Article pubs.acs.org/ac
Cite This: Anal. Chem. 2018, 90, 8478−8486
Approach for Downscaling of Electromembrane Extraction as a Lab on-a-Chip Device Followed by Sensitive Red-Green-Blue Detection Mahroo Baharfar,† Yadollah Yamini,*,† Shahram Seidi,‡ and Muhammad Balal Arain§ †
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran Department of Analytical Chemistry, Faculty of Chemistry, K.N. Toosi University of Technology, 19697 Tehran, Iran § Department of Chemistry, Abdul Wali Khan University, Mardan, Khyber Pakhtunkhwa Pakistan, 23200 ‡
Anal. Chem. 2018.90:8478-8486. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/06/19. For personal use only.
S Supporting Information *
ABSTRACT: A design of electromembrane extraction (EME) as a lab on-achip device was proposed for the extraction and determination of phenazopyridine as the model analyte. The extraction procedure was accomplished by coupling EME and packing a sorbent. The analyte was extracted under the applied electrical field across a membrane sheet impregnated by nitrophenyl octylether (NPOE) into an acceptor phase. It was followed by the absorption of the analyte on strong cation exchanger as a sorbent. The designed chip contained separate spiral channels for donor and acceptor phases featuring embedded platinum electrodes to enhance extraction efficiency. The selected donor and acceptor phases were 0 mM HCl and 100 mM HCl, respectively. The on-chip electromembrane extraction was carried out under the voltage level of 70 V for 50 min. The analysis was carried out by two modes of a simple red-green-blue (RGB) image analysis tool and a conventional HPLC-UV system. After the absorption of the analyte on the solid phase, its color changed and a digital picture of the sorbent was taken for the RGB analysis. The effective parameters on the performance of the chip device, comprising the EME and solid phase microextraction steps, were distinguished and optimized. The accumulation of the analyte on the solid phase showed excellent sensitivity and a limit of detection (LOD) lower than 1.0 μg L−1 achieved by an image analysis using a smartphone. This device also offered acceptable intra- and interassay RSD% ( 0.9969) for HPLC-UV and RGB analysis, respectively. To investigate the applicability of the method in complicated matrixes, urine samples of patients being treated with phenazopyridine were analyzed.
D
amounts of organic solvents and chemicals, time-consuming steps, and difficult handling.8−10 Therefore, the necessity of developing more environmentally and user-friendly techniques as well as improving sensitivity, selectivity, and efficiency has eclipsed many research incentives in the field of analytical chemistry. Owing to these extensive efforts, several innovative sample preparation methods have been established. Among them, membrane-based techniques are well-known sample preparation methods because of their advantages over other similar techniques, benefiting from high sample cleanup, favorable extraction efficiency, and low required amounts of organic solvents.11 Electromembrane extraction (EME) is one of the relatively new membrane-based extraction methods which was introduced at first by Pedersen Bjergaard et al.12 The basis of this electrically driven method is related to the fact that the mass transfer of charged species is enhanced by the application of an electrical field. Therefore, in this method, two platinum
esign and application of the new generation of miniaturized chemical devices is a current, interesting topic in a broad range of research fields from biology and chemistry to engineering.1,2 The concept behind these microscale lab on-a-chip (LOC) devices is integrating one or several processes into a single chip.3 LOC systems have undergone extensive promotions since their advent, thanks to the numerous advantages associated with these systems, including higher efficiency and sample throughput, low consumption of reagents and solvents, portability, feasibility of parallelization and automation, and shortening the process time.4−6 Utilization of LOC devices is also a superior strategy to benefit chemical analysis and integration of, for instance, sample preparation, separation, and detection processes over a small platform.7 Among these steps, sample preparation is a critical step before a chemical analysis. This helps to reduce the matrix effect, provide a suitable form of sample for instrumental analysis, concentrate analytes, eliminate interferences, and warrant reliable measurements. For this purpose, plenty of conventional sample preparation techniques have been proposed during the several past decades, entailing excessive © 2018 American Chemical Society
Received: March 19, 2018 Accepted: May 30, 2018 Published: May 30, 2018 8478
DOI: 10.1021/acs.analchem.8b01224 Anal. Chem. 2018, 90, 8478−8486
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Analytical Chemistry Table 1. Chemical Structure and Corresponding pKa and log KO/W Values of PP
analysis methods, HPLC-UV system and RGB analysis, were compared.
electrodes connected to a power supply are responsible for providing an electrical field across a thin layer of an organic solvent immobilized in the pores of a porous polypropylene sheet or a hollow fiber. Analytes are extracted by the electrical force through the supported liquid membrane into the acceptor phase in their ionized form.13−15 Since the initiation of EME, much advancement has been accomplished to make this technique more efficacious.16−18 Due to the fascinating and advantageous features of LOC devices, one of the most promising strategies for further development of EME is the exploitation of this technique on chip platforms. The early studies on this issue go back to 2010 and since then diverse designs of EME as downscaled LOC systems have been proposed.19,20 However, designing an integrated, sensitive, and portable system without any special instrumental analysis still remains a challenge and few works on this topic have been reported in the literature.21 In the present study, a design of EME as a LOC device followed by a microextraction step accomplished by a packed solid phase was proposed for extraction and preconcentration of phenazopyridine (PP) as a model analyte. The chip device consisted of spiral channels for both donor and acceptor phase compartments to obtain higher extraction efficiency. It is expected that the higher amount of the acceptor solution prevents the early saturation of the acceptor phase and improves the efficiency of the process. The desired electrical field was applied homogeneously throughout the length of the spiral extraction channels. This homogeneity is achieved by the constant distance between the electrodes and their fixed position. In order to integrate the whole analysis process on a single platform, the EME procedure was followed by the adsorption of the target analyte on a solid phase. Then, a RGB technique was used as the detection method. The accumulation of the analyte on the solid phase caused the color change of the sorbent due to the orange color of the target compound. To assess the reliability of the RGB method, the HPLC-UV analysis was additionally carried out and the obtained results were compared. To do the RGB analysis, a digital image of the solid phase was taken by a smartphone, and its three main color components (red, green, and blue) were analyzed by an image analysis software. The relations between variation in concentration and different color components were determined, and the best concentration-dependent one was selected as the analytical signal. Finally, the applicability of the method was evaluated by the analysis of the real urine samples of patients being treated with PP, and then the results of both
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EXPERIMENTAL SECTION Chemicals and Reagents. Phenazopyridine (PP) was kindly donated by Tehran Shimi Pharmaceutical Co (Tehran, Iran). The structure and physicochemical properties of PP is shown in Table 1. All chemicals were of analytical grade. Nitrophenyl octylether (NPOE), tris(2-ethylhexyl) phosphate (TEHP), and di(2-ethylhexyl) phosphate (DEHP) were supplied from Fluka (Buchs, Switzerland). 1-Octanol was obtained from Merck (Darmstadt, Germany). The Accurel 2E HF (R/P) polypropylene membrane sheet with a thickness of 100 μm and a pore size of 0.2 μm was purchased from Membrana (Wup- pertal, Germany). The ultrapure water used in this work was prepared by a Younglin 370 series aqua MAX purification instrument (Kyounggi-do, Korea). The utilized sorbent was strong cation exchange resin (SCX) with the particle diameter of 50 μm and pore size of 60 Å, provided by Analytica International (New York, NY). The stock solution of PP was prepared at the concentration of 1.0 mg mL−1 in methanol. Standard solutions were freshly prepared from the stock solution by dilution with ultrapure water. All of the standard solutions were stored at 4 °C and protected from light. Real Samples. To plot the calibration curves and assess figures of merit, all experiments were conducted in a drug-free urine sample of a 24-year-old healthy female volunteer. For this purpose, 2 mL of the urine sample was spiked with proper amounts of the standard solution to obtain the desirable concentration without any pretreatment or dilution. The real urine samples were collected from volunteer patients being treated with PP. One of the patients was a 53-year-old male volunteer, taking a regular dose of 100 mg three times a day, and the other one was a 43-year-old female volunteer, taking a single dose of 100 mg. All samplings were carried out based on the guidelines for research ethics, and the protocol was approved by an internal review board. Chromatographic Apparatus. Separation and detection of PP was accomplished by an Agilent 1260 HPLC system equipped with a quaternary pump, degasser, a 20 μL sample loop, and UV−vis detector (Waldbornn, Germany). Elution of PP was carried out by an isocratic program at the flow rate of 1.0 mL min−1. The mobile phase consisted of acetonitrile and a 10 mmol L−1 acetate buffer with the pH of 5.5 (60:40, v/v). For detection and quantification, the detector wavelength was set at 395 nm. The results were recorded and analyzed using ChemStation for the LC system software (version B.04.03). 8479
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Figure 1. (A) Equipment used for EME chip device. (B) Schematic illustration of the whole extraction and detection procedure.
Figure 2. Effect of SLM composition on extraction efficiency. PP was extracted by the voltage of 60 V, flow rate of 50 μL min−1, 0 and 100 mM HCl for the donor and acceptor phases, respectively.
The elution was performed on an ODS-3 column (150 mm × 4.6 mm, with 5 μm particle size) provided by MZAnalysenteknik (Mainz, Germany). Preparation of the Chip Device. The chip device consisted of two poly(methyl methacrylate) (PMMA) plates as substrates. In each part, the same pattern of spiral channels was carved with the width of 2.0 mm and the depth of 500 μm. The chip device for electromembrane extraction is shown in Figure 1A. The upper plate was a compartment for the stagnant acceptor phase and the lower plate was dedicated to the flow pass of the sample solution. In each spiral pattern, three individual holes were drilled (i.d. of 0.5 mm) to provide inlet and outlet tubes and insert the platinum electrodes. As shown in Figure 1B, holes a and b were exploited to provide the inlet and outlet tubes, and hole c was used to mount the platinum electrodes. These platinum electrodes with the diameter of 0.2 mm were purchased from Pars Platin (Tehran, Iran). The electrodes were fixed by bending and embedding along the length of the channels. To mill all channels and holes, a SMG-
302 CNC micromilling machine from Sadrafan Gostar Industries (Tehran, Iran) was used. PMMA plates were separated by a piece of NPOE impregnated polypropylene sheet, which was located between the two parts. The whole structure was fixed by the aid of bolts and nuts, and the membrane sheet was renewed after each extraction. The sample solution was pumped into the corresponding channels by a syringe pump obtained from Fanavaran Nano-Meghyas (Tehran, Iran), and the acceptor solution was injected into the dedicated channel and withdrawn from that by a microsyringe. A transparent silicon tube was packed with 2 mg of SCX, and both ends of the tube were manually fitted by two frit pieces. Then, it was connected to the outlet tube of the acceptor phase. On-Chip EME Coupled with the Solid Phase Microextraction Step. A piece of polypropylene sheet, with the dimension of 3.5 mm × 3.5 cm, was cut and impregnated by NPOE. The desired organic solvent was immobilized in the pores of the porous propylene sheet, and the excess amount of that was removed by a small piece of Kleenex. The resulted 8480
DOI: 10.1021/acs.analchem.8b01224 Anal. Chem. 2018, 90, 8478−8486
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Figure 3. Optimization of (A) donor phase composition, (B) acceptor phase composition, (C) voltage, and (D) flow rate.
sheet was located between the two parts of the chip. After fixing the whole device, the sample solution was introduced to the dedicated channel by a syringe pump. A volume of 500 μL of 100 mM HCl was injected into the upper channel by a microsyringe as the acceptor phase. After the fulfillment of the EME, the acceptor phase was conducted into the solid phase by conducting the excess air at a certain flow rate with the same syringe pump. Then, the digital image, captured from the solid phase, was decomposed to three main color components (RGB analysis). Through another approach, the tube containing the solid phase was mounted and fixed on the needle of a microsyringe, and the target analyte was desorbed by the ejection and withdrawal of a 20 μL methanol solution containing 40% of 0.2 mol L−1 NaOH. These steps were repeated certain times to ensure the complete desorption of the analyte. The final solution was injected to the HPLC-UV system for further analysis. A diagram of the whole procedure is illustrated in Figure 1. After each extraction, the chip device was carefully washed and dried by ultrapure water and methanol, and the packed silicon tube was replaced with a new one. RGB Analysis. To design a portable device capable of an in situ analysis, an online RGB analysis was performed by a smart phone (iPhone SE 2016). For this purpose, the obtained solid phase was placed in a certain position, and the smart phone was mounted at a fixed place, 12 cm above the solid phase. A digital photo of the solid phase was taken and was analyzed via an iDropper tool, which is an image analysis application available for iOS operating systems. The correlation between concentration and intensity of color components, as a concentrationdependent signal, was investigated by subtraction of the RGB intensities from white. All experiments were carried out with
respect to a blank analysis, and optical conditions were the same in all cases.
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RESULTS AND DISCUSSION Composition of the Supported Liquid Membrane. Composition of the supported liquid membrane (SLM) has a Table 2. Evaluation of Calibration Curves against Different RGB Components Analysis component Linear dynamic range (μg L−1)
R2
Green Blue Green + blue Green + red Blue + red
50.0−1000.0 50.0−1000.0 30.0−1000.0 50.0−1000.0 50.0−1000.0
0.9686 0.9700 0.9969 0.9657 0.9649
crucial effect on the EME efficiency. Nonvolatile organic solvents that have definite conductivity, compatibility with polypropylene, less toxicity, immiscibility in water, and certain viscosity are worthy of investigation as the SLM in the EME. Considering these criteria and based on previous studies, NPOE and 1-octanol are among the widely utilized solvents in the EME. In addition, numerous reports in the literature have revealed that modification of SLM with DEHP and TEHP as carriers can benefit the extraction recovery of the EME, especially in the case of polar compounds.11 Therefore, 1octanol and NPOE containing a known amount of DEHP and TEHP were evaluated. This effect is shown in Figure 2, 8481
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Table 3. Analytical Performance of LOC Device for Determination of PP from Urine Samples by Both the Detection Approach of HPLC-UV and the RGB Analysis RSD %b
a
−1
−1
−1
2
Analysis method
LOD (μg L )
LOQ (μg L )
Linearity (μg L )
R
HPLC-UV RGB
0.2 1.0
5.0 30.0
5.0−1000.0 30.0−1000.0
0.9980 0.9969
PF
a
25
Inter-assay
Intra-assay
5.2 6.3
7.1 7.5
Preconcentration based on four-replicate measurements at 250 μg L−1 for HPLC. bBased on four-replicated measurements.
Figure 4. (A) Final digital images of the solid phase after the analyte absorption as a medium for the RGB analysis at different concentration levels of the case. (B) Utilized zones for the RGB analysis.
Table 4. Comparison of Analytical Performance of the Proposed Method with Other Reported Methods for PP Determination method a
LLE/GC -MS LLE/HPLC-UVb EME/HPLC-UVc OC-EME/HPLC-UVd OC-EME/RGB analysise
Matrix
LOD (μg L−1)
LDR (μg L−1)
R2
RSD %
Ref
plasma plasma tablets urine plasma urine urine
0.3 10.0 10.0 0.5 1.0 0.2 1.0
5.0−500.0 50.0−1000.0 2000.0−20000.0 1.0−1000.0 5.0−1000.0 5.0−1000.0 50.0−1000.0
0.9992 0.9986 0.9999 0.9960 0.9985 0.998 0.9934
2.7 8.3
24 25 26 27 27 this work this work
4.6 5.9 5.2 6.3
a Liquid−liquid extraction−gas chromatography mass spectrometry detection. bLiquid−liquid extraction−liquid chromatography ultraviolet detection. cElectromembrane extraction. dOn-chip electromembrane extraction. eOn-chip electromembrane extraction−RGB image analysis detection.
is defined as ion balance. In line with the reported studies, analyte mass transfer increases with the reduction of ion balance.22 This fact is attributed to the competition between proton ions and target ionized compounds, feasibility of electrolysis reactions, bubble formation, Joule heating phenomenon, and other instability problems.23 These effects were investigated, and according to Figure 3A, B, 0 mM (ultrapure water) and 100 mM HCl were chosen as the composition of the donor and acceptor phases, respectively.
according to which, NPOE had the best performance and thus was selected as the SLM for further studies. Effect of Acceptor and Donor Phase Composition. In EME, extraction mechanism is mainly based on electrokinetic migration, and thus it is of great importance for the donor phase composition to keep the target analytes in their ionized form. In addition, the high concentration of HCl in the acceptor phase facilitates the release of the analytes at the interface of the SLM and the acceptor phase. The ratio of HCl concentration in the donor phase to that in the acceptor phase 8482
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the sample solution, corresponds to an extraction time of 50 min. Solid Phase Microextraction Parameters. The effective parameters on the extraction efficiency of the solid phase microextraction step, including the adsorption flow rate, the type of desorption solvent, the number of draw-eject cycles for desorption, and the desorption volume, were taken into account. The effect of these parameters on the extraction efficiency of the solid phase microextraction step can be found in Figures S1−S4 in the Supporting Information. Studies were carried out at the flow rate of 120 μL min−1, at which adsorption equilibrium was acquired. After the RGB detection for the subsequent analysis by the HPLC-UV system, the desorption step is necessary and the related parameters should be investigated. Due to the ionized form of the analyte in the EME and the nature of the target drug, SCX was chosen as the solid phase material. For desorption of the analyte, a basic desorption solvent was required, and 20 μL of methanol containing 40% NaOH of 0.2 mol L−1 showed the best performance. After 11 cycles of draw-eject, the analyte was successfully desorbed. Method Evaluation. To assess the analytical performance of the proposed method, figures of merit were determined in the blank urine samples by the addition of known amounts of the standard solution. In terms of the RGB analysis, the calibration curves were established against different color components. Considering the obtained results shown in Table 2, the sum of green and blue components showed the best linearity and was thus selected for further studies. Linearity, repeatability, and LODs were studied under the optimal conditions. The results are summarized in Table 3. In
Table 5. Determination of PP in Real Samples Sample Urine 1a Urine 2b a
Analysis method HPLCUV RGB HPLCUV RGB
Creal (μg L−1)
Cadded (μg L−1)
Cfound (μg L−1)
RSD % (n = 4)
Error %
237.3
200.0
433.7
5.8
−1.8
234.4 33.3
200.0 40.0
440.1 72.8
7.2 6.6
2.8 −1.2
39.2
40.0
75.9
7.6
−8.2
After 9 h. bAfter 12 h.
Effect of Applied Voltage. In EME, the electrical field accounts for the main driving force to transfer the target analyte across the SLM into the acceptor phase. Basically, the higher the applied voltage, the more the extraction efficiency in the EME. Nevertheless, at high voltage values, extraction efficiency deteriorates as a result of increasing the current level across the SLM. To reach the optimum level of the applied voltage, this parameter was studied within the range of 50 to 90 V. The results are shown in Figure 3C. At the voltage level of 70 V, the best performance was achieved and was selected as the optimum value. Effect of Sample Solution Flow Rate. The main characteristic of the nonexhaustive extraction methods, such as EME, is that extraction recoveries are limited by the time in which the system reaches equilibrium. However, in EME, the extraction time should be adequately short due to the unsustainability of the SLM when the voltage is applied. This parameter was studied, and as can be seen in Figure 3D, at the optimum flow rate of 40 μL min−1, the system reached the desired equilibrium. This flow rate value, regarding 2000 μL of
Figure 5. (A) Typical chromatogram of PP extraction from urine sample of a patient after and before the addition of 200 μg L−1. (B) Corresponding images used for the RGB analysis. 8483
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Figure 6. (A) Obtained chromatograms after the LOC procedure (a) nonspiked and (b) spiked at the concentration level of 40 μg L−1. (B) Corresponding images for the RGB analysis.
Table 6. Validation of the RGB Method by t-Test Urine 1 −1
PP concn (μg L )
Average standard deviation Spooled ttest tTable(4,0.05) = 2.776
Urine 2
HPLC
RGB
252.6 234.1 225.2 237.3 13.9 15.4 0.23
252.9 230.1 220.2 234.4 16.7
ttest
ard deviations (RSDs), indicative of method precision, were evaluated as inter- and intra-assay values based on four replicate measurements. The method showed acceptable repeatability with inter- and intra-assay RSDs% values less than 6.3 and 7.5, respectively. Table 4 is provided to compare the characteristics of the proposed method with other reported studies in the literature. Compared with the conventional LLE methods, which are associated with large amounts of organic solvents and samples and difficult handling, the chip device is much more user- and environmental-friendly. The chip-based device also needs lower amounts of samples and enjoys less power consumption in comparison with conventional EME. Moreover, as opposed to the conventional extraction methods, in this procedure samples can be used without any pretreatment owing to the remarkable cleanup of the first membrane-based step, which can be a prominent strategy to eliminate pretreatments in the case of solid phase methods. More importantly, the main advantage of the proposed system is the simplicity of the instrumentation and detection. Detection by the image analysis tool is capable of direct and sensitive analyte quantification in complicated matrices. Additionally, elimination of the liquid chromatography decreases the cost of analysis and organic solvents used for the elution. It goes without saying that detection via smartphone is an available approach which allows in-field analysis. It is noteworthy that in the case of detecting several colorful species, the analytes can be accumulated in different distinct zones of the solid phase by further elution similar to what we have in the chromatographic columns. Another fascinating
HPLC
36.2 30.6 33.1 33.3 2.8 2.9 2.44 < tTable(4,0.05), not significant
RGB 35.6 40.9 41.1 39.2 3.1
accordance with the obtained results, the level of concentration as low as 1.0 μg L−1 can be detected by a simple smartphone image analysis, while the corresponding LOD by HPLC-UV was 0.2 μg L−1. In comparison with the instrumental analysis, RGB detection by smartphones normally lacks sensitivity, but in this work, the concentrated accumulation of the analyte in the solid phase caused outstanding sensitivity. The variation in the color of the solid phase at different concentration levels of the case is illustrated in Figure 4A. In all experiments, the middle region, just above the first frit, in which the analyte was accumulated more homogeneously, was exploited. In this region, there is the least variation in the RGB components against the location of the analysis. Corresponding utilized areas for the RGB analysis are also shown in Figure 4B. Calibration curves were linear within the range of 30−1000 μg L−1 (r2 = 0.9969) and 10−1000 μg L−1 (r2 = 0.9980) for RGB detection and HPLC-UV analysis, respectively. Relative stand8484
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Analytical Chemistry approach for eliminating interferences or multidetection is benefiting from the curve resolution methods. This device is a simple prototype, paving a new way for sensitive portable detection by LOC devices. Additionally, in the case of colorless analytes, derivtization and tagging by a chromophore would be helpful, and the interaction between a fluorophore and desired analytes can be even used as an analytical signal for quantification. Further studies are also being conducted on this issue, and the results will be reported in the not too distant future. Analysis of Real Samples. The LOC system was used for the analysis of the target drug in urine samples of patients being treated with PP to assess the applicability of the method. The results are presented in Table 5. Accuracy was evaluated as the parameter of error %. The obtained values were less than 8.2% for both analysis modes. Furthermore, RSD % values are indicative of acceptable precision related to the proposed method. Figures 5A and 6A are typical chromatograms, resulting from extraction of PP from patients’ urine samples before and after the addition of certain concentration levels. As can be seen, excellent cleanup was acquired considering the fact that the samples were used without any pretreatment. In addition, the corresponding image analysis of the blank and spiked samples is illustrated below each chromatogram (Figures 5B and 6B). To validate the RGB method and compare the obtained results by both detection modes, the extraction of each real urine sample was repeated three times. The acquired average concentration level in each urine sample by the two analysis modes were assessed by a t test at the confidence level of 95%. The related results are shown in Table 6. According to the performed t test, there was no significant difference between the calculated concentration values by the RGB method and the HPLC-UV system and both confirmed each other. Therefore, the RGB method can be utilized as a reliable, accurate detection mode for sensitive and in situ analysis of the model analyte with the minimum cost.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +98-21-82883449. Fax: +98-21-82883460. ORCID
Yadollah Yamini: 0000-0002-2484-9477 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant from the Program of Cooperation (PoC) in Science and Technology between the Ministry of Science and Technology, Government of the Islamic Republic of Pakistan and the Ministry of Science, Research, and Technology, Government of the Islamic Republic of Iran for the years 2017−2019 (Project No. 962.201.19503). The authors also gratefully acknowledge the support of University of Sistan and Baluchestan.
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CONCLUSIONS In the present research, a LOC device based on the combination of EME with a solid phase as the medium for RGB analysis was proposed. Although RGB analysis via smartphones normally suffers from poor sensitivity, in this method utilization of the solid phase as a medium for concentrated accumulation of the model analyte caused remarkable sensitivity as compared with the conventional chromatographic methods. It was shown that RGB intensities can be used as a reliable concentration-dependent signal to quantify analytes from the complicated matrixes. The proposed device can be considered as an encouraging strategy for further developments in smartphone-based LOC systems for in-field and inexpensive detections.
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3, number of draw-eject cycles for desorption on the extraction efficiency; and Figure S-4, influence of desorption volume on the extraction efficiency (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b01224. Calculations of preconcentration factor and relative recovery; Figure S-1, effect of the absorption flow rate on the extraction efficiency; Figure S-2, effect of type of desorption solvent on the extraction efficiency; Figure S8485
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