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. 2026 Sep 25. Online ahead of print. doi: 10.1039/d6ra06405a

Pyrrole-modified gold nanoparticle structure for dual UV-vis and smartphone-based colorimetric detection of hexavalent chromium in food and environmental samples

Md Ramzan Ali a,b, Ruksana Suraia Akhi a, Md Ruhul Amin a, Alamin Howlader c, Gang Chen b, Fatema-Tuj Zohra d, Mohamed Aly Saad Aly a,e,f,✉, Hermine Gharagulyan g,h, Md Zaved H Khan a,✉
PMCID: PMC13614017  PMID: 42799455

Abstract

As a potent, human-carcinogenic toxin, hexavalent chromium, Cr(vi), causes aggressive cancers and severe organ failure. It poses a major environmental threat due to its mobility in water and easy cellular penetration. Therefore, sensitive detection methods are essential for monitoring and managing Cr(vi) levels in food, soil, water, and cosmetics. Herein, a gold nanoparticles-pyrrole (AuNPs@pyrrole) based novel colorimetric sensing platform is developed for the rapid quantification of Cr(vi). The mechanism of the developed sensor is driven by the bridging effect of Cr(vi) ions. By binding to the pyrrole on distinct gold nanoparticles, the ions link the particles together, triggering aggregation and an observable color change. The colorimetric response was quantitatively analyzed using UV-vis spectroscopy and a smartphone RGB detector, providing a portable and user-friendly Cr(vi) monitoring approach, with a detection limit of 0.01 ppm over a linear range of 0.1–100 ppm. Furthermore, the developed sensor exhibited outstanding specificity toward Cr(vi). The detection process remained immune to the presence of competing aqueous metal ions, notably Fe(ii), Cd(ii), As(iii), Pb(ii), Cr(iii), Hg(ii), and Fe(iii). The efficiency of the proposed sensing platform was validated against a reference coupled plasma optical emission spectrometry (ICP-OES) scheme, demonstrating the detector's high precision and robustness. The practical utility of the AuNPs@pyrrole structure was confirmed through the successful detection of Cr(vi) in food and environmental water samples. These findings emphasize the potential of the developed sensing platform as an intuitive, user-friendly colorimetric sensor for on-site pollution monitoring, providing a cost-effective and dependable solution for both environmental and industrial applications.

As a potent, human-carcinogenic toxin, hexavalent chromium, Cr(vi), causes aggressive cancers and severe organ failure.graphic file with name d6ra06405a-ga.webp

1. Introduction

Chromium is a heavy metal essential to many industries, utilized in leather tanning, stainless steel manufacturing, metal plating, and as pigments.1 Despite its industrial significance, chromium constitutes a significant hazard to ecological systems and public health due to its toxic nature.2–5 Chromium can be found in two main forms: essential trace nutrient trivalent chromium (Cr(iii)) and highly toxic, carcinogenic hexavalent chromium (Cr(vi)), which primarily results from industrial processes like chrome plating and welding.6 Cr(iii) is a critical trace component for lipid and glucose metabolism,7 while Cr(vi) is a highly lethal, Group 1 human carcinogen categorized by the International Agency for Research on Cancer.8 Cr(vi) can give rise to critical health concerns such as, respiratory issues, kidney and liver damage, skin irritation, and a greater chance of lung cancer.9 Therefore, tracking Cr(vi) levels in industrial and environmental samples is crucial to ensure public safety and regulatory compliance.10–12

Conventional approaches for the determination of Cr(vi), like atomic absorption spectroscopy,13 inductively coupled plasma mass spectrometry,14 and chromatography,15 provide superior accuracy and sensitivity. But, these techniques demand costly equipment, skilled operators, and complex sample preparation, limiting their use for rapid on-site monitoring. In contrast, colorimetric sensing provides a simple, affordable, and easy to use alternative, allowing for visual detection through hue shifts that are measurable using portable spectrophotometers or even smartphone cameras.16–21

Because of their special localized surface plasmon resonance (LSPR) characteristics, which allow them to display intense and adjustable colors according to their size, shape, and aggregation state, gold nanoparticles (AuNPs) have attracted a lot of interest in colorimetric sensing.22–25 When dispersed uniformly, AuNPs in colloidal suspension exhibit distinct size- and aggregation-dependent colors, typically ranging from red to purple and blue.26 This unique optical behavior is attributed to LSPR, a phenomenon resulting from the collective oscillation of surface conduction electrons upon interaction with incident visible light.27 However, when they aggregate, the LSPR coupling between adjacent nanoparticles causes widening of the absorption peak, leading to a visible color change.28 This optical phenomenon makes AuNPs highly suitable for developing selective and sensitive colorimetric detectors for metal ions, including Cr(vi).29

Despite the advantages of AuNPs-based sensors, their stability and selectivity can be compromised by non-specific aggregation and interference from other ions present in environmental samples. To overcome these limitations, composites of AuNPs with functional polymers have been explored to enhance stability and provide selective binding sites for target analytes. Pyrrole, a monomer of a conducting polymer, is an attractive candidate for such composites due to its high chemical stability, biocompatibility, and strong interaction with metal ions.30 Ayyob M. Bakry et. al., (2020) provides experimental evidence of adsorption of Cr(iv) with nitrogen of pyrrolic structure.31 Pyrrole contains nitrogen atoms with lone pairs of electrons, which can coordinate with Cr(vi) ions, facilitating selective detection.32 Additionally, pyrrole molecules can stabilize gold nanoparticles, preventing non-specific aggregation and improving the sensor's overall sensitivity and selectivity.33

Combining AuNPs with pyrrole offers several synergistic advantages. Pyrrole molecules not only stabilizes the AuNPs but also provides functional groups for specific interaction with Cr(vi).34,35 This interaction cause the accumulation of Cr(vi) ions around the AuNPs, triggering electron transfer and a corresponding LSPR shift.36 The result is a discernible shift in color, which can be simply observable by the unaided eye or measured spectroscopically.37 Furthermore, the conductivity of pyrrole molecules enhances the transfer of electrons between Cr(vi) and AuNPs, facilitating the reduction of Cr(vi) to Cr(iii), which further amplifies the colorimetric response.38 Numerous reports have studied the use of sensors based on AuNPs for the determination of Cr(vi),39–42 but none of those have explored the potential of AuNP-Pyrrole structures. Ghayyem et. al. (2021) developed (polyethylene glycol-functionalized poly(N-phenylglycine) (PNPG-PEG) nanoparticles for colorimetric Cr(vi) detection using 3,3′,5,5′- tetramethylbenzidine (TMB) substrate and of hydrogen peroxide (H2O2). The research group claimed the sensor is highly selective towards other heavy metal ions but they suffer for the detection limit of sensor which is in around 1 ppm43. Karn-orachai et. al. (2024) developed colorimetric sensor for detection of Cr(vi) utilizing a mixture of citrate-capped silver nanoparticles and sodium borohydride-coated gold (Na-AuNPs/cit-AgNPs) which can identify Cr(vi) in ppb level, but color change pattern is not same over the concentration of analyte. This may create confusion during the real sample analysis.44

In this work, the fabrication of a gold nanoparticles-pyrrole (AuNPs@pyrrole) is reported for the colorimetric quantification of Cr(vi). The structure was synthesized through a simple chemical route, ensuring uniform coating of pyrrole on the surface of the AuNPs. The sensing mechanism of the developed platform relies on the interaction between pyrrole nitrogen atoms and Cr(vi) ions, which triggers the aggregation of AuNPs and a clear visual color change. The efficiency of the sensor was characterized by its reliability, response time, sensitivity, and selectivity. Additionally, its viability for real-world applications was established by measuring Cr(vi) concentrations in environmental water samples. The primary innovation of this study stems from the synergistic pairing of AuNPs and pyrrole to optimize the colorimetric sensing. By simultaneously stabilizing the nanoparticles and establishing targeted binding sites, pyrrole enables highly selective Cr(vi) detection with minimal interference from competing metal ions. Furthermore, the conductive properties of pyrrole promote electron transfer, thus accelerating Cr(vi) reduction and magnifying the colorimetric signal. Furthermore, combining UV-vis spectrophotometry with smartphone-based RGB analysis provides a complementary dual-mode strategy. UV-vis enables reliable quantitative analysis, while smartphone-based RGB detection offers a simple, low-cost, and portable approach for rapid on-site screening. Thus, the combined strategy provides both analytical reliability and field applicability for Cr(vi) detection.

2. Chemicals and methods

2.1. Materials and equipment

Each reagent and substance employed was used as received. Chloroauric acid (HAuCl4, ≥99%) and hydrochloric acid (HCl, 37%) were acquired from Merck (Germany). Pyrrole (analytical grade) was sourced from China. Trisodium citrate (≥99.7%), potassium dichromate (≥99%), potassium chromate (≥99%), and standard solutions of chromium, iron, lead, and mercury were procured from suppliers in India. A chromium standard solution (≥99%) was also acquired from an Indian supplier. PBS (100 mM, pH 7.4) and ethanol were bought from Sigma-Aldrich (Shanghai, China). Ultrapure water (Type I, Evoqua, Germany) used in the studies had a resistivity of 18.2 MΩ cm. Environmental samples, including wastewater and vegetables, were collected for analysis. Wastewater samples were gathered from residential areas and the university campus. Vegetable samples were obtained from a local market in Jashore, Bangladesh.

A comprehensive characterization of the nanostructure ‘s properties was conducted using the following analytical techniques. ICP-OES was utilized for accurate elemental investigation. The optical characteristics and colorimetric sensing performance were evaluated using a UV-vis spectrophotometer (PG Instruments T80+) and an RGB analysis system through mobile application named RGB color detector which is available in Google Play Store. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy (PerkinElmer spectrum II) was used to determine chemical functional groups. Morphological and elemental composition studies were performed using a ZEISS GeminiSEM 500 field-emission scanning electron microscope (FE-SEM) supplied with Bruker energy-dispersive X-ray spectroscopy (EDS).

2.2. Synthesis of AuNPs

A slightly modified version of the Turkevich method and other previously published literature on trisodium citrate reduction preparative techniques were used to synthesize the AuNPs.45 In brief, a 237 mL of 0.268 mM HAuCl4 solution was prepared using ultrapure (Type I) water. The solution was heated under stirring with reflux conditions to prevent air oxidation and loss of volume. The continuous stirring and closed conditions were maintained throughout the synthesis of gold nanoparticles. When the liquid reached the boiling point, 12.5 mL of trisodium citrate (10 mg L−1) was added dropwise. This mixture kept boiling for 30 minutes with vigorous stirring. Next, the resulting AuNP solution keeps at room temperature for cool, collected, and filtered several times to remove impurities. Finally, purple colored colloidal AuNPs were then gathered in a sealed vial and kept for later use at 4 °C. In this synthesize method, citrate acts as both a reducing and stabilizing agent to produce citrate-stabilized gold nanoparticles, termed as AuNPs.

2.3. Synthesis of AuNPs@pyrrole structure

The AuNPs@pyrrole nanostructure was synthesized through an in situ chemical oxidation process. In a typical procedure, 1 mL of as synthesized gold nanoparticle (AuNPs) colloid was combined with 500 µL of 1 M hydrochloric acid (HCl) and 50 µL of 3 M pyrrole in a sealed vial. The mixture was subsequently sonicated for 5 minutes to facilitate the uniform binding of pyrrole onto the surface of the AuNPs. The resulting nanostructure was left to cool down to ambient temperature and subjected to repeated filtration to eliminate unbind compounds.

2.4. Colorimetric detection of chromium

The sensing capability of the AuNPs@pyrrole structure for chromium ions was evaluated through a colorimetric assay. For the determination experiment, 45 µL of the synthesized AuNPs@pyrrole structure stock solution was added to 4.5 mL of aqueous solutions containing Cr(vi) ions across a concentration range of 0.01 to 100 ppm prepared in ultrapure water (Type I, 18.2 MΩ cm). The resulting working assay medium maintained an acidic pH environment of 3.5 and an ionic strength governed solely by the target analyte. Under this pH regime, Cr(vi) speciation is dominated by HCrO4− and Cr2O72− ions, optimizing electrostatic binding with the protonated pyrrole molecules. A distinct visual color change was observed upon the addition of the analyte, and then the RGB value was recorded using smart phone. Following, the absorption spectra of each solution was documented utilizing a UV-vis spectrophotometer to obtain quantitative optical data.

2.5. Real sample analysis

Real-world water and produce samples were examined to validate the practical application of the synthesized nanoprobe. Samples included laboratory tap water, surface water from the Bhairab River (Jashore, Bangladesh), pond water from Jashore University of Science and Technology, and extracts from locally sourced vegetables and fruits (Jashore, Bangladesh) were used. For the analysis, 4.5 mL of each sample was aliquoted into a vial and mixed with 45 µL of the AuNPs@pyrrole structure. The absorption spectra of each solution were subsequently documented via a UV-vis spectrophotometer to detect the presence of the target analytes.

3. Findings and discussions

3.1. Characterization of AuNPs@pyrrole

To confirm the successful fabrication of the AuNPs@pyrrole structure, SEM and UV-vis analysis were investigated. Fig. 1 represent the SEM images and UV-vis spectra of AuNP and the AuNPs@pyrrole structure. As seen in Fig. 1(A), the AuNPs appear as uniformly distributed spherical particles with a narrow size distribution. The particles are well-separated with minimal aggregation, indicating good colloidal stability. This uniform dispersion is crucial for maintaining consistent optical properties. The surface of the AuNPs is smooth, suggesting efficient synthesis and stabilization, likely because of the existence of a capping agent that prevents particle agglomeration. The particle size is relatively consistent, with an estimated diameter of around 20–30 nm. This uniformity is beneficial for reproducible optical sensing.

Fig. 1. SEM image of (A) AuNP, (B and C) AuNPs@pyrrole, (D) UV-vis spectra of AuNP and AuNPs@pyrrole.

Fig. 1

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The image in Fig. 1(B) shows a layered, flake-like morphology, indicating that pyrrole molecules form a continuous film encapsulating the AuNPs. This structure enhances the stability of AuNPs by preventing aggregation and shielding them from environmental factors. The surface is relatively smooth with visible cracks, which may result from the drying process of pyrrole. These cracks do not affect the uniformity of nanoparticle distribution but may influence the mechanical properties of AuNPs@pyrrole. The layered arrangement suggests a strong interaction between pyrrole and AuNPs, potentially through π–π interactions or hydrogen bonding. This interaction is crucial for ensuring uniform nanoparticle dispersion and enhancing the sensitivity of the structure in colorimetric sensing. At higher magnification, AuNPs are clearly visible as bright spots embedded within the pyrrole film as illustrated in Fig. 1(C). The particles are homogeneously dispersed without significant aggregation, confirming the successful incorporation of AuNPs into the pyrrole film. The intimate contact between AuNPs and the pyrrole film indicates strong interfacial interactions. These interactions are likely because of the functional groups that contains nitrogen in pyrrole, which can coordinate with the Au surface. This enhances the stability and sensitivity of the sensor.

The UV-vis spectrum of AuNPs displays an apparent absorbance peak around 520 nm,46 which is specific to the well-dispersed gold nanoparticles as shown in Fig. 1(D). The UV-vis spectrum of AuNPs@Pyrrole exhibits a modified and broader absorption profile compared with that of bare AuNPs. In addition to the characteristic localized surface plasmon resonance (LSPR) absorption of AuNPs, the UV-region absorption is associated with the electronic transitions of the conjugated pyrrole backbone. The extended and modified absorption profile of AuNPs@pyrrole can therefore be attributed to the combined optical response of pyrrole and AuNPs, as well as their interfacial interactions. These spectral changes provide evidence for the formation of the AuNPs@pyrrole structure and demonstrate that pyrrole contributes to the overall optical response. These spectral changes provide evidence for the formation of the AuNPs@pyrrole structure and demonstrate that pyrrole contributes to the overall optical response. This peak results from the cumulative vibration of light-excited conduction electrons at the surface of the nanoparticle. Compared to the bare AuNPs, the absorption peak for AuNPs@pyrrole shows a redshift and broadening. This shift towards a shorter wavelength is attributed to the shift in the local refractive index surrounding the AuNPs due to the pyrrole molecules. The redshift indicates strong electronic interaction between the AuNPs and the nitrogen-containing functional groups of pyrroles. Broadening of the peak suggests slight aggregation or close packing of AuNPs within pyrrole. This could be due to the formation of a more compact structure. The decrease in absorbance intensity is likely due to the partial shielding of AuNPs by the pyrrole layer, which affects the light interaction with the nanoparticle surface.

Dynamic light scattering (DLS) analysis further confirmed the successful modification of the AuNPs with pyrrole. The hydrodynamic size increased from approximately 42.6 nm for AuNPs to 47.0 nm for AuNPs@pyrrole, indicating an increase in particle diameter following pyrrole coating. In addition, AuNPs@pyrrole exhibited a relatively narrow size distribution (PDI = 0.128), suggesting good particle-size uniformity. These results are consistent with the formation of a pyrrole layer on the AuNP surface and support the successful preparation of AuNPs@pyrrole. The corresponding DLS size-distribution spectra are provided in Fig. S1 and S2 of the SI.

Fig. 2(A) EDX spectrum of AuNPs@pyrrole. Energy-dispersive X-ray (EDX) spectroscopy was performed to investigate the elemental composition of the synthesized AuNPs@pyrrole nanostructure. The EDX spectrum displays characteristic signals corresponding to C, N, O, Si, and Au. The quantitative elemental analysis yielded approximately 33.19% C, 8.09% N, 4.09% O, 44.33% Si, and 10.30% Au. The presence of carbon and nitrogen is attributed primarily to pyrrole, whereas the characteristic Au signals confirm the incorporation of gold nanoparticles. The oxygen signal may originate from surface oxygen-containing species or environmental contributions. The relatively high Si content is associated with the silicon-based substrate/support used for EDX measurement. The simultaneous detection of Au, C, and N provides compositional evidence for the formation of the AuNPs@Pyrrole nanostructure.

Fig. 2. (A) EDX spectrum of AuNPs@pyrrole, (B) FTIR spectra of AuNP, pyrrole and AuNPs@pyrrole.

Fig. 2

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The FTIR spectra shown in Fig. 2(B) display distinct differences among AuNP (blue line), pyrrole (green line), and the AuNPs@pyrrole structure (red line). The spectrum of AuNP displays a wide peak around 3400 cm−1, which resembles the O–H stretching, likely due to adsorbed water or hydroxyl groups on the nanoparticle surface, with minimal other peaks observed due to the absence of organic functional groups. In the FTIR spectrum of AuNP, the Au–O peak usually appears within the 500–700 cm−1 region.47,48 This is because metal–oxygen-metal vibrations occur at lower wavenumbers due to the heavy atomic mass of gold and the strong metal–oxygen bonding. Importantly, the absorption near ∼1389 cm−1 and 1633 cm−1 attributable to the symmetric stretching vibration of the carboxylate (–COO−) group of citrate, supporting the presence of citrate-derived species associated with the AuNP surface.49,50 In contrast, the pyrrole spectrum exhibits characteristic peaks, including N–H stretching near 3400 cm−1, aromatic C–H stretching around 3000 cm−1, and prominent C Created by potrace 1.16, written by Peter Selinger 2001-2019 C and C–N stretching bands between 1500–1600 cm−1, reflecting its conjugated ring structure.51 Additionally, out-of-plane C–H bending vibrations are noted in the range of 700–900 cm−1. In the AuNPs@pyrrole structure spectrum, the N–H and C–N stretching vibrations are shifted compared to pure pyrrole, suggesting strong interactions between pyrrole and AuNPs, possibly through nitrogen coordination or π–π interactions. The broadening of peaks and changes in intensity indicate strong interfacial bonding, confirming successful structure formation. These spectral changes suggest structural modifications due to encapsulation of AuNPs within the pyrrole film, enhancing the structure's stability and sensitivity, which is beneficial for colorimetric sensing applications.

3.2. Working mechanism of Cr(vi) detection

The synthesized AuNPs initially displayed a purple color, which transformed into bluish upon the addition of pyrrole in an acidic medium, confirming the formation of the AuNPs@pyrrole structure. The addition of 45 µL of AuNPs@pyrrole to a light-yellow Cr(vi) solution resulted in a marked enhancement of the yellow color intensity. Furthermore, the color intensity increased proportionally with increasing Cr(vi) concentration, indicating the potential of the AuNPs@pyrrole system for concentration-dependent colorimetric detection of Cr(vi). Here, the pyrrole film facilitates the adsorption of Cr(vi) species through electrostatic interactions between protonated pyrrole sites and chromate/dichromate ions under the sensing conditions.52 The intimate contact between the pyrrole film and AuNPs facilitates efficient electron transfer, enabling the reduction of Cr(vi) to Cr(iii) at the AuNP surface, as shown in Fig. 3. This interfacial redox process redistributes the surface electron density of the AuNPs and alters the local dielectric environment surrounding the nanoparticles. Gold nanoparticles exhibit localized surface plasmon resonance (LSPR), which is greatly sensible to variations in the near electronic and dielectric environment. Consequently, the adsorption and reduction of Cr(vi) induce a concentration-dependent modulation of the LSPR characteristics, resulting in a measurable decrease in absorbance intensity together with a visible color change. Since the AuNPs are immobilized within the pyrrole film, the optical response is attributed primarily to interfacial charge transfer and LSPR modulation rather than physical aggregation of the nanoparticles. The sensor high sensitivity arises from the strong plasmonic response of AuNPs to subtle changes in their local environment, allowing the determination of trace levels of Cr(vi). The selectivity results from the combined effects of preferential adsorption of chromate species by the protonated pyrrole molecules, favorable reduction kinetics of Cr(vi) at the AuNP surface, and the optimized sensing conditions. Other tested metal ions exhibit significantly weaker adsorption and electron-transfer behavior under identical conditions and therefore produce negligible optical responses.

Fig. 3. Detection mechanism of Cr(vi).

Fig. 3

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3.3. Cr(vi) detection

The colorimetric determination of Cr(vi) was investigated by mixing different concentrations of Cr(vi) standard solutions with the AuNPs@pyrrole structure and monitoring the resulting optical and colorimetric variations. Following the inclusion of Cr(vi), a noticeable color change was monitored with cumulative Cr(vi) concentration, which is indicative of the nanoparticle aggregation triggered by the interaction between pyrrole functional groups in the structure and the Cr(vi) ions.

Fig. 4(A) displays the UV-vis absorption spectra of the AuNPs@pyrrole structure at varying Cr(vi) concentrations. As the Cr(vi) concentration increased, the characteristic absorption peak at approximately 465 nm significantly decreased in intensity and became broader. These changes are attributed to the interaction of Cr(vi) with the pyrrole/pyrrole functional groups and the resulting modification of the local electronic environment of the AuNP-pyrrole structure. No distinct red-shift of the AuNP plasmon band toward 600–700 nm was observed; therefore, the response is not assigned to conventional aggregation-induced plasmon coupling. The colorimetric response was rapid and concentration-dependent, with clear differentiation visible even at low Cr(vi) levels. As shown in Fig. 4(B), a calibration graph was created by charting the absorbance magnitude at 465 nm versus Cr(vi) concentrations at a range of 0.01–100 ppm. The sensor exhibited a linear output within the window of 0.1–100 ppm, and a correlation coefficient (R2) exceeding 0.9867, indicating excellent linearity and sensitivity. The limit of detection was determined using the 3σ/slope method and was found to be 0.01 ppm, demonstrating the sensor's high sensitivity for trace-level detection of Cr(vi).

Fig. 4. (A) UV-vis absorption spectra of Cr(vi) solutions at concentrations ranging from 0.01 to 100 ppm after the addition of the AuNPs@pyrrole structure; (B) corresponding calibration curve; (C) digital images showing the colorimetric response of Cr(vi) solutions at concentrations ranging from 0.1 to 100 ppm before and after the addition of the AuNPs@pyrrole structure; and (D) plot of RGB intensity versus Cr(vi) concentration (0.1–100 ppm). Data are presented as the mean ± standard deviation (±SD) from three independent experiments (n = 3).

Fig. 4

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The visual color change before and after adding the AuNPs@pyrrole structure to Cr(vi) solution of different concentrations is presented in Fig. 4(C). The color transition with increasing Cr(vi) levels allows for immediate visual estimation of chromium contamination in water samples, supporting the feasibility of the sensor for on-site applications. In parallel, a smartphone-based RGB analysis was conducted by capturing images of the sensor solutions after interacting with various concentrations of Cr(vi). The RGB quantities indicated a significant decrease with raising the concentration of Cr(vi), correlating well with the UV-vis responses. The corresponding RGB vs. concentration plot is shown in Fig. 4(D), where the RGB value exhibited a linear decline within the 0.1–100 ppm range. This result confirms the potential of the smartphone-assisted method as a portable, effective, and easy to use detection platform for on-site Cr(vi) analysis. Overall, the AuNPs@pyrrole sensor demonstrated high sensitivity, rapid response, and good visual discernibility for Cr(vi) detection, validated by both spectroscopic and smartphone-based colorimetric methods.

Table 1 contrasts the analytical activity of the proposed AuNPs@pyrrole detection platform with previously reported colorimetric sensors for Cr(vi) determination. The comparison includes the sensing material, chromogenic medium/support, detection method, limit of detection (LOD), and linear detection range. The proposed AuNPs@pyrrole structure exhibits a wide linear detection range of 0.01–100 ppm using UV-vis spectroscopy with a low LOD of 10 ppb, while the RGB-based analysis provides a practical linear range of 0.1–100 ppm with an LOD of 100 ppb. These results demonstrate that the developed sensor offers a broad working range, satisfactory sensitivity, and the additional advantage of simple smartphone-assisted colorimetric detection without the need for external chromogenic reagents such as TMB.

Table 1. Comparative analysis of recently developed colorimetric sensors for the determination of Cr(vi) based on UV-vis spectrophotometry and RGB digital image analysisa.

Sensing materials External agent Detection medium Linear range (ppm) LOD (ppb) Ref.
Fe3O4@pyrrole TMB UV-vis & RGB 0.028–22.04 2 53
Starch-AgNPs — UV-vis 0.551–4.408 600 54
PNPG-PEG NPs TMB UV-vis 0.002–0.668 28 55
AgNPs@starch TMB UV-vis 0–5.51 11.3 56
MOF-919-Fe-Cu TMB UV-vis 0.055–1.102 11.5 57
Fe-QueNPs@ZIF-67 TMB UV-vis 0.028–3.38 11.57 58
AuNPs@pyrrole — UV-vis RGB 0.01–100, 0.1–100 10 & 100 This work
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a

TMB -3,3′,5,5′-tetramethylbenzidine, PNPG-PEG-polyethylene glycol-functionalized poly(N-phenylglycine), QueNPs@ZIF-67- zeolitic imidazolate framework-67 encapsulated iron-quercetin nanocomposite.

3.4. Sensor reproducibility and selectivity

The duplicability of the sensor was evaluated by conducting five consecutive measurements under identical conditions. The sensor consistently showed nearly identical responses in each trial, indicating excellent reproducibility. The relative standard deviation of the readings was calculated to be lower than 5%, demonstrating high precision and stability of the sensor. This consistency confirms that the sensor's performance is reliable and repeatable, making it suitable for practical applications in chromium detection. The reproducibility can be attributed to the uniform distribution of AuNPs within the pyrrole film, ensuring consistent interaction with the chromium ions in each measurement.

The sensor selectivity towards Cr(vi) was assessed by comparing its response to other potentially interfering metal ions, including Fe(ii), Cr(iii), Cd(ii), Hg(ii), As(iii), Pb(ii), and Fe(iii) as demonstrated in Fig. 5. The UV-vis absorption spectra of the sensor exposed to each metal ion is illustrated in Fig. 5(A). A notable peak appears in the absorption spectra only when Cr(vi) is present; while Cr(iii) produces a small peak, it is negligible by comparison. Furthermore, the other metal ions cause very slight alterations. This indicates that the sensor shows a high selectivity in favorite of Cr(vi), likely due to the specific interaction between the Cr(vi) and the AuNPs@pyrrole structure. Fig. 5(B) further confirms the selectivity by comparing the RGB values for each metal ion. While Cr(vi) displays a significantly lower RGB value than the other metal ions, Cr(iii) and the remaining metals exhibit RGB values similar to pure water. The error bars, showing minimal variation, confirming the reproducibility of the measurements. The sensor's high selectivity for Cr(vi) in the presence of potentially interfering substances highlights its potential for precise environmental monitoring.

Fig. 5. (A) UV-vis spectra, (B) RGB vs. concentration plot of 100 ppm Pb(ii), Fe(iii), Cd(ii), Fe(ii), Hg(ii), As(iii), Cr(iii), and Cr(vi) solution after adding AuNP@pyrrole.

Fig. 5

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3.5. Real sample analysis and result validation

Real-sample evaluation was performed to assess the applicability of the developed sensor for Cr(vi) detection in environmental water and food samples, including river water, pond water, tap water, vegetables, and fruit. As shown in Table 2, the RGB and UV-vis methods provided concentration-dependent responses for several of the investigated samples, with the detected concentrations showing reasonable correspondence with the ICP-OES results. For example, the river-water sample showed concentrations of 688, 661, and 656 ppb by RGB, UV-vis, and ICP-OES analysis, respectively, while Vegetable-1 and the fruit sample also exhibited comparable concentrations among the analytical methods. Some samples showed no detectable response by one or more methods, indicating that the measured response can vary depending on the sample matrix and analytical technique. Overall, the results demonstrate the potential of the AuNPs@pyrrole-based platform for rapid screening of Cr(vi) in real environmental and food samples.

Table 2. Detection of Cr from real sample and result validation ICP-OES.

Sample name RGB analysis method UV vis analysis ICP-OES analysis
RGB Conc. (ppb) Peak height (Abs.) Conc. (ppb) Conc. (ppb)
River water 537.8 688 0.202 661 656
Pond water 540.0 Not detected — Not detected Not detected
Tap water 549.0 Not detected — Not detected 90.2
Vegetable – 1 538.0 539 0.198 521 459
Vegetable – 2 546.0 Not detected — Not detected Not detected
Fruit 538.3 314 0.192 311 307
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4. Conclusion

In this study, a gold nanoparticles-pyrrole (AuNPs-pyrrole) structure was developed as a colorimetric detector for the sensitive and selective determination of Cr(vi). The sensor exhibited a low detection limit of 0.01 ppm and a linear response window of 0.1–100 ppm. In this study, visual observation, UV-vis spectrophotometry, and smartphone-based RGB analysis were combined to establish a highly effective dual-mode strategy for on-site, point-of-care (POC) testing application. Selectivity analysis confirmed minimal interference from Fe, Cd, Pb, Hg, As, Cr(iii). The sensing performance was justified via ICP-OES, showing good correlation and accuracy. The practical utility of the AuNPs@pyrrole structure was confirmed through the successful detection of Cr(vi) in food and environmental water samples. These results highlight the developed platform as a user-friendly, colorimetric sensor for on-site pollution monitoring, offering a cost-effective and dependable solution across environmental and industrial applications. Its great selectivity, sensitivity, and portability make it a promising platform for environmental pollution control and regulatory compliance.

Author contributions

Md. Romzan Ali: Fabrication of AuNPs and AuNPs@pyrrole structure, investigation, data, curation, writing, first draft, and formal analysis; Ruksana Suraia Akhi: colorimetric detection of chromium, data curation, writing first draft; Md. Ruhul Amin: real sample analysis, formal analysis, and writing first draft; Alamin Howlader: data curation, and characterization; Gang Chen: reviewing and editing, supervision, Fatema Tuj Zohra: Reviewing and editing, and formal analysis; Mohamed Aly Saad Aly: discussion and analysis, methodology, conceptualization, project management, reviewing and editing, and supervision; Hermine Gharagulyan: formal analysis, methodology, and reviewing and editing; Md. Zaved H. Khan: methodology, conceptualization, project management, supervision.

Conflicts of interest

The authors declare that there is no conflicts of interest.

Supplementary Material

RA-OLF-D6RA06405A-s001

Acknowledgments

The authors recognize the financial support provided by Georgia Tech Shenzhen Institute (GTSI). Furthermore, the authors recognize the financial support provided by the research grant from Jashore University of Science and Technology.

Data availability

The data of this article have been included within the manuscript and its supplementary information (SI). Further information and data will be made available upon request from the corresponding author. Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra06405a.

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Supplementary Materials

RA-OLF-D6RA06405A-s001
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The data of this article have been included within the manuscript and its supplementary information (SI). Further information and data will be made available upon request from the corresponding author. Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra06405a.

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