LETTER pubs.acs.org/ac
Highly Selective Colorimetric Detection of Hydrochloric Acid Using Unlabeled Gold Nanoparticles and an Oxidizing Agent Suraj Kumar Tripathy, Ju Yeon Woo, and Chang-Soo Han* School of Mechanical Engineering, Korea University, Seoul, 136713, South Korea
bS Supporting Information ABSTRACT: We report a colorimetric system for the detection of HCl in aqueous environments using unlabeled gold nanoparticle (AuNP) probes. This nonaggregation-based detection system relies on the ability of chloro species to cause rapid leaching of AuNPs in an aqueous dispersion containing a strong oxidizing agent, such as HNO3 or H2O2. The leaching process leads to remarkable damping of the surface plasmon resonance peak of the AuNP dispersion. This method works only with AuNPs of a particular size (∼30 nm diameter). It is highly selective for HCl over several common mineral acids, salts, and anions. This simple and cost-effective sensing system provides rapid and simple detection of HCl at concentrations as low as 500 ppm (far below the hazard limit) in natural water systems.
W
ater pollution has become one of the most challenging issues for mankind. As rapid industrialization has increased the release of toxic chemicals into natural water systems,16 both the illegal dumping and accidental leakage of wastes from various industries have raised concern among environmentalists. Chlorine-containing inorganic acids in general, and hydrochloric acid (HCl) in particular, are among the most abundant water pollutants.7,8 Because of its extensive use, excellent stability, and high mobility in aqueous environments, HCl has the potential to acidify wetlands and water resources.9 However, a rapid and selective detection technique for HCl has not yet been developed. Here, we describe a colorimetric system for the detection of HCl in aqueous environments using unlabeled gold nanoparticle (AuNP) probes. This nonaggregation-based detection system exploits the fact that aqueous chloro species induce the rapid leaching of AuNPs in an aqueous dispersion containing a strong oxidizing agent, such as HNO3 or H2O2.10 This process leads to the marked damping of the surface plasmon resonance (SPR) peak of the AuNP dispersion. The simple and cost-effective technique is highly selective for HCl over several common mineral acids, salts, and anions and is useful for the rapid detection of HCl at concentrations as low as 500 ppm, far less than that deemed the hazardous limit in the near or long-term (3000 ppm and 1000 ppm, respectively).11 Colorimetric sensors are molecular or ionic sensors that signal analyte interaction through a change in color. They offer a simple and convenient platform for analyte detection, and AuNP sensors have received much interest as unique and tunable colorimetric sensors.12 The optical properties of AuNPs are dominated by SPR, the collective oscillation of electrons at their r 2011 American Chemical Society
surfaces, in resonance with the incident electromagnetic radiation. The SPR absorption wavelengths of AuNPs are highly dependent upon their size, shape, and refractive index, and thus any minor perturbation in their chemical environment, surface structure, or aggregation may lead to irreversible or reversible colorimetric changes of their dispersions.12 Appropriately functionalized ligands on the AuNP surface can induce a shift in the absorbance peak or can quench fluorescence in the presence of an analyte or environmental response.13,14 Current AuNP sensor applications include the detection of glucose, protein modifications, or metal ions.1517 Although these optical chemosensors have been of great value in ionic and molecular sensing, further research is needed to enable their practical application as environmental sensors. The simplicity and cost-effectiveness of colorimetric, nonaggregation-based sensors using unlabeled metal nanoparticles makes them very user-friendly.18 In one of the earliest attempts to develop a colorimetric sensor, Chen et al.19 developed a system for the detection of lead ions based on the leaching of unlabeled AuNPs. This idea was further extended by Wu et al.,20 who used the leaching kinetics of unlabeled AuNPs in the presence of pyrophosphate to develop an Fe3+ sensor. Because they are free of the requirements for complex labeling agents and special working environments, similar techniques should be a primary focus of sensor research. However, the development of molecular sensors for environmental systems is complicated due to the presence of multiple ions in the analytical system and the Received: September 21, 2011 Accepted: November 10, 2011 Published: November 10, 2011 9206
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for further use. For each sample tested, a 1 mL aliquot was added to each vial containing the AuNP dispersion. The vials were shaken well and then incubated for 30 min without manipulation. The colorimetric change was assessed visually and then by UVvis spectroscopy. The relative sensitivity is expressed by the following equation: S ¼ ðAbs0 AbsA Þ=Abs0 where the absorbance intensity at 530 nm (A530) of the oxidantcontaining AuNP dispersion in the absence and presence of the analyte is represented by Abs0 and AbsA, respectively. A schematic view of the reaction was expected to occur between AuNPs and HCl in the presence of a strong oxidizing agent as shown in Figure 1a. As a noble metal, gold is invulnerable to attack by any strong acid or base, but in the presence of a strong oxidant and chloride ions, it can be dissolved (leached) as the gold chloride species [Au(Cl)2h] or [Au(Cl)4h] in the following reaction:13 Au0 ðaqÞ þ nHþ þ nCl f ½AuðClÞx
Figure 1. (a) Leaching of Au NPs by HCl in the presence of strong oxidizing agents, (b) effect of addition of HCl and possible interfering chemicals on the Au NPs dispersion in presence of HNO3, and (c) effect of addition of HCl and possible interfering chemicals on the Au NPs dispersion in presence of H2O2.
possibility of various interfering reactions with common molecular species, such as salts and minerals. In the present study, we explored the utility of unlabeled AuNPs for the selective colorimetric detection of HCl in aqueous environmental systems. AuNPs dispersed in an aqueous medium were procured from Mijitech (Korea). HCl, HF, and H2S were from Junshi chemicals. Zn chloride, Sn chloride, Na perchlorate, Zn nitrate, and Na phosphate were from Sigma. All chemicals were of analytical grade and used without further purification. Deionized water (18.2 MΩ cm) was prepared using a Milli-Q water system. Ultravioletvisible (UVvis) spectra were recorded using a 1 mL quartz cuvette in a UVvis spectrophotometer (Scinco-SD 1000). The sensing process was performed as follows: 200 μL of the AuNP dispersion was placed in a 5 mL vial and diluted with 1 mL of deionized water. A known amount of the oxidizing agent (HNO3 or H2O2) was added, and the mixture was incubated for 30 min. UVvis spectra were taken to confirm that the addition of the oxidizing agent did not affect the SPR spectrum. Standard solutions of various mineral acids and other salts (100, 200, 500, 800, 1000, 2000, and 5000 ppm) were prepared and preserved
where x = 2 or 4 depending upon the concentration of chloride ion, pH, and nature of the oxidizing agent. The stability of the gold chloride ions depends upon the Au and HCl concentrations and the electrochemical potential (Eh) of the system.13 The above leaching reaction is precisely what occurs when Au is leached in aqua regia, which contains concentrated HNO3. The use of chlorine as an oxidant is well documented, with chlorination being a common method of recovering gold prior to the invention of cyanidation.21 However, this simple technique has not been previously exploited for the development of a chlorine or HCl sensor, possibly because AuNPs are stable at higher pH; the technique suggested by Chen et al.21 is applicable only at alkaline pH (pH 712). Furthermore, aqueous dispersions of AuNPs in the size range normally used for chemosensor applications (212 nm in diameter) are unstable at low pH; they are rapidly destroyed by strong acids. As shown in Figure 1S (see the Supporting Information), AuNPs (∼12 nm diameter) are readily dissolved by any of the mineral acids even in the absence of an oxidizing agent. Expecting that the stability of the AuNPs would be enhanced by increasing their size, we examined the stability of AuNP dispersions (2030 nm diameter) in the presence of different mineral acids (including HCl) and oxidizing agents and found that they were indeed more stable (Figure 1S in the Supporting Information). However, larger AuNPs (∼90 nm in diameter) yielded a less intense color, limiting their utility as sensing probes. The intensity of color due to the SPR effect of a AuNP dispersion decreases sharply as the particle size increases.22,23 As shown in Figure 1S in the Supporting Information, a dispersion containing ∼90 nm particles is light pink, whereas those containing ∼12 or ∼30 nm particles are much deeper in color, appearing wine-red. In addition, the larger particles do not disperse stably; they show a tendency to settle as the pH of the medium changes.24 Addition of analyte molecules to the larger AuNPs greatly reduces the intensity of the dispersion color, either because of the decreased intensity of the SPR phenomenon or because the larger particles aggregate at the lower pH.2224 Taking these findings into consideration, we selected AuNPs ∼30 nm in diameter as the basis of our HCl sensing probe. The colorimetric test for HCl detection was carried out at room temperature under atmospheric pressure. As shown in 9207
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Figure 2. UVvisible absorbance spectrum of Au NPs aqueous dispersion in the presence of a different concentration of HCl (a) 100, (b) 200, (c) 500, (d) 1000, (e) 2000, (f) 5000 ppm), challenged with (a) HNO3 and (b) H2O2. Effect of the the addition of different concentrations of HCl on the absorbance of Au NPs and relative sensitivity (c) in the presence of HNO3 and (d) in the presence of H2O2.
Figure 1a, the intense wine-red color of an aqueous dispersion of 30 nm AuNPs gradually decreased in intensity as increasing amounts of HCl were added in the presence of HNO3. The color change also occurred more rapidly as the HCl concentration
increased. At HCl concentrations greater than 2000 ppm, the dispersion became colorless within 1520 min. Experiments using H2O2 rather than HNO3 as the strong oxidizer yielded similar observations (Figure 1b), although the magnitude of the 9208
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Figure 3. (a) XRD pattern of test Au NPs before and after adding HCl, respectively. (b) XPS spectrum of test Au NPs with and without HCl, respectively.
Figure 4. Parts a, c, and e show the TEM images of test Au NPs and test Au NPs with 1000 and 2000 ppm of HCl, respectively. Parts b, d, and e show the corresponding particle size distribution graph.
color change was slightly decreased, and the color change process was slower, requiring up to 60 min for completion. At very low HCl concentrations, the process might be even further delayed. The reddish color of the AuNP dispersion in the HNO3 oxidant system was unaffected by the presence of mineral acids other than HCl or commonly encountered salts (Figure 1a); none of the other chemical species tested induced a color change comparable to that caused by HCl. These results demonstrate that the presence of the aqueous hydrochloric acid species leads to specific and rapid leaching of the AuNPs in the presence of a strong oxidant. To quantitatively evaluate the colorimetric change induced by HCl, we investigated the UVvis spectrum of the AuNP dispersion. As shown in Figure 2a, an aqueous dispersion of AuNPs displayed a distinct SPR absorbance peak at 530 nm. The addition of HCl red-shifted the absorbance peak to 535 nm and sharply decreased its intensity. We attribute the sharp decrease in
peak intensity to the leaching of the metallic AuNPs by HCl. With an increase in the concentration of this acid, the reduction in the peak intensity is more pronounced. A similar result was obtained when HNO3 was replaced with H2O2 (Figure 2b), but the peak was red-shifted to 540 nm rather than 535 nm because the H2O2 oxidant system exhibits less SPR damping. Liu et al.25 reported similar observations for a detection system using protein-capped, fluorescent AuNPs. In the presence of cyanide ions, the fluorescence spectrum was quenched by the leaching of the AuNPs in the aqueous system.25 In addition to a rapid decrease in the fluorescence intensity, they also observed a slight red-shift in the fluorescence spectrum, similar to our present findings. Comparative plots of acid concentration vs absorbance (and relative sensitivity) for our system using HNO3 or H2O2 as the oxidizing agent are shown in Figure 2c,d. The data were well fit by Hill’s sigmoid equation and yielded sensitivity values of 0.08, 0.27, 9209
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the peaks corresponding to metallic Au suggests the leaching of the metal in the presence of HCl and a strong oxidizing agent. To confirm these findings, we used X-ray photoelectron spectroscopy to examine the interaction between HCl and the AuNPs. The Au 4f7/2 spectrum of the virgin AuNPs (in the presence of HNO3) could be deconvoluted into two peaks centered at 81.0 and 84.5 eV, corresponding to the binding energies of Au0 and Au+, respectively;25 the addition of 5000 ppm HCl with HNO3 caused the intensity of both peaks to dramatically decrease and shifted them to 81.2 and 86.5 eV, respectively. This result indicates that HCl induced the oxidation of the AuNPs from the Au0 to the Aun+ state in the presence of the HNO3 oxidant (Figure 3b). Wang et al.26 observed a similar change in peak position during the formation of Au3+ ions. We investigated the morphological change associated with the sensing reaction using transmission electron microscopy. As shown in Figure 4 (boxes), before the addition of HCl, near spherical AuNPs ranging in size from 25 to 35 nm in diameter were clearly visible. However, after the addition of 1000 or 2000 ppm HCl, both the size and number of the AuNPs decreased significantly. All the above experimental results are in good agreement with the suggested leaching mechanism. Two common approaches are employed to detect HCl in the aqueous environment, neither of which can discriminate among chloride ion sources.27,28 One approach is Mohr’s test, a two-step process in which the pH of the system is first adjusted to an acidic pH and then AgNO3 is added. In the presence of HCl, the AgNO3 forms a AgCl precipitate in the following reaction: AgNO3 þ Mx Cly ðaqÞ f AgClðprecipitateÞ
Figure 5. (a) Effects of different concentrations of HCl and other commonly encountered chemicals on the relative damping of the SP spectrum of Au NPs dispersion (b) Effects of a concentration of HCl (5000 ppm) and other commonly encountered chemicals on the relative damping of the SP spectrum in various water samples.
and 0.98 for HCl concentrations of 500, 1000, and 5000 ppm, respectively. The lower limit for the detection of HCl is about 500 ppm, which is less than the hazardous limit in environmental water sources.11 To better understand the sensing mechanism of our system, we used X-ray diffraction (XRD) to examine the structural changes associated with the sensing reaction. After HNO3-containing AuNP dispersions were carefully prepared with and without HCl, equal amounts were deposited on a quartz slide for analysis. As shown in Figure 3a, the virgin AuNPs showed three clear peaks at 2θ = 37.8, 44.51, and 64.60 corresponding to (111), (200), and (220) planes of metallic Au with a face-centered cubic structure. Analysis using Scherrer’s formula yielded a mean crystallite diameter of 27 nm. As clearly shown in Figure 3a, the XRD pattern of the samples changed dramatically with the sensing reaction. The disappearance of
where M can be H, Sn, Zn, Cu, or another metal and x and y represent the valences of elements M and Cl, respectively. The other common approach to HCl detection is quantitative analysis by acidbase titration.29,30 The time-consuming and nonspecific nature of both of these traditional detection techniques justifies the development of alternative, nanoparticle-based detection techniques. Most optical chemosensors are readily disturbed by the presence of contaminating salts or ions (anions, in particular). To investigate the specificity of our system for HCl, we measured its absorbance response to three other acids (H2S, HF, and CH3COOH), five common salts (Zn chloride, Sn chloride, Na chloride, Fe3+ chloride, and Na iodide), and three anions (phosphate, chlorate, and nitrate). As clearly shown in Figure 5a, none of these ions or salts induced remarkable change in the intensity and/or position of the SPR peak AuNP dispersion. Only HCl induced a notable damping of the absorbance spectrum (Figure 5b), whereas no significant effects were noted from the presence of any of the other chemical species. However, we should note that the SPR-based spectral damping event was slightly affected by the presence of Fe3+ chloride. A similar result was observed by Zhang et al.31 during their detection of iodide using CuAu core/shell nanoparticles. The color change induced by Fe3+ ions was easily detectable by the naked eye. To evaluate whether the AuNP-based sensing system discussed here is applicable to natural systems, we collected local samples of ground, tap, river, and seawater for analysis with this system. In the absence of added HCl, the addition of these realworld water samples had no significant effect on the signal (Figure 5a,b); thus, like deionized water, these water samples did not appear to affect the performance of the sensing system. 9210
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of HCl in marine ecosystems prone to industrial pollution. Furthermore, this technique could be extended to the detection of HNO3 or H2O2 in aqueous environments (see Figure 3S in the Supporting Information). We are currently developing systems for the selective detection of other mineral acids and common anions.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by Basic Research Fund of Korea Institute of Machinery & Materials (KIMM), and partially supported by Center for Advanced Soft Electronics in Global Frontier Program and Center for Nanoscale Mechatronics and Manufacturing in Frontier Program from MEST, Korea. ’ REFERENCES
Figure 6. (a) Effect of the addition of artificial effluent to the test Au NPs followed by the addition of blind HCl samples and (b) determination of the amount of HCl from the graph.
On the other hand, the addition of real-world water samples charged with 5000 ppm of HCl led to a significant change in the absorbance value; interestingly, all of the water samples yielded comparable absorbance changes in this experiment. (SP damping data for several water samples are shown in Figure 2S in the Supporting Information.) In further investigations, we attempted to demonstrate the quantitative analysis of HCl in artificial wastewater samples prepared by mixing together 500 ppm of each of the various chemical species investigated in this paper. As shown in Figure 6a, addition of this artificial effluent did not change the SPR peak of the AuNPs even after 30 min. We then used our sensing system to analyze two different artificial effluent samples containing an unknown amount of HCl (blind HCl samples). The addition of the blind HCl samples damped the SPR absorbance peaks, as expected. A fit of the absorbance values to Hill’s sigmoid curve (Figure 6b) showed the concentrations of HCl to be 900 and 1050 ppm. In summary, we have demonstrated a technique for the selective detection of HCl in an aqueous environment based upon the damping of AuNP SPR absorbance by HCl-triggered leaching. As this technique does not involve any labeling, it is simple and user-friendly. It demonstrates good selectivity in the presence of commonly encountered salts and anions and can be used to directly analyze natural water systems. These features make this system a potentially powerful tool for the investigation
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