A novel immunosensor utilizing multi-walled carbon nanotube as label on a lateral flow system for simple and quantitative electrical detection was investigated. Multi-walled carbon nanotubes (MWCNTs) were first modified with polyvinylpyrrolidone (PVP) for uniform dispersion in aqueous solution. Then, the MWCNTs were conjugated with human immunoglobulin G (IgG) using 1-(3-(dimethylamino)-propyl)-3-ethylcarbodiimide hydrochloride (EDC) coupling chemistry. The lateral flow immunosensor was made of nitrocellulose membrane that transports sample and reagents through a porous structure by capillary action. The performance of the immunosensor was demonstrated using human IgG as a model analyte competing with the conjugate (MWCNT-labeled human IgG) at the capture zone. As a result of binding reaction between the conjugate and the immobilized Protein A at the capture zone, the conjugated MWCNTs formed a conducting network at the capture zone providing conductance measurement corresponding to the amount of captured conjugate. Quantitative immunoassay response for the target human IgG was demonstrated in the range of 25 to 200 mg ml À1 without an additional amplification step. The presented immunosensing technique could be expanded for the detection of various analyte-specific biomolecules with a potential for simple and rapid tests suitable for point-of-care diagnostics.
This paper presents a novel method for direct detection of Plasmodium falciparum histidine rich protein-2 (PfHRP-2) antigen using carbon nanofiber (CNF) forests grown on glass microballoons (NMBs). Secondary antibodies specific to PfHRP-2 densely attached to the CNFs exhibit extraordinary ability for the detection of minute concentrations of Plasmodium species. A sandwich immunoassay protocol was employed, where a glass substrate was used to immobilize primary antibodies at designated capture zones. High signal amplification was obtained in both colorimetric and electrical measurements due to the CNFs through specific binding. As a result, it was possible to detect PfHRP-2 levels as low as 0.025 ng/mL concentration in phosphate buffered saline (PBS) using a visual signal within only 1 min of test duration. Lower limits of 0.01 ng/mL was obtained by measuring the electrical resistivity of the capture zone. This method is also highly selective and specific in identifying PfHRP-2 and other Plasmodium species from the same solution. In addition, the stability of the labeling mechanism eliminates the false signals generated by the use of dyes in current malaria rapid diagnostic test kits (MRDTs). Thus, the rapid, sensitive and high signal amplification capabilities of NMBs is a promising tool for early diagnosis of malaria and other infectious diseases.
Early diagnosis of diseases is critical in its effective management. Traditional disease detection methods require specialized equipment and trained personnel. With the introduction of rapid diagnostic test kits (RDTs), disease detection has become easier and faster. However, these RDTs have failed to compete with the specialized laboratory equipment due to their high detection limits and false alarm rates. This paper presents a novel method of using carbon nanofibers (CNFs) grown on glass microballoons (NMBs) to achieve ultra-low detection limits in RDTs. The NMBs have millions of nanosized CNFs grown on each microballoon, with each CNF having a strong bonding affinity for antibodies. The NMBs conjugated with secondary antibodies have therefore a significantly higher probability of capturing minute antigen concentrations in solution. Furthermore, the dark color formation at the capture zone makes visual disease detection possible. Human Immunoglobulin G (IgG) was selected as the model analyte to study the performance of NMBs using a sandwich immunoassay protocol. Ultra-low electrical detection limit of (4 pg/ml) and rapid response (~1 minute) was achieved using this method.
A recent approach in disease diagnosis and viral epidemics is aimed at point-of-care tests that could be administered near the patient rather than time-consuming processes involving centralized laboratories. Point-of-care devices provide rapid results in simple and low-cost manner requiring only small sample volumes. These devices will strongly benefit from advanced materials and fabrication methods to improve their efficiency and sensitivity. We report a functionalized carbon nanotube label for an immunosensor application. Carbon nanotube label was prepared by modifying the carbon nanotube surface to anchor biomolecules. First, the carboxylic acid treated multi-walled carbon nanotubes (MWCNTs) were uniformly dispersed with polyvinylpyrrolidone (PVP) by sonication in aqueous solution. PVP partially wraps around the carbon nanotubes and exposes the surface of the nanotubes for further functionalization. The MWCNTs were then conjugated with human immunoglobulin G (IgG) using EDC/Sulfo-NHS coupling chemistry, where the antibodies occupied sites not covered by PVP. The dispersion, surfactant modification, and antibody conjugation of the MWCNTs were also confirmed using SEM and TEM images. The successful functionalization of the MWCNTs and reactivity of the covalent attached antibodies were demonstrated for specific antigen binding on the microelectrode device. The carbon nanotube-based detection mechanism could be tailored for screening various analyte specific molecules. Furthermore, the reported technique could easily be integrated in various microfluidic and lab-on-a-chip devices for the development of functional electronic sensors providing quantitative, sensitive, and low-cost detection in pointof- care setup.
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