Ag Nanoparticles@Au Nanograting Array as a 3D Flexible and Effective Surface-Enhanced Raman Scattering Substrate

Surface-enhanced Raman scattering (SERS) is a powerful analytical technique for chemical identification, but it remains a great challenge to realize the large-scale and well-controlled fabrication of sensitive and repeatable SERS substrates. Here, we report a facile strategy to fabricate centimeter-sized periodic Au nanograting (Au-NG) decorated with well-arranged Ag nanoparticles (Ag-NPs) (denoted as Ag-NPs@Au-NG) as a three-dimensional (3D) flexible hybrid SERS substrate with high sensitivity and good reproducibility. The Au-NG patterns with periodic ridges and grooves are fabricated through nanoimprint lithography by employing a low-cost digital versatile disc (DVD) as a master mold, and the Ag-NPs are assembled by a well-controlled interface self-assembly method without any coupling agents. Multiple coupling electromagnetic field effects are created at the nanogaps between the Ag-NPs and Au-NG patterns, leading to high-density and uniform hot spots throughout the substrate. As a result, the Ag-NPs@Au-NG arrays demonstrate an ultrahigh SERS sensitivity as low as 10-13 M for rhodamine 6G with a high average enhancement factor (EF) of 1.85 × 108 and good signal reproducibility. For practical applications, toxic organic pollutants including crystal violet, thiram, and melamine have been successfully detected with high sensitivity at a low detection limit, showing a good perspective in the rapid detection of toxic organic pollutants.

Enhanced ultrasonic spray ionization for direct mass spectrometry analysis of aqueous solution and complex samples using a single-orifice piezoelectric atomizer

An efficient and superior soft ionization approach for direct mass spectrometry analysis of a variety of samples such as aqueous solution, raw biological sample and proteins, was developed based on commercially available piezoelectric atomizers. A single conical orifice (5 μm in diameter) was created on the atomizer, which resulted in generation of uniform fine droplets and long-duration of MS signal. The two electrodes of piezoelectric atomizer were connected to the two sides of ceramic ring which was insulated from the metallic substrate. The unique design allowed an additional high voltage input towards the spray reagents, which facilitated direct analysis of more complex samples without sample pre-treatment, such as biological samples (tomato tissue). The ionization was driven by an extremely low electrical power (3.5 V rechargeable battery) yet providing an efficient and superior soft ionization. The method displayed a better thermal and pH stability than nano electrospray ionization (nanoESI) and electrospray ionization (ESI) on direct analysis of Vitamin B and protein aqueous solutions. Quantitative analysis of Vitamin B and Rhodamine B aqueous solutions was also investigated, showing a good linearity (R² > 0.99). In addition, our results suggested that compared with ESI and nanoESI, the method not only could be used for direct analysis of intact protein, but also provide more information concerning the association between intact protein and the subunits.

The development and application of matrix assisted laser desorption electrospray ionization: The teenage years

In the past 15 years, ambient ionization techniques have witnessed a significant incursion into the field of mass spectrometry imaging, demonstrating their ability to provide complementary information to matrix-assisted laser desorption ionization. Matrix-assisted laser desorption electrospray ionization is one such technique that has evolved since its first demonstrations with ultraviolet lasers coupled to Fourier transform-ion cyclotron resonance mass spectrometers to extensive use with infrared lasers coupled to orbitrap-based mass spectrometers. Concurrently, there have been transformative developments of this imaging platform due to the high level of control the principal group has retained over the laser technology, data acquisition software (RastirX), instrument communication, and image processing software (MSiReader). This review will discuss the developments of MALDESI since its first laboratory demonstration in 2005 to the most recent advances in 2021.

A Lanthanide Complex Fluorescent Probe for the Detection of Melamine

Melamine has been illegally adulterated in dairy food because of the rich nitrogen content and stable chemical properties in recent years. Therefore, the detection of melamine is of great significance for food safety supervision and human health protection. As melamine is a weak fluorescent substance, it is difficult to detect melamine directly by fluorescence spectroscopy. In this work, we found that melamine can significantly enhance the emission of the tetracycline-europium (EuTC) complex at 616 nm. Therefore, we took EuTC complex as a fluorescent probe to detect melamine. According to the characterizations of absorption spectra, molecular electrostatic potential distribution, and the time-resolved spectra, we speculated that tetracycline and melamine may form a complex through hydrogen bonding interaction in the melamine-EuTC reaction system, causing the melamine closer approach to Eu³⁺ and reducing the non-radiative energy loss of water molecules to Eu³⁺, which significantly enhanced the fluorescence intensity of EuTC. The fluorescence intensity of EuTC complex with melamine concentration in the range of 0.5-40.0 μM shows a good linear relationship, and the correlation coefficient is 0.9951 with the detection limit of 7.85 × 10^-8M. It shows a high sensitivity for the EuTC complex as a fluorescent probe to detect melamine, which provides a supplement and extension for the detection of melamine by fluorescence spectroscopy.

Nebulization dielectric barrier discharge ionization mass spectrometry: Rapid and sensitive analysis of acenaphthene

A rapid, simple and sensitive method was proposed for low-polar acenaphthene analysis by coupling nebulization with dielectric barrier discharge ionization (N-DBDI). The sample solution was nebulized followed by heating and converted to be gas-phase analyte molecules prior to DBDI. This boosts the collision efficiency of analyte molecules with reactive species and thus the sensitivity, and the high-velocity gas from nebulization guides ions directed to the MS inlet without deflection. The dependence of sensitivity on the operation parameters was systematically investigated. The LOD and LOQ of acenaphthene were determined to be 0.61 ng/L and 2.05 ng/L, respectively, which were superior approximately 30 folds compared to those obtained by other methods. Parameters, including accuracy, precision, reproducibility and utility, were tested to further evaluate the performance of N-DBDI. Real environmental samples, including river water, initial rainwater and mineral water, were analyzed with good accuracy (93.61-103.50%) and satisfactory precision (RSD ≤ 8.92%). These findings suggest that the N-DBDI allows the determination of non/low-polar species at sub-pg/mL possible, and would benefit for the non/low-polar species analysis in real environmental samples.

High-sensitive detection of fluorene by ambient ionization mass spectrometry

High sensitive analysis for fluorene at the sub-ng L⁻¹ level in real water samples was achieved by nebulization-dielectric barrier discharge ionization.

Sonodecoration of magnetic phosphonated-functionalized sporopollenin as a novel green nanocomposite for stir bar sorptive dispersive microextraction of melamine in milk and milk-based food products

The novel green magnetic phosphonated-functionalized sporopollenin nanocomposite (MPSP-nanocomposite) was synthetized and used for stir bar sorptive dispersive microextraction (SBSDME) of melamine in milk and milk-based food products. TEM, SEM-EDX, FT-IR, VSM techniques were applied for characterization of MPSP-nanocomposite. The influential parameters including pH, extraction time, stirring rate, elution solvent type and volume, sample volume, desorption time, and ionic strength were studied and at optimum conditions, the linear range of 1-500 (µg L^-1), the LOD (S/N = 3) of 0.30 (µg L^-1), and the LOQ (S/N = 10) of 0.95 (µg L^-1) were achieved. The intra-day precision values (RSD (%), n = 7) of 3.5% for the melamine concentration of 25 (µg L^-1). The relative recoveries of 95.8% to 99.6% were acquired for the real samples which confirmed that the proposed method could be successfully utilized in complex matrixes with high matrix effects.

Inexpensive Ultrasonic Nebulization Coupled with Direct Current Corona Discharge Ionization Mass Spectrometry for Liquid Samples and Its Fundamental Investigations

The concept of direct mass-spectrometric analysis, especially exploited by ambient desorption/ionization (ADI) methods, provides numerous means for convenient sample analysis. While many simple and versatile ionization sources have been developed, challenges lay in achieving efficient sample introduction. In previous work, a sample introduction method employing direct current corona discharge (CD) coupled to a surface acoustic wave nebulization (SAWN) device enhanced sampling performance for both polar and nonpolar analytes by up to 4 orders of magnitude. In fact, the SAWN-CD method generated a multiply charged peptide ion signal comparable to that of conventional ESI. Unfortunately, the high cost of the SAWN devices themselves limits their accessibility. Herein, we report on an analogous implementation of CD with an inexpensive ultrasonic nebulizer (USN) on the basis of a commercial room humidifier demonstrating equivalent exemplary performance. We subsequently compare the two methods of SAWN-CD and USN-CD in a screening application of milk for the detection of two antibiotic drugs, ciprofloxacin and ampicillin. Finally, we further investigate the relative softness of these CD-coupled acoustic nebulization methods in comparison to that of ESI using a survival yield study of the thermometer ion nitrobenzylpyridinium.

Atmospheric Solids Analysis Probe Coupled to a Portable Mass Spectrometer for Rapid Identification of Bulk Drug Seizures

The emergence of ambient ionization techniques and their combination with smaller, cheaper mass spectrometers is beginning to make real the possibility of mass spectrometry measurements being made routinely outside of traditional laboratory settings. Here, we describe the development of an atmospheric solids analysis probe (ASAP) source for a commercially available miniaturized, single-quadrupole mass spectrometer and subsequent modification of the instrument to allow it to run as a deployable system; we further go on to describe the application of this instrument to the identification of the contents of drug seizures. For the drug seizure analysis, a small quantity of the material (powder, tablet, resin, etc.) was dissolved in ethanol and shaken to extract the analytes, the resulting solutions were then sampled by dipping a sealed glass capillary into the solution prior to analysis by ASAP-MS. Identification of the contents of the seizures was carried out using a NIST searching approach utilizing a bespoke spectral library containing 46 compounds representative of those most commonly encountered in UK forensic laboratories. In order to increase confidence in identification the library sample and subsequent analyses were carried out using a four-channel acquisition method; each channel in this method used a different cone voltage (15, 30, 50, and 70 V) inducing differing levels of in-source fragmentation in each channel; the match score across each channel was then used for identification. Using this developed method, a set of 50 real-life drug samples was analyzed with each of these being identified correctly using the library searching method.

Rapid desorption of low-volatility compounds in liquid droplets accompanied by the flash evaporation of solvent below the Leidenfrost temperature

RATIONALE

The objective of this work is to study the interaction of methanol droplets with the heated surface for the improved detection of low-volatility and thermally labile compounds by the flash evaporation that occurs below the Leidenfrost temperature.

METHODS

5 μL solutions of low-volatility compounds in methanol were introduced into the heated tube. Desorbed analytes were ionized in the sealed atmospheric pressure chemical ionization (APCI) source by direct current (DC) corona discharge using air as the reagent gas.

RESULTS

The rapid desorption of low-volatility compounds accompanied by the flash evaporation of methanol solvent was observed in the temperature range of 60-100°C. Linear relationships between the signal intensities and the solute concentrations in the range of 0.01-5 ppm for morphine, cocaine, methamphetamine, and amphetamine were obtained at 95°C.

CONCLUSIONS

The observed rapid desorption of low-volatility compounds below the Leidenfrost temperature would provide useful information in many fields, e.g., the interaction of liquid droplets with heated matter, liquid sample introduction into the injection port of a gas chromatograph, coupling of the flash evaporation with pulse valve operated miniaturized mass spectrometer, etc.

Scale-up of microdroplet reactions by heated ultrasonic nebulization

Dramatically higher rates for a variety of chemical reactions have been reported in microdroplets compared with those in the liquid bulk phase. However, the scale-up of microdroplet chemical synthesis has remained a major challenge to the practical application of microdroplet chemistry. Heated ultrasonic nebulization (HUN) was found as a new way for scaling up chemical synthesis in microdroplets. Four reactions were examined, a base-catalyzed Claisen-Schmidt condensation, an oximation reaction from a ketone, a two-phase oxidation reaction without the use of a phase-transfer-catalyst, and an Eschenmoser coupling reaction. These reactions show acceleration of one to three orders of magnitude (122, 23, 6536, and 62) in HUN microdroplets compared to the same reactions in bulk solution. Then, using the present method, the scale-up of the reactions was achieved at an isolated rate of 19 mg min-1 for the product of the Claisen-Schmidt condensation, 21 mg min-1 for the synthesis of benzophenone oxime from benzophenone, 31 mg min-1 for the synthesis of 4-methoxybenzaldehyde from 4-methoxybenzyl alcohol, and 40 mg min-1 for the enaminone product of the Eschenmoser coupling reaction.

Designing a sustainable mobile phase composition for melamine monitoring in milk samples based on micellar liquid chromatography and natural deep eutectic solvent

Modified micellar liquid chromatography (MLC) with a natural deep eutectic solvent (NADES), produced from choline chloride (ChCl) and ethylene glycol (EG), was employed for melamine (MEL) monitoring in milk matrix. This sustainable mobile phase was attained through chemometrical optimization of crucial variables including concentration of sodium dodecyl sulphate ([SDS]) along with volume percentages of both NADES and glacial acetic acid (GAC). The desirability function and central composite design were utilized as chemometrical tools. Retention time (t_R-MEL), and chromatographic peak width of MEL at 50% of its height (W_50%-MEL) were considered for finding the best possible arrangement of the influential factors in the configuration of the mobile phase. Under the optimal experimental conditions of 0.10 mol L^-1 SDS, 4% (v/v) NADES, and 4% (v/v) GAC, the results showed that both t_R-MEL and W_50%-MEL drastically decreased when NADES was a part of the mobile phase composition. This indicated that ChCl-EG-based NADES had a significant impact on improving the chromatographic behaviour of an ionizable polar compound, MEL. At the optimal point, MEL was eluted in approximately 10 min without being interfered by coexisting proteins and endogenous species in milk. The practical performance of the mobile phase was established through direct injection of milk samples into the MLC system. The eligibility criteria of the United State-Food and Drug Administration (US-FDA) were considered for validation of the introduced methodology.

Direct Analysis of Aqueous Solutions and Untreated Biological Samples Using Nanoelectrospray Ionization Mass Spectrometry with Pipette Tip in Series with High-Ohmic Resistor as Ion Source

Commercially available disposable plastic pipette tip with the inner diameter of ca. 120 μm in series with a high-ohmic resistor (10 GΩ) was adapted as a low-cost alternative ion source for high-throughput nanoelectrospray mass spectrometry (nESI-MS) analysis of a variety of samples, especially aqueous solutions, without sample pretreatment. The use of high-ohmic resistor enabled the formation of stable electrospray of aqueous solutions at ambient conditions. In addition, corona discharge was avoided even with a high voltage applied. Quantitative analysis of vitamin B in water was successfully conducted by tip-ESI. The results exhibited a good linearity (R ˃ 0.9983), a low detection limit (0.25 ng/mL), and a wide dynamic response range (0.25-1000 ng/mL). Our study revealed that tip-ESI not only performed equally well to capillary nESI in terms of flow rate (˂ 100 nL/min), signal sensitivity, and sample consumption, but also offered a number of additional advantages, including better signal duration, tolerance to high analyte concentration (> 100 μg/mL) and high ionizing voltage (up to 6 kV), and obviation of tip clogging and corona discharge. High compatibility of tip-ESI with various kinds of samples (aqueous, viscous, solid, or bulk biological samples) makes it a promising tool for direct MS analysis.

Low-cost fabrication of a paper-based microfluidic using a folded pattern paper

Despite that microfluidic paper-based analytical devices (μPADs) provide effective analytical platforms for point-of-care diagnosis in resource-limited areas, it remains challenging to achieve simple and low-cost fabrication of μPADs. A novel method for fabrication of μPADs is developed in this study using a folded polydimethylsiloxane (PDMS)-coated paper mask with a specific pattern to form a sandwich structure with inserted chromatographic paper. PDMS penetrates the target paper from the front and the back sides, and then is cured in the target paper to form legible channels. This method for prototyping μPADs has many favorable merits including simple operation without the need of trained personnel, fast fabrication and low cost. We further investigated colorimetric detection of melamine in the μPADs, and it showed a remarkable measurement with a detection limit of 0.1 ppm in aqueous solutions and liquid milk discriminated by the naked eye, which meets the detection limit required by USA and China. The fabricating strategy developed in this study is very promising and attractive for the development of simple μPADs for point-of-care applications, including diagnostic testing, food safety control and environmental monitoring.

Melamine-Induced Decomposition and Anti-FRET Effect from a Self-Assembled Complex of Rhodamine 6G and DNA-Stabilized Silver Nanoclusters Used for Dual-Emitting Ratiometric and Naked-Eye-Visible Fluorescence Detection

In this work, blue-emitting silver nanoclusters (AgNCs) were prepared in a matrix of single-stranded deoxyribonucleic acid (DNA) on the basis of ambient hydrothermal reactions. DNA acted as the stabilizer or coating agent, and NaBH₄ was used as the reducing agent. Through the interactions between rhodamine 6G (Rh6G) and the synthesized DNA-AgNCs, the self-assembled complex of DNA-AgNC-Rh6G was generated. Meanwhile, fluorescence emission of AgNCs was weakened as a result of fluorescence-resonance-energy transfer (FRET) from AgNCs (donor) to Rh6G (acceptor). In the DNA-AgNC-Rh6G complex aqueous suspension, the addition of melamine induced obvious emission recovery of AgNCs and fluorescence decrease of Rh6G, attributable to melamine-induced decomposition of the self-assembled complex and anti-FRET effects. There was a well-plotted linear relationship of ratiometric fluorescence intensities ( I_AgNCs/ I_Rh6G) versus melamine concentration in the range of 0.1-10 μM, with a low detection limit of 25 nM. Responses of I_AgNCs/ I_Rh6G to melamine were highly selective and sensitive over potential interferents. A novel dual-emitting ratiometric fluorescence sensor of melamine was facilely constructed on the basis of the DNA-AgNC-Rh6G complex. In particular, the sensor enabled visual fluorescence detection of melamine both in aqueous solution and on wetted filter paper. Superior detection results of the sensor were experimentally obtained and confirmed its high feasibility for melamine detection in practical samples.

Reaction screening and optimization of continuous-flow atropine synthesis by preparative electrospray mass spectrometry

Preparative electrospray (ES) exploits the acceleration of reactions in charged microdroplets to perform a small scale chemical synthesis. In combination with on-line mass spectrometric (MS) analysis, it constitutes a rapid screening tool to select reagents to generate specific products. A successful reaction in preparative ES triggers a refined microfluidic reaction screening procedure which includes the optimization for stoichiometry, temperature and residence time. We apply this combined approach for refining a flow synthesis of atropine. A successful preparative ES pathway for the synthesis of the phenylacetyl ester intermediate, using tropine/HCl/phenylacetyl chloride, was optimized for solvent in both the preparative ES and microfluidics flow systems and a base screening was conducted by both methods to increase atropine yield, increase percentage conversion and reduce byproducts. In preparative ES, the first step yielded 55% conversion (judged using MS) to intermediate and the second step yielded 47% conversion to atropine. When combined in two discrete steps in continuous-flow microfluidics, a 44% conversion of the starting material and a 30% actual yield of atropine were achieved. When the reactions were continuously telescoped in a new form of preparative reactive extractive electrospray (EES), atropine was synthesized with a 24% conversion. The corresponding continuous-flow microfluidics experiment gave a 55% conversion with an average of 34% yield in 8 min residence time. This is the first in depth study to utilize telescoped preparative ES and the first use of dual ESI emitters for multistep synthesis.

Triplex DNA formation-mediated strand displacement reaction for highly sensitive fluorescent detection of melamine

Since melamine is a strong hazard to human health, the development of new methods for highly sensitive detection of melamine is highly desirable. Herein, a novel fluorescent biosensing strategy was designed for sensitive and selective melamine assay based on the recognition ability of abasic (AP) site in triplex towards melamine and signal amplification by Mg²⁺-dependent DNAzyme. In this strategy, the melamine-induced formation of triplex DNA was employed to trigger the strand displacement reaction (SDR). The SDR process converted the specific target recognition into the release and activation of Mg²⁺-dependent DNAzyme, which could catalyze the cleavage of fluorophore/quencher labeled DNA substrate (FQ), resulting in a significantly increased fluorescent signal. Under the optimal conditions, the fluorescent signal has a linear relationship with the logarithm of the melamine concentration in a wide range of 0.005-50 μM. The detection limit was estimated to be 0.9 nM (0.1ppb), which is sufficiently sensitive for practical application. Furthermore, this strategy exhibits high selectivity against other potential interfering substances, and the practical application of this strategy for milk samples reveals that the proposed strategy works well for melamine assay in real samples. Therefore, this strategy presents a new method for the sensitive melamine assay and holds great promise for sensing applications in the environment and the food safety field.

A Sensitive "Turn-On" Fluorescent Sensor for Melamine Based on FRET Effect between Polydopamine-Glutathione Nanoparticles and Ag Nanoparticles

In this work, Ag nanoparticles (AgNPs) were synthesized quickly by a one-step method utilizing polydopamine-glutathione nanoparticles (PDA-GNPs) as a reducing agent. The PDA-GNPs and the generated AgNPs acted as the energy donor and acceptor, respectively. Accordingly, the fluorescence of PDA-GNPs was quenched on the basis of fluorescence resonance energy transfer (FRET). In the presence of melamine, the preferential combination of Ag(I) and melamine to form Ag(I)-melamine complex prevents Ag(I) from forming AgNPs, together with fluorescence enhancement compared with the absence of melamine. Under the optimal conditions including the concentration of AgNO₃, reaction time, reaction temperature, and pH, the fluorescence enhancement efficiency has a linear response to the concentration of melamine from 0.1 to 40 μM with a detection limit of 23 nM for melamine. The proposed method is simple, time-saving, and low-cost, which was further applied to detect melamine in real milk products with satisfactory results.

Convenient and sensitive colorimetric detection of melamine in dairy products based on Cu(ii)-H2O2-3,3′,5,5′-tetramethylbenzidine system

The illegal adulteration of melamine in dairy products for false protein content increase is a strong hazard to human health. Herein, a simple and sensitive colorimetric method was developed for the quantification of melamine in dairy products based on a Cu²⁺-hydrogen peroxide (H₂O₂)-3,3′,5,5′-tetramethylbenzidine (TMB) system. In this strategy, Cu²⁺ exhibits peroxidase-like activity and can catalyze the oxidation of TMB to oxidized TMB (oxTMB) in the presence of H₂O₂ with a blue colour change of the solution. However, the presence of melamine quickly interacts with H₂O₂ leading to the consumption of H₂O₂ and thus strongly hinders the oxidation of TMB. Under the optimal conditions, the absorbance change of oxTMB has a linear response to the concentration of melamine from 1 to 100 μM with a detection limit of 0.5 μM for melamine. The proposed method has many merits including more simplicity, good selectivity, and more cost-effectiveness without using any nanomaterials. The method was further successfully applied to detect melamine in dairy products including milk and infant formula powder.

Convenient and sensitive colorimetric detection of melamine in dairy products based on a Cu(ii)-H₂O₂-3,3′,5,5′-tetramethylbenzidine system was reported.

Rapid and selective DNA-based detection of melamine using α-hemolysin nanopores

We have developed a rapid and selective approach for the detection of melamine based on simple DNA probes and α-hemolysin nanopores.

Multi-channel microfluidic chip coupling with mass spectrometry for simultaneous electro-sprays and extraction

Considering the advantages and research status of microfluidic chip coupling with mass spectrometry (MS), a microfluidic chip-based multi-channel ionization (MCMCI) for the extraction of untreated compounds in complex matrices without sample pretreatments was developed. Quantitative analysis of human urine spiked with various rhodamine B concentrations was also performed, and good linearity was obtained. Comparing to the macro ionization device, MCMCI significantly improved the integration of ionization source, simplified the operation of such a device, and greatly increased the signal intensity with much lower gas pressure. Comparison of our MCMCI with two and three gas channels indicated that the liquid–liquid extraction process before spraying and after spraying produced similar MS results. Moreover, this MCMCI with three gas channels also implemented simultaneous dual sprays with high DC voltages, the interference of two samples was minor and ion suppression effect was drastically alleviated. Such advantages may easily enable internal calibration for accurate mass measurement. Furthermore, dual extraction can be implemented by integrating such multi-spray configuration, which can improve the extracted signal intensity and sensitivity. These technologies open up new avenues for the application of microfluidic chip coupling with MS.

Triplex DNA-based Bioanalytical Platform for Highly Sensitive Homogeneous Electrochemical Detection of Melamine

Melamine detection has attracted much attention since the discovery of the damage of melamine to human health. Herein, we have developed a sensitive homogeneous electroanalytical platform for melamine detection, which is relied on the formation of triplex molecular beacon integrated with exonuclease III (Exo III)-mediated signal amplification. The formation of triplex molecular beacon was triggered by the recognition and incorporation of melamine to the abasic (AP) site contained in the triplex stem. The stem of the triplex molecular beacon was designed to have a protruding double-strand DNA, which can be recognized and hydrolyzed by Exo III for releasing methylene blue (MB)-labeled mononucleotide. These released MB molecules exhibit high diffusivity toward indium tin oxide electrode with negative charge, thus producing a significantly increased electrochemical response. Taking advantages of the high binding affinity of the DNA triplex structure containing AP sites towards melamine and the unique features of Exo III, this sensing platform is capable for sensitive and selective melamine assay with a detection limit as low as 8.7 nM. Furthermore, this strategy shows good applicability for melamine assay in real samples. Therefore, this strategy broadens the application of triplex DNA and presents a new method for sensitive detection of melamine.

Colorimetric detection of melamine in milk by using gold nanoparticles-based LSPR via optical fibers

A biosensing system with optical fibers is proposed for the colorimetric detection of melamine in liquid milk samples by using the localized surface plasmon resonance (LSPR) of unmodified gold nanoparticles (AuNPs). The biosensing system consists of a broadband light source that covers the spectral range from 200 nm to 1700 nm, an optical attenuator, three types of 600 μm premium optical fibers with SMA905 connectors and a miniature spectrometer with a linear charge coupled device (CCD) array. The biosensing system with optical fibers is low-cost, simple and is well-proven for the detection of melamine. Its working principle is based on the color changes of AuNPs solution from wine-red to blue due to the inter-particle coupling effect that causes the shifts of wavelength and absorbance in LSPR band after the to-be-measured melamine samples were added. Under the optimized conditions, the detection response of the LSPR biosensing system was found to be linear in melamine detection in the concentration range from 0μM to 0.9 μM with a correlation coefficient (R²) 0.99 and a detection limit 33 nM. The experimental results obtained from the established LSPR biosensing system in the actual detection of melamine concentration in liquid milk samples show that this technique is highly specific and sensitive and would have a huge application prospects.

Applications of DART-MS for food quality and safety assurance in food supply chain

Direct analysis in real time (DART) represents a new generation of ion source which is used for rapid ionization of small molecules under ambient conditions. The combination of DART and various mass spectrometers allows analyzing multiple food samples with simple or no sample treatment, or in conjunction with prevailing protocolized sample preparation methods. Abundant applications by DART-MS have been reviewed in this paper. The DART-MS strategy applied to food supply chain (FSC), including production, processing, and storage and transportation, provides a comprehensive solution to various food components, contaminants, authenticity, and traceability. Additionally, typical applications available in food analysis by other ambient ionization mass spectrometers were summarized, and fundamentals mainly including mechanisms, devices, and parameters were discussed as well. © 2015 Wiley Periodicals, Inc. Mass Spec Rev. 36:161-187, 2017.

Desorption in Mass Spectrometry

In mass spectrometry, analytes must be released in the gas phase. There are two representative methods for the gasification of the condensed samples, i.e., ablation and desorption. While ablation is based on the explosion induced by the energy accumulated in the condensed matrix, desorption is a single molecular process taking place on the surface. In this paper, desorption methods for mass spectrometry developed in our laboratory: flash heating/rapid cooling, Leidenfrost phenomenon-assisted thermal desorption (LPTD), solid/solid friction, liquid/solid friction, electrospray droplet impact (EDI) ionization/desorption, and probe electrospray ionization (PESI), will be described. All the methods are concerned with the surface and interface phenomena. The concept of how to desorb less-volatility compounds from the surface will be discussed.

A new dual-emission fluorescence sensor based on carbon nanodots and gold nanoclusters for the detection of melamine

A new dual-emission fluorescence sensor is developed to detect melamine, in which the gold nanoclusters serve as the signal section and the carbon nanodots serve as the internal reference.