Yuangen Wu

Label-free fluorescent sensor for lead ion detection based on lead(II)-stabilized G-quadruplex formation.

A label-free fluorescent DNA sensor for the detection of lead ions (Pb(2+)) based on lead(II)-stabilized G-quadruplex formation is proposed in this article. A guanine (G)-rich oligonucleotide, T30695, was used as a recognition probe, and a DNA intercalator, SYBR Green I (SG), was used as a signal reporter. In the absence of Pb(2+), the SG intercalated with the single-stranded random-coil T30695 and emitted strong fluorescence. While in the presence of Pb(2+), the random-coil T30695 would fold into a G-quadruplex structure and the SG could barely show weak fluorescence, and the fluorescence intensity was inversely proportional to the involving amount of Pb(2+). Based on this, a selective lead ion sensor with a limit of detection of 3.79 ppb (parts per billion) and a detection range from 0 to 600 ppb was constructed. Because detection for real samples was also demonstrated to be reliable, this simple, low-cost, sensitive, and selective sensor holds good potential for Pb(2+) detection in real environmental samples.

A simple fluorescent assay for lead(II) detection based on lead(II)-stabilized G-quadruplex formation

A fluorescence quenching result was obtained when an FAM-labelled G-rich oligonucleotide T30695 was treated with Pb2+, owing to the formation of a lead(II)-stabilized G-quadruplex. Based on this phenomenon a selective fluorescent assay for lead(II) detection with a limit of detection of 0.77 ppb and a detection range from 0 to 200 ppb was constructed.

A silver-specific DNA-based bio-assay for Ag(I) detection via the aggregation of unmodified gold nanoparticles in aqueous solution coupled with resonance Rayleigh scattering

We report a label-free, silver-specific DNA-based bio-assay for the detection of silver ion (Ag+) in aqueous solution. The bio-assay relies on the structural change of cytosine (C)-rich silver-specific DNA and the status transition of gold nanoparticles (AuNPs). In the absence of Ag+, unfolded silver-specific DNA stabilizes the dispersed AuNPs at high salt concentrations, which induces low resonance Rayleigh scattering (RRS) intensity. However, in the presence of Ag+, silver-specific DNA folds into a hairpin conformation through C–Ag+–C mismatches, and cannot coat on the surface of AuNPs. Thus, the uncoated AuNPs aggregate at high salt concentrations and cause a significant increase in RRS intensity, which reflects the amount of Ag+ in the system. Under the optimized conditions, the RRS intensity at 550 nm increased linearly with the concentration of Ag+ ranging from 0.20 μM to 1.00 μM, and the limit of detection for Ag+ was determined as 0.202 μM. The bio-assay also shows high selectivity against coexisting cations.

Ultrasensitive Resonance Scattering (RS) Spectral Detection for Trace Tetracycline in Milk Using Aptamer-Coated Nanogold (ACNG) as a Catalyst

This paper reports an ultrasensitive resonance scattering (RS) method to detect tetracycline (TET) in milk based on the competition of aptamers between nanogold and TET, aggregation of naked nanogold, nanocatalytic Fehling reaction, and RS signals of catalytic product Cu₂O cubic. The detection principle was confirmed by the nanoparticle size analyzer (NANOS), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The variations of RS intensity had good linear correlation with TET concentrations, and the limit of detection was calculated as 11.6 nM. The proposed method was successfully applied for analysis of TET in milk, with total recoveries ranging from 105 to 109%.

A Simple and Sensitive Colorimetric Detection of Silver Ions Based on Cationic Polymer-Directed AuNPs Aggregation

This paper describes a simple and sensitive colorimetric sensor employing single-stranded DNA (ssDNA) ligand, cationic polymer, and gold nanoparticles (AuNPs) to detect silver ions. The positively charged polymer can electrostatically interact with ssDNA and destroy the charge balance leading to induction of AuNP aggregation. Silver ions (Ag+) can bind to cytosine (C)-rich nucleic acids to form the C-Ag+-C hair-pin structure, which can prevent its interaction with polymers. The resulting cationic polymer could aggregate AuNPs causing a remarkable change in colour. The concentration of Ag+ can be determined visually. This sensing platform exhibits high sensitivity and selectivity towards Ag+ versus other metal ions, with a detection limit of 48.6 nM. The assay did not require any labelling or modifying steps. This method is simple, effective, and convenient and can in principle be used to detect other metal ions or small molecules.

Sensitive colorimetric detection of melamine in milk with an aptamer-modified nanogold probe

We report a rapid and facile colorimetric sensing method for melamine detection in milk using aptamer based nanoparticles (AuNPs). Aptamers can absorb on the surface of AuNPs and electrostatically interact with poly(diallyldimethylammonium chloride) (PDDA), which protects the AuNPs from aggregation. The introduction of melamine causes a melamine–aptamer complex to form via hydrogen bonding, thus the resulting cationic polymer can aggregate the AuNPs and cause a remarkable change in color. As a result of this, the presence of melamine can be determined by the naked eye and by measuring absorbance. This sensor is selective for the detection of melamine in milk samples and has a limit of detection of 34 nM. Since aptamer-modified AuNPs are more stable and selective than unmodified AuNPs in the sensing of melamine, we hope that this type of detection method will be helpful for the detection of other small molecules.

Regulation of hemin peroxidase catalytic activity by arsenic-binding aptamers for the colorimetric detection of arsenic(III)

The catalytic activity of higher concentration of hemin can be temporarily inhibited by arsenic-binding aptamers, and it will recover in the presence of As(III) due to the exhaustion of aptamers, so the subsequent TMB molecules can be oxidized thoroughly and cause a remarkable increase of absorbance value at 442 nm, which enables the colorimetric detection of As(III) with high selectivity and a detection limit of 6 ppb.

A Colorimetric Aptamer Biosensor Based on Gold Nanoparticles for the Ultrasensitive and Specific Detection of Tetracycline in Milk

This paper proposes a sensing strategy which employs an aptamer, unmodified gold nanoparticles (AuNP), and hexadecyltrimethylammonium bromide (CTAB) to detect tetracycline (TET) in raw milk. The method is based on the colorimetric assay of aggregating AuNP. In the absence of TET, the CTAB and aptamer form a complex which allows the aggregation of AuNP. In the presence of TET, the TET aptamer is exhausted first due to the formation of aptamer-TET complexes, which prevents assembly of the CTAB–aptamer supramolecule, causing a colour change and no aggregation of AuNP. This mechanism for the detection of TET proved to be sensitive and convenient. The colorimetric assay has a detection limit of 122 nM TET. This sensor has great potential for the sensitive, colorimetric detection of a wide range of molecular analytes.

A highly sensitive resonance scattering based sensor using unmodified gold nanoparticles for daunomycin detection in aqueous solution

A sensitive sensor for detection of daunomycin (DNR) was developed by using free-labeled gold nanoparticles (AuNPs) and DNR aptamer. In the absence of DNR, free aptamer could absorb onto the surface of AuNPs to prevent them from aggregating in high salt solution. In the presence of DNR, the aptamer was firstly exhausted due to the formation of a DNR/aptamer complex, which lead to the remarkable change of resonance scattering (RS) intensity at 550 nm. Such sensor showed high selectivity and a detection limit of 17.1 nM. Based on the advantage of the sensor, the proposed sensing method is believed to be implemented to the on-site and real-time DNR detection in food safety and other applications.

Preparation and Characterization of Artificial Antigens for Cadmium and Lead

Cadmium and lead were conjugated to two carrier proteins using a bifunctional chelator [2-(4-aminobenzyl)-diethylenetriaminepentaacetic acid] to synthesize artificial antigens for cadmium and lead. The techniques, including ultraviolet spectrometry, circular dichroism, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and inductively coupled plasma optical emission spectroscopy, were utilized for characterizing the artificial antigens. The results of ultraviolet spectrometry showed characteristic absorption peak shifts between conjugates and carrier proteins. Circular dichroism resulted that the second structure of the conjugates was α-helix. The sodium dodecyl sulfate polyacrylamide gel electrophoresis results revealed the differences of band migration and molecular weight among antigens, chelator protein conjugate, and carrier proteins. The result of coupling ratios revealed that the metal content of the antigens was much higher than that of carrier proteins. These results indicated that the artificial antigens of cadmium and lead were synthesized successfully and had potential application in immunoassays of cadmium and lead ions.