Current contributions of novel nanoparticles-based colorimetric sensors for detection of SCN ions in different aqueous models. A review

1.1 The challenges and future prospects in novel colorimetric response-based sensors (NCRS)

1. Sensitivity and Selectivity: One of the primary challenges in colorimetric sensors is achieving high sensitivity and selectivity for target analytes. Designing sensor systems that can detect low concentrations of analytes and differentiate them from interfering substances is crucial.

2. Response Time: Rapid response is desirable in many sensing applications. Developing colorimetric sensors with fast response times is essential to enable real-time monitoring and detection.

3. Stability and Durability: Ensuring the stability and durability of colorimetric sensors is crucial for long-term applications. Sensors should maintain their response characteristics over time and exhibit robustness in different environmental conditions.

4. Signal Interpretation and Quantification: Accurate interpretation and quantification of colorimetric signals pose challenges due to the subjective nature of color perception. Developing robust algorithms and analytical methods to convert color changes into quantitative data is necessary for reliable sensing.

5. Sensing Mechanisms: Exploring and understanding different sensing mechanisms is an ongoing challenge. Novel approaches and materials are being investigated to enhance sensor performance and broaden the range of detectable analytes.

Future Prospects in novel colorimetric response-based sensors:

1. Multiplexed Sensing: Advancements in colorimetric sensors may enable the development of multiplexed sensing platforms, allowing simultaneous detection of multiple analytes. This capability would be valuable in various fields, such as medical diagnostics, environmental monitoring, and food safety.

2. Portable and Point-of-Care Applications: There is a growing demand for portable and user-friendly colorimetric sensors that can be used in point-of-care settings. Future prospects include the development of handheld devices or smartphone-based systems for rapid on-site analysis.

3. Integration with Internet of Things (IoT): Integration of colorimetric sensors with IoT technology can enable real-time data transmission, remote monitoring, and data analysis. This integration could revolutionize applications in environmental monitoring, industrial processes, and personalized healthcare.

4. Enhanced Sensitivity and Selectivity: Ongoing research aims to improve the sensitivity and selectivity of colorimetric sensors by utilizing advanced materials, nanostructures, and signal amplification strategies. This progress would broaden the range of detectable analytes and improve overall sensor performance.

5. Smart Materials and Responsive Systems: Future prospects involve the development of smart materials that exhibit reversible or tunable color changes in response to specific analytes. Additionally, responsive systems that can be triggered by external stimuli (e.g., light, heat, pH) offer opportunities for more versatile and adaptable colorimetric sensors.

Herein, we present a comprehensive review article, aiming to provide researchers and readers with an overview of the current advancements in colorimetric response-based nanomaterial sensors for the detection of thiocyanate ions in various water models. This review encompasses discussions on the fabrication, diagnosis, and performance of colorimetric nanosensors utilizing silver, copper, gold, and iron nanoparticles as effective probes for thiocyanate ion detection in diverse aqueous environments. The detection mechanisms for SCN^- ions are explained, along with the assessment of key quality assurance parameters including sensitivity, selectivity, and efficiency. The advantages and disadvantages of these colorimetric nanosensors are also discussed. Additionally, we highlight the contributions made thus far and future prospects in this field, with the aim of developing novel nanometal-based colorimetric tools, encompassing both noble and non-noble metal nanoparticles. Furthermore, we have compiled an extensive collection of works, reports, studies, and literature related to the detection and estimation of thiocyanate ions using various nanometallic sensors (Ag, Cu, Au, and Fe) within this review article. To date, there is limited literature or published work available on this specific topic, making this paper a valuable and unique resource for readers and researchers in the field of colorimetric assays for pollutant sensing, particularly regarding the detection of the toxic thiocyanate ion in various aquatic environments. This paper serves as a comprehensive review, providing valuable insights into the fabrication and construction of efficient, user-friendly, and visually observable color sensors for detecting thiocyanate ions in water models.

3.1 Nano gold sensor (AuNSs)

In the field of nanotechnology, gold nanoparticles have garnered significant attention due to their remarkable properties and ease of fabrication. These nanoparticles exhibit high stability and possess diverse characteristics that make them an ideal material for various studies. One notable feature is their ability to assemble with different materials, which can be precisely controlled by manipulating their sizes. Additionally, gold nanoparticles display exceptional optical behavior attributed to their surface plasmon resonance (SPR), which spans a wide range from the visible to the infrared spectrum. This property arises from the collective oscillation of conducting electrons in the metal and is influenced by factors such as the shape and size of the gold nanoparticles (Lu et al., 2015). Numerous reproducible methods have been developed for the fabrication of gold nanoparticles. These methods can be further customized by incorporating various molecules and reagents with different functional groups to synthesize gold nanoparticles. Recently, gold nanoparticles have found extensive applications in sensitive and specific assay principles, including the detection of contaminants through colorimetric assays.

The chemical reduction method is the most commonly used and popular approach for preparing gold nanoparticles (AuNPs). In this method, gold ions in chloroauric acid (HAuCl₄) salt are reduced using various chemical compounds, derivatives, and solvents, such as citrate derivatives. Qiang and colleagues developed a simple, rapid, sensitive, and selective colorimetric procedure for sensing thiocyanate (SCN^-) ions based on the anti-agglomeration of gold nanoparticles (AuNPs) (Song et al., 2016). The agglomerated nanoparticles were prepared using citrate as a chemical capping and reducing agent. The prepared nanoparticles were characterized using UV–Vis and TEM analysis, confirming their unique plasmon resonance wavelength and spherical shape with a size of 38 nm. Subsequently, the sensor was further enhanced by the addition of CTAB to obtain an aggregated nanoparticle solution with a red color. The red AuNPs sensor showed a decrease in color intensity upon increasing the SCN^- concentration, resulting in a blue solution (Fig. 3). This color change was monitored using a UV–Vis spectrophotometer and was visible to the naked eye. The AuNPs sensor demonstrated excellent detection capability for SCN^- ions within 15 min, with a color conversion from red to blue. Moreover, the detection limit (DL) for this reaction was determined to be 6.5 nM. The authors suggested that this AuNPs sensor could also be used for the estimation of SCN^- in various aqueous samples with satisfactory results.

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Fig. 3

UV–Vis spectra, colors and TEM images of AuNPs in the absence (a) and presence of CTAB (b), after the addition of CTAB and SCN^- (c). [cited from reference (Song et al., 2016)].

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Similarly, a new method involved the room temperature reduction of HAuCl₄ with trisodium citrate in an aqueous medium to fabricate gold nanoparticles (AuNPs) (Rashidiani et al., 2017). This protocol resulted in the formation of a purple solution containing well-sized AuNPs. Techniques such as TEM micrograph analysis, dynamic light scattering (DLS) study, and UV–Vis measurement were employed to characterize the obtained purple solution. The data from these techniques confirmed the excellent morphology of the nanoparticles, with a size of 32.5 nm and a spherical shape. Although this preparation protocol is easy, simple, and yields good results, it is worth noting that the toxic agent used can affect the quality of the preparation.

A colorimetric sensor based on the anti-aggregation of citrate-capped gold nanoparticles (AuNPs) has been developed for the detection of thiocyanate (SCN^-) ions (Deng et al., 2014). The synthesized sensor was characterized using TEM image analysis and UV–Vis analysis, revealing that the prepared sensor exhibited localized surface plasmon resonance (LSPR) properties, with a size of 13 nm and a spherical morphology.

The authors found that the citrate-capped AuNPs agglomerated in a sulfuric acid medium, causing a color change in the solution from red to blue. However, they discovered that this aggregation could be prevented by the presence of SCN^- ions, which exhibited excellent protective properties on the Au colloid. The effectiveness of the fabricated sensor was evaluated for SCN^- ion sensing, demonstrating significant sensitivity even at a concentration of 1 mM of SCN^- within 5 min, as confirmed by naked-eye observation. The authors stated that this simple sensor can be effectively applied for SCN^- assays in different effluent samples using a straightforward, selective, and sensitive approach without the need for complicated steps.

In a separate study, Pakkanen et al. developed a highly ultrasensitive and recyclable catalyst, Au-SiO₂@Ag wire (W) superstructure, for the detection of SCN^- in various media (Ankudze et al., 2019). This novel sensor, Au-SiO₂@AgW, was created by incorporating nanoscale patches of SiO₂ onto micron-sized AgW and decorating the patches with 30 nm-sized AuNPs using mercaptopropyl trimethoxysilane. The ultrasensitive catalyst was assessed for its ability to detect SCN^- in different aqueous models, demonstrating low detection limits of 0.001 mM in water and 0.01 mM in urine and human serum. Notably, the authors highlighted the simplicity of regenerating and recycling the SCN^- bound Au-SiO₂@AgW catalyst in the tested media. The reported benefits from this work present a promising approach for developing active and recyclable catalysts for the detection of SCN^- ions.

In another notable study, Pienpinijtham et al. investigated a novel sensor and technique for detecting thiocyanate ions using starch-reduced gold nanoparticles (SAuNPs) and surface-enhanced Raman scattering (SERS) (Pienpinijtham et al., 2011). The synthesized nanoparticles were characterized and confirmed using UV–Vis, FE-SEM, TEM, AFM, and Raman spectrum analysis. UV–Vis analysis demonstrated a maximum absorbance spectrum at 550 nm, indicating a high active surface area with localized surface plasmon resonance (LSPR) property. Further analysis of FESEM, TEM, and AFM images confirmed the spherical shape of the nanoparticles with a size of 80 nm. Moreover, SAuNPs exhibited a distinct Raman peak at 2125 cm⁻¹, attributed to the stretching mode of the C = C group in the prepared nanoparticles. The SAuNPs sensor showed excellent performance in detecting thiocyanate ions. Due to the strong adsorptivity of thiocyanate ions on the SAuNPs surface, a new peak appeared in the surface-enhanced Raman scattering spectrum at around 2100 cm⁻¹, corresponding to the stretching vibration of the C = N group associated with the SAuNPs sensor. The authors successfully utilized this result to determine SCN^- concentrations both individually and in a mixture system. Furthermore, they reported a detection limit of 0.05 μM for thiocyanate ions within a range of 0.05–50 μM. The proposed procedure demonstrated high selectivity for thiocyanate ions without interference from other coexisting anions in the reaction mixture, such as sulfate, halides, and carbonate.

A significant challenge faced by researchers is distinguishing thiocyanate ions from other common ions and their interferences. In addressing this issue, Zhao Hengxin et al. developed flawless gold nanoclusters (AuNCs) as fluorescent probes for the specific detection of thiocyanate ions, utilizing poly-cytosine DNAs as a template (Mo et al., 2019). The authors demonstrated that the fluorescence efficiency of the prepared AuNCs sensor was diminished in the presence of SCN^- ions due to the interaction between thiocyanate ions and Au atoms, resulting in quenching at a wavelength of 425 nm. They proposed that AuNCs could obtain energy support from DNA molecules, enhancing their emission intensity. However, in the presence of thiocyanate ions, the association with gold atoms disrupted the energy transfer between the template (nucleobases) and the structure of AuNCs, leading to luminescence quenching. The method exhibited a detection limit of 4.2 × 10^-7 mol/L, with the decrease in fluorescence intensity proportional to the concentration of thiocyanate ions within the range of 8.0 × 10^-7 to 1.5 × 10^-5 mol/L. The authors concluded that this approach is suitable for detecting thiocyanate ions in natural water samples and can yield excellent results.

The utilization of effective agents to achieve greater accuracy is highly desirable, especially in sensing processes. To address this, Giang FIE et al. employed the seed-mediated method to synthesize core–shell Au@Pt nanoparticles (Au@PtNPs) with peroxidase catalytic activity (Wang et al., 2020). The prepared nanoparticles were characterized using UV–Vis, FT-IR, XPS, and TEM-EDX analysis. TEM images confirmed the stability of the Au@PtNPs, revealing a pure spherical shape with a diameter of 13 nm. The fabricated sensor (Au@PtNPs) was employed to catalyze the luminol-H₂O₂ reaction, enhancing the luminescence intensity. However, the catalytic activity of the colored Au@PtNPs was significantly inhibited upon the addition of thiocyanate ions. This behavior established a detection method for thiocyanate ions, utilizing Au@PtNPs in conjunction with the catalytic luminol- H₂O₂ system. The protocol demonstrated several advantages, including simplicity, cost-effectiveness, convenience, and an ultra-low detection limit. The authors reported that the detection mechanism for thiocyanate ions involved the association or adsorption of SCN^- ions on the surface of Au@PtNPs, occupying the active sites of the Pt structure, which resulted in a reduction in the amount of Pt⁰ and subsequent loss of the catalytic activity of Au@PtNPs. After optimizing the conditions of the procedure, the detection protocol achieved a low detection limit of 2.9 nM within a linear range of 5–180 nM. Finally, the suggested procedure was successfully applied to tap-water samples for the detection of thiocyanate ions, highlighting its practical application value. In a similar study, Peng et al. fabricated a facile sensor consisting of core–shell Au@Pt nanocatalysts (Au@PtNCs) with high peroxidase-like activity (Peng et al., 2017). The detection of thiocyanate ions (SCN^-) was monitored by inhibiting the peroxidase-like activity of Au@PtNCs, which effectively occurred (Fig. 4). The inhibition mechanism of the fabricated sensor was monitored using various diagnostic techniques, including XPS, DLS, TEM, and EPR. The recorded data confirmed the formation of nanoparticles with excellent morphology (uniform spherical shapes). Inhibition of the catalytic efficiency of Au@PtNCs by SCN^- ions was evaluated, showing a decreased ability of the catalyst to capture hydroxyl radicals (•OH) and an increased ratio of Pt(II) to Pt(0) on the catalyst surface. Based on this principle, a sensitive colorimetric detection of SCN^- ions was achieved through the inhibition of Au@PtNCs' activity by SCN^-. Additionally, the prepared catalyst was further enhanced by cysteine molecules, which significantly improved the selectivity of SCN^- detection. After optimization, a colorimetric test for SCN^- detection yielded a limit of detection (LOD) of 5.0 nM within a broad linear calibration range of 20–40 µM. The authors highlighted the protocol's advantages, including its low cost, efficient sensitivity, and selectivity. Moreover, this approach exhibited high potential for the quantitative determination of thiocyanate ions in various aqueous models, such as water and raw milk.

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Fig. 4

The assumed mechanism for detection of SCN ions by Au@PtNCs sensor explained with TEM images [cited from reference (Peng et al., 2017)].

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In the development of strategies for the detection of heavy pollutants like thiocyanate, the work of Zhihu Dai et al. stands out with their facile strategy for rapid, highly sensitive, and selective detection of SCN^- ions using an electrochemical oxidation approach (Cui et al., 2020). They prepared small-sized gold nanostars (AuNSs) with uniform and sharp tips, confirmed by TEM analysis. To enhance performance, zinc phthalocyanine (ZnPc) molecules were bound to the AuNSs, resulting in a selective electrode for thiocyanate ions. The ZnPc-AuNSs electrode exhibited rapid and selective response to thiocyanate ions under electrochemical oxidation, causing a color change in the reaction mixture from dark blue to red. With the wide wavelength range of AuNSs, the ZnPc-AuNSs sensor achieved a broader detection range for SCN^- ions (10 nM to 80 mM) compared to other reports, with a low detection limit of 3 nM. The colorimetric sensor for SCN^- detection was successfully applied to a milk sample, demonstrating excellent sensing capabilities. The authors suggested that the electrochemical oxidation strategy could play a critical role in the development of a promising colorimetric sensor with high selectivity and detection performance for monitoring environmental pollution.

Another remarkable colorimetric and fluorometric approach for the detection of thiocyanate (SCN^-) was proposed using a new, sensitive, and selective dual-readout sensor fabricated from carbon dots (CDts) and gold nanoparticles (AuNPs) (Dan Zhao, Chuanxia Chen, Lixia Lu, 2015). The NPs were synthesized using citrate molecules as reducing and stabilizing agents, confirmed by TEM imaging techniques. Amino-functionalized CDts were attached to the surface of citrate-stabilized AuNPs through Au-N interactions, resulting in the agglomeration of AuNPs and the nonfluorescent off-state of CDts. This agglomeration allowed for potential fluorescence resonance energy transfer (FRET). However, upon the addition of thiocyanate ions, they competitively bound with AuNPs, preventing aggregation and fluorescence quenching. As a result, both colorimetric and fluorometric signals remained “light-on” with a red color. A discernible color change was observed at an SCN^- concentration of 1 µM, and a low detection limit of 0.036 µM was achieved through fluorescence spectroscopy. Both colorimetric and fluorometric sensors exhibited excellent selectivity for SCN^- over other metallic ions and anions. This sensing assay offered simplicity, rapidity, cost-effectiveness, and ease of operation without requiring further modification. The accuracy and precision of the method were evaluated by quantitatively detecting SCN^- in tap water and saliva samples, yielding satisfactory results.

Similarly, Yuqi Zhaw et al. utilized 2-aminopyridine as an efficient agent to develop a sensitive gold nanoparticle (C-AuNPs) colorimetric sensor for SCN^- detection (Zhao et al., 2020). C-AuNPs were fabricated using citrate as a chemical reducing and stabilizing agent, resulting in a red-colored solution. Characterization techniques including XRD, FESEM, FT-IR, and UV–Vis analyses confirmed the spherical shape and unique optical properties of the NPs. Efficient aggregation of the AuNPs was achieved by employing 2-aminopyridine (2-AP) due to its high electrostatic attraction. Detection of thiocyanate ions involved an anti-aggregation process. Upon the addition of SCN^- to the sensor solution, thiocyanate ions formed an S-Au bond with the gold nanoparticles, preventing aggregation. The colorimetric detection mechanism showed a color change from blue to red, and UV–Vis spectroscopy demonstrated linear correlation between absorbance values and SCN^- concentration in the range of 0.4–1.2 μmol/L. The detection limit in this protocol was 0.37 μmol/L. The authors also reported significant selectivity and high anti-interference performance. Importantly, this approach was successfully applied for the sensing and determination of thiocyanate ions in real environmental water samples, yielding good results. (Fig. 5).

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Fig. 5

UV.Vis spectra, and TEM images of AuNPs in the absence (a) and presence of 2-AP (b), after the addition of SCN^- ion (c), and with different ions (d) [cited from reference (Zhao et al., 2020)].

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3.2 Nano silver sensor (AgNSs)

Silver is one of the brightest, most famous, and widely used metals worldwide. Due to its unique properties, the demand for silver in general, and in the nanoparticle (AgNPs) phase in particular, is increasing across various fields such as mining factories, alloy design, pharmaceutical companies, healthcare, medical cosmetics, agriculture, food, and consumer products. The wide range of functionalities offered by AgNPs has led to their utilization in numerous pathways and applications, including coating purposes, anti-humidifiers, water pipes, the food industry, medical devices, drugs, wound dressing, diagnostics, orthopedics, as antibacterial and anticancer agents, and in sensing applications (Lv et al., 2020). The nanoscale of silver reveals unique features across all these fields, and its ability to alter its physical, chemical, and biological properties makes it highly versatile in various applications, including sensing (Zainal et al., 2019).

In general, the synthesis routes for nanosensors, including metallic nanosensors, can be categorized into chemical, physical, and biological methods (Yang et al., 2014).

Among these methods, the most common and conventional approach for fabricating AgNPs involves the chemical reduction of silver ions in AgNO₃ salt using various chemical reagents, derivatives, and solvents, including derivatives of citrate molecules. Chengyoung Li and his group utilized sodium citrate as both a reducing agent and a capping agent to produce well-defined AgNPs (Feng et al., 2019). The resulting product obtained through this method was in the form of a colloidal solution. Several analytical techniques, such as UV–Vis, TEM, and Raman spectra, were employed to characterize the product. The analysis confirmed the presence of uniformly spherical nanoparticles with an average size of 55 nm at a wavelength of 405 nm. Although the synthesis method employed in this study may not be sustainable for catalyst preparation, the sensor demonstrated excellent performance in detecting thiocyanate ions in water-milk samples, achieving a detection limit as low as 0.004 mg/l using a colorimetric tube (Fig. 6). However, one drawback of this sensing method was the use of multiple chemical surfactants (i.e., trichloroacetic acid and ferric nitrate) for determining the target ion.

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Fig. 6

a) tem image analysis and supported with uv–Vis analysis of fabricated AgNPs. b) The detection of SCN ions in the milk sample at different concentrations. c) Relation curve between recorded absorbance and SCN concentrations [cited from reference (Feng et al., 2019)].

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In another similar preparation, Hepieng Li et al. employed the chemical reduction method using trisodium citrate to fabricate small AgNPs. The prepared sensor was utilized for detecting thiocyanate ions through both the adsorption–desorption method and the pressure approach in a diamond anvil cell (DAC) apparatus with quartz pressure sensors at room temperature (Wang et al., 2019). The as-prepared AgNPs were characterized by FESEM image analysis, which revealed the formation of uniformly small nanospherical particles measuring 50 nm. To evaluate the sensor's performance, the catalyst was cast onto a silicon wafer using an assembly process. The detection process demonstrated rapid estimation of SCN^- ions in the sample vial at room temperature, with immediate results. Despite the positive outcomes obtained using this methodology within a short timeframe, it still suffers from limitations such as the use of expensive and complex apparatus as well as the employment of harmful chemical reagents for ion detection.

Furthermore, Xiang et al. (Bai et al., 2021) preferred the seed-based growth method for synthesizing AuNPs decorated with small AgNPs. In this approach, a mixture of sodium citrate and OPE3 was used to fabricate small AuNPs (30 nm in size). Subsequently, the AuNPs were used as templates (seeds) for the synthesis of well-defined Au@AgNPs sensors by combining the seeds with trisodium citrate and ascorbic acid. The resulting product from the growth reaction facilitated successful nucleation and growth of Au@AgNPs with a size of 15 nm. After confirming the prepared AuNPs, efforts were made to assess the effectiveness of the preparation method, as well as the optical properties (shape and size), stability, surface enhancement, and application. According to Fig. 7, the prepared Au@AgNPs sensor exhibited high sensitivity and efficiency in detecting SCN^- ions within 15 min, with a detection limit of 10 nM.

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Fig. 7

Illustrative scheme of the synthesized AgNPs-adorned AuNPs surface and its performance in the determination of toxic thiocyanate ions [Cited from reference (Bai et al., 2021)].

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Despite their detrimental effects and lack of environmental sustainability, the reduction of metal salts using fast and effective agents has gained prominence in industrial processes and chemical reactions. Wang et al. reported the fabrication of silver nanoparticles (AgNPs) through the reduction of silver ions using AgNO₃ salt with sodium borohydride (NaBH₄) in an aqueous solution, specifically for the estimation assay of SCN^- ions (Wang et al., 2004). This approach proved useful in producing perfectly sized AgNPs (21 nm) with a spherical monodispersed shape. The resulting catalyst exhibited excellent sensing capabilities for thiocyanate ions, achieving a detection limit of 4 × 10^-8 mol/L.

Currently, green synthesis methods have gained significant attention in the fabrication of metal and metal oxide nanoparticles due to their safety, cleanliness, low cost, and high efficiency. The importance of this technique lies in its utilization of biological sources such as plant components, microorganism bodies, and yeasts, which serve as efficient stabilizers and reducing agents for nanoparticle synthesis. These green agents contain functional groups that effectively reduce metal ions in an eco-friendly manner without the need for toxic agents. The application of this technology extends to various metals, including copper, titanium, and gold, enabling the acquisition of new properties and improved surface functionalization. Moreover, this technique provides nanometals with remarkable capabilities, making them suitable for application in diverse fields such as biology, industry, technology, and sensing processes (Askari et al., 2020). In this context, Kumar and Praveen proposed a green protocol for detecting thiocyanate ions using silver nanoparticles synthesized via green synthesis using Achyranthes aspera L. extract (Durga Praveena and Vijaya Kumar, 2015). The nanoparticles were further capped with 1% (w/v) chitosan/CH₃CO₂H to enable rapid, efficient, and selective sensing of SCN^- ions in polluted water samples under ambient conditions. The synthesized sensor was characterized using FTIR, confirming the presence of active functional groups on the NP surfaces. The interaction between AgNPs and SCN^- ions was studied using a UV–Vis spectrophotometer. The optical sensor demonstrated excellent detection performance within three seconds, with a detection limit of 1 ppm at room temperature. The authors suggested that this optical thiocyanate sensor could be employed with different models to detect low levels of thiocyanate, such as in wastewater or in human samples like saliva, plasma, and sweat.

3.3 Nano iron sensor (FeNSs)

Dissolved thiocyanate ions in used water models, including fresh water, pose significant health and safety concerns for all living organisms. To address this problem, many researchers have turned to nanoparticles that can break down or remove this type of pollutant. Iron nanoparticles, titanium nanoparticles, and zinc nanoparticles are among the effective and cost-efficient options. These metals are known for their affordability, non-toxic nature, and widespread applications, making them attractive for water purification processes. They play a crucial role in capturing and removing thiocyanate ions due to their active surfaces, which readily associate with the target ion, forming red or colorless M−SCN compounds (where M represents Fe, Ti, and Zn) (Pari and Reddy, 2020). Recently, these nanometals have shown successful results in removing SCN^- ions from various real water samples, including drinking water, river water, and industrial water, through chemical reduction or electrical approaches. Most of these engineered nanoparticles have demonstrated good sensing capabilities, high removal efficiency, and strong reusability for thiocyanate pollutants in aqueous models (C. Li et al., 2018; Nemakal et al., 2021; Zargaran et al., 2020). However, their utilization in colorimetric assays has further advanced sensing technologies (Poursaberi, Tahereh., Akbar, Vahide., Shoja, 2015). Designing fluorescent sensors for the selective detection of single anions in aqueous models remains one of the most challenging tasks for researchers and scientists. Furthermore, understanding the simultaneous detection mechanism and capturing of a single ion among multiple anions continues to be a major challenge for these sensors. In this context, Fengjuan and colleagues reported the synthesis of a white emission nanoprobe based on dipicolinic acid (dpa)-PEG-Fe₃O₄ nanoparticles decorated with the Coumarin-Rhodamine CR-Eu complex [Fe@CR@EuNPs], which was used for the selective detection of SCN^- ions and ClO^- (Lihua Zhi et al., 2015). The authors noted that the prepared nanoprobe exhibited three distinct colors (green, blue, and red) from a total of 15 primary colors, as well as white emission under different excitation energies. Although this approach utilized various complex compounds and agents, this nanosensor demonstrated a pronounced change in emission color from red to violet in the presence of ClO^-, while displaying a reversible change from violet to rose-red emission color in the presence of SCN^- ions (Fig. 8). With detection limits of 0.037 and 0.250 nM, respectively, this catalyst exhibited high selectivity and sensitivity (20) for ClO^- and SCN^- ions. Furthermore, the redox reaction between ClO^- and SCN^- allowed for simultaneous degradation of these ions.

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Fig. 8

a) preparation mechanism of fe@cr@eunps sensor. b) the white emission spectrum of the fe@cr@eunps sensor excited under the illumination of a 269 nm lamp. c) Mechanism description of the detection of SCN^- and ClO^- by Fe@CR@EuNPs [Cited from reference (Lihua Zhi et al., 2015)].

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3.4 Nano copper sensor (CuNSs)

The fabrication of metallic nanoparticles, such as Au, Ag, and Cu, possessing localized surface plasmon resonance (LSPR) properties, has garnered significant attention across various applications and fields. The unique performance, efficiency, and optical characteristics of these metals make them highly sought-after candidates in areas such as biotechnology, medical treatments, catalysts, agriculture, optical devices, and sensing applications (Easow et al., 2016). Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) have played crucial roles as effective catalysts in numerous biological applications and colorimetric detection of various pollutants since their discovery. However, due to their limited availability and high costs, researchers have explored alternative metals that could offer better effectiveness at a lower price, such as coppe (Guo et al., 2016). The structural properties of nanocopper, including its d orbitals, exhibit excellent physicochemical behavior at the scale of 1–100 nm. Another advantage is that copper precursors are widely available, inexpensive, and possess optical properties that can be comparable to those of gold and silver (Mohindroo et al., 2016). However, a significant challenge in using copper nanoparticles as sensors is their susceptibility to oxidation in ambient air. Consequently, preventing oxidation and enhancing their stability has become a major research focus. Different molecules, such as CTAB, amino acids, and proteins, have been employed as reducing and capping agents to produce stable and effective Cu nanosensors (Hatamie et al., 2014; Soomro et al., 2014). Despite achieving distinctive and applicable Cu nanosensors, these methods often utilize expensive chemical stabilizing agents, and in some cases, highly toxic surfactants are employed. Recently, phytochemicals have garnered attention from researchers due to their availability, safety, affordability, rich bioactive components, and high ratio of active functional groups (Samie Yaseen Sharaf Zeebaree, 2019). Tree gum, which exhibits versatile properties, can be utilized in various applications and fields such as medicine, food protection, and industry.

Plant bio-compositions are considered sustainable, environmentally friendly, and efficient stabilizing reagents for nanoparticle synthesis. In this context, Samie Zeebaree and Aymn Zeebaree investigated the biosynthesis of a novel type of nanosized CuNPs using almond gum extract as a clean, suitable, sustainable, and stabilizing agent (Zeebaree et al., 2022). The process of preparing the green nanosensor involved the utilization of a constant volume of gum extract, copper salt solution, a weak POH medium, and continuous stirring with heating for 30 min. The sustainable synthesis of CuNPs was analyzed and characterized using UV–Vis spectroscopy, XPS study, XRD measurement, FESEM, HR-TEM, and EDX analysis. The almond gum-prepared nanoparticles exhibited uniform spherical shapes with an average size of 33 nm. The authors confirmed the formation of nanoparticles through the observation of a red solution. The colorimetric assay efficiency of AG@CuNPs for the detection of SCN ions was monitored using UV–Vis spectroscopy. The green CuNPs sensor exhibited a significant color change from red to yellowish, indicating a strong interaction between Cu(I) and SCN(I) ions and the formation of the CuSCN phase (Fig. 9). The formation of CuSCN was further confirmed by XPS and XRD studies. The AG@CuNPs sensor demonstrated high detection performance for thiocyanate ions within 5 min, with a low limit of detection (0.225 mg/L) and without interference from other cations or anions.

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Fig. 9

Formation steps of AG@CuNPs sensor and their mechanism in detection of thiocyanate ions [Cited from reference (Zeebaree et al., 2022)].

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