# A Comparative Cyclic Voltametric Study on Rare Earth (Eu, Sm, Dy, and Tb) Ions Doped La10Si6O27 Nanophosphors for Sensor Application

Authors: A. Naveen Kumar; D.M. Jnaneshwara; C.R. Ravikumar; H.C. Ananda Murthy; S.C. Prashantha; M.R. Anil Kumar; K.M. Ajay
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

The rare earth (RE = Eu, Sm, Dy, and Tb) ions doped La10Si6O27 nanophosphor was synthesized by a simple solution combustion method. The prepared La10Si6O27:RE3+ nanophosphors (LNPs) were subjected to diverse technical tools for exploring their structural, optical, morphological, and electrochemical features. The structural analysis using powder X-ray diffraction (PXRD) patterns revealed the hexagonal oxy apatite phase for LNPs with a crystallite size in the range of 25–50 nm, and the equivalent was affirmed by image analysis via transmission electron microscopy (TEM). Utilizing DRS data, the bandgap energy (Eg) values were recorded for LNPs. Cyclic voltametric (CV), electrochemical impedance spectroscopy (EIS), and sensor studies were performed using a modified carbon paste electrode of LNPs. The modified LNP electrodes were found to be highly effective in sensing paracetamol in acidic medium with a quick response time of 3 secs for sensing the drugs at 1 mM concentration. All the RE ions Eu3+, Sm3+, Dy3+, and Tb3+ (5 mol%)-doped LNPs exhibited the most promising electrochemical sensing characteristics.


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## Body

## 1. Introduction

Nanomaterials offer promising opportunities for improved and tailored properties for applications in various fields due to their unique physicochemical properties, caused by their nanosized dimensions and large surface to volume ratios [1]. Nanomaterials are of special interest not only for basic research, but also for their interesting applications in various fields like flat panel displays, solar energy converters, optical amplifiers, electroluminescent devices, photodiodes, bio-detectors, in lamp industries, radiation dosimetry, colour display, catalysts, host for solid-state lasers, solid electrolytes, chemical sensors, magnetic refrigeration materials, substrates for high-temperature superconductor deposition, and thermal barrier coatings [2]. Novel nanomaterials have brought revolutionary transformations in the recent years due to their versatile nature with significant commercial impact [3]. In addition, these nanomaterials were applied in many varieties of sectors such as energy, catalysis, cosmetics, electronics, and environment [4–6].La10Si6O27 is an exceptionally important wide band gap insulator and attracted both fundamental and application studies for use in catalysis, toxic waste remediation, refractory, paint, translucent ceramics, absorbent for many pollutants and superconductor products. Furthermore, it has a homomorphous compound with rock salt structure, the Lanthanum ions occupying oxyapatite sites within the anion closed-packed structure and its ionic constituents comprise a relatively small number of electrons. Due to their simple crystal structure and perfect ionicity, La10Si6O27 appears to form outstanding building blocks for the construction of functional nanostructures, or to serve as a suitable model compound for the investigation of surface reactivity on silicates. Its nanostructures are expected to have novel properties superior to their bulk counterparts due to the quantum confinement effect [7–10].Generally, phosphor materials were prepared via the solution combustion method by mixing of desired ingredients and followed by high-temperature sintering (>1400°C). In this method, many problems occur such as poor sintering and nonhomogeneity and difficult to control cation stoichiometry. However, these techniques usually include many sophisticated processes and consume much longer time. Moreover, they have not received much commercial attention due to lacking of reproducibility, reliability, and cost-effectiveness. Therefore, simple synthesis method was required to overcome these problems. The combustion technique emerged in the last decade as an important synthesis technique for oxide nanoparticles for being very simple experimental setup, provides molecular level of mixing and high degree of homogeneity, fast, cost-effective, ultrapure, and large surface area. Also, at the same time, solution combustion synthesis facilitates the fine-tuning of the properties by controlling crucial parameters [11–13].We studied sensors using drug paracetamol (650 mg) for different dopants. Paracetamol is a popular and effective antipyretic and analgesic, widely used without prescription to treat a variety of symptoms including headache, fever, arthritis, colds, and pain. Paracetamol is also considered a good alternative treatment for those with an intolerance to aspirin and presenting lower risk and minimize the adverse health effects when it is taken in the recommended dose [14]. Hence, there is a need for a simple and rapid method for early detection and monitoring of paracetamol for the prevention and diagnosis of overdose [15].Electrochemical sensors are an alternative preferred for applications in the field due to their simplicity, fast response, portability, cost effectiveness, and easy adaptation to drugs detection. The most common sensors in the market are the glucometers, alcoholmeters, and pregnancy test. However, the key advantage of electrochemical sensors is the possibility of miniaturization and portability, and thus, smaller devices can be fabricated. Of all the electrochemical techniques, voltammetry has been the most extensively used for quantification of drugs [16, 17]. Applying these methods, several biosensors with nanomaterials have been used for the analysis of paracetamol samples [18, 19]. In the present work, we report the synthesis of the rare earth Eu3+, Sm3+, Dy3+, and Tb3+ ions-doped La10Si6O27 nanophosphor via the solution combustion method. The as synthesized LNPs were characterized by using various advanced technical tools. The carbon paste electrode of LNPs was employed towards the detection of paracetamol through CV and electrochemical impedance studies.


## 2. Experimental

### 2.1. Synthesis of La10Si6O27: RE3+ (RE = Eu, Sm, Dy, and Tb)

La10Si6O27:RE3+ (5 mol %) nanophosphors were made by first converting rare earth oxides to rare earth nitrates through a reaction with 1 : 1 HNO3, then evaporating the surplus nitric acid to obtain clear rare earth solution at 80°C [20–22]. By using the solution combustion approach, La10Si6O27:RE3+ NPs (5 mol %) were created (the duration of synthesis is 5 min at 450°C). Lanthanum nitrate (La(NO3)3.6H2O: 99.99%, Sigma Aldrich Ltd.) and fumed silica (SiO2 (AR grade)) were mixed with oxalyl-dihydrazide (ODH; C2H6N4O2-fuel) and dissolved in a minimum quantity of doubled distilled and mixed for about 5 min (Solution A). Rare earth ions, RE3+ (Eu3+, Sm3+, Dy3+, and Tb3+) (5 mol %) were added separately, in the form of nitrate to a Petri dish containing the above Solution A. These mixes were then put in a muffle furnace that was preheated and kept at 450 °C. Initial dehydration of the reaction mixture is followed by gas evolution and combustion. The obtained foamy product was crushed into a fine powder and calcined at 950°C for 3 h [23, 24].The chemical reaction for the synthesis process was as below:(1)995LaNO33+6SiO2+1791C2H6N4O2+5.0EuNO3=La995Si6O27Eu005+5373H2O+3582CO2+5082N2995LaNO33+6SiO2+1791C2H6N4O2+5.0SmNO3=La995Si6O27Sm005+5373H2O+3582CO2+5082N2995LaNO33+6SiO2+1791C2H6N4O2+5.0DyNO3=La995Si6O27Dy005+5373H2O+3582CO2+5082N2995LaNO33+6SiO2+1791C2H6N4O2+5.0TbNO3=La995Si6O27Tb005+5373H2O+3582CO2+5082N2


### 2.2. Characterizations

Crystalline Shimadzu X-ray diffractometer (CuK-1.541) with a scan rate of 20 per minute was used to do the structural investigation. Using a Perkin Elmer FTIR (Spectrum-1000) spectrometer in the 4000–400 cm−1 range, the functional groups in the produced compounds were detected. The morphology, polycrystallinity, and interplanar distance was determined using JEOL transmission electron microscopy (TEM) (JEM-2100 (accelerating voltage up to 200 kV, LaB6 filament). To calculate the energy band gap using the K–M function, the diffuse reflectance spectral (DRS) studies were performed using Shimadzu UV-Vis spectrophotometer model 2600 in the range of 200–800 nm. The trielectrode setup and an electrochemical analyzer CHI608 E potentiostat were used to conduct the cyclic voltametric, impedance, and galvanostatic charge-discharge tests.


### 2.3. Preparation of an Electrode Using Carbon Paste

La10Si6O27:RE3+(5 mol percent) nanoparticles, graphite powder (conducting carbon), and silicon oil (binder) were combined in a weight ratio of 15 : 70 : 15 to create the active material for the working electrode, which was then physically ground into a uniform slurry. The steps for making the working electrode for LNPs were derived from our earlier publications [25–27]. The conventional three-electrode configuration (half-cell) was utilized in various electrolytes to investigate the electrochemical behavior of La10Si6O27:RE3+(5 mol percent) electrode material.


## 2.1. Synthesis of La10Si6O27: RE3+ (RE = Eu, Sm, Dy, and Tb)

La10Si6O27:RE3+ (5 mol %) nanophosphors were made by first converting rare earth oxides to rare earth nitrates through a reaction with 1 : 1 HNO3, then evaporating the surplus nitric acid to obtain clear rare earth solution at 80°C [20–22]. By using the solution combustion approach, La10Si6O27:RE3+ NPs (5 mol %) were created (the duration of synthesis is 5 min at 450°C). Lanthanum nitrate (La(NO3)3.6H2O: 99.99%, Sigma Aldrich Ltd.) and fumed silica (SiO2 (AR grade)) were mixed with oxalyl-dihydrazide (ODH; C2H6N4O2-fuel) and dissolved in a minimum quantity of doubled distilled and mixed for about 5 min (Solution A). Rare earth ions, RE3+ (Eu3+, Sm3+, Dy3+, and Tb3+) (5 mol %) were added separately, in the form of nitrate to a Petri dish containing the above Solution A. These mixes were then put in a muffle furnace that was preheated and kept at 450 °C. Initial dehydration of the reaction mixture is followed by gas evolution and combustion. The obtained foamy product was crushed into a fine powder and calcined at 950°C for 3 h [23, 24].The chemical reaction for the synthesis process was as below:(1)995LaNO33+6SiO2+1791C2H6N4O2+5.0EuNO3=La995Si6O27Eu005+5373H2O+3582CO2+5082N2995LaNO33+6SiO2+1791C2H6N4O2+5.0SmNO3=La995Si6O27Sm005+5373H2O+3582CO2+5082N2995LaNO33+6SiO2+1791C2H6N4O2+5.0DyNO3=La995Si6O27Dy005+5373H2O+3582CO2+5082N2995LaNO33+6SiO2+1791C2H6N4O2+5.0TbNO3=La995Si6O27Tb005+5373H2O+3582CO2+5082N2


## 2.2. Characterizations

Crystalline Shimadzu X-ray diffractometer (CuK-1.541) with a scan rate of 20 per minute was used to do the structural investigation. Using a Perkin Elmer FTIR (Spectrum-1000) spectrometer in the 4000–400 cm−1 range, the functional groups in the produced compounds were detected. The morphology, polycrystallinity, and interplanar distance was determined using JEOL transmission electron microscopy (TEM) (JEM-2100 (accelerating voltage up to 200 kV, LaB6 filament). To calculate the energy band gap using the K–M function, the diffuse reflectance spectral (DRS) studies were performed using Shimadzu UV-Vis spectrophotometer model 2600 in the range of 200–800 nm. The trielectrode setup and an electrochemical analyzer CHI608 E potentiostat were used to conduct the cyclic voltametric, impedance, and galvanostatic charge-discharge tests.


## 2.3. Preparation of an Electrode Using Carbon Paste

La10Si6O27:RE3+(5 mol percent) nanoparticles, graphite powder (conducting carbon), and silicon oil (binder) were combined in a weight ratio of 15 : 70 : 15 to create the active material for the working electrode, which was then physically ground into a uniform slurry. The steps for making the working electrode for LNPs were derived from our earlier publications [25–27]. The conventional three-electrode configuration (half-cell) was utilized in various electrolytes to investigate the electrochemical behavior of La10Si6O27:RE3+(5 mol percent) electrode material.


## 3. Results and Discussion

### 3.1. Powder X-Ray Diffraction Studies

Figure1 depicts the powder X-ray diffraction (PXRD) patterns of La10Si6O27:RE3+(5 mol%) NPs. The presence of peaks at 2θ values of 21.38°, 22.39°, 24.98°, 27.23°, 28.35°, 31.05°, 31.27°, 32.29°, 39.03°, 42.40°, 43.64°, 45.33°, 46.67°, 47.80°, 49.15°, 50.05°, and 50.61° corresponding to miller indices of crystal planes (200), (111), (002), (102), (210), (211), (112), (300), (103), (113), (400), (222), (312), (213), (321), (410), and (402) is in compliance with hexagonal oxyapatite structure (P63/m) with JCPDS card no.53–0291 [7, 28, 29]. Rietveld analysis revealed the values of lattice constant values, a = b = 9.7375Ao and c = 7.1826Ao. For all the dopants, the highest diffraction peak is at 2θ = 31.05°. The crystallite sizes of the LNPs were estimated using Scherrer’s formula,(2)D=kλβcosθ,where Β = full width at half maximum (FWHM) of the diffraction peak, λ = wavelength of X-ray (1.5418 A0), θ = Bragg’s angle, and K = Scherrer’s constant varying from (0.88–0.92).Figure 1
PXRD patterns of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb -5 mol %) nanophosphors.The average value of 0.9 was taken for the calculation. The average crystallite size was found to be between 25 and 50 nm. The estimated particle size of La10Si6O27:RE3+ (5 mol%) for (211) planes is shown in Table 1 [30].Table 1
Estimated particle size (nm) of La10Si6O27:RE3+ (5 mol%) NPs for (211) plane.
SamplesPlanesParticle sizes (nm)La10Si6O27:Eu3+ (5 mol%)(211)0.274La10Si6O27:Sm3+ (5 mol%)(211)0.275La10Si6O27:Dy3+ (5 mol%)(211)0.280La10Si6O27:Tb3+ (5 mol%)(211)0.273


### 3.2. Fourier Transform Infrared Spectroscopy

Fourier transform infrared (FTIR) spectra of undoped La10Si6O27 and RE3+ and doped La10Si6O27 (5 mol %) samples were recorded in the wavenumber range between 400 and 4000 cm−1 (Figure 2.). The bending modes of the Si-O-Si band vibrations are responsible for the IR band between 440 and 490 cm−1. The Si-O symmetric stretch presents an extreme band centered from 630 to 645 cm−1. The band appeared in the range of 854 to 870 cm−1 is due to the Si-O-Si asymmetric stretch. Similarly, the bands at 910 to 1063 cm−1 corresponds to the SiO6 group and 2353 cm−1 corresponds to stretching of the hydroxyl group [31].Figure 2
FTIR spectra of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb -5 mol %) nanophosphors.


### 3.3. Morphological Studies

The morphological characteristics of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb) (5 mol% (100 μm)) were explored by using scanning electron microscopy (SEM). Micrographs revealed fogy and grain shape-like morphology for the synthesized LNPs with a little agglomeration as shown in Figures 3(a)–3(d)). TEM measurements were made to examine the size and morphology of the synthesized La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb) (5 mol% (0.5 μm)) nanoparticles. Figures 4(a)–4(d) depicts TEM images of La10Si6O27:RE3+ (5 mol %) NPs. The micrographs revealed the presence of nearly cubical, hexagonal, and irregular shapes with a little agglomeration having an average particle size ranging between 25 nm and 35 nm [32, 33].Figure 3
SEM image of the La10Si6O27:RE3+nanophosphors.Figure 4
TEM image of the La10Si6O27:RE3+nanophosphors.The crystallite sizes and the particle sizes show a minor discrepancy as estimated from XRD and TEM examinations. When using TEM analysis, the observed size is the outcome of an average weighted by the sixth power of the crystal radius, however when using XRD, the intensity is proportional to the square of the volume. The diffraction signals are stronger for larger diameters than for the smallest ones for crystals that are imperfectly monodisperse. As a result, the size estimated by XRD will always be less than the one examined by the TEM technique [34–36].


### 3.4. Diffuse Reflectance Spectroscopy (DRS) Analysis

The diffused reflectance spectra of La10Si6O27:RE3+ (5 mol %) NPs are shown in Figure 5. Two bands, a high absorption peak and a weak absorption peak could be seen in the spectra of the LNPs doped with rare earth elements. It was discovered that the intensity of the absorption peak increases with the concentration of Tb3+ ions and slightly shifts towards the longer wavelength side [37]. The host LNPs are represented by both the absorption peaks.Figure 5
DRS patterns of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb -5 mol %) nanophosphors.Figure6 shows the estimated direct band gap energy by plotting (f(R) (hv))2 vs energy gap (eV). The energy band gap was determined by using the following equation [38].(3)Dαhϑ=Ahϑ−Egn,where hυ = photon energy, α = absorption coefficient (α = 4πk/λ), k = absorption index or absorbance, k = wavelength in nanometer), Eg = energy band gap, A = constant, and n = 1/2 for the allowed direct band gap [39, 40].Figure 6
Energy band gap spectra of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb -5 mol %) nanophosphors.According to the type of transition, the exponent ‘n’ can have values of 1/2, 2, 3/2, and 3, which correspond to the permitted direct, permitted indirect, prohibited direct, and prohibited indirect transitions, respectively [41]. The estimated eV values are presented in Table 2.Table 2
Estimated energy gap (eV) of La10Si6O27:RE3+ (5 mol%) NPs.
SamplesEnergy gap (eV)Strong absorption peakWeak absorption peakLa10Si6O27:Eu3+ (5 mol%)5.0218.56535.13La10Si6O27:Sm3+ (5 mol%)5.36251.24533.66La10Si6O27:Dy3+ (5 mol%)5.67257.09535.61La10Si6O27:Tb3+ (5 mol%)5.79227.09534.64


### 3.5. Electrochemical Study

A three-electrode cell in 0.1 M HCl aqueous electrolytes at room temperature was used to study the electrochemical characteristics of the synthesized LNPs electrodes using CV and EIS. Utilizing CV studies, researchers can gain insight into the reversibility and charge efficiency of the electrode response. The voltammograms of the Eu3+, Sm3+, Dy3+, and Tb3+ ions-doped La10Si6O27 NPs electrodes were obtained at a scan rate of 30 mV/s in 0.1 M HCl Figures 7(a), 7(b), 7(c) and 7(d), respectively. Due to faradaic reactions, the voltammograms displayed two reversible oxidation-reduction peaks (Figure 7) [42, 43]. The LNPs electrode was found to have pseudocapacitive behaviour. The capacitance of the electrodes is thought to be significantly influenced by the ionic conductivity of the electrolyte. The ionic size of the electrolyte must be less than or equal to the pore size of the electrode material in order to achieve high power density and big capacitance values [44]. It is thought that the specific capacitance and ionic conductivity are both influenced by the size of the hydrated ions. (4) was used to compute the specific capacitance of the electrodes [45].(4)Csp=S2mkΔV,where S = area under the CV curve, m = weight of the deposited material on electrode, ΔV = voltage window, and.k = scan rate.Figure 7
CV plots of La10Si6O27:RE3+ (5 mol %) nanophosphors with 30 mV/s scan rates.
(a)(b)(c)(d)The specific capacitance of an electrode is inversely proportional to the area under its CV curve, which represents the total charge (Q) stored. Therefore, a large-area electrode yields a large capacitance at known electrode material loading and potential window masses [46]. Formula (4) was used to calculate the specific capacitance values of La10Si6O27 electrodes containing 5 mol% Eu, Sm, Dy, and Tb. These values were determined to be 115.2, 128.5, 165, and 143.6 Fg− 1, respectively, in 0.1 M HCl at a scan rate of 30 mV/s.The measured ΔEa,c value serves as a clear indicator of how reversible the redox reaction is [47]. The computed ΔEa,c values for LNP electrodes containing 5 mol% of Eu, Sm, Dy, and Tb are shown in Table 3. This could be as a result of the protons moving easily , inside and outside the oxide lattice during oxidation and reduction, respectively [48].Table 3
Oxidation potential (Epa), reduction potential (Epc), and the difference between Epa and Epc of La10Si6O27:RE3+ (5 mol%) NPs electrodes.
Sample namesEpa (V)Epc (V)ΔEac (V)La10Si6O27:Eu3+ (5 mol%)0.3660.3270.039La10Si6O27:Sm3+ (5 mol%)0.3710.3330.038La10Si6O27:Dy3+ (5 mol%)0.3570.3280.029La10Si6O27:Tb3+ (5 mol%)0.3640.3320.032


#### 3.5.1. EIS Measurements

In the frequency range between 1 Hz and 1 MHz at 5 mV amplitude, the charge transfer properties of the LNP electrodes with 5 mol percent of Eu, Sm, Dy, and Tb were investigated (Figure8(a)). The diameters of the semicircles on Nyquist plots were used to measure the resistance at the interface between the active electrode and electrolyte. Low charge transfer resistance (RCt) and high capacitance (C) of the electrode were confirmed by the impedance spectrum of the electrode produced from Dy-doped La10Si6O27 NPs, which showed a reduced arc with a smaller diameter in the region of high frequency [49, 50]. The doped LNP electrodes, on the other hand, were arranged according to their capacitance in the following order Dy > T b > Sm > Eu. The charge transfer resistance (Rct) and capacitance (C) measurements obtained from the equivalent circuit fitting method for the Nyquist plots (Figure 8(b)) are further evidence for the observations made above. These values are reported in Table 4.Figure 8
(a) Nyquist, (b) equivalent circuit, and (c) bode plots of La10Si6O27:RE3+ (5 mol %) nanophosphor.
(a)(b)(c)Table 4
Impedance parameters of different La10Si6O27:RE3+ (5 mol%) NP electrodes in the 0.1 M HCl electrolyte by using the equivalent circuit (Figure 8(b)).
Names of the electrodeRCt (Ω)Cdl (F)RS (Ω)R1 (Ω)Q1 (rad/S)Q2 (F)La10Si6O27:Eu3+ (5 mol%)406.40.0005670.6546542.57.65 × 10−8 0.67618.977 × 10−5 0.8865La10Si6O27:Sm3+ (5 mol%)262.20.0008850.7784754.95.68 × 10−8 0.9616.286 × 10−4La10Si6O27:Dy3+ (5 mol%)60.280.0065340.9216798.44.887 × 10−8 0.784.715 × 10−3 0.5La10Si6O27:Tb3+ (5 mol%)69.560.00014240.6784568.55.967 × 10−8 0.5567.448 × 10−8 0.890In the Warburg element “W” is represented by a straight line in the low-frequency area of the Nyquist plots. It shows that electrolyte ions and electrons have diffused into the LNP electrode’s surface pores [51]. The transition of the plot from the high-frequency to the mid-frequency region resulted in the diffusion of the electrolyte ions. The constant phase element (Q1) and the low-frequency capacitance (Q2) are both parallel to the charge-transfer resistance (Rct) and leakage resistance (R1), respectively [52]. The phase angle and frequency of La10Si6O27:RE3+ (5 mol %) electrodes are related, as shown by the Bode graphs in Figure 8(c). Comparing the Dy3+-doped La10Si6O27 samples to those doped with other rare earth ions, it was found that the phase angle was 57o, which is near to the behaviour of an ideal capacitor (−90o) [53].The detection of paracetamol was carried out using cyclic voltammograms of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb - 5 mol %) NPs (Figures 9–12). The effectiveness of the carbon paste electrodes made from La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb - 5 mol %) NPs for sensor application in acidic medium from 1 mM to 5 mM quantity is supported by the sharp shift in the locations of the oxidation and reduction peak. The figures show that for the constructed electrode, a cathode reduction peak and an anodic oxidation peak occurred at varied potential values. Table 5 displays the peak appearance at various potentials during the detection of paracetamol using La10Si6O27 electrode material with various dopants (Eu, Sm, Dy, and Tb - 5 mol %) [54].Figure 9
Cyclic voltammogram of La10Si6O27:Eu3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 10
Cyclic voltammogram of La10Si6O27:Sm3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 11
Cyclic voltammogram of La10Si6O27:Dy3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 12
Cyclic voltammogram of La10Si6O27:Tb3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Table 5
Peak appearance at various potentials during the detection of paracetamol using the La10Si6O27 electrode material with various dopants.
Sample namesOxidation peak potential (V)Reduction peak potential (V)La10Si6O27:Eu3+ (5 mol%)0.62, 0.370.34La10Si6O27:Sm3+ (5 mol%)0.78, 0.510.48La10Si6O27:Dy3+ (5 mol%)0.62, 0.350.34La10Si6O27:Tb3+ (5 mol%)0.6, 0.62, 0.360.32The reduced peak current response as a function of paracetamol concentration is shown in Figure13. The plots reveal a more or less linear trend, and the La10Si6O27:RE3+ (5 mol%) sensor exhibits an improved reduction current with an increase in paracetamol concentration [25–27, 54]. These findings support the increased sensitivity of modified La10Si6O27:RE3+ electrodes (5 mol percent) to the electrochemical detection of paracetamol.Figure 13
Peak concentration of La10Si6O27:RE3+ (5 mol %) nanophosphor.


## 3.1. Powder X-Ray Diffraction Studies

Figure1 depicts the powder X-ray diffraction (PXRD) patterns of La10Si6O27:RE3+(5 mol%) NPs. The presence of peaks at 2θ values of 21.38°, 22.39°, 24.98°, 27.23°, 28.35°, 31.05°, 31.27°, 32.29°, 39.03°, 42.40°, 43.64°, 45.33°, 46.67°, 47.80°, 49.15°, 50.05°, and 50.61° corresponding to miller indices of crystal planes (200), (111), (002), (102), (210), (211), (112), (300), (103), (113), (400), (222), (312), (213), (321), (410), and (402) is in compliance with hexagonal oxyapatite structure (P63/m) with JCPDS card no.53–0291 [7, 28, 29]. Rietveld analysis revealed the values of lattice constant values, a = b = 9.7375Ao and c = 7.1826Ao. For all the dopants, the highest diffraction peak is at 2θ = 31.05°. The crystallite sizes of the LNPs were estimated using Scherrer’s formula,(2)D=kλβcosθ,where Β = full width at half maximum (FWHM) of the diffraction peak, λ = wavelength of X-ray (1.5418 A0), θ = Bragg’s angle, and K = Scherrer’s constant varying from (0.88–0.92).Figure 1
PXRD patterns of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb -5 mol %) nanophosphors.The average value of 0.9 was taken for the calculation. The average crystallite size was found to be between 25 and 50 nm. The estimated particle size of La10Si6O27:RE3+ (5 mol%) for (211) planes is shown in Table 1 [30].Table 1
Estimated particle size (nm) of La10Si6O27:RE3+ (5 mol%) NPs for (211) plane.
SamplesPlanesParticle sizes (nm)La10Si6O27:Eu3+ (5 mol%)(211)0.274La10Si6O27:Sm3+ (5 mol%)(211)0.275La10Si6O27:Dy3+ (5 mol%)(211)0.280La10Si6O27:Tb3+ (5 mol%)(211)0.273


## 3.2. Fourier Transform Infrared Spectroscopy

Fourier transform infrared (FTIR) spectra of undoped La10Si6O27 and RE3+ and doped La10Si6O27 (5 mol %) samples were recorded in the wavenumber range between 400 and 4000 cm−1 (Figure 2.). The bending modes of the Si-O-Si band vibrations are responsible for the IR band between 440 and 490 cm−1. The Si-O symmetric stretch presents an extreme band centered from 630 to 645 cm−1. The band appeared in the range of 854 to 870 cm−1 is due to the Si-O-Si asymmetric stretch. Similarly, the bands at 910 to 1063 cm−1 corresponds to the SiO6 group and 2353 cm−1 corresponds to stretching of the hydroxyl group [31].Figure 2
FTIR spectra of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb -5 mol %) nanophosphors.


## 3.3. Morphological Studies

The morphological characteristics of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb) (5 mol% (100 μm)) were explored by using scanning electron microscopy (SEM). Micrographs revealed fogy and grain shape-like morphology for the synthesized LNPs with a little agglomeration as shown in Figures 3(a)–3(d)). TEM measurements were made to examine the size and morphology of the synthesized La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb) (5 mol% (0.5 μm)) nanoparticles. Figures 4(a)–4(d) depicts TEM images of La10Si6O27:RE3+ (5 mol %) NPs. The micrographs revealed the presence of nearly cubical, hexagonal, and irregular shapes with a little agglomeration having an average particle size ranging between 25 nm and 35 nm [32, 33].Figure 3
SEM image of the La10Si6O27:RE3+nanophosphors.Figure 4
TEM image of the La10Si6O27:RE3+nanophosphors.The crystallite sizes and the particle sizes show a minor discrepancy as estimated from XRD and TEM examinations. When using TEM analysis, the observed size is the outcome of an average weighted by the sixth power of the crystal radius, however when using XRD, the intensity is proportional to the square of the volume. The diffraction signals are stronger for larger diameters than for the smallest ones for crystals that are imperfectly monodisperse. As a result, the size estimated by XRD will always be less than the one examined by the TEM technique [34–36].


## 3.4. Diffuse Reflectance Spectroscopy (DRS) Analysis

The diffused reflectance spectra of La10Si6O27:RE3+ (5 mol %) NPs are shown in Figure 5. Two bands, a high absorption peak and a weak absorption peak could be seen in the spectra of the LNPs doped with rare earth elements. It was discovered that the intensity of the absorption peak increases with the concentration of Tb3+ ions and slightly shifts towards the longer wavelength side [37]. The host LNPs are represented by both the absorption peaks.Figure 5
DRS patterns of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb -5 mol %) nanophosphors.Figure6 shows the estimated direct band gap energy by plotting (f(R) (hv))2 vs energy gap (eV). The energy band gap was determined by using the following equation [38].(3)Dαhϑ=Ahϑ−Egn,where hυ = photon energy, α = absorption coefficient (α = 4πk/λ), k = absorption index or absorbance, k = wavelength in nanometer), Eg = energy band gap, A = constant, and n = 1/2 for the allowed direct band gap [39, 40].Figure 6
Energy band gap spectra of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb -5 mol %) nanophosphors.According to the type of transition, the exponent ‘n’ can have values of 1/2, 2, 3/2, and 3, which correspond to the permitted direct, permitted indirect, prohibited direct, and prohibited indirect transitions, respectively [41]. The estimated eV values are presented in Table 2.Table 2
Estimated energy gap (eV) of La10Si6O27:RE3+ (5 mol%) NPs.
SamplesEnergy gap (eV)Strong absorption peakWeak absorption peakLa10Si6O27:Eu3+ (5 mol%)5.0218.56535.13La10Si6O27:Sm3+ (5 mol%)5.36251.24533.66La10Si6O27:Dy3+ (5 mol%)5.67257.09535.61La10Si6O27:Tb3+ (5 mol%)5.79227.09534.64


## 3.5. Electrochemical Study

A three-electrode cell in 0.1 M HCl aqueous electrolytes at room temperature was used to study the electrochemical characteristics of the synthesized LNPs electrodes using CV and EIS. Utilizing CV studies, researchers can gain insight into the reversibility and charge efficiency of the electrode response. The voltammograms of the Eu3+, Sm3+, Dy3+, and Tb3+ ions-doped La10Si6O27 NPs electrodes were obtained at a scan rate of 30 mV/s in 0.1 M HCl Figures 7(a), 7(b), 7(c) and 7(d), respectively. Due to faradaic reactions, the voltammograms displayed two reversible oxidation-reduction peaks (Figure 7) [42, 43]. The LNPs electrode was found to have pseudocapacitive behaviour. The capacitance of the electrodes is thought to be significantly influenced by the ionic conductivity of the electrolyte. The ionic size of the electrolyte must be less than or equal to the pore size of the electrode material in order to achieve high power density and big capacitance values [44]. It is thought that the specific capacitance and ionic conductivity are both influenced by the size of the hydrated ions. (4) was used to compute the specific capacitance of the electrodes [45].(4)Csp=S2mkΔV,where S = area under the CV curve, m = weight of the deposited material on electrode, ΔV = voltage window, and.k = scan rate.Figure 7
CV plots of La10Si6O27:RE3+ (5 mol %) nanophosphors with 30 mV/s scan rates.
(a)(b)(c)(d)The specific capacitance of an electrode is inversely proportional to the area under its CV curve, which represents the total charge (Q) stored. Therefore, a large-area electrode yields a large capacitance at known electrode material loading and potential window masses [46]. Formula (4) was used to calculate the specific capacitance values of La10Si6O27 electrodes containing 5 mol% Eu, Sm, Dy, and Tb. These values were determined to be 115.2, 128.5, 165, and 143.6 Fg− 1, respectively, in 0.1 M HCl at a scan rate of 30 mV/s.The measured ΔEa,c value serves as a clear indicator of how reversible the redox reaction is [47]. The computed ΔEa,c values for LNP electrodes containing 5 mol% of Eu, Sm, Dy, and Tb are shown in Table 3. This could be as a result of the protons moving easily , inside and outside the oxide lattice during oxidation and reduction, respectively [48].Table 3
Oxidation potential (Epa), reduction potential (Epc), and the difference between Epa and Epc of La10Si6O27:RE3+ (5 mol%) NPs electrodes.
Sample namesEpa (V)Epc (V)ΔEac (V)La10Si6O27:Eu3+ (5 mol%)0.3660.3270.039La10Si6O27:Sm3+ (5 mol%)0.3710.3330.038La10Si6O27:Dy3+ (5 mol%)0.3570.3280.029La10Si6O27:Tb3+ (5 mol%)0.3640.3320.032


### 3.5.1. EIS Measurements

In the frequency range between 1 Hz and 1 MHz at 5 mV amplitude, the charge transfer properties of the LNP electrodes with 5 mol percent of Eu, Sm, Dy, and Tb were investigated (Figure8(a)). The diameters of the semicircles on Nyquist plots were used to measure the resistance at the interface between the active electrode and electrolyte. Low charge transfer resistance (RCt) and high capacitance (C) of the electrode were confirmed by the impedance spectrum of the electrode produced from Dy-doped La10Si6O27 NPs, which showed a reduced arc with a smaller diameter in the region of high frequency [49, 50]. The doped LNP electrodes, on the other hand, were arranged according to their capacitance in the following order Dy > T b > Sm > Eu. The charge transfer resistance (Rct) and capacitance (C) measurements obtained from the equivalent circuit fitting method for the Nyquist plots (Figure 8(b)) are further evidence for the observations made above. These values are reported in Table 4.Figure 8
(a) Nyquist, (b) equivalent circuit, and (c) bode plots of La10Si6O27:RE3+ (5 mol %) nanophosphor.
(a)(b)(c)Table 4
Impedance parameters of different La10Si6O27:RE3+ (5 mol%) NP electrodes in the 0.1 M HCl electrolyte by using the equivalent circuit (Figure 8(b)).
Names of the electrodeRCt (Ω)Cdl (F)RS (Ω)R1 (Ω)Q1 (rad/S)Q2 (F)La10Si6O27:Eu3+ (5 mol%)406.40.0005670.6546542.57.65 × 10−8 0.67618.977 × 10−5 0.8865La10Si6O27:Sm3+ (5 mol%)262.20.0008850.7784754.95.68 × 10−8 0.9616.286 × 10−4La10Si6O27:Dy3+ (5 mol%)60.280.0065340.9216798.44.887 × 10−8 0.784.715 × 10−3 0.5La10Si6O27:Tb3+ (5 mol%)69.560.00014240.6784568.55.967 × 10−8 0.5567.448 × 10−8 0.890In the Warburg element “W” is represented by a straight line in the low-frequency area of the Nyquist plots. It shows that electrolyte ions and electrons have diffused into the LNP electrode’s surface pores [51]. The transition of the plot from the high-frequency to the mid-frequency region resulted in the diffusion of the electrolyte ions. The constant phase element (Q1) and the low-frequency capacitance (Q2) are both parallel to the charge-transfer resistance (Rct) and leakage resistance (R1), respectively [52]. The phase angle and frequency of La10Si6O27:RE3+ (5 mol %) electrodes are related, as shown by the Bode graphs in Figure 8(c). Comparing the Dy3+-doped La10Si6O27 samples to those doped with other rare earth ions, it was found that the phase angle was 57o, which is near to the behaviour of an ideal capacitor (−90o) [53].The detection of paracetamol was carried out using cyclic voltammograms of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb - 5 mol %) NPs (Figures 9–12). The effectiveness of the carbon paste electrodes made from La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb - 5 mol %) NPs for sensor application in acidic medium from 1 mM to 5 mM quantity is supported by the sharp shift in the locations of the oxidation and reduction peak. The figures show that for the constructed electrode, a cathode reduction peak and an anodic oxidation peak occurred at varied potential values. Table 5 displays the peak appearance at various potentials during the detection of paracetamol using La10Si6O27 electrode material with various dopants (Eu, Sm, Dy, and Tb - 5 mol %) [54].Figure 9
Cyclic voltammogram of La10Si6O27:Eu3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 10
Cyclic voltammogram of La10Si6O27:Sm3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 11
Cyclic voltammogram of La10Si6O27:Dy3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 12
Cyclic voltammogram of La10Si6O27:Tb3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Table 5
Peak appearance at various potentials during the detection of paracetamol using the La10Si6O27 electrode material with various dopants.
Sample namesOxidation peak potential (V)Reduction peak potential (V)La10Si6O27:Eu3+ (5 mol%)0.62, 0.370.34La10Si6O27:Sm3+ (5 mol%)0.78, 0.510.48La10Si6O27:Dy3+ (5 mol%)0.62, 0.350.34La10Si6O27:Tb3+ (5 mol%)0.6, 0.62, 0.360.32The reduced peak current response as a function of paracetamol concentration is shown in Figure13. The plots reveal a more or less linear trend, and the La10Si6O27:RE3+ (5 mol%) sensor exhibits an improved reduction current with an increase in paracetamol concentration [25–27, 54]. These findings support the increased sensitivity of modified La10Si6O27:RE3+ electrodes (5 mol percent) to the electrochemical detection of paracetamol.Figure 13
Peak concentration of La10Si6O27:RE3+ (5 mol %) nanophosphor.


## 3.5.1. EIS Measurements

In the frequency range between 1 Hz and 1 MHz at 5 mV amplitude, the charge transfer properties of the LNP electrodes with 5 mol percent of Eu, Sm, Dy, and Tb were investigated (Figure8(a)). The diameters of the semicircles on Nyquist plots were used to measure the resistance at the interface between the active electrode and electrolyte. Low charge transfer resistance (RCt) and high capacitance (C) of the electrode were confirmed by the impedance spectrum of the electrode produced from Dy-doped La10Si6O27 NPs, which showed a reduced arc with a smaller diameter in the region of high frequency [49, 50]. The doped LNP electrodes, on the other hand, were arranged according to their capacitance in the following order Dy > T b > Sm > Eu. The charge transfer resistance (Rct) and capacitance (C) measurements obtained from the equivalent circuit fitting method for the Nyquist plots (Figure 8(b)) are further evidence for the observations made above. These values are reported in Table 4.Figure 8
(a) Nyquist, (b) equivalent circuit, and (c) bode plots of La10Si6O27:RE3+ (5 mol %) nanophosphor.
(a)(b)(c)Table 4
Impedance parameters of different La10Si6O27:RE3+ (5 mol%) NP electrodes in the 0.1 M HCl electrolyte by using the equivalent circuit (Figure 8(b)).
Names of the electrodeRCt (Ω)Cdl (F)RS (Ω)R1 (Ω)Q1 (rad/S)Q2 (F)La10Si6O27:Eu3+ (5 mol%)406.40.0005670.6546542.57.65 × 10−8 0.67618.977 × 10−5 0.8865La10Si6O27:Sm3+ (5 mol%)262.20.0008850.7784754.95.68 × 10−8 0.9616.286 × 10−4La10Si6O27:Dy3+ (5 mol%)60.280.0065340.9216798.44.887 × 10−8 0.784.715 × 10−3 0.5La10Si6O27:Tb3+ (5 mol%)69.560.00014240.6784568.55.967 × 10−8 0.5567.448 × 10−8 0.890In the Warburg element “W” is represented by a straight line in the low-frequency area of the Nyquist plots. It shows that electrolyte ions and electrons have diffused into the LNP electrode’s surface pores [51]. The transition of the plot from the high-frequency to the mid-frequency region resulted in the diffusion of the electrolyte ions. The constant phase element (Q1) and the low-frequency capacitance (Q2) are both parallel to the charge-transfer resistance (Rct) and leakage resistance (R1), respectively [52]. The phase angle and frequency of La10Si6O27:RE3+ (5 mol %) electrodes are related, as shown by the Bode graphs in Figure 8(c). Comparing the Dy3+-doped La10Si6O27 samples to those doped with other rare earth ions, it was found that the phase angle was 57o, which is near to the behaviour of an ideal capacitor (−90o) [53].The detection of paracetamol was carried out using cyclic voltammograms of La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb - 5 mol %) NPs (Figures 9–12). The effectiveness of the carbon paste electrodes made from La10Si6O27:RE3+ (Eu, Sm, Dy, and Tb - 5 mol %) NPs for sensor application in acidic medium from 1 mM to 5 mM quantity is supported by the sharp shift in the locations of the oxidation and reduction peak. The figures show that for the constructed electrode, a cathode reduction peak and an anodic oxidation peak occurred at varied potential values. Table 5 displays the peak appearance at various potentials during the detection of paracetamol using La10Si6O27 electrode material with various dopants (Eu, Sm, Dy, and Tb - 5 mol %) [54].Figure 9
Cyclic voltammogram of La10Si6O27:Eu3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 10
Cyclic voltammogram of La10Si6O27:Sm3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 11
Cyclic voltammogram of La10Si6O27:Dy3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Figure 12
Cyclic voltammogram of La10Si6O27:Tb3+ (5 mol%, scan rate at 30 mV/s) in the sensing of paracetamol sensor concentration range (1–5 mM).
(a)(b)(c)(d)(e)Table 5
Peak appearance at various potentials during the detection of paracetamol using the La10Si6O27 electrode material with various dopants.
Sample namesOxidation peak potential (V)Reduction peak potential (V)La10Si6O27:Eu3+ (5 mol%)0.62, 0.370.34La10Si6O27:Sm3+ (5 mol%)0.78, 0.510.48La10Si6O27:Dy3+ (5 mol%)0.62, 0.350.34La10Si6O27:Tb3+ (5 mol%)0.6, 0.62, 0.360.32The reduced peak current response as a function of paracetamol concentration is shown in Figure13. The plots reveal a more or less linear trend, and the La10Si6O27:RE3+ (5 mol%) sensor exhibits an improved reduction current with an increase in paracetamol concentration [25–27, 54]. These findings support the increased sensitivity of modified La10Si6O27:RE3+ electrodes (5 mol percent) to the electrochemical detection of paracetamol.Figure 13
Peak concentration of La10Si6O27:RE3+ (5 mol %) nanophosphor.


## 4. Conclusions

Rare-earth (RE = Eu, Sm, Dy, and Tb) ions-doped lanthanum silicate (La10Si6O27) nanophosphors were synthesized by a solution combustion method. The diffraction patterns of as prepared La10Si6O27:RE3+ (5 mol %) nanophosphors can be indexed as the hexagonal oxyapatite phase. The average crystallite size was calculated to be in the range between 25 nm and 50 nm, and the equivalent was affirmed by TEM. The TEM micrographs revealed the presence of nearly cubical, hexagonal, and irregular shapes with a little agglomeration. The energy band gap (Eg) values were found to be in the range from 5.0 eV to 5.8 eV. The cyclic voltametric measurement confirmed the high effectivity of LNPs in sensing of paracetamol in acidic medium, with a short response time. The doped LNP electrodes with different dopants followed the order of their capacitance as Dy˃Tb˃Sm˃Eu. The results suggest that RE-doped lanthanum silicate can be a simple and cost-effective electrode material for sensing applications.



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*Source: 7429133-2022-09-23.xml*