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Colorimetric speciation of Cr on paper-based analytical devices based on field amplified stacking
Journal Pre-proof Colorimetric speciation of Cr on paper-based analytical devices based on field amplified stacking Hui-Min Zhai, Ting Zhou, Fang Fang, Zhi-Yong Wu PII:
S0039-9140(19)31268-8
DOI:
https://doi.org/10.1016/j.talanta.2019.120635
Reference:
TAL 120635
To appear in:
Talanta
Received Date: 4 September 2019 Revised Date:
6 December 2019
Accepted Date: 8 December 2019
Please cite this article as: H.-M. Zhai, T. Zhou, F. Fang, Z.-Y. Wu, Colorimetric speciation of Cr on paper-based analytical devices based on field amplified stacking, Talanta (2020), doi: https:// doi.org/10.1016/j.talanta.2019.120635. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Colorimetric speciation of Cr on paper-based analytical devices based on field amplified stacking Hui-Min Zhai#, Ting Zhou#, Fang Fang, Zhi-Yong Wu*
Research Center for Analytical Sciences, Chemistry Department, College of Sciences, Northeastern University, Shenyang 110819, China
Correspondence to: Zhi-Yong Wu, Dr & Prof., email: [email protected], Tel/Fax: +86 (0)2483687659 #
These two authors contribute equally to this work.
Highlights 1. Fast and instrument-free speciation detection of Cr was demonstrated on a paper-based analytical device 2. Colorimetric detection of Cr (VI) was assured by the charge selectivity of stacking and selective visualization reaction of 1,5-diphenylcarbazide with CrO423. Signal intensity of this method was increased impressively to about 30 times in µPADs, thanks to the online stacking effect
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Abstract In this work, a paper-based analytical device (PAD) for sensitive speciation detection of chromium (Cr) by smartphone camera was introduced. In anion electrokinetic stacking mode, Cr (VI) in the form of CrO42- was firstly enriched into a narrow band on a paper fluidic channel, and then the band was visualized by 1,5-diphenylcarbazide (DPC). Colorimetric detection of Cr (VI) was based on the gray intensity of stacking bands. Detection of Cr (III) was achieved by subtraction of the detection results of Cr (VI) before and after the sample was oxidized. Under optimized conditions, the detection limit of Cr (VI) and Cr (III) of 0.20 µM (~10.4 µg L-1) and 0.30 µM (~14.6 µg L-1) were achieved with a linear response in the range of 0.67~5.00 µM and 0.93~6.00 µM, respectively. The detection performance of PAD is close to that obtained by desktop spectrophotometry, thanks to the online stacking effect. Meanwhile, this PAD also exhibited a good selectivity over coexisting cations and anions, thanks to the charge selectivity of the stacking mode and the selective visualization reaction of DPC. Speciation detection of Cr from tap water, river water, surface water, and electroplating wastewater was demonstrated. This work shows the potential of PADs for on-site detection of Cr (VI) in environmental water samples.
Abstract graph
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Key words: chromium; speciation detection; paper-based analytical device; field-amplified stacking; colorimetric detection, smartphone camera
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‐ 1. Introduction The pollution of heavy metals on aquatic and terrestrial ecosystems has been a serious environment issue for a long time [1]. Chromium is one of hazardous heavy metals [2], which has two main stable oxidation states, i.e. Cr (III) and Cr (VI). The toxicity of chromium depends much on its states. As an essential trace element for humans, Cr (III) plays an important role in maintaining normal glucose, cholesterol and fatty acid metabolism [3]. On the other hand, Cr (VI) is highly carcinogenic, which causes important chromosomic aberration through modifying the DNA transcription process [4-6]. The bioaccumulation of Cr (VI) causes damage to livers, kidneys and other organs through respiratory and skin [7]. With the widespread use of chromium in the industrial process such as electroplating, metallurgy, leather tanning and pigment production, chromium-containing wastewater or slag is discharged into environment, posing a serious threat to human health. Therefore, Cr (VI) emission in industrial wastewater and its concentration in wastewater are strictly restricted by most countries and regions. Thus, a simple and sensitive speciation analysis of chromium is necessary for environmental monitoring. At present, the reported methods for quantitatively detecting Cr are mainly AAS [8] , XRF [9], ICP-MS [10], HPLC [11], electrochemical method [12] and chemiluminescence [13], etc. Among them, AAS is the priority for detecting transition metal elements due to its high elemental selectivity and sensitivity, with which low detection limit (FAAS from mg L-1 to µg L-1, ET-AAS from µg L-1 to ng 4
L-1) is easy to achieve [14]. However, due to its poor valence state discrimination performance, speciation analysis for trace elements by AAS depends much on sample pretreatment, for instance, liquid–liquid microextraction [15] and solid-phase extraction [16]. Furthermore, AAS is not convenient for field analysis. Paper-based analysis devices (PADs) are simple, low-cost, portable, and instrument-free. Meanwhile, it is possible to read test results on-site with PADs by CCD camera or even naked eyes as has been demonstrated by many applications [17-21]. Digital camera [22] and smartphone [23] can be used to capture the color intensity signals in real-time for colorimetric analysis. The signal intensity can also be enhanced by online sample preconcentration methods such as evaporation concentration [24], isoelectric focusing (IEF) [25], ion concentration polarization (ICP) [26] and field-amplified stacking (FAS) [27]. FAS is a simple and effective online preconcentration approach, especially for charged target components. Ma [28] et al. successfully introduced FAS in a PAD, and demonstrated three orders of magnitude signal enhancement effect on the detection of dsDNA with inverted fluorescence microscope. With FAS, Zhang [29] et al demonstrated sensitive colorimetric detection of nitrite on a PAD, and the detection limit similar to that of desktop spectrophotometry (~ 0.86 µM) was achieved by smartphone camera. Sensitivity of PADs can also be enhanced by introducing colorimetric detection [30], metal nanoparticles [31], quantum dot [32], etc.. Ghavami group [33] demonstrated sensitive colorimetric determination of Cr (III) on PADs by using 2,2′-thiodiacetic acid modified gold nanoparticles as a probe, and the detection limit as low as 0.64 nM 5
was achieved. Zhuang et al [34] developed a chemosensor for sensitive colorimetric detection of Cr (VI) with a limit of detection of 0.1 µM as demonstrated on the paper-based chemosensor. The detection was implemented by Cr (VI)-stimulated catalytic activity of GA-AuNPs, resulting in the color switch of methylene blue. Chen et al [35] prepared a type of rotary 3D PAD with wax printing technology, through which colorimetric analysis of Cr (VI) was realized with a home-made refractive index detection system, and a detection limit of 0.31 µM was reported. In this paper, cost-effective and instrument-free detection of Cr (VI) and Cr (III) was presented on a PAD by combining FAS and selective chromogenic reaction. The selectivity of this method was optimized by virtue of anion electrokinetic stacking mode, and the chromogenic reaction between Cr (VI) with DPC was also contributed to the outstanding selectivity. This method was suitable for on-site determination of Cr (VI) and speciation analysis of Cr in the industrial water. Hence, the applicability of the method was demonstrated by the detection of Cr (VI) and Cr (III) from the samples of river water, lake water, tap water as well as electroplating wastewater.
2 Experimental section 2.1 Chemicals and material Hydroxyethyl cellulose (HEC), tris (hydroxymethyl) aminomethane (Tris), 1,5-diphenylcarbazide (DPC) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). All Hydrochloric acid (HCl), phosphoric acid (H3PO4), sodium hydroxide (NaOH), sodium chromate (Na2CrO4), hydrogen peroxide (30%, V/V), absolute ethanol, methanol, and acetone were purchased from Sinopharm Chemical 6
Reagent Co., Ltd (Shanghai, China). K2SO4, CH3COONa, K2CO3, Na3PO4, Fe (NO3)3·9H2O, and NaCl were purchased from Tianjin Yongda Chemical Reagent Co., Ltd (Tianjin, China). All reagents in this work were analytical-reagent grade and used without further pretreatment. Unless extra noted, deionized water was used to prepare the solutions. A portable power supply was assembled by combing a miniature DC–DC booster converter unit (ESSO Latham Electronic Technology Co., Ltd, Shenzhen, China) with a portable battery power bank (PH 50, Romoss Technology Co., Ltd, Shenzhen, China).The booster converter generates a constant output voltage of 400 V when input from the battery is a voltage of 9 V. The digital multimeter (MT-1232, ProsKit, Taiwan) was connected to the circuit to monitor the change of current in this experiment. In this work, 5 µL microsyringe (Gaoge Industrial and trading Co., Ltd., Shanghai, China) was used to add chromogenic agents to the paper strip. Microfocus lens (China Shenzhen Maipuluo Technology Co., Ltd., Shenzhen, China) was connected to the Samsung smartphone for clearer images. Glass fiber filter (Dezhou Kejia Environmental Products Co., Ltd., Dezhou, China) was cut into strips (35 mm ×4 mm) with a laser cutter. 0.5 mm diameter platinum wires were used as the electrodes. A light emitting diode is used as light source. Spectroscopy was carried out by a UV Professional V1.11.0 spectrophotometer. 2.2 Preparation of chromogenic reagent 0.02 g of DPC powder was accurately weighed, and dissolved in 5 mL of solvent (acetone, methanol, ethanol). After it was completely dissolved, DPC solution was 7
transferred to a 10 mL volumetric flask and diluted to the scale with deionized water. The DPC solution in a brown bottle could be preserved in refrigerator for three days. The H3PO4 solution was mixed with the DPC solution (V:V = 1:1) to form the chromogenic agent. 2.3 Operation procedure For PADs determination of Cr (VI), a paper strip was placed above the two reservoirs with an interval of 30 mm. First of all, 300 µL background electrolyte (BGE) of 150 mM Tris-HCl (pH = 8.1) was added into the anode reservoir and the paper stirp was wetted with this solution. And then 300 µL sample solution was added to the cathode reservoir. Then a DC voltage of 400 V from the battery-driven power supply was applied to the pair of electrodes, which are put into two reservoirs. The power was disconnected after three minutes of electrokinetic stacking. 5 µL of the chromogenic agent was added to the strip, where a colored stacking zone appeared that can be recorded by the smartphone. The smartphone connected to the microfocus lens continuously captured the stacking band in the camera mode, taking a picture every 60s, and taking photos was no more than 180s in total. The photos were taken in the box that was covered with black cloth to avoid the influence of external light source. Finally, images acquired by the smartphone were processed using ImageJ software (Version 1.49v, National Institutes of Health, Bethesda, MD). The region of stacking band was selected by the color threshold tool of ImageJ. And then the mean gray value of the selected zone was measured as the color intensity for quantitative analysis. Note that each band intensity was corrected by subtracting mean gray value 8
of the BGE zone near the band. Detection of Cr (III) was achieved by subtraction of the detection results of Cr (VI) before and after the sample was oxidized. The Oxidation steps were carried out following procedures reported by Ahmad research group [36]. Briefly, 2 mL 0.10 M NaOH, and 200 µL 30% H2O2 were added to 50 mL sample solution. After 20 minutes, the solution was boiled to 10 mL or so, and recalibrated to 50 mL with deionized water for detection of total Cr. 2.4 Preparation of water samples 2.4.1 Spiking experiments
Tap water, river water and surface water were collected and stored at 4
for spiking
experiments to prove the universal applicability of PADs method. These samples were used directly without additional purification steps after three levels (1.00, 2.00, 3.00 µM) of Cr (VI) and Cr (III) were added into these samples.
2.4.2 Contrast experiment
Electroplating wastewater was determined by ICP-OES, spectrophotometry and PADs method respectively in this paper. Electroplating wastewater was firstly filtered with a radius of 0.2 µm to remove impurities for ICP-OES and spectrophotometry respectively. Afterwards, the filtrate as stock solution was placed in a 50 mL centrifuge tube stored in a refrigerator at 4°C until use. The stock solution diluted 100-fold with 2% hydrochloric acid was detected by ICP-OES. And then the rest of stock solution also diluted 4000-fold with deionized water was detected by 9
spectrophotometry. Electroplating wastewater can be directly tested by diluting 4000-fold with deionized water in a PAD. When spectrophotometry and PAD were used to detect total chromium, the oxidation step of the wastewater was referred to Section 2.3.
3 Results and discussion The schematic diagram of anionic enrichment process in FAS is shown in Fig 1. According to the previous report [37, 38], when a DC voltage is applied to the pair of electrodes, a stable electric field gradient is established on paper channel based on the neutralization reaction, where H+ from electrolysis of BGE (TrisH+) reacts with OHfrom electrolysis of H2O, namely, OH- + TrisH+ = Tris + H2O. Negatively charged components (CrO42-) at the cathode can be stacked as narrow bands in the paper-based fluid channel to preconcentrate sample online which can significantly enhance sensitivity of detection. This stacking process not only enables the electric enrichment of anion Cr (VI), but also effectively eliminates the interference of other coexisting cations (such as Cu2+, Ni2+, etc.). DPC can be oxidized to diphenylcarbazone (DPCO) by Cr (VI) under acidic conditions, and then violet-red chelate brighter than Cr (VI) can be found on the paper strip [39]. The color intensity of the violet-red complex has a linear relationship with Cr (VI) content so that this method can realize the quantitative determination of Cr (VI). The detection of Cr (III) was performed by subtraction of Cr (VI) detection results with and without oxidation of the sample. Therefore, PADs can realize the speciation analysis of Cr. 10
Figure 1 Paper-based FAS for sample preconcentration. (a) Schematic illustration of enriching Cr (VI). (b) The stacking process with the change of voltage in the glass fiber paper strip. (c) Enrichment of Cr (VI) on the glass fiber paper strip in the experiment. (d) Speciation analysis process of Cr.
3.1 Optimization In the process of conventional capillary or chip electrophoresis, the concentration of Tris-HCl solution, a kind of BGE, has a great influence on the stacking effect of FAS. Figure 2(a) shows the concentration of Tris-HCl versus the intensity of stacking bands. It can be seen from the figure that the signal intensity of stacking bands increases as the concentration of BGE increases when the concentration of Tris-HCl is between 50 and 150 mM, but the signal strength does not significantly increased when the concentration of Tris-HCl is higher than 150 mM, so all the following experiments select 150 mM as the optimum concentration. The chromogenic agent also has a significant effect on stacking band intensity 11
and needs to be optimized (Fig S1). The chromogenic agent is manually added to the stacking band to cause a distinct color change in the enrichment zone, which takes a certain period, therefore the color developing time is also a parameter that needs to be optimized. The variation curve of signal strength over time and color developing process of stacking bands is shown in Fig 2(b), where the value of the band intensity in 0 to 120s is positively correlated with time. It is found that the signal intensity after 120s tends to be smooth, but there is a tendency to decrease after 140s because of the spreading of stacking band under the capillary force of paper, therefore 120s is selected as the best developing time.
Figure 2 (a) The effect of Tris-HCl concentration on the intensity of stacking band. The BGE was composed of Tris-HCl solution with different concentrations (50, 100, 150, 200, 250 mM) and 0.3% HEC. (b) The curve of stacking bands intensity over time and the color developing process. The concentration of Tris-HCl was 150 mM. The experimental conditions for (a) and (b): ethanol as a solvent, and the concentration of H3PO4 and CrO42- were 7 M and 10 µM, respectively. Noticeably, every data point was the average of four parallel experiments and corrected by the background intensity.
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3.2 Enrichment factor
To evaluate the enrichment effect of FAS mentioned above in PADs, the linear curves of Cr (VI) were obtained in the range of 0.25~5.00 µM with FAS and 7.00~50.00 µM without FAS, respectively, as shown in Fig 3. Under the same conditions, the intensity of stacking band without FAS was detected on a circular glass fiber filter paper with a diameter of 1 cm and photographed by the same smartphone. As can be seen from Fig 3, when the intensity of stacking band is 16.78 a.u., according to the linear curve
y = -1.32 + 0.40x, R2 = 0.996
without FAS, the
concentration of Cr (VI) is 45.00 µM, and from the linear curve (y = 12.98 + 11.68x, R2 = 0.990) with FAS, the concentration is 0.32 µM, so FAS can increase sensitivity of PADs by nearly two orders of magnitude. Calculated by another method, namely, the ratio of the slope of the two lines, the EF is about 30 times.
Figure 3 Calibration curves of Cr (VI) with and without FAS. The blue curve, red curve represented the intensity of stacking band with and without FAS, respectively. Noticeably, every data point was the average of four parallel experiments and corrected by the background intensity.
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3.3 Selectivity In anion electrokinetic stacking mode, only negatively charged components are enriched, so the influence of other coexisting cations, such as Cu2+, Ni2+, Co2+, Cd2+, Cr3+ on stacking band intensity can be excluded, as shown in Fig 4. Furthermore, these anions (NO3-, CO32-, PO43-, Ac-, Cl-) have no obvious influence on Cr (VI) detection neither, which is mainly attributed to the high selectivity of the colorimetric reaction.
Figure 4 Stacking band intensity values of Cr (VI) (10 µM) with co-existing metal ion (Cr3+, Cu2+, Ni2+, Co2+, Cd2+: 100 µM) and anion (NO3-, CO32-, PO43-, Ac-, Cl-: 1 mM). The experimental conditions consistent with Fig 2(b), and every data point was the average of four parallel
experiments.
3.4 Calibration curves
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Figure 5 (a) Under optimal experimental conditions, the signal response curve for Cr (VI) concentration ranging from 0.20 to 20.00 µM and the standard curve (0.67~5.00 µM) is embedded. (b) The signal response curve for Cr (III) concentration ranging from 0.30 to 13.00 µM and the standard curve (0.93~6.00 µM) is embedded. Every data point was the average of four parallel experiments.
In the optimal experimental conditions, the concentration range of 0.20 ~ 20.00 µM of Cr (VI) and 0.30~13.00 µM of Cr (III) were prepared for exploring quantitative performance of this method, whose intensity of stacking bands was shown in Fig 5. For the detection of Cr (VI), the linear range was 0.67~5.00 µM and the linear standard curve was y = 12.98 + 11.68x (R2 = 0.990). Employing the same PAD, Cr (III) within the scope of 0.93 to 6.00 µM had a good linear relationship, and the linear standard curve was y = 15.18 + 8.64x (R2 = 0.995). Based on LOD = 3 σ/k, it was concluded that Cr (VI) and Cr (III) detection limit was 0.20 µM, (~10.40 µg L-1) and 0.28 µM (~14.56 µg L-1) respectively in Table 1. As shown in Table S1, compared with other reported colorimetric methods, this method has a lower detection limit by smartphone. In addition, the detection performance of this PAD is close to that 15
obtained by desktop spectrophotometry. The standard curve of spectrophotometry was y = 0.013 + 0.032x, R2 = 0.998, the linear range was 0.37~10.00 µM, and the detection limit was 0.11 µM (the standard deviation of blank sample σ = 0.00123). Experiments show that PADs method improves the detection level and achieves the detection ability similar to instruments after combining FAS and selective colorimetric detection.
Table 1. Quantitative analysis performance of spectrophotometry and PADs method parameters
spectrophotometry
PAD
Cr (VI)
Cr (III)
Cr (VI)
Cr (III)
0.37~10.00
0.47~10.00
0.67~5.00
0.93~6.00
Standard curve
y = 0.032x + 0.013
y = 0.027x - 0.0017
y = 11.68x + 12.98
y = 8.64x + 15.18
Linearity (R2)
0.998
0.998
0.990
0.995
LOD (µM)
0.11
0.14
0.20
0.28
Linear range (µM)
3.5 Detection of samples To prove the feasibility of PADs, we analyzed Cr (VI) and Cr (III) from tap water, river water, and surface water in this method. In these samples with three levels (1.00, 2.00, 3.00 µM) of Cr (VI) and Cr (III), standard addition recovery experiment was carried out, the standard addition recovery rate of Cr (VI) is 80%~107%, as shown in Table S2. The results proved that PADs method is potential in the quantitative analysis of Cr from environmental samples. The concentration of Cr (VI) 16
in above three water samples is lower than LOQ of PADs, so we also detected electroplating wastewater after spike tests. The conclusion, as shown in Table 2, further illustrated the great potential of PADs method for environmental water sample detection.‐
Table 2. Electroplating wastewater detected by ICP-OES, PADs method and spectrophotometry methods
ICP-OES a
Spectrophotometry b
PAD b
Concentration
Total Cr
Total Cr
Cr (VI)
Total Cr
Cr (VI)
(mM)
8.44
8.12
6.00
8.08
5.88
RSD (%)
0.6
1.2
1.6
4.2
7.9
*a: Relative standard deviation (RSD) was obtained from three parallel experiments b: Relative standard deviation (RSD) was obtained from four parallel experiments
4 Conclusion In this work, Cr (VI) in the form of CrO42- was firstly enriched into a narrow band on a paper fluidic channel, and then the band was visualized by DPC in anion electrokinetic stacking mode. Paper-based FAS combined with DPC-specific colorimetric detection is suitable for speciation analysis of Cr in the detection of environmental water and also can effectively eliminate the interference of coexisting cations and anions. In addition, due to the electricstacking effect, the detection sensitivity is greatly enhanced, and the detection limit of this method is similar to that of spectrophotometry. Compared with reported colorimetric detection methods of Cr, 17
the detection limit is also improved, and the results show that a simple and environmentally benign PAD method is potential for on-site rapid detection of Cr (VI) in environmental water samples as well.
Acknowledgements Financial support from National Natural Science Foundation of China (21575019) is acknowledged. Prof. Lin-shan Wang and Miss Mei-Ying Liu are thanked for providing the water samples, and Miss Di Ke is thanked for the assistance in ICP-OES tests.
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readout, J Microchimica Acta 185(8) (2018) 374. [34] Y.T. Zhuang, S. Chen, R. Jiang, Y.L. Yu, J.H. Wang, An Ultrasensitive Colorimetric Chromium Chemosensor Based on Dye Color Switching under the Cr(VI)-Stimulated Au NPs Catalytic Activity, J Analytical Chemistry 91(8) (2019) 5346-5353. [35] X. Sun, B. Li, A. Qi, C. Tian, J. Han, Y. Shi, B. Lin, L. Chen, Improved assessment of accuracy and performance using a rotational paper-based device for multiplexed detection of heavy metals, J Talanta
(2017).
[36] W. Ahmad, A.S. Bashammakh, A.A. Al-Sibaai, H. Alwael, M.S. El-Shahawi, Trace determination of Cr(III) and Cr(VI) species in water samples via dispersive liquid-liquid microextraction and microvolume UV–Vis spectrometry. Thermodynamics, speciation study, Journal of Molecular Liquids 224 (2016) 1242-1248. [37] Y.Z. Song, X.X. Zhang, B. Ma, Z.Y. Wu, Z.Q. Zhang, Performance of electrokinetic stacking enhanced paper-based analytical device with smartphone for fast detection of fluorescent whitening agent, J Analytica Chimica Acta 995 (2017) 85-90. [38] B. Ma, S.-F. Xie, L. Liu, F. Fang, Z.-Y. Wu, Two orders of magnitude electrokinetic stacking of proteins in one minute on a simple paper fluidic channel, Analytical Methods 9(18) (2017) 2703-2709. [39] L. Li, Y. Leng, H. Lin, Photometric and visual detection of Cr(VI) using gold nanoparticles modified with 1,5-diphenylcarbazide, J Microchimica Acta 183(4) (2016) 1367-1373.
21
Figure captions Figure 1 Paper-based FAS for sample preconcentration. (a) Schematic illustration of enriching Cr (VI). (b) The stacking process with the change of voltage in the glass fiber paper strip. (c) Enrichment of Cr (VI) on the glass fiber paper strip in the experiment. (d) Speciation analysis process of Cr.
Figure 2 (a) The effect of Tris-HCl concentration on the intensity of stacking band. The BGE was composed of Tris-HCl solution with different concentrations (50, 100, 150, 200, 250 mM) and 0.3% HEC. (b) The curve of stacking bands intensity over time and the color developing process. The concentration of Tris-HCl was 150 mM. The experimental conditions for (a) and (b): ethanol as a solvent, and the concentration of H3PO4 and CrO42- were 7 M and 10 µM, respectively. Noticeably, every data point was the average of four parallel experiments and corrected by the background intensity.
Figure 3 Calibration curves of Cr (VI) with and without FAS. The blue curve, red curve represented the intensity of stacking band with and without FAS, respectively. Noticeably, every data point was the average of four parallel experiments and corrected by the background intensity. Figure 4 Stacking band intensity values of Cr (VI) (10 µM) with co-existing metal ion (Cr3+, Cu2+, Ni2+, Co2+, Cd2+: 100 µM) and anion (NO3-, CO32-, PO43-, Ac-, Cl-: 1 mM). The experimental conditions consistent with Fig 2(b), and every data point was the average of four parallel
experiments. Figure 5 (a) Under optimal experimental conditions, the signal response curve for Cr (VI)
22
concentration ranging from 0.20 to 20.00 µM and the standard curve (0.67~5.00 µM) is embedded. (b) The signal response curve for Cr (III) concentration ranging from 0.30 to 13.00 µM and the standard curve (0.93~6.00 µM) is embedded. Every data point was the average of four parallel experiments.
23
Table 1. Quantitative analysis performance of spectrophotometry and PADs method parameters
spectrophotometry
PAD
Cr (VI)
Cr (III)
Cr (VI)
Cr (III)
0.37~10.00
0.47~10.00
0.67~5.00
0.93~6.00
Standard curve
y = 0.032x + 0.013
y = 0.027x - 0.0017
y = 11.68x + 12.98
y = 8.64x + 15.18
Linearity (R2)
0.998
0.998
0.990
0.995
LOD (µM)
0.11
0.14
0.20
0.28
Linear range (µM)
Table 2. Electroplating wastewater detected by ICP-OES, PADs method and spectrophotometry methods
ICP-OES a
Spectrophotometry b
PAD b
Concentration
Total Cr
Total Cr
Cr (VI)
Total Cr
Cr (VI)
(mM)
8.44
8.12
6.00
8.08
5.88
RSD (%)
0.6
1.2
1.6
4.2
7.9
*a: Relative standard deviation (RSD) was obtained from three parallel experiments b: Relative standard deviation (RSD) was obtained from four parallel experiments
Declaration of Interest Statement On behalf of all authors, I state here that this manuscript is neither published nor submitted to other journals, and the authors declare no conflict of interest.
S0039-9140(19)31268-8
DOI:
https://doi.org/10.1016/j.talanta.2019.120635
Reference:
TAL 120635
To appear in:
Talanta
Received Date: 4 September 2019 Revised Date:
6 December 2019
Accepted Date: 8 December 2019
Please cite this article as: H.-M. Zhai, T. Zhou, F. Fang, Z.-Y. Wu, Colorimetric speciation of Cr on paper-based analytical devices based on field amplified stacking, Talanta (2020), doi: https:// doi.org/10.1016/j.talanta.2019.120635. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Colorimetric speciation of Cr on paper-based analytical devices based on field amplified stacking Hui-Min Zhai#, Ting Zhou#, Fang Fang, Zhi-Yong Wu*
Research Center for Analytical Sciences, Chemistry Department, College of Sciences, Northeastern University, Shenyang 110819, China
Correspondence to: Zhi-Yong Wu, Dr & Prof., email: [email protected], Tel/Fax: +86 (0)2483687659 #
These two authors contribute equally to this work.
Highlights 1. Fast and instrument-free speciation detection of Cr was demonstrated on a paper-based analytical device 2. Colorimetric detection of Cr (VI) was assured by the charge selectivity of stacking and selective visualization reaction of 1,5-diphenylcarbazide with CrO423. Signal intensity of this method was increased impressively to about 30 times in µPADs, thanks to the online stacking effect
1
Abstract In this work, a paper-based analytical device (PAD) for sensitive speciation detection of chromium (Cr) by smartphone camera was introduced. In anion electrokinetic stacking mode, Cr (VI) in the form of CrO42- was firstly enriched into a narrow band on a paper fluidic channel, and then the band was visualized by 1,5-diphenylcarbazide (DPC). Colorimetric detection of Cr (VI) was based on the gray intensity of stacking bands. Detection of Cr (III) was achieved by subtraction of the detection results of Cr (VI) before and after the sample was oxidized. Under optimized conditions, the detection limit of Cr (VI) and Cr (III) of 0.20 µM (~10.4 µg L-1) and 0.30 µM (~14.6 µg L-1) were achieved with a linear response in the range of 0.67~5.00 µM and 0.93~6.00 µM, respectively. The detection performance of PAD is close to that obtained by desktop spectrophotometry, thanks to the online stacking effect. Meanwhile, this PAD also exhibited a good selectivity over coexisting cations and anions, thanks to the charge selectivity of the stacking mode and the selective visualization reaction of DPC. Speciation detection of Cr from tap water, river water, surface water, and electroplating wastewater was demonstrated. This work shows the potential of PADs for on-site detection of Cr (VI) in environmental water samples.
Abstract graph
2
`
Key words: chromium; speciation detection; paper-based analytical device; field-amplified stacking; colorimetric detection, smartphone camera
3
‐ 1. Introduction The pollution of heavy metals on aquatic and terrestrial ecosystems has been a serious environment issue for a long time [1]. Chromium is one of hazardous heavy metals [2], which has two main stable oxidation states, i.e. Cr (III) and Cr (VI). The toxicity of chromium depends much on its states. As an essential trace element for humans, Cr (III) plays an important role in maintaining normal glucose, cholesterol and fatty acid metabolism [3]. On the other hand, Cr (VI) is highly carcinogenic, which causes important chromosomic aberration through modifying the DNA transcription process [4-6]. The bioaccumulation of Cr (VI) causes damage to livers, kidneys and other organs through respiratory and skin [7]. With the widespread use of chromium in the industrial process such as electroplating, metallurgy, leather tanning and pigment production, chromium-containing wastewater or slag is discharged into environment, posing a serious threat to human health. Therefore, Cr (VI) emission in industrial wastewater and its concentration in wastewater are strictly restricted by most countries and regions. Thus, a simple and sensitive speciation analysis of chromium is necessary for environmental monitoring. At present, the reported methods for quantitatively detecting Cr are mainly AAS [8] , XRF [9], ICP-MS [10], HPLC [11], electrochemical method [12] and chemiluminescence [13], etc. Among them, AAS is the priority for detecting transition metal elements due to its high elemental selectivity and sensitivity, with which low detection limit (FAAS from mg L-1 to µg L-1, ET-AAS from µg L-1 to ng 4
L-1) is easy to achieve [14]. However, due to its poor valence state discrimination performance, speciation analysis for trace elements by AAS depends much on sample pretreatment, for instance, liquid–liquid microextraction [15] and solid-phase extraction [16]. Furthermore, AAS is not convenient for field analysis. Paper-based analysis devices (PADs) are simple, low-cost, portable, and instrument-free. Meanwhile, it is possible to read test results on-site with PADs by CCD camera or even naked eyes as has been demonstrated by many applications [17-21]. Digital camera [22] and smartphone [23] can be used to capture the color intensity signals in real-time for colorimetric analysis. The signal intensity can also be enhanced by online sample preconcentration methods such as evaporation concentration [24], isoelectric focusing (IEF) [25], ion concentration polarization (ICP) [26] and field-amplified stacking (FAS) [27]. FAS is a simple and effective online preconcentration approach, especially for charged target components. Ma [28] et al. successfully introduced FAS in a PAD, and demonstrated three orders of magnitude signal enhancement effect on the detection of dsDNA with inverted fluorescence microscope. With FAS, Zhang [29] et al demonstrated sensitive colorimetric detection of nitrite on a PAD, and the detection limit similar to that of desktop spectrophotometry (~ 0.86 µM) was achieved by smartphone camera. Sensitivity of PADs can also be enhanced by introducing colorimetric detection [30], metal nanoparticles [31], quantum dot [32], etc.. Ghavami group [33] demonstrated sensitive colorimetric determination of Cr (III) on PADs by using 2,2′-thiodiacetic acid modified gold nanoparticles as a probe, and the detection limit as low as 0.64 nM 5
was achieved. Zhuang et al [34] developed a chemosensor for sensitive colorimetric detection of Cr (VI) with a limit of detection of 0.1 µM as demonstrated on the paper-based chemosensor. The detection was implemented by Cr (VI)-stimulated catalytic activity of GA-AuNPs, resulting in the color switch of methylene blue. Chen et al [35] prepared a type of rotary 3D PAD with wax printing technology, through which colorimetric analysis of Cr (VI) was realized with a home-made refractive index detection system, and a detection limit of 0.31 µM was reported. In this paper, cost-effective and instrument-free detection of Cr (VI) and Cr (III) was presented on a PAD by combining FAS and selective chromogenic reaction. The selectivity of this method was optimized by virtue of anion electrokinetic stacking mode, and the chromogenic reaction between Cr (VI) with DPC was also contributed to the outstanding selectivity. This method was suitable for on-site determination of Cr (VI) and speciation analysis of Cr in the industrial water. Hence, the applicability of the method was demonstrated by the detection of Cr (VI) and Cr (III) from the samples of river water, lake water, tap water as well as electroplating wastewater.
2 Experimental section 2.1 Chemicals and material Hydroxyethyl cellulose (HEC), tris (hydroxymethyl) aminomethane (Tris), 1,5-diphenylcarbazide (DPC) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). All Hydrochloric acid (HCl), phosphoric acid (H3PO4), sodium hydroxide (NaOH), sodium chromate (Na2CrO4), hydrogen peroxide (30%, V/V), absolute ethanol, methanol, and acetone were purchased from Sinopharm Chemical 6
Reagent Co., Ltd (Shanghai, China). K2SO4, CH3COONa, K2CO3, Na3PO4, Fe (NO3)3·9H2O, and NaCl were purchased from Tianjin Yongda Chemical Reagent Co., Ltd (Tianjin, China). All reagents in this work were analytical-reagent grade and used without further pretreatment. Unless extra noted, deionized water was used to prepare the solutions. A portable power supply was assembled by combing a miniature DC–DC booster converter unit (ESSO Latham Electronic Technology Co., Ltd, Shenzhen, China) with a portable battery power bank (PH 50, Romoss Technology Co., Ltd, Shenzhen, China).The booster converter generates a constant output voltage of 400 V when input from the battery is a voltage of 9 V. The digital multimeter (MT-1232, ProsKit, Taiwan) was connected to the circuit to monitor the change of current in this experiment. In this work, 5 µL microsyringe (Gaoge Industrial and trading Co., Ltd., Shanghai, China) was used to add chromogenic agents to the paper strip. Microfocus lens (China Shenzhen Maipuluo Technology Co., Ltd., Shenzhen, China) was connected to the Samsung smartphone for clearer images. Glass fiber filter (Dezhou Kejia Environmental Products Co., Ltd., Dezhou, China) was cut into strips (35 mm ×4 mm) with a laser cutter. 0.5 mm diameter platinum wires were used as the electrodes. A light emitting diode is used as light source. Spectroscopy was carried out by a UV Professional V1.11.0 spectrophotometer. 2.2 Preparation of chromogenic reagent 0.02 g of DPC powder was accurately weighed, and dissolved in 5 mL of solvent (acetone, methanol, ethanol). After it was completely dissolved, DPC solution was 7
transferred to a 10 mL volumetric flask and diluted to the scale with deionized water. The DPC solution in a brown bottle could be preserved in refrigerator for three days. The H3PO4 solution was mixed with the DPC solution (V:V = 1:1) to form the chromogenic agent. 2.3 Operation procedure For PADs determination of Cr (VI), a paper strip was placed above the two reservoirs with an interval of 30 mm. First of all, 300 µL background electrolyte (BGE) of 150 mM Tris-HCl (pH = 8.1) was added into the anode reservoir and the paper stirp was wetted with this solution. And then 300 µL sample solution was added to the cathode reservoir. Then a DC voltage of 400 V from the battery-driven power supply was applied to the pair of electrodes, which are put into two reservoirs. The power was disconnected after three minutes of electrokinetic stacking. 5 µL of the chromogenic agent was added to the strip, where a colored stacking zone appeared that can be recorded by the smartphone. The smartphone connected to the microfocus lens continuously captured the stacking band in the camera mode, taking a picture every 60s, and taking photos was no more than 180s in total. The photos were taken in the box that was covered with black cloth to avoid the influence of external light source. Finally, images acquired by the smartphone were processed using ImageJ software (Version 1.49v, National Institutes of Health, Bethesda, MD). The region of stacking band was selected by the color threshold tool of ImageJ. And then the mean gray value of the selected zone was measured as the color intensity for quantitative analysis. Note that each band intensity was corrected by subtracting mean gray value 8
of the BGE zone near the band. Detection of Cr (III) was achieved by subtraction of the detection results of Cr (VI) before and after the sample was oxidized. The Oxidation steps were carried out following procedures reported by Ahmad research group [36]. Briefly, 2 mL 0.10 M NaOH, and 200 µL 30% H2O2 were added to 50 mL sample solution. After 20 minutes, the solution was boiled to 10 mL or so, and recalibrated to 50 mL with deionized water for detection of total Cr. 2.4 Preparation of water samples 2.4.1 Spiking experiments
Tap water, river water and surface water were collected and stored at 4
for spiking
experiments to prove the universal applicability of PADs method. These samples were used directly without additional purification steps after three levels (1.00, 2.00, 3.00 µM) of Cr (VI) and Cr (III) were added into these samples.
2.4.2 Contrast experiment
Electroplating wastewater was determined by ICP-OES, spectrophotometry and PADs method respectively in this paper. Electroplating wastewater was firstly filtered with a radius of 0.2 µm to remove impurities for ICP-OES and spectrophotometry respectively. Afterwards, the filtrate as stock solution was placed in a 50 mL centrifuge tube stored in a refrigerator at 4°C until use. The stock solution diluted 100-fold with 2% hydrochloric acid was detected by ICP-OES. And then the rest of stock solution also diluted 4000-fold with deionized water was detected by 9
spectrophotometry. Electroplating wastewater can be directly tested by diluting 4000-fold with deionized water in a PAD. When spectrophotometry and PAD were used to detect total chromium, the oxidation step of the wastewater was referred to Section 2.3.
3 Results and discussion The schematic diagram of anionic enrichment process in FAS is shown in Fig 1. According to the previous report [37, 38], when a DC voltage is applied to the pair of electrodes, a stable electric field gradient is established on paper channel based on the neutralization reaction, where H+ from electrolysis of BGE (TrisH+) reacts with OHfrom electrolysis of H2O, namely, OH- + TrisH+ = Tris + H2O. Negatively charged components (CrO42-) at the cathode can be stacked as narrow bands in the paper-based fluid channel to preconcentrate sample online which can significantly enhance sensitivity of detection. This stacking process not only enables the electric enrichment of anion Cr (VI), but also effectively eliminates the interference of other coexisting cations (such as Cu2+, Ni2+, etc.). DPC can be oxidized to diphenylcarbazone (DPCO) by Cr (VI) under acidic conditions, and then violet-red chelate brighter than Cr (VI) can be found on the paper strip [39]. The color intensity of the violet-red complex has a linear relationship with Cr (VI) content so that this method can realize the quantitative determination of Cr (VI). The detection of Cr (III) was performed by subtraction of Cr (VI) detection results with and without oxidation of the sample. Therefore, PADs can realize the speciation analysis of Cr. 10
Figure 1 Paper-based FAS for sample preconcentration. (a) Schematic illustration of enriching Cr (VI). (b) The stacking process with the change of voltage in the glass fiber paper strip. (c) Enrichment of Cr (VI) on the glass fiber paper strip in the experiment. (d) Speciation analysis process of Cr.
3.1 Optimization In the process of conventional capillary or chip electrophoresis, the concentration of Tris-HCl solution, a kind of BGE, has a great influence on the stacking effect of FAS. Figure 2(a) shows the concentration of Tris-HCl versus the intensity of stacking bands. It can be seen from the figure that the signal intensity of stacking bands increases as the concentration of BGE increases when the concentration of Tris-HCl is between 50 and 150 mM, but the signal strength does not significantly increased when the concentration of Tris-HCl is higher than 150 mM, so all the following experiments select 150 mM as the optimum concentration. The chromogenic agent also has a significant effect on stacking band intensity 11
and needs to be optimized (Fig S1). The chromogenic agent is manually added to the stacking band to cause a distinct color change in the enrichment zone, which takes a certain period, therefore the color developing time is also a parameter that needs to be optimized. The variation curve of signal strength over time and color developing process of stacking bands is shown in Fig 2(b), where the value of the band intensity in 0 to 120s is positively correlated with time. It is found that the signal intensity after 120s tends to be smooth, but there is a tendency to decrease after 140s because of the spreading of stacking band under the capillary force of paper, therefore 120s is selected as the best developing time.
Figure 2 (a) The effect of Tris-HCl concentration on the intensity of stacking band. The BGE was composed of Tris-HCl solution with different concentrations (50, 100, 150, 200, 250 mM) and 0.3% HEC. (b) The curve of stacking bands intensity over time and the color developing process. The concentration of Tris-HCl was 150 mM. The experimental conditions for (a) and (b): ethanol as a solvent, and the concentration of H3PO4 and CrO42- were 7 M and 10 µM, respectively. Noticeably, every data point was the average of four parallel experiments and corrected by the background intensity.
12
3.2 Enrichment factor
To evaluate the enrichment effect of FAS mentioned above in PADs, the linear curves of Cr (VI) were obtained in the range of 0.25~5.00 µM with FAS and 7.00~50.00 µM without FAS, respectively, as shown in Fig 3. Under the same conditions, the intensity of stacking band without FAS was detected on a circular glass fiber filter paper with a diameter of 1 cm and photographed by the same smartphone. As can be seen from Fig 3, when the intensity of stacking band is 16.78 a.u., according to the linear curve
y = -1.32 + 0.40x, R2 = 0.996
without FAS, the
concentration of Cr (VI) is 45.00 µM, and from the linear curve (y = 12.98 + 11.68x, R2 = 0.990) with FAS, the concentration is 0.32 µM, so FAS can increase sensitivity of PADs by nearly two orders of magnitude. Calculated by another method, namely, the ratio of the slope of the two lines, the EF is about 30 times.
Figure 3 Calibration curves of Cr (VI) with and without FAS. The blue curve, red curve represented the intensity of stacking band with and without FAS, respectively. Noticeably, every data point was the average of four parallel experiments and corrected by the background intensity.
13
3.3 Selectivity In anion electrokinetic stacking mode, only negatively charged components are enriched, so the influence of other coexisting cations, such as Cu2+, Ni2+, Co2+, Cd2+, Cr3+ on stacking band intensity can be excluded, as shown in Fig 4. Furthermore, these anions (NO3-, CO32-, PO43-, Ac-, Cl-) have no obvious influence on Cr (VI) detection neither, which is mainly attributed to the high selectivity of the colorimetric reaction.
Figure 4 Stacking band intensity values of Cr (VI) (10 µM) with co-existing metal ion (Cr3+, Cu2+, Ni2+, Co2+, Cd2+: 100 µM) and anion (NO3-, CO32-, PO43-, Ac-, Cl-: 1 mM). The experimental conditions consistent with Fig 2(b), and every data point was the average of four parallel
experiments.
3.4 Calibration curves
14
Figure 5 (a) Under optimal experimental conditions, the signal response curve for Cr (VI) concentration ranging from 0.20 to 20.00 µM and the standard curve (0.67~5.00 µM) is embedded. (b) The signal response curve for Cr (III) concentration ranging from 0.30 to 13.00 µM and the standard curve (0.93~6.00 µM) is embedded. Every data point was the average of four parallel experiments.
In the optimal experimental conditions, the concentration range of 0.20 ~ 20.00 µM of Cr (VI) and 0.30~13.00 µM of Cr (III) were prepared for exploring quantitative performance of this method, whose intensity of stacking bands was shown in Fig 5. For the detection of Cr (VI), the linear range was 0.67~5.00 µM and the linear standard curve was y = 12.98 + 11.68x (R2 = 0.990). Employing the same PAD, Cr (III) within the scope of 0.93 to 6.00 µM had a good linear relationship, and the linear standard curve was y = 15.18 + 8.64x (R2 = 0.995). Based on LOD = 3 σ/k, it was concluded that Cr (VI) and Cr (III) detection limit was 0.20 µM, (~10.40 µg L-1) and 0.28 µM (~14.56 µg L-1) respectively in Table 1. As shown in Table S1, compared with other reported colorimetric methods, this method has a lower detection limit by smartphone. In addition, the detection performance of this PAD is close to that 15
obtained by desktop spectrophotometry. The standard curve of spectrophotometry was y = 0.013 + 0.032x, R2 = 0.998, the linear range was 0.37~10.00 µM, and the detection limit was 0.11 µM (the standard deviation of blank sample σ = 0.00123). Experiments show that PADs method improves the detection level and achieves the detection ability similar to instruments after combining FAS and selective colorimetric detection.
Table 1. Quantitative analysis performance of spectrophotometry and PADs method parameters
spectrophotometry
PAD
Cr (VI)
Cr (III)
Cr (VI)
Cr (III)
0.37~10.00
0.47~10.00
0.67~5.00
0.93~6.00
Standard curve
y = 0.032x + 0.013
y = 0.027x - 0.0017
y = 11.68x + 12.98
y = 8.64x + 15.18
Linearity (R2)
0.998
0.998
0.990
0.995
LOD (µM)
0.11
0.14
0.20
0.28
Linear range (µM)
3.5 Detection of samples To prove the feasibility of PADs, we analyzed Cr (VI) and Cr (III) from tap water, river water, and surface water in this method. In these samples with three levels (1.00, 2.00, 3.00 µM) of Cr (VI) and Cr (III), standard addition recovery experiment was carried out, the standard addition recovery rate of Cr (VI) is 80%~107%, as shown in Table S2. The results proved that PADs method is potential in the quantitative analysis of Cr from environmental samples. The concentration of Cr (VI) 16
in above three water samples is lower than LOQ of PADs, so we also detected electroplating wastewater after spike tests. The conclusion, as shown in Table 2, further illustrated the great potential of PADs method for environmental water sample detection.‐
Table 2. Electroplating wastewater detected by ICP-OES, PADs method and spectrophotometry methods
ICP-OES a
Spectrophotometry b
PAD b
Concentration
Total Cr
Total Cr
Cr (VI)
Total Cr
Cr (VI)
(mM)
8.44
8.12
6.00
8.08
5.88
RSD (%)
0.6
1.2
1.6
4.2
7.9
*a: Relative standard deviation (RSD) was obtained from three parallel experiments b: Relative standard deviation (RSD) was obtained from four parallel experiments
4 Conclusion In this work, Cr (VI) in the form of CrO42- was firstly enriched into a narrow band on a paper fluidic channel, and then the band was visualized by DPC in anion electrokinetic stacking mode. Paper-based FAS combined with DPC-specific colorimetric detection is suitable for speciation analysis of Cr in the detection of environmental water and also can effectively eliminate the interference of coexisting cations and anions. In addition, due to the electricstacking effect, the detection sensitivity is greatly enhanced, and the detection limit of this method is similar to that of spectrophotometry. Compared with reported colorimetric detection methods of Cr, 17
the detection limit is also improved, and the results show that a simple and environmentally benign PAD method is potential for on-site rapid detection of Cr (VI) in environmental water samples as well.
Acknowledgements Financial support from National Natural Science Foundation of China (21575019) is acknowledged. Prof. Lin-shan Wang and Miss Mei-Ying Liu are thanked for providing the water samples, and Miss Di Ke is thanked for the assistance in ICP-OES tests.
18
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Figure captions Figure 1 Paper-based FAS for sample preconcentration. (a) Schematic illustration of enriching Cr (VI). (b) The stacking process with the change of voltage in the glass fiber paper strip. (c) Enrichment of Cr (VI) on the glass fiber paper strip in the experiment. (d) Speciation analysis process of Cr.
Figure 2 (a) The effect of Tris-HCl concentration on the intensity of stacking band. The BGE was composed of Tris-HCl solution with different concentrations (50, 100, 150, 200, 250 mM) and 0.3% HEC. (b) The curve of stacking bands intensity over time and the color developing process. The concentration of Tris-HCl was 150 mM. The experimental conditions for (a) and (b): ethanol as a solvent, and the concentration of H3PO4 and CrO42- were 7 M and 10 µM, respectively. Noticeably, every data point was the average of four parallel experiments and corrected by the background intensity.
Figure 3 Calibration curves of Cr (VI) with and without FAS. The blue curve, red curve represented the intensity of stacking band with and without FAS, respectively. Noticeably, every data point was the average of four parallel experiments and corrected by the background intensity. Figure 4 Stacking band intensity values of Cr (VI) (10 µM) with co-existing metal ion (Cr3+, Cu2+, Ni2+, Co2+, Cd2+: 100 µM) and anion (NO3-, CO32-, PO43-, Ac-, Cl-: 1 mM). The experimental conditions consistent with Fig 2(b), and every data point was the average of four parallel
experiments. Figure 5 (a) Under optimal experimental conditions, the signal response curve for Cr (VI)
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concentration ranging from 0.20 to 20.00 µM and the standard curve (0.67~5.00 µM) is embedded. (b) The signal response curve for Cr (III) concentration ranging from 0.30 to 13.00 µM and the standard curve (0.93~6.00 µM) is embedded. Every data point was the average of four parallel experiments.
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Table 1. Quantitative analysis performance of spectrophotometry and PADs method parameters
spectrophotometry
PAD
Cr (VI)
Cr (III)
Cr (VI)
Cr (III)
0.37~10.00
0.47~10.00
0.67~5.00
0.93~6.00
Standard curve
y = 0.032x + 0.013
y = 0.027x - 0.0017
y = 11.68x + 12.98
y = 8.64x + 15.18
Linearity (R2)
0.998
0.998
0.990
0.995
LOD (µM)
0.11
0.14
0.20
0.28
Linear range (µM)
Table 2. Electroplating wastewater detected by ICP-OES, PADs method and spectrophotometry methods
ICP-OES a
Spectrophotometry b
PAD b
Concentration
Total Cr
Total Cr
Cr (VI)
Total Cr
Cr (VI)
(mM)
8.44
8.12
6.00
8.08
5.88
RSD (%)
0.6
1.2
1.6
4.2
7.9
*a: Relative standard deviation (RSD) was obtained from three parallel experiments b: Relative standard deviation (RSD) was obtained from four parallel experiments
Declaration of Interest Statement On behalf of all authors, I state here that this manuscript is neither published nor submitted to other journals, and the authors declare no conflict of interest.