Electrochemical Detection of Circulating Tumor Cells Based on DNA Generated Electrochemical Current and Rolling Circle Amplification

In vitro detection of circulating tumor cells using the nicking endonuclease-assisted lanthanide metal luminescence amplification strategy

The in vitro detection of circulating tumor cells (CTCs) has been proven as a vital method for early diagnosis and evaluation of cancer metastasis, since the existence and number fluctuation of CTCs have shown close correlation with clinical outcomes. However, it remains difficult and technically challenging to realize accurate CTCs detection, due to the rarity of CTCs in the blood samples with complex components. Herein, we reported a CTCs in vitro detection strategy, utilizing a loop amplification strategy based on DNA tetrahedron and nicking endonuclease reaction, as well as the anti-background interference based on lanthanide metal luminescence strategy. In this work, a detection system (ATDN-MLLPs) composed of an aptamer-functionalized tetrahedral DNA nanostructure (ATDN) and magnetic lanthanide luminescent particles (MLLPs) was developed. ATDN targeted the tumor cells via aptamer-antigen recognition and extended three hybridizable target DNA segments from the apex of a DNA tetrahedron to pair with probe DNA on MLLPs. Then, the nicking endonuclease (Nt.BbvCI) recognized the formed double-strand DNA and nicked the probe DNA to release the target DNA for recycling, and the released TbNps served as a high signal-to-noise ratio fluorescence signal source for CTCs detection. With a detection limit of 5 cells/mL, CTCs were selectively screened throughout a linear response range of low orders of magnitude. In addition, the ATDN-MLLPs system was attempted to detect possible existence of CTCs in biological samples in vitro.

Efficient detection of single circulating tumor cell in blood using Raman mapping based on Aptamer-SERS bio-probe coupled with micropore membrane filtration

Rapid and accurate detection of rare circulating tumor cells (CTCs) in human blood still remains a challenge. We present a surface enhanced Raman spectroscopy (SERS) method based on aptamer-SERS bio-probe recognition coupled with micropore membrane filtration capture for the detection of CTCs at single cell level. The parylene micropore membrane with optimized micropore size installed on a filtration holder could capture bio-probe labeled CTCs by gravity in less than 10 s, and only with very less white blood cells (WBCs) residual. In order to facilitate the synthesis of the aptamer-SERS bio-probe, ethyl acetate dehydration method was established. The bio-probe can be rapidly synthesized within 2 h by binding SH-aptamer to 4- mercaptobenzoic acid (4-MBA) modified AuNPs with the help of ethyl acetate. The SERS bio-probe with selected specific aptamer could distinguish single human non-small cell lung cancer A549 cells from residual WBCs on membrane efficiently and reliably based on their Raman signal intensity difference at 1075 cm-1. Through the filter membrane coupled with aptamer-SERS bio-probe system, even 20 A549 cells in blood solution simulating CTCs sample can be detected, which the recovery rate and recognition rate are more than 90%. This method is rapid, reliable and cost-effective, which indicates a good prospect in clinical application for CTCs detection.

Transistor-based immunosensor using AuNPs-Ab2-HRP enzyme nanoprobe for the detection of antigen biomarker in human blood

Alpha-fetoprotein (AFP) is inextricably linked to various diseases, including liver cancer. Thus, detecting the content of AFP in biology has great significance in diagnosis, treatment, and intervention. Motivated by the urgent need for affordable and convenient electronic sensors in the analysis and detection of aqueous biological samples, we combined the solution-gated graphene transistor (SGGT) with the catalytic reaction of enzyme nanoprobes (HRP-AuNPs-Ab2) to accurately sense AFP. The SGGT immunosensor demonstrated high specificity and stability, excellent selectivity, and excessive linearity over a range of 4 ng/mL to 500 ng/mL, with the lower detection limit down to 1.03 ng/mL. Finally, clinical samples were successfully detected by the SGGT immunosensor, and the results were consistent with chemiluminescence methods that are popular in hospitals for detecting AFP. Notably, the SGGT immunosensor is also recyclable, so it has excellent potential for use in high-throughput detection.

Nanosensor Chemical Cytometry: Advances and Opportunities in Cellular Therapy and Precision Medicine

With the definition of therapeutics now encompassing transplanted or engineered cells and their molecular products, there is a growing scientific necessity for analytics to understand this new category of drugs. This Perspective highlights the recent development of new measurement science on label-free single cell analysis, nanosensor chemical cytometry (NCC), and their potential for cellular therapeutics and precision medicine. NCC is based on microfluidics integrated with fluorescent nanosensor arrays utilizing the optical lensing effect of a single cell to real-time extract molecular properties and correlate them with physical attributes of single cells. This new class of cytometry can quantify the heterogeneity of the multivariate physicochemical attributes of the cell populations in a completely label-free and nondestructive way and, thus, suggest the vein-to-vein conditions for the safe therapeutic applications. After the introduction of the NCC technology, we suggest the technological development roadmap for the maturation of the new field: from the sensor/chip design perspective to the system/software development level based on hardware automation and deep learning data analytics. The advancement of this new single cell sensing technology is anticipated to aid rich and multivariate single cell data setting and support safe and reliable cellular therapeutics. This new measurement science can lead to data-driven personalized precision medicine.

Application of Biosensors in Detecting Breast Cancer Metastasis

Breast cancer has garnered global attention due to its high incidence worldwide, and even more noteworthy is that approximately 90% deaths due to breast cancer are attributed to cancer metastasis. Therefore, the early diagnosis of breast cancer metastasis holds significant importance for reducing mortality outcomes. Biosensors play a crucial role in the early detection of metastatic breast cancer due to their advantages, such as ease of use, portability, and real-time analysis capabilities. This review primarily described various types of sensors for detecting breast cancer metastasis based on biomarkers and cell characteristics, including electrochemical, optical, and microfluidic chips. We offered detailed descriptions of the performance of these various biosensors and made comparisons between them. Furthermore, we described the pathology of breast cancer and summarized commonly used biomarkers for metastatic breast cancer. Finally, we discussed the advantages of current-stage biosensors and the challenges that need to be addressed, as well as prospects for their future development.

Bifunctional electrochemical biosensor based on PB-MXene films for the real-time analysis and detection of living cancer cells

Circulating tumor cells (CTCs) are important prognostic markers for cancer diagnosis and metastasis, and their detection is an important means to detect cancer metastasis. Herein, we construct a novel bifunctional electrochemical biosensor based on the PB-MXene composite films. A simple electrostatic self-assembly approach was employed to prepare a film composed of PB nanocubes on the MXene substrates. Given that the PB is an artificial peroxidase for H2O2 sensing, the PB-MXene films can realize the real-time monitoring of H2O2 secretion from living CTCs. Besides, the anti-CEA attached biosensors can be utilized to quantify the corresponding CTCs. The synergic effects of the MXene with a large specific area and PB with enzyme-free catalysis for H2O2 resulted in PB-MXene films exhibiting high electrocatalytic and low cytotoxicity for both H2O2 sensing and living CTCs capturing. As a result, the biosensor shows a low detection limit of 0.57 μM towards H2O2 with a wide linear range (1 μM to 500 μM), as well as an excellent sensing performance for CTCs (an extremely low detection limit of 9 cells/mL in a wide linear range of 1.3 ×101 to 1.3 ×106 cells/mL). Moreover, the prepared biosensor showed satisfactory stability and anti-interference ability for potential applications in clinical cancer diagnosis and tumor metastasis.

Circulating Tumor Cells Adhesion: Application in Biosensors

The early and non-invasive diagnosis of tumor diseases has been widely investigated by the scientific community focusing on the development of sensors/biomarkers that act as a way of recognizing the adhesion of circulating tumor cells (CTCs). As a challenge in this area, strategies for CTCs capture and enrichment currently require improvements in the sensors/biomarker's selectivity. This can be achieved by understanding the biological recognition factors for different cancer cell lines and also by understanding the interaction between surface parameters and the affinity between macromolecules and the cell surface. To overcome some of these concerns, electrochemical sensors have been used as precise, fast-response, and low-cost transduction platforms for application in cytosensors. Additionally, distinct materials, geometries, and technologies have been investigated to improve the sensitivity and specificity properties of the support electrode that will transform biochemical events into electrical signals. This review identifies novel approaches regarding the application of different specific biomarkers (CD44, Integrins, and EpCAm) for capturing CTCs. These biomarkers can be applied in electrochemical biosensors as a cytodetection strategy for diagnosis of cancerous diseases.

Single-atom nanozyme-based electrochemical sensors for health and food safety monitoring

Electrochemical sensors and biosensors play an important role in many fields, including biology, clinical trials, and food industry. For health and food safety monitoring, accurate and quantitative sensing is needed to ensure that there is no significantly negative impact on human health. It is difficult for traditional sensors to meet these requirements. In recent years, single-atom nanozymes (SANs) have been successfully used in electrochemical sensors due to their high electrochemical activity, good stability, excellent selectivity and high sensitivity. Here, we first summarize the detection principle of SAN-based electrochemical sensors. Then, we review the detection performances of small molecules on SAN-based electrochemical sensors, including H₂O₂, dopamine (DA), uric acid (UA), glucose, H₂S, NO, and O₂. Subsequently, we put forward the optimization strategies to promote the development of SAN-based electrochemical sensors. Finally, the challenges and prospects of SAN-based sensors are proposed.

Typing of cancer cells by microswimmer based on Co-Fe-MOF for one-step simultaneously detect multiple biomarkers

Capturing, identifying, and counting CTCs cancer cells that have escaped from the tumor and wandered into the bloodstream is a major challenge. We proposed a noval microswimmer dual-mode aptamer (electrochemical and fluorescent)-(Mapt-EF) homogeneous sensor with active capture/controlled release double signaling molecule/separation and release cell based on the Co-Fe-MOF nanomaterial for simultaneous one-step detection of multiple biomarkers protein tyrosine kinase-7 (PTK7), Epithelial cell adhesion molecule (EpCAM), and mucin-1 (MUC1) for diagnosis of multiple cancer cell types. The Co-Fe-MOF is a nano-enzyme capable of catalyzing the decomposition of hydrogen peroxide to release bubbles of oxygen, producing a driving force to conduct hydrogen peroxide through the liquid, and has the capacity to self-decompose during the catalytic process. Phosphoric acid is present in the aptamer chains of PTK7, EpCAM, and MUC1, and the aptamer chains are adsorbed to the surface of the Mapt-EF homogeneous sensor in the form of a gated switch to inhibit the catalytic decomposition activity of hydrogen peroxide. The Mapt-EF homogeneous sensor has the capability to actively target biomarkers that can be entrained by oxygen bubbles without being degraded. The detection time of the sensor was 20 min, the detection limits were 9.6 fg/mL, 8.4 fg/mL and 7.7 fg/mL with the linear range was 0-20 pg/mL, respectively. The Mapt-EF homogeneous sensor has high detection sensitivity, and its detection limit can reach the level of single cell at the lowest. The Mapt-EF homogeneous sensor has great application potential in clinical detection and analysis of tumor cells.

Biosensors for healthcare: current and future perspectives

Biosensors are utilized in several different fields, including medicine, food, and the environment; in this review, we examine recent developments in biosensors for healthcare. These involve three distinct types of biosensor: biosensors for in vitro diagnosis with blood, saliva, or urine samples; continuous monitoring biosensors (CMBs); and wearable biosensors. Biosensors for in vitro diagnosis have seen a significant expansion recently, with newly reported clustered regularly interspaced short palindromic repeats (CRISPR)/Cas methodologies and improvements to many established integrated biosensor devices, including lateral flow assays (LFAs) and microfluidic/electrochemical paper-based analytical devices (μPADs/ePADs). We conclude with a discussion of two novel groups of biosensors that have drawn great attention recently, continuous monitoring and wearable biosensors, as well as with perspectives on the commercialization and future of biosensors.

Recent progress in aptamer-based microfluidics for the detection of circulating tumor cells and extracellular vesicles

Liquid biopsy is a technology that exhibits potential to detect cancer early, monitor therapies, and predict cancer prognosis due to its unique characteristics, including noninvasive sampling and real-time analysis. Circulating tumor cells (CTCs) and extracellular vesicles (EVs) are two important components of circulating targets, carrying substantial disease-related molecular information and playing a key role in liquid biopsy. Aptamers are single-stranded oligonucleotides with superior affinity and specificity, and they can bind to targets by folding into unique tertiary structures. Aptamer-based microfluidic platforms offer new ways to enhance the purity and capture efficiency of CTCs and EVs by combining the advantages of microfluidic chips as isolation platforms and aptamers as recognition tools. In this review, we first briefly introduce some new strategies for aptamer discovery based on traditional and aptamer-based microfluidic approaches. Then, we subsequently summarize the progress of aptamer-based microfluidics for CTC and EV detection. Finally, we offer an outlook on the future directional challenges of aptamer-based microfluidics for circulating targets in clinical applications.

Malachite Green-Based Detection of SARS-CoV-2 by One-Step Reverse Transcription Loop-Mediated Isothermal Amplification

The pandemic of severe acute respiratory syndrome 2 (SARS-CoV-2) revealed the necessity of diagnosis of the infected people to prevent the prevalence infection cycle. Many commercial pathogen diagnosis methods are based on the detection of genomic materials. Isothermal amplification methods such as loop-mediated-isothermal amplification (LAMP) are the method of choice in these cases. Reverse transcription steps are efficiently coupled to LAMP for the detection of pathogens with genomic RNAs such as SARS-CoV-2. Many detection systems for LAMP include fluorescent readout systems. Although such systems result in desirable limits of detection, the need for special instrumentation is the main dispute of such systems to become real point of care assays. In contrast, colorimetric detection methods would reduce costs and improve the applicability of the system. In this study one-step reverse transcription-LAMP reaction was established that enables visual detection of the SARS-CoV-2 genome. Nasopharyngeal RNA samples were first validated by reverse transcription quantitative polymerase chain reaction and then subjected to RT-LAMP. To lower the cost associated with the readout system equipment, malachite green (MG) was used. The color change of MG to blue allowed visual detection of the virus. Firstly, experiments were set up as two-step RT-LAMP reaction to identify the best primer sets. In addition, MG concentration was optimized with the significant colorimetric signal for the positive samples. Next, a one-step colorimetric method was developed for the detection of SARS-CoV-2 based on MG color shift in 2 h.

Hierarchal polyaniline-folic acid nanostructures act as a platform for electrochemical detection of tumor cells

The fabrication of electrochemical sensing platforms for cancer monitoring by quantifying circulating tumor cells (CTCs) in blood holds promise for providing a low-cost, rapid, feasible, and safe approach for cancer diagnosis. Here, we isolate cancer cells using CoFe₂O₄ nanoparticles functionalized with folic acid and chitosan as an inexpensive magnetic nanoprobe. This electrochemical cytosensing platform was realized using polyaniline-folic acid nanohybrids with a three-dimensional hierarchical structure that presents abundant affinity sites toward overexpressed folate bioreceptors on cancer cells, in addition to retaining satisfied conductivity. Furthermore, 3D modeling and simulation of the polyaniline-folic acid structures were conducted to investigate the stable complex between aniline and folate, and the interaction between the polyaniline-folate complex and folate receptor alpha1, a bioreceptor on MCF-7 was revealed for the first time. The limit of detection was calculated to be 4 cells mL^-1 with a linear range from 50 to 10⁶ cells mL^-1.

Enzyme-Assisted Nucleic Acid Amplification in Molecular Diagnosis: A Review

Infectious diseases and tumors have become the biggest medical challenges in the 21st century. They are driven by multiple factors such as population growth, aging, climate change, genetic predispositions and more. Nucleic acid amplification technologies (NAATs) are used for rapid and accurate diagnostic testing, providing critical information in order to facilitate better follow-up treatment and prognosis. NAATs are widely used due their high sensitivity, specificity, rapid amplification and detection. It should be noted that different NAATs can be selected according to different environments and research fields; for example, isothermal amplification with a simple operation can be preferred in developing countries or resource-poor areas. In the field of translational medicine, CRISPR has shown great prospects. The core component of NAAT lies in the activity of different enzymes. As the most critical material of nucleic acid amplification, the key role of the enzyme is self-evident, playing the upmost important role in molecular diagnosis. In this review, several common enzymes used in NAATs are compared and described in detail. Furthermore, we summarize both the advances and common issues of NAATs in clinical application.

Aptamer-Engineered Cu2O Nanocubes as a Surface-Modulated Catalytic Optical Sensor for Lung Cancer Cell Detection

Herein, fine and homogeneous Cu₂O nanocubes are synthesized and sensitized with a hairpin-structured AS1411 aptamer for the establishment of a biosensor for lung cancer cell detection. The Apt-Cu₂O nanocubes feature a recognition function in identifying a cancer-associated surface nucleolin protein. The intrinsic reduction catalytic ability is also confirmed by the use of two benchmark substrates, methylene blue (MB) and 4-nitrophenol (4-NP). The aptamer grafting on Apt-Cu₂O nanocubes is able to greatly prevent nonspecific-protein binding and to show specificity toward the nucleolin protein. The specific binding resulting from nucleolin protein leads to less exposure of the active area of the Apt-Cu₂O nanocubes, so the catalytic ability of Apt-Cu₂O nanocubes is thus diminished. The modulated catalytic ability led to less generation of the reduced 4-AP product, and the change in absorption of 4-AP allows the quantification of the nucleolin protein with a detection limit of 0.47 nM. The as-developed biosensor is applied to the detection of nucleolin-overexpressed A549 lung cancer cells, presenting a sensitive detection limit down to 20 cells. This may be ascribed to the clustering of surface nucleolin protein in a lipid raft membrane of cancer cells, as evidenced by a notable binding of Apt-Cu₂O nanocubes on the cancer cell surface. Real human serum samples spiked with cancer cells were also investigated, and a recovery rate of 87 ± 2.4% for 20 extracted cells validates the surface-modulated Apt-Cu₂O nanocubes-based catalytic optical biosensor as a promising tool for the detection of circulating tumor cells. The establishment of the Apt-Cu₂O nanocubes may allow for further studies on their use as a potential theranostics tool for cancer therapy.

Photoelectrochemical biosensor based on DNA aptamers and dual nano-semiconductor heterojunctions for accurate and selective sensing of chloramphenicol

Nanosheets of anatase TiO₂ and CdS quantum dots modified with thioglycolic acid (TGA-CdS QDs) were prepared and hierarchically modified on the indium tin oxides (ITO) electrodes. The heterojunction structure is formed to improve the light capture ability and carrier migration, significantly enhancing the sensitivity of photoelectrochemical (PEC) biosensors. Specific DNA sequences labeled with TGA-CdS QDs were placed on the electrodes to prepare a biosensor for the detection of chloramphenicol with ultrahigh selectivity. In addition, the heterojunction structure and the principle of photocurrent signal amplification on the electrode are described in detail. Under the optimal conditions, the photoelectrochemical biosensors showed good reproducibility and stability for chloramphenicol with a linear response in the range 10-10,000 pM and a limit of detection (LOD) of 0.23 pM. Due to the specific recognition of base pairs, the sensor has excellent anti-interference ability in practical applications. An effective method was developed for the accurate detection of antibiotics with far reaching prospects.

Overview on the Design of Magnetically Assisted Electrochemical Biosensors

Electrochemical biosensors generally require the immobilization of recognition elements or capture probes on the electrode surface. This may limit their practical applications due to the complex operation procedure and low repeatability and stability. Magnetically assisted biosensors show remarkable advantages in separation and pre-concentration of targets from complex biological samples. More importantly, magnetically assisted sensing systems show high throughput since the magnetic materials can be produced and preserved on a large scale. In this work, we summarized the design of electrochemical biosensors involving magnetic materials as the platforms for recognition reaction and target conversion. The recognition reactions usually include antigen-antibody, DNA hybridization, and aptamer-target interactions. By conjugating an electroactive probe to biomolecules attached to magnetic materials, the complexes can be accumulated near to an electrode surface with the aid of external magnet field, producing an easily measurable redox current. The redox current can be further enhanced by enzymes, nanomaterials, DNA assemblies, and thermal-cycle or isothermal amplification. In magnetically assisted assays, the magnetic substrates are removed by a magnet after the target conversion, and the signal can be monitored through stimuli-response release of signal reporters, enzymatic production of electroactive species, or target-induced generation of messenger DNA.

Recent Advances in Signal Amplification to Improve Electrochemical Biosensing for Infectious Diseases

The field of infectious disease diagnostics is burdened by inequality in access to healthcare resources. In particular, “point-of-care” (POC) diagnostics that can be utilized in non-laboratory, sub-optimal environments are appealing for disease control with limited resources. Electrochemical biosensors, which combine biorecognition elements with electrochemical readout to enable sensitive and specific sensing using inexpensive, simple equipment, are a major area of research for the development of POC diagnostics. To improve the limit of detection (LOD) and selectivity, signal amplification strategies have been applied towards these sensors. In this perspective, we review recent advances in electrochemical biosensor signal amplification strategies for infectious disease diagnostics, specifically biosensors for nucleic acids and pathogenic microbes. We classify these strategies into target-based amplification and signal-based amplification. Target-based amplification strategies improve the LOD by increasing the number of detectable analytes, while signal-based amplification strategies increase the detectable signal by modifying the transducer system and keep the number of targets static. Finally, we argue that signal amplification strategies should be designed with application location and disease target in mind, and that the resources required to produce and operate the sensor should reflect its proposed application, especially when the platform is designed to be utilized in low-resource settings. We anticipate that, based on current technologies to diagnose infectious diseases, incorporating signal-based amplification strategies will enable electrochemical POC devices to be deployed for illnesses in a wide variety of settings.

Prospects and challenges of dynamic DNA nanostructures in biomedical applications

The physicochemical nature of DNA allows the assembly of highly predictable structures via several fabrication strategies, which have been applied to make breakthroughs in various fields. Moreover, DNA nanostructures are regarded as materials with excellent editability and biocompatibility for biomedical applications. The ongoing maintenance and release of new DNA structure design tools ease the work and make large and arbitrary DNA structures feasible for different applications. However, the nature of DNA nanostructures endows them with several stimulus-responsive mechanisms capable of responding to biomolecules, such as nucleic acids and proteins, as well as biophysical environmental parameters, such as temperature and pH. Via these mechanisms, stimulus-responsive dynamic DNA nanostructures have been applied in several biomedical settings, including basic research, active drug delivery, biosensor development, and tissue engineering. These applications have shown the versatility of dynamic DNA nanostructures, with unignorable merits that exceed those of their traditional counterparts, such as polymers and metal particles. However, there are stability, yield, exogenous DNA, and ethical considerations regarding their clinical translation. In this review, we first introduce the recent efforts and discoveries in DNA nanotechnology, highlighting the uses of dynamic DNA nanostructures in biomedical applications. Then, several dynamic DNA nanostructures are presented, and their typical biomedical applications, including their use as DNA aptamers, ion concentration/pH-sensitive DNA molecules, DNA nanostructures capable of strand displacement reactions, and protein-based dynamic DNA nanostructures, are discussed. Finally, the challenges regarding the biomedical applications of dynamic DNA nanostructures are discussed.

Novel Detection Method for Circulating EGFR Tumor DNA Using Gravitationally Condensed Gold Nanoparticles and Catalytic Walker DNA

The detection of circulating tumor DNA is a major challenge in liquid biopsies for cancer. Conventionally, quantitative polymerase chain reactions or next-generation sequencing are used to detect circulating tumor DNA; however, these techniques require significant expertise, and are expensive. Owing to the increasing demand for a simple diagnostic method and constant monitoring of cancer, a cost-effective detection technique that can be conducted by non-experts is required. The aim of this study was to detect the circulating tumor DNA containing the epidermal growth factor receptor (EGFR) exon 19 deletion, which frequently occurs in lung cancer. By applying walker DNA to a catalytic hairpin assembly and using the differential dispersibility of gold nanoparticles, we detected EGFR exon 19 deletion mutant #2 DNA associated with lung cancer. Our sensing platform exhibited a limit of detection of 38.5 aM and a selectivity of 0.1% for EGFR exon 19 wild-type DNA. Moreover, we tested and compared EGFR exon 19 deletion mutants #1 and #3 to evaluate the effect of base pair mismatches on the performance of the said technique.

Ratiometric electrochemical aptasensor for the sensitive detection of carcinoembryonic antigen based on a hairpin DNA probe and exonuclease I-assisted target recycling

A novel ratiometric electrochemical aptasensor was constructed for the detection of carcinoembryonic antigen (CEA) based on a hairpin DNA (hpDNA) probe and exonuclease Ⅰ (Exo Ⅰ)-assisted target recycling amplification strategy. A thiolated methylene blue (MB)-labeled hpDNA as the internal control element was fixed on the surface of the gold nanoparticles (AuNPs)-modified gold electrode (AuE) through Au-S bonds. A ferrocene (Fc)-modified aptamer DNA (Fc-Apt) was partially hybridized with hpDNA to form a Fc-Apt/hpDNA duplex. Due to the specific recognition of Fc-Apt to CEA, the presence of CEA caused dissociation of Fc-Apt from the duplex, and further triggered the degradation process of Exo Ⅰ and recycling of CEA. Hence, the Fc tags were released from the electrode surface and the oxidation peak current of Fc (I_Fc) decreased while that of MB (I_MB) remained stable owing to the distance between MB tags and the electrode unchanged. A linear relationship was observed between I_Fc/I_MB and the logarithm of CEA concentration from 10 pg mL^-1 to 100 ng mL^-1 with a detection limit of 1.9 pg mL^-1. Moreover, the developed aptasensor had been applied to detect CEA in diluted human serum with satisfactory results, indicating its great potential in practical applications.

Glassy Carbon Electrode Modified with N-Doped Reduced Graphene Oxide Sheets as an Effective Electrochemical Sensor for Amaranth Detection

Amaranth is one of the synthetic azo colorants used to improve the appearance and to increase the appeal of some foods and soft drinks. The excessive consumption of amaranth can be associated with health side effects, emphasizing the need to monitor this food dye. Accordingly, the present study aimed to introduce an electrochemical sensor of glassy carbon electrode (GCE) modified with N-doped reduced graphene oxide (N-rGO), N-rGO/GCE, to detect the amaranth sensitively and rapidly. Several electrochemical techniques such as differential pulse voltammetry (DPV), linear sweep voltammetry (LSV), chronoamperometry (CHA), and cyclic voltammetry (CV) are exploited for the evaluation of the efficiency of the developed electrode for the detection of amaranth. We found that N-rGO/GCE enhanced amaranth oxidation, thus significantly elevating the current signal. Amaranth showed that calibration curves ranged from 0.1 to 600.0 µM, and the limit of detection (LOD) (S/N = 3) was 0.03 μM. Finally, the developed sensor was effectively applied for real samples (tap water, apple juice, and orange juice) with acceptable recovery values from 96.0 to 104.3%.

Cell membrane-coated nanoparticles as peroxidase mimetics for cancer cell targeted detection and therapy

The development of novel and efficient recognition molecules that can be easily modified by nanomaterials to achieve ultra-sensitive and specific cancer cell analysis is of great significance for its early diagnosis and timely prognosis. Herin, a new nanostructured hybrid based on cell membrane-coated Au cores- ultrathin Pt skins composite nanoparticles (Au@Pt@CM NPs) were developed for in vitro detection and treatment of cancer cells. In this strategy, the Au@Pt NPs acted as the signal transducer, and the cell membrane were used as the cancer-cell recognition tool. The synthesized Au@Pt@CM NPs could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of the hydrogen peroxide and were demonstrated to have excellent peroxidase-like activity. Coated with the source cancer cell membrane, the nanoparticles achieved highly specific self-recognition to the source cell. Therefore, the colorimetric method based on Au@Pt@CM NPs could detect the cancer cells in the linear range from 50 to 100000 cells/mL with a limit of detection of 5 cells/mL, which is much lower than other colorimetric detection methods. Afterwards, the nanoparticles as a mimetic enzyme were used for therapeutics of cancer cells through the ROS-mediated oxidative damage. Due to the change of the redox state in the cells by the Au@Pt@CM NPs, the hybrid can achieve the growth inhibitory effect and the selective killing effect on cancer cells. It can be expected that this novel hybrid membrane coating method will bring new insight into developing targeted nanomaterials for tumor treatment and detection.

A highly sensitive electrochemical cytosensor based on a triple signal amplification strategy using both nanozyme and DNAzyme

The development of a sensitive cytosensor is beneficial for the early diagnosis and treatment of cancer. Herein, highly sensitive cytosensing was achieved by applying triple signal amplification strategies with Fe₃O₄@Au nanozymes and DNAzyme hybrids as electrochemical nanoprobes and toluidine blue (Tb) as the electron transfer medium. The Fe₃O₄@Au nanocomposites not only acted as nanozymes with excellent catalytic performance towards H₂O₂ reduction but also served as promising scaffolds to carry massive electroactive substances and DNA probes. The dual-functional DNA probes were designed with the sequence of hemin/G-quadruplex to serve as the DNAzyme and the sequence of aptamer to recognize cancer cells. Furthermore, Tb was also conjugated to the surface of the Fe₃O₄@Au nanohybrids, working as the electron transport medium to magnify the electrochemical response. With the above design, the Fe₃O₄@Au nanozymes and hemin/G-quadruplex DNAzyme efficiently co-catalyzed the reduction of H₂O₂ to accelerate the electron transfer of Tb, which realized triple signal amplification and finally improved the performance of the electrochemical cytosensor. The proposed cytosensor achieved a sensitive detection of HepG2 cells with a low detection limit of 20 cells mL^-1, and could be potentially used as an effective analysis tool in early cancer diagnosis in the future.

Fluorescence detection of Staphylococcus aureus using vancomycin functionalized magnetic beads combined with rolling circle amplification in fruit juice

Staphylococcus aureus is a common foodborne pathogen that can cause a suppurative infection after eating contaminated food. Detection of S. aureus plays an important role in the food industry. In this study, a strategy for the detection of S. aureus using magnetic separation (MS) technology combined with rolling circle amplification (MS-RCA) was proposed. The strategy used antibiotics to capture bacteria and employed RCA products as signal output probes. Vancomycin (Van), as a commonly used antibiotic, can recognize peptidoglycan on the cell wall of Gram-positive bacteria and can effectively identify target bacteria. Therefore, we prepared BSAylated-Van functionalized magnetic beads (Van-MBs) for the pre-enrichment of S. aureus. To ensure the selectivity of this method, we used biotin-pig IgG to bind S. aureus. In addition, to amplify the output signal of the MS-RCA strategy, we introduced streptavidin (SA) and successfully obtained the Van-MBs@S. aureus@biotin-pig IgG@SA@biotin-RCA probe complex and used the biotin-avidin-system (BAS) by combining magnetic separation technology and RCA technology to realize the enrichment and specific detection of S. aureus. Furthermore, by optimizing the experimental conditions such as the magnetic separation time and the amount of Van-MBs, the detection performance of this method was improved. Under the optimal conditions, the detection limit of this method for S. aureus was 3.3 × 10² CFU/mL in fruit juice, and it was less affected by other bacteria.

Functional nucleic acid biosensors utilizing rolling circle amplification

Functional nucleic acids regulate rolling circle amplification to produce multiple detection outputs suitable for the development of point-of-care diagnostic devices.

Formation of Visible Aggregates between Rolling Circle Amplification Products and Magnetic Nanoparticles as a Strategy for Point-of-Care Diagnostics

Visual detection of rolling circle amplification products (RCPs) has been achieved by specific aggregation with magnetic nanoparticles. The method presented here reliably generates aggregates in 1.5 h; these are visible to the naked eye in samples containing at least 0.4 fmol of RCPs. In addition, alternate current susceptometry and absorbance spectroscopy have also been used to quantify the amplified products. The specificity of the detection method was tested, and no non-specific aggregation was detected in samples containing up to 20 fmol of non-complementary amplified DNA. This method is a versatile tool for detecting pathogenic DNA in point-of-care diagnostics, with no readout equipment required. However, chips and automated assays can be used in conjugation with the developed method since detection and quantification can be achieved by commercially available readout instruments.

Capture, Detection, and Simultaneous Identification of Rare Circulating Tumor Cells Based on a Rhodamine 6G-Loaded Metal-Organic Framework

Circulating tumor cells (CTCs) play a key role in the development of tumor metastasis. It will be a big step forward for CTC application as a reliable clinical liquid biopsy marker to be able to identify the captured CTCs while achieving a high capture efficiency within one analytical system. Herein, in this work, a stimuli-responsive and rhodamine 6G (Rho 6G)-entrapped fluorescent metal-organic framework (MOF) probe, named MOF-Rho 6G-DNA, was designed to capture, detect, and subsequently identify CTCs from blood samples of cancer patients. The probe was fabricated by modifying the epithelial cell adhesion molecule (EpCAM) hairpin DNA aptamer with Rho 6G enclosed and an Arm-DNA-attached UiO-66-NH2 MOF by sequence complementation. CTCs could be captured by the EpCAM hairpin DNA on the probe; as a result, Rho 6G loaded in the probe was released, and the number of CTCs was positively related to the concentration of released Rho 6G. An excellent correlation of fluorescence intensities with CTC numbers was obtained from 2 to 500 cells/mL. More importantly, the MOF-Rho 6G-DNA probe simultaneously realized rapid identification of the captured cells within 30 min by only relying on the residue Rho 6G in the MOF cavity. The captured target cells can be conveniently released from the probe using the complementary DNA sequence. These performance features of the probe were further verified by blood samples from patients of various types of tumor.

Multivalent Duplexed-Aptamer Networks Regulated a CRISPR-Cas12a System for Circulating Tumor Cell Detection

Although circulating tumor cells (CTCs) have great potential to act as the mini-invasive liquid biopsy cancer biomarker, a rapid and sensitive CTC detection method remains lacking. CRISPR-Cas12a has recently emerged as a promising tool in biosensing applications with the characteristic of fast detection, easy operation, and high sensitivity. Herein, we reported a CRISPR-Cas12a-based CTC detection sensor that is regulated by the multivalent duplexed-aptamer networks (MDANs). MDANs were synthesized on a magnetic bead surface by rolling circle amplification (RCA), which contain multiple duplexed-aptamer units that allow structure switching induced by cell-binding events. The presence of target cells can trigger the release of free "activator DNA" from the MDANs structure to activate the downstream CRISPR-Cas12a for signal amplification. Furthermore, the 3D DNA network formed by RCA products also provided significantly higher sensitivity than the monovalent aptamer. As a proof-of-concept study, we chose the most widely used sgc8 aptamer that specifically recognizes CCRF-CEM cells to validate the proposed approach. The MDANs-Cas12a system could afford a simple and fast CTC detection workflow with a detection limit of 26 cells mL^-1. We also demonstrated that the MDANs-Cas12a could directly detect the CTCs in human blood samples, indicating a great potential of the MDANs-Cas12a in clinical CTC-based liquid biopsy.

Proximity ligation assay mediated rolling circle amplification strategy for in situ amplified imaging of glycosylated PD-L1

Glycosylated PD-L1 is a more reliable biomarker for immune checkpoint therapy and plays important roles in tumor immunity. Glycosylation of PD-L1 hinders antibody-based detection, which is partially responsible for the inconsistency between PD-L1 immunohistochemical results and therapeutic treatment response. Herein, we present a proximity ligation assay mediated rolling circle amplification (PLA-RCA) strategy for amplified imaging of glycosylated PD-L1 in situ. The strategy relies on a pair of DNA probes: an aptamer probe to specifically recognize cellular surface protein PD-L1 and a glycan conversion (GC) probe for metabolic glycan labeling. Upon proximity ligation of sequence binding to the two probes, the proximity ligation-triggered RCA occurs. The feasibility of the as-proposed strategy has been validated as it realized the visualization of PD-L1 glycosylation in different cancer cells and the monitoring of the variation of PD-L1 glycosylation during drug treatment. Thus, we envision the present work offers a useful alternative to track protein-specific glycosylation and potentially advances the investigation of the dynamic glycan state associated with the disease process.

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.

Photoelectrochemical assay for the detection of circulating tumor cells based on aptamer-Ag2S nanocrystals for signal amplification

In this work, we developed a photoelectrochemical assay for circulating tumor cells (CTCs) detection based on hexagonal carbon-nitrogen tubes (HCNT) as visible light-sensitive materials. The MCF-7 cell was selected as the model CTC and was captured through specific recognition between epithelial cell adhesion molecules (EpCAM) on the cell surface and anti-EpCAM antibodies. Anti-EpCAM antibody-modified magnetic nanoparticles were used to enrich and separate MCF-7 cells from samples. The detection signal was amplified by Ag₂S nanocrystals, which can compete with HCNTs for absorbing visible light, leading to a decrease of photocurrent intensity. The linear range of the assay for MCF-7 cells is from 10 to 5000 cells mL^-1, with a detection limit of 3 cells mL^-1 (S/D = 3). The assay has good selectivity for MCF-7 detection over HeLa cells. The assay was successfully applied for the detection of MCF-7 in human whole blood, which indicates the potential for clinical application.

The mechanism and improvements to the isothermal amplification of nucleic acids, at a glance

A comparative review of the most common isothermal methods is provided. In the last two decades, the challenge of using isothermal amplification systems as an alternate to the most extensive and long-standing nucleic acids-amplifying method-the polymerase chain reaction-has arisen. The main advantage of isothermal amplification is no requirement for expensive laboratory equipment for thermal cycling. Considerable efforts have been made to improve the current techniques of nucleic acid amplification and the development of new approaches based on the main drawbacks of each method. The most important and challenging goal was to achieve a low-cost, straightforward system that is rapid, specific, accurate, and sensitive.

Ultrasensitive Nanopore Sensing of Mucin 1 and Circulating Tumor Cells in Whole Blood of Breast Cancer Patients by Analyte-Triggered Triplex-DNA Release

The characterization of circulating tumor cells (CTCs) by liquid biopsy has a great potential for precision medicine in oncology. Here, a universal and tandem logic-based strategy is developed by combining multiple nanomaterials and nanopore sensing for the determination of mucin 1 protein (MUC1) and breast cancer CTCs in real samples. The strategy consists of analyte-triggered signal conversion, cascaded amplification via nanomaterials including copper sulfide nanoparticles (CuS NPs), silver nanoparticles (Ag NPs), and biomaterials including DNA hydrogel and DNAzyme, and single-molecule-level detection by nanopore sensing. The amplification of the non-DNA nanomaterial gives this method considerable stability, significantly lowers the limit of detection (LOD), and enhances the anti-interference performance for complicated samples. As a result, the ultrasensitive detection of MUC1 could be achieved in the range of 0.0005-0.5 pg/mL, with an LOD of 0.1 fg/mL. Moreover, we further tested MUC1 as a biomarker for the clinical diagnosis of breast cancer CTCs under double-blind conditions on the basis of this strategy, and MCF-7 cells could be accurately detected in the range from 5 to 2000 cells/mL, with an LOD of 2 cells/mL within 6 h. The detection results of the 19 clinical samples were highly consistent with those of the clinical pathological sections, nuclear magnetic resonance imaging, and color ultrasound. These results demonstrate the validity and reliability of our method and further proved the feasibility of MUC1 as a clinical diagnostic biomarker for CTCs.

Electrochemical sensing technology for liquid biopsy of circulating tumor cells-a review

In recent years, a lot of new detection techniques for circulating tumor cells (CTCs) have been developed. Among them, electrochemical sensing technology has gradually developed because of its advantages of good selectivity, high sensitivity, low cost and rapid detection. Especially in the latest decade, the field of electrochemical biosensing has witnessed great progress, thanks to the merging of biosensing research area with nanotechnology, immunotechnology, nucleic acid technology, and microfluidic technology. In this review, the recent progress for the detection of CTCs according to the principle of detection was summarized and how they can contribute to the enhanced performance of such biosensors was explained. The latest electrode construction strategies such as rolling circle amplification reaction, DNA walker and microfluidic technology and their advantages were also introduced emphatically. Moreover, the main reasonswhy the existing biosensors have not been widely used clinically and the next research points were clearly put forward.

Nondestructive capture, release, and detection of circulating tumor cells with cystamine-mediated folic acid decorated magnetic nanospheres

Circulating tumor cell (CTC) detection and enumeration have been considered as a noninvasive biopsy method for the diagnosis, characterization, and monitoring of various types of cancers. However, CTCs are exceptionally rare, which makes CTC detection technologically challenging. In the past few decades, much effort has been focused on highly efficient CTC capture, while the activity of CTCs has often been ignored. Here, we develop an effective method for nondestructive CTC capture, release, and detection. Folic acid (FA), as a targeting molecule, is conjugated on magnetic nanospheres through a cleavable disulfide bond-containing linker (cystamine) and a polyethylene glycol (PEG2k) linker, forming MN@Cys@PEG2k-FA nanoprobes, which can bind with folate receptor (FR) positive CTCs specifically and efficiently, leading to the capture of CTCs with an external magnetic field. When approximately 150 and 10 model CTCs were spiked in 1 mL of lysis blood, 93.1 ± 2.9% and 80.0 ± 9.7% CTCs were recovered, respectively. In total, 81.3 ± 2.6% captured CTCs can be released from MN@Cys@PEG2k-FA magnetic nanospheres by treatment with dithiothreitol. The released CTCs are easily identified from blood cells for specific detection and enumeration combined with immunofluorescence staining with a limit of detection of 10 CTC mL-1 lysed blood. Moreover, the released cells remain healthy with high viability (98.6 ± 0.78%) and can be cultured in vitro without detectable changes in morphology or behavior compared with healthy untreated cells. The high viability of the released CTCs may provide the possibility for downstream proteomics research of CTCs; therefore, cultured CTCs were collected for proteomics. As a result, 3504 proteins were identified. In conclusion, the MN@Cys@PEG2k-FA magnetic nanospheres prepared in this study may be a promising tool for early-stage cancer diagnosis and provide the possibility for downstream analysis of CTCs.

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.