Development of a high-sensitivity immunoassay for amyloid-beta 1-42 using a silicon microarray platform

Sensor-integrated brain-on-a-chip platforms: Improving the predictive validity in neurodegenerative research

Affecting millions of individuals worldwide, neurodegenerative diseases (NDDs) pose a significant and growing health concern in people over the age of 60 years. Contributing to this trend are the steady increase in the aging population coupled with a persistent lack of disease-altering treatment strategies targeting NDDs. The absence of efficient therapeutics can be attributed to high failure rates in clinical trials and the ineptness of animal models in preceding preclinical studies. To that end, in recent years, significant research effort has been dedicated to the development of human cell-based preclinical disease models characterized by a higher degree of predictive validity. However, a key requirement of any in vitro model constitutes the precise knowledge and replication of the target tissues' (patho-)physiological microenvironment. Herein, microphysiological systems have demonstrated superiority over conventional static 2D/3D in vitro cell culture systems, as they allow for the emulation and continuous monitoring of the onset, progression, and remission of disease-associated phenotypes. This review provides an overview of recent advances in the field of NDD research using organ-on-a-chip platforms. Specific focus is directed toward non-invasive sensing strategies encompassing electrical, electrochemical, and optical sensors. Additionally, promising on- and integrable off-chip sensing strategies targeting key analytes in NDDs will be presented and discussed in detail.

Microfluidics as diagnostic tools

The challenges in the management of human diseases are largely determined by the precision, speed and ease of diagnostic procedures available. Developments in biomedical engineering technologies have greatly helped in transforming human health care, especially for disease diagnosis which in turn lead to better patient outcomes. One such development is in the form of microfluidic chip technology which has transformed various aspects of human health care. We present in this review, a comprehensive account on the utility of microfluidic chip technologies for the diagnosis of autoimmune disorders, cardiovascular diseases (CVDs), infectious diseases, and neurodegenerative conditions. We have included the diseases posing global threat such as rheumatoid arthritis, diabetes, pernicious anemia, tuberculosis, COVID-19, influenza, alzheimer's, multiple sclerosis, and epilepsy. Apart from discussing the ways of microfluidic chip in diagnosis, we included a section presenting electrochemical, electrical, optical, and acoustic detection technologies for the precise diagnosis of CVDs. Microfluidics platforms have thus revolutionized novel capabilities in addressing the requirements of point-of-care diagnostics enabling miniaturization by integrating multiple laboratory functions into a single chip resulting in "one flow - one solution" systems. Hence, the precision and early diagnoses of diseases are now possible due to the advancements of microfluidics-based technology.

Chiral nanomaterial-based approaches for diagnosis and treatment of protein-aggregated neurodiseases: current status and future opportunities

Protein misfolding and its aggregation, known as amyloid aggregates (Aβ), are some of the major causes of more than 20 diseases such as Parkinson's disease, Alzheimer's disease, and type 2 diabetes. The process of Aβ formation involves an energy-driven oligomerization of Aβ monomers, leading to polymerization and eventual aggregation into fibrils. Aβ fibrils exhibit multilevel chirality arising from its amino acid residues and the arrangement of folded polypeptide chains; thus, a chirality-driven approach can be utilized for the detection and inhibition of Aβ fibrils. In this regard, chiral nanomaterials have recently opened new possibilities for various biomedical applications owing to their stereoselective interaction with biological systems. Leveraging this chirality-driven approach with chiral nanomaterials against protein-aggregated diseases could yield promising results, particularly in the early detection of Aβ forms and the inhibition of Aβ aggregate formation via specific and strong "chiral-chiral interaction." Despite the advantages, the development of advanced theranostic systems using chiral nanomaterials against protein-aggregated diseases has received limited attention so far because of considerably limited formulations for chiral nanomaterials and lack of information of their chiroptical behavior. This review aims to present the current status of chiral nanomaterials explored for detecting and inhibiting Aβ forms. This review covers the origin of chirality in amyloid fibrils and nanomaterials and different chiral detection methods; furthermore, different chiral nanosystems such as chiral plasmonic nanomaterials, chiral carbon-based nanomaterials, and chiral nanosurfaces, which have been used so far for different therapeutic applications against protein-aggregated diseases, are discussed in detail. The findings from this review may pave the way for the development of novel approaches using chiral nanomaterials to combat diseases resulting from protein misfolding and can further be extended to other disease forms.

Combination of Ternary Electrochemiluminescence System of BNQDs/AgMOG-K2S2O8 and Electrochemiluminescence Resonance Energy Transfer Strategy for Ultrasensitive Immunoassay of Amyloid-β Protein

In this work, based on boron nitride quantum dots (BNQDs) as energy donors and MnO2@MWCNTs-COOH as energy receptors, we designed an efficient electrochemiluminescence resonance energy transfer (ECL-RET) immunosensor for the detection of amyloid-β (Aβ42) protein, a biomarker of Alzheimer's disease (AD). First, the signal amplification of a ternary ECL system composed of BNQDs (as the ECL emitter), K2S2O8 (as the coreactant), and silver metal-organic gels (AgMOG, as the coreaction accelerator) was realized, and PDDA as stabilizer was added, a strong and stable initial ECL signal was obtained. AgMOG could not only support a large amount of BNQDs and Aβ42 capture antibody (Ab1) through Ag-N bond but also exhibit excellent ECL catalytic performance and enhance the luminescent intensity of BNQDs@PDDA-K2S2O8 system. In addition, due to the broad absorption spectrum of MnO2@MWCNTs-COOH and the extensive overlap with the ECL emission spectrum of BNQDs, the quenching probe Ab2-MnO2@MWCNTs-COOH could be introduced into the ternary system through a sandwich immune response. On this basis, the signal on-off ECL immunosensor was constructed to achieve the ultrasensitive detection of Aβ42 through signal transformation. Under the optimal conditions, the prepared ECL biosensor manifested a wide linear range (10 fg/mL-100 ng/mL) with a detection limit of 2.89 fg/mL and showed excellent stability, selectivity, and repeatability, which provided an effective strategy for the ultrasensitive detection of biomarkers in clinical analysis.

Biosensor integrated brain-on-a-chip platforms: Progress and prospects in clinical translation

Because of the brain's complexity, developing effective treatments for neurological disorders is a formidable challenge. Research efforts to this end are advancing as in vitro systems have reached the point that they can imitate critical components of the brain's structure and function. Brain-on-a-chip (BoC) was first used for microfluidics-based systems with small synthetic tissues but has expanded recently to include in vitro simulation of the central nervous system (CNS). Defining the system's qualifying parameters may improve the BoC for the next generation of in vitro platforms. These parameters show how well a given platform solves the problems unique to in vitro CNS modeling (like recreating the brain's microenvironment and including essential parts like the blood-brain barrier (BBB)) and how much more value it offers than traditional cell culture systems. This review provides an overview of the practical concerns of creating and deploying BoC systems and elaborates on how these technologies might be used. Not only how advanced biosensing technologies could be integrated with BoC system but also how novel approaches will automate assays and improve point-of-care (PoC) diagnostics and accurate quantitative analyses are discussed. Key challenges providing opportunities for clinical translation of BoC in neurodegenerative disorders are also addressed.

Metal-Organic Frameworks as Sensors for Human Amyloid Diseases

Metal-organic frameworks (MOFs) are versatile compounds with emergent applications in the fabrication of biosensors for amyloid diseases. They hold great potential in biospecimen protection and unprecedented probing capabilities for optical and redox receptors. In this Review, we summarize the main methodologies employed in the fabrication of MOF-based sensors for amyloid diseases and collect all available data in the literature related to their performance (detection range, limit of detection, recovery, time of analysis, among other parameters). Nowadays, MOF sensors have evolved to a point where they can, in some cases, outperform technologies employed in the detection of several amyloid biomarkers (amyloid β peptide, α-synuclein, insulin, procalcitonin, and prolactin) present in biological fluids, such as cerebrospinal fluid and blood. A special emphasis has been given by researchers on Alzheimer's disease monitoring to the detriment of other amyloidosis that are underexploited despite their societal relevance (e.g., Parkinson's disease). There are still important obstacles to overcome in order to selectively detect the various peptide isoforms and soluble amyloid species associated with Alzheimer's disease. Furthermore, MOF contrast agents for imaging peptide soluble oligomers in living humans are also scarce (if not nonexistent), and action in this direction is unquestionably required to clarify the contentious link between the amyloidogenic species and the disease, guiding research toward the most promising therapeutic strategies.

Detection of β-amyloid peptide aggregates by quartz crystal microbalance based on dual-aptamer assisted signal amplification

β-amyloid peptide (Aβ) aggregates are regarded as a typical neuropathology hallmark for the diagnosis of Alzheimer's disease (AD). Aβ₄₀ aggregates include soluble oligomers (Aβ₄₀O) and insoluble fibrils (Aβ₄₀F). Both of them can simultaneously bind to two different kinds of its aptamer (Apt1 and Apt2). As a mass-sensitive sensing platform, quartz crystal microbalance (QCM) converts changes in mass on the Au chip surface into frequency shift. Here, a dual-aptamer assisted Aβ₄₀ aggregates assay was developed. Taking Aβ₄₀O detection as an example, Apt2 was modified on the surface of Au chip by Au-S bond. Subsequently, the solution consisted of Aβ₄₀O and gold nanoparticles-Apt1 (AuNPs-Apt1) were injected into the QCM chamber. As a result, Aβ₄₀O was specifically recognized and captured by Apt2. AuNPs-Apt1 were also combined on the surface of the Au chip because Aβ₄₀O can simultaneously bind to Apt1. Then, a significant frequency shift occurred because of the large weight of AuNPs. Similarly, this procedure can be used to detect Aβ₄₀F. This QCM biosensor was able to detect Aβ₄₀O with a range of 0.2-10 pM with a detection limit of 0.11 pM, while the linear range for Aβ₄₀F was 0.1-10 pM with a detection limit of 0.02 pM. This QCM biosensor was simple and highly sensitive, which provided a new method for Aβ₄₀ aggregates detection.

Optimization of Functional Group Concentration of N, N-Dimethylacrylamide-based Polymeric Coatings and Probe Immobilization for DNA and Protein Microarray Applications

We report here a deep investigation into the effect of the concentration of a polymeric coating's functional groups on probe density immobilization with the aim of establishing the optimal formulation to be implemented in specific microarray applications. It is widely known that the ideal performance of a microarray strictly depends on the way probes are tethered to the surface since it influences the way they interact with the complementary target. The N, N-dimethylacrylamide-based polymeric coating introduced by our research group in 2004 has already proven to offer great flexibility for the customization of surface properties; here, we demonstrate that it also represents the perfect scaffold for the modulation of probe grafting. With this aim in mind, polymers with increasing concentrations of N-acryloyloxysuccinimide (NAS) were synthesized and the coating procedure optimized accordingly. These were then tested not only in DNA microarray assays, but also using protein probes (with different MWs) to establish which formulation improves the assay performance in specific applications. The flexibility of this polymeric platform allowed us also to investigate a different immobilization chemistry-specifically, click chemistry reactions, thanks to the insertion of azide groups into the polymer chains-and to evaluate possible differences generated by this modification.

Detection of Amyloid-β(1–42) Aggregation With a Nanostructured Electrochemical Sandwich Immunoassay Biosensor

Amyloid-β(1–42) [Aβ(1–42)] oligomer accumulations are associated with physiologic alterations in the brains of individuals with Alzheimer’s disease. In this study, we demonstrate that a nanostructured gold electrode with deposited gold nanoparticles, induced via electrochemical impedance spectroscopy (EIS), may be used as an Aβ(1–42) conformation biosensor for the detection of Alzheimer’s disease. Monoclonal antibodies (12F4) were immobilized on self-assembled monolayers of the electrochemical sandwich immunoassay biosensor to capture Aβ(1–42) monomers and oligomers. Western blot and fluorescence microscopy analyses were performed to confirm the presence of Aβ(1–42) monomers and oligomers. EIS analysis with an equivalent circuit model was used to determine the concentrations of different Aβ(1–42) conformations in this study. We identified conformations of Aβ(1–42) monomers and Aβ(1–42) oligomers using probe antibodies (12F4) by employing EIS. RAβ(1−42) indicates the sum resistance of impedance measured during Aβ(1–42) immobilization. ΔR12F4 refers to the concentration of probe antibody (12F4) binding with Aβ(1–42). The concentration of Aβ(1–42) oligomer was defined as the percentage of Aβ(1–42) aggregation R12F4/RAβ(1−42). The experimental results show that the biosensor has high selectivity to differentiate Aβ(1–40) and Aβ(1–42) monomers and Aβ(1–42) oligomers and that it can detect Aβ(1–42) oligomer accurately. The linear detection range for Aβ(1–42) oligomers was between 10 pg/ml and 100 ng/ml. The limit of detection was estimated to be 113 fg/ml.

A β-Amyloid(1-42) Biosensor Based on Molecularly Imprinted Poly-Pyrrole for Early Diagnosis of Alzheimer's Disease

Background:

Alzheimer’s disease (AD) is a common form of dementia, characterized by production and deposition of β-amyloid peptide in the brain. Thus, β-amyloid peptide is a potentially promising biomarker used to diagnose and monitor the progression of AD.

Objective:

The study aims to develop a biosensor based on a molecularly imprinted poly-pyrrole for detection of β-amyloid.

Material and Methods:

In this experimental study, an imprinted poly-pyrrole was employed as an artificial receptor synthesized by electro-polymerization of pyrrole on screen-printed carbon electrodes in the presence of β-amyloid. β-amyloid acts as a molecular template within the polymer. The biosensor was evaluated by cyclic voltammetry using ferro/ferricyanide marker. The parameters influencing the biosensor performance, including electro-polymerization cycle umbers and β-amyloid binding time were optimized to achieve the best biosensor sensitivity.

Results:

The β-amyloid binding affinity with the biosensor surface was evaluated by the Freundlich isotherm, and Freundlich constant and exponent were obtained as 0.22 ng mL^-1 and 10.60, respectively. The biosensor demonstrated a detection limit of 1.2 pg mL^-1. The biosensor was applied for β-amyloid determination in artificial cerebrospinal fluid.

Conclusion:

The biosensor is applicable for early Alzheimer’s disease detection.

Graphene based electrochemical immunosensor for the ultra-sensitive label free detection of Alzheimer's beta amyloid peptides Aβ(1-42)

An immunosensor capable of high sensitivity detection of beta-amyloid peptides, shown to be a reliable biomarker for Alzheimer's disease, has been developed using screen printed graphene electrodes (SPGEs) modified with ultra-thin layers of polymerised 1,5-diaminonaphthalene (pDAN). Electropolymerization of 1,5-diaminonaphthalene (DAN) was performed to coat the graphene screen printed electrodes in a continuous polymer layer with controlled thickness. The surface characteristics of pristine graphene and polymer modified graphene electrodes were examined using Raman and X-ray photoelectron spectroscopy. The effects of polymer thickness on the electron transfer rates were investigated. An immunosensor for selective detection of beta amyloid peptides Aβ(1-42) was developed via biofunctionalization of the pDAN modified SPGE with the anti-beta amyloid antibody used as the peptide bioreceptor. The immunosensor has been used for specific detection of Aβ(1-42) with a linear range of 1 pg mL^-1 to 1000 pg mL^-1 and showed 1.4 pg mL^-1 and 4.25 pg mL^-1 detection and quantification limit, respectively. The biosensor was further validated for the analysis of spiked human plasma. The immunosensor enables rapid, accurate, precise, reproducible and highly sensitive detection of Aβ(1-42) using a low-cost SPGE platform, which opens the possibilities for diagnostic ex vivo applications and research-based real time studies.

Nanoconstructs as a versatile tool for detection and diagnosis of Alzheimer biomarkers

The current review focuses towards the advancements made in the past decade in the field of nanotechnology for the early Alzheimer's disease (AD) diagnosis. This review includes the application of nanomaterials and nanosensors for the early detection of the main AD biomarkers (amyloid beta, phosphorylated tau, apolipoprotein E4 allele or APOE4, microRNAs, cholesterol, hydrogen peroxide etc) in biological fluids, to detect the biomarkers at a very low concentration ranging in pico, femto and even atto molar concentrations. The field of drug development has always aimed and is constantly working on developing disease modifying drugs, but these drugs will only succeed when given in the early disease stages. Thus, developing efficient diagnostic tools is of vital importance. Various nanomaterials such as liposomes; dendrimers; polymeric nanoparticles; coordination polymers; inorganic nanoparticles such as silica, manganese oxide, zinc oxide, iron oxide, super paramagnetic iron oxides; quantum dots, silver nanoparticles, gold nanoparticles, and carbon based nanostructures (carbon nanotubes, graphene oxide, nanofibres, nanodiamonds, carbon dots); Up-conversion nanoparticles; 2D nanomaterials; and radioactive nanoprobes have been used in constructing and improving efficiency of nano-sensors for AD biosensing at an early stage of diagnosis.

Flexible nanopillar-based immunoelectrochemical biosensor for noninvasive detection of Amyloid beta

The noninvasive early detection of biomarkers for Alzheimer's disease (AD) is essential for the development of specific treatment strategies. This paper proposes an advanced method for fabricating highly ordered and flexible nanopillar-based electrochemical biosensors by the combination of soft/photolithography and metal evaporation. The nanopillar array (NPA) exhibits high surface area containing 1500 nm height and 500 nm diameter with 3:1 ratio. In regard with physical properties of polyurethane (PU) substrate, the developed NPA is sustainable and durable to external pressure such as bending and twisting. To manipulate the NPA surface to biocompatible, the gold was uniformly deposited on the PU substrate. The thiol chemistry which is stably modified on the gold surface as a form of self-assembled monolayer was employed for fabricating the NPA as a biocompatible chip by covalently immobilize the antibodies. The proposed nanopillar-based immunoelectrochemical biosensor exhibited good and stable electrochemical performance in β-amyloid (Aβ) detection. Moreover, we successfully confirmed the performance of the as-developed sensor using the artificial injection of Aβ in human tear, with sensitivity of 0.14 ng/mL and high reproducibility (as a standard deviation below 10%). Our findings show that the developed nanopillar-based sensor exhibits reliable electrochemical characteristics and prove its potential for application as a biosensor platform for testing at the point of care.

A Probeless Capacitive Biosensor for Direct Detection of Amyloid Beta 1-42 in Human Serum Based on an Interdigitated Chain-Shaped Electrode

Amyloid beta (aβ) 1-42, a peptide that is 1-42 amino acids long, is a major component of senile plaques in the brains of patients with Alzheimer's disease. Aβ detection has become an essential antecedence to predict the declining mental abilities of patients. In this paper, a probeless capacitive biosensor for the non-Faradaic detection of aβ 1-42 peptide was developed by immobilizing a specific anti-aβ antibody onto a self-assembled monolayer functionalized interdigitated chain-shaped electrode (anti-aβ/SAM/ICE). The novelty and difference of this article from previous studies is the direct detection of aβ peptide with no redox probe ((Fe(CN)6)3-/4-), which can avoid the denaturation of the protein caused by the metallization (binding of aβ to metal ion Fe which is presented in the redox couple). The direct detection of aβ with no redox probe is performed by non-Faradaic capacitive measurement, which is greatly different from the Faradaic measurement of the charge transfer resistance of the redox probe. The detection of various aβ 1-42 peptide concentrations in human serum (HS) was performed by measuring the relative change in electrode interfacial capacitance due to the specific antibody-aβ binding. Capacitance change in the anti-aβ/SAM/ICE biosensor showed a linear detection range between 10 pg mL-1 and 104 pg mL-1, and a detection limit of 7.5 pg mL-1 in HS, which was much lower than the limit of detection for CSF aβ 1-42 (~500 pg mL-1) and other biosensors. The small dissociation constant Kd of the antibody-antigen interaction was also found to be 0.016 nM in HS, indicating the high binding affinity of the anti-aβ/SAM/ICE biosensor in the recognizing of aβ 1-42. Thus, the developed sensor can be used for label-free and direct measurement of aβ 1-42 peptide and for point-of-care diagnosis of Alzheimer's disease without redox probe.

Hollow core photonic crystal fiber-assisted Raman spectroscopy as a tool for the detection of Alzheimer's disease biomarkers

SIGNIFICANCE

Alzheimer's disease (AD) is an irreversible and progressive disorder that damages brain cells and impairs the cognitive abilities of the affected. Developing a sensitive and cost-effective method to detect Alzheimer's biomarkers appears vital in both a diagnostic and therapeutic perspective.

AIM

Our goal is to develop a sensitive and reliable tool for detection of amyloid β (1-42) peptide (Aβ42), a major AD biomarker, using fiber-enhanced Raman spectroscopy (FERS).

APPROACH

A hollow core photonic crystal fiber (HCPCF) was integrated with a conventional Raman spectroscopic setup to perform FERS measurements. FERS was then coupled with surface-enhanced Raman spectroscopy (SERS) to further amplify the Raman signal thanks to a combined FERS-SERS assay.

RESULTS

A minimum 20-fold enhancement of the Raman signal of Aβ42 as compared to a conventional Raman spectroscopy scheme was observed using the HCPCF-based light delivery system. The signal was further boosted by decorating the fiber core with gold bipyramids generating an additional SERS effect, resulting in an overall 200 times amplification.

CONCLUSIONS

The results demonstrate that the use of an HCPCF-based platform can provide sharp and intense Raman signals of Aβ42, in turn paving the way toward the development of a sensitive label-free detection tool for early diagnosis of AD.

On-Chip Detection of the Biomarkers for Neurodegenerative Diseases: Technologies and Prospects

Alzheimer's disease (AD), Parkinson's disease (PD) and glaucoma are all regarded as neurodegenerative diseases (neuro-DDs) because these diseases are highly related to the degeneration loss of functions and death of neurons with aging. The conventional diagnostic methods such as neuroimaging for these diseases are not only expensive but also time-consuming, resulting in significant financial burdens for patients and public health challenge for nations around the world. Hence early detection of neuro-DDs in a cost-effective and rapid manner is critically needed. For the past decades, some chip-based detection technologies have been developed to address this challenge, showing great potential in achieving point-of-care (POC) diagnostics of neuro-DDs. In this review, chip-based detection of neuro-DDs' biomarkers enabled by different transducing mechanisms is evaluated.

A label-free biosensor based on graphene and reduced graphene oxide dual-layer for electrochemical determination of beta-amyloid biomarkers

A label-free biosensor is developed for the determination of plasma-based Aβ_1–42 biomarker in Alzheimer’s disease (AD). The platform is based on highly conductive dual-layer of graphene and electrochemically reduced graphene oxide (rGO). The modification of dual-layer with 1-pyrenebutyric acid N-hydroxysuccinimide ester (Pyr-NHS) is achieved to facilitate immobilization of H31L21 antibody. The effect of these modifications were studied with morphological, spectral and electrochemical techniques. The response of the biosensor was evaluated using differential pulse voltammetry (DPV). The data was acquired at a working potential of ~ 180 mV and a scan rate of 50 mV s⁻¹. A low limit of detection (LOD) of 2.398 pM is achieved over a wide linear range from 11 pM to 55 nM. The biosensor exhibits excellent specificity over Aβ_1–40 and ApoE ε4 interfering species. Thus, it provides a viable tool for electrochemical determination of Aβ_1–42. Spiked human and mice plasmas were used for the successful validation of the sensing platform in bio-fluidic samples. The results obtained from mice plasma analysis concurred with the immunohistochemistry (IHC) and magnetic resonance imaging (MRI) data obtained from brain analysis.

Graphical abstract

Biosensors on the road to early diagnostic and surveillance of Alzheimer's disease

Alzheimer's disease is a debilitating and largely untreatable condition with subtle onset and slow progression over an extensive period of time, which culminate in increasing levels of disability. As Alzheimer's disease prevalence is expected to grow exponentially in the upcoming decades, there is an urgency to develop analytical technologies for the sensitive, reliable and cost-effective detection of Alzheimer's disease biomarkers. Biosensors are powerful analytical devices that translate events of biological recognition on physical or chemical transducers into electrical, thermal or optical signals. The high sensitivity and selectivity of biosensors associated with easy, rapid and low-cost determination of analytes have made this discipline one of the most intensively studied in the past decades. This review centers on recent advances, challenges and trends of Alzheimer's disease biosensing particularly in the effort to combine the unique properties of nanomaterials with biorecognition elements. In the last decade, impressive progresses have been made towards the development of biosensors, mainly electrochemical and optical, for detection of Alzheimer's disease biomarkers in the pico- and femto-molar range. Nonetheless, advances in multiplexed detection, robustness, stability and specificity are still necessary to ensure an accurate and differentiated diagnosis of this disease.

Nanomaterials towards Biosensing of Alzheimer's Disease Biomarkers

Alzheimer's disease (AD) is an incurable and highly debilitating condition characterized by the progressive degeneration and/or death of nerve cells, which leads to manifestation of disabilities in cognitive functioning. In recent years, the development of biosensors for determination of AD's main biomarkers has made remarkable progress, particularly based on the tremendous advances in nanoscience and nanotechnology. The unique and outstanding properties of nanomaterials (such as graphene, carbon nanotubes, gold, silver and magnetic nanoparticles, polymers and quantum dots) have been contributing to enhance the electrochemical and optical behavior of transducers while offering a suitable matrix for the immobilization of biological recognition elements. Therefore, optical and electrochemical immuno- and DNA-biosensors with higher sensitivity, selectivity and longer stability have been reported. Nevertheless, strategies based on the detection of multiple analytes still need to be improved, as they will play a crucial role in minimizing misdiagnosis. This review aims to provide insights into the conjugation of nanomaterials with different transducers highlighting their crucial role in the construction of biosensors for detection of AD main biomarkers.

Highly sensitive quantification of Alzheimer's disease biomarkers by aptamer-assisted amplification

Alzheimer's disease (AD), a chronic neurodegenerative disease associated with the loss of neurons in the brain, is the most pervasive type of dementia; 47 million people are affected, and the number is expected to increase to more than 131 million by 2050, according to Alzheimer's Disease International. Both early diagnosis and continuous monitoring are crucial for early intervention, symptomatic treatment, monitoring of the efficacy of intervention and improved patient function. Beta-amyloid peptide, tau, and phosphorylated tau are useful for screening and diagnosis; meanwhile, simultaneous assessment of multiple biomarkers is of paramount importance for accurate disease diagnosis.

Methods: Herein, we report a direct, inexpensive and ultrasensitive aptamer-based multiplex assay for the quantification of trace amounts of AD biomarkers in both human serum and cerebrospinal fluid (CSF) samples. In this newly developed assay, molecular recognition of an antibody-aptamer pair provides high specificity in target detection, and the use of a DNA amplification strategy affords high sensitivity, allowing quantification of AD biomarkers in both biological fluids in 1.5 h with only a diminutive amount of the sample consumed. A tailor-made turn-on fluorophore, namely, SPOH, was employed to label the antibody-aptamer hybrids and provide a strong fluorescence signal, which was then detected with a total internal reflection fluorescence microscopy electron-multiplying charge-coupled device (TIRFM-EMCCD) imaging system. The simultaneous detection of biomarkers was achieved by a direct shape-coded method in which the nanoplatforms can be distinguished from one another by their morphologies.

Results: This assay demonstrated a lower detection limit (in the femtomolar range) for AD biomarkers than the previously reported antibody-antibody method.

Conclusion: The developed assay holds tremendous clinical potential for early diagnosis of AD and monitoring of its progression.

A novel sandwich-type photoelectrochemical immunosensor based on Ru(bpy)32+ and Ce-CdS co-sensitized hierarchical ZnO matrix and dual-inhibited polystyrene@CuS-Ab2 composites

A novel and sensitive sandwich-type photoelectrochemical (PEC) immunosensor was developed for the quantitative detection of β-amyloid protein (Aβ). A ITO electrode was sequentially coated with hierarchical porous zinc oxide (ZnO) microspheres with a large specific area, sensitized with tris(bipyridine)ruthenium(II) ion (Ru(bpy)₃²⁺) to achieve high visible light absorption, and modified with cerium-doped cadmium sulfide (Ce-CdS) nanoparticles to enhance the PEC response. Under the stimulation of visible light and ascorbic acid as an efficient electron donor, the photoelectric signal of ZnO/Ru(bpy)₃²⁺/Ce-CdS was 70 times that of pure ZnO. The amino-functionalized polystyrene (PS) microspheres coated with copper sulfide (CuS) was linked with a secondary antibody (Ab₂) for the first time for the Aβ detection by the immunosensor. The good insulation and steric resistance of the as-prepared polystyrene@CuS-Ab₂ (PS@CuS-Ab₂) composite significantly weakened the photocurrent response of the immunosensor in the specific immune recognition. Under the optimal conditions, the quantitative detection of Aβ was achieved within the range of 0.001-100 ng/mL with the detection limit of 0.37 pg/mL. In addition, the PEC immunosensor is easy to make, stable and selective, which has provided a good experimental platform for the detection of disease biomarkers.

Towards precision medicine: the role and potential of protein and peptide microarrays

Techniques to comprehensively analyze protein signatures are pivotal to unravel disease mechanisms, develop novel biomarkers and targeted therapies. In this frame, protein and peptide microarrays can play a major role in fuelling precision medicine.

Quantum dots-based sandwich immunoassay for sensitive detection of Alzheimer's disease-related Aβ1-42

Amyloid-beta peptide 1-42 (Aβ_1-42) is known as a component of amyloid plaques in association with Alzheimer's disease. Herein, we developed a reliable and remarkably sensitive sandwich immunoassay to detect the Aβ_1-42 using quantum dots (QDs) as fluorescent label. In the presence of Aβ_1-42, the biotinylated Anti-beta Amyloid 1-16 (N-Ab) recognized the target and formed C-Ab-Aβ_1-42-N-Ab sandwich immunocomplexes. Then Streptavidin-QDs conjugated to biotinylated N-Ab and the concentration of Aβ_1-42 was determined by detecting the fluorescence intensity in the supernatant. This method is faster and more efficient than the previous approach we reported. It also has reasonable sensitivity and selectivity. Under the optimized conditions, the linear range is 5.0 to 100 pM (0.023-0.45 ng/mL) and the detection limit is 1.7 pM (7.6 pg/ mL). In addition, this method has been successfully applied to detect the Aβ_1-42 in human cerebrospinal fluid sample.

An impedimetric micro-immunosensing assay to detect Alzheimer's disease biomarker: Aβ40

A miniaturized biosensing platform, based on monoclonal amyloid-beta antibodies (mAβ_ab) that were immobilized on a disc-shaped platinum/iridium (Pt/Ir) microelectrode surface coupled with an impedimetric signal transducer, was developed for the label-free and sensitive detection of amyloid-beta peptide fragment 1-40 (Aβ40); a reliable biomarker for early diagnosis of Alzheimer's disease (AD). A Pt/Ir microelectrode was electropolymerized with poly (ortho-phenylenediamine), a conducting free amine-containing aromatic polymer; followed by crosslinking with glutaraldehyde for subsequent coupling of mAβ_ab on the microelectrode surface. This modification strategy efficiently improved the impedimetric detection performance of Aβ40 in terms of charge transfer resistance (∼400-fold difference) and normalized impedance magnitude percentage change (∼40% increase) compared with a passive adsorption-based immobilization method. The sensitivity of the micro-immunosensing assay was found to be 1056 kΩ/(pg/mL)/cm² and the limit of detection was found to be 4.81 pg/mL with a dynamic range of 1-10⁴ pg/mL (R² = 0.9932). The overall precision of the assay, as measured by relative standard deviation, ranged from 0.84 to 5.15%, demonstrating its reliability and accuracy; while in respect to assay durability and stability, the immobilized mAβ_ab were able to maintain 80% of their binding activity to Aβ40 after incubation for 48 h at ambient temperature (25 °C). To validate the practical applicability, the assay was tested using brain tissue lysates prepared from AD-induced rats. Results indicate that the proposed impedimetric micro-immunosensing platform is highly versatile and adaptable for the quantitative detection of other disease-related biomarkers.

Rapid multiplexed detection of beta-amyloid and total-tau as biomarkers for Alzheimer's disease in cerebrospinal fluid

This paper reports the multiplexed monitoring of two promising biomarkers, beta-amyloid (Aβ42) and total tau (T-tau), in both buffer and cerebrospinal fluid (CSF) for Alzheimer's disease (AD) using label-free optical nanosensors. It has been found that 7.8 pg/ml of Aβ42 in buffer and 15.6 pg/ml of T-au in buffer can be readily detected with very good specificity. Based on our measurements, the purchased CSF itself contains Aβ42, whose concentration is estimated to be about 400 pg/ml. Aβ42 and T-tau in the mixtures of Aβ42 and T-tau spiked in CSF have been detected successfully, indicating the feasibility of the optical nanosensors to detect these biomarkers in clinical samples. For the measurements, only a small amount (~1 μl) of the samples is required. This type of sensor is suitable for point-of-care application to diagnose the AD due to its low cost and ease-of-operation.

Emerging Biosensing Technologies for Neuroinflammatory and Neurodegenerative Disease Diagnostics

Neuroinflammation plays a critical role in the onset and progression of many neurological disorders, including Multiple Sclerosis, Alzheimer's and Parkinson's diseases. In these clinical conditions the underlying neuroinflammatory processes are significantly heterogeneous. Nevertheless, a common link is the chronic activation of innate immune responses and imbalanced secretion of pro and anti-inflammatory mediators. In light of this, the discovery of robust biomarkers is crucial for screening, early diagnosis, and monitoring of neurological diseases. However, the difficulty to investigate biochemical processes directly in the central nervous system (CNS) is challenging. In recent years, biomarkers of CNS inflammatory responses have been identified in different body fluids, such as blood, cerebrospinal fluid, and tears. In addition, progress in micro and nanotechnology has enabled the development of biosensing platforms capable of detecting in real-time, multiple biomarkers in clinically relevant samples. Biosensing technologies are approaching maturity where they will become deployed in community settings, at which point screening programs and personalized medicine will become a reality. In this multidisciplinary review, our goal is to highlight both clinical and recent technological advances toward the development of multiplex-based solutions for effective neuroinflammatory and neurodegenerative disease diagnostics and monitoring.

Ultra-sensitive detection of protein biomarkers for diagnosis of Alzheimer's disease

Beta amyloid peptide, tau, and phosphorylated tau are well recognized as promising biomarkers for the diagnosis of Alzheimer's disease (AD). In this work, we developed a direct, versatile, and ultrasensitive multiplex assay for the quantification of trace amounts of these protein biomarkers for AD in different types of biological fluids including cerebrospinal fluid, serum, saliva, and urine. The detection assay is based on the immunoreaction between the target proteins and their corresponding pair of antibodies followed by fluorescence labelling with a newly developed indolium-based turn-on fluorophore, namely SIM. SIM was tailor-made as a reporter to provide a high signal-to-noise ratio for the detection assay. An exceptionally low limit of detection down to the femto-molar level was achieved in this assay with minute consumption of the sample. This versatile detection assay is capable of reliably quantifying not only the target proteins simultaneously from a CSF sample in an hour but also trace amounts of protein biomarkers in saliva and urine. This assay has a high potential to serve as a practical tool for the diagnosis of AD.

Graphene quantum dots for detecting monomeric amyloid peptides

Detection of the concentration of amyloid monomers is of great importance in the diagnosis of amyloidogenesis. Herein, we propose a method to detect the concentration of amyloid β (Aβ) peptide monomers by utilizing the fluorescence characteristics of graphene quantum dots (GQDs). The linear dependence of the photoluminescence (PL) intensity of GQDs on the Aβ monomer concentration can be identified. It can be further illustrated that both monomeric and fibrillar Aβ peptides can be monitored by using GQDs. Conventional fluorescent dyes, such as thioflavin T (ThT), usually undergo co-incubation with amyloid peptides, which could lead to disturbance of the aggregation because of their inhibitory effect. Similar Aβ aggregation dynamics observed by using GQDs and ThT demonstrated the feasibility of the GQD-based detection method without co-incubation with soluble amyloid peptide monomers. The utilization of GQDs as a novel probe monitoring amyloid monomers could be applied in pathological detection and diagnosis of degenerative diseases and other conformational disorders.

Nano-biosensors to detect beta-amyloid for Alzheimer's disease management

Beta-amyloid (β-A) peptides are potential biomarkers to monitor Alzheimer's diseases (AD) for diagnostic purposes. Increased β-A level is neurotoxic and induces oxidative stress in brain resulting in neurodegeneration and causes dementia. As of now, no sensitive and inexpensive method is available for β-A detection under physiological and pathological conditions. Although, available methods such as neuroimaging, enzyme-linked immunosorbent assay (ELISA), and polymerase chain reaction (PCR) detect β-A, but they are not yet extended at point-of-care (POC) due to sophisticated equipments, need of high expertize, complicated operations, and challenge of low detection limit. Recently, β-A antibody based electrochemical immuno-sensing approach has been explored to detect β-A at pM levels within 30-40 min compared to 6-8h of ELISA test. The introduction of nano-enabling electrochemical sensing technology could enable rapid detection of β-A at POC and may facilitate fast personalized health care delivery. This review explores recent advancements in nano-enabling electrochemical β-A sensing technologies towards POC application to AD management. These analytical tools can serve as an analytical tool for AD management program to obtain bio-informatics needed to optimize therapeutics for neurodegenerative diseases diagnosis management.

Combined mass quantitation and phenotyping of intact extracellular vesicles by a microarray platform

The interest towards extracellular vesicles (EVs) has grown exponentially over the last few years; being involved in intercellular communication and serving as reservoirs for biomarkers for tumors, they have a great potential for liquid biopsy development, possibly replacing many costly and invasive tissue biopsies. Here we propose, for the first time, the use of a Si/SiO2 interferometric, microarray platform for multiparametric intact EVs analysis combining label-free EVs mass quantitation and high sensitivity fluorescence based phenotyping. Label free interferometric measurement allows to quantify the amount of vesicles captured by printed antibodies while, on the same chip, EVs are also detected by fluorescence in a sandwich immunoassay. The proposed method simultaneously detects, quantify and phenotype intact EVs in a microarray format.

Magnetically-actuated, bead-enhanced silicon photonic immunosensor

Magnetic actuation has been introduced to an optical immunosensor technology resulting in improvements in both rapidity and limit of detection for an assay quantitating low concentrations of a representative protein biomarker. For purposes of demonstration, an assay was designed for monocyte chemotactic protein 1 (MCP-1), a small cytokine which regulates migration and infiltration of monocytes and macrophages, and is an emerging biomarker for several diseases. The immunosensor is based on arrays of highly multiplexed silicon photonic microring resonators. A one-step sandwich immunoassay was performed and the signal was further enhanced through a tertiary recognition event between biotinylated tracer antibodies and streptavidin-coated magnetic beads. By integrating a magnet under the sensor chip, magnetic beads were rapidly directed towards the sensor surface resulting in improved assay performance metrics. Notably, the time required in the bead binding step was reduced by a factor of 11 (4 vs 45 min), leading to an overall decrease in assay time from 73 min to 32 min. The magnetically-actuated assay also lowered the limit of detection (LOD) for MCP-1 from 124 pg mL^-1 down to 57 pg mL^-1. In sum, the addition of magnetic actuation into bead-enhanced sandwich assays on a silicon photonic biosensor platform might facilitate improved detection of biomarkers in point-of-care diagnostics settings.

Magneto-immunocapture with on-bead fluorescent labeling of amyloid-β peptides: towards a microfluidized-bed-based operation

Novel combination of Magneto-immunocapture and on-beads fluorescent labeling of Aβ peptides for their sensitive determination in cerebro spinal fluid samples.

Chemical engineering of self-assembled Alzheimer's peptide on a silanized silicon surface

The aim of this work is to develop a sensitive and specific immune-sensing platform dedicated to the detection of potential biomarkers of Alzheimer's disease (AD) in biological fluids. Accordingly, a controlled and adaptive surface functionalization of a silicon wafer with 7-octenyltrichlorosilane has been performed. The surface has extensively been characterized by atomic force microscopy (AFM; morphology) and X-ray photoelectron spectroscopy (XPS; chemical composition) and contact angle measurements. The wettability of the grafted chemical groups demonstrated the gradual trend from hydrophilic to hydrophobic surface during functionalization. XPS evidenced the presence of silanes on the surface after silanization, and even carboxylic groups as products from the oxidation step of the functionalization process. The characterization results permitted us to define an optimal protocol to reach a high-quality grafting yield. The issue of the quality of controlled chemical preparation on bioreceiving surfaces was also investigated by the recognition of one AD biomarker, the amyloid peptide Aβ 1-42. We have therefore evaluated the biological activity of the grafted anti Aβ antibodies onto this silanized surface by fluorescent microscopy. In conclusion, we have shown, both qualitatively and quantitatively, the uniformity of the optimized functionalization on slightly oxidized silicon surfaces, providing a reliable and chemically stable procedure to determine specific biomarkers of Alzheimer disease. This work opens the route to the integration of controlled immune-sensing applications on lab-on-chip systems.