Agglomerated polymer monoliths with bimetallic nano-particles as flow-through micro-reactors

Polymer monoliths in capillary format have been prepared as solid supports for the immobilisation of platinum/palladium bimetallic nano-flowers. Optimum surface coverage of nano-flowers was realised by photografting the monoliths with vinyl azlactone followed by amination with ethylenediamine prior to nano-particle immobilisation. Field emission SEM imaging was used as a characterisation tool for evaluating nano-particle coverage, together with BET surface area analysis to probe the effect of nano-particle immobilisation upon monolith morphology. Ion exchange chromatography was also used to confirm the nature of the covalent attachment of nano-flowers on the monolithic surface. In addition, EDX and ICP analyses were used to quantify platinum and palladium on modified polymer monoliths. Finally the catalytic properties of immobilised bimetallic Pd/Pt nano-flowers were evaluated in flow-through mode, exploiting the porous interconnected flow-paths present in the prepared monoliths (pore diameter~1–2?μm). Specifically, the reduction of Fe (III) to Fe (II) and the oxidation of NADH to NAD⁺ were selected as model redox reactions. The use of a porous polymer monolith as an immobilisation substrate (rather than aminated micro-spheres) eliminated the need for a centrifugation step after the reaction.

Colloidal stability evolution and completely reversible aggregation of gold nanoparticles functionalized with rationally designed free radical initiators

A series of molecular adsorbates having various chain lengths of terminal poly(ethylene glycol methyl ether) (PEG) moieties, thiol head groups, and intervening free radical initiator moieties was used to functionalize the surface of gold nanoparticles (AuNPs). The bulky PEG groups stabilized the functionalized AuNPs by providing steric hindrance against AuNP aggregation, such aggregation being a major problem in the modification and manipulation of metal nanoparticles. UV–vis spectroscopy was used to evaluate the stability of the adsorbate-functionalized AuNPs as a function of AuNP size (～15, 40, and 90 nm in diameter) and PEG chain length (Mn 350, 750, and 2,000). The longer PEG chains (Mn 750 and 2,000) afforded stability to AuNPs with smaller gold cores (～15 and 40 nm in diameter) for up to several days without any marked aggregation. In contrast, the adsorbate-functionalized AuNPs with the largest gold cores (～90 nm) were noticeably less stable than those with the smaller gold cores. Importantly, the adsorbate-functionalized AuNPs could be isolated in solvent-free “dried” form and readily dispersed in aqueous buffer solution (both acidic and basic) and various organic solvents (protic and aprotic). This isolation–redispersion (i.e., aggregation/deaggregation) process was completely reversible. The chemisorption of the PEG-terminated initiator on the surface of the AuNPs was verified by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). As a whole, the strategy reported here affords colloidally stable, free radical initiator-functionalized AuNPs and offers a promising general method for encapsulating metal nanoparticles within polymer shells.

Electrochemiluminescent detection of the hybridization of oligonucleotides using an electrode modified with nanocomposite of carbon nanotubes and gold nanoparticles

We report on an electrochemiluminescent (ECL) sensing technique for the detection of the hybridization between oligonucleotides. A glassy carbon electrode was first functionalized with a composite prepared from gold nanoparticles and carbon nanotubes, and a sensor was then constructed by immobilizing the probing oligonucleotide. The ECL of luminol acts as the sensing signal. It is quenched, to a different degree, by the hybridized double strands of the oligonucleotide depending on the match status. The slope of the ECL response as a function of the status of hybridization drops with increasingly matched hybridization. The response is attributed to the interaction between luminol and the strands of oligomers, and also related to the reduction of reactive oxygen species.

Sensing Lorazepam with a glassy carbon electrode coated with an electropolymerized-imprinted polymer modified with multiwalled carbon nanotubes and gold nanoparticles

We report on an electrode for the amperometric determination of lorazepam. A glassy carbon electrode was coated with a molecular imprint made by electropolymerization of ortho-phenylenediamine and filled with multiwalled carbon nanotubes and gold nanoparticles, which enhances the transmission of electrons. The sensor was studied with respect to its response to hexacyanoferrate (III) as a probe and by electrochemical impedance spectroscopy, cyclic voltammetry and square wave voltammetry. The linear response range to Lorazepam is from 0.5 nM to 1.0 nM and from 1.0 nM to 10.0 nM, with a detection limit of 0.2 nM (at an S/N of 3). The electrode was successfully applied to determine Lorazepam in spiked human serum.

Electrochemical sensor for nitrobenzene based on carbon paste electrode modified with a poly(3,4-ethylenedioxythiophene) and carbon nanotube nanocomposite

A sensitive and selective electrochemical sensor for the determination of nitrobenzene (NB) was developed based on a carbon paste electrode (CPE) modified with a nanocomposite prepared from the conducting polymer poly(3,4-ethylenedioxythiophene) and carbon nanotubes. The modified CPE exhibits good conductivity, a large surface area, and excellent catalytic activity towards the electrochemical reduction of NB. Under optimal conditions, the modified CPE is capable of detecting NB in the 0.25 to 43 μM concentration range and with a detection limit at 83 nM. Moreover, the sensor is highly stable and reusable, and free of interferences by other commonly present nitro compounds. It was used to determine NB in wastewater samples.

Hemin-adsorbed carbon felt for sensitive and rapid flow-amperometric detection of dissolved oxygen

Hemin was physically adsorbed onto porous carbon felt (CF), a microelectrode ensemble of micro-carbon fiber (ca. 7 μm in diameter) and possessing a three-dimensional random structure. The hemin-CF exhibited a well-defined redox wave that is due to Fe(III)/Fe(II) redox process in hemin, with a formal potential of ?0.32 V (vs. Ag/AgCl) in deoxygenated buffer solution of pH 7.0. The surface coverage of the electroactive hemin molecules on the surface of the CF was calculated to be 5.0?×?10^?11 mol cm^?2, and the apparent heterogeneous electron transfer rate constant is 3.35 s^?1. The hemin-CF electrode displays excellent electrocatalytic activity for the reduction of dissolved oxygen (DO), and the magnitude of the cathodic current increases with increasing concentrations of DO in the sample solution. The electrode was used as a flow-through detector for sensitive and rapid consecutive determination of DO. Deoxygenated pH 7.0 solutions were analyzed at a flow rate of 8.0 mL min^?1 at an applied potential of ?0.2 V, and highly reproducible cathodic peak current responses to DO were observed in the 0.72 to 13.3 mg L^?1 concentration range. The maximum throughput is 170 samples h^?1. The hemin-CF-based amperometric flow-sensor was applied to determine the concentration of DO in environmental water samples.

Novel ion-imprinted polymer coated on nanoporous silica as a highly selective sorbent for the extraction of ultratrace quantities of gold ions from mine stone samples

We have developed a gold ion-imprinted polymer (GIP) by incorporating a dipyridyl ligand into an ethylene glycol dimethacrylate matrix which then was coated onto porous silica particles. The material was used for the selective extraction of ultratrace quantities of gold ion from mine stones, this followed by its quantitation by FAAS. The effects of concentration and volume of eluent, pH of the solution, flow rates of sample and eluent, and effect of potentially interfering ions, especially palladium and platinum, was investigated. The limit of detection is <0.2 ng?mL^?1, the precision (RSD%) is 1.03 %, and recoveries are >99 %. In order to show the high selectivity and efficiency of the new sorbent, the results were compared to those obtained with more simple sorbents possessing the same functional groups. The accuracy of the method was demonstrated by the accurate determination of gold ions in a certified reference material. To the best of our knowledge, there is no report so far on an imprint for gold ions that has such a selectivity over Pd(II) and Pt(II) ions.

Centri-voltammetric determination of glutathione

Centri-voltammetry is a method for concentrating an analyte on an electrode with the aid of a centrifuge. It can be performed in the presence or the absence of a solid carrier/support. This is followed by a voltammetric (analytical) scan. Specifically, we describe here an application of the technique to the determination of glutathione (GSH). Silica gel is used as the carrier precipitate to which gold nanoparticles were added in order to improve accumulation as a result of their strong affinity for thiols. Voltammetry was performed with a carbon paste electrode modified with multi-wall carbon nanotubes. The response to GSH is linear in the 25 and 800 μM concentration range (the correlation coefficient being 0.9915) and the relative standard deviation is 3.40 % (at 250 μM of GSH and n?=?6). The procedure was successfully applied to the determination of GSH in wine and in synthetic plasma using the standard addition method. The recoveries are 100.8 % and 100.0 %, respectively.

Continuous flow synthesis and characterization of tailor-made bare gold nanoparticles for use in SERS

We describe a method for the synthesis of gold nanoparticles in a stainless steel continuous flow tubular reactor using tetrachloroauric acid as a precursor but without using a classical reducing agent. Gold(III) ion is reduced by stainless steel to form gold nanoparticles which are collected at the end of the coil. A single-phase system is introduced that generates dispersed nanoparticles in the absence of reducing agents on their surface. By controlling flow rates and temperature, the size of the nanoparticles can be tuned in the range from 24 nm to 36 nm. The reproducibility of the preparation was investigated, relative standard deviation of both the wavelength of the peak and the intensity of the plasmonic absorption band were determined and found to vary by 0.15 % and 6.5 %, respectively. Flow synthesis is found to be an excellent alternative to chemical methods to produce stable gold nanoparticles of varying size in an efficiently way. The particles obtained also perform very well when used as a substrate in surface enhanced Raman scattering as shown by the characterization of carboxylated single walled carbon nanotubes.

Surface molecularly imprinted polydopamine films for recognition of immunoglobulin G

We have prepared a surface imprinted polymer (SIP) film for label-free recognition of immunoglobulin G (IgG). The IgG-SIPs were obtained by covalent immobilization of IgG via a cleavable covalent bond and a suitable spacer unit to a gold electrode, followed by electrodepostion of a nm-thin film of polydopamine (PDA). The IgG was then removed by destruction of the cleavable bond so that complementary binding sites were created on the surface of the film. IgG-SIPs with various thicknesses of the PDA films were compared with respect to their affinity to IgG using a quartz crystal microbalance combined with flow injection analysis. The films were also characterized by cyclic voltammetry and scanning electron microscopy. The IgG-SIPs with a film thickness of around 17 nm showed the most pronounced imprinting effect (IF 1.66) and a binding constant of 296 nM.

Simple optical determination of silver ion in aqueous solutions using benzo crown-ether modified gold nanoparticles

We describe a method for the modification of gold nanoparticles (Au-NPs) with benzo-15-crown-5 that led to the development of a colorimetric assay for Ag(I) ion. The brown color of a solution of the modified Au-NPs turns to purple on addition of Ag(I) ion. The ratio of the UV–vis absorption at 600 nm and 525 nm is proportional to the concentration of Ag(I) ions in the range from 20 to 950 nM, and the detection limit is 12.5 nM. Other metal ions do not interfere if present in up to millimolar concentrations. The method enables a rapid determination of Ag(I) in lake and drinking water and is amenable to bare-eye readout.

Preparation of a boronate-functionalized affinity hybrid monolith for specific capture of glycoproteins

A novel strategy for preparation of a boronate affinity hybrid monolith was developed using a Cu(I)-catalyzed 1,3-dipolar azide–alkyne cycloaddition (CuAAC) reaction of an alkyne–boronate ligand with an azide-functionalized monolithic intermediate. An azide-functionalized hybrid monolith was first synthesized via a single-step procedure to provide reactive sites for click chemistry; then the alkyne–boronate ligands were covalently immobilized on the azide-functionalized hybrid monolith via an in-column CuAAC reaction to form a boronate affinity hybrid monolith under mild conditions. The boronate affinity monolith was characterized and evaluated by means of elemental analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy. The boronate affinity hybrid monolith exhibited excellent specificity toward nucleosides and glycoproteins, which were chosen as test cis-diol-containing compounds under neutral conditions. The binding capacity of the monolith for the glycoprotein ovalbumin was 2.36 mg?·?g^-1 at pH 7.0. The practicability of the boronate affinity hybrid monolithic material was demonstrated by specific capture of the glycoproteins ovalbumin and ovotransferrin from an egg sample.

Self-assembly of ultra-small gold nanoparticles on an indium tin oxide electrode for the enzyme-free detection of hydrogen peroxide

A novel enzyme-free electrochemical sensor for H₂O₂ was fabricated by modifying an indium tin oxide (ITO) support with (3-aminopropyl) trimethoxysilane to yield an interface for the assembly of colloidal gold. Gold nanoparticles (AuNPs) were then immobilized on the substrate via self-assembly. Atomic force microscopy showed the presence of a monolayer of well-dispersed AuNPs with an average size of ~4 nm. The electrochemical behavior of the resultant AuNP/ITO-modified electrode and its response to hydrogen peroxide were studied by cyclic voltammetry. This non-enzymatic and mediator-free electrode exhibits a linear response in the range from 3.0?×?10^?5 M to 1.0?×?10^?3 M (M?=?mol?·?L^?1) with a correlation coefficient of 0.999. The limit of detection is as low as 10 nM (for S/N?=?3). The sensor is stable, gives well reproducible results, and is deemed to represent a promising tool for electrochemical sensing.

Ultra-sensitive electrochemical DNA biosensor based on signal amplification using gold nanoparticles modified with molybdenum disulfide,graphene and horseradish peroxidase

We have developed an ultra-sensitive electrochemical DNA biosensor by assembling probe ssDNA on a glassy carbon electrode modified with a composite made from molybdenum disulfide, graphene, chitosan and gold nanoparticles. A thiol-tagged DNA strand coupled to horseradish peroxidase conjugated to AuNP served as a tracer. The nanocomposite on the surface acts as relatively good electrical conductor for accelerating the electron transfer, while the enzyme tagged gold nanoparticles provide signal amplification. Hybridization with the target DNA was studied by measuring the electrochemical signal response of horseradish peroxidase using differential pulse voltammetry. The calibration plot is linear in the 5.0?×?10^?14 and 5.0?×?10^?9 M concentration range, and the limit of detection is 2.2?×?10^?15 M. The biosensor displays high selectivity and can differentiate between single-base mismatched and three-base mismatched sequences of DNA. The approach is deemed to provide a sensitive and reliable tool for highly specific detection of DNA.

Impedimetric aptasensor for Staphylococcus aureus based on nanocomposite prepared from reduced graphene oxide and gold nanoparticles

We describe here an aptasensor for the ultrasensitive detection of Staphylococcus aureus by electrochemical impedance spectroscopy (EIS). Single-stranded DNA was linked to a nanocomposite prepared from reduced graphene oxide (rGO) and gold nanoparticles (AuNP). Thiolated ssDNA was covalently linked to the AuNPs linked to rGO, and probe DNA was immobilized on the surface of an AuNP-modified glassy carbon electrode to capture and concentrate Staph. aureus. The probe DNA of the aptasensor selectively captures the target bacteria in its three-dimensional space, and these results in a dramatic increase in impedance. Scanning electron microscopy, cyclic voltammetry and EIS were used to monitor the single steps of the electrode assembly process. The effect was utilized to quantify the bacteria in the concentration range from 10 to 10⁶ cfu mL^?1 and with a detection limit of 10 cfu mL^?1 (S/N?=?3). The relative standard deviation of Staphylococcus aureus detection was equal to 4.3 % (10⁵ cfu mL^?1, n?=?7). In addition to its sensitivity, the biosensor exhibits high selectivity over other pathogens.

Amperometric determination of sulfite using screen-printed electrodes modified with metallic nanoparticles

We report on the amperometric determination of sulfite using screen-printed carbon electrodes (SPCEs) modified with gold and silver nanoparticles that were deposited on the electrode to improve the capabilities of detection. The electrode is fairly selective and responds to sulfite with an oxidation current (at 300 mV and pH 6) in the 9.80 to 83.33 μM concentration range. The precision in terms of repeatability and reproducibility is 14.4 % and 10.7 % in the case of SPCEs modified by gold nanoparticles. The method was applied to the determination of sulfite in drinking water, pickle juice and vinegar. Recoveries ranged from 96 % to 104 %.

Facile synthesis of a porous network-like silver film for electrocatalytic detection of nitrate

We have developed a method for in-situ construction of a porous network-like silver film on the surface of a glassy carbon electrode (GCE). It is based on a galvanic replacement reaction where a layer of copper nanoparticles is first electrodeposited as a sacrificial template. The silver film formed possesses a porous network-like structure and consists of an assembly of numerous nanoparticles with an average size of 200 nm. The electrode displays excellent electrocatalytic activity, good stability, and fast response (within 2 s) toward the reduction of nitrate at a working potential of ?0.9 V. The catalytic currents linearly increase with the nitrate concentrations in the range of 0.08–6.52 mM, with a detection limit of 3.5 μM (S/N?=?3) and a repeatability of 3.4 % (n?=?5).

Development of monolith-based stir bar sorptive extraction and liquid chromatography tandem mass spectrometry method for sensitive determination of ten sulfonamides in pork and chicken samples

A highly sensitive method was developed for the simultaneous determination of ten sulfonamides in pork and chicken samples by monolith-based stir bar sorptive extraction (SBSE) coupled to high-performance liquid chromatography tandem mass spectrometry. The samples were freeze-dried and extracted by acetonitrile, then enriched and further extracted by SBSE which was based on poly(vinylphthalimide-co-N,N-methylenebisacrylamide) monolith (SBSE-VPMB) as coating. To achieve optimum extraction performance of SBSE for sulfonamides, several parameters, including pH value and ionic strength in the sample matrix and extraction and desorption time, were investigated in detail. Under the optimal conditions, the limits of detection (S/N?=?3) for target sulfonamides were 1.2–6.1 ng/kg in pork and 2.0–14.6 ng/kg in chicken, respectively. Real samples spiked at the concentration of 0.5 and 5.0 μg/kg showed recoveries above 55 % and relative standard deviations below 12 %. At the same time, the extraction performances of target sulfonamides on SBSE-VPMB were compared with other SBSE based on porous monolith and commercial SBSE.

New method for preparation of polyoxometalate-capped gold nanoparticles, and their assembly on an indium-doped tin oxide electrode

Functionalized gold nanoparticles capped with polyoxometalates were prepared by a simple photoreduction technique where phosphododecamolybdates serve as reducing reagents, photocatalysts, and as stabilizers. TEM images of the resulting gold nanoparticles show the particles to have a relative narrow size distribution. Monolayer and multilayer structures of the negatively charged capped gold nanoparticles were deposited on a poly(vinyl pyridine)-derivatized indium-doped tin oxide (ITO) electrode via the layer-by-layer technique. The surface plasmon resonance band of the gold nanoparticles displays a blue shift on the surface of the ITO electrode. This is due to the substrate-induced charge redistribution in the gold nanoparticles and a change in the electromagnetic coupling between the assembled nanoparticles. The modified electrode exhibits the characteristic electrochemical behavior of surface-confined phosphododecamolybdate and excellent electrocatalytic activity. The catalysis of the modified electrode towards the model compound iodate was systematically studied. The heterogeneous catalytic rate constant for the electrochemical reduction of iodate was determined by chronoamperometry to be ca. 1.34?×?10⁵ mol^?1·L·s^?1. The amperometric method gave a linear range from 2.5?×?10^?6 to 1.5?×?10^?3 M and a detection limit of 1.0?×?10^?6 M. We believe that the functionalized gold nanoparticles prepared by this photoreduction technique are advantageous in terms of fabrication of sensitive and stable redox electrodes.

A glassy carbon electrode modified with poly(anthranilic acid), poly(diphenylamine sulfonate) and CuO nano-particles for the sensitive determination of hydrogen peroxide

We report on a modified glassy carbon electrode (GCE) for sensing hydrogen peroxide (H₂O₂). It was constructed by consecutive electrochemical deposition of poly(anthranilic acid) and poly(diphenylamine sulfonate) on the GCE, followed by the deposition of copper oxide (CuO). The morphology and electrochemistry of the modified electrode was characterized by atomic force microscopy, X-ray diffraction, cyclic voltammetry, and electrochemical impedance spectroscopy. The catalytic performance of the sensor was studied with the use of differential pulse voltammetry under optimized conditions. This sensor displayed significantly better electrocatalytic activity for the reduction of H₂O₂ in comparison to a GCE without or with modification with CuO or polymer films alone. The response to H₂O₂ is linear in the range between 0.005 to ~11 mM, and the detection limit is 0.18 μM (at an S/N of 3).