Ultrasensitive simultaneous detection of four biomarkers based on hybridization chain reaction and biotin-streptavidin signal amplification strategy

Biotin-functionalized nanoparticles: an overview of recent trends in cancer detection

Electrochemical bio-sensing is a potent and efficient method for converting various biological recognition events into voltage, current, and impedance electrical signals. Biochemical sensors are now a common part of medical applications, such as detecting blood glucose levels, detecting food pathogens, and detecting specific cancers. As an exciting feature, bio-affinity couples, such as proteins with aptamers, ligands, paired nucleotides, and antibodies with antigens, are commonly used as bio-sensitive elements in electrochemical biosensors. Biotin-avidin interactions have been utilized for various purposes in recent years, such as targeting drugs, diagnosing clinically, labeling immunologically, biotechnology, biomedical engineering, and separating or purifying biomolecular compounds. The interaction between biotin and avidin is widely regarded as one of the most robust and reliable noncovalent interactions due to its high bi-affinity and ability to remain selective and accurate under various reaction conditions and bio-molecular attachments. More recently, there have been numerous attempts to develop electrochemical sensors to sense circulating cancer cells and the measurement of intracellular levels of protein thiols, formaldehyde, vitamin-targeted polymers, huwentoxin-I, anti-human antibodies, and a variety of tumor markers (including alpha-fetoprotein, epidermal growth factor receptor, prostate-specific Ag, carcinoembryonic Ag, cancer antigen 125, cancer antigen 15-3, etc.). Still, the non-specific binding of biotin to endogenous biotin-binding proteins present in biological samples can result in false-positive signals and hinder the accurate detection of cancer biomarkers. This review summarizes various categories of biotin-functional nanoparticles designed to detect such biomarkers and highlights some challenges in using them as diagnostic tools.

Sensitive Method To Analyze Cell Surface GPI-Anchored Proteins Using DNA Hybridization Chain Reaction-Mediated Signal Amplification

GPI-anchored proteins (GPI-APs) are ubiquitous and essential but exist in low abundances on the cell surface, making their analysis and investigation especially challenging. To tackle the problem, a new method to detect and study GPI-APs based upon GPI metabolic engineering and DNA-facilitated fluorescence signal amplification was developed. In this context, cell surface GPI-APs were metabolically engineered using azido-inositol derivatives to introduce an azido group. This allowed GPI-AP coupling with alkyne-functionalized multifluorophore DNA assemblies generated by hybridization chain reaction (HCR). It was demonstrated that this approach could significantly improve the detection limit and sensitivity of GPI-APs, thereby enabling various biological studies, including the investigation of live cells. This new, enhanced GPI-AP detection method has been utilized to successfully explore GPI-AP engineering, analyze GPI-APs, and profile GPI-AP expression in different cells.

Multiplexed electrochemical liposomes applied to the detection of nucleic acids for Influenza A, Influenza B and SARS-CoV-2

Multiplexing is a relevant strategy for biosensors to improve accuracy and decision-making due to the increased amount of simultaneously obtained information. Liposomes offer unique benefits for label-based multiplexing since a variety of different marker molecules can be encapsulated, leading to intrinsic signal amplification and enabling a variety of detection formats. We successfully developed an electrochemical (EC) liposome-based platform technology for the simultaneous detection of at least three analytes by studying parameters to ensure specific and sensitive bioassay performance. Influenza A and B and SARS-CoV-2 sequences served as model system in a standard sandwich hybridization assay. Studies included encapsulants, probe distribution on liposomes and capture beads, assay setup and interferences between liposomes to also ensure a generalization of the platform. Ruthenium hexamine(III), potassium hexacyanoferrate(II) and m-carboxy luminol, when encapsulated separately into a liposome, provided desirable long-term stability of at least 12 months and no cross-signals between liposomes. Through the optimization process, low limits of detections of 1.6 nmol L-1, 125 pmol L-1 and 130 pmol L-1, respectively, were achieved in a multiplexed assay setup, which were similar to singleplex assays. Non-specific interactions were limited to 25.1%, 7.6% and 7.5%, respectively, through sequential liposome incubations and singleplex capture bead designs. Here, ruthenium hexamine liposomes had only mediocre performance so that low overall signal strength translated into higher LODs and worse specificity. A different marker such as ferroin may be an option in the future. The identification of further electrochemical markers will provide new opportunities for liposomes to function as multiplex, orthogonal or internal standard labels in electrochemical bioassays.

Aptasensor for ovarian cancer biomarker detection using nanostructured gold electrodes

The development of an electrochemical aptasensor for the detection of CA125 as an ovarian cancer biomarker using gold nanostructures (GNs) modified electrodes is reported. The GNs were deposited on the surface of fluorine-doped tin oxide electrodes using a simple electrochemical method and the effects of pH and surfactant concentration on the topography and electrochemical properties of the resulting GNs modified electrodes were investigated. The electrodes were characterized using field-emission scanning electron microscopy and X-ray diffraction, cyclic voltammetry, and electrochemical impedance spectroscopy. The best electrode, in terms of the uniformity of the deposited GNs and the increase in electroactive surface area, was used for development of an aptasensor for CA125 tumor marker detection in human serum. Signal amplification was done by using aptamer-conjugated gold nanorods resulting in the detection limit of 2.6 U/ml and a linear range of 10 to 800 U/ml. The results showed that without the need for expensive antibodies, the developed aptasensor could specifically measure the clinically relevant concentrations of the tumor marker in human serum.

A colorimetric aptasensor for CA125 determination based on dual catalytic performance of CeO2 nanozyme confined in macroporous silica foam

A portable colorimetric aptasensor was constructed based on the dual catalytic performance of CeO2 nanozyme to determine carbohydrate antigen 125 (CA125). Firstly, CeO2 nanozyme was synthesized by calcination and ultrasonically dispersed in a macroporous silica foam (MSF) to form CeO2@MSF. Then the aptamer of CA125 (apt) and complementary DNA (c-DNA) were successively assembled on the CeO2@MSF to construct a CeO2@MSF/apt/c-DNA colorimetric aptasensor, which exhibited excellent oxidase-mimic performance and phosphatase-mimic activity simultaneously. In the presence of CA125, the apt specifically binds to target CA125, and the single-strand c-DNA leaves the CeO2@MSF/apt surface, which is catalytically hydrolyzed by exonuclease I. The produced phosphate ions inhibit the phosphatase-mimic activity of CeO2 nanozyme. Thus, the absorbance at 652 nm of 3,3',5,5'-tetramethylbenzidine solution containing ascorbic acid-2-phosphate increases with the concentration of CA125. The response is linearly related to the logarithm of CA125 concentration from 1.0 to 10.0 U/mL under optimal experimental conditions. Based on this, the constructed colorimetric aptasensor has a high sensitivity, good selectivity, and high accuracy for CA125 determination in real human serum sample.

Single-molecular protein-based bioelectronics via electronic transport: fundamentals, devices and applications

Biomolecular electronics is a rapidly growing multidisciplinary field that combines biology, nanoscience, and engineering to bridge the two important fields of life sciences and molecular electronics. Proteins are remarkable for their ability to recognize molecules and transport electrons, making the integration of proteins into electronic devices a long sought-after goal and leading to the emergence of the field of protein-based bioelectronics, also known as proteotronics. This field seeks to design and create new biomolecular electronic platforms that allow for the understanding and manipulation of protein-mediated electronic charge transport and related functional applications. In recent decades, there have been numerous reports on protein-based bioelectronics using a variety of nano-gapped electrical devices and techniques at the single molecular level, which are not achievable with conventional ensemble approaches. This review focuses on recent advances in physical electron transport mechanisms, device fabrication methodologies, and various applications in protein-based bioelectronics. We discuss the most recent progress of the single or few protein-bridged electrical junction fabrication strategies, summarise the work on fundamental and functional applications of protein bioelectronics that enable high and dynamic electron transport, and highlight future perspectives and challenges that still need to be addressed. We believe that this specific review will stimulate the interdisciplinary research of topics related to protein-related bioelectronics, and open up new possibilities for single-molecule biophysics and biomedicine.

Electrochemiluminescent immunosensor for detection of carcinoembryonic antigen using luminol-coated silver nanoparticles

Recently, electrochemiluminescent (ECL) immunosensors have received much attention in the field of biomarker detection. Here, a highly enhanced ECL immunosensing platform was designed for ultrasensitive detection of carcinoembryonic antigen (CEA). The surface of the glassy carbon electrode was enhanced by applying functional nanostructures such as thiolated graphene oxide (S-GO) and streptavidin-coated gold nanoparticles (SA-AuNPs). The selectivity and sensitivity of the designed immunosensor were improved by entrapping CEA biomolecules using a sandwich approach. Luminol/silver nanoparticles (Lu-SNPs) were applied as the main core of the signaling probe, which were then coated with streptavidin to provide overloading of the secondary antibody. The highly ECL signal enhancement was obtained due to the presence of horseradish peroxidase (HRP) in the signaling probe, in which the presence of H₂O₂ further amplified the intensity of the signals. The engineered immunosensor presented excellent sensitivity for CEA detection, with limit of detection (LOD) and linear detection range (LDR) values of 58 fg mL^-1 and 0.1 pg mL^-1 to 5 pg mL^-1 (R² = 0.9944), respectively. Besides its sensitivity, the fabricated ECL immunosensor presented outstanding selectivity for the detection of CEA in the presence of various similar agents. Additionally, the developed immunosensor showed an appropriate repeatability (RSD 3.8%) and proper stability (2 weeks). Having indicated a robust performance in the real human serum with stated LOD and LDR, the engineered immunosensor can be considered for the detection and monitoring of CEA in the clinic.

A novel electrochemical immunosensor that amplifies Poly(o-phenylenediamine) signal by pH-driven cascade reaction used for alpha-foetoprotein detection

The present protocol develops an electrochemical immunosensor with poly(o-phenylene diamine) attached gold nanoparticles (PPD@Au NPs) as the immune platform, polydopamine-loaded cobalt ions (Co²⁺-PDA) as the immune probe, and K₂S₂O₈ as the signal amplifying substance with pH-driven cascade reaction. The application of conventional immunosensors often leads to easy leakage of the current signal and increases the impedance due to assembly. However, this new immunosensor offers the following advantages: (1) The signal substance PPD is modified on the electrode surface, effectively reducing the signal loss and leakage of the immunosensor; (2) The pH response reduces the impedance of the immunosensor while destroying the Co²⁺-PDA secondary antibody label; (3) The pH response releases a small amount of Co²⁺, leading to SO₄^-· generation by K₂S₂O₈ through a cascade reaction, further amplifying the PPD response current signal; (4) The pH response generates excess Co²⁺ and the by-product PDA fragments can consume the SO₄^-· generated by K₂S₂O₈, so that the final response signal decreases with the increasing antigen concentration. The experimental results showed that the immunosensor exhibited good selectivity, long-term stability, and reproducibility for AFP detection in the range of 1 pg/mL-100 ng/mL, with a detection limit of 0.214 pg/mL. Interestingly, it is expected to be used for detecting AFP in actual blood samples.

Triple Amplification Ratiometric Electrochemical Aptasensor for CA125 Based on H-Gr/SH-β-CD@PdPtNFs

A triple-amplified and ratiometric electrochemical aptasensor for CA125 was designed based on hemin-graphene/SH-β-cyclodextrin@PdPt nanoflower (H-Gr/SH-β-CD@PdPtNF) composites and an exonuclease I (Exo I)-assisted strategy. In the nanocomposite, hemin acts as an internal reference signal owing to the reversible hemin_ox/hemin_red pair. PdPtNFs can significantly improve the electron transfer rate. SH-β-CD can efficiently enrich quercetin probes through host-guest recognition and increase the second indicator signal. In the presence of CA125, due to the specific binding between the aptamer and CA125, the conformational change of dsDNA (designed by the CA125 aptamer and its complementary DNA) results in the release of quercetin embedded in dsDNA. Subsequently, the free quercetin and DNA fragments are enriched on the H-Gr/SH-β-CD@PdPtNF-modified electrode. Thus, an enhanced oxidation peak from quercetin (I_Q) and a reduced peak from hemin (I_hemin) can indicate the same biological identification event. In addition, the recycling amplification of CA125 by Exo I can effectively assist the increase of the quercetin signal. The value of I_Q/I_hemin is linear with the concentration of CA125 in the range from 6.0 × 10^-4 to 1.0 × 10³ ng/mL, and the limit of detection is 1.4 × 10^-4 ng/mL. The recovery of CA125 in human blood serum samples was from 99.2 to 104.4%. The proposed sensor is sensitive and reliable, which provides an avenue for the development of triple amplification and ratiometric signal strategies for detecting tumor markers in clinical diagnostics.

Recent advances in nanotechnology-enhanced biosensors for α-fetoprotein detection

α-Fetoprotein (AFP) is a kind of fetal protein that is related to tumor, the increasing concentration of which gives birth to a large variety of diseases, such as liver cancer. Therefore, the detection method with super sensitivity, high selectivity, and less time consumption under trace concentrations in early stage of diseases is becoming a necessity. In recent years, nanomaterials have been regarded as significant resources for the exploration of efficient biosensors with high sensitivity, selectivity, speed, as well as simple process, due to their excellent optical, electrical, and chemical properties. In this paper, we reviewed the research progress of AFP biosensors with enhanced sensitivity and selectivity by nanoparticles. Representative examples have also been displayed in this paper to expound the nanotechnologies utilized in the early detection of AFP. Furthermore, challenges of the clinical application of AFP biosensors based on nanotechnology have been elaborated, as well as the development opportunity in this field in the future. This review provides a comprehensive overview on the various nano-biosensor for AFP detection based on functional nanotechnology.

Preparation of a pH-responsive controlled-release electrochemical immunosensor based on polydopamine encapsulation for ultrasensitive detection of alpha-fetoprotein

To accomplish ultra-sensitive detection of alpha-fetoprotein(AFP), a novel electrochemical immunosensor using polydopamine-coated Fe₃O₄ nanoparticles (PDA@Fe₃O₄ NPs) as a smart label and polyaniline (PANI) and Au NPs as substrate materials has been created. The sensor has the following advantages over typical immunoassay technology: (1) The pH reaction causes PDA@Fe₃O₄ NPs to release Prussian blue (PB) prosoma while also destroying the secondary antibody label and immunological platform and lowering electrode impedance; (2) PB has a highly efficient catalytic effect on H₂O₂, allowing for the obvious amplification of electrical impulses; (3) PANI was electrodeposited on the electrode surface to avoid PB loss and signal leakage, which effectively absorbed and fixed PB while considerably increasing electron transmission efficiency. The sensor's detection limit was 0.254 pg·mL^-1 (S/N = 3), with a detection range of 1 pg·mL^-1 to 100 ng·mL^-1. The sensor has a high level of selectivity, repeatability, and stability, and it is predicted to be utilized to detect AFP in real-world samples.

Electrochemical Immunosensor Modified with Nitrogen-Doped Reduced Graphene Oxide@Carboxylated Multi-Walled Carbon Nanotubes/Chitosan@Gold Nanoparticles for CA125 Detection

Lung cancer is one of the malignant tumors with the highest mortality rate, and the detection of its tumor marker carcinoma antigen 125 (CA125) is significant. Here, an electrochemical immunoassay for CA125 was described. Nitrogen-doped reduced graphene oxide (N-rGO), carboxylated multi-walled carbon nanotubes (CMWCNTs) and gold nanoparticles (AuNPs) were applied to co-modify glassy carbon electrode (GCE), after incubation with Anti-CA125, the modified electrode was employed for the specific detection of CA125. The N-rGO@CMWCNTs (Nitrogen-doped reduced graphene oxide@carboxylated multi-walled carbon nanotubes) were used as a matrix, while CS@AuNPs (Chitosan@gold nanoparticles) with high conductivity and biocompatibility was immobilized on it through the reaction between carboxyl groups from CMWCNTs and amino groups, hydroxyl groups from chitosan (CS), resulting in the effect of double signal amplification. The immunosensor demonstrated excellent electrochemical performance with a linear detection range of 0.1 pg mL−1–100 ng mL−1, and the detection limit was as low as 0.04 pg mL−1 (S/N = 3). It had been verified that this method had good precision and high accuracy, and the immunosensor could remain stable for 10 days. This research provided a new method for the detection of CA125 in serum.

Supramolecular Enzymatic Labeling for Aptamer Switch-Based Electrochemical Biosensor

Here we report a novel labeling strategy for electrochemical aptasensors based on enzymatic marking via supramolecular host–guest interactions. This approach relies on the use of an adamantane-modified target-responsive hairpin DNA aptamer as an affinity bioreceptor, and a neoglycoconjugate of β-cyclodextin (CD) covalently attached to a redox enzyme as a labeling element. As a proof of concept, an amperometric aptasensor for a carcinoembryonic antigen was assembled on screen-printed carbon electrodes modified with electrodeposited fern-like gold nanoparticles/graphene oxide and, by using a horseradish peroxidase-CD neoglycoenzyme as a biocatalytic redox label. This aptasensor was able to detect the biomarker in the concentration range from 10 pg/mL to 1 ng/mL with a high selectivity and a low detection limit of 3.1 pg/mL in human serum samples.

Biomarkers for Early Diagnosis of Ovarian Carcinoma

The leading cause of gynecological cancer-related morbidity and mortality is ovarian cancer (OC), which is dubbed a silent killer. Currently, OC is a target of intense biomarker research, because it is often not discovered until the disease is advanced. The goal of OC research is to develop effective tests using biomarkers that can detect the disease at the earliest stages, which would eventually decrease the mortality, thereby preventing recurrence. Therefore, there is a pressing need to revisit the existing biomarkers to recognize the potential biomarkers that can lead to efficient predictors for the OC diagnosis. This Perspective covers an update on the currently available biomarkers used in the triaging of OC to gain certain insights into the potential role of these biomarkers and their estimation that are crucial to the understanding of neoplasm progression, diagnostics, and therapy.

Rapid detection of Salmonella in milk by a nuclear magnetic resonance biosensor based on the streptavidin-biotin system and O-carboxymethyl chitosan target gadolinium probe

Rapid and sensitive detection of foodborne pathogens is of great importance for food safety. Here, a set of nuclear magnetic resonance (NMR) biosensors based on a O-carboxymethyl chitosan target gadolinium (Gd) probe was developed to quickly detect Salmonella in milk by combining NMR technology and bioimmunotechnology with membrane filtration technology. First, O-carboxymethyl chitosan (O-CMC) was biotinylated to prepare biotinylated O-carboxymethyl chitosan (biotin-O-CMC) through amide reaction, and biotinylated magnetic complexes (biotin-O-CMC-Gd) were obtained by using O-CMC, which has strong chelating adsorption on Gd. The target probe was obtained by combining biotin-O-CMC-Gd with the biotinylated antibody (biotin-antibody) via streptavidin (SA) by introducing the SA-biotin system. Then, Salmonella was captured by the target probe through antigen-antibody interaction. Finally, NMR was used to measure the longitudinal relaxation time (T₁) of the filtrate collected by membrane filtration. This NMR biosensor with good specificity and high efficiency can detect Salmonella with the sensitivity of 1.8 × 10³ cfu/mL within 2 h; in addition, it can realize the detection of complex samples because of its strong anti-interference capability and may open up a new method for rapid detection of Salmonella, which has a great application potential.

Biosensors Coupled with Signal Amplification Technology for the Detection of Pathogenic Bacteria: A Review

Foodborne disease caused by foodborne pathogens is a very important issue in food safety. Therefore, the rapid screening and sensitive detection of foodborne pathogens is of great significance for ensuring food safety. At present, many research works have reported the application of biosensors and signal amplification technologies to achieve the rapid and sensitive detection of pathogenic bacteria. Thus, this review summarized the use of biosensors coupled with signal amplification technology for the detection of pathogenic bacteria, including (1) the development, concept, and principle of biosensors; (2) types of biosensors, such as electrochemical biosensors, optical biosensors, microfluidic biosensors, and so on; and (3) different kinds of signal amplification technologies applied in biosensors, such as enzyme catalysis, nucleic acid chain reaction, biotin-streptavidin, click chemistry, cascade reaction, nanomaterials, and so on. In addition, the challenges and future trends for pathogenic bacteria based on biosensor and signal amplification technology were also discussed and summarized.

Inner filter effect between upconversion nanoparticles and Fe(ii)-1,10-phenanthroline complex for the detection of Sn(ii) and ascorbic acid (AA)

Dual-function and multi-function sensors can use the same material or detection system to achieve the purpose of detection of two or more substances. Due to their high sensitivity and specificity, dual-function and multi-function sensors have potential applications in many fields. In this article, we designed a dual-function sensor to detect Sn(ii) and ascorbic acid (AA) based on the inner filter effect (IFE) between NaYF₄:Yb,Er@NaYF₄@PAA (UCNPs@PAA) and Fe(ii)–1,10-phenanthroline complex. Fe(ii)–1,10-phenanthroline complex has strong absorption in most of the ultraviolet-visible light range (350 nm–600 nm), and this absorption band overlaps with the green emission peak of UCNPs@PAA at 540 nm; Fe(ii)–1,10-phenanthroline complex can significantly quench the green light emission of UCNPs@PAA. When Sn(ii) or AA is added to the UCNPs@PAA/Fe(iii)/1,10-phenanthroline, they can reduce Fe(iii) to Fe(ii). Fe(ii) can react with 1,10-phenanthroline to form an orange complex, thereby quenching the green light emission of UCNPs@PAA. And the quenching efficiency is related to the concentration of Sn(ii) and AA; there is a linear relationship between quenching efficiency and the concentration of Sn(ii) and AA, within a certain concentration range the detection limits of this dual-function sensor for Sn(ii) and AA are 1.08 μM and 0.97 μM, respectively. In addition, the dual-function sensor can also detect Sn(ii) and AA in tap and spring water.

Based on the inner filter effect (IFE), we use UCNPs to develop a dual-function sensors, which can realize sensitive and selective detection for the Sn(ii) and ascorbic acid (AA).

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.

Label-Free Electrochemical Immunosensor for Ultrasensitive Detection of Carbohydrate Antigen 125 Based on Antibody-Immobilized Biocompatible MOF-808/CNT

In this work, a nanocomposite of Zr-trimesic acid MOF (MOF-808) with carbon nanotube (CNT) was synthesized through an in situ formation of MOF-808 on the activated CNT. The synthesized materials were characterized by powder X-ray diffraction, transmission electron microscopy, X-ray photoluminescence spectroscopy, Brunauer-Emmett-Teller, Fourier transform infrared spectroscopy, and Raman spectroscopy. The protein compatible nature with high surface area and electrocatalytic ability of MOF-808 was utilized to construct an immunosensor for ultra low-level detection of the ovarian cancer biomarker, carbohydrate antigen 125 (CA 125). The mutual benefit of each constituent of the MOF-808/CNT composite was capable of producing highly enhanced electrochemical properties. A glassy carbon electrode modified with MOF-808/CNT was used as a platform to fabricate a label-free electrochemical immunosensor. The antibody binding sites of MOF-808/CNT were enriched by functionalization with streptavidin. The immunosensor exhibited two linear determination ranges of 0.001-0.1 and 0.1-30 ng·mL^-1, and the calculated limit of detection was 0.5 pg·mL^-1 (S/N 3). The immunosensor showed excellent reproducibility and selectivity. The patient serum sample analysis was cross-verified with the electrochemiluminescence method with a relative error of 105-110%.

Construction of electrochemical immunosensors based on redox hydrogels for ultrasensitive detection of carcinoembryonic antigens

The introduction of cellulose nanocrystals (CNCs) endows a redox hydrogel with a larger specific surface area and better adhesion to an electrode.

Nanobody-based electrochemical competitive immunosensor for the detection of AFB1 through AFB1-HCR as signal amplifier

A novel nanobody (Nb)-based voltammetric immunosensor coupled with horseradish peroxidase concatemer-modified hybridization chain reaction (HRP-HCR) signal amplifying system is described to realize the rapid and ultrasensitive detection of AFB₁. To design such an immunoassay, anti-AFB₁ Nbs with smaller molecular size were coated densely onto the surface of Au nanoparticle-tungsten disulfide-multi-walled carbon nanotubes (AuNPs/WS₂/MWCNTs) functional nanocomposites as an effective molecular recognition element, whereas AFB₁-streptavidin (AFB₁-SA) conjugates were ingeniously bound with biotinylated HCR dsDNA nanostructures as the competitor, amplifier, and signal report element. In the presence of AFB₁ targets, a competitive immunoreaction was performed between the analyte and AFB₁-SA-labeled HCR (AFB₁-HCR) platform. Upon the addition of SA-modified polyHRP (SA-polyHRP), AFB₁-HCR nanostructures containing abundant biotins were allowed to cross-link to a quantity of HRP by streptavidin-biotin chemistry for signal amplification and signal conversion. Under optimal conditions, the immunosensor displayed a good linear correlation toward AFB₁ ranging from 0.5 to 10 ng mL^-1 with a sensitivity of 2.7 μA • (mL ng^-1) and an ultralow limit of detection (LOD) of 68 fg mL^-1. The specificity test showed that the AFB₁ immunosensor had no obvious cross-reaction with OTA, DON, ZEN, and FB₁. The signal of this sensor decreased by 10.18% in 4 weeks indicating satisfactory stability, and its intra- and inter-laboratory reproducibility was 3.42~10.35% and 4.03%~12.11%, respectively. This biosensing system will open up new opportunities for the detection of AFB₁ in food safety and environmental analysis and extend a wide range of applications in the analysis of other small molecules. Graphical abstract.

Multitag-Regulated Cascade Reaction: A Generalizable Ultrasensitive MicroRNA Biosensing Approach for Cancer Prognosis

Ultrasensitive PCR-free microRNA (miR) analysis based on biosensors with enzyme-free nucleic acid amplification and reusable surface has great clinical significance in cancer prognosis. However, building such a biosensing strategy has long been challenging due to uncontrollable miR-triggered cascade amplifiers and insufficient sensing surface regeneration capability. To meet the challenge, for the first time, a general approach, named enzyme-free multitag-regulated cascade reaction (MCR), is developed to fabricate reliable trace miR biosensors. As a proof of concept, miR let-7a is detected on an evanescent wave fluorescent optical-fiber biosensing platform. The size and morphology of well-formed MCR assemblies (∼1 μm in length) are characterized by atomic force microscopy. This MCR method achieves a 30 000-fold improved sensitivity (detection limit 0.8 fM) compared to the MCR-free system and can detect abnormal urinary miR levels in lung cancer patients. Moreover, the biosensor is robust enough to be reused for over 100 cycles, which greatly reduces the cost of single detection. In sum, MCR is developed as a generalizable ultrasensitive miR biosensing approach for cancer prognosis, which opens a broad field for facile enzyme-free biosensing applications by nucleic acid assembling regulation.

2D Materials in Development of Electrochemical Point-of-Care Cancer Screening Devices

Effective cancer treatment requires early detection and monitoring the development progress in a simple and affordable manner. Point-of care (POC) screening can provide a portable and inexpensive tool for the end-users to conveniently operate test and screen their health conditions without the necessity of special skills. Electrochemical methods hold great potential for clinical analysis of variety of chemicals and substances as well as cancer biomarkers due to their low cost, high sensitivity, multiplex detection ability, and miniaturization aptitude. Advances in two-dimensional (2D) material-based electrochemical biosensors/sensors are accelerating the performance of conventional devices toward more practical approaches. Here, recent trends in the development of 2D material-based electrochemical biosensors/sensors, as the next generation of POC cancer screening tools, are summarized. Three cancer biomarker categories, including proteins, nucleic acids, and some small molecules, will be considered. Various 2D materials will be introduced and their biomedical applications and electrochemical properties will be given. The role of 2D materials in improving the performance of electrochemical sensing mechanisms as well as the pros and cons of current sensors as the prospective devices for POC screening will be emphasized. Finally, the future scopes of implementing 2D materials in electrochemical POC cancer diagnostics for the clinical translation will be discussed.

Nanostructures for nonlabeled and labeled electrochemical immunosensors: Simultaneous electrochemical detection of cancer markers: A review

The simultaneous electrochemical determination of multiple tumor antigens has attracted a great deal of attention, which can effectively enhance the capability and accuracy of the analysis. Nanostructured materials mostly played a key major role in the electrochemical immunosensors fabrication and operation improvement. This review focused mainly on the protocols for using nanostructures to fabricate electrochemical (nonlabeled@label-free and labeled@sandwich-type) immunosensors. Furthermore, this review has also described the diverse classes of electroactive nanospecies which are a complementary part of any immunosensor that assists to reach the selectivity for the target antigen. Finally, the important analytical characteristics of the published immunosensors were discussed (electrochemical detection technique, linear range, and detection limit). Studies published between the years 2009-2018 have been included in this review.

A novel impedance enhancer for amperometric biosensor based ultrasensitive detection of matrix metalloproteinase-2

The detection range and sensitivity is crucial to the determination of tumor markers. For amperometric biosensors, the detection range relies on the initial current signals (I₀) and the sensitivity is tightly related to the current signals difference (ΔI) caused by per unit concentration target. Herein, an amperometric biosensor was fabricated using polyaniline gel as substrate and CS-AuNPs-Pb²⁺ as impedance enhancer. The sensing substrate exhibited strong current signal in [Fe(CN)₆]^3-/4- due to the excellent conductivity and large specific surface area of polyaniline gel. CS-AuNPs-Pb²⁺ with large hindrance effect can significantly increase interfacial resistance, resulting in the enhancement of ΔI. In addition, Pb²⁺ can react with sodium tartrate to produce non-conducting sodium tartrate gel on sensing interface, leading to further amplification of ΔI. Matrix metalloproteinase-2 (MMP-2) was analysed to prove the feasibility of this strategy. The proposed amperometric biosensor reveal both wide linear detection range from 1 pg mL^-1 to 1 μg mL^-1 and high sensitivity of 28.4 μA·(LgC_MMP-2)^-1. Therefore, this strategy will be of great significance to design other ultrasensitive amperometric biosensors.

Graphene- and Graphene Oxide-Based Nanocomposite Platforms for Electrochemical Biosensing Applications

Graphene and its derivatives such as graphene oxide (GO) and reduced GO (rGO) offer excellent electrical, mechanical and electrochemical properties. Further, due to the presence of high surface area, and a rich oxygen and defect framework, they are able to form nanocomposites with metal/semiconductor nanoparticles, metal oxides, quantum dots and polymers. Such nanocomposites are becoming increasingly useful as electrochemical biosensing platforms. In this review, we present a brief introduction on the aforementioned graphene derivatives, and discuss their synthetic strategies and structure-property relationships important for biosensing. We then highlight different nanocomposite platforms that have been developed for electrochemical biosensing, introducing enzymatic biosensors, followed by non-enzymatic biosensors and immunosensors. Additionally, we briefly discuss their role in the emerging field of biomedical cell capture. Finally, a brief outlook on these topics is presented.

SERS-based cascade amplification bioassay protocol of miRNA-21 by using sandwich structure with biotin-streptavidin system

In our bioassay protocol, the Ag@4MBA@DNA-biotin probes were synthesized by linking biotin-modified DNA and 4-mercaptobenzoic acid-covered Ag nanoparticles, and the Si@Ag@anti-digoxin/digoxin-DNA substrate was fabricated by immune linking of digoxin-DNA and anti-digoxin immobilized on a Ag-coated wafer. Then, the probes, miRNA-21 and the substrate were constructed into a "sandwich structure" to detect the variation in the SERS signals with respect to miRNA-21 concentrations. Next, streptavidin and extra probes were alternately introduced to implement the cascade amplification of the SERS signal to increase the detection sensitivity. The results show that the limit of detection (LOD) with cascade amplification is remarkably improved from 97.81 pM to 38.02 fM, which is three orders of magnitude higher than the original data without cascade amplification. Furthermore, the SERS-based cascade amplification mechanism was analyzed and is attributed to the "hot spots effect" of the noble metal nanostructure. The biotin-streptavidin (B-S) system was introduced into the SERS detection platform, and the novel SERS-based cascade amplification bioassay protocol has significant creativity for the detection of nucleic acids.

A ratiometric electrochemical sensor for multiplex detection of cancer biomarkers using bismuth as an internal reference and metal sulfide nanoparticles as signal tags

Ratiometric electrochemical sensors can provide a relatively accurate analysis of target analytes due to their self-calibration function. Herein, we report a simple ratiometric strategy for achieving the electrochemical detection of Cd(ii), Hg(ii), Pb(ii) and Zn(ii), as well as multiple cancer biomarkers by using metal sulfide nanoparticles as signal tags. A conductive polymer film of poly(2-amino terephthalic acid) (ATA) was electrochemically produced on a glassy carbon electrode (GCE) and doped with carbon nanotubes (CNTs) and mercaptosuccinic acid (MSA). Using Bi(iii) as an enhancer and internal reference in anodic stripping voltammetry, the MSA-CNT-ATA/GCE exhibited sensitive and distinguishable voltammetric responses to Cd(ii), Hg(ii), Pb(ii) and Zn(ii), with detection limits of 0.13, 0.49, 0.16 and 0.089 μg L^-1, respectively. By using CdS, HgS, PbS and ZnS labeled secondary antibodies as the signal tags, alpha-fetoprotein, carbohydrate antigen 19-9, carbohydrate antigen 125, and carcinoembryonic antigen were determined simultaneously according to the amounts of metal sulfide in the sandwich-type complexes, with detection limits of 0.11 pg mL^-1, 0.68 mU mL^-1, 1.4 mU mL^-1 and 0.23 pg mL^-1, respectively. This ratiometric approach has a wide scope in the electrochemical detection of heavy metal ions as well as immunoassays with metal ions serving as signal tags.

A label-free immunosensor based on PHEMA/graphene oxide nanocomposite for simultaneous electrochemical determination of alpha fetoprotein

An electrochemical immunosensor based on poly(2-hydroxyethyl methacrylate) (PHEMA)/graphene oxide (GO) nanocomposite was designed in a simple way for the ultrasensitive detection of tumor makers (alpha-fetoprotein, AFP as a model). PHEMA with excellent biocompatibility, provides a large number of sites for connecting signal molecules. After modification with signal molecules, the functional PHEMA significantly improved the sensitivity of electrochemical detection. In order to immobilize antibodies, GO was introduced and used to construct a nanocomposite as a substrate. The designed AFP immunosensor showed favorable selectivity and excellent stability. Meanwhile, it has a low detection limit of 0.403 pg mL-1. Furthermore, the immunosensor was used to detect target AFP in human serum, demonstrating the feasibility of clinical diagnosis.

Nitrogen doped carbon dots: mechanism investigation and their application for label free CA125 analysis

Nitrogen-doped CDs (N-CDs) were firstly prepared by using pear juice as the carbon source and ethanediamine as a nitrogen doping precursor with a microwave assisted pyrolysis technique. Based on the fluorescence recovery induced by competitive adsorption and desorption, a label-free “turn on” fluorescence assay with high sensitivity and selectivity was proposed for the analysis of CA125.

Employing AgNPs doped amidoxime-modified polyacrylonitrile (PAN-oxime) nanofibers for target induced strand displacement-based electrochemical aptasensing of CA125 in ovarian cancer patients

In this study, a high-performance biosensing nanoplatform based on amidoxime-modified polyacrylonitrile nanofibers decorated with Ag nanoparticles (AgNPs-PAN-oxime NFs) is described. The AgNPs-PAN-oxime NFs were prepared by the combination of electrospinning technique and chemical modification of nitrile group in the PAN. The proposed signal amplifiying nanoplatform was applied in the fabrication of an electrochemical aptasensor for the sensitive detection of CA 125 based on aptamer-cDNA duplex and target induced strand displacement recognition mechanism. The aptasensing interface offers high sensitivity and selectivity for detection of tumor marker due to inherent advantages such as high specific surface area of NFs, good conductivity by doping AgNPs into the polymer NFs and especially the ideal selectivity of anti CA 125 aptamer to its target. The electrochemical aptasensor revealed a wide dynamic linear range (DLR) from 0.01 to 350 U mL^-1 with a correlation coefficient of 0.991 and limit of detection (LOD) of 0.0042 U mL^-1. Additionally, the designed aptasensor showed acceptable selectivity, reproducibility, repeatability and stability. The satisfactory results for determination of CA 125 in serum samples compared to ELISA method (p-value > 0.05) indicated the potential application of aptasensor in clinical monitoring of tumor biomarker for early diagnosis and management of ovarian cancer.

A rapid and efficient technique for liposomal and nonliposomal drug pharmacokinetics studies using magnetic nanoprobes and its application to leakage kinetics of liposomes

Currently, the pharmacokinetics of liposomes was researched in vivo by measuring the total amount of drug in plasma. This method of using the total drug amount instead of the free drug amount virtually increase the apparent exposure and apparent biological distribution. To solve this problem, we developed a rapid and efficient method by using well-established streptavidin-functional Fe₃O₄@PDA as the separation nanoprobes to efficiently isolate biotin-labeled DTX-liposomes over 75% from plasma in the presence of magnetic field. The isolation procedure takes only 20 min and the concentration of DTX in liposomes from plasma was determined by LC-MS/MS. The method for the determination of DTX in plasma was linear in the range of 5-5000 ng/mL, and the correlation coefficient was 0.9989. Results obtained in this study clearly demonstrated that the pharmacokinetic parameters of non-liposomal drug and total drug are different in vivo. Therefore, traditional method for studying the pharmacokinetics of liposomes in vivo is unreasonable, and the new method mentioned here provided a strategy for studying the pharmacokinetics of liposomes.

Sandwich pair nanobodies, a potential tool for electrochemical immunosensing serum prostate-specific antigen with preferable specificity

Prostate-Specific Antigen (PSA) is a crucial biomarker for screening prostate cancer, but a sensitive and selective immunosensor for rapid quantification of serum PSA remains to be developed. In this study, a sandwich pair of nanobodies (Nbs) (i.e., Nb2 and Nb40) against PSA surface antigen was obtained from an alpaca-derived immune phage display library. A sandwich-type immunosensor for the sensitive and selective detection of PSA in serum samples was ingeniously designed based on the pair of Nbs. The small size of Nb40 allowed high capture densities on the surface of reduced graphene oxide (rGO) nanocomposed with massive Au nanoparticles (rGO@AuNPs), which significantly improved the conductivity and provided a large area to anchor many primary antibodies. The secondary antibody Nb2 fused with streptavidin -binding peptide (SBP) cooperated with Nb40 for PSA sandwiching. Accompanying introduction of horseradish peroxidase-streptavidin (HRP-SA) coupled with Nb2-SBP, the faradaic current was linearly correlated with the logarithm of PSA concentration in a range of 0.1-100 ng mL^-1. More importantly, this immunosensor exhibited excellent selectivity, stability, and reproducibility due to the sandwich pair Nbs. The proposed immunosensor was successfully applied in determining PSA in serum samples and could be used for the sensitive and specific detection of PSA.