Colorimetric detection of melamine in milk based on Triton X-100 modified gold nanoparticles and its paper-based application

A nanogold sensor test for tire wear chemicals based on the plasmon ruler approach

The release of tire wear substances in the environment is raising concerns about potential impacts on aquatic ecosystems. The purpose of this study was to develop a quick and inexpensive screening test for the following tire wear substances: 6-phenylphenyldiamine quinone (6-PPD quinone), hexamethoxymethylmelamine (HMMM), 1-3-diphenylguanidine (1,3-DPG), and melamine. A dual strategy consisting of nanogold (nAu) signal intensity and the plasmonic ruler principle was used based on the spectral shift from the unaggregated free-form nAu from 525 nm to aggregated nAu at higher wavelengths. The shift in resonance corresponded to the relative sizes of the tire wear substances at the surface of nAu: 6-PPD (560 nm), HMMM (590 nm), 1,3-DPG (620 nm), and melamine (660 nm) in a concentration-dependent manner. When present in mixtures, a large indiscriminate band between 550 and 660 nm with a maximum corresponding to the mean intermolecular distance of 0.43 nm from the tested individual substances suggests that all compounds indiscriminately interacted at the surface of nAu. An internal calibration methodology was developed for mixtures and biological extracts from mussels and biofilms and revealed a proportional increase in absorbance at the corresponding resonance line for each test compound. Application of this simple and quick methodology revealed the increased presence of melamine and HMMM compounds in mussels and biofilms collected at urban sites (downstream city, road runoffs), respectively. The data also showed that treated municipal effluent decreased somewhat melamine levels in mussels.

Paper-based sensors: affordable, versatile, and emerging analyte detection platforms

Paper-based sensors, often referred to as paper-based analytical devices (PADs), stand as a transformative technology in the field of analytical chemistry. They offer an affordable, versatile, and accessible solution for diverse analyte detection. These sensors harness the unique properties of paper substrates to provide a cost-effective and adaptable platform for rapid analyte detection, spanning chemical species, biomolecules, and pathogens. This review highlights the key attributes that make paper-based sensors an attractive choice for analyte detection. PADs demonstrate their versatility by accommodating a wide range of analytes, from ions and gases to proteins, nucleic acids, and more, with customizable designs for specific applications. Their user-friendly operation and minimal infrastructure requirements suit point-of-care diagnostics, environmental monitoring, food safety, and more. This review also explores various fabrication methods such as inkjet printing, wax printing, screen printing, dip coating, and photolithography. Incorporating nanomaterials and biorecognition elements promises even more sophisticated and sensitive applications.

Microfluidic paper analytic device (μPAD) technology for food safety applications

Foodborne pathogens, food adulterants, allergens, and toxic chemicals in food can cause major health hazards to humans and animals. Stringent quality control measures at all stages of food processing are required to ensure food safety. There is, therefore, a global need for affordable, reliable, and rapid tests that can be conducted at different process steps and processing sites, spanning the range from the sourcing of food to the end-product acquired by the consumer. Current laboratory-based food quality control tests are well established, but many are not suitable for rapid on-site investigations and are costly. Microfluidic paper analytical devices (μPADs) are a fast-growing field in medical diagnostics that can fill these gaps. In this review, we describe the latest developments in the applications of microfluidic paper analytic device (μPAD) technology in the food safety sector. State-of-the-art μPAD designs and fabrication methods, microfluidic assay principles, and various types of μPAD devices with food-specific applications are discussed. We have identified the prominent research and development trends and future directions for maximizing the value of microfluidic technology in the food sector and have highlighted key areas for improvement. We conclude that the μPAD technology is promising in food safety applications by using novel materials and improved methods to enhance the sensitivity and specificity of the assays, with low cost.

Transfer Printing of Ordered Plasmonic Nanoparticles at Hard and Soft Interfaces with Increased Fidelity and Biocompatibility Supports a Surface Lattice Resonance

Transfer printing, the relocation of structures assembled on one surface to a different substrate by adjusting adhesive forces at the surface-substrate interface, is widely used to print electronic circuits on biological substrates like human skin and plant leaves. The fidelity of original structures must be preserved to maintain the functionality of transfer-printed circuits. This work developed new biocompatible methods to transfer a nanoscale square lattice of plasmon resonant nanoparticles from a lithographed surface onto leaf and glass substrates. The fidelity of the ordered nanoparticles was preserved across a large area in order to yield, for the first time, an optical surface lattice resonance on glass substrates. To effect the transfer, interfacial adhesion was adjusted by using laser induction of plasmons or unmounted adhesive. Optical and confocal laser scanning microscopy showed that submicron spacing of the square lattice was preserved in ≥90% of transfer-printed areas up to 4 mm2. Up to 90% of ordered nanoparticles were transferred, yielding a surface lattice resonance measured by transmission UV-vis spectroscopy.

Progress in the design of portable colorimetric chemical sensing devices

The need for precise determination of heavy metals, anions, biomolecules, pesticides, drugs, and other substances is vital across clinical, environmental, and food safety domains. Recent years have seen significant progress in portable colorimetric chemical sensing devices, revolutionizing on-the-spot analysis. This review offers a comprehensive overview of these advancements, covering handheld colorimetry, RGB-based colorimetry, paper-based colorimetry, and wearable colorimetry devices. It explores the underlying principles, functional materials (chromophoric reagents/dyes and nanoparticles), detection mechanisms, and their applications in environmental monitoring, clinical care, and food safety. Noble metal nanoparticles (NPs) have arisen as promising substitutes in the realm of sensing materials. They display notable advantages, including heightened sensitivity, the ability to fine-tune their plasmonic characteristics for improved selectivity, and the capacity to induce visible color changes, and simplifying detection. Integration of NPs fabricated paper device with smartphones and wearables facilitates reagent-free, cost-effective, and portable colorimetric sensing, enabling real-time analysis and remote monitoring.

Recent advancements in nanomaterial based optical detection of food additives: a review

Food additives have become a critical component in the food industry. They are employed as preservatives to decelerate the negative effects of environmental and microbial factors on food quality. Currently, food additives are used for a variety of purposes, including colorants, flavor enhancers, nutritional supplements, etc., owing to improvements in the food industry. Since the usage of food additives has increased dramatically, the efficient monitoring of their acceptable levels in food products is quite necessary to mitigate the problems associated with their inappropriate use. The traditional methods used for detecting food additives are generally based on standard spectroscopic and chromatographic techniques. However, these analytical techniques are limited by their high instrumentation cost and time-consuming procedures. The emerging field of nanotechnology has enabled the development of highly sensitive and specific sensors to analyze food additives in a rapid manner. The current article emphasizes the need to detect various food additives owing to their potential negative effects on humans, animals, and the environment. In this article, the role of nanomaterials in the optical sensing of food additives has been discussed owing to their high accuracy, ease-of-use, and excellent sensitivity. The applications of nanosensors for the detection of various food additives have been elaborated with examples. The current article will assist policymakers in developing new rules and regulations to mitigate the adverse effects of toxic food additives on humans and the environment. In addition, the prospects of nanosensors for the optical detection of food additives at a commercial scale have been discussed to combat their irrational use in the food industry.

An off-on fluorescent nanoprobe for L-cysteine sensing based on the FRET effect

A self-assembled fluorescent nanosensor for the determination of L-cysteine (Cys) was constructed based on the mechanism of fluorescence resonance energy transfer (FRET). In this system, CdTe/ZnS QDs serve as the energy donor while AuNPs serve as the receptor, resulting in the occurrence of FRET with dramatic fluorescence quenching of the QDs (turn off). Once Cys is added, AuNPs can adsorb Cys, leading to the release of the QDs. The process would inhibit the FRET, which contributed to the recovery of fluorescence (turn on) and an off-on fluorescence aptasensor for Cys detection was constructed accordingly. The linear response range of the fluorescence sensor is from 0.8 to 50 μM, and the detection limit is 0.24 μM. The sensor demonstrates great sensitivity and selectivity to Cys. More importantly, the QD-based sensing platform was successfully used for the detection of Cys in milk samples with high precision and accuracy, indicating the potential of the probe in practical applications.

eXtreme gradient boosting-based classification of bacterial mixtures in water and milk using wireless microscopic imaging of quorum sensing peptide-conjugated particles

Numerous bacteria can cause water- and foodborne diseases and are often found in bacterial mixtures, making their detection challenging. Specific bioreceptors or selective growth media are necessary for most bacterial detection methods. In this work, we collectively used five quorum sensing-based peptides identified from bacterial biofilms to identify 10 different bacterial species (Bacillus subtilis, Campylobacter jejuni, Enterococcus faecium, Escherichia coli, Legionella pneumophila, Listeria monocytogenes, Pseudomonas aeruginosa, Salmonella Typhimurium, Staphylococcus aureus, Vibrio parahaemolyticus) and their mixtures in water and milk. Four different machine learning classification methods were used: k-nearest neighbors (k-NN), decision tree (DT), support vector machine (SVM), and eXtreme Gradient Boosting (XGBoost). Peptides were crosslinked to submicron particles, and peptide-bacteria interactions on paper microfluidic chips caused the particle aggregation. A wireless, pocket fluorescence microscope (interfaced with a smartphone) counted such particle aggregations. XGBoost showed the best accuracy of 83.75% in identifying bacterial species from water samples using 320 different datasets and 91.67% from milk samples using 140 different datasets (5 peptide features per dataset). Each peptide's contribution to correct classification was evaluated. The results were concentration-dependent, allowing the identification of a dominant species from bacterial mixtures. Using XGBoost and the previous milk database, we tested 14 blind samples of various bacterial mixtures in milk samples, with an accuracy of 81.55% to predict the dominant species. The entire process could be completed within a half hour. The demonstrated system can provide a handheld, low-cost, easy-to-operate tool for potential hygiene spot-checks, public health, or personal healthcare.

Bombax ceiba flower extract mediated synthesis of Se nanoparticles for antibacterial activity and urea detection

Plant mediated synthesis of metallic nanomaterials has emerged as a non-toxic and economical approach to their applications in diverse fields especially in biomedical sciences. Herein, this study first time reporting the use of Bombax ceiba flower extract for synthesis of selenium nanoparticles (SeNPs). Initially, SeNPs were confirmed by turning the color of reaction mixtures from light yellow to brick-red. Scanning electron microscope (SEM) and Transmission electron microscopy (TEM) images showed spherical shaped nanoparticles with smooth surface, size ranges between 30 and 150 nm. Dynamic light scattering (DLS) showed 100-150 nm for the distribution of particle size. X-ray diffraction (XRD) analysis revealed SeNPs crystallinity and confirmed by matching with selenium JCPD card No. 06-362. Energy-dispersive X-ray (EDX) spectra showed presence of pure Se peaks that corroborate the conversion of selenium ions into its elemental form by bio-reduction. Fourier-transform infrared spectroscopy (FTIR) spectra demonstrated that involvement of -OH, C-H, C=C, and C=O functional groups for SeNPs formation. Raman Spectra peaks at 250 cm^-1 represent asymmetric trigonal selenium (t-Se). Ultraviolet-visible spectrophotometer (UV-Vis) peaks at 296 and 306 nm which is an indication of surface plasmon resonance (SPR). Moreover, maximum antibacterial activity of SeNPs were observed against Staphylococcus aureus- a gram positive bacteria that possess zone of inhibition (ZOI) 20 mm and Klebsiella pneumonia and Pseudomonas aeruginosa-gram negative bacteria with ZOI 28 mm, respectively, at concentration 100 µg/ml. In addition, the surface functionalities induced through extract components adhere over Se binds with urea and give its detection up to 1mM in milk sample. Conclusively, synthesized SeNPs may function as a potential antibacterial pharmaceutical candidate.

Latex-Based Paper Devices with Super Solvent Resistance for On-the-Spot Detection of Metanil Yellow in Food Samples

The following paper presents a construct for a paper-based device which utilizes latex as the hydrophobic material for the fabrication of its hydrophobic barrier, which was deposited onto the cellulose surface either by free-hand or stenciled drawing. This method demands the least amount of expertise and time from its use, enabling a simple and rapid fabrication experience. Several properties of the hydrophobic material were characterized, such as the hydro head and penetration rate, with the aim of assessing its robustness and stability. The presented hydrophobic barriers fabricated using this approach have a barrier width of 4 mm, a coating thickness of 208 µm, and a hydrophilic resolution of 446.5 µm. This fabrication modality boasts an excellent solvent resistance with regard to the hydrophobic barrier. These devices were employed for on-the-spot detection of Metanil Yellow, a banned food adulterant often used in curcumin and pigeon peas, within successful limits of detection (LOD) of 0.5% (w/w) and 0.25% (w/w), respectively. These results indicate the great potential this fabricated hydrophobic device has in numerous paper-based applications and other closely related domains, such as diagnostics and sensing, signalling its capacity to become commonplace in both industrial and domestic settings.

Paper-based microfluidic devices for food adulterants: Cost-effective technological monitoring systems

Analytical sciences have witnessed emergent techniques for efficient clinical and industrial food adulterants detection. In this review, the contributions made by the paper-based devices are highlighted for efficient and rapid detection of food adulterants and additives, which is the need of the hour and how different categories of techniques have been developed in the past decade for upgrading the performance for point-of-care testing. A simple strategy with an arrangement for detecting specific adulterants followed by the addition of samples to obtain well-defined qualitative or quantitative signals for confirming the presence of target species. The paper-based microfluidics-based technology advances and prospects for food adulterant detection are discussed given the high-demand from the food sectors and serve as a valued technology for food researchers working in interdisciplinary technological frontiers.

Gold nanoparticle-based optical nanosensors for food and health safety monitoring: recent advances and future perspectives

Modern society has been facing serious health-related problems including food safety, diseases and illness. Hence, it is urgent to develop analysis methods for the detection and control of food contaminants, disease biomarkers and pathogens. As the traditional instrumental methods have several disadvantages, including being time consuming, and having high cost and laborious procedures, optical nanosensors have emerged as promising alternative or complementary approaches to those traditional ones. With the advantages of simple preparation, high surface-to-volume ratio, excellent biocompatibility, and especially, unique optical properties, gold nanoparticles (AuNPs) have been demonstrated as excellent transducers for optical sensing systems. Herein, we provide an overview of the synthesis of AuNPs and their excellent optical properties that are ideal for the development of optical nanosensors based on local surface plasmon resonance (LSPR), colorimetry, fluorescence resonance energy transfer (FRET), and surface-enhanced Raman scattering (SERS) phenomena. We also review the sensing strategies and their mechanisms, as well as summarizing the recent advances in the monitoring of food contaminants, disease biomarkers and pathogens using developed AuNP-based optical nanosensors in the past seven years (2015-now). Furthermore, trends and challenges in the application of these nanosensors in the determination of those analytes are discussed to suggest possible directions for future developments.

Assessment of Melamine in Different Water Samples with ZnO-doped Co3 O4 Nanoparticles on a Glassy Carbon Electrode by Differential Pulse Voltammetry

In this investigation, a melamine electrochemical sensor has been developed by using wet-chemically synthesized low-dimensional aggregated nanoparticles (NPs) of ZnO-doped Co₃ O₄ as sensing substrate that were decorated onto flat glassy carbon electrode (GCE). The characterization of NPs such as UV-Vis, FTIR, XRD, XPS, EDS, and FESEM was done for detailed investigations in optical, functional, structural, elemental, and morphological analyses. The ZnO-doped Co₃ O₄ NPs decorated GCE was used as a sensing probe to analyze the target chemical melamine in a phosphate buffer at pH 5.7 by applying differential pulse voltammetry (DPV). It exhibited good performances in terms of sensor analytical parameters such as large linear dynamic range (LDR; 0.15-1.35 mM) of melamine detection, high sensitivity (80.6 μA mM^-1  cm^-2 ), low limit of detection (LOD; 0.118±0.005 mM), low limit of quantification (LOQ; 0.393 mM), and fast response time (30 s). Besides this, the good reproducibility (in several hours) and repeatability were investigated under identical conditions. Moreover, it was implemented to measure the long-time stability, electron mobility, less charge-transfer resistance, and analyzed diffusion-controlled process for the oxidation reaction of the NPs assembled working GCE electrode, which showed outstanding chemical sensor performances. For validation, real environmental samples were collected from various water sources and investigated successfully with regard to the reliability of the selective melamine detection with prepared NPs coated sensor probe. Therefore, this approach might be introduced as an alternative route in the sensor technology to detect selectively unsafe chemicals by an electrochemical method with nanostructure-doped materials for the safety of environmental, ecological, healthcare fields in a broad scale.

Recent Advances in Microfluidic Devices for Contamination Detection and Quality Inspection of Milk

Milk is a necessity for human life. However, it is susceptible to contamination and adulteration. Microfluidic analysis devices have attracted significant attention for the high-throughput quality inspection and contaminant analysis of milk samples in recent years. This review describes the major proposals presented in the literature for the pretreatment, contaminant detection, and quality inspection of milk samples using microfluidic lab-on-a-chip and lab-on-paper platforms in the past five years. The review focuses on the sample separation, sample extraction, and sample preconcentration/amplification steps of the pretreatment process and the determination of aflatoxins, antibiotics, drugs, melamine, and foodborne pathogens in the detection process. Recent proposals for the general quality inspection of milk samples, including the viscosity and presence of adulteration, are also discussed. The review concludes with a brief perspective on the challenges facing the future development of microfluidic devices for the analysis of milk samples in the coming years.

Paper-based DNA sensor enabling colorimetric assay integrated with smartphone for human papillomavirus detection

A colorimetric paper-based DNA sensor that relies on the inhibition of PNA-induced AuNPs aggregation was combined with a simple smartphone readout for the point-of-care detection of HPV type 16 DNA.

Colorimetric detection of insulin in human serum using GO/AuNPs/TX-100 nanocomposite

In this study, graphene oxide/gold nanoparticles/Triton X-100 nanocomposites (GO/AuNPs/TX-100) were synthesized using the sonochemical method and their ability in ultrasound-assisted colorimetric detection of insulin was investigated. The synthesized GO/AuNPs/TX-100 nanocomposites were characterized by UV-visible absorption spectroscopy and TEM analysis. The interaction between the nanocomposites and insulin was observed by both naked eye and optical absorption spectroscopy. The GO/AuNPs/TX-100 nanocomposites displayed apparent color changes (red to blue) and absorption spectra changes (decreasing of the band around 528 nm and appearance of a new red-shifted band at 640 nm) in presence of insulin. The interaction mechanism of the nanocomposites and insulin was discussed. It is based on the special structure of insulin, that insulin can be easily self-assemble into the GO/AuNP/TX-100 nanocomposites and can also play the role of a bridge between two different GO/AuNPs/TX-100 nanocomposites by peptide chains. The effective parameters for insulin detection were optimized. The colorimetric method was used for quantification of insulin in the range of 2-300 ng mL^-1 with a detection limit of 0.1 ng mL^-1. Moreover, the relative standard deviation of the method was 3.1 and 2.7% (n = 10) at concentrations of 50 and 200 ng mL^-1, respectively on the same day and 4.8% at a concentration (200.0 ng mL^-1) on five consecutive days. The present method was utilized for insulin assay in human blood serums with satisfactory results.

New Ag-Doped COF Catalytic Amplification Aptamer Analytical Platform for Trace Small Molecules with the Resonance Rayleigh Scattering Technique

Covalent organic frameworks (COFs) and Ag-doped COFs (AgCOFs) are prepared by the polycondensation procedure and characterized by electron microscopy and molecular spectral techniques. Their catalysis of the Cu₂O particle reaction of glucose (GL)-Cu(II) was examined by resonance Rayleigh scattering (RRS), and AgCOFs were found to exhibit the strongest catalysis. The melamine (ML) aptamers (Apt_ML) can attach to the surface of AgCOF and inhibit its catalytic activity. When melamine (ML) is added to this reacting solution, Apt_ML-ML complexes are formed and the Apts are desorbed from the surface of AgCOF. As the concentration of ML increased, the catalytic activity of AgCOF increased and the RRS signal enhanced due to the increase in Cu₂O particles. When the ML concentration was in the range of 0.79-13.2 nmol/L, the RRS intensity increased linearly, with a detection limit of 0.72 nmol/L. When the Apts of urea and bisphenol A (BPA) were replaced by the Apt_ML, 66.7-1333 nmol/L urea and 0.33-2.7 nmol/L BPA, respectively, could also be determined, with detection limits of 30.4 nmol/L urea and 0.15 nmol/L BPA. Based on this, a new AgCOF amplification RRS method was established.

Controllable and robust dual-emissive quantum dot nanohybrids as inner filter-based ratiometric probes for visualizable melamine detection

The ratiometric fluorescence technique is of great interest due to its visualization characteristics. The construction of a reliable fluorescent ratiometric nanoprobe for high-sensitivity visual quantification is highly sought after but it is limited by poor stability and controllability. Herein, we report a robust dual-emissive quantum dot nanohybrid with precise color tunability and demonstrate its potential as a two-signal-change ratiometric probe for visual detection. A novel assembly strategy was developed for spatially implanting hydrophobic green and red quantum dots (QDs) into a silica scaffold to form a dual-emissive hierarchical fluorescent silica nanohybrid. The fluorescence intensity ratio and color of the nanohybrid were precisely tailored by altering the amounts of green and red QDs. Particularly, after the alkylsilane-mediated phase transfer and exterior silica shell growth, the nanohybrid exhibited the well-preserved fluorescence features of the original QDs and robust optical/colloid stability. An inner filter-based ratiometric nanoprobe for the visual determination of melamine was ultimately devised by combining the spectra-overlapped two-colored fluorescent nanohybrid with analyte-specific gold nanoparticles. Furthermore, based on the reversible fluorescence signal changes in two-colored QDs induced by melamine, a logic gate strategy for melamine monitoring was constructed. The newly developed fluorescent ratiometric nanoprobe shows great prospects for the visual and quantitative determination of analytes in a complex biological matrix.

Switch on fluorescence mode for determination of l-cysteine with carbon quantum dots and Au nanoparticles as a probe

Citric acid and urea were used as precursors for the preparation of carbon quantum dots (CQDs) which exhibited a maximum emission wavelength at 515 nm when excited at 410 nm. Upon addition of citrate-stabilized Au nanoparticles (AuNPs) with the maximum absorption wavelength at 520 nm, the fluorescence of the CQDs could be efficiently quenched, attributed to the energy transfer between CQDs and AuNPs. However, the further introduction of l-cysteine (Cys) could cause the aggregation of AuNPs along with a drop in absorption at 520 nm, resulting in the fluorescence recovery of the CQDs–AuNPs system. Therefore, a simple and reliable switch on fluorescence sensing platform for determination of Cys was constructed. The significant factors, such as pH and incubation time, that affected the detection of Cys were optimized with the AuNP concentration set as 2.50 nM at room temperature. Under the optimized conditions, the fluorescence recoveries (ΔF) were strongly correlated with Cys concentration in the 0.20 to 4.0 μM range, and the detection limit is 0.012 μM. More importantly, our CQD-based sensing platform was successfully used for the detection of Cys in milk samples with high precision and accuracy, indicating the potential of the probe in practical applications.

We demonstrated a fluorescence probe composed of CQDs and AuNPs for switch on detection of Cys, where CQDs as fluorescence reporters and AuNPs as fluorescence quenchers were chosen.

Reshaping of triangular silver nanoplates by a non-halide etchant and its application in melamine sensing

Although triangular silver (Ag) nanoplates are intrinsically unstable, this characteristic has been taken advantage of in the development of a novel sensing platform. However, most of these applications have relied on halide ions as etchants. In the current work, we used sodium 4-vinylbenzenesulfonate (Na-VBS) as a new powerful etchant of triangular silver (Ag) nanoplates. When aged with Na-VBS at room temperature, Na-VBS etched Ag nanoplates nearly as powerfully as halides did, and these nanoplates rapidly transformed into oblate nanospheroids. This shape evolution permitted tuning of the corresponding localized surface plasmon resonance (LSPR) features of the Ag nanostructures. Interestingly, passivation of the Ag nanoplate surface with melamine was shown to protect the nanoplates from Na-VBS-induced etching. The rate of change of the color and spectral features of the Ag nanoplate solution exposed to Na-VBS was found to be strongly correlated with the concentration of melamine in the solution. This association allowed us to apply this system to the development of a novel platform for sensing melamine.

Aquamarine blue emitting silver nanoparticles as fluorescent sensor for melamine detection

Designing an exceptional probe to detect minute quantities of melamine is of huge importance for the safety and health of the human race. In this study, an aquamarine blue and rapid fluorescent sensor was devised by chemical reduction synthesis method for highly selective and sensitive detection of melamine in milk using polyethyleneimine protected silver nanoparticles (PEI-AgNPs). The optical properties of PEI-AgNPs were studied as a function of reactive material concentration, incubation time, temperature and pH. The formation of PEI-AgNPs was verified by using transmission electron microscopy (TEM) and found that the PEI-AgNPs were spherical in shape with an average diameter of 15 nm. The prepared PEI-AgNPs were aquamarine blue luminescent under UV light and show a maximum emission at 470 nm, when excited at 348 nm. The fluorescence of the PEI-AgNPs can be effectively quenched by the addition of melamine due to its ability to interact with citric acid (CA) through hydrogen bonding and form silver-melamine complexes with Ag⁺ via coordination bonds, while the addition of other nitro compounds induced feeble quenching of PEI-AgNPs, suggesting the high selectivity of PEI-AgNPs. The present sensor system exhibits a good linear response to melamine ranging from 0.16 to 56 μM, and the limit of detection was found to be 132 nM (Sb/K = 3). Significantly, the fluorescence sensor exhibited good selectivity and acted as a potential probe for detection of melamine in milk samples.

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.

Review on Nanomaterial-Based Melamine Detection

Illegal adulteration of milk products by melamine and its analogs has become a threat to the world. In 2008, the misuse of melamine with infant formula caused serious effects on babies of China. Thereafter, the government of China and the US Food and Drug Administration (FDA) limited the use of melamine of 1 mg/kg for infant formula and 2.5 mg/kg for other dairy products. Similarly, the World Health Organization (WHO) has also limited the daily intake of melamine of 0.2 mg/kg body weight per day. Many sensory schemes have been proposed by the scientists for carrying out screening on melamine poisoning. Among them, nanomaterial-based sensing techniques are very promising in terms of real-time applicability. These materials uncover and quantify the melamine by means of diverse mechanisms, such as fluorescence resonance energy transfer (FRET), aggregation, inner filter effect, surface-enhanced Raman scattering (SERS), and self-assembly, etc. Nanomaterials used for the melamine determination include carbon dots, quantum dots, nanocomposites, nanocrystals, nanoclusters, nanoparticles, nanorods, nanowires, and nanotubes. In this review, we summarize and comment on the melamine sensing abilities of these nanomaterials for their suitability and future research directions.

Application of Gold-Nanoparticle Colorimetric Sensing to Rapid Food Safety Screening

Due to their unique optical properties, narrow size distributions, and good biological affinity, gold nanoparticles have been widely applied in sensing analysis, catalytic, environmental monitoring, and disease therapy. The color of a gold nanoparticle solution and its maximum characteristic absorption wavelength will change with the particle size and inter-particle spacing. These properties are often used in the detection of hazardous chemicals, such as pesticide residues, heavy metals, banned additives, and biotoxins, in food. Because the gold nanoparticles-colorimetric sensing strategy is simple, quick, and sensitive, this method has extensive applications in real-time on-site monitoring and rapid testing of food quality and safety. Herein, we review the preparation methods, functional modification, photochemical properties, and applications of gold nanoparticle sensors in rapid testing. In addition, we elaborate on the colorimetric sensing mechanisms. Finally, we discuss the advantages and disadvantages of colorimetric sensors based on gold nanoparticles, and directions for future development.

Paper-Based Analytical Methods for Smartphone Sensing with Functional Nanoparticles: Bridges from Smart Surfaces to Global Health

In this Feature, the most recent developments as well as "pros and cons" in smartphone sensing, which have been developed using various functional nanoparticles in paper-based sensing systems, will be discussed. Additionally, smart phone sensing and POC combination as a potential tool that opens a gate for knowledge flow "from lab scale data to public use" will be evaluated.

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.