Optical Tagging

We demonstrate the fabrication of novel functional gel coatings with randomized physical and chemical patterns that enable dual encoding ability to realize unclonable optical tags. This design is based on swelling-mediated massive reconstruction of an ultrathin responsive gelatinous polymer film uniformly adsorbed with plasmonic nanostructures into a randomized network of interacting folds, resulting in bright electromagnetic hotspots within the folds. We reveal a strong correlation between the topology and near-field electromagnetic field enhancement due to the intimate contact between two plasmonic surfaces within the folds, each of them representing a unique combination of local topography and chemical distribution caused by the formation of electromagnetic hotspots. Because of the efficient trapping of the Raman reporters within the uniquely distributed electromagnetic hotspots, the surface enhanced Raman scattering enhancement from the morphed plasmonic gel was found to be nearly 40 times higher compared to that from the pristine plasmonic gel. Harnessing the nondeterministic nature of the folds, the folded plasmonic gel can be employed as a multidimensional (with dual topo-chemical encoding) optical taggant for prospective anticounterfeiting applications. Such novel optical tags based on the spontaneous folding process are virtually impossible to replicate because of the combination of nondeterministic physical patterns and chemical encoding.