A new colorimetric platform for ultrasensitive detection of protein and cancer cells based on the assembly of nucleic acids and proteins

A Highly Selective Schiff Base Based Chemodosimeter for the Detection of Perfluorooctanoic Acid by Optical Biosensor

A simple imine derivative based sensor (IDP) has been synthesized and characterized by 1 H NMR, 13 C NMR and mass spectral techniques. IDP is more capable of detecting perfluorooctanoic acid (PFOA) in a selective and sensitive manner. The PFOA as a biomarker interacts with IDP and shows "TURN-ON" response by colorimetric and fluorimetric method. Under optimized experimental observations, the selective determination of PFOA using IDP among other competitors as biomolecules has been noticed. The detection limit is 0.31 × 10- 8 mol/L. The practical applications of the IDP is effectively evaluated in human biofluids and water samples.

Anti-CD44 antibodies grafted immunoaffinity Fe3O4@MnO2 nanozymes with highly oxidase-like catalytic activity for specific detection of triple-negative breast cancer MDA-MB-231 cells

Cell-enzyme-linked immunosorbent assay (CELISA) is extensively applied for cancer diagnosis and screening because of its simple operation, high sensitivity, and intuitive color change. However, the unstable horseradish peroxidase (HRP), hydrogen peroxide (H₂O₂) and non-specificity have led to a high false negative rate, which limits its application. In this study, we have developed an innovative immunoaffinity nanozyme aided CELISA based on anti-CD44 monoclonal antibodies (mAbs) bioconjugated manganese dioxide-modified magnetite nanoparticles (Fe₃O₄@MnO₂ NPs) for the specific detection of triple-negative breast cancer MDA-MB-231 cells. The ^CD44FM nanozymes were fabricated to replace unstable HRP and H₂O₂ to counteract possible negative effects in conventional CELISA. Results suggested that ^CD44FM nanozymes displayed remarkable oxidase-like activities over an extensive pH and temperature range. The bioconjugation of CD44 mAbs enabled ^CD44FM nanozymes to enter MDA-MB-231 cells selectively via over-expressed CD44 antigens on the membrane surface of these cells, and then catalyzed oxidation of the chromogenic substrate TMB, further achieving specific detection of these cells. Additionally, this study exhibited high sensitivity and low detection limit for MDA-MB-231 cells with a quantitation range of just 186 cells. To sum up, this report developed a simple, specific and sensitive assay platform based on ^CD44FM nanozymes, which could provide a promising strategy for targeted diagnosis and screening of breast cancer.

Nanomaterial-Based Fluorescent Biosensor for Food Safety Analysis

Food safety issues have become a major threat to public health and have garnered considerable attention. Rapid and effective detection methods are crucial for ensuring food safety. Recently, nanostructured fluorescent materials have shown considerable potential for monitoring the quality and safety of food because of their fascinating optical characteristics at the nanoscale. In this review, we first introduce biomaterials and nanomaterials for food safety analysis. Subsequently, we perform a comprehensive analysis of food safety using fluorescent biosensors based on nanomaterials, including mycotoxins, heavy metals, antibiotics, pesticide residues, foodborne pathogens, and illegal additives. Finally, we provide new insights and discuss future approaches for the development of food safety detection, with the aim of improving fluorescence detection methods for the practical application of nanomaterials to ensure food safety and protect human health.

Functional nucleic acids in glycobiology: A versatile tool in the analysis of disease-related carbohydrates and glycoconjugates

As significant components of the organism, carbohydrates and glycoconjugates play indispensable roles in energy supply, cell signaling, immune modulation, and tumor cell invasion, and function as biomarkers since aberrance of them has been proved to be associated with the emergence and development of certain diseases. Functional nucleic acids (FNAs) have properties including easy-to-synthesize, good stability, good biocompatibility, low cost, and high programmability, they have attracted significant research attention and been incorporated into biosensors for detecting disease-related carbohydrates and glycoconjugates. This review summarizes the construction strategies and biosensing applications of FNAs-based biosensors in glycobiology in terms of target recognition and signal transduction. By illustrating the mechanisms and comparing the performances, the challenges and development opportunities in this area have been critically elaborated. We believe that this review will provide a better understanding of the role of FNAs in the analysis of disease-related carbohydrates and glycoconjugates, and inspire further discovery in fields that include glycobiology, chemical biology, clinical diagnosis, and drug development.

In Situ Assembly of Nanomaterials and Molecules for the Signal Enhancement of Electrochemical Biosensors

The physiochemical properties of nanomaterials have a close relationship with their status in solution. As a result of its better simplicity than that of pre-assembled aggregates, the in situ assembly of nanomaterials has been integrated into the design of electrochemical biosensors for the signal output and amplification. In this review, we highlight the significant progress in the in situ assembly of nanomaterials as the nanolabels for enhancing the performances of electrochemical biosensors. The works are discussed based on the difference in the interactions for the assembly of nanomaterials, including DNA hybridization, metal ion-ligand coordination, metal-thiol and boronate ester interactions, aptamer-target binding, electrostatic attraction, and streptavidin (SA)-biotin conjugate. We further expand the range of the assembly units from nanomaterials to small organic molecules and biomolecules, which endow the signal-amplified strategies with more potential applications.

A sensitive platform for DNA detection based on organic electrochemical transistor and nucleic acid self-assembly signal amplification

Highly sensitive detection of DNA is of great importance for the detection of genetic damage and errors for the diagnosis of many diseases. Traditional highly sensitive organic electrochemical transistor (OECT)-based methods mainly rely on good conductivity materials, which may be limited by complex synthesis and modification steps. In this work, DNA biosensor based on OECT and hybridization chain reaction (HCR) signal amplification was demonstrated for the first time. Au nanoparticles were electrochemically deposited on the Au gate electrode to increase the surface area. Then, the HCR products, long negatively charged double-stranded DNA, were connected to the target by hybridization, which can increase the effective gate voltage offset of OECT. This sensor exhibited high sensitivity and even 0.1 pM target DNA could be directly detected with a significant voltage shift. In addition, it could discriminate target DNA from the mismatched DNA with good selectivity. This proposed method based on HCR in DNA detection exhibited an efficient amplification performance on OECT, which provided new opportunities for highly sensitive and selective detection of DNA.

A new method has been developed for DNA detection by integrating hybridization chain reaction signal amplification with organic electrochemical transistor device for the first time.

MnO2-graphene oxide hybrid nanomaterial with oxidase-like activity for ultrasensitive colorimetric detection of cancer cells

Robust and sensitive cell-based enzyme-linked immunosorbent assay (CELISA) is of great significance in the diagnosis and screening of cancer. However, the method is limited by the high rate of negative results attributed to the instability of horseradish peroxidase (HRP), H₂O₂, and antibody. Here, we construct a folic acid-functionalized in situ-grown MnO₂ nanosheet/graphene oxide hybrid (FA-MnO₂/GO) with oxidase-like activity instead of the anti-folate receptor antibody in traditional CELISA to resist the possible negative interference arising from unstable HRP, H₂O₂, and antibodies for more robust colorimetric detection of cancer cells. The functionalization of FA enables the selective binding between hybrid and cancer cells through the over-expressed folate receptor, and then the binding events are converted into quantitative colorimetric signals though the oxidation of the chromogenic substrate TMB catalyzed by MnO₂, allowing the detection of cancer cells with colorimetric method. Moreover, the construction of MnO₂/GO hybrid can synergistically enhance the oxidase-like activity of MnO₂ and promote its dispersion in water, further ensuring the accuracy and sensitivity of the detection. A detection limit of 20 cancer cells is obtained by a plate reader, which is lower than those obtained by most reported CELISA methods for cancer cell detection, and as few as 75 cancer cells can be identified by the naked eye. This study not only provides a multifunctional sensing platform for robust and sensitive cancer cell detection, but also offers a promising oxidase-like mimic in the field of bioanalysis.

Hybridization chain reaction-enhanced enzyme biomineralization for ultrasensitive colorimetric biosensing of a protein biomarker

By employment of an aptamer-initiated hybridization chain reaction (HCR) to enhance the enzyme biomineralization of cupric subcarbonate, this work develops a novel colorimetric biosensing method for protein analysis. The HCR product was used to specifically attach a large amount of urease-functionalized gold nanoparticles (Au NPs) for the preparation of a gold nanoprobe. After the sandwich biorecognition reactions, this nanoprobe could be quantitatively captured onto the antibody-functionalized magnetic bead (MB) platform. Then, numerous copper ions would be enriched onto the MB surface through the urease-induced biomineralization of cupric subcarbonate. Based on the complete release of Cu2+ ions for the sensitive copper chromogenic reaction, convenient colorimetric signal transduction was thus achieved for the quantitative analysis of the target analyte of the carcinoembryonic antigen. The HCR product provides a large number of biotin sites for the attachment of Au NP nanotags. The biomineralization reaction of high-content urease loaded onto Au NPs leads to highly efficient Cu2+ enrichment for signal amplification. So this method features excellent performance including a very wide linear range and a low detection limit down to 0.071 pg mL-1. In addition, the satisfactory results of real sample experiments reveal that this method possesses huge potential for practical applications.

The synthesis of a smart streptavidin-functionalized poly(N-isopropylacrylamide) composite and its application in the separation and detection of virus nucleic acid

A new kind of polymeric material (PNIPAAm-co-SA) was prepared by conjugating a thermosensitive polymer, Poly (N-isopropylacrylamide) (PNIPAAm) with streptavidin (SA). This smart prepared composite displayed a controllable conformation change between an expanded and a collapsed form, below or above its lower critical solution temperature (LCST). Differential scanning calorimetry (DSC) analysis demonstrated that the PNIPAAm-co-SA bioconjugate showed the same LCST as the original synthetic polymer, PNIPAAm, which was also 32°C. Based on the specific interaction between SA and biotin, a higher capture efficiency of PNIPAAm-co-SA, which was almost 100% in PBS buffer solution and above 70% in serum was obtained, respectively. And the high affinity between PNIPAAm-co-SA and biotin was still maintained after three heating cycles. Subsequently, the variola virus (small pox, VV) oligonucleotide sequence was chosen as a model to demonstrate the sensitivity of the biosensor which was fabricated based on PNIPAAm-co-SA. The biosensor exhibited the ability to separate and enrich targets from complicated system with its phase transition ability, and high sensitivity toward VV-targets were achieved. Moreover, other types of targets such as proteins and cells, could be detected by changing the biotin-captures, which indicated the broad applicability of biosensors based on this smart polymer material.

A cationic conjugated polymer coupled with exonuclease I: application to the fluorometric determination of protein and cell imaging

A strategy is described for the detection of protein by using a cationic fluorescent conjugated polymer coupled with exonuclease I (Exo I). Taking streptavidin (SA) as model protein, it is observed that Exo I can digest single-stranded DNA conjugated with biotin and carboxyfluorescein (P1) if SA is absent. This leads to the formation of small nucleotide fragments and to weak fluorescence resonance energy transfer (FRET) from the polymer to P1. If, however, SA is present, the high affinity of SA and biotin prevents the digestion of P1 by Exo I. This results in the sorption of P1 on the surface of the polymer through strong electrostatic interaction. Hence, efficient FRET occurs from the fluorescent polymer to the fluorescent label of P1. Fluorescence is measured at an excitation wavelength of 370 nm, and emission is measured at two wavelengths (530 and 425 nm). The ratio of the two intensities (I₅₃₀/I₄₂₅) is directly related to the concentration of SA. Under the optimal conditions, the assay has a detection limit of 1.3 ng·mL^-1. The method was also applied to image the folate receptor in HeLa cells, thus demonstrating the versatility of this strategy. Graphical abstract A fluorometric strategy is described for protein detection and cell imaging based on a cationic conjugated polymer (PFP) coupled with exonuclease I (Exo I) trigged fluorescence resonance energy transfer (FRET).

"Turn-on" fluorescent probe detection of Ca2+ ions and applications to bioimaging

Ca²⁺ is intracellular divalent cation with the largest concentration variations and involved in many biological phenomena and often acted as a second messenger in signaling pathway. Therefore, the development of probes for specific Ca²⁺ detection is of great importance. Herein, a novel turn-on fluorescent probe for the detection of Ca²⁺ in MeCN-aqueous medium was designed and synthesized. The probe displayed responses to Ca²⁺ with a fluorescence enhancement at 525nm, accompanying with a distinct fluorescence change from nearly colorless to bright yellow-green. Besides, the probe exhibited a rapid signal response time (within 25s), a good linearity range and a lower detection limit (2.70×10^-7M). In addition, the ability of the probe to detect Ca²⁺ in living cells (HeLa cells) via an enhancement of the fluorescence has also been demonstrated.

Universal ratiometric electrochemical biosensing platform based on mesoporous platinum nanocomposite and nicking endonuclease assisted DNA walking strategy

The occurrence and development of many complex diseases are associated with various molecules, whose contents are rarely in the early stage of the disease. Thus a universal platform for the ultrasensitive detection of multilevel biomarkers should be developed. In this study, we introduced an electrochemical biosensing system based on nicking endonuclease (Nt.BbvCI) assisted DNA walking strategy. We successfully constructed a universal signal-off-on ratiometric electrochemical biosensor for various biomolecules, including small molecules, nucleic acids, and proteins, by progressively optimizing the schematics (schemes 1, 2, and 3). The MB-hairpin probes (MB-HPs) acted as a signal-off probe, and nanocomposites (MPNs@DOX@DNA2) acted as a conventional signal-on probe (scheme 3). With the aid of the MPNs@DOX@DNA2 and Nt.BbvCI assisted DNA walking mechanism, the designed ratiometric electrochemical biosensor showed a high sensitivity and broad detection range. In addition, the proposed method can be utilized to detect diverse targets quantitatively by changing the sequence of aptamers under optimum experimental conditions. Furthermore, it has been widely proved to realize well-accepted signal response in identifying complex samples, thereby resulting in an wide prospect for bioanalysis and clinical diagnosis.

Fluorescence Sensing of Circulating Diagnostic Biomarkers Using Molecular Probes and Nanoparticles

The interplay of photonics, nanotechnology, and biochemistry has significantly improved the identification and characterization of multiple types of biomarkers by optical biosensors. Great achievements in fluorescence-based technologies have been realized, for example, by the advancement of multiplexing techniques or the introduction of nanoparticles to biochemical and clinical research. This review presents a concise overview of recent advances in fluorescence sensing techniques for the detection of circulating disease biomarkers. Detection principles of representative approaches, including fluorescence detection using molecular fluorophores, quantum dots, and metallic and silica nanoparticles, are explained and illustrated by pertinent examples from the recent literature. Advanced detection technologies and material development play a major role in modern biosensing and consistently provide significant improvements toward robust, sensitive, and versatile platforms for early detection of circulating diagnostic biomarkers.

Quencher-Free Fluorescence Method for the Detection of Mercury(II) Based on Polymerase-Aided Photoinduced Electron Transfer Strategy

A new quencher-free Hg²⁺ ion assay method was developed based on polymerase-assisted photoinduced electron transfer (PIET). In this approach, a probe is designed with a mercury ion recognition sequence (MRS) that is composed of two T-rich functional areas separated by a spacer of random bases at the 3′-end, and a sequence of stacked cytosines at the 5′-end, to which a fluorescein (FAM) is attached. Upon addition of Hg²⁺ ions into this sensing system, the MRS folds into a hairpin structure at the 3′-end with Hg²⁺-mediated base pairs. In the presence of DNA polymerase, it will catalyze the extension reaction, resulting in the formation of stacked guanines, which will instantly quench the fluorescence of FAM through PIET. Under optimal conditions, the limit of detection for Hg²⁺ ions was estimated to be 5 nM which is higher than the US Environmental Protection Agency (EPA) standard limit. In addition, no labeling with a quencher was requiring, and the present method is fairly simple, fast and low cost. It is expected that this cost-effective fluorescence method might hold considerable potential in the detection of Hg²⁺ ions in real biological and environmental samples.

Surface plasmon resonance biosensor for sensitive detection of microRNA and cancer cell using multiple signal amplification strategy

A sensitive and versatile surface plasmon resonance (SPR) biosensor was proposed for the detection of microRNA (miRNA) and cancer cell based on multiple signal amplification strategy. Thiol-modified hairpin probe, including a sequence complementary to the target miRNA, was first immobilized on the Au film. In the presence of target miRNA, the stem-loop structure of hairpin probe was unfolded, and then DNA-linked Au nanoparticles (AuNPs) were hybridized with the terminus of the unfolded hairpin probe. Subsequently, DNA-linked AuNPs initiated the formation of DNA supersandwich structure through the addition of two report DNA sequences. Owing to the electronic coupling between localized plasmon of the AuNPs and the surface plasmon wave, as well as the enhancement of the refractive index of the medium over the Au film induced by DNA supersandwich structure, the SPR response was significantly enhanced. Next, numerous positively charged silver nanoparticles (AgNPs) were absorbed onto the long-range DNA surpersandwich equably, resulting in a further increase of SPR response. Due to the enzyme-free multiple signal amplification strategy, as low as ca. 0.6 fM miRNA-21 could be detected. In addition, this biosensor showed high selectivity toward single-base mismatch. More importantly, this SPR biosensor was also used for cancer cell detection coupled with the cell-specific aptamer modified magnetic nanoparticles. Given that the biosensor avoided enzyme introduction, the limitation of the enzyme was overcome. The versatile biosensor has great potential for the broad applications in the field of clinical analysis.