Layer Of AuNPs Research Articles

Solid-State Probe Based Electrochemical Aptasensor for Cocaine: A Potentially Convenient, Sensitive, Repeatable, and Integrated Sensing Platform for Drugs

Aptamers, which are artificial oligonucleotides selected in vitro, have been employed to design novel biosensors (i.e., aptasensors). In this work, we first constructed a label-free electrochemical aptasensor introducing a probe immobilization technique by the use of a layer-by-layer (LBL) self-assembled multilayer with ferrocene-appended poly(ethyleneimine) (Fc-PEI) on an indium tin oxide (ITO) array electrode for detection of cocaine. The Fc-PEI and gold nanoparticles (AuNPs) were LBL assembled on the electrode surface via electrostatic interaction. Then, cocaine aptamer fragments, SH-C2, were covalently labeled onto the outermost AuNP layer. When the target cocaine and cocaine aptamer C1 were present simultaneously, the SH-C2 layer hybridized partly with C1 to bind the cocaine, which led to a decreased differential pulse voltammetry (DPV) signal of Fc-PEI. This DPV signal change could be used to sensitively detect cocaine with the lowest detectable concentration down to 0.1 microM and the detection range up to 38.8 microM, which falls in the the expected range for medical use of detecting drug abuse involving cocaine. Meanwhile, the sensor was specific to cocaine in complex biologic fluids such as human plasma, human saliva, etc. The sensing strategy had general applicability, and the detection of thrombin could also be realized, displayed a low detection limit, and exhibited worthiness to other analytes. The aptasensor based on the array electrode held promising potential for integration of the sensing ability in multianalysis for simultaneous detection.

Designing nanomaterial-enhanced electrochemical immunosensors for cancer biomarker proteins

Detection of multiple cancer biomarker proteins in human serum and tissue at point-of-care is a viable approach for early cancer detection, but presents a major challenge to bioanalytical device development. This article reviews recent approaches developed in our laboratories combining nanoparticle decorated electrodes and multilabeled secondary antibody labeled particles to achieve high sensitivity for the detection of cancer biomarker proteins. Two nanomaterial-based sensor platforms were used: (a) upright single wall carbon nanotube forests and (b) layers of densely packed 5 nm gold nanoparticles. Both platforms feature pendant carboxylate groups for easy attachment of enzymes or antibodies by amidization. In quality performance tests, the biocatalytic responses for determination of hydrogen peroxide of AuNP layers with attached horseradish peroxidase (HRP) on electrodes gave somewhat better detection limit and sensitivity than single wall carbon nanotube (SWNT) forest platforms with HRP attached. Evaluation of these sensors as platforms for sandwich immunoassays for cancer biomarker prostate specific antigen (PSA) in serum showed that both approaches gave accurate results for human serum samples from cancer patients. The best detection limit (0.5 pg mL − 1 ) and sensitivity were obtained by combining the AuNP immunosensors with binding of 1 μm diameter magnetic particles decorated with secondary antibodies and 7500 HRP labels.

Ellipsometric Studies of Optical Properties of Local Surface Plasmon Resonance for Au Nanoparticles on the Substrate

Few ellipsometric studies have been conducted on Au self-assembled monolayers (SAM), with large discrepancies obtained for refractive index values. Because the Au NPs layer is a kind of composite layer, Au NPs and void, the effective media approximate (EMA) model was employed to investigate the optical properties of Au NPs. The reflective coefficient (approximately 1.2) was smaller than the extinction coefficient (approximately 2.5), which is different from the previous reports and corresponds to the Drude model. The absorption peak of surface plasmon resonance for Au NPs on the glass substrate was shifted from 580 nm to 480 nm.

Layer-by-Layer Self-Assembled Mutilayer Films of Gold Nanoparticles for Surface-Assisted Laser Desorption/Ionization Mass Spectrometry

Layer-by-layer (LBL) self-assembled multilayer films of gold nanoparticles (AuNPs) on a silicon wafer were demonstrated to be promising substrates for surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) of peptides and environmental pollutants for the first time. LBL multilayer films, (AuNPs/PAHC)n, consisting of alternating layers of ammonium citrate capped AuNPs and poly(allylamine hydrochloride) (PAHC) were prepared on a silicon surface. Silicon plates with aggregated AuNPs were more suitable than those with dispersed AuNPs for the SALDI-MS of peptides. The number of particle layers had a significant effect on the laser desorption/ionization of angiotensin I; the peak intensity of the peptide (molecular ion amount) increased with an increase in the number of layers of AuNPs. As a result, the (AuNPs/PAHC)5 multilayer films increased the sensitivity of the angiotensin I to subfemtomoles and raised the useful analyte mass range, thus making it possible to detect small proteins (a 12 kDa cytochrome c). The signal enhancement when using (AuNPs/PAHC)5 may be due to (i) the high absorption of the UV laser light at 337 nm by the AuNP layers, (ii) the low thermal conductivity due to the AuNPs being covered with a thin monolayer of PAHC, and (iii) the increase in the surface roughness (approximately 100 nm) with the number of AuNP layers. Thus, laser-induced rapid high heating of AuNPs for effective desorption/ionization of peptides is possible. In addition, it was found that (AuNPs/PAHC)5 could be used to extract environmental pollutants (pyrene and dimethyldistearylammonium chloride) from very dilute aqueous solutions with concentrations less than 10(-10) mg/mL, and the analytes trapped in the LBL film could be identified by introducing the film directly into the SALDI mass spectrometer without needing to elute the analytes out of the film.

Bisthiol‐Assisted Multilayers' Self‐Assembly of Gold Nanoparticles: Synthesis, Characterization, Size Control and Electrocatalytic Applications

AbstractThe layer‐by‐layer (LBL) approach has been utilized to self‐assemble multilayers films of citrate‐stabilized gold nanoparticles (AuNPs) on polycrystalline gold (poly‐Au) substrates. 1,4‐benzenedimethanethiol (BDMT) was used as a cross linker to bind every two successive AuNPs layers. The transmission electron microscopy (TEM), and X‐ray photoelectron spectroscopy (XPS) techniques were employed to confirm the existence of the alternative layers of AuNPs and BDMT in the self‐assembled multilayers film. The AuNPs modified poly‐Au electrode showed a significant electrocatalysis towards the reduction of oxygen (a positive potential shift of about 100 mV was observed in the reduction peak potential compared to that obtained at the bare poly‐Au electrode). The AuNPs tend to aggregate during their assembling to different extent which markedly depends on the substrate geometry and roughness. An effort was dedicated to understand and control the aggregation phenomenon.

Signal Enhancement and Tuning of Surface Plasmon Resonance in Au Nanoparticle/Polyelectrolyte Ultrathin Films

Investigations on optical and dielectric properties of hybrid ultrathin layer-by-layer (LbL) films of gold nanoparticles (AuNPs) and polyelectrolytes under different pH conditions resulted in surface plasmon resonance (SPR) signal enhancement under attenuated total reflection (ATR) spectroscopy conditions. Theoretical considerations on the basis of the Maxwell−Garnett theory were made to compare with experimental results. LbL films with different layer architectures were fabricated from AuNPs and two polyelectrolytes: poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) on Au and Ag thin film substrates for ATR. UV−vis spectroscopy and SPR spectroscopy were applied to study LbL film growth and monitor SPR shift upon pH switching. The (PAH/AuNPs)x multilayers showed an interesting dual-responsive SPR change as a function of pH and distance between AuNP layers and metal film. The addition of (PAH/PSS)y layers was found to act as an effective cushion to enhance this SPR response due to the significant swelling/shrinking of the film. In the case of [(PAH/PSS)y + (PAH/AuNPs)x], both theoretical calculations and experimental results showed that the SPR response can either (a) move toward lower incident angle with a sharp peak shape upon swelling in pH = 2 or (b) shift to a higher angle with a broadened peak shape after contraction in pH = 10. This effect was found to be opposite to what is expected from LbL films made up of the polyelectrolytes alone. Moreover, increasing the distance between AuNPs and the metal films also decouples this enhancement effect. A two-wavelength experiment (red and green lasers) was used to quantitatively demonstrate this SPR response to pH switching. Finally, SPR imaging was employed to monitor the SPR change with pH switching between 2 and 10 on the [(PAH/PSS)4 + (PAH/AuNPs)2] film. Thus, on the basis of the SPR spectroscopic and SPR imaging pH response, a reproducible and stable sensing system can be successfully fabricated with these films.

Gold Nanoparticle Enhanced Charge Transfer in Thin Film Assemblies of Porphyrin−Fullerene Dyads

Photoinduced vectorial electron transfer in a molecularly organized porphyrin-fullerene (PF) dyad film is enhanced by the interlayer charge transfer from the porphyrin moiety of the dyad to an octanethiol protected (dcore approximately 2 nm) gold nanoparticle (AuNP) film. By using the time-resolved Maxwell displacement charge (TRMDC) method, the charge separation distance was found to increase by 5 times in a multilayer film structure where the gold nanoparticles face the porphyrin moiety of the dyad, that is, AuNP|PF, compared to the case of the PF layer alone. Films were assembled by the Langmuir-Blodgett (LB) method using octadecylamine (ODA) as the matrix compound. Atomic force microscopy (AFM) images of the monolayers revealed that AuNPs are arranged into continuous, islandlike structures and PF dyads form clusters. The porphyrin reference layer was assembled with the AuNP layer to gain insight on the interaction mechanism between porphyrin and gold nanoparticles. Interlayer electron transfer was also observed between the AuNPs and porphyrin reference, but the efficiency is lower than that in the AuNP|PF film. Fluorescence emission of the reference porphyrin is slightly quenched, and fluorescence decay becomes faster in the presence of AuNPs. The proposed mechanism for the electron transfer in the AuNP|PF film is thus the primary electron transfer from the porphyrin to the fullerene followed by a secondary hole transfer from the porphyrin to the AuNPs, resulting in an increased charge separation distance and enhanced photovoltage.

Multilayered Gold-Nanoparticle/Polyimide Composite Thin Film through Layer-by-Layer Assembly

A novel type of composite thin film consisting of gold nanoparticles (AuNPs) and polymide (PI) was fabricated through layer-by-layer (LBL) assembly. To fabricate such films, bare AuNPs and a poly (amic acid) bearing pendant amine groups, namely, amino poly (amic acid) or APAA, were synthesized and assembled in an LBL fashion. Without any organic encapsulation layer on their surface, AuNPs were bound directly to APAA chains at the amine sites; X-ray photoelectron spectroscopy study suggested that the binding was based on a combined effect of metal-ligand coordination and electrostatic interaction, with the former dominating over the latter. An approximately linear growth of the film started from the second layer of AuNP as revealed by the UV-vis spectroscopy, and the degree of particle aggregation was higher in the first AuNP layer than in the subsequent layers due to the differences in the density of binding sites. The resultant assembly was heated to imidize the APAA, thereby creating a robust composite structure.

Layer-by-layer Au nanoparticles as a Schottky barrier in a water-based dye-sensitized solar cell

We fabricated layer-by-layer gold nanoparticles (Au NPs) onto an electrode as a Schottky barrier in a water-based dye-sensitized solar cell. The Maxwell–Garnett equation was used to fit the characteristics of the Au film. The short-circuit current for four layers of Au NPs in a water-based dye-sensitized solar cell (DSSC) ranges from 2.76 mA/cm2 to 5.96 mA/cm2, which is due to the Schottky barrier reducing the number of electrons going from the semiconductor back to either the dye or the electrolyte. The efficiency of photo-electric conversion for four layers of Au NPs in water-based DSSC can be enhanced from 0.26% up to 0.95%.

Fabrication and Electrochemical Application of Three-Dimensional Gold Nanoparticles: Self-Assembly

Multilayers film of nanostructured citrate-stabilized gold particles (AuNPs) has been fabricated based on the layer-by-layer (LBL) technique using a self-assembled monolayer of 1,4-benzenedimethanethiol (BDMT). The formation of AuNPs and BDMT self-assemblies as alternative multilayers was confirmed by transmission electron microscopy (TEM), X-ray photoelectron spectroscope (XPS), and quartz crystal microbalance (QCM). The formation of uniform AuNP layers with an average monolayer thickness of 5-6 nm was obvious in the TEM images. The existence of BDMT molecules as cross linkers for the AuNPs' layers was proved by XPS measurements. The greater affinity of AuNPs' layers to bind BDMT molecules in comparison with the bare Au bulk electrode was revealed by QCM measurements. Electrochemically, the AuNPs' layers on the electrode surface did not only catalyze the reduction of oxygen (ca. 100-mV positive shift of the reduction peak potential compared with that at the bare Au bulk electrode) but also showed a fascinating nature of working as a renewed activated-electrode surface; a zigzag response was observed for oxygen reduction during alternative immobilization of BDMT and the AuNP layer. The self-assembly of a new AuNPs layer restored the catalytic activity that was entirely blocked by the preceding BDMT layer.

Electrical Detection of Protein Using Gold Nanoparticles and Nanogap Electrodes

A method of electrically detecting of protein described is developed using self-assembled multilayer gold nanoparticles (AuNPs) on a SiO2/Si substrate between gold electrodes. Electrical measurements are performed at room temperature using a probe station. A monoclonal antibody is immobilized on the top surface of the first layer of AuNPs (14 nm). The second layer of AuNPs is formed through specific binding among a target antigen [hepatitis C virus, (HCV)], the monoclonal antibody, and the conjugate of a AuNP-polyclonal antibody. Once the specific binding among the monoclonal antibody, target antigen, and polyclonal antibody occurs, a significant electric current is detected through multilayer self-assembled gold nanoparticles between nanogap electrodes. No significant current (<1 pA) can be measured through a monolayer of AuNPs. A significant difference between the IV curves of the monolayer and the multilayer of AuNPs is used to identify whether the target antigen exists in the tested sample.

An array-based CMOS biochip for electrical detection of DNA with multilayer self-assembly gold nanoparticles

This paper presents an array-based CMOS biochip for DNA detection using self-assembly multilayer gold nanoparticles (AuNPs). The biochip is fabricated by a TSMC 0.35 μm standard CMOS process and post-CMOS micromachining processes. Before taking DNA detection measurements, self-assembly monolayer of AuNPs is established on SiO 2 surface between two microelectrodes. The gap distance between the two microelectrodes in this study is less than 800 nm. First, capture oligonucleotide strands are immobilized on the top surface of self-assembly monolayer of AuNPs. On each measuring spot, selective binding occurs among thiol-modified probe oligonucleotide strands, capture oligonucleotide strands, and target oligonucleotide strands if the target oligonucleotide strands are complementary with the probe and capture oligonucleotide strands in the sample solution. Finally, through a self-assembly process between suspended gold nanoparticles and the thiol-modified end of probe oligonucleotide strand, a multilayer of AuNPs on the nanogap surface can be established. The DNA hybridization can be determined by a sharp decrease in impedance over the nanogap between the two microelectrodes, which results from an increase in the number of AuNPs layers. Measured results show that the electric current of multilayer AuNPs is about 1000 times greater than that of monolayer AuNPs. The effect of nanogap distance is also investigated. It is shown that the electrical conductivity decreases with the increase of gap distance.

Bimetallic Clusters by Underpotential Deposition on Layered Au Nanoparticle Films

In this paper, we describe the well-defined underpotential deposition systems consisting of Cu, Ag, Pb, and Tl on layered gold nanoparticle (AuNP) thin films linked with bifunctional 1,6-hexanedithiol (HDT). Scanning electron microscopy informs the morphological structures of layered AuNPs accumulated by stepwise immersion in citrate-stabilized AuNP and HDT solutions. Although cyclic voltammograms for typical Au oxide formation show that colloidal AuNP frameworks are polycrystalline, (111) reflection is the major character in X-ray diffraction measurements. As the AuNP layers are piled up on indium tin oxide, faradaic currents for the underpotential deposition adsorption increase linearly, indicating that entire AuNPs are electrochemically accessible; as a result, the mass transport and the electron transfer are plausible in AuNP frameworks. Voltammetric profiles of Ag on AuNP and Cu on AuNP exhibit similar features with those on the bulk metal film. On the other hand, heavy metal systems of Pb/AuNP and T...