Effect of metals on silver electrodeposition: Application to the detection of cisplatin

In the present work, the influence of several metals (Co, Ru, Pd, Os, Pt, Cu, Pb), deposited on a carbon paste electrode, towards silver electrodeposition was tested. First, adequate conditions for the electrodeposition of metals on the electrode were found. Then, the cyclic voltammograms registered (silver deposition curves and analytical signals) showed that Co, Cu, Pt and Pd were able to accelerate silver electrodeposition. Finally, a valid methodology for the detection of cisplatin was established. It is based on the deposition of silver on a Pt (from cisplatin) modified electrode and the analytical signal corresponds to the anodic stripping of the deposited silver. A limit of detection of 3.2 × 10⁻⁹ mol dm⁻³ (1 ng cm⁻³) cisplatin was obtained.     

Relative Nuclease Resistance of a DNA Aptamer Covalently Conjugated to a Target Protein

A major obstacle to the therapeutic application of an aptamer is its susceptibility to nuclease digestion. Here, we confirmed the acquisition of relative nuclease resistance of a DNA-type thrombin binding aptamer with a warhead (TBA₃) by covalent binding to a target protein in the presence of serum/various nucleases. When the thrombin-inhibitory activity of TBA₃ on thrombin was reversed by the addition of the complementary strand, the aptamer was instantly degraded by the nucleases, showing that the properly folded/bound aptamer conferred the resistance. Covalently binding aptamers possessing both a prolonged drug effect and relative nuclease resistance would be beneficial for in vivo translational applications.     

Electrochemical detection of avian influenza virus H5N1 gene sequence using a DNA aptamer immobilized onto a hybrid nanomaterial-modified electrode

A sensitive electrochemical method for the detection of avian influenza virus (AIV) H5N1 gene sequence using a DNA aptamer immobilized onto a hybrid nanomaterial-modified electrode was developed. To enhance the selectivity and sensitivity, the modified electrode was assembled with multi-wall carbon nanotubes (MWNT), polypyrrole nanowires (PPNWs) and gold nanoparticles (GNPs). This electrode offered a porous structure with a large effective surface area, highly electrocatalytic activities and electronic conductivity. Therefore, the amount of DNA aptamer immobilized onto the electrode was increased while the accessibility of the detection target was maintained. The biosensor is based on the hybridization and preferred orientation of a DNA aptamer immobilized onto a modified electrode surface with its target (H5N1 specific sequence) present in solution. It is selective for the H5N1 specific sequence, and the signal of the indicator was approximately linear to log(concentration) of the H5N1 specific sequence from 5.0 × 10⁻¹² to 1.0 × 10⁻⁹ M (R = 0.9863) with a detection limit of 4.3 × 10⁻¹³ M. These studies showed that the new hybrid nanomaterial (MWNT/PPNWs/GNPs) and the DNA aptamer could be used to fabricate an electrochemical biosensor for gene sequence detection. Furthermore, this design strategy is expected to have extensive applications in other biosensors.     

Colloidal silver nanoparticles modified electrode and its application to the electroanalysis of Cytochrome c

A colloidal silver nanoparticles (CSNs) chemically modified electrode was prepared and its application to the electroanalysis of Cytochrome c (Cyt. c) was studied. The CSNs were prepared by reduction of AgNO₃ with NaBH₄, and were stabilized by oleate. They could be efficiently immobilized on the surface of a silver electrode. The result showed that the CSNs could clearly enhance the electron transfer process between Cyt. c and the electrode compared with bulk silver electrode. Linear sweep voltammetric measurement of Cyt. c at the chemical modified electrode indicated that the oxidative peak current of Cyt. c was linear to its concentration ranging from 8.0 nmol L⁻¹ to 3.0 μmol L⁻¹ with the calculated detection limit was about 2.6 nmol L⁻¹. The direct electrochemistry of Cyt. c was also studied by cyclic voltammetry.     

Aptasensors for rapid detection of Escherichia coli O157:H7 and Salmonella typhimurium

Herein we reported the development of aptamer-based biosensors (aptasensors) based on label-free aptamers and gold nanoparticles (AuNPs) for detection of Escherichia coli (E. coli) O157:H7 and Salmonella typhimurium. Target bacteria binding aptamers are adsorbed on the surface of unmodified AuNPs to capture target bacteria, and the detection was accomplished by target bacteria-induced aggregation of the aptasensor which is associated as red-to-purple color change upon high-salt conditions. By employing anti-E. coli O157:H7 aptamer and anti-S. typhimurium aptamer, we developed a convenient and rapid approach that could selectively detect bacteria without specialized instrumentation and pretreatment steps such as cell lysis. The aptasensor could detect as low as 10⁵colony-forming units (CFU)/ml target bacteria within 20 min or less and its specificity was 100%. This novel method has a great potential application in rapid detection of bacteria in the near future.     

A sensitive electrochemical aptasensor based on water soluble CdSe quantum dots (QDs) for thrombin determination

A novel aptamer biosensor with easy operation and good sensitivity, specificity, stability and reproducibility was developed by immobilizing the aptamer on water soluble CdSe quantum dots (QDs) modified on the top of the glassy carbon electrode (GCE). Methylene blue (MB) was intercalated into the aptamer sequence and used as an electrochemical marker. CdSe QDs improved the electrochemical signal because of their larger surface area and ion centers of CdSe QDs may also had a major role on amplifying the signal. The higher ion concentration caused more combination of aptamer which caused larger signal. The thrombin was detected by differential pulse voltammetry (DPV) quantitatively. Under optimal conditions, the two linear ranges were obtained from 3 to 13 μg mL⁻¹ and from 14 to 31 μg mL⁻¹, respectively. The detection limit was 0.08 μg mL⁻¹ at 3σ. The constructed biosensor had better responses compared with that in the absence of the CdSe QDs immobilizing. The control experiment was also carried out by using BSA, casein and IgG in the absence of thrombin. The results showed that the aptasensor had good specificity, stability and reproducibility to the thrombin. Moreover, the aptasensor could be used for detection of real sample with consistent results in comparison with those obtained by fluorescence method which could provide a promising platform for fabrication of aptamer based biosensors.     

Quality Control Certification of RNA Aptamer‐Based Detection

Aptamers are single‐stranded DNA or RNA molecules with a defined tertiary structure for molecular recognition. Numerous RNA aptamers with excellent binding affinity and specificity have been reported; they constitute an attractive reservoir of molecular recognition elements for biosensor development. However, RNA is relatively unstable owing to spontaneous hydrolysis and nuclease degradation. Thus, RNA aptamer‐based biosensors are prone to producing false‐positive signals. Here, we present an RNA aptamer biosensor design strategy that utilises an internal control to distinguish target binding from false‐positive signals. The sequence of a chosen RNA aptamer is expanded so that it can form three consecutive short RNA–DNA duplexes with 1) a quencher‐labelled DNA strand (Q₁DNA), 2) a dual‐fluorophore‐labelled DNA strand (F₁DNAF₂) and 3) another quencher‐labelled DNA strand (Q₂DNA). The addition of a target releases Q₂DNA from the duplex assembly, and produces the expected positive signal from F₂. However, the authenticity of target recognition is validated only if no signal is generated from F₁. We have successfully engineered two fluorescent reporters by using an RNA aptamer that binds thrombin and one that binds theophylline. Both reporters show the expected binding affinity and specificity, and are capable of reporting system malfunction when treated with nucleases and chemical denaturants. This strategy provides a simple and reliable way to ensure high‐quality detection when RNA aptamers are employed as molecular‐recognition elements.     

A label-free DNA sensor based on impedance spectroscopy

This paper describes a label-free detection system for DNA strands based on gold electrodes and impedance measurements. A single-stranded 18 mer oligonucleotide (ssDNA) was immobilised via a thiol linker on gold film electrodes and served as probe DNA. Residual binding places were filled with mercaptobutanol. The sensor surface clearly distinguished between complementary and non-complementary target ssDNA. Additionally, detection of single base pair mismatches was possible. The electrode was impedimetrically characterised in the presence of the redox system ferri/ferrocyanide before and after DNA hybridisation. Impedance analysis showed that the charge transfer resistance, R_ct, was increasing after DNA duplex formation, whereas the capacitive properties remain rather unaltered. The relative change of R_ct was used as sensor parameter. Concentrations in the nanomolar range have been detected by the system. The sensor was reusable because a denaturation protocol allowed effective double strand dissociation without changing the surface properties of the electrode substantially. The time for DNA detection have been reduced to about 15 min including regeneration. The sensor signal was amplified by about 20% after binding of a negatively charged molecule to the formed DNA duplex. The sensor was also capable of sensing longer target ssDNA strands as shown with 25 mer and 37 mer oligonucleotides.     

Enhanced sensitivity of surface plasmon resonance phase-interrogation biosensor by using oblique deposited silver nanorods

Sensitivity of surface plasmon resonance phase-interrogation biosensor is demonstrated to be enhanced by oblique deposited silver nanorods. Silver nanorods are thermally deposited on silver nanothin film by oblique angle deposition (OAD). The length of the nanorods can be tuned by controlling the deposition parameters of thermal deposition. By measuring the phase difference between the p and s waves of surface plasmon resonance heterodyne interferometer with different wavelength of incident light, we have demonstrated that maximum sensitivity of glucose detection down to 7.1 × 10^-8 refractive index units could be achieved with optimal deposition parameters of silver nanorods.     

Electrochemical tolazoline sensor based on gold nanoparticles and imprinted poly-o-aminothiophenol film

Amperometric detection of tolazoline (TL) was carried out on a gold nanoparticles (AuNPs)/poly-o-aminothiophenol (PoAT)-modified electrode by a molecular imprinting technique and electropolymerization method. The modification procedure was characterized via electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The recognition between the imprinted sensor and target molecule was observed by measuring the variation of amperometric response of the oxidation-reduction probe, K₃Fe(CN)₆ on electrode. Under the optimal experimental conditions, the peak currents were proportional to the concentrations of tolazoline in two ranges of 0.05-5.0 μg mL⁻¹ and 5.0-240 μg mL⁻¹ with the detection limit of 0.016 μg mL⁻¹. Meanwhile the prepared sensor showed sensitive and selective binding sites for tolazoline. The enhancement of sensitivity was attributed to the presence of AuNPs which decreased the electron-transfer impedance.     

Fractal gold modified electrode for ultrasensitive thrombin detection

We report a label-free and ultrasensitive aptasensor based on a fractal gold modified (FracAu) electrode for thrombin detection with a femtomolar detection limit. The FracAu electrode was prepared by electrodeposition of hydrogen tetrachloroaurate (HAuCl(4)) onto a bare indium tin oxide (ITO) electrode surface. After this process the electrode was characterized by SEM. A thiol-modified aptamer against thrombin was immobilized on the FracAu electrode through a self-assembling process. Upon thrombin binding, the interfacial electron transfer of the FracAu electrode was perturbed by the formation of an aptamer-thrombin complex. The concentration of thrombin in the sample solution was determined by measuring the change in the oxidation peak current of hydroxymethyl ferrocene (C(11)H(12)FeO) with differential pulse voltammetry (DPV). The current response (reduced peak current) had a linear relationship with the logarithm of thrombin concentrations in the range of 10(-15) to 10(-10) M with a detection limit of 5.7 fM. Furthermore, the as-prepared FracAu electrode exhibited high selectivity. The application of FracAu electrodes may be extended to prepare other types of biosensors, such as immunosensors, enzyme biosensors and DNA biosensors. These results show that FracAu electrodes have great promise for clinical diagnosis of disease-related biomarkers.     

Anodic stripping voltammetric measurement of trace heavy metals at antimony film carbon paste electrode

The antimony film carbon paste electrode (SbF-CPE) was prepared in situ on the carbon paste substrate electrode as a “mercury-free” electrochemical sensor. Its aptitude for measuring some selected trace heavy metals has been demonstrated in combination with square-wave anodic stripping voltammetry in non-deaerated model solutions of 0.01 M hydrochloric acid with pH 2. Some important operational parameters, such as deposition potential, deposition time, and concentration of antimony ions were optimized, and the electroanalytical performance of the SbF-CPE was critically compared with both bismuth film carbon paste electrode (BiF-CPE) and mercury film carbon paste electrode (MF-CPE) using Cd(II) and Pb(II) as test metal ions. In comparison with BiF-CPE and MF-CPE, the SbF-CPE exhibited superior electroanalytical performance in more acidic medium (pH 2) associated with favorably low hydrogen evolution, improved stripping response for Cd(II), and moreover, stripping signals corresponding to Cd(II) and Pb(II) at the SbF-CPE were slightly narrower than those observed at bismuth and mercury counterparts. In addition, the comparison with antimony film electrode prepared at the glassy carbon substrate electrode displayed higher stripping current response recorded at the SbF-CPE. The newly developed sensor revealed highly linear behavior in the examined concentration range from 5 to 50 μg L⁻¹, with limits of detection (3σ) of 0.8 μg L⁻¹ for Cd(II), and 0.2 μg L⁻¹ for Pb(II) in connection with 120 s deposition step, offering good reproducibility of ±3.8% for Cd(II), and ±1.2% for Pb(II) (30 μg L⁻¹, n = 10). Preliminary experiments disclosed that SbF-CPE and MF-CPE exhibit comparable performance for measuring trace concentration levels of Zn(II) in acidic medium with pH 2, whereas its detection with BiF-CPE was practically impossible. Finally, the practical applicability of SbF-CPE was demonstrated via measuring Cd(II) and Pb(II) in a real water sample.     

Real time protein recognition in a liquid-gated carbon nanotube field-effect transistor modified with aptamers

The combination of optimized and passivated Field Effect Transistors (FETs) based on carbon nanotubes (CNTs) together with the appropriate choice and immobilization strategy of aptamer receptors and buffer concentration have allowed the highly sensitive and real time biorecognition of proteins in a liquid-gated configuration. Specifically we have followed the biorecognition process of thrombin by its specific aptamer. The aptamer modified device is sensitive enough to capture a change in the electronic detection mechanism, one operating at low protein concentrations and the other in a higher target concentration range. The high sensitivity of the device is also sustained by the very low detection limits achieved (20 pM) and their high selectivity when other target proteins are used. Moreover, the experimental results have allowed us to quantify the equilibrium constant of the protein-aptamer binding and confirm its high affinity by using the Langmuir equation.     

The electroreduction kinetics of silver sulfite complexes

The electroreduction kinetics of silver sulfite complexes was investigated by rotation disk electrode (RDE) voltammetry, chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS). The stability constants of the silver sulfite complexes, pβ₂ = 7.9 and pβ₃ = 8.53 were determined. For the series of isopotential solutions investigated, a reaction order of 0.67 was obtained, the diffusion coefficient of the silver complexes varies in the range of 3.36 × 10⁻⁶ to 5.54 × 10⁻⁶ cm² s⁻¹ and the silver degree of complexation (2.31-2.67) were found. The analysis of the RDE, CP data and EIS spectra indicate the existence of a slow stage of the silver electrocrystallization in the region of the equilibrium potential and at stronger polarization of the electrode at initial time moments.     

Impedimetric thrombin aptasensor based on chemically modified graphenes

Highly sensitive biosensors are of high importance to the biomedical field. Graphene represents a promising transducing platform for construction of biosensors. Here for the first time we compare the biosensing performance of a wide set of graphenes prepared by different methods. In this work, we present a simple and label-free electrochemical impedimetric aptasensor for thrombin based on chemically modified graphene (CMG) platforms such as graphite oxide (GPO), graphene oxide (GO), thermally reduced graphene oxide (TR-GO) and electrochemically reduced graphene oxide (ER-GO). Disposable screen-printed electrodes were first modified with chemically modified graphene (CMG) materials and used to immobilize a DNA aptamer which is specific to thrombin. The basis of detection relies on the changes in impedance spectra of redox probe after the binding of thrombin to the aptamer. It was discovered that graphene oxide (GO) is the most suitable material to be used as compared to the other three CMG materials. Furthermore, the optimum concentration of aptamer to be immobilized onto the modified electrode surface was determined to be 10 μM and the linear detection range of thrombin was 10-50 nM. Lastly, the aptasensor was found to demonstrate selectivity for thrombin. Such simply fabricated graphene oxide aptasensor shows high promise for clinical diagnosis of biomarkers and point-of-care analysis.     

Electro-recovery of gold and silver from a cyanide leaching solution using a three-dimensional reactor

The selective electro-recovery of gold and silver values from cyanide leaching solutions containing copper was accomplished in a three-dimensional (3D) electrochemical reactor. This case let to contrast three different points of view when dealing with a composed metallic solution: First, the thermodynamic predictions; second, the microelectrolysis approach and finally, the macroelectrolysis experiments. Standard electrode potentials for the study solution would indicate a tendency for gold to deposit first. However, microelectrolysis studies of the three-metallic solution indicated that gold and silver are co-deposited onto a Vitreous carbon (VC) electrode without copper interference in a narrow potential range. Mass balances during the macroelectrolysis experiments (batch model assuming mass transfer control) indicated a preferential deposition of silver during the first ten minutes, even if gold deposition also occurred. On the other hand, values of Stanton (St) for different linear flow velocity corroborated that metals concentration gradients may establish a limit to make profitable the fluid velocity increase in an electrochemical flow cell. Electrolysis experiments were carried out under potentiostatic (at −1400 mV versus SCE) and galvanostatic (at −3.9 Am⁻²) conditions in the FM-01 LC flow cell.     

Silver-zinc electrodeposition from a thiourea solution with added EDTA or HEDTA

This paper shows the study of silver-zinc electrodeposition from a thiourea solution with added (ethylenedinitrilo)tetraacetic acid (EDTA), disodium salt and N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), trisodium salt. Voltammetric results indicated that silver-zinc alloy can be obtained applying overpotential higher than 0.495 V, in Tu solution containing 1.0 × 10⁻¹ mol L⁻¹ Zn(NO₃)₂ + 2.5 × 10⁻² mol L⁻¹ AgNO₃. This was due to silver(I) ion complexation with thiourea, which shifted the silver deposition potential to more negative value and due to silver-zinc alloy deposition, which occurred at potentials more positive than the potential to zinc deposition alone. EDTA or HEDTA did not significantly affect the silver and zinc deposition potentials, but decreased the current density for silver-zinc deposition. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses of the silver-zinc deposits showed that the morphology and composition changed as a function of the conditions of deposition, viz, deposition potential (E_d), deposition charge density (q_d) and solution composition (silver, EDTA and HEDTA concentrations). EDS analysis of the deposits showed sulphur (S) incorporated into the silver-zinc deposit, while SEM images showed that this sulphur content seemed to improve the silver-zinc morphology, as did the presence of EDTA and HEDTA in the solution, which enhanced the sulphur incorporation into the silver-zinc deposit. X-ray diffraction (XRD) analysis of the silver-zinc deposit showed that it was amorphous, irrespective of its composition and morphology.     

Electrochemical performance of silver-modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathodes prepared via electroless deposition

Silver-modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathodes for intermediate-temperature solid-oxide fuel cells (IT-SOFCs) were prepared by an electroless deposition process using N₂H₄ as the reducing agent at room temperature. This fabrication technique together with tailored electrode porosity, modified the BSCF electrodes with silver content that varied from 0.3 to 30 wt.% without damaging the electrode microstructure. Both the Ag loading and firing temperatures were found to have a significant impact on the electrode performance, which could facilitate or block the electrochemical processes of the BSCF-based cathodes, processes that include charge-transfer, oxygen adsorption and oxygen electrochemical reduction. At an optimal Ag loading of 3.0 wt.% and firing temperature of 850 °C, an area specific resistance of only 0.042 Ω cm² at 600 °C was achieved for a modified BSCF cathode.     

Electrodeposition of macroporous silver films from ionic liquids and assessment of these films in the electrocatalytic reduction of nitrate

Macroporous silver films, ordered or fragmented, were fabricated by electrodeposition of silver into the interstitial spaces of templates formed by polystyrene (PS) latex spheres that had been self-assembled onto bare indium tin oxide (ITO) electrodes or onto gold-coated ITO (ITO/Au) electrodes (in which the electrode had been coated by gold sputtering deposition) from two room-temperature ionic liquids (ILs): N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) and N-butyl-N-methylpyrrolidinium dicyanamide (BMP-DCA), respectively, under normal atmospheric conditions. After electrodeposition of silver, the PS spheres were removed by dissolution in tetrahydrofuran (THF) to leave a macroporous silver structure. The higher wettability of ILs onto PS spheres leads to improved penetration of the ILs into the cavities of the PS templates. Electrodeposition is easier if an electrolyte that has a good penetration into the interstitial spaces between the PS spheres. The macroporous silver electrode exhibited much better electrocatalytic performance with respect to nitrate reduction than a regular silver wire electrode. Quantitative determination of nitrate was also examined.     

Effect of hydroxyl ions on chloride penetration depth measurement using the colorimetric method

In this paper, the effect of hydroxyl ions on chloride penetration depth measurement using the colorimetric method was studied. Equivalent silver nitrate solution (i.e. Ag⁺ = Cl⁻) was added to the NaCl + NaOH solution with different concentrations, then the amount of precipitated silver chloride and silver oxide were determined by chemical methods, and the color of the precipitated products was examined. Results show that the amount of silver chloride formed decreases linearly as OH⁻ to Cl⁻ ratio (r) increases. Thus, the chloride concentration at color change boundary changes with the pH value of the concrete. AgCl has a white color, while Ag₂O has a dark brown color. When the value of r exceeds 4, the color of the mixture looks brown, and color change boundary cannot be easily distinguished.