Self-Assembling Allochroic Nanocatalyst for Improving Nanozyme-Based Immunochromatographic Assays

Nanolabels Prepared by the Entrapment or Self-Assembly of Signaling Molecules for Colorimetric and Fluorescent Immunoassays

Nanomaterials have attracted significant attention as signal reporters for immunoassays. They can directly generate detectable signals or release a large number of signaling elements for readout. Among various nanolabels, nanomaterials composed of multiple signaling molecules have shown great potential in immunoassays. Generally, signaling molecules can be entrapped in nanocontainers or self-assemble into nanostructures for signal amplification. In this review, we summarize the advances of signaling molecules-entrapped or assembled nanomaterials for colorimetric and fluorescence immunoassays. The nanocontainers cover liposomes, polymers, mesoporous silica, metal-organic frameworks (MOFs), various nanosheets, nanoflowers or nanocages, etc. Signaling molecules mainly refer to visible and/or fluorescent organic dyes. The design and application of immunoassays are emphasized from the perspective of nanocontainers, analytes, and analytical performances. In addition, the future challenges and research trends for the preparation of signaling molecules-entrapped or assembled nanolabels are briefly discussed.

Nanozyme-Enabled Biomedical Diagnosis: Advances, Trends, and Challenges

As nanoscale materials with the function of catalyzing substrates through enzymatic kinetics, nanozymes are regarded as potential alternatives to natural enzymes. Compared to protein-based enzymes, nanozymes exhibit attractive characteristics of low preparation cost, robust activity, flexible performance adjustment, and versatile functionalization. These advantages endow them with wide use from biochemical sensing and environmental remediation to medical theranostics. Especially in biomedical diagnosis, the feature of catalytic signal amplification provided by nanozymes makes them function as emerging labels for the detection of biomarkers and diseases, with rapid developments observed in recent years. To provide a comprehensive overview of recent progress made in this dynamic field, here an overview of biomedical diagnosis enabled by nanozymes is provided. This review first summarizes the synthesis of nanozyme materials and then discusses the main strategies applied to enhance their catalytic activity and specificity. Subsequently, representative utilization of nanozymes combined with biological elements in disease diagnosis is reviewed, including the detection of biomarkers related to metabolic, cardiovascular, nervous, and digestive diseases as well as cancers. Finally, some development trends in nanozyme-enabled biomedical diagnosis are highlighted, and corresponding challenges are also pointed out, aiming to inspire future efforts to further advance this promising field.

Scalable synthesis of Au@CeO2 nanozyme for development of colorimetric lateral flow immunochromatographic assay to sensitively detect heart-type fatty acid binding protein

Nanozymes with core@shell nanostructure are considered promising biolabeling materials for their multifunctional properties. In this work, a simple one-pot strategy has been proposed for scalable synthesis of gold@cerium dioxide core@shell nanoparticles (Au@CeO2 NPs) with strong localized surface plasmon resonance (LSPR) absorption and high peroxidase-like catalytic activity by redox reactions of Ce3+ ions and AuCl4- ions in diluted ammonia solution under room temperature. A colorimetric lateral flow immunochromatographic assay (LFIA) has been successfully fabricated for sensitive detection of heart-type fatty acid binding protein (H-FABP, an early cardiac biomarker) by using the Au@CeO2 NPs as reporters. The as-developed LFIA with Au@CeO2 NP reporter (termed as Au@CeO2-LFIA) exhibits a dynamic range of nearly two orders of magnitude, and a limit of detection (LOD) as low as 0.35 ng mL-1 H-FABP with nanozyme-triggered 3,3',5,5'-tetramethylbenzidine (TMB) colorimetric amplification. Furthermore, the practicality of Au@CeO2-LFIA has been demonstrated by profiling the concentrations of H-FABP in 156 blood samples of acute myocardial infarction (AMI) patients, and satisfactory results are obtained.

Linker-Preserved Iron Metal-Organic Framework-Based Lateral Flow Assay for Sensitive Transglutaminase 2 Detection in Urine Through Machine Learning-Assisted Colorimetric Analysis

A groundbreaking demonstration of the utilization of the metal-organic framework MIL-101(Fe) as an exceptionally perceptive visual label in colorimetric lateral flow assays (LFA) is described. This pioneering approach enables the precise identification of transglutaminase 2 (TGM2), a recognized biomarker for chronic kidney disease (CKD), in urine specimens, which offers a remarkably sensitive naked-eye detection mechanism. The surface of MIL-101(Fe) was modified with oxalyl chloride, adipoyl chloride, and poly(acrylic) acid (PAA); these not only improved the labeling material stability in a complex matrix but also achieved a systematic control in the detection limit of the TGM2 concentration using our LFA platform. The advanced LFA with the MIL-101(Fe)-PAA label can detect TGM2 concentrations down to 0.012, 0.009, and 0.010 nM in Tris-HCl buffer, urine, and desalted urine, respectively, which are approximately 55-fold lower than those for a conventional AuNP-based LFAs. Aside from rapid TGM2 detection (i.e., within 20 min), the performance of the MIL-101(Fe)-PAA-based LFA on reproducibility [coefficients of variation (CV) < 2.9%] and recovery (95.9-103.2%) along with storage stability within 25 days of observation (CV < 6.0%) shows an acceptable parameter range for quantitative analysis. A sophisticated sensing method grounded in machine learning principles was also developed, specifically aimed at precisely deducing the TGM2 concentration by analyzing immunoreaction sites. More importantly, our developed LFA offers potential for clinical measurement of TGM2 concentration in normal human urine and CKD patients' samples.

Ultrabright Fluorescent Nanorod-Based Immunochromatographic with Low Background for Advancing Detection Performance

Nanomaterials-based immunochromatographic assays (ICAs) are of great significance in point-of-care testing (POCT), yet it remains challenging to explore low background platforms and high chromogenic intensity probes to improve detection performance. Herein, we reported a low interference and high signal-to-noise ratio fluorescent ICA platform based on ultrabright persistent luminescent nanoparticles (PLNPs) Zn2GeO4: Mn, which could produce intense photoluminescence at 254 nm excitation to reduce background interference from ICA substrates and samples. The prepared immunosensor was successfully applied in T-2 toxin detection with a remarkable limit of detection of 0.025 ng/mL, which was 22-fold more sensitive compared with that of traditional gold nanoparticles. Ultimately, a portable 3D-printed detection device equipped with a smartphone analyzing application was fabricated for quantitative readout in POCT, achieving favorable recoveries in practical sample detection. This work provides a creative attempt for ultrabright PLNP-based low background ICA, and it also guarantees its feasibility in practical POCT.

Signal Amplification Strategy Design in Nanozyme-Based Biosensors for Highly Sensitive Detection of Trace Biomarkers

Nanozymes show great promise in enhancing disease biomarker sensing by leveraging their physicochemical properties and enzymatic activities. These qualities facilitate signal amplification and matrix effects reduction, thus boosting biomarker sensing performance. In this review, recent studies from the last five years, concentrating on disease biomarker detection improvement through nanozyme-based biosensing are examined. This enhancement primarily involves the modulations of the size, morphology, doping, modification, electromagnetic mechanisms, electron conduction efficiency, and surface plasmon resonance effects of nanozymes for increased sensitivity. In addition, a comprehensive description of the synthesis and tuning strategies employed for nanozymes has been provided. This includes a detailed elucidation of their catalytic mechanisms in alignment with the fundamental principles of enhanced sensing technology, accompanied by the presentation of quantitatively analyzed results. Moreover, the diverse applications of nanozymes in strip sensing, colorimetric sensing, electrochemical sensing, and surface-enhanced Raman scattering have been outlined. Additionally, the limitations, challenges, and corresponding recommendations concerning the application of nanozymes in biosensing have been summarized. Furthermore, insights have been offered into the future development and outlook of nanozymes for biosensing. This review aims to serve not only as a reference for enhancing the sensitivity of nanozyme-based biosensors but also as a catalyst for exploring nanozyme properties and their broader applications in biosensing.

One-pot synthesis of AuPt@FexOy nanoparticles with excellent peroxidase-like activity for development of ultrasensitive colorimetric lateral flow immunoassay of cardiac troponin I

Detection of cardiac troponin I (cTnI) plays a critical role in diagnosing acute myocardial infarction (AMI). In this report, a new kind of spherical AuPt@FexOy core@shell nanoparticles (termed as AuPt@FexOy NPs) were one-pot synthesized by a redox interaction-engaged strategy (RIES) without the addition of any surfactants or reducing agents. The as-synthesized AuPt@FexOy NPs not only retain the plasmonic activity of gold nanoparticles (AuNPs), but also possess excellent catalytic activities of platinum nanoparticles (PtNPs) and FexOy nanoclusters. The features of AuPt@FexOy NPs enable greatly enhance the colorimetric detection sensitivity of lateral flow immunoassay (LFIA) through integrating AuPt@FexOy NPs labeling procedure and catalyzing oxidation of chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) signal amplification strategy. The as-developed colorimetric LFIA (termed as AuPt@FexOy-LFIA) exhibits the limit of detection (LOD) as 26.0 pg mL-1 cTnI under the TMB signal amplification mode. In particular, the detection results of cTnI in 40 clinical seral samples by AuPt@FexOy-LFIA are correlated well with those of cTnI in the same samples by commercial enzyme-linked immunosorbent assay (ELISA) detection kit (R2 = 0.97, slope = 1), demonstrating the highly reliable analytical performance and good application prospect of AuPt@FexOy-LFIA.

Advancements in Biosensors Based on the Assembles of Small Organic Molecules and Peptides

Over the past few decades, molecular self-assembly has witnessed tremendous progress in a variety of biosensing and biomedical applications. In particular, self-assembled nanostructures of small organic molecules and peptides with intriguing characteristics (e.g., structure tailoring, facile processability, and excellent biocompatibility) have shown outstanding potential in the development of various biosensors. In this review, we introduced the unique properties of self-assembled nanostructures with small organic molecules and peptides for biosensing applications. We first discussed the applications of such nanostructures in electrochemical biosensors as electrode supports for enzymes and cells and as signal labels with a large number of electroactive units for signal amplification. Secondly, the utilization of fluorescent nanomaterials by self-assembled dyes or peptides was introduced. Thereinto, typical examples based on target-responsive aggregation-induced emission and decomposition-induced fluorescent enhancement were discussed. Finally, the applications of self-assembled nanomaterials in the colorimetric assays were summarized. We also briefly addressed the challenges and future prospects of biosensors based on self-assembled nanostructures.

Recent Advances in the Immunoassays Based on Nanozymes

As a rapid and simple method for the detection of multiple targets, immunoassay has attracted extensive attention due to the merits of high specificity and sensitivity. Notably, enzyme-linked immunosorbent assay (ELISA) is a widely used immunoassay, which can provide high detection sensitivity since the enzyme labels can promote the generation of catalytically amplified readouts. However, the natural enzyme labels usually suffer from low stability, high cost, and difficult storage. Inspired by the advantages of superior and tunable catalytic activities, easy preparation, low cost, and high stability, nanozymes have arisen to replace the natural enzymes in immunoassay; they also possess equivalent sensitivity and selectivity, as well as robustness. Up to now, various kinds of nanozymes, including mimic peroxidase, oxidase, and phosphatase, have been incorporated to construct immunosensors. Herein, the development of immunoassays based on nanozymes with various types of detection signals are highlighted and discussed in detail. Furthermore, the challenges and perspectives of the design of novel nanozymes for widespread applications are discussed.

Fiber-integrated WGM optofluidic chip enhanced by microwave photonic analyzer for cardiac biomarker detection with ultra-high resolution

Cardiac troponin I (cTnI) plays an important role in emergency diagnosis of cardiovascular diseases, which exists predominately in the form of cardiac troponin I-C (cTnI-C) complex. We proposed a fiber-integrated optofluidic chip immunosensor with time-delay-dispersion based microwave photonic analyzer (MPA) for cTnI-C detection. The whispering gallery mode (WGM) fiber probe was fabricated by embedding a polydopamine functionalized hollow glass microsphere (HGMS) into the etched capillary-fiber structure, and the WGMs could be excited through the efficient coupling between the thin-wall capillary and the HGMS. The reflective WGM optofluidic chip functioned as a wavelength tuner to construct fiber ring laser cavity, whose laser output wavelength was cTnI-C concentration-dependent. The tiny wavelength variation of sensing laser was converted into a radio-frequency (RF) response, which was retrieved by measuring the change of RF-domain free spectrum range (FSR) in time-delay-dispersion based MPA, and the quantitative detection of cTnI-C complex can be achieved with high resolution. Experimental results show that this immunosensor had a limit of detection (LOD) of 0.59 ng/mL, and a detection resolution of 1.2 fg/mL. The relative resolving power was 10²-10⁴-fold higher than that of others optical fiber cTnI biosensors. The proposed fiber-integrated optofluidic chip provides an innovative lab-on-chip diagnostic tool for myocardial damage.

An Overview for the Nanoparticles-Based Quantitative Lateral Flow Assay

The development of the lateral flow assay (LFA) has received much attention in both academia and industry because of their broad applications to food safety, environmental monitoring, clinical diagnosis, and so forth. The user friendliness, low cost, and easy operation are the most attractive advantages of the LFA. In recent years, quantitative detection has become another focus of LFA development. Here, the most recent studies of quantitative LFAs are reviewed. First, the principles and corresponding formats of quantitative LFAs are introduced. In the biomaterial and nanomaterial sections, the detection, capture, and signal amplification biomolecules and the optical, fluorescent, luminescent, and magnetic labels used in LFAs are described. The invention of dedicated strip readers has drawn further interest in exploiting the better performance of LFAs. Therefore, next, the development of dedicated reader devices is described and the usefulness and specifications of these devices for LFAs are discussed. Finally, the applications of LFAs in the detection of metal ions, biotoxins, pathogenic microorganisms, veterinary drugs, and pesticides in the fields of food safety and environmental health and the detection of nucleic acids, biomarkers, and viruses in clinical analyses are summarized.

Recent advances in enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.