The Case for Human Serum as a Highly Preferable Sample Matrix for Detection of Anthrax Toxins

Dual-mode detection of 2,6-pyridinedicarboxylic acid based on the enhanced peroxidase-like activity and fluorescence property of novel Eu-MOFs

2,6-Pyridinedicarboxylic acid (DPA) is a significant biomarker of anthrax, which is a deadly infectious disease for human beings. However, the development of a convenient anthrax detection method is still a challenge. Herein, we report a novel europium metal-organic framework (Eu-MOF) with an enhanced peroxidase-like activity and fluorescence property for DPA detection. The Eu-MOF was one-step synthesized using Eu3+ ions and 2-methylimidazole. In the presence of DPA, the intrinsic fluorescence of Eu3+ ions is sensitized, the fluorescence intensity linearly increases with an increase in DPA concentration, and the fluorescence color changes from blue to purple. Simultaneously, the peroxide-like activity of the Eu-MOF is enhanced by DPA, which can promote the oxidation of TMB to oxTMB. The absorbance values increase linearly with DPA concentrations, and the colorimetric images change from colorless to blue. The dual-mode detection of DPA has good sensitivity with a colorimetric detection limit of 0.67 μM and a fluorescent detection limit of 16.67 nM. Moreover, a simple detection method for DPA was developed using a smartphone with the RGB analysis system. A portable kit with standard color cards was developed using paper test strips. The proposed methods have good practicability for DPA detection in real samples. In conclusion, the developed Eu-MOF biosensor offers a valuable and general platform for anthrax diagnosis.

Biosensors for the Detection of Bacillus anthracis

Bacillus anthracis, present in two forms of vegetative cells and spores, is a pathogen that infects humans through contact with infected animals or contaminated animal products and is also maliciously used in terrorist acts. Therefore, a rapid and sensitive test for B. anthracis is necessary but challenging. The challenge comes from the following aspects: an accurate distinction of B. anthracis from other Bacillus species due to their high genomic similarity and the horizontal gene transfer between Bacillus members; direct detection of the B. anthracis spores without damaging them for component extraction to avoid the risk of spore atomization; and the rapid detections of B. anthracis in complex samples, such as soil and suspicious powders, without sample pretreatments and expensive large-scale equipment. Although culturing B. anthracis from samples is the conventional method for the detection of B. anthracis, it is time-consuming and the detection results would not be easy to interpret because many Bacillus species share similar phenotypic features such as a lack of motility and hemolysis, resistance to gamma phages, and so on. Intensive and extensive effort has been expended to develop reliable detection technologies, among which biosensors exhibit comprehensive advantages in terms of sensitivity, specificity, and portability. Here, we briefly review the research progress, providing highlights of the latest achievements and our own practice and experience. The contents can be summarized in three aspects: the discovery of detection targets, including genes, toxins, and other components; the creation of molecular recognition elements, such as monoclonal antibodies, single-chain antibody fragments, specific peptides, and aptamers; and the design and construction of biosensing systems by the integration of appropriate molecular recognition elements and transducer devices. These sensor devices have their own characteristics and different principles. For example, the surface plasmon resonance biosensor and quartz crystal microbalance biosensor are very sensitive, while the multiplex PCR-on-a-chip can detect multitargets. Biosensors for direct spore detection are highly recommended because they are not only fast but also avoid contamination from aerosol-containing spores. The introduction of nanotechnology has significantly improved the performance of biosensors. Superparamagnetic nanoparticles and phage-displayed gold nanoparticle ligand peptides have made the results of spore detection visible to the naked eye. Because of space constraints, many advanced biosensors for B. anthracis are not described in detail but are cited as references. Although biosensors provide a variety of options for various application scenarios, the challenges have not been fully addressed, which leaves room for the development of more advanced and practical B. anthracis detection means.

A digital single-molecule nanopillar SERS platform for predicting and monitoring immune toxicities in immunotherapy

The introduction of immune checkpoint inhibitors has demonstrated significant improvements in survival for subsets of cancer patients. However, they carry significant and sometimes life-threatening toxicities. Prompt prediction and monitoring of immune toxicities have the potential to maximise the benefits of immune checkpoint therapy. Herein, we develop a digital nanopillar SERS platform that achieves real-time single cytokine counting and enables dynamic tracking of immune toxicities in cancer patients receiving immune checkpoint inhibitor treatment - broader applications are anticipated in other disease indications. By analysing four prospective cytokine biomarkers that initiate inflammatory responses, the digital nanopillar SERS assay achieves both highly specific and highly sensitive cytokine detection down to attomolar level. Significantly, we report the capability of the assay to longitudinally monitor 10 melanoma patients during immune inhibitor blockade treatment. Here, we show that elevated cytokine concentrations predict for higher risk of developing severe immune toxicities in our pilot cohort of patients.

There is a clinical need to monitor immune-related toxicities of immune checkpoint blockade therapy. Here, the authors develop a digital SERS platform for multiplexed single cytokine counting to track immune-toxicities and demonstrate the ability to use pre-screening to identify patients at higher risk.

Molecular-Imprinting-Based Surface-Enhanced Raman Scattering Sensors

Molecularly imprinted polymers (MIPs) receive extensive interest, owing to their structure predictability, recognition specificity, and application universality as well as robustness, simplicity, and inexpensiveness. Surface-enhanced Raman scattering (SERS) is regarded as an ideal optical detection candidate for its unique features of fingerprint recognition, nondestructive property, high sensitivity, and rapidity. Accordingly, MIP based SERS (MIP-SERS) sensors have attracted significant research interest for versatile applications especially in the field of chemo- and bioanalysis, showing excellent identification and detection performances. Herein, we comprehensively review the recent advances in MIP-SERS sensors construction and applications, including sensing principles and signal enhancement mechanisms, focusing on novel construction strategies and representative applications. First, the basic structure of the MIP-SERS sensors is briefly outlined. Second, novel imprinting strategies are highlighted, mainly including multifunctional monomer imprinting, dummy template imprinting, living/controlled radical polymerization, and stimuli-responsive imprinting. Third, typical application of MIP-SERS sensors in chemo/bioanalysis is summarized from both small and macromolecular aspects. Lastly, the challenges and perspectives of the MIP-SERS sensors are proposed, orienting sensitivity improvement and application expanding.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Enhancing biosensing sensitivity of metal nanostructures through site-selective binding

The localised surface plasmon resonance (LSPR) at the surface of metal nanostructures can induce a highly intense electromagnetic (EM) field, which is confined to the edges with big curvature or at narrow gaps between nanostructures. Therefore, the localisation of target molecules at these sites is crucial to achieve high sensitivity in LSPR-based biosensors. To this end, we fabricated a 40 nm high gold nano-truncated cone (GNTC) array using thermal nanoimprint lithography. As the EM field is most intense at the side surface and relatively weak at the top surface of GNTC, we improved the detection sensitivity by blocking the top surface with oxides to limit adsorption of antibodies and antigens to the top surface. We observed the difference in sensitivity by detecting α-fetoprotein (AFP) on the oxide-capped and uncapped GNTC arrays through sandwich immunoassay and enzymatic precipitation. The capped GNTC array exhibited higher detection sensitivity than the uncapped one. Particularly, six-fold enhancement of sensitivity was achieved in the serum sample. We used atomic force microscopy and electron microscopy to validate that the deposition of the oxides on the top surface of GNTC effectively blocked the adsorption of the biomolecules and the target molecules were preferentially adsorbed on the side surfaces.

Antibodies Irreversibly Adsorb to Gold Nanoparticles and Resist Displacement by Common Blood Proteins

Gold nanoparticles (AuNPs) functionalized with proteins to impart desirable surface properties have been developed for many nanobiotechnology applications. A strong interaction between the protein and nanoparticle is critical to the formation of a stable conjugate to realize the potential of these emerging technologies. In this work, we examine the robustness of a protein layer adsorbed onto gold nanoparticles while under the stress of a physiological environment that could potentially lead to protein exchange on the nanoparticle surface. The adsorption interaction of common blood plasma proteins (transferrin, human serum albumin, and fibrinogen) and anti-horseradish peroxidase antibody onto AuNPs is investigated by nanoparticle tracking analysis. Our data show that a monolayer of protein is formed at saturation for each protein, and the maximum size increase for the conjugate, relative to the AuNP core, correlates with the protein size. The binding affinity of each protein to the AuNP is extracted from a best fit of the adsorption isotherm to the Hill equation. The antibody displays the greatest affinity (K_d = 15.2 ± 0.8 nM) that is ∼20-65 times stronger than the affinity of the other plasma proteins. Antibody-AuNP conjugates were prepared, purified, and suspended in solutions of blood plasma proteins to evaluate the stability of the antibody layer. An enzyme-mediated assay confirms that the antibody-AuNP interaction is irreversible, and the adsorbed antibody resists displacement by the plasma proteins. This work provides insight into the capabilities and potential limitations of antibody-AuNP-enabled technologies in biological systems.