This study presents a method for the selective determination of Hg(II) using electromembrane extraction (EME), followed by square wave anodic stripping voltammetry (SWASV), using a gold nanoparticle-modified glassy carbon electrode, (AuNP/GCE). By applying an electrical potential of typically 60 V for 12 min through a thin supported liquid membrane (1-octanol), Hg(II) ions are extracted from a donor phase (i.e., the sample solution) to an acidic acceptor solution (15 μL) placed in the lumen of a hollow fiber. The influences of experimental parameters during EME were optimized using face-centered central composite design. The calibration plot, established at a working voltage of 0.55 V (vs. Ag/AgCl), extends from 0.2 to 10 μg.L?1 of Hg(II). The limit of detection, at a signal to noise ratio of 3, is 0.01 μg.L?1 and the relative standard deviations (for 5 replicate determinations at 3 concentration levels) are between 7.5 and 8.7 %. The method was successfully applied to the determination of Hg(II) in spiked real water samples to give recoveries ranging from 89 to 97 %. The results were validated by cold vapor atomic absorption spectroscopy.
Graphical abstract Hg(II) ions were extracted from a donor phase into an acidic acceptor phase (15 μL) placed in the lumen of a hollow fiber using electromembrane extraction. The acceptor phase was then analyzed using anodic stripping voltammetry.
The authors describe an electrochemical strategy for highly sensitive determination of ATP that involves (a) aptamer-based target recognition, (b) enzyme-free dendritic DNA nanoassembly amplification with multiplex binding of the biotin-strepavidin system, and (c) enzyme-amplified differential pulse voltammetric readout. In the presence of ATP, binding of ATP to the aptamer releases trigger DNA from the double-stranded complex between ATP aptamer and trigger DNA. The single-stranded thiolated capture probe, chemisorbed on the gold electrode surface, captures the released trigger DNA via hybridization. The toehold of the trigger DNA is recombined with one end of the first substrate DNA (1) which is on its other end biotinylated and blocked, with loops, by a counterstrand. The latter is removed by a complementary single-stranded helper (1) exposing two toeholds and two identical complimentary sequences for a second biotinylated substrate DNA (2). The latter, which is double-stranded except for the toehold, binds to one of these two sites. It is then stripped from its counter strand by another single-stranded helper DNA 2, exposing a toehold to bind another substrate DNA 1. On this substrate, another cycle with dentrimeric bransching can start.Substrate 1 with its two binding sites for substrate 2 initiates the assembly of dendritic DNA on the surface of the gold electrode, which finally possesses numerous biotins at the terminal ends of both of the associated substrate DNAs. Subsequent multiplex binding of streptavidinylated alkaline phosphatase and enzyme-amplified electrochemical readout leads to a highly sensitive electrochemical ATP aptasensor. If operated in the DPV mode, the current as measured at a typical working potential of 0.25 V (vs. Ag/AgCl) increases linearly over the 10 nM to 10 μM logarithmic ATP concentration range, and the detection limit is 5.8 nM (at an S/N ratio of 3). The assay is highly specific and reproducible. It was successfully applied to the detection of ATP in spiked human serum samples.
Graphical Abstract Schematic of the electrochemical strategy for adenosine triphosphate detection using aptamer-based target recognition and dendritic DNA nanoassembly amplification
A method is described for the colorimetric determination of mercury(II). In the absence of Hg(II), aminopropyltriethoxysilane (APTES) which is positively charged at pH 7 is electrostatically absorbed on the surface of gold nanoparticles (AuNPs). This neutralizes the negative charges of the AuNPs and leads to NP aggregation and a color change from red to blue-purple. However, in the presence of Hg(II), reduced Hg (formed through the reaction between Hg(II) and citrate on the AuNP surface) will replace the APTES on the AuNPs. Hence, the formation of aggregates is suppressed and the color of the solution does not change. The assay is performed by measuring the ratio of absorbances at 650 and 520 nm and can detect Hg(II) at nanomolar levels with a 10 nM limit of detection. The specific affinity between mercury and gold warrants the excellent selectivity for Hg(II) over other environmentally relevant metal ions.
Graphical Abstract Schematic of the method for determination of Hg2+ based on the gold amalgam-induced deaggregation of gold nanoparticles in the presence of APTES with the LOD of 10.1 nM.
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.
It is shown that triangular silver nanoplates (TAgNPs) are viable colorimetric probes for the fast, sensitive and selective detection of Hg(II). Detection is accomplished by reducing Hg(II) ions to elemental Hg so that an Ag/Hg amalgam is formed on the surface of the TAgNPs. This leads to the inhibition of the etching TAgNPs by chloride ions. Correspondingly, a distinct color transition can be observed that goes from yellow to brown, purple, and blue. The color alterations extracted from the red, green, and blue part of digital (RGB) images can be applied to the determination of Hg(II). The relationship between the Euclidean distances (EDs), i.e. the square roots of the sums of the squares of the ΔRGB values, vary in the 5 nM to 100 nM Hg(II) concentration range, and the limit of detection is as low as 0.35 nM. The color changes also allow for a visual estimation of the concentrations of Hg(II). The method is simple in that it only requires a digital camera for data acquisition and a Photoshop software for extracting RGB variations and data processing.
Graphical abstract Hg2+ detection was achieved by anti-etching of TAgNPs caused by the formation of silver amalgam, along with vivid multicolor variations from yellow to brown, purple, and eventually to be blue.
The authors describe an electrochemical method for the determination of the single-stranded DNA (ssDNA) oligonucleotide with a sequence derived from the genom of hepatitis B virus (HBV). It is making use of circular strand displacement (CSD) and rolling circle amplification (RCA) strategies mediated by a molecular beacon (MB). This ssDNA hybridizes with the loop portion of the MB immobilized on the surface of a gold electrode, while primer DNA also hybridizes with the rest of partial DNA sequences of MB. This triggers the MB-mediated CSD. The RCA is then initiated to produce a long DNA strand with multiple tandem-repeat sequences, and this results in a significant increase of the differential pulse voltammetric response of the electrochemical probe Methylene Blue at a rather low working potential of ?0.24 V (vs. Ag/AgCl). Under optimal experimental conditions, the assay displays an ultrahigh sensitivity (with a 2.6 aM detection limit) and excellent selectivity. Response is linear in the 10 to 700 aM DNA concentration range.
Graphical abstract Schematic of a voltammetric method for the determination of attomolar levels of target DNA. It is based on molecular beacon mediated circular strand displacement and rolling circle amplification strategies. Under optimal experimental conditions, the assay displays an ultrahigh sensitivity with a 2.6 aM detection limit and excellent selectivity.
The authors describe a fluorometric method for improving the determination of the cancer biomarker 8-hydroxy-2′-deoxyguanosine (8-OHdG). A nicking endonuclease (NEase)-powered 3-D DNA nanomachine was constructed by assembling hundreds of carboxyfluorescein-labeled single strand oligonucleotides (acting as signal reporter) and tens of swing arms (acting as single-foot DNA walkers) on a gold nanoparticle (AuNP). The activity of this DNA nanomachine was controlled by introducing the protecting oligonucleotides. In the presence of aptamer against 8-OHdG, the protecting oligonucleotides are removed from the swing arms by toehold-mediated strand displacement reaction. In the next step, detached DNA walker hybridizes to the labelled DNA so that the DNA nanomachine becomes activated. Special sequences of signal reporter in the formed duplex can be recognized and cleaved by NEase. As a result, the DNA walker autonomously and progressively moves along the surface of the AuNP, thereby releasing hundreds of signal reporters and causing a rapid increase in green fluorescence. This 3-D nanomachine is highly efficient because one aptamer can release hundreds of signal reporters. These unique properties allowed for the construction of a DNA nanomachine-based method for sensitively detecting 8-OHdG in concentrations as low as 4 pM. This is three orders of magnitude lower compared to previously reported methods.
Graphical abstract Schematic of a fluorometric method for determination of the cancer biomarker 8-hydroxy-2′-deoxyguanosine. A nicking endonuclease powered 3D-DNA nanomachine was used to improve the sensitivity. Limit of detection is three orders of magnitude lower than reported methods.
The authors describe a fluorometric assay for microRNA. It is based on two-step amplification involving (a) strand displacement replication and (b) rolling circle amplification. The strand displacement amplification system is making use of template DNA (containing a sequence that is complementary to microRNA-21) and nicking enzyme sites. After hybridization, the microRNA strand becomes extended by DNA polymerase chain reaction and then cleaved by the nicking enzyme. The DNA thus produced acts as a primer in rolling circle amplification. Then, the DNA probe SYBR Green II is added to bind to ssDNA to generate a fluorescent signal which increases with increasing concentration of microRNA. The method has a wide detection range that covers the10 f. to 0.1 nM microRNA concentration range and has a detection limit as low as 1.0 fM. The method was successfully applied to the determination of microRNA-21 in the serum of healthy and breast cancer patients.
Graphical abstract Schematic of a fluorometric microRNA assay based on two-step amplification involving strand displacement replication and rolling circle amplification. DNA probe SYBR Green II is then bound to ssDNA to generate a fluorescent signal which increases with increasing concentration of microRNA.
MicroRNAs (miRNAs) are considered as being promising biomarkers for hematological malignancies, their aging, progression and prognosis. The authors have developed a method for the detection of miRNA-155 by using surface plasmon resonance (SPR) imaging coupled to a nucleic acid-based amplification strategy using gold nanoparticles (AuNPs). The target miRNA-155 is captured by surface-bound DNA probes. After hybridization, DNA-AuNP are employed for signal amplification via DNA sandwich assembly, resulting in a large increase in the SPR signal. This method can detect miRNA-155 in concentrations down to 45 pM and over dynamic that extends from 50 pM to 5 nM. The assay is highly specific and can discriminate even a single base mismatch. It also is reproducible, precise, and was successfully applied to the determination of miRNA-155 in spiked real samples where it gave recoveries in the range between 86% and 98%. This biosensor provides an alternative approach for miRNA detection in biomedical research and clinical diagnosis, which is highly effective and efficient.
Graphical abstract Schematic of a surface plasmon resonance imaging biosensor for detection of miRNA-155 using strand displacement amplification and gold nanoparticle.
A DNAzyme-embedded hyperbranched DNA dendrimer is used as a colorimetric signal amplifier in an ultrasensitive detection scheme for nucleic acids. The hyperbranched DNA dendrimers were constructed by single-step autonomous self-assembly of three structure-free DNA monomers. A cascade of self-assembly reactions between the first and second strands leads to the formation of linear DNA concatemers containing overhang flank fragments. The third strand, which bears a peroxidase-mimicking DNAzyme domain, serves as a bridge to trigger self-assembly between the first and second strands across the side chain direction. This results in a chain branching growth of the DNAzyme-embedded DNA dendrimer. This signal amplifier was incorporated into the streptavidin-biotin detection system which comprises an adaptor oligonucleotide and a biotinylated capture probe. The resulting platform is capable of detecting a nucleic acid target with an LOD as low as 0.8 fM. Such sensitivity is comparable if not superior to most of the reported enzyme-free (and even enzyme-assisted) signal amplification strategies. The DNA dendrimer based method is expected to provide a universal platform for extraordinary signal enhancement in detecting other nucleic acid biomarkers by altering the respective sequences of adaptor and capture probe.
Graphical abstract Schematic of an assembly of a DNAzyme-embedded hyperbranched DNA dendrimer which operates as a signal amplifier for nucleic acids detection. The nanostructure is constructed by autonomous self-assembly of three DNA monomers. Colored letters represent each domain, and complementary domains are marked by asterisk. Domain d represents the DNAzyme sequence.
The authors describe an upconversion nanoparticle-based (UCNP–based) fluorometric method for ultrasensitive and selective detection of Cu2+. The UCNPs show a strong emission band at 550 nm under near-infrared excitation at 980 nm. The principle of the strategy is that gold nanoparticles (AuNP) can quench the fluorescence of UCNP. In contrast, the addition of L-cysteine (Cys) can induce the aggregation of AuNP, resulting in a fluorescence recovery of the UCNPs. On addition of Cu2+, it oxidizes Cys to cystine and is reduced to Cu+. The Cu+ thusformed can be oxidized cyclically to Cu2+ by dissolved O2, which catalyzes and recycles the whole reaction. Thus, the aggregation of AuNP is inhibited and the fluorescence recovered by Cys is quenched. Under the optimal condition, the quenching efficiency shows a good linear response to the concentrations of Cu2+ in the 0.4–40 nM range. The limit of detection is 0.16 nM, which is 5 orders of magnitude lower than the U.S. Environmental Protection Agency limit for Cu2+ in drinking water (20 μM). The method has been further applied to monitor Cu2+ levels in real samples. The results of detection are well consistent with those obtained by atomic absorption spectroscopy.
Graphical abstract Gold nanoparticles (AuNP) as a high efficient fluorescence quenching reagent of upconversion nanoparticles (UCNP) were used in a fluorometric method for detection of Cu2+ based on a cyclic catalytic oxidation amplification strategy.
The authors describe a highly sensitive and selective photoelectrochemical (PEC) assay for mercury(II) ions. It is based on a dual signal amplification strategy. The first enhancement results from the surface plasmon resonance (SPR) of Au@Ag nanoparticles (NPs) absorbed on MoS2 nanosheets. Here, the injection of hot electrons of Au@Ag NPs into MoS2 nanosheets produces a strong photocurrent, while background signals are strongly reduced. The second enhancement results from the use of a thymine rich ct-DNA aptamer attached to the Au@Ag-MoS2 nanohybrid. The DNA specifically binds Hg(II) ions to form thymine-Hg(II)-thymine (T-Hg-T) complexes. This leads to the formation of a hairpin-shaped dsDNA structure. The use of a CdSe quantum dot label at the terminal end of the ct-DNA further facilitates electron–hole separation. The photocurrent of the detector is measured as a function of Hg(II) concentration at a bias voltage of 0.1 V and under irradiation of 430 nm light. Due to the two-fold amplification strategy presented here, the linear range extends from 10 pmol·L?1 to 100 nmol·L?1, with a detection limit of 5 pmol·L?1 (at S/N?=?3).
Graphical Abstract The injection of hot electrons of Au@Ag into MoS2 produces a strong photocurrent, and the formation of thymine-Hg(II)-thymine further facilitates electron–hole separation by CdSe. This dual signal amplification strategy is used to detect Hg(II) ions via a photoelectrochemical assay.
The authors describe a colorimetric method for the determination of Hg(II) ions by exploiting the peroxidase-lile activity of few-layered MoS2 nanosheets (MoS2-NSs). These were prepared by sonication-induced exfoliation of bulk MoS2 crystals in aqueous surfactant solution. The MoS2-NSs were found to acts as a peroxidase mimic that is capable of oxidizing the substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2) to give a blue product with an absorption maximum at 652 nm. The addition of Hg(II) strongly accelerates the kinetics of this reaction. It is shown that the enzyme mimic possesses a high affinity for TMB and a lower pseudo-Michaelis-Menten constant. The stimulating effect of Hg(II) is seriously influenced by the change of surface charge. The use of nanosheets covered with (negatively charged) polystyrene sulfonate results in a decrease in the formation of blue dye, while those covered with (cationic) poly(diallyldimethyl ammonium) ions cause a small increase. Under optimal conditions, the peroxidase-like activity of MoS2-NSs is affected by Hg(II) in the 2.0 to 200 μM concentration range. The method has a detection limit (LOD) of 0.5 μM which is much below the allowed level in cosmetics (1 ppm; ca. 5 μM). The method display excellent sensitivity, selectivity and stability. It was applied to the determination of total mercury in cosmetic samples, and results compared well with results obtained by ICP-AES.
Graphical abstract A spectrophotometric assay for mercury?-?(II) determination is reported that is based on Hg2+-stimulation effect on the 3,3′,5,5′-tetramethylbenzidine (TMB)-H2O2 reaction system catalyzed by MoS2 nanosheets.
The authors describe a method for DNA target recognition and signal amplification that is based on the target-induced formation of a three way junction. The subsequent assembly of two DNA probes releases the inhibitory strand and triggers a downstream strand displacement amplification. This causes the formation of a G-rich single sequence that binds to a hemin monomer with its peroxidase-mimicking properties. The resulting peroxidase (POx) activity is quantified by using H2O2 and TMB as the substrate. In the presence of an inhibitor, in contrast, the POx-like activity is strongly reduced. This forms the basis for a highly sensitive DNA assay. It has a 0.8 pM detection limit when operated at a wavelength of 450 nm and was applied to the isothermal determination of target DNA with high selectivity.
Graphical abstract Schematic of the assay: Introduction of target results in the formation of a three way junction. The subsequent assembly of two probes releases the inhibitory strand and triggers a downstream strand displacement amplification, generating amount of G-rich single sequence which causes peroxidase-mimicking activity on binding to a hemin monomer.
We report on a method for highly sensitive and selective colorimetric determination of Hg(II) via a signal amplification strategy. Cu@Au nanoparticles (NPs) are found to exhibit intrinsic peroxidase-like activity and can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine by H2O2. This is accompanied by a solution color change from colorless to green (with an absorption peak at 655 nm). The catalytic capability of the Cu@Au NPs (pale green) is strongly enhanced by a Cu@Au-Hg trimetallic amalgam (bluish), and this effect can be applied directly to the determination of Hg(II). The limit of detection as observed with the unaided eye is 10 nM, which is at least one order of magnitude lower than that of the known AuNP-based colorimetric assay. Due to excellent specificity of the amalgamation process, the assay is highly selective for Hg(II) and is not interfered by other metal ions in up to 0.5 μM concentrations. This assay was successfully applied to the determination of Hg(II) in tap water. In view of these advantages, we expect this colorimetric method to become an attractive tool for the quantitation of Hg(II) in biological, environmental, and food samples.
Graphical Abstract Cu@Au nanoparticles (NPs) exhibit intrinsic peroxidase-like activity and can catalyze the oxidation of tetramethylbenzidine (TMB) by H2O2. This is accompanied by a color change of the solution from colorless to green.
A fluorometric ATP assay is described that makes use of carbon dots and graphene oxide along with toehold-mediated strand displacement reaction. In the absence of target, the fluorescence of carbon dots (with excitation/emission maxima at 360/447 nm) is strong and in the “on” state, because the signal probe hybridizes with the aptamer strand and cannot combine with graphene oxide. In the presence of ATP, it will bind to the aptamer and induce a strand displacement reaction. Consequently, the signal probe is released, the sensing strategy will change into the “off” state with the addition of graphene oxide. This aptasensor exhibits selective and sensitive response to ATP and has a 3.3 nM detection limit.
Graphical abstract Schematic of signal amplification by strand displacement in a carbon dot based fluorometric assay for ATP. This strategy exhibits high sensitivity and selectivity with a detection limit as low as 3.3 nM.
The authors describe an oligonucleotide-based lateral flow test for visual detection of Ag(I). The assay is based on cytosine-Ag(I)-cytosine [C-Ag(I)-C] coordination chemistry to capture gold nanoparticle (AuNP) tags in the test zone. A thiolated C-rich oligonucleotide probe was immobilized on the AuNPs via gold-thiol chemistry, and a biotinylated C-rich oligonucleotide probe was immobilized on the test zone. The AuNPs labelled with C-rich oligonucleotides are captured by Ag(I) ions in the test zone through the C-Ag(I)-C coordination. The resulting accumulation of AuNPs produces a readily visible red band in the test zone. Under optimized conditions, the test is capable of visually detecting 1.0 ppb of Ag(I) which is 50 times lower than the maximum allowable concentration as defined by the US Environmental Protection Agency for drinking water. Hence, the test is inexpensive and highly sensitive. It was applied to the detection of Ag(I) in spiked samples of tap water and river water. In our perception, the test is a particularly valuable tool in limited resource settings.
The authors describe a fluorometric assay for ochratoxin A (OTA) that is based on the use of graphene oxide and RNase H-aided amplification. On addition of OTA, cAPT is replaced from the APT/cAPT hybridization complex and then hybridizes with RNA labeled with a fluorophore at the 5′-end. Eventually, the fluorophore is released by RNase H cleavage. As the concentration of OTA increases, more cAPTs are displaced, this leading to fluorescence enhancement (best measured at excitation/emission wavelengths of 495/515 nm). This RNase H-assisted cycle response results in strong signal amplification. The limit of detection, calculated on the basis of a signal to noise ratio of 3, is 0.08 ng·mL?1. Response is linear in the 0.08–200 ng·mL?1 OTA concentration range. The method is highly selective for OTA over ochratoxin B and aflatoxin B1. It was applied to the determination of OTA in red wine samples spiked at levels of 1, 7, and 50 ng·mL?1, and the recoveries ranged from 90.9 to 112%.
A simple method is described for the determination of copper(II) ions based on the cathodic electrochemiluminescence (ECL) of lucigenin which is quenched by Cu(II). The blue ECL is best induced at ?0.45 V (vs. Ag/AgCl) at a scan rate of 50 mV·s?1. Under optimum conditions, the calibration plot is linear in the 3.0 to 1000 nM Cu(II) concentration range. The limit of detection is 2.1 nM at a signal-to-noise ratio of 3. Compared to other analytical methods, the one presented here is simple, fast, selective and cost-effective. It has been successfully applied in the analysis of copper ions in spiked tap water samples with recoveries ranging from 93.0% (at 50 nM concentration) to 105.7% (at 150 nM).
Graphical abstract The inhibitory effect of Cu(II) on the cathodic electrochemiluminescence of lucigenin enables determination of Cu(II) with a 2.1 nM detection limit.
The article describes a bienzyme visual system for aptamer-based assay of Hg(II) at nanomolar levels. The detection scheme is based on the finding that Hg(II) ions captured by aptamer-functionalized magnetic beads are capable of inhibiting the enzymatic activity of uricase and thus affect the formation of H2O2 and the blue product, i.e., oxidized tetramethylbenzidine. This strategy allows for a visual detection of Hg(II) at nanomolar levels without additional amplification procedure. Measuring the absorbance at 650 nm, the logarithmic calibration plot is linear in the concentration range of 0.5–50 nM and the limit of detection (LOD) is 0.15 nM. This is as low as the LOD obtained by atomic fluorescence spectrometry (AFS). The ions K+, Mg2+, Na+, Ca2+, Cu2+, Zn2+, Fe3+, Al3+, Co2+, AsO2?, Ni2+, Cd2+ and Pb2+ do not have a significant effect on color formation. The method was applied to the analysis of (spiked) river water, lake water, mineral water, tap water and certified reference water samples, and the results agreed well with those obtained by AFS or certified values, with recoveries ranging from 97% to 109%. The relative standard deviation for five parallel detections at a 10 nM Hg(II) level is 5.2%.
Graphical abstract A bienzyme-based visual aptasensor was fabricated for label-free detection of nanomolar Hg2+ in water samples without any amplification or enrichment procedure.
