Ultrasensitive Multiplexed Immunoassay for Tumor Biomarkers Based on DNA Hybridization Chain Reaction Amplifying Signal

Facile and sensitive detection of mercury ions based on fluorescent structure-switching aptamer probe and exonuclease Ⅲ-assisted signal amplification

Hg2+ is highly toxic to human health and ecosystem. In this work, based on the unique fluorescent property of 2-Aminopurine (2-AP), the formation of T-Hg2+-T mismatch structure and the signal amplification of exonuclease III (Exo III) assisted target cycle, a fluorescent probe for facile and sensitive detection of Hg2+ is constructed. The hairpin-looped DNA probe is rationally designed with 2-AP embedded in the stem and thymine-rich recognition overhangs extended at the termini. The cleavage of the double stranded DNA stem with stable T-Hg2+-T pairs catalyzed by Exo III is prompted to happen upon recognition of trace Hg2+. Under the optimal reaction conditions, there is an excellent linear relationship between Hg2+ concentration and fluorescence intensity in the range of 7.5-200 nM with a detection limit of 0.38 nM. In addition, the detection results of Hg2+ in Songhua River water and fish samples are satisfactory. The fluorescent probe avoids labeling additional quenchers or quenching materials and has strong anti-interference ability. Thus, the fluorescent probe has a broad prospect in practical application.

High-sensitive ferrocene labeled aptasensor for the detection of Mucin 1 by tuning the sequence constitution of complementary probe

A strand displacement-based "signal-off" electrochemical aptasensor is reported for the detection of Mucin 1 (MUC 1) based on a high original signal. Different from the conventional "signal-off" electrochemical biosensors where electrochemical substances are dispersed in electrolyte solution, here the current signal was generated by the complementary probe (CP) associated with ferrocene (Fc) labeled aptamer (Apt.-Fc). Because Apt.-Fc and MUC 1 have a higher affinity, Apt.-Fc dissociates from CP in the presence of MUC 1, resulting in a reduction of detection current signal generated by oxidation of labeled Fc. In this system, high detection signal is necessary to improve the sensor's performance. For this aim, a strategy is proposed for changing the modalities of electron transport and the quantity of Apt.-Fc introduced by simply tuning the sequence constitution of CP. As expected, a high detection current signal was obtained after selecting CP(Apt.-Fc)-TTT as the optimal CP. The aptasensor was then employed to detect MUC 1, and satisfactory detection results with a low detection limit (LOD) of 0.087 pM (S/N = 3), good specificity, good stability, and feasibility of detection of MUC 1 in artificial serum (recovery of 92-101%, RSD of 1.36-5.23%) were obtained.

Highly sensitive magnetic relaxation sensing method for aflatoxin B1 detection based on Au NP-assisted triple self-assembly cascade signal amplification

Highly sensitive detection of aflatoxin B₁ (AFB₁) is of great significance because of its high toxicity and carcinogenesis. We propose a magnetic relaxation sensing method based on gold nanoparticles (Au NPs)-assisted triple self-assembly cascade signal amplification for highly sensitive detection of AFB₁. Both AFB₁ antibody and initiator DNA (iDNA) are labeled on Au NPs to form Ab-Au-iDNA probe. iDNA is enriched by Au NPs to achieve first signal amplification. Different amounts of Ab-Au-iDNA were bound with AFB₁ antigen by indirect competitive immunoassay, and then hybridization chain reaction event was initiated by iDNA to produce long hybridization chain reaction products to enrich more horseradish peroxidase-streptavidin for the second signal amplification. Dopamine could be rapidly converted to polydopamine by HRP catalysis, which is used as the third signal amplification. The Fe³⁺ solution, providing paramagnetic ions with a strong magnetic signal, could be adsorbed by the polydopamine due to the formation of coordination bonds of phenolic hydroxyl groups with Fe³⁺. This effective interaction between polydopamine and Fe³⁺ significantly changes the transverse relaxation time signal of Fe³⁺ supernatant solution, which can be used as a magnetic probe for highly sensitive detection of AFB₁. The sensor exhibited high specificity and sensitivity with a detection limit of 0.453 pg/mL owing to the Au NP-assisted triple self-assembly cascade signal amplification strategy. It has been successfully employed for AFB₁ detection in animal feed samples with consistent results of enzyme linked immune sorbent assay and high-performance liquid chromatography.

Recent Progress in DNA Hybridization Chain Reaction Strategies for Amplified Biosensing

With the continuous development of DNA nanotechnology, various spatial DNA structures and assembly techniques emerge. Hybridization chain reaction (HCR) is a typical example with exciting features and bright prospects in biosensing, which has been intensively investigated in the past decade. In this Spotlight on Applications, we summarize the assembly principles of conventional HCR and some novel forms of linear/nonlinear HCR. With advantages like great assembly kinetics, facile operation, and an enzyme-free and isothermal reaction, these strategies can be integrated with most mainstream reporters (e.g., fluorescence, electrochemistry, and colorimetry) for the ultrasensitive detection of abundant targets. Particularly, we select several representative studies to better illustrate the novel ideas and performances of HCR strategies. Theoretical and practical utilities are confirmed for a range of biosensing applications. In the end, a deep discussion is provided about the challenges and future tasks of this field.

Ultrasensitive zero-background photoelectrochemical biosensor for analysis of organophosphorus pesticide based on in situ formation of DNA-templated Ag2S photoactive materials

Herein, a novel signal-on photoelectrochemical (PEC) biosensor with nearly zero background noise (ZBN) was first fabricated to determine the presence of organophosphorus pesticide based on in situ formation of DNA-templated Ag₂S photoactive materials, accompanied by hybridization chain reaction (HCR) signal amplification. The capture probe (S1) on the gold nanoparticle-modified electrode can hybridize with the aptamer molecule to generate a simple PEC biosensor. In the presence of a target molecule, the aptamer molecule is released on the double-stranded DNA (dsDNA)-modified PEC biosensor. Meanwhile, the capture probe remains on the electrode and can open the DNA hairpins (H1, H2) which are rich in cytosine, to trigger the HCR reaction. The rich "C" strands are uncovered after formation of a long dsDNA polymer strand, which can assemble multiple silver ions (Ag⁺) by means of by C-Ag⁺-C chelation. Then, a large number of Ag₂S can be generated by challenging with S^2- solution, producing a satisfactory photocurrent signal. The photoactive material is formed in situ, which eliminates the laborious operation. Moreover, the signal can be highly amplified with nearly zero background noise and HCR signal amplification. Under optimal conditions, the ZBN aptasensor exhibited high sensitivity and selectivity, with a low detection limit of 2 pg mL^-1 for malathion. Importantly, the sensing platform can also be applied to determine the presence of malathion in real samples. In this assay, a novel signal-on photoelectrochemical biosensor with nearly zero background noise was first fabricated to determine the presence of organophosphorus pesticide based on in situ formation of DNA-templated Ag₂S photoactive materials, accompanied by hybridization chain reaction signal amplification.

A Novel Electrochemical Aptasensor for the Ultrasensitive Detection of Adenosine Triphosphate Based on DNA-Templated Copolymers

Adenosine triphosphate (ATP) is a small but significant biological molecule that plays an important role in regulating cellular metabolism processes. Accurate and sensitive analytical techniques for detecting ATP are urgently needed. Herein, a new electrochemical aptasensor was designed in light of DNA-templated copolymers that parallel to the electrode for sensitive gauging of ATP. The ATP aptamer decorated by the electronic medium ferrocene can be regarded as a ″bridge″ connecting two DNA-templated copolymers. When ATP exists, because of the extraordinary binding selectivity of DNA-templated copolymers for target ATP, the rapid electron transfer of ferrocene was beneficial to the sensitive detection of target ATP. Specifically, our parallel DNA copolymers are more stable than upright DNA copolymers and have a faster signal transmission because of the close distance to the electrode; meanwhile, the nonspecific pollution is less. Consequently, the developed novel aptasensor exhibited a wide range of linear response toward ATP that was from 0.1 to 10 000 pM and high selectivity with a detection limit down to 11.5 fM. The electrochemical aptasensor has a broad application prospect in the detection of small biological molecules.

Functional Micro-/Nanomaterials for Multiplexed Biodetection

When analyzing biological phenomena and processes, multiplexed biodetection has many advantages over single-factor biodetection and is highly relevant to both human health issues and advancements in the life sciences. However, many key problems with current multiplexed biodetection strategies remain unresolved. Herein, the main issues are analyzed and summarized: 1) generating sufficient signal to label targets, 2) improving the signal-to-noise ratio to ensure total detection sensitivity, and 3) simplifying the detection process to reduce the time and labor costs of multiple target detection. Then, available solutions made possible by designing and controlling the properties of micro- and nanomaterials are introduced. The aim is to emphasize the role that micro-/nanomaterials can play in the improvement of multiplexed biodetection strategies. Through analyzing existing problems, introducing state-of-the-art developments regarding relevant materials, and discussing future directions of the field, it is hopeful to help promote necessary developments in multiplexed biodetection and associated scientific research.

Nucleic Acid Amplification Techniques in Immunoassay: An Integrated Approach with Hybrid Performance

An immunoassay is mostly employed for the direct detection of food contaminants, and a molecular assay for targeting nucleic acids employs amplification techniques for distinguishing genes. The integration of an immunoassay with nucleic acid amplification techniques inherits the direct and rapid performance of an immunoassay and the ultrasensitive merit of a molecular assay. Enthusiastic attention has been attracted in recent years on the utilization of isothermal amplification techniques in an immunoassay, as well as the employment of a lateral flow immunoassay in a molecular assay. Thus, this Review discussed these kinds of approaches from two categories: immuno-nucleic acid amplification (I-NAA) and nucleic acid amplification-immunoassay (NAA-I). The advantages, drawbacks, and future developments were discussed for a comprehensive understanding.

Electrochemical prostate-specific antigen biosensors based on electroconductive nanomaterials and polymers

Prostate cancer (PCa), the second most malignant neoplasm in men, is also the fifth leading cause of cancer-related deaths in men globally. Unfortunately, this malignancy remains largely asymptomatic until late-stage emergence when treatment is limited due to the lack of effective metastatic PCa therapeutics. Due to these limitations, early PCa detection through prostate-specific antigen (PSA) screening has become increasingly important, resulting in a more than 50% decrease in mortality. Conventional assays for PSA detection, such as enzyme-linked immunosorbent assay (ELISA), are labor intensive, relatively expensive, operator-dependent and do not provide adequate sensitivity. Electrochemical biosensors overcome these limitations because they are rapid, cost-effective, simple to use and ultrasensitive. This article reviews electrochemical PSA biosensors using electroconductive nanomaterials such as carbon-, metal-, metal oxide- and peptide-based nanostructures, as well as polymers to significantly improve conductivity and enhance sensitivity. Challenges associated with the development of these devices are discussed thus providing additional insight into their analytic strength as well as their potential use in early PCa detection.

Double ion-exchange reaction-based photoelectrochemical immunoassay for sensitive detection of prostate-specific antigen

This work developed a double ion-exchange reaction-based photoelectrochemical (PEC) immunoassay with the split-type detection mode for sensitive detection of prostate-specific antigen (PSA, used as a model). The nanocomposite of cadmium sulfide and nickel sulfide (CdS@NiS nanocomposite), as the photoactive material, was rapidly synthesized by two-step hydrothermal treatment. In the presence of target PSA, the cupric oxide nanoparticle (CuO NP) labeled detection antibody was introduced into the detection system by sandwich immunoreaction and the copper (Cu²⁺) ions was released from CuO nanoparticles by acid to participate in double ion-exchange reaction. The double ion-exchange reaction on the photoelectric sensing interface between Cu²⁺ and CdS@NiS nanocomposites formed the weak photoactive material Cu_xS (x = 1, 2) to reduce the photocurrent. Under optimal conditions, the double ion-exchange reaction-based PEC immunoassay exhibited good photocurrent responses toward target PSA within the dynamic working range from 0.01 ng mL^-1 to 50 ng mL^-1 at a low limit of detection (LOD) of 2.9 pg mL^-1. Besides, our work could achieve good reproducibility and high specificity under the split-type detection mode. Compared with human PSA ELISA kit, the accuracy obtained by our strategy was satisfactory. Importantly, this Cu²⁺-activated double ion-exchange reaction-based PEC immunoassay provides a promising platform for the detection of biomarkers.

Isothermal Nucleic Acid Amplification Techniques and Their Use in Bioanalysis

Recently, there has been a rapid progress in the development of techniques for isothermal amplification of nucleic acids as an alternative to polymerase chain reaction (PCR). The advantage of these methods is that the nucleic acids amplification can be carried out at constant temperature, unlike PCR, which requires cyclic temperature changes. Moreover, isothermal amplification can be conducted directly in living cells. This review describes the principles of isothermal amplification techniques and demonstrates their high efficiency in designing new highly sensitive detection methods of nucleic acids and enzymes involved in their modifications. The data on successful application of isothermal amplification methods for the analysis of cells and biomolecules with the use of DNA/RNA aptamers are presented.

Advances in prostate specific antigen biosensors-impact of nanotechnology

Prostate cancer is one of the most dangerous and deadly cancers in elderly men. Early diagnosis using prostate-specific antigen (PSA) facilitates disease detection, management and treatment. Biosensors have recently been used as sensitive, selective, inexpensive and rapid diagnostic tools for PSA detection. In this review, a variety of PSA biosensors such as aptasensors, peptisensors and immunesensors are highlighted. These use aptamers, peptides and antibodies in the biorecognition element, respectively, and can detect PSA with very high sensitivity via electrochemical, electrochemiluminescence, fluorescence and surface-enhanced Raman spectroscopy. To improve the sensitivity of most of these PSA biosensors, different nanostructured materials have played a critical role.

Target-Driven Cascade-Amplified Release of Loads from DNA-Gated Metal-Organic Frameworks for Electrochemical Detection of Cancer Biomarker

In this paper, a label-free and stimuli-responsive electrochemical biosensing platform was fabricated based on target-driven load release from DNA-gated metal-organic frameworks (MOFs) with cascade amplification. By using MOFs (UiO-66-NH₂) as a nanocarrier of electroactive molecules (methylene blue; MB) and the programmably assembled DNA acted as the gatekeeper, the biofunctionalized MOFs (MB@DNA/MOFs) were not only used as an amplified signal label but also worked as three-dimensional tracks for biosensing. In the presence of a target, the nicking endonuclease cleavage process was triggered, leading to the generation of two strands (S1 and S2). Both S1 and S2 act as stimuli to participate in the strand displacement reaction on the MB@DNA/MOFs, which caused the unlocking of the pore to release MB, resulting in the decrease of the signal. Using carcinoembryonic antigen (CEA) as a model target, the cascade-amplified biosensor presented good performance for CEA detection, ranging from 50 fg/mL to 10 ng/mL with a detection limit of 16 fg/mL. The stimuli-responsive DNA-gated MOF-based electrochemical platform exhibited three-dimensional biosensing tracks with rational utilization of the cascade amplification, providing an effective method for cancer biomarker detection.

Dual-Signaling Amplification Electrochemical Aptasensor Based on Hollow Polymeric Nanospheres for Acetamiprid Detection

In this work, we first reported a dual-signaling electrochemical aptasensor based on layer-by-layer template technology and catalytic amplification for acetamiprid detection. Herein, the signal probe of the ferrocene (Fc)-based hollow polymeric nanospheres (Fc-HPNs) were prepared with repeated electrostatic adsorption between anionic poly(acrylic acid) and hyperbranched cationic polyethylenimine. In addition, ascorbic acid (AA) as an enhancer can catalyze the reduction of Fc-HPNs, which results in significant enhancement of the oxidation peak current of Fc-HPNs. Remarkably, the Fc-HPNs played dual roles: as nanocarriers to significantly increase the load amount of Fc and as nanoreducers to effectively catalyze reduction by AA for further signal amplification. Therefore, because of the special nanostructures of Fc-HPNs and the effective catalytic effect of AA, a dual-signaling electrochemical aptasensor was proposed. Surprisingly, this proposed assay for trace amounts of target detection exhibits excellent sensitivity with a linear range from 10 nM to 1 fM and a limit of detection down to 0.33 fM (S/N = 3), which opened a novel avenue and versatile strategy for monitoring of acetamiprid.

A nanozyme tag enabled chemiluminescence imaging immunoassay for multiplexed cytokine monitoring

We report a new concept of a chemiluminescence imaging nanozyme immunoassay (CINIA), in which nanozymes are exploited as catalytic tags for simultaneous multiplex detection of cytokines. The CINIA provides a novel and universal nanozyme-labeled multiplex immunoassay strategy for high-throughput detection of relevant biomarkers and further disease diagnosis.

DNA concatemer-silver nanoparticles as a signal probe for electrochemical prostate-specific antigen detection

A ultrasensitive electrochemical detection of prostate-specific antigen was reported based on hybridization chain reaction amplifying silver nanoparticles response signal.

Chemical Redox-Cycling for Improving the Sensitivity of Colorimetric Enzyme-Linked Immunosorbent Assay

Herein, a redox-cycling was proposed to amplify the signal of enzyme-linked immunosorbent assay (ELISA), which was performed in a polystyrene microplate based on a classic sandwich-type. After the sandwich immunoreactions were finished, the alkaline phosphatase captured on a microplate triggered the hydrolyzation of l-ascorbic acid 2-phosphate to generate ascorbic acid (AA), which then reduced colorless tris(bathophenanthroline) iron(III) (Fe(BPT)₃³⁺) encapsulated in the micelle of TX-100 to pink red tris(bathophenanthroline) iron(II) (Fe(BPT)₃²⁺). In the presence of tris(2-carboxyethyl)phosphine, the oxidation product, dehydroascorbic acid, was transformed to AA quickly which then reduced Fe(BPT)₃³⁺ again and again, resulting in the generation of abundant Fe(BPT)₃²⁺ that could be read out conveniently by a commercial microplate reader or the naked eye. Because the negative charged TCEP with large size could hardly pass through the micelle, the reduction of Fe(BPT)₃³⁺ by TCEP directly was negligible. Experiment results for assay of alpha-fetoprotein (a model antigen) showed the cycling greatly improved the detection limit to 5 pg/mL, 2 orders of magnitude lower than that of conventional ELISA. The cycling also exhibited the advantages of simplicity and high reproducibility, implying its great potential for practical applications in biological and clinical diagnosis.

Multiplexed electrochemical immunoassay for two immunoglobulin proteins based on Cd and Cu nanocrystals

Herein, a simple and feasible electrochemical immunosensing method for simultaneous voltammetric detection of two immunoglobulin proteins, human IgG (HIgG) and rabbit IgG (RIgG), was developed using two distinguishable signal-generation tags on the same electrode. The immunosensor was prepared by immobilizing two Fab antibody fragments on a gold electrode. After this, Cu and Cd nanocrystals, as nanotags, were synthesized and functionalized with identical detection antibodies. Transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the Cu and Cd nanocrystals. The covalently modified electrode with the Fab antibody fragments through the Au-thiolate bond (to dispel the non-specific adsorption) was investigated via scanning electron microscopy (SEM). After the sandwiched immunoreaction, the antibody-modified nanocrystals were captured on the immunosensor, which could be interrogated in pH 3.5 HCl using square-wave anodic stripping voltammetry. Experimental results also indicated that the multiplexed immunoassay enabled the simultaneous detection of HIgG and RIgG in a single run with the similar linear range from 0.01 to 10 ng mL^-1, and the limits of detection (LODs) towards two analytes could be as low as 3.4 pg mL^-1 (at 3σ). Acceptable assay results on precision, reproducibility, specificity, and method accuracy were also acquired. Importantly, the newly designed strategy avoided cross-talk and enzymatic introduction as compared to conventional electrochemical immunoassays, thus exhibiting a promising potential in clinical applications.

Spectrum-Resolved Triplex-Color Electrochemiluminescence Multiplexing Immunoassay with Highly-Passivated Nanocrystals as Tags

Nanocrystals (NCs) were extensively employed in optical-multiplexing with their size-dependent and narrow waveband photoluminescence (PL). Herein, to achieve NCs-based novel optical-multiplexing strategy with improved sensitivity, a spectrum-resolved triplex-color electrochemiluminescence (ECL) multiplexing immunoassay (MIA) was proposed for the first time by extensively exploring the color-different monochromatic ECL from dual-stabilizers-capped CdTe and CdSe NCs and the inherent lower background of ECL than PL. As a proof of concept, carcinoembryonic antigen (CEA), prostate specific antigen (PSA), and alpha fetoprotein (AFP) were adopted as model analytes and formed three antigen-NCs pairs at the glassy carbon electrode surface via a one-pot immune-reaction with CdSe(550), CdTe(650), and CdTe(776) NCs as tags, respectively. The spectral ECL responses of three antigen-NCs pairs were simultaneously measured via one-pot ECL assay, and the maximum ECL intensity of each tag was directly utilized to determine corresponding antigen without any assistance from either optical filters or signal deconvolution. This ECL-MIA displayed negligible cross-reactivity, high color-selectivity, and excellent sensitivity with limits of detection down to 1, 10, and 0.01 pg/mL for CEA, PSA, and AFP, respectively, which might provide a promising alternative to NCs PL multiplexing.

An ultrasensitive flow cytometric immunoassay based on bead surface-initiated template-free DNA extension

An ultrasensitive flow cytometric immunoassay (TdT-FCI) is developed based on bead surface-initiated template-free DNA extension.

Proteins lack the duplication mechanism like nucleic acids, so the connection of immunoassays with effective nucleic acid amplification techniques has become a powerful way for the detection of trace protein biomarkers in biological fluids. However, such immunoassays generally suffer from rather stringent DNA sequence design and complicated operations. Herein, we propose a simple but highly sensitive flow cytometric immunoassay (FCI) by employing on-bead terminal deoxynucleotidyl transferase (TdT)-initiated template-free DNA extension as an effective signal amplification pathway (TdT-FCI), and gold nanoparticles (AuNPs) co-functionalized with both the detection antibody and a 3′-OH oligonucleotide (ODN) as the transducer to bridge the immunoassay and subsequent TdT-mediated DNA amplification. The target antigen can sandwich with the capture antibody immobilized on the magnetic beads (MBs) and the detection antibody on the AuNPs to bring a lot of ODNs onto the surface of MBs. Each ODN on the MBs can be effectively elongated by TdT in a template-free manner to produce a long poly(T) tail, which will then bind to many 6-carboxyfluorescein (FAM)-labeled poly(A)25. Since each AuNP can carry multiple ODNs and each extended ODN can ultimately capture numerous FAM-poly(A)25, efficiently amplified fluorophore accumulation on the MBs can be achieved. The fluorescent MBs can be individually interrogated with a flow cytometer and thus quantitative analysis of the target antigen can be realized. Coupled with the powerful flow cytometry analysis, the simple but efficient TdT-based signal amplification mechanism has pushed the detection limit of prostate specific antigen (PSA) down to a low level of 0.5 pg mL^–1. Furthermore, based on an elegant bead size-encoding principle, we have further advanced the TdT-FCI for multiplexed antigen detection in a single reaction. Sharing the unique merits of simple design and operation, efficient signal amplification, powerful signal readout and the capability for multiplexed analysis, this TdT-FCI provides a versatile tool for detecting trace antigen biomarkers towards clinical diagnosis as well as prognosis.

Amplified electrochemiluminescence detection of CEA based on magnetic Fe3O4@Au nanoparticles-assembled Ru@SiO2 nanocomposites combined with multiple cycling amplification strategy

In this work, we designed a new strategy for ultrasensitive detection of CEA based on efficient electrochemiluminescence (ECL) quenching of Ru(bpy)₃²⁺-doped SiO₂ nanocomposite by ferrocene using target recycling amplification technique. A large number of Ru@SiO₂ ECL signal probe were firstly assembled on the novel magnetic core-shell Fe₃O₄@Au nanoparticles (NPs), then the ferrocene-labeled ECL quenching probe (Fc-probe) was linked to the magnetic NPs. Finally, numerous DNA1 sequences were produced by target CEA-triggered multiple recycling amplification and displaced the Fc-probe on the magnetic NPs, leading to significantly enhanced ECL signal for CEA detection. Because of the designed cascade signal amplification strategy, the newly developed method achieved a wide linear range of 10 fg/mL to 10 ng/mL with a low detection limit of  3.5 fg/mL. Furthermore, taking advantages of the magnetic Fe₃O₄@Au NPs for carring abundant signal probes, sensing target and ECL detection, the developed ECL strategy is convenient, rapid and displayed high sensitivity for CEA detection, which has great potential for analyzing the clinical samples in practical disease diagnosis applications.

Chemical and Biological Sensing Using Hybridization Chain Reaction

Since the advent of its theoretical discovery more than 30 years ago, DNA nanotechnology has been used in a plethora of diverse applications in both the fundamental and applied sciences. The recent prominence of DNA-based technologies in the scientific community is largely due to the programmable features stored in its nucleobase composition and sequence, which allow it to assemble into highly advanced structures. DNA nanoassemblies are also highly controllable due to the precision of natural and artificial base-pairing, which can be manipulated by pH, temperature, metal ions, and solvent types. This programmability and molecular-level control have allowed scientists to create and utilize DNA nanostructures in one, two, and three dimensions (1D, 2D, and 3D). Initially, these 2D and 3D DNA lattices and shapes attracted a broad scientific audience because they are fundamentally captivating and structurally elegant; however, transforming these conceptual architectural blueprints into functional materials is essential for further advancements in the DNA nanotechnology field. Herein, the chemical and biological sensing applications of a 1D DNA self-assembly process known as hybridization chain reaction (HCR) are reviewed. HCR is a one-dimensional (1D) double stranded (ds) DNA assembly process initiated only in the presence of a specific short ssDNA (initiator) and two kinetically trapped DNA hairpin structures. HCR is considered an enzyme-free isothermal amplification process, which shows substantial promise and offers a wide range of applications for in situ chemical and biological sensing. Due to its modular nature, HCR can be programmed to activate only in the presence of highly specific biological and/or chemical stimuli. HCR can also be combined with different types of molecular reporters and detection approaches for various analytical readouts. While the long dsDNA HCR product may not be as structurally attractive as the 2D and 3D DNA networks, HCR is highly instrumental for applied biological, chemical, and environmental sciences, and has therefore been studied to foster a variety of objectives. In this review, we have focused on nucleic acid, protein, metabolite, and heavy metal ion detection using this 1D DNA nanotechnology via fluorescence, electrochemical, and nanoparticle-based methodologies.

Proximity aptasensor for protein detection based on an enzyme-free amplification strategy

A novel electrochemical aptasensor for the detection of trace protein is proposed based on proximity binding-induced strand displacement and hybridization chain reaction. This method is proven to be highly selective and has potential practical utility, and offers new opportunities for the convenient detection of proteins with an enzyme-free amplification process.

Tumor target amplification: Implications for nano drug delivery systems

Tumor cells overexpress surface markers which are absent from normal cells. These tumor-restricted antigenic signatures are a fundamental basis for distinguishing on-target from off-target cells for ligand-directed targeting of cancer cells. Unfortunately, tumor heterogeneity impedes the establishment of a solid expression pattern for a given target marker, leading to drastic changes in quality (availability) and quantity (number) of the target. Consequently, a subset of cancer cells remains untargeted during the course of treatment, which subsequently promotes drug-resistance and cancer relapse. Since target inefficiency is only problematic for cancer treatment and not for treatment of other pathological conditions such as viral/bacterial infections, target amplification or the generation of novel targets is key to providing eligible antigenic markers for effective targeted therapy. This review summarizes the limitations of current ligand-directed targeting strategies and provides a comprehensive overview of tumor target amplification strategies, including self-amplifying systems, dual targeting, artificial markers and peptide modification. We also discuss the therapeutic and diagnostic potential of these approaches, the underlying mechanism(s) and established methodologies, mostly in the context of different nanodelivery systems, to facilitate more effective ligand-directed cancer cell monitoring and targeting.

A dual-cycling biosensor for target DNA detection based on the toehold-mediated strand displacement reaction and exonuclease III assisted amplification

Based on the toehold-mediated strand displacement reaction and exonuclease III assisted amplification, a sensitive and simple target DNA biosensor was established.

Biofunctionalized mesoporous silica nanospheres for the ultrasensitive chemiluminescence immunoassay of tumor markers

Mesoporous silica nanospheres (SiO₂) are synthesized and biofunctionalized for the development of an ultrasensitive chemiluminescent (CL) immunosensor for tumor markers.

Ultrasensitive detection of prostate specific antigen by electrochemical aptasensor using enzyme-free recycling amplification via target-induced catalytic hairpin assembly

Based on the target-induced catalytic hairpin assembly and bimetallic catalyst, the enzyme-free recycling amplification strategy for sensitive detection of prostate specific antigen (PSA) has been designed. The aptamer and its complementary DNA (C-apt) are modified on the magnetic particles. The aptamer-PSA binding event can release the C-apt that triggers the catalytic assembly between hairpin capture DNA and hairpin help DNA. Then the catalytic hairpin assembly leads to cyclic reuse the C-apt and the generation of many opened hairpin capture DNA, which can associate with the prepared Au/Pt-polymethylene blue (PMB) probes to yield electrochemical signal. Meanwhile, the Au/Pt-PMB probes exhibit excellent electrocatalytic ability for H₂O₂ to magnify the response current. The designed sensor possesses a wide dynamic range of 10fgmL^-1 to 100ngmL^-1 and ultra-low detection limit of 2.3fgmL^-1. The present method has good performance in real serum sample analysis. This strategy is promising to be extended to provide a highly sensitive platform for various target analytes.

Ultrasensitive microRNA-21 detection based on DNA hybridization chain reaction and SYBR Green dye

It is extremely important for quantifying trace microRNAs in the biomedical applications. In this study, an ultrasensitive, rapid and efficient label-free fluorescence method was proposed and applied for detecting microRNA-21 in serum of gastric cancer patients based on DNA hybridization chain reaction (HCR). DNA H1 and DNA H2 were designed and used as hairpin probes, the HCR was proceeded in the presence of target microRNAs. Amounts of SYBR Green І dyes were used as signal molecules to intercalate long DNA concatemers from HCR, which guaranteed the model of label-free fluorescence and strong fluorescence density. The detection method showed a wide linear region from 1 fM to 10⁵ fM, and the limit of detection was 0.2554 fM (at S/N = 3) for microRNAs. The results showed that this method had an excellent specificity and reproducibility. Furthermore, the label-free fluorescence strategy exhibited a sensitive response to microRNA-21 in real serum samples of gastric cancer patients and the results obtained were in accordance with reference method (R² = 0.994). Overall, the proposed strategy could be satisfactory for rapid, ultrasensitive and efficient detection of microRNA-21, and held great potentials in clinic diagnosis of gastric cancer.

Shell-encoded Au nanoparticles with tunable electroactivity for specific dual disease biomarkers detection

The exploration of electroactive labelling with tailorable and strong differential pulse voltammetry (DPV) responses is of great importance in accurate and sensitive screening of a panel of biomarkers related to cancer. Herein, shell-encoded gold nanoparticles (Au NPs) are fabricated and give rise to shell species-dominated DPV peak potentials. Two independent DPV peaks appear at -0.08V for Au@Cu₂O core-shell NPs and 0.26V for Au@Ag core-shell NPs. Shell-encoded Au NPs drastically exhibit shell thickness-tunable amplified peak currents. The non-interfering and amplified DPV responses enable shell-encoded Au NPs to be an alternative electrochemical signal amplifier for dual screening of carcinoembryonic antigen (CEA) and alpha-fetoprotein (AFP). The limits of detection (LODs) are calculated to be 1.8pg/mL for CEA and 0.3pg/mL for AFP. In comparison to the parallel single-analyte assays, shell-encoded Au NPs engineered electrochemical aptasensors offer multiplexing capability and show significant prospects in biomedical research and early diagnosis of diseases.

Fluorescent aptasensor for antibiotic detection using magnetic bead composites coated with gold nanoparticles and a nicking enzyme

Antibiotic abuse has been bringing serious pollution in water, which is closely related to human health. It is desirable to develop a new strategy for antibiotic detection. To address this problem, a sensitive fluorescent aptasensor for antibiotic detection was developed by utilizing gold nanoparticles modified magnetic bead composites (AuNPs/MBs) and nicking enzyme. AuNPs/MBs were synthesized with the help of polyethylenimine (PEI). The prepared AuNPs/MBs acted as dual-functional scaffolds that owned excellent magnetic separation capacity and strong covalent bio-conjugation. The non-specifically absorbed aptamers in AuNPs/MBs were less than that in MBs. Hence, the fluorescent aptasensor based on AuNPs/MBs show a better signal to background ratio than that based on carboxyl modified magnetic beads (MBs). In this work, ampicillin was employed as a model analyte. In the presence of ampicillin, the specific binding between ampicillin and aptamer induced structure-switching that led to the release of partial complementary DNA (cDNA) of aptamer. Then, the released cDNA initiated the cycle of nicking enzyme assisted signal amplification (NEASA). Therefore, a large amount of taqman probes were cleaved and fluorescence signal was amplified. The prepared fluorescent aptasensor bring sensitive detection in range of 0.1-100 ng mL^-1 with the limit of detection of 0.07 ng mL^-1. Furthermore, this aptasensor was also successfully applied in real sample detection with acceptable accuracy. The fluorescent aptasensor provides a promising method for efficient, rapid and sensitive antibiotic detection.

SERS- and luminescence-active Au-Au-UCNP trimers for attomolar detection of two cancer biomarkers

Alpha-fetoprotein (AFP) and mucoprotein1 (mucin-1) are two important disease biomarkers. Self-assembled gold nanoparticles (AuNPs) and upconversion nanoparticle (Au-Au-UCNP) trimers based on aptamers were developed for the ultrasensitive detection of AFP and mucin-1. The Au-Au-UCNP trimers produced ideal optical signals, with prominent Raman enhancement and fluorescence quenching effects. The surface-enhanced Raman scattering (SERS) intensity decreased in the presence of mucin-1 and the luminescence intensity increased in the presence of AFP. A limit of detection (LOD) of 4.1 aM and a wide linear range of 0.01-10 fM for the detection of mucin-1 were obtained with this SERS-encoded sensing system. Using the luminescence-encoded sensing system, a LOD of 0.059 aM and a wide linear range of 1-100 aM for the detection of AFP were obtained. These LODs are the lowest values reported so far. This approach has the advantage of detecting two disease biomarkers simultaneously.

An Electrochemical Strategy using Multifunctional Nanoconjugates for Efficient Simultaneous Detection of Escherichia coli O157: H7 and Vibrio cholerae O1

The rapid and accurate quantification of the pathogenic bacteria is extremely critical to decrease the bacterial infections in all areas related to health and safety. We have developed an electrochemical strategy for simultaneous ultrasensitive detection of E. coli O157:H7 and Vibrio cholerae O1. This approach was based on the specific immune recognition of different pathogenic bacteria by multifunctional nanoconjugates and subsequent signal amplification. By employing the proposed biosensor, the concentrations of these pathogenic bacteria could be established on a single interface in a single run with improved sensitivity and accuracy. The successful approach of the simultaneous detection and quantification of two bacteria by an electrochemical biosensor demonstrated here could be readily expanded for the estimation of a variety of other pathogenic bacteria, proteins, and nucleotides. Because of their high sensitivity, electrochemical biosensors may represent a new avenue for early diagnosis of diseases.