Article pubs.acs.org/acssensors
Reduced Graphene Oxide-Poly(3,4-ethylenedioxythiophene) Polystyrenesulfonate Based Dual-Selective Sensor for Iron in Different Oxidation States Ashok K. Sundramoorthy, Bhagya S. Premkumar, and Sundaram Gunasekaran* Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry Mall, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: A dual-selective sensor platform for detection of iron in ferrous (Fe2+) and ferric (Fe3+) oxidation states was developed. Upon dispersing reduced graphene oxide (rGO) sheets into poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) polymer, we deposited a hybrid film of rGO-PEDOT:PSS onto a glassy carbon electrode (GCE) surface. Effective intercalation of rGO sheets in PEDOT:PSS film was observed by Raman spectroscopy, UV−vis-NIR spectroscopy, and scanning electron microscopy. The rGO-PEDOT:PSS/ GCE sensor showed high electrocatalytic activity for Fe2+/Fe3+ redox reaction. Using amperometry with controlled applied potential, we demonstrated selective detection of both Fe2+ and Fe3+ with rGOPEDOT:PSS/GCE. The sensor responded linearly to Fe2+ from 20−833 μM (at 0.6 V) and Fe3+ from 1−833 μM (at 0.4 V) in 0.5 M KCl + 0.05 M HCl. The presence of several common metal and organic interferents such as Cu2+, Co2+, Ag+, Pb2+, Cd2+, Zn2+, Mn2+, Ni2+, Hg2+, L-glycine, L-cysteine, L-tyrosine, glucose, KCN, guanine, uric acid, xanthan, salicylate, tartrazine, and naphthol yellow did not affect the selective detection of Fe2+ and Fe3+. In addition, detection of Fe2+ and Fe3+ ions in a red wine sample and iron supplement tablets were performed with satisfactory results. The sensor was also useful in determining oxidation kinetics of Fe2+ using hydrogen peroxide and measuring Fe3+ by differential pulse voltammetry. Thus, rGO/PEDOT:PSS hybrid film based electrode we developed can serve as a practical sensor for detecting iron in its different oxidation states in real samples for different applications. KEYWORDS: heavy metals, iron detection, oxidation kinetics, PEDOT:PSS, reduced graphene oxide
I
hemochromatosis, a disease which can damage organs such as the liver, heart, and pancreas.10 High iron concentration in the brain is attributed to diseases such as Huntington’s, Parkinson’s, and Alzheimer’s.11 Therefore, detection and quantification of iron in its different oxidation states is critically important in various fields including agriculture, biology, environment, geology, food, and nutrition.12−14 The bioavailability and the extent of metabolism of iron in ferrous (Fe2+) and ferric (Fe3+) oxidation states are different. Since the Fe3+ is reduced to Fe2+ within the body, all iron preparations/supplements on the market are in the form of Fe2+. To enhance the absorption efficiency and bioavailability of iron utilized by the metabolism, iron in the Fe2+ form should be consumed. However, Fe2+ is readily oxidized to Fe3+ by light or air (oxygen) depending on how it is stored.15,16 Hence, it is necessary to be able to simultaneously detect both Fe2+ and
ron is an essential element and a vital component of many proteins that are involved in oxygen transport,1 energy metabolism,2 immune response,3 and DNA synthesis.4 Iron is a reactive catalytic center in many metalloproteins and metalloenzymes (hemoglobin and myoglobin). The average iron content in average human adults is about 40 to 50 mg kg−1 of body weight. Iron deficiency is the most common and widespread nutritional disorder in the world,5 which arises when dietary iron absorption is insufficient to meet physiological requirements.6 The main consequence of iron deficiency is anemia,7 affecting more than two billion people worldwide. Iron deficiency can lead to stunted growth, lower resistance to infections, long-term impairment in mental function, decreased productivity and food-energy conversion, and impaired neural motor development and can cause medical complications in unborn babies.8 The concentration of iron in the blood, i.e., serum ferritin (SF), is different in males (10−220 μg L−1) and females (10− 85 μg L−1). SF values below 12 μg L−1 usually represents iron deficiency.9 On the other hand, abnormally high levels (>220 μg L−1 for males and >85 μg L−1 for females) of iron absorption and storage in the body could lead to © 2015 American Chemical Society
Received: August 11, 2015 Accepted: November 25, 2015 Published: November 25, 2015 151
DOI: 10.1021/acssensors.5b00172 ACS Sens. 2016, 1, 151−157
Article
ACS Sensors Fe3+ ions in iron preparations and supplementary products to determine their status and bioavailability. Significant steps have been taken to develop a simple, fast, and accurate detection of both Fe2+ and Fe3+. Recent reports include colorimetric detection of Fe2+ based on its reaction with 1,10-phenanthroline.17 There are also several methods for the detection of Fe3+. They include fluorescent sensors based on N(rhodamine-6G)lactam-ethylenediamine/polyethylene glycolFe3O4 nanoparticles (NPs)18 and carbon NPs,19 a colorimetric method using N-acetyl-L-cysteine-stabilized silver NPs,20 titanium dioxide/iron organochelator,21 and deferoxamine immobilized MCM-41 mesoporous silica.22 However, these methods are suitable for the detection of either Fe3+ or Fe2+, not both. In some methods total iron content is determined after reducing Fe3+ to Fe2+ with ascorbic acid as reducing agent.17,23 Wansapura et al. demonstrated a quantitative spectro-electrochemical method for sensing of Fe3+ and Fe2+ using optically transparent indium tin oxide electrode coated with a thin film of Nafion preloaded with ligand 2,2′-bipyridine (bipy).24 Herein, we report fabrication of a facile, simple, and selective electrochemical sensor for the detection of both Fe3+ and Fe2+ using the same electrode, but at different applied potentials. The electrode was fabricated by dispersing reduced graphene oxide (rGO) sheets in poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) polymer aqueous solution by ultrasonication. UV−vis-NIR spectroscopy, Raman spectroscopy and scanning electron microscopy (SEM) confirmed successful intercalation of rGO sheets in the PEDOT:PSS solution. The rGO-PEDOT:PSS dispersion was applied on a glassy carbon electrode (GCE) to form a thin film, which was used as a highly selective and sensitive sensing platform for Fe3+/Fe2+ ions in 0.5 M KCl + 0.05 M HCl solution. The selectivity of rGO-PEDOT:PSS film toward Fe3+ and Fe2+ was further investigated by testing the electrocatalytic current using amperometry in the presence of other metal ions and organic inteferents including copper (Cu2+), cobalt (Co2+), silver (Ag+), lead (Pb2+), cadmium (Cd2+), zinc (Zn2+), manganese (Mn2+), nickel (Ni2+), and mercury (Hg2+), L-glycine, L-cysteine, Ltyrosine, glucose, potassium cyanide (KCN), guanine, uric acid, xanthan, salicylate, tartrazine, naphthol yellow, ascorbic acid (vitamin C), and oxalic acid. The rGO intercalated PEDOT:PSS hybrid film showed improved catalytic current response for Fe3+ and Fe2+ ions compared to PEDOT:PSS film (because of high conductivity, large-surface-to-volume ratio, etc.).25 In addition, oxidation of Fe2+ was carried out using hydrogen peroxide (H2O2) in the acidic electrolyte solution. The oxidation product of Fe2+ was monitored using rGOPEDOT:PSS hybrid film coated electrode as a sensor over different durations, which indicates that Fe2+ oxidation kinetics can be studied using our rGO-PEDOT:PSS sensor platform.
1100 nm (Figure 1a). The later peak is due to the PEDOT:PSS polymer backbone.26 This result clearly indicates that rGO sheets were well dispersed in PEDOT:PSS by ultrasonication.
Figure 1. (a) UV−vis-NIR spectra of PEDOT:PSS and rGOPEDOT:PSS dispersions in water. Insets show (i) enlarged view of PEDOT:PSS peak and (ii) spectrum of rGO−DMF dispersion. (b) Raman spectra (under green light excitation, 532 nm) of PEDOT:PSS, rGO, and rGO-PEDOT:PSS films. (c, d) FE-SEM images of rGOPEDOT:PSS film.
Raman spectra of rGO, PEDOT:PSS film, and rGOPEDOT:PSS hybrid film are shown in Figure 1b. For the rGO, a strong D (disorder) band (at ∼1343 cm−1) was observed due to the high level of defects on the graphene layers, the ID/IG ≈ 1.07 (Figure 1b, black curve).27 The spectrum for PEDOT:PSS film showed several distinct peaks, which could be assigned to the PEDOT polymer structure: peaks at 1575 and 1510 cm−1 to the symmetric Cα = Cβ stretching, peak at 1369 cm−1 to Cβ−Cβ stretching, and peak at 1249 to Cα−Cα inter-ring stretching (Figure 1b, blue curve).28,29 For rGO-PEDOT:PSS, the peaks were observed at 1343 (D band due of rGO), 1436, 1597, 1585, 2665 cm−1 (2D band of rGO). Comparing with the rGO spectrum, additional peaks corresponding to PEDOT:PSS polymer were present in the spectrum of rGO-PEDOT:PSS, which indicated intercalation of rGO within PEDOT:PSS. The strong G band at 1597 cm−1 is attributed to the merging peaks of rGO and polymer, which is the upshifted G band of rGO at 1585 cm−1. In addition, the 2D band of rGO observed in the spectrum of rGO-PEDOT:PSS has downshifted from 2682 to 2665 cm−1. From this, it was evident that the electronic and vibrational properties of PEDOT:PSS were significantly modified after intercalation of rGO sheets. The PEDOT:PSS and rGOPEDOT:PSS were further studied with SEM (Figure 1c,d and Figure S1b). In the micrograph of rGO-PEDOT:PSS, rGO sheets of ∼500 nm to 4 μm in size are observed, which further confirms intercalation of rGO sheets within the PEDOT:PSS polymer matrix. Electrochemical Characterization and Impedance Studies. To characterize chemically modified electrode surface, we measured cyclic voltammograms (CVs) in 0.5 mM [Fe(CN)6]3‑/4‑ + 0.1 M KCl solution using GCEs coated with thin-film of rGO-PEDOT:PSS, PEDOT:PSS, and rGO. The redox peak separation (ΔEp = Epc − Epa) of [Fe(CN)6]3‑/4‑ was calculated as 66.7 (anodic peak current, Ipa = 12.23 μA), 62.4
■
RESULTS AND DISCUSSION UV−vis-NIR, Raman, and SEM Characterizations. The rGO, PEDOT:PSS, and rGO-PEDOT:PSS dispersions are shown in Figure S1a (see Supporting Information for Experimental Section). The rGO dispersed PEDOT:PSS polymer solution is dark black in color, which is due to high dispersibility of rGO sheets in the polymer solution. UV−visNIR spectra of rGO−DMF dispersion shows a sharp absorption peak at 273 nm, but rGO-PEDOT:PSS and PEDOT:PSS dispersions show two absorption peaks, one sharp peak at 271 nm and a broad peak in the range ∼650− 152
DOI: 10.1021/acssensors.5b00172 ACS Sens. 2016, 1, 151−157
Article
ACS Sensors (Ipa = 9.88 μA), 123.5 (Ipa = 8.96 μA), and 130.4 mV (7.78 μA) for rGO-PEDOT:PSS/GCE, PEDOT:PSS/GCE, rGO/GCE, and bare GCE, respectively (Figure 2a). The electrochemical
Cyclic Voltammetry of Fe2+and Fe3+ at rGO-PEDOT:PSS/GCE and PEDOT:PSS/GCE. Figure 3 shows CVs
Figure 2. (a) Cyclic voltammetry and (b) Nyquist plots recorded in 0.1 M KCl with 0.5 mM [Fe(CN)6]3‑/4‑ using bare and differently modified glassy carbon electrodes (GCEs). Scan rate = 20 mV s−1.
Figure 3. Cyclic voltammetry of bare and modified glassy carbon electrodes (GCEs) in 0.5 M KCl + 0.05 M HCl containing 98 μM (a) Fe2+ and (b) Fe3+. Scan rate = 20 mV s−1.
active surface area of the electrodes were estimated as 1.112, 0.902, 0.818, and 0.707 cm2 for rGO-PEDOT:PSS/GCE, PEDOT:PSS/GCE, rGO/GCE, and bare GCE, respectively.30 The highest Ipa and relatively low peak separation were observed with rGO-PEDOT:PSS nanocomposite film-coated electrode, which corroborates the existence of high electrocatalytic effect with an increased electrochemically active surface area. In addition, the backbone of PEDOT:PSS is positively charged, which might attract negatively charged [Fe(CN)6]3‑/4‑ toward the electrode surface.31 We note that rGO- and PEDOT:PSS-modified electrodes exhibited higher peak currents compared to bare GCE; however, for rGOPEDOT:PSS hybrid film, the redox peak currents were even higher, perhaps due to synergistic effect between rGO and PEDOT:PSS film (Figure 2a). The Nyquist plots were recorded for all coated electrodes in 0.5 mM [Fe(CN)6]3‑/4‑ + 0.1 M KCl solution (Figure 2b). We observed that the diameter of the semicircle in the high frequency region, which corresponds to the charge-transfer resistance (R ct ) at the electrode/electrolyte interface, changed.32 The Rct values were measured as 111, 108, 136, and 143 Ω for bare and PEDOT:PSS-, rGO-, and rGOPEDOT:PSS-modified GCE, respectively (Figure 2b). The PEDOT:PSS film formation on the surface did not affect the Rct of the electrode because PEDTO:PSS is highly conducting. However, rGO and rGO-PEDOT:PSS layers increased the Rct value which points to increased hindrance to electron transfer at rGO and rGO-PEDOT:PSS films. This is consistent with literature reports that added layers on electrodes tend to increase Rct value,32−34 and confirms successful deposition of nanocomposite film on the electrode surface.
of rGO-PEDOT:PSS/GCE in the absence and presence of 98 μM Fe2+ or Fe3+. After the addition of 98 μM Fe3+ or Fe2+ both PEDOT:PSS/GCE and rGO-PEDOT:PSS/GCE exhibited highly enhanced oxidation/reduction peaks at 0.571 V/0.479 and 0.571 V/0.475 V, respectively, which indicate improved electrocatalytic activities for both Fe3+ or Fe2+. The oxidation peak currents (Ipa) of Fe2+ (23%) and Fe3+ (29%) increased in the presence of rGO. As expected, bare GCE did not produce well-defined redox peaks for both Fe3+ and Fe2+ ions. However, there were ill-defined oxidation/reduction peaks for Fe2+ with rGO/GCE compared to with bare GCE, which indicates increased electrochemically active surface area of the electrode upon rGO coating (Figure 3a). This means that due to synergistic effect between rGO and PEDOT:PSS film, it has high electrochemical catalytic activity toward Fe2+ and Fe3+. As reported earlier, PSS functional groups of PEDOT:PSS polymer film had high affinity to multivalent cations such as Fe3+. We hypothesize that due to strong electrostatic attraction, Fe2+ and Fe3+ ions are drawn to the electrode surface of PEDOT:PSS film, which in return produces high catalytic current35 (Scheme 1). To study the effect of accumulation time on electrode response, we recorded CVs using PEDOT:PSS/GCE in 0.5 M KCl + 0.05 M HCl containing 455 μM Fe2+. We did not observe any change in electrocatalytic peak current at PEDOT:PSS/GCE over 0 to 60 min, which indicated there was no accumulation effect on the electrode response (Figure S2a). Furthermore, to ascertain whether Fe2+ forms complex with PEDOT:PSS film, we recorded CVs in 0.5 M KCl + 0.05 M HCl (without Fe2+) using PEDOT:PSS/GCE which was previously used for accumulation study. The lack of redox peak 153
DOI: 10.1021/acssensors.5b00172 ACS Sens. 2016, 1, 151−157
Article
ACS Sensors Scheme 1. Representation of Electrostatic Attraction between PEDOT:PSS and Fe2+ or Fe3+ and Their Electrochemical Redox Reaction at Different Applied Potentials
of Fe2+ confirmed that Fe2+ did not form a complex with the PEDOT:PSS film on the electrode surface (Figure S2b). Herein, for the first time, we demonstrated selective detection of multivalent cations such as Fe2+ and Fe3+ using rGO-PEDOT:PSS hybrid film based sensor. In the 0.5 M KCl + 0.05 M HCl, both rGO/PEDOT:PSS/GCE and PEDOT:PSS/GCE yielded a redox peak at 0.63 V (Figure S3a,b). To determine if any impurity of Fe compounds present in the purchased PEDOT:PSS, the elemental composition of the rGO/PEDOT:PSS, PEDOT:PSS, and rGO films were measured using X-ray photoelectron spectroscopy (XPS). No Fe impurity was found in any of the films (Table S1), indicating that the observed redox peak at 0.63 V is not due to Fe. Some monomers got oxidized around 0.588 to 0.652 V when PEDOT:PSS/GCE was cycled between 0.0 and 1.0 V (Figure S3a). Further studies are required to understand the contribution of this small redox peak at 0.63 V (Figure S3b). The effect of scan rate on the redox peak current of 192 μM Fe2+ and Fe3+ in 0.5 M KCl + 0.05 M HCl was investigated using rGO-PEDOT:PSS/GCE. Both anodic and cathodic peak currents (Ipa and Ipc) of Fe2+ and Fe3+ increased linearly with scan rate from 20 to 300 mV s−1, which suggests that the electrode reaction is controlled by the adsorption process (Figure S4).36 Amperometric Detection of Fe2+ and Fe3+ at rGOPEDOT:PSS/GCE. We believe that functional groups of PEDOT:PSS preferentially attracts Fe3+ and Fe2+ compared to other metal cations. This was investigated by testing the sensor response to different metal ions via amperometry under constant stirring (650 rpm) in 0.5 M KCl + 0.05 M HCl. Amperograms obtained with consecutive additions of different metal ions (Fe2+, Fe3+, Cu2+, Co2+, Ag+, Pb2+, Cd2+, Zn2+, Mn2+, Ni2+, and Hg2+) showed a significant (oxidation current) response only for Fe2+ addition when the onset potential was set at +0.6 V (Figure 4a), and only for Fe3+ addition (reduction current) when the onset potential was set at +0.4 V (Figure 4b). Thus, our sensor can be used to selectively detect either Fe2+ or Fe3+ simply by adjusting the detection potential.
Figure 4. Amperometric detection of Fe3+ and Fe2+ with rGOPEDOT:PSS/GCE in 0.5 M KCl + 0.05 M HCl. Amperograms recorded at (a) 0.6 V and (b) 0.4 V with different metal ions at various concentrations (μM): 49.5 Fe3+, 49.01 Fe2+, 48.54 Cu2+, 48.07 Co2+, 47.61 Ag+, 47.16 Pb2+, 46.72 Cd2+, 46.29 Zn2+, 45.87 Mn2+, 45.45 Ni2+, 45.04 Hg2+, 44.64 Fe2+, and 44.24 Fe3+. Amperogram obtained at rGOPEDOT:PSS/GCE for successive additions of (c) Fe2+ from 20 to 833 μM at 0.6 V and (d) Fe3+ from 1 to 833 μM at 0.4 V.
Further, the electrode current response (μA) measured changed linearly with amount of Fe2+ or Fe3+ added (μM) at their respective detection potentials. The linear range for Fe2+, fitted to Ipa = −0.028 Fe − 0.013 (R2 = 0.9997), is 20 to 833 μM (Figure 4c and Figure S5a); similarly, the linear range for Fe3+, fitted to Ipc = 0.217 Fe + 0.012 (R2 = 0.9973), is 1 to 833 μM (Figure 4d and Figure S5b). We also recorded amperograms using bare GCE and rGO/GCE (Figure S6a,b). These results show that rGO/PEDOT:PSS/GCE is more sensitive than rGO/GCE and bare GCE. The limit of detection (LOD) values were estimated as 0.1 μM and 20 μM for Fe3+ and Fe2+, 154
DOI: 10.1021/acssensors.5b00172 ACS Sens. 2016, 1, 151−157
Article
ACS Sensors
μm ascorbic acid, and 185 μm oxalic acid (Figure S8a,b). The rGO-PEDOT:PSS/GCE did not respond to most of these molecules. However, from this study we found that salicylate, ascorbic acid (vitamin C), and oxalic acid were relatively strongly interfered at this potential in this given condition (Figure S8a,b). Furthermore, to demonstrate the potential application of this sensor, we tested Fe3+ and Fe2+ content in red wine samples by amperometry (Figures S9 and S10). Amperograms were recorded with the standard addition of 192 μM Fe3+ in 500 μL wine sample (California Cabernet Sauvignon, Two Vines, Paterson, WA) and followed by 192 μM Fe3+ in 0.5 M KCl + 0.05 M HCl. The electrode responded very well with each Fe3+ addition, only changing about 2%, which indicated that Fe3+ detection can be performed in wine samples (Figure S9a,b). Similar results were obtained with addition of 98 μM Fe2+ (Figure S10). The sensor response was stable for all additions with a relative standard deviation (RSD) of 0.88. Next, one tablet each from Blood Builder, Mega Food (Tablet A), and Finest Nutrition Iron Supplement 28 mg (Tablet B) were separately dissolved in two 10 mL vials in water by bath sonication. After complete dissolution, the tablet solutions (Fe2+ form) were centrifuged at 4000 rpm for 20 min and the supernatant was used to detect the presence of Fe2+ concentration by CV (Figure S11). A calibration curve was prepared using a series of standard Fe2+ solutions. The CV data were converted into the corresponding iron concentration using the calibration curve. As shown in Table 1, Fe2+ values
respectively, using 3SD/b, where SD is standard deviation of the lowest concentration and b is the slope of the calibration graph.37 The performance of our sensor, in terms of low LOD and wide linear range for both Fe3+ and Fe2+, compares very favorably with that of other analytical methods reported in the literature14,38−40 (Table S2). Interaction of Fe2+and Fe3+ with PEDOT:PSS. The interaction between Fe3+ or Fe2+ and PEDOT:PSS solution was investigated using UV−vis-NIR spectroscopy. As shown in Figure 5a, absorption peak of PEDOT:PSS decreased and red-
Table 1. Measurement of Fe2+ Content in Commercial Iron Supplement Tablets samplea
labeled Fe2+ value (mg tablet−1)
measured value (Fe2+)b (mg tablet−1)
recovery (%)
Tablet A Tablet B
28 26
26 ± 3b 27 ± 1b
92.9 103.8
Tablet A − Blood Builder Mega Food, Tablet B − Finest Nutrition Iron Supplement. bThree replicate measurements each.
a
Figure 5. UV−vis-NIR spectra of 125 μM iron solution and PEDOT:PSS dispersion with and without 125 μM iron (a) in Fe3+ form and (b) in Fe2+ form. Insets show expanded views of the highlighted areas.
were determined as 26 ± 3 mg and 27 ± 1 mg, compared to the expected values of 28 and 26 mg, respectively, for Tablets A and B. Other constituents in the tablets (listed in the product labels) such as dicalcuium phosphate, calcium carbonate, cellulose, croscarmellose, stearic acid, magnesium stearate, folate, and vitamin B12 did not interfere with the measurement. Further, the sensor was tested by spiking the tablet solutions with a known amount of Fe2+. The measurement data for the spiked samples (Table 2) show a high recovery rate indicating that the sensor performed very satisfactorily. Thus, our sensor is potentially useful for the detection of Fe2+ in various real samples. Sensor Repeatability/Reliability. The repeatability/reliability of electrode fabrication was tested by preparing seven rGO-PEDOT:PSS/GCEs. The redox peak currents were
shifted from 271 to 279 nm immediately after adding Fe3+, suggesting a strong charge-transfer interaction between Fe3+ and PEDOT:PSS (Scheme 1).41 With the addition of Fe2+, there was a small decrease in the absorbance peak of PEDOT:PSS, without any significant peak shift. This suggests that interaction between Fe2+ and PEDOT:PSS functional groups is not as strong as between Fe3+ and PEDOT:PSS (Figure 5b). This is the reason for the lower LOD of 0.1 μM for Fe3+ compared ∼20 μM for Fe2+ (Figure 4c,d). Sensor Stability and Real Sample Analyses. Electrochemical stability of rGO/PEDOT:PSS/GCE was tested in 0.5 M KCl + 0.05 M HCl electrolyte by potential sweeping from 0.0 to 1.0 V using cyclic voltammetry. The electrode background current was changed only about 5% after 100 cycles, indicating excellent stability (Figure S7). Furthermore, we studied the effect of common organic interferents on the rGO-PEDOT:PSS/GCE. Figure S8 shows sequential addition of Fe2+ (at 0.6 V) or Fe3+ (at 0.4 V) with 490 μm glycine, 260 μm cysteine, 48 μm L-tyrosine, 4.63 μm glucose, 261 μm KCN, 624 μm guanidine, 46.3 μm uric acid, 0.01% xanthan, 137.6 μm salicylate, 45.45 μm tartrazine, 45.05 μm naphthyl yellow, 109.8
Table 2. Detection of spiked Fe2+ in Commercial Iron Supplement Tablet Samples
a
155
Fe2+ spiked (mg mL‑1)
Fe2+ measureda (mg mL‑1)
recovery (%)
0.028 0.055 0.082
0.030 ± 0.0010 0.052 ± 0.0015 0.081 + 0.0015
107.1 94.5 98.8
Mean ± standard deviation of three measurements each. DOI: 10.1021/acssensors.5b00172 ACS Sens. 2016, 1, 151−157
Article
ACS Sensors recorded with 455 μM Fe3+ or Fe2+ using each electrode individually by CV. The current values obtained with all the electrodes were almost constant with RSD of 0.517, which attests high repeatability of our electrode fabrication procedure. Fe2+ Oxidation Kinetics. Fe2+ is readily oxidized by oxygen or other oxidizing agents to the thermodynamically stable Fe3+ form. However, Fe3+ is insoluble in water and unavailable for biological use. Therefore, it is necessary to study the stability of Fe2+.42 We studied the oxidation kinetics of Fe2+ in 0.5 M KCl + 0.05 M HCl at rGO-PEDOT:PSS/GCE using differential pulse voltammetry by recording changes in the concentration of Fe3+ species. Figure 6a shows the differential pulse voltammo-
been reported. 42 This experiment confirms that rGOPEDOT:PSS-modified electrode can be used to monitor oxidation of Fe2+.
■
CONCLUSIONS We have developed and tested a highly selective electrochemical sensor platform using rGO/PEDOT:PSS film based electrode for the detection of iron in both Fe3+ and Fe2+ oxidation states with controlled applied potentials. The LODs of the sensor were 0.1 μM for Fe3+ and 20 μM for Fe2+ and the linear range was up to 833 μM for both. The sensor response was unaffected by several metal and organic interferents tested and by other constituents present in commercial iron supplement tablets and wine samples. The sensor could be useful in monitoring oxidation of Fe2+ to Fe3+ using H2O2 in acidic electrolyte by DPV. Thus, our rGO/PEDOT:PSS film based electrode system is potentially suitable for testing iron content in food, environmental, and biological samples.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.5b00172. Experimental details, photographic images of dispersions, SEM image of PEDOT:PSS, accumulation study CV data, CVs of PEDOT:PSS, and rGO/PEDOT:PSS, effect of scan rate, calibration graph of Fe2+ and Fe3+, comparison amperograms, stability CV data, organic interference study, real sample analysis, elemental composition, and performance characteristics of different Fe sensors (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +1 (608) 262-1019. Fax: +1 (608) 262-1228.
Figure 6. (a) DPVs of rGO-PEDOT:PSS/GCE in 0.5 M KCl + 0.05 M HCl at a scan rate of 20 mV s−1 with (i) 0 and (ii−vii) 38 Fe2+ (μg mL−1); with 38 μg mL−1 Fe2+ and 56 mM H2O2 at different times after H2O2 addition (iii) 0, (iv) 5, (v) 10, (vi) 20, and (vii) 130 min. (b) Similar experiments as in (a) but without H2O2 for (i) 0 and (ii−v) 38 μg mL−1 Fe2+ measured at different times (ii) 0, (iii) 5, (iv) 35, and (v) 65 min.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported in part by funding from the National Institute of Food and Agriculture, United States Department of Agriculture, under ID number WIS01644. The Authors acknowledge the use of instrumentation supported by the UW MRSEC (DMR-1121288) and the UW NSEC (DMR0832760).
grams (DPVs) recorded without and with Fe2+ ions (curves i and ii). A strong reduction peak at 0.512 V was observed, which is due to the electrochemical reduction of Fe3+ to Fe2+ (Figure 6a curve ii). However, to oxidize Fe2+, 56 mM H2O2 was added. To verify the oxidation of Fe2+, we recorded DPVs in different time intervals after H2O2 addition (Figure 6a, curves i−vii). Cathodic reduction peak of Fe3+ increased immediately after H2O2 addition (curve iii) and steadily increase until 15 min. After 15 min, the cathodic peak remained stable and began to decline, which indicated the complete oxidation of Fe2+ to Fe3+ (Figure 6a curves i−vii). Since the oxidized Fe3+ form is insoluble, it led to decreased cathodic peak current after 130 min (Figure 6a, curves i−vii). This data set indicates that our sensor can be used to study oxidation kinetics of Fe2+ and hence help monitor the bioavailability of Fe2+. DPVs obtained without the addition of H2O2 show no significant changes in Fe3+ reduction peak over the 65 min of test duration (Figure 6b). Thus, without H2O2, Fe2+ is stable in acidic solution, as has
■
REFERENCES
(1) Milman, N. Anemia-still a major health problem in many parts of the world! Ann. Hematol. 2011, 90, 369−377. (2) Ganz, T. Molecular control of iron transport. J. Am. Soc. Nephrol. 2007, 18, 394−400. (3) Johnson, E. E.; Wessling-Resnick, M. Iron metabolism and the innate immune response to infection. Microbes Infect. 2012, 14, 207− 216. (4) Pra, D.; Franke, S. I. R.; Henriques, J. A. P.; Fenech, M. Iron and genome stability: An update. Mutat. Res., Fundam. Mol. Mech. Mutagen. 2012, 733, 92−99. (5) WHO; FAO, Guidelines on food fortification with micronutrients; World Health Organization: Geneva. 2006. (6) Zimmermann, M. B.; Hurrell, R. F. Nutritional iron deficiency. Lancet 2007, 370, 511−520.
156
DOI: 10.1021/acssensors.5b00172 ACS Sens. 2016, 1, 151−157
Article
ACS Sensors
a three-dimensional surfactant 4-sulfocalix[4]arene. Sci. Rep. 2015, 5, 10716. (28) Han, Y.-K.; Chang, M.-Y.; Huang, W.-Y.; Pan, H.-Y.; Ho, K.-S.; Hsieh, T.-H.; Pan, S.-Y. Improved Performance of Polymer Solar Cells Featuring One-Dimensional PEDOT Nanorods in a Modified Buffer Layer. J. Electrochem. Soc. 2011, 158, K88−K93. (29) Chang, S. H.; Chiang, C. H.; Kao, F. S.; Tien, C. L.; Wu, C. G. Unraveling the Enhanced Electrical Conductivity of PEDOT:PSS Thin Films for ITO-Free Organic Photovoltaics. IEEE Photonics J. 2014, 6, 8400307. (30) Trasatti, S.; Petrii, O. A. Real surface-area measurements in electrochemistry. Pure Appl. Chem. 1991, 63, 711−734. (31) Vasantha, V. S.; Chen, S.-M. Electrochemical preparation and electrocatalytic properties of PEDOT/ferricyanide film-modified electrodes. Electrochim. Acta 2005, 51, 347−355. (32) Barreira, S. V. P.; García-Morales, V.; Pereira, C. M.; Manzanares, J. A.; Silva, F. Electrochemical Impedance Spectroscopy of Polyelectrolyte Multilayer Modified Electrodes. J. Phys. Chem. B 2004, 108, 17973−17982. (33) Zhang, J.; Xu, J.; Wen, Y.; Wang, Z.; Zhang, H.; Ding, W. Voltammetric determination of phytoinhibitor maleic hydrazide using PEDOT:PSS composite electrode. J. Electroanal. Chem. 2015, 751, 65−74. (34) Stöcker, T.; Köhler, A.; Moos, R. Why does the electrical conductivity in PEDOT:PSS decrease with PSS content? A study combining thermoelectric measurements with impedance spectroscopy. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 976−983. (35) Lefebvre, M.; Qi, Z.; Rana, D.; Pickup, P. G. Chemical Synthesis, Characterization, and Electrochemical Studies of Poly(3,4ethylenedioxythiophene)/Poly(styrene-4-sulfonate) Composites. Chem. Mater. 1999, 11, 262−268. (36) Chen, K.; Zhang, Z.-L.; Liang, Y.-M.; Liu, W. A Graphene-Based Electrochemical Sensor for Rapid Determination of Phenols in Water. Sensors 2013, 13, 6204−6216. (37) Shrivastava, A.; Gupta, V. B. Methods for the determination of limit of detection and limit of quantitation of the analytical methods. Chron. Young Sci. 2011, 2, 21. (38) Donato, N.; Aloisio, D.; Leonardi, S. G.; Neri, G. Ink-Jet Printed Colorimetric Sensor for the Determination of Fe(II). IEEE Sens. J. 2015, 15, 3196−3200. (39) Xiang, X.; Zhang, Z.; Shi, J.; Huang, F. Paper-based analytical device with colorimetric assay application to the determination of phenolic acids and recognition of Fe3+. RSC Adv. 2015, 5, 2615−2619. (40) Sobkowiak, M.; Gabrielsson, R.; Inganas, O.; Milczarek, G. Amperometric detection of iron (III) on electroconductive hydrogel based on polypyrrole and alkoxysulfonated poly(3,4-ethylenedioxythiophene) (PEDOT-S). Synth. Met. 2014, 194, 170−175. (41) Yoo, D.; Son, W.; Kim, S.; Lee, J. J.; Lee, S. H.; Choi, H. H.; Kim, J. H. Gradual thickness-dependent enhancement of the thermoelectric properties of PEDOT:PSS nanofilms. RSC Adv. 2014, 4, 58924−58929. (42) Spolaor, A.; Vallelonga, P.; Gabrieli, J.; Cozzi, G.; Boutron, C.; Barbante, C. Determination of Fe2+ and Fe3+ species by FIA-CRCICP-MS in Antarctic ice samples. J. Anal. At. Spectrom. 2012, 27, 310− 317.
(7) Patrick, G.; Brady, M. D. Iron Deficiency Anemia: A Call for Aggressive Diagnostic Evaluation. South. Med. J. 2007, 100, 966−967. (8) Welch, R. M. The impact of mineral nutrients in food crops on global human health. Plant Soil 2002, 247, 83−90. (9) Kohgo, Y.; Ikuta, K.; Ohtake, T.; Torimoto, Y.; Kato, J. Body iron metabolism and pathophysiology of iron overload. Int. J. Hematol. 2008, 88, 7−15. (10) Crisponi, G.; Remelli, M. Iron chelating agents for the treatment of iron overload. Coord. Chem. Rev. 2008, 252, 1225−1240. (11) Burdo, J.; Connor, J. Brain iron uptake and homeostatic mechanisms: An overview. BioMetals 2003, 16, 63−75. (12) Chen, S.; Li, N.; Zhang, X.; Yang, D.; Jiang, H. Online spectrophotometric determination of Fe(II) and Fe(III) by flow injection combined with low pressure ion chromatography. Spectrochim. Acta, Part A 2015, 138, 375−380. (13) Bowie, A. R.; Achterberg, E. P.; Mantoura, R. F. C.; Worsfold, P. J. Determination of sub-nanomolar levels of iron in seawater using flow injection with chemiluminescence detection. Anal. Chim. Acta 1998, 361, 189−200. (14) Peña, R. C.; de Souza, A. P. R.; Bertotti, M. Determination of Fe(III) in wine samples using a ruthenium oxide hexacyanoferrate modified microelectrode. J. Electroanal. Chem. 2014, 731, 49−52. (15) Senyuva, H. Z.; Sarica, D. Y.; Ozden, T. Simultaneous determination of Fe(II) and Fe(III) in pharmaceutical samples by post-column derivatization/HPLC. Turk. J. Chem. 2002, 26, 425−430. (16) Lu, M.; Compton, R. G. Voltammetric Determination of Iron(III) in Water. Electroanalysis 2013, 25, 1123−1129. (17) Apilux, A.; Dungchai, W.; Siangproh, W.; Praphairaksit, N.; Henry, C. S.; Chailapakul, O. Lab-on-Paper with Dual Electrochemical/Colorimetric Detection for Simultaneous Determination of Gold and Iron. Anal. Chem. 2010, 82, 1727−1732. (18) Wang, B. D.; Hai, J.; Liu, Z. C.; Wang, Q.; Yang, Z. Y.; Sun, S. H. Selective Detection of Iron(III) by Rhodamine-Modified Fe3O4 Nanoparticles. Angew. Chem., Int. Ed. 2010, 49, 4576−4579. (19) Qu, K. G.; Wang, J. S.; Ren, J. S.; Qu, X. G. Carbon Dots Prepared by Hydrothermal Treatment of Dopamine as an Effective Fluorescent Sensing Platform for the Label-Free Detection of Iron(III) Ions and Dopamine. Chem. - Eur. J. 2013, 19, 7243−7249. (20) Gao, X.; Lu, Y.; He, S.; Li, X.; Chen, W. Colorimetric detection of iron ions (III) based on the highly sensitive plasmonic response of the N-acetyl-L-cysteine-stabilized silver nanoparticles. Anal. Chim. Acta 2015, 879, 118−125. (21) Becuwe, M.; Rouge, P.; Gervais, C.; Courty, M.; DassonvilleKlimpt, A.; Sonnet, P.; Baudrin, E. A new sensitive organic/inorganic hybrid material based on titanium oxide for the potentiometric detection of iron(III). J. Colloid Interface Sci. 2012, 388, 130−136. (22) Alberti, G.; Emma, G.; Colleoni, R.; Pesavento, M.; Nurchi, V. M.; Biesuz, R. Novel DFO-functionalized mesoporous silica for iron sensing. Part 2. Experimental detection of free iron concentration (pFe) in urine samples. Analyst 2014, 139, 3940−3948. (23) Peng, B.; Shen, Y.; Gao, Z.; Zhou, M.; Ma, Y.; Zhao, S. Determination of total iron in water and foods by dispersive liquidliquid microextraction coupled with microvolume UV-vis spectrophotometry. Food Chem. 2015, 176, 288−293. (24) Wansapura, C. M.; Seliskar, C. J.; Heineman, W. R. Spectroelectrochemical Sensing Based on Multimode Selectivity Simultaneously Achievable in a Single Device. 20. Detection of Metal Ions in Different Oxidation States. Anal. Chem. 2007, 79, 5594− 5600. (25) Sundramoorthy, A. K.; Gunasekaran, S. Applications of graphene in quality assurance and safety of food. TrAC, Trends Anal. Chem. 2014, 60, 36−53. (26) Yasri, N.; Sundramoorthy, A. K.; Chang, W.-J.; Gunasekaran, S. Highly Selective Mercury Detection at Partially Oxidized Graphene/ Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Nanocomposite Film Modified Electrode. Front. Mater. 2014, 1, 33. (27) Sundramoorthy, A. K.; Wang, Y.; Wang, J.; Che, J.; Thong, Y. X.; Lu, A. C. W.; Chan-Park, M. B. Lateral assembly of oxidized graphene flakes into large-scale transparent conductive thin films with 157
DOI: 10.1021/acssensors.5b00172 ACS Sens. 2016, 1, 151−157
