Resonance scattering detection of trace melamine using aptamer-modified nanosilver probe as catalyst without separation of its aggregations

Development of a new phosphorescence sensor based on surface molecularly imprinted Mn-doped ZnS quantum dots for detection of melamine in milk products

This paper presents a novel room temperature phosphorescence sensor (IMIPs-ZnS QDs RTP sensor) based on inorganic surface molecularly imprinted polymers and Mn-doped ZnS quantum dots (QDs) for the rapid detection of trace melamine (MEL) in commercial milk products. The surface of Mn-ZnS QDs was modified with 3-(mercaptopropyl) trimethoxy silane (MPTS). Then, MEL, 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) were used as a template/target molecule, functional monomer, and cross-linker, respectively. IMIPs-ZnS QDs RTP sensor was characterized using spectrofluorimeter, UV-Vis spectrophotometer, FT-IR, transmission electron microscope (TEM), and X-ray photoelectron spectrometer (XPS). Detection time and linear range for IMIPs-ZnS QDs RTP sensor were 30 min and 4.0-79.2 µM with a correlation coefficient value of 0.9946, respectively. Furthermore, LOD and LOQ values were calculated using Stern-Volmer equation as 0.29 and 0.97 µM, respectively. Thus, IMIPs-ZnS QDs RTP sensor was successfully applied for the detection of MEL residue in milk samples. Recovery values were in the range of 88.62-90.22 % with relatively high precision values (0.57-0.92 % RSD). Our findings indicate that the developed IMIPs-ZnS QDs RTP sensor exhibits high sensitivity and selectivity towards the MEL in milk sample containing potentially relatively high number of interfering compounds.

Future perspectives on aptamer for application in food authentication

Food fraudulence and food contamination are major concerns, particularly among consumers with specific dietary, cultural, lifestyle, and religious requirements. Current food authentication methods have several drawbacks and limitations, necessitating the development of a simpler, more sensitive, and rapid detection approach for food screening analysis, such as an aptamer-based biosensor system. Although the use of aptamer is growing in various fields, aptamer applications for food authentication are still lacking. In this review, we discuss the limitations of existing food authentication technologies and describe the applications of aptamer in food analyses. We also project several potential targets or marker molecules to be targeted in the SELEX process. Finally, this review highlights the drawbacks of current aptamer technologies and outlines the potential route of aptamer selection and applications for successful food authentication. This review provides an overview of the use of aptamer in food research and its potential application as a molecular reporter for rapid detection in food authentication process. Developing databases to store all biochemical profiles of food and applying machine learning algorithms against the biochemical profiles are urged to accelerate the identification of more reliable biomarker molecules as aptamer targets for food authentication.

Can microplastics from personal care products affect stream microbial decomposers in the presence of silver nanoparticles?

Microplastics (MPs) are emerging contaminants of great concern due to their abundance and persistence over time in aquatic environments. However, studies on their impacts on freshwater organisms are scarce. In resemblance, silver nanoparticles (Ag-NPs) are incorporated into textiles and personal care products and are also classified as emerging contaminants. We used the leaf litter decomposition model system to investigate the effects of MPs from a commercially used personal care product, alone or in mixture with Ag-NPs, on the diversity and activities of freshwater microbial decomposers. We exposed stream microbial communities associated with leaf litter to increasing concentrations of MPs (polyethylene extracted from a personal care product; 100 μg L^-1 up to 1 g L^-1 5 concentrations plus 1 control) for 27 days in the absence or presence of Ag-NPs (0.1 mg L^-1 and 1 mg L^-1). The exposure to MPs, alone or in mixture with Ag-NPs, negatively affected fungal diversity and sporulation, with a reduction in leaf litter decomposition (Cohen's d > 1.5; r> 0.8; Bonferroni, P < 0.01). Shifts in community structure of sporulating fungi were observed, and effects were more pronounced in mixtures with Ag-NPs at the highest concentration. Mixtures of MPs with Ag-NPs (at the higher concentration) had the strongest impacts on extracellular enzymatic (β-glucosidase, Cohen's d > 1; r > 0.5; phenol oxidase, Cohen's d > 1; r > 0.4) activities (ANOVAs, P < 0.05). Apart from sporulation rates, observed toxicity in mixtures was lower than that expected based on individual toxicity effects, mainly for higher concentrations (Bonferroni, P < 0.05). Our study provided evidence of the potential harmful effects of MPs, alone or in mixtures with Ag-NPs, on the activities of aquatic fungi and on a key ecosystem process, determinant to organic matter turnover in streams.

Semiconducting CuxNi3-x(hexahydroxytriphenylene)2 framework for electrochemical aptasensing of C6 glioma cells and epidermal growth factor receptor

A 2D CuNi metal-organic framework (MOF) named CuxNi3-x(HHTP)2 was synthesized with 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) as the linker and was used as a sensitive scaffold to adsorb aptamer strands for the electrochemical detection of living C6 glioma cells and one of their biomarkers, epidermal growth factor receptor (EGFR). Different from conventional MOFs, the CuxNi3-x(HHTP)2 MOF comprises long-range delocalized electrons, a graphene-analog nanostructure, multiple metal states (Cu0/Cu+/Cu2+ and Ni2+/Ni3+), and abundant oxygen vacancies. With these features, the CuxNi3-x(HHTP)2 MOF anchored a large amount of C6 cell-targeted aptamer strands via coordination among metal centers, oligonucleotides, π-π stacking, and van der Waals force. The CuxNi3-x(HHTP)2-based cytosensor showed a low limit of detection (LOD) of 21 cells mL-1 toward C6 glioma cells within a wide range from 50 cells mL-1 to 1 × 105 cells mL-1. Moreover, the proposed aptasensor displayed high selectivity, good stability, acceptable reproducibility, and a low LOD of 0.72 fg mL-1 for detecting EGFR with the concentration ranging from 1 fg mL-1 to 1 ng mL-1. The aptasensor based on the CuxNi3-x(HHTP)2 MOF exhibited superior sensing performance over those based on its monometallic analogues such as Cu3(HHTP)2 MOF and Ni3(HHTP)2 MOF. Hence, this sensing strategy based on a bimetallic semiconducting MOF shows great potential for cancer diagnosis.

High-performance liquid chromatography associated with resonance Rayleigh scattering spectral study of the interactions of palladium(ii)–antimigraine chelates with Erythrosin B and their analytical applications

For the first time, a highly sensitive and selective detection technology of high-performance liquid chromatography associated with resonance Rayleigh scattering spectra (HPLC-RRS) is applied to analyze migraine drugs including zolmitriptan (ZON) and rizatriptan (RIN).

Facile detection of melamine by a FAM-aptamer-G-quadruplex construct

The development of a novel method for melamine detection that uses a FAM-aptamer-G-quadruplex construct due to the efficient quenching ability of an aptamer-linked G-quadruplex is reported herein. The construct, which is labeled with the fluorescent dye 6-carboxyfluorescein (FAM), consists of two parts: a melamine-binding aptamer and a G-rich sequence that can form a G-quadruplex structure. Because of the specific recognition of melamine by the T-rich aptamer, this aptamer folds into a hairpin structure in the presence of melamine, which draws the G-quadruplex closer to the FAM fluorophore, leading to the quenching of the fluorescence of FAM. Thus, a highly sensitive and selective fluorescence strategy for assaying melamine was established. Under optimal conditions, the fluorescence quenching is proportional to the concentration of melamine within the range 10-90 nM, and the method has a detection limit of 6.32 nM. Further application of the method to plastic cup samples suggested that it permitted recoveries of between 97.15% ± 1.02 and 101.92% ± 2.07. The detected amounts of melamine spiked into the plastic cup samples and the corresponding amounts measured by HPLC were in good accordance, indicating that this fluorescent method is reliable and practical. Owing to its high sensitivity, excellent selectivity, and convenient procedure, this strategy represents a promising alternative method of melamine screening. Graphical abstract.

Phosphodiesters quaternary ammonium nanoparticles as label-free light scattering probe for turn-off detection of tyrosine

In this contribution, a new highly sensitive and selective sensor of the determination of tyrosine has been proposed based on the downturn effect of light scattering (LS) using phosphodiesters quaternary ammonium nanoparticles (PQANPs). Phosphodiesters quaternary ammonium (PQA), one of Gemini zwitterionic surfactants, self-aggregated into the micelle named as PQANPs, which generated strong LS signal in aqueous solution under the optimum condition. Interestingly, the powerful LS intensity of PQANPs with the maximum peak located at 391 nm significantly decreased after introducing trace amount of tyrosine. The decreased value of the LS intensity of the PQA-tyrosine system (ΔI_LS) was in proportion to tyrosine concentration in the ranges from 5.5 × 10^-8 mol/L to 4.68 × 10^-6 mol/L, with a detection limit of 1.38 × 10^-8 mol/L. Based on this decreased LS situation, the novel approach of the determination of tyrosine was first developed. The reaction mechanism for the interaction between PQANPs and tyrosine was also investigated. Moreover, the proposed LS assay was applied to the detection of tyrosine concentration in human serum and urine samples with satisfactory results.

Target-switched triplex nanotweezer and synergic fluorophore translocation for highly selective melamine assay

This paper describes a triplex DNA nanotweezer to specifically capture melamine (MEL). The triplex-forming oligonucleotide (TFO) arm can be switched from the open state to the closed state once MEL binds to the abasic site (AP site) in duplex via the bifacial hydrogen bonding with thymines. Following this nanotweezer operation, the AP site-bound fluorophore is translocated to the terminal triplet to subsequently light up the nanotweezer. The TFO arm is found to be pivotal for permitting the AP site binding. The synergic processes of target competition and fluorophore translocation support a high selectivity for the MEL assay even against the inherent adenosine and the MEL hydrolysis products. Chelerythrine is employed as the fluorescent probe. The detection limit of MEL was estimated to be about 140 nM assuming a signal-to-noise ratio of 3. It was applied to the determination of MEL in spiked milk samples without any separation procedure. Conceivably, this method opens a new avenue towards highly selective triplex-based sensors by making use of other commercially available DNA modifications for recognizing other analytes. Graphical abstract Schematic presentation of a triplex nanotweezer with an open-to-close conversion upon the abasic site binding of melamine. The assay is based on a synergic fluorophore translocation. The corresponding duplex otherwise shows no binding with melamine. Chelerythrine (CHE) with a yellow-green emission peaking at 544 nm is employed as the fluorescent probe.

A Label-free Resonance Rayleigh Scattering Sensor for Detection of Thrombin Based on Aptamer Recognizing

The interaction between thrombin binding aptamer (TBA) and thrombin (TB) was studied by resonance Rayleigh scattering (RRS). In neutral medium, TBA is present in a balanced form between a G-quadruplex structure and a random coil structure, and the TBA can be induced by metal ions to form a G-quadruplex structure. Upon addition of thrombin, the G-quadruplex selectively bound to TB, which resulted in enhanced resonance Rayleigh scattering. The scattering intensities increased proportionally with the concentration of TB from 10 to 50 nM. The method had very high sensitivity and good selectivity, and the detection limit (3δ/s) was 1 nM. In this work, the spectral characteristics of RRS, the optimum conditions of the reaction, and influencing factors for the RRS intensities were investigated. Furthermore, the structure of the TBA-TB complex and the sensing mechanism were explored. The TB sensor was applied to a diluted human serum sample with satisfactory results, indicating the potential of this method to be applied to biological samples. A selective and simple RRS sensor for the detection of trace amounts of TB is proposed based on conformational change of TBA.

Plasmonic Janus hybrids for the detection of small metabolites

Janus hybrids with amphiphilic structures were used for the sensitive detection of small metabolites.

How do physicochemical properties influence the toxicity of silver nanoparticles on freshwater decomposers of plant litter in streams?

AgNP physicochemical properties may affect AgNP toxicity, but their effects on plant litter decomposition and the species driving this key ecosystem process in freshwaters have been poorly investigated. We assessed the impacts of AgNPs with different size and surface coating (100nm PVP (polyvinylpyrrolidone)-dispersant, 50-60nm and 35nm uncoated) on freshwater decomposers of leaf litter by exposing leaf associated microbial assemblages to increasing concentrations of AgNPs (up to 200mgL^-1) and of AgNO₃ (up to 25mgL^-1). We further conducted a feeding preference experiment with a common invertebrate shredder, Limnephilus sp., which was allowed to feed on microbially-colonized leaves previously exposed to AgNPs and AgNO₃. Leaf decomposition and microbial activity and diversity were inhibited when exposed to increased concentrations of 100nm AgNPs (≥25mgL^-1), while microbial activity was stimulated by exposure to 35nm AgNPs (≥100mgL^-1). Invertebrate shredders preferred leaves exposed to 35nm AgNPs (25mgL^-1) and avoided leaves exposed to AgNO₃ (≥2mgL^-1). Results from the characterization of AgNPs by dynamic light scattering revealed that AgNps with PVP-dispersant were more stable than the uncoated AgNPs. Our results highlight the importance of considering the physicochemical properties of NPs when assessing their toxicity to litter decomposers in freshwaters.

The fluorescence and resonance Rayleigh scattering spectra study on the interactions of palladium (II)-Nootropic chelate with Congo red and their analytical applications

A highly sensitive detection approach of resonance Rayleigh scattering spectra (RRS) is firstly applied to analyzing nootropic drugs including piracetam (PIR) and oxiracetam (OXI). In HCl-NaAc buffer solution (pH=3.0), the OXI chelated with palladium (II) to form the chelate cation [Pd₂·OXI]²⁺, and then reacted with Congo red (CGR) by virtue of electrostatic attraction and hydrophobic force to form binary complex [Pd₂·OXI]. CGR₂, which could result in the great enhancement of RRS. The resonance Rayleigh scattering signal was recorded at λ_ex=λ_em=375nm. This mixture complex not only has higher RRS, but also makes contribution to significant increase of fluorescence, and the same phenomena also were discovered in PIR. The enhanced RRS intensity is in proportion to the PIR and OXI concentration in the range of 0.03-3.0μgmL^-1, and the detection limit (DL) of RRS method for PIR and OXI is 2.3ngmL^-1 and 9.7ngmL^-1. In addition, the DL of fluorescence method for PIR and OXI is 8.4μgmL^-1 and 19.5μgmL^-1. Obviously, the RRS is the highly sensitive method, and the recoveries of the two kinds of nootropic drugs were range from 100.4% to 101.8.0% with RSD (n=5) from 1.1% to 3.1% by RRS method. This paper not only investigated the optimum conditions for detecting nootropics with using RRS method, but also focused on the reasons for enhancing RRS intensity and the reaction mechanism, which in order to firm and contract the resultant. Finally, The RRS method has been applied to detect nootropic drugs in human urine samples with satisfactory results.

Silver nanoparticles in aquatic environments: Physiochemical behavior and antimicrobial mechanisms

Nanosilver (silver nanoparticles or AgNPs) has unique physiochemical properties and strong antimicrobial activities. This paper provides a comprehensive review of the physicochemical behavior (e.g., dissolution and aggregation) and antimicrobial mechanisms of nanosilver in aquatic environments. The inconsistency in calculating the Gibbs free energy of formation of nanosilver [ΔGf(AgNPs)] in aquatic environments highlights the research needed to carefully determine the thermodynamic stability of nanosilver. The dissolutive release of silver ion (Ag(+)) in the literature is often described using a pseudo-first-order kinetics, but the fit is generally poor. This paper proposes a two-stage model that could better predict silver ion release kinetics. The theoretical analysis suggests that nanosilver dissolution could occur under anoxic conditions and that nanosilver may be sulfidized to form silver sulfide (Ag2S) under strict anaerobic conditions, but more investigation with carefully-designed experiments is required to confirm the analysis. Although silver ion release is likely the main antimicrobial mechanism of nanosilver, the contributions of (ion-free) AgNPs and reactive oxygen species (ROS) generation to the overall toxicity of nanosilver must not be neglected. Several research directions are proposed to better understand the dissolution kinetics of nanosilver and its antimicrobial mechanisms under various aquatic environmental conditions.

Soil-pore water distribution of silver and gold engineered nanoparticles in undisturbed soils under unsaturated conditions

Release of engineered nanoparticles (ENPs) to soil is well documented but little is known on the subsequent soil-pore water distribution of ENPs once present in soil. In this study, the availability and mobility of silver (Ag) and gold (Au) ENPs added to agricultural soils were assessed in two separate pot experiments. Pore water samples collected from pots from day 1 to 45 using porous (<0.17 μm) membrane samplers suggest that both Ag and Au are retained almost completely within 24 h with less than 13% of the total added amount present in pore water on day 1. UV-Vis and TEM results showed that AuENPs in pore water were present as both homoaggregates and heteroaggregates until day 3 after which the concentration in pore water was too low to detect the presence of aggregates. A close relation between the concentration of Au and Fe in pore water suggests that the short term solubility of Au is partly controlled by natural soil colloids. Results suggest that under normal aerated soil conditions the actual availability of Ag and AuENPs is low which is relevant in view of risk assessment even though the impact of environmental conditions and soil properties on the reactivity of ENPs (and/or large ENPs aggregates) retained in the solid matrix need to be addressed further.

Portable Detection of Melamine in Milk Using a Personal Glucose Meter Based on an in Vitro Selected Structure-Switching Aptamer

Melamine detection in milk and other foods has attracted much attention since the discovery that melamine-adulterated food causes severe kidney damage. Although many methods have been developed to detect melamine, few methods can provide quantitative results using an affordable and portable device that is suitable for home use or field application. To achieve this goal, we herein report the first in vitro selection of a melamine responsive aptamer using a structure-switching method. A personal glucose meter (PGM) based melamine sensor was designed and subsequently tested using the newly isolated aptamer. Conversion of melamine concentration to glucose amount was achieved by including an invertase-conjugated DNA that is complementary to part of the aptamer. Melamine binding triggers the release of the invertase-DNA conjugate, which hydrolyzes sucrose into glucose. The glucose produced is then measured directly using an off-the-shelf PGM. The described sensor shows high selectivity for melamine against several closely related melamine analogues, such as cyanuric acid, ammeline, and ammelide, and has low detection limits of 0.33 μM (or 41.1 ppb) in buffer and 0.53 μM (or 67.5 ppb) in 80% whole milk without any pretreatment. The detection limits meet the threshold of 2.5 ppm for non-infant-formula products and 1 ppm for melamine in infant milk products as defined by the U.S. Food and Drug Administration (FDA). In addition to the PGM sensor demonstrated here, the same aptamer can be converted into other types of sensors with different signal outputs, allowing portable detection of melamine under a variety of conditions.

Recent advances in nanoparticle based aptasensors for food contaminants

Food safety and hazard analysis is a prime concern of human life, thus quality assessment of food and water is the need of the day. Recent advances in nano-biotechnology play a significant role in providing possible solutions for developing highly sensitive and affordable detection tools for food analysis. Nanomaterials based aptasensors hold great potential to overcome the drawbacks of conventional analytical techniques. Aptamers comprise a novel class of highly specific bio-recognition elements which are produced by SELEX (systematic evolution of ligands by exponential enrichment) process. They bind to target molecules by folding into 3D structures that can discriminate different chiral compounds. The flexibility in making modifications in aptamers contribute to the design of biosensors, enabling the generation of bio-recognition elements for a wide variety of target molecules. Nanomaterials such as metal nanoparticles, metal nanoclusters, metal oxide nanoparticles, metal and carbon quantum dots, graphene, carbon nanotubes and nanocomposites enable higher sensitivity by signal amplification and introduce several novel transduction principles such as enhanced chemiluminescence, fluorescence, Raman signals, electrochemical signals, enhanced catalytic activity, and super-paramagnetic properties to the biosensor. Although there are a few reviews published recently which deal with the potential of aptamers in various fields, none are devoted exclusively to the potential of aptasensors based on nanomaterials for the analysis of food contaminants. Hence, the current review discusses several transduction systems and their principles used in aptamer based nanosensors which have been developed in the past five years, the challenges faced in their designing, along with their strengths and limitations.

Resonance Rayleigh scattering method for determination of 2-mercaptobenzothiazole using gold nanoparticles probe

A sensitive, simple and novel method was developed to determine 2-mercaptobenzothiazole (2MBT) in water samples. This method was based on the interaction between gold nanoparticles (AuNPs) and 2MBT followed by increasing of the resonance Rayleigh scattering (RRS) intensity of nanoparticles. The change in RRS intensity (ΔIRRS) was linearly correlated to the concentration of 2MBT over the ranges of 5.0-100.0 and 100.0-300.0 μg L(-1). 2MBT can be measured in a short time (5 min) without any complicated or time-consuming sample pretreatment process. Parameters that affect the RRS intensities such as pH, concentration of AuNPs, standing time, electrolyte concentration, and coexisting substances were systematically investigated and optimized. Interference tests showed that the developed method has a very good selectivity and could be used conveniently for determination of 2MBT. The limit of detection (LOD) and limit of quantification (LOQ) were 1.0 and 3.0 μg L(-1), respectively. Relative standard deviations (RSD) for 20.0 and 80.0 μg L(-1) of 2MBT were 1.1 and 2.3, respectively. Possible mechanisms for the RRS changes of AuNPs in the presence of 2MBT were discussed and the method was successfully applied for the analysis of real water samples.

One-step, room temperature, colorimetric melamine sensing using an in-situ formation of silver nanoparticles through modified Tollens process

We have developed a rapid, sensitive, one-step, and selective colorimetric detection method for melamine (MEL) in milk powder based upon an in-situ formation of silver nanoparticles (AgNPs) through modified Tollens process at room temperature. The triazine ring N atoms of MEL molecule were strategically designed to complex the Ag(+) through electron donor-acceptor interaction. During the AgNPs formation procedure, the MEL molecule, which has been covalently bonded with the Ag(+) ions, was adsorbed to the surface of as-prepared AgNPs, resulting in the aggregation of the adjacent AgNPs with detectable decreases of absorption signal. The concentration of MEL can be determined with the naked eye or a UV-vis spectrometer at which the yellow-to-brown color change associated with aggregate enhancement takes place. This method enables rapid (less than 30 min) and sensitive (limit of detection, LOD, 10 nM) detection, and it was also able to discriminate MEL from sixteen other milk relevant coexisting compounds. This assay does not utilize organic cosolvents, enzymatic reactions, light-sensitive dye molecules, lengthy protocols, or sophisticated instrumentation thereby overcoming some of the limitations of conventional methods.

A simple resonance Rayleigh scattering method for determination of trace CA125 using immuno-AuRu nanoalloy as probe via ultrasonic irradiation

AuRu nanoalloy (GR) with Au/Ru molar ratio of 32/1 was prepared by the sodium borohydride reduction method. It was used to label the CA125 antibody (Ab) to obtain an immunonanoprobe (GRAb) for cancer antigen 125 (CA125). In pH 7.0 citric acid-Na2HPO4 buffer solution and irradiation of ultrasound, the probes were aggregated nonspecifically to big clusters that showed a strong resonance Rayleigh scattering (RRS) peak at 278 nm. Upon addition of CA125, GRAb reacted specifically with CA125 to form dispersive immunocomplexes of CA125-GRAb in the solution and this process enhanced by the ultrasonic cavitation effect, which led to the RRS intensity decreased greatly. The decreased RRS intensity was linear to the concentration of CA125 in the range of 1.3-80 U/mL, with a detection limit of 0.6 U/mL. The proposed method was applied to detect CA125 in real sample, with satisfactory results.

A nanogold resonance Rayleigh scattering method for determination of trace As based on the hydride nanoreaction

In H2 SO4 solution, As(III) was reduced to arsine (AsH3 ) by NaBH4 , and was absorbed in HAuCl4 solution to form nanogold particles (NGs) that exhibited a resonance Rayleigh scattering (RRS) effect at 370 nm. Under the selected conditions, when the As(III) concentration increased the RRS peak also increased due to the formation of more NGs. There was a linear correlation between RRS intensity and As(III) concentration in the range 6-1000 ng/mL, with a detection limit of 3 ng/mL. This new hydride generation-nanogold reaction RRS (HG-NG RRS) method was applied to determine trace amounts of As in milk samples, with satisfactory results.

Aptamer-based technology for food analysis

Aptamers are short and functional single-stranded oligonucleotide sequences selected from systematic evolution of ligands by exponential enrichment (SELEX) process, which have the capacity to recognize various classes of target molecules with high affinity and specificity. Various analytical aptamers acquired by SELEX are widely used in many research fields, such as medicine, biology, and chemistry. However, the application of this innovative and emerging technology to food safety is just in infant stage. Food safety plays a very important role in our daily lives because varieties of poisonous and harmful substances in food affect human health. Aptamer technique is promising, which can overcome many disadvantages of existing detection methods in food safety, such as long detection time, low sensitivity, difficult, and expensive antibody preparation. This review provides an overview of various aptamer screening technologies and summarizes the recent applications of aptamers in food safety, and future prospects are also discussed.

Aptamer and its potential applications for food safety

Accompanied by industrial globalization, rapid urbanization, and population increment, mass production and staple trading for food consumption are upsoaring continuously, foodborne disease resulted from various food safety issues is currently a crucial public health concern worldwide, which has not only created a great burden on both economy and society, but also greatly threatened the sustainability of mankind's livelihood and human reproduction. In order to better ensure food safety and thus effectively curb the occurrence of foodborne diseases, the development and evolving of inspection strategies are indispensable measures for quality assurance and conformity assessment. Nowadays, as complementary measures to and with advantageous merits over classic analytical methods, highly specific and selective aptamer-based assays have found their increasingly important roles in various domains of food analysis. This critical review summarizes the advantages of aptamer as compared with antibody, introduces important evolving variants of systematic evolution of ligands by exponential enrichment (SELEX), and presents an overview of potential aptamer applications for food safety.

Escherichia coli growth and transport in the presence of nanosilver under variable growth conditions

Nanosilver (silver nanoparticles) has the ability to anchor to the bacterial cell membrane and subsequently penetrate it, thereby causing structural changes (i.e., permeability) in the cell membrane and death of the cell. The bacterial responses to the presence of nanosilver usually vary depending on the concentration of nanosilver particles, exposure time and the bacterial physiological stage. Since bacterial anabolism dependents upon a stoichiometric ratio of carbon and inorganic elements (nutrients), the macronutrient ratio, i.e. carbon to nitrogen ratio (C/N) thus plays an important role of bacterial responses to the exposure of nanosilver. This study investigated the responses of Escherichia coli to the exposure of nanosilver under variable growth conditions. It was discovered that E. coli grown under different growth conditions had different responses to the presence of nanosilver. E. coli had least resistance to the toxicity of nanosilver when cultured under carbon-limited conditions. However, the presence of rhamnolipid, a commonly utilized biosurfactant for soil remediation increased the resistance of E. coli to nanosilver. The transport of E. coli cultured under carbon-limited conditions was further studied in silica sand columns. E. coli adsorption in silica sand increased when cultured in the presence of nanosilver. On the contrary, E. coli adsorption in silica sand was significantly reduced when cultured in the presence of rhamnolipid.

A new silver nanorod SPR probe for detection of trace benzoyl peroxide

The stable silver nanorod (AgNR) sol in red was prepared by the two-step procedure of NaBH₄-H₂O₂ and citrate heating reduction. The AgNR had a transverse and a longitudinal surface plasmon resonance (SPR) absorption peak at 338 nm and 480 nm. Meanwhile, two transverse and longitudinal SPR Rayleigh scattering (SPR-RS) peaks at 340 nm and 500 nm were observed firstly using common fluorescence spectrometer. The SPR absorption, RS, surface enhanced Raman scattering (SERS) and electron microscope technology were used to study the formation mechanism of red silver nanorods and the SERS enhancement mechanism of nano-aggregation. The AgNR-BPO SPR absorption and AgNR-NaCl-BPO SPR-RS analytical systems were studied to develop two new simple, rapid, and low-cost SPR methods for the detection of trace BPO.

Autocatalytic oxidization of nanosilver and its application to spectral analysis

The stable yellow nanosilver (AgNP) and blue nanosilver (AgNPB) sols were prepared by the NaBH₄ procedure. The new nanocatalytic reaction of AgNP-NaCl-H₂O₂ was investigated by surface plasmon resonance (SPR) absorption, resonance Rayleigh scattering (RRS), surface-enhanced Raman scattering (SERS) and scanning electron microscope (SEM) techniques. The autocatalytic oxidization of Ag on AgNP surface by H₂O₂ was observed firstly and the AgNP/AgCl nanoparticles were characterized. The [Ag⁺] from AgNP is different to the Ag⁺ from AgNO₃ that adsorb on the AgNP surface. An autocatalytic oxidization mechanism was proposed to explain experimental phenomena. The relationship between the SPR absorption peaks and the RRS peaks of AgNPB was studied, and three characteristic RRS peaks called as out-of-plane quadrupole, out-of-plane dipole and in-plane dipole RRS peaks were observed firstly. Using AgNP as nanoprobe, a simple, sensitive and selective RRS method was developed for assay of H₂O₂ in the range of 2.0 × 10⁻⁸-8.0 × 10⁻⁵ mol/L.

A new resonance Rayleigh scattering spectral method for determination of O3 with victoria blue B

Ozone (O3) could be absorbed by boric acid-potassium iodide (BKI) absorbent solution to produce tri-iodine ion (I3(-)) that react with victoria blue B (VBB) to form the associated particle (VBB-I3)n and exhibited a strong resonance Rayleigh scattering (RRS) peak at 722 nm. Under the chosen conditions, the RRS peak intensity was linear with O3 concentration in the range of 0.2-50 μmol/L, with a linear regression equation of ΔI722=17.9c-45.4 and detection limit of 0.057 μmol/L. Accordingly, a simple, rapid and sensitive RRS spectral method was set up for determination of trace O3 in air, with satisfactory results.

Cerium-based M4L4 tetrahedrons containing hydrogen bond groups as functional molecular flasks for selective reaction prompting

Metal–organic tetrahedrons with abundant hydrogen bond groups work as “molecular flasks” to prompt Knoevenagel condensation and cyanosilylation reactions.

Aptamer-based analysis: a promising alternative for food safety control

Ensuring food safety is nowadays a top priority of authorities and professional players in the food supply chain. One of the key challenges to determine the safety of food and guarantee a high level of consumer protection is the availability of fast, sensitive and reliable analytical methods to identify specific hazards associated to food before they become a health problem. The limitations of existing methods have encouraged the development of new technologies, among them biosensors. Success in biosensor design depends largely on the development of novel receptors with enhanced affinity to the target, while being stable and economical. Aptamers fulfill these characteristics, and thus have surfaced as promising alternatives to natural receptors. This Review describes analytical strategies developed so far using aptamers for the control of pathogens, allergens, adulterants, toxins and other forbidden contaminants to ensure food safety. The main progresses to date are presented, highlighting potential prospects for the future.

A simple and sensitive resonance Rayleigh scattering method for determination of As(III) using aptamer-modified nanogold as a probe

A simple and selective aptamer (ssDNA)-modified nanogold probe (AussDNA) was prepared for the determination of trace As(III) in HEPES buffer solution (pH 8.2) containing 0.05 mol/L NaCl. The method coupled the aptamer reaction of AussDNA-As(III) and the resonance Rayleigh scattering (RRS) of nanogold aggregations at 278 nm. When the As(III) concentration increased, the RRS intensity at 278 nm increased to form more nanogold aggregation and a stable As(III)-ssDNA complex. Under selected conditions, the increased RRS intensity (ΔI) was linear to the concentration of As(III) in the range 3.8-230.4 ng/mL, with a detection limit of 1.9 ng/mL. This RRS method was applied to detect As(III) in water samples, with simplicity, sensitivity and selectivity.

Highly sensitive detection of melamine based on gemini surfactant using enhanced resonance Rayleigh scattering signals

We present here a resonance Rayleigh scattering (RRS) spectrum method for the determination of melamine at the nanogram level using a gemini surfactant (disodium 4-dodecyl-2,4'-oxydibenzenesulfonate, DDOF). It was found that DDOF could react with cationic melamine to form an ion-association complex, which induced the enhancement of RRS intensity and the appearance of a new RRS spectrum in acetate buffer (pH 3.6). The RRS spectral characteristics of the melamine-DDOF system, the optimum conditions of the reaction, and the influencing factors have been investigated. Under optimum conditions, the enhanced RRS intensity was proportional to the concentration of melamine in the range of 0.38-6.30 μg/mL. The method has high sensitivity, and the detection limit for melamine is 8.48 ng/mL. Furthermore, the reaction mechanism and the reasons of RRS enhancement were evaluated.