Biosensors Based on the Au–Se Bond

Gold nanoparticles-doped MXene heterostructure for ultrasensitive electrochemical detection of fumonisin B1 and ampicillin

Early transition metal carbides (MXene) hybridized by precious metals open a door for innovative electrochemical biosensing device design. Herein, we present a facile one-pot synthesis of gold nanoparticles (AuNPs)-doped two-dimensional (2D) titanium carbide MXene nanoflakes (Ti3C2Tx/Au). Ti3C2Tx MXene exhibits high electrical conductivity and yields synergistic signal amplification in conjunction with AuNPs leading to excellent electrochemical performance. Thus Ti3C2Tx/Au hybrid nanostructure can be used as an electrode platform for the electrochemical analysis of various targets. We used screen-printed electrodes modified with the Ti3C2Tx/Au electrode and functionalized with different biorecognition elements to detect and quantify an antibiotic, ampicillin (AMP), and a mycotoxin, fumonisin B1 (FB1). The ultralow limits of detection of 2.284 pM and 1.617 pg.mL-1, which we achieved respectively for AMP and FB1 are far lower than their corresponding maximum residue limits of 2.8 nM in milk and 2 to 4 mg kg-1 in corn products for human consumption set by the United States Food and Drug Administration. Additionally, the linear range of detection and quantification of AMP and FB1 were, respectively, 10 pM to 500 nM and 10 pg mL-1 to 1 µg mL-1. The unique structure and excellent electrochemical performance of Ti3C2Tx/Au nanocomposite suggest that it is highly suitable for anchoring biorecognition entities such as antibodies and oligonucleotides for monitoring various deleterious contaminants in agri-food products.

Integrating Reliable Pt-S Bond-Mediated 3D DNA Nanomachine with Magnetic Separation in a Homogeneous Electrochemical Strategy for Exosomal MicroRNA Detection with Low Background and High Sensitivity

Precise and sensitive analysis of exosomal microRNA (miRNA) is of great importance for noninvasive early disease diagnosis, but it remains a great challenge to detect exosomal miRNA in human blood samples because of their small size, high sequence homology, and low abundance. Herein, we integrated reliable Pt-S bond-mediated three-dimensional (3D) DNA nanomachine and magnetic separation in a homogeneous electrochemical strategy for the detection of exosomal miRNA with low background and high sensitivity. The 3D DNA nanomachine was easily prepared via a facile and rapid freezing method, and it was capable of resisting the influence of biothiols, thus endowing it with high stability. Notably, the as-developed magnetic 3D DNA nanomachine not only enabled the detection system to have a low background but also coupled with liposome nanocarriers to synergistically amplify the current signal. Consequently, by ingeniously combining the low background and multiple signal-amplification strategies in homogeneous electrochemical biosensing, highly sensitive detection of exosomal miRNA was successfully achieved. More significantly, with good anti-interference ability, the as-proposed method could effectively discriminate plasma samples from cancer patients and healthy subjects, thus showing a high potential for application in the nondestructive early clinical diagnosis of disease.

Stimuli-responsive probes for amplification-based imaging of miRNAs in living cells

MicroRNAs (miRNAs) have emerged as important biomarkers in biomedicine and bioimaging due to their roles in various physiological and pathological processes. Real-time and in situ monitoring of dynamic fluctuation of miRNAs in living cells is crucial for understanding these processes. However, current miRNA imaging probes still have some limitations, including the lack of effective amplification methods for low abundance miRNAs bioanalysis and uncontrollable activation, leading to background signals and potential false-positive results. Therefore, researchers have been integrating activatable devices with miRNA amplification techniques to design stimuli-responsive nanoprobes for "on-demand" and precise imaging of miRNAs in living cells. In this review, we summarize recent advances of stimuli-responsive probes for the amplification-based imaging of miRNAs in living cells and discuss the future challenges and opportunities in this field, aiming to provide valuable insights for accurate disease diagnosis and monitoring.

Multi-stimuli-responsive molecular fluorescent probes for bioapplications

Stimuli-responsive fluorescent probes have been widely utilized in detecting the physiological and pathological states of living systems. Numerous stimuli-responsive fluorescent probes have been developed due to their advantages of good sensitivity, high resolution, and high contrast fluorescent signals. In this feature article, the progress of multi-stimuli-responsive probes, including organic molecules and metal complexes, for the detection of various biomarkers for bio-applications is summarized. The feature article focuses on the applications of organic-molecule- and metal-complex-based molecular probes in biological systems for detecting different biomarkers of cancer or other diseases. The current challenges and potential future directions of these probes for applications in biological systems are also discussed.

Photo-Assisted Robust Anti-Interference Self-Powered Biosensing of MicroRNA Based on Pt-S Bonds and the Inorganic-Organic Hybridization Strategy

Photo-assisted biofuel cell-based self-powered biosensors (PBFC-SPBs) possess the advantages of no need for external power supply, ease of sensing design, and simple instruments. In this work, a robust anti-interference PBFC-SPB for microRNA detection was constructed based on the Pt-S bond and the inorganic-organic hybridization strategy. The organic semiconductor [6,6]-phenyl-C61-butyric acid methylester@anthraquinone (PCBM@anthraquinone) served as an efficient light-harvesting material, and gold nanoparticle@Pt (AuNP@Pt) nanomaterials were immobilized on the surface via electrostatic adsorption for the binding of DNA. Notably, compared to Au-S bonds for DNA immobilization, the Pt-S bond exhibited better anti-interference ability. Ingeniously, cadmium sulfide quantum dots (CdS QDs) were close to the PCBM@anthraquinone substrate electrode to form sensitization structures, which was beneficial to enhance the photocurrent signal. Combining with the laccase-mimicking activity Cu²⁺/carbon nanotubes (Cu²⁺/CNTs) cathode, the PBFC-SPB for microRNA detection was achieved. Once the target existed, the identical sequence complementary microRNA would make DNA2/CdS dissociate and break away from the electrode, leading to a low signal. The linear detection range was 10 fM-100 pM, with the limit of determination of 2.4 fM (3S/N). The as-proposed strategy not only paves a new way for the design of photoelectrochemical biosensing but also opens a door for the construction of robust anti-interference bioassay for microRNA detection.

Real-Time In Situ Sequential Fluorescence Activation Imaging of Cyt c and Caspase-9 with a Gold-Selenium-Bonded Nanoprobe

Apoptosis, as a very important mode of programmed death, is closely associated with many diseases. Real-time in situ monitoring of the dynamic change of the apoptotic process remains a great challenge. Herein, a nanoprobe based on the gold-selenium (Au-Se) bond was developed for a sequential fluorescence activation imaging of cytochrome c (Cyt c) and caspase-9, two important apoptotic signaling molecules, to monitor the progression of apoptosis. The Cyt c aptamer and caspase-9-cleavable peptide chains labeled with two dyes were modified onto the surface of gold nanoparticles (Au NPs) by the Au-Se bond, which can be activated by upstream Cyt c and downstream caspase-9 to trigger fluorescence recovery. The Au-Se nanoprobe exhibited good specificity and stability. Compared with the traditional nanoprobe based on the gold-sulfur (Au-S) bond, the interference of biological thiols on the Au-Se nanoprobe can be effectively avoided. Importantly, the Au-Se nanoprobe can image the sequential changes of the two markers in situ in real time during cell apoptosis. This work provides an effective tool for the accurate and real-time detection of apoptosis and is conducive to the in-depth study of the relationship between apoptosis and disease.

Polyvalent spherical aptamer engineered macrophages: X-ray-actuated phenotypic transformation for tumor immunotherapy

Spatiotemporally activatable immune cells are promising for tumor immunotherapy owing to their potential high specificity and low side effects. Herein, we developed an X-ray-induced phenotypic transformation (X-PT) strategy through macrophage engineering for safe and efficient tumor immunotherapy. Without complex genetic engineering, the cell membranes of M0-type macrophages were chemically engineered with AS1411 aptamer-based polyvalent spherical aptamer (PSA) via the combination of metabolic glycan labelling and bioorthogonal click reaction. Owing to the superior specificity, affinity and polyvalent binding effects of the high-density AS1411 aptamers, the engineered macrophages could easily recognize and adhere to tumor cells. With further X-ray irradiation, reactive oxygen species (ROS) generated by the Au-based PSA could efficiently transform the accumulated macrophages in situ from biocompatible M0 into antitumoral M1 phenotype via activating the nuclear factor κB signaling pathway, thereby achieving tumor-specific killing. In vitro and in vivo experiments confirmed the high tumor recognition and X-ray-induced polarization effect of the engineered macrophages. Compared to natural macrophages, our engineered macrophages significantly inhibited tumor growth in mice even if the radiation dose was reduced by three-fold. We believe this X-PT strategy will open a new avenue for clinical immune cell-based therapy.

An X-ray-induced phenotypic transformation strategy (X-PT) through macrophage engineering was developed for safe and effective immunotherapy.

Multicolor Covalent Organic Framework-DNA Nanoprobe for Fluorescence Imaging of Biomarkers with Different Locations in Living Cells

Precisely detecting biomarkers in living systems holds tremendous promise for disease diagnosis and monitoring. Herein, we developed a covalent organic framework (COF)-based tricolor fluorescent nanoprobe for simultaneously imaging biomarkers with different spatial locations in living cells. Briefly, a TAMRA-labeled survivin mRNA antisense nucleotide and a Cy5-labeled transmembrane glycoprotein mucin 1 (MUC1) aptamer were adsorbed on a nanoscale fluorescent COF. To enhance the interactions between COF nanoparticles (NPs) and nucleic acid molecules, a freezing method was employed for improving the nucleic acid loading density and ensuring detection performance. The fluorescence signals of dyes on DNAs were first quenched by the COF NPs. Internalization and distribution of the nanoprobes can be real-time visualized by the autofluorescence of COF NPs. In living cells, recognition between MUC1 with MUC1 aptamers causes fluorescence signal recovery of Cy5, while hybridization between survivin mRNA and its antisense DNA induces the signal recovery of TAMRA. Therefore, this COF-based multicolor nanoprobe could be employed for visualizing MUC1 on the cell membrane and survivin mRNA in the cytoplasm. Cancer cell-specific diagnostic imaging and monitoring of the process of cancer cell exosomes infecting normal cells using the nanoprobe were achieved. This work not only offers a versatile nanoprobe for bioanalysis but also provides new insights for developing novel COF-based nanoprobes.

Covalent Organic Framework-Derived Carbonous Nanoprobes for Cancer Cell Imaging

Covalent organic frameworks (COFs) have emerged as promising materials for biomedical applications, but their functions remain to be explored and the potential toxicity concerns should be resolved. Herein, it is presented that carbonization significantly enhances the fluorescence quenching efficiency and aqueous stability of nanoscale COFs. The probes prepared by physisorbing dye-labeled nucleic acid recognition sequences onto the carbonized COF nanoparticles (termed C-COF) were employed for cell imaging, which could effectively light up biomarkers (survivin and TK1 mRNA) in living cells. The C-COF has enhanced photothermal conversion capacity, indicating that the probes are also promising candidates for photothermal therapy. The potential toxicity concern from the aromatic rigid building units of COFs was detoured by carbonization. Overall, carbonization is a promising strategy for developing biocompatible and multifunctional COF-derived nanoprobes for biomedical applications. This work may inspire more versatile COF-derived nanoprobes for bioanalysis and nanomedicine.

Covalent Organic Framework-Based Spherical Nucleic Acid Probe with a Bonding Defect-Amplified Modification Strategy

Developing spherical nucleic acids with new structures holds great promise for nanomedicine and bioanalytical fields. Covalent organic frameworks (COFs) are emerging promising materials with unique properties for a wide range of applications. However, devising COF-based spherical nucleic acid is challenging because methods for the preparation of functionalized COFs are still limited. We report here a bonding defect-amplified modification (BDAM) strategy for the facile preparation of functionalized COFs. Poly(acrylic acid) was employed as the defect amplifier to modify the surface of COF nanoparticles by the formation of amide bonds with amino residues, which successfully converted and amplified the residues into abundant reactive carboxyl groups. Then, amino terminal-decorated hairpin DNA was densely grafted onto the surface of COF nanoparticles (NPs) to give rise to a spherical nucleic acid probe (SNAP). A series of experiments and characterizations proved the successful preparation of the COF-based SNAP, and its application in specifically lighting up RNA biomarkers in living cells for cancer diagnostic imaging was demonstrated. Therefore, the COF-based SNAP is a promising candidate for biomedical applications and the proposed BDAM represents a useful strategy for the preparation of functionalized COFs for diverse fields.

CRISPR/Cas-Based In Vitro Diagnostic Platforms for Cancer Biomarker Detection

Timely diagnosis is of great benefit to improve the survival rate of cancer patients. Body fluid cancer biomarker detection is a critical kind of noninvasive method for cancer diagnosis. Nevertheless, traditional methods for cancer biomarker detection always rely on a large-scale instrument and involve sophisticated operation. Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas)-based in vitro diagnosis can simplify the detection procedures and improve sensitivity and specificity, holding great promise as the next-generation molecular diagnostic technology. In this Feature, we introduce the working mechanisms of different kinds of CRISPR/Cas systems for biosensing and CRISPR/Cas-mediated detection strategies for different kinds of cancer biomarkers including nucleic acids, proteins, and extracellular vesicles. In addition, the perspective and challenges of CRISPR/Cas-based strategies for cancer biomarkers are discussed.

Nucleic Acid-Gated Covalent Organic Frameworks for Cancer-Specific Imaging and Drug Release

Developing nanoplatforms that simultaneously integrate diagnostic imaging and therapy functions has been a promising but challenging task for cancer theranostics. Herein, we report the rational design of a smart nucleic acid-gated covalent organic framework (COF) nanosystem for cancer-specific imaging and microenvironment-responsive drug release. Cy5 dye-labeled single-stranded DNA (ssDNA) for mRNA recognition was adsorbed on the surface of doxorubicin (Dox)-loaded COF nanoparticles (NPs). Dox loaded in the pores of COF NPs could strengthen the interactions between ssDNA and COF and enhance the fluorescence quenching effect toward Cy5, while the densely coated ssDNA could prevent the leakage of Dox from COF NPs. The obtained nanosystem exhibited low fluorescence signal and Dox release in normal cells; however, the ssDNA could be released by the overexpressed TK1 mRNA in cancer cells to recover the intense fluorescence signal of Cy5, and the loaded Dox could be further released for chemotherapy. Therefore, cancer cell-specific diagnostic imaging and drug release were realized with the rationally developed nanosystem. This work offers a universal nanoplatform for cancer theranostics and a promising strategy for regulating the interaction between COFs and biomolecules.

Protease Biosensor by Conversion of a Homogeneous Assay into a Surface-Tethered Electrochemical Analysis Based on Streptavidin-Biotin Interactions

This work proposed a new sensing strategy for protease detection by converting a homogeneous assay into a surface-tethered electrochemical analysis. Streptavidin (SA), a tetramer protein, was used as the sensing unit based on the SA-biotin coupling chemistry. Caspase-3 was used as the model analyte, and a biotinylated peptide with a sequence of biotin-GDEVDGK-biotin was designed as the substrate. Specifically, the peptide substrate could induce an assembly of SA to form (SA-biotin-GDEVDGK-biotin)_n aggregates through SA-biotin interactions, which was confirmed by atomic force microscopy (AFM). The peptide substrate-induced assembly of SA was facilely initiated on an electrode-liquid surface by modification of the electrode with SA. The in situ formation of (SA-biotin-GDEVDGK-biotin)_n aggregates created an insulating layer, thus limiting the electron transfer of ferricyanide. Once the peptide substrate was cleaved into two shorter fragments (biotin-GDEVD and GK-biotin) by caspase-3, the resulting products would compete with biotin-GDEVDGK-biotin to bind SA proteins immobilized on the electrode surface and distributed in a solution, thus preventing the in situ formation of (SA-biotin-GDEVDGK-biotin)_n assemblies. With the simple principle of the substrate-induced assembly of SA, a dual-signal amplification was achieved with improved sensitivity. Taking advantage of high sensitivity, simple principle, and easy operation, this method can be augmented to design various surface-tethered biosensors for practical applications.

A COF-based anti-interference nanoplatform for intracellular nucleic acid imaging

A porphyrin covalent organic framework-based nanoplatform was demonstrated. The nanoplatform could effectively avoid the interference of proteins and biothiols and realize high-fidelity mRNA imaging in living cells.