We have investigated the gas sensing properties of ZnO thin films (100 to 200 nm thickness) deposited by room-temperature radio frequency magnetron sputtering. The sensitivity of the films to ethanol vapor was measured in the 10 to 50 ppm concentration range at operating temperatures between 200 and 400 °C. A synergetic effect of decreasing grain size and increasing operating temperature was observed towards the improvement of the sensitivity, reaching a value of 54 and a limit of detection as low as 0.61 ppm. The decrease in the grain size resulted in prolonged response time but faster recovery. In any case, both response time and recovery time are < 400 s. The results demonstrate that room-temperature magnetron sputtering is a viable approach to enhance the performances of ZnO films in sensors for ethanol vapor.
Sensor response for ZnO films in presence of 50 ppm ethanol as a function grain size and temperature
We report on a method for the determination of the herbicide atrazine in tap water samples using melamine-modified gold nanoparticles (Mel-AuNPs). If a solution containing atrazine is added to a solution of such NPs, a color change occurs from wine-red to blue. This is due to a transition from monodisperse to aggregated Mel-AuNPs and caused by strong hydrogen bonding between atrazine and melamine. The color change can be monitored by a UV–vis spectrophotometer or with bare eyes. The ratio of the absorbances at 640 and 523 nm is linearly related to the logarithm of the atrazine concentration in the 0.165 to 16.5 μM range, and (with different slope) in the 16.5 μM to 330 μM range. The detection limit of atrazine is as low as 16.5 nM (S/N = 3). The method was successfully applied to the determination of atrazine in spiked tap water and gave recoveries that ranged from 72.5 % to 102.3 %.
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A label-free and single-step method is reported for rapid and highly sensitive detection of bisphenol A (BPA) in aqueous samples. It utilizes an aptamer acting as a probe molecule immobilized on a commercially available array of interdigitated aluminum microelectrodes. BPA was quantified by measuring the interfacial capacitance change rate caused by the specific binding between bisphenol A and the immobilized aptamer. The AC signal also induces an AC electrokinetic effect to generate microfluidic motion for enhanced binding. The capacitive aptasensor achieves a limit of detection as low as 10 fM(2.8 fg ⋅ mL − 1) with a 20 s response time. The method is inexpensive, highly sensitive, rapid and therefore provides a promising technology for on-site detection of BPA in food and water samples.
A. AC electrokinetics effect plays a vital role in BPA detection by introducing microfluidic movement to accelerate the molecular transport to the electrode surface.
B. The ACEK capacitive aptasensor has a limit of detection as low as 10 fM (2.8 fg ⋅ mL − 1) with a 20-s response time.
We have investigated the possibility of sampling ammonium ion using the diffusive-gradients-in-thin-films technique (DGT) by introducing a novel binding agent that is based on micro-sized zeolite. The performance of zeolite-DGT was characterized by measurement of the following parameters: (1) the diffusion coefficient of ammonium ion in hydrogel; (2) the adsorption rate of ammonium ion by the zeolite binding gel; (3) the elution efficiency, and (4) the effects of pH, ionic strength and interfering ions on DGT. The method was validated by studying the uptake of ammonium ion from in freshwaters by zeolite gels which was found to be fast enough to meet the requirements of DGT. The concentrations determined via DGT agreed well with the concentrations determined in bulk solutions. Sampling of ammonium ion using zeolite-DGT was consistent over the pH 3 to 8 range and the 0.001 to 10 mM ionic strength range. The method also performs predictably in natural waters containing various metal ions. The technique is considered to be a viable passive tool for sampling ammonium from aqueous solutions.
Schematic representation of the principle of DGT and the determination of mass accumulated on the binding gel.
We report on the preparation of fluorescent silica nanoparticles (SiNPs) modified with chitosan and lucigenin by using a reverse microemulsion method. The introduction of chitosan to the lucigenin doped SiNPs is shown to improve the fluorescence quantum yield. The modified SiNPs were used as fluorescent markers in an aptamer-based method for selective determination of thrombin. In this protocol, thrombin was sandwiched between streptavidin-coated magnetic beads and the fluorescent SiNPs modified with a thrombin-binding aptamer. The method was successfully applied to the determination of thrombin in human serum and showed a detection limit as low as 0.02 nM. In our perception, the protocol presented here is promising in that such SiNPs may be applied to the sensitive fluorescent detection of other analytes by changing the corresponding aptamer.
The introduction of chitosan to the lucigenin doped SiNPs is shown to improve the fluorescence quantum yield. The modified SiNPs were used as fluorescent markers in an aptamer-based method for selective determination of thrombin. The effect of chitosan concentration on fluorescence intensity of lucigenin/SiO2 nanoparticles (the volume of chitosan solution is 100 μL)
We describe an electrochemical sensor for nitric oxide that was obtained by modifying the surface of a nanofiber carbon paste microelectrode with a film composed of hexadecyl trimethylammonium bromide and nafion. The modified microelectrode displays excellent catalytic activity in the electrochemical oxidation of nitric oxide. The mechanism was studied by scanning electron microscopy and cyclic voltammetry. Under optimal conditions, the oxidation peak current at a working voltage of 0.75 V (vs. SCE) is related to the concentration of nitric oxide in the 2 nM to 0.2 mM range, and the detection limit is as low as 2 nM (at an S/N ratio of 3). The sensor was successfully applied to the determination of nitric oxide released from mouse hepatocytes.
NO electrochemical sensor based on CTAB-Nafion/CNFPME was fabricated through a simple method and applied to detect NO released from mouse hepatocytes successfully.
Magnetic Fe3O4@SiO2 core shell nanoparticles containing diphenylcarbazide in the shell were utilized for solid phase extraction of Hg(II) from aqueous solutions. The Hg(II) loaded nanoparticles were then separated by applying an external magnetic field. Adsorbed Hg(II) was desorbed and its concentration determined with a rhodamine-based fluorescent probe. The calibration graph for Hg(II) is linear in the 60 nM to 7.0 μM concentration range, and the detection limit is at 23 nM. The method was applied, with satisfying results, to the determination of Hg(II) in industrial waste water.
Microparticles were synthesized by suspension copolymerization of the synthetic ionic liquid (IL) 1-allyl-3-methyl-imidazolium bromide with ethylene glycol dimethacrylate. The particles have a regular spherical shape and an average diameter of 65 ± 24 μm. Their affinity for the fluoroquinolone antibiotics ofloxacin (OFL), lomefloxacin (LOM) and ciprofloxacin (CIP) is much higher than that of the blank polymer (not containing an IL), of polymers using methacrylic acid as functional monomer, of hydrophilic-lipophilic balanced sorbents, and of C18 sorbents. The microparticles were applied to the solid-phase extraction and rapid preconcentration of the fluoroquinolones from urine which then were quantified by HPLC. The calibration plot covers the 0.05 to 20 μg mL−1 concentration range, and the average recoveries at three spiking levels range from 93.6 to 103.7 %, with RSD of ≤5.7 %. The method was successfully applied to the determination of fluoroquinolones in spiked urine.
Microparticles covalently functionalized with an ionic liquid ([Amim][Br]) were synthesized by suspension copolymerization and show higher affinity for fluoroquinolones than other sorbents. The microparticles were used as a sorbent for solid-phase extraction and preconcentration of three fluoroquinolones from urine.
An ultrasensitive electrochemical glucose biosensor has been developed by depositing C60-fullerene functionalized with tetraoctylammonium bromide (C60-TOAB+) on the surface of a glassy carbon electrode (GCE). The glucose-binding protein concanavalin A (Con A) was then linked to the surface. Binding of glucose by Con A affects the electroactivity of the reversible redox couple C60/C60 −, and this finding forms the basis for a quantitative glucose assay over the 10 to 10 mM concentration range and with a lower detection limit of 3.3 nM (at an S/N ratio of 3). The sensitivity of this sensor allowed glucose to be determined in saliva. This biosensor possesses excellent selectivity, outstanding reproducibility and good long-term stability.
An ultrasensitive electrochemical glucose biosensor has been developed by depositing C60-fullerene functionalized with tetraoctylammonium bromide (C60-TOAB+) on the surface of a glassy carbon electrode (GCE). The glucose-binding protein concanavalin A (Con A) was then linked to the surface. Binding of glucose by Con A affects the electroactivity of the reversible redox couple C60/C60 −, and this finding forms the basis for a quantitative glucose assay over the 10 to 10 mM concentration range and with a lower detection limit of 3.3 nM (at an S/N ratio of 3). The sensitivity of this sensor allowed glucose to be determined in saliva.
Co@Pt core-shell nanoparticles (NPs) were synthetized by a two-step reductive method using carbon (Vulcan XC-72) as a solid support. The NPs were characterized by X-ray diffraction, field emission gun scanning electron microscopy, energy dispersive X-ray spectroscopy, and transmission electron microscopy. Their electrochemical performance was evaluated by cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry, and these showed that the Co@Pt NPs display an electrocatalytic activity towards the oxidation of glucose that is much better than that of plain Pt NPs. Under optimized conditions and at pH 7.0, the oxidation current of glucose at a working potential of −50 mV (vs. SCE) is linearly related to its concentration in the 1.0 to 30 mM range, and the detection limit is 0.3 mM (S/N = 3). It therefore covers the clinical range. The sensor also exhibits excellent stability and repeatability.
Co@Pt core-shell nanoparticles (NPs) display an electrocatalytic activity towards the oxidation of glucose that is much better than that of plain Pt NPs. The oxidation current for glucose is linearly related to its concentration in the 1.0 to 30 mM range, and the detection limit is 0.3 mM (S/N =3).
Alloy nanoparticles of the type PtxFe (where x is 1, 2 or 3) were synthesized by coreduction with sodium borohydride in the presence of carbon acting as a chemical support. The resulting nanocomposites were characterized by scanning electron microscopy and X-ray diffraction. The nanocomposite was placed on a glassy carbon electrode, and electrochemical measurements indicated an excellent catalytic activity for the oxidation of glucose even a near-neutral pH values and at a working voltage as low as 50 mV (vs. SCE). Under optimized conditions, the sensor responds to glucose in the 10.0 μM to 18.9 mM concentration range and with a 3.0 μM detection limit (at an S/N ratio of 3). Interferences by ascorbic acid, uric acid, fructose, acetamidophenol and chloride ions are negligible.
Nonenzymatic sensing of glucose is demonstrated at neutral pH values and low working potential using a glassy carbon electrode modified with platinum-iron alloy nanoparticles on a carbon support.
This article describes an electrochemical sensor for the dye additive Sunset Yellow (SY). It consists of a carbon paste electrode modified with nanostructured resorcinol-formaldehyde (RF) resin. The RF resin warrants strong signal enhancement and a strongly increased oxidation peak currents of SY at 0.66 V (vs. SCE). The effects of pH value, amount of RF polymer, accumulation potential and time were optimized. The sensor has a linear response to SY in the 0.3 to 125 nM concentration range, and the limit of detection is 0.09 nM after a 2-min accumulation time. The electrode was applied to the analysis of samples of wastewater and drinks, and the results are consistent with those obtained by HPLC.
Nanostructured resorcinol-formaldehyde (RF) resin was prepared and used as a material for electrochemical determination of Sunset Yellow.
We describe a simple and homogenous fluorimetric method for sensitive determination of DNA. It is based on target-triggered isothermal cycling and a cascade exponential amplification reaction that generates a large amount of a G-quadruplex. This results in strong fluorescence signal when using thioflavin T as a G-quadruplex-specific light-up fluorescent probe. Tedious handling after amplification is widely eliminated by the addition of thioflavin T. No other exogenous reagent is required. This detection platform is inexpensive and rapid, and displays high sensitivity for target DNA, with a detection limit as low as 91 pM.
The addition of target DNA can trigger the isothermal exponential amplification reaction to generate a large amount of G-quadruplex sequence oligonucleotides and then employ thioflavin T (Th T) (a G-quadruplex-specific light-up dye) as signal output for sensitive DNA detection.
Gold nanoclusters (AuNCs) stabilized with bovine serum albumin were utilized as a fluorescent probe for ferrous ion. The detection scheme is based on the quenching of the fluorescence of the modified AuNCs by hydroxyl radical (•OH) that is generated in the Fenton reaction between Fe(II) and H2O2. Fe(II) can be quantified in the 0.08 to 100 μM concentration range, and the limit of detection is as low as 24 nM. The method also displays good accuracy and high sensitivity when employed to the determination of Fe(II) in rat cerebrospinal fluids (CSFs). When applied to CSFs of a rat model of Alzheimer’s disease, it revealed enhanced levels of Fe(II) compared to a control, thereby showing the important physiological role of iron(II) in this disease.
BSA-stabilized gold nanoclusters (BSA-AuNCs) were utilized for the determination of ferrous ion in rat cerebrospinal fluids. The method is based on the quenching of the fluorescence by hydroxyl radical (•OH) which is generated in the Fenton reaction between Fe(II) and H2O2.
A molecularly imprinted polymer (MIP) was prepared by self-polymerization of dopamine in the presence of bovine hemoglobin (BHb) and then deposited on the surface of an electrode modified with gold nanoparticles (AuNPs). Scanning electron microscopy, cyclic voltammetry, and differential pulse voltammetry were employed to characterize the modified electrode using the hexacyanoferrate redox system as an electroactive probe. The effects of BHb concentration, dopamine concentration, and polymerization time were optimized. Under optimized conditions, the modified electrode selectively recognizes BHb even in the presence of other proteins. The peak current for hexacyanoferrate, typically measured at + 0.17 V (vs. SCE), depends on the concentration of BHb in the 1.0 × 10−11 to 1.0 × 10−2 mg mL−1 range. Due to the ease of preparation and tight adherence of polydopamine to various support materials, the present strategy conceivably also provides a platform for the recognition and detection of other proteins.
Gold nanoparticles and molecularly imprinted self-polymerization dopamine were modified on gold electrode surface to recognize and determine bovine hemoglobin. Under the optimized conditions, the modified electrode showed specific adsorption, selective recognition, and sensitive detection of bovine hemoglobin.
We describe a method for the synthesis of SERS-active silver nanoparticles (AgNPs) directly on the surface of bacteria (bacteria@AgNPs), specifically of E. coli cells. This straightforward strategy allows for the sensitive determination of bacteria on a microarray platform. Antibodies were used as selective receptors on the microarray surface. The Raman signal of bacteria@AgNPs is about 10 times higher than that obtained previously with microarrays based on mixing bacteria and AgNPs (bacteria+AgNPs). The optimum SERS enhancement of bacteria@AgNPs is obtained under 633-nm laser excitation, and this most likely is due to the plasmonic interaction of aggregated AgNPs. The method allows for an identification and quantification even of single E. coli bacteria. In our perception, this straightforward approach represents a most valuable tool for the detection of E. coli and, conceivably, of other bacteria, and thus has a large potential in environmental monitoring, medical diagnosis, and in food safety and quality control.
Synthesizing AgNPs directly on the surface of bacteria is demonstrated to be a highly efficient approach for a label-free readout of bacteria microarrays by surface-enhanced Raman scattering (SERS), resulting in signals about 10 times higher than previously reported results.
Platinum nanoparticles (Pt-NPs) with sizes in the range from 10 to 30 nm were synthesized using protein-directed one-pot reduction. The model globular protein bovine serum albumin (BSA) was exploited as the template, and the resulting BSA/Pt-NPs were studied by transmission electron microscopy, energy dispersive X-ray spectroscopy, and resonance Rayleigh scattering spectroscopy. The modified nanoparticles display a peroxidase-like activity that was exploited in a rapid method for the colorimetric determination of hydrogen peroxide which can be detected in the 50 μM to 3 mM concentration range. The limit of detection is 7.9 μM, and the lowest concentration that can be visually detected is 200 μM.
Pt-NPs were synthesized using BSA-directed one-pot reduction and BSA/Pt-NPs composite can effectively catalyze the oxidation of TMB producing blue solution in the presence of H2O2.
We describe the preparation of carbon dots (CDs) from glucose that possess high stability, a quantum yield of 0.32, and low toxicity (according to an MTT assay). They were used, in combination with the fluorogenic zinc(II) probe quercetin to establish a fluorescence resonance energy transfer (FRET) system for the determination of Zn(II). The CDs are acting as the donor, and the quercetin-Zn(II) complex as the acceptor. This is possible because of the strong overlap between the fluorescence spectrum of CDs and the absorption spectrum of the complex. The method enables Zn(II) to be determined in the 2 to 100 μM concentration range, with a 2 μM detection limit. The method was applied to image the distribution of Zn(II) ions in HeLa cells.
Based on the fluorescence resonance energy transfer (FRET) between carbon dots and quercetin (QCT)-Zn2+, the fluorescence indicator was established, which displays high sensitivity and selectivity in the detection of Zn2+. The method was also applied to image the distribution of Zn(II) ions in HeLa cells.
A jelly-like form of carbon dots (C-dots) was prepared by microwave-assisted synthesis from citric acid in the presence of tetraoctylammonium bromide. The effect of the concentration of tetraoctylammonium bromide was examined. The synthesized carbon dots were characterized by UV–vis, XRD, FTIR, fluorescence and HR-TEM. Fluorescence extends from 350 to 600 nm, and the corresponding excitation wavelengths range from 300 to 460 nm. Quantum yields are at around 0.11. A cytotoxicity study showed carbon dots to be cell permeable and biocompatible which renders them appropriate for imaging applications. The dots were used to image HeLa cell lines via the blue fluorescence of the dots.
C-dots were synthesized from citric acid by microwave heating in presence of varying concentrations of tetraoctylammonium bromide (TOAB) as a micellar template. The excellent optical properties of the nanoparticles make them well suitable for bio-imaging of HeLa cells.
We describe a square wave anodic stripping voltammetric (SWASV) platform for the determination of Cu(II). It is based on the use of amino-reduced graphene oxide (NH2-rGO) and β-cyclodextrin (β-CD) that were self-assembled on the surface of a glassy carbon electrode (GCE). The hydrophilicity and electrochemical performance of the resulting modified GCE were investigated by measurement of static contact angles, cyclic voltammetry and electrochemical impedance spectroscopy. Cu(II) was reduced at −1.1 V and then reoxidized at −0.012 V. Under optimum experimental conditions, the modified GCE exhibited excellent SWASV response in that the stripping peak currents (when sweeping between −0.3 and +0.25 V) depends on the concentration of Cu(II) in the 30 nM to 100 μM range. The limit of detection is 2.8 nM (at 3σ/slope). The modified GCE displaying good reproducibility, is stable, highly sensitive and selective. It was successfully applied to the determination of Cu(II) in synthetic and real water samples. The fast electron transfer rate and simple preparation of the NH2-rGO/β-CD composite makes it a promising electrode material for applications in sensing of heavy metal ions.
Amino-modified rGO and β-cyclodextrin form an attractive material for use in an electrochemical platform for highly sensitive and selective determination of Cu(II).