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. 2021 May 21;6(22):14379–14393. doi: 10.1021/acsomega.1c01242

A Fluorescence Switching Sensor for Sensitive and Selective Detections of Cyanide and Ferricyanide Using Mercuric Cation-Graphene Quantum Dots

Niradchada Kongsanan †, Nipaporn Pimsin †, Chayanee Keawprom †, Phitchan Sricharoen ‡, Yonrapach Areerob §, Prawit Nuengmatcha ∥, Won-Chun Oh ⊥, Saksit Chanthai †,*, Nunticha Limchoowong #,*
PMCID: PMC8190883  PMID: 34124460

Abstract

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This study aims to use graphene quantum dots (GQDs) as a fluorescence switching sensor (turn on–off) for the simultaneous detection of cyanide (CN–) and ferricyanide [Fe(CN)6]3– in wastewater samples. The GQDs were synthesized by pyrolyzing solid citric acid. The intrinsic blue color of the solution was observed under ultraviolet irradiation. The fluorescence spectrum was maximized at both excitation and emission wavelengths of 370 and 460 nm, respectively. The fluorescence intensity of GQDs decorated with Hg2+ (turn-off mode as the starting baseline) could be selectively turned on in the presence of CN– and once back to turn-off mode by [Fe(CN)6]3–. The fluorescence switching properties were used to develop a fluorescence turn-on–off sensor that could be used to detect trace amounts of CN– and [Fe(CN)6]3– in water samples. For highly sensitive detection under optimum conditions (Britton–Robinson buffer solution in the pH range of 8.0–9.0, linearity ranges of 5.0–15.0 μM (R2 = 0.9976) and 10.0–50.0 μM (R2 = 0.9994), respectively, and detection limits of 3.10 and 9.48 μM, respectively), good recoveries in the ranges of 85.89–112.66% and 84.88–113.92% for CN– and [Fe(CN)6]3–, respectively, were recorded. The developed methods were successfully used for the simultaneous and selective detection of CN– and [Fe(CN)6]3– in wastewater samples obtained from local municipal water reservoirs.

1. Introduction

Cations and anions are ubiquitous in nature and play a critical role in daily life. They also find their utility in the industrial, biological, and environmental systems.1 The absorption of ions through the skin, gastrointestinal tract, and lungs results in health hazards. The side effects include vomiting, convulsions, loss of consciousness, anesthetization of the central nervous system, and inhibition of the aerobic respiration of cells. These can potentially cause dyspnea and eventual death.2−5 The high toxicity of CN– is caused by its binding with iron present in cytochrome c oxidase. The interaction between CN– and iron in cytochrome c oxidase can prevent the transfer of electrons from cytochrome c oxidase to oxygen, which results in hypoxia.6 However, CN– also plays a critical role in multifunctional reactions in many industrial processes such as chemical synthesis, metallurgy, electroplating, plastic production, and extraction of gold and silver. As a result, CN– is released into the environment.7 According to the World Health Organization (WHO), the permitted concentration of CN– in drinking water should be lower than 1.9 μM. A concentration of CN– in the range of 0.5–3.5 mg/kg body weight is lethal to human beings.8−10

It is known that transition metal cations are very dangerous. In the following sections, the role of ferric ions (Fe3+) has been discussed. The Fe3+ ion is one of the most abundant elements. This transition metal ion plays an important role in maintaining the proper function of the environmental and biological systems and is an indispensable trace element present in living organisms. It plays an important role in many physiological processes such as DNA synthesis, hemoglobin generation, oxygen transport, and storage. It also helps coordinate brain activity, etc.11,12 It is important to maintain the proper concentration of Fe3+ in the human body. Fe3+ is incorporated into the daily diet for the growth and development of the body.13 Fe3+ ions mostly accumulate within the liver, spleen, and bone marrow cells. They can also bind to ferritin. However, both its deficiency and overload can potentially cause serious disorders and diseases in humans.14 The deficiency of Fe3+ can induce iron deficiency anemia (IDA) and reduce the activity of the iron enzyme in the body, resulting in metabolic disorders and diabetes. It also damages the liver and kidney. An excessive amount of Fe3+ in the human body can also cause hepatic and renal injury. It can also result in Parkinson’s and Alzheimer’s diseases. Iron deficiency is attributed to coronary heart disease, cancer, immune system dysfunction, and diabetes.15−17 Moreover, in the field of environmental quality assessment, the content of Fe3+ is an important indicator of water quality as the iron ions are present in their stable forms in solution (present as Fe2+, Fe3+, complex anions, and/or other organo-iron complexes). However, the US Environmental Protection Agency has reported that the limit of detection was lower than the maximum level (0.3 mg/L, equivalent to 5.4 μmol/L) of Fe3+ permitted in drinking water.18 If Fe3+ is intentionally used as ferricyanide ([Fe(CN)6]3–), then it can be found in water reservoirs as a highly stable complex is formed. Substances toxic to human beings and/or the environment and physiological systems are formed. Therefore, it is important to develop a simple, fast, and convenient method for the simultaneous detection of ultra-trace levels of CN– and [Fe(CN)6]3– ions in such an aqueous environment.

Fluorescence analysis is a method where fluorescence probes are used to measure the changes in fluorescence intensity, life, and anisotropy of fluorescent materials. In situ and quantitative detection of the target pollutants helps in rapid and highly sensitive detection.19,20 The process is technically simple. Graphene quantum dots (GQDs) are carbon nanoparticles with a diameter of <10 nm and are emerging as fluorescence materials.21,22 They can be fabricated as single-, double-, or multi-layered compounds (3 to <10). They also exhibit luminescence properties. These are cost-efficient and exhibit good optical stability, low toxicity, good biocompatibility, and unique electronic properties.23 To achieve tunable fluorescence emissions, good photostabilities, favorable biocompatibilities, easy functionalization, and robust chemical inertness, it can also be used as an electron acceptor or donor.24−26 Some reports have revealed that the fluorescence intensity of GQDs can be quenched by Hg2+ and Fe3+ ions.27,28 The detection process of turn-off sensors involves the interference of background fluorescence and is inconveniently applied down to the sensing field. The turn-on sensor is superior to the turn-off sensor. It is more valuable and can find its applications in various fields.29−31

In this study, a simple and low-cost synthetic strategy to prepare Hg2+-quenched graphene quantum dots (Hg2+-GQDs) as a fluorescence turn-on–off sensor has been reported that can be used for the simultaneous selective detection of CN– and [Fe(CN)6]3– ions. The complex [Fe(CN)6]3– is a complex ligand involved in food anticaking. The uncoordinated nitrogen atoms of the CN– groups can act as efficient hydrogen bond acceptors and are considered as chemically “evergreen”, which is also true for Berlin Blue.32,33 Generally, GQDs emit a strong fluorescence in an aqueous solution. Hg2+ ions could significantly quench the fluorescence intensity of the GQDs. When CN– ions were added to the Hg2+-GQD solution, they could selectively interact with Hg2+. This can potentially result in the destruction of the Hg2+-GQD complex, and the turn-on fluorescence mode can be recovered. However, the turn-off fluorescence mode could be re-established when [Fe(CN)6]3– was selectively added to the test solution. The fluorescence intensity of the Hg2+-GQDs was linearly related to their concentration of the CN– and [Fe(CN)6]3– ions. The fluorescence switching sensor can be potentially used for the simultaneous and selective determination of CN– and [Fe(CN)6]3– ions with different sensitivity ranges in real water samples.

2. Results and Discussion

2.1. Characterization of GQDs

The ATR-FTIR spectra of GQDs are shown in Figure 1. The bands corresponding to the stretching vibration of the hydroxyl groups were observed at approximately 3338 cm–1. The C=C stretching bands of the polycyclic aromatic groups appeared at 1554 cm–1,34,35 and the C–O stretching bands (COH/COC (epoxy) groups) appeared in the range of 1377–1107 cm–1.36 The results revealed that the GQDs could be successfully synthesized by pyrolyzing citric acid that provided the structure identity of graphene.

Figure 1.

Figure 1

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FTIR spectrum of GQDs.

The obtained sample was characterized using the transmission electron microscopy (TEM) technique to determine the particle size of the GQDs. Figure 2 reveals that the GQDs exhibit a uniform and consistent spherical shape (diameter, 7.12 ± 3.8 nm). The nano GQDs exhibited characteristic fluorescence properties.

Figure 2.

Figure 2

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TEM image of GQDs.

The elemental composition of the samples was determined using the energy-dispersive X-ray spectroscopy (EDX) technique (Figure 3). The compositional elements were determined (34.1% O, 31.2% C, and 34.7% Na). Their peaks (GDQ samples) were commonly found in association with C (0.277 keV) and O (0.525 keV). The presence of the Na peak could be attributed to the use of the NaOH solution as the GQD stabilizer.

Figure 3.

Figure 3

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EDX spectrum of GQDs.

The XPS technique was used to determine the detailed chemical composition and the chemical bond environment of the GQD samples (Figure 4). In most of the cases, GQDs are composed of C, O, Na, and H (Figure 4a). The high-resolution spectrum of C 1s (Figure 4b) could be deconvoluted into three surface components, corresponding to sp2 C (C=C/C–C) at a binding energy of 284.8 eV, sp3 C (C–OH) at a binding energy of 286.2 eV, and C=O at a binding energy of 288.3 eV. The presence of the sp3 C peak in the XPS spectral profile suggests a significant reduction in the planarity of the GQDs and the appearance of defects in the prepared GQDs. The high-resolution spectrum of O 1s (Figure 4c) confirmed the presence of the C=O (531.1 eV), C–OH (532.3 eV), and O=C–OH (535.7 eV) bonds. These results suggest that the prepared GQDs are rich in oxygen-containing functional groups. The C/O ratio in GQDs, calculated from XPS spectra, was 51.51/48.49.

Figure 4.

Figure 4

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(a) XPS survey spectrum of GQDs, (b) high-resolution C 1s XPS spectra of GQDs, and (c) high-resolution O 1s XPS spectra of GQDs.

2.2. Spectral Properties of GQDs

The UV–visible absorption spectral profiles of the GQD samples are shown in Figure 5. A typical absorption band at 365 nm can be observed, which can be ascribed to the π–π* transition in the graphitic sp2 domains.37 The fluorescence spectrum of the GQDs shows a strong emission peak at 460 nm when excited at 370 nm. The full width at half maximum (FWHM) was at 100 nm, which resembles that of most of the graphene quantum dots.30 The fluorescence quantum yield of GQDs can also be determined by comparing the data with those obtained using quinine sulfate (quantum yield = 0.54 at 360 nm) as a reference. The quantum yield of the synthetic GQDs was calculated to be approximately 28.7%. It is calculated using the slope of the regression line generated by plotting the integrated fluorescence intensity at the emission wavelength of 460 nm versus the UV–visible absorption at 370 nm for multiple concentrations of the GQDs and quinine sulfate solutions. The relative Q (%) was calculated as follows

2.2. 1

where Q is the quantum yield, m is the slope of the plot of integrated fluorescence intensity vs absorbance, and n is the refractive index (taken here as 1.33, assuming both solutions exhibit the same refractive index of distilled water). The subscripts S and R refer to the GQD and reference fluorophore (quinine sulfate) solutions, respectively.38

Figure 5.

Figure 5

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UV–visible absorption spectrum and FL excitation and emission spectra of the GQDs.

2.3. Linear Quenching Effect Exhibited by GQDs in the Presence of Hg2+

The fluorescence intensity of the GQDs was quenched using various concentrations of Hg2+ to adjust the signal window of the Hg2+-based GQD system. Figure 6a shows the quenching effect of Hg2+ concentration on the fluorescence intensity of GQDs. The fluorescence intensity decreases with increasing Hg2+ concentration. A linear correlation was obtained in the range of 0–40 μM when the maximum fluorescence intensity at 460 nm was plotted against the Hg2+ concentration (Figure 6b). The calibration curve was plotted between F0 – F and the concentration of Hg2+ using 20 μM Hg2+. The fluorescence intensity of the GQDs could be quenched with a relative quenching effect of 88.75%. Therefore, this concentration of Hg2+ was used for further experiments. The Hg2+ quenching effect was also studied using the Stern–Volmer equation36 (Figure 6c). The fluorescence intensities of the GQD samples were almost quenched and could be turned on. The suitable concentration of Hg2+ was found to be 30 μM.

Figure 6.

Figure 6

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(a) FL in the emission spectra of the GQDs at different Hg2+ concentrations (0–40 μM), (b) FL in the emission intensity at 460 nm versus the Hg2+ concentration in 0.05 M Britton–Robinson buffer (pH 8), and (c) linear relationship of F0 – F versus the concentration of Hg2+ over the range 0–20 μM.

2.4. Optimum Conditions for the Use of Hg2+-GQD Fluorescence Sensors for the Selective Detection of CN– and [Fe(CN)6]3–

A water sample was used as a model to simplify the interactions present in the system when Hg2+-GQDs were used as the substrate sensors to simultaneously detect CN– and [Fe(CN)6]3– present in the same sample. A graphical scheme for the fluorescence turn-on–off sensor for the detection of CN– and [Fe(CN)6]3– using Hg2+-GQDs as the background substrate is shown in Scheme 1. Generally, the GQDs exhibit significantly strong blue fluorescence in a basic solution. Following the addition of Hg2+, the fluorescence of GQDs gets quenched. The ionic interactions present in Hg2+-GQDs might be reversible as labile Hg2+ ions are complexed with surface functional groups in GQDs. The fluorescence intensities of GQDs will be self-recovered, which can be used to quantify CN– as Hg2+-GQDs can easily interact with the ligand CN– to form Hg(CN)2 in solution. The fluorescence intensity of the GQDs could be recovered. Similarly, when [Fe(CN)6]3– was added into a solution of Hg2+-GQDs in the presence of CN–, the fluorescence intensity of the GQDs was quenched as the [Fe(CN)6]3– ions could selectively interact with the GQDs (common ion effect of the ferricyanide anion) under the optimal conditions. The experimental conditions, including solution pH, reaction time, sonication time, and concentrations of both ionic strength and masking agents were optimized, to improve the CN– and [Fe(CN)6]3– detection performance.

Scheme 1. A Graphical Scheme for the Fluorescence Turn-On–Off Sensor of CN– and [Fe(CN)6]3– Detection Using Hg2+-GQDs.

Scheme 1

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2.4.1. Effect of Solution pH

The effect of pH on the fluorescence intensity was investigated as pH-sensitive functional groups were present in GQDs (Figure 7). A series of Britton–Robinson buffer solutions (each 0.05 M) at different pH values were prepared, and the GQD solution together with the Hg2+, CN–, and [Fe(CN)6]3– was added successively to each of the buffer solutions (pH range, 2.0–12.0). The fluorescence intensity of the GQDs at 460 nm was recorded using the buffer solutions. The fluorescence intensity of the GQDs increased sharply within the pH range of 3.0–5.0. Following this, the intensity stabilized under alkaline conditions in the pH range of 6.0–12.0. This implies that the total turn-off–on–off of the Hg2+, CN–, and [Fe(CN)6]3– would be at pH > 6.0. However, it was found that the pH of Hg2+-GQDs + CN– was stabilized in the pH range of 6–9. Thus, the stability of the GQDs was sensitive to solution pH that was set in the range of 8.0–9.0 (Figure 7c).

Figure 7.

Figure 7

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Effect of pH on (a) GQDs, (b) Hg2+-GQDs, (c) Hg2+-GQDs + CN–, and (d) Hg2+-GQDs + CN– + [Fe(CN)6]3–.

2.4.2. Effect of Sonication Time and Reaction Time

Figure 8 reveals the sonication time after adding CN– to Hg2+-GQDs. The fluorescence intensity recorded at 460 nm was plotted against the sonication time, as shown in Figure 8c. After 5 min, the turn-on signal of CN– decreased, following which it increased rapidly and remained steady for 30 min. However, in the absence of sonication, the signal remained constant for another 30 min. The effect of reaction time was also studied, as shown in Figure 9. It was observed that when CN– or [Fe(CN)6]3– was added to Hg2+-GQDs (Figure 9c), the turn-on mode of CN– increased rapidly and remained constant for 5 min. The turn-off mode of [Fe(CN)6]3– decreased and then remained constant for 5 min (Figure 9d). The results revealed that the reaction time of the turn-on–off sensor was approximately 5 min.

Figure 8.

Figure 8

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Effect of sonication time on the fluorescence intensity of the Hg2+-GQDs by CN– and [Fe(CN)6]3– ions in 0.05 M Britton–Robinson buffer (pH 8): (a) GQDs, (b) Hg2+-GQDs, (c) Hg2+-GQDs + CN–, and (d) Hg2+-GQDs + CN– + [Fe(CN)6]3–.

Figure 9.

Figure 9

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Effect of reaction time on the fluorescence intensity of the Hg2+-GQDs by CN– and [Fe(CN)6]3– ions in 0.05 M Britton–Robinson buffer (pH 8): (a) GQDs, (b) Hg2+-GQDs, (c) Hg2+-GQDs + CN–, and (d) Hg2+-GQDs + CN– + [Fe(CN)6]3–.

2.4.3. Effects of the Concentration of NaCl in Adjusting the Ionic Strength and EDTA as a Masking Agent

The effect of NaCl (concentrations: 0.05, 0.10, 0.15, 0.20, and 0.25 M) as an ionic strength adjuster was investigated, as shown in Figure 10. The effect of EDTA (concentrations: 0.01, 0.02, 0.03, 0.04, and 0.05 M) as a masking agent was also studied, as shown in Figure 11. It was observed that the ionic strength did not affect the fluorescence intensity of the Hg2+-GQDs. This is another aim to provide a strong electrolyte to the reactive species of the mixed nanoparticle solution, which could be done in conjunction with the addition of EDTA to mask the interference of some metal ions present in wastewater samples. However, the EDTA masking agent had an effect on the fluorescence intensity of the Hg2+-GQDs. When EDTA solution was added, the fluorescence intensity increased rapidly, and then, its slope decreased slowly when the concentration of the EDTA solution reached 0.01 M. Therefore, in the process of decorating the surface of the GQDs with Hg2+, EDTA should be absent in the matrix system. Other interfering metal ions in water samples should be masked with EDTA prior to conducting the tests with the Hg2+-GQD sensor.

Figure 10.

Figure 10

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Effect of concentration of NaCl as an ionic strength adjuster: (a) GQDs, (b) Hg2+-GQDs, (c) Hg2+-GQDs + CN–, and (d) Hg2+-GQDs + CN– + [Fe(CN)6]3–.

Figure 11.

Figure 11

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Effect of concentration of EDTA as a masking agent: (a) GQDs, (b) Hg2+-GQDs, (c) Hg2+-GQDs + CN–, and (d) Hg2+-GQDs + CN– + [Fe(CN)6]3–.

2.4.4. Detection of CN– Using Hg2+-GQDs as a Fluorescence Turn-On Sensor

The Hg2+-GQD solutions were added successively to varying concentrations of CN– (0–40 μM). At first, the quenching of GQD fluorescence was observed due to the presence of the Hg2+ ions present on the surface of the GQDs. Upon the addition of CN–, it would react with the Hg2+ in the GQD solution, resulting in the increase in the fluorescence intensity of Hg2+-GQDs with the increase in the CN– concentration, as shown in Figure 12a. The fluorescence intensities of each solution were recorded, and a linear relationship was obtained in the concentration range of 5–40 μM, when the maximum fluorescence intensity at 460 nm was plotted against the CN– concentration, as shown in Figure 12b. The calibration curve was plotted between F – F0 and the concentration of CN– (μM) (Figure 12c). It was noted that the slope in the CN– concentration range of 5–15 μM significantly increased with an increase in the cyanide concentration. Following this, the intensity remained constant. An ionic interaction between Hg2+ and CN– was expected, resulting in the formation of Hg(CN)2. In most of the cases, cyanide–mercury can form different complexes such as Hg(CN)2, Hg(CN)3–, and Hg(CN)24–. In the pH range of 9–14, mercury is present in the form of the Hg(CN)2 complex.39

Figure 12.

Figure 12

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(a) FL in the emission spectra of the Hg2+-GQDs at different CN– concentrations (0–40 μM), (b) FL in the emission intensity at 460 nm versus CN– concentration in 0.05 M Britton–Robinson buffer (pH 8), and (c) linear relationship of F – F0 versus the concentration of CN– over the range 5–15 μM.

To study the selectivity of the Hg2+-GQD sensor for CN– detection, the intensity ratios of F/F0 for the Hg2+-GQDs in the presence of various anions, including iodide (I–), chloride (Cl–), bromide (Br–), hydroxide (OH–), thiocyanate (SCN–), acetate (CH3COO–), nitrate (NO3–), iodate (IO3–), cyanate (CN–), carbonate (CO32–), and sulfate (SO42–), were obtained, as shown in Figure 13. The concentration of each anion was 100 μM, the same as that of the CN– in the assay solution. The addition of CN– to those solutions resulted in an apparent recovery of the fluorescence intensity (turn-on mode), whereas the other remaining anions exerted no effect under the same optimal conditions.

Figure 13.

Figure 13

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Selectivity of the Hg2+-GQD fluorescent sensor for various anions (100 μM) in the absence of some anions in 0.05 M Britton–Robinson buffer (pH 8).

2.4.5. Detection of [Fe(CN)6]3– Using Hg2+-GQDs-CN– as the Fluorescence Turn-Off Sensor

It was found that when [Fe(CN)6]3– was added into the mixed solution containing Hg2+-GQDs and CN–, the fluorescence intensity decreased (turn-off), as shown in Figure 14a,b. The calibration curve was plotted between F0 – F and the concentration of [Fe(CN)6]3– (μM), as shown in Figure 14c. The slope in the concentration range of 10–50 μM [Fe(CN)6]3– significantly decreased with the increase in the concentration of [Fe(CN)6]3–. Following this, the slope gradually decreased and remained constant. [Fe(CN)6]3– is capable of generating higher oxidation states of iron via the Fenton reaction. Ferricyanide is slowly reduced under a dry air atmosphere, forming cyanogen and ferricyanide in small amounts, according to reactions 2–440

2.4.5. 2
2.4.5. 3
2.4.5. 4
Figure 14.

Figure 14

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(a) FL in the emission spectra of the Hg2+-GQDs at different [Fe(CN)6]3– concentrations (0–400 μM), (b) FL in the emission intensity at 460 nm versus [Fe(CN)6]3– concentration in 0.05 M Britton–Robinson buffer (pH 8), and (c) linear relationship of F0 – F versus the concentration of [Fe(CN)6]3– over the range 10–50 μM.

The fluorescence intensities of the GQDs in the presence of various metal ions, such as Hg2+, Fe3+, Mg2+, Ni2+, Mn2+, Co2+, Cu2+, Zn2+, Cr3+, Cd2+, Pb2+, and Fe2+, were investigated. The values of F/F0 were plotted using 100 μM of each metal. Figure 15 shows that the maximum extent of fluorescence quenching was observed when Hg2+ (decorating the surface of GQDs) was added. Similar observations were made when Fe3+ (Fe3+ from [Fe(CN)6]3–) was added to the reaction mixture containing the Hg2+-quenched GQDs. The other cations had no significant effect under the same experimental conditions.

Figure 15.

Figure 15

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Quenching of the fluorescence intensity of the GQDs by different metal ions. All the ions were at 100 μM concentration. F and F0 are the emission intensities of the GQDs at 460 nm either in the presence or in the absence of some metal ions in 0.05 M Britton–Robinson buffer (pH 8).

2.5. Method Validation

As shown in Table 1, the analytical characteristics of the developed method were validated under the optimized conditions in terms of linearity, the limit of detection (LOD), the limit of quantification (LOQ), and precision (expressed as the relative standard deviation (%RSD) of the calibration slope obtained from both intraday and interday analysis) to estimate the efficiency and feasibility of the method for use with water samples. The linearity ranges were found to be 5–15 μM (R2 = 0.9976) and 10–50 μM (R2 = 0.9994) for CN– and [Fe(CN)6]3–, respectively. The linear calibration graphs are as follows: y = 28.047x – 68.911 and y = 3.4616x + 53.658 (where y is the fluorescence intensity and x is the concentration of CN– and [Fe(CN)6]3–). The limit of detection (LOD) is defined as 3SD/m (where SD is the standard deviation of a very low concentration of CN– or [Fe(CN)6]3– and m is the slope of the calibration graph), and the values were 3.10 and 9.48 μM for CN– and [Fe(CN)6]3–, respectively. The limit of quantification (LOQ) is defined as 5SD/m, and the values were 5.17 and 15.81 μM for CN– and [Fe(CN)6]3–, respectively. The precision values, which were evaluated in terms of repeatability (intraday precision, n = 3 × 3), were 0.47 and 1.94% for CN– and [Fe(CN)6]3–, respectively, and the reproducibilities (work performed during 5 × 3 consecutive days, interday RSD) were 4.47 and 9.72% for CN– and [Fe(CN)6]3–, respectively. The values demonstrated an acceptable reproducibility of the developed method.

Table 1. Analytical Characteristics of the Hg2+-GQD System for the Simultaneous Determination of CN– and [Fe(CN)6]3–

  analytical
analytical parameter CN– [Fe(CN)6]3–
linear range (μM) 5–15 10–50
linear equation y = 28.047x – 68.911 y = 3.4616x + 53.658
correlation coefficient (R2) 0.9976 0.9994
limit of detection (μM, n = 11) 3.10 9.48
limit of quantification (μM, n = 11) 5.17 15.81
relative standard deviation (%) for intraday analysis (n = 9) 0.47 1.94
relative standard deviation (%) for interday analysis (n = 11) 4.47 9.72
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2.6. Real Sample Analysis

Both CN– and [Fe(CN)6]3– were determined in wastewater samples collected from municipal water reservoirs in Khon Kaen City. To demonstrate the applicability and reliability of the developed method, it was successfully used to analyze the natural water samples under optimum conditions. The contents of CN– and [Fe(CN)6]3– in these samples were determined (Table 2). The results showed that trace amounts of CN– and [Fe(CN)6]3– were not detectable in the water samples under study. The accuracy of the developed method was verified by calculating the recovery in the real samples to evaluate the matrix effect. Each sample was spiked with three concentrations (5.0, 50.0, and 100.0 μM) of the standard solution of CN– and with three concentrations (5.0, 50.0, and 250.0 μM) of the standard solution of [Fe(CN)6]3–. The relative percentage recoveries were calculated as follows41

2.6. 5

where Cfound, Creal, and Cadded are the concentration of an analyte after addition of a known amount of a standard in the real sample, the concentration of an analyte in the real sample, and the concentration of a known amount of a standard that was spiked in the real sample, respectively. The recoveries were expressed as the mean values of three independent determinations. The recovery ranged from 85.89 to 112.66% and 84.88 to 113.92% for CN– and [Fe(CN)6]3–, respectively, indicating that they were free of matrix interferences. The methods provide acceptable recovery, demonstrating that they can be efficiently used for CN– and [Fe(CN)6]3– determination in the real samples. The results using the developed method were also compared with previously reported results, as shown in Table 3.42−48

Table 2. The Contents and Their Recoveries of CN– and [Fe(CN)6]3– in Water Samples Using the Hg2+-GQD System (n = 3).

  CN–
 [Fe(CN)6]3–
water sample added (μM) found (μM) recovery (%) ± SD added (μM) found (μM) recovery (%) ± SD
drinking water brand 1 5.0 4.29 85.89 ± 0.32 5.0 5.10 102.09 ± 2.52
  50.0 47.32 94.64 ± 0.47 50.0 50.33 100.65 ± 1.64
  100.0 95.52 95.52 ± 0.64 250.0 248.35 99.34 ± 2.18
drinking water brand 2 5.0 4.83 96.64 ± 0.96 5.0 5.20 104.08 ± 3.11
  50.0 53.18 106.36 ± 1.09 50.0 50.89 101.78 ± 2.32
  100.0 112.66 112.66 ± 2.0 250.0 245.18 98.07 ± 0.84
drinking water brand 3 5.0 4.62 92.42 ± 2.38 5.0 4.32 86.33 ± 2.47
  50.0 52.34 104.68 ± 1.22 50.0 44.52 89.04 ± 2.32
  100.0 106.36 106.36 ± 0.95 250.0 232.07 92.83 ± 1.46
drinking water brand 4 5.0 4.73 94.89 ± 1.19 5.0 5.44 108.72 ± 2.34
  50.0 51.12 102.23 ± 2.48 50.0 55.23 110.46 ± 1.43
  100.0 104.42 104.42 ± 2.14 250.0 258.15 103.26 ± 1.66
drinking water brand 5 5.0 4.78 95.67 ± 1.21 5.0 4.31 86.27 ± 2.42
  50.0 52.21 104.42 ± 2.11 50.0 42.44 84.88 ± 1.37
  100.0 101.43 101.43 ± 1.22 250.0 225.92 90.37 ± 2.06
tap water 1 5.0 4.42 88.49 ± 2.37 5.0 4.71 94.21 ± 6.72
  50.0 52.45 104.90 ± 1.76 50.0 50.36 100.71 ± 1.74
  100.0 109.86 109.86 ± 2.55 250.0 242.20 96.88 ± 1.35
tap water 2 5.0 4.98 99.64 ± 2.54 5.0 5.37 107.34 ± 2.56
  50.0 52.12 104.32 ± 2.51 50.0 55.36 110.72 ± 2.34
  100.0 110.02 110.02 ± 2.49 250.0 264.95 105.98 ± 2.11
tap water 3 5.0 4.75 94.97 ± 2.14 5.0 5.70 113.92 ± 6.38
  50.0 51.82 103.63 ± 1.27 50.0 55.17 110.34 ± 2.49
  100.0 107.48 107.48 ± 1.30 250.0 269.20 107.68 ± 2.85
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Table 3. Comparison of Different Methods for the Detection of CN– and [Fe(CN)6]3– Ions.

carbon-based material analytes LOD (μM) reference
azoic dye with benzophenone cyanide 20 (42)
C-dots ferric 9.97 (43)
PVA-containing azoic dye with trifluoroacetyl cyanide 10 (44)
SBA-15-DNPH ferric 39 (45)
Rhodamine-based arylpropenone azo dyes ferric 0.91 (46)
N-acetylamino aldehyde hydrazone azo dye cyanide 1.29 (47)
CDs ferric 10.98 (48)
Hg2+-GQDs cyanide 3.10 this work
  ferricyanide 9.48  
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3. Conclusions

In summary, we reported a new method for the fabrication of a fluorescence turn-on–off sensor using the Hg2+-quenched GQDs for the simultaneous determination of CN– and [Fe(CN)6]3– present in wastewater samples obtained from municipal water reservoirs. The advantage of the proposed method is its simplicity. The proposed method can be used for the rapid detection of both anions present in an aqueous solution. Interestingly, the Hg2+-GQD sensor could selectively detect [Fe(CN)6]3– in the presence of the acceptable levels of CN– under the optimum conditions. This detection procedure was not affected by other interfering ions. The Hg2+-GQD fluorescence sensor was highly sensitive with limits of detection of 3.10 and 9.48 μM for CN– and [Fe(CN)6]3–, respectively. The fluorescence switching sensor can be effectively used for the simultaneous detection of trace amounts of CN– and [Fe(CN)6]3– in real water samples.

4. Materials and Methods

4.1. Chemicals

All of the chemicals used were of analytical grade. Citric acid, mercury chloride, potassium cyanide, and boric acid were obtained from Qrec (New Zealand). Potassium ferricyanide was purchased from BOH Chemicals Ltd. (England). Sodium hydroxide was obtained from Carlo Erba (Italy). Phosphoric acid and acetic acid were purchased from RCI Labscan Limited (Thailand). Ethylene diaminetetraacetic acid was purchased from UNIVAR (Australia). Paraffin oil was purchased from Ajex Finechem (Australia). Deionized water (Simplicity Water Purification System, Model Simplicity 185, Millipore, U.S.A.) was used for all the experiments.

4.2. Instruments and Apparatuses

A spectrofluorophotometer (Shimadzu RF-5301PC, Japan) with an excitation and emission slit width of 5 nm was primarily used to conduct the experiments. A quartz cuvette (1 cm path length) was used in both the UV–visible absorption and fluorescence experiments. UV–visible absorption spectra were obtained using an Agilent 8453 spectrophotometer (Agilent, Germany). A pH meter (Model Proline B21, Becthai Equipment & Chemical, Thailand) and an analytical balance (Model LX 220A, Precisa, Thailand) were also used. A round-bottom flask (Pyrex, England), a hot plate equipped with a magnetic stirrer, and a paraffin oil bath were used to pyrolyze citric acid.

4.3. Preparation of GQDs and Hg2+-Quenched GQDs

Graphene quantum dots (GQDs) were prepared following the citric acid pyrolysis method.49 Citric acid (1.0 g) was transferred to a 100 mL round-bottom flask, which was heated at 220 °C for approximately 5 min using a paraffin oil bath. The solid citric acid was liquated, and its color changed to yellow. Following this, 50 mL of NaOH (0.25 M) was added to the round-bottom flask, and the mixture was continuously stirred for 30 min at room temperature. The obtained solution of GQDs was refrigerated (4 °C) for further experiments.

A fresh solution of Hg2+-quenched GQDs was also prepared. One milliliter of approximately 1.0 mg mL–1 GQD solution and Britton–Robinson buffer (0.05 mol L–1, pH 8) were mixed in a 10 mL volumetric flask. Subsequently, various concentrations of Hg2+ solution were added to an aliquot of the buffered GQD solution (10 mL final volume) at room temperature. The Hg2+-quenched fluorescence spectrum of each GQD solution was recorded immediately at λex/λem = 370/460 nm. The spectral data were used to plot the quenching linear curve to determine the optimum condition for setting the turn-off baseline of the Hg2+-GQD sensor.

4.4. Simultaneous Detection and Method Validation of CN– and [Fe(CN)6]3–

Various concentrations of either CN– and/or [Fe(CN)6]3– ions and 30 μL of the Hg2+-quenched GQD solution were mixed with Britton–Robinson buffer solution (0.05 mol L–1) adjusted to various pH values (10 mL final volume) at room temperature. For the selective determination of CN– and [Fe(CN)6]3– ions, a suitable and known concentration of CN– solution (turn-on mode) was used to generate the linear curve of the [Fe(CN)6]3– solution before recording the fluorescence spectrum (turn-off mode) of each solution (at λex/λem = 370/460 nm). Such spectral measurements were used to plot the increasing calibration curve for CN– and the quenching calibration curve for [Fe(CN)6]3–.

The optimum conditions for the dual detection of CN– and [Fe(CN)6]3– were also investigated in detail. Moreover, to evaluate the performance of the developed method, the optimized conditions were validated in terms of linearity, limits of detection, limits of quantification, and precision (expressed as the relative standard deviation (%RSD) of the calibration slope obtained from intraday and interday analysis). Finally, the Hg2+-GQD-based switching sensor was used to detect trace amounts of CN– and [Fe(CN)6]3– in wastewater samples obtained from municipal water reservoirs in Khon Kaen, Thailand.

Acknowledgments

The authors thank the Materials Chemistry Research Center, Department of Chemistry, and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Khon Kaen University, the Department of Chemistry, Faculty of Science, Srinakharinwirot University, and the Thailand Institute of Nuclear Technology (Public Organization) for support.

The authors declare no competing financial interest.

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