Multilayer film of thiourea and gold nanoparticles as an effective platform for immobilization of activated non-labeled-DNA and construction of an ultrasensitive electrochemical DNA biosensor

In this work, self assembly of thiourea and gold nano-particle multilayer built up on a thiourea modified gold nanoparticles Au electrode, has been used as a platform for immobilization of activated ss-DNA. Two NH₂ group of thiourea on a multilayer surface can interact with an activated phosphate group of non-labeled ss-DNA. Activated non-labeled ss-DNA was prepared using N-(3 dimethylaminopropyl)-N-ethyl-carbodiimide hydrochloride (EDC) and N-hydroxy-succinimide (NHS). The whole DNA biosensor fabrication process was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods with the use of ferricyanide as an electrochemical redox indicator. Methylene Blue (MB) was used as the electrochemical indicator for monitoring the hybridization reaction after hybridized with the target ssDNA and the reduction current of MB intercalation decreased with increasing the concentration of target DNA, ranging from 7.9 × 10^–13 to 1.2 × 10^–8 M with a very low detection limit of 3.8 × 10^–13 M (S/N = 3).     

A New Porphyrin for the Preparation of Functionalized Water-Soluble Gold Nanoparticles with Low Intrinsic Toxicity

A potential new photosensitizer based on a dissymmetric porphyrin derivative bearing a thiol group was synthesized. 5-[4-(11-Mercaptoundecyloxy)-phenyl-10,15,20-triphenylporphyrin (PR-SH) was used to functionalize gold nanoparticles in order to obtain a potential drug delivery system. Water-soluble multifunctional gold nanoparticles GNP-PR/PEG were prepared using the Brust–Schiffrin methodology, by immobilization of both a thiolated polyethylene glycol (PEG) and the porphyrin thiol compound (PR-SH). The nanoparticles were fully characterized by transmission electron microscopy and ¹H nuclear magnetic resonance spectroscopy, UV/Vis absorption spectroscopy, and X-ray photoelectron spectroscopy. Furthermore, the ability of GNP-PR/PEGs to induce singlet oxygen production was analyzed to demonstrate the activity of the photosensitizer. Cytotoxicity experiments showed the nanoparticles are nontoxic. Finally, cellular uptake experiments demonstrated that the functionalized gold nanoparticles are internalized. Therefore, this colloid can be considered to be a novel nanosystem that could potentially be suitable as an intracellular drug delivery system of photosensitizers for photodynamic therapy.     

Construction of simple gold nanoparticle aggregates with controlled plasmon–plasmon interactions

We have developed a colloidal assembly for the study of plasmon–plasmon interactions between gold nanoparticles. Colloidal aggregates of controlled size and interparticle spacing were synthesized on silica nanoparticle substrates. Following the immobilization of isolated gold nanoparticles onto silica nanoparticles, the surfaces of the adsorbed gold nanoparticles were functionalized with 4-aminobenzenethiol. This molecular linker attached additional gold nanoparticles to the ‘parent' gold nanoparticle, forming small nanoparticle aggregates. The optical absorption spectrum of these clusters differed from that of gold colloid in a manner consistent with plasmon–plasmon interactions between the gold nanoparticles.     

Enzyme-encapsulated silica monolayers for rapid functionalization of a gold surface

We report a simple and rapid method for the deposition of amorphous silica onto a gold surface. The method is based on the ability of lysozyme to mediate the formation of silica nanoparticles. A monolayer of lysozyme is deposited via non-specific binding to gold. The lysozyme then mediates the self-assembled formation of a silica monolayer. The silica formation described herein occurs on a surface plasmon resonance (SPR) gold surface and is characterized by SPR spectroscopy. The silica layer significantly increases the surface area compared to the gold substrate and is directly compatible with a detection system. The maximum surface concentration of lysozyme was found to be a monolayer of 2.6 ng/mm(2) which allowed the deposition of a silica layer of a further 2 ng/mm(2). For additional surface functionalization, the silica was also demonstrated to be a suitable matrix for immobilization of biomolecules. The encapsulation of organophosphate hydrolase (OPH) was demonstrated as a model system. The silica forms at ambient conditions in a reaction that allows the encapsulation of enzymes directly during silica formation. OPH was successfully encapsulated within the silica particles and a detection limit for the substrate, paraoxon, using the surface-encapsulated enzyme was found to be 20 microM.     

Synthesis and characterization of platinum monolayer oxygen-reduction electrocatalysts with Co–Pd core–shell nanoparticle supports

We synthesized Pt monolayer electrocatalysts for oxygen-reduction using a new method to obtain the supporting core–shell nanoparticles. They consist of a Pt monolayer deposited on carbon-supported Co–Pd core–shell nanoparticles with the diameter of 3–4 nm. The nanoparticles were made using a redox-transmetalation (electroless deposition) method involving the oxidation of Co by Pd cations, yielding a Pd shell around the Co core. The quality of the thus-formed core–shell structure was verified using transmission electron microscopy and X-ray absorption spectroscopy, while cyclic voltammetry was employed to confirm the lack of Co oxidation (dissolution). A Pt monolayer was deposited on the Co–Pd core–shell nanoparticles by the galvanic displacement of a Cu monolayer obtained by underpotential deposition. The total noble metal mass-specific activity of this Pt monolayer electrocatalyst was ca. 3-fold higher than that of commercial Pt/C electrocatalysts.     

Synthesis and characterization of the water-soluble silica-coated ZnS:Mn nanoparticles as fluorescent sensor for Cu ions

Silica-coated ZnS:Mn nanoparticles were synthesized by coating hydrophobic ZnS:Mn nanoparticles with silica shell through microemulsion. The core–shell structural nanoparticles were confirmed by X-ray diffraction (XRD) patterns, high-resolution transmission electron microscope (HRTEM) images and energy dispersive spectroscopy (EDS) measurements. Results show that each core–shell nanoparticle contains single ZnS:Mn nanoparticle within monodisperse silica nanospheres (40 nm). Photoluminescence (PL) spectroscopy and UV–vis spectrum were used to investigate the optical properties of the nanoparticles. Compared to uncoated ZnS:Mn nanoparticles, the silica-coated ZnS:Mn nanoparticles have the improved PL intensity as well as good photostability. The obtained silica-coated ZnS:Mn nanoparticles are water-soluble and have fluorescence sensitivity to Cu²⁺ ions. Quenching of fluorescence intensity of the silica-coated nanoparticles allows the detection of Cu²⁺ concentrations as low as 7.3 × 10⁻⁹ mol L⁻¹, thus affording a very sensitive detection system for this chemical species. The possible quenching mechanism is discussed.     

Fabrication of a Sensitive Impedance Biosensor of DNA Hybridization Based on Gold Nanoparticles Modified Gold Electrode

In this work, we report on the preparation of a simple, sensitive DNA impedance sensor. Firstly gold nanoparticles were electrodeposited on the surface of a gold electrode, and then probe DNA was immobilized on the surface of gold nanoparticles through a 5′‐thiol‐linker. Electrochemical impedance spectroscopy (EIS) was used to investigate probe DNA immobilization and hybridization. Compared to the bare gold electrode, the gold nanoparticles modified electrode could improve the density of probe DNA attachment and the sensitivity of DNA sensor greatly. The difference of electron transfer resistance (ΔR_et) was linear with the logarithm of complementary oligonucleotides sequence concentrations in the range of 2.0×10^?12 to 9.0×10^?8 M, and the detection limit was 6.7×10^?13 M. In addition, the DNA sensor showed a fairly good reproducibility and stability during repeated regeneration and hybridization cycles.     

Colloid Au-enhanced DNA immobilization for the electrochemical detection of sequence-specific DNA

We use colloidal Au to enhance the DNA immobilization amount on a gold electrode and ultimately lower the detection limit of our electrochemical DNA biosensor. Self-assembly of approximately 16-nm diameter colloidal Au onto a cysteamine modified gold electrode resulted in an easier attachment of an oligonucleotide with a mercaptohexyl group at the 5′-phosphate end, and therefore an increased capacity for nucleic acid detection. Quantitative results showed that the surface densities of oligonucleotides on the Au colloid modified gold electrode were approximately (1–4)×10¹⁴ molecules cm⁻². Hybridization was induced by exposure of the ssDNA-containing gold electrode to ferrocenecarboxaldehyde labeled complementary ssDNA in solution. The detection limit is 5×10⁻¹⁰ mol l⁻¹ of complementary ssDNA, which is much lower than our previous electrochemical DNA biosensors. The Au nanoparticle films on the Au electrode provide a novel means for ssDNA immobilization and sequence-specific DNA detection.     

Novel potentiometric immunosensor for determination of diphtheria antigen based on compound nanoparticles and bilayer two-dimensional sol–gel as matrices

A novel method for fabrication of a diphtheria potentiometric immunosensor has been developed by means of self-assembling compound nanoparticles to a thiol-containing sol–gel network. A cleaned gold electrode was first immersed in a 3-mercaptopropyltrimethoxysilane (MPS) sol–gel solution to assemble a silica sol–gel monolayer. The silane entities were then polymerized into a two-dimensional sol–gel network (2D network) by dipping into aqueous NaOH. The second silane layer was formed by re-immersion in the MPS sol–gel solution overnight. The compound nanoparticles (nanocompounds) containing gold nanoparticles and silver nanoparticles were then chemisorbed on to the thiol groups of the second silane layer. Finally, diphtheria antibody (anti-Diph) was adsorbed on to the surface of the compound nanoparticles. The modified process was characterized by cyclic voltammetry (CV). Detection is based on the change in the potentiometric response before and after the antigen–antibody reaction. A direct potentiometric response to diphtheria antigen (Diph) was obtained from the immobilized diphtheria antibody. The potentiometric response of the resulting immunosensor was rapid and the linear range was from 22 to 800 ng mL^–1 with the linear regression equation E=–79.5+69.4 log [Diph] and a detection limit of 3.7 ng mL^–1 (at 3). Up to 19 successive assay cycles with retention of sensitivity were achieved for probes regenerated with 0.2 mol L^–1 glycine–hydrochloric acid (Gly–HCl) buffer solution. Moreover, analytical results from several serum samples obtained using the developed technique were in satisfactory agreement with those given by the ELISA method, implying a promising alternative approach for detecting diphtheria antigen in clinical diagnosis.     

Enzyme-modified nanoparticles using biomimetically synthesized silica

The entrapment of enzymes within biomimetic silica nanoparticles offers unique and simple immobilization protocols that merge the stability of proteins confined in solid phases with the high loading and reduced diffusion limitations inherent to nano-sized structures. Herein, we report on the biomimetic silica entrapment of chemically derivatized horseradish peroxidase for amperometric sensing applications. Scanning electron microscopy shows evidence of the formation of enzyme-modified nanospheres using poly(ethylenimine) as a template for silicic acid condensation. When these nanospheres are directly deposited on graphite electrodes, chemically modified anionic peroxidase shows direct electron transfer at 0 mV vs Ag|AgCl. Microgravimetric measurements as well as SEM images demonstrate that negatively charged peroxidase is also entrapped when silica precipitates at gold electrodes are modified with a self-assembled monolayer of poly(ethylenimine). Electrostatic interactions may play a crucial role for efficient enzyme entrapment and silica condensation at the PEI template monolayer. The in-situ biomimetically synthesized peroxidase nanospheres are catalytically active, enabling direct bioelectrocatalysis at 0 mV vs Ag|AgCl with long-term stability.