Magnetic Separation-Assistant Fluorescence Resonance Energy Transfer Inhibition for Highly Sensitive Probing of Nucleolin

An ultrasensitive paper-based SERS sensor for detection of nucleolin using silver-nanostars, plastic antibodies and natural antibodies

A state-of-the-art, ultrasensitive, paper-based SERS sensor has been developed using silver nanostars (AgNSs) in combination with synthetic and natural antibodies. A key component of this innovative sensor is the plastic antibody, which was synthesized using molecularly imprinted polymer (MIP) technology. This ground-breaking combination of paper substrates/MIPs with AgNSs, which is similar to a sandwich immunoassay, is used for the first time with the aim of SERS detection and specifically targets nucleolin (NCL), a cancer biomarker. The sensor device was carefully fabricated by synthesizing a polyacrylamide-based MIP on cellulose paper (Whatman Grade 1 filter) by photopolymerization. The binding of NCL to the MIP was then confirmed by natural antibody binding using a sandwich assay for quantitative SERS analysis. To facilitate the detection of NCL, antibodies were pre-bound to AgNSs with a Raman tag so that the SERS signal could indicate the presence of NCL. The composition of the sensory layers/materials was meticulously optimized. The intensity of the Raman signal at ∼1078 cm-1 showed a linear trend that correlated with increasing concentrations of NCL, ranging from 0.1 to 1000 nmol L-1, with a limit of detection down to 0.068 nmol L-1 in human serum. The selectivity of the sensor was confirmed by testing its analytical response in the presence of cystatin C and lysozyme. The paper-based SERS detection system for NCL is characterized by its simplicity, sustainability, high sensitivity and stability and thus embodies essential properties for point-of-care applications. This approach is promising for expansion to other biomarkers in various fields, depending on the availability of synthetic and natural antibodies.

"Light-On" Fluorescent Nanoprobes for Monitoring Dynamic Distribution of Cellular Nucleolin During Pyroptosis

Nucleolin (NCL) is a multifunctional nuclear protein that plays significant roles in regulating physiological activities of the cells. However, it remains a challenge to monitor the dynamic distribution and expression of nucleolin within living cells during cell stress processes directly. Here, we designed "turn-on" fluorescent nanoprobes composed of specific AS1411 aptamer and nucleus-targeting peptide on gold nanoparticles (AuNPs) to effectively capture and track the NCL distribution and expression during pyroptosis triggered by electrical stimulation (ES). The distribution of nucleolin in the cell membrane and nucleus can be easily observed by simply changing the particle size of the nanoprobes. The present strategy exhibits obvious advantages such as simple operation, low cost, time saving, and suitability for living cell imaging. The ES can induce cancer cell pyroptosis controllably and selectively, with less harm to the viability of normal cells. The palpable cell nuclear stress responses of cancerous cells, including nucleus wrinkling and nucleolus fusion after ES at 1.0 V were obviously observed. Compared with normal cells (MCF-10A), NCL is overexpressed within cancerous cells (MCF-7 cells) using the as-designed nanoprobes, and the ES can effectively inhibit NCL expression within cancerous cells. The developed NCL sensing platform and ES-based methods hold great potential for cellular studies of cancer-related diseases.

Dual-functionalization of fluorescent carbon dots via cyclodextrin and aminosilane for visual detection of β-glucuronidase and bioimaging

A sensitive method for the detection of β-glucuronidase was established using functionalized carbon dots (β-CD-SiCDs) as fluorescent probes. The β-CD-SiCDs were found to be obtained through in situ autopolymerization by mixing the solutions of methyldopa, mono-6-ethylenediamine-β-cyclodextrin and N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane at room temperature. The method has the characteristics of low energy consumption, simple and rapid. β-CD-SiCDs exhibited green fluorescence at 515 nm emission with a quantum yield of 7.9 %. 4-nitrophenyl-β-D-glucuronide was introduced as a substrate for β-glucuronidase to generate p-nitrophenol. Subsequently, p-nitrophenol self-assembled with β-CD-SiCDs through host-guest recognition to form a stable inclusion complex, resulting in the fluorescence quenching of β-CD-SiCDs. The linear range of β-CD-SiCDs for detecting β-glucuronidase activity was 0.5-60 U L-1 with a detection limit of 0.14 U L-1. For on-site detection, gel reagents were prepared by a simple method and the images were visualized and quantified by taking advantage of smartphones, avoiding the use of large instrumentation. The constructed fluorescence sensing platform has the benefits of easy operation and time saving, and has been successfully used for the detection of β-glucuronidase activity in serum and cell imaging.

Atomic layer deposition (ALD)-constructed TaS2nanoflakes for cancer-related nucleolin detection

Sensitive detection of nucleolin (NCL) is of great significance for the early diagnosis of cancer. In this work, as a new type of two-dimensional (2D) transition metal dichalcogenides (TMDCs), TaS₂nanoflakes (NFs) were precisely constructed by atomic layer deposition (ALD) on carbon fiber paper (CFP) with high specific surface area.In situobservation showed that the nucleation and growth of TaS₂nanoflakes were precisely controlled by the number of ALD cycles, thereby regulating their electrochemical properties. The electrochemical performance of TaS₂NFs was observed in depth, and compared with that of traditional 2D TMDCs. Due to the high surface area and conductivity, anodic/cathodic current of ∼1570μA of TaS₂NFs/CFP can be obtained. Subsequently, an electrochemical biosensor based on ALD-constructed TaS₂NFs/CFP for cancer-related NCL detection was fabricated. Due to the excellent electrochemical performance of TaS₂NFs/CFP, ultrasensitive detection of NCL in the linear range of 0.1 pM-10 nM with a detection limit of 0.034 pM was achieved.

A universal, portable, and ultra-sensitive pipet immunoassay platform for deoxynivalenol detection based on dopamine self-polymerization-mediated bioconjugation and signal amplification

Deoxynivalenol (DON) is highly toxic to the environment and human health. It is important to detect DON with ultra-high sensitivity, ease of operation, and low cost. Inspired by the excellent stability and biocompatibility of polydopamine, a universal, portable and ultra-sensitive pipet immunoassay platform was reported for DON detection based on dopamine self-polymerization (polydopamine coating and polydopamine nanoparticles). The polydopamine coating acted as an effective strategy for biomolecule immobilization on the pipet to improve the coating efficiency that significantly reduced the required concentration of biomolecules. Performing the ELISA in pipets saved nearly 67% of the antigen amount and 83% of the antibody amount, which reduced the detection cost and simplified the experimental steps. The dual signal amplification in this pipet immunoassay enabled ultra-high sensitivity. Polydopamine nanoparticles acted as the enrichment carrier of horseradish peroxidase-goat anti-mouse IgG for the first-round signal amplification, followed by the tyramine-mediated loading of streptavidin-HRP for the second-round signal amplification. The dual-enriched HRP catalyzed the color-developing substrate to achieve highly sensitive colorimetric DON detection with a limit of detection of 0.435 ng/mL.

Metal-Enhanced Aggregation-Induced Emission Strategy for the HIV-I RNA-Binding Ligand Assay

The HIV-Ι trans-activation responsive (TAR) RNA-trans-activator of transcription (Tat) protein complex is crucial for the efficient transcription of the integrated human immunodeficiency virus-I genome and is an established therapeutic target for AIDS diagnosis and treatment. Developing a sensitive strategy for the TAR RNA-binding ligand assay could provide antiviral leads with a radically new mechanism for the treatment of AIDS. Herein, a new TAR RNA-binding ligand assay platform was established using a signal amplification strategy that combines aggregation-induced emission (AIE) with a metal-enhanced fluorescence (MEF) concept. The tetraphenylethylene (TPE) derivative was labeled on the Tat peptide as a fluorescent molecule, while the TAR RNA was immobilized on the surface of the Fe₃O₄@Au@Ag@SiO₂ nanoparticles (NPs) to specifically bind the TPE-Tat peptide. The TPE-Tat peptide was weakly emissive itself while emitting strongly in the NP-TAR-TPE-Tat complex by the AIE and MEF signal amplification effect. It was confirmed by known Tat peptide competitors that this system could be applied to the screening and detection of TAR RNA-binding ligands because they could replace the TPE-Tat peptide from the complex and make the system fluorescence decrease. When this system was adopted to test four candidate ligands, it was found that bisantrene had a favorable TAR RNA-binding ability. The proposed AIE-MEF strategy not only provides a sensitive and reliable method for the TAR RNA-binding ligand assay but also can avoid the influence of ligands on fluorescent detection in the conventional displacement assay.

A Thrombin-Responsive Nanoprobe for In Vivo Visualization of Thrombus Formation through Three-Dimensional Optical/Computed Tomography Hybrid Imaging

Early spontaneous detection of thrombin activation benefits precise theranostics for thrombotic vascular disease. Herein, a thrombin-responsive nanoprobe conjugated by a FITC dye, PEGylated Fe₃O₄ nanoparticles, and a thrombin-sensitive peptide (LASG) was constructed to visualize thrombin activation and subsequent thrombosis in vivo. The FITC dye was linked to the LASG coated on the Fe₃O₄ nanoparticles for sensing the thrombin activity via the Förster resonance energy transfer effect. In vitro fluorescence imaging showed that the fluorescence signal intensity increased significantly after incubation with thrombin in contrast to that of the control group (p < 0.05), and the signal intensity was enhanced with the increase in thrombin concentration. Further in vivo fluorescence imaging also revealed that the signal elevated markedly in the left common carotid artery (LCCA) lesion of the mice thrombosis model after nanoprobe injection, in contrast to that of the control + nanoprobe group (p < 0.05). Moreover, the thrombin inhibitor bivalirudin could decrease the filling defect of the LCCA. Three-dimensional fusion images of micro-CT and fluorescence confirmed that filling defects in the LCCA were nicely colocalized with fluorescence signal caused by nanoprobes. The nanoplatform based on a thrombin-activatable visualization system could provide smart responsive and dynamic imaging of thrombosis in vivo.

Seedless synthetic branched gold nanoshells for chemo-thermal antitumor therapy

Gold nanomaterials (GNMs) are used in photothermal therapy due to their superior optical properties and excellent biocompatibility. However, the complex preparation process involving seed-mediated growth limits further clinical applications of GNMs. Herein, a novel one-pot approach to rapidly prepare liposome-based branched gold nanoshells (BGNS) as an antitumor drug nanocarrier is reported. This efficient seedless synthesis realized tunable absorption peaks of BGNS through controlling the concentration of the Au precursor solution, obtaining high absorbance in the near-infrared (NIR) window to achieve a superior photothermal effect. Hyperthermia during NIR laser irradiation can ablate the tumor and trigger drug release to achieve combined treatment. After laser irradiation, the nanocarriers disintegrated into individual gold nanoparticles (size: about 8 nm), which can be metabolized by the kidneys. Cell experiments in vitro and experiments involving mice with tumors have confirmed that the nanodrugs have strong antitumor effects. Such a flexible method provides a universal approach for rapidly preparing liposome-based gold nanoshells, which have the potential for large-scale preparation for further clinical applications.

Microwave-assisted ultrafast fabrication of high-performance polypyrrole nanoparticles for photothermal therapy of tumors in vivo

Photothermal therapy with minimal invasiveness and high selectivity has been regarded as a powerful technique for tumor therapy to overcome the risks of toxic side effects and limited therapeutic efficacy of clinic cancer treatments. Among various photothermal therapeutic agents, polypyrrole (PPy) nanoparticles show a promising prospect in tumor ablation in vivo due to their admirable biocompatibility and outstanding photothermal performance. Besides, polypyrrole nanoparticles are extensively applied in biosensors, electrochemical sensors, tissue engineering, flexible microelectronics, and so on. However, the available synthesis methods of PPy nanoparticles are all time-consuming and seriously hindered their highly efficient production for diverse applications. Here we present a microwave-assisted strategy for the fabrication of PPy nanoparticles in 2 min, and the required synthesis time is shortened by 120-720 times compared to that in traditional ways. The prepared PPy nanoparticles possess uniform size, favorable aqueous solubility, and enhanced photothermal performance derived from the stronger near-infrared absorbance. Low cytotoxicity and in vivo toxicity of the nanoparticles were confirmed via comprehensive assessments. The PPy nanoparticle-based photothermal therapy in vitro led to a remarkable death of tumor cells, and in vivo tumor ablation using the nanoparticles was achieved under mild laser irradiation with a FDA-approved safe power. The proposed microwave-assisted synthesis strategy opens up a facile and ultrafast way for the construction of organic nanoparticles and facilitates a wide range of applications of them in biomedicine and other fields.

Afterglow Resonance Energy Transfer Inhibition for Fibroblast Activation Protein-α Assay

Traditional photoluminescence resonance energy transfer (PRET)-based sensors are widely applied, but still suffer from the severe background interference from in situ excitation. The afterglow nature of the persistent luminescence nanoparticles (PLNPs) allows optosensing after the stoppage of in situ illumination, and thus subtly overcomes that interference. We proposed a simple strategy for functionalizing PLNPs for bioanalytical applications and the new afterglow resonance energy transfer (ARET)-based assay for quantitative determination and imaging of fibroblast activation protein-alpha (FAPα) in live cells using Au-decorated Cr³⁺_0.004:ZnGa₂O₄ as donor and Cy5.5-KGPNQC-SH as acceptor. The ARET between the donor and acceptor quenches the afterglow of the donor, and the cleavage of peptide KGPNQC by FAPα inhibits the ARET and restores the afterglow of the donor. The ARET-based assay of FAPα, with the linear range of 0.1-2.0 mg·L^-1 (1.2-22.9 nM), LOD of 11 μg·L^-1 (115 pM), and RSD of 3.9% (for 0.5 mg·L^-1 FAPα, n = 5), displays higher sensitivity, lower limit of detection (LOD), and better anti-interference capability than the corresponding PRET-based assay. Besides, the ARET-based sensors are lighted up by the FAPα-positive U87MG and MDA-MB-435 cells, but kept in the dark when incubated in the FAPα-negative AD293 cells. The proposed ARET-based sensor can detect FAPα of U87MG and MDA-MB-435 living cells in human serum with the spiked recoveries of 95.6-103%. Our data demonstrated a simple and effective strategy for bridging PLNPs to bioanalytical applications, and an attractive ARET assay of FAPα.

Nanopipette-Based SERS Aptasensor for Subcellular Localization of Cancer Biomarker in Single Cells

Single cell analysis is essential for understanding the heterogeneity, behaviors of cells, and diversity of target analyte in different subcellular regions. Nucleolin (NCL) is a multifunctional protein that is markedly overexpressed in most of the cancer cells. The variant expression levels of NCL in subcellular regions have a marked influence on cancer proliferation and treatments. However, the specificity of available methods to identify the cancer biomarkers is limited because of the high level of subcellular matrix effect. Herein, we proposed a novel technique to increase both the molecular and spectral specificity of cancer diagnosis by using aptamers affinity based portable nanopipette with distinctive surface-enhanced Raman scattering (SERS) activities. The aptamers-functionalized gold-coated nanopipette was used to capture target, while p-mercaptobenzonitrile (MBN) and complementary DNA modified Ag nanoparticles (AgNPs) worked as Raman reporter to produce SERS signal. The SERS signal of Raman nanotag was lost upon NCL capturing via modified DNA aptamers on nanoprobe, which further helped to verify the specificity of nanoprobe. For proof of concept, NCL protein was specifically extracted from different cell lines by aptamers modified SERS active nanoprobe. The nanoprobes manifested specifically good affinity for NCL with a dissociation constant Kd of 36 nM and provided a 1000-fold higher specificity against other competing proteins. Furthermore, the Raman reporter moiety has a vibrational frequency in the spectroscopically silent region (1800-2300 cm-1) with a negligible matrix effect from cell analysis. The subcellular localization and spatial distribution of NCL were successfully achieved in various types of cells, including MCF-7A, HeLa, and MCF-10A cells. This type of probing technique for single cell analysis could lead to the development of a new perspective in cancer diagnosis and treatment at the cellular level.

Label-free fluorescence turn-on aptasensor for prostate-specific antigen sensing based on aggregation-induced emission-silica nanospheres

Fluorescent light-up probes based on aggregation-induced emission (AIE)-active molecules have recently attracted great research interest due to the intelligent fluorescence activation mechanism and high sensitivity. In this work, an AIE-silica nanosphere (SiO₂ NP)-based label-free fluorescent aptasensor for the sensitive "turn-on" detection of prostate-specific antigen (PSA) is reported for the first time. The positively charged amino-functionalized SiO₂ NPs were used as efficient nanocapturer to electrostatically adsorb single-stranded PSA aptamer (PA) to form SiO₂ NP-PA nanocomposite as well as adsorb negatively charged tetraphenylethylene derivative 3 (TPE3) to form AIE-SiO₂ NP nanocomposite. The binding of the aptamer to the target PSA could induce a rigid aptamer conformation, resulting in the release of the PA away from the surface of SiO₂ NPs. This made the AIE molecules TPE3 aggregate on the SiO₂ NP surface and emit high fluorescence. With the advantages of simple design and rapid responses, the proposed aptasensor showed high sensitivity and selectivity for PSA with a detection limit of 0.5 ng/mL. The aptasensor was further applied in human serum samples with satisfactory results. Given its versatility, high selectivity, and sensitivity, the proposed method could be extended to other targets by varying the recognition probes. Graphical abstract An AIE-SiO₂ NP-based label-free fluorescent aptasensor for the sensitive "turn-on" detection of PSA is reported for the first time.

General Bioluminescence Resonance Energy Transfer Homogeneous Immunoassay for Small Molecules Based on Quantum Dots

Here, we describe a general bioluminescence resonance energy transfer (BRET) homogeneous immunoassay based on quantum dots (QDs) as the acceptor and Renilla luciferase (Rluc) as the donor (QD-BRET) for the determination of small molecules. The ratio of the donor-acceptor that could produce energy transfer varied in the presence of different concentrations of free enrofloxacin (ENR), an important small molecule in food safety. The calculated Förster distance (R0) was 7.86 nm. Under optimized conditions, the half-maximal inhibitory concentration (IC50) for ENR was less than 1 ng/mL and the linear range covered 4 orders of magnitude (0.023 to 25.60 ng/mL). The cross-reactivities (CRs) of seven representative fluoroquinolones (FQs) were similar to the data obtained by an enzyme-linked immunosorbent assay (ELISA). The average intra- and interassay recoveries from spiked milk of were 79.8-118.0%, and the relative standard deviations (RSDs) were less than 10%, meeting the requirement of residue detection, which was a satisfactory result. Furthermore, we compared the influence of different luciferase substrates on the performance of the assay. Considering sensitivity and stability, coelenterazine-h was the most appropriate substrate. The results from this study will enable better-informed decisions on the choice of Rluc substrate for QD-BRET systems. For the future, the QD-BRET immunosensor could easily be extended to other small molecules and thus represents a versatile strategy in food safety, the environment, clinical diagnosis, and other fields.

Aggregation-Induced Emission Luminogen-Embedded Silica Nanoparticles Containing DNA Aptamers for Targeted Cell Imaging

Conventional fluorophores usually undergo aggregation-caused quenching (ACQ), which limits the loading amount of these fluorophores in nanoparticles for bright fluorescence imaging. On the contrary, fluorophores with aggregation-induced emission (AIE) characteristics are strongly fluorescent in their aggregate states and have been an ideal platform for developing highly fluorescent nanomaterials, such as fluorescent silica nanoparticles (FSNPs). In this work, AIE luminogens based on salicylaldehyde hydrazones were embedded in silica nanoparticles through a facile noncovalent approach, which afforded AIE-FSNPs emitting much brighter fluorescence than that of some commercial fluorescein-doped silica and polystyrene nanoparticles. These AIE-FSNPs displaying multiple fluorescence colors were fabricated by a general method, and they underwent much less fluorescence variation due to environmental pH changes compared with fluorescein-hybridized FSNPs. In addition, a DNA aptamer specific to nucleolin was functionalized on the surface of AIE-FSNPs for targeted cell imaging. Fluorescent microscopy and flow cytometry studies both revealed highly selective fluorescence staining of MCF-7 (a cancer cell line with nucleolin overexpression) over MCF-10A (normal) cells by the aptamer-functionalized AIE-FSNPs. The fluorescence imaging in different color channels was achieved using AIE-FSNPs containing each of the AIE luminogens, as well as photoactivatable fluorescent imaging of target cells by the caged AIE fluorophore.