MercuryII-mediated construction of DNA capsules for turn-on fluorescence detection of melamine

Researchers have shown significant interest in three-dimensional DNA building blocks due to their potential applications in biomedicine and biosensing. This study focuses on the synthesis of an HgII ion-stabilized DNA capsule with T-HgII-T pairs for the purpose of detecting melamine (MA). MA reacts with HgII to form a MA-HgII-MA complex, which causes HgII to leave the capsule shell, ultimately leading to capsule collapse and release of fluorescent cargo as output signal. Density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) were used to demonstrate the ability of MA to extract HgII from the T-HgII-T adducts. The DNA capsules were characterized using TEM, SEM, DLS, zeta-potential, and melting curve analysis, which indicated the successful construction of the HgII-intercalated DNA shell. The MA-triggered destruction of the DNA capsules was visualized by confocal microscopy, and the dynamics of decapsulation were evaluated through fluorescent cargo release. The HgII-stabilized DNA capsules enable MA detection with a detection limit of 0.037 µM and are insensitive to potential interfering ions and amino acids. The tests conducted using MA spiked milk solution resulted in recoveries ranging from 109 to 113% (0.1 µM) and 94.5 to 96% (0.5 µM). These results suggest that the system is promising for highly accurate and reproducible monitoring of MA adulteration.

A new difunctional liquid crystal nanosurface molecularly imprinted polyitaconic acid nanoprobe for SERS/RRS determination of ultratrace melamine

In this paper, a new dimode scattering spectral method for rapid detection of ultratrace melamine (ML) in dairy products was established by coupling nanosurface molecular imprinting technology with nanocatalytic amplification reaction of liquid crystal particles. It was found that liquid crystal cholesteryl butyrate (CBU) nanosurface imprinted polymers (CBU@MIP) not only recognized ML but also catalyzed the nano indicator reaction of HAuCl4-sodium formate to produce gold nanoparticles with surface-enhanced Raman scattering (SERS) and resonance Rayleigh scattering (RRS) effect. When ML was added, it specifically combined with CBU@MIP to form CBU@MIP-ML conjugates with strong catalytic activity, and SERS and RRS signals increased linearly with the detection limits of 0.0072 pmol/L and 0.093 pmol/L respectively. The method was applied to the determination of ML in dairy products and plastic tablewares with relative standard deviation (RSD) of 2.2-4.4 % and 1.6-4.7 %, and recovery of 95.4 %-108.3 % and 95.9-108.6  % respectively.

Novel rapid detection of melamine based on the synergistic aggregation of gold nanoparticles

A simple and rapid colorimetric method for the detection of melamine in milk samples is described. Polythymidine oligonucleotide was adsorbed on to the surface of gold nanoparticles (AuNPs), protecting it from aggregation. In the presence of melamine, polythymidine oligonucleotide combined with melamine formed a double-strand DNA-like structure, allowing AuNPs aggregation. In the presence of positively charged SYBR Green I (SG I), AuNPs were further aggregated. In the presence of melamine and SG I, aggregation of AuNPs was synergistic. Thus, in this principle, melamine can be detected visually. Plasmon resonance peak changes enabled detection of melamine quantitatively using UV-vis spectroscopy. The limit of detection for this colorimetric method was 16 μg L^-1 with a good linear range from 19.5 μg L^-1 to 1.25 × 10³ μg L^-1, and detection took only 1 min. The method was successfully applied for detection of melamine in milk samples.

A novel polymeric fluorescence sensor based on acrylated citric acid for detection of melamine adulteration: Application in milk powder

Melamine selective acrylate citric acid (ACA) based polymeric membrane sensor was prepared by radical polymerization method and the sensor was characterized. The sensor showed a selective fluorescent response to melamine (λ_ex/λ_em = 388/425 nm). The sensor response is linear in the concentration range of 3.96 × 10^-9 to 7.93 × 10^-8 mol L^-1, the optimum pH value is 6.0 and response time is less than 1 min. Limit of detection (LOD) and limit of quantification (LOQ) were calculated as 2.32 × 10^-10 mol L^-1 and as 7.74 × 10^-10 mol L^-1, respectively. The sensor showed great selectivity towards melamine in the presence of a large number of molecules and ions. The performance of sensor was also confirmed by determining of melamine in milk powder sample and the results were compared with HPLC results and acceptable results were obtained. As a conclusion, the results revealed that the proposed sensor is an interesting alternative for melamine determination.

The creation of selective imprinted cavities on quartz crystal microbalance electrode for the detection of melamine in milk sample

Molecularly imprinted polymer based nanofilms specific to melamine were synthesized on quartz crystal microbalance (QCM) electrode surface. Contact angle measurements, atomic force microscopy, ellipsometry and scanning electron microscopy were used for characterizations process. Some of the findings of the study are as follows: pH 6.0 was found as optimal working pH for melamine detection. Prepared MIP QCM electrode showed a linearity of 99.53% in the concentration range of 50-1000 ng/mL. Langmuir-Freundlich hybrid model was the best fitted isotherm for whole concentration range. The performance of MIP QCM electrode was also confirmed by determining of melamine in melamine spiked milk samples. As a conclusion, the results figured out that not only QCM nanosensor for specific melamine detection but also polymerization strategy were classified as an intriguing alternative for developing new melamine sensing platforms. Limit of detection (LOD) and limit of quantification (LOQ) were calculated as 2.3 ng/mL and 7.8 ng/mL, respectively.

A label-free ratiometric method to detect Hg2+ based on structural change of DNA

In this work, a simple ratiometric method has been designed to detect Hg²⁺ based on the structural change between double-stranded DNA (dsDNA) and its G-quadruplex structure. When Hg²⁺ was added, the designed G-quadruplex structure could change into the corresponding dsDNA by forming the T-Hg²⁺ -T mismatch. This kind of variation resulted in a decrease in the fluorescence of the G-quadruplex/N-methyl mesoporphyrin IX (NMM) complex and an increase in the fluorescence from the dsDNA/SYBR Green I (SG I) pair. The secondary excitation wavelength of SG I was used to excite NMM and SG I simultaneously. The titration experiment indicated that the new method had a linear response within 0.7-2.5 μM Hg²⁺ with a limit of detection of 9.3 nM. Because using the T-Hg²⁺ -T mismatch to recognize Hg²⁺ was very specific, the selectivity of the new method was also satisfactory. The recoveries ranged from 92.8% to 110.2% suggested that this new method could achieve a potential application for Hg²⁺ detection in real environmental samples.

Enhancing the Sensitivity of DNA and Aptamer Probes in the Dextran/PEG Aqueous Two-Phase System

Increasing the local concentration of DNA-based probes is a convenient way to improve the sensitivity of biosensors. Instead of using organic solvents or ionic liquids that phase-separate with water based on hydrophobic interactions, we herein studied a classic aqueous two-phase system (ATPS) comprising polyethylene glycol (PEG) and dextran. Polymers of higher molecular weights and higher concentrations favored phase separation. DNA oligonucleotides are selectively enriched in the dextran-rich phase unless the pH was increased to 12. A higher volume ratio of PEG-to-dextran and a higher concentration of PEG also enrich more DNA probes in the dextran-rich phase. The partition efficiency of the T₁₅ DNA was enriched around seven times in the dextran phase when the volume ratio of dextran and PEG reached 1:10. The detection of limit improved by 3.6-fold in a molecular beacon-based DNA detection system with the ATPS. The ATPS also increased the sensitivity for the detection of Hg²⁺ and adenosine triphosphate, although these target molecules alone distributed equally in the two phases. This work demonstrates a simple method using water soluble polymers to improve biosensors.