Rapid industrialization and various anthropogenic activities have led to the discharge of various types of toxic chemicals, which contaminate the environment and adversely affect the ecosystem. Among various toxic chemicals, the nitrophenols and organic dyes are the most emerging environmental pollutants. In particular, p-nitrophenol (PNP) is commercially very useful and finds application in various fields such as use as an intermediate in the synthesis of paracetamol and an important base material for the synthesis of commonly used pesticides such as acetophenetidine; it also plays an important role in the leather industry, etc. [1
]. The release of PNP into the environment takes place through a number of ways, such as release as a side product of many pharmaceutical industries, the disintegration of other pesticides and the transformation of phenol into nitrophenol molecules. PNP is very stable in the environment under normal conditions and is a highly toxic chemical that leads to bioaccumulation as well as cytotoxic/carcinogenic effects on living organisms [4
]; due to all these reasons, the United States Environmental Protection Agency (USEPA) has considered PNP as a priority contaminant [6
]. PNP is considered as the root cause of different problems such as irritation and inflammation in the eyes and nose, respiratory tract disorders, cardiac disorders, hematological problems, digestive problems, musculoskeletal issues, renal disease, and dermal/ocular problems in living organisms. It easily reacts with blood and leads to the formation of methemoglobin, which is mainly responsible for cyanosis confusion and unconsciousness [5
]. Different environmental organizations such as the Brazilian Environmental Council, USEPA and European Commission have declared 100, 60 and 0.1 ppb of PNP as maximum permitted limits [5
], respectively. Therefore, the detection of PNP at trace levels is required.
Hence, it is the need of the hour to design a highly reliable and robust chemical sensor for the rapid detection of PNP. Several methods are reported in the literature for the effective and selective determination of PNP such as gas chromatography, high-performance liquid chromatography (HPLC), spectrophotometry, fluorescence, electrophoresis/capillary electrophoresis etc. [8
]. However, expensive columns and a lot of reagents are required in HPLC and electrophoresis, aqueous samples cannot be used directly in gas chromatography, and interference is a major problem in spectrophotometric and fluorometric methods. In other words, all these techniques are either very expensive or time consuming or suffer from some other problems. Therefore, there is a need to develop a simple, cost-effective and quick technique that can effectively monitor the presence of PNP in a sample. Recently, several attempts have been made to synthesize a PNP sensor using various metal oxide nanomaterials such as copper oxide nanoparticles [13
], a reduced graphene oxide/Au nanoparticle composite [14
], a gold–ZnO–SiO2
], a CeO2
/ZnO nano-composite [16
], Ag doped Nd2
] and CeO2
–ZnO nanoellipsoids [18
Among the various transition metal oxides, zinc oxide is a very versatile, multifunctional semiconductor with a direct band gap of 3.37 eV. Zinc oxide is preferred over other metal oxide nanomaterials because it has a high exciton binding energy (60 meV), high catalytic efficiency, high-electron communication property, and cost-effective environmental sustainability [19
]. Moreover, it has been observed that we can alter the properties of ZnO nanostructures for desired applications by manipulating the shape, grain size and morphology. However, the doping of ZnO nanostructures with transition metals, such as Mn, Ga, Fe, In, Co, Mg, Al, Sb, Sn, Ag, etc., has been found to deliver better control over the band gap [24
]. Among the various transition metals, Ag ions have become the interest of many research groups [27
], because of their suitable band gap, as they reduces the electron hole recombination and hence result in improvement in the photocatalytic properties, as well as proving to be an effective sensor material for toxic chemicals [29
In this paper, we report the solution combustion synthesis of silver-doped zinc oxide nanostructures, Agx
O (x = 0, 0.01, 0.05 and 0.10). The solution combustion synthesis route was utilized in the present paper for the production of nanostructure materials in a single step, a time- and energy-saving process [30
]. As-synthesized Agx
O nanostructures were utilized for the formation of a highly sensitive luminescence sensor for the detection of PNP in an aqueous medium. The limit of detection (LOD) for PNP was calculated for ZnO as well as Ag-doped ZnO samples, by using the Stern–Volmer equation.
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), tartaric acid (C4H6O6), silver nitrate (AgNO3) and p-nitrophenol (C6H5NO3) (PNP) were all of analytical grade and obtained from Merck, Germany and these chemicals were directly used as received without further purification. Deionized (DI) water obtained from a Millipore Elix system was used for all the experiments.
2.2. Synthesis of ZnO Nanoparticles
Pure ZnO nanostructures were synthesized from zinc nitrate hexahydrate (oxidizer) and tartaric acid (fuel) using a typical reaction procedure of solution combustion by optimizing the stoichiometric ratio (ϕ = 1). In the synthesis process, 4.0 g of Zn(NO3)2·6H2O and 2.0 g of C4H6O6 were dissolved in 50 mL of deionized water, separately. Then, the tartaric acid solution was added drop-wise to the Zn(NO3)2·6H2O solution with continuous stirring at room temperature. The resultant solution was thoroughly stirred at room temperature for 30 min, and the resulting solution was heated till the evaporation of the excess water content to obtain the gel-like materials. The resulting gel-like material was transferred into a muffle furnace in a silicate crucible at 500 °C for 1 h. In the muffle furnace, tartaric acid (fuel) and zinc nitrate hexahydrate (oxidizer) were combusted for the formation of the crystalline ZnO nanostructures, at high temperature. The synthesized nanostructures were further washed with distilled water followed by ethanol, and finally, the precipitates were dried in an oven at 60 °C.
2.3. Synthesis of Ag-Doped ZnO Nanostructures
The same procedure was adopted for the synthesis of Ag-doped ZnO by taking an appropriate amount of AgNO3
. For the synthesis of 1%, 5% and 10% Ag-doped ZnO, 0.022, 0.114 and 0.228 g of AgNO3
dissolved in 50 mL of water was added, respectively; after that, the same procedure was adopted as in Section 2.2
The synthesis of the nanostructures was confirmed by characterizing them for their morphological, structural and optical properties. The morphological investigation was carried out by using field emission scanning electron microscopy (SEM) with a FEI Quanta 200 FEG (Pittsburgh, PA, USA) with the energy dispersive X-ray spectrometer Bruker XFlash 4030 (EDS), while the structural characterization was performed using X-ray diffractometer (D8 Discover, Bruker AXS, Karlsruhe, Germany). The optical properties of the as-synthesized nanostructures were examined at room temperature by using UV–visible spectroscopy (AvaSpec-2048). The XPS study of the ZnO powder and Ag-doped ZnO powder was carried out by employing X-ray energy dispersion spectrometer (Bruker XFlash 4030, Karlsruhe, Germany). The sensing properties of the pure ZnO and Ag-doped ZnO nanostructures were recorded using room-temperature photoluminescence spectra (Perkin Elmer-LS55 fluorescence spectrophotometer, Waltham, MA, USA).
2.5. Luminescent Sensor Evaluation of ZnO and Ag-ZnO Nanoparticles
The Perkin Elmer LS55 fluorescence spectrophotometer was used at room temperature to determine the sensing properties of the pure ZnO and Ag-ZnO nanostructure. In this investigation, the pure ZnO and Ag-ZnO nanostructures were utilized to fabricate the luminescent sensor interface for the sensing of PNP. The obtained ZnO and Ag-ZnO nanostructures were dispersed in DI water (1.5 mg in 50 mL of H2O), and a 5 mM stock solution of PNP was also prepared for the entire experiment. All the solutions were prepared 30 min before performing the experiments. The sensing experiments were performed at different concentrations of PNP in the range of 0.833–9.980 μM.
In summary, we successfully synthesized ZnO and Ag-doped ZnO nanostructures via a solution combustion process. The synthesized ZnO and Ag-ZnO nanostructures were characterized by using various characterization techniques, which revealed the large-scale synthesis of well-crystalline nanostructures and successful incorporation of Ag ions in the ZnO matrix, showing excellent chemical sensing properties. ZnO was found to show a low value of the limit of detection, i.e., 2.175 µM/L, for p-nitrophenol sensing; moreover, a sharp decrease in the limit of detection was observed with an increase in the concentration of silver ions, and the LOD value decreased to 0.669 µM/L for the 10% silver-doped zinc oxide. It is therefore concluded that Ag-doped ZnO shows a lower limit of detection as compared to pure ZnO for p-nitrophenol sensing. We observed a decrease in the particle size with an increasing Ag ion concentration, as well as the presence of silver also decreasing the electron hole recombination rate, which led to the enhancement of the surface area and Ag also behaving as an electron scavenger. Hence, there was an increase in the chemical sensing efficiency owing to the separation of charge carriers, resulting in the restriction of their recombination. It is concluded that Ag-doped ZnO nanostructures are promising candidates for the detection of p-nitrophenol.