Gelsolin bound β-amyloid peptides(1-40/1-42): electrochemical evaluation of levels of soluble peptide associated with Alzheimer's disease

Recent Advancements in Biosensors for the Detection and Characterization of Amyloids: A Review

Modern medicine has increased the human lifespan. However, with an increase in average lifespan risk of amyloidosis increases. Amyloidosis is a condition characterized by protein misfolding and aggregation. Early detection of amyloidosis is crucial, yet conventional diagnostic methods are costly and lack precision, necessitating innovative tools. This review explores recent advancements in diverse amyloid detection methodologies, highlighting the need for interdisciplinary research to develop a miniaturized electrochemical biosensor leveraging nanotechnology. However, the diagnostics industry faces obstacles such as skilled labor shortages, standardized selection processes, and concurrent multi-analyte identification challenges. Research efforts are focused on integrating electrochemical techniques into clinical applications and diagnostics, with the successful transition of miniaturized technologies from development to testing posing a significant hurdle. Label-free transduction techniques like voltammetry and electrochemical impedance spectroscopy (EIS) have gained traction due to their rapid, cost-effective, and user-friendly nature.

Classification of binding property of amyloid β to lipid membranes: Membranomic research using quartz crystal microbalance combined with the immobilization of lipid planar membranes

A biomembrane-related fibrillogenesis of Amyloid β from Alzheimer' disease (Aβ) is closely related to its accumulation behavior. A binding property of Aβ peptides from Alzheimer' disease to lipid membranes was then classified by a quartz crystal microbalance (QCM) method combined with an immobilization technique using thiol self-assembled membrane. The accumulated amounts of Aβ, Δfmax, was determined from the measurement of the maximal frequency reduction using QCM. The plots of Δfmax to Aβ concentration gave the slope and saturated value of Δfmax, (Δfmax)sat that are the parameters for binding property of Aβ to lipid membranes. Therefore, the Aβ-binding property on lipid membranes was classified by the slope and (Δfmax)sat. The plural lipid system was described as X + Y where X = L1, L1/L2, and L1/L2/L3. The slope and (Δfmax)sat values plotted as a function of mixing ratio of Y to X was classified on a basis of the lever principle (LP). The LP violation observed in both parameters resulted from the formation of the crevice or pothole, as Aβ-specific binding site, generated at the boundary between ld and lo phases. The LP violation observed only in the slope resulted from glycolipid-rich domain acting as Aβ-specific binding site. Furthermore, lipid planar membranes indicating strong LP violation favored strong fibrillogenesis. Especially, lipid planar membranes indicating the LP violation only in the slope induced lateral aggregated and spherulitic fibrillar aggregates. Thus, the classification of Aβ binding property on lipid membranes appeared to be related to the fibrillogenesis with a certain morphology.

GSN gene frameshift mutations in Alzheimer's disease

BACKGROUND

The pathogenic missense mutations of the gelsolin (GSN) gene lead to familial amyloidosis of the Finnish type (FAF); however, our previous study identified GSN frameshift mutations existed in patients with Alzheimer's disease (AD). The GSN genotype-phenotype heterogeneity and the role of GSN frameshift mutations in patients with AD are unclear.

METHOD

In total, 1192 patients with AD and 1403 controls were screened through whole genome sequencing, and 884 patients with AD were enrolled for validation. Effects of GSN mutations were evaluated in vitro. GSN, Aβ42, Aβ40 and Aβ42/40 were detected in both plasma and cerebrospinal fluid (CSF).

RESULTS

Six patients with AD with GSN P3fs and K346fs mutations (0.50%, 6/1192) were identified, who were diagnosed with AD but not FAF. In addition, 13 patients with AD with GSN frameshift mutations were found in the validation cohort (1.47%, 13/884). Further in vitro experiments showed that both K346fs and P3fs mutations led to the GSN loss of function in inhibiting Aβ-induced toxicity. Moreover, a higher level of plasma (p=0.001) and CSF (p=0.005) GSN was observed in AD cases than controls, and a positive correlation was found between the CSF GSN and CSF Aβ42 (r=0.289, p=0.009). Besides, the GSN level was initially increasing and then decreasing with the disease course and cognitive decline.

CONCLUSIONS

GSN frameshift mutations may be associated with AD. An increase in plasma GSN is probably a compensatory reaction in AD, which is a potential biomarker for early AD.

Surface Plasmon Resonance Biosensors with Magnetic Sandwich Hybrids for Signal Amplification

The conventional signal amplification strategies for surface plasmon resonance (SPR) biosensors involve the immobilization of receptors, the capture of target analytes and their recognition by signal reporters. Such strategies work at the expense of simplicity, rapidity and real-time measurement of SPR biosensors. Herein, we proposed a one-step, real-time method for the design of SPR biosensors by integrating magnetic preconcentration and separation. The target analytes were captured by the receptor-modified magnetic nanoparticles (MNPs), and then the biotinylated recognition elements were attached to the analyte-bound MNPs to form a sandwich structure. The sandwich hybrids were directly delivered to the neutravidin-modified SPR fluidic channel. The MNPs hybrids were captured by the chip through the neutravidin-biotin interaction, resulting in an enhanced SPR signal. Two SPR biosensors have been constructed for the detection of target DNA and beta-amyloid peptides with high sensitivity and selectivity. This work, integrating the advantages of one-step, real-time detection, multiple signal amplification and magnetic preconcentration, should be valuable for the detection of small molecules and ultra-low concentrations of analytes.

Potentiometric Biosensor Based on Artificial Antibodies for an Alzheimer Biomarker Detection

This paper presents a potentiometric biosensor for the detection of amyloid β-42 (Aβ-42) in point-of-care analysis. This approach is based on the molecular imprint polymer (MIP) technique, which uses covalently immobilised Aβ-42 to create specific detection cavities on the surface of single-walled carbon nanotubes (SWCNTs). The biosensor was prepared by binding Aβ-42 to the SWCNT surface and then imprinting it by adding acrylamide (monomer), N,N’-methylene-bis-acrylamide (crosslinker) and ammonium persulphate (initiator). The target peptide was removed from the polymer matrix by the proteolytic action of an enzyme (proteinase K). The presence of imprinting sites was confirmed by comparing a MIP-modified surface with a negative control (NIP) consisting of a similar material where the target molecule had been removed from the process. The ability of the sensing material to rebind Aβ-42 was demonstrated by incorporating the MIP material as an electroactive compound in a PVC/plasticiser mixture applied to a solid conductive support of graphite. All steps of the synthesis of the imprinted materials were followed by Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The analytical performance was evaluated by potentiometric transduction, and the MIP material showed cationic slopes of 75 mV-decade−1 in buffer pH 8.0 and a detection limit of 0.72 μg/mL. Overall, potentiometric transduction confirmed that the sensor can discriminate Aβ-42 in the presence of other biomolecules in the same solution.

Ultrasensitive capacitance sensor to detect amyloid-beta 1-40 in human serum using supramolecular recognition of β-CD/RGO/ITO micro-disk electrode

In this paper, we developed a new ultrasensitive capacitance sensor for detection of amyloid beta 1-40 (aβ40) protein (one of Alzheimer's disease core biomarkers) in human serum based on the high supramolecular recognition of the β-cyclodextrin/reduced graphene oxide (β-CD/RGO) nanohybrid toward the anti-aβ40 antibody molecule. The sensor was established by immobilizing specific anti-aβ40 antibody onto the β-CD/RGO nanohybrid functionalized on indium tin oxide micro-disk electrode (anti-aβ40/β-CD/RGO/ITO). Detection of aβ40 in the human serum (HS) using the sensor anti-aβ40/β-CD/RGO/ITO is carried out by capacitance measurement without a redox probe to prevent protein denaturation, serving as a convenient strategy for point-of-care diagnosis. In comparison with other studies, the sensor shows a very low limit of detection of 0.69 fg mL^-1 in HS, demonstrating its ability for the ultrasensitive detection of aβ40. Using this sensor, the dissociation constant K_D of the binding interaction between anti-aβ40 and aβ40 in HS is found to be 2.9 × 10^-7 nM, indicating the high binding affinity of antibody-antigen and the suitability of the anti-aβ40/β-CD/RGO/ITO sensor for aβ40 protein detection. The good selectivity of the anti-aβ40/β-CD/RGO/ITO sensor in the presence of differential analytes was also performed in this paper.

Ultrasensitive detection of multiple Alzheimer's disease biomarkers by SERS-LFA

A lateral flow assay, based on Surface-enhanced Raman scattering nanotags (SERS-LFA), is proposed for simultaneous quantification of multiple AD biomarkers with high sensitivity.

Development of three-dimensional semi-solid hydrogel matrices for ratiometric fluorescence sensing of Amyloid β peptide and imaging in SH-SY5 cells: Improvement of point of care diagnosis of Alzheimer's disease biomarker

Alzheimer's is a neurodegenerative disease with high morbidity and mortality in the elderly, so, detection of its biomarker for definite diagnosis of Alzheimer's in the early stage of disease is a challenge. Amyloid beta peptide (Aβ) chosen as an Alzheimer's biomarker. Here, we developed novel, semi-solid, three-dimensional hydrogel matrices for ratiometric fluorescence detection of Aβ. This assay's great performance stems from the employment of a hybrid conjugate composed of Rhodamine B (RB), Carbon dots (CDs), and an Aβ probe entrapped in Polyvinyl alcohol (PVA), and then detection of fluorescence resonance energy transfer (FRET) that occurs in the presence of AuNP/target-Aβ, as a result of hybridization. The RB-CDs' fluorescence (at 582 nm and 675 nm under 430 nm excitation) is quenched in the presence of AuNPs, while the ratio of fluorescence (I₅₈₂/I₆₇₅) is increased by the addition of Aβ target, and shows a linear relationship in the range of 75 pM-250 nM, with a detection limit of 0.5 pM. Furthermore, the assay possesses strong selectivity for Aβ compared to other proteins, and different quantities of a human serum sample successfully analyzed with excellent sensitivity, satisfactory precision, and reliability. Due to distribution of Aβ in SH-SY5 human neuroblastoma cells, extending this UV-Vis-NIR full-range responsive CDs bio-probe to imaging of Aβ in cells. In both fixed and living SH-SY5 cells, the nanoprobe delivers a clear signal to the Aβ target. Because of its high sensitivity, selectivity, biocompatibility and affordability, this nanoprobe is a good option for early Alzheimer's disease diagnosis.

Extracellular protein components of amyloid plaques and their roles in Alzheimer's disease pathology

Alzheimer's disease (AD) is pathologically defined by the presence of fibrillar amyloid β (Aβ) peptide in extracellular senile plaques and tau filaments in intracellular neurofibrillary tangles. Extensive research has focused on understanding the assembly mechanisms and neurotoxic effects of Aβ during the last decades but still we only have a brief understanding of the disease associated biological processes. This review highlights the many other constituents that, beside Aβ, are accumulated in the plaques, with the focus on extracellular proteins. All living organisms rely on a delicate network of protein functionality. Deposition of significant amounts of certain proteins in insoluble inclusions will unquestionably lead to disturbances in the network, which may contribute to AD and copathology. This paper provide a comprehensive overview of extracellular proteins that have been shown to interact with Aβ and a discussion of their potential roles in AD pathology. Methods that can expand the knowledge about how the proteins are incorporated in plaques are described. Top-down methods to analyze post-mortem tissue and bottom-up approaches with the potential to provide molecular insights on the organization of plaque-like particles are compared. Finally, a network analysis of Aβ-interacting partners with enriched functional and structural key words is presented.

Clinically oriented Alzheimer's biosensors: expanding the horizons towards point-of-care diagnostics and beyond

The development of minimally invasive and easy-to-use sensor devices is of current interest for ultrasensitive detection and signal recognition of Alzheimer's disease (AD) biomarkers. Over the years, tremendous effort has been made on diagnostic platforms specifically targeting neurological markers for AD in order to replace the conventional, laborious, and invasive sampling-based approaches. However, the sophistication of analytical outcomes, marker inaccessibility, and material validity strongly limit the current strategies towards effectively predicting AD. Recently, with the promising progress in biosensor technology, the realization of a clinically applicable sensing platform has become a potential option to enable early diagnosis of AD and other neurodegenerative diseases. In this review, various types of biosensors, which include electrochemical, fluorescent, plasmonic, photoelectrochemical, and field-effect transistor (FET)-based sensor configurations, with better clinical applicability and analytical performance towards AD are highlighted. Moreover, the feasibility of these sensors to achieve point-of-care (POC) diagnosis is also discussed. Furthermore, by grafting nanoscale materials into biosensor architecture, the remarkable enhancement in durability, functionality, and analytical outcome of sensor devices is presented. Finally, future perspectives on further translational and commercialization pathways of clinically driven biosensor devices for AD are discussed and summarized.

Development of liquid crystal biosensor for the detection of amyloid beta-42 levels associated with Alzheimer's disease

This study represents the development of a biosensor which is based on the liquid crystal (LC) orientation as a function of the peptide concentration to detect an amyloid-beta-42 (Aβ42) antibody-antigen binding events. The Aβ42 peptide binds to the Aβ42 antibody forming an immunocomplex which is immobilized on the Dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) coated surface. The disturbed orientation of LCs as a result of the binding of the formed immunocomplex was observed using the polarized optical microscope (POM) as a function of decreasing Aβ42 peptide concentration from 1000 to 1 pg/ml. The concentration, as low as 1 pg/ml of Aβ42 peptide was able to be successfully detected in our system. Apolipoprotein E4 (ApoE4), that specifically bound to the Aβ42 peptide, was added into the system and a remarkable change in reflection spectra of samples was observed with increasing Aβ42 peptide concentration. The concentration of ApoE4 protein was detected in the range of 0.1-30 nM by this system due to the interaction between the two proteins.

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.

The application of multifunctional nanomaterials in Alzheimer's disease: A potential theranostics strategy

By virtue of their small size, nanomaterials can cross the blood-brain barrier and, when modified to target specific cells or regions, can achieve high bioavailability at the intended site of action. Modified nanomaterials are therefore promising agents for the diagnosis and treatment of neurodegenerative diseases such as Alzheimer's disease (AD). Here we review the roles and mechanisms of action of nanomaterials in AD. First, we discuss the general characteristics of nanomaterials and their application to nanomedicine. Then, we summarize recent studies on the diagnosis and treatment of AD using modified nanomaterials. These studies indicate that using nanomaterials is a potential strategy for AD treatment by slowing the progression of AD through enhanced therapeutic effects.

Electrochemical Biosensors Based on Nanomaterials for Early Detection of Alzheimer's Disease

Alzheimer's disease (AD) is an untreatable neurodegenerative disease that initially manifests as difficulty to remember recent events and gradually progresses to cognitive impairment. The incidence of AD is growing yearly as life expectancy increases, thus early detection is essential to ensure a better quality of life for diagnosed patients. To reach that purpose, electrochemical biosensing has emerged as a cost-effective alternative to traditional diagnostic techniques, due to its high sensitivity and selectivity. Of special relevance is the incorporation of nanomaterials in biosensors, as they contribute to enhance electron transfer while promoting the immobilization of biological recognition elements. Moreover, nanomaterials have also been employed as labels, due to their unique electroactive and electrocatalytic properties. The aim of this review is to add value in the advances achieved in the detection of AD biomarkers, the strategies followed for the incorporation of nanomaterials and its effect in biosensors performance.

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

Au/NiFe2O4 nanoparticle-decorated graphene oxide nanosheets for electrochemical immunosensing of amyloid beta peptide

In the present work, an electrochemical immunosensor has been fabricated for the detection of amyloid beta peptide (βA_1--42) based on a gold nanoparticle/nickel ferrite decorated graphene oxide-chitosan nanocomposite (Au/NiFe₂O₄@GO-Ch) modified glassy carbon electrode (GCE) as an effective sensing platform. βA_1-42 has been analyzed as a potential biomarker for its application in Alzheimer's disease monitoring. The combination of highly conducting Au and NiFe₂O₄ nanoparticles on two-dimensional GO nanosheets provides an excellent platform for sensitive and selective sensing applications. A miniaturized Au/NiFe₂O₄@GO-Ch/GCE immunosensor was prepared by immobilization of βA antibody onto Au//NiFe₂O₄@GO-Ch/GCE via carbodiimide coupling. Various characterization techniques were utilized in the study to estimate the morphological and electronic attributes of the components used to fabricate the immunosensor. Differential pulse voltammetry (DPV) was performed to study the amperometric response of the developed immunosensor as a function of βA_1-42 concentration. The DPV results confirmed that the immunosensor detected βA_1-42 selectively and demonstrated a wide linear range from 1 pg mL^-1 to 1 ng mL^-1 and a detection limit of 3.0 pg mL^-1. Furthermore, the immunosensor also indicated its clinical viability by detecting βA_1-42 in cerebrospinal fluid.

Carbon dots sensitized lanthanide infinite coordination polymer nanoparticles: Towards ratiometric fluorescent sensing of cerebrospinal Aβ monomer as a biomarker for Alzheimer's disease

Herein, a novel ratiometric fluorescent probe based on CDs@Eu/GMP ICP nanoparticles was developed for the detection of Aβ monomer in rat as a biomarker for Alzheimer's disease (AD) by fully exploring the competitive coordination interaction and by taking advantage of excellent optical property of carbon dots sensitized lanthanide infinite coordination polymer (ICP) nanoparticles. The carbon dots (CDs) with abundant functional groups were encapsulated into Eu/GMP ICPs through self-adaptive chemistry, which could not only sensitize the red fluorescence of Eu/GMP ICPs effectively, but also act as an internal reference for self-correction. In the absence of Cu²⁺, the as-formed CDs@Eu/GMP ICPs exhibited the characteristic emission of CDs at 400 nm and strong emission of Eu³⁺ at 592 nm, 615 nm, 650 nm and 694 nm. With the addition of Cu²⁺, the red fluorescence of Eu³⁺ decreased due to the coordination interaction between CDs and Cu²⁺, thus destroyed the antenna effect. After the subsequent addition of Aβ monomer, the specific binding occurred between Cu²⁺ and Aβ monomer, and then the red fluorescence of Eu³⁺ restored again. During this process, the fluorescence of CDs remained unchanged, thus could be used as an internal reference to cancel out the environmental fluctuation and was more adaptive for the detection of Aβ monomer in biological fluids. The method demonstrated here was highly sensitive, free from the interference of other species in rat brain, the in vivo analysis of Aβ monomer in CSF and different brain regions from normal rats and Alzheimer's rats could be realized, which was of great significance for better understanding the mechanism of AD and paving the way to understand the chemical essence involved in AD.

An aptamer based fluorometric assay for amyloid-β oligomers using a metal-organic framework of type Ru@MIL-101(Al) and enzyme-assisted recycling

Amyloid-beta (Aβ) oligomers causing neuron damage are regarded as potential therapeutic targets and diagnostic markers for Alzheimer's disease (AD). A homogeneous turn-on fluorometric aptasensor is described for Aβ oligomers. It is highly selective and non-invasive and based on (a) the use of a luminescent metal-organic framework carrying aptamer-modified AuNPs (L-MOF/Apt-Au) as tracking agent, and (b) enzyme-assisted target recycling signal amplification. The tracking agent does not emit fluoresce by fluorescence resonance energy transfer (FRET) between the luminescent MOF as donor and Apt-Au as the acceptor under the excitation wavelength of 466 nm. When Aβ oligomers are added to the tracking agent solution, the Apt-Au on tracking agent can preferentially bind with Aβ oligomers and then be released. This turns the "off" signal of the luminescent MOF tracer to the "on" state. The enzyme (Rec Jf exonuclease) added into the supernatant further improves sensitivity due to enzyme-assisted target-recycling signal amplification. The assay has an excellent linear response to Aβ oligomers from 1.0 pM to 10 nM, with a detection limit of 0.3 pM. This homogeneous turn-on fluorometric method is expected to have potential and applications in clinical diagnosis. Graphical abstractSchematic representation of fluorometric assay for amyloid-β oligomers based on luminescence metal-organic framework nanocomposites as tracking agent with exonuclease-assisted target recycling.

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.

Chemical sensing platforms for detecting trace-level Alzheimer's core biomarkers

This review provides an overview of recent advances in optical and electrical detection of Alzheimer's disease biomarkers in clinically relevant fluids.

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.

Transition metal-coordinated graphitic carbon nitride dots as a sensitive and facile fluorescent probe for β-amyloid peptide detection

Herein, we developed a sensitive graphitic carbon nitride quantum dot (gCNQD)-based fluorescent strategy for β-amyloid peptide monomer (Aβ) determination down to the ng mL-1 level for the first time. To realize this goal, the nanostructured gCNQDs were firstly coordinated with four transition metal ions (Cu2+, Cu+, Fe3+, Zn2+). Our findings showed that the fluorescence (FL) intensity of gCNQDs was quenched in the presence of these metal ions possibly due to the effective chelation with the nitrogen element in gCNQDs and subsequent photoinduced electron transfer (PET) of gCNQDs. The degree of fluorescence quenching was found to be the most intense with the addition of Cu2+ and therefore, we selected Cu2+ as the quencher for the following Aβ determination. Through binding to Cu2+, the introduction of Aβ unexpectedly induced a further decline of FL intensity. Importantly, on account of different peptide sequences coexisting in the same cerebral system, including Aβ1-11, Aβ1-16, Aβ1-38, Aβ1-40 and Aβ1-42, their affinities to Cu2+ could be reflected by the distinguished declining extent of FL intensity. The possible mechanism of Aβ sensing by the probe was clarified by TEM characterization. The developed fluorescent biosensor was demonstrated to give a wide linear range from 1 to 700 ng mL-1 and a low detection limit of 0.18 ng mL-1 for Aβ1-42. In the end, the proposed fluorescence approach was successfully applied to monitoring of Aβ1-42 variations in the cortex and hippocampus of AD rats.

Simultaneous Monitoring of Amyloid-β (Aβ) Oligomers and Fibrils for Effectively Evaluating the Dynamic Process of Aβ Aggregation

Herein, we provide a proof of concept for a novel strategy that targets the assessment of the aggregation of amyloid-β (Aβ) by simultaneously determining its oligomers (Aβo) and fibrils (Aβf) in one analytical system. By fabricating and combining two immunosensors for Aβo and Aβf, respectively, we constructed a two-channel electrochemical system. The ratio of Aβf to Aβo was calculated and taken as a possible criterion for evaluating the extent of aggregation. Thereby, the presence of and transformation between oligomers and fibrils were accurately probed by incubating the Aβ monomer for different times and then calculating the ratios of Aβf to Aβo. The applicability of this method was further validated by tracking the dynamic progress of Aβ aggregation in the cerebrospinal fluid and tissues of Alzheimer's disease (AD) rats, which revealed that the ratio of Aβf to Aβo in rat brain gradually increased with the progression of AD, which was indicative of the severity of peptide aggregation during this process. Overall, this study represents the first example of a quantitative strategy for precisely evaluating the aggregation process that is related to pathological events in AD brain.

Structure, regulation and related diseases of the actin-binding protein gelsolin

Gelsolin (GSN), one of the most abundant actin-binding proteins, is involved in cell motility, shape and metabolism. As a member of the GSN superfamily, GSN is a highly structured protein in eukaryotic cells that can be regulated by calcium concentration, intracellular pH, temperature and phosphatidylinositol-4,5-bisphosphate. GSN plays an important role in cellular mechanisms as well as in different cellular interactions. Because of its participation in immunologic processes and its interaction with different cells of the immune system, GSN is a potential candidate for various therapeutic applications. In this review, we summarise the structure of GSN as well as its regulating and functional roles, focusing on distinct diseases such as Alzheimer's disease, rheumatoid arthritis and cancer. A short overview of GSN as a therapeutic target in today's medicine is also provided.

Combined determination of copper ions and β-amyloid peptide by a single ratiometric electrochemical biosensor

Copper ions (Cu²⁺) play a critical role in biological processes and are directly involved in β-amyloid peptide (Aβ) aggregation, which is responsible for the occurrence and development of Alzheimer's disease (AD). Therefore, combined determination of Cu²⁺ and Aβ in one analytical system is of great significance to understand the exact nature of the AD event. This work presents a novel ratiometric electrochemical biosensor for the dual determination of Cu²⁺ and Aβ_1-42. This unique sensor is based on a 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS) and poly(diallyldimethylammonium chloride) (PDDA)-bi functionalized single-walled carbon nanotubes (ABTS-PDDA/CNTs) composite. The inclusion of ABTS not only enhanced the sensitivity, but it also acted as an inner reference molecule to improve detection accuracy. The specific recognition of Cu²⁺ was realized by neurokinin B (NKB) coatings on the ABTS-PDDA/CNTs surface to form a [Cu^II(NKB)₂] complex with Cu²⁺. The ABTS-PDDA/CNTs-NKB modified electrode also displayed an excellent electrochemical response toward the Aβ_1-42 monomer, when a certain amount of the Aβ_1-42 monomer was added to Cu²⁺-contained PBS buffer, which was due to the release of Cu²⁺ from the [Cu^II(NKB)₂] complex through Aβ binding to Cu²⁺. Meanwhile, our work showed that Cu²⁺ bound Aβ_1-42 was concentration-dependent. Consequently, the presented electrochemical approach was capable of quantifying two important biological species associated with AD by one single biosensor, with the detection limits of 0.04 μM for Cu²⁺ and 0.5 ng mL^-1 for Aβ_1-42, respectively. Finally, the ratiometric electrode was successfully applied for monitoring Cu²⁺ and Aβ_1-42 variations in plasma and hippocampus of normal and AD rats.

Highly sensitive/selective 3D nanostructured immunoparticle-based interface on a multichannel sensor array for detecting amyloid-beta in Alzheimer's disease

Accumulation of β-amyloid (Aβ) peptides is highly associated with Alzheimer's disease (AD) progression in prevailing studies. The successful development of an ultrasensitive detection assay for Aβ is a challenging task, especially from blood-based samples.

Methods: We have developed a one-step electrophoresis/electropolymerization strategy for preparing a CSIP hierarchical immunoelectrochemical interface that is easily integrated into a PoCT device. The interface includes conductive silk fibroin-based immunoparticles (CSIPs) via electropolymerized Poly(3,4-ethylenedioxythiophene) (PEDOT) bridging to enable on-site electrochemical detection of serum amyloid-β₄₂ (Aβ₄₂) and -β₄₀ (Aβ₄₀) peptides from an AD blood test. In addition, micro-positron emission tomography (microPET) neuroimaging and behavioral tests were simultaneously performed.

Results: This nanostructured conductive interface favors penetration of water-soluble biomolecules and catalyzes a redox reaction, providing limits of detection (LOD) of 6.63 pg/mL for Aβ₄₀ and 3.74 pg/mL for Aβ₄₂. Our proof-of-concept study confirms that the multi-sensing electrochemical immunosensor array (MEIA) platform enables simultaneous measurement of serum Aβ₄₂ and Aβ₄₀ peptide levels and is more informative in early stage AD animals than amyloid-labeling Aβ plaque PET imaging and behavioral tests.

Conclusion: We believe this study greatly expands the applications of silk fibroin-based materials, is an important contribution to the advancement of biomaterials, and would also be valuable in the design of new types of multichannel electrochemical immunosensor arrays for the detection of other diseases.

Biosensors for Alzheimer's disease biomarker detection: A review

Alzheimer's disease (AD) is a chronic disease amongst people aged 65 and older. Increasing evidence has illustrated that early diagnosis holds the key to effective treatment of AD. A variety of detection techniques have been developed. Biosensors are excellent analytical tools which have applications in detecting the biomarkers of AD. This review includes appropriate bioreceptors to achieve highly sensitive and selective quantification of AD biomarkers by using transducers. AD biomarkers such as tau protein, amyloid β peptides and apolipoprotein E4, are firstly summarized. The most commonly used bioreceptors, including aptamers and antibodies, are also reviewed. We introduce aptamers specific to AD biomarkers, list the sequences of aptamers designed to capture AD biomarkers and compare the properties of aptamers with those of antibodies with regard to their efficiency as bio-recognition elements. We discuss the recent progress of aptamer systems' applications in AD biomarkers in biosensing. The review also discusses novel strategies used for signal amplification in sensing AD biomarkers.

A highly sensitive plasma-based amyloid-β detection system through medium-changing and noise cancellation system for early diagnosis of the Alzheimer's disease

We developed an interdigitated microelectrode (IME) sensor system for blood-based Alzheimer's disease (AD) diagnosis based on impedimetric detection of amyloid-β (Aβ) protein, which is a representative candidate biomarker for AD. The IME sensing device was fabricated using a surface micromachining process. For highly sensitive detection of several tens to hundreds of picogram/mL of Aβ in blood, medium change from plasma to PBS buffer was utilized with signal cancellation and amplification processing (SCAP) system. The system demonstrated approximately 100-folds higher sensitivity according to the concentrations. A robust antibody-immobilization process was used for stability during medium change. Selectivity of the reaction due to the affinity of Aβ to the antibody and the sensitivity according to the concentration of Aβ were also demonstrated. Considering these basic characteristics of the IME sensor system, the medium change was optimized in relation to the absolute value of impedance change and differentiated impedance changes for real plasma based Aβ detection. Finally, the detection of Aβ levels in transgenic and wild-type mouse plasma samples was accomplished with the designed sensor system and the medium-changing method. The results confirmed the potential of this system to discriminate between patients and healthy controls, which would enable blood-based AD diagnosis.

Electrochemical sandwich-type biosensors for α-1 antitrypsin with carbon nanotubes and alkaline phosphatase labeled antibody-silver nanoparticles

A novel sandwich-type biosensor was developed for the electrochemical detection of α-1 antitrypsin (AAT, a recognized biomarker for Alzheimer's disease). The biosensor was composed of 3, 4, 9, 10-perylene tetracarboxylic acid/carbon nanotubes (PTCA-CNTs) as a sensing platform and alkaline phosphatase-labeled AAT antibody functionalized silver nanoparticles (ALP-AAT Ab-Ag NPs) as a signal enhancer. CNTs offer high surface area and good electrical conductivity. Importantly, Ag NPs could increase the amount of ALP on the sensing surface and the ALP could dephosphorylate 4-amino phenyl phosphate (APP) enzymatically to produce electroactive species 4-aminophenol (AP). For detecting AAT based on the sandwich-type biosensor, the results show that the peak current value of AP using ALP-AAT Ab-Ag NPs as signal enhancer is much higher than that by using ALP-AAT Ab bioconjugate (without Ag NPs), the biosensor exhibited desirable performance for AAT determination with a wide linearity in the range from 0.05 to 20.0pM and a low detection limit of 0.01pM. Finally, the developed sensor was successfully applied to the analysis of AAT concentration in serum samples.

Functionalized poly (ionic liquid) as the support to construct a ratiometric electrochemical biosensor for the selective determination of copper ions in AD rats

An efficient ratiometric electrochemical biosensor for Cu²⁺ determination was constructed using dual hydroxyl-functionalized poly (ionic liquid) (DHF-PIL) as the catalyst support. The DHF-PIL exhibited typical macroporous structure, which provided a high surface area of 39.31m²/g for the sufficient loading of biomolecules. The specific recognition of Cu²⁺ was accomplished by employing neurokinin B (NKB) for the first time, which could bind to Cu²⁺ to form a [Cu^II(NKB)₂] complex with high specificity. Meanwhile, a common redox mediator, 2, 2'-Azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) was modified into DHF-PIL by electrostatic interactions to act as an inner reference molecule, which provided a built-in correction for environmental effects and improving the detection accuracy. With this strategy, the developed electrochemical biosensor was capable of determining Cu²⁺ with a linear range between 0.9 and 36.1μM and low detection limit (LOD) and quantification limit (LOQ) of 0.24 and 0.6μM, respectively. The sensor also displayed a satisfactory selectivity against a series of interferences in the brain, including metal ions, amino acids and other endogenous compounds. Accordingly, the present biosensor was successfully applied to evaluate Cu²⁺ levels in normal and AD rats.

Application of Nanomedicine to the CNS Diseases

Drug delivery to the brain is a challenge because of the many mechanisms that protect the brain from the entry of foreign substances. Numerous molecules which could be active against brain disorders are not clinically useful due to the presence of the blood-brain barrier. Nanoparticles can be used to deliver these drugs to the brain. Encapsulation within colloidal systems can allow the passage of nontransportable drugs across this barrier by masking their physicochemical properties. It should be noted that the status of the blood-brain barrier is different depending on the brain disease. In fact, in some pathological situations such as tumors or inflammatory disorders, its permeability is increased allowing very easy translocation of carriers. This chapter gathers the promising results obtained by using nanoparticles as drug delivery systems with the aim to improve the therapy of some CNS diseases such as brain tumor, Alzheimer's disease, and stroke. The data show that several approaches can be investigated: (1) carrying drug through a permeabilized barrier, (2) crossing the barrier thanks to receptor-mediated transcytosis pathway in order to deliver drug into the brain parenchyma, and also (3) targeting and treating the endothelial cells themselves to preserve locally the brain tissue. The examples given in this chapter contribute to demonstrate that delivering drugs into the brain is one of the most promising applications of nanotechnology in clinical neuroscience.

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.

Integration of a miniature quartz crystal microbalance with a microfluidic chip for amyloid beta-Aβ42 quantitation

A miniature quartz crystal microbalance (mQCM) was integrated with a polydimethylsiloxane (PDMS) microfluidic device for on-chip determination of amyloid polypeptide–Aβ₄₂. The integration techniques included photolithography and plasma coupling. Aβ₄₂ antibody was immobilized on the mQCM surface using a cross-linker method, and the resonance frequency of mQCM shifted negatively due to antibody-antigen binding. A linear range from 0.1 µM to 3.2 µM was achieved. By using matrix elimination buffer, i.e., matrix phosphate buffer containing 500 µg/mL dextran and 0.5% Tween 20, Aβ₄₂ could be successfully detected in the presence of 75% human serum. Additionally, high temperature treatments at 150 °C provided a valid method to recover mQCM, and PDMS-mQCM microfluidic device could be reused to some extent. Since the detectable Aβ₄₂ concentration could be as low as 0.1 µM, which is close to cut-off value for Alzheimer patients, the PDMS-mQCM device could be applied in early Alzheimer’s disease diagnosis.