Ratiometric fluorescence immunoassay of SARS-CoV-2 nucleocapsid protein via Si-FITC nanoprobe-based inner filter effect

Dual-emission Si dots-based sensing array for identification of metal ions

Achieving rapid recognition and sensitive detection of multiple heavy metal ions simultaneously is of great significance for the monitoring of public health and environment. Herein, a fluorescence sensing array was constructed using Si dots with two emission centers for efficient discrimination of seven metal ions. Each metal ion had different binding capacities with -OH/-NH2 groups present on the surface of Si dots and thereby resulting in different changes in fluorescence intensity of the two emission peaks. The changes in fluorescence signals were transformed into unique "fingerprints" and "Euclidean distances" through linear discriminant analysis and hierarchical cluster analysis. The constructed sensing array provided multi-dimensional information to distinguish seven metal ions through a single material and a signal mode and show good linearity for individual metal ion and good performance in the discrimination of metal mixtures, effectively reducing the cost and response time and simplifying the experimental process. This system achieved accurate discrimination of metal ions in actual water samples and has broad application prospects in environmental monitoring.

Engineered DR/NIR dual-emission carbonized polymer dots for simultaneous tracking of lipid droplets and lysosomes

Developing near-infrared fluorescent probes for simultaneous tracking of lipid droplets (LDs) and lysosomes is highly desirable for studying cell metabolism. In this work, deep-red/near-infrared dual-emission carbonized polymer dots (DN-CPDs) were prepared for ratiometric monitoring of the intracellular polarity. Detailed structural analysis revealed that the deep-red emission and near-infrared peak of DN-CPDs originate from the molecular state and surface state, respectively. The surface-state emission was derived from the intraparticle charge-transfer (ICT) effect of the donor-bridge-acceptor (D-π-A) structure of DN-CPDs. The obtained DN-CPDs exhibited excellent dual-labeling ability, large Stokes shifts, ratiometric polarity sensitivity, high selectivity, and satisfactory photostability. Moreover, with the polarity distinction between LDs and lysosomes, DN-CPDs nanoprobes were successfully used to observe the dynamic changes of the two aforementioned organelles during starvation-induced lipophagy and drug-induced lipophagy inhibition processes. This work not only provides a useful tool for LD-lysosome related studies but is also valuable for the preparation of CPDs with long wavelength emission.

Intrinsic dual-emitting Si dots for high-precision and broad-range pH detection

BACKGROUND

High-precision and broad-range pH detection is critical for health status assessment, such as signal transduction, enzyme activity, endocytosis, and cell proliferation and apoptosis. Although pH-responsive ratiometric fluorescent probes offer an effective pH monitoring strategy, their preparation often requires multi-step modification and decreases fluorescence efficiency and stability. Herein, we developed a simple method to prepare fluorescent Si dots with dual emission centers for high-precision and broad-range pH monitoring, and the detection of urease based on pH-responsive Si dots and pH monitoring in living cell was further explored.

RESULTS

The dual-emitting Si dots had two emission centers at 427 nm and 500 nm. The emission at 427 nm showed a fluorescence enhancement and effective quenching in the pH ranges of 3.0-10.0 and 10.0-12.0, respectively. While the emission at 500 nm increased gradually with increasing pH from 3.0 to 12.0. Two linear relationships were obtained between the synchronous fluorescence intensity ratio and pH in the ranges of 7.0-9.5 and 10.0-12.0 with a △pH of 0.1, indicating a broad detection range and high precision. Then the Si dots were used to detect urease activity via urea hydrolysis-mediated pH changes. A linear range of 1-80 U/L was established with a detection limit of 0.28 U/L. Furthermore, the Si dots were used for pH imaging in living HeLa cells. The cells changed from green to blue when the pH of HeLa cells increased from 6.0 to 10.0.

SIGNIFICANCE

These findings collectively suggest that the intrinsic dual-emitting Si dots may offer a simple and versatile platform for developing pH-relevant biosensors and bioimaging applications. Additionally, this approach provides reliable methods for preparing intrinsic dual-emitting probe and constructing fluorescence ratiometrics, which widely used in health status assessment.

Fluorescence turn-on recognition of trace acetamiprid in seawater using selective molecularly imprinted polymer-based nanoprobe

A novel imprinted composite nanoprobe for fluorescence turn-on recognition of acetamiprid was fabricated and applied to rapidly and sensitively detect trace-level acetamiprid in seawater. The fluorescent probe was prepared using modified fluorescein isothiocyanate as a response unit to improve the sensitivity of signal transmission. The quantitative analysis of acetamiprid was obtained by measuring fluorescence enhancement efficiency of the probe. Under optimal conditions, a good linear relationship with a determination coefficient of 0.9988 was demonstrated in the range of 0-45 μg L-1 and the limit of detection was 1.5 μg L-1. The developed fluorescence-enhancing nanoprobe was utilized in determination of acetamiprid in seawater samples and achieved recoveries from 96.00 % to 104.00 % with the relative standard deviations <5.88 % (n = 3). This study offered a promising strategy for simple, reliable and sensitive detection of acetamiprid by embedding fluorescent dye in molecularly imprinted material as highly selective probes.

Ultra-Sensitive Detection of the SARS-CoV-2 Nucleocapsid Protein via a Clustered Regularly Interspaced Short Palindromic Repeat/Cas12a-Mediated Immunoassay

Tracking trace protein analytes in precision diagnostics is an ongoing challenge. Here, we developed an ultrasensitive detection method for the detection of SARS-CoV-2 nucleocapsid (N) protein by combining enzyme-linked immunosorbent assay (ELISA) with the clustered regularly interspaced short palindromic repeat/CRISPR-associated protein (CRISPR/Cas) system. First, the SARS-CoV-2 N protein bound by the capture antibody adsorbed on the well plate was sequentially coupled with the primary antibody, biotinylated secondary antibody, and streptavidin (SA), followed by biotin primer binding to SA. Subsequently, rolling circle amplification was initiated to generate ssDNA strands, which were targeted by CRISPR/Cas12a to cleave the FAM-ssDNA-BHQ1 probe in trans to generate fluorescence signals. We observed a linear relationship between fluorescence intensity and the logarithm of N protein concentration ranging from 3 fg/mL to 3 × 107 fg/mL. The limit of detection (LOD) was 1 fg/mL, with approximately nine molecules in 1 μL of the sample. This detection sensitivity was 4 orders magnitude higher than that of commercially available ELISA kits (LOD: 5.7 × 104 fg/mL). This method was highly specific and sensitive and could accurately detect SARS-CoV-2 pseudovirus and clinical samples, providing a new approach for ultrasensitive immunoassay of protein biomarkers.

Highly efficient dual-mode detection of AFB1 based on the inner filter effect: Donor-acceptor selection and application

BACKGROUND

The utilization of inner filter effect (IFE) brings more opportunities for construction of fluorescence immunoassays but remains a great challenge, especially how to select best donor in the face of extensive fluorescent nanomaterials. Aflatoxin B1 possesses high toxicity among mycotoxins and is frequently found in agricultural products that may significantly threaten to human health. Therefore, with the help of signal transduction mechanism of IFE to develop a convenient and sensitive approach for AFB1 detection is of great significance in ensuring food safety.

RESULTS

Herein, the classical alkaline phosphatase (ALP) catalyzes hydrolysis of p-nitrophenylphosphate to produce p-nitrophenol (PNP) was employed as a model reaction, which intends to explore tunable multicolor fluorescence of gold nanoclusters (AuNCs) for matching PNP to maximize IFE efficiency. The luminescent green-emitting AuNCs were selected as an optimal donor in terms of excellent spectral overlap, high photoluminescence, and adequate system adaptability, thus achieving a 22-fold increase in sensitivity improvement compared to colorimetric method for ALP detection. The fluorescence quenching mechanism between PNP and AuNCs was validated as IFE by studying ultraviolet absorption, zeta potentials and fluorescence lifetime. In light of this, we integrated a highly specific antibody-antigen recognition system, efficient enzymatic reaction and excellent optical characteristics of AuNCs to develop dual-mode immunoassay for AFB1 monitoring. The sensitivity of fluorometric immunoassay was lower to 0.06 ng/mL, which obtained a 3.5-fold improvement compared to "gold standard" ELISA. Their practicability and applicability were confirmed in the tap water, corn, wheat and peanuts samples.

SIGNIFICANCE

This work provides an easy-to-understand screening procedure to select optimal donor-acceptor pairs in IFE analysis. Furthermore, we expect that integration of IFE-based signal conversion strategy into mature immunoassay not only extends the signal types, simplifies signal amplification steps, and reduces the false-positive/false-negative rates, but also provides a simple, convenient, and versatile strategy for monitoring of trace other contaminants.

Full-color-emitting fluids from carbon dots stabilized in nonconventionally fluorescent micelles

Red-emitting carbon dots from pyrolysis of citric acid in formamide, which are intrinsically water-insoluble, were solubilized and stabilized by the fluorescent micelles formed by a nonionic silicone surfactant and an anionic surfactant, leading to the formation of full-color-emitting colloids with good biocompatibility and a variety of potential applications.

Ultrasensitive Ratiometric Fluorescence Bioassay for Accurate Detection of Covid-19-Specific Nucleocapsid Protein in Clinical Serum Samples Using Modified Cleavable Mesoporous SiO2 Satellite-Enriched Carbon Dots

Due to the presence of various autofluorescent compounds in biological samples like serum and the photobleaching of organic fluorophores, fluorescence sensing has limited practical applicability. This study describes the development of an improved ratiometric fluorescence assay to determine the nucleocapsid protein (N protein), one of the most conserved biomarkers of Covid-19 in spiked and serum samples using highly stable buffer-based near IR-dual emission carbon dots (CDs) encapsulated into the cavities of cleavable silica nanocapsule (SNCs) nanocomposite. The cavities of cleavable silica nanocapsules (SNCs) and the formed core-shell CDs@ SNCs were used as a superior reservoir of fluorescent markers produced by cohydrolyzing tetraethyl orthosilicate and diiminosilane linker, which held hundreds of CDs in silica shell frameworks. The SiO2 nanocomposite was modified with an N protein antibody that specifically paired to the receptor binding region of the Cov-19 spike protein subunit. CDs were taken out of SNCs by NaBH4 reduction, and the released CDs exhibited dual emission at 475 and 675 nm when excited at 400 nm. Ratiometric detection is completed over a binding-induced, concentration-dependent immuno-affinity of the N protein that drives the fluorescence quenching phenomenon between the CDs as fluorophore and the AuNPs as quencher. As the N protein concentration increased, the intensity of the red emission (675 nm) dropped, whereas the intensity of the green emission (475 nm) already remained constant, which is due to sandwich immunoassays of CDs around AuNPs. Using the exceptional fluorescent characteristics of CDs and the high selectivity of nanocomposite functionalized with N-protein antibody, the developed assay efficiently eliminates the autofluorescence background interference of serum samples. The fluorescence ratio (I475/I675) provides a limit of detection of 2 pg mL-1 over a linear range of 0.01 to 5 ng mL-1 and exhibits an amplified sensitivity of 54 times compared to conventional immunoassay using CDs as fluorescent labels. With one-step signal amplification and requiring small sample quantities (only 20 μL), this sensing platform can be effectively used for the accurate detection of N protein, and no cross-reactivity is detected in the presence of different interfering agents.

Fluorescence and Colorimetric Analysis of African Swine Fever Virus Based on the RPA-Assisted CRISPR/Cas12a Strategy

It is well-established that different detection modes are necessary for corresponding applications, which can effectively reduce matrix interference and improve the detection accuracy. Here, we reported a magnetic separation method based on recombinase polymerase amplification (RPA)-assisted clustered regularly interspaced short palindromic repeats (CRISPR)/Cas12a for dual-mode analysis of African swine fever virus (ASFV) genes, including colorimetry and fluorescence. The ASFV gene was selected as the initial RPA template to generate the amplicon. The RPA amplicon was then recognized by CRISPR-associated RNA (crRNA), activating the trans-cleavage activity of Cas12a and leading to the nonspecific cleavage of ssDNA as well as a significant release of alkaline phosphatase (ALP) in the ALP-ssDNA modified magnetic bead. The released ALP can catalyze para-nitrophenyl phosphate to generate para-nitrophenol, resulting in substantial changes in absorbance and fluorescence, both of which can be used for detection with the naked eye. This strategy allows the sensitive detection of ASFV DNA, with a 20 copies/mL detection limit; no cross-reactivity with other viruses was observed. A good linear relationship was obtained in serum. In addition, this sensor displayed 100% specificity and sensitivity for clinical sample analysis. This method integrates the high sensitivity of fluorescence with easy readout of colorimetry and enables a simple, low-cost, and highly sensitive dual-mode detection of viral nucleic acid, thereby providing a broad prospect for the practical application in the diagnosis of virus infection.