Plasmonic MoO3-x nanosheets by anodic oxidation of molybdenum for colorimetric sensing of hydrogen peroxide
Z Ahmadzadeh, M. Ranjbar 1
a Department of Physics, Isfahan University of Technology, Isfahan, 841568311, Iran
Abstract
Hydrogen peroxide sensing is crucial for industrial and health applications. Recently O vacancy doped transition metal oxides have emerged as cost-effective plasmonic colorimetric hydrogen peroxide sensors. In this paper, highly stable plasmonic MoO3-x nanosheets with intense blue color and strong NIR absorption band were obtained by a facile electrochemical anodizing method for hydrogen peroxide sensing applications. Based on X-ray photoelectron spectroscopy (XPS), the plasmonic behavior of the nanoparticles was attributed to the formation of O vacancy. The plasmonic nanosheets could be oxidized by interaction with hydrogen peroxide leading to fading the blue color and weakening of the localized surface plasmon resonance (LSPR) absorption, which was further studied for colorimetric sensing. The influence of anodizing potential (10, 20 and 30 V) and reaction time on LSPR peak was investigated. The linear detection range and limit of detection (LOD) for hydrogen peroxide, calculated from calibration curves, were found to be dependent on anodizing parameters. An increasing trend in LSPR wavelength as a function of analyte concentration was observed, which was explained by the plasmonic behavior model in doped semiconductors. MoO3-x nanosheets were immobilized successfully on felt fiber substrates to be used as a hydrogen peroxide colorimetric assay. The stabilization of nanosheets on fibers was verified by scanning electron microscope (SEM). The feasibility of the assay was confirmed in 800 μm to 100 mM hydrogen peroxide concentration range. By calculating the mean gray value, a LOD as low as ? mM was achieved by digital image analysis. Overall, our study develops molybdenum oxide nanosheets obtained by anodic oxidation for detecting hydrogen peroxide in the level of human positive diabetes (2.8-5.6 mM).
Keywords: Hydrogen peroxide, sensing, colorimetric, MoO3-x, anodic oxidation, XPS.
Introduction
Hydrogen peroxide (H2O2) has many applications in various industries such as food, textile and pharmaceutical industries [1,2]. In addition, it plays an important role in a variety of biological and medical phenomena, so that its side production in biological systems can indicate the development of various diseases, membrane damage and genetic mutations [3,4]. It is therefore important to develop inexpensive sensors that can accurately measure hydrogen peroxide. H2O2 sensing platforms
So far, variety of sensing platforms have been explored for an accurate detection of hydrogen peroxide, such as electrochemical [5], fluorimetry [6], fluorescence [7], and the colorimetric eye-readable methods [8]. The colorimetric methods have advantages over other methods such as inexpensiveness, sensitivity, simplicity, and easy-to-use operation for naked-eye detection, and can be designed in small size with simple methods for medical diagnostics [9–11]. Calorimetric methods that are often reported are based on the agglomeration of plasmonic nanoparticles of noble metals such as gold and silver by analytes or enzymes which needs combination with other materials [12]. However, there are limitations to these methods because noble metal nanoparticles can aggregate due to changes in surface charges in living cells without hydrogen peroxide []. On the other hand, substoichiometric transition metal oxide semiconductors (TMOSs) have been classified into a new class of plasmonic materials due to the tunable collective oscillation of free electrons donated from oxygen defects. In these materials, the plasmonic properties depend more on oxygen vacancies than aggregation of nanoparticles so that the spectral variation of LSPR bands allows determining the concentration of oxygen vacancy [13–16]. The appearance of these materials is also colored because their absorption band often extends to the visible region, which can be used for naked-eye sensing [17]. Hydrogen peroxide as a strong oxidant can affect the oxygen vacancy in a doped metal oxide and will directly affect the LSPR and the visual color. Therefore, LSPR metal oxides are desirable tools colorimetric detection of hydrogen peroxide. LSPR MoO3-x
Tungsten oxide and molybdenum oxide bronzes (AxMO3, A=H, Li, Na, K, etc, and M=W or Mo) have plasmonic property due to interstitial doping of electrons into the conduction band []. important oxide materials that have a strong tendency to become into sub-stoichiometry by ion injection even at room temperature. This feature has led to a variety of applications in the fields of energy storage [18–23], catalytic [?], electrochromic [24], photochromic [25], gasochromic [26] and thermochromic [27]. Recently, substoichiometric TMOSs have been highlighted also for colorimetric detection of hydrogen peroxide due to the dependence of their optical properties on the oxidation states. MoO3-x nanosheets exhibit LSPR absorption in the visible and NIR spectral region, with high sensitivity to lateral thickness, aspect ratio and more importantly to O vacancy content, x, [28,29]. In the case of molybdenum oxide, due to the presence of weak van der Waals bonds in its structures, it is easily produced in the form of 2D nanosheets [30].
There are numerous reports on preparation plasmonic MoO3-x nanosheets through increasing oxygen vacancy. In many of them, UV irradiation of MoO3 lead to a controlled substoichiometric MoO3-x state [31]. Also, various reducing agents such as NaBH4 have been widely used for preparation of MoO3-x from MoO3[32]. However, due to the presence of other oxidizing agents such as dissolved oxygen in these methods, the produced MoO3-x does not have long-term stability and convert to MoO3. In addition, the conventional methods for preparation of MoO3 are multi-step and need heating process to control the compositions
Anodizing method
Anodic production of Mo oxide has been used as green, single-step and efficient liquid-based synthesis method []. For example, Ch. Ni et.al. have anodized molybdenum metal in ethanol at 3 KV under ambient condition with a gaseous microplasma electrode and obtained a mixture of Mo oxyethoxide and MoO3 nanocrystals [http://dx.doi.org/10.1021/acs.cgd.9b00646]. Previously, we have reported anodic oxidation of Mo and W electrodes under low voltage (up to 60 V) in a HCl electrolyte and obtained mixtures of nanocrystals and nanosheets of W and Mo oxides [33–35]. The colloidal solutions exhibited pronounced gasochromic or photochromic properties. We found an essential difference between the obtained products: the as-prepared tungsten oxide was colorless, which turned into a deep blue by H2 or NaBH4 with a short-term stability. However, as-prepared molybdenum oxide, without any reducing agent, had initially an intense blue color along with long-term stability. This difference can be related a lot of van der Waals bonds in Mo oxide structure, which lead to easily formation of 2D structures with deficient lateral edges []. Electrochemical anodizing is also accompanied by proton formation which promote exfoliation of particles into nanosheets via breaking the weak van der Waals bonds. Gaps
Since few reports exist on molybdenum oxide for sensing hydrogen peroxide[36,37], this material should be more explored. In addition to the widespread use of nanocalcogenide 2D plates in catalytic applications, their optical properties have also been widely used in sensing reactions due to the high correlation between optical absorption and their chemical state. In this case, it has been shown that the mechanism of light absorption and color change in MoO3-x is more directly dependent on the oxidation state [] though the shape and size of the colloids have role in spectral properties such as intensity and maximum wavelength of absorption []. This property, combined with the long-term stability of MoO3-x nanosheets obtained by anodic oxidation, promise detection of a wide range of oxidants with different concentrations. Accordingly, the main contribution of this work is to use the electrochemical anodic oxidation as a simple, rapid and cost-effective method for detection of hydrogen peroxide. The role of anodizing parameters, like applied potential and time, is studied to tailor the oxygen vacancy. Since colorimetric devices play an important role in everyday life, the produce MoO3-x nanosheets were immobilized on felt fiber substrates and were examined for naked-eye detection of hydrogen peroxide. Materials and methods
Experimental
Molybdenum rods (Mo) and Hydrogen peroxide (H2O2) were purchased from ?? and were used without further purification. Preparation of MoO3-x colloidal NPs. MoO3-x nanosheets was synthesized using anodizing exfoliation method, reported in our previous papers [36]. firstly, ?? HCl was dissolved in 100 ml equal volume of DI water. The obtained solution was then used as electrolyte. The Mo wires were inserted into the electrolyte at a different applied potentials and different anodizing time. Samples were named according to these parameters, for example S10V-15 min for potential of 10 V and anodizing time of 15 min. In order to investigate the sensitivity of our colloids, hydrogen peroxide was successively diluted by adding water. The initial examination for colorimetric detection was performed by adding 100 mM H2O2 into ? ml of as-prepared blue colloidal MoO3-x solution (total mixing volume of ?? ml). This was followed by adding 300 ml of as prepared MoO3-x in a total reaction volume of 1000 ml. The mixture was vortexed and incubated for 10 min at room temperature after which the
Fig.1.Schematic representation of the synthesis procedure of blue MoO3-x nanosheets by anodizing method and hydrogen peroxide detecting mechanism. The initial deep blue color turns pale blue/colorless due to oxidation of Mo states by filling oxygen vacancies giving rise to a H2O2 platform.
Graphical representation: Comparing the sensitivity of colorimetric methods earlier reported for detection of H2O2 [53e63]. corresponding absorbance was recorded in UV-Vis spectrophotometer model Perkin Elmer Lamda25.. immobilization of was performed by immersing felt fiber into colloidal solution for ??? min of MoO3-x followed by drying in air. Photograph images of felt fibers were taken in similar lighting and camera to object distance condition for different hydrogen peroxide concentrations. The color intensity of the samples was obtained by ImageJ software using the mean gray values quantity for a constant image area. Instrumentation
The MoO3-x nanoparticles were observed on transmission electron microscope (TEM)?. X-ray photoelectron spectroscopy (XPS) was conducted using an ESCA/AES system (Specs model EA10 plus). UV–Vis spectroscopy was performed by the Perkin Elmer spectrophotometer (Lambda 25). XRD patterns were recorded using a Philips XPERT X-ray diffractometer unit with Cu Kα radiation. Results & discussions
Characterization of MoO3-x
As shown in Fig.2(a), ultrathin molybdenum oxide nanosheets can be achieved by anodic oxidation of Mo wire. The nanosheets are nearly transparent with lateral size of about several hundred nanometers. A typical UV/vis-NIR absorption spectrum and photograph of the MoO3-x sample are shown in Figure 2 (b). The as-prepared MoO3-x nanosheets have an intense blue color, which can be interpreted by absorption spectra. With a plasmonic absorption band centered at …? nm, the spectrum displays strong visible light absorption. According to literature, the LSPR peak of the MoO3-x nanosheets is due to oxygen vacancies and varies from 680-950 nm [Plasmonic Catalysis From Fundamentals to Applications p238]. The appearing of the blue color is therefore due to the removal of the red part of the scattered light. The formation of oxygen vacancies in anodic MoO3-x is further confirmed by XPS. It was also used to determine the O/Mo molar ratio at the surface of nanosheets. The high-resolution Mo3d and O1s XPS spectra specify the details of the chemical states (Figure 2 (c and d)). The Mo3d XPS spectrum of nanosheets (Fig.2(c)) was deconvoluted into two separate doublets: the first one with the major contribution is located in the binding energies of 232.8 and 235.9 eV, which corresponds to Mo6+ oxidation state [38]. The other one with smaller contribution at binding energies of 231.7 and 234.8 eV is related to the Mo5+ state [39]. The binding energy of O1s shows two peaks at 530.7 and 533.5 eV with FWHM of 2.0 and 2.1 eV, respectively. The former peak is corresponding to the lattice oxygen of the Mo-O bond [40], and the later one reveals the presence of surface hydroxyl groups and also oxygen vacancies [41]. Taking into account the peak area of O1s (corresponding to Mo-O bond) and Mo3d and considering the corresponding XPS sensitivity factors, the stoichiometry of as-prepared molybdenum oxide nanoparticles is estimated to be MoO2.5, which is consistent with the electrochemical estimations in the next section. The presence of Mo5+ cations and hydroxyl states and/or oxygen vacancy confirms the defectiveness of the nanosheets. These cations provide delocalized electrons to d-band below the Fermi energy level in conduction band and sustain the LSPR.
Fig.2 (a) typical TEM image of MoO3-x nanosheets, (b) and (c) high resolution XPS spectra of Mo3d and O1s, respectively.
These spectra reveal tunable oxygen vacancies
Mechanism of MoO3-x formation
During anodization of Mo, the following oxidation process occurs:
Mo〖Mo〗^(5+)+5e^-
The oxygen-bearing species (OH- and O2-) migrate inward and metal ions migrate outward to reach the oxide-electrolyte interface leading to growth of an oxide layer, which leads to the oxidation of anode, developing of pores at the metal-oxide interface and growth of a porous molybdenum oxide layer [42]. Doping of oxide layer formed on anode surface is possible via self-doping or hydrogen doping. Following equation represents the formation of a self-doped anodic oxide film.
Mo+y/2H2OMoOy/2+yH++ye-
In addition, hydrogen atoms are able to migrate into the interstitial sites of the molybdenum oxide without inducing considerable structural variations [plasmonic catalyst book] to form a hydrated anodic layer according to the following equation:
Mo+yH2OMo(OH)y+yH++ze-
Depending of doping level, doped molybdenum oxide has different visual color. For example, blue HxMoO3 phases have different hydrogen content (0.23<x<0.4 or 0.85<x<1.04) []. Accumulation of surface stress facilitates ejection of oxide layer formed on the anode into the electrolyte, leading to formation of a blue colloidal solution. In addition, presence of weak van der Waals bonds in Mo oxide structure causes formation of nanosheets [43,44]. Doped molybdenum oxide nanosheets have LSPR properties due to interstitial doping of outer-d valence electrons into the conduction band [] due to formation of lattice oxygen vacancy during the anodizing process [45,46]. On the other hand, doped MoO3-x allows it to have more active sites for reaction than pristine samples, and can be promising for plasmonic chemical sensing. The impact of applied potential and reaction time
The anodizing procedure is influenced by different factors including current density, applied potential, electrolyte composition and time. Since the shape and chemical composition are factors that can affect the LSPR of doped molybdenum oxide, we study the impact of anodization conditions on optical properties of the obtained nanosheets. We kept the electrolyte composition constant and investigated the effect of reaction time at different voltages of 10, 20 and 30 V on the absorption spectra. The corresponding absorption spectra nm are shown in Figure 3 (a). LSPR peak intensity (Fig.?) increases with reaction time, which according to the Beer Lambert law is due to formation of more anodic nanosheets. It seems, nanosheet production rate decreases over time because the current density often drops gradually in the anodizing process. Fig.1 shows a typical anodizing current in terms of time at 30 V and a duration of 5 min, which shows that the electrical current of anodizing increases in the early anodizing and then shows a decreasing trend. When the potential is applied, the surface of the molybdenum metal begins to oxidize and release metal ions. The higher the potential, the more current and consequently more electrons are created on the metal surface by oxidation. The formation of a metal oxide achieves a saturation at the metal-solution interface after a certain time, which results in the formation of an oxide layer on the anode. As the electrical resistance increases, the electrode conductivity decreases due to oxidation. The area under the current-time curve also provides the total charge transferred in the anodizing process. The total number of molybdenum atoms released can be estimated with a good approximation of x in the MoO3-x composition. From Fig.1, the total change was calculated to be 162.41 C, which can be used to estimate to total number of O2- species produced in the anodizing process. Also, the mass released from the molybdenum sheet, measured by a digital microbalance, was 0.0343 gr. It is worth noting that the mass of the oxide layer accumulated on the surface of the anode is neglected. According to the mass corrosion, we could estimate the total number of Mo atoms released in the electrolyte and finally have an approximate estimate of the stoichiometry
Our calculations showed the stoichiometry of MoO2.5, which is in good agreement with XPS results. Figure 3 also shows that a minimum anodizing time is required to achieve a colloidal solution with a measurable optical absorption by spectrometer, which decreases with increasing voltage. For example, for applied potential of 10 V, it takes at least 7 min to obtain a solution with measurable LSPR absorption (see arrow in Fig3(a), 10 V), indicating a minimum anodizing time is required at low voltages. By increasing the voltage to 30 V, an effective anodizing can proceed within a 3 min. Another interesting point in the absorption spectra is the spectral shift of the LSPR position. At 10 V, a pronounced blue shift in LSPR wavelength from 810 nm to 720-740 nm also occurs. For voltages of 20 and 30 V, a blue shift from about 790 to about 740 nm with anodizing time (Fig.3(c)). Mechanism of LSPR
As mentioned, the filling of oxygen vacancies in molybdenum oxide is responsible for the spectral shift in optical absorption. Because according to existing models, the resonant frequency depends on the number of free charge carriers (and thus the oxygen vacancy). Eq.1 shows the LSPR relation of angular frequency (ꞷLSPR), where N is the free carrier concentration, e is electron charge, ε0 is the dielectric constant of vacuum, ε∞ is the high frequency dielectric constant, εm is the medium dielectric constant, me is the effective mass of the free carriers [15].
ω_LSPR=1/2π √((Ne^2)/((ε_ω (ε_∞+2ε_m )) ))
According to Eq.1, the LSPR frequency is directly related to the square of the number of charge carriers. Increasing the anodizing time leads to a higher density of O vacancy, while increasing the voltage increases the number of particles released at the same time. This is to be expected because with increasing voltage, the current and consequently the total charge transferred in the anodizing process increases. But increasing the anodizing time may cause the particles released in the environment to be further reduced by the reaction of the environment. However, the LSPR wavelength remains almost constant at about 740 nm for anodizing times of more than 8 min. Formation of Mo5+ states shown by XPS results (Fig.2(b)) and the variation in the optical properties with anodizing parameters demonstrate successful synthesis of MoO3-x nanosheets with tunable oxygen vacancies.
Fig.3 (a) Optical absorption spectra of MoO3-x colloids prepared at different times of and applied potentials of 10, 20 and 30 V DC. (b)
Fig.? Typical current-time curve observed for the anodizing of Mo
H2O2 sensing
The experimental conditions including anodizing parameters (time and voltage) and hydrogen peroxide concentration had direct impact on the LSPR of MoO3 hence colorimetric performance.
To explore the capability of MoO3-x nanosheets for sensing of hydrogen peroxide, samples S10V-15, S20V-15 and S30V-15, prepared at different potentials but similar anodizing time were selected. These samples were selected for investigation of hydrogen peroxide sensing because they have similar LSPR wavelength for 15 min anodizing time. Sample S30V-25 was also included for study the effect of anodizing time in order to obtain a wider detection range. Also, for a correct comparison of spectral changes, corresponding blank samples were prepared by adding the same volume of water used in all hydrogen peroxide dilution processes. Fig.4(a) represents the optical absorption spectra of the MoO3-x samples mixed with variable concentrations of H2O2 in the range of 0-100 mM. The photographs of sample S30V-25 in the presence of different concentrations of hydrogen peroxide are shown in Fig.4(b), which indicates a pronounced visual change in the appearance of the sample, evidencing oxidation of the MoO3-x particles to MoO3 by hydrogen peroxide. All the LSPR bands with peak at around 730 nm begin to drop with increasing concentration of hydrogen peroxide and diminish at above certain ranges of concentration. The normalized absorptions at LSPR peaks of different samples in terms of concentration of hydrogen peroxide (calibration curves) are plotted in Fig.5. The linear range and limit of detection (LOD) are presented in Table.2. The LOD is calculated using the calibration curves according to the following relation:
LOD=3.3(σ/S)
Where σ and S are the standard deviation and the slope of calibration curves. As shown in Fig.7a, a linear response was observed in concentration ranges that are depending on the sample fabrication conditions. The linear detection range increases from 0.001-0.1 to 0.01-0. 6 mM as the anodizing voltage increase from 10 to 30 V at constant 15 min anodizing time but it increases to 0.04-1 mM when anodizing time increase to 25 min at constant potential of 30 V (see vertical arrows in Fig.5). The wider detection range of S30V-25 is attributed to the higher MoO3-x to hydrogen peroxide ratio, because anodizing at 30 V for a period of 25 min produces relatively much MoO3-x nanosheets (see also Fig.3). However, the LOD is lower at lower anodizing potentials
Fig.4 (a) LSPR absorption spectra of the MoO3-x colloidal solutions mixed with variable concentrations of H2O2 in the range of 0-100 mM. (b) Photographs of sample S30V-25 after adding varying concentration of hydrogen peroxide.
Fig.5 Relative absorption curves at LSPR wavelength (~740 nm) for
different samples at mM concentration range of hydrogen peroxide.
Sample name Linear range (mM) LOD (mM)
S10V-15 0.001-0.08 0.022
S20V-15 0.01-0.1 0.025
S30V-15 0.01-0.6 0.044
S30V-25 0.04-1 0.283
Fig.7 LSPR wavelength as a function of hydrogen peroxide concentration for sample S30V-25.
The absorption wavelength in terms of hydrogen peroxide concentration is plotted for sample S30V-25 in Fig.7, which increases from about 743 to 749 nm with hydrogen peroxide concentration. As a strong oxidizer, H2O2 fills the oxygen vacancies created in the anodizing process, causing the free charge carriers to reduce, and the LSPR band to diminish and the deep blue color to transform to pale blue. According to Eq.1, as the number of charge carriers decreases due to the oxidation of MoO3-x, ꞷLSPR decreases and as a result the LSPR red-shifts.
Application as strip
Fig.8 SEM images of (a) uncoated (b) MoO3-x coated and (c) after dipping in hydrogen peroxide (….mM)
Fig.9 Photograph images of color change of (a) MoOx coated felt fibers and (b) commercial glucose detector strip for varying hydrogen peroxide concentrations.
MoO3-coated fabrics as colorimetric ready-to-use sensor
Since detection and sensing of hydrogen peroxide is crucial for health applications, we designed a fabric-based analytical assay for detection of H2O2 using immobilization of molybdenum oxide nanosheets on fiber substrates. Pure felt fabrics were used as the substrate to fabricated portable colorimetric sensors. The porosity of felt fibers provide sufficient specific surface area and have rapid rehydration for performing H2O2-MoO3-x reaction leading to detect hydrogen peroxide rapidly by naked eye. To the best of our knowledge, colorimetric detection of H2O2 using MoO3-x based paper assay has rarely been reported yet [47]. We show the ability of felt fabrics coated with the blue MoO3-x nanosheets to act as ready-to-use colorimetric sensors. Felt fabrics with 1×1 cm2 dimension were immersed in the MoO3-x (sample S30V-25) solution for 1 hour which was repeated three times, then they were allowed to dry in ambient air. The surface morphology of the fabrics was characterized by SEM before and after coating with MoO3-x nanosheets. SEM micrograph of the initial untreated felt fabrics (Fig.?(a)) shows the matrix is composed of the same 10 μm monofibers which have smooth surface in a rather tight network. After the immersion treatment process, presence of a new layer of immobilized nanoparticles around the monofibers is clearly observable which completely covered the fibrous structure (Fig.?(b). These results prove that MoO3 nanosheets has a high level of adhesion to monofibers which causes a stabilized MoO3-x matrix, which can improve the efficiency of H2O2-MoO3-x reaction. Moreover, according to SEM micrograph in Fig.?(c) it seems that the layers of MoO3-x remain stable after reaction of the matrix with hydrogen peroxide due to the strong affinity between nanosheets and fibers. This indicates that the bleaching of the fabrics is only due to oxidation of MoO3-x layer and no detachment of the MoO3-x layer occurs due to washing with drops of hydrogen peroxide. The coated felts were dipped in H2O2 with different concentrations ranging from 800 μM to 100 mM. The coated fabrics demonstrate distinguishable color change upon interaction with hydrogen peroxide, which can be detected within a few second by the naked eye. The blue color entirely disappears for hydrogen peroxide concentration above 2 mM. Fig.9(b) shows the effect of different concentrations of hydrogen peroxide on a commercial glucose detection strip based on hydrogen peroxide concentrations. It can be seen that in comparison, our sample provides better color change contrast for different concentrations of oxygen water. To evaluate the sensitivity of the MoO3-x coated fibers, different concentration of hydrogen peroxide was used. The quantification of hydrogen peroxide was possible by an image analysis using mean gray value of the photographs. The mean gray value of digital photographs measured in the range of …-… mM is plotted as a function of hydrogen peroxide concentration, as shown in Fig.?. The linear range of the calibration curve using general form of y=b+ax gives a=?? and b=?? with R2=…? and is valid for ??<x<???. which shows an exponential behavior. A LOD is as low as ..mM was obtained, which is in the diabetic diagnostic range. Therefore, our samples are capable of detecting hydrogen peroxide in the level of human positive diabetes (2.8-5.6 mM).
Selectivity
The selectivity test using several common interfering species demonstrated excellent anti-interfering…..
Conclusion
Low voltage electrochemical anodic oxidation was developed as a rapid, facile and mass-productive method for fabrication of doped molybdenum oxide colloidal solutions. The LSPR properties which depend strongly on oxygen vacancy level can be tuned by anodizing parameters. The obtained nanosheets have a rapid reactivity with hydrogen peroxide with diminishing behavior of LSPR and visual color, suitable for a colorimetric detection platform. Using UV-vis spectroscopy of colloidal nanoparticles in liquid a detection limit as low as .. could be obtained. The MoO3-x nanosheets can be also immobilized on a porous substrate easily by immersion in molybdenum oxide colloidal solution. The obtained colorimetric assay can be used as naked-eye detector or can be quantified by digital image analysis method for diabetes diagnostic applications.
Acknowledgement
Appendix A. Supplementary data
References
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