Label-free and sensitive faradic impedance aptasensor for the determination of lysozyme based on target-induced aptamer displacement

A label-free and sensitive faradic impedance spectroscopy (FIS) aptasensor based on target-induced aptamer displacement was developed for the determination of lysozyme as a model system. The aptasensor was fabricated by self-assembling the partial complementary single strand DNA (pcDNA)–lysozyme binding aptamer (LBA) duplex on the surface of a gold electrode. To measure lysozyme, the change in interfacial electron transfer resistance of the aptasensor using a redox couple of [Fe(CN)₆]^3−/4− as the probe was monitored. The introduction of target lysozyme induced the displacement of the LBA from the pcDNA–LBA duplex on the electrode into the solution, decreasing the electron transfer resistance of the aptasensor. The decrease in the FIS signal is linear with the concentration of lysozyme in the range from 0.2 nM to 4.0 nM, with a detection limit of 0.07 nM. The fabricated aptasensor shows a high sensitivity, good selectivity and satisfactory regeneration. This work demonstrates that a high sensitivity of the fabricated aptasensor can be obtained using a relatively short pcDNA. This work also demonstrates that the target-induced aptamer displacement strategy is promising in the design of an electrochemical aptasensor for the determination of lysozyme with good selectivity and high sensitivity.     

Determination of nanomolar uric and ascorbic acids using enlarged gold nanoparticles modified electrode

Individual and simultaneous determination of 50 nM uric acid (UA) and ascorbic acid (AA) using enlarged, citrate-stabilized gold nanoparticles (AuNPs) self-assembled to 2,5-dimercapto-1,3,4-thiadiazole (DMT) monolayer modified Au (Au/DMT) electrode by an amperometric method is described for the first time. Self-assembly of AuNPs on the electrode surface was confirmed by atomic force microscopy (AFM), attenuated total reflectance FT-IR and diffuse reflectance spectral measurements. The electron transfer reaction (ETR) of [Fe(CN)₆]^3−/4− was blocked at Au/DMT electrode, whereas it was restored with a peak separation of 200 mV after the attachment of AuNPs on the Au/DMT (Au/DMT/AuNPs) electrode, which was confirmed from the ETR of the [Fe(CN)₆]^3−/4− redox couple. When the self-assembled AuNPs were enlarged by hydroxylamine seeding, the ETR of [Fe(CN)₆]^3−/4− was improved significantly with a peak separation of 100 mV. Tapping mode AFM showed that the average size of the enlarged-AuNPs (E-AuNPs) was 50-70 nm. The E-AuNPs modified electrode catalyzes the oxidation of AA and UA, separates their voltammetric signals by 200 mV, and has excellent sensitivity towards AA and UA with a detection limit of 50 nM. The practical application of the modified electrode was demonstrated by measuring the concentration of UA in blood serum and urine.     

Design of ultrasensitive bisphenol A–aptamer based on platinum nanoparticles loading to polyethyleneimine-functionalized carbon nanotubes

Here, a highly sensitive electrochemical aptasensor based on a novel signal amplification strategy for the determination of bisphenol A (BPA) was developed. Construction of the aptasensor began with the deposition of highly dispersed platinum nanoparticles (PtNPs)/acid-oxidized carbon nanotubes (CNTs–COOH) functionalized with polyethyleneimine (PEI) at the surface of glassy carbon (PtNPs/PEI/CNTs–COOH/GC) electrode. After immobilizing the amine-capped capture probe (ssDNA1) through the covalent amide bonds formed by the carboxyl groups on the nanotubes and the amino groups on the oligonucleotides, we employed a designed complementary BPA–aptamer (ssDNA2) as a detection probe to hybridize with the ssDNA1. By adding BPA as a target, the aptamer specifically bound to BPA and its end folded into a BPA-binding junction. Because of steric/conformational restrictions caused by aptamer–BPA complex formation at the surface of modified electrode, the interfacial electron transfer of [Fe(CN)₆]^3−/4− as a probe was blocked. Sensitive quantitative detection of BPA was carried out by monitoring the decrease of differential pulse voltammetric responses of [Fe(CN)₆]^3−/4− peak current with increasing BPA concentrations. The newly developed aptasensor embraced a number of attractive features such as ease of fabrication, low detection limit, excellent selectivity, good stability and a wide linear range with respect to BPA.     

Entropies of Redox Reactions between Proteins and Mediators: The Temperature Dependence of Reversible Electrode Potentials in Aqueous Buffers

The temperature dependencies of the reversible electrode potentials for a number of charge transfer reactions of redox mediators were used to evaluate the corresponding charge transfer entropies in Tris–HCl (pH 8) buffer. The redox mediator thermodynamic data, along with reaction enthalpy data for mediator redox protein electron transfer, were used to evaluate the charge transfer entropy for the cytochrome c redox couple [(cytc)_ox/(cytc)_red] in Tris–HCl (pH 8) buffer and were found to be equal to −16 cal/°K mol. Reversible electrode potentials at 298°K for the redox mediator half-reactions were observed to vary from −528 to +657 mV (vs NHE). Charge transfer entropies were observed to depend upon the structure of the redox mediators and to vary from −13.8 to −29.7 cal/°K mol for a closely related series of organic dications (viologens) and a value of −43.6 cal/°K mol was observed for the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻couple under the same conditions. A procedure for determining charge transfer entropies of protein redox couples which cannot be studied by direct electrochemical methods is outlined. The factors contributing to the magnitude of the charge transfer entropies are discussed.     

A novel label-free electrochemical aptasensor based on graphene-polyaniline composite film for dopamine determination

A novel label-free electrochemical aptasensor based on graphene-polyaniline (GR-PANI) nanocomposites film for dopamine (DA) determination was reported. The resulting GR-PANI layer exhibited good current response for DA determination. The good electron transfer activity might be attributed to the effect of GR and PANI. The highly conductive and biocompatible nanostructure of GR-PANI nanocomposites was characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). To quantify the amount of DA, the peaks of square-wave voltammetry (SWV) were monitored using the redox couple of an [Fe(CN)(6)](4-/3-) probe. The electrochemical aptasensor showed a linear response to DA in the range 0.007-90 nmol/L and a limit of detection of 0.00198 nmol/L (S/N=3). The electrochemical aptasensor was successfully tested on human serum samples.     

An aptasensor for ochratoxin A based on grafting of polyethylene glycol on a boron-doped diamond microcell

A novel strategy for the fabrication of an electrochemical label-free aptasensor for small-size molecules is proposed and demonstrated as an aptasensor for ochratoxin A (OTA). A long spacer chain of polyethylene glycol (PEG) was immobilized on a boron-doped diamond (BDD) microcell via electrochemical oxidation of its terminal amino groups. The amino-aptamer was then covalently linked to the carboxyl end of the immobilized PEG as a two-piece macromolecule, autoassembled at the BDD surface, forming a dense layer. Due to a change in conformation of the aptamer on the target analyte binding, a decrease of the electron transfer rate of the redox [Fe(CN)₆]^4–/3– probe was observed. To quantify the amount of OTA, the decrease of the square wave voltammetry (SWV) peak maximum of this probe was monitored. The plot of the peak maximum against the logarithm of OTA concentration was linear along the range from 0.01 to 13.2 ng/L, with a detection limit of 0.01 ng/L. This concept was validated on spiked real samples of rice.     

Chemical evolution of iron containing enzymes: Mixed ligand complexes of iron as intermediary steps

Activities of the iron complexes of evolutionary importance like K₄[Fe(CN)₆], K₄[Fe(CN)₅(gly)], and K₄[Fe(CN)₅(trigly)] have been tested towards some redox reactions of biological significance, namely, decomposition of hydrogen peroxide, dehydrogenation of NADH and ascorbic acid both coupled with reduction of methylene blue. It has been observed that the catalytic activities of iron (II) complexes towards the redox reactions studied at pH 9.18 followed the order, K₄[Fe(CN)₆]4[Fe(CN)₅(gly)]4[Fe(CN)₅(trigly)]. Decomposition of H₂O₂ catalysed by cyanocomplexes of iron (II) has been discussed through the formation of an innersphere complex in which loosly bound ligands like, glycine and triglycine are replaced by hydroperoxide ion. A tentative mechanism for the catalysed decomposition of H₂O₂ has been discussed.Based upon the experimental observations a hypothesis on the evolution of iron containing enzymes has been envisaged as: iron(II) ion iron(II) cyanide complexes mixed ligand iron(II) cyanide and amino acid complexes iron(II) complexes of macromolecules proenzyme or early enzyme containing iron(II).     

Development of DNA electrochemical biosensor based on immobilization of ssDNA on the surface of nickel oxide nanoparticles modified glassy carbon electrode

A sensitive electrochemical method for DNA hybridization based on immobilization of DNA probe and [Ru(NH₃)₅Cl]PF₆ complex onto nickel oxide nanomaterials (NiOx_np) modified glassy carbon electrode was developed. Due to strong affinity of NiOx_np for phosphate groups, oligonucleotides probe with a terminal 5′-phosphate group was attached to the surface of the modified electrode. DNA immobilization and hybridization were characterized by electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry using K₃Fe(CN)₆/K₄Fe(CN)₆ and [Ru(NH₃)₅Cl]PF₆ as probe and indicator, respectively. The Ru-complex current response indicates only the complementary sequence showing an obvious current signal in comparison to non-complementary and three or single point mismatched sequences. The fabricated biosensor possessed good selectivity and sensitivity for complementary probe, taxon: 32630 tumor necrosis factor (TNF). The linear dynamic range, sensitivity and detection limit of the proposed biosensor were 4 × 10⁻¹⁰ M to 1 × 10⁻⁸ M, 34.32 nA nM⁻¹ and 6.8 × 10⁻¹¹ M, respectively. Excellent reproducibility and stability, quite simple and inexpensive preparation are the other advantages of proposed biosensor.     

Photoinhibition as a function of the ambient redox potential in Tris-washed PS II membrane fragments

The mode of photoinhibition as a function of the ambient redox potential (E_ambient) in suspensions of Tris-washed PS II membrane fragments has been analyzed by monitoring flash-induced absorption changes at 830 nm. It was found: (a) the detectable initial amplitude, ΔA^total ₈₃₀, as a measure of the capacity to form the `stable' radical pair, P680^+ċ Q^−ċ _A, drastically decreases during a 10 min photoinhibition at E_ambient values below +350 mV; (b) conversely, the normalized extent of the 18 μs relaxation kinetics, ΔA^{18 μ s} ₈₃₀ as a measure of the electron transfer from Y_Z to P680^+ċ becomes highly susceptible to light stress when E_ambient exceeds values of about +350 mV; (c) effects of the ambient redox potentials are highly pronounced during light exposure under anaerobic conditions, while much smaller differences arise under aerobic conditions; (d) the extent of damage does not correlate with the total concentration of K₃[Fe(CN)₆] and K₄[Fe(CN)₆] in the suspension during photoinhibition but rather depends on the E_m-values; (e) qualitatively similar features are observed when the redox buffer system K₃[Fe(CN)₆]/Na₂S₂O₄ is replaced by K₂[IrCl₆]/Na₂S₂O₄; (f) the characteristic E_ambient-dependence of photoinhibition is observed only under anaerobic conditions. The results are discussed with respect to different redox components that might be involved, including brief comments on a possible role of Cyt b559. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sensitive and antifouling impedimetric aptasensor for the determination of thrombin in undiluted serum sample

A highly sensitive and attractive antifouling impedimetric aptasensor for the determination of thrombin in undiluted serum sample was developed. The aptasensor was fabricated by co-assembling thiol-modified anti-thrombin binding aptamer, dithiothreitol and mercaptohexanol on the surface of gold electrode. The performance of aptasensor was characterized by atomic force microscopy, contact angle and electrochemical impedance spectroscopy. In the measurement of thrombin, the change in interfacial electron transfer resistance of aptasensor was monitored using a redox couple of Fe(CN)(6)(3-/4-). The increase in the electron transfer resistance was linearly proportional to the concentration of thrombin in the range from 1.0 to 20ng/mL and a detection limit of 0.3ng/mL thrombin was achieved. The fabricated aptasensor displayed attractive antifouling properties and allowed direct quantification of extrinsic thrombin down to 0.08ng/mL in undiluted serum sample. This work provides a promising strategy for clinical application with impressive sensitivity and antifouling characteristics.     

Synthesis, magnetic properties and crystal structures of two compounds: [Cu(en)(H2O)]2[Fe(CN)6]·4H2O and [Cu(en)2][KFe(CN)6] (en=ethylenediamine)

Two new heterometallic complexes, [Cu(en)(H₂O)]₂[Fe(CN)₆]·4H₂O (1) and [Cu(en)₂][KFe(CN)₆] (2), have been isolated from the reactions of CuCl₂ and en with K₃[Fe(CN)₆] in different molar ratios. Both complexes have been characterized by X-ray analyses, IR spectra and elemental analyses. Complex 1 is a cyanide bridged bimetallic assembly, its crystal structure consists of a two-dimensional polymeric sheet with two different rings, one a four-membered square ring and another a 12-membered hexagonal ring. The Fe(II) ion of 1 has two terminal, two linear bridging and two 1,1 en-on bridging cyanide groups. In the crystal structure of 2, the neighboring [Fe(CN)₆]³⁻ units are bridged by the K⁺ and the [K[Fe(CN)₆]]²⁻ units forming a three-dimensional network structure. The [Cu(en)₂]²⁺ units fill in the holes of the network acting as counter cations and charge compensations. Variable temperature magnetic susceptibility studies of 1 indicate that the complex exhibits ferromagnetic interaction between the Cu(II) ions.     

Stress Distortion Restraint to Boost the Sodium Ion Storage Performance of a Novel Binary Hexacyanoferrate

Mn‐based hexacyanoferrate Na_xMnFe(CN)₆ (NMHFC) has been attracting more attention as a promising cathode material for sodium ion storage owing to its low cost, environmental friendliness, and its high voltage plateau of 3.6 V, which comes from the Mn²⁺/Mn³⁺ redox couple. In particular, the Na‐rich NMHFC (x > 1.40) with trigonal phase is considered an attractive candidate due to its large capacity of ≈130 mAh g^?1, delivering high energy density. Its unstable cycle life, however, is holding back its practical application due to the dissolution of Mn²⁺ and the trigonal‐cubic phase transition during the charge–discharge process. Here, a novel hexacyanoferrate (Na_1.60Mn_0.833Fe_0.167[Fe(CN)₆], NMFHFC‐1) with Na‐rich cubic structure and dual‐metal active redox couples is developed for the first time. Through multiple structural modulation, the stress distortion is minimized by restraining Mn²⁺ dissolution and the trigonal‐cubic phase transition, which are common issues in manganese‐based hexacyanoferrate. Moreover, NMFHFC‐1 simultaneously retains an abundance of Na ions in the framework. As a result, Na_1.60Mn_0.833Fe_0.167[Fe(CN)₆] electrode delivers high energy density (436 Wh kg^?1) and excellent cycle life (80.2% capacity retention over 300 cycles), paving the way for the development of novel commercial cathode materials for sodium ion storage.     

Graphene-promoted 3,4,9,10-perylenetetracarboxylic acid nanocomposite as redox probe in label-free electrochemical aptasensor

Graphene/3,4,9,10-perylenetetracarboxylic acid (GPD) with three-dimensional porous structure has been successfully synthesized and served as redox probe to construct ultrasensitive electrochemical aptasensor. The GPD nanocomposite shows promoted electrochemical redox-activity of 3,4,9,10-perylenetetracarboxylic acid (PTCA) with an obvious well-defined cathodic peak from -0.7 to 0 V that never been seen from graphene or PTCA, which avoids miscellaneous redox peaks of PTCA in electrochemical characterization, offering a novel redox probe for electrochemical sensors with highly electrochemical active area and conductivity. To the best of our knowledge, this is the first study that utilizes PTCA self-derived redox-activity as redox probe in electrochemical sensors. Moreover, the interesting GPD possesses the advantages of membrane-forming property, providing a direct immobilization of redox probes on electrode surface. This simple process not only diminishes the conventional fussy immobilization of redox probes on the electrode surface, but also reduces the participation of the membrane materials that acted as a barrier of the electron propagation in redox probe immobilization. With thrombin as a model target, the redox probe-GPD based label-free electrochemical aptasensor shows a much higher sensitivity (a detection range from 0.001 nM to 40 nM with a detection limit of 200 fM) to that of analogous aptasensors produced from other redox probes.     

Kinetics and mechanism of the reduction of the molybdatopentaamminecobalt(III) ion by aqueous sulfite and aqueous potassium hexacyanoferrate(II)

A detailed investigation on the oxidation of aqueous sulfite and aqueous potassium hexacyanoferrate(II) by the title complex ion has been carried out using the stopped-flow technique over the ranges, 0.01≤[S(IV)]_T≤0.05 mol dm⁻³, 4.47≤pH≤5.12, and 24.9≤θ≤37.6 °C and at ionic strength 1.0 mol dm⁻³ (NaNO₃) for aqueous sulfite and 0.01≤[Fe(CN)₆ ⁴⁻]≤0.11 mol dm⁻³, 4.54≤pH≤5.63, and 25.0≤θ≤35.3 °C and at ionic strength 1.0 or 3.0 mol dm⁻³ (NaNO₃) for the hexacyanoferrate(II) ion. Both redox processes are dependent on pH and reductant concentration in a complex manner, that is, for the reaction with aqueous sulfite, k_obs={(k₁K₁K₂K₃+k₂K₁K₄[H⁺])[S(IV)]_T]/([H⁺]²+K₁[H⁺]+K₁K₂) and for the hexacyanoferrate(II) ion, k_obs={(k₁K₃K₄K₅+k₂K₃K₆[H⁺])[Fe(CN)₆ ⁴⁻]_T)/([H⁺]²+K₃[H⁺]+K₃K₄). At 25.0 °C, the value of k₁′ (the composite of k₁K₃) is 0.77±0.07 mol⁻¹ dm³ s⁻¹, while the value of k₂′ (the composite of k₂K₄) is (3.78±0.17)×10⁻² mol⁻¹ dm³ s⁻¹ for aqueous sulfite. For the hexacyanoferrate(II) ion, k₁′ (the composite of k₁K₅) is 1.13±0.01 mol⁻¹ dm³ s⁻¹, while the value of k₂′ (the composite of k₂K₆) is 2.36±0.05 mol⁻¹ dm³ s⁻¹ at 25.0 °C. In both cases there was reduction of the cobalt(III) centre to cobalt(II), but there was no reduction of the molybdenum(VI) centre. k₂₂, the self-exchange rate constant, for aqueous sulfite (as SO₃ ²⁻) was calculated to be 5.37×10⁻¹² mol⁻¹ dm³ s⁻¹, while for Fe(CN)₆ ⁴⁻, it was calculated to be 1.10×10⁹ mol⁻¹ dm³ s⁻¹ from the Marcus equations.     

Nafion/lead nitroprusside nanoparticles modified carbon ceramic electrode as a novel amperometric sensor for l-cysteine

This work describes the electrochemical and electrocatalytic properties of carbon ceramic electrode (CCE) modified with lead nitroprusside (PbNP) nanoparticles as a new electrocatalyst material. The structure of deposited film on the CCE was characterized by energy dispersive X-ray (EDX), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM). The cyclic voltammogram (CV) of the PbNP modified CCE showed two well-defined redox couples due to [Fe(CN)₅NO]³⁻/[Fe(CN)₅NO]²⁻ and Pb^IV/Pb^II redox reactions. The modified electrode showed electrocatalytic activity toward the oxidation of l-cysteine and was used as an amperometric sensor. Also, to reduce the fouling effect of l-cysteine and its oxidation products on the modified electrode, a thin film of Nafion was coated on the electrode surface. The sensor response was linearly changed with l-cysteine concentration in the range of 1 × 10⁻⁶ to 6.72 × 10⁻⁵ mol L⁻¹ with a detection limit (signal/noise ratio [S/N] = 3) of 0.46 μM. The sensor sensitivity was 0.17 μA (μM)⁻¹, and some important advantages such as simple preparation, fast response, good stability, interference-free signals, antifouling properties, and reproducibility of the sensor for amperometric determination of l-cysteine were achieved.     

Fe‐Based Tunnel‐Type Na0.61[Mn0.27Fe0.34Ti0.39]O2 Designed by a New Strategy as a Cathode Material for Sodium‐Ion Batteries

Sodium‐ion batteries are promising for grid‐scale storage applications due to the natural abundance and low cost of sodium. However, few electrodes that can meet the requirements for practical applications are available today due to the limited routes to exploring new materials. Here, a new strategy is proposed through partially/fully substituting the redox couple of existing negative electrodes in their reduced forms to design the corresponding new positive electrode materials. The power of this strategy is demonstrated through the successful design of new tunnel‐type positive electrode materials of Na_0.61[Mn_0.61‐xFe_xTi_0.39]O₂, composed of non‐toxic and abundant elements: Na, Mn, Fe, Ti. In particular, the designed air‐stable Na_0.61[Mn_0.27Fe_0.34Ti_0.39]O₂ shows a usable capacity of ≈90 mAh g^?1, registering the highest value among the tunnel‐type oxides, and a high storage voltage of 3.56 V, corresponding to the Fe³⁺/Fe⁴⁺ redox couple realized for the first time in non‐layered oxides, which was confirmed by X‐ray absorption spectroscopy and Mössbauer spectroscopy. This new strategy would open an exciting route to explore electrode materials for rechargeable batteries.     

Determination of uric acid in human urine and serum by capillary electrophoresis with chemiluminescence detection

A simple and sensitive method based on capillary electrophoresis (CE) with chemiluminescence (CL) detection has been developed for the determination of uric acid (UA). The sensitive detection was based on the enhancement effect of UA on the CL reaction between luminol and potassium ferricyanide (K₃[Fe(CN)₆]) in alkaline solution. A laboratory-built reaction flow cell and a photon counter were deployed for the CL detection. Experimental conditions for CL detection were studied in detail to achieve a maximum assay sensitivity. Optimal conditions were found to be 1.0 × 10⁻⁴ M luminol added to the CE running buffer and 1.0 × 10⁻⁴ M K₃[Fe(CN)₆] in 0.2 M NaOH solution introduced postcolumn. The proposed CE-CL assay showed good repeatability (relative standard deviation [RSD] = 3.5%, n = 11) and a detection limit of 3.5 × 10⁻⁷ M UA (signal/noise ratio [S/N] = 3). A linear calibration curve ranging from 6.0 × 10⁻⁷ to 3.0 × 10⁻⁵ M UA was obtained. The method was evaluated by quantifying UA in human urine and serum samples with satisfactory assay results.     

Trimetallic complexes featuring Group 10 tetracyanometallate dianions as bridging ligands

The trimetallic complexes {Ru(PPh₃)₂Cp}₂{μ-M(CN)₄} and {Ru(dppe)Cp*}₂{μ-M(CN)₄} (M = Ni, Pd, Pt) have been prepared from reactions of RuCl(PPh₃)₂Cp or RuCl(dppe)Cp* with the appropriate tetracyanometallate salt, and structurally characterised. While a similar reaction of FeCl(dppe)Cp with K₂[Pt(CN)₄] afforded {Fe(dppe)Cp}₂{μ-Pt(CN)₄}, the iron cyanide complex Fe(CN)(dppe)Cp was isolated as the only iron containing product from reaction of FeCl(dppe)Cp with K₂[Ni(CN)₄]. The trimetallic complexes can be oxidised in two sequential one-electron steps. Spectroelectrochemical experiments reveal weak NIR absorption bands in the mono-oxidised complexes which are not present in the binuclear complex K[Ru(dppe)Cp*{Pt(CN)₄}], and are therefore attributed to Ru^II → Ru^III charge transfer processes. The coupling parameter, V_ab, extracted using Hush-style analysis falls in the range 250 ± 50 cm⁻¹, consistent with the weak interaction between the Group 8 metal centres. The energy of the IVCT process is dominated by reorganisation energy of the Group 8 metal-ligand fragment.     

Strategy of red blood cells immobilisation onto a gold electrode: Characterization by electrochemical impedance spectroscopy and quartz crystal microbalance

This study describes the grafting of red blood cells (RBC) onto a gold electrode. The erythrocytes were immobilised using antigen/antibody crosslinking based on the bonding of anti-D with the corresponding antigen of the RBC membrane that is shared by all erythrocytes from the positive rhesus group (O⁺). To optimise the reproducibility of the modified electrode and to avoid nonspecific interactions, the anti-D layer was deposited onto a protein G layer. The bridge between the protein G and the gold transducer was formed using mixed thiol-based self-assembled monolayer (SAM) (a mixture of 11-mercaptoundecanoic acid and 6-mercapto-1-hexanol in a 1/10 ratio). Each layer deposited was characterized, firstly, with a quartz crystal microbalance to obtain the deposited mass and the corresponding number of moles per square centimetres and secondly, by electrochemical impedance spectroscopy (EIS) using a redox couple Fe(CN)₆^3−/4− (1:1) as an EIS probe. Subsequent modelling with appropriate circuitry enabled the values for each component representing the interface (electrode/film/solution) to be calculated at each step of the grafting process. From these results the surface coverage has been calculated to range from 95 to 98%.     

A multi-amplification aptasensor for highly sensitive detection of thrombin based on high-quality hollow CoPt nanoparticles decorated graphene

In this work, we have successfully demonstrated a facile strategy to incorporate high-quality hollow CoPt bimetal alloy nanoparticles (HCoPt) onto reduced graphene oxide sheet (HCoPt-RGs). An advanced sandwich-type electrochemical aptasensor for thrombin was proposed by using the HCoPt-RGs conjugates as secondary label. The formed conjugates provided large surface area for loading plentiful redox probe thionine (Thi), horseradish peroxidase (HRP) and secondary aptamer (Apt II) with good stability and friendly biocompatibility, indicating their superior properties in electroactive mediator enrichment and biomolecule immobilization. Furthermore, activated by glutaraldehyde (GA), the chitosan-hollow CoPt alloy nanoparticle (CS-HCoPt) film can greatly facilitate the capture of primary aptamer (Apt I) and dramatically reduce the nonspecific binding. Excellent sensitivity was obtained by detecting the conspicuously enhanced electrochemical signal of Thi, which was amplified by HCoPt alloy nanoparticles and HRP toward the catalytic reduction of H₂O₂. The aptasensor displayed excellent performance for thrombin with a wide linearity in the range from 1.0 × 10⁻¹² to 5.0 × 10⁻⁸ M and a relatively low detection limit of 3.4 × 10⁻¹³ M. Moreover, the resulted aptasensor also exhibited good specificity, acceptable reproducibility and stability, indicating that the present strategy could pave a promising way for the wide application of graphene in clinical research.