Chemo-sensors development based on low-dimensional codoped Mn2O3-ZnO nanoparticles using flat-silver electrodes
© Rahman et al.; licensee Chemistry Central Ltd. 2013
Received: 18 January 2013
Accepted: 19 March 2013
Published: 28 March 2013
Semiconductor doped nanostructure materials have attained considerable attention owing to their electronic, opto-electronic, para-magnetic, photo-catalysis, electro-chemical, mechanical behaviors and their potential applications in different research areas. Doped nanomaterials might be a promising owing to their high-specific surface-area, low-resistances, high-catalytic activity, attractive electro-chemical and optical properties. Nanomaterials are also scientifically significant transition metal-doped nanostructure materials owing to their extraordinary mechanical, optical, electrical, electronic, thermal, and magnetic characteristics. Recently, it has gained significant interest in manganese oxide doped-semiconductor materials in order to develop their physico-chemical behaviors and extend their efficient applications. It has not only investigated the basic of magnetism, but also has huge potential in scientific features such as magnetic materials, bio- & chemi-sensors, photo-catalysts, and absorbent nanomaterials.
The chemical sensor also displays the higher-sensitivity, reproducibility, long-term stability, and enhanced electrochemical responses. The calibration plot is linear (r2 = 0.977) over the 0.1 nM to 50.0 μM 4-nitrophenol concentration ranges. The sensitivity and detection limit is ~4.6667 μA cm-2 μM-1 and ~0.83 ± 0.2 nM (at a Signal-to-Noise-Ratio, SNR of 3) respectively. To best of our knowledge, this is the first report for detection of 4-nitrophenol chemical with doped Mn2O3-ZnO NPs using easy and reliable I-V technique in short response time.
As for the doped nanostructures, NPs are introduced a route to a new generation of toxic chemo-sensors, but a premeditate effort has to be applied for doped Mn2O3-ZnO NPs to be taken comprehensively for large-scale applications, and to achieve higher-potential density with accessible to individual chemo-sensors. In this report, it is also discussed the prospective utilization of Mn2O3-ZnO NPs on the basis of carcinogenic chemical sensing, which could also be applied for the detection of hazardous chemicals in ecological, environmental, and health care fields.
KeywordsDoped Mn2O3-ZnO nanoparticles Wet-chemical method Powder X-ray diffraction 4-nitrophenol I-V technique X-ray photoelectron spectroscopy Sensitivity
Semiconductor codoped nanomaterials have received significant interest due to their electronic, optoelectronic, magnetic, catalytical, electro-chemical, mechanical behaviors and their potential applications in different research areas. Semiconductor nanomaterials might be a promising due to their high-specific surface-area, low-resistances, high-catalytic activity, attractive electrochemical and optical properties [1, 2]. Nanomaterials are also scientifically important codoped nanostructure materials owing to their extraordinary mechanical, optical, electrical, electronic, thermal, and magnetic characteristics. Lately, it has attained significant attention in manganese doped-semiconductor materials in order to develop their physic-chemical behaviors and extend their efficient applications [3–5]. It has not only investigated the basic of magnetism, but also has huge potential in scientific features such as magnetic materials, bio & chemi-sensors, photo-catalysts, and absorbent nanomaterials [6–9]. Recently, very few articles are published based on transition-metal doped semiconductor nanomaterials synthesis and investigated the magnetic behaviors and potential applications only [10–13]. Here, it is prepared codoped Mn2O3-ZnO NPs by easy, facile, economical, non-toxic, repeatable, and reliable low-temperature wet-chemical technique. The nanostructure and morphology of the codoped Mn2O3-ZnO NPs were examined and potentially applied for the enhancement of higher-sensitive 4-nitrophenol chemo-sensor at room condition. Generally, chemo-sensing exploration have been developed with the transition-metal oxides nanostructures for the recognition and quantification of various toxic-chemicals such as phenyl-hydrazine, methanol, formaldehyde, ethanol, chloroform, dichloromethane etc., which are not ecologically safe and friendly [14–18]. The sensing mechanism with doped semiconductor metal oxides thin-film used primarily the properties of meso-porous thin-film generated by the physi-sorption and chemisorptions methods. The hazardous chemical detection is depended on the current responses of the fabricated thin-film, which cause by the presence of chemical components in the reaction-format in aqueous phase [19–21]. The key efforts are based on recongnition the least amount of 4-nitrophenol necessary for the fabricated Mn2O3-ZnO NPs chemo-sensors for electrochemical investigation.
Phenolic compounds have attained significant interest in last decade owing to their eco-toxic effects on human health, ecological, and environmental fields. These toxic compounds (i.e., 4-nitrophenol) are prepared using a number of polluting techniques, such as industry-related ways of plastic, pesticides, paint, drugs, composites, antioxidant, petroleum, and paper production . The 4-nitrophenol is recognized for its hazardous nature, carcinogenetic, toxicity, and persistence in the environment, which is become a common pollutant in nature and waste water . Because of its high solubility and stability in water, it has been also found in freshwater, sea environments and has been detected in industrial wastewaters and is difficult to degrade by conventional method. It is concerned in most of the degradation pathways of organo-phosphorous pesticides, which are decomposed in soil and water to form 4-nitrophenol as an intermediate or final-product in the reaction systems [24, 25]. Therefore, 4-nitrophenol is integrated in the Environmental Protection Agency List of Priority Pollutants (EPALPP) . Therefore, it is straight away desirable to fabricate a chemo-sensor for the detection of organic pollutants to accumulate the environment and human health. There is focused a significant attention for the development of simple, reliable, and ultra-sensitive in various detection methodology based on codoped nanomaterials. Generally, the detection of toxic 4-nitrophenol is consummated using chromatographic techniques, such as gas-chromatography [27, 28], high-performance liquid chromatography [29, 30], liquid chromatography connected with mass-spectroscopy , and capillary-electrophoresis . Electrochemical technique, which can offer fast, reliable, and direct real-time monitoring is one of the most utilized methods in the determination of nitro-phenolic stuffs. Electro-analytical techniques have been performed for 4-nitrophenol detection and quantification with a modified glassy carbon electrode [33, 34] hanging mercury drop electrode  and boron-doped diamond electrode . The analytical signal is derived from the four-electron reduction of the nitro-group  or by the direct two-electron oxidation of phenol to the corresponding o-benzoquinone [38–40]. Electrochemical chemo-sensors have attained huge interest in the recognition and quantification of environmental unsafe chemicals due to their reliable and fast response and determination [41–44]. Chemo-sensor technology plays a significant task in ecological protection that usually caused by environmental contamination and unintended seepage of harmful chemicals, which is a huge-menace for eco-systems. Thus for the attention of ecological and health monitoring, it is important to fabricate easy, simple, reproducible, reliable, and inexpensive chemo-sensors to detect toxic chemicals in aqueous systems. The sensitivity and low-detective of electrochemical chemo-sensor energetically dependent on the size, structure and properties of fabricated electrode doped nanomaterials. Hence doped nanostructure materials have received much attention and have widely been used as a redox mediator in chemo-sensors [45–48].
Codoped nanomaterial is largely established for the recognition of toxic chemicals in electro-chemical control method owing to their numerous benefits over conventional chemical methods in term of large-surface area for examining in medical, health-care and environmental fields [49–56]. In general electro-analytical technique, it was executed the slower responses, surface-fouling, noises, flexible-responses, and smaller dynamic-range and lower-sensitivity with bared codoped nanomaterials surfaces for chemical recognition. Therefore, the modification of the chemo-sensor surface with doped metal oxides nanostructure materials is urgently required to achieve higher sensitive, repeatability, and stable responses. Therefore, an easy and reliable I-V electrochemical approach is immediately needed for relatively simple, appropriate, and economical instrumentation which displays higher-sensitivity and lower-detection limits compared to general techniques. Here, a consistent, large-scale, and highly responsive I-V method is applied for detection of 4-nitrophenol chemical by codoped Mn2O3-ZnO NPs. The present approach represents a consistent, sensitive, low-sample volume, ease to handle, and specific electrochemical methods over the existing UV, CV, LC-MS, LSV, FL, and HPLC methods [57–60]. The simple coating technique for preparation of nanomaterials thin-film with conducting coating agents is developed for the fabrication of doped Mn2O3-ZnO NPs films. Here, low-dimensional doped Mn2O3-ZnO NPs films with conducting coating agents are synthesized and detected 4-nitrophenol in phosphate buffer solution (PBS) phase by reliable I-V method. To best of our knowledge, this is the first report for detection of 4-nitrophenol chemical with doped Mn2O3-ZnO NPs using easy and reliable I-V technique in short response time.
Materials and methods
Manganese chloride (MnCl2.4H2O), zinc chloride (ZnCl2), 4-nitrophenol, ammonium hydroxide (25%), Ethyl cellulose (EC), Disodium phosphate, Butyl carbitol acetate (BCA), Ethanol, Monosodium phosphate, and all chemicals utilized were of analytical grade and obtained from Sigma-Aldrich Company. Stock solution of 1.0 M 4-nitrophenol was synthesized in double distilled water. The doped Mn2O3-ZnO NPs was investigated with UV/visible spectroscopy (Lamda-950, Perkin Elmer, Germany). FT-IR spectra were recorded for Mn2O3-ZnO NPs with a spectrophotometer (Spectrum-100 FT-IR) in the mid-IR range, which was acquired from Perkin Elmer, Germany. Raman station 400 (Perkin Elmer, Germany) was exploited to investigate the Raman shift of Mn2O3-ZnO NPs using radiation source (Ar+ laser line, λ: ~513.4 nm). The XPS measurements were executed on a Thermo Scientific K-Alpha KA1066 spectrometer (Germany). Monochromatic AlKα x-ray radiation sources were used as excitation sources, where beam-spot size was kept in 300.0 μm. The spectrum was recorded in the fixed analyzer of transmission mode, where pass-energy was kept at 200.0 eV. The scanning of the spectra was performed at lower pressures (<10−8 Torr). The X-ray powder (XRD) diffraction prototypes were measured with X-ray diffractometer (XRD; X’Pert Explorer, PANalytical diffractometer) prepared with Cu-Kα1 radiation (λ = 1.5406 nm) by a generator voltage (~40.0 kV) and current (~35.0 mA) applied for the measurement. Morphology of codoped Mn2O3-ZnO NPs was evaluated on FE-SEM instrument (FESEM; JSM-7600 F, Japan). Elemental analysis (EDS) was investigated for doped Mn2O3-ZnO NPs using from JEOL, Japan. I-V technique was used for sensing NPs modified sensor electrode by Electrometer (Kethley, 6517A, Electrometer, USA) at room conditions.
Synthesis and growth mechanism of codoped Mn2O3-ZnO NPs
Initially manganese chloride (MnCl2.4H2O) and zinc chloride (ZnCl2) were gradually dissolved into the de-ionized water to prepare 0.1 M concentration separately at room temperature. After addition of NH4OH into the mixture of metal chloride solution, it was stirred slowly for several minutes at room condition. Mn2O3-ZnO NPs have been synthesized by adding equi-molar concentration of manganese chloride and zinc chloride as starting (reducing) materials into reaction-cell (in Teflon-line auto-clave) for 12 hours. Then the solution pH is attuned (at 10.5) by using prepared NH4OH and put into the auto-clave cell. The starting materials of MnCl2 and ZnCl2 were employed without further purification for co-precipitation method to codoped Mn2O3-ZnO nanoparticles composition. Again reducing agent (NH4OH) was added drop-wise into the vigorously stirred MnCl2 and ZnCl2 solutions mixture to produce a significant doped precipitate.
The precursors of MnCl2 and ZnCl2 are soluble in alkaline medium (NH4OH reagent) according to the equation of (i) - (iii). After addition of NH4OH into the mixture of metal chlorides solution, it was strongly stirred for several minutes at room temperature. The reaction is development gradually according to the equation (iv). Then the resultant solution was washed systematically with ethanol, acetone and kept for drying at room temperature. During the total preparative procedure, NH4OH acts a pH buffer to control the pH value of the solution and slow donate of OH– ions. When the concentrations of the Mn2+, Zn2+, and OH- ions are reached above the critical value, the precipitation of doped Mn2O3-ZnO nuclei begin to start. As there is higher concentration of Zn2+ ion in the solution, the nucleation of doped Mn2O3-ZnO crystals become easier due to the lower-activation energy barrier of heterogeneous nucleation. However, as the concentration of Zn2+ subsistence, a number of larger doped Mn2O3-ZnO crystals with a spherical particle-shape morphology form in nano-level. The shape of codoped Mn2O3-ZnO NPs is approximately consistent with the growth pattern of codoped Mn2O3-ZnO crystals [61, 62]. Finally, the as-grown codoped Mn2O3-ZnO NPs products were calcined at 400.0°C for 4 hours in the furnace (Barnstead Thermolyne, 6000 Furnace, USA). The calcined doped nanomaterials were synthesized in detail in terms of their morphological, structural, optical properties, and applied for 4-nitrophenol chemical sensing.
Fabrication of AgE using doped Mn2O3-ZnO NPs
Results and discussions
Where Ebg is the band-gap energy and λmax is the wavelength (~284.0 nm) of the doped Mn2O3-ZnO NPs. No extra peak related with contaminants and structural defects were found in the spectrums, which confirmed that the prepared NPs control crystallinity of codoped Mn2O3-ZnO NPs [63, 64].
The codoped Mn2O3-ZnO NPs are also investigated from the atomic and molecular vibrations. To investigate the vibration of materials, FT-IR spectrum mostly in the area of 450.0-4000.0 cm-1 is measured. Figure 2b displays the FT-IR spectrum of the Mn2O3-ZnO NPs. It represents band at 521.0, 1407.0, 1631.0, and 3427.0 cm-1. These executed wide vibration band (at 521.0 cm-1) could be assigned as metal-oxygen (Mn-O and Zn-O modes) stretching vibrations [65, 66], which verified the pattern of doped Mn2O3-ZnO NPs. The additional experimental vibration bands may be allocated to O-H stretching (3427 cm−1), C-O stretching vibration (1631.0 cm−1), and O-H bending vibration (1407 cm−1). The absorption bands at 1407, 1631 and 3427 cm-1 usually displays from water and carbon dioxide, which generally semiconductor nanostructure materials absorbed from the surroundings owing to their meso-porous nature. Finally, the resultant vibration bands at lower-frequencies areas recommended the formation of codoped Mn2O3-ZnO NPs.
Raman spectroscopy is a spectroscopic method employs to reveal vibrational, rotational and other low-frequency phases in Raman active compounds. Figure 2c confirms the Raman spectrum, where key features of the wave number are accomplished at about ~368.0 cm-1 and 667 cm-1 for metal-oxygen (Mn-O and Zn-O) stretching vibrations. These bands can be allocated to a codoped Mn2O3-ZnO NPs .
The electron dispersive spectroscopy (EDS) evaluation of calcined Mn2O3-ZnO NPs assigns the existence of Mn, Zn, and O composition in the pure calcined Mn2O3-ZnO materials. It is clearly employed that NP materials controlled with only manganese, zinc, and oxygen elements, which is shown in Figure 3b. The composition of Mn, Zn, and O is 33.07%, 18.81%, and 48.12% respectively. No other peak related with any impurity has been found in the EDS, which demonstrates that the doped Mn2O3-ZnO NPs are composed only with Mn, Zn, and O. High resolution FESEM images of calcined Mn2O3-ZnO NPs are exhibited in Figure 3c-d. The FESEM images displayed of codoped materials with aggregated nano-particles shapes. The average diameter of doped Mn2O3-ZnO NPs is calculated in the range of 22.7 nm to 50.0 nm, which is close to ~37.5 nm. It is displayed noticeably from the FESEM images that the simple wet-chemical method of prepared crystalline nanomaterials are nanostructure of codoped Mn2O3-ZnO NPs, which executed in aggregated shape, higher-density, and attained nanostructure in spherical nano-particle shapes. It is also suggested that nanomaterials composed in spherical particle-like morphology of the combined codoped Mn2O3-ZnO NPs [71, 72].
Applications: detection of 4-nitrophenol using codoped Mn2O3-ZnO NPs
Comparison the performances of 4-nitrophenol detection based on doped Mn 2 O 3 -ZnO NPs using various reported methods
1.0 nM to 1.0 mM
4.50 μAcm-2 mM-1
Poly(safranine) Film Electrode
8.0 × 10−8 to 4.0 × 10−5 M
3.0 × 10−8 M
Mn-Doped ZnS QDs
0.1 to 40 μM
Fluorescence Spectroscopy (FL)
5 and 1000 μg/L
Graphene Oxide sensors
0.1 to 120 μM
B-doped diomond Electrodes
Doped Mn 2 O 3 -ZnO NPs/AgE
0.1 nM to 50.0 μM
~4.6667 μAcm -2 μM -1
By reliable I-V techniques for fabricating, assembling and integrating structural semiconductor doped Mn2O3-ZnO NPs onto conductive flat-silver electrodes has been investigated in details for the detection of toxic 4-nitrophenol compound. Codoped Mn2O3-ZnO NPs fabricated sensor executed the potential applications in providing 4-nitrophenol chemo-sensors and encouraging improvement has been consummated in this investigation. Besides the development of codoped nanomaterials, there are still a number of significant subjects that are required for additional examination before this nanomaterial can be moved into the profitable uses for the mentioned applications. As for the doped nanostructures, NPs are introduced a route to a new generation of toxic chemo-sensors, but a premeditate effort has to be applied for doped Mn2O3-ZnO NPs to be taken comprehensively for large-scale applications, and to achieve higher-potential density with accessible to individual chemo-sensors.
This paper was funded by King Abdulaziz University, under grant No. (31-3-1432/HiCi). The authors, therefore, acknowledge technical and financial support of KAU.
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