A gold nanoparticle-based label free colorimetric aptasensor for adenosine deaminase detection and inhibition assay

A label-free G-quadruplex aptamer/gold nanoparticle-based colorimetric biosensor for rapid detection of bovine viral diarrhea virus genotype 1

Bovine viral diarrhea virus (BVDV) is the cause of bovine viral diarrhea disease, one of the most economically important livestock diseases worldwide. The majority of BVD disease control programs rely on the detection and then elimination of persistent infection (PI) cattle, as the continuing source of disease. The main purpose of this study was to design and develop an accurate G-quadruplex-based aptasensor for rapid and simple detection of BVDV-1. In this work, we utilized in silico techniques to design a G-quadruplex aptamer specific for the detection of BVDV-1. Also, the rationally designed aptamer was validated experimentally and was used for developing a colorimetric biosensor based on an aptamer-gold nanoparticle system. Firstly, a pool of G-quadruplex forming ssDNA sequences was constructed. Then, based on the stability score in secondary and tertiary structures and molecular docking score, an aptamer (Apt31) was selected. In the experimental part, gold nanoparticles (AuNPs) with an average particle size of 31.7 nm were synthesized and electrostatically linked with the Apt31. The colorimetric test showed that salt-induced color change of AuNPs from red to purple-blue occurs only in the presence of BVDV-Apt31 complex, after 20 min. These results approved the specificity of Apt31 for BVDV. Furthermore, our biosensor could detect the virus at as low as 0.27 copies/ml, which is an acceptable value in comparison to the qPCR method. The specificity of the aptasensor was confirmed through cross-reactivity testing, while its selectivity was confirmed through plasma testing. The sample analysis showed 90% precision and 94% accuracy. It was concluded that the biosensor was adequately sensitive and specific for the detection of BVDV in plasma samples and could be used as a simple and rapid method on the farm.

Cordycepin: A review of strategies to improve the bioavailability and efficacy

Cordycepin is a bioactive compound extracted from Cordyceps militaris. As a natural antibiotic, cordycepin has a wide variety of pharmacological effects. Unfortunately, this highly effective natural antibiotic is proved to undergo rapid deamination by adenosine deaminase (ADA) in vivo and, as a consequence, its half-life is shortened and bioavailability is decreased. Therefore, it is of critical importance to work out ways to slow down the deamination so as to increase its bioavailability and efficacy. This study reviews recent researches on a series of aspects of cordycepin such as the bioactive molecule's pharmacological action, metabolism and transformation as well as the underlying mechanism, pharmacokinetics and, particularly, the methods for reducing the degradation to improve the bioavailability and efficacy. It is drawn that there are three methods that can be applied to improve the bioavailability and efficacy: to co-administrate an ADA inhibitor and cordycepin, to develop more effective derivatives via structural modification, and to apply new drug delivery systems. The new knowledge can help optimize the application of the highly potent natural antibiotic-cordycepin and develop novel therapeutic strategies.

Plasmonic Nanomaterials for Colorimetric Biosensing: A Review

In the last few decades, plasmonic colorimetric biosensors raised increasing interest in bioanalytics thanks to their cost-effectiveness, responsiveness, and simplicity as compared to conventional laboratory techniques. Potential high-throughput screening and easy-to-use assay procedures make them also suitable for realizing point of care devices. Nevertheless, several challenges such as fabrication complexity, laborious biofunctionalization, and poor sensitivity compromise their technological transfer from research laboratories to industry and, hence, still hamper their adoption on large-scale. However, newly-developing plasmonic colorimetric biosensors boast impressive sensing performance in terms of sensitivity, dynamic range, limit of detection, reliability, and specificity thereby continuously encouraging further researches. In this review, recently reported plasmonic colorimetric biosensors are discussed with a focus on the following categories: (i) on-platform-based (localized surface plasmon resonance, coupled plasmon resonance and surface lattice resonance); (ii) colloid aggregation-based (label-based and label free); (iii) colloid non-aggregation-based (nanozyme, etching-based and growth-based).

Sensing Adenosine and ATP by Aptamers and Gold Nanoparticles: Opposite Trends of Color Change from Domination of Target Adsorption Instead of Aptamer Binding

The 27 mer DNA aptamer for adenosine and adenosine 5'-triphosphate (ATP) is a popular model system for designing biosensors. Various strategies have been reported for label-free colorimetric detection using gold nanoparticles (AuNPs). It is generally accepted that free aptamers can protect AuNPs against salt-induced aggregation, whereas target-bound aptamers cannot. However, these studies only considered the aptamer binding to its target, and the adsorption of the aptamer on AuNPs, but none considered the adsorption of target molecules by AuNPs. We herein report that the adsorption of adenosine destabilized citrate-capped AuNPs with an apparent K_d of just 7.7 μM adenosine, whereas that of ATP stabilized the AuNPs because of the negative charges from the triphosphate group. The adsorbed ATP inhibited the adsorption of DNA. Using the aptamer and a nonbinding mutant, ATP and guanosine 5'-triphosphate (GTP) had the same colorimetric response, and so did adenosine and guanosine, regardless of the DNA sequence, indicating that the color change mainly reflected the adsorption of the nucleosides and nucleotides instead of aptamer binding. The related literature examples using this aptamer were classified into three types and individually analyzed, where the reported color changes can all be explained by the adsorption of target analytes.

A rapid assay to screen adenosine deaminase inhibitors from Ligustri Lucidi Fructus against metabolism of cordycepin utilizing ultra-high-performance liquid chromatography-tandem mass spectrometry

Cordycepin has recently received increased attention owing to its extensive pharmacological activity. Adenosine deaminase (ADA) is widely distributed in mammalian blood and tissues; as a result, cordycepin is quickly metabolized upon entering into the body and converted into the inactive metabolite 3'-deoxyinosine, thus limiting its activity when administered alone. We herein present a novel ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method for screening ADA inhibitors against the metabolism of cordycepin. Cordycepin and 3'-deoxyinosine were chosen as substrate and product, respectively. A proper separation was achieved for all analytes within 3 min. 3'-Deoxyinosine was quantified in the presence or absence of potential ADA inhibitors to evaluate ADA activity. The assay can simultaneously determine substrate and product, with the endogenous substance and ADA inhibitors added not interfering in its activity. After optimizing the enzymatic incubation and UHPLC-MS/MS conditions, Km and Vmax values for ADA deamination of cordycepin were 95.18 ± 7.85 μm and 363.90 ± 12.16 μmol/min/unit, respectively. Oleanolic acid and ursolic acid from Ligustri Lucidi Fructus were chosen as ADA inhibitors with half maximal inhibitory concentration values of 21.82 ± 0.39 and 18.41 ± 0.14 μm, respectively. A non-competitive inhibition model was constructed and this assay can be used to screen other potential ADA inhibitors quickly and accurately.

Development of a Simple Assay Method for Adenosine Deaminase via Enzymatic Formation of an Inosine-Tb3+ Complex

Adenosine deaminase (ADA), which catalyzes the irreversible deamination of adenosine to inosine, is related to various human diseases such as tuberculous peritonitis and leukemia. Therefore, the method used to detect ADA activity and screen the effectiveness of various inhibitor candidates has important implications for the diagnosis treatment for various human diseases. A simple and rapid assay method for ADA, based on the enzymatic formation of a luminescent lanthanide complex, is proposed in this study. Inosine, an enzymatic product of ADA with stronger sensitization efficiency for Tb³⁺ than adenosine, produced a strong luminescence by forming an inosine-Tb³⁺ complex, and it enabled the direct monitoring of ADA activity in real-time. By introducing only Tb³⁺ to adenosine and ADA in the buffer, the enhancement of luminescence enabled the detection of a low concentration of ADA (detection limit 1.6 U/L). Moreover, this method could accurately determine the inhibition efficiency (IC₅₀) of the known ADA inhibitor, erhythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), and the inhibition of ADA could be confirmed by the naked eye. Considering its simplicity, this assay could be extended to the high-throughput screening of various ADA inhibitor candidates.

A Label-Free Fluorescent Assay for the Rapid and Sensitive Detection of Adenosine Deaminase Activity and Inhibition

Adenosine deaminase (ADA), able to catalyze the irreversible deamination of adenosine into inosine, can be found in almost all tissues and plays an important role in several diseases. In this work, we developed a label-free fluorescence method for the detection of adenosine deaminase activity and inhibition. In the presence of ADA, ATP has been shown to be hydrolyzed. The ATP aptamer was shown to form a G-quadruplex/thioflavin T (ThT) complex with ThT and exhibited an obvious fluorescence signal. However, the ATP aptamer could bind with ATP and exhibited a low fluorescence signal because of the absence of ADA. This assay showed high sensitivity to ADA with a detection limit of 1 U/L based on an SNR of 3 and got a good linear relationship within the range of 1–100 U/L with R² = 0.9909. The LOD is lower than ADA cutoff value (4 U/L) in the clinical requirement and more sensitive than most of the reported methods. This technique exhibited high selectivity for ADA against hoGG I, UDG, RNase H and λexo. Moreover, this strategy was successfully applied for assaying the inhibition of ADA using erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA) and, as such, demonstrated great potential for the future use in the diagnosis of ADA-relevant diseases, particularly in advanced drug development.

Gold nanoparticle-based colorimetric biosensors

Gold nanoparticles (AuNPs) provide excellent platforms for the development of colorimetric biosensors as they can be easily functionalised, displaying different colours depending on their size, shape and state of aggregation. In the last decade, a variety of biosensors have been developed to exploit the extent of colour changes as nano-particles (NPs) either aggregate or disperse, in the presence of analytes. Of critical importance to the design of these methods is that the behaviour of the systems has to be reproducible and predictable. Much has been accomplished in understanding the interactions between a variety of substrates and AuNPs, and how these interactions can be harnessed as colorimetric reporters in biosensors. However, despite these developments, only a few biosensors have been used in practice for the detection of analytes in biological samples. The transition from proof of concept to market biosensors requires extensive long-term reliability and shelf life testing, and modification of protocols and design features to make them safe and easy to use by the population at large. Developments in the next decade will see the adoption of user friendly biosensors for point-of-care and medical diagnosis as innovations are brought to improve the analytical performances and usability of the current designs. This review discusses the mechanisms, strategies, recent advances and perspectives for the use of AuNPs as colorimetric biosensors.

From Interface to Solution: Integrating Immunoassay with Netlike Rolling Circle Amplification for Ultrasensitive Detection of Tumor Biomarker

Both the 3D solution and the 2D interface play important roles in bioanalysis. For the former, reactions can be carried out adequately; while for the latter, interfering substance can be eliminated simply through wash. It is a challenge to integrate the advantages of solution-based assays and the interface-based assays. Here, we report an immuno-NRCA (netlike rolling circle amplification) strategy, which integrates immunoassay with NRCA for the ultrasensitive detection of tumor biomarker, by taking the assay of a tumour marker as an example. In this strategy, immunoreactions occur on interface, while the target-induced signal amplification can be completed totally in solution. As a result, this system has the merits of both solution- and interface-based assays. The whole procedure of this novel strategy is similar to the conventional ELISA, inheriting the usability. But in comparison with ELISA, the performance is greatly improved. The detection limit can be lowered to 5.5 fg/L, making it possible to detect the target tumour marker in one drop of blood. Also, in comparison with established immuno-PCR method, which integrates immunoassay with the commonly used nucleic acid amplification approach, this system has no requirement for thermal cycler owing to the isothermal amplification, and it has the ability to retain the immunoreactivities. So, the new immunoassay method proposed in this study may have more feasible applications in the future.

Gold nanoparticles as sensitive optical probes

Recent advances in Au NP based optical sensing systems for various analytes based on absorption, fluorescence and SERS are summarized.

Nucleic Acid Aptamers: An Emerging Tool for Biotechnology and Biomedical Sensing

Detection of small molecules or proteins of living cells provides an exceptional opportunity to study genetic variations and functions, cellular behaviors, and various diseases including cancer and microbial infections. Our aim in this review is to give an overview of selected research activities related to nucleic acid-based aptamer techniques that have been reported in the past two decades. Limitations of aptamers and possible approaches to overcome these limitations are also discussed.