Target-Induced Displacement Reaction Accompanying Cargo Release from Magnetic Mesoporous Silica Nanocontainers for Fluorescence Immunoassay

A Review of Sensing Applications of Molecularly Imprinted Fluorescent Carbon Dots for Food and Biological Sample Analysis

Molecularly imprinted fluorescent carbon dots (MI-FCDs) find numerous applications in analytical chemistry due to their outstanding photoluminescent properties and having specific pockets for the recognition of target molecules. Despite significant advances, practical applications of MI-FCDs-based fluorescent sensors are still in their initial stages. Therefore, the topical developments in the synthesis, working, and application of MI-FCDs for sensing various target species (e.g., pharmaceuticals, biomolecules, pesticides, food additives, and miscellaneous species) in food and biological media have been highlighted. Moreover, a careful evaluation has been made to select the best methods based on their performance in terms of analytical parameters. To expand the horizons of this field, important challenges and future directions for developing MI-FCDs for practical use are also presented. This review will highlight important aspects of MI-FCDs-based fluorescent sensors for their applicability in food science, material science, environmental science, nanoscience, and biotechnology.

Surface Plasmon Resonance (SPR) biosensor for detection of mycotoxins: A review

Mycotoxin is one of the most important natural pollutants, which poses a global threat to food safety. However, the pollution of mold in food production is inevitable. The detection technology of mycotoxins in food production is an important means to prevent the damage of mycotoxins, so rapid detection and screening to avoid pollution diffusion is essential. The focus of this review is to update the literature on the detection of mycotoxins by surface plasmon resonance (SPR) technology, rather than just traditional chromatographic methods. As a relatively novel and simple analytical method, SPR has been proved to be fast, sensitive and label-free, and has been widely used in real-time qualitative and quantitative analysis of various pollutants. This paper aims to give a broad overview of the sensors for detection and analysis of several common mycotoxins.

Recent Progress in Rapid Determination of Mycotoxins Based on Emerging Biorecognition Molecules: A Review

Mycotoxins are secondary metabolites produced by fungal species, which pose significant risk to humans and livestock. The mycotoxins which are produced from Aspergillus, Penicillium, and Fusarium are considered most important and therefore regulated in food- and feedstuffs. Analyses are predominantly performed by official laboratory methods in centralized labs by expert technicians. There is an urgent demand for new low-cost, easy-to-use, and portable analytical devices for rapid on-site determination. Most significant advances were realized in the field bioanalytical techniques based on molecular recognition. This review aims to discuss recent progress in the generation of native biomolecules and new bioinspired materials towards mycotoxins for the development of reliable bioreceptor-based analytical methods. After brief presentation of basic knowledge regarding characteristics of most important mycotoxins, the generation, benefits, and limitations of present and emerging biorecognition molecules, such as polyclonal (pAb), monoclonal (mAb), recombinant antibodies (rAb), aptamers, short peptides, and molecularly imprinted polymers (MIPs), are discussed. Hereinafter, the use of binders in different areas of application, including sample preparation, microplate- and tube-based assays, lateral flow devices, and biosensors, is highlighted. Special focus, on a global scale, is placed on commercial availability of single receptor molecules, test-kits, and biosensor platforms using multiplexed bead-based suspension assays and planar biochip arrays. Future outlook is given with special emphasis on new challenges, such as increasing use of rAb based on synthetic and naïve antibody libraries to renounce animal immunization, multiple-analyte test-kits and high-throughput multiplexing, and determination of masked mycotoxins, including stereoisomeric degradation products.

A controllable staining colorimetric method based on carboxylated cellulose membranes for early-warning and semi-quantitative detection of aflatoxins in water

A controllable staining colorimetric method was proposed for antibody-free detection of AFs by exploiting controllable electrostatic-staining of CCMs with Hg2+-capped AuNPs.

4-Nitrophenol-Loaded Magnetic Mesoporous Silica Hybrid Materials for Spectrometric Aptasensing of Carcinoembryonic Antigen

Aptamer- or antibody-based sensing protocols have been reported for detecting carcinoembryonic antigen (CEA), but most exhibit complicated procedures or multiple reactions. In this work, we developed a one-step aptasensing protocol for the spectrometric determination of CEA based on 4-nitrophenol (4-NP)-loaded magnetic mesoporous silica nanohybrids (MMSNs) for bioresponsive controlled-release applications. To fabricate such a responsive–controlled sensing system, single-stranded complementary oligonucleotides relative to the CEA-specific aptamer were first modified on the aminated MMSN. Thereafter, 4-NP molecules blocked the pores with the assistance of the aptamers via a hybridization reaction. The introduced target CEA specifically reacted with the hybridized aptamer, thus detaching from the MMSN to open the gate. The loaded 4-NP molecules were released from the pores, as determined using ultraviolet–visible (UV–vis) absorption spectroscopy after magnetic separation. Under optimum conditions, the absorbance increased with an increase in the target CEA in the sample and exhibited a good linear relationship within the dynamic range of 0.1–100 ng mL⁻¹, with a detection limit of 46 pg mL⁻¹. Moreover, this system also displayed high specificity, good reproducibility, and acceptable accuracy for analyzing human serum specimens, in comparison with a commercialized human CEA-enzyme-linked immunosorbent assay (ELISA) kit.

Micro flow photochemical synthesis of Ca-sensitive fluorescent sensor particles

Fluorescence probes have widely been used for detecting and imaging Ca2+-enriched parts of cells but more rarely for quantitative determination of concentrations. In this study we show how this can be achieved by a novel approach using hydrogel particles. In a microfluidic co-flow arrangement spherical droplets were generated from an aqueous solution of acrylamide, N,N'-methylenebisacrylamide crosslinker and photoinitiator and subsequently photo-cured in situ yielding gel particles in a sub millimeter range. These particles were separated, dried under reduced pressure and re-swollen in water containing Rhod-5N tri potassium salt as calcium ion selective fluorescence probe. After that the particles were dried again and stored for further investigations. Upon exposure of dried particles to calcium chloride solutions they swell and take up Ca2+-ions forming a strong fluorescing complex with Rhod-5N. Thus, fluorescence intensity increases with calcium ion concentration. Up to ca. 0.50 mM the enhancement effect is strong and then becomes considerably weaker. The intensity-concentration-dependence is well described by an equation derived from the equilibrium of the formation of a 1:1 Ca2+:Rhod-5N complex. The particles allow for a fast optical determination of Ca2+-concentrations up to 0.50 mM in analyte volumes down to below 10 μL.

SERS Sensors Based on Aptamer-Gated Mesoporous Silica Nanoparticles for Quantitative Detection of Staphylococcus aureus with Signal Molecular Release

This work describes a simple and novel biosensor for the quantitative determination of Staphylococcus aureus (S. aureus) based on target-induced release of signal molecules from aptamer-gated aminated mesoporous silica nanoparticles (MSNs) coupled with surface-enhanced Raman scattering (SERS) technology. MSNs were synthesized and then modified with amino groups by (3-aminopropyl) triethoxysilane to make them positively charged. Next, signal molecules (4-aminothiophenol, 4-ATP) were loaded into the pores of MSNs. Then, negatively charged aptamers of S. aureus were assembled on the surface of MSNs through electrostatic interactions. Upon the addition of S. aureus, the assembled aptamers were specifically bound to the bacteria. Consequently, the "gates" were opened, resulting in the release of 4-ATP from the pores of MSNs. The released molecules were measured by a Raman spectrometer, and the intensity of 4-ATP at 1071 cm^-1 was linearly related to the S. aureus concentration. A silver nanoflower silica core-shell structure (Ag NFs@SiO₂) was prepared and it served as a SERS substrate. Under optimized experimental conditions, a good linear relationship (y = 2107.93 + 1536.30x, R² = 0.9956) in the range from 4.7 × 10 to 4.7 × 10⁸ cfu/mL was observed with a limit of detection of 17 cfu/mL. The method was successfully applied for the analysis of S. aureus in fish samples and the recovery rate was 91.3-109%.

Sensitive Metal Oxide-Clay Nanocomposite Colorimetric Sensor Development for Aflatoxin Detection in Foods: Corn and Almond

The work reports on zinc oxide bentonite nanocomposite (ZnOBt) chemical route synthesis, characterization, and investigation of curcumin (Cur) functionalization for a label-free colorimetric detection of total aflatoxins (AFs) in foods. XRD of ZnO nanoparticles (NPs) confirmed the wurtzite structure (2θ = 36.2°) and that of ZnOBt showed the intercalated interlayer composite phase. The Debye-Scherrer relation calculated the crystallite size as 20 nm (ZnO) and 24.4 nm (ZnOBt). Surface morphology by SEM exhibited flower-like hexagonal, rod-shaped ZnO NPs on the bentonite surface. Colorimetric reaction involved two-stage redox reactions between ZnOBt and dye Cur followed by AFs phenolic group and Zn(Cur)OBt. Cur gets oxidized at its diketone moiety in the presence of ZnOBt to form a red colored complex Zn(Cur)OBt, which further scavenge protons from AFs phenolic group, and gets oxidized to AFs-Zn(Cur)OBt (yellow). Binding of AFs-Zn(Cur)OBt is characterized by FT-IR ascribed to C-H bending (1966.615 cm-1), O-H stretching (3256.974 cm-1), and C=O stretching (1647.362 cm-1). 1H NMR chemical shifts (δ) (ppm) showed an increase in proton at the aliphatic region (0 to 4.4) while removal of proton in ether at 4.4 to 6 regions. Job plot calculation using UV-Vis data resulted in a higher total AF binding coefficient of Zn(Cur)OBt (K a = 3.77 × 106 mol-1 L) compared to Zn(Cur)O (K a = 0.645 × 106 mol-1 L) as well as a molar ratio of 1:1 by the Benesi-Hildebrand plot equation. Corn and almond food samples showed the total AFs LOD of 2.74 and 4.34 ppb, respectively. The results are validated with standard LC/MS-MS in compliance with MRL value as per the regulatory standard (EU).The NP-based method is facile and rapid and hence can be utilized for onsite detection of total AFs in foods.

Proximity hybridization triggered hybridization chain reaction for label-free electrochemical homogeneous aptasensors

A label-free homogeneous electrochemical aptasensor was developed for detection of thrombin based on proximity hybridization triggered hybridization chain reaction induced G-quadruplex formation. Thrombin promoted the formation of a complex via the proximity hybridization of the aptamer DNA strands, which unfolded the molecular beacon, the stem part of molecular beacon as a primer to initiate the hybridization chain reaction process. Thus, with the electrochemical indicator hemin selectively intercalated into the multiple G-quadruplexes, a significant electrochemical signal drop is observed, which is dependent on the concentration of the target thrombin. Thus, using this"signal-off" mode, label-free homogeneous electrochemical strategy for sensitive thrombin assay with a detection limit of 44 fM is realized. Furthermore, this method also exhibits additional advantages of simplicity and low cost, since both expensive labeling and sophisticated probe immobilization processes are avoided. Its high sensitivity, acceptable accuracy, and satisfactory versatility of analytes led to various applications in bioanalysis.

Multiplexed Detection of Analytes on Single Test Strips with Antibody-Gated Indicator-Releasing Mesoporous Nanoparticles

Rapid testing methods for the use directly at a point of need are expected to unfold their true potential especially when offering adequate capabilities for the simultaneous measurement of multiple analytes of interest. Considering the unique modularity, high sensitivity, and selectivity of antibody‐gated indicator delivery (gAID) systems, a multiplexed assay for three small‐molecule explosives (TATP, TNT, PETN) was thus developed, allowing to detect the analytes simultaneously with a single test strip at lower ppb concentrations in the liquid phase in <5 min using a fluorescence reader or a smartphone for readout. While the TNT and PETN systems were newly developed here, all the three systems also tolerated harsher matrices than buffered aqueous model solutions. Besides a single‐track strip, the outstanding modularity of the hybrid biosensor materials in combination with strip‐patterning technologies allowed us to obtain a multichannel strip in a straightforward manner, offering comparable analytical performance while allowing to be tailored even more to the user's need.

Facile dispersive solid-phase extraction based on humic acid for the determination of aflatoxins in various edible oils

Aflatoxins (AFs), as the secondary metabolites of the toxigenic fungi Aspergillus flavus and Aspergillus parasiticus, are well known to be extremely harmful to humans and animals because of their high toxicity, mutagenicity, carcinogenicity, and teratogenicity. Recurring and increasing studies on AF ingestion incidents indicate that AF contamination is a serious food safety issue worldwide. Currently, immunoaffinity chromatography (IAC) has become the most conventional sample clean-up method for determining AFs in foodstuffs. However, the IAC method may be limited to some laboratories because it requires the use of expensive disposable cartridges and the IA procedure is time-consuming. Herein, to achieve the cost-effective determination of AFs in edible oils, we developed a dispersive solid-phase extraction (DSPE) clean-up method based on humic acids (HAs), which is followed by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) analysis. HAs could be directly used as a DSPE sorbent after simple treatment without any chemical modification. In the HA-DSPE, AFs could remain on the HA sorbent by both hydrophobic and hydrophilic interactions, whereas the oil matrix was retained on HA via only hydrophobic interactions. The oil matrix could be sufficiently washed off by n-hexane, whereas the AFs could still be retained on HA; thus, the selective extraction of AFs and clean-up of oil matrices were achieved. Under the optimal conditions of HA-DSPE, satisfactory recoveries ranging from 81.3% to 106.2% for four AFs (B1, B2, G1, and G2) were achieved in various oil matrices i.e. blended oil, mixed olive oil, tea oil, sunflower seed oil, rapeseed oil, sesame oil, soybean oil, rice oil, corn oil, and peanut oil. Minor matrix effects ranging from 89.3% to 112.9% were obtained for the four AFs, which were acceptable. Moreover, the LODs of AFs between 0.063 and 0.102 μg kg-1 completely meet the regulatory levels fixed by the Food and Drug Administration (FDA), the European Union (EU), China, or other countries. The proposed methodology was further validated using a naturally contaminated peanut oil, and the results indicated that the accuracy of the HA-DSPE could match the accuracy of the referenced IAC. In addition, HA-DSPE can be used to directly treat diluted edible oil without liquid-liquid extraction and HA is cheap and can be easily obtained from the market worldwide; these advantages make the proposed methodology simple, low-cost, and accessible for the determination of AFs in edible oils.

Indirect Competitive Immunoassay on a Blu-ray Disc for Digitized Quantitation of Food Toxins

We report herein a Blu-ray disc technology enabled immunoassay (namely, assay-on-a-Blu-ray) protocol for the quantitation of food toxins. In particular, commercial Blu-ray discs (BDs) are activated as substrates to create indirect competitive immunoassays with the aid of microfluidic channel plates for the quantitation of aflatoxins; an unmodified Blu-ray drive is employed to read the digitized signal (error counts generated from gold/silver-particle-enhanced binding sites); and a free disc-quality control software is adapted to process the raw data. The performance of this BD-based digital detection platform has been tested for the quantitation of aflatoxin B1 (AFB1) in spiked corn samples and validated with standard high-performance liquid chromatography measurements. The detection limit attained is as low as 0.27 ppb with a dynamic response range up to 200 ppb, which meets the standards established by government agencies worldwide for food products. We truly believe that the application potential of such a BD-technology-based, portable device for multiplex on-site quantitative analysis of food products as well as environmental and biomedical samples in real time is unlimited.

Optimization of analytical assay performance of antibody-gated indicator-releasing mesoporous silica particles

An in-depth study of the chemical tuning modes of antibody-gated indicator delivery (gAID) systems revealed the importance of size matching, localisation of grafting and loading sequence for obtaining high-performance small-molecule sensor materials.

Development of a lateral flow test for rapid pyrethroid detection using antibody-gated indicator-releasing hybrid materials

The employment of type-I pyrethroids for airplane disinfection in recent years underlines the necessity to develop sensing schemes for the rapid detection of these pesticides directly at the point-of-use.

Proximity hybridization triggered rolling-circle amplification for sensitive electrochemical homogeneous immunoassay

A new homogeneous electrochemical immunoassay strategy was developed for ultrasensitive detection of carcinoembryonic antigen (CEA) based on target-induced proximity hybridization coupled with rolling circle amplification (RCA). The immobilization-free detection of CEA was realized by the use of an uncharged peptide nucleic acid (PNA) probe labeled with ferrocene (Fc) as the electroactive indicator on a negatively charged indium tin oxide (ITO) electrode. In the presence of a target protein and two DNA-labeled antibodies, the proximate complex formed in homogeneous solution could unfold the molecular beacon, and a part of the unfolded molecular beacon as a primer hybridized with the RCA template to initiate the RCA process. Subsequently, the detection probe modified Fc (Fc-PNAs) hybridized with the long amplified DNA products. The consumption of freely diffusible Fc-PNAs (neutrally charged) resulted in a significant reduction of the Fc signal due to the fact that long amplified DNA/Fc-PNA products were electrostatically repelled from the ITO electrode surface. The reduction of the electrochemical signal (signal-off) could indirectly provide the CEA concentration. Under the optimal conditions, CEA detection was implemented in a wide range from 1 pg mL^-1 to 10 ng mL^-1, with a low detection limit of 0.49 pg mL^-1. The proposed strategy exhibited advantages of good selectivity, high sensitivity, acceptable accuracy, and favorable versatility of analytes. Moreover, the practical application value of the system was confirmed by the assay of CEA in human serums with satisfactory results.

Direct Immunoassay for Facile and Sensitive Detection of Small Molecule Aflatoxin B1 based on Nanobody

Aflatoxin B₁ (AFB₁ ), one of the most toxic mycotoxins, is classified as a group I carcinogen and ubiquitous in various foods and agriproducts. Thus, accurate and sensitive determination of AFB₁ is of great significance to meet the criteria of food safety. Direct detection of AFB₁ is difficult by monoclonal antibody (mAb) with large molecular size (≈150 kD) since the target is too small to produce a detectable signal change. Herein, by combining the electrochemical properties of nanomaterials and the advantages of nanobodies, we developed a direct, highly selective and sensitive electrochemical immunosensor for small molecule detection. The proposed immunosensor had a wide calibration range of 0.01 to 100 ng mL^-1 and a low detection limit of 3.3 pg mL^-1 (S/N=3). Compared with the immunosensor prepared with mAb which was applied in the typical indirect immunoassay, the immunosensor in this work possessed two orders of magnitudes wider linear range and 10-fold more sensitivity. The as-obtained immunosensor was further successfully applied for sensing AFB₁ in real samples. This proposed assay would provide a simple, highly sensitive and selective approach for the direct immunoassay of small molecule AFB₁ , and is extendable to the development of direct immunosensing systems for other small molecules detection by coupling nanocarbon and nanobody.

Novel quartz crystal microbalance immunodetection of aflatoxin B1 coupling cargo-encapsulated liposome with indicator-triggered displacement assay

A simple and sensitive quartz crystal microbalance (QCM) immunosensing platform was designed for the high-efficient detection of aflatoxin B₁ (AFB₁) in foodstuff. Initially, phenoxy-derived dextran molecule was immobilized on the surface of QCM gold substrate by using thiolated β-cyclodextrin based on the supramolecular host-guest chemistry between phenoxy group and cyclodextrin. Then, AFB₁-bovine serum albumin (AFB₁-BSA)-conjugated concanavalin A (Con A) was assembled onto the QCM probe through the dextran-Con A interaction. Glucose-loaded nanoliposome, labeled with monocolonal anti-AFB₁ antibody, was used for the amplification of QCM signal. Upon target AFB₁ introduction, the analyte competed with the immobilized AFB₁-BSA on the probe for the labeled anti-AFB₁ antibody on the nanoliposome. Based on specific antigen-antibody reaction, the amount of the conjugated nanoliposomes on the QCM probe gradually decreased with the increment of target AFB₁ in the sample. Upon injection of Triton X-100 in the detection cell, the carried nanoliposome was lysed to release the encapsulated glucose molecules. Thanks to the stronger affinity of Con A toward glucose than that of dextran, AFB₁-BSA-labeled Con A was displaced from the QCM probe, resulting in the change of the local frequency. Under the optimum conditions, the shift of the functionalized QCM immunosensing interface in the frequency shift was proportional to the concentration of target AFB₁ within a dynamic range from 1.0 ng kg^-1 to 10 μg kg^-1 at a low detection limit of 0.83 ng kg^-1. In addition, the acceptable assayed results on precision, reproducibility, specificity and method accuracy for the analysis of real samples were also acquired. Importantly, our strategy can provide a signal-on competitive immunoassay for the detection of small molecules, e.g., mycotoxins and biotoxins, thereby representing a versatile sensing schemes by controlling the corresponding antibody or hapten in the analysis of food safety.

Dual-competitive lateral flow aptasensor for detection of aflatoxin B1 in food and feedstuffs

A novel dual-competitive lateral flow aptasensor (LFA) benefited from aptamer and lateral flow strips was first designed by using aflatoxin B₁ (AFB₁) as the model target. In this LFA assay, the target AFB₁ competed with AFB₁-hapten at T line for binding to Cy5-labeled AFB₁ aptamer and the complementary strand competed with the target AFB₁ for binding to Cy5-labeled AFB₁ aptamer at C line. The ratio of their fluorescent intensities at the T line and C line (ST/SC ratio) was employed in order to increase the sensitivity for target AFB₁ detection. This technique has the limit of detection (LOD) of 0.1ng/mL for AFB₁ within the linear range from 0.1ng/mL to 1000ng/mL. Subsequently, the LFA approach was validated using 11 kinds of food and feedstuff samples with a simple aqueous extraction protocol. The test results with different naturally contaminated feedstuffs indicated a good correlation between this LFA and a commercial ELISA kit. The assay can be completed within 20min and its sensitivity, specificity and reproducibility are highly satisfactory. This is the first LFA that has been rigorously validated, which will be greatly beneficial to development of commercial aptamer-based biosensors for food safety, environmental analysis, particularly in clinical diagnosis.

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.

An On-Site, Ultra-Sensitive, Quantitative Sensing Method for the Determination of Total Aflatoxin in Peanut and Rice Based on Quantum Dot Nanobeads Strip

An on-site, ultra-sensitive, and quantitative sensing method was developed based on quantum dot nanobeads (QDNBs) and a test strip for the determination of total aflatoxins (AFTs) in rice and peanuts. The monoclonal antibody against AFT (mAb_AFT) was homemade and labeled with QDNB. After the pre-coating of the AFT antigen on the test line (T line), the competitive immunoreactions were conducted between AFT and AFT antigen on the T line with QDNBs-mAb_AFT. Under optimal conditions, this approach allowed a rapid response towards AFT with a considerable sensitivity of 1.4 pg/mL and 2.9 pg/mL in rice and peanut matrices, respectively. The put-in and put-out durations were within 10 min. The recoveries for AFT in rice and peanut sample matrices were recorded from 86.25% to 118.0%, with relative deviations (RSD) below 12%. The assay was further validated via the comparison between this QDNB strip and the conventional HPLC method using spiked samples. Thus, the design provided a potential alternative for on-site, ultra-sensitive, and quantitative sensing of AFT that could also be expanded to other chemical contaminants for food safety.

Low-cost humic acid-bonded silica as an effective solid-phase extraction sorbent for convenient determination of aflatoxins in edible oils

Aflatoxins (AFs) are highly toxic, mutagenic, carcinogenic, and teratogenic secondary metabolites produced by the toxigenic fungi Aspergillus flavus and Aspergillus parasiticus. AFs tend to contaminate a wide range of foods which is a serious and recurring food safety problem worldwide. Currently, immunoaffinity chromatography (IAC) has become the most conventional sample clean-up method for determining AFs in foodstuffs. However, IAC method is limited in the large-scale food analysis because it requires the use of expensive disposable cartridges and the IA procedure is time-consuming. Herein, to achieve the cost-effective determination of AFs in edible oils, we developed a promising solid-phase extraction (SPE) method based on commercially available humic acid-bonded silica (HAS) sorbent, followed by high performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS) analysis. In HAS-SPE, AFs can be captured by the HAS sorbent with both hydrophobic and hydrophilic interactions, whereas the oil matrix was captured only with the hydrophobic interactions. The oil matrix can be sufficiently washed off with isopropanol, while the AFs were still retained on the SPE packing, thus achieving selective extraction of AFs and clean-up of oil matrices. Under the optimal conditions of HAS-SPE, satisfactory recoveries ranging from 82% to 106% for four AFs (B₁, B₂, G₁, and G₂) were achieved in various oil matrices, containing blended oil, tea oil, rapeseed oil, peanut oil, sunflower seed oil, corn oil, blended olive oil, rice oil, soybean oil, and sesame oil. Only minor matrix effects ranging from 99% to 105% for four AFs were observed. Moreover, the LODs of AFs between 0.012 and 0.035 μg/kg completely meet the regulatory levels fixed by the EU, China or other countries. The methodology was further validated for assaying the naturally contaminated peanut oils, and consistent results between the HAS-SPE and the referenced IAC were obtained. In addition, HAS-SPE can directly treat diluted oil sample without liquid-liquid extraction and is automatable, thus making it simple and convenient for the large-scale determination of AFs in edible oils. Using this method, we successfully detected four AFs in the naturally contaminated peanut oils, which is, to the best of our knowledge, the first report about the determination of AFs in edible oils using HA-based SPE.

Application of Porous Materials to Carbohydrate Chemistry and Glycoscience

There is a growing interest in using a range of porous materials to meet research needs in carbohydrate chemistry and glycoscience in general. Among the applications of porous materials reviewed in this chapter, enrichment of glycans from biological samples prior to separation and analysis by mass spectrometry is a major emphasis. Porous materials offer high surface area, adjustable pore sizes, and tunable surface chemistry for interacting with glycans, by boronate affinity, hydrophilic interactions, molecular imprinting, and polar interactions. Among the materials covered in this review are mesoporous silica and related materials, porous graphitic carbon, mesoporous carbon, porous polymers, and nanoporous gold. In some applications, glycans are enzymatically or chemically released from glycoproteins or glycopeptides, and the porous materials have the advantage of size selectivity admitting only the glycans into the pores and excluding proteins. Immobilization of lectins onto porous materials of suitable pore size allows for the use of lectin-carbohydrate interactions in capture or separation of glycoproteins. Porous material surfaces modified with carbohydrates can be used for the selective capture of lectins. Controlled release of therapeutics from porous materials mediated by glycans has been reported, and so has therapeutic targeting using carbohydrate-modified porous particles. Additional applications of porous materials in glycoscience include their use in the supported synthesis of oligosaccharides and in the development of biosensors for glycans.

Electrochemical immunosensor for detection of prostate specific antigen based on an acid cleavable linker into MSN-based controlled release system

A mesoporous silica nanoparticle (MSN)-based controlled release system with acid cleavable linkage was developed to fabricate an electrochemical immunosensor for the quantitative detection of the prostate-specific antigen (PSA). 3,9-Bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro[5.5]undecane functionalized mesoporous silica nanoparticles (MSN-Acetal) were used to immobilize the electron mediator thionine (Th). The encapsulation of Th molecules was achieved by capping the pores of MSN-Acetal with carboxylic acid modified Au nanoparticles (defined as MSN-Th-Au). Under the acidic conditions, the capped Au nanoparticles were removed from MSN-Th-Au through the hydrolysis of the acid-labile acetal linker, resulting in the release of encapsulated Th. In this work, the pH-responsive cargo release system was firstly used as the label of secondary anti-PSA for developing an electrochemical immunosensor, and amination Fe3O4 was used as the sensing matrix for immobilizing primary anti-PSA on magnetic carbon electrode surfaces. The specific recognition of PSA resulted in the attachment of MSN-Th-Au-secondary anti-PSA (MSN-Th-Au-Ab2) onto the electrode surfaces. Subsequently, the released Th was detected by differential pulse voltammetry under the acidic conditions. The developed cargo release system provided an innovative and reliable method for the detection of PSA because the response signal was correlated with the concentration of PSA. Under the optimal conditions, the electrochemical immunosensor exhibited a wide linear range of 0.001-5.0ng/mL with a low detection limit of 0.31pg/mL. Moreover, the developed immunosensor showed superior reproducibility and long-term stability, which has promising applications in bioassay and biosensing.

Gold nanoparticle-catalyzed uranine reduction for signal amplification in fluorescent assays for melamine and aflatoxin B1

A multifunctional fluorescence platform has been constructed based on gold nanoparticle (AuNP)-catalyzed uranine reduction. The catalytic reduction of uranine was conducted in aqueous solution using AuNPs as nanocatalyst and sodium borohydride as reducing reagent, which was monitored by fluorescence and UV-vis spectroscopy. The reaction rate was highly dependent on the concentration, size and dispersion state of AuNPs. When AuNPs aggregated, their catalytic ability decreased, and thereby a label-free fluorescent assay was developed for the detection of melamine, which can be used for melamine determination in milk. In addition, a fluorescent immunoassay for aflatoxin B1 (AFB1) was established using the catalytic reaction for signal amplification based on target-induced concentration change of AuNPs, where AFB1-BSA-coated magnetic beads and anti-AFB1 antibody-conjugated AuNPs were employed as capture and signal probe, respectively. The detection can be accomplished in 1 h and acceptable recoveries in spiked maize samples were achieved. The developed fluorescence system is simple, sensitive and specific, which could be used for the detection of a wide range of analytes.

Ni@Au nanoparticles for surface enhanced Raman spectroscopy based ultrasensitive magnetic immunoassay on aflatoxin B1

A facile approach for the fabrication of Ni@Au magnetic nanoparticles was developed as the immune substrates for the competitive magnetic immunoassay. The approach was used for quasi-quantitatively detection on the trace residue of AFB₁in foods.

Gated Silica Mesoporous Materials in Sensing Applications

Silica mesoporous supports (SMSs) have a large specific surface area and volume and are particularly exciting vehicles for delivery applications. Such container-like structures can be loaded with numerous different chemical substances, such as drugs and reporters. Gated systems also contain addressable functions at openings of voids, and cargo delivery can be controlled on-command using chemical, biochemical or physical stimuli. Many of these gated SMSs have been applied for drug delivery. However, fewer examples of their use in sensing protocols have been reported. The approach of applying SMSs in sensing uses another concept-that of loading pores with a reporter and designing a capping mechanism that is selectively opened in the presence of a target analyte, which results in the delivery of the reporter. According to this concept, we provide herein a complete compilation of published examples of probes based on the use of capped SMSs for sensing. Examples for the detection of anions, cations, small molecules and biomolecules are provided. The diverse range of gated silica mesoporous materials presented here highlights their usefulness in recognition protocols.

Lectin-gated, mesoporous, photofunctionalized glyconanoparticles for glutathione-responsive drug delivery

A stimuli-responsive drug delivery system based on fluorescent, lectin-gated, mesoporous glyconanoparticles has been developed and evaluated in normal- and cancer lung epithelial cells. The gating process proved efficient, exhibiting good sealing properties in the absence of the glutathione redox trigger, avoiding premature release in normal cells. In the presence of higher levels of glutathione in cancer cells, the lectin gate was rapidly opened and the anticancer drug released.

Rapid on-site sensing aflatoxin B1 in food and feed via a chromatographic time-resolved fluoroimmunoassay

Aflatoxin B1 poses grave threats to food and feed safety due to its strong carcinogenesis and toxicity, thus requiring ultrasensitive rapid on-site determination. Herein, a portable immunosensor based on chromatographic time-resolved fluoroimmunoassay was developed for sensitive and on-site determination of aflatoxin B1 in food and feed samples. Chromatographic time-resolved fluoroimmunoassay offered a magnified positive signal and low signal-to-noise ratio in time-resolved mode due to the absence of noise interference caused by excitation light sources. Compared with the immunosensing performance in previous studies, this platform demonstrated a wider dynamic range of 0.2-60 μg/kg, lower limit of detection from 0.06 to 0.12 µg/kg, and considerable recovery from 80.5% to 116.7% for different food and feed sample matrices. It was found to be little cross-reactivity with other aflatoxins (B2, G1, G2, and M1). In the case of determination of aflatoxin B1 in peanuts, corn, soy sauce, vegetable oil, and mouse feed, excellent agreement was found when compared with aflatoxin B1 determination via the conversational high-performance liquid chromatography method. The chromatographic time-resolved fluoroimmunoassay affords a powerful alternative for rapid on-site determination of aflatoxin B1 and holds a promise for food safety in consideration of practical food safety and environmental monitoring.

Controlled growth of immunogold for amplified optical detection of aflatoxin B1

A simple, sensitive and cost-effective method for the analysis of the mycotoxin aflatoxin B1 (AFB1) has been established based on controlled growth of immunogold.