Introduction

Among neurodegenerative disorders, Alzheimer’s disease (AD) is regarded as highly common, varied, and intricate. It stands as the most common type of dementia that presents a significant global challenge, especially for the elderly population1. On a global scale, exceeding 55 million individuals are impacted by dementia, with approximately 10 million novel instances arising annually2. In AD, one can observe the formation of β-amyloid (Aβ) plaques outside of cells alongside neurofibrillary tangles (NFT) that consist of hyperphosphorylated tau protein (P-tau) located in neuronal cytoplasm3. Aβ deposits are optimally characterized as insoluble and conglomerated manifestations of the Aβ peptide4. Aβ is generated from the erroneous cleavage of the amyloid precursor protein (APP), a single pass-membrane protein that is articulated in the cerebrum. This procedure encompasses successive fragmentation by β-secretase succeeded by γ-secretase, which constitutes a multiprotein assembly incorporating either PS1 or PS2 as enzymatic components. Furthermore, various lengths of Aβ fragments are produced, with monomeric Aβ40 and Aβ42 being the predominant forms that aggregate and form oligomeric Aβ, eventually developing into Aβ fibrils and plaques that initiate disease progression3,5. The function of Aβ in the etiology of AD remains ambiguous. In recent decades, the approaches adopted for AD have primarily focused on Aβ, yet they have yielded only marginal success. One obstacle in managing Alzheimer’s Disease is the effective administration and localization of pharmacological agents, attributable to the intricate neuropathology of the Alzheimer’s-affected brain and the safeguarding barriers of the central nervous system6,7,8. The blood-brain barrier functions as a protective mechanism for the central nervous system, excluding harmful toxins and microorganisms that traverse the bloodstream, and concurrently regulating the transit of pharmaceutical molecules9.

The use nanoparticles (NP) with dimensions smaller than 100 nm has surfaced as a vital element of contemporary, pioneering medicine with extensive utilizations. The distinct attributes of NPs, characterized by their small size and large surface area, play an essential role in enhancing nanotechnology-based pharmacological delivery systems in current therapeutic approaches. NPs have proven successful in their usefulness as drug carriers, delivering therapeutic compounds. Selenium nanoparticles (SeNPs) have surfaced as a significant subject in current conversations, primarily due to their adaptable uses that range from biomedicine to environmental science and industry. Advances in synthesis methods, particularly green and sustainable approaches, are expected to enhance their potential for practical use. So far, SeNPs have the potential for significant biomedical applications in pharmaceuticals and medical nano-engineering, particularly for treating arthritis, cancer, diabetes, and inflammatory diseases10. They function as potential drug delivery carriers characterized by low toxicity, strong biocompatibility, as well as antioxidant and anti-inflammatory properties11. SeNPs in AD treatment show promising potential to improve penetration ability through the BBB in neurological disorders and reach targeted regions in the brain12. The beneficial effects of selenium are primarily attributed to its crucial role as an antioxidant trace element in regulating brain function13. Most lines of evidence suggest that Se depletion, followed by decreased activity of Se-dependent enzymes, is probably an important factor connected with pathologies of AD. Previous research demonstrated a strong direct correlation between lower plasma Se concentration and cognitive decline in AD patients compared to healthy individuals. Compounds containing selenium may act as antioxidants, reducing oxidative stress in various neurodegeneration diseases such as AD by the capacity to inhibit Aβ aggregation14.

Green nanotechnology connects the divide between nanotechnology and the natural world, promoting the advancement and execution of ecologically sustainable substitutes for the synthesis of nanomaterials. The bio-mediated synthesis of NPs from plant extracts offers distinct advantages over chemical synthesis methods due to its safety and non-toxic production15. Purple sweet potatoes (Ipomoea batatas), characterized by their profound purple or violet interior, are not solely palatable but also replete with essential nutrients that enhance holistic health and well-being. Purple sweet potato (PSP) contains dietary fiber, vitamins, minerals, carbohydrates, fat, and so on, which can replenish daily energy for humans and animals16. It is considered a green food that helps people’s daily intake of cereals and potatoes17. Currently, anthocyanins derived from PSP are widely utilized as food additives in China18. PSP is rich in anthocyanin, phenolic compounds with high antioxidant activity, and can positively impact the properties of SeNPs synthesized from it19. The caffeoylquinic acids (CQA)and their derivatives encompass 70% of the total phenolic in PSP20. These CQA derivatives are known to reduce Aβ deposition and improve cognitive function. Investigations show that 4,5-di-O-caffeoylquinic acid (4,5-di-CQA) and 3,4,5-tri-O-caffeoylquinic acid (3,4,5-tri-CQA) greatly affect Aβ42 peptide aggregation, revealing concentration-dependent differences. The presence of the caffeoyl moiety is essential for their inhibitory efficacy. They also inhibit the transformation of Aβ peptide into β-sheet21,22.

In this study, SeNPs are functionalized with CQA extracted from PSP extract, and their antioxidant, anticholinesterase, and anti-amyloidogenic potentials are studied. We have utilized molecular dynamic simulations for the first time to investigate the mechanism underlying SeNP functionalization and their anti-amyloidogenic effects. Molecular dynamics simulation is one of the most widely used method to obtain interaction trajectories23. Molecular dynamic simulation aids in modulating the folding of Aβ peptides both in the absence and presence of SeNPs at the molecular level. It also enhances the understanding of the mechanism that impedes the formation of β-sheet characterized Aβ plaques, which are typical in AD.

Materials and methods

Extraction procedure

Purple sweet potato (PSP) was sourced from a local market in Shantou, Guangdong province, China. The PSP extract was prepared using the method described by Sasaki et al.24. To create the extract, the PSP was first cleaned, chopped into small pieces, and then processed in a compression machine to concentrate the extract. After filtering, the extract was then steamed for 40 min, pulverized, and dried. The dried extract was then crushed in 80% ethanol, filtered, and concentrated using an evaporator. Finally, caffeoylquinic acids (CQA) were purified through ethyl acetate liquid/liquid extraction method, after which they were utilized for SeNPs synthesis.

Synthesis of selenium nanoparticles

For the synthesis of 5‑caffeoylquinic acid capped SeNPs, 80 mL of 1 mM sodium selenite (Sigma-Aldrich, Germany) was added to 250 mL beaker, followed by the addition of 20 mL of sweet potato extracts, which were sonicated for 10 min. Next, 4 mM ascorbic acid (Macklin Chemicals, China) was added, and the reaction mixture was again sonicated for 1 h. The color changes from light yellow to red, indicating the synthesis of SeNPs. The reaction mixtures were transferred to 50 mL Falcon vessels and subjected to centrifugation at 6000 revolutions per minute for 30 min to aggregate the SeNPs. The supernatants were discarded, and the pellets containing SeNPs were washed 5 times with distilled water. Finally, the SeNPs were dried by lyophilization (EYELA, FDU-1200)25.

Characterization of synthesized AgNPs

The synthesis of SeNPs were confirmed with the help of UV-Vis spectroscopy (Thermo Fisher, GENESYS-150). FT-IR spectral measurements were completed with the help of spectrophotometer (BRUKER A220/D-01) within a frequency range of 4000–600 cm−1. XRD analysis was carried out to study the crystalline nature of samples (Rigaku, SmartLab (9KW)). The electronic state and elemental compositions of each individual element were verified with X-ray Photoelectron Spectroscopy (XPS) (PHI 500 VersaProbe III). Powder samples were mounted by pressing them into indium foil, which was secured with double-sided carbon tape to a paper label affixed to the XPS sample mount. Survey scans were recorded between 1200 and 0 eV binding energy, using 160 eV pass energy at 1 eV intervals, and a 300-second duration per sweep. High-resolution O 1s, C 1s, and Se 3d XPS spectra were collected at 20 eV pass energy and 0.1 eV intervals over the appropriate energy range, with one 300-second sweep for all elements.

The morphology and elemental composition analysis of the purified SeNPs were examined using scanning electron microscopy (Zeiss, Gemini 450), and Transmission electron microscopy (TEM) equipped with energy dispersive X-ray (EDX). The structural characterization of the SeNPs was conducted using selected-area electron diffraction (SAED)26.

In vitro antioxidant assay

  1. (a)

    Free radical scavenging activity

The antioxidant activity of the synthesized SeNPs (20–100 µg/mL) was assessed by evaluating the in vitro scavenging potential of samples against the free radical 2,2-diphenyl 1-picrylhydrazyl (DPPH). Briefly, a 0.1 mM DPPH solution in methanol was devised, utilizing ascorbic acid as the benchmark. The reaction mixture consisted of 20 µL of SeNPs and 180 µL of DPPH solution. Post a one-hour resting phase, the absorbance of every reaction mixture was determined at 517 nm through the use of a spectrophotometer. The scavenging result was calculated as:

$${\text{Scavenging}}\;{\text{effect}}\left( \% \right) = \left[ {\left( {{\text{A}}_{0} - {\text{A}}_{{\text{1}}} } \right)/{\text{A}}_{0} } \right] \times {\text{1}}00$$

where A0 is the absorbance of the control reaction and A1 is the absorbance in the presence of the standard27.

  1. (b)

    Metal chelating activity

The method described by Haro-Vicente et al.28 is used to determine the metal chelating ability of synthesized SeNPs. Briefly, SeNPs (10–50 µg/ml) were mixed with 0.15 mM ferrous sulfate, and the reaction was commenced by the incorporation of ferrozine (0.5 mM). The reaction mixture was agitated and incubated in obscurity for 20 min at ambient temperature; ultimately, the absorbance was quantified at 562 nm.

$${\text{Inhibition}}\;{\text{of}}\;{\text{complex}}\;{\text{formation}}\left( \% \right) = \left[ {\left( {{\text{A}}_{0} - {\text{A}}_{{\text{1}}} } \right)/{\text{A}}_{0} } \right] \times {\text{1}}00$$

where A0 is the absorbance of the control reaction and A1 is the absorbance in the presence of the standard.

Cholinesterase (AChE/BChE) inhibition assay

Modified Elman’s method was employed to investigate the inhibitory potency of synthesized SeNPs against acetyl-cholinesterase (AChE) and butyryl-cholinesterase (BChE)29. The investigation incorporated acetylcholinesterase, sourced from Electrophorus electricus (CAS number 9000-81-1) and acquired from Sigma Aldrich located in St. Louis, Missouri, alongside butyrylcholinesterase (CAS number: 9001-08-5) sourced from equine serum from Sigma Aldrich GmbH, Germany. For the assay, the synthesized SeNPs, and dispersed in PBS, and other substrate solutions, including DTNB (5,5-dithiobisnitrobenzoic acid) at 100 mM, butyrylcholine iodide at 100 mM and acetylcholine iodide at 100 mM, were prepared in distilled water and maintained at 80C. Both AChE and BChE exhibited enzyme concentrations of 0.4 U/mL each. The role of Galantamine hydrobromide was that of a positive control, made in methanol at a concentration of 10 mg/mL, whereas the reaction mixture lacking a test specimen acted as a negative control. The DTNB complexes displayed a yellow colour, which was subsequently used to measure absorbance at 412 nm using a UV-VIS spectrophotometer. The enzyme inhibition was calculated as:

$$\% {\text{Inhibition}} = \left( {\left[ {\Delta {\text{Abs}}\;{\text{control}} - \Delta {\text{Abs}}\;{\text{sample}}} \right]/\left[ {\Delta {\text{Abs}}\;{\text{control}}} \right]} \right) \times {\text{1}}00$$

Aβ peptide activity

  1. (a)

    Aβ polymerization assay

To obtain uniform non-aggregated Aβ (1–40) peptide, Aβ was dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), sonicated, and incubated for 2 h, then subsequently dried under reduced pressure using a lyophilizer. The obtained thin film of Aβ (1–40) was stored at − 800C until use. The Aβ (1–40) peptides were later dissolved in DMSO at a concentration of 1 mM before conducting the assay30. As the Aβ (1–40) monomer synthesized recently was placed in Tris-Cl buffer at a pH of 7.4 and 37 °C, oligomers formed steadily over the course of 20 h. This oligomeric blend saw the introduction of SeNPs (50 µg/ml), which were then incubated for an extra 48 h. From the incubation mixture, 20 µl aliquots were extracted at 20 and 48 h, respectively, intended for spectroscopic examination, confocal microscopy, and FTIR analysis.

  1. (b)

    Measurement of fibril destabilization activity

The destabilization potential of synthesized SeNPs on fibrils was quantified by first creating mature Aβ (1–40) peptide fibrils through 96 h of incubation, which was succeeded by a long incubation period with SeNPs (50 µg/ml) lasting 9 days. Aliquots were drawn for spectroscopic, FTIR, and confocal microscopic studies29.

  1. (c)

    Thioflavin T assay

Fluorescence alterations of Thioflavin T were assessed to quantify amyloid fibril synthesis utilizing a spectrofluorometer. Preparations containing Aβ (1–40) alongside or apart from SeNPs were standardized with 300 µl of 50 mM glycine-NaOH buffer at pH 8.5, including 5 µM thioflavin T obtained from Sigma-Aldrich. The evaluation of fluorescence intensity occurred at 450 nm for excitation and at 485 nm for emission. The background fluorescence intensity of thioflavin T was deducted from the experimental readings, and each assay was executed in triplicate31.

  1. (d)

    Microfluorescence assay

A volume of 2.5 µl of fibrillated Aβ (1–40) peptide, concentrated at 100 µM, was diluted two-fold with 5 µM thioflavin T in a 50 mM glycine-NaOH buffer solution, with pH precisely calibrated to 8.5. The fluorescent signals (488 nm) were observed using a confocal laser microscope system (CLSM 710, Carl Zeiss, Germany) and processed with Zen 2011. The fluorescence intensity was visualized in each of three random samples30.

  1. (e)

    FTIR

To detect the existence of β-sheet secondary structure and distinguish its formation prior to and following aggregation, FTIR was utilized. The FTIR spectra were recorded at room temperature using a BRUKER A220/D-01 FTIR spectrophotometer, equipped with OPUS version 6.5 IR software. The spectrum signifies the average of 64 scans with a scanning range from 4000 to 400 cm−1 and a resolution of 4 cm−1. Background scans were conducted before recording samples32.

Molecular simulation

Molecular Dynamics (MD) simulations were conducted using the CHARMM37 force field to examine the interactions and dynamics of SeNPs capped with CQA in relation to amyloid β (Aβ) monomers. The CHARMM37 force field was chosen for its capacity to accurately model biomolecular systems, including proteins and nanoparticles. In these simulations, the protonation state of CQA was adjusted to reflect physiological conditions at pH 7. The force field parameters specific to CQA in Gromacs format were acquired via the Automated Topology Builder (ATB) web server, a resourceful tool that guarantees accurate parameter assignment and consistency for molecular modeling. The initial structural model of amyloid β (Aβ (1–40) monomers in their unfolded state was sourced from the Protein Data Bank (PDB), specifically using entry 2M9R. This entry provides a high-resolution structural framework for the amyloid protein in an unfolded configuration, making it suitable for investigation its conformational behaviour and aggregation potential in simulation environments. Standard protonation states were assigned to all ionizable amino acid residues within the amyloid protein, aligning with pH 7, to simulate realistic physiological pH conditions and maintain a relevant biochemical environment.

The structure of SeNPs was prepared using the CHARMM-GUI Nanomaterial modeler33. As the the structure of SeNP is unavailable, we prepared the spherical Gold (Au) NP using CHARMM-GUI Nanomaterial modeler34. The AuNP topology was modified by replacing the atomic mass and charge of Au with those of Se, referencing the GROMACS topology of selenocysteine35. The Lennard-Jones non-bonded parameters were also substituted for the Au non-bonded parameters in AuNPs topology36. As the bonded parameters of Se were not available, we did not alter them. However, to maintain the spherical shape of the SeNPs we applied 1000 (KJ mol−1 nm−2) restraining force on each Se atom during the simulation. The prepared SeNP topology was then input to GROMACS for molecular dynamics simulation studies.

To replicate experimental conditions, CQA molecules were added as a capping layer over the selenium core. This involved placing a single a single selenium core in the simulation box and systematically adding CQA molecules tom completely encapsulate the nanoparticle core. This step was essential to ensure that the simulation model accurately reflected the experimental setup, where CQA molecules stabilize and alter the properties of the SeNP through surface interactions. The final configuration changes underwent a 100-nanosecond (ns) MD simulation to investigate the stability, conformational changes, and interaction dynamics within the SeNP–CQA complex and with the Aβ (1–40) monomers. This timescale allowed sufficient sampling to observe significant molecular events and understand the potential role of capped SeNPs in influencing amyloid aggregation, thereby enhancing our insights into therapeutic applicability. After the MD simulation, the resulting trajectories were utilized for various dynamics analysis, including, root mean square deviation (RMSD), radius of gyration (Rg), root mean square fluctuation (RMSF), number of hydrogen bonds, Gibbs free landscape and secondary structural analysis using various built-in tools in GROMACS37.

Results and discussion

Synthesis of senps

A rich source of diverse nutrients, including vitamins, minerals, and polyphenols makes PSP more nutritious and beneficial to health than most foods. The biological capabilities of PSP encompass antioxidative, anti-inflammatory, and neuroprotective attributes. The neuroprotective effects of PSP are primarily linked to CQA derivatives found within it21. This study mainly focuses on the anti-amyloidogenic properties of CQA capped SeNPs. The SeNPs were synthesized from PSP extracts through phytochemical-mediated reduction of sodium selenite solution. A change in color of the reaction mixture from yellow to red within 10 min of the reaction indicates the formation of SeNPs (Fig. 1a inset). This color change results from the vibration of the plasmon base, an optical property unique to noble metals.

Characterizations

The UV–VIS spectra of the synthesized SeNPs from the PSP extract displayed maximum light absorption at 260 nm (Fig. 1a). XRD patterns provided detailed information about the crystal structure of the synthesized SeNPs. The XRD analysis of the synthesized SeNPs did not reveal any sharp peaks, indicating their amorphous nature (Fig. 1b). In vivo Studies has revealed that amorphous nano formulations have more bioavailability than crystalline one, thus synthesised SeNPs will able to show better activity in vivo studies also38. Functional groups on the surface of synthesized SeNPs were determined by FTIR spectroscopy; the CQA-rich extract of PSP exhibited characteristic vibrations at (Fig. 1c) 3412 cm−1, which are ascribed to the -OH stretching vibration. Bands at 2912 cm−1 correspond to the C-H vibration modes, and the band at 1656 cm−1 is assigned to the stretching of the carbonyl group (C = O)39,40. Moreover, the bands at 1480 cm−1 are attributed to olefinic C-C and aromatic stretching. Thus, the signal at 1217 cm−1 is ascribed to aromatic bending C-H modes. Frequencies below 1120 cm−1 might be due to the C-C-C bending modes of the aromatic system, except for the signals at 840 and 634 cm−1, which are assigned to the bending modes of the carbonyl group41.

The reduction, stabilization and capping process causes significant changes in the FTIR spectra of CQA – SeNPs, such as changes in spectral intensity and/or shifts in the position of the peaks. The synthesized SeNPs exhibited an intense peak at 3318 cm−1 attributed to the intermolecular hydrogen-bonded network formed by the hydroxyl groups (–OH) of phenolic acids on the nanoparticle’s surface. The increased intensity of these bands in CQA–SeNPs compared to extract may be due to the higher concentration of the oxidized CQA molecules forming hydrogen bonds on the surface of SeNPs. The increased signal intensity of the peak at 1640 cm−1 indicates the out-of-phase C = O stretching vibrations of CQA near the surface of the CQA–SeNPs42,43,44. The peak at 1382 cm−1 represents the vibrations of aromatic bond. Similarly, the peaks 1288 and 1046 cm−1 correspond to vibrations of primary alcohol (–OH) and polyols, while 2182 cm−1 is assigned to C-H vibrations. Therefore, based on the IR studies, it is proposed that CQA–SeNPs are stabilized through electrostatic interactions between the carboxylate group of CQA and its oxidized quinine form (Fig. 1c). The zeta potential was calculated to be 24.3 ± 2.1 mV, reflecting that the created SeNPs remain stable in an aqueous setting (Fig. 1d). Both SEM and TEM (Fig. 1e and f) confirm the spherical shape of synthesized SeNPs. The size of SeNPs ranges from 50 to 60 nm (Fig. 1e inset), which in the appropriate for crossing the blood brain barrier9. EDX analysis confirms that NPs are primarily composed of selenium, carbon and oxygen (Fig. 1e inset). The selected area electron diffraction (SAED) pattern also illustrates the amorphous nature of the synthesised SeNPs, consistent with the XRD results (Fig. 1f inset). High angular annular dark field scanning transmission electron microscopy (HAADF-STEM) (Fig. 1g-j) further demonstrates the capping of CQA over the core Se.

After evaluating the microstructural aspects, an elaborate X-ray photoelectron spectroscopy (XPS) assessment was performed to uncover insights regarding the electronic states and elemental compositions of every single element independently. The XPS survey spectra confirm the presence of the expected elements, including Se, O, and C, as depicted in Fig. 1k. The Se 3d spectra were analysed to identify two distinct spin-orbit peaks: Se 3d3/2 and the Se 3d5/2. Literature suggests that the binding energies for Se0 is at 3d5/2 at 55.1 eV; however, due to the capping of the Se with CQA, Se0 can be become polarised, leading to slightly lower binding energies 54.5 eV45,46. The binding energy at 55.2 eV (Fig. 1l) confirmed the presence of Se0. Conversely, the C1s core-level spectra (Fig. 1m) show two prominent peaks at 283.4 eV, corresponding to C=C bonds, and at 285 eV (C-C/C-H), confirming the presence of organic carbon. The high-resolution O1s spectrum exhibits distinct peaks that correspond to the metal lattice oxide and oxygen vacancies in synthesised SeNPs at approximately 530.8 eV, (Se-O) and 532 (C-O, organic bond) eV, respectively (Fig. 1n)47.

Fig. 1
figure 1

Characterization (a) UV-spectra of synthesized SeNPs (insect -color change of reaction from light yellow to red) (b) X-ray diffraction pattern of synthesized SeNPs (c) FTIR spectrum of CAQ rich PSP extract and synthesized SeNPs (d) Zeta potential analysis (e) FESEM image of synthesized SeNPs (insect EDX and particle size distribution). (f) TEM analysis and SAED pattern (g–j) HAADF images for element mapping analysis (k–n) X-ray photoelectron spectroscopy (XPS) analysis, (k) survey spectrum of synthesised SeNPs (l) Se3d (m) C 1s, and (n) O 1s.

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Antioxidant activity

The DPPH assay is the simplest method for assessing the free radical scavenging ability any compound48. When the test compound interacts with the DPPH reagent, the purple color of the mixture becomes colorless as the compound scavenges free radicals. This study looked into the antioxidative effectiveness of assorted levels of synthesized SeNPs (10–50 µg/ml) together with standard ascorbic acid (10–50 µg/ml) employing the DPPH method. The results presented in Table 1 indicated that the antioxidant activity of synthesised SeNPs increases with increasing concentration. Studies confirmed that the IC50 for SeNPs, meaning the concentration essential to block half of the free radicals, was documented at 8.01 + 1.21 µg/ml, whereas the ascorbic acid’s IC50 was established as 5.01 + 1.11 µg/ml. Previous studies reported IC50 values of 125 µg/ml for spherical-shaped SeNPs synthesised from ginger extract, measuring 100–150 nm49. A different study found that biosynthesized SeNPs, with a particle size of 4–16 nm, have an IC50 of 0.225 µg/ml50.

Metal chelating activity

Metals, due to their redox activity and their affinity for binding properties to amyloid β fibrils, promote Aβ aggregation, leading to its accumulation and deposition outside neurons. Metal chelation is essential in averting and managing Alzheimer’s disease (AD). The iron-chelating ability of synthesized SeNPs was examined at concentrations of 10–50 µg/ml, revealing that they effectively chelate ferrous (Fe2+) ions. The IC50 value, denoting the concentration requisite to obstruct 50% of the Ferrozine-Fe2 + complex, was determined to be 42.61 + 1.62 µg/ml, whereas for the reference EDTA, it was 21.32 + 1.32 µg/ml. Prior investigations have also demonstrated that chitosan-stabilised SeNPs impede metal-induced Aβ aggregation28,51.

Table 1 Antioxidant activities of synthesized senps.
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Acetylcholinesterase and butyrylcholinesterase Inhibition

Acetylcholine (ACh) and butyrylcholine (BCh) both play a significant role in memory learning; however, the elevated levels of AchE and BchE enzymes in an AD patient’s brain, reduce the half-life of ACh and BCh, contributing to the progression of AD52. Numerous additional investigations also documented that β-amyloid peptide and phosphorylated tau elevated the expression of AchE in the cerebral tissues of subjects afflicted with AD53. BChE is essential in Aβ peptide clustering, resulting in neuritic deposits and neurofibrillary entanglements in vivo54. Therefore, current therapeutic approaches for AD focus on inhibiting of AChE and BChE.

The findings from this investigation elucidated that produced SeNPs (10–50 µg/ml) exhibited AChE inhibitory efficacy in a concentration-dependent manner (Fig. 2a). The synthesized SeNPs reached the highest inhibitory effect of 92.31 ± 0.0043% at a concentration of 50 µg/ml (IC50 value of.

3.70 ± 0.02 µg/ml), compared to the equivalent dose of galantamine (78.17 ± 0.16%) (IC50 17.24 ± 0.04 µg/ml) (Fig. 2a). Similarly, for BChE, the synthesized SeNPs revealed an inhibition efficacy of 38.25 ± 0.13% at 50 µg/ml, with an IC50 figure of 72.05 ± 0.05 µg/ml, against the benchmark positive control galantamine (86.4 ± 0.022%; IC50 6.53 ± 0.022).The precise mechanism by which SeNPs inhibit AChE remains unknown. Certain scholars propose that the adsorption of Acetylcholinesterase onto the nanoparticle interface elicits conformational alterations in the protein architecture, and the liberation of metal ions from the nanoparticles may additionally interfere with the affinity to the substrate. The inactivation of the enzyme by various NPs depends on their physicochemical properties, including size, shape, curvature, and surface functional groups55.

Inhibition of Aβ fibrillization

Numerous studies have highlighted that the deposition of senile plaques (Aβ aggregates) and Aβ oligomer is crucial in triggering brain astrocytes and microglia, leading to inflammation-mediated neurotoxicity that eventually results in cognitive impairment and synaptic dysfunction. Consequently, it is profoundly essential to inhibit the Aβ aggregation and/or eliminate the preformed oligomers and fibrils for efficacious management of AD56. The present investigation evaluates the capacity of synthesized SeNPs to inhibit the coalescence of Aβ oligomers and the destabilization of preformed fibrils. First, we began by evaluating the fibrillization kinetics of Aβ using Th-T assay. Fibrillization pertains to the mechanism of transforming soluble peptide monomers into insoluble conglomerates characterized by a fibrillar architecture. Th-T is a standard amyloid dye widely accepted for measuring the fibrillization process due to its strong fluorescence after attaching to Aβ-fibril. Th-T fluorescence is directly proportional to the amount and length of the Aβ-fibrils57. Figure 2b presents a typical sigmoidal growth pattern, illustrating an initial lag phase that represents nucleation, followed by rapid growth indicating aggregation, which reaches an equilibration plateau at 96 h, signifying the formation of mature fibrils from Aβ monomers.

In order to investigate how synthesized SeNPs (50 µg/ml) impact previously formed oligomers, we blended the SeNPs and the positive control galantamine (50 µg/ml) with oligomers that were preformed for a duration of 48 h, followed by measuring fluorescence intensity. The results of the Th-T assay (Fig. 2c) exhibited an elevation in fluorescence intensity in a temporal-dependent manner (from 20 to 48 h) in the untreated sample; however, in the SeNPs and galantamine treated specimens, a reduction in fluorescence intensity was noted, signifying that SeNPs inhibit the β-sheet proliferation rate, thereby obstructing the fibrillation process. Synthesised SeNPs may be directly bind with the precisely with residues of monomeric Aβ and prevent β-sheet formation. Confocal microscopy investigations additionally corroborate these findings, demonstrating a reduction in green fluorescence in SeNPs and galantamine-administered specimens in comparison to the untreated specimen (Fig. 2d).

Further validation of results of the Th-T assay was conducted using FTIR analysis. FTIR is commonly employed to monitor changes in secondary structure changes during the aggregation process (α- to β-transition), which leads to amyloid formation32. A notable peak of absorption can be identified in the amide-I (∼1600–1700 cm−1), amide-II (1550 cm−1), and amide-III (1250 cm−1) areas of the infrared spectra, denoting vibrations of C=O, C–N, and N–H bonds occurring in the polypeptide and highlighting its secondary structure32. Based on the amide I band analysis, Aβ (1–40) incubated for 48 h primarily exhibits a β-sheet structure, with a prominent peak around 1630 cm−1. The relatively narrow width of this peak indicates stable and/or long β-strands, which are also typical for extremely stable structures like amyloid fibers. However, the SeNPs and galantamine-treated samples show lower absorbance at 1630 cm−1, and there is also a peak around 1655 cm−1, similar to the Aβ (1–40) oligomers, representing a random coil structure (Fig. 2e). The results of the IR spectra align with Th-T assay, indicating that SeNPs are effective in preventing the aggregation of Aβ oligomers into mature fibrils.

Fig. 2
figure 2

(a) Percentage inhibition of AChE and d BChE by synthesized SeNPs at different doses (10–50 µg/ml) (b) aggregation kinetics of Aβ (1–40) from monomers to mature fibrils (c) Thioflavin T fluorescence assay (d) confocal microscopic images after 20 and 48 h (e) FTIR absorption spectra of amide − 1 region.

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Destabilization of mature Aβ fibrils

The results of the destabilization of preformed mature Aβ (1–40) are presented in Fig. 3. In the confocal microscopic study, green fluorescence signals increased in untreated group, while only a faint fluorescence signal was observed in the SeNPs and galantamine-treated groups (Fig. 3a). The fluorescence intensity in the untreated group rose in a time-dependent manner from 96 h to 9 days, indicating plaque production. Nevertheless, the SeNP-treated cohort displayed a significant decrease in fluorescence intensity, illustrating the dispersal of mature fibrils and the prevention of plaque development (Fig. 3b).

FTIR results in amide-1(1630 cm−1) region show a sharp increase in absorbance in the untreated group incubated for 9 days, suggesting a strong β-sheet structure (Fig. 3c). No peak was observed at 1655 cm−1 (antiparallel-sheet), indicating the absence of oligomers. The SeNPs-treated group displayed lower absorbance at 1630 cm−1 and a small peak at 1655 cm−1 indicating the presence of oligomers. When comparing the untreated groups to the SeNPs-treated, both after 96 h and 9 days, the results strongly suggested that synthesized SeNPs not only effectively prevented the formation of mature fibrils but also destabilized preformed ones.

Fig. 3
figure 3

(a) confocal images of collapse of preformed mature Aβ (1–40) fibrils by SeNPs and positive standard galantamine (b) Thioflavin T fluorescence assay (c) FTIR analysis after 9-day treatment.

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Molecular simulation

Earlier research indicates that the prevention of Aβ fibrillization by NPs is affected by factors such as surface chemistry, surface charge, and size. Additionally, the role of hydrophilic interactions is vital, as they denote the phase that restricts the rate in amyloid progression58,59. Studies also demonstrate the effectiveness of functionalized SeNPs in AD, both in vitro and in vivo; however, the actual mechanism of binding of SeNPs and the Aβ peptide is not well in the literature60. To better understanding the actual mechanism of Aβ fibrillization prevention, we have utilized atomistic molecular dynamic simulation. Molecular dynamics simulation is typically employed to obtain interaction trajectories that enhance our understanding of biological processes. Computer simulations serve as a complementary approach to experimental studies and help in acquiring interaction trajectories for a deeper understanding of biological processes61.

To accurately replicate the experimental process, we initially capped the Se core with CQA a 100 ns simulation. At the end of the simulation, we found that 80 CQA molecules were capped on the surface of Se core (Fig. 4a). The CQA forms well-known metal accepter interaction with Se and also established hydrogen bonding and Pi-Pi stacking, as well as hydrogen bond, and electrostatic interaction among them (Fig. 4d). Figure 4b displays a snapshot from the 100 ns simulation of the Aβ peptide (1–40). At the simulation’s conclusion, it exhibits a secondary structure similar to what was reported in the experimental section and by other researchers62. Figure 4c represents the snapshot of the 100 ns simulation of Aβ peptide (1–40) with CAQ capped SeNPs. It shows that Aβ peptide (1–40) is attached to the capped SeNPs and does not exhibit any secondary structure. The hydrophobic region of the fibril interacts directly with CAQ at the surface of the SeNP (Fig. 4e). The Aβ-peptides do not lie flat on the SeNP surface, as it is heavily covered with CQA, with only a few residues pointing towards the SeNPs.

Hydrophobic engagement serves an essential function in the creation of fibrils in Aβ (1–40). Studies have reported that the hydrophobic contact between Phe19 and Leu34 has been observed in nearly all structural studies of Aβ (1–40) fibrils (10–14). Modifying Phe19 with proline significantly prolonged the fibrillation times, as demonstrated by the ThT fluorescence assay63. In the present study, the hydrophobic residues of Aβ (1–40) primarily attached to the CQA-capped SeNPs, which resulted in less aggregation in the CQA-SeNPs-treated sample. Furthermore, CQA-SeNPs bind with Leu34, forming a Pi-Alkyl interaction (Fig. 4f), and with Phe19, they form a hydrogen bond (Fig. 4f), thus rendering both residue inaccessible for the conventional hydrophobic interactions observed in the Aβ (1–40) fibril64. This also indicated the prevention of Aβ (1–40) fibrillation.

Fig. 4
figure 4

(a) 100 ns snapshot of CAQ capped Se core (b) 100 ns snapshots of Aβ (1–40) peptide (c) 100 ns snapshots of the adsorption of Aβ (1–40) peptide on CAQ-capped SeNP (d) interaction of CAQ with SeNP (e) Interaction of hydrophobic amino acid (blue) of Aβ (1–40) peptide with CAQ-capped SeNP (f) interaction Leu34 (purple)residue in Aβ (1–40) peptide with capped CAQ-SeNP (g) interaction Phe19 (cyan) residue in Aβ (1–40) peptide with capped CAQ-SeNP.

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Structural deviations and compactness

Root-mean-square deviation (RMSD) plays a crucial role in understanding structural variations and the dynamic behaviour of protein structures37. Figure 5a illustrates the RMSD trend for Aβ (1–40) peptide in both the apo and Aβ (1–40)-CQA-SeNP complex, respectively. The RMSD plot indicates that the Aβ (1–40) peptide stabilises after binding with CQA-SeNP compared to the free Aβ (1–40) peptide. The attachment of CQA-SeNP results in reduced structural deviations of the Aβ (1–40) peptide from its native conformation, thereby stabilizing it throughout the simulation trajectory. In contrast, the apo simulation shows significant changes in the native structure of the Aβ (1–40) peptide.

The radius of gyration (Rg) is closely associated with the density, stability, and conformation of protein architectures65. We calculated the stability of Aβ (1–40) peptide and the Aβ (1–40)-CQA-SeNP complex by computing the Rg of both systems (Fig. 5b). The Rg plot shows an increase in Rg values for the Aβ (1–40)-CQA-SeNP complex, suggesting that the peptide does not fold. In contrast, a lowering of Rg for the Aβ (1–40) peptide, denoting the formation and quantity of Aβ (1–40) peptide was observed.

Solvent-accessible surface area (SASA) provides an estimate of the attachment of nearby solvent to the protein based on its electrostatic and surface interactions66. The behaviour of solvent molecules varies under different conditions and can be used to investigate the conformational changes in protein under solvent conditions. We calculated the SASA of the Aβ (1–40) peptide in the apo state and Aβ (1–40)-CQA-SeNP complex to explore conformational changes during the simulation (Fig. 5c). The SASA of Aβ (1–40) peptide during the apo simulation shows decreasing trend due to the folding of Aβ (1–40) peptide. In contrast, the SASA in the presence of CQA-SeNP does not exhibit significant variations throughout the simulation, indicating the unfolded state of the Aβ (1–40) peptide. This suggests the structural stability of Aβ (1–40) peptide in the presence of CQA-SeNP.

Intramolecular hydrogen interactions within a protein macromolecule are crucial for the integrity of its tertiary conformation67. We have calculated the kinetics of intramolecular hydrogen interactions in the Aβ (1–40) peptide within the apo and in the Aβ (1–40)-CQA-SeNP complex (Fig. 5d). The intramolecular hydrogen bond in the apo simulation Aβ (1–40) peptide increases, whereas in the presence of CQA-SeNP, it remains nearly the same. This again confirms that CQA-SeNP prevents secondary structure formation in Aβ (1–40).

Fig. 5
figure 5

(a) Root-mean-square deviation (RMSD) plot of Aβ (1–40) peptide before and after binding of SeNP (b) Time evolution of the radius of gyration. (c) SASA plot of Aβ (1–40) peptide as a function of time. (d) Estimation of hydrogen bond of Aβ (1–40) peptide before and after binding of SeNP. (e and f) Gibbs free energy landscape plot of Aβ (1–40) peptide before and after binding of SeNP. The values were obtained from the 100 ns molecular dynamic (MD)simulation time scale. Gray and maroon represent values obtained for Aβ (1–40) peptide apo and Aβ (1–40) peptide-SeNP complex, respectively.

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Free energy landscape analysis

The Gibbs free energy landscape (FEL) depicts the global minimum energy conformation for a structure, such as a receptor-ligand complex. Dark blue areas signify energy minima and energetically favorable structural conformation, while the red and yellow areas denote unfavourable structural confirmation. The FEL for the Aβ (1–40) peptide in its apo form and the and Aβ (1–40) peptide-CQA-SeNP complex has been generated by calculating the first two principal components (PC1 and PC2) as reaction coordinates via GROMACS built-in scripts67 (Fig. 5e and f). The Aβ (1–40) peptide exhibits a single global minimum confined within a local basin. However, in the presence of CQA-SeNP, the Aβ (1–40) peptide demonstrates different conformational motion with noticeable changes and does not evolve into multiple stable global minima. The analysis indicates that CQA-SeNP influences the size and position of the sampled essential space of Aβ (1–40) molecules, which maintain a stable single global minimum.

Conclusion

This study demonstrates the potential of CQA-capped SeNPs as a multi-faceted therapeutic agent against Alzheimer’s disease. Synthesized using a green chemistry approach with CQA-rich PSP extract, these nanoparticles exhibited significant antioxidant, acetylcholinesterase inhibitory, free radical scavenging, and metal chelating activities. Critically, CQA-SeNPs effectively reduced the propensity for Aβ fibrillization and destabilized preformed mature fibrils, suggesting a dual mechanism of action against Aβ aggregation.

Molecular dynamics simulations furnished additional elucidations regarding the fundamental mechanisms, disclosing that CQA-SeNPs engage with hydrophobic residues of the Aβ peptide, inhibiting the formation of β-sheet conformations and ensuing aggregation. This interaction is mediated by the capping CQA molecules, which form Pi-Alkyl interactions with Leu34 and hydrogen bonds with Phe19 residues of Aβ, effectively disrupting the conventional hydrophobic interactions essential for fibril formation.

These findings highlight the potential of CQA-SeNPs as a promising therapeutic strategy for AD. By targeting multiple pathological hallmarks of the disease, including oxidative stress, cholinergic dysfunction, metal ion dysregulation, and Aβ aggregation, CQA-SeNPs offer a comprehensive approach to AD treatment. Subsequent in vivo investigations are warranted to corroborate these outcomes and evaluate the effectiveness and safety of CQA-SeNPs in a clinical environment. This investigation establishes a basis for the progression of cutting-edge, nanotechnology-focused strategies aimed at preventing and treating Alzheimer’s disease.