Imaging the Pore Structure of Geological Materials with Bifunctional

Article Cite This: Anal. Chem. 2017, 89, 12550-12555

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Imaging the Pore Structure of Geological Materials with Bifunctional Nanoparticles Na Li,*,† Feng Yi,‡ Peng Liao,‡ Wenling Chen,† Meiyan Fu,† Jun Zheng,† Lin Du,† Jia-liang Liu,‡ and Chong-ying Li*,‡ †

College of Energy Resources, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, Sichuan 610059, China ‡ College of Materials and Chemistry & Chemical Engineering Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu University of Technology, Chengdu, Sichuan 610059, China S Supporting Information *

ABSTRACT: Analysis of complex pore structure of geomaterials is a fundamental issue in geoscience. Here bifunctional nanoparticles with magnetic and fluorescent properties are introduced as novel markers for optical imaging of pore structure in geomaterials. Using the paramagnetic property, powder of the nanoparticle is driven into pores under an external magnetic field, avoiding a tedious sample preparation and eliminating artificial damage of sample preparation in conventional methods. Meanwhile, the fluorescent nanoparticle marker offers a sharp contrast imaging between the rock matrix (black) and pores (bright) under microscopy. Furthermore, fluorescent nanoparticles with different sizes and colors are designed to demonstrate the potential of the method for describing pore throat sizes. Combining the merits of the paramagnetic and fluorescent properties of nanoparticles, a convenient and practical sample preparation is proposed to promote optical imaging analysis of the pore structure in geomaterials.

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analysis of polished planar sections, is probably the most widespread imaging method because it is cheap, handy, and widely available. In petrographic microscopy, pores are transparent under planar polarized light and black in crosspolarized light, which can be utilized to identify the pores of rocks. Since some isotropic minerals (e.g.: fluorite, garnet) may have overlap with pores in petrographic microscopy,1 before thin section manufacturing, pore markers (e.g.: pigments, fluorchrome molecules), often impregnate into the void space of rocks to aid in analysis. But the embedding of pore markers is performed with pressure impregnation, which is an inconvenient sample preparation with artificial influences due to high temperature and pressure damage. Therefore, a convenient and practical sample preparation has been desired to promote optical imaging analysis of the pore structure in geomaterials. Nanoparticles have been widely studied for their excellent properties, and their potential applications in biomedicine, optical, electronic, and magnetic devices as well as catalysts.15−18 In particular, bifunctional nanoparticles with magnetic and fluorescent properties have drawn special attention in applications of biological imaging as markers for proteins, cells, and biological tissues.19 With bifunctional nanoparticles, magnetic resonance imaging, which could offer

he complex pore structure of natural rocks (an inhomogeneous geological material) significantly influences their bulk physical and mechanical properties, including permeability, compressibility, elastic moduli, strength, seismic velocity, electrical conductivity, thermal conductivity, and failure behavior. An array of geoscience applications have stood to benefit notably from a better understanding of the relationship between rock pore structure and their properties, such as hydrology, petroleum engineering, CO2-sequestration, subsurface storage of nuclear waste, geothermal energy generation, and building stone performance.1 The basic characteristic of pore structure in rocks is porosity, which can be measured on bulk samples using the gas porosimetry technique based on Boyle’s Law. However, pore structure in natural rocks is so complex that porosity is not sufficient to characterize them in all aspects. With the exception of porosity, pore throat size distribution and connectivity can affect the physical and mechanical properties of rocks. Many statistical description methods of the pore structure have been developed in this field, including mercury injection,2 smallangle scattering,3 nuclear magnetic resonance,4−6 and adsorption analyses.7 All of them are indirect methods that are based on certain geometric assumptions and require complicated interpretation of experimental data. Imaging methods offer the direct strategy to characterize the pore structure in rocks unambiguously, such as optical light microscopy,1,8−10 electron microscopy,11,12 atomic force microscope, X-ray computed tomography,13,14 and etc. Among them, conventional petrographic microscopy, which is based on the © 2017 American Chemical Society

Received: September 16, 2017 Accepted: October 25, 2017 Published: November 7, 2017 12550

DOI: 10.1021/acs.analchem.7b03794 Anal. Chem. 2017, 89, 12550−12555

Article

Analytical Chemistry

three nanoparticles were 20.0 ± 10.0, 60.0 ± 30.0, and 120.0 ± 40.0 nm, respectively.

sensitive deep tissue imaging, has been combined with fluorescence imaging techniques, which could provide higher spatial resolution for visualizing cellular structure and quantifying molecular events. Bifunctional nanoparticles, due to their extraordinary merits, have been popular in biological imaging, but they have not yet exhibited their applicability in geological science. Herein, a bifunctional nanoparticle powder with magnetic and fluorescent properties is introduced as a novel marker for optical imaging analysis of the pore structure in rocks. With their magnetic property, the nanoparticles are efficiently controlled by an external magnetic field, which offers a facile strategy to drive nanomaterial markers in rocks, avoiding a multistep sample preparation and eliminating the high temperature and pressure damage of sample preparation in conventional methods. With fluorescent properties, the nanoparticles are employed as a novel marker and provide sharp contrast imaging between the material (black) and the porosity (bright). Combining the merits of superparamagnetic and fluorescence properties of the nanoparticles, a convenient and credible sample preparation was proposed to promote imaging analysis of the pore structure of geomaterials.

Figure 1. (a−c) TEM and (d−f) SEM image and the corresponding size distributions of silica capsulation nanoparticles of magnetic Fe3O4 nanoparticles and rhodamine B.

The concentrations of rhodamine B and iron of the bifunctional nanoparticles were determined after chemical decomposition in hydrochloric acid medium (pH 1.0, 6h). Rhodamine B was quantified by a model fluorescence spectrophotometer (F-4600, Hitachi, Japan) after dilution in phosphate buffer (pH 7.0). Atomic absorption spectrophotometry (AA-7000, Beijing East and West Analysis Instrument, China) was employed to determine the concentration of iron. The fluorescent properties of the bifunctional nanoparticles were examined using a fluorescence spectrophotometer (F4600, Hitachi, Japan). The magnetic properties of the bifunctional nanoparticles were measured using a vibrating magnetometer (MPMS-XL-7, Quantum Design, U.S.A.). Geological Materials. Two types of rock materials were used in the study: sandstone from the Sichuan Basin and shale rock from the Ordos Basin. The porosity of the rocks was measured using gas porosimetry techniques. Sandstones with a variable porosity ranging from 3.77% to 15.17% and two shales with porosities of 3.33% and 5.12% were selected. Cores of 25 mm D × 25 mm H were analyzed in gas porosimetry. The cores were cut and prepared as geological thin sections (thickness = 30 μm) for further impregnation with nanomarkers of pore structure. Impregnation Agents and Processes. Bifunctional nanoparticles were used as impregnation agents in this study. The preparation of thin sections with nanoparticle-marked pore structure was based on magnetic control. The nanoparticle powder was placed on the top of geological thin sections without coverslips. A square neodymium N48 magnet (50 × 50 × 10 mm3), which is significantly bigger than the geological thin sections (25 mm D × 30 μm H), was placed at the back of the thin sections to drive nanoparticles into the pore structures of the rocks at room temperature. After the pores were impregnated with nanoparticle markers, residual nanoparticles on the surface of the thin sections were cleaned with an alcohol-soaked cotton ball. The impregnation time of 0.2, 0.5, 1, 3, 6, and 9 h were investigated on a sandstone with the porosity of 15.17% (gas porosimetry). With the increase of the applied impregnation time, the fluorescence intensity increased obviously, and the area porosity reached at a saturation point at 6 h (Figure S1 of the Supporting Information, SI), indicating that 6 h could be an optimized time to finish the impregnation of the nanoparticles into the rock pores.

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EXPERIMENTAL SECTION Chemicals. Fe3O4 nanoparticle was purchased from Aladdin Inc. (Shanghai, China). Poly vinylpyrrolidone (PVP, Mw = 36 000), absolute ethyl alcohol, aqueous ammonia, rhodamine B, fluorescein isothiocyanate, and tetraethyl orthosilicate (TEOS) were obtained from Shanghai Reagent (Shanghai, China). All reagents were of analytical grade, and used asreceived. Ultrapure water obtained by a Direct-Q 3 UV water purification system (Millipore, U.S.A.) was used. Preparation of Bifunctional Silica Nanoparticles. As the Fe3O4 nanoparticle could hardly be dispersed in ultrapure water, PVP was involved in its surface coating. In brief, 0.06 g Fe3O4 nanoparticle was added in 300.0 mL water by ultrasonic dispersion for about 60 min. 1.0 g PVP was dissolved in the resulting suspension and mechanically stirred for 24 h. Then, the PVP-coated Fe3O4 nanoparticles were separated by a permanent magnet and rinsed with alcohol by a dispersion/ preciptation cycle in 80.0 mL absolute ethyl alcohol to remove excess PVP. The well-dispersed Fe3O4 colloid was obtained and employed in the further process. Bifunctional silica nanoparticles were synthesized by a typical Stöber method.20 5.0 mL absolute ethyl alcohol with 5.0 × 10−6 mol/L rhodamine B (or fluorescein isothiocyanate) and 5.0 mL ammonia aqueous (NH3·H2O, 25 wt %) with 2.0 mL water were added into the prepared Fe3O4 colloid (80.0 mL). Subsequently, a TEOS/ethanol solution (v/v = 1/10) was added dropwise, and the mixture was stirred for 2 h. The resulting particles formed a flocculate, which was easily separated from the supernatant with a permanent magnet and washed three times with ethyl alcohol. The amounts of the TEOS ethanol solution, such as 5.5, 11.0, and 16.5 mL, were utilized to control the dimension of the bifunctional silica nanoparticles, and three sizes of the nanoparticles were obtained. Characterization of Bifunctional Silica Nanoparticles. The sizes of the synthesized nanoparticles were characterized by high resolution transmission electron microscopy (HRTEM, Tecnai G2 F20, FEI, U.S.A.) and scanning electron microscopy (SEM, Zeiss G500, Germany) as shown in Figure 1. Statistical analysis of SEM data revealed that the average diameters of the 12551

DOI: 10.1021/acs.analchem.7b03794 Anal. Chem. 2017, 89, 12550−12555

Article

Analytical Chemistry Image Analysis Procedure. Image analysis of porosity was conducted with an optical microscope (Leica DM2700p, Germany), using fluorescent and polarized light. The excitation wavelength was from 450 to 490 nm. The paralleled polarized and cross-polarized images were overlaid with fluorescent images to validate where the pore structure were formed. Pore structures were segmented and porosity was calculated by using a threshold technique based on RGB color space from the fluorescent images. The same threshold values were utilized for all images. More than five fluorescent images, giving an area of ∼20 mm2, were utilized in the statistical analysis of porosity for each rock sample.

nanoparticles shows the magnetic behavior corresponding to a superparamagnetic comportment. The saturation magnetization (Ms) of the bifunctional nanoparticles is in the range of 32.0− 35.9 emu g−1 (with difference sizes) less than that of Fe3O4 nanoparticles (112.7 emu g−1), which might be due to the lower concentration of Fe3O4 in the prepared nanoparticles. The superparamagnetic properties endow the bifunctional nanoparticles with the capability of an efficient marker, which can be easily controlled under an external magnetic field. The fluorescence spectrum of the magnetic fluorescent particles is shown in Figure 3. Emission spectra of bifunctional

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RESULTS AND DISCUSSION Silica capsulation nanoparticles of magnetic Fe3O4 nanoparticles and rhodamine B were utilized as an example of bifunctional nanoparticles for common optical imaging of pore structure in rocks. The nanoparticles were synthesized by a modified Stöber method in a one-pot reaction. The sizes of the nanoparticles were controlled during the synthesis of the silica shell capsulation. Figure 1 shows SEM and TEM images of the nanoparticles with three different diameters. Statistical analysis of SEM data reveals that the average diameters of the three nanoparticles are 20.0 ± 10.0 nm (S), 60.0 ± 30.0 nm (M), and 120.0 ± 40.0 nm (L), respectively. Components of the prepared bifunctional nanoparticles were examined. The Fe3O4 wt % in the nanoparticles with different sizes were 8.2% (S), 6.4% (M), and 3.8% (L), respectively. Since the addition of Fe3O4 nanoparticles is constant in the synthetic process, Fe3O4 wt % in the nanoparticles decreases when the nanoparticle size increases due to the growth of the SiO2 proportion. Rhodamine B wt % of the three nanoparticles with different sizes are 0.050% (S), 0.056% (M), and 0.051% (L), respectively, demonstrating there is slight influence of nanoparticle size on the concentration of rhodamine. The magnetic properties of the nanoparticles were studied using a vibrating sample magnetometer with the magnetic field cycle between −20 000 and 20 000 Oe at room temperature (Figure 2). The field-dependent magnetization curve of the

Figure 3. Emission spectra of the bifunctional nanoparticles with diameters of 20.0 nm (S), 60.0 nm (M), and 120.0 nm (L). The inset is the fluorescent image of the bifunctional nanoparticles.

nanoparticles occur at ∼580 nm. With the increase of the nanoparticle size, the blue shift (4 nm) of the maximum emission wavelength is observed, which may suggest that there are interactions between the dye molecule and the SiO2 substrate or aggregation of the dye molecule in the SiO2 host.21,22 Figure 3 also shows that the fluorescence intensity increases with the increase of nanoparticle size. Since there is slight influence of particle size on the concentration of rhodamine in the nanoparticles, the increase of fluorescence intensity may be attributed to the decrease of fluorescence quenching by Fe3O4,21,22 in accord with the decrease of Fe3O4 wt % with the increase of the nanoparticle size. The inset in Figure 3 is the fluorescent image of the bifunctional nanoparticles and shows that the color of fluorescence is yellow, which was used to identify the pore situation in fluorescent imaging analysis. Because of the higher fluorescent intensity, a rhodaminedoped magnetic silica nanoparticle with a size of ∼120 nm (Large size) was used as a marker to fill into the pores and cracks of the rocks. The nanomarkers were driven into pore structure under magnetic control. The fluorescence of the nanomarker is yellow with a high value of red and green in the RGB color space, which is easily identified in dark field for pore imaging because most minerals exhibit no or weak natural fluorescence and they are black in dark field. Before the impregnation with the fluorescent nanoparticle, all of the geological thin sections were examined to confirm the blank fluorescence. Examples of thin sections using fluorescence light microscopy with rhodamine-doped magnetic silica nanoparticle as markers of pore structure are shown in Figure 4. The paralleled polarized and cross-polarized images were overlaid with

Figure 2. Field-dependent magnetization curves of Fe3O4 nanoparticles (a) and bifunctional nanoparticles with diameters of 20.0 nm (green line), 60.0 nm (red line), and 120.0 nm (blue line). The inset (upper left) shows the photograph of the bifunctional colloid before (right) and after (left) magnetic separation by an external magnetic field. The inset (lower right) shows the enlarged magnetization curves when the magnetic field is from −400 to 400 Oe. 12552

DOI: 10.1021/acs.analchem.7b03794 Anal. Chem. 2017, 89, 12550−12555

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with nanomarkers are lower than the results from gas porosimetry. Although the heterogeneity of the rocks and our segmentation technique could lead to the random difference between the two methods, the one-sided results in our experiment may be attributed to the difference of probe size between the nanoparticles and the gas molecules. The diameter of the nanoparticles is ∼120 nm, and the diameter of the nitrogen molecules is ∼0.3 nm. Nitrogen molecules can through pore throats with sizes ranging from 0.3−120 nm, but nanoparticles cannot. Thus, the higher results of porosity from gas porosimetry is likely to be derived from the additional contribution of pore throats with sizes ranging from 0.3−120 nm. The porosity is dependent on the probe size, which indicates the potential of the technique for describing the pore size distribution based on enough nanomarkers with different sizes. Moreover, the recent popularity of gas shale has improved understanding of the importance of microstructure in shale. Shale is commonly recognized as a seal for conventional gas reservoirs or a source rock of oil and gas. However, for gas shales, the shale itself is the reservoir.23 Micrometer- and nanometer-size pores have been widely considered as a significant part of the storage and flow space of gas in shale.23−25 In this work, we compared the result of porosity from gas porosimetry, the lower results of porosity from image analysis, which relates to pore throats with sizes ranging from 0.3−120 nm, are calculated and listed in Table 1 to describe the amount of microstructure in rocks. For the two shale samples with porosities of 3.33% and 5.12%, the percentage of microstructure (0.3−120 nm) in the pore systems are 19% and 13%, respectively. For the two sandstone samples (sandstone1, Φ = 3.77%; sandstone 2, Φ = 6.18%) with similar porosity to the two shales, the percentage of microstructure (0.3−120 nm) in the pore systems are 15% and 9%, respectively. Under similar porosity conditions, the percentage of microstructure (0.3−120 nm) in the two shales is higher than that in the two sandstones, indicating the advantage of micrometer- and nanometer-size pores in shales. Until now, porecast, molecule markers (e.g.: pigments, fluorchrome molecules) mixed with epoxy, was primarily the domain of pore markers for imaging analysis in geology. We have introduced a novel marker, magneto-fluorescent nanoparticles, which not only offer a convenient and practical sample preparation for promoting optical imaging analysis of the pore structure, but also widen the application of nanoparticles in the basic geological research. Compared to the conventional molecule markers, the proposed imaging method with nanomarkers offers several advantages. First, our technique is nondestructive. Generally, optical light microscopy relies on the analysis of polished planar sections. In the conventional technique, before the manufacture of the thin section, a melting epoxy mixed with molecular markers (e.g.: pigments, fluorchrome molecules) is often involved in the impregnation of the void space of rocks with pressure injection so that rock pore structures are correctively identified through the marked color or fluorescence. It is reported that high temperature, which is utilized to melt the epoxy resin, can initiate changes in the porosity of rocks due to the thermal expansion.26 Additionally, the pressure used for impregnation of pores with mixture of marker molecules and epoxy may lead to artificial cracks and damage accumulation, which will significantly change the pore geometry. However, attributed to the magnetic injection, our technique successfully

Figure 4. Thin sections of sandstone (a−c) and shale (d−f) under optical microscopy using parallel polarized light (left), cross-polarized light (center), and fluorescence light microscopy (right).

fluorescent images to validate where the pore structure were formed. In the sandstone sample, primary intergranular pores (Figure 4c), which are the main pore spaces, are visibly observed under a fluorescence light microscopy. Most intergranular pores are polygonal and connected with a tubular throat, the profiles of which are clear. Additionally, SEM was performed in the same visual field to confirm that the bifunctional nanoparticles filled in the pores (Figure 5). In

Figure 5. Thin sections of sandstone under fluorescence light microscopy (left) and scanning electron microscopy (middle and right).

the shale sample, the main pore structure is microcracks. Because of the low light transmittance, the paralleled polarized (Figure 4d) and cross-polarized images (Figure 4e) of the shale sample perform limited information and the difference of gray between the crack and the shale is not obvious. But in the fluorescent image (Figure 4f), as the color of the nanomarkers is yellow, zones of yellow florescence are identified as to where the microcracks are situated, indicating the advantage of the fluorescent image technique. Seven rock samples, five sandstones with widely porosity range from 3.77% to 15.17% and two shales with porosity of 3.33% and 5.12% were selected to compare the result of porosity from gas porosimetry with the proposed imaging method (Table 1). The results from the microscopic analyses Table 1. Comparison of Porosity from Gas Porosimetry and the Proposed Image Analysis

sample shale 1 shale 2 sandstone sandstone sandstone sandstone sandstone

1 2 3 4 5

porosity from image analysis (ΦA/%)

porosity from gas porosimetry (ΦV/%)

difference between the two porosities (1− ΦA/ΦV)

± ± ± ± ± ± ±

3.33 5.12 3.77 6.18 10.45 11.82 15.17

0.19 0.13 0.15 0.09 0.22 0.14 0.12

2.70 4.46 3.21 5.62 8.18 10.12 13.39

0.40 0.68 0.50 1.69 0.52 1.01 3.00

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easily extracted by Photoshop. The area porosity of yellow color (Figure 6c) and green color (Figure 6d) are calculated to be 14.87% and 0.43%, respectively, which are successfully utilized to characterize the pores marked by nanoparticles with different sizes. Most pore structures are yellow with ∼150 of red and green value in RGB space, which represents that the sizes of these pore throats are more than 120 nm. Some pore structures are green with ∼250 of green value in RGB space, which represents that sizes of these pore throats are ranging from 20 to 120 nm. Similarly, the pore structure marked with two nanoparticles of different sizes and colors are also successfully applied to describe the pore throat size in a shale (Figure S2). As nanoparticles with different fluorescent colors were designed with different particle sizes, different fluorescent colors of nanoparticles represented their different particle sizes, which corresponded to the sizes of the pore throats.

avoids the problems of artificial damage from high temperature and pressure. Thus, the results would be more reliable. Second, our technique is much more convenient to operate than conventional methods. Nanomarkers are driven into geological thin sections directly by a permanent magnet placed at the back of the thin sections. The impregnation process is simple and does not need any equipment other than a magnet, while the conventional technique requires a tedious and multistep sample preparation including melt (epoxy resin), mix (epoxy resin and molecule markers), and press impregnation, which requires comprehensive equipment with high temperature and high pressure systems. Moreover, an improper preparation would lead to false conclusions, including over- or underestimation of porosity or crack density when the pigments or fluorescent molecules are mixed inhomogeneously with the epoxy resin.1 In this aspect, bifunctional nanoparticles are not the simple mixture of entities, and they are homogeneity products from chemical synthesis, which could solve the problem of inhomogeneity and eliminate the lengthy mixing process. Besides, regular shapes of geological materials (e.g.: cylinder) are required in the conventional technique to reduce the fracture from stress anisotropy. In contrast, any irregular geological materials, after being polished, could be examined with our technique. Remarkably, benefiting from reflection fluorescence, our technique is not even constrained by the light transmittance property of the materials, which can be applied in pore analysis of cores and small pieces of rock directly. Third, as nanoparticles could enter the pore or fracture, it provides a direct correlation between the particle size and pore throats. Hence, two bifunctional nanoparticles with different sizes and colors were prepared to generate size-dependent fluorescence to confirm the potential of our technique in describing the pore throat size. A green-marker (fluoresceindoped magnetic silica nanoparticles) with a size of ∼20 nm was served in the model study and added into rocks after the impregnation of yellow-marker (rhodamine B-doped magnetic silica nanoparticles) with a size of ∼120 nm. The pore structure in a sandstone marked with two nanoparticles of different sizes and colors is shown in Figure 6. Different color zones could be

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CONCLUSIONS Silica capsulation nanoparticles of magnetic Fe3O4 nanoparticles and dyes, which served as an example of bifunctional nanoparticles, were designed, synthesized, characterized, and introduced as the fluorescent marker for common optical imaging of the pore structure in rocks. As the fluorescence intensity of the nanoparticles increases with the increase of the nanoparticle size, large nanoparticles (120 nm) were chosen in the imaging application. Pore structure of sandstones and shale were successfully imaged, and various pores were examined by the novel method. With the magnetic property, nanoparticles were controlled by an external magnetic field and thus avoided the risks of high temperature and pressure damage, which offers a novel strategy to load nanomaterial markers into rocks and develop a facile sample preparation method to promote optical imaging of pore structures. Compared with other optical microscopy methods, fluorescence imaging provides a clearer contrast imaging between geological materials (black) and porosity (bright), which makes use of automated image analysis technique much more straightforward to determine the porosity of the samples. Remarkably, with designed sizedependent fluorescence, bifunctional nanoparticles could directly describe the pore throat size of rocks through a multicolor fluorescence imaging system. To profit from a great boom of nanoscience and nanomaterials, more nanoparticles would be utilized as pore markers in the geological application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b03794. The optimization of impregnation time and double color image of pore structure in a shale with two nanomarkers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.L.). *E-mail: [email protected] (C.-y.L.). ORCID

Figure 6. Double color image of pore structure in sandstone with two nanomarkers. The same field of vision under parallel polarized light (a), fluorescence light (b), yellow (c), and green (d) area extracted from fluorescence image.

Na Li: 0000-0002-5787-6332 Notes

The authors declare no competing financial interest. 12554

DOI: 10.1021/acs.analchem.7b03794 Anal. Chem. 2017, 89, 12550−12555

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Analytical Chemistry

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ACKNOWLEDGMENTS The financial support of the research by the National Natural Science Foundation of P. R. China (Grant No. 41502139), Scientific Research Project Funding of Department of Education in Sichuan Province (16ZB0102), Open Fund of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (PLC201608), and Chengdu University of Technology “Youth Backbone Teacher Training Project” (2014−2018) are gratefully acknowledged.

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DOI: 10.1021/acs.analchem.7b03794 Anal. Chem. 2017, 89, 12550−12555