# Biosensors-based polypyrrole/metal-organic framework core-shell for mycotoxins detection

Mostafa Fytory, Fatma Besbes, Diana Dragoe, Rym Mlika, Waleed M A El

Rouby, Ahmed A Farghali, Hassan M E Azzazy, Hafsa Korri-Youssoufi

► To cite this version:

Mostafa Fytory, Fatma Besbes, Diana Dragoe, Rym Mlika, Waleed M A El Rouby, et al.. Biosensors-based polypyrrole/metal-organic framework core-shell for mycotoxins detection. Analytica Chimica Acta, 2025, 1372, pp.344334. 10.1016/j.aca.2025.344334. hal-05403780

HAL Id: hal-05403780

https://hal.science/hal-05403780v1

Submitted on 8 Dec 2025

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# Biosensors-based polypyrrole/metal-organic framework core-shell for mycotoxins detection

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Mostafa Fytory$^{a,b,*}$, Fatma Besbes$^{a,c}$, Diana Dragoe$^{a}$, Rym Mlika$^{c}$, Waleed M.A. El Rouby$^{b}$, Ahmed A. Farghali$^{b}$, Hassan M.E. Azzazy$^{d}$, Hafsa Korri-Youssoufi$^{a,**}$

## HIGHLIGHTS

* Developing advanced biomaterials with enhanced sensing properties is crucial for food safety control.

* We design innovative **core-shell NMOF-polypyrrole** biomaterials functionalized with DNA aptamers.

* The immobilized aptamer enable selective target recognition through a specific binding process.

* The detection is achieved by monitoring the induced variations in the electrical properties of the biomaterial.

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

Handling Editor: Professor Chuck Henry

**Keywords:**
Ochratoxin A
Zr-NMOF
Polypyrrole (Ppy)
Aptamer
Electrochemical
Biosensors

## ABSTRACT

**Background:** Ochratoxin A (OTA) is a naturally occurring carcinogenic mycotoxin that poses critical risks to human health due to its presence in our daily consumed foods and beverages. Electrochemical aptasensor-based assays have been developed as an alternative to classical methods such as HPLC. However, they need many steps for their construction and labeling, which leads to constrained signal output and reduced large-scale production. It's clear that a highly sensitive, selective, and rapid autosensing platform with high stability and printing technology for large-scale production is still needed for mycotoxins detection.

**Results:** Here, we showed that biosensor based on core-shell nanostructure formed with conductive polypyrrole (Ppy) and Zirconium nanosized metal-organic framework (Zr-NMOF) allows an electrochemical detection of an ultra-trace of OTA in foods. The nanostructure was formed by controlled chemical polymerization of Ppy on Zr-

<footnote>$^a$ Université Paris-Saclay, Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO) UMR-CNRS Orsay, 17 avenue des sciences, 91405, Orsay, France</footnote>

<footnote>$^b$ Material Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, 62511, Beni-Suef, Egypt</footnote>

<footnote>$^c$ Laboratory of Interfaces and Advanced Materials, Faculty of Science of Monastir, University of Monastir, Tunisia</footnote>

<footnote>$^d$ Department of Chemistry, School of Sciences & Engineering, The American University in Cairo, AUC Avenue, P.O. Box 74, New Cairo, 11835, Egypt</footnote>

<footnote>* Corresponding author. Université Paris-Saclay, Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO) UMR-CNRS Orsay, France.</footnote>

<footnote>** Corresponding author.
E-mail address: hafsa.korri-youssoufi@universite-paris-saclay.fr (H. Korri-Youssoufi).</footnote>
NMOF to form a core-shell nanostructure followed by DNA aptamers interaction through electrostatic interaction thanks to the positively charged nanostructure provided by P-doping Ppy. We demonstrated that the aptasensor detects OTA through the modification of the electrical properties of the nanostructure thanks to the synergetic effect of the two nanomaterials. A limit of detection of 0.24 pM (0.1 ng/L), and high dynamic linear range were demonstrated. The biosensor also shows high selectivity when compared to OTB detection and can determine OTA directly in red wine with high recovery compared to the standard method.

**Significance:** This work presents the first design of a biosensor based on Ppy/Zr-NMOF/DNA with improved electrical properties and stable properties that can be used as a bioelectronics interface to follow the detection of mycotoxins at a very low concentration of 0.1 ng/L and allows the detection directly in food without any sample treatment. This is an innovative approach that could be adapted for the detection of numerous contaminants by tailoring the specific aptamers.

# 1. Introduction

Ochratoxin A (OTA) is a mycotoxin produced by different strains of *Aspergillus ochraceus* and *Penicillium verrucosum*, and it is prevalent in various food sources, including wheat, coffee, corn, nuts, and wine [1]. OTA poses a significant threat to human health, given its hepatotoxic, nephrotoxic, and carcinogenic effects. Furthermore, its resistance to common metabolic processes in living organisms leads to its accumulation within the body [2]. Due to these considerations, governments and regulatory bodies have implemented regulations on OTA content. For instance, the European Union has set specific limits for OTA in dried fruits (10 µg kg⁻¹), processed cereals (3 µg kg⁻¹), beverages (2 µg kg⁻¹), and red wine (2 µg kg⁻¹). Therefore, the highly sensitive identification of OTA within different food matrices holds significant importance from clinical and health perspectives. Conventional analytical approaches for OTA detection, such as high-performance liquid chromatography (HPLC) [3], mass spectrometry (MS) [4], and enzyme-linked immunosorbent assay (ELISA) [5], have been widely employed. Nonetheless, these approaches are often hindered by drawbacks such as time-intensive processes, elevated expenses, intricate pretreatment procedures, and the need for trained personnel. Consequently, there is a critical need for the exploration of novel analytical methods that offer heightened sensitivity, selectivity, simplicity, rapid response times, and economical solutions for OTA detection.

A variety of biosensors were developed to measure different types of mycotoxins, using unique interactions between the target molecules and detection elements like antibodies, aptamers, or DNA. Among these, aptamers, which are artificial nucleotide sequences with a high affinity for their specific targets, stand out [6-8]. In recent studies, aptasensors have been integrated with various methodologies to detect OTA, including colorimetric [9], fluorescent [10], and electrochemical methods [11-13]. The electrochemical method has gained prominence as a viable alternative due to its notable advantages, encompassing cost effectiveness, fast response, uncomplicated operation, and remarkable compatibility. In a classic electrochemical aptasensor, the aptamer is immobilized as a biolayer on the electrode surface. The interaction between the aptamer and the target leads to detectable electrochemical signals, providing a basis for analysis [14]. Although electrochemical aptasensors exhibit notable selectivity, sensitivity, and simple operation, they have certain limitations. These include the requirement of labeling at a single point with electroactive molecules, such as ferrocene [15] or methylene blue [16], which leads to constrained signal output and reduced sensitivity. Therefore, exploring direct detection methods and pursuing alternative approaches for constructing biosensing systems using simple manufacturing techniques that offer biocompatibility, conductivity, and strong binding capabilities for aptamer probes have garnered significant attention.

Numerous nanomaterials have been used as sensitive substrates for aptasensors, including [17] carbon-related hybrids, inorganic nanomaterials [18], Metal-organic frameworks (MOFs), and polymers [19-21]. MOFs are fabricated by combining metal-based nodes with organic ligands, offering inherent advantages such as elevated porosity, effective absorbability, and the ability to undergo controlled

modification synthesis [22]. The MOF's exceptional suitability as a sensing materials candidate arises from its substantial surface area [23], convenient modifiability [24], and plentiful metal active sites for creating aptasensors and immobilizing aptamer strands to increase detection efficiency [25,26]. Currently, various types of MOFs have been utilized as amplifiers or redox indicators for the detection of OTA. These include Cd-MOF [27], Co/Fe-MOFs [28], Zr-UiO-66 [1], and Co-MOFs [29]. In a recent study, an electrochemical aptasensor was designed for the measurement of OTA and aflatoxin B1 (AFB1) based on a nanocomposite of carbon nanodots, gold nanoparticles, and MOF named HKUST. This study used carbon nanodots loaded with gold nanoparticles as the sensing substrate, while HKUST-1 MOF was used as redox probe-labeled aptamers. HKUST-1 loaded with ferrocene and hemin days used as electroactive bioconjugates for simultaneous detection of AFB1 and OTA [30]. All the above-mentioned studies for electrochemical aptasensors using MOFs to enhance and generate redox signals. Despite achieving a very low limit of detection (LOD) in the femtogram per milliliter (fg·mL⁻¹) range, these biosensing approaches are hindered by complicated construction procedures involving labeled aptamer strands and additional signal amplification steps. Additionally, MOFs are occasionally utilized as signal probes rather than an immobilized matrix, significantly restricting the broader applications of MOFs in aptasensors [29]. However, when used as sensitive substrates for aptasensors, MOFs may have issues including low conductivity and poor stability [31], which would impair their overall sensing performance. To some extent, this limitation can be resolved by integration of the MOF materials with the conductive matrix. For instance, the introduction of conjugated polymers can enhance conductivity and electrochemical activity [32]. A potential alternative strategy involves the selective introduction of guest molecules to create a charge transport pathway while preserving significant porosity. This aims to strike a balance between electrical and thermal conductivity. Various studies have investigated MOFs with guests like conducting polymers (e.g., polypyrrole (Ppy)) [33,34], poly3,4-ethylenedioxythiophene (PEDOT), and redox-active molecules (e.g., 7,7,8,8-tetracyanoquinodimethane (TCNQ)) [35] inside nanochannels, revealing enhanced electrical conductivity but often at the expense of substantial porosity loss.

Zirconium-based MOFs known for their exceptional redox activity and biocompatibility were selected to enhance the electrochemical properties of aptasensor. Zr-based MOF conjugated with OTA-specific DNA aptamers using EDC/NHS chemistry was employed as signal probes for OTA measurement [36]. Conducting polymers such as polypyrrole (Ppy) stands out due to its catalytic activity, incorporation of several functional groups, robust stability, heightened effect of electron delocalization, and the π electron clouds within its conjugated structure. Notably, Zr-MOF and Ppy were gathered to fabricate the Ppy-NMOF hybrid resulting in the UiO-66-Ppy nanocomposite exhibiting multivalent properties from active Zr and Ppy conjugated structure and used for NO⁻² detection. The findings indicated that Ppy microspheres are uniformly coated the exterior surface of the Zr-MOF, enhancing the material's surface characteristics and significantly boosting Zr MOF conductivity [37].

In this work, we reported the synthesis of MOF-Ppy nanocomposites
to obtain core-shell nanoparticles with high conductivity and positively charged surface provided by a thin layer of Ppy on the shell with catalytic properties and high surface area provided by NMOF. The nanocomposites provided a positively charged surface related to p-doping of Ppy leading to aptamer attachment through ionic and hydrogen bonding interaction. The detection of OTA is followed through the change of electrical properties by electrochemical impedance spectroscopy of the nanocomposite after the G-quadruplex structured complex OTA/ aptamer formation on the surface. The synergetic properties of the nanocomposite are sensitive to any modification of the conformation and charge on the surface measured through variation of charge transfer resistance. This strategy is prompting a novel impedimetric aptasensor for OTA detection in various environments (Scheme 1). The Ppy-NMOF core-shell nanoparticles were formed by embedding Ppy within the UiO-66 network following optimized chemical synthesis polymerization approach. The aptasensors are formed by the adsorption of OTA-aptamers to the nanomaterials. The nanomaterials and biosensors properties were studied through various methods to highlight the chemical and structural properties as well as the mechanism of detection. Mycotoxin detection in buffer and real samples was achieved by measuring the electrical characteristics of nanocomposites and compared to HPLC as a gold standard method.

## 2. Materials and methods

### 2.1. Reagents

1,4-benzenedicarboxylic acid ($H_2BDC$) (99 %) was purchased from Alfa Aesar (Germany). Zirconium tetrachloride ($ZrCl_4$, 98 %) was obtained from Acros Organics (New Jersey, USA). Ochratoxin A $N-\{[(3R)-(5-chloro-8-hydroxy-3-methyl-1-oxo-7-isochromanyl) carbonyl]-l-phenylalanine\}$ (OTA, from A. ochraceus) and Ochratoxin B (OTB, from A. ochraceus) were obtained from Santa Cruz Biotechnology (Germany). Pyrrole (Py) was purchased from Sigma-Aldrich and argon distillation treatment before use. DNA aptamer again OTA were selected purchased from Eurogentec with the sequence (C5-GAT-CGG-GTG-TGG-GTG-GCG-TAA-AGG-GAG-CAT-CGG-ACA-3'). This aptamer was selected as its

forms structure switched after OTA interaction, inducing conformational change [38], and also high affinity with $K_d$ at nM range [39]. Phosphate-buffered saline (PBS) was prepared from 1.8 mM $KH_2PO_4$, 2.7 mM KCl, 10 mM $Na_2HPO_4$, and 137 mM NaCl at pH 7.4. Additional reagents and solvents, of analytical grade, were utilized in their original form without any undergoing additional purification.

### 2.2. Synthesis of UiO-66 NMOF

The UiO-66 NMOF was synthesized using the solvothermal method, as reported in a previous report [40]. In summary, $ZrCl_4$ was mixed with 15 ml DMF and 4 ml HCl, followed by sonication for 20 min. Subsequently, 0.536 g (2.9 mol) of $H_2BDC$ dissolved in 15 ml DMF was introduced, and the mixture was heated at 80 °C for 24 h. After the solution was air-cooled, the resulting product underwent multiple washes with DMF and a solvent exchange in acetonitrile. The synthesized NMOF was then dried under vacuum at 60 °C overnight and activated before use by heating at 150 °C for 4 h.

### 2.3. Fabrication of Ppy-NMOF

Ppy nanoparticles were coated onto UiO-66 NMOF using the in situ oxidative polymerization method, as reported elsewhere with some modification to obtain a thin layer of conductive polypyrrole [37]. In a nutshell, 100 mg of the prepared NMOF was placed in an aqueous solution and stirred for 30 min. Subsequently, a dropwise injection of $FeCl_3$ in (360 mg in 10 mL of DI $H_2O$) of aqueous solution occurred, followed by 30 min of stirring at room temperature. Afterward, the reaction mixture was supplemented with 120 µl of pyrrole solution and stirred in an ice bath for various durations of 20, 60, 120, 180, and 240 min, resulting in samples denoted as Ppy-NMOF 20 min, Ppy-NMOF -1h, Ppy-NMOF -2h, Ppy-NMOF -3h, and Ppy-NMOF -4h, respectively. The control Ppy-NPs (labeled as Ppy-20 min) was also synthesized according to the same procedure for 20 min without adding NMOF. The resulting products were centrifuged, washed with acetone and deionized water, and dried under vacuum overnight at 50 °C.

<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Schematic diagram of the preparation of Ppy-UiO-66 nanocomposite and the fabrication of Ppy-UiO-66-based aptasensor for OTA detection.</figcaption></figure>
## 2.4. Construction of NMOF based biosensor

The Ppy-UiO-66 NMOF nanocomposites with varying synthesis durations (20 min, 1h, 2h, 3h, and 4h) were individually dispersed in Milli-Q water and thoroughly mixed through ultrasonication to create homogeneous suspensions. Taking the construction of the aptasensor based on Ppy-UiO-66 NMOF synthesized for 20 min as an example, suspension of 2.0 µL of the Ppy-UiO-66 NMOF 20 min with a concentration of (1.0 mg mL⁻¹) was drop casted to the bare screen-printed electrode (pre-treated) surface and air-dried for 2 h at room temperature. The modified electrode, referred to as Ppy-NMOF 20 min, underwent three rinses with ethanol and Milli-Q water. Subsequently, Ppy-NMOF 20 min was incubated with an aptamer solution (10 µM) for 30 min. The aptamer was treated at 90 °C for 3 min to obtain unfolded form and to improve the association with the nanoparticles through ionic interaction. The SPEs surface modified with aptamer are designated as Apt-Ppy-NMOF 20 min. The resulting Apt-Ppy-NMOF was then exposed to several concentrations of OTA solutions during 30 min, followed by washing with PBS solution to remove nonspecific OTA binding. For comparison, aptasensors based on UiO-66 NMOF and Ppy-NMOF were also fabricated with the same method.

## 2.5. Real sample measurements

The red wine samples were purchased from the local market without any pretreatment. Before analysis, each red wine sample was diluted using PBS (50 µL red wine to 950 µL PBS). Several concentrations of OTA, ranging from (150 pM–5 nM (60 ng L⁻¹ to 2µgL⁻¹)) were spiked into a small volume (1 mL) of the diluted red wine samples. The biosensor was then exposed to the prepared samples before being introduced into the electrochemical cell, and analyzed using the developed biosensor. The signal obtained after binding of OTA was quantified using EIS techniques. For comparison, the HPLC technique was used to determine OTA in the red wine samples. Firstly, the red wine was analyzed and then various concentration of OTA was added from (0.25–10 mg L⁻¹) by using dilution factor of 10 and injection volume (10 µL). HPLC has used InfinityLab Poroshell 120 EC-C18 column (Injection with needle wash 10.00 µL) at 25 °C with a flow rate of 0.500 mL/min. The Solvent Composition (Acetonitrile 50 %: H₂O 0.1 %TFA 50 %).

## 2.6. Physiochemical characterization

The FT-IR spectra in the range of 4000–600 cm⁻¹ were collected using a Bruker VERTEX spectrometer. The measurements were conducted with an Attenuated Total Reflection (ATR) technique, employing a Germanium crystal and an MCT detector, with a nominal resolution set at 4 cm⁻¹. The examination of the size and shape of NMOFs and Ppy-NMOF were conducted using a transmission electron microscope (TEM; CEM902A; Carl Zeiss, Oberkochen, Germany). Field emission scanning electron microscopy (FESEM, Zeiss Sigma 500 VP Analytical Carl Zeiss, Germany) was utilized to examine the chosen core-shell nanoparticles of NMOF and Ppy-NMOF. The NMOF and Ppy-NMOF samples underwent a heating process in nitrogen gas, beginning at room temperature and reaching 800 °C at a rate of 10 °C/min for thermogravimetric analyses (TGA, DSC) conducted with a PerkinElmer STA6000 thermogravimetric analyzer. The surface area of NMOFs and Ppy-NMOF were assessed through Brunauer-Emmett-Teller (BET) by employing nitrogen absorption/desorption isotherms. The ζ-potential on the nano-formulations surfaces of nanomaterials was detected at 25 °C using electrophoretic light scattering (ELS) techniques, utilizing a Malvern zeta sizer from Malvern Instruments, U.K.

XPS measurements were performed on a K Alpha spectrometer from ThermoFisher, equipped with a monochromated X-ray Source (Al Kα, 1486.68 eV) with a spot size of 400 µm, corresponding to an irradiated area of ~1 mm². Samples were measured after drop casting on the

surface of screen printed. The base pressure was 3 × 10⁻⁹ mbar. The hemispherical analyzer was operated in CAE (Constant Analyzer Energy) mode, with a pass energy of 200 eV and a step of 1 eV for the acquisition of surveys spectra, and a pass energy of 50 eV and a step of 0.1 eV for the acquisition of narrow spectra. A “dual beam” flood gun was used to neutralize the charge build-up. The binding energies were calibrated against the neutral carbon binding energy set at 284.8 eV. The precision in binding energy is ±0.2 eV. The spectra were recorded and treated using Avantage and Casa XPS software [41]. The fitting procedure implied the use of either lineshapes extracted from reference spectra (MOF and Ppy) or Gauss-Lorentz lineshapes with 30 % lorentian character after the extraction of a Shirley-type background.

## 2.7. Electrochemical measurements

Electrochemical measurements were conducted using the AUTOLAB PGSTAT 12 potentiostat-galvanostat controlled by Nova 2.1.5 Software. In this study, a conventional three-electrode setup was employed through Screen-Printed Carbon Electrodes (SPCE) provided by Metrohm dropsens ref 110 (in this SPCE, the working electrode (WE) and counter electrode (CE) are made with carbon, and the reference electrode with silver). The WE with has 4 mm diameter and surface area of 12.5 mm². The reference electrode was modified by chemical treatment to obtain Ag/AgCl. All the electrochemical measurement was made with the same SPCE electrodes. Cyclic voltammetry (CV) were performed in 10 mM [Fe(CN)₆]³⁻/⁴⁻ or 10 mM of [Ru(NH₃)₆]³⁺/²⁺ in 0.1 M aqueous KCl. Electrochemical Impedance Spectroscopy (EIS) measurements were performed in [Fe(CN)₆]³⁻/⁴⁻. All impedance curves were recorded at open circuit potential with amplitude of 10 mV, in frequency range from 100 kHz to 0.1 Hz.

# 3. Results and discussion

## 3.1. Design concept and sensing mechanism

The electrochemical aptasensor, designed with a simplified synthesis approach and offering sensitive, stable, and reproducible detection, has broad applications beyond toxin detection, including the detection of various molecules and biomolecules. OTA detection was selected as a proof of concept due to its high carcinogenicity as a food contaminant.

The developed aptasensor leverages nanomaterials with enhanced electrochemical properties, providing a positively charged surface that enables stable electrostatic interaction with the DNA bioreceptor through phosphate groups. A similar approach was previously explored with UiO-66, where Zr⁴⁺ ions in the MOF coordinated with the phosphate groups of aptamers, resulting in a reversible interaction-based aptasensor using methylene blue (MB) as a detection probe [42].

In this work, we present a novel aptasensor based on Ppy-UiO-66 NMOF core-shell nanostructures. The PPy shell introduces a positively charged surface due to the p-doped polaron effect, facilitating stable aptamer immobilization while enabling electrochemical signal transduction by detecting changes in conductivity upon complex formation and structural switch of the aptamer [43]. This platform offers a simple yet effective electrochemical aptasensor for detecting trace amounts of OTA in diverse environments (Scheme 1).

The biosensor is fabricated by drop-casting PPy-UiO-66 NMOF onto the surface of a screen-printed electrode (SPE), followed by aptamer immobilization. The PPy-UiO-66 NMOF exhibits exceptional properties, including multivalent interactions, a UiO-66 framework with a high specific surface area, superior electrochemical activity, and strong bio-affinity provided by Ppy. These properties promote effective aptamer adsorption and immobilization onto the PPy-UiO-66 NMOF surface.

Subsequently, the Apt/PPy-UiO-66 NMOF/SPE configuration was used for OTA detection via specific molecular recognition. The aptasensor construction process—comprising electrode modification with Ppy-UiO-66 NMOF, aptamer immobilization, and OTA
binding—induces structural aptamer modification leading to electrochemical signal variations, such as changes in electrical properties of the nanomaterials, which were measured using EIS techniques.

## 3.2. Nanostructures synthesis and characterization

The synthesis of the nanoparticle MOF/Ppy was performed by chemical polymerization of Ppy on the Zr-MOF surface using $FeCl_3$ as oxidant. The chemical polymerization was obtained thanks to the nucleus formation of py monomer on nanoparticles surface followed by growth phenomena. The time of the reaction and concentration of pyrrole monomer determine the film thickness and its conductivity. These conditions were optimized to obtain the electrochemical properties of the nanocomposite. To demonstrate the nanohybrids formation and their structural, chemical, and electrical properties various physicochemical characterizations were performed.

### 3.2.1. Morphological and structural characterization (TEMs and BET)

Electron microscopy techniques were utilized to investigate the intricate morphology of nanostructures. High-resolution transmission electron microscopy (HRTEM) images of UiO-66 NMOF (Fig. 1a) and Ppy-UiO-66NMOF (Fig. 1b, c) reflect the difference in the surface morphological patterns of UiO-66 NMOF before and after polymerization of the conductive polymer on the surface of NMOF (Fig. 1). The image shows that Ppy-UiO-66NMOF displays the core-shell nanostructure of Ppy on the NMOF matrix (Fig. 1b). The magnified image (inset Fig. 1c) reveals a very thin Ppy conductive polymer shell, promoting electron transfer between the NMOF core and Ppy shell [32]. Moreover, the results illustrate that the time of the polymerization reaction influences the thickness of the Ppy shell on the MOF surface. As depicted in Fig. S1, the long-time of the Ppy polymerization reaction results in an increased thickness of the shell. For further investigation of nanostructure morphology, FESEM was applied (Figure S2 (a–c)), and the results were in good agreement with previously prepared nanocomposites [44]. We investigated the elemental mapping of Ppy-NMOF core-shell nanostructure using Energy-dispersive X-ray spectroscopy (EDXS) (Fig. 2) and the distributions of the carbon, oxygen, nitrogen, and Zr in composites. From the figure, we show that the Zr and nitrogen (N) serving as markers for UiO-66 and Ppy have the same distribution in the composites confirming Ppy polymerization on the MOF surface confirming core-shell structure.

To highlight the porosity of nanocomposites and surface area, we explore BET measurement. Fig. 3A illustrates an examination of the surface area in both the pristine UiO-66 NMOF and the Ppy-UiO-66NMOF nanocomposites through N2 physisorption at 77 K. The pristine UiO-66 NMOF exhibited a characteristic type-I isotherm indicative of micropore adsorption and showcased a notable high BET surface area of 1022 m²/g. In contrast, the Ppy-NMOF sample displayed significantly reduced BET surface areas of 365 m²/g compared to the pristine UiO-66 NMOF. This occurs because the small Ppy particles scatter throughout both the exterior and the internal structure of the UiO-66 framework. This dispersion is likely due to the thin layer formation Ppy shell on the surface of NMOF, consequently reducing the adsorption capacity of the Ppy-UiO-66 nanocomposites.

### 3.2.2. Chemical and thermal characteristics (FT-IR, XPS, Zeta potential, thermal stability)

FTIR spectra were performed to highlight the chemical structure of the nanocomposite. The spectra of the nanocomposite were recorded compared to MOF and Ppy alone (Fig. 3B). In the case of MOF UiO-66 (Zr), the spectrum shows the asymmetric and symmetrical vibrations at 1579 cm⁻¹ and 1400 cm⁻¹ corresponding to a carbonyl group [45]. The Zr–O–Zr vibration frequency is observed at 659 cm⁻¹ and 745 cm⁻¹ [24]. Ppy exhibits a C–N stretching vibration peak at 1170 cm⁻¹ [46], and the C=C vibration band at 1548 cm⁻¹ corresponding to the aromatic ring [47]. The nanocomposite shows the presence of a band associated with MOF and Ppy where the additional characteristic peak related to C–N vibration at 1166 cm⁻¹ from Ppy is observed which indicates the successful nanocomposite formation.

To check the stability of the nanocomposites (Ppy-NMOF) the thermal characteristics were examined using thermogravimetric analysis (TGA) and compared to UiO-66 and Ppy. Differential scanning calorimetry (DSC) was employed to yield the essential data of the thermal decomposition processes (Fig. S3). The weight loss profiles of Ppy-NMOF nanocomposite and UiO-66 NMOF (Fig. S3a) show the profile of decomposition within the temperature. UiO-66 NMOF shows an initial weight loss of 7.89 % at 100 °C attributed to the decomposition of physically adsorbed water, followed by a subsequent 17.58 % weight loss from approximately 100 to 450 °C, which resulted from the reduction in DMF solvent. The 25.81 % weight loss above 450 °C was identified as the decomposition of Zr–O in UiO-66 NMOF [48]. In the case of Ppy UiO-66 NMOF nanocomposite, the first stage of weight loss

<figure><img src="image_6.png" /><figcaption>**Fig. 1.** HRTEM images of UiO-66-NMOF with inset for single NMOF crystal showing the triangle structure of UiO-66 NMOF(a) and Ppy-UiO-66 NMOF (b) showing core-shell nanocomposite formation of Ppy in NMOF matrix, (c) inset picture indicating the thin layer formation of conductive polymer on the surface of NMOF. The red line inside the (b) and (c) indicating the thin layer of Ppy shell.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 2. FE-SEM and elemental maps for Ppy-NMOF illustrate the homogenous distribution of N for Ppy-NMOF due to N of Ppy confirming oxidative polymerization.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 3. (A) N2 adsorption isotherms, and BET surface area measurement, (B) FTIR spectra of Ppy, UiO-66 NMOF, and Ppy-UiO-66 NMOF.</figcaption></figure>

at 100 °C is 9.65 % attributed to the evaporation of water followed by subsequent 31.25 % weight loss at higher temperature resulted from the thermal decomposition of Ppy, caused by the disruption of the bond affinity between the pyrrole ring and the doped Cl⁻ in Ppy [49]. However, it's noted that the nanocomposite Ppy-UiO-66 N MOF exhibits incomplete decomposition even when subjected to temperatures higher than 600 °C. From these results, it's clear that the addition of Ppy enhance the thermal stability of the nanocomposite.

The surface charge of nanoparticles is important to highlight the nature of Ppy coated on MOF if it is in p-doped or over-oxidated state. The nanoparticles surface charge was measured through Zeta potential in a dispersed aqueous solution. The charge potential of NMOF is measured at +37 ± 4.8 mV, indicating a relatively high positive surface charge. This positive ζ-potential suggests good stability in aqueous solutions, as particles with significant positive or negative ζ-potentials tend to repel each other, thus minimizing aggregation and enhancing overall stability [50]. Ppy nanoparticles possess a highly positive charge, measuring +40 ± 4.1 mV, which aligns well with previously reported results [51]. When NMOF is coated with polypyrrole, the resulting

nanocomposite exhibits a more positive ζ-potential compared to its uncoated NMOF as shown in Table 1. The increase in positivity can be attributed to the p doped Ppy conductive polymer coating NMOF. These findings underscore the potential of the designed biosensor as an ideal platform for attaching negatively charged DNA aptamers.

X-ray Photoelectron Spectroscopy (XPS) was employed to examine changes in the chemical composition throughout various stages of the synthesis process and the charge transfer between in the nanocomposite. Fig. 4A illustrates the survey spectra for the nanocomposite in comparison with Ppy and MOF. The survey spectrum of the UiO-66 NMOF nanocomposite exclusively features C1s, O1s, and Zr (3d, 3p, and 3s)

<table><caption>**Table 1**

Zeta potential results of Ppy NPs, UiO-66 NMOF, and Ppy-UiO-66 NMOF.</caption><thead><tr><td>Nanomaterials</td><td>Zeta Potential</td><td>SD ±</td></tr></thead><tbody><tr><td>UiO-66 NMOF</td><td>37</td><td>3.84</td></tr><tr><td>Ppy NPs</td><td>40.7</td><td>4.1</td></tr><tr><td>Ppy-UiO-66 NMOF</td><td>43.8</td><td>5.9</td></tr></tbody></table>
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 4.** Comparison survey for the XPS spectra of NMOF, Ppy NPs and Ppy-NMOF(A), The corresponding high-resolution Ppy-NMOF peaks, C1s peak (B), N1s peak (C) and O1s peak (D), and Core-level spectra of Cl2p and Zr3d (E) and P2p after aptamer attachment (F).</figcaption></figure>

peaks, whereas the Ppy spectrum displays the predominant C1s and N1s peaks with additional traces of Fe and Cl alongside coming from the reagent. Moreover, a small O1s peak is present in the Ppy spectrum, indicating slight oxidation, as expected for a sample exposed to air. The survey spectrum of the core-shell sample reveals the presence of peaks from both Ppy and MOF, including C1s, N1s, O1s, and Zr3d. To begin, we conducted measurements of the Ppy-UiO-66 NMOF nanocomposite spectra for C1s, N1s, and O1s in both the reference NMOF and Ppy NPs

samples. Fig. 4B illustrates the C1s spectra obtained from the nano-composites along with those from reference samples [52]. For the nanocomposite, we employed real lineshapes extracted from both Ppy NPs and NMOF based on the guidelines outlined in reference [53]. The discrepancy between the nanocomposite curve and the sum of Ppy and NMOF contributions was then addressed by fitting it with synthetic line shapes of the LG (30) type. The same methodology was employed for the N1s and O1s spectra, as depicted in Fig. 4(C and D).
The outcome of this fitting process led us to the conclusion that in the nanocomposite there is a high probability of an increase in positively charged N species, specifically polarons (C-N⁺) and bipolarons (C=N⁺), situated at energy levels of 286.6 and 288.3 eV, respectively. This interference finds support in the observation of a component in the Cl2p core-level spectrum as shown in Fig. 4E at 197.1 eV, which can be ascribed to negatively charged Cl. Additionally, the component at 199.5 eV can be attributed to Zn-Cl [54,55]. Although the quantification lacks precision, it demonstrates that the proportion of carbon originating from Ppy is roughly double that of MOF, thus confirming Ppy presence in the NMOF matrix.

### 3.2.3. Electrochemical characterization

#### 3.2.3.1. CV and EIS comparing NMOF and Ppy-NMOF.
The Electrochemical measurements were performed to demonstrate the improved charge transfer properties of the nanocomposites compared to MOF.

The electrochemical characterization of the biosensor fabrication through a stepwise assembly was conducted using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with [Fe(CN)₆]³⁻/⁴⁻ and [Ru(NH₃)₆]³⁺/²⁺ serving as the redox probe. SPE was modified with the suspension of the solution of Ppy-MOF composite by drop casting and compared with the deposit suspension of polypyrrole nanoparticles. The polypyrrole nanoparticles were synthesized under the same conditions as with MOF.

Firstly, we investigated the effect of the thickness of the Ppy shell on the electrochemical properties of the nanocomposites by controlling the duration of chemical polymerization. This is because the electrical and sensing properties of the nanocomposites can be influenced by the thickness of the Ppy layer. Indeed, the Ppy layer should be sufficiently thick to ensure a heightened surface-to-volume ratio while providing

uniform coverage across the NMOF surface without losing its conducting properties due to the overoxidation of polypyrrole. Thus, the Ppy thickness was controlled by optimizing the oxidative polymerization reaction time of Ppy on the NMOF surface.

Nanocomposites were formed by varying the chemical polymerization time from 20 min to 240 min. To highlight the effect of polymerization time on the properties of the composite, the electrochemical properties were monitored after drop casting on various SPEs using CV in [Fe(CN)₆]³⁻/⁴⁺ (Fig. 5A). The findings reveal that a higher redox peak current was obtained with SPCE modified with a nanocomposite formed with 20 min of polymerization reaction compared to longer polymerization times. The composite obtained with a reaction time of 20 min demonstrated a current intensity three times higher than that of the other composites. In addition, a shift in the oxidative potential by 32 mV and improved reversibility were observed compared to the nanocomposites obtained with longer polymerization times (see SI, Table S1). These results indicate that a thin layer of conductive polypyrrole enhances the electron transfer ability of the redox marker. This improvement is attributed to the electrical conductivity of the polypyrrole layer on the shell of the framework surface, which facilitates effective charge transport throughout the nanocomposite.

Nevertheless, extending the Ppy oxidative reaction time leads to the formation of a thick Ppy layer on the surface, which exhibits reduced electroactivity. The thick Ppy film is less conductive because prolonged polymerization may favor the overoxidation of Ppy, resulting in decreased conductivity. Consequently, based on the obtained results, the Ppy-NMOF nanocomposite was synthesized using a 20-min oxidative polymerization reaction time for the designated biosensor to be used in subsequent OTA detection measurements.

The nanocomposite Ppy-NMOF obtained under optimized conditions was then analyzed by CV using various redox markers, [Fe(CN)₆]³⁻/⁴⁺.

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 5.** (A) CV of the Ppy-NMOF obtained at various oxidative polymerization reaction time after drop casting on SPE and analyzed in solution containing 10 mM of [Fe (CN)₆]³⁻/⁴⁻; (B) CVs of SPEs modified with Ppy-NMOF, Ppy NPs, and NMOF analysis is performed in solution of 10 mM of [Fe (CN)₆]³⁻/⁴⁻, (C) CVs of SPEs modified with Ppy-NMOF, NMOF and SPCE analysis are performed in solution of 10 mM [Ru (NH₃)₆]³⁺/²⁺, all CV are at the same scan rate of 50 mV s⁻¹. (D) Nyquist plots for bare SPE, NMOF, and Ppy-NMOF measured 10 mM of [Fe (CN)₆]³⁻/⁴⁻, at open circuit potential and amplitude of 10mv in frequency range of 100 kHz to 0.1Hz.</figcaption></figure>
and [Ru(NH₃)₆]³⁺/²⁺, in comparison with non-modified SPE and SPE modified with NMOFs or Ppy nanoparticles synthesized under the same chemical polymerization conditions, to highlight the properties of the nanocomposites (Fig. 5B) and (Fig. 5C). The CVs presented in Fig. 5B and C shows a significant increase in the peak current for SPE modified with the Ppy-NMOF nanocomposite compared to the unmodified SPE, Ppy NPs, and NMOF-modified SPCEs.

Notable modifications in the oxidation peaks of both redox markers are observed when comparing the various modified electrodes. For the [Fe(CN)₆]³⁻/⁴⁻ redox marker, bare SPE, UiO-66 NMOF/SPE, Ppy/SPE, and Ppy-UiO-66/SPE were compared, and the CV data are summarized (see SI, Table S2). The data show that both Ppy and UiO-66 NMOF exhibit higher oxidation peak currents than bare SPE. However, when Ppy/UiO-66 NMOF/SPE is used as the working electrode, it displays a significant increase in the oxidation peak current.

The electroactive surface area of the various modified electrodes was calculated from the peak current using the Randles-Sevcik equation (Eq. (1)).

$$I_p = 2.69 \times 10^5 \text{ n}^{3/2} \text{ A} \text{ D}^{1/2} \text{ C} \text{ v}^{1/2} \qquad \text{Eq. 1}$$

where n is the number of electrons that takes place in the redox reaction, D is the diffusion coefficient, v¹/² is the square root of the scan rate, and Ip is the peak current. The surface area (A) can be determined from the slope between Ip and v¹/².

The electroactive surface area was calculated at 11 mm² and 22 mm² for NMOF and Ppy-NMOF composite, respectively, which shows an increase for the nanocomposite. This enhancement can be attributed to both the increased surface area provided by the high surface-to-volume ratio of the NMOF and the high surface area of the conductive Ppy coating on the NMOF nanoparticles. This improvement results from the combination of UiO-66 with a thin conductive polypyrrole film.

These findings were further confirmed by analyzing the modified surface using another redox marker, [Ru(NH₃)₆]³⁺/²⁺ (Fig. 5C), which demonstrated similar behavior and an improvement in current for Ppy/UiO-66 compared to the MOF alone. Furthermore, this nanocomposite material offers a larger specific surface area and a highly positive charge, promoting the adsorption and electrocatalytic redox processes of the redox markers.

These conclusions are further supported by EIS characterization, as illustrated in Fig. 5D. Furthermore, an EIS Nyquist plot consists of a semicircle section and a linear section. The high-frequency semi-circular part corresponds to the electron transfer process, with its diameter representing the electron transfer resistance (Rct). The low-frequency linear part is attributed to the diffusion process [56]. The fitting of the Nyquist plot was performed with Randles equivalent circuit model as it can give information on the kinetic of charge transfer and diffusion of redox markers in the interface (Fig. S12). The fitting data gives the Rct values for the bare SPE, and NMOF/SPE at 967 Ω and 598 Ω respectively, where we found a decrease in Rct upon the surface modification. The decrease in resistance is related to electrocatalytic NMOF properties which facilitates the electron transfer. In the case of Ppy-NMOF, EIS shows a straight line corresponding to conductive properties of the nanomaterials.

The Ppy-NMOF nanocomposites gains in conductivity compared to NMOF which increases the electron transfer ability of redox markers, which is very important in the sensing mechanism.

#### 3.2.3.2. Kinetic characterizations.
The investigation of the Ppy-NMOF platform through cyclic voltammetry (CV) at various scan rates (v) was conducted. In Fig. S4, the CV curves of NMOF and Ppy-NMOF in [Fe(CN)₆]³⁻/⁴⁻ are presented, with scan rates varying from 10 to 100 mV. s⁻¹. Notably, the redox peaks exhibited a gradual enhancement with increasing scan rates. The correlation between redox peak currents (Iₚ) and scan rate (v) is illustrated (Figure S4 C, D). Linear regression of logIₚₐ versus log (scan rate) gives a slope of 0.5 which confirming that

the electrochemical process is controlled by diffusion [57].

The kinetic of electron transfer were also be calculated using Nicolson method by measuring the peak-to-peak variation at various scan rate and applying equation (2).

$$\psi = k^0 \sqrt{\frac{RT}{\pi n F D}} \frac{1}{\sqrt{V}} \qquad \text{Eq 2}$$

where ψ is the dimensionless kinetic parameter calculated from ΔEₚ, R is the universal gas constant, T is the thermodynamic temperature, n is the number of electrons involved in the reaction, F is the Faraday constant, D is the diffusion coefficient of the electroactive species and v is the scan rate. Plot of the ψ versus V⁻¹/² lead to the determination of the heterogenous electron transfer.

The heterogenous rate of electron transfer was calculated with the redox marker Fe (CN)₆]³⁻/⁴⁻ by comparing the modified electrode NMOF and Ppy-NMOF nanocomposites following Nicolson's formalism and by plotting ψ versus (V⁻¹/²) (see SI, Figure S5 C, D).

The obtained value of rate of electron transfer are 11 × 10⁻³ cm s⁻¹ and 14 × 10⁻³ cm s⁻¹ for NMOF and Ppy-NMOF, respectively.

These results confirm the EIS data and clearly demonstrate an increased electron transfer rate in the composite compared to the NMOF, highlighting the effect of the conductive properties of polypyrrole. The enhanced electron transfer capability is a crucial factor in electrochemical sensors based on conductive surfaces, where any modification in interfacial properties significantly impacts the sensing response.

### 3.3. Biolayer formation and analytical performance

The Ppy-UiO-66 NMOF biolayer formation is presented in (Scheme 1). The dispersion of Ppy-UiO-66 NMOF was drop casted onto the surface of SPCE. Subsequently, the DNA aptamer specific to OTA was attached to the nanocomposites through electrostatic interaction between the positively charged provided by Ppy shell on the nanocomposite surface and negatively charge of phosphate DNA aptamers.

To confirm the attachment of the DNA aptamer, XPS spectra were recorded and are presented in Fig. 4, and (SI, Fig. S13, Table S8). In the survey spectrum of DNA-incorporated sample, the characteristic peaks of the Ppy-UiO-66 NMOF nanocomposite are observed, along with additional P 2p and Na 1s peaks (Fig. 4A). The presence of DNA on the surface of the nanocomposite is confirmed by the detection of the P 2p signal. The P 2p core-level spectrum shown in Fig. 4F consists of two spin-orbit coupling components, with the P 2p₃/₂ peak at 133.7 eV, which can be unequivocally attributed to the PO₄³⁻ group [51,58]. High-resolution peaks of nitrogen and carbon were measured before and after DNA attachment. The C 1s and N 1s signals tend to lose their tailing shape and become broader and more symmetric, due to the presence of various functional groups introduced by the attached DNA (see Fig. S13). Furthermore, DNA loading was evaluated by quantifying the zirconium (Zr) and phosphate content in the Ppy-UiO-66 NMOF-DNA sample (see SI Table S8), corresponding to approximately 0.85 aptamer per MOF.

These results provide compelling evidence that DNA molecules have been successfully grafted onto the surface of the nanocomposite. The positively charged nitrogen within Ppy, as well as the additional positive charge introduced by the p-doping process of conductive Ppy, facilitates DNA immobilization through ionic interactions with the PO₄³⁻ groups of DNA.

EIS experiments were conducted to monitor the development of the biosensor. The introduction of aptamer modifies the interfacial properties of the biolayer. This is observed by measuring Nyquist plot (see SI, Fig. S6) providing resistive properties where Rct measured after fitting increased. This change be connected to changes in the nanocomposite's electrical characteristics, where the DNA aptamer's anionic charge act as repulsive interaction with the negatively charge redox probe [Fe(CN)₆]³⁻/⁴⁻ causing a hindrance in charge transfer processes.
### 3.3.1. Design principles and sensing mechanism of the OTA biosensor

EIS was employed to investigate the detection of ochratoxin A (OTA) using aptasensors based on UiO-66 NMOF and 66 (PPy-NMOF). Upon OTA detection, the obtained OTA/APT/PPy-NMOF/SPE reveals an increase in impedance and the charge transfer resistance ($R_{ct}$) value. This variation is attributed to changes in the electrical properties of the biolayers, including modifications in capacitance and charge transfer resistance. The formation of the Aptamer/OTA complex induces structural modifications in the biolayer, which are attributed to the formation of a G-quadruplex structure. This structure introduces greater steric hindrance at the electrode surface, thereby hindering the diffusion of the redox probe to the electrode. Consequently, charge transfer at the electrode/electrolyte interface is impeded, and the capacitance value decreases—likely due to an increase in the interfacial layer thickness. This structural change in the aptamer upon OTA binding was previously demonstrated in our earlier work using circular dichroism (CD) measurements [60], where the CD spectrum after OTA interaction exhibited characteristic positive and negative peaks at 295 nm and 260 nm, respectively, corresponding to the typical signature of an anti-parallel G-quadruplex structure.

To highlight the advantage of using the nanocomposite, same biosensors were prepared with NMOF alone and their response to 5 nM OTA concentration was recorded. The NMOF-APT based biosensor was formed in the same way as previously. The immobilization of aptamer on NMOF leads to an increase of $R_{ct}$ from 405 Ω to $R_{ct}$ = 2.75 kΩ. The increase is attributed to the interaction of DNA aptamer with $Zr^{4+}$ of UiO-66 NMOF and $PO_4^{3-}$ of DNA. However, upon interaction with the OTA, the $R_{ct}$ value decreases to 861 Ω. The response of the NMOF-based biosensor after OTA addition, reveals a 1.8-fold decrease in $R_{ct}$ after OTA interaction (Fig. S7). This can be explained by the low stability of aptamer attached to MOF where the presence of OTA leads to their dissociation away from the MOF surface resulting in a decrease in impedance.

In summary, comparative experiments suggest that the biosensor

utilizing UiO-66 NMOF as the signal-amplification platform inherits the excellent target recognition ability of the aptamer. However, it is not stable for OTA recognition due to the rapid dissociation between Apt/NMOF and OTA [33]. The introduction of Ppy in this nanocomposite demonstrates a strong interaction and bioaffinity between Ppy-NMOF and the aptamer facilitated stable and substantial adsorption of the aptamer strands onto the biosensor surface. Importantly, this strategy eliminates the need for a blocking agent, streamlining the biosensor construction process.

Thus, the Ppy-NMOF exhibit a sensing platform, including multi-valent characteristics, with a large specific surface area for aptamer immobilization and strong bioaffinity provided by Ppy, superior electrochemical activity. The proposed Ppy-NMOF-based electrochemical biosensing strategy is introduced as a promising approach for electro-chemical biosensors and holds the potential for detecting various targets by anchoring different aptamer strands.

### 3.3.2. Calibration curve and LOD

The performance of the Ppy-NMOF biosensor in detecting OTA was assessed across OTA concentrations ranging from 150 pM to 5 nM. Upon forming the Apt-Ppy-NMOF complex, the electrode underwent washing to eliminate any unbound OTA aptamer. Then, in a solution containing the redox marker [Fe(CN)6] $^{3-/4-}$, the biosensor response was monitored using (EIS). The EIS show from Nyquist plot a semicircle change after Aptamer/OTA complex formation after addition of each concentration (Fig. 6A).

Fitting data using the Randles circuit involved adjusting capacitance via a constant phase element to accommodate the inhomogeneous charge distribution associated with the Ppy-NMOF structure (Fig. 6A). Notably, an increase in charge transfer resistance ($R_{ct}$) was observed with rising OTA concentrations (Table S4). A calibration curve was created using the $R_{ct}$ changes from the fitted data. The average $R_{ct}$ variations were plotted against the logarithm of the OTA concentrations, showing a broad dynamic range at higher concentrations (Fig. 6B).

<figure><img src="image_19.png" /><figcaption>**Fig. 6.** (A) EIS Nyquist plots of the Ppy-NMOF based biosensor for measuring OTA in PBS solution containing 10 mM [Fe(CN)6] $^{3-/4-}$ redox probe at various OTA concentrations (a-150 pM, b-300 pM, c-600, d-1.2 nM, e- 2 nM, f-5 nM. (B) Calibration curve depicting the average change in $\Delta R_{ct}/R_{ct0}$ relative to the logarithmic concentrations of OTA. (C) The selectivity of Apt/Ppy-NMOF biosensor toward OTB.</figcaption></figure>
According to the standard deviation of blank tests performed on four independently modified electrodes, the slope of the linear part was utilized to determine the limit of detection (LOD), which was found to be 0.24 pM (0.1 ng·L⁻¹).

### 3.3.3. Selectivity

The selectivity of the Apt-Ppy-NMOF biosensor was confirmed through exposure to Ochratoxin B (OTB), a structural analog of OTA that differs only by the absence of a chlorine atom. Electrochemical impedance spectroscopy (EIS) analysis was performed using equal concentrations of OTA and OTB (Fig. 6C). Despite their structural similarity, the biosensor exhibited minimal impedance change upon exposure to OTB, indicating a lack of significant interaction between OTB and the OTA-specific aptamer immobilized on the biosensor surface.

In contrast, a notable increase in charge transfer resistance (Rct) was observed upon OTA binding (Table S5), confirming the specific recognition and interaction between OTA and the aptamer. These results highlight the excellent selectivity of the biosensor, capable of distinguishing OTA from structurally related analogs.

Additionally, the selectivity of this aptamer toward OTA in the presence of structurally distinct food-related mycotoxins, such as aflatoxin M1, Aflatoxin B1 and Zearalenone, has been previously validated [59]. These prior studies support the aptamer's high specificity and reinforce the biosensor's potential for reliable OTA detection in complex food matrices.

### 3.3.4. Reproducibility and repeatability

The reproducibility of the designed biosensor was assessed by employing four freshly prepared electrodes, with Nyquist plots demonstrating consistent performance (Fig. S8).

The reproducibility of our developed biosensor was tested in both PBS and red wine using four separate electrodes in red wine under the same conditions as in PBS. To assess reproducibility for OTA detection, we conducted four parallel experiments within a concentration range of 150 pM–5 nM. The results demonstrated high reproducibility (Tables S4 and S7) with the average Δ(RCT) values and standard deviations (SD) calculated across the electrodes in PBS and red wine, respectively.

Regarding repeatability, the Apt-Ppy-NMOF biosensor showed outstanding performance throughout the experiment, successfully completing multiple cycles of OTA detection and subsequent regeneration. The biosensor could be reset by cleansing it with an appropriate binding solution after each use, effectively removing any unbound molecules. This characteristic is highly beneficial because it allows for repeated use of the same biosensor, saving time and resources by avoiding the need to create a new one for each measurement.

### 3.3.5. Biosensor stability

The stability of the designed nanocomposite as well as the biolayer was scrutinized over two weeks. The nanocomposite was stored at room temperature and biolayer at 4 °C. Regarding the PPy-MOF-Apta the ESI obtained after 14 days shows a small variation the Rct measured after fitting gives of 5 % deviation (Fig. S9A, Table S6). The EIS was measured for surface Py-MOF prepared freshly and after 30 days (Fig. S9B, Table S6), the obtained data showed the same performance of the freshly and after one-month measurement. This underlines the robust stability of the composite as well as the aptamer attachment through ionic interaction. The nanomaterials show very good stability which can be maintained for 2 years after synthesis without distorting their properties.

## 3.4. Applicability of the biosensor for real samples analysis

To evaluate the effectiveness of this biosensor in actual samples, we conducted tests on wine samples to detect the OTA present using the standard addition method. For this purpose, the wine was diluted 10 time with buffer and was enriched with various concentrations of OTA

from 150 pM to 5 nM (15ng·L⁻¹ - 2 µg·L⁻¹). Initially, we examined how the wine's composition influenced the properties of the biolayer. Subsequently, the biolayer was subjected to wine samples containing varying concentrations of OTA. After each addition, EIS data was collected for analysis. The result reveals an increase in the Nyquist semi-circle diameter when wine samples are enriched with various concentrations of OTA (Fig. 7).

The EIS data were fitted using the same equivalent circuit model to obtain the charge transfer resistance values, and they showed similar behaviour to those seen in the buffer solution scenario. Subsequently, the Apt-Ppy-NMOF-OTA binding was quantified using these data. A calibration curve displayed a linear connection with the OTA concentration's logarithm (Fig. 7B).

Additionally, the wine matrix had no effect since the Limit of Detection (LOD) achieved using the same approach was found to be 0.3 pM, which was almost equal to that in the PBS solution (0.24 pM). The minimal impact of the matrix is likely due to the hydrophobic nature of the nanocomposite, prevent the attachment of other molecules and microorganisms like yeast found in wine. The LOD value in wine was notably lower than the data published in the literature, confirming the Apt-Ppy-NMOF biosensor's competitive sensing capabilities and dependability for OTA detection even in complex matrices like wine.

To measure the value of OTA in wine, the linear part of the calibration curve was plotted (Fig. 7C). The value of OTA in wine is calculated to be (0.2 nM), considering the dilution factor of 10, the concentration of OTA in wine measured is 0.8 µg·L⁻¹.

To compare the measurement results of the designed biosensor in red wine samples, HPLC as standard method was employed in the same conditions without any red wine treatment. HPLC was performed with known concentration in PBS and then by doping the wine with various concentration of OTA. HPLC results show that OTA signal increases accordingly with increasing OTA concentration in PBS (Figure S10 (a, b)). Regarding the doped wine with OTA any signal were obtained with the lower concentration of OTA in the range of µg·L⁻¹. However, HPLC signal was obtained in the range of concentration of mg·L⁻¹ and linear detection range in the range of 0.25–1 mg/L was obtained (Fig. S10c). The obtained HPLC results display LOD for spiked red wine samples at a concentration of 0.25 mg/L without any preconcentration experiment.

These results highlight superior sensitivity of biosensor based on aptamer Ppy-NMOF compared to HPLC for OTA detection in wine without any pre-treatment and preconcentration able to detect the concentration lower than that preconised by WHO of 2 µg·L⁻¹ leading to their possible application of the direct control of the quality of wine during all the chain line fabrication process without any further pre-treatment steps.

## 4. Discussion

The detection of Ochratoxin A (OTA) remains a critical challenge due to its low permissible limits in food and beverages, necessitating highly sensitive, selective, and rapid detection methods. Electrochemical aptasensors have emerged as a promising alternative, leveraging aptamer-target interactions for high specificity while offering fast and cost-effective detection. However, existing aptasensors often require signal amplification steps and complex labeling procedures, which can limit reproducibility and scalability.

The developed novel Ppy-NMOF-based aptasensor for OTA detection, addressing these limitations through, amplification-free approach. The incorporation of conducting polypyrrole (Ppy) within the UiO-66 MOF matrix significantly improved electrical conductivity, ensuring effective electron transfer and a stable sensing platform. The combination of high porosity and electroactive sites within the nanocomposite allowed for the direct immobilization of OTA-specific aptamers, enhancing sensitivity while eliminating the need for chemical cross-linking agents.

Compared with other OTA electrochemical aptasensors that require
<figure><img src="image_20.png" /><figcaption>**Fig. 7.** (A) Nyquist plots of the SPE modified with Apt-Ppy-NMOF after exposure to OTA in red wine at concentrations ranging from 150 pM to 5 nM, with concentration varying logarithmically. The black line represents OTA-free biosensors, while colored lines denote different OTA concentrations (400 pM, 600 pM, 900 pM, 1.2 nM, 2 nM, 5 nM). (B) Calibration curves depict the average change in ΔRct/Rct0 versus OTA concentrations, measured using three independent electrodes. (C) The linear part of the calibration curve used for determining the LOD of the biosensor and the OTA present in red wine.</figcaption></figure>

signal probes various process of fabrication, the proposed biosensor, based on Ppy-NMOF strategy reduces complexity while maintaining high sensitivity and biosensing efficacy, as summarized in Table 2. Previous MOF-based aptasensors were often used as signal amplifiers rather than as an integrated conductive sensing matrix. In contrast, Ppy-NMOF hybrid developed here acts as both a stable substrate and an electroactive signal transducer. The incorporation of conducting polypyrrole (Ppy) within the UiO-66 MOF matrix significantly improved electrical conductivity, ensuring effective electron transfer and a stable sensing platform. The combination of high porosity and electroactive sites within the nanocomposite allowed for the direct immobilization of OTA-specific aptamers, enhancing sensitivity while eliminating the need for chemical cross-linking agents.

The Ppy-NMOF-based biosensor achieved ultra-low detection limits

<caption>**Table 2**</caption>

Comparison with other reported biosensors-based detection methods for OTA detection. fGO = functionalized Graphene Oxide, Ppy = Polypyrrole, CF = Carbon nanofiber, dsDNA = double-stranded DNA, PAMAM = polyamidoamine, DPV = Differential pulse voltammetry.

<table><thead><tr><th>Materials</th><th>Detection method</th><th>LOD (ng. L<sup>-1</sup>)</th><th>Detection range (ng.mL<sup>-1</sup>) µg. L<sup>-1</sup></th><th>Real samples</th><th>Refs.</th></tr></thead><tbody><tr><td>Ppy</td><td>EIS</td><td>2000</td><td>2–6000</td><td>Wine</td><td>[60]</td></tr><tr><td>fGO/<br>Fe<sub>3</sub>O<sub>4</sub>NP/<br>Ppy</td><td>EIS</td><td>5</td><td>0.01–200</td><td>Wheat</td><td>[61]</td></tr><tr><td>dsDNA</td><td>EIS</td><td>50</td><td>0.05–10</td><td>Malt</td><td>[62]</td></tr><tr><td>MnCO<sub>3</sub>NS/<br>CF</td><td>DPV</td><td>4.9</td><td>0.024–2.47</td><td>Apple<br>Juice</td><td>[63]</td></tr><tr><td>PAMAM</td><td>DPV</td><td>0.31</td><td>0.005–100</td><td>wine</td><td>[64]</td></tr><tr><td>Ppy-UiO-66</td><td>EIS</td><td>0.1</td><td>0.00012–2 µg. L<sup>-1</sup></td><td>Wine</td><td>This work</td></tr></tbody></table>

(0.1 ng.L<sup>-1</sup>), surpassing most previously reported OTA aptasensors and a broad dynamic range (0.12 ng.L<sup>-1</sup> to 2 µg.L<sup>-1</sup>) covering regulatory limits set by international bodies such as the European Union (2 µg kg<sup>-1</sup> in wine). The biosensor exhibited high specificity, effectively discriminating OTA from structurally similar mycotoxins (OTB). Additionally, it showed excellent long-term stability at room temperature, a key advantage for field-deployable sensors. Unlike traditional biosensors that degrade over time or require cold storage, the Ppy-NMOF nanocomposite maintains structural integrity and electrochemical performance for extended periods without significant loss of sensitivity. This is attributed to the strong binding interactions between Ppy and UiO-66, preventing degradation and enhancing durability. These results indicate that Ppy-NMOF nanocomposites provide a stable, highly conductive matrix capable of supporting direct electrochemical sensing, making them ideal candidates for food safety applications.

The modular design of the Ppy-NMOF hybrid platform allows for adaptation beyond OTA detection. By altering the aptamer sequence, this strategy can be extended to detect other mycotoxins, pesticides, or biomarkers, broadening its applicability in food safety, environmental monitoring, and medical diagnostics. Additionally, the ability to integrate this nanomaterial into printable sensor technologies offers exciting opportunities for low-cost, rapid, and on-site monitoring.

Overall, this study introduces a novel electrochemical aptasensor based on a Ppy-NMOF hybrid for OTA detection, offering a highly sensitive, selective, and rapid sensing platform. The integration of PPy-NMOFs into printing technology represents a significant step forward in biosensor fabrication, providing a scalable and cost-effective method for mass production. Furthermore, the high stability of Ppy-NMOF at room temperature ensures long-term usability, making this sensor highly attractive for practical applications. By overcoming key limitations of existing methods, this work paves the way for the development of printed, field-deployable, and disposable bioelectronics interfaces for
food safety monitoring and beyond.

The developed biosensors are adaptable for advancing technology such as integration with smart systems, such as smartphone-based readout systems and wireless sensing platforms and scalable industrial manufacturing processes. These developments will enhance user accessibility, facilitate real-time monitoring, and create opportunities for IoT-connected food safety monitoring solutions, making the sensor highly attractive for both industry and consumer applications. Additionally, ensuring compatibility with automated large-scale fabrication methods and compliance with food safety regulations will be key factors in transitioning this sensor from the laboratory to real-world applications, making it highly attractive for both industry and consumer markets.

## 5. Conclusion

We developed porous semiconducting nanocomposites by integrating conducting polypyrrole (Ppy) within the voids of UiO-66 using an optimized chemical polymerization method. This nanocomposite was successfully employed to construct an OTA aptasensor. Our results demonstrate that the Ppy-NMOF hybrid exhibits significantly improved electronic properties compared to MOFs alone, allowing for stable aptamer immobilization via electrostatic interactions while eliminating the need for chemical cross-linking.

This biosensor enables direct OTA trace detection in food samples without requiring complex purification or amplification steps. The Ppy-NMOF nanocomposite combines the robust framework and high surface area of UiO-66 with the bioaffinity, reliable electrochemical performance, and physicochemical stability of polypyrrole (Ppy). The substantial improvement in electrical conductivity is attributed to the interaction between the conjugated polymer and the host framework, providing enhanced electron transfer and ample active sites for aptamer binding.

As a result, the Ppy-NMOF-based biosensor exhibits exceptional performance in OTA detection, with an ultra-low detection limit of 0.24 pM (0.1 ng·L⁻¹), a broad detection range (0.24 p.m.-5 nM) (0.12 ng·L⁻¹-2 µg·L⁻¹), and high selectivity against OTB, along with good reproducibility, repeatability, and outstanding stability. Its simplicity, rapid preparation process, and direct detection capability without amplification make it a strong candidate for practical OTA detection in wine and other food matrices.

Beyond OTA detection, the versatile design of this platform enables its adaptation to detect a wide range of small molecules or macromolecules by modifying the aptamer used. This adaptability positions the Ppy-NMOF hybrid as a highly scalable and cost-effective biosensing technology, contributing to the future development of portable, real-time electrochemical sensors for food safety, environmental monitoring, and medical diagnostics.

### CRediT authorship contribution statement

**Mostafa Fytory:** Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Fatma Besbes:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation.
**Diana Dragoe:** Writing – review & editing, Writing – original draft, Validation, Software, Investigation, Formal analysis, Data curation.
**Rym Mlika:** Writing – review & editing, Writing – original draft, Validation, Supervision, Software, Formal analysis.
**Waleed M.A. El Rouby:** Writing – review & editing, Validation, Supervision, Funding acquisition.
**Ahmed A. Farghali:** Writing – review & editing, Validation, Supervision, Funding acquisition.
**Hassan M.E. Azzazy:** Writing – review & editing, Validation, Supervision, Resources.
**Hafsa Korri-Youssoufi:** Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors are grateful to Dr. Mahmoud Abdel-Hafiez for his insightful comments and scientific advice in connection with this study.

MF acknowledges the Erasmus+ European fund for the scholarship. HKY and R.M acknowledge CNRS for funding IEA project and the Ministries MEAE and MESR of France an Tunisia for funding PHC project 22MAG20.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.aca.2025.344334.

### Data availability

Data will be made available on request.

### References

[1] W. Li, X. Zhang, X. Hu, Y. Shi, N. Liang, X. Huang, X. Wang, T. Shen, X. Zou, J. Shi, 1-Simple design concept for dual-channel detection of ochratoxin A based on bifunctional metal-organic framework, ACS Appl. Mater. Interfaces 14 (2022) 5615–5623, https://doi.org/10.1021/acsami.1c22809.

[2] Y. Song, L. He, S. Zhang, X. Liu, K. Chen, Q. Jia, Z. Zhang, M. Du, Novel impedimetric sensing strategy for detecting ochratoxin A based on NH2-MIL-101 (Fe) metal-organic framework doped with cobalt phthalocyanine nanoparticles, Food Chem. 351 (2021), https://doi.org/10.1016/j.foodchem.2021.129248.

[3] Y. Nonaka, K. Saito, N. Hanioka, S. Narimatsu, H. Kataoka, HPLC-Determination of aflatoxins in food samples by automated on-line in-tube solid-phase microextraction coupled with liquid chromatography-mass spectrometry, J. Chromatogr. A 1216 (2009) 4416–4422, https://doi.org/10.1016/j.chroma.2009.03.035.

[4] M. Numako, T. Toyo'oka, I. Noge, Y. Kitagawa, H. Mizuno, K. Todoroki, mass-Risk assessment of diabetes mellitus using dried saliva spot followed by ultra-performance liquid chromatography with fluorescence and mass spectrometry, Microchem. J. 142 (2018) 202–207, https://doi.org/10.1016/j.microc.2018.06.037.

[5] D.D. Sun, X. Gu, J.G. Li, T. Yao, Y.C. Dong, ELISA-Quality evaluation of five commercial enzyme linked immunosorbent assay kits for detecting aflatoxin B1 in feedstuffs, AJAS (Asian-Australas. J. Anim. Sci.) 28 (2015) 691–696, https://doi.org/10.5713/ajas.14.0868.

[6] J. Chang, X. Wang, J. Wang, H. Li, F. Li, Nucleic acid-functionalized metal-organic framework-based homogeneous electrochemical biosensor for simultaneous detection of multiple tumor biomarkers, Anal. Chem. 91 (2019) 3604–3610, https://doi.org/10.1021/acs.analchem.8b05599.

[7] N. Colozza, K. Kehe, G. Dionisi, T. Popp, A. Tsoutsoulopoulos, D. Steinritz, D. Moscone, F. Arduini, A wearable origami-like paper-based electrochemical biosensor for sulfur mustard detection, Biosens. Bioelectron. 129 (2019) 15–23, https://doi.org/10.1016/j.bios.2019.01.002.

[8] Y. Li, D. Liu, C. Zhu, X. Shen, Y. Liu, T. You, Sensitivity programmable ratiometric electrochemical aptasensor based on signal engineering for the detection of aflatoxin B1 in peanut, J. Hazard. Mater. 387 (2020), https://doi.org/10.1016/j.jhazmat.2019.122001.

[9] C. Wang, J. Qian, K. Wang, X. Yang, Q. Liu, N. Hao, C. Wang, X. Dong, X. Huang, Colorimetric aptasensing of ochratoxin A using Au@Fe3O4 nanoparticles as signal indicator and magnetic separator, Biosens. Bioelectron. 77 (2016) 1183–1191, https://doi.org/10.1016/j.bios.2015.11.004.

[10] H. Xu, C. Xiao, F. Zhao, Z. Suo, Y. Liu, M. Wei, B. Jin, Ratiometric fluorescent aptasensor based on DNA-gated Fe3O4@UiO-66-NH2 and Exo I-assisted signal amplification, Anal. Chim. Acta 1340 (2025), https://doi.org/10.1016/j.aca.2025.343665.

[11] M. Wang, C. Xiao, F. Zhao, Z. Suo, Y. Liu, M. Wei, B. Jin, A label-free electrochemical sensor based on π-structured bipedal DNA walker-triggered hybridization chain reaction and AuPt NPs/Zr-MOF for OTA detection, Anal. Chim. Acta 1334 (2025), https://doi.org/10.1016/j.aca.2024.343424.

[12] X. Wu, C. Geng, W. Cheng, Z. Wang, Y. Zhang, D. Wu, X. Tang, An ultrasensitive homogeneous electrochemical strategy for ochratoxin a sensing based on nanoscale PCN-224@MB@Apt, Talanta 287 (2025), https://doi.org/10.1016/j.talanta.2025.127695.
[13] M. Wei, W. Zhang, A novel impedimetric aptasensor based on AuNPs-carboxylic porous carbon for the ultrasensitive detection of ochratoxin A, RSC Adv. 7 (2017) 28655-28666, https://doi.org/10.1039/c7ra04209d.

[14] Z. Sun, J. Lv, X. Liu, Z. Tang, X. Wang, Y. Xu, B.D. Hammock, Development of a nanobody-AviTag fusion protein and its application in a streptavidin-biotin-amplified enzyme-linked immunosorbent assay for ochratoxin A in cereal, Anal. Chem. 90 (2018) 10628-10634, https://doi.org/10.1021/acs.analchem.8b03085.

[15] X. Zhang, C. Song, K. Yang, W. Hong, Y. Lu, P. Yu, L. Mao, Photoinduced regeneration of an aptamer-based electrochemical sensor for sensitively detecting adenosine triphosphate, Anal. Chem. 90 (2018) 4968-4971, https://doi.org/10.1021/acs.analchem.7b05442.

[16] R. Ren, K. Shi, J. Yang, R. Yuan, Y. Xiang, DNA three way junction-mediated recycling amplification for sensitive electrochemical monitoring of uracil-DNA glycosylase activity and inhibition, Sensor. Actuator. B Chem. 258 (2018) 783-788, https://doi.org/10.1016/j.snb.2017.11.164.

[17] X. Chen, D. Gao, F. Sun, Z. Li, Y. Wang, C. Qiu, K. He, J. Wang, Nanomaterial-based aptamer biosensors for ochratoxin A detection: a review, Anal. Bioanal. Chem. 414 (2022) 2953-2969, https://doi.org/10.1007/s00216-022-03960-5.

[18] K.Y. Goud, S.K. Kalisa, V. Kumar, Y.F. Tsang, S.E. Lee, K.V. Gobi, K.H. Kim, Progress on nanostructured electrochemical sensors and their recognition elements for detection of mycotoxins: a review, Biosens. Bioelectron. 121 (2018) 205-222, https://doi.org/10.1016/j.bios.2018.08.029.

[19] Y.F. Yang, Y.J. Feng, X.P. Hu, H.W. Gu, X.F. Yan, X.L. Yin, Smartphone-based visual sensing platform based on molecular imprinted polymers and aptamers synergistic recognition for simultaneous detection of aflatoxin B1 and ochratoxin A, Microchem. J. 212 (2025) 113361, https://doi.org/10.1016/j.microc.2025.113361.

[20] W.P. Lustig, S. Mukherjee, N.D. Rudd, A.V. Desai, J. Li, S.K. Ghosh, Metal-organic frameworks: functional luminescent and photonic materials for sensing applications, Chem. Soc. Rev. 46 (2017) 3242-3285, https://doi.org/10.1039/c6cs00930a.

[21] X. Zhang, M. Zhu, Y. Jiang, X. Wang, Z. Guo, J. Shi, X. Zou, E. Han, Simple electrochemical sensing for mercury ions in dairy product using optimal Cu2+-based metal-organic frameworks as signal reporting, J. Hazard. Mater. 400 (2020), https://doi.org/10.1016/j.jhazmat.2020.123222.

[22] M.R. Saber, M.H. Fytory, S.M. Hamdy, A.A. Farghali, R. Ludwig, M.R. Saber, MOF Modified Pencil Electrode for Biohydrogen Peroxide Detections, 2018.

[23] M. Bagheri, M.Y. Masoomi, Sensitive ratiometric fluorescent metal-organic framework sensor for calcium signaling in human blood ionic concentration media, ACS Appl. Mater. Interfaces 12 (2020) 4625-4631, https://doi.org/10.1021/acsami.9b20489.

[24] J. Shanahan, D.K. Kissel, E. Sullivan, PANI@UiO-66 and PANI@UiO-66-NH2Polymer-MOF hybrid composites as tunable semiconducting materials, ACS Omega 5 (2020) 6395-6404, https://doi.org/10.1021/acsomega.9b03834.

[25] J. Tang, Y. Li, Y. Yao, R. Hu, Colorimetric/electrochemical dual-mode aptasensor with phosphatase-like ceria nanozyme for fast and ultrasensitive detection of Ochratoxin A, Sensor. Actuator. B Chem. 432 (2025), https://doi.org/10.1016/j.snb.2025.137482.

[26] H. Cai, Y.L. Huang, D. Li, Biological metal-organic frameworks: structures, host-guest chemistry and bio-applications, Coord. Chem. Rev. 378 (2019) 207-221, https://doi.org/10.1016/j.ccr.2017.12.003.

[27] D.L. Li, X. Zhang, Y. Ma, Y. Deng, R. Hu, Y. Yang, Preparation of an OTA aptasensor based on a metal-organic framework, Anal. Methods 10 (2018) 3273-3279, https://doi.org/10.1039/c8ay00758f.

[28] Z. Wang, H. Yu, J. Han, G. Xie, S. Chen, Rare Co/Fe-MOFs exhibiting high catalytic activity in electrochemical aptasensors for ultrasensitive detection of ochratoxin A, Chem. Commun. 53 (2017) 9926-9929, https://doi.org/10.1039/c7cc05327d.

[29] Y. Guan, P. Bin Si, T. Yang, Y. Wu, Y.H. Yang, R. Hu, A novel method for detection of ochratoxin A in foods-Co-MOFs based dual signal ratiometric electrochemical aptamer sensor coupled with DNA walker, Food Chem. 403 (2023), https://doi.org/10.1016/j.foodchem.2022.134316.

[30] F.J. Dehaghani, H.R. Zare, Z. Shekari, Talanta Simultaneous measurement of ochratoxin A and aflatoxin B1 using a duplexed-electrochemical aptasensor based on carbon nanodots decorated with gold nanoparticles and two redox probes hemin @ HKUST-1 and ferrocene @ HKUST-1, Talanta 266 (2024) 124947, https://doi.org/10.1016/j.talanta.2023.124947.

[31] Y. Song, M. Xu, X. Liu, Z. Li, C. Wang, Q. Jia, Z. Zhang, M. Du, A label-free enrofloxacin electrochemical aptasensor constructed by a semiconducting CoNi-based metal-organic framework (MOF), Electrochim. Acta 368 (2021), https://doi.org/10.1016/j.electacta.2020.137609.

[32] A. Jadhav, K. Gupta, P. Ninawe, N. Ballav, Imparting multifunctionality by utilizing bipolarity in a zirconium-based metal-organic framework, Angew. Chem. 132 (2020) 2235-2239, https://doi.org/10.1002/ange.201910625.

[33] W. Qiu, F. Gao, N. Yano, Y. Kataoka, M. Handa, W. Yang, H. Tanaka, Q. Wang, Specific coordination between Zr-MOF and phosphate-terminated DNA coupled with strand displacement for the construction of reusable and ultrasensitive aptasensor, Anal. Chem. 92 (2020) 11332-11340, https://doi.org/10.1021/acs.analchem.0c02018.

[34] B. Dhara, S.S. Nagarkar, J. Kumar, V. Kumar, P.K. Jha, S.K. Ghosh, S. Nair, N. Ballav, Increase in electrical conductivity of MOF to billion-fold upon filling the nanochannels with conducting polymer, J. Phys. Chem. Lett. 7 (2016) 2945-2950, https://doi.org/10.1021/acs.jplett.b601236.

[35] A.A. Talin, A. Centrone, A.C. Ford, M.E. Foster, V. Stavila, P. Haney, R.A. Kinney, V. Szalai, F. El Gabaly, H.P. Yoon, F. Léonard, M.D. Allendorf, Tunable electrical conductivity in metal-organic framework thin-film devices, Science 343 (2014) 66-69, https://doi.org/10.1126/science.1246738.

[36] W. Li, X. Zhang, X. Hu, Y. Shi, N. Liang, X. Huang, X. Wang, T. Shen, X. Zou, J. Shi, Simple design concept for dual-channel detection of ochratoxin A based on bifunctional metal-organic framework, ACS Appl. Mater. Interfaces 14 (2022) 5615-5623, https://doi.org/10.1021/acsami.1c22809.

[37] M. Xu, H. Zhang, J. Zheng, Polypyrrole microsphere modified porous UiO-66 for electrochemical nitrite sensing, J. Electrochem. Soc. 169 (2022) 047515, https://doi.org/10.1149/1945-7111/ac644c.

[38] F. Liu, A. Ding, J. Zheng, J. Chen, B. Wang, A label-free aptasensor for ochratoxin a detection based on the structure switch of aptamer, Sensors (Switzerland) 18 (2018), https://doi.org/10.3390/s18061769.

[39] J.A. Cruz-Aguado, G. Penner, Determination of ochratoxin A with a DNA aptamer, J. Agric. Food Chem. 56 (2008) 10456-10461, https://doi.org/10.1021/jf801957h.

[40] M.J. Katz, Z.J. Brown, Y.J. Colón, P.W. Siu, K.A. Scheidt, R.Q. Snurr, J.T. Hupp, O. K. Farha, A facile synthesis of UiO-66, UiO-67 and their derivatives, Chem. Commun. 49 (2013) 9449-9451, https://doi.org/10.1039/c3cc46105j.

[41] N. Fairley, V. Fernandez, M. Richard-Plouet, C. Guillot-Deudon, J. Walton, E. Smith, D. Flahaut, M. Greiner, M. Biesinger, S. Tougaard, D. Morgan, J. Baltrusaitis, Systematic and collaborative approach to problem solving using X-ray photoelectron spectroscopy, Appl. Surf. Sci. Adv. 5 (2021) 100112, https://doi.org/10.1016/j.apsadv.2021.100112.

[42] W. Qiu, F. Gao, N. Yano, Y. Kataoka, M. Handa, W. Yang, H. Tanaka, Q. Wang, Specific coordination between Zr-MOF and phosphate-terminated DNA coupled with strand displacement for the construction of reusable and ultrasensitive aptasensor, Anal. Chem. 92 (2020) 11332-11340, https://doi.org/10.1021/acs.analchem.0c02018.

[43] J. Zhang, X. Zhang, G. Yang, J. Chen, S. Wang, A signal-on fluorescent aptasensor based on Tb3+ and structure-switching aptamer for label-free detection of Ochratoxin A in wheat, Biosens. Bioelectron. 41 (2013) 704-709, https://doi.org/10.1016/j.bios.2012.09.053.

[44] Y. Wang, L. Li, P. Dai, L. Yan, L. Cao, X. Gu, X. Zhao, Missing-node directed synthesis of hierarchical pores on a zirconium metal-organic framework with tunable porosity and enhanced surface acidity: via a microdroplet flow reaction, J. Mater. Chem. A 5 (2017) 22372-22379, https://doi.org/10.1039/c7ta06060b.

[45] M. Fytory, A. Mansour, W.M.A. El Rouby, A.A. Farghali, X. Zhang, F. Bier, M. Abdel-Hafiez, I.M. El-Sherbiny, Core-shell nanostructured drug delivery platform based on biocompatible metal-organic framework-ligated polyethyleneimine for targeted hepatocellular carcinoma therapy, ACS Omega 8 (2023) 20779-20791, https://doi.org/10.1021/acsomega.3c01385.

[46] Z. Zhang, L. Liu, H. Huang, L. Li, Y. Wang, J. Xu, J. Xu, Encapsulation of CsPbBr3 perovskite quantum dots into PPy conducting polymer: exceptional water stability and enhanced charge transport property, Appl. Surf. Sci. 526 (2020), https://doi.org/10.1016/j.apsusc.2020.146735.

[47] J. Li, H. Wang, Z.Y. Qi, C. Ma, Z. Zhang, B. Zhao, L. Wang, H. Zhang, Y. Chong, X. Chen, X. Cheng, D.D. Dionysiou, Kinetics and mechanisms of electrocatalytic hydrodechlorination of diclofenac on Pd-Ni/PPy-rGO/Ni electrodes, Appl. Catal. B Environ. 268 (2020), https://doi.org/10.1016/j.acpatb.2020.118696.

[48] H.R. Abid, H.M. Ang, S. Wang, Effects of ammonium hydroxide on the structure and gas adsorption of nanosized Zr-MOFs (UiO-66), Nanoscale 4 (2012) 3089-3094, https://doi.org/10.1039/c2nr30244f.

[49] D. Liu, Y. Li, C. Liu, Y. Zhou, Facile preparation of UiO-66@PPy nanostructures for rapid and efficient adsorption of fluoride: adsorption characteristics and mechanisms, Chemosphere 289 (2022), https://doi.org/10.1016/j.chemosphere.2021.133164.

[50] D.B. Trushina, A.Y. Sapach, O.A. Burachevskia, P.V. Medvedev, D.N. Khmelenin, T.N. Borodina, M.A. Soldatov, V.V. Butova, Dorubicin-loaded core-shell UiO-66@SiO2 metal-organic frameworks for targeted cellular uptake and cancer treatment, Pharmaceutics 14 (2022), https://doi.org/10.3390/pharmaceutics14071325.

[51] J. Wen, Y. Tian, Z. Mei, W. Wu, Y. Tian, Synthesis of polypyrrole nanoparticles and their applications in electrically conductive adhesives for improving conductivity, RSC Adv. 7 (2017) 53219-53225, https://doi.org/10.1039/c7ra09725e.

[52] C. Malitesta, I. Losito, L. Sabbatini, P.G. Zambonin, New findings on polypyrrole chemical structure by XPS coupled to chemical derivatization labeling, J. Electron. Spectrosc. Relat. Phenom. 76 (1995) 629-634, https://doi.org/10.1016/0368-2048(95)02438-7.

[53] J. Walton, P. Wincott, A. Carrick, N. Fairely, CasaXPS processing software for XPS spectra, CASA Orig. Man (2009) 1-177.

[54] J.L. Bredas, G.B. Street, Polarons, bipolarons, and solitons in conducting polymers, Acc. Chem. Res. 18 (1985) 309-315, https://doi.org/10.1021/ar00118a005.

[55] E. De Giglio, M.R. Guascito, L. Sabbatini, G. Zambonin, Electropolymerization of pyrrole on titanium substrates for the future development of new biocompatible surfaces, Biomaterials 22 (2001) 2609-2616, https://doi.org/10.1016/S0142-9612(00)00449-X.

[56] M.C. Rodriguez, A.N. Kawde, J. Wang, Aptamer biosensor for label-free impedance spectroscopy detection of proteins based on recognition-induced switching of the surface charge, Chem. Commun. (2005) 4267-4269, https://doi.org/10.1039/b506571b.

[57] R. Hajian, Z. Tayebi, N. Shams, Fabrication of an electrochemical sensor for determination of doxorubicin in human plasma and its interaction with DNA, J. Pharm. Anal. 7 (2017) 27-33, https://doi.org/10.1016/j.jpha.2016.07.005.

[58] B. Saoudi, N. Jammul, M.M. Chehimi, A.S. Jaubert, C. Arkam, M. Delamar, XPS study of the adsorption mechanisms of DNA onto polypyrrole particles, Spectroscopy 18 (2004) 519-535, https://doi.org/10.1155/2004/943729.

[59] S. Li, Y. Kang, M. Shang, Y. Cai, Z. Yang, Highly sensitive and selective detection of Ochratoxin a using modified graphene oxide-aptamer sensors as well as
application, Microchem. J. 179 (2022), https://doi.org/10.1016/j.microc.2022.107449.

[60] N. Mejri-Omrani, A. Miodek, B. Zribi, M. Marrakchi, M. Hamdi, J.L. Marty, H. Korri-Youssoufi, Direct detection of OTA by impedimetric aptasensor based on modified polypyrrole-dendrimers, Anal. Chim. Acta 920 (2016) 37–46, https://doi.org/10.1016/j.aca.2016.03.038.

[61] M. Liaqat, S. Riaz, M.H. Nawaz, M. Badea, A. Hayat, J.L. Marty, Fabrication of electro-active nano-trans surfaces to design label free electrochemical aptasensor for ochratoxin A detection, Electrochim. Acta 379 (2021), https://doi.org/10.1016/j.electacta.2021.138172.

[62] Y. Hou, N. Long, B. Jia, X. Liao, M. Yang, L. Fu, L. Zhou, P. Sheng, W. Kong, Development of a label-free electrochemical aptasensor for ultrasensitive detection of ochratoxin A, Food Control 135 (2022), https://doi.org/10.1016/j.foodcont.2022.108833.

[63] M. Mavioğlu Kaya, H.A. Deveci, İ. Kaya, N. Atar, M.L. Yola, The electrochemical detection of ochratoxin A in apple juice via MnCO3 nanostructures incorporated into carbon fibers containing a molecularly imprinting polymer, Biosensors 13 (2023), https://doi.org/10.3390/bios13080760.

[64] X. Chen, D. Gao, J. Chen, X. Wang, C. Peng, H. Gao, Y. Wang, Z. Li, H. Niu, A polyamidoamine-based electrochemical aptasensor for sensitive detection of ochratoxin A, Biosensors 13 (2023) 955, https://doi.org/10.3390/bios13110955.