IEEE SENSORS JOURNAL, VOL. 15, NO. 6, JUNE 2015
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A Selective and Sensitive Sensor for Determination of Sulfide in Aquatic Environment Afsaneh Safavi and Alireza Abi
Abstract— A cost-effective, sensitive, and selective electrochemical sensor for the analysis of sulfide based on the high affinity of sulfide to metallic silver is proposed. To achieve high sensitivity while lowering the cost of the sensor, silver nanoparticles (Ag NPs), which possess larger metal surface area-to-volume ratio compared with bulk silver, were employed for the development of the sensor. The sensor was prepared through electrodeposition of Ag NPs on the surface of a carbon ionic liquid electrode using potentiostatic double-pulse technique, and it showed a wide linear range from 25 to 2500 µM of sulfide with a detection limit of 6.02 µM. Since the analysis of sulfide was performed at a remarkably low detection potential, the presented sensor offered high selectivity against common interferents and was successfully applied to the analysis of sulfide in a hot spring water sample. Index Terms— Carbon ionic liquid electrode, double-pulse technique, silver nanoparticles, sulfide sensor.
I. I NTRODUCTION YDROGEN sulfide (H2 S) is a hazardous and toxic compound, which is produced both naturally and as a result of industrial activities. In nature, hydrogen sulfide is produced through decomposition of sulfur-containing organic compounds and can be found in natural gases, crude petroleum, volcanoes and hot springs. H2 S is soluble in water and, depending on pH of the solution, can dissociate to hydrosulfide anion (HS− ) and sulfide ion (S2− ). Due to the high toxicity of these species as well as their capability to remove dissolved oxygen, the level of total sulfide is considered as an important pollution index for water. Consequently, developing a simple, cost-effective, sensitive, and selective method for monitoring total sulfide in water samples has drawn considerable attention among scientific community [1]–[3]. The level of sulfide concentration in natural waters and industrial wastewaters depends on a variety of variables, including geographical location and the type of natural sources or industry. According to the existing literature, the typical concentration range of sulfide in hot spring waters and wastewaters are 9-440 μM [4]–[8] and 2-4750 μM [9]–[13],
H
Manuscript received September 1, 2014; revised November 6, 2014; accepted November 18, 2014. Date of publication January 20, 2015; date of current version May 4, 2015. This work was supported by the Research Council through the Shiraz University, Shiraz, Iran. The associate editor coordinating the review of this paper and approving it for publication was Prof. Venkat R. Bhethanabotla. A. Safavi is with the Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71348, Iran (e-mail:
[email protected]). A. Abi was with the Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71348, Iran. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2014.2379696
respectively. These levels of sulfide concentration can be determined by a number of analytical techniques, such as iodimetric [12], [14], spectrophotometric [15]–[22], chromatographic [23], [24], capillary electrophoretic [25], and electrochemical methods [13], [26]–[31]. Among them, electrochemical methods are of particular interest for analysis of sulfide, which is due to their prominent advantages of procedural simplicity, high sensitivity, short analysis time, capability of miniaturization, and low instrumental cost. The high affinity of sulfide species to metallic silver has been previously exploited as a route for sensitive and selective quantification of these compounds. Schiavon et al. utilized a porous silver electrode for electrochemical detection of trace hydrogen sulfide in gaseous atmospheres [32]. In another study, Shimizu and Osteryoung proposed a method for determination of sulfide based on cathodic stripping voltammetry of Ag2 S films formed at a rotating silver disk electrode upon electrode anodization [33]. Recently, Contreira-Pereira et al. has developed an electrochemical sensor for monitoring of sulfide in deep sea vent habitats based on electroreduction of silver sulfide layers formed on a bare silver electrode [34]. Although the use of bulk silver electrodes for analysis of sulfide has led to the development of sensors with good performances, utilization of silver nanoparticles (Ag NPs) supported on inexpensive electrodes can be advantageous, as the larger metal surface area to volume ratio of these electrodes may enhance the sensitivity while lowering the cost of the developed sensors [35]. Electrochemical deposition is considered as an easy and inexpensive practical approach for preparation of arrays of metallic NPs on conductive supports. In this method, size, shape, and morphology of the formed nanostructures can be easily controlled by adjusting electrodeposition variables such as current density, applied potential and time. It was previously demonstrated that metallic NPs with narrow particle size distribution can be fabricated using potentiostatic double-pulse, which is an electrodeposition technique that performs nucleation and growth processes in two separate steps [36]–[38]. In the first step, an extremely short pulse of high cathodic potential is applied to the electrode in order to initiate nucleation. The second step includes application of a much longer pulse of a lower cathodic overpotential, which causes the NPs to grow in size while overlapping of the depletion zones of adjacent NPs is prevented. In this work, we applied and compared different electrodeposition techniques for fabrication of Ag NPs on a carbon ionic liquid electrode (CILE), which was previously
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introduced as a high performance electrode with relatively good features [39]. The Ag NPs-modified CILE (Ag NPs/CILE) obtained by the best electrodeposition method was then employed for sensitive and selective determination of sulfide at a low detection potential based on electrooxidation of Ag NPs in sulfide solutions. Finally, practical utility of the proposed sensor was evaluated by analyzing sulfide in a hot spring water sample. II. E XPERIMENTAL A. Chemicals All chemicals were of analytical grade and used without further purification. A stock solution of sulfide was prepared daily by dissolving an appropriate amount of Na2 S.9H2 O in distilled water and standardized by iodometric titration. Working solutions were prepared from the stock solution through dilution with Britton–Robinson buffer (0.1 M acetic acid, 0.1 M phosphoric acid and 0.1 M boric acid, pH was adjusted by addition of concentrated sodium hydroxide). Due to the instability of sulfide solutions, the working solutions were analyzed immediately after preparation. Doubly distilled water was used throughout the experiments.
Fig. 1. Consecutive CVs of CILE in 1 mM AgNO3 /0.1 M KNO3 at 100 mV s−1 . Curves a, b, and c represent the first, second, and third cycles, respectively. Arrow shows direction of the first scan.
B. Instrumentation All electrochemical measurements were performed using a conventional three-electrode cell linked to a μ-Autolab electrochemical system (Eco-Chemie, Utrecht, The Netherlands), which was equipped with GPES software. The cell setup was consisted of Ag NPs/CILE as working, an Ag/AgCl (3 M KCl) as reference and a platinum disk as auxiliary electrode. Pulse amplitude of 200 mV, pulse width of 7 ms, and step potential of 15 mV (apparent scan rate of 21 mV s−1 ) were employed in differential pulse voltammetric (DPV) measurements. Surface morphology of the modified electrodes was visualized using a Leica Cambridge S360 scanning electron microscope (SEM). All the experiments were carried out at ambient temperature (25 ± 2 °C).
Fig. 2. SEM images of (a) bare CILE and (b-d) CILE modified with Ag NPs using different electrodeposition techniques: cyclic voltammetry (b), potentiostatic single-pulse (c), and potentiostatic double-pulse (d). Inset in panel (d) is a higher magnification view of (d).
C. Preparation of CILE CILE was obtained through mixing of graphite powder and n-octyl pyridinium hexafluoro phosphate ([C8 Py][PF6 ]) with a ratio of 50/50 (w/w) as described elsewhere [39]. The obtained composite was packed into the cavity of a Teflon tube (1.8-mm diameter) and heated by a hair dryer for a few minutes in order to achieve better composite homogeneity and lower background current. Electrical contact was established via a stainless steel handle and electrode surface was renewed by polishing the electrode onto a weighing paper. D. Preparation of Ag NPs/CILE Electrodeposition of Ag NPs on CILE surface was performed in a solution containing 1 mM AgNO3 and 0.1 M KNO3 using three different electrodeposition techniques: cyclic voltammetry, potentiostatic single-pulse, and potentiostatic double-pulse. In cyclic voltammetry, the potential of the bare CILE was cycled (10 cycles) between
0.7 V and −0.1 V (cyclic scans were begun at 0.7 V, reversed at −0.1 V and ended at 0.7 V) at scan rate, ν, of 100 mV s−1 . In potentiostatic single-pulse, a constant potential pulse of 0.04 V was applied to the electrode for 50 s. Potentiostatic double-pulse was performed by applying a short (5 ms) potential pulse of 0.04 V followed by a longer (50 s) potential pulse of 0.24 V. After electrodeposition step, the modified electrodes (Ag NPs/CILE) were washed with distilled water and used in electrochemical measurements. Electrodes surfaces were renewed and modified with Ag NPs before each experiment. III. R ESULTS AND D ISCUSSION A. Electrochemical Behavior of Silver on CILE Fig. 1 represents consecutive cyclic voltammograms (CVs) of CILE in 1 mM AgNO3 /0.1 M KNO3 solution. In the first cycle (curve a), the cathodic peak observed at 0.17 V corresponds to the reduction of Ag+ to Ag, whereas the
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Fig. 3. (a) CVs recorded with CILE (a and b ) and Ag NPs/CILE (c and d ) in the absence (a and c ) and presence (b and d ) of 0.5 mM Na2 S. (b) CVs of Ag NPs/CILE in solutions containing 0 (a ), 0.25 (b ), and 0.5 (c ) mM of Na2 S. Britton-Robinson buffer (pH = 9) was used as supporting electrolyte, and voltammograms were recorded at scan rate of 50 mV s−1 .
anodic peak at 0.47 V (in the anodic sweep) is attributed to the oxidation of the electrochemically deposited Ag to Ag+ . As can be observed from curves b and c in Fig. 1, the cathodic peak shifted to more positive potentials in the second and third cycles, which is indicative of more facile electroreduction of Ag+ to Ag during these cycles. Indeed, residual Ag NPs remaining on the surface after stripping sweeps (anodic scans) facilitate further electroreduction of silver in the following cycles, which leads to the accumulation of more and more NPs on the surface as cycling is continued. Cyclic voltammetry can thus be employed as a technique for electrodeposition of Ag NPs on CILE. Fig. 2b shows SEM image of the Ag NPs prepared by consecutive cycling (10 cycles) of CILE in 1 mM AgNO3 / 0.1 M KNO3 solution. As can be seen, the application of this procedure did not result in a surface with high NPs surface density necessary for development of a sensitive sensor. The poor NPs surface density can be ascribed to the electrooxidation of some of the Ag NPs during the last anodic scan (from −0.1 V to 0.7 V) of the employed procedure (see Experimental section). In order to gain higher surface density of Ag NPs we used potentiostatic single-pulse method, which employs a pulse of high cathodic overpotential for a period of time. As can be seen in Fig. 2c, Ag NPs with mean diameter of ∼153 nm and high surface density were obtained in this case. Nevertheless, the produced NPs possessed large particle size distribution (the relative standard deviation of the particle diameter or RSDdia. = 33%), which may adversely affect the reproducibility of the sensor. The polydispersity in the Ag NPs array prepared using the single-pulse method is ascribed to the increasing overlap of the depletion zones of adjacent particles as they grow in size, which is a consequence of the high overpotential employed. In order to achieve narrower particle size heterogeneity, we used double-pulse technique in which a short pulse of high cathodic overpotential (nucleation pulse) is followed by a longer pulse of lower overpotential (growth pulse) [36], [37]. As can be observed in Fig. 2d, hemispherical-shape
nanoparticles with mean diameter of ∼180 nm and narrow particle size distribution (RSDdia. = 16%) were obtained by this method. Therefore, we chose potentiostatic double-pulse as the electrodeposition method for further experiments. B. Electrochemical Response of Sulfide at Ag NPs/CILE Fig. 3a compares cyclic voltammogram of bare CILE with that of Ag NPs/CILE recorded in 0.5 mM Na2 S solution. While broad redox peaks with E 0 = −0.20 V were observed at bare CILE (curve b ), Ag NPs/CILE exhibited different voltammetric features with a pair of redox peaks at E 0 = −0.65 V (curve d ). Electrochemical response of AgNPs/CILE in solutions of different sulfide concentrations showed that, upon increasing of sulfide concentration the intensity of the redox peaks at E 0 = 0.25 V (corresponding + to the Ag/Ag oxidoreduction) diminished, while the intensity of the peaks at E 0 = −0.65 V enhanced (Fig. 3b). The redox 0 peaks at E = −0.65 V are ascribed to the oxidoreduction of Ag NPs in the presence of sulfide according to the following reaction [33]: 2Ag + HS− + OH− Ag2 S + H2 O + 2e−
(1)
In fact, the high affinity of sulfide to metallic silver to form Ag2 S (K sp [Ag2 S] = 6 × 10−50 ) causes the oxidoreduction of Ag NPs to take place at more negative potentials (i.e., at E 0 = −0.65 V) in sulfide-containing solutions. Consequently, less Ag becomes available to undergo the oxidoreduction reaction at E 0 = 0.25 V, and thus the intensity of the Ag/Ag+ peaks decreases (Fig. 3b). Fig. 4a (inset) represents the plot of the Ag2 S formation peak current as a function of the square root of scan rate (ν 1/2 ), which was obtained using Ag NPs/CILE in 0.5 mM Na2 S solution. The anodic peak current was found to be linearly dependent on ν 1/2 in the range of 5-500 mV s−1 and suggested that the electrochemical oxidation of Ag to Ag2 S was limited by the diffusion of HS− toward the electrode surface. An investigation into the effect of pH on the Ag2 S formation
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Fig. 4. (a) CVs of Ag NPs/CILE in Britton-Robinson buffer (pH = 9)/0.5 mM Na2 S at different potential scan rates (scan rates of 5, 25, 50, 100, 200, and 400 mV s−1 are shown). Inset shows the plot of the anodic peak current versus the square root of scan rate (error bar represents standard deviation for five replicate experiments). (b) CVs of Ag NPs/CILE in 0.5 mM Na2 S/Britton-Robinson buffer at different pH values (CVs at pH 3, 6, 9, and 12 are shown; scan rate is 50 mV s−1 ). Inset shows the plot of the formal potential, E 0, versus pH (error bar represents standard deviation for three replicate experiments).
peak current and potential was carried out over the pH range of 3-12 by cyclic voltammetry, and the results are shown in Fig. 4b. Under acidic conditions, low anodic peak currents were detected, as volatile hydrogen sulfide (H2 S) gas is the dominant form of the analyte in acidic solutions. However, the anodic peak current increased with increasing the solution pH from 7 to 9, but decreased again upon further increasing of pH. In the case of the peak potential, the plot of the E 0 vs. pH showed two linear segments in the pH ranges from 3 to 7 and from 7 to 12 with slopes of −0.0537 and −0.0346 V pH−1 , respectively (inset of Fig. 4b). The relative number of electrons (n) to protons (m) involved in the electrochemical reaction in each range of pH can be estimated from the slope of the linear segments, given by −0.0591m/n (at 25 °C). Based on that, the Ag/Ag2 S redox reaction in the pH range between 3 and 7 was accompanied with the transfer of two electrons and two protons, whereas in the pH range from 7 to 12 two electrons and a single proton were involved in the electrochemical reaction. C. Determination of Sulfide With Ag NPs/CILE Fig. 5 represents differential pulse voltammograms recorded with Ag NPs/CILE in solutions of different sulfide concentrations. In order to achieve high sensitivity, the measurements were performed at pH 9, where the most intense Ag/Ag2 S anodic signal was observed (Fig. 4b). Under this condition, the plot of the DPV peak current versus Na2 S concentration was linear in the range of 25-2500 μM of sulfide (inset of Fig. 5). The regression equation was I = 0.1333C − 2.8607, where the current (I ) and the concentration (C) were measured in μA and μM, respectively. The theoretical detection limit (3σ ) was estimated as 6.02 μM, and the RSD for 10 replicate experiments was found to be 4.7%, which demonstrated the good reproducibility of the developed sensor. Table I summarizes analytical figures of merits of the proposed sensor and some of the recently reported electrochemical-based sulfide sensors. As it is apparent, the
Fig. 5. Differential pulse voltammograms of Ag NPs/CILE in Britton-Robinson buffer (pH = 9) containing different concentrations of Na2 S (25-2500 μM). Inset shows the corresponding calibration plot. Means of five replicate experiments are represented.
sensor developed in this work is superior to many of the previously reported sensors in terms of detection potential, sensitivity, and linearity of the calibration range. However, the detection limit reported in this work is high compared to that of the other sensors, which can be improved by employing stripping voltammetric techniques (where sulfide is first accumulated on the surface and then the formed silversulfide array is reduced during a stripping step). It is worth mentioning that the sensitivity of Ag NPs/CILE for sulfide analysis was higher than that of the bare-Ag electrode reported in [34], though the detection potential of the bare-Ag electrode was lower (less positive) than the detection potential of our sensor (Table I). D. Interference Study Selectivity is a critical factor in practical application of sulfide sensors. In order to assess the selectivity of the
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TABLE I A NALYTICAL F IGURES OF M ERITS FOR S OME OF THE R ECENTLY R EPORTED E LECTROCHEMICAL -BASED M ETHODS FOR D ETERMINATION OF S ULFIDE
TABLE II R ESULTS OF I NTERFERENCE S TUDY FOR D ETERMINATION OF 1 × 0−3 M S ULFIDE
proposed sensor, the effect of a number of inorganic and organic species (sulfite, thiosulfate, fluoride, chloride, bromide, iodide, thiocyanate, disulfate, cyanide, L-cysteine, and glutathione) on the electrochemical response of 1 mM sulfide was investigated. The change in the Ag/Ag2 S peak current upon individual addition of sulfite, thiosulfate, fluoride, chloride, and bromide at 100-fold concentration, thiocyanate, and disulfate at 10-fold concentration, and iodide, cyanide, L-cysteine, and glutathione at 1-fold concentration was
