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. 2026 Jun 23;11(26):38934–38944. doi: 10.1021/acsomega.6c02494

A Colorimetric Sensor for Cyanide Ion Detection Using a Poly(TCDA)-Based Material Sensor

Ruttayapon Potai 1,*
PMCID: PMC13347342  PMID: 42428900

Abstract

This study presents a colorimetric sensor for the rapid visual detection of highly toxic cyanide ions (CN–) utilizing poly­(TCDA) assemblies and poly­(TCDA)/Zn2+/ZnO nanocomposites. The pristine polydiacetylene (PDA) and nanocomposites exhibit a distinct naked-eye observable color transition from a non-fluorescent blue phase to a highly fluorescent red phase upon exposure to CN–. The incorporation of 0.5 wt % ZnO into the nanocomposites yielded optimal sensing performance, demonstrating a more rapid colorimetric response than pure PDA or nanocomposites with higher ZnO concentrations. The color change was visible to the naked eye when the RSC exceeded 50%, with a limit of detection for cyanide of 0.6 mM. The spectroscopic and structural characterizations reveal that CN– intercalates into the PDA crystal, interacting with the carboxylate headgroups and inducing conformational changes in both the conjugated polymer backbone and the alkyl side chains. Furthermore, the fabrication of poly­(TCDA)-based hydrogel beads, particularly those with 0% and 0.5% ZnO, was successfully demonstrated as a practical platform for effective naked-eye CN– detection.

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1. Introduction

Chemical substances have become inseparable from daily human life. Many chemicals pose serious risks to human health and the environment. Especially, hazardous chemical contamination of the environment is an important problem affecting human health and ecosystems. For example, cyanide ions (CN–), a highly toxic substance, inhibit the function of cytochrome oxidase enzymes in cellular respiration, leading to oxygen deprivation and potentially death. − The World Health Organization (WHO) recommended a tolerable CN– limit in drinking water of 1.9 μM. Likewise, the US Environmental Protection Agency recommends a CN– limit of 7.8 μM. Several analytical techniques are used for the detection of cyanide, such as fluorescence, electrochemistry, flow injection analysis, , atomic absorption spectrometry, UV–visible spectroscopy, and gas chromatography–mass spectrometry analysis. , Notably, fluorescent and colorimetric measurements of cyanide are more widely used. During the decade, the synthesis of molecular probes has been developed to detect CN–, such as triphenylamine derivatives, , benzothiazolium-based molecules, boron dipyrromethane (BODIPY), coumarins, pyrimidinium salts, , hydroxyanthraquinone, naphthalene derivatives, azo-azomethine, and fluorene derivatives.

Polydiacetylenes (PDAs), a class of conjugated polymers, exhibit a distinct color transition from blue to red upon exposure to environmental stimuli, such as temperature, pH, heavy metal ions, biomolecules, and environmental pollutants. In its pristine state, PDA typically exhibits a deep blue color with a maximum absorption wavelength (λmax) of 640 nm and shifts to 540 nm under external perturbations. − Despite these advantageous optical properties, the development of PDAs-based sensors for CN– detection has remained relatively limited. The diacetylene monomer, 10,12-tricosadiynoic acid (TCDA), relies on a carboxylic acid headgroup, which may not possess specific recognition sites for direct CN– detection. Consequently, previous studies have focused on the structural modification of PDA headgroups to incorporate specific CN– reactive moieties. For example, the PDA headgroup modified with o-hydroxybenzaldehyde or dicyanovinyl groups, coated on polyvinylidene fluoride (PVDF) membranes, has yielded a limit of detection (LOD) of 0.6 μM. Similarly, the functionalization of PDAs with (dicyanovinyl)­phenyl groups enabled CN– detection at concentrations as low as 0.55 μM. Utilizing metal ions, such as Co2+, to fabricate highly stacked nanocrystals significantly enhances the selectivity of CN– detection. However, these conventional functionalization methods require complex, time-consuming, and multistep synthetic procedures to modify the PDA structure.

To address these synthetic barriers, this work proposes a facile approach to cyanide detection that avoids structural modification of the PDA monomer by utilizing a solution-mixing method to fabricate poly­(TCDA)/Zn2+/ZnO nanocomposites. The strong coordination between the carboxylate headgroups and the Zn2+/ZnO network stabilizes the PDA structure. ,, This proposed that the nucleophilic CN– ion can effectively intercalate into the PDA crystal lattice, interacting with the carboxylate headgroup via an ionic interaction. This specific interaction disrupts local stabilization, inducing conformational distortions in both the conjugated polymer backbone and the alkyl side chains. The poly­(TCDA)/Zn2+/ZnO nanocomposites undergo a pronounced blue-to-red color transition.

Therefore, this study presents a novel, facile, and straightforward approach for the rapid colorimetric detection of CN–. By fabrication of poly­(TCDA)/Zn2+/ZnO nanocomposites via a solution-mixing method in a water/n-propanol system, a highly responsive nanosheet morphology was achieved. Preliminary findings demonstrate that an optimized poly­(TCDA)/Zn2+/ZnO nanocomposite containing 0.5 wt % effectively detects cyanide at a concentration of 0.6 mM, enabling rapid, naked-eye visual identification without specialized instrumentation. Furthermore, this approach extends the novelty and practical applicability of the sensor by successfully encapsulating these nanocomposites into hydrogel beads. This method offers a robust, portable, and solid-state platform for on-site environmental cyanide monitoring.

2. Experimental Section

2.1. Materials

The 10,12-tricosadiynoic acid (TCDA) monomer was purchased from Sigma-Aldrich, as well as n-propanol in AR grade. Potassium cyanide, calcium chloride, and other inorganic salts were purchased from Alfa Aesar. Sodium alginate powder was purchased from Loba Chemie (India). The ZnO nanoparticles were commercially available from Kemaus. Milli-Q water was used in all of the experiments.

2.2. Preparation of Poly­(TCDA) Assembly and the Poly­(TCDA)/Zn2+/ZnO Nanocomposite

The preparation of the poly­(TCDA) assembly and poly­(TCDA)/Zn2+/ZnO nanocomposite using the solution mixing method (SMM) was adapted from previous studies. − For poly­(TCDA) assembly, the 0.05 M TCDA monomer solution was dissolved in n-propanol and then filtered using a 0.45 μm pore-size nylon membrane. Then, the TCDA solution was injected into a mixed solvent of water and 10% v/v propanol while stirring the mixture. For poly­(TCDA)/Zn2+/ZnO nanocomposites, the ZnO suspensions were dispersed in 20 mL of deionized water assisted by ultrasonication for 10 min. The TCDA solution was injected into a mixed solvent, followed by the ZnO suspensions. The ZnO/TCDA ratios were varied from 0.5 to 2 wt %. After that, all mixtures were ultrasonicated for 10 min. The samples were allowed to co-assemble in a binary solvent overnight at 4 °C. The polymerization of TCDA was carried out by irradiation with UV light (λ∼254 nm, 10 W) for 5 min, yielding a blue suspension of poly­(TCDA) assemblies and a poly­(TCDA)/Zn2+/ZnO nanocomposite. Their morphologies were characterized by using a field-emission scanning electron microscope (FE-SEM, Thermo Scientific, Apreo S). The local interactions were also explored by Fourier transform infrared spectroscopy (FT-IR) (Bruker, Tensor II). The measurements were performed in attenuated total reflectance (ATR) mode. A Raman microscope (Horiba, XploRA PLUS model, combined with LabSpec6 software, 785 nm laser) was used to examine the PDA-conjugated backbone and alkyl side chain for conformational changes. X-ray diffraction (XRD) (Bruker AXS Model D8 Discover, λ­(CuKα) = 1.54 Å) was carried out to explore the molecular structure.

2.3. Colorimetric Detection for Cyanide Ion and Other Anions

The colorimetric detection of the CN– ion of the poly­(TCDA) assembly and poly­(TCDA)/Zn2+/ZnO nanocomposite was measured using a UV–vis spectrophotometer (AE Lab, AE-S90-MD) and a smartphone camera (Galaxy S25 ultra, Samsung, Korea). The gradually increasing concentration of CN– solution was added to the blue solution of the poly­(TCDA) assembly and poly­(TCDA)/Zn2+/ZnO nanocomposite. The mixed solutions were stirred and incubated for about 30 s at room temperature, and then the color transition was measured using UV–visible spectroscopy and photographed. The colorimetric response (%CR) was calculated according to the equation: %CR = [(PB0–PB)/PB0]×100. PB was the percentage of the blue phase calculated from PB = A 640/(A 640 + A 540), where A 640 and A 540 were the absorbance of blue (λ = 640 nm) and red (λ = 540 nm) phases, respectively. The initial PB0 value was determined before exposure to the stimulus.

To further examine specificity, 0.2 mL of 0.01 M anion solutions were added to the blue solution of the poly­(TCDA) assembly and poly­(TCDA)/Zn2+/ZnO nanocomposites. The various kinds of anions include potassium salts: Cl–, I–, SCN–, IO3 –, NO3 –, S2O8 2–, and H2PO4 –; sodium salts: HCO3 –, S2O3 2–, S2O4 2–, C7H5O2 –, HSO3 –, CH3COO–, and F–; magnesium salt: SO4 2–; and ammonium salt: HSO4 –. The PDA sensing reactions were allowed to equilibrate for 5 min before taking the absorbance measurement and photographing.

2.4. Fabrication of Poly­(TCDA) and Poly­(TCDA)/Zn2+/ZnO Hydrogel Beads

The fabrication of the PDA-based hydrogel bead was adapted from previous studies. − For poly­(TCDA) hydrogel beads, first, a 0.05 M TCDA solution was mixed with a 4 wt % sodium alginate solution in a 1:2 vol ratio. The mixture was stored in a refrigerator at 4 °C for 30 min to allow co-assembly of TCDA monomers in an alginate medium. The mixture was then injected dropwise into a 2.5 wt % calcium chloride solution using a 10 mL syringe under vigorous stirring. After 20 min, the colorless hydrogel beads were washed three times with deionized water. The hydrogel beads were irradiated with UV light (λ∼254 nm, 10 W) for 5 min to achieve photopolymerization. This changes the color of the hydrogel bead to blue. The poly­(TCDA)/Zn2+/ZnO hydrogel beads could be fabricated as in the above experiment by substituting the 0.05 M TCDA solutions with the mixture of TCDA and the ZnO suspension. The colorimetric sensor of PDAs-based hydrogel beads for detecting CN– solution was examined. The hydrogel beads were immersed in glass vials (3–5 beads per vial) containing CN– solutions at various concentrations (0.001, 0.01, 0.1, 1, 5, 10, and 100 mM). The color transition of hydrogel beads was monitored over time, and photographs were taken at 0, 30, and 60 min using a smartphone camera (Galaxy S25 ultra). The fluorescence microscopic images were obtained on a ZEISS microscope, LSM 910 Lightfield 4D.

3. Results and Discussion

3.1. Characteristics of Poly­(TCDA) Assembly and Poly­(TCDA)/Zn2+/ZnO Nanocomposite

The poly­(TCDA) assembly and poly­(TCDA)/Zn2+/ZnO nanocomposite were fabricated using the SMM in a binary solvent system of water and 10% v/v n-propanol. Figure a illustrates the absorption spectra of the poly­(TCDA)/Zn2+/ZnO nanocomposites with ZnO loadings ranging from 0 to 3 wt %. These absorption spectra exhibit a prominent band at approximately 635 nm, which is the characteristic spectral signature of the PDA blue phase. For samples containing 4, 5, and 10 wt % ZnO, the absorption maxima shifted to 645 nm, accompanied by the emergence of a secondary peak at 675 nm. This red shift suggests that ZnO nanoparticles facilitate a more ordered monomer alignment prior to photopolymerization. This enhanced molecular organization results in a polymer backbone with an extended conjugation length and improved structural stability. The morphology characterization via SEM, as shown in Figure b, revealed that the poly­(TCDA) assemblies comprise a mixture of spherical, sheet-like, and ribbon-like structures. Notably, the poly­(TCDA)/Zn2+/ZnO nanocomposites with 0.5–2 wt % ZnO displayed similar morphological features, as further detailed in the Supporting Information, Figure S1.

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(a) Absorption spectra of poly­(TCDA)/Zn2+/ZnO nanocomposites with various ratios of ZnO in 0–10 wt %; the inset is the blue-shaded suspensions of samples. (b) SEM image of poly­(TCDA) assembly.

The local interactions of poly­(TCDA) assembly and poly­(TCDA)/Zn2+/ZnO nanocomposites were explored by FT-IR in an ATR mode in Figure (lines (1) and (2)), respectively. The FT-IR spectra exhibit vibrational peaks at around 2850, 2920–1, and 2950 cm–1, indicating the presence of aliphatic C–H stretching vibrations of νs(CH2), νas(CH2), and νas(CH3), respectively. The peaks around 1470–1420 cm–1 are assigned to methylene scissoring, δ­(CH2), which is common in polymeric materials. − The strong vibration peak at about 1693 cm–1 indicates the presence of the carboxylic headgroup. The coordination between the carboxylate headgroup of TCDA and the Zn2+ ion, released from ZnO nanoparticles, is indicated by the peaks at 1540 cm–1 and 1395 cm–1, corresponding to νas(COO–) and νs(COO–) stretching vibrations of the carboxylate anion, respectively. −

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Photographs showing the color change of (a) poly­(TCDA) assembly and (b) poly­(TCDA)/ Zn2+/ZnO nanocomposites with various CN– concentrations. (c) The RSC values of poly­(TCDA)-based materials as a function of CN– concentration.

3.2. Colorimetric Detection of Cyanide (CN–)

In this section, the effect of ZnO ratios (0, 0.5, 1, and 2 wt %) on the sensing performance of PDA materials was determined. The experimental procedure involved the incremental addition of cyanide solution into the blue-phase poly­(TCDA)/Zn2+/ZnO nanocomposite solutions, followed by absorption measurements and photography. The titration was continued until a complete color transition from blue to red was observed, and the spectral profile reached a steady state, ensuring that the reaction was complete.

As illustrated in Figure a, the poly­(TCDA) assembly in its initial blue phase exhibited a λmax at 640 nm, with a characteristic vibrational shoulder at approximately 580 nm (0 mM CN–). Upon the addition of CN–, the intensity of the 640 nm peak gradually decreased, accompanied by the emergence of a new absorption band at 540 nm. Significant spectral alterations were first detectable at a CN– concentration of 0.283 mM. However, a visual color transition from blue to purple was observed at a concentration of 0.614 mM (Figure a). Upon further increases in cyanide concentration to 1.15 mM and above, the solution turned a distinct red, corresponding to a sharp increase in the 540 nm peak and the depletion of the original 640 nm band.

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Absorption spectra of (a) poly­(TCDA) assembly and (b) poly­(TCDA)/Zn2+/ZnO nanocomposite measured with increasing concentration of CN–. (c) The plot of %CR versus concentration of CN–.

The system of the poly­(TCDA)/Zn2+/ZnO nanocomposite, containing 0.5 wt % ZnO, exhibited a faster colorimetric response compared to the pure poly­(TCDA) assembly. Specifically, spectral modifications characterized by an increase in the absorption peak at 540 nm were discernible at a cyanide concentration of as low as 0.146 mM. The visual detection of the blue-to-purple transition was observed at 0.370 mM (Figure b). Upon increasing the cyanide concentration to 0.614 mM and beyond, the solution underwent a distinct transition to red, accompanied by a sharp increase in the 540 nm peak and the nearly complete depletion of the 640 nm band. However, as the ZnO ratio increased to 1–2 wt %, the chromatic behavior of the nanocomposites became comparable to that of the pure poly­(TCDA) assembly (corresponding spectra are provided in the Supporting Information, Figure S2). Furthermore, at higher ZnO ratios (3–5 wt %), the cyanide-responsive capability of the nanocomposites was found to significantly diminish.

To compare the color transition behavior in detail, the %CR was plotted as a function of CN– concentration, as shown in Figure c. The %CR analysis indicates that all samples exhibited consistent and comparable chromatic behavior toward cyanide ions. The color transition depends on the ZnO content in the nanocomposites. The 0.5 wt % ZnO nanocomposite yielded a superior response rate, as evidenced by the markedly steeper slope. At a concentration of 1.67 mM CN–, this system achieved the highest % CR value among all other systems. The pure poly­(TCDA) system also exhibited a rapid initial slope transition. Its overall %CR values remained consistently lower than those of the 0.5 wt % ZnO system. For the systems with 1 and 2 wt % ZnO, a relatively rapid chromatic transition was discernible at CN– concentrations below 1.00 mM. However, as the CN– concentration further increased, the rate of color transition slowed, suggesting that the systems had approached a state of chemical equilibrium or reached a plateau phase. This result suggests that 0.5 wt % ZnO is the optimal ZnO content, providing the best ZnO distribution on the PDA surface for detecting CN–.

Furthermore, the LOD was calculated using spectroscopic data according to the following equation: LOD = 3δ/S, where δ is the standard deviation of the blank measurement, and S is the slope of the calibration curve (Figure S3 in Supporting Information). The LOD was calculated as approximately 0.6 mM for poly­(TCDA)-based materials. Although this LOD exceeds the WHO safety standards for drinking water, the sensor remains highly effective for monitoring industrial effluents. For example, in metal plating and metallurgy, cyanide concentrations typically surpass those found in potable water. , Consequently, the proposed poly­(TCDA)-based sensor serves as a robust early warning tool for industrial leaks, offering rapid, naked-eye detection through a blue-to-red colorimetric transition without requiring expensive instrumentation. While existing sensors often achieve ultralow detection limits, they frequently necessitate complex instruments, multistep syntheses of receptor dyes, or expensive materials. Instead, our approach avoids complex changes to the PDA monomer structure. This method simply uses a straightforward mixing method to prepare the poly­(TCDA)/Zn2+/ZnO nanocomposites. The main value of this work is the rapid, naked-eye visual identification, which removes the need for expensive equipment or expert staff for initial field testing. The analytical performance of the poly­(TCDA)-based sensor was compared with that of previously reported methods, as summarized in Table . In contrast to chemosensors requiring complex synthesis procedures, the proposed sensor was fabricated via a straightforward synthetic route employing a water/propanol mixed solvent system, which offers both environmental compatibility and operational safety relative to conventional organic solvent-based methodologies.

1. Comparison of the Poly­(TCDA)-Based Sensor with Some Previously Reported Methods for CN– Detection.

sensor method of color transition LOD solvent Ref.
2-benzoylamino-1-hydroxyanthraquinone colorimetric yellow to red 0.22 μM MeCN–H2O mixture (95:5 v/v)
α-ethoxynaphthalene-dicyanovinyl conjugate colorimetric pale yellow to colorless and nonemissive to blue fluorescence 1.1 μM THF/H2O (9:1 v/v) containing 0.01 M HEPES, pH = 7.3
azo-azomethine chemosensor colorimetric colorless to yellow and turn-on fluorescence (475 nm) 21 nM DMSO/H2O
(6:4 v/v)
fluorene based probe with N,N-dimethylbenzaldehyde substituted colorimetric yellow to dark red and fluorescence emission enhancement 1.34 ppb (27.3 nM) H2O/DMF (4:1 v/v)
tetraseleno-BODIPY fluorescence turn-on (blue to yellow fluorescent) 26.26 nM THF
benzo-hemicyanine-based fluorescent probe (PBiSMe) colorimetric yellow to colorless and turn-off red fluorescence (570 nm) 0.43 μM 50% aq DMSO
spiropyran-functionalized polydiacetylene (SPFPDA) vesicles colorimetric blue to red and turn-on fluorescence 0.5 μM HEPES-buffered aqueous solution (pH)
poly(PCDA-DCV) @PVDF colorimetric blue to red 0.6 μM aqueous solution
PDA-BMN (polydiacetylene incorporating a receptor) colorimetric blue to orange/red 0.55 μM water
highly stacked PCDA–Co nanoplate film colorimetric blue to red and green-to-red luminescence change 15 mM water
poly(TCDA)/Zn2+/0.5 wt % ZnO nanocomposite colorimetric blue to red color transition 0.6 mM water this work
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Figure summarizes the colorimetric response of the poly­(TCDA) assembly and poly­(TCDA)/Zn2+/ZnO nanocomposite (0.5 wt %) with various CN– concentrations. The gradual change from blue to red was observed as the concentration of CN– increased. The investigation of poly­(TCDA)/Zn2+/ZnO nanocomposites with 1–5 wt % provides consistent results. The chromatic transition of the investigated poly­(TCDA) photographs was quantified using ImageJ 1.54g to extract the red-green-blue (RGB) values. Each experiment was repeated three times to get an average value. The red intensity can be expressed in terms of red chromaticity (r): r = R/(R + G + B), where R (red), G (green), and B (blue) are the values of three primary colors. Then, the convention for the red chromatic shift can be calculated as follows: RSC(%) = [(r 1 – r 0)/(r max – r 0)]×100. Here, r sample represented the red chromaticity of poly­(TCDA)-based materials reacted with CN– solution, r0 was measured from the blue phase of poly­(TCDA), and r max was measured from the red phase of the poly­(TCDA)/Zn2+/ZnO nanocomposite (0.5 wt %) after treatment with CN– at 1.67 mM. , The RSC (%) values of poly­(TCDA)-based materials increase with increasing CN–concentration and reach a plateau at above 1.0 mM CN– (Figure c). In the case of the poly­(TCDA)/Zn2+/ZnO nanocomposite (0.5 wt %), the color change occurred more rapidly. The color change was visible to the naked eye when the RSC value exceeded 50%. This corresponds to a LOD for cyanide of 0.6 mM. Furthermore, fluorescence imaging of the red-phase PDA following the addition of CN– revealed distinct red emission across all investigated systems (Figure S4 in Supporting Information). This transition confirms the shift from the non-fluorescent blue phase to the highly fluorescent red phase upon interaction with the cyanide.

Local interactions between PDA materials and CN– were characterized by FT-IR (Figure ). As previously discussed, the blue-phase poly­(TCDA) assemblies and nanocomposites exhibit a prominent vibrational band at 1693 cm–1, assigned to the hydrogen-bonded carbonyl stretching of the carboxylic acid (−COOH) headgroups. In the ZnO-incorporated systems, the emergence of peaks at 1540 cm–1 and 1395 cm–1 is attributed to the asymmetric (νas) and symmetric (νs) stretching vibrations of carboxylate anions (COO–), respectively, confirming the coordination between the polymer and ZnO. The addition of CN– induces a distinct blue-to-red chromatic transition in the poly­(TCDA)-based material. For the system of red phase, the peaks at 1693 cm–1 (for 0% ZnO) and 1537 cm–1 (for 2% ZnO) diminish, while new peaks are detected. A new vibrational peak at 1620 cm–1 is assigned to the CN stretching mode, providing clear evidence of the chemical interaction between cyanide ions and the TCDA headgroups. The two peaks at 1388 and 1004 cm–1 correspond to the C–N stretching mode. The broad vibrational band observed at 3390 cm–1 is assigned to the stretching modes of NH group vibrations. , The appearance of this band indicates that CN– ions have successfully interacted with the molecular network, leading to chemical perturbation within the system. The FT-IR peak at about 1560 cm–1 corresponds to the presence of carboxylate (−COO–). The νs(CH2), νas(CH2), and νas(CH3) peaks of the red phase remain at the same position.

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FT-IR spectra of poly­(TCDA)/Zn2+/ZnO nanocomposites in blue and red phases after the addition of 0.005 M CN–. Blue phases are (1) 0 and (2) 2 wt % ZnO. Red phases are (3) 0 and (4) 2 wt % ZnO.

The conformational change of the PDA-conjugated backbone and alkyl side chain during the color transition was explored by Raman spectroscopy (Figure ). The stretching vibrations of CC and CC bonds in the blue phase of poly­(TCDA) assembly occur at peaks of 2077 and 1447 cm–1, respectively. The observation of vibrational bands in the 1150–1000 cm–1 region reflects the all-trans conformation of the alkyl side chain. In the red phase, the vibrational bands of CC and CC shift to a higher energy region at 2114 and 1512 cm–1, respectively, after adding CN– solution into the system of PDA. ,, These results indicate that the relaxation of some PDA-conjugated backbone into a new local environment. The bands in the 1150–1000 cm–1 region were also observed, corresponding to conformational changes in some alkyl side chains. Some peaks become broader in the red phase, reflecting the transformation of trans to gauche conformation of some alkyl side chains. The Raman peak at 691 cm–1, assigned to the C–C bending mode of the conjugated poly­(TCDA) backbone in the blue phase, shifts to 724 cm–1 in the red phase. This spectral shift confirms the structural reorganization of the polymer backbone during the phase transition. − Raman spectral analysis revealed that perturbation by CN– could induce conformational distortion in both the conjugated main chain and the aliphatic side chain of PDAs, leading to a distinct blue-to-red color change. ,,

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Raman spectra of poly­(TCDA) assemblies in the blue phase and red phase after the addition of 0.005 M CN–. The expanded 1150–950 cm–1 region of Raman spectra and the all-trans and gauche conformations of alkyl side chains are shown on the right side of the spectra.

To investigate the interaction of CN– into the poly­(TCDA)-based materials, XRD experiment was carried out, as shown in Figure . It is well-known that pristine PDAs with a monocarboxylic headgroup form a bilayer lamellar structure in the solid state, where the alkyl chain length determines their interlamellar d-spacing value. , In the blue phase of poly­(TCDA) assembly, the XRD profiles show the diffraction pattern of the bilayer lamellar structure with a d-spacing of 4.08 nm (2θ = 2.16). After adding CN– solution, the XRD pattern of the red phase still shows a lamellar structure. However, the diffracting peaks shift to lower angles (2θ = 1.85), corresponding to the increase of lamellar d-spacing to 4.77 nm. The peaks are also broader, indicating the decrease in molecular ordering. , From this result, it may be proposed that CN– should intercalate into the PDA crystal by ionic interaction with the COOH group in the PDA side chain. This interaction in the side chains effectively altered the conjugation length of the PDA main chain, resulting in a pronounced change in the UV–vis absorption properties. The change in local interactions within poly­(TCDA)-based materials induces structural relaxation, causing a blue-to-red color transition, as suggested in Figure .

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XRD profiles of poly­(TCDA) assemblies in the blue phase and red phase after the addition of 0.005 M CN–.

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Schematic representation of the poly­(TCDA)-based materials for the CN– sensing process.

Furthermore, the selectivity of the poly­(TCDA)-based sensor toward cyanide was evaluated by the presence of various interfering anions, including Cl–, I–, F–, SCN–, IO3 –, NO3 –, HCO3 –, S2O8 2–, H2PO4 –, SO4 2–, S2O3 2–, C7H5O2 –, HSO3 –, HSO4 –, and CH3COO–. Figure a illustrates the absorption spectra and corresponding colorimetric change upon the addition of these anions at concentrations as high as 0.01 M. The sensing reaction was allowed to equilibrate for 5 min before taking the measurement. Compared with other anions, the addition of CN– to the poly­(TCDA)-based sensor resulted in a significant change in the absorption spectra. The CR value also quantified the chromatic change from blue to red. In alignment with the change in absorption spectra, CN– induced an increase in the CR value (Figure c), whereas other anions elicited a negligible response. As a result, the poly­(TCDA)-based sensor described in this work demonstrates selectivity for cyanide over a broad concentration range of common anions typically found in water.

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(a) Colorimetric response of poly­(TCDA) assembly obtained after the addition of different anions at 0.01 M. The response times were about 5 min. (b) Absorption spectra, and (c) the related CR values.

The effect of pH on the poly­(TCDA)-based sensor was further investigated by varying the pH over the range of 2–10, as presented in Figure . At pH 2–7, no colorimetric change was observed in the solution, and the absorption spectra remained unchanged in both shape and peak position. At pH 8, the poly­(TCDA)-based sensor solution underwent a transition from blue to purple, while at pH 10, a further shift to a red-purple color was observed, accompanied by the appearance of an absorption band at 540 nm. Upon addition of 5 mM KCN to the poly­(TCDA)-based sensor solutions across all pH conditions examined, a colorimetric transition to red was consistently induced. These results indicate that the poly­(TCDA)-based sensing system is operable under acidic conditions (pH 2–7).

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(a) Colorimetric response of the poly­(TCDA)/Zn2+/0.5 wt % ZnO nanocomposite upon exposure to various pH solutions and after adding a 5 mM KCN solution. (b) The corresponding absorption spectra of the poly­(TCDA)/Zn2+/0.5 wt % ZnO nanocomposite following the addition of pH solution and KCN solution.

3.3. Poly­(TCDA) and Poly­(TCDA)/Zn2+/ZnO Hydrogel Beads for CN– Detection

The colorimetric properties of the poly­(TCDA)-based hydrogel bead sensor were evaluated upon exposure to various concentrations of CN– across different time intervals. Figure illustrates the chromatic response of the hydrogel bead sensors at 30 min and 1 h. The results demonstrate a clear transition from blue to red, which became increasingly pronounced over time. At the 30 min exposure interval (Figure a), the 0% ZnO system initiated a color change to purple at a CN– concentration of 5 mM, achieving a complete transition to red at 100 mM. In contrast, the 0.5 wt % ZnO system exhibited a purple phase only at a concentration of 100 mM. Meanwhile, no significant difference was observed at 1 h (Figure b). Prolonged exposure for 24 h revealed a distinct red transition in both the 0 and 0.5 wt % ZnO systems at a concentration as low as 1 mM. Conversely, systems with 1–2 wt % ZnO loadings showed no discernible chromatic response. These findings indicate that poly­(TCDA)-based hydrogel bead sensors with 0 and 0.5 wt % ZnO contenta are effective candidates for cyanide detection. Analysis of the red chromaticity percentage (%r) showed that %r values increased proportionally with the cyanide concentration. The %r exhibits a significant response reaching approximately 35% (Figure S5 in Supporting Information). Additionally, the red-phase hydrogel beads exhibited red fluorescence emission (Figure c).

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Colorimetric response of poly­(TCDA)-based hydrogel beads after (a) 30 min and (b) 1 h exposure to CN– solutions at different concentrations. (c) Fluorescence image of the poly­(TCDA) assembly hydrogel bead after 1 h exposure to a 0.1 M CN– solution.

4. Conclusions

In summary, a PDA-based material was developed as a colorimetric sensor for the detection of highly toxic CN– ions. The poly­(TCDA)/Zn2+/ZnO nanocomposite containing 0.5 wt % ZnO exhibited the best sensing performance, demonstrating a significantly faster color-change response than both pure poly­(TCDA) assemblies and nanocomposites with higher ZnO contents. The structural and spectroscopic analyses elucidated the sensing mechanism, revealing that cyanide ions intercalate into the PDA network and interact with the carboxylate head groups. This interaction induces conformational distortion in both the conjugated backbone and the aliphatic side chains. The PDA sensor demonstrated selectivity for cyanide over a wide range of common interfering anions. PDA-based hydrogel beads are an example validated through the practical utility of this system. The nanocomposite hydrogels with 0 and 0.5 wt % ZnO have potential for use as a portable platform for cyanide detection.

Supplementary Material

ao6c02494_si_001.pdf (688.7KB, pdf)

Acknowledgments

This research was supported by the Research and Innovation Fund, Nakhon Phanom University, Fundamental Fund 2568 (Project code 204661). R.P. would like to thank the Faculty of Science, Nakhon Phanom University, for supporting the research facilities. The author also thanks the Mahidol University-Frontier Research Facility (MU-FRF) for instrumentation support of XRD and Raman measurements. Thank you to the Science Lab Centre at the Faculty of Science, Naresuan University, for assistance with operating the FESEM. Special thanks to Rushmore Precision Co., Ltd, Thailand, for assistance with operating the fluorescence microscope.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c02494.

  • SEM image of poly­(TCDA)/Zn2+/ZnO nanocomposites with various ZnO ratios; absorption spectra of poly­(TCDA)/Zn2+/ZnO nanocomposites with 1 and 2 wt % ZnO measured with increasing concentration of CN–; calibration curve of the poly­(TCDA)/Zn2+/ZnO nanocomposite as a function of CN– concentration; fluorescence image of poly­(TCDA)/ Zn2+/ZnO nanocomposites in the red phase; and the plot of the red chromaticity level (%r) of poly­(TCDA)/Zn2+/ZnO nanocomposite hydrogels as a function of CN– concentration (PDF)

The author declares no competing financial interest.

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