Detection of lysozyme magnetic relaxation switches based on aptamer-functionalized superparamagnetic nanoparticles

A sensitive Ag+-mediated magnetic relaxation and colorimetry dual-mode sensing platform

To address the relatively low sensitivity of current redox reagent-mediated magnetic relaxation sensing methods, we present a novel Ag+-mediated magnetic sensing platform that enhances the sensitivity by three orders of magnitude. The new sensing platform is based on Ag+-catalyzed oxidation of Mn2+ to KMnO4, accompanied by a distinct color change, which facilitates colorimetric detection. In the case of insufficient Ag+ ions, MnO2 is an additional oxidation product and the KMnO4/MnO2 ratio is dependent on the concentration of Ag+. When combined with a specific quantity of reducing agent, both KMnO4 and MnO2 are reduced to Mn2+ with a large relaxivity, and the concentration of Mn2+ in the resultant solution inversely correlates with the amount of KMnO4 since KMnO4 consumes more reductant during reduction. Consequently, the transverse relaxation rate of the solution exhibits a negative correlation with the Ag+ concentration. Thus, by coupling this Ag+-mediated Mn2+ to KMnO4 transformation with reactions that modulate Ag+ concentration, a dual-mode sensing platform for magnetic relaxation and colorimetry can be realized. Herein, we take H2O2 as an example to verify the detection performance of this sensing platform since H2O2 can oxidize Ag0 in Ag@Fe3O4 nanoparticles to Ag+. Experimental findings demonstrate detection limits of 10 nM and 20 nM for the magnetic relaxation and colorimetry modes, respectively, affirming the excellent sensitivity and the potential practical application of this strategy.

Targeted-activation superparamagnetic spherical nucleic acid nanomachine for ultrasensitive SERS detection of lysozyme based on a bienzymatic-mediated in situ amplification strategy

Lysozyme (LYS) is a widely used bacteriostatic enzyme. In this paper, we built a sensitive and accurate Raman biosensing platform to detect trace amounts of LYS. The method is based on magnetic spherical nucleic acid formed by a combination of LYS aptamer (Apt) and magnetic beads (MBs). Meanwhile, this method utilizes a dual enzyme-assisted nucleic acid amplification circuit and surface-enhanced Raman scattering (SERS). In this sensing strategy, which is based on the specific recognition of Apt, magnetic spherical nucleic acids were associated with SERS through a nucleic acid amplification circuit, and the low abundance of LYS was converted into a high-specificity Raman signal. Satellite-like MB@AuNPs were formed in the presence of the target, which separated specifically in a magnetic field, effectively avoided the interference of complex sample environment. Under the optimal sensing conditions, the concentration of LYS exhibited a good linear relationship between 1.0 × 10-14 and 5.0 × 10-12 M and the limit of detection was as low as 8.3 × 10-15 M. In addition, the sensor strategy shows excellent accuracy and sensitivity in complex samples, providing a new strategy for the specific detection of LYS.

Magnetic relaxation switch sensor based on aptamer-modified poly-L-lysine-ferroferric oxide magnetic nanoparticles and graphene oxide for the determination of insecticides in vegetables

A simple and effective graphene oxide-magnetic relaxation switch (GO-MRS) sensor that combines graphene oxide (GO) and aptamer-modified poly-L-lysine(PLL)-Fe₃O₄ nanoparticles (Fe₃O₄@PLL-Apt NPs) was designed for the detection of acetamiprid (ACE). In this sensor, Fe₃O₄@PLL-Apt NPs acted as a relaxation signal probe and GO facilitated the generation of relaxation signal changes (dispersion/aggregation shift), while the aptamer is a molecular component that recognizes ACE. This GO-assisted magnetic signal probe improves the stability of magnetic nanoparticles in solution and enhances their sensitivity to small molecules while avoiding cross-reactions. Under optimal conditions, the sensor exhibits a wide working range (10-80 nM) and low detection limit (8.43 nM). The spiked recoveries ranged from 96.54 to 103.17%, with a relative standard deviation (RSD) of less than 2.3%. In addition, the performance of the GO-MRS sensor matched that of the standard method (liquid chromatography-mass spectrometry (LC-MS)), indicating that the GO-MRS sensor is suitable for the detection of ACE in vegetables.

Recent Advances in Biomolecular Detection Based on Aptamers and Nanoparticles

The fast, accurate detection of biomolecules, ranging from nucleic acids and small molecules to proteins and cellular secretions, plays an essential role in various biomedical applications. These include disease diagnostics and prognostics, environmental monitoring, public health, and food safety. Aptamer recognition (DNA or RNA) has gained extensive attention for biomolecular detection due to its high selectivity, affinity, reproducibility, and robustness. Concurrently, biosensing with nanoparticles has been widely used for its high carrier capacity, stability and feasibility of incorporating optical and catalytic activity, and enhanced diffusivity. Biosensors based on aptamers and nanoparticles utilize the combination of their advantages and have become a promising technology for detecting of a wide variety of biomolecules with high sensitivity, reliability, specificity, and detection speed. Via various sensing mechanisms, target biomolecules have been quantified in terms of optical (e.g., colorimetric and fluorometric), magnetic, and electrical signals. In this review, we summarize the recent advances in and compare different aptamer-nanoparticle-based biosensors by nanoparticle types and detection mechanisms. We also share our views on the highlights and challenges of the different nanoparticle-aptamer-based biosensors.

Magnetic Relaxation Switch Sensor Based on Magnetophoresis and "T-Hg(II)-T" Signal Amplification

In this study, we designed a magnetic relaxation switch (MRS) sensor combined with magnetophoresis technology (MS-MRS), which helps solve the problems of traditional MRS sensors. The sensor is based on a new combined magnet and is composed of small magnetic blocks and iron sheets that can rapidly separate magnetic nanoparticles of different sizes within 5 min. The MS-MRS sensor consists of aptamer-functionalized magnetic nanoparticles (diameter: 200 nm) (MNP₂₀₀-Apt), complementary DNA-functionalized magnetic nanoparticles (diameter: 20 nm) (MNP₂₀-cDNA), and a combined magnet ("M2" magnet). The MNP₂₀₀-Apt probe could be separated by an "M2" magnet but the MNP₂₀-cDNA probe could not. To further improve the sensitivity of the sensor, we successfully constructed an MS-MRS-Hg sensor based on the "T-Hg(II)-T" specific recognition that aggregated MNP₂₀-cDNA probes to amplify the relaxation signal. The detection working range of the MS-MRS sensor is 0.5-100 ng/mL and that of the MS-MRS-Hg sensor is 0.05-100 ng/mL. Their limit of detection (LOD) values are 0.15 and 0.01 ng/mL, respectively. The relative recoveries of the MS-MRS and MS-MRS-Hg sensors are 95.2-119.5% and 93.1-113.1%, respectively. These results indicate that the proposed sensors have a high accuracy level.

Selective isolation and sensitive detection of lysozyme using aptamer based magnetic adsorbent and a new quartz crystal microbalance system

Magnetic chitosan beads and quartz crystal microbalance chip were decorated with lysozyme specific aptamer for isolation and detection of lysozyme, respectively. The lysozyme specific aptamer was immobilized on poly (dopamine) coated magnetic chitosan beads and the chip via Schiff base reaction. The percentage of the removal efficiency and purity of the isolated lysozyme from egg white were 87.6% and 91.8%, respectively. Further, the sensor system was contacted with different concentrations of lysozyme and other test proteins. This sensor system provided a method for the label-free, concentration-dependent, and selective detection of lysozyme with an observed detection limit of 17.9 ± 0.6 ng/mL. The sensor system was very selective and not significantly responded to the other tested proteins such as ovalbumin, trypsin, cytochrome C, and glucose oxidase. The prepared new sensor system showed a good durability and a high sensitivity for determination of lysozyme from solutions and whole egg white.

Recent advances in engineering iron oxide nanoparticles for effective magnetic resonance imaging

Iron oxide nanoparticle (IONP) with unique magnetic property and high biocompatibility have been widely used as magnetic resonance imaging (MRI) contrast agent (CA) for long time. However, a review which comprehensively summarizes the recent development of IONP as traditional T₂ CA and its new application for different modality of MRI, such as T₁ imaging, simultaneous T₂/T₁ or MRI/other imaging modality, and as environment responsive CA is rare. This review starts with an investigation of direction on the development of high-performance MRI CA in both T₂ and T₁ modal based on quantum mechanical outer sphere and Solomon–Bloembergen–Morgan (SBM) theory. Recent rational attempts to increase the MRI contrast of IONP by adjusting the key parameters, including magnetization, size, effective radius, inhomogeneity of surrounding generated magnetic field, crystal phase, coordination number of water, electronic relaxation time, and surface modification are summarized. Besides the strategies to improve r₂ or r₁ values, strategies to increase the in vivo contrast efficiency of IONP have been reviewed from three different aspects, those are introducing second imaging modality to increase the imaging accuracy, endowing IONP with environment response capacity to elevate the signal difference between lesion and normal tissue, and optimizing the interface structure to improve the accumulation amount of IONP in lesion. This detailed review provides a deep understanding of recent researches on the development of high-performance IONP based MRI CAs. It is hoped to trigger deep thinking for design of next generation MRI CAs for early and accurate diagnosis.

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T₂ contrast capacity of iron oxide nanoparticles (IONPs) could be improved based on quantum mechanical outer sphere theory.

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IONPs could be expand to be used as effective T₁ CAs by improving q value, extending τ_s, and optimizing interface structure.

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Environment responsive MRI CAs have been developed to improve the diagnosis accuracy.

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Introducing other imaging contrast moiety into IONPs could increase the contrast efficiency.

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Optimizing in vivo behavior of IONPs have been proved to enlarge the signal difference between normal tissue and lesion.

Graphene-Coated Iron Nitride Streptavidin Magnetic Beads: Preparation and Application in SARS-CoV-2 Enrichment

In this study, we prepared a streptavidin magnetic bead based on graphene-coated iron nitride magnetic beads (G@FeN-MB) and tried to use it for the enrichment of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). The outer shell of our magnetic bead was wrapped with multiple graphene sheets, and there is no report on the application of graphene to the magnetic-bead-coating material. First, the graphene shell of G@FeN-MB was oxidized by a modified Hummer method so as to generate the carboxyl groups required for the coupling of streptavidin (SA) on the surface of the magnetic beads. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) were used to characterize the oxidized G@FeN-MB (GO@FeN-MB). Streptavidin was then linked to the surface of the GO@FeN-MB by coupling the amino of the streptavidin with the carboxyl on the magnetic beads by carbodiimide method; thus, the streptavidin magnetic beads (SAMBs) were successfully prepared. To prove the practicality of the SAMBs, biotinylated SARS-CoV-2 S1 antibody was linked with it to respectively capture SARS-CoV-2 Spike-protein-coupled polystyrene beads (S-PS) and pseudovirus with S-protein expressed. Microplate reader and fluorescence microscope results show that the SAMBs can effectively enrich viruses. In conclusion, the preparation of SAMBs with G@FeN-MB is feasible and has potential for application in the field of virus enrichment.

Magnetoelastic Immunosensor via Antibody Immobilization for the Specific Detection of Lysozymes

Lysozymes in human urine have crucial clinical significance as an indicator of renal tubular and glomerular diseases. Most lysozyme detection methods rely on the enzyme-linked immunosorbent assay (ELISA), which is usually a tedious procedure. Meanwhile, aptamer sensors and fluorescence-based techniques for lysozyme detection have emerged in recent studies. However, these methods are time-consuming and highly complex in operation, and some even require exorbitant reagents and instruments, which restricts real-time clinical monitoring as diagnostic approaches. Therefore, a rapid and low-cost lysozyme detection method with facile preparation is still in demand for modern precision medicine. Herein, we propose a magnetoelastic (ME) immunosensor for lysozyme detection by detecting changes in resonance frequency under a magnetostrictive effect. The detection system is composed of a magnetoelastic chip with an immobilized lysozyme antibody, a solenoid coil, and a vector network analyzer. Since the ME sensor is ultrasensitive to mass change, the frequency offset caused by mass change can be utilized to detect the content of lysozyme. The immunosensor is evaluated to possess superior sensitivity of 138 Hz/μg mL^-1 in terms of the resonance frequency shift (RFS). In addition, our sensor displays an outstanding performance in specificity experiments and shows a relatively lower detection limit (1.26 ng/mL) than other conventional lysozyme detection methods (such as ELISA, chemiluminescence assay, fluorescence, and aptamer biosensors).

Horseradish peroxidase-catalyzed formation of polydopamine for ultra-sensitive magnetic relaxation sensing of aflatoxin B1

Aflatoxin B₁ as one of the most toxic mycotoxins poses a major health risk to humans and animals. Highly sensitive detection methods of aflatoxin B₁ are urgently required because of its low abundance in biological samples. In this work, we developed a magnetic relaxation sensing strategy using enzyme-catalyzed formation of polydopamine for signal amplification. Horseradish peroxidase can catalyze the reaction to generate polydopamine that assembles magnetic nanoparticles for magnetic relaxation sensing with a high signal-to-noise ratio. Combined with the specific antigen-antibody interaction, this magnetic sensor enables fast and ultra-sensitive detection of aflatoxin B₁ by using transverse relaxation time (T₂) as a readout. Under optimized conditions, the linear range of this magnetic sensor for detecting aflatoxin B₁ is from 10 pg/mL to 10 ng/mL, and the limit of detection is 0.35 pg/mL. This sensor has been challenged for the quantitative analysis of aflatoxin B₁ in animal feed samples that is promising for real-world applications.

Magnetic field-enhanced agglutination as a readout for rapid serologic assays with human plasma

Recent virus outbreaks have revealed a critical need for large scale serological assays. However, many available tests either require a cumbersome, costly apparatus or lack the availability of full automation. In order to address these limitations, we describe a homogeneous assay for antibody detection via measurement of superparamagnetic particles agglutination. Application of a magnetic field permits to overcome the limitations governed by Brownian translational diffusion in conventional assays and results in an important acceleration of the aggregation process as well as an improvement of the limit of detection. Furthermore, the use of protein-concentrated fluid such as 5 times-diluted human plasma does not impair the performances of the method. Screening of human plasma samples shows a strict discrimination between seropositive and seronegative samples in an assay duration as short as 14 s. The sensitivity of this method, combined with its quickness and simplicity, makes it a promising diagnostic tool.

Magnetic graphene quantum dots facilitate closed-tube one-step detection of SARS-CoV-2 with ultra-low field NMR relaxometry

The rapid and sensitive diagnosis of the highly contagious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is one of the crucial issues at the outbreak of the ongoing global pandemic that has no valid cure. Here, we propose a SARS-CoV-2 antibody conjugated magnetic graphene quantum dots (GQDs)-based magnetic relaxation switch (MRSw) that specifically recognizes the SARS-CoV-2. The probe of MRSw can be directly mixed with the test sample in a fully sealed vial without sample pretreatment, which largely reduces the testers' risk of infection during the operation. The closed-tube one-step strategy to detect SARS-CoV-2 is developed with home-made ultra-low field nuclear magnetic resonance (ULF NMR) relaxometry working at 118 μT. The magnetic GQDs-based probe shows ultra-high sensitivity in the detection of SARS-CoV-2 due to its high magnetic relaxivity, and the limit of detection is optimized to 248 Particles mL^‒1. Meanwhile, the detection time in ULF NMR system is only 2 min, which can significantly improve the efficiency of detection. In short, the magnetic GQDs-based MRSw coupled with ULF NMR can realize a rapid, safe, and sensitive detection of SARS-CoV-2.

Magnetic graphene quantum dots facilitate closed-tube one-step detection of SARS-CoV-2 with ultra-low field NMR relaxometry

The rapid and sensitive diagnosis of the highly contagious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is one of the crucial issues at the outbreak of the ongoing global pandemic that has no valid cure. Here, we propose a SARS-CoV-2 antibody conjugated magnetic graphene quantum dots (GQDs)-based magnetic relaxation switch (MRSw) that specifically recognizes the SARS-CoV-2. The probe of MRSw can be directly mixed with the test sample in a fully sealed vial without sample pretreatment, which largely reduces the testers' risk of infection during the operation. The closed-tube one-step strategy to detect SARS-CoV-2 is developed with home-made ultra-low field nuclear magnetic resonance (ULF NMR) relaxometry working at 118 μT. The magnetic GQDs-based probe shows ultra-high sensitivity in the detection of SARS-CoV-2 due to its high magnetic relaxivity, and the limit of detection is optimized to 248 Particles mL^‒1. Meanwhile, the detection time in ULF NMR system is only 2 min, which can significantly improve the efficiency of detection. In short, the magnetic GQDs-based MRSw coupled with ULF NMR can realize a rapid, safe, and sensitive detection of SARS-CoV-2.

Recent Advances in Aptamer-Based Biosensors for Detection of Pseudomonas aeruginosa

Increasing concerns about nosocomial infection, food and environmental safety have prompted the development of rapid, accurate, specific and ultrasensitive methods for the early detection of critical pathogens. Pseudomonas aeruginosa is one of the most common pathogens that cause infection. It is ubiquitous in nature, being found in water, soil, and food, and poses a great threat to public health. The conventional detection technologies are either time consuming or readily produce false positive/negative results, which makes them unsuitable for early diagnosis and spot detection of P. aeruginosa. To circumvent these drawbacks, many efforts have been made to develop biosensors using aptamers as bio-recognition elements. Various aptamer-based biosensors for clinical diagnostics, food, and environmental monitoring of P. aeruginosa have been developed in recent years. In this review, we focus on the latest advances in aptamer-based biosensors for detection of P. aeruginosa. Representative biosensors are outlined according to their sensing mechanisms, which include optical, electrochemical and other signal transduction methods. Possible future trends in aptamer biosensors for pathogen detection are also outlined.

Aptamer-Modified Nanoparticles in Medical Applications

Since aptamers have been selected against a broad range of target structures of medical interest and nanoparticles are available with diverse properties, aptamer-modified nanoparticles can be used in various diagnostic and therapeutic applications. While the aptamer is responsible for specificity and affinity of the conjugate, the nanoparticles' function varies from signal generation in diagnostic approaches to drug loading in drug delivery systems. Within this chapter different medical applications of aptamer-modified nanoparticles will be summarized and underlying principles will be described.

Magnetic-Separation-Assisted Magnetic Relaxation Switching Assay for Mercury Ion Based on the Concentration Change of Oligonucleotide-Functionalized Magnetic Nanoparticle

Magnetic-separation-assisted magnetic relaxation switching (MRS) assay based on the concentration change of magnetic nanoparticles switches has been designed for bacteria, biological macromolecules, and small molecules detection because of its better analysis performance. As one of the most hazardous pollutants and highly dangerous elements, mercury ion (Hg²⁺) was employed as a model to further investigate the applicability of nanoparticle switches concentration change-based MRS assay mode for detecting metal ions in this study. The principle is based on the specific and strong interaction between mercury ion with the thymine-thymine(T-T) mismatch in double-stranded DNA duplexes by employing oligonucleotide functionalized magnetic nanoparticle as magnetic capture probe and MRS signal probe, respectively. The result shows that magnetic nanoparticles concentration-dependent MRS sensing mode could be facile applied to detect metal ion of Hg²⁺ in tap water, lake water and serum with wider detection range and higher accuracy. The as-presented magnetic-separation-assisted MRS assay of Hg²⁺ in complicated samples shows potential application values for Hg²⁺ assay in clinical and environmental monitoring, which broadens its application.

Broad-Range Magnetic Relaxation Switching Bioassays Using Click Chemistry-Mediated Assembly of Polystyrene Beads and Magnetic Nanoparticles

Magnetic relaxation switching assays with a broad and tunable detection range can greatly improve current magnetic sensors for biochemical detections, but it remains challenging in terms of the limited detection range and low sensitivity. Herein, we report a methodology that uses click chemistry to assemble different sizes of polystyrene beads and magnetic nanoparticles to prepare versatile magnetic probes for broad-range bioassays with high sensitivity. Small magnetic nanoparticles can be controllably assembled on different sizes of polystyrene beads to form core-satellite structures, acting as broad-range probes that enable the magnetic relaxation switching assays with high sensitivity because different sizes of polystyrene beads can conjugate different numbers of small magnetic nanoparticles. On the basis of click chemistry, we assemble polystyrene beads and magnetic nanoparticles to develop a biosensing technique for analyzing three different antibiotics, with a high sensitivity and a tunable detection range from pg/mL to μg/mL.

Assay of miRNA in cell samples using enhanced resonance light scattering technique based on self aggregation of magnetic nanoparticles

AIMS

miRNAs are regarded as potential biomarkers correlated with the development and progression of many diseases. However, it is a challenge to construct a sensitive method to detect them without using time-consuming radioactive labeling or complex amplification strategies.

METHODS

A facile resonance light scattering (RLS) system was developed for the detection of miRNA employing magnetic nanoparticles (MNPs) as RLS probes. MNPs were coated with streptavidin. DNA probes were modified on the surface of MNPs based on the specific interaction of streptavidin and biotin forming MNPs@DNA probes. MNPs@DNA probes dispersed in homogeneous media causing low RLS signal.

RESULTS & CONCLUSION

miRNA hybridized with DNA probes resulting in the aggregation of MNPs and inducing the enhancement of RLS intensity. miRNAs were determined successfully with limit of detection at 0.9 picomole per liter (pM). The potential clinical application of the present biosensor was also demonstrated by measuring miRNAs in human normal and cancer cells, and human serum samples.

A magnetic nanoparticle-based aptasensor for selective and sensitive determination of lysozyme with strongly scattering silver nanoparticles

Qualitative and quantitative determination of lysozyme concentrations in urine and serum with high selectivity and sensitivity is important for diagnosing the progression of several diseases. In this report, we devised an improved method for specifically detecting lysozyme by combining magnetic nanoparticles (for separation and enrichment), an aptamer (for selective binding of lysozyme) and strongly scattering silver nanoparticles (AgNPs, for detection by light scattering, but also providing another level of selectivity due to their electrostatic binding with lysozyme). In this system, 0.4-30 nM lysozyme could be simply detected owing to the decreased light scattering of AgNPs in solution after magnetic separation, with a detection limit of 100 pM. In addition, lysozyme was also able to be semi-quantified by using the dark-field light scattering images of AgNPs after enrichment by the MNP-apt-lysozyme complex. Moreover, this design shows great promise for the robust and reliable detection of lysozyme in real samples, with a recovery rate ranging from 98.6% to 101.3% in human serum samples. Therefore, this assay provided robust measurements with good specificity, sensitivity, and tolerance of changes in the sample matrix. We expect that this MNP-based aptasensor may find utility in the accurate diagnosis of lysozyme-related diseases.

Development and characterization of DNA aptamers against florfenicol: Fabrication of a sensitive fluorescent aptasensor for specific detection of florfenicol in milk

Specific ssDNA aptamers for the antibiotic florfenicol (FF) were developed from an enriched nucleotide library using magnetic beads-based SELEX (Systematic Evolution of Ligands by EXponential enrichment) technique with high-binding affinity. After 12 rounds of selection, thirty-six sequences were obtained that were then divided into five major families, according to the primary sequence similarity. Binding affinity analyses of three fluorescently tagged aptamers belonging to different families demonstrated that the dissociation constants (K_d) were in the low nanomolar range (K_d = 52.78-211.4 nmol L^-1). Furthermore, to verify the potential application of the aptamers, a fluorescent aptasensor was fabricated for detecting the FF residue in raw milk samples based on the energy transfer between graphene oxide as the acceptor and fluorescently tagged FF-specific aptamer as the donor. Under optimal conditions, the aptasensor displayed a wide linear range from 5 to 1200 nmol L^-1 and a detection limit of 5.75 nmol L^-1 with excellent selectivity in milk. The recovery rate in the milk was between 101% ± 0.14% and 110% ± 2.8%, indicating high accuracy. This fluorescent aptasensor possessed considerable potential for rapid analysis of FF in raw milk because of its simplicity of detection. Moreover, the interaction between the aptamer and FF was studied using molecular modeling.

Gold Nanoparticles Decorated Hematite Photoelectrode for Sensitive and Selective Photoelectrochemical Aptasensing of Lysozyme

Photoelectrochemical aptasensor (PECAS) is a new and promising detection platform with both high sensitivity and good selectivity. Exploration of new photoelectrode materials and establishment of effective charge transfer channel between photoelectrode and aptamer are the main challenges in this field. In this work, an efficient PECAS based on Au nanoparticles (NPs) decorated Fe₂O₃ nanorod photoelectrode is rationally designed, fabricated, and exhibited excellent sensitivity and selectivity for detection of lysozyme (Lys) with an ultralow detection limit of 3 pM and wide detection range from 10 pM to 100 nM. The Au NPs not only act as anchor to establish an efficient charge transfer channel between the photoelectrode and the aptamer, but also help to enhance the PEC performance through adjusting the carrier density of Fe₂O₃. The rationally designed photoelectrode opens up a distinctive avenue for promoting the PECAS to be a versatile analysis method.

Multiplexed aptasensor for simultaneous detection of carcinoembryonic antigen and mucin-1 based on metal ion electrochemical labels and Ru(NH3)63+ electronic wires

In this paper, a dual-target electrochemical aptasensor has been developed for simultaneous detection of carcinoembryonic antigen and mucin-1 based on metal ion electrochemical labels and Ru(NH₃)₆³⁺ electronic wires. When targets are present, the interaction between targets and their respective aptamers leads to the dissociation of double-strand DNA because the targets have higher affinity to its aptamer than the complementary strand. And the qualitative and quantitative analyses of the two targets are realized by the differential pulse voltammetry (DPV) peaks generated by metal ion electrochemical labels. For the effective loading of a large number of metal ions, Au/bovine serum albumin (Au/BSA) nanospheres are employed as carriers to develop Au/BSA-metal ions. After Ru(NH₃)₆³⁺ complexes are embedded into double-strand DNA to form the electronic wires, the electrical conductivity and the electron transfer of the detection system are greatly improved. The detection limit of the proposed assay was calculated as 3.33fM ranging from 0.01pM to 100nM. Therefore, this novel sensing assay provides a new and sensitive platform for detecting several targets simultaneously in biochemical research and clinical diagnosis.

Nanotechnology-Enhanced No-Wash Biosensors for in Vitro Diagnostics of Cancer

In vitro biosensors have been an integral component for early diagnosis of cancer in the clinic. Among them, no-wash biosensors, which only depend on the simple mixing of the signal generating probes and the sample solution without additional washing and separation steps, have been found to be particularly attractive. The outstanding advantages of facile, convenient, and rapid response of no-wash biosensors are especially suitable for point-of-care testing (POCT). One fast-growing field of no-wash biosensor design involves the usage of nanomaterials as signal amplification carriers or direct signal generating elements. The analytical capacity of no-wash biosensors with respect to sensitivity or limit of detection, specificity, stability, and multiplexing detection capacity is largely improved because of their large surface area, excellent optical, electrical, catalytic, and magnetic properties. This review provides a comprehensive overview of various nanomaterial-enhanced no-wash biosensing technologies and focuses on the analysis of the underlying mechanism of these technologies applied for the early detection of cancer biomarkers ranging from small molecules to proteins, and even whole cancerous cells. Representative examples are selected to demonstrate the proof-of-concept with promising applications for in vitro diagnostics of cancer. Finally, a brief discussion of common unresolved issues and a perspective outlook on the field are provided.

Recent Advances on Magnetic Relaxation Switching Assay-Based Nanosensors

Magnetic relaxation switching assay (MRSw)-based nanosensors respond to the changes of transverse relaxation time (T₂) of water molecules resulted from the analyte-induced aggregation and disaggregation of magnetic nanoparticles (MNPs). This strategy has been widely applied to the detections of various substrates from heavy metal ions to organic pollutants, proteins, nucleic acids, bacteria and viruses, and specific cells. Compared with other nanosensors, MRSw-based nanosensors not only are free from the background interferences, signal bleaching, and quenching but also overcome light scattering from samples without pretreatments. Therefore, MRSw-based nanosensors have been developed as real-time and on-site detection platforms for environmental protection, food safety, and risk assessment. This review summarizes the latest developments of the principles, the applicable magnetic nanoparticles, and the exploited environmental and biological applications of MRSw-based nanosensors.

CdTe/CdSe quantum dot-based fluorescent aptasensor with hemin/G-quadruplex DNzyme for sensitive detection of lysozyme using rolling circle amplification and strand hybridization

Lysozyme with a small monomeric globular enzymatic protein is part of the innate immune system, and its deficiency can cause the increased incidence of disease. Herein, we devise a new signal-enhanced fluorescence aptasensing platform for quantitative screening of lysozyme by coupling with rolling circle amplification (RCA) and strand hybridization reaction, accompanying the assembly of CdTe/CdSe quantum dots (QDs) and hemin/G-quadruplex DNzyme. Initially, target-triggered release of the primer was carried out from DNA duplex via the reaction of the aptamer with the analyte, and the released primer could be then utilized as the template to produce numerous repeated oligonucleotide sequences by the RCA reaction. Following that, the formed long-stranded DNA simultaneously hybridized with the CdTe/CdSe QD-labeled probe and hemin/G-quadruplex DNzyme strand in the system, thereby resulting in the quenching of QD fluorescent signal through the proximity hemin/G-quadruplex DNzyme on the basis of transferring photoexcited conduction band electrons of quantum dots to Fe(III)/Fe(II)-protoporphyrin IX (hemin) complex. Under optimal conditions, the fluorescent signal decreased with the increasing target lysozyme within the dynamic range from 5.0 to 500nM with a detection limit (LOD) of 2.6nM at the 3s_blank criterion. Intra-assay and interassay coefficients of variation (CVs) were below 8.5% and 11.5%, respectively. Finally, the system was applied to analyze spiked human serum samples, and the recoveries in all cases were 85-111.9%.

Quantitative Detection of MicroRNA in One Step via Next Generation Magnetic Relaxation Switch Sensing

One-step, quantitative and rapid detection of microRNA (miRNA) in tumor cells or tissues can provide critical information for clinical diagnosis and cancer treatment. In this work, we develop a magnetic relaxation switch sensing (MRS)-based miRNA sensor using magnetic microparticle (1 μm in diameter, MM1000)-DNA probe-magnetic nanoparticle (30 nm in diameter, MN30) conjugates (MM1000-DNA-MN30). In the presence of target miRNA, DSN enzyme selectively cleaves the DNA tether after miRNA/DNA hybridization to release MN30 and leaves the miRNA intact to lead to the declustering of more MN30 than before. In contrast to conventional MRS by measuring the change of transverse relaxation time (ΔT2) induced by the aggregation or dissociation of magnetic particles in the presence of target, we use the cleaved MN30 from conjugates as the direct readout of ΔT2, which is more sensitive and stable. This MRS-based assay allows for one-step detection of 5 fM of miR-21 in urine samples, quantification of miR-21 from 100 cancer cells, and differentiation of the expression of miR-21 in tumor and surrounding tissues. The merits of this assay, rapidity, ability for quantitation, high sensitivity, and one-step operation, ensure a promising future in diagnostic technology.

Homogeneous Biosensing Based on Magnetic Particle Labels

The growing availability of biomarker panels for molecular diagnostics is leading to an increasing need for fast and sensitive biosensing technologies that are applicable to point-of-care testing. In that regard, homogeneous measurement principles are especially relevant as they usually do not require extensive sample preparation procedures, thus reducing the total analysis time and maximizing ease-of-use. In this review, we focus on homogeneous biosensors for the in vitro detection of biomarkers. Within this broad range of biosensors, we concentrate on methods that apply magnetic particle labels. The advantage of such methods lies in the added possibility to manipulate the particle labels by applied magnetic fields, which can be exploited, for example, to decrease incubation times or to enhance the signal-to-noise-ratio of the measurement signal by applying frequency-selective detection. In our review, we discriminate the corresponding methods based on the nature of the acquired measurement signal, which can either be based on magnetic or optical detection. The underlying measurement principles of the different techniques are discussed, and biosensing examples for all techniques are reported, thereby demonstrating the broad applicability of homogeneous in vitro biosensing based on magnetic particle label actuation.

Aptamer-nanoparticle complexes as powerful diagnostic and therapeutic tools

Correct diagnosis and successful therapy are extremely important to enjoy a healthy life when suffering from a disease. To achieve these aims, various cutting-edge technologies have been designed and fabricated to diagnose and treat specific diseases. Among these technologies, aptamer-nanomaterial hybrids have received considerable attention from scientists and doctors because they have numerous advantages over other methods, such as good biocompatibility, low immunogenicity and controllable selectivity. In particular, aptamers, oligonucleic acids or peptides that bind to a specific target molecule, are regarded as outstanding biomolecules. In this review, several screening techniques for aptamers, also called systematic evolution of ligands by exponential enrichment (SELEX) methods, are introduced, and diagnostic and therapeutic aptamer applications are also presented. Furthermore, we describe diverse aptamer-nanomaterial conjugate designs and their applications for diagnosis and therapy.

A simple and sensitive resonance light scattering method based on aggregation of gold nanoparticles for selective detection of microRNA-21

A novel resonance light scattering method based on analyte-induced aggregation of gold nanoparticles for the determination of microRNAs was developed.

A luminescent G-quadruplex-selective iridium(iii) complex for the label-free detection of lysozyme

A novel Ir(iii) complex 1 displays high selectivity for the G-quadruplex, and was used to establish a label-free G-quadruplex-based detection platform for lysozyme in buffer.

Blu-ray based optomagnetic aptasensor for detection of small molecules

This paper describes an aptamer-based optomagnetic biosensor for detection of a small molecule based on target binding-induced inhibition of magnetic nanoparticle (MNP) clustering. For the detection of a target small molecule, two mutually exclusive binding reactions (aptamer-target binding and aptamer-DNA linker hybridization) are designed. An aptamer specific to the target and a DNA linker complementary to a part of the aptamer sequence are immobilized onto separate MNPs. Hybridization of the DNA linker and the aptamer induces formation of MNP clusters. The target-to-aptamer binding on MNPs prior to the addition of linker-functionalized MNPs significantly hinders the hybridization reaction, thus reducing the degree of MNP clustering. The clustering state, which is thus related to the target concentration, is then quantitatively determined by an optomagnetic readout technique that provides the hydrodynamic size distribution of MNPs and their clusters. A commercial Blu-ray optical pickup unit is used for optical signal acquisition, which enables the establishment of a low-cost and miniaturized biosensing platform. Experimental results show that the degree of MNP clustering correlates well with the concentration of a target small molecule, adenosine triphosphate (ATP) in this work, in the range between 10µM and 10mM. This successful proof-of-concept indicates that our optomagnetic aptasensor can be further developed as a low-cost biosensing platform for detection of small molecule biomarkers in an out-of-lab setting.

A simple strategy based on upconversion nanoparticles for a fluorescent resonant energy transfer biosensor

A novel biosensor was fabricated for lysozyme and DNA detection based on fluorescence resonance energy transfer between UCNPs and TAMRA-aptamer.

Gold-Coated Fe3O4 Nanoroses with Five Unique Functions for Cancer Cell Targeting, Imaging and Therapy

The development of nanomaterials that combine diagnostic and therapeutic functions within a single nanoplatform is extremely important for molecular medicine. Molecular imaging with simultaneous diagnosis and therapy will provide the multimodality needed for accurate diagnosis and targeted therapy. Here, we demonstrate gold-coated iron oxide (Fe₃O₄@Au) nanoroses with five distinct functions, which integrate aptamer-based targeting, magnetic resonance imaging (MRI), optical imaging, photothermal therapy and chemotherapy into one single probe. The inner Fe₃O₄ core functions as an MRI agent, while the photothermal effect is achieved through near-infrared absorption by the gold shell, causing a rapid rise in temperature and also resulting in a facilitated release of the anticancer drug doxorubicin carried by the nanoroses. Where the doxorubicin is released is monitored by its fluorescent. Aptamers immobilized on the surfaces of the nanoroses enable efficient and selective drug delivery, imaging and photothermal effect with high specificity. The five-function-embedded nanoroses show great advantages in multimodality.

Post-genomics nanotechnology is gaining momentum: nanoproteomics and applications in life sciences

The post-genomics era has brought about new Omics biotechnologies, such as proteomics and metabolomics, as well as their novel applications to personal genomics and the quantified self. These advances are now also catalyzing other and newer post-genomics innovations, leading to convergences between Omics and nanotechnology. In this work, we systematically contextualize and exemplify an emerging strand of post-genomics life sciences, namely, nanoproteomics and its applications in health and integrative biological systems. Nanotechnology has been utilized as a complementary component to revolutionize proteomics through different kinds of nanotechnology applications, including nanoporous structures, functionalized nanoparticles, quantum dots, and polymeric nanostructures. Those applications, though still in their infancy, have led to several highly sensitive diagnostics and new methods of drug delivery and targeted therapy for clinical use. The present article differs from previous analyses of nanoproteomics in that it offers an in-depth and comparative evaluation of the attendant biotechnology portfolio and their applications as seen through the lens of post-genomics life sciences and biomedicine. These include: (1) immunosensors for inflammatory, pathogenic, and autoimmune markers for infectious and autoimmune diseases, (2) amplified immunoassays for detection of cancer biomarkers, and (3) methods for targeted therapy and automatically adjusted drug delivery such as in experimental stroke and brain injury studies. As nanoproteomics becomes available both to the clinician at the bedside and the citizens who are increasingly interested in access to novel post-genomics diagnostics through initiatives such as the quantified self, we anticipate further breakthroughs in personalized and targeted medicine.

Using metal nanoparticles as a visual sensor for the discrimination of proteins

The fluorescence of metal NPs is changed differently upon binding to a protein-in gel, forming a visual sensor for protein discrimination.

Amplification of resonant Rayleigh light scattering response using immunogold colloids for detection of lysozyme

A strategy for attomolar-level detection of small molecule-size proteins is reported based on Rayleigh light scattering spectroscopy of individual nanoplasmonic aptasensors by exploiting the outstanding characteristics of gold colloids to amplify the nontransparent resonant signal at ultralow analyte concentrations. The fabrication method utilizes thiol-mediated adsorption of a DNA aptamer on the immobilized Au nanoparticle surface, the interfacial binding characteristics of the aptamer with its target molecules, and the antibody-antigen interaction through plasmonic resonance coupling of the Au nanoparticles. Using lysozyme as a model analyte for disease detection, the detection limit of the aptasensor is ∼7 × 10(3) aM, corresponding to the LSPR λmax shift of ∼2.25 nm. Up to a 380% increase in the localized resonant λmax shift is demonstrated upon antibody binding to the analyte compared to the primary response during signal amplification using immunogold colloids. This enhancement leads to a limit of detection of ∼7 aM, which is an improvement of three orders of magnitude. The results demonstrate substantial promise for developing coupled plasmonic nanostructures for ultrasensitive detection of various biological and chemical analytes.