Simultaneous Chemical and Refractive Index Sensing in the 1-2.5 μm Near-Infrared Wavelength Range on Nanoporous Gold Disks

Ultrasensitive Photothermal Spectroscopy: Harnessing the Seebeck Effect for Attogram-Level Detection

Molecular-level spectroscopy is crucial for sensing and imaging applications, yet detecting and quantifying minuscule quantities of chemicals remain a challenge, especially when they surface adsorb in low numbers. Here, we introduce a photothermal spectroscopic technique that enables the high selectivity sensing of adsorbates with an attogram detection limit. Our approach utilizes the Seebeck effect in a microfabricated nanoscale thermocouple junction, incorporated into the apex of a microcantilever. We observe minimal thermal mass exhibited by the sensor, which maintains exceptional thermal insulation. The temperature variation driving the thermoelectric junction arises from the nonradiative decay of molecular adsorbates' vibrational states on the tip. We demonstrate the detection of photothermal spectra of physisorbed trinitrotoluene (TNT) and dimethyl methylphosphonate (DMMP) molecules, as well as representative polymers, with an estimated mass of 10-18 g.

Predicting Concentrations of Mixed Sugar Solutions with a Combination of Resonant Plasmon-Enhanced SEIRA and Principal Component Analysis

The detection and quantification of glucose concentrations in human blood or in the ocular fluid gain importance due to the increasing number of diabetes patients. A reliable determination of these low concentrations is hindered by the complex aqueous environments in which various biomolecules are present. In this study, we push the detection limit as well as the discriminative power of plasmonic nanoantenna-based sensors towards the physiological limit. We utilize plasmonic surface-enhanced infrared absorption spectroscopy (SEIRA) to study aqueous solutions of mixtures of up to five different physiologically relevant saccharides, namely the monosaccharides glucose, fructose, and galactose, as well as the disaccharides maltose and lactose. Resonantly tuned plasmonic nanoantennas in a reflection flow cell geometry allow us to enhance the specific vibrational fingerprints of the mono- and disaccharides. The obtained spectra are analyzed via principal component analysis (PCA) using a machine learning algorithm. The high performance of the sensor together with the strength of PCA allows us to detect concentrations of aqueous mono- and disaccharides solutions down to the physiological levels of 1 g/L. Furthermore, we demonstrate the reliable discrimination of the saccharide concentrations, as well as compositions in mixed solutions, which contain all five mono- and disaccharides simultaneously. These results underline the excellent discriminative power of plasmonic SEIRA spectroscopy in combination with the PCA. This unique combination and the insights gained will improve the detection of biomolecules in different complex environments.

A Multi-Layered Borophene-Silica-Silver Based Refractive Index Sensor for Biosensing Applications Operated at the Infrared Frequency Spectrum

We have presented the borophene based refractive index sensor for the infrared frequency spectrum of 188 to 250 THz (1.2–1.6 µm) range. The proposed structure was formed by using the Silver-borophene-silica-Ag layered structure. The behaviour of the different analyte (with a different refractive index) material is numerically calculated by placing it on the top of the structure. The behaviour of the structure is identified in terms of absorption, reflectance, physical parameter variation, and oblique angle incident conditions. The presented results provide the basic idea of selecting optimized structure dimensions to get the specific resonating response. This sensor offers the Figure of Merit (FOM) of 444 RIU−1 with high sensitivity of 660 THz/RIU (4471 nm/RIU). The refractive index sensor also provides wide-angle stability for (0° to 80°) for the wide frequency range (239 to 245 THz and 207 to 209 THz). This sensor is developed on the silver metal layer (not required to separate borophene from its origin metal deposition process) and easily fabricated using standard boron fabrication and layered deposition techniques. The results of the proposed structure make it possible to design a basic biosensor structure. This device is also applicable for various THz and biomedical applications.

Exploring the synergy of radiative coupling and substrate undercut in arrayed gold nanodisks for economical, ultra-sensitive label-free biosensing

We report radiatively coupled arrayed gold nanodisks on invisible substrate (AGNIS) as a cost-effective, high-performance platform for nanoplasmonic biosensing. By substrate undercut, the electric field distribution around the nanodisks has been restored to as if the nanodisks were surrounded by a single medium, thereby provides analyte accessibility to otherwise buried enhanced electric field. The AGNIS substrate has been fabricated by wafer-scale nanosphere lithography without the need for costly lithography. The LSPR blue-shifting behavior synergistically contributed by radiative coupling and substrate undercut have been investigated for the first time, which culminates in a remarkable refractive index sensitivity increase from 207 nm/RIU to 578 nm/RIU. The synergy also improves surface sensitivity to monolayer neutravidin-biotin binding from 7.4 nm to 20.3 nm with the limit of detection (LOD) of neutravidin at 50 fM, which is among the best label-free results reported to date on this specific surface binding reaction. As a potential cancer diagnostic application, extracellular vesicles such as exosomes excreted by cancer and normal cells were measured with a LOD within 112-600 (exosomes/μL), which would be sufficient in many clinical applications. Using CD9, CD63, and CD81 antibodies, label-free profiling has shown increased expression of all three surface antigens in cancer-derived exosomes. This work demonstrates, for the first time, strong synergy of arrayed radiative coupling and substrate undercut can enable economical, ultrasensitive biosensing in the visible light spectrum where high-quality, low-cost silicon detectors are readily available for point-of-care applications.

Locally excited surface plasmon resonance for refractive index sensing with high sensitivity and high resolution

An angle-interrogated surface plasmon resonance (SPR) sensor based on a prism-coupled configuration has been extensively applied in biomedicine, environment monitoring, and food safety. Yet, the low sensitivity and low spatial resolution impede its further development. In this Letter, we investigated objective-coupled locally excited SPR for refractive index (RI) sensing with high sensitivity and high resolution. Through theoretical analysis, the SPR angle was retrieved from back focal plane imaging, which was highly correlated to the RI of the surrounding medium. Experimentally, a RI sensitivity of 77.41° refractive index unit (RIU)^-1 was achieved with a detection range of 0.068 RIU when using glucose solutions for the demonstration. Furthermore, we acquired the spatial resolution of the configuration being 290 nm, and the RI measurement to a polydimethylsiloxane droplet with high spatial resolution was implemented. As a result, compared with the classical prism-coupled configuration, the locally excited SPR provides a method to achieve RI sensing with high sensitivity and high resolution.

Advances in differentiation and identification of foodborne bacteria using near infrared spectroscopy

Rapid and sensitive detection of foodborne bacteria is a growing concern for ensuring safe food supply and preventing human foodborne infections. It is difficult for conventional methods to meet these detection requirements because they are often tedious and time-consuming. In the recent years, near infrared (NIR) spectroscopy has been found to be a promising method for all sorts of analyses in microbiology due to its highly specific absorption signature and non-destructive measurements. In this review, we first briefly introduce the fundamental and basic operational procedure of NIR spectroscopy for foodborne bacteria detection. Then we summarize the main advances and contributions of this technique in the study of foodborne bacteria. Finally, we conclude that much work still remains to be done before NIR spectroscopy really becomes a viable alternative in the field of microbiological characterization.

Nucleation and Growth-Controlled Facile Fabrication of Gold Nanoporous Structures for Highly Sensitive Surface-Enhanced Raman Spectroscopy Applications

The fabrication of porous metal structures usually involves complicated processes such as lithography or etching. In this study, a facile and clean method based on thermal evaporation at high pressure is introduced, by which a highly porous, black colored structure of Au can be formed through the control of homogeneous nucleation and growth during evaporation. The deposited films have different morphologies, from columnar to nanoporous structures, depending on the working pressure. These porous structures consist of Au nanoparticle aggregates, and a large number of nano-gaps are found among the nanoparticles. Thus, these structures indicate a much higher intensity of surface-enhanced Raman spectroscopy (SERS) when compared with commercial SERS substrates. The SERS intensity depends on the working pressure and thickness. Even circumstances that can induce agglomeration of nanoparticle aggregates do not deteriorate the sensitivity of SERS. These nanoporous structures based on high-pressure thermal evaporation are expected to provide a new platform for the development of low-cost and highly sensitive chemical sensors.

Extraordinary sensitivity enhancement of Ag-Au alloy nanohole arrays for label-free detection of Escherichia Coli

Alloy nanostructures unveil extraordinary plasmonic phenomena that supersede the mono-metallic counterparts. Here we report silver-gold (Ag-Au) alloy nanohole arrays (α-NHA) for ultra-sensitive plasmonic label-free detection of Escherichia Coli (E. coli). Large-area α-NHA were fabricated by using nanoimprint lithography and concurrent thermal evaporation of Ag and Au. The completely miscible Ag-Au alloy exhibits an entirely different dielectric function in the near infra-red wavelength range compared to mono-metallic Ag or Au. The α-NHA demonstrate substantially enhanced refractive index sensitivity of 387 nm/RIU, surpassing those of Ag or Au mono-metallic nanohole arrays by approximately 40%. Moreover, the α-NHA provide highly durable material stability to corrosion and oxidation during over one-month observation. The ultra-sensitive α-NHA allow the label-free detection of E. coli in various concentration levels ranging from 10³ to 10⁸ cfu/ml with a calculated limit of detection of 59 cfu/ml. This novel alloy plasmonic material provides a new outlook for widely applicable biosensing and bio-medical applications.

Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis

The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.

Directed Concentrating of Micro-/Nanoparticles via Near-Infrared Laser Generated Plasmonic Microbubbles

Directed concentrating of micro- and nanoparticles via laser-generated plasmonic microbubbles in a liquid environment is an emerging technology. For effective heating, visible light has been primarily employed in existing demonstrations. In this paper, we demonstrate a new plasmonic platform based on nanoporous gold disk (NPGD) array. Thanks to the highly tunable localized surface plasmon resonance of the NPGD array, microbubbles of controlled size can be generated by near-infrared (NIR) light. Using NIR light provides several key advantages over visible light in less interference with standard microscopy and fluorescence imaging, preventing fluorescence photobleaching, less susceptible to absorption and scattering in turbid biological media, and much reduced photochemistry, phototoxicity, and so forth. The large surface-to-volume ratio of NPGD further facilitates the heat transfer from these gold nanoheaters to the surroundings. While the microbubble is formed, the surrounding liquid circulates and direct microparticles randomly dispersed in the liquid to the bottom NPGD surface, which can be made to yield a unique collection of 3D hollow dome microstructures with bubbles larger than 5 μm. Such capability can also be employed in concentrating suspended colloidal nanoparticles at desirable sites and with the preferred configuration enhancing the sensor performance. Specifically, the interaction among concentrated nanoparticles and their interactions with the underlying substrate have been investigated for the first time. These collections have been characterized using optical microscopy, scanning electron microscopy, hyperspectral localized surface plasmon resonance imaging, and hyperspectral Raman imaging. In addition to various micro- and nanoparticles, the plasmonic microbubbles are also shown to collect biological cells and extracellular nanovesicles such as exosomes. By using a spatial light modulator to project the laser in arbitrary patterns, parallel concentrating can be achieved to fabricate an array of clusters.

Nanoporous Gold-Based Sensing

In recent years, the field of nanoporous metals has undergone accelerated developments as these materials possess high specific surface areas, well-defined pore sizes, functional sites, and a wide range of functional properties. Nanoporous gold (NPG) is, surely, the most attractive system in the class of nanoporous metals: it combines several desired characteristics as occurrence of surface plasmon resonances, enormous surface area, electrochemical activity, biocompatibility, in addition to feasibility in preparation. All these properties concur in the exploitatiton of NPG as an efficient and versatile sensong platform. In this regard, NPG-based sensors have shown exceptional sensitivity and selectivity to a wide range of analytes ranging from molecules to biomolecules (and until the single molecule detection) and the enormous surface/volume ratio was shown to be crucial in determining these performances. Thanks to these characteristics, NPG-based sensors are finding applications in medical, biological, and safety fields so as in medical diagnostics and monitoring processes. So, a rapidly growing literature is currently investigating the properties of NPG systems toward the detection of a multitude of classes of analytes highlighting strengths and limits. Due to the extension, complexity, and importance of this research field, in the present review we attempt, starting from the discussion of specific cases, to focus our attention on the basic properties of NPG in connection to the main sensing applications, i.e., surface enhanced Raman spectroscopy-based and electrochemical-based sensing. Owing to the nano-sized pore channels and Au ligaments, which are much smaller than the wavelength of visible light (400–700 nm), surface plasmon resonances of NPG can be effectively excited by visible light and presents unique features compared with other nanostructured metals, such as nanoparticles, nanorods, and nanowires. This characteristics leads to optical sensors exploiting NPG through unique surface plasmon resonance properties that can be monitored by UV-Vis, Raman, or fluorescence spectroscopy. On the other hand, the catalytic properties of NPG are exploited electrochemical sensors are on the electrical signal produced by a specific analyte adsorbed of the NPG surface. In this regard, the enourmous NPG surface area is crucial in determining the sensitivity enhancement. Due to the extension, complexity, and importance of the NPG-based sensing field, in the present review we attempt, starting from the discussion of specific cases, to focus our attention on the basic properties of NPG in connection to the main sensing applications, i.e., surface enhanced Raman spectroscopy-based and electrochemical-based sensing. Starting from the discussion of the basic morphological/structural characteristics of NPG as obtained during the fabrication step and post-fabrication processes, the review aims to a comprehensive schematization of the main classes of sensing applications highlighting the basic involved physico-chemical properties and mechanisms. In each discussed specific example, the main involved parameters and processes governing the sensing mechanism are elucidated. In this way, the review aims at establishing a general framework connecting the processes parameters to the characteristics (pore size, etc.) of the NPG. Some examples are discussed concerning surface plasmon enhanced Uv-Vis, Raman, fluorescence spectroscopy in order to realize efficient NPG-based optical sesnors: in this regard, the underlaying connections between NPG structural/morphological properties and the optical response and, hence, the optical-based sensing performances are described and analyzed. Some other examples are discussed concerning the exploitation of the electrochemical characteristics of NPG for ultra-high sensitivity detection of analytes: in this regard, the key parameters determing the NPG activity and selectivity selectivity toward a variety of reactants are discussed, as high surface-to-volume ratio and the low coordination of surface atoms. In addition to the use of standard NPG films and leafs as sensing platforms, also the role of hybrid NPG-based nanocomposites and of nanoporous Au nanostructures is discussed due to the additional increase of the electrocatalytic acticvity and of exposed surface area resulting in the possible further sensitivity increase.

Commercial and emerging technologies for cancer diagnosis and prognosis based on circulating tumor exosomes

Exosomes are nano-sized extracellular vesicles excreted by mammalian cells that circulate freely in the bloodstream of living organisms. Exosomes have a lipid bilayer that encloses genetic material used in intracellular communication (e.g. double-stranded DNA, micro-RNAs, and messenger RNA). Recent evidence suggests that dysregulation of this genetic content within exosomes has a major role in tumor progression in the surrounding microenvironment. Motivated by this discovery, we focused here on using exosomal biomarkers as a diagnostic and prognostic tool for cancer. In this review, we discuss recently discovered exosome-derived proteomic and genetic biomarkers used in cancer diagnosis and prognosis. Although several genetic biomarkers have been validated for their diagnostic values, proteomic biomarkers are still being actively pursued. We discuss both commercial technologies and emerging technologies for exosome isolation and analysis. Emerging technologies can be classified into optical and non-optical methods. The working principle of each method is briefly discussed as well as advantages and limitations.

Ultraviolet Interband Plasmonics With Si Nanostructures

Although Si acts as an electrical semiconductor, it has properties of an optical dielectric. Here, we revisit the behavior of Si as a plasmonic metal. This behavior was previously shown to arise from strong interband transitions that lead to negative permittivity of Si across the ultraviolet spectral range. However, few have studied the plasmonic characteristics of Si, particularly in its nanostructures. In this paper, we report localized plasmon resonances of Si nanostructures and the observation of plasmon hybridization in the UV (∼250 nm wavelength). In addition, simulation results show that Si nanodisk dimers can achieve a local intensity enhancement greater than ∼500-fold in a 1 nm gap. Lastly, we investigate hybrid Si-Al nanostructures to achieve sharp resonances in the UV, due to the coupling between plasmon resonances supported by Si and Al nanostructures. These results will have potential applications in the UV range, such as nanostructured devices for spectral filtering, plasmon-enhanced Si photodetectors, interrogation of molecular chirality, and catalysis. It could have significant impact on UV photolithography on patterned Si structures.

Differential extinction of vibrational molecular overtone transitions with gold nanorods and its role in surface enhanced near-IR absorption (SENIRA)

Resonant coupling between plasmonic nanoantennas and molecular vibrational excitations is employed to amplify the weak overtone transitions that reside in the near-infrared. We explore for the first time the differential extinction of forbidden molecular overtone transitions coupled to the localized surface plasmons. We show a non-trivial interplay between the molecular absorption enhancement and suppression of plasmonic absorption in a coupled system. When the resonance conditions are met at 1.5 μm, two orders of magnitude enhancement of differential extinction as compared to the extinction of the same amount of free probe molecules is achieved. Our results pave a road toward a new class of surface enhanced near-infrared absorption-based sensors.

Vibrational Sensing Using Infrared Nanoantennas: Toward the Noninvasive Quantitation of Physiological Levels of Glucose and Fructose

Monosaccharides, which include the simple sugars such as glucose and fructose, are among the most important carbohydrates in the human diet. Certain chronic diseases, e.g., diabetes mellitus, are associated with anomalous glucose blood levels. Detecting and measuring the levels of monosaccharides in vivo or in aqueous solutions is thus of the utmost importance in life science, health, and point-of-care applications. Noninvasive sensing would avoid problems such as pain and potential infection hazards. Here, with the help of surface enhanced infrared absorption (SEIRA) spectroscopy, we demonstrate the reliable optical detection in the mid-infrared spectral range of pure glucose and fructose solutions as well as mixtures of both in aqueous solution. We utilize a reflection flow cell geometry with physiologically relevant concentrations as small as 10 g/L. As significant improvement over the standard baseline correction employed in SEIRA applications, we utilize principal component analysis (PCA) as machine learning algorithm, which is ideally suited for the extraction of vibrational data. We anticipate our results as important step in biosensing applications that will stimulate efforts to further improve the employed SEIRA substrates, the noise level of the spectroscopic light source, as well as the flow cell environment en route to significantly higher sensitivities and quantitative analysis, even in tear drops.

Adaptive Method for Quantitative Estimation of Glucose and Fructose Concentrations in Aqueous Solutions Based on Infrared Nanoantenna Optics

In life science and health research one observes a continuous need for new concepts and methods to detect and quantify the presence and concentration of certain biomolecules-preferably even in vivo or aqueous solutions. One prominent example, among many others, is the blood glucose level, which is highly important in the treatment of, e.g., diabetes mellitus. Detecting and, in particular, quantifying the amount of such molecular species in a complex sensing environment, such as human body fluids, constitutes a significant challenge. Surface-enhanced infrared absorption (SEIRA) spectroscopy has proven to be uniquely able to differentiate even very similar molecular species in very small concentrations. We are thus employing SEIRA to gather the vibrational response of aqueous glucose and fructose solutions in the mid-infrared spectral range with varying concentration levels down to 10 g/l. In contrast to previous work, we further demonstrate that it is possible to not only extract the presence of the analyte molecules but to determine the quantitative concentrations in a reliable and automated way. For this, a baseline correction method is applied to pre-process the measurement data in order to extract the characteristic vibrational information. Afterwards, a set of basis functions is fitted to capture the characteristic features of the two examined monosaccharides and a potential contribution of the solvent itself. The reconstruction of the actual concentration levels is then performed by superposition of the different basis functions to approximate the measured data. This software-based enhancement of the employed optical sensors leads to an accurate quantitative estimate of glucose and fructose concentrations in aqueous solutions.

Massive Enhancement of Optical Transmission across a Thin Metal Film via Wave Vector Matching in Grating-Coupled Surface Plasmon Resonance

We demonstrate how distinct surface plasmon resonance modes on opposite sides of a metal-coated grating can be coupled across the metal film. This coupling occurs by matching the resonance conditions on each side of the grating by tuning the refractive index directly adjacent to the metal film. In the first example, we deposited a high refractive index layer of tin oxide on top of the grating to red-shift the front side surface plasmon until it coupled with the backside surface plasmon across a semitransparent ∼45 nm thin silver grating. By shifting the resonance condition of the nearby surface plasmon, this high refractive index coating creates an effective matching of wave vectors across the metal film, allowing them to couple and enhance the optical response. A massive increase in the magnitude of enhanced transmission is observed, increasing from a 6-fold transmission enhancement through a bare silver grating to a near 100-fold enhancement after deposition of a tin oxide layer of appropriate thickness (∼310 nm). This optical transmission enhancement is then probed through computational modeling and by experiments with liquids of various refractive index values. The matched system shows an increased amplitude sensitivity with respect to refractive index changes and a waveguide like behavior within the tin oxide film. As an alternative configuration, we also demonstrate coupling the front and back-side plasmon modes by using a lower refractive index substrate in order to blue-shift the back-side surface plasmon. Coupling between the two plasmon modes is then demonstrated by introducing aqueous solutions of various refractive index values. Under the proper conditions, this matched system also shows a substantial enhancement in transmission. This technique of wave vector matching provides a route to substantially increasing the plasmon enhanced optical transmission through metal gratings, which has potential application in improved plasmonic sensing, spectroscopy, and plasmon-based optical devices.

Synthesis and characterization of size controlled bimetallic nanosponges

Metallic and bimetallic nanosponges with well-defined size and form have attracted increasing attention due to their unique structural properties and their potential for many applications. In this chapter, the recently developed methods for the synthesis and preparation of metallic and bimetallic nanosponges are presented. These methods can be mainly cataloged in two groups: dealloying-based methods and reduction reaction-based methods. Different topographical reconstruction methods for the investigation of their structural properties are then reviewed briefly. The optical properties of the metallic nanosponges are clearly different from those of the solid counterparts due to the tailored disordered structure. The recent advances in the exploration of the distinct linear and non-linear optical properties of the nanosponges are summarized.

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Gap-mode excitation, manipulation, and refractive-index sensing application by gold nanocube arrays

The challenges in fabricating two-dimensional metallic nanostructures over large areas, which normally involve expensive and time-consuming nanofabrication techniques, have severely limited the exploration of the related applications based on plasmon-induced effects. Here, we cost-efficiently prepared large-area Au nanocube arrays (NCAs) using only the electrostatic forces between colloidal Au nanocubes and polyelectrolyte layers. This method provides a flexible way for obtaining controlled Au NCAs with various fill fractions and single-cube sizes. When the Au NCAs were arranged to be coupled with a continuous Au film, the plasmonic gap mode could be excited and manipulated, leading to significant and tunable light absorbance from the visible to the near-infrared parts of the spectrum. Besides, the as-prepared Au NCAs were used to construct a prototype refractive-index (RI) sensor, which exhibited excellent stability and sensitivity over 560 nm per RI unit.

Advancements in fractal plasmonics: structures, optical properties, and applications

Fractal nanostructures exhibit optical properties that span the visible to far-infrared and are emerging as exciting structures for plasmon-mediated applications.

Sizing and identification of nanoparticles by a tapered fiber

There is a strong desire for sizing and identification of nanoparticles in fields of advanced nanotechnology and environmental protection. Although existing approaches can size the nanoparticles, or identify nanoparticles with different refractive indexes, a fast and simple method that combines the two functions still remains challenges. Here, we propose a versatile optical method to size and identify nanoparticles using an optical tapered fiber. By detecting reflection signals in real time, 400-600 nm SiO₂ nanoparticles can be sized and 500 nm SiO₂, PMMA, PS nanoparticles can be identified. This method requires only an optical tapered fiber, avoiding the use of elaborate nanostructures and making the device highly autonomous, flexible and compact. The demonstrated method provides a potentially powerful tool for biosensing, such as identification of nano-contaminant particles and biological pathogens.

Strong Spatial and Spectral Localization of Surface Plasmons in Individual Randomly Disordered Gold Nanosponges

Porous nanosponges, percolated with a three-dimensional network of 10 nm sized ligaments, recently emerged as promising substrates for plasmon-enhanced spectroscopy and (photo)catalysis. Experimental and theoretical work suggests surface plasmon localization in some hot-spot modes as the physical origin of their unusual optical properties, but so far the existence of such hot-spots has not been proven. Here we use scattering-type scanning near-field nanospectroscopy on individual gold nanosponges to reveal spatially and spectrally confined modes at 10 nm scale by recording local near-field scattering spectra. High quality factors of individual hot-spots of more than 40 are demonstrated, predicting high Purcell factors up to 10⁶. The observed field localization and enhancement make such nanosponges an appealing platform for a variety of applications ranging from nonlinear optics to strong-coupling physics.

Highly-Sensitive Refractive Index Sensing by Near-infrared Metatronic Nanocircuits

In this work, we present a highly-sensitive refractive index sensor based on metatronic nanocircuits operating at near-infrared spectral range. The structure is designed based on simple nanorod geometry and fabricated by nanopatterning of transparent conducting oxides. The functionality of these polarization dependent metatronic nanocircuits is enhanced by applying tunable response. This feature is investigated by depositing NH₂ (Amine) groups via plasma polymerization technique on top of indium-tin-oxide nanorods. The dielectric constant of Amine groups is a function of their thickness, which can be controlled by the RF power and the time duration of the applied plasma polymerization process. The resonance wavelengths of nanocircuits shift to higher wavelength, as the dielectric constant of the deposited material increases. An excellent agreement between the design and experimental results are obtained. Our metatronic based nanosensor offers a high-sensitive performance of 1587 nm/RIU with a satisfactory figure of merit for this class of sensors.

Photothermal generation of programmable microbubble array on nanoporous gold disks

We present a novel technique to generate microbubbles photothermally by continuous-wave laser irradiation of nanoporous gold disk (NPGD) array covered microfluidic channels. When a single laser spot is focused on the NPGDs, a microbubble can be generated with controlled size by adjusting the laser power. The dynamics of both bubble growth and shrinkage are studied. Using computer-generated holography on a spatial light modulator (SLM), simultaneous generation of multiple microbubbles at arbitrary locations with independent control is demonstrated. A potential application of flow manipulation is demonstrated using a microfluidic X-shaped junction. The advantages of this technique are flexible bubble generation locations, long bubble lifetimes, no need for light-adsorbing dyes, high controllability over bubble size, and relatively lower power consumption.

Photothermal generation of programmable microbubble array on nanoporous gold disks

We present a novel technique to generate microbubbles photothermally by continuous-wave laser irradiation of nanoporous gold disk (NPGD) array covered microfluidic channels. When a single laser spot is focused on the NPGDs, a microbubble can be generated with controlled size by adjusting the laser power. The dynamics of both bubble growth and shrinkage are studied. Using computer-generated holography on a spatial light modulator (SLM), simultaneous generation of multiple microbubbles at arbitrary locations with independent control is demonstrated. A potential application of flow manipulation is demonstrated using a microfluidic X-shaped junction. The advantages of this technique are flexible bubble generation locations, long bubble lifetimes, no need for light-adsorbing dyes, high controllability over bubble size, and relatively lower power consumption.

Nanoporous metals by alloy corrosion: Bioanalytical and biomedical applications

Nanoporous metals obtained by dealloying have attracted significant attention for their unusual catalytic properties, and as model materials for fundamental studies of structure-property relationships in a variety of research areas. There has been a recent surge in the use of these metals for biomedical and bioanalytical applications, where many exciting opportunities exist. The goal of this article is to provide a review of recent progress in using nanoporous metals for biological applications, including as biosensors for detecting biomarkers of disease and multifunctional neural interfaces for monitoring and modulating the activity of neural tissue. The article emphasizes the unique properties of nanoporous gold and concludes by discussing its utility in addressing important challenges in biomedical devices.

Gold nanorod-based multifunctional nanocarrier for synergistic chemo-photothermal therapy in tumors

Synergistic photothermal therapy (PTT) and chemotherapy is an efficient strategy for tumor therapy.

Far-field plasmonic coupling in 2-dimensional polycrystalline plasmonic arrays enables wide tunability with low-cost nanofabrication

We report the experimental observation and numerical modeling study of far-field plasmonic coupling (FFPC) in 2-dimensional polycrystalline plasmonic arrays consisting of "single crystalline" domains of a random size and orientation. Even though polycrystalline plasmonic arrays are routine products of low-cost nanosphere lithography (NSL), their FFPC behavior has not been well understood. Herein, FFPC observed from gold nanodisk (AuND) arrays fabricated using NSL appears, qualitatively, to be in keeping with that of highly regular nanoparticle arrays, where they induced cyclic modulations on the peak position and linewidth of the localized surface plasmon resonance (LSPR). Remarkable blue shifts as large as 1000 nm with nearly doubled linewidth were observed experimentally. Numerical modeling was systematically carried out and showed quantitative agreement with the experimental results. Using the modeling approach, the influences of array randomness and particle size on FFPC have been studied independently for the first time. Finally, two potential applications have been developed for FFPC-based LSPR tuning. Firstly, when AuND arrays are fabricated on flexible substrates, a novel transduction mechanism can be established between the LSPR peak position and the substrate strain. Owing to the far-field propagating nature, FFPC-based transduction can effectively extend the strain-tuning displacement range by an order of magnitude compared with those based on near-field coupling. Secondly, we show that FFPC leads to an LSPR peak within 1 μm for nanoporous gold disk arrays, which otherwise have a single particle LSPR peak beyond 1.5 μm. Such a significant FFPC-induced blue shift is critically important for compatibility with the use of silicon-based detectors.

Gold-rich ligament nanostructure by dealloying Au-based metallic glass ribbon for surface-enhanced Raman scattering

A new method to fabricate an Au-rich interconnected ligament substrate by dealloying the Au-based metallic glass ribbon for surface-enhanced Raman scattering (SERS) applications was investigated in this study. Specifically, three substrates, Au film, Au-based metallic glass ribbon, and dealloyed Au-based metallic glass ribbon, were studied. The dealloyed surface showed ligament nanostructure with protruding micro-islands. Based on the field emission scanning electron microscopy, reflection and scattering measurements, the dealloyed Au-based metallic glass provided a large surface area, multiple reflections, and numerous fine interstices to produce hot spots for SERS enhancements. The SERS signal of analyte, p-aminothiophenol, in the micro-island region of dealloyed Au-based metallic glass was about 2 orders of magnitude larger than the flat Au film. Our work provides a new method to fabricate the inexpensive and high SERS enhancements substrates.

Catalytic assembly of DNA nanostructures on a nanoporous gold array as 3D architectures for label-free telomerase activity sensing

Telomerase, an enzyme known to catalyze telomere elongation by adding TTAGGG [thymine (T), adenine (A), and guanine (G)] repeats to the end of telomeres, is vital for cell proliferation. Overexpression of telomerase has been found in most tumor cells, resulting in telomere dysfunction and uncontrolled cellular proliferation. Thus, telomerase has been considered as a potential cancer biomarker, as well as a potential target in cancer therapy. In this study, telomerase-catalyzed growth of tandem G-quadruplex (G4) assembled on a nanoporous gold array (NPGA) resulted in the formation of three-dimensional hybrid nanoarchitectures. The generated nanostructure then captured malachite green (MG) (reporter molecule) without the need of a complicated labeling process. Upon laser irradiation, the captured MG molecules produced a surface-enhanced Raman scattering (SERS) signal that was generated by an abundant amount of plasmonic hot spots in the NPGA substrates. A limit of detection (LOD) of 10^-10 IU along with a linear range, which was 3 orders of magnitude, was achieved, which was equivalent to the telomerase amount extracted from 20 HeLa cells. The LOD is 2 orders of magnitude better than that of the commercial enzyme-linked immunosorbent assay (ELISA), and it approaches that of the most sensitive technique, telomeric repeat amplification protocols (TRAP), which require a laborious and equipment-intensive polymerase chain reaction (PCR). In addition, X-ray photoelectron spectroscopy (XPS) was used to chemically identify and quantify the telomerase activity on the sensitized NPGA surface. Furthermore, the sensor was applied to screen the effectiveness of anti-telomerase drugs such as zidovudine, thus demonstrating the potential use of the sensor in telomerase-based diagnosis and drug development. Moreover, the framework represents a novel paradigm of collaborative plasmonic intensification and catalytic multiplication (c-PI/CM) for label-free biosensing.

Nanoporous Gold Nanocomposites as a Versatile Platform for Plasmonic Engineering and Sensing

Plasmonic metal nanostructures have shown great potential in sensing applications. Among various materials and structures, monolithic nanoporous gold disks (NPGD) have several unique features such as three-dimensional (3D) porous network, large surface area, tunable plasmonic resonance, high-density hot-spots, and excellent architectural integrity and environmental stability. They exhibit a great potential in surface-enhanced spectroscopy, photothermal conversion, and plasmonic sensing. In this work, interactions between smaller colloidal gold nanoparticles (AuNP) and individual NPGDs are studied. Specifically, colloidal gold nanoparticles with different sizes are loaded onto NPGD substrates to form NPG hybrid nanocomposites with tunable plasmonic resonance peaks in the near-infrared spectral range. Newly formed plasmonic hot-spots due to the coupling between individual nanoparticles and NPG disk have been identified in the nanocomposites, which have been experimentally studied using extinction and surface-enhanced Raman scattering. Numerical modeling and simulations have been employed to further unravel various coupling scenarios between AuNP and NPGDs.

Surface-Enhanced Infrared Spectroscopy Using Resonant Nanoantennas

Infrared spectroscopy is a powerful tool widely used in research and industry for a label-free and unambiguous identification of molecular species. Inconveniently, its application to spectroscopic analysis of minute amounts of materials, for example, in sensing applications, is hampered by the low infrared absorption cross-sections. Surface-enhanced infrared spectroscopy using resonant metal nanoantennas, or short "resonant SEIRA", overcomes this limitation. Resonantly excited, such metal nanostructures feature collective oscillations of electrons (plasmons), providing huge electromagnetic fields on the nanometer scale. Infrared vibrations of molecules located in these fields are enhanced by orders of magnitude enabling a spectroscopic characterization with unprecedented sensitivity. In this Review, we introduce the concept of resonant SEIRA and discuss the underlying physics, particularly, the resonant coupling between molecular and antenna excitations as well as the spatial extent of the enhancement and its scaling with frequency. On the basis of these fundamentals, different routes to maximize the SEIRA enhancement are reviewed including the choice of nanostructures geometries, arrangements, and materials. Furthermore, first applications such as the detection of proteins, the monitoring of dynamic processes, and hyperspectral infrared chemical imaging are discussed, demonstrating the sensitivity and broad applicability of resonant SEIRA.