Real-time label-free quantitative fluorescence microscopy-based detection of ATP using a tunable fluorescent nano-aptasensor platform

Organelle-Targeted Fluorescent Probes for Sulfane Sulfur Species

Sulfane sulfurs, which include hydropersulfides (RSSH), hydrogen polysulfides (H2Sn, n > 1), and polysulfides (RSnR, n > 2), play important roles in cellular redox biology and are closely linked to hydrogen sulfide (H2S) signaling. While most studies on sulfane sulfur detection have focused on sulfane sulfurs in the whole cell, increasing the recognition of the effects of reactive sulfur species on the functions of various subcellular organelles has emerged. This has driven a need for organelle-targeted detection methods. However, the detection of sulfane sulfurs, particularly of RSSH and H2Sn, in biological systems is still a challenge due to their low endogenous concentrations and instabilities. In this review, we summarize the development and design of organelle-targeted fluorescent sulfane sulfur probes, examine their organelle-targeting strategies and choices of fluorophores (e.g., ratiometric, near-infrared, etc.), and discuss their mechanisms and ability to detect endogenous and exogenous sulfane sulfur species. We also present the advantages and limitations of the probes and propose directions for future work on this topic.

Label-free bioassay with graphene oxide-based fluorescent aptasensors: A review

Bioassays using a fluorophore and DNA aptamer have been extensively developed due to the ultrasensitivity of fluorophores and recognition ability of DNA aptamers. Conventional fluorescent aptamer-based sensors (aptasensors) require chemical labeling between the fluorophore and aptamer and is technologically impracical for various sensing and assay applications. A simple "mix and go" strategy has been introduced that uses label-free technology as a platform for sensor development. The biosensors comprise a fluorophore, a ssDNA aptamer, and eco-friendly graphene oxide (GO). In the absence of the sensor target, GO quenches the fluorescence of the fluorophore and single-strand DNA aptamer complex. When the target is added, the DNA aptamer conformationally turns into a duplex, G-quadruplexe, or other secondary structure. This structure change leads to release of GO by the fluorophore-aptamer-target complex, generating dramatic fluorescence recovery and amplification. With this sensing method, the DNA aptamer does not need to be chemically labeled. Therefore, flexible fluorophore indicators and ssDNA aptamers can be used in this label-free aptasensing strategy. In this review, we discuss various unlabeled fluorophores, including synthetic small molecular fluorophores and genetically encoded fluorescent proteins, as indicators for generating GO-based fluorescent DNA aptasensors for label-free bioassay.

Advancement in Salmonella Detection Methods: From Conventional to Electrochemical-Based Sensing Detection

Large-scale food-borne outbreaks caused by Salmonella are rarely seen nowadays, thanks to the advanced nature of the medical system. However, small, localised outbreaks in certain regions still exist and could possess a huge threat to the public health if eradication measure is not initiated. This review discusses the progress of Salmonella detection approaches covering their basic principles, characteristics, applications, and performances. Conventional Salmonella detection is usually performed using a culture-based method, which is time-consuming, labour intensive, and unsuitable for on-site testing and high-throughput analysis. To date, there are many detection methods with a unique detection system available for Salmonella detection utilising immunological-based techniques, molecular-based techniques, mass spectrometry, spectroscopy, optical phenotyping, and biosensor methods. The electrochemical biosensor has growing interest in Salmonella detection mainly due to its excellent sensitivity, rapidity, and portability. The use of a highly specific bioreceptor, such as aptamers, and the application of nanomaterials are contributing factors to these excellent characteristics. Furthermore, insight on the types of biorecognition elements, the principles of electrochemical transduction elements, and the miniaturisation potential of electrochemical biosensors are discussed.

Recent progress, challenges, and prospects of fully integrated mobile and wearable point-of-care testing systems for self-testing

The rapid growth of research in the areas of chemical and biochemical sensors, lab-on-a-chip, mobile technology, and wearable electronics offers an unprecedented opportunity in the development of mobile and wearable point-of-care testing (POCT) systems for self-testing. Successful implementation of such POCT technologies leads to minimal user intervention during operation to reduce user errors; user-friendly, easy-to-use and simple detection platforms; high diagnostic sensitivity and specificity; immediate clinical assessment; and low manufacturing and consumables costs. In this review, we discuss recent developments in the field of highly integrated mobile and wearable POCT systems. In particular, aspects of sample handling platforms, recognition elements and sensing methods, and new materials for signal transducers and powering devices for integration into mobile or wearable POCT systems will be highlighted. We also summarize current challenges and future prospects for providing personal healthcare with sample-in result-out mobile and wearable POCT.

Quenched sandwich-type photoelectrochemical aptasensor for protein detection based on exciton energy transfer

This work proposes a quenched photoelectrochemical sensing method for highly selective and sensitive detection of protein via Energy Transfer (ET) effect between the AuNPs and CdS:Mn quantum dots. This detection was performed on a sandwich-type aptamer sensing interface. Chitosan modified CdS:Mn/TiO₂/ITO electrode was used to immobilize capture DNA (S1) via -CONH- bond. In the presence of target protein, AuNPs labeled DNA (AuNPs-S2) was further bonded to the protein to fabricate sandwich sensing platform, which forced the AuNPs away from the electrode surface. In this state, the photocurrent was greatly depressed, mainly due to two factors: (a) the ET effect produced by interparticle distance between CdS:Mn and AuNPs; (b) the steric hindrance of AuNPs-S2 partly obstructs the diffusion of the electron donor. The photocurrent decreased with the increasing concentration of the target protein. Using thrombin as a target, this sensitized method showed a detectable range of 0.1 pM to 8 nM and a detection limit of 30 fM. It possessed high selectivity and good stability for detection of thrombin. This method is extremely flexible and can be extended to varieties of protein targets.

Selection and preliminary application of a single stranded DNA aptamer targeting colorectal cancer serum

Colorectal cancer is one of the common causes of malignant tumors in recent years, thus the discovery of potential compounds that detect the occurrence of colorectal cancer by efficient approaches is necessary. In this study, the method of systematic evolution of ligands by exponential enrichment (SELEX) was used for recognizing serum from colorectal cancer patients by a single-stranded DNA library of aptamers assisted by single-walled carbon nanotubes (SWCNTs) to remove single-stranded DNA with low affinity. Ten rounds of selection were applied using colorectal cancer serum as a target with the serum of healthy individuals as a control. As the result, we have successfully identified four candidate aptamers after high-throughput genome sequencing analysis, comparison analysis and secondary structure prediction. Among them, aptamer Seq-2 exhibited the highest affinity and the strongest selectivity with an equilibrium dissociation constant (K_d) of 11.31 ± 3.25 nM and a C_t difference value of 4.25 ± 0.38 between the colorectal cancer group and the healthy group. Moreover, with fifty negative control serum samples, the positive detection rate of fifty positive serum samples tested by aptamer Seq-2 was over 90%. In particular, aptamer Seq-2 can strongly bind the colorectal cancer serum, less strongly bind the non-colon cancer serum and hardly bind the healthy serum. Therefore, aptamer Seq-2 presents enormous potential for exploring as a tumor diagnostic kit and detecting unknown tumor markers in serum to reflect colorectal cancer.

Aptamer Seq-2 with high affinity and selectivity was screened against colorectal cancer serum directly for clinical application.

Direct Screening for Cytometric Bead Assays for Adenosine Triphosphate

Cytometric bead assays have caught much attention because of their many exceptional advantages. Unfortunately, the immobilization of existing molecular recognition elements including monoclonal antibodies and aptamers onto solid particles may lead to the functional failure of the molecular recognition elements since they are generally obtained in free state. Herein we develop a powerful screening approach for direct and rapid discovery of aptamer based cytometric bead assays (AB-CBAs) by individually measuring the functional activity of every aptamer particles in a library and sorting them at rates of up to 10⁸ particles per hour. The strategy is based on the transformation of molecular libraries into pools of monoclonal aptamer particles so that one individual particle displays ∼10⁵ copies of an identical aptamer sequence. Our library design incorporates a two-color fluorescent reporter system in which changes in aptamer structure generate an optical readout, such that we can use fluorescence-activated cell sorting to rapidly and selectively separate the individual aptamer particles that exhibit large fluorescent signal change upon target binding. For demonstration, we isolated AB-CBA aptamer particles with high signaling performance for ATP after just 3 rounds of screening. We believe that the rapid and direct screening features of this strategy make it an excellent platform for generating AB-CBAs for for a wide range of important analytes.

SELEX methods on the road to protein targeting with nucleic acid aptamers

Systematic evolution of ligand by exponential enrichment (SELEX) is an efficient method used to isolate high-affinity single stranded oligonucleotides from a large random sequence pool. These SELEX-derived oligonucleotides named aptamer, can be selected against a broad spectrum of target molecules including proteins, cells, microorganisms and chemical compounds. Like antibodies, aptamers have a great potential in interacting with and binding to their targets through structural recognition and are therefore called "chemical antibodies". However, aptamers offer advantages over antibodies including smaller size, better tissue penetration, higher thermal stability, lower immunogenicity, easier production, lower cost of synthesis and facilitated conjugation or modification with different functional moieties. Thus, aptamers represent an attractive substitution for protein antibodies in the fields of biomarker discovery, diagnosis, imaging and targeted therapy. Enormous interest in aptamer technology triggered the development of SELEX that has underwent numerous modifications since its introduction in 1990. This review will discuss the recent advances in SELEX methods and their advantages and limitations. Aptamer applications are also briefly outlined in this review.

Culture-free, highly sensitive, quantitative detection of bacteria from minimally processed samples using fluorescence imaging by smartphone

A critical unmet need in the diagnosis of bacterial infections, which remain a major cause of human morbidity and mortality, is the detection of scarce bacterial pathogens in a variety of samples in a rapid and quantitative manner. Herein, we demonstrate smartphone-based detection of Staphylococcus aureus in a culture-free, rapid, quantitative manner from minimally processed liquid samples using aptamer-functionalized fluorescent magnetic nanoparticles. The tagged S. aureus cells were magnetically captured in a detection cassette, and then fluorescence was imaged using a smartphone camera with a light-emitting diode as the excitation source. Our results showed quantitative detection capability with a minimum detectable concentration as low as 10 cfu/ml by counting individual bacteria cells, efficiently capturing S. aureus cells directly from a peanut milk sample within 10 min. When the selectivity of detection was investigated using samples spiked with other pathogenic bacteria, no significant non-specific detection occurred. Furthermore, strains of S. aureus from various origins showed comparable results, ensuring that the approach can be widely adopted. Therefore, the quantitative fluorescence imaging platform on a smartphone could allow on-site detection of bacteria, providing great potential assistance during major infectious disease outbreaks in remote and resource-limited settings.

A smartphone imaging-based label-free and dual-wavelength fluorescent biosensor with high sensitivity and accuracy

The accuracy of a bioassay based on smartphone-integrated fluorescent biosensors has been limited due to the occurrence of false signals from non-specific reactions as well as a high background and low signal-to-noise ratios for complementary metal oxide semiconductor image sensors. To overcome this problem, we demonstrate dual-wavelength fluorescent detection of biomolecules with high accuracy. Fluorescent intensity can be quantified using dual wavelengths simultaneously, where one decreases and the other increases, as the target analytes bind to the split capture and detection aptamer probes. To do this, we performed smartphone imaging-based fluorescence microscopy using a microarray platform on a substrate with metal-enhanced fluorescence (MEF) using Ag film and Al₂O₃ nano-spacer. The results showed that the sensitivity and specificity of the dual-wavelength fluorescent quantitative assay for the target biomolecule 17-β-estradiol in water were significantly increased through the elimination of false signals. The detection limit was 1pg/mL and the area under the receiver operating characteristic curve of the proposed assay (0.922) was comparable to that of an enzyme-linked immunosorbent assay (0.956) from statistical accuracy tests using spiked wastewater samples. This novel method has great potential as an accurate point-of-care testing technology based on mobile platforms for clinical diagnostics and environmental monitoring.

ABCs of DNA aptamer and related assay development

This review is intended to guide the novice in aptamer research and development to understand virtually all of the aptamer development options and currently available assay modalities. Aptamer development topics range from discussions of basic and advanced versions of Systematic Evolution of Ligands by EXponential Enrichment (SELEX) and SELEX variations involving incorporation of exotic unnatural nucleotides to expand library diversity for even greater aptamer affinity and specificity to improved next generation methods of DNA sequencing, screening and tracking aptamer development throughout the SELEX process and characterization of lead aptamer candidates. Aptamer assay development topics include descriptions of various colorimetric and fluorescent assays in microplates or on membranes including homogeneous beacon and multiplexed Fluorescence Resonance Energy Transfer (FRET) assays. Finally, a discussion of the potential for marketing successful aptamer-based assays or test kits is included.

Molybdenum disulfide sphere-based electrochemical aptasensors for protein detection

In this work, we report the development of an ultrasensitive sandwich-type electrochemical aptasensor for protein detection.

Optical Aptasensors for Adenosine Triphosphate

Nucleic acids are among the most researched and applied biomolecules. Their diverse two- and three-dimensional structures in conjunction with their robust chemistry and ease of manipulation provide a rare opportunity for sensor applications. Moreover, their high biocompatibility has seen them being used in the construction of in vivo assays. Various nucleic acid-based devices have been extensively studied as either the principal element in discrete molecule-like sensors or as the main component in the fabrication of sensing devices. The use of aptamers in sensors - aptasensors, in particular, has led to improvements in sensitivity, selectivity, and multiplexing capacity for a wide verity of analytes like proteins, nucleic acids, as well as small biomolecules such as glucose and adenosine triphosphate (ATP). This article reviews the progress in the use of aptamers as the principal component in sensors for optical detection of ATP with an emphasis on sensing mechanism, performance, and applications with some discussion on challenges and perspectives.

A sensitive sandwich-type electrochemical aptasensor for thrombin detection based on platinum nanoparticles decorated carbon nanocages as signal labels

In this work, a novel and sensitive sandwich-type electrochemical aptasensor has been developed for thrombin detection based on platinum nanoparticles (Pt NPs) decorated carbon nanocages (CNCs) as signal tags. The morphological and compositional of the Pt NPs/CNCs were examined using transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. The results showed that the Pt NPs with about 3-5nm in diameter were well dispersed on the surface of CNCs. The thiolated aptamer was firstly immobilized on the gold electrode to capture the thrombin molecules, and then aptamer functionalized Pt NPs/CNCs nanocomposites were used to fabricate a sandwich sensing platform. Then, the high-content Pt NPs on carbon nanocages acting as hydrogen peroxide-mimicking enzyme catalyzed the reduction of H2O2, resulting in significant electrochemical signal amplification. Differential pulse voltammetry is employed to detect thrombin with different concentrations. Under optimized conditions, the approach provided a good linear response range from 0.05 pM to 20nM with a low detection limit of 10fM. This Pt NPs/CNCs-based aptasensor shows good precision, acceptable stability and reproducibility, which provided a promising strategy for electrochemical aptamer-based detection of other biomolecules.