Additional molecular biological amplification strategy for enhanced sensitivity of monitoring low-abundance protein with dual nanotags

A target-triggered ultra-sensitive aptasensor for simultaneous detection of Cd2+ and Hg2+ using MWCNTs-Au NPs modified electrode

A sensitive electrochemical assay for simultaneously detecting cadmium ion (Cd2+) and mercury ion (Hg2+) with the aptamer as recognition unit was established, in which methylene blue (MB) and target-triggered in-situ generated Ag nanoclusters (Ag NCs) were identified as signal reporters. Multi-walled carbon nanotubes and gold nanoparticles composites were prepared with polyethyleneimine to amplify electrical signals of screen-printed electrodes. Due to the particular base sequences, MB labeled Cd2+ aptamer paired with ssDNA through T-Hg-T structure with Hg2+. Notably, the C-rich structure in ssDNA acted as a template for the generation of Ag NCs, which could induce differential pulse voltammetry signals corresponding to Hg2+ concentrations. This electrochemical aptasensor exhibited detection limits of 94.01 pg/mL and 15.74 pg/mL for Cd2+ and Hg2+, respectively. The developed aptasensor allowed for practical application to tea and vegetable samples with satisfactory accuracy. This work possesses potential in developing biosensing technologies for simultaneous determination of multiple heavy metals.

Construction of an electrochemical-fluorescent dual-mode sensor with a dual-mode signal AgNC probe synthesized from cytosine-rich DNA for OTA detection

Herein, we have used DNA-silver nanocluster (DNA-AgNC) signal probes with both electrochemical and fluorescent signals for the first time to construct an electrochemical-fluorescent dual-mode sensor. The sensor has an easy-to-prepare dual-signal property combined with the magnetic separation technique for dual-mode detection of ochratoxin A (OTA). In the absence of OTA, the DNA strand used to synthesize AgNCs was not available in the system after magnetic separation. DNA-AgNCs probes could not be synthesized in the system, resulting in low fluorescence and electrochemical signals. In the presence of OTA, it led to the shedding of sulfhydryl-modified and cytosine-rich DNA (C-DNA). DNA-AgNCs probes with high fluorescence and electrochemical signals were formed by adding AgNO3 and NaBH4 to the supernatant after magnetic separation. Dual-mode detection of OTA was achieved by the signal response of fluorescence and electrochemistry. The detection ranges were 2.5 × 10-4-50 ng mL-1 and 2.5 × 10-4-25 ng mL-1 in the fluorescence mode and electrochemical mode with detection limits of 0.11 pg mL-1 and 0.025 pg mL-1, respectively. Meanwhile, the dual-mode sensor displayed better specificity, repeatability and reproducibility than conventional electrochemical and fluorescent single-mode sensors. The results of the spiked peanut and wheat flour detection showed that the fluorescence and electrochemical modes of the sensor exhibited satisfactory average recoveries.

Electrospun Nanofibers including Organic/Inorganic Nanohybrids: Polystyrene- and Clay-Based Architectures in Immunosensor Preparation for Serum Amyloid A

Diagnostic techniques based on biomolecules have application potential that can be realized in many fields, such as disease diagnosis, bioprocess imaging, food/beverage industries, and environmental pollutant imaging. Successful surface immobilization of biomolecules is critical to increasing the stabilization, sensitivity, and selectivity of biomolecules used in bioassay systems. Nanofibers are good candidates for the immobilization of biomolecules owing to many advantages such as morphology and pore size. In this study, montmorillonite (MMT) clay is modified with poly(amidoamine) (PAMAM) generation 3 (PAMAM_G3) and added to polystyrene (PS) solutions, following which PS/MMT-PAMAM_G3 nanofibers are obtained using the electrospinning method. The nanofibers are obtained by testing PS% (wt%) and MMT-PAMAM_G3% (wt%) ratios and characterized with scanning electron microscopy. Antiserum amyloid A antibody (Anti-SAA) is then conjugated to the nanofibers on the electrode surface via covalent bonds using a zero-length cross linker. Finally, the obtained selective surface is used for electrochemical determination of serum amyloid A (SAA) levels. The linear range of PS/MMT-PAMAM/Anti-SAA is between 1 and 200 ng/mL SAA, and the detection limit is 0.57 ng/mL SAA. The applicability of PS/MMT-PAMAM_G3/Anti-SAA is investigated by taking measurements in synthetic saliva and serum both containing SAA.

Recent advances in biological detection with rolling circle amplification: design strategy, biosensing mechanism, and practical applications

Rolling circle amplification (RCA) is a simple and isothermal DNA amplification technique that is used to generate thousands of repeating DNA sequences using circular templates under the catalysis of DNA polymerase.

Ion-selective electrode-based potentiometric immunoassays for the quantitative monitoring of alpha-fetoprotein by coupling rolling cycle amplification with silver nanoclusters

This work reports an ion-selective electrode-based potentiometric immunoassay for AFP detection coupling rolling cycle amplification with silver nanoclusters.

DNAzyme-Based Biosensors: Immobilization Strategies, Applications, and Future Prospective

Since their discovery almost three decades ago, DNAzymes have been used extensively in biosensing. Depending on the type of DNAzyme being used, these functional oligonucleotides can act as molecular recognition elements within biosensors, offering high specificity to their target analyte, or as reporters capable of transducing a detectable signal. Several parameters need to be considered when designing a DNAzyme-based biosensor. In particular, given that many of these biosensors immobilize DNAzymes onto a sensing surface, selecting an appropriate immobilization strategy is vital. Suboptimal immobilization can result in both DNAzyme detachment and poor accessibility toward the target, leading to low sensing accuracy and sensitivity. Various approaches have been employed for DNAzyme immobilization within biosensors, ranging from amine and thiol-based covalent attachment to non-covalent strategies involving biotin-streptavidin interactions, DNA hybridization, electrostatic interactions, and physical entrapment. While the properties of each strategy inform its applicability within a proposed sensor, the selection of an appropriate strategy is largely dependent on the desired application. This is especially true given the diverse use of DNAzyme-based biosensors for the detection of pathogens, metal ions, and clinical biomarkers. In an effort to make the development of such sensors easier to navigate, this paper provides a comprehensive review of existing immobilization strategies, with a focus on their respective advantages, drawbacks, and optimal conditions for use. Next, common applications of existing DNAzyme-based biosensors are discussed. Last, emerging and future trends in the development of DNAzyme-based biosensors are discussed, and gaps in existing research worthy of exploration are identified.

Protein detection based on rolling circle amplification sensors

Rolling circle amplification (RCA) is an isothermal process under the action of DNA polymerases. Large-scale DNA templates have been generated using RCA for target detection. Some signal amplification strategies including optical sensors and electrochemical sensors based on RCA have been applied to achieve sensitive detection. Sensors based on RCA have attracted increasing interest. Advances in RCA-based sensors for protein detection are reviewed in this paper. The advantages and detection mechanisms of sensors based on RCA are revealed and discussed. Finally, possible challenges and future perspectives are also outlined.

Determination of plasma β-amyloids by rolling circle amplification chemiluminescent immunoassay for noninvasive diagnosis of Alzheimer's disease

A rolling circle amplification chemiluminescence immunoassay (RCA-CLIA) was developed for precise quantitation of Aβ in plasma. Capture antibodies conjugated with magnetic beads and detection antibodies with collateral single-stranded DNA (ssDNA) were bound to Aβ42/Aβ40 antigens to form a typical double-antibody sandwich structure. The RCA reaction was triggered by the addition of ssDNA, which generated products with a large number of sites for the binding of acridinium ester (AE)-labeled detection probes, thereby realizing the purpose of the amplification. The RCA-CLIA method had higher sensitivity than conventional CLIA without loss of specificity. Under optimum conditions, the linear range of Aβ42 and Aβ40 detection was 3.9-140 pg/mL and 3.9-180 pg/mL, respectively, with corresponding low detection limits of 1.99 pg/mL and 3.14 pg/mL, respectively. Plasma Aβ42 and Aβ40 were detected in the blood of 21 AD patients and 22 healthy people, wherein this ratio could significantly distinguish AD patients from healthy individuals with a sensitivity of 90.48% and specificity of 63.64% for a cutoff value of 154. The Aβ42/Aβ40 ratio of plasma acts as an accurate indicator for AD diagnosis; therefore, detection of plasma Aβ using the RCA-CLIA exhibits great potential in noninvasive diagnosis and progressive assessment of AD.

Circular Nucleic Acids: Discovery, Functions and Applications

Circular nucleic acids (CNAs) are nucleic acid molecules with a closed-loop structure. This feature comes with a number of advantages including complete resistance to exonuclease degradation, much better thermodynamic stability, and the capability of being replicated by a DNA polymerase in a rolling circle manner. Circular functional nucleic acids, CNAs containing at least a ribozyme/DNAzyme or a DNA/RNA aptamer, not only inherit the advantages of CNAs but also offer some unique application opportunities, such as the design of topology-controlled or enabled molecular devices. This article will begin by summarizing the discovery, biogenesis, and applications of naturally occurring CNAs, followed by discussing the methods for constructing artificial CNAs. The exploitation of circular functional nucleic acids for applications in nanodevice engineering, biosensing, and drug delivery will be reviewed next. Finally, the efforts to couple functional nucleic acids with rolling circle amplification for ultra-sensitive biosensing and for synthesizing multivalent molecular scaffolds for unique applications in biosensing and drug delivery will be recapitulated.

Design principles and fundamental understanding of biosensors for amyloid-β detection

Aβ as biomarker in Alzheimer’s disease (AD) drives the significant research efforts for developing different biosensors with different sensing strategies, materials, and mechanisms for Aβ detection.

Advances in DNA/RNA detection using nanotechnology

Specific nucleic acid detection in vitro or in vivo has become increasingly important in the discovery of genetic diseases, diagnosing pathogen infection and monitoring disease treatment. One challenge, however, is that the amount of target nucleic acid in specimens is limited. Furthermore, direct sensing methods are also unable to provide sufficient sensitivity and specificity. Fortunately, due to advances in nanotechnology and nanomaterials, nanotechnology-based bioassays have emerged as powerful and promising approaches providing ultra-high sensitivity and specificity in nucleic acid detection. This chapter presents an overview of strategies used in the development and integration of nanotechnology for nucleic acid detection, including optical and electrical detection methods, and nucleic acid assistant recycling amplification strategies. Recent 5 years representative examples are reviewed to demonstrate the proof-of-concept with promising applications for DNA/RNA detection and the underlying mechanism for detection of DNA/RNA with the higher sensitivity and selectivity. Furthermore, a brief discussion of common unresolved issues and future trends in this field is provided both from fundamental and practical point of view.

Real-Time FO-SPR Monitoring of Solid-Phase DNAzyme Cleavage Activity for Cutting-Edge Biosensing

DNA nanotechnology has a great potential in biosensor design including nanostructuring of the biosensor surface through DNA origami, target recognition by means of aptamers, and DNA-based signal amplification strategies. In this paper, we use DNA nanotechnology to describe for the first time the concept of real-time solid-phase monitoring of DNAzyme cleavage activity for the detection of specific single-stranded DNA (ssDNA) with a fiber optic surface plasmon resonance (FO-SPR) biosensor. Hereto, we first developed a robust ligation strategy for the functionalization of the FO-SPR biosensing surface with ssDNA-tethered gold nanoparticles, serving as the substrate for the DNAzyme. Next, we established a relation between the SPR signal change, due to the cleavage activity of the 10-23 DNAzyme, and the concentration of the DNAzyme, showing faster cleavage kinetics for higher DNAzyme concentrations. Finally, we implemented this generic concept for biosensing of ssDNA target in solution. Hereto, we designed a DNAzyme-inhibitor complex, consisting of an internal loop structure complementary to the ssDNA target, that releases active DNAzyme molecules in a controlled way as a function of the target concentration. We demonstrated reproducible target detection with a theoretical limit of detection of 1.4 nM, proving that the presented ligation strategy is key to a universal DNAzyme-based FO-SPR biosensing concept with promising applications in the medical and agrofood sector.

Multiplex Analysis on a Single Porous Hydrogel Bead with Encoded SERS Nanotags

Bead-based assays have drawn more and more attention in biomedical fields. Herein, we proposed a novel approach to achieve multiplex analysis on a single porous hydrogel bead (PHB) with Raman dyes (RDs) encoded core-shell surface-enhanced Raman scattering (SERS) nanotags. Because of the amplified signal of RDs by core-shell metal structure of the nanotag and the high surface area to volume ratio (SVR) of the PHB, the sensitivity and linear dynamic range (LDR) of the as-proposed multiplex analysis method are significantly improved. We anticipate this approach to be widely used in the multiplex assays and biosensors.

Electrochemical Detection of Amyloid-β Oligomers Based on the Signal Amplification of a Network of Silver Nanoparticles

Amyloid-β oligomers (AβOs) are the most important toxic species in the brain of Alzheimer's disease (AD) patient. AβOs, therefore, are considered reliable molecular biomarkers for the diagnosis of AD. Herein, we reported a simple and sensitive electrochemical method for the selective detection of AβOs using silver nanoparticles (AgNPs) as the redox reporters and PrP(95-110), an AβOs-specific binding peptide, as the receptor. Specifically, adamantine (Ad)-labeled PrP(95-110), denoted as Ad-PrP(95-110), induced the aggregation and color change of AgNPs and the follow-up formation of a network of Ad-PrP(95-110)-AgNPs. Then, Ad-PrP(95-110)-AgNPs were anchored onto a β-cyclodextrin (β-CD)-covered electrode surface through the host-guest interaction between Ad and β-CD, thus producing an amplified electrochemical signal through the solid-state Ag/AgCl reaction by the AgNPs. In the presence of AβOs, Ad-PrP(95-110) interacted specifically with the AβOs, thus losing the capability to bind AgNPs and to induce the formation of an AgNPs-based network on the electrode surface. Consequently, the electrochemical signal decreased with an increase in the concentration of AβOs in the range of 20 pM to 100 nM. The biosensor had a detection limit of 8 pM and showed no response to amyloid-β monomers (AβMs) and fibrils (AβFs). On the basis of the well-defined and amplified electrochemical signal of the AgNPs-based network architecture, these results should be valuable for the design of novel electrochemical biosensors by marrying specific receptors.

Emerging technologies for biomedical analysis

Options for biomedical analysis continue to evolve from many fields of study, employing diverse detection and quantification methods. New technologies in this arena focus on improving the sensitivity of analysis and the speed of testing, as well as producing systems at low cost which can be used on site as a point-of-care device for telemedicine applications. In this article, the most important original experimental platforms as well as current commercial approaches to biomedical analysis are critically chosen and reviewed, covering January 2010 to January 2014. While literature is quite broad and numerous, there is clear emphasis on biological recognition and imaging for the most impactful works. The analytical approaches are discussed in terms of their utility in diagnostics and biomedical testing.

In situ DNA-templated synthesis of silver nanoclusters for ultrasensitive and label-free electrochemical detection of microRNA

On the basis of the use of silver nanoclusters (AgNCs) in situ synthesized by cytosine (C)-rich loop DNA templates as signal amplification labels, the development of a label-free and highly sensitive method for electrochemical detection of microRNA (miRNA-199a) is described. The target miRNA-199a hybridizes with the partial dsDNA probes to initiate the target-assisted polymerization nicking reaction (TAPNR) amplification to produce massive intermediate sequences, which can be captured on the sensing electrode by the self-assembled DNA secondary probes. These surface-captured intermediate sequences further trigger the hybridization chain reaction (HCR) amplification to form dsDNA polymers with numerous C-rich loop DNA templates on the electrode surface. DNA-templated synthesis of AgNCs can be realized by subsequent incubation of the dsDNA polymer-modified electrode with AgNO3 and sodium borohydride. With this integrated TAPNR and HCR dual amplification strategy, the amount of in situ synthesized AgNCs is dramatically enhanced, leading to substantially amplified current response for highly sensitive detection of miRNA-199a down to 0.64 fM. In addition, the developed method also shows high selectivity toward the target miRNA-199a. Featured with high sensitivity and label-free capability, the proposed sensing scheme can thus offer new opportunities for achieving sensitive, selective, and simple detection of different types of microRNA targets.

Wrapping DNA-gated mesoporous silica nanoparticles for quantitative monitoring of telomerase activity with glucometer readout

This work reports a simple and sensitive sensing protocol for the quantitative monitoring of telomerase activity based on target-responsive release of cargo from wrapping DNA-capped mesoporous silica nanoparticles (MSNs) by coupling with a portable personal glucometer (PGM).

Fabrication of a simple and sensitive QDs-based electrochemiluminescence immunosensor using a nanostructured composite material for the detection of tumor markers alpha-fetoprotein

A simple label-free electrochemiluminescence immunosensor was fabricated for detection of alpha-fetoprotein (AFP) based on a nanostructured composite material (PICA–MWNT) with 2-aminoethanethiol modified CdSe nanoclusters as luminescent particles.

Metal sulfide-functionalized DNA concatamer for ultrasensitive electronic monitoring of ATP using a programmable capillary-based aptasensor

A new flow-through electrochemical aptasensor was designed for ultrasensitive monitoring of adenosine triphosphate (ATP) by coupling microvalve-programmable capillary column with CdS-functionalized DNA concatamer for signal amplification. Initially, a layer of primary DNA-conjugated polyacrylamide hydrogel was covalently linked onto the internal surface of capillary column, and then an automated sequenctial injection format with a syringe pump was employed for development of the programmable capillary-based aptasensor. In the presence of target DNA aptamer, the immobilized primary DNA hybridized with partial bases of the aptamer. The excess aptamer fregment could trigger the formation of DNA concatamer between auxiliary DNA1 and CdS-labeled auxiliary DNA2. Upon target ATP introduction, a specific ATP-aptamer reaction was excuated, thereby resulting in the release of CdS-functionalized DNA concatamer from the capillary. Subsenquent anodic stripping voltammetric detection of cadmium released under acidic conditions from the released CdS nanoparticles could be conducted in a homemade detection cell. Under optimal conditions, the dynamic concentration range spanned from 0.1 pM to 10nM ATP with a detection limit of 0.06 pM ATP. The electrochemical aptasensor showed good reproducibility, selectivity, and stability. In addition, the methodology was evaluated for the analysis of ATP spiked serum samples, and the recoveries was 81-140%.