A strategy for constructing sensitive and renewable molecularly imprinted electrochemical sensors for melamine detection

Advanced Urea Precursors Driven NiCo2O4 Nanostructures Based Non-Enzymatic Urea Sensor for Milk and Urine Real Sample Applications

The electrochemical performance of NiCo₂O₄ with urea precursors was evaluated in order to develop a non-enzymatic urea sensor. In this study, NiCo₂O₄ nanostructures were synthesized hydrothermally at different concentrations of urea and characterized using scanning electron microscopy and X-ray diffraction. Nanostructures of NiCo₂O₄ exhibit a nanorod-like morphology and a cubic phase crystal structure. Urea can be detected with high sensitivity through NiCo₂O₄ nanostructures driven by urea precursors under alkaline conditions. A low limit of detection of 0.05 and an analytical range of 0.1 mM to 10 mM urea are provided. The concentration of 006 mM was determined by cyclic voltammetry. Chronoamperometry was used to determine the linear range in the range of 0.1 mM to 8 mM. Several analytical parameters were assessed, including selectivity, stability, and repeatability. NiCo₂O₄ nanostructures can also be used to detect urea in various biological samples in a practical manner.

Development of an ultrasensitive molecularly imprinted poly-(ortho-phenylenediamine) based sensor for the determination of melamine adulteration in milk and infant formula

A sensitive molecularly imprinted poly-(ortho-phenylenediamine) electrochemical sensor was fabricated for selective melamine detection in milk and infant formula. The pencil graphite electrode (PGE) was modified by deposition of Au nanoparticles and reduced graphene oxide (RGO) on its surface. The fabrication of the electrode in various stages was monitored using cyclic voltammetry. The immobilized RGO, MIP, and gold nanoparticles on the PGE surface were morphologically characterized by field-emission scanning electron microscopy (FESEM). Under the optimized conditions, the linear range and the limit of detection (LOD) were 10-17-10-8 M and 2.64 × 10-16 M (S/N = 3), respectively. The prepared sensor exhibited a good reproducibility and repeatability response. The recovery range of melamine-spiked milk and infant formula was 92.7%-103.9% and 93.5%-105.8%, respectively. The sensor could apply successfully for melamine determination in milk and infant formula samples.

A renewable electrochemical sensor based on a self-assembled framework of chiral molecules for efficient identification of tryptophan isomers

Molecular self-assembly provides a reasonably effective strategy for the design and construction of chiral sensors. Here, Cu²⁺ was connected to β-cyclodextrin (β-CD) through coordination to synthesize Cu₂-β-CD, subsequently assembled with ammoniated chitosan-MWCNTs (NH₂-CS-MWCNTs) by the effect of coordination driver to form a chiral sensing interface Cu₂-β-CD/NH₂-CS-MWCNTs. Using the electrochemical method, the valid recognition of tryptophan (Trp) isomers was achieved on the self-assembly interface. Under the optimal experimental conditions, the developed sensor exhibited good linearity and satisfactorily renewable ability. Cu₂-β-CD/NH₂-CS-MWCNTs/GCE showed the capacity to predict the ratio of D-Trp and L-Trp in racemic mixtures and the possibility of qualitative and quantitative determination for Trp isomers. Finally, the electrochemical sensor was used to detect the Trp enantiomers in rat serum, further verifying the feasibility of the sensor in the determination of actual samples. Therefore, the electrochemical chiral sensor not only is used for the recognition of Trp enantiomers but shows great potential in practical applications.

Sensor Based on a Poly[2-(Dimethylamino)ethyl Methacrylate-Co-Styrene], Gold Nanoparticles, and Methylene Blue-Modified Glassy Carbon Electrode for Melamine Detection

Melamine has been used as a non-protein nitrogenous additive in food products to artificially increase the apparent “false” protein content. Melamine is known as a dangerous and poisonous substance for human health and it causes diverse diseases. An electrochemical sensor for melamine detection has been developed by modification of a glassy carbon electrode using copolymer poly[DMAEMA-co-styrene], gold nanoparticles, and methylene blue. The characterization of the modified electrode was conducted using several analysis techniques including cyclic voltammetry (CV), differential pulse voltammetry (DPV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The electrochemical detection of melamine was performed by impedance spectroscopy. Obtained results revealed that the developed sensor has a large detection range from 5.0 × 10⁻¹³ to 3.8 × 10⁻⁸ M with a low detection limit of 1.8 × 10⁻¹² M (at S/N = 3). Various interfering species such as phenol, hydroquinone, and bisphenol A have been used and their behavior on modified electrode has been studied.

Food safety and quality assessment: comprehensive review and recent trends in the applications of ion mobility spectrometry (IMS)

Ion mobility spectrometry (IMS) is an analytical separation and diagnostic technique that is simple and sensitive and a rapid response and low-priced technique for detecting trace levels of chemical compounds in different matrices. Chemical agents and environmental contaminants are successfully detected by IMS and have been recently considered to employ in food safety. In addition, IMS uses stand-alone or coupled analytical diagnostic tools with chromatographic and spectroscopic methods. Scientific publications show that IMS has been applied 21% in the pharmaceutical industry, 9% in environmental studies and 13% in quality control and food safety. Nevertheless, applications of IMS in food safety and quality analysis have not been adequately explored. This review presents the IMS-related analysis and focuses on the application of IMS in food safety and quality. This review presents the important topics including detection of traces of chemicals, rate of food spoilage and freshness, food adulteration and authenticity as well as natural toxins, pesticides, herbicides, fungicides, veterinary, and growth promoter drug residues. Further, persistent organic pollutants (POPs), acrylamide, polycyclic aromatic hydrocarbon (PAH), biogenic amines, nitrosamine, furfural, phenolic compounds, heavy metals, food packaging materials, melamine, and food additives were also examined for the first time. Therefore, it is logical to predict that the application of the IMS technique in food safety, food quality, and contaminant analysis will be impressively increased in the future. HighlightsCurrent status of IMS for residues and contaminant detection in food safety.To assess all the detected contaminants in food safety, for the first time.Identified IMS-related parameters and chemical compounds in food safety control.

Rapid Detection of Adulterants in Whey Protein Supplement by Raman Spectroscopy Combined with Multivariate Analysis

The growing demand for whey protein supplements has made them the target of adulteration with cheap substances. Therefore, Raman spectroscopy in tandem with chemometrics was proposed to simultaneously detect and quantify three common adulterants (creatine, l-glutamine and taurine) in whey protein concentrate (WPC) powder. Soft independent modeling class analogy (SIMCA) and partial least squares discriminant analysis (PLS-DA) models were built based on two spectral regions (400-1800 cm-1 and 500-1100 cm-1) to classify different types of adulterated samples. The most effective was the SIMCA model in 500-1100 cm-1 with an accuracy of 96.9% and an error rate of 5%. Partial least squares regression (PLSR) models for each adulterant were developed using two different Raman spectral ranges (400-1800 cm-1 and selected specific region) and data pretreatment methods. The determination coefficients (R2) of all models were higher than 0.96. PLSR models based on typical Raman regions (500-1100 cm-1 for creatine and taurine, the combination of range 800-1000 cm-1 and 1300-1500 cm-1 for glutamine) were superior to models in the full spectrum. The lowest root mean squared error of prediction (RMSEP) was 0.21%, 0.33%, 0.42% for creatine, taurine and glutamine, and the corresponding limit of detection (LOD) values for them were 0.53%, 0.71% and 1.13%, respectively. This proves that Raman spectroscopy with the help of multivariate approaches is a powerful method to detect adulterants in WPC.

Necklace-like Molecularly Imprinted Nanohybrids Based on Polymeric Nanoparticles Decorated Multiwalled Carbon Nanotubes for Highly Sensitive and Selective Melamine Detection

In this study, molecularly imprinted nanohybrids with "necklace-like" nanostructures were developed based on self-assembled polymeric nanoparticles decorated multiwalled carbon nanotubes (MWCNTs) by employing melamine as template molecules. An amphiphilic copolymer poly(acrylic acid- co-(7-(4-vinylbenzyloxy)-4-methyl coumarin)- co-ethylhexyl acrylate) (poly(AA- co-VMc- co-EHA), PAVE) containing photosensitive coumarin units was synthesized first. Then, the PAVE copolymers were co-assembled with MWCNTs in the presence of template molecules, generating photosensitive molecularly imprinted nanohybrids (MIP-MWCNTs) with necklace-like structures. Subsequently, the MIP-MWCNTs nanohybrids were used to modify electrode surface followed by photo-polymerization of the coumarin units in the nanohybrids, leading to a network architectured complex film. After extracting melamine molecules by electrolysis, a melamine MIP sensor was successfully developed. The as-prepared sensor exhibited a significantly wide linear range (1.0 × 10^-12-1.0 × 10^-6 mol L^-1) and a low detection limit (5.6 × 10^-13 mol L^-1) for melamine detection. High selectivity of the sensor toward melamine was well demonstrated with respect to other melamine analogues and interferents. Furthermore, the MIP sensor showed high stability and reproducibility. The excellent performance of the MIP sensor can be attributed to the unique nanostructure of the complex film provided by these necklace-like nanohybrids. On the one hand, the nanosized polymeric MIP nanoparticles along the MWCNTs increase the effective electrode surface area and thus offer a high melamine-binding capacity. On the other hand, the MWCNTs in MIP-MWCNTs nanohybrids serve as "electronic bridges" to accelerate the electron transfer among the complex MIP film. More importantly, the MIP sensor was practically used to monitor melamine in milk samples, demonstrating a promising feature for applications in the analysis of food like milk and other food products including milk powder, infant formula, and animal feed. Considering the ease of polymeric nanoparticles functionalization, the necklace-like nanohybrids would be extended to wider applications in many other sensors and devices.

Enzymatic Fuel Cell-Based Self-Powered Homogeneous Immunosensing Platform via Target-Induced Glucose Release: An Appealing Alternative Strategy for Turn-On Melamine Assay

Enzymatic fuel cell (EFC)-based self-powered biosensors have attracted considerable attention because of their unique feature of no need for extra power sources during the entire detection process, which endows them with the merits of simplicity, rapidness, low cost, anti-interference, and ease of use. Herein, we proposed, for the first time, an EFC-based self-powered homogeneous immunosensing platform by integrating the target-induced biofuel release and bioconjugate immunoassay for ultrasensitive melamine (ME) detection. In this design, the biofuel, i.e., glucose molecules, was entrapped in the pores of positively charged mesoporous silica nanoparticles and capped by the biogate AuNPs-labeled anti-ME antibody (AuNPs-Ab). The presence of the target ME triggered the entrapped glucose release due to the removal of the biogate via immunoreaction, which resulted in the transfer of electrons produced by glucose oxidation at the bioanode to the biocathode, and thus, the open-circuit voltage of the EFC-based self-powered immunosensor dramatically increased, realizing the ultrasensitive turn-on assay for ME. The limit of detection for ME assay was down to 2.1 pM (S/N = 3), superior to those previously reported in the literature. Notably, real milk samples need no special sample pretreatment for the detection of ME because of the good anti-interference ability of EFC-based self-powered biosensors and the excellent selectivity of the homogeneous immunoassay. Therefore, this appealing self-powered homogeneous immunosensing platform holds great promise as a successful prototype of portable and on-site bioassay in the field of food safety.

Molecularly Imprinted Polymers Based Electrochemical Sensor for 2,4-Dichlorophenol Determination

A molecularly imprinted polymers based electrochemical sensor was fabricated by electropolymerizing pyrrole on a Fe₃O₄ nanoparticle modified glassy carbon electrode. The sensor showed highly catalytic ability for the oxidation of 2,4-dichlorophenol (2,4-DCP). Square wave voltammetry was used for the determination of 2,4-DCP. The oxidation peak currents were proportional to the concentrations of 2,4-DCP in the range of 0.04 to 2.0 µM, with a detection limit of 0.01 µM. The proposed sensor was successfully applied for the determination of 2,4-DCP in water samples giving satisfactory recoveries.

Molecularly imprinted nanohybrids based on dopamine-modified poly(γ-glutamic acid) for electrochemical sensing of melamine

A voltammetric sensor for melamine (MEL) was prepared from molecularly imprinted nanohybrids (MINBs). A dopamine modified poly-γ-glutamic acid copolymer (γ-PGA-DA) and MEL were self-assembled into MEL/γ-PGA-DA nanoparticles (NPs) in aqueous solution via weak interactions, followed by adding an aqueous AgNO3 solution into the mixture. The Ag(+) was adsorbed in the MEL/γ-PGA-DA NPs and spontaneously reduced to Ag NPs by the dopamine moieties of γ-PGA-DA, forming Ag/MEL/γ-PGA-DA MINBs, which were then cast on a gold electrode to form a MINBs film. The MEL was removed by electrolysis via catalysis of Ag NPs at a constant potential of 1.4V in phosphate buffer saline solution, to obtain a voltammetric sensor for MEL. The sensor responded linearly to MEL in the concentration range of 5×10(-18) to 5×10(-7)molL(-1). Compared to other published molecularly imprinted polymer sensors for sensing MEL, the prepared MINBs sensor had much wider detection range with lower detection limit.

Chemical sensors and biosensors for the detection of melamine

Melamine is an emerging contaminant in milk, infant formula and pet food.

Molecular imprinting science and technology: a survey of the literature for the years 2004-2011

Herein, we present a survey of the literature covering the development of molecular imprinting science and technology over the years 2004-2011. In total, 3779 references to the original papers, reviews, edited volumes and monographs from this period are included, along with recently identified uncited materials from prior to 2004, which were omitted in the first instalment of this series covering the years 1930-2003. In the presentation of the assembled references, a section presenting reviews and monographs covering the area is followed by sections describing fundamental aspects of molecular imprinting including the development of novel polymer formats. Thereafter, literature describing efforts to apply these polymeric materials to a range of application areas is presented. Current trends and areas of rapid development are discussed.

3D porous graphene-porous PdCu alloy nanoparticles-molecularly imprinted poly(para-aminobenzoic acid) composite for the electrocatalytic assay of melamine

In this work, a three-dimensional hybrid film with in- and out-of-plane pores was fabricated by using porous graphene as framework structure and porous PdCu alloy nanoparticles as building blocks. The porous PdCu alloy nanoparticles were prepared by chemical dealloying with acetic acid. The hierarchical pores had abundant active catalytic sites, and the material exhibited remarkable catalytic activity toward the oxidation of hydrazine. Based on this hybrid film, an electrochemical sensor of melamine was developed by further introducing melamine imprinted electro-polymer of para-aminobenzoic acid. Melamine was detected by differential pulse voltammetry using hydrazine as electrochemical probe. The detection signal was amplified due to the catalytic oxidation of hydrazine at this hybrid film. The linear determination range was 0.01-1 μM and the detection limit was 2 nM (S/N = 3). The sensor displayed high recognition capacity toward melamine and also showed good reproducibility and stability. It is promising in the determination of melamine in real samples.

Doubly imprinted polymer nanofilm-modified electrochemical sensor for ultra-trace simultaneous analysis of glyphosate and glufosinate

A rapid, selective, and sensitive double-template imprinted polymer nanofilm-modified pencil graphite electrode was fabricated for the simultaneous analysis of phosphorus-containing amino acid-type herbicides (glyphosate and glufosinate) in soil and human serum samples. Since both herbicides respond overlapped oxidation peaks and only glyphosate is prone to nitrosation, n-nitroso glyphosate and glufosinate were used as templates for obtaining the well-resolved quantitative differential pulse anodic stripping voltammetric peaks on the proposed sensor. Toward sensor fabrication, a nano-structured polymer film was first grown directly on the electrode via initial immobilization of gold nanoparticles at its surface. This was followed by linking of monomeric (N-methacryloyl-l-cysteine) molecules through S-Au bonds. Subsequently, these molecules were subjected to free radical polymerization, in the presence of templates, cross linker, initiator, and multiwalled carbon nanotubes as pre-polymer mixture. The modified sensor observed wide linear ranges (3.98-176.23 ng mL(-1) and 0.54-3.96 ng mL(-1)) of simultaneous analysis with detection limits as low as 0.35 and 0.19 ng mL(-1) (S/N=3) for glyphosate and glufosinate, respectively, in aqueous samples. The respective oxidation peak potentials of both analytes were found to be substantially apart by 265 mV. This enabled the simultaneous determination of one target in the presence of other, without any cross reactivity, interferences, and false-positives, in real samples.

Toxicity of melamine: the public health concern

Melamine contamination in food has resulted in sickness and deaths of human infants, pets, and farm animals in the past decade. The majority of the victims suffered from acute kidney injury, nephrolithiasis, and urolithiasis. Since then, animal studies have revealed the possible target organs of the melamine toxicity and the extent of the adverse effects of the contaminant. State-of-the-art analytical methods have been developed to achieve the "zero tolerance" aim for such economically motivated adulteration. These studies provide in-depth understanding of the melamine toxicity and promising analytical methods, which can help us safeguard our dairy food source.

Recent developments in the detection of melamine

In recent years, there were two reported outbreaks of food borne illness associated with melamine. The presence of melamine and its related compounds in milk, feed, and other foods has resulted in the need for reliable methods for the detection and accurate quantification of this class of contaminants. The sample pretreatment for melamine in a complex matrix usually involves a liquid extraction by a polar solvent, followed by a further clean-up with solid phase extraction. Analyses of melamine and related compounds are commonly carried out by liquid or gas chromatographic methods conjugated with mass spectrometry. Other innovative screening methods, which use antibodies, molecularly imprinted polymers, capillary electrophoresis, and gold nanoparticles, are also used to develop assays and biosensors to melamine. However, many of these methods have been hindered by matrix effects, the solubility of melamine-cyanuric acid complex, and background contamination. This article reviews recent developments for detecting melamine and discusses future directions.