Impedimetric graphene-based biosensors for the detection of polybrominated diphenyl ethers.

Single-layer graphene, decorated with Au nanoparticles, and a specially designed peptide are used for the first time in the detection of decabrominediphenyl ether using impedance spectroscopy. Biosensor calibration is presented, showing a good linear response from 5% to saturated dissolutions (100 ppt). Selectivity towards brominated species is demonstrated by lack of response to molecules with similar structures but without any bromines.

Semiconductor nanowires directly grown on graphene--towards wafer scale transferable nanowire arrays with improved electrical contact.

We present for the first time the growth of dense arrays of silicon and silicon carbide nanowires directly on graphene as well as methods of transferring these novel hybrids to arbitrary substrates. Improved electrical contact for SiC nanowire/graphene hybrid is demonstrated in the application of a robust supercapacitor electrode.

Gold-coated silver dendrites as SERS substrates with an improved lifetime.

Nanostructured silver is known to yield the highest signal-enhancement factors in surface-enhanced Raman spectroscopy, but its low chemical stability toward oxidation presents a challenge in the realization of Ag-based SERS substrates with long operating lifetimes. Here, a study of the long-term stability of silver dendrites as SERS substrates is reported. SERS spectra of 1,2-benzenedithiol monolayers on Ag dendrites, acquired over a period of time in excess of 1 year, shows appreciable degradation with time. However, no degradation is observed in the spectra of monolayers deposited on Ag dendrites that were coated with a monolayer-thin Au film deposited by an immersion plating process. X-ray photoelectron spectra confirm the oxidation of the uncoated Ag dendrites whereas no chemical changes are detected in the Au-coated ones. These results suggest that the galvanic displacement of Au on preformed Ag nanostructures provides a suitable route to producing SERS-active substrates with long operating and/or shelf lifetimes.

Single crystal silicon nanopillars, nanoneedles and nanoblades with precise positioning for massively parallel nanoscale device integration.

Arrays of precisely positioned single crystal silicon nanopillars, nanoneedles, and nanoblades with minimum feature sizes as small as 30 nm are fabricated using entirely scalable top-down fabrication techniques. Using the same scalable technologies, devices consisting of electrically connected silicon nanopillars with multiple addressable electrodes for each nanostructure are realized. The arrays of nanopillars, nanoneedles, and nanoblades are shown to exhibit Raman signal enhancement on 1,2-benzenedithiol monolayers, opening a path to nanodevices that manipulate, position, detect and analyze molecules.

Single-layer CVD-grown graphene decorated with metal nanoparticles as a promising biosensing platform.

A new approach to the development of a single-layer graphene sensor decorated with metal nanoparticles is presented. Chemical vapor deposition is used to grow single layer graphene on copper. Decoration of the single-layer graphene is achieved by electroless deposition of Au nanoparticles using the copper substrate as a source of electrons. Transfer of the decorated single-layer graphene on glassy carbon electrodes offers a sensitive platform for biosensor development. As a proof of concept, 10 units of glucose oxidase were deposited on the surface in a Nafion matrix to stabilize the enzyme as well as to prevent interference from ascorbic acid and uric acid. Amperometric linear response calibration in the µmoll(-1) is obtained. The presented methodology enables highly sensitive platforms for biosensor development, providing a scalable roll-to-roll production with a much more reproducible scheme when compared to the graphene biosensors reported previously based on drop-cast of multi-layer graphene suspensions.

Graphene decoration with metal nanoparticles: towards easy integration for sensing applications.

A simple and versatile method for the decoration of CVD grown graphene with metal nanoparticles is presented. The mechanism of nanoparticle formation is galvanic displacement resulting in physically adsorbed clusters. The single layer graphene obtained by this method can be easily transferred. Integration onto a gas sensing transducer is presented as proof of concept.

Ultrasmooth gold thin films by self-limiting galvanic displacement on silicon.

Galvanic displacement (GD), a type of electroless deposition, has been used to obtain ultrasmooth gold thin films on silicon <111>. The novel aspect of the method presented herein is the absence of fluoride ions in the liquid phase, and its principal advantage when compared to previous efforts is that the process is inherently self-limiting. The self-limiting factor is due to the fact that in the absence of fluorinated species, no silicon oxide is removed during the process. Thus, the maximum gold film thickness is achieved when elemental silicon is no longer available once the surface is oxidized completely during the galvanic displacement process. X-ray photoelectron spectroscopy has been used as a tool for thickness measurement, using the gold to silicon ratio as an analytical signal. Three gold plating solutions with different concentrations of KAuCl4 (2, 0.2, and 0.02 mM) have been used to obtain information about the formation rate of the gold film. This XPS analysis demonstrates the formation of gold films to a maximum thickness of ∼3.5 Å. Atomic force microscopy is used to confirm surface smoothness, suggesting that the monolayer growth does not follow the Volmer-Weber growth mode, in contrast to the GD process from aqueous conditions with fluorinated species.

Silver dendrites from galvanic displacement on commercial aluminum foil as an effective SERS substrate.

A silver galvanic displacement process on commercial aluminum foil has been carried out to produce cost-effective SERS substrates. The process is based on an extremely simple redox process where aluminum is oxidized while silver ions are reduced, yielding a final silver dendritic structure that offers a large surface area-to-volume ratio. XPS measurements confirmed the metallic nature of the formed dendrites. SERS substrates were fabricated by spreading of the dendrites on double side Scotch tape attached to a paper slide. Three different thiols were incubated to achieve SAM formation on the Ag dendrites and measured by Raman spectroscopy. The obtained spectra presented well resolved bands and provided valuable information regarding the orientation of the thiols. The high Raman intensity also demonstrates the high enhancement capacities of the produced silver structures. The overall method is cost-effective and allows the use of silver dendrite paste for the mass production of SERS-active substrates, including on flexible substrates and/or via screen printing.

Long-chain alkylthiol assemblies containing buried in-plane stabilizing architectures.

A series of alkylthiol compounds were synthesized to study the formation and structure of complex self-assembled monolayers (SAMs) consisting of interchanging structural modules stabilized by intermolecular hydrogen bonds. The chemical structure of the synthesized compounds, HS(CH(2))(15)CONH(CH(2)CH(2)O)(6)CH(2)CONH-X, where X refers to the extended chains of either -(CH(2))(n)CH(3) or -(CD(2))(n)CD(3), with n = 0, 1, 7, 8, 15, was confirmed by NMR and elemental analysis. The formation of highly ordered, methyl-terminated SAMs on gold from diluted ethanolic solutions of these compounds was revealed using contact angle goniometry, null ellipsometry, cyclic voltammetry, and infrared reflection absorption spectroscopy. The experimental work was complemented with extensive DFT modeling of infrared spectra and molecular orientation. New assignments were introduced for both nondeuterated and deuterated compounds. The latter set of compounds also served as a convenient tool to resolve the packing, conformation, and orientation of the buried and extended modules within the SAM. Thus, it was shown that the lower alkyl portion together with the hexa(ethylene glycol) portion is stabilized by the two layers of lateral hydrogen bonding networks between the amide groups, and they provide a structurally robust support for the extended alkyls. The presented system can be considered to be an extension of the well-known alkyl SAM platform, enabling precise engineering of nanoscopic architectures on the length scale from a few to approximately 60 A for applications such as cell membrane mimetics, molecular nanolithography, and so forth.

Silver nanodesert rose as a substrate for surface-enhanced Raman spectroscopy.

Silver galvanic displacement on silicon has been employed to produce large-area reproducible substrates, with morphology similar to that of the natural desert rose but on the micrometer scale. The process is based on an extremely simple wet chemistry approach using only AgF and KF, as silver and fluoride sources. A key element is the absence of HF in the deposition solution, which has been commonly used in previous silver galvanic displacement processes. The new process affords a higher degree of control in the redox reaction than those reported previously. The structures formed in this manner possess a large area-to-volume ratio with a high density of rough silver flakes uniformly distributed across the substrate. The silver morphology on the nanometer scale is shown to provide an excellent platform for surface-enhanced Raman spectroscopy (SERS), yielding detection levels for trans-1,2-bis(4-pyridyl)ethylene, 4-mercaptopyridine, and Rhodamine 6G in solution down to ppb, ppt, and ppq limits, respectively. The SERS reproducibility on the substrate was verified by monitoring the signal intensity variations across the sample. The simplicity of the substrate fabrication process, as well as the excellent uniformity, opens up opportunities for the quantitative and in-field chemical trace analysis using these substrates.

Wavelet neural networks to resolve the overlapping signal in the voltammetric determination of phenolic compounds.

Three phenolic compounds, i.e. phenol, catechol and 4-acetamidophenol, were simultaneously determined by voltammetric detection of its oxidation reaction at the surface of an epoxy-graphite transducer. Because of strong signal overlapping, Wavelet Neural Networks (WNN) were used in data treatment, in a combination of chemometrics and electrochemical sensors, already known as the electronic tongue concept. To facilitate calibration, a set of samples (concentration of each phenol ranging from 0.25 to 2.5mM) was prepared automatically by employing a Sequential Injection System. Phenolic compounds could be resolved with good prediction ability, showing correlation coefficients greater than 0.929 when the obtained values were compared with those expected for a set of samples not employed for training.