Quantum dot-based electrochemical DNA biosensor using a screen-printed graphite surface with embedded bismuth precursor

This work reports the development of screen-printed quantum dots (QDs)-based DNA biosensors utilizing graphite electrodes with embedded bismuth citrate as a bismuth precursor. The sensor surface serves both as a support for the immobilization of the oligonucleotide and as an ultrasensitive voltammetric QDs transducer relying on bismuth nanoparticles. The utility of this biosensor is demonstrated for the detection of the C634R mutation through hybridization of the biotin-tagged target oligonucleotide with a surface-confined capture complementary probe and subsequent reaction with streptavidin-conjugated PbS QDs. The electrochemical transduction step involved anodic stripping voltammetric determination of the Pb(II) released after acidic dissolution of the QDs. Simultaneously with the electrolytic accumulation of Pb on the sensor surface, the embedded bismuth citrate was converted in situ to bismuth nanoparticles enabling ultra-trace Pb determination. The biosensor showed a linear relationship of the Pb(II) peak current with respect to the logarithm of the target DNA concentrations from 0.1 pmol L^− 1 to 10 nmol L^− 1, and the limit of detection was 0.03 pmol L^− 1. The biosensor exhibited effective discrimination between a single-base mismatched sequence and the fully complementary target DNA. These “green” biosensors are inexpensive, lend themselves to easy mass production, and hold promise for ultrasensitive bioassay formats.     

An aptamer-based biosensor for sensitive thrombin detection

A sensitive aptamer-based sandwich-type sensor is presented to detect human thrombin using quantum dots as electrochemical label. CdSe quantum dots were labeled to the secondary aptamer, which were determined by the square wave stripping voltammetric analysis after dissolution with nitric acid. The aptasensor has a lower detection limit at 1 pM, while the sample consumption is reduced to 5 μl. The proposed approach shows high selectivity and minimizes the nonspecific adsorption, so that it was used for the detection of target protein in the human serum sample. Such an aptamer-based biosensor provides a promising strategy for screening biomarkers at ultratrace levels in the complex matrices.     

Electrochemical immunoassay of cotinine in serum based on nanoparticle probe and immunochromatographic strip

A disposable sensor for the determination of cotinine in human serum was developed based on immunochromatographic test strip and quantum dot label. In this assay, cotinine linked with quantum dot competes with cotinine in sample to bind to anti-cotinine antibody in the test strip and the quantum dots serve as signal vehicles for electrochemical readout. Some parameters governing the performance of the sensor were optimized. The sensor shows a wide linear range from 1 ng mL^?1 to 100 ng mL^?1 cotinine with a detection limit of 1.0 ng mL^?1. The sensor was validated with spiked human serum samples and it was found that this method was reliable in measuring cotinine in human serum. The results demonstrate that this sensor is rapid, accurate, and less expensive and has the potential for point of care (POC) detection of cotinine and fast screening of tobacco smoke exposure.     

Efficient stripping voltammetric detection of organophosphate pesticides using NanoPt intercalated Ni/Al layered double hydroxides as solid-phase extraction

We developed a simple strategy for designing a sensitive electrochemical stripping voltammetric sensor for organophosphate pesticides (OPs) based on solid-phase extraction (SPE) using nanosized Pt intercalated Ni/Al layered double hydroxides (labeled as NanoPt-LDHs). By assembling NanoPt with LDHs together, the resulting NanoPt-LDHs are highly efficient to capture OPs. It dramatically facilitates the enrichment of OPs onto their surface and realizes the sensitive stripping voltammetric detection of methyl parathion (MP) as a model of OPs. The stripping analysis shows highly linear over MP concentration ranges of 0.001–0.15 and 0.3–1.0 μg mL^? 1 with a detection limit of 0.6 ng mL^–1 (S/N = 3). The combination of NanoPt, LDHs, SPE, and square-wave voltammetry (SWV) provides a fast, simple, and sensitive electrochemical method for OPs.     

Stripping voltammetric analysis of organophosphate pesticides based on solid-phase extraction at zirconia nanoparticles modified electrode

A sensitive electrochemical stripping voltammetric method for analyzing organophosphate (OP) compounds was developed based on solid-phase extraction (SPE) at zirconia (ZrO₂) nanoparticles modified electrode. ZrO₂ nanoparticles were proved as a new sorbent for SPE of OP pesticides. Because of the strong affinity of ZrO₂ for the phosphoric group, nitroaromatic OPs can strongly bind to the ZrO₂ nanoparticle surface. The combination of SPE with square-wave voltammetry (SWV) provided a fast, sensitive, and selective electrochemical method for nitroaromatic OP compounds using methyl parathion (MP) as a model. The stripping response was highly linear over the MP range of 0.003–2.0 μg/mL, with a detection limit of 0.001 μg/mL. The fast extraction ability of ZrO₂ nanoparticles makes it promising sorbent for various solid-phase extractions.     

Quantum-dots based electrochemical immunoassay of interleukin-1α

We describe a quantum-dot (QD, CdSe@ZnS) based electrochemical immunoassay to detect a protein biomarker, interleukin-1α (IL-1α). QD conjugated with anti-IL-1α antibody was used as a label in an immunorecognition event. After a complete sandwich immunoreaction among the primary IL-1α antibody (immobilized on the avidin-modified magnetic beads), IL-1α, and the QD-labeled secondary antibody, QD labels were attached to the magnetic-bead surface through the antibody-antigen immunocomplex. Electrochemical stripping analysis of the captured QDs was used to quantify the concentration of IL-1α after an acid-dissolution step. The streptavidin-modified magnetic beads and the magnetic separation platform were used to integrate a facile antibody immobilization (through a biotin/streptavidin interaction) with immunoreactions and the isolation of immunocomplexes from reaction solutions in the assay. The voltammetric response is highly linear over the range of 0.5–50 ng ml⁻¹ IL-1α, and the limit of detection is estimated to be 0.3 ng ml⁻¹ (18 pM). This QD-based electrochemical immunoassay shows great promise for rapid, simple, and cost-effective analysis of protein biomarkers.     

Selective determination of drug resistant cancer cells on indium tin oxide electrode modified with nano titanium dioxide

A new electrochemical cell sensor, with low cost, simple fabrication, high selectivity and sensitivity was developed in this study. Titanium dioxide nanoparticles (nano-TiO₂) were assembled on the disposable indium tin oxide (ITO) electrodes for the immobilization of the drug sensitive leukemia K562/B.W. cells and drug resistant leukemia K562/ADM cells to fabricate the relative cell sensors. The different electrochemical behaviors of the probe allowed us to differentiate one type of leukemia cells from another. Furthermore, the results of electrochemical impedance spectroscopy indicated that the detection limit of the new cell sensor is 1.3 × 10³ cells ml^?1 with a linear range of 1.6 × 10⁴ to 1.0 × 10⁷ cells ml^?1. These results suggested the promising application of this nano-TiO₂ interface to construct the non-labeling potential-discriminative cell biosensors for clinical uses.     

Multiplex electrochemical immunoassay using gold nanoparticle probes and immunochromatographic strips

We describe a multiplex electrochemical immunoassay based on the use of gold nanoparticle (Au-NP) probes and immunochromatographic strips (ISs). The approach takes advantage of the speed and low cost of the conventional IS tests and the high sensitivities of the nanoparticle-based electrochemical immunoassays. Rabbit IgG (R-IgG) and human IgM (H-IgM) were used as model targets for the demonstration of the proof of concept. The Au-NPs based sandwich immunoreactions were performed on the IS, and the captured gold nanoparticle labels on the test zones were determined by highly sensitive stripping voltammetric measurement of the dissolved gold ions (III) with a carbon paste electrode. The detection limits are 1.0 and 1.5 ng ml⁻¹ with the linear range of 2.5–250 ng ml⁻¹ for quantitative detection of R-IgG and H-IgM, respectively. The total assay time is around 25 min. Such multiplex electrochemical immunoassay could be readily highly multiplexed to allow simultaneous parallel detection of numerous proteins and is expected to open new opportunities for protein diagnostics and biosecurity.     

Enzymatic glucose biosensor based on flower-shaped copper oxide nanostructures composed of thin nanosheets

Well-crystallized flower-shaped copper oxide nanostructures composed of thin nanosheets has been synthesized by simple low-temperature hydrothermal process and used to fabricate highly sensitive amperometric glucose biosensor which exhibited a high and reproducible sensitivity of 47.19 μA mM^?1 cm^?2, response time less than 5 s, linear dynamic range from 0.01 to 10.0 mM, correlation coefficient of R = 0.9986, and limit of detection (LOD), based on S/N ratio (S/N = 3) of 1.37 μM. This work opens a way to utilize simply-grown CuO nanostructures as an efficient electron mediator to fabricate efficient glucose biosensors.     

Bioelectrocatalytic oxidation of glucose by hexose oxidase directly wired to graphite

Glucose-oxidizing enzymes are widely used in electrochemical biosensors and biofuel cells; in most applications glucose oxidase, an enzyme with non-covalently bound FAD and low capability of direct electronic communications with electrodes, is used. Here, we show that another glucose-oxidizing enzyme with a covalently bound FAD center, hexose oxidase (HOX), adsorbed on graphite, exhibits a pronounced non-catalytic voltammetric response from its FAD, at − 307 mV vs. Ag/AgCl, pH 7, characterized by the heterogeneous electron transfer (ET) rate constant of 29.2 ± 4.5 s^− 1. Direct bioelectrocatalytic oxidation of glucose by HOX proceeded, although, with a 350 mV overpotential relative to FAD signals, which may be connected with a limiting step in biocatalysis under conditions of the replacement of the natural redox partner, O₂, by the electrode; mediated bioelectrocatalysis was consistent with the potentials of a soluble redox mediator used. The results allow development of HOX-based electrochemical biosensors for sugar monitoring and biofuel cells exploiting direct ET of HOX, and, not the least, fundamental studies of ET non-complicated by the loss of FAD from the protein matrix.     

Electrochemical sensing of hydroxylamine by gold nanoparticles on single-walled carbon nanotube films

The arrays of gold nanoparticles (AuNPs) were fabricated on flexible and transparent single-walled carbon nanotube (SWCNT) films using the electrochemical deposition method, and the patterned nanotubes were then used as electrodes for hydroxylamine detection. The sizes and densities of the AuNPs could easily be controlled by varying the amount of charge deposited, and the gold-deposited area showed a homogeneous distribution on the exposed SWCNT film surface. X-ray diffraction analysis of the AuNPs shows a face-centered cubic structure that is dominated by the lowest energy {111} facets. The oxidation of the hydroxylamine on the AuNP-deposited SWCNT films depended strongly on the solution pH, and the maximum catalytic current was observed at a pH of 9.0. A linear electrical response was observed for concentrations ranging from 0.016 to 0.210 mM, and the detection limit and the sensitivity were 0.72 μM and 165.90 μAmM^?1 cm^?2, respectively. Moreover, the amperometric response in hydroxylamine showed a stable response for a long time (300 s), during which time it retained 94% of its initial value. In the long-term storage stability test, the current response to hydroxylamine decreased slightly, with only 17% leakage after 30 days.     

Selective determination of trichloroacetic acid using silver nanoparticle coated multi-walled carbon nanotubes

Silver nanoparticle coated multi-walled carbon nanotubes (Ag/MWCNT) were prepared and used to fabricate a modified electrode. The Ag/MWCNT composites were observed by a transmission electron microscope (TEM), and the electrochemical properties of the Ag/MWCNT composite modified glassy carbon electrode were characterized by electrochemical measurements. The results showed that these composites had a favorable catalytic ability for the reduction of trichloroacetic acid (TCAA). Square wave voltammetric (SWV) technique was applied to detect TCAA. Under optimum conditions, the voltammetric determination of TCAA was performed with a linear range of 5.0 × 10^? 6–1.2 × 10^? 4 mol L^? 1 and a detection limit of 1.9 × 10^? 6 mol L^? 1 (S/N = 3).     

Using flowerlike polymer–copper nanostructure composite and novel organic–inorganic hybrid material to construct an amperometric biosensor for hydrogen peroxide

A new type of amperometric hydrogen peroxide biosensor was fabricated by entrapping horseradish peroxidase (HRP) in the organic–inorganic hybrid material composed of zirconia–chitosan sol–gel and Au nanoparticles (ZrO₂–CS–AuNPs). The sensitivity of the biosensor was enhanced by a flowerlike polymer–copper nanostructure composite (pPA–FCu) which was prepared from co-electrodeposition of CuSO₄ solution and 2,6-pyridinediamine solution. Several techniques, including UV–vis absorption spectroscopy, scanning electron microscopy, cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy were employed to characterize the assembly process and performance of the biosensor. The results showed that this pPA–FCu nanostructure not only had excellent redox electrochemical activity, but also had good catalytic efficiency for hydrogen peroxide. Also the ZrO₂–CS–AuNPs had good film forming ability, high stability and good retention of bioactivity of the immobilized enzyme. The resulting biosensors showed a linear range from 7.80 × 10^?7 to 3.7 × 10^?3 mol L^?1, with a detection limit of 3.2 × 10^?7 mol L^?1 (S/N = 3) under optimized experimental conditions. The apparent Michaelis–Menten constant was determined to be 0.32 mM, showing good affinity. In addition, the biosensor which exhibits good analytical performance, acceptable stability and good selectivity, has potential for practical applications.     

Electrochemiluminescence immunosensor based on nanocomposite film of CdS quantum dots-carbon nanotubes combined with gold nanoparticles-chitosan

A novel and sensitive electrochemiluminescence (ECL) immunosensor based on CdS quantum dots (QDs)-carbon nanotubes (CNTs) and gold nanoparticles-chitosan (GNPs-CHIT) was presented. CdS QDs ECL was much enhanced by combing poly(diallyldimethylammonium chloride) functionalized CNTs. GNPs-CHIT nanohybrids was used to construct an effective antibody immobilization matrix with excellent stability and bioactivity. The principle of ECL detection for target human IgG is based on the increment of steric hindrance after immunoreaction, which resulted in the decrease in ECL intensity. The linear response range was between 0.006 and 150 ng mL^?1, and the detection limit was 0.001 ng mL^?1. This approach offers obvious advantages of being simpler, faster, and more stable compared with other immunosensors, which possesses great potential for protein detection in clinical laboratory.     

Novel poly (neutral red) nanowires as a sensitive electrochemical biosensing platform for hydrogen peroxide determination

Poly (neutral red) nanowires (PNRNWs) have been synthesized for the first time by the method of cyclic voltammetric electrodeposition using porous anodic aluminum oxide (AAO) template and were examined by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, horseradish peroxidase (HRP) was encapsulated in situ in PNRNWs (denoted as PNRNWs–HRP) by electrochemical copolymerization for potential biosensor applications. The PNRNWs showed excellent efficiency of electron transfer between the HRP and the glassy carbon (GC) electrode for the reduction of H₂O₂ and the PNRNWs–HRP modified GC electrode showed to be excellent amperometric sensors for H₂O₂ at −0.1 V with a linear response range of 1 μM to 8 mM with a correlation coefficient of 0.996. The detection limit (S/N = 3) and the response time were determined to be 1 μM and <5 s and the high sensitivity is up to 318 μA mM⁻¹ cm⁻².     

A simple fluorescence quenching method for berberine determination using water-soluble CdTe quantum dots as probes

A novel method for the determination of berberine has been developed based on quenching of the fluorescence of thioglycolic acid-capped CdTe quantum dots (TGA-CdTe QDs) by berberine in aqueous solutions. Under optimum conditions, the relative fluorescence intensity was linearly proportional to the concentration of berberine between 2.5 × 10^?8 and 8.0 × 10^?6 mol L^?1 with a detection limit of 6.0 × 10^?9 mol L^?1. The method has been applied to the determination of berberine in real samples, and satisfactory results were obtained. The mechanism of the proposed reaction was also discussed.     

A novel method for the anodic stripping voltammetry determination of Sb(III) using silver nanoparticle-modified screen-printed electrodes

Carbon screen-printed electrodes (CSPE) modified with silver nanoparticles present an interesting alternative in the determination of antimony using differential pulse anodic stripping voltammetry.Metallic silver nanoparticle deposits have been obtained by direct electrochemical deposition. Scanning electron microscopy measurements show that the electrochemically synthesized silver nanoparticles are deposited in aggregated form. Any undue effects caused by the presence of foreign ions in the solution were also analyzed to ensure that common interferents in the determination of antimony by ASV, such as bismuth, do not influence the electrochemical response of the latter element. The detection limit for Sb(III) obtained was 6.79 × 10⁻¹⁰ M. In terms of reproducibility, the precision of the above mentioned method in %RSD values was calculated at 3.50%. The method was applied to determine levels of antimony in seawater samples and pharmaceutical preparations.     

Ultrasensitive electrochemical detection of nucleic acid based on the isothermal strand-displacement polymerase reaction and enzyme dual amplification

We describe an ultrasensitive electrochemical detection of DNA protocol based on the isothermal strand-displacement polymerase reaction (ISDPR) and enzyme dual amplifications. Target DNA triggered an ISDPR to produce numerous bi-functionalized duplex DNA complexes. Following an immuno-magnetic collection via an immunoreaction between the attached digoxin on the duplex DNA and the anti-digoxin antibody on the magnetic bead, horseradish (HRP) tracers were bound to the duplex DNA through a biotin–streptavidin interaction. The quantification of DNA was realized by square wave voltammetric detection of the enzymatic products with a screen-printed gold electrode. The voltammetric response was proportional to the concentration of DNA in the range of 0.1 fM–0.5 pM, and the limit of detection was estimated to be 0.06 fM. The new protocol showed great promise for simple, cost-effective, and quantitative gene analysis.     

Development of urease based amperometric biosensors for the inhibitive determination of Hg (II)

Enzymatic amperometric procedures for measurement of Hg (II), based on the inhibitive action of this metal on urease enzyme activity, were developed. Screen-printed carbon electrodes (SPCEs) and gold nanoparticles modified screen-printed carbon electrodes (AuNPs/SPCEs) were used as supports for the cross-linking inmobilization of the enzyme urease. The amperometric response of urea was affected by the presence of Hg (II) ions which caused a decreasing in the current intensity. The optimum working conditions were found using experimental design methodology. Under these conditions, repeatability and reproducibility for both types of biosensors were determined, reaching values below 6% in terms of residual standard deviation. The detection limit obtained for Hg (II) was 4.2 × 10^?6 M for urease/SPCE biosensor and 5.6 × 10^?8 M for urease/AuNPs/SPCE biosensor. Analysis of the possible effect of the presence of foreign ions in the solution was performed. The method was applied to determine levels of Hg (II) in spiked human plasma samples.     

Selective determination of uric acid with DNA doped polymers modified carbon fiber microelectrode

New biocomposite materials, based on the incorporation of DNA doped p-aminobenzensulfonic acid, was fabricated by electrochemical method. A carbon fiber microelectrode modified by this thin film was fabricated for selective determination of uric acid (UA) in the presence of a larger amount of ascorbic acid (AA). It was found that the voltammetric oxidation peak separation between UA and AA is about 260 mV at the modified electrode. A linear response of the peak current versus the concentration was found in the range of 8 × 10⁻⁷–6 × 10⁻⁴ M with correlation coefficient of 0.9991 and the detection limit was 5 × 10⁻⁷ M (s/n = 3) at the 5 × 10⁻⁴ M AA. The presence of high concentration AA did not interference the determination. The electropolymerized film was characterized by SEM techniques. The modified electrode shows good sensitivity, selectivity and stability.