Facile Synthesis of Fe-Doped Hydroxyapatite Nanoparticles from Waste Coal Ash: Fabrication of a Portable Sensor for the Sensitive and Selective Colorimetric Detection of Hydrogen Sulfide

In this work, a new strategy has been reported for the portable detection of H2S based on Fe-doped hydroxyapatite nanoparticles (Fe-HA) using a colorimetric paper test strip integrated with a smartphone platform. Fe-HA NPs were fabricated successfully via recycling waste coal ash. The obtained probe response toward H2S was through a distinct visual color change. The sensing mechanism is based on the displacement reaction, in which PO43- is replaced by S2-. The prepared test strip shows high selectivity, and the other compounds containing thiol and sulfur anion have a negligible effect on the detection of H2S. The designed scheme is applied for H2S detection in the concentration range of 0.5–130 ppm with a limit of detection of 70 ppb. Furthermore, such a disposable sensor was used as a practical system for monitoring H2S in actual water samples, suggesting the promising potential of this platform for suitable analysis of H2S in an aqueous environment.


■ INTRODUCTION
Owing to increasing population and lack of raw materials, numerous studies are acting on reusing industrial waste as a source for the synthesis of nanomaterials. 1,2 Coal ash is a major byproduct of coal combustion produced in large quantities during the energy production in the power plants. 3 This solid waste is mainly disposed of in lagoons and landfill, which lead to groundwater and soil pollution. 4 In recent years, numerous works have been conducted to recycle coal ash including manufacturing cement and ceramics, geopolymer production, zeolite synthesis, and stabilizing soils. 5−8 However, compared to the amount of waste ash generated, the recycling scale is still low. 8 Thus, it is required to find more ways to exploit the potential applications of this byproduct. Recently, many efforts have been made to synthesize diverse nanomaterials from fly ash such as iron oxide, 9 magnesium oxide, 10 mesoporous nanosilica, 11 and carbon nanofiber. 12 Abdelbasir et al. synthesized nanocuprous oxide from waste electric cables for the electrochemical detection of dopamine. 13 An electrochemical sensor was prepared by anchoring cuprous oxide nanoparticles to flexible graphene electrode. In another study, an aluminosilicate framework was prepared from agro-waste material and introduced as a substrate and hosting material for AuNi bimetallic nanoparticles. 14 The aluminosilicate framework possesses large porosity and high specific surface area for the construction of a suitable catalyst. The designed sensing material was efficiently used for non-enzymatic glucose detection. Green production methods of sensor fabrication with recovered waste material meet the urgent need for sustainable, low-cost, and facile sensors.
Biological reports have confirmed that H 2 S plays a significant role in health and disease by regulating physiological processes in the heart, liver, nervous system, and kidneys. 15−17 Therefore, abnormal level of H 2 S is associated with several illnesses including diabetes, hypertension cardiovascular diseases. 18−20 Accordingly, designing an effective sensor for sensitive and selective determination of H 2 S is critical. Numerous conventional methods such as chromatography, 21 electrochemistry, 22 fluorescence, 23 and colorimetric detection 24 have been built for H 2 S detection. Among them, the colorimetric sensor is powerful tool as a detection platform due to its high sensitivity, excellent selectivity, and simple design without requiring a power source. 25−27 Several colorimetric sensors have been employed for H 2 S analysis based on organic dyes and or plasmonic metal nanoparticles. 28,29 While these detection techniques are accurate and sensitive, but, the application of those methods is hindered because they mostly rely on liquid media. 30 Lead acetate paper is a commercial sensor widely used for the colorimetric detection of H 2 S. Upon the addition of H 2 S, the white paper turns black from the formation of PbS. 31,32 Over other analytical methods, paper test strips have rapidly developed in different areas because of low-cost and easy-touse analytical tools. 33 Although this commercial test strip is popular and readily available, it has various drawbacks including the toxicity of lead metal and high detection limit. 34 Hence, a sensitive and green probe for the colorimetric detection of H 2 S in the solid phase is highly desirable. Hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 , HA] is an excellent biocompatible and low toxic nanostructure and has gained greater attention for construction of sensors. For instance, HA has been used for the detection of glucose, 35 hydrogen peroxide, 36 carbon dioxide, 37 cyanide 38 and phenolic compounds. 39 However, HA has been not reported to date for the detection of H 2 S.
Here, a very simple method is reported for the synthesis of Fe-HA nanoparticles to be used in colorimetric test strips for smartphone-based detection of H 2 S. The application of smartphones as a portable color sensor for chemical analysis reduces the expense and simplifies the operation. 40 In the presence of H 2 S, the color of the paper strip turned green. This obvious color change could be easily observed by the naked eyes and quantitively monitored by the smartphone. The detection mechanism was proposed as a displacement reaction between PO 4 3− and S 2− . In contrast to the neurotoxic commercial lead (II) acetate-based test papers, this nanomaterial is used due to its non-toxic properties. The sensing properties of the paper strips were examined against increasing H 2 S concentration between 0.5 and 130 ppm with a LOD of 70 ppb. Furthermore, the experimental results of the selectivity test display that the colorimetric sensor is not affected by thiolcontaining interferences. Considering these characteristics, this proposed sensor a simple, portable, and inexpensive qualitative detector of H 2 S for practical applications.

Materials and Instruments.
Sodium sulfide hydrate (Na 2 S · xH 2 O), monosodium phosphate (NaH 2 PO 4 ), and sodium hydroxide (NaOH) were analytical grades purchased from Sigma-Aldrich. All solutions were supplied by deionized water from a Milli-Q Plus system (Millipore). Scanning electron microscopy (SEM) images were recorded on MIRA3 TESCAN HV: 20.0 kV (Czech Republic). X-ray diffraction (XRD) patterns were acquired by a Bruker D8 Advance diffractometer using a copper source and a general area detector diffraction system (GADDS) (Netherlands). X-ray photoelectron spectroscopy (XPS) was obtained by a Kratos AXIS Supra spectrometer equipped with a monochromatic Al K (alpha) source (15 mA, 15 kV). The FT-IR spectra were recorded by a SPECTROD 250-Analytik Jena spectrophotometer (Germany).

Synthesis of Fe-HA Nanoparticles.
The coal ashes received from the power plant were used as a precursor for the preparation of Fe-HA NPs (Scheme 1a). Ca 2+ and (PO 4 ) 3− ions arranged around the OH− ion in a hexagonal crystal structure. 10 calcium cations decorated in two non-equivalent positions with relative abundance of Ca(I)/Ca(II) = 2/3 and surrounded by phosphates. This arrangement in hexagonal unit cells makes HA a stable and flexible structure for cation substitution. 41 Iron-substituted HA was synthesized by a facile method at room temperature. First, 1.0 g of coal ash was dispersed in 3 mL of water and then 3 mL of HCl was added as the leaching agent. The coal ashes contain high concentrations of metallic cations, particularly Ca 2+ and Fe 3+ . The mixed slurry was stirred at 60°C for 1 h. After leaching, the sample was centrifuged and the liquid phase was separated. Here, 20 mL of NaH 2 PO 4 solution (0.5 M, pH = 10) was slowly added to the leachate taken to precipitate out the cations. After vigorous stirring for 15 min, the mixture was washed with deionized water several times. Finally, the precipitate product was dried and stored for further analysis.

Procedure for Colorimetric Studies.
The stock solution of Fe-HA NPs (3 mM) was prepared in deionized water. A sheet of filter paper was cut into 2 × 1 pieces, and each piece was immersed in Fe-HA aqueous solution. The colorimetric tests were performed by dropping different concentrations of Na 2 S aqueous solutions on paper strips. Afterward, a photograph was taken with the smartphone (Galaxy A31) and the green color intensity of each piece was measured using Color Picker application installed on the smartphone. The registered color intensity of papers was utilized to make the calibration curve which defines the analytical performance of the designed sensor. For real sample analysis, the water samples of the tap water and river used to prepare Na 2 S solution. After filtering, samples were spiked with different concentrations of Na 2 S solution. The color intensity of all samples was recorded, and concentrations were calculated using the standard curve method.

Characterization of Fe-HA NPs.
To identify the structure of the Fe-HA nanocomposites, the SEM image is presented in Figure 1 A, B. The EDS analysis presented the atomic compositions of the prepared Fe-HA NPs in Figure 1C. The result reveals that the Fe/Ca atomic ratio value is 0.57. It displays spherical morphology and an average size of about 20 nm. Powder XRD patterns were recorded over a 2θ range between 20 and 60°. As shown in Figure 1 D, the XRD peaks showed that the system crystallizes in the hexagonal P6 3 /m HA structure, matching the standard JCPDS card (no. 09− 0432). 42 No signals were observed related to metallic iron or iron oxide phases. The XPS spectra of the Fe-HA are shown in Figure 2, with Ca 2p, O 1s, and P 2p signals at 348, 531, and 135 eV, respectively. The chemical state of Ca, O, and P elements are accordance with the previous reports. 43,44 The binding energies at 707 and 720.6 eV with an energy difference of 13.5 eV are attributed to Fe 2p 3/2 and Fe 2p 1/2 , corresponding to the +3 oxidation state. 45 The FTIR graph

Colorimetric Detection of H 2 S by Fe-HA NPs.
The colorimetric behavior of Fe-HA NPs was evaluated on the test strip by naked-eye detection and smartphone. Upon H 2 S addition, the color of the test strip turned from yellow to green (Scheme 1b).
The detection mechanism is based on the replacement of PO 4 3by S 2and the formation of the Fe 2 S 3 product. The green solid of Fe 2 S 3 produced as follows As seen in Figure 4A, with the increase in H 2 S concentration, the probe response increased. The curves of the dynamic response of the Fe-HA NP sensor indicated nearly linear increase from 0.5 ppm to 130 ppm. The detection limit was measured as low as 70 ppb in aqueous solution. In comparison with other previously sensing platform for detection of H 2 S, the proposed assay had a wide linear range  and low detection limit, which confirms the good potential of this method for the sensing of H 2 S (Table 1).
In order to investigate the selectivity of the colorimetric sensor for the detection of H 2 S, competitive species (GSH, cysteine, ...) were dropped onto the paper sensor platform to obverse the color intensity changes under the same conditions. Figure 4B showed that, all interference compounds of the 10fold concentration had no effect on the colorimetric sensor.
Therefore, Fe-HA NPs would be appropriate for the detecting of H 2 S in complex samples.
The practical applications of Fe-HA NPs were checked on the test papers. To this end, a paper sensor was prepared by immersing a filter paper in the Fe-HA NP solution. When the filter paper dried, river water with different concentrations of H 2 S was dripped on the Fe-HA NPs@test paper. As can be seen in Table 2, the recovery and RSD ranges were 97−103.5% and 0.9−2.6%, respectively, indicating the excellent potential of this colorimetric method for H 2 S detection in real water samples. Furthermore, the colorimetric response of Fe-HA NPs for H 2 S detection was also tested in various pH solutions ( Figure 4C). It can be seen that in the presence of H 2 S, the color change of Fe-HA was increased with the rise of pH level and reached a maximum at pH 8. These results indicated that alkaline reaction condition is better for the formation of iron sulfide. 47

CONCLUSIONS
In summary, for the first time, Fe-HA NPs were successfully synthesized from coal ash by a simple method. The synthesized Fe-HA NPs were applied for the fabrication of a novel colorimetric paper-based platform for specific detection of H 2 S. The interaction between H 2 S and Fe-HA NPs on colorimetric paper strips makes a color change from yellow to green. The sensing mechanism was on the basis of the anion replacement reaction between Fe-HA and H 2 S. Quantitative detection of H 2 S on the colorimetric paper test strip obtained from a smartphone application (Color picker). With the increase of H 2 S concentration (0.5−130 ppm), the colorimetric response of Fe-HA NPs was gradually increased, and the LOD was obtained as 70 ppb. Furthermore, this colorimetric sensor applies to monitoring H 2 S in environmental real samples. Taken together, the developed paper sensor integrated with the smartphone, provide simple, low cost, portable, and easy operation strategy for on-site detection of H 2 S.