An Emerging Fluorescent Carbon Nanobead Label Probe for Lateral Flow Assays and Highly Sensitive Screening of Foodborne Toxins and Pathogenic Bacteria

Tuning the Excited-State Intramolecular Proton Transfer in Carbon Dots via Coordination with Metal Ion

Although carbon dots (CDs) are widely used in the detection of heavy metal ions, rationally designing high-performance CDs for metal ion detection remains a significant challenge due to the limited understanding of their interaction mechanism. Here, we reveal the excited-state intramolecular proton transfer (ESIPT) in CDs and provide an effective strategy for switching the ESIPT by coordinating with metal ions. This work demonstrates that Zn2+ has strong coordination capacity toward CDs, while Mn2+ has the mildest coordination mode that can effectively sense other metal ions. Reasonable design of CDs with unique responses to heavy metal ions is achieved by doping Zn2+ or Mn2+ to regulate the ESIPT process. Therefore, a fluorescent sensor array for the sensitive identification of 10 kinds of heavy metal ions in real samples is constructed, which shows consistent detection results with ICP-OES. This work deepens the understanding of the fundamental fluorescence mechanism of CDs, opens new avenues for the rational tuning the emissive process, and provides a simple yet reliable platform to design nanoprobes for heavy metal ions as well as other molecules.

MOF scaffold for anchoring platinum-nickel nanoparticles with enhanced oxidase-like activity to improve lateral flow immunoassay diagnosis

Noble metal nanoparticles have attracted tremendous attention as the promising signal reporters for catalytic-colorimetric lateral flow immunoassay (LFIA). However, it remains great challenges for improving their stability and catalytic activity. Herein, first, a kind of porphyrinic based metal-organic framework (MOF) was used as a carrier for loading platinum (Pt) nanoparticles to avoid its aggregation. Moreover, nickel (Ni) atoms were dopped into Pt nanoparticles to adjust crystal structure, thus greatly improving catalytic activity. The resulting MOF@PtNi nanocomposite showed enhanced colorimetric signal brightness and excellent oxidase-like activity, which can improve sensitivity via amplifying the color signal. The catalytic mechanism was further studied by scavenger and electron paramagnetic resonance analysis. Furthermore, integrated with the competitive immunization LFIA platform, the high sensitivity colorimetric detection of human immunoglobulin G was realized with a detection limit of 0.378 ng/mL and 0.269 ng/mL for pre- and post-catalytic detection, respectively. In addition, this MOF@PtNi based catalytic-colorimetric LFIA was used for detection of clinical serum samples and the results agreed well with that measured by the standard method. Therefore, this study helps open up the application of proposed catalytic-colorimetric nanocomposite in the ultrasensitive LFIA for point-of-care diseases diagnosis.

Zero Background Visualizing Phosphorescence Lateral Flow Immunoassay of Cardiac Troponin I for Rapid and Accurate Diagnosis of Myocardial Infarction

Fluorescence lateral flow immunoassays (FL-LFIA) have attracted considerable attention in clinical diagnosis due to their outstanding merits of affordable, sensitive, on-site, and quick detection. However, they are still plagued by significant signal interference, such as autofluorescence and scattered light. The development of high-performance and robust phosphors, i.e., label probes featuring with the character of low/no optical background, remains a great challenge. Herein, we report a novel visualized phosphorescence LFIA (Phos-LFIA), where the composite microspheres, i.e., carbon dots (CDs) covalently embedded in dendritic mesoporous silicon nanoparticles (DMSNs), were designed and selected as the report probes. The obtained CDs@DMSNs revealed uniform morphologies and particle sizes, as well as ultralong (lifetime: 1.14 s, visible for over 8 s to naked eyes) room temperature phosphorescence (RTP) in aqueous solution. As competitive nanotags, CDs@DMSNs were designed for an ultralong phosphorescence-based time-gated LFIA for cardiac troponin I (cTnI) without optical interference. The fabricated Phos-LFIA test strips demonstrated zero-background signal and were applied for highly sensitive cTnI detection in both buffer and a complex serum matrix, with corresponding limits of detection (LODs) of 0.19 and 0.21 ng/mL, respectively. For a clinical validation, the proposed Phos-LFIA revealed an excellent clinical analytical performance (sensitivity: 95.45%, specificity: 88.9%, κ value: 0.85), demonstrating its potential for rapid and accurate diagnosis of myocardial infarction. This work provided a promising background-free probe for FL-LFIA, and it would also open an opportunity for developing highly sensitive screening platforms for other targets through modifying different recognition ligands onto CDs@DMSNs.

Polydopamine modified colloidal gold nanotag-based lateral flow immunoassay platform for highly sensitive detection of pathogenic bacteria and fast evaluation of antibacterial agents

Bacterial infection is a great threat to human health. Lateral flow immunoassays (LFIAs) with the merits of low cost, quick screening, and on-site detection are competitive technologies for bacteria detection, but their detection limits depend on the optical performance of the adopted nanotags. Herein, we presented a LFIA platform for bacteria detection using polydopamine (PDA) functionalized Au nanoparticles (denoted as Au@PDA) as the nanotag. The introduction of PDA could provide enhanced light absorption of Au, as well as numerous functional groups for conjugation. Small recognition molecules i.e. vancomycin (Van) and p-mercaptophenylboronic acid (PMBA) were covalently anchored to Au@PDA, and selected as the specific probes towards Gram-positive (G+) and Gram-negative (G-) bacteria, respectively. Taken Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) as the representative targets of G+ and G- bacteria, two LFA strips were successfully constructed based on the immuno-sandwich principle. They could quantitatively detect S. aureus and E. coli both down to 102 cfu/mL, a very competitive detection limit in comparison with other colorimetric or luminescent probes-based LFIAs. Furthermore, the proposed two strips were applied for the quantitative, accurate, and rapid detection of S. aureus and E. coli in food and human urine samples with good analytical results obtained. In addition, they were integrated as a screening platform for quick evaluation of diverse antibacterial agents within 3 h, which is remarkably shortened compared with that of the two traditional methods i.e. bacterial culture and plate-counting.

Lateral flow assays: Progress and evolution of recent trends in point-of-care applications

Paper based point-of-care (PoC) detection platforms applying lateral flow assays (LFAs) have gained paramount approval in the diagnostic domain as well as in environmental applications owing to their ease of utility, low cost, and rapid signal readout. It has centralized the aspect of self-evaluation exhibiting promising potential in the last global pandemic era of Covid-19 implementing rapid management of public health in remote areas. In this perspective, the present review is focused towards landscaping the current framework of LFAs along with integration of components and characteristics for improving the assay by pushing the detection limits. The review highlights the synergistic aspects of assay designing, sample enrichment strategies, novel nanomaterials-based signal transducers, and high-end analytical techniques that contribute significantly towards sensitivity and specificity enhancement. Various recent studies are discussed supporting the innovations in LFA systems that focus upon the accuracy and reliability of rapid PoC testing. The review also provides a comprehensive overview of all the possible difficulties in commercialization of LFAs subjecting its applicability to pathogen surveillance, water and food testing, disease diagnostics, as well as to agriculture and environmental issues.

Innovative Wearable Sweat Sensor Array for Real-Time Volatile Organic Compound Detection in Noninvasive Diabetes Monitoring

Wearable sweat sensors are reshaping healthcare monitoring, providing real-time data on hydration and electrolyte levels with user-friendly, noninvasive devices. This paper introduces a highly portable two-channel microfluidic device for simultaneous sweat sampling and the real-time detection of volatile organic compound (VOC) biomarkers. This innovative wearable microfluidic system is tailored for monitoring diabetes through the continuous and noninvasive tracking of acetone and ammonia VOCs, and it seamlessly integrates with smartphones for easy data management. The core of this system lies in the utilization of carbon polymer dots (CPDs) and carbon dots (CDs) derived from monomers such as catechol, resorcinol, o-phenylenediamine, urea, and citric acid. These dots are seamlessly integrated into hydrogels made from gelatin and poly(vinyl alcohol), resulting in an advanced solid-state fluorometric sensor coating on a cellulose paper substrate. These sensors exhibit exceptional performance, offering linear detection ranges of 0.05-0.15 ppm for acetone and 0.25-0.37 ppm for ammonia, with notably low detection limits of 0.01 and 0.08 ppm, respectively. Rigorous optimization of operational parameters, encompassing the temperature, sample volume, and assay time, has been undertaken to maximize device performance. Furthermore, these sensors demonstrate impressive selectivity, effectively discerning between biologically similar substances and other potential compounds commonly present in sweat. As this field matures, the prospect of cost-effective, continuous, personalized health monitoring through wearable VOC sensors holds significant potential for overcoming barriers to comprehensive medical care in underserved regions. This highlights the transformative capacity of wearable VOC sweat sensing in ensuring equitable access to advanced healthcare diagnostics, particularly in remote or geographically isolated areas.

A novel paper-based electrochemiluminescence biosensor for non-destructive detection of pathogenic bacteria in real samples

The demand for sensitive, portable, and non-destructive analysis of pathogenic bacteria is of significance in point-of-care diagnosis. Herein, we constructed a smart electrochemiluminescence (ECL) biosensor by integrating a flexible paper-based sensing device and a disposable three-electrode detecting system. Staphylococcus aureus (S. aureus)-responsive cellulose paper was prepared by employing aptamer as recognition element and a probe DNA (probe DNA-GOD) tagged with glucose oxidase (GOD) as a signal amplification unit. The formation of aptamer-S. aureus complex mediated the quantitative release of probe DNA-GOD. The remaining probe DNA-GOD on the paper-based aptasensor was then activated by glucose, which resulted in a significant decrease in ECL signal. To further improve the ECL performance of biosensor, a large number of Ru(bpy)32+ molecules were embedded into porous zinc-based metal-organic frameworks (MOFs) to form Ru(bpy)32+ functionalized MOF nanoflowers (Ru-MOF-5 NFs). Such biosensor enabled accurate, non-destructive, and real-time monitoring of S. aureus-contaminated food samples, opening a new avenue for sensitive recognition of pathogenic bacteria.

Improvement of the sensitivity of lateral flow systems for detecting mycotoxins: Up-to-date strategies and future perspectives

Mycotoxins are dangerous human and animal health-threatening secondary fungal metabolites that can be found in various food and agricultural products. Several countries have established regulations to restrict their presence in food and agricultural products destined for human and animal consumption. Consequently, the need to develop highly sensitive and smart detection systems was recognized worldwide. Lateral flow assay possesses the advantages of easy operation, rapidity, stability, accuracy, and specificity, and it plays an important role in the detection of mycotoxins. Nevertheless, strategies to comprehensively improve the sensitivity of lateral flow assay to mycotoxins in food have rarely been highlighted and discussed. In this article, a comprehensive overview was presented on the application of lateral flow assay in mycotoxin detection in food samples by highlighting the principle of lateral flow assay, presenting a detailed discussion on various analytical performance-improvement strategies, such as the development of high-affinity recognition reagents, immunogen immobilization methods, and signal amplification. Additionally, a detailed discussion on the various signal analyzers and interpretation approaches was provided. Finally, current hurdles and future perspectives on the application of lateral flow assay in the detection of mycotoxins were discussed.

High extinction coefficient material combined with multi-line lateral flow immunoassay strip for ultrasensitive detection of bacteria

Lateral flow immunoassay strips (LFIAs) are a reliable and point-of-care detection method for rapid monitoring of bacteria, but their sensitivity was limited by the low extinction coefficient of colloidal gold nanoparticles (Au NPs) and low capture efficiency of test-line. In this study, polydopamine nanoparticles (PDA NPs) were employed to replace Au NPs, due to their high extinction coefficient. And the amount of test-line was increased to 5 for further improving the efficiency of bacteria capture. Thus, under visual observation, the detection limits of PDA-based LFIAs (102 CFU/mL) were about 2 orders of magnitude lower than Au-based LFIAs (104 CFU/mL). Furthermore, the invisible signal could be collected by Image J and the detection limit can reach 10 CFU/mL. The proposed test strips were successfully applied for the quantitative, accurate, and rapid screening of E. coli in food samples. This study provided a universal approach to enhance the sensitivity of bacteria LFIAs.

Enhanced sensitivity and accuracy via gold nanoparticles based multi-line lateral flow immunoassay strip for Salmonella typhimurium detection in milk and orange juice

Food borne pathogens threaten food safety and affect human health. The lateral flow immunoassays (LFIAs) are widely concerned because of simplicity, low cost and user friendliness, and have broad application prospects in pathogen detection. However, the sensitivity of LFIAs is limited. Herein, multi-line LFIAs are introduced into pathogen detection for the first time. Compared with traditional single-line LFIAs, the overall signal strength of multi-line LFIAs has been significantly improved. It is particularly noteworthy that multi-line LFIAs detection accuracy of 10³ CFU/mL pathogen has been improved by about 55%. The proposed multi-line LFIAs reduce the possibility of judging a positive result as a false negative result. The LFIAs strip was validated in real samples of milk and orange juice. This strategy has great potential for rapid detection of pathogens in real samples, and provides new insights for improving the accuracy and sensitivity of LFIAs strips.

No-cost ballpoint pen dispenser for lateral flow assays

We have developed a no-cost, lightweight, human-powered dispenser using an empty ballpoint pen. Used in lateral flow assays, this dispenser restricts antibody deposition to narrow zones, allowing freehand drawing of test and control lines. The lines can be drawn in widths ranging from 0.15 to 1.00 mm. Naphthol green B, a compatible stain, was used to label antibody solutions and certify handwriting traces. Using human chorionic gonadotropin (HCG) as a model antigen, we demonstrated that the pen dispenser can imprint antibodies on nitrocellulose membranes without affecting their microstructure and chromatographic function. A lateral flow assay using the pen dispenser detected HCG at 0.1 μg/mL, comparable to the sensitivity of standard tests using traditional benchtop dispensers.

Mycobacteriophage Derived Lipoarabinomannan Binding Protein for Recognizing Non-Tuberculosis Mycobacteria

Non-tuberculosis mycobacteria (NTM) is one family of pathogens usually leading to nosocomial infections. Exploration of high-performance biological recognition agent plays a pivotal role for the development of point-of-care testing device and kit for detecting NTM. Mycobacterium smegmatis (M. smegmatis) is a NTM which has been frequently applied as an alternative model for highly pathogenic mycobacteria. Herein, a recombinant tail protein derived from mycobacteriophage SWU1 infecting M. smegmatis was expressed with Escherichia coli system and noted as GP89. It shows a fist-like structure according to the results of homology modeling and ab initio modeling. It is confirmed as a lipoarabinomannan (LAM) binding protein, which can recognize studied NTM genus since abundant LAM constructed with d-mannan and d-arabinan is distributed over the mycobacterial surface. Meanwhile an enhanced green fluorescent protein (eGFP)-fused GP89 protein was acquired with a fusion expression technique. Then GP89 and eGFP-fused GP89 were applied to establish a sensitive and rapid method for fluorescent detection of M. smegmatis with a broad linear range of 1.0 × 10² to 1.0 × 10⁶ CFU mL^-1 and a low detection limit of 69 CFU mL^-1. Rapid and reliable testing of antimicrobial susceptibility was achieved by the GP89-based fluorescent method. The present work provides a promising recognition agent for studied NTM and opens an avenue for clinical diagnosis of NTM-induced infections.

Emerging and Versatile Platforms of Metal-Ion-Doped Carbon Dots for Biosensing, Bioimaging, and Disease Therapy

Metal ions possess abundant electrons and unoccupied orbitals, as well as large atomic radii, whose doping into carbon dots (CDs) is a facile strategy to endow CDs with additional physicochemical characteristics. After being doped with metal ions, CDs reveal obvious changes in their optical, electronic, and magnetic properties by adjustments to their electron density distribution and the energy gaps, leading them to be promising and competitive candidates as labeling probes, imaging agents, catalysts, nanodrugs, and so on. In this review, we summarize the fabrication methods of metal-ion-doped CDs (M-CDs), and highlight their biological applications including biosensing, bioimaging, tumor therapy, and anti-microbial treatment. Finally, the challenging future perspectives of M-CDs are analyzed. We hope this review will provide inspiration for further development of M-CDs in various biological aspects, and help readers who are interested in M-CDs and their biological applications.