Centrifugal microfluidic system for colorimetric sample-to-answer detection of viral pathogens.

We describe a microfluidic system for conducting thermal lysis, polymerase chain reaction (PCR) amplification, hybridization, and colorimetric detection of foodborne viral organisms in a sample-to-answer format. The on-chip protocol entails 24 steps which are conducted by a centrifugal platform that allows for actuating liquids pneumatically during rotation and so facilitates automation of the workflow. The microfluidic cartridge is fabricated from transparent thermoplastic polymers and accommodates assay components along with an embedded micropillar array for detection and read-out. A panel of oligonucleotide primers and probes has been developed to perform PCR and hybridization assays that allows for identification of five different viruses, including pathogens such as norovirus and hepatitis A virus (HAV) in a multiplexed format using digoxigenin-labelled amplicons and immunoenzymatic conversion of a chromogenic substrate. Using endpoint detection, we demonstrate that the system can accurately and repetitively (n = 3) discriminate positive and negative signals for HAV at 350 genome copies per µL. As part of the characterization and optimization process, we show that the implementation of multiple (e.g., seven) micropillar arrays in a narrow fluidic pathway can lead to variation (up to 50% or more) in the distribution of colorimetric signal deriving from the assay. Numerical modeling of flow behaviour was used to substantiate these findings. The technology-by virtue of automation-can provide a pathway toward rapid detection of viral pathogens, shortening response time in food safety surveillance, compliance, and enforcement as well as outbreak investigations.

On-the-Fly Phase Transition and Density Changes of Aqueous Two-Phase Systems on a Centrifugal Microfluidic Platform.

This paper describes on-the-fly physical property changes of aqueous two-phase systems (ATPS) in microfluidic devices. The properties and phases of the ATPS are modulated on-demand by using a centrifugal microfluidic device filled with poly(ethylene glycol) (PEG) and dextran (DEX) solutions. By use of the centrifugal force and active pneumatic controls provided by a centrifugal microfluidic platform (CMP), PEG-DEX mixtures are manipulated and processed inside simple thermoplastic microfluidic devices. First, we experimentally demonstrate an on-chip ATPS transition from two phases to a single phase and vice versa by dynamically changing the concentration of the solution to bring ATPS across the binodal curve. We also demonstrate a density modulation scheme by introducing an ATPS solution mixed with sodium diatrizoate hydrate, which allows to increase the liquid density. By adding precisely metered volumes of water, we spontaneously change the density of the solution on the CMP and show that density marker microbeads fall into the solution according to their corresponding densities. The measured densities of ATPS show a good agreement with densities of microbeads and analytical plots. The results presented in this paper highlight the tremendous potential of CMPs for performing complex on-chip processing of ATPS. We anticipate that this method will be useful in applications such as microparticle-based plasma protein analysis and blood cell fractionation.

An automated centrifugal microfluidic assay for whole blood fractionation and isolation of multiple cell populations using an aqueous two-phase system.

Fractionating whole blood and separating its constituent components one from another is an essential step in many clinical applications. Currently blood sample handling and fractionation processes remain a predominantly manual task that require well-trained operators to produce reliable and reproducible results. Herein, we demonstrate an advanced on-chip whole human blood fractionation and cell isolation process combining (i) an aqueous two-phase system (ATPS) to create complex separation layers with (ii) a centrifugal microfluidic platform (PowerBlade) with active pneumatic pumping to control and automate the assay. We use a polyethylene glycol (PEG) and dextran (DEX) mixture as the two-phase density gradient media and our automated centrifugal microfluidic platform to fractionate blood samples. Different densities of precisely tuned PEG-DEX solutions were tested to match each of the cell types typically targeted during blood fractionation applications. By employing specially designed microfluidic devices, we demonstrate the automation of the following steps: loading of a whole blood sample on-chip, layering of the blood on the ATPS solution, blood fractionation, precise radial repositioning of the fractionated layers, and finally extraction of multiple, selected fractionated components. Fractionation of up to six distinct layers is shown: platelet-rich plasma, buffy coat, PEG, DEX with neutrophils, red blood cells (RBCs) and high density gradient media (HDGM). Furthermore, through controlled dispensing of HDGM to the fractionation chamber, we show that each of the fractionated layers can be repositioned radially, on-the-fly, without disturbing the interfaces, allowing precise transfer of target fractions and cell types into external vials via a chip-to-world interface. Cell counting analysis and cell viability studies showed equivalence to traditional, manual methods. An overall cell viability greater than 90% of extracted cells demonstrates that the proposed approach is suitable for cell isolation applications. This proof-of-principle demonstration highlights the utility of the proposed system for automated whole blood fractionation and isolation for blood cell applications. We anticipate that the proposed approach will be a useful tool for many clinical applications such as standard cell isolation procedures and other bioanalytical assays (e.g., circulating tumor cells, and cell and gene therapy).

Multifunctional magnetic nanoparticle cloud assemblies for in situ capture of bacteria and isolation of microbial DNA.

We investigate the formation of suspended magnetic nanoparticle (MNP) assemblies (M-clouds) and their use for in situ bacterial capture and DNA extraction. M-clouds are obtained as a result of magnetic field density variations when magnetizing an array of micropillars coated with a soft ferromagnetic NiP layer. Numerical simulations suggest that the gradient in the magnetic field created by the pillars is four orders of magnitude higher than the gradient generated by the external magnets. The pillars therefore serve as the sole magnetic capture sites for MNPs which accumulate on opposite sides of each pillar facing the magnets. Composed of loosely aggregated MNPs, the M-cloud can serve as a porous capture matrix for target analyte flowing through the array. The concept is demonstrated by using a multifunctional M-cloud comprising immunomagnetic NPs (iMNPs) for capture of Escherichia coli O157:H7 from river water along with silica-coated NPs for subsequent isolation and purification of microbial DNA released upon bacterial lysis. Confocal microscopy imaging of fluorescently labeled iMNPs and E. coli O157:H7 reveals that bacteria are trapped in the M-cloud region between micropillars. Quantitative assessment of in situ bacterial capture, lysis and DNA isolation using real-time polymerase chain reaction shows linear correlation between DNA output and input bacteria concentration, making it possible to confirm E. coli 0157:H7 at 103 cells per mL. The M-cloud method further provides one order of magnitude higher DNA output concentrations than incubation of the sample with iMNPs in a tube for an equivalent period of time (e.g., 10 min). Results from assays performed in the presence of Listeria monocytogenes (at 106 cells per mL each) suggest that non-target organisms do not affect on-chip E. coli capture, DNA extraction efficiency and quality of the eluted sample.

Centrifugal microfluidic lab-on-a-chip system with automated sample lysis, DNA amplification and microarray hybridization for identification of enterohemorrhagic Escherichia coli culture isolates.

The development of technology for the rapid, automated identification of bacterial culture isolates can help regulatory agencies to shorten response times in food safety surveillance, compliance, and enforcement as well as outbreak investigations. While molecular methods such as polymerase chain reaction (PCR) enable the identification of microbial organisms with high sensitivity and specificity, they generally rely on sophisticated instrumentation and elaborate workflows for sample preparation with an undesirably high level of hands-on engagement. Herein, we describe the design, operation and performance of a lab-on-a-chip system integrating thermal lysis, PCR amplification and microarray hybridization on the same cartridge. The assay is performed on a centrifugal microfluidic platform that allows for pneumatic actuation of liquids during rotation, making it possible to perform all fluidic operations in a fully-automated fashion without the need for integrating active control elements on the microfluidic cartridge. The cartridge, which is fabricated from hard and soft thermoplastic polymers, is compatible with high-volume manufacturing (e.g., injection molding). Chip design and thermal interface were both optimized to ensure efficient heat transfer and allow for fast thermal cycling during the PCR process. The integrated workflow comprises 14 steps and takes less than 2 h to complete. The only manual steps are related to loading of the sample and reagents on the cartridge as well as fluorescence imaging of the microarray. On-chip lysis and PCR amplification both provided results comparable to those obtained by bench-top instrumentation. The microarray, incorporating a panel of oligonucleotide probes for multiplexed detection of seven enterohemorrhagic E. coli priority serotypes, was implemented on a cyclic olefin copolymer substrate using a novel activation scheme that involves the conversion of hydroxyl groups (derived from oxygen plasma treatment) into reactive cyanate ester using cyanogen bromide. On-chip hybridization was demonstrated in a non-quantitative fashion using fluorescently-labelled gene markers for E. coli O157:H7 (rfbO157, eae, vt1, and vt2) obtained through a multiplexed PCR amplification step.

Buoyancy-driven step emulsification on pneumatic centrifugal microfluidic platforms.

We present here a new method for controlling the droplet size in step emulsification processes on a centrifugal microfluidic platform, which, in addition to the centrifugal force, uses pneumatic actuation for fluid displacement. We highlight the importance of the interplay between buoyancy effects and the flow rate at the step junction, and provide a simple analytical model relating these two quantities to the size of the droplets. Numerical models as well as experiments with water-in-oil emulsions are performed in support of the proposed model.

Polymer Micropillar Arrays for Colorimetric DNA Detection.

We describe the use of periodic micropillar arrays, produced from cyclic olefin copolymer using high-fidelity microfabrication, as templates for colorimetric DNA detection. The assay involves PCR-amplified gene markers for E. coli O157:H7 (rfbO157, eae, vt1, and vt2) incorporating a detectable digoxigenin label, which is revealed through an immunoenzymatic process following hybridization with target-specific oligonucleotide capture probes. The capacity of micropillar arrays to induce wicking is used to distribute and confine capture probes with spatial control, making it possible to achieve a uniform signal while allowing multiple, independent probes to be arranged in close proximity on the same substrate. The kinetic profile of color pigment formation on the surface was followed using absorbance measurements, showing maximum signal increase between 20 and 60 min of reaction time. The relationship between microstructure and colorimetric signal was investigated through variation of geometric parameters, such as pitch (10-50 µm), pillar diameter (5-40 µm), and height (16-48 µm). Our findings suggest that signal intensity is largely influenced by the edges of the pillars and less by their height such that it deviates from a linear relationship when both aspect ratio and pillar density become very high. A theoretical model used to simulate the changes in surface composition at the molecular level suggests that differences in the temporal and spatial accumulation of assay components account for this observation.

Extraction of nucleic acids from blood: unveiling the potential of active pneumatic pumping in centrifugal microfluidics for integration and automation of sample preparation processes.

This paper describes the development of an on-chip nucleic acid (NA) extraction assay from whole blood using a centrifugal microfluidic platform that allows for pneumatic actuation of liquids during rotation. The combination of pneumatic and centrifugal forces makes it possible to perform fluidic operations without the need for integrating active control elements on the microfluidic cartridge. The cartridge is fabricated from thermoplastic polymers (e.g., Zeonor 1060R) and features a simple design that is compatible with injection molding. In addition, the cartridge is interfaced with two external vials for off-chip storage of the blood sample and retrieval of the eluted NA solution, respectively. On-chip capture of NAs is performed using an embedded solid-phase extraction matrix composed of commercial glass microfiber filters (Whatman GF/D and GF/F). The yield of the automated, on-chip extraction protocol, determined by measuring absorbance at 260 nm, is comparable to some of the best manually operated kits (e.g., Qiagen QIAamp DNA Mini Kit) while providing low assay-to-assay variability due to the high level of control provided by the platform for each processing step. The A260/A280 and A260/A230 ratios of the absorbance spectra also reveal that protein contamination of the sample is negligible. The capability of the pneumatic platform to circulate air flux through the microfluidic conduit was used to dry leftover ethanol residues retained in the capture matrix during washing. This method, applied in combination with localized heating, proved effective for reducing ethanol contamination in eluted samples from ∼12% to 1% (v/v).

Microfluidic Integration of a Cloth-Based Hybridization Array System (CHAS) for Rapid, Colorimetric Detection of Enterohemorrhagic Escherichia coli (EHEC) Using an Articulated, Centrifugal Platform.

We describe the translation of a cloth-based hybridization array system (CHAS), a colorimetric DNA detection method that is used by food inspection laboratories for colony screening of pathogenic agents, onto a microfluidic chip format. We also introduce an articulated centrifugal platform with a novel fluid manipulation concept based on changes in the orientation of the chip with respect to the centrifugal force field to time the passage of multiple components required for the process. The platform features two movable and motorized carriers that can be reoriented on demand between 0 and 360° during stage rotation. Articulation of the chip can be used to trigger on-the-fly fluid dispensing through independently addressable siphon structures or to relocate solutions against the centrifugal force field, making them newly accessible for downstream transfer. With the microfluidic CHAS, we achieved significant reduction in the size of the cloth substrate as well as the volume of reagents and wash solutions. Both the chip design and the operational protocol were optimized to perform the entire process in a reliable, fully automated fashion. A demonstration with PCR-amplified genomic DNA confirms on-chip detection and identification of Escherichia coli O157:H7 from colony isolates in a colorimetric multiplex assay using rfbO157, fliCH7, vt1, and vt2 genes.

Twin tubular pinch effect in curving confined flows.

Colloidal suspensions of buoyancy neutral particles flowing in circular pipes focus into narrow distributions near the wall due to lateral migration effects associated with fluid inertia. In curving flows, these distributions are altered by Dean currents and the interplay between Reynolds and Dean numbers is used to predict equilibrium positions. Here, we propose a new description of inertial lateral migration in curving flows that expands current understanding of both focusing dynamics and equilibrium distributions. We find that at low Reynolds numbers, the ratio δ between lateral inertial migration and Dean forces scales simply with the particle radius, coil curvature and pipe radius as (Rp(3)R)/a(4). A critical value δc = 0.148 of this parameter is identified along with two related inertial focusing mechanisms. In the regime below δc, coined subcritical, Dean forces generate permanently circulating, twinned annuli, each with intricate equilibrium particle distributions including eyes and trailing arms. At δ > δc (supercritical regime) inertial lateral migration forces are dominant and particles focus to a single stable equilibrium position.

Active pneumatic control of centrifugal microfluidic flows for lab-on-a-chip applications.

This paper reports a novel method of controlling liquid motion on a centrifugal microfluidic platform based on the integration of a regulated pressure pump and a programmable electromechanical valving system. We demonstrate accurate control over the displacement of liquids within the system by pressurizing simultaneously multiple ports of the microfluidic device while the platform is rotating at high speed. Compared to classical centrifugal microfluidic platforms where liquids are solely driven by centrifugal and capillary forces, the method presented herein adds a new degree of freedom for fluidic manipulation, which represents a paradigm change in centrifugal microfluidics. We first demonstrate how various core microfluidic functions such as valving, switching, and reverse pumping (i.e., against the centrifugal field) can be easily achieved by programming the pressures applied at dedicated access ports of the microfluidic device. We then show, for the first time, that the combination of centrifugal force and active pneumatic pumping offers the possibility of mixing fluids rapidly (~0.1 s) and efficiently based on the creation of air bubbles at the bottom of a microfluidic reservoir. Finally, the suitability of the developed platform for performing complex bioanalytical assays in an automated fashion is demonstrated in a DNA harvesting experiment where recovery rates of about 70% were systematically achieved. The proposed concept offers the interesting prospect to decouple basic microfluidic functions from specific material properties, channel dimensions and fabrication tolerances, surface treatments, or on-chip active components, thus promoting integration of complex assays on simple and low-cost microfluidic cartridges.

Enhancing the Detection of Giardia duodenalis Cysts in Foods by Inertial Microfluidic Separation.

The sensitivity and specificity of current Giardia cyst detection methods for foods are largely determined by the effectiveness of the elution, separation, and concentration methods used. The aim of these methods is to produce a final suspension with an adequate concentration of Giardia cysts for detection and a low concentration of interfering food debris. In the present study, a microfluidic device, which makes use of inertial separation, was designed and fabricated for the separation of Giardia cysts. A cyclical pumping platform and protocol was developed to concentrate 10-ml suspensions down to less than 1 ml. Tests involving Giardia duodenalis cysts and 1.90-µm microbeads in pure suspensions demonstrated the specificity of the microfluidic chip for cysts over smaller nonspecific particles. As the suspension cycled through the chip, a large number of beads were removed (70%) and the majority of the cysts were concentrated (82%). Subsequently, the microfluidic inertial separation chip was integrated into a method for the detection of G. duodenalis cysts from lettuce samples. The method greatly reduced the concentration of background debris in the final suspensions (10-fold reduction) in comparison to that obtained by a conventional method. The method also recovered an average of 68.4% of cysts from 25-g lettuce samples and had a limit of detection (LOD) of 38 cysts. While the recovery of cysts by inertial separation was slightly lower, and the LOD slightly higher, than with the conventional method, the sample analysis time was greatly reduced, as there were far fewer background food particles interfering with the detection of cysts by immunofluorescence microscopy.

Microfluidic filtration and extraction of pathogens from food samples by hydrodynamic focusing and inertial lateral migration.

Detecting pathogenic bacteria in food or other biological samples with lab-on-a-chip (LOC) devices requires several sample preparation steps prior to analysis which commonly involves cleaning complex sample matrices of large debris. This often underestimated step is important to prevent these larger particles from clogging devices and to preserve initial concentrations when LOC techniques are used to concentrate or isolate smaller target microorganisms for downstream analysis. In this context, we developed a novel microfluidic system for membrane-free cleaning of biological samples from debris particles by combining hydrodynamic focusing and inertial lateral migration effects. The microfluidic device is fabricated using thermoplastic elastomers being compatible with thermoforming fabrication techniques leading to low-cost single-use devices. Microfluidic chip design and pumping protocols are optimized by investigating diffusive losses numerically with coupled Navier-Stokes and convective-diffusion theoretical models. Stability of inertial lateral migration and separation of debris is assessed through fluorescence microscopy measurements with labelled particles serving as a model system. Efficiency of debris cleaning is experimentally investigated by monitoring microchip outlets with in situ optical turbidity sensors, while retention of targeted pathogens (i.e., Listeria monocytogenes) within the sample stream is assessed through bacterial culture techniques. Optimized pumping protocols can remove up to 50 % of debris from ground beef samples while percentage for preserved microorganisms can account for 95 % in relatively clean samples. However, comparison between inoculated turbid and clean samples (i.e., with and without ground beef debris) indicate some degree of interference between debris inertial lateral migration and hydrodynamic focusing of small microorganisms. Although this interference can lead to significant decrease in chip performance through loss of target bacteria, it remains possible to reach 70 % for sample recovery and more than 50 % for debris removal even in the most turbid samples tested. Due to the relatively simple design, the robustness of the inertial migration effect itself, the high operational flow rates and fabrication methods that leverage low-cost materials, the proposed device can have an impact on a wide range of applications where high-throughput separation of particles and biological species is of interest.

From cellular lysis to microarray detection, an integrated thermoplastic elastomer (TPE) point of care Lab on a Disc.

We present an all-thermoplastic integrated sample-to-answer centrifugal microfluidic Lab-on-Disc system (LoD) for nucleic acid analysis. The proposed CD system and engineered platform were employed for analysis of Bacillus atrophaeus subsp. globigii spores. The complete assay comprised cellular lysis, polymerase chain reaction (PCR) amplification, amplicon digestion, and microarray hybridization on a plastic support. The fluidic robustness and operating efficiency of the assay were ensured through analytical optimization of microfluidic tools enabling beneficial implementation of capillary valves and accurate control of all flow timing procedures. The assay reliability was further improved through the development of two novel microfluidic strategies for reagents mixing and flow delay on the CD platform. In order to bridge the gap between the proof-of-concept LoD and production prototype demonstration, low-cost thermoplastic elastomer (TPE) was selected as the material for CD fabrication and assembly, allowing the use of both, high quality hot-embossing and injection molding processes. Additionally, the low-temperature and pressure-free assembly and bonding properties of TPE material offer a pertinent solution for simple and efficient loading and storage of reagents and other on-board components. This feature was demonstrated through integration and conditioning of microbeads, magnetic discs, dried DNA buffer reagents and spotted DNA array inserts. Furthermore, all microfluidic functions and plastic parts were designed according to the current injection mold-making knowledge for industrialization purposes. Therefore, the current work highlights a seamless strategy that promotes a feasible path for the transfer from prototype toward realistic industrialization. This work aims to establish the full potential for TPE-based centrifugal system as a mainstream microfluidic diagnostic platform for clinical diagnosis, water and food safety, and other molecular diagnostic applications.

Integrated air stream micromixer for performing bioanalytical assays on a plastic chip.

This paper describes the design, functioning and use of an integrated mixer that relies on air flux to agitate microliter entities of fluid in an embedded microfluidic cavity. The system was fabricated from multiple layers of a thermoplastic elastomer and features circuits for both liquid and air supply along with pneumatic valves for process control. Internally-dyed polymer particles have been used to visualize flow within the fluid phase during agitation. Numerical modelling of the micromixer revealed an overall efficacy of 10(-1) to 10(-2) for momentum transfer at the air-water interface. Simulation of air vortex dynamics showed dependency of the flow pattern on the velocity of the flux entering the cavity. Three bioanalytical assays have been performed as proof-of-concept demonstrations. In a first assay, cells of Listeria monocytogenes were combined with magnetic nanoparticles (NPs), resulting in high-density coverage of the bacteria's surface with NPs after 1 min of agitation. This finding is contrasted by a control experiment without agitation for which interaction between bacteria and NPs remains low. In a second one, capture and release of genomic DNA from fungi through adsorption onto magnetic beads was tested and shown to be improved by agitation compared to non-agitated controls. A third assay finally involved fluorescently-labelled target oligonucleotide strands and polystyrene particles modified with DNA capture probes to perform detection of nucleic acids on beads. Excellent selectivity was obtained in a competitive hybridization process using a multiplexed micromixer chip design.

Thermo-active elastomer composite for optical heating in microfluidic systems.

Single-walled carbon nanotubes are used as doping agents to form thermo-active composites with an elastomeric block-copolymer. Thermal imaging reveals that the temperature response upon irradiation with NIR laser light is dependent (among other things) on the mass fraction of the nanotubes in the polymer matrix.

All-thermoplastic nanoplasmonic microfluidic device for transmission SPR biosensing.

Early and accurate disease diagnosis still remains a major challenge in clinical settings. Biomarkers could potentially provide useful tools for the detection and monitoring of disease progression, treatment safety and efficacy. Recent years have witnessed prodigious advancement in biosensor development with research directed towards rapid, real-time, label-free and sensitive biomarker detection. Among emerging techniques, nanoplasmonic biosensors pose tremendous potential to accelerate clinical diagnosis with real-time multiplexed analysis, rapid and miniaturized assays, low sample consumption and high sensitivity. In order to translate these technologies from the proof-of-principle concept level to point of care clinical diagnosis, integrated, portable devices having small footprint cartridges that house low-cost disposable consumables are sought. Towards this goal, we developed an all-polymeric nanoplasmonic microfluidic (NMF) transmission surface plasmon resonance (SPR) biosensor. The device was fabricated in thermoplastics using a simple, single step and cost-effective hot embossing technique amenable to mass production. The novel 3D hierarchical mold fabrication process enabled monolithic integration of blazed nanogratings within the detection chambers of a multichannel microfluidic system. Consequently, a single hard thermoplastic bottom substrate comprising plasmonic and fluidic features allowed integration of active fluidic elements, such as pneumatic valves, in the top soft thermoplastic cover, increasing device functionality. A simple and compact transmission-based optical setup was employed with multiplexed end-point or dual-channel kinetic detection capability which did not require stringent angular accuracy. The sensitivity, specificity and reproducibility of the transmission SPR biosensor was demonstrated through label-free immunodetection of soluble cell-surface glycoprotein sCD44 at clinically relevant picomolar to nanomolar concentrations.

3D thermoplastic elastomer microfluidic devices for biological probe immobilization.

Microfluidics has emerged as a valuable tool for the high-resolution patterning of biological probes on solid supports. Yet, its widespread adoption as a universal biological immobilization tool is still limited by several technical challenges, particularly for the patterning of isolated spots using three-dimensional (3D) channel networks. A key limitation arises from the difficulties to adapt the techniques and materials typically used in prototyping to low-cost mass-production. In this paper, we present the fabrication of thin thermoplastic elastomer membranes with microscopic through-holes using a hot-embossing process that is compatible with high-throughput manufacturing. The membranes provide the basis for the fabrication of highly integrated 3D microfluidic devices with a footprint of only 1 × 1 cm(2). When placed on a solid support, the device allows for the immobilization of up to 96 different probes in the form of a 10 × 10 array comprising isolated spots of 50 × 50 µm(2). The design of the channel network is optimized using 3D simulations based on the Lattice-Boltzmann method to promote capillary action as the sole force distributing the liquid in the device. Finally, we demonstrate the patterning of DNA and protein arrays on hard thermoplastic substrates yielding spots of excellent definition that prove to be highly specific in subsequent hybridization experiments.

Multiple surface plasmon resonances and near-infrared field enhancement of gold nanowells.

Arrays of Au nanowells (NWs) were fabricated by electron-beam lithography (EBL) and characterized by surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS). It is revealed that these Au NW arrays exhibit multiple SP resonances that can be tuned by adjusting the geometrical characteristics of the NWs. SERS activity of Au NWs was confirmed for a range of excitation wavelengths and a number of model compounds including rhodamine 6G (R6G), phthalazine, and single-stranded oligonucleotides. According to numerical simulations based on the discrete dipole approximation (DDA), SERS enhancement originates from high electromagnetic fields (hot spots) localized both inside and outside individual NWs. In addition, far-field intercoupling effects between NWs have been observed experimentally in arrays with subwavelength pitch sizes. We show that the SERS enhancement factors can also be tuned and optimized by adjusting the geometry of NWs.

Surface enhanced Raman scattering on long-range ordered noble-metal nanocrescent arrays.

Long-range ordered noble-metal nanocrescent arrays of different sizes and shapes have been successfully fabricated by using both nanoimprint lithography and e-beam lithography techniques. Large surface enhanced Raman scattering (SERS) enhancements in the detection of rhodamine 6G (R6G) molecules on these arrays have been observed and attributed to the enhancement of the local electromagnetic (EM) fields near individual nanocrescents. Electromagnetic enhancement factors for crescents of different shapes are computed using the discrete dipole approximation and compared with experimental measurements of the R6G Raman intensities. It is found that the maximum values of SERS intensity appear at an intermediate value of the crescent eccentricity and the observed behaviour is related to the spatial distributions of the enhancement of the local EM field (hot spots).