Although blood-biomarker-based tests help in the early diagnosis of Alzheimer’s disease, the low abundance of blood protein biomarkers related to Alzheimer’s disease and the complexity of the human serum environment pose a challenge in diagnosis and treatment.

Due to the small size of nanomaterials, they are considered promising candidates for constructing biosensors with high sensitivity. Moreover, fabricating nanomaterials-containing transistor-based biosensors is challenging on a large scale and often lacks sensitivity and reproducibility in complex physiological fluids.

In an article recently published in the journal ACS Sensors, mass production of field-effect transistor (FET) biosensors based on carbon nanotube thin films was reported to realize high selectivity and sensitivity toward the detection of Alzheimer’s disease blood biomarkers of beta (β)-amyloid (Aβ).

The mass-produced carbon nanotube FET-sensors were combined with oligonucleotide aptamers to achieve efficient bioreceptors that enable reproducible and reliable Aβ₄₂ and Aβ₄₀ peptide detection in human serum with sub-femtomolar detection sensitivity, thus outperforming other existing detection methods towards Alzheimer’s disease.

Application of Carbon Nanotube Network Films Toward Alzheimer’s Disease Detection

Alzheimer’s disease is the common cause of dementia and leads to progressive cognitive decline. Moreover, Alzheimer’s disease is incurable and irreversible after the outset of cognitive symptoms. Hence, early diagnosis and disease-modifying treatments before the onset of symptoms are critical for Alzheimer’s disease treatment.

In Alzheimer’s disease diagnosis, Aβ protein level measurement in cerebrospinal fluid (CSF) and Aβ positron emission tomography (PET) imaging are currently applied detection methods. However, these are not extensively applied because obtaining CSF fluid involves an invasive lumbar puncture, and PET imaging is an expensive procedure.

To this end, diagnosis based on Alzheimer’s disease blood biomarkers can be a better alternative to previously mentioned methods due to simple peripheral blood sample requirement, minimal invasion, and the low cost of the procedure. However, the low concentration of Alzheimer’s disease proteins in the blood, the significant presence of interferants in the blood that cause masking effects, and protein’s large dynamic range are a few challenges posed by Alzheimer’s disease blood biomarkers-based diagnosis.

Nanomaterials-based FET sensors are next-generation, highly sensitive, and label-free methods with high- integration capability. Despite the outstanding performance of nano-FET sensors, their clinical application for blood-based biomarkers detection remains challenging to facilitate their application in diagnosing Alzheimer’s disease.

Semiconducting carbon nanotube network films are promising nanomaterials to construct FET biosensors with ultra-sensitivity. However, large-scale fabrication of carbon nanotube FET-based biosensors to detect ultralow biomarker concentrations with quick response in complex physiological environments remains unexplored.

Aptamer-Functionalized Carbon Nanotube FET Biosensors for Alzheimer’s Disease Diagnosis

In the present work, carbon nanotube FETs functionalized with nucleic acid aptamers were manufactured via a high-throughput fabrication process. In doing so, the team was able to achieve a highly selective and sensitive biosensor array for detecting Aβ proteins in serum at low concentrations and thereby promoting the clinical application of carbon nanotube FET-based sensor in early diagnosis of Alzheimer’s disease.

This FET sensor leverages the advantages of semiconducting carbon nanotube network film and DNA aptamers that are modified on a floating gate (FG) insulator. These modified DNA aptamers serve as Aβ₄₀ and Aβ₄₂ peptide selective receptors for the early detection of Alzheimer’s disease.

DNA aptamer-functionalized carbon nanotube FETs can detect Aβ₄₀ and Aβ₄₂ peptides in undiluted serum and single-strength phosphate buffer saline with a limit of detection (LoD) of 50 attomoles. The DNA aptamer-functionalized carbon nanotube FET sensors were highly selective towards Aβ peptides, immunoglobin G (IgG), and albumin masking proteins. Thus, carbon nanotube FET sensors outperform other Alzheimer’s disease sensing methods.

Moreover, the biological substrate’s adsorption to carbon nanotube FET biosensor has enhanced the selectivity ratios of up to 800% (Aβ₄₂) and 730% (Aβ₄₀). The DNA aptamer-functionalized carbon nanotube FET biosensor showed a quick response, wide dynamic range, low variation, and reduced overall cost, making this rapid detection method applicable for mass screening and early diagnosis of Alzheimer’s disease.

Conclusion

To summarize, aptamer-functionalized carbon nanotube FET biosensors were fabricated and explored for early diagnosis of Alzheimer’s disease by detecting the corresponding serum biomarkers in undiluted and single-strength PBS human serum. The aptamer-functionalized carbon nanotube FETs showed a broad analytical range with LoDs as low as 45 attomoles for Aβ₄₂ and 55 attomoles for Aβ₄₀.

The multi-blocking step reduced the nonspecific adsorption of biological matrix on aptamer-functionalized carbon nanotube FETs. Despite the presence of structurally similar proteins in the biological matrix solution, the selectivity ratio was improved by 800%. Moreover, these aptamer-functionalized carbon nanotube FETs showed a high recovery rate of 88 to 108%, high accuracy, serum-based long-term stability, and high reproducibility with less than 10% device-to-device variation.

Due to the high reliability and outstanding sensing properties, aptamer-functionalized carbon nanotube FET biosensors showed great potential as a cost-effective, reliable, and quick clinical platform, contributing toward advancements in mass screening tests and early diagnosis for Alzheimer’s disease.