Near-Infrared Responsive Ag@Au Nanoplates with Exceptional Stability for Highly Sensitive Colorimetric and Photothermal Dual-Mode Lateral Flow Immunoassay.

Lateral flow immunoassay (LFIA) has played a vital role in point-of-care (POC) testing on account of its simplicity, rapidity, and low cost. However, the low sensitivity and difficulty of quantitation limit its further development. Sensitive markers with new detection modes are being developed to dramatically improve LFIA's performance. Herein, a ligand-complex approach was proposed to uniformly coat a thin layer of Au onto Ag triangular nanoplates (Ag TNPs) without etching the Ag cores, which not only retain the unique optical properties from Ag TNPs but also acquire the surface stability and biocompatibility of gold. The localized surface plasmon resonance absorption of these Ag@Au TNPs could be finely adjusted from visible (550 nm) to the second near-infrared region (NIR-II) (1100 nm), and even longer, by simply adjusting the ratio between edge length and thickness. Utilizing the Ag@Au TNPs as new markers for LFIA, a highly sensitive colorimetric and photothermal dual-mode detection of the SARS-CoV-2 nucleocapsid protein was achieved with a very low background. The Ag@Au TNPs showed an exceedingly high photothermal conversion efficiency of 61.4% (ca. 2 times higher than that of Au nanorods), endowing the LFIA method with a low photothermal detection limit (40 pg/mL), which was 25-fold lower than that of the colorimetric results. The generality of the method was further verified by the sensitive and accurate analysis of cardiac troponin I (cTnI). This method is robust, reproducible, and highly specific and has been successfully applied to SARS-COV-2 detection in 35 clinical samples with satisfactory results, demonstrating its potential for POC applications.

Colorimetric and photothermal dual-mode lateral flow immunoassay based on Au-Fe3O4 multifunctional nanoparticles for detection of Salmonella typhimurium.

Au-Fe3O4 multifunctional nanoparticles (NPs) were synthesized and integrated with lateral flow immunoassay (LFIA) for dual-mode detection of Salmonella typhimurium. The Au-Fe3O4 NPs not only combined excellent local surface plasmon resonance characteristics and superparamagnetic properties, but also exhibited good photothermal effect. In the detection, antibody-conjugated Au-Fe3O4 NPs first captured S. typhimurium from complex matrix, which was then loaded on the LFIA strip and trapped by the T-line. By observing the color bands with the naked eyes, qualitative detection was performed free of instrument. By measuring the photothermal signal, quantification was achieved with a portable infrared thermal camera. The introduction of magnetic separation achieved the enrichment and purification of target bacteria, thus enhancing the detection sensitivity and reducing interference. This dual-mode LFIA achieved a visual detection limit of 5 × 105 CFU/mL and a photothermal detection limit of 5 × 104 CFU/mL. Compared with traditional Au-based LFIA, this dual-mode LFIA increased the detection sensitivity by 2 orders of magnitude and could be directly applied to unprocessed milk sample. Besides, this dual-mode LFIA showed good reproducibility and specificity. The intra-assay and inter-assay variation coefficients were 3.0% and 7.9%, and with this dual-mode LFIA, other bacteria hardly produced distinguishable signals. Thus, the Au-Fe3O4 NPs-based LFIA has potential to increase the efficiency of pandemic prevention and control. Au-Fe3O4 nanoparticle proved to be a promising alternative reporter for LFIA, achieving multifunctions: target purification, target enrichment, visual qualitation, and instrumental quantification, which improved the limitations of traditional LFIA.

Ag Nanoparticles with Ultrathin Au Shell-Based Lateral Flow Immunoassay for Colorimetric and SERS Dual-Mode Detection of SARS-CoV-2 IgG.

Immunoglobulin detection is essential for diagnosing progression of SARS-CoV-2 infection, for which SARS-CoV-2 IgG is one of the most important indexes. In this paper, Ag nanoparticles with ultrathin Au shells (∼2 nm) embedded with 4-mercaptobenzoic acid (MBA) (AgMBA@Au) were manufactured via a ligand-assisted epitaxial growth method and integrated into lateral flow immunoassay (LFIA) for colorimetric and SERS dual-mode detection of SARS-CoV-2 IgG. AgMBA@Au possessed not only the surface chemistry advantages of Au but also the superior optical characteristics of Ag. Moreover, the nanogap between the Ag core and the Au shell also greatly enhanced the Raman signal. After being modified with anti-human antibodies, AgMBA@Au recognized and combined with SARS-CoV-2 IgG, which was captured by the SARS-CoV-2 spike protein on the T line. Qualitative analysis was achieved by visually observing the color of the T line, and quantitative analysis was conducted by measuring the SERS signal with a sensitivity four orders of magnitude higher (detection limit: 0.22 pg/mL). The intra-assay and inter-assay variation coefficients were 7.7 and 10.3%, respectively, and other proteins at concentrations of 10 to 20 times higher than those of SARS-CoV-2 IgG could hardly produce distinguishable signals, confirming good reproducibility and specificity. Finally, this method was used to detect 107 clinical serum samples. The results agreed well with those obtained from enzyme-linked immunosorbent assay kits and were significantly better than those of the colloidal gold test strips. Therefore, this dual-mode LFIA has great potential in clinical practical applications and can be used to screen and trace the early immune response of SARS-CoV-2.

Magnetic Relaxation Switching Immunoassay Based on Hydrogen Peroxide-Mediated Assembly of Ag@Au-Fe3 O4 Nanoprobe for Detection of Aflatoxin B1.

Magnetic relaxation switching (MRS) sensors have shown great potential in food safety monitoring due to their high signal-to-noise ratio and simplicity, but they often suffer from insufficient sensitivity and stability due to the lack of excellent magnetic nanoprobes. Herein, dumbbell-like Au-Fe3 O4 nanoparticles are designed as magnetic nanoprobes for developing an aflatoxin B1-MRS immunosensor. The Fe3 O4 portion in the Au-Fe3 O4 nanoparticles functions as the magnetic probe to provide transverse relaxation signals, while the Au segments serve as a bridge to grow Ag shell and assemble the Au-Fe3 O4 nanoparticles, thus modulating transverse relaxation time of surrounding water molecular. The formation of Ag@Au-Fe3 O4 is triggered by hydrogen peroxide. After degraded by horseradish peroxidase, hydrogen peroxide reduces Ag+ to Ag nanoparticles which assemble dispersed Au-Fe3 O4 to aggregated Ag@Au-Fe3 O4 , thus dramatically improving the sensitivity of traditional MRS sensor. Combined with competitive immunoreaction, this Ag@Au-Fe3 O4 -MRS immunosensor can detect aflatoxin B1 with a high sensitivity (3.81 pg mL-1 ), which improved about 21 folds and 9 folds than those of enzyme-linked immunosorbent assay and high-performance liquid chromatography (HPLC), respectively. The good consistency with HPLC in real samples detection indicates the good accuracy of this immunosensor. This Ag@Au-Fe3 O4 -MRS immunosensor offers an attractive tool for detection of harmful substances.

Toward ultrasensitive and fast colorimetric detection of indoor formaldehyde across the visible region using cetyltrimethylammonium chloride-capped bone-shaped gold nanorods as "chromophores".

Plasmonic nanostructures have been broadly used for chemical detections, but their applications are limited by slow detection rates, insufficient visual resolution and sensitivity due to the chemical and structural stability of conventional plasmonic nanomaterials. It is thus essential to develop strategies to enhance the detection kinetics while promoting their excellent plasmonic properties. In this work, a colorimetric assay for HCHO measurement is developed based on the fact that HCHO can react with Tollens' reagent to anisotropically deposit a layer of silver shells onto the bone-shaped gold nanorod (Au NR) cores. Compared to the routine rod-shaped Au NRs, the bone-shaped Au NRs facilitate the deposition of Ag onto the sunken section due to their unique concave structures, giving rise to fast reaction kinetics and detection rate. It is also important to point out that the surface ligand exchange from CTAB to CTAC is helpful to accelerate the deposition of silver onto Au NRs, which significantly shortens the reaction time. The preferential deposition of Ag on the concave Au NRs induces more dramatic morphology changes and therefore promotes the plasmonic shift of the bone-shaped Au NRs and improves the sensing efficiency. Correspondingly, the apparent color of the solution changes from light gray to dark blue, purple, red, orange and finally to yellow as the longitudinal localized surface plasmon resonance (LSPR) band shifts from 710 to 500 nm along with the emergence of a new LSPR band at 400 nm almost covering the full visible region. The colorimetric method developed enables sensitive detection of HCHO with a low detection limit (1 nM), wide linear range (0.1-50 µM), high visual resolution and good specificity against other common indoor gases. It was successfully applied to the detection of gaseous HCHO present in the air collected from a furniture plaza, showing its potential practicality for on-site HCHO analysis.

Aptamer-functionalized magnetic and fluorescent nanospheres for one-step sensitive detection of thrombin.

A one-step sandwich method is described for detecting proteins with magnetic nanospheres (MNs) and fluorescent nanospheres (FNs). Thrombin is selected as a model analyte to validate the method. Two DNA aptamers (Apt 29 and Apt 15 targeting two different exosites of thrombin) are chosen as recognition elements to modify MNs and FNs. The superparamagnetic MN-Apt 29 conjugate is used to separate and concentrate thrombin. The FN-Apt 15 conjugate encapsulates hundreds of fluorescent quantum dots and is used as reporter to provide a stable signal. Magnetic capture and fluorescence identification are performed simultaneously to form a sandwich complex (MN-Apt 29-thrombin-FN-Apt 15) for fluorescence determination (at excitation/emission wavelengths of 380/622 nm). The method is convenient, time saving, and gives a strong signal (compared to the two-step method where capture and identification are performed in two steps). The one-step method presented here is completed within 30 min and has a 3.5 ng·mL-1 (97 pM) detection limit. The method is reproducible, has an intra-assay variability of 1.5%, and an inter-assay variability of 4.9%. Other serum proteins (HSA, CEA, PSA, and AFP) do not interfere. The method was also applied to analyze serum samples. Almost the same fluorescence intensity was measured when analyzing 1% serum samples (compared to buffer samples). Graphical abstract Magnetic nanospheres with excellent superparamagnetic property and fluorescent QD-based nanospheres were prepared and used in a one-step sensitive method for detecting thrombin. The method exhibits good reproducibility, high specificity, and good selectivity.

Determination of the Absolute Number Concentration of Nanoparticles and the Active Affinity Sites on Their Surfaces.

Number concentration of nanoparticles is a critical and challenging parameter to be identified. Recently, gravimetric strategy is a fundamental method for absolute quantification, which is widely accepted and used by researchers, yet limited by the inaccuracy in measuring related parameters (e.g, density). Hence, we introduced isopycnic gradient centrifugation to determine the nanopartices' density and improved the current gravimetric method for more accuracy. In this work, polymer nanospheres were used as a model to validate this method. Through isopycnic gradient centrifugation, nanospheres finally reached the zone of equal density as them. By measuring the density of the medium solution in this zone, the nanospheres' density was identified. Then, the density was multiplied by the volume of a single nanosphere characterized by transmission electron microscopy (TEM), and the average weight of a single nanosphere was obtained. Using total weight of the nanospheres divided by the unit weight, their number concentration was quantified. Directly using the real density of the nanoparticles achieved more accurate quantification than the current gravimetric method which used the density of the bulk material counterparts for calculation. Besides, compared with the viscosity/light scattering method and the high-sensitivity flow cytometry (HSFCM) method (another two kinds of typical methods respectively based on light measurements and single particle counting), the improved gravimetric method showed better reproducibility and more convenience. Further, we modified the nanospheres with streptavidin (SA) and antibody, and through biorecognition interaction, we determined the amount of the active affinity sites on each biofunctional nanosphere. Moreover, their bioactivity in different storage conditions was monitored, which showed good stability even in PBS at 4 °C over one year. Our work provided a promising method for more accurately determining the absolute number concentration of nanoparticles and the active affinity sites on their surfaces, which would greatly facilitate their downstream applications.

Sensitive and Quantitative Detection of C-Reaction Protein Based on Immunofluorescent Nanospheres Coupled with Lateral Flow Test Strip.

Sensitive and quantitative detection of protein biomarkers with a point-of-care (POC) assay is significant for early diagnosis, treatment, and prognosis of diseases. In this paper, a quantitative lateral flow assay with high sensitivity for protein biomarkers was established by utilizing fluorescent nanospheres (FNs) as reporters. Each fluorescent nanosphere (FN) contains 332 ± 8 CdSe/ZnS quantum dots (QDs), leading to its superstrong luminescence, 380-fold higher than that of one QD. Then a detection limit of 27.8 pM C-reaction protein (CRP) could be achieved with an immunofluorescent nanosphere (IFN)-based lateral flow test strip. The assay was 257-fold more sensitive than that with a conventional Au-based lateral flow test strip for CRP detection. Besides, the fluorescence intensity of FNs and bioactivity of IFNs were stable during 6 months of storage. Hence, the assay owns good reproducibility (intra-assay variability of 5.3% and interassay variability of 6.6%). Furthermore, other cancer biomarkers (PSA, CEA, AFP) showed negative results by this method, validating the excellent specificity of the method. Then the assay was successfully applied to quantitatively detect CRP in peripheral blood plasma samples from lung cancer and breast cancer patients, and healthy people, facilitating the diagnosis of lung cancer. It holds a good prospect of POC protein biomarker detection.

Rapid and Quantitative Detection of Avian Influenza A(H7N9) Virions in Complex Matrices Based on Combined Magnetic Capture and Quantum Dot Labeling.

Avian influenza A(H7N9) virus, which emerged in China in the spring of 2013, has infected hundreds of people and resulted in many deaths. Herein, a rapid and quantitative assay is proposed for the one-step detection of H7N9 virions. Immunomagnetic nanospheres (IMNs) and antibody-conjugated quantum dots (Ab-QDs) are simultaneously employed to capture and identify the target virus, leading to a high efficiency, good specificity, and strong anti-interference ability. Moreover, this reliable detection assay, which combines the efficient magnetic enrichment and the unique photophysical properties of QDs, can achieve a high sensitivity for a low detection limit. At the same time, this detection strategy shows great flexibility for employment in a variety of fluorescence detectors, including fluorescence spectrometry, microscope assays, and handheld UV lamp tests. Furthermore, our one-step detection strategy induces very little change in the integrity of the vulnerable virions, which enables additional genotyping testing following the fluorescence detection. The present study, thus, reports a rapid and quantitative approach for the detection of H7N9 virions based on simultaneous magnetic capture and QD labeling, thereby providing a higher probability for detection and therefore faster diagnosis of H7N9-infected patients.

Recognition kinetics of biomolecules at the surface of different-sized spheres.

Bead-based assay is widely used in many bioanalytical applications involving the attachment of proteins and other biomolecules to the surface. For further understanding of the formation of a sphere-biomolecule complex and easily optimizing the use of spheres in targeted biological applications, it is necessary to know the kinetics of the binding reaction at sphere/solution interface. In our presented work, a simple fluorescence analysis method was employed to measure the kinetics for the binding of biotin to sphere surface-bound FITC-SA, based on the fact that the fluorescence intensity of FITC was proportionally enhanced by increasing the binding amount of biotin. By monitoring the time-dependent changes of FITC fluorescence, it was found that the binding rate constant of biotin to sphere surface-immobilized FITC-SA was much smaller than that of biotin to freely diffusing FITC-SA. This can be attributed to the decreased encounter frequency of the reaction pair, restricted motion of the attached biomolecule, and the weakened steric accessibility of the binding site. These factors would become more obvious when increasing the size of the sphere upon which the FITC-SA was immobilized. Additionally, the effect of nanoparticles on the diffusion-controlled bimolecular binding reaction was more evident than that on the chemical recognition-controlled binding reaction.

Engineered decomposable multifunctional nanobioprobes for capture and release of rare cancer cells.

Early detection and isolation of circulating tumor cells (CTCs) can provide helpful information for diagnosis, and functional readouts of CTCs can give deep insight into tumor biology. In this work, we presented a new strategy for simple isolation and release of CTCs using engineered nanobioprobes. The nanobioprobes were constructed by Ca(2+)-assisted layer-by-layer assembly of alginate onto the surface of fluorescent-magnetic nanospheres, followed by immobilization of biotin-labeled anti-EpCAM. As-prepared anti-EpCAM-functionalized nanobioprobes were characterized with integrated features of anti-EpCAM-directed specific recognition, fluorescent magnetic-driven cell capture, and EDTA-assisted cell release, which can specifically recognize 10(2) SK-BR-3 cells spiked in 1 mL of lysed blood or human whole blood samples with 89% and 86% capture efficiency, respectively. Our proof-of-concept experiments demonstrated that 65% of captured SK-BR-3 cells were released after EDTA treatment, and nearly 70% of released SK-BR-3 cells kept their viability, which may facilitate molecular profiling and functional readouts of CTCs.

Optically encoded multifunctional nanospheres for one-pot separation and detection of multiplex DNA sequences.

In this study, we report a simple method for simultaneous detection of multiplex DNA sequences, including complementary DNA (cDNA) sequences of HIV and HCV, DNA sequence of HBV, with QDs-encoded fluorescent nanospheres and nano-γ-Fe2O3-coated magnetic nanospheres. Detection was achieved on a fluorescence spectrophotometer without additional auxiliary instruments, and the detection limit was about 100 pM. Here, QDs-encoded fluorescent nanospheres (FNS) with different photoluminescent properties, and magnetic nanospheres (MNS) were separately fabricated by stepwise assembly of hydrophobic QDs or nano-γ-Fe2O3 on the surface of branched poly(ethylene imine) (PEI)-coated nanospheres in precisely controlled amounts, finally followed by silica encapsulation. FNS-labeled probe DNAs and MNS-labeled capture DNAs were used to hybridize with the corresponding targets at the same time. After magnetic separation, the sandwich-structured adducts were measured by fluorescence spectrophotometry. The results indicated that the targets could be detected with high sensitivity. This method is convenient, fast enough, and capable of high anti-interference. Therefore, it is expected to be used for simultaneous detection and separation of multiple targets at high levels of purity and throughput.

One-step sensitive detection of Salmonella typhimurium by coupling magnetic capture and fluorescence identification with functional nanospheres.

Sensitive, rapid, and reliable detection of bacteria has always been pursued due to the great threat of the bacteria to human health. In this study, a convenient one-step strategy for detecting Salmonella typhimurium was developed. Immunomagnetic nanospheres (IMNS) and immunofluorescent nanospheres (IFNS) were used to specifically capture and recognize S. typhimurium simultaneously. After magnetic separation, the sandwich immune complexes (IMNS-bacteria-IFNS) were detected under a fluorescence microscope with a detection limit as low as ca. 10 CFU/mL. When they were detected by fluorescence spectrometer, a linear range was exhibited at the concentration from 10(5) to 10(7) CFU/mL with R(2) = 0.9994. Compared with the two-step detection strategy, in which the bacteria were first captured with the IMNS and subsequently identified with the IFNS, this one-step strategy simplified the detection process and improved the sensitivity. Escherichia coli and Shigella flexneri both showed negative results with this method, indicating that this method had excellent selectivity and specificity. Moreover, this method had strong anti-interference ability, and it had been successfully used to detect S. typhimurium in synthetic samples (milk, fetal bovine serum, and urine), showing the potential application in practice.

High-density Au nanoshells assembled onto Fe3O4 nanoclusters for integrated enrichment and photothermal/colorimetric dual-mode detection of SARS-CoV-2 nucleocapsid protein.

Traditional lateral flow immunoassays (LFIA) suffer from insufficient sensitivity, difficulty for quantitation, and susceptibility to complex substrates, limiting their practical application. Herein, we developed a polyethylenimine (PEI)-mediated approach for assembling high-density Au nanoshells onto Fe3O4 nanoclusters (MagAushell) as LFIA labels for integrated enrichment and photothermal/colorimetric dual-mode detection of SARS-CoV-2 nucleocapsid protein (N protein). PEI layer served not only as "binders" to Fe3O4 nanoclusters and Au nanoshells, but also "barriers" to ambient environment. Thus, MagAushell not only combined magnetic and photothermal properties, but also showed good stability. With MagAushell, N protein was first separated and enriched from complex samples, and then loaded to the strip for detection. By observation of the color stripes, qualitative detection was performed with naked eye, and by measuring the temperature change under laser irradiation, quantification was attained free of sophisticated instruments. The introduction of Fe3O4 nanoclusters facilitated target purification and enrichment before LFIA, which greatly improved the anti-interference ability and increased the detection sensitivity by 2 orders compared with those without enrichment. Moreover, the high loading density of Au nanoshells on one Fe3O4 nanocluster enhanced the photothermal signal of the nanoprobe significantly, which could further increase the detection sensitivity. The photothermal detection limit reached 43.64 pg/mL which was 1000 times lower than colloidal gold strips. Moreover, this method was successfully applied to real samples, showing great application potential in practice. We envision that this LFIA could serve not only for SARS-CoV-2 detection but also as a general test platform for other biotargets in clinical samples.

An achromatic colorimetric nanosensor for sensitive multiple pathogen detection by coupling plasmonic nanoparticles with magnetic separation.

Rapid screening of multiple pathogens will greatly improve the efficiency of pandemic prevention and control. Colorimetric methods exhibit the advantages of convenience, portability, low cost, time efficiency, and free of sophisticated instruments, yet usually have difficulties in simultaneous detection and suffer from monotonous color changes with low visual resolution and sensitivity. Hence, coupled three kinds of plasmonic nanoparticles (NPs) with magnetic separation, we developed an achromatic colorimetric nanosensor with highly enhanced visual resolution for simultaneous detection of SARS-CoV-2, Staphylococcus aureus, and Salmonella typhimurium. The achromatic nanosensor was composed of SARS-CoV-2-targeting red gold NPs, S. aureus-targeting yellow silver NPs and S. typhimurium-targeting blue silver triangle NPs mixed as black color. In the detection, three corresponding magnetic probes were added into the above mixture. In the presence of a target pathogen, it would be recognized and combined with corresponding colored reporters and magnetic probes to form sandwich complexes, which were removed by magnetic separation, and the sensor changed from black to a chromatic color (the color of the reporters remained in supernatant). Consequently, different target pathogen induced different color. For example, SARS-CoV-2, S. aureus, and S. typhimurium respectively produced green, purple, and orange. While coexistence of S. aureus and S. typhimurium produced red, and coexistence of S. aureus and SARS-CoV-2 produced blue, etc. Therefore, by observing the color change or measuring the absorption spectra, multiple pathogen detection was achieved conveniently. Compared with most colorimetric sensors, this achromatic nanosensor involved rich color change, thus significantly enhancing visual resolution and inspection sensitivity. Therefore, this sensor opened a promising avenue for efficient monitoring and early warning of food safety and quality.

Fluorescent-magnetic dual-encoded nanospheres: a promising tool for fast-simultaneous-addressable high-throughput analysis.

Bead-based optical encoding or magnetic encoding techniques are promising in high-throughput multiplexed detection and separation of numerous species under complicated conditions. Therefore, a self-assembly strategy implemented in an organic solvent is put forward to fabricate fluorescent-magnetic dual-encoded nanospheres. Briefly, hydrophobic trioctylphosphine oxide-capped CdSe/ZnS quantum dots (QDs) and oleic acid-capped nano-γ-Fe2O3 magnetic particles are directly, selectively and controllably assembled on branched poly(ethylene imine)-coated nanospheres without any pretreatment, which is crucial to keep the high quantum yield of QDs and good dispersibility of γ-Fe2O3. Owing to the tunability of coating amounts of QDs and γ-Fe2O3 as well as controllable fluorescent emissions of deposited-QDs, dual-encoded nanospheres with different photoluminescent emissions and gradient magnetic susceptibility are constructed. Using this improved layer-by-layer self-assembly approach, deposition of hydrophobic nanoparticles onto hydrophilic carriers in organic media can be easily realized; meanwhile, fluorescent-magnetic dual-functional nanospheres can be further equipped with readable optical and magnetic addresses. The resultant fluorescent-magnetic dual-encoded nanospheres possess both the unique optical properties of QDs and the superparamagnetic properties of γ-Fe2O3, exhibiting good monodispersibility, huge encoding capacity and nanoscale particle size. Compared with the encoded microbeads reported by others, the nanometre scale of the dual-encoded nanospheres gives them minimum steric hindrance and higher flexibility.

Au-Fe3O4 heterodimer multifunctional nanoparticles-based platform for ultrasensitive naked-eye detection of Salmonella typhimurium.

In this work, we developed an ultrasensitive colorimetry for Salmonella typhimurium detection with multifunctional Au-Fe3O4 dumbbell-like nanoparticles (DBNPs) which possessed easy bio-modifiability, excellent LSPR characteristics, superparamagnetic properties and super peroxidase-like activity. In the detection, the anti-S. typhimurium antibody modified DBNPs (IDBNPs) bound with S. typhimurium and aggregated on their surfaces in a large number, which showed much quicker magnetic response than free IDBNPs. By controlling appropriate separation conditions, IDBNPs@S. typhimurium composites were captured, while free IDBNPs were remained in the supernatant. Therefore, by detecting the absorbance of the supernatant, quantitative detection was achieved from 10 to 1000 CFU/mL. Moreover, utilizing the peroxidase-like activity of IDBNPs, we further realized semi-quantitative naked-eye detection. By adding ABTS into the above supernatant, which was oxidized to green chelate (OxABTS), colorimetric signal was amplified significantly, and meanwhile, the green chelates and the wine-red IDBNPs engendered mixed color, enhancing the range of color gradation and greatly improving the visual resolution. Finally, a detection limit (10 CFU/mL) comparable with that of above spectrum measurement was achieved. Besides, our method exhibited efficient capture capability (nearly 100% even for rare S. typhimurium), and had good stability and specificity, and acceptable anti-interference ability in fetal bovine serum and milk samples.

Noble metal nanomaterial-based aptasensors for microbial toxin detection.

Microbial toxins generated by bacteria, fungi and algae cause serious food-safety problems due to the frequent contamination of foodstuffs and their poisonous nature. Becoming acquainted with the contamination condition of foodstuffs is highly dependent on developing sensitive, specific, and accurate methods for targeting microbial toxins. Aptamers, obtained from systematic evolution of ligands by exponential enrichment (SELEX), have significant advantages for microbial toxin analysis, such as small size, reproducible chemical synthesis, and modification, as well as high binding affinity, specificity, and stability. Besides, aptamers have a predictable structure and can be tailored using biomolecular tools (e.g., ligase, endonuclease, exonuclease, polymerase, and so on), which is conducive to the development of flexible and variable amplification methods. Recent studies revealed that the combination of aptamers and noble metal nanomaterials offers unprecedented opportunities for microbial toxin detection. Noble metal nanomaterials with outstanding physical and chemical properties facilitate the detection process and improve the sensitivity and specificity. In this review, we discuss current progress in the development of various noble metal nanomaterial-based aptasensors for microbial toxin detection. These noble metal nanomaterials include gold nanoparticles, gold nanorods, gold nanoclusters, silver nanoparticles, silver nanoclusters, and bimetallic nanomaterials. Aptasensors based on noble metal nanomaterials exhibiting high selectivity and sensitivity represent a promising tool for microbial toxin detection.

Magnetic nanospheres for convenient and efficient capture and release of hepatitis B virus DNA.

Nucleic acid isolation and purification are essential steps in molecular biology. Currently-used isolation methods focus on the extraction of all the nucleic acids from crude samples, yet ignore the specific nucleic acids of interest, which may induce the loss of the specific nucleic acids and hinder their analyses. Herein, a magnetic nanospheres (MNs)-based strategy for efficient capture and release of specific nucleic acids is developed. The DNA sequence of hepatitis B virus (HBV) is taken as a model to validate this method. The MNs are modified with the complementary strand of HBV DNA for specific capture based on hybridization reaction. Then, by melting at high temperature, the captured DNAs are detached from the MNs to achieve release. The capture and release process are performed conveniently with magnetic separation. High capture efficiency (over 80%) and nearly 100% release efficiency for HBV DNA are achieved respectively via 40 min and 5 min interaction. While non-target DNAs are hardly captured, indicative of good selectivity. Moreover, after releasing DNAs, the MNs are directly regenerated and can be reused without degrading performance, which greatly reduces the operation costs. Finally, this method is applied to serum samples without any pretreatment, which exhibits similar capture and release capacity with those in the ideal samples, indicating its great application potential in practice.

Electro-enhanced solid-phase microextraction of bisphenol A from thermal papers using a three-dimensional graphene coated fiber.

Three-dimensional (3-D) graphene was synthesized by the assembly of graphene oxide with phenolic resin, followed by carbonization in argon. The as-synthesized 3-D graphene has excellent conductivity, good thermal stability, large specific surface area (1511 m² g-1) and pore volume (0.90 cm3 g-1). By immobilizing the 3-D graphene onto the stainless steel wire (SSW), we obtained a 3-D graphene coated fiber that was then used as a working electrode for electro-enhanced SPME, which shows a 3.2-fold improvement of extraction efficiency for bisphenol A (BPA) over that of traditional SPME. Coupled to gas chromatography, the method was developed to the determination of BPA with good linearity (R2 = 0.9935) in the range of 0.1-10 µg mL-1. The limit of detection was calculated to be 0.006 µg mL-1 based on the signal-to-noise of 3. The proposed method was applied for the analysis of three kinds of thermal papers with BPA being detected in all samples (0.696-3.78 mg g-1). Recovery tests were performed to validate the reliability of the method, and the recoveries were found between 81.9% and 119% with relative standard deviations lower than 4.8%.