Plasmonic Substrates Research Articles

Microfluidic platform for integrated plasmonic detection in laminal flow

In this work, we propose a novel approach to design robust microfluidic devices with integrated plasmonic transducers allowing portability, reduced analysis time through dynamic measurements and high sensitivity. Specifically, the strategy we apply involves two steps: (i) the controlled deposition of gold bipyramidal nanoparticles (AuBPs) onto a functionalized solid glass substrate and (ii) the integration of the as-fabricated plasmonic substrate into a polydimethylsiloxane (PDMS) microfluidic circuit. The localized surface plasmon resonance (LSPR) sensitivity of the plasmonic-microfluidic device was evaluated by monitoring the optical responses at refractive index changes, proving a bulk sensitivity of 243 nm RIU−1 for the longitudinal LSPR band of isolated AuBPs and 150 nm RIU−1 for the band assigned to end-to-end linked nanoparticles. A strong electric field generated in the gaps between AuBPs—due to the generation of the so-called extrinsic ‘hot-spots’—was subsequently proved by the volumetric surface enhanced Raman scattering (SERS) detection of molecules in continuous flow conditions by loading the analyte into the microfluidic channel via a syringe pump. In conclusion, our miniaturized portable microfluidic system aims to detect and identify, in real-time with high accuracy, analyte molecules in laminal flow, thus providing a groundwork for further complex biosensing applications.

Plasmonic Properties of Gold Nanostructures on Gold Film

This paper reports on a systematic study of the plasmonic properties of periodic arrays of gold cylindrical nanoparticles in contact with a gold thin film. Depending on the gold film thickness, it observes several plasmon bands. Using a simple analytical model, it is able to assign all these modes and determine that they are due to the coupling of the grating diffraction orders with the propagating surface plasmons travelling along the film. With finite difference time domain (FDTD) simulations, it demonstrates that large field enhancement occurs at the surface of the nanocylinders due to the resonant excitation of these modes. By tilting the sample, it also observes the evolution of the spectral position of these modes and their tuning through nearly the whole visible range is possible. Such plasmonic substrates combining both advantages of the propagative and localised surface plasmons could have large applications in enhanced spectroscopies.

Ag/Au alloy entangled in a protein matrix: A plasmonic substrate coupling surface plasmons and molecular chirality

Plasmonic circular dichroism enables chirality detection in the visible range.Here, it is shown that growing plasmonic achiral Ag/Au nanostructures in a handed-peptide dielectric matrix caused a strong chiral optical coupled plasmonic signal. The complex architecture of the matter has been designed for enhancing the localized surface plasmon resonance. However, a more accurate investigation displayed the amplification of the optical field localized at the plasmonic resonators and strong coupling of the molecular dipoles and the metallic electrons. Then, the enhancement of the factor E/E0 provided evidence that the handed matrix modified the propagation constant of the surface plasmon polaritons and demonstrated the existence of plasmonic hotspots at the interparticle gap.Finally, the Ag/Au nanoalloy was used to attain the enantioselectivity of the l- and d- forms of the biomolecules on-resonance. The sensitivity to l- and d- penicillamine racemic mixture was measured measured to be 10-6 M.

Quasi-3D Plasmonic Nanowell Array for Molecular Enrichment and SERS-Based Detection.

We report on a quasi-three-dimensional (3D) plasmonic nanowell array with high structural uniformity for molecular detection. The quasi-3D plasmonic nanowell array was composed of periodic hexagonal Au nanowells whose surface is densely covered with gold nanoparticles (Au NPs), separated by an ultrathin dielectric interlayer. The uniform array of the Au nanowells was fabricated by nanoimprint lithography and deposition of Au thin film. A self-assembled monolayer (SAM) of perfluorodecanethiol (PFDT) was coated on the Au surface, on which Au was further deposited. Interestingly, the PFDT-coated Au nanowells were fully covered with Au NPs with an ultra-high density of 375 μm−2 rather than a smooth film due to the anti-wetting property of the low-energy surface. The plasmonic nanogaps formed among the high-density Au NPs led to a strong near-field enhancement via coupled localized surface plasmon resonance and produced a uniform surface-enhanced Raman spectroscopy (SERS) response with a small relative standard deviation of 5.3%. Importantly, the highly uniform nanostructure, featured by the nanoimprint lithography and 3D growth of densely-packed Au NPs, minimizes the spatial variation of Raman intensity, potentially providing quantitative analysis. Moreover, analyte molecules were highly concentrated and selectively deposited in nanowells when a water droplet containing the analyte was evaporated on the plasmonic substrate. The analyte formed a relatively thick overcoat in the nanowells near the triple line due to the coffee-ring effects. Combining 3D plasmonic nanowell substrates with molecular enrichments, highly sensitive detection of lactic acid was demonstrated. Given its combination of high sensitivity and signal uniformity, the quasi-3D plasmonic nanowell substrate is expected to provide a superior molecular detection platform for biosensing applications.

Ultrasensitive and Multi-Functional Rapid Screening Plasmonic Ag-Based SERS Substrate

Motivation The complex interpretation of the cause for some diseases and the lack of specificity in clinical manifestations lead to the difficulty in making early diagnosis and treatment. So far, several analytical methods, including radiation immunoassay (RIA), fluorescence immunoassay (FIA)[1] and enzyme-linked immunoassay (ELISA) [2], have been widely applied in the field of biomedical research. However, the above-mentioned techniques still suffer some disadvantages. For example, these immunoassay take at least 4~6 hours to complete the incubation and detection process. Also, attenuation of the fluorescence signal-to-noise ratio is low which may cause inaccurate detection easily. [3]As a consequence, it is necessary to develop a new and effective method to obtain the ‘3S’ goal, which are the sensitive, specific and speedy detection of biomarkers. Introduction With the mentioned motivations in mind, a plasmonic Ag-based SERS substrate is proposed to implement the ultrasensitive detection of biomarkers while cardiac troponin I (cTnI), which is the most reliable biomarker for cardiovascular disease (CVD), is chosen as the analyte in this work. The proposed substrate is mainly composed of silicon nanowires array decorated with silver nanoparticles. By exploiting this substrate, the appropriate length and interspace of silicon nanowires may offer a high surface-to-volume ratio for wider area of detection. On the other hand, the silver nanoparticles can contribute localized surface plasmon resonance (LSPR) for SERS enhancement as well. The superiority of this substrate includes (1) facile fabrication, (2) low detection limit, (3) wide linear range of concentration (4) time-saving and (5) label-free detection. Apart from this, the cost of this SERS substrate is very low while it can achieve multiple and repeated measurements by one sampling, hence shows great potential for practical application in clinical diagnosis. Experimental Procedures To begin with, the silicon nanowires scaffold was fabricated by metal-assisted chemical etching (MaCE) procedure. As shown in Fig. 1, after the first absorption of silver nanoparticles on a cleaned silicon substrate, it was then being immersed into a HF/H2O2 solution for etching. Next, the residual silver nanoparticles were removed by concentrated HNO3 solution and then immersed the prepared substrate into a KOH/IPA solution to enlarge the interspace between silicon nanowires. Finally, silver nanoparticles were deposited on the silicon nanowires by electroless metal deposition (EMD). After applying 5 mL of different concentrations of cTnI on each fabricated substrate by using pipette, a Confocal Micro-Raman Spectroscopy equipped with 633nm laser source and 17 mW of power was utilized to excite the SERS substrates, while the spectra were collected in the continuous mode with the integration time of 10 s. Results and Conclusions The morphology of silver nanoparticles deposited on silicon nanowires with different KOH etching time were characterized by scanning electron microscopy (SEM). As shown in Fig. 2, silicon nanowires are free-standing on the substrate with silver nanoparticles distribute uniformly on the tips, while the interspaces of the silicon nanowires are significantly enlarged after the KOH etching. To startup, a preliminary Raman measurement of 1.616 ppb cTnI was obtained and consistent to the referred Raman spectrum as shown in Fig. 3 [4]. By eliminating the overlapped signals from silicon on 520 cm-1 and 900 cm-1, the more significant characteristic peaks of cTnI which located at around 1300 cm-1 and 1600 cm-1 are mainly considered for further analysis. Also, taking into account the discrepancy in the state of focus and power for each measurement, every Raman spectrum was normalized by the intensity of the silicon peak at 520.6 cm-1. Fig. 4 shows the Raman results of substrates with different KOH etching time for three different concentrations of cTnI, while all of the results show that the one with 1 min KOH etching gives the most enhancement. Furthermore, Fig. 5 shows both the Raman spectra of blank substrate that without applying any analyte and that of cTnI with different concentrations from 0.012 ppb to 1.616 ppb. Herein, a clean background signal was successfully obtained for the blank substrate, demonstrating that the proposed substrate might be applicable for not only cTnI but also other chemical molecules such as food additives and drug. Importantly, the plot for the concentration-dependent SERS intensity of the peaks at 1300 cm-1 and 1600 cm-1 (Fig. 6) show that, both of them obey a good linear relationship, with R2=0.93368 and R2=0.91904 respectively. These experiment results demonstrate that the proposed strategy displays high detection sensitivity for cTnI up to 0.012 ppb in a wide and linear detection range from 0.012 ppb to 1.616 ppb, which exhibits the potential to perform quantitative analysis. Meanwhile, the specificity of the analyte is also verified by the unique Raman fingerprint of cTnI, which presents the superior ability to neglect the complex incubation process needed in many immunoassay methods.

Reusable Plasmon-Decorated Chestnut-like Copper Oxide Nanostructures for SERS Sensing

Reusable plasmonic substrates are crucial for the development of biosensing applications using surface-enhanced Raman scattering (SERS), as they can provide unique advantages for ultrafast and accurate single-molecule recognition of different species. In this research, we employed thermally annealed cupric CuO and cuprous oxide Cu2O heterostructures to serve as highly stable nanotextured surfaces for designing robust 3D plasmonic biochips.Figure 1: Schematic representation of a research outline. Step 1 – controllable surface texturing by thermal annealing at oxygen-enriched atmosphere. Step 2 – plasmonic activation by ion spattering of Au/Pd alloy film. Step 3 – SERS performance monitoring by means of R6G dye fingerprint detection. Step 4 – Recovery test under ultra-fast mild cleaning in reactive oxygen/argon plasma. Step 5 – Simplified scheme of Rhodamine degradation inside plasma followed by the vanishing of the characteristic peaks in the Raman spectrum.Figure 2: (A-E) – SEM images showing the cuprous oxide Cu2O surface morphology altering with Au/Pd alloy thickening. (F) – Broadband Raman spectra representing vibrational features of chestnut Cu2O structures decorated by different plasmonic thicknesses together with ethanol diluted Rhodamine 10-5 M absorbed on the surface. (G) – Normalized intensity of the in-plane C-H bending vibrations of R6G 10-5 M as a function of bimetallic film thickness.The influence of bimetallic layer thickness on SERS performance has been studied for the chestnut patterned substrate. Figure 2 F depicts Raman spectra of Rhodamine used as a model biomarker (600-1800 cm-1) complemented by a signal originated from the Cu2O layers (100-800 cm-1). During Au/Pd film thickness reduction from 80 nm down to 20 nm, the R6G peak intensities undergo a gradual decrease, simultaneously, acoustic bands originated from Cu2O nanoparticles located in a range of 100-800 cm-1 become dominant. In accordance with Figure 2 G, that displays a signal intensity dropping for in-plane C-H bending vibrations as a function of bimetallic layer thicknesses, the analytical enhancement factor AEF acquires the following values: for 60 nm ~ 9.8×104, for 40 nm ~ 8.4×104 and for 20 nm ~ 1.1×104, respectively. The analogous finding was observed for Au decorated CuO nanoflakes, where EF increases with a noble metal film thickening [1]. Due to prominent anti-reflection properties, CuO/Cu2O heterostructures [2] appear to be an extremely suitable material for designing SERS-active substrates. Moreover, the high chemical inertness of Au/Pd alloy plasmonic layer allows a non-destructive substrate recovery with the assistance of reactive plasma species. Furthermore, the generation of reactive oxygen species-rich plasma by introducing oxygen gas into argon plasma drastically reduces cleaning session to less than 1 minute. For our best knowledge, this is the shortest recovery time for any 3D plasmonic SERS substrate reported in the literature. Additionally, results indicate that the nano-roughness is more important for achieving optimal plasmonic activity than micro-roughness. Overall, the designed chestnut-like Au/Pd@/Cu2O substrate is a ready-to-use chip that possesses a decent signal enhancement ~ 105 and reveals remarkable robustness under multiple plasma treatment showing nearly 100% of self-recovery with no degradation of a plasmonic layer. These findings are of great significance for the development of novel reliable SERS substrates with low-cost manufacture which can be utilised with great success in a wide area of plasmonic biosensing. REFERENCES [1] A. Balčytis, M. Ryu, G. Seniutinas, J. Juodkazyte, B. C. C. Cowie, P. R. Stoddart, M. Zamengo, J. Morikawa, S. Juodkazis, “Black-CuO: Surface-enhanced Raman scattering and infrared properties”, Nanoscale, 7, 43, October 2015, 18299-18304.[2] J. Yu, M. Yang, C. Zhang, S. Yang, Q. Sun, M. Liu, Q. Peng, X. Xu, B. Man, F. Lei, “Capillarity-Assistant Assembly: A Fast Preparation of 3D Pomegranate-Like Ag Nanoparticle Clusters on CuO Nanowires and Its Applications in SERS”, Advanced Materials Interfaces, 5, 19, July 2018, 1-8. Figure 1

Lab-On-Capillary Platform for On-Site Quantitative SERS Analysis of Surface Contaminants Based on Au@4-MBA@Ag Core–Shell Nanorods

A portable and highly reproducible lab-on-capillary surface-enhanced Raman scattering (SERS) platform was developed using a specially designed homemade device for rapid on-site SERS measurement. In particular, this platform was composed of a capillary with a tiny orifice, which allows an effective and lossless sample extraction, resulting in high SERS performance. The capillary-based plasmonic substrate was prepared by compactly assembling Au@Ag core-shell nanorods (NRs) embedded with the 4-mercaptobenzoic acid (4-MBA) molecule as an internal standard onto the inner wall of a capillary tube. The fabrication process is facile and convenient with no requirement for complicated procedures. The exclusively prepared nanoparticles were able to significantly improve the signal consistency and overcome the limitations of reliable quantitative SERS analysis compared with conventional methods. Importantly, it was found that this capillary-based substrate with higher sensitivity was essentially attributed to more valid nanoparticles in the effective laser excitation region derived from the unique structure of the capillary. Furthermore, the applicability of the Au@4-MBA@Ag nanorod-decorated capillary for the quantitative identification of fungicides (malachite green and crystal violet) on the shell was demonstrated. As a result, this proposed lab-on-capillary sensor holds promising practical potential for rapid on-site analysis, especially for various contaminants on an uneven surface.

Spatial localization of hotspots in Fano-resonant plasmonic oligomers for surface-enhanced coherent anti-Stokes Raman scattering

Realization of Fano resonance in plasmonic oligomers is often exploited to design efficient plasmonic substrates for surface-enhanced coherent anti-Stokes Raman scattering. Disk-type Fano-resonant plasmonic oligomers are widely used to enhance the Raman signal of the probe material. Generally, hot spots are generated in those oligomers at different spatial locations at different wavelengths and only a few spatially overlapping hot spots at multiple wavelengths can be achieved with oblique incidence of excitation light. In this work, we proposed hexagonal gold nanoparticle based Fano-resonant plasmonic oligomers that can yield higher number of spatially overlapped hot spots compared to the disk type oligomers even with the normal incidence of excitation light. The oligomers were numerically modelled and optimized for surface-enhanced coherent anti-Stokes Raman scattering with 780 nm pumping and 500–1800 cm− 1 Raman signature region. The Fano lineshape was engineered to ensure near-field energy coupling at pump while enhancing the coherent anti-Stokes Raman signal at the far field. Our computational studies explored the purely electric origin of Fano resonance in those oligomers and provided maximum Raman enhancements of 1012–1013 from them to enable single-molecular level applications. Our findings provide a way to realize fabrication-friendly nanostructures with higher number of spatially localized hotspots for improving the Raman detection sensitivity.

Chiral and plasmonic hybrid dimer pair: reversal of both near- and far-field optical binding forces

In both the near-field (around 10 to 250 nm interparticle distance) and far-field (around 1 µm to higher interparticle distances) regions, controlling the mutual attraction and repulsion between chiral and plasmonic hybrid dimers using light has not been reported so far to the best of our knowledge. Such control is called controlling the reversal of the optical binding force. In most setups, the reversal of the optical binding force between plasmonic heterodimers vanishes with an interparticle distance of around 100 nm and above due to the disappearance of the Fano resonance. In this paper, we have demonstrated a possible optical setup, illuminated by a linearly polarized plane wave: chiral and plasmonic hybrid dimers over a plasmonic substrate, which supports the reversal of the optical binding force in both the near- and far-field regions. First, by varying the light wavelengths, we have shown that the optical binding force does not reverse for either the chiral homodimers set and or the plasmonic homodimer set for different interparticle distances. Later, we created a hybrid dimer system by placing a plasmonic and a chiral nanoparticle together. Interestingly, at the far-field region, a strong plasmonic resonance is observed, but a reversal of the optical binding force does not occur. Finally, we have placed the same chiral–plasmonic hybrid dimer setup over a plasmonic substrate and the desired result—a reversal of the binding force—is observed due to the induced lateral force on the chiral object (in the presence of the substrate) and the Fano-type resonance in the system. Controlling such near- and far-field optical binding forces can be an important aspect for particle clustering, accumulation, crystallization, and the organization of templates for biological and colloidal sciences in the near future.

SERS study on the synergistic effects of electric field enhancement and charge transfer in an Ag2S quantum dots/plasmonic bowtie nanoantenna composite system

Localized surface plasmon resonance (LSPR) of nanostructures and the interfacial charge transfer (CT) of semiconductor materials play essential roles in the study of optical and photoelectronic properties. In this paper, a composite substrate of Ag 2 S quantum dots (QDs) coated plasmonic Au bowtie nanoantenna (BNA) arrays with a metal–insulator–metal (MIM) configuration was built to study the synergistic effect of LSPR and interfacial CT using surface-enhanced Raman scattering (SERS) in the near-infrared (NIR) region. The Au BNA array structure with a large enhancement of the localized electric field (E-field) strongly enhanced the Raman signal of adsorbed p-aminothiophenol (PATP) probe molecules. Meanwhile, the broad enhanced spectral region was achieved owing to the coupling of LSPR. The as-prepared Au BNA array structure facilitated enhancements of the excitation as well as the emission of Raman signal simultaneously, which was established by finite-difference time-domain simulation. Moreover, Ag 2 S semiconductor QDs were introduced into the BNA/PATP system to further enhance Raman signals, which benefited from the interfacial CT resonance in the BNA / Ag 2 S - QDs / PATP system. As a result, the Raman signals of PATP in the BNA / Ag 2 S - QDs / PATP system were strongly enhanced under 785 nm laser excitation due to the synergistic effect of E-field enhancement and interfacial CT. Furthermore, the SERS polarization dependence effects of the BNA / Ag 2 S - QDs / PATP system were also investigated. The SERS spectra indicated that the polarization dependence of the substrate increased with decreasing polarization angles ( θ pola ) of excitation from p-polarized ( θ pola = 90 ° ) excitation to s-polarized ( θ pola = 0 ° ) excitation. This study provides a strategy using the synergistic effect of interfacial CT and E-field enhancement for SERS applications and provides a guidance for the development of SERS study on semiconductor QD-based plasmonic substrates, and can be further extended to other material-nanostructure systems for various optoelectronic and sensing applications.

Reusable Au/Pd-coated chestnut-like copper oxide SERS substrates with ultra-fast self-recovery

Reliable and reusable plasmonic substrates are crucial for the development of biosensing applications using surface-enhanced Raman scattering (SERS), as they can provide unique advantages for ultrafast and accurate single-molecule recognition of different species. These properties are unrevealed in this paper, where thermally annealed cupric CuO and cuprous oxide Cu2O heterostructures were used as templates for highly stable nanotextured surfaces and design of robust 3D plasmonic biochips. Differently tailored nano/micro-roughness provided outstanding light trapping abilities that lead to significant SERS performance improvement. It was found that Cu2O chestnut-like substrate activated with 80 nm Au/Pd alloy film reveals impressive 3.7-fold Raman signal increment in respect to grainy-like structure and about twice larger amplification than that of nanowires enriched platform decorated in the same manner. Large enhancement factor AEF ~5 × 105 of a chestnut-like Au/Pd@/Cu2O chip allows adding it up to the list of the most effective oxide-based plasmonic substrates. Moreover, the substrate shows unprecedented durability during repetitive plasma-cleaning, demonstrating a remarkable 100% self-recovery in less than 1 min, accompanied by virtually no thickness degradation of the plasmonic layer.

Photochemical Printing of Plasmonically Active Silver Nanostructures.

In this paper, we demonstrate plasmonic substrates prepared on demand, using a straightforward technique, based on laser-induced photochemical reduction of silver compounds on a glass substrate. Importantly, the presented technique does not impose any restrictions regarding the shape and length of the metallic pattern. Plasmonic interactions have been probed using both Stokes and anti-Stokes types of emitters that served as photoluminescence probes. For both cases, we observed a pronounced increase of the photoluminescence intensity for emitters deposited on silver patterns. By studying the absorption and emission dynamics, we identified the mechanisms responsible for emission enhancement and the position of the plasmonic resonance.

Phase-sensitive optical neural recording of cerebellum tissue on a flexible interface

Knowing an increased number of patients suffering from mental disorders, neural signal recording and imaging have become highly prerequisite challenges for providing healing procedures. Despite the fact that novel optical techniques provide highly resolved imaging/recording of large neuron population, most of them suffer from insertion damage, tethering connection, labeling, and photobleaching deficiencies, among which plasmonic ellipsometry is a highly sensitive and label-free platform for detecting neural activity both quantitatively and qualitatively. In this paper, a flexible patterned plasmonic substrate is used as a sensing surface for phase-sensitive neural recording of a cerebellum tissue slice under electrical and chemical stimulations. Although the traditional reflection spectrum cannot represent the changes in neural activity with high precision, phase-sensitive neuroplasmonics can not only reveal the neural activity level but also distinguish different electrical and chemical stimulation types with a considerable phase splitting factor. This study can open up a new insight into label-free and flexible biological sensors with neuroscience applications.

Fabrication of Hybrid Silver Microstructures from Vermiculite Templates as SERS Substrates.

There is great interest in developing complex, 3D plasmonic materials with unusual structural properties. This can be achieved via template-assisted approaches exploiting scaffold elements to engineer unique plasmonic substrates, which would be otherwise impossible to synthesize. Herein, we present a novel, simple, and low-cost template-assisted method for producing interconnected 3-D silver microstructures by utilizing vermiculite, a well-known silicate, as both in-situ reductant and template for silver growth. The silicate network of the vermiculite can be easily removed by dissolution with hydrofluoric acid, which, simultaneously, leads to the formation of a magnesium fluoride skeleton supporting a plasmonically active silver film. Optical, morphological, and chemical properties of the materials were extensively investigated, revealing, for example, that hybrid silver microstructures can be exploited as valuable SERS substrates over a broad spectral range of excitation wavelengths.

Multiplex SERS Detection of Metabolic Alterations in Tumor Extracellular Media

AbstractThe composition and intercellular interactions of tumor cells in the tissues dictate the biochemical and metabolic properties of the tumor microenvironment. The metabolic rewiring has a profound impact on the properties of the microenvironment, to an extent that monitoring such perturbations could harbor diagnostic and therapeutic relevance. A growing interest in these phenomena has inspired the development of novel technologies with sufficient sensitivity and resolution to monitor metabolic alterations in the tumor microenvironment. In this context, surface‐enhanced Raman scattering (SERS) can be used for the label‐free detection and imaging of diverse molecules of interest among extracellular components. Herein, the application of nanostructured plasmonic substrates comprising Au nanoparticles, self‐assembled as ordered superlattices, to the precise SERS detection of selected tumor metabolites, is presented. The potential of this technology is first demonstrated through the analysis of kynurenine, a secreted immunomodulatory derivative of the tumor metabolism and the related molecules tryptophan and purine derivatives. SERS facilitates the unambiguous identification of trace metabolites and allows the multiplex detection of their characteristic fingerprints under different conditions. Finally, the effective plasmonic SERS substrate is combined with a hydrogel‐based three‐dimensional cancer model, which recreates the tumor microenvironment, for the real‐time imaging of metabolite alterations and cytotoxic effects on tumor cells.

Plasmonic Gold Templates Enhancing Single Cell Lipidomic Analysis of Microorganisms.

Single cell lipid profiling is a powerful tool to connect membrane composition and its changes within individual cells to specific biochemical functions or stimuli, but current approaches are inadequate due to the complex nature of the cells and technical limitation in analysis. Herein we report a new method with plasmonic substrates capable of cell localization and enhanced lipid ionization through thin-gold-film MALDI-MS. We performed lipidomic profiling of algae single cells with a 120-well microarray and identified more than 50 lipids in C. reinhardtii without an extraction process. The substrate was used for probing toxicological effect of herbicide atrazine on the algae's lipidome, demonstrating molecular changes in glycerol lipid profiles. Fast location of cells with metal-enhanced fluorescence (MEF) and subsequent precise and direct ionization of the LDI process contribute to the enhanced performance, allowing for assessment of lipid changes concurrent with atrazine affected populations. This method that combines microarrays, MEF, and MALDI-MS presents an effective platform for lipidomic study of single cells and for environmental toxicity study with microorganisms.

Plasmonic Manipulation of DNA using a Combination of Optical and Thermophoretic Forces: Separation of Different-Sized DNA from Mixture Solution

We demonstrate the size-dependent separation and permanent immobilization of DNA on plasmonic substrates by means of plasmonic optical tweezers. We found that a gold nanopyramidal dimer array enhanced the optical force exerted on the DNA, leading to permanent immobilization of the DNA on the plasmonic substrate. The immobilization was realized by a combination of the plasmon-enhanced optical force and the thermophoretic force induced by a photothermal effect of the plasmons. In this study, we applied this phenomenon to the separation and fixation of size-different DNA. During plasmon excitation, DNA strands of different sizes became permanently immobilized on the plasmonic substrate forming micro-rings of DNA. The diameter of the ring was larger for longer DNA (in base pairs). When we used plasmonic optical tweezers to trap DNA of two different lengths dissolved in solution (φx DNA (5.4 kbp) and λ-DNA (48.5 kbp), or φx DNA and T4 DNA (166 kbp)), the DNA were immobilized, creating a double micro-ring pattern. The DNA were optically separated and immobilized in the double ring, with the shorter sized DNA and the larger one forming the smaller and larger rings, respectively. This phenomenon can be quantitatively explained as being due to a combination of the plasmon-enhanced optical force and the thermophoretic force. Our plasmonic optical tweezers open up a new avenue for the separation and immobilization of DNA, foreshadowing the emergence of optical separation and fixation of biomolecules such as proteins and other ncuelic acids.

Uniting Top-Down and Bottom-Up Strategies Using Fabricated Nanostructures as Hosts for Synthesis of Nanomites

Gold nanomites on gold nanotriangles were produced using a combination of bottom-up synthesis and top-down fabrication. Modified gold nanostar protocols were the basis for the synthesis to form the nanomite features on the fabricated structures. Gold nanostars produce some of the strongest signal enhancements in plasmonics due to the large number of sharp features per particle. However, the tendency for the high-aspect-ratio features of nanostars to undergo structural changes and the prevalence of particle aggregation in different solution conditions present challenges for research as well as applications of these particles. As an alternative to solution-based synthesis, top-down fabrication methods have been used to produce plasmonic structures with sharp features with greater stability in different environments and elimination of aggregation issues. These advantages tend to be at the cost of decreased sensitivity compared to synthesized particles with a higher density of high-aspect-ratio features such as nanostars. Here, top-down fabrication is united with bottom-up synthesis to add a high density of surface features to fabricated gold nanotriangles in the form of nanomites. This approach combines advantageous features of both nanomaterial strategies. The nanomite nucleation and growth processes were controlled by altering the reagents and incubation temperatures used in the one-pot synthesis. Fabricated gold nanostructures with different shapes and sizes could be hosts for the nanomites making this hybrid bottom-up/top-down approach a highly versatile route to tunable plasmonic substrates with a high density of hotspot regions for plasmon-enhanced spectroscopy applications.

Dynamic Actuation of DNA-Assembled Plasmonic Nanostructures in Microfluidic Cell-Sized Compartments.

Molecular motor proteins form the basis of cellular dynamics. Recently, notable efforts have led to the creation of their DNA-based mimics, which can carry out complex nanoscale motion. However, such functional analogues have not yet been integrated or operated inside synthetic cells toward the goal of realizing artificial biological systems entirely from the bottom-up. In this Letter, we encapsulate and actuate DNA-assembled dynamic nanostructures inside cell-sized microfluidic compartments. These encapsulated DNA nanostructures not only exhibit structural reconfigurability owing to their pH-sensitive molecular switches upon external stimuli but also possess optical feedback enabled by the integrated plasmonic probes. In particular, we demonstrate the power of microfluidic compartmentalization for achieving on-chip plasmonic enantiomer separation and substrate filtration. Our work exemplifies that the two unique tools, droplet-based microfluidics and DNA technology, offering high precision on the microscale and nanoscale, respectively, can be brought together to greatly enrich the complexity and diversity of functional synthetic systems.

Label-Free Surface-Enhanced Raman Spectroscopy Biosensor for On-Site Breast Cancer Detection Using Human Tears.

Surface-enhanced Raman scattering (SERS) is an ultrasensitive molecular screening technique with greatly enhanced Raman scattering signals from trace amounts of analytes near plasmonic nanostructures. However, research on the development of a sensor that balances signal enhancement, reproducibility, and uniformity has not yet been proposed for practical applications. In this study, we demonstrate the potential of the practical application for detecting or predicting asymptomatic breast cancer from human tears using a portable Raman spectrometer with an identification algorithm based on multivariate statistics. This potentiality was realized through the fabrication of a plasmonic SERS substrate equipped with a well-aligned, gold-decorated, hexagonal-close-packed polystyrene (Au/HCP-PS) nanosphere monolayer that provided femtomole-scale detection, giga-scale enhancement, and <5% relative standard deviation for reliability and reproducibility, regardless of the measuring site. Our results can provide a first step toward developing a noninvasive, real-time screening technology for detecting asymptomatic tumors and preventing tumor recurrence.