A Au particle-on-wire system that can be used as a specific, sensitive, and multiplex DNA sensor is developed. A pattern formed by multiple Au nanowire sensors provides positional address and identification for each sensor. By using this system, multiplex sensing of target DNAs was possible in a quantitative manner with a detection limit of 10 pM. Target DNAs from reference bacteria and clinical isolates were successfully identified by this sensor system, enabling diagnostics for infectious diseases.KEYWORDS DNA, multiplex, pathogen, pattern, surface-enhanced Raman scattering M ultiplex, sensitive, and specific DNA detection is of great demand for various biological and biomedical studies including gene profiling, drug screening, and clinical diagnostics because it has a potential to provide the most information from a small sample volume at low cost.1 In this regard, simple, reliable, and highthroughput methods that allow detection of multiple DNAs in one assay have been developed by taking various sensing approaches such as the measurement of fluorescence, [1][2][3] surface plasmon resonance (SPR), 4 electric signals, 5 and mass changes.6 Among these DNA sensing methods, fluorescence-based assay is currently the most preferred technique for multiplex DNA detection.7 Surface-enhanced Raman scattering (SERS) has also been considered as an attractive method for label-free multiplex DNA detection because of its single molecule level sensitivity, 8-10 molecular specificity, 11 and insensitivity to quenching. 7,12 These distinct advantages have led to the development of a number of ingenious SERS sensing platforms. [13][14][15][16] However, achieving optimum reproducibility of SERS signals and detecting various target molecules with a very small sample volume in one assay still remain as challenging tasks for practical multiplex SERS sensors. It has been shown that hot spots at nanoscale gaps between a nanowire (NW) and nanoparticles (NPs) can become highly SERS-active. [17][18][19][20][21][22] We have recently reported a new biomolecule detection method that provides reproducible SERS signals by using nanoscale gaps between Au NW and NPs. 23Here we present a multiplex DNA detection method employing multiple Au particle-on-wire systems as a SERS sensing platform. The system operates by the self-assembly of Au NPs onto Au NW in the presence of target DNAs, providing reproducible SERS signals in proportion to the concentrations of target DNAs. Multiple pathogen DNAs could be successfully detected by employing this method, demonstrating that this multiplex SERS sensor can be used a convenient system for clinical diagnostic and biomolecular interaction studies.Raman signal can be dramatically enhanced by placing the signal carrying molecules in the interstices between the assembled nanostructures. [24][25][26][27][28][29] To fabricate a SERS-active nanostructure that can be turned on by biomolecular binding, we adopted an Au particle-on-wire structure constructed by self-assembly of Au NPs onto Au NW through DN...

Fabricating well-defined and highly reproducible platforms for surface-enhanced Raman scattering (SERS) is very important in developing practical SERS sensors. We report a novel SERS platform composed of a single metallic nanowire (NW) on a metallic film. Optical excitation of this novel sandwich nanostructure provides a line of SERS hot spots (a SERS hot line) at the gap between the NW and the film. This single nanowire on a film (SNOF) architecture can be easily fabricated, and the position of hot spots can be conveniently located in situ by using an optical microscope during the SERS measurement. We show that high-quality SERS spectra from benzenethiol, brilliant cresyl blue, and single-stranded DNA can be obtained on a SNOF with reliable reproducibility, good time stability, and excellent sensitivity, and thus, SNOFs can potentially be employed as effective SERS sensors for label-free biomolecule detection. We also report detailed studies of polarization- and material-dependent SERS enhancement of the SNOF structure.

We report vapor-phase synthesis of single-crystalline freestanding Ag nanowires (NWs) and polarized surface-enhanced Raman scattering (SERS) of a single NW. To the best of our knowledge, vapor-phase synthesis of single-crystalline Ag NWs and polarized SERS on an individual NW have not yet been reported.Silver has the highest electrical and thermal conductivity among all metals. In addition, because of the nature of its optical constants, silver shows the most effective SERS among all metals in the visible region. 1 Since the intensity of the SERS signal depends strongly on the detailed morphology of the Ag nanostructures, it is most important to have a well-defined and well-characterized system with a clean surface for the preparation of reliable sensors based on SERS detection. Current fabrication processes for the production of SERSactive substrates, however, are often irreproducible to a certain degree, thereby leading to inconsistent optical properties and, thus, significantly fluctuating enhancement factors for substrates prepared by apparently the same procedure. Irreproducible SERS enhancement would be detrimental to quantitative and reliable sensing, which would be most critical for medical diagnosis.As a first step toward fabrication of well-controlled nanobiosensors employing SERS technique, we searched for a method to synthesize well-characterized free-standing single-crystalline Ag NWs with a clean surface. This led us to the adoption of vaporphase synthesis rather than solution phase. While vapor-phase synthesis is one of the most widely used methods for the synthesis of 1D nanostructures, it has been used mainly for the synthesis of semiconductor NWs and nanotubes. Only a few metallic NWs have been synthesized through vapor phase, 2 and most of the reported methods for the synthesis of Ag NWs are wet chemical methods involving templates, surfactants, or capping agents. 3 Our synthetic method is unique in that it uses only a single reactant, Ag 2 O, without using any templates or catalysts. In a typical synthesis, 0.2 g of Ag 2 O powder was placed in an alumina boat in the middle of a 1 in. diameter horizontal quartz tube furnace. The NWs were grown at a few centimeters downstream from the precursor on a Si substrate. At high temperature (T 1 ) 900-1000°C ), the precursor vapor was carried downstream by the flow of 500 sccm of Ar at 5-10 Torr to a lower temperature zone (T 2 ) 500°C), where Ag NWs were grown. The noncatalytic growth of the NWs and the preferred formation of large particles over NWs at higher precursor temperatures demonstrated that the Ag NWs were formed by a vapor-solid mechanism. 2a A representative SEM image in Figure 1a shows good density of straight NWs tens of micrometers long on a Si substrate. The insets are SEM and TEM images of Ag NWs, showing that the tips of NWs are round, and NWs have clean surfaces and diameters of 80-150 nm.The XRD patterns of as-grown NW ensembles (Supporting Information Figure S1) are indexed perfectly to the face-centered cubic (fcc) crystal str...