Article pubs.acs.org/ac
Colorimetric Method for Determining Pb2+ Ions in Water Enhanced with Non-Precious-Metal Nanoparticles Jeffrey Yan† and Erik M. Indra* YTC (Yazaki Technology Center) America Inc., 3401 Calle Tecate, Camarillo, California 93012, United States ABSTRACT: Sulfur anions and their derivatives have long been recognized for their high selectivity and reactivity toward Pb2+ ions and formation of highly absorptive yet water-insoluble compounds with both acid and base media. This phenomenon has been used for qualitative analysis of lead ions in water. We demonstrate a new method to quantitatively determine the Pb2+ concentration in the range of 0.5− 500 ppm in water using colorimetric measurement, based on forming “soluble” lead sulfide in water enhanced with non-precious-metal nanoparticles. This method has inherent high selectivity for lead over other alkali-metal and alkaline-earth-metal ions. The colorimetric measurements of the absorptive solutions provide accurate determination of the lead concentration in water comparable to that measured using inductively coupled plasma mass spectrometry. To our knowledge, this is the simplest, lowest cost, and easiest-to-use method for detecting and determining the lead concentration in water.
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the incident electromagnetic radiation.16,17 The resonance frequency of AuNPs, governed by their bulk dielectric constant, lies in the visible region of the electromagnetic spectrum. The Plasmon frequency is exquisitely sensitive to any surface change of these particles and results in a color change of the dispersions when AuNP interacts with other species. The selectivity to specific heavy metal ions was improved significantly by modifying the AuNP surface with DNAzyme, mercaptonicotinic acid, cysteine, or fluorescent dyes.18−21 Although this method provides a high sensitivity for detecting metal ions, it comes with a high cost for materials, and its application is currently limited to certain researchers in highly specific fields. We report, for the first time, a low-cost and easy-to-use colorimetric method to determine lead in solution with a lead content as low as 0.5 ppm to as high as 500 ppm. This sensitivy range encompasses the lead leachable limit set by the FDA for testing ceramic dishware. This sensitive and easy-to-use method is based on forming a very stable and dispersible lead sulfide solution by reacting lead with sulfide ions in the presence of stabilizing non-precious-metal nanoparticles. Lead sulfide has a distinct dark color and is highly insoluble in water and organic solutions at different pH ranges. Lead sulfide has almost the lowest solubility product constant of 7 × 10−28 in comparison to other lead salts.22 Therefore, it was thought impossible to detect lead, by spectrophotometrically measuring absorption of lead sulfide, due to the high degree of precipitation of the compound in water.
f water is contaminated with a trace amount of heavy metal ions, such as Pb2+, Cd2+, or Hg2+, the contaminated water is colorless and tasteless yet poses dangerous health hazards to both children and adults.1−3 The U.S. Food and Drug Administration (FDA) set guidelines for the maximum amount of lead that can be leached out from ceramics, mugs, and flat dish plates to be between 0.5 and 30 ppm.4 Currently the amount of lead and heavy metal ions in solutions is mainly determined by atomic absorption or emission spectrometry methods, such as atomic adsorption spectroscopy (AAS) and inductively coupled plasma (ICP).5−8 However, due to the high costs for both the equipment and measurement, determining the heavy metal in solutions by using AAS or ICP is impractical in certain situations. A simpler and less expensive method for detecting and determining lead ions has been reported by using the colorimetric method. Lead ions are invisible in the visible spectral region due to its filled d orbital. As a result, it can only be probed by coordination to an active chelate reagent.6,9 The main reagents available for the spectrophotometric or colorimetric determination of lead are dithiazone, diethyldithiocarbamate, diphenylcarbazone, and porphyrin compounds. Although each chromogenic system has its advantages and disadvantages with respect to sensitivity, selectivity, stability, and reliability, most of them require extraction using organic solvents, surfactants, or cyanide to increase the sensitivity or selectivity.10 Most of the reagents are light sensitive, and the quantitative analysis is time dependent. Recently a renewed interest in detecting heavy metal ions using the colorimetric method has emerged using functionalized Au nanoparticles (AuNPs).11−15 The optical properties of AuNPs are dominated by collective oscillation of electrons at the surface known as localized surface Plasmon resonance that is in resonance with © 2012 American Chemical Society
Received: April 17, 2012 Accepted: June 11, 2012 Published: June 11, 2012 6122
dx.doi.org/10.1021/ac301018y | Anal. Chem. 2012, 84, 6122−6127
Analytical Chemistry
Article
inhaled and irritating to the eyes and skin, so personal protective equipment (PPE), such as respirators, lab coat, chemical-resistant gloves, and safety goggles, should be worn to minimize the chances of exposure during handling. Solution mixing and handling were done in a ventilated fume hood, and all chemicals were stored in tightly closed containers when not in use. A Varian Cary 500 ultraviolet−visible−near-infrared (UV− vis−NIR) spectrophotometer was used for all colorimetric measurements of Pb2+ in water. The sample holder used for the measurement was a 10 × 10 × 35 mm quartz cell which has ≥90% transmittance of light in the wavelength range of 300− 1000 nm. The baseline was collected on a cell containing the same amount of nanoparticle stabilizing solution and sodium sulfide in 4% by volume acetic acid solution. Inductively coupled plasma mass spectrometery (Perkin-Elmer Elan 9000) was used to validate the lead concentration measured by the UV−vis−NIR spectrophotometer. The ICPMS analysis was performed by NSL Analytical Services, Inc. (Cleveland, OH). About 1 g of leachate solution was acidified with a HNO3/HF/ H2O mixture and diluted to a final volume of 100 mL. A fivepoint calibration curve obtained from National Institute of Standards and Technology (NIST) traceable standard solutions was used for the analysis. Scanning electron microscopy (SEM) samples were prepared by placing a drop of lead sulfide solution stabilized with nanoparticles onto a holey carbon film coated 300 mesh copper grid (HC300-Cu, Electron Microscopy Sciences). The lead sulfide coated grid was dried at room temperature overnight. SEM imaging at up to 950000× was performed using a JEOL JSM-7500F field emission (FE) scanning electron microscope equipped with an energydispersive X-ray (EDX) spectrometer (Noran System 6, Thermo Electron Corp.). Images were acquired under a working voltage of 20 kV and working distance of 10 mm. Colorimetric Method for Determining the Lead Concentration in Water. Testing solutions (5 mL) with different amounts of lead ions were added to a 50 mL beaker and mixed with 500 μL of nanoparticle stabilizing solution, and the resulting solution was stirred at 200 rpm for 2 min. Sodium sulfide solution (250 μL, 0.2 M) was added, and the solution was continuously stirred for 5 min. A 3.5 mL portion of the solution was transferred into a quartz cell and placed in the UV−vis−NIR spectrophotometer compartment for absorption measurement (Figure 2). A calibration equation was established
If equal molar amounts of sulfide and lead ions are present in water, the soluble amount of lead ions will be as low as 5.5 × 10−9 mg/L (ppm). Reaction with sulfide converts almost 100% of the colorless lead ions into highly absorptive lead sulfide. If the absorption intensity of the lead sulfide could be measured, it would provide a very sensitive method for detecting lead compounds in solution. We report, for the first time, a method to determine the lead concentration by forming stable and dispersible lead sulfide in situ in the test solutions in the presence of nanoparticles. Normally, precipitation of lead sulfide in a solution occurs rapidly within a few seconds to minutes; however, the addition of nanoparticles to the testing solution significantly increases the surface area and results in formation of dispersible lead sulfide in the solution (Figure 1).
Figure 1. Three solutions containing lead were first stabilized with silica nanoparticles and then reacted with sulfide to form absorptive, transparent, and stable lead sulfide in water (left). Solutions with the same concentration instantly precipitated in water with acid (center) and base (right) media without nanoparticles. Each set of three solutions contained 17.6 (left), 88.7 (center), and 433 (right) ppm lead sulfide.
Adding nanoparticles avoids the aggregation and precipitation of lead sulfide and consequently enables sensitive and reliable detection of lead by using spectrophotometric or colorimetric methods. The nanoparticles used for this application are silica, alumina, or polystyrene nanoparticles. These non-preciousmetal nanoparticles are readily available from different suppliers at a very low cost, compared to gold (Au) or silver (Ag) nanoparticles. These nanoparticles are stable in acid and base solutions and a variety of organic solvents.
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EXPERIMENTAL SECTION Materials and Instrumentation. Acetic acid (4% by volume) in water was prepared by adding 105 g of analytical grade acetic acid (99.9%, Aldrich) in 2400 g of deionized water and stirred at 200 rpm for 20 min. Water containing 4% acetic acid has a pH of about 2.3. Eight lead calibration solutions, varying from 0.1 to 501.7 ppm, were prepared by dissolving Pb(NO3)2 (99.9%, Aldrich) in 4% by volume acetic acid solution. The 4% by volume acetic acid solution is the standard leaching solution used for the heavy metal ion leaching test for ceramic dishware set by the FDA.23 The nanoparticle stabilizing solution was prepared by adding 30 g of 31 wt % silica nanoparticles (10−15 nm) in 2-propanol to 70 g of 4% acetic acid solution and stirring at 200 rpm for 20 min. The silica nanoparticles were IPA-ST supplied by Nissan Chemical Inc., and the pH of the solution was about 2.7 in 2-propanol. Sodium sulfide (0.2 M) was prepared by adding 5.1966 g of sodium sulfide (60 wt %, Aldrich) to 196.679 g of deionized water and stirring at 200 rpm for 30 min. Pb(II) compounds are known to be harmful (toxic to the nervous system) if
Figure 2. Procedure for measuring lead sulfide absorption with a nanoparticle stabilizing solution. 6123
dx.doi.org/10.1021/ac301018y | Anal. Chem. 2012, 84, 6122−6127
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for the absorption intensity at fixed wavelength versus the lead concentration in the solutions. For unknown lead-containing solutions, the spectrophotometric measurement was conducted in exactly the same way. The lead concentration was determined by using the absorption intensity at the given wavelength and comparing with those obtained by ICPMS measurements (refer to the section “Measuring the Lead Content in Real Tests and Comparing with ICPMS Results”). All spectrophotometric measurements were carried out about 8−10 min after addition of sodium sulfide solution to the nanoparticle-stabilized lead solutions. Measuring the Lead Content in Real Tests and Comparing with ICPMS Results. Two aqueous solutions were obtained by leaching lead-containing ceramic dishware with 4% by volume acetic acid in deionized water at 20 and 90 °C for 24 h. For each leaching test, the dishware was filled with 200 mL of acetic acid solution in a sealed quartz container. Leaching at 20 °C is the FDA standard test condition, and the test at 90 °C is an aggressive test to simulate the conditions of a slow-cooking process. The glaze composition and thickness of the ceramic dishware were analyzed with EDX and SEM. One set of the two leachate solutions containing 10 mL of solution each was evaluated for lead concentration using the ICPMS method. The other set of solutions was stabilized with colloidal nanoparticles, and the lead content was measured using the colorimetric method.
Figure 4. Visual appearance of 0.1−443 ppm lead ions reacted with sodium sulfide in the presence of nanoparticle stabilizing solution. The light path is 1 and 3.5 mm for the bottom and top pictures, respectively. The lead sulfide concentration from left to right is 0.1, 0.4, 4.3, 17.6, 44.7, 88.7, 175, and 433 ppm.
lead reacted with sodium sulfide in the presence of nanoparticles in the solution. The 0.4 ppm lead sulfide solution produced a visible light gray color. With increasing lead concentration, the solution changed color from light gray to deep gray and dark black. Since no large particles were produced in the solution, and consequently the solution did not cause light scattering, all testing solutions were transparent with different levels of light absorption in the tested wavelength range. Figure 5 shows the absorption spectra of 0.1−443 ppm lead ions reacted with sodium sulfide in the presence of the nanoparticles in the solutions. The absorption intensity at 425 nm, as demonstrated in Figure 3, is independent of the reaction time and shows a linear increase with an increase in the lead concentration. The lower wavelength also provided a relatively high signal-to-noise (S/N) ratio. For example, the S/N ratio is >60 at 425 nm for 0.1 ppm lead compared to the lead-free blank solution. However, the absorption becomes quickly saturated with a lead concentration of 115 ppm. The absorption intensity at lower wavelength (
