Horseradish Peroxidase-Mediated, Iodide-Catalyzed Cascade Reaction for Plasmonic Immunoassays

This report outlines an enzymatic cascade reaction for signal transduction and amplification for plasmonic immunoassays by using horseradish peroxidase (HRP)-mediated aggregation of gold nanoparticles (AuNPs). HRP-catalyzed oxidation of iodide and iodide-catalyzed oxidation of cysteine is employed to modulate the plasmonic signals of AuNPs. It agrees well with the current immunoassay platforms and allows naked-eye readout with enhanced sensitivity, which holds great promise for applications in resource-constrained settings

A Plasmonic Nanosensor for Immunoassay <i>via</i> Enzyme-Triggered Click Chemistry

Current techniques for plasmonic immunoassay often require the introduction and additional conjugation of enzyme, and thus cannot accommodate conventional immunoassay platforms. Herein, we develop a plasmonic nanosensor that well accommodates conventional immunoassays and dramatically improves their sensitivity and stability. This plasmonic nanosensor directly employs alkaline phosphatase-triggered click chemistry between azide/alkyne functionalized gold nanoparticles as the readout. This straightforward approach broadens the applicability of nanoparticle-based immunoassays and has great potential for applications in resource-constrained settings

Versatile T₁‑Based Chemical Analysis Platform Using Fe³⁺/Fe²⁺ Interconversion

We report a versatile analytical platform for assaying multiple analytes relying on changes in longitudinal relaxation time (T₁) as a result of Fe³⁺/Fe²⁺ interconversion. The T₁ of water protons in Fe³⁺ aqueous solution differs significantly from that of Fe²⁺, allowing for the development of a generally applicable T₁-based assay since many redox reactions enable the interconversion between Fe²⁺ and Fe³⁺ that can result in the change of T₁. Compared with conventional magnetic biosensors, this T₁-based assay is free of magnetic nanoparticles (MNPs), and the stability of T₁-based assay is better than conventional magnetic sensors that suffer from nonspecific adsorption and aggregation of MNPs. This T₁-based assay simultaneously enables “one-step mixing” assays (such as saliva sugar) and “multiple-washing” immunoassays with good stability and sensitivity, offering a promising platform for convenient, stable, and versatile biomedical analysi

Point-of-Care Detection of β‑Lactamase in Milk with a Universal Fluorogenic Probe

The illegal addition of β-lactamase (Bla) in milk to disguise β-lactam antibiotics has been a serious issue in the milk industry worldwide. Herein, we report a method for point-of-care detection of Bla based on a probe, Tokyo Green-tethered β-lactam (CDG-1), as a common substrate of various Blas (Bla A, B...) which can enzymatically convert CDG-1 (low fluorescence) to Tokyo Green (high fluorescence). This approach allows rapid screening of a broad spectrum of Blas in real milk samples within 15 min without any pretreatment. Combined with the immuno-magnetic separation, we achieved sensitive and quantitative detection of Bla (10^–5 U/mL), which provides a universal platform for screening and determining Blas in complex samples with high efficiency and accuracy

Cascade Reaction-Mediated Assembly of Magnetic/Silver Nanoparticles for Amplified Magnetic Biosensing

Conventional magnetic relaxation switching (MRS) sensor suffers from its relatively low sensitivity when it comes to the analysis of trace small molecules in complicated samples. To meet this challenge, we develop a cascade reaction-mediated magnetic relaxation switching (CR-MRS) sensor, based on the assembly of silver nanoparticles (Ag NPs) and magnetic nanoparticles (MNPs) to improve the sensitivity of conventional MRS. The cascade reaction triggered by alkaline phosphatase generates ascorbic acid, which reduces Ag⁺ to Ag NPs that can assemble the initially dispersed MNPs to form magnetic/silver nanoassemblies, thus modulating the state of MNPs to result in the change of transverse relaxation time. The formed magnetic/silver nanoassemblies can greatly enhance the state change of MNPs (from dispersed to aggregated) and dramatically improve the sensitivity of traditional MRS sensor, which makes this CR-MRS sensor a promising platform for highly sensitive detection of small molecules in complicated samples

One-Step Detection of Pathogens and Viruses: Combining Magnetic Relaxation Switching and Magnetic Separation

We report a sensing methodology that combines magnetic separation (MS) and magnetic relaxation switching (MS-MRS) for one-step detection of bacteria and viruses with high sensitivity and reproducibility. We first employ a magnetic field of 0.01 T to separate the magnetic beads of large size (250 nm in diameter) from those of small size (30 nm in diameter) and use the transverse relaxation time (<i>T</i>₂) of the water molecules around the 30 nm magnetic beads (MB₃₀) as the signal readout of the immunoassay. An MS-MRS sensor integrates target enrichment, extraction, and detection into one step, and the entire immunoassay can be completed within 30 min. Compared with a traditional MRS sensor, an MS-MRS sensor shows enhanced sensitivity, better reproducibility, and convenient operation, thus providing a promising platform for point-of-care testing

Enzymatic Assay for Cu(II) with Horseradish Peroxidase and Its Application in Colorimetric Logic Gate

We report an ultrasensitive and colorimetric assay for Cu­(II) via enzymatic amplification strategy. The enzymatic activity of horseradish peroxidase (HRP) is strongly inhibited by Cu­(I), which can be used indirectly to assay Cu­(II). The limit of detection (LOD) is 0.37 nM, and the detection of 20 nM Cu­(II) in solution can be achieved with naked eyes. This assay can be used to construct a colorimetric logic gate

Peptide-Mediated Controllable Cross-Linking of Gold Nanoparticles for Immunoassays with Tunable Detection Range

The colorimetric immunoassay based on gold nanoparticles (AuNPs) can hardly enable simultaneous detection of multiple biomarkers in vastly different concentrations (e.g., pg/mL−μg/mL) because of its narrow dynamic range. In this work, we demonstrate an immunoassay with tunable detection range by using peptide-mediated controlled aggregation of surface modification-free AuNPs. Alkaline phosphatase (ALP) removes the phosphate group of the peptide to yield a positively charged product, which triggers the aggregation of negatively charged AuNPs and the color change of the AuNPs solution from red to blue with naked-eye readout. We design and screen 20 kinds of phosphorylated peptides to obtain a broad and controllable detection range for ALP sensing and apply them for detecting multiple inflammatory biomarkers in clinical samples. Our assay realizes straightforward, multiplexed, and simultaneous detection of multiple clinical biomarkers with tunable detection range (from pg/mL to μg/mL) in the same run and holds great potential for chemical/biochemical analysis

T₁‑Mediated Nanosensor for Immunoassay Based on an Activatable MnO₂ Nanoassembly

Current magnetic relaxation switching (MRS) sensors for detection of trace targets in complex samples still suffer from limitations in terms of relatively low sensitivity and poor stability. To meet this challenge, we develop a longitudinal relaxation time (T₁)-based nanosensor by using Mn²⁺ released from the reduction of a MnO₂ nanoassembly that can induce the change of T₁, and thus can greatly improve the sensitivity and overcome the “hook effect” of conventional MRS. Through the specific interaction between antigen and the antibody-functionalized MnO₂ nanoassembly, the T₁ signal of Mn²⁺ released from the nanoassembly is quantitatively determined by the antigen, which allows for highly sensitive and straightforward detection of targets. This approach broadens the applicability of magnetic biosensors and has great potential for applications in early diagnosis of disease biomarkers