Highly sensitive pH dependent acetone sensor based on ultrananocrystalline diamond materials at room temperature

Abstract

Diamond-based materials often considered inappropriate for sensor device applications, however these robust materials exhibit unpredictable electrochemical sensing properties. Herein, we report a high-performance novel acetone sensor based on diamond materials such as nanocrystalline diamond (NCD) films and ultrananocrystalline diamond (UNCD) films. A new approach has been undertaken for the selective detection of liquid acetone in water using an extended gate field effect transistor (EGFET), since these two liquids are miscible with a pH value of 7. Systematic studies revealed that ultra-high acetone sensing properties were observed from the UNCD based sensor. A higher proportion of grain boundaries exists in the UNCD film with the presence of graphitic phase (sp² content) built conduction paths for electron transportation, as revealed by tunneling electron microscopy (TEM). The excellent sensing behavior of these UNCD based sensors was achieved owing to the reduction of the diamond-to-Si interfacial resistance, which increased the conductivity as analyzed by electrochemical impedance spectroscopy (EIS) investigations. Thus the electrical conductivity of UNCD material shows an enhanced stability and reproducibility towards acetone.

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1. Introduction

Diamond semiconductor materials have a wide band gap of 5.4 eV, and are very hard with a long stable life time in several electronic applications.^1–4 Moreover, diamond materials have various unique properties such as chemical inertness, large electro-chemical potential, and high stability in chemically harsh atmospheres.^5,6 In recent decades, electrochemical sensors have been fabricated using oxide materials, with superior sensing properties. However oxide material based electrochemical sensors are temperature dependent, which imparts difficulty in device fabrication and causes low device life time.^7–9 Hence, commercial manufactures are keen to develop novel and better performance devices. Synthetic diamonds are normally used as an electronic material because of their highly conductive nature¹⁰ and are mainly used as potential candidates in the detection of molecular interactions between liquid and solid interfaces.¹¹ Correspondingly, nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) are two of the most studied forms of diamond materials and have distinct properties and growth conditions.

The grain size of diamond films could be diverse with different growth parameters and different gas mixtures. Large sized diamond grains were synthesized using higher amount of CH₄/H₂ in plasma, which exhibits a large electronic bandgap.¹² However, the addition of Ar in the CH₄/H₂ plasma has been discovered to trigger the formation of small-sized grains called nanocrystalline diamond (NCD) films.¹³ However, the higher amount of H₂ plasma in conventional MPE-CVD method for growing MCD or NCD films on Si-substrates generally forms amorphous carbon (a-C) layer. The atomic H₂ in the plasma specifically etch the graphite amount and forms large-sized grains.¹⁴ The CH₄/Ar plasma without the addition of H₂, markedly increases the richness of diamond growing species (C₂) and non-diamond content (graphite) forming species (CH). Additionally Ar rich plasma stimulates the charged species during the growth of diamond films, and forms ultra nanocrystalline diamond (UNCD) with size of less than 10 nm.¹⁵ The reduction in diamond grain size increases the proportion of UNCD grain boundaries, which has sp² (non-diamond-graphite). The ultra nanosized grains along graphite phase play crucial role in the enhanced electrical properties of UNCD films. These superior aspects enable UNCD films to be more conductive than other form of diamond films such as MCD and NCD.¹²

In accordance with the survey of EPA (Environmental Protection Agency) and National health foundations, the detection of volatile organic compounds (VOCs) is an essential concern for ecological regulations.^16,17 Moreover as stated by the Clean Air Act amendments of 1990, acetone is listed in the U.S. EPA's SNAP (Significant New Alternatives Program) as an adequate substitute to ozone depleting compounds and hazardous pollutant.^18,19 It is well known that VOCs vaporize rapidly and are precarious to human health. One such VOC is acetone, which is generally used in laboratories and chemical reagent industries.²⁰ Additionally, if acetone blends with water, there is less chance to vaporize completely and leave huge amount of toxicity.²¹ Hence, the detection of acetone in water is essential as well as indispensable because it may cause some health issues such as eye and nose irritation (short term effects), allergies, reproductive defects, weight loss and even unconsciousness (long term effects).²² Nevertheless, corresponding to the clean water act (CWA), acetone is not a priority pollutant. But still health organizations elucidates that the detection of acetone is essential if the surroundings exposed to high concentrations of acetone.²³ Hence, acetone sensors have been used extensively for commercial applications such as environmental safety, chemical, biological, and also medical applications.²⁴

Ion sensitive field effect transistors (ISFET) is one of the pH dependent electrochemical sensors, which were used to detect pH values and liquid chemicals in the last few decades.²⁵ However, they have some disadvantages related to hysteresis drift, thermal and long-term drift. The device accuracy is also damaged when these devices are utilized in long time period measurement results poor sensitivity.²⁶ Extended gate field effect transistor (EGFET) is developed to overcome such drawbacks of ISFET and also offers several advantages such as high sensitivity, stability and device flexibility.²⁷ The EGFET configuration normally have two parts as sensor structure (with sensitive membrane) and a metal-oxide-semiconductor field-effect transistor (MOSFET) structure. Thus the EGFET configuration gives a fast linear response with an applied gate voltage, which is a significant factor to calculate the sensitivity of the sensor. Moreover, in EGFET, the source and drain current was directly connected to the ions' in the chemical solution, which is specifically suitable for detecting liquid chemicals.^28,29

In this study, we have fabricated and observed the electrochemical sensing properties of two different diamond films for the selective detection of liquid acetone in water. We used nanocrystalline diamond films and ultrananocrystalline diamond films for sensing liquid acetone at room temperature. Correspondingly, the curves of sensitivity, linearity, and reliability were measured by constant I–V characteristics. The highly enhanced sensing properties illustrate that the UNCD based materials are noble candidates for electrochemical sensor applications.

2. Experimental methods

2.1. Sample preparation

The NCD and UNCD films were grown on Si substrate by using Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) system. Prior to NCD and UNCD film growth, pre-seeding of Si substrates were done by ultrasonication for 45 min in a methanol solution, which has nano-sized Ti powders (32.5 nm) and diamond powders (5 nm). The Si substrates were ultrasonicated again for 1 min in methanol solution to remove any loosely bound diamond particle from Si surface. Subsequently, the NCD films were grown by the composition of 2% CH₄, 48% Ar, and 50% H₂ plasma along with a 1200 W microwave power and a 65 Torr pressure. Similarly, the UNCD films were grown by 2% CH₄/98% Ar plasma with a 1000 W microwave power with 150 Torr pressure for 30 min.

2.2. Characterization techniques

The surface morphology of NCD and UNCD films were characterized using field emission scanning electron microscopy (FESEM, JSM-6500F) with an acceleration voltage of 15 kV. The crystalline value and bonding structure of the films were characterized by Raman spectroscopy with an excitation wavelength of 514 nm. The binding energies of the NCD and UNCD films were taken by X-ray Photoelectron Spectroscopy (XPS). The crystal structure of the sample was characterized by X-ray diffraction (XRD) (D2 PHASER-X-ray Powder Diffraction, BRUKER) using Cu Kα1 radiation (λ = 1.54056 Å). The comprehensive bonding and microstructure of the samples were examined using TEM (Joel 2100F).

2.3. Sensor fabrication and detection method of acetone in water

The electrochemical properties of NCD and UNCD thin films were characterized using a source measure unit (Keithley 237) and EGFET, which is shown in Fig. 1. For acetone detection studies, the NCD and UNCD thin films were bound to metal wire with silver paste and packed with epoxy resin. The NCD and UNCD electrodes (sensing membrane) were used as working electrode and the reference electrode was Ag/AgCl. Both the electrodes were connected to a commercially available standard MOSFET device (CD4007UB) and dipped in the chemical solution (mixture of acetone and water in mL). The electrodes were connected to source–drain and the sensing membrane is connected to the gate, which is known as EGFET. Each acetone sensors were connected to a computer controlled source meter (Keithley 237) and, different ratios of water/acetone mixtures [1 : 0 (1 mL : 0 mL), 1 : 1 (1 mL : 1 mL), 1 : 2 (1 mL : 2 mL) and 1 : 3 (1 mL : 3 mL)] in a dark box were measured by current–voltage (I–V) characteristics at room temperature. The source–drain voltage (V_DS) was fixed at 0.3 V whereas the reference electrode voltage (V_GS) swept from 0 to 3.5 V. The acetone sensitivity can be measured by plotting the slope of the V_GS as a function of the acetone concentrations when the source drain current is fixed at 400 μA linear region is chosen from the linear region of the I–V curve. On the other hand, the electrochemical impedance spectroscopy (EIS) investigations of the UNCD and NCD films were executed in 0.1 M KCl solution, containing 5.0 mM [Fe(CN)₆]^3−/4− electrolyte. The UNCD or NCD films act as the working electrode with Ag/AgCl (sat. KCl) as a reference electrode and Pt wire as a counter electrode.

Fig. 1 The experimental set-up of EGFET-acetone sensor.

3. Results and discussion

Fig. 2a and b shows the surface morphology of the NCD and UNCD films obtained by FESEM micrographs, respectively. The surface morphology of NCD to UNCD films shows a decrease in their grain size and, it is seen that the grain size of NCD film is comparatively larger than UNCD films due to their different growth condition. However, from FESEM image, the exact grain size and the surface roughness cannot be observed. Hence, in order to obtain the surface topology, grain size and surface roughness of the NCD and UNCD films, AFM has performed. The AFM images of Fig. 2c and d displays the cauliflower like structure of both NCD and UNCD films. The observed surface roughness of NCD film is 46.2 nm whereas UNCD film is 8.17 nm. It is known that a decrease in the surface roughness results a decrease in the grain size and increase in the grain boundary of the diamond films, simultaneously.¹² Thus the UNCD films exhibit smallest surface roughness as compared to NCD film.

Fig. 2 FESEM micrographs of (a) NCD films and (b) UNCD films; AFM micrographs of (c) NCD films and (d) UNCD films.

In order to differentiate the chemical bonding of NCD and UNCD films, Fig. 3a shows the Raman spectra of NCD and UNCD films, which contains sharp Raman peak at ∼1320 cm⁻¹ (D*-band), ∼1350 cm⁻¹ (D-band) and ∼1580 cm⁻¹ (G-band). Also some diffuse Raman peaks at ∼1140 cm⁻¹ (ν₁-band), ∼1475 cm⁻¹ (ν₃-band) and ∼1580 cm⁻¹ (G-band). The sharp D*-band represents large sized diamond grains in NCD films (Curve I), however the D*-band completely disappeared in the Raman spectra of UNCD films (Curve II), which correspond to ultra-small sized diamond grains (Curve II).^30,31 The ν₁-band and ν₃-band originate from the vibration of trans-polyacetylene (t-pa) segments present in the grain boundaries of the films.³² The D-band and G-band resonance peaks characterizes the disordered carbon and graphite phases.³³ Besides, the G-band resonance peak of UNCD film shifts slightly towards ∼1600 cm⁻¹, revealing the stimulation of the nanographitic phase.^31–34

Fig. 3 (a) Raman spectra of NCD films and UNCD films; XPS spectra of NCD films and UNCD films (b) C 1s peak of NCD films (c) C 1s peak of UNCD films; (d) XRD spectra of NCD and UNCD films.

Furthermore, XPS spectra were studied to estimate the chemical bonding of the NCD and UNCD films using Al Kα-line. In Fig. 3b and (c), the C 1s peak of NCD and UNCD films were fitted by using the Lorentzian peaks at their consequent binding energies of sp² C=C (∼284.01 eV), sp³ C–C (∼284.65 eV), and sp³ C–O–C (∼285.92 eV).³⁵ As reported, the UNCD film grown by CH₄/Ar plasma has large number of sp² bonds with excellent conductivity.³⁶ The XPS results of UNCD film shows that sp² C=C peak and sp³ C–O peak is markedly larger than NCD films. Hence, the UNCD film surface could be slightly hydrogen terminated and also it has some oxygen adsorbates due to the high density of grain boundaries in it. However, the NCD film shows much lesser concentration of sp³ C–O bond owing to its growth condition of 2% CH₄, 48% Ar, and 50% H₂ plasma. Hence, the surface of NCD film is fully hydrogen terminated. The XRD spectra were carried out to elucidate the crystalline grain size of NCD and UNCD films, shown in Fig. 3d. The observed diffraction peaks at 2θ values of 43.95° (111) and 75.75° (220) represents a cubic diamond, which is in accord with JCPDS 00-011-1249. It's observed that the diffraction peak (111) FWHM value of NCD film (0.34) is markedly lower than UNCD film (1.05), which is correlated to decrease in the crystalline size and increase in the grain boundaries of the UNCD films.^12,37

Fig. 4a and b shows the source drain current (I_DS) and the source drain voltage (V_DS) of the EGFET with NCD and UNCD films operated at reference voltage of 3.5 V. The electrochemical measurements were carried out to investigate the detection of acetone under the ratio of water/acetone mixtures at room temperature. The detection measurement of acetone was implemented by changing the concentration of acetone ranging from (1 mL to 3 mL) with constant water volume (1 mL). As mentioned earlier, the sensitivity of EGFET is calculated by an applied gate voltage according to the site binding theory. Thus, the number of binding sites presenting on the sensing film (sensing membrane) leads to change in the surface potential voltage between the electrolyte interface and the sensing layer. Fig. 4c shows the linearity curves (obtained from the I_DS–V_DS with applied gate voltage and reference voltage) of NCD and UNCD films, respectively. The change in the surface potential voltage is normally depend on the pH value of the electrolytic solution, thus if the chemical solution is pH dependent, then the Nernst equation³⁸ can be adopted to reveal their sensitivity in terms of mV per chemical concentration. In Fig. 5c the obtained sensitivity of NCD is approximately 51 mV per acetone concentration and UNCD is approximately 88 mV per acetone concentration, which is significantly higher than NCD films.

Fig. 4 I–V characteristics of sensitivity curves for (a) NCD sensor, (b) UNCD sensor, (c) linearity curves of (I) NCD sensors and (II) UNCD sensors and (d) test for other VOCs mixture in water.

Fig. 5 Constant voltage (I–t) of reliability graph for (a) UNCD sensors, and (b) NCD film sensors.

Selectivity and stability is a significant factor for all real-time sensor. To confirm the selectivity of water : acetone mixture, we have performed the test for other VOCs such as ethanol, methanol and isopropanol (IPA) in water. The sensitivity is also calculated for every water : VOCs mixture (water : ethanol, water : methanol and water : IPA) and the obtained sensitivities are given in the respective bar diagram, which is shown in Fig. 4d. However, acetone shows high sensitivity among other VOCs. Fig. 5a and b displays the real time response of NCD and UNCD sensor dipped for 60 s in pH dependent chemical solution of water : acetone mixture. The source drain current (I_DS) decreases linearly as the ratio of water : acetone increases from 1 : 0 to 1 : 4 and increases linearly as ratio of water : acetone decreases from 1 : 4 to 1 : 0. This kind of curve normally shows the real time response and reliability of the pH dependent electrochemical sensors.²⁸ It is seen that the UNCD film sensor reveals good sensitivity and reliability as compared with NCD films. As mentioned earlier, the UNCD film has oxygen adsorbates on their surface because of high density of grain boundaries. However, SEM and AFM cannot give the sufficient information about the exact grain size and grain boundaries of the diamond films. Hence, TEM micrographs were performed to analyze the grain boundaries of NCD and UNCD sensing membranes.

Fig. 6a and b shows the bright and dark field area TEM microstructure of NCD films, which indicate that the films consist of diamond grains ranging from 50 to 100 nm in size. The result observed from NCD films discloses that these materials are predominately diamond with few hundred nanometers in grain size. The comprehensive microstructure of the diamond grains in NCD films was examined using higher solution TEM (HRTEM) as shown in Fig. 6c. The microstructure of the NCD films is better illustrated using a Fourier-transformed (FT₀) diffractogram corresponding to the diamond phase as marked in area 1 (ft₁ image) and area 2 (ft₂ image) to reveal the a-C phase along with no grain boundaries between grains due to the large grain size.

Fig. 6 TEM images of NCD films. (a) Bright field area NCD films, (b) dark field area of NCD films, and (c) the selective area electron diffraction (SAED) patterns of NCD films.

Fig. 7a and b shows the bright and dark field TEM of UNCD films, which have small grains of around 5–10 nm in size besides containing a greater proportion of sp² phase along the more grain boundaries. The films comprise of diamond grains along with graphitic phases as highlighted as yellow (diamond) and green (graphitic phases) color as shown in the Fig. 7b. The HRTEM microstructure reveals that the diamond aggregates in UNCD films, which are apparently made by combination of the ultra-small diamond grains, with higher proportion of grain boundaries as compared with the NCD films. The FT_0b image corresponds to the whole UNCD film structure image shows in Fig. 7c, which comprises donut-shaped ring in the middle. The diffuse ring is indicating the presence of sp²-bonded carbon, graphitic phase. Furthermore region 4 and 5 (ft₄) and (ft₅) reveals the diamond materials in the films. There was a fine donut shaped region comprising sp²-(graphitic) bonded phase, which is illustrated by area 6 (ft₆) and area 7 (ft₇) image as shown in Fig. 7c. Earlier reports discovered that the graphitic phases are more conducting than the a-C phases, hence the UNCD with sp² graphitic films are better than NCD with a-C films. The grain boundary allows the electron flow in the whole films. Also the formation of sp²-bonded graphitic phases at the grain boundaries generates conduction channels for the easy transportation of electrons.^36,39

Fig. 7 TEM images of UNCD films. (a) Bright field area of UNCD films, (b) dark field area of UNCD films and (c), selective area electron diffraction (SAED) patterns of UNCD films.

From the results of TEM micrographs, it is revealed that the UNCD film has higher proportion of grain boundary with smaller grain size as compared with the NCD films. As reported Debabrata et al. the decrease in the grain size from NCD to UNCD film increase the density of grain boundary, which is filled with graphitic carbon (non-diamond) and enhancing the electrical conductivity of the UNCD films.¹² Furthermore, we have performed the impedance spectroscopy to assure the electrical resistivity of the NCD and UNCD films, which is shown in Fig. 8. It should be noted that the UNCD films consist of ultra-small diamond grains separated by grain boundaries, which are too small to be resolved by EIS measurements. As to the interface located in between the diamond (UNCD or NCD) and the Si substrate, the interfacial resistance R_i can be very large if there exists non-conducting a-C phase formation prior to diamond nuclei. Also the interfacial capacitance C_i is large due to the small thickness of this interfacial layer (i.e., a-C phase). The interfacial layer can also be modelled as a lump circuit, which is parallel combination of R_i and C_i (inset, Fig. 8), where equivalent circuit demonstrates the Si substrate resistance (R_Si), diamond resistance (R_dia).

Fig. 8 The Cole–Cole plots of NCD and UNCD films.

In principle, the lump circuits form semicircle in Cole–Cole plots, which plots the imaginary part of impedance z′′(ω) against the real part of impedance z′(ω) at various measuring frequencies. The interfacial resistance/capacitance are relatively large compared with those of the substrate and diamond, the behavior of interfacial lump circuit R_i–C_i dominates the Cole–Cole plots, viz. the semicircle observed in Fig. 8 are predominately the frequency response of the lump circuit corresponding to the interfacial layer. The right hand side interception of semi-circle with abscissa, which correspond to low measuring frequency in EIS measurement, represents the interfacial resistance R_i, whereas the left hand side interception represents the Si substrate resistance. Curves I and II in Fig. 8 shows the Cole–Cole plots of UNCD and NCD films, respectively. Moreover, these curves indicate that the interface resistance of UNCD films is around (R_i)_UNCD 8954 Ω, whereas the interface resistance of NCD films is larger interfacial resistance as (R_i)_NCD = 33 090 Ω. Apparently, the large interfacial resistance (R_i) in NCD films hindered the transport of electrons from substrates, crossing the diamond-to-Si interface to diamond.⁴⁰ It is proven that UNCD sensor has higher electrical conductivity than NCD sensor. The enhanced sensing properties of UNCD sensor is achieved by their electrical conductivity and, the sensor mechanism of UNCD towards acetone will be discussed.

The schematics of possible mechanism according to the binding sites of UNCD (sensing membrane) and electrolyte solution is shown in Fig. 9a and c. These refer to the detection mechanism between diamond and adsorbate layer of an electrolyte solution that contains water : acetone mixtures. Owing to their oxygen adsorbates on surfaces, diamond films can adsorb electrolyte solution.⁴¹ Fig. 10a and c shows before contact, the sensing membrane of UNCD consist of free electron on their surface because of non-diamond C phases (nano-graphitic phases) residing on it. The reaction is determined by the difference in Fermi level (E_F) of UNCD sensing membrane and the chemical potential of electrons in the electrolyte solution (μ_e). On condition that μ_e is below E_F, the free electron in the valence band were transferred to the chemical potential of electrolyte solution (μ_e) when a required voltage is given. After the free electron transferred to μ_e, some of the oxygen adsorbates residing on the UNCD sensor membrane contact with electrolyte solution and thereby reduce CH₃C⁺O to CO₂ and H₂O to OH⁻. Then, the accumulation layer is formed by the compensating holes in the UNCD sensor membrane. It is because of few hydrogen terminated region residing on the UNCD sensor membrane and the space charge induces a chemical potential that raises μ_e.^42–44 Thus, μ_e and E_F reaches equilibrium at the interface as shown in Fig. 9b and d. The dissociation of CH₃C⁺O to CO₂ occurs in water, because water and acetone has pH value of 7, which is a beneficial factor for using EGFET method as electrochemical acetone sensor.

Fig. 9 Possible sensing mechanism between UNCD films and acetone. (a, c) Before contact and Fermi energy diagram of UNCD films to detect acetone, and (b, d) after contact and Fermi energy diagram of UNCD films to detect acetone.

Fig. 10 Chemical reaction between UNCD films and acetone concentrations. (a) Formation of oxygen species and (b) influence of oxygen species react with acetone.

A more intriguing explanation is that the O–H bond exists in water due to the intermolecular forces, which helps to sense acetone in water since acetone does not have O–H bonds. It is because, a pH dependent electrochemical sensor normally requires H⁺ and OH⁻ ions in the electrolyte solution. Hence water/acetone mixture can work under this condition since both are miscible and gives superior response when the applied gate voltage is given to UNCD sensing membrane. However, the other VOCs mixture in water exhibits poor sensitivity as compared to water : acetone mixtures. It is because of other VOCs such as ethanol, methanol and IPA has O–H bond similar to water according to the intermolecular force. Hence, when the UNCD sensing membrane is in contact with such mixtures, they start to dissociate with water and thereby reduce the sensing ability of ethanol, methanol and IPA in water. Similarly, Fig. 10 exhibits the chemical reaction existed between UNCD sensing membrane and acetone. The reaction in Fig. 10a shows how the oxygen species is formed. Thus, sp³ C–O bond of UNCD sensor obtained from high density grain boundaries, have oxygen adsorbates on their surface. On the other hand, the higher amount of nano-graphitic phases (conductive phases) results the high electrical conductivity of UNCD sensor, as revealed by impedance spectroscopy. The chemical reaction between acetone and oxygen species of UNCD sensor in Fig. 10b shows how the acetone electrochemical sensor has been achieved. From the every characterization studies of NCD and UNCD, it is revealed that UNCD has higher grain boundaries as compared to NCD films, which created a nanographitic phase. Such a nano-graphite phase can transport the electrons efficiently along the ultra-small diamond grains, thus efficiently enhance sensitivity and reproducibility towards acetone.

4. Conclusion

In conclusion, the work demonstrates the acetone sensing behavior of two different diamond films at room temperature. The sensing membrane of NCD and UNCD films were developed under different conditions using a MPE-CVD system. The films were characterized using SEM, AFM, TEM, Raman, XRD and XPS to confirm their unique structures and properties. Furthermore, the sensing properties of NCD and UNCD sensor was observed by water : acetone mixtures using EGFET. It was found that both the sensor exhibits electrochemical sensing properties, however, UNCD sensors exhibit excellent sensitivity and reproducibility towards acetone as compared with NCD sensor. It is achieved due to the manifestation of sp² graphitic phase with a large grain boundary structure of UNCD sensor, which influences the acetone sensing properties, as revealed by TEM microstructures. The sp³ C–O bond of UNCD exposed by XPS spectra also plays a significant role in acetone detection, which incorporates the real sensing mechanism of UNCD acetone sensor. This study provides a cost effective sensor fabrication and an easy detection of acetone in water, which is a new perspective and a promising method for electrochemical acetone sensors.

Notes

The authors declare no competing financial interest.

Acknowledgements

The authors like to thank the financial support of Ministry of Science and Technology of Republic of China through the project no. MOST 104-2221-E-011-011.

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