Gold Disk Research Articles

Photothermal inactivation of heat-resistant bacteria on nanoporous gold disk arrays

A rapid photothermal bacterial inactivation technique has been developed by irradiating near-infrared (NIR) light onto bacterial cells (Escherichia coli, Bacillus subtilis, Exiguobacterium sp. AT1B) deposited on surfaces coated with a dense, random array of nanoporous gold disks (NPGDs). With the use of cell viability tests and SEM imaging results, the complete inactivation of the pathogenic and heat-resistant bacterial model strains is confirmed within ~25 s of irradiation of the NPGD substrate. In addition to irradiation control experiments to prove the efficacy of the bacterial inactivation, thermographic imaging showed an immediate averaged temperature rise above 200 °C within the irradiation spot of the NPGD substrate. The light-gated photothermal effects on the NPGD substrate offers potential applications for antimicrobial and nanotherapeutic devices due to strong light absorption in the tissue optical window, i.e., the NIR wavelengths, and robust morphological structure that can withstand high instantaneous thermal shocks.

Properties of gold electrode modified with dimercaptosuccinic acid and electrochemical behavior of copper histidine complex

A self-assembled monolayer of meso-2,3-dimercaptosuccinic acid was prepared on the surface of gold disc electrode. The modified electrode was characterized using cyclic voltammetry in copper(II) solution and cyclic voltammetry and electrochemical impedance spectroscopy in the presence of potassium hexacyanoferrate( II)/(III) and hexaammineruthenium (II)/(III) chloride. Binding of copper(II) histidine complex (Cu–His) onto the electrode was successfully achieved for a wide range of tested concentrations, as shown with adsorption transfer stripping voltammetry. Electrode response (logΔIp) was linearly proportional to logc(Cu–His) with correlation coefficient R32 = 0.9839.

A high sensitive impedimetric salbutamol immunosensor based on the gold nanostructure-deposited screen-printed carbon electrode

Salbutamol (SAL) is a β2-agonist which is illegally used as a growth-promoting agent in livestock, which can result in health risks to consumers. Highly sensitive label-free impedimetric immunosensors based on gold nanostructure (AuNS)-deposited screen-printed carbon electrodes (SPCEs) are used to detect SAL. Two-step templateless electrodeposition creates the AuNS in a sea urchin shape with submicrometer-scaled pyramidal structures on micrometer-scaled particles, and the roughness of the resulting AuNS/SPCEs is 4.9 times greater than that of polished gold disk electrodes (AuDE). Anti-SAL monoclonal antibody was immobilized on the 1-ethyl-3-(3-dimethylamino-propyl) carbodiimide hydrochloride/N-hydroxysuccinimide-activated 3-mercaptoproponic acid-modified AuNS/SPCEs. Electron transfer resistance was measured using electrochemical impedance spectroscopy (EIS) to quantify the SAL concentration. Label-free AuNS/SPCE-based immunosensors presented a wide linear range from 0.1pg/mL to 1μg/mL and the limit of detection was as low as 4fg/mL, which is much lower than that (3pg/mL) obtained using the AuDE-based immunosensors. Moreover, the AuNS/SPCE-based immunosensors could practically quantify the concentration of SAL in 1000 time-diluted serum samples with a good recovery and are sufficiently sensitive to meet EU minimum required performance limit (<3ng/mL). The AuNS/SPCEs have promise for the development of disposable and label-free EIS-based immunosensors for the detection of small molecule chemicals, such as SAL.

Near-field engineering of Fano resonances in a plasmonic assembly for maximizing CARS enhancements.

Surface enhanced coherent anti-Stokes Raman scattering (SECARS) is a sensitive tool and promising for single molecular detection and chemical selective imaging. However, the enhancement factors (EF) were only 10~100 for colloidal silver and gold nanoparticles usually used as SECARS substrates. In this paper, we present a design of SECARS substrate consisting of three asymmetric gold disks and strategies for maximizing the EF by engineering near-field properties of the plasmonic Fano nanoassembly. It is found that the E-field “hot spots” corresponding to three different frequencies involved in SECARS process can be brought to the same spatial locations by tuning incident orientations, giving rise to highly confined SECARS “hot spots” with the EF reaching single-molecule sensitivity. Besides, an even higher EF of SECARS is achieved by introducing double Fano resonances in this plasmonic nanoassembly via further enlarging the sizes of the constituent disks. These findings put an important step forward to the plasmonic substrate design for SECARS as well as for other nonlinear optical processes.

Electrically Tunable Quasi-3-D Mushroom Plasmonic Crystal

This paper reports an electrically tunable plasmonic crystal incorporating a nematic liquid crystal (LC) layer on the top surface of quasi-3-D mushroom plasmonic nanostructures. The presented plasmonic crystal is formed by an array of polymeric mushroom nanoposts with gold disks at the top and perforated nanoholes in a gold thin film at the bottom. The coupling between surface plasmon polariton (SPP) and Rayleigh anomaly (RA) is observed in experiments with quasi-3-D plasmonic crystals, and verified by simulations. The coupled SPP-RA resonance mode has its electric field vector prominently normal to the surface of the plasmonic nanostructures, and extends into the surrounding medium. This feature makes the coupled resonance sensitive to molecular reorientation in LC, and thus, is useful for designing index modulation-based tunable plasmonic crystal devices. Therefore, by applying external voltages across the LC layer, the SPP-RA resonance mode shows a redshift of 8 nm with a 35% change in the amplitude.

Voltammetric study of the interactions of alkaline earth metal ions and melamine at different ph

The electrochemical redox behavior of alkaline earth metal ions at different pH in acetate buffer solution has been investigated using cyclic voltammetric method at gold disk electrode (GDE). In the entire studied pH (3.68-6.23), metal ions show two cathodic and an anodic peak. The method has also been employed to observe the interaction of metal ions with melamine in acetate buffer solution. At all pH, a reasonably strong interaction occurs between metal ions and melamine at different metal ions/melamine molar ratio. However, maximum interaction takes place at 1:4 molar ratios of metal ions and melamine at pH 6.23. This is possibly the most suitable condition for alkaline earth metal-melamine interaction.Bangladesh J. Sci. Res. 27(1): 39-50, June-2014

Electrochemical Determination of Hg2+ Using Electrodes Modified with Peptide Nucleic Acid

The development of gold disk electrodes modified with single-stranded PNA (peptide nucleic acid) for the determination of Hg2+ is described. PNA is the most useful and popular example of nucleic acid analogues. Due to the presence of glycine units linked via peptide bond instead of sugar-phosphate backbone, its interaction with molecules like proteins or metal ions is possible. The performance of anionic (AQMS) and cationic (MB and RuHex) redox markers for the generation of current signal is compared. It is shown that the use of anionic marker results in a significantly better analytical parameters, as compared to measurements with cationic markers. In the proposed system, the AQMS current difference increases proportionally within the Hg2+ concentration range 0.05 to 10 μmol·L−1, while for higher concentrations, the saturation of PNA layer with Hg2+ is observed. The optimized sensor distinguishes itself with high selectivity toward Hg2+ and lower detection limit of 3.33 nmol·L−1. The interactions between mercury ion and PNA strands were studied using electrochemical impedance spectroscopy (EIS) and spectrophotometry (UV-VIS).

Visualization and Measurement of Natural Convection from Electrochemically-Generated Density Gradients at Concentric Microdisk and Ring Electrodes in a Microfluidic System

Natural convection actuated by electrochemically-generated density gradients at microelectrodes was investigated under different conditions by simultaneously visualizing fluid flow with the electrochemical response. The studies elucidate deviations of electrochemical behavior from theoretical expectations and parameters that control natural convection, which can be exploited in electroanalysis, microfluidics, and electrodeposition. Experiments involved an enclosed, small volume containing 0.00475-0.095 M each of K3Fe(CN)6 and K4Fe(CN)6 in 0.095 M KCl, over concentric gold disk (radius: 16–156 μm) and ring (inner radius: 200–1600 μm, outer radius: 250–2000 μm) microelectrodes. Fluid velocities were obtained with video microscopy by tracking 10-μm beads added to the solution. Flow radiates near the disk either inwardly or outwardly at the bottom of the cell and reverses direction at the top, producing a vertical circulation. Maximum velocities of ∼10 μm/s were measured for the 156-μm disk in 0.095 M. After application of potential or current, the onset of natural convection occurred at shorter times (6 s) than measurable affects in electrochemical current/voltage responses (tens of seconds). Convection from density gradients occurred without corresponding changes in electrochemical responses for the 78-μm disk at the lowest concentration (0.00475 M) and for the smallest, 16-μm disk at the highest concentration (0.095 M).

Development of Surface Plasmon Resonance-Based Immunosensor for Detection ofBrucella melitensis

A label-free optical immunosensor was developed based on surface plasmon resonance (SPR) method for rapid and selective detection of Brucella melitensis. This immunosensor was constructed by immobilizing capture antibody on 11-mercaptoundecanoic acid modified gold disk. This fabricated immunosensor detected B. melitensis at a concentration range from 103 to 107 cell mL-1 (R2 = 0.998) and a detection limit of 100 cell mL-1. Additionally, the kinetic equilibrium dissociation constant (KD) for assessment of the interaction affinity was calculated as 1.1 × 10-9 mol L-1 that can be considered as high affinity interaction. This SPR immunosensor provided advantages in term of fast response, label free and accurate detection of B. melitensis in analytical systems.

A Label-Free Electrochemical DNA Aptasensor for the Detection of Dopamine

Application of aptamers as the sensing layers in biosensors offers several advantages for the detection of various analytes. Herein, an electrochemical label - free biosensor for dopamine (DA) was developed with the use of 57-mer DNA aptamer. The proposed aptasensor operates via specific interaction between dopamine and aptamer sequence tethered to the gold disk electrode via thiol group. DA concentration was determined without use of any external redox indicator and the dopamine oxidation current serves as the analytical signal. Electrodes modified with DNA aptamer show selectivity toward dopamine within micromolar range with the lower detection limit of 26.8 μmol·L−1. The developed assay exhibits minor response in the presence of various interferents and its utility was tested in sample of diluted serum. Furthermore, the performance of proposed sensor was improved with LOD of 3.36 μmol·L−1 and the range of linear response from 5 to 75 μmol·L−1 by utilization of DNA aptamer/AuNP/reduced graphene oxide - modified glassy carbon electrode.

Application of DNA aptamers as sensing layers for electrochemical detection of potassium ions

Determination of potassium level in organism is of special importance due to its vital role in the maintenance of physiological activities in organism. Herein, an electrochemical sensor for K+ ions was developed with recognition layer based on DNA aptamers. The elaborated assay was formed by tethering of thiolated aptamer probes to gold disk electrode via Au-S bond. Potassium concentration was determined with the use of voltammetric techniques by comparison of current change of redox indicators—methylene blue (MB), AQMS and Na4Fe(CN)6. The studies revealed that among aptamer sequences, which were previously shown to bind with potassium ions, 15-mer thrombin binding aptamer (TBA) exhibited highest affinity towards potassium with Kd=1.13μmolL−1 when used as a sensing layer. The aptasensor demonstrated linear response within the range from 10−8 to 10−5molL−1 with a LOD of 2.31×10−9molL−1 and good selectivity towards K+ ions with the use of MB electroactive marker.

Direct Measurement of Optical Force Induced by Near-Field Plasmonic Cavity Using Dynamic Mode AFM.

Plasmonic nanostructures have attracted much attention in recent years because of their potential applications in optical manipulation through near-field enhancement. Continuing experimental efforts have been made to develop accurate techniques to directly measure the near-field optical force induced by the plasmonic nanostructures in the visible frequency range. In this work, we report a new application of dynamic mode atomic force microscopy (DM-AFM) in the measurement of the enhanced optical force acting on a nano-structured plasmonic resonant cavity. The plasmonic cavity is made of an upper gold-coated glass sphere and a lower quartz substrate patterned with an array of subwavelength gold disks. In the near-field when the sphere is positioned close to the disk array, plasmonic resonance is excited in the cavity and the induced force by a 1550 nm infrared laser is found to be increased by an order of magnitude compared with the photon pressure generated by the same laser light. The experiment demonstrates that DM-AFM is a powerful tool for the study of light induced forces and their enhancement in plasmonic nanostructures.

Thermal characteristics of temperature-controlled electrochemical microdevices

The development of novel, miniaturized sensing systems is driven by the demand for better and faster chemical measurements with lower power consumption and smaller sample sizes. Emerging miniature sensors, or microsensors, also offer rapid thermal and diffusive transport characteristics. For instance, temperature changes, during both heating and cooling, can be achieved on micrometer-scale surfaces much more rapidly than on bulk, macro-scale surfaces. While these rapid thermal characteristics have been most successfully exploited to date in gas-phase sensing devices, the prospect of developing analogous microfabricated, temperature-controlled microsensors for use in aqueous, or solution-phase, environments has been less explored. In this work, electrodes with underlying microheaters were designed and fabricated, and thermal characterization was performed using temperature imaging, transient temperature measurements, and theoretical modeling to determine temperature distributions and thermal response times in both gas- and solution-phase environments. These results will guide the development of solution-phase electrochemical sensors. Temperature-controlled electrochemical characterization was performed using cyclic voltammetry of a model analyte, hexaamineruthenium(III) chloride, to demonstrate the use of the multilayer, microfabricated devices, which consisted of a gold disk electrode and an underlying microheater. Electrochemical signals were enhanced by up to a factor of three at elevated temperatures (up to 81°C) compared to those measured at room temperature (21°C). This improved signal at elevated temperatures was explained by finite element method calculations that accounted for both temperature-dependent diffusion and thermal convection near the heated electrode surface.

Amplified electrochemical DNA sensor based on hemin/G-quadruplex DNAzyme as electrocatalyst at gold particles modified heated gold disk electrode

A new method for electrochemical detection of DNA hybridization with elevated electrode temperature at gold particles modified heated gold disk electrode (Au-HAuDE) is demonstrated. This method employs a hairpin DNA probe for target DNA capture and incorporates a specific DNAzyme sequence into the probe as electrocatalytic label. The hairpin DNA probes are immobilized on the Au-HAuDE. The presence of target DNA opens the hairpin structure, releases the DNAzyme sequence, and forms the hemin/G-quadruplex peroxidase-mimicking DNAzyme (HRP-DNAzyme) with hemin addition. With elevated electrode temperature, the electrocatalytic activity of HRP-DNAzyme for H2O2 reduction by oxidation of hydroquinone (used as an electron transfer mediator) can be significantly improved, resulting in an enhanced electrochemical reduction current of benzoquinone for DNA detection. With an electrode temperature of 50°C, the detection limit for target DNA measurements can be decreased ca. one magnitude compared with that at 0°C.

Unidirectional scattering by nanoparticles near substrates: generalized Kerker conditions.

Starting from a general description of light scattering by a nanoparticle in homogeneous surroundings and situated near a substrate, we outline the connection to multipole expansion of scattered light and derive conditions and limits on achievable half-space scattering asymmetry, including the possibility of unidirectional scattering along the propagation direction of the incident light (i.e., generalized Kerker conditions). As a way of realizing strongly asymmetric scattering, we perform a parametric study of the optical properties of disk-shaped gap-surface plasmon (GSP) resonators, consisting of a glass spacer sandwiched between two gold disks, with numerical calculations that corroborate the conditions derived from the multipole expansion. Finally, we present proof-of-principle experiments of asymmetric scattering by GSP-resonators on a glass substrate.

Detection of prostate specific antigen based on electrocatalytic platinum nanoparticles conjugated to a recombinant scFv antibody

Highly sensitive and label free detection of prostate specific antigen (PSA) still remains a challenge in prostate cancer diagnosis. In this paper, we propose a sensitive electrochemical immunosensor based on electrocatalytic platinum nanoparticles conjugated to a recombinant scFv antibody. Gold disc electrodes functionalised with a l-Cysteine (Cys) self-assembled monolayer (SAM) were used to covalently bind PSA specific monoclonal antibody (anti-PSA) using N-ethyl-N′-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide (EDC/NHS) chemistry. Immunosensing was completed using sandwich-type immunoreaction of the PSA–antigen (1–30ng/mL) between anti-PSA immobilized on the l-Cys modified electrode using label free electrochemical impedance (EIS) technique. Furthermore, highly specific in-house generated scFv fragments as receptor proteins were utilised for one step site-directed immobilisation on the surface of platinum nanoparticles (PtNPs). To improve the sensitivity of the immunoassay, these scFV labelled electrocatalytic PtNPs were then used for covalent hybridisation to the PSA modified electrode and then applied in a hybridisation assay to determine the concentration of the PSA by measuring the faradaic current associated with reduction of peroxide in solution. Semi-log plots of the PSA concentration vs. faradaic current are linear from 1 to 30ng/mL and pM concentrations can be detected without the need for molecular, e.g., PCR or NASBA, amplification.

Electrochemical genosensor based on disc and screen printed gold electrodes for detection of specific DNA and RNA sequences derived from Avian Influenza Virus H5N1

The genosensors based on thiolated ssDNA probe deposited on the two types of gold electrodes: screen-printed (miniaturized) and disc electrodes destined for determination of specific sequences of DNA and RNA derived from Avian Influenza Virus H5N1 have been proposed. The working principle of genosensor is based on the ion-channel mechanism. The analytical signals generated upon hybridization processes were recorded using electrochemical technique – Osteryoung square wave voltammetry in the presence of a redox active marker [Fe(CN)6]3−/4− in the sample solution. The miniaturized genosensor based on screen printed gold electrodes was able to detect the 20-mer complementary DNA oligonucleotide sequence as well as ∼280-mer RNA sequences containing the complementary 20-mer sequence in various positions: at 3′-terminus, at 5′-terminus and in the middle of the RNA transcript at the 1pM concentration. The measuring systems were selective. Non-complementary 20-mer oligonucleotide sequence as well as RNA transcript without complementary region generated weak response. The RNA transcripts were also tested with gold disc electrodes modified in the same manner. This device was able to detect ∼280-mer RNA sequences, but at higher concentration of 10pM. The good discrimination of the position of complementary part in the ∼280-mer RNA sequences was observed with using both types of modified electrodes.

Cover Picture: Label‐free, zeptomole cancer biomarker detection by surface‐enhanced fluorescence on nanoporous gold disk plasmonic nanoparticles (J. Biophotonics 10/2015)

Cover Picture: Label‐free, zeptomole cancer biomarker detection by surface‐enhanced fluorescence on nanoporous gold disk plasmonic nanoparticles (J. Biophotonics 10/2015)

Sequence and Temperature Influence on Kinetics of DNA Strand Displacement at Gold Electrode Surfaces.

Understanding complex contributions of surface environment to tethered nucleic acid sensing experiments has proven challenging, yet it is important because it is essential for interpretation and calibration of indispensable methods, such as microarrays. We investigate the effects of DNA sequence and solution temperature gradients on the kinetics of strand displacement at heated gold wire electrodes, and at gold disc electrodes in a heated solution. Addition of a terminal double mismatch (toehold) provides a reduction in strand displacement energy barriers sufficient to probe the secondary mechanisms involved in the hybridization process. In four different DNA capture probe sequences (relevant for the identification of genetically modified maize MON810), all but one revealed a high activation energy up to 200 kJ/mol during hybridization, that we attribute to displacement of protective strands by capture probes. Protective strands contain 4 to 5 mismatches to ease their displacement by the surface-confined probes at the gold electrodes. A low activation energy (30 kJ/mol) was observed for the sequence whose protective strand contained a toehold and one central mismatch, its kinetic curves displayed significantly different shapes, and we observed a reduced maximum signal intensity as compared to other sequences. These findings point to potential sequence-related contributions to oligonucleotide diffusion influencing kinetics. Additionally, for all sequences studied with heated wire electrodes, we observed a 23 K lower optimal hybridization temperature in comparison with disc electrodes in heated solution, and greatly reduced voltammetric signals after taking into account electrode surface area. We propose that thermodiffusion due to temperature gradients may influence both hybridization and strand displacement kinetics at heated microelectrodes, an explanation supported by computational fluid dynamics. DNA assays with surface-confined capture probes and temperature gradients should not neglect potential influences of thermodiffusion as well as sequence-related effects. Furthermore, studies attempting to characterize surface-tethered environments should consider thermodiffusion if temperature gradients are involved.

Determination of trace mercury in water based on N-octylpyridinium ionic liquids preconcentration and stripping voltammetry

A novel method for determination of trace mercury in water is developed. The method is performed by extracting mercury firstly with ionic liquids (ILs) and then detecting the concentration of mercury in organic media with anodic stripping voltammetry. Liquid-liquid extraction of mercury(II) ions by four ionic liquids with N-octylpyridinium cations ([OPy]+) was studied. N-octylpyridinium tetrafluoroborate and N-octylpyridinium trifluoromethylsulfonate were found to be efficient and selective extractant for mercury. Temperature controlled dispersive liquid phase microextraction (TC-DLPME) technique was utilized to improve the performance of preconcentration. After extraction, precipitated IL was diluted by acetonitrile buffer and mercury was detected by differential pulse stripping voltammetry (DPSV) with gold disc electrode. Mercury was enriched by 17 times while interfering ions were reduced by two orders of magnitude in the organic media under optimum condition. Sensitivity and selectivity for electrochemical determination of mercury were improved by using the proposed method. Tap, pond and waste water samples were analyzed with recoveries ranging from 81% to 107% and detection limit of 0.05μg/L.