Ultrasensitive Fluorescence Detection of Alzheimer's Disease based

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Ultrasensitive Fluorescence Detection of Alzheimer’s Disease based on Polyvalent Directed Peptide Polymer Coupled to a Nanoporous ZnO Nanoplatform Sang-Choon Lee, Hyun-Hee Park, Sang-Heon Kim, Seong-ho Koh, Sung-Hwan Han, and Moon Young Yoon Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03735 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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Analytical Chemistry

Ultrasensitive Fluorescence Detection of Alzheimer’s Disease based on Polyvalent Directed Peptide Polymer Coupled to a Nanoporous ZnO Nanoplatform

Sang-Choon Lee†,‡, Hyun-Hee Park§, Sang-Heon Kim†, Seong-Ho Koh§, Sung-Hwan Han||, and Moon-Young Yoon†,*

†, ||Department

of Chemistry and Research Institute for Natural Sciences, Hanyang University,

Seoul 04763, Republic of Korea. ‡Department

of Chemistry, Georgia State University, Atlanta, GA 30303, USA.

§Department

of Neurology, Hanyang University College of Medicine, Seoul 04763, Republic

of Korea.

*Corresponding

author:

Tel: +82-2-2220-0946 Address: Department of Chemistry, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 04763, Republic of Korea. E-mail address: [email protected] (M. Y. Yoon)

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ABSTRACT Amyloid-beta 42 (Aβ42), the key biomarker of Alzheimer ’s disease (AD), aggregates to form neurotoxic amyloid plaques. In this work, we modified two fluorescein isothiocyanate-labeled Aβ42-targeting peptides and designed an Aβ42-specific ultrasensitive polyvalent-directed peptide polymer (PDPP) to enhance AD diagnosis sensitivity. The dissociation constant of Aβ42 by PDPP was 103-fold higher than the single-site directed peptide. The improved binding was due to the ability of PDPP to detect multiple receptors on the target. The power of the PDPP diagnostic probe was verified in its application to detect Aβ42 in cerebrospinal fluid (CSF), which showed a lower limit of detection (LOD) in the fg mL-1 range, which is more sensitive than detection by antibodies or single peptides. In addition, we present a novel ultrasensitive diagnostic system using an array of nanoporous ZnO nanoparticles, which play a role in fluorescence signal amplification, to further improve AD diagnosis sensitivity. We enhanced the signal based on the properties of nanoporous ZnO nanoparticles and measured and quantified an ultra-low concentration (ag mL-1 range) of Aβ42. This PDPP coupled to the nanoporous ZnO-based system is a novel approach to AD diagnosis that might also be useful for the detection of other target biomarkers and clinical applications.


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INTRODUCTION Alzheimer’s disease (AD) is a neurodegenerative disorder that is one of the leading causes of dementia. The primarily neurotoxic and abundant biomarker of the AD is that amyloid-beta aggregates to form amyloid plaque.1 Symptoms for AD were confirmed via various diagnostic methods such as the detection of AD-specific biomarkers (Aβ42 and various tau types) in cerebrospinal fluid (CSF), the imaging of patient brains via magnetic resonance imaging (MRI) or positron emission tomography (PET) and identifying the mild cognitive impairment (MCI) stage.2 Levels of CSF analytes related to AD such as Aβ42, total tau (t-tau), and phosphorylated tau (p-tau181) are associated with pathologic AD diagnosis.3 Particularly, individuals with late dementia and aggregation of Aβ plaques in the brain have lower CSF Aβ42 level but higher CSF tau or p-tau level.4,5 While these analytes are promising markers for the improvement of early and more accurate AD diagnosis, the development of various effective diagnostic approaches to detect AD at the early pre-symptomatic stages has long been a challenge.6 To treat AD patients at their cognitively normal or pre-symptomatic stage, more advanced and highly sensitive and selective diagnostic probes to detect these primary biomarkers must be developed. With the increased need for reliable methods to detect accurate biomarkers of the AD before the first pathologic symptoms are observed, ultrasensitive and highly target-specific diagnostic tools are required. The detection of amyloidogenicity by Aβ42 has focused on the application of Aβ42-specific antibodies,7,8 but recent research has focused on the use of peptides instead of antibodies due to their smaller size, higher stability in harsh conditions due to lower immunogenicity, lower cost, and less time-consuming synthesis.9-11 However, the use of a single detection peptide has considerable limitations such as lack of target specificity and low binding affinities toward the site of interest.12 Therefore, a probe with an improved binding affinity with a great capacity for detecting multiple receptors on a target and increased target-



specificity is required in addition to the advantages of single peptide13 use for early diagnosis of the AD. Various detection systems have been proposed with many methods linked to nanomaterials such as nanowire- (NW),14 carbon nanotube- (CNT), and graphene-based platforms using field effect transistors (FET),15,16 silver nanoparticle-based signal amplification,17 and scanning tunneling microscopy (STM)-based electrical detection.18 In this study, we modified two Aβ42-specific peptides developed by Larbanoix et al.19 and designed a multivalent peptide probe (polyvalent directed peptide polymer, PDPP) that enhance a binding sensitivity and specificity by cooperative binding to the target molecule. To fabricate a novel ultrasensitive diagnostic system, we further applied the use of a nanomaterial (nanoporous zinc oxide (ZnO))-based fluorescent detection method for AD diagnosis at the preclinical stage (Figure 1). This technique aims to bind multiple target sites to enhance binding affinity, sensitivity, and selectivity. The polyvalent exposure of PDPP to Aβ42, the main component of amyloid, enhanced sensitivity by approximately 104-fold compared to a FITClabeled single peptide. Interestingly, the binding affinity values of each peptide in the PDPP form to the target were also considerably increased relative to single peptide binding to Aβ42. To further enhance binding sensitivity, the PDPP-based detection system was further applied to a ZnO-based nanoplatforms. We confirmed the efficiency of the nanoporous ZnO platform compared to nanorods. Nanoporous ZnO showed efficient enhancement of fluorescence that might be due to its high surface area. In addition, the probe was tested on CSF samples from AD patients to test its ability as a potent early-stage AD diagnostic approach. Binding of PDPP on the nanoporous ZnO system significantly increased the limit of detection (LOD) for Aβ42 to 12 ag mL-1, whereas the LOD of the well-known antibody-based system typically used in hospitals is 100 pg mL-1.7 Consequently, our novel diagnostic system supports the ultrasensitive detection of Aβ42 with approximately a 104-fold higher LOD than the current diagnostic system for AD diagnosis.


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EXPERIMENTAL SECTION Animals. All experiments involving animals were performed in accordance with the Hanyang University guidelines for the care and use of laboratory animals. As much as possible, we reduced the number of animals used and minimized suffering. Every animal was used only once. Wild-type mice and Alzheimer’s disease (AD) transgenic (Tg) mice (3x Tg-AD) were purchased from Jackson Laboratory (Bar Harbor, ME, USA). The mice were maintained at 60 ± 10% relative humidity and 12 h light and dark cycles at 23 ± 2 ℃.20,21 Preparation of Brain Tissue Sections. To confirm the accumulation of Aβ42 in the mouse model, 5- and 15- month-old 3x Tg-AD mice were used for in vivo experiments. All samples are prepared from Dr. Koh’s laboratory, Department of Neurology, Hanyang University College of Medicine. The mice were sacrificed and perfused with 0.9% saline (Frenz, Sejong, South Korea) for exsanguination. After blood removal, the mice were fixed with 4% paraformaldehyde (PFA) for 24 h followed by harvesting of the brains. The collected brains were incubated in 30% sucrose (Amresco, OH, USA) for 2 to 3 days until the brain samples sank to the bottom of a tube. Finally, the brains were fixed with optimal cutting temperature (OCT) compound (SAKURA, CA, USA) at -20 °C and cut across coronal sections at 20-µm intervals.22 Verification of Synthetic Peptide Binding Capacity on Tissue Samples via Immunohistochemistry. Sectioned brain tissues mounted on glass slides were outlined with a super pap pen (Life Technology, MD, USA) to avoid cross contaminations between samples. The fixed tissues were permeabilized with 50% ethanol in 10 mM PBS for 30 min. Endogenous peroxidase activity was blocked with 0.3% H2O2 in PBS for 10 min and the tissues were incubated with 10% normal serum in PBS for 60 min. The tissues were then incubated overnight in 0.5% normal serum in PBS containing the following primary antibody and probes:



anti-oligomer β-amyloid (1:100, EMD Millipore, MA, USA) and 2.5 µM of FITC-labeled P1 and P2 peptide. The next day, the tissues were incubated with 0.5% normal serum in PBS containing anti-rabbit tetramethylrhodamine (TRITC) (Thermo Fisher Scientific Inc., MA, USA) as a secondary antibody for 2 h. The tissues were washed several times with PBS and mounted with mounting medium containing a 4',6-diamidino-2-phenylindole (DAPI, Vector Laboratories, CA, USA). The tissues were observed using a fluorescence microscope (Eclipse Ti, Nikon, NY, USA).22,23 Synthesis and Binding Affinity of the PDPP. The fabrication protocol of the polyvalentdirected peptide polymer (PDPP) follows the modification of flexible poly-d-lysine hydrobromide (PDL, MW 30-70 kDa, Sigma, MO, USA). First, PDL was activated with Nsuccinimidyl-3-(2-pyridyldithio) propionate (SPDP, Thermo Fisher Scientific Inc., MA, USA) as a bifunctional linker dissolved in dimethyl sulfoxide (DMSO, Sigma) with 75 mM sodium acetate buffer (pH 8.5) for 1 h at RT. After gel filtration of the PDL-SPDP backbone, it was conjugated with the FITC-labeled peptide (Figure S1) The yield of synthesized PDPP was determined by comparing with FITC-labeled single peptide using fluorescence intensity value as described previously.24 The binding affinity of the synthesized PDPP toward Aβ42 and Aβ42 in CSF sample was tested by the same protocol of peptide binding affinity described in the Supporting Information. Fabrication of ZnO Nanoporous Platform for Aβ42 Detection in CSF and LOD Estimation. For the detection of Aβ42 in CSF with probes (peptides and PDPPs) on the nanostructure platform, we used the ZnO nanoporous platform from Dr. Han’s laboratory, Department of Chemistry, Hanyang University. Briefly, CdO porous films were dipped in Zn2+ ion solution at 60 ºC for various time durations. Ion exchange of Cd2+ with Zn2+ occurs rapidly and was clearly visible due to change in color from dark brown (CdO) to white (ZnO). The reaction was carried out for 6 h to ensure complete ion exchange. The characterization of crystal structure


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and phases of the ZnO nanoporous platform was confirmed using X-ray diffraction patterns (XRD) obtained using an X-ray diffractometer (Rigaku D/MAX 2500 V, Cu-Kα, λ = 0.15418 nm). The morphology of the crystals was monitored using scanning electron microscopy (SEM, Hitachi S-4200).25,26 To identify properties such as binding efficiency and limit of detection (LOD) for the probes toward Aβ42 in CSF on the ZnO nanoporous platform, the experiment was carried out via a sandwich-binding assay using the same probes (P2 single peptide and P1 or P1, 2-PDPP). The test was initiated by adding 1 µM capture peptide (P2) to immobilize on the ZnO nanoporous platform for 4h at RT. After washing with PBST, the next measurement was performed with the same procedure as described above. The signal was analyzed using fluorescence microscopy (Olympus, Japan).27-28

RESULTS AND DISCUSSIONS Analysis of Binding Efficiency and Cytotoxicity for Individual Aβ42 Diagnostic Probes. To verify the binding affinity of the modified peptide compared with previously reported cyclic peptide binding, we evaluated the dissociation constant (Kd) values for the biotin-labeled P1 and P2 on human Aβ42 by enzyme-linked immunosorbent assay (ELISA, Kd for P1-Biotin = 35.3 µM; Kd for P2-Biotin = 34.7 µM, data not shown). These Kd values are comparable to the reference values for the circular form of the biotinylated peptides (Kd for PHIb = 22.5 µM; Kd for PHOb = 24.1 µM).19 The low binding affinity values of the biotinylated peptides indicate limitations for the use of a single peptide to recognize the target of interest. To apply the Aβ42 binding peptide to a different type of diagnostic system, both Aβ42-binding peptides were labeled with fluorescein isothiocyanate (FITC) instead of biotin (FITC-P1: AcFRHMTEQGCGK-FITC; FITC-P2: Ac-IPLPFYNGCGK-FITC). Due to the highly sensitive properties of FITC, its reduced time for detection and reduced chance for interactions in



between probes (i.e, the target to the primary antibody or the primary antibody to the secondary antibody),18 the Kd values of FITC-P1 and P2 were measured to show approximately 40-fold lower binding affinity toward Aβ42 (Kd for FITC-P1 = 796 nM; Kd for FITC-P2 = 949 nM) relative to the biotinylated probes (Figure S2), suggesting that the fluorescent measurement method is an efficient tool for Aβ42 diagnosis. To confirm the toxicity for modified peptides, we carried out a cytotoxicity test via MTT assay using a RAW 264.7 macrophage cell. Consequently, peptides are shown to be non-toxic to the cells. Relatively, negative control (DMSO and SDS known to be toxic materials in the cell) induced cell toxicity with approximately 85% decreased cell viability (Figure S2C). We further designed a novel flexible polyvalent-directed peptide polymer (PDPP) via peptide and polymer combination using an SPDP linker to enhance sensitivity and specificity. Determination of the binding affinity of P1-PDPP, P2-PDPP, and P1, 2-PDPP to Aβ42 showed enhanced binding sensitivity relative to each peptide bound to the target (Figure S3, Kd for P1PDPP = 983 pM; Kd for P2-PDPP = 23.4 pM; Kd for P1, 2-PDPP = 3.13 pM). The PDPP form dramatically increased Aβ42 detection relative to the binding affinity values of the Aβ42-binding single site-directed peptides (approximately 103 and 105 times higher sensitivity, respectively; Table S1). The enhanced binding affinity of PDPP is likely due to the highly favorable gap distance between each peptide connected to the polymer backbone, which can be best-fit with the gaps of each Aβ42 peptide binding site. In addition, the hydrophobic amino acid-rich PDPP peptide form (P2-PDPP) showed a 42-fold higher binding affinity than that obtained by the hydrophilic amino acid-rich peptide (P1-PDPP). This phenomenon can be explained by the highly hydrophobic properties of Aβ42,19 where the multiple hydrophobic amino acid-rich peptides (P2) attached to the polymer could form abundant hydrophobic bonds with Aβ42. When the two peptides were randomly conjugated to the flexible PDL backbone, both the hydrophilic and hydrophobic properties of peptides presumably bound to each peptide’s


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corresponding target-specific site, which strengthened the binding affinity to greater than those of P1-PDPP and P2-PDPP. These polyvalent interactions can be explained by the simultaneous binding of multiple ligands to multiple receptors, thus enhancing PDPP sensitivity.24 We carried out the probe binding test with a different type of Aβ42 formation to probe the biological selectivity. Aβ42 oligomerization was constructed by incubation time-dependent and tested the activity by treated ThT. The result is shown in Figure S4A. As increased incubation time, the fluorescece intestity by ThT binding to β-sheet was relatively increased due to Aβ42 oligomerization. At the condition of 80 and 180 min reaction, the Aβ42 formed oligomer and fibril. Based on this, we tested probe binding affinity using P1, 2-PDPP in a concentrationdependent manner. As shown in Figure S4B, the probe was detected to monomer, oligomer, and fibril of Aβ42 with similar Kd value (data not shown). The peptides used in this system were screened against monomer Aβ42 and these are also shown to be bound to plaque in Larbanoix’s work. Based on this information and our data, we can explain that the peptides are not able to detect specific formation of Aβ42, because they interact with the specific domain of Aβ42, not structurally bind. Two peptides are bind to N- or C-terminus of Aβ42 that has hydrophilic and hydrophobic properties, respectively. That is why the peptides can bind to monomer, oligomer, fibril plaque of Aβ42. Verification of Probe Binding Ability on 3x Tg-AD Mouse Brain Tissue via Immunohistochemistry. To confirm modified peptide binding in AD mouse brain tissue, we obtained brain tissue from 5- and 15-month-old 3x Tg-AD (APP-PS1 tauP301L) mice with three mutant alleles, including homozygous mutations for Psen1 and co-injected with APPSwe and tau 301L transgenes. In the 3x Tg-AD mouse model, amyloid-beta peptide deposition in brain tissue is indicated early at 3 to 4 months, and aggregates of conformationally altered and hyperphosphorylated tau are detectable between 12 to 15 months. Here, we focused on amyloid-beta deposition and first conducted immunohistochemistry (IHC) test with the anti-



oligomer β-amyloid antibody. As shown in Figure S5, the accumulation of Aβ42 as an oligomer, fibril, and plaque formation was only detected in the brain tissue of 15-month-old mice and was rarely detected in 5-month-old mice. After confirmation of accumulated Aβ42 in mouse brain tissue, we tested it with single modified P1 or P2 peptide and compared the results with those of original probes previously reported. Both peptides detected accumulated Aβ42 in the 15-month-old brain and not in the 5-month-old brain. P1 peptide clearly detected Aβ42 with fibril formation (Figure 2). Thus, the modified peptides showed the similar binding ability to Aβ42 as the original probes in 3x Tg-AD mouse brain tissue. Aβ42 Detection in CSF from the AD and preAD Patients. We measured the concentration of Aβ42 in CSF samples from four moderate to late dementia (AD) patients and seven very mild to mild dementia (preAD) patients using a multiplex xMAP Luminex platform with Innogenetics immunoassay kit-based reagents.29 Among the AD patients, three (N-17, 20, and 33) were diagnosed with an AD with low levels of Aβ42 in the CSF and the rest as presymptomatic stage AD. The patient cognitive performance was characterized by levels based on the Mini-Mental Status Examination (MMSE), Clinical Dementia Rating (CDR), Aβ42, t-tau, and p-tau181. Cognitive decline rates were estimated by the slope of MMSE score linear regression over time (Table S2). Based on CSF Aβ42 concentration obtained by following the above protocol, a comparison of each Aβ42 targeting probe, FITC-conjugated monoclonal antibody, modified single peptide, and PDPP, was performed with N-6 CSF samples that were representative of AD patients (group A). The N-6 CSF sample was used in this test because the concentration of Aβ42 and CDR level are similar with the mean concentration of CSF Aβ42 (485.76 pg mL-1) in group A. Estimation of LOD for CSF Aβ42 via the commercial monoclonal antibody CSF ELISA kit was set to 100 pg mL-1.7 However, the highly sensitive properties of FITC induced a lower LOD of CSF Aβ42 by the FITC-conjugated monoclonal antibody.30 Compared to the LOD obtained by


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the FITC-conjugated monoclonal antibody to Aβ42, the single peptide could detect CSF Aβ42 at similar or higher CSF Aβ42 concentration (Figure 3A), demonstrating the limitations of a single peptide in recognizing a specific target due to low binding affinity toward the site of interest. When both peptides were mixed together (P1, 2-PDPP), Aβ42 detection in a 1:500 dilution CSF sample was confirmed due to the ability of each peptide to bind to specific sites in Aβ42. The binding affinity of each probe tested on CSF Aβ42 validated the efficiency of PDPP by showing that the highest affinity toward the target was found with the mixture of both P1 and P2 in the PDPP form. The reason for the enhancement of binding affinity by PDPP than single peptide is caused by existed a high number of a peptide having a different binding function which can be supported a binding opportunity. This is a simultaneous binding of multiple ligands to multiple receptors and it thus affected in an increase of the PDPP’s sensitivity. That is why the PDPP shows high sensitivity in CSF Aβ42 detection. Using P1 and 2-PDPP as the diagnostic probe, we examined CSF Aβ42 detection for early to late dementia (group B) compared to moderate to late dementia (N-17, N-20, N-33 and N-34) and very mild to mild dementia (N-9, N-21, O-16, and N-36; group C, Table S2). As mentioned previously, patients with moderate to late dementia have lower Aβ42 concentrations in CSF in contrast to patients with very mild to mild dementia. Therefore, higher concentrations of CSF Aβ42 were detected in very mild to mild dementia patients based on higher fluorescence intensity values obtained from these patients. Gaps in fluorescence intensity values between the two groups at low CSF Aβ42 concentrations are shown in Figure 3B. The graph shows that patients with moderate to late dementia tend to have lower fluorescence intensity levels than patients with very mild to mild dementia. This result suggests that using P1 and 2-PDPP is an accurate diagnostic strategy for the categorization of patients based on lower concentrations of CSF Aβ42 (moderate to late dementia) compared to those who have higher concentrations (very mild to mild dementia). As shown in Figure 3, P1, 2-PDPP has higher sensitivity and higher



binding affinity toward Aβ42 in human fluid samples than any other probe tested here. Therefore, the P1 and 2-PDPP probe with two different peptide characteristics attached to the flexible polymer backbone yields ultrasensitive binding that can be used a biotechnological tool for early AD diagnostics for the treatment of onset AD. Fabrication of a Nanoporous ZnO Nanoplatform and Verification of Aβ42 Fluorescence Signal. ZnO nanostructures are used as a functional material because of their novel properties such as transparency in the visible range, electrical stability, and energy band gap. It is also a unique material that exhibits both semiconducting and piezoelectric properties.31 For these reasons, ZnO nanomaterials function to enhance fluorescent signals in the detection of fluorescent-labeled biomolecules32,33 for use in biosensors and biomedical applications. In previous research, ZnO nanomaterials were confirmed to increase fluorescence signals.34 Herein, a nanoporous ZnO platform was obtained by cation exchange (Zn+2) with CdO architectures, and its XRD pattern is shown in Figure 4A. Cubic phase CdO (JCPDS 73-2245) architectures were transformed to hexagonal ZnO (JCPDS 36-1451) architectures within 30 min of ion exchange. The formation of hexagonal ZnO from cubic CdO architectures can be explained based on the relative stability of the hexagonal phase over the cubic phase of ZnO. Scanning electron microscopy (SEM) images of the CdO and ZnO products formed after ion exchange are shown in Figure 4B. To verify fluorescence by the biomolecule on the ZnO nanostructures (nanorods and nanoporous), we used 0.5 µg mL-1 Aβ42 with 1 µM single peptide (P1). As shown in Figure 4C, the fluorescence signal was detected with high intensity on both nano-structures. In terms of sensitivity between the two structures, nanoporous ZnO was visibly clearer and resulted in high fluorescence intensity due to biomolecule localization on the nanostructures. The nanorod structure has a relatively horizontal space between the rods, which is why small biomolecule such as Aβ42 can bind to both the top and the bottom of the rods. When we used fluorescent microscopy to measure the fluorescence signal, the laser


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source was reflected in a top-down approach. The signal measured depending on the number of biomolecules present on the surface of the nanostructure. Thus, the nanoporous ZnO structure, which had abundant Aβ42 bound to the surface, can support a high fluorescence signal and can be useful in the detection of Aβ42. Detection of Aβ42 in CSF with Peptide/PDPP on the Nanoporous ZnO Platform and Estimating the LOD. To determine optimized Aβ42 (in CSF) and probe concentrations, we first tested single peptide concentrations in the presence or absence of target using a sandwichbinding assay on the ZnO nanoporous platform. As shown in Figure S6, the fluorescence signal was detected with high intensity with increased detection peptide (P1, detection peptide) concentration, but was not detected in the absence of target or probe. Next, we optimized peptide binding efficiency with P1 serial dilution at a constant target concentration (Figure S7A) and then with 100-fold diluted-Aβ42 using the optimized P1 concentration (Figure S7B). The optimized P1 concentration and limit of detection (LOD) for Aβ42 on the ZnO nanoporous platform were estimated at 100 nM and 5 fg mL-1, respectively. To verify probe binding to Aβ42, we further performed the same test using CSF samples (N17 and O-16 representative of AD and preAD patient samples, respectively), commercialized Aβ42 (cAβ42), and recombinant Aβ42 (rAβ42), which was produced from a recombinant plasmid construct consisting of an inserted Aβ42 gene in the pET28a expression vector (Figure S8A and S8B) and purified rAβ42 protein via affinity chromatography and gel filtration (Figure S8C, detailed information for experimental methods is described in the Supporting Information). The binding ability of rAβ42 was confirmed with FITC-conjugated His-antibody (Figure S9A) and Aβ42 monoclonal antibody (Figure S9B) and compared with cAβ42 binding in 96-wellplates via ELISA. In the probe binding test with the different Aβ42, the detection values for each probe corresponded with Aβ42 in a concentration-dependent manner, in which we used the same diluted Aβ42 ratio from each stock (Figure S10A). When applied to the ZnO



nanoporous platform, fluorescence images and signals were clearly detectable and increased with a corresponding Aβ42 concentration in CSF. The LOD values were determined at 12.8 fg mL-1 for N-17 CSF and 14.5 fg mL-1 for O-16 CSF (Figure S10B). These LODs are similar to the LODs for cAβ42. We further demonstrated an aligned concentration of cAβ42 and Aβ42 in CSF (Figure 5A). As Aβ42 in CSF increased in concentration, the fluorescence intensity values were enhanced depending on the same Aβ42 concentration ratio. The selectivity for probe was further verified by tested toward tau (one of crucial biomarker in neurodegenerative diseases) and CD44 (glycoprotein existed in various mammalian cell surface). As shown in Figure 5B, the probe was confirmed to be selectively bound to Aβ42, not in tau and CD44. To confirm PDPP specificity, we used bovine serum albumin (BSA), and the signal was not observed. With the above results, we verified peptide binding efficiency to Aβ42 in CSF on the ZnO nanoporous platform. The FITC-labeled peptide was relatively detectable with accurate and enhanced fluorescence intensity on the ZnO nanoporous platform for the detection of Aβ42 in CSF relative to detection on 96-well-plates given the high error values at the low Aβ42 concentration in CSF. The LOD value for Aβ42 in CSF based on FITC-labeled peptide was not the highest value that has been reported in the literature. Therefore, we designed a novel probe, a polyvalent directed peptide polymer (PDPP), and applied that to the nanoporous ZnO platform for AD diagnosis to enhance sensitivity and LOD due to the interaction of PDPP with the ZnO nanoporous system. The use of the ZnO nanoporous system combined with PDPP resulted in a wider detection range, as low as attogram level, for Aβ42 in CSF, which was at least 100-fold more sensitive than the single peptide LOD (Figure S11). This detection value is further verified by measuring a relative fluorescence intensity with quantifying the detection of Aβ42 in CSF in Figure S12. The fluorescence signals according to the Aβ42 in three different samples were similarly detected, indicating the concentration of Aβ42 in CSF can be verified by showing a


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detection value similar to the quantified cAβ42. We thus estimated the LOD for PDPP at approximately 12.8 ag m-1, which is 103-fold lower than that of the single peptide (Table 1). This results consistently demonstrate that the sensitivity of the ZnO nanoporous-PDPP system approaches attogram concentration. Furthermore, we performed image analysis to directly compare detectable intensity by treated probe dependent as shown in Figure 6. The PDPP drive to enhanced fluorescence intensity than the single peptide or anti-Aβ42 and the rate of increased intensity were indicated to correspond with estimated each probe binding affinity in Table 1. In comparing with other previously reported diagnostic systems for the AD, this PDPP-ZnO nanoporous detection system is significantly more sensitive and more efficient in the ultralow detection range. The ultrasensitive fluorescence imaging of this system can be explained by the change in photonic node density and the reduction in self-quenching of the fluorophores due to the presence of traps in the energy levels.35 It was reported that when protein deposits on ZnO nanostructures, the fluorescence signal is enhanced due to the inherent properties of ZnO that are not present on poly-methyl methacrylate (PMMA), silicon, glass, or quartz surfaces.36 Consequently, the PDPP-ZnO nanoporous-based detection system supports ultrasensitive diagnostic ability and has potential as a biotechnological tool for the detection and treatment of early-onset disease and can be a powerful diagnosis tool for not only the AD but other diseases as well.

CONCLUSIONS In conclusion, the detection and quantification of ultralow target molecule concentrations play a critical role in basic drug discovery and clinical applications of disease diagnosis. Here, we report advanced target binding affinity with exceptional ultra-sensitivity in vitro and a lowered human fluid target detection limit via simultaneous, multiple sites-targeted binding with PDPP on a ZnO nanoporous platform relative single-site directed peptide or antibody assays. This



study suggests important possibilities for the utilization of this PDPP-ZnO nanoporous system in human disease diagnosis that could replace antibody-based diagnostic systems (Table S3). Furthermore, the PDPP-ZnO nanoporous system for Aβ42 described here could be a key breakthrough in AD diagnosis with detection of ultralow concentrations of Aβ42 in CSF lower than any currently reported system. PDPP-ZnO nanoporous fabrication of the selected Aβ42specific peptides with high binding affinity shows great potential as an ultrasensitive diagnostic system and provides the possibility for further development of monitoring AD via improved molecular imaging skills.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 201800000001222).

ASSOCIATED CONTENT Supporting Information The supporting information includes specifics regarding instrumentation, additional experimental methods, data for PDPP synthetic mechanism, preparation of CSF samples, characterization of probe binding affinities via fluorometry, immunohistochemistry imaging using anti-Aβ42 in 3x Tg-AD mouse brain tissue, characterization and optimization of probe binding efficiency for Aβ42 in CSF on the ZnO nanoporous platform, gene cloning and purification of recombinant protein for Aβ42, information for CSF samples, and comparison of methods for detection of Aβ42 (in CSF).

AUTHOR INFORMATION Corresponding Author


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*E-mail:

[email protected] (M. Y. Yoon)

Note The authors declare no competing financial interests.

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Figure 1. Description of the Aβ42 detection method on ZnO nanoporous platform using two different probes, single peptide (P2 as a capture probe) and multivalent peptide (P1, 2-PDPP as detection probe), compared with the current detection method via sandwich-binding assay.


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Figure 2. Verification of binding efficiency for synthetic peptides on brain tissue from 3x TgAD mice (5- and 15-month-old) via immunohistochemistry. A 2.5 µM concentration of FITClabeled (A) P1-peptide, (B) P2-peptide and anti-oligomer β-amyloid (1:100 dilution) as the primary antibody, and anti-rabbit tetramethylrhodamine (TRITC) as secondary antibody were used for detection of accumulated plaque-forming Aβ42 in mouse brain tissue. For nuclear staining, 4',6-diamidino-2-phenylindole (DAPI) was added to the mounting medium. The sample signals were observed using a fluorescence microscopy.



Figure 3. Characterization of single peptides and PDPPs binding properties to Aβ42 in AD patient CSF samples. To determine which probe can detect Aβ42 with high sensitivity, (A) the test was conducted with 100 nM peptides, PDPPs, and anti-Aβ42 (1:2000) after immobilization of diluted N-6 AD patient CSF samples on opaque 96-well plates. After verification of probe sensitivity, P1, 2-PDPP was selected as the highly sensitive detection probe. (B) To investigate probe selectivity in real samples, P1, 2-PDPP was tested with two different AD patient CSF sample groups: N-17, N-20, N-33, and N-34 samples contain low concentrations of Aβ42 and are from AD patients, and N-9, N-21, N-36 and O-16 samples contain high concentration of Aβ42 and are from preAD patients. All experiments followed the process described in the experimental section. The signal was measured using a fluorescence microplate reader (excitation at 495 nm and emission at 520 nm).


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Figure 4. Synthetic ZnO nanoporous structures. (A) X-ray diffraction spectra patterns of pristine CdO and ZnO obtained by ion exchange. (B) High and low magnification SEM images of CdO (a and c) and ZnO (b and d) crystals obtained after ion exchange of CdO with Zn(NO3)2 solution for 30 min at 60 ºC on a glass surface. (C) SEM images for comparison of fabricated ZnO nanorods and nanoporous structures (scale bar = 2 µm) of treated 0.5 µg mL-1 of Aβ42 and 1 µM of the P1 peptide. Its corresponding fluorescence images are displayed in Figure S5.



Figure 5. Verification of peptide binding efficiency. To determine (A) the limit of detection (LOD), the probe was tested toward Aβ42 in CSF samples. (B) Confirmation of the selectivity for probe, the test was carried out using CD44 and tau proteins. The experiment was performed with 1 µM capture peptide (P2), 100 nM detection peptide (P1), and different protein concentrations (commercialized Aβ42 (cAβ42)), Aβ42 in N-17 CSF from AD patient, Aβ42 in O16 CSF (from preAD patient), CD44, and tau on 96-well plates. The signal was measured using a fluorescence microplate reader with excitation at 495 nm and emission at 520 nm.


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Figure 6. Direct comparison of fluorescence intensity with probe dependent for detection of Aβ42 (in CSF) on a ZnO nanoporous platform. The experiment was performed with 1 µM capture peptide (P2), 100 nM detection peptides (P1-2 and P1, 2-PDPP), anti-oligomer βamyloid (1:100) and 1.28 pg mL-1 of commercialized Aβ42 (cAβ42), Aβ42 in CSF (N-17 from AD patient and O-16 from preAD patient). The images were obtained using the fluorescence microscopy (scale bar = 10 µm).



Table 1. Comparison of the limit of detection (LOD) for single peptide and P1, 2-PDPP. Single peptide

P1, 2-PDPP

Fold ratio

Aβ42

5 fg mL-1

50 ag mL-1

1.0 × 102 ↑

N-17 CSF

12.8 fg mL-1

12.8 ag mL-1

1.0 × 103 ↑

O-16 CSF

14.5 fg mL-1

12.8 ag mL-1

8.8 × 102 ↑


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