A label-free activatable aptamer probe for colorimetric detection of cancer cells based on binding-triggered in situ catalysis of split DNAzyme

A Hemin-Graphene Nanocomposite-Based Aptasensor for Ultrasensitive Colorimetric Quantification of Leukaemia Cells Using Magnetic Enrichment

Diagnostic blood cell counting is of limited use in monitoring a minimal number of leukaemia cells, warranting further research to develop more sensitive and reliable techniques to identify leukaemia cells in circulation. In this work, a hemin-graphene nanocomposite-based aptasensor was developed for ultrasensitive colorimetric detection of leukaemia cells (CEM) using magnetic enrichment. Hemin-conjugated graphene oxide nanocomposites (HGNs) were prepared by hydrazine reduction using graphene oxide nanosheets and hemins. Hence, the prepared HGNs become able to absorb single-stranded DNA and acquire peroxidase-like activity. The aptamer sgc8c, which recognizes a specific target on leukaemia cells, was absorbed onto HGNs to capture the target CEM cancer cells. The captured target cells that associated with the HGNs were then concentrated and separated by magnetic beads (MBs) coated with sgc8c aptamers, forming a HGN-cell-MB sandwich structure. These sandwich structures can be quantified via an oxidation reaction catalysed by HGNs. By utilizing dual signal amplification effects generated by magnetic enrichment and the improved peroxidase activity of HGNs, the biosensor allowed for highly sensitive detection of 10 to 10⁵ CEM cells with an ultra-low limit of detection (LOD) of 10 cells under optimal conditions. It is expected that the proposed aptasensor can be further employed in monitoring the minimal residual disease during the treatment of leukaemia.

Paving the way towards continuous biosensing by implementing affinity-based nanoswitches on state-dependent readout platforms

Combining affinity-based nanoswitches with state-dependent readout platforms allows for continuous biosensing and acquisition of real-time information about biochemical processes occurring in the environment of interest.

Recent Development of Nanomaterials-Based Cytosensors for the Detection of Circulating Tumor Cells

The accurate analysis of circulating tumor cells (CTCs) holds great promise in early diagnosis and prognosis of cancers. However, the extremely low abundance of CTCs in peripheral blood samples limits the practical utility of the traditional methods for CTCs detection. Thus, novel and powerful strategies have been proposed for sensitive detection of CTCs. In particular, nanomaterials with exceptional physical and chemical properties have been used to fabricate cytosensors for amplifying the signal and enhancing the sensitivity. In this review, we summarize the recent development of nanomaterials-based optical and electrochemical analytical techniques for CTCs detection, including fluorescence, colorimetry, surface-enhanced Raman scattering, chemiluminescence, electrochemistry, electrochemiluminescence, photoelectrochemistry and so on.

Analyzing Criteria Affecting the Functionality of G-Quadruplex-Based DNA Aptazymes as Colorimetric Biosensors and Development of Quinine-Binding Aptazymes

A DNA aptazyme consists of an aptamer domain and a DNAzyme module, in which the DNAzyme activity can be regulated by the aptamer-target interaction. The complex of G-quadruplex (GQ) and hemin is a peroxidase-mimicking DNAzyme and has become increasingly popular as a reporter system for biosensing applications. The development of GQ-based aptazymes is of high interest as they can be used as label-free biosensors for the real-time detection of pathogens. Herein, we rationally designed ca. 200 GQ-based aptazyme candidates and evaluated the suitability of 14 aptamers targeting quinine, Protein A, Staphylococcus enterotoxin B, and ATP for this detection concept. As a result, six novel aptazymes were developed for the specific detection of quinine based on two quinine-binding aptamers. The rest of designed probes, however, hardly showed significant functionality. To uncover the reasons, we performed enzyme-linked oligonucleotide assays to find how the affinity of aptamers is affected once conjugated to the DNAzyme sequence or upon integration into the aptazyme probe. Furthermore, we investigated the impact of the structure-switching functionality in the parent aptamer and the effect of the reaction matrix on the efficiency of probes.

Aptamer-Based Detection of Circulating Targets for Precision Medicine

The past decade has witnessed ongoing progress in precision medicine to improve human health. As an emerging diagnostic technique, liquid biopsy can provide real-time, comprehensive, dynamic physiological and pathological information in a noninvasive manner, opening a new window for precision medicine. Liquid biopsy depends on the sensitive and reliable detection of circulating targets (e.g., cells, extracellular vesicles, proteins, microRNAs) from body fluids, the performance of which is largely governed by recognition ligands. Aptamers are single-stranded functional oligonucleotides, capable of folding into unique tertiary structures to bind to their targets with superior specificity and affinity. Their mature evolution procedure, facile modification, and affinity regulation, as well as versatile structural design and engineering, make aptamers ideal recognition ligands for liquid biopsy. In this review, we present a broad overview of aptamer-based liquid biopsy techniques for precision medicine. We begin with recent advances in aptamer selection, followed by a summary of state-of-the-art strategies for multivalent aptamer assembly and aptamer interface modification. We will further describe aptamer-based micro-/nanoisolation platforms, aptamer-enabled release methods, and aptamer-assisted signal amplification and detection strategies. Finally, we present our perspectives regarding the opportunities and challenges of aptamer-based liquid biopsy for precision medicine.

Recent advances in nanomaterials for colorimetric cancer detection

The early diagnosis of cancer can significantly improve patient survival rates. Colorimetric methods for real-time naked-eye detection have aroused growing interest owing to their low cost, simplicity, and practicability. With the rapid development of nanotechnology, compared with conventional diagnostic methods, nanomaterials with unique physical and chemical properties were applied to improve selectivity and sensitivity in colorimetric detection of cancer biomarkers, such as MUC1 aptamer conjugated PtAuNPs to specifically recognize MUC1 proteins on the cancer cell surfaces, etching of silver nanoprisms to detect prostate-specific antigen, and aggregation or dispersion of AuNPs to sense prostate cancer antigen gene 3 or glutathione, by which the limit of detection (LOD) could approach values down to a few cancer cells per mL, several fg per mL proteins, several ng of nucleic acids, or even tens of nM of organic molecules. Herein, we review the recent progress achieved in developing colorimetric nanosensors for cancer diagnosis, particularly providing an overview of the sensing principles, target biomarkers, advanced nanomaterials employed in the fabrication of sensing platforms, and strategies for improving signal sensitivity and specificity. Finally, we sum up the nanomaterial-based colorimetric cancer detection as well as existing challenges that should be resolved to extend their clinical application.

Earlier diagnoses of acute leukemia by a sandwich type of electrochemical aptasensor based on copper sulfide-graphene composite

Due to high affinity and specificity of aptamers, they are widely considered for construction of aptasensor to specific recognizing of analytes in biological complex matrix. So, in this work we design a high selective and sensitive aptasensor for leukemia cancer cells (CCRF-CEM) via superior catalytic effect of copper sulfide-graphene (CuS-GR) nanocomposite as label and Au-GR nanocomposite as sensing platform. The CuS-GR nano-composite (label component) is CuS nanoparticles that wrapping on graphene sheets. Its catalytic activity (CuS-GR) increases the current of sensor in parallel with adding of CCRF-CEM and provide sensitive detection of analytes. The detailed of signal amplification and effect on the aptasensor performance completely discussed. This sensor has a linear range of 50-1 × 10⁶ cell mL^-1, with a limit of detection of 18 cell mL^-1. Also, the developed aptasensor has a significance specificity, high sensitivity and accuracy. It was used for the identification of CCRF-CEM cells in blood samples.

Aptamers and Antisense Oligonucleotides for Diagnosis and Treatment of Hematological Diseases

Aptamers or chemical antibodies are single-stranded DNA or RNA oligonucleotides that bind proteins and small molecules with high affinity and specificity by recognizing tertiary or quaternary structures as antibodies. Aptamers can be easily produced in vitro through a process known as systemic evolution of ligands by exponential enrichment (SELEX) or a cell-based SELEX procedure. Aptamers and modified aptamers, such as slow, off-rate, modified aptamers (SOMAmers), can bind to target molecules with less polar and more hydrophobic interactions showing slower dissociation rates, higher stability, and resistance to nuclease degradation. Aptamers and SOMAmers are largely employed for multiplex high-throughput proteomics analysis with high reproducibility and reliability, for tumor cell detection by flow cytometry or microscopy for research and clinical purposes. In addition, aptamers are increasingly used for novel drug delivery systems specifically targeting tumor cells, and as new anticancer molecules. In this review, we summarize current preclinical and clinical applications of aptamers in malignant and non-malignant hematological diseases.

Short Duplex Module Coupled to G-Quadruplexes Increases Fluorescence of Synthetic GFP Chromophore Analogues

Aptasensors became popular instruments in bioanalytical chemistry and molecular biology. To increase specificity, perspective signaling elements in aptasensors can be separated into a G-quadruplex (G4) part and a free fluorescent dye that lights up upon binding to the G4 part. However, current systems are limited by relatively low enhancement of fluorescence upon dye binding. Here, we added duplex modules to G4 structures, which supposedly cause the formation of a dye-binding cavity between two modules. Screening of multiple synthetic GFP chromophore analogues and variation of the duplex module resulted in the selection of dyes that light up after complex formation with two-module structures and their RNA analogues by up to 20 times compared to parent G4s. We demonstrated that the short duplex part in TBA25 is preferable for fluorescence light up in comparison to parent TBA15 molecule as well as TBA31 and TBA63 stabilized by longer duplexes. Duplex part of TBA25 may be partially unfolded and has reduced rigidity, which might facilitate optimal dye positioning in the joint between G4 and the duplex. We demonstrated dye enhancement after binding to modified TBA, LTR-III, and Tel23a G4 structures and propose that such architecture of short duplex-G4 signaling elements will enforce the development of improved aptasensors.

Optical and electrochemical-based nano-aptasensing approaches for the detection of circulating tumor cells (CTCs)

More recently, detection of circulating tumor cells (CTCs) has been considered as an appealing prognostic and diagnostic approach for cancer patients. CTCs as a type of tumor-derived cells are secreted by the tumor and released into the blood circulation. Since the migration of CTCs is an early event in cancer progression, patients who still have tumor-free lymph nodes have to be well examined for the CTCs presence in their blood circulation. Nowadays, there is a broad range of detection methods available to identify CTCs. As artificial RNA oligonucleotides or single-stranded DNA with receptor and catalytic characteristics, aptamers have been standing out, owing to their target-induced conformational modifications, elevated stability, and target specificity to be implemented in biosensing techniques. To date, several sensitivity-enhancement methods alongside smart nanomaterials have been used for the creation of new aptasensors to address the limit of detection (LOD), and improve the sensitivity of numerous analyte identification methods. The present review article supports a focused overview of the recent studies in the identification and quantitative determination of CTCs by aptamer-based biosensors and nanobiosensors.

A novel label-free terbium(iii)-aptamer based aptasensor for ultrasensitive and highly specific detection of acute lymphoma leukemia cells

Acute leukemia is a malignant clonal disease of hematopoietic stem cells with a high prevalence and mortality rate. However, there are no efficient tools to facilitate early diagnosis and treatment of leukemia. Therefore, development of new methods for the early diagnosis and prevention of leukemia, especially non-invasive diagnosis at the cellular level, is imperative. Here, a label-free signal-on fluorescence aptasensor based on terbium(iii)-aptamer (Tb³⁺-apt) was applied for the detection of leukemia. The aptamer sensitizes the fluorescence of Tb³⁺ and forms the strong fluorescent Tb³⁺-apt probe. The target cells, the T-cell acute lymphoblastic leukemia cell line (CCRF-CEM) combined with the Tb³⁺-apt probe to form the Tb³⁺-apt-CEM complex, were removed by centrifugation, and the supernatant containing a small amount of the Tb³⁺-apt probe was detected using a fluorescence spectrophotometer. The logarithm of cell concentration showed a good linear relationship (R² = 0.9881) with the fluorescence signal. The linear range for CCRF-CEM detection was 5-5 × 10⁶ cells per ml, while the detection limit was 5 cells per ml of the binding buffer. Clinical samples were collected from 100 cases, and the specificity and positive rates detected by this method were up to 94% and 90%, respectively. Therefore, a single-stranded DNA-sensitized terbium(iii) luminescence method diagnostic was developed which is rapid, sensitive, and economical and can be used for diagnosis of various types of leukemia at the early stage.

DNAzyme Catalyzed Tyramide Depositing Reaction for In Situ Imaging of Protein Status on the Cell Surface

Effective characterization of protein biomarkers status on the cell surface has important value in the diagnosis and treatment of diseases. Traditional immunohistochemistry can only assess the protein expression level rather than accurately reflect their interaction and oligomerization, resulting in inevitable problems for personalized therapy.

Methods: Herein, we developed a novel DNAzyme-catalyzed tyramide depositing reaction (DCTDR) for in situ amplified imaging of membrane protein status. By using human epidermal growth factor receptor 2 (HER2) as model, the binding of HER2 proteins with specific aptamers induced the formation of activated hemin/G-quadruplex (G4) DNAzyme on the cell surface to catalyze the covalent deposition of fluorescent tyramide on the membrane proteins for fluorescence imaging.

Results: The DCTDR-based imaging can conveniently characterize total HER2 expression and HER2 dimerization on the breast cancer cell surface with the application of aptamer-G4 probes and proximity aptamer-split G4 probes, respectively. The designed DCTDR strategy was successfully applied to quantitatively estimate total HER2 expression and HER2 homodimer on clinical breast cancer tissue sections with high specificity and accuracy.

Conclusion: The DCTDR strategy provides a simple, pragmatic and enzyme-free toolbox to conveniently and sensitively analyze protein status in clinical samples for improving clinical research, cancer diagnostics and personalized treatment.

Duplexed aptamers: history, design, theory, and application to biosensing

Nucleic acid aptamers are single stranded DNA or RNA sequences that specifically bind a cognate ligand. In addition to their widespread use as stand-alone affinity binding reagents in analytical chemistry, aptamers have been engineered into a variety of ligand-specific biosensors, termed aptasensors. One of the most common aptasensor formats is the duplexed aptamer (DA). As defined herein, DAs are aptasensors containing two nucleic acid elements coupled via Watson-Crick base pairing: (i) an aptamer sequence, which serves as a ligand-specific receptor, and (ii) an aptamer-complementary element (ACE), such as a short DNA oligonucleotide, which is designed to hybridize to the aptamer. The ACE competes with ligand binding, such that DAs generate a signal upon ligand-dependent ACE-aptamer dehybridization. DAs possess intrinsic advantages over other aptasensor designs. For example, DA biosensing designs generalize across DNA and RNA aptamers, DAs are compatible with many readout methods, and DAs are inherently tunable on the basis of nucleic acid hybridization. However, despite their utility and popularity, DAs have not been well defined in the literature, leading to confusion over the differences between DAs and other aptasensor formats. In this review, we introduce a framework for DAs based on ACEs, and use this framework to distinguish DAs from other aptasensor formats and to categorize cis- and trans-DA designs. We then explore the ligand binding dynamics and chemical properties that underpin DA systems, which fall under conformational selection and induced fit models, and which mirror classical SN1 and SN2 models of nucleophilic substitution reactions. We further review a variety of in vitro and in vivo applications of DAs in the chemical and biological sciences, including riboswitches and riboregulators. Finally, we present future directions of DAs as ligand-responsive nucleic acids. Owing to their tractability, versatility and ease of engineering, DA biosensors bear a great potential for the development of new applications and technologies in fields ranging from analytical chemistry and mechanistic modeling to medicine and synthetic biology.

Human serum albumin templated MnO2 nanosheets are oxidase mimics for colorimetric determination of hydrogen peroxide and for enzymatic determination of glucose

This paper reports on a colorimetric assay for H₂O₂ and glucose. It is based on the use of human serum albumin-templated MnO₂ nanosheets that possess oxidase-like activity. They are capable of oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) with oxygen to give a blue product (oxTMB) with an absorbance maximum at 652 nm. When H₂O₂ is introduced, the MnO₂ nanosheets are reduced to Mn(II) ions, and this inhibits the formation of oxTMB. Based on these findings, a colorimetric assay was established for H₂O₂ that has a 0.56 μM detection limit. If glucose is oxidized by glucose oxidase under formation of H₂O₂, the nanosheets can be used to quantify H₂O₂ and thereby to sense glucose. Response is linear in the 0.5 μM to 50 μM glucose concentration range, and the detection limit is 0.32 μM. The method was applied to the determination of glucose in spiked serum samples and gave satisficatory results. Graphical abstract Human serum albumin (HSA) is used as a template for the synthesis of MnO₂ nanosheet. These possess oxidase mimicking activity. H₂O₂ can reduce the nanosheets. The effect is exploited in colorimetric assays for H₂O₂ and glucose using tetramethylbenzidine (TMB) as a chromogenic substrate.

A Graphene Oxide-Based Fluorescent Aptasensor for the Turn-on Detection of CCRF-CEM

A convenient, low-cost, and highly sensitive fluorescent aptasensor for detection of leukemia has been developed based on graphene oxide-aptamer complex (GO-apt). Graphene oxide (GO) can absorb carboxyfluorescein-labeled Sgc8 aptamer (FAM-apt) by π-π stacking and quench the fluorescence through fluorescence resonance energy transfer (FRET). In the absence of Sgc8 target cell CCRF-CEM, the fluorescence is almost all quenched. Conversely, when the CCRF-CEM cells are added, the quenched fluorescence can be recovered rapidly and significantly. Therefore, based on the change of fluorescence signals, we can detect the number of CCRF-CEM cells in a wide range from 1 × 102 to 1 × 107 cells/mL with a limit of detection (LOD) of 10 cells/mL. Therefore, this strategy of graphene oxide-based fluorescent aptasensor may be promising for the detection of cancer.

Temperature-responsive split aptamers coupled with polymerase chain reaction for label-free and sensitive detection of cancer cells

A label-free and general thermo-controlled split apta-PCR strategy was first developed for the sensitive and specific detection of cancer cells. By integrating the temperature-responsive function of split aptamers with PCR amplification, a facile fluorescence assay of liver cancer SMMC-7721 cells was successfully realized with the detection of as low as 100 cells.

Aptamer-integrated DNA nanoassembly: A simple and sensitive DNA framework to detect cancer cells

The development of powerful techniques to detect cancer cells at early stages plays a notable role in diagnosing and prognosing cancer patients and reducing mortality. This paper reports on a novel functional DNA nanoassembly capable of detecting cancer cells based on structural DNA nanotechnology. DNA nanoassemblies were constructed by the self-assembly of a DNA concatemer to a plenty of sticky-ended three-way junctions. While an aptamer moiety guided the nanoassembly to the target cancer cell, the peroxidase-mimicking DNAzymes embedded in the nanoassemblies were used as the sensing element to produce colorimetric signals. As proof-of-concept, as low as 175 cancer cells were detected by the assay, and color change was clearly distinguished by the naked eyes. The proposed system enjoys potential applications for point-of-care cancer diagnosis, with its excellent sensitivity and selectivity.

A label-free and high-efficient GO-based aptasensor for cancer cells based on cyclic enzymatic signal amplification

A label-free and high-efficient graphene oxide (GO)-based aptasensor was developed for the detection of low quantity cancer cells based on cell-triggered cyclic enzymatic signal amplification (CTCESA). In the absence of target cells, hairpin aptamer probes (HAPs) and dye-labeled linker DNAs stably coexisted in solution, and the fluorescence was quenched by the GO-based FÖrster resonance energy transfer (FRET) process. In the presence of target cells, the specific binding of HAPs with the target cells triggered a conformational alternation, which resulted in linker DNA complementary pairing and cleavage by nicking endonuclease-strand scission cycles. Consequently, more cleaved fragments of linker DNAs with more the terminal labeled dyes could show the enhanced fluorescence because these cleaved DNA fragments hardly combine with GOs and prevent the FRET process. Fluorescence analysis demonstrated that this GO-based aptasensor exhibited selective and sensitive response to the presence of target CCRF-CEM cells in the concentration range from 50 to 10⁵ cells. The detection limit of this method was 25 cells, which was approximately 20 times lower than the detection limit of normal fluorescence aptasensors without amplification. With high sensitivity and specificity, it provided a simple and cost-effective approach for early cancer diagnosis.

Tumor cell-specific split aptamers: target-driven and temperature-controlled self-assembly on the living cell surface

An activatable split aptamer probe with target-induced shape change and thermosensitivity was developed. Triggered by proteins on the cell surface, the probe could assemble into a desired binding shape, thus affording a FRET-based tumor cell assay. Moreover, a reversible cell catch/release strategy was realized through mild temperature switching (4°C/37°C).

A triple-amplification colorimetric assay for antibiotics based on magnetic aptamer-enzyme co-immobilized platinum nanoprobes and exonuclease-assisted target recycling

Herein, an ultrasensitive and selective colorimetric assay for antibiotics, using chloramphenicol (CAP) as the model analyte, was developed based on magnetic aptamer-HRP-platinum composite probes and exonuclease-assisted target recycling. The composite probes were prepared through immunoreactions between the double stranded DNA antibody (anti-DNA) labeled on core-shell Fe3O4@Au nanoparticles (AuMNP-anti-DNA) as the capture probe, and the double stranded aptamer (aptamer hybrid with its complementary oligonucleotides) labeled on Pt@HRP nanoparticles as the nanotracer (ds-Apt-HRP-PtNPs). When the CAP samples were incubated with the probes for 30 min at room temperature, they could be captured by the aptamer to form a nanotracer-CAP complex, which was then released into the supernatant after magnetic separation. This is because the anti-DNA on the capture probes cannot recognize the single strand aptamer-CAP complex. The exonuclease I (Exo I) added into the supernatant can further digest the aptamer-CAP from the 3'-end of the aptamer and the CAP in the aptamer-CAP complex can be released again, which can further participate in a new cycling process to react with the probes. Pt and HRP in the nanotracer could both catalyze and dual amplify the absorbance at 650 nm ascribed to the 3,3',5,5'-tetramethylbenzidine (TMB)-H2O2 system. Moreover, Exo I can assist the target recycling, which can further amplify the signal. Thus, the triple amplified signal can be quantified by ultraviolet-visible spectroscopy. The experimental results showed that the CAP detection possessed a linear range of 0.001-10 ng mL(-1) and a detection limit of 0.0003 ng mL(-1) (S/N = 3). The assay was successfully employed to detect CAP in milk, which is much more facile, time saving, and sensitive than the commercial ELISA kits.

Investigation of newly identified G-quadruplexes and their application to DNA detection

G-quadruplexes are guanine-rich nucleic acid sequences that can act as universal signal-transducers and generate colorimetric, fluorescence, and chemiluminescence signals when complexed with different ligands. Due to their merits including easy modification and low cost, it is of great importance to explore new G-quadruplexes with improved performance. Herein the properties of newly identified G-quadruplexes 9th-3-35 and 10th-2-40 were investigated in detail with UV-vis spectra, circular dichroism (CD) spectra and fluorescence spectra. The results indicated that 9th-3-35 and 10th-2-40 exhibited excellent peroxidase-like activity, as well as fluorescence enhancement of thioflavin T (ThT). Furthermore, the application of G-quadruplexes to DNA detection was performed on account of the ThT fluorescence enhancement, and the limit of detection was as low as 8 pM. This study implied that 9th-3-35 and 10th-2-40 are competitive candidates as signal-transducers in the design of bioassays.

Aptamers: versatile molecular recognition probes for cancer detection

In the past two decades, aptamers have emerged as a novel class of molecular recognition probes comprising uniquely-folded short RNA or single-stranded DNA oligonucleotides that bind to their cognate targets with high specificity and affinity. Aptamers, often referred to as "chemical antibodies", possess several highly desirable features for clinical use. They can be chemically synthesized and are easily conjugated to a wide range of reporters for different applications, and are able to rapidly penetrate tissues. These advantages significantly enhance their clinical applicability, and render them excellent alternatives to antibody-based probes in cancer diagnostics and therapeutics. Aptamer probes based on fluorescence, colorimetry, magnetism, electrochemistry, and in conjunction with nanomaterials (e.g., nanoparticles, quantum dots, single-walled carbon nanotubes, and magnetic nanoparticles) have provided novel ultrasensitive cancer diagnostic strategies and assays. Furthermore, promising aptamer targeted-multimodal tumor imaging probes have been recently developed in conjunction with fluorescence, positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI). The capabilities of the aptamer-based platforms described herein underscore the great potential they hold for the future of cancer detection. In this review, we highlight the most prominent recent developments in this rapidly advancing field.

A polydopamine nanosphere based highly sensitive and selective aptamer cytosensor with enzyme amplification

With CCRF-CEM as the model cell, a highly sensitive and selective cytosensor was developed by taking advantage of polydopamine nanospheres for the first time. The strategies of aptamer/membrane protein recognition and Exonuclease III assisted cycle amplification were used for improving selectivity and sensitivity. The detection of limit reached was as low as 15 cells per mL.

Sensitive electrochemical aptamer cytosensor for highly specific detection of cancer cells based on the hybrid nanoelectrocatalysts and enzyme for signal amplification

Human cancer is becoming a leading cause of death in the world and the development of a straightforward strategy for early detection of cancer is urgently required. Herein, a sandwich-type electrochemical aptamer cytosensor was developed for detection of human liver hepatocellular carcinoma cells (HepG2) based on the hybrid nanoelectrocatalysts and enzyme for signal amplification. The thiolated TLS11a aptamers were used as a selective bio-recognition element, attached to the gold nanoparticles (AuNPs) modified the glassy carbon electrode (GCE) surface. Meanwhile, the electrochemical nanoprobes were fabricated through the G-quadruplex/hemin/aptamer complexes and horseradish peroxidase (HRP) immobilized on the surfaces of Au@Pd core-shell nanoparticle-modified magnetic Fe3O4/MnO2 beads (Fe3O4/MnO2/Au@Pd). After the target cells were captured, the hybrid nanoprobes were further assembled to form an aptamer-cell-nanoprobes sandwich-like system on the electrode surface. Then, hybrid Fe3O4/MnO2/Au@Pd nanoelectrocatalysts, G-quadruplex/hemin HRP-mimicking DNAzymes and the natural HRP enzyme efficiently catalyzed the oxidation of hydroquinone (HQ) with H2O2, amplifying the electrochemical signals and improving the detection sensitivity. This electrochemical cytosensor delivered a wide detection range of 1×10(2)-1×10(7)cellsmL(-1), high sensitivity with a low detection limit of 15cellsmL(-1), good selectivity and repeatability. Finally, an electrochemical reductive desorption method was performed to break gold-thiol bond and desorb the components on the AuNPs/GCE for regenerating the cytosensor. These results have demonstrated that the electrochemical cytosensor has the potential to be a feasible tool for cost-effective cancer cell detection in early cancer diagnosis.

A structure-switchable aptasensor for aflatoxin B1 detection based on assembly of an aptamer/split DNAzyme

An ultrasensitive, colorimetric and homogeneous strategy for aflatoxin B1 (AFB1) detection, which uses a DNA aptamer and two split DNAzyme halves, has been developed. Split halves of a hemin-binding DNAzymes is combined with an AFB1 aptamer to generate a homogeneous colorimetric sensor that undergoes an AFB1 induced DNA structural change. In the absence of AFB1, the split probes have peroxidase mimicking DNAzyme activity associated with catalysis of a color change reaction. Specific recognition of AFB1 by the aptamer component leads to structural deformation of the aptamer-DNAzyme complex, which causes splitting of the DNAzyme halves and a reduction in peroxidase mimicking activity. Therefore, a decrease of colorimetric signal arising from the catalytic process takes place upon in the presence of AFB1 in a concentration dependent manner in the 0.1-1.0 × 10(4) ng/mL range and with a colorimetric detection limit of 0.1 ng/mL. The new assay system exhibits high selectivity for AFB1 over other mycotoxins and can be employed detect the presence of AFB1 in ground corn samples. Overall, the strategy should serve as the basis for the development of rapid, simple and low-cost methods for detection of mycotoxins.

A repeatable assembling and disassembling electrochemical aptamer cytosensor for ultrasensitive and highly selective detection of human liver cancer cells

In this work, a repeatable assembling and disassembling electrochemical aptamer cytosensor was proposed for the sensitive detection of human liver hepatocellular carcinoma cells (HepG2) based on a dual recognition and signal amplification strategy. A high-affinity thiolated TLS11a aptamer, covalently attached to a gold electrode through Au-thiol interactions, was adopted to recognize and capture the target HepG2 cells. Meanwhile, the G-quadruplex/hemin/aptamer and horseradish peroxidase (HRP) modified gold nanoparticles (G-quadruplex/hemin/aptamer-AuNPs-HRP) nanoprobe was designed. It could be used for electrochemical cytosensing with specific recognition and enzymatic signal amplification of HRP and G-quadruplex/hemin HRP-mimicking DNAzyme. With the nanoprobes as recognizing probes, the HepG2 cancer cells were captured to fabricate an aptamer-cell-nanoprobes sandwich-like superstructure on a gold electrode surface. The proposed electrochemical cytosensor delivered a wide detection range from 1×10(2) to 1×10(7) cells mL(-1) and high sensitivity with a low detection limit of 30 cells mL(-1). Furthermore, after the electrochemical detection, the activation potential of -0.9 to -1.7V was performed to break Au-thiol bond and regenerate a bare gold electrode surface, while maintaining the good characteristic of being used repeatedly. The changes of gold electrode behavior after assembling and desorption processes were investigated by electrochemical impedance spectroscopy and cyclic voltammetry techniques. These results indicate that the cytosensor has great potential in disease diagnostic of cancers and opens new insight into the reusable gold electrode with repeatable assembling and disassembling in the electrochemical sensing.

A signal-on fluorescent aptasensor based on single-stranded DNA-sensitized luminescence of terbium (III) for label-free detection of breast cancer cells

Breast cancer is the most common type of malignant tumor in women. Recently, it has been shown that detection of breast cancer tumor cells outside the primitive tumor is an effective early diagnosis with great prognostic and clinical utility. For this purpose, we developed a signal-on fluorescence aptasensor for label-free, facile and sensitive detection of MCF-7 breast cancer cells. Due to target-aptamer specific recognition and single-stranded DNA-sensitized luminescence of terbium (III), the proposed aptasensor exhibits excellent sensitivity with detection limit as low as 70 cells mL(-1). Compared with common organic dyes and the emerging nano-technological probes, the combination of terbium (III) and single-stranded DNA signal probe (Tb(3+)-SP) serves as a more powerful bio-probe because of its stable optical property, good biocompatibility and free from complex synthesis. The feasibility investigations have illustrated the potential applicability of this aptasensor for selective and sensitive detection of MCF-7 breast cancer cells. Moreover, this proposed aptasensor can be also extended for the determination of other tumor cancers or bio-molecules by altering corresponding aptamers. Taken together, this easy-to-perform aptasensor may represent a promising way for early screening and detection of tumor cancers or other bio-molecules in clinical diagnosis.

Designing activatable aptamer probes for simultaneous detection of multiple tumor-related proteins in living cancer cells

We report a novel strategy for high specific and simultaneous detection of multiple tumor-related proteins in cancer cells based on the activated fluorescence signal, which is triggered by specific-binding-induced conformation alteration of the designed activatable aptamer probe. The activatable aptamer probe consists two fragments: a target-protein-recognized aptamer sequence for specifically recognizing the protein, and an extending spacer making the aptamer in hairpin structure to enable the close proximity of quencher to fluorophore, which is labeled at 5'- and 3'-terminus, respectively, of the probes. Before interaction with cell, the fluorescence of the probe is quenched due to fluorescent resonance energy transfer (FRET) between the fluorophore and quencher. After interaction events, the fluorescence signal is activated through specific binding of the probe with target protein in cell, causing the conformation alteration and forcing the separation of fluorophore from the quencher. We achieve simultaneous detection of multiple tumor-related proteins in cells by designing the different activatable aptamer probes with various fluorophore/quencher combinations. Moreover, it can also achieve a high detection sensitivity (for example, detecting MCF-7 cells at a low abundance of ~(10±5) cells mL(-1)) and specific discrimination of the subtype of cancers. The advantage of this approach is that it has high detection sensitivity because of the significant suppression of background with use of the designed activatable aptamer probe. In addition, it has ability of avoiding false signals arising due to the nonspecific adsorption of interferents because it operates via monitoring the activated fluorescence signals of the designed activatable aptamer probe.