A sensitive thrombin-linked sandwich immunoassay for protein targets using high affinity phosphorodithioate modified aptamer for thrombin labeling

Advances in Aptamer-Based Conjugate Recognition Techniques for the Detection of Small Molecules in Food

Small molecules are significant risk factors for causing food safety issues, posing serious threats to human health. Sensitive screening for hazards is beneficial for enhancing public security. However, traditional detection methods are unable to meet the requirements for the field screening of small molecules. Therefore, it is necessary to develop applicable methods with high levels of sensitivity and specificity to identify the small molecules. Aptamers are short-chain nucleic acids that can specifically bind to small molecules. By utilizing aptamers to enhance the performance of recognition technology, it is possible to achieve high selectivity and sensitivity levels when detecting small molecules. There have been several varieties of aptamer target recognition techniques developed to improve the ability to detect small molecules in recent years. This review focuses on the principles of detection platforms, classifies the conjugating methods between small molecules and aptamers, summarizes advancements in aptamer-based conjugate recognition techniques for the detection of small molecules in food, and seeks to provide emerging powerful tools in the field of point-of-care diagnostics.

Role of pretty nanoflowers as novel versatile analytical tools for sensing in biomedical and bioanalytical applications

In recent years, an encouraging breakthrough in the synthesis of immobilized enzymes in flower-shaped called "organic-inorganic hybrid nanoflowers (hNFs)" with greatly enhanced catalytic activity and stability were reported. Although, these hNFs were discovered by accident, the enzymes exhibited highly enhanced catalytic activities and stabilities in the hNFs compared with the free and conventionally immobilized enzymes. Herein, we rationally utilized the catalytic activity of the hNFs for analytical applications. In this comprehensive review, we covered the design and use of the hNFs as novel versatile sensors for electrochemical, colorimetric/optical and immunosensors-based detection strategies in analytical perspective.

Gold single atom-based aptananozyme as an ultrasensitive and selective colorimetric probe for detection of thrombin and C-reactive protein

An ultra-efficient biocatalytic peroxidase-like Au-based single-atom nanozyme (Au-SAzymes) has been synthesized from isolated Au atoms on black nitrogen doped carbon (Au-N-C) using a simple complexation-adsorption-pyrolysis method. The atomic structure of AuN4 centers in black carbon was revealed by combined high-resolution transmission electron microscopy/high-angle annular dark-field scanning transmission electron microscopy. The Au-SAzymes showed a remarkable peroxidase activity with 1.7 nM as Michaelis-Menten constant, higher than most previously reported SAzyme activity. Density functional theory and Monte Carlo calculations revealed the adsorption of H2O2 on AuN4 with formation of OH* and O*. Molecular recognition was greatly enhanced via label-free integration of thiol-terminal aptamers on the surface of single Au atoms (Aptamer/Au-SAzyme) to design off-on ultrasensitive aptananozyme-based sensor for detecting thrombin and CRP with 550 pM and 500 pg mL-1 limits of detection, respectively. The Aptamer/Au-SAzyme showed satisfactory accuracy and precision when applied to the serum and plasma of COVID-19 patients. Due to the maximum Au atom utilization, approximately 3636 samples can be run per 1 mg of gold, highlighting the commercialization potential of the developed Aptamer/Au-SAzyme approach.

Development of a Gold Nanoparticle-Linked Immunosorbent Assay of Staphylococcal Enterotoxin B Detection with Extremely High Sensitivity by Determination of Gold Atom Content Using Graphite Furnace Atomic Absorption Spectrometry

Highly sensitive staphylococcal enterotoxin B (SEB) assay is of great importance for the prevention of toxic diseases caused by SEB. In this study, we present a gold nanoparticle (AuNP)-linked immunosorbent assay (ALISA) for detecting SEB in a sandwich format using a pair of SEB specific monoclonal antibodies (mAbs) performed in microplates. First, the detection mAb was labeled with AuNPs of different particle sizes (15, 40 and 60 nm). Then the sandwich immunosorbent assay for SEB detection was performed routinely in a microplate except for using AuNPs-labeled detection mAb. Next, the AuNPs adsorbed on the microplate were dissolved with aqua regia and the content of gold atoms was determined by graphite furnace atomic absorption spectrometry (GFAAS). Finally, a standard curve was drawn of the gold atomic content against the corresponding SEB concentration. The detection time of ALISA was about 2.5 h. AuNPs at 60 nm showed the highest sensitivity with an actual measured limit of detection (LOD) of 0.125 pg/mL and a dynamic range of 0.125-32 pg/mL. AuNPs at 40 nm had an actual measured LOD of 0.5 pg/mL and a dynamic range of 0.5 to 128 pg/mL. AuNPs at 15 nm had an actual measured LOD of 5 pg/mL, with a dynamic range of 5-1280 pg/mL. With detection mAb labeled with AuNPs at 60 nm, ALISA's intra- and interassay coefficient variations (CV) at three concentrations (2, 8, and 20 pg/mL) were all lower than 12% and the average recovery level was ranged from 92.7% to 95.0%, indicating a high precision and accuracy of the ALISA method. Moreover, the ALISA method could be successfully applied to the detection of various food, environmental, and biological samples. Therefore, the successful establishment of the ALISA method for SEB detection might provide a powerful tool for food hygiene supervision, environmental management, and anti-terrorism procedures and this method might achieve detection and high-throughput analysis automatically in the near future, even though GFAAS testing remains costly at present.

Ultrasensitive thrombin sensing platform based on three-way junction initiated dual signal amplification

The sensitive and precise quantitative determination of thrombin is important for both fundamental research and clinical diagnostics of hypoxic ischemic encephalopathy because it is a key biological molecule in hemostasis and hemolysis. Herein, we depict a sensitive and label-free thrombin detection approach by taking the advantages of aptamer’s superior capability to bind with thrombin and the high efficiency of three-way junction initiated dual signal recycle. In this method, a capture probe which is inserted with an aptamer sequence is designed to specifically identify thrombin molecule and facilitate the signal amplification. Based on the DNA polymerase and endonuclease Nb.BbvCI-assisted chain extension, a large amount of single-strand DNA sequences that can fold into G-quadruplex are produced to specifically recognize commercial fluorescent dye thioflavin T for signal generation. Consequently, the approach exhibits a high detection sensitivity with the limit of detection as low as 768 fM, holding a great promise for detection of thrombin and disease diagnosis in the clinic.

Nanozyme and Stimulated Fluorescent Cu-Based Metal-Organic Frameworks (Cu-MOFs) Functionalized with Engineered Aptamers as a Molecular Recognition Element for Thrombin Detection in the Plasma of COVID-19 Patients

An essential tool in the management and control of the COVID-19 pandemic is the development of a fast, selective, sensitive, and inexpensive COVID-19 biomarkers detection method. Herein, an ultrasensitive and label-free biosensing strategy was described for the colorimetric and fluorimetric detection of thrombin. A dual-mode aptasensing method based on integrating engineered ssDNA with a stimulated fluorescent enzyme-mimetic copper-based metal-organic framework (Cu-MOF) as a molecular recognition element for thrombin was investigated. Cu-MOFs displayed stimulated fluorescence and enzyme-mimetic peroxidase activities that oxidize the chromogenic colorless substance TMB to blue-colored oxTMB. The thrombin-based aptamer (ssDNA) can be immobilized on the Cu-MOF surface to form a functionalized composite, ssDNA/MOF, and quench the stimulated fluorescence emission and the enzymatic activity of the Cu-MOF. Later, addition of thrombin recovers the fluorescence and enzymatic activity of the MOF. Thus, a turn-on colorimetry/fluorimetry aptasensing probe was designed for the detection of thrombin. Based on colorimetric assay, 350 pM was recorded as the lower limit of detection (LOD), while based on the fluorescence mode, 110 fM was recorded as the LOD (when S/N = 3). The label-free aptasensing probe was used successfully for the detection of thrombin in COVID-19 patients with satisfactory recoveries, 95-98%. Since the detection time of our aptasensor is relatively rapid (45 min) and due to the low-cost precursors and easy-to-operate characteristics, we believe that it has great potential to be used in point-of-care testing (POCT).

Cell-specific aptamers as potential drugs in therapeutic applications: A review of current progress

Cell-specific aptamers are a promising emerging player in the field of disease therapy. This paper reviews the multidimensional research progress made in terms of their classification, modification, and application. Based on the target location of cell-specific aptamers, it is defined and classified cell-specific aptamers into three groups including aptamers for cell surface markers, aptamers for intracellular components, and aptamers for extracellular components. Moreover, the modification methods of aptamers to achieve improved stability and affinity are concluded. In addition, recent advances in the application of cell-specific aptamers are discussed, mainly focusing on the increasing research attraction of cell state improving helpers and cell recruitment mediators in the improvement of cellular microenvironments to achieve successful disease therapy. This review also highlights 11 types of clinical aptamer drugs. Finally, the challenges and future directions of potential clinical applications are presented. In summary, we believe that cell-specific aptamers are promising drugs in disease therapy.

A bifunctional immunosensor based on osmium nano-hydrangeas as a catalytic chromogenic and tinctorial signal output for folic acid detection

As a neglected member of the platinum group elements, osmium, the metal with the highest density in the earth, is very suitable for the preparation of a peroxidase with high catalytic activity and stability, and can also be associated with the development of a sensor. In this study, we accessed Os nano-hydrangeas (OsNHs) with one-pot synthesis and utilized them in a bifunctional immunosensor that can present both catalytic chromogenic and tinctorial signal for nanozyme-linked immunosorbent assay (NLISA) and lateral flow immunoassay (LFIA) for use in folic acid (FA) detection. In the OsNHs-NLISA, the linear range is from 9.42 to 167.53 ng mL^-1. The limit of detection (LOD) is 4.03 ng mL^-1 and the IC₅₀ value is 39.73 ng mL^-1. In OsNHs-LFIA, the visual cut-off value and limit of detection (v-LOD) are 100 ng mL^-1 and 0.01 ng mL^-1, respectively. Additionally, the outcome from the specificity and spiked sample analysis offered recovery from the spiked milk powder sample ranging from 93.9 to 103.6% with a coefficient of variation under 4.9%, compared with UPLC-MS/MS for a correlation of R² = 0.999 and admirable validation. The promising application of the OsNHs can also be used in other bioprobes, and this bifunctional immunosensor analysis mode is suitable for diversified analytes.

Visual detection of tropomyosin, a major shrimp allergenic protein using gold nanoparticles (AuNPs)-assisted colorimetric aptasensor

A gold nanoparticle-based label-free colorimetric assay was developed to detect the shrimp allergenic protein tropomyosin (TM), an important biomarker responsible for severe clinical reactivity to shellfish. In a gold nanoparticles (AuNPs)-tropomyosin-binding aptamer (TMBA) complex, the aptamer adsorbs onto the surface of AuNPs and dissociates in the presence of TM. In addition, AuNPs tend to aggregate in the presence of ionic salt, revealing a color change (i.e., wine-red to purple/blue) with a shift in the maximum absorption peak from 520 nm. In the presence of specific binding TM, the aptamer folds into a tertiary structure where it more efficiently stabilizes AuNPs toward the salt-induced aggregation with a hypsochromic shift in the absorption spectra compared to the stabilized AuNPs by aptamer alone. Based on the aggregation and sensitive spectral transformation principle, the AuNPs-based colorimetric aptasensor was successfully applied to detect TM with a range of 10-200 nmol/L and a low detection limit of 40 nmol/L in water samples. The reliability, selectivity, and sensitivity of the aptasensor was then tested with food samples spiked with TM. The observed detection limit was as low as 70 nmol/L in shrimp, 90 nmol/L in tofu, and 80 nmol/L in eggs, respectively. We anticipate the proposed AuNPs-based colorimetric aptasensor assay possesses a high potential for the easy and efficient visual colorimetric detection of TM.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-020-00085-5.

Recent Progress in Aptamer Discoveries and Modifications for Therapeutic Applications

Aptamers are oligonucleotide sequences with a length of about 25-80 bases which have abilities to bind to specific target molecules that rival those of monoclonal antibodies. They are attracting great attention in diverse clinical translations on account of their various advantages, including prolonged storage life, little batch-to-batch differences, very low immunogenicity, and feasibility of chemical modifications for enhancing stability, prolonging the half-life in serum, and targeted delivery. In this Review, we demonstrate the emerging aptamer discovery technologies in developing advanced techniques for producing aptamers with high performance consistently and efficiently as well as requiring less cost and resources but offering a great chance of success. Further, the diverse modifications of aptamers for therapeutic applications including therapeutic agents, aptamer-drug conjugates, and targeted delivery materials are comprehensively summarized.

Aptamer-based fluorescent sensors for the detection of cancer biomarkers

Aptamers are short single-stranded RNA or DNA molecules that can recognize a series of targets with high affinity and specificity. Known as "chemical antibodies", aptamers have many unique merits, including ease of chemical synthesis, high chemical stability, low molecular weight, lack of immunogenicity, and ease of modification and manipulation compared to their protein counterparts. Using aptamers as the recognition groups, fluorescent aptasensors provide exciting opportunities for sensitive detection and quantification of analytes. Herein, we give an overview on the recent development of aptamer-based fluorescent sensors for the detection of cancer biomarkers. Based on various nanostructured sensor designs, we extended our discussions on sensitivity, specificity and the potential applications of aptamer-based fluorescent sensors in early diagnosis, treatment and prognosis of cancers.

The application of antibody-aptamer hybrid biosensors in clinical diagnostics and environmental analysis

The growing number of various diseases and the increase of environmental contamination are the causes for the development of novel methods for their detection. The possibility of the application of affinity-based biosensors for such purposes seems particularly promising as they provide high selectivity and low detection limits. Recently, the usage of hybrid antibody-aptamer sandwich constructs was shown to be more advantageous in terms of working parameters in comparison to aptamer-based and immune-based biosensors. This review is focused on the usage of hybrid antibody-aptamer receptor layers for the determination of clinically and environmentally important target molecules. In this work, antibodies and aptamer molecules are characterized and the methods of their immobilization as well as analytical signal generation are shown. This is followed by the critical presentation of examples of hybrid sandwich biosensors that have been elaborated in the past 12 years.

Construction of electrochemical aptasensor of carcinoembryonic antigen based on toehold-aided DNA recycling signal amplification

Carcinoembryonic antigen (CEA), serves as a broad-spectrum tumor marker, and plays an important role in reflecting the existence, therapeutic evaluation, development, monitoring and prognosis of many types of cancer. An electrochemical aptasensor was designed for CEA detection based on toehold-aided DNA recycling. A partially hybridized Probe-4 (i.e. P2/P3/P4) was self-assembled on the surface of a gold electrode serving as the sensing platform. For CEA detection, CEA can bind with aptamer and free probe-1 (P1) can hybridize with P4, triggering toehold-aided DNA recycling. This enables the hybridization of more probe-5 (P5) (labeled with methylene blue (MB)) with P4, causing more methylene blue (MB) to be brought close to the electrode surface. An amplified current signal was thus generated due to more MB in the electrode surface. The proposed design showed good linearity between current response and log CEA concentration ranging from 0.1 to 50 ng·mL^-1, with a detection limit of 20 pg mL^-1. This aptasensor also showed high specificity for CEA detection, and was successfully used in spiked biological samples.