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.     

Flow‐injection chemiluminescence and electrogenerated chemiluminescence determination of escitalopram oxalate in tablet form

Rapid, simple and highly sensitive flow‐injection (FI) chemiluminescence (CL) and flow‐injection electrogenerated chemiluminescence (ECL) methods were developed for the determination of escitalopram oxalate (ESC), a selective serotonin reuptake inhibitor used as an antidepressant drug. The CL method was based on the CL reaction of ESC with acidic cerium(IV) and tris(2,2'‐bipyridyl)ruthenium(II) (Ru). Various experimental parameters affecting CL intensity were carefully studied and optimised. The method enabled the determination of 0.001‐50 µg/mL of ESC in bulk form with a correlation coefficient r = 0.9999. The limit of detection (LOD) was 0.01 ng/mL (S/N = 3). The ECL method was based on the ECL reaction of Ru with the drug in an acidic medium, permitting the determination of ESC in the range of 0.00001‐70 µg/mL with r = 0.9999 and LOD of 1 x 10^‐4 ng/mL. The proposed methods were applied to the determination of ESC in commercial tablets. The results were compared statistically with those obtained from a published method using t‐ and F‐tests. Copyright © 2012 John Wiley & Sons, Ltd.     

Sonochemical synthesis of Ag nanoclusters: electrogenerated chemiluminescence determination of dopamine

We report a facile one‐pot sonochemical approach to preparing highly water‐soluble Ag nanoclusters (NCs) using bovine serum albumin as a stabilizing agent and reducing agent in aqueous solution. Intensive electrogenerated chemiluminescence (ECL) was observed from the as‐prepared Ag (NCs) and successfully applied for the ECL detection of dopamine with high sensitivity and a wide detection range. A possible ECL mechanism is proposed for the preparation of Ag NCs. With this method, the dopamine concentration was determined in the range of 8.3 × 10^–9 to 8.3 × 10^–7 mol/L without the obvious interference of uric acid, ascorbic acid and some other neurotransmitters, such as serotonin, epinephrine and norepinephrine, and the detection limit was 9.2 × 10^–10 mol/L at a signal/noise ratio of 3. Copyright © 2013 John Wiley & Sons, Ltd.     

A three-way junction aptasensor for lysozyme detection

A well-designed three-way junction (TWJ) aptasensor for lysozyme detection was developed based on target-binding-induced conformational change of aptamer-complementary DNA (cDNA) as probe. A ferrocene (Fc)-tagged cDNA is partially hybridized with an anti-lysozyme aptamer to form a folded structure where there is a coaxial stacking of two helices and the third one at an acute angle. In addition, the fabrication of the sensor was achieved via the single-step method, which offered a good condition for sensing. In the absence of lysozyme, electron transfer (eT), through the coaxial two helices called "conductive path", is allowed between Fc-labeled moiety and the electrode. The binding of lysozyme to the aptamer blocks eT, leading to diminished redox signal. This aptasensor with an instinct signal attenuation factor shows a high sensitivity to lysozyme, and the response data is fitted by nonlinear least-squares to Hill equation. Detection limit is 0.2nM with a dynamic range extending to 100nM. Compared with existing electrochemical impedance spectroscopy (EIS)-based approaches, TWJ-DNA aptasensor was demonstrated to be more specific for detection and simpler for regeneration procedure.     

Flow-injection electrogenerated chemiluminescence determination of fluoroquinolones based on its sensitizing effect.

A simple and sensitive flow-injection electrogenerated chemiluminescence (ECL) method for the determination of fluoroquinolones was developed. The method is based on the sensitizing effect of fluoroquinolones on the weak ECL signal of electrochemical oxidation of luminol on the surface of the platinum flake electrode in the medium of 0.1 mol/L Na2CO3-NaHCO3. At the optimum experimental conditions, the relative ECL intensity increased linearly with increasing fluoroquinolones concentration, in the ranges 1.0 x 10(-8)-2.0 x 10(-4) g/mL for norfloxacin, 5.0 x 10(-9)-6.0 x 10(-6) g/mL for oxfloxacin, 2.0 x 10(-8)-1.4 x 10(-5) g/mL for ciprofloxacin, 1.0 x 10(-8)-1.4 x 10(-5) g/mL for pefloxacin, and 1.0 x 10(-9)-1.0 x 10(-5) g/mL for enoxacin, with detection limits of 4.0 x 10(-9) g/mL, 2.0 x 10(-9) g/mL, 1.0 x 10(-8) g/mL, 8.0 x 10(-9) g/mL, and 8.0 x 10(-10) g/mL, respectively. The relative standard deviations were all less than 2.5% for the determination of 2.0 x 10(-6) g/mL fluoroquinolones (n = 11). The method was used to determine these medicines in pharmaceutical samples with satisfactory results.     

Coreactant electrogenerated chemiluminescence of ruthenium porphyrins

The coreactant electrogenerated chemiluminescence (ECL) of 5,10,15,20-tetraphenyl-21H,23H-porphine ruthenium(II) carbonyl (Ru(TPP)(CO))), and 2,3,7,8,12,13,17, 18-octaethyl-21H,23H-porphine ruthenium(II) carbonyl (Ru(OEP)(CO)) in acetonitrile is reported. Both complexes have absorption maxima in the visible region of the spectrum and emit in fluid solution at room temperature around 650 nm in acetonitrile. Photoluminescence efficiencies (?_em) were between 1.5 × 10⁻⁴ and 4.0 × 10⁻⁴ when compared to (bpy = 2,2′-bipyridine) with ?_em = 0.042. The complexes show two-electrochemically reversible oxidations via cyclic voltammetry. ECL was generated using tri-n-propylamine (TPrA) as an oxidative-reductive coreactant and the ECL peaks at a potential corresponding to oxidation of both the TPrA and both of the porphyrin oxidations. ECL efficiencies (?_ecl) were 0.65 for Ru(TPP)(CO) and 0.58 for Ru(OEP)(CO) when compared to (?_ecl = 1). Also, qualitative studies using transmission filters suggest that both complexes emit ECL in approximately the same region as their photoluminescence, indicating that the same excited state is formed in both experiments.     

Label-free electrochemical aptasensor for thrombin detection based on the nafion@graphene as platform

A sensitive label-free electrochemical aptasensor was successfully fabricated for thrombin detection with nafion@graphene as platform. With electrostatic interaction between nafion and methylene blue (MB), positive charged MB was successfully assembled on nafion@graphene modified electrode surface, which provided amounts of redox probes for electrochemical aptasensor. In the presence of thrombin, the thrombin aptamer (TBA) on the electrode surface would catch the target on the electrode interface, which made a barrier for electrons and inhibits the electro-transfer, resulting in the decreased differential pulse voltammetry signals of MB. As a result, the proposed approach showed a high sensitivity and a wider linearity to thrombin in the range 0.01–50 nM with a detection limit of 6 pM.     

A conformational switch‐based aptasensor for the chemiluminescence detection of microRNA

A simple microRNA (miRNA) aptasensor has been developed combining the conformational switch of a streptavidin aptamer and isothermal strand displacement amplification. In the presence of its target miRNA, the allosteric molecular beacon (aMB) probe immobilized on the plate can be ‘switched on' and release the streptavidin aptamer. At the same time, Klenow fragment (3′→5′ exo‐) is utilized to initiate DNA‐strand displacement, which starts the target recycling process. Based on the aptamer' high binding affinity and subsequent catalytic chemiluminescence (CL) detection, this CL strategy is highly specific in distinguishing mature miRNAs in same family. It exhibits a dynamic range of four orders of magnitude with a detection limit of 50 fM, and shows great potential for miRNA‐related clinical practices and biochemical research.     

Upconversion luminescence resonance energy transfer-based aptasensor for the sensitive detection of oxytetracycline

In this work, a biosensor based on luminescence resonance energy transfer (LRET) from NaYF₄:Yb,Tm upconversion nanoparticles (UCNPs) to SYBR Green I has been developed. The aptamers are covalently linked to UCNPs and hybridized with their complementary strands. The subsequent addition of SYBR Green allows SYBR Green I to insert into the formed double-stranded DNA (dsDNA) duplex and brings the energy donor and acceptor into close proximity, leading to the fluorescence of UCNPs transferred to SYBR Green I. When excited at 980 nm, the UCNPs emit luminescence at 477 nm, and this energy is transferred to SYBR Green I, which emits luminescence at 530 nm. In the presence of oxytetracycline (OTC), the aptamers prefer to bind to its corresponding analyte and dehybridize with the complementary DNA. This dehybridization leads to the liberation of SYBR Green I, which distances SYBR Green I from the UCNPs and recovers the UCNPs' luminescence. Under optimal conditions, a linear calibration is obtained between the ratio of I₅₃₀ to I₄₇₇ nm (I₅₃₀/I₄₇₇) and the OTC concentration, which ranges from 0.1 to 10 ng/ml with a limit of detection (LOD) of 0.054 ng/ml.     

Homogeneous electrogenerated chemiluminescence immunoassay for the determination of digoxin employing Ru(bpy)(2)(dcbpy)NHS and carrier protein.

A highly sensitive homogeneous electrogenerated chemiluminescence (ECL) immunoassay for the determination of anti-digoxin antibody and digoxin hapten was developed employing Ru(bpy)(2)(dcbpy)NHS (bpy = 2,2'-bipyridyl; dcbpy = 2,2'-bipyridine-4,4'-dicarboxylic acid; NHS = N-hydroxysuccinimide ester) as an electrochemiluminescent label and bovine serum albumin (BSA) as a carrier protein. A digoxin hapten was indirectly heavily labelled with Ru(bpy)(2)(dcbpy)NHS through BSA to form Ru(bpy)(2)(dcbpy)NHS-BSA-digoxin conjugate. The ECL intensity of the immunocomplex of the conjugate with anti-digoxin antibody markedly decreased when the immunoreaction between Ru(bpy)(2)(dcbpy)NHS-BSA-digoxin conjugate and anti-digoxin antibody took place. Two formats, direct homogeneous immunoassay for anti-digoxin antibody and competitive immunoassay for digoxin, were developed to determine anti-digoxin antibody and digoxin, respectively. The anti-digoxin antibody concentration in the range 7.6 x 10(-8)-7.6 x 10(-6) g/mL was determined by direct homogeneous format. Digoxin hapten was determined throughout the range 4.0 x 10(-10)-1.0 x 10(-7) g/mL with a detection limit of 1.0 x 10(-10) g/mL by competitive format. The relative standard derivation for 6.0 x 10(-9) g/mL was 4.3%. The method has been applied to assaying digoxin in control human serum.     

Label-free electrochemical aptasensor for thrombin detection with platinum nanoparticles and blocking reagent horseradish peroxidase as enhancers

A sensitive label-free electrochemical aptasensor was successfully fabricated for thrombin detection with platinum nanoparticles (Pt) and blocking reagent horseradish peroxidase (HRP) as enhancers. A Nafion?-graphene-coated electrode was first modified with an electrochemical probe of methylene blue (MB) through electrostatic interaction. Then Pt was electrodeposited onto an electrode for immobilization of the thrombin aptamer (TBA). Subsequently, HRP served as blocking reagent instead of bovine serum albumin (BSA). With the synergistic effect between Pt and HRP, the prepared aptasensor showed a superior catalytic efficiency toward H(2) O(2) in the presence of MB. After the combination of target thrombin on electrode surface, the TBA-thrombin complex made a barrier for electrocatalysis of Pt and HRP and inhibited the electrotransfer, resulting in a greater decrease in MB signals. As a result, the proposed approach showed a high sensitivity and a much wider linearity to thrombin in the range from 0.005 to 50 nM with a detection limit of 1 pM.     

A new sensitive flow-injection chemiluminescence method for the determination of H(2)-receptor antagonists.

Based on the chemiluminescence (CL) intensity generated from the potassium ferricyanide [K(3)Fe(CN)(6)]-rhodamine 6G system in sodium hydroxide (NaOH) medium, a new sensitive flow-injection chemiluminescence (FI-CL) method has been developed, validated and applied for the determination of three kinds of H(2)-receptor antagonists: cimetidine (CIMT), ranitidine (RANT) hydrochloride and famotidine (FAMT). Under the optimum conditions, the linear range for the determination was 1.0 x 10(-9)-7.0 x 10(-5) g/ml for CIMT, 1.0 x 10(-9)-5.0 x 10(-5) g/mL for RANT hydrochloride and 5.0 x 10(-9)-7.0 x 10(-5) g/mL for FAMT. During 11 repeated measurements of 1.0 x 10(-6) g/mL sample solutions, the relative standard deviations (RSDs) were all <5%. The detection limit was 8.56 x 10(-10) g/mL for CIMT, 8.69 x 10(-10) g/mL for RANT hydrochloride and 2.35 x 10(-9) g/mL for FAMT (S:N = 3). This method has been successfully implemented for the analysis of H(2)-receptor antagonists in pharmaceuticals.     

Determination of norfloxacin using a terbium-sensitized electrogenerated chemiluminescence method.

A simple electrogenerated chemiluminescence (ECL) analysis method for the determination of norfloxacin (NFLX) is reported. It is based on ECL produced by Na(2)SO(3), which is sensitized by the Tb-NFLX complex. The relative ECL intensity of the Tb(3+)-NFLX-Na(2)SO(3) system is proportional to the amount of NFLX. The optimized experimental conditions were investigated. The linear range and detection limit for NFLX were 1.0 x 10(-10)-8.0 x 10(-7) mol/L and 2.8 x 10(-11) mol/L, respectively. This method was successfully applied to the determination of NFLX in a capsule. NFLX in urine can be directly detected without pretreatment or separation.     

A highly sensitive aptasensor towards Plasmodium lactate dehydrogenase for the diagnosis of malaria

Finding a highly sensitive diagnostic technique for malaria has challenged scientists for the last century. In the present study, we identified versatile single-strand DNA aptamers for Plasmodium lactate dehydrogenase (pLDH), a biomarker for malaria, via the Systematic Evolution of Ligands by EXponential enrichment (SELEX). The pLDH aptamers selectively bound to the target proteins with high sensitivity (K(d)=16.8-49.6 nM). The selected aptamers were characterized using an electrophoretic mobility shift assay, a quartz crystal microbalance, a fluorescence assay, and circular dichroism spectroscopy. We also designed a simple aptasensor using electrochemical impedance spectroscopy; both Plasmodium vivax LDH and Plasmodium falciparum LDH were selectively detected with a detection limit of 1 pM. Furthermore, the pLDH aptasensor clearly distinguished between malaria-positive blood samples of two major species (P. vivax and P. falciparum) and a negative control, indicating that it may be a useful tool for the diagnosis, monitoring, and surveillance of malaria.     

A fluorescent aptasensor for sensitive analysis oxytetracycline based on silver nanoclusters

A fluorescent aptasensor for detection of oxytetracycline (OTC) was presented based on fluorescence quenching of DNA aptamer‐templated silver nanoclusters (AgNCs). The specific DNA scaffolds with two different nucleotides fragments were used: one was enriched with a cytosine sequence fragment (C12) that could produce DNA–AgNCs via a chemical reduction method, and another was the OTC aptamer fragment that could selectively bind to the OTC antibiotic. Thus, the as‐prepared AgNCs could exhibit quenched fluorescence after binding to the target OTC. The fluorescence ratio of the DNA–AgNCs was quenched in a linearly proportional manner to the concentration of the target in the range of 0.5 nM to 100 nM with a detection limit of 0.1 nM. This proposed nanobiosensor was demonstrated to be sensitive, selective, and simple, introducing a viable alternative for rapid determination of toxin OTC in honey and water samples. Copyright © 2016 John Wiley & Sons, Ltd.     

Development of a sensitive flow‐injection–chemiluminescence detection method for the determination of laevodopa

A simple chemiluminometric method using flow injection has been developed for the determination of laevodopa, based on its sensitizing effect on the weak chemiluminescence (CL) reaction between Na₂SO₃ and acidic KMnO₄. Under optimum experimental conditions, the CL intensity was linearly related to the concentration of laevodopa from 3.4 × 10^–8 to 2.4 × 10^–5 mol/L and the detection limit was 1.1 × 10^–8 mol/L (s:n = 3). The relative standard deviation (RSD) of the proposed method calculated from 20 replicate injection of 3 × 10^–7 mol/L laevodopa was 3.3%. The correlation coefficient was 0.997. The method was successfully applied to the determination of laevodopa in commercial pharmaceutical formulations and spiked urine samples. Copyright © 2008 John Wiley & Sons, Ltd.     

Development of a highly sensitive and selective microplate chemiluminescence enzyme immunoassay for the determination of free thyroxine in human serum

A microplate chemiluminescence enzyme immunoassay (CLEIA) with high sensitivity, selectivity and reproducibility was developed for the determination of free thyroxine (FT4) in human serum. A competitive assay has been utilized with horseradish peroxidase (HRP) labeled thyroxine analog in the chemiluminescence (CL) detection. The CL signal produced by the emission of photons from luminol was directly proportional to the amount of analyte. The linear range was 0.45-7.5 ng dL(-1 )and the detection limit was 0.09 ng dL(-1). Experimental conditions, such as temperature, pH, incubation time, titration level and other relevant variables upon the CL signal have been examined and optimized. A coefficient of variance of less than 16% was obtained for intra- and inter-assay precision. The present method has been successfully applied to the analysis of FT4 in human serum. The positive and negative coincidence ratios are satisfactory. Good correlations were obtained between the results by the proposed method and radioimmunoassay (RIA), as well as a Bayer ACS-180SE detection system.