Direct Laser Writing of Chitosan-Borax Composites: Toward Sustainable Electrochemical Sensors.

In this study, the laser-induced graphitization process of sustainable chitosan-based formulations was investigated. In particular, optimal lasing conditions were investigated alongside the effect of borax concentration in the chitosan matrix. In all cases, it was found that the obtained formulations were graphitizable with a CO2 laser. This process gave rise to the formation of high surface area, porous, and electrically conductive laser-induced graphene (LIG) structures. It was found that borax, as a cross-linker of chitosan, enabled the graphitization process when its content was ≥30 wt % in the chitosan matrix, allowing the formation of an LIG phase with a significant content of graphite-like structures. The graphitization process was investigated by thermogravimetric analysis (TGA), Raman, X-ray photoemission (XPS), and Fourier transform infrared (FTIR) spectroscopies. LIG electrodes obtained from CS/40B formulations displayed a sheet resistance as low as 110 Ω/sq. Electrochemical characterization was performed after a 10 min electrode activation by cycling in 1 M KCl. A heterogeneous electron transfer rate, k0, of 4 × 10-3 cm s-1 was determined, indicating rapid electron transfer rates at the electrode surface. These results show promise for the introduction of a new class of sustainable composites for LIG electrochemical sensing platforms.

Silver nanoparticles - laser induced graphene (Ag NPs - LIG) hybrid electrodes for sensitive electrochemical-surface enhanced Raman spectroscopy (EC-SERS) detection.

This paper presents a novel approach for the fabrication of low cost Electrochemical-Surface Enhanced Raman Scattering (EC-SERS) sensing platforms. Laser Induced Graphene (LIG) electrodes were readily fabricated by direct laser writing of polyimide tapes and functionalized with silver nanoparticles (Ag NPs) to obtain hybrid Ag NPs - LIG electrodes suitable for EC-SERS analysis. Detection was achieved by coupling a handheld potentiostat with a Raman spectrograph, enabling measurement of SERS spectra of target analytes generated during voltage sweeps in the 0.0 to -1.0 V interval range. The sensing capabilities of the fabricated system were first tested with model molecule 4-aminobenzenethiol (4-ABT). Following sensitive detection of 4-ABT, EC-SERS analysis of food contaminant melamine in milk and antibiotic difloxacin hydrochloride (DIF) in river water was demonstrated, achieving sensitive detection of both analytes without pre-treatment steps. The easiness of fabrication, versatility of design, rapid analysis time and potential miniaturization of the system make Ag NPs - LIG electrodes suitable for a large range of in situ applications in the field of food monitoring and for environmental analysis.

Highly Sensitive and Ultra-Responsive Humidity Sensors Based on Graphene Oxide Active Layers and High Surface Area Laser-Induced Graphene Electrodes.

Ultra-sensitive and responsive humidity sensors were fabricated by deposition of graphene oxide (GO) on laser-induced graphene (LIG) electrodes fabricated by a low-cost visible laser scribing tool. The effects of GO layer thickness and electrode geometry were investigated. Sensors comprising 0.33 mg/mL GO drop-deposited on spiral LIG electrodes exhibited high sensitivity up to 1800 pF/% RH at 22 °C, which is higher than previously reported LIG/GO sensors. The high performance was ascribed to the high density of the hydroxyl groups of GO, promoted by post-synthesis sonication treatment, resulting in high water physisorption rates. As a result, the sensors also displayed good stability and short response/recovery times across a wide tested range of 0-97% RH. The fabricated sensors were benchmarked against commercial humidity sensors and displayed comparable performance and stability. Finally, the sensors were integrated with a near-field communication tag to function as a wireless, battery-less humidity sensor platform for easy read-out of environmental humidity values using smartphones.

Direct-write formation of integrated bottom contacts to laser-induced graphene-like carbon.

We report a simple, scalable two-step method for direct-write laser fabrication of 3D, porous graphene-like carbon electrodes from polyimide films with integrated contact plugs to underlying metal layers (Au or Ni). Irradiation at high average CO2laser power (30 W) and low scan speed (∼18 mm s)-1leads to formation of 'keyhole' contact plugs through local ablation of polyimide (initial thickness 17µm) and graphitization of the plug perimeter wall. Top-surface laser-induced graphene (LIG) electrodes are then formed and connected to the plug by raster patterning at lower laser power (3.7 W) and higher scan speed (200 mm s)-1. Sheet resistance data (71 ± 15 Ω sq.)-1indicates formation of high-quality surface LIG, consistent with Raman data which yield sharp first- and second-order peaks. We have also demonstrated that high-quality LIG requires a minimum initial polyimide thickness. Capacitance data measured between surface LIG electrodes and the buried metal film indicate a polyimide layer of thickness ∼7µm remaining following laser processing. By contrast, laser graphitization of polyimide of initial thickness ∼8µm yielded devices with large sheet resistance (>1 kΩ sq.)-1. Raman data also indicated significant disorder. Plug contact resistance values were calculated from analysis of transfer line measurement data for single- and multi-plug test structures. Contacts to buried nickel layers yielded lower plug resistances (1-plug: 158 ± 7 Ω , 4-plug: 31 ± 14 Ω) compared to contacts to buried gold (1-plug: 346 ± 37 Ω , 4-plug: 52 ± 3 Ω). Further reductions are expected for multi-plug structures with increased areal density. Proof-of-concept mm-scale LIG electrochemical devices with local contact plugs yielded rapid electron transfer kinetics (rate constantk0 âˆ¼ 0.017 cm s-1), comparable to values measured for exposed Au films (k0 âˆ¼0.023 cm s)-1. Our results highlight the potential for integration of LIG-based sensor electrodes with semiconductor or roll-to-roll manufacturing.

Design of Experiments and Optimization of Laser-Induced Graphene.

Realization of graphene-based sensors and electronic devices remains challenging, in part due to integration challenges with current fabrication and manufacturing processes. Thus, scalable methods for in situ fabrication of high-quality graphene-like materials are essential. Low-cost CO2 laser engravers can be used for site-selective conversion of polyimide under ambient conditions to create 3-D, rotationally disordered, few-layer, porous, graphene-like electrodes. However, the influences of non-linear parameter terms and interactions between key parameters on the graphitization process present challenges for rapid, resource-efficient optimization. An iterative optimization strategy was developed to identify promising regions in parameter space for two key parameters, laser power and scan speed, with the goal of optimizing electrode performance while maximizing scan speed and hence fabrication throughput. The strategy employed iterations of Design of Experiments Response Surface (DoE-RS) methods combined with choices of readily measurable parameters to minimize measurement resources and time. The initial DoE-RS experiment set employed visual response parameters, while subsequent iterations used sheet resistance as the optimization parameter. The final model clearly demonstrates that laser graphitization through raster scanning is a highly non-linear process requiring polynomial terms in scan speed and laser power up to fifth order. Two regions of interest in parameter space were identified using this strategy: Region 1 represents the global minimum for sheet resistance for this laser (∼16 Ω/sq), found at a low scan speed (70 mm/s) and a low average power (2.1 W) . Region 2 is a local minimum for sheet resistance (36 Ω/sq), found at higher values for scan speed (340 mm/s) and average power (3.4 W), allowing ∼5-fold reduction in write time. Importantly, these minima do not correspond to constant ratios of average laser power to scan speed. This highlights the benefits of DoE-RS methods in rapid identification of optimum parameter combinations that would be difficult to discover using traditional one-factor-at-a-time optimization. Verification data from Raman spectroscopy showed sharp 2D peaks with mean full-width-at-half-maximum intensity values <80 cm-1 for both regions, consistent with high-quality 3D graphene-like carbon. Graphene-based electrodes fabricated using the parameters from the respective regions yielded similar performance when employed as capacitive humidity sensors with hygroscopic dielectric layers. Devices fabricated using Region 1 parameters (16 Ω/sq) yielded capacitance responses of 0.78 ± 0.04 pF at 0% relative humidity (RH), increasing to 31 ± 7 pF at 85.1% RH. Region 2 devices (36 Ω/sq) showed comparable responses (0.88 ± 0.04 pF at 0% RH, 28 ± 5 pF at 85.1% RH).

Fabrication and Electrochemical Properties of Three-Dimensional (3D) Porous Graphitic and Graphenelike Electrodes Obtained by Low-Cost Direct Laser Writing Methods.

The development of three-dimensional (3D) porous graphitic structures is of great interest for electrochemical sensing applications as they can support fast charge transfer and mass transport through their extended, large surface area networks. In this work, we present the facile fabrication of conductive and porous graphitic electrodes by direct laser writing techniques. Irradiation of commercial polyimide sheets (Kapton tape) was performed using a low-cost laser engraving machine with visible excitation wavelength (405 nm) at low power (500 mW), leading to formation of 3D laser-induced graphene (LIG) structures. Systematic correlation between applied laser dwell time per pixel ("dwell time") and morphological/structural properties of fabricated electrodes showed that conductive and highly 3D porous structures with spectral signatures of nanocrystalline graphitic carbon materials were obtained at laser dwell times between 20 and 110 ms/pix, with graphenelike carbon produced at 50 ms/pix dwell time, with comparable properties to LIG obtained with high cost CO2 lasers. Electrochemical characterization with inner and outer sphere mediators showed fast electron transfer rates, comparable to previously reported 2D/3D graphene-based materials and other graphitic carbon electrodes. This work opens the way to the facile fabrication of low-cost, disposable electrochemical sensor platforms for decentralized assays.

Flexible and transparent Surface Enhanced Raman Scattering (SERS)-Active Ag NPs/PDMS composites for in-situ detection of food contaminants.

The fabrication of flexible and transparent Surface Enhanced Raman Scattering (SERS) substrates enabling fast, sensitive and on site detection is relevant for the practical application of SERS for real world analysis, such as food safety and organic pollutants monitoring. In this work novel Ag NPs/PDMS composites were fabricated and employed for the SERS detection of food contaminants directly on food surfaces. Ag NPs/PDMS composites were obtained by self-assembly of organic Ag nanoparticle solutions on flexible PDMS surfaces. Preliminary evaluation of the suitability of Ag NPs/PDMS probes for SERS analysis showed that composites were characterized by a SERS enhancement factor (EF) of 3.1 × 105, good stability and resistance to harsh conditions as well as good uniformity and batch to bach reproducibility. The "sticky" nature of Ag NPs/PDMS composites was exploited to "paste" them on irregular analytical surfaces, thus enabling the detection in situ of food contaminant crystal violet (CV) and pesticide thiram, respectively. Specifically, CV and thiram concentrations as low as 1 × 10-7 M and 1 × 10-5 M were measured on contaminated fish skin and orange peel, respectively. Furthermore, efficient SERS detection by micro-extraction of CV from fish skin and thiram from fruit surfaces was achieved, showing the analytical versatility of the fabricated SERS composites.

Asymmetric Pentagonal Metal Meshes for Flexible Transparent Electrodes and Heaters.

Metal meshes have emerged as an important class of flexible transparent electrodes. We report on the characteristics of a new class of asymmetric meshes, tiled using a recently discovered family of pentagons. Micron-scale meshes were fabricated on flexible polyethylene terephthalate substrates via optical lithography, metal evaporation (Ti 10 nm, Pt 50 nm), and lift-off. Three different designs were assessed, each with the same tessellation pattern and line width (5 µm), but with different sizes of the fundamental pentagonal unit. Good mechanical stability was observed for both tensile strain and compressive strain. After 1000 bending cycles, devices subjected to tensile strain showed fractional resistance increases in the range of 8-17%, while devices subjected to compressive strain showed fractional resistance increases in the range of 0-7%. The performance of the pentagonal metal mesh devices as visible transparent heaters via Joule heating was also assessed. Rapid response times (∼15 s) at low bias voltage (≤5 V) and good thermal resistance characteristics (213-258 °C cm2/W) were found using measured thermal imaging data. Deicing of an ice-bearing glass coupon on top of the transparent heater was also successfully demonstrated.

Low-cost silver capped polystyrene nanotube arrays as super-hydrophobic substrates for SERS applications.

In this paper, we describe the fabrication, simulation and characterization of dense arrays of freestanding silver capped polystyrene nanotubes, and demonstrate their suitability for surface enhanced Raman scattering (SERS) applications. Substrates are fabricated in a rapid, low-cost and scalable way by melt wetting of polystyrene (PS) in an anodized alumina (AAO) template, followed by silver evaporation. Scanning electron microscopy reveals that substrates are composed of a dense array of freestanding polystyrene nanotubes topped by silver nanocaps. SERS characterization of the substrates, employing a monolayer of 4-aminothiophenol (4-ABT) as a model molecule, exhibits an enhancement factor of ∼1.6 × 10(6), in agreement with 3D finite difference time domain simulations. Contact angle measurements of the substrates revealed super-hydrophobic properties, allowing pre-concentration of target analyte into a small volume. These super-hydrophobic properties of the samples are taken advantage of for sensitive detection of the organic pollutant crystal violet, with detection down to ∼400 ppt in a 2 µl aliquot demonstrated.

Gold nanowire electrodes in array: simulation study and experiments.

Recent developments in nanofabrication have enabled fabrication of robust and reproducible nanoelectrodes with enhanced performance, when compared to microelectrodes. A hybrid electron beam/photolithography technique is shown that permits discrete gold nanowire electrode arrays to be routinely fabricated at reasonable cost. Fabricated devices include twelve gold nanowire working electrode arrays, an on-chip gold counter electrode and an on-chip platinum pseudo reference electrode. Using potential sweep techniques, when diffusionally independent, these nanowires exhibit measurable currents in the nanoAmpere regime and display steady-state voltammograms even at very high scan rates (5000 mV s(-1)) indicative of fast analyte mass transport to the electrode. Nanowire electrode arrays offer the potential for enhancements in electroanalysis including increased signal to noise ratio and increased sensitivity while also allowing quantitative detection at much lower concentrations. However, to achieve this goal a full understanding of the diffusion profiles existing at nanowire arrays is required. To this end, we simulate the effects of altering inter-electrode separations on analyte diffusion for a range of scan rates at nanowire electrode arrays, and perform the corresponding experiments. We show that arrays with diffusionally independent concentration profiles demonstrate superior electrochemical performance compared to arrays with overlapping diffusion profiles when employing sweep voltammetric techniques. By contrast, we show that arrays with diffusionally overlapping profiles exhibit enhanced performance when employing step voltammetric techniques.

Mechanical constraint and release generates long, ordered horizontal pores in anodic alumina templates.

We describe the formation of long, highly ordered arrays of planar oriented anodic aluminum oxide (AAO) pores during plane parallel anodization of thin aluminum 'finger' microstructures fabricated on thermally oxidized silicon substrates and capped with a silicon oxide layer. The pore morphology was found to be strongly influenced by mechanical constraint imposed by the oxide layers surrounding the Al fingers. Tractions induced by the SiO(2) substrate and capping layer led to frustrated volume expansion and restricted oxide flow along the interface, with extrusion of oxide into the primary pore volume, leading to the formation of dendritic pore structures and meandering pore growth. However, partial relief of the constraint by a delaminating interfacial fracture, with its tip closely following the anodization front, led to pore growth that was highly ordered with regular, hexagonally packed arrays of straight horizontal pores up to 3 µm long. Detailed characterization of both straight and dendritic planar pores over a range of formation conditions using advanced microscopy techniques is reported, including volume reconstruction, enabling high quality 3D visualization of pore formation.

Scanning the potential energy surface for synthesis of dendrimer-wrapped gold clusters: design rules for true single-molecule nanostructures.

The formation of true single-molecule complexes between organic ligands and nanoparticles is challenging and requires careful design of molecules with size, shape, and chemical properties tailored for the specific nanoparticle. Here we use computer simulations to describe the atomic-scale structure, dynamics, and energetics of ligand-mediated synthesis and interlinking of 1 nm gold clusters. The models help explain recent experimental results and provide insight into how multidentate thioether dendrimers can be employed for synthesis of true single-ligand-nanoparticle complexes and also nanoparticle-molecule-nanoparticle "dumbbell" nanostructures. Electronic structure calculations reveal the individually weak thioether-gold bonds (325 ± 36 meV), which act collectively through the multivalent (multisite) anchoring to stabilize the ligand-nanoparticle complex (∼7 eV total binding energy) and offset the conformational and solvation penalties involved in this "wrapping" process. Molecular dynamics simulations show that the dendrimer is sufficiently flexible to tolerate the strained conformations and desolvation penalties involved in fully wrapping the particle, quantifying the subtle balance between covalent anchoring and noncovalent wrapping in the assembly of ligand-nanoparticle complexes. The computed preference for binding of a single dendrimer to the cluster reveals the prohibitively high dendrimer desolvation barrier (1.5 ± 0.5 eV) to form the alternative double-dendrimer structure. Finally, the models show formation of an additional electron transfer channel between nitrogen and gold for ligands with a central pyridine unit, which gives a stiff binding orientation and explains the recently measured larger interparticle distances for particles synthesized and interlinked using linear ligands with a central pyridine rather than a benzene moiety. The findings stress the importance of organic-inorganic interactions, the control of which is central to the rational engineering and eventual large-scale production of functional building blocks for nano(bio)electronics.

Controllable selective exfoliation of high-quality graphene nanosheets and nanodots by ionic liquid assisted grinding.

Bulk quantities of graphene nanosheets and nanodots have been selectively fabricated by mechanical grinding exfoliation of natural graphite in a small quantity of ionic liquids. The resulting graphene sheets and dots are solvent free with low levels of naturally absorbed oxygen, inherited from the starting graphite. The sheets are only two to five layers thick. The graphene nanodots have diameters in the range of 9-29 nm and heights in the range of 1-16 nm, which can be controlled by changing the processing time.

Single nanoskived nanowires for electrochemical applications.

In this work, we fabricate gold nanowires with well controlled critical dimensions using a recently demonstrated facile approach termed nanoskiving. Nanowires are fabricated with lengths of several hundreds of micrometers and are easily electrically contacted using overlay electrodes. Following fabrication, nanowire device performance is assessed using both electrical and electrochemical characterization techniques. We observe low electrical resistances with typical linear Ohmic responses from fully packaged nanowire devices. Steady-state cyclic voltammograms in ferrocenemonocarboxylic acid demonstrate scan rate independence up to 1000 mV s(-1). Electrochemical responses are excellently described by classical Butler-Volmer kinetics, displaying a fast, heterogeneous electron transfer kinetics, k(0) = 2.27 ± 0.02 cm s(-1), α = 0.4 ± 0.01. Direct reduction of hydrogen peroxide is observed at nanowires across the 110 pM to 1 mM concentration range, without the need for chemical modification, demonstrating the potential of these devices for electrochemical applications.

Ion-transfer electrochemistry at arrays of nanointerfaces between immiscible electrolyte solutions confined within silicon nitride nanopore membranes.

Ion transfer across interfaces between immiscible liquids provides a means for the nonredox electrochemical detection of ions. Miniaturization of such interfaces brings the benefits of enhanced mass transport. Here, the electrochemical behavior of geometrically regular arrays of nanoscale interfaces between two immiscible electrolyte solutions (nanoITIES arrays) is presented. These were prepared by supporting the two electrolyte phases within silicon nitride membranes containing engineered arrays of nanopores. The nanoITIES arrays were characterized by cyclic voltammetry of the interfacial transfer of tetraethylammonium cation (TEA(+)) between the aqueous phase and the gelled organic phase. Effects of pore radius, pore center-to-center separation, and number of pores in the array were examined. The ion transfer produced apparent steady-state voltammetry on the forward and reverse sweeps at all experimentally accessible scan rates and at all nanopore array designs. However, background-subtraction of the voltammograms revealed the evolution of a peak-shaped response on the reverse sweep with increasing scan rate, indicative of pores filled with the organic phase to a certain extent. The steady-state voltammetric behavior at the nanoITIES arrays on the forward sweep for arrays with significant diffusion zone overlap between adjacent nanoITIES is indicative of the dominance of radial diffusion to interfaces at the edge of the arrays over linear diffusion to interfaces within the arrays. This implies that nanoITIES arrays, which occupy an overall area of micrometer dimensions, behave like a single microITIES of corresponding area to the nanoITIES array.

Fabrication of nanopore array electrodes by focused ion beam milling.

Single nanopore electrodes and nanopore electrode arrays have been fabricated using a focused ion beam (FIB) method. High aspect ratio pores (approximately 150-400-nm diameter and 500-nm depth) were fabricated using direct-write local ion milling of a silicon nitride layer over a buried platinum electrode. This local milling results in formation of a recessed platinum electrode at the base of each nanopore. The electrochemical properties of these nanopore metal electrodes have been characterized by voltammetry. Steady-state voltammograms were obtained for a range of array sizes as well as for single nanopore electrodes. High-resolution scanning electron microscopy imaging of the arrays showed that the pores had truncated cone, rather than cylindrical, conformations. A mathematical model describing diffusion to an electrode located at the base of a truncated conical pore was developed and applied to the analysis of the electrode geometries. The results imply that diffusion to the pore mouth is the dominant mass transport process rather than diffusion to the electrode surface at the base of the truncated cone. FIB milling thus represents a simple and convenient method for fabrication of prototype nanopore electrode arrays, with scope for applications in sensing and fundamental electrochemical studies.

Hysteresis of charge tunneling in assemblies of carboxylic acid-modified gold nanoparticles.

We report on charge transport measurements through laterally contacted assemblies of Au nanoparticles capped with 11-mercaptoundecanoic acid ligands. Both alternating- and direct-current data indicate that although the nanoparticles behave as electrically isolated metallic islands, there is a significant influence from the nanoparticle environment, indicating the existence of a slow reorganization process linked to charge transport. On the basis of the observation of temperature-dependent hysteresis of charge tunneling, we propose that this process is due to proton transfer between the carboxylic acid tails of the ligands.