The authors describe an aptamer based assay for determination of ractopamine (RAC) by using PicoGreen (PG) as a fluorescent probe specific for dsDNA. In the absence of RAC, the aptamer forms a duplex structure with a complementary sequence that results in enhanced PG fluorescence. Upon binding to RAC, the aptamer undergoes a structural switch. This reduces the number of DNA duplexes formed and causes a reduction of fluorescence intensity of PG as measured at excitation/emission wavelengths of 480/520 nm. Under optimized conditions, the dynamic calibration plot covers the 50 pM to 50 μM concentration range, with a 50 pM detection limit. This meets the safety supervision regulations of the European Commission in terms of residue limits of RAC in food. The method displays high selectivity over other β-adrenergic agonists including clenbuterol, dopamine and salbutamol. The assay was successfully applied to samples of swine urine at spiking levels of 7.4 nM, 22.2 nM and 37 nM. Average recoveries ranged from 95 to 110%, with an RSD of <1.5%. The method is expected to represent a promising tool for simple, rapid and sensitive on-site detection of RAC in animal products.
Graphical abstract An aptamer based fluorescent assay for determination of ractopamine was developed with a dynamic range of 50 pM to 50 μM. The average recovery from spiked urine samples ranged from 95 to 110%, with an RSD of <1.5%.
The authors describe a fluorescence based aptasensor for adenosine (AD), a conceivable biomarker for cancer. The assay is based on the immobilization of capture DNA on newly synthesized quaternary CuInZnS quantum dots (QDs) and the conjugation of probe DNA on gold nanoparticles (AuNPs). The capture DNA is an adenosine-specific aptamer that is partly complementary to the probe DNA. Once the capture aptamer hybridizes probe DNA, the fluorescence of the QDs (measured at excitation/emission wavelengths of 522/650 nm) is quenched by the AuNPs. However, when AD is added, it will bind to the aptamer and restrain the hybridization between capture DNA and probe DNA. Therefore, the fluorescence of the QDs will increase with increasing AD concentration. Under optimal conditions, fluorescence is linearly related to the AD concentration in the range from 50 to 400 μM, the detection limit being 1.1 μM. This assay is sensitive, selective, reproducible and acceptably stable. It was applied to the determination of AD in spiked human serum samples where it gave satisfactory results.
A colorimetric method is described for the determination of Pt(II). It is based on the use of gold nanoparticles (AuNPs) which are known to aggregate in the presence of a cationic polymer such as poly(diallyldimethylammonium chloride) (PDDA). If, however, a mismatched aptamer (AA) electrostatically binds to PDDA, aggregation is prevented. Upon the addition of Pt(II), it will bind to the aptamer and induce the formation of a hairpin structure. Hence, interaction between aptamer and PDDA is suppressed and PDDA will induce the aggregation of the AuNPs. This is accompanied by a color change from red to blue. The effect can be observed with bare eyes and quantified by colorimetry via measurement of the ratio of absorbances at 610 nm and 520 nm. Response is linear in the 0.24–2 μM Pt(II) concentration range, and the detection limit is 58 nM. The assay is completed within 15 min and selective for Pt(II) even in the presence of other metal ions. It was successfully applied to the rapid determination of Pt(II) in spiked soil samples.
Graphical abstract Schematic representation of the method for detection of Pt(II) based on the use of a cationic polymer and gold nanoparticles. In the presence of Pt(II), aptamer interacts with the Pt(II) and prevents the interaction between aptamer and cationic polymer. Hence, cationic polymer induce the aggregation of the AuNPs and lead to the color change from red to blue.
An aptamer based assay is described for the colorimetric detection of adenosine. The presence of adenosine triggers the deformation of hairpin DNA oligonucleotide (HP1) containing adenosine aptamer and then hybridizes another unlabeled hairpin DNA oligonucleotide (HP2). This leads to the formation of a double strand with a blunt 3′ terminal. After exonuclease III (Exo III)-assisted degradation, the guanine-rich strand (GRS) is released from HP2. Hence, the adenosine-HP1 complex is released to the solution where it can hybridize another HP2 and initiate many cycles of the digestion reaction with the assistance of Exo III. This leads to the generation of a large number of GRS strands after multiple cycles. The GRS stabilize the red AuNPs against aggregation in the presence of potassium ions. If, however, GRS forms a G-quadruplex, it loses its ability to protect gold nanoparticles (AuNPs) from salt-induced AuNP aggregation. Therefore, the color of the solution changes from red to blue which can be visually observed. This colorimetric assay has a 0.13 nM detection limit and a wide linear range that extends from 5 nM to 1 μM.
Graphical abstract Schematic presentation of a colorimetric aptamer biosensor for adenosine detection based on DNA cycling amplification and salt-induced aggregation of gold nanoparticles.
An electrochemical nanoaptasensor is described that is based on the use of a glassy carbon electrode (GCE) modified with electrodeposited silver nanoparticles (AgNPs). An aptamer (Apt) against trinitrotoluene (TNT) was then immobilized on the AgNPs. The addition of TNT to the modified GCE leads to decrease in peak current (typically measured at a potential of ?0.45 V vs. Ag/AgCl) of riboflavin which acts as an electrochemical probe. Even small changes in the surface (as induced by binding of Apt to TNT) alter the interfacial properties. As a result, the LOD is lowered to 33 aM, and the dynamic range extends from 0.1 fM to 10 μM without sacrificing specificity.
Graphical abstract Schematic presentation of a nanoaptasensor which is based on a glassy carbon electrode (GCE) modified with electrodeposited silver nanoparticles (AgNPs) and aptamer (Apt). It was applied to the detection of 2,4,6-trinitrotoluene (TNT) with the help of riboflavin (RF) as a redox probe.
The authors describe a surface plasmon resonance (SPR) based aptasensor for the carcinogenic mycotoxin aflatoxin B1 (AFB1) in a direct assay format. The aptamer is immobilized on the surface of a commercial sensor chip, and the SPR signal increases on binding of AFB1. The sensor chip can be fully regenerated by passing a flow of buffer over it upon which bound AFB1 dissociates from the aptamer. The biosensor works in the 0.4 nM to 200 nM AFB1 concentration range and has a 0.4 nM detection limit. It allows AFB1 to be determined in complex samples such as diluted red wine and beer. The assay is sensitive, and the chip is easily regenerated and stable. The method therefore overcomes certain limitations of antibody-based SPR assays and of competitive SPR assays for AFB1.
Graphical abstract Schematic presentation of the assay: Aptamer is coated on the chip of SPR, and the binding between aflatoxin B1 (AFB1) and the aptamer on chip causes SPR responses, allowing sensitive detection of AFB1.
The authors describe the fabrication of an interconnected edge-exposed graphene nanostructure via chemical vapor deposition (CVD) of foliated graphene onto a network of alumina nanofibers. The fibers such obtained are shown to enable ultra-sensitive voltammetric determination of dopamine (DA), uric acid (UA) and ascorbic acid (AA). The electrode displays powerful and persistent electro oxidative behavior and excellent electron transport properties. Cyclic voltammetry and differential pulse voltammetry demonstrate excellent selectively and sensitivity for AA, DA and UA, with typical peaks at ?0.08 V, +0.19 V, and +0.34 V (vs. SCE), respectively. Under optimum conditions, the calibration plots are linear in the 1–80 μM range for DA, in the 1–60 μM range for UA, and in the 0.5–60 μM range for UA, with detection limits of 0.47 μM, 0.28 μM and 0.59 μM, respectively. The sensor was successfully applied to the simultaneous determination of DA and UA in the presence of AA in spiked urine sample.
Graphical abstract Material with high density of graphene foliates grown over highly aligned nano-dimensional ceramic fibers is used as electrode for simultaneous highly sensitive electrochemical determination of DA in the presence of UA and AA with a considerably low limit of detection.
A glassy carbon electrode (GCE) was anodically oxidized by cyclic voltammetry (CV) in 0.05 M sulfuric acid to introduce hydroxy groups on its surface (GCEox). Next, an imidazolium alkoxysilane (ImAS) is covalently tethered to the surface of the GCEox via silane chemistry. This electrode is further modified with graphene oxide (GO) which, dispersed in water, spontaneously assembles on the electrode surface through electrostatic interaction and π-interaction to give an electrode of type GO/ImAS/GCE. Electroreduction of GO and GCEox by CV yields electroreduced GO (erGO) and an electrode of the type erGO/ImAS/GCE. This electrode displays excellent electrocatalytic activity for the oxidation of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Three fully resolved anodic peaks (at ?50 mV, 150 mV and 280 mV vs. Ag/AgCl) are observed during differential pulse voltammetry (DPV). Under optimized conditions, the linear detection ranges are from 30 to 2000 μM for AA, from 20 to 490 μM for UA, and from 0.1 to 5 μM and from 5 μM to 200 μM (two linear ranges) for DA. The respective limits of detection (for an S/N of 3) are 10 μM, 5 μM and 0.03 μM. The GCE modified with erGO and ImAS performs better than a bare GCE or a GCE modified with ImAS only, and also outperforms many other reported electrodes for the three analytes. The method was successfully applied to simultaneous analysis of AA, DA and UA in spiked human urine.
Graphical abstract Differential pulse voltammetric simultaneous determination of ascorbic acid, dopamine and uric acid is achieved on a glassy carbon electrode modified with electroreduced graphene oxide and imidazolium groups, through anodic treatment of glassy carbon and silane chemistry.
Carbon polymer dots (CPDs) were prepared by a one-pot aqueous synthetic route from ascorbic acid and diethylenetriamine at room-temperature. The CPDs under 350-nm excitation exhibit blue fluorescence peaking at 430 nm with a quantum yield of 47%. Other features include an average diameter of 5 nm, a fluorescence that is independent of the excitation wavelength, good water dispersibility and photostability, and excellent biocompatibility. The CPDs are shown to be viable fluorescent probes for ferric ion which acts as a strong quencher. The response to Fe(III) is linear in the 0.2 to 10 μM concentration range, and the detection limit is 0.1 μM. The probe was applied to the determination of Fe(III) in environmental waters and to intracellular imaging of ferric ions in HeLa cells.
Graphical abstract Carbon polymer dots (CPDs) are prepared from ascorbic acid and diethylenetriamine (DETA) at room-temperature (RT). The RT-CPDs exhibit excellent optical performance, biocompatibility and selectivity of quenching by ferric ions. This can be applied for determination and intracellular imaging of ferric ion.
A method is described for the synthesis of a nanocomposite containing FeOOH and N-doped carbon nanosheets. The nanocomposite was synthesized by a hydrothermal method using a Fe3O4/chitosan nanocomposite as the precursor. The nanocomposite displays peroxidase-like activity and catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H2O2. This results in the formation of a blue colored product with an absorption maximum at 652 nm in the UV-vis spectra. Based on these findings, colorimetric assays were worked out for both hydrogen peroxide and glucose. The H2O2 assay works in the 5 to 19 μM concentration range, and the limit of detection is 5 nM. The glucose assay works in the 8 μM to 0.8 mM concentration range and has a 0.2 μM detection limit. The method was successfully applied to the determination of glucose in human urine.
Graphical abstract Schematic of the hydrothermal synthesis of a FeOOH/N-doped carbon nanocomposite. It was used to replace peroxidase enzyme for the catalytic oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in a visual colorimetric test for glucose in human urine.
Core-shell Au@Ag nanorods (Ag@GNRs) were synthesized and utilized to construct a voltammetric biosensor for trichloroacetic acid (TCA). The biosensor was prepared by immobilizing hemoglobin (Hb) on a glassy carbon electrode (GCE) that was modified with the Ag@GNRs. Cyclic voltammetry revealed a pair of symmetric redox peaks, indicating that direct electron transfer occurs at the Hb on the Ag@GNR-film. The electron transfer rate constant is as high as 2.32 s?1. The good electrocatalytic capability and large surface area of the Ag@GNR-film is beneficial in terms of electron transfer between Hb and the underlying electrode. The modified GCE, best operated at ?0.4 V (vs. SCE), exhibits electrocatalytic activity toward TCA in the 0.16 μM to 1.7 μM concentration range, with a 0.12 μM detection limit (at an S/N ratio of 3).
Graphical abstract Core-shell Au@Ag nanorods (Ag@GNRs) were synthesized and used to immobilize hemoglobin to construct an effective biosensor for trichloroacetic acid.
A voltammetric analytical assay for the selective quantification of vanillin is described. It is based on the use of a gold nanoparticle-modified screen-printed carbon electrode (SPCE) modified with graphene quantum dots (GQD) in a Nafion matrix. The GQD were synthesized by an acidic thermal method and characterized by UV-Vis, photoluminescence, and FTIR spectroscopy. The modified SPCE displays a strongly enhanced response to vanillin. Linear sweep voltammetry (LSV) and differential pulse voltammetry (DPV) were applied to optimize the methods. The analytical assay has linear responses in the 13 to 660 μM and 0.66 to 33 μM vanillin concentration ranges. The detection limits are 3.9 μM and 0.32 μM when using LSV and DPV, respectively. The analytical assay is selective and stable. It was applied to the determination of vanillin in several food samples with satisfactory results. Recoveries from spiked samples ranged between 92.1 and 113.0%.
Graphical abstract The selective and sensitive quantification of vanillin is carried out by the use of a gold nanoparticle-modified screen-printed carbon electrode modified with graphene quantum dots in a Nafion matrix.
The authors describe a voltammetric sensor for simultaneous determination of dopamine (DA), uric acid (UA), L-tyrosine (Tyr), and the diuretic drug hydrochlorothiazide (HCTZ). The assay is based on the use of graphene nanowalls deposited on a tantalum substrate. The nanowalls are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy, and cyclic voltammetry. The nanowalls are vertically grown on the substrate by direct-current arc plasma jet chemical vapor deposition. The modified electrode is shown to enable simultaneous differential pulse voltammetric determination of DA, UA, Tyr, and HCTZ. The graphene nanowalls display a large specific surface, high conductivity, and a large number of catalytically active sites for oxidation of analytes. Simultaneous detection is performed best at a pH value of 7.0 and at peak potentials of 0.124 V (vs. SCE) for DA, 0.256 V for UA, 0.536 V for Tyr and 0.708 V for HCTZ. The respective detection limits are 0.04 μM, 0.1 μM, 0.6 μM and 0.4 μM. The results show that this graphene wall modified electrode is a promising tool for the design of sensitive, selective, and stable sensors.
Graphical abstract The graphene-based differential pulse voltammetric sensor for simultaneous determination of dopamine, uric acid, L-tyrosine, and hydrochlorothiazide exhibits high selectivity, sensitivity, and stability.
A multifunctional fluorescent probe is synthesized for the determination of adenosine 5′-triphosphate (ATP). The 6-carboxyfluorescein-labeled aptamer (FAM-aptamer) was bound to the surface of magnetite nanoparticles coated with polydopamine (Fe3O4@PDA) by π-π stacking interaction to form the multifunctional probe. The probe has three functions including recognition, magnetic separation, and yielding a fluorescent signal. In the presence of ATP, FAM-aptamer on the surface of the probe binds to ATP and returns to the solution. Thus, the fluorescence of the supernatant is enhanced and can be related to the concentration of ATP. Fluorescence intensities were measured at excitation/emission wavelengths of 494/526 nm. Response is linear in the 0.1–100 μM ATP concentration range, and the detection limit is 89 nM. The probe was applied to the quantitation of ATP in spiked human urine and serum samples, with recoveries ranging between 94.8 and 102%.
Graphical abstract A multifunctional fluorescent probe based on the use of FAM-aptamer and Fe3O4@PDA is described for the determination of ATP in spiked human urine and serum samples. FAM-aptamer: 6-carboxyfluorescein-labeled aptamer; Fe3O4@PDA: magnetite nanoparticles coated with polydopamine. ATP: adenosine 5′-triphosphate.
Glutathione coated gold and silver nanoclusters (GSH-Au/AgNCs) were synthesized by one-pot reduction methods and are found to be viable fluorescent nanoprobes for cysteine (Cys) and arginine (Arg), with good selectivity over other amino acids. The GSH-Au/AgNCs have two emissions at 616 nm and 412 nm when excited at 360 nm. With the increased concentration of Cys, the ratio of the emission intensities (I616/I412) linearly decreases with Cys in concentration ranging from 0.05 to 10 μM and from 10 to 50 μM, respectively. With increased concentrations of Arg, the ratio of I616/I412 linearly decreases with Arg concentration ranging from 0 to 50 μM and from 50 to 100 μM, respectively. The probe was applied to the determination of Cys and Arg in spiked samples of serum and urine where it gave good recoveries.
Graphical abstract Glutathione-coated gold and silver nanoclusters (GSH-Au/AgNCs) were synthesized by one-pot reduction and are found to be viable fluorescent nanoprobes for cysteine (Cys) and arginine (Arg).
The authors describe a method for amperometric determination of thiodiglycol (TDG), the main hydrolysis product of sulfur mustard. The electrode consists of a mixture of graphene nanosheets, silver nanoparticles and the ionic liquid octylpyridinium hexafluorophosphate. Electrochemical oxidation of TDG was performed by cyclic voltammetry at pH 4 and revealed a pair of well-defined redox peaks at potentials of 0.43 and 0.19 V (vs. Ag/AgCl). Amperometric detection was accomplished over a dynamic range that is linear in the 10–3700 μM concentration range. The detection limit (at an S/N of 3) is 6 μM. The electrode was applied to the determination of TDG in (spiked) waste water and gave recoveries that ranged from 98.2 to 103.3 %.
Graphical abstract The article describes an amperometric sensor for the determination of thiodiglycol, the main hydrolysis product of sulfur mustard. The electrode was constructed by using graphene nanosheets, silver nanoparticles and an ionic liquid electrode, and it was successfully applied to the determination of thiodiglycol in (spiked) waste water samples.
Cobalt oxyhydroxide (CoOOH) nanosheets are efficient fluorescence quenchers due to their specific optical properties and high surface area. The combination of CoOOH nanosheets and carbon dots (CDs) has not been used in any aptasensor based on fluorescence quenching so far. An aptamer based fluorometric assay is introduced that is making use of fluorescent CDs conjugated to the aptamer against methamphetamine (MTA), and of CoOOH nanosheets which reduce the fluorescence of the CDs as a quencher. The results revealed that the conjugated CDs with aptamers were able to enclose the CoOOH nanosheets. Consequently, fluorescence is quenched. If the aptamer on the CD binds MTA, the CDs are detached from CoOOH nanosheets. As a result, fluorescence is restored proportionally to zhe MTA concentration. The fluorometric limit of detection is 1 nM with a dynamic range from 5 to 156 nM. The method was validated by comparing the results obtained by the new method to those obtained by ion mobility spectroscopy. Theoretical studies showed that the distance between CoOOH nanosheet and C-Ds is approximately 7.6 Å which can illustrate the possibility of FRET phenomenon. The interactions of MTA and the aptamer were investigated using molecular dynamic simulation (MDS).
Graphical abstract Carbon dots (C-Ds) were prepared from grape leaves, conjugated to aptamer, and adsorbed on CoOOH nanosheets. So, the fluorescence of C-Ds is quenched. On addition of MTA, fluorescence is restored.
The authors describe a method for amperometric determination of chloramine-T that is based on the indirect detection of chloramine-T by detecting p-quinone imine (p-QI) that is generated by oxidation of p-aminophenylboronic acid by chloramine-T. p-QI can be detected with excellent selectivity and at low potential by using a glassy carbon electrode. Hence, the method displays attractive features such as high sensitivity, wide detection range and excellent selectivity. The electrode has two linear responses in the 50 nM to 100 μM concentration range and a 6 nM detection limit. Compared to other electrochemical methods, this assay has a detection limit that is better by three orders of magnitude. The relative standard deviation is 3.4% for the determination of 10 μM of the medical chloramine-T sample, and the recovery of a samples containing chloramine-T at a level of 10 μM is 115%.
Graphical abstract Highly sensitive electrochemical detection of chloramine-T is achieved based on the reaction of chloramine-T with p-aminophenylboronic acid with a detection limit of 6 nM.
A voltammetric sensor is presented for the simultaneous determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA). It is based on a gold electrode (GE) modified with carboxyl-functionalized graphene (CFG) and silver nanocube functionalized DA nanospheres (AgNC@PDA-NS). The AgNC@PDA-NS nanocomposite was characterized by scanning electron microscopy and UV-Vis spectroscopy. The electrochemical behavior of the modified electrode was evaluated by electrochemical impedance spectroscopy, cyclic voltammetry and differential pulse voltammetry. The modified electrode displays good electrocatalytic activity towards DA (typically at 0.14 V vs. Ag/AgCl) and UA (typically at 0.29 V vs. Ag/AgCl) even in the presence of ascorbic acid. Response to DA is linear in the concentration range of 2.5 to 130 μM with a detection limit of 0.25 μM. Response to UA is linear in the concentration range of 10 to 130 μM with a detection limit of 1.9 μM. In addition, the sensitivity for DA and UA is 0.538 and 0.156 μA μM?1 cm?2, respectively. The modified electrode also displays good stability, selectivity and reproducibility.
Graphical abstract The gold electrode modified with polydopamine nanospheres functionalized with silver nanocube and carboxylated graphene is used for simultaneous determination of DA and UA in the presence of AA, with wide linear range and low detection limit.
The authors describe a highly chemiluminescent metal-organic framework (MOF) that was obtained by loading the pores of MIL-101(Cr) with luminol. Immobilization is based on Lewis acid-base interactions between the coordinatively unsaturated metal sites of MIL-101(Cr) and the amino groups of luminol. The luminol-loaded MOF displays strong chemiluminescence (CL) in the presence of hydrogen peroxide (H2O2) in alkaline solution. Pyrophosphate (PPi), in turn, acts as a quencher of this CL. These findings have been exploited in the design of a CL based method for the determination of either H2O2 or PPi. The assays for H2O2 works in the 3 to 100 μM concentration range and has a detection limit of 0.5 μM. The quenchometric assay for PPi works in the 5 to 70 μM concentration range and has a 1.2 μM detection limit.
Graphical abstract Luminol-embedded metal-organic frameworks (MOFs) with high chemiluminescent activity were prepared and used for the sensitive determination of either H2O2 or pyrophosphate ions.
