Oxime-Functionalized Polydiacetylene Nanocomposite Sensor for Equipment-Free On-Site Detection Of Organophosphate Insecticide Chlorpyrifos Ethyl

Abstract

A novel oxime-incorporated polydiacetylene-functionalized molecule (PCDA-OX) was synthesized for real-time, rapid, and selective detection of the organophosphate insecticide chlorpyrifos ethyl, which is widely used in agricultural fields globally. Nanocomposites containing PCDA-OX molecules were fabricated within a cellulose acetate matrix using an electrospinning technique, followed by chemical hydrolysis to obtain regenerated cellulose-based sensing materials. Comprehensive characterization was performed using scanning electron microscopy, nuclear magnetic resonance, Fourier-transform infrared spectroscopy, and an Instron tensile tester. Exposure to the chlorpyrifos ethyl triggered an instantaneous chromatic transition from blue to red/pink, enabling naked eye detection without instrumentation. The proposed sensor exhibited high selectivity against other classes of insecticides and achieved a limit of detection of 40 mmol. The sensing platform offers a simple, rapid, and field-deployable approach for organophosphate monitoring, offering a promising strategy for protecting human health and saving lives.

1. Introduction

Insecticides are employed worldwide extensively to safeguard crops from insect infestations and to enhance agricultural productivity. Among these, organophosphate (OP) insecticides represent one of the most prevalent insecticides currently used globally. Their application spans agriculture, domestic, horticultural, and veterinary settings. Despite their effectiveness in pest management, OP insecticides have raised significant concerns because of their adverse effects on environmental sustainability and human health. These compounds, while effective in pest control, pose a significant risk of contamination, even when residues persist in agricultural products. Exposure to these chemicals predominantly occurs in agricultural settings during various stages of handling such as production, transportation, preparation (including mixing and loading), and application (spraying). , Research has associated exposure to such compounds with various diseases, including cancer, leukemia, diabetes, Parkinson’s disease, depression, autism, asthma, and alterations in DNA methylation, which cause 250,000 annual deaths globally. , Consequently, there is an urgent need for robust, easy-to-use, and sensitive sensing techniques capable of rapidly detecting and quantifying the exposure and presence of organophosphate insecticides.

Traditional methods for organophosphate insecticide detection including gas chromatography, liquid chromatography, and mass spectrometry are lab-based techniques and are often limited by time-consuming and labor-intensive procedures, the need for trained operators, and expensive equipment. , Moreover, they are not available in many parts of the world. These methods, although accurate, are not suitable for on-site or real-time monitoring, hindering their practicality for applications where immediate and precise detection is needed. Therefore, developing alternative strategies for simple, quick, sensitive, selective, accurate, less expensive, and user-friendly techniques would be highly beneficial in detecting harmful insecticides. Among the alternatives, optical sensors are a facile, rapid, and low-cost approach to the sensitive detection of OP insecticides. Typically, an optical sensor contains a recognition element to interact with the target insecticide and a transducer component for signaling the binding event. Colorimetric sensors, a subclass of optical sensors, have been highly attractive due to their ease of use, quick response, and equipment-free detection, enabling them for field-level applications. Moreover, the chemical interaction is transmitted via a color change, which is visible by the naked eye, omitting the need for equipment. However, a significant challenge in creating a colorimetric sensor lies in translating the response to a visible color change. Researchers around the globe have been investigating functional polymers such as polydiacetylene for this purpose.

Polydiacetylenes (PDAs) are polymers with conjugated π-bonds, known for their intriguing optical properties and extensive exploration in various sensing applications. After photopolymerization, PDAs display a blue color with maximum absorption (λmax) at 640 nm. They undergo a color transition from blue to red, with the red phase exhibiting an absorption peak (λmax) around 540 nm. PDAs have been functionalized with diverse functional groups to detect a wide range of analytes, including surfactants, enzymes, nerve agents, and pesticides. , Lee et al. developed a polydiacetylene-based molecular sensor for the detection of Diisopropyl Florophospahte (DFP) at pH 9.0 or above. These molecules, when used in pairs, detected the DFP in liquid media. when used as a pair of sensing molecules. Zhang et al. developed a polydiacetylene-based sensing system for the detection of malathion at pH 9.0. The sensing system is slow and takes minutes to respond to the stimulus. None of these systems work at a normal pH of 7.0. Literature on fiber-based sensing systems suitable for detecting OP insecticide chlorpyrifos is scarce. Therefore, our research aims to develop a composite-fiber-based sensing system for this purpose.

To enhance sensitivity toward organophosphate insecticides, PDAs were functionalized with a nucleophilic oxime group known for its strong reactivity with phosphorus electrophiles. The convergence of nanotechnology and sensor development was realized in developing composite-fiber-based innovative solutions for chemical sensing. Employing electrospinning technology, where electrostatic forces are used to fabricate fibers, PDA-oxime (PDA-OX) nanocomposites were fabricated for composite sensors, harnessing their unique properties, including high surface area, enhanced chemical reactivity, and tunable optical properties. These attributes render nanocomposite materials ideal for the fabrication of highly sensitive and selective sensors with an elevated sensitivity and selectivity in detecting organophosphate insecticides. This paper presents a comprehensive exploration of the design, development, and characterization of a novel nanocomposite fiber sensor tailored for the rapid and precise detection of the commercially used organophosphate insecticide chlorpyrifos. The textile-based nanocomposite sensor offers several advantages such as flexibility, portability, and potential for integration into wearable clothing.

2. Experimental Section

2.1. Materials

10, 12-Pentacosadiynoic acid (PCDA, 98%) was purchased from GFS Organics (Columbus, OH). Cellulose acetate (CA) with 39% acetyl content and of 30 kDa, determined by Gel Permeation Chromatography (GPC), was purchased from Sigma-Aldrich (St. Louis, MO). 2,4-dihydroxybenzaldehyde (98%), 4-(Dimethylamino) pyridine (DMAP, ≥ 99%), N, N′-dicyclohexylcarbodiimide (DCC, 99%), and sodium hydroxide (≥97%) was also purchased from Sigma-Aldrich. Certified ACS grade acetone (≥99.9%), acetonitrile (≥99.9%), ethyl acetate (99.5%), hexane (95%), hydrochloric acid (38.8%), deionized water (HPLC grade), and methylene chloride (99.8%) were purchased from Fisher Chemical (Fair Lawn, NJ).

2.2. Synthesis of PCDA- OX Polymer

Figure shows the synthesis of PCDA-OX polymer. Hydroxylamine hydrochloride (11.1 mmol, 0.771 g) and KOH (11.4 mmol, 0.64 g) were both dissolved separately in 5 mL of water. The KOH solution was added dropwise to the reaction mixture of Hydroxylamine hydrochloride at 0 °C. Ten mmol (1.38 g) 2,4-dihydroxybenzaldehyde was added to the mixture. The reaction mixture was heated to 90 °C for 3 min to reflux, followed by cooling down to room temperature. The precipitate was filtered off. The filtrate was evaporated under a vacuum. The crude product was recrystallized from water to obtain 2,4-dihydroxybenzaldehyde oxime.

1.

Synthesis of PCDA- OX polymer.

10,12 Pentacosadiynoic acid (PCDA) (8.16 mmol, 1. 3.036 g), 2,4 dihydroxybenzaldehyde oxime (8.0 mmol, 1.25 g), and dimethyl aminopyridine (DMAP) (2.04 mmol, 0.25 g) were combined in a light-protected round-bottom flask. The mixture was dissolved in 200 mL of dichloromethane (DCM) and sparged with argon. A 30 mL solution of DCM containing 0.86 g of N,N-dicyclohexylcarbodiimide (DCC) was added to the solution by syringe and continued stirring for 24 h. The solution was then filtered by gravity filtration. The solvent was removed under vacuum on a rotary evaporator and yielded a white solid. The crude product was recrystallized overnight with a solvent system containing 50:50 = ethyl alcohol/water. The final product was dried under high pressure for 45 min.

2.3. Electrospinning of CA-PCDA-OX Nanocomposite Membranes

2.3.1. Finding a Suitable Solvent System

The PCDA-OX sensing polymer possesses limited solubility; they do not dissolve in any known solvent system. The following mixed solvent systems were studied to find a suitable solvent system for efficient electrospinning to ensure defect-free continuous fibers: Acetone: Et OH= 50:50 (v/v), Acetone: Et OH= 70:30 (v/v), and Acetone: Et OH= 85:15 (v/v).

2.3.2. Preparation of Electrospinning Solutions

8.5 mL of acetone and 1.5 mL of Et OH were mixed with 0.15 g of PCDA-OX and kept on stirring at 300 rpm on a hot plate stirrer for half an hour away from sunlight. 1.5 g (15 w/v %) of cellulose acetate was added to the mixture and continued stirring for 24 h to be ready for electrospinning.

2.3.3. Optimization of Electrospinning Parameters

The following experimental design (Table ) was followed to optimize the electrospinning parameters to ensure the fabrication of defect-free fibers.

1. Optimization of Electrospinning Parameters.

trial #	injection speed (mL.hr–1)	voltage (KV)	distance (cm)
1	3	20	15
2	3	15	15
3	2	20	15
4	2	15	15
5	2	15	12
6	1	15	15

2.3.4. Electrospinning of CA-PCDA-OX Composite Membranes

A 15 (w/v %) solution of cellulose acetate (CA) was chosen for the nanocomposite fabrication based on our previous experiments. A mixed solvent system containing 85% (v/v) acetone and 15% (v/v) ethyl alcohol was used based on our solvent optimization experiments as described in Section 2a. The concentration of PCDA-OX was varied in three levels as depicted in Table . The electrospinning was conducted at room temperature for 4 h on Al foil to obtain a membrane suitable for the subsequent chemical treatments.

2. Parameters of Electrospinning.

CA conc (w/v %)	PCDA-OX (w/w %) of CA	infusion rate (mL hr–1)	potential difference (kV)	collector distance (cm)
15	3	2	15	12
15	5	2	15	12
15	10	2	15	12

2.4. Development of RC-OX Composites

2.4.1. Chemical Treatment of the CA-PCDA-OX Composites

The composites were chemically treated to remove acetyl groups following previously reported methods.15 0.1 M NaOH solution comprising 4:1 NaOH/Et OH was made for deacetylation to obtain regenerated cellulosic RC-PCDA-OX nanocomposite fiber membranes. CA-PCDA-OX nanocomposite fiber membranes sandwiched between glass fiber mesh were soaked in a glass bowl in the deacetylation solution. A magnetic stirring bar was placed on the mesh and was kept rotating at 150 rpm for 30 h at room temperature on a hot plate. The membranes were then thoroughly rinsed with deionized water to complete neutralization, as confirmed by pH paper. They were then air-dried overnight in a light-protected room. The RC-PCDA-OX nanocomposite fiber membranes were then stored in a dark place away from light.

2.4.2. Photopolymerization of RC-PCDA-OX Composites

The CA-PCDA-OX nanocomposite fiber membranes were photopolymerized to RC-PDA-OX with 254 nm UV-light (Spectroline, Longlife filter, New York) irradiation for 15 s on both sides of the membranes. During UV irradiation, the colorless fibers turned blue within 15 s.

2.5. Characterization

The ^HNMR spectra of 10,12 pentacosadiynoic acid (PCDA) and PCDA-OX were recorded on a Varian 400 M spectrometer, a narrow bore 9.4 T/400 MHz magnet equipped with a one NMR pulse-field gradient probe using DMSO-d6 as the solvent. Chemical shifts were reported in ppm from the solvent resonance, following the internal standard. Multiplicity is indicated as follows: s (singlet), d (doublet), t (triplet), and m (multiplet).

A field-emission scanning electron microscope (FEI Quanta 250 FE-SEM) was employed to study the size and surface morphology of the ENF membranes. The membranes were kept under a vacuum overnight to evaporate any residual solvents or moisture. They were then sputter-coated with a 5 nm layer of gold to improve the conductivity of the samples and improve imaging. ImageJ software (National Institute of Health) was used to calculate the diameter and distribution of the fibers. The average and distribution of diameters were determined by measuring 50 representative fibers from SEM images.

The functional group and chemical structure of RC-PCDA-OX composites were studied by an Agilent Cary 630 Fourier-transform infrared (FT-IR) spectrometer (Agilent Technologies, Inc., Danbury, CT) equipped with a smart DATR accessory. The nanocomposites were dried overnight in an Isotemp (Model 282A) programmable vacuum oven (Fisher Scientific, Waltham, MA) at room temperature before FT-IR analysis. At first, the air was used as a background to run the experiments. Then, the samples were scanned 32 times at a resolution of 4 cm^–1 and an interval of 1 cm^–1 over the wavenumbers ranging from 550 to 4000 cm^–1.

The tensile properties of the nanocomposite membranes were tested in accordance with ASTM D638–10 standards, utilizing an Instron 5966 tensile testing machine equipped with a 250 N load cell. Test specimens measuring 75 × 10 mm², conditioned at ambient temperature for 24 h, were subjected to a gauge length of 40 mm during testing. The specimens were stretched at a crosshead speed of 10 mm/min. Each sample underwent five replications. Stress–strain curves were generated based on the load and extension values obtained. The thickness of the specimens was measured using a digital caliper from Electron Microscopy Sciences (Hatfield, PA).

The colorimetric properties were measured with a UV–vis spectrophotometer. The solvent was removed using a lyophilizer to obtain dry, solid materials, and the samples were analyzed using a Nicolet Evolution 300 UV–vis spectrophotometer in a diffuse reflectance mode. The colorimetric response (% CR) was calculated using eq as below

$$\%\mathrm{CR}=\left[\frac{\mathrm{PB}\mathrm{before}-\mathrm{PB}\mathrm{after}}{\mathrm{PB}\mathrm{before}}\right]\times 100$$ 1

where the PB _before, and PB _after is the respective percent blue (PB) before and after the color change due to the exposure.

The PB values will be calculated following eq

$$\mathrm{PB}=\left[\frac{\mathrm{A}\mathrm{blue}}{\mathrm{A}\mathrm{blue}+\mathrm{A}\mathrm{red}}\right]\times 100$$ 2

where A _blue, and A _red is the absorbance at 640 and 540 nm respectively.

For a complete blue-to-red transition, PB_after equals 0 and CR is therefore 100%.

2.6. Test of Sensing

RC-PDA-OX nanocomposite fiber membranes encapsulated with 10% PCDA-OX were exposed to different insecticides at various concentrations in a fume hood, as depicted in Table . 100 μL of diluted insecticide solution were poured onto the nanocomposite membranes with a pipet and observed for color transition. The response was recorded, and the samples were left in the fume hood for air drying. Three replications were done for each of the tests.

3. Test of Sensing of RC-PDA-OX Nanocomposite to Chlorpyrifos Ethyl Insecticide.

insecticide	0.01 M	0.1 M	0.2 M	0.4 M
organophosphate	chlorpyrifos	chlorpyrifos	chlorpyrifos	chlorpyrifos
organochlorine	aldrin	aldrin	aldrin	aldrin
pyrethroids	bifenthrin	bifenthrin	bifenthrin	bifenthrin

2.7. Calculation of Limit of Detection (LOD)

The limit of detection (LOD) was calculated from the calibration curve of colorimetric response following published literature. , 50 mL high-concentration solution of chlorpyrifos ethyl insecticide was initially prepared as a standard solution and then diluted to other concentrations as in Table . A concentration range from 0.001 to 0.5 M (as in Table ) was tested for linearity and limit of detection (LOD). The LOD was statistically determined from the slope of the calibration curve and is defined as LOD = 3 × SD ÷ slope, where SD is the standard deviation of the noise.

3. Results and Discussion

3.1. Synthesis of PCDA- OX Polymer

The synthesis was carried out in two stages: the first stage was synthesizing 2,4 dihydroxybenzaldehyde oxime, followed by synthesizing polydiacetylene oxime. The respective ^1H NMR spectra are presented in Figures and below.

2.

^1H NMR of 2,4-dihydroxybenzaldehyde oxime in DMSO-d6.

3.

^1H NMR of PCDA-OX in DMSO D_6..

^1H NMR of oxime (400 MHz, DMSO-d6): δ (ppm) = 10.97 (s, 1H, CN–OH), 10.17 (s, 1H, CN–OH), 9.90 (s, 1H, OH), 8.23 (s, 1H, HCN), 7.23 (d, 1H, Ph), 6.45 (s, 1H, Ph), (Figure ). The Spectron matches with the published literature of oxime.

^1H NMR of polydiacetylene oxime (400 MHz, DMSO-d6): δ (ppm)= δ: 0.86 (3H, m), 1.25–1.46 (32H, m), 2.11 (2H, t), 2.26–2.32­(6H, m), 3.56 (2H, t), 6.56 (2H, m), 7.35 (1H, t), 8.60 (1H, s). The Spectron is similar to the published literature of a similar product. The peak around 8.6 ppm belongs to the −OH group which is an indication of the presence of -NOH group in the structure due to the integration of the oxime in the PDA structure. The structure was further verified with mass-spectroscopy and found the material in accordance with its structure (Figure ).

4.

Mass spec spectra of PCDA-OX.

3.2. Finding a Suitable Solvent System

PCDA-OX is not easily dissolved in acetone or ethyl alcohol alone. Therefore, mixed solvent systems were studied to prepare a homogeneous electrospinning solution suitable for fabricating fibers via electrospinning. The 50:50 (v/v) acetone/ethyl alcohol mixture became too viscous to spin into fibers. To reduce the viscosity of the solution, the ethyl alcohol content was decreased to 30%. However, the 70:30 acetone/ethyl alcohol mixture solution was not good enough to make thin and regular fibers. Finally, the amount of ethyl alcohol was further reduced to 15% (v/v), and the 85:15 acetone/ethyl alcohol emulsion was able to make fine spinnable fibers. On the basis of these experiments, a solvent system containing 85% acetone and 15% ethyl alcohol was chosen for the study, since this solvent system produced a more homogeneous solution that was able to produce defect-free continuous fine fibers.

3.2.1. Optimization of Electrospinning Parameters

The fabrication of nanocomposite membranes via electrospinning requires precise control of the chemical composition, solution properties, and processing parameters. The polymer properties, solvent properties, electrospinning parameters, and ambient conditions strongly influence fiber formation and morphology of the membranes. Electrospinning of PCDA-OX alone is highly challenging because of its inherently low solution viscosity. Therefore, the use of a high-viscosity supporting polymer as a matrix is essential for electrospinning. Cellulose acetate (CA) was selected for this study as matrix polymer due to its excellent mechanical and chemical properties, biodegradability, nontoxicity, and low cost. −

Optimization experiments were conducted to identify the optimum electrospinning parameters that yield bead-free CA-PCDA-OX nanocomposites. Table summarizes the experimental design and the corresponding SEM images are presented in Figure . On the basis of preliminary trials, the applied voltage (15 kV) and tip-to-collector distance (12 cm) were kept fixed, as these values consistently produced continuous fiber deposition. The CA concentration was maintained at 15%, which provided the optimal viscosity for forming defect-free fibers. PCDA-OX was incorporated at three loading levels (3%, 5%, and 10% w/w relative to CA), and the effect of injection speed on fiber formation was evaluated at two levels, i.e., 2 and 3 mL.h^–1.

5.

Optimization of electrospinning parameters.

Analysis of the SEM images reveals that the variation in PCDA-OX loading in the CA solution resulted in variation in viscosity, resulting in the formation of fibers with varying diameters and morphologies. Electrospinning of CA containing 10% PCDA-OX at 2 mL.h-1 injection speed formed the most uniform defect-free fibers. Since fine and uniform fiber morphology enhances chromatic response, the optimized parameters (10% PCDA-OX, 2 mL h^–1 injection speed, 15 kV potential difference, and 12 cm tip-to-collector distance) were selected for all subsequent sensing experiments.

3.2.2. Electrospinning of CA-PCDA-OX Composite Membranes

On the basis of the optimization experiments, electrospinning of the membranes was continued at injection speeds of 2 and 3 mL.h^–1 at a potential difference of 15 kV. The collector plate was placed at 12 cm, and the spinning was conducted on Al foil under ambient conditions for four (4) h. It can be seen from Figure that finer fibers are formed at a lower injection speed (2 mL.hr^–1). Conversely, at any injection speed, finer fibers are obtained with solutions having a high oxime content (Figure ). Finer fibers are more sensitive for sensing applications due to their higher surface area per unit volume. , Therefore, PCDA-OX-10 was selected for sensing experiments afterward.

6.

High resolution (5000X) SEM images of membranes developed at 2 mL.hr^–1 injection speed.

3.3. Regeneration of the Membrane into Cellulosic Oxime RC-OX Membranes

The regeneration of cellulose nanocomposites (RC-OX) from the as-spun cellulose acetate nanocomposites (CA-OX) was done via a chemical hydrolysis treatment. FT-IR was conducted to compare the OX crystal with nanocomposite fiber membranes to check whether the matrix polymer and the hydrolysis treatment affected the conjugated macromolecules that might hinder the sensing properties of the RC-OX nanocomposite fibers.

The FT-IR spectra of the PCDA crystal, oxime crystal, cellulose acetate nanocomposite, and regenerated cellulose nanocomposite fiber containing PCDA-Oxime (PCDA-OX) are depicted in Figure . Spectroscopic data of PCDA molecules on spectrum A showed the following resonance features: IR ν_max cm^–1: 2955 cm^–1 (H–CC), 2918–2847 cm^–1 (H–C–H), 1690 cm^–1 (CO), and 721 cm^–1 (CC). The resonance features were similar to the previously reported results for the PCDA polymers.

7.

FT-IR spectra of PCDA (A), oxime crystal (B), CA-PCDA-OX composite (C), and RC-PCDA-OX composite (D).

The following resonance features were identified in the oxime spectrum (B): IR νmax (cm^–1): 956 (N–O bending), 1303 (aromatic CC bending), and 1623 (oxime CN characteristic stretching), 3275 (aliphatic H–CC stretching), and 3350 (aromatic H–CC stretching). These spectral signatures align with the predicted structure of oxime, and the resonance features are consistent with previously reported oxime structures. Notably, the distinctive band around 1623 cm^–1 suggests the presence of an oxime group around a benzene ring in the structure.

The following resonance features were observed in the PCDA-OX (D) encapsulated regenerated cellulose nanocomposite membranes: IR νmax (cm^–1): 985 (N–O bending), 1104 (C–O bend), 1505 (CO stretching), 1635 (oxime CN characteristic stretching), 2925–2849 (H–C– H stretching), 3058 (H–CC stretching), and 3360 (−OH stretching). Comparing the resonance features of CA-PCDA-OX (C) and RC-PCDA-OX (D), it is evident that the characteristic peaks either decreased in intensity, disappeared, or shifted to a lower wavenumber, indicating the successful conversion of CA composites into RC composites. These findings are consistent with previous studies on PDA-containing compounds. ,,

The FT-IR spectrum of PCDA-OX reveals the retention of characteristic functional groups from the polydiacetylene macromolecule and oxime molecule within the matrix cellulosic polymer structure following the deacetylation process. Notably, the carbonyl (CO) peak in the PCDA-OX structure shifted toward a lower wavenumber, indicative of decreased CO groups because of hydrolysis. Moreover, the broad absorption peak at 3350 cm^–1 suggests the formation of a large amount of hydroxyl groups resulting from a successful deacetylation process. Therefore, the deacetylation process did not affect the functional groups and the PCDA-OX polymer presents evidence of the incorporation of oxime molecules with PCDA macromolecules.

3.4. Tensile Properties of the Composites

The tensile properties of the composite membranes were evaluated to assess their suitability for textile applications. Table summarizes the tensile strength, elongation at break, and Young’s modulus of the PDA-OX composites (thicknesses ∼ 0.10 mm), calculated from five replicates per sample.

4. Tensile Properties of the PDA-OX Composites.

composite	tensile strength (MPa)	elongation (%)	Youngs’s modulus (MPa)
PDA-OX-3	6.73 ± 0.46	6.55 ± 2.09	416 ± 85
PDA-OX-5	5.01 ± 1.86	4.99 ± 1.96	382 ± 110
PDA-OX-10	5.02 ± 0.87	4.02 ± 1.01	358 ± 79

The typical stress–strain curves of PDA-OX-3, PDA-OX-5, and PDA-OX-10 composite membranes are plotted in Figure . The curves exhibit the characteristic plateau-like behavior commonly observed in the structure of textile fibers. Incorporation of PDA-OX into the regenerated cellulose matrix resulted in a gradual reduction in tensile strength and elongation compared to pristine regenerated cellulose membranes (6.85 ± 0.6 MPa strength, 6 ± 1.7% elongation, and 392 ± 71 MPa modulus). This reduction is attributed to the bulky aromatic structure of PDA-OX, which behave as inert fillers rather than a reinforcing component within the cellulose network. Similar behavior was reported by other researchers in composite systems where the added components does not chemically interact with the matrix polymer. These results suggest that a lower loading of PCDA-OX is better for tensile properties. Despite the decrease in mechanical performance at higher loadings, all membranes remain within the acceptable range for textile applications, which typically require moderate tensile strength and sufficient flexibility to withstand bending and handling.

8.

Typical stress–strain curves of the composites.

3.5. Test of Sensing of Chlorpyrifos Ethyl Insecticide

In this study, dihydroxybenzaldehyde oxide was synthesized and conjugated with polydiacetylene monomers to develop PCDA-OX crystals, which were subsequently encapsulated within a cellulose acetate matrix via electrospinning. The as-spun nanocomposites were chemically hydrolyzed to convert cellulose acetate into regenerated cellulose, yielding colorless PCDA-OX composites. The resulting regenerated cellulose-based PCDA-OX composites were then photopolymerized with 254 nm UV-light irradiation for 15s on each side, resulting in blue phase PDA-OX composites. The color transformation from colorless to blue within seconds of irradiation indicated the formation of the π-conjugated backbone of the PDA structure within the cellulose matrix, rendering the composite a transducer for sensing the organophosphate insecticide chlorpyrifos ethyl in its surrounding environment.

Preliminary screenings of composite formulations revealed that composites containing 10% PDA-OX (PDA-OX-10) exhibited the most pronounced chromatic response among all compositions investigated in this study, identifying the PDA-OX-10 formulation as optimal for sensor development. The enhanced sensitivity of PCDA-OX-10 is attributed to the formation of finer fibers during electrospinning, which provide an increased surface area and improved analyte accessibility compared with the other composites. ,

The sensing performance of PDA-OX-10 was evaluated by exposure of the composites to chlorpyrifos ethyl solutions of varying concentrations in a fume hood. Upon contact with the insecticide, the composites exhibited an immediate blue-to-red/pink color transition, observable to the naked eye even at a concentration of 0.01 M (100 ppm) (Figure ). Given that the acute dermal lethal dose (LD₅₀) of chlorpyrifos is approximately 202 ppm for test animal rats, this rapid visual response highlights its high sensitivity and practical relevance of the first solid-state nanocomposite fiber sensing platform.

9.

Colorimetric response of PDA-OX-10 composites to different classes of insecticides at various concentrations.

Quantitative analysis of the chromatic transition was performed using a UV–vis spectrophotometer. Figure shows the representative spectra of the composites when exposed to different concentrations of chlorpyrifos ethyl insecticide. The peaks around 640 nm represent the blue state of the PDA-OX composites. Because of the interaction with the insecticide upon exposure, the peak around 640 nm gradually diminished or decreased in intensity. On the other hand, peaks around 540 nm gradually increased, representing the color shift to red/pink upon exposure to the chlorpyrifos ethyl insecticide. An index of the color change (% CR) was calculated from the spectra using eqs and , following the established protocol in the literature. On the basis of these measurements, the limit of detection (LOD) was calculated to be 40 mmol. Collectively, these results demonstrate that the pronounced and quantifiable blue-to red/pink chromatic transition of PDA-OX composites provides a robust and highly promising strategy for field-level detection of organophosphate insecticide chlorpyrifos ethyl.

10.

Colorimetric response of the composites when exposed to field-use insecticide chlorpyrifos ethyl at various concentrations,.

3.6. Selective Detection of Organophosphate Insecticide Chlorpyrifos Ethyl

Selectivity toward the target analyte is a critical concern for the reliable sensor performance. To assess the specificity of the sensing platform, the colorimetric response of the PDA-oxime composites toward chlorpyrifos ethyl was compared with their response to insecticides from other chemical classes. Aldrin (organochlorine) and bifenthrin (pyrethroid/pyrethrin) were selected as representative potential interferents to evaluate the likelihood of false-positive signals during detection. As shown in Figure , no measurable chromatic transition was observed upon exposure of the composites to aldrin or bifenthrin across the tested concentration range, confirming the selectivity of the proposed sensor toward organophosphate insecticide chlorpyrifos ethyl.

The high selectivity arises from the specific chemical interaction between the oxime functional groups incorporated within the polydiacetylene framework and the electrophilic phosphoryl center (PO) characteristic of organophosphate compounds, such as chlorpyrifos ethyl. The oxime moiety can engage in nucleophilic attack or strong-dipole–dipole interactions with the phosphoryl groups, inducing localized perturbations in the PDA backbone. These perturbations trigger the characteristic blue-to red/pink chromatic transition.

In contrast, aldrin and bifenthrin lack phosphoryl functionalities or comparable electrophilic functional groups are engaged in comparable chemical interactions. Aldrin is a polycyclic organochlorine structure dominated by C–C and C–Cl bonds, while bifenthrin contains ester and aromatic functionalities but lacks an electrophilic phosphorus center. Because these molecules cannot participate in oxime-phosphoryl interactions, they fail to induce conformational or electronic changes required to trigger the PDA chromatic transition. This structural incompatibility explains the absence of colorimetric response and reinforces that the sensing mechanism is chemically induced and highly selective toward organophosphate insecticides, further demonstrating its robustness against interference from nonorganophosphate insecticides.

4. Conclusions

We have developed a novel oxime-incorporated polydiacetylene-functionalized nanocomposite sensing platform (PDA-OX) that enables equipment-free real-time visual detection of field-use organophosphate insecticide chlorpyrifos ethyl. Nanocomposites containing varying loadings of PCDA-OX were fabricated via electrospinning into cellulose acetate matrix, followed by an alkaline hydrolysis treatment to convert cellulose acetate into regenerated cellulose. Higher PCDA-OX content promoted finer fibers, and the hydrolysis treatment generated abundant hydroxyl groups in the composites, enhancing the sensing performance. Tensile testing results demonstrated that the composites retained mechanical properties suitable for use in textile structures. Upon exposure to insecticide chlorpyrifos ethyl, the composites exhibited an immediate and pronounced color transition from blue to red/pink, consistent with a PDA backbone conformational shift induced by oxime–phosphoryl interactions. The color transition was rapid and visible to naked eye without auxiliary instrumentation. The sensor displayed a limit of detection (LOD) of 40 mmol. This work establishes a chemically selective, mechanically robust, and equipment-free field-deployable sensing platform for organophosphate insecticides, offering a promising pathway toward wearable and field-deployable sensors for organophosphate monitoring and protecting human health and lives.

A.K.M.M.A.: Conceptualization, conducting experiments, investigation, writing the original draft. D.A.F.: Helping with the PDA synthesis. G.A.K.: Conceptualization, supervision. C.X.: Conceptualization, supervision, project administration, writing review, and editing.

The authors declare no competing financial interest.